Isotopic transient analysis of the ethanol coupling reaction over magnesia

• Published in: Journal of catalysis 2013-02, Vol.298, p.130-137
• Main Authors: Birky, Theodore W., Kozlowski, Joseph T., Davis, Robert J.
• Format: Article
• Language: English
• Subjects: Acetaldehyde; active sites; adsorption; Butanol; carbon dioxide; Carbon dioxide adsorption; Catalysis; catalysts; Chemistry; condensation; Ethanol; Exact sciences and technology; General and physical chemistry; Guerbet reaction; IR spectroscopy; Isotopes; Magnesia; magnesium oxide; Microcalorimetry; Reaction kinetics; Surface physical chemistry; temperature; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2012.11.014

Isotopic transient analysis of the ethanol coupling reaction to butanol on MgO at 673K revealed that half of the surface is covered with adsorbed ethanol, identified as ethoxide by DRIFTS, whereas the surface coverage of reactive intermediates leading to butanol was an order of magnitude lower than that of ethoxide. The high coverage of ethoxide and hydroxide inhibited CC coupling reactions, such as aldol addition reactions, that occur readily on MgO and lead to deactivation. [Display omitted] ► Acetaldehyde, butanol, and ethene were produced from ethanol on MgO at 673K. ► Ethanol dissociatively adsorbed on MgO forming surface ethoxide and hydroxide species. ► The coverage of adsorbed ethanol was about 50% of surface MgO pairs. ► The coverage of coupling intermediates was an order of magnitude lower than ethanol. ► High coverage of ethoxide and hydroxide inhibited coupling reactions. Isotopic transient analysis of ethanol coupling to butanol over MgO in a fixed-bed reactor at 673K revealed a surface coverage of adsorbed ethanol equivalent to about 50% of the exposed MgO atomic pairs. DRIFTS of ethanol reaction at 673K confirmed that the surface was populated primarily with adsorbed ethoxide and hydroxide, presumably from the dissociative adsorption of ethanol. The coverage of reactive intermediates leading to butanol was an order of magnitude lower than that of adsorbed ethanol, and about half the surface base sites counted by adsorption of CO2. The intrinsic turnover frequency for the coupling reaction at 673K determined by isotopic transient analysis was 0.04s−1, which is independent of any assumptions about the nature of the active sites. Although the ethanol coupling reaction appears to involve aldol condensation of an aldehyde intermediate, the high coverage of ethanol under steady-state conditions apparently inhibits unproductive CC coupling reactions that deactivate the catalyst at high temperature.