Preparation Of Metal-organic Frameworks-based Materials As Heterogeneous Catalysts For Liquid-phase Hydrogenation And Oxidation Reactions

Metal-organic frameworks(MOFs) are anew type of ordered network materials assembled with metal ions and organic linkers. Owing to the characteristics of high specific surface areas and pore volumes, low crystal densities, and flexible pore sizes and functional structures, MOFs have recently attracted continuously increasingresearch interest. Presently, MOFs have been used in various areas, such as gas separation and storage, catalysis, drug delivery and fluorescent materials. Especially, MOFs are identified as a class of promising materials for heterogeneous catalysis.In this thesis, we aimed to develop novel and highly efficient MOFs-based catalysts for some industrially important liquid-phase hydrogenation and oxidation reactions by: introducing highly dispersed metal sitesonto MOFs, using MOFs alone with special active sites,and employing MOFs as sacrificialtemplates to prepare homogenously dispersed metal catalysts. The main contents of this thesisare as follows:1. Au-Pd alloy nanoparticles were deposited on MIL-101 and used as catalyst for selective oxidation of saturated hydrocarbons. A simple colloidal deposition method has been developed to prepare highly dispersed Au-Pd nanoparticles on MIL-101 with HAu Cl4 and Pd Cl2 as Au and Pd precursorsrespectively, Na BH4 as reductant, and PVP as protecting agent. TEM characterization revealed that the Au-Pd alloy nanoparticles were homogeneously dispersed on the support with an average size of 2.40 ± 0.63 nm. Au-Pd/MIL-101 afforded a very high TOF of 19000 h-1 for the liquid-phase selective oxidation cyclohexane to KA-oil at 150 oC, 1.0 MPa O2 without the use of solvents. The Au-Pd alloy catalyst exhibited higher reactivity than their pure metal counterparts and a Au + Pd physical mixture. The high activity and selectivity of Au-Pd/MIL-101 in cyclohexane aerobic oxidation may be correlated to the synergistic alloying effect of bimetallic Au-Pd nanoparticles. Moreover, Au-Pd/MIL-101 exhibited a wide applicability for the selective oxidation of saturated C-H bonds with molecule oxygen, including primary, secondary, and tertiary saturated C-H bonds. Furthermore, a possible reaction pathway was also proposed for the liquid-phase aerobic oxidation of cyclohexane over the Au-Pd/MIL-101 catalysts.2. A multifunctional Pt/Ni-MOF catalyst was prepared and used for the selective hydrogenation of nitriles to imines. The DABCO ligand on the surface of Ni-MOF contained some un-coordinated N sites. The preparation of Pt/Ni-MOF employed a simple colloidal deposition method in methanol with H2 Pt Cl6·6H2O as Pt precursor, PVP as protecting agent and Na BH4 as reductant. The catalyst showed excellent synergy in promoting the hydrogenation of a variety of nitriles, giving significantly improved activity and selectivity(up to >99% yield) even under atmospheric pressure of H2. Moreover, it was suggested that the Lewis base(DABCO) sites on the Ni-MOF inhibited further hydrogenation of the imines thereby improve the selectivity of the desired product. The influences of H2 pressure, reactant concentration, stirring speed, and reaction temperature were investigated. The kinetics and mechanism coupling of selective hydrogenation of benzonitrile(BN) to imine by the Pt/Ni-MOF catalyst have been studied. The reaction showed a first-order dependence on both BN concentration and H2 pressure. A kinetic model was proposed based on the mechanism of nitriles hydrogenation and compared with experimental observations.3. Activation of molecular oxygen was achieved by a metal–organic framework(MOF-253) containing open N sites for selective oxidation of saturated hydrocarbons without the assistance of transition metals. MOF-253 with open 2,2’-bipyridine sites could efficiently catalyze liquid-phase selective oxidation of cyclohexane to KA-oil. Under150 oC, 1.0MPa O2 and solvent-free conditions,the catalyst showed excellent activity for this reaction, furnishing 40% yield for KA-oil. A theoretical calculation at the DFT level of B3LYP/6-31G(d, p) was performed, obtaining the HOMO and LUMO energies of MOF-253. It was suggested that the MOF-253 catalyst with a low H–L gap might facilitate the excitation of electrons from HOMO to LUMO, where the excited electrons could activate molecular oxygen to form ·O2-, thereby promoting the oxidation of cyclohexane.4. Co@C-N composites were synthesized by using Co-MOF as sacrificial template, and the materials were employed as catalysts for transfer hydrogenation of unsaturated bonds in the absence of base additives. The choice of the Co-MOF as template was attributed to the presence of triethylenediamine basic sites(even after simple thermolysis) which could be favorable for transfer hydrogenation reactions. Co@C–N showed similar particles shape as that of parent Co-MOF. TEM and elemental mapping characterization revealed that the Co nanoparticles were uniformly distributed in the Co@C–N materials. The transfer hydrogenation reactions of various unsaturated bonds were investigated usingthe Co@C–N catalyst with i-Pr OH as both hydrogen donor and solvent. The catalyst system exhibited excellent reaction activity and selectivity for transfer hydrogenation a variety of unsaturated C-H bonds, including ketones, aldehydes, C=C, C≡N, and N=O bonds, under base-free reaction condition.5. A series of highly dispersed bimetallic heterogeneous alloys catalysts were prepared using a variety of heterodinuclear MOFs as sacrificial templates,and these materials were employed as catalysts for transfer hydrogenation of nitriles and electrochemical oxygen reduction reaction. Co-Ni-MOF, Co-Cu-MOF, Ni-Cu-MOF and Co-Ni-Cu-MOF with different metal ratios have been used as template materials to synthesize Co-Ni@C-N,Co-Cu@C-N, Ni-Cu@C-N and Co-Ni-Cu@C-N alloy materials, respectively. SEM and element mapping characterization showed that metals, C and N elements were homogeneously distributed in these MOFs. HAADF-STEM images showed that metals, C and N were highly dispersed in every alloy nanoparticles. All the materials were tested in transfer hydrogenation coupling of nitriles to imines and electrochemical oxygen reduction reaction. The results showed that Co-Ni(3:1)@C-N exhibited the best reactivity in transfer hydrogenation of nitriles. The yield afforded by Co-Ni(3:1)@C-N was 5 times greater than the single-metal catalysts.For the electrochemical oxygen reduction reaction(ORR) Co-Cu@C-N(3:1) exhibited an excellent activity, which could comparable to commercial Pt/C electrode under basic condition.