The Fabrication Of ZIFs Derived Efficient Metal Catalysts And Study On Their Catalytic Reaction In Conversion Of Cellulose And Its Derivatives

The critical issues of the energy crisis and increasingly serious environmental impact caused by the continuous consumption of fossil resources have attracted tremendous research interest to develop clean and renewable energy.Among them,biomass resources,especially lignocellulose,are considered to be the most potential sustainable feedstock to produce fuel and fine chemicals in the substitution of fossil resources.A valuable green route for"biorefining"using renewable resources is first production platform compounds from cellulose,which is the main component of lignocellulose,followed by the conversion of these compounds into high value-added chemicals.The preparation of high value-added chemicals from cellulose includes the following methods:（1）Acid-catalyzed hydration of cellulose to levulinic acid（LA）,after which LA can be hydrogenated into high value-addedγ-valerolactone（GVL）;（2）Direct conversion of cellulose to diols.The preparation of highly efficient metal catalysts is the key to realize the conversion of cellulose to high value-added chemicals.In this thesis,four catalyst systems were designed using zeolite-like imidazole frameworks（ZIFs）as precursors,through the modification of support,control of composition,and design of the nanostructure.The structure-activity relationship between the size of metal active site,electronic structure,content,synergy and the reaction performance was investigated in the aqueous-phase LA hydrogenation to GVL,and cellulose conversion to diols,which provides a new idea and theoretical basis for the design of highly efficient metal catalysts for biomass conversion.The main research results of this thesis are summarized as follows:（1）ZIF-67 was grown in situ on the surface of sodium dodecyl sulfate（SDS）modified wipe fiber.Subsequently,Co NPs were uniformly embedded in nitrogen-doped mesoporous carbon nanofibers after high temperature pyrolysis.The SDS modification improved the dispersion of Co NPs as well as the interaction between Co NPs and the support.Moreover,the dual active sites of Co⁰ and Co-N_x in the catalyst acted synergistically,which enhanced the activity and stability of the aqueous-phase LA hydrogenation reaction.（2）By constructing the Ru-containing ZIF-67 and turning the amount of Ru doping,a series of Ru Co single-atom alloy（SAA）catalysts with different Ru contents（0.03%<Ru wt.%<0.74）were synthesized.Aberration-corrected scanning transmission electron microscopy（AC-STEM）and X-ray absorption fine structure（XAFS）characterization results indicated that Ru atoms were dispersed in the crystal lattice of the Co substrate in the Ru Co SAA,and its electronic structure could be modulated at the atomic scale.The electron-rich Ru in Ru Co SAA boosted the adsorption C=O/H₂ and the dissociation of H₂to H atoms,hence lowered the reaction barrier ofγ-C hydrogenation,which was the rate-determining step for LA hydrogenation.Lowered the reaction barrier contributed to an extremely large turnover frequency value of 3500 h^-1.Additionally,Ru Co SAA has good reusability and can be recycled nine times without notable activity loss.（3）A"yolk-shell"structured catalyst with Co NPs and Co single atoms（SA）supported on a nitrogen-doped carbon core was prepared,synthesized by a layer-by-layer assembly supplemented by a chemical conversion strategy.The molar ratio of Zn to Co in the bimetallic ZIFs precursor was modulated to realize the regulation of the size of Co NPs and the contents of Co NPs and Co SA in the catalyst.When the molar ratio of Zn to Co was 1,the catalyst possessed the best catalytic performance for the aqueous-phase LA hydrogenation,in which the Co NPs promoted the dissociation and activation of H₂,and the Co SA was propitious to the adsorption and activation of C=O.The two components synergistically enhanced the adsorption and activation capacity for the reactants,and thereby increased the activity of the aqueous-phase LA hydrogenation（4）The nitrogen-doped hollow carbon sphere support was prepared through a pyrolysis strategy using ZIF-8 coated with modified chitosan as the precursor.The active site content of the catalyst was adjusted by the sulfonation temperature and Ru loading.The catalyst owned a high specific surface area(487 m² g^-1)plus a large pore volume(0.859 cm³ g^-1)when the preferred sulfonation temperature is 180 ^oC and the Ru loading reached 2.87 wt.%.In addition,the sulfonic acid group in the catalyst provided abundant acid active sites,the nitrogen-doped carbon support possessed high alkalinity,and the highly dispersed Ru NPs had a moderate H₂ dissociation and activation ability.The rates of cascade reactions of retro-aldol condensation,isomerization and hydrogenation matched well due to the synergistic effect of acid,alkali and metal active sites.The total yield of diols（ethylene glycol,1,2-propylene glycol）attained as high as 50%when the mass feed ratio of cellulose to catalyst was 7.5.