Synthesis And Catalytic Properties Of MOFs And Pd/MOF Composites For(De) Hydrogenation Reactions

With the development of modern industry,various important reactions have been explored.Dehydrogenation and hydrogenation reactions,acting as the vital parts,have attracted wide interests.Heterogenous catalysis takes an important part in the industrial chemistry,therefore,it is necessary to study the de/hydrogenation reactions over the heterogenous catalysts.Metal nanoparticles(MNPs)are widely applied as the active sites for the de/hydrogenation reactions,which is also urgent to generate an efficient surface for outstanding catalytic performance.However,the reactions are usually occurred under the high temperature and pressure resulting in lower selectivity.Thus,it is necessary to fabricate the MNPs catalysts with high catalytic performance under mild conditions.Metal-organic frameworks(MOFs)are known as the crystalline porous material featuring high surface area,tunability,and porosity,which also shows outstanding catalytic efficiency comparable to the homogeneous catalyst in de/hydrogenation reaction.Moreover,metal nanoparticles(MNPs)/MOF composite is also fabricated reasonably to regulate the MNPs catalytic properties.It can not only maintain the high activity of metal active sites but also inherit the properties of MOFs for the positive effects on catalytic performance,like the tailorability and the porosity.Herein,the efficient MOF catalytic materials and palladium-based/MOF composite catalytic materials are designed and synthesized for the important dehydrogenation and hydrogenation reactions.Moreover,the reaction mechanism is further studied to get insight into the structure-function relationship in the development of MOF based materials for the efficient de/hydrogenation reaction.The specific research contents and main results are as follows:(1)Cyclohexanol and cyclohexanone,known as the KA oil,both are the important feedstock for preparing the Nylon-6 and Nylon-66.Oxidation of cyclohexane is an efficient way to produce the KA oil.Herein,the mesoporous MOF with a pore size of 3.7 nm,PCN-222,is applied for the selective oxidation of cyclohexane,which includes the Fe active sites in the ligands.To improve the catalytic performance,three kinds of acid molecule,trifluoroacetic acid(F3),pentafluoropropionic acid(F5)and heptafluorobutyric acid(F7),are grafted on the cluster of PCN-222 via post-synthetic modification,respectively.Fortunately,the modified PCN-222-Fx showed an enhanced hydrophobic property with the increased molecular chains,in which the water contact angles increase from 51°to 137°.Compared with the parent materials,the PCN-222-Fx exhibited enhanced catalytic performance,among which PCN-222-F7 displayed the superior catalytic performance in cyclohexane dehydrogenation reaction with 50.2%conversion and 90.1%selectivity to KA oil.Moreover,all of the modified PCN-222-Fx displayed higher activity than the homogeneous catalysis.From the results of the liquid adsorption experiments,it indicates that the high catalytic performance is due to the enrichment of the cyclohexane around the Fe active sites.Importantly,the stability and recyclability of PCN-222-Fx can be maintained after 3 consecutive cycles.(2)Dehydrogenation or hydrogenation reactions are normally activated over the surface of metal nanoparticles(MNPs).Therefore,regulating the microenvironment around MNPs with good accessibility is important and urgent.Herein,the surface clean Pd NPs are introduced into the MIL-101-NH2 via the double-solvent method(DSM).After modificating the-NH2 group with different chain lengths of perfluorinated carboxylic acid,the hydrophobic environment around Pd NPs is created successfully.Through the hydrophobic treatment on the pore wall of MIL-101-NH2,the obtained Pd@MIL-101-Fx displayed superior dehydrogenation reaction of silane.Particularly,the Pd@MIL-101-F5 can complete the full conversion with 100%selectivity to silanol due to the enrichment of the substrates.Moreover,the selective hydrogenation of p-nitrchlorobenzene can also be accomplished to produce p-chloroaniline.All the modified Pd@MIL-101-Fx with good accessibility of Pd active sites displayed higher catalytic performance than the parent material.Moreover,the Pd@MIL-101-F3 showed the surperior selectivity(98%)to p-chloroaniline.According to the DRIFTS detection,it is certified that the high selectivity to p-chloroaniline is ascribed to the selective adsorption of-NO2 group rather than the-Cl group over Pd@MIL-101-F3.Importantly,the reaction activity has no apparent decrease even after 10 consecutive runs.(3)Selective hydrogenation of phenylacetylene is an important approach to produce the styrene monomer of polystyrene.Pd-catalyst is the commercial catalyst for the hydrogenation reaction.However,its selectivity tends to decrease in a prolonged time with the increasing conversion and produce the over-hydrogenation product.Herein,the CuPd@ZIF-8 composite is obtained via encapsulating the pre-synthesized CuPd nanocube into ZIF-8.The presence of Cu nanocube can not only generate the photo-thermal effects but also disperse the Pd active sites and regulate their electronic properties for high chemoselectivity.ZIF-8 shell is introduced to prevent the Cu nanocube from the further chemical etching,the steric hindrance generated from the pore size of ZIF-8 also contributes to the high regioselectivity.Surprisingly,the CuPd@ZIF-8 has displayed more than 96%selectivity to phenylethylene within 3 minutes under irradiation condition with NH3BH3 as the hydrogen source.Importantly,the selectivity was able to be maintained even after reacting for 8 hours.The DFT and 1H NMR results indicated that the polar H species were generated during the reaction in CH3OH solvent in the presence of NH3BH3 via the dissociation of B-H bond,which contributed to the constant high selectivity.Moreover,the tolerance of CuPd@ZIF-8 was certified to be outstanding in both the selective hydrogenation reaction of aromatic and aliphatic alkyne.With the steric hindrance of ZIF-8,the selective hydrogenation of middle alkyne is obstacled,resulting in the high regio-selectivity to alkynol in the hydrogenaiotn of phenylpropiolaldehyde.Fortunately,the catalytic performance of CuPd@ZIF-8 has no apparent decrease after 5 runs cycle and the structure is well maintained after reaction.