Preparation Of The Chlorine-free CuY Catalyst By Solid State Reaction And Heterogeneous Catalysis In The Oxidative Carbonylation Of Methanol

Dimethyl carbonate（DMC） is considered as an environmentally benign chemical due to its negligible ecotoxicity and low bioaccumulation. And chemical character of DMC is very active due to its more functional groups,such as methyl, methoxy, carbanyl group and ester, so it can be used as a carbonylation agent in place of toxic phosgene and a methoxycarbonylation agent for the production of carcinogenic polycarbonates and polyurethane. In addition, DMC could be used as oil additives, thus to reduce the environmental pollution. Among the many synthetic processes, oxidative carbonylation of methanol to DMC occurs under mild reaction conditions with high atom usage and byproduct water, so it accords with the principles of green chemistry, and then has a bright prospect for DMC product.Supported Cu-based catalysts on the Y zeolite（Cu Y） shows an excellent catalytic activity for oxidative carbonylation of methanol to DMC. And the extraframe bound-Cu₊ located in the supercage of Y zeolite is the active center. In the thesis, cupric acetylacetonate Cu（acac）₂ as copper source and NH₄ Y zeolite as support, under solvent-free conditions, by heat treatment of the mixture of NH₄ Y and Cu（acac）₂, to make sublimate, disperse and decompose on the NH₄ Y zeolite, thus the solid state occurs and Cu₊ locates in the supercage of Y zeolite. Finally, a chloride-free Cu Y catalyst with high activity was successfully prepared. By systematically studying solid state reaction between Cu（acac）₂ and NH₄ Y zeolite, the influence factors, catalytic activity and the solid state reaction characteristic of between other copper source and NH₄ Y zeolite, main conclusions of the study are as follows:（1） During heat treatment of the mixture of Cu（acac）₂ and NH₄ Y zeolite, when the temperature is 160oC, Cu（acac）₂ begins to sublimate and gradually disperse on the NH₄ Y zeolite; when the temperature is 174oC, the ion exchange between Cu（acac）₂ and NH₄ Y begin to occur; when the temperature is 280oC, the ion-exchanged Cu²⁺ begins to auto-reduce to form the active center Cu₊ in the supercage. In the meantime, Cu（acac）₂ in the supercage decomposes to Cu O species, which contribute the lattice oxygen and further improve the catalytic activity of Cu₊.（2） For Y zeolite support, NH₄⁺ of NH₄ Y zeolite and H+ of HY zeolite can ion-exchange with Cu²⁺ of Cu（acac）₂, and followed by subsequent decomposition of Cu（acac）₂ and auto-reduction of Cu²⁺, thus there are Cu²⁺, Cu O and Cu₊ species on the catalyst. The Cu Y catalyst from NH₄ Y zeolite exhibits the better activity. However, Na+ of Na Y does not ion-exchange with Cu²⁺ of Cu（acac）₂, there is only Cu O species on the as-prepared Cu Y catalyst, resulting in no catalytic activity.（3） During preparation of precursor by heating the mixture of Cu（acac）₂ and NH₄ Y, as the temperature rises, the ion-exchanged degree between Cu（acac）₂ and NH₄ Y increases. At the temperature of 247oC, Cu（acac）₂ decoposes while ion exchange between Cu（acac）₂ and NH₄ Y occurs. Above 250oC, decomposition of Cu（acac）₂ have become a priority. So with increase of the preparation temperature of precursor, the exchange degree of copper ion increases followed by a decrease and the activitis have the same tendency of changes. When the preparature temperature of precursor is 250oC, the Cu₊ content in the supercage is highest and the as-prepared Cu Y catalyst shows the best catalyst activity.（4） The activation temperature is a direct factor of influencing the Cu₊ content and catalytic activity. When the activation temperature is 280°C, the active center Cu₊ begins to form,but the as-prepared Cu Y catalyst has no catalytic activity. When the activation temperature is 650°C, the content of the active center Cu₊ reaches to the maximum and the activity is the best. When the activation temperature is 700°C, the crystal structure of Y zeolite is completely damaged and the as-prepared Cu Y catalyst loses its activity.（5） Cu loading is one of major factors of affecting the Cu Y catalyst activity. With the increase of Cu loading, the amount of active center Cu₊ in the supercage gradually increases. Once Cu loading increases to 10 wt%, the corresponding Cu O species from decomposion of Cu（acac）₂ generates and the content gradually increases. The moderate Cu O can contribute the lattice oxygen which is beneficial to improve the catalytic activity.But too much Cu O species will partly cover active center even block the pores and channels of Y zeolite, resuling in decrease of catalytic activity on the Cu Y catalyst. When Cu loading is 12 wt%, the as-prepared Cu Y catalyst exhibits an excenllent catalytic activity with 267.3 mg·g-1·h-1 STYDMC, 6.9% XCH₃ OH and 69.2% DMC selectivity.（6） With the increase of the NH₄⁺ exchange degree, Cu O species on the Cu Y catalyst is gradually transformed into copper ion and the catalytic activity of the Cu Y catalyst also increase. When the NH₄⁺ exchange degree is zero, the copper species on the Cu Y catalyst exists in the form of Cu O, and the as-prepared Cu Y catalyst has no catalytic activity. When the NH₄⁺ exchange degree reaches to the extreme limit of 68.1%, copper species mainly exists in the form of copper ion besides small amounts of Cu O, and the as-prepared Cu Y catalyst shows the best catalytic activity.（7）The activation atmosphere of the Cu Y catalyst is one of the most important factors that influence distribution of copper species on the Cu Y catalyst and catalytic activity of the Cu Y catalyst. At 650°C, under nitrogen, the surface of the catalyst deposited carbon black and the carbon black covers part catalytic activity even blocks the pores and channels of Y zeolite, resuling in a lower catalytic activity, STYDMC, XCH₃ OH and DMC selectivity is 163.3 mg·g-1·h-1, 5.1% and 59.8% respectivly. Under oxygen, there is no carbon black on the surface of the Cu Y catalyst. Although oxidising atmosphere eliminates the influence of carbon black, this is unfavorable for reduction of Cu²⁺. So the Cu₊ content of the as-prepared Cu Y catalyst is low, resulting in less-than-ideal catalytic catalyst, STYDMC, XCH₃ OH and DMC selectivity is 293.2 mg·g-1·h-1, 8.0% and 64.8% respectivly. When a mixture of nitrogen and oxygen is used as activation atmosphere, with the decrease of oxygen, the Cu²⁺ content of the Cu Y catalyst decreases and the Cu₊ content increase, and the catalytic activity also improves. The Cu Y catalyst activated in nitrogen dopped little oxygen（1.8 vol%） shows the most optimal catalytic activity, STYDMC, XCH₃ OH and DMC selectivity is 370.7 mg·g-1·h-1, 10.5% and 63.5% respectivly.（8） Under the optimal preparing condition of the solid state reaction between Cu（acac）₂ and NH₄ Y zeolite, the Cu Y catalysts prepared by solid state reaction were further studied with other copper species such as Cu SO4, Cu（NO3）₂, Cu（CH₃COO）₂ and Cu Cl2, increasing sequence of catalytic activity is as follow: Cu Y-S < Cu Y-AC < Cu Y-C < Cu Y-N. Cu（NO3）₂ and Cu（CH₃COO）₂ as copper source, although there is Cu O species to contribute the lattice oxygen to DMC formation, the Cu₊ content in the supercage is lower than that of the Cu Y catalyst from Cu（acac）₂. Cu Cl2 as copper source, the as-prepared Cu Y catalyst does not contain Cu O species. Cu SO4 as copper source, the crystal structure of Y zeolite is completely damaged. So the catalytic activity is inferior to the Cu Y catalyst from Cu（acac）₂.