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Scheme 2 Reaction pathways for PG transformation.
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Scheme 3 Plausible mechanism of PG intramolecular dehydration to
PO over alkali-loaded silica catalysts.
the selectivity to di-PG first increases to a maximum, and
then decreases with the increase of alkali loadings. Other
by-products, such as propionaldehyde, acetone and allyl
alcohol, may be formed via the PO isomerization in addition
to the PG transformation.14
Based on the above results and the formation mechanisms
of other ethers,13 the PO formation mechanism catalyzed by
alkali-loaded silica can be proposed as follows (see Scheme 3).
On the surface of alkali-loaded silica, the adsorbed PG
molecule is converted to an effective nucleophile (structure I)
via the abstracting of the primary hydroxyl proton by the base
site; at the same time the secondary hydroxyl group becomes a
good leaving group. Subsequently, the C2 atom is attacked by
the nucleophilic C1–OÀ, accompanied with the dehydroxyl-
ation of the secondary OH group (the concerted nucleophilic
substitution), to produce PO and water.
J. H. Robson, US Pat.,
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In summary, we have proposed a novel and sustainable
route for the production of PO from bio-glycerol via the PG
intermediate. Alkali-loaded silicas are active and selective
catalysts for this process. The weak base sites may play crucial
roles in the intramolecular dehydration of PG to PO.
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This journal is The Royal Society of Chemistry 2009
3936 | Chem. Commun., 2009, 3934–3936