快速氧离子传导功能的纳米储氧材料在异丁烷选择氧化反应中与催化剂间协同作用本质的研究

Efficient heterogeneous catalysts for selective oxidation of light alkanes are still of substantial importance in catalytic technology. The selective oxidation of isobutane to methacrylic acid is a reaction of high interest that therefore deserves research efforts due to the low cost of teh main raw material, the lower negative impacts on the environment and the lower amounts of co- or by-products compared to the traditional acetone-cyanohydrin route. However, the challenge remains the design and development of an efficient and stable catalyst.. The Keggin form of polyoxometalates (POMs) such as molybdophosphoric acid and its derivatives are widely acknowledged to be active and selective catalysts for the direct oxidation of isobutane. One further aspect is that POMs are more selective to methacrylic acid exclusively when the reaction is carried out under isobutane-rich conditions with selectivities to methacrylci acid and methaacrolein higher than 70%. However, the isobutane conversion is inevitably very low (in general,< 15%). The main reason is that under the isobutane-rich conditions, oxygen in gas phase is not enough to supply the lattice oxygen consumed during the reaction, and leads to a restructuring of the surface active center on the POMs and a partial destruction of the anions, which is detrimental to the catalytic activity and stability. In order to improve the conversion of isobutane and stability of the POMs catalysts, it is necessary to increase the capability of the catalysts for transporting lattice oxygen. One way to improve the efficiency of the POM catalysts is to search for synergetic effects between these compounds and other phases with a strong capability for storing oxygen and fast oxide-ions conductivity. In this manner, the lattice oxygen in the oxygen-storage materials can be readily transferred to the surface of the POMs catalyst, creating and/or regenerating efficient reoxidation centers capable of dioxygen dissociation and reincorporation of oxygen into the catalytic cycle, but keeping its favorable selectivity intact.. Fast oxide-ion conductors attract much interest due to their high mobility of oxygen ions. Especially, a new family of oxide-ion conductors, β-La2Mo2O9 material with nanostructure exhibits an ionic conductivity as high as 6×10-2 S cm-1 at about 400 oC. Because of its unique structure of possessing layers, through its channels, a ready supply of lattice oxygen to the POMs catalyst can be realized. . According to the above, it is prospected that β-La2Mo2O9 material with nanostructure can be responsible for optimizing the catalytic efficiency of the POMs-based catalytic site by facilitating the electron transfers and increasing the mobility of oxygen ions. The present task will focus on the way to rationalize the catalytic behavior of POMs catalysts in the isobutane oxidation by supplying of the lattice oxygen to the active centers. Another important objective of this project is to proceed a step forward in the analysis of phase cooperation in catalytic oxidation. More precisely, we shall attempt to show the transfer of lattice oxygen of β-La2Mo2O9-based nano-material and the reoxidation of active sites of POMs catalysts and further to explain phase cooperation. Moreover, much effort has been devoted to advancing our understanding of theses catalysts, including the activation of C-H bind and the insertion of lattice oxygen, the nature of phase cooperation and reaction mechanism, the functions of the active phases and the nature of active sites. And we expected that an excellent catalyst with the high activity, selectivity and stability could be developed.

低碳烷烃选择活化和氧化转化已被发达国家列为化学化工和催化发展的优先研究方向。本项目针对异丁烷选择氧化生成甲基丙烯酸过程中杂多酸盐催化剂在富烃体系下再氧化能力（即可逆储氧能力）差的缺点，从低碳烷烃选择氧化的共性问题出发，首次将两相协同作用机理应用于异丁烷选择氧化反应的富烃体系中，创新性地将具有低温快速晶格氧传递功能的纳米储氧材料作为辅助活性相引入到活性相磷钼基杂多酸盐催化剂体系中，利用纳米结构β-La2Mo2O9-基辅助活性相所特有的低温快速晶格氧传递功能来快速重构磷钼基杂多酸盐催化剂表面反应活性中心，提高反应过程中催化剂活性和稳定性。探讨低碳烷烃选择氧化反应过程中C-H键的选择性活化规律以及晶格氧的生成、两相传递、产物中晶格氧选择性插入的影响因素；加深对磷钼基杂多酸盐催化剂表面多功能活性中心位本质以及两相协同作用机理的认识：开发出高活性、高选择性和高稳定性的异丁烷选氧化反应的高效催化剂。

低碳烷烃选择活化和氧化转化已被发达国家列为化学化工和催化发展的优先研究方向。我国目前C4馏分中烷烃化工利用率<10%；而炼厂气C4馏分中异丁烷含量高，利用异丁烷选择氧化制甲基丙烯酸(MAA)不仅提高C4烷烃化工利用率，也可以实现MAA生产的变革性技术替代。.项目针对异丁烷选择氧化生成甲基丙烯酸催化剂研发，获得了下列主要研究结果和结论：.1、.首先考察了含钒前驱体对不同V掺杂量的磷钼酸铯盐催化剂异丁烷选择氧化性能的影响，结果表明，含钒前驱体种类明显影响催化剂的表面酸性、V物种的掺杂位置以及表面V4+/V5+和Mo5+/Mo6+比值（氧化还原性）。硫酸氧钒前驱体制备的催化剂，VO2+位于二级结构中导致催化剂表面酸性增加，同时表面适度的V4+/V5+和Mo5+/Mo6+比值（氧化还原性），使得该催化剂表现出最佳的异丁烷选择氧化性能。.2、.考察了不同过渡金属离子和VO2+离子共取代的磷钼酸铯盐的异丁烷选择氧化性能，研究发现，Cu2+离子与在二级结构中的VO2+离子之间存在协同相互作用，这种协同作用促进了Keggin结构中Mo6+与Cu2+之间的电荷转移以及催化剂在贫氧反应条件下的再氧化能力，从而大大提高了Cs2.0V0.3Cu0.2PMo12O40催化剂的异丁烷选择氧化活性和选择性。.3、.考察了不同类型载体负载的(NH4)3HPMo11VO40 (APMV)催化剂的异丁烷选择氧化性能，研究发现，当活性APMV负载在与其具有相同或相似Keggin结构的载体，如Cs3PMo12O40, Cs2.5H0.5PMo12O40, Cs4PMo11VO40 and Cs3HPMo11VO40上时，其异丁烷选择氧化活性、特别是MAA选择性远远高于负载在与活性APMV结构完全不同的载体上的催化剂性能。这是因为，活性组分与载体结构的高度匹配性有助于二者在界面处形成共格晶界（coherent boundary），这种共格晶面的形成有助于电子与活性晶格氧在界面处的传递，从而大大提高了选择氧化过程中晶格氧在异丁烷反应中间物中的氧插入能力，导致产物MAA的选择性大幅度提高。.上述研究结果和结论加深了对异丁烷选择氧化过程中C-H键的选择性活化规律以及晶格氧的生成、两相传递、产物中晶格氧选择性插入规律的认识，有助于开发出高活性高选择性和高稳定性的异丁烷选氧化反应的高效催化剂。