Experimental adsorption isotherms of CO2 and CH4 on STT zeolite: comparison with high- and pure-silica zeolites

Abstract

Adsorption isotherms of CO2 and CH4 on a high-silica STT zeolite (Si/Al = 27), an odd zeolite which consists of 7- and 9-MR window openings, were measured over pressure range of 0–100 kPa at temperatures 298, 323 and 348 K. The isotherms follow typical Type-I according to the BDDT classification. Experimental isotherms data of CH4 and CO2 are well described by Langmuir-type isotherms. Calculation of isosteric heats of adsorption along with the isotherms types revealed that CH4 adsorption system is energetically homogeneous while that is heterogeneous for CO2 adsorption. Furthermore, comparison of STT adsorptive performance with high- and pure-silica zeolites led to development of a trade-off plot, i.e. CO2/CH4 selectivity against CO2 adsorption, by ideal adsorbed solution theory (IAST) calculations. The results revealed that Silicalite-1 has the lowest CO2/CH4 selectivity while SSZ-45 is the most selective adsorbent. Additionally, DD3R has the minimum CO2 adsorption amount whereas STT has the highest capacity. CO2/CH4 selectivity of STT is just ~ 12% smaller than that of SSZ-45 (13.30 against 15.12) but its CO2 adsorption is 63% greater than that of SSZ-45 (1.42 against 0.867 mol kg−1). Thus, STT zeolite is superior to the other high- and pure silica zeolite as it has simultaneously both high selectivity and capacity in CO2/CH4 separation.