Effect of transition metal cations on the commensurate freezing of n-hexane confined in micropores of ZSM-5

Besides its importance concerning fundamental studies on gas adsorption in narrow pores, investigation of the commensurate freezing of a fluid within a zeolite is of practical importance in the application of zeolites in the processes of adsorption, separation and catalysis. In this work, the adsorption of n-hexane on HZSM-5 and its transition metal ion-exchanged modified forms was studied at 303 K by means of microcalorimetry. The thermal molar entropies changes of adsorption were calculated and thereby, the freezing-like behaviour of n-hexane inside the structure of the zeolite as a confinement media was noticed. This effect is governed by the attractive interactions between n-hexane molecules and the pore walls, and is influenced by the length of the pores and the nature of the charge-balancing cations. Among the investigated zeolites, a solid-like phase of n-hexane in the pores of zeolites with Fe(II) ions was the most similar to solid bulk n-hexane, while Cu(II) ions contributed to the lowest ordering of the obtained solid-like n-hexane phase.