Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHODS FOR PREPARING INTEGRAL CATALYSTS WHILE MAINTAINING
ZEOLITE ACIDITY AND CATALYSTS MADE THEREBY
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No.
61/503,990,
filed July 1, 2011.
BACKGROUND
The present disclosure relates to methods for the preparation of catalysts
containing a
catalytically active transition metal component and an acidic zeolite
component and further
relates to catalysts prepared by the methods. More particularly, the present
disclosure relates
to methods for the preparation of catalysts which avoid ion exchange of the
transition metal
component with the ions within the channels of the acidic zeolite component.
Bifunctional catalysts prepared by depositing at least one catalytically
active
transition metal component onto an acidic component such as a zeolite are
known for use in
catalytic processes including, for example, synthesis gas conversion and
hydrotreating. Such
uses may benefit from the acid function of the zeolite. For instance, the acid
component may
catalyze skeletal isomerization, cracking and alkylation reactions.
Fischer-Tropsch (FT) catalysts and their preparation methods are known. FT
catalysts
are typically based on Group 8-10 metals such as, for example, iron, cobalt,
nickel and
ruthenium, also referred to herein as "FT components," "FT active metals" or
simply "FT
metals," with iron and cobalt being the most common. The product distribution
over such
catalysts is non-selective and is generally governed by the Anderson-Schulz-
Flory (ASF)
polymerization kinetics. Recent developments have led to so-called "hybrid FT"
or "integral
FT" catalysts having improved properties involving an FT component bound on an
acidic
component, typically a zeolite component. The catalytic functionality of
hybrid or integral FT
catalysts allows conversion of synthesis gas to desired liquid hydrocarbon
products by
minimizing product chain growth, thus precluding the need for further
hydrocracking to
obtain desired products. Thus, the combination of an FT component displaying
high
selectivity to short-chain a-olefins and oxygenates with zeolite(s) results in
an enhanced
selectivity for pourable, wax free liquid products by promoting
oligomerization, cracking,
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isomerization, and/or aromatization reactions on the zeolite acid sites.
Hybrid or integral FT
catalysts for the conversion of synthesis gas to liquid hydrocarbons have been
described, for
example, in co-pending U.S. Patent Application Number 12/343,534 and U.S.
Patent
7,943,674 issued May, 17, 2011 (Kibby et al.), which are herein incorporated
by reference.
Hybrid or integral FT catalysts are typically prepared by wet impregnation
methods
using aqueous or non-aqueous solutions of metal salts. During the course of
this impregnation
and the resultant drying and calcination, a portion of the FT metal ions
(cations) migrate into
the zeolite channels and essentially titrate the acid sites through ion
exchange with protons in
the zeolite channels. Ion exchange of the FT metal for protons within the
zeolite has two
disadvantages. First, zeolite acidity necessary to crack or isomerize FT
olefins and to avoid
making a solid wax component is neutralized. Second, ion-exchanged FT metal is
non-
reducible by virtue of strong metal-support interactions thus decreasing the
activity of the
catalyst and the overall productivity of the FT reaction. For cobalt FT metal,
the ion exchange
sites are quite stable positions and cobalt ions in these positions are not
readily reduced
during normal activation procedures. The reduction in the amount of reducible
cobalt
decreases the activity of the FT component in the catalyst.
A method is needed to prepare a bifunctional catalyst in which a metal is
deposited
onto a zeolite surface while minimizing ion exchange of metal cations with
protons within the
zeolite channels, such that both the zeolite acid capacity and metal activity
are maintained.
SUMMARY
In one aspect, a method is provided for preparing a catalyst which includes
the steps
of conducting ion exchange of a zeolite with an ammonium cation to form an ion
exchanged
zeolite, depositing a catalytically active component comprising a transition
metal onto the ion
exchanged zeolite to form an intermediate integral catalyst, and heating the
intermediate
integral catalyst at sufficient temperature to decompose the catalytically
active component
and generate the H+ form of the zeolite.
In another aspect, a method is provided for preparing a catalyst which
includes the
steps of conducting initial ion exchange of a zeolite with a cation selected
from the group
consisting of Na, K, Ca, Li, Rb, Be, Mg, Sr, Ca, Ba and ammonium ions and
mixtures thereof
to form an ion exchanged zeolite, depositing a catalytically active component
comprising a
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transition metal onto the ion exchanged zeolite to form an intermediate
integral catalyst, and
heating the intermediate integral catalyst at sufficient temperature to
decompose the
catalytically active component. The method further includes conducting
secondary ion
exchange of the intermediate integral catalyst with ammonium to form an
ammonium treated
catalyst wherein ammonium ions exchange with the cation of the ion exchanged
zeolite, and
heating the ammonium treated catalyst at sufficient temperature to decompose
the ammonium
to ammonia and generate the H+ form of the zeolite.
DETAILED DESCRIPTION
The present disclosure relates to methods for the preparation of bifunctional
catalysts
comprising a transition metal supported by a zeolite without any appreciable
ion exchange of
the transition metal cations with the protons within the zeolite channels. The
protons bound to
the zeolite acid sites within the zeolite channels are protected from exchange
with metal
cations by first protecting the zeolite acid sites with protecting cations
prior to deposition of
the metal. The metal can then be decomposed to a stable oxide, and the
protecting cations can
subsequently be removed under conditions which do not promote migration of the
metal into
the zeolite channels.
As used herein, the terms "bifunctional catalyst" and "integral catalyst"
refer
interchangeably to a catalyst containing at least a catalytically active metal
component and a
zeolite component.
In some embodiments, the catalysts of the present disclosure are useful as
hydrotreating catalysts and contain at least one transition metal component
selected from
Groups 8-11 of the IUPAC Periodic Table (2011) deposited onto a zeolite
component. For
example, the transition metal component can be platinum or palladium.
In some embodiments, the catalysts of the present disclosure are hybrid
Fischer-
Tropsch (FT) catalysts. The phrases "hybrid FT catalyst," "integral FT
catalyst" and "integral
synthesis gas conversion catalyst" refer interchangeably to a catalyst
containing at least one
FT metal component selected from the group consisting of cobalt, iron,
ruthenium and
mixtures thereof as well as a zeolite component containing the appropriate
functionality to
convert the primary Fischer-Tropsch products into desired products, i.e.,
minimize the
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amount of heavier, C21+ products. The FT component is preferably cobalt. Thus,
the
combination of a FT component displaying high selectivity to short-chain a-
olefins and
oxygenates with a zeolite component, interchangeably referred to as an "acidic
component,"
results in an enhanced C5+ selectivity by promoting combinations of
oligomerization,
cracking, isomerization, and/or aromatization reactions on the zeolite acid
sites. Desired
hydrocarbon mixtures, including, for example, diesel range products, can be
produced in a
single reactor using hybrid FT catalysts by combining a FT component with an
acidic zeolite
component. Primary waxy products, when formed on the FT component, are
cracked/
hydrocracked (i.e., by the acidic zeolite component) into mainly branched
hydrocarbons with
limited formation of aromatics. In particular, the presently disclosed hybrid
FT catalyst can
be run under certain FT reaction conditions to provide liquid hydrocarbon
mixtures or
products containing less than about 10 weight % CH4 and less than about 5
weight % C21+.
The products formed can be substantially free of solid wax, i.e., C21+
paraffins, by which is
meant that there is minimal soluble solid wax phase at ambient conditions,
i.e., 20 C. at 1
atmosphere. As a result, there is no need to separately treat a wax phase in
hydrocarbons
effluent from a reactor.
In the integral FT catalyst according to the present disclosure, the FT metal
is
distributed as small crystallites on a binder such as alumina in combination
with the zeolite
component. The FT metal content of the integral FT catalyst can depend on the
alumina
content of the zeolite. For example, for a binder content of about 20 weight %
to about 99
weight % based upon the weight of the binder and zeolite, the catalyst can
contain, for
example, from about 1 to about 20 weight % FT metal, preferably 5 to about 15
weight % FT
metal, based on total catalyst weight, at the lowest binder content. At the
highest binder
content, the catalyst can contain, for example, from about 5 to about 30
weight % FT metal,
preferably from about 10 to about 25 weight % FT metal, based on total
catalyst weight. By
way of example and not limitation, suitable binder materials include alumina,
silica, titania,
magnesia, zirconia, chromia, thoria, boria, beryllia and mixtures thereof
A zeolite is a molecular sieve or crystalline material having regular passages
(pores)
that contains silica in the tetrahedral framework positions. Examples include,
but are not
limited to, silica-only (silicates), silica-alumina (aluminosilicates), silica-
boron
(borosilicates), silica-germanium (germanosilicates), alumina-germanium,
silica-gallium
(gallosilicates) and silica-titania (titanosilicates), and mixtures thereof If
examined over
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several unit cells of the structure, the pores will form an axis based on the
same units in the
repeating crystalline structure. While the overall path of the pore will be
aligned with the
pore axis, within a unit cell, the pore may diverge from the axis, and it may
expand in size (to
form cages) or narrow. The axis of the pore is frequently parallel with one of
the axes of the
crystal. The narrowest position along a pore is the pore mouth. The pore size
refers to the
size of the pore mouth. The pore size is calculated by counting the number of
tetrahedral
positions that form the perimeter of the pore mouth. A pore that has 10
tetrahedral positions
in its pore mouth is commonly called a 10 membered ring pore. Pores of
relevance to
catalysis in this application have pore sizes of 8 tetrahedral positions
(members) or greater. If
a molecular sieve has only one type of relevant pore with an axis in the same
orientation to
the crystal structure, it is called 1-dimensional. Molecular sieves may have
pores of different
structures or may have pores with the same structure but oriented in more than
one axis
related to the crystal.
In the acid form of a zeolite, also referred to as the H+ form, the acid sites
are formed
since a charge balancing cation is needed due the presence of aluminum in the
Si02
framework. If the cation is a proton, as is the case for suitable zeolites for
use in the present
method and catalyst, the zeolite will have Bronsted acidity.
Small pore molecular sieves are defined herein as those having 6 or 8 membered
rings; medium pore molecular sieves are defined as those having 10 membered
rings; large
pore molecular sieves are defined as those having 12 membered rings; extra-
large molecular
sieves are defined as those having 14+ membered rings.
Mesoporous molecular sieves are defined herein as those having average pore
diameters between 2 and 50 nm. Representative examples include the M41 class
of
materials, e.g. MCM-41, in addition to materials known as SBA-15, TUD-1, HMM-
33, and
FSM-16.
Exemplary medium pore molecular sieves include, but are not limited to,
designated
EU-1, ferrierite, heulandite, clinoptilolite, ZSM-11, ZSM-5, ZSM-57, ZSM-23,
ZSM-48,
MCM-22, NU-87, SSZ-44, SSZ-58, SSZ-35, SSZ-46 (MEL), SSZ-57, SSZ-70, SSZ-74,
SUZ-4, Theta-1, TNU-9, IM-5 (IMF), ITQ-13 (ITH), ITQ-34 (ITR), and
silicoaluminophosphates designated SAPO-11 (AEL) and SAPO-41 (AFO). The three
letter
designation is the name assigned by the IUPAC Commission on Zeolite
Nomenclature.
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Exemplary large pore molecular sieves include, but are not limited to,
designated Beta
(BEA), CIT-1, Faujasite, H-Y, Linde Type L, Mordenite, ZSM-10 (MOZ), ZSM-12,
ZSM-18
(MET), MCM-68, gmelinite (GME), cancrinite (CAN), mazzite/omega (MAZ), SSZ-26
(CON), MTT (e.g., SSZ-32, ZSM-23 and the like), SSZ-33 (CON), SSZ-37 (NES),
SSZ-41
(VET), SSZ-42 (IFR), SSZ-48, SSZ-55 (ATS), SSZ-60, SSZ-64, SSZ-65 (SSF), ITQ-
22
(IWW), ITQ-24 (IWR), ITQ-26 (IWS), ITQ-27 (IWV), and silicoaluminophosphates
designated SAPO-5 (AFT), SAPO-40 (AFR), SAPO-31 (ATO), SAPO-36 (ATS) and SSZ-
51
(SFO).
Exemplary extra large pore molecular sieves include, but are not limited to,
designated CIT-5, UTD-1 (DON), SSZ-53, SSZ-59, and silicoaluminophosphate VPI-
5
(VFI).
The zeolite of the catalysts of the present disclosure may be herein referred
to as the
"acidic component" which may encompass the above zeolitic materials. The Si/A1
ratio for the
zeolite can be 10 or greater, for example, between about 10 and 100. The
acidic component may
also encompass non-zeolitic materials such as by way of example, but not
limited to,
amorphous silica-alumina, tungstated zirconia, non-zeolitic crystalline small
pore molecular
sieves, non-zeolitic crystalline medium pore molecular sieves, non-zeolitic
crystalline large
and extra large pore molecular sieves, mesoporous molecular sieves and non-
zeolite analogs.
According to one embodiment, the zeolite is initially in the form of an
extrudate
comprising zeolite in a binder matrix. Such zeolite materials can be made by
known extrusion
means or may be purchased. Suitable binder matrix materials useful for forming
the extrudate
include, for example, solids of alumina, silica, titania, magnesia, zirconia,
chromia, thoria,
boria, beryllia and mixtures thereof The zeolite extrudate can have an
external surface area of
between about 10 m2/g and about 300 m2/g, a porosity of between about 30 and
80%, and a crush
strength of between about 1.25 and 5 lb/mm.
In one embodiment, a suitable zeolite extrudate is subjected to an initial ion
exchange
step with a suitable protecting cation to effect ion-exchange of the acidic
protons with the
protecting cation, thus forming an ion exchanged zeolite extrudate.
Suitable protecting cations include, for example, Na, K, Ca, Li, Rb, Be, Mg,
Sr, Ca, Ba
and ionic ammonium complexes and mixtures thereof in soluble solution.
Solutions of
sodium ions are preferred such as may be found as sodium chloride solutions.
The solution of
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the protecting cation will typically be in the range of from about 0.01 M to
the limit of
solubility, preferably about 0.1 M to about 10 M, more preferably about 0.5 M
to about 5 M
and most preferably from about 0.5 M to about 1.0 M. Generally, the zeolite
and protecting
cation(s) are brought into contact in a vessel suitable for this purpose with
stirring. Heat may
be added as necessary for any suitable length of time to effect the ion
exchange of the
protecting cation. Most often when heat is employed, less than about 100 C
will be effective.
The condition in which this step is carried is not restrictive and a skilled
artisan will be able
to determine any appropriate conditions to achieve the desired reaction.
In an alternative embodiment, a suitable zeolite extrudate which is already in
the ion
exchanged form, i.e. in the Na + form, can be obtained commercially, thus
obviating the need to
conduct the initial ion exchange step in order to protect the acid sites.
Protection of the acid sites on the zeolite is followed by deposition of the
transition
metal by any suitable technique well known to those skilled in the art so as
to distend the
metal in a uniform thin layer on the catalyst zeolite support which may
include, but not
limited to, precipitation, impregnation and the like. For example, a method to
deposit the
metal onto the zeolite support may involve an impregnation technique using an
aqueous or
nonaqueous solvent solution containing a soluble metal salt and, if desired, a
soluble
promoter metal, in order to achieve the necessary metal loading and
distribution required to
provide a highly selective and active catalyst. For example, for the
deposition of cobalt in the
preparation of a hybrid FT catalyst, suitable cobalt salts include, but are
not limited to, cobalt
nitrate, cobalt acetate, cobalt carbonyl, cobalt acetylacetonate, or the like.
Suitable promoters
include platinum, palladium, rhenium, iridium, silver, copper, gold,
manganese, ruthenium
and combinations thereof
In one preferred embodiment, a ruthenium promoter is included with a primary
cobalt
FT component in the preparation of a hybrid FT catalyst. These catalysts have
very high
activities due to easy activation at low temperatures. In the preparation of
ruthenium
promoted catalysts, any suitable ruthenium salt, such as ruthenium nitrate,
chloride, acetate or
the like can be used. Descriptions of known methods for preparing hybrid FT
catalysts
including cobalt and ruthenium are described in U.S. Patents 4,088,671to
Kobylinski, and
5,756,419 and 5,939,350 to Chaumette et al. For a catalyst containing about 10
weight %
cobalt, the amount of ruthenium can be from about 0.01 to about 0.50 weight %,
for example,
from about 0.05 to about 0.25 weight % based upon total catalyst weight. The
amount of
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ruthenium would accordingly be proportionately higher or lower for higher or
lower cobalt
levels, respectively. A catalyst level of about 10 weight % has been found to
best for 80
weight % ZSM-5 zeolite and 20 weight % alumina binder. The amount of cobalt
can be
increased as amount of alumina increases, up to about 20 weight % cobalt.
The transition metal along with the promoter can be deposited on the zeolite
support
material by the "incipient wetness" technique for instance. Such technique is
well known and
requires that the volume of solvent solution be predetermined so as to provide
the minimum
volume which will just wet the entire surface of the zeolite support, with no
excess liquid.
Alternatively, the excess solution technique can be utilized if desired. If
the excess solution
technique is utilized, then the excess solvent present, e.g., acetone, is
merely removed by
evaporation. Alternatively, vapor deposition or any other suitable means for
depositing the
transition metal can be used as would be apparent to one skilled in the art.
Suitable solvents include, for example, water; ketones, such as acetone,
butanone
(methyl ethyl ketone); the lower alcohols, e.g., methanol, ethanol, propanol
and the like;
amides, such as dimethyl formamide; amines, such as butylamine; ethers, such
as diethylether
and tetrahydrofuran; hydrocarbons, such as pentane and hexane; and mixtures of
the
foregoing solvents. For example, the solvents can be acetone or
tetrahydrofuran.
Next, the solvent solution and zeolite extrudate can be stirred while
evaporating the
solvent at a temperature of from about 25 C. to about 50 C. until "dryness."
The impregnated catalyst is slowly dried at a temperature of from about 110
C. to
about 120 C. for a period of about 1 hour so as to spread the metals over the
entire zeolite
extrudate to form an intermediate integral catalyst. The drying step is
conducted at a very
slow rate in air.
The dried catalyst, i.e., the intermediate integral catalyst, may be reduced
directly in
hydrogen or it may be calcined first. A single calcination step to decompose
nitrates is
simpler if multiple impregnations are needed to provide the desired metal
loading. Reduction
in hydrogen requires a prior purge with inert gas, a subsequent purge with
inert gas and a
passivation step in addition to the reduction itself, as described later as
part of the activation.
However, impregnation of the transition metal salt should be carried out in a
dry, oxygen-free
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atmosphere and it should be decomposed directly, then passivated, if the
benefits of its lower
oxidation state are to be maintained.
The dried catalyst is calcined by heating slowly in flowing air, for example,
about 10
cc/gram/minute, to a temperature between about 100 C and about 500 C., even
in the range
of from about 200 C. to about 350 C, for example, from about 250 C. to
about 300 C.,
that is sufficient to decompose the metal salts and fix the metals as metal
oxides. The
aforesaid drying and calcination steps can be done separately or can be
combined. However,
calcination should be conducted by using a slow heating rate of, for example,
about 0.5 C. to
about 3 C. per minute or from about 0.5 C. to about 1 C. per minute and the
catalyst
should be held at the maximum temperature for a period of about 1 to about 20
hours, for
example, for about 2 hours.
The foregoing impregnation steps are repeated with additional substantially
aqueous
or non-aqueous solutions in order to obtain the desired metal loading.
Promoter metal oxides
are conveniently added together with the transition metal, but they may be
added in other
impregnation steps, separately or in combination, either before, after, or
between
impregnations of transition metal.
Next, a secondary ion exchange step is conducted with an ionic ammonium
complex or
salt, also referred to herein as simply "ammonium," to exchange the protecting
cations in the
zeolite with the ionic ammonium complex to form an NH4 + form of the zeolite.
For the
purposes of the present disclosure, the secondary ion exchange step
encompasses not only an
ion exchange process as previously described, involving stirring the zeolite
with a cation-
containing solution containing the ionic ammonium complex, but also contacting
the zeolite
with the cation-containing solution containing the ionic ammonium complex by
incipient wet
impregnation or excess solution such that ion exchange occurs with the cations
in the zeolite.
Suitable ionic ammonium complexes can be selected from ammonium nitrate,
ammonium
chloride, ammonium carbonate and the like.
Following the secondary ion exchange step, the ammonium treated catalyst is
dried and
subjected to heating at a temperature sufficient to decompose the ammonium to
ammonia
which is released and H+, which restores the acidity of the zeolite, i.e.
generates the acid or H+
form of the zeolite, since a cation is required for each aluminum atom for
charge balance. This
temperature can be less than about 500 C., even between about 350 C. and 500
C.
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In an alternative embodiment, the protecting cation used is ammonium. Thus,
the initial
ion exchange step occurs by exchanging ammonium cations with the zeolite
extrudate protons.
In this embodiment, following the transition metal deposition and drying, the
intermediate
integral catalyst is subjected to mild heat treatment at a temperature less
than about 500 C.,
even between about 350 C. and 500 C., upon which the ammonium decomposes to
ammonia
and H. Again, the decomposition of ammonium converts the zeolite back to the
acid or H+
form of the zeolite. In this embodiment, advantageously, no secondary ion
exchange step is
required.
While the above methods have assumed starting with the zeolite in the form of
an
extrudate comprising zeolite in a matrix of binder, the scope of the present
disclosure
includes alternative methods for forming the catalyst. For example, according
to one
embodiment, the initial form of the zeolite can be a powder. The zeolite
powder can be
subjected to an initial ion exchange step, or a commercial zeolite already in
the Na + form can
be obtained. To this can be added the transition metal (with optional
promoters) deposited
onto a binder material. Suitable binder materials have previously been
described. Suitable
methods for depositing the metal onto the binder material are the same as
those described for
depositing the metal onto a zeolite extrudate, i.e., by wet impregnation,
excess solution or
vapor deposition techniques and the like. The combination of ion exchanged
zeolite powder
and metal/binder can then be formed into an integral catalyst by extrusion.
According to yet another embodiment, the transition metal can be deposited
directly
onto a zeolite powder by any of the previously described deposition methods,
and the
resulting metal/zeolite particles can be combined with a binder matrix and
formed into an
integral or bifunctional catalyst by extrusion.
Integral or bifunctional catalysts prepared according to any of the methods
disclosed
herein maintain full zeolite acidity after transition metal deposition with
the metal highly
dispersed and of optimum particle size for good catalytic activity.
Substantially all of the metal
is in the form of reduced crystallites of metal located outside the zeolite
channels with little or
none of the metal located within the zeolite channels. No appreciable ion
exchange of the metal
therefore occurs within the zeolite channels. As a result, the percentage of
residual acid sites is
at least about 50%, even at least about 80%, even at least about 90%, even at
least about 95%
and even about 100%. As defined herein, "percentage of residual acid sites"
refers to the
percentage of acidity of the integral catalyst as measured by FTIR
spectrometer in umol
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Bronsted acid sites per gram zeolite relative to the acidity of the zeolite
component used in the
integral catalyst having no additional components thereon. In other words, the
acid site density
of the integral catalyst as measured by FTIR spectrometer in nmol Bronsted
acid sites per
gram is at least about 50%, even at least about 80%, even at least about 90%,
even at least
about 95% and even about 100% of the acid site density of the zeolite
component used in the
integral catalyst having no additional component. The high percentage of
residual acid sites
allows for maximum utilization of metal for catalytic activity, since any
metal that exchanges
will not be available for catalysis. No separate, undesirable aluminate phase
is formed.
The integral catalyst prepared according to any of the foregoing methods can
optionally be further activated prior to use in a synthesis gas conversion
process by either
reduction in hydrogen or successive reduction-oxidation-reduction (ROR)
treatments. The
reduction or ROR activation treatment is conducted at a temperature
considerably below
about 500 C. in order to achieve the desired increase in activity and
selectivity of the integral
catalyst. Temperatures of 500 C. or above reduce activity and liquid
hydrocarbon selectivity
of the catalyst. Suitable reduction or ROR activation temperatures are below
500 C.,
preferably below 450 C. and most preferably, at or below 400 C. Thus, ranges
of about
100 C. or 150 C. to about 450 C, for example, about 250 C. to about 400
C. are suitable
for the reduction steps. The oxidation step should be limited to about 200 C.
to about 300
C. These activation steps are conducted while heating at a rate of from about
0.1 C. to about
5 C, for example, from about 0.10 C to about 2 C.
The catalyst can be slowly reduced in the presence of hydrogen. If the
catalyst has
been calcined after each impregnation, to decompose nitrates or other salts,
then the reduction
may be performed in one step, after an inert gas purge, with heating in a
single temperature
ramp (e.g., 1 C./min.) to the maximum temperature and held at that
temperature, from about
250 C. or 300 C. to about 450 C, for example, from about 350 C. to about
400 C., for a
hold time of 6 to about 65 hours, for example, from about 16 to about 24
hours. Pure
hydrogen is preferred in the first reduction step. If nitrates are still
present, the reduction is
preferably conducted in two steps wherein the first reduction heating step is
carried out at a
slow heating rate of no more than about 5 C. per minute, for example, from
about 0.1 C. to
about 1 C. per minute up to a maximum hold temperature of about 200 C. to
about 300 C.,
for example, about 200 C. to about 250 C., for a hold time of from about 6
to about 24
hours, for example, from about 16 to about 24 hours under ambient pressure
conditions. In
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the second treating step of the first reduction, the catalyst can be heated at
from about 0.5 C.
to about 3 C. per minute, for example, from about 0.1 C. to about 1 C. per
minute to a
maximum hold temperature of from about 250 C. or 300 C. up to about 450 C.,
for
example, from about 350 C. to about 400 C. for a hold time of 6 to about 65
hours, for
example, from about 16 to about 24 hours. Although pure hydrogen is preferred
for these
reduction steps, a mixture of hydrogen and nitrogen can be utilized.
Thus, the reduction may involve the use of a mixture of hydrogen and nitrogen
at
about 100 C. for about one hour; increasing the temperature about 0.5 C. per
minute until a
temperature of about 200 C; holding that temperature for approximately 30
minutes; and
then increasing the temperature about 1 C. per minute until a temperature of
about 350 C. is
reached and then continuing the reduction for approximately 16 hours.
Reduction should be
conducted slowly enough and the flow of the reducing gas maintained high
enough to
maintain the partial pressure of water in the offgas below 1%, so as to avoid
excessive
steaming of the exit end of the catalyst bed. Before and after all reductions,
the catalyst
should be purged in an inert gas such as nitrogen, argon or helium.
The reduced catalyst can be passivated at ambient temperature (about 25 C. to
about
35 C.) by flowing diluted air over the catalyst slowly enough so that a
controlled exotherm
of no larger than +50 C. passes through the catalyst bed. After passivation,
the catalyst is
heated slowly in diluted air to a temperature of from about 300 C. to about
350 C.,
preferably 300 C., in the same manner as previously described in connection
with
calcination of the catalyst.
The temperature of the exotherm during the oxidation step should be less than
about
100 C., and will be about 50 C. to about 60 C. if the flow rate and/or the
oxygen
concentration are dilute enough.
Next, the reoxidized catalyst is then slowly reduced again in the presence of
hydrogen, in the same manner as previously described in connection with the
initial reduction
of the catalyst. Since nitrates are no longer present, this reduction may be
accomplished in a
single temperature ramp and held, as described above for reduction of calcined
catalysts.
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EXAMPLES
The methods and catalysts of the present disclosure will be further
illustrated by the
following examples, which set forth particularly advantageous method
embodiments. While
the Examples are provided to illustrate the invention, they are not intended
to limit it. This
application is intended to cover those various changes and substitutions that
may be made by
those skilled in the art without departing from the spirit and scope of the
present disclosure.
Analytical Methods
Zeolite Acidity was measured using a Nicolet 6700 FTIR spectrometer with MCT
detector (available from Thermo Fisher Scientific Inc.). Materials were
pressed into self
supporting wafers (about 5 to about 15 mg/cm2) and degassed by heating under
vacuum at
about 1 C./min to about 350 C. and held at that temperature for about 1 hr
before measuring
spectra at about 80 C. in transmission mode. Spectra were recorded with 128
scans from
about 400 to about 4000 cm-1 with a resolution of about 4 cm-1. Total acidity
was estimated
using the integrated area of acidic OH resonance centered near 3610 cm-1 and
correcting for
the pellet weight and Co concentration.
Percentage of Residual Acid Sites was calculated by dividing the acidity
measurement
of an integral FT catalyst sample by the acidity measurement of the zeolite
component having
no additional component, i.e., no FT metal component, thereon. In other words,
percentage of
residual acid sites is the percentage of retained acidity in the integral
catalyst relative to the
zeolite. For example, an extrudate consisting of about 80 wt% H-ZSM-5 and
about 20 wt%
A1203 would have an acidity of 100%. A cobalt exchanged catalyst prepared on
this support
would have an acidity of 100% if it retained all of the acid sites. The error
for this
measurement is less than 10% absolute.
Example 1
A zeolite extrudate was obtained from Zeolyst International, Conshohocken,
Pennsylvania (CBV-8014) containing about 80 wt% H-ZSM-5 and about 20 wt% A1203
and
having 252 umol Bronsted acid sites per gram of zeolite. The extrudate was ion-
exchanged
twice with sodium cations. Each ion exchange used 10 g extrudate that was
stirred in a 0.5 M
aqueous NaC1 solution at about 80 C. for about 1 hr. The zeolite was filtered
and washed
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with 2 L of deionized water after each exchange. A cobalt solution was
prepared by
dissolving about 15.07 g Co(NO3)2 6H20 in about 20 g deionized water. The
zeolite
containing sodium cations was dried in a box furnace at about 120 C. with
flowing dry air. It
was impregnated with the above solution by adding about 2.04 g dropwise to
about 3.94 g
zeolite extrudate. The material was then heated to about 120 C. in air at
about 1 C./min and
held at that temperature for about 1 hr, then heated to about 350 C. at about
2.3 C./min and
held at that temperature for about 5 hr. The cobalt impregnated extrudate was
then ion-
exchanged with about 0.5 M aqueous NH4NO3 solution at about 80 C for about
1.5 hr. Next
the material was heated to about 120 C. in air at about 1 C./min and held at
that temperature
for about 1 hr, then heated to about 500 C. at about 1 C./min and held at
that temperature
for about 5 hr. The acidity measurement and percentage residual acid sites are
shown in Table
1.
Example 2
The Co impregnated zeolite from Example 1, after the first ion exchange with
about
0.5 M aqueous NH4NO3, was ion exchanged two more times with about 0.5 M
aqueous
NH4NO3 at 80 C. Next the product was heated to about 120 C. in air at about
1 C./min and
held at that temperature for about 1 hr, then heated to about 500 C. at about
1 C./min and
held at that temperature for 5 hr. The acidity measurement and percentage
residual acid sites
are shown in Table 1.
Example 3
A zeolite extrudate was obtained from Zeolyst International (CBV-8014) that
contained about 80% H-ZSM-5 and about 20% A1203. The extrudate was ion-
exchanged
three times with ammonium cations. Each ion exchange uses 10 g extrudate that
is stirred in a
0.5 M aqueous NH4NO3 solution at about 80 C. for about 1 hr. The zeolite was
filtered and
washed with 2 L of deionized water after each exchange. A cobalt solution was
prepared by
dissolving about 15.07 g Co(NO3)2 6H20 in about 20 g deionized water. The
zeolite
containing ammonium cations was dried in a box furnace at about 120 C. with
flowing dry
air. It was impregnated with the above solution by adding about 2.3 g dropwise
to about 4 g
zeolite extrudate. The material was then heated to about 120 C. in air at
about 1 C./min and
held at that temperature for about 1 hr, then heated to about 350 C. at about
2.3 C./min and
held at that temperature for about 5 hr. The acidity of the resulting material
was measured
using FTIR and the results are shown in Table 1.
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Comparative Example
A cobalt solution was prepared by dissolving about 15.07 g Co(NO3)2 6H20 in
about
20 g deionized water. A zeolite extrudate was obtained from Zeolyst
International (CBV-
8014) that contained about 80 wt% H-ZSM-5 and about 20 wt% A1203. The zeolite
was dried
in a box furnace at about 120 C. with flowing dry air. The support was
impregnated with the
above solution by adding about 1.45 g dropwise to about 2.87 g zeolite
extrudate. The
product was then heated to about 120 C. in air at about 1 C./min and held at
that
temperature for about 1 hr, then heated to about 350 C. at about 2.3 C./min
and held at that
temperature for about 5 hr. The acidity measurement and percentage residual
acid sites are
shown in Table 1.
Table 1
Sample Acidity measurement, % Residual Acid
Sites
umol Bronsted acid
sites per gram zeolite
Example 1 204 81
Example 2 224 89
Example 3 189 75
Comparative Example 156 62
Where permitted, all publications, patents and patent applications cited in
this
application are herein incorporated by reference, to the extent such
disclosure is not
inconsistent with the present invention.
Unless otherwise specified, the recitation of a genus of elements, materials
or other
components, from which an individual component or mixture of components can be
selected,
is intended to include all possible sub-generic combinations of the listed
components and
mixtures thereof Also, "comprise," "include" and its variants, are intended to
be non-
limiting, such that recitation of items in a list is not to the exclusion of
other like items that
may also be useful in the materials, compositions, methods and systems of this
invention.
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From the above description, those skilled in the art will perceive
improvements,
changes and modifications, which are intended to be covered by the appended
claims.
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