Construction And Surface Regulation Of Vanadium Based Catalysts For Selective Oxidation Of 2-methylnaphthalene To Naphthaldehyde

As an important fine chemical product,2-naphthaldehyde(2-NA)is widely used in the fields of medicine,perfume,pesticide and so on.The gas phase catalytic oxidation of 2-methylnaphthalene(2-MN)to 2-NA is a continuous,green and economical production route.However,the oxidation path of 2-MN is a complex and strong exothermic process.How to control the catalytic oxidation process to improve the selectivity and yield of 2-NA is the key to the development of catalysts.In this paper,based on the previous research of our group and from the perspective of vanadium-based catalyst modification,the modified vanadium-based catalysts were systematically studied to screen out the highly efficient selective oxidation catalysts and ultimately to improve the 2-NA yield.The mechanism of gas phase oxidation of 2-MN as well as the catalyst structure and surface properties that affect this reaction were studied in depth.The process conditions were optimized and the intrinsic kinetic equations were established.The reaction paths of 2-MN over VOx with different polymerization degrees and the effects of metal doping on vanadium-based catalysts were investigated by density functional theory(DFT)calculations.The effects of supports TiO2,MgO,ZSM-5,NaY and Mordenite on the selective oxidation reaction of 2-MN were investigated under the condition of the same active component.Results showed that the strong interactions between TiO2 and ZSM-5 supports and active components could promote the production of monolayer amorphous active substances on the catalyst surface as well as enhance the synergy between different active components,which was beneficial to the selectivity of 2-NA.However,the weak interactions between NaY and Mordenite supports and the active components led to the appearance of surface oxide crystals.What is more,the weak interaction might result in the separation of the active components and further reduce the catalyst reducibility and surface oxygen vacancy density,which was unfavorable to the selectivity of 2-NA.The microporous structures in the three zeolites could inhibit the diffusion of reactants and intermediates with larger diameters.Strong acid sites of zeolites also had certain negative adsorption effects.The comprehensive analysis showed that TiO2 is a more suitable catalyst carrier in this catalyst system.On the basis of the TiO2 support,the modification of V/TiO2 catalyst was studied.V-Cu(1)/TiO2 catalyst showed excellent catalytic activity and 2-NA selectivity in a series of V-M/TiO2 catalysts prepared by introducing thirteen metal auxiliaries.Further studies showed that although Cu doping increased the dispersion of the active component to some extent and expanded the pore size of the catalyst,these structural changes were not the determinants of the catalytic performance.On the surface of the catalyst,VOx mainly existed in monolayer amorphous form.The terminal oxygen and bridging oxygen were the active sites for the oxidation of 2-MN.The introduced structural defects and enhanced oxygen mobility by promoter Cu accelerated the formation of nucleophilic oxygen and inhibited the direct reaction of electrophilic oxygen with aromatic ring,which had great contribution to the optimal catalytic performance of V-Cu/TiO2 catalyst,especially in 2-NA selectivity.On the other hand,the synergy between V,Cu and O through V-O-Cu bridging bond was inclined to improve weak acid ratio,redox ability and the reactivity of active sites.In-situ DRIFT and XPS characterizations had confirmed that the oxidation of 2-MN by molecular oxygen obeyed the Mars-van Krevelene mechanism and the reaction process involved the reduction of V5+and the reoxidation of V4+and V3+.In order to further reduce the reaction temperature of the reaction on the basis of maintaining the high yield of 2-NA,the studies were carried out by modifying V-Mo/TiO2 and V-Cu/TiO2 catalysts with promoters Ag,Ce,Co,Fe and Y.It was found that the doping of Co and Y could significantly increase the selectivity of 2-NA over the two kinds of catalysts,but at the same time,they had a strong inhibitory effect on the activity of the catalysts.While Ag enhanced the catalyst activity accompanied by different degree drop of 2-NA selectivity on both catalysts.Ce doping could limit the activity of O2-on V-Mo/TiO2 surface and block the reaction path of phthalic anhydride formation.Its synergistic effect with MoOx could maintain the high selectivity of 2-NA at high temperature.However,the co-doping of Ce and Cu was not conducive to this reaction.The doping of 7 wt.%Mo in V-Cu(1)/TiO2 could interact with the carrier and affect the surface VOx properties.At the same time,the right amount of Mo could also be used as an active component to improve the catalytic activity and low temperature selectivity.This V-Cu-Mo/TiO2 catalyst showed a highest yield of 2-NA at the reaction temperature of 380℃.Single factor and response surface experiments were carried out based on the selected V-Cu-Mo/TiO2 catalyst.The optimum reaction conditions were obtained as follows:reaction temperature 385℃,space velocity 11,000 h-1,feed flow rate 3.87×10-4 mol·h-1.The yield of 2-NA was 42.95%.The catalyst maintained high catalytic stability in the continuous 130 h temperature-variable stability experiment.On the basis of theoretical research and process optimization,the intrinsic kinetics of 2-MN gas phase oxidation was studied.The intrinsic dynamics models were established through experimental data fitting and parameter estimation.Residual analysis and model test showed that the dynamic models were suitable.The dehydrogenation of 2-MN and aldehyde formation processes over V2O5(001)and V/TiO2(001)surfaces were studied in detail by density functional theory(DFT)calculations.It was found that 2-MN was physically adsorbed on the surface of the two catalysts;the most feasible mechanism of methyl dehydrogenation was free radical mechanism.The O1 site of V2O5(001)surface was the most active.The dehydrogenation activation energies of O1 and O2 sites on the V/TiO2(001)surface had little difference,both could be regarded as reactive sites.The adsorbed methylene on the surface of both catalysts could form chemisorbed aldehydes by transferring an H atom to the adjacent O or OH sites.The energy barriers of this H transfer and the first C-H bond activation steps were both higher.Cu doping could promote the electron transfer in the catalyst and significantly reduce the energy barrier of C-H bond activation.