Molecular simulation of methane adsorption in slit-like quartz pores

The adsorption behaviors and structural features of methane in quartz nanopores were investigated using GCMC and MD methods, and the influences of the different pore sizes, temperatures, and water content on methane adsorption on quartz are discussed herein. The results show that the isosteric heats of methane decreased with increasing pore size. The methane molecules, affected by the stronger interaction forces with the pore walls, gathered near to the pore walls to form the adsorbed phase. However, the methane molecules in the free phase were dispersed in the area away from the pore walls, due to weaker interaction forces. The proportion of adsorbed gas in the quartz pores decreased with increasing pressure for the same pore size, and decreased with increasing pore size under the same pressure; the methane existed mainly as free gas in the quartz pores when the pore size was over 6 nm. The adsorption sites of the methane molecules gradually changed from higher energy adsorption sites to lower ones, with increasing pressure or decreasing pore size, resulting in the increase of the methane adsorption capacity. The methane adsorption capacity in micropores increased as the pore size increased, unlike in the mesopores. With the increase in temperature, the isosteric heat of adsorption of methane decreased and the adsorption sites of methane molecules gradually changed from lower energy adsorption sites to higher ones, resulting in the reduction of the methane adsorption capacity. The water molecules in the quartz pores occupied the pore wall directionally, acted upon by van der Waals interactions, Coulomb force interactions and hydrogen bonding interactions, causing the water molecules in the pores to accumulate. The relationship between methane molecules and water molecules in the quartz pores might not be adsorption site competitive, but adsorption space competitive, indicating that the water molecules occupy only the adsorption space of methane molecules, resulting in the reduction of the methane adsorption capacity.