Microspheres of carbon xerogel: an alternative Pt-support for the selective hydrogenation of citral

Abstract

A new carbon xerogel (A8) was obtained in powder by polymerization of resorcinol-formaldehyde using a stirred batch reactor, microwave drying and carbonization in inert atmosphere. The ability of this material as Pt-support regarding SiO2, TiO2 and Al2O3 to develop selective catalysts for the citral hydrogenation was analyzed on the basis of their morphologic, textural and acid characteristics. Inorganic supports are crystalline and mesoporous materials while the carbon xerogel is exclusively microporous and is formed by spherical nanoparticles of around 250 nm in diameter of amorphous carbon. The supports acidity (pHpzc) vary in the sense Al2O3 > TiO2> SiO2 > A8. The Pt-dispersion depends on the support nature and pretreatment conditions used; in general, an increase of Pt-particle size favors the selectivity to unsaturated alcohols but the catalytic activity decrease. The conversion also strongly depends on the acidity of supports, but selectivity is more specifically influenced by the strength of the acid sites and pore size distribution. Cracking reactions are favored by Brönsted acid sites present in Al2O3 and the high mesopore volume of SiO2 induces mainly secondary cyclization and hydrogenation reactions. Pt-supported on carbon xerogel (Pt/A8) provided selectivity values to unsaturated alcohols of around 80%, a very high value for monometallic Pt-catalyst and only comparable with those obtained with Pt/TiO2 pretreated in H2-flow.