Zeolitic imidazolate frameworks (ZIFs) are a fascinating class of metal–organic frameworks that exhibit robust chemical and thermal stability as a result of their metal–imidazolate linkages. While several of these materials have exhibited interesting breathing and/or framework contortions, ZIF-7 has garnered possibly the most attention of this class of materials due to the unique Type V isotherm exhibited by multiple different gases at pressures that may be of relevance for industrial separations. The origination of the phase contraction in ZIF-7 has been the focus of study by several groups, both at ExxonMobil as well as in the broader scientific community. Through a molecular-level insight of the driving forces of this transformation, we have discovered EMM-36, a multivariate zeolitic imidazolate framework in the sodalite topology comprising a solid solution of benzimidazole and 4,5,6,7-tetrahydro-1H-benzimidazole coordinated to zinc cations. Using this bi-linker approach, we can precisely tune the linker–linker interactions in the framework, altering the thermodynamic driving force for phase contraction. This results in a series of materials that have Type V isotherm steps and profiles that are dependent purely on material composition. The amount of control is truly impressive as the gate-opening effect upon, for example, methane exposure can be tuned over a pressure region of over two orders of magnitude. Indeed, we not only can tune this gate opening effect but entirely eliminate it, resulting in materials with traditional Type I adsorption isotherms for carbon dioxide as well as for light hydrocarbon gases. Moreover, unlike ZIF-7, with its completely non-porous narrow-pore phase, EMM-36—when comprising between 0-30% tetrahydrobenzimidazole—collapses into a narrow pore phase that is still permeable to carbon dioxide. This narrow pore phase exhibits extremely high adsorption energies for CO2, reaching values at low loadings of up to 80 kJ/mol.

Structure-property relationships in the flexible zeolitic imidazolate framework EMM-36: Controlling the thermodynamic driving force for framework phase transformation

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.