阐明铜物种在甲醇氧化羰基化法制dmc在cuy碳酸二甲酯原位光谱和催化研究.pdf

Applied Catalysis A: General 382 (2010) 303–311Contents lists available at ScienceDirectApplied Catalysis A: Generaljournal homepage: the role of Cu species in the oxidative carbonylation of methanol to dimethyl carbonate on CuY: An in situ spectroscopic and catalytic studyJana Engeldinger, Christine Domke, Manfred Richter, Ursula Bentrup∗Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germanya r t i c l ei n f oArticle history: Received 3 February 2010 Received in revised form 7 April 2010 Accepted 10 May 2010 Available online 16 May 2010Keywords: CuY zeolite Dimethyl carbonate FTIR spectroscopya b s t r a c tCuY catalysts with varied Cu content obtained by incipient-wetness-impregnation of commercial NH4-Y zeolite with copper nitrate solution and calcination at 400◦C in air were examined by in situ FTIR inves-tigations to characterize the Cu species and elucidate their specific role in the oxidative carbonylation of methanol. The adsorption of CO was applied to check the state of Cu(I) species whereas the acidity was characterized by pyridine adsorption. Depending on the Cu content different intensity ratios of the characteristic Cu(I)–CO bands at 2160 and 2146cm−1were observed which give information about the distribution of Cu(I) cations at different positions near the supercage. Lewis sites of different strengths are created by introduction of Cu resulting from Cu(I)/Cu(II) cations positioned in the super cages. MeOH adsorbs dissociatively on preferable Cu(I) Lewis sites by formation of methoxy species without additional supply of oxygen. Methoxy species and CO adsorb at the same Cu sites. Oxygenated products (monomethyl carbonate, dimethyl carbonate, CO2) are formed during simultaneous adsorption of MeOH and CO with and without oxygen which indicates a participation of lattice oxygen of CuOxaggregates in the oxidation process and points to a Mars-van-Krevelen mechanism. Adsorbed formate species more pronounced at higher Cu loadings were additionally found. CO reacts with adsorbed methoxy species by formation of mainly monomethyl carbonate whereas the simultaneous presence of formate-like speciespromotes the formation of dimethyl carbonate. This implicates a specific role of adsorbed carbonate-like species for dimethyl carbonate formation. High Cu loadings were found to be beneficial because the additional formation of CuOxagglomerates in the super cages favors oxidation and oxocarbonylationreactions of methanol and enhances the formation of dimethyl carbonate which was also confirmed by the catalytic tests. © 2010 Elsevier B.V. All rights reserved.1. IntroductionDimethyl carbonate (DMC) is an attractive chemical with ver- satile applications [1,2] which require an increasing production capacity. However, the traditional synthesis route of DMC com- prises the use of toxic, hazardous phosgene as reagent. Insofar, the development of an environmentally benign preferable gas phase process for DMC synthesis is necessary. Cu-containing zeolites, in particular Cu–Y zeolites, have been found to be attractive catalysts, because they promote the oxidative carbonylation of methanol to DMC in the gas phase [3–10]. For improving the catalytic perfor- mance of suitable catalysts knowledge about reaction mechanism and kinetics is needed. Thus, several studies have been reported describing possible mechanisms of the oxidative carbonylation of methanol to DMC over Cu-containing zeolites [3–6,9–13]. King [3] investigated ion-exchanged Cu(II)Y and Cu(I)Y zeo- lite obtained by solid-state ion exchange. While the Cu(I)Y zeolite∗Corresponding author. E-mail address: ursula.bentrup@catalysis.de (U. Bentrup).revealed a good performance in the oxidative carbonylation reac- tion the ion-exchanged sample Cu(II)Y showed only little activity for the same reaction. In situ FTIR spectroscopy was applied to elu- cidatethereactionmechanism.TheoxidationofbothmethanolandCu(I) to form Cu(II) methoxide was proposed to be the first step of reaction while the insertion of CO into the Cu(II) methoxide form- ing a carbomethoxide species is the rate-limiting step. The latter reacts with methanol and oxygen to form DMC.These findings agree with results of Anderson and Root [6], who showed by kinetic analysis that the insertion of gaseous CO into surface methoxide to form a carbomethoxide via an Eley-Rideal pathway is the rate-determining step for DMC production. By com- parison of Cu(I)X and Cu(I)ZSM-5 they found a weaker adsorption of CO onto Cu(I)X than on Cu(I)ZSM-5 which seems to be advan- tageous for the formation of DMC. Strong adsorption of CO blocks the sites for methoxide formation but does not participate in DMC formation. The role and nature of Cu sites involved in the reaction was not discussed. ZhangandBell[12]investigatedthemechanismofDMCsynthe- sis from oxidative carbonylation of methanol over Cu-exchanged Y zeolite using in situ FTI。