WO2012078277A1

WO2012078277A1 - CONVERSION OF SYNGAS TO MIXED ALCOHOLS ON CHROMIUM-PROMOTED, SUPPORTED CoMoSx CATALYSTS

Info

Publication number: WO2012078277A1
Application number: PCT/US2011/059512
Authority: WO
Inventors: Gerolamo Budroni; Rainer Bruening; Daniela Ferrari; Davy Nieskens; Neelesh Rane
Original assignee: Dow Global Technologies Llc
Priority date: 2010-12-06
Filing date: 2011-11-07
Publication date: 2012-06-14

Abstract

As a catalyst for converting synthesis gas to an alcohol, use a combination of molybdenum, cobalt, an alkali metal and chromium on a carbon support.

Description

CONVERSION OF SYNGAS TO MIXED ALCOHOLS ON CHROMIUM-PROMOTED,

SUPPORTED CoMoS_x CATALYSTS

This application is a non-provisional application claiming priority from the U.S. Provisional Patent Application No. 61/419,938, filed on December 06, 2010, entitled "CONVERSION OF SYNGAS TO MIXED ALCOHOLS ON CHROMIUM-PROMOTED, SUPPORTED CoMoS_x CATALYSTS" the teachings of which are incorporated by reference herein, as if reproduced in full hereinbelow.

This invention relates generally to catalysts used in converting synthesis gas (also known as "syngas", a mixture of carbon monoxide (CO) and hydrogen (H₂)) to a mixture of alcohols that comprises at least ethanol and n-propanol, and more particularly to supported cobalt-molybdenum sulfide (CoMoS_x) catalysts used in such conversion wherein at least a portion of the cobalt is replaced by chromium (Cr).

Skilled artisans recognize that unsupported CoMoS_x catalysts have utility in converting syngas to such a mixture of alcohols. Skilled artisans also recognize that hydrocarbons such as methane and ethane constitute undesirable byproducts of syngas conversion and seek to minimize production of such undesirable byproducts.

Lower alcohols, such as methanol, ethanol and propanol, constitute important commodity chemicals. The lower alcohols can be converted to olefins, with methanol conversion via a process known as methanol to olefins or MTO being one example, and conversion of ethanol and propanol to, respectively, ethylene and propylene via dehydration being another example. The olefins, in turn, are important raw materials in, for example, a variety of polymerization processes such as those used to make polyethylene (e.g. low density polyethylene, high density polyethylene and linear low density polyethylene), polypropylene, and copolymers that contain both ethylene and propylene (e.g. ethylene- propylene rubber (EPR) and ethylene-propylene-conjugated diene (EPDM) interpolymers).

United States Patent Application Publication (USPAP) 2007/0004588 (Wang et al.) discloses a catalyst composition, its preparation and its use in converting syngas to alcohols. The catalyst comprises an anionic clay, especially hydrotalcite clay, and at least one catalytically active metal component selected from Groups 3 to 15 of the Periodic Table of the Elements (e.g. rhodium (Rh), manganese (Mn), cobalt (Co), and copper (Cu)). Wang et al. place catalysts used for mixed alcohol synthesis into three groups, referring to publications such as P. Forzatti et al., Catalysis Reviews - Science Engineering, Volume 33, Number 1-2, pages 109-168 (1991). One group includes mixed metal oxides made via co- precipitation and promoted with alkali metals such as sodium (Na), potassium (K), rubidium (Rb) or cesium (Cs). A second group is cobalt-molybdenum sulfide promoted with K (Co- M0S₂/K). Noble metals supported on an oxide support such as alumina or silica constitute a third group. Wang et al. suggests that catalyst compositions may be prepared by any of a number of methods, but provides a general method wherein one first dissolves a compound of each desired catalytically active metal in a solvent (e.g. water, a mineral acid, or an organic solvent such as acetone or methanol) to form a solution, combining the solutions with a support, either by impregnation or slurry mixing, and then drying the resultant mixture. Subsequent to drying, one typically forms the dried material into a suitable shape, then calcines or heat treats the shaped material.

J. Iranmahboob et al., in "Alcohol synthesis from syngas over K₂CO₃/C0S/M0S₂ on activated carbon", Catalysis Letters, volume 78, Numbers 1-4 (2002), pages 49-55, teach preparation of supported K₂CO₃/C0-M0S₂ on activated carbon catalyst by co-impregnation (using an aqueous mixture of ammonium molybdate tetrahydrate, potassium carbonate and cobalt (Π) sulfide) with testing of the catalyst under higher alcohol synthesis conditions of 2000 pounds per square inch gauge (psig) (13.79 megapascals (MPa)) to 2400 psig (16.55 MPa) and a temperature of from 270 degrees Celsius (°C) to 330 °C. An increase in reaction temperature leads to a decrease in selectivity to methanol and an increase in selectivity of hydrocarbons. An increase in gas hourly space velocity (GHSV) provides an increase in total alcohol productivity. Page 49 includes a reference to work by Murchison (C. Murchison et al., Proceedings 9^th International Congress on Catalyst, Volume 2 (1988), page 6) that tests cobalt-molysulfide on both alumina and carbon and shows that the carbon support is superior to alumina.

J. Nunan et al., in "Higher Alcohol and Oxygenate Synthesis over Cs/Cu/ZnO/M₂0₃ (M = Al, Cr) Catalysts", Journal of Catalysis 116 (1989), pages 222-229, teaches that surface doping of Cu/ZnO/M20₃ (M = Al, Cr) catalysts made from hydrotalcite precursors with cesium (Cs) significantly enhances alcohol synthesis rates under higher alcohol synthesis conditions. With aluminum (Al) doping, product selectivity shifts toward methanol while Cr doping shifts product toward ethanol. J. Christensen et al., in "Effects of H₂S and process conditions in the synthesis of mixed alcohols from syngas over alkali promoted cobalt-molybdenum sulfide", Applied Catalysis A: General 366 (2009), pages 29-43, investigates how process conditions influence synthesis of mixed alcohols from syngas over a K2CO₃/C0/M0S2/C catalyst, with emphasis upon effects of hydrogen sulfide (H₂S) in the syngas feed as well as temperature and partial pressures of hydrogen (H₂) and carbon monoxide (CO).

Z. Li et al., in "Effect of cobalt promoter on Co-Mo-K/C catalysts used for mixed alcohol synthesis", Applied Catalysis: A General, volume 1-2, (2001), pages 21-30, characterizes structures of sulfided Co-Mo-K/C catalysts by means of X-ray diffraction (XRD), laser Raman spectra (LRS) and X-ray absorption fine structure (XAFS) and assesses activities for alcohol synthesis via CO hydrogenation. .

J. Bao et al., in "Mixed Alcohol Synthesis from K-Co-Mo/C Catalyst Prepared by a Sol-Gel Method", Topics in Catalysis 52 (2009), pages 789-794, compare, in Table 1, catalytic performance of K-Co-Mo, K-Co-Mo/SiO and K-Co-Mo/C catalysts for synthesis of mixed alcohols. Table 1 shows that carbon (C) as a support provides better CO conversion percentile, better alcohol selectivity percentile and better selectivity to all of methanol, ethanol, propanol and butanol than silica (Si0₂) as a support.

D. Li et al., in "Ni/ADM: a high activity and selectivity to C₂₊ OH catalyst for catalytic conversion of synthesis gas to C1-C5 mixed alcohols", Topics in Catalysis, Volume 32, Numbers 3-4 (2005), pages 233-239, teaches that nickel (Ni), a well-known methanation catalyst, shows an excellent promotional effect to enhance both the activity and selectivity to C₂₊ OH over the ADM (alkali doped molybdenum sulfide) catalyst for mixed alcohols synthesis. Li et al. refers to prior work on the use of transition metals, especially cobalt (Co), iron (Fe), Ni, rhodium (Rh) and palladium (Pd), that suggests possible activity and C₂₊ OH selectivity enhancements stemming from their addition.

K. Fang et al., in "A short review of heterogeneous catalytic process for mixed alcohols synthesis via syngas", Catalysis Today, Volume 147 (2009), pages 133-138, refers to division of heterogeneous catalysts used for mixed alcohol synthesis (MAS) into two broad categories, noble metal-based and non-noble metal-based. The non-noble metal- based catalysts include modified Fischer-Tropsch catalysts, ADM catalysts and modified methanol catalysts. The ADM catalysts include cobalt-modified and nickel-modified molybdenum sulfide catalysts, each with possible further modifications with iron or lanthanum.

X. Ma et al., in "Co-decorated carbon nanotube-supported Co-Mo-K sulfide catalyst for higher alcohol synthesis", Catalysis Letters, Volume 111, Numbers 3-4 (2006), pages 141-151, teaches Co deposition on multi-walled carbon nanotubes (MWCNTs) as a support for Co-Mo-K sulfide catalysts. Catalyst preparation involves conventional incipient wetness methodology.

V. Surisetty et al., in "Synthesis of higher alcohols from syngas over alkali promoted MoS₂ catalysts supported on multi-walled carbon nanotubes", Applied Catalysis A: General 365 (2009), pages 243-251, reports results of a study of higher alcohol synthesis from syngas using K promoted molybdenum sulfide supported on MWCNTs with loadings of up to 20 wt% Mo and 9 wt% K.

In some aspects, this invention is a synthesis gas conversion catalyst comprising a combination of molybdenum, cobalt, an alkali metal and chromium on a carbon support, the catalyst providing a greater carbon monoxide conversion than the same catalyst save for replacing the chromium with equal molar amount of cobalt at identical conditions of temperature and gas hourly space velocity.

The aforementioned catalysts have utility in converting synthesis gas to a mixture of alcohols such as ethanol and propanol.

The catalyst comprises a combination of molybdenum, cobalt, an alkali metal and chromium on a carbon support.

The catalyst has a molybdenum content within a range of from 1 percent by weight (wt ) to 30 wt , preferably from 5 wt to 25 wt and more preferably from 10 wt to 20 wt , a cobalt content within a range of from 0.1 wt to 8 wt , preferably from 1 wt to 5 wt and more preferably from 1.5 wt to 3 wt , a chromium content within a range of from 0.1 wt to 8 wt , preferably from 1 wt to 5 wt and more preferably from 1.5 wt to 3 wt , an alkali metal within a range of from 0.1 wt to 20 wt , preferably from 5 wt to 15 wt and more preferably from 8 wt to 13 wt , and carbon support content within a range of from 10 wt to 90 wt , preferably from 20 wt to 80 wt and more preferably from 50 wt to 75 wt , each wt being based upon combined weight of molybdenum, cobalt, chromium, alkali metal and carbon support. The alkali metal may be any element in Group 1 (new notation) of the Periodic Table of the Elements which includes sodium, lithium, potassium, rubidium, cesium and francium, but is preferably sodium.

The molybdenum is present in a molar ratio to cobalt and chromium within a range of 0.5: 1 to 4:1, preferably from 1:1 to 3:1, and more preferably from 1.5:1 to 2.5:1.

The carbon support can be activated carbon or another carbon based support such as carbon nanotubes, carbon nanowires and other high surface area or structured carbon materials. Activated carbon constitutes a preferred support, especially activated carbon with a surface area between 500 square meters per gram (m²/g) and 2500 m²/g, more preferably between 750 m²/g and 1500 m²/g and most preferably between 1000 m²/g and 1300 m²/g.

In succeeding paragraphs, Arabic numerals designate examples (Ex) of the invention and capital letters designate comparative examples (CEx).

Ex 1

Prepare a mixed solution of ammonium heptamolybdate tetrahydrate (97.74 mg in 879.7μ1 of water), and use 400 μΐ to impregnate 400 milligrams (mg) of sieved/sized (40-80 mesh) activated carbon support material (Deltrex Chemicals) by incipient wetness. Dry the impregnated carbon in air at 80 °C for 3 hours. Effect a second impregnation with 223.4 μΐ of the same ammonium heptamolybdate solution and repeat drying. Prepare a solution containing 58.7 mg of ammonium heptamolybdate tetrahydrate, 59.8 mg of cobalt nitrate hexahydrate and 82.5 mg of chromium nitrate nonahydrate dissolved in 1.46 ml of water. Impregnate the carbon support material three times with 400 μΐ of this solution and follow each impregnation by drying in air at 80 °C for 3 hours. Prepare a solution with 207.2 mg of sodium nitrate dissolved in 948.4 μΐ of water. Impregnate the carbon support material three times with 400 μΐ of this solution and follow each impregnation by drying in air at 80 °C for 3 hours. Calcine the multiply impregnated, and dried carbon support material under nitrogen flow (200 ml/min) according to the following temperature program: 120 °C for 2 hours (ramp rate 5 °C/min); 210 °C for 2 hours (ramp rate 5 °C/min); 350 °C for 3 hours (ramp rate 5 °C/min). The calcined catalyst has a nominal composition of 15.5 wt molybdenum, 2.3 wt cobalt, 2 wt chromium, and 10.8 wt sodium (Na/Mo molar ratio of 3) on a carbon support, each wt being based upon combined weight of molybdenum, cobalt, chromium, sodium and carbon. The molybdenum, chromium and cobalt are present in a molar ratio of molybdenum to a combination of chromium and cobalt of 2: 1.

Load 100 microliters (μΐ) of the catalyst into a stainless steel reactor 3 mm internal diameter. Dry the catalyst in situ by heating reactor contents to a temperature of 150 °C (5 °C/min) and passing a flow of nitrogen of 5 ml/min while pressurizing to 35 bar. Following, pre-sulfurize the catalyst in situ in a hydrogen atmosphere at a pressure 35 bars (3.5 MPa) by passing a solution of 10 wt of dimethyl disulfide (DMDS) in pentane through the catalyst at a liquid hourly space velocity (LHSV) of 2 hr^"1 while increasing temperature from 150 °C to 400 °C at a rate of 1 °C/min. Maintain the condition at 400 °C for six hours, then cool the catalyst to a temperature of 310 °C at a rate of 5°C/min under flowing nitrogen (N₂) (5 ml/min). Before starting catalyst evaluation with a syngas feedstream, stabilize the catalyst under a flow of syngas (H₂:CO ratio of 1 : 1) with 50 parts by volume per million parts by volume (ppm) of syngas of hydrogen sulfide (H₂S) for 60 hours. After stabilizing the catalyst, evaluate the catalyst under the conditions shown in Table 1 below at a reaction pressure of 90 bars (9 MPa).

Analyze effluent gas from the reactor via gas chromatography (GC) to determine product composition and amount of CO converted. Raise catalyst temperature to 330 °C at a rate of 5 °C/min, holding catalyst temperature at 330 °C to collect 3 measurements of catalyst performance at each condition using GC analysis. Table 1 below summarizes operating conditions and the relative catalyst performance.

Ex 2

Replicate Ex 1, but replace the sodium solution with a potassium solution (292 mg of potassium nitrate in 902 μΐ of water) to yield a catalyst having a nominal composition of 15.5 wt molybdenum, 4.6 wt cobalt, and 18.5 wt potassium (K Mo molar ratio of 3) on a carbon support, each wt being based upon combined weight of molybdenum, cobalt, sodium and carbon. The molybdenum and cobalt are present in a molar ratio of molybdenum to a combination of chromium and cobalt of 2: 1.

CEx A

Replicate Ex 1, but eliminate the chromium solution to yield a catalyst having a nominal composition of 15.5 wt molybdenum, 4.6 wt cobalt, and 10.8 wt sodium (Na/Mo molar ratio of 3) on a carbon support, each wt being based upon combined weight of molybdenum, cobalt, sodium and carbon. The molybdenum and cobalt are present in a molar ratio of molybdenum to cobalt of 2:1.

Table 1

A comparison of the data in Table 1 for Ex 1 and CEx A shows that replacing part of the cobalt with chromium (Ex 1) in combination with sodium promotes CO conversion at the four reaction condition combinations of temperature and GHSV. At approximately the same CO conversion (9.7% and 7.4% for Ex 1 and, respectively, 9.5% and 7.0% for CEx A), Ex 1 shows that Cr decreases selectivity to CH4 and increases selectivity to n-propanol relative to CEx A which lacks chromium.

The improving effect is not observed when sodium is replaced by potassium (Ex 2) giving lower CO conversion relative to Ex 1 at given reaction condition. A comparison of the data in Table 1 for Ex 2 and CEx A shows that at same CO conversion the combination of Cr and K (Ex 2) produces more methane than CEx A.

Claims

WHAT IS CLAIMED IS:

1. A synthesis gas conversion catalyst comprising a combination of molybdenum, cobalt, an alkali metal and chromium on a carbon support, the catalyst providing a greater carbon monoxide conversion than the same catalyst save for replacing the chromium with equal molar amount of cobalt at identical conditions of temperature and gas hourly space velocity.

2. The catalyst of Claim 1, wherein the alkali metal is sodium.

3. The catalyst of Claim 1 or Claim 2, wherein the carbon is activated carbon.

4. The catalyst of any of Claim 1, Claim 2 or Claim 3, wherein the molybdenum, cobalt and chromium are present in a molar ratio of molybdenum to a combination of cobalt and chromium within a range of from 0.5:1 to 4:1.

5. The catalyst of any of Claims 1-4, wherein the catalyst has a molybdenum content within a range of from 1 percent by weight to 30 percent by weight, a cobalt content within a range of from 0.1 percent by weight to 8 percent by weight, a chromium content within a range of from 0.1 percent by weight to 8 percent by weight, an alkali metal within a range of from 0.1 weight percent to 20 weight percent, and a carbon support content within a range of from 10 percent by weight to 90 percent by weight, each percent by weight being based upon combined weight of molybdenum, cobalt, chromium, alkali metal and carbon support.