(445g) Highly Selective Conversion of Methanol to Propylene: Design of a MFI Zeolite with Selective-Blockage of (010) Surfaces

Cai, D., Tsinghua University
Methanol-to-propylene (MTP) is one of the most important processes in modern chemical industry. ZSM-5 is widely used in MTP process, and its anisotropy of diffusion path lead to a varied product distribution. The diffusion of aromatics products results in a lower olefin selectivity. Herein, for the first time, we designed a surface-specific silica deposition method to selectively block straight channels. The surface-specific coated sample shows a significantly lower aromatic selectivity (3%) compared to non-specific coated counterpart (12%), and paraffin selectivity is also suppressed. This study provide new methodology for zeolite structure construction and will be greatly beneficial to industrial MTP process.

As one of a 10-membered ring zeolite, a larger pore opening of ZSM-5 can lead to a lower selectivity for ethylene and propylene, as well as a higher selectivity of aromatic species as compared to SAPO-34. According to the mass balance, as the formation of one aromatic molecule are accompanied by the formation of three alkane molecules, a high aromatics selectivity leads to a high of paraffin selectivity, resulting in a loss of olefin selectivity. Aromatics and paraffin, as by-products, are not only less-valuable but also influence the cycling of C4-C8 olefins.

The diffusion of aromatics products (benzene, toluene and para-xylene (PX)) out of MFI zeolite are not through sinusoidal channel, but through the straight channels. So, blocking straight channels without affecting sinusoidal ones is speculated to prevent the generation of aromatics products, thereby improving the performance of the MTO reaction. In previous literature, much work has been done on surface modification of ZSM-5 to improve the product selectivity in relevant methanol conversion process. For example, in methanol-to-aromatics (MTA) reaction, by removing the surface acid sites, isomerization reaction of xylene can be effectively suppressed. However, surface-selective modification can barely be achieved, facing difficulties in orientation. Recently, we found that during the MTA reaction, the (100) surfaces of spent Zn/ZSM-5 was covered with a thick coke layer, which blocked the sinusoidal channels of ZSM-5 and prevented the diffusion of paraffin and olefin out of zeolite channels, while the coke on (010) surfaces was much smaller, leading to ultrahigh aromatic selectivity (>98%).

Here, based on this interesting phenomenon, we designed a silica deposition process, which can block straight pores without affecting sinusoidal channels on ZSM-5 crystals. By trapping carbon catalysis species inside the zeolite channels, the selectivity of both aromatics and paraffin were significantly suppressed, and the olefin selectivity would be greatly benefited.