(495d) Shape Selective Separation of n- and Iso-Alkanes Using Carbon-Silica Composites

Porous activated carbon materials are major workhorses in adsorptive separations, are easily produced and modifiable, able to adsorb most components from gas and liquid phase given their wide pore size range. This disordered structure is however often a disadvantage in catalysis and adsorption, and not much shape selectivity is present in contrast to zeolites. Ordered mesoporous carbons can be produced using a replication process starting from mesoporous ordered silicas filled with carbon, followed by dissolution of the template silica. An alternative approach [1] has been proposed to obtain ordered porous carbon materials with adjustable smaller pores. A carbon precursor (ex. furfuryl alcohol) is deposited in a mesoporous template (ex. MCM-41), polymerized and subsequently carbonized. The resulting material is a carbon-silica composite (CSC). In this work, materials with different carbon loading were prepared and their shape selective properties were investigated.

Carbon-silica composites were prepared using increasing amounts of carbon precursor to impregnate a template MCM-41 material with a 2.1 nm pore diameter. The materials were characterized and adsorption properties were determined. Linear and branched alkanes are used as probe molecules.

Pulse chromatography was performed on the Carbon-Silica particles, pelletized as a 500 to 630 µm sized fraction packed into a column. Henry constants and heats of adsorption were measured and low coverage separation factors calculated. Adsorption isotherms of n-octane and iso-octane were measured at 60 °C using the gravimetric technique. Separation of a light naphta mixture was studied at 110 °C in column breakthrough experiments.

The low coverage separation factors between hexane and 2,2-dimethylbutane and between hexane and 3,3-dimethylpentane increase up to 8, similar to zeolites, with the carbon loading of the composite material and effective pore size decreases. The optimal carbon loading lies around 75% of furfuryl alcohol precursor.

Adsorption isotherms of n-octane and iso-octane nearly coincide on the parent MCM-41 material, whereas with increasing loading, the more bulky iso-octane molecule is adsorbed less in comparison to n-octane on the composite material, still yielding some separation. Breakthrough experiments with a mixture of 5 C5 and C6 isomers demonstrate the shape selective properties of these materials. At a carbon loading of 50% or more, the dibranched chain elutes first from the column.

Modification of silica based mesoporous materials by controlled carbon deposition allows to tune the shape selective properties of the material. This could be interesting for specific separations or catalytic reactions in which shape selectivity is desired.

References

[1] T. G. Glover, K. I. Dunne, R. J. Davis, M. D. LeVan, Microporous and Mesoporous Materials: 2008, 111, 1-11