多孔氧化锌支撑体上合成MAF-4膜及其H_2/CO_2分离性能

Fabrication of MAF-4 membrane on porous ZnO support for H_2/CO_2 separation

为了简化多孔配位聚合物(porous coordination polymer,PCP)或金属-有机框架(metal-organic framework,MOF)膜分离器件的制备流程,本文研究了以多孔氧化锌作为含锌离子MOF膜的支撑体,使用简易的二次生长法合成了SOD-[Zn(mim)2](MAF-4,Hmim=2-甲基咪唑,2-methylimidazole)的致密连续、厚度约为4μm的膜层.在100℃和100 kPa压差下,膜对H_2/CO_2的理想选择性达到25,H2的渗透速率达到2.0×10^(-7) mol/(m^2s Pa);在25℃和100k Pa压差下,等摩尔H_2/CO_2混合气的选择性达到5.7.本工作表明多孔氧化锌可能是基于锌离子的MOF膜材料的理想载体.

Membrane separation is a cost-effective and energy-saving technology compared with physical adsorption,chemical absorption,distillation,and other conventional methods.With high surface area,uniform and adjustable pore aperture size/shape,etc.,porous coordination polymers or metal-organic frameworks（MOFs） have demonstrated outstanding potentials for storage,separation,catalysis,sensing and many other applications.MOFs are also emerging as promising membrane materials in recent years.Nevertheless,most MOF-based membranes still suffer from complicated synthesis,low stability,low permeability,and/or low selectivity.SOD-[Zn（mim）2]（MAF-4 or ZIF-8,Hmim=2-methylimidazole） with a very small aperture size（0.34 nm） is a zeolitic MOF material having high chemical stability and thermal stability.Besides adsorption,catalysis,and many other functions,MAF-4 has also been extensively studied as a membrane material,and many membrane synthesis methods and membrane supports have been reported.As a Zn（II）-based MOF,MAF-4 crystals/membranes can grow on Zn O surface by partial conversion of the oxide to the MOF component.Therefore,porous Zn O may be a suitable support for a MAF-4 membrane.However,for known MAF-4 membrane synthesis methods,the Zn O layers need to be coated on the surface of conventional supports such as porous α-Al2O3,silica,and titania,by complicated procedures.To utilize the advantage of Zn O and simplify the membrane preparation process,porous ZnO supports were used in this work,and MAF-4 membranes were successfully grown by the facile secondary growth method.Powder X-ray diffraction,energy dispersive X-ray spectroscopy and scanning electron microscope showed that the obtained membranes were compact and continuous with thickness of 4 μm.Single-component gas permeation experiments gave an H2/CO2 ideal selectivity of 7.5 and an H2 permeance of 2.1×10^（-7） mol/（m^2 Pa s） under a pressure drop of 100 k Pa at 25°C.When the temperature increased to 100°C,the H2/CO2 ideal selectivity increased to 25,while the H2 permeance remained 2.0×10-7 mol/（m^2 Pa s）.For equimolar H2/CO2 mixture,the obtained MAF-4 membrane showed a selectivity of 5.7 for H2/CO2 and a H2 permeance of 1.2×10^（-7） mol/（m^2 Pa s） under a pressure drop of 100 k Pa at 25°C.This work indicates porous ZnO as a suitable support for fabrication of other Zn-based MOF membranes.