Synthesis And Catalysis Of Rigid Modulator-Induced Hierarchically Porous MOFs

Metal-organic frameworks(MOFs)are a class of crystalline porous materials composed of metal ions/clusters as nodes and organic linkers.Based on their advantages such as well-defined crystalline structure,highly tunable pore geometry and diverse components,MOFs have been developed rapidly in the past two decades and have been applied in many fields such as gas adsorption/separation,catalysis,etc.Traditionally,most of MOFs present intrinsic microporous characteristics,endowing them with unique features for small-molecule access,realizing molecular sieving and size-selective catalysis.However,the micropores are unfavorable to the diffusion of large molecules,thus hampering the application of MOFs involving large-size molecules.Although it is possible to elongate the length of bridging linkers and directly widen the corresponding micropores to mesopores,the synthesis of linkers precursors is usually very complicated and the subsequent synthesis of MOFs is also a challenge.In addition,the stability of MOFs decreases sharply with the prolongation of organic linkers.Therefore,a feasible approach is to generate additional mesopores/macropores in stable microporous MOFs,obtaining hierarchical MOFs(HP-MOFs).Currently,there are many methods to obtain HP-MOFs,including but not limited to soft/hard templates,template-free,ligand fragment,thermal fracture,laser photolysis,etc.However,most of these reported methods are uncontrollable and only applicable to specific MOF systems.Among them,the modulator-inducing method can overcome the mentioned problems.By using alkyl monocarboxylic acids as modulators in the growth process of MOFs,microporous MOFs can subsequently generate mesopores and the method presents the well generality.However,the size of introduced mesopores is limited by the small steric hindrance and weak coordination ability from the modulators.In addition,the length of the used alkyl modulators is limited within a length range,and the situation in which the linkers replace the pre-coordinated modulators on metal clusters should be avoided during the synthesis.In view of the above problems,it is necessary to develop a simple and universal method to obtain HP-MOFs with controllable and large-sized mesopores.Herein,we developed a simple and efficient rigid modulator-assisted strategy.By introducing a large number of rigid modulators with obvious steric hindrance in the MOFs synthesis process,the microporous MOFs synthesized by the modulators could be converted into the corresponding hierarchically porous MOFs with mesopores and the well stability by acid etching.The introduction of mesopores limitation in the modulator-induced method could be overcome.The resulting HP-MOFs can be encapsulated with large-size guest molecules and present the same stability as the original microporous MOFs.Taking hierarchical UiO-66-NH2(HP-UiO-66-NH2)as a representative,it presents an uncoordinated carboxyl group hanging on the skeleton.The trifluoromethanesulfonic acid group(OTf)can be modified to the Zr-oxo cluster to obtain HP-UiO-66-NH2-OTf.The resulting HP-UiO-66-NH2-OTf exhibits excellent catalytic activity and selectivity in multi-component deacetalization-acetalization tandem reaction due to the synergistic advantages of rich micropores and defective mesopores.