Impact of Cations and Framework on Trapdoor Behavior: A Study of Dynamic and In Situ Gas Analysis.

Due to their distinct and tailorable internal cavity structures, zeolites serve as promising materials for efficient and specific gas separations such as the separation of /CO2 from N2. A subset of zeolite materials exhibits trapdoor behavior which can be exploited for particularly challenging separations, such as the separation of hydrogen, deuterium, and tritium for the nuclear industry. This study systematically delves into the influence of the chabazite (CHA) and merlinoite (MER) zeolite frameworks combined with different door-keeping cations (K+, Rb+, and Cs+) on the trapdoor separation behavior under a variety of thermal and gas conditions. Both CHA and MER frameworks were synthesized from the same parent Y-zeolite and studied using in situ X-ray diffraction as a function of increasing temperatures under 1 bar H2 exposures. This resulted in distinct thermal responses, with merlinoite zeolites exhibiting expansion and chabazite zeolites showing contraction of the crystal structure. Simultaneous thermal analysis (STA) and gas sorption techniques further demonstrated how the size of trapdoor cations restricts access to the internal porosities of the zeolite frameworks. These findings highlight that both the zeolite frameworks and the associated trapdoor cations dictate the thermal response and gas sorption behavior. Frameworks determine the crystalline geometry, the maximum porosities, and displacement of the cation in gas sorption, while associated cations directly affect the blockage of the functional sites and the thermal behavior of the frameworks. This work contributes new insights into the efficient design of zeolites for gas separation applications and highlights the significant role of the trapdoor mechanism.

In situ visualization of loading-dependent water effects in a stable metal-organic framework.

Competitive water adsorption can have a significant impact on metal-organic framework performance properties, ranging from occupying active sites in catalytic reactions to co-adsorbing at the most favourable adsorption sites in gas separation and storage applications. In this study, we investigate, for a metal-organic framework that is stable after moisture exposure, what are the reversible, loading-dependent structural changes that occur during water adsorption. Herein, a combination of in situ synchrotron powder and single-crystal diffraction, infrared spectroscopy and molecular modelling analysis was used to understand the important role of loading-dependent water effects in a water stable metal-organic framework. Through this analysis, insights into changes in crystallographic lattice parameters, water siting information and water-induced defect structure as a response to water loading were obtained. This work shows that, even in stable metal-organic frameworks that maintain their porosity and crystallinity after moisture exposure, important molecular-level structural changes can still occur during water adsorption due to guest-host interactions such as water-induced bond rearrangements.

Room-Temperature Synthesis of Metal-Organic Framework Isomers in the Tetragonal and Kagome Crystal Structure.

Two metal-organic framework (MOF) isomers with the chemical formula Zn2(X)2(DABCO) [X = terephthalic acid (BDC), dimethyl terephthalic acid (DM), 2-aminoterephthalic acid (NH2), 2,3,5,6-tetramethyl terephthalic acid (TM), and anthracene dicarboxylic acid (ADC); DABCO = 1,4-diazabicyclo[2.2.2]octane] have been synthesized via a fast, room-temperature synthesis procedure. The synthesis solvent was found to play a vital role in directing the formation of the Kagome lattice (ZnBD) versus tetragonal topology (DMOF-1). When N, N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) was used as the synthesis solvent, the reaction resulted in the formation of ZnBD, whereas methanol, ethanol, acetone, N, N-diethylformamide (DEF), and acetonitrile each produced DMOF-1. Water adsorption isotherms of ZnBD and DMOF-1 were collected, and the materials were found to have similar adsorption characteristics and stabilities. Both MOFs degraded upon exposure to water at a relative pressure ( P/ Po) of 0.5 at 25 °C, but both are hydrophobic below a P/ Po of 0.4, displaying very little water adsorption. Additionally, CO2 adsorption isotherms of ZnBD were collected and compared to those previously reported for DMOF-1. ZnBD adsorbs less CO2 at low pressure compared to DMOF-1 but reaches a similar capacity at 20 bar. This adsorption behavior can be explained by the structural features of the materials, where ZnBD possesses large hexagonal pores (15 Å) compared to the smaller pore opening (7.5 Å) in DMOF-1. The heat of adsorption of CO2 on ZnBD was calculated to be ∼22 kJ/mol at zero coverage. Attempts to functionalize the Kagome lattice proved to be unsuccessful but instead resulted in a new method for producing functionalized DMOF-1 at room temperature. This was hypothesized to be a result of the steric effects imposed by the functional groups that prevent the formation of the Kagome lattice.