Design Of MOF/CeO₂ Loaded Noble Metal Tandem Catalysts And Their Performance Study In Liquid Phase Hydrogenation Reactions

Tandem catalysis is a catalytic reaction in which reactants are converted into target products by two or more catalytic processes in a reactor.It simplifies or even eliminates the separation,purification,storage and transportation of intermediates as well as the consequent loss of time and materials,with higher efficiency,greater economic benefits and more environmentally friendly.In terms of reaction kinetics,tandem catalysis can promote the first reaction by in-situ consumption of its products as the reactants of the next reaction according to Le Chatelier’s law.The tandem catalysis changes the reaction path through the unstable intermediate and makes its thermodynamics more favorable.Heterogeneous tandem catalysts have more advantages in industrial production due to their easy separation from catalytic systems.Nanostructured materials have unique structural characteristics and their own physical and chemical properties,especially in heterogeneous tandem catalytic reactions.In this paper,the effects of porous/hollow nanostructure,crystal plane effect of support and acid property of metal organic framework（MOF）on the liquid phase tandem reaction were studied respectively,and a reasonable explanation of catalytic performance was proposed based on series of characterizations.1.CeO₂nanotube/Pd@MIL-53（Al）sandwich catalyst synthesis and their tandem catalysts for hydrodehydrogenation of ammonoborane with phenyacetyleneHow to prevent the aggregation and leaching of active metal nanoparticles（e.g.,Pd）while achieving high conversion and selectivity under mild conditions has always been a focus of scientists in liquid phase catalytic reactions.To address these problems,we have prepared CeO₂nanotube/Pd@MIL-53（Al）sandwich-structured catalysts.The stability of Pd particles is greatly improved due to MOF shell protection.Moreover,due to the promotional effect of CeO₂and enrichment/size-sieving effect of MIL-53（Al）,the CeO₂nanotube/Pd@MIL-53（Al）catalyst exhibited a higher catalytic performance in terms of conversion and selectivity than its CeO₂nanotube/Pd and Pd/MIL-53（Al）counterparts.More importantly,because the hydrogen for the reaction with phenylacetylene is generated in situ from the dehydrogenation of ammonia borane,the gas/solid/liquid phase contact and related mass transfer are more favorable.As a result,100%conversion and high selectivity（96.2%）were achieved within a short time（1 min）,even at room temperature and atmospheric pressure.2.Effect of CeO₂morphology on CeO₂/Pd catalytic hydrogenation of ammonia borane and phenylacetylene in tandem hydrogenationPd nanoparticles were loaded on different morphology of CeO₂（cube,rod and octahedral）for the study crystal plane effect on the catalytic perfromance towards the tandem reaction between the dehydrogenation of ammonia borane and benzene acetylene selective hydrogenation.The catalysts with different morphologies were characterized by a variety of catalytic characterization tools,and ammonia borane hydrolysis,phenylacetylene hydrogenation and their tandem catalytic reactions were fully studied and compared.The results demonstrated that the catalytic performances are highly dependent on the crystal plane of the CeO₂support.Combined with the results of different catalytic reactions,the structure-activity relationships between individual catalytic reaction and multiple catalytic reactions（tandem reactions）were clarified.3.Effect of acidic property of MOF on hydrogenation activity of furfural acetone in tandem condensationConsidering the effect of catalyst acidity on catalytic performance,we assemble MOF with different functional groups by changing the types of organic ligands and inorganic nodes,and then regulate the acid sites of catalysts,in order to explore the acidic effect on the tandem catalytic performance.Specifically,we have designed and synthesized UiO-66-X,with different functional groups（ligands are terephthalic acid,2-amino-terephthalic acid and2-nitro-terephthalic acid）.Finally,Pd nanoparticles were supported onto the MOF,and the Pd@UiO-66-X（X=H,NH₂,NO₂）were evaluated by furfural acetone condensation and4-（2-furan）-butene-2-one hydrogenation tandem reactions.The experimental results showed that Pd@UiO-66-X（X=H,NH₂,NO₂）showed different performance in furfural acetone condensation and 4-（2-furan）-butene-2-one hydrogenation tandem catalytic reactions,in which Pd@UiO-66-NH₂showed optimal catalytic activity and selectivity.