Small pore SAPO-14-based zeolites with improved propylene selectivity in the methanol to olefins process

Abstract

The methanol to olefins (MTO) process is an important non-petroleum route to prepare light olefins, however, it still remains a challenge to improve the one-pass propylene selectivity over the industrial SAPO-34 catalyst. The acidity (Si content) and pore structure (pore size and cage) of the catalyst are the vital factors for product selectivity. In this work, SAPO-14 zeolite (AFN zeotype) with a small-pore and ultra-small cage structure, as well as a high Si content (about 9%) has been successfully synthesized. It presents a short catalytic lifetime of 26 min, but high propylene selectivity with a P/E ratio up to 4.17 at 450 °C and WHSV = 0.5 h⁻¹. Furthermore, for the first time, SAPO-34/SAPO-14 composite zeolites are prepared using traditional SAPO-34 zeolites as seeds and varying the seed amount in the reaction system of SAPO-14. Such zeolites exhibit improved MTO performance, and the catalytic lifetime and selectivity for light olefins can reach 226 min and 82% at 400 °C and WHSV = 2 h⁻¹, and the P/E ratio reaches 2.7 or 1.3. This work demonstrates a feasible method for the design and synthesis of composite zeolite catalysts for controlling the product selectivity for MTO conversion.