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confirmed by the time monitored concentration data of benzyl
alcohol, benzaldehyde and benzoic acid (Fig. 1a). However, by
increasing the substrate to catalyst concentration substantially
(from 300:1 to 1500:1), benzaldehyde with TON > 800 and
selectivity ~ 96% may be obtained (Table 1, Entry 4).
The catalytic activity of 1 has been evaluated for the oxidation
of other representative aliphatic and benzylic alcohols (Table 2).
All primary alcohols are oxidized mainly to the acid while
the secondary alcohols expectedly yield ketones. As mentioned
earlier, in benzyl alcohol oxidation by increasing the substrate
to catalyst concentration substantially, benzaldehyde with >95%
selectivity may be obtained. Such a strategy may also work for
the selective conversion of other primary alcohols to the cor-
responding aldehydes. Good TONs and TOFs are obtained in
all the cases and no adverse effect due to olefinic functionalities
are observed (Table 2, Entries 10, 12, 15). For substrates with
such functionalities only the alcohol functionality is selectively
oxidised. It may also be noted that as most of the reactions in
Tables 1 and 2 were set to a fixed time, the TON and especially
TOF given for the reactions that went to completion correspond
to lower limits (Entries 1 and 2 in Table 1; entries 1 and 3 in
Table 2).
The recycling and deactivation studies on 1 have been carried
out under conditions where high turnovers are obtained per
batch. As shown in Table 3, on recycling, partial deactivation of
1 is observed, from the first to the fifth batch the TONs drop from
300 to 183 and 44 to 26 for benzyl alcohol and 1-phenylethanol,
respectively. The total turnovers over the five batches are 1150
and 165, respectively. This is to be contrasted with the reported
data on recycling for ARP-Pt and Gly-Pt. For these two catalysts
total turnovers ~80–90 over five batches were obtained but no
deactivation was reported.
Interestingly and significantly, on treatment with dihydrogen
(30 bar, 3 h, in water) the used 1 regains its original activity
completely. This is evident from the conversion and TON data of
the regenerated catalyst (Table 3, Entries 6, 12). The regenerated
catalyst has been tested for more than one recycle for both
the model substrates, benzyl alcohol and 1-phenyl ethanol. Its
behaviour has been found to be identical to that of the fresh
catalyst. As catalytic activity is very often found to be strongly
correlated with the particle size, the TEM studies of the fresh,
used and regenerated 1 have been performed (see Experimental).
As reported in our earlier publications, freshly prepared 1 has
CO-protected platinum nanoparticles within the size range of 2–
8 nm with ca. 70% being within 3–5 nm.28a,b After five catalytic
runs no small (≤6 nm) particles could be seen in the partially
deactivated catalyst (Fig. 2a) which consists mainly of large
particles (8–10 nm). However, the growth in particle size is
reversible as the hydrogen treated regenerated catalyst shows
a reduction in the particle size (Fig. 2b). The nanoparticle
size measurements with the TEM facilities at our disposal (see
Experimental) are approximate. However, the overall trend,
that of an increase in the particle size in the used catalyst
and a reduction of particle size in the regenerated catalyst, is
unambiguous.
Fig. 1 (a) Time monitored conversion of benzyl alcohol to benzalde-
hyde and benzoic acid by 1 in water. Reaction Conditions: Catalyst (1) =
100 mg (5.4 ¥ 10-3 mmol of Pt), benzyl alcohol = 175 mg (. Substrate
to platinum molar ratio: 300). Solvent (water) = 5 mL, temperature =
353 K, O2 = 1 atm., stirring speed = 900 rpm. (b) Comparative time
monitored conversion of 1-phenylethanol to acetophenone by 1 in water
with and without activation. Reaction Conditions: Catalyst (1) = 100 mg
(5.4 ¥ 10-3 mmol of Pt), 1-phenyl ethanol = 33 mg, (Substrate to platinum
molar ratio: 50). Solvent (water) = 5 mL, temperature = 353 K, O2 = 1
atm., stirring speed = 900 rpm.
The time monitored concentration profiles of 1-phenylethanol
oxidation by using freshly prepared 1, and activated 1 (1 fully
decarbonylated by heating under vacuum) as catalysts have
been recorded (Fig. 1b). The concentration vs. time plots are
practically identical and both catalysts show induction periods
~5 h. As reported for hydrogenation reactions in water, in
oxidation reactions also the CO ligands are quickly (< 5 min)
lost. This has been confirmed by carrying out a separate
experiment and monitoring the disappearance of nCO. These
observations are significant from a mechanistic point of view,
as they show that the long induction time is independent of the
time required for CO loss.
The X-ray photoemission spectra (Fig. 3) of fresh and used
1 have also been recorded. Freshly prepared 1 shows signals for
the Pt 4f core levels with the binding energy (BE) of Pt 4f 7/2
~71.0 eV being very close to the literature reported value of the
free cluster.28a,d,30 The XPS signal of the five times used partially
deactivated catalyst on deconvolution shows a strong peak at
~72.5 eV, and a much weaker one at ~71.0 eV. This indicates that
the predominant oxidation state of platinum in the deactivated
catalyst is +2.
This journal is
The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 554–561 | 557
©
Maity, Prasenjit
