Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR PRODUCING A METHANATION CATALYST AND A PROCESS FOR THE
METHANATION OF SYNTHESIS GAS
The invention relates to a process for producing a methanation catalyst and a
process for the
methanation of CO- and/or CO2-comprising gas streams, preferably at high
temperatures. To
produce the catalyst, hydrotalcite-comprising starting material is brought
into contact with a
fusible metal salt, preferably a salt comprising nickel nitrate, intimately
mixed and subjected to
a.) a thermal treatment step and b.) a calcination step.
The use of methanation for producing synthetic natural gas has been of great
economic and
industrial interest for half a century. The synthetic natural gas which can be
produced by
methanation is frequently referred to as substituted natural gas or SNG.
To present the prior art in the field of methanation, a brief review of the
development of
methanation processes and methanation catalysts will be given below.
Catalysts based on nickel-comprising active components have been used for
methanation for
many decades. In many of these catalysts, the nickel is present together with
an oxidic support
composed of aluminum oxide. The catalysts are often produced by precipitation
of the active
components in the presence of an aluminum-comprising support component or by
coprecipitation of active component and support component. The products
obtained in the
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precipitation are firstly dried and subsequently calcined. To obtain the
catalyst in a suitable
particle size, a shaping process is frequently inserted between drying and
calcination.
Thus, for example, US 3,912,775 describes the production of a precipitation
product having the
composition Ni6Al2(OH)16CO3.4H20 which is obtained by precipitation of nickel
nitrate and
aluminum nitrate from aqueous solution by means of sodium carbonate solution.
The
precipitation can also be carried out in the presence of a support component.
Furthermore, it is
disclosed that the precipitation product is dried at a temperature in the
range from 80 to 180 C
and calcined at a temperature in the range from 300 to 550 C. In the
production process, the
temperature increase between the drying process and the calcination process is
effected using
a temperature gradient by means of a controlled heating rate. To produce the
methane-
comprising product gas, naphtha and steam are used as starting materials and
are brought into
contact with the active composition at a temperature in the range from 270 C
to 460 C and a
pressure in the range 15.8 ¨ 29.6 bar.
The efficiency of nickel-comprising catalysts for methanation can, according
to US 3,865,753,
be increased by additionally adding magnesium species to the aluminum-
comprising synthesis
system. This synthesis and subsequent heat treatment gives a nickel-comprising
magnesium
aluminate as active composition which displays a high activity and stability
in respect of
methanation. As regards the precipitation product, it is advised that the
divalent metals
(magnesium and nickel) and the trivalent aluminum be present in a molar ratio
of at least 1:1,
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with a preferred molar ratio of M2+ to M3+ being in the range from 2.5:1 to
3:1. The increased
activity of the catalyst is also explained by, after drying, calcination and
reduction, a magnesium
spinel being formed during the reaction.
US 3,988,262 discloses an improved catalyst which is obtained by the nickel-
comprising
component being deposited on the aluminum-comprising support in the presence
of zirconium
oxide. The catalyst according to the invention has a nickel oxide content of
from 15 to 40% by
weight, with a large part of the nickel oxide being reduced to nickel before
commencement of
the methanation.
According to DE 26 24 396, the thermal stability of the methanation catalysts
can be increased
by the catalyst having a certain proportion of molybdenum oxide. A molybdenum
content of from
0.25 to 8% by weight of molybdenum or molybdenum oxide has been found to be
advantageous.
EP 2 308 594 A2 discloses a nickel-comprising catalyst for producing synthesis
gas from
methane, water and carbon dioxide in a ratio in the range of 1.0/1.0-2.0/0.3-
0.6. The improved
stability of the catalyst is achieved by addition of Ce and/or Zr. In the
experimental examples, a
synthesis using magnesium-aluminum hydrotalcite as starting material is also
disclosed. An
impregnation process in which hydrotalcite as support is impregnated with an
aqueous nickel
nitrate solution, with the water subsequently being removed at 70 C in a
vacuum evaporator, is
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disclosed. In the process disclosed in EP 2 308 594 A2 for producing a
synthesis gas, the feed
stream used has a minimum content of 1 mol of water per mole of methane and
the process is
carried out at a pressure in the range from 0.5 to 20 atm. EP 2 308 594 A2
discloses an
example in which the catalytic tests for producing synthesis gas were carried
out at 10 atm.
EP 031 472 A2 discloses and claims a catalyst for methane production, which is
produced using
thermally decomposable salts of nickel, of cobalt and of magnesium which are
fixed on a
support. The support is converted by thermal treatment into the metal oxides.
DE 29 52 683 discloses a methanation catalyst which comprises Co and Ni
species as active
components. Aluminum oxides or mixed oxides of aluminum oxide and silicon
dioxide or silicon
dioxide are used as support materials, with the catalytic properties of the
catalyst being
improved by addition of magnesium-comprising salts to the synthesis mixture.
In the context of
the thermal treatment of the catalyst precursor material, the formation of a
spinel-comprising
phase is reported. The catalysts are used for methanation reactions which are
carried out at
temperatures below 500 C and in which the pressure is in the region of
atmospheric pressure.
One of the objects of the invention is to provide an improved process and an
improved catalyst
for the methanation of CO- and/or CO2-comprising synthesis gas. In particular,
a catalyst
material whose thermal and mechanical stability is superior to that of the
materials known from
the prior art should be provided.
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The formation of methane by reaction of carbon monoxide and/or carbon dioxide
with hydrogen
is a strongly exothermic process. In the presence of a suitable catalyst, the
reaction normally
proceeds to equilibrium. The catalytic formation of methane is carried out
under adiabatic
process conditions. The temperature increase within the reactor associated
with the adiabatic
process conditions is determined, inter alia, by the gas composition, the
temperature of the gas
fed in and the working pressure. The temperature increase when carrying out
the methanation
is typically in the range from 200 to 500 C.
The temperature of the gas fed into the reactor is selected so that the
effectiveness of the
catalyst with a high degree of conversion can be utilized. For this purpose,
the gas fed in has to
be preheated to a suitable inlet temperature. When carrying out the
methanation process, it
needs to be noted that the formation of methane within the catalyst bed is
limited to a narrow
reaction zone. The location of the reaction zone depends on the time for which
the methanation
process has been operated. At the beginning of the methanation process,
methane formation
initially extends to the region of the catalyst bed which is in the vicinity
of the introduction of feed
gas. With increasing time of operation and progressive deactivation of the
catalyst within the
reaction zone, this then moves in the direction of gas flow from the inlet
region to the outlet
region of the catalyst bed.
,
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The inlet temperature and the process parameters should be selected so that
the formation of
Ni(C0)4 is prevented. For example, in the methanation of CO-comprising feed
gas by means of
nickel-comprising catalysts, an inlet temperature of greater than 250 C is
required. The
methanation of CO2-comprising feed gas can also be carried out at lower inlet
temperatures,
e.g. at a temperature of 200 C or even below 200 C. Methanation using feed gas
which has a
lower inlet temperature is also possible in conjunction with nickel-free
catalysts.
Owing to the mode of operation mentioned here, the part of the catalyst bed
which is located in
the vicinity of the reactor outlet is subjected to higher thermal stress than
the part of the catalyst
bed which is located in the vicinity of the reactor inlet. The higher thermal
stress on the catalyst
material which is located in the catalyst bed in the vicinity of the reactor
outlet occurs before this
material is utilized for methanation. To limit the thermal stress on the
catalyst, the temperature
of the gas stream which leaves the reactor at the downstream end is monitored.
Correspondingly, the operating parameters when carrying out the methanation
process are set
so that the temperature of the product mixture at the reactor outlet does not
exceed an upper
temperature limit. This can be achieved, for example, by the feed stream being
diluted with a
certain proportion of product stream (recycle). The dilution reduces the CO
and CO2 content in
the feed stream and the temperature increase caused by the exothermic reaction
is limited.
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It may be pointed out that all temperatures mentioned in the present
disclosure in respect of the
methanation process of the invention always relate to the temperature of the
gas mixture
obtained at the outlet end of the reaction space, unless indicated otherwise.
The objects mentioned here and also further objects which are not mentioned
here are achieved
by a process for producing a catalyst for the methanation of CO- and/or 002-
comprising
synthesis gases being provided. The process relates to impregnation of a
starting material with
a fusible metal salt, wherein the production process comprises the following
steps:
(i) contacting of a fusible metal salt and finely divided hydrotalcite-
comprising starting
material,
(ii) intimate mixing of the fusible metal salt and the hydrotalcite-
comprising starting material,
(iii) thermal treatment of the fusible metal salt and hydrotalcite-comprising
starting material
and heating of the mixture under conditions under which the metal salt is
present in the
form of a metal salt melt, preferably at a temperature in the range from 30 to
250 C, more
preferably at a temperature in the range from 50 to 140 C,
(iv) low-temperature calcination of the mixture at a temperature of < 500 C,
preferably at a
temperature in the range from 250 to 500 C, with the duration of the low-
temperature
calcination preferably being in the range from 0.1 to 24 hours, preferably
less than 2
hours, in the case of a continuous process preferably 5 1 hour,
(v) molding or shaping,
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(vi) high-temperature calcination of the mixture at a temperature of 500 C,
preferably at a
temperature in the range from 500 to 1000 C, with the duration of the high-
temperature
calcination preferably being in the range from 0.1 to 24 hours, preferably
less than 2
hours, in the case of a continuous process preferably 1 hour.
In a preferred embodiment, the calcination in process steps (iv) and (vi) is
carried out using a
defined heating rate and/or cooling rate, with the heating rate and/or cooling
rate preferably
being in the range from 0.01 to 10 C per minute, more preferably in the range
from 0.1 to 5 C
per minute.
In a preferred embodiment of the process, the shaping step (v) is followed by
a sieving step.
Further preference is given to the metal salt fraction used in (i) comprising
a nickel salt,
preferably nickel nitrate hexahydrate.
The hydrotalcite-comprising starting material preferably has defined
proportions of magnesium
and aluminum, preferably at least 10 mol /0 of magnesium and at least 10 mol%
of aluminum.
The invention also provides a catalyst for the methanation of CO- and/or 002-
comprising
synthesis gas, wherein this catalyst can be obtained by the following steps:
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(i) contacting of a fusible metal salt and finely divided hydrotalcite-
comprising starting
material,
(ii) intimate mixing of the metal salt and the hydrotalcite-comprising
starting material,
(iii) thermal treatment of the fusible metal salt and hydrotalcite-comprising
starting material
and heating of the mixture under conditions under which the metal salt is
present in the
form of a melt, preferably at a temperature in the range from 30 to 250 C,
more preferably
at a temperature in the range from 50 to 140 C,
(iv) low-temperature calcination of the mixture at a temperature of < 500 C,
preferably at a
temperature in the range from 250 to 500 C, with the duration of the low-
temperature
calcination preferably being in the range from 0.1 to 24 hours, preferably
less than 2
hours, in the case of a continuous process preferably 5. 1 hour,
(v) molding or shaping,
(vi) high-temperature calcination of the mixture obtained in the preceding
steps at a
temperature of 500 C, preferably at a temperature in the range from 500 to
1000 C,
with the duration of the high-temperature calcination preferably being in the
range from
0.1 to 24 hours, preferably less than 2 hours, in the case of a continuous
process
preferably 5- 1 hour.
In the catalyst of the invention, the nickel is present in very highly
disperse form on the support
oxide and the support oxide consists of or comprises very small particles of
MgA1204. This
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results in catalysts having an improved property profile which is reflected
both in an improved
sintering stability at high temperatures and in an improved carbonization
behavior.
The production process of the invention has advantages over production
processes based on
precipitation methods. The process of the invention forms no significant
amount of process
water or the process of the invention can also be carried out in such a way
that absolutely no
process water is formed. At the same time as avoiding the formation of process
water,
precipitation reagents can also be saved. The problems associated with
precipitation reagents,
namely introduction of contamination, can be prevented.
As regards the synthesis of the catalysts of the invention, it may also be
emphasized that an
extremely energy-efficient and environmentally friendly process is provided
because of the
largely water-free production process.
Based on the total pore volume of the hydrotalcite-comprising support used,
preferably
hydrotalcite, the amount of water used is preferably 5 100%, more preferably 5
90%, even more
preferably 5 70%, more preferably 5 50%, even more preferably 5 40%,
particularly preferably
5 30% and more preferably 5 20%, of the total pore volume of the support. In a
further preferred
embodiment of the invention, the catalyst can be produced without addition of
water since the
water necessary for the synthesis is in this case supplied solely by the water
of hydration of the
salt.
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In addition, a high metal loading or deposition of metal-containing phase on
the support oxide or
precipitation on a material which is a precursor of the support oxide can be
achieved by means
of the process of the invention.
The manner of mixing and the resulting combination of the hydrotalcite-
comprising starting
materials with the metal salt melt as per the process of the invention is
extremely effective as
regards the application and introduction of active components into the
framework structure.
Without wishing to restrict the present invention by theoretical
considerations, the following
explanation of the formation of the catalyst of the invention appears
plausible to us on the basis
of structural studies on the formation mechanism: the treatment according to
the invention of the
hydrotalcite-comprising starting material with the nickel-comprising nitrate
melt at a temperature
of less than or equal to 500 C leads to nanostructuring of the material.
Magnesium is leached
from the preformed layer-like carbonate-comprising precursor material.
Together with the nickel,
a nanocrystalline mixed crystal phase NixMg(i_x)O having a periclase-bunsenite
structure is
formed from the hydrotalcite. In addition, an Mg spinel phase and aluminum
oxide phases which
are partly amorphous and are transformed into crystalline spinels in which the
particles* are
nanocrystalline only at relatively high calcination temperatures are formed.
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Catalysts which at temperatures up to 1000 C have nickel crystallites which
are smaller than
100 nm, preferably smaller than or equal to 70 nm and particularly preferably
smaller than or
equal to 40 nm, and have a high resistance to sintering and carbonization
processes are
obtained. The present nanostructuring of the material is particularly
advantageous in respect of
the catalytic properties thereof. In particular, the material according to the
invention has been
found to be an advantageous catalyst compared to the prior art which is also
particularly
suitable for the methanation of CO- and/or 002- comprising synthesis gases.
In a preferred embodiment of the invention, the catalyst support comprises a
magnesium spinel
which is in intimate contact with a mixed oxide phase of nickel and magnesium.
In this catalyst
or catalyst precursor according to the invention, both the nickel-comprising
phase and the
spinel-comprising phase have very small crystallite sizes. In the case of the
spinel-comprising
phase, the average crystallite size is < 100 nm, preferably 5 70 nm, more
preferably 5. 40 nm.
In a further preferred embodiment of the invention, the phase composition of
the catalyst of the
invention is distinguished by the intensity of the diffraction reflection at
43.15 0.15 20
(2 theta) (d = 2.09 0.01 A) being less than or equal to the intensity of the
diffraction reflection
at 44.83 0.20 20 (d = 2.02 0.01 A), with the intensity of the diffraction
reflection at 43.15
0.15 20 (2 theta) (d = 2.09 0.01 A) more preferably being less than the
intensity of the
reflection at 44.83 0.20 20 (d = 2.02 0.01 A) and the intensity ratio of
the two diffraction
reflections 1(43.15 )/1(44.83 ) even more preferably being in a range from 0.3
to 1.0, preferably
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from 0.5 to 0.99, more preferably from 0.6 to 0.97 and particularly preferably
from 0.7 to 0.92.
An illustrative depiction of a typical diffraction pattern (5-800 20) of a
catalyst according to the
invention is shown in figure I.
The presence of small amounts of Ni spinel phase and possibly also NiO in the
catalyst material
of the invention or the catalyst precursor material is not ruled out. However,
if an Ni spinel phase
is present in the precursor material of the invention, it can be assumed that
this will be
transformed at the high pressures and the high temperatures of the use
according to the
invention of the catalysts.
The process of the invention enables all active metals which are present as
metal salt melt in
the temperature range from 30 C to 250 C and result in catalysts which display
catalytic activity
as methanation catalyst to be applied to hydrotalcite or to hydrotalcite-
comprising starting
material. In a preferred embodiment, promoters can be added to the metal salt
melt and/or
further support oxides, pore-forming agents or binders can be introduced into
the synthesis
system in addition to the hydrotalcite-comprising starting material.
To produce the catalyst of the invention, preference is given to using metal
salts which do not
decompose during melting or in the case of which the decomposition is greatly
inhibited
kinetically. Examples of such metal salts are, inter alia, nitrates, nitrites,
halides, chlorates,
bromates, iodates, sulfates, sulfites. Particular preference is given to
nitrates, nitrites and salt
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melts comprising nitrates and nitrites. The addition of particular additives
to the melts, for
example urea, ethylene glycol, is encompassed.
The fusible metal salts can comprise, for example, Na, K, Ca, Mg, Sr, Ba, Al,
La, Y, Mo, W, Nb,
Zr, Ti, Fe, Co, Ni, Cu, a platinum metal and/or Ce as cationic species.
Possible anionic species
are, in particular, nitrogen-comprising anions such as nitrates and nitrites.
However, other
anions such as halogens, sulfates and sulfites and other inorganic and organic
anions known to
those skilled in the art can in principle be used. The metal salts preferably
comprise at least one
nickel-comprising or cobalt-comprising component, preferably nickel nitrate
hydrate or cobalt
nitrate hydrate, for example hexahydrate. Particular preference is given to
nickel nitrate
hexahydrate.
The term hydrotalcite-comprising starting material as used in the present
disclosure means that
the material used comprises at least one hydrotalcite-like compound as
significant constituent
and can optionally comprise oxidic additive and/or secondary constituents. The
total proportion
of the hydrotalcite-like compound and the oxidic additive is greater than 50%
by weight,
preferably greater than 70% by weight and particularly preferably greater than
90% by weight. In
addition to hydrotalcite-like compounds and oxidic additives, the hydrotalcite-
comprising starting
material can also comprise secondary constituents which comprise, for example,
metal salts
and serve, for example, to adapt the metal concentration of trivalent to
divalent metal salt. Such
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secondary metal salt constituents are present in amounts of less than or equal
to 10% by
weight, preferably less than or equal to 5% by weight.
Hydrotalcite-like compounds are mixed hydroxides of divalent and trivalent
metals which are
made up of polycations and have a layer structure. Hydrotalcite-like compounds
are also
referred to in the literature as anionic clays, layered double hydroxides
(=LDHs), Feitknecht
compounds or double layer structures. Divalent metals which can be used are,
for example,
metals from the group consisting of Mg, Zn, Cu, Ni, Co, Mn, Ca and Fe and
trivalent metals
which can be used are, for example, metals from the group consisting of Al,
Fe, Co, Mn, La, Ce
and Cr.
In a preferred embodiment, the hydrotalcite-like compound is composed of
hydrotalcite. The
hydrotalcites used for the process of the invention preferably comprise
magnesium as divalent
metal and aluminum as trivalent metal. The metals of the hydrotalcites used
preferably comprise
predominantly magnesium and aluminum.
The oxidic additive can also be a mixture, preferably a mixture comprising
aluminum-comprising
compounds. Examples of such aluminum-comprising oxidic additives are, inter
alia, gibbsite,
boehmite and pseudoboehmite. Typical contents of such aluminum oxides,
hydroxides or oxide
hydrates can be in the range from 30 to 95 percent by weight calculated on the
basis of
aluminum oxide. This corresponds to a molar proportion of aluminum based on
total metal of
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from 26 to 84 mol%. Particular preference is given to the range from 50 to 80
percent by weight
calculated on the basis of aluminum oxide. This corresponds to a molar
proportion of aluminum
based on total metal of from 44 to 70 mol%. Very particular preference is
given to the range
from 60 to 75 percent by weight calculated on the basis of aluminum oxide.
This corresponds to
a molar proportion of aluminum based on total metal of from 53 to 66 mol%.
The hydrotalcite-like compounds and the oxidic additive also display very
intimate mixing. The
same also applies to secondary constituents should these be comprised in the
hydrotalcite-
comprising starting material.
Such mixing can be effected, for example, by physical mixing of hydrotalcite-
like and aluminum
hydroxide-comprising powders. For example, powder mixing can be carried out in
suitable
industrial apparatuses such as mixers. Such mixing processes are known to
those skilled in the
art. A further possibility is to mix the hydrotalcite-like powder and the
aluminum hydroxide-
comprising powder in suitable dispersion media. As dispersion media, it is
possible to use, for
example, water, alcohols such as methanol, ethanol, propanol, butanol,
ethylene glycol and/or
butanediol and ketones such as acetone or methyl ethyl ketone. It is also
possible for the
dispersion media to be present as mixtures and comprise surface-active agents
such as
surfactants. Examples of such surfactants are, inter alia, polyethylene
glycols, Mersolates,
carboxylates, long-chain ammonium compounds such as CTAB.
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Another possible way of achieving intimate mixing is the direct synthesis of a
mixture of
hydrotalcite-like and aluminum hydroxide-comprising substances by
precipitation reactions.
Such processes can be carried out, inter alia, as described in DE 195 03 522
Al by hydrolysis
of water-sensitive precursors, which allows many possible compositions. Other
alternative
processes for producing mixtures of hydrotalcite-comprising and aluminum
hydroxide-
comprising substances can be carried out on the basis of precipitation
reactions from aqueous
media. For example, it is possible to use carbonate-comprising precipitates or
carbon dioxide-
comprising gas mixtures can be allowed to act under pressure on suitable
precursor solutions of
metal salts or metal hydroxides.
Examples of hydrotalcite-comprising starting materials used for the purposes
of the invention
are products from Sasol which are marketed under the trade name Pural MG
(Pural MG5 to
Pural MG70 are commercially available, where Pural MG70 is an Mg-Al
hydrotalcite without
addition of aluminum hydroxide). Intimate mixing of magnesium- and aluminum-
comprising
hydrotalcites with other carbonates, hydroxides or hydroxycarbonates is also
encompassed by
the invention.
Preference is given to using hydrotalcites or hydrotalcite-like compounds
having a particular
purity for the process of the invention. The process for producing these
hydrotalcite-like
compounds which are particularly preferably used in the process of the
invention is disclosed by
J.P. van Berge et al. in DE 195 03 522 Al.
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According to DE 195 03 522 Al, the hydrotalcites or hydrotalcite-like
compounds are formed by
hydrolysis of metal alkoxides by means of water and subsequent drying of the
hydrolysis
products obtained as precipitate. The metal alkoxides are formed by reaction
of monovalent,
divalent and/or trivalent alcohols with one or more divalent metals and/or one
or more trivalent
metals. The water used for the hydrolysis preferably comprises water-soluble
anions selected
from the group consisting of hydroxide anions, organic anions, in particular
alkoxides, alkyl ether
sulfates, aryl ether sulfates and glycol ether sulfates and inorganic anions,
in particular
carbonate, hydrogencarbonate, chloride, nitrate, sulfate and/or
polyoxometalate anions.
Ammonium is preferably used as counterion.
As hydrotalcite-comprising materials which are particularly suitable as
starting materials for
producing the catalyst and have been prepared by hydrolysis of metal
alkoxides, mention may
be made of materials which can be procured from Sasol under the trade names
Pural MG5,
Pural MG20, Pural MG30, Pural MG50 and Pural MG70. According to the
information provided
by the manufacturer, the numerical value in the product names is the
percentage by weight of
MgO present in the product. To obtain a total weight of 100%, the A1203
content has to be added
to the proportion by weight of MgO. It should be noted that the figures here
are based on the
oxides, although the samples also comprise hydroxide groups and water. It is
also possible in
this case for the samples to be able to also comprise further anions, such as
carbonate anions.
It is also possible to procure materials which have other MgO to A1203 ratios.
Particularly in
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those products or materials which have low magnesium contents, it is possible
for these to
comprise not only magnesium-aluminum-comprising hydrotalcite but also
proportions of finely
divided aluminum hydroxide or oxide hydrate.
A particularly preferred hydrotalcite-comprising starting material, viz. Pural
MG30, comprises,
for example, a mixture of hydrotalcite (i.e. a component having the
composition
Mg6Al2(OH)18*4H20 or Mg6Al2(OH)16003*4H20) and boehmite, with the mixture
having an
overall A1203/MgO ratio close to seventy to thirty % by weight. This number in
the trade name of
the product used here relates to the calcined material and means that in this
particularly
preferred example, the starting material has a boehmite content of about 55%
by weight.
Instead of hydrotalcite, which is particularly preferred as constituent of the
hydrotalcite-
comprising starting material in the production process of the invention, it is
also possible to use
other metal hydroxides or hydroxycarbonates as starting materials. Particular
preference is
given to those which can be produced by the same synthesis process as
hydrotalcites and
hydrotalcite-like compounds.
It is also important for the purposes of the invention for the hydrotalcite-
like starting material to
have a preferred Al/Mg ratio. In a description of the composition of the
hydrotalcite-like starting
material in terms of the oxides comprised therein (in ignited form), the
preferred
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alumina/magnesia ratio (i.e. the A1203/MgO ratio) is in the range from 0.5 to
20 on a weight
basis, with an alumina/magnesia ratio of from 1 to 10 on a weight basis being
more preferred.
The preferred Al/Mg ratio is in the range from 1.5 to 2.5 on a molar basis,
with an Al/Mg ratio of
from 1.7 to 2.3 on a molar basis being more preferred. The preferred
hydrotalcite-comprising
starting material should be able to be converted preferably in significant
proportions or
particularly preferably virtually completely into a material having spinel or
spinel-related
structures or phase mixtures of such structures by high-temperature
calcination at temperatures
above 500 C.
Another important aspect of the invention is very intimate mixing of the
hydrotalcite-comprising
starting material with the fusible metal salt which gives close contact
between the nickel species
and the support precursor component and leads to unexpectedly good
stabilization of the nickel
species. After calcination, this leads, as mentioned above, to a mixed oxide
phase having the
composition NixMg(i_x)0 where x = 0.3-0.7, preferably 0.4-0.6. (The content
range of x = 0.3-0.7
corresponds to an NiO content of about 44-81% by weight and in the case of x =
0.4-0.6 the
NiO content is about 55-73.5% by weight.) Furthermore, a certain proportion of
Ni spinel could
be detected by means of XRD analyses after calcination.
The XRD results indicate that depletion of Mg species occurs in the mixed
oxide phase
NixMg(1_8)0. The Mg species replace Ni species in the Ni spinel. A possible
explanation, which
CA 02854914 2014-05-07
21
does not constitute a restriction of the invention, would be that a proportion
of the aluminum
continues to be present as aluminum oxide hydrate even at high temperatures.
Under reductive
conditions at high temperatures, elimination of metallic nickel from the mixed
oxide phase
NixMg(1_x)0 could occur, with the magnesium liberated then reacting with the
aluminum oxide
hydrate to form magnesium-aluminum spinel.
As regards the molar ratio of metal species in the hydrotalcite-comprising
starting material MHT
and metal species in the salt melt Ms, it can be stated that the molar ratio
of metals MHT/Ms is
always greater than 1. The molar ratio MHT/Ms is preferably in the range from
15 to 1.5 and
more preferably in the range from 10 to 3. The use of a preferred ratio is
important to ensure the
conditions for good mixing of the components and homogeneous coating of the
hydrotalcite and
thus ensure the nanostructuring, in particular the high dispersion and finely
divided nature of the
nickel and of the mixed oxide composed of Ni and Mg and the finely divided
nature of the Mg
spinel, of the material according to the invention.
In a preferred embodiment, the pulverulent hydrotalcite-comprising material is
heated before
contacting with the fusible metal salt and on being brought into contact with
the metal salt has a
temperature in the range from 30 to 250 C, preferably in the range from 50 C
to 140 C.
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The temperature required for melting the metal salt depends on the properties
of the metal salt
or metal salt mixture used in each case. Metal salts which are particularly
suitable for the
process of the invention have a melting point in the range from 30 to 250 C.
In one of the preferred embodiments of the process of the invention, the
hydrotalcite-comprising
starting material is brought into contact with the metal salt melt. To
suppress solidification of the
metal salt melt during contacting and mixing with the hydrotalcite, it is
advantageous to preheat
the metal salts to a temperature which is at least 10 C above, preferably 20 C
above, the
temperature of the melting point of the salts or salt mixture used in each
case.
In selecting the process parameters for contacting of the powder with the
melt, it has to be taken
into account that the water of crystallization of the hydrotalcite and of the
metal salt melt is
subjected to evaporation. This evaporation depends on the temperature, the gas
exchange, the
gas atmosphere and the duration of the process. Complete evaporation of the
water of
crystallization can be undesirable since decomposition of the salt or of the
hydrotalcite can then
occur before homogenization of the mixture. Solidification of a region in the
melt which has not
yet been intimately mixed with the hydrotalcite-comprising material adversely
affects the
homogeneity of the distribution of the metal species on the solid hydrotalcite-
comprising starting
material.
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The duration of contacting should be very short, i.e. preferably less than or
equal to 30 minutes.
The gas atmosphere should preferably comprise a certain proportion of water in
order to
suppress the decomposition of metal salt or the hydrotalcite-comprising
starting material during
mixing. The content of water vapor here can be, for example, in the range from
0 to 10% by
volume.
It is advantageous to heat the hydrotalcite-comprising starting material to a
temperature which
corresponds approximately to the temperature of the salt melt before bringing
it into contact with
the salt melt in order to avoid uncontrolled solidification of the salt melt.
I. Contacting and mixing of hydrotalcite with metal salt
It firstly has to be pointed out that the process step of contacting of the
hydrotalcite-comprising
starting material with the metal salt is not subject to any limitation.
However, a number of
embodiments of contacting which are advantageous are indicated below.
For example, the hydrotalcite-comprising starting material can firstly be
combined and mixed
with the pulverulent metal salt at a temperature below the melting point of
the salt before the
latter is melted. The substances are firstly combined cold. The combining and
mixing can be
carried out in a plurality of steps or in a single step.
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In another preferred embodiment of the process of the invention, the
pulverulent hydrotalcite-
comprising starting material is placed in a vessel and the metal salt melt is
added thereto while
agitating the solid. The melt can be added to the hydrotalcite a little at a
time in a plurality of
steps or in a single step.
In still another embodiment, which is likewise preferred, the hydrotalcite-
comprising starting
material is first coated with the metal salt before the latter is then melted.
Here, it is possible, for
example, firstly to suspend the hydrotalcite-comprising starting material in
water and combine it
with a metal salt solution. The mixture of the hydrotalcite-comprising
starting material and the
metal salt solution forms a suspension which can, for example, be dried by
spray drying.
To ensure intimate mixing of the fusible metal salt and the hydrotalcite-
comprising starting
material, the components which have been brought into contact with one another
have to be
mixed and homogenized by means of mechanical mixing elements. As mixers, it is
possible to
use, for example, powder mixers, tumblers, kneaders, etc. The suitable
industrial means for
mixing should be known to a person skilled in the art. The duration of the
mixing step is
preferably 2 minutes, more preferably 10 minutes and even more preferably ?.
30 minutes.
The mixing as per step (ii) and the thermal treatment as per step (iii) are
preferably carried out
simultaneously. The material to be mixed is preferably heated during the
mixing process in
order to prevent solidification or crystallization of the salt melt.
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II. Further process steps for producing the catalyst
(a) The homogenized mixture of metal salt and hydrotalcite (or the
hydrotalcite-comprising
starting material) is subjected to a low-temperature calcination. The low-
temperature
calcination is carried out by thermal treatment of the homogenized mixture in
a
temperature range from 100 C to 500 C for a time in the range from 0.1 h to 24
h. The
material is preferably heated using a controlled heating rate. The heating
rate is preferably
less than 20 C/min, preferably less than 10 C/min and more preferably less
than 5 C/min.
The material obtained after the low-temperature calcination can be present as
a finely
divided powder or as coarsely particulate loose material. To be able to use
the material as
loose particulate catalyst, a shaping process can be necessary. As shaping
step, it is
possible to carry out, for example, comminution, milling, tableting or
extrusion.
(b) The material which has been calcined at low temperature is preferably
subjected to a
shaping process in order to obtain a molded material. This shaping process can
comprise
one or more of the following steps:
b.i) compacting, b.ii) comminution, b.iii) sieving and/or b`) tableting.
In a further process variant, the shaping process is an extrusion process. The
melt-
impregnated catalyst composition is, for example, processed by means of an
extruder with
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additional additives to give the desired shaped bodies. When a shaping process
by means
of extrusion is used, it is conceivable that the process step of low-
temperature calcination
(iv) does not have to be carried out. The process can be carried out with the
calcination
occurring only after extrusion in the form of a high-temperature calcination
step. In general,
a precalcination is carried out before extrusion.
(c) The molded material always has to be subjected to a high-temperature
calcination
process. The target temperature in the high-temperature calcination is in the
region of
greater than or equal to 500 C, preferably in the range from 500 to 1000 C.
The duration of
the high-temperature calcination, i.e. the heating of the sample at the target
temperature, is
in the range from 0.1 to 24 h.
(d) The high-temperature calcination can be carried out in the presence of an
oxygen-
comprising atmosphere, preferably air. The heating of the sample to the target
temperature
is preferably carried out using a controlled heating rate, preferably a
heating rate of less
than 20 /min and more preferably less than 10 C/min.
In the production of the catalyst of the invention, it can be preferred for at
least individual
substeps of the production process to be carried out continuously. For
example, particular
preference is given to carrying out the low-temperature calcination in a
continuously operated
rotary tube furnace.
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In a further process step, the calcined catalyst can be exposed to a reductive
gas atmosphere
while being heated in order to reduce at least part of the metal species,
preferably of the nickel.
This thermal treatment under a reductive gas atmosphere is preferably carried
out in the same
reactor in which the catalytic process is carried out.
In a particularly preferred embodiment, the invention provides a catalyst for
the catalysis of
heterogeneous reactions, preferably the reaction of methane, carbon dioxide
and water to form
synthesis gas, which comprises at least the three phases nickel-magnesium
mixed oxide,
magnesium spinel and aluminum oxide hydroxide and in which the nickel-
magnesium mixed
oxide has an average crystallite size of < 100 nm, preferably < 70 nm, more
preferably <40 nm,
and the magnesium spinel phase has an average crystallite size of < 100 nm,
preferably
<70 nm, more preferably <40 nm, the proportion of nickel is in the range 7-28
mol%, that of
magnesium is in the range 8-26 mol%, that of aluminum is in the range 50-70
mol% and the
BET surface area is in the range 10-200 m2/g.
Particular preference is also given to an embodiment of the catalyst of the
invention which has a
proportion of nickel in the range 6-30 mol% and a proportion of magnesium in
the range from
8-38 mol%, preferably in the range 23-35 mol%. The proportion of aluminum is
preferably in the
range 50-70 mol%.
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It should be emphasized that particularly high-performance catalysts and thus
particularly
preferred embodiments of the invention are obtained when the physicochemical
properties of
the catalysts have particular values.
In a preferred embodiment, the physicochemical properties selected from the
group consisting
of phase composition according to XRD, BET surface area, average pore diameter
and/or
tamped density of the catalyst of the invention have preferred values.
The phase composition of a particularly preferred catalyst is distinguished by
the intensity of the
diffraction reflection at 43.15 0.15 20 (2 theta) (d = 2.09 0.01 A)
being less than or equal to
the intensity of the diffraction reflection at 44.83 0.200 20 (d = 2.02
0.01 A), with the intensity
of the diffraction reflection at 43.15 0.15 20 (2 theta) (d = 2.09 0.01
A) more preferably
being less than the intensity of the reflection at 44.83 0.20 20 (d = 2.02
0.01 A) and the
intensity ratio of the two diffraction reflections 1(43.15 )/1(44.83 ) even
more preferably being
from 0.3 to 1.0, preferably from 0.5 to 0.99, more preferably from 0.6 to 0.97
and particularly
preferably from 0.7 to 0.92. A diffraction pattern (5-80 20) of a catalyst
according to the
invention having a molar ratio of Ni/Mg/AI of 14/29/57 is depicted by way of
example in figure I.
A particularly preferred embodiment of the catalyst has a BET surface area in
the range from 10
to 200 m2/g, preferably from 15 to 150 m2/g, more preferably from 20 to 100
m2/g, even
preferably from 30 to 80 m2/g, very particularly preferably from 30 to 78 m2/g
and in particular
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preferably from 30 to 78 m2/g. The determination of the BET specific surface
area was carried
out in accordance with DIN 66131.
Furthermore, a preferred embodiment of the catalyst also has a characteristic
tamped density
which is preferably < 1500 g/I, more preferably < 1350 g/I and even more
preferably .. 1100 g/I.
The determination of the characteristic tamped density was carried out by
means of an STAV
2003 tamped volumeter from JEL. A 0.5-1.0 mm fraction of crushed catalyst was
used for the
measurement.
III. Methanation process
A further and important aspect of the invention relates to a methanation
process, preferably
high-temperature methanation, which has the features indicated in claims 7 to
14. The
production of the catalyst according to the invention is carried out according
to any of claims 1
to 4 or the methanation catalyst according to the invention can be produced
according to either
claim 5 or 6.
The methanation process of the invention can be carried out over a temperature
range from
300 C to 900 C. The methanation process of the invention is preferably carried
out in a
temperature range above 500 C, more preferably in a temperature range from 500
C to 800 C,
even more preferably in a temperature range from 600 C to 750 C.
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Here, the high thermal stability of the catalyst when carrying out the
methanation process of the
invention is particularly remarkable compared to when the process is carried
out using a catalyst
material known from the prior art. Owing to the high thermal stability of the
catalyst of the
invention, its deactivation is relatively low even under high thermal stress.
The operating lives of
the catalyst can be considerably lengthened by means of the process of the
invention, which
leads to an improvement in the economics of the process.
In addition to the improved thermal stability of the catalyst, the catalyst of
the invention also has
a higher mechanical hardness compared to comparable catalysts from the prior
art. Owing to
the increased mechanical stability, the process of the invention can be
carried out at high
process pressures. The process pressures can be in the range from 10 to 50
bar, usually from
20 to 30 bar, e.g. 25 bar.
The carbonization tendency in the process of the invention is low, which is
necessary for
maintenance of the high activity.
A preferred embodiment of the process of the invention relates to the
methanation of synthesis
gas having an Hz/CO ratio in the range from 2.5 to 4, more preferably in the
range from 3 to 3.5.
In a particularly preferred embodiment of the process of the invention, the
synthesis gas is
provided, for example, from coal gasification (e.g. Lurgi process).
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This synthesis gas originating from coal gasification is usually firstly
purified before the
methanation. For example, the sulfur-comprising components and a large part of
the CO2 are
removed before carrying out the methanation.
It is a characteristic of the Lurgi process that a relatively high proportion
of methane is present in
the synthesis gas. After purification, the dry synthesis gas comprises the
following main
components in the following, typical concentrations in proportions by volume:
¨35% by volume
of CH4, ¨45% by volume of H2 and ¨15% by volume of CO; secondary components
can be: CO2
in particular, and also nitrogen or higher hydrocarbons, for example ethane.
The reactor outlet
temperature in the process is limited by the fact that the CO content in the
synthesis gas is
reduced by recirculation of part of the product stream in order to limit the
heat evolved in the
overall reaction.
When the synthesis gas originates from the Lurgi process, the catalyst is
accordingly supplied
with a synthesis gas having the following composition: CH4 content in the
range from 36 to 42%
by volume, H2 content in the range from 35 to 45% by volume, CO content in the
range from 9
to 12% by volume, H2O content in the range from 8 to 12% by volume and CO2
content in the
range from 0 to 3% by volume.
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Owing to the high thermal stability of the catalyst of the invention, the
recycle stream can be
dispensed with in a further embodiment and the purified synthesis gas from
coal gasification,
which has previously been subjected to the usual purification steps, can be
used directly.
The synthesis gas can also comprise further components, for example nitrogen,
argon, which
themselves do not participate in the methanation reaction. The sulfur content
of the synthesis
gas should be as low as possible in order to avoid poisoning of the nickel
sites by sulfidation.
The process of the invention makes it possible to achieve a high methane yield
per nickel atom
present on the catalyst support or comprised therein. It can be assumed that
this is associated
with the special structure of the catalyst material and the good accessibility
of the active sites.
The mode of operation of the process of the invention is such that the GHSV
thereof is in a
=
range from 500 to 50 000 h-1, preferably in a range from 1000 to 15 000 h-1
and especially
preferably in a range from 1000 to 5000 h-1.
The use of the catalyst of the invention in the form of shaped bodies is also
particularly
advantageous for carrying out methanation reactions since it is possible to
achieve a lower
pressure drop within the reactor when using the shaped bodies than when using
a catalyst in
the form of a bed of unshaped material. The catalyst material is particularly
suitable when
employed in methanation reactions because of the high mechanical stability of
the material.
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With regard to shaped bodies, it may be said that these have a virtually
identical shape and a
particular minimum dimension in any direction of the three axes in space, with
the dimension in
each direction of an axis in space being greater than 2 mm.
Examples
Production process
The process of the invention for producing the catalyst is illustrated by
example Ml. Firstly,
411.4 g of pulverulent nickel nitrate hexahydrate which had previously been
crushed to a fine
powder by means of a mortar and pestle and 600 g of hydrotalcite (Pural MG30
from Sasol)
were intimately mixed to produce a premix of metal salt and hydrotalcite and
introduced into the
rotary tube of a rotary tube furnace. The premix was heated to 80 C in the
rotary tube furnace
and maintained there at 80 C for 1 hour, with the rotary tube and the premix
present therein
being rotated at two revolutions per minute and an air stream of 150 l/h being
passed through
the rotary tube. The weight of the premix obtained after cooling was 886 g.
400 g of the sample obtained during premixing were subsequently subjected to a
low-
temperature calcination. For this purpose, the sample was introduced into a
fused silica flask,
this was fastened in a rotary bulb furnace and heated therein at a heating
rate of 5 C/min to a
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34
target temperature of 425 C and heated at 425 C for one hour. During the
thermal treatment of
the sample, the fused silica flask was rotated at a rotary speed of 12
revolutions per minute,
with air simultaneously being passed through the flask at a flow rate of 1
l/min.
The sample obtained in the low-temperature calcination was mixed with graphite
powder and
pressed by means of a punch press to produce pellets. The graphite powder
serves as lubricant
and it is also possible to use stearic acid or magnesium stearate instead of
graphite. The pellets
produced by means of the press used here had a diameter of 4.75 mm and a
thickness of about
4 - 5 mm. The lateral compressive strength of the pellets was 60 - 70 N.
The pellets were comminuted by means of a screen mill and pressed through a
sieve in order to
obtain a size fraction of < 1.6 mm. The precompacted material was tableted
again to give pellets
having a diameter of 4.75 mm and a thickness of 3 - 4 mm. The pellets had a
lateral
compressive strength of 130 - 150 N.
The sample material obtained in this way was calcined at 850 C for one hour in
a muffle furnace
through which air was passed and subsequently cooled to room temperature. The
sample
material placed in the muffle furnace was heated at a heating rate of 5 C/min
from room
temperature to 850 C. The air which was passed through the furnace during the
heating-up
phase, the calcination and the cooling phase had a flow rate of 6 l/min.
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The calcined sample material was subjected to chemical and physical
characterization. In the
elemental analysis, the following composition was found: 21% by weight of NiO,
53% by weight
of A1203 and 23% by weight of MgO, with the figures being based on the oxides.
In the XRD
analysis, magnesium spine! (MgA1204) and MgNi02 were detected as phases. The
average
crystallite size of the phases could be determined from the reflections using
the Scherrer
equation. The result was that the spinel particles had a crystallite size of
9.0 nm and the mixed
oxide particles had a crystallite size of 16.5 nm.
The sample material was characterized by nitrogen sorption and Hg porosimetry.
The sample
material had a BET surface area of 67 m2/g. The sample material had an Hg pore
volume of
0.31 ml/g and a pore surface area of 83 m2/g, with the sample material having
a monomodal
pore structure. The pores of the sample material had an average pore diameter
of about 15 nm.
A further catalyst M2 was produced in a manner analogous to M1 but was
calcined at a
temperature of 950 C. The following composition was found by elemental
analysis: 21% by
weight of NiO, 53% by weight of A1203 and 23% by weight of MgO, where the
figures are based
on the oxides. In the XRD analysis, magnesium spinel (MgA1204) and MgNi02 were
detected as
phases (see figure I). From the reflections, the average crystallite size of
the phases was
determined more precisely using the Scherrer equation. The result was that the
spinel particles
had a crystallite size of 14 nm and the mixed oxide particles had a
crystallite size of 13 nm.
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The sample material was characterized by means of nitrogen sorption and Hg
porosimetry. The
sample material had a BET surface area of 58 m2/g. The sample material had an
Hg pore
volume of 0.41 ml/g and a pore surface area of 48 m2/g, with the sample
material having a
monomodal pore structure. The pores of the sample material had an average pore
diameter of
about 34 nm.
Comparative example
As comparative example CM1, the catalytic properties of a catalyst which had
been produced
by means of a precipitation process were tested. To produce this catalyst,
175.7 g of
hydrotalcite (Pural MG30 from Sasol having a loss on ignition of 34.2% by
weight) were firstly
placed in a vessel comprising 6 I of deionized water which had been preheated
to 48 C. In a
separate vessel, a solution of nickel nitrate and aluminum nitrate was
produced by dissolving
612.8 g of nickel nitrate hexahydrate and 313.1 g of aluminum nitrate
nonahydrate in 509.2 g of
deionized water and the solution was heated to 48 C. An aqueous sodium
carbonate solution
which had a sodium carbonate content of 20% by weight and had likewise been
preheated to
48 C was used as precipitation reagent.
To precipitate the metal species, the solution of the metal nitrate salts and
the sodium carbonate
solution were simultaneously introduced dropwise into the vessel comprising
the aqueous
hydrotalcite dispersion. The initial charge of the aqueous hydrotalcite
dispersion was heated to
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37
48 C and the dispersion was mixed by means of a stirrer. During the addition
of the salt solution
and the precipitation reagent to the aqueous initial charge, the pH of the
aqueous dispersion
was monitored and the addition of the sodium carbonate solution was controlled
so that the pH
in the initial charge was maintained at a value of 8Ø After all the metal
salt solution had been
transferred to the vessel with the initial charge, a total of 3.5 I of sodium
carbonate solution had
been used as precipitation reagent.
After the precipitation was complete, the suspension obtained by the
precipitation process was
stirred for another 15 minutes and the precipitated product was subsequently
filtered off by
means of a suction filter. The filter cake was washed with deionized water,
with the nitrate
content of the filtrate being simultaneously determined. The temperature of
the water used for
washing was 20 C. The washing procedure was stopped as soon as nitrate ions
could no longer
be detected in the filtrate (the nitrate content was thus below the detection
limit of 10 ppm). 350 I
of water were required for washing the filter cake. The washed filter cake was
subsequently
dried at 120 C in a drying oven for a period of 16 h.
The dried solid was heated at 700 C in a muffle furnace for 5 h. The muffle
furnace and the
solid comprised therein were heated to 700 C at a controlled heating rate and
an air stream
having a volume flow of 20 I/min was passed through the muffle furnace during
heating. The
solid obtained in this calcination was mixed with 3% by weight of graphite
powder and the
mixture was pressed by means of a punch press to produce pellets. The pellets
obtained here
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38
had a diameter of 4.75 mm and a thickness of about 2 mm. The pellets were
comminuted by
means of a screen mill and pressed through a sieve having mesh opening of 1 mm
in order to
obtain a size fraction comprising particles smaller than 1 mm.
The particle fraction obtained after precompacting was admixed with 10% by
weight of Puralox
(boehmite from Sasol) and 3% by weight of graphite, intimately mixed and
subjected to
tableting. The pellets obtained here had a diameter of 4.75 mm and a thickness
of about 3-
4 mm. The lateral compressive strength of the pellets was 100 N.
The composition of the pellets or the calcined samples was determined by means
of chemical
analysis which showed that the material had an Ni content of 29.8% by weight,
an Al content of
21.1% by weight, an Mg content of 4.7% by weight and a carbon content of 3.1%
by weight. At
a temperature of 900 C, the samples displayed a loss on ignition of 7.3% by
weight. Based on
the oxides, the following composition was determined for the calcined
precipitation product: 41%
by weight of NiO, 43% by weight of A1203, 8.4% by weight of MgO and 3.3% by
weight of C.
In the XRD analysis of the calcined sample, nickel oxide (NiO) and nickel
spinel (NiA1204) were
identified. The nickel oxide particles had an average crystallite size of 5.0
nm determined by
analysis of the corresponding reflections using the Scherrer equation.
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39
The sample material had a BET surface area of 165 m2/g. The sorption study was
carried out
using nitrogen. In analysis of the pore material by Hg porosimetry, a pore
volume of 0.33 ml/g
was found. The sample material displayed a bimodal pore structure: the major
part of the pores
had an average pore diameter of 6 nm and the smaller part of the pores had an
average pore
diameter of 30 nm. An average pore diameter of 11 nm was determined.
Calculation of the
surface area of the sample material on the basis of Hg analysis gave a surface
area of
123 m2/g.
Catalyst testing
The catalysts of example Ml, example M2 and comparative example CM1 were
subjected to
the process conditions of CO methanation in succession in an experimental
reactor to produce
synthetic natural gas in order to characterize the performance properties of
the catalysts in
respect of CO methanation. The experimental reactor was equipped with a
reaction tube which
before the individual tests had been charged with 50 ml of the respective
catalyst sample (i.e.
example Ml, example M2 or comparative example CM1). In the charge, the
catalyst samples
were present in the form of pellets.
The catalyst of comparative example CM1 installed in the tube reactor and the
test set-up was
firstly subjected to activation. For this purpose, the catalyst CM1 was heated
to 280 C in the
presence of a nitrogen stream and subsequently exposed to a reductive
atmosphere for
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16 hours by mixing 5% by volume of H2 into the nitrogen stream. The catalysts
according to the
invention, example M1 and example M2, were not activated but instead installed
and started up
directly in the oxidic form. An advantage which may be mentioned is that it is
possible to
implement the process of the invention even without activating the catalysts.
After the activation under the reducing atmosphere in the case of CM1 or
directly after
installation in the case of M1 or M2, the methanation reaction was started,
with the catalyst
being exposed to a feed gas stream which had been pretreated to 280 C. The
feed gas stream
had a volume flow of 1202 standard l/h and comprised the six components
hydrogen, CO, 002,
CH4, N2 and H20 in the following ratio of the respective individual volume
flows: 468 standardl/h
of hydrogen, 132 standard l/h of CO, 12 standard l/h of 002, 456 standard I/h
of CH4,
24 standard l/h of N2 and 110 standard l/h of H20. The experimental parameters
chosen here
and the plant configuration led to a reaction temperature in the range from
600 to 620 C being
established in the reactor while carrying out the methanation.
The feed gas stream and the product gas stream were each characterized in the
water-free
state by GC analysis. The characterization of the feed gas stream was carried
out before the
addition of water and the characterization of the product gas stream was
carried out after the
water had been condensed out. Tables 1 and 2.A give a summary of the measured
data for the
gas compositions of the feed and product streams. The respective values
represent the average
values determined from the individual values during the total time of the
experiment.
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41
The catalyst according to the invention (example M1) displayed a CO conversion
of 93% and
the comparative example (CM1) displayed a CO conversion of 88%. The CO
conversion of
example M1 was thus 5% above the conversion achieved using comparative example
CM1. In
addition, the catalyst according to the invention (example M1) could give the
high conversion
over a period of more than 1200 h, while the catalyst from comparative example
CM1 displayed
a significant decrease in activity after only about 300 h, leading to
termination of the experiment.
It is also noteworthy that the catalyst from example M1 not only displayed a
higher activity and
long-term stability but also had a significantly higher mechanical strength
than the catalyst from
comparative example CM1.
The catalysts of example Ml, example M2 and comparative example CM1 were
removed from
the reaction tube after the methanation study had been carried out and
subjected to
characterization. The samples were the used catalyst from example Ml, the used
catalyst from
example M2 and the used catalyst from comparative example CM1.
The catalyst according to the invention (example M2) displayed a higher CO
conversion of 95%.
The CO conversion achieved over the catalyst from example M2 was thus 2% above
the
conversion achieved using the catalyst from example Ml. In addition, the
catalyst according to
the invention (example M2) was able to give a high conversion over a period of
more than
480 h, while the catalyst from comparative example CM1 displayed a significant
decrease in
CA 02854914 2014-05-07
42
activity even after about 300 h, leading to the experiment being stopped. In
addition, the catalyst
from example M2 had a lateral compressive strength of 168 N and thus a greater
mechanical
strength than the catalyst from example Ml.
Table 1 shows the composition of the feed gas before it was brought into
contact with the
catalyst and of the product gas obtained after contacting with the inventive
catalyst example Ml.
The figures for the individual components are in % by volume. The duration of
the catalysis
experiment was 1200 h.
Feed gas Product gas
CH4 37.1 53.0
H2 41.0 22.8
CO 9.7 0.7
CO2 1.1 3.0
H20 9.1 18.2
N2 2.0 2.3
Table 2.A shows the feed gas and product gas composition in the methanation
test of the
comparative catalyst (comparative example CM1) over a period of 300 h. After
this time, a
decrease in the conversion was observed, i.e. H2 and CO content in the product
stream
CA 02854914 2014-05-07
43
increased and the CH4 content decreased greatly. The figures for the
individual components are
given in % by volume.
Feed gas Product gas
CH4 37.7 53.3
H2 40.5 22.2
CO 9.6 1.2
CO2 1.1 2.6
H20 9.1 18.4
N2 2.0 2.3
Table 2.B shows the composition of the feed gas before it was brought into
contact with the
catalyst and of the product gas obtained after contacting with the catalyst
according to the
invention from example M2. The figures for the individual components are % by
volume. The
duration of the catalysis experiment was 480 h.
CA 02854914 2014-05-07
44
Feed gas Product gas
CH4 37.9 55.1
H2 41.7 21.2
CO 9.9 0.5
CO2 1.1 2.4
H20 8.5 18.2
N2 1.9 2.6
Table 3 shows the parameters determined by analysis of used catalyst from
example 1 (after
testing for 1200 h) and used catalyst CM1 (after testing for 300 h) by means
of XRD, nitrogen
sorption and Hg porosimetry. The Scherrer equation was used for estimating the
crystallite size.
,
CA 02854914 2014-05-07
U example M1 U
comparative
example CM1
XRD MgA1204 Content [mol%] 82 /
MgA1204 Crystallite size 24.5 /
[nm]
NiMg02 Content [mol%] 10 /
NiMg02 Crystallite size 28.5 /
[nm]
Ni metal Content [mol%] 8 /
Ni metal Crystallite size 54.5 40.5
[nm]
BET surface area using N2 [1-n2/m 19 39
Hg porosimetry [g/m1] 0.31 0.41
Pore structure monomodal
bimodal
average pore [nm] 58 - 60 60, 30
diameter
Pore surface area Hg [m2ig] 21 41
measurement
Lateral [N] 85 12
compressive
strength
i
CA 02854914 2014-05-07
46
Physical characterization
The XRD analyses were carried out by means of a D8 Advance Series 2 from
Bruker/AXS using
a CuK-alpha source (having a wavelength of 0.154 nm at 40 kV and 40 mA). The
measurements were carried out over the measurement range 5-800 (2-theta) in
0.02 steps at
4.8 seconds/step. The structure analysis software TOPAS (Bruker AXS) was used
to determine
the average crystallite sizes of the individual phases.
Figure 1 shows a representation of the powder diffraction pattern
which was taken on the
catalyst sample example M2 after the high-temperature calcination.
