Modification Of ZSM-5Zeolites And Their Catalytic Performances In The Isomerization Of Styrene Oxide To Phenylacetaldehyde

Aldehydes are important chemical raw materials or intermediates. In theirtraditional synthetic processes (such as oxidation and reduction methods), formationof alcohols or carboxylic acids is inevitable, usually leading to low selectivities ofaldehydes. While aldehydes can also be obtained via isomerization of epoxides overacid catalysts, which is a higher atom economy reaction and especially avoids thesubsequent oxidation and reduction of aldehydes. However, the competitive aldolcondensation or polymerization of aldehydes and fast deactivation of catalysts bycoke formation are still difficult problems. In this dissertation, the effects of acidityand pore structure of HZSM-5zeolites on the isomerization of styrene oxide tophenylacetaldehyde was investigated under a gas-phase atmosphere free of solvents.Phosphorus modification and alkali treatment methods were used to modify acidityand pore structure of zeolites, respectively.The isomerization was mainly catalyzed by the strong acid sites of HZSM-5zeolites, and HZSM-5with SiO2/Al2O3=25~360can almost completely catalyzethis reaction. The catalyst lifetimes were influenced by both acid strength andconcentration. Increasing the acid concentration and simultaneously decreasing theacid strength can prolong the catalyst lifetimes. The main by-products in this reactionwere dimer (2,4-diphenyl-2-butenal) and trimer (2,4,6-tribenzyl-s-trioxane) ofphenylacetaldehyde. The dimer was formed through aldol condensation ofphenylacetaldehyde, which was not sensitive to the varying of acidity with itsselectivity at the same level (1~3%) on all the catalysts. The trimer was formedthrough trimerization of phenylacetaldehyde at the external acid sites, resulting in anobvious decrease of phenylacetaldehyde selectivity.Introduction of phosphorus in HZSM-5(P/Al≤1.0) leaded to a decrease of strongaicd strength and concentration, making the rate of coke formation reduced. Thus, thecatalyst lifetimes increased nearly one-fold. Moreover, external acid sites can besignificantly deactivated by phosphorus modification, thus improvingphenylacetaldehyde selectivity by suppressing its trimerization occurring at externalacid sites. Alkali treatment brought some intracrystalline mesoporosity (about14nm), whichimproved the diffusion rate of reactants into and out of HZSM-5zeolites. Thus,Phenylacetaldehyde could be separated from the catalysts as soon as possible. Thecatalyst lifetimes increased nearly four-fold. But alkali treatment exposed some acidsites which initially existed in the micropore on the external surface, leading to adecrease of phenylacetaldehyde selectivity.