Kinetics and enthalpies of methane adsorption in microporous materials AX-21, MIL-101 (Cr) and TE7

Methane is touted as a replacement for fossil fuels in transport applications due to its lower costs of production and cleaner combustion. Storage of methane is still a problem and different technologies have been considered, including compression and liquefaction. Adsorption in a porous material is a potential alternative for methane storage, as it can increase densities at moderate pressures and temperatures. For practical applications, in addition to the quantities stored and working capacities, it is important to equally consider aspects such as kinetics of storage and thermal management of the storage system. In this paper, the kinetics and enthalpies of adsorption of methane in activated carbons AX-21 and TE7, and metal-organic framework MIL-101 (Cr) are extracted from readily available gas sorption data. The adsorption kinetics at 300 K and 325 K are analysed and fitted with the linear driving force (LDF) model, and mass transfer coefficients (MTC) and effective diffusivities are estimated. The effective diffusivities have a range of values from 1.79 × 10⁻¹³ m² s⁻¹ for the MIL-101 (Cr) at 300 K to 9.36 × 10⁻¹⁰ m² s⁻¹ for the TE7 at 325 K. The activation energies for the effective diffusivities based on an Arrhenius-type temperature dependence are calculated as 7.42, 7.09 and 5.38 kJ mol⁻¹ for the AX-21, the MIL-101 (Cr) and the TE7, respectively. The enthalpies of adsorption are calculated with the Clausius-Clapeyron equation and the differences observed when calculating these with excess and absolute amounts are presented and discussed, with the results showing that enthalpies can have up to 10% differences if using excess amounts instead of absolute quantities. The isosteric enthalpies are also compared with enthalpies at zero-coverage obtained from differential calorimetry experiments for the MIL-101 (Cr), and a ∼3.5 kJ mol⁻¹ difference is observed, which underlines the importance of refining calculation methods and bridging the gap between direct and indirect methods for calculating enthalpies of adsorption.