Preparation And Properties Of Metal-Organic Framework-derived Carbon Nanomaterials

As a youthful family of crystalline organic-inorganic hybrid porous materials,metal-organic frameworks（MOFs）,assembled by inorganic vertices（metal ions or clusters）and polydentate organic ligands,featuring the structural diversity,large internal surface areas,high porosities,regular and tunable pore structures,along with the controllable chemistry and functionality,making them outstanding candidates for a variety of applications,including gas storage and separation,catalysis,luminescence and sensors,proton conduction,etc.Recently,the newly emerging metal-organic frameworks have been widely used as self-sacrificed precursors/templates to fabricate porous carbon materials.The well-distributed metal centers and organic linkers in MOFs can be in situ transformed into metal/metal oxide-loaded porous carbons and even metal-free porous carbons by pyrolysis,and the well-defined structures,adjustable pore topology can be easily functionalized,which may pave an avenue to the further development of functional applications of MOF derivatives.This thesis devoted to prepare metal/metal oxide@carbon composites and porous carbons from direct pyrolysis of metal-organic frameworks and investigate their applications in energy,environment and catalytic related fields.The effect of textural properties on the performance of the derived porous carbons was studied systematically.This thesis mainly includes the following four sections:In chapter Ⅱ,we prepared various N-doped hierarchical porous carbons by using a new mixed-ligand ZIF（JUC-160）as the precursor.A systematic study was conducted to investigate the effect of the precursor’s crystal size on the resultant porous carbon materials.We proposed facile synthetic routes for preparing two kinds of JUC-160crystals with regulated crystal sizes ranging from the nanometre to micrometre scales.The micron-sized crystals of JUC-160 were synthesised using a solvothermal reaction,while nanocrystals of JUC-160 were synthesised at room temperature.Micropores are dominant in porous carbons obtained from micron-sized JUC-160 crystals,however,more mesopores were presented in porous carbons derived from nanometre JUC-160crystals.mJUC160-900,which possess the high nitrogen concentration combined with favourable microporosity,exhibited the best CO₂ uptakes of 5.50 and 3.50 mmol g^-1 at 273 and 298 K,and shows high adsorption selectivity for CO₂/N₂ at 298 K and 1bar.Different size of MOF crystals derived porous carbon materials with different structures,which may provide some reference of precursor size selection for the practical application.In chapter Ⅲ,the active K⁺was introduced into the pores of bio-MOF-1precursors,and various N-doped hierarchical porous carbons were prepared.Pore size distribution of the BM-T samples in a wide range of 0.5-9 nm,while the KBM-T samples shows a narrow range of 0.5-2 nm.The effect of the K⁺in-situ activation on the textural properties of the derived porous carbons was studied systematically,and the CO₂ capture properties and electrochemical performance for supercapacitors of these porous carbons were enhanced.The KBM-700 sample derived from K@bio-MOF-1（potassium-ion-exchanged bio-MOF-1）,which had high nitrogen content（10.16%）and micropore volume（73%）,exhibited good CO₂ uptakes(3.29 mmol g^-1),high adsorption selectivity for CO₂/N₂ at 298 K and 1 bar(S_ads=99.1)as well as high specific capacitance(230 F g^-1).To introduce the functional metal sites in the MOF scaffold by a facile ion-exchange strategy,which provide a general method to make full use of the porous structure of the MOFs.In chapter Ⅳ,Co²⁺exchanged MOF-derived nitrogen-doped porous carbons as highly efficient electrocatalysts for oxygen reduction reaction were introduced.A typical anionic MOF（bio-MOF-1）was selected as the“proof of concept”model due to its ion exchange feature.Bio-MOF-1 has a rigid anionic framework with Me₂NH₂⁺cations as counter ions in the channels,which can be readily exchanged with Co²⁺.We prepared a series of N-doped different metal content Co/CoO@C via one-step carbonisation Co²⁺exchanged bio-MOF-1 under the same conditions.The resulting carbon materials show high ORR performance than pristine bio-MOF-1 derived porous carbons.We introduced transition-metal ions into the channels of MOFs by a facile ion-exchange strategy,which may provide a general method to ensure the facile synthesis of porous carbons and pave an avenue to the further development of functional applications of porous MOF materials.In chapter Ⅴ,we propose a straightforward one-step method for the preparation of hierarchical porous carbons by using simple coordination polymer（K₄PTC）as precursors.During thermal conversion under an inert atmosphere,the organic ligands within the coordination polymers serve as carbon precursors and the uniform distributed active K⁺centres act as activating agents to react and etch with the carbon framework,generating highly porous carbons with well-define pore structures.A series of porous carbons was obtained by simply adjust the carbonization temperature,KC-850,which possess the highest specific surface area(1779 m² g^-1)and large pore volume(1.69 cm³ g^-1),exhibited high capacity and excellent electrochemical cycling stability.The inner-connected micro-/mesopore structures of KC-T provide the advantage of quick transportation and rapid diffusion of electrolyte ions to the interface of the electrode.This work presented a one-step and efficient synthesis strategy to produce porous carbons with hierarchical porosity,without any additional carbon sources or activating agents,which pave a new avenue for the further functional application of coordination polymers.In summary,we used metal-organic frameworks as self-sacrificed precursors/templates to fabricate porous carbon materials.The effect of the guest,metal vertices,crystal size and morphology of the MOFs on the performance of the derived porous carbons was studied systematically.This thesis provides a new avenue for the further functional application of MOF materials and their derivatives.