6
F. Bucciol et al. / Journal of Catalysis xxx (xxxx) xxx
ꢂ1
activity than monometallic Ru/TiO
selectivity to GVL.
2
, while also maintaining full
is centred at ~1445 cm . These bands are associated with the 19b
ring mode of py, assigned to the py species adsorbed on Brønsted
protonic sites (py-B), and to py species on coordinatively unsatu-
To the best of our knowledge, this is the first time that LA has
been converted to 1,4 PDO over Au-based catalysts. A comparison
with the results of Huo et al. is shown in Table 2 [17]; this paper
deals with complete ethyl levulinate conversion and yields 1,4-
PDO after 6 h at 160 °C over a CuZnAl catalyst, using 1,4-dioxane
as the solvent and a 1:50 substrate/catalyst ratio (mmol/mg).
The effect of other parameters, such as reaction time, the nature
of the employed substrate and the use of water as the solvent, were
investigated by carrying out LA hydrogenation in the presence of
the AuRO catalyst. The data reported in Table 3 clearly indicate
that the reaction must be performed for 4 h, because LA conversion
decreased from 100% to 58% and the 1,4-PDO yield was lowered
to 19.6% when the reaction time was reduced to 2 h. Moreover,
the choice to start from GVL did not lead to any improvement
in catalytic performance; it gave conversions and yields that
were lower than those accomplished when using LA as the
substrate.
n+
rated M Lewis sites (py-Lewis), respectively [54,55]. Therefore,
the exposed acid sites of the supports were probed by Py adsorp-
tion and subsequent desorption. The experiments were performed
at room temperature on the as-prepared catalysts, which had pre-
viously been submitted to outgassing at room temperature for
30 min. Fig. 3 compares the in situ FTIR spectra of py adsorbed
on the AuRO (a) and AuZ (b) catalysts in the 1700–1400 cm fre-
quency range of ring-breathing (mCCN) modes of vibration. In both
cases, the bands ascribable to liquid-like (physisorbed) py species
are observed at 1592 (8a mode) and 1440 (shoulder) cm . In
the case of the AuRO catalyst, a component was detected at about
ꢂ1
ꢂ1
ꢂ1
1480 cm
.
The intensities of the bands related to the py-Lewis species
appear to be higher than those of the bands due to py-B species.
In both cases, the peak located at 1603–1605 cm can be assigned
to the 8a mode of a stronger form of adsorbed py interacting with
surface OH groups via H-bonding. No bands related to the forma-
tion of pyridinium surface species were detected at higher frequen-
ꢂ1
The use of water as the solvent strongly inhibited the reaction
over the AuRO catalyst. It led to a lower yield to 1,4-PDO (18.3%)
than under solvent-free conditions, and a partial stop of hydro-
genation to GVL (4.6%). This effect was even more evident when
performing GVL hydrogenation, since no conversion was observed.
There are reports that aqueous phase GVL dehydrogenation can
ꢂ1
cies (~1630 cm ), which indicates that strong Brønsted acid sites
were absent on both supports. After evacuation at room tempera-
ture, the observed bands remain, though with decreased intensity,
at 1603 (py coordinated to Lewis acid sites), 1576-7 (8b mode
lead to
a
-angelica lactone [47] and/or LA [48]. This indicates that
common to all adsorbed py species), 1492–1490, and 1445–
ꢂ1
inhibition by water can play a role in the reversibility of these reac-
tions, as they proceed through an hydration/dehydration equilib-
rium step (Scheme 1). In a paper dealing with LA hydrogenation
to GVL using formic acid as the H-donor in the presence of a num-
ber of 5 wt% Ru catalysts supported on carbon, the authors
observed that higher conversion values were attained when lower
amounts of water were used. This was due to easier diffusion to the
active sites of the catalysts, resulting in the faster decomposition of
the H-donor [49]. Given the same Au-particle size and exposed
gold sites in the AuRO and AuZ catalysts, these findings indicate
that the sites of the support have a role to play.
1442 cm
.
Water was then adsorbed on preadsorbed py to investigate the
effect of the presence of H O on the availability of the acid sites of
the supports. A further decrease in the intensity of the bands was
observed upon the adsorption of 10 mbar H O on preadsorbed py
2
2
at room temperature (blue lines in Fig. 3c and 3d) and further out-
gassing at increasing times (black fine lines) up to 30 min (cyan
lines).
This behaviour (much more evident for AuZ) is a clear indica-
tion that water molecules compete with adsorbed pyridine for
the same adsorption sites on the supports. It is worth noting that
GVL hydrogenation to 1,4-PDO is strongly inhibited by the pres-
ence of water [56], which highlights the difficulty of performing
this reaction in water.
3.2.1. Effect of the nature of the support
Information on the nature, amount and strength of the acid sites
exposed at the surface is required to investigate the role that the
support plays in MW-assisted LA hydrogenation. For the sake of
clarity, the surface of metal oxides consists of coordinatively unsat-
urated (cu) cations (M ) and oxide (O ) ions [50]. In order to
reduce the coordinative unsaturation of the surface sites, water
dissociative adsorption occurs, forming terminal OH-groups [51].
In this frame, the in situ FTIR spectroscopy of small adsorbed base
molecules, such as pyridine (py), is a powerful tool with which to
characterise the nature, strength and concentration of acid sites
3.2.2. LA hydrogenation mechanism
LA catalytic hydrogenation has been reported to produce differ-
ent products, depending on the catalyst and reaction conditions
[57]. LA transformation into GVL and then 1,4-PDO is made up of
consecutive hydrogenation/dehydration steps, which both occur
via the formation of well-defined, but short-lived intermediates
[58]. The proposed steps for LA reduction are shown in Scheme 1:
the LA keto group is first reduced to 4-hydroxypentanoic acid
(4-HPA) by H species that are formed upon the activation of molec-
ular hydrogen on the metallic sites. This intermediate promptly
dehydrates by undergoing intramolecular esterification to give
GVL. It is known that Lewis and Brønsted acid sites promote cat-
alytic dehydration [59]. This step therefore takes place on the
Lewis sites of the titania and zirconia supports. The C@O bond is
n+
2ꢂ
[
52]. In particular, the py probe can distinguish between Lewis
and Brønsted acidity, giving rise to specific IR absorption bands
[
53]. It has been reported that the adsorption/desorption of py at
ꢂ1
room temperature produces two bands in the 1700–1400 cm
mid-IR range. The former (broad, and of medium intensity) is
observed at ~1540 cm and the latter (sharp, and of low intensity)
ꢂ1
Table 3
Effect of the reaction time and of the substrate in the microwave-assisted LA hydrogenation carried out in the presence of H
2
over the AuRO catalyst.
Yield, GVL (%)
Catalyst
AuRO
Substrate
LA
t (h)
T (°C)
H
2
(bar)
Water (mL)
Conversion (%)
Yield, 1,4-PDO (%)
4
2
4
4
4
4
200
200
200
150
200
200
50
50
50
50
50
50
solvent-free
solvent-free
2.5
100
58
100
20
88
0
0
100
19.6
18.3
27.7
3.7
9.2
4.6
0
0
0
AuRO
GVL
solvent-free
solvent-free
2.5
0