Preparation And Properties Of Methane/nitrogen Separation Membranes Based On Metal-organic Frameworks

Coal-bed methane（CBM）,which is rich in reserves,clean and efficient,plays an important role in the energy structure of China.Removing the main impurity N₂ to enrich CH₄ is very important for the efficient utilization of CBM.Membrane separation technology has many advantages,such as low energy consumption,high efficiency and small occupied area.The development of CH₄/N₂ membrane materials with high permeability and high selectivity has been one of the research focuses in the field of membrane separation.MIL-100（Cr）is a metal-organic framework material（MOFs）composed of metal ions Cr（III）and 1,3,5-Benzenetricarboxylic acid.It has good chemical stability,high specific surface area and abundant unsaturated metal sites,the organic ligands in the framework have high affinity and preferential adsorption ability to CH₄,which has potential advantages in preparing mixed matrix membranes with high separation performance of CH₄/N₂.In this paper,MIL-100（Cr）was used as nano-filling unit and polyether block copolyamide（Pebax）was used as polymer matrix to construct the mixed MOF membrane for CH₄ separation,in order to separate CH₄/N₂efficiently from CH₄/N₂,functional group modification and channel adjustment were carried out by using the coordination unsaturated metal sites in the framework of MIL-100（Cr）.The main results are as follows:（1）MIL-100（Cr）was synthesized by solid-state method and was added into Pebax matrix to prepare Pebax/MIL-100（Cr）mixed matrix membrane.The effects of MIL-100（Cr）addition on the physical and chemical properties and the separation performance of CH₄/N₂ were investigated.The results show that MIL-100（Cr）can be uniformly dispersed in Pebax polymer matrix,and the thermal stability of Pebax/MIL-100（Cr）mixed matrix membrane is improved,with the increase of MIL-100（Cr）,the solubility of the mixed matrix membrane for CH₄gradually improved,and the osmotic selectivity of CH₄/N₂ increased first and then decreased.When the addition of MIL-100（Cr）was 15 wt%,Pebax/MIL-100（Cr）mixed matrix membrane had the best performance,with the permeability coefficient of CH₄ remaining at 20.01 barrer and the separation selectivity of CH₄/N₂ reaching 3.71.（2）By ligating the unsaturated metal sites in MIL-100（Cr）structure with naphthalyl alcohol,ethylene glycol,2-nitroethanol and propanol,hydroxyl,nitro,methyl and naphthalene groups were introduced into MIL-100（Cr）to prepare MIL-100（Cr）modified by different functional groups,which were added into Pebax matrix to prepare mixed matrix membrane.The effects of modified functional group structure on the adsorption performance of MIL-100（Cr）gas and CH₄/N₂ separation performance of mixed matrix membrane were investigated.The results show that the original morphology and crystal structure of MIL-100（Cr）are not changed by functional group modification at the unsaturated metal sites.The adsorption selectivity of CH₄/N₂ to MIL-100（Cr）was 3.71,25%higher than that of unmodified MIL-100（Cr）,and the adsorption selectivity of CH₄/N₂ was 3.71.At the same time,the mixed matrix membrane prepared with MIL-100（Cr）modified by naphthalene group,methyl group and hydroxyl group has better CH₄/N₂ separation performance.When the particle content is 15wt%,the selectivity of the mixed matrix membrane CH₄/N₂ reaches 4.01,which is 22%higher than that of pure Pebax membrane.The osmotic flux of CH₄ remained basically unchanged.（3）NO₂-MIL-100（Cr）and its mixed matrix membranes were prepared by using 2-nitro-ethanol as the grafting agent,and the effects of the nitro content on the gas adsorption and CH₄/N₂ separation properties of the membranes were investigated.The results show that with the increase of the dosage of 2-nitroethanol,the nitro content in NO₂-MIL-100（Cr）structure increases gradually,the adsorption selectivity for CH₄/N₂ increases gradually,but the pore size decreases gradually.When the concentration of 2-nitroethanol was 6 mmol,the adsorption selectivity of NO₂-MIL-100（Cr）particles for CH₄/N₂ reached 3.85.In addition,the separation performance of Pebax/NO₂-MIL-100（Cr）mixed matrix membrane for CH₄/N₂ increased first and then decreased with the increase of the dosage of 2-nitroethanol.When the dosage was 4mmol and the addition amount of NO₂-MIL-100（Cr）was 15 wt%,Pebax/NO₂-MIL-100（Cr）mixed matrix membranes obtained the best separation performance of CH₄/N₂,with a CH₄permeation flux of 18.5 barrer and a selectivity of 4.11.