Sol-Gel Synthesis of High-Density Zeolitic Imidazolate Framework Monoliths via Ligand Assisted Methods: Exceptional Porosity, Hydrophobicity, and Applications in Vapor Adsorption

Monolithic ZIF-8 and ZIF-67 adsorbents are synthesized at room temperature using a novel, ligand-assisted method. Despite reductions in crystallinity within some of the samples, monolithic zeolitic imidazolate frameworks (ZIFs) have superior volume-relative microporosity, total porosity, and surface areas relative to their particulate counterparts due to increased density. Samples synthesized using a single modulator, n-butylamine, have a hierarchical porosity resulting in improved adsorption capacities in mid- to high- sorbate pressure regions. ZIF-67 monoliths produced through mixed-modulator synthesis, n-butylamine and 1-methylimidazole, are almost entirely microporous. Vapor adsorption isotherms find that, whilst their amorphous content results in increased water uptake, monolithic ZIFs are found to possess higher surface and adsorption hydrophobicity than traditional non-polar adsorbents. Cosorption measurements with a common VOC toluene, under humid conditions, find that these monolithic ZIF samples outperform powder equivalents, with the mixed-modulator ZIF-67 monolith capturing 28% more VOC compared to the powder ZIFs studied due to its superior volumetric efficiency. This study provides insights into the benefits of modulator-based tuning of porosity within monolithic ZIFs which, combined with their hydrophobicity, may facilitate their application for industrial organic vapor recovery or indoor air cleaning, where efficient hydrophobic adsorbents which can operate in humid environments are essential.

Automated summary

Monolithic ZIF-8 and ZIF-67 adsorbents synthesized at room temperature using a novel ligand-assisted method exhibit superior porosity and hydrophobicity, with the ZIFs possessing higher surface areas and adsorption properties. Additionally, in humid conditions, these monolithic ZIF samples outperform their powder equivalents in terms of adsorption capacity.