Hydrophobic shell structured NH2-MIL(Ti)-125@mesoporous carbon composite via confined growth strategy for ultra-high selective adsorption of toluene under highly humid environment

Competitive adsorption of volatile organic compounds (VOCs) under high humidity is a critical but challenging issue in the applications of metal-organic frameworks (MOFs). In this work, hydrophobic-shell structured NH2- MIL(Ti)-125@mesoporous carbon composite was designed to enhance selective adsorption towards VOCs under humid conditions via confined growth strategy. Ti-clusters were first anchored into pores of ordered mesoporous carbon (OMC), and then confined grown into NH2-MIL(Ti)-125 via 'ship-in-the-bottle' approach. Hydrophobic shell of OMC concurrently protected the adsorption sites on NH2-MIL(Ti)-125 from H2O occupation and enhanced affinity towards non-polar toluene. Moreover, the resulting composited supplied abundant diffusion channels for toluene thereby accelerated the mass transfer though mesopores (OMC) and micropores (MOFs). As expected, the hydrophobic-shell NH2-MIL(Ti)-125@OMC composite efficiently enhanced hydrophobic property and toluene adsorption affinity. It obtained a dramatical increase in toluene adsorption capacity (3.86 mmol/g at 0.001P/P-0) about 7.4 times of NH2-MIL(Ti)-125, and a 29% decrease in water vapor adsorption capacity (0.30 g/g at 1 mbar), which much superior than many reported expensive adsorbents. In addition, the composite induced more confined micropores to mesopores interconnected structure in MIL(Ti)@OMCTi, and hence facilitated toluene diffusion. The toluene rate constant of pseudo-second-order adsorption (k(a)) on the MIL(Ti)@OMCTi was up to 0.12 g/(mmol.min), which was 1.2-2.0 times higher than those of the MIL(Ti) species. Moreover, breakthrough curve indicated that MIL(Ti)@OMCTi showed 1.5 times of toluene working capacity with faster diffusivity at 80% RH compared to pure NH2-MIL(Ti)-125, while the latter exhibited much lower value of Q(w)/Q(e) than that of the former. This work provides a novel composite strategy for hydrophobic MOFs construction, and deeper understanding for VOCs/H2O competitive adsorption on MOFs composites in large scale applications.