Ammonium Modification Of Heteropolyacid Catalysts For Oxidation Of Methacrolein To Methacrylic Acid

Selective oxidation of methacrolein?MAL?to methacrylic acid?MAA?is one of the key steps leading to methyl methacrylate?MMA?in industry.Various catalyst systems have been developed,wherein the Keggin-type heteropoly catalysts composed of P,Mo,and V behave relatively well.It has been demonstrated that the type of counter-ions has an important impact on the morphology,structure,and properties of catalysts,thereby enhancing the reaction activity.In this regard,introducing NH₄⁺ into the Keggin-type heteropolyacid catalyst has a positive effect on the catalytic oxidation efficiency.However,systematic studies on the NH₄⁺-introduction modes and function mechanism of heteropolyacid in the catalytic performance of heteropolyacid are highly demanded.Our study was,therefore,devoted to find the relationship between the introduction modes of ammonium ion and the structure of heteropoly materials,such as crystal structure,surface morphology,acidity and redox property,thereby providing efficient strategies of modification.The catalytic behavior of the newly prepared materials in the oxidative conversion of MAL to MAA is also investigated.The contents could be categorized into three sections as the following:?1?A series of Cs?NH₄?_xH_3-xPMo₁₁VO₄₀ with different x value were synthesized.' The influence of loading amount of ammonium?x?on the structure of heteropoly materials was investigated.In addition,their catalytic performance in the selective oxidation of MAL to MAA was evaluated.Mechanistic studies suggest it helps to adjust not only the crystal structure,surface area and morphology of the heteropoly materials but also the amount of acid sites and redox properties on its surface by varying the x value.As a result,Cs?NH₄?_1.5H_1.5PMo₁₁VO₄₀?x = 1.5?provided the optimum reaction efficiency.A high conversion of MAL?80%?and selectivity to MAA?90%?can still be reached even after reaction for 100 h.?2?Two hybrid heteropoly catalysts,CTAB-CsH₃PMo₁₁VO₄₀?CTA-CsPAV?and Chitin-CsH₃PMo₁₁VO₄₀?Ch-CsPAV?,were prepared using cetyltimethyl ammonium bromide?CTAB?and chitin functioning as the modified precursor,respectively.Each of them has an uniform and well-defined nanoparticle structure and was successfully employed as the catalyst in the oxidative transformation of MAL to MAA.Specifically,Ch-CsPAV shows good stability during the long-term stability test,wherein a 80%conversion of MAL and a 90%selectivity to MAA was maintained for100 h.Mechanistic studies demonstrated that the formation of ammonium group upon calcination is promising no matter what kind of organic nitrogen precursor was introduced,which can thereby modify the redox property of heteropolyacid hybrids.The improved crystal structure,surficial morphology and catalytic performance were attributed to the linear sequential nitrogen atom in the organic molecules launched.In this regard,both CTAB and chitin can provide the optimized surface acidity,?3?A series of heteropolyacid hybrids of[C₈H₁₄N₂O₂?EMIM?-H₄PMo₁₁VO₄₀]were synthesized using imidazole-bsed ionic liquids bearing different anion?NO₃^-?AC^-?BF₄?as the modifying agent.The influence of ionic liquid's structure on the morphology,crystal structure,redox property and catalytic performance of the resulted hybrids was investigated.The optimized structure of EMIMAC-HPAV was obatined,which provided a high catalytic selectivity?98%?in the aerobic oxidation of MAL to MAA.As compared to[EMIM][NO₃]and[EMIM][BF₄],[EMIM][AC]shows relatively strong H-bond basicity,which is considered as the main reason for the high activity upon hybridization with HPAV.The positive effects of[EMIM][AC]on the HPAV hybrids were categized as the following:???the H-bonding interaction between EMIM⁺ and PAV^-provided a self-assembling structure of IL-HPAV precursor,which,thereby,leading to an uniform morphology of hybrids upon calcination;???the formation of ammonium group during the calcination was attributed to the potential charge transfer between EMIM⁺ and PAV^-;???The enhanced redox property of EMIMAC-HPAV hybrids was rationized by the formation of NH₄⁺ group and the high H-bond basicity of[EMIM][AC].