Design,Preparation And Catalytic Performance Of MOFs-Based Hierarchical Porous Materials

Metal-organic frameworks（MOFs）,also known as porous coordination polymers,have emerged as a promising class of porous crystalline materials,constructed by metal cations/clusters,and polydentate organic ligands with coordination-type connections.By virtue of their structural diversity,high porosities,excellect tunability and flexibility,MOFs have been widely applied in catalytic fields in recent years.However,there are still many problems and challenges in the progressing research on the MOFs.For example,precise control of morphology,porositiy and sizes of MOFs still remain challenges during the synthesis processes.In addition,the poor electrical conductivity and insufficient stability of MOFs also limit their catalytic applications.To solve the above problems,a series of MOFs-based hierarchical porous materials with adjustable structure and morphology,high catalytic activity and good stability have been fabricated by synthesising MOFs under confined spaces or pyrolyzing MOFs with pre-designed structures.Subsequently,the above as-prepared MOFs-based materials were employed as efficient catalysts for important carbon-carbon coupling reactions and energy conversion application.Not only the preparation and regulation mechanism for the materials but also the relationships between the structure properties of the catalyst and the catalytic peformance are investigated in detail.The main contents and findings of the thesis are as follows:A three-dimensionally ordered（3DO）macroporous polystyrene was employed as a confined template.The confined growth under a double-solvent condition in the above template enables the successful fabrication of the 3DO sphere-assembled ZIF-8 single crystals（3DOSA-ZIF-8）or 3DO single-crystalline ZIF-8 sphere arrays（3DOSC-ZIF-8）by using a low or high concentration precursor solution as a feedstock,respectively.The formation of 3DO-ZIF-8 with different structures in low concentration or high concentration of precursor solution,is attributed to the fact the dry precursors in templates present discontinuous and continuous distribution,which are subsequently in situ crystallization,respectively.Furthermore,our strategy can be extended to the preparation of other 3DO-MOFs single crystals,including ZIF-67 and HKUST-1 with similar controllable hierarchical nanoarchitectures.With the succeed in synthesizing a series of monodisperse ZIF-8 single-crystalline spheres with diameters ranging from 200 to 610 nm,we further unravel their catalytic performance is inversely proportional to its size by using the Knoevenagle condensation between benzaldehyde and malononitrile as a probe reaction,wherein the smallest spheres show the fastest first-order reaction kinetics.A unique ZIF-8@SiO₂/ZIF-67（x）with core@hybrid shell architecture constructed from a ZIF-8 core and a ZIF-67 hybrid shell encapsulated with SiO₂ spheres is first prepared,followed by carbonizing and etching process to obtain a series of Co/N co-doped hollow carbon cage with controllable hierarchically porous shells（H-Co/NC-HS（x））.The optimized H-Co/NC-HS（90）material simultaneously possesses the advantages of high specific surface area,large pore volume,high degree of graphitization,highly dispersed Co nanoparticles and high active Co-N_x sites.Moreover,the unique hollow structure with hierarchical porous shell can benefit the provision of abundant exposed active sites that could accelerate the transfer and diffusion of reactants and improve the utilization efficiency of the material.Consequently,when employed as an ORR catalyst in alkaline solution,H-Co/NC-HS（90）exhibits superior electrocatalytic performance,with a half-wave potential of 0.86 V,far exceeding those of solid-structure NC and pure hollow-structure H-Co/NC,comparable to that of commercial Pt/C.More importantly,it also exhibits much better methanol tolerance and stability than commercial Pt/C.It has been reported the facile synthesis of Fe,N-doped carbon nanorods（denoted as Fe-N/C-NR）with abundant hierarchical pores and highly active sites by the pyrolysis of one-dimensional（1D）Fe-doped zeolitic imidazolate framework（Fe-ZIF-8）as a self-sacrificing template.The unique 1D nanoarchitecture of the Fe-N/C-NR can provide fast electron and electrolyte transport towards the exposed active sites,and their hierarchical porous structures with large surface areas can efficiently facilitate mass diffusion and increase the density of exposed active sites.Beside,the coexistence of highly-dispersed Fe-N_x sites and Fe₃C/Fe nanoparticles in these electrocatalysts can provide a large number of desired catalytic centers with highly intrinsic activity and structural stability.As a result,the optimized 5Fe-N/C-NR exhibits admirable catalytic activity for ORR,with a high half-wave potential(E_1/2)of 0.90 V vs.RHE in alkaline medium,superior to that of commercial Pt/C（0.86 V vs.RHE）,and also a high E_1/2 of 0.81 V vs.RHE in acidic medium,comparable to that of commercial Pt/C（0.81 V vs.RHE）.Moreover,In addition,5Fe-N/C-NR also exhibits long-term stability and methanol resistance can far surpass that of commercial Pt/C in both acidic and alkaline media.Our findings provide a new strategy for rational design and preparation of cost-effective and highly-efficient non-Pt-based ORR catalysts.