Molecular simulation for adsorption and separation of CH<sub>4</sub>/H<sub>2</sub> in zeolites

Abstract

Based on the high-throughput calculation method of molecular simulation, except the structures with zero surface area and less than zero adsorption capacity, four geometric descriptors (largest cavity diameter, specific surface area, pore volume and porosity), an energy descriptor (heat of adsorption), adsorption capacity, and adsorption selectivity coefficient of 199 zeolites are obtained. By studying the correlation between structural characteristics and adsorption separation performance, the result shows that when the largest cavity diameter is 6 Å, the surface area is 1400–2100 m<sup>2</sup>·g<sup>–1</sup>, and the pore volume is in a range of 0.2–0.3 cm<sup>3</sup>·g<sup>–1</sup>, the zeolite has the greatest influence on the adsorption capacity and adsorption selectivity for methane molecules. At the same time, it is found that the largest cavity diameter and porosity of zeolite molecular sieves have a positive correlation, and there is also an obvious linear relationship between the CH<sub>4</sub> adsorption selectivity coefficient of the equimolar CH<sub>4</sub>/H<sub>2</sub> mixed component and the single-component CH<sub>4</sub> adsorption capacity. By using the grand canonical Monte Carlo simulation method, physical quantities such as adsorption isotherms and isosteric heats of adsorption for CH<sub>4</sub> and H<sub>2</sub> of three channel-shaped zeolites are obtained. The result shows that the pore structure (surface area and pore volume) of the channel-shaped zeolite has a greater influence on the CH<sub>4</sub> adsorption capacity than the energy effect (heat of adsorption), under the same external environment. Combining with the industrial background of steam methane reforming hydrogen production, the separation and selectivity performance of the CH<sub>4</sub>/H<sub>2</sub> mixed system under different components are further studied. The result reveals that there is no correlation between adsorption selectivity of ultra-microporous zeolite material for CH<sub>4</sub> and bulk pressure or feed ratio. According to the centroid distribution density of gas molecules, it is found that CH<sub>4</sub> preferentially occupies the space of smaller pore windows in the channel-shaped zeolite, while the distribution range of H<sub>2</sub> is larger but there is no unambiguous preferential adsorption site.