Kinetics of hydrogen adsorption in MIL-101 single pellets

Abstract

A new type of MIL-101: aluminum composites with improved thermal conductivity is reported and their hydrogen adsorption rate investigated in a systematic study of hydrogen adsorption/desorption kinetics of MIL-101 single pellets, filling the gap of data on hydrogen sorption kinetics by compressed monoliths in the cryo-adsorption storage conditions.Kinetic curves were measured within a wide range of pressure in the temperature range 77–159 K and the hydrogen diffusivity were estimated from the rate constants obtained by fitting the fractional uptake with the kinetic equations. In order to compare MIL-101 with other metal-organic frameworks (MOFs), measurements on pellets of MIL-100(Fe) and HKUST-1 were also performed. As expected, the results, together with some data available in literature, reveal that the hydrogen diffusivity obtained from kinetic studies is strongly related to the pore diameter. The activation energy of hydrogen diffusion in MIL-101, calculated from the rate constants, is 1.6 kJ mol−1, close to the literature values reported for other MOFs. Kinetic studies on composite pellets of MIL-101 with Al tapes (8–10%) with improved thermal conductivity (∼0.5 W m−1K−1) are also reported. They show indeed higher uptake rates, consistent with the requirement of fast filling time (less than 3 min) required for applications. The adsorption/desorption rate at 77 K allows ∼90% fractional uptake in about 40 s, promising for applications of cryo-adsorption storage. The results show that the time required to reach saturation and thermal equilibrium depends on pellet geometry and hydrogen diffusivity.