Preparation And CO₂/CH₄ Separation Performance Of Functionalized 2D Zeolite Nanosheets Mixed Matrix Membrane

Carbon dioxide（CO₂）separation from gas mixtures has gained increasing interests in energy and environmental fields since the soaring increase in CO₂ emission from energy-related industrial processes constitutes a major concern of global warming.Among various separation techniques,membrane technology could offer a promising solution to achieve highly efficient,low-cost and environmentally friendly CO₂ separation.Mixed matrix membranes（MMMs）,made up by polymer continuous phase and nanomaterials dispersed phase,hope to marry the robust processing properties of polymers with the outstanding permeability and separation properties of advanced nanomaterials,which has been applied to break the permeability-selectivity trade-off.However,MMMs suffer from some limitations such as the non-uniform distribution of fillers and interfacial incompatibility.This study focuses on the structure optimization and mechanism investigation of MMMs based on two-dimensional（2D）zeolite nanosheets as fillers,aiming to strengthen CO₂/CH₄ separation performance.The main research contents are as follows:（1）Traditional MFI zeolite crystals and 2D MFI nanosheets were introduced as fillers into Pebax matrix to prepare MMMs.The effect of filler morphology on the CO₂separation performance of the membrane was investigated.Compared with traditional crystals,the large specific surface area of 2D MFI nanosheets provides abundant surface silanols,which improves the compatibility between fillers and polymer matrices.The fast diffusion path provided by the ultrathin thickness of nanosheets facilitates the rapid transport of CO₂.Under dry gas feed conditions,the optimized MMM showed the ideal CO₂/CH₄ selectivity of 29.9 with the CO₂ permeability of 188.9 Barrer.（2）Small-sized APDEMS-modified 2D MFI nanosheets were introduced as fillers into Pebax matrix to prepare MMMs.The size of the filler was tuned to achieve the uniform distribution of the fillers in the matrix from the macro scale.APDEMS was grafted on the surface of 2D MFI nanosheets to regulate interface compatibility between the filler and the matrix from the micro scale.Under dry gas feed conditions,the optimized MMM showed the ideal CO₂/CH₄ selectivity of 43.1 with the CO₂permeability of 190.2 Barrer.（3）Ag⁺and H⁺ion-exchanged Na-ZSM-5 nanosheets were introduced as fillers into Pebax matrix to prepare MMMs,and the effects of three different cations on the CO₂ separation performance of the membrane were investigated.Ag⁺ions in the pore channel act as a carrier to promote CO₂ facilitated transport and adjust the pore size.By synergistically strengthening the facilitated transport and diffusion selective mechanisms,the optimized MMM showed the ideal CO₂/CH₄ selectivity of 38.9 with the CO₂ permeability of 185.4 Barrer.（4）Modified by carboxyl groups,the traditional Na-ZSM-5 zeolite crystals and 2D Na-ZSM-5 nanosheets were introduced as fillers into Pebax matrix to prepare MMMs,respectively.Carboxyl modification of the fillers improves its CO₂ affinity and the interaction between fillers and polymer matrices,synergistically strengthening the solution and diffusion selective mechanisms of MMMs.Under the same conditions,the 2D Na-ZSM-5 nanosheets provided more surface silanols as the grafting sites for carboxyl modification,which greatly improved the positive effect of carboxyl groups.Under dry gas feed conditions,the optimized MMM showed the ideal CO₂/CH₄selectivity of 47.7 with the CO₂ permeability of 200.3 Barrer.