Microwave-Assisted Synthesis Of Al-Porphyrinic Metal-Organic Framework For Photocatalytic Hydrogen Production

Metal-organic frameworks(MOFs),featuring large specific surface area,high porosity and outstanding tunability,are promising in catalysis.MOFs are able to catalytically drive many important reactions,including organic transformation,photocatalytic and electrocatalytic reactions.As most of the above reactions take place in liquid-phase,the respective challenges of homogeneous or heterogeneous catalysts should be considered.Owing to the excellent dispersibility in liquid-phase,homogeneous catalysts can well expose active sites to interact with substrates,which leads to higher activity.However,they are usually not easy to be recycled.As a class of heterogeneous catalysts,MOFs are readily recyclable,but the bulky MOF powders with large particle size synthesized by conventional hydrothermal route suffer from poor dispersity,ease of sedimentation and less exposed active sites in liquid-phase reactions,which may result in unsatisfied activity.Fortunately,due to their designable structures and tunable functionalities,MOFs can act as a platform to integrate the strengths of both homogeneous and heterogeneous catalysts.Till now,there are mainly two strategies for the integration:one is using MOFs as a support to realize the heterogenization of homogeneous active molecules,and the other is synthesizing soluble MOFs for the homogenization of heterogeneous MOF catalysts.The former has been well established,but the latter is rarely reported.Microwave-assisted synthesis is a newly developed synthetic method in the last decade,the principle of which is heating the reactants under microwave irradiation.Because microwave can directly interact with molecules,microwave heating is much more efficient and faster than the heating of external source.Microwave-assisted synthesis can effectively accelerate the nucleation and growth of MOFs,which cannot be achieved by conventional hydrothermal method.In view of the above,we have successfully synthesized a soluble porphyrinic MOF,denoted as S-Al-PMOF,via microwave-assisted synthetic approach.The as-synthesized S-Al-PMOF can completely dissolve in acetonitrile to give a clear and transparent colloidal solution,which can remain clear without any aggregation after standing for one month.However,the Bulk-Al-PMOF synthesized by conventional hydrothermal route can only be dispersed in solvent to form a turbid suspension liquid with sedimentation in a short time after standing.Furthermore,S-Al-PMOF can easily be separated from solution by suction filtration and then re-dissolved in acetonitrile.This separation and re-dissolution process can be repeated several times to prove its good recovery and recycling.To demonstrate the superiority of S-Al-PMOF in liquid-phase catalytic reaction,photocatalysis water splitting for H2 generation has been adopted to test the activity of both S-Al-PMOF and Bulk-Al-PMOF.As a result,S-Al-PMOF shows much higher activity than Bulk-Al-PMOF,and the subsequent photoelectrochemical characterization analysis further verifies the advantages of S-Al-PMOF in liquid-phase photocatalysis.Moreover,the recyclability of S-Al-PMOF is examined by the filtration and re-dissolution process.In the consecutive three cycles of reaction,the activity of S-Al-PMOF can be well maintained.Experimental results have shown that S-Al-PMOF has the strengths of both homogeneous and heterogeneous catalysts.We believe that the successful synthesis of soluble Al-PMOF opens a new avenue to the homogenization of heterogeneous catalysts.