Design,Preparation And Catalytic Oxidation Performance Of Heteropolyacids-Based Heterogeneous Catalysts

Selective oxidation is an important class of reactions in organic chemistry.Oxidation products are widely used in food,flavors fragrances,biology,medicine and other fields.Numerous selective oxidation can be catalyzed by heteropolyacids?HPAs?due to their exceptional redox properties.However,HPAs suffer from the low surface area and high solubility in polar reaction media,with an unsatisfied activity in redox reactions.Thus,to achieve the highly active,conveniently recoverable and steadily reusable HPA-based catalysts,and to realize the liquid-solid biphasic catalysis,are crucial for the clean synthesis of fine organic chemicals.Because different selective oxidation reactions require the catalysts to have different redox abilities,how to adjust the redox ability of the HPAs becomes a difficult problem in the design of catalyst.Therefore,this paper is intended to prepare a class of highly reactive heteropolyacids-based heterogeneous catalysts that catalyze different selective oxidation reactions.In conclusion,the redox ability of heteropolyacids-based heterogeneous catalysts can be modulated by functional ionic liquids,and the redox capacity of them can also be changed by changing the intrinsic structure of HPAs through high-temperature carbonation.In this dissertation,the heteropolyacids-based heterogeneous catalysts from the molecular level is designed and prepared,and their catalytic performance of different selective oxidation reactions is studied.The specific research contents are as follows:?1?A series of heteropolyacids-based chitosan?CS?hybrids was prepared by the anion-exchange of a newly task-specific designed cross-linked CS ionic polymer with a Keggin H₃PW₁₂O₄₀.The catalyst was fully characterized by ¹HNMR,FT-IR,TG,SEM,ICP-OES and CHN elemental analysis.The obtained CS_1.5-BPA-PW hybrid can be used as highly efficient solid catalyst for H₂O₂-based epoxidation of alkenes and oxidation of alcohols,affording high activity.The unique cross-linked catalyst framework and the appropriate intramolecular electron interaction between amino groups and heteropolyanions play an important role in achieving efficient catalytic performance.Control experiment further demonstrated that the amino functional group can promote the epoxidation of olefins.Compared with other catalysts,the catalyst CS_1.5-BPA-PW has excellent catalytic properties.After repeated use of the catalyst for five times,no significant reduction in the efficiency of the catalyst was observed.?2?Since the ionic liquid with basic structural feature and novel physicochemical properties are ideal precursors for the preparation of porous N-doping carbon materials.The N-doped carbon material can promote the oxidation reaction.And high temperature carbonization will improves the stability of the catalyst.The V-doped molybdenum carbides?MoCV?encapsulated in porous N-doped carbon were developed through annealing phosphovanadomolybdate(PMo₁₀V₂O₄^5-)-paired cyano-functionalized imidazole ionic hybrid.The catalyst was fully characterized by ¹HNMR,FT-IR,TG-DTG,SEM,EDS,TEM,HTEM,XRD,XPS and Raman.The synthesized MoCV@CN_t?t stands for the annealing temperature:450 ^oC,600 ^oC,800 ^oC?showed a pomegranate-like structure with MoCV nanocrystals well-dispersed in the mesoporous N-doped carbon shell.The annealing temperature played an important role in the pore-structure,BET surface and chemical structure area of materials,which would further influence the catalytic performance of catalysts.MoCV@CN_t has a different catalytic activity in the oxidative coupling of benzylamine.MoCV@CN₈₀₀ exhibited high catalytic activity in the oxidative coupling of benzylamine,the conversion reached 99.9%at a mild reaction condition.The MoCV nanocrystals in MoCV@CN₈₀₀00 are the main active sites for the oxidative coupling of benzylamine reaction.Meanwhile,the mesoporous carbon material would expose a large quantity of active metal sites for the oxidative coupling of benzylamine reactions,and offer abundant channels for the mass transfer.After repeated use of the catalyst for five times,no significant reduction in the efficiency of the catalyst was observed.?3?Based on the above studies,the MoCV@CN₈₀₀ was obtained by annealing phosphovanadomolybdate(PMo₁₀V₂O₄^5-)-paired mononuclear imidazole ionic hybrid.The MoCV encapsulated in thin carbon material leads to exfoliation of the active center MoCV.Therefore,we report the molybdenum carbide??-Mo₂C?encapsulated in N-doped carbon as efficient and stable catalyst for oxidant-free dehydrogenation of alcohols,prepared by anion exchange of H₃PMo₁₂O₄₀?PMo?with ionic copolymer?DIM-AN?of dicationic imidazole ionic liquid and acrylonitrile,followed by a two-step carbonization at 400°C in air and800°C in Ar,respectively.The catalysts were characterized by ¹HNMR,FT-IR,TG-DTG,SEM,EDS,TEM,HTEM,XRD,XPS and Raman.It was found that different carbonization methods resulted in different carbonization products.In the two-step carbonization process,the organic polymer DIM-AN is transformed into a N-doped carbon material through continuous cross-link polymerization at 400°C in air,and then PMo is pyrolyzed at 800°C in Ar to generate?-Mo₂C encapsulated in N-doped carbon material.The synthesized catalyst(PMo@DIM-AN_400/800)exhibits outstanding catalytic activity and selectivity for dehydrogenation of various alcohols under oxidant-free conditions,and can be steadily reused at least five times for dehydrogenation of benzyl alcohol without changing its crystalline structure.The characterizations and comparison experiments results demonstrate that the active sites for dehydrogenation stem from the?-Mo₂C,as well as the abundant N atoms in the carbon shell,which contribute to the enhancement of catalytic performance.After repeated use of the catalyst for five times,no significant reduction in the efficiency of the catalyst was observed.