Carbonylation Of Dimethyl Ether To Methyl Acetate Over Modified Mordenite Zeolites

Ethanol is becoming one of the important worldwide clean fuels,fuel-additives and industrial chemicals.Thus,it is urgent to explore efficient synthesis routes to meet the significantly increased demands for ethanol.Industrially,dimethyl ether(DME)can be easily synthesized from syngas(CO+H₂),which is produced from coal,natural gas,biomass and shale gas.Moreover,DME can also be directly produced from biomass,and the Bio DME project has been industrially built and operated in Sweden.On the basis of this material,a green tandem ethanol synthesis route,composed of DME carbonylation to methyl acetate(MA)and MA hydrogenation to ethanol,has received extensive attentions,because of its high atomic efficiency and potential industrial application value.Currently,the production process of MA hydrogenation to ethanol is very mature with the high activity and selectivity to ethanol.H-mordenite(H-MOR)shows a high catalytic activity for DME carbonylation to MA.However,the H-MOR catalyst for DME carbonylation has the problem of low activity and stability,limiting further industrial applications.Herein,this work successful encapsulates single Cu sites in the 8-MR side pocket of H-MOR,named as Cu-isolate,with only 0.3%Cu loading.The Cu-isolate catalyst shows a surprisingly high stability,apparent activity and selectivity to MA.Combining the in-situ characterizations,this work successfully proves that the main reason accounting for the increased catalytic activity is that the atomic Cu⁺ can facilitate the DME carbonylation reaction by activating CO.Compared with H-MOR and traditional ion-exchanged Cu-MOR catalysts,the Cu-isolate catalyst is more advantageous to enhance the apparent activity of DME carbonylation by 15%and improves catalytic stability.Moreover,pyridine modification of H-MOR can improve its stability,but undesirably lose catalytic activity.Herein,this work report the intrinsic impact of the pyridine adsorption behaviour on H-MOR and spacial hindrance of zeolite frameworks on dimethyl ether carbonylation at a molecular level.The acid sites at O2 positions,locating on common walls of eight-membered ring(8-MR)side pockets and 12-MR channels,are active for DME carbonylation,but are unfortunately poisoned during pyridine modification.The density functional theory(DFT)calculation reveals that the pyridine-poisoned acid sites on O2 positions are easily regenerated due to spacial hindrance of zeolite frameworks.Accordingly,this work can facilely regenerate them by proper thermal treatment,which induces 60%promotion in the catalytic activity accompanying with the high stability.This work demonstrates the determining role of O2positions in H-MOR for DME carbonylation.In addition,this work also reports the effect of the hot-water pretreatment on the catalytic activity of the H-mordenite zeolite for DME carbonylation.With the increase of the duration of the hot-water pretreatment,the catalytic activity shows a volcano-type variation.The H-MOR catalyst pretreated by hot water for 10 min at 573 K displays the highest catalytic activity among all of the catalysts.The appropriate duration of the hot-water pretreatment can generate new medium strong and strong Br?nsted acid sites,which changes the microenvironment in the pores of H-MOR.Additionally,the newly generated Br?nsted acid sites will accelerate the first step of DME carbonylation,that is,DME reacts with the Br?nsted acid sites of the zeolite forming surface methoxy groups and methanol,improving the catalytic activity of the catalysts.However,too long duration of the hot-water pretreatment will remain water molecules in H-MOR,which inhibits DME carbonylation.This work provides a new avenue to rational design of other efficient zeolite-relevant catalytic systems.