Synthesis Of Metal Nanocatalysts Supported On MIL-101and Their Applications In Green Synthesis

Metal-organic frameworks （MOFs） are a new class of porous organic-inorganic hybridmaterials formed by the self-assembly of organic bridging ligands and metal ions. Theypossess large specific surface area and porosity as well as tunable structure and propertiescompared to other porous materials. Due to these outstanding properties, MOFs could behighly desirable and promising materials for applications in heterogeneous catalysis. However,the design and preparation of highly-efficient MOF-based catalysts remains a challengingresearch target. Based on the structural features and chemical properties of MOFs, a numberof efficient preparation methods have been developed in this thesis for tuning the catalyticactive sites of MOFs to synthesize new and highly active MOF-based catalysts. Theircatalytic performances in green synthesis reactions were investigated in detail and thestructure-performance relationships were explored. The main research contents andexperimental results are as follow:1) Au/MIL-101（CD/PVP） was prepared by a simple colloidal method withpolyvinylpyrrolidone （PVP） as protecting agent using HAuCl₄as the Au precursor. Thesupported gold catalyst has been shown to be highly efficient for liquid-phase aerobic alcoholoxidation in the absence of base. Au/MIL-101（CD/PVP） afforded a very high TOF of25000h-1for the conversion of1-phenylethanol under the solvent-free conditions at160°C, whichwas even higher than the values obtained on the most active Au catalysts reported in thecurrent literature under the same reaction conditions. This work also represents the firstexample of an active MOF supported Au catalyst prepared by a liquid-phase synthesis method.Moreover, the catalyst was highly stabilized against metal agglomeration and leaching,maintaining the high activities during a number of recycles. In contrast, the Au/MIL-101catalysts prepared by CD/glucose （colloidal deposition with glucose as protecting agent）,DPSH （deposition-precipitation with sodium hydroxide）, and IMP （impregnation） caused theagglomeration of Au nanoparticles, which were inactive for liquid-phase aerobic alcoholoxidation under base-free condition. Mechanistic studies indicate that the high catalyticperformance may be attributed to the high dispersion of Au NPs as well as the electron donation effects of aryl rings of MIL-101to the Au NPs in the large cages of the MIL-101support.2) The deposition of Pd nanoparticles on MIL-101with a strong Lewis acidity couldrender a bifunctional catalyst that combines Lewis acidity and hydrogenation activity. Theresults showed that Pd/MIL-101was able to effectively catalyze the selective hydrogenationof phenol to cyclohexanone in water even at atmospheric pressure and room temperaturewith>99.9%selectivity to cyclohexanone at phenol conversions>99.9%. Moreover, thecatalyst was found to be highly reusable, giving identical activities and selectivities after>5uses. Mechanistic studies indicated that Lewis acids activated aromatic rings of phenol whilePd activated H2, thereby facilitating the hydrogenation of phenol. Furthermore, acid-baseinteraction between the Lewis acid and cyclohexanone inhibited further hydrogenation tocyclohexanol. The exceptional catalytic activity and selectivity in phenols hydrogenationcould be attributed to the synergistic effect between the Lewis acidic sites and Pd on thecatalyst.3) Based on our previous findings on the effect of Lewis acidity on the activation ofaromatic rings, we designed and prepared a multifunctional Au-Pd/MIL-101catalyst by usinga colloidal method for the selective oxidation of SP³C-H bonds to aromatic ester. The resultsshowed that Au-Pd/MIL-101was particularly active for aromatic hydrocarbons oxidation witha broad substrate tolorance. The TONs achieved on this catalyst were3-100times greater thanthose of previously reported catalysts under milder or at least comparable conditions. Inaddition, neither benzyl alcohol nor benzoic acid was detected in Au-Pd/MIL-101catalyzedtoluene. Mechanistic studies indicated that the synergistic effect for the Au-Pd made it morefavorable for the activation of O₂to form superoxo-like species than the correspondingmonometallic Au or Pd. At the same time, an interaction between the aromatic ring of toluenewith the Lewis acidic sites rendered toluene to be more easily attacked by the activatedoxygen species to form benzyl alcohol. On the other hand, the acid-base interaction betweenthe Lewis acid and benzaldehyde effectively suppressed the formation of benzoic acid, andalso facilitated a nucleophilic attack of the carbon atom of the carbonyl group by benzylalcohol to form the corresponding hemiacetal. Finally, the hemiacetal was further oxidized tobenzyl benzoate. 4) Au-Pd/MIL-101could effectively catalyze the direct oxidative synthesis of aromaticmethyl esters from methyl aromatics with alcohols. The results showed that Au-Pd/MIL-101was able to accomplish the oxidative esterification of various methyl aromatics with alcoholto the corresponding ester products, up to99%conversions and98%selectivity to esters.Mechanistic studies indicated that free radical wasn’t involved in this transformation.Moreover, possible pathway for this oxidative esterification reaction involved four steps.Firstly, toluene was oxidised to benzyl alcohol via SP³C-H bond activation over thebimetallic Au-Pd nanoparticles. The formed benzyl alcohol then undergoes a β-hydrideelimination to generate benzaldehyde. The intermediate aldehyde reacted with methanol toform the corresponding hemiacetal species which upon further oxidation provided thearomatic methyl esters.5) The catalytic performance of MOF-253in the direct arylation of unactivated areneswas studied. The results showed that the efficiency of bpydc ligands was dramaticallyincreased by assembling them within a MOF-253structure, and the yield of4-methoxybiphenyl increased from1%to50%. MOF-253afforded higher chemo-and regioselectivitiescompared to the other catalysts reported in related literature. Mechanistic studies indicatedthat the electron configuration of the2,2’-bipyridine moieties was changed after the ligandwas assembled into the MOF, and hence catalytic activity was improved. The improvement ofchemo-selectivity could be attributed to the steric hindrance of the micropores as well as theπ,π-stacking interactions between the2,2’-bipyridine moieties of MOF-253and the aromaticring of anisole. The pore size of the MOF-253and the dimensions of reactant moleculescaused reactant molecules to enter the pores along a certain direction, and hence theregioselectivities could be enhanced.