Characteristics of Natural Zeolites using Nitrogen and Methane as Adsorption Probes

Abstract

Adsorption isotherms of nitrogen and methane were measured on natural clinoptilolite and montmorillonite/mordenite zeolites as adsorbents. IR spectra of methane adsorbed by the natural zeolites were also measured. Analyses of isotherms of nitrogen showed that small surface area (S_BET; 20∼26m²g^-1) and pore volume (V_p; 0.060∼0.062mlg^-1) as well as small micro porosity (internal surface area, S_INT; 3∼4m²g^-1, micro pore volume, V_micro; 0.008∼0.011mlg^-1), while those of larger values (S_BET; 112m²g^-1, V_p; 0.24mlg^-1, S_INT; 17m²g^-1, V_micro; 0.048mlg^-1) for montmorillonite/mordenite zeolite. Adsorption data of methane on clinoptilolite zeolites and on montmorillonite/mordenite zeolite were not found to obey a Langmuir-type adsorption equation even in the range of lower pressures (p<200mmHg). Isosteric heats of adsorption (q_st) of methane on montmorillonite/ mordenite zeolite were larger than those on clinoptilolite zeolites. The values of q_st decreased with an increase in the amount of methane adsorbed, indicating that surfaces of the natural zeolites are not homogeneous and adsorption of methane is likely to proceed on stronger adsorption sites. IR spectra of the ν_{1, CH4}, for the natural zeolites-methane system suggested that methane molecules are adsorbed on strong cationic sites which corresponds to Na⁺ and K⁺ sites for clinoptilolite zeolites and to Ca²⁺ and Na⁺ sites for montmorillonite/mordenite zeolite.