Preparation Of 2-D Continuous And Stable ZIF-8 Membranes By Zn(?)-PDA Induced Growth

ZIF-8,as the typical representative of ZIFs,with a pore size of 0.34 nm,high porosity,superb chemical and thermal stability,is often used to make continuous composite membranes for gas separation,but this kind of membranes is subjected to the poor interfacial adhesion between the ZIF-8 skin and its supporting substrate.Such a composite structure is prone to be fragmenting,once a slight deformation is caused by mechanical action or shear stress.In addition,the absence in nucleation sites on the surface of the carrier makes the crystal nucleate first in solution,and the all-round growth of ZIF-8 crystals is a time-consuming process,consequently,discrete ZIF-8 islands are often formed on the supporting layer.Therefore,it is significant to ameliorate the substrate surface so that a continuous ZIF-8 film can be tightly anchored on the substrate.In this paper,Zn???-PDA,used as an intermediate modified layer,was activated by 2-methylimidazole?Hmim?to form nucleus inside the coating,thus induce the synthesis of continuous and stable ZIF-8 composite membranes.First,the plate polyvinylidene fluoride membrane?PVDF?was modificated by Zn???-PDA,activated by Hmim,and then was used to synthesize the continuous and stable ZIF-8 composite membrane at room temperature in water.The doping of Zn???into PDA coatings enhances the stability of the coating in water and the uniformity on PVDF membranes;the Zn???-PDA coating activated by Hmim can preferentially form the crystal nucleus,promoting the rapid growth of the crystal,and the final synthetic time is determined to be 5 h.In this case,the permeation flux of H₂ is 6.42×10⁴ GPU and the ideal selectivity of H₂/N₂,H₂/CO₂ and H₂/CH₄ is 7.0,6.5 and 8.0,respectively,and the membrane has good preparation repeatability by different batch tests;after ultrasonic processing for 2 h,the membrane still maintains a complete composite structure without defects and cracks,thus it has good mechanical stability.Secondly,stainless steel nets?SSNs?with high mechanical strength were modificated by Zn???-PDA,activated by Hmim,and then was used to synthesize continuous and stable ZIF-8composite membrane via thermomethanol method.The doping of Zn???into PDA coatings enhances the stability of the coating in thermomethanol solvent and the uniformity on SSNs;Compared with PDA coating,Zn???-PDA coating can make the ZIF-8 crystal nucleus uniformly distributed in the coating,grow rapidly,and shorten the synthesis time by 1/3.The membrane shows excellent gas separation performance with J_H2=3980 GPU,?_H2/CO2=9.9,?_H2/N2=15.1,and?_H2/CH4=26.0 at 100?,18 h and the membrane has good preparation repeatability by different batch tests;after ultrasonic processing for 2 h,the membrane still maintains a complete composite structure without defects and cracks,thus it has good mechanical stability.Based on the two experimental results,the nucleation growth mechanism of ZIF-8 on Zn???-PDA coating was investigated deeply.It turned out that strong alkaline conditions are conducive to the deprotonation of dopamine,which enables O atoms to have higher electron cloud density and stronger coordination ability with Zn???,while there is no Zn?OH?₂precipitation formed;there are a large number of Zn???clusters and Zn???-O coordination bonds in the Zn???-PDA coating,and these Zn???clusters can be as the starting points for ZIF-8 nucleation growth.When they are activated by Hmim,there will be crystal nucleus formed in the coating,inducing stable film formation rapidly;in addition,Zn doping along with polymerization can provide abundant starting points,so the corresponding ZIF-8 film is very stable.