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E. Sadeghmoghaddam et al. / Applied Catalysis A: General 405 (2011) 137–141
Table 2
Results of the catalytic reactions of the mixture of two allyl alcohols using poisoned Pd nanoparticles.
a
Reactants
Time (h)
Isomerization yields (%)
TOFiso
4
24
100 + 30
100 + 80
475 + 24
79 + 63
Prop-2-en-1-ol
(1) + but-2-en-1-ol (5)
4
24
70 + 20
100 + 35
333 + 95
79 + 28
Prop-2-en-1-ol
(1) + 3-methylbut-2-en-1-ol (6)
4
20 + 0
95 + 0
Pent-1-en-3-ol
24
100 + 25
79 + 20
(2) + but-2-en-1-ol (5)
a
Turnover frequencies (initial TOFs) of fresh Pd nanoparticles (5 mol% in Pd atoms) are based on the mole isomerized per mol Pd atoms per hour.
was likely a result of the increased thermodynamic stability of enol
intermediate, compared to the intermediates generated from com-
pounds 5 and 6, with the presence of a CH3 in R1 position. Overall,
the results demonstrated that the isomerization reaction involves
the formation of enol intermediates and the catalytic reactions are
of prop-2-en-1-ol required more time (24 h) when less reactive 3-
methyl-but-2-en-1-ol was mixed together. The presence of bulky
3-methyl-but-2-en-1-ol on the surface of Pd NPs likely prevented
the facile approach of prop-2-en-1-ol to the active sites of Pd NP cat-
alysts, causing a decrease in the rate of the catalytic reaction. Similar
results were also observed from the reaction of mixtures contain-
ing pent-1-en-3-ol (2) and but-2-en-1-ol (5). This result was in a
good agreement with the one from the catalytic reaction in C6D6
solvent. Organic compounds containing aromatic or olefin groups
may be able to further poison the active sites of Pd NPs by surface
adsorption [10].
The catalytic reactions of the mixtures of allyl alcohols (one
reactive allyl alcohol in the presence of one less reactive, more
substituted allyl alcohol) were also investigated and the results
are shown in Table 2. When prop-2-en-1-ol (1) and but-2-en-1-ol
(5) were mixed together, the results showed that the isomeriza-
The isomerization yields for but-2-en-1-ol were 30% after 4 h and
80% after 24 h, while the isomerization of prop-2-en-1-ol was com-
pleted in just 4 h. Considering that the catalytic reaction of pure
but-2-en-1-ol resulted in only 4% isomerization (Table 1), the high
yield (80%) for the catalytic isomerization of but-2-en-1-ol in the
presence of prop-2-en-1-ol was a quite interesting one. This result
demonstrated that the isomerization of but-2-en-1-ol is promoted
in the presence of prop-2-en-1-ol. However, when the ratio of prop-
2-en-1-ol to but-2-en-1-ol was at or less than a half (1:2) during
the catalytic reaction, the isomerization of but-2-en-1-ol to butanal
did not take place. This indicated that the reactive allyl alcohol
(prop-2-en-1-ol) had a major role in the reactivity enhancement
of more substituted, less reactive allyl alcohol (but-2-en-1-ol) and
must be presenting at a high concentration during the reaction to
be effective.
4. Conclusion
The lower density of alkanethiolate ligands provides both good
enough protection and high selectivity (by poisoning the parti-
cle surface) for Pd nanoparticles. Isomerization of allyl alcohols
depends on both thermodynamic effect (the stability of enol inter-
mediate) and kinetic effect (the steric inhibition by alkanethiolate
ligands). The results demonstrate that controlling the density and
the structure of well-defined alkanethiolate ligands on nanopar-
ticles might bring about the development of highly selective and
efficient catalytic materials. The utilization of well known strategy
associated with the assembly of various thiolate monolayers [32]
on metal nanoparticles in the future will likely enable us to further
control the activity and selectivity of metal nanoparticle catalysts.
Acknowledgement
The enhancement of the isomerization of less reactive com-
pound 6 was also witnessed (albeit in a less extent; 20% after 4 h
and 35% after 24 h) from the catalytic isomerization of the mix-
ture containing prop-2-en-1-ol (1) and 3-methylbut-2-en-1-ol (6).
This is a notable improvement in the reactivity of highly sub-
stituted compound 6, which did not undergo any isomerization
as a pure form. The isomerization of but-2-en-1-ol (5) was also
slightly enhanced in the presence of pent-1-en-3-ol (2), another
reactive allyl alcohol. Although the mechanism is not completely
clear, the involvement of intermediate (Pd alkyl or enol intermedi-
ates) and/or carbonyl product generated from more reactive allyl
alcohols seems to be a major reason for such improvements in the
isomerization of more substituted allyl alcohols. To briefly exam-
ine an involvement of carbonyl products, the catalytic reaction of
but-2-en-1-ol was attempted in the presence of an equimolar con-
centration of acetaldehyde. However, any notable improvement in
the isomerization of but-2-en-1-ol was not observed, suggesting an
insignificant contribution of carbonyl products during the isomer-
in reactivity of more substituted allyl alcohols in the presence of
reactive, less substituted allyl alcohols are currently under investi-
gation.
This research was supported in part by a grant from the ACS-PRF
(PRF49407-UR7).
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Another important observation from Table 2 is that the isomer-
ization activity of prop-2-en-1-ol (1) was slightly decreased in the
presence of 3-methyl-but-2-en-1-ol (6). The complete conversion