6
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F. Menegazzo et al. / Journal of Catalysis 319 (2014) 61–70
species, simultaneously to the increase in the bands related to
methylfuroate, while the others do not change significantly in
intensity. The observed behaviour can be an indication that meth-
oxy species, possibly bridged on Zr4+ and Au sites at the perimeter
of the metal nanoparticles close to the support [23,24,26,27], are
directly involved in the furfural esterification reaction. Moreover,
the intensity of the band related to this species is more abundant
on AuZ than on AuTWGC after interaction with methanol at room
temperature (data not shown) in agreement with the observed cat-
alytic trend.
sites; therefore, the band is unchanged upon interaction with these
molecules. On the contrary, no bands are present on AuTWGC (black
curve). This feature could be related to the high and stable in time
selectivity for the AuZ sample. This is a confirmation of previous
2
studies on Au/ZrO stability in the furfural oxidative esterification
[14].
3.1.2. HMF oxidative esterification
Similarly, we investigated the mechanism for the reaction of
HMF under our reaction conditions as well. A possible oxidation–
esterification pathway has been previously supposed by Corma
and co-workers for gold on nanoparticulated ceria [11]. In princi-
ple, the reaction can follow two different pathways (Scheme 3).
Catalytic tests, analogous to those performed in the case of fur-
fural, were carried out in order to elucidate the HMF oxidation–
esterification pathway under our reaction conditions and in the
presence of the AuZ catalyst. We obtained the results illustrated
À1
Focusing on the 1750–1200 cm spectroscopic range, bands
related to adsorbed methyl-2-furoate, whose frequencies are
reported in grey in the figure, are formed on AuTWGC (grey curve
in Fig. 3, section a) only upon increasing the temperature at
1
20 °C. On the contrary, these bands are observed already at RT
on AuZ (black curve in section b) and further increase in intensity
when the temperature reached 120 °C (grey curve).
The spectroscopic findings confirm that the formation of the
product occurs without the formation of any adsorbed intermedi-
ate species and confirm that in the presence of AuZ, the reaction
proceeds through direct selective oxidation of furfural to the
desired furoate, as previously discussed (see Scheme 2). Moreover,
AuZ is able to catalyse the furfural esterification already at RT, giv-
ing an almost complete selectivity to methyl-2-furoate. On the
other hand, AuTWGC is less active, corroborating the catalytic
results reported in Fig. 1 (section a). Indeed, no reaction takes place
at RT and it is necessary to heat at 120 °C to observe some
reactivity.
2
in Scheme 1, having R = ACH AOH). In the presence of AuZ, the
reaction follows the direct way of the oxidation from HMF into
the desired FDMC (way 1), involving oxygen activation on the gold
clusters. On the contrary, the reaction goes completely through the
second way and leads to the acetal formation when performed in
the presence of the bare support (way 2). Due to acetal stability
under reaction conditions, it cannot be further transformed.
In Fig. 1 (section b), the HMF conversion and selectivity to
FDMC for AuZ and AuTWGC catalysts are shown. Again, the AuZ
sample shows much better catalytic performances than AuTWGC
reference sample. However, the gap between the two catalysts is
much higher than for the reaction with furfural. In fact, the pres-
ence of the ring substituent in the HMF substrate makes more dif-
ficult the reaction, decreasing the oxidation rate [11].
After reaction, both catalysts were submitted to simple outgas-
sing of the reaction mixture at RT followed by CO adsorption at the
same temperature. The results are reported in Fig. 5.
À1
After reaction, a band at 2083 cm due to CO on gold sites [19]
is produced upon CO adsorption at RT on AuZ (grey curve). The
adsorption of CO on the same sample before reaction (dashed grey
curve) experiments reveals that a fraction of the Au sites present
on AuZ remains available after reaction. Moreover, this band is
modified after reaction, but not upon interaction with furfural
and methylfuroate, possibly due to the temperature (the reaction
has been carried out at 120 °C, whilst both furfural and methylf-
uroate adsorption have been performed at RT) as well as to the
presence of oxygen in the reaction mixture. In fact, the highly dis-
3.2. Oxidative esterification of furfural and HMF: optimisation of
reaction conditions
3.2.1. Reaction time
First of all, the effect of the reaction time on the oxidative ester-
ification of furfural over the AuZ catalyst was investigated. We per-
formed different catalytic tests by varying only the time of reaction
(Fig. 6).
For all catalytic tests, selectivity is very high (almost 100%) and
it is constant for the time range that was investigated. These data
indicate that the transformation of furfural into the corresponding
furoate happens without any side reactions on the AuZ catalyst. On
the contrary, as expected, the conversion increases with the reac-
tion time. In particular, conversion rises until the maximum after
2
persed gold species react with O producing atomic oxygen spe-
cies, which can activate methanol [13,14,19]. Upon furfural and
methylfuroate contacting, we did not find spectroscopic evidences
of the participation to the adsorption by the uncoordinated gold
3
h of reaction.
As reported in Fig. 6, the conversion reaches 90% after 90 min of
reaction. It is therefore more convenient to perform catalytic tests
at 90 min of reaction. For a batch process, the halving of reaction
time is fundamental: Small plants can be exploited to the maxi-
mum, allowing more cycles. After this screening, the reaction time
was fixed at 90 min for all subsequent tests of furfural oxidative
esterification.
A screening on the reaction times for the oxidative esterification
of HMF with the AuZ catalyst is reported Fig. 7, where the concen-
trations of the reagent, the intermediate, and the product are
shown. It can be seen that the HMF is completely converted after
3
h. The formed monoester alcohol is the intermediate of the reac-
tion and its concentration increases up to reach a maximum at
about 3 h of reaction, whereas the FDMC is produced linearly with
time.
Fig. 5. FTIR absorbance spectra of 20 mbar CO adsorbed at RT on AuZ (grey curve)
and AuTWGC (black curve) outgassed at RT after reaction. The spectrum of CO
adsorbed on the AuZ catalyst before reaction is also reported for comparison
3
.2.2. Effect of the pressure and of the nature of the oxidant
The research work was then addressed to the investigation of
(
dashed grey curve).
the effect of the oxygen pressure on the furfural esterificative