Hydrophobic Modification Of Pd/Metal-Organic Framework Nanocomposites For Enhanced Catalysis

Emerging as a relatively new class of porous materials,metal-organic frameworks(MOFs),possessing crystalline nature,tailorable structures as well as high surface area,have aroused wide research interest and shown potential multifunctional applications in many fields in recent years.Amongst them,MOFs are very suitable for catalysis,especially being used as supports or hosts for metal nanoparticles(NPs)to afford heterogeneous catalysts.In this thesis,we have successfully synthesized MOF-stabilized Pd/MOF nanocomposites,which can easily be hydrophobically modified to regulate the surface environment of catalysts.In addition,by using several types of MOFs as hard-templates and precursors,porous carbon materials as efficient metal-free catalysts with homogeneous distribution of N atoms have also been successfully synthesized.The main research results are as follows:1.There have been quite a few reports on metal nanoparticles(NPs)/MOF in recent years,almost all of which are focused on the stabilization of metal NPs by MOF for catalysis.However,the investigation of surface environment effect of active metal sites on the catalytic performance is extremely rare,and study on surface hydrophilic/hydrophobic modification to regulate catalytic activity/selectivity over metal NPs/MOF has not been reported thus far.In this work,surface hydrophobization modification of a MOF-stabilized Pd NPs,Pd/UiO-66,has been realized via a facile polydimethysiloxane(PDMS)coating.The resultant PDMS-coated Pd/UiO-66(denoted as Pd/UiO-66@PDMS)exhibits superior catalytic activity and recyclability to the pristine Pd/UiO-66.Moreover,Pd/UiO-66@PDMS possesses additional selectivity in sieving reactants with different wettability.More importantly,similar catalytic activity/selectivity/recyclability improvement can be achieved by coating PDMS for diverse Pd nanoparticulate catalysts,suggesting the generality of PDMS coating in catalysis.2.Heterogeneous catalysis is well-known to be of paramount importance in many areas including fundamental and industrial chemistry,and the rational design of catalysts is an important research field.Herein,the famous MOF,MIL-101-NH2,has been chosen to be a host to synthesize a heterogeneous catalyst Pd@MIL-101-NH2,in which Pd NPs have been encapsulated in MIL-101-NH2 in the absence of surfactant to generate exposed actived sites.Besides,as MIL-101-NH2 can easily be modified to regulate the surface environment of catalysts for the improvement of the synergistic interaction between catalyst and reactants,the hydrophobic group of aryl has been successfully grafted to yield Pd@MIL-101-ToUr(ToUr = 1-(p-tolyl)urea)by means of a post-synthetic modification.Compared with Pd@MIL-101-NH2,Pd@MIL-101-ToUr presented an obvious enhancement of catalytic activity under the same conditions in the hydrogenation of the different substrates,such as styrene,cinnamaldehyde and nitrobenzene,due to the fact that the hydrophobicity of catalyst facilitates the enrichment of hydrophobic substrates.Moreover,Pd@MIL-101-ToUr catalyst exhibits good recycling performance with the assistance of the encapsulation of MOF.3.Metal-free catalysts are of great importance and alternative candidates to conventional metal-based catalysts for many reactions.Carbon materials are one class of important metal-free catalysts and doping with heteroatoms into carbon materials has been demonstrated to be an effective method to improve their catalytic performances.Uniform doping of N atom into porous carbons is important for various reactions but still remains a significant challenge.To tackle this problem,metal-organic frameworks(MOFs),involving different types/contents of nitrogen in their well organized structures,should be ideal templates and precursors.We have successfully synthesized porous carbon materials with homogeneous distribution of N atoms using several types of MOFs as hard-templates and precursors.The resultant metal-free catalyst exhibits low activation energy and superior activity to most of metallic catalysts in the catalytic reduction of 4-nitrophenol(4-NP)to 4-aminophenol(4-AP).Theoretical investigations suggest that,the content and type of the N dopant play crucial roles in determining the catalytic performance,and the pyrrolic N species dominates the contributions to the activity.