Benchtop In Situ Measurement of Full Adsorption Isotherms by NMR

Physisorption using gas or vapor probe molecules is the most common characterization technique for porous materials. The method provides textural information on the adsorbent as well as the affinity for a specific adsorbate, typically through equilibrium pressure measurements. Her; we demonstrate how low-field NMR can be used to measure full adsorption isotherms, and how by selectively measuring H-1 spins of the adsorbed probe molecules, rather than those in the vapor phase, this NMR-relaxorption technique provides insights about local dynamics beyond what can be learned from physisorption alone. The potential of this double-barreled approach was illustrated for a set of microporous metal-organic frameworks (MOFs). For methanol adsorption in ZIF-8, the method identifies multiple guest molecules populations assigned to MeOH clusters in the pore center, MeOH bound at cage windows and to MeOH adsorption on defect sites. For UiO-66(Zr), the sequential pore filling is demonstrated and accurate pore topologies are directly obtained, and for MIL-53(Al), structural phase transitions are accurately detected and linked with two populations of dimeric chemical species localized to specific positions in the framework.

Automated summary

The use of gas or vapor probe molecules for physisorption is a common technique for characterizing porous materials, while low-field NMR can offer additional insights into local dynamics. By selectively measuring H-1 spins of adsorbed molecules, NMR-relaxation technique provides information beyond physisorption alone. This approach was demonstrated to be effective for studying microporous metal-organic frameworks.