Interplay of Linker Functionalization and Hydrogen Adsorption in the Metal–Organic Framework MIL-101

Abstract

Functionalization of metal-organic frameworks results in higher hydrogen uptakes owing to stronger hydrogen-host interactions. However, it has not been studied whether a given functional group acts on existing adsorption sites (linker or metal) or introduces new ones. In this work, the effect of two types of functional groups on MIL-101 (Cr) is analyzed. Thermal-desorption spectroscopy reveals that the -Br ligand increases the secondary building unit's hydrogen affinity, while the -NH2 functional group introduces new hydrogen adsorption sites. In addition, a subsequent introduction of -Br and -NH2 ligands on the linker results in the highest hydrogenstore interaction energy on the cationic nodes. The latter is attributed to a push-and-pull effect of the linkers.

The Impact of Post-Synthetic Linker Functionalization of MOFs on Methane Storage: The Role of Defects

“…Dynamic effects, cf., ligand rotation, will be disregarded as they are not formed upon or affected by SALE. Furthermore, synchrotron X-ray diffraction patterns and BET surface areas ) (Szilágyi et al, 2014) reveal an identical crystallographic structure in these samples, highlighting the lack of volume defects. As N2, H2, and CH4 molecules are of similar size and polarity, surface and line defects on MOFs should affect their adsorption similarly.…”

“…In fact, MOFs were practically viewed as ideal crystalline solids, and gas adsorption has been discussed in terms of fairly rigid and perfectly ordered crystal lattices (Simon et al, 2015;Smit, 2015). Most defects reported are related to the heterogeneity of building blocks, such as mixed cationic units (Brozek and Dincă, 2013;Szilágyi et al, 2013) or linkers (Deng et al, 2010;Karagiaridi et al, 2012;Szilágyi et al, 2014). It has, however, recently emerged that, similar to other materials classes, MOFs display a range of lattice defects (Fang et al, 2015), such as vacancies (Vermoortele et al, 2012a;Wu et al, 2013), surface defects (Choi et al, 2008;Ameloot et al, 2013), dislocations (Shöâeè et al, 2008;Walker and Slater, 2008), and defect voids (Whittington et al, 2014).…”

“…SALE should thus be an ideal tool to introduce defects in a MOF. MIL-101(Cr) has been chosen as the material under scrutiny, as it has been demonstrated that its linkers may be exchanged post-synthetically (Szilágyi et al, 2014); it will thus allow the verification of (i) the engineering of defect sites through SALE and (ii) its impact on methane adsorption. It has been reported that −NH2 groups (Wood et al, 2012), and in general N sites (Li et al, 2015), may enhance methane storage on porous materials.…”

“…It has been reported that −NH2 groups (Wood et al, 2012), and in general N sites (Li et al, 2015), may enhance methane storage on porous materials. To take advantage of this improvement, the PS exchange of the BDC linker to its NH2-BDC has been chosen and SALE exchange was carried out, resulting in the post-synthetically linkerexchanged sample, PSM (Szilágyi et al, 2014). Furthermore, in order to distinguish between the effect of functional groups and that of defects the pristine NH2-MIL-101 was also synthesized.…”

“…Synthesis of NH2-MIL-101(Cr) was carried out by the chemical reduction of NO2-MIL-101(Cr), as described by Stock using a longer reaction time (16 h) (Bernt et al, 2011;Juan-Alcañiz et al, 2013). SALE was carried out by infusing an aqueous solution of 2-amino-1,4-benzendicarboxylic acid (NH2-BDC) into MIL-101(Cr), described in Szilágyi et al (2014). UV-vis spectra were measured with a Perkin-Elmer Lambda 900 spectrophotometer equipped with an integrating sphere ("Labsphere") in the 200-800 nm range.…”