Synthesis and characterization of fibrous silica zsm-5 for methanol to olefin reaction

Abstract

Olefins are one of the most important chemicals and raw materials in the petrochemical industry. However, because of the rapid increase in the price of crude oil and the oil shortage in the foreseeable future, alternative routes for production of light olefins from non-oil sources are desired. Catalytic conversion of methanol to light olefins (MTO) provides an alternative route for production of light olefins from a non- petroleum source. Protonated commercial ZSM-5 (HZSM-5) zeolite has been widely used in the MTO reaction. However, fast deactivation of HZSM-5 due to coke deposition has always been one of the key problems in MTO reaction. A novel ZSM- 5 catalyst with silica fibrous morphology (HSi@ZSM-5) was successfully prepared using a microemulsion system with ZSM-5 seed assisted crystallization followed by protonation for MTO reaction. X-ray diffraction and field emission scanning electron microscopy analyses showed that the HSi@ZSM-5 possesses ZSM-5 structure and a spherical morphology with evenly distributed dendrimeric silica fibers. In addition, HSi@ZSM-5 exhibited intrinsic mesopores at 3-5 and 10-20 nm, which led to an increase in the surface area up to 22% compared with HZSM-5. Ammonia Fourier transform infrared spectroscopy result showed a remarkable reduction of Brønsted acid sites in HSi@ZSM-5. This reduction of Brønsted acid sites suppressed side reactions which led to increased olefin selectivity. These were proven in the catalytic activity as the propylene selectivity of HSi@ZSM-5 was almost two-fold higher than that of HZSM-5. Besides, the catalytic lifetime was improved significantly up to 80 hours for HSi@ZSM-5 compared to about 30 hours for HZSM-5. The high selectivity towards propylene and long catalyst lifetime of HSi@ZSM-5 could be attributed to the unique morphology of HSi@ZSM-5 which facilitates the diffusion of reactant and product into and out of the catalyst. Lowering diffusion limitation reduces the possibility of coke accumulation on the catalyst that lead to the deactivation of the catalyst. This new protonated silica fibrous ZSM-5 catalyst opens a big potential in general heterogeneous catalytic reaction.