Catalyst for the synthesis of methyl mercaptan and process for
producing methyl mercaptan from synthesis gas and hydrogen
sulphide
5 The work that led to this invention received financing from the
European Union as part of the 7th Framework Programme (FP?/2007-
2013) under project number No. 241718 EUROBIOREF.
The present invention relates to a specific molybdenum- and
potassium-based catalyst that is useful for producing methyl mercaptan
10 from synthesis gas and hydrogen sulfide, and to its preparation
process.
The invention also relates to a process for producing methyl
mercaptan that uses this catalyst.
The invention lastly relates to the use of hydroxyapatite as a
15 . support for a catalyst for producing methyl mercaptan.
Methyl mercaptan has great industrial interest, particularly as a
raw material for synthesizing methionine, an essential amino acid that
is in widespread use in animal food. Methyl mercaptan is also a raw
material for many other molecules, in particular dimethyldisulfide
20 (DMDS), a sulfidation additive for hydrotreating catalysts m
petroleum fractions, among other applications.
Methyl mercaptan is commonly produced m large tonnages
industrially from methanol and hydrogen sulfide. It may prove
economically interesting to want to produce methyl mercaptan directly
25 from carbon monoxide, hydrogen and hydrogen sulfide according to
the following reaction scheme:
C0+2H2+H2S-->CH3SH+H20 (I)
The main by-product from this synthesis is carbon dioxide.
Carbonyl sulfide (COS) is considered to be the reaction intermediate,
30 which leads to methyl mercaptan after hydrogenation according to the
following reaction schemes:
CO+HzS .._. COS+H2 (2)
wo 2014/154885 PCT /EP20 14/056343
2
COS+3H2 ~ CH3SH+H20 (3)
The carbon dioxide comes from two side reactions:
CO+H20 ~ C02+H2 (4) and
COS+H20 ~ C02+H2S (5)
5 These two side reactions, which consume the mam raw
material: carbon monoxide, and the reaction intermediate: carbonyl
sulfide, are due to the inescapable presence of water, coproduced
during methyl mercaptan synthesis. The carbon dioxide can optionally
be recycled to produce methyl mercaptan as well according to the
10 following scheme:
C02+3H2+H2S ~ CH3SH+2H20 (6)
But this reaction is known to be slower than that from carbon
monoxide. Therefore there is incentive to make carbon dioxide
production as low as possible at the outlet of the methyl mercaptan
15 reactor.
From document W02005/040082 several catalysts are known
·for the synthesis of methyl mercaptan from synthesis gas and hydrogen
sulfide.
In particular, this document discloses the use of a catalyst
20 comprising a Mo-0-K based active component, an active promoter and
optionally a support. The catalysts exemplified have different
chemical natures, such as K2Mo04/Fe20 3/NiO or
K2Mo04/CoO/Ce02/Si02, each supported on silica. This leads to a
C02/MeSH selectivity ratio of 0.88 at 333°C.
25 A family of catalysts composed of a porous support onto which
a metal has been deposited electrolytically is also known from
document US2010/0286448. K2Mo04 and another metal oxide as
promoter were then impregnated onto this support. Example 15 of this
document describes the preparation of K2Mo04/Ni0/CoSi02. The
30 C02/MeSH selectivity ratio with this complex catalyst is 0.65.
Lastly, US document 2010/0094059 describes supported
K2Mo04 based catalysts, where the porous support used alone or in
mixtures is chosen from Si02, AhOJ, Ti02, Ah03/Si0 2, Zr02, zeolites
or carbon-containing materials. Tellurium oxide (Te02) is used as
wo 2014/154885 PCT /EP20 14/056343
3
promoter. The C02/MeSH selectivity ratios are comprised between
0.60 and 0.77 measured at 300°C.
From the teaching of these documents it has been observed that
combining catalysts with specific structures, promoters and supports,
5 each being carefully selected, means that interesting selectivity ratios
can be achieved.
There is a current need for a catalyst that is simply synthesized
and leads to very good selectivity. This technical problem has been
resolved by a molybdenum- and potassium- based catalyst supported
10 by hydroxyapatite.
It has been observed that the catalyst according to the
invention is easier to prepare, given that the presence of a promoter is
not indispensable. It is less costly than those disclosed in the
previously cited documents. And lastly, it leads to very good
15 C02/MeSH selectivities.
The invention also relates to the preparation process for this
·catalyst.
The invention also relates to a process for producing methyl
mercaptan from synthesis gas and hydrogen sulfide using the catalyst
20 according to the invention.
The invention also relates to the use of the catalyst as defined
above for the synthesis of methyl mercaptan from synthesis gas and
hydrogen sulfide.
Lastly the invention relates to the use of hydroxyapatite as
25 support for preparing a catalyst for producing methyl mercaptan, and
in particular in a catalytic process by reacting carbon oxide, sulfur
and/or hydrogen sulfide and hydrogen.
Oth.er characteristics, features, subjects and benefits of the
present invention will emerge even more clearly on reading the
30 description and the examples that follow.
Any range of values denoted by the expression "between a and b"
represents the values ranging from more than a to less than b (i.e. limits
a and b excluded), while any range of values denoted by the expression
5
wo 2014/154885 PCT /EP20 14/056343
4
"from a to b" means the values rangmg from a to b (i.e. including the
limits a and b).
Catalyst
The present invention relates to a catalyst.
This catalyst comprises a molybdenum- and potassium-based
active component and a hydroxyapatite-based support.
Active component
The active component present in the catalyst according to the
invention comprises molybdenum and potassium within a single
10 component.
Preferably, the molybdenum- and potassium-based active
component is chosen from compounds based on Mo-S-K, compounds
based on Mo-0-K, and their mixtures.
The Mo-S-K based active component may be obtained by
15 deposit and calcination of K2MoS4 or (NH4)2 MoS4 precursors with
impregnated K2C03 added separately to the support.
20
The Mo-0-K based active component may be obtained by
deposit and calcination of K2Mo04 or (NH4)2 Mo04 precursors with
impregnated K2C03 added separately to the support.
it IS also possible to use ammonium heptamolybdate
(NH4)6Mo70 24.4H20 as reagent, in the presence of a potassium salt
such as for instance potassium nitrate KN03 , potassium carbonate
K2C03 or potassium hydroxide KOH.
These compounds are precursors of Mo-S-K and Mo-0-K based
25 active phases respectively. The active phases are obtained after in situ
precursor pretreatment, with for example a procedure consisting in a
first step of drying in nitrogen at 250°C, followed by sulfidation with
hydrogen sulfide at the same temperature for 1 hour, then a step of
reduction/sulfidation with H2/H2S at 3 50°C for 1 hour.
30 Support
The catalyst support
hydroxyapatite having formula
stoichiometric hydroxyapatite.
according to the invention 1s
Cato(P04)6(0H)2, advantageously a
wo 2014/154885 PCT/EP2014/056343
5
Preferably, hydroxyapatite that is useful according to the
present invention has a Ca/P molar ratio ranging from 1.5 to 2.1, and
more preferably 1.67, corresponding to the expected value for
stoichiometric hydroxyapatite.
5 Preferably, the weight ratio of the catalyst according to the
invention is:
K2MoS4/Ca1 o(P04)6(0H)2=31.311 00
K2Mo04/Ca1o(P04)6(0H)2= 50.71100
The catalytic activity may be improved by usmg a support
10 material having a specific area greater than 25 m2/g.
15
Preferably, the hydroxyapatite supports according to the
invention have a specific area of at least 40 m2/g, more specifically
the specific area ranges from 40 m2/g to 300 m2/g and a Ca/P molar
ratio of 1.67.
The structure of the support may be three dimensional,
spherical, cylindrical, ring-shaped, star-shaped, granulates or any
other three dimensional shape, or in the form of a powder, which can
be pressed, extruded, granulated or in a three dimensional shape.
Preferably, the catalyst particles have uniform particle s1ze
20 distribution with diameter from 0.1 mm to 20.0 mm measured by sieve
analysis.
Promoter
Preferably, the catalyst according to the invention consists m a
molybdenum- and potassium-based active component and a
25 hydroxyapatite-based support.
30
However, it is possible to env1sage the presence of a promoter
known to the person skilled in the art, such as tellurium oxide, nickel
oxide or iron oxide.
Catalyst preparation process
The invention also relates to the preparation process for the
catalyst according to the invention. This process compnses the
following successive steps:
preparing the precursor for the active phase
preparing the support, and
5
wo 2014/154885 PCT /EP2014/056343
6
dry impregnating the support with the active phase
precursor.
Preparing the precursor (or the active phase
1/Mo-0-K
1. The K2Mo04 salt is a commercial salt. To prepare the Mo-0-
K based-catalyst, a fixed quantity of K2Mo04 is dissolved in a volume
of water to obtain a solution with desired concentration, such as for
example a concentration ranging from 0.5-1.0 g/mL.
2. It is also possible to begin with separated molybdenum and
10 potassium salts, i.e. that are not part of the same compound. For this
synthesis, a molybdenum-based solution is prepared by adding
ammonium heptamolybdate in water to obtain a Mo03 concentration
ranging from 22 to 33% by weight.
In parallel, a potassium-based solution is prepared by adding
15 potassium nitrate in water to obtain a K20 concentration ranging from
31 to 43 % by weight.
2/Mo-S-K
The K2MoS4 synthesis is generally done in two steps.
The first step involves preparing ammonium tetrathiomolybdate
20 (A TTM); the second step IS the synthesis of potassium
tetrathiomolybdate (K2MoS4 ) from the salt prepared in the first step.
To prepare ATTM, hydrogen sulfide IS left to bubble
continuously in a 25% aqueous ammonia solution, in which ammonium
heptamolybdate (HMA) has been dissolved. The solution temperature
25 increases, indicating an exothermic reaction. The hydrogen sulfide
bubbling is stopped when the temperature falls (generally after one
hour).
30
The. solution then contains red crystals with green reflections,
which correspond to ammonium tetrathiomolybdate.
The second step consists 1ll an Ion exchange between
ammonium ions in the ammonium tetrathiomolybdate salt obtained and
potassium ions, which come from a potassium hydroxide solution. The
salts obtained are then stored under vacuum. A quantity of potassium
tetrathiomolybdate is dissolved in water.
wo 2014/154885 PCT /EP20 14/056343
7
The potassium salt· useful in the catalyst according to the
present invention may come from the following compounds: potassium
acetate (KAc), potassium oxalate (K2C20 4), potassium hydroxide
(KOH), potassium carbonate (K2C03), potassium nitrate (KN03), and
5 potassium bicarbonate (KHC03).
Support preparation
The catalyst support, constituted of hydroxyapatite, is prepared
by a coprecipitation method. An aqueous solution of calcium nitrate
Ca(N03)2 was added dropwise to an ammonium hydrogenphosphate
10 (NH4)H2P04 solution with stirring. The temperature is held at I 00°C
and the pH is held at 10 with addition of an ammonia solution (25%).
The resulting white precipitate is filtered, washed, dried at
80°C overnight and calcinated at 400°C. The hydroxyapatite
Ca1 0(P04)6(0H)2 was obtained with a Ca/P molar ratio of 1.67
15 corresponding to the expected value for · a stoichiometric
hydroxyapatite.
Dry impregnating the support with the active phase precursor
1/Mo-0-K
The dry impregnation method is used to prepare the catalyst.
20 The K2Mo04 solution is impregnated in one step on the support. When
the solutions containing potassium and molybdenum are distinct, the
impregnation is done in 2 steps.
2/Mo-S-K
A potassium tetrathiomolybdate solution is then impregnated
25 onto hydroxyapatite. The molybdate content in the catalyst depends on
the K2MoS4 or K2Mo04 solubility and the support's porous volume.
The K2MoS4 solubility is between 0.25 g/mL and 0.50 g/mL
(0.35 g/mL) and the K2Mo04 solubility is between 0.50 g/mL and
1.50 g/mL (0.90 g/mL). The support's porous volume is between
30 0.8 mL/g and 2.2 mL!g.
Consequently, the volume of solution used is calculated to
obtain the desired weight ratio, and preferably the weight ratio as
defined above.
wo 2014/154885 PCT/EP2014/056343
8
After impregnation, the solid undergoes a maturation step for 2
hours, then oven drying at 80 °C for 24 hours, and calcination under
gas flow (typically air) at 490 oc for 4 hours. If a second impregnation
step is necessary, the solid undergoes the maturation, drying and
5 calcination steps again.
Production process (or methyl mercaptan
The invention relates to a production process for methyl
mercaptan in a catalytic process by reacting carbon oxide, sulfur
and/or hydrogen sulfide and hydrogen, comprising the use of a catalyst
10 as defined above.
The CO or C02/H2S/H2 molar ratios range from 1/1/0 to 1/8/8,
or when sulfur is used to replace hydrogen sulfide, the molar ratios of
CO or C02/H2S/H2/S reagents range from I II/OII to 1/8/8/8.
Preferably, the CO or COz/lhS/Hz molar ratios range from
15 1/2/1 to 1/4/4, when sulfur is used to replace hydrogen sulfide, the
molar ratios of CO or C02/H2S/H2/S reagents from 1/2/2/1 to 1/4/4/4.
These molar ratios take C02 into account. Therefore, they
consider both reaction scheme (I) and reaction scheme (6).
Preferably, the reaction may occur m fixed tubular,
20 multitubular, catalytic wall micro-channel or fluid bed reactors.
The invention also relates to the use of the catalyst as defined
above for the production of methyl mercaptan from synthesis gas and
hydrogen sulfide.
Lastly the invention relates to the use of hydroxyapatite as
25 support for preparing a catalyst for producing methyl mercaptan, and
in particular in a catalytic process by reacting carbon oxide, sulfur
and/or hydrogen sulfide and hydrogen.
The present invention will now be described in the examples
below, these examples being given only for illustration, and are of
30 course not limiting.
EXAMPLES
Example I
5
10
15
20
wo 2014/154885 PCT/EP2014/056343
9
The catalyst according to the invention is prepared according to
the dry impregnation method, as defined above.
The resulting catalyst has the following characteristics:
Catalyst
Chemical composition
(% by weight) Mo K s N
K2MoS4/Hap 9.9 8.1 13.3 <0.10
Table 1: Elemental analysis of the catalyst
Example 2: The catalyst used is K2Mo04 on hydroxyapatite
Example 3: The catalyst tested is K2Mo04 on Si02
Example 4: The catalyst tested is K2MoS4 on Ah03
Example 5: The catalyst tested is K2Mo04 on Ah03 .
Evaluating the catalysts
The catalysts are evaluated In a reaction to produce methyl
mercaptan in a fixed-bed reactor in the following conditions:
Temperature: 280°C,
Pressure: 1 0 bars,
Composition of CO!H2/H2S=l/2/1 feed gas (v/v),
GHSV (Gas Hourly Space Velocity)= 1333 h- 1
The reagents and products were analyzed in-line by gas
25 chromatography.
Before the test, the catalysts were activated in situ with a first
procedure consisting in a first step of drying in nitrogen at 250°C,
followed by sulfidation with hydrogen sulfide at the same temperature
5
wo 2014/154885 PCT/EP2014/056343
10
for I hour, then a step of reduction/sulfidation with H2/H2S at 350°C
for I hour.
The results are in table 2 below.
Molar selectivities (%)
Examples Catalyst C02/CH3SH ratio
CH3SH cos C02
1 (inv) K2MoS4/Hap 44.1 23.3 32.6 0.74
2 (inv) K2Mo04/Hap 43.3 23.6 31.9 0.74
3 (comp) K2Mo04/Si02 48.8 5.3 45.3 0.93
4 (comp) K2MoS4/Ab03 45.0 7.3 46.6 1.04
5 (comp) K2Mo04/Ah03 47.0 3.4 49.6 1.06
TABLE 2: Results of catalytic tests
The results presented in table 2 show that the catalysts
according to the invention (examples 1 and 2) give much lower C02
10 (undesired product) selectivities than catalysts on the supports in the
prior art (silica: example 3 or alumina: examples 4 and 5).
15
20
The selectivities are compared uslllg carbon monoxide
isoconversion, where this conversion is expressed by m2 of specific air
in the catalyst.
By comparing the results obtained with catalysts 1 and 4, we
observe a 30% improvement in ratio, and this improvement is linked to
choosing hydroxyapatite as support.
The same observation is seen when comparing example 2
according to the invention and examples 3 and 5.
We observe increased methyl mercaptan selectivity compared
to the carbon dioxide produced according to a side reaction.
It should be noted that this selectivity is obtained without aid
from the promoter such as tellurium oxide, nickel oxide or iron oxide
as described in the prior art.

CLAIMS
1. A catalyst compnsmg a molybdenum- and potassium-based
active compqnent and a hydroxyapatite-based support. ·
. 2. Thle catalyst as claimed in claim 1, characterized in that the
catalyst su~lport is hydroxyapatite having stoichiometric for.mula
Ca1o(P04)6(~Hh. .
. · 3. The catalyst as claimed in claim l or 2, characterized in that
the m.olybddnum- and potassium-based activ. e component is chosen
from compoJnds based on Mo~S-K, compounds based on Mo-0-K, and
h
. . I . t eir miXtUr(jS.
4. T:le catalyst as claimed in claim 3, characterized in that the
precursor f r the Mo-S-K based active component has structure
K2MoS4
'
5. T]le catalyst as claimed in claim 4, charact'"rized in that the·
weight ratio for the catalyst according to the invention is
. K2M S4/Ca1o(P04)6(0H)2= 31.3/100 ,
6. Tlje catalyst as claimed in claim 3, characterized in that the
precursor fjr the Mo-0-K based active component has structure
K2Mo04.
7. T e catalyst as claimed in claim 6, characterized in that the
weight ratio for the catalyst according to the invention is:
K2M • 04/Ca1o(P04)6(0Hh= 50.71100
8. A preparation process for the catalyst as defined in any of
claims 1 to , characterized in that it comprises the following steps:
- prfparing the precursor for the active phase
- preparing the support, and
- drt impregnating the support with the active phase
precursor. 1
9. AI production pro.cess for methyl mercaptan in a catalytic
process by leacting carbon oxide, sulfur and/or hydrogen sulfide and
hydrogen, c mprising the use of a catalyst as defined in any of claims
l to 7.
10. The use of hydroxyapatite as a support for preparing the
catalyst for· toducing .methyl mercaptan.