Morphology Control And Performance Study Of Palladium-based Catalysts For The Reaction Of CO And MN Coupling To Dimethyl Oxalate

Ethylene glycol(EG)is an important basic organic chemical raw material,and the traditional industry usually rehydrates ethylene oxide obtained by epoxidation of ethylene.The hydrogenation of dimethyl oxalate prepared from carbonylation of coal-based synthesis gas(CO)to produce ethylene glycol is an emerging industrial technology.This new technology accords with the current fossil energy status of China's "relatively coal-rich,oil-poor and gas-free".Therefore,it has attracted much attention from our country.This article takes CO and methyl nitrite(MN)oxidative coupling reaction as the object to develop the morphology control of palladium-based catalyst,the crystal plane effect on the catalyst performance,then explores the inherent law between morphology-control and catalyst performance,which lay the foundation for the further development of the catalyst.Pd nanoparticles with different morphologies are synthesized by liquid phase synthesis and the composition of the crystal planes is determined by TEM,HRTEM and other characterization methods.The synthesized Pd nanoparticles are supported on ?-Al2O3 and obtain a series of Pd/?-Al2O3 catalysts,then perform characterization analysis and evaluate their catalytic activities.The results show that the Pd nanoparticles enclosed by(111)facets have better catalytic performance for CO and MN coupling reaction.Among them,the Pd icosahedra catalyst performs best and the conversion of MN is 90%,the selectivity of DMO is 98.2%.In-situ DRIFTS results demonstrate that CO molecules are more likely to generate bridge adsorption on catalysts enclosed by(111)facets,and this adsorption is beneficial for the coupling reaction.CO has a higher adsorption peak on Pd icosahedron-based catalyst which illustrate that Pd icosahedron-based catalyst can enhance the concentration of the bridged CO and thereby facilitate the reaction.The CO stripping characterization results show that CO adsorption is more stable on the Pd(111)facets,and that the Pd icosahedra have more active sites,which is beneficial to the adsorption of CO.The DFT calculation results demonstrate that after CO adsorbing on different facets of Pd,the charge density around it is also different.When CO is adsorbed on the surface of Pd(111)facets,only a small amount of negative charge migrates to the inside,and a large amount of charge is concentrated on the surface.With the increase of the surface electron density of Pd,its catalytic performance will also be improved.In addition,the binding energy of CO adsorbed on the Pd(111)facets is 0.45 eV higher than that on the(100)and(730)facets,which indicates that CO adsorbed on the Pd(111)facets can obtain a more stable adsorption structure,and CO oxidative coupling reaction takes place on the Pd(111)surface having a lower activation energy.Based on the Pd icosahedron nanoparticles,the effects of the following conditions such as load,ultrasonic time and impregnation time on the catalytic performance of the catalysts are investigated.The results illustrate that the catalyst has the best catalytic performance when the Pd loading is 0.9%wt,the ultrasonic time is 10 min,and the impregnation time consists of 1.5 h of stirring time and 1.5 h aging time.Under this condition,the conversion of MN is 90%and thee selectivity of DMO is 98%.Based on the above conclusion of loading conditions,the effects of process conditions such as temperature,feed gas ratio,space velocity,and operation time on catalyst performance are further investigated.The relatively suitable operating conditions finally determined:when reaction temperature is 140?,the feed gas ratio CO:MN=2.5 and the space velocity is4000 ml·h-1·g-1,the conversion of MN and the selectivity of DMO remained at about 92%and 98%respectively after 50 hours of continuous operation of the catalyst.