Equilibrium adsorption of linear and branched C6 alkanes on silicalite-1 studied by the tapered element oscillating microbalance

Abstract

The equilibrium adsorption of linear and branched C₆ alkanes n-hexane, 2-methylpentane, 3-methylpentane and 2,3-dimethylbutane on silicalite-1 has been investigated using a novel technique—the tapered element oscillating microbalance (TEOM). For n-hexane, a small “kink ” in the isotherm is observed at about 4 molecule (unit cell of silicalite-1)⁻¹. The measured isotherms of both 2-methylpentane and 3-methylpentane at 303 K for the first time show a second-step adsorption at loadings over 4 molecule (unit cell)⁻¹. A two-step adsorption behavior is confirmed for single branched C₆ alkanes. This observation is in good agreement with the picture of two distinct adsorption locations for single branched alkanes in silicalite-1 indicated by other techniques. The maximum loading for 2,3-dimethylbutane is about 4 molecule (unit cell)⁻¹ under the conditions investigated and the molecules reside completely in the intersections. A dual-site Langmuir expression appropriately describes the equilibrium data for n-hexane, 2-methylpentane and 3-methylpentane, while the isotherms of 2,3-dimethylbutane can be described by the Langmuir model. The derived thermodynamic properties such as adsorption enthalpy and entropy agree with those available, determined by other techniques. The observed two-step adsorption behavior for single branched C₆ alkanes on silicalite-1 is attributed to the large difference in the adsorption entropy between the molecular locations in the channel intersections and in the zigzag channels.