Conversion of furfuryl alcohol into 2-methylfuran at room temperature using Pd/TiO2 catalyst

Article abstract of DOI:10.1039/c4cy00184b

The selective hydrogenation of furfuryl alcohol into 2-methylfuran was investigated at room temperature using palladium supported catalysts. We have shown that Pd-TiO₂ catalysts can be very effective for the synthesis of 2-methylfuran at room t

Full text of DOI:10.1039/c4cy00184b

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Conversion of furfuryl alcohol into 2-methylfuran
at room temperature using Pd/TiO2 catalyst
Cite this: Catal. Sci. Technol., 2014,
4, 2280
Sarwat Iqbal,a Xi Liu,a Obaid F. Aldosari,a Peter J. Miedziak,a Jennifer K. Edwards,a
Gemma L. Brett,a Adeeba Akram,a Gavin M. King,a Thomas E. Davies,b
a
a
a
*
David J. Morgan, David K. Knight and Graham J. Hutchings
Received 12th February 2014,
Accepted 14th April 2014
The selective hydrogenation of furfuryl alcohol into 2-methylfuran was investigated at room temperature
using palladium supported catalysts. We have shown that Pd–TiO2 catalysts can be very effective for the
synthesis of 2-methylfuran at room temperature and low pressure of hydrogen (1–3 bar). The effect of
various reaction conditions (pressure, catalyst amount, and solvent) was studied.
DOI: 10.1039/c4cy00184b
www.rsc.org/catalysis
Furfuryl alcohol is an important bio-derivative obtained from
hydrogenation of furfural-a product of xylose. This compound
Introduction
One of the most plentiful resources of renewable energy that
exists in the world is biomass. The current challenge for
researchers in both industry and academia is the development
of efficient technologies that utilize biomass or biomass-
derived chemicals to a large extent.1–4 The platform molecules
that feature in a large proportion of the current research are
based on intermediates such as 5-hydroxymethylfurfural
(HMF), furfural, furfuryl alcohol (FA), gamma valerolactone
(GVL), and attractive bio fuels e.g. 2-methyl furan (2-MF), and
2-methyltetrahydrofuran (2-MTHF).1,5–7 HMF is considered as
one of the most versatile molecule for a number of reasons. All
the 6 carbon atoms originally present in hexoses are retained
in this molecule, and can be prepared with high selectivity
from fructose. It is a parent molecule of many useful chemicals
like caprolactone,8 2,5-dimethylfuran,9 2,5-furandicarboxylic
acid10 etc.
Furan derivatives are considered to be important interme-
diates because of their rich chemistry with carbohydrates
being the most dominant source of these platform molecules.
Recently, efforts have been made in the conversion of carbo-
hydrates into furan derivatives in the form of furfural and
furfuryl alcohol.11–14 One of the methods used to get fuels
from the renewable feedstock resource is by its pre-treatment
through hydrolysis of the cellulosic and hemicellulosic com-
ponents to produce sugar alcohols, such as sorbitol, xylitol,
mannitol, lactitol or maltitol15–20 which are consequently
converted into fuel components via chemical reactions.21–24
has shown a variety of applications in chemical industry.25–32
A schematic pathway is shown in Fig. 1 for the synthesis of
various derivatives from lignocellulose. There are some studies
reported on conversion of furfuryl alcohol into various fuel
derivatives e.g. 1,5-pentanediols,33 tetrahydrofurfuryl alcohol,34
and methyl furan.4 All these derivatives are important in terms
of their application, but their synthesis under green conditions
represents a significant challenge. 2-MF is obtained from the
hydrogenation of FA and furfural. It is mainly used for the syn-
thesis of crysanthemate pesticides, perfume intermediates, and
chloroquine lateral chains in medical applications.35,36 Almost
all the reported work on FA hydrogenation is performed using
harsh reaction conditions. Adkins et al.33 in 1931, for the first
time reported the hydrogenolysis of FA using a copper chro-
mite catalyst. The reaction was carried out in the liquid phase
for 11.5 h at 100 °C and 100–150 bar pressure hydrogen. They
reported a yield of 70% to pentanediol under these conditions.
Pure Energy Corporation patented a process for the synthesis
of 2-MF using chromium-based catalysts.37 The main problem
with chromium catalysts is toxicity which causes severe envi-
ronmental pollution.25 Dunlop and Schegulla34 patented a
process for transforming FA into tetrahydrofurfuryl alcohol
with a nickel-based catalyst. Nickel catalysts have been
reported to be selective for the synthesis of tetrahydrofurfuryl
alcohol from FA under very harsh reaction conditions29,38,39
(180 °C, and 3.5 h). Sitthisa et al.4 have reported hydrogena-
tion of FA with Ni–Fe/SiO2 catalyst at 250 °C, 1 bar, with high
selectivity to 2-MF and furan. Increasing the loading of nickel
increased the selectivity to 2-MF. Zhu et al.40 have reported an
efficient synthesis of 2-MF and GBL over copper-based catalysts
from furfural at 240 °C. Ruthenium metal is known to be an
active hydrogenation catalyst for a number of hydrocarbons.
Zhang et al.29 have reported the catalytic activity of Pt, Pd and
a Cardiff Catalysis Institute, School of chemistry, Main building, Park Place,
Cardiff, CF10 3AT, UK. E-mail: hutch@cardiff.ac.uk; Fax: +44 (0)2920874059;
Tel: +44 (0)2920874059
b Stephenson Institute for Renewable Energy, Chemistry Department,
The University of Liverpool, Crown Street, Liverpool, L69 7ZD, UK
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Fig. 1 Reaction pathway for the formation of various derivatives from lignocellulose biomass.
Ru supported on MnOx for the hydrogenation of FA at 120 °C,
30–60 bar, for 4 h in an aqueous phase. A high selectivity of
tetrahydrofurfuryl alcohol was achieved using Pd/MnOx and
Rh/MnOx catalysts. The Ru/MnOx catalyst was reported to be
more selective towards 1,2-pentanediol under these reaction
conditions. Pd, Rh, and Pt supported catalysts are reported to
be less active compared with nickel even at higher temperature
and pressure.41,42 Zhong et al.35 have reported 85% yield of
2-MF from furfural at 212 °C with copper-based catalysts. These
observations show that a process for the selective synthesis of
2-MF under green reaction conditions is a challenge, because
of the possibility of formation of various by products. In the
current work we report an application of Pd-supported catalyst
synthesized by wet impregnation method in selective biomass
hydrogenation and we discuss the catalytic performance of
Pd catalysts under different reaction conditions such as
variation in pressure, solvent and amount of catalyst on the
hydrogenation of FA at room temperature. The unique catalytic
activity and preferable selectivity to hydrogenated deoxygen-
ation of CO, and not the reduction of CC in this kind of
furan substrates has not been reported before, under green
conditions.
Experimental
Materials
Furfuryl alcohol (98%), 2-methylfuran (98%), 1, 2-dichloroethane
(98%), and all the intermediates were purchased from Sigma
Aldrich and used as received. Palladium chloride was pur-
chased from Johnson Matthey. Titania was purchased from
Degussa. Pure hydrogen (99.9%) was from BOC, 5% Pd/Al2O3
was purchased from Johnson Matthey.
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Catalyst preparation
Catalytic testing
Incipient wetness impregnation method. Catalysts supported
on titania were prepared using the standard wet impregnation
method. The preparation of desired loading of 1, 2.5, and 5%
Pd on titania is described as follows (all quantities stated are
per g of finished catalyst). PdCl2 (0.0166 g, Johnson Matthey)
was added to deionised water and stirred at 80 °C until the Pd
dissolved completely. The support (0.99 g; titania (Degussa))
was added to the solution and stirred to form a paste. The paste
was dried (110 °C, 16 h) and calcined in static air (400 °C, 3 h).
The MP-AES analysis showed less amount of Pd on support, so
the actual loading is discussed in all relevant sections.
Furfuryl alcohol hydrogenation. The reactor was charged
with furfuryl alcohol (1 g), dichloroethane (20 ml) and
catalyst (0.1 g). The autoclave was sealed, pressurised with
hydrogen (1–3 bar, continuously controlled or constant
pressure), and stirred (1000 rpm) for 30–120 min at room
temperature. The reaction mixture (after centrifuging the
sample to isolate the catalyst) was analysed by GC (Varian
3800 fitted with CP wax column). Products were identified by
comparison with authentic samples. For the quantification of
the amounts of reactant consumed and products generated,
an external calibration method was used. External standard
was acetonitrile.
Characterization
Results and discussion
Effect of palladium loading
Microwave plasma atomic emission spectroscopy (MP-AES).
The actual metal loadings were performed using Agilent 4100
MP-AES (Microwave Plasma Atomic Emmision Spectroscopy)
for all catalysts. About 5 mg catalysts were digested using 20%
aqua regia, and all the particles in the solution were removed
before the analysis.
X-ray diffraction (XRD). Powder XRD was carried out using
a PANalytical X'Pert Pro with a CuKα X-ray source run at
40 kV and 40 mA fitted with an X'Celerator detector. Each
sample was scanned from 2θ = 10 to 80 for 30 min. The
catalysts were ground into fine powder form and loaded on a
silicon wafer. The results obtained were compared with the
information in ICDD library for each catalyst.
Various loadings of palladium metal supported on titania
have been studied and a variation in activity of catalysts is
observed with respect to conversion and selectivity both. The
results are presented in Table 1. An increase in loading of Pd
metal from 0.50% to 2.58% increases the activity of catalyst
and selectivity to 2-MF. The reactions were performed at
room temperature and hydrogen pressure was 1 bar (constant
pressure). The side product was tetrahydrofurfuryl alcohol.
High resolution transmission electron microscopy
(HRTEM). Transmission electron microscopy (TEM) was
performed on a Jeol 2100 microscope operated at 200 kV
fitted with an Oxford Instruments EDX analyser. Samples
were prepared by dispersion in methanol and dropped on to
holey carbon film, 300 mesh copper grids. Particle size
distributions (PSD) were determined by counting 150 particles
using Image J software.
X-ray photoelectron spectroscopy (XPS). Samples were
characterized using a Kratos Axis Ultra-DLD photoelectron
spectrometer, using monochromatic Al Kα radiation, at 144 W
(12 mA × 12 kV) power. High resolution and survey scans were
performed at pass energies of 40 and 160 eV respectively.
Spectra were calibrated to the C (1s) signal for adventitious
carbon at 284.7 eV and quantified using CasaXPS v2.3.15,
utilizing sensitivity factors supplied by the manufacturer.
Fig. 2 XRD patterns of TiO2 supported Pd with different loading.
a. Pd = 0.50%, b. Pd = 1.26%, c. Pd = 2.58%.
Table 1 Effect palladium loading on catalysis for hydrogenation of furfuryl alcohola
Selectivity (%)
Catalysts
Conversion (%)
2-Methylfuran
Tetrahydrofurfuryl alcohol
TON h−1
b
0.50% Pd/TiO2
1.26% Pd/TiO2
2.58% Pd/TiO2
21.4
46.2
65.1
100
49
5.1
7.5
13.8
—
76
124
80
92.5
85.2
17
b
5% Pd/Al2O3
12
a
b
Reaction conditions: substrate (1 g), catalyst (100 mg), C2H4Cl2 (20 ml), autoclave reactor, 25 °C, 3 h, 1000 rpm, 1 bar H2. Rest of the
products are dimers. TON = Turn over number.
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Blank reaction, and the reaction with pure titania showed no
conversion under these conditions. One commercial catalyst
5% Pd/Al2O3 was also tested in order to make a comparison
with titania but it showed very poor selectivity to the desired
product and the main products were polymerized hydrocarbons.
The XRD pattern of supported Pd catalysts with different
loadings is shown in Fig. 2. No significant difference was
observed from 0.50 wt% to 2.58 wt% Pd loading and no peak
can be assigned to diffraction of crystalline Pd particles. The
TiO2 used in present work is mixture of anatase and rutile,
which keeps stable during preparation process. It suggests no
big particles of Pd compounds (metallic Pd or PdO) are
formed during preparation.
1.26% and 2.58% Pd/TiO2. We did not find any large palla-
dium particles in microscopic images. It keeps a good agree-
ment with XRD results. In the HRTEM images, very small
particles were observed in abundance in all three samples,
which cannot be identified by XRD due to their lower mean
particle size (<2 nm). This combined with the XRD results,
means the impregnation method produces very small nano-
particles (<2 nm). In this case we tried to identify detailed
information about the smaller particles we observed in the
localized TEM images considering that the smaller particles
might have higher activity for the catalytic results.43–45 The
particle size distribution of the three Pd catalysts with differ-
ent loading were analysed. As can be observed in Fig. 3 there
is very little difference in the particles size with increased
loading, with all catalysts displaying an average particle size
in the order of 1 nm. Given the uniformity of particle size
Particle size of the metal nanoparticles in these supported
catalysts is an important parameter which can affect the
activity. Fig. 3 shows TEM images for catalysts with 0.5%,
Fig. 3 Transmission electron micrographs and particle size distribution for titania supported palladium catalysts; a) 0.5% PdTiO2, b) 1.26% PdTiO2,
c) 2.58% PdTiO2.
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the results are shown in Table 4. There is no remarkable
leaching observed in all three catalysts.
Reaction conditions
Effect of hydrogen pressure. 2.58% Pd/TiO2 was tested at
varying pressures of hydrogen from 1 to 3 bar at room
temperature. Separate studies were performed with constant
pressure (reactor was charged at desired pressure without
refilling during the reaction) and continuously controlled
pressure (pressure maintained throughout reaction). The
reactions were performed for 30 min. In constant pressure
reactions less conversion was observed overall compared with
the continuously controlled pressure reactions as shown in
Table 2. This could be because of the fact that more
hydrogen was available in the latter case and the amount of
hydrogen was limited in constant pressure reactions.
Fig. 4 XPS profiles of TiO2 supported Pd with different Pd loading.
a. Pd = 0.50%, b. Pd = 1.26%, c. Pd = 2.58%.
A further increase in pressure from 1 to 3 bar in case of
continuously controlled pressure showed an enhancement in
activity of FA hydrogenation in both cases. At 3 bar continu-
ous pressure almost full conversion was observed with very
high selectivity to 2-MF. The only by-product detected was
tetrahydrofurfuryl alcohol. So the amount of hydrogen avail-
able in the environment is an important factor for achieving
higher selectivity.
distribution the increase in catalytic activity can be linked to
the increase in active metal concentration and consequently
the availability of active sites.
Fig. 4 shows deconvoluted XPS profiles of TiO2-supported
Pd catalysts with different loadings of Pd. Clearly, two species
of Pd are observed to be present on the surface of these cata-
lysts, one is Pd2+ and another is Pd0, with binding energies of
337.6 eV and 335.8 eV, respectively. In all Pd/TiO2 catalysts
Pd2+ is the majority species and there is only trace amount of
metallic Pd. In different samples, the ratio of Pd2+ to Pd0 is
maintained, ranging from 12 to 15. The predominant
presence of Pd2+ suggests active sites might not be metallic
Pd, but Pd cations, which might have a similar function like
co-ordinated metal centres for H2 activation and reduction of
carbonyl group.46 It also explains why CO, not CC, is
preferably reduced since metallic Pd is preferably active
for CC bond reduction. Despite of different loading, a simi-
larity in chemical nature of three supported Pd samples was
confirmed by XRD, HRTEM and XPS. It suggests the change
in catalytic performance should only be related with the
amount of same activity sites.
Fig. 5 Effect of catalyst mass on the conversion of FA. Reaction
conditions: 1000 mg of substrate, 20 ml C2H4Cl2, autoclave reactor,
25 °C, 30 min. 1000 rpm, 3 bar.
In order to examine the stability of catalyst during reaction
an analysis of metal loading was performed using MP-AES,
Table 2 Effect of hydrogen pressure on palladium catalysts for the hydrogenation of furfuryl alcohola
Selectivity (%)
Pressure (bar)
Time (min)
Conversion (%)
2-Methylfuran
Tetrahydrofurfuryl alcohol
Constant pressure
1
2
3
1
2
3
30
30
30
30
30
30
28.5
50.3
73.1
54.9
53.1
93.7
97.7
97.7
96.6
98.2
96.8
93.6
2.3
2.3
2.8
1.8
3.2
5.7
Continuously controlled pressure
a
Reaction conditions: substrate (1 g), 2.58% Pd/TiO2 (100 mg), C2H4Cl2 (20 ml), autoclave reactor, 25 °C, 1000 rpm
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Table 4 MP-AES data of fresh and used catalysts
Theoretical loading
Fresh
Used
Pd (wt%)
1
Pd (wt%)
0.50
Pd (wt%)
0.51
2.5
5
1.26
2.58
1.36
2.5
1,2-dichloroethane. Very limited reduction was observed
when using acetonitrile and toluene as solvents. These
observed solvent effect can be related with stability of the
surface adsorbed H species generated from H2 dissociation
in presence of the solvents. Interaction between H2 and H
with the solvents could manipulate activity of the H species
and then influence different reaction pathways.48,49
Fig. 6 Effect of catalyst mass on the selectivity of 2-MF from FA.
Reaction conditions: substrate (1 g), C2H4Cl2 (20 ml), autoclave reactor,
25 °C, 30 min, 1000 rpm, 3 bar. (▲) 2-Methytetrahydrolfuran, (♦)
2-methylfuran, (■) tetrahydrofurfuryl alcohol.
Conclusions
We have demonstrated that FA can be converted selectively
into 2-MF at room temperature using very low pressure of
hydrogen with Pd supported catalysts. In this work highly
active Pd catalysts with an abundance of very small particles
(<2 nm) were prepared via a very simple impregnation
method. By using these Pd catalysts, only O–H hydrogenated
deoxygenation in furfuryl alcohol happened, CC was very
less reduced under these reaction conditions, wherein the
only byproduct detected was tetrahydrofurfuryl alcohol <6%.
The simpler preparation method and higher catalytic activity
Effect of the catalyst amount. Increasing the amount of
catalyst improved the overall catalytic performance, as shown
in Fig. 5 and 6. The reaction was found to be very selective to
2-MF, when we used 100 to 125 mg 2.58 wt% Pd/TiO2. The
conversion kept increasing, but selectivity decreased slightly
when 150 mg of catalyst was used. This can be attributed to
side reactions due to the excess amount of catalyst. It confirms
the very high selectivity observed using our Pd catalyst for the
reduction of CO to produce hydrocarbons, whereas the CC
reduction is almost inhibited even with the higher catalyst
amounts. It suggests a different reaction pathway on the
surface of the supported Pd catalysts, compared with the gas
phase reduction of unsaturated hydrocarbons or aldehyde by
using Pd catalysts.47
Effect of solvent. A comparison among different solvents
was conducted and the results are shown in Table 3.
Dichloroethane gives the best catalytic performance, and is
very selective to 2-MF with very low production of tetrahydro-
furfural alcohol. However, the reaction with octane shows
very high activity but is not very selective for the formation of
2-MF. There are number of products which could not be
identified properly. The GC-MS traces have indicated longer
chain alcohols, and some dimers as well, and clearly the
reaction was not very selective to 2-MF as is observed with
of the Pd catalysts suggests
a great potential in this
important area. The selective synthesis of 2-MF is a particu-
larly attractive approach. The current study has reported a
process which provides a possibility of synthesis of 2-MF, an
important petroleum derivative using reaction conditions
that minimise the consumption of energy in terms of heating
and hydrogen, utilizing a very simple catalyst preparation
method.
Acknowledgements
The authors would like to thank EPSRC (EP/K014854/1) and
the Research Campus at Harwell for access to the transmis-
sion electron microscope. Obaid F. Aldosari would like to
thank Royal embassy of Saudi Arabia for funding his PhD.
We are thankful to Samuel Pattison for MP-AES analysis.
Table 3 Effect of solvent on furfuryl alcohol hydrogenationa
Selectivity (%)
Solvents
Conversion (%)
2-Methylfuran
Tetrahydrofurfuryl alcohol
Unidentified products
Acetonitrile
Toluene
14.9
3.4
100
100
0
0
0
0
Octane
1,2-Dichloroethane
88.4
93.7
58.04
93.6
2.2
5.7
39.8
0
a
Reaction conditions: substrate (1 g), 2.58% Pd/TiO2 (0.1 g), solvent (20 ml), autoclave reactor, 25 °C, 30 min, 1000 rpm, 3 bar H2.
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27 M. Hronec, K. Fulajtarova and T. Sotak, J. Ind. Eng. Chem.,
2014, 20, 650–655.
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