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Under identical reaction conditions, Au NPs (3–5 nm in diame-
ter as determined from TEM images) supported on silica-alumi-
na (Si-Al) are particularly active and the activity (butanol con-
version) decreases in the order Si-Al (95%)@TiO2 (50%)>g-
Al2O3 (42%)>MgO (35%)>HT (32%)>HAP (18%) (Figure S1
in the Supporting Information). Aldehyde selectivity also de-
pends on the support composition. Supports that are weakly
acidic and basic, such as HT and hydroxyapatite (HAP), are
more selective towards n-butanol dehydrogenation to n-buta-
nal, whereas more strongly acidic supports, such as Si-Al or g-
Al2O3, catalyze the formation of butenes and n-butyl ether
(Figure 1). TiO2, though selective to C8+ alcohols, showed sig-
or by varying the aging time used during the DP synthesis pro-
cedure (Figure S3–S4). HAuCl4 was used as the Au precursor.
Improved Au NP dispersion was possible when the point of
zero charge (PZC) of the support material was higher than
five.[12] HTs have a PZC of ~10, suggesting that a good disper-
sion of Au NPs can be achieved on the surfaces of HT.[13]
Figure 2 shows the effect of average particle size on the turn-
Figure 2. Dependence of TOF and mean particle size of Au/HT in n-butanol
dehydrogenation reactions. Reaction conditions: T=423 K, Pn-butanol ꢀ2 kPa,
M
cat =0.1 g, Qtot =150 cm3 minÀ1
.
over frequency (TOF) for n-butanol dehydrogenation. As can
be seen, the activity per surface Au atom, the TOF, decreases
strongly with increasing particle size, confirming that alcohol
dehydrogenation is structure sensitive.[14] Previous experimen-
tal and theoretical studies show that the surface of Au NPs is
composed of planar sites, perimeter sites, and low coordinated
edge and corner sites.[15] As particle size increases, the ratio of
low-coordinated sites to high coordinated sites decreases. This
suggests that the dehydrogenation of n-butanol occurs prefer-
entially on coordinatively unsaturated sites (CUS). Consistent
with this reasoning, calculations of the TOF based solely on
the corner/edge sites (Figure S5) leads to a single value
(~145 hÀ1) for the reaction conditions reported in Figure 2, in-
dependent of the Au NP size.
Figure 1. Selectivity patterns of various supported Au catalysts in n-butanol
dehydrogenation reactions. Reaction conditions: T=473 K, partial pressure
of n-butanol Pn-butanol ꢀ2 kPa, mass of catalyst Mcat =0.05–0.1 g (3 wt% Au),
total gas flow rate (Qtot) adjusted to maintain the butanol conversion at ca.
30–35%.
nificant selectivity to esters. Purely basic supports, such as
MgO, produced significant yields of Guerbet products, such as
ethyl-2-hexanol, suggesting that the aldehydes undergo aldol
condensation and hydrogenation on this support (Scheme 1).
Thus, amphoteric supports, containing weakly acidic and basic
groups, such as HT and HAP, exhibit the most promising prop-
erties for non-oxidative dehydrogenation of aliphatic alcohols.
Comparison in synthesis procedures using similar weight
loadings of Au showed that the DP method leads to better ac-
tivity than catalysts prepared by impregnation of the support
with an Au precursor. However, changing the precipitating
agent from urea to NaOH has a negligible effect on the catalyt-
ic activity (Figure S2). Control reactions demonstrated that HT
and Au/SiO2 are inactive for n-butanol dehydrogenation at
473 K. However, a physical mixture of Au/SiO2 and HT showed
an intermediate level of conversion, albeit significantly lower
than that obtained from the Au/HT (Figure S2). These results il-
lustrate the necessity of having the metal and basic sites in
close proximity to realize co-operative catalytic enhancement
and suggest that the Au–HT interface plays a crucial role in en-
hancing the catalytic activity.
For comparison, Cu/HT and Pd/HT catalysts having similar
metal loadings to that of Au/HT were prepared by the DP
method. The results presented in Figure 3 demonstrate that
while Pd/HT is more active than Au/HT for n-butanol dehydro-
genation, it is much less selective for aldehyde formation and
exhibits significant selectivity towards aldehyde decabonyla-
tion to form propene. Cu/HT is slightly more active than Au/
HT, but tends to form esters (see Figure 3). Both Pd/HT and Cu/
HT deactivated with time on stream (Figure S6). In the case of
Pd/HT, CO produced by aldehyde decarbonylation poisons the
surface of Pd, whereas in the case of Cu/HT, butanoic acid pro-
duced by hydrolysis of the ester possibly poisons the basic
sites on HT. By contrast, Au/HT was stable under the reaction
conditions for 5 h and TEM images taken before and after reac-
tion showed no evidence for an increase in Au particle size
(Figure S7). The absence of sintering is attributed to the inter-
action of Au NPs with Al present on the support surfaces, in
agreement with previous studies showing that Al provides
strong anchoring sites for stabilization of Au NPs.[12] This inter-
Particle size and particle size distribution are often critical
characteristics for tuning the activity/selectivity of metal sup-
ported catalysts.[11] The size of Au NPs on the HT support was
altered by either regulating the concentration of Au precursor
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