Influence of the Support of Bimetallic Platinum Tungstate Catalysts on 1,3-Propanediol Formation from Glycerol

Article abstract of DOI:10.1002/cctc.201701067

Three aluminium oxide materials and a HZSM-5 zeolite were used as supports of bimetallic Pt-WO_x catalysts to establish structure–activity relationships in the glycerol hydrogenolysis reaction. The surface W density and the intimate contact between Pt and WO_x were key parameters. Surface W density controls the formation of polytungstates, the only species able to produce the weak Br?nsted acidity that is required to produce 1,3-propanediol selectively. The comparison between the HZSM-5 and the Al₂O₃ supports demonstrated that an increment of the medium Br?nsted acidity is detrimental for the selective 1,3-propanediol formation as it promotes reactions that yield 1-propanol and propane. An increase of the dispersion of Pt on the Pt/WO_x/Al₂O₃ catalysts led to higher glycerol conversions but also promoted the hydrogenolysis routes that lead to 1,2- and 1,3-propanediol similarly. On the contrary, an increase of the Pt metal content favoured the hydrogenolysis route that leads to 1,3-propanediol significantly. A more intimate contact between Pt and WO_x promoted the hydrogenation of the intermediate carbocation, formed and stabilised on a polytungstate active site, into 1,3-propanediol.

Full text of DOI:10.1002/cctc.201701067

Accepted Article
Title: Influence of the support of bimetallic Pt-WOx catalysts on the 1,3-
propanediol formation from glycerol
Authors: Sara García-Fernández, Inaki Gandarias, Yaiza Tejido-
Núñez, Jesus Requies, and Pedro Luis Arias
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Influence of the support of bimetallic Pt–WOx catalysts on the 1,3-
propanediol formation from glycerol
S. García-Fernández[a], I. Gandarias*[a], Y. Tejido-Núñez[b], J. Requies[a] and P.L. Arias[a]
Abstract: Three aluminium oxide materials and a HZSM-5 zeolite
were used as supports of bimetallic Pt–WOx catalysts, in order to
the most effective approach so far is the use of heterogeneous
catalysts that combine a noble metal with an oxophilic metal,
especially the combination of Pt, Rh and Ir with Re[7] and Pt with
W.[8] Much efforts have been devoted to identify the reaction
parameters and the specific sites which play an important role in
the selective formation of 1,3-PDO. Although most of these
works directly correlate the acidity and the Brönsted to Lewis
ratio with the selective formation of each PDO,[8c,d-9] this
correlation might not be straightforward as it is explained in this
report.
In our previous studies on supported Pt–WOx species, a reaction
mechanism for the glycerol hydrogenolysis to 1,3-PDO was
proposed[8a,10] (Figure 1). Tungsten surface density (ρW) was
reported to be a crucial parameter in the reaction since it
controls the kind of WOx species present on the catalyst surface:
monotungstates (WO4), polytungstates (WO5/WO6) and WO3
nanoparticles (NPs).[11] Only polytungstates (polymeric WOx)
were able to produce the Brönsted acid sites required for the
protonation of the internal OH group of glycerol, which is
adsorbed by its terminal OH group/s. It is worth mentioning that
1,3-PDO yield increased with ρW until WO3 NPs started to
appear, when a significant yield decrease occurred. According to
the mechanism of Figure 1, after dehydration of the protonated
alkoxide a secondary carbocation is formed, which needs to be
readily stabilised by a hydride coming from the hydrogen
activation on adjacent Pt sites.
Thus, the aim of this work is to better understand the effect of
three key parameters that affect the selective formation of 1,3-
PDO: the ρW, the Brönsted acidity and the effect of the Pt–WOx
proximity. For this purpose, three different aluminium oxide
materials and a HZSM-5 zeolite were used as supports. The
prepared catalysts were tested in the glycerol hydrogenolysis
reaction and characterised by inductively coupled plasma optical
emission (ICP-OES), N2 physisorption, CO chemisorption,
temperature programmed reduction of H2 (TPR-H2), diffused
reflectance infrared Fourier transform (DRIFT) spectroscopy of
adsorbed pyridine, Raman spectroscopy, scanning electron
microscopy (SEM), high-angle annular dark field scanning
transmission electron microscopy (HAADF-STEM), X-ray
diffraction (XRD) and X-ray photoelectron spectroscopy (XPS)
techniques.
establish
structure-activity
relationships
in
the
glycerol
hydrogenolysis reaction. The tungsten surface density and the
intimate contact between Pt and WOx showed to be key parameters.
Tungsten surface density controls the formation of polytungstates,
the only species able to produce the weak Brönsted acidity that is
required to selectively produce 1,3-propanediol. The comparison
between the HZSM-5 and the Al2O3 supports demonstrated that an
increment on the medium Brönsted acidity is detrimental for the
selective 1,3-propanediol formation, as it promotes the reactions that
yield 1-propanol and propane. Increasing the dispersion of Pt for the
Pt/WOx/Al2O3 catalysts led to larger glycerol conversion values, but
promoted the hydrogenolysis routes that lead to 1,2 and 1,3-
propanediol similarly. On the contrary, increasing the Pt metal
content significantly favoured the hydrogenolysis route leading to
1,3-propanediol. A more intimate contact between Pt and WOx
promoted the hydrogenation of the intermediate carbocation, formed
and stabilized on a polytungstate active site, into 1,3- propanediol.
Introduction
The depletion of fossil fuels, global warming and pollution make
the use of biomass an attractive alternative as feedstock for the
production of fuels and chemicals.[1] The biomass-derived
glycerol can be obtained as a by-product in the biodiesel
manufacture through the transesterification of vegetable oils,[2]
and through hydrogenation/hydrogenolysis reactions of sugars
and sugar alcohols, largely derived from the renewable
lignocellulosic biomass.[3] Due to its wide availability, glycerol is
considered as one of the top 12 biomass-derived building blocks
in the biorefinery field.[4] An interesting glycerol transformation
process is the aqueous-phase hydrogenolysis to 1,3-propanediol
(1,3-PDO). 1,3-PDO can, in turn, be used in the production of
polytrimethylene terephthalate (PTT) polymers, which makes it
one of the most interesting compounds that can be obtained
from glycerol. Unfortunately, the formation of the other diol, 1,2-
PDO, is more favoured with most of the catalytic systems
reported,[5] and very low 1,3-PDO/1,2-PDO ratios are normally
obtained.[6] In order to obtain relatively high yields of 1,3-PDO,
Results and Discussion
[a]
Dr. S. García-Fernández, Dr. I. Gandarias, Dr. J. Requies, Prof. P.L.
Arias
Effect of the Al-based support on the characteristics of the
supported Pt-WOx catalysts
Department of Chemical and Environmental Engineering, School of
Engineering.
University of the Basque Country (UPV/EHU)
Plaza Ingeniero Torres Quevedo 1, 48013 Bilbao, Spain
E-mail: inaki_gandarias@ehu.eus
Y. Tejido-Nuñez,
Ceit-IK4 and Tecnun
Three types of Al-based supports were used: a commercial γ-
Al2O3 (Alcom), and a γ-Al2O3 and a pseudo γ-AlO(OH) prepared
by the SG method (AlSG and AlSG’, respectively). The Pt loading
was kept constant (2 wt%). The ρW of the Pt/WOx/Al-based
catalysts was modified in order to study the effect of this
parameter on the activity and selectivity towards the reaction
[b]
Paseo de Manuel Lardizabal, 15, 20018 San Sebastián, Spain
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201701067
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Figure 1. Reaction mechanism proposed for the glycerol hydrogenolysis to 1,3-PDO (reproduced from ref. [8a]).
products. Three different strategies were carried out for this
purpose:
Table 1 Textural properties of the samples obtained by N2-physisorption.
1. Synthesis of catalysts with a nominal W content of 10 wt%
supported on the three different Al-based materials. In spite of
their similar W content, they have a dissimilar ρW due to their
remarkable different SBET (see Table 1).
2. Synthesis of Pt/WOx samples with the highest possible ρW
before the formation of the undesired WO3 NPs. For Alcom
support, a 9 wt% W loading was required, whereas higher W
loadings of 15 wt% and 23 wt% were necessary in the case of
AlSG and AlSG’, respectively (more details in the Raman
spectroscopy section).
3. Increment of the ρW by raising the calcination temperature
from 450 to 800 ºC and, therefore, decreasing the SBET of the
catalysts. For this purpose, two Pt-WOx catalysts with a nominal 11W AlSG’ 800 (2.4)
W content of 10 wt% supported on AlSG and AlSG’ were used.
The results of the characterisation techniques and their
correlation with the results of the activity tests are discussed in
the following sections.
Catalyst
Alcom
AlSG
SBET (m2 g-1)
140
Vp (cm3 g-1)
0.26
rp (Å)
38.4
23.1
12.6
13.8
286
0.32
AlSG’
275
0.14
HZa
423
0.18
9W Alcom (2.3)
110
0.20
38.3
9W AlSG (0.8)
307
181
204
288
126
208
107
267
93
0.27
0.25
0.22
0.28
0.25
0.15
0.19
0.09
0.16
18.2
28.5
44.1
20.3
30.5
31.5
30.4
25.7
34.4
9W AlSG 800 (1.4)
15W AlSG (1.9)
11W AlSG’ (1.1)
23W AlSG’ (2.6)
2Pt9W Alcom (2.3)
2Pt15W HZ (1.4)
9Pt8W Alcom (2.3)
a HZ = HZSM-5
Chemical composition and textural properties
It is widely reported that the appearance of crystalline WO3 NPs
may be an evidence that a close packed monolayer of surface
WOx has been achieved and that no additional exposed alumina
sites remain for the anchoring of WOx species.[12] However,
Raman spectroscopy shows that a small fraction of WO3 begins
to form three-dimensional clusters at ρW values below the
theoretical monolayer content of 4-7 W at. nm-2 predicted in the
literature.[13] In addition, these models assume that the
structures of WOx are similar to the one corresponding to the
crystalline WO3. Therefore, the theoretical monolayer
determinations based on such structural calculations are rough
approximations, as it occurs with similar oxophilic metal
systems.[14] All in all, it is possible that the emergence of WO3
NPs takes place before a full monolayer formation, but what was
clearly observed in our previous works is that these NPS hinder
the 1,3-PDO production.[10] Therefore, their formation should be
avoided.
Table
1 summarises the textural properties of bare
supports, as well as of WOx and Pt-WOx supported
catalysts, obtained by N2 physisorption at -196 ºC.
The incorporation of WOx on Alcom (9W Alcom) reduced the SBET
and the pore volume (Vp) with respect to the bare support. This
suggests the blockage or filling of some of the pores, especially
the smallest ones, as the rise in the average pore radio (rp)
indicated. By contrast, the opposite effect on SBET was found
when WOx was incorporated on AlSG and AlSG’. In view of the
results, these supports did not suffer the pore blockage by the
WOx species; on the contrary, higher surface areas were
generated. Comparing the calcined 9W AlSG and 11W AlSG’
samples, they showed similar textural properties, Vp and rp, with
the exception of a slightly lower SBET for the latter (307 vs. 288
m2 g-1). When the calcination temperature was incremented from
450 to 800 ºC, the SBET decreased from 307 to 181 m2 g-1 for the
9W AlSG and from 288 to 126 m2 g-1 for the 11W AlSG’ catalyst.
Similarly to the incorporation of WOx on Alcom, a decrease in the
SBET and Vp was observed after the addition of Pt over
WOx/Al2O3 (9Pt8W Alcom) catalyst which can be explained
through the same assumptions listed above.
The Raman spectroscopy technique was selected in order to
establish the highest ρW values before the appearance of WO3
lim
NPs (ρWlim) for each Al-based support. The ρW
for WOx
supported on Alcom was previously found at ~ 2.3 W at. nm-2.[10]
The spectra of the calcined WOx over AlSG and AlSG’ catalysts
are shown in Figure 2 (a and b). It can be observed that the
band corresponding to the symmetric stretching of W=O bonds
Raman spectroscopy
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of polytungstates[13b] appears at ~ 950 cm-1 for 9W AlSG (1.1 W at. accommodate a remarkably higher W loading before the
Figure 2. Raman spectra of the calcined WOx/Al-based catalysts: AlSG (a), AlSG’ (b) and comparison between Raman spectra of 9W AlSG and 11W AlSG’ calcined at
450 ºC and 800 ºC (c). In the latter case the calcination temperature was indicated in the catalyst name.
nm-2) and 11W AlSG’ (1.5 W at. nm-2) catalysts, respectively.
This band shifted towards higher wavenumbers with the rise of
ρW, as the polytungstate concentration increased;[12a,15] reaching
the 970 cm-1 position for different ρW: 1.9 W at. nm-2 over AlSG
support (15W AlSG) and 2.6 W at. nm-2 over AlSG’ (23W AlSG’).
At higher ρW very strong typical bands of WO3 NPs were
observed at ~ 800, 700 and 270 cm-1, which are assigned to the
W−O stretching, W−O bending and W−O−W deformation modes
of octahedrally coordinated WO3 NPs.[11b] These NPs appeared
at ρW ~ 2.7 for AlSG support (18W AlSG) and at ρW ~ 4.7 W at.
nm-2 for AlSG’ (30W AlSG’). Bulk WO3 crystallites could also be
visually identified by their light yellow colour, in contrast to the
white well-dispersed WOx surface species (see Figure S1).
appearance of WO3 NPs (25W AlSG’), leading to the highest ρW
value (2.6 W at. nm-2). Some authors suggested that the surface
WOx species primarily anchor to the γ-Al2O3 support by titrating
the surface hydroxy groups.[16] The higher hydroxy concentration
on γ-AlO(OH) with respect to γ-Al2O3 support agrees with this
hypothesis. Besides incorporating a higher WOx loading, another
way to increase the ρW is to decrease the SBET of the catalyst.
Thus, two other catalysts with 10 wt% W nominal content were
prepared over AlSG (1.4 W at. nm-2) and AlSG’ (2.4 W at. nm-2)
and calcined at higher temperatures, 800 ºC instead of 450 ºC.
Raman spectra of catalysts with a 10 wt% W nominal content at
both calcination temperatures are compared in Figure 2 (c). The
symmetric stretching W=O band of polytungstates shifted
towards higher wavenumbers (from ~ 950 to 970 cm-1) and the
lim
lim
At first sight, it can be concluded that the ρW depends on the
Al-based starting material. As it can be observed in Table 2, the
AlSG support could incorporate some higher WOx loading before
peak suffered a significant intensity increase, when the
calcination temperature increased up to 800 ºC. From this
qualitatively comparison, it may be deduced that polytungstates
to monotungstates ratio increased with the calcination
temperature, since the ρW of the catalysts increased
the appearance of WO3 NPs (15W AlSG), but it did not achieve a
lim
higher ρW
than Alcom support (9W Alcom). However, it is
interesting to highlight that the AlSG’ support, which according to
XRD analysis showed a pseudo γ-AlO(OH) structure, can
XRD
Table 2 Summary of the WOx/Al-based samples investigated by Raman
The structure of Pt-WOx with a nominal W content of 10
wt% (real contents: 9-11 wt%) over the three Al-based
supports employed (Alcom, AlSG and AlSG’) were studied by
XRD. These samples were named 2Pt9W Alcom, 2Pt9W AlSG
and 2Pt11W AlSG’. The diffraction patterns of these reduced
catalysts, before (fresh) and after been utilised in the
glycerol hydrogenolysis (used), are shown in Figure 3.
Only diffraction peaks attributed to γ-Al2O3 were found in
the fresh catalysts. However, a fraction of this phase was
transformed into its hydrate form (γ-AlO(OH)) during the
reaction for those samples with AlSG and AlSG’ supports. This
was demonstrated by the appearance of very sharp peaks
characteristic of well-crystallised γ-AlO(OH), the most
spectroscopy and the main findings achieved.
Tcalc.
(ºC)
450
ρW
(W at. nm-2)
2.3
Stretching W=O
position (cm-1)
Presence of
Catalyst
NPs
No
9W Alcom
9W AlSG
15W AlSG
18W AlSG
973
953
970
970
450
0.8
No
450
1.9
No
450
2.7
Yes
11W AlSG’
23W AlSG’
30W AlSG’
9W AlSG 800
11W AlSG’ 800
1.1
2.6
4.7
1.4
2.4
956
970
970
968
965
No
No
450
450
450
800
800
Yes
No
No
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Morphological analyses of the reduced samples with the highest
ρWlim (2Pt9W Alcom, 2Pt15W AlSG and 2Pt23W AlSG’) were carried
out by SEM in order to determine the influence of the Al-based
supports.
The 2Pt9W Alcom sample was composed by multiple layers of
close-packed spheres (Figure S2). WOx species were generally
well-dispersed over the Alcom support. Nevertheless, some
agglomerates were found. By contrast, Raman spectroscopy
and the XRD analyses showed the absence of WO3 NPs. This
contradiction could be attributed to the difference in the analysis
depth; while Raman spectroscopy only investigates the surface
of the sample, SEM reaches a deeper layer under the catalyst
surface.[20]
Pt particles were found well-dispersed, which is in agreement
with the relatively small Pt size (about 5 nm) obtained by CO
chemisorption (see Table 4).
Pt/WOx supported on AlSG (2Pt15W AlSG) and AlSG’ (2Pt23W
AlSG’) showed a similar morphology (Figure S3). The absence of
a plate-like structure characteristic of layered γ-AlO(OH)[21]
verifies the phase transformation observed by XRD, from a
pseudo γ-AlO(OH) to a γ-Al2O3 phase, that occurs during the
calcination in air at temperatures higher than 450 °C.
Although the 2Pt23W AlSG’ catalyst showed
a
more
homogeneous distribution, both samples formed aggregates
with a wide range of morphologies. Quite big WOx agglomerates
were found in 2Pt15W AlSG, and poorly dispersed Pt in the
2Pt23W AlSG’ sample.
Figure 3. XRD diffraction patterns of 2Pt9W Alcom (a), 2Pt9W AlSG (b),
2Pt11W AlSG’ (c), and 2PtCVI 9W Alcom (d). The subscripts indicate the state
of the catalyst: fresh reduced (1), and used (2).
TPR-H2
intense located at 2θ = 14.4º, 28.2º, 38.1º and 49.4º.[17] The
aggressive hydrothermal conditions employed (200 ºC, 25
bar of H2 and 95 wt% liquid water) favoured this
transformation. The same phenomenon was reported in the
literature under similar conditions.[18] Hence, the Al-based
supports prepared by SG are not stable under the reaction
conditions employed, unlike the commercial γ-Al2O3 (Alcom).
The SG preparation method must, therefore, be improved in
order to avoid these phenomena.
The study of the reducibility of surface species was carried out
by TPR-H2, within the temperature range of 40-1000 °C. The
TPR technique can also provide information about the
dispersion of the metallic components, as well as about the
metal-support and metal-metal interactions. The TPR profiles of
the most relevant Al-supported Pt-WOx and WOx catalysts are
shown in Figure 4.
Within the investigated temperature range, WOx/Al-based
samples showed a broad reduction peak with a maximum ~
900 °C for 11W AlSG’ and 11W AlSG’ 800, and higher than 1000
ºC for 9W Alcom. Despite the fact that their reduction started at
around 740 °C, they required very high temperatures (T > 1000
ºC) to achieve it entirely. According to the literature, those peaks
at elevated temperatures are attributed to the reduction of the
WO3 (W6+→W0) strongly bonded to the γ-Al2O3 support.[22] The
difference between the maxima of ~ 100 ºC could indicate a
higher interaction between WO3 and the Alcom support. The
absence of TPR peaks at lower temperatures indicates that
surface WO3 species were well-dispersed over the respective
support and, therefore, WO3 NPs were not formed,[10,23] as it was
previously confirmed by Raman spectroscopy and XRD.
Diffractograms of fresh samples showed the presence of
metallic Pt (reflection angles of 39.9º, 46.4º and 67.4º)[19]
which indicated that it was completely reduced under the
pre-treatment conditions employed, and remained at its
metallic state after the glycerol hydrogenolysis reaction.
Qualitatively, the most intense reflection of Pt (39.9º) for the
2Pt9W Alcom catalyst was significantly sharper in the used
sample, suggesting some sintering of Pt crystallites during
the reaction. This effect was not observed for those
samples supported on AlSG and AlSG’
.
SEM
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Figure 4. TPR-H2 of the fresh calcined catalysts: WOx and Pt/WOx supported on Alcom (a) and AlSG’, WOx/AlSG’ calcination at 450 and 800 ºC (b and c, respectively). A
catalyst supported on AlSG was also showed for comparison in Figure 4b.
All Pt-containing catalysts showed multiple reduction peaks in
the 40 - 270 ºC range, related to the reduction of different PtOx
species.[24] It can be observed that the incorporation of Pt has a
significant effect on the WOx reducibility. It facilitates its
reduction probably through a H2 spillover effect, which was also
found in similar catalytic systems.[25] In addition, according to Jia
et al.[26], the peaks whose maximum appeared between 420 and
480 ºC could be assigned to the reduction of PtO2 in high
interaction with the support. However, the TPR profile of 2Pt
Alcom only showed a small shoulder in this region (at 455 ºC).
Therefore, there are not clear evidences to rule out whether the
reduction peaks in the 400 - 500 ºC temperature range could be
attributed to PtOx–WOx or to PtO2 species. Moreover, the 2Pt
Alcom sample also showed three main peaks with maxima at 83,
211 and 366 ºC. With the presence of WOx (2Pt9W Alcom), the
temperature required for the reduction of PtOx species generally
increased (peak maxima at 73, 260 and 427 ºC). The visible
higher H2 consumption of 2Pt Alcom over 2Pt9W Alcom catalyst is
well explained by its notably higher Pt dispersion (see Table 4).
This implies that the incorporation of WOx hinders the Pt
anchoring, worsening its dispersion.
peaks for PtOx reduction were observed for the 2Pt23W AlSG’
catalyst at lower temperatures (40 - 150 ºC).
The unexpected low temperature peaks are indicative of a quite
weak PtOx interaction with the support. The low intensity, due to
small H2 consumption, is consistent with the poor Pt dispersion
observed by SEM. It is important to note that the peak at ~ 400
ºC observed in the previous cases, and probably corresponding
to PtOx–WOx species, was not observed there.
In addition, the reduction of WOx did not start until 600 ºC. Very
low Pt–WOx proximity in the 2Pt23W AlSG’ catalyst is suggested
by all these evidences, the lowest of all the investigated samples.
Hence, for this catalyst, the interactions between the hydrogen
species activated in the Pt and the WOx sites are lower.
Activity tests
The effect of Al-based support and the ρW parameter on the
aqueous phase glycerol hydrogenolysis were investigated at 200
ºC, 25 bar of H2 (initial pressure) and 16 h reaction time.
The ρW played a key role in the formation of 1,2 or 1,3-PDO. At
low ρW values, 0.8 and 1.1 W at. nm-2 for 2Pt9W AlSG and
2Pt11W AlSG’, respectively, negligible amounts of 1,3-PDO were
formed in the reaction (see Table 3 entries 2 and 3). These
catalysts acted as common monometallic Pt/Al2O3 catalytic
systems, producing 1,2-PDO as the main product (selectivity >
88 %).At higher ρW, 1.9 W at. nm-2 (2Pt15W AlSG) and 2.6 W at.
nm-2 (2Pt23W AlSG’), glycerol conversion increased, and the
product distribution changed drastically (Table 3, entries 4 and
5).
Comparing the TPR profiles of 2Pt9W Alcom, 2Pt11W AlSG’ and
2Pt11W AlSG’ 800, it seems that the reduction of PtOx, in the 40 -
300 ºC range, required temperatures slightly higher when Alcom
was used as support. This indicates a possible higher interaction
between PtOx species and the commercial support. The 2Pt11W
AlSG’ and 2Pt15W AlSG samples showed similar peaks related to
the reduction of PtOx species. However, only three very small
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The selectivity towards 1,2-PDO decreased at the same time as
the 1,3-PDO selectivity increased. In addition to this, the C–C
bond cleavage reactions, which mainly leaded to EG and
methanol, were almost suppressed. Nevertheless, due to the
over-hydrogenolysis of the PDOs, quite high amounts of 1-PO
(selectivity values > 14 %) were produced.
Unlike other works reported,[9] W5+ species were not observed by
XPS in the Pt/WOx/Al2O3 catalytic system, neither in the fresh
reduced sample (Figure S4) nor in the used sample (Figure S5).
This fact was previously reported by our group for similar
catalysts.[10] Nonetheless, the in-situ formation of reduced W5+
during the reaction cannot be rejected.
The higher concentration of polytungstate species as the ρW
increased, according to Raman spectroscopy results, affects the
1,3-PDO/1,2-PDO ratio. Dispersed monotungstates cannot
accommodate the negative charge required to stabilise the
Brönsted acid sites, unlike the extended network of WOx formed
by polytungstate species.[11b,13b,27] In the absence of certain
Brönsted acidity 1,3-PDO is not produced (Table 3 Entries 2 and
3). Polytungstates might undergo a slight reduction according to
the following equation (eq. 4):
In order to verify that ρW is actually the controlling parameter in
the selective production of 1,3-PDO, instead of the W loading,
two Pt/WOx/Al-based catalysts with similar W content (9-11 wt%),
but higher ρW than their homologous, were tested.
The relationship between the ρW and the yield of 1,3-PDO is
depicted in Figure 5. The 2Pt9W Alcom (2.3 W at. nm-2) sample
provided the best result: 8.2 % yield of 1,3-PDO (Table 3 entry
1). The higher stability of the commercial support in the reaction
media can explain its better performance. As previously
discussed, XRD patterns of used samples showed, under the
operating conditions the Al-based supports prepared by SG are
transformed into γ-AlO(OH).
2W6+ + H2 → 2W5+ + 2H+
Eq. 4
Table 3. Activity test results obtained in the glycerol hydrogenolysis after 16 h, 200 ºC and 25 bar of H2 (initial pressure).
Glycerol conv.
Product select. (%)
Entry
Catalysta
1,3-PDO/1,2-PDO
1,3-PDOb
1,2-PDO
1-PO
Crackingc
(%)
15.7
3.8
1
2
3
4
5
6
7
2Pt9W Alcom (2.3)
2Pt9W AlSG (0.8)
52.7 (8.2)
0.0 (0.0)
16.9
88.1
89.0
36.4
43.9
41.8
33.4
17.8
6.4
20.8
0.0
0.0
11.2
11.0
3.8
3.1
0
2Pt11W AlSG’ (1.1)
2Pt15W AlSG (1.9)
2Pt23W AlSG’ (2.6)
2Pt9W AlSG 800 (1.4)
2Pt11W AlSG’ 800 (2.4)
2PtCVI9W Alcom (2.3)
9Pt8W Alcom (2.3)
3.4
0.0 (0.0)
0.0
0
9.2
45.4 (4.2)
36.2 (1.7)
40.6 (3.1)
42.9 (4.3)
46.8 (3.8)
55.9 (33.0)
14.4
19.9
12.2
14.4
26.6
25.8
1.2
0.8
1.0
1.3
2.6
8.7
4.7
0.0
7.6
0.0
10.1
8.1
0.0
0.0
8
9d
59.4
0.0
a ρW in brackets (expressed in W at. nm-2 of support), b yield of 1,3-PDO in brackets (%), c cracking products: ethylene glycol (EG) and methanol, above all, d
extrapolated values at 16 h reaction time from data in Figure 6. The error of the carbon balance was within 5 % in all the experiments.
high concentration of polytungstate species, without the
Noteworthy, despite the fact that 2Pt23W AlSG’ was the sample
formation of WO3 NPs, and with a close proximity between the
that showed the highest ρWlim, it achieved an unexpected low
metallic sites of Pt and the acid sites of WOx. Two strategies
yield of 1,3-PDO (Table 3 entry 5). These results show that,
were followed in order to enhance the interaction between the Pt
besides the ρW, there are other catalytic properties that are also
and WOx active sites: (i) to improve the Pt dispersion of the
important in the selective production of 1,3-PDO. In regard to
catalyst which showed the best results so far, 2Pt9W Alcom, with
this, TPR-H2 profiles suggested that all the analysed and tested
the use of a non-conventional method for Pt deposition: the CVI
catalysts showed some interactions between the hydrogen
method; and (ii) to increase the Pt loading.
activated in the Pt sites and the WOx species. Nonetheless,
these interactions were almost negligible for the 2Pt23W AlSG’
sample. The absence of Pt and WOx sites with intimate contact
seems to explain the low yield of 1,3-PDO obtained in spite of its
high ρW value.
Effect of the Pt dispersion
According to the literature, the CVI method (sometimes found as
CVD, chemical vapour deposition) has some advantages over
conventional preparation techniques: (i) it facilitates a high metal
dispersion through the enhancement of interactions (adsorption)
between the organometallic precursor and the support,[28] and (ii)
the lack of a solvent decreases the likelihood of the surface
contamination from residual solvent or by-products of solvent
decomposition during the heat treatment.[29]
Effect of the Pt dispersion and the Pt–WOx interactions of
the catalysts supported on commercial γ-Al2O3
The previous findings showed that in order to obtain 1,3-PDO
selectively from glycerol it seems necessary a catalyst with a
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The Pt particle size and metal dispersion of 2PtCVI9W Alcom and
Once it was established that the 2Pt9W Alcom was the catalyst
which showed the largest 1,3-PDO yield, the Pt loading was
risen up to 9 wt % in order to boost the proximity and
interactions between the Pt and the WOx sites. Figure 6 shows
the time-evolution of glycerol hydrogenolysis using the 9Pt8W
Alcom catalyst at 200 ºC and 25 bar of H2.
Glycerol conversion reached a maximum of 65.8 % at 24 h.
Regarding the selectivity towards 1,2-PDO, a constant drop was
found as the selectivity towards 2-PO increased, indicating that
2-PO was formed through the over-hydrogenolysis of 1,2-PDO.
The selectivity towards 1,3-PDO remained quite stable (> 50 %),
showing a little initial increase until 12 h, from which it slightly
decreased. Relatively high 1-PO selectivity values were
observed at low glycerol conversions, and this selectivity did not
change significantly with glycerol conversion. This trend suggest
another reaction route besides 1,3-PDO over-hydrogenolysis for
the production of 1-PO.
Extrapolating the results obtained with 9Pt8W Alcom catalyst at
16 h, and comparing with those obtained with the catalyst with 2
wt% Pt (Table 3 entries 9 and 1), glycerol conversion increased
Figure 5 Relationship between ρW and the yield of 1,3-PDO for the different
Pt/WOx/Al-based catalysts. The results are divided by the corresponding
support.
2Pt9W Alcom catalysts were measured by CO chemisorption. The
results displayed in the Table 4 indicate that the conventional WI
method led to the formation of smaller Pt particles than the CVI:
a Pt size of 5.1 nm for the 2Pt9W Alcom sample and 7.0 nm for
the 2PtCVI9W Alcom sample. Thus, the CVI method was not
effective to enhance the Pt dispersion (22.4 for WI and 17.3 %
for CVI) on this catalytic system. The reason of this unexpected
behaviour could reside in the higher reduction temperatures
used for the sample prepared by CVI, 400 ºC instead of 300 ºC.
However, this high temperature was required in order to fully
pyrolyse the Pt-precursor employed in the synthesis.
Table 4. Pt particle size and dispersion obtained for the fresh reduced
samples by CO chemisorption at 35 °C.
Pt dispersion
-1
Catalyst
Pt size (nm)
mmol CO·gcat
(%)
73.7
22.4
17.3
6.3
2Pt Alcom
1.5
5.1
6.9·10-2
2.1·10-2
1.6·10-2
5.9·10-3
7.2·10-2
2Pt9W Alcom
2PtCVI 9W Alcom
2Pt15W HZ
9Pt8W Alcom
7.0
Figure 6. Time-evolution of glycerol conversion and main product selectivities
obtained with 9Pt8W Alcom catalyst at 200 ºC and 25 bar of H2.
14.7
7.2
15.8
with Pt content: from 15.7 to 59.4 %. As it can be observed in
Table 4, although significant dispersion was lost by increasing
the Pt loading, the amount of active sites (expressed as mmol
CO·gcat-1) increased by a factor of three. The increment in the
glycerol conversion can again be ascribed to a larger amount of
active sites leading to a higher H2 activation rate. More
interestingly, the yield of 1,3-PDO noticeably increased from 8.2
to 33.0 % and the 1,3-PDO/1,2-PDO ratio from 3.1 to 8.7. This
means that, besides the increment in the Pt active sites, another
effect is induced by increasing the Pt content
A considerably lower glycerol conversion for the 2PtCVI 9W Alcom
catalyst was obtained as compared to that obtained with 2Pt9W
Alcom (Table 3, entries 8 and 1, respectively). For the same metal
content a larger dispersion, and as a consequence of this a
higher number of active sites, promotes the conversion of
glycerol through a higher H2 activation. Nonetheless, both
hydrogenolysis routes (1,2-PDO or 1,3-PDO) are similarly
promoted.
Effect of the Pt loading
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Figure 7. TEM (first left) and HAADF-STEM images of 2Pt9W Alcom (top), and 9Pt8WAlcom (bottom). Element scanning images from left to right: Al
(red), Pt (blue), W (green), and Pt and W superposition.
The preponderance of the 1,3-PDO route with the increase of
the Pt loading could arise from the enhancement of the Pt–WOx
interactions. HAADF-STEM images (Figure 7) showed well-
dispersed Pt and W particles over the support for 2Pt9W Alcom
catalyst. Worse Pt metal dispersion can be observed for 9Pt8W
Alcom sample, which was in good agreement with CO
chemisorption results (Table 4). Interestingly, the superposed
images of Pt and W elemental mapping revealed intimate
contact between Pt–WOx for this catalyst.
catalysts[10], the ρW did not remarkably change the selectivity
towards 1,3-PDO. However, the formation of 1-PO and propane
seemed to be favoured for the three ρW values. Glycerol
conversion reached a maximum with 2Pt15W HZ catalyst (1.4 W
at. nm-2) and so did the yield of 1,3-PDO (Table 5, entry 2).
According to the mechanism proposed in our previous study[8a]
(Figure 1), the secondary carbocation formed by the protonation
of the internal OH group with a Brönsted acid site and
subsequent dehydration, needs to be fast stabilised by a hydride
attack. A more intimate contact between Pt and WOx seems to
promote this hydride transfer. Otherwise, the intermediate
carbocation might interact with another proton and be
Comparison between HZSM-5 and Alcom supports
Taking into account that 1,3-PDO is only selectively formed from
glycerol when the catalytic system presents Brönsted acidity, a
HZSM-5 zeolite was selected as support, and Pt/WOx over
HZSM-5 with different ρW values were prepared and tested. The
results are shown in Table 5. Unlike γ-Al2O3 supported
dehydrated to
acrolein,
which could easily suffer
hydrogenation/hydrogenolysis reactions to form 1-PO and
propane, respectively.
Table 5 Activity test results obtained after 16 h, 200 ºC and 25 bar of H2 (initial pressure) with HZSM-5 support.
Product selectivity (%)
Glycerol conv.
Entry
Catalysta
1,3-PDOb
20.6 (2.3)
20.6 (3.6)
20.4 (2.1)
1,2-PDO
10.0
1-PO
46.0
35.1
49.1
Propane
23.4
(%)
11.3
17.3
10.3
1
2
3
2Pt5W HZ (0.4)
2Pt15W HZ (1.4)
2Pt25W HZ (2.8)
11.8
32.5
0.0
30.5
a ρW in brackets (expressed in W at. nm-2 of support), b yield of 1,3-PDO in brackets (%).
1,3-PDO was used as reactant in order to shed light into the
reaction routes for the formation of 1-PO and propane. The 1,3-
PDO hydrogenolysis results with 2Pt15W HZ catalyst can be
observed in Figure 8. Only ~ 10 % of 1,3-PDO was transformed
into 1-PO and propane. Thus, the high amounts of these
products found when glycerol was the reactant (79.6 % total
selectivity) should also be formed through another reaction route
besides 1,3-PDO over-hydrogenolysis.
Some reports correlate increasing Brönsted acidity with higher
yields of 1,3-PDO.[9,30] In this work, TPD-NH3 measurements and
DRIFT spectroscopy of adsorbed pyridine (Figure S6) were
Figure 8. Glycerol and 1,3-PDO conversion and product selectivities obtained
after 16 h, 200 ºC and 25 bar of H2 (initial pressure) with 2Pt15W HZ catalyst.
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carried out to evaluate the effect of the acidity of 2Pt9W Alcom Conclusions
and 2Pt15W HZ samples on the glycerol hydrogenolysis. The
results given in Table 6 show that 2Pt15W HZ had a notable
higher amount of weak strength acid sites and considerably
The results of this work shed light on the main activity-structure
relationships involved in the glycerol hydrogenolysis to 1,3-PDO
when using the Pt-WOx catalytic system. The ρW was pointed
out to be a relevant feature because it controls the kind of WOx
species formed on the catalyst surface. The presence of
polytungstate species, that generate a relatively low amount of
weak/medium Brönsted sites, are necessary for the selective
formation of 1,3-PDO. Nonetheless, when the amount and
strength of Brönsted acidity is increased, by using an HZSM-5
zeolite as support, the selectivity and yield of 1,3-PDO
significantly decreased due to the formation of 1-PO and
propane through glycerol dehydration-hydrogenation (acrolein
route).
The pseudo-AlO(OH) support showed a high concentration of
hydroxy groups on its surface as compared to γ-Al2O3 and,
therefore, it allowed the incorporation of larger W loadings
leading to higher ρW without the emergence of the undesired
WO3 NPs (ρWlim). Although a higher ρW was achieved, the small
interactions between the hydrogen species activated in the Pt
sites and the WOx, as TPR-H2 showed, and the support
instability made this catalyst less effective than Pt/WOx over
commercial γ- Al2O3.
higher amount of medium Brönsted acid sites than 2Pt9W Alcom
.
Medium strength acid sites are known to catalyse the glycerol
dehydration to acrolein.[31]
Focusing again on the reaction mechanism of Figure 1, an
increment on the Brönsted acidity can promote the dehydration
of the intermediate carbocation into acrolein, instead of its
hydrogenation into 1,3-PDO. Acrolein can easily suffer
hydrogenation/hydrogenolysis reactions to form 1-PO and
propane, respectively. Hence, Brönsted acid sites are involved
in two different reaction routes. The selective formation of 1,3-
PDO seems to be related to the presence of a relatively low
amount of weak/medium Brönsted sites and not to the total
Brönsted acidity of the catalyst, as previously suggested.
It is worth adding that the higher H2 uptake and further
dissociation in the catalyst with higher Pt loading could also be
involved in the generation and maintenance of the Brönsted acid
sites.[13b]
Table 6 Acid properties of fresh reduced samples measured by TPD-
NH3.
Acidity (µmol NH3 g-1)
B/L acid ratioc
By keeping constant the metal content, improving the dispersion
of Pt increased the conversion of glycerol, but promoted both
hydrogenolysis routes similarly. On the contrary, increasing the
Pt metal content significantly favored the hydrogenolysis route
leading to 1,3-PDO. A more intimate contact between Pt and
WOx promoted the hydrogenation of the intermediate secondary
carbocation into 1,3-PDO.
Catalysta
Weakb
Mediumb
Totalb
200 ºC
300 ºC
9W Alcom (2.3)
2Pt9W Alcom(2.3)
157
162
158
162
269
426
443
696
646
-
-
282
0.07
-
0.04
-
15W HZ (1.4)
538
2Pt15W HZ (1.4)
484
1.00
0.82
a ρW in brackets (expressed in W at. nm-2 of support).
b
Acid strength distribution: weak at 150 – 300 ºC, medium at 300 –
450 ºC. It cannot guarantee the stability of the catalyst at T > 450 ºC.
c Obtained by DRIFT of adsorbed pyridine.
Experimental Section
Catalyst preparation
Table 7 Pt and W metal leaching in the reaction solution based on the
catalyst used.
Three Al-based materials were used as supports: commercial γ-
Al2O3 (Merck, ≥ 99.9%), denoted as Alcom, and other two supports
prepared by acid sol-gel (SG) method, in a similar way described in
other works published by our group.[32] The SG material obtained by
this procedure showed a pseudo-boehmite structure (pseudo γ-
AlO(OH)) according to the XRD analysis (ESI). A fraction of this
material was used as support without any treatment and was
denoted as AlSG’ (see Figure S7 (a)). The other fraction was
calcined at 450 ºC in air at 2 ºC min-1 for 4 h, and was converted
into γ-Al2O3, named as AlSG (Figure S7 (b)). The thermal evolution
can be observed in Figure S8. Ammonium metatungstate
Catalyst
Pt loss (%)
0.3
W loss (%)
2.3
2Pt9W Alcom (2.3)
2Pt15W HZ (1.4)
1.3
8.2
Regarding the catalyst stability, negligible Pt leaching was
observed for the 2Pt9W Alcom sample (Table 7), whereas the
tungsten leaching was 2.3%. In the case of the 2Pt15W HZ
catalyst, a Pt leaching of 1.3% and a substantial W leaching of
8.2% were measured. In spite of the considerable leaching that
the zeolite based catalyst suffered in the reaction medium, a low
glycerol conversion and an insignificant 1,3-PDO yield were
obtained (Table 5, entry 2). These results suggest that the effect
of the dissolved metal species in the reaction mechanism
(homogeneous catalysis) is not relevant. Nonetheless,
reusability studies of the Alcom supported catalysts should be
performed to study the effect of the lost of tungsten in their
stability.
((NH4)6(H2W12O40)·nH2O,
Sigma-Aldrich,
≥
99.99%)
and
tetraammineplatinum (II) nitrate (Pt(NH3)4(NO3)2, Sigma-Aldrich, ≥
99.995%) were used as precursors for W and Pt, respectively.
These metals were incorporated into the support using the
sequential wetness impregnation method (WI), which was
extensively described in our previous work.[8a] After the W precursor
incorporation, the resulted catalysts were calcined in air at 450 ºC
or 800 ºC. In the latter case, the temperature is specified in the
catalyst nomenclature. The Pt precursor was then incorporated
following the same procedure and the final catalyst was calcined at
300 ºC.
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Zeolite ZSM-5 ammonium (Alfa Aesar, SiO2:Al203 = 80:1) was also
used as support. The purchased support was calcined at 550 ºC in
air for 4 h prior to the W and Pt incorporation by WI, in order to
convert it into its protonated form HZSM-5 (HZ).
The prepared Pt/WOx/support samples are named xPtyW support,
where x refers to the Pt content in the final catalyst (Pt wt%), y to
the W content before Pt incorporation (W wt%), both measured by
ICP-OES, and support specifies the material used in each case (i.e.
Alcom, AlSG, AlSG’ or HZ).
DRIFT spectroscopy of adsorbed pyridine was carried out in a
Vertex 70 spectrometer (Bruker) equipped with an external sample
chamber XSA and a mercury cadmium telluride (MCT) detector.
Samples were ground and mixed with KBr (1:20 wt/wt) and loaded
in a high temperature reaction chamber (Praying Mantis from
Harrick). The background was then collected at 40 ºC. After
exposure to pyridine (Scharlan, 99.5%) vapour, the samples were
evacuated under vacuum at different temperatures (200 ºC and 300
ºC) for 15 min. Samples spectra were recorded at 40 ºC with a
spectral resolution of 4 cm-1 and 200 scans per spectrum. Brönsted
to Lewis ratio (B/L) at different pyridine desorption temperatures
was determined by integrating the absorbance bands
corresponding to Brönsted at 1545 cm-1 and Lewis acid sites at
1454 cm-1.
The chemical vapour impregnation (CVI) technique was used for
preparing highly dispersed Pt nanoparticles over WOx/Al2O3
samples and was used for studying the effect of the Pt dispersion
on the glycerol hydrogenolysis. Information regarding this
preparation method can be found at[33]. This catalyst was known as
xPtCVIyW Alcom
.
Raman spectra of calcined catalysts were obtained using a
Tungsten surface density (ρW) values were calculated using the
Renishaw InVia microscope with Ar ion laser (Modu-Laser) and 514
nm laser excitation. Spectra of the fresh catalysts were recorded at
ambient temperature within the 150 – 1500 cm-1 region.
SEM was employed to examine the morphology of some reduced
Pt/WOx/Al catalysts. For this purpose, a JEOL JSM-6400 apparatus
equipped with a W filament was used. Fresh catalyst samples were
previously reduced (similar to the reduction before the activity
tests).
formula shown in the Eq. 1.[34]
푥
(
푊 )푁퐴
푊
푀
−2
[ ]
휌푊 푊 푎푡표푚푠 푛푚 표푓 푠푢푝푝표푟푡 =
Eq. 1
⁄
푆푠푎푚푝푙푒 (1−푥푊푂3
)
Where xw and xWO3 are the mass fraction of the W and WO3 species
in the final catalyst, NA is the Avogadro number, Mw is the W atomic
weight, and Ssample is the SBET of the calcined sample.
TEM and HAADF-STEM images of reduced catalysts were obtained
on a TECNAI G2 20 TWIN equipped with a LaB6 filament, at 200
kV.
Catalyst characterisation
XRD patterns of some reduced catalysts were obtained with an
Xpert-Pro instrument with a PW3050/60 goniometer and a Cu
anode at current of 40 mA and voltage of 40 kV, in a 2θ range from
10º to 90º with a 0.026º step size. The in-situ XRD was carried out
on the same system using a high-temperature cell in which the
sample was heated at 2 ºC min-1 under stationary air. The patterns
were compared with the power diffraction files (PDF) by Xpert-Pro
High Score tool.
XPS spectra of the fresh reduced sample and after used in the
reaction were obtained on a SPECS system equipped with a
Phoibos 150 1D-DLD analyser and Al Kα (1486.6 eV) X-ray source.
Binding energy (BE) values were referred to the C1s peak at 284.6
eV.
The chemical composition of the calcined catalysts was determined
by inductively coupled plasma optical emission (ICP-OES)
spectroscopy using
a Perkin-Elmer Optima 2000 instrument.
Previous to the analysis, solid samples were digested in
a
microwave oven with an acid mixture of HNO3, HCl and HF. Metal
leaching in the reaction mixtures were also examined by ICP-OES.
The metal loss was quantified comparing the metal content of the
fresh catalysts with the metal content in the reaction solutions.
Textural properties were measured by N2 physisorption at -196 ºC
using a Quantachrome AUTOSORB-1C-TCD device. Prior to the
measurements, the samples were dried at 200 ºC overnight under
high vacuum. The surface area was calculated by the Brunauer-
Emmett-Teller method (BET) and the pore volume by the Barrett-
Joyner-Halenda (BJH) method.
Activity Tests
CO chemisorption measurements were carried out in
a
Micromeritics AutoChem II 2920 instrument. Calcined samples were
reduced under H2 for 1 h. After 2 h in He, CO chemisorption
uptakes were measured by pulses of pure CO at 35 ºC. The CO/Pt
adsorption stoichiometry of 1 was employed for the estimation of
the Pt size.
TPR-H2 profiles were obtained using the same device. Samples
were heated at 250 ºC under a He stream for 2 h prior to the
analysis, in order to eliminate the water and impurities from the
surface. Afterwards, they were reduced in a stream of 5 vol% H2/Ar
(50 mL min-1) from 40 ºC up to 1000 ºC at 10 ºC min-1.
(TPD-NH3 measurements were also carried out using the Autochem
II equipment. Calcined samples were in-situ reduced (similar to the
reduction before the activity tests) previous to the adsorption of
NH3. After cooling down to 100 ºC, the sample was saturated in a
stream of 10 vol% NH3/He (50 mL min-1) for 30 min. Physically
adsorbed NH3 was removed by purging with He at 150 ºC for 1 h.
Afterwards, the sample was heated up to 900 ºC at a heating rate of
10 ºC min-1 while the desorbed NH3 was monitored using a TCD
detector. The number of acid sites was calculated from the peak
area under the NH3 desorption curve. The strength is usually
classified as weak acid (150 – 300 ºC), medium acid (300 –
500 ºC), and strong acid (500 – 650 ºC) sites.[35]
Activity tests were carried out in 4 Hastelloy autoclaves of 50 mL
capacity. Typically, 10 mL of 5 wt% glycerol (Sigma-Aldrich,
99.9%), or 1,3-PDO (Sigma-Aldrich, 98.0%), in water and 0.08 g of
catalyst (0.166 gcatalyst·greactant-1) were placed in each autoclave.
Prior to the activity test, catalysts were ex-situ reduced for 1 h under
a stream of pure H2 (100 mL/min) at 300 ºC, with the exception of
the sample prepared by CVI which required a higher reduction
temperature (400 ºC). Autoclaves were then purged, pressurised up
to 25 bar of H2 and placed in preheated heating plates (up to
200 ºC) with a magnetic stirring of 550 rpm. After 16 h reaction time,
autoclaves were cooled down to room temperature and the gas
phase was analysed with
a GC-TCD-FID (7890 A, Agilent
Technologies) equipped with a HP-Molesieve (30 m x 535 µm x 25
µm) and HP-PLOT/Q (30 m x 320 µm x 20 µm) columns. A known
amount of 1,4-butanediol (Sigma–Aldrich, 99.99%) was added to
the reaction mixture as external standard, and the liquid sample
was analysed in a GC-TCD-FID (6890 N, Agilent Technologies)
equipped with a MetaWax capillary column (30 m x 530 µm x 1.2
µm).
For the monitoring of glycerol hydrogenolysis evolution with
intermediate sampling a 50 mL stainless steel autoclave (PID
Eng&Tech, Spain) was used. Reactant conversion and selectivity
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10.1002/cctc.201701067
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FULL PAPER
towards products were calculated based on the following equations
(Eq. 2 and 3):
[10] S. García-Fernández, I. Gandarias, J. Requies, M. B. Güemez, S.
Bennici, A. Auroux, P. L. Arias J. Catal. 2015, 323, 65–75.
[11] a) X. Wu, L. Zhang, D. Weng, S. Liu, Z. Si, J. Fan J. Hazard. Mater. 2012,
225–226, 146–154; b) D. G. Barton, M. Shtein, R. D. Wilson, S. L. Soled,
E. Iglesia J. Phys. Chem. B 1999, 103, 630–640.
∑
퐶−푏푎푠푒푑 푚표푙 표푓 푎푙푙 푝푟표푑푢푐푡푠
퐶−푏푎푠푒푑 푚표푙 표푓 푟푒푎푐푡푎푛푡푡=0
푡=푡 × 100
Eq. 2
( )
퐶표푛푣. % =
퐶−푏푎푠푒푑 푚표푙 표푓 푡ℎ푒 푝푟표푑푢푐푡푡=푡
( )
푆푒푙푒푐푡. % =
× 100 Eq. 3
∑
퐶−푏푎푠푒푑 푚표푙 표푓푎푙푙 푝푟표푑푢푐푡푠푡=푡
[12] a) I. E. Wachs, T. Kim, E. I. Ross Catal. Today 2006, 116, 162–168; b) I.
E. Wachs Catal. Today 1996, 27, 437–455.
[13] a) N. Naito, N. Katada, M. Niwa J. Phys. Chem. B 1999, 103, 7206–7213;
b) D. G. Barton, S. L. Soled, G. D. Meitzner, G. A. Fuentes, E. Iglesia J.
Catal. 1999, 181, 57–72.
Acknowledgements
This work was supported by University of the Basque Country
(UPV/EHU), European Union through the European Regional
Development Fund (ERDF) (Spanish MICIN Project: CTQ2015-
64226-C3-2R), and the Basque Government (Researcher
Training Program of the Department of Education, Universities
and Research). The authors also gratefully acknowledge all
members of SuPrEn research group for their human support and
SGIker (UPV/EHU) for the technical assistance.
[14] I. E. Wachs, B. M. Weckhuysen Appl. Catal. A Gen 157, 1997, 157, 67–
90.
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The closer the better: an intimate
contact between Pt and
polytungstate sites favours the
selective production of 1,3-
S. García-Fernández*, I. Gandarias, Y.
Tejido-Núñez, J. Requies and P.L.
Arias*
Page No. – Page No.
propanediol. Weak Brönsted acidity
is required, but an enhancement on
medium Brönsted sites promotes the
formation of 1-PO and propane
through the acrolein route.
Influence of the support of
bimetallic Pt–WOx catalysts on the
1,3-propanediol formation from
glycerol.
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