Novel solid acid materials based on polyoxometalates

Strong solid acids have found various applications as environmentally benign heterogeneous acid catalysts for chemical and petrochemical processes, proton conductive membranes in fuel cells, selective adsorbents of basic impurities in separation and purification technologies, etc. The efficiency of solid acids in such applications depends primarily on their acid strength and thermal stability. The choice of effective solid acids possessing strong acid sites as well as high stability is limited, hence strong demand for such materials in many fields of application. Following promising preliminary experiments, we propose a collaborative programme bringing together experts in heterogeneous catalysis, materials chemistry and solid-state NMR to develop the capability for the synthesis of novel functionalised materials possessing strong acidity and high thermal stability based on polyoxometalates. The acidic forms of typical polyoxometalates, known as heteropoly acids, are very strong protonic acids, however their stability is relatively low. The proposed programme aims at the preparation of solid acid materials by a transformation of multicomponent oxidic systems. Preliminary experiments indicate the formation of a highly stable and strongly acidic two-dimensional heteropoly acid phase in such systems, which could potentially be effective in catalytic and other applications. In the proposed work, the fundamental blending and mounting procedures will be used to prepare the acidic materials. The structure of materials thus made will be thoroughly characterised at different levels. Structural understanding will be essential for the elaboration of the general strategy towards the synthesis of novel materials. We also wish to establish the relationship between the structural features of these materials and their acidity, catalytic activity and ionic conductivity. The catalytic performance of materials will be studied in detail using model reactions in comparison with standard solid acid catalysts. The materials will also be tested in a range of acid-catalysed reactions of commercial interest. The novel strong solid acids derived from multicomponent oxidic systems will have important advantages over the conventional solid acids (mixed oxides, zeolites) and typical heteropoly acids. They will have stronger acidity compared to mixed oxides and zeolites. As compared to the typical soluble heteropoly acids, they will have higher thermal stability and stability towards leaching into liquid phase, allowing effective use of these materials as regenerable green solid acid catalysts, conductive membranes and reusable selective adsorbents in polar solvents.