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

Abstract

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 mixedmodulator 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 nonpolar 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.

Polydimethyl siloxane/MIL-101 Composites for Enhanced Toluene Adsorption in the Presence of Humidity

“…An established metric, hydrophobicity index (HI), was used in this study as a comprehensive and quantitative descriptor of pore hydrophobicity. HI takes into account a material's adsorption capacity ratio for a hydrophobic molecule such as toluene 55 or cyclohexane 56 relative to water, as obtained from specific single or dual component adsorption experiments. 23,57,58 Alternatively, some authors have estimated HI through pore volume measurement from N 2 adsorption and volume of desorbed water from TGA analysis.…”

Evaluation of ZIF-8 and ZIF-90 as Heat Storage Materials by Using Water, Methanol and Ethanol as Working Fluids

“…However, most of the stable ZIFs are hydrophobic and, therefore, alternative working fluids have been considered. The idea of using the adsorption Crystals 2021, 11, 1422 2 of 11 of small chain alcohols in ZIFs has already been evaluated in recent years [6,[10][11][12][13][14][15][16]. The adsorption of methanol and ethanol in 18 MOFs (including ZIF-8) for TES applications was reported by De Lange et al [6].…”

“…Finally, the paper concludes that the use of methanol or ethanol is dependent on the evaporation temperature required for the application used [6]. Hunter-Sellars et al examined ZIF-8 and ZIF-67 monoliths via ligand-assisted methods for water, toluene and methanol adsorption [10]. For the monoliths, n-butylamine was used as the single modulating ligand while 1-methylimidazole and n-butylamine were used for the multiple modulating ligand monoliths.…”

“…Both unmodified ZIF-8 and ZIF-67 had a similar uptake for methanol and toluene. However, the ZIF-8 monolith using a single modulating ligand and both ZIF-67 monoliths showed a significant improvement in the uptake of methanol and toluene, favouring toluene [10]. Many studies have examined the alcohol adsorption of ZIFs for applications such as organic vapour recovery, indoor air cleaning, adsorption-based heat pumps and separation/selectivity in alcohols [6,10,[15][16][17][18][19].…”

“…Both ZIFs have the sample topology and pore entrance, and only have a difference of 0.4 Å in their pore/cage capacities [22][23][24]. ZIF-8 uses the hydrophobic 2-methylimidazolate linker while ZIF-90 uses the 2-carboxaldehyde imidazolate linker, which is hydrophilic [10,25]. ZIF-8 and ZIF-90 are considered the most widely researched ZIFs, with half of all articles in recent years focusing on either or both of these ZIFs [25].…”