Study On The Activity Of Heterogeneous Polyoxometalates In Oxidative Reaction Over H₂O₂

As a green oxidant,H₂O₂ is cheap,easily available,and water is the only by-product,which is widely used in the oxidation of sulfides and olefins.Mustard gas andα-pinene are two representative compounds in oxidation reaction.In view of the competitive reactions in the catalytic conversion of H₂O₂（including the disproportionation of H₂O₂ into O₂ and H₂O,the generation of·OH by H₂O₂,the transfer of peroxy bonds and epoxidation reaction,etc.）,it is of great significance to catalyze H₂O₂ with high selectivity to realize the conversion of organic substrates.We designed and synthesized functional polyoxometalate（POMs）heterogeneous,took the degradation of mustard gas simulants（2-Chloroethyl ethyl sulfide,CEES）and epoxidation ofα-pinene as the research objects in H₂O₂ system catalysts and carried out the following research:1.The phase transfer catalyst（CTA）₃PW₄O₂₄(CTA=C₁₆H₃₃（CH₃）₃N⁺)was synthesized,and its catalytic oxidation activity was investigated in H₂O₂ system.Under the reaction conditions of 2 m L of methanol,31μL of CEES,25 mg of catalyst,26μL 30%of H₂O₂,and40 min at room temperature,the conversion of CEES was 84.8%and the selectivity of 2-Chloroethyl ethyl sulfoxide（CEESO）was 99.7%.Under the reaction conditions of 10 m L of ethyl acetate,0.1 g of catalyst,310μL 30%of H₂O₂,and 60℃for 120 min,the conversion ofα-pinene was 68.4%and the selectivity of epoxide was 79.8%.In order to improve the conversion ofα-pinene,the continuous process was used to improve the experiment,and the reaction was reacted for 300 min,the conversion ofα-pinene could reach～100%and the selectivity of epoxide was 82.2%.（CTA）₃PW₄O₂₄ showed good stability in two kinds of reactions,and could be reused with little change in activity.2.A series of surfactant（CTA）_xH_5-xPW₁₁Ni（x=1-5）were synthesized by adjusting the molar ratio of H₅PW₁₁Ni O₃₉ and CTA bromide).The structure and morphology were characterized by elemental analysis,IR,XRD,DR-UV and SEM,and its catalytic oxidation activity was investigated in H₂O₂ system.In the oxidation disinfection of CEES,CTAH₄PW₁₁Ni showed the highest reactivity among a series of composite catalysts,and CEES could be converted into CEESO by～100%at room temperature for 90 min.The high activity of the reaction mainly came from the adsorption of the catalyst to the substrate and the high redox of the catalyst.In the epoxidation ofα-pinene,（CTA）₃H₂PW₁₁Ni had low acidity,suitable redox property and high catalytic activity,and overcame the problem of difficult mass transfer in the reaction process.Reacting at 60℃for 120 min,the conversion ofα-pinene was 34.4%and the selectivity of epoxide was 80.5%.In the continuous epoxidation reaction ofα-pinene,the conversion ofα-pinene increased to 97.8%and the selectivity of epoxide was 81.7%after 420min.In the reaction,H₂O₂ reacted with PW₁₁Ni to form a peroxide active intermediate,which promoted the reaction.The composite catalyst had good stability,the activity and selectivity of the catalyst could not be significantly reduced after being reused for ten times in two kinds of reactions.3.Using EB-COF（Ethidium bromide-Covalent Organic Framework）as a two-dimensional covalent organic framework material,PW₁₁Ni@EB-COF（n=5,10 and 15wt%）(PW₁₁Ni=H₅PW₁₁Ni O₃₉)composite catalysts were prepared by ion exchange method,and its structure was characterized by elemental analysis,IR,XRD,¹³C NMR and SEM,which were used to determine structure and morphology of the compounds.It was found that PW₁₁Ni@EB-COF（15）showed the best activity in the degradation of CEES and epoxidation ofα-pinene.Using H₂O₂ as oxidant,the complete conversion of CEES could be transformed in 70 min,and the only nontoxic CEESO could be produced.Theπ-πconjugate structure of EB-COF was beneficial to enrich CEES molecules,made CEES adsorb around the active site and promoted the reaction.For the epoxidation ofα-pinene at 60℃for 180 min,the conversion ofα-pinene was 38.2%and the epoxide selectivity was 87.7%.EB-COF contained basic imine groups,which inhibited the ring opening of epoxide and was beneficial to the improvement of epoxide selectivity.In the continuous epoxidation reaction ofα-pinene,the conversion ofα-pinene increased to 96.7%and the selectivity of epoxide was 88.2%after 480 min.Moreover,the catalyst showed good stability and heterogeneity in both kinds of reactions,and the activity of the catalyst did not decrease obviously after being reused for 10 times.