Investigation Of The Application Of The Gold Catalyst For The Synthesis Of Methyl Methacrylate

Methyl methacrylate（MMA）is a monomer of polymethyl methacrylate（PMMA）,which is also an important chemical material,used for various industries.Presently,the major industrial process for synthesis of MMA is acetone cyanohydrin（ACH）route.However,due to the supply problems of raw materials of hydrocyanic acid（HCN）,the acid corrosion and environment pollution,this method may be obsoleted in the near further.The process of synthesizing MMA from isobutylene is a clean,environmentally friendly route,which also owns high atomic utilization.In this route,isobutylene is firstly oxidized to form methyl acrolein（MAL）,and then MAL is oxidized and esterified simultaneously to form the MMA.The selectivity of traditional Pd-catalyst is very low because the reaction routs involve multiple steps of oxidation and esterification.The addition of Pb and Bi additives can improve the selectivity to some certain extent,but it will inevitably cause the environmental pollution.Supported gold catalyst has the advantage of higher selectivity than the Pd-based catalyst,but the activity is greatly affected by the particle size and the support.Therefore,in this study,different oxides and hydroxyapatite supported gold catalysts have been synthesized and utilized in oxidation esterification reaction of MAL and methanol to synthesis of MMA.At mean time.the particle size effect,support effect and other influences on catalytic performance of gold were investigated accordingly.The main research contents are as follows:（1）Different oxides（Zn O,Si O₂,Al₂O₃,Ce O₂,Ti O₂,Zn Al O_x,etc.）supported Au catalysts were prepared by sol-gel method and deposition-precipitation（DP）method.The results from XRD and TEM characterizations reveal that the high dispersed,small particle size of gold were obtained on the supports after high temperature calcination at 400℃.The XPS and CO Drifts results show that the valence state of gold were Au⁰.While the intensity of CO adsorption of gold was stronger in the catalyst with monodispersed Au₂₅ clusters as the precursor than the traditional gold catalysts prepared by DP method.In oxidation esterification reaction of MAL and methanol,it is found that the catalyst prepared with Zn-Al hydrotalcite as the precursor of support and Au₂₅ clusters as the precursor of gold exhibits good activity and selectivity（Conv.93.1%&Sel.94.8%）.Combined the characterizations results with the comparison tests,the small particle size of gold,the interaction between gold with the support and the alkalinity of the support appear to be the key points for high activity and selectivity.The substrate expansion experiments and stability test show that the catalyst has good universality,but the stability still needs to be improved.（2）The supported gold catalysts were successfully prepared with Au₂₅ nanoclusters as the precursor of gold,and different functional groups(VO₄^3-/F^-/Ce⁴⁺)substituted hydroxyapatite（HAP:Ca₅（PO₄）₃（OH））as the support.The effects of particle size,support and other factors on the one-step oxidation esterification of MAL were systematically studied on the gold catalysts.XRD and Raman results show that the catalysts have different structure characteristics of（replaced）hydroxyapatites.TEM,HRTEM results show that the gold particles in vanadium substituted hydroxyapatite（HVP）has the smallest average particle size（2.8 nm）and distribution.XPS and CO Drift results show that the valence state of Au was mainly Au⁰.But the Au₂₅/HVP possesses the strongest adsorption of CO.In oxidation esterification reaction of MAL and methanol,the Au₂₅/HVP catalyst showed the best activity and selectivity.What’s more,the adsorption of reactants and products on the above catalysts was further studied by in-situ infrared experiments.It was found that the C=O bond in MAL could be rapidly converted to MMA only on the Au₂₅/HVP catalyst,but the same phenomenon was not observed on other catalysts.The stability of Au₂₅/HVP catalyst was tested in fixed bed reactor under optimized conditions.It was found that the activity and selectivity of the catalyst did not change significantly after continuous testing for 150 h,which indicated the catalyst has good thermal stability.The above results provide a method for preparation of highly efficient and stable supported gold catalysts,supply a good way for the in-depth understanding of the effects of particle size and support effect on catalytic oxidation esterification reaction over the supported gold catalysts.