Pt/ZrO2 Prepared by Atomic Trapping: An Efficient Catalyst for the Conversion of Glycerol to Lactic Acid with Concomitant Transfer Hydrogenation of Cyclohexene

Article abstract of DOI:10.1021/acscatal.9b02139

A series of heterogeneous catalysts consisting of highly dispersed Pt nanoparticles supported on nanosized ZrO₂ (20 to 60 nm) was synthesized and investigated for the one-pot transfer hydrogenation between glycerol and cyclohexene to produce lactic acid and cyclohexane, without any additional H₂. Different preparation methods were screened, by varying the calcination and reduction procedures with the purpose of optimizing the dispersion of Pt species (i.e., as single-atom sites or extra-fine Pt nanoparticles) on the ZrO₂ support. The Pt/ZrO₂ catalysts were characterized by means of transmission electron microscopy techniques (HAADF-STEM, TEM), elemental analysis (ICP-OES, EDX mapping), N₂-physisorption, H₂ temperature-programmed-reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Based on this combination of techniques it was possible to correlate the temperature of the calcination and reduction treatments with the nature of the Pt species. The best catalyst consisted of subnanometer Pt clusters (<1 nm) and atomically dispersed Pt (as Pt²⁺ and Pt⁴⁺) on the ZrO₂ support, which were converted into extra-fine Pt nanoparticles (average size = 1.4 nm) upon reduction. These nanoparticles acted as catalytic species for the transfer hydrogenation of glycerol with cyclohexene, which gave an unsurpassed 95% yield of lactic acid salt at 96% glycerol conversion (aqueous glycerol solution, NaOH as promoter, 160 °C, 4.5 h, at 20 bar N₂). This is the highest yield and selectivity of lactic acid (salt) reported in the literature so far. Reusability experiments showed a partial and gradual loss of activity of the Pt/ZrO₂ catalyst, which was attributed to the experimentally observed aggregation of Pt nanoparticles.

Full text of DOI:10.1021/acscatal.9b02139

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Article
Pt/ZrO2 prepared by atomic trapping: an efficient
catalyst for the conversion of glycerol to lactic acid with
concomitant transfer hydrogenation of cyclohexene
Zhenchen Tang, Pei Liu, Huatang Cao, Sara Bals, Hero Jan Heeres, and Paolo P. Pescarmona
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02139 • Publication Date (Web): 16 Sep 2019
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Pt/ZrO2 Prepared by Atomic Trapping: an Efficient
Catalyst for the Conversion of Glycerol to Lactic
Acid with Concomitant Transfer Hydrogenation of
Cyclohexene
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Zhenchen Tang,a Pei Liu, b Huatang Cao,c Sara Bals, b Hero J. Heeres a and Paolo P.
Pescarmona a,*
a Chemical Engineering Group, Engineering and Technology Institute Groningen, University of
Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
b
Electron Microscopy for Materials Science, University of Antwerp, Gronenenborgerlaan 171,
2020 Antwerp, Belgium
c Advanced Production Engineering Group, Engineering and Technology Institute Groningen,
University of Groningen, Nijenborgh 4, 9747AG, The Netherlands
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ABSTRACT: A series of heterogeneous catalysts consisting of highly dispersed Pt nanoparticles
supported on nanosized ZrO2 (20 to 60 nm) was synthesized and investigated for the one-pot
transfer hydrogenation between glycerol and cyclohexene to produce lactic acid and cyclohexane,
without any additional H2. Different preparation methods were screened, by varying the
calcination and reduction procedures with the purpose of optimizing the dispersion of Pt species
(i.e. as single-atom sites or extra-fine Pt nanoparticles) on the ZrO2 support. The Pt/ZrO2 catalysts
were characterized by means of transmission electron microscopy techniques (HAADF-STEM,
TEM), N2-physisorption, H2 temperature-programmed-reduction (H2-TPR), X-ray photoelectron
spectroscopy (XPS) and X-ray diffraction (XRD). Based on this combination of techniques it was
possible to correlate the temperature of the calcination and reduction treatments with the nature
of the Pt species. The best catalyst consisted of sub-nanometer Pt clusters (< 1 nm) and
atomically-dispersed Pt (as Pt2+ and Pt4+) on the ZrO2 support, which were converted into extra-
fine Pt nanoparticles (average size = 1.4 nm) upon reduction. These nanoparticles acted as
catalytic species for the transfer hydrogenation of glycerol with cyclohexene, which gave an
unsurpassed 96% yield of lactic acid salt at 96% glycerol conversion (aqueous glycerol solution,
NaOH as promoter, 160 °C, 4.5 h at 20 bar N2). This is the highest yield and selectivity of lactic
acid (salt) reported in the literature so far. Reusability experiments showed a partial and gradual
loss of activity of the Pt/ZrO2 catalyst, which was attributed to the experimentally observed
aggregation of Pt nanoparticles.
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KEYWORDS: glycerol, lactic acid, Pt catalyst, transfer hydrogenation, cyclohexene
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Introduction
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Biomass is a renewable and, therefore, sustainable alternative to fossil resources like oil, gas
and coal for the production of bulk and fine chemicals.1-3 Glycerol is one of the most attractive
bio-based platform molecules due to the broad scope of chemical products that can be derived
from it (e.g. lactic acid, acrolein, acrylic acid, 1,2- and 1,3-propanediol) and for its availability,
which is a consequence of being the main side product of the manufacturing of biodiesel through
transesterification of triglycerides from vegetable oils with methanol.4-5 Therefore, the production
of valuable fine and bulk chemicals from glycerol has attracted a lot of interest from both
academia and industry.6-8 Among the products that can be obtained from glycerol, lactic acid
(LA) and alkyl lactates are attractive bio-based platform molecules with several applications,
ranging from the synthesis of the biodegradable poly-lactic acid to the use as green solvents.9
Lactic acid and lactates can be produced from glycerol through a dehydrogenation-rearrangement
pathway (Scheme 1), which would provide a viable alternative to the current production of lactic
acid by fermentation of carbohydrates.4, 9-12 The dehydrogenation-rearrangement of glycerol
implies the nominal formation of H2 (Scheme 1) and in this sense can be correlated to the use of
glycerol as feedstock for the sustainable production of H2 through a aqueous phase reforming
(APR).8, 13-14 Hydrogen is widely used in the chemical industry (e.g. ammonia synthesis, Fischer-
Tropsch process, steel industry and various hydrogenation reactions) and in the fuel cell systems
as a clean power source.2, 8, 15 Clearly, routes that allow producing H2 from a renewable source
such as biomass represent a sustainable alternative to the current production from fossil fuels
through methane steam reforming, which requires extremely harsh conditions.2, 16
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The first reports on the conversion of aqueous glycerol into a lactic acid salt employed strongly
basic solutions (NaOH and KOH in stoichiometric excess relative to glycerol) at high
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temperature (300 C).17-19 The combination of an excess of base and high temperature promoted
the rate of dehydrogenation in the first step of the reaction network, and the rate of the
rearrangement in the second step by neutralizing the formed lactic acid. However, these
conditions are not desirable for practical application nor from the point of view of green
chemistry. The reaction temperature can be lowered to 180 °C under a He atmosphere by using a
noble metal catalyst (Pt/C or Ir/C) in combination with a homogeneous base, reaching 95%
conversion of glycerol and 55% selectivity towards lactic acid.20-21 The oxidative
dehydrogenation of glycerol can also be carried out in the presence of O2, in which case water is
the side-product. In this context, supported Au and its alloy catalysts (AuPt/TiO2) in combination
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with NaOH gave 30% glycerol conversion and 86% selectivity to lactic acid at 90 C.17 More
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recent reports proved that the presence of a base is not essential under this oxidative atmosphere,
with a bifunctional catalyst consisting of Pt supported on a zeolite (Sn-MFI) achieving an
excellent 81% selectivity towards lactic acid at 90% conversion of glycerol under O2 (6 bar) at a
relatively mild temperature (90 °C).10 Between the employed approaches, the path in the absence
of O2 allows generating valuable H2 but requires relatively high temperature and a base to
promote the dehydrogenation, whereas the reactions conducted under O2 do not require a base
(though at the cost of the turnover frequency per metal site) but the hydrogen atoms removed
from glycerol react with oxygen to form a low-value product as water.10
A third approach that provides an attractive alternative to those described above consists in
combining the dehydrogenation of glycerol with the hydrogenation of another compound. Few
reports described the hydrogenation of cyclohexene or nitrobenzene using glycerol as hydrogen
source, but focusing only on the efficiency of the hydrogenation step and not on that of the
conversion of glycerol.22-25 Here, we report a catalytic system that combines the dehydrogenation
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of glycerol and hydrogenation of cyclohexene over a Pt/ZrO2 heterogeneous catalyst in a one-pot
batch reaction under N2 atmosphere (Scheme 1). We show that the careful design of the catalyst
enables the efficient conversion of glycerol to lactic acid with much higher selectivity and under
significantly milder conditions compared to those previously reported for the dehydrogenation of
glycerol in the absence of O2. Additionally, by performing the reaction in the presence of a model
hydrogen acceptor as cyclohexene, we combined a very high lactic acid yield with the production
of cyclohexane. In our catalyst design we selected Pt as active species, since this metal is highly
active and, thus, widely used in hydrogenation and dehydrogenation reactions.26 Since Pt is a
very expensive element, it is of crucial importance to maximize the activity per gram of metal
(and thus the TON). For this purpose, several preparation methods have been developed to obtain
highly and uniformly dispersed Pt nanoparticles, such as wet-impregnation, sol-immobilization
and deposition-precipitation.27-30 Very recently, a method based on atomic trapping was
developed for the preparation of highly dispersed Pt, even at atomic level.31-33 It was reported that
the oxidized Pt species can disperse as single atoms on a CeO2-x support upon calcination at 800
oC under air. The obtained catalyst showed excellent catalytic performance in the low-
temperature oxidation of CO and in the conversion of methane into C2 hydrocarbons.34-35 Only a
few kinds of supports (i.e., CeO2-x, TiO2 and nitrogen doped carbon) were found to promote such
dispersion of Pt species at atomic level.31, 35-36 Here, these concepts have been extended to the
preparation of highly dispersed Pt species (as Pt2+ and Pt4+) on a nanosized ZrO2 (particle size
between 20 and 60 nm; average size: 32 nm). This oxide was chosen because of the similarity of
its coordination geometry (zirconium is coordinated to 7 oxygens) to that found in CeO2-x (with 0
< x < 0.5 as a consequence of the presence of Ce(III) along Ce(IV) species),37 but its potential as
support for atomically dispersed Pt species has not been explored yet.38 The Pt species were
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converted into extra-fine Pt nanoparticles upon reduction by H2, and this system was studied for
the first of time for the combined dehydrogenation of glycerol and hydrogenation of cyclohexene,
achieving unsurpassed yield and selectivity of lactic acid.
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OH
O
HO
HO
O
OH
Pt/ZrO2
NaOH
HO
OH
OR
rearrangement
O
OH
glycerol
transfer-hydrogenation
lactic acid
(R = H or Na)
OH
cyclohexane
cyclohexene
Scheme 1. Catalytic route from glycerol to lactic acid using cyclohexene as hydrogen acceptor.
Experimental
Materials
Glycerol (99%), 1,3-dihydroxyacetone dimer (97%), glyceraldehyde (90%), glycolic acid
(99%), lactic acid (98%), pyruvic aldehyde (40 wt% in H2O), cyclohexene (99%), cyclohexane
(99.5%), sodium hydroxide (98%), benzene (99.9%), hydro platinic acid (H2PtCl4·xH2O, 99.9%),
zirconium oxide (nanopowder, <100 nm), cerium oxide (nanopowder), titanium oxide (P25) were
purchased from Sigma Aldrich. Glyceric acid (20 wt% in H2O) was purchased from TCI
Chemicals. The H2O used in this work was always of MilliQ grade. All chemicals were used
without further purification.
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Catalyst synthesis
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A wet-impregnation method was used for the preparation of the Pt/ZrO2 catalysts. Typically,
ZrO2 (2.0 g) was mixed with an aqueous solution of H2PtCl4 (4.5 g Pt per L), with the volume of
the latter being tuned to the target loading of Pt, and the slurry was stirred at room temperature
until the water was evaporated. The solid mixture was then dried at 100 oC in air overnight in an
oven. The resulting solid was milled into a fine powder and then calcined in air (400, 550 or 800
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o
oC, heating rate 3 C/min). The calcined catalysts were reduced in a tubular oven under H2 flow
(99.9%; 200 mL/min) at a selected temperature (100, 250 or 400 oC, heating rate 3 oC/min) for 2
h. The gas flow was switched to N2 for 1 h to remove adsorbed H2 from the catalyst surface. A
typical catalyst prepared by this method was named as aPt/ZrO2-b-Rc, in which a, b and c stand
for the wt% loading of Pt (a), the calcination temperature (b) and the reduction temperature (c).
In addition, the catalyst prepared with 2 wt% of Pt was also reduced directly at 250 oC under a H2
flow for 2 h after overnight drying (without calcination). This catalyst was named 2Pt/ZrO2-
DR250.
Characterization of the catalysts
Transmission electron microscopy (TEM) images were obtained using a CM12 (Philips)
electron microscope operating at 120 keV. The samples were prepared by ultra-sonication in
ethanol, after which a droplet of the suspension was added to a carbon coated 400 mesh copper
grid. The images were taken with a slow scanning CCD camera.
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)
images and EDX mapping were collected using an aberration-corrected cubed FEI Titan
microscope operating at an acceleration voltage of 300 kV. The samples were prepared by
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suspending the material in ethanol and depositing drops of the suspension on a copper grid
covered with a holey carbon film.
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Nitrogen physisorption isotherms were measured at -196 oC using a Micromeritics ASAP 2420
apparatus. The Brunauer-Emmet-Teller (BET) method was used to calculate the specific surface
area. The Barrett-Joyner-Halenda (BJH) method was used to calculate the pore volume.
Elemental analysis by inductively-coupled plasma optical emission spectrometry (ICP-OES)
was performed using a Perkin Elmer Optima 7000 DV instrument to determine the actual Pt
loadings in the catalysts.
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X-ray Photoelectron Spectroscopy (XPS) analysis was carried out by mounting the catalysts on
a conductive tape adhered to the XPS sample holder. No further treatment was carried out prior to
the XPS measurement. The sample was loaded into the device and the pressure was reduced
below 1·10-7 mbar. The XPS measurements were performed using a Surface Science SSX-100
ESCA instrument equipped with a monochromatic Al Kα X-ray source (h = 1486.6 eV). During
the measurement, the pressure was kept below 2·10-9 mbar in the analysis chamber. For acquiring
the data, a spot diameter of 600 µm was used. The neutralizer was turned on, to avoid charging
effects. All XPS spectra were analyzed using the Winspec software package developed by LISE
laboratory, University of Namur, Belgium including Shirley background subtraction and peak
deconvolution.
Hydrogen-temperature programmed reduction (H2-TPR) measurements were performed on an
Autochem II 2920 from Micromeritics. In a typical experiment, 80 mg of sample was pre-treated
at 500 oC (heating rate 10 oC/min) for 1 h in a flow of He (30 mL/min). Subsequently, the sample
was cooled to 50 oC under the same flow of He. The reduction analysis was performed from 50 to
900 oC (10 oC/min) in a 30 mL/min flow of 5 vol.% H2 in He.
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X-ray diffraction (XRD) measurements were performed on a D8 Advance Bruker
diffractometer with a Cu Kα1 radiation (λ = 1.5418 Å). The XRD patterns were collected under
40 kV and 40 mA in the range of 10o-80o.
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Catalytic tests
The catalytic tests were carried out in a 100 mL Parr stainless steel autoclave reactor equipped
with a Teflon liner and an overhead stirrer. In a typical test, a predetermined amount of the
Pt/ZrO2 catalyst was loaded into the reactor together with an aqueous solution of glycerol (0.5 M
in 20 ml), NaOH (0.015 mol) and cyclohexene (0.02 mol, as organic phase). The reaction was
performed under N2 (20 bar) for 4.5 h at 160 ᵒC (heating time 0.5 h not counted) at a stirring
speed of 800 rpm. Then, the reactor was depressurized and the biphasic liquid was separated into
an aqueous and an organic phase, which were filtered to remove the catalyst. The organic phase
was analyzed by gas chromatography using a Thermo Trace GC equipped with a Restek
Stabilwax-DA column (30 m × 0.32 mm × 1 μm) and a FID detector. The aqueous phase was
first neutralized and diluted by aqueous H2SO4 (1 M), then analyzed by high performance liquid
chromatography [HPLC, Agilent Technologies 1200 series, Bio-Rad Aminex HPX-87H 300 ×
7.8 mm column, T = 60 °C, with 0.5 mM aqueous H2SO4 as eluent (flow rate: 0.55 mL/min)
using a combination of refractive index detector and UV detector]. Each component was
calibrated using solutions of the individual component at 4 different concentrations. Selected
catalytic tests were performed on three different batches of 2Pt/ZrO2-550-R250, showing good
reproducibility of the results (deviation in the lactic acid yield value within ± 4%). For these
experiments, the average value of the yield is reported.
For the catalyst recycling test, a small amount of the reaction mixture was collected for analysis
and the remaining mixture was filtered to recover the catalyst. The catalyst was washed first with
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H2O (20 mL), then with ethanol (20 mL), and this procedure was repeated 3 times, after which
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the solid was dried overnight at 100 ᵒC. The obtained solid was used for the next run in the
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recycling test.
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Definitions
The glycerol conversion (Conv./%) is defined by equation 1:
퐶(푔,0) ― 퐶(푔) × 100%
Conv. =
(1)
퐶(푔,0)
in which C(g) is the molar concentration of glycerol after a certain reaction time and C (g,0) is the
initial glycerol concentration.
The yield of lactic acid (YLA) is defined by equation 2:
퐶(퐿퐴) × 100%
YLA = 퐶(푔,0)
(2)
in which C(LA) is the molar concentration of lactic acid after a certain reaction time and C (g,0) is
the initial molar concentration of glycerol.
The product selectivity for a compound P is defined by equation 3:
퐶(푝)
Sp =
(3)
퐶(푔,0) ― 퐶(푔) × 100%
in which C(p) is the molar concentration of a product after a certain reaction time.
The selectivity towards the transfer hydrogenation is defined by equation 4:
푛 ∗ 푦(푐푦푐푙표ℎ푒푥푎푛푒) ― 2 ∗ 푦(푏푒푛푧푒푛푒) × 100%
Stransfer-H
=
(4)
∑
푖푥 ∗ 푦(푝푖)
in which n is the molar ratio between cyclohexene and glycerol in the reaction mixture,
y(cyclohexane) is the yield of cyclohexane, y(benzene) is the yield of benzene - which is obtained
from the dehydrogenation of cyclohexene, which most likely occurs as a disproportionation with
formation of two cyclohexane molecules per benzene molecule; y(pi) is the yield of each product
that is obtained from the dehydrogenative oxidation of glycerol, e.g. lactic acid, glyceric acid or
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glycolic acid, and x is the number of H2 that can be removed from glycerol by dehydrogenation to
each possible product (i.e. x = 1 for lactic acid; x = 2 for glyceric acid; x = 3 for glycolic acid).
The term “lactic acid” is used in this article to describe the product obtained from the reaction
mixture, which actually is sodium lactate (mixed with small portion of lactic acid from
hydrolysis).
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Results and Discussion
Synthesis and characterization of the Pt/ZrO2 catalysts
With the aim of developing an active and selective heterogeneous catalyst for the
dehydrogenation of glycerol combined with the hydrogenation of cyclohexene at relatively mild
temperature, we designed a system in which the Pt actives species would be highly dispersed on
nanosized ZrO2 as support, possibly even as single atoms.31-32, 35 A series of Pt/ZrO2 catalysts
with different loading of the noble metal was prepared by wet-impregnation, calcination and then
reduced by H2 in a tubular oven. The actual loadings of Pt determined by ICP-OES measurement
(Table 1) was very similar to the theoretical ones, and ranged between 0.6% and 8.4%. The BET
surface area decreased only slightly after loading the ZrO2 support with 2 wt% Pt (~10%, from 32
to 29 m2/g), which indicates that the presence of Pt did not affect significantly the textural
properties of ZrO2.
Table 1. Pt loading on the Pt/ZrO2 catalysts and surface area before and after supporting Pt on
ZrO2.a
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Entry
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Material
ZrO2
Pt loading/wt%
Surface area/(m2/g)
32
Pt particle size/nm
n.a.
0
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1Pt/ZrO2-550-R250
2Pt/ZrO2-550-R250
5Pt/ZrO2-550-R250
9Pt/ZrO2-550-R250
0.6
1.1
2.1
4.8
8.4
n.d.
n.d.
29
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1.4
2.0
2.6
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n.d.
n.d.
a n.d. = not determined; n.a. = not applicable.
HAADF-STEM and TEM were used to characterize the presence and size of Pt particle on ZrO2
(Figure 1 and 2). Remarkably, clusters at the sub-nanometer scale and single atomic Pt were
observed on the ZrO2 support after calcination at 550 °C in air (2Pt/ZrO2-550, Figure 1.A, Figure
2.A, B and Figure S1). This demonstrates that the Pt species were highly and even atomically
dispersed on ZrO2 after calcination in air.31, 34 Upon subsequent reduction of this catalyst in H2
(250 °C, 1 h), very small, well-dispersed Pt nanoparticles with an average particle size of 1.4 nm
formed on the ZrO2 surface (2Pt/ZrO2-550-R250, Figure 1.C and Figure 2.C). On this sample, the
size of Pt ranges from sub-nanometer clusters (from 0.35 nm) to nanoparticles (up to 2.5 nm).
The dispersion behavior of Pt on ZrO2 is very similar to that reported for Pt/CeO2-x, Pt/CN
(nitrogen doped carbon) and Rh/ZrO2 systems characterized by atomically dispersed Pt or Rh
species.31-32, 39-40 The presence of atomically dispersed cationic Pt species in the material obtained
by calcination but prior to reduction (2Pt/ZrO2-550) is further supported by XPS analysis (Figure
3.A), which evidenced that all Pt in this material was in oxidized state, mainly as Pt2+ (at 72.7 eV,
84% peak area) and to a lesser extent as Pt4+ (75.0 eV, 16% peak area). No Pt0 species were
observed, confirming the absence of metallic nanoparticles. The oxidized Pt species existing as
highly dispersed single atoms and very small clusters are probably coordinated to ZrO2 through
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Pt-O-Zr bonds.31, 33, 35, 39, 41-43 After reduction at 250 C, the majority of the oxidized Pt species
were reduced to Pt0 (at 71.2 eV, 62% peak area, Figure 3.B), in agreement with the formation of
metallic Pt nanoparticles observed in 2Pt/ZrO2-550-R250. However, Pt2+ (at 72.7 eV, 33% peak
area) and Pt4+ (at 75.0 eV, 5% peak area) species were still present in this catalyst.31, 43
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Figure 1. TEM images of 2Pt/ZrO2 catalysts prepared by different calcination and reduction
procedures. (A) 2Pt/ZrO2-550; (B) 2Pt/ZrO2-550-R100, average particle size of Pt: 1.4 nm; (C)
2Pt/ZrO2-550-R250, average particle size of Pt: 1.4 nm; (D) 2Pt/ZrO2-550-R400, average particle
size of Pt: 1.8 nm; (E) 2Pt/ZrO2-DR250, average particle size of Pt: 3.7 nm. Note: the resolution
of these TEM images does not allow identification of nanoparticles < 0.5 nm.
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Figure 2. HAADF-STEM images of : (A, B) the 2Pt/ZrO2 catalyst calcined at 550 oC (2Pt/ZrO2-
550); and (C) the same material after reduction at 250 oC (2Pt/ZrO2-550-R250). In image (B), the
arrows point to the single atom Pt species.
Raw data
2Pt/ZrO2-550-R250
Raw data
2Pt/ZrO2-550
Fitting of raw data
Fitting of raw data
Pt0
Pt2+
Pt4+
Pt2+
Pt4+
Background
Background
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Binding Energy/eV
Binding Energy/eV
Figure 3. Pt 4f XPS spectra of various Pt/ZrO2 catalysts. (A) 2Pt/ZrO2-550, area of Pt2+ peak,
84%; area of Pt4+ peak, 16%; (B) 2Pt/ZrO2-550-R250, area of Pt0 peak, 62%; area of Pt2+ peak,
33%; area of Pt4+ peak, 5%.
To investigate in more detail the effect of the temperature of the reduction step, 2Pt/ZrO2-550
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was also reduced at 100 and 400 C under H2 flow (Figure 1.B and D). By increasing the
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increased from 1.4 nm (100 oC) to 1.8 nm (400 oC). This result can be explained considering that
a higher reduction temperature leads to a reduction of a larger fraction of the oxidized Pt species
to metallic Pt, though it also promotes the growth of larger Pt nanoparticles. In the Pt/ZrO2
catalyst that was reduced directly at 250 oC by H2 without previous calcination step, only few Pt
particles were observed on the surface of ZrO2 (Figure 1.E), and these display a significantly
larger average size (3.7 nm) compared to those in 2Pt/ZrO2-550-R250 (1.4 nm). This suggests
that the calcination process strongly enhances the chemical interaction between oxidized Pt
species and ZrO2, which is critical for the subsequent formation of extra-fine Pt nanoparticles
upon reduction.
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Considering the crucial role exerted by the calcination step on the state and size of the Pt
species on the catalyst, we decided to study the effect of different calcination temperatures (400
and 800 oC, compared to 550 oC discussed above) on the dispersion of Pt species on ZrO2. In all
cases, the catalysts were reduced at 250 oC in a H2 flow after the calcination. The TEM images of
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2Pt/ZrO2 calcined at 400 C did not show any particles before reduction, while Pt nanoparticles
with an average size of 2.0 nm were observed after reduction (2Pt/ZrO2-400-R250, Figure S2.A),
which is slightly larger than the size of the nanoparticles in 2Pt/ZrO2-550-R250 (Figure 1.C and
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2.B). On the other hand, when the calcination was carried out at 800 C, only very large Pt
particles (around 50 nm, determined by EDX-mapping) were observed before reduction
(2Pt/ZrO2-800, Figure 4.A). Though not detected, a small fraction of Pt as single atoms might
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also be present, as suggested by the fact that after reduction at 400 C a small amount of very
fine Pt nanoparticles (~ 1 nm) was observed alongside the large Pt nanoparticles (2Pt/ZrO2-800-
R400, Figure 4.C). These results are in agreement with previous reports, which showed that the
calcination temperature can largely affect the interaction between Pt atoms and the oxide used as
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support.33, 36, 40, 44 The effect of decreasing the Pt loading to 0.5% on the nature of the Pt species
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0.5Pt/ZrO2-550, sub-nanometer Pt clusters and even single atoms similar to those in 2Pt/ZrO2-
550 were observed, though obviously in a lower amount. On the other hand, no Pt species could
be detected in 0.5Pt/ZrO2-800 (Figure S3 and S4), which indicates that the lower Pt loading
prevents the formation of the large Pt nanoparticles observed in the case of 2Pt/ZrO2-800 and
that Pt most likely exists only as single atoms in this material.
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The reducibility of 2Pt/ZrO2 as a function of the calcination temperature was investigated
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further by H2-TPR from 50 to 800 C (Figure 5). The intense peak in the 50-220 °C range and
centered at 110 °C, which is visible in the TPR profiles of the materials calcined at 400 and 550
°C but is absent in that of the parent ZrO2, is attributed to the reduction of oxidized Pt species.40-
41, 45-48
The area of this peak becomes smaller when the calcination temperature increases. In the
material calcined at 800 °C, the intensity of this peak is further decreased and its position is
shifted to higher temperature (150 °C). This much lower tendency of 2Pt/ZrO2-800 to be reduced
below 250 °C is in agreement with what observed by TEM and HAADF-STEM (see Figure S2.B
and 4). This supports the hypothesis that the calcination at 800 °C promotes more efficiently the
formation of highly dispersed and fully anchored oxidized Pt species in ZrO2.36, 39 All H2-TPR
profiles present a broad signal ranging from 250 to 450 °C, which stems from two overlapping
peaks centered at 325 and 380 oC (Figure 5). The peak at 325 °C can be ascribed to the reduction
of remaining, oxidized Pt species (Pt2+ and Pt4+), which based on the XPS data (vide supra)
account for 38% of the Pt atoms in 2Pt/ZrO2-550-R250, i.e. after reduction at 250 oC.41, 48-49 The
peak at 380 oC is ascribed to the reduction peaks of coordinatively unsaturated Zr4+ species at the
surface of ZrO2.41 This peak is slightly shifted to lower temperature compared to the
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corresponding peak of the parent ZrO2 (at 410 oC), which suggests that the presence of Pt species
promotes the reduction of ZrO2, possibly by hydrogen spillover.41, 50-52 The peak at 600 °C, which
appeared in the profiles of all the samples, is ascribed to the reduction of (nearly) coordinatively
saturated Zr4+ at surface terraces or in the bulk of ZrO2.41, 52-53
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Figure 4. HAADF-STEM images (left) and EDX-mapping (right) of 2Pt/ZrO2 calcined at 800 oC
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(A and B, 2Pt/ZrO2-800); and the same material after reduction at 400 C (C and D, 2Pt/ZrO2-
800-R400). EDX-mapping: Zr: green; Pt: red.
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110oC
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380oC
325oC
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2Pt/ZrO2-400
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2Pt/ZrO2-550
2Pt/ZrO2-800
ZrO2
100
200
300
400
500
600
700
800
Temp./oC
Figure 5. TPR profile of 2Pt/ZrO2 catalysts calcined at different temperature (400, 550 and 800
oC) and the support ZrO2.
Next, we monitored the effect of the Pt loading on the features of the final catalyst, while
keeping the temperature of the calcination (550 °C) and that of the reduction (250 °C) constant.
Before reduction, Pt is present in the form of highly dispersed species with sub-nanometer size
for Pt loading between 0.5 and 2% (Figure 2.A,B and Figure S5.A, B). With higher loadings (5%
and 9%), small Pt nanoparticles with average size of 1.3 and 1.5 nm, respectively, were detected
by TEM (Figure S5.C and D). This suggests that a Pt loading ≥ 5% exceeds the maximum
capacity of ZrO2 surface of hosting highly dispersed Pt species. The Pt atoms that cannot interact
strongly with ZrO2, aggregate as small nanoparticles.40, 54 After reduction at 250 °C under H2
flow, Pt nanoparticles were observed for all samples (0.5-9Pt/ZrO2-550-R250, Figure S6), and
the average particle size gradually increased from 0.8 nm to 2.6 nm. This series of materials with
different loading of Pt on ZrO2 (0.5-9Pt/ZrO2) was further characterized by XRD (Figure S7). All
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XRD patterns display the characteristic peaks of monoclinic ZrO2. No diffraction peaks due to Pt
were observed on the Pt/ZrO2 catalysts (before or after reduction) when the Pt loading was ≤ 5
wt%, whereas the characteristic peaks of metallic Pt (face centered cubic crystal) are observed for
9Pt/ZrO2. This is due both to the small size and low loading of the Pt nanoparticles, which
implies that the diffraction peaks of Pt are too broad and have too low intensity to be detected.
In summary, by systematically studying the effect of the Pt loading, the calcination temperature
and the reduction temperature, we can conclude that the formation of atomically dispersed Pt
species on ZrO2 is promoted by lower Pt loadings and by higher calcination temperatures. These
conditions also lead to the formation of smaller nanoparticles upon reduction with H2, with a
smaller size being also favored by lower reduction temperature.
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Catalytic Performance of the Pt/ZrO2 Catalysts
The prepared Pt/ZrO2 catalysts were tested for the conversion of glycerol to lactic acid using
cyclohexene as the hydrogen acceptor (Scheme 1 and Table 2). Initially, 2Pt/ZrO2-550 (without
reduction) was tested as catalyst for this reaction and showed 57% conversion of glycerol with
55% yield of lactic acid. The selectivity of the transfer hydrogenation (S(Transfer-H) in Table 2) was
31%, which means 31% of the hydrogen generated from the oxidation of glycerol was employed
to reduce cyclohexene to cyclohexane (Entry 1 in Table 2). When the catalyst was reduced at 100
oC in H2 flow, the catalytic results were very similar to the “unreduced” one (Entry 2, Table 2).
The TEM analysis showed a clear difference between the two catalysts (Figure 1), with no Pt
nanoparticle being visible on the unreduced sample and very small, well-dispersed Pt
nanoparticles appearing upon reduction at 100 °C in H2 flow. Combining this with the catalytic
results, we infer that the 2Pt/ZrO2-550 catalyst is reduced during the reaction. The catalyst
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prepared by reduction at 250 C, 2Pt/ZrO2-550-R250, showed significantly higher conversion of
glycerol (96%) and yield of lactic acid (96%), which is the highest yield of lactic acid from
glycerol in the state of art (Entry 3, Table 2).9-10, 17, 55 The selectivity in the transfer hydrogenation
also increased, reaching 36%. When the reduction temperature of the catalyst was increased to
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400 C, the activity slightly decreased, with 88% glycerol conversion and 86% lactic acid yield
(Entry 4, Table 2). All reactions produced minor amounts (< 2%) of glyceric acid, glycolic acid
and propanediol as side products. For what concerns the conversion of cyclohexene, very high
selectivity towards the hydrogenation to cyclohexane was observed, with no or minor
dehydrogenation to benzene (Table 2). Combining the catalytic performance with the
characterization results, it can be concluded that the highly dispersed oxidized Pt species, Pt2+
and Pt4+ (as determined by XPS and TPR), which are most abundant on the unreduced catalyst
2Pt/ZrO2-550, are probably not the active sites for the dehydrogenation of glycerol. These
oxidized species are easily reduced to metallic Pt nanoparticles (Pt0), which are highly active in
catalyzing the dehydrogenation of glycerol, in line with several literature reports proving the
hydrogenation/dehydrogenation activity of these species with a variety of substrates. 32-33, 35, 49-50,
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On the other hand, when the catalyst was directly reduced at 250 C after wet-impregnation
without prior calcination (2Pt/ZrO2 DR250, Entry 5, Table 2), it showed much lower catalytic
performance compared to the 2Pt/ZrO2 550-R250. Combining these catalytic results with the
TEM characterization, the activity trend can be correlated to much larger and fewer Pt
nanoparticles present on the surface of 2Pt/ZrO2-DR250. This underlines the importance of the
calcination step in the preparation of highly dispersed, very small and thus highly active Pt
nanoparticles supported on ZrO2.
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The suitability of ZrO2 a support for Pt active species was further demonstrated by comparing
the activity of 2Pt/ZrO2 550-R250 to that of analogous Pt-based catalysts prepared using TiO2 or
CeO2 as support. Particularly, CeO2 has been reported to promote the formation of atomically-
dispersed Pt species.32-33, 35 Our results indicate that the catalyst prepared using TiO2 has
significantly lower activity than its counterpart on ZrO2, whereas the Pt catalyst supported on
CeO2 has slightly lower activity than the one on ZrO2 (Table S1). Therefore, we focused the rest
of our study on Pt/ZrO2 catalysts.
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Table 2. Catalytic conversion of glycerol to lactic acid using a Pt/ZrO2 prepared with different
reduction methods.a
Selectivity in the conversion of Yield in the conversion
glycerol (%)
of cyclohexene (%)b
Conv.GLY YLA S(transfer-H)
Lactic Glyceric Glycolic Propane-
Cyclohexane Benzene
Entry Catalyst
(%)
(%)
(%)
acid
acid
acid
diol
30
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60
1
2
3
4
5
2Pt/ZrO2-550
57
55
35
97
0.9
0.5
1.8
8.9
7.7
17
0.5
0
2Pt/ZrO2-550-R100
2Pt/ZrO2-550-R250
2Pt/ZrO2-550-R400
2Pt/ZrO2-DR250
56
96
88
71
54
95
85
69
27
36
41
15
97
99
97
97
1.2
0.5
0.9
1.0
0.6
0.2
0.5
0.0
1.0
0.7
1.4
1.6
0
18
0
5.1
0
a
Reaction conditions: aqueous glycerol solution: 10 mmol (0.5 M, 20 mL); cyclohexene: 20
mmol; nominal Pt/glycerol ratio = 1/1950; NaOH: 15 mmol; temperature: 160°C; reaction time:
4.5 h; N2 pressure: 20 bar. b Under the employed reaction conditions (molglycerol : molcyclohexene = 1
: 2) the maximum theoretical yield of cyclohexane is 50%.
The calcination temperature of Pt/ZrO2 was found to affect significantly the size and dispersion
of Pt species on the support (vide supra). These differences have a clear effect on the catalytic
o
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performance (Table 3). The Pt/ZrO2 prepared by calcination at 400 C and reduction at 250 C
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(2Pt/ZrO2-400-R250), showed 46% conversion of glycerol and 46% yield of lactic acid (Entry 1,
Table 3), while 2Pt/ZrO2-550-R250 showed significant higher conversion of glycerol (96%) and
yield of lactic acid (95%) (Entry 2, Table 3). The improved activity is ascribed to the smaller size
of the Pt nanoparticles in the latter catalyst (compare Figure 1.C and Figure S2.A). Further
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increase in the calcination temperature to 800 C (2Pt/ZrO2-800-R250) caused a drastic drop in
activity (24% glycerol conversion, Entry 3, Table 3). Keeping the calcination temperature at
800°C but increasing the reduction temperature to 400 oC (2Pt/ZrO2-800-R400), which was used
to further reduce the Pt species, led to even worse activity (12% glycerol conversion, Entry 4,
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Table 3). It should be noted that for the two catalysts prepared by calcination at 800 C, the
efficiency of transfer hydrogenation was just around 1%, which means that almost none of the
hydrogen from glycerol was transferred to cyclohexene. The low activity of these two catalysts
can be correlated to the observed large Pt nanoparticles present in the material already prior to
reduction (see Figure 4), which implies that a low fraction of the Pt atoms are exposed on the
surface and thus available for catalyzing the reaction. Based on these results, it can be concluded
that a calcination temperature of 550 °C leads to an intermediate interaction between oxidized Pt
species and ZrO2, which then allows their reduction leading to the formation of very small Pt
nanoparticles that are highly dispersed on the surface of the support and that thus display high
catalytic activity.
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Table 3. Catalytic conversion of glycerol to lactic acid using Pt/ZrO2 catalysts calcined at
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Selectivity in the conversion of Yield in the conversion
glycerol (%)
of cyclohexene (%)a
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Glycolic
acid
Conv.GLY
(%)
YLA
(%)
S(transfer-H)
(%)
Lactic Glyceric
Propane-
diol
Cyclohexane Benzene
acid
acid
Entry Catalyst
1
2
3
4
2Pt/ZrO2 400-R250
46
96
24
12
46
95
23
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35
36
0.9
0
99
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0.2
0.0
0.7
0.8
0.7
1.9
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17
0.1
0
0
0
0
0
2Pt/ZrO2 550-R250
2Pt/ZrO2 800-R250
2Pt/ZrO2 800-R400
99
98
87
0.5
0.3
0.7
a
Reaction conditions: aqueous glycerol solution: 10 mmol (0.5 M, 20 mL); cyclohexene: 20
mmol; nominal Pt/glycerol ratio = 1/1950; NaOH: 15 mmol; temperature: 160°C; reaction time:
4.5 h; N2 pressure: 20 bar. a Under the employed reaction conditions (molglycerol : molcyclohexene = 1
: 2) the maximum theoretical yield of cyclohexane is 50%.
Optimization of Pt loading, NaOH Amount and Reaction Temperature
Catalysts with different loading of Pt on ZrO2 were also tested to investigate the effects of this
parameter on the catalytic performance. The same nominal molar ratio Pt/glycerol (1/1950) was
used in all reactions, i.e. different weights of Pt/ZrO2 catalyst were employed. The conversion of
glycerol and the yield of lactic acid improved upon an increase in the loading of Pt from 0.5 to
2%, whereas further increase of the loading to 5 and 9% caused a drop of activity (Figure 6). In
all these tests, the selectivity towards lactic acid was higher than 97%, with very minor yields (≤
0.2%) of side products, i.e. glyceric acid, glycolic acid and propanediol. The yield of cyclohexane
from the transfer hydrogenation reaction was in the same range with all catalyst (between 16 and
20%). The results can be rationalized considering that at lower loading of Pt (0.5 and 1%), the
larger total mass of support employed might hinder the accessibility of the Pt nanoparticles,11
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whereas at higher loading of Pt (5 and 9%) the larger size and worse dispersion of the Pt
nanoparticles on the ZrO2 surface (Figure S6) account for the lower activity of the catalysts. The
intermediate loading of Pt (2%) provided the best balance between these two factors, leading to
the observed highest activity with 2Pt/ZrO2 550-R250. An alternative or additional explanation
for the better performance of the 2% catalyst compared to those with lower loading is that the 2%
Pt catalyst (with Pt particle size of 1.4 nm) displays the largest fraction of suitable metallic sites
for the dehydrogenation of glycerol.
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Figure 6. Catalytic performance of Pt/ZrO2 550-R250 catalysts with different Pt loading (0.5-
9%). Reaction conditions: aqueous glycerol solution: 10 mmol (0.5 M, 20 mL); cyclohexene: 20
mmol; nominal Pt/glycerol ratio = 1/1950; NaOH: 15 mmol; temperature: 160°C; reaction time:
4.5 h; N2 pressure: 20 bar.
All the catalytic tests were conducted in the presence of NaOH. The role of this strong base
was studied by varying the molar ratio between NaOH and glycerol (from 0 to 2) in the reaction
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mixture (Figure S8). Without addition of NaOH, both the conversion of glycerol and the
selectivity to lactic acid were very low (conversion of glycerol, 1.3%). When increasing the
molar ratio between NaOH and glycerol, the conversion of glycerol gradually increased reaching
96% with 99% selectivity towards lactic acid at NaOH/glycerol = 1.5. However, a further
increase in the NaOH/glycerol molar ratio to 2 caused a decrease in the conversion of glycerol to
73%, suggesting that excess NaOH can inhibit the activity of the catalyst. These results confirm
that the presence of a NaOH in the reaction mixture is critical to promote the deprotonation of
one of the hydroxyl groups of glycerol, thus promoting the dehydrogenation of glycerol on the
surface of the Pt nanoparticles.9, 17, 58-59 Moreover, NaOH can catalyze the isomerization of
glyceraldehyde and dihydroxyacetone, and reacts with the formed lactic acid to yield sodium
lactate (which is very stable in the reaction system) thus shifting the equilibrium concentrations
towards the products (Scheme 1) and granting very high selectivity towards the lactic acid.9, 17-18,
21 A reference reaction with only NaOH and no Pt catalyst gave very low conversion of glycerol
(7.3%), which is significantly lower compared to the conversion achieved in the presence of the
2Pt/ZrO2-550-R250 catalyst (96%). This demonstrates the crucial role played by the Pt/ZrO2
catalyst under the relatively mild reaction conditions employed here.17-21
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The 2Pt/ZrO2-550-R250 catalyst was further tested at different temperatures (from 120 to 180
°C). By increasing the reaction temperature, the expected trend of increasing glycerol conversion
was observed, going from 25% at 120 °C to full conversion at 180 °C. The selectivity to lactic
acid was nearly constant and always > 95% with very similar side products distribution in all
cases (Table S2). Also the conversion of cyclohexene and the yield of cyclohexane increased
with the temperature. On the other hand, the selectivity of transfer hydrogenation did not show a
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clear trend as a function of the reaction temperature, reaching the highest efficiency in the
reaction carried out at 140 °C.
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Detailed Study of Trans-hydrogenation Process
The catalytic results presented in Table 2 and 3 show that the only a fraction of the hydrogen
atoms removed from glycerol are employed in the transfer hydrogenation of cyclohexene. This is
probably due to the intrinsic low reactivity of the double bond in cyclohexene but also to the
hydrophilicity of the Pt/ZrO2 catalysts, which causes them to be preferentially located in the
aqueous phase of the reaction mixture (consisting of water and glycerol), thus limiting the contact
with cyclohexene (which together with cyclohexane constitutes the organic phase). As a
consequence, the rate of the hydrogenation step is lower than that of the dehydrogenation.
Though combining the conversion of glycerol to the transfer hydrogenation of cyclohexene is
attractive, it is also interesting to evaluate the catalytic performance of the best catalyst identified
in this work (2Pt/ZrO2 550-R250) in the absence of cyclohexene. The test was carried out under
conditions (reaction at 140 °C) at which the conversion of glycerol would be far from being
complete. The results demonstrate that the presence of cyclohexene as hydrogen acceptor does
not affect the catalytic performance, as both reactions showed nearly the same conversion of
glycerol and yield of lactic acid (Table S3). To further investigate the formation of free H2 in our
system, we conducted an additional experiment with no hydrogen acceptor, higher amount of
glycerol and lower N2 pressure (3 bar) (Table S4). Under these conditions, the reaction generated
10 bar free H2 after reaction, which correspond to 98% yield relative to the glycerol conversion.
This experiment demonstrated that the dehydrogenation of glycerol did lead to the formation of
free H2.
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Additionally, a linear alkene and a linear alkyne were also studied as hydrogen acceptors,
instead of cyclohexene (Table S5). When 1-decene was used as hydrogen acceptor, the 2Pt/ZrO2
550-R250 catalyst exhibited very high activity (glycerol conversion 97%) and selectivity towards
the lactic acid salt (99%). Moreover, the selectivity of the transfer-hydrogenation (92%) was
much higher than with cyclohexene as hydrogen acceptor. This is probably due to the better
accessibility of the C-C double bond in the linear 1-decene. On the other hand, when 1-decyne
was used as the hydrogen acceptor, the catalytic activity of 2Pt/ZrO2 550-R250 was almost fully
inhibited (1.3% glycerol conversion). This indicates that 1-decyne cannot be used as hydrogen
acceptor in this system (i.e. under basic hydrothermal conditions), possibly due to poisoning of
the Pt catalyst.
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Kinetic Test
The reaction was monitored as a function of reaction time with catalyst 2Pt/ZrO2-550-R250.
This test showed that under the employed conditions the reaction behaves as being first order
with respect to glycerol (Figure 7). The turnover frequency for the conversion of glycerol based
on the amount of Pt and calculated from the linear part of the kinetic curve (i.e. the first 1.5 h)
was 995 h-1. The selectivity towards lactic acid was > 95% at all stages, which can be related to
the rapid conversion of the dihydroxyacetone and/or glyceraldehyde formed from the
dehydrogenation of glycerol into the lactic acid salt (see Scheme 1). The yield of cyclohexane via
the transfer hydrogenation reaction increased within the first 1.5 h, after which it remained nearly
constant.
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Figure 7. Conversion of glycerol and transfer hydrogenation over the 2Pt/ZrO2 550-R250
catalyst as a function of reaction time (A); and linear fitting of the natural logarithm of the
concentration of glycerol as a function of the reaction time (B). Reaction conditions: aqueous
glycerol solution: 10 mmol (0.5 M, 20 mL); cyclohexene: 20 mmol; nominal Pt/glycerol ratio =
1/1950; NaOH: 15 mmol; temperature: 160°C; N2 pressure: 20 bar.
Reusability
The best catalyst identified in this work, 2Pt/ZrO2-550-R250, was selected for a reusability test
(Figure 8A). The catalyst was reused in 5 consecutive runs, displaying a partial and gradual loss
of activity, corresponding to a decrease in glycerol conversion from 96% in the first run to 54%
in the fifth run. The selectivity towards the lactic acid remained very high (> 97%) in all runs. In
addition, the yield of cyclohexane from transfer hydrogenation was about constant for the 5 runs.
The observed decrease in activity in the conversion of glycerol is attributed to an increase in the
size of the Pt nanoparticles from 1.4 nm in the fresh catalyst to 4.6 nm after the fifth run, as
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evidenced by TEM analysis (Figure 8B). Such aggregation of Pt nanoparticles led to lower
exposed Pt surface and thus to the observed decrease in activity.
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Figure 8. Reusability test of the 2Pt/ZrO2 550-R250 catalyst for the conversion of glycerol and
transfer hydrogenation: (A) Catalytic performance upon recycling; the solid catalyst was
recovered by filtration, washed with water and ethanol and dried at 100 °C after each run. (B)
TEM picture of the catalyst after 5 recycles; average particle size of Pt: 4.6 nm. Reaction
conditions: aqueous glycerol solution: 10 mmol (0.5 M, 20 mL); cyclohexene: 20 mmol; nominal
Pt/glycerol ratio = 1/1950; NaOH: 15 mmol; temperature: 160°C; reaction time: 4.5 h; N2
pressure: 20 bar.
Conclusions
We developed a novel catalytic system based on highly dispersed Pt species supported on
nanosized ZrO2 with high activity and selectivity for the one-pot conversion of glycerol into
lactic acid (salt), with concomitant transfer hydrogenation of cyclohexene to cyclohexane.
Careful tuning of the synthesis method through optimization of the calcination temperature, the
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