Sorption of CO₂ and CH₄ on Raw and Calcined Halloysite—Structural and Pore Characterization Study

Abstract

The article presents comparative characteristics of the pore structure and sorption properties of raw halloysite (R-HAL) and after calcination (C-HAL) at the temperature of 873 K. Structural parameters were determined by optical scanning and transmission electron microscopy methods as well as by mercury porosimetry (MIP, Hg) and low-pressure nitrogen adsorption (LPNA, N2, 77 K). The surface area parameter (LPNA) of halloysite mesopores before calcination was 54−61 m2/g. Calcining caused the pore surface to develop to 70−73 m2/g. The porosity (MIP) of halloysite after calcination increased from 29% to 46%, while the surface area within macropores increased from 43 m2/g to 54 m2/g. The total pore volume within mesopores and macropores increased almost twice after calcination. The course of CH4 and CO2 sorption on the halloysite was examined and sorption isotherms (0−1.5 MPa, 313 K) were determined by gravimetric method. The values of equilibrium sorption capacities increased at higher pressures. The sorption capacity of CH4 in R-HAL was 0.18 mmol/g, while in C-HAL 0.21 mmol/g. CO2 sorption capacities were 0.54 mmol/g and 0.63 mmol/g, respectively. Halloysite had a very high rate of sorption equilibrium. The values of the effective diffusion coefficient for methane on the tested halloysite were higher than De > 4.2 × 10−7 cm2/s while for carbon dioxide De > 3.1 × 10−7 cm2/s.

Keywords

• halloysite
• pore structure
• sorption capacity of ch₄ and co₂
• kinetics of diffusion
• effective diffusion coefficient