Molten Salt Assisted Preparation Of Metal-organic Framework Based Hierarchical Composites For Heterogeneous Catalysis

Faced with the energy crisis and severe environmental problems,the transformation of renewable biomass resources into biofuels and biochemicals has attracted much research interest.Compared with the thermal technology and biotechnology（such as pyrolysis,gasification and enzyme/bacteria assisted strategy）,chemical catalysis is considered as a more effective protocol for biomass valorization.Among the large variety of the developed catalysts（e.g.,metal-carbon composites,zeolites,silicates,and metal oxides）for biomass conversion,metal-carbon composites,offering ample possibilities for the integration of large specific surface areas,tunable pore sizes,and variable components,have received considerable research attentions.Currently,metal-carbon composites are most commonly prepared by impregnation or ion exchanging,which however suffer from the unsatisfied catalytic performances.On the other hand,the newly developed template-assisted methods,which possess strong morphological modulating abilities,are energy consuming and environment unfriendly.Therefore,exploring facile,environmental benign and cost effective strategies for the scale-up preparation of metal-carbon composites for biomass transformation still remains a challenge.In recent years,metal-organic frameworks（MOFs）,as a class of typical coordination polymers composed of metal nodes and organic ligands,have shown great potential in the preparation of metal carbon derivatives via pyrolysis owing to the periodic structures and adjustable components.However,the direct pyrolysis of MOFs often leads to the collapse of the porous frameworks and the sintering of metals,resulting in low catalytic activities.Based on these premises,a novel molten salt-assisted pyrolysis strategy based on MOFs is proposed in this thesis.Hollow hierarchical porous carbons loaded with ultrafine transition-metal nanoparticles are prepared in one step via this strategy,which are further examined as catalysts for the selective oxidation of biomass-derived 5-hydroxymethylfurfural,and hydrogenation of furfural.In this preparation,Cu-BDC was used as the sacrificing template,in which the KCl-KBr molten salts were added to prepare the porous carbon-based materials doped with ultrafine metal nanoparticles upon thermal treatment.A number of characterization techniques including BET,XRD,TEM,SEM,and XPS were used to analyze the physicochemical properties of the yielded materials.The results show that the materials have abundant micropores and mesopores,and the hollow skeleton is decorated with Cu nanoparticles with an average size of ca.2 nm.Further study on the materials prepared at different calcination temperature,time and amount of molten salt shows that the molten salt plays an important role in expanding the pore size and inhibiting metal sintering of the materials.In the catalytic reactions of selective oxidation of biomass-derived 5-hydroxymethylfurfural and hydrogenation of furfural,benefit from the encapsulation effect of carbon skeleton and the ultrafine metal active sites,Cu@HHC shows remarkable catalytic performances,affording almost complete conversions of 5-hydroxymethylfurfural and furfural and>99%product selectivity,with TOF values as high as1.67 h^-1 and 40 h^-1 respectively.The material could maintain the morphological structure and catalytic performances even after five recycles of reaction.In addition,the preparation method is general which could be extended to the synthses of derived materials of other types of MOFs including Cu-BTC,Ni-BDC,Mn-BDC and ZIF8.