| Polyoxometalates?POMs?are a diverse family of anionic metal-oxygen clusters mainly constructed with the early transition metals at their highest oxidation states?W?,Mo?,V?,Nb?and Ta??.POMs exhibit an overwhelming diversity in component and structure with outstanding and tunable physicochemical properties,which have been applied in many fields such as catalysis,drug discovery,biomedicine and material science.POMs exhibit fast reversible multielectron redox transformations under mild conditions,which can be used as homogeneous and heterogeneous catalysts for various oxidation reactions,such as water oxidation,olefins epoxidation and oxidative desulfurization,etc.Normally,POMs possess high activity as homogenous catalysts,but suffer from the difficulty in catalyst separation and reusability.For this reason,the recycling of POMs can be realized by modifying them to be heterogeneous,nonetheless,POMs-based heterogeneous catalysts are still limited by low catalytic activity and slow mass transfer.At present,more attention has been paid to the temperature-controlled POMs-based catalysts which are homogeneous in the reaction process and become heterogeneous after the reaction.In addition,many POMs are very unstable and undergo rapid and complex hydrolytic equilibria especially in aqueous systems,which makes it difficult to study the catalytic active centers and interaction mechanism in the catalytic reactions.Based on the above analysis,a temperature-controlled POM-based catalyst was designed and synthesized by immobilization method,which was successfully applied in catalytic oxidation desulfurization?ODS?.In order to give an insight into the stability of POMs in aqueous solution,we applied capillary electrophoresis?CE?to study the solution stability and hydrolytic equilibria of pentamolybdobis?phenylphosphonate?,[?C6H5PO3?2Mo5O15]4–,which is a Strandberg-type of POMs.The research was carried out as follows:1.Intelligenttemperature-controlledPOM-basedcatalyst[C16H33N?CH3?3]3[PO4[WO?O2?2]4]/poly?N-isopropylacrylamide?(abbreviatedas C16PW?O2?2/PNIPAM)hasbeenpreparedusingthermo-responsivepolymer Poly?N-isopropylacrylamide??PNIPAM?asasupportandusing[C16H33N?CH3?3]3[PO4[WO?O2?2]4](abbreviated as C16PW?O2?2)as the catalytic center.The thermo-responsive hybrid exhibits novel switchable property based on the change of temperature,while its solubility in reaction medium is reversibly controllable through an external temperature stimulus linking the gap between heterogeneous catalysis and homogeneous one.This catalyst has been successfully used in catalyzing the oxidation of refractory sulfur-containing compound dibenzothiophene?DBT?into its corresponding sulfone with 100%efficiency in the presence of H2O2.Besides,this catalyst can be reused.Before reaction,the temperature-controlled catalyst was heterogeneous.The polymer carrier turned to be homogeneous with the increase of reaction temperature.Then,non-polar organic substrates?DBT?were accumulated around the catalytic sites by two synergistic effects of amphiphilic POM molecules and the existence of PNIPAM and oxidized to be sulfone?DBTO2?by H2O2.When the reaction was finished,the catalyst could adsorb sulfone?DBTO2?and precipitate from the reaction solution with the decrease of reaction temperature.Thus,the temperature-controlled POM-based catalyst C16PW?O2?2/PNIPAM has the advantages of both homogeneous and heterogeneous catalyst,can not only be recovered and reused,but also play the role of extractant,which can achieve high efficiency,green and economical ultra-deep desulfurization process.2.In this study,we applied CE to investigate the solution stability and hydrolytic equilibria of[?C6H5PO3?2Mo5O15]4–,which is a Strandberg-type of POMs.According to the results of CE electropherogram,we found that[?C6H5PO3?2Mo5O15]4–is unstable in aqueous solution and undergo rapid hydrolytic equilibria,which leads to the formation of three components,that is,[?C6H5PO3?2Mo5O15]4–,[Mo7O24]6–and[Mo8O26]4–.Besides,we investigated different experimental conditions including sample concentration,counterion,and solution pH that may affect[?C6H5PO3?2Mo5O15]4–equilibria.The results showed that the equilibria of[?C6H5PO3?2Mo5O15]4–will move if the experimental conditions changed.Moreover,the POMs anion in the solution can interact with the charged amino acid,which leads to the shift of the hydrolytic equilibria of[?C6H5PO3?2Mo5O15]4–.The results proved that CE has great potential as a simple,fast,economical,and efficient method to monitor the synthesis,purity,stability,and hydrolytic equilibria of POMs. |
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