Aromatic
alcohols are essential components of many solvents, coatings,
plasticizers, fine chemicals, and pharmaceuticals. Traditional manufacturing
processes involving the oxidation of petroleum-derived aromatic hydrocarbons
suffer from low selectivity due to facile overoxidation reactions
which produce aromatic aldehydes, acids, and esters. Here we report
a Co-containing hydroxyapatite (HAP) catalyst that converts ethanol
directly to methylbenzyl alcohols (MB–OH, predominantly 2-MB–OH)
at 325 °C. The dehydrogenation of ethanol to acetaldehyde, which
is catalyzed by Co2+, has the highest reaction barrier.
Acetaldehyde undergoes rapid, HAP-catalyzed condensation and forms
the key intermediate, 2-butenal, which yields aromatic aldehydes through
self-condensation and then MB–OH via hydrogenation. In the
presence of Co2+, 2-butenal is selectively hydrogenated
to 2-butenol. This reaction does not hinder aromatization because
cross-coupling between 2-butenal and 2-butenol leads directly to MB–OH
without passing through MBO. Using these insights a dual-bed
catalyst configuration was designed for use in a single reactor to
improve the aromatic alcohol selectivity. Its successful use supports
the proposed reaction mechanism.
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