The selective hydrogenation of furfural over supported palladium nanoparticle catalysts prepared by sol-immobilisation: Effect of catalyst support and reaction conditions

Article abstract of DOI:10.1039/c7cy02110k

The selective hydrogenation of bio-derived furfural was investigated under mild conditions using a series of supported palladium catalysts prepared by a sol-immobilisation technique. Catalysts using alumina and titania supports were more selective towards tetrahydrofurfuryl alcohol. The catalytic activity of 1.19% Pd/TiO₂ was evaluated under different reaction conditions and higher selectivity towards tetrahydrofurfuryl alcohol was observed when using 2-propanol as a solvent, and the yield of tetrahydrofurfuryl alcohol decreased as reaction temperature increased. The performance of the Pd catalyst was enhanced by the addition of Pt and a 95% yield of tetrahydrofurfuryl alcohol was achieved. The catalysts were characterised using a range of techniques, and the synergistic effect of adding Pt to Pd was due to an electronic promotional effect.

Full text of DOI:10.1039/c7cy02110k

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The Selective Hydrogenation of Furfural over Supported Palladium Nanoparticle
Catalysts Prepared by Sol-Immobilisation: Effect of Catalyst support and Reaction
Conditions
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Reem Albilali,*1 Mark Douthwaite,2 Qian He2 and Stuart H. Taylor*2
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1 Department of Chemistry, College of Science, Imam Abdulrahman Bin Faisal University,
P.O. Box 1982, Dammam 31441, Saudi Arabia.
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Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park
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Place, Cardiff, CF10 3AT, United Kingdom.
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* Corresponding authors eꢀmail: taylorsh@cardiff.ac.uk, ralblali@iau.edu.sa
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Abstract
The selective hydrogenation of bioꢀderived furfural was investigated under mild conditions using a
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series of supported palladium catalysts prepared by a solꢀimmobilisation technique. Catalysts using
alumina and titania supports were more selective towards tetrahydrofurfural alcohol. The catalytic
activity of 1.19 % Pd/TiO2 was evaluated under different reaction conditions and higher selectivity
towards tetrahydrofurfural alcohol was observed when using 2ꢀpropanol as a solvent, and the yield of
tetrahydrofurfural alcohol decreased as reaction temperature increased. The performance of the Pd
catalyst was enhanced by the addition of Pt and a 95 % yield of tetrahydrofurfural alcohol was
achieved. Catalysts were characterised by a range of techniques, and the synergistic effect of adding
Pt to Pd was due to an electronic promotional effect.
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Keywords: Hydrogenation, furfural, heterogeneous catalysis, biomass, tetrahydrofurfuryl
alcohol
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1. Introduction
The social, political and environmental issues surrounding the manufacture of 1st generation
biofuels has limited their global application somewhat.1 As a consequence, is that interest in
the synthesis of liquid fuels and fineꢀchemicals from lignocellulosic feedstocks has increased,
due to its large natural abundance and sustainability.2, 3 However, the valorisation of such
feedstocks are currently economically limited due to the extensive processing steps required.
As such, interest in the development of technologies to facilitate the conversion of
lignocellulosic feedstocks to highꢀvalue compounds has increased significantly in recent
years.4 Furfural (FF) is produced from the hydrolysis and dehydration of lignocellulosic
feedstocks.2 It has very few direct applications, but is considered to harbour a lot of potential
as a highly desirable platform chemical.5
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The selective hydrogenation of FF over heterogeneous catalysts has been investigated in
significant detail in recent years, the progress of which has been documented in some current
reviews.6, 7 The products of this hydrogenation reaction have a variety of different industrial
applications. Furfuryl alcohol (FA), which is produced from the chemoselective
hydrogenation of the formyl group, is used as a monomer for the formation of furanic resins,8
in addition to it’s potential as a fuel.9 Furthermore, the total hydrogenation of FF gives
tetrahydrofurfuryl alcohol (THFA), which is considered to be an environmentally benign
green solvent due to its biodegradable nature10. The hydrodeoxygenation of FF produces 2ꢀ
methylfuran (MF), which also has potential to be utilised as a fuel additive.11 In addition to
the above, the hydrogenation of FF can also lead to the formation of numerous other products
via ringꢀopening, decarbonylation and rearrangement pathways.12 Accordingly, tuning the
reaction selectivity towards a specific product is challenging but can be achieved through
catalyst design.
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It is clear from studies of FF hydrogenation that the selection of catalytic metal, support,
reaction conditions and solvent choice can drastically influence the product distribution. Polar
solvents have been found to significantly promote the rate of these hydrogenation reactions,
but can also lead to the formation of acetyl and/or ester adducts.13 It has been shown that the
formation of these byꢀproducts can be suppressed through the utilization of some nonꢀpolar
solvents, but solvents of this nature also significantly reduce the reaction rate. Further drastic
changes in the selectivity profile can also be observed; water facilitates the reꢀarrangement of
FF to cyclopentanone14, 15 and the ring opening of furfuryl alcohol (FA) to produce diols.16
Dichloroethane was found to promote the hydrodeoxygenation (HDO) of FF to methyl furan
(MF).17
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The hydrogen pressure in the system can also significantly affect the product distribution.
LuO et al.18 showed recently that increasing the hydrogen pressures in the presence of a 10 %
Pt/C catalyst increased selectivity to furfuryl alcohol, and that low pressures favoured
hydrogenolysis reactions to produce furan. This is consistent with a previous DFT study by
Vlachos and coꢀworkers19 who showed that the product distribution is typically dependent on
the energy and conformation of adsorbed FF, which in turn is driven by the concentration of
hydrogen adsorbed to the surface.
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Additional work has investigated the effect of different support and metal combinations. A
study by Sitthisa et al.20 showed that Cu, Pd and Ni supported independently on SiO2, each
favoured different reaction pathways for the hydrogenation of FF in a continuous flow
system. A considerable number of recent studies have focussed on developing supported
metal catalysts with mono and bimetallic combinations of transition metals, and through
optimisation of the materials and reaction conditions, extremely high yields of FA are
obtainable.21ꢀ24 Particle size effects are also known to have a profound effect on
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chemoselectivity, with an example being the hydrogenation of α,βꢀunsaturated aldehydes,25ꢀ27
an effect which is likely to be due to an alteration of the substrates mode of adsorption.28
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Whilst a substantial amount of literature reports highly effective systems for the selective
hydrogenation and transfer hydrogenation of FF to FA, a much smaller quantity of
publications focus on the total hydrogenation of FF to THFA. Table 1 contains some catalytic
systems which are reported for the total hydrogenation of FF to FA.
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Although there are numerous examples of catalytic systems which report exceptionally high
yields to THFA from FF, in most cases either a high reaction temperature, high H2 pressure
or both are required to achieve them. As such, it would clearly be advantageous to develop a
system which could produce similar yields of THFA but under milder reaction conditions.
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The preparation of supported metal catalysts by the solꢀimmobilisation method can have a
dramatic effect on the physicochemical properties of the supported metal nanoparticles.37
This technique utilizes a stabilising agent such as polyvinyl alcohol (PVA) or polyvinyl
pyrolidone (PVP) to control the growth of the nanoparticles and subsequently leads to the
formation of colloidal metal nanoparticles of a wellꢀdefined particle size.38 Once the colloids
are formed, the metal is subsequently reduced, typically using a reducing agent such as
NaBH4. The support material is subsequently added and the pH of the solution is often
adjusted to ensure full immobilisation of the metal. The adsorption of the stabilised
nanoparticles are typically dependent on the nature of the stabiliser and the isoelectric point,
surface functionality and surface area of the support.39, 40 It is the defined and often tuneable
metal particle sizes that make this technique so desirable.
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To the best of our knowledge, studies investigating the activity of supported metal catalysts
prepared via solꢀimmobilization method for the liquid phase hydrogenation of FF are limited.
Only a study conducted by Rogers et al.34 highlighted the potential of using Pd/TiO2 catalysts
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prepared by a solꢀimmobilization method for the selective hydrogenation of FF, determining
that the size of the Pd nanoclusters and the nature of the reaction sites have a significant
influence on the reaction selectivity. In this study, we investigate the effect of reaction
conditions and support materials on the performance of supported Pd nanoparticles for the
selective hydrogenation of FF. All the catalysts were prepared by the solꢀimmobilisation
method to produce catalysts with small and narrow particle size distributions and tested under
very mild conditions for the liquid phase hydrogenation of FF. We also present results
showing how the catalytic performance of Pd nanoparticles can be enhanced by the addition
of Pt to produce supported bimetallic catalysts.
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2. Experimental
2.1 Materials (Source and Purity)
Potassium tetrachloropalladate(II) (99.99 %, Alfa Aesar), sodium borohydride (98 %, Sigmaꢀ
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Aldrich) polyvinyl alcohol (99 %, SigmaꢀAldrich), furfuryl alcohol (98 %, SigmaꢀAldrich),
furfural (99 %, SigmaꢀAldrich), γꢀvalerolactone (99 %, SigmaꢀAldrich), magnesium oxide
(99.9 %, BDH) and iron (II, III) oxide (99.99 %, SigmaꢀAldrich), titania P.25 (99.9 %,
Degussa), activated charcoal (DARCO Gꢀ60, ACROS). All experiments were carried out
using 2ꢀpropanol (99.5 %, Fisher Scientific) as a solvent, unless otherwise stated.
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2.2 Catalyst Preparation
The experimental method for the preparation of the monoꢀ and bimetallic catalysts by solꢀ
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immobilisation is as follows: Desired quantities of K2PdCl4 (0.0183 mol) and/or H2PtCl6
(0.0184 mol) was added to H2O (800 mL gꢀ1 of catalyst prepared) and stirred. To this
solution, polyvinyl alcohol (metal/PVA = 0.65 weight ratio, weight average molecular weight
Mw ¼ 9000–10 000 g molꢀ1, 80 % hydrolysed) was added. Subsequently, NaBH4
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(NaBH4/metal (mol/mol) = 5) was then introduced. After 30 minutes of sol generation, the
colloid was immobilised by adding support (0.99 g) and the solution was acidified to pH 2
(0.1 M, H2SO4) under vigorous stirring. After 1 hour, the slurry was filtered, the catalyst
washed thoroughly with distilled water, and dried at 110 ºC for 16 h.
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The experimental method for the preparation of the monometallic Pd/TiO2 catalyst by an
impregnation method is as follows: K2PdCl4 (0.00915 mol) was dissolved in 1.6 mL of
deionised water and stirred slowly at 80 ºC until the salt dissolved completely. The support
(0.495 g, TiO2 commercial P25) was subsequently added to the solution under vigorous
stirring at 80 ºC until a thick paste was formed. The paste was dried at 110 ºC for 16 h and
the subsequent material was finely ground and calcined at 400 ºC under static air for 3 hours
with a ramp rate of 20 ºC minꢀ1.
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2.3 Catalyst Testing
Furfural hydrogenation was performed between 30 °C and 60 °C in a Colaver glass reactor.
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The reactor was charged with FF solution (15 mL; 0.3 M in 2ꢀpropanol) and the desired
amount of catalyst, typically FF/metal molar ratio = 500, was added to the solution. The
reactor was sealed, and purged three times with H2 (3 bar) whilst stirring (800 rpm), before
finally being charged with H2 (1ꢀ3 bar). A continuous reaction pressure was maintained
throughout the reaction in order to replenish any H2 consumed. The reaction was conducted
for 120 minutes, unless otherwise stated.
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Catalyst reusability was investigated after the hydrogenation reaction of FF, the catalyst was
separated using a simple filtration method and washed with 50 mL of solvent 2ꢀpropanol. The
separated catalyst was dried at room temperature for 24 hours before being reused. The
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recycling experiments were continued using the same catalyst and same experimental
protocol for five cycles.
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2.4 Quantification of Products
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The reaction effluent was analysed by GC (Agilent 7890B) fixed with a CPꢀWax 52 CB
column (25 m x 0.53 mm x 2.0 µm) and a FID. nꢀOctane was used as an external standard for
product quantification. A combination of GC and GCꢀMS was used to determine the
chemical identity of the products, and comparison with authentic standards was carried out to
ensure correct identification. The reactant conversion, products selectivity and product yield
were calculated on mole basis using the following formulae:
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% Conversion of (FF) = [(Initial moles of FF ꢀ Final moles of FF) / Initial mole of FF)] x 100
% Selectivity = (Moles of product formed / (Total moles of products identified) x 100
% Yield = [Moles of product formed / Moles of FF(Start)] x 100
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Attempts were made to calculate the Pd dispersion of the catalysts and determine the initial
rate of each catalyst directly from the number of Pd sites detected by CO chemisorption.
However, it has been reported previously that the presence of a stabiliser can significantly
reduce the quantity of CO that can adsorb to the surface of catalysts prepared by the solꢀ
immobilisation method.41 As such, in this study, the initial rate measurements were estimated
by dividing the moles of substrate converted by the mass of metal in the reaction per unit
time (h) after 30 minutes.
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2.5 Catalyst Characterisation
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Powder X-ray diffraction (XRD) patterns were collected using a PANalytical X’pert Pro
diffractometer with a Niꢀfiltered CuKα radiation source operating at 40ꢁkV and 40ꢁmA. The
diffraction patterns were recorded over the 2θ range 10 – 80º, and phases identified by
matching to entries in the International Centre for Diffraction Data (ICDD) database.
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X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis Ultra DLD
spectrometer using a monochromatic Al Kα Xꢀray source (100 W). Spectra were recorded at
analyser pass energies of either 160 eV (survey scans) or 40 eV (detailed scans). Binding
energies were referenced to the C(1s) binding energy of adventitious carbon contamination at
284.7 eV, and data were quantified using CasaXPSTM v2.3.15 software, using sensitivity
factors supplied by the manufacturer.
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Transmission electron microscopy (TEM) was carried out using a JEOL JEMꢀ2100 with a
LaB6 filament operating at 200 kV. Powdered catalyst samples were dry dispersed onto lacey
carbonꢀcoated 400 mesh copper grids. The mean particle size distributions (based on 300
particles) were subsequently measured using Image J software.
Scanning Transmission electron microscopy (STEM) was carried out using an aberration
corrected JEOL JEM ARM 200CF microscope operating at 200kV. Powdered catalyst
samples were dry dispersed onto lacey carbonꢀcoated 300 mesh copper grids. Xꢀray energy
dispersive (XEDS) spectra were acquired from individual metal nanoparticles by rastering the
beam over the entire metal particle, while using a JEOL Centurio 0.9sr silicon drift detector.
Scanning electron microscopy (SEM) was conducted on a Hitachi TM3030PLUS
microscope equipped with a Quantax70 energyꢀdispersive Xꢀray spectroscope (EDX,
Microanalysis System, Oxford Instruments).
Microwave plasma-atomic emission spectroscopy (MP-AES) was carried out using an
Agilent 4100 instrument to determine the metal loading. A known mass of catalyst was added
to a dilute aqua regia solution (50 mL of 10 % aqueous solution) and left to digest for 16 h.
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The digested solution was subsequently filtered using PTFE filters (Acrodisc PVDF 0.45 ꢂl)
to eradicate any remaining residual support material from the solution. Samples were
introduced into a stream of nitrogen plasma via a single pass spray chamber at a pressure of
120 kPa in the absence of any air injection. The instrument was calibrated with 2.5 ppm, 5
ppm and 10 ppm solutions of the metal which were prepared from analytical standard
solutions (Agilent, 1000 ppm). Samples were tested three times, and an average of the three
results used to calculate elemental compositions.
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Inductively Coupled Plasma-mass spectrometry (ICP-MS) analysis was performed in
order to quantify any metal leaching during the test reactions to determine catalyst
performance. The analysis was performed using an Agilent ICPꢀMS 7900 instrument.
Quantitative analysis of the postꢀreaction effluents were performed in duplicate, and metal
concentration determined following calibration with certified standards.
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3. Results and Discussion
3.1 Effect of the support on catalytic performance and reaction mechanism
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In order to expand on the recent work by Rogers et al.,34 a series of monometallic Pd
catalysts were prepared using a similar colloidal method. Different support materials were
utilised in order to investigate how the support affects the activity and selectivity of the Pd
nanoparticles. Each reaction was conducted under mild conditions (3 bar hydrogen, 30 °C) in
2ꢀpropanol solvent. The results from the initial catalytic screening experiments are displayed
in Table 2. The products expected from the hydrogenation of FF using these catalysts are
presented in Figure 1.
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It is evident that all the catalysts are active for this reaction and that the support material
significantly influences the performance of Pd nanoparticles. The Pd supported on TiO2, C,
MgO and Al2O3 achieved full conversion after 180 min, while only 64.2 % conversion was
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observed with Pd/Fe3O4. The initial rates revealed that the Pd/C catalyst exhibited the highest
activity of the catalysts tested. An initial rate of 5.21mol gꢀ1 hꢀ1 was calculated for this
catalyst, which was approximately 85 % greater than the initial rate observed for Pd
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supported on TiO2, MgO and Al2O3. Despite the high activity associated with the Pd/C
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catalyst, the observed product distribution was undesirable, as many unknown products were
formed. Previous work has shown that some catalytic systems lead to the formation of ring
opening products such as 1,2ꢀpentandiol.16 The formation of many of these ring opening
products can be discounted here, as none of them were observed in the post reaction effluent.
Another potential explanation is the participation of the 2ꢀpropanol solvent in competitive
reactions, leading to the formation of the corresponding ether or acetyl adduct. Merlo and coꢀ
workers22 reported that the formation of the corresponding isopropanol ether is possible
during the catalysed liquid phase hydrogenation of FF in the presence of 2ꢀpropanol, but we
did not observe such products. Interestingly, the Pd/C appears to facilitate hydrogenolysis of
FF to MF, and some of the unknown products may be either intermediates or additional
products formed during this pathway. Nevertheless, the conclusion is that the Pd/C catalyst
did not show the control over selectivity required.
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The other supports tested were more selective to a smaller range of products, and they did not
produce significant quantities of unknown products. The Al2O3 and TiO2 supports led to the
formation of γꢀvalerolactone (GVL). It is possible that Bronsted acidic sites in the catalyst
are promoting the ring opening of FA to levulinic acid (LA), which can subsequently be
reduced and cyclise to give GVL.
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Interestingly, chemoselective hydrogenation is observed with the Fe3O4 and MgO based
catalysts, as they appear to efficiently hydrogenate the unsaturated functional groups in the
furan ring. Consequently, high quantities of tetrahydrofurfural (THFF) are observed in the
reactions using these catalysts.
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Comparison of selectivity at relatively low conversions for catalysts with different supports
can also be made from the data in Table 2. For all catalysts, the selectivity after 30 minutes
reaction time favours the formation of FA over all other products (THFA, 2MF, GVL, and
THFF), suggesting that C=O hydrogenation is favoured over the hydrogenation of the
unsaturated groups within the furan ring. As FA is consumed, an increase in the quantity of
THFA in the system is observed. Whilst the majority of FA consumed between 30 and 120
minutes is likely to be a result of sequential hydrogenation to THFA, the increase in the yield
of THFA does not account for all the FA consumed during this time. This suggests that there
are additional competitive reaction pathways proceeding from FA. As discussed previously,
Bronsted acidity can promote the formation of GVL from FA which may account for perhaps
some of the GVL observed in the system. Another interesting observation that can be made is
that THFF appears to be relatively stable in the system; only small quantities of THFF are
consumed from 120 to 180 minutes. This implies that the C=O moiety in THFF is more
difficult to hydrogenate than the furan ring in FA to form THFA. A possible explanation for
this is that the C=O in THFF forms a hydrogen bond with the polar OꢀH moiety in the 2ꢀ
propanol solvent, while the conjugated πꢀbond in the furan ring in FA cannot interact with the
solvent in the same manor. It’s possible that this solventꢀsubstrate interaction would reduce
the affinity of the C=O moiety to adsorb onto the surface of the catalyst which would
ultimately reduce the rate of the C=O hydrogenation. Hu et. al.42 observed a similar
observation for the catalytic liquid phase hydrogenation of furfural in methyl formate. It was
suggested that the high quantities of THFF observed in their system was a result of the ability
of the C=O moiety in furfural to form hydrogen bonds with the methyl formate, ultimately
making the carbonyl group more difficult to hydrogenate than the furan ring in furfural.
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The comparison of the selectivity towards THFA (the desired product) at an initial time of 30
minutes shows that the Pd/TiO2 catalyst has the higher selectivity towards THFA (20.2 %)
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compared with 4.2 %, 2.3 %, 5.8 % and 13.3 % for Pd/C, Pd/MgO, Pd/Al2O3 and Pd/Fe2O3
respectively (Table 2). Furthermore, it is clear that the yield of THFA produced increases
significantly with reaction time in the presence of Pd/TiO2 and Pd/Al2O3. This increase in the
yield of THFA is associated with a decrease in the selectivity towards FA. No FA was
detected after 180 minutes of this reaction, in which the selectivity towards THFA reached a
maximum of 42 % and 36.4 % with the Pd/TiO2 and Pd/Al2O3 catalysts respectively. As
such, we can conclude that FA is an intermediate in the conversion of FF to THFA.
Furthermore, the yield of THFA is influenced by the type of support used. Figure 2 shows
that the maximum yield of 42 % was observed for Pd/TiO2. Given that THFA is the desired
product in this study, further reaction optimisation and investigation was focussed on the
Pd/TiO2 catalyst.
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The powder Xꢀray diffraction patterns for catalysts on different supports are displayed in
Figure S1. The data implies that the Pd metal is well dispersed on all supports, as no
diffraction from Pd phases is observable. MPꢀAES was used to determine the Pd metal
loadings associated with each of the catalysts, the results of which are shown in Table 2 and
indicate that significant loadings of Pd are present. The range of the Pd loading on different
supports varied slightly, but in each case the Pd metal loading fell within the range 1.19 to
1.57 wt.%. Thus, we can conclude that the successful immobilisation of Pd onto each support
material was achieved.
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In order to determine whether reducing the metal loading influences the activity of the Pd
nanoparticles, an additional Pd/TiO2 catalyst was prepared with a lower Pd loading and was
subsequently tested under the same reaction conditions. The comparison of these two
Pd/TiO2 catalysts is displayed in Table S1. MPꢀAES confirmed that an appropriate reduction
in the Pd loading was achieved as the Pd loading in this catalyst was measured to be 0.57 %.
Transmission electron microscopy (TEM) was subsequently utilised to assess whether
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reducing the metal loading has an influence on the Pd particle size distribution. TEM images
and corresponding particle size distributions of the two Pd/TiO2 catalysts with different Pd
loadings are displayed in Figure 3. It can be noted that the palladium particles were well
dispersed on the titania support for both catalysts. A small decrease in the mean particle size
was observed when decreasing the palladium loading, as the mean particle size reported for
the 1.19 % Pd/TiO2 and 0.57 % Pd/TiO2 catalysts was 3.32 nm and 2.97 nm respectively. The
evaluation of catalytic performance for these catalysts (Table S1) reveals that the conversion
after 30 min decrease from 36.4 % to 28.2 % when the Pd loading was reduced from 1.19 %
to 0.57 %. This decrease in conversion corresponds to a 15 % decrease in the initial reaction
rate, decreasing from 2.75 to 2.32 mol gꢀ1 hꢀ1. Assessment of the corresponding product
distributions after 30 minutes (conversions of 28.2 % and 36.4 %) shows that there was no
significant change in the quantity of FA observed, with values of 38.4 % and 41.9 % for the
reaction catalysed by the 0.57 % Pd/TiO2 and 1.19 % Pd/TiO2 respectively. In contrast, the
yield of THFA decreases significantly from 20 % to 11 % when reducing the Pd loading from
1.19 % to 0.57 %. This decrease in the yield of THFA observed upon reducing the Pd loading
is associated with a 17 % increase in the yield of THFF observed, as the selectivity increased
from 23.6 % to 40.7 % when the Pd loading was reduced. This is consistent with a previous
observation, that the hydrogenation of the C=O species may be more difficult when the furan
ring is saturated, accordingly, the catalyst with low metal loading shows higher quantities of
THFF. Increasing the metal loading can enhance the hydrogenation of C=O over Pd
nanoparticles, which is more difficult to hydrogenate using the 0.57 % Pd/TiO2, resulting in
more selectivity towards THFA at higher Pd loading.
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Figure 4 shows the yield of THFA and FA produced after a reaction time of 120 minutes. In
the presence of the 1.19 % Pd/TiO2, no FA was observed and the yield of THFA reached 43
%, while only 20 % yield of THFA was observed for the 0.57 % Pd/TiO2 catalyst with a
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substantial quantity of FA still present in the post reaction effluent (12%). These results
reveal that the rate of sequential hydrogenation of FA to THFA decreases by reducing the Pd
loading, which is consistent with the decrease of the initial rate of overall hydrogenation.
4
5
There are numerous examples of similar catalytic systems which facilitate the transfer
hydrogenation of furfural in the presence of 2ꢀpropanol.43ꢀ46 In each of these examples
however, significantly higher reaction temperatures (110 to 200 ºC) were required in order to
facilitate the transfer hydrogenation. Nevertheless, it was important to eradicate the
possibility that these reactions were taking place within this catalytic system. For this,
additional reactions were conducted under N2 using the same reaction conditions and catalyst,
the results of which are displayed in Table S2. In the absence of H2, no furfural conversion
was observed suggesting that the reductions taking place require H2. Furthermore, no acetone
was observed in the post reaction solutions for the reactions in the presence of both H2 and
N2, providing further evidence that the isopropanol does not provide hydrogen in the reaction.
An additional reaction was subsequently conducted under N2 in the presence of the TiO2
support material in order to eradicate any possibility that this material was facilitating a
transfer hydrogenation reaction. As with the reaction conducted in the presence of Pd/TiO2,
no conversion of FF or production of acetone was observed.
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SEMꢀEDX analysis was carried out to provide more information about the morphology of
these two catalysts. The results of which are displayed in Figure S2. Both catalysts exhibit
similar morphologies. The Pd loading calculated using EDX for the 0.57 % Pd/TiO2 and 1.19
% Pd/TiO2 are 0.59% and 1.10% respectively, which is consistent with the Pd loading
reported from MPꢀAES. The metal elemental mapping images for both catalysts are shown in
Figure S3. It can be noted that Pd was less densely distributed in 0.57 % Pd/TiO2 compared
with the 1.19 % catalyst, this is most likely expected, as there is only a slight change in Pd
particle size, from 2.97 nm to 3.32 nm for 0.57 % Pd/TiO2 and 1.19 Pd/TiO2 respectively. As
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such, it can be concluded that the density of Pd in the catalyst is predominantly related to the
Pd content. Considering that the applied FF/Pd molar ratio of 500:1 remained equal during
catalyst testing, and there is no significant change in the mean particle size obtained from
TEM images for both catalysts, hence we can conclude that the main factor which enhances
the catalytic activity in 1.19 % Pd/TiO2 catalyst is having a greater number of surface Pd sites
available.
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Xꢀray photoelectron spectroscopy (XPS) was conducted to obtain further information
regarding the oxidation state of Pd and elemental distribution on the surface of each TiO2
supported catalyst. The corresponding spectroscopic data is presented in Figure 5 and Table
S3. Two dominant peaks with binding energies of 335.3 and 336.6 eV are observed in the
spectra of each catalyst, and correspond to the Pd(3d5/2) spinꢀorbit coupled peaks of
palladium. It is clear that Pd is present in both its metallic state (Pd0) and oxidised state (Pd2+)
in all the catalyst samples. As shown in Figure 5, the Pd(3d5/2) peak can be fitted to two
dominant peaks, with binding energies of 335.3 and 336.6 eV. The peak at 335.3 eV can be
assigned to the metallic state of palladium, while the other peak at 336.6 eV can be assigned
to Pd2+ in PdO.47, 48 The Pd0/Pd2+ ratio does not appear to correlate with differences in
catalyst performance. The XPS spectrum for the used 1.19 % Pd/TiO2 displayed in Figure 6
shows that only Pd0 is present in the used catalyst, which indicates that PdO supported on
TiO2 readily reduces in the presence of H2 at ambient temperature.49
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To evaluate the leaching of Pd from each catalyst prepared using different supports, ICPꢀMS
analysis was conducted on the post reaction effluent of each reaction (Table S4). Pd metal
was detected in the post reaction effluents with each of the catalysts, but the concentration of
Pd was low. In each case, less than 0.1 % of Pd was leached from each of the catalysts, which
indicates that the binding of the metal nanoparticles to the different surfaces was a stable
process under our reaction conditions.
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As the 1.19 % Pd/TiO2 catalyst was found to provide the highest yields of THFA, an
additional investigation was conducted in order to enphasise the benefit of using the solꢀ
immobilisation as a means of preparing the Pd supported catalysts. For this, an additional
Pd/TiO2 catalyst (Pd loading calculated to be 1.10 wt.%) was prepared using a conventional
impregnation method and tested for the hydrogenation of furfural under the same reaction
conditions. This results are displayed in Figure S4. It was determined that the catalyst
prepared by the solꢀimmobilisation method displayed an initial activity approximately 80 %
higher than the catalyst prepared by the conventional impregnation method. Interestingly, an
18 % reduction in selectivity to THFA was also observed with the catalyst prepared by the
impregnation method. It is known that the solꢀimmobilisation method typically yields
supported metal catalysts with a significantly smaller metal particle size with a narrower
particle size distribution than identical catalysts prepared by impregnation methods.50, 51 As
such, the significant difference in the activity of these catalysts can be attributed to a particle
size effect.
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In order to evaluate the stability of the 1.19 % Pd/TiO2 catalyst prepared by the solꢀ
immobilisation method, a series of recycling experiments were conducted. The results from
this study are displayed in Figure 7 and Table S5. The catalyst preserved catalytic activity for
2 cycles, as the initial rate of 2.74 mol gꢀ1 hꢀ1 remained constant during the first two runs. A
slight decrease in the yield of THFA was observed in the second cycle, as the yield of THFA
decrease from 43 % to 36 %. This decrease in the THFA is associated with a 14 % increase in
the yield of FA, which indicates that the rate of the sequential hydrogenation of FA to THFA
in the second run was reduced. In the third cycle, the catalytic activity in terms of initial rate
decreased by 20 % from its original value with a 56 % decrease in the yield of THFA
compared with the THFA yield from the first cycle. A further drop in the activity was
observed during the fourth and fifth run, as the initial rate decreased by approximately 70 %
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with values of to 0.88 and 0.81 mol gꢀ1 hꢀ1 for the fourth and fifth cycles respectively, with a
continuous decrease in the yield of THFA observed upon subsequent reactions. This decrease
in activity may be attributed to a number of possibilities, including leaching of Pd, a change
of Pd particle size, and deactivation of some active sites on the surface of the catalyst.
Leaching of Pd seems unlikely considering the low Pd leaching measured previously.
Nevertheless, Pd leaching was measured after each run. The concentration of Pd in solution
after each use was negligible, and was found to lie in the range 0.048 – 0.066 ppm. This
corresponds to a maximum leaching of 0.1 % of available Pd in each use, and hence loss of
active metal through leaching was not considered to be an important factor in the loss of
activity on reuse.
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The particle size distribution obtained from TEM images for the used 1.19 % Pd/TiO2
catalyst after five cycles of use is shown in Figure S5. There was a slight increase in the mean
particle size, with a value of 3.82 nm for the catalyst after five cycles, compared to 3.32 nm
for the fresh catalyst. The slight increase of particle size will result in some loss of metal
surface area, however the more drastic decrease of activity is greater than would be expected
for the minor loss of metal area. Hence, we conclude that the loss of activity also has a
contribution attributed to the deactivation of some active sites on the surface of the catalyst.
Roger et al 34
. concluded that deactivation of 1% Pd/TiO2 prepared using solꢀimmobilization
and used for the liquid phase hydrogenation of FF under mild conditions (5 bar hydrogen,
25°C) was due to the formation of palladium carbide, which affected hydrogen dissociation
and influenced the adsorption of C=O and C=C functionalities.
3.2 Effect of reaction conditions on catalyst performance
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The reaction solvent usually has an impact on the catalytic activity in a liquidꢀphase reaction.
A comparison of 2ꢀpropanl, 2ꢀbutanol, toluene and 1,2 dichloroethene as solvents was
conducted to investigate the influence on the catalytic performance for FF hydrogenation.
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The experiments were carried out using a 1.19 % Pd/TiO2 catalyst under 3 bar H2, FF/Pd
molar ratio = 500 and a temperature of 30 °C. The results of which are represented in Figure
8. It can be observed that variation of the solvent can influence both the product distribution
and reaction rate. Among the solvents tested, 2ꢀbutanol was found to facilitate the highest rate
of FF hydrogenation with a conversion of 49 % after 30 minutes, compared with 36 %, 32 %
and 13 % when using 2ꢀpropanol, 1,2 dichloroethene and toluene respectively. Despite the
high rate of FF hydrogenation observed with 2ꢀbutanol, the selectivity towards THFA and FA
decreased by 40 % and 45 % respectively, when compared to the reaction conducted in 2ꢀ
propanol. This decrease in the THFA and FA selectivities was associated with a 30 %
increase in the selectivity towards THFF in the presence of 2ꢀbutanol, which indicates that
the hydrogenation of unsaturated bonds within the furan ring is more favourable than the
hydrogenation of C=O on FF in 2ꢀbutanol. In contrast, the conversion decreased significantly
by 62 % when the reaction was conducted in toluene instead of 2ꢀpropanol. Hu et.al.42
reported that a shielding effect resulting from an interaction between the conjugated π system
in both furfural and toluene can be a factor which negatively affects the rate of hydrogenation
of both the furan ring and carbonyl group in furfural when the reaction was conducted in
toluene. An additional explanation was proposed by Merlo et al.22 who also observed a
significant reduction in catalytic activity when comparing the rate of reaction in polar and
nonꢀpolar solvents. It was noted that a significant reduction in catalytic activity was observed
for a supported Pd catalyst when the reaction was conducted in the presence of toluene
instead of isopropanol, where the dielectric constant (ɛ) of each equal 2.0 and 18.3
respectively.
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Interestingly, a significant change in the product distribution is observed when the reaction
was conducted in 1,2ꢀdichloroethane. Large quantities of 2ꢀMF and much lower quantities of
FA, THFA were formed, suggesting that 1,2ꢀdichloroethane favours hydrodeoxygenation
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(HDO) of FF over hydrogenation. The promotional effect of 1,2ꢀdichloroethane on the HDO
of FF has already been reported for reactions conducted in the presence of RuꢀPd/TiO2
bimetallic catalysts.17, 52
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5
Further experiments were conducted in order to evaluate the influence of the reaction
temperature on the product distribution. The experiments were carried out using 1.19 %
Pd/TiO2 catalysts (conditions: 3 bar H2, solvent 2ꢀpropanol). The results from these
experiments are displayed in Table 3. It is evident that the initial rate increases as the
temperature increases. However, the final conversion after 120 minutes decreased as the
temperature was raised from 50 to 60 ºC. This decrease in catalytic activity could be due to
formation of additional products which inhibit the catalytic performance, indeed the higher
temperature resulted in an increased amount of unknown products, which are most likely of
the type discussed earlier. Examination of the corresponding product distributions supports
this theory, as less THFA and THFF and more FA were observed as the reaction temperature
was increased. If we assume that the majority of the THFA is produced by the sequential
hydrogenation of FA and THFF, it is likely that this reduction in THFA selectivity is also a
result of this product inhibition. Another possible cause for this drop in performance may be a
result of the high temperatures facilitating the hydrogenolysis of FF. This would likely result
in the presence of residual C1 species on the surface of the catalyst and would supplement the
work by Rogers et al.34, who recently identified that the formation of a Pdꢀcarbide species is
responsible for catalyst deactivation, reducing the efficiency of FF adsorption on the surface
of the Pd cluster.
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The catalytic performance of 1.19 %Pd/TiO2 was evaluated at varying H2 pressure from 1 to
3 bar at 30 °C. The results are summarised in Table 4. It is evident that the hydrogen
pressure has a significant impact on the catalytic activity as the initial rate decreases
markedly when the H2 pressure was reduced from 3 to 1 bar. This decrease in the catalytic
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activity was also accompanied with a decrease in the yield of THFA and FA observed. These
results can be attributed to a decrease in the availability of hydrogen in the system, which is a
result of a decrease in dissolved H2 as the reaction pressure decreases. Interestingly, at 3 bar
hydrogen, we reached 100 % conversion of FF with 44 % selectivity to THFA after only 120
minutes reaction time. A significant increase in the yield of unknown products was also
observed as the H2 pressure was reduced, which could indicate the reaction pathways, which
typically lead to the formation of these unknowns, compete with the hydrogenation reaction
pathways. Given that previous work has shown that the hydrogenolysis of FF to furan
typically increases at the expense of the hydrogenation products as the H2 pressure
decreases,18, 19 it is possible that this undesirable reaction leads to the formation of some of
the unknown products in the system. If this is to be the case, we must assume that any furan
produced from the hydrogenolysis of FF subsequently partakes in another reaction, as no
furan is observed in these reactions. Assuming that the Pd carbide formation is a result of the
hydrogenolysis of FF as suggested previously, it can be postulated that the formation of this
species may also contribute to the drop in catalytic activity observed at lower H2 pressures.
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3.3 Bimetallic Pd-Pt/TiO2 catalyst
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In some cases, the addition of a second metal or a promoter can improve the catalytic activity
and/or the selectivity to the desired compound. Recent work has highlighted the potential of
using supported PdPt bimetallic catalysts in hydrogenation reactions.53 For this reason, an
equimolar bimetallic 0.97 % PdPt/TiO2 catalyst was prepared by the same preparation
method and tested for the hydrogenation of FF under our standard conditions (3 bar H2, 30
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°C). The catalytic performance of synthesised bimetallic 0.97 % PdPt/TiO2 was compared
with the monometallic 1.19 .% Pd/TiO2 and 1 % Pt/TiO2 catalysts as shown in Table 5. The
results reveal that the bimetallic 0.97 % PdPt/TiO2 catalyst was far more active than both the
1.19 % Pd/TiO2 and 1 % Pt/TiO2 monometallic catalysts, as the initial rate calculated after
30 minutes increased to 11.03 mol gꢀ1 hꢀ1 compared with 2.75 and 0.66 mol gꢀ1 hꢀ1 for 1.19
t% Pd/TiO2 and 1 % Pt/TiO2 respectively. Taking into account that the Pt/TiO2 catalyst was
significantly less active than the Pd/TiO2 catalyst, it is clear that there is a synergistic
interaction between the Pd and Pt in the system. Furthermore, the comparison of the product
distribution after 240 min for the three catalysts (Figure 9) shows that a 95 % yield of THFA
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can be achieved when utilizing the bimetallic 0.97 % PdPt/TiO2, while only 43 % and 0.3 %
was achieved for the 1.19 % Pd/TiO2 and 1 %Pt/TiO2 respectively.
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The initial activity of these catalysts is also reflected in the corresponding product
distribution. The poorly active Pt/TiO2 displayed exceptionally high selectivity to FA with
only trace quantities of the sequential hydrogenation product (THFA) observed. Clearly, Pt
favours the hydrogenation of the formyl species over the hydrogenation of the furan ring, as
no THFF was observed in the presence of this monometallic catalyst. Furthermore, the
selectivity to THFF observed with the bimetallic system is also significantly lower than that
observed with the Pd/TiO2. This may also suggests that the 0.97 % PdꢀPt/TiO2 catalyst was
able to hydrogenate the THFF to THFA more effectively, which resulted in an increase in the
yield of THFA, reaching 95% after 240 minutes. Another possibility is that Pt in the
bimetallic PdPt/TiO2 catalyst affects the chemoselectivity of the catalyst and as a result, a
larger quantity of FA is produced from FF. Considering the mild reaction conditions used in
this system, the exceptionally high yield of THFA highlights the potential of using bimetallic
PdPt/TiO2 catalysts for the valorisation of FF.
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In order to gain more insight into the structureꢀactivity relationship for the bimetallic 0.97 %
PdPt/TiO2 catalyst, characterization by TEM and XPS was conducted. Figure 10 represents
TEM images of the PdPt/TiO2 catalyst and the corresponding particle size distribution.
Interestingly, a mean particle size of 1.94 nm was observed for the 0.97 PdPt/TiO2 catalyst,
which is notably smaller than 3.32 nm reported for the 1.19 % Pd/TiO2 catalyst. As shown in
Figure 11, the PdPt/TiO2 catalyst was characterized by aberration corrected scanning
transmission electron microscopy (ACꢀSTEM). The nanoalloy formation between Pd and Pt
within individual particles was confirmed using Xꢀray energy dispersive spectra obtained
from individual particles. No segregation or coreꢀshell formation was detected using Zꢀ
contrast high angle annular dark field (HAADF) imaging. The formation of random alloy Pdꢀ
Pt nanoalloys was consistent with our previous report.54 Hence it can be inferred that the
improved activity of the bimetallic catalyst is influenced by an increased dispersion of the
active metal species, and a combination of electronic promotional effects, as there is a
synergistic combination of Pd and Pt.
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XPS was subsequently utilised in order to further assess the surface nature of the PdꢀPt
nanoparticles and oxidation state of Pd and Pt in the catalyst. Figure 12 shows the XPS
spectrum for the 0.97 % PdꢀPt/TiO2 catalyst and confirms the existence of metallic Pd and
metallic Pt on the surface layers. As shown in Figure 12, the Pd(3d5/2) peak can be fitted to a
single peak, with a binding energy of 334.6 eV which can be assigned to the metallic state of
palladium, whilst there was no evidence for a Pd2+ peak, which is expected at 336.9 eV. In
addition, the Pt(4f) signal can be fitted to two peaks, at 70.6 eV and 73.95 eV. These two
peaks are attributed to Pt0 and Pt2+ species, respectively.55, 56 The elemental composition
obtained from the XPS data is shown in Table S6. The single metallic oxidation state of Pd in
the bimetallic catalyst is in clear contrast to the combination of Pd0 and Pd2+ in the
monometallic TiO2 supported catalyst, and indicates there is an electronic modification of the
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surface Pd through interaction with Pt. We believe that this interaction is important for
promoting the activity of FF hydrogenation and indicates that the wealth of bimetallic
nanoparticles available for catalyst preparation are now worthey of much further study.
2
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Conclusions
6
In this study, we have highlighted the potential of using solꢀimmobilisation as a method of
preparing supported Pdꢀbased catalysts for the liquid phase hydrogenation of FF. We have
determined that the support material, reaction conditions and solvent system can have a
significant effect on both the activity and product selectivity of Pd nanoparticles. Of the
supports tested, TiO2 provided the highest selectivity to THFA. Further tests conducted with
this catalyst revealed how vital the reaction conditions are to control the reaction selectivity.
Subsequent testing conducted in the presence of a PdPt/TiO2 catalyst revealed that a yield of
THFA in excess of 95 % could be achieved. It was suggested that this increase in
performance was a result of an electronic promotional effect from the synergistic
combination of Pd and Pt.
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Acknowledgements
We would like to thank Professor Christopher J. Kiely from Lehigh University and the
Cardiff Catalysis Institute, Cardiff University, for kind support with the aberration corrected
STEM characterization. AlBilali gratefully acknowledges Imam Abdulrahman Bin Faisal
University for the financial support, and Cardiff Catalysis Institute, School of Chemistry, for
access to the facilities. Authors would like to thank Dr Greg Shaw for useful discussion and
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experimental assistance, Dr Thomas Davies for TEM collection, and Dr David Morgan for
collection of XPS data.
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Figures and Tables:
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Figure 1. Schematic representation of the possible reaction pathways for the liquid phase
hydrogenation of FF under mild conditions in the presence of supported Pd nanoparticles.
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100
80
60
40
20
0
Conversion
Yield of THFA
1.57%Pd/Fe O
Pd/Fe2O3
1.51%Pd/Al O
1.47 %Pd/C
Pd/C
1.19%Pd/TiO
Pd/TiO2
1.34%Pd/MgO
Pd/MgO
Pd/Al2O3
3
4
2
3
2
5
6
7
Figure 2. Effect of support on the yield of THFA produced in the liquid phase hydrogenation
of FF. Reaction conditions: 0.3 M FF (15 ml), substrate/metal molar ratio = 500, 3 bar H2, 30
°C, 180 min.
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Figure 3. TEM images and corresponding particle size distribution for: (a) 0.57 % Pd/TiO2
and (b) 1.19 % Pd/TiO2
.
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