Synthesis Of Hierarchical VAPO Molecular Sieves By Dynamic Hydrothermal Method And Its Catalytic Mechanism For Aromatic Hydrocarbon Oxidation

Aluminum phosphate molecular sieves with hierarchical porous structure overcome the limitation of single pore,reduce the mass transfer resistance,and enable the reactant macromolecules to enter the pore,thereby improving the catalytic performance and expanding its application in the catalytic fields such as alkylation reaction,isomerization reaction and redox reaction.In this paper,a dynamic hydrothermal method was proposed to synthesize hierarchical D-VAPO molecular sieves.The effects of crystallization conditions,template agents and other factors on the synthesis of D-VPAO molecular sieves were investigated,and they were employed as catalysts for the oxidation of aromatic hydrocarbons to evaluate their catalytic performance.Subsequently,the mechanism of benzene to phenol over VAPO-5molecular sieve was studied by ONIOM method.Firstly,aluminum phosphate molecular sieves were prepared by single template and composite template in the fluorine-containing system where pseudo-boehmite,phosphoric acid and ammonium metavanadate was employed as aluminum,phosphorus and vanadium sources,respectively.The effects of dynamic crystallization conditions on the crystallinity and morphology of D-VAPO-5 molecular sieves were investigated,and they were used as catalysts for the preparation of phenol by benzene oxidation.The results showed that the catalytic performance of D-VAPO-5 synthesized with composite template（TEA and CTAB）in benzene oxidation was better than that of microporous W-VAPO-5synthesized with single template（TEA）.Compared with the static method,D-VAPO-5 synthesized by dynamic method has higher crystallinity and better catalytic performance.The optimum synthesis condition of D-VAPO-5 had been attained at crystallization temperature of 180°C,crystallization time of 6 h,dynamic rotation speed of 400 rpm respectively.In this case,the conversion of benzene was 27.7%,and the selectivity of phenol was 80.7%.Then,the hierarchical VAPO catalysts were synthesized by dynamic hydrothermal method using various organic amines as template agents to investigate the effects of template agents on the structure of the synthesized molecular sieves,and the catalytic properties of D-VAPO molecular sieves were evaluated by toluene oxidation reaction as a probe reaction.In addition,the effects of reaction conditions on toluene oxidation were also investigated.Characterization analysis showed that the samples with AFI structure,were synthesized with triethylamine and N,N-diisopropylethylamine as template.But if the template was replaced by dipropylamine,diisopropylamine and dibutylamine,the samples present AEL structure.The results showed that the catalytic performance of D-VAPO-5 in toluene oxidation was better than that of D-VAPO-11.The optimal settings were:m（acetonitrile）:m（toluene）of 6:1,n（hydrogen peroxide）:n（toluene）of 2:1,m（D-VAPO-5）:m（toluene）of 0.1:1,reaction temperature 60°C,reaction time 5 h.Under the condition,the conversion of toluene was 24.1%,the selectivity of benzaldehyde,benzyl alcohol and cresol were 32.1%,8.8%and 27.4%,respectively.Finally,the mechanism of benzene to phenol over VAPO-5molecular sieve was studied using ONIOM method.The transition state,intermediate and energy barrier in the reaction process were analyzed,and the effect of solvent acetonitrile on the benzene oxidation over VAPO-5 molecular sieve was investigated.The results show that one O in H₂O₂ forms a ternary ring with two C on the benzene ring,and then forms phenol in the absence of catalyst.The formation of phenol is a rate-determining step with an energy barrier of 296.8 kJ/mol.In the presence of VAPO-5,benzene oxidation to phenol is divided into four steps:the first step is the formation of V-OOH between H₂O₂ and V active site in VAPO-5;in the second step,OH in V-OOH was attacked with H at the proton acid site in VAPO-5 to form H₂O;the third step is the formation of phenol;the fourth step is the desorption of phenol from VAPO-5.The third step is the rate-determining step,and the energy barrier is 204.8 kJ/mol.Compared with the system without solvent,the four-step energy barriers of the system with solvent acetonitrile changed by-2.0 kJ/mol,+1.9 kJ/mol,-30.0 kJ/mol and-23.7 kJ/mol.The third step is still the rate-determining step,and the reaction energy barrier is 174.8kJ/mol.The addition of solvent acetonitrile reduced the activation energy barrier required for benzene oxidation.