Note: Descriptions are shown in the official language in which they were submitted.
CA 02400046 2002-08-13
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CHROMIUM-BASED CATALYSTS AND PROCESSES
FOR CONVERTING HYDROCARBONS TO SYNTHESIS GAS
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit under 35 U.S.C. ~ 119(e) of U.S.
Provisional
Application No. 60/183,423 filed February 18, 2000 and is a continuation-in-
part of co-
pending U.S. Application No, 09/703,701 filed November 1, 2000. This
application is
also related to U.S. Provisional Application No. 60/183,575 filed February 18,
2000,
which corresponds to co-pending U.S. Non-Provisional Patent Application No.
filed
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to catalysts and processes for the catalytic
conversion
of hydrocarbons (e.g., natural gas) using chromium-based catalysts to produce
carbon
monoxide and hydrogen. More particularly, the invention relates to such
catalysts and
their manner of making, and to processes employing the catalysts.
Description of Related Art
Large quantities of methane, the main component of natural gas, are available
in
many areas of the world, and natural gas is predicted to outlast oil reserves
by a significant
margin. However, most natural gas is situated in areas that are geographically
remote
from population and industrial centers. The costs of compression,
transportation, and
storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on
methane as a starting material for the production of higher hydrocarbons and
hydrocarbon
liquids. The conversion of methane to hydrocarbons is typically carned out in
two steps.
In the first step, methane is reformed with water to produce carbon monoxide
and
hydrogen (i.e., synthesis gas or "syngas"). In a second step. the syngas is
converted to
hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels
that boil in
the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon
waxes.
Current industrial use of methane as a chemical feedstock proceeds by the
initial
conversion of methane to carbon monoxide and hydrogen by either steam
reforming,
which is the most widespread process, or by dry reforming. Steam reforming
currently is
the major process used commercially for the conversion of methane to synthesis
gas,
proceeding according to Equation 1.
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CH,~ + HBO t~ CO + 3H2 (1)
Although steam reforming has been practiced for over five decades, efforts to
improve the energy efficiency and reduce the capital investment required for
this
technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane
to
syngas is also a process known in the art. While currently limited as an
industrial process,
partial oxidation has recently attracted much attention due to significant
inherent
advantages, such as the fact that significant heat is released during the
process, in contrast
to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched
air, or
oxygen, and introduced to a catalyst at elevated temperature and pressure. The
partial
oxidation of methane yields a syngas mixture with a HZ:CO ratio of 2:1, as
shown in
Equation 2.
CH4 + 1/202 r~ CO + 2H2 (2)
This ratio is more useful than the HZ:CO ratio from steam reforming for the
downstream conversion of the syngas to chemicals such as methanol and to
fuels. The
partial oxidation is also exothermic, while the steam reforming reaction is
strongly
endothermic. Furthermore, oxidation reactions are typically much faster than
reforming
reactions. This allows the use of much smaller reactors for catalytic partial
oxidation
processes. The syngas in turn may be converted to hydrocarbon products, for
example,
fuels boiling in the middle distillate range, such as kerosene and diesel
fuel, and
hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.
The selectivities of catalytic partial oxidation to the desired products,
carbon
monoxide and hydrogen, are controlled by several factors, but one of the most
important
of these factors is the choice of catalyst composition. Difficulties have
arisen in the prior
art in making such a choice economical. Typically, catalyst compositions have
included
precious metals andlor rare earths. The large volumes of expensive catalysts
needed by
prior art catalytic partial oxidation processes have placed these processes
generally outside
the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation
process
must be able to achieve a high conversion of the methane feedstock at high gas
hourly
space velocities, and the selectivity of the process to the desired products
of carbon
monoxide and hydrogen must be high. Such high conversion and selectivity must
be
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achieved without detrimental effects to the catalyst, such as the formation of
carbon
deposits ("coke") on the catalyst, which severely reduces catalyst
performance.
Accordingly, substantial effort has been devoted in the art to the development
of catalysts
allowing commercial performance without coke formation.
A number of process regimes have been described in the art for the production
of
syngas via catalyzed partial oxidation reactions. The noble metals. which
typically serve
as the best catalysts for the partial oxidation of methane, are scarce and
expensive. The
widely used, less expensive. nickel-based catalysts have the disadvantage of
promoting
coke formation on the catalyst during the reaction, which results in loss of
catalytic
activity.
U.5. Pat. No. 5,149.516 discloses a process for the partial oxidation of
methane
comprising contacting methane and a source of oxygen with a perovskite of the
formula
AB03, where B can be a variety of metals including Cr. In the example shown,
the
perovskite that was used is LaCo03. M. Stojanovic et al., (J. Catal. (1997)
166 (2), 324-
332) disclose the use of chromium-containing ternary perovskite oxides,
LaCr~_,~NiX03 (x
= 0 to 1.0) as catalysts for the partial oxidation of methane to syngas. The
catalytic
activity was found to increase monotonically with the value of x, i.e., LaCrO~
was found
to be the least active catalyst.
U.S. Pat. No. 5,447.705 also discloses a process for the partial oxidation of
methane to syngas by contacting the starting materials with a catalyst having
a perovskite
crystalline structure and having the composition LnXA,_YBY03, in which x is a
number such
that 0<x<10, y is a number such that 0<y<1, Ln is at least one of a rare
earth, strontium or
bismuth, A is a metal of groups IVb, Vb, VIb, VIIb or VIII, A is a metal of
groups IVb,
Vb, VIb, VIIb or VIII and A and B are two different metals. Various
combinations of La,
Ni and Fe were exemplified.
U.S. Pat. No. 5,149,464 discloses a method for selectively converting methane
to
syngas at 650°C to 950°C by contacting the methane/oxygen
mixture with a solid catalyst,
which is either: (a) a catalyst of the formula MxM'yOZ where: M is at least
one
element selected from Mg, B. Al, Ln, Ga, Si, Ti, Zr and Hf; Ln is at least one
member of
lanthanum and the lanthanide series of elements, M' is a d-block transition
metal, and each
of the ratios x/z and y/z and (x+y)/z is independently from 0.1 to 8; or (b)
an oxide of a d
block transition metal; or (c) a d-block transition metal on a refractory
support ; or (d) a
catalyst formed by heating a) or b) under the conditions of the reaction or
under non
oxidizing conditions. The d-block transition metals are selected from those
having atomic
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number 21 to 29, 40 to 47 and 72 to 79, the metals Sc, Ti, Va, Cr, Mn, Fe, Co,
Ni, Cu, Zr,
Nb, Mo, Tc, Ru, Rh, Pa, Ag, Hf, Ta, W, Re, Os, Ir, Pt and Au. Preferably M' is
selected
from Fe, Os, Co, Rh, Ir, Pd, Pt and particularly Ni and Ru.
U.S. Pat. No. 5,431,855 describes a catalyst which catalyzes the combined
partial
oxidation-dry reforming reaction of a reactant gas mixture comprising CO~, OZ
and CH4 to
for a product gas mixture comprising CO and H~. Related patent U.S. Pat. No.
5,500,149
describes similar catalysts and methods for production of product gas mixtures
comprising
HZ and CO.
U.S. Pat. No. 2,942,958 discloses an improved method for converting methane to
carbon monoxide and hydrogen employing a reforming catalyst for the steam-
methane
reaction. Although it is stated that any reforming catalyst is suitable for
the process, the
preferred catalysts are nickel, chromium and cobalt, or their oxides.
U.S. Pat. No. 4,843,181 discloses a process for preparing Cr~03 that includes
pyrolysis of ammonium dichromate. The chromium oxide is employed in a process
for
manufacturing 1,1,1-trifluorodichloroethane and 1,1,1,2-
tetrafluorochloroethane. U.S. Pat.
No. 5,036,036 discloses an improved Cr203 catalyst composition, prepared by
pyrolysis of
ammonium dichromate, which is useful in hydrofluorination reactions.
An example of the previous attempts at synthesis gas production by catalytic
partial oxidation to overcome some of the disadvantages and costs of steam
reforming are
described in EP303438 entitled "Production of Methanol from Hydrocarbonaceous
Feedstock." The asserted advantages of EP303438 are relatively independent of
catalyst
composition, i.e., "partial oxidation reactions will be mass transfer
controlled.
Consequently, the reaction rate is relatively independent of catalyst
activity, but dependent
on surface area-to-volume ratio of the catalyst." A monolith catalyst is used
with or
without metal addition to the surface of the monolith at space velocities of
20,000-500,000
hr-1. The suggested metal coatings of the monolith are selected from the
exemplary list of
palladium, platinum, rhodium, iridium, osmium, ruthenium, nickel, chromium,
cobalt,
cerium, lanthanum, and mixtures thereof in addition to metals of the groups
IA, IIA, III,
IV, VB, VIB, or VIIB. An exemplary catalyst comprises alumina on cordierite,
with a
coating comprising platinum and palladium. Steam is required in the feed
mixture to
suppress coke formation on the catalyst. Products from the partial oxidation
of methane
employing these catalysts results in the production of significant quantities
of carbon
dioxide, steam, and CZ+ hydrocarbons.
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None of the prior art processes or catalysts describes a completely
satisfactory
catalyst or process capable of high conversion and high selectivity for CO and
HZ products
and which are capable of operation with very low coke formation. Accordingly,
there
remains a need for a process and catalyst for converting hydrocarbons,
particularly
methane, that have low coke formation, high conversions of methane and high
selectivities
to CO and H2, and which are economically feasible at commercial-scale
conditions.
SUMMARY OF THE INVENTION
Many of the shortcomings of conventional syngas manufacturing methods are
overcome by the processes and catalysts of the present invention. The
preferred
chromium-based catalysts provide higher levels of activity (i.e., conversion
of CH4) and
high selectivity to CO and H~ reaction products than is typically available
with
conventional catalytic systems designed for commercial-scale use. Another
advantage of
the catalytic compositions and syngas production processes of the invention is
that no
appreciable coking occurs with use of many of the chromium-containing catalyst
compositions. Still another advantage of the new catalysts and processes is
that they are
more economically feasible for use in commercial-scale conditions than
conventional
catalysts now used for producing syngas. Some catalyst compositions containing
higher-
melting-point pure ceramic oxides instead of metals, demonstrate improved
catalyst life
when used for production of syngas.
In accordance with one aspect of the invention, a process for the catalytic
conversion of a hydrocarbon feedstock to syngas is provided. Conversion of the
hydrocarbon is achieved by contacting a feed stream comprising the hydrocarbon
feedstock and an oxygen-containing gas with a chromium-based catalyst in a
reaction zone
maintained at conversion-promoting conditions effective to produce an effluent
stream
comprising carbon monoxide and hydrogen. In accordance with another aspect of
the
invention is provided catalyst compositions comprising a chromium-containing
compound
optionally combined with at least one metal selected from the group consisting
of Group 1,
Group 2, Group 11 and Group 12 of the periodic table of the elements; a metal
with an
atomic number of 57 through 71; Co, Ru, Rh, Pd, Ir, Pt, Al, Ti, Y and Zr, and
optionally
Si. The preferred compositions do not have a perovskite structure. Yet another
aspect of
the present invention includes methods of making the new chromium-based
catalytic
compositions.
As discussed in more detail below, many of the new chromium-based catalysts
exhibit high methane oxidation activities and selectivities to syngas (CO and
HZ) in a
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millisecond contact time reactor. The low light-off temperatures of these
materials (i.e.,
less than 650°C) and superior performance are indicative of the more
preferred catalytic
compositions. Pure chromium oxide catalysts, and chromium catalysts containing
rare
earth oxides show little or no carbon or coke build-up after reaction with
CH~/O2. Trends
S in light-off temperature appear to correlate with the basicity or ionicity
of the rare earth
components, which may, in turn, relate to trends in C-H activation. Chromium
oxide-
based catalysts containing cobalt show carbon deposition on the reduced cobalt
metal
particles which are formed in situ.
In accordance with certain embodiments of the present invention, a chromium-
based composition for catalyzing the conversion of a C1-C5 hydrocarbon to form
a product
gas mixture containing CO and HZ is provided. The composition comprises about
0.1-
100 mole % of chromium or chromium-containing compound per total moles of
metal or
metal ion in the composition. The composition also includes at least one other
elemental metal or metal-containing compound, the metal of which is Li, Na, K,
Rb, Cs,
Mg, Ca, Sr, Ba, Cu, Ag, Au, Zn, Cd, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,
Er, Tm, Yb,
Lu, Co, Ni, Ru or Rh. In some embodiments the composition may also contain an
oxidatively and thermally stable porous support. Preferably the chromium-based
composition does not have a perovskite structure. In certain embodiments which
include a
porous material, or support, the porous material may include at least one
oxide or
oxyhydroxide of a metal such as magnesium, silicon, titanium, tantalum,
zirconium or
aluminum.
In some embodiments of the catalyst compositions the chromium or chromium-
containing compound comprises about 10-100 mole % of the total moles of metal
or metal
ion in said composition. In some embodiments the catalyst composition
initially
comprises a catalyst precursor comprising a mixed metal oxide, and after
reaction in a
syngas reactor, the catalyst finally comprises reduced metal and metal oxide.
In some of
these embodiments, the catalyst precursor comprises CoCrz04, the reduced metal
is zero
valent cobalt metal, and the metal oxide is Cr203. Some of these compositions
finally
comprise, after exposure to reaction conditions for a period of time, metal
oxide and
substantially no deposited carbon.
In some embodiments, the composition comprises a matrix structure which is a
xerogel or an aerogel. In some embodiments the matrix structure comprises at
least one
oxide or oxyhydroxide of a metal such as magnesium, silicon, titanium,
tantalum,
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zirconium or aluminum. Certain chromium-based compositions of the invention
have a
matrix structure comprising at least 30 wt %, preferably about 30-99.9 mole %,
and more
preferably about 50-97.5 mole % of the total moles (of metal) of the
composition. In
some embodiments the matrix structure comprises titanium oxideioxyhydroxide,
or
S magnesium oxideioxyhydroxide and silicon oxide/oxyhydroxide. In some
embodiments
the chromium-based composition also contains cobalt or a cobalt-containing
compound.
In some embodiments, the composition also includes lanthanum or a lanthanum-
containing compound. Certain catalytic chromium-based compositions contain
magnesium or a magnesium-containing compound and silicon oxideioxyhydroxide.
In certain embodiments, the chromium-based composition contains cerium or
samarium, or compounds containing those metals. There are some embodiments
that
include gold and aluminum oxide/oxyhydroxide, in addition to chromium or a
chromium-
containing compound. In other embodiments, the chromium-based catalytic
composition
comprises gold or a gold-containing compound and magnesium oxide/oxyhydroxide.
Still
other embodiments contain lanthanum and lithium, or compounds containing those
elements, and a-Ah03, in addition to chromium or a chromium-containing
compound.
Some embodiments of the chromium-based catalytic compositions comprise a
catalyst support, which may be oxidatively and thermally stable. The catalyst
support may
also be in the form of a porous three-dimensional monolith or it could be a
reticulated
ceramic or ceramic foam.
In accordance with another aspect of the invention, a process in provided for
preparing a chromium-based composition for catalyzing the partial oxidation of
a C~-C;
hydrocarbon to form a product gas mixture comprising CO and H,. This process
comprises combining about 0.1-100 mole % elemental chromium or chromium-
containing
compound per total moles of metal in the composition, together with,
optionally, at least
one other metal or metal oxide the metal component of which is Li, Na, K, Rb,
Cs, Mg,
Ca, Sr, Ba, Cu, Ag, Au, Zn, Cd, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,
Tm, Yb, Lu,
Co, Ni, Ru or Rh. Optionally the composition also contains at least one matrix-
forming
material such as an alkoxide of magnesium, silicon, titanium, tantalum,
zirconium or
aluminum. The process also includes forming the combination into a porous
solid. The
matrix-forming material may be at least 30 wt % of the total weight of said
composition
with said chromium compound and said at least one other metal compound. In
some
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embodiments the matrix-forming material comprises titanium or titanium oxide,
or a
combination of oxides or alkoxides of magnesium and silicon.
In some embodiments, the process also includes preparing an intermediate
composition containing the chromium or chromium-containing compound and at
least one
other metal or metal-containing compound. In this embodiment, the process
includes
applying the intermediate composition to a porous matrix material comprising
at least 30
wt % of the total weight of the composition. The porous matrix material may
comprise a
porous monolith support and the intermediate composition may be in the form of
a liquid
which is applied to the porous matrix material by impregnation. In some
embodiments,
the intermediate composition is dried, or calcined. Certain embodiments of the
process
provide for calcining the composition in situ under reaction conditions. In
some
embodiments the composition is formed by freeze-drying, spray drying or spray
roasting
the intermediate composition. In some embodiments, a powder is formed, which
may be
compressed into a pellet. Other embodiments of the process for making a
chromium-
based catalyst composition include forming an extrudate, or a gel such as a
xerogel or
aerogel.
In some embodiments, the process of making a chromium-based catalytic
composition employs a matrix-forming material comprising at least one metal
alkoxide.
The metal alkoxide may contain 1 to 20 carbon atoms, and in some embodiments
contains
1 to 5 carbon atoms. Some embodiments combine with the chromium or chromium-
containing compound at least one metal alkoxide that is a C1-C4 alkoxide such
as
tantalum n-butoxide, titanium isopropoxide or zirconium isopropoxide. In
certain
embodiments of the process of making the chromium-based catalyst composition,
the
process includes dissolving at least one of the metal alkoxides in a non-
aqueous medium
to form a metal alkoxide solution. In some of these embodiments, the metal
alkoxide
solution is mixed with a protic solvent, such as water, whereby the
alkoxide(s) reacts)
with the protic solvent to form a gel. Some embodiments include first
dissolving the
chromium or chromium-containing compound in the protic solvent to form a
protic
catalytic metal solution. In some alternative embodiments, the process may
include
dissolving or suspending the matrix material in the non-aqueous liquid medium
to form a
non-aqueous matrix solution or colloidal suspension. In some embodiments, the
process
may include dissolving at least one other elemental metal or metal-containing
compound
and one or more matrix-forming component in a non-aqueous medium.
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Certain embodiments of the process for making a chromium-based catalytic
composition provide for combining a protic solvent and an alkoxide in a molar
ratio of
about 5:1 to 53.1 or about 26.5:1. The process may include the gradual
addition and
mixing of sufficient protic solution to induce hydrolysis and condensation of
the metal
alkoxide(s). In certain embodiments the mixing comprises combining water and
the
alkoxide in a molar ratio of about 0.1:1 to 10:1 water:alkoxide. Some
embodiments
include combining water and zirconium alkoxide or titanium alkoxide in a molar
ratio of
about 4:1.
In accordance with yet another aspect of the present invention, processes are
provided for converting a Cl-CS hydrocarbon to form a product gas mixture
containing CO
and HZ. In certain embodiments the process comprises mixing a C~-CS
hydrocarbon-
containing feedstock and an oxygen-containing feedstock to provide a reactant
gas
mixture. The process includes contacting said reactant gas mixture with a
cataly~tically
effective amount of one of the above-described chromium-based catalyst
compositions.
During the catalyst/reactant gas contacting period, the composition and the
reactant gas
mixture are maintained at a temperature of about 600-1,100°C or about
700-1,000°C. The
catalyst composition/reactant gas system is also maintained at a pressure of
about 100-
12,500 kPa, preferably about 130-10,000 kPa, and the reactant gas mixture is
passed over
the catalyst composition at a continuous flow rate of about 20,000 to about
100,000,000
NL/kg/h, preferably about 50,000 - 50,000,000 NL/kg/h. In the most preferred
embodiments the reactant gas/catalyst composition contact time is 10
milliseconds or less.
Some embodiments of the syngas manufacturing process include mixing a methane
containing gas feedstock and an oxygen-containing gas feedstock to provide a
reactant gas
mixture having a carbon:oxygen ratio of about 1.25:1 to about 3.3:1, or about
1.3:1 to
about 2.2:1, or about 1.5:1 to about 2.2:1, preferably about 2:1.
In some embodiments of the hydrocarbon conversion processes, the oxygen-
containing gas further comprises steam, CO2, or a combination thereof. In some
embodiments the process comprises mixing a hydrocarbon feedstock and a gas
comprising
steam and/or COZ to provide a reactant gas mixture. In some embodiments the C,-
C;
hydrocarbon comprises at least about 50 % methane by volume of the reactant
gas
mixture, preferably at least about 75 %, and more preferably at least about 80
% methane
by volume of the reactant gas mixture. Certain embodiments of the processes of
making
syngas provide for preheating the hydrocarbon feedstock and/or the oxygen-
containing
feedstock before contacting the catalyst composition. In some embodiments the
reactant
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gases are preheated to temperatures up to about 700°C. In some
embodiments the catalyst
composition is in a fixed bed reaction zone.
One embodiment of the process of converting a hydrocarbon to methane and
hydrogen employs a particularly highly active and selective catalyst system.
This process
includes mixing a C~-CS hydrocarbon-containing feedstock and an oxygen-
containing
feedstock to provide a reactant gas mixture. The reactant gas mixture is
contacted with a
catalytically effective amount of a CoCr20a cubic spinel precursor dispersed
in a
chromium oxide matrix. During this contacting, the catalyst composition and
the reactant
gas mixture are maintained at a temperature of about 600-1,100°C. and
at a pressure of
about 100-12,500 kPa. The reactant gas mixture is passed over the catalyst
composition at
a continuous flow rate of about 20,000 to about 100,000,000 NL/kg/h. At least
a portion
of the catalyst precursor is reduced to cobalt metal (in a chromium oxide
matrix) by the
heated gases of the reactant stream. Some of the more preferred embodiments of
the
syngas production process achieve greater than 95 % CH4 conversion of the
hydrocarbon
in the reactant gas mixture, and at least about 97-98 % selectivity to CO and
Hz products.
Certain embodiments provide a process for converting a C~-CS hydrocarbon that
contains at least about 80 vol% methane to form a product gas mixture
comprising CO and
H2. This process may include mixing a methane-containing gaseous feedstock and
an
oxygen-containing gaseous feedstock to provide a reactant gas mixture having a
carbon:oxygen ratio of about 1.25:1 to about 3.3:1. The gaseous feedstocks are
preheated
and combined, and the reactant gas mixture is then contacted with a
catalytically effective
amount of a chromium-based composition containing about 10-100 mole % (as the
metal)
chromium or chromium-containing compound per total moles of metal or metal ion
in the
catalyst composition. The catalyst composition also contains 0-90% cobalt or
cobalt-
containing compound, and optionally, an oxidatively and thermally stable
porous support
supporting the chromium or chromium-containing compound and the cobalt or
cobalt-
containing compound. In preferred embodiments the catalyst composition
comprises a
structure other than a perovskite structure. During the gas/catalyst
contacting period, the
composition and reactant gas mixture are maintained at a temperature of about
600-
1,100°C and at a pressure of about 100-12,500 kPa. The reactant gas
mixture is passed
over the catalytic composition at a continuous flow rate of about 20,000 to
100,000,000
NL/kg/h, preferably ensuring a reactant gas/catalyst composition contact time
of no more
than about 10 milliseconds.
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In some embodiments of the syngas manufacturing methods the catalytic
composition is nominally 0.8 mole % in elemental chromium or chromium ion and
0.2
mole % in elemental cobalt or cobalt ion. In other embodiments the composition
is
nominally 0.2 mole % in elemental chromium or chromium ion and 0.8 mole % in
elemental cobalt or cobalt ion. In still other embodiments the composition is
nominally
0.5 mole % in elemental chromium or chromium ion and 0.5 mole % in elemental
cobalt
or cobalt ion.
Certain embodiments of the processes for converting a hydrocarbon to yield CO
and H~ employ a catalytic composition that is nominally 2-10 mole % chromium
or
chromium ion, 1 mole % in lithium or lithium ion and 27 mole % lanthanum or
lanthanum
ion and also includes an alpha-alumina support. Such processes preferably
provide at least
90% conversion of CH.~ and at least 90% selectivities for CO and H~ products.
Other
embodiments, features and advantages of the present invention will become
apparent with
reference to the following figures and description.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a bar graph comparing the catalyst performance of three pure
chromium
oxide systems. The black bars indicate the % CH4 conversion and the cross-
hatched bars
indicate the % CO selectivity.
Fig. 2A is a transmission electron microscopy photomicrograph showing the
crystal structure of a representative freeze-dried chromium oxide catalyst as
prepared.
Fig. 2B is similar to Fig. 2A but was taken after the catalyst was employed 6
hours
on stream.
Fig. 3 is a graph showing trends in light-off temperature and
basicity/ionicity of
representative "support'' matrix compositions.
Fig. 4 is a graph showing the results of thermal gravimetric analysis (TGA)
studies
of a representative rare earth oxide based chromium catalyst.
Fig. 5 is a graph showing the reaction chemistry for several representative
ternary
freeze dried chromium oxides containing chromium and cobalt.
Fig. 6A is X-ray diffraction data for the catalyst precursor Co0.2Cr0.gOx
following reaction in situ (i.e., on stream). Fig. 6B is like Fig. 6A, except
the catalyst
specimen was taken before reactor evaluation.
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Fig. 7 A shows the X-ray diffraction data for Co0.2Cr0.gOx after reactor
evaluation. Fig. 7 B shows the X-ray diffraction data for Co0.5 CrO.gOx after
reactor
evaluation, and Fig. 7 C shows the X-ray diffraction data for CoO.g Cr0_2Ox
after reactor
evaluation.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Catalyst Preparation
The chromium-containing catalysts useful for catalyzing the partial oxidation
of
methane to CO and H~ are prepared by employing a variety of known art
techniques such
as impregnation, xerogel or aerogel formation, freeze-drying, spray drying,
and spray
roasting. In addition to catalyst powders, extrudates and pellets, monoliths
can be used as
supports provided that they have sufficient porosity for reactor use. The
supports used
with some of the catalyst compositions may be in the form of monolithic
supports or other
configurations having longitudinal channels or passageways permitting high
space
1 S velocities with a minimal pressure drop. Such configurations are known in
the art and
described in, for example, Structured Catalysts and Reactors, A. Cybulski and
J.A.
Moulijn (Eds.), Marcel Dekker, Inc., 1998, p. 599-615 (Ch. 21, X. Xu and J.A.
Moulijn,
"Transformation of a Structured Carrier into Structured Catalyst").
Additionally, some of
the preferred three-dimensional forms of these new catalysts include chromium
oxide
reticulated ceramics or ceramic foams, and directly deposited materials on
three-
dimensional monoliths, which are needed for millisecond contact time reactors
and for
commercial use. The impregnation techniques preferably comprise contacting the
support
with a solution of a compound of the catalytically active material, or a
solution of
compounds of the catalytically active materials or their precursors. The
contacting is
followed by drying and calcining, or transforming or thermally treating the
supported
materials under reaction conditions; in some cases this thermal treatment can
be
accomplished in situ under reaction conditions.
A key component of the most preferred catalysts is chromium, and optionally at
least one other metal selected from the group consisting of Group 1 (i.e., Li,
Na, K, Rb and
Cs); Group 2 (i.e., Mg, Ca, Sr and Ba); Group 11 (i.e., Cu, Ag and Au); Group
12 (i.e., Zn
and Cd); metals with atomic numbers of 57 through 71 (i.e., La, Ce, Pr, Nd,
Sm, Eu, Gd,
Tb, Dy, Ho, Er, Tm, Yb and Lu), Co, Ni, Ru and Rh. The catalyst, or catalytic
composition, must contain a catalytically effective amount of the metal
component(s).
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The amount of catalytic metal present in the composition may vary widely.
Preferably the
catalyst comprises from about 0.1 mole % to about 100 mole % (as the metal) of
chromium per total moles of catalytic metal and matrix metal, and more
preferably from
about 10 mole % to about 100 mole %. A matrix is a skeletal framework of
oxides and
oxyhydroxides. One or more of the catalytic components may also serve as a
matrix
material in which another catalytic metal or metal-containing compound is
dispersed. For
example, a catalyst composition may include a CoCrZOa cubic spinet catalyst
precursor
dispersed in a chromium oxide matrix. This catalyst precursor is then reduced
to cobalt
metal in a chromium oxide matrix by the hot gases of the reactant stream. A
suitable
matrix can also be obtained from the hydrolysis and condensation of alkoxides
and/or
other reagents. Alternatively, or additionally, an oxidatively and thermally
stable material
may serve as a matrix or a support for the catalyst composition. For example,
a
composition containing (in wt%) 10% Cr, 1 % Li, 27% La and a-A1Z03 may be
used.
Xerogels and Aerogels from Metal Alkoxides
For the purposes of this disclosure, the term "gel" refers to a coherent,
rigid three-
dimensional polymeric network. As described in more detail below, the present
gels are
formed in a liquid medium, usually water, alcohol, or a mixture thereof. The
term
"alcogel" refers to gels in which the pores are filled with predominantly
alcohol. Gels
whose pores are filled primarily with water may be referred to as aquagels or
hydrogels.
A "xerogel" is a gel from which the liquid medium has been removed and
replaced by a
gas. In general, the structure is compressed and the porosity reduced
significantly by the
surface tension forces that occur as the liquid is removed. As soon as liquid
begins to
evaporate from a gel at temperatures below the critical temperature, surface
tension creates
concave menisci in the gel's pores. As evaporation continues, the menisci
retreat into the
gel body, compressive forces build up around its perimeter, and the perimeter
contracts,
drawing the gel body inward. Eventually surface tension causes significant
collapse of the
gel body and a reduction of volume, often as much as two-thirds or more of the
original
volume. This shrinkage causes a significant reduction in the porosity, often
as much as 90
to 95 percent depending on the system and pore sizes.
In contrast to a xerogel, an "aerogel" is a gel from which the liquid has been
removed in such a way as to prevent significant collapse or change in the
structure as
liquid is removed. This is typically accomplished by heating the liquid-filled
gel in an
autoclave while maintaining the prevailing pressure above the vapor pressure
of the liquid
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until the critical temperature of the liquid has been exceeded, and then
gradually releasing
the vapor, usually by gradually reducing the pressure either incrementally or
continuously,
while maintaining the temperature above the critical temperature. The critical
temperature
is the temperature above which it is impossible to liquefy a gas, regardless
of how much
pressure is applied. At temperatures above the critical temperature, the
distinction
between liquid and gas phases disappears and so do the physical manifestations
of the
gas/liquid interface. In the absence of an interface between liquid and gas
phases, there is
no surface tension and hence no surface tension forces to collapse the gel.
Such a process
may be termed "supercritical drying." Aerogels produced by supercritical
drying typically
have high porosities, on the order of from 50 to 99 percent by volume.
The new xerogels or aerogels preferably comprise a matrix material that is
essentially derived from a solution of one or more matrix components and
incorporate the
active catalyst component(s). The active catalyst components are preferably
derived from
one or more dissolved component. The matrix is a skeletal framework of oxides
and
oxyhydroxides derived from the hydrolysis and condensation of alkoxides and/or
other
reagents. This framework preferably comprises 30% or more, by weight, of the
total
catalyst composition. The matrix material comprises magnesium, silicon,
titanium,
zirconium or aluminum, oxide/hydroxide xerogels or aerogels, or mixtures
thereof,
totaling from 30 to 99.9 mole %, preferably 50-97.5 mole% of the catalyst
composition.
Especially preferred are combinations where the matrix metal is Ti and
combinations
where the matrix metal is a combination of Mg and Si.
In preparing a chromium-based catalyst, one or more metal alkoxides (e.g.,
titanium n-butoxide) may be used as starting material for preparing the gels.
Suitable
metal alkoxides are any alkoxide that contains from 1 to 20 carbon atoms,
preferably 1 to
5 carbon atoms, in the alkoxide group. It is also preferred that the alkoxide
is soluble in
the liquid reaction medium. C1-C4 alkoxides such as tantalum n-butoxide,
titanium
isopropoxide and zirconium isopropoxide are especially preferred. Commercially
available alkoxides can be used, if desired. In addition, suitable alkoxides
can be prepared
by other routes. Some examples include direct reaction of zero valent metals
with
alcohols in the presence of a catalyst. Many alkoxides can be formed by
reaction of metal
halides with alcohols. Alkoxy derivatives can be synthesized by the reaction
of the
alkoxide with alcohol in a ligand interchange reaction. Direct reactions of
metal
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dialkylamides with alcohol also form alkoxide derivatives. Additional examples
are
disclosed in ''Metal Alkoxides" by D.C. Bradley et al.. Academic Press, (
1978).
The first step in the synthesis of the gels containing alcohol, or alcogels,
consists of
first preparing non-aqueous solutions of the alkoxides and other reagents and
separate
solutions containing protic solvents such as water. When the alkoxide
solutions are mixed
with the solutions containing the protic solvents, the alkoxides will react
and polymerize
to form a gel.
The medium utilized in the process generally should be a solvent for the
alkoxide
or alkoxides which are utilized and the additional metal reagents and
promoters which are
added in the single step synthesis. Solubility of all components in their
respective media
(aqueous and non-aqueous) is preferred to produce highly dispersed materials.
By
employing soluble reagents in this manner, mixing and dispersion of the active
metals and
promoter reagents can be near atomic, in fact mirroring their dispersion in
their respective
solutions. The precursor gel thus produced by this process will contain highly
dispersed
active metals and promoters. High dispersion results in catalyst metal
particles in the
nanometer size range.
It is preferred that the catalytic metal component of the gel is dissolved in
a
separate protic solvent (e.g., water) and this solution of catalytic metal
compounds) is
then mixed with the non-aqueous solution of the matrix component(s).
Alternatively, the
catalytic metal component is dissolved in the same non-aqueous solution as the
matrix
component(s), and an aqueous supplement is used.
The concentration or amount of solvent used is linked to the alkoxide content.
A
molar ratio of 26.5:1 ethanolaotal alkoxide can be used, although the molar
ratio of
ethanolaotal alkoxide can be from about 5:1 to 53:1, or even greater. If a
large excess of
alcohol is used, gelation will not generally occur immediately; some solvent
evaporation
will be needed. At lower solvent concentrations, it is thought by the
inventors that a
heavier gel will be formed having less pore volume and surface area.
The process continues with adding to the alcohol soluble alkoxide and other
reagents, water and any aqueous solutions, in a dropwise fashion, to induce
hydrolysis
and condensation reaction. Depending on the alkoxide system, a discernible gel
point can
be reached in minutes or hours. The molar ratio of the total water added to
total Mg, Si,
Ti, Zr, and Al added (including water present in aqueous solutions) varies
according to the
specific alkoxide being reacted. Preferably, a molar ratio of water:alkoxide
from about
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0.1:1 to 10:1 is used. However, ratios close to 4:1 for zirconium(alkoxide)4
and
titanium(alkoxides)4 can also be used with success. The amount of water
utilized in the
reaction is that calculated to hydrolyze the alkoxide in the reaction mixture.
A ratio lower
than that needed to hydrolyze the alkoxide species will result in a partially
hydrolyzed
material, which in most cases will reach a gel point at a much slower rate,
depending on
the aging procedure and the presence of atmospheric moisture.
The addition of acidic or basic reagents to the alkoxide medium can have an
effect
on the kinetics of the hydrolysis and condensation reactions, and the
microstructure of the
oxide/hydroxide matrices derived from the alkoxide precursor which entraps or
incorporates the soluble metal and promoter reagents. It is preferred that a
pH within the
range of from 1 to 12 is used, with a pH range of from 1 to 6 being more
preferred.
After reacting to form an alcogel, as described above, it may be necessary to
complete the gelation process with some aging of the gel. This aging can range
from one
minute to several days. Generally, the alcogels are aged at room temperature
in air for at
1 S least several hours.
Removal of solvent from the alcogels is accomplished by several methods.
Removal by vacuum drying or heating in air results in the formation of a
xerogel. An
aerogel of the material can typically be formed by charging in a pressurized
system such
as an autoclave. The solvent-containing gel is placed in an autoclave where it
can be
contacted with a fluid above its critical temperature and pressure by allowing
supercritical
fluid to flow through the gel material until the solvent is no longer being
extracted by the
supercritical fluid. In performing this extraction to produce an aerogel
material, various
fluids can be utilized at their critical temperature and pressure. For
instance,
fluorochlorocarbons typified by Freon~ fluorochloromethanes (e.g., Freon~ 11
(CC13F),
12 (CC12F2) or 114 (CC1F2CC1F2), ammonia and carbon dioxide are all suitable
for this
process. Typically, the extraction fluids are gases at atmospheric conditions,
so that pore
collapse due to the capillary forces at the liquid/solid interface are avoided
during drying.
The resulting material should, in most cases, possess a higher surface area
than the non-
supercritically dried materials.
The xerogels and aerogels thus produced may be described as precursor salts
dispersed in an oxide or oxyhydroxide matrix. The hydroxyl content is at this
point
undefined; a theoretical maximum corresponds to the valence of central metal
atom. The
molar H20:alkoxide ratio can also impact the final xerogel stoichiometry so
that there will
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be residual -OR groups in the unaged gel. However, reaction with atmospheric
moisture
will convert these to the corresponding -OH, and -O groups upon continued
polymerization and dehydration. Aging, even under inert conditions, can also
effect the
condensation of the -OH, eliminating HBO, through continuation of cross
linking and
polymerization, i.e., gel formation.
Xerogels and Aerogels from Inorganic 1><etal Colloids
Alternatively, one or more inorganic metal colloids may be used as starting
material for preparing the gels. These colloids include colloidal alumina
sols, colloidal
ceria sots, colloidal zirconia sots or their mixtures. The colloidal sols are
commercially
available from well-known suppliers. There are also several methods of
preparing
colloids, as described in "Inorganic Colloid Chemistry", Volumes 1, 2 and 3,
J. Wiley and
Sons, Inc., 1935. Colloid formation involves either nucleation and growrth, or
subdivision
or dispersion processes. For example, hydrous titanium dioxide sots can be
prepared by
adding ammonia hydroxide to a solution of a tetravalent titanium salt,
followed by
peptization (re-dispersion) by dilute alkalis. Zirconium oxide sol can be
prepared by
dialysis of sodium oxychlorides. Cerium oxide sol can be prepared by dialysis
of a
solution of ceric ammonium nitrate.
Commercially available alkoxides, such as tetraethylorthosilicate and Tyzor~
organic titanate esters, may be used. However, alkoxides may also be prepared
by various
well-known routes. Examples include direct reaction of zero valent metal with
alcohols in
the presence of a suitable catalyst; and the reaction of metal halides with
alcohols. Alkoxy
derivatives can be synthesized by the reaction of the alkoxide with alcohol in
a ligand
interchange reaction. Direct reactions of metal dialkamides with alcohol also
form
alkoxide derivatives. Additional examples are described in D. C. Bradley et
al., "Metal
Alkoxides" (Academic Press, 1978).
In one especially preferred method of preparing a chromium-based catalyst, pre-
formed colloidal sols in water, or aquasols, are used. The aquasols are
comprised of
colloidal particles ranging in size from 2 to 50 nanometers. In general, the
smaller
primary particle sizes (2 to 5 nm) are preferred. The pre-formed colloids
contain from 10
to 35 weight % of colloidal oxides or other materials, depending on the method
of
stabilization. Generally, after addition of the active (for the partial
oxidation reactions,
either as a catalyst or promoter) metal components, the final de-stabilized
colloids can
possess from about 1 to 35 weight % solids, preferably from about 1 to 20
weight percent.
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The colloidal oxides or their mixtures are destabilized during the addition of
soluble salts of the primary and promoter cation species by the addition of
acids or bases
or by solvent removal, both of which alter pH. These changes modify the
colloidal
particle's electrical double layer. Each colloidal particle possesses a double
layer when
suspended in a liquid medium. For instance, a negatively charged colloid
causes some of
the positive ions to form a firmly attached layer around the surface of a
colloid.
Additional positive ions are still attracted by the negative colloid, but now
they are
repelled by the primary positive layer as well as the positive ions, and form
a diffuse layer
of counterions. The primary layer and the diffuse layer are referred to as the
double layer.
The tendencies of a colloid to either agglomerate (flocculate and precipitate)
or
polymerize when destabilized will depend on the properties of this double
layer. The
double layer, and resulting electrostatic forces can be modified by altering
the ionic
environment, or pH, liquid concentration, or by adding a surface active
material directly to
affect the charge of the colloid.
Once the particles come in close enough contact when destabilized,
polymerization
and crosslinking reaction between surface functional groups, such as surface
hydroxyls,
can occur. In this invention, the colloids, which are originally stable
heterogeneous
dispersions of oxides and other species in solvents, are destabilized to
produce colloidal
gels. Destabilization is induced, in some cases, by the addition of soluble
salts, e.g.,
chlorides or nitrates, which change the pH and the ionic strength of the
colloidal
suspensions; by the addition of acids or bases; or by solvent removal. pH
changes
generally accompany the addition of soluble salts; in general, this is
preferred over solvent
removal. Generally, a pH range of from about 0 to about 12 can be used to
destabilize the
colloids; however, very large extremes in pH (such as pH 12) can cause
flocculation and
precipitation. For this reason, a pH range of from about 2 to 8 is preferred.
The medium utilized in this process is typically aqueous, although non-aqueous
colloids can also be used. The additional metal or inorganic reagents (i.e.,
salts of Cr or
promoters) used should be soluble in the appropriate aqueous and non-aqueous
media.
Freeze Drying to Form the Solid Catalyst Composition
Removal of solvent from the gels can be accomplished by several methods as
described above to prepare either an aerogel or xerogel. Freeze drying
procedures can
accommodate several catalyst compositions, and are useful if the catalyst
precursors are
soluble in water or other solvent which can be rapidly (<1 minute) frozen.
Precursor salts
are dissolved in an appropriate amount of solvent to form a solution or fine
colloid. The
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solution is then rapidly cooled and frozen by immersion in a suitable medium,
such as
liquid nitrogen. If the solution is rapidly frozen, the salts and other
components will
remain intimately mixed and will not segregate to any significant decree. The
frozen solid
is transferred to a freeze drying chamber. The solution is kept frozen while
water vapor is
removed by evacuation. In the present studies, a two section Virtis freeze
drying unit was
employed. Refrigerated shelves were used to prevent thaw-out of the frozen
solids during
evacuation.
Spray Drying to Form the Solid Catalyst Composition
Spray drying procedures involve the use of solutions, colloids or slurnes
containing catalyst precursors or catalyst compounds. The technique consists
of
atomization of these liquids (usually but not exclusively aqueous) into a
spray, and contact
between spray and drying medium (usually hot air) resulting in moisture
evaporation. The
drying of the spray proceeds until the desired moisture content in the dried
particles is
obtained, and the product is recovered by suitable separation techniques
(usually cyclone
separation). A detailed description of spray drying methods can be found in
"Spray
Drying Handbook", 4th edition by K Masters (Longman Scientific and Technical,
John
Wiley and Sons, N.Y) c. 1985.
Spray Roasting to Form the Solid Catalyst Composition
Spray roasting also involves the use of solutions or colloids, but generally
involves
drying and calcination (at higher temperatures) in one process step to produce
catalyst
powders. Suitable spray roasting techniques are described in U.S. Pat. No.
5,707,910.
EXAMPLES
The catalyst compositions are given in atomic ratios except where otherwise
noted.
Example 1: Cr0,1La0,9 Ox
An aqueous solution of Cr3(OH)2(CH3C02)~ (Aldrich 31,810-8) (2.22 mL, 2.5603
M in Cr) and aqueous La(N03)3 (42.78 mL, 1.1955 M) were simultaneously added
to a
150 mL petri dish with gentle swirling. The entire solution was rapidly frozen
with liquid
nitrogen and dried as a frozen solid under vacuum for several days to produce
a freeze
dried powder. The freeze dried material was heated in air at 350°C for
~ hours prior to
pelletization and use. The final catalyst had a nominal composition of CrO.1
La0.9 Ox.
Example 2: Cr0.p25Mg0.975 ~x
A magnesium methoxide solution (68.767 mL, 0.3495 M) diluted with 50
volume % ethanol (punctilious) was added to a 150 mL petri dish with gentle
swirling
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under an inert N~ atmosphere. In a subsequent addition, aqueous
Cr~(OH)2(CH3C02)~
solution (1.233 mL, 0.5 M in Cr) was introduced to the petri dish while it was
gently
swirled. Following the addition of the aqueous solutions, a gel point was
realized and a
homogeneous gel formed which was nearly white in color. The gel was allowed to
age 8
days in air and then dried under vacuum at 120°C prior to use. The
final xerogel had a
nominal composition of Cr0_025Mg0.975 Ox.
Example 3: Crp.2Mgp.4Si0.4 Ox
A magnesium methoxide solution (57.474 mL, 0.669 M) and a
tetraethylorthosilicate (TEOS) solution (diluted with ethanol to 60 volume %
TEOS, 40
volume % ethanol) were simultaneously added to a 150 mL petri dish with gentle
swirling
under a nitrogen atmosphere. In a subsequent step, aqueous Cr;(OH)Z(CH~COZ)~
solution
(7.846 mL, 2.5603 M) was added. A white gel formed, and was aged for 5 days,
dried
under vacuum at 120°C for 5 hours prior to use. The final xerogel had a
nominal
composition of Cr0.2Mg0.4/Si0.4 Ox.
Example 4: Crp,iCep,9 Ox
An aqueous solution of Cr3(OH)Z(CH3C02)7 (1.375 mL, 2.560 M in Cr) and
aqueous solution of Ce(N03)3~6H20 (43.625 mL, 0.7261 M) were simultaneously
added
to a 150 mL pyrex petri dish with gentle swirling. The entire solution was
rapidly frozen
with liquid nitrogen and dried as a frozen solid under vacuum for several days
to produce
a freeze dried powder. The freeze dried material was heated in air at
350°C for 5 hours
prior to pelletization and use. The final catalyst had a nominal composition
of Cr0_ 1 Cep.9
Ox.
Example 5: Crp,lSmp.9 Ox
An aqueous solution of Cr3(OH)2(CH3C0z)~ (0.969 mL, 2.560 M in Cr) and an
aqueous solution of samarium nitrate (44.031 mL, 0.5069 M), the solution was
formed
using water and nitric acid to bring the final pH to 0.24 to dissolve
Sm(N03)3~6HZ0, were
simultaneously added to a 150 mL pyrex petri dish with gentle swirling. The
entire
solution was rapidly frozen with liquid nitrogen and dried as a frozen solid
under vacuum
for several days to produce a freeze dried powder. The freeze dried material
was heated in
air at 350°C for 5 hours prior to pelletization and use. The final
catalyst had a nominal
composition of Crp.l Smp.9 Ox.
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Example 6: Cr0,25Co0.25T~0.~ Ox
A titanium n-butoxide solution in ethanol (2.67 mL, 60 volume %) was added to
a
150 mL petri dish under an inert nitrogen atmosphere with gentle swirling. In
a second
step, an ethanolic solution of anhydrous CoCl2 (2.342 mL, 1.00 M), glacial
acetic acid
(0.140 mL), H20 (1.182 mL) and an ethanolic solution of chromium (III)
acetylacetonate
(93.667 mL, 0.03 M) were simultaneously added. A gel point was realized
following the
addition of the aqueous reagents, and the red, opaque gel which formed and was
aged for
at least 24 hours prior to drying under vacuum at 120°C for five hours.
The final xerogel
had a nominal composition of Cr0_25Co0.25Ti0.50x.
.10 Example 7: Cr0,2Au0,025A10.775 ~x
An aqueous AuCl3 solution (28.822 mL, 0.03 M) was combined with an aqueous
solution of Cr3(OH)z(CH3C02)~ (4.095 mL, 1.689 M in Cr) and an aqueous A120~
colloid
(5.742 mL, 4.668 M (as Al)) in a 150 mL petri dish with gentle swirling. A
NaOH
solution (1.340 mL, 0.01 M) was added both for Na content and to destabilize
the colloid
and induce gellation by altering pH. A red brown gel formed, and the material
was aged
for at least two days prior to drying under vacuum at 120°C for five
hours. The final
xerogel had a nominal composition of Cr0.2Au0,25A10.7750x.
Example 8: Cr0,025Au0.025Mg0.95 ~x
A magnesium methoxide (Mg(OCH3)~) solution (47.353 mL, 0.3495 M) formed by
combining magnesium methoxide with 50 volume % ethanol and 0.02 M AuCl3
(21.776
mL) in absolute ethanol were simultaneously added to a 150 mL petri dish with
gentle
swirling under a nitrogen atmosphere. In a subsequent addition, an aqueous
solution
containing Cr3(OH)2(CH3C00)~ (0.871 mL, 0.5M in Cr) was added. A gel formed,
and
the dark gel, after aging for at least three days, was dried at 120°C
under vacuum for 5
hours. The final xerogel had a nominal composition of Crp.025Au0.025Mg0.95 Ox.
Example 9: 10% Cr 1 % Li 27% La /a-A1203
An aqueous solution of LiN03 (1.762 g) in distilled water was added by the
incipient wetness technique to an alpha-alumina support (19.723 g, calcined at
900°C
overnight before use). The solids were dried at 110°C for two hours. An
aqueous solution
of La(N03)3~6H20 (22.134 g) in distilled water was added by the incipient
wetness
technique to the dried solids. The solids were again dried at 110°C for
two hours. An
aqueous solution of Cr(N03)3~9H20 (23.087 g) in distilled water was added by
the
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incipient wetness technique to the dried solids. Finally, the material was
dried at 110°C
for two hours followed by calcination at 900°C overnight. The final
catalyst had a
nominal composition of (in wt%) 10% Cr 1 % Li 27% La Ox/a-AIzO~.
Example 10: 2% Cr 1 % Li 27% La /a-A1z03
An aqueous solution of LiN03 ( 1.762 g) in distilled water was added by the
incipient wetness technique to an alpha-alumina support (22.123 g. calcined at
900°C
overnight before use). The solids were dried at 110°C for two hours. An
aqueous solution
of La(N03)3~6H20 (22.134 g) in distilled water was added by the incipient
wetness
technique to the dried solids. The solids were again dried at 110°C for
two hours. An
aqueous solution of Cr(N03)3~9H20 (4.617 g) in distilled water was added by
the incipient
wetness technique to the dried solids. Finally, the material was dried at
110°C for two
hours followed by calcination at 900°C overnight. The final catalyst
had a nominal
composition of(wt%) 2% Cr 1 % Li 27% La Ox oc-A1203.
Example 11: Freeze-Dried Cr203
An aqueous solution of Cr3(OH)Z(CH3C02)~ (100 mL, 2.5603 M in Cr) was added
to a 150 mL petri dish and rapidly frozen with liquid nitrogen. The frozen
solid was dried
under vacuum for several days (approximately 7 days) to produce a freeze dried
powder.
The freeze dried material was heated in air at 350°C for 5 hours and
525°C for 1 hour
prior to use.
Example 12: Aerogel Cr203
Aerogel synthesis using a sol gel chemistry (Cr203 derived from the reaction
of
Cr03 and methanol to produce a Crz03 gel, followed by supercritical extraction
to produce
a high surface area oxide (>500 mz/g). 16 g of chromium trioxide (Cr03,
Aldrich 23, 265-
7) was dissolved in 24 ml of water, and added to 420 ml of methanol and 36 ml
of
additional water. Three of these combined solutions were loaded into a 1 liter
autoclave,
which was sealed and heated over a four hour time period to 300°C and
3400 psig. After
holding at this temperature and pressure for 120 minutes, the pressure was
vented to 1000
psig over 2 hours while maintaining 300°C. Pressure was finally vented
to 1 atmosphere
by bleeding at a rate of 10 psig per minute while maintaining 300°C,
and the material was
allowed to cool overnight. A Cr203 aerogel is formed by this procedure
(reaction of Cr03
+ CH30H ~ Cr203 + other oxidation products of methanol (e.g., formaldehyde)).
The
surface area of materials formed by this procedure 537 m2/g, as determined by
N~ BET
analysis, and is X-ray (diffraction) amorphous.
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Example 13: "Newport Chrome" Chromium Oxide (Comparative Example)
A commercially prepared catalyst manufactured by DuPont at the Holly Run site,
by the pyrolysis of ammonium dichromate, (NH.~)~Cr~O~, was tested for
comparative
purposes.
Example 14: Cop, Crp.g Ox
12 ml of Co(N03)~.6Hz0 1.0826M was combined with 20.30 ml of an aqueous
solution of Cr3(OH)~(CH~COZ)~(2.5603M in Cr) to form an aqueous solution which
was
frozen, freeze-dried and calcined as described in Example 11.
Example 15: Cop.g Crp.2 Ox
48 ml of Co(N03)~.6Hz0 (Aldrich, 23,037-5), 1.0826M was combined with 5.07
ml of an aqueous solution of Cr3(OH)2(CH,CO~)~ (2.5603M in Cr) to form an
aqueous
solution which was then frozen, freeze-dried and calcined as described in
Example 11.
Example 16: Cop.S Crp_5 Ox
30 ml of Co(N03)2.6H20 (Aldrich, 23,037-5), 1.0826M was combined with 12.69
ml of an aqueous solution of Cr3(OH)Z(CH;COZ)~ (2.5603M in Cr) to form an
aqueous
solution which was then frozen, freeze-dried and calcined as described in
Example 11.
Example 17: CoOx (Comparative Example)
60 ml of a solution containing Co(N03)z.6H~0 (Aldrich, 23,037-5) 1.0826 M
was rapidly frozen in liquid nitrogen, placed in a freeze dryer (Virtis
Corporation, shelves
refrigerated to 0°C) and evacuated to dryness over a period of 5-7
days, or until
completely dry. The material was calcined in air according to the following
schedule:
5°C/min to 525°C, 525°C for 1 hour; 10°C/min to
room temperature. The material was
pelletized and sieved prior to the reactor evaluation. The cobalt oxide
catalyst was
evaluated as described in the following section entitled "Test Procedure."'
This
composition is identified in Table 9, and the performance results are shown in
Table 10.
The beneficial effects of adding chromium to cobalt catalyst compositions, as
described
below, are also apparent in Table 10. The preferred Cr-Co Ox catalysts which
exhibit
reduced carbon deposition are designed with lower cobalt levels and the
addition of basic
rare earth, alkaline or alkaline earth components.
Test Procedure
Catalysts were evaluated in a 25 cm long quartz tube reactor equipped with a
co-axial, quartz thermocouple well, resulting in a 4 mm, reactor i.d. The void
space within
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the reactor was packed with quartz chips. The catalyst bed was positioned with
quartz
wool at approximately mid-length in the reactor. A three point, K type,
thermocouple was
used with the catalyst's "hot spot"", read-out temperature reported as the run
temperature.
The catalyst bed was heated with a 4 inch (10.2 cm), 600 W band furnace at 90%
electrical
output. Mass flow controllers and meters regulated the feed composition and
flow rate.
Prior to start-up, the flows were checked manually with a bubble meter and
then the feed
composition was reconfirmed by gas chromatographic. analysis. The flow rates
of all the
meters were safety interlocked and their measurements were checked
electronically by the
mass flow meters every second. All runs were performed at a CH4:0~ feed ratio
of 2:1,
safely outside of the flammable region. Specifically, the feed contained (in
volume %)
30% CH4, 15% O~ and 55% N~. Experiments were conducted at 5 psig (136 kPa).
The
reactor effluent was analyzed by a gas chromatograph (g.c.) equipped with a
thermal
conductivity detector. The feed components (CHa, O~, NZ) and potential
products (CO,
H2, COZ, and H20) were all well resolved and reliably quantified by two
chromatography
columns in series consisting of 5A molecular sieve and Haysep T. Mass balances
of C, H,
and O all closed at 98-102%. Runs were conducted over two operating days, each
with 6
hours of steady state, run time.
The results of testing the catalyst compositions of Examples 1-16 are shown in
Table 1. Catalyst performance is reported at steady state and showed no
evidence of
catalyst deactivation after 12 hours, according to g.c. analysis.
Table 1. Catalyst Performance
Example Catalyst Temp. GHSV %CH~/O~ %CO/H, H~:CO
%Coke
No. Composition V(mL) Wt.(g) °C x10' Conv. Sel.
1 Cro_,La°_9 Ox 2 2.1417 770 6.1 58/100 83/73 1.8
0.08
2 Cr°.o~sM$°.9~s Ox 2 0.9024 710 6.1 45/100 74/48 1.3
2.99
3 Cr°.~Mg°_4Si°.a Ox 2 1.3851 875 6.1 64/100 93/50 1.1
1.83
4 Cr°.,Ce°.9 Ox 0.4 0.5972 860 3.045 36/100 49/45 1.8
n.d.
5 Cr°,,Srr~.9 Ox 0.4 0.5350 870 3.045 48/100 65/66 2.0
0.17
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6 Coo.~SCro_~STio_5 Ox 2 1.0605 980 6.1 82/100 93/92 2.0
n.d.
7 Auo.o~sCro.zAlo.»5 Ox 0.7 0.5685 911 17.4 28/92 60/25 0.8
9.98
8 Aua.o~sCro.o~sMgo.9s2 0.9560915 6.1 48/10084/44 1.0
Ox
22.19
9 10%Cr,l%Li/27%La 0.9 1.0235850 6.1 90/10097/90 1.9
/a-A1~0;'
n.d.
2%Cr,l%Li,27%La /a-A1~0;'0.4 0.5327830 3.045 90/10096/93 1.9
10 2.69
11 Cr~03 (freeze dried)2 2.3529670 6.1 72/10091/85 1.9
n.d.
12 Cr~03 (Aerogel) 0.4 0.2180550 3.045 21/99 0/0 n/a
n.d.
13 Crz03 (Newport) 0.4 0.0778655 30.5 29/64 70/30 0.86
n.d.
14 Coo,ZCro.B Ox 2 2.1793630 6.1 96/10098/97 1.98
0.39
15 Coo.BCro,~ Ox 2 2.2806670 6.1 93/10096/96 2.00
n.d.
16 Coo_SCro.s Ox 2 1.9917650 6.1 95/10098/97 1.98
n.d.
n.d. = none detected
* Wt
The recovered (after use) Cr203 catalyst of Example 11 showed no weight loss
after thermal gravimetric analysis in air at 600°C to 700°C; no
significant carbon
deposition (coking) is apparent using this analytical method.
With reference to Fig. 1, comparing catalyst performance of the catalysts from
Examples 11, 12 and 13, it can be appreciated that the catalyst preparation
procedure has a
major impact on catalyst performance. All three of these catalysts are
nominally chromium
oxide, yet demonstrate major differences in performance which can be seen in
Table 1.
The catalyst of Example 11 was prepared by freeze drying chromium oxide
precursors
(e.g., chromium hydroxide acetate), followed by calcination at 525°C,
which appears to
produce a catalyst precursor with superior (best) performance (i.e., highest
conversion and
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selectivity). The catalyst of Example 12 was prepared by aerogel synthesis
using a sol gel
chemistry (Cr203 derived from the reaction of Cr03 and methanol to produce a
Cr~03 gel,
followed by supercritical extraction to produce a high surface area oxide
(>500m~: g). The
catalyst of Example 13 is a commercially available catalyst. "Newport Chrome,"
manufactured by DuPont at the Holly Run site. It is commercially prepared by
pvrolyzing
ammonium dichromate, (NH.~)~Cr20~. Chromium oxide prepared by freeze drying an
aqueous solution of chromium hydroxide acetate, followed by calcination in air
at 525°C,
is clearly the most active and selective catalyst, as shown in Fig. 1 and in
Table 2,
although this result could not have been predicted from previous work with
chromium
catalysts. Other major differences in performance between the catalysts of
Examples 11-
13 are also noted in Table 2. The "coke" and carbon content of the catalysts
were
determined on samples which were evaluated, and indicated in the last column
of the
table. The first number is derived from TGA analysis, and is the percentage
weight loss of
the sample above 600°C in air. The second number is determined by an
elemental
analysis technique involving combustion of the catalyst and analysis of CO/COZ
which is
produced.
Table 2. Millisecond Contact Time Reactor Data,a Chromium Oxide
Catalyst Vol Wt. Temp. GHSV %CH4/OZ %CO/HZ H~:CO % Coke
(mL) (g) (C) x 104 Conv. Sel. Ratio
Cr203 (freeze dried)2.3529670 6.1 72/100 91/85 1.90 n.d./0.3:
2.0 ~
Cr203 (Aerogel) 0.4 0.2180550 3.0 21/99 0/0 nia n.d.~
'
Cr203 (Newport) 0.4 0.0778655 30 29/64 70/30 0.86 0.05
Co0.2 CrO.g Ox 2.0 2.1793630 6.1 96/100 98/97 1.98 0.39/2.2
Co0.5 Cr0,5 Ox 2.0 1.9917650 6.1 95/100 98/97 1.98 n.d./5.2
CoO.g Cr0.2 Ox 2.0 2.2806670 6.1 93/100 96/96 2.00 n.d.b/21.c
'Feed: 30% CH4, 15% O~, 55% N
bnot detected
The conventional view is that chromium promoters or additives promote non-
selective reaction pathways for alkane oxidation reactions using molecular
oxygen, OZ.
Therefore, the selective behavior of chromium oxide-based compositions as
catalysts for
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WO 01/60740 PCT/USO1/04976
the partial oxidation of methane to CO and H2, as disclosed herein, is
unexpected and even
surprising. In one inventor's experience with n-butane oxidation, for example,
it was
observed that chromium promoters in vanadium phosphorus oxide catalysts
increased
catalyst activity at the expense of selectivity. In these cases the catalysts
were compared
at the same percent conversion of reactant. A similar trend was also noted by
Oganowski,
W. et al. ("Promotional Effect of Molybdenum. Chromium and Cobalt on a V-Mg-O
catalyst in oxidative dehydrogenation of ethylbenzene to styrene." Applied
Catalysis A:
General 136 (1996) 143-159.) At page 156 of that reference, the reaction
chemistry is the
oxidative dehydrogenation of ethylbenzene to styrene: ''The molybdenum,
chromium or
cobalt doped V-Mg-O catalyst changes its activity and selectivity in the
oxidative
dehydrogenation of ethylbenzene. The specific activity decreases in the
direction
Cr,Co>Cr>Co>Mo while the selectivity increases in the direction:
Cr»Co,Cr,Co>Mo.''
This suggests that Cr would not serve as a selective catalyst for a process
involving C-H
activation, such as CH4 partial oxidation, and is contrary to the inventors'
present findings.
Although the reasons for these performance differences are under
investigation, the
selectivity changes may be related to the surface areas of the catalyst
generated. Catalysts
possessing the highest surface areas (i.e., a chromium oxide aerogel) also
possess the
lowest light-off temperatures and exhibit no selectivity to CO/H~. This
suggests that an
increase in the number of sites for CO adsorption results in slower desorption
from the
catalyst surface, allowing for oxidation of CO. In the millisecond contact
time reaction
regime, a lower surface area catalyst possessing a limited number of active
sites may
actually be preferred for selective oxidation pathways.
Another important observation for the freeze dried catalyst systems is the
near
absence of carbon deposition on the catalyst surfaces. This is clearly
indicated for the
pure chromium oxide freeze dried catalyst in the transmission electron
microscopy (TEM)
studies. Fig. 2A shows the crystal structure of the chromium oxide catalyst as
prepared in
Example 11. The freeze dried chromium oxide is comprised of highly crystalline
powder
containing well-faceted chromium oxide crystallites. Powder X-ray diffraction
confirms
the well-crystallized nature of this material. No change in catalyst
appearance is apparent
after reactor evaluation during an eight hour period, indicating stability of
the material on
stream over short time intervals. Fig. 2B shows the crystal structure of a
sample taken
after 6 hours on stream, indicating that there was no apparent change in
crystallite size or
morphology with time on stream in a reactor. After 6 hours on stream there is
little carbon
build-up (coking). A carbon deposit is indicated by the arrow in Fig. 2B. The
surprisingly
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WO 01/60740 PCT/USO1/04976
low carbon deposition may also be related to the lower surface area of the
catalyst and the
highly faceted, defect-free nature of the catalyst surface. Low carbon
deposition and
stability on stream (i.e., lack of sintering of chromium oxide particles) are
very favorable
catalyst properties for syngas catalysts.
X-ray diffraction (XRD) analysis of the Co/Cr materials of the representative
catalysts of Examples 14-16 revealed that the Co0.2 Cr0_g Ox catalyst
comprised, in the
catalyst as calcined (heated in air), a mixture of CoCr204 (cubic spinet
phase) and Cr~O~
(eskolaite, hexagonal/rhombohedral phase). In the final catalyst, after an 8
hour
evaluation as described in Table 2, XIRD analysis revealed Cr~O~ and possibly
Co metal
or a chromium carbide phase. Low carbon formation is a very unusual,
unexpected, and
advantageous feature of many of the Cr20~ catalyst systems described herein.
Additional catalyst systems were investigated to study trends in C-H
activation. In
this case, a series of rare earth promoted catalysts were synthesized and
tested. Fig. 3 is a
graph showing trends in light-off temperature and basicity/ionicity of
representative
"support" matrix compositions (i.e., Cr0.025 Mg0.975 Ox; CrO, l La0,9 Ox;
CrO,1 Ce0.9
Ox; and Cr0.1 Sm0.9 Ox from Examples 1, 2, 4 and 5.). The predicted ionicity
or basicity
of the compositions increases from right to left along the x-axis of the
graph. These
systems were chosen for their thermal stability. In addition, rare earth oxide
base catalysts
have been reported for methane coupling-type reactions. The basicity of these
rare earth
oxide systems may facilitate C-H activation. Trends in light-off temperature,
or ignition
temperature, suggest that this may be the case. A lanthanum chromium oxide
compound
(comprised of LaZCr20~ + CrzO~ in powder X-ray diffraction studies) possesses
the lowest
light-off or ignition temperature. A plot of the light-off temperature versus
the expected
basicity or ionicity of the rare earth component shows a correlation which
suggests C-H
activation may be related to this property.
Thermogravimetric analysis (TGA) studies also indicate low carbon deposition
for
the rare earth oxide based chromium catalysts, as shown in Fig. 4 for La0.1
Crp.9 Ox
(CrZ03 + LazCr206 by X-ray diffraction), prepared similarly to the method
described in
Example 11. In Fig. 4, the arrow at about 300°C indicates a temperature
region where the
catalyst undergoes carbonate decomposition and appreciable weight loss occurs.
Carbon
deposition, as indicated by the weight loss at about rt-350°C in N~ is
6.548% (0.6889 mg).
The weight loss from about 350-600°C is 2.897% (0.3048 mg), and from
about 600-700°C
is 0.08311 % (0.008744 mg). TGA analysis of weight loss in air (>600°C)
indicates « 1
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WO 01/60740 PCT/USO1/04976
wt % carbon deposition for these catalyst systems after eight hours on stream
(i.e., <0.07
wt % upon oxidation in air from 600-700°C for La0.1 Cr0.9 Ox).
A series of cobalt-containing chromium oxides are particularly interesting for
this
reaction chemistry. Initial studies on a Co0,25 Cr0.25 T10.5 Ox system
prepared using sol
S gel methods indicated that this catalyst was a promising catalyst system. A
''freeze-dried"
variant of this catalyst system, without titanium oxide, and prepared similar
to the
procedure in Example 1 l, exhibits the highest activity and selectivity, as
shown in Fig. 5
and Table 2. Fig. 5 is a graph showing the reaction chemistry for several
representative
ternary freeze dried chromium oxides containing chromium and cobalt, and for
chromium
oxide and cobalt oxide alone. The reaction conditions included 30 % methane
feed, 15%
Oz feed, 55% NZ atmosphere, 6 hours on stream. X-ray diffraction analysis
indicated the
presence of Cr203 matrix and varying proportions of cobalt in the catalyst
compositions
tested. The low conversion and selectivity demonstrated by cobalt oxide in
Fig. 5 shows
the beneficial effect of including chromium in the catalyst composition. This
figure also
indicates an optimal composition range for the specified reaction conditions
(i.e., feed
composition and flow rate of about 6.1 x 104 GHSV).
As shown in the X-ray diffraction data of Figs. 6 and 7, the cobalt chromium
oxide
materials consist of the cubic spinet (CoCr204) dispersed in a Cr203 matrix
(hexagonal/rhombohedral phase, eskolaite). Before reactor evaluation (Fig. 6B)
X-ray
diffraction indicates a representative cobalt chromium oxide composition
contains
CoCr204 cubic spinet and Crz03. Following reaction in situ (i.e., on stream)
the catalyst
precursor Co0.2 CrO,g Ox is reduced to cobalt metal and chromium oxide, as
shown in
Fig. 6A. This is not surprising considering the higher temperatures used in
this methane
oxidation reaction. Figs 7A-C contain the X-ray diffraction data for ''reactor
evaluated"
catalysts having the following compositions, respectively: Co0.2 Cr0_g Ox,
Co0.5 Crp,g
Ox, and CoO.g Cr0.2 Ox. These compositions were prepared as described in
Examples 14-
16. Each of these samples show Co metal and Cr203, or pure Co.
These materials are highly active and selective catalyst systems (>95 % CH4
conversion, 97-98 % selectivity to CO and HZ). As mentioned above, the low
light-off
temperatures of these materials (< 650°C) and superior performance make
these catalyst
systems favorable candidates for further improvement and commercialization. It
was
observed that the chromium oxide-based catalysts containing cobalt show carbon
deposition on the reduced cobalt metal which is formed in situ. The amount of
carbon
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WO 01/60740 PCT/LTSO1/04976
deposition directly correlated with the cobalt metal in the composition.
Transmission
electron microscopy (TEM) studies indicate that turbostratic carbon is
deposited mostly at
the cobalt centers, and not on the chromium oxide support or matrix.
Example 18: Ni0,2 CrO,g Ox
13.93 ml of Ni (N03)2 of 1.068 M solution (prepared by dissolving
Ni(N03)2.6H20 in water.) Stoichiometry was determined by elemental analysis,
ICP) was
combined with 119 rnl of an aqueous solution of Cr~(OH)2(CH~COO)~ (0.5M in
Cr),
prepared by diluting a 2.5603 M solution of chromium hydroxide acetate
(prepared by
dissolving chromium hydroxide acetate in water). The mixed solution was
rapidly frozen
in liquid nitrogen. It was placed in a freeze dryer (Virus Corporation,
shelves refrigerated
to 0°C) and evacuated to dryness over a period of 5-7 days, or until
completely dry. The
material was calcined in air according to the following schedule:
5°C/min to 350°C, 5
hour soak at 350 °C, 5°C/min to 525°C, 525°C soak
for 1 hour; 10°C /min to room
temperature. The material was sieved prior to the reactor evaluation. The
Nio.~ Cro.g Ox
powder was evaluated as described above under "Test Procedure," and the
results are
shown in Table 4.
Example 19: Ni0.1 Cr0,9 Ox
27.149 ml of an aqueous solution of Cr3(OH)Z(CH3C00)~ (1.6575 M in Cr), and
4.682 ml of 1.068 M Ni (N03)2 , prepared by dissolving Ni(N03)2.6H~0 in water
(stoichiometry determined by elemental analysis (ICP)) were simultaneously
added to a
150 ml pyrex petri dish with gentle swirling. The entire solution was rapidly
frozen with
liquid nitrogen and dried as a frozen solid under vacuum for several days in a
Virtis 25EL
"Freezemobile" equipped with a Unitop 800 L unit (with refrigerated shelves)
to produce a
freeze dried powder. The freeze dried material was heated or calcined in air
at 350°C for
5 hrs prior to pelletization and use in a microreactor, as described above
under "Test
Procedure." Test results are shown in Table 4.
Example 20: Ni0.p1 Cr0.99 Ox
The same procedure was usedas described in Example 19 except that 29.864 ml
of the chromium hydroxide acetate solution and 0.468 ml of the nickel nitrate
solution
were used.
Example 21: Y0.1 CrO,~ Ni0.2 Ox
8.554 ml of 0.9352 M yttrium nitrate solution (prepared by dissolving Y(NO3)3
hydrate, Alfa 12898 in water) was combined with 33.786 ml of an aqueous
solution of
CA 02400046 2002-08-13
WO 01/60740 PCT/USO1/04976
Cr3(OH)~(CH~COO)- (1.6575 M in Cr), and 14.981 ml of 1.068 M Ni (N03)~ of
solution
(prepared by dissolving Ni(N03)~~6H20 in water). The solution was freeze
dried, calcined
and prepared for testing as described in Example 19.
Example 22: La0.1 Cr0,7 Ni0.2 Ox
An identical procedure as described in Example 20 was used to prepare
La0.lCrp.7Ni0.2Ox, except that 6.692 ml of 1.1955 M lanthanum nitrate
((La(N03)3
aqueous solution prepared by dissolving 503.02 g of La(N03)~xHzO (Aldrich
23,855-4) in
water to make a solution with La content 33.0 wrt %, ) was combined with
33.786 ml of an
aqueous solution of Cr3(OH)~(CH3C00)~ (1.6575 M in Cr), and 14.981 ml of 1.068
M
Ni(NO~), prepared as described above.
Example 23: Ce0.1 Cr0.7 Ni0.2 Ox
An identical procedure as described in Example 20 was used to make
Ce0,1Cr0.7Ni0_2Ox, except that 8.00 ml of 1.00 M cerium nitrate (Ce(N03)3
aqueous
solution prepared by dissolving 503.02 g of Ce(N03)~ 6 Hz0 (Alfa 11329) in
sufficient
water to make a 1 M solution) was combined with 33.786 ml of an aqueous
solution of
Cr3(OH)Z(CH3C00)~ (1.6575 M in Cr), and 14.981 ml of Ni (N03)z of 1.068 M
solution
(prepared by dissolving Ni(N03)~~ 6Hz0 in water; molarity determined by ICP,
elemental
analysis).
Table 3. Ni-Cr Series Catalysts
Example No. Composition
18 Nio.2 Cro.B Ox
19 Nio.~ Cro.9 Ox
Nio,o, Cro.99 Ox
21 ~'o.~ Cro.~ Nio.z Ox
22 Lao_~ Cro.~ Nio.2 Ox
23 Ceo.l Cro.~ Nio., Ox
Table 4. Performance of Ni-Cr Series Catalysts
Ex. Vol.Wt. Temp. GHSV % CH.,% %C0 % HZ HZ:CO% Coke
OZ
No. (mL)(g) (C) x 10' Conv. Conv.Sel. Sel.
18 2.0 2.6096 686 6.1 94 100 97 98 2.02 12.8
787* 4.6 91 100 97 98 2.02 12.8
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571 7.6 93 100 99 99 2.00 12.8
*
599* 12.2 91 100 98 98 2.00 12.8
571 15.2 90 100 98 98 2.00 23.8**
*
19 2.0 2.2551 746 6.1 90 ~ 100 96 9~ 1.98 0.83
20 2.0 2.1817 804 6.1 80 100 91 87 1.91 0.43
21 2.0 2.0049 748 6.1 95 100 97 97 2.00 2.62
22 2.0 2.1250 758 6.1 96 100 98 97 1.98 1.93
23 2.0 2.4859 753 6.1 96 100 98 97 1.98 1.67
Compositions were evaluated for 6 hrs., except where noted otherwise.
* Feed composition 90% CH4, 30% O~ and 10% N
** Evaluated for 25 hrs.
As illustrated above, by choosing catalyst compounds or catalyst precursor
materials which provide higher melting point pure ceramic oxides instead of
metals,
longer-life catalysts are obtained. By appropriate choice of catalyst
composition, as
demonstrated herein, sintering phenomena, which typically result in loss of
catalytic
surface area and eventually activity during use, can be diminished at high
temperatures,
thereby extending catalyst life. Some compositions which form metal plus
ceramic oxide
in situ, such as certain Co/Cr and Ni/Cr oxide compositions, may not share
this
advantage, however. With catalytic use oxides of Co and Ni tend to sinter and
to
contribute to coking, decreasing catalyst performance and catalyst life. This
behavior is
more problematic at higher operating temperatures.
In light of the above-described problem, a particularly interesting finding by
the
inventors was that coke formation is suppressed in the rare earth-containing
Ni Cr
compounds (e.g., the Ao.~ Cro_~ Nio_2 Ox series of Examples 21-23), even
though the
activities demonstrated in these tests appeared to be comparable to that of
other Cr-
containing compositions, as shown in Table 4. The percent coking with Ni0.2
CrO.g Ox
(Example 18), evaluated for 6 hrs, was 12.8%. The same composition evaluated
for 25
hrs. experienced 23.8% coke formation. By comparison, the rare earth compounds
showed markedly less carbon build-up during a 6 hr evaluation, indicating the
desirable
longer life of these catalyst compositions. Although not wishing to be limited
to any one
theory, it is thought that the action of the rare earth oxide may be one of
moderating (i.e.,
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lowering) the surface acidity of the oxide, which suppresses some of the acid
catalyzed
carbon forming reactions.
Example 24: Cr0,025 (Mg01_X (OH)x)0.975 (xerogel)
68.767 ml of 0.3495 M magnesium methoxide solution ((Aldrich 33,565-7),
diluted with 50 volume % ethanol (punctilious)), was added to a 150 ml petri
dish with
gentle swirling under an inert Nz atmosphere. In a subsequent addition, 1.233
ml of an
aqueous solution of Cr3(OH)~(CH3C00)~ (0.5 M in Cr) was introduced to the
petri dish
while it was gently swirled. Following the addition of the aqueous solutions,
a gel point
was realized and a homogeneous gel formed which was nearly white in color. The
gel was
allowed to age 8 days in air and then dried under vacuum at 120°C prior
to use. This
catalyst was evaluated as described in the section entitled "Test Procedure,'"
and the results
are shown in Table 6.
Example 25: Cr0.2 (Mg01_x (OH)x)0.4 ( si02_x (OH)x)0.4 (Xerogel)
57.474 ml of 0.669 M magnesium methoxide solution (Aldrich 33,565-7) and
14.935 ml of tetraethylorthosilicate (TEOS) solution (Aldrich, 13,190-3)
diluted with
ethanol to 60 volume % TEOS, 40 volume % ethanol), were simultaneously added
to a
150 ml petri dish with gentle swirling under a nitrogen atmosphere. In a
subsequent step,
7.846 ml of an aqueous solution of Cr3(OH)z(CH3C00)~ (2.5603 M in Cr) was
added. A
white gel formed, and was allowed to age for 5 days prior to drying in vacuum
at 120°C
for 5 hours. This catalyst was evaluated as described in the section entitled
"Test
Procedure," and the results are shown in Table 6.
Table 5. Cr Powder Catalysts
Example Catalyst Composition
No. No.
1 92 Cro.l Lao.9 Ox
5 93 Cro. ~ Smo.90x
4 94 Cro.~Ceo.90x
95 Auo.ozs Cr0.025
Mg0.95 ~x
96 Auo.ozs Cro.z
Alo.~~s Ox
24 97 Cro.ozs Mgo.9~s
Ox
98 Cro.z Mgo.a Sio.a
Ox
99 Coo.zs Cro.zs
Tio.s Ox
11 100 Crz03 (freeze
dry)
12 101 Crz03(Aerogel)
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Example No. Catalyst No. Composition
102 Cro.sTio,sOX(Xerogel)
103 Cro.zAlo,4Sio.40X
104 Coo.4Lio.oosTiOz
Table 6. Performance of Cr-containing Powder Catalysts
_
CatalystVol.Wt. Temp GHSV %CH4 %0z %C0 %Hz H_~:CO
No. (mL)(g) (C) x 104 Conv. Cony. 5e1. Sel.
92 2.0 2.1417 770 6.1 58 100 83 73 1.76
93 0.4 0.5350 870 3.0 48 100 65 66 2.03
94 0.4 0.5972 860 3.0 36 100 49 45 1.84
95 2.0 0.9560 915 6.1 48 100 84 44 1.05
96 0.7 0.5685 911 17.4 28 92 60 25 0.83
97 2.0 0.9024 710 6.1 45 100 74 48 1.30
98 2.0 1.3851 875 6.1 64 100 93 50 1.08
99 2.0 1.0605 980 6.1 82 100 93 92 1.98
100 2.0 2.3529 670 6.1 72 100 91 85 1.87
101 0.4 0.2180 550 3.0 21 99 0 0
102 2.0 1.2818 631 6.1 0 0 0 0
103 0.5 0.3300 845 24.4 3 6 23 0
104 2.0 2.0573 640 6.1 3 2 0 2
Some non-chromium containing catalyst powders were prepared substantially as
described in the foregoing examples. Examples of these compositions are
identified in
Table 7. Their catalytic performance was evaluated according to the "Test
Procedure"
described above and the results are reported in Table 8
Table 7. Non-Cr Powder Catalysts
Catalyst No. Composition
105 Ruo,ozs Coo.izs Ceo.ss Ox
106 Auo.ozs Mgo.9~s Ox
34
hrs. experienced 23.8% coke formation. By compa
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Catalyst Composition
No.
107 Auo.o~ Lao.99 Ox
108 Coo..~Lio.oosTi02
109 Ruo.oz;Coo.~zs
Ceo.asOX
110 Auo.o~;Mgo.9~sOx
111 Auo.o~ 510.992
112 Auo_o ~ Lao.990X
113 Auo,oICeo.994X
114 Auo.o, Snp.99Ox
Table 8. Performance of Non-Cr Powder Catalysts
_
Catalyst Vol. Wt. Temp GHSV %CH.~ %O~ %C0 %H2 Hz:CO
ID No. (mL) (g) (C) x 10'~ Cony. Cony. Sel. Sel.
105 0.4 0.2516700 3.0 0 0 0 0
106 0.4 0.2291870 3.0 24 73 60 1 0.03
107 0.4 0.2500850 3.0 23 82 33 1 0.06
108 2.0 2.0573640 6.1 0 0 0 0
109 2.0 3.3520655 6.1 14 45 58 11 0.38
110 1.7 0.8465900 7.2 29 82 78 13 0.33
111 1.3 0.5255650 9.4 0 0 0 0
112 0.2 0.2484625 61 10 27 54 6 0.22
113 0.2 0.2090650 81 0 0 0 0
114 0.1 0.0692600 122 0 0 0 0
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As shown in Table 8, these non-chromium containing compositions were not as
reactive in the syngas production tests, compared to the Cr-containing
catalysts identified
above and tested under similar conditions.
Additional cobalt-chromium catalyst powders were prepared as described in the
following examples. These compositions are identified in Table 9 and their
performance
when evaluated according to the "Test Procedure" described above is reported
in Table 10.
Table 9. Co-Cr Series Catalysts
Example No. Catalyst No. Composition
17 115 Co Oxide
14 116 Coo.ZCro.B
Ox
117 Coo_RCro,2
Ox
16 118 Coo.sCro.s
Ox
119 Coo.osCro.9s Ox
120 Coo.oiCro.99 Ox
121 Coo.~Cro.9 Ox
122 Coo_ZCro.g Ox
26 123 Coo., Cro.gLao_, Ox
Example 26: Cop.lCrO,g La0.1 Ox
6.692 ml of 1.1955 M lanthanum nitrate (La(N03)~ aqueous solution prepared by
dissolving 503.02 g of La(N03) x H20 (Aldrich 23,855-4) in sufficient water to
make a
33.0 wt % La solution)) was combined with 38.612 ml of an aqueous solution of
Cr3(OH)2(CH3C00)~ (1.6575 M in Cr), and 7.39 ml of 1.0826 M Co(N03)~ solution,
(prepared by dissolving Co(N03)2~6H20 (Alfa 11341) in water). The La, Cr and
Co
solutions were simultaneously added to a 150 ml pyrex petri dish with gentle
swirling.
The entire solution was rapidly frozen with liquid nitrogen and dried as a
frozen solid
under vacuum for several days in a Virtis 25EL "Freezemobile" equipped with a
Unitop
800 L unit (with refrigerated shelves) to produce a freeze dried powder. The
freeze dried
material was heated or calcined in air at 350°C for 5 hrs prior to
pelletization and use in a
microreactor. This catalyst (Catalyst No. 123) was evaluated as described in
the section
entitled "Test Procedure," and the results are shown in Table 10.
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Table
10.
Performance
of
Co-Cr
Series
Catalysts
Catalyst Vol.Wt. Temp GHSV %CH~ %0Z %C0 %H~ H~:CO
%Coke
No. (mL)(g) (C) x 104 Conv. Cony. Sel. Sel.
115 0.4 0.6826 765 30.5 42 100 70 36
1.03 0.61
116 2.0 2.0793630 6.1 96 100 98 97 1.98
0.4 0.5277735 22.1 93 100 98 98 2.00
700 30.5 90 100 97 96 1.98
2.17
117 2.0 2.2806670 6.1 93 100 96 96 2.00
21.6
118 2.0 1.9917650 6.1 95 100 98 97 1.98
5.20
119 2.0 2.7451678 6.1 86 100 93 97 2.09
786 4.4* 84 100 91 95 2.09
0.80
120 2.0 2.3837727 6.1 77 100 88 91 2.08
825 4.4* 77 100 86 90 2.09
0.42
121 2.0 2.9071621 6.1 92 100 96 97 2.02
737 4.4* 85 100 95 95 2.00
571 7.5* 91 100 99 99 2.00
2.74
122 2.0 2.2923655 6.1 91 100 97 98 2.02
689 4.4* 93 100 98 99 2.02
123 2.0 2.0155660 6.1 89 100 96 97 2.02
624 7.6* 91 100 98 98 2.00
533 12.2* 89 100 98 98 2.00
492 15.2* 88 100 98 98 2.00
*OZ feed
37
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Example 27: La~03 (Comparative Example)
100 ml of a Lanthanum Nitrate solution (1.1955 M, prepared by dissolving
414.13 g of La(N03)~6 HBO (Alfa 12915) in water) was rapidly frozen in liquid
nitrogen.
The frozen solution was placed in a freeze dryer (Virtis Corporation, shelves
refrigerated
to 0°C) and evacuated to dryness over a period of S-7 days, or until
completely dry. The
material was calcined in air according to the following schedule: 5
°C/min to 350°C,
350°C for 5 hours, 5°C/min to 525°C, 525°C 1 hour;
10°C /min to room temperature. The
material was pelletized and sieved prior to the reactor evaluation as
described above.
Example 28: Cr0,1 La0,9 Ox
95.068 ml of a Lanthanum Nitrate solution (1.1955 M, prepared by dissolving
414.13 g of La(N03)~6 HZO (Alfa 12915) in sufficient water to make a 1.1955 M
solution)
was simultaneously added to 4.3932 ml of an aqueous solution of chromium
hydroxide
acetate solution (2.5603 M in Cr, as determined by ICP analysis). The solution
was
rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis
Corporation,
shelves refrigerated to 0°C) and evacuated to dryness over a period of
5-7 days, or until
completely dry. The material was calcined in air according to the following
schedule:
5°/min to 350°C, 350°C for 5 hours, 5°C/min to
525°C, 525°C 1 hour; 10°C /min to room
temperature. The material was pelletized and sieved prior to the reactor
evaluation as
described above.
Example 29: Cr0.25Lx0.75 Ox
86.532m1 of a 1.1955 M aqueous lanthanum nitrate solution , prepared as
described in the foregoing Example, was added to a 13.468 ml of an aqueous
solution of
chromium hydroxide acetate (2.5603 M in Cr). The solution was rapidly frozen
in liquid
nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves
refrigerated to 0°C)
and evacuated to dryness over a period of 5-7 days, or until completely dry.
The material
was calcined in air according to the following schedule: 5°C/min to
350°C, 350°C for 5
hours, S°C/min to 525°C, 525°C 1 hour; 10°C/min to
room temperature. The material was
pelletized and sieved prior to the reactor evaluation as described above.
Example 30: Cr0,5 La0.5 Ox
68.169 ml of the previously described 1.1955 M lanthanum nitrate solution was
added to 31.831 ml of an aqueous solution of chromium hydroxide acetate
(2.5603 M in
38
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Cr). The solution was rapidly frozen in liquid nitrogen, placed in a freeze
dryer (Virus
Corporation, shelves refrigerated to 0°C), and evacuated to dryness
over a period of 5-7
days, or until completely dry. The freeze-dried material was then calcined in
air according
to the following schedule: 5°C/min to 350°C, 350°C for 5
hours, 5°C/min to 525°C,
525°C 1 hour; 10°C /min to room temperature. The material was
pelletized and sieved
prior to the reactor evaluation as described above.
Example 31: Crp,75Lap.25 Ox
41.653 ml of an aqueous lanthanum nitrate solution ( 1.1955 M) was added to
58.347 ml of aqueous chromium hydroxide acetate solution (2.5603 M in Cr). The
solution was rapidly frozen in liquid nitrogen, freeze-dried, and calcined as
described in
the foregoing example. The calcined material was then pelletized and sieved
prior to the
reactor evaluation as described above.
Example 32: Crp.9 Lap.l Ox
19.222 ml of aqueous lanthanum nitrate solution ( 1.1955 M) was added to
80.778
ml of an aqueous solution of chromium hydroxide acetate(2.5603 M in Cr)). The
solution
was rapidly frozen in liquid nitrogen, freeze-dried, calcined, pelletized and
sieved, as
described in the foregoing example, after which it was tested in the reduced-
scale reactor
as described under "Test Procedure."
Table 11. La-Cr Ox Series Catalysts
Example No. Composition
27 La~03
28 Cr~.~Lao_9
Ox
29 Cro.Z;Lao.~S
Ox
Cro.SLao_5
Ox
31 Cro.~SLao.~S
Ox
32 Cro_9La~.,
Ox
39
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Table 12. Performance of La-Cr Ox Series Catalysts
Example Vol. Wt. Temp GHSV %CH4 %O~ %C0 %H_~ Hz:CO
No. (mL) (g) (°C) x 104 Conv. Cony. Sel. Sel
27 2.0 2.3996 860 6.1 55 100 70 5~ 1.57
28 2.0 2.2106 830 6.1 43 100 63 46 1.46
29 2.0 1.5846 95~ 6.1 51 100 67 52 1.55
30 2.0 2.2184 746 6.1 59 100 85 72 1.69
31 2.0 1.9359 834 6.1 69 100 87 78 1.79
32 2.0 1.9517 873 6.1 69 100 90 80 1.78
Example 33: Znp.2 Crp.g Ox
14.88 ml of Zn (N03)~ of 1.0 M solution was combined with 119 ml of an
aqueous solution of Cr3(OH)2(CH3C00)~ (0.5 M in Cr) prepared by diluting an
aqueous
2.5603 M chromium hydroxide acetate (Aldrich 31,810-8) solution with
sufficient water to
make a solution that is O.SM in chromium. The mixed solution was rapidly
frozen in
liquid nitrogen. It was placed in a freeze dryer (Virus Corporation, shelves
refrigerated to
0°C) and evacuated to dryness over a period of 5-7 days, or until
completely dry. The
material was calcined in air according to the following schedule:
5°C/min to 350°C, 5
hour soak at 350°C, 5°C/min to 525°C, 525°C soak
for 1 hour; 10°C lmin to room
temperature. The material was sieved prior to the reactor evaluation.
Example 34: Cup,2Crp,g Ox
14.88 ml of Cu (N03)z.3H20, 1.0 M aqueous solution was combined with 119 ml
of an aqueous solution of Cr3(OH)~(CH3C00)~ (0.5M in Cr), prepared as
described in
Example 39. The mixed solution was rapidly frozen in liquid nitrogen. It was
placed in a
freeze dryer (Virus Corporation, shelves refrigerated to 0°C) and
evacuated to dryness
over a period of 5-7 days, or until completely dry. The material was calcined
in air
according to the following schedule: 5°C/min to 350°C, 5 hour
soak at 350°C, 5°C/min to
525°C, 525°C soak for 1 hour; 10°C /min to room
temperature. The material was sieved
prior to the reactor evaluation.
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Example 35: Fe0,2 CrO,g Ox
14.88 ml of Fe(N03)~.6Hz0, 1.0 M aqueous solution was combined with 119 ml
of an aqueous solution of Cr3(OH)z (CH3C00)~ (0.5M in Cr), prepared as
described
above. The mixed solution was rapidly frozen in liquid nitrogen. It was placed
in a freeze
dryer (Virtis Corporation, shelves refrigerated to 0°C) and evacuated
to dryness over a
period of 5-7 days, or until completely dry. The material was calcined in air
according to
the following schedule: 5°C/min to 350°C, 5 hour soak at
350°C, 5°C/min to 525°C,
525°C soak for 1 hour; 10°C /min to room temperature. The
material was sieved prior to
the reactor evaluation.
Example 36: Vp,2Cr0,g Ox
194.66 ml of ammonium metavandate (NH4V03, solution concentration
determined by elemental, ICP analysis to be 0.07644 M in V) was combined with
119 ml
of an aqueous solution of Cr3(OH)z(CH3C00); (0.5M in Cr), prepared as
previously
described. The mixed solution was rapidly frozen in liquid nitrogen. It was
placed in a
freeze dryer (Virtis Corporation, shelves refrigerated to 0°C) and
evacuated to dryness
over a period of 5-7 days, or until completely dry. The material was calcined
in air
according to the following schedule: 5°C/min to 350°C, 5 hour
soak at 350°C, 5°C/min to
525°C, 525°C soak for 1 hour; 10°C /min to room
temperature. The material was sieved
prior to the reactor evaluation.
Example 37: Mnp,2 CrO,g Ox
6.103 g of Mn(N03)z (Alfa, 87848, Mn(N03)z. xH20, contains 22.56 wt % as
Mn) was added to 200 ml of an aqueous solution of Cr3(OH)z(CH~COO)~ (0.5M in
Cr),
prepared as previously described. The mixed solution was rapidly frozen in
liquid
nitrogen. It was placed in a freeze dryer (Virtis Corporation, shelves
refrigerated to 0°C)
and evacuated to dryness over a period of 5-7 days, or until completely dry.
The material
was calcined in air according to the following schedule: 5°C/min to
350°C, 5 hour soak at
350°C, 5 °C/min to 525°C, 525°C soak for 1 hour;
10°C/min to room temperature. The
material was sieved prior to the reactor evaluation.
Example 38: Cop,2 W0.8 Ox
6.0 ml Co(N03)z~6H20 (Alfa, 11341) aqueous solution of 1.0 M solution was
simultaneously combined with 109.639 ml of an aqueous solution of
(NH4)~oW~zOa~ ~5Hz0
(Alfa, 10899) (0.2189 M in W). The mixed solution was rapidly frozen in liquid
nitrogen.
41
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It was placed in a freeze dryer (Virus Corporation, shelves refrigerated to 0
C) and
evacuated to dryness over a period of 5-7 days, or until completely dry. The
material was
calcined in air according to the following schedule: 5°C/min to
350°C, 5 hour soak at
350°C, 5°C/min to 525°C, 525°C soak for 1 hour;
10°C/min to room temperature. The
material was sieved prior to the reactor evaluation.
Example 39: Ni0.2 WO.g Ox
4.682 ml Ni(NO=)~ aqueous solution of 1.068 M solution (determined by ICP
analysis)was simultaneously combined with 91.366 ml of (NHa)~oWlzO.~,.5H~0
(Alfa.
10899) (0.2189M in W). The mixed solution was rapidly frozen in liquid
nitrogen. It was
placed in a freeze dryer (Virtis Corporation, shelves refrigerated to
0°C) and evacuated to
dryness over a period of 5-7 days, or until completely dry. The material was
calcined in
air according to the following schedule: 5°C/min to 350°C, 5
hour soak at 350°C, 5°C/min
to 525°C, 525°C soak for 1 hour; 10°C /min to room
temperature. The material was
sieved prior to the reactor evaluation.
Example 40: Cu0,2W0.8 Ox
1.5177 ml of 3.9535 M Cu(N03)z.sH20 (Aldrich, 2239-5) aqueous solution was
simultaneously combined with 109.639 ml of an aqueous solution of
(NHa)~oW~z~an5Hz0 (Alfa, 10899) (0.2189 M in W). The mixed solution was
rapidly
frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis
Corporation, shelves
refrigerated to 0°C) and evacuated to dryness over a period of 5-7
days, or until
completely dry. The material was calcined in air according to the following
schedule:
5°C/min to 350°C, 5 hour soak at 350°C, 5°C/min to
525°C, 525°C soak for 1 hour; 10°C
/min to room temperature. The material was sieved prior to the reactor
evaluation.
Example 41: Ni0,2 MoO,g Ox
9.363 ml Ni(N03)z aqueous solution of 1.068 M solution (determined by ICP
analysis) was simultaneously combined with 80.0 ml of an aqueous solution of
(NHa)6Mo~04o.4H20 (Alfa, 11831) (0.5M in Mo). The mixed solution was rapidly
frozen
in liquid nitrogen. It was placed in a freeze dryer (Virtis Corporation,
shelves refrigerated
to 0°C) and evacuated to dryness over a period of 5-7 days, or until
completely dry. The
material was calcined in air according to the following schedule:
5°C/min to 350°C, 5
hour soak at 350°C, 5°C/min to 525°C, 525°C soak
for 1 hour; 10°C /min to room
temperature. The material was sieved prior to the reactor evaluation.
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Example 42: Co0,2 Mop,g Ox
10.0 ml Co(N03)Z~6H~0 (Alfa, 11341) aqueous solution of 1.068 M solution
(determined by ICP analysis) was simultaneously combined with 80.0 ml of an
aqueous
solution of (NH,~)6Mo~0ao 4H~0 (Alfa, 11831) (0.5M in Mo). The mixed solution
was
rapidly frozen in liquid nitrogen. It was placed in a freeze dryer (Virtis
Corporation,
shelves refrigerated to 0°C) and evacuated to dryness over a period of
5-7 days, or until
completely dry. The material was calcined in air according to the following
schedule:
5°C/min to 350°C, 5 hour soak at 350°C, 5°C/min to
525°C, 525°C soak for 1 hour;
10°C/min to room temperature. The material was sieved prior to the
reactor evaluation.
Example 43: Cu0,2 Mop,g Ox
2.5295 ml of an aqueous 3.9535 M Cu (N03)~.5 HBO (Aldrich, 2239-5) was
simultaneously combined with 80.0 ml of an aqueous solution of
(I~TH.~)~Mo~O.~o~4H20
(Alfa, 11831 ) (0.5M in Mo). The mixed solution was rapidly frozen in liquid
nitrogen. It
was placed in a freeze dryer (Virtis Corporation, shelves refrigerated to
0°C) and
evacuated to dryness over a period of 5-7 days, or until completely dry. The
material was
calcined in air according to the following schedule: 5°C/min to
350°C, 5 hour soak at
350°C, 5°C/min to 525°C, 525°C soak for 1 hour;
10°C/min to room temperature. The
material was sieved prior to the reactor evaluation.
Table 13. Mp,2Crp,g Ox Series Catalysts
Example No. Composition
33 Zno.2 Cro,g
Ox
34 Cuo.~ Cro.g
Ox
35 Feo.z Cro,B
Ox
36 Vo_2 Cro.g Ox
37 Mno.2 Cro.g
Ox
38 Coo_2 Wo.B Ox
39 Nio.z Wo.g Ox
40 Cuo.2 Wo.g Ox
41 Nio_~ Moo.g
Ox
42 Coo_2 Moo,g
Ox
43 Cuo.z Moo_g
Ox
43
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Table 14. Performance of Mp,2Crp,g Ox Series Catalysts
Example Vol. Wt. Temp GHSV %CH,~ %O~ %C0 %H_, Hz:CO
%Coke
No. (mL) (g) (°C) x 10~ Cony. Cony. Sel. 5e1.
33 2.0 2.4031 696 6.1 73 100 89 85 1.91 0.21
34 2.0 2.4873 748 6.1 62 100 82 74 1.81 17.4
35 2.0 2.6567 778 6.1 69 100 88 84 1.91 0.25
36 2.0 2.0431 703 6.1 59 100 83 74 1.78 0.30
37 2.0 2.4971 777 6.1 66 100 87 82 1.89 0.21
38 2.0 3.4235 604 6.1 little conversion
39 2.0 3.6498 650 6.1 little conversion
40 2.0 3.4032 623 6.1 no conversion
41 2.0 2.5265 652 6.1 little conversion
42 2.0 2.3329 643 6.1 no light-off
43 2.0 2.4575 670 6.1 no light-off
In the series of compositions shown in Table 14, it can be seen that the non-
Cr containing
systems (Examples 39-43) showed no light off when tested as described in the
section
entitled "Test Procedure."
Process of Producing Syngas
Any suitable reaction regime is applied in order to contact the reactants with
the
catalyst. One suitable regime is a fixed bed reaction regime, in which the
catalyst is
retained within a reaction zone in a fixed arrangement. Supported or self
supporting
catalysts may be employed in the fixed bed regime, retained using fixed bed
reaction
techniques well known in the art. Preferably a millisecond contact time
reactor is
employed. Several schemes for carrying out catalytic partial oxidation (CPOX)
of
hydrocarbons in a short contact time reactor have been described in the
literature. For
example, L.D. Schmidt and his colleagues at the University of Minnesota
describe a
millisecond contact time reactor in U.S. Pat. No. 5,648,582 and in J.
Catalysis 138, 267
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282 (1992) for use in the production of synthesis gas by direct oxidation of
methane over a
catalyst such as platinum or rhodium. A general description of major
considerations
involved in operating a reactor using millisecond contact times is given in
U.S. Patent No.
5,654,491. The disclosures of the above-mentioned references are incorporated
herein by
reference.
Accordingly, a feed stream comprising a hydrocarbon feedstock and an oxygen-
containing gas is contacted with one of the above-described chromium-based
catalysts in a
reaction zone maintained at conversion-promoting conditions effective to
produce an
effluent stream comprising carbon monoxide and hydrogen. The hydrocarbon
feedstock
may be any gaseous hydrocarbon having a low boiling point, such as methane,
natural gas,
associated gas, or other sources of light hydrocarbons having from 1 to 5
carbon atoms.
The hydrocarbon feedstock may be a gas arising from naturally occurring
reserves of
methane which contain carbon dioxide. Preferably, the feed comprises at least
50% by
volume methane, more preferably at least 75% by volume, and most preferably at
least
80% by volume methane.
The hydrocarbon feedstock is contacted with the catalyst as a gaseous phase
mixture with an oxygen-containing gas, preferably pure oxygen. The oxygen-
containing
gas may also comprise steam and/or COZ in addition to oxygen. Alternatively,
the
hydrocarbon feedstock is contacted with the catalyst as a mixture with a gas
comprising
steam and/or CO2. Preferably, the methane-containing feed and the oxygen-
containing gas
are mixed in such amounts to give a carbon (i.e., carbon in methane) to oxygen
(i.e.,
oxygen) ratio from about 1.25:1 to about 3.3:1, more preferably, from about
1.3:1 to about
2.2:1, and most preferably from about 1.5:1 to about 2.2:1, especially the
stoichiometric
ratio of 2:1.
The process is operated at atmospheric or superatmospheric pressures, the
latter
being preferred. The pressures may be from about 100 kPa to about 12,500 kPa,
preferably from about 130 kPa to about 10,000 kPa. The process of the present
invention
may be operated at temperatures of from about 600°C to about
1,100°C, preferably from
about 700°C to about 1,000°C. The hydrocarbon feedstock and the
oxygen-containing gas
are preferably pre-heated before contact with the catalyst. The hydrocarbon
feedstock and
the oxygen-containing gas are passed over the catalyst at any of a variety of
space
velocities.
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Gas hourly space velocities (GHSV) for the process, stated as normal liters of
gas
per kilogram of catalyst per hour, are from about 20000 to at least about
100.000,000
NL/kg/h, preferably from about 50,000 to about 50,000,000 NL/kg/h. Preferably
the
catalyst is employed in a millisecond contact time reactor for syngas
production. The
process preferably includes maintaining a catalyst residence time of no more
than 10
milliseconds for the reactant gas mixture. Residence time is the inverse of
the space
velocity, and high space velocity equates to low residence time on the
catalyst. The
effluent stream of product gases, including CO and HZ, emerges from the
reactor.
Although not wishing to be bound by any particular theory, the inventors
believe that the
primary reaction catalyzed by the preferred catalysts described herein is the
partial
oxidation reaction of Equation 2, described above in the background of the
im~ention.
Additionally, other chemical reactions may also occur to a lesser extent,
catalyzed by the
same catalyst composition. For example, in the course of syngas generation,
intermediates
such as COZ + H20 may occur as a result of the oxidation of methane, followed
by a
reforming step to produce CO and HZ. Also, particularly in the presence of
carbon
dioxide-containing feedstock or COZ intermediate, the reaction CH4 + COz -~ 2
CO +
2H2 (3) may also occur during the production of syngas.
While the preferred embodiments of the invention have been show and
described, modifications thereof can be made by one skilled in the art without
departing
from the spirit and teachings of the invention. The embodiments described
herein are
exemplary only, and are not intended to be limiting. Many variations and
modifications of
the invention disclosed herein are possible and are within the scope of the
invention.
Accordingly, the scope of protection is not limited by the description set out
above, but is
only limited by the claims which follow, that scope including all equivalents
of the subject
matter of the claims. The disclosures of U.S. Provisional Application Nos.
60/183,423
and 60/183,575, and the disclosures of all patents and publications cited
herein are
incorporated by reference.
46
