Zeolitic imidazolate frameworks (ZIFs) have been foregrounded as structures with exceptional, intrinsic chemical and thermal stability. However, there has yet to be a systematic study of the isothermal stability of ZIFs, specifically the well-studied ZIF-8. In this work, ZIF-8 isothermal TGA decomposition kinetics were studied in air, argon, H 2 /CO 2 , and nitrogen environments by exposing ZIF-8 to each gas for 20 h at temperatures of 200, 250, and 300 °C, respectively. ZIF-8 crystallinity was preserved under the experimental isothermal conditions at 200 °C in each atmosphere, but crystallinity was increasingly eliminated at higher temperatures. Decomposition kinetics data show that the rate of ZIF-8 carbonization significantly increases at temperatures above 200 °C irrespective of environment. ZIF-8 decomposition in the H 2 /CO 2 reducing mixture exhibits the slowest decomposition kinetics at all temperatures and the greatest morphological change. At 300 °C, oxidative effects enhance ZIF-8 decomposition in air. At lower temperatures the decomposition rate in air behaves more similarly to that of nitrogen and argon. Arrhenius activation energy parameters enable postulation that the temperature dependency of ZIF-8 thermal decomposition after carbonization at 300 °C is more similar upon decomposition in inert and reducing environments as compared to decomposition in oxidizing atmosphere. Four chemical equations inferring the residual carbonized ZIF structure after decomposition at 300 °C were developed based upon EDS quantification and FTIR/azirine formation models. The FTIR/azirine derived model postulates a heterogeneous carbonized ZIF-8 structure containing 2-methylimidazole and azirine rings coordinated to zinc and more precisely agreed with TGA weight decomposition data than the EDS derived model.

The most common products obtained in the synthesis of zirconium-based metal−organic frameworks (ZrMOFs) are fine powders. The particle size of a typical ZrMOF UiO-66 was first reported to be around 200 nm, so the original crystal structure was only solved by powder XRD coupled with Rietveld refinement due to the incapability of single crystal XRD to solve such small crystals with poor crystallinity. One may ask the reason why the particle size of UiO-66 is so small compared to that of other common MOFs and what the key factor terminating the growth of UiO-66 is. In this work, we try to answer this question by proposing a hypothesis that the partially deprotonated ligand caused by the accumulated protons in the reaction solution is the key factor preventing the continuous growth of the UiO-66 crystal. The hypothesis is verified by growth reactivation with the addition of a deprotonating agent in an in situ biphase solvothermal reaction. As long as the protons were sufficiently coordinated by the deprotonating agent, the continuous growth of UiO-66 is guaranteed. Moreover, the modulation effect can impact the coordination equilibrium and nucleation so that an oriented attachment growth of UiO-66 film was achieved in membrane structures.

The thermal stability of ZIF membranes is important for high temperature separation applications but has not been systematically studied. This work highlights the results of a thermal stability study of ZIF-8 membranes in terms of material structure, H 2 /CO 2 gas permeation and separation characteristics. During binary and single gas temperature dependent permeance tests conducted from 25-250 o C, both H 2 and CO 2 permeances decrease as a function of temperature. In the binary test, H 2 /CO 2 selectivity increases between 25-225 ○ C, and then decreases as temperature is further increased between 225-275 o C. The results can be explained by the adsorption/diffusion mechanism. Beyond 275 ○ C, H 2 /CO 2 permeance and selectivity drastically increase with respect to temperature and is indicative of ZIF-8 membrane partial carbonization during the dynamic 30 hour temperature dependent test. The time/temperature dependency of the onset of ZIF-8 thin film structural change was deconvoluted in isothermal transient permeation experiments. Transient tests performed at 50, 100, 150 and 300 ○ C for 24 hours indicate that ZIF-8 thin films maintain their crystallinity and structural integrity below 150 ○ C. However, at temperatures of 150 ○ C and greater the framework undergoes increased magnitudes of thermally induced carbonization as a function of temperature. Thermomechanically induced stresses between the ZIF-8 membrane thin film and α-alumina support may account for differences in static thermal stability observed when comparing ZIF-8 membranes and ZIF-8 crystalline powders.

Zeolitic imidazolate framework-8 or ZIF-8 membranes have shown great promise in separating propylene/propane (C3) mixtures; however far fewer works have analyzed ethylene/ethane (C2) transport behavior in ZIF-8 membranes. This work studies C2 permeation behavior, transport properties, and selectivity as a function of temperature and pressure in single and binary gas mixtures. In single and binary separation tests conducted from 25 to 100 °C, the permeances of ethylene and ethane show a negative correlation with temperature attributable to activation energies of diffusion (E d) for ethylene and ethane (11.7 and 13.2 kJ/mol) that are lower than their respective heats of adsorption (16.2 and 17.1 kJ/mol). Low E d values are observed for C2 molecules in ZIF-8 due to pore flexibility. C2 diffusive selectivity is limited in ZIF-8 due to the similar size of C2 molecules which are both smaller than the effective ZIF-8 pore aperture (low energetic selectivity) and sizable entropic selectivity is limited by the zeolitic pore shape. Binary selectivity is 20% lower than ideal selectivity due to cooperative adsorption, which enhances ethane adsorption in the presence of ethylene. The presence of relatively stronger adsorbing C2 molecules in mixture with hydrogen decreases H2 permeability and inverts the H2 temperature dependency of permeation from adsorption controlled to diffusion controlled. In single and binary C2 pressure dependent experiments performed between 1 and 4 atm, starkly contrasting ethylene/ethane separation profiles are observed due to differences in single and binary adsorption isotherms for C2 molecules. The ZIF-8 structure is amenable to adsorption/pressure induced distortions which greatly affect C2 permeation behavior.

SignificanceGas separation by metal-organic framework (MOF) membranes is an emerging research field. Their commercial application potential is, however, still rarely explored due in part to unsatisfied separation characteristics and difficulty in finding suitable applications. Herein, we report "sharp molecular sieving" properties of high quality isoreticular MOF-1 (IRMOF-1) membrane for CO 2 separation from dry, CO 2 enriched CO 2 /CH 4 , and CO 2 /N 2 mixtures. The IRMOF-1 membranes exhibit CO 2 /CH 4 and CO 2 /N 2 separation factors of 328 and 410 with CO 2 permeance of 2.55 3 10 27 and 2.06 3 10 27 mol m 22 s 21 Pa 21 at feed pressure of 505 kPa and 298 K, respectively. High grade CO 2 is efficiently produced from the industrial or lower grade CO 2 feed gas by this MOF membrane separation process. The demonstrated "sharp molecular sieving" properties of the MOF membranes and their potential application in production of valueadded high purity CO 2 should bring new research and development interest in this field.

Zeolitic imidazolate frameworks (ZIFs) are promising materials for gas and liquid separations due to their unique pore structures and tunable surface properties. Recent studies show that ZIF crystalline powders, especially membranes, are not stable in aqueous solutions. The present work shows the improvement of ZIF-8 crystal and membrane stability in water through utilization of a ligand exchange post-modification method which replaces methylimidazole ligands on the outer surface of ZIF-8 crystals or membranes by the more hydrophobic, bulkier 5,6-dimethylbenzimidazole. Ligand exchange modification does not change the crystal structure, morphology or gas permeance as shown ZIF-8 powder and membrane characterization experiments. The modified ZIF-8 powders and membranes retain identical morphologies and crystallinities after static and dynamic water immersion. The modified ZIF-8 membrane exhibits stable water pervaporation flux controlled by the ZIF-8 layer while the unmodified ZIF-8 membrane experiences dissolution of the ZIF-8 layer during water pervaporation. This ligand exchange strategy enables hydrostable ZIF-8 membranes for practical applications involving aqueous solutions.

The ability to tailor the pore structure of metal− organic framework (MOF) membranes enables synthesis of new or modified MOF membranes with enhanced separation characteristics. This work employs a modified version of solvent-assisted ligand exchange, termed membrane surface ligand exchange (MSLE), to modify the pore structure of zeolitic imidazolate framework-8 (ZIF-8) membranes. This paper is the first to perform a time-based, ex situ characterization and gas permeation study of ZIF-8 MSLE with 5,6-DBIM (DBIM, dimethylbenzimidazole) to effectively narrow the ZIF-8 pores, enhance light hydrocarbon gas-phase separations, and give insight into the exchange mechanism with respect to time and temperature. The results show that relatively fast exchange kinetics occur mainly at the outer surface of the ZIF-8 membrane during the initial 30 min of exchange and enables significant (40−70%) increases in propylene/propane selectivity with minimal (10−20%) propylene permeance losses for the modified ZIF-8 membranes. We postulate as the reaction time proceeds, the ligand-exchange rate slows as the DBIM linker diffuses into the ZIF-8 membrane beyond the external surface, exchanges with the original linker, disrupts the original framework's crystallinity, and then increases long-range order/crystallinity as the reaction proceeds. The H 2 /C 2 separation factor increases with increased 5,6-DBIM content in the ZIF-8 framework which is facilitated by increased MSLE time and reaction temperature.

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 μm in thickness and were determined to have limited nonselective 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, ditert-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.