Application Of Multifunctional Catalyst Based On MOFs In Tandem Reaction

In the field of chemical industry,seeking green,efficient and economically sustainable synthetic methods has always been one of the core research issues in the chemical industry.Tandem reaction is a typical process-focused method.Two or more single-step reactions are concentrated in one reaction system and occur continuously.This method is commonly known as the one-pot method.After a series of reactions in the reaction system,the final required product was directly obtained,reducing energy consumption steps,such as the isolation and purification of intermediates,cost-effective and greatly shortening the reaction time.This method can be hopefully achieved using minimal energy consumption while maximize the output of time and space,which can achieve green,environmentally friendly and become one kind of sustainable reaction.However,in tandem reaction,the active sites required for each single-step reaction are often different.When these different type active sites exist in the same reaction system,chemical incompatibility may occur.At the same time,due to the occurrence of multiple reactions,the problem of mass transfer in the reaction is particularly important.This poses challenges for catalysts used in tandem reaction.Metal organic frameworks（MOFs）,as a kind of catalyst material with high specific surface area,uniform pore size distribution,high porosity,and easy modification,also with the excellent characteristics of supporting acid,alkali,and metal active sites,etc.,has great potential in tandem reaction.In view of this,combining MOF and tandem reaction,the following researches have been done in this paper:（1）Two MOFs materials（HSO₃-UiO-66（Zr） and NH₂-MIL-125（Ti））with different lattice parameters have been successfully used to build a pizza shape MOF-on-MOF heterogeneous structure by using the epitaxial growth method.This catalyst has both acidic sites（sulfonic acid group-HSO₃ on UiO-66）and basic sites（amino-NH₂ on MIL-125）.The presence of the structure directing agent polyvinylpyrrolidone（PVP）allows the acid-base sites to be independently distributed on two different MOF materials,avoiding acid-base neutralization leading to catalyst deactivation.Secondly,this pizza shape MOF-on-MOF heterogeneous structure instead of the core-shell structure has solved the mass transfer difficulties in tandem reaction,showing high efficiency catalytic performance with conversion and final products yields which are close to 100% in the catalytic Deacetylation-Knoevenage condensation tandem reaction.（2）Encouraged by the successful synthesis of acid-base dual funcational catalysts,we use ZIF-8 source nitrogen-doped porous carbon（N-ZDC）as the core and mesoporous silica（mSiO₂）as the shell of the core-shell structure to achieve precise spatial isolation and assembly of acid-base-Pd three different active sites.Among them,the acid sites are provided by phosphotungstic acid molecules（PTA）,which are located in the pores of the mesoporous silica shell.The base active sites acted by the species with N-H species,pyridine nitrogen and pyrrole nitrogen which are originated from ZIF-8 derived nitrogen doped porous carbon core material（N-ZDC）,and ultra-fine palladium nanoparticles（Pd）are in the microporous pore of N-ZDC.We named this structure Pd@N-ZDC@PTA-mSiO₂.The catalysts unique core-shell structure with hierarchical pore not only ensures the smooth progress of tandem reaction mass transfer,but also achieves successful spatial isolation of the three different types active sites while ensuring communication.The catalyst showed reaction conversion nearly up to100% and final product yield of 96% in the three-step tandem Deacetalization-Knoevenagel-Hydrogenation（D-K-H）reaction,and maintained good recycle stability with 95% conversion and 91% yield after five cycles.