The Study Of Base Catalysis Of Polyoxometalates

Polyoxometalates?POMs?,as metal–oxide nanoclusters,have attracted muchinterest in diverse fields such as magnetism,pharmacy,optics,electricity,especially in catalysis due to the controllable and variable structures along with high thermal and oxidative stability.POMs as the efficient catalysts have exhibited excellent catalytic performance in acid catalysis,oxidation catalysis,photocatalysis and electrocatalysis due to their well-defined structure and tunable redox behavior.However,there were few reports which deals on the basic catalytic performance of POMs.Herein,the investigation of DFT calculations and catalytic experiments was performed to compare the basicity of the various POMs.On the basis of the above results,the POMs,which showed the strong basicity,have exhibited superior catalysis for Knoevenagel Condensation,CO₂ cycloaddition and transesterification.1.Different POMs were employed as the catalysts for Knoevenagel Condensation.As it can be seen from the obtained results,the lacunary POTs(Na10[?-SiW9O34]·nH2O and K₁₂[?₂-P₂W₁₂O₄₈]·nH₂O)are much more efficient compared to plenary ones.By contrast,fully-occupied Lindqvist K₇HNb₆O₁₉·nH₂O?Nb₆?has shown better basic catalysis than the POTs,which is well consistent with the obvservation from DFT calculations.Besiders,efficient Nb₆ can be employed as a heterogeneous catalyst with good recyclability in the transformation.2.From the results of NBO calculation,Lindqvist K₈Ta₆O₁₉·nH₂O?Ta₆?exhibits superior basicity than Nb₆,which implied the better basic catalysis of Ta₆.In order to validate the theoretical results,a series of amphiphilic[(C_nH_2n+1)N?CH₃?₃]₈[M₆O₁₉]?n=14,16,18,M=Ta;n=16,M=Nb?was synthesized via combination of the POMs and surfactant,which were employed as the catalysts for CO2 cycloaddition.On the basis of the results,[(C₁₆H₃₃)N?CH₃?₃]₈[Ta₆O₁₉]presented excellent catalysis?over 85%yield of product?for the transformation along without co-catalysts under atmosphere conditions,which proved that[(C₁₆H₃₃)N?CH₃?₃]₈[Ta₆O₁₉]exhibited superior basicity.3.POMs with strong basicity were employed as the catalysts for the synthesis of dimethyl carbonate via alcoholysis of ethylene carbonate.Compared to the other alkalis including SiW₉ and organic amines,Lindqvist Nb₆ has shown superior catalysis but it is poor in stability.And K₃Na₂H₉?H₂en?₂[Fe₃Nb₂₅O₇₆]·17H₂O has exhibited superior catalysis and good stability in recyclability.