CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 U.S.C. § 371 of International Application No. PCT/CN2018/102072, filed Aug. 24, 2018, which claims priority to International Application No. PCT/CN2017/098861, filed Aug. 24, 2017, the disclosures of which are explicitly incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates in general to a dehydration process and in particular to a process for the dehydration of methanol in the presence of a solid Brønsted acid catalyst and a promoter compound.

BACKGROUND OF THE INVENTION

Industrial processes for the dehydration of methanol to dimethyl ether using catalysts such as alumina are known. Such processes employing alumina catalysts are described, for example in EP-A-1396483. Although these alumina catalysts are solid acid catalysts they are not Brønsted acid catalysts.

Processes for the dehydration of alcohols such as methanol employing zeolite catalysts in dehydration reactions of methanol are described, for example in WO 2004/074228.

WO 2004/074228 describes a process for preparing dimethyl ether in high yield by employing a dual-catalyst system. Methanol is initially dehydrated over a hydrophilic solid acid catalyst such as gamma-alumina; unreacted methanol is then dehydrated over a second solid acid catalyst, a hydrophobic zeolite such as ZSM-5.

EP-A-1396483 and WO 2004/074228 exemplify the use of high reaction temperatures, typically 250° C. and higher. Whilst the use of such high reaction temperatures may be desirable to achieve acceptable reaction rates, a disadvantage is that at temperatures, typically in excess of 250° C., hydrocarbons are co-produced with the dimethyl ether product and this typically leads to a reduction in catalytic performance.

WO 2011/027105 describes a process for the simultaneous dehydration of methanol and hydrolysis of methyl acetate. The process can be conducted at reaction temperatures below 250° C. by employing a zeolite having a 2-dimensional framework structure comprising at least one channel having a 10-membered ring. Examples of such zeolites are zeolites of framework type FER typified by ferrierite and HEU typified by clinoptilolite.

Processes for the co-production of acetic acid and dimethyl ether by the dehydration of methanol and hydrolysis of methyl acetate in the presence of zeolites having a 2-dimensional framework structure are also described, for example in WO 2013/124404 and WO 2013/124423.

Processes in which methanol-containing streams are dehydrated over various types of solid acid catalyst such as heteropolyacids, gamma-aluminas or zeolites are described, for example in WO 2015/193186 and WO 2015/193188.

SUMMARY OF THE INVENTION

Applicant has now found that compounds which are (i) ketones of Formula I R1COR2wherein R1and R2are identical or different and are each a C1-C11alkyl group, and furthermore R1and R2together with the carbonyl carbon atom to which they are bonded may form a cyclic ketone or (ii) ketal derivatives of ketones of Formula I, have a beneficial effect on the rate of dehydration of methanol reactions carried out in the presence of solid Brønsted acid catalysts which are aluminosilicate zeolites or heteropolyacids.

Accordingly, the present invention provides a process for dehydrating methanol to dimethyl ether product in the presence of a catalyst and a promoter, wherein the catalyst is at least one solid Brønsted acid catalyst selected from aluminosilicate zeolites or heteropolyacids, and the promoter is at least one

(i) ketone of formula R1COR2(Formula I)

wherein R1and R2are identical or different and are each a C1-C11alkyl group and furthermore R1and R2together with the carbonyl carbon atom to which they are bonded may form a cyclic ketone; or

(ii) ketal derivative of a ketone of Formula I; and

wherein the molar ratio of promoter to methanol is maintained at 0.5 or less.

Advantageously, the promoters of the present invention allow productivity to dimethyl ether product to be improved in methanol dehydration reactions which are carried out in the presence of solid Brønsted acid catalysts which are aluminosilicate zeolites or heteropolyacids.

Also, according to the present invention there is provided a method of improving the productivity to dimethyl ether product in a process for dehydrating methanol in the presence of a catalyst and a promoter, wherein the catalyst is at least one solid Brønsted acid catalyst selected from aluminosilicate zeolites or heteropolyacids, and the promoter is at least one

(i) ketone of formula R1COR2(Formula I)

wherein R1and R2are identical or different and are each a C1-C11alkyl group and furthermore R1and R2together with the carbonyl carbon atom to which they are bonded may form a cyclic ketone; or

(ii) ketal derivative of a ketone of Formula I; and

wherein the molar ratio of promoter to methanol is maintained at 0.5 or less.

Yet further according to the present invention there is provided the use of a promoter in a process for the catalytic dehydration of methanol to dimethyl ether to improve productivity to dimethyl ether product wherein the catalyst is at least one solid Brønsted acid catalyst selected from aluminosilicate zeolites or heteropolyacids, and the promoter is at least one

(i) ketone of formula R1COR2(Formula I)

wherein R1and R2are identical or different and are each a C1-C11alkyl group and furthermore R1and R2together with the carbonyl carbon atom to which they are bonded may form a cyclic ketone; or

(ii) ketal derivative of a ketone of Formula I; and

wherein the molar ratio of promoter to methanol is maintained at 0.5 or less.

More advantageously, use of the ketones of Formula I in which at least one of R1and R2is a branched alkyl chain group may also mitigate deactivation of a zeolite catalyst thereby improving stability of a zeolite catalyst.

A further aspect of the present invention provides a process for dehydrating methanol to dimethyl ether product in the presence of a catalyst, wherein the catalyst is at least one solid Brønsted acid catalyst selected from aluminosilicate zeolites or heteropolyacids, and wherein prior to using the catalyst in the dehydration process, the catalyst has been impregnated with a promoter, wherein the promoter is at least one

(i) ketone of formula R1COR2 (Formula I)

wherein R1 and R2 are identical or different and are each a C1-C7 C11 alkyl group and furthermore R1 and R2 together with the carbonyl carbon atom to which they are bonded may form a cyclic ketone; or

(ii) ketal derivative of a ketone of Formula I.

