PROCESS FOR THE PREPARATION OF SYNTHESIS GAS FROM METHANOL

Abstract: A process for the preparation of synthesis gas from methanol by converting methanol at an elevated tempera­ ture into synthesis gas in the presence of a catalyst which has been obtained by depositing on aluminium oxide a substantially nickel-free metal compound comprising an alkali metal in a quantity corresponding with at least 2.5 mmol of alkali metal per gram of aluminium oxide, and subjecting the loaded aluminium oxide to a heat treatment which causes the alkali metal compound to decompose.

Description: PROCESS FOR THE PREPARATION OF SYNTHESIS GAS FROM METHANOL The invention relates to a process for the preparation of synthesis gas from methanol. It is known in the art to prepare synthesis gas by means of partial combustion from hydrogen- and carbon-containing materials, such as lignite, crude petroleum and fractions thereof, oils recovered from tar sand and bituminous shale, brown coal, anthracite and coke. A known method is the coal gasification process. Since sources of synthesis gas are often in places other than those where it is needed, it has to be transported and if necessary, stored. This calls for technically sophisticated means of transportation and storage. Consequently, storage and transport of synthesis gas is a costly business. Under certain circumstances, it may therefore be advantageous to convert the synthesis gas at the source of its production into a substance which is easier and less expensive to store and transport and subsequently, after transporting it to where synthesis gas is required, to reconvert said substance into synthesis gas. It has now been found that methanol, for which processes are known to prepare it from synthesis gas and for which the cost of storage and transportation is considerably lower than for synthesis gas, can be converted advantageously into synthesis gas while achieving a high conversion of the methanol and a high selectivity in respect of the desired product - the synthesis gas. It will be appreciated that for the preparation of synthesis gas from methanol, one may also use methanol which has been obtained in a manner other than starting from synthesis gas. This may be an attractive option in particular when the need for synthesis gas is relatively small and infrequent. The invention therefore relates to a process for the preparation of synthesis gas from methanol wherein methanol is converted into synthesis gas at an elevated temperature in the presence of a catalyst which has been obtained by depositing on aluminium oxide a substantially nickel-free metal compound comprising an alkali metal in a quantity corresponding with at least 2.5 mmol of alkali metal per gram of aluminium oxide, and subjecting the loaded aluminium oxide to a heat treatment which causes the alkali metal compound to decompose. The preparation of the synthesis gas is preferably carried out in such a manner that during the reaction the methanol is in the vapour phase. The temperature at which the reaction is carried out preferably lies between 300 and 6009C and in particular between 350 and 550"C, while the pressure, which in this connection is of lesser importance, may vary within wide limits. The reaction can suitably be carried out at atmospheric presure or slightly below. The methanol vapour will usually be passed over the catalyst in a simple manner known per se in which the methanol vapour space velocity may vary within wide limits, e.g. between 20-1500 1 (S.T.P.) methanol per 1 of catalyst per hour, depending on the temperature used. Although the methanol vapour may be converted batchwise into synthesis gas by the process of the invention, preference is given to a continuous process in which a catalyst bed in fixed or fluidized state can suitably be employed. If desired, a carrier gas may be used when passing the methanol vapour over the catalyst. This carrier gas should be inert under the conditions at which the methanol vapour is converted into synthesis gas. A suitable carrier gas is, for example, nitrogen or helium. Moreover, water vapour may be passed over the catalyst together with the methanol vapour, if desired. The water vapour reacts with the carbon monoxide formed to yield hydrogen and carbon dioxide. It is thus possible, if desired, to change the hydrogen/carbon monoxide ratio in the synthesis gas formed. The aluminium oxide, which in the catalyst acts as the carrier, may in principle be any form of aluminium oxide. Preference is given to the use of gamma-aluminium oxide with a specific surface area of at least 25 m2/g and in particular at least 100 m2/g. The metal compound comprising alkali metal which is deposited on the aluminium oxide prior to the heat treatment is suitably chosen from the group consisting of alkali metalhydroxides, -nitrates, -carbonates, -bicarbonates, -alcoholates, -carboxylic acid salts and -dicarboxylic acid salts; a combination of two or more of said compounds may also be used. Preference is given, however, to the use of a single compound chosen from the group consisting of alkali metal -carbonates, -bicarbonates, -alcoholates, -carboxylic acid salts and dicarboxylic acid salts. Most preference is given to alkali metal carbonates. Of the above-mentioned alkali metal compounds, potassium compounds are preferred, special preference being given to such compounds as potassium carbonate, potassium bicarbonate, potassium formate, potassium acetate and potassium oxalate. The most preferred compound is potassium carbonate. Deposition of nickel, a compund having substantial hydrogenating activity, appears to have a negative influence on the selectivity of synthesis gas preparation from methanol. The quantity of alkali metal compound deposited on the aluminium oxide preferably corresponds with 2.5-10 mmol alkali metal per gram of aluminium oxide; most preference is given to a quantity of 3-6 mmol per gram of aluminium oxide. The deposition of the alkali metal compound(s) on the aluminium oxide may be effected by any method known in the art of depositing compounds on carriers. The alkali metal compound(s) can be deposited on the aluminium oxide carrier in a simple manner by impregnating the carrier with an aqueous solution of the alkali metal compound(s), followed by drying at, for example, 100-300 C. The temperature at which the heat treatment of the aluminium oxide on which the alkali metal compound has been deposited is carried out may vary within wide limits, for example 350-8000C; excellent results can be achieved at temperatures from 375-7000C. As regards the duration of the heat treatment, the higher the chosen temperature, the shorter need the duration be. A duration of 1-50 hours will generally be sufficient. The heat treatment is preferably carried out in the presence of an inert gas, i.e. a gas which under the conditions at which the heat treatment is carried out will not react with one or more of the substances present during the heat treatment. Hydrogen, helium, nitrogen or mixtures thereof, and in particular nitrogen, can be used for the purpose. The heat treatment can be suitably effected by passing a heated inert gas over the aluminium oxide on which the alkali metal compound has been deposited. It may be advantageous to precede the heat treatment in the inert gas by a heat treatment in an oxygen-comprising gas (e.g. air or pure oxygen), preferably within the same temperature range and in the same manner as described above for the heat treatment in an inert gas. An advantageous operation can be achieved by introducing the aluminium oxide on which an alkali metal compound has been deposited into the reactor in which the preparation of synthesis gas from methanol is subsequently to be carried out, then sub ing it to a heat treatment by passing heated nitrogen over it, optionally preceded by a heat treatment by passing a heated oxygen-comprising gas over it, and finally passing methanol vapour over the catalyst obtained. Although the catalyst will remain active for a long time, an unacceptable drop in activity will nevertheless be observed in the course of time. The catalyst can then be regenerated by methods known per se by burning off the material deposited on catalyst during the conversion of methanol into synthesis gas, the burning off being carried out with the use of an oxygen-comprising gas. Subsequently, the burnt-off catalyst should be subjected to a heat treatment in an inert gas as described above before it can be used again in the process according to the invention. The invention will now be illustrated with the aid of the following Examples. EXAMPLE 1 20 g K2CO3 was dissolved in 40 ml H2O. Then 80 g gamma g was 2 A1203 with a specific surface area of 329 m2/g was impregnated with this solution. The impregnated A1203 was dried by heating to 2500C and then calcined for 4 h at 500"C. The amount of potassium atoms on the aluminium oxide was 3.6 mmol/g Awl203. 4 g of the loaded A1203 was introduced into a reaction tube having an inner diameter of 10 mm. Then N2 was passed through the reaction tube for 2 h at a flow rate of 5 l(S.T.P.)/h, at 400"C and at atmospheric pressure. Next, CH3OH was added to the N2 stream and passed over the catalyst for 16 h at a space velocity of 500 l(S.T.P.)/h. 97.3%w of the throughput of methanol was found to have been converted. The converted product had the following composition expressed as %v: H2 62.5; CO 28.0; C1 2.4; CO2 2.1; CH30CH3 4.9 and others 0.1. EXAMPLE 2 The expriment was carried out in a similar manner as described in Example 1, except that: (i) the heat treatment was carried out in the presence of nitrogen at 5000C; and (ii) methanol vapour was passed over the catalyst at a temperature of 4500C for 7 days, at a space velocity of 300 l(S.T.P.)/l/h with helium being used as carrier gas (1 l(S.T.P.)/h). All of the methanol was converted and, in addition to synthesis gas, the product gas contained only 0.5 Zv of CO2 and 0.1 Zv of CH4. After 7 days, there was still no sign of any decrease in catalyst activity. When instead of methanol a mixture of methanol and water (volume ratio 1:1) was passed over the catalyst, this likewise resulted in complete conversion of the methanol.

Claims: CLAIMS 1. A process for the preparation of synthesis gas from methanol wherein methanol is converted into synthesis gas at an elevated temperature in the presence of a catalyst which has been obtained by depositing on aluminium oxide a substantially nickel-free metal compound comprising an alkali metal in a quantity corresponding with at least 2.5 mmol of alkali metal per gram of aluminium oxide, and subjecting the loaded aluminium oxide to a heat treatment which causes the alkali metal compound to decompose. 2. A process as claimed in claim 1 wherein the conversion of methanol into synthesis gas is carried out at a temperature from 300-600 C. 3. A process as claimed in claim 1 or 2 wherein a catalyst is used which has been subjected to a heat treatment at a temperature from 350-800 C. 4. A process as claimed in any of the preceding claims wherein the heat treatment is carried out in the presence of an inert gas. 5. A process as claimed in claim 4 wherein the inert gas is nitrogen. 6. A process as claimed in claim 4 or 5 wherein the heat treatment in the presence of an inert gas is preceded by a heat treatment in the presence of an oxygen-comprising gas. 7. A process as claimed in any of the preceding claims wherein a catalyst is used which has been obtained by heat-treating aluminium oxide on which there has been deposited a quantity of an alkali metal compound corresponding with 2.5-10 mmol alkali metal per gram of aluminium oxide. 8. A process as claimed in claim 7 wherein a quantity of the alkali metal compound is used corresponding with 3-6 mmol alkali metal per gram of aluminium oxide. 9. A process as claimed in any of the preceding claims wherein the metal compound comprising alkali metal is chosen from the group consisting of the alkali metal-hydroxides, -nitrates, -carbonates, -bicarbonates, -alcoholates, -carboxylic acid salts and -dicarboxylic acid salts. 10. A process as claimed in claim 9, wherein the alkali metal is potassium. 11. A process as claimed in claim 10, wherein potassium carbonate has been used. 12. A process for the preparation of synthesis gas from methanol substantially as described hereinbefore with reference to the Examples. 13. Synthesis gas when ever prepared by a process as claimed in any of the preceding claims.

Inventors: KRAMER, PETRUS ANTHONIUS, VAN DEN BERG, FRANCISCUS GONDULFUS ANTHONIUS
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1985
Main classification: C01B3
IPC codes: C10J3, C01B3, B01J23