A Study Of N-Doped Activated Carbon Supported Ag Catalysts For The Hydrogenation Of Dimethyl Oxalate To Methyl Glycolate

Methyl glycolate(MG)is a kind of simplest alkyd ester,possessing chemical properties similar to alcohols and esters.The molecular structure of MG contains a-H,hydroxyl,and ester group,so MG can process reactions like hydrogenation,hydrolysis,ammonolysis and so on.Thus,MG is an important chemical intermediate and high-quality solvent with broad applications.Several approaches to MG synthesis have been developed.Among them,the chemoselective hydrogenation of dimethyl oxalate(DMO)based on syngas chemistry is of a highly competitive route for the MG synthesis.This dissertation focused on the preparation and performance of nitrogen-doped activated carbon supported silver catalysts(Ag/AC-N-x,x represents the weight percentage of N content)for the vapor phase hydrogenation of DMO to MG.The catalysts were characterized by means of various techniques for detailed composition,structure,and properties.The structure-performance relationships had been carefully investigated.The main achievements are summarized as following.The AC-N-x supports with different N contens were readily synthesized through a catalyst-free approach by annealing the mixture of AC and melamine.Then the Ag/AC-N-x catalysts were prepared by a conventional impregnation method and subjected to the DMO hydrogenation.The results showed that the catalysts doped with N could significantly improve the activity of DMO hydrogenation reaction,affording MG as the main product with selectivity close to 100%.When the N content in AC-N was 12.74%,catalyst Ag/AC-N-12 performed the best activity,giving the MG yiled as high as 96%.The catalyst had good thermal stability and stability in the process of DMO hydrogenation.To elucidate the essence of Ag/AC-N catalysts with enhanced performance for the DMO hydrogenation to MG,a series of characterizations such as X-ray diffraction,Raman,transmission electron microscopy,and X-ray photoelectron spectroscopy(XPS)have been conducted to study the structures of the supports and the catalysts.The results indicated that the introduction of N could increase the defects of AC surfaces and thus enhance the interactions between AC and Ag species.Furthermore,there were electronic interactions betweem the lone pair electrons of N species and Ag species,making the Ag species tend to Ag0.Those interactions are believed to benifit the anchoring of Ag species on the surface of AC,so as to reduce the size and depress the sintering of Ag particles.The further XPS results revealed that the pyridine N content in AC-N significantly influenced the activity of catalyst,that is,the catalyst activity was proportionally related to the pyridine N content.Therefore,the enhanced performance of Ag/AC-N catalysts for the DMO hydrogantion to MG is essentially ascribed to the strong interactions(including electronic interactions)betweem N species and Ag particles,where the interactions between the pyridine N species and Ag are particularly crucial for the catalysis.