IR study of dynamic bahaviour of 2-propanol on alumina

Abstract

Dynamic behavior of surface complexes of 2-propanol on aluminia during adsorption and dehydration was studied using IR spectroscopy. Good agreement was found between the first-order rate constants of alkene formation in pulse-flow experiments and that of disapearance of a reactive surface complex. It was shown that the simple surface alkoxide 2-Pr-O-Al remains on the surface constant temperature, but it can be completely removed by hydrolysis, or by heating up to 200 °C. The surface carboxylates are formed very slowly and are stable even in the presence of water vapour at 300 °C.

Theoretical investigation of the adsorption of methanol on the (110) surface of γ -alumina

“…Thus, the C-O bond becomes stronger and exhibits a double bond character as deduced from the analysis of the bond distances (Tables 1 and 3). These results are in good agreement with the dissociative adsorption mechanism of methanol on g -alumina suggested by Moravek [37] (see Fig. 4).…”

“…As a conclusion, the present study supports the dissociative mechanism proposed by Moravek [37] for the adsorption of alcohols on g -alumina. Methanol adsorbs on a tetrahedral aluminum ion forming a covalent bond.…”

“…Consequently, we assumed, in our calculation, that a previous deprotonation of methanol happens, leading to the formation of a water molecule, and this step is not considered here. As Porchet et al have observed the release of water probably as a result of the deprotonation of methanol during its adsorption, this gives undoubtedly credibility to the reaction scheme suggested by Moravek [37]. Therefore we assumed in the present work that this mechanism prevails and chose to calculate this model of adsorption.…”

Elimination and Addition Reactions

“…In the pulse-flow regime, a pulse of an alcohol on alumina first yields a wave of water, followed by a wave of the alkene [62,63]. Further investigations using other transient and infra-red (IR) techniques showed [64][65][66] that the surface hydroxyl groups can be replaced by alkoxyl groups via the reactions with alcohols and ethers. These transformations are reversible, and the formed surface alkoxides are intermediates in alkene or ether formation and in the hydrolysis of ethers.…”

“…This failure is probably caused by the simplified assumptions adopted for the derivation of the Langmuir-Hinshelwood kinetics and the strong interactions of the reacting substances with the surface of alumina. Transient kinetic studies indicate [61][62][63][64][65][66] that the number of active sites is not constant but rather depends on the partial pressures of the alcohol, ether, and water (i.e., in a steady-state situation) on conversion.…”