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Enhanced H2 Uptake of n-Alkanes Confined in Mesoporous Materials

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journal contribution
posted on 22.05.2014, 00:00 by S. Clauzier, L. Ngoc Ho, M. Pera-Titus, D. Farrusseng, B. Coasne
The hydrogen uptake in hybrid sorbents consisting of n-alkane solvents confined in mesoporous silica aerogel is measured at different temperatures from 273 to 313 K and pressures up to 40 bar. An apparent “oversolubility” effect is observed as the H2 uptake in the hybrid sorbents is much larger than that in bulk solvents. The H2 uptake in the hybrid sorbents is found to increase with increasing temperature, which suggests that the flexibility and conformation of n-alkane molecules confined in the aerogel play a crucial role; high-entropy (disordered) alkane configurations lead to the creation of numerous cavities which make it possible to solubilize a larger number of H2 molecules. This departs from adsorption-driven solubility effects for which the number of solubilized molecules decreases with increasing temperature. For a given temperature and pressure, it is found that the number of solubilized H2 molecules per unit volume increases with decreasing alkane chain length. Such an effect, which is observed for both the bulk alkanes and the alkanes confined in the silica aerogel, can be rationalized by considering the number density of CHx (x = 2 or 3) groups; for a given temperature, the latter number density decreases with decreasing alkane chain length so that the free volume available to solubilize H2 molecules increases.