Optimisation of the synthesis of spherical ZSM-5 for the dehydration of methanol to dimethyl ether

Abstract

In the presented work, spherical zeolite ZSM-5 was tested as a catalyst for the synthesis of a biofuel, dimethyl ether (DME) from methanol. Conventional zeolites with strong acid sites are very active in this reaction but are rapidly deactivated by coke deposits. To overcome this problem, spherical, hierarchical ZSM-5 was synthesised using mesoporous silica spheres (MSS) as a 'reactive' template, providing the source of silica templating the final morphology of the catalyst. A series of samples, subjected to hydrothermal treatment for 7–15 days, was prepared. At the beginning of the synthesis gel ageing, the MSS surface dissolute under basic conditions and, over time, crystallisation of ZSM-5 zeolite was observed on a spherical matrix. Prolonged crystallisation led to the complete ‘consumption’ of MSS and the formation of fully developed zeolite crystals. A 12-day hydrothermal treatment was found to be optimal to obtain a catalyst with very good characteristics of ZSM-5 zeolite (verified by X-ray diffraction and N₂-sorption) and a spherical grain shape (SEM microscopy). The activity of the samples was compared to the conventional commercially available ZSM-5 zeolite. The spherical shape of the zeolite grains was found to have a positive effect on catalytic activity. The spherical MSS@ZSM-5_12d sample was more stable in the long-term catalytic test compared to the commercial ZSM-5 (a methanol conversion of about 90 % remained unchanged for about 15 h). Furthermore, in conventional ZSM-5, a higher amount of coke deposit was detected after the stability test (8.9 %, vs 7.2 % for the spherical sample, TG analysis, FT-IR spectroscopy).