Pervaporation Separation of Organic Mixtures by MOF‑5 Membranes Amr Ibrahim and Y. S. Lin* School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, Arizona 85287-6006, United States ABSTRACT: Metal organic framework (MOF) membranes have received much attention for gas separation applications, with however limited information about their liquid separation properties. This paper reports a study of permeation and separation of liquid organics by a MOF-5 membrane in pervaporation mode. Multiple high quality MOF-5 membranes were reproducibly prepared by the secondary growth method for various experimental runs. The pervaporation of pure toluene, o- xylene, and 1,3,5-triisopropylbenzene (TIPB) and the separation of their binary mixtures were studied. The permeation flux and separation factors decrease with pervaporation on-stream time and steady state permeation flux could not be reached even after 10 h of pervaporation. The fouling effects do not change the crystalline structure of the MOF-5 membrane. The pervapora- tion flux with the mixture feed is lower than the pure component flux, and the reduction in the flux decreases with decreasing affinity of the permeating species with MOF-5. The mixture maximum separation factors for toluene/TIPB and o-xylene/TIPB are respectively about 26.7 and 14.6, significantly higher than the pure component ideal separation factor. The fluxes and separation factors cannot be restored to their original values upon membrane activation at 100 °C in vacuum. ■ INTRODUCTION Metal-organic frameworks (MOFs) are a group of crystalline, microporous materials consisting of metal ions linked together by organic ligands. 1 One representative MOF is IRMOF-1 (or more commonly referred to as MOF-5) with a highly crystalline cubic structure consisting of a network of zinc oxide tetrahedra connected by terephthalic acid linkers creating inner cavities of about 12 and 15 Å in diameter and aperture opening of 8 Å in width. 2 MOF-5 has been widely studied as adsorbents for gas storage, gas purification, and separation applications as well as heterogeneous catalysis. 3 MOF-5 membranes were also prepared and studied for gas separation. 4-6 However, MOF-5 is known to be unstable in humid air, 7-9 which hinders its gas- phase applications involving a trace amount of water vapor. MOF-5 may find applications in which the contact with the humid atmosphere is negated, such as adsorption 10-12 and pervaporation of organic liquids. 6,13 Recent studies have shown that MOF-5 can separate three pentane isomers 10 and some aromatics of different molecular size. 14 The adsorption strength of aromatics on MOF-5 decreases in the order p-xylene > ethylbenzene > toluene > benzene. 14 Experimental data showed that a fixed-bed MOF-5 adsorber can separate a xylene isomer from ethylbenzene but not the three xylene isomers. 15 The MOF-5 crystals are structurally stable in organic liquids. Lin and co-workers 6 studied pervaporation of several organic liquids including some aromatics with molecular sizes close to the pore size of MOF-5 membranes. They found that the pervaporation fluxes for a 14 μm thick MOF-5 membrane decreases with increasing kinetic diameter of the permeating organic compounds, with a sharp (2 order of magnitude) drop in the flux for the two larger molecules, as summarized in Table 1. Also, it was found that MOF-5 membranes are structurally stable upon pervaporation of organic liquids. The data in Table 1 show molecular sieving separation characteristics of the MOF-5 membrane for the molecules with the size significantly larger than the aperture size of MOF-5 crystals. Molecules with a size smaller or slightly larger than the MOF-5 pore size can permeate through the membrane. Similar results were found for some of zeolitic imidazolate framework materials. For example, from crystallographic data, ZIF-8 pore opening is estimated to be 3.4 Å. However, several studies show that even molecules with a size larger than that of ZIF-8 pores, Received: May 21, 2016 Revised: July 16, 2016 Accepted: July 21, 2016 Published: July 21, 2016 Table 1. Pervaporation Fluxes of Pure Organic Compounds through a MOF-5 Membrane 6 permeating molecule kinetic diameter (Å) flux (10 -4 mol/(m 2 ·s)) p-xylene 5.8 9.00 o-xylene 6.8 7.25 tri-isopropylbenzene 8.4 2.08 1,3-di-tert-butylbenzene 11 1.71 2-dicyclohexyl-phosphino-2′-(N,N- dimethylamino)-biphenyl 12 0.014 Article pubs.acs.org/IECR © 2016 American Chemical Society 8652 DOI: 10.1021/acs.iecr.6b01965 Ind. Eng. Chem. Res. 2016, 55, 8652-8658