Supported Pt-Re catalysts for the selective hydrogenation of methyl and ethyl esters to alcohols

Article abstract of DOI:10.1016/j.cattod.2016.03.043

The effect of Re-addition to Pt/TiO₂ catalysts for the hydrogenation of carboxylic acids and esters to their corresponding alcohols was investigated. The highest catalytic activity was observed for the materials with Pt:Re molar ratio of 1:2, which allowed for a complete conversion of hexanoic acid under mild conditions. The hydrogenation of esters over the same catalysts was much more difficult. The reactions showed only moderate alcohol yields which also depended strongly on the alkoxy moiety of the ester substrate. The highest yield of ca. 25% was achieved by the hydrogenation of ethyl hexanoate. Conversion of methyl hexanoate was much less efficient. This was attributed to the inhibiting effect of methanol by-product. In situ FTIR spectroscopy suggests that the decarbonylation of methanol and the resulting CO poisoning of the catalyst surface in the course of the reaction is the most likely cause of the low activity of Pt and Pt-Re/TiO₂ towards methyl ester hydrogenation.

Full text of DOI:10.1016/j.cattod.2016.03.043

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Supported Pt-Re catalysts for the selective hydrogenation of methyl
and ethyl esters to alcohols
James Pritcharda, Aysegul Ciftcia,1, M.W.G.M. (Tiny) Verhoevena, Emiel J.M. Hensena,b,∗
,
Evgeny A. Pidkoa,b,∗
a Inorganic Materials Chemistry group, Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
b Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of Re-addition to Pt/TiO2 catalysts for the hydrogenation of carboxylic acids and esters to their
corresponding alcohols was investigated. The highest catalytic activity was observed for the materials
with Pt:Re molar ratio of 1:2, which allowed for a complete conversion of hexanoic acid under mild
conditions. The hydrogenation of esters over the same catalysts was much more difficult. The reactions
showed only moderate alcohol yields which also depended strongly on the alkoxy moiety of the ester
substrate. The highest yield of ca. 25% was achieved by the hydrogenation of ethyl hexanoate. Conversion
of methyl hexanoate was much less efficient. This was attributed to the inhibiting effect of methanol by-
product. In situ FTIR spectroscopy suggests that the decarbonylation of methanol and the resulting CO
poisoning of the catalyst surface in the course of the reaction is the most likely cause of the low activity
of Pt and Pt-Re/TiO2 towards methyl ester hydrogenation.
Received 14 October 2015
Received in revised form 14 March 2016
Accepted 17 March 2016
Available online xxx
Keywords:
Hydrogenation
Platinum
Rhenium
Carboxylic acid
Catalyst deactivation
Ester hydrogenation
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
a range of Cu/Cr/Ba oxide-containing catalysts and their appli-
cation in hydrogenation of lactic acid esters to 1,2-propanediol,
The hydrogenation of carboxylic acids and esters derived from
bio-based feedstocks is an important reaction used to obtain fatty
alcohols and other bulk chemicals [1,2]. Fatty alcohols can serve
as intermediates in the manufacture of a wide range of chemi-
cals including flavours, fragrances, nutrients and pharmaceuticals
[3,4]. The particular application is largely determined by the chain
length; many commercially available detergents and surfactants
comprise C12–14 fatty alcohols while longer chain fatty alcohols may
find use as fuels and lubricants. Global fatty alcohol consumption
reached 2.2 million tonnes in 2012 [5] and is forecast to continue
growing year-on-year until 2020 with the highest production and
consumption levels reported for Southeast Asia [6].
␣-amino and hydroxy esters to amino alcohols and glycols, methyl
and n-butyl caproates to n-hexanol. Due to the intrinsic resis-
tance of carboxylic acids and esters in particular towards reduction
[14,15], current heterogeneously-catalysed processes are operated
under extremely harsh conditions with temperatures in the range
200–300 ◦C and 140–300 bar H2 pressures [16]. The introduction of
bimetallic catalysts allowed to carry out the reaction under much
milder reaction conditions. For example, the use of bimetallic Au-Ag
[17] and Au-Cu [18] catalysts allowed carrying out the hydrogena-
tion of dialkyl oxalates with high selectivity to ethylene glycol at
temperatures as low as 145 ◦C.
A bimetallic catalyst formulation of longstanding interest for
hydrogenation of methyl esters is Ru-Sn. Narasimhan and co-
workers [19,20] reported that methyl oleate could be selectively
hydrogenated to oleyl alcohol over a NaBH4-reduced Ru-Sn-
B/Al2O3 catalyst at 270 ◦C and 44 bar H2 pressure. The B and Sn
components were proposed to enhance the electron density of Ru
species and the dispersion of metallic Ru, respectively. Regarding
the reaction mechanism, it was proposed that Ru sites promote the
dissociation of H2 while adjacent Lewis acid (Sn2+/Sn4+) sites polar-
ize the carbonyl group of methyl oleate. This is followed by hydride
transfer from adjacent Ru-H sites to form a carbanion species that is
rapidly hydrogenated to give alcohol. However, the contribution of
Beginning in the early-1930s with seminal studies by Adkins and
co-workers [7,8], numerous works describing catalytic hydrogena-
tion of carboxylic acid derivatives to their alcohols have appeared
in the literature [9–13]. Adkins spearheaded the development of
∗
Corresponding authors at: Inorganic Materials Chemistry group, Schuit Institute
of Catalysis, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
E-mail addresses: e.j.m.hensen@tue.nl (E.J.M. Hensen), e.a.pidko@tue.nl
(E.A. Pidko).
1
Current address: Hydrocarbons R&D, Dow Benelux B.V., P.O. Box 48, 4530 AA
Terneuzen, The Netherlands.
http://dx.doi.org/10.1016/j.cattod.2016.03.043
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Pritchard, et al., Supported Pt-Re catalysts for the selective hydrogenation of methyl and ethyl esters
to alcohols, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.043

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different Ru/Ru-Sn phases, Sn species, support-effects and whether
the reaction proceeds via an aldehyde or hemiacetal intermedi-
ate remains the source of much controversy. Further investigations
[21–24] demonstrated that the relative concentrations of metallic
and ionic Sn species can be controlled by the support material. The
support itself may also participate in ester hydrogenation and, for
an 8 wt% Ru/ZrO2·xH2O catalyst applied in the hydrogenation of
methyl propionate to 1-propanol [25], it was claimed that hydro-
gen bonding between surface hydroxyl groups of the support and
the substrate can induce polarization of the carbonyl group of
the ester and facilitate hydride insertion steps. Mendes et al. [26]
described the importance of strong metal support interactions for
the hydrogenation of carboxylic acid derivatives over Ru/TiO2. It
was proposed that TiO2, in addition to stabilizing a high disper-
sion of active Ru particles, also directly participates in the catalytic
reaction by polarizing the carbonyl group through surface Tin+ sites.
The potential of other bimetallic catalysts including Rh-Re, Rh-
Mo and Ru-Re for the hydrogenation of a range of carboxylic acids
and their esters was investigated by Fuchikami and co-workers
[27]. Much interest has been expressed in using Re in combina-
tion with other noble metals in aqueous phase reforming reactions,
[28,29], hydrogenation of amides [30,31] and hydrogenation of
C6–C18 fatty acids [32,33]. Pt-Re catalysts in particular are known
to be highly active and selective catalysts in hydrogenation of
amides to amines [34]. The activity of 2% Pt-10% Re/graphite cata-
lysts towards hydrogenation of 108 substrates under moderate to
mild conditions (c.a. 160 ◦C, 5–70 bar H2) was assessed by Stein and
Breit [30]. For a series of cyclic amides including N-acetyl piperi-
dine, 99% conversion with 100% selectivity to the corresponding
tertiary amine could be achieved. The promoting effect of Re in Pd-
catalysed hydrogenation of aqueous succinic acid to 1,4-butanediol
was also considered by Especel and co-workers. [35]. Evaluating
both impregnation and surface redox reduction preparation meth-
ods, addition of Re to 2 wt% Pd/TiO2 gave rise to increased activity
and selectivity of the catalytic reaction. Characterisation of catalysts
prepared via surface redox reduction highlighted good dispersion
of Pd and Re species, though few comments were made regard-
ing the metal oxidation states. Previous characterisation studies
[36,37] have discussed the difficulty in completely reducing Re
to its metallic state given its high oxophilicity, making it likely
that ReOx species co-exist with metallic Pd being present on TiO2.
Depending on the preparation method, the amount of Re needed
to induce synergy can vary from 0.6 to 0.8 wt.% (surface redox) to
3.5 wt.% Re (impregnation), confirming that the extent of Pd-Re
interactions is influenced by the preparation method. Tomishige
and co-workers [32] demonstrated the versatility of Pd-Re catalysts
in hydrogenation of fatty acids including hexanoic, capric, palmitic
and stearic acids. Using a ReOx-Pd/SiO2 (Re/Pd = 8) catalyst, fatty
alcohol yields in the range 92–99% were obtained at 140 ◦C and
80 bar H2. Hardacre and co-workers [33] were successful in hydro-
genating stearic acid to stearyl alcohol using passivated 4 wt% Pt-x
wt% Re/TiO2 catalysts under unprecedentedly mild conditions of
120 ◦C and 20 bar H2. In-situ EXAFS measurements performed by
the same group [38] indicated that more efficient reduction and
stabilization of alloyed Pt-Re species takes place on TiO2. Finally,
detailed mechanistic analysis by Dumesic and co-workers [39] has
underpinned the importance of bifunctional acid sites derived from
combination of a highly reducible metal with an oxophilic metal for
selective CO hydrogenolysis reactions.
Scheme 1. General reaction for the hydrogenation of esters to alcohols and the
substrate scope considered in this work.
Table 1
Summary of quantified XPS data for Pt:Re/TiO2 catalysts.
Pt:Re
Composition (atom%)
Atom ratios
Re/Ti
Pt
Re
Ti
Pt/Ti
Pt/Re
1:0
5:1
2:1
1:1
1:2
1:5
0.72
0.57
0.54
0.51
0.40
0.35
–
24.50
28.99
28.34
24.50
22.85
22.38
–
0.2
0.3
0.2
0.2
0.2
0.1
–
1.91
2.06
5.98
6.49
15.66
0.09
0.07
0.17
0.28
0.70
0.35
0.30
0.12
0.06
0.01
function can be derived from metal–metal and/or metal-support
interactions. In this work, we aimed at investigating the promo-
tional effect of Re on the catalytic performance of Pt catalysts in
the hydrogenation of model esters and carboxylic acids including
methyl and ethyl hexanoate, methyl benzoate and hexanoic acid
(Scheme 1). Catalysts were characterised by transmission electron
microscopy, X-ray photoelectron spectroscopy and temperature
programmed reduction with the aim of understanding how
addition of Re to Pt/TiO2 modifies catalyst structure and activity.
Additionally, in-situ FTIR was used to study the origin of catalyst
inhibition in the hydrogenation of methyl esters.
2. Experimental
2.1. Catalyst preparation
Catalysts precursors, reactants and solvents were obtained
from Sigma-Aldrich: perrhenic acid, HReO4 (75 wt.%, aqueous),
hexachloroplatinic acid, H2PtCl6·6H2O, methyl hexanoate (99.9%),
ethyl hexanoate (99.9%), methyl benzoate (99.9%), hexanoic acid
(99.9%) and hexane (99%) were used as received without addi-
tional purification. Catalyst supports were obtained from the
following sources: ␣-Al2O3 (Condea), Carbon (RX-3, Norit), CeO2
(Sigma Aldrich), SiO2 (Grace), TiO2 (P-25, Degussa). Supported
3 wt% Pt-x wt% Re (x = 0–14.3) catalysts were synthesized by wet-
impregnation with all quantities listed per gram of catalyst. HReO4
(10–128 ␮L) was added to a solution of H2PtCl6·6H2O (3.98 mL, 1 g
in 25 mL H2O). The appropriate amount of support (125–250 ␮m)
was then added to the solution while stirring at 80 ◦C. The mate-
rial was dried (110 ◦C, 16 h), ground using a pestle and mortar and
reduced at 300 ◦C in 10% H2/He (100 mL/min) for 2 h.
2.2. Catalyst characterisation and testing
2.2.1. Fourier transform infrared spectroscopy (FTIR) of adsorbed
methanol
FTIR spectra of the samples were recorded in the range of
4000–400 cm−1 by a Bruker Vertex V70 v instrument. The spec-
tra were acquired at a 2 cm−1 resolution and averaged over 32
scans. The samples were prepared as thin self-supporting wafers of
25–35 mg/cm2 and placed inside a controlled environment infrared
transmission cell, capable of heating and cooling, gas dosing, and
evacuation. Prior to the measurements, all catalysts were reduced
at 300 ◦C by flowing 10 vol.% H2 in N2 (total flow 100 mL/min) for
We recently emphasized the importance of bifunc-
tional/cooperative character for both heterogeneous and
homogeneous ester hydrogenation catalysts [2]. Acid-base
non-innocent ligands are known to provide the cooperative func-
tion in homogeneous transition metal catalysts and consequently
activity can be improved through careful ligand design [40–43].
For heterogeneous ester hydrogenation catalysts, cooperative
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Fig. 1. Representative TEM images and corresponding particle-size distribution histograms for 3 wt% Pt and Re/TiO2 catalysts.
2 h. The cell was evacuated at room temperature and the sample
2.2.4. X-ray photoelectron spectroscopy (XPS)
was then pulse-dosed with either 12C or 13C-labelled methanol.
The cell was subsequently heated to 180 ◦C with a 2 ◦C/min heat-
ing rate. The temperature was held for 2 min at each temperature
interview and a spectrum was recorded. All spectra presented were
normalized to the weight of the pellet sample.
XPS measurements were carried out with a Thermo Scien-
tific K-Alpha, equipped with a monochromatic small-spot X-ray
source and a 180◦ double focusing hemispherical analyzer with a
128-channel detector. Spectra were obtained using an aluminium
anode (Al K␣ = 1486.6 eV) operating at 72 W and a spot size of
400 ␮m. Survey scans were measured at a constant pass energy
of 200 eV and region scans at 50 eV. The background pressure
was 2 × 10−9 mbar and during measurement 3 × 10−7 mbar Argon
because of the charge compensation by the dual-beam low energy
electron/Argon ion source. Data analysis was performed using
CasaXPS software. The binding energy was corrected for surface
charging by taking the C 1s peak of adventitious carbon as a refer-
ence at 284.6 eV.
2.2.2. Transmission electron microscopy (TEM)
Transmission electron micrographs were acquired on a FEI Tec-
nai 20 transmission electron microscope at an acceleration voltage
of 200 kV with a LaB6 filament. Typically, an amount of the sample
was ground and suspended in pure ethanol, sonicated and dis-
persed over a Cu grid with a holey carbon film. Samples were first
reduced in 10 vol.% H2 in He (total flow 100 mL min−1) at 300 ◦C for
2 h, followed by passivation in 1 vol.% O2 in He for 2 h.
2.2.5. Catalyst activity measurements
All hydrogenation experiments were prepared inside a large,
two-hand glove bag (AtmosBag with zipper lock, Sigma Aldrich).
The glove bag was purged five times with nitrogen after which a
constant flow rate was applied to give a suitable working over-
pressure. A pre-reduced catalyst (3–18 mg) was then loaded into a
small stainless steel autoclave followed by the addition of solvent
(3.5 mL), substrate (0.44 mmol) and internal standard (dodecane,
35.2 ␮L) after which the autoclave was sealed and removed from
the glove bag. The autoclave was subsequently purged five times
with hydrogen (2 MPa) before filling the vessel to the desired pres-
2.2.3. Temperature-programmed reduction (TPR)
H2-TPR experiments were performed in
a Micromeritics
Autochem II 2920 equipped with a fixed-bed reactor, a computer-
controlled oven, and a thermal conductivity detector. Typically,
50 mg catalyst was loaded in a tubular quartz reactor. The sam-
ple was reduced in 4 vol.% H2 in N2 at a flow rate of 10 mL/min,
while heating from room temperature up to 700 ◦C at a ramp rate
of 2 ◦C/min. The H2 signal was calibrated using a CuO/SiO2 reference
catalyst.
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Fig. 2. Representative TEM images and corresponding particle-size distribution histograms of Pt-Re (5:1, 2:1, 1:2)/TiO2 catalysts.
sure (5 MPa). Using a hot plate equipped with magnetic stirring
autoclave were filtered using an organic-based polymer frit and
a sample was prepared for GC-FID analysis (Shimadzu GC-FID
equipped with RX-5 carbowax column). Calibration was performed
using known concentrations of all reactants and products in order
to determine correct response factors.
capability, the autoclave was gradually heated to 180 ◦C (unless
otherwise stated) before stirring at 1000 rpm was commenced
(1–8 h). On completion of the reaction, both heating and stirring
were stopped, the autoclave was removed from the hot plate and
then cooled in ice-cold water. Once cooled, the contents of the
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Fig. 4. Comparison of different supported Pt-Re (1:2) catalysts in hydrogenation of
methyl hexanoate.
Table 2
Fig. 3. TPR profiles of Pt-Re TiO2-supported catalysts Pt, Pt-Re and Re/TiO2 catalysts.
Evaluation of Pt-Re (x:y)/TiO2 catalysts in hydrogenation of methyl- and ethyl hex-
anoate. Conditions: catalyst (3 mg), methyl hexanoate (0.44 mmol), hexane (3.5 mL),
dodecane (35.2 ␮L), 180 ◦C, 50 bar H2, 8 h, 1000 rpm. n.a. = too low to determine.
3. Results and discussion
Pt:Re Re (% wt.) Methyl hexanoate
Conv. (%) Sel. (%) 1-hexanol Conv. (%)
95
Ethyl hexanoate
Sel. (%) 1-hexanol
8 91
3.1. Catalyst characterisation
1:0
5:1
2:1
1:1
1:2
1:5
0:1
0
4
Fig. 1 shows representative TEM images of TiO2-supported Pt
and Re catalysts. The average sizes for nanoparticles in 3 wt% Pt
and Re/TiO2 samples are 1.3 and 1.1 nm, respectively. Both catalysts
exhibit a relatively narrow particle size distribution ( 0.4 nm).
The population of sub-nanometre particles is greatest for Re/TiO2
(>50%), which is typical when strong metal support interactions
exist [44]. The effect of Re addition to Pt/TiO2 is shown in Fig. 2; both
an extended particle size distribution and increased average parti-
cle size were observed for all bimetallic Pt-Re samples compared
to their monometallic counterparts.
On increasing addition of Re to Pt, there is a notable decrease
in average particle size. At the same time, there is a broader dis-
tribution and particles up to 3 nm in size could be observed in
all three samples. Consequently, Pt-Re (1:2)/TiO2 comprises the
largest population of sub-nanometre particles (35%) among all
Pt-based catalysts studied here. A similar effect was previously
reported [44] for a series ZrO2-supported Pt and Pt-Re catalysts.
This effect was proposed to contribute to superior activity of the
bimetallic catalysts in aqueous phase glycerol reforming.
Fig. 3 summarizes the results of the H2-TPR characterisation of
the catalysts. For all samples, reduction was observed at relatively
low temperatures. Reduction features range from sharp peaks cen-
tred around 100 ◦C to broad features with shoulders up to 250 ◦C.
The low temperature reduction of Re alone on TiO2 is a somewhat
surprising result given that reduction of Re in the Re/C counter-
parts [29] only started at around 300 ◦C and reached completion
only close to 500 ◦C. This points to the support-assisted reduction
of Re by TiO2. The consumption of H2 upon the TPR experiment
expressed as H2 to metal ratios exceeds the theoretical values for
all materials investigated, thereby providing an indication of (i) H2-
spillover effects in reduction of Re/Pt-Re species on TiO2, and (ii)
reduction of the support surface. The presence of broad shoulders
starting at approximately 150 and 230 ◦C in TPR profiles for Pt-Re
(1:5) and Re/TiO2 samples, respectively, suggests the presence of
some residual isolated ReOx species attached to the TiO2 surface of
the Re-rich catalysts [28].
0.6
1.5
2.9
5.7
14.3
3
5
87
15
87
90
93
92
87
n.a.
10
10
11
6
94
95
98
88
n.a.
16
19
23
11
<1
<1
to a surface Re enrichment. Even the most Re-poor catalyst with the
bulk Pt:Re ratio of 5:1 shows a more than 3-fold excess of surface
Re, while the active component on the surface of Pt-Re (1:5)/TiO2
catalyst is almost entirely Re. The percentage of surface exposed Pt
is also consistently low and decreases by approximately 50% with
increasing Re content. On addition of Re to Pt/TiO2, a slight shift
of the Pt features to higher binding energies (71.0 to 71.7 eV) is
observed in all samples suggesting that Pt and Re are in good con-
tact (Supporting information, Fig. S1). The binding energy of the Re
4f7/2 peaks in XPS spectra of the Re 4f region for bimetallic samples
decreases from 41.8 to 40.0 eV with increasing Re content. Further-
more, a higher binding energy of 42.4 eV is observed for Re/TiO2
which suggests that the Pt component in Pt-Re/TiO2 catalysts pro-
motes the reduction of ReOx oxide species on the surface.
3.2. Catalytic activity measurements
Optimization of the reaction conditions in hydrogenation of
methyl benzoate and methyl hexanoate substrates was first under-
taken (Supporting information, Table S1). The hydrogenation of
esters to alcohols over supported Pt-Re catalysts was determined
to be a highly-activated process. As a result, the catalytic results
discussed below (unless otherwise stated) were obtained after
8 h reaction time at 180 ◦C and under 50 bar H2 pressure. Solvent
screening identified hexane as the optimal reaction solvent.
Analysis of the influence of the support on the activity of Pt
and Pt-Re (1:2) catalysts evidences the promoting role of the
metal oxide supports and Re doping on the conversion of methyl
hexanoate to 1-hexanol and methanol (Fig. 4). In addition to 1-
hexanol and methanol (and potentially in turn CO), other possible
reaction products include (i) n-hexane, formed via hydrogenoly-
sis of 1-hexanol, (ii) methane, pentane and CO2, formed through
the decarboxylation of methyl hexanoate, (iii) hexyl-hexanoate,
We further employed X-ray photoelectron spectroscopy (XPS)
to analyse the chemical composition and the nature of species on
the surface of TiO2-supported catalysts. Quantified XPS data sum-
marized in Table 1 clearly shows that Re-addition to Pt/TiO2 leads
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Fig. 6. Influence of catalyst mass on activity of Pt and Pt-Re (1:2)/TiO2 catalysts
in hydrogenation of methyl benzoate (Me-Benz) and methyl hexanoate (Me-Hex),
respectively.
Fig. 5. Evaluation of the performance Pt, Re and Pt-Re (x:y)/TiO2 catalysts for the
hydrogenation of methyl and ethyl hexanoate.
formed by transesterification of methyl hexanoate with 1-hexanol
produced in-situ. Selectivity to 1-hexanol is also influenced by the
support material with highest values over 90% obtained for Pt-
Re (1:2)/CeO2, TiO2 and CeZrO2 catalysts. Alcohol selectivity in
the case of their carbon and alumina-supported counterparts was
only around 70% (Supplementary information, Table S2). Based
on these initial results, further optimization was undertaken for
TiO2-supported Pt-Re catalysts. The results in Fig. 5 evidence the
emergence of a synergy between catalyst components upon the
addition of Re to Pt. The activity increases up to a Pt:Re ratio of
1:2 resulting in alcohol yield improvement of approximately 3-fold
for both methyl- (4–11%) and ethyl (8–23%) hexanoate. Activity
declines by around 50% on moving to Pt-Re (1:5)/TiO2 and we
believe that this is due to the substantial decrease in surface Pt
to Re ratio (Table 1), leading to a partial loss of synergy.
Fig. 7. Effect of direct methanol addition on the activity of a Pt-Re (1:2)/TiO2 catalyst
in hydrogenation of methyl- and ethyl hexanoate.
Crucially, Re/TiO2 is inactive towards ester hydrogenation
adding support to the notion that Re plays a secondary role in Pt-
Re catalysts. The contribution of TiO2 in the reaction mechanism
is also likely in view of the fact that Pt/TiO2 exhibits some activ-
ity in ester hydrogenation [26,45]. Pt and Pt-Re/TiO2 catalysts were
found to be 1.6–3 times more active in hydrogenation of ethyl hex-
anoate compared to methyl hexanoate. A maximum conversion of
23% was achieved over Pt-Re (1:2)/TiO2. Changing the substrate
however did not dramatically affect product distribution showing
a high selectivity of over 90% to 1-hexanol with minor products
constituting alkanes (Table 2). For our most active Pt-Re catalyst,
selectivity to 1-hexanol reached 98%. The lack of a clear relationship
between selectivity and Re content was previously acknowledged
by Hardacre and co-workers [33] in hydrogenation of stearic acid,
also over Pt-Re/TiO2 catalysts. Our results suggest that increasing
the amount of Re in Pt-Re/TiO2 catalysts leads to an increase in the
number of available sites capable of polarizing the carbonyl group
of alkyl esters as opposed to modifying the extent and/or mech-
anism of hydrogenation. This is partly supported by the XPS data
presented in Table 1, which shows a pronounced increase in surface
Re content for higher Re loadings (3.6–14.3 wt%).
The increase of catalyst loading (Fig. 6) has only a minimal effect
on methyl hexanoate conversion. In the case of Pt-Re (1:2)/TiO2, the
6-fold increase of the catalyst content leads to double conversion
of the ester, while in the case of the monometallic Pt/TiO2 cata-
lyst this conversion increased only by 50%. We conclude that this
effect cannot be attributed to mass-transfer limitations given the
fact that when methyl benzoate was used as a substrate, the con-
version was complete with these catalysts already after 3 h (see
Fig. 6, blue triangle). The conversion of the aromatic ester methyl
benzoate, however, exhibited poor chemoselectivity, as aromatic
ring hydrogenation was facile. The major reaction products were
methyl cyclohexanoate, cyclohexanol and methanol. A factor that
cannot be eliminated at this stage is the possibility of catalyst deac-
tivation by the reaction products. This hypothesis is supported by
comparison of the performance of Pt, Re and Pt-Re/TiO2 catalysts
in hydrogenation of methyl and ethyl hexanoate.
It is also concluded from Fig. 5 that simply changing the alkoxy
moiety in the ester from methyl to ethyl dramatically improves the
rate of ester hydrogenation over all Pt-based catalysts. Therefore,
to check the possibility of catalyst deactivation by the reaction by-
product, two additional experiments were performed in which one
mole equivalent of methanol was added to the reaction medium
(Fig. 7). Activity is significantly hindered in the presence of excess
methanol and points towards deactivation due to in-situ formed
methanol.
In response to the above findings, a series of time online mea-
surements were performed using our most active catalyst, Pt-Re
(1:2)/TiO2 (Fig. 8). In addition to methyl hexanoate, the hydrogena-
tion of hexanoic acid was performed to directly assess the effect of
short chain alcohol formation on catalyst performance. It is now
possible to fully convert hexanoic acid under the same reaction
conditions with 75% conversion after 2 h reaction (compared to
just 6% methyl hexanoate conversion). In conjunction with results
shown in Fig. 7, this categorically proves that ester hydrogenation
is hindered in the presence of short chain alcohols, in particular,
methanol.
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Fig. 8. Time online measurements for a Pt-Re (1:2)/TiO2 catalyst in hydrogenation
of hexanoic acid and methyl hexanoate.
Fig. 10. FTIR spectra of adsorbed methanol on Pt-Re (1:2)/TiO2 recorded at 10 ◦
intervals.
C
assignments [51]. For 3 wt% Re/TiO2, we did not observe any major
features in recorded spectra and, given this catalyst is inactive in
ester hydrogenation, we infer that catalyst deactivation is related to
the poisoning of Pt. However, factors including the size, shape, co-
ordination number and dispersion of Pt species can be modified by
Re-addition. Lastly, for Pt/TiO2, formation of a broad band centred
at 2046 cm−1 was observed (Fig. 11(a)). While we also attribute this
band to CO, it is situated 27 cm−1 lower than its Pt-Re (1:2) counter-
part, which is presumably due to the band positions being sensitive
to factors including alloying and Pt dispersion [52]. Referring back
to TEM images in Fig. 1, there is indeed a lower population of sub-
nanometre particles in Pt/TiO2. What is very interesting here is that
only CO, formaldehyde and formate species are formed, highlight-
ing that decarboxylation and other complex reactions may only
proceed over bimetallic Pt-Re/TiO2 catalysts; Re-addition can thus
modify the behaviour of Pt. To conclude, comparison of normalized
spectra presented in Fig. 11 suggests CO binding is strongest for
Pt/TiO2 (a) and also more facile at lower temperatures compared
to Pt-Re (1:2)/TiO2 (b).
Fig. 9. Influence of catalyst mass on the activity of a Pt-Re (1:2)/TiO2 catalyst in
hydrogenation of hexanoic acid and methyl hexanoate.
Next the influence of catalyst mass on activity is compared for
hexanoic acid and methyl hexanoate substrates in Fig. 9. For the
hydrogenation of hexanoic acid (in which no short chain alcohol is
produced), a first order behaviour with respect to catalyst mass is
observed. The conversion rises uniformly with increasing catalyst
mass. It is plausible that increasing the mass of Pt or Pt-Re (x:y)/TiO2
used in methyl ester hydrogenation improves the initial rate of
1-hexanol and methanol production. As a result, a higher concen-
tration of methanol may be in contact with the catalyst at shorter
reaction times which could lead to faster deactivation in-situ and
help explain the limited response in Fig. 6.
We further employed in situ FTIR spectroscopy to understand
the deactivating role of methanol for Pt-Re (x:y)/TiO2 catalysts dur-
ing ester hydrogenation. The interaction of methanol with 3 wt%
Re, Pt and Pt-Re (1:2)/TiO2 catalysts was investigated. For Pt-Re
(1:2)/TiO2, multiple features were detected with increasing tem-
perature (Fig. 10). The gradual formation of a broad band centred
at 2073 cm−1 was attributed to CO [46], formed via decarbonyla-
tion of methanol. This interaction was reported by Rochester [47]
and Igarashi [48] groups to be that of Pt-CO and we consider this
species to be primarily responsible for the catalyst deactivation in
our study. Formation of another band at 2360 cm−1 in addition to
multiple features in the region 1800–1350 cm−1 were also observed
and attributed to carbon dioxide, formic acid, formaldehyde, water
and formate species, respectively [49,50]. Additional experiments
(Supplementary information, Figs. S2 and S3) support the above
4. Discussion
It is well-known that supported Pt-Re catalysts are effective in
the hydrogenation of a range of aromatic and aliphatic carboxylic
acids [32] and amides [30,31] and, in this study, we showed that
Pt and Pt-Re systems can also be used to hydrogenate methyl and
ethyl esters to their corresponding alcohols under mild conditions
(180 ◦C, 50 bar H2). Evaluation of different support materials in
hydrogenation of methyl hexanoate revealed reducible supports,
in particular TiO2, to favour both higher ester conversion and selec-
tivity to 1-hexanol compared to inert supports such as carbon and
silica. A key aspect of our investigation was to better understand
the effect of Re-addition to Pt/TiO2. First, we found Re/TiO2 to be
inactive in hydrogenation of methyl hexanoate. It was next deter-
mined that on increasing the amount of Re in Pt-Re/TiO2 up until
a Pt to Re molar ratio of 1:2, methyl hexanoate conversion could
be improved by a factor of approximately 3. Further increasing
the amount of Re decreased conversion. All Pt-Re/TiO2 catalysts
exhibited high selectivity towards 1-hexanol, implying the effect of
Re-addition is therefore to enhance the rate of reaction by provid-
ing sites at which activation via interaction with the oxygen atom
of the carbonyl group of the ester can be facilitated.
Characterisation of the catalysts by TEM reported the aver-
age particle sizes of bimetallic Pt-Re catalysts as being slightly
larger (1.3–1.5 nm) than their monometallic (1.1–1.3 nm) counter-
Please cite this article in press as: J. Pritchard, et al., Supported Pt-Re catalysts for the selective hydrogenation of methyl and ethyl esters
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tem. We eliminated the former possibility given that supporting
experiments in which methyl benzoate was hydrogenated over Pt-
Re (1:2)/TiO2 under the same mild conditions allowed for complete
conversion (due to non-selective hydrogenation of the aromatic
ring) and therefore mass-transfer limitations cannot be responsi-
ble for the hindered performance described above. More support
comes from hexanoic acid hydrogenation experiments in which it
was possible to achieve full conversion and first order behaviour
with respect to catalyst mass.
As water is produced in carboxylic acid hydrogenation as
opposed to methanol/ethanol when hydrogenating methyl/esters,
it came to the attention that a functional group effect may explain
why there is limited ester conversion. This hypothesis is confirmed
by FTIR spectroscopy. On approaching a reaction temperature
of 180 ◦C, methanol undergoes decarbonylation, dehydrogenation
and, depending on the type of catalyst, decarboxylation in-situ to
yield carbon monoxide, formic acid, formaldehyde, water, formate
species and carbon dioxide, respectively. It is decarbonylation of
methanol to CO that is responsible for catalyst deactivation by the
poisoning of active Pt sites. The formation of surface carbonyl has
been confirmed by the appearance of new IR bands upon methanol
adsorption attributed to Pt-CO observed at 2046 cm−1 for Pt/TiO2
and 2073 cm−1 for Pt-Re (1:2)/TiO2 catalysts. With reference to the
literature [52], we propose that the difference of 27 cm−1 could be
due to the CO band position being sensitive to factors including
alloying and Pt dispersion. Such deactivation phenomena was pre-
viously disclosed in 2006 by Merck & Co [53]. for several classes
of hydrogenation reactions performed in short chain alcohol sol-
vents and over Raney-Ni and supported Pd catalysts. Critically,
none of the aforementioned species were observed on Re/TiO2
which indicates that methanol interacts and undergoes reaction
specifically over Pt sites; however, the presence of Re can indeed
modify Pt behaviour. Pt-Re (1:2)/TiO2 can facilitate decarbony-
lation, decarboxylation and dehydrogenation reactions whereas
decarbonylation is the only significant process over Pt/TiO2. Decar-
bonylation is however more facile over Pt/TiO2 with CO formed
even upon the room temperature adsorption of methanol and nor-
malization of spectra recorded for Pt and Pt-Re (1:2)/TiO2 pellets
shows decarbonylation having taken place to greater extent over
the former catalyst.
Fig. 11. FTIR spectra of adsorbed methanol on (a) Pt/TiO2 and (b) Pt-Re (1:2)/TiO2
recorded at 10 ◦C intervals and normalized to the weight of the pellet sample.
parts. Simultaneously, the population of sub-nanometre particles
was observed to increase by as much as 50% moving from Pt to
Pt-Re (1:2)/TiO2. Most interestingly, the largest population of sub-
nanometre particles was detected for Re/TiO2 and therefore it is
very likely, based on this information, that alloying between Pt and
5. Conclusions
Re metals has occurred. Referring to the XPS spectra of the Pt (4f7/2
)
and Re (4f7/2) regions, good contact between Pt and Re was sug-
gested by a slight shift of the Pt features to higher binding energies
(71.0 to 71.7 eV) with increasing Re content. Alongside this, the Re
4f7/2 peaks became progressively more metallic in nature (41.8 to
40.0 eV) with almost 20 times as much surface-exposed Re detected
on Pt-Re (1:2)/TiO2 compared to Pt/TiO2. Finally, bulk characteri-
sation of Pt, Re and Pt-Re/TiO2 catalysts by TPR indicated that it
is also easier to reduce Re in the presence of Pt and on TiO2 due
to H2-spillover effects. Taking these factors into account, we pro-
pose that catalytic activity in hydrogenation of esters is associated
with alloyed and largely metallic sub-nanometre Pt-Re particles
comprising a high Re/Pt surface ratio.
Changing the alkoxy group of the ester substrate can have a sig-
nificant effect on catalytic activity. On substituting methyl for ethyl
hexanoate, conversion over Pt and all Pt-Re/TiO2 catalysts was sig-
nificantly improved (reaching a maximum at Pt:Re = 1:2) and with
high selectivity to 1-hexanol maintained. However, the maximum
conversion of ethyl hexanoate is still only 23% (11% for methyl
hexanoate) and, in addition, methyl and ethyl hexanoate conver-
sion did not show uniform response with respect to catalyst mass
and time online parameters, respectively, suggesting that transport
limitations and/or catalyst deactivation may be present in the sys-
Supported Pt, Re and Pt-Re catalysts prepared via impregna-
tion were evaluated in hydrogenation of hexanoic acid, methyl
benzoate and methyl/ethyl hexanoate substrates under mild con-
ditions. Characterisation of catalysts by TEM, TPR and XPS provided
evidence of synergy between Pt and Re in terms of decreasing par-
ticle size and particle size distribution, contribution of H2-spillover
effects during reduction and concentration of surface Pt/Re species,
respectively. Full conversion of hexanoic acid over supported Pt-
Re catalysts and a first order dependence on catalyst mass were
observed. In ester hydrogenation, optimal activity is achieved over
Pt-Re (1:2)/TiO2 in the hydrogenation of methyl/ethyl hexanoate
to 1-hexanol and methanol/ethanol. On substituting methyl hex-
anoate for ethyl hexanoate, activity could be improved by as much
as a factor of 3 depending on the Pt to Re ratio in the catalyst. Addi-
tional experiments in which either the catalyst mass or reaction
time were varied indicated that catalyst deactivation was respon-
sible for this difference in activity. Direct addition of methanol to
the reaction medium was found to be highly detrimental to activ-
ity suggesting that the diminished catalytic performance observed
upon the hydrogenation of methyl esters is due to the inhibit-
ing role of the methanol by-product formed in the course of the
reaction. In situ FTIR spectroscopy measurements of Pt, Re and
Please cite this article in press as: J. Pritchard, et al., Supported Pt-Re catalysts for the selective hydrogenation of methyl and ethyl esters
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Pt-Re (1:2)/TiO2 catalysts supported both the decarbonylation of
methanol and resulting CO poisoning of Pt surface sites, thereby
explaining the observed catalyst deactivation in situ. While cata-
lysts were subject to deactivation related to Pt-CO interactions, the
beneficial effects of Re and TiO2 as a support were recognised and
could form the basis of future studies aimed at developing alter-
native Re-containing catalysts with improved tolerance for methyl
ester hydrogenation.
[18] Y. Wang, X. Duan, J. Zheng, H. Lin, Y. Yuan, H. Ariga, S. Takakusagi, K. Asakura,
Catal. Sci. Technol. 2 (2012) 1637.
[19] V.M. Deshpande, W.R. Patterson, C.S. Narasimhan, J. Catal. 121 (1990) 165.
[20] V.M. Deshpande, K. Ramnarayan, C.S. Narasimhan, J. Catal. 121 (1990) 174.
[21] S.R. De Miguel, M.C. Roman-Martinez, E.L. Jablonski, J.L.G. Fierro, D.
Cazorla-Amoros, O.A. Scelza, J. Catal. 184 (1999) 514.
[22] A. Auroux, D. Sprinceana, A. Gervasini, J. Catal. 195 (2000) 140.
[23] N. Nava, T. Viveros, Hyperfine Interact. 122 (1999) 147.
[24] Y. Pouilloux, F. Autin, C. Guimon, J. Barrault, J. Catal. 176 (1998) 215.
[25] G. Fan, Y. Zhou, H. Fu, X. Ye, R. Li, H. Chen, X. Li, Chin. J. Chem. 29 (2011) 229.
[26] M.J. Mendes, O.A.A. Santos, E. Jordão, A.M. Silva, Appl. Catal. A: Gen. 217
(2001) 253.
Acknowledgements
[27] D.-H. He, N. Wakasa, T. Fuchikami, Tetrahedron Lett. 36 (1995) 1059.
[28] A. Ciftci, D.A.J.M. Ligthart, A.O. Sen, A.J.F. van Hoof, H. Friedrich, E.J.M. Hensen,
J. Catal. 311 (2014) 88.
This work has been performed within the framework of the
CatchBio program. The authors gratefully acknowledge the support
of the Smart Mix Program of the Netherlands Ministry of Eco-
nomic Affairs and the Netherlands Ministry of Education, Culture
and Science. E.A.P. thanks the Technology Foundation STW and the
Netherlands Organization for Scientific Research (NWO) for his per-
sonal VENI grant. We thank Mr. Arno J.F. van Hoof and Mr. Anton
Litke for TEM measurements.
[29] A. Ciftci, D.A.J.M. Ligthart, E.J.M. Hensen, Green Chem. 16 (2014) 853.
[30] M. Stein, B. Breit, Angew. Chem. Int. Ed. 52 (2013) 2231.
[31] R. Burch, C. Paun, X.M. Cao, P. Crawford, P. Goodrich, C. Hardacre, P. Hu, L.
McLaughlin, J. Sa, J.M. Thompson, J. Catal. 283 (2011) 89.
[32] Y. Takeda, T. Shoji, H. Watanabe, M. Tamura, Y. Nakagawa, K. Okumura, K.
Tomishige, ChemSusChem 8 (2015) 1170.
[33] H.G. Manyar, C. Paun, R. Pilus, D.W. Rooney, J.M. Thompson, C. Hardacre,
Chem. Commun. 46 (2010) 6279.
[34] A.M. Smith, R. Whyman, Chem. Rev. 114 (2014) 5477.
[35] B.K. Ly, D.P. Minh, C. Pinel, M. Besson, B. Tapin, F. Epron, C. Especel, Top. Catal.
55 (2012) 466.
[36] C. Bolivar, H. Charcosset, R. Frety, M. Primet, L. Tournayan, C. Betizeau, G.
Leclercq, R. Maurel, J. Catal. 39 (1975) 249.
Appendix A. Supplementary data
[37] C. Betizeau, G. Leclercq, R. Maurel, C. Bolivar, H. Charcosset, R. Frety, L.
Tournayan, J. Catal. 45 (1976) 179.
[38] J. Sa, C. Kartusch, M. Makosch, C. Paun, J.A. van Bokhoven, E. Kleymenov, J.
Szlachetko, M. Nachtegaal, H.G. Manyar, C. Hardacre, Chem. Commun. 47
(2011) 6590.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.03.
043.
[39] M. Chia, Y.J. Pagan-Torres, D. Hibbitts, Q.-H. Tan, H.N. Pham, A.K. Datye, M.
Neurock, R.J. Davis, J.A. Dumesic, J. Am. Chem. Soc. 133 (2011) 12675.
[40] S. Werkmeister, K. Junge, M. Beller, Org. Process Res. Dev. 18 (2014) 289.
[41] J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem. Int. Ed. 45 (2006)
1113.
References
[1] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411.
[2] J. Pritchard, G.A. Filonenko, R. van Putten, Emiel J.M. Hensen, Evgeny A. Pidko,
Chem. Soc. Rev. 44 (2015) 3808.
[3] H. Mimoun, G. Holzner, J. Pastori, US Patent 6, 806, 250 (2014).
[4] M. Eini, D. Tamarkin, US Patent 6, 994, 863 (2006).
[5] http://www.oleoline.com/index.php/news/2013.
[6] http://www.hexaresearch.com/research-report/global-oleochemicals-
industry/.
[7] H. Adkins, K. Folkers, J. Am. Chem. Soc. 53 (1931) 1095.
[8] H. Adkins, H.I. Cramer, R. Connor, J. Am. Chem. Soc. 53 (1931) 1402.
[9] H.S. Broadbent, C.W. Whittle, J. Am. Chem. Soc. 81 (1959) 3587.
[10] A.K. Agarwal, N.W. Cant, M.S. Wainwright, D.L. Trimm, J. Mol. Catal. 43 (1987)
79.
[11] A.K. Agarwal, M.S. Wainwright, D.L. Trimm, N.W. Cant, J. Mol. Catal. 45 (1988)
247.
[12] R. Connor, K. Folkers, H. Adkins, J. Am. Chem. Soc. 54 (1932) 1138.
[13] K. Folkers, H. Adkins, J. Am. Chem. Soc. 54 (1932) 1145.
[14] P.A. Dub, T. Ikariya, ACS Catal. 2 (2012) 1718.
[15] H.I. Adkins, A.A. Pavlic, J. Am. Chem. Soc. 69 (1947) 3039.
[16] T. Turek, D.L. Trimm, D.S. Black, N.W. Cant, Appl. Catal. A 116 (1994) 137.
[17] J. Zheng, H. Lin, Y.-N. Wang, X. Zheng, X. Duan, Y. Yuan, J. Catal. 297 (2013) 110.
[42] E. Alberico, P. Sponholz, C. Cordes, M. Nielsen, H.-J. Drexler, W. Baumann, H.
Junge, M. Beller, Angew. Chem. Int. Ed. 52 (2013) 14162.
[43] G.A. Filonenko, M.J.B. Aguila, E.N. Schulpen, R. van Putten, J. Wiecko, C. Müller,
L. Lefort, E.J.M. Hensen, E.A. Pidko, J. Am. Chem. Soc. 137 (2015) 7620.
[44] A. Ciftci, S. Eren, D.A.J.M. Ligthart, E.J.M. Hensen, ChemCatChem 6 (2014) 1260.
[45] W. Rachmady, M.A. Vannice, J. Catal. 192 (2000) 322.
[46] T. Yajima, H. Uchida, M. Watanabe, J. Phys. Chem. B 108 (2004) 2654.
[47] J.A. Anderson, F.-K. Chong, C.H. Rochester, J. Mol. Catal. A: Chem. 140 (1999)
65.
[48] H. Iida, A. Igarashi, Appl. Catal. A: Gen. 303 (2006) 48.
[49] J. Aran˜a, C. Garriga i Cabo, J.M. Dona-Rodrıguez, O. González-Dıaz, J.A.
Herrera-Melián, J. Pérez-Pen˜a, Appl. Catal. B: Environ. 53 (2004) 221.
[50] J. Aran˜a, C. Garriga i Cabo, J.M. Dona-Rodrıguez, O. González-Dıaz, J.A.
Herrera-Melián, J. Pérez-Pen˜a, Appl. Surf. Sci. 239 (2004) 60.
[51] F.K. Chong, J.A. Anderson, C.H. Rochester, J. Catal. 190 (2000) 327.
[52] F. Solymosi, T. Bangasi, J. Phys. Chem. 96 (1992) 1349.
[53] U.K. Singh, S.W. Krska, Y. Sun, Org. Proc. Res. Dev. 10 (2006) 1153.
Please cite this article in press as: J. Pritchard, et al., Supported Pt-Re catalysts for the selective hydrogenation of methyl and ethyl esters
to alcohols, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.043