PNNL

Abstract

Understanding the quantitative interactions between zeolite pore walls, Brønsted acid sites, and molecules with both polar and nonpolar regions is essential for scoping the potential of zeolites as sorbents and catalysts. Purely siliceous zeolites (MFI and Beta in the present study) are hydrophobic, whereas those containing aluminum are considered hydrophilic, preferentially adsorbing organic molecules even in aqueous environments. To characterize these interactions, we use primary alcohols of increasing molecular weight, quantifying their specific interactions in the confined pore space the alkyl (CHx) and the OH groups. Three types of interactions were identified: (i) CHx groups interacting with the zeolite pore walls (approximately 10 kJ·mol-1 per carbon), (ii) alcohol OH groups interacting with the pore walls (30–35 kJ·mol-1), and (iii) alcohol OH groups interacting with Brønsted acid sites (67–72 kJ·mol-1). All three interactions were well mirrored by computational simulations. The contribution of the alkyl CHx groups was inferred from the incremental increase in sorption enthalpy with increasing molecular weight; the interaction strength of the OH groups was determined by extrapolating the global adsorption enthalpy of the alcohols to a hypothetical OH group without an alkyl group. This value was identical with the adsorption enthalpy of water. The experiments demonstrated that only water has an adsorption enthalpy on zeolite pore walls lower than its condensation enthalpy (30–35 kJ·mol-1 vs. 45 kJ·mol-1), limiting the concentration of water that can be adsorbed.