Pd-Fe/α-Alumina Catalysts For CO Oxidative Coupling To Diethyl Oxalate

CO gas phase coupling to diethyl oxalate（DEO）is a green chemical process,with mild reaction conditions,high atom economy and environmental friendliness.In order to further improve the utilization efficiency of precious metal Pd and reduce the production cost of the catalyst,methyl cellulose and plant tannin were introduced into Pd-Fe/α-Al₂O₃ catalysts and the effect of promoters on catalysts structure and catalytic performance for DEO synthesis by CO oxidative coupling was investigated.A novel Pd-Fe/α-Al₂O₃ catalyst was prepared by using incipient wetness impregnation method with addition of methyl cellulose.The impregnation thickness of Pd nanoparticles on the commercial Raschig hollow cylindricalα-Al₂O₃ support and its dispersion were the main factors affecting the catalytic performance of Pd-Fe/α-Al₂O₃.Since Pd ions can be stabilized by methyl cellulose in aqueous solution to form highly dispersed nanoparticles,they are well dispersed inα-Al₂O₃ carrier.Methyl cellulose also enhanced the interaction between Pd and Fe to partially form Pd-Fe alloy.Due to the incomplete filling of the valence band of iron,the electrons were transferred to void d-orbital of Pd,which increased electron density of Pd on the surface and triggered Pd-Fe synergistic effect.This synergistic effect enhanced the adsorption of CO.Therefore,the modified Pd-Fe/α-Al₂O₃ catalyst with 0.085 wt.%Pd and 0.91wt.%Fe loading showed better catalytic activity and selectivity than the traditional Pd-Fe/α-Al₂O₃ cordierite catalyst with 1.0 wt.%Pd and 1.0 wt.%Fe loading.Commercial Raschig’s hollow columnarα-Al₂O₃ as a carrier and bayberry tannin as a chelating promoter,Pd-Fe/α-Al₂O₃ catalyst with low Pd loading was prepared by incipient wetness impregnation method.Plentiful phenolic hydroxyl groups in bayberry tannin can effectively anchor the active palladium and reduce the particle size,resulting in a dendritic polycyclic distribution of Pd on the commercialα-Al₂O₃ carrier.Moreover,there was a strong interaction between Pd and Fe and the existence of Fe Pd₃ alloy,O₄Pd_3.5,Fe_1-xO and Pd⁰ species at the interface was discovered,which resulted in the increase of electron density of Pd on the surface and enhancement the adsorption of CO.The synergistic effect endowed the modified catalysts with excellent catalytic performance.Under the reaction conditions of 130℃and0.04 MPa,the Pd-Fe/α-Al₂O₃ catalyst with 0.23 wt.%Pd and 0.22 wt.%Fe loading showed excellent catalytic activity and selectivity and the space-time yield of DEO,CO conversion and selectivity of DEO reached 978 g·L^-1·h^-1,44%and 95.5%,respectively.