Synthesis And Application Of Co-based Zeolitic Imidazolate Frameworks

Co-based zeolite imidazolate framework?ZIFs?materials not only have the excellent characteristics of traditional metal–organic frameworks?MOFs?,including high specific surface area,diverse aperture,unsaturated metal sites,etc.,but also have the outstanding chemical stability and thermal stability of zeolites.The appropriate pore size and the particularity of the ligand of the imidazole derivatives make it suitable for specific gas adsorption and separation,and the metal Co catalytic active center also provides more possibilities for its application in catalysis.The porous N-doped carbon-base materials and their derivatives obtained by pyrolysis of Co-ZIF under inert atmosphere maintain the advantages of the precursor to a large extent and are widely used in many fields,such as catalysis and energy.In addition,Co-ZIF materials with large aperture can also achieve functional composite with active noble metal nanoparticles.Porous N-doped carbon stable bimetallic catalyst based on the MNPs@Co-ZIF can keep most of good qualities of its precursor to a certain extent.Above all,both active noble metal nanoparticles and Co nanoparticles derived from Co-ZIF can be evenly dispersed in the carbon skeleton resulting from the pyrolysis of the organic ligand,and play a synergistic catalytic role in catalytic reaction.In this paper,a series of studies were carried out on the preparation of Co-ZIF membrane,carbon-based materials derived from the pure Co-ZIF and MNPs@Co-ZIF composite,as well as their applications in gas separation and catalysis.The main research results are as follows:1.ZIF-9 membrane:ZIF-9,one of the Co-ZIF materials,has an effective pore diameter of 0.32 nm,which is just between the kinetic diameter of H₂ and CO₂.Moreover,its organic ligand called benzimidazole molecules contain nitrogen atom which has a strong interaction with CO₂.Therefore,it is an ideal membrane material for H₂/CO₂ separation.However,compared with Zn-ZIF membrane,the Co-ZIF membrane is much more difficult to prepare,especially on the tubular substrate which is beneficial to industrial application due to the fact that Co-ZIF nanocrystals prefer to nucleate and grow in mother solution rather than on the substrate.Thus,in order to obtain a dense and defect-free tubular Co-ZIF membrane,it is necessary to strengthen the interaction between the tubular?-Al₂O₃ substrate and Co-ZIF membrane.Thus,we utilized 3-aminopropyltriethoxysilane?APTES?as a covalent linker to modify the?-Al₂O₃ tube.Due to the formation of covalent bonds between APTES and ZIF-9,ZIF-9 nutrients are easy to be attached to the support surface,thus promoting the nucleation and growth of ZIF-9 on the substrate surface so as to obtain a dense and phase-pure ZIF-9 membrane with a thickness of 4?m.Subsequently,the gas separation performance of tubular ZIF-9 membrane was evaluated by single gas permeation and mixed gas separation test based on common small molecular gases,such as H₂,CO₂,N₂ and CH₄.Among of them,the mixed separation factors of H₂/CO₂,H₂/N₂ and H₂/CH₄ of ZIF-9 membrane are 21.5,8.2 and 14.7,respectively,all of which far exceed the corresponding Knudsen diffusion coefficient,indicating that as-prepared APTES-modified ZIF-9 membrane has a good hydrogen selectivity.In addition,the influence of operating temperature and pressure on H₂/CO₂ separation performance of ZIF-9 membrane and the stability of ZIF--9 membrane were also investigated.It was found that ZIF-9 membrane could maintain excellent H₂/CO₂separation performance within a wide operating temperature range,which was very beneficial to its application in industrial hydrogen separation and membrane catalytic reactors.2.Although Pt-based catalysts have exhibited high catalytic activity in propane dehydrogenation.However,not only is the cost of Pt expensive,but also encounters deactivation of the catalyst under high reaction temperature.It is a good alternative to find suitable transition metal catalysts to completely replace or dope transition metal to prepare bimetallic or polymetallic catalysts for propane dehydrogenation.Studies have shown that transition metal cobalt has certain catalytic activity in the activation of C-H bond.We chose Co-ZIF-9 containing metal Co catalytic active center as precursor to be calcined at 700?,800?and 900?directly to obtain three kinds of N-doping carbon-stabilized cobalt-base catalysts?Co/NC-T,T means pyrolysis temperature?for the direct dehydrogenation from propane to propylene.The results showed that the catalytic performance of Co/NC-800 was the best among three catalysts in terms of the yield of propylene and the stability of the catalyst,which could be attributed to the combination effect of the appropriate specific surface area and the suitable relative metal Co cobalt content.3.The large pore size of ZIF materials is the key to realize the functionalized composite of the MNPs and ZIF material.Compared to Co-ZIF-9,the effective aperture of the Co-ZIF-67 is 0.67 nm,which is almost twice than that of the ZIF-9.In addition,the preparation of ZIF-67 is simple and could be realized at room temperature.The supported Pt-based catalysts have shown good catalytic activity in propane dehydrogenation,and the common supporters of active metal Pt include?-Al₂O₃,SiO₂,zeolites and so on.However,the preparation of Pt nanoparticles supported by MOF has not been reported for catalytic dehydrogenation of propane.Thus,we chose ZIF-67 as the carrier and prepared the Pt@ZIF-67 composite materials by dip-coating method and one-step method as precursors to be calcined at800?for 5 h in nitrogen atmosphere so as to obtain N-doped carbon-stabilized bimetallic catalyst for propane dehydrogenation,respectively.And we investigated the influence of the loading method of the Pt content on the catalytic performance of the catalyst.The results showed that the catalyst obtained from the pyrolysis of the composite by one-step method had better catalyst activity than that obtained from the pyrolysis of pure ZIF-67.However,the catalytic performance of the catalyst on propane dehydrogenation obtained from the pyrolysis of the composite prepared by impregnation method was almost the same as that of the catalyst obtained by the pyrolysis of pure ZIF-67,and there was no significant improvement.The propane dehydrogenation performance of several catalysts were tested under the same experimental conditions,and the propane conversion rate of the bimetallic catalyst?E-PtCo/NC?through pyrolysis of the composite prepared by one-step method reached 38%,which was 21%higher than that of Co/NC.Meanwhile,both the yield of the target product,propylene and the stability of the catalyst were all improved to a certain extent.And the propane conversion rate of 38%and the propylene selectivity of 46%could remain stable for more than 10 h in the case of carbon deposition on the catalyst,which could be ascribed to the synergism of active Pt nanoparticles and Co nanoparticles obtained from the pyrolysis of ZIF-67.In addition,the catalyst prepared by using ZIF-67 as a template to complete the encapsulation of Pt NPs had more advantages in propane dehydrogenation than the catalyst prepared by using Co/NC derived from the pyrolysis of ZIF-67 as a template to encapsulate Pt NPs.