Regulation Of Surface Cu Species On Cu-based Zeolite And Catalytic Performance Of Methanol Oxidative Carbonylation

CH₃OH reacts with CO and O₂ under oxidative carbonylation to produce CH₃OCOCH₃(DMC)which offers high atom economy and environmentally benign process.It is one of the important ways to consuming converted coal that has attracted extensive attention from the researchers in recent years.A Cu loaded Y zeolite is important catalyst in this reaction,but the microporous structure constructed by small cages of Y zeolite limits the diffusion of reactant molecules and affects the catalytic performance of active sites.In addition,it is difficult to control the state of surface Cu species after high temperature activation,which hinders the preparation of high performance catalysts.In order to improve the diffusion of reactant molecules,to optimize the Cu distribution,to regulate the formation process of active Cu species to modify the support pore structure and to promote the catalytic efficiency of Cu species,the regulation of the Cu species state and the controllability of the activation process of active substances were studied in this work.In addition,the microstructure of catalysts was studied,especially for the mechanistic action of the surface-active Cu species on oxidative carbonylation of methanol.The main results and conclusions obtained are as follow:(1)Y zeolite treated with Na OH solution for desilication could adjust the microporous structure which indeed improved the catalytic activity.By adjusting the concentration of Na OH solution,the amorphous silicon and aluminum species in the micropores were removed and partially removed framework silicon of the small cage eventually opens the cage structure which promoted the Cu exchange and subsequent Cu dispersion in the catalysts.Furthermore,the accessibility between Cu species and the reactant molecules in the small cage was promoted which improved the catalytic activity.The time space yield of DMC for Cu-loaded on the Y zeolite treated with 0.6 mol L^-1 Na OH solution was 151.4mg·g^-1·h^-1 which is 2.8-fold higher than that of Cu Y catalyst loaded on untreated support.(2)Increasing the dispersion of Cu species was found to be beneficial to the catalytic activity.The interconnected mesoporous channels of the Cu catalyst support,KIT-6 promoted the diffusion of reactant molecules and improved the catalytic activity.Under the similar Cu loading,the time space yield of DMC was 2-fold higher that that of Cu Y catalyst.Upon introduction of Al into the KIT-6 support,the formed surface acid sites provided the stable exchange sites,which facilitated the dispersion of Cu on the catalyst surface.The optimal Cu dispersion promoted the time space yield to increase further by 83%.(3)By treating the zeolite with oxalic acid solution and then with Na OH solution,the pore structure of the Y zeolite was optimized,which was beneficial to the formation and dispersion of Cu active species.After oxalic solution treatment,the specific surface area of Y zeolite was decreased and the quantity of exchanged Cu was also reduced however the quantity of adsorbed Cu on the surface was increased.These Cu species formed inactivated agglomerated Cu O and the activated[Cu-O-Cu]²⁺clusters during the high temperature activation,and the time space yield of DMC was 220.4 mg·g^-1·h^-1,which was 4.1 times higher than that of Cu Y catalyst loaded on untreated support.After the treatment with alkali for dealuminated Y zeolite,the surface area was restored and the ability of cationic exchange was promoted,which further promoted to the catalytic activity.(4)The Cu species were reduced to low states Cu species under high temperature activation,which gradually formed the catalytic active species.On activating Cu Y catalysts under 600?in N₂ atmosphere,the exchanged Cu species were reduced to activated Cu⁺at the low Cu loading(Cu<6.4 wt%).Upon increasing the Cu loading,a part of these exchanged Cu²⁺ions formed activated[Cu-O-Cu]²⁺clusters.At the same time,the formed Cu O on the catalyst surface were gradually self-reduced to active Cu₂O,which was conductive for the catalytic activity.Moreover,Cu²⁺species were reduced to active Cu⁺during further activation in H₂atmosphere,which improved the catalytic activity.But for the catalysts with high Cu loading,the active Cu₂O species on the external surface were reduced to agglomerated Cu⁰ particles resulting in the declined catalytic activity.