Water adsorption and hydrothermal stability of CHA zeolites with different Si/Al ratios and compensating cations

Abstract

[EN] Zeolites are well known crystalline aluminosilicates, which may be used in processes that take advantage of their molecular sieving effect, such as natural gas drying. They are often used in cyclic processes that swing pressure and/or temperature to perform adsorption and desorption steps. It is recognized that thermal stress may decrease process performance upon prolonged use. In this work, chabazite (CHA) zeolite with two different Si/Al ratios and compensating cations was investigated by thermally aging the samples using a laboratory-scale protocol. A Premature Aging Protocol - PAP was proposed that took into account the conditions which the adsorbent is exposed to in Temperature Swing Adsorption (TSA) process for natural gas drying. The sample was previously saturated with water and n-heptane vapors (as a reference hydrocarbon) followed by pressurization (30 bar) and heating (573 K) with a mixture of CO2 and CH4 (1:4, v/v). The Si/Al ratios of the CHA samples under study were 2 and 5 and the compensating cations were Na and K. Aged materials presented a lower CO2 and water vapor adsorption capacity with an increasing content of carbon in the bulk composition. The sample with higher Si/Al ratio (approximate to 5) had a larger pore volume but adsorbed less water. Despite having the highest carbon content after aging and modest acidity, it was the most thermally stable sample, together with the sample containing potassium. Even with a lower Si/Al ratio (approximate to 2), the presence of potassium provided the sample a protective effect against aging.