Deactivation Mechanism Of Dimethyl Ether Carbonylation Over Mordenite

As a clean energy,ethanol can be used as blending agent in fuels to improve combustion properties and reduce emissions.The development of ethanol will conform the energy structure in China,and alleviate the concern for the security of the petroleum supply.From syngas to dimethyl ether(DME),then carbonylation of DME to methyl acetate(MA)and subsequent hydrogenation of MA to ethanol is a novel route to produce ethanol,with the advantages of high atom economy,excellent MA selectivity,low cost and mild reaction conditions.In terms of carbonylation of DME to MA reaction,mordenite(MOR)is the most promising catalyst so far,while it suffers serious deactivation during the reaction.To date there is no systematical research on deactivation,in this thesis we focus on the nature and location of coke,deactivation kinetics,the coking mechanism and the effect of copper on coke formation to investigate the deactivation of MOR.The nature and evolution of the coke have been fully characterized using TG,GC-MS,EELS,UV-Vis,¹³C MAS NMR and FTIR.The results reveal that the coke is mainly alkylated aromatic species and the coke content increases with time on stream.Moreover,it has been demonstrated that the coke mainly deposits in 12-MR with no significate change of 8-MR Br?nsted acid sites,leading to the deactivation of MOR.The effects of reaction temperature,pressure,space time and reaction time on reaction performance and deactivation were investigated by the evaluation of H-MOR at different reaction conditions.The results reveal that the reaction performance was improved with increasing reaction temperature and pressure,but the increase of temperature and pressure will accelerate the deactivation as well.Based on the fitting results and in situ FTIR analysis,the most suitable model was established.In this model,coke is derived from DME and CO,which will generate coke intermediates in a consecutive reaction.The establishment of deactivation kinetics will be conductive to elucidate the coke formation mechanism and optimize the process conditions.In order to elucidate the coke formation mechanism,the Br?nsted acid sites in 8-MR was selectively ion exchanged by Na⁺,the characterizations of coke indicate that the Br?nsted acid sites in 8-MR will accelerate coke formation as well.Several possible coke pathways were investigated by FTIR,GC-MS,TPO-MS and TG.The results reveal that DME,MA and ketene can lead to coke,and the coke pathway from ketene conforms best with the formation of coke during the reaction.Furthermore,a kinetic model for coke formation is proposed to investigate the contributions of different channels to coke formation:the coke precusors are firstly formed in 8-MR,then diffuse into 12-MR to generate coke species.Compared with H-MOR,the introduction of Cu presented a higher carbonylation activity accompanied by a more rapid deactivation.A series of characterizations(e.g.TG,UV-Vis,¹³C MAS NMR and FTIR)indicate that there is no intrinsic difference in coke nature between spent Cu/H-MOR and H-MOR catalysts.Furthermore,a kinetic model for coke formation was proposed to compare the deactivation behaviors of H-MOR with Cu/H-MOR.In terms of activation energies for the formation of carbonaceous species,the presence of Cu accelerates the formation of non-condensed single aromatics species,but inhibits them to be more condensed.