Adsorption of hydrogen, methane, CO2 and their binary mixtures in silicalite-1: role of pore characteristics revealed by molecular simulations.

Physisorption in nanoporous materials is an important alternative for storing hydrogen fuel. Use of methane and CO2 as cushion gases can aid in keeping hydrogen stored in geological repositories. For this, an understanding of the coadsorption of these gases and the underlying mechanisms that are determined by the pore characteristics of the repository rocks is essential. Here we use grand canonical Monte Carlo (GCMC) simulations to understand coadsorption of hydrogen, methane, CO2 and their binary mixtures in microporous silicalite-1. To understand the effects of pore characteristics like surface area, connectivity, and tortuosity, we progressively block some channel-like pores of the adsorbent by filling it with immobile organic material (represented by methane). At 100 atm in unmodified silicalite the adsorption amounts of hydrogen, methane and CO2 in pure state are respectively (1.08 ± 0.11) mmol g-1; (2.86 ± 0.06) mmol g-1 and (3.52 ± 0.03) mmol g-1. Both CO2 and methane are found to suppress hydrogen adsorption from an equimolar mixture, while presence of hydrogen in the mixture has no discernible impact on the adsorption of these gases. Suppression of hydrogen adsorption by CO2 is stronger by an order of magnitude. This is because the adsorption energy of CO2 in silicalite is about four times that for hydrogen and 1.5 times that for methane. Adsorption of all gases in pure state as well as the carbon fluids in the mixtures exhibits clear dependence on the ratio of surface area to volume of the pores. Information obtained in this study can guide the design of a future study that recreates the UHS scenario of a cushion gas being pushed on top of hydrogen present in a reservoir.