Synthesis Of MIL-based MOFs Catalysts And Application In Acid-base Tandem Reaction

The replacement of multi-step and high-cost synthesis reactions with efficient catalytic reactions pursuing atomic economy and the search for efficient,green,and economically sustainable synthesis methods have been one of the key problems in the field of chemical engineering.Compared to stepwise reactions,tandem reactions have attracted extensive attention because of their simplicity,efficiency,and decreased purification steps.As an important branch of tandem reactions,acid-base tandem reactions played a pivotal role in the development of drug synthesis,fine chemicals,and so on.However,the introduction of acidic and basic catalytic active sites on the same carrier and ensuring that the acid-base catalytic sites do not undergo neutralization reactions is still a hot research topic.Therefore,the synthesis of heterogeneous catalysts containing acid-base active sites or adjustable acid-base site ratios has been one of the important research areas for researchers.It is still a challenging task to avoid the neutralization of acid-base sites,achieve the spatial separation of acid-base sites,and regulate the amount of acid-base sites during the synthesis of acid-base bifunctional catalysts.Also,the synthesis of easily separable catalysts that are more suitable for industrial applications is a pressing issue.To address this problem,in this paper,we designed and synthesize heterogeneous acid-base catalysts with acid-base site separation and novel structures based on metal-organic frameworks（MOFs）,one kind of porous crystalline material,for the tandem Deacetalization-Knoevenagel（D-K）condensation reaction,and investigated the effects of MOFs-based acid-base catalysts with different composition structures and different morphologies on this tandem reaction.The IRA900（OH）-MIL-101（Al）-NH₂ composites were synthesized through an ion-exchange solvothermal method by using millimeter-scale IRA900（Cl）anion exchange resin as the carrier and loaded MIL-101（Al）-NH₂ nanocrystals and used to catalyze the acid-base tandem D-K condensation reaction.The as-prepared composites not only exhibited the spatially separated acid-base catalytic active sites in the MIL-101（Al）-NH₂backbone structure but also achieved a 1.67-fold increase in turnover frequency（turnover frequency,TOF）value,which promoted the D-K condensation reaction.Meanwhile,the catalysts maintained the millimeter size of the IRA900（Cl）macroporous resin,which can be rapidly separated from the reaction solution without additional separation means such as centrifugation,filtration,etc,which greatly saved labor costs and provided the possibility of industrial production applications.The heterostructure MIL-88B-on-Ui O-66-NH₂ materials with the regulated ratio of acid-base sites were synthesized by the epitaxial growth method,which contained both the backbone structural features and pore structure types of MIL-88B and Ui O-66-NH₂,overcoming the drawback of single MOFs materials.The catalyst MIL-88B-on-Ui O-66-NH₂-6（6 represents the reaction time during the synthesis process was 6 h）exhibited good catalytic activity in the acid-base tandem D-K condensation reaction,owing to the mesopore capacity as high as 2.02 cm³ g^-1,which was much higher than that of MIL-88B(0.67 cm³ g^-1)and Ui O-66-NH₂(0.62 cm³ g^-1).It was also shown that the highest catalytic reaction rate of 0.11 mol L^-1 h^-1 was achieved when the Fe/N mole ratio was 1:0.77.The example of heterostructure MIL-88B-on-Ui O-66-NH₂ materials in acid-base tandem reactions provided a new strategy for the MOFs-based materials in the field of catalysis.By adjusting the ratio of Al/Fe metal salts in the reaction system,a series of bimetallic MIL-101（Al/Fe）-NH₂ materials were successfully prepared by a one-step fast reflux method.The synergistic effect between bimetallic Al³⁺and Fe³⁺not only significantly accelerated the reaction rate of the first step of the acid-base tandem D-K reaction but also greatly increased the reaction rate of the second step of the Knoevenagel condensation reaction.The results showed that the bimetallic MIL-101（Al/Fe）-NH₂catalysts exhibited excellent catalytic activity in the acid-base tandem reaction with a yield of about 100%at 90^oC for 1.5 h.The catalytic activity was about 10%higher than that of single MIL-101（Al）-NH₂ and MIL-101（Fe）-NH₂.In addition,the catalysts still maintained good catalytic activity after five cycles.The synthesis of bimetallic MIL-101（Al/Fe）-NH₂ materials provide a reference for the application of bimetallic MOF materials in heterogeneous catalytic reactions.