Synthesis And Catalytic Performance Of Polyoxovanadate-based Metal–organic Framework

Polyoxovanadates(POVs),as an important branch of polyoxometalates(POMs),have been lagging behind molybdenum-and tungsten-containing POMs in basic and applied research.Nevertheless,POVs have unique catalytic performance,which makes it is expected to become a novel catalyst to replace noble metals in some oxidative catalytic reactions.In this paper,using imidazole derivatives as organic ligands,a series of POVs-based MOFs were synthesized by one-step hydrothermal method.According to the structure and composition characteristics of these compounds,the catalytic performances of these compounds in different organic reactions were studied,and plausible catalytic reaction mechanisms were proposed based on the experimental results.1.A mixed-valence {V10} cluster-based Cu ^I–MOF,[Cu^I(bbi)]₂{[Cu^I(bbi)]₂V₂^IVV₈^VO₂₆}·2H₂O(1),was facilely synthesized using flexible diimidazole ligand bbi(bbi=1,1'-(1,4-butanediyl)bis(imidazole)).Using 1 as a heterogeneous catalyst without any co-catalyst,one-step oxidative cleavage of?-O-4 lignin into high value-added aromatics with high catalytic activity and selectivity was realized under an oxygen atmosphere.No obvious decrease in activity was observed after five cycles,which indicates the excellent stability of 1.The perfect catalytic performance of 1 can be attributed to the multi-site synergistic effect of the mixed-valence V^V–O–V^IV sites on polyoxovanadate for the oxidation of?-O-4 alcohol to?-O-4 ketone and the Cu ^I sites on the framework for the rapid cleavage of the C?–C?bond of?-O-4 ketone.To the best of our knowledge,this system represented the first co-catalyst-free example for the one-step selective degradation of lignin catalyzed by a well-defined crystalline catalyst with definite composition and structure in a single solvent.2.Three POVs-based MOFs,[Ni(bib)2]{V2O6}(2),[Co(bib)]{V2O6}(3)and[Ni(en)(bib)]{V2O6}·2H2O(4,en=ethylenediamine),were facilely prepared using rigid diimidazole ligand bib(bib=1,4-bis(1H-imidazoly-1-yl)benzene).Although all three compounds contain V sites with oxidative catalytic activity,they exhibit very different catalytic activity in the oxidation detoxification experiment of the mustard gas simulant(2-chloroethyl ethyl sulfide,CEES)due to the difference in the coordination environment of transition metal ions(Co²⁺and Ni²⁺)in the framework.3 shows the highest catalytic activity,it converts 100%of CEES to no-toxic 2-chloroethyl ethyl sulfoxide(CEESO)within 10 min,whereras 2 and 4 only convert 48.4%and 47.5%of CEES under indentical conditions,respectively.This can be explained by the fact that the Co²⁺ion in 3 presents a{Co O2N2}distorted tetrahedral configuration with coordinatively unsaturated sites,that is,a coordinatively unsaturated metal center;while Ni²⁺ions in 2 and 4 adopt{Ni O2N4}octahedral configuration,and the metal centers are coordinatively saturated.It is well known that the coordinatively unsaturated metal centers can usually be used as active sites for adsorption and activation of substrate molecules in catalytic reactions.The coordinatively unsaturated Co center in 3 coordinates with the S atom in CEES to activate CEES and shortens the molecular size distance between CEES and the V site with oxidation catalytic activity,thereby greatly improving the rate of the catalytic reaction.3.A{V2O6}-based Zn ^II–MOF,[Zn(bix)]{V2O6}(5),was successfully synthesized using semi-rigid diimidazole ligand bix(bix=1,4-bis(imidazol-1-ylmethyl)benzene).Single crystal X-ray analysis reveals that 5 contains rich Lewis base({V₂O₆}_n^2n-)and acid(Zn²⁺)sites.Considering that Lewis acid and base can usually be utilized as active centers for catalytic reactions,we explore catalytic activity of 5 for Knoevenagel condensation and cyanosilylation of aromatic aldehydes.The experimental results indicate that 5 can efficiently and selectively catalyze both reactions of the Knoevenagel condensation and cyanosilylation of aromatic aldehydes under solvent-free conditions,which is attributed to two-site synergetic effect:(1)the open zinc site as Lewis acid catalyst can electrophilically activate the carbonyl group of aldehydes;(2)the{V2O6}n^2n-polyanion as Lewis base catalyst to activate methylene or cyanating reagents through nucleophilic reaction.In addition,5 exhibits good sustainability,no obvious decrease in catalytic activity is observed in both catalytic reactions after ten cycles.4.A{V4}cluster-based copper(I)–organic framework,[Cu ^I(bib)]4{V^V4O12}(6),is facilely synthesized under mild hydrothermal conditions.The structure of 6 is constructed from a cyclic{V₄O₁₂}^4-polyanion cluster and a 1D chain Cu^I–MOF([Cu ^I(bib)]⁺).The presence of the{V4O12}^4-cluster stabilizes the Cu^I–MOF with Cu ^I as the center,thereby improving the stability of 6 and enabling it to stably exist in various solvents and p H=2–12solutions.Additionally,6 as a heterogeneous catalyst can catalyze the carboxylative cyclization of CO2 and propargylic alcohols to high value-added?-alkylidene cyclic carbonates at room temperature,and the conversion and selectivity are almost 100%.And 6shows good sustainability,no obvious decrease in the yield of the?-alkylidene cyclic carbonate is observed after ten cycles.