Controllable Growth of Zeolitic Imidazolate Framework Composite Membranes for Gas Separation

Abstract

As an emerging class of hybrid organic-inorganic nanoporous material, metal organic frameworks (MOFs) with tunable pore size and chemistry are very attractive for integration into membranes and thin films for gas separation applications. Zeolitic imidazolate frameworks (ZIFs), a subfamily of MOFs, are known for their permanent porosity and exceptional chemical and thermal stability. Among a number of available ZIF materials, ZIF-8 is particularly interesting owing to its relative ease of synthesis as well as its great potential in separating small gas molecules. However, the progress on the fabrication of ZIF-8 membranes with satisfactory gas separation performance is very limited and there is no report of ZIF membranes being used in industrial scale so far. Therefore, development of simple and more effective methods to fabricate high quality ZIF molecular sieving membranes with high gas selectivity is still required. The new processing approaches require the advantages of being rapid, reproducible, scalable, and economically and environmentally viable while simultaneously producing high quality ZIF membranes. The ultimate goal of this PhD research program is to address challenges that hinder the facile synthesis of supported-ZIF membranes in a reproducible and scalable manner. In this thesis, three new approaches are demonstrated to potentially address these challenges. First, a novel scalable strategy of using vapor phase to chemically modify the polymer support for ZIF membrane fabrication is developed. Such surface modification enabled fast formation of a continuous ZIF-8 ultrathin layer after only 3 minutes. The resulting ZIF-8 membranes exhibited exceptional H2 permeance as high as 2.05 ×10-6 mol m-2 s-1 Pa-1 with high H2/N2 and H2/CO2 selectivities (9.7 and 12.8, respectively). Next, based on the chemical vapor modification, a simple, effective, and environmentally friendly method is described for the fabrication of high-quality ZIF-8 membranes with controllable positioning on a polymer substrate in aqueous solution. The ZIF-8 membrane exhibited a propylene permeance of 1.50 × 10–8 mol m–2 s–1 Pa−1 and excellent selective permeation properties; after post heat-treatment, the membrane showed ideal selectivities of C3H6/C3H8 and H2/C3H8 as high as 27.8 and 2259, respectively. The new synthesis approach holds promise for further development of the fabrication of high-quality polymer-supported ZIF membranes for practical separation applications. Finally, a new concept for the use of one-dimensional material (e.g. CNT) as nano-scaffolds and pseudo-seeds for the fabrication of molecular sieving membranes supported on a porous substrate is introduced. To demonstrate the potential for universal applicability of the proposed pseudo-seeding and nano-scaffolding method, ZIF-8/CNTs membranes were prepared on both polymeric and inorganic substrates. At 25 °C and 1 bar, the ideal separation selectivities of H2/CO2, H2/N2, H2/CH4, H2/C3H6, and H2/C3H8 are 14, 18, 35, 52.4 and 950.1, respectively, with H2 permeance as high as 2.87 × 10−5 mol m−2 s−1 Pa−1. This high hydrogen permselectivity combined with its mechanically reinforced structure shows that the ZIF-8/CNT membrane is a promising candidate for hydrogen separation and purification. Finally, it is anticipated that the novel strategies developed in this research may be further developed for the fabrication of other MOF and zeolite molecular sieve membranes.