Study On Copper Base Supported Hβ Zeolite Catalysts For The Synthesis Of Diethyl Carbonate By Oxidative Carbonylation Of Ethanol

Diethyl carbonate (DEC) has attracted increased attention as an environmentally friendly green chemical. DEC has also been used as a substitute for methyl tert-butyl ether (MTBE), which is widely used as an oxygen-containing fuel additive. Although various catalytic systems for DEC synthesis been investigated in the past decades, the active site and structure of active components remains elusive. Achieving high selectivity toward DEC also remains a challenge.In this dissertation, Cu-based catalysts supported on Hβzeolite were studied for the vapor phase synthesis of DEC via the oxidative carbonylation of ethanol. Specifically, Cuβcatalyst showed better catalytic selectivity to DEC with the negligible amount of the byproducts 1,1-diethoxyethane (DEE) which was observed in CuY catalyst systems. The computational results largely agreed with the experimental ones, which indicated that DEC (with a small kinetic diameter and the lowest energy barrier) was preferentially formed over both zeolites. It is primarily due to the fact that the special structure ofβzeolite prohibits the diffusion of DEE molecules through its pores. The effect of chemical treatment on the reactivity of the catalysts was also studied. For cupric based catalysts, those using CuCl as the active component showed better catalytic performance. The favorable temperature for catalyst preparation was 450℃at which the gaseous CuCl could preferablely exchange with H+ species in acidic Br?nsted sites on the surfaces. The loadings of copper was closely interrelated with the active center of the catalyst system and the best catalytic performance were observed with 40wt.% copper loading. Results from XPS, ICP and IR indicated that the treatment ofβzeolite facilitated the extraction of extra-framework silicon and changed Si/Al ratio in the bulk. DFT calculations showed that the activation barrier of the formation of the active center for Cuβwithout extra-framework silicon was lower than that with extra-framework silicon. This suggests that the extraction of extra-framework silicon promoted the active center of Cuβforming.Effect of various quaternary ammonium salt (QAS) promoters on the performance of the PdCl₂-CuCl₂/Hβcatalyst was investigated. Hexadecyl trimethyl ammonium Chloride (CTAC) promoter chemically modified the functional groups on the surface of Hβ, which facilitated the cooperation between PdCl₂ and CuCl₂. A possible modification mechanism could be the one that the halogen groups of QAS interacted with the polar hydrophilic OH groups on the surfaces, leading to the formation of a film composed of hydrophobic nonpolar groups by organizing alkyl groups of QAS in the ethanol system. The presence of the film could facilitate the dispersion of active species but also avoid the strong interaction between active species and the support. I have also shown that the coupling reactions, hydrolysis of diethyl ether (DEE) and oxidative carbonylation of ethanol, can enhance STY of DEC, and decrease the water content from 3% to 4% in the system. The performance of the catalyst was robust and the life time maintained more than 30 hours.