Study On The Catalysts For Selective Oxidation Of Isobutane To Methacrolein And Methacrylic Acid

Methacrolein?MAL?and methacrylic acid?MAA?are important chemical intermediates which are mostly produced through acetone-cyanohydrin?ACH?route.Though the yield of desired products is quite high,the utilization of highly toxic reactant HCN and disposal of large quantities of ammonium bisulfate waste are two main problems,considering the requirement of green chemistry.With a broad variety of feedstock,the selective oxidation of isobutane to methacrolein and methacrylic acid is considered as a simple and clean process.This route proves to be important and promising for the use of light alkanes and production of MAL and MAA.This paper mainly focuses on the catalysts for selective oxidation of isobutane to methacrolein and methacrylic acid.Mixed metal oxides and polyoxometalates?POMs?catalysts were prepared and characterized by FT-IR,TG-DTA,XRD and XPS techniques to investigate their physical and chemical properties.The results of characterization were related to the evaluation results on a fixed bed microreactor and optimal reaction conditions were obtained.The roles of acidity and redox properties were also investigated in isobutane oxidation.Besides,a detailed redox mechanism was proposed for isobutane oxidation over POM catalysts.WO₃-CoO catalyst was prepared by conventional co-precipitation method.Both the catalytic activity and selectivity to desired products of WO₃-CoO catalyst were superior to single metal oxide WO₃ or CoO,which was attributed to the effect of phase cooperation.However,the catalytic performance of WO₃-CoO was relatively poor compared with the POM catalysts.The thermal stability of CsFe_0.2 phosphomolybdic salt catalyst was relatively good and the active Keggin structure remained stable in the range of reaction temperature.Introduction of Cs could increase the surface area and enhance the redox property,leading to the improvement of catalytic activity.Moreover,Cs addition neutralized part of acidity of POM catalyst and moderate acidity was obtained,contributing to the formation of desired products.Besides,Fe substitution improved both the acidity and oxidizing ability of phosphomolybdic salt catalyst,which greatly enhanced the catalytic activity.Supported POM catalysts exhibited an obvious increase of surface area,but the decrease of acidity and oxidizing ability resulted in inferior catalytic performance than bulk POM catalysts.Partially reduced catalysts(containing Mo⁵⁺)were formed after calcination under N2 and isobutane atmospheres,and a larger extent of reduction was obtained for the isobutane calcined catalyst.It was demonstrated that the catalysts calcined under N2 atmosphere showed the highest isobutane conversion and yield of MAL and MAA?7.6%?.However,a worse product distribution was obtained over the catalyst calcined in isobutane.Hence,the appropriate amounts of Mo⁵⁺ facilitated redox reaction and were necessary for the enhancement of catalytic activity.The interaction between oxidizing sites of catalyst and C-H bond contributed to the activation of isobutane.Furthermore,the reduced Mo⁵⁺ formed during the activation of isobutane could easily be reoxidized by Fe3+ and the rapid transfer of charge greatly enhanced the redox reaction of active molybdenum species.