High Propylene Selectivity in Methanol-to-olefin Reaction over H-ZSM-5 Catalyst Treated with Phosphoric Acid

Abstract

H-ZSM-5 zeolite was treated with phosphorus acid by impregnating H-ZSM-5 with aqueous solutions of phosphoric acid at various concentrations. H-ZSM-5 (P-HZSM-5) modified with phosphoric acid was used as a catalyst for the methanol-to-olefin reaction. The molar ratios of P/Si and Si/Al in H-ZSM-5 and P-HZSM-5 were measured by EDX analysis. The Si/Al molar ratios of P-HZSM-5 increased with higher concentration of H₃PO₄ in the solution, which might be caused by partial dealumination of H-ZSM-5 by the H₃PO₄ treatment. The P/Si molar ratio of P-HZSM-5 after washing was proportional to the H₃PO₄ concentrations in the aqueous solutions. The remaining phosphorus species after the washing must be strongly adsorbed by interaction with the pore surface of H-ZSM-5 zeolite. The P-HZSM-5 catalyst showed very high propylene selectivity up to 57% with methanol conversion of 100%. Furthermore, catalyst stability was significantly improved for the P-HZSM-5 catalysts. Ammonia TPD spectra showed that the strong acid sites of H-ZSM-5 disappeared after the phosphoric acid treatment. Consequently, the formation of aromatics and coke was inhibited, resulting in higher light olefin selectivity and catalyst stability.