Surface-Passivated Hierarchically Structured ZSM5 Zeolites: High-Performance Shape-Selective Catalysts for para-Xylene Production

Hierarchically structured ZSM-5 zeolites (HSZ) were synthesized and used as high-performance catalysts for para-xylene (p-X) production by tuning their pore structures and external surface acidity, which was achieved by two independent passivation approaches: dealumination in oxalic acid solution and chemical liquid deposition of tetra-ethoxysilane (CLD of TEOS). The mesoporous structures of HSZ were well-preserved after dealumination or CLD of TEOS, and the external surfaces of HSZ were passivated significantly. Compared to dealumination, CLD is more efficient because not only the external surface could be passivated, but also the pore-openings of HSZ had been effectively narrowed, which both favored the enhancement of product p-X selectivity in ortho-xylene (o-X) isomerization. The constraint index (CI) of as-synthesized catalysts derived from the competitive reactions of ethylbenzene (EB) dealkylation and meta-xylene (m-X) isomerization and gravimetric adsorption measurements were introduced to investigate the extent of pore-narrowing by CLD of TEOS. Benefitting from the auxiliary mesopores and surface-passivation treatments, the optimized catalyst HSZ(Si0.5) showed remarkably enhanced catalytic activity over the microporous ZSM-5 counterpart in the model reaction of o-X isomerization.