β Zeolite-based Catalysts For The Vapor Phase Beckmann Rearrangement Of Cyclohexanone Oxime

ε-Caprolactam is a valuable starting material for the manufacture of nylon 6 and resins. Beckmann rearrangement of cyclohexanone oxime toε-caprolactam is one of the most important processes in the production ofε-caprolactam. The currently used technology for the Beckmann rearrangement, catalyzed by liquid acids, suffers several drawbacks: anticorrosive equipment being required and the production of large amounts of low value byproducts. So, it is of great importance to find low pollution, more active and more stability solid acid catalysts, which can render the reaction to be operated in the vapor phase.In the present work, zeoliteβwas first synthesized by a hydrothermal method; B₂O₃/Hβ, MoO3/Hβcatalysts were prepared by impregnation method; Hβwas treated by alkaline; H-Tiβwas prepared by isomorphous substitution of Al inβzeolite by Ti. All the catalysts obtained were characterized by XRD, BET, IR, XPS and TPD etc., and studied in the vapor phase Beckmann rearrangement of cyclohexanone oxime. Also, H-ZSM-5 zeolite with different ratios of SiO2/Al2O3 and the mesoporous H-Si-MCM-41 were compared with Hβin this reaction.For the Hβ-supported B₂O₃ catalysts, the supported B₂O₃ with < 20.33 wt.% loadings mainly increased the amount of the weak Bronsted acid sites and showed better catalytic stability, while the loadings of B₂O₃ > 20.33 wt.% covered the acid sites and decreased the catalytic stability. Loading 3.66 wt.% MoO3 on Hβalso mainly increased the amount of the weak Bronsted acid sites and showed better catalytic stability. However, with loadings of 16.46 wt. % MoO3 led to the structural breakage of zeoliteβto some extent. The framework Ti in the Hβzeolite increased the amount of weak Bronsted acid sites and improved the catalytic stability, while the amorphous TiO2 favored the side reactions. The catalysts treated in ammonia, ethanolamine, or ethylenediamine solutions showed better catalytic stability due to the formation of more weak Bronsted acid sites.The results revealed that the active sites for the vapor phase Beckmann rearrangement of cyclohexanone oxime were not identical on different catalysts. On zeoliteβ-based catalysts, the active sites might be the weak Bronsted acid sites. On the catalysts having the MFI structure, the neutral Si-OH might be responsible for theBeckmann rearrangement, while on the mesoporous H-Si-MCM-41, the neutral Si-OH favored the formation of by-products.The analysis of coke deposited on the catalysts showed that a portion of the coke produced on the outside, while a portion deposited on the inside, surfaces of the catalysts. When the amounts of coke increased and blocked the strong acid sites, the selectivity would increase. If the coke blocked the weak acid sites, the conversion would decrease.