GB2099327A - Catalyst for production of dimethyl ether - Google Patents

Abstract

A process for producing dimethyl ether (DME) from CO and H2, and possibly also CO2, is disclosed in which the gaseous reactants are reacted in the presence of a catalytic composition constituted overall by a mixture of: (a) crystalline silica in which some silicon atoms in the crystal lattice have been replaced by aluminium, and which corresponds to the general formula: Si1Al (0.0012 &cirf& 0.0052)Oy where y varies between 2.0018 and 2.0075; and (b) a mixture of Cu/Zn/Cr and/or Cu/Zn/Al oxides and/or salts; the Al can also be present together with the Cr. Good yields of DME can be achieved at temperatures lower than those normally associated with the dehydration of methyl alcohol to produce DME.

Description

SPECIFICATION Process for producing dimethyl ether This invention relates to a processforthe production of dimethyl ether, which compound is often referred to herein by the abbraviation DME.

Dimethyl ether is conventionally prepared in two stages, first by synthesising methanol from CO and H2 and possibly also CO2, and then by dehydrating the resulting methanol to give DME.

Some processes are, however, known in which the DME synthesis is carried out in a single stage, by coupling a methanol synthesis to a dehydration catalyst, which is generally alumina.

In the known processes, the methanol synthesis catalyst generally is a composition constituted by Cu, Zn and Cr or by Cu, Zn and Al, whereas the dehydration catalyst is generally alumina.

One embodiment described in German Patent Application No.2757778 (corresponding to United States Patent No.4177167) involves a DME synthesis catalyst constituted by metal oxides and/or metal salts which have been stabilised by treatment with a silicon compound, the stabilisation making the catalytic material obtained after the treatment capable of resisting thermal and mechanical forces and the action of steam at high temperature. The metal oxides and/or metal salts used according to that German patent application are generally Al, Cr, La, Mn, Cu orZn oxides and/salts or their mixtures.

A problem associated with the catalysts of the known art, and thus with the catalyst of that German patent application, is related to the fact that the dehydrating agent used exhibits its activity to a significant extent only at relatively high temperatures at which copper-based methanol synthesis catalysts (which are able to operate at lowtemperatures and low pressure) are unstable, thereby giving rise to sintering phenomena and thus to a loss of activity with time.

The optimum temperature for the dehydration in accordance with the prior art is of the order of 280-300"C, whereas copper-based catalysts have a long life only if used at a temperature of from 200 to 260"C.

According to the present invention, there is provided a process for producing dimethyl ether from CO and H2 and optionally also CO2, as gaseous reactants, wherein the dimethyl ether is formed by reacting the gaseous reactants in the presence of a composition constituted by a mixture of: (a) crystalline silica in which some silicon atoms of the crystal lattice have been replaced by aluminium, and which satisfies the general formula Sit Al(o.c012-o.ooSo)Oy where y is in the range from 2.0018 to 2.0075; and (b) a mixture of oxides and/or salts of Cu, Zn and Cr or Cu, Zn and Al, in which Al can also be present together with Cr.

It has now been surprisingly found possible to operate at a much lower average temperature while at the same time obtaining good DME conversion yields by using a catalytic composition formed by intimately mixing together powdered crystalline siliceous material substituted with aluminium, and powdered oxides or salts of Cu, Zn and Cr (or Al).

In the process according to the present invention CO and H2, and possibly CO2, are fed to a reaction zone filled with the above-mentioned catalytic composition.

The crystalline silica used in the composition employed in the process of the present invention, and its preparation, may be as described in German Patent Application No. 2924870 to which reference should be made.

Preferably 40 to 70% of all the Si, Al, Cu, Zn and Cr atoms present in the catalystic composition (i.e. (a) and (b)) are silicon atoms.

Preferably, the percentage of copper atoms in the total of Si, Al, Cu, Zn and Cr atoms (a+ b) varies from 15 to 30%, the zinc from 8 to 15%, the chromium from 0 to 10% (the value 0 only being obtained when aluminium is present), and the aluminium from 0.1 to 10%.

The DME production process according to the present invention is convenientiy carried out at a temperature in the range from 150 to 250 C and at a pressure in the range from 4000 to 15000 kiloPascals (kPa).

The present invention will be illustrated by the following non-limiting examples.

EXAMPLES 1-2 A catalyst was prepared based on Cu, Zn and Al, and silica modified with aluminium, according to the following procedure. 676 Grams of Cu(NO3)2.3H20, 327 g of Zn(NO3)2.6H20 and 57 g of sodium aluminate were dissolved in 10 1 of water. The resulting solution was heated to 85"C, and a 10% solution of NaOH in water was added under stirring until the pH reached 7.5. The precipitate was allowed to settle during cooling, the liquid was decanted, and the precipitate washed repeatedly with water until the sodium and nitrates disappeared, using decantation and finally filtration.

The washed precipitate was dried in an oven at 1200C in air. The material was ground to granules not larger than 20 mesh ASTM, and was mixed with 325 g of silica modified with aluminium prepared as described in Example 5 of German Patent Application No.2,924,870. The resulting powder was compressed into pellets having a diameter of 4 mm and a length of 6 mm.

The Cu, Zn, Al and Si were present in the catalyst pellets in the atomic ration of 28:11 :7:54.

Then 100 cm3 of catalyst were placed in atubular reactor with an internal diameter of 2.54 cm.

A sheath with an external diameter of 8 millimetres to house a thermocouple was located axially at the centre of the reactor.

The temperature was gradually increased while feeding a mixture of H2 and N2 to the reactor in order to reduce the catalyst under controlled conditions.

When the temperature had reached 220"C and catalyst reduction was complete, the pressure was reduced to 7000 kPa, and the mixture of H2 and N2 was gradually replaced by a 1:1 mixture of CO and H2 at a gaseous hourly space velocity (GHSV) of 2100 h-1.

The catalyst temperature was stabilised at 200"C (Example 1) and 230"C (Example 2).

The water, methanol and part of the dimethyl ether produced in the reaction were condensed in a condenser iocated downstream of the reactor; and the condensed water, methanol and DME were withdrawn under pressure and analysed by gas chromatography.

The gas leaving the reactor was fed to a sample valve of a gas chromatograph and analysed, and then fed to an integrating flowmeter in order to measure the gas quantity.

The following Table 1 shows the results obtained undertheforegoing conditions. By-products present in a quantity of less than 1% were not considered.

TABLE 1 GHSV R T P Molar Selectivity of the h-' H2/CO "C kPa conversion CO converted into DME CH3OH CO2 Example 1 2100 1:1 200 7000 22 61.8 4.1 34.1 Example 2 2100 1:1 230 7000 69 63.4 3.0 33.6 EXAMPLES 3 - 4 A catalyst was prepared based on oxides of Cu, Zn, Cr, Si and Al, the components being present in the atomic ratios of20:12:8:60:0.2.

First 1600 g of Cu(NO3)2.3H2O, 1182 g of Zn(NO3)2.6H2O and 1060 g ofCr(NO3)3.9H2Owere dissolved in 20 1 of distilled water. The resulting solution was heated to 95"C, and 20 litres of an aqueous solution of 1300 g of NaOH were then added under stirring. The precipitate which was formed was cooled, washed with water by decanting, filtered and washed repeatedly with water. The washed precipitate was dried in an oven at 1200C. The material was ground to granules less than 20 mesh ASTM, then mixed with 1192 g of silica modified with aluminium (as in Example 5 of German Patent Application No.

2,924,870).

Pellets were formed having a diameterof4 mm and length 6 mm.

Then 100 cm3 of catalyst were examined under the same conditions as in Example 1-2, the results being given in the following Table 2.

TABLE 2 GHSV R T P Molar Selectivity of the h-' H2/CO C kPa conversion CO converted into DME CH3OH CO3 Examble 3 2100 1 200 7000 12 62.6 3.8 33.6 Example 4 2100 1 230 7000 45 64.7 1.8 33.5 EXAMPLES 5-6 (comparative) The catalyst of Example 1 of United States Patent No. 4,177,167, in which the atomic ratios of Cu :Zn :Cr:Al were 20:12:8:60, was examined under the same conditions as in Examples 1 and 2.

The results are given in the following Table 3.

TABLE 3 GHSV R T P Molar Selectivity of the h-1 H2/CO "C kPa conversion CO converted into DME CH3OH CO2 Example 5 2100 1 200 7000 11 11.9 54.0 34.1 Example 6 2100 1 230 7000 42 18.5 47.0 35.0 These comparison examples show that at low operating temperatures (200-230"C), the known catalyst based on stabilised aluminium does not exhibit sufficient dehydrating activity, so that the DME is present only in small quantities.

Claims (7)

1. A process for producing dimethyl ether from CO and H2 and optionally also CO2, as gaseous reactants, wherein the dimethyl ether is formed by reacting the gaseous reactants in the presence of a composition constituted by a mixture of: (a) crystalline silica in which some silicon atoms of the crystall lattice have been replaced by aluminium, and which satisfies the general formula Si,AIt0.00,2~0 QosoOy where y is in the range from 2.0018 to 2.0075; and (b) a mixture of oxides and/or salts of Cu, Zn and Cr or Cu, Zn and Al, in which Al can also be present together with Cr.

2. A process according to Claim 1, wherein the respective percentage ranges of atoms of the various elements in the overall composition, based on the total number of atoms of Si, Al, Cu, Zn and Cr, are as follows: 40 to 70% of the total atoms in the case of Si 15 to 30% of the total atoms in the case of Cu 8 to 15% of the total atoms in the case of Zn 0 to 10% ofthe total atoms in the case of Cr 0.1 to 10% of the total atoms in the case of Al.

3. A process according to Claim 1 or 2, wherein the reaction is carried out at a pressure in the range from 4000 to 15000 kPa.

4. A process according to Claim 1,2 or3, wherein the reaction is carried out at a temperature in the range from 150"C to 250"C.

5. A process according to Claim 1, substantiaily as described in any one of the foregoing Examples 1 to4.

6. Dimethyl ether whenever produced by a process according to any preceding Claim.

7. A catalytic composition as defined in Claim 1.