Fabrication and orientation of Ni-LAB membranes by linker salt approach

Abstract Metal-organic framework (MOF) materials have exhibited exponential growth over the last two decades and MOF membranes already show great potential in separation application. Herein the linker salt method was firstly applied to the preparation of MOF membranes using Ni-LAB [Ni 2 (L-asp) 2 (bipy)] membranes as a model. This method is based on the utilization of organic linker salts instead of its protonated forms, thus their solubility and deprotonation step could be greatly enhanced, leading to a higher supersaturation that is beneficial for the formation of MOF membranes. This method was effective and led to the greener and milder synthesis for Ni-LAB membranes. In this work, all of the Ni-LAB membranes were fabricated on α-Al 2 O 3 tubes by secondary growth method using ball-milled Ni-LAB seeds. By using the linker salt potassium l -aspartate hydrate and soluble metal source nickel nitrate hexahydrate, continuous and compact Ni-LAB membrane can be prepared by crystallizing at 150 °C for only 1 h, in which both the soluble metal precursor and the organic linker salt played important roles in the rapid preparation of Ni-LAB membrane. Effect of crystallization temperature on membrane preparation was investigated, it was found that Ni-LAB membrane can be synthesized even at 70 °C and the membranes prepared at different temperatures showed different morphologies. The Ni-LAB membrane could also be prepared in a dilute solution and the concentration of synthetic solution greatly affected the orientation of Ni-LAB membrane. Ni-LAB membrane exhibited preferential (200) orientation when prepared with a low concentration of synthetic solution whereas it showed preferential (101) & (002) orientation when prepared with a relative high concentration. The generation of corrosive nitric acid during the preparation process can be greatly reduced and replaced by eco-friendly potassium nitrate. Single gas permeation tests were carried out to evaluate the quality of the as-synthesized Ni-LAB membranes.