Catalytic synthesis of dimethyl sulfide from dimethyl disulfide and methanol

Abstract

The reaction of dimethyl disulfi de with methanol was studied at atmospheric pressure and temperature of 350°C in the presence of catalysts containing acid and basic sites.Oxidation processes are extensively used today for removing mercaptans from hydrocarbon mixtures at oil-and gas-processing plants. These processes give a mixture of lower dialkyl disulfi des dominated by dimethyl disulfi de (DMDS) [1] suitable for synthesis of a number of valuable thio compounds. In this context, of particular interest is preparation of dimethyl sulfi de (DMS), an important organosulfur compound which is used for preparation of dimethyl sulfoxide and also serves as extractant and gas odorant.Solid-catalyzed conversion of DMDS in an inert medium at atmospheric pressure and T = 190-350°C leads to DMS and also to large quantities of byproducts [2][3][4]. Previously [2] we studied this reaction in the presence of acid-base catalysts and presumed that formation of DMS requires that the catalyst surface contain strong acid sites and medium-strength basic sites. They cause disruption of the S-S and C-S bonds in disulfi de, with the resulting CH 3 and CH 3 S groups reacting to form DMS. The appearance of the surface CH 3 groups originated from decomposition of DMDS is accompanied by formation of sulfur, hydrogen sulfi de, carbon disulfi de, C 1 -C 2 hydrocarbons, and resins. As a result, DMS is produced with a low selectivity of ≤50%. As found earlier [5,6], it is possible to suppress the side processes and increase the DMS yield by introducing into the reaction system of methanol able of more effi cient, compared to DMDS, methoxylation of the alumina surface.Here, we studied the reaction of DMDS with methanol in the presence of catalysts possessing different surface acid-base properties with a view to obtaining active catalysts for selective formation of DMS. EXPERIMENTALIn our experiments we used chemically pure-grade methanol and pure-grade DMDS. As support and catalyst served off-the-shelf γ-Al 2 O 3 material with S sp = 275 m 2 g -1 ; also we used off-the-shelf SiO 2 with S sp = 310 m 2 g -1 as support. As catalysts served zeolites NaX, HNaY (S sp = 800 m 2 g -1 ), HZSM-5 (Si/Al 17 and 35; S sp = 500 m 2 g -1 ), and HZSM-5 (Si/Al 17) with 20% binder, as well as massive Cr 2 O 3 .The supported catalysts were prepared by impregnation of precalcined supports with aqueous solutions of phosphoric, sulfuric, and tungstosilicic (HSiW) acids, as well as chromic acid anhydride, sodium hydroxide, ammonium tungstate, potassium tungstate, cobalt acetate, and ammonium heptamolybdate. Bimetallic catalyst NiMo/Al 2 O 3 was obtained by impregnating alumina with an aqueous solution containing a mixture of nickel nitrate and ammonium heptamolybdate. Catalyst CoHZSM-5 was prepared by the procedure described in [3].After impregnation, all the samples were maintained in air at room temperature for 12 h and dried at 110°C (5 h). The samples based on phosphoric and tungstophosphoric