Impact of the Environment of BEA-Type Zeolites for Sorption of Water and Cyclohexanol

The (mutual) interactions of water and cyclohexanol with the pore walls and functional groups of Brønsted acidic zeolites of the BEA type (H-BEA) have been investigated. Upon reaction with Brønsted acid sites, water forms hydrated hydroxonium ions limited in size by the sorption free energy, creating in this way domains occupied by water. Organic molecules, such as cyclohexanol, occupy the remaining unoccupied volume. The pore size of the zeolite H-BEA stabilizes hydrated hydroxonium ions (H⁺(H₂O)₁₀) that are two H₂O molecules larger than those formed in the smaller pore zeolite H-ZSM-5. Increasing the density of hydroxonium ions by increasing the concentration of aluminum in the zeolite gradually leads to less negative standard free energy of adsorbed cyclohexanol. The increasing proximity of positive charges of the hydroxonium ions induces a higher excess chemical potential of the sorbed molecule, which is manifested in a weakened interaction strength with the zeolite pores.