Adsorption of hexane isomers on MFI type zeolites at ambient temperature: Understanding the aluminium content effect

We investigate the influence of the aluminium content on the adsorption of hexane isomers at 293 K on MFI type zeolites (Si/Al ratios – SAR of 25, 100 and ∞). Adsorption isotherms were assessed with a homemade volumetric system. Temporal up-take curves were recorded using a manometric set-up combined with a Rubotherm suspension balance. All three isomers show dual site adsorption behaviour that is well described with a Dual Site Sips (DSS) model. The Si/Al ratio influences strongly the adsorption equilibrium properties. Regardless of the isomer type, the sample with SAR = 100 shows the highest saturation loadings. At the studied pressures the sorption order in the three samples is: n-hexane > 2-methylpentane > 2,3-dimethylbutane > 2,2-dimethylbutane. Generally, we found that the Henry constant increases with aluminium content. Clear diffusional limitations are observed for the di-branched hexane isomers on all three zeolite samples. The sample with SAR = 100 gives the highest diffusion rates for the hexane isomers. Silicalite-1 is the most promising for enhancing gasoline octane numbers at 293 K, as it gives the highest equilibrium 2,3-dimethylbutane/2-methylpentane ratios.

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► Di-branched isomers only adsorb on the channel intersections.
► n-Hexane adsorbs on the channel intersections and on the channels.
► Separation between mono-branched and di-branched isomers has a selectivity about 20.
► Strong diffusional limitations are prominent for the di-branched hexane isomers.
► Operating at lower temperatures increases the equilibrium and kinetic selectivity.