Liquid phase chemo-selective catalytic hydrogenation of furfural to furfuryl alcohol

Article abstract of DOI:10.1016/j.apcata.2012.12.010

Novel Cu:Zn:Cr:Zr based catalysts were developed for the hydrogenation of furfural to furfuryl alcohol. Physio-chemical characterizations of the catalysts were performed by using XRD, BET, FTIR, TPR, NH₃-TPD, ICP-MS, SEM, TEM, CO-chemisorption, and XANES techniques. Among all the catalysts prepared, the catalysts Cu(3):Zn(2):Cr(1):Zr(3) and Cu(3):Zn(2):Cr(1):Zr(4), referred as Cat-C and Cat-D, respectively are the best ones to demonstrate high activity and selectivity profile. Cat-C and Cat-D exhibited 100% conversion and 96% selectivity at 170 ± 2 °C and 2 MPa of hydrogen pressure. The role of constituent metals in the catalyst was delineated. Incorporation of Zn increases the activity for furfural conversion whereas Zr contributes significantly to the selectivity of furfuryl alcohol. It was also found that Zr loading not only increases the acidity of the catalyst but also helps in the dispersion of metallic Cu. The particle size of metallic Cu was found to be in the range of 17-19 nm as confirmed by TEM, XRD and CO chemisorption techniques. XANES analysis confirmed the presence of copper in Cu⁰ and Cu ²⁺ oxidation states in Cat-C (freshly reduced) and Cat-C (fresh), respectively. Hydrogenation of furfural to furfuryl alcohol follows a pseudo-first order reaction with an the apparent activation energy of 24.4 kcal/mol. Cat-C was recycled at least 4 times for the hydrogenation of furfural with no loss of activity and selectivity when compared to the fresh catalyst.

Full text of DOI:10.1016/j.apcata.2012.12.010

Applied Catalysis A: General 454 (2013) 127–136
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Liquid phase chemo-selective catalytic hydrogenation of furfural to
furfuryl alcohol
Rajesh V. Sharma, Umashankar Das, Ramaswami Sammynaiken, Ajay K. Dalai∗
Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, SK,
S7N 5A9, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel Cu:Zn:Cr:Zr based catalysts were developed for the hydrogenation of furfural to furfuryl alcohol.
Physio-chemical characterizations of the catalysts were performed by using XRD, BET, FTIR, TPR, NH3-
TPD, ICP-MS, SEM, TEM, CO-chemisorption, and XANES techniques. Among all the catalysts prepared, the
catalysts Cu(3):Zn(2):Cr(1):Zr(3) and Cu(3):Zn(2):Cr(1):Zr(4), referred as Cat-C and Cat-D, respectively
are the best ones to demonstrate high activity and selectivity profile. Cat-C and Cat-D exhibited 100%
conversion and 96% selectivity at 170 2 ◦C and 2 MPa of hydrogen pressure. The role of constituent
metals in the catalyst was delineated. Incorporation of Zn increases the activity for furfural conversion
whereas Zr contributes significantly to the selectivity of furfuryl alcohol. It was also found that Zr loading
not only increases the acidity of the catalyst but also helps in the dispersion of metallic Cu. The particle size
of metallic Cu was found to be in the range of 17–19 nm as confirmed by TEM, XRD and CO chemisorption
techniques. XANES analysis confirmed the presence of copper in Cu0 and Cu2+ oxidation states in Cat-C
(freshly reduced) and Cat-C (fresh), respectively. Hydrogenation of furfural to furfuryl alcohol follows a
pseudo-first order reaction with an the apparent activation energy of 24.4 kcal/mol. Cat-C was recycled
at least 4 times for the hydrogenation of furfural with no loss of activity and selectivity when compared
to the fresh catalyst.
Received 17 July 2012
Received in revised form
26 November 2012
Accepted 1 December 2012
Available online 21 December 2012
Keywords:
Furfural
Furfuryl alcohol
Heterogeneous catalyst
Hydrogenation
Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction
[4,5,9]. Baijun et al. [10] reported that the addition of heteropoly-
acids improves selectivity and conversion profile of furfural to
Chemicals derived from biomass have attracted a consider-
able interest in the recent years. A wide range of chemicals such
as furfural, 5-hydroxymethyl furfural, sorbitol, levulinic acid, and
glucaric acid are obtained from agricultural waste. The acid degra-
dation of agricultural waste at higher temperatures produces
furfural as a major product which is used as a key intermedi-
ate in the preparation of several non-petroleum based chemicals
[1–3]. Furfuryl alcohol produced by the hydrogenation of furfural
is widely used in fine chemical and polymer industries. The major
applications of furfuryl alcohol are the production of thermostatic
resins, acid proof bricks, corrosion resistant fiber glass, corrosion
resistant polymer concrete and liquid resins for galvanic bath tub.
Furfuryl alcohol is also an important intermediate chemical in the
synthesis of lysine, vitamin C, lubricants, dispersing agents, plasti-
cizer and tetrahydrofurfuryl alcohol [4–8].
furfuryl alcohol. Cu based catalysts are often more preferred for
high activity and selectivity for hydrogenation of furfural to fur-
furyl alcohol, however it generates a low yield of 2-methyl furan as
a by-product [11–13]. Other catalysts such as Raney nickel, amor-
phous nickel alloys, Cu–Zn mixed oxide with Al, Fe, Mn, Cu–MgO
and Pt on different supports have also been used for hydrogenation
of furfural [14–16]. Pd based catalysts trigger decarbonylation of
furfural to produce furan and also, generate further hydrogenated
by-products. However, major disadvantages of these catalysts are
that these catalysts promote unwanted side reactions and also,
cannot be reused. The Cu–Cr catalyst, at high temperature and
pressure, is used in industry for the hydrogenation of furfural
with moderate activity and selectivity to produce furfuryl alcohol
[17–20]. Rao et al. reported that copper chromite catalyst reduced
at 300 ◦C provides 60% furfural conversion and 70% furfuryl alcohol
selectivity [21].
Liquid phase hydrogenation of furfural to furfuryl alcohol has
been extensively studied with Ni, Co, Ru, Pt and Pd based catalysts
Fig. 1 illustrates the formation of furfuryl alcohol and sev-
eral other byproducts from the hydrogenation of furfural which
is dependent on the type of catalyst used. Primary reactions occur
by the reduction of C O group or furan ring, whereas the secondary
reactions include several by-products such as hydrogenolysis of
∗
Corresponding author. Tel.: +1 306 966 4771; fax: +1 306 966 4777.
E-mail address: ajay.dalai@usask.ca (A.K. Dalai).
0926-860X/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.12.010

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R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
same procedure and by using zirconium nitrate 5.1 g, 10.2 g and
20.4 g, Cat-A, Cat-B, and Cat-D were prepared with the elemen-
tal molar ratios of Cu(3):Zn(2):Cr(1):Zr(1), Cu(3):Zn(2):Cr(1):Zr(2),
and Cu(3):Zn(2):Cr(1):Zr(4), respectively. The catalysts Cu:Cr (3:1),
Cu:Zn:Cr (3:1:1), Cu:Zn:Cr (3:2:1), and Cu:Zn:Cr (3:3:1) were also
prepared as per the above mentioned procedure using the corre-
sponding metal nitrates.
2.3. Catalyst characterization
2.3.1. X-ray diffraction (XRD) analysis
All the catalysts were characterized by a large angle X-ray
scattering to determine different phases present. The X-ray diffrac-
tion patterns were recorded on a D8 diffractometer using Cu K␣
radiation and a Bruker Smart 6000 CCD detector. The X-ray diffrac-
tograms were obtained with a step rate of 0.005◦ with 300 s/step
from 10 to 100◦.
Fig. 1. Formation of furfuryl alcohol and other by-products from hydrogenation of
furfural.
C
O bond, decarbonylation and hydrogenation of furan ring. In
addition, formation of a condensation product such as furfuryl ether
is also reported in the literature [5,6].
2.3.2. N2 adsorption–desorption isotherms
Nitrogen physisorption isotherms were obtained using
a
In this study, liquid phase hydrogenation of furfural to furfuryl
alcohol is reported by using novel Cu:Zn:Cr:Zr based catalysts with
elemental molar ratios of 3:2:1:x where x = 1–4, respectively. The
main objective was to investigate the influence of zirconium metal
in this catalyst matrix on the activity and selectivity of the forma-
tion of furfuryl alcohol. In addition, various reaction parameters
such as catalyst composition, speed of agitation, catalyst loading,
hydrogen pressure, furfural concentration, and reaction temper-
ature were studied to optimize the furfural conversion, furfuryl
alcohol selectivity, and to determine kinetic parameters for cataly-
sis reaction.
Micromeritics ASAP 2000 analyzer. The catalyst was out-gassed
in vacuum at 200 ◦C prior to the analysis until the static pressure
remained less than 6.6 × 10−4 Pa. The Brunauer–Emmett–Teller
(BET) method was used to calculate the surface area in the range of
relative pressures between 0.01 and 0.20 Pa. In all cases, the corre-
lation coefficients were above 0.999. All the samples were analyzed
in triplicate and the results were within 2 error range.
2.3.3. FT-IR spectroscopy (FTIR)
Fourier transform infrared (FTIR) spectroscopy was carried out
by using a PerkinElmer (Wellesley, MA, USA) Spectrum GX instru-
ment equipped with DTGS detector and a KBr beam splitter. The
spectrum for each analysis was averaged over 16 scans with a res-
olution of 4 cm−1 by using spectroscopic grade potassium bromide
(KBr) in the range of 375–4000 cm−1 wave numbers.
2. Experimental
2.1. Chemicals and gases
The following chemicals were purchased and used without
further purification: Furfural (reagent grade, 99.99% purity from
VWR, Canada), copper (II) nitrate trihydrate (purity ≥ 99% from
VWR, Canada), zinc (II) nitrate hexahydrate (reagent grade, 98%
purity from VWR, Canada), chromium nitrate nonahydrate (reagent
grade ≥ 98% purity, from VWR, Canada), zirconium nitrate (reagent
grade ≥ 98% purity, from VWR, Canada), potassium carbonate
(lab grade ≥ 98% from VWR, Canada), furfuryl alcohol (reagent
grade ≥ 99% purity from VWR, Canada) and 1-octanol (reagent
grade ≥ 99% purity from Sigma–Aldrich). Hydrogen, helium, nitro-
gen and air with high purity grade, 99.99% were purchased from
Praxair, Saskatoon, Canada.
2.3.4. Temperature programmed reduction (TPR)
H2-temperature programmed reduction (H2-TPR) profile of the
catalysts was conducted by using a TPD/TPR Quantachrome (USA)
instrument for analyzing the nature of reducible metal species
present in the samples. For a typical experiment, 100 mg sample
was taken in a quartz tube and purged with helium at 200 ◦C for
1 h to remove impurities on the surface of the sample. The sample
was cooled down to the room temperature under flowing helium
and the TPR was carried out using 3% H2/N2 (v/v) mixture at a flow
rate of 20 mL/min. The heating rate was 10 ◦C/min starting from
room temperature to 800 ◦C. The TPR profile was recorded using a
thermal conductivity detector.
2.2. Catalyst synthesis
2.3.5. Temperature programmed desorption (TPD)
NH3-temperature programmed desorption (NH3-TPD) of the
catalysts was carried out using a TPD/TPR Quantachrome (USA)
instrument for analyzing the acidity present in the catalyst. 100 mg
of the sample was placed in a quartz tube and purged with helium
at 550 ◦C for 1 h. The sample was cooled down to the room tem-
perature under flowing helium and saturated with 3 wt% NH3 in N2
at a flow rate of 30 mL/min for 120 min. Physisorbed ammonia was
removed by flowing helium at 100 ◦C for 1 h. Analysis was carried
out by heating the sample at 10 ◦C/min from 100 to 600 ◦C. The TPD
profile of the catalysts was recorded using a thermal conductivity
detector.
Co-precipitation method was used to prepare the catalysts with
different elemental molar ratios. Copper nitrate (Cu(NO3)2·3H2O,
23 g), chromium nitrate (Cr(NO3)2·9H2O, 15.4 g), zinc nitrate
(Zn(NO3)2·6H2O, 18.2 g) and zirconium nitrate (Zr(NO3)2·xH2O,
15.3 g) were dissolved in 1000 mL of distilled water. The solu-
tion was precipitated with 1 M solution of potassium carbonate
with continuous vigorous stirring. Potassium carbonate solution
was added till pH of the solution reached the range of 9–10. The
precipitate was aged at 70 2 ◦C for 5 h. The reaction mixture
was cooled to 25 2 ◦C, the precipitate was filtered and washed
thoroughly with distilled water till neutral pH. The solid cake
was dried at 120 2 ◦C for 12 h in an oven. Finally, the solid
material was calcined at 650 10 ◦C for 3 h to provide 22 g of
the catalyst Cu(3):Zn(2):Cr(1):Zr(3), referred as Cat-C (elemen-
tal molar ratios are presented in parenthesis). Following the
2.3.6. Scanning electron microscopy (SEM)
The powder samples were grounded in an agate mortar. A few
particles were then deposited on 200 mesh copper grids covered

R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
129
with a holey carbon film. The scanning electron microscopy (SEM)
micrographs were obtained on a Hitachi-S4700 microscope.
2.3.7. Transmission electron microscopy (TEM)
The morphological features of the catalysts were studied from
electron micrographs obtained with a Philips CM10 (1990) scan-
ning transmission electron microscope. The powder samples were
grounded softly in an agate mortar and dispersed in ethanol in
an ultrasonic bath for 1 min. A few drops of this suspension were
deposited on 200 mesh copper grids. The electron micrographs
were recorded on films. The films were developed to get the images.
2.3.8. Inductive coupled plasma-mass spectroscopy (ICP-MS)
The elemental molar ratios of the fresh and regenerated cata-
lysts were measured by ICP-MS. A sample of the catalyst (0.1 g)
was dissolved in concentrated hydrofluoric acid (48–51%) at a tem-
perature of 100–150 ◦C for two days. After cooling, concentrated
HNO3 was added to it to ensure the complete dissolution of the
metals. The final solution was prepared by using 0.2 N HNO3 acid
and analyzed with a mass spectrometer.
Fig. 2. XRD patterns of the catalysts: (1) Cat-A, (2) Cat-B, (3) Cat-C, (4) Cat-D, (5)
used Cat-C, (6) freshly reduced Cat-C, and (7) regenerated Cat-C.
desired value and aliquot of 1 mL of the sample was collected at an
interval of 15 min. A constant hydrogen pressure was maintained
throughout the reaction. A standard reaction was carried out with
using furfural (12.43 mL, 0.15 mol), isopropyl alcohol (87.57 mL),
catalyst (1.5 g) at 170 ◦C, H2 pressure of 2 MPa and agitation speed
of 1000 rpm. All the experiments were carried out in triplicate for
reproducibility of the data and the results were within an error of
5%.
2.3.9. CO chemisorption
A Micromeritics ASAP 2020 Chemisorption system was used
to measure the carbon monoxide uptake on the oxide catalysts.
Prior to the CO chemisorption measurements, the sample was put
under vacuum and backfilled with helium at 35 ◦C and then evac-
uated at 110 ◦C for 60 min. The sample was then heated to 350 ◦C
at 10 ◦C/min in the presence of hydrogen and was maintained at
350 ◦C for 2 h, and was then cooled to 35 ◦C and evacuated until the
static pressure remained less than 1.3–10.5 Pa. The chemisorption
analysis was performed by passing pulses of CO over the sample to
measure total CO uptake at 35 ◦C. The following formula was used
to calculate the percentage of metal dispersion of the active metal
available for interaction with the adsorbate.
2.5. Method of analysis
The samples were analyzed using a gas chromatograph system
(Agilent 7890A) with flame ionizing detector (FID). The stabilwax
capillary column with 30 m length and 0.25 mm inside diameter
with 0.5 ␮m film thickness was used. The oven temperature was
programmed to begin at 80 ◦C for 4 min and ramped to 240 ◦C at
20 ◦C/min with a final hold time of 4 min. One microliter of the
sample was injected into the column with a split ratio of 10:1.
Helium was used as a carrier gas. The conversion of furfural and
selectivity of furfuryl alcohol was monitored by analyzing the sam-
ples at 30 min intervals. The results have an error of 3% throughout
the experiments. The formation of furfuryl alcohol was confirmed
by GC–MS, 1H NMR and 13C NMR. The selectivity of furfural alco-
hol found to be same throughout the progress of the reaction. Eqs.
(1)–(3) were used to calculate conversion, product selectivity and
yield, respectively.
1
V × SFCALC
%MDisp
=
22, 414
weight fraction of Cu/WATOMIC,Cu
where % MDisp is metal dispersion (%), V is volume intercept derived
from the best line fit to the volume differences between the selected
points of the first analysis and the repeat analysis (cm3/g STP), and
SFCALC is the calculated stoichiometry factor.
2.3.10. X-ray absorption near-edge structure (XANES)
The copper K-edge XANES of the Cat-C (fresh) and Cat-C
(reduced) were recorded in the transmission mode using two nitro-
gen has filled detectors, at the VESPERS beam line at the Canadian
Light Source. The transmission mode of detection was chosen over
fluorescence because it measures the absorption near the surface
and through the sample. The samples were tightly packed in an
absorption holder to avoid pinholes and at a thickness close to one
absorption length.
initial moles of furfural − final moles of furfural
conversion(%) =
× 100
(1)
(2)
initial moles of furfural
moles of desired product formed
total moles of all products formed
product selectivity(%) =
× 100
yield(%) = % conversion(starting material)
× % selectivity(desired product)
2.4. Experimental set up and reaction procedure
(3)
Experiments were performed in a 300 mL stainless steel Parr
autoclave. A four bladed pitch turbine impeller was used for agita-
tion. Calculated quantity of the catalyst was reduced at 300 1 ◦C
by H2 pressure of 2 MPa for 2 h. The reactor was cooled to room tem-
perature, then hydrogen gas was released. The calculated quantities
of furfural and isopropyl alcohol were charged into the autoclave
which was flushed with N2 and further pressurized with H2 gas
to the desired pressure. The reaction temperature was raised to
the desired value and agitation was started. The total pressure in
the reactor was observed to be higher than the set value because
of autogenous pressure of the reaction mixture. An initial sample
was collected when the temperature in the autoclave reached the
3. Results and discussion
3.1. Characterization of the catalysts
The XRD patterns for the Cat A-D, Cat-C (used), Cat-C (freshly
reduced) and Cat-C (regenerated) are presented in Fig. 2. The pres-
ence of CuO was evident from the peaks at 2ꢀ = 35◦, 38◦ and 47◦
[ICDD no. 00-048-1548], ZrO2 at 2ꢀ = 28◦ and 49◦ [ICDD no. 00-050-
1089], ZnO at 2ꢀ = 31◦, 34◦, 47◦ [ICDD no. 01-070-8070], ZnCr2O4
at 2ꢀ = 28◦ and 38◦ [ICDD no. 00-022-1107] and Cu at 43.16◦ and

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R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
Table 1
BET surface area of various catalysts.a
Catalysts
BET (m2/g)
Cat-A
Cat-B
Cat-C
Cat-D
21.0
20.3
23.1
23.5
38.3
39.5
24.1
Cat C (used)
Cat-C (freshly reduced)
Cat-C (regenerated)
a
The data presented has an error of 2.
50.37◦ [ICDD no. 01-070-3038]. The presence of sharp signals con-
firmed crystalline structure of the catalysts. The intensity of the
ZrO2 peak increased from Cat-A to Cat-D due to an increase in the
zirconium metal contents in Cat A–D (Fig. 2). The XRD patterns
for used and freshly reduced catalyst show the peaks correspond-
ing to metallic Cu. The crystalline size of metallic Cu calculated
by Scherrer equation was found to be 16.9 nm. The intense peak
observed at 2ꢀ = 43◦ in XRD corresponding to metallic Cu in Cat-C
(used) and Cat-C(freshly reduced) was not observed for the regen-
erated Cat-C, indicating that metallic Cu was converted into CuO
during calcination.
The BET surface area of Cat A–D, Cat-C (used), Cat-C (freshly
reduced) and Cat-C (regenerated) are presented in Table 1. The
BET surface areas of the metal oxide Cat A–D were in the range
of 21.0–23.5 m2/g. The BET surface area of the used Cat-C was
dramatically increased to 38.3 m2/g compared to the fresh Cat-
C (23.1 m2/g). The BET surface area of the freshly reduced Cat-C
(39.5 m2/g) was found to be very similar to that of the used Cat-
C which suggests that the reduction of CuO to metallic copper
increases the BET surface area. This was confirmed by XRD (Fig. 2).
The regenerated catalyst (24.1 m2/g) showed very similar BET sur-
face area to that of the fresh Cat-C confirming that metallic copper
is again converted back into CuO by calcination at 650 ◦C. This result
was also supported by the XRD data.
The FTIR spectra of Cat A-D and pure ZrO2 are shown in Fig. 3. A
broad band in the region of 3500 cm−1 could be as a result of mois-
ture absorbed on the catalyst. The IR bands observed at 740 and
541 cm−1 are the characteristics peaks of crystalline zirconia [25].
Yadav et al. reported that zirconium hydroxide undergoes transfor-
mation to crystalline zirconia during calcination at 650 ◦C [26]. The
presence of crystalline zirconia in the catalysts was also observed
from the XRD data. The IR band between 700 and 550 cm−1 was
broad due to an increase in the zirconia content from Cat A–D. This
observation also correlates with by XRD data which indicates that
the peak intensity at 2ꢀ = 29◦ and 49◦ increases with an increase in
the zirconia content.
Fig. 4. TPR profile of Cat-C (fresh), Cat-C (regenerated), Cat-C (used) and Cat-C
(freshly reduced).
Fig. 4 shows the temperature programmed reduction (TPR) pro-
file of Cat-C (fresh), Cat-C (regenerated), Cat-C (used) and Cat-C
(freshly reduced). A single peak observed in the range of 200–400 ◦C
for the fresh catalyst was due to the hydrogen consumption for the
reduction of copper oxide to metallic copper which was in accor-
dance with the literature report [27]. The shape of the peak was
narrow and symmetrical, indicating homogeneous distribution of
copper oxide [28]. The TPR profile of the freshly reduced catalyst
did not show any peak at 200–400 ◦C which confirms the presence
of only metallic Cu in the catalyst Therefore, in the used catalyst,
no peak should be observed at this temperature range. However,
TPR profile of the used catalyst shows two small peaks at the tem-
perature range of 225–265 ◦C (CuO) and 290–400 ◦C (Cu2O). The
presence of a small amount of copper oxides (i.e. CuO and Cu2O)
in the used catalyst was believed to be formed due to the oxida-
tion of metallic copper into copper oxides during recovery of the
catalyst after first run and drying at 100 ◦C. The TPR profile of the
regenerated catalyst shows an additional peak at 340–420 ◦C along
with the main peak which is due to the presence of Cu2O. This
result shows that during calcination process in the regeneration of
the used catalyst, a sequential oxidation process Cu → Cu2O → CuO
takes place which is in agreement with the literature report [29].
The presence of metallic copper in the used and freshly reduced
catalyst was also supported by the XRD data which shows intense
peaks corresponding to metallic copper at 2ꢀ of 43.16◦ and 50.37◦
[ICDD no. 01-070-3038].
Acidic strength of Cat A–D was studied by NH3-TPD and com-
pared against ZrO2 (Fig. 5). Ammonia is a suitable probe molecule
Fig. 3. FTIR spectra: (1) ZrO2, (2) Cat-D, (3) Cat-C, (4) Cat-B, and (5) Cat-A.
Fig. 5. NH3-TPD profile of Cat-A, Cat-B, Cat-C, Cat-D and ZrO2.

R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
131
Fig. 6. SEM micrographs of Cat-C (fresh), Cat-C (used), and Cat-C (regenerated).
Fig. 7. TEM images of Cat-C (fresh), Cat-C (used) and Cat-C (freshly reduced).
to measure acidity of the catalyst due to its small size and basic-
ity. Ammonia interacts at the acidic sites present in the catalyst.
Ammonia desorption temperature is an indicator of the strength of
the acidic sites of the catalyst. Fig. 5 shows that most of ammonia
was desorbed in the range of 250–350 ◦C, indicating the presence of
moderate acidic sites [30]. Cat-C and D demonstrated high ammo-
nia desorption as compared to Cat-A and B, indicating that Cat-C
and D possess higher acidity than Cat-A and B. NH3-TPD profile of
Cat A–D revealed that the acidity of the catalyst increases with an
increase in the zirconia content which influences the selectivity of
furfuryl alcohol. The results in Table 3 indicate that the selectivity
of furfural alcohol increases with an increase in the Zr content from
Cat A–C. Therefore, it is apparent that the selectivity is influenced
by the acidity of the catalyst, i.e. selectivity increases as the acidity
of the catalyst increases.
morphology and particle size of the used catalyst with respect to
the fresh catalyst were observed. This was further explained by
the TEM images (Fig. 7), CO-chemisorption and XRD profile that
indicate the presence of metallic Cu in the used catalyst. The regen-
erated catalyst has some metal agglomerates due to calcination at
650 ◦C, which converts metallic Cu to CuO and was confirmed by
XRD. It was found that the regenerated catalyst retains its external
morphology with an average particle size of 3 ␮m after calcination
at 650 ◦C.
The TEM images of fresh, used and freshly reduced Cat-C are pre-
sented in Fig. 7. It shows that the fresh Cat-C has CuO clusters, while
used and freshly reduced catalysts have well dispersed metallic Cu.
The average particle size for metallic Cu was calculated to be 19 nm
which was in agreement with the average particle size of metallic
Cu obtained from CO-chemisorption studies, i.e. 17 nm and XRD,
i.e. 16.9 nm.
CO-chemisorption results for the dispersion of Cu in Cat-A, Cat-
B, Cat-C and Cat-D are portrayed in Table 7. It was concluded
that the dispersion of copper increased with an increase in Zr
loading, i.e. from Cat-A to Cat-D. Cu crystalline size measured by
CO-chemisorption for Cat-C was found to be 17 nm which was
in agreement with the value obtained from XRD and TEM. CO
adsorbed on the catalysts was increased from Cat-A to Cat-D,
indicating that metallic Cu is more dispersed in Cat-C and Cat-D
compared to Cat-A and B. As the composition of other metals such
as Cu, Cr and Zn was found to be the same in Cat A–D, except Zr, it
was apparent that an increase in Zr loading helps in the dispersion
of Cu metal which might be the reason for increase in selectivity
for furfuryl alcohol.
Bulk elemental molar compositions of Cat-C (fresh) and Cat-
C (regenerated) were analyzed by using ICP-MS and the analysis
results presented in Table 2. The results show that the elemental
molar ratios of the regenerated catalyst were very similar to the
values for the fresh catalyst.
SEM micrographs of Cat-C (fresh), Cat-C (used) and Cat-C (regen-
erated) are represented in Fig. 6. Very little differences in the
Table 2
Elemental analysis of Cat-C by ICP-MS.
Elements
Elemental molar composition
Cu
Zn
Cr
Zr
Cat-C (fresh)
Cat-C (regenerated)
3.04
3.10
2.12
1.91
1.19
0.98
2.91
3.04
X-ray Absorption Near Edge Spectroscopy (XANES) measures
the density of the unoccupied states in the final state of a dipolar

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R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
10% (Table 3). From these experiments, it was concluded that the
Cu:Zn:Cr catalyst with an elemental molar ratio of 3:2:1, respec-
tively was the best catalyst to demonstrate 100% conversion and
moderate selectivity (i.e. 70%) for the hydrogenation of furfural
to furfuryl alcohol. Zirconium metal is known to enhance acidity
of the catalyst, and also increase selectivity of the hydrogenation
reaction [34,35]. Therefore, a decision was made to add Zr to the
Cu(3):Zn(2):Cr(1) catalyst with a view to enhance the selectivity
of furfural alcohol formation. A series of Cu(3):Zn(2):Cr(1):Zr(1–4)
were prepared by varying elemental ratio of Zr from 1 to 4 and
investigated for their potential for furfural conversion and selectiv-
ity of furfural alcohol. The results show that the addition of Zr did
not bring any advantage in terms of conversion of furfural as com-
pared to Cu(3):Zn(2):Cr(1). In all cases, the conversion was 100%
irrespective of the elemental molar ratios of Zr. However, addition
of Zr to Cu(3):Zn(2):Cr(1) influenced the selectivity of the forma-
tion of furfuryl alcohol significantly. An increase in the selectivity of
the reaction was observed with an increase in the elemental molar
ratios of Zr from 1 to 3; however no further increase in the selec-
tivity was observed by increasing the elemental molar ratio of Zr
from 3 to 4. From this study, Cu(3):Zn(2):Cr(1):Zr(3) (Cat C) and
Cu(3):Zn(2):Cr(1):Zr(4) (Cat D) were found to be the best catalysts
for maximum furfural conversion and furfuryl alcohol selectivity.
The potential of Cat-C and Cat-D to display highest conversion and
selectivity is believed from a combined effect of (1) Zn, Cu and
Zr metal, (2) the dispersion of Cu; and (3) the acidity of the cat-
alyst. Based on the elemental molar ratio of Zr used in the catalysts
and the results of activity and selectivity of the reaction, Cat-C was
chosen as the best catalyst for further studies.
Fig. 8. XANES spectrum: (A) Cu foil, (B) Cat-C (freshly reduced), and (C) Cat-C (fresh).
transition. The copper K edge XANES is a measure of the 1s→4p
transition in copper. Oxidized copper (Cu2+) is expected to have
the threshold binding at higher energy than the zero valent copper
(Cu0). Fig. 8 shows that the absorption energy for the Cat-C (freshly
reduced) is 8979 eV, indicating the presences of Cu0 state and has
the similar absorption energy pattern as that of the copper edge of
a copper foil. Cat-C (fresh) has absorption energy at 8984 eV that is
due to the presence of Cu2+. These results are also in agreement with
the XRD data which shows the presence of Cu2+ oxidation state in
Cat-C (fresh) as indicated by the presence of CuO and Cu0 oxidation
state in Cat-C (freshly reduced) as confirmed by the presence of
metallic copper.
3.3. Optimization studies using Cat-C
3.3.1. Effect of speed of agitation
3.2. Efficacy study for various catalysts
The reactions were carried out in a batch reactor with a speed
of agitation in the range of 800, 1000, 1200 and 1400 rpm to over-
come the influence of external mass transfer resistance between
the bulk liquid phase and catalyst surface. The experimental con-
dition for optimization of the speed of agitation is reported in
Table 4. The maximum conversion of furfural to furfuryl alcohol
was found to be 91% with the selectivity of 96% at 1000 rpm using
Cat-C beyond which no change in the conversion and selectivity
was observed, indicating that there was no external mass transfer
resistance on the overall rate of reaction above 1000 rpm (Table 4).
This is in consistent with the report of Vaidya and Mahajani [9] that
1000 rpm is sufficient to eliminate mass transfer resistance. Thus
the speed of agitation was kept at 1000 rpm for further experiments
for the assessment of the effect of other variable parameters on the
reaction.
The catalysts with varying elemental molar ratios such as Cu:Cr
(3:1), Cu:Zn:Cr (3:1:1), Cu:Zn:Cr (3:2:1), Cu:Zn:Cr (3:3:1), Cat-A,
Cat-B, Cat-C and Cat-D were prepared by co-precipitation method
as discussed earlier and used in the hydrogenation of furfural.
The reactions were carried out in isopropyl alcohol, since this sol-
vent was reported to be a good solvent for furfural hydrogenation
[9,22–24]. We also investigated the need of a solvent by carrying
out a reaction without any solvent using Cat-C. The reaction with-
out any solvent yielded only 31% conversion with 80% selectivity,
whereas the reaction in isopropyl alcohol increased the conversion
of furfural to 100% with 96% selectivity in the formation of fur-
furyl alcohol. Hydrogenation of furfural catalyzed by Cu:Cr catalyst
produces furfuryl alcohol and 2-methyl furan as two major prod-
ucts [21] (Fig. 1). Copper helps to improve the selectivity of furfuryl
alcohol [11–13]. Therefore, a novel Cu:Cr catalyst was prepared by
increasing the elemental molar ratio of Cu and investigated for
selectivity of furfural alcohol. In this study, Cu(3):Cr(1) catalyst
provided 75% conversion with 60% selectivity for furfural alco-
hol. Bruch et al. reported that ZnO has an important role as an
enabling support, act as a reservoir for atomic hydrogen and pro-
mote hydrogen spillover for the reactions [31]. The role of ZnO is
known to increase the dispersion of Cu which improves the per-
formance of the catalyst [32,33]. Therefore, in order to enhance
conversion and selectivity of the hydrogenation of furfural to fur-
fural alcohol, Zn was incorporated into the Cu(3):Cr(1) catalyst. As
anticipated, addition of Zn with elemental molar ratio of 1 and 2
leads to an improvement in the conversion of furfural from 75%
to 83% and 100% and furfuryl alcohol selectivity from 60% to 68%
and 70%, respectively. When the elemental molar ratio of Zn was
further increased to 3 with respect to Cu(3):Cr(1), the reaction pro-
ceeded with 100% conversion, but the selectivity was decreased by
3.3.2. Effect of catalyst loading
The effect of catalyst loading on the reaction was evaluated by
varying the catalyst amount in the range from 0.5 to 2.0 g. The per-
centage conversion of furfural to furfuryl alcohol increased with
an increase in the catalyst loading (Fig. 9), which was due to a
proportional increase in the active site of the catalyst. The selec-
tivity of furfuryl alcohol was not affected by catalyst loading and
was found to be in the range of 94–96% in all the cases. Additional
hydrogenated product such as 2-methylfuran was not observed in
the reaction. Highest conversion of furfural to furfural alcohol was
observed with 1.5 and 2 g of Cat-C loading. The reaction in the case
of 2 g catalyst loading was found to be faster compared to a load-
ing of 1.5 g; however, no much difference in the total conversion
of furfural to furfuryl alcohol was observed. Taking the cost of the
catalyst in consideration, 1.5 g of catalyst loading was chosen for
further optimization studies.

R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
133
Table 3
Effect of various catalysts on % conversion, product selectivity and yield of hydrogenation of furfural to furfural alcohol.
Sr. no.
Catalystsa
Conversion,% (furfural)
Selectivity, % (furfuryl alcohol)
Yield, % (furfuryl alcohol)
1
2
3
4
5
6
7
8
Cu:Cr (3:1)
Cu:Zn:Cr (3:1:1)
Cu:Zn:Cr (3:2:1)
Cu:Zn:Cr (3:3:1)
Cu:Zn:Cr:Zr (3:2:1:1) (Cat-A)
Cu:Zn:Cr:Zr (3:2:1:2) (Cat-B)
Cu:Zn:Cr:Zr (3:2:1:3) (Cat-C)
Cu:Zn:Cr:Zr (3:2:1:4) (Cat-D)
75
83
60
68
70
60
78
85
96
96
45
56
70
60
78
85
96
96
100
100
100
100
100
100
Reaction condition: furfural (12.43 mL, 0.15 mol), isopropyl alcohol (87.57 mL), catalyst (1.5 g), temperature (170 ◦C), H2 pressure (2 MPa) and time (3.5 h).
a
Elemental molar ratios are presented in parenthesis.
Table 4
Effect of speed of agitation on % conversion, product selectivity and yield of hydrogenation of furfural to furfural alcohol using Cat-C.
Sr. no.
Speed of agitation (rpm)
Conversion, mole% (furfural)
Selectivity,% (furfuryl alcohol)
Yield, % (furfuryl alcohol)
1
2
3
4
800
1000
1200
1400
86
91
90
91
96
96
96
96
82.5
87.3
86.4
87.3
Reaction condition: furfural (12.43 mL, 0.15 mol), isopropyl alcohol (87.57 mL), catalyst (1.5 g), temperature (170 ◦C), H2 pressure (2 MPa) and time (3 h).
3.3.3. Effects of hydrogen pressure
furfural was decreased without affecting selectivity of furfuryl alco-
hol (94–96%) because of availability of less number of active sites to
convert furfural to furfuryl alcohol (Fig. 11). Highest conversion was
achieved by using 0.75 and 1.5 M of furfural; however the reaction
was very fast at 0.75 M. Therefore furfural concentration of 1.5 M
was chosen for further studies.
The percentage conversion of furfural and selectivity of fur-
furyl alcohol were investigated by varying the hydrogen pressure
from 1 to 4 MPa. Autogeneous pressure of the reaction was gen-
erated at 170 ◦C. Total pressure of the reaction was maintained
at 2–6 MPa with additional hydrogen supply. It was observed that
with an increase in the hydrogen pressure, the conversion of fur-
fural increased without affecting selectivity of furfuryl alcohol
(Fig. 10). The increased solubility of hydrogen in the reaction mix-
ture at increased hydrogen pressure can account for the increased
conversion. The conversion of furfural to furfuryl alcohol was very
fast at a hydrogen pressure of 3–4 MPa using Cat-C. At 2 MPa hydro-
gen pressure, the conversion attained 96% but at a slower rate.
For kinetic studies, a slow reaction is preferred over a fast reac-
tion. Therefore, further studies were carried out at 2 MPa hydrogen
pressure.
3.3.5. Effects of temperature
The reactions were carried out using Cat-C with a variation of
temperature from 140 to 170 ◦C using 2 MPa hydrogen pressure to
investigate its effect on the furfural conversion and furfuryl alco-
hol selectivity. During the experiments, the samples were collected
periodically and after each sample, the total pressure of the reaction
was maintained constant by the addition of extra hydrogen. It was
observed that the furfural conversion increased with an increase
in the temperature (Fig. 12). The percentage selectivity of furfuryl
alcohol was in the range of 94–96%.
3.3.4. Effects of furfural concentration
It
was
assumed
that
the
reaction
follows
Furfural concentration was varied in the range of 0.75–3.0 M
to estimate its effect on the conversion and selectivity of fur-
furyl alcohol by using Cat-C. It was observed that with an increase
in the furfural concentration from 0.75 to 3.0 M, conversion of
Longmuir–Hinshelwood–Hougen–Watson (LHHW) type mecha-
nism with the involvement of two similar catalytic sites. A plot of
−ln(1 − XA) vs. time (min) gave a straight line that passes through
Fig. 9. Effect of catalyst loading on the conversion of furfural to furfural alcohol
using Cat-C. Reaction condition: furfural (12.43 mL, 0.15 mol), isopropyl alco-
hol (87.57 mL), temperature (170 ◦C), H2 pressure (2 MPa), speed of agitation
(1000 rpm), and reaction time (3 h).
Fig. 10. Effect of hydrogen pressure on the conversion of furfural to furfuryl
alcohol using Cat-C. Reaction condition: furfural (12.43 mL, 0.15 mol), isopropyl
alcohol (87.57 mL), temperature (170 ◦C), H2 pressure (2 MPa), speed of agitation
(1000 rpm), and reaction time (3 h).

134
R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
Table 5
Cat-C reusability study on % conversion, product selectivity and yield of hydrogenation of furfural to furfural alcohol.
Sr. no.
Reusability
Conversion, mole% (furfural)
Selectivity,% (furfuryl alcohol)
Yield, % (furfuryl alcohol)
1
2
3
4
5
Fresh
91
90
89
88
90
96
96
95
94
95
87.3
86.4
84.5
82.7
85.5
1st reuse
2nd reuse
3rd reuse
4th reuse
Reaction condition: furfural (12.43 mL, 0.15 mol), isopropyl alcohol (87.57 mL), catalyst (1.5 g), temperature (170 ◦C), H2 pressure (2 MPa) and time (3 h).
Fig. 11. Effect of furfural concentration on the conversion of furfural to furfuryl
alcohol using Cat-C. Reaction condition: furfural (0.075–0.3 mol), isopropyl alcohol
Fig. 13. Plot of −ln(1 − XA) vs. time (min) to calculate the rate constant.
(made up to 100 mL), catalyst (1.5 g), temperature (170 ◦C), H2 pressure (2 MPa),
speed of agitation (1000 rpm) and reaction time (3 h).
and also to convert metallic copper into copper oxide which was
confirmed by XRD for the regenerated catalyst (Fig. 2). This obser-
vation was also supported by TPR analysis (Fig. 4). The XRD and
TPR results of fresh, used, regenerated and freshly reduced Cat-C
have been discussed in details in Section 3.1. In a batch reaction,
there was an inevitable loss of particles during filtration and hand-
ling. Hence, the actual amount of catalyst used in the next batch
was almost 5% less than the previous batch. The loss of the catalyst
was made up with fresh catalyst. Catalyst was reduced with H2 at
300 1 ◦C for 3 h prior to use. It was observed that the conversion of
furfural decreased by 3–5% and the selectivity of furfuryl alcohol
was not affected which was in the range of 94–96% (Table 5). ICP-
MS data revealed that the regenerated Cat-C has a similar elemental
composition as of the fresh catalyst (Table 2). The TPR data also sup-
ported the reduction profile of the fresh and regenerated catalysts
(Fig. 4). Surface area of the used catalyst was higher than the fresh
the origin. This confirms that the reaction is pseudo first order with
respect to furfural (Fig. 13). Rao et al. [7], Vaidya and Mahajani [9]
and Sitthisa et al. [13] reported that this reaction follows LHHW
type mechanism [7,9,36]. Table 6 shows the rate constants at
different temperatures. Arrhenius plot was obtained by correlating
ln k vs. 1/T (Fig. 14). The value of apparent activation energy was
found to be 24.4 kcal/mol. This activation energy suggests that the
reaction was controlled by the surface reaction.
3.3.6. Catalyst reusability
Catalyst recyclability in heterogeneous reactions is very impor-
tant from the cost point of view. The reusability of Cat-C was carried
out by conducting four runs. After each reaction, the catalyst was
filtered and refluxed with 100 mL of methanol at 60 ◦C and dried at
100 ◦C for 30 min. Further, it was calcined at 650 10 ◦C for 3 h to
remove any adsorbed material from the active sites of the catalyst
Fig. 12. Effect of temperature on the conversion of furfural to furfuryl alcohol
using Cat-C. Reaction condition: furfural (12.43 mL, 0.15 mol), isopropyl alcohol
(87.57 mL), catalyst (1.5 g), temperature (140–170 ◦C), H2 pressure (2 MPa), speed
of agitation (1000 rpm) and reaction time (5 h).
Fig. 14. Arrhenius plot for the conversion of furfural to furfural alcohol using Cat-
C. Reaction condition: furfural (12.43 mL, 0.15 mol), isopropyl alcohol (87.57 mL),
catalyst (1.5 g), temperature (140–170 ◦C), H2 pressure (2 MPa), speed of agitation
(1000 rpm), and reaction time (5 h).

R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
135
O
O
O
O
O
O
H
H
H
H
H
H
H
2
+
H
Catalyst
Catalyst
Furfural
Hydrogen
O
H
H
H
o
Catalyst
Catalyst
O
H
O
HO
H
H
H
HO
Catalyst
Furfuryl alcohol
Fig. 15. LHHW type of reaction mechanism for the hydrogenation of furfural to furfural alcohol.
Table 6
Rate constant (k) for hydrogenation of furfural using Cat-C at different temperatures.
Sr. no.
Temperature (◦C)
T (Kelvin)
Rate constant k, (min−1
)
1/T (kelvin−1
)
ln k
1
2
3
4
140
150
160
170
413
423
433
443
0.0014
0.0048
0.0048
0.0107
0.002421
0.002364
0.002309
0.002257
−6.023
−6.047
−6.070
−6.093
Table 7
CO-chemisorption study for Cat A–D.
Catalyst
Cu metal dispersion (%)
Cu metallic surface area (m2/g)
Cu crystallite size (nm)
CO adsorbed (␮mole/g)
Cat-A
Cat-B
Cat-C
Cat-D
3.2
4.2
5.8
6.1
0.61
0.81
1.12
1.19
32.6
24.5
17.0
16.6
15.1
19.9
27.8
28.4
catalyst due to reduction of the catalyst during the reaction. The
presence of metallic Cu was confirmed by XRD and XANES, which
was also supported by TPR profile of the used catalyst. This justifies
that Cat-C catalyst is reusable and has the prospective for industrial
applications.
4. Conclusion
In the present work, Cu–Cr based catalyst was modified with
the addition of Zn and Zr as promoters. It was found that the addi-
tion of Zn increases the activity of the catalyst that resulted in
100% conversion of furfural. The selectivity for furfuryl alcohol was
increased by addition of Zr in the catalyst matrix. CO chemisorption
data shows that an increase in Zr content from Cat-A to Cat-D, not
only increased the percentage dispersion but also reduced the crys-
talline size of the active Cu metal. The particle size of metallic Cu
was found to be 16.9 nm by XRD, 17 nm by CO-chemisorption and
19 nm by TEM. NH3-TPD results show that with an increase in the Zr
content in the catalyst, acidity of the catalyst increases. It was also
found that the Zr loading in the catalyst not only increased the acid-
ity of the catalyst, but also helped in the dispersion of metallic Cu.
Elemental molar composition of the regenerated Cat-C was found to
be same as that of the fresh Cat-C as per the ICP-MS results, suggest-
ing that this catalyst can be reused for subsequent reactions. XANES
analysis confirmed the presence of copper in the oxidation states of
Cu0 and Cu2+ in the freshly reduced Cat-C and fresh Cat-C, respec-
tively. The effect of various reaction parameters such as catalyst
screening, speed of agitation, catalyst loading, hydrogen pres-
sure, furfural concentration and temperature on the conversion of
3.3.7. Reaction mechanism
The exact mechanistic pathway for the reduction of aldehydic
C
O of furfural to alcohol is not fully understood. Fig. 15 depicts
a suggested mechanism for catalytic hydrogenation of furfural to
furfuryl alcohol. It is proposed that hydrogen molecule undergoes
dissociative adsorption as hydrogen atoms on the catalyst surface.
It is assumed that in the first step, the acidity of the catalyst attracts
the lone pair of aldehydic oxygen that helps adsorption of furfural
on the catalyst surface in a preferred orientation [22]. Hence, the
choice of active metal is the key for chemoselective hydrogenation
of furfural. Second step involves the addition of hydrogen atom to
carbonyl oxygen atom to form hydroxyl alkyl intermediate, which
on further addition of hydrogen atom gives furfuryl alcohol. This
mechanism was formulated based on the potential energy diagram
delineated by Alcala et al. [37] for the hydrogenation of acetone and
propanal to their corresponding alcohols.

136
R.V. Sharma et al. / Applied Catalysis A: General 454 (2013) 127–136
furfural were carried out to investigate optimized reaction condi-
tions. This study reveals a novel Cu(3):Zn(2):Cr(1):Zr(3) catalyst
which demonstrates 100% conversion and 96% chemo-selectivity
for the hydrogenation of furfural to furfuryl alcohol without form-
ing any significant by-products. The reaction was pseudo-first order
with respect to furfural and the apparent activation energy was
found to be 24.4 kcal/mol. The catalyst was reused for four runs;
hence expands its scope for commercial use.
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Acknowledgments
The funding for this research was supported by the Canadian
Research Chair Program awarded to Dr. Dalai from the Govern-
ment of Canada, the Province of Saskatchewan and the University
of Saskatchewan. The XANES experiments described in this paper
were performed using a VESPERS beam line at the Canadian Light
Source, which is funded by the Natural Sciences and Engineer-
ing Research Council of Canada, the National Research Council
Canada, the Canadian Institutes of Health Research, the Province
of Saskatchewan, Western Economic Diversification Canada, and
the University of Saskatchewan.
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