Adsorption microcalorimetry of small molecules on various metal–organic frameworks

Abstract

Adsorption microcalorimetry of small molecules such as CO2, CH4, O2 and NH3 was studied on three commercially available metal–organic frameworks (MOFs), namely: Basolite™ A100, Basolite™ C300 and Basolite™ F300. All studies were conducted by interfacing a volumetric adsorption apparatus, Micromeritics ASAP 2020, with a differential scanning calorimeter, which runs isothermally. The CO2 adsorption studies were performed on A100 and C300, at four different temperatures, and the uptakes increased with rise in temperature with significantly higher uptakes (approximately double) observed on C300. The differential heats on the A100 sample were confined in a low range of 18–36 kJ mol−1 and appeared to have similar profiles irrespective of the temperature. Comparably, low differential heats in the range of 21–27 kJ mol−1 were also observed for CO2 adsorption on C300. However, unlike in A100, these heat profiles appeared to be relatively discrete and tend to decrease with increasing temperature. Adsorption of CH4 on A100 showed increase in uptakes with temperature and very similar heats of adsorption profiles (16.5–19.5 kJ mol−1). O2 adsorption studies on C300 indicated an increase in uptakes with temperature and scattered heat values (11.5–16.5 kJ mol−1) at all temperatures. NH3 adsorption on F300 at 40 °C using the “two-isotherm” method yielded an irreversible uptake of ~4 mmol g−1 and an initial heat of 102 kJ mol−1. Overall, it appears that MOFs have appreciable amount of adsorption capacity for small molecule adsorbates within a very low range of adsorption heats.