Preparation And CO₂ Separation Performance Of LDH-based Mixed Matrix Membranes

Recent years,it is a major issue of global concern that the greenhouse effect caused by the continuous increase in the concentration of CO₂ in the atmosphere.Among various technologies for separating and capturing carbon dioxide,membrane separation is widely concerned because of its simple operation,more energy saving and environmental protection,etc.Generally speaking,polymer membranes and inorganic membranes have been widely used in gas separation processes,but each has its own advantages and disadvantages.Layered double hydroxide（LDH）is a type of two-dimensional sheet material with a unique structure.Therefore,this topic proposes to use two-dimensional LDH as a filler to explore the influence of its structure on the CO₂ separation performance of MMMs;In order to further improve the CO₂permeability of LDH-based MMMs,a constract of three-dimensional hollow LDH were proposed and the influence law and mechanism of hollow structure on gas permeation were explored.First,a two-dimensional layered ZnAl-LDH material was synthesized by a hydrothermal method,which has a hexagonal smooth sheet structure with a uniform shape and a size of about200 nm.It was further blended with the polymer Pebax1657 to prepare Zn Al-LDH/Pebax mixed matrix membranes,and the influence of different loadings on the gas separation performance was explored.The results show that the CO₂ permeation of MMMs filled with LDH was significantly improved.This is due to the abundant hydroxyl groups on the LDH laminates,which have an affinity for CO₂.At the same time,the lamellar structure provides a gas transmission channel,which can reduce the CO₂ transmission resistance,thereby increasing the CO₂ permeation rate.When the fillers loading is 3%,MMMs show the best performance.At25°C and an absolute pressure of 0.4 MPa,it has a CO₂ permeability of 88.20 Barrer and a CO₂/N₂ selectivity of 67.01,which are 66.41%and 21.82%higher than pure Pebax membranes.However,two-dimensional LDH-based materials are dense and non-porous materials with a large degree of stacking and a small specific surface area.The effective hydroxyl sites provided in the membrane are limited,which limits the further improvement of its CO₂ permeability.Three-dimensional hollow structure has a larger specific surface area,which is beneficial to provide more hydroxyl sites,and the porous structure can reduce the gas transmission resistance,thereby effectively increasing the CO₂ permeation rate.In order to further improve the contribution of LDH-based materials to the CO₂ permeability of MMMs,ZIF-67 was used as a precursor to successfully prepare three-dimensional hollow CoNi-LDH materials by in-situ conversion method,and the suitible conversion conditions were explored.The nitrogen adsorption and desorption test results show that the three-dimensional hollow CoNi-LDH has both microporous and mesoporous structures,which is beneficial to construct low-resistance gas transmission channels,and the specific surface area is greatly increased（329.77 m²/g）,which can provide more for CO₂.The hydroxyl affinity site is conducive to CO₂ transfer.Three-dimensional hollow CoNi-LDH with different conversion time was added as a filler to Pebax to prepare MMMs.The effect of different conversion time materials on the membrane performance was explored,and CoNi-LDH with the best conversion time was selected as fillers to prepare a series of MMMs with different filler loading.The results show that the three-dimensional hollow CoNi-LDH/Pebax membranes with a loading of 1 wt%and conversion time of 2h has the best performance.It has a CO₂ permeability of 111.05 Barrer,which is 109%higher than the pure Pebax membrane,and the selectivity is 71.11,which also has an increase of 29.3%.