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.

Automated summary

A novel composite material was designed via confined growth strategy to enhance selective adsorption of volatile organic compounds (VOCs) under high humidity conditions. The composite material showed excellent hydrophobic property, increased toluene adsorption affinity, and provided abundant diffusion channels for enhanced mass transfer. It exhibited higher toluene adsorption capacity and lower water vapor adsorption capacity compared to other reported adsorbents.