GB2148881A

GB2148881A - Preparation of dialkyl carbonates

Info

Publication number: GB2148881A
Application number: GB08328906A
Authority: GB
Inventors: Suzanne Fiona Davison
Original assignee: BP Chemicals Ltd
Current assignee: BP Chemicals Ltd
Priority date: 1983-10-28
Filing date: 1983-10-28
Publication date: 1985-06-05
Also published as: GB8328906D0; GB2148881B

Abstract

Dialkyl carbonates, e.g. dimethyl carbonate are prepared by oxidative carbonylation of the appropriate alcohol using a Group VIII metal catalyst together with a heteropoly acid as a reversible oxidant, the carbonylation being conducted in the presence of oxygen, or the reversible oxidant being contacted with oxygen in a step separate from the carbonylation step. Preferred heteropoly acids are those based on molybdenium, tungsten vanadium.

Description

SPECIFICATION Preparation of dialkyl carbonates The invention relates to a process for the oxidative carbonylation of alcohols to form dialkyl carbonates.

The reaction of alcohols with oxygen and carbon monoxide to form dialkyl carbonates is known and a variety of different catalyst systems have been previously described, for example, palladium and platinum chlorides and bromides with which the corresponding divalent copper and trivalent iron salts are employed as a reversible oxidant, sometimes referred to as a redox system. It has now been found that heteropolyacids can be employed as reversible oxidants with metals of Group VIII of the Periodic Table for the above described reaction.

Thus according to the present invention a process for the oxidative carbonylation of alcohols to dialkyl carbonates comprises reacting an alcohol with carbon monoxide in the presence of a catalyst system comprising a Group VIII metal and, as a reversible oxidant, a heteropolyacid, the reaction either being effected in the presence of free oxygen or the reversible oxidant being contacted with free oxygen in a separate step.

The term heteropolyacid is known in the art and is described in UK Patent No 1,025,679. The term refers to a large class of salts and free acids containing complex anions the structure of which are based on MO6 octahedra. The MO6 octahedra can be joined by sharing corners, edges or faces to form a co-ordination sphere around a central heteroatom. The central atom can either be tetrahedral, octahedral or icosahedral. While up to as many as forty elements have been claimed as central heteroatoms, only niobium, vanadium, molybdenum and tungsten are known to form the outer MO6 octahedra, although some other transition elements can be substituted into the structures in a few cases. The range of heteropolymolybdate and heteropolytungstate compounds is much greater than that for the other two elements, probably as a result of relative stabilities.

In the present invention particularly suitable heteropolyacids are those of formulae Xt+M,204o'8-n)- and X2n+M,8062':6-n'- where M is W, Mo or V or mixture thereof and X is P, As, Si, Ge, B, Al, Fe, Co, Cu, Zn, Cr, Mn, Te, Ga, Ti or Zr.

These can be preprepared by methods described by G.A. Tsigdinos in Top. Curr. Chem. 1978. 76, 1 and the references cited therein, or made in situ from readily available starting materials as described in British Patent No 1,508,331.

Of the Group VIII metals, platinum group metals e.g. divalent palladium and platinum are preferred.

The catalyst and heteropolyacid are conveniently employed in solution, the concentrations of which can vary widely, for example, the amount of Group Vlil metal can be from 10-6 to 0.5 g atom/litre and the heteropolyacid from 10-4 to 1 molar.

It can be beneficial to remove water (which is produced in the process) by the use of a drying agent such as an orthoformate, ketal or molecular sieve, since water can cause reduction of the divalent palladium to a lower oxidation state.

The drying can be effected as a separate step.

Conveniently the temperature is in the range 15 to 1509C preferably 20 to 100 C and pressures of carbon monoxide can be from atmospheric upwards.

The reaction of the alcohol with the carbon monoxide can be carried out in the presence of free oxygen or the catalyst can be reoxidized with oxygen in a separate step.

Preferred alcohols which can be employed as a starting material are C, to C6 monohydric alcohols such as methanol and ethanol, although dihydric alcohols such as ethylene glycol can also be employed.

The invention is illustrated by the following Example.

Example - Carbonylation of methanol to dimethyl carbonate 0.138 g K2Pd(NO2)4 and 4.165 g H,Mo16V2O4620H20 in 20 mls methanol was stirred under a partial pressure of CO of 100 psi at 25"C. The K2Pd(NO2)4 was obtained from Johnson Matthey Ltd.

The H5PMo1OV204020H20 was prepared as described by G.A. Tsigdinos and C.J. Hallada: Inorg Chem 1968 7 p437. After 1 hour gas liquid chromatography (g.l.c.) analysis showed the formation of 0.16 g of dimethyl carbonate. Absorption of carbon monoxide ceased indicating that the catalyst was inactive.

The solution was then stirred at 50"C under a partial pressure of air of 150 psi for 1h, followed by treatment with CO at 100 psi and 25"C for a further 1h. Analysis by g.l.c. showed the formation of 0.2119 dimethyl carbonate.

This examble illustrates that the above catalyst system is active for the carbonylation of methanol to methyl carbonate and that on contacting with oxygen the catalyst system is reactivated in which state it will convert more methanol to dimethyl carbonate.

Comparative Example Absence of heteropolyacid 0.138 g K2Pd (NO2)4 in 20 mls methanol was stirred under a partial pressure of CO of 100 psi at 25"C. After 1h gas liquid chromatography analysis (glc) showed the formation of 2.24 x 10-3 g of dimethyl carbonate.

The solution was the stirred at 50"C under a partial pressure of air of 150 psi for a further 1h, followed by treatment with CO at 100 psi and 25"C for a further 1h. Analysis by gic showed that no additional dimethyl carbonate had been formed.

This comparative example shows that, without the heteropolyacid, the palladium is only able to give a yield of dimethyl carbonate equivalent to a turnover of 0.2 on palladium. Treatment with O2 does not reactivate the catalyst if no heteropolyacid is present.

Claims

CLAIMS:

1. A process for the oxidative carbonylation of alcohols to dialkylcarbonates which process comprises reacting an alcohol with carbon monoxide in the presence of a catalyst system comprising a Group VIII metal and, as a reversible oxidant, a heteropolyacid, the reaction either being effected in the presence of free oxygen or the reversible oxidant being contacted with free oxygen in a separate step.

2. A process as claimed in claim wherein the heteropolyacid is of the series XnsM12 40 (Bn)- or X2n+Ml5062'6-n where M is tungsten, molybdemn or vanadium or mixtures thereof and X is P, As, Si, Ge, B, Fe, Al, Co, Cu, Zn, Cr, Mn, Te, Ga, Ti or Zr.

3. A process as claimed in either claim 1 or 2 wherein the catalyst and the heteropolyacid are employed in solution.

4. A process as claimed in any one of the preceding claims where the temperature is from 15 to 150"C and the presssure of carbon monoxide is at least atmospheric.