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Synthesis of ZIF-68 Membrane on a ZnO Modified #Alumina Support by a Modified Reactive Seeding Method Alexandra Kasik, Joshua B. James, and Jerry Y.S. Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00236 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on March 4, 2016
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Synthesis of ZIF-68 Membrane on a ZnO Modified α-Alumina Support by a Modified Reactive Seeding Method Alexandra Kasik, Joshua James and Y.S. Lin* School of Engineering Matter, Transport and Energy Arizona State University Tempe, AZ 85287-6006, USA Abstract: Large-pore ZIF-68 membranes offer adsorption-based selectivity for separation of gas mixtures or molecular sieving characteristics for the separation of large liquid molecules. ZIF-68 membranes can be grown on ZnO modified α-alumina supports by a modified reactive seeding method. The resultant membranes were around 40 microns in thickness and were determined to have limited non-selective defects given their adherence to Knudsen diffusion during single gas permeation measurements.
Further pervaporation experiments showed that the ZIF-68
membranes synthesized via the modified reactive seeding method had a p-xylene pervaporation flux approximately 5.4 times as large as that reported for similar pore-sized MOF-5 membranes, however pervaporation flux of larger molecule, di-tert-butylbenzene, through the same two MOF membranes showed the flux of the ZIF-68 membrane was 3.4 times smaller than that reported for MOF-5. This reversal in pervaporation flux indicates the ZIF-68 structure is more readily accessible to molecules smaller than its pore size, but larger molecules are subjected to a staunch cut off in flux.
Keywords: metal-organic frameworks, zeolitic imidazolate frameworks, microporous membranes, synthesis, reactive seeding *Correspondence Author: email:
[email protected], phone 480-965-7769 1 ACS Paragon Plus Environment
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1 Introduction Zeolitic imidazolate frameworks (ZIFs) are a relatively new extension of the widely studied metal organic frameworks (MOFs). Both MOFs and ZIFs are microporous materials characterized by metallic ion secondary building units (SBUs) tetrahedrally coordinated with organic linkers. ZIFs differ, however, as the bond angle created in ZIFs when an imidazole linker joins two tetrahedrally coordinated metal ions, M-Im-M, is almost identical to the Si-O-Si bond angle formed in zeolites.1–3,4
Due to this, an extensive cross-section of traditional
aluminosilicate zeolite topologies has been replicated in ZIFs, and much like zeolites, the ZIF materials show similar chemical and thermal stability.1,2,4 The research on ZIF membranes is saturated with smaller pore ZIF materials. Those reported on are ZIF-7 (3.0 Å), ZIF-9-67 (< 3.4 Å), ZIF-8 (3.4 Å), ZIF-90 (3.5 Å), ZIF-78 (3.8 Å), ZIF-71 (4.2 Å) and ZIF-69 (4.4 Å).1,3,5 While the pore size and functionality of these membrane materials has rendered them front running candidates for applications involving the separation of smaller molecules, such as H2 purification and CO2 sequestration, there are, however, marked differences between the smaller, more widely researched ZIFs, and those with larger pores.1 Larger pore ZIFs, while still retaining the characteristic desirability afforded to ZIF materials, exhibit perm-selectivity that is driven by adsorption properties of the material. The isolation of solely adsorption based separation has been used to effectively separate a number of mixtures in other large pore MOF and zeolite materials.6,7 To isolate the inherent adsorption properties of the ZIF membrane material, without having to suffer any losses in selectivity due to secondary sieving effects, could prove invaluable for a number of industrially relevant separation processes.
Further, another motivation behind studying large pore ZIF
membranes is the potential they offer for large molecule gas and liquid separation. It is by virtue
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of this, work was carried out to synthesize the 7.5Å pore size ZIF-68 in useable membrane form.8 Given no other large-pore ZIFs have been synthesized in membrane form as of yet, initial synthesis attempts for ZIF-68 were modeled after a smaller pore ZIF, ZIF-69, that was chosen due to its similar dual ligand nature and P63/mmc structure.9
The two reports on ZIF-69
membrane synthesis outline a seeded secondary growth method and an in situ method utilizing porous alumina supports.10,11 In initial synthesis attempts of ZIF-68, these methods were used as a general rubric, and modifications were made as necessary. The findings, however, were that regardless of the parameters altered, heterogeneous nucleation of ZIF-68 was not attainable through in situ synthesis, nor was seeding effective. Images obtained using scanning electron microscopy showed exceptionally poor coverage of the alumina support, and it was hypothesized that the rate at which ZIF-68 grows along the c-axis yields crystals that are far too large for effective seeding without the addition of a surface modification or anchor. In lieu of this, reactive seeding on a ZnO support was attempted. The reactive seeding method, which has been reported in literature previously for MIL53, MIL-96 and ZIF-71, utilizes the unsatiated terminal metallic atoms at the surface of the oxide support to then react with the organic precursors in solution to create a closely bound seeds layer. 12,13,14
Following a secondary growth step with the relevant organic precursors and an external
source of the needed metallic ion, a continuous, well-intergrown membrane layer can be obtained. It was found, however, during the characterization of the ZnO supported ZIF-68 membranes synthesized via reactive seeding that great care was needed to maintain a transmembrane pressure drop of no more than 0.68 atm, or catastrophic cracking of the support would occur.8 Given the weaker nature of the ZnO support, and its potential hindrance to the 3 ACS Paragon Plus Environment
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applications for which ZIF-68 membranes synthesized upon it are suited, developing a new manner of synthesis on an alumina support was necessary. The objective of this work was to synthesize ZIF-68 membranes on more robust alumina supports by a method that utilizes the principles behind reactive seeding. 2 Experimental 2.1 Surface Modification of Alumina Support with ZnO The alumina (Al2O3) supports used in the synthesis of ZIF-68 via the modified reactive seeding method were made in house from a press-sintering method reported elsewhere.15 Once sintered, the supports were polished with progressively finer grit sandpaper (#500, #800 and #1200) using an MTI Unipol-320 polisher. Following an ultrasonic cleaning in ethanol and distilled water and an overnight drying at 100°C, the supports were then ready for use. 2.1.1 Surface Modification via Dip Coating and Sintering The first method of surface modification attempted was dip coating. A solution of 2 wt% zinc oxide (ZnO, 99.9%, Sigma Aldrich) in dimethylformamide (DMF, 98%, Alfa Aesar) was agitated until the
