Post-Modification Of Two-Dimensional Imino-Based COFs And Their Photocatalytic Hydrolysis Of Ammonia Borane

Covalent organic frameworks(COFs)are a new class of crystalline porous materials assemblied by organic building units through covalent bonds connection.Due to their unprecedented combination of low density,large specific surface area,tunable pore size and structure,long-range order,rich variety of organic building units,and high stability,COFs have been widely used in the fields of gas adsorption and separation,chemical sensing,heterogeneous catalysis,proton conduction and drug delivery.In addition,COFs also have the following advantages:i)abundant organic building units render COFs with tunable bandgap;ii)the extended π-conjugated framework in COFs can improve the mobility of photogenerated carriers;iii)The tunable pores of COFs can facilitate the transport of reaction substrates.Therefore,COFs have been widely used in photocatalytic water splitting,photocatalytic CO2 reduction,and photocatalytic pollutant degradation.Moreover,COFs can be composited with other materials to form heterostructures through post-modification strategies.Furthermore,functional groups can be introduced into COFs to enhance the photocatalytic performance of COFs.Therefore,the structural characteristics of COFs can be used to prepare catalysts with specific properties and expand their application in the field of photocatalysis.It is well-known that ammonia borane(AB,NH3BH3)is a leading contender for chemical hydrogen storage mainly due to its high hydrogen content of 19.6 wt%.Besides,by virtue of its remarkable stability and high solubility in solution under ambient conditions,AB is an excellent transfer hydrogenation agent for various hydrogenation reactions.However,the existing photocatalysts for NH3BH3 hydrolysis are relatively simple.Therefore,it is necessary to develop new photocatalysts for NH3BH3 hydrolysis.COFs are ideal photocatalytic materials for designing novel NH3BH3 hydrolysis photocatalysts with high stability.This is very meaningful for broadening the application scope of COFs photocatalysts and realizing the large-scale utilization of hydrogen energy.In this paper,the main work is as follows:(1)Ti-MOF@TpTt hybrid coated with ultra-thin COF nanobelt,which was different from the fibrillar-like parent COF,was successfully synthesized through a sequential growth strategy.The as-obtained Pd decorated Ti-MOF@TpTt catalyst exhibited much higher photocatalytic performance than those of Ti-MOF,TpTt-COF,and Ti-MOF@TpTt hybrids with fibrillar-like COF shell for the photocatalytic cascade reactions of ammonia borane(AB)hydrolysis and nitroarenes hydrogenation.These can be attributed to its high BET surface area,core-shell structure,and type Ⅱheterojunction,which offered more accessible active sites and improved the separation efficiency of photo-generated carriers.(2)The amino groups in defective COF were converted to sulfonic acid groups through the post-modification strategy.The sulfonic acid sites can adsorb metal ions through electrostatic interaction,which is beneficial to the uniform dispersion of metal active sites.The COF-SO3--20-Pd prepared by post-modification has strong light absorption ability and high specific surface area,which improved the separation efficiency of photogenerated electrons and holes.In addition,the sulfonic acid group sites can enable Pd NPs to be highly dispersed on COF-SO3H-20,which effectively suppressed the agglomeration of Pd NPs during the photoreduction process and improved the utilization of metal nanoparticles.Therefore,the COF-SO3--20-Pd exhibited good catalytic activity and stability in the photocatalytic hydrolysis of ammonia borane reaction,which was better than the COF-NH2-20-Pd catalyst without post-modification treatment.