Deep Eutectic Solvents Assistant Synthesis Of Vanadium Phosphorus Oxide Catalyst

Deep eutectic solvents?DESs?as a new type of designable green solvent exhibits the excellent performance in the field of catalysis,separation,adsorption and the synthesis of material,especially in the structure regulation of catalysts.The selective oxidation of n-butane to maleic anhydride?MA?is a typical representative of selective oxidation reaction of low carbon alkanes.And vanadium phosphate?VPO?catalyst is the most effective catalyst for this reaction at present.However,the catalyst still suffer from many problems in terms of the poor crystallization,high impurity phase,low specific surface area and serious pollution etc.All these cause the low conversion of n-butane and poor selectivity of MA.In this thesis,a series of hydroxy-rich bio-based DESs were designed and developed to optimize and overcome the above-mentioned problems,and the physicochemical properties of different DESs were explored.The new VPO catalysts with high activity and selectivity were obtained by using DESs as auxiliary agents.Meanwhile,the influences of DESs in the structure and surface characteristics of VPO catalysts were investigated.Furthermore,the relationships between the DESs type,amount of DESs,physicochemical characteristics of the catalyst and catalytic activity of the catalyst were established,respectively.The works are primarily summarized as follows:?1?A series of bio-based alcohols DESs were designed and synthesized by using choline chloride?Ch Cl?as hydrogen bond acceptor and polyols?Ethylene glycol?EG?,1,4-Butanediol?BDO?,Glycerol?GL?,and Xylitol?XYL??as hydrogen bond donor?HBD?.The study of DESs physicochemical properties shows that the reduction of the freezing point of alcohol DESs was positively correlated with the number of hydroxyl groups in HBD.This may be ascribed to the fact that the cohesion of DESs strengthened with the number of hydroxyl groups increased,which affected the lattice energy of DESs.The viscosity of DESs gradually increases as the DESs hydroxyl groups number increases.This was owing to the decreases of cavity size of DESs decreases with the enhancement of DESs hydrogen bond force,which was inconducive to the movement of molecules/ions.In addition,the viscosity and density of DESs were negatively correlated with temperature.?2?The above DESs were used as promoters to regulate the synthesis process of VPO catalyst.The results show that DESs as an inducer promote the growth of the active crystal plane?001?and?020?,which lead to the significant increase for the relative intensity of the active crystal plane?001?/?020?.The nucleation and growth of precursors were affected by the interactions between the rich-hydrogen bond networks of DESs with vanadium?V?.Correspondingly,the morphologies of precursors were changed from rose-like cluster to a stacked structure of two-dimensional nanosheets.DESs played the role of"shape memory"promoter during the precursors topologically transformed into the catalysts to obtain platy-like and porous catalysts with high crystallinity.Furthermore,DESs affected the formation of heterogeneous VOPO₄,thereby reduced the average valence of V,and increasing the P/V ratio on the surface of the catalysts.?3?Combined with the performances of the catalysts,the strengthening mechanism of DESs was studied and the structural relationship was analyzed.The results shown that the synergistic effect between the hydrogen bond acceptor and hydrogen bond cause an important impact on the conversion of n-butane and the MA selectivity.The performances of VPO catalysts were significantly improved with the addition of polyols DESs.Especially,for the catalysts regulated by Ch Cl-BDO DES,the MA selectivity could reach up to 66.05%under the n-butane conversion of 92.28%,the yield of MA was increased by 14.78%by compared with that of VPO-Blank.The strengthening process by DESs makes the VPO catalyst overcome the“trade off”effect of the n-butane conversion and MA selectivity.The catalyst structure and catalytic performance are correlated.The results show that the conversion of n-butane is positively correlated with the I_?020?/I_?204?relative intensity of the VPO catalyst,while the selectivity of MA is negatively correlated with the average valence state of V on the catalyst surface.