Simultaneously Improving the Selectivity and Stability of HZSM-5 Zeolite by NaOH Treatment in Aqueous Ethanol-to-Propylene Reactions

Mesoporous nanoscale HZSM-5 zeolites were prepared by alkali treatment and characterized by XRD, SEM, NMR, ICP-OES, N2 adsorption/desorption, NH3-TPD, Py-IR, and TG techniques. The effects of NaOH concentration on pore structure, acidity, and catalytic performance of nanoscale HZSM-5 zeolites were systematically investigated for selective conversion of aqueous ethanol to propylene. The results showed that the newly developed mesopores on HZSM-5 zeolite were enhanced with increasing NaOH concentrations. By treating nanoscale HZSM-5 zeolite using NaOH solutions with appropriate concentrations (0.2 mol/L), AZ-0.2 showed simultaneously higher propylene selectivity and better stability because of its larger mesopore volume, higher B/L ratios, and suitable acidity. However, excessive treatment by a high-concentration NaOH (0.4 mol/L) solution led to serious desilication, which remarkably increased the strength and amount of strong acid sites on AZ-0.4, resulting in a remarkable decrease in propylene selectivity and catalyst stability.