(113b) Zeolite and Metal Oxide Catalysts for the Production of Dimethyl Sulfide and Methanethiol

Methanol and hydrogen sulfide react in the presence of a metal oxide or zeolite catalyst at 613-673 K to produce dimethyl sulfide (DMS) along with methanethiol (MT), dimethyl ether (DME), and hydrocarbons. Desired products are DMS and/or MT. Other reactions to produce sulfides follow a similar pattern.

Aside from activity/cost ratio, minimal formation of methane and other hydrocarbons and resistance to deactivation at optimal H₂S/methanol ratios are necessary catalyst characteristics. Mashkina et al. suggested that the most active and selective metal oxide catalysts have both strong Lewis acid sites and moderately basic sites, with g-Al₂O₃ being suitable [1]. Both H₂S and methanol dissociatively adsorb in the presence of both of these sites while methanol can be weakly activated by the Bronsted acid sites. They also found that supported oxides of group VI metals were active and selective for DMS production. Mashkin et al. proposed a serial mechanism in which MT is formed initially and then reacts with methanol to produce DMS [2]. Concurrently, DME is also formed which can react with MT or H₂S to produce S-containing products. Mashkina et al. studied the reaction on zeolites and found that H-MFI was very active for DMS production at low conversion [3]. They proposed that MT and DMS are produced by parallel paths. Ziolek et al. found that the more acidic zeolites were more active and selective, but also deactivated faster [4].

We have examined several mixed metal oxides and zeolites for the condensation of methanol and H₂S, including deactivation behavior. Methanol/ 12% H₂S/N₂ feeds were metered to fixed bed reactors at atmospheric pressure containing 1-3 g catalyst; analysis was by GC-MS. SAPO-18 and AlPO-18 were synthesized using the procedure of Chen et al. [5]; other catalysts were either commercial products or synthesized by wet impregnation of commercial supports. The range of space velocities (methanol basis) was 0.25 ? 1.60 h^-1, while the optimal molar ratios of methanol to H₂S were found to be 1.5?2.5 for DMS production and 0.3?0.8 for MT production. Acid sites were characterized by the thermal desorption (TPR) of propanamine, using the method of Kanazirev et al [6]. The relative strength of sites could be determined by the temperature at which propanamine and its Hoffmann elimination products desorb.

The catalysts investigated fell into three groups based on activity and selectivity. The most active group consisted of, in order of activity to sulfur products, WO₃/ZrO₂ > La₂O₃/Al₂O₃ > g-Al₂O₃ > MFI. WO₃/ZrO₂ showed considerable activity to hydrocarbons at temperatures above 613 K. For all of these catalysts, it was found that increasing temperature and feed ratio increased selectivity of DMS over MT, while increasing space velocity did the opposite. Except for MFI, the catalysts in this group converted most of the methanol to DME at short residence times, and this DME then further to form sulfur products as contact time increased. The second group consisted of, in order of activity, WO₃/Al₂O₃ > MoO₃/SiO₂ > SAPO-18 > AlPO-18; this group was more selective to MT. In particular, WO₃/Al₂O₃ showed moderate activity, with selectivity to MT over DMS greater than 80%. SAPO-18 and AlPO-18 were more selective to DME; MoO₃/SiO₂ was selective to carbonyl sulfide at higher temperatures, while SAPO-18 was active to C3 - C4 olefins after about an hour on stream. Other catalysts such as TiO₂/SiO₂ were mostly inactive.

La₂O₃/Al₂O₃, g-Al₂O₃, MFI and WO₃/Al₂O₃ showed negligible deactivation after 60 h on stream. However, WO₃/ZrO₂ gave ~10% reduction in DMS yield in 40 h on stream. It was partly regenerated by heating in air at 673 K.

The catalysts in the first group were characterized by higher concentrations of acid sites in the propanamine TPR range 573-673 K. The concentration of sites desorbing between 573-623 K correlated well with overall activity to sulfur products. Catalysts with high selectivity to DMS over MT had more sites desorbing in the next higher (623-673 K) range of temperatures. There was no correlation between activity to sulfur products and sites desorbing at even higher (>673 K) temperatures.

References [1] Mashkina, A.V., Paukstis, E.A., Yakovleva, V.N. Kinet. Katal. 29, 596 (1988). [2] Mashkin, V.Y., Kudenkov, V.M., Mashkina, A.V. Ind. Eng. Chem. Res. 34, 2964 (1995). [3] Mashkina, A.V., Yakovleva, V.N. Kinet. Katal. 32, 636 (1991). [4] Ziolek, M., Czyzniewska, J., Kujawa, J., Travert, A., Mauge, F., Lavalley, J.C. Microporous and Mesoporous Materials 23, 45 (1998). [5] Chen, J., Wright, P.A., Thomas, J.M., Natarajan, S., Marchese, L., Bradley, S.A., Sankar, G., Catlow, C.R.A. J. Phys. Chem. 98, 10216 (1994). [6] Kanazirev, V., Dooley, K., Price, G.L. Journal of Catalysis, 146, 228 (1994).