Preparation and Permeation Performance of Co-based ZIF Membranes

Author:Nian Pei

Supervisor:zhang xiong fu


Degree Year:2019





Owing to their unique pore structures,high porosity and rich functionality,metal organic frameworks(MOFs)is one of the good membrane materials.Zeolite imidazole frameworks(ZIFs)as an important branch of MOFs are referred as ideal membrane materials duo to their ultra-micropores and exceptional thermal/chemical stabilities.Particularly,cobalt-based ZIF membranes not only could separate some gas mixtures but also have the potential to be used as perm-selective membrane reactors duo to the presence of redox catalytic cobalt centers.However,to date,it is still a big challenge to prepare high-performance Co-based ZIF membranes.The reason is mainly attributed to the fact that Co-based ZIF nucleates and grows too fast in the solution and the limitation of the preparation method so far.Here,based on the growth characteristics of Co-based ZIFs,ZnO induced heteroepitaxial growth and vapor phase transformation were developed to prepared Co-based ZIF(ZIF-67,ZIF-9 and 2D Co2(bim)4 nanosheet)membranes on alumina tubular substrates,and the permeation performance of the prepared membranes were also studied.The main contents are as follows:(1)ZIF-67 membrane was prepared by ZnO induced heteroepitaxial growth.Conventional methods including in situ growth and same metal source induced synthesis were also investigated to prepare ZIF-67 membrane,it was hard to get a continuous ZIF-67 membrane.While,the ZnO nanorod layer was grown on the porous tubular substrates as induction sites for membrane growth,which could promote the heterogeneous nucleation of ZIF-67 crystals and resulting in uniform ZIF-67 membrane.The prepared ZIF-67 membrane possesses excellent hydrophilicity,stability and molecular sieving function duo to the special microstructure of ZnO induced strategy.For the separation of 15 wt.%MeOH/MTBE mixture at 40 ℃,the ZIF-67 membrane exhibited the separation factor of exceeding 3000 for MeOH/MTBE with high flux of 1.84 kg—m-2-h-1,which is superior to almost all the reported membranes for the pervaporation separation of MeOH/MTBE mixture,showing great potential for practical applications.(2)ZIF-9 membrane was prepared by ZnO induced heteroepitaxial growth for gas separation,and the ZnO induced heteroepitaxial growth method was extended to prepare other types of MOF membranes.Combined with the characteriazation results of XRD,SEM and FT-IR for the Co-ZIF membranes formation process,a membrane growth mechanism of(Zn,Co)hydroxy nitrate salts(hydroxy double salts,HDS)driving the Co-ZIFs formation was proposed and discussed.The H2/CO2 ideal selectivity of the prepared ZIF-9 m embrane reached 23.8,and the membrane achieved could exhibit excellent stability during the long-term operation,even after a series of temperature changes.Furthermore,three types of MOF membranes including Cu-based CuBDC and HKUST-1 and Ni-based USO-2-Ni were achieved by the ZnO induced method,demonstrating the excellent universality of the ZnO-induced heteroepitaxial growth of MOF membranes.(3)2D Co2(bim)4 nanosheet membrane was prepared via vapor phase transformation of the Co-based gel.The same metal source solvothermal method was also investigated to prepare Co2(bim)4 nanosheet membrane,the results showed that it was difficult to obtain continuous and oriented Co-ZIF nanosheet membrane owing to the intertness of Co3O4 and fast speed of Co2(bim)4 homogeneous nucleation.For the vapor phase transformation process,the uniform Co-based gel layer acts as a multifunctional role such as providing active metal source,guiding the oriented growth of the nanosheets owing to the network structure and controlling the membrane thickness in the form ation of the nanosheet membrane.The achieved membrane with the thickness of 57 nm show excellent gas separation performance,the ideal selectivities of H2/CO2,H2/N2 and H2/CH4 were as high as 58.7,49.7,68.6,respectively,and the H2/CO2 separation selectivity was increased to about 63 at 150 ℃.Moreover,the Co2(bim)4 nanosheet membrane showed an excellent stability without any degradation in performance during the long-time tests.The vapor phase transformation strategy possesses some obvious advantages such as solvents-free,environmentally friendly and scalable,which can solve the problem of complication and difficulty for the conventional operation process,thus showing the potential for the separation of H2/CO2.