Piao Liu,Jianxin Cao,Zheng Xu,Chunliang Yang,Xiaodan Wang,Fei Liu
Core-shell structured ZSM-5@t-ZrO 2 catalysts were prepared by a hydrothermal coating method, and their physicochemical properties, catalytic performance, and deactivation behavior in thiolation reactions were studied using a ZSM-5/t-ZrO 2 physically-blended catalyst as a comparison. Reactions using the core-shell ZSM-5@t-ZrO 2 catalyst were carried out at a pressure of 0.7 MPa, 370 °C, a H 2 S:CH 3 OH molar ratio of 2:1, and a N 2 flow rate of 90 ml·min −1 . The conversion of CH 3 OH, and the selectivity and yield of CH 3 SH under these conditions were 92.07%, 90.89%, and 83.68%, respectively, and the catalytic lifetime was longer than 30 h. Compared with the ZSM-5/t-ZrO 2 physically-blended catalyst, the CH 3 SH yield and the catalytic lifetime of the core-shell structured ZSM-5@t-ZrO 2 catalyst were higher by 5.37% and 150%, respectively. The unique reaction pathway of the shell (t-ZrO 2 , Lewis (L) base active site)-core (ZSM-5, Brǿnsted (B) acid active site) and the Lewis-Brønsted acid active center based on a composite solid-solution interface were mainly responsible for the high activity of the thiolation reaction. Meanwhile, the formation of a micro-mesoporous system increased the diffusion of molecules and reduced the deactivation rate of carbon- and sulfur-containing deposits. Generally, carbon- and sulfur-containing substances deposited on catalysts were mainly composed of polymethylbenzene, polymethyl naphthalene, cyclic elemental sulfur, and other compounds containing C S and S S bonds. Finally, the core-shell structured catalysts showed excellent resistance to deactivation by carbon- and sulfur-containing deposits.
The catalytic properties of bifunctional ZSM-5@t-ZrO 2 catalyst for methanol thiolation were revealed by using methanol and H 2 S as probe molecules. Taking the ZSM-5/t-ZrO 2 physically-blended catalyst, pure ZSM-5 and t-ZrO 2 as control catalysts, we investigated the characteristics of adsorption and transformation of different reaction molecules on or over different catalysts by combination of in-situ diffuse reflectance Fourier transform infrared spectroscopy (Drifts) and a variety of techniques including XRD, XPS, XRF, FT-IR, N 2 adsorption-desorption, CO 2 /NH 3 -TPD, and DSC. The results showed that the synergistic effect of acid sites between ZSM-5 phase and t-ZrO 2 phase in the bifunctional catalyst enhanced the adsorption and dissociation of methanol molecules, while the base sites in t-ZrO 2 phase were mainly responsible for the adsorption and dissociation of H 2 S molecules. Due to the small specific surface area of pure t-ZrO 2 catalyst, precursor species for sulfur deposition and carbon deposition were easily formed. Presulfurization could improve the initial activity of methanol thiolation and shorten the induction period. The dissociation of H 2 S at the base sites was the rate-determining step of the bimolecular reaction, and the appropriate increase of base sites in the bifunctional components could construct the matching formation rate of sulfhydryl and methoxy groups. At the same time, the special composite structure and meso-microporous system of ZSM-5@t-ZrO 2 could effectively reduce the formation rate of carbon and sulfur deposits.
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