GB2083469A - Methanol/higher alcohol mixtures - Google Patents

Abstract

Mixtures of methanol and higher alcohols to be used in particular as fuels, either alone or in admixture with petrol, are produced by reacting H2 and CO, possibly in the presence of CO2, in a molar ratio H2:CO of from 0.1:1 to 20:1, preferably from 0.5:1 to 5:1, at a temperature of from 300 to 500 DEG C, preferably from 350 to 450 DEG C, and at a pressure of from 2000 to 16000 KPa, in the presence of a catalyst based on chromium, zinc and at least one alkali metal. The Zn:Cr weight ratio of the catalyst, considered as oxides thereof, is from 5:1 to 1:1 and the alkali metals or the alkali metals as oxides are present in an amount of 0.5 to 5% by weight relative to the total of the elements considered as oxides.

Description

SPECIFICATION Methanol/higher alcohol mixtures This invention relates to a process for preparing methanol/higher alcohol mixtures. The mixtures are in particular useful as substitutes for petrol, and can also be mixed therewith in various amounts for use as fuels in internal combustion engines.

Many processes are known for producing methanol in admixture with higher alcohols. They, however, suffer from the disadvantage of requiring the use of comparatively high working pressures, generally between 1 7000 and 35000 KPa. This is a considerable defect both from the point of view of initial costs and running costs. The need to use high pressures is well known from the literature. For example, Natta in "Catalysis", Vol. V, Reinhold Publishing Corp. New York, (1957), on page 136, clearly states that, due to the considerable volume reduction which takes place when producing higher alcohols, the synthesis must be effected at high pressures. Obviously, the greater the pressure, the greater is the amount of higher alcohols produced. This is corroborated by Table VII of the paper presented by P. G.Laux at the International Symposium on Alcohol Fuel Technology, Wolfsburg, 21 to 23 November,1977, and having the title "The Catalytic Production and Mechanism of Formation of Methyl Fuel", where it is stated that the amount of isobutanol increases from 1.86% at 3900C and 1 76 kg/cm2, to 6.14% at 3900C and 302 kg/cm2.

The prior art thus clearly shows that the production of mixtures of methanol and higher alcohols is possible only by using high pressures, in view of thermodynamic considerations.

According to the present invention, there is provided a process for preparing a methanol/higher alcohol mixture, which comprises reacting hydrogen and carbon monoxide in a H 2:CO molar ratio of from 0.1:1 to 20:1 at a temperature of from 300 to 5000C and at a pressure of from 2000 to 16000 KPa, in the presence of a catalyst comprising chromium, zinc and at least one alkali metal.

Thus, it has now surprisingly been found that the production of mixtures of methanol and higher alcohols can be carried out under lower pressures and that such mixtures have a content of higher alcohols which is considerably higher than that which is obtained under the high pressures used in the prior art. The mixtures fulfil, better than the known mixtures, the task of solubilizing water, due to their higher content of higher alcohols and act in the same way as the known mixtures when added to petrol, while their production cost is considerably lower due to the reduced pressures under which they are produced.

The reason why mixtures of methanol and higher alcohols, rather than methanol alone are added to petrol arises from the fact that a certain amount of water is always present in petrol, this water deriving most usually from the water used for washing the pipelines in refineries, or from the moisture in the air. Methanol, when used alone, mixes with the water which is present in the petrol and separates from the petrol, so that in petrol tanks two layers are formed, one of petrol and the other of methanol and water. Under these circumstances, the engines run with great difficulties when the layer of methanol and water is dispensed instead of petrol.

It is known that the presence of higher alcohols permits methanol to solubilize when water is present in petrol, thoroughly homogeneous mixtures then being formed. The mixtures obtained according to the invention usually have a content of higher alcohols of from 25 to 65% by weight and permit methanol to be solubilized even if the water content of the petrol is extremely high.

With the mixtures of the prior art, which generally contain 10% by weight of higher alcohols, it is possible, at -1 80C, to tolerate 1000 parts per million of water when the alcohol/hydrocarbon ratio is 20:80. With the same ratio of 20:80 and at --18 OC, the mixtures obtained according to the invention usually permit from 2500 ppm to more than 5000 to 13000 KPa, respectively.

In the process of the invention, the reaction may take place in the presence of carbon dioxide and/or inert gas. The preferred H2:CO molar ratio is from 0.5:1 to 5:1. The preferred temperatures and pressures are from 350 to 4500C and from 500 to 13000 KPa, respectively.

Herein, the term "higher alcohols" includes other oxygen-containing compounds which, in use as fuels in internal combustion engines, behave like alcohols. Such compounds, if present, are usually present in small amounts.

The catalyst used according to the invention comprises chromium, zinc and at least one alkali metal. The ratio of Zn to Cr by weight, considered as the oxides thereof, is preferably from 5:1 to 1:1.

The alkali metal is preferably potassium and the amount of alkali metal, considered as oxide, is preferably from 0.5% to 5% by weight relative to the total weight of the elements considered as oxides.

The preparation of the catalyst may be effected in a number of ways. By way of example, there can be mentioned the precipitation with NH3 from solutions of nitrates of chromium and zinc, or the attack with chromic acid of aqueous suspensions of zinc oxide. The catalyst can be dried in an oven or by atomization and then subjected to firing. The catalyst can be extruded, pelletized or granulated to various sizes and shapes, depending upon the characteristics of the reactor in which it is to be used,jts porosity being appropriately regulated.

The alkali metal may be introduced by impregnating an already formed zinc-chromium catalyst with an aqueous solution of an alkali metal compound such as a hydroxide, carbonate, acetate,formate or other organic salt thereof. As an alternative, the catalyst can be prepared by reacting zinc oxide with mixtures of bichromates of ammonium and of alkali metals in such amounts that the final catalyst contains the desired quantity of alkali metal oxide(s).

Care should be taken in the reduction of the catalyst, which is carried out before or after the introduction of the alkali metals, by diluting the reducing gas, which is preferably hydrogen, with an inert gas such as nitrogen, and controliing the temperature of the catalytic bed so that it does not exceed 350"C.

A preferred mixture produced according to the invention comprises from 35 to 75% by weight of methanol, from 2 to 5% by weight of ethanol, from 3 to 12% by weight of n-propanol, from 10 to 30% by weight of isobutanol and from 5 to 25% by weight of alcohol(s) having five or more carbon atoms, the percentages being based on the weight of the mixture excluding the weight of any water therein.

All percentages are calculated on an anhydrous basis, that is desregarding any water which is present due to the fact that it has been produced in the reaction as a by-product.

It should be observed, in connection with the concentration ranges given above, that the lower values of the amount of higher alcohols, and correspondingly the higher values of the content of methanol, are obtained with the use of higher pressures of the pressure range used according to the invention. Consequently, the higher values of the content of the higher alcohols and the lower values of the content of methanol are obtained with the use of lower pressures in the range.

The invention will now be illustrated by the following Example.

EXAMPLE 242.5 g of chromium trioxide were dissolved in distilled water so as to obtain a solution having a concentration of 30% by weight. An aqueous slurry of 736 g of zinc oxide in two litres of distilled water was separately prepared and was kept under vigorous stirring. The solution of chromium trioxide was added, with stirring, to the zinc oxide slurry and stirring was continued for many hours so as to achieve complete homogenization. The basic zinc chromate obtained was collected on a filter, the resulting dry powder was admixed with zinc stearate binder and the mixture was pelletized. The pellets, having a diameter of 6 mm, were soaked by an aqueous solution of potassium acetate in such an amount that, in the finished and reduced catalyst, the content of K20 was about 2.5% by weight.

After drying to remove the water of impregnation, the catalyst was reduced. Thus, 100 cm3 of pellets were introduced into a stainless steel tubular reactor immersed in a bath of fluidized sand and heated to 3000C in a nitrogen stream containing about 2% of hydrogen, care being taken to ensure that the temperature during the reduction never exceeded 3500C. The reduction required a comparatively long time, about 24 hours. Once the catalyst had been reduced, it was not exposed to air. Chemical analysis of the reduced catalyst showed that it consisted of 77.3% by wt of ZnO,19.0% by wt of Cr2O3 and 2.4% by wt of K20. Its loss on firing at 4000C was 1.3% by wt. Its specific surface area was 1 25 m 2g.

The catalyst obtained was used for the preparation of mixtures of methanol and higher alcohols. In the reactor described above, containing 100 cm3 of the catalyst, a synthesis gas was introduced, which gas had the following composition, on a molar basis: H2 69.0% CO 30.5% CO2 0.1% CH4 0.1% N2 0.3% The temperature of the catalytic bed was maintained between 3900C and 4200C. Four tests are carried out, under pressures of 5000 KPa, 7000 KPa, 9000 KPa and 13000 KPa, respectively. In each test the liquid reaction product was separated from the gas by cooling and condensation. Analyses of average samples collected after a 24 hour test were carried out gas chromatographically, on the anhydrous products, and gave the results tabulated in the following Table.

TABLE

Pressure (KPa) 5000 7000 9000 13000 Temperature ("C) 409-415 410-422 400-415 390-410 Gas space hourly velocity (h-') 5500 11700 14400 14400 Methanol (% by wt.) 43.0 46.8 57.8 68.8 Ethanol (% by wt.) 3.7 3.9 3.4 2.9 n-Propanol (% by wt.) 9.1 9.7 8.2 6.5 Isobutanol (% by wt.) 23.2 22.4 17.4 12.1 Alcohols having more than 5 C atoms (% by wt.) 21.0 17.7 13.8 9.7 Total content of C2+ alcohols (% by wt.) 57.0 53.7 42.8 31.2

Claims (14)

1. A process for preparing a methanol/higher alcohol mixture, which comprises reacting hydrogen and carbon monoxide in a H2:CO molar ratio of from 0.1:1 to 20:1 at a temperature of from 300 to 5000C and at a pressure of from 2000 to 16000 KPa, in the presence of a catalyst comprising chromium, zinc and at least one alkali metal.

2. A process according to claim 1, wherein the H2:CO molar ratio is from 0.5:1 to 5:1.

3. A process according to claim 1 or 2, wherein the temperature is from 350 to 45000.

4. A process according to any of claims 1 to 3, wherein the pressure is from 5000 to 13000 KPa.

5. A process according to any of claims 1 to 4, wherein the reaction takes place in the presence of carbon dioxide and/or inert gas.

6. A process according to any of claims wherein the chromium, zinc and alkali metal(s) of the catalyst are present, at least in part, as oxides thereof.

7. A process according to claim 6, at least one of the oxides of the catalyst having been obtained by reduction of a respective compound of chromium, zinc or alkali metal.

8. A process according to claim 7, the chromium oxide of the catalyst having been obtained by reduction of a chromate, zinc oxide or alkali metal oxide being simultaneous obtained in the event that the chromate is zinc chromate or alkali metal chromate, respectively.

9. A process according to any of claims 1 to 8, wherein the Zn:Cr weight ratio of the catalyst, calculated as the oxides thereof, is from 5:1 to 1:1.

10. A process according to any of claims 1 to 9, wherein the alkali metal content of the catalyst, calculated as oxide thereof, is from 0.5 to 5% by weight based on the total weight of the elements calculated as oxides.

11. A process according to any of claims 1 to 10, wherein the alkali metal is potassium.

12. A methanol/higher alcohol mixture prepared by a process according to any of claims 1 to 11.

13. A mixture according to claim 12 having a methanol content of from 35 to 75% by weight, based on the weight of the mixture excluding the weight of any water therein.

14. A mixture according to claim 13, comprising from 35 to 75% by weight of methanol, from 2 to 5% by weight of ethanol, from 3 to 12% by weight of n-propanol, from 10 to 30% by weight of isobutanol and from 5 to 25% by weight of alcohol(s) having five or more carbon atoms, the percentages being based on the weight of the mixture excluding the weight of any water therein.

1 5. A mixture according to claim 12, substantially as describe in the foregoing Example.