Selective transformation of glycerol into 1,2-propanediol on several Pt/ZnO solids: Further insight into the role and origin of catalyst acidity

Article abstract of DOI:10.1016/j.cattod.2014.11.014

Microemulsion technique allowed us to synthesize different ZnO solids with similar particle sizes and textural properties. Platinum was subsequently incorporated by deposition-precipitation and impregnation methods and solids tested for glycerol selective transformation into 1,2-PDO. Incorporation of platinum led to the creation of new (mainly Lewis) acid sites. A good correlation between conversion and acidity of Pt/ZnO solids was obtained. Interestingly, despite exhibiting some acidity, supports alone were inactive in the process which evidenced the role of the metal in dehydration of glycerol into acetol. Furthermore, as the reaction proceeded some chlorine coming from the precursor (H₂PtCl₆) was leached which led to the disappearance of the strongest acid sites, associated to side reactions (catalytic cracking) thus resulting in an increase in selectivity to 1,2-PDO. Eventual formation of Pt-Zn alloy upon reduction of the systems at ca. 400°C was beneficial to 1,2-PDO selectivity.

Full text of DOI:10.1016/j.cattod.2014.11.014

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Contents lists available at ScienceDirect
Catalysis Today
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Selective transformation of glycerol into 1,2-propanediol on
several Pt/ZnO solids: Further insight into the role
and origin of catalyst acidity
V. Montesa, M. Boutonnetb, S. Järåsb, A. Marinasa,∗, J.M. Marinasa, F.J. Urbanoa
a Organic Chemistry Department, University of Córdoba, Campus de Excelencia Internacional Agroalimentario, ceiA3, Marie Curie Building,
E-14014 Córdoba, Spain
b KTH (Royal Institute of Technology), Chemical Technology, Teknikringen 42, SE-100 44 Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Microemulsion technique allowed us to synthesize different ZnO solids with similar particle sizes and
textural properties. Platinum was subsequently incorporated by deposition–precipitation and impreg-
nation methods and solids tested for glycerol selective transformation into 1,2-PDO. Incorporation of
platinum led to the creation of new (mainly Lewis) acid sites. A good correlation between conversion
and acidity of Pt/ZnO solids was obtained. Interestingly, despite exhibiting some acidity, supports alone
were inactive in the process which evidenced the role of the metal in dehydration of glycerol into acetol.
Furthermore, as the reaction proceeded some chlorine coming from the precursor (H2PtCl6) was leached
which led to the disappearance of the strongest acid sites, associated to side reactions (catalytic cracking)
thus resulting in an increase in selectivity to 1,2-PDO. Eventual formation of Pt–Zn alloy upon reduction
of the systems at ca. 400 ◦C was beneficial to 1,2-PDO selectivity.
Received 9 May 2014
Received in revised form
11 September 2014
Accepted 3 November 2014
Available online xxx
Keywords:
Glycerol transformation
Microemulsion technique
Pt/ZnO
1,2-Propanediol
Role of acid sites
Effect of chlorine
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
particle size [15,16] or the support [17,18], just to cite some of
them.
Glycerol is a by-product from biodiesel production (ca. 100 kg of
glycerol per ton of biodiesel produced). Therefore, its valorization
through transformation into other valuable chemicals is of great
interest. One of those valuable products is 1,2-propanediol (1,2-
PDO), which is used in food industry, as a less toxic alternative
to 1,2-ethanediol in antifreeze and as a deicer or as a feedstock
in the preparation of polyester resins, just to cite some exam-
ples of applications [1]. This chemical is traditionally obtained
through the petrochemical route via hydration of propylene oxide.
Alternatively, 1,2-PDO could be produced through a biomass route
from glycerol via dehydration of primary hydroxyl group (thus
forming acetol) followed by hydrogenation of acetol into 1,2-PDO
[2].
As for the mechanistic studies, there are some discrepancies
in the literature concerning the nature of active sites responsible
for selective transformation of glycerol into 1,2-PDO, in particular
for initial dehydration of glycerol into acetol. Selective dehydrox-
ylation of polyols can proceed through 3 different mechanisms.
(i) E1 (acid-catalyzed), involving protonation of a hydroxyl group
which is then expelled as water, the resulting carbocation being
neutralized by the elimination of a neighboring proton; (ii) E2
(base-catalyzed) involving simultaneous H+ removal, loss of the
OH and formation of C C bond and (iii) homolytic cleavage of a
C
O bond on a metallic surface (hydrogenolysis). Therefore, on
acidic systems E1 mechanism is followed. In principle, dehydration
of glycerol could take place through the primary or the secondary
hydroxyl group, the former being thermodynamically favored [19].
According to Zhu et al. [20] Brønsted acid sites catalyze
1,3-propanediol formation whereas Lewis acid sites lead to 1,2-
propanediol. On the contrary, Peng et al. [21] speculate on Brønsted
acid sites being responsible for glycerol dehydration processes in
aqueous medium given the fact that Lewis acid sites would be
converted into Brønsted centers. As for the strength needed for
the process, in a study on gas phase hydrogenolysis of glycerol
There are different features affecting activity and selectivity of
glycerol transformation on metals, such as the metal of choice
(e.g. Pt [3,4], Rh [5,6], Pd [5,7], Ir [8], Cu [9–11]) the addition of a
second metal [12,13], of acid or basic additives [5,14], the metal
∗
Corresponding author. Tel.: +34 957218622; fax: +34 957212066.
E-mail address: alberto.marinas@uco.es (A. Marinas).
http://dx.doi.org/10.1016/j.cattod.2014.11.014
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: V. Montes, et al., Selective transformation of glycerol into 1,2-propanediol on several Pt/ZnO solids:
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catalyzed by Cu/ZnO/MOx (MOx = Al2O3, TiO2, and ZrO2) solids,
Feng et al. [22] concluded that strong acid sites were responsible for
1,2-PDO formation whereas weak acid sites led to 1,3-PDO. In the
liquid phase, Vasiliadou et al. [18] found that moderate acid sites
are sufficient to activate glycerol dehydration. Finally, some studies
give support to the role of the metal not only in hydrogenation of
acetol but also in glycerol activation [18,23,24].
In a previous paper, a screening of different partially reducible
oxides to be used as supports for platinum was described, ZnO being
selected for subsequent studies [4]. Moreover, systems reduced at
200 ◦C exhibited better catalytic performance than those reduced
at 400 ◦C, a temperature at which Pt–Zn alloy was formed which
was detrimental to activity. In a follow-up study [24], different
solids consisting in a noble metal supported on ZnO were syn-
thesized through the microemulsion method. This allowed us to
obtain quite similar metal (Pt, Rh, Pd) particle sizes. Under our
experimental conditions, reactivity followed the order Rh > Pt > Pd.
Furthermore, the presence of some remaining surfactant seemed to
somehow hinder hydrogenation activity of the metal, thus leading
to an unusually high selectivity to acetol.
In the present paper, the good control of particle size ensured
through microemulsion (ME) technique is used to synthesize
diverse ZnO solids (either alone or modified with Al, Ce or Zr) with
a view to tune acidity of the support. Platinum is subsequently
incorporated onto the systems through deposition–precipitation
technique or impregnation from H2PtCl6 aqueous solutions. For
comparative studies, a system starting from a different precursor
(platinum nitrate) was also synthesized. The final goal is to cast
further light on the nature and origin of active sites responsible for
the initial dehydration step of glycerol into acetol.
microemulsions (ME) was surfactant: synperonic 13/6.5 (18.8 wt%),
oil: organic precursor of metal (10 wt% of Zn) dissolved in n-hexane
(24.5%), water: 56.7 wt%. In the case of doping of ZnO with Al, Ce or
Zr, the oil is formed by 10 wt% Zn + (Al, Ce or Zr). Moreover, Al, Ce
or Zr content was calculated to have 5 wt% of these metals in the
resulting ZnO solid.
Once the microemulsion had been obtained in the presence of
the Zn(II) ethylhexanoate aqueous solution, pH was increased up
to 11 with NH4OH in order to precipitate ZnO [26]. Resulting solids
were aged under stirring for 7 h, centrifuged and carefully washed
with 3 portions of 100 mL n-hexane. The solids were dried at 70 ◦C
for 12 h and calcined at 400 ◦C for 2 h at a rate of 10 ◦C/min with a
synthetic air flow of 2 L/h.
For comparative purposes, a commercial ZnO solid was also used
as the support in the present study.
2.2.2. Incorporation of platinum
2.2.2.1. Deposition precipitation method. The synthetic procedure
was as follows: a volume of 6.57 mL of chloroplatinic acid solu-
tion (or 1.67 mL of Pt(NO3)4 solution) was diluted to 200 mL with
Milli-Q water and adjusted to pH 7 by adding 0.1 M NaOH. Then, an
amount of 4.75 g of support was added and the mixture readjusted
to pH 7 with 0.1 M HCl. The solution containing the support was
refluxed at 70 ◦C under vigorous stirring for 2 h. Then, a volume of
10 mL of isopropanol was added, the temperature raised to 110 ◦C
and refluxing continued for 30 min, after which the mixture was
vacuum filtered and the filtrate washed with 3 portions of 25 mL
of water each. The resulting solid was dried in a muffle furnace at
110 ◦C for 12 h, ground and calcined at 400 ◦C for 4 h with a rate
of 1 ◦C/min. After calcination, the solid was ground again, sieved
through a mesh of 0.149 mm pore size and stored in a flask.
2. Experimental
2.2.2.2. Impregnation method. 200 mL of water containing the
metal precursor (chloroplatinic acid) was adjusted to pH 7 with
NaOH. Then, the corresponding amount of ZnO solid (in order to
obtain 5 wt% Pt/ZnO in final systems) was suspended and pH re-
adjusted to 7 with HCl. Suspensions were stirred for 5 h at room
temperature and then the solvent was rota-evaporated and cal-
cined at 400 ◦C. After calcination, the solid was ground, sieved
through a mesh of 0.149 mm pore size and stored in a flask.
The nomenclature of the solids includes an N or Cl prefix indi-
cating the platinum precursor (platinum nitrate or chloroplatinic
acid, respectively), followed by the method of incorporation (dp
or im for deposition–precipitation or impregnation, respectively)
and the origin of the ZnO used (com or ME for commercial or
synthesized through microemulsion, respectively). In the latter
case, when applicable, Al, Ce or Zr refers to the metal doping ZnO.
Finally, the name is followed by the reduction treatment. Therefore,
for instance, a catalyst synthesized by deposition–precipitation
method from chloroplatinic acid on an Al-doped ZnO solid synthe-
sized through microemulsion and pre-reduced at 200 ◦C is denoted
as Cl-dp-ME-Al-200 whereas N-dp-com-unred would indicate that
platinum nitrate was incorporated on a commercial ZnO through
deposition–precipitation method and tested in the reaction with-
out any reduction pre-treatment.
2.1. Materials
Synperonic 13/6.5 was a gift from Croda. Zn(II)-2-ethylhexa-
noate (89%) dissolved in mineral spirit, Al(III)-2-ethylhexanoate,
Zr(IV)-2-ethylhexanoate, Ce(IV)-2-ethylhexanoate, and 15% (w/w)
Pt(IV) nitrate solution were purchased from Alfa Aesar. 8 wt%
of H2PtCl6 aqueous solution, ZnO nanopowder, acetone (techni-
cal grade), glycerol 99%, 1,2-propanediol 99.5%, 1,3-propanediol
98%, (hydroxyacetone) acetol 95%, ethylenglycol 99.5%, n-propanol
99.5%, n-hexane > 99%, HCl 33% in water, and NaOH > 99% were pur-
chased from Sigma–Aldrich. Milli-Q water was used for preparation
of water solutions.
2.2. Synthesis of the solids
2.2.1. Synthesis of ZnO solids through ME technique
The solids, ZnO (either alone or doped with 5 wt% of Al, Zr or
Ce) were synthesized using the commonly known method of oil
in water (O/W) microemulsion (ME) [25]. The internal structure of
the ME is determined by the relative fractions of three constituents:
surfactant, oil and water. The ME is only formed for certain ratios
of the constituents, outside which a two-phase system is formed.
The first step was to determine the relative fractions of compo-
nents where the ME was stable. So, different composition mixtures
of surfactant and water were prepared at different temperatures.
Then a solution of organometallic precursor was added dropwise
in order to know the maximum soluble amount. Determination
of this amount is easy because the microemulsions are isotropic
and transparent, and when they destabilize the transparent disso-
lution turns into a cloudy system. These experiments allowed us to
determine the region of relative fractions of constituents to form
microemulsion. Under optimized conditions, the composition of
2.3. Characterization
Elemental analysis of metal-containing samples was performed
by the staff at the Central Service for Research Support (SCAI) of the
University of Córdoba. It was performed using inductively coupled
plasma mass spectrometry (ICP-MS). Measurements were made on
a Perkin-Elmer ELAN DRC-e instrument following dissolution of the
sample in a 1:3 HNO3/HCl mixture with a soft heating. Calibration
was done by using PE Pure Plus atomic spectroscopy standards, also
from Perkin-Elmer.
Please cite this article in press as: V. Montes, et al., Selective transformation of glycerol into 1,2-propanediol on several Pt/ZnO solids:
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Thermogravimetric analyses (TGA–DTA) were performed on a
Setaram SetSys 12 instrument. An amount of 20 mg of sample was
placed in an alumina crucible and heated at temperatures from 30
to 1000 ◦C at a rate of 10 ◦C/min under a stream of synthetic air at
40 mL/min in order to measure weight loss, heat flow and derivative
weight loss.
EDX measurements were performed on a JEOL JSM-6300
scanning electron microscope (SEM) equipped with an energy-
dispersive X-ray (EDX) detector. It was operated at an acceleration
voltage of 20 keV with a resolution of 65 eV.
Surface areas of the solids were determined from nitro-
gen adsorption–desorption isotherms obtained at liquid nitrogen
temperature on a Micromeritics ASAP-2010 instrument, using
the Brunnauer–Emmett–Teller (BET) method. All samples were
degassed to 0.1 Pa at 120 ◦C prior to measurement.
2.4. Catalytic tests
Hydrogenolysis of glycerol was conducted in a Berghof HR-100
stainless steel high-pressure autoclave equipped with a 75 mL PTFE
vessel and a magnetic stirrer. Under standard conditions, 10 mL of
a 1.36 M solution of glycerol in water and 100 mg catalysts were
introduced in the vessel. Reactor was then purged with the selected
atmosphere (H2 or N2), and temperature (180 ◦C) and pressure (2 or
6 bar) adjusted. The stirring rate was 1200 rpm. After 15 h of reac-
tion, stirring was stopped and the vessel cooled with an ice bath.
The reaction mixture was centrifuged to separate the catalyst and
the liquid passed through a filter of PTFE 0.45 ␮m. Then it was ana-
lyzed by GC-FID (Agilent Technologies 7890, with a Supelco 25357
NukolTM capillary column). Quantification was carried out through
the corresponding calibration curves.
Transmission electron microscopy (TEM) images were obtained
using a JEOL JEM 1400 microscope. All samples were mounted on
3 mm holey carbon copper grids. Particle sizes were obtained by
counting 100 particles.
X-ray patterns of the samples were obtained on a Siemens D-
5000 diffractometer equipped with a DACO-MP automatic control
and data acquisition system. The instrument was equipped with a
graphite monochromator and used Cu K␣ radiation. Metal particle
sizes were calculated using Scherrer equation.
X-ray photoelectron spectroscopy (XPS) data were recorded on
4 mm × 4 mm pellets 0.5 mm thick that were obtained by gently
pressing the powdered materials following outgassing to a pressure
below about 2 × 10−8 Torr at 150 ◦C in the instrument pre-chamber
to remove chemisorbed volatile species. The main chamber of the
Leibold–Heraeus LHS10 spectrometer used, capable of operating
down to less than 2 × 10−9 Torr, was equipped with an EA-200MCD
hemispherical electron analyser with a dual X-ray source using
AlK␣ (hꢀ = 1486.6 eV) at 120 W, at 30 mA, with C (1s) as energy
reference (284.6 eV).
Temperature-programmed reduction (TPR) measurements
were made with a Micromeritics TPD-TPR 2920 analyser. An
amount of 100 mg of catalyst was placed in the sample holder
and reduced in a 10:90 H2/Ar stream flowing at 20 mL/min. The
temperature was ramped from 50 to 350 ◦C.
3. Results and discussion
3.1. Characterization of support
TG-DTA profiles of all uncalcined solids obtained through ME
method are quite similar. Fig. 1 shows that of ME system. There are
two main weight losses centered at ca. 140 ◦C and 330 ◦C, respec-
tively. The first one could be due to water whereas the second one
could be attributed to the decomposition of remaining organic com-
pounds (e.g. surfactant, organic precursor) [27,28]. In fact, the loss
weight % (41.6%) is much higher than the theoretical one corre-
sponding to the conversion of Zn(OH)2 into ZnO (ca. 18.1%) thus
evidencing the presence of remaining organics. Interestingly, there
is no significant weight loss at temperatures above calcination tem-
perature (400 ◦C). In fact, TG-DTA profile of the solid calcined at
400 ◦C (Fig. 1) did not exhibit any weight loss thus ensuring thermal
stability of the obtained solid.
The main features concerning characterization of the supports
are given in Table 1. As can be seen, and as expected, ME method
ensured quite similar particle sizes (22–26 nm) for all ZnO-based
solids, BET surface areas being in the 27–36 m2/g range. For com-
parative purposes, commercial ZnO solid has also being included in
Table 1, its surface area being significantly lower (15 m2/g). In all
Surface acidity in the catalysts were determined by thermal
programmed desorption (TPD) of a pre-adsorbed probe molecule,
pyridine (Py) monitored by TCD. An amount of 50 mg of sample
was placed under a He stream flowing at 75 mL/min in a reac-
tor 10 mm in diameter that was placed inside an oven. The He
stream was used to clean the solids by heating to 350 ◦C at a rate
of 10 ◦C/min and then cooling to 50 ◦C. At that point, the surface of
the solid was saturated with the Py for 30 min. Pyridine was sup-
plied by bubbling the He stream through liquid pyridine at room
temperature over the samples. After saturation, excess physisorbed
Py was removed increasing the temperature up to 50 ◦C and pass-
ing a He stream at 75 mL/min for 60 min. Then temperature was
increased up to 400 ◦C at 10 ◦C/min, holding the final level for
30 min. Desorbed pyridine was quantified against a calibration
graph previously constructed from variable injected volumes of
pyridine.
The above-described overall acidity study by TPD-Py was
supplemented with one by diffuse reflectance infrared (DRIFT)
spectroscopy of the pyridine-saturated solids intended to identify
the specific types of acid sites present. Measurements were made
with an ABB Bomen MB Series IR spectrophotometer equipped
with a SpectraTech P/N 0030-100 environmental chamber includ-
ing a diffuse reflectance device capable of performing 258 scans
at 8 cm−1 resolution at an adjustable temperature. Prior to analy-
sis, each catalyst was thermally cleaned at 400 ◦C for 30 min. The
last few minutes of the thermal treatment were used to record
reference spectra.
400°C
0
70
60
DTG
TG ME uncal
Heat Flow
TG ME cal 400
50
40
30
20
10
0
-10
-20
-30
-40
-10
-20
-30
-40
-50
0
100 200 300 400 500 600 700 800
Temperature(°C)
Fig. 1. TG-DTA profile obtained in synthetic air for the ME uncalcined system. Dotted
line corresponds to weight loss of ME calcined at 400 ◦C.
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Table 1
Some features concerning characterization of ZnO supports.
Support
N2 isotherms
Particle size diameter (XRD)
Metal content (%)
Acidity (␮mol py
per mg catalyst)
BET area (m2/g)
Mean pore diameter (nm)
ICP-MS
EDX
XPS
Com
ME
ME-Al
ME-Ce
ME-Zr
15
29
27
31
36
11
8
9
7
8
50
26
23
22
24
–
–
0.36
0.53
0.19
–
–
0.46
3.67
1.26
–
–
1.65
3.44
1.61
0.05
0.08
0.15
0.15
0.12
cases, ZnO-systems are mesoporous solids, with mean pore diam-
eters ranging 7–11 nm. As regards elemental analyses of samples,
bulk analyses (ICP-MS and EDX) reveal that the metals (Al, Ce or
Zr) have been incorporated below the nominal value (5 wt%). One
possible reason could be the partial re-dissolution of precipitated
hydroxides at the high pH (11) used in the synthesis [29,30]. X-
ray diffractograms of ZnO supports (not shown) revealed that in all
cases ZnO have a zincite (wurzite) structure with the typical peaks
at 2ꢁ values of 31.7, 34.4, 36.2◦ corresponding to (1 0 0), (0 0 2) and
(1 0 1) reflections, respectively [31]. No signals corresponding to
ceria, zirconia or alumina were observed which is hardly surpris-
ing considering the above-mentioned low incorporation. Acidity
of the solids was determined by TPD of pre-adsorbed pyridine
(Table 1). Interestingly, despite the relatively low metal-doping
content, incorporation of Al, Ce or Zr led to a significant increase in
acidity, in the 50–88% range. Moreover, acidity of commercial ZnO
solid is lower than that of the pure ZnO system obtained through
microemulsion technique. No change in acidity of the supports was
observed after treatment under H2 flow for 2 h at 200 ◦C or 400 ◦C.
environment (e.g. presence of chloride species coming from the
precursor) or when using partially reducible oxides as the sup-
port, as it is the case of the present study, existence of strong
metal–support interactions [35–37]. Therefore, smaller metal par-
ticles are more difficult to be reduced than larger ones thus
resulting in a shift to higher temperatures in the TPR profile. More-
over, the presence of chloride ions at the metal–support interface
has been described to hinder electron exchange between the metal
and the oxide support thus leading to higher reduction tempera-
tures. Finally, reduction peaks appearing at the highest reduction
temperatures are typically associated to those platinum particles
strongly interacting with the support. Fig. 3 confirms that at the
temperature selected for catalytic experiments (180 ◦C) all systems
are reduced. Furthermore, TPR signals of all the systems obtained
through ME exhibit a single relatively narrow peak which sug-
gests a homogeneous platinum particle size distribution. As regards
the solids based on commercial ZnO, results suggest the influ-
ence of both the synthetic method and the metal precursor on
metal dispersion. Therefore, a more homogeneous distribution of
platinum particle sizes would be expected for Cl-dp-com than for
Cl-im-com judging by the narrower TPR peak in the former case.
Comparing the effect of the employed Pt precursors (Cl-dp-com
vs N-dp-com), larger and more heterogeneous in size platinum
particles seem to have been obtained from platinum nitrate, as
suggested by the wider TPR profile appearing at lower reduc-
tion temperatures. The influence of chlorine residues shifting TPR
peaks to higher temperatures in the case of Cl-dp-com, cannot be
ruled out. TEM micrographs (Figs. 4–6) cast further light on these
issues. The first conclusion that can be drawn from this study is
that the average particle sizes for the samples reduced at 200 ◦C
are quite similar for all types of solids (in the 1.3–2.0 nm range,
Table 2), the exception being N-dp-com for which larger metal
particles (2.8 nm, Fig. 6) were obtained. This could be ascribed to
the different metal precursor used and is in line with TPR pro-
files which evidenced an easier reduction of platinum particles
in that solid. If TEM figures of Cl-im-com-200 and Cl-dp-com-
200 are compared (Fig. 5), quite similar particle size distributions
are obtained. This suggests that above-commented observed dif-
ferences in TPR and XRD profiles could be ascribed to different
metal–support interactions. Finally, in all cases, increase in reduc-
tion temperature from 200 ◦C up to 400 ◦C resulted in metal
sintering.
3.2. Characterization of platinum-containing solids
Some features concerning characterization of Pt-containing sys-
tems are summarized in Table 2. ICP-MS results confirm a good
incorporation of platinum, quite close to the nominal content
(5 wt%).
XRD profiles of the samples (Fig. 2) reveal that in general, reduc-
tion at 200 ◦C results in the appearance of a band at ca. 39.8◦
attributed to (1 1 1) crystal plane of the Pt0 face-centered-cubic
phase [32] whereas subsequent reduction at 400 ◦C leads to the
shift of the band to higher 2ꢁ values (ca. 40.9◦) which is indicative
of the formation of Pt–Zn alloy [4,33]. The exceptions are Cl-dp-
ME-Ce and Cl-im-com systems. In the former case, unred system
already exhibits a signal at ca. 39.8◦ which could be indicative of
some kind of Pt–support interaction (Pt–Zn or Pt–Ce [34]). In the
latter case, Pt–Zn alloy seems to be already present in the solid
reduced at 200 ◦C (see signal at ca. 45.9◦). In any case, the influence
of other factors on the appearance of Pt bands (e.g. metal particle
size, oxidation state) should be studied by other techniques (e.g.
TEM, XPS).
H2 TPR profiles are shown in Fig. 3. There are different fac-
tors affecting reducibility of metal particles such as size, the metal
Table 2
Some features concerning characterization of the different Pt/ZnO solids.
Catalyst
Pt (wt%)
ICP-MS
AveragePtparticlesize(TEM, nm)
Total acidity (␮mol py/mg catalyst)
EDX
200
400
Unred
200
400
Used
Cl-dp-com
Cl-dp-ME
4.2
4.7
4.5
6.1
4.8
4.5
4.3
2.7
3.0
3.9
4.0
5.5
2.9
6.0
1.9
1.8
1.7
1.3
1.5
2.0
2.8
3.6
3.0
3.3
2.2
3.2
3.3
4.2
0.23
0.29
0.35
0.30
0.47
0.20
0.07
0.14
0.20
0.21
0.16
0.31
0.11
0.06
0.14
0.14
0.17
0.15
0.16
0.08
0.07
0.11
0.15
0.13
0.11
0.12
0.0
Cl-dp-ME-Al
Cl-dp-ME-Ce
Cl-dp ME-Zr
Cl-im-com
N-dp-com
0.0
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5
Fig. 2. X-ray diffractograms of the different Pt/ZnO systems unreduced or reduced at 200 ◦C and 400 ◦C. In some cases the profiles of unreduced systems after 15 h of reaction
have also been included.
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XPS spectra of unreduced samples in the Zn 2p3/2 region showed
a peak centered at ca. 1022 eV which can be ascribed to ZnO. In the
case of solids obtained from H2PtCl6, reduction at 200 ◦C resulted in
the appearance of a second peak at ca. 1023 eV whose relative inten-
sity decreased upon subsequent reduction at 400 ◦C (see Fig. 7).
This second peak can be associated to oxychlorinated Zn2+ species
[38]. As regards Pt 4f5/2, unreduced systems are formed by Pt2+
(72.4–72.8 eV) and Pt4+ (73.7–74.9 eV) whereas no signals corre-
sponding to Pt0 at ca. 70.6–71.0 eV were observed. Interestingly,
the highest Pt2+ percentage (ca. 44%) corresponded to Cl-dp-ME-Ce
solid which could be ascribed to Ce interacting with Pt (remember
XRD profiles, Fig. 2). Reduction of the solids at 200 ◦C resulted in the
formation of Pt0 which is concordant with XRD and TPR profiles. As
regards XPS spectra of metal-doping species (Fig. 8), Al 2s signals of
Cl-dp-ME-Al unreduced and reduced at 200 and 400 ◦C remained at
118.5 eV which suggests an Al2O3 environment [39]. Similarly, Zr
3d5/2 signal of Cl-dp-ME-Zr-unred, 200 and 400 ◦C solids appeared
at 182 eV which suggests the presence of ZrO2 [40]. In contrast,
Ce 3d5/2 signal of Cl-dp-ME-Ce-unred solid differed from those of
the solid reduced at 200 ◦C and 400 ◦C thus suggesting different
Ce(III)/Ce(IV) ratios in those samples depending on the reduction
treatment [34].
180°C
N-dp-Com
Cl-im-Com
Cl-dp-Com
Cl-dp-ME
Cl-dp-ME-Al
Cl-dp-ME-Ce
Cl-dp-ME-Zr
50 100 150 200 250 300 350
Temperature (°C)
TPD profiles of pre-adsorbed pyridine are depicted in Fig. 9 and
acidity data are summarized in Table 2. From Fig. 9 it is evident that
the incorporation of platinum using H2PtCl6 as the precursor led
to the creation of acidity. Therefore, there are two main pyridine
Fig. 3. TPR profiles of the different Pt/ZnO systems. The vertical line indicates reac-
tion temperature in catalytic experiments of glycerol transformation (180 ◦C).
Fig. 4. TEM micrographs of Cl-dp-ME-Al, Cl-dp-ME-Ce and Cl-dp-ME-Zr reduced at 200 and 400 ◦C.
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Fig. 5. TEM micrographs of Cl-dp-ME, Cl-dp -com and Cl-im-com reduced at 200 and 400 ◦C.
Fig. 6. TEM micrographs of N-dp-com and Cl-dp-com reduced at 200 and 400 ◦C.
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Cl-dp-ME-Zr-unred
Cl-dp-ME-Zr-200
Cl-dp-ME-Zr-400
1015
1019
1023
1027
1015 1019 1023 1027
1015
1019
1023
1027
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
Fig. 7. XPS profiles in the Zn (2p3/2) region of Cl-dp-ME-Zr system unreduced and reduced at 200 ◦C or 400 ◦C.
Al 2s
Zr 3d
Ce 3d
Al2O3 118.5
ZrO2 182
A
B
C
400
400
400
200
200
200
unred
unred
unred
176 178 180 182 184 186 188
Binding Energy (eV)
880
890
900
910
920
114 116 118 120 122
Binding Energy (eV)
Binding Energy (eV)
Fig. 8. XPS profiles of Cl-dp-ME-Al (A), Cl-dp-ME-Zr (B) and Cl-dp-ME-Ce (C) unreduced and reduced at 200 ◦C and 400 ◦C in the XPS Al (2s). Zr (3d) and Ce (3d) regions,
respectively.
desorption peaks centered at ca. 100 ◦C and 300 ◦C, respectively.
Reduction of the solids at 200 ◦C results in the disappearance of the
high-temperature peak whereas the low-temperature one remains
even after reduction at 400 ◦C. In the case of N-dp-com solid, on
the contrary, incorporation of platinum did not result in new acid
sites, thus confirming that new acid sites are somehow associated
to the presence of chlorine. In order to cast further light on the
nature (Lewis or Brønsted) of acid sites, acidity tests were com-
plemented with DRIFT studies. Fig. 10 presents the DRIFT spectra
of pyridine chemisorbed on Cl-dp-com solids at different tempera-
tures (100–400 ◦C) though similar results are obtained for the other
systems coming from H2PtCl6 precursor. In all cases, the spectra
of pyridine adsorbed at 100 ◦C on unreduced Pt-containing solids
exhibit two main bands centered at ca. 1454 and 1610 cm−1 which
are associated to Lewis acid sites, together with some other minor
ones at ca. 1486 (Brønsted + Lewis), 1547 (Brønsted) [41,42]. Acid-
ity of solids reduced at 200 ◦C is lower (as evidenced by the decrease
in intensity of all bands) whereas subsequent reduction at 400 ◦C
hardly changes acidity (which is consistent with Fig. 9). Interest-
ingly, N-dp-com-unred solid also exhibited in DRIFT studies the
above-mentioned Lewis acid sites which retained pyridine up to
100 ◦C whereas no bands due to pyridine adsorption were observed
for reduced systems (not shown). All these results suggest that on
introduction of platinum (either using H2PtCl6 or Pt(NO3)4 as the
precursor) some new acid sites, mainly Lewis-type, are created. In
the case of Cl-series some of those sites are kept in reduced solids
(probably associated to the presence of chlorine) whereas for N-dp-
com reduction at 200 ◦C leads to their disappearance, which could
be due to reduction of Pt(II) and Pt(IV) to Pt0 as evidenced by XPS
and H2 TPR studies.
3.3. Catalytic activity
Production of 1,2-PDO from glycerol is known to occur via inter-
mediate production of acetol. Therefore, dehydration of primary
hydroxyl group in glycerol yields acetol whose hydrogenation leads
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Pt-unred
Pt-200
Supports
Pt-400
Cl-dp-ME-Zr
Cl-dp-ME-Ce
Cl-dp-ME-Al
ME-Zr
ME-Ce
ME-Al
ME
Cl-dp-ME
Cl-dp-com
Cl-im-com
com
N-dp-com
200 300
200 300
200 300
200 300
100 400
100
400
100
400
100
400
Isothermal
Isothermal
Isothermal
Isothermal
Temperature ºC
Fig. 9. Temperature-programmed desorption (TPD) profiles of pyridine pre-adsorbed at 50 ◦C for supports and Pt/ZnO systems (unreduced and reduced at 200 ◦C and 400 ◦C).
to 1,2-PDO (Scheme 1). The different catalysts were tested for their
activity for glycerol transformation into 1,2-PDO. Under standard
conditions (see Section 2.4), supports (unred or reduced at 200 ◦C
or 400 ◦C) were not active in the process. As for the Pt-containing
solids, results obtained for conversion and selectivity after 15 h
reaction are summarized in Fig. 11. In terms of conversion, val-
ues achieved with unreduced samples are in general quite similar
to those obtained for solids reduced at 200 ◦C. This is hardly sur-
prising since at the working temperature (180 ◦C) platinum can
be in situ reduced (remember TPR profiles). Subsequent reduc-
tion at 400 ◦C led to a dramatic decrease in conversion. Increase in
particle size (as evidenced by TEM, Table 2) and formation of Pt–Zn
alloy (remember XRD results) could account for that. An additional
point to take into account is that total acidity decreases with reduc-
tion temperature (see Table 2) which will be further commented in
the mechanistic discussion. It is also worth noting that conversion
values for N-dp-com systems are significantly lower than those of
their Cl-dp-com counterparts which again could be related to the
larger particles. In fact, if TOF values (expressed as moles of glycerol
converted per mole of Pt per hour) are represented (Fig. 12), N-dp-
com and Cl-dp-com perform quite similarly either unreduced and
reduced at 200 ◦C.
Cl dp com unred
Cl dp com 200
Cl dp com 400
100 C
100 C
100 C
200 C
400 C
200 C
400 C
200 C
400 C
1400 1450 1500 1550 1600 1650 1700
1400 1450 1500 1550 1600 1650 1700
1400 1450 1500 1550 1600 1650 1700
Wavenumbers (cm 1)
Fig. 10. Diffuse reflectance infrared Fourier transformed spectra (DRIFT) of Cl-dp-com systems saturated with pyridine at 50 ◦C upon thermal treatment at different
temperatures (100, 200 and 400 ◦C).
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Scheme 1. Mechanism proposed for transformation of glycerol under our experimental conditions.
In terms of TOF, Cl-dp-ME-Zr-unred and Cl-dp-Al-unred (the
most acidic solids, as determined by TPD of pre-adsorbed pyridine)
are the systems exhibiting the highest values whereas Cl-dp-
ME-Ce-unred, for which some kind of Pt–support interaction had
already been detected by XRD in unred system and Cl-im-com
(which apparently also evidenced Pt–support interaction at lower
temperatures than its ME counterparts) are the least active solids.
Fig. 13 shows a dependence of conversion on acidity and platinum
average particle size. From that figure it is evident that the presence
of the metal is necessary for the reaction since supports, despite
exhibiting some acidity, are inactive in the reaction. This backs
the idea of the metal participating in dehydration of glycerol to
acetol.
As far as selectivity to 1,2-PDO is concerned (Fig. 11B), reduction
at 400 ◦C resulted in a significant increase up to values of ca. 90%, the
exception being N-dp-com and Cl-im-com. However, appropriate
study of effect of reduction temperature on selectivity to 1,2-PDO
requires the performance of reactions with unred and 200 systems
at lower reaction times in order to get similar conversions to those
achieved with solids reduced at 400 ◦C after 15 h (i.e. selectivity val-
ues must be compared at iso-conversions). This will be performed
in the following section.
Fig. 11. Results for catalytic transformation of glycerol expressed in terms of conversion (A) and selectivity to ethylene glycol (EG), acetol, 1,2-PDO or others (B) for t = 15 h.
Reaction conditions: 100 mg catalysts, 10 mL 1.36 M water solution of glycerol. 180 ◦C and 6 bar of initial hydrogen pressure. Reaction time: 15 h.
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25
20
15
10
5
0
Fig. 12. Results obtained for catalytic transformation of glycerol expressed in terms of turnover frequencies (TOF) for t = 15 h. Reaction conditions: 100 mg catalysts, 10 mL
1.36 M water solution of glycerol. 180 ◦C and 6 bar of initial hydrogen pressure. Reaction time: 15 h. Data correspond to systems unreduced, reduced at 200 ◦C and 400 ◦C. In
some cases results for the first reutilization of unreduced solids have been included (r1 suffix).
3.4. Mechanistic discussion
(Scheme 1). Firstly, experiments at variable reaction times (1–15 h)
were conducted starting from glycerol. The results showed that for
Cl-containing solids, the selectivity to 1,2-PDO was particularly low
during the first hours of reaction (see data for Cl-dp-com-unred in
In order to cast further light on the process, some further
studies were performed and a reaction mechanism suggested
45
40
support
red400
unred_used
red200
N-dp-com
35
30
25
20
15
10
5
unred
0
0.00
0.10
0.20
0.30
0.40
0.50
Acidity (micromol py/mg cat)
45
40
35
30
25
20
15
10
5
red400
red200
0
0
1
2
3
4
5
Average Pt parꢀcle size (nm)
Fig. 13. Glycerol conversion as a function of acidity (micromoles of pyridine per mg of catalyst) or average particle size (in nm, as determined by TEM) for the different catalysts
used in the present manuscript. Reaction conditions: 100 mg catalysts, 10 mL 1.36 M water solution of glycerol. 180 ◦C and 6 bar of initial hydrogen pressure. Reaction time:
15 h.
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Table 3
Results obtained for glycerol transformation on Cl-dp-com and N-dp-com under standard conditions expressed as glycerol conversion % and selectivity to 1,2-PDO, acetol,
ethylene glycol (EG) and other products. In some cases, results for characterization of the solids have been included.
Catalyst
GC-FID
ICP-MS
% Pt
XPS
TPD pyridine
TEM
Gly conv
Selectivity
1.2-PDO
Selectivity
acetol
Selectivity
EG
Selectivity
others
Pt/Zn
Cl/Pt
Cl/Zn
Acidity
(␮mol/mg cat)
Particle
size (nm)
Cl-dp-com unred (fresh)
Cl-dp-com unred (1 h)
Cl-dp-com unred (5 h)
Cl-dp-com unred (10 h)
Cl-dp-com unred (15 h)
Cl-dp-com unred r1 (15 h)
Cl-dp-com unred r2 (15 h)
–
2.3
–
–
–
–
4.2
ND
ND
ND
4.3
ND
ND
0.42
0.28
ND
ND
0.24
ND
0.72
0.11
ND
ND
0.11
ND
0.29
0.03
ND
ND
0.03
ND
0.23
0.11
ND
ND
0.11
ND
1.9
ND
ND
ND
1.9
ND
ND
35.8
35.0
63.0
69.1
71.9
79.2
6.5
4.0
4.3
3.8
2.7
3.2
0.0
2.5
1.7
1.6
0.6
1.0
57.7
58.5
25.5
25.5
24.5
16.5
10.5
24.5
31.0
20.3
18.0
ND
ND
ND
ND
N-dp-com unred (fresh)
N-dp-com unred (5 h)
N-dp-com unred (15 h)
N-dp-com unred r1
–
–
–
–
–
4.3
ND
3.3
ND
ND
0.25
ND
0.14
ND
0
ND
0
ND
ND
0
ND
0
ND
ND
0.06
ND
0.06
ND
2.8
ND
2.8
ND
ND
11.3
22.5
8.8
53.9
66.0
80.7
87.3
4.8
1.5
5.3
4.8
1.6
0.9
0.3
0.0
39.7
31.6
13.7
7.9
N-dp-com unred r2
7.3
ND
ND
Table 3). This could be due to the presence of Cl ions on the surface
favoring excessive hydrogenolysis and C C cleavage [17,18]. More-
over, aqueous phase reforming (APR) is known to be favored on
highly-dispersed catalysts as it is the case [16] (Scheme 1). Appar-
ently, after 5 h of reaction the extent of C C cleavage decreased thus
resulting in an increase in selectivity to 1,2-PDO. Complementary
XPS and TPD studies of pre-adsorbed pyridine (Table 3) showed
that chlorine was almost lost from the catalyst surface within
1 h of reaction and that acidity had significantly decreased. These
results suggest the possibility of acidic cracking occurring on strong
acid sites which deactivate as reaction proceeds thus resulting in
an increase in selectivity to 1,2-PDO. Interestingly, reutilization
studies (Table 3) showed a drop in conversion which could be
associated to the deactivation of those strong acid sites which
could catalyze both C C cleavage and dehydration of glycerol into
acetol. The drop in conversion is accompanied by a significant
increase in selectivity to 1,2-PDO. From Table 3 it is also evi-
dent that as reaction proceeds, there is a catalyst restructuration
involving chlorine leaching and decoration of platinum particles
by Zn (see decrease in Pt/Zn ratio), whereas Pt particle size remains
constant. In the case of N-dp-com system, initial selectivity to 1,2-
PDO is higher than that achieved on Cl-series, which again could
be related to the absence of strong acid sites associated to chlo-
rine as well as the higher Pt particle size which is detrimental to
APR. Reuse of the systems led to a significant decrease in conver-
sion which could be associated to the Pt leaching (compare %Pt as
determined by ICP-MS of N-dp-com unred (fresh) and N-dp-com
unred (15 h), Table 3), and decoration of Pt particles by Zn. The
increase in selectivity with reuses could suggest some Pt–Zn strong
metal–support interaction leading to active sites more selective
to 1,2-PDO.
85.2%. When the reaction time is decreased to 3 h, the conver-
sion and selectivity values on Cl-dp-com-unred are lower (74% and
85.5%, respectively). Reducing acetol initial concentration to 1/3
of its value, conversion after 3 h is 93% and selectivity 97%. In the
case of acetol, GC–MS studies evidenced the formation of several
C4 C9 liquid oxygenates, their relative percentage increasing with
the reduction of hydrogen pressure or the increase in acetol initial
concentration.
Finally, some reactions were carried out under nitrogen atmo-
sphere. Activity order was coincident with that obtained under
H2 atmosphere (e.g. the most acidic solids, Cl-dp-com-Al-unred
and Cl-dp-com-Zr-unred, were also the most active ones) though
selectivity to 1,2-PDO decreased significantly because of APR and
formation of C4 C9 liquid oxygenates.
4. Conclusions
Different ZnO solids (either alone or doped with Al, Zr or Ce)
were synthesized through the microemulsion technique which
allowed us to obtain similar particle sizes and textural properties.
Platinum was subsequently incorporated from H2PtCl6 through
deposition–precipitation or impregnation method. For compara-
tive purposes, a system from Pt(NO3)4 was also obtained through
deposition–precipitation method. Incorporation of platinum led to
the creation of new (mainly Lewis) acid sites, particularly impor-
tant in the case of chlorine-containing solids. Moreover, acidity is
partly lost during reduction treatment or as the reaction proceeds
which could be ascribed to both chlorine release and platinum dec-
oration by the support (as evidenced by XPS). A direct relationship
between acidity and glycerol conversion was found. Interestingly,
supports were not active in the process which evidences the par-
ticipation of the metal in the dehydration of glycerol to acetol. As
regards selectivity to 1,2-PDO, it increases as reaction proceeds and
acidity of solids decreases to the detriment of acidic cracking. This
suggests that strong acid sites associated to chlorine are responsible
for C C cleavage and excessive hydrogenolysis, whereas dehydra-
tion of glycerol into acetol requires moderate acidity. Moreover,
formation of Pt–Zn strong metal interaction is beneficial to 1,2-PDO
selectivity.
For similar conversion values, solids reduced at 400 ◦C exhib-
ited a higher selectivity to 1,2-PDO than unred systems (Fig. 11
and Table 3). This suggests that changes occurring in the catalyst
as reduction temperature is increased (Pt–Zn alloy formation, loss
of acidity, increase in Pt particle size) leads to the prevalence or
the formation of specific active sites for 1,2-PDO generation. Some
other experiments were conducted starting from acetol. Produc-
tion of 1,2-PDO from glycerol is known to occur through acetol,
via dehydration of a primary hydroxyl group in glycerol. More-
over, dehydration of glycerol to acetol is slower than hydrogenation
of acetol to 1,2-PDO [19]. In fact, if we start from 1.36 M acetol
(the same concentration as for glycerol studies), conversion after
15 h is total, with selectivity to 1,2-PDO in the 83–86% range. The
exception is Cl-im-com (91% conversion, 68% selectivity). Taking
the example of Cl-dp-com-unred, selectivity at full conversion is
Acknowledgements
The authors are thankful to Junta de Andalucia and FEDER funds
(P08-FQM-3931 and P09-FQM-4781 projects) for financial support.
SCAI at the University of Cordoba is also acknowledged for ICP-MS
measurements and the use of TEM and XPS. Finally, the authors are
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grateful to COST Action CM0903 for financial support, including a
short-term scientific mission (STSM) of V. Montes.
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Please cite this article in press as: V. Montes, et al., Selective transformation of glycerol into 1,2-propanediol on several Pt/ZnO solids:
Further insight into the role and origin of catalyst acidity, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.11.014