Synthesis of ZIF-68 Crystals and Membranes On ZnO Supports Using Reactive Seeding Method

In recent years, hybrid materials known as metal-organic frameworks (MOFs) have begun garnering a sizeable amount of attention.  Similar to zeolite membranes, large pore MOF-5 membranes exhibit good adsorption-controlled perm-selectivity for carbon dioxide over less adsorbing gases such as nitrogen.  However, MOF-5 is amenable to degradation in humid atmosphere.   Zeolitic imidazolate frameworks (ZIFs), a specific MOF wherein the topologies mimic those of zeolites, except with the advantages of the hybrid structure and organic ligands, are structurally more stable than MOFs.  This paper reports the synthesis of crystals and membranes of ZIF-68, a highly crystalline, larger pore (pore diameter of 0.75 nm) ZIF analog to MOF-5.   The work is the first effort to synthesize ZIF-68 membranes.

ZIF-68 crystals were synthesized solvothermally over a range of temperatures and times to optimize synthesis conditions and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and N₂ porosimetry.    The results indicate that at 85°C, the development of crystals was proportional to sintering time.  From 42 to 72 hours, there was an increase in crystal size, as seen in SEM photomicrographs, an increase in crystallinity, as seen in XRD spectra, and a general increase in N₂ adsorption capacity and surface area, as seen in N₂ porosimetry measurements.  This trend was not continued past 72 hours, and the crystals synthesized up to 96 hours showed a decrease in quality.        

Attempts to utilize the optimized crystals synthesized to create the seeding layer on macroporous alumina support did not yield the coverage and intergrowth needed, so a reactive seeding method was used to coat the seed layer.  Macroporous zinc oxide supports were prepared by press and sintering of zinc oxide powder.  The zinc support surface was brought in contact with imidazolate precursors in solution to create intimately attached ZIF-68 seeds.  Following seeding, a secondary growth step was incorporated to facilitate enhanced crystal growth along the c-axis, as well as complete interpenetration of the seed layers. Continuous, c-oriented ZIF-68 membranes of 50-100 mm were synthesized on porous zinc oxide supports.   The helium permeance for the ZIF-69 membrane is about 2 x 10^-7 mol/m².s.Pa, a 10 time reduction from the 2 x 10^-6 mol/m².s.Pa helium permeance of the zinc oxide support.  Gas permeance of He, N₂, Ar and CO₂ for the ZIF-68 membranes will be presented to examine the quality of the membranes prepared in this work.