Hydrogen bonding also occurs during the adsorption of hazardous materials (especially organics) over MOFs. Liu et al. studied the adsorption of phenol and p-nitrophenol (PNP) from aqueous solutions over several MOFs (MIL-100-Fe, Cr and NH2-MIL-101-Al) and AC [117]. In phenol adsorption, all three MOFs showed similar and limited adsorption capacities. However, NH2-MIL-101-Al showed exceptionally high adsorption of PNP, 4.3 and 1.9 times higher than those of MIL-100-Fe and MIL-100-Cr, respectively. The negligible effect of metal ions and the CUSs of MIL-100-Fe/Cr indicated similar adsorption affinities for phenol and PNP. However, NH2-MIL-101-Al showed a higher adsorption capacity for PNP and an exceptionally higher adsorption selectivity for PNP over phenol than AC. This remarkable adsorption affinity is attributed to hydrogen bonding between PNP and the amino groups in NH2-MIL-101-Al (Fig. 4). Xie et al. reported efficient adsorption of nitrobenzene from wastewater over two aluminum-based MOFs, CAU-1 and MIL-68-Al; the adsorption capacities are 970 ± 10 and 1130 ± 10 mg/g respectively, which are much higher than the experimental values for other porous materials reported so far [86]. Both the MOFs have μ-OH groups in Alsingle bondOsingle bondAl units, which play a key role in very high adsorption by forming hydrogen bonds between the μ-OH groups (of the MOFs) and nitrogen atom of nitrobenzene. Moreover, hydrogen bonding in the adsorption of ammonia and carbon dioxide with MOFs or functionalized MOFs [118] and [119] has been reported, even though the adsorptions were performed in gaseous phases.