Preparation And Permeation Of ZnO Induced Zeolitic Imidazolate Framworks Membranes For Gas Separation

Metal-organic frameworks(MOFs)show huge application prospect in fields of absorption and isolation,energy and sensing due to the high specific surface area,controllable pore size and favorable stability of their unique framework structures.Zeolite imidazole frameworks(ZIFs)represent an important branch of MOFs.ZIFs are frequently used as the membrane materials for gas or liquid separation,thus becoming one of the hotspots in the current membrane research field.Moreover,using the conventional ZIF membrane for gas permeation is generally restricted by the trade-off between permeability and selectivity,which is also referred to as the Robertson upper limit.In this paper,different structures of ZIF films were successfully prepared by different synthetic method such as activation induction,confinement induction and self-conversion of ZnO with different organic ligands.The main research content is as follows:(1)The ZnO nanorod arrays on the support surface were activated using the DMF solution of diethyl imidazole,which was used to induce the growth of ZIF-14 membrane.Activation had reduced the nucleation difficulty of ZIF-14 on the support surface.Moreover,the ZnO nano arrays could modify the support surface and enhance the connection between the ZIF-14 membrane and the support,which could connect the ZIF-14 membrane with the support,thus improving the stability of the membrane layer.The synthesized ZIF-14 membrane had favorable gas separation performance.The ideal separation factor of H2/CO2?H2/N2 and H2/CH4 is 20.4?27.8 and 30.3,respectively.The ZnO nano array could strength the connection between the membrane layer and the support,so that the prepared ZIF-14 membrane layer had good stability,which had high separation property after loop test at 250? for 100 h,and the H2/CO2 separation factor maintained at about 18.(2)The ZnO nanorod arrays were self-transformed into intact ZIF-8 membrane in the diethyl imidazole aqueous solution(Hmim),duing the transformation,the zinc source of the ZIF-8 membrane were completely provided by the ZnO arrays.It was discovered that the consumed amount of Hmim in the ZnO self-transformation for the preparation ZIF-8 membrane was less than 10%of that in traditional method,while the consumed solvent was only 2%of the original one.Therefore,ZnO self-transformation for the preparation of ZIF-8 membrane could greatly reduce the preparation costs.The prepared membrane layer possessed good gas separation property and thermal stability and the separation factor of binary H2/CH4 mixture can reach 40.2.Besides,it was found through deduction in the tester and experimental verification that,when the[Hmim]?0.23 M in the self-transformation solution,the ZnO nano arrays on the support surface could be self-transformed into the intact and condense ZIF-8 membrane,which was of guiding significance to future membrane preparation.(3)Based on the universal Robertson upper limit in conventional membrane separation,a bottom-up strategy was employed to self-transform the ZnO nano particles for the direct growth of the nanosheet membrane.The ZnO nano particles in the organic ligand benzimidazole(bIm)solution can be self-transformed into the Zn2(bim)4 nanosheet with the thickness of 22±2 nm and the horizontal-to-vertical ratio of 40.The ZnO nano particle layer was introduced onto the substrate surface,which can provide the anchoring sites and zinc source for the orientated growth of the nanosheet.The ZnO nano particles were self-transformed to prepare the membrane in bIm solution.Besides,the ligand solution was used to wash the support surface during the reaction,which could improve the precursor orientation of the nanosheet,so that 2D nanosheet membrane with high orientation could be prepared during the subsequent membrane formation process.H2 permeances of the highly oriented nanosheet membranes reached 2.04×10-7 mol·m-1·s-1·Pa-1,and the ideal selectivities for H2/CO2,H2/N2 and H2/CH4 were as high as 53,67 and 90,respectively,which were far beyond their corresponding Knudsen selectivities(4.7,3.7 and 2.8,respectively).The membrane exhibit excellent gas separation performance after was heated up to 150 ? and tested for 100 h,which demonsetrate the membrane has excellent thermal and operating stability.(4)As the disordered orientation of the nanosheet during the preparation of the nano sheets were inevitably formed membrane,the graphite oxide with abundant oxygen-containing groups was designed to wrap the ZnO nanoparticles on the support surface,which could induce the nanosheet nucleate and confine the randomly growth of the nanosheet.Nanosheet membrane with high orientation and high gas separation property had been successfully prepared on the porous ceramic tube support.The H2 permeances of the highly oriented nanosheet membranes was 1.5×10-7 mol·m-2·s-1·Pa-1,and the ideal selectivities were as high as 106,126 and 256 for H2/CO2,H2/N2 and H2/CH4,respectively.The membrane exhibit excellent gas separation performance after was heated up to 150 ? and tested for 400 h.Our research has opened up a simple synthetic strategy for the large scale preparation of nanosheet MOF membrane with high orientation.