Abstract
A series of zirconium doped mesoporous silica, with different Si/Zr molar ratios, has been synthesized and tested in the gas-phase dehydration of glycerol. The surface characterization of these solids by using NH3-TPD and pyridine adsorption coupled to FTIR spectroscopy has revealed the existence of well dispersed acid sites, mainly of Lewis type, associated to Zr(IV) species deficiently coordinated located on the pore walls of the siliceous framework. These acid catalysts are active in the glycerol dehydration, increasing the conversion with the zirconium content until values higher than 90mol% for a Si/Zr molar ratio of 4, at 325°C after 5h of reaction. However, the catalysts suffer deactivation, which is more important when zirconium oxide is incorporated by impregnation of mesoporous MCM-41 silica. The main reaction products were acrolein, acetaldehyde and acetol. Moreover, the catalysts with a SiZr molar ratio higher than 5 are more selective to acetaldehyde. The acrolein yield was, in all cases, lower than 15mol% after 24h of TOS, but a pretreatment under a helium flow saturated with water vapour allows reaching an acrolein yield of 28mol% and ameliorates the stability of catalysts. The selectivity towards acrolein and hydroxyacetone can be explained by considering the influence of the nature of active sites.
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