Role of acid modulators in improving electrochemical hydrogen storage in the MIL-53(Cr) framework on the Ag/f-MWCNT substrate via defect engineering

Abstract

Porous metal–organic frameworks (MOFs) are recognized as important materials in green chemistry and environmentally friendly adsorbents owing to their potential for regeneration and reusability. However, efforts are being made to achieve lower toxicity and higher sustainability to reduce the need for work-up and energy consumption while minimizing side products and waste, with the goal of commercialization. Therefore, a systematic and comprehensive study was conducted on the synthetic insertion of missing-cluster defects within the MIL-53(Cr) framework through nitric acid addition under the most commonly employed conditions. The MIL-53(Cr) framework obtained from the modulated synthesis exhibited high thermal stability and increased catalytic activity in order to improve hydrogen storage. Upon removing some of the BDC linkers and subsequently the complete coordination of the metal ions of the framework, the metal centers became more accessible, and consequently, more active for Lewis acid-catalysed reactions. We clearly demonstrate that this phenomenon was significantly dependent on the extent of altering the concentration and/or acidity of the moderator. Therefore, the improved adsorption in this study was related to the new interface between the modifiers and MOF units/defects, which provided additional active sites that were formed as a result of the surface groups of the modifiers acting as MOF linkers.