Study On Controlling Cu Catalytic Active Center And Catalytic Performace Of CuY Catalysts

Dimethyl carbonate, as an environmentally friendly green chemical products, can be used as a fuel additive, a phosgene alternative in polycarbonate, a non-toxic carbonylating and methylating agent to substitute for toxic and corrosive phosgene and dimethyl sulfate. This and other applications leads to an enormous effort in the investigation of low-cost and not toxic synthesis of DMC. The direct vapor-phase oxidative carbonylation of methanol with CO and O2had been considered as a promising way of new coal chemical industry technology because of avoidng the use of NO poisonous gas in the indirect gas-phase method and the corroisive problem in the liquid-phase reaction. At present, active carbon supported chloride-containing catalysts, CuCl2-PdCl2bimetallic chlorides and chlorine-free zeolite-based catalysts were the main catalysts. Because of the corroisive problem, the development of a new kind of good performance and environmental catalysts for DMC synthesis is necessary.Cu-containing zeolites, in particular CuY zeolites, had been found to be potential catalyst system, on the one hand this kind of catalysts showed better catalytic activity of the oxidative carbonylation of MeOH to DMC in the gas phase than other chlorine-free zeolite-based catalysts, on the other hand CuY catalysts contained Cu2+and Cu+species, and the role of different value state Cu in the catalytic recation is controversially discussed. Preparation of chlorine-free CuY and good catalytic activity, further investigation of different value state Cu in the catalytic recation had been became the important research directions. Our preliminary investigations about CuY prepared by ion exchange and incipient wetness impregnation showed different catalytic activity because of the differences of the location of Cu species, besides, introducing Ce, La and Cs ion into the zeolites exhibited higher catalytic activity. On this bases, we discussed that the content of different value state Cu had an effect on the oxidation carbonylation by reduction treatment, that CuHY-3preparated by firstly adopted ion exchange, then Cu-impregnated, were different from the catalysts preparated by single method on the microstructure, and that the rule of exchange between NaY and Cu ammonia solution, and the regularity of Cu species with the increasing of the content of Cu. The characterization of AAS, XRD, TPR, TPD, XPS and TEM were used to study the microstructure, activity center and catalytic activity. The main conclusions are listed as follows:(1) Reducing treatment at different temperature was used to prepare CuY catalysts. Cu2+located in the supercage of zeolite were more easily reduced to Cu+than Cu2+located in the small cage of zeolite, but a part of Cu2+were reduced to Cu°by means of Cu2+â†'Cu+â†'Cu°during reducing treatment at low temperature for a long time.(2) After reducing treatment, the content of Cu°located in the cage inceased from0.3%of CuY-200catalyst to1.8%CuY-500catalyst, achieved33.4%of the whole Cu content. The change of Cu°and Cu2+led to the strong adsorption of CO, which was not good for the substitution of CO, and the catalytic activity decreased.correspondingly.(3) Three different preparation methods:liquid-phase ion exchange, incipient wetness impregnation and firstly adopted ion exchange, then Cu-impregnated, were applied to prepare CuHY catalysts. It was found that CuHY catalyst, prepared by ion exchange and Cu-impregnated shown high catalytic activity because of different Cu species distributed on the bulk and surface of zeolite.(4) CuHY-3catalyst, prepared by ion exchange and Cu-impregnated had different Cu species. To analyse the Cu species on bulk and surface through H2-TPR and XPS, the amount of Cu2+located in super cages of CuHY-3catalyst were more than CuHY-2catalyst under the similar Cu content condition, which was the reason why CuHY-3have better catalytic activity than CuHY-2.(5) The form and valence state of Cu species changed regularly with the increase in the Cu content. The results of XRD, H2-TPR and XPS indicated that different Cu specises (Cu2+, Cu+, Cu°, CuO and Cu2O) had been appeared in turn with the increase in the Cu content. In addition, the appearance of Cu2O was the main reason why CuNaY-4catalyst was found to more catalytic activity than the anothers.(6) It can be seen that a small fraction of Cu specises deviated from Y zeolites in CuNaY-4, CuNaY-5and CuNaY-6catalyst, and the deviated Cu specises of CuNaY-6were more than the others. In order to gain information as detailed as possible on deviated Cu specises, The hexagonal crystal particle of Cu specises were detectived by HR-TEM, and the0.208nm of lattice distance was attributed to the Cu°. More and more Cu°paticles were depart from Y zeolites with the increase in Cu content, and this phenomena corresponded to the results of H2-TPR that the content of Cu°decreased slightly in the CuNaY-5and CuNaY-6.