Synthesis of Mofs-based Composites and Their Oxidation and Hydrogenation Catalytic Performance

Author:Qiu Zuo

Supervisor:li ying wei


Degree Year:2018





Metal-organic frameworks(MOFs)are a new kind of organic-inorganic zeolite-type porous hybrid materials which are self-assembled by organic ligands and inorganic metal ions.Compared with other hybrid materials,MOFs have excellent properties such as large surface areas,high porosities and diversification of structures.These properties endow them with wide and promising applications.The MOFs-based composites have also become a research hotspot as the combination of MOFs and other materials afford the composites with new promising properties.However,how to design a stable and high-performance MOFs-based catalyst is still a challenge in this field.Based on the structures and properties of MOFs,this thesis focuses on developing new strategies for the synthesis of novel MOFs-based composites and investigating the relationship between the structure and catalytical properties.The main contents and achievements of this thesis are as follows:A new and efficient hydrophilicity-directed approach(HDA)was developed to encapsulate large guest molecules beyond the aperture size limitation in the nanospace of MOFs,as exemplified by the self-assembly of a metal-organic polyhedral(MOP,M6L4 in this case)into MOFs(MIL-101 in this case).This strategy was based on the different hydrophilicities between inner and outer surfaces of the pre-formed MOFs that might direct the self-assembly of MOP in the MOFs pores by using a two-solvent system.Compared to pure M6L4,the catalytic property of M6L4 after encapsulation(denoted as M6L4?MIL-101)was significantly improved.In the selective oxidation of benzyl alcohol transformation,the benzaldehyde yield in the M6L4?MIL-101 system was 3.5 times of that in M6L4 system.More importantly,in the fifth run,the benzaldehyde yield obtained on the M6L4?MIL-101system was almost 20 times of that of the M6L4 system.The stability of M6L4?MIL-101 was also greatly enhanced because the MOP aggregation and leaching were effectively prevented by the MIL-101,endowing M6L4?MIL-101 with significantly enhanced reactivity and stability in the catalytic transformations as compared to the pristine MOP.A new and simple synthetic strategy was proposed to modulate the growth and distribution of MOFs on GO.In this synthesis procedure,GO was allowed to be mixed with metal ions for a long time before organic ligand was added,enabling a sufficient coordination between the surface groups of GO with metal ions,which would provide dense and homogeneous nucleation nods for the formation of MOFs.Thus,MOFs/GO hybrids with high-density and ordered MOFs particles were achieved,which featured different sizes and morphologies as compared to parent MOFs as well as similar composites prepared by using the reported methods.Furthermore,metal-containing/rGO nanomaterials were also obtained after pyrolysis by applying MOFs/GO as sacrificing templates.As compared to the counterpart materials,the TOF of metal-containing/rGO nanomaterials was enhanced by a factor of ca.2.2.The significantly enhanced catalytic performance was believed to be related to the higher dispersion and smaller size of the particles by employing the newly developed recipe for the preparation of MOFs/GO composites.The present method might bring new insight and opportunities for the synthesis of growth-controllable MOFs/GO and metal-containing/rGO nanohybrids for advanced functional applications.An innovative transition metal-free heterogeneous catalytic alternative was proposed for the selective oxidative coupling of amines to imines under mild solvent-free conditions.As indicated by the theoretical calculation,the HOMO-LUMO gaps of 2,2’-bipyridine(bpy)moieties reduced greatly if being assemblied into the MOF-253 structure,which were suggested to be responsible for the activation of molecular oxygen to superoxide radical anion species(·O2-).The·O2-species was believed to be active for the oxidative coupling of both aromatic and aliphatic amines to obtain the imines.As expected,the experimental results showed that the MOF-253 could efficiently catalyze the oxidative coupling of the amine under solventless condition,and the conversion and yield were as high as 90%.It was worth mentioning that the MOF-253 system was also active for the cross-coupling with selectivity up to 98%.Catalytic performances of the counterpart materials(H2bpydc,bpy and ZrMOF-BIPY)were also studied.However,as compared to MOF-253,these catalysts had much lower catalytic activities,mainly due to the different HOMO-LUMO gaps steming from structural differences.For the first time,highly dispersed C-N-decorated single atom/metal sub-nanoparticles were fabricated by employing MOPs as sacrificing template,which were further encapsulated and stabilized in the pores of a MOF.Firstly,the MOP was encapsulated into the MOF pores by the HDA strategy.After low-temperature thermolysis under hydrogen,the MOP guest collapsed to form C-N-decorated single atom/metal sub-nanoparticles while the MOF host reserved its crystalline frameworks.As revealed by the HAADF-STEM,XPS,EXAFS and XANES measurements,the residual C-N fragments derived from the organic components of MOP were supposed to act as in-situ formed stabilizers to prevent the aggregation of single atom/metal sub-nanoparticles.These metal particles were further stabilized by the confinement effect offered by the MOF cages.This new hybrid was highly stable and efficient for the nitrobenzene hydrogenation and furfural selective hydrogenation to cyclopentanone.The conversion and selectivity were up to 99%.Furthermore,after reusing for five runs,the catalytic performance was still retained.