Promotional effect of Fe on the performance of supported Cu catalyst for ambient pressure hydrogenation of furfural

Article abstract of DOI:10.1039/c5ra24742j

A noble-metal free FeCu based bimetallic catalyst system prepared by facile co-impregnation method was found to be highly admirable for vapour phase selective hydrogenation of furfural to furfuryl alcohol at ambient pressure. Monometallic Cu/γ-Al₂O₃, Fe/γ-Al₂O₃ and bimetallic FeCu/γ-Al₂O₃ catalysts with different Fe loadings were prepared. Structural and morphological features of the catalysts were thoroughly investigated by several physico-chemical characterization techniques. The influence of various reaction parameters, such as Fe loading, reaction temperature and flow of reactants was examined with respect to furfural conversion and furfuryl alcohol yield. The results clearly showed that an optimum amount of Fe is necessary to enhance the catalytic activity of monometallic Cu/γ-Al₂O₃ for the selective hydrogenation of furfural. The catalyst FC-10 with 10 wt% Fe exhibited excellent activity which led to high furfural conversion (>93%) and furfuryl alcohol selectivity (>98%) under mild reaction conditions. The higher activity of bimetallic FeCu/γ-Al₂O₃ compared to monometallic Cu/γ-Al₂O₃ is ascribed to the formation of FeCu bimetallic particles and the existence of oxygen vacancies in the Fe oxide system. The superior activity after Fe loading on the Cu-based catalyst was attributed to the synergy between Cu and Fe. A plausible mechanism is proposed to explain the promoting effect of Fe, which involves synergism between Fe sites with strong oxophilic nature and Cu sites with a high ability for hydrogen activation. Based on the activity results, prolonged catalytic activity and spent catalyst analysis, the developed FeCu/γ-Al₂O₃ catalyst is inexpensive, eco-benign and robust, which makes it a promising candidate for the efficient conversion of biomass-derived substrates to fine chemicals and drop-in biofuels.

Full text of DOI:10.1039/c5ra24742j

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Promotional effect of Fe on performance of supported Cu catalyst
for ambient pressure hydrogenation of furfural
Received 00th January 20xx,
Accepted 00th January 20xx
Marimuthu Manikandan, Ashok Kumar Venugopal, Atul S. Nagpure, SatyanarayanaChilukuri* and
Thirumalaiswamy Raja*
DOI: 10.1039/x0xx00000x
Noble-metal free FeCu based bimetallic catalyst system prepared by facile co-impregnation method was found to be a
highly admirable for vapour phase selective hydrogenation of furfural to furfuryl alcohol at ambient pressure.
Monometallic Cu/γ-Al2O3, Fe/γ-Al2O3 and bimetallic FeCu/γ-Al2O3 catalysts with different Fe loadings were
prepared.Structural and morphological features of the catalysts were thoroughly investigated by several physico-chemical
characterization techniques. The influence of various reaction parameters, such as Fe loading, reaction temperature and
flow of reactantswas examined with respect to furfuralconversion and furfuryl alcohol yield.The results clearly showed
that the optimum amount of Fe is necessary to enhance the catalytic activity of monometallic Cu/γ-Al2O3 for the selective
hydrogenation of furfural. The catalyst FC-10 with a 10 wt% of Fe exhibited an excellent activity which led to high furfural
conversion (>93%) and furfuryl alcohol selectivity (>98%) under mild reaction conditions. The higher activity of bimetallic
Fe-Cu/γ-Al2O3 compared to monometallic Cu/γ-Al2O3 is ascribed by the formation of FeCu bimetallic particles and the
existence of oxygen vacancies in Fe oxide system.The superior activity after Fe loading on the Cu-based catalyst was
attributed to the synergy between Cu and Fe. A plausible mechanism is proposed to explain the promoting effect of Fe,
which involves the synergism between Fe sites with strong oxophilic nature and Cu sites with the high ability for hydrogen
activation. Based on the activity results, prolonged catalytic activity and spent catalyst analysis,the developed CuFe
catalyst is inexpensive, eco-benign and robust,which makes it a promising candidate for the efficient conversion of
biomass-derived substrates to fine chemicals and drop-in fossil fuels.
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Introduction
of lignocellulosic biomass.In recent years, FAL has been
Climate change, population growth and depletion of fossil fuel
suggest that renewable resources will need to play a bigger
role in the near future. The fundamental understanding and
the efficient utilization of renewable biomass resources has
become more and more attractive research area in industry
and academia.1 Lignocellulosic biomass, an unique renewable
and alternative resource, can be converted into platform
chemicals, value added products and fuels.1-3 The selective
conversion of biomass derived substrates to fine chemicals
and fuels has been examined extensively over the past few
decades and many studies have been shown that
lignocellulosic biomass offers a great potential to be used as a
sustainable feedstock.4,5 The immense attention has been
received to develop the process for the efficient conversion of
biomass to liquids, solids, gases and fuels.6
received renewed attention as
a
potential platform
moleculefor the production of biomass-derived chemicalsand
drop-in biofuels.7 FAL can be transformed into a series of liquid
fuels, fuel additives and other chemicals like furfuryl alcohol
(FOL),
2-methylfuran
(2-MF),methyltetrahydrofuran
(MTHF),furoic acid, furan andlevulinic acid, etc (Scheme 1).8
FOL, an essential intermediate finds many applications as
solvent, ingredient in the manufacture of various chemical
products such as resins, rubbers, adhesives, synthetic fibres
and wetting agents,impregnating solutions, carbon
binders,lubricant, dispersing agent and plastisizer.7It is also
used as an inert diluent for epoxy resins, resins for corrosion-
resistant mortars and furan polymer concrete,modifier for
urea and phenolic resins.8,9Importantly, FOL is used in the
production
useful
organic
solvents
like
Furfural (FAL) is an important platform molecule, which is
obtained from the dehydration of xylose, a monosaccharide
often available in huge quantities in the hemicellulose fraction
tetrahydrofurfurylalcohol (THFA), tetrahydrofuran (THF) and
others.7-9In industry, the crucial moleculeFOL is generally
produced by the catalytic reduction of FAL and industries
adopt vapour phase hydrogenation most often due to obviate
the high-priced operating costs of using batch reactors and
mandatory of exorbitant apparatus for generating high
pressure in the liquid phase process. Recently, efforts have
Catalysis & Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Dr.
Homi Bhabha Road,Pune - 411 008, India.Fax: (+) 91-20-25902633 ; Tel: (+) 91-20-
25902006 ; E-mail: t.raja@ncl.res.in
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been made to carry out the conversion of FAL to FOL in minimize the undesired products by adding another metal in
vapour-phase continuous processes. Cr2O3-promoted Cu-based order to enhance its activity, stability and selectivity.8,14 As
system is the conventional catalyst for the production of FOL mentioned earlier, it is possible to tune the reactivity and
with moderate activity and selectivity at stringent reaction selectivity of Cu bythe addition of another element like Fe.
conditions.10Due to its toxicity, it causes serious environmental Because Fe has redox behavior like cerium and zirconium,
issues. Indeed, the evolution of innocuous Cr-free green which are usually accounted for the efficientperformance by
catalysts is crucial for the efficient production of FOL.
providing the synergistic interaction. In addition, Fe has a
Over the past few decades, a large number of reports are higher oxygen affinity and oxygen vacancies which can
available in the literature with Cr-free catalysts for selective facilitate the reaction by binding and subsequently activating
hydrogenation of FAL to FOL. Generally, most catalystscan the oxygenates.23 Recently, Sitthisa et al. showed superior
hydrogenate both C=C and C-O bonds, but the control of the activityof FeNi/SiO2 catalysts in the conversion of FAL to 2-
selective hydrogenation activity over C=C/C-O hydrogenation MF.24 Also, the incorporation of inexpensive Fe on Cu-based
has one of the most important tasksfor the design of an heterogeneous catalysts for various catalytic transformations
efficient hydrogenation catalyst.Cu-based catalysts are the have been studied in many reports.23-25Addition of Fe could
most utilized anddeliberated in industry as well as in inhibit the sintering of Cu and also enhances the active surface
literatures, due to its preferential C=O bond hydrogenation area of Cu. Moreover, it can also promote the catalytic activity
ability in carbonyl compounds.11 B. M. Nagaraja et al. reported due to the synergistic interaction between Cu and
an excellent activity of Cu/MgO and Cu-MgO-Cr2O3 catalystsin Fe.26,27However, the systematic investigation ofCuFe catalyst
FAL hydrogenation.12K. Yan et al. achieved 95% yield of FOL system for hydrogenation of FAL to FOL is rarely disclosed.
overCu–Cr catalyst at ahigh reaction temperature (120–220 °C)
Herein, we presentthe work aiming toward the efficient
and high hydrogen pressure (35–70 bar).13Although, these development of high-performance catalyst for the selective
catalysts showed superior performance, still it is far from hydrogenation of FAL to FOL at ambient pressure. The efforts
satisfactory due to the toxicity and deactivation within a short have been made to prepare γ-Al2O3 supported Cu, Fe and FeCu
period of time. Meanwhile, several heterogeneous catalysts, catalysts with different Fe wt% and investigated the effect of
including both noble and non-noble metal catalysts have been Fe on the catalyst performance in the selective hydrogenation
reported for vapour phase as well as liquid phase of FAL. The excellent yield (~92%) of FOL was obtained over
hydrogenation of FAL to FOL.13-16 Merlo et al. reported FC-10 catalyst under optimized reaction conditions, which is
bimetallic PtSn catalysts for the liquid phase selective much higher than that of the commercial catalyst (Cu-1800P),
hydrogenation of FAL to FOL with selectivity of 96–98% at high which gave FAL conversion of 65% with 99% selectivity
pressure.17Kijenski et al. reported the use of Pt deposited on towards FOL.12 We found that the product distribution was
monolayer supports for hydrogenation of FAL to FOL with highly influenced by the Fe loading, reaction temperature and
93.8% of yield. Recently, more number of results are reported contact time of the reactant. Moreover, the effect of various
with noble metal catalysts.15-18 Regrettably, the industrial scale reaction parameters and catalytic stability were also studied.
process of these reported catalysts is away from reality due to Mainly, the results are contributing to understanding the
their cost and abundance towards commercialization. In view origin of the promotional effect of Fe and the design of greatly
of the innovative aspect of the heterogeneous bimetallic admirable catalyst for selective hydrogenation of FAL to FOL.
catalysts,especially those based on non-precious and eco- The results reported in the present investigation may inspire to
benign, are highly demanded for various chemical processes start focusing on the inexpensive non-noble metal based
such as hydrogenation, dehydrogenation, catalytic reforming catalysts for the selective conversion of biomass derived
and so on.19
furanic compounds to value added chemicals and fuels.
Bimetallic catalysts often show electronic and chemical
behavior, which are significantly distinct from their
corresponding monometallic catalysts that offer the greater
opportunity to tailor a novel catalyst with enhanced catalytic
performance. Incorporating a second metal could be markedly
altered the activity of the first one due to electronic, chemical
and geometric effects.20 The perceiving of these surpassing
properties of bimetallic catalysts has inspired many extensive
investigations on using these in catalysis. Indeed, the designing
of effective supported inexpensive, non-noble metal catalysts
with outstanding activity isa strenuous task for
researchers.21The applications of first row transition metals in
Scheme 1. Reaction routes for the hydrogenation of FAL to FOL and other
possible products. (a) Hydrogenation, (b) hydrogenolysis, and (c)
decarbonylation.
a
variety of chemical transformations arewell-known
phenomenon. For example, Cu serves an exceptional behavior,
which is competing for noble metals based catalysts in salient
research fields.22In this work, we have chosen Cu as an active
component based on the reported literatures and aimed to
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Results and Discussion
Characterizations of pre-catalysts
Structural characteristics of the pre-catalysts were examined
by powder XRD analysis. Fig. 1 shows the XRD patterns of the
pre-catalysts. All diffraction features were indexed with both γ-
Al2O3 and Fe2O3. γ-Al2O3 exhibits strong diffraction peaks at
2θ=33°, 37.4°, 46°and 66.7°.28 For F-10 pre-catalyst, the weak
reflections at 33.34°, 35.66°, 49.8° and 54.4°are ascribed to
Fe2O3 (JCPDS # 65-7002).27The diffraction peaks related to FeO
and Fe3O4 were not observed in XRD. No apparent peaks
attributable to CuO were observed for the C-10 and other Cu
containing pre-catalysts suggesting that Cu is present as
amorphous or in highly dispersed form. Likewise, the patterns
for pre-catalysts from FC-2.5 to FC-10, absence of the
corresponding peaks for both Fe2O3 and CuO demonstrated
that both species are also present as highly dispersed
nanocrystalline, or amorphous form.28 Moreover, the pre-
catalyst with maximum Fe loading (FC-12.5) shows the
reflections for corresponding Fe2O3 features, which evidences
the formation of large crystallites after certain
loading.According to the obtained results, it is found that there
was an interaction between iron and copper oxides, promoting
the dispersion of both species. This is further substantiated by
H2-TPR results
Fig.1 XRD patterns of the γ- Al2O3 supported Fe, Cu, and FC-X pre-catalysts.
800 °C is associated with the reduction of magnetite to
metallic Fe.26,31In contrast to the F-10 pre-catalyst,bimetallic
FC-X pre-catalysts did not display any peaks above 400°C. This
inference is evidence that the simultaneous reduction of iron
oxide species with copper oxides. Clearly, Cu promoted the
reducibility of iron oxides, consistent with other
investigations.31This is further established by the increasing H2
consumption with the increasing Fe loading in the bimetallic
samples (Table 1), since Cu loading is kept constant and
complete reduction of F-10 sample is much difficult below 500
°C. The promotion effect might be due to facile H2 dissociation
on Cu followed by spillover of hydrogen to magnetite
phase.31Furthermore, as the Fe content increases, the high-
H2-TPR profiles of all calcined catalysts are depicted in Fig.
2. H2-TPR profile for C-10 pre-catalyst exhibited a symmetrical
reduction peak at about 181.4 °C which could be attributed to
the reduction of CuO species. Literature reports suggested that
the supported Cu catalysts show two H2 consumption peaks,
which are assigned to the reduction of well dispersed and Cu2+
species having a strong interaction withthe matrix.29,30The
observed result for C-10 pre-catalyst is much lower than the
reported values which might be due to the presence of
isolated highly-dispersed CuO species. For F-10 pre-catalyst,
H2-TPR is distinctively different from those of others. The first
peak located maximum at 396 °C is attributed to the reduction
of Fe2O3 to Fe3O4 and the second peak (not shown here) above
temperature peak shift to
a low temperature region,
evidenced the weakened interaction between Cu and the
support, which is also indicating the strong interaction
between Fe and Cu. This could be also attributed to the
synergistic electronic interaction between Fe and Cu. For FC-
12.5 pre-catalyst,
a slight peak shift towards higher
temperature along with a new peak around 300 °C suggested
that in higher Fe loading causes large crystallites after a certain
Table 1. Physico-chemical properties of the γ-Al2O3 supported metal catalysts
Entry
Catalyst
Surface
Average
pore size[b]
(nm)
Total pore
volume[b]
(cc/g)
Cu particle
Size[c]
Cu metal
Cu metal
H2 consumption
from TPR
area ( m2g-1)[a]
dispersion[d] Surface area
(nm)
(%)
---
(m2g-1)[d]
(mmol g-1 cat)
1
γ-Al2O3
182.0
11.3
1.0
---
---
---
2
3
C-10
FC-2.5
202.6
216.1
219.7
224.6
205.8
151.8
217.8
172.4
193.8
11.0
11.1
11.4
10.9
10.8
7.6
0.71
0.57
0.59
0.61
0.58
0.48
0.64
0.42
0.46
8.4
8.1
26.1
32.3
33.5
34.9
36.6
30.8
---
10.8
12.0
13.4
13.7
14.6
10.2
---
13.4
13.5
13.9
14.2
14.6
14.4
0.67
n.d
4
FC-5
7.9
5
FC-7.5
7.9
6
FC-10
8.2
7
FC-12.5
F-10
10.3
7.1
8
10.7
7.9
9
Spent C-10
Spent FC-10
11.7
9.1
n.d
n.d
10
9.4
n.d
n.d
n.d
[a] Specific surface area calculated using the BET method. [b] Calculated from BJH method. [c] Calculated from TEM analysis. [d] Determined by H2-N2O
titration.
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limit, as evidenced by XRD results. Finally, the H2-TPR profiles patterns, the absence of metallic copper or cuprous oxide
allow us to conclude that Fe and Cu species forming bimetallic signals suggested that either well dispersion or overlapped
system.
peaks with Fe3O4 at 35.6°.As shown in Table 1, the Cu
dispersion was measured by H2–N2O titration, which increases
monotonically from 26.1 to 36.6% with the increasing Fe
loading up to 10 wt%, then it showed a marginal decrease.The
observed results are highly consistent with the change in the
size of the Cu particles determined by the TEM analysis. As a
result, the addition of a large amount of Fe is not helpful for
the higher dispersion, which is reflected as the decrease in
dispersion at higher Fe loading (FC-12.5, Table 1, entry 7).
Generally, a high specific surface area can result in a good
metal dispersion.11 In the present catalyst system, the BET
specific surface area of the FeCu/γ-Al2O3samples increases
gradually with the Fe loading (Table 1),which is associated
mainly with the increase in the contentof the Fe. Meanwhile,
Cu surface areas exhibit a slight increasingtrend with the Fe
loading from 2.5 to 10 wt%, indicativeof the increase in the
surface metallic Cu sites. However,the FC-12.5 sample has the
Fig.2 TPR profiles of the γ-Al2O3 supported F-10 (dash line), C-10 and FC-X pre- smallest Cu surface area (10.2m2 g−1), which should be
catalysts.
ascribed to the agglomeration or surface coverage of Fe
species.
The textural characteristics of γ-Al2O3 based catalysts were
examined by N2 adsorption-desorption, pore-size distribution
analysis and the results are shown in Fig.S1 (ESI) and Table 1.
All catalysts show type IV isotherm with H4 hysteresis loops
(Fig. S1a) which is typical for mesoporous materials. Pore size
distribution (Fig. S1b) of the catalysts shows thatthe pores are
falling within the uniform range (8-12 nm). The BET surface
area increases marginally from 182 to 202.6m2g-1 for γ-Al2O3
and C-10 respectively, which might be due well-dispersed Cu
oxides.The BET surface area of FC-2.5 and FC-5 pre-catalysts
were slightly increased with the increase of Fe content and
then marginally decreased for FC-7 and FC-10. The change in
particle size in the same trend is accounting for the transition
in surface area of the pre-catalysts. The pre-catalyst containing
highest Fe loading exhibited the decrease in surface area
which might be due to the agglomeration. Compared to the C-
10 sample, bimetallic samples (FC-X) showed more surface
area indicated that the addition of Fe promoted the surface
area of monometallic samples. XRD of the FC-10 catalysts also
substantiated the high dispersion of Fe and Cu species which
leads to give more surface area. Moreover, increasing surface
area after the metal loadings of the promoter is a well-known
phenomenon, whichwas explained elsewhere.32,33
To study the reduction behavior of both Fe and Cu in more
detail, the most active FC-10 catalyst was examined for the in-
situ XRD analysis with different temperatures (27-400 °C) in
the presence of H2 flow (30 mL min-1). As shown in Fig. 3, the
diffraction patterns were recorded at 27, 200, 250, 300, 350
and 400 °C with the scanning rate of 0.4°min-1, temperature
was allowed to stabilize prior to each scan. XRD profile
emphasized that the CuO was started reduced to
Cu2O/metallic copper at 200 °C (2θ=43.2 and 50.3°), which is
accordance with the literature values.35 No corresponding
peaks were observed for reduced Fe species at this
temperature, which might be due to the insufficient
temperature for reduction of Fe3+ to Fe0. The appearance of
metallic phases by increasing temperature unveils that the
reduction of both Fe and Cu at 400 °C. The obtained results
concluded that the bimetallic FeCu species were formed upon
pre-treatment at 400 °C.36
Characterization of catalysts
The XRD patterns of reduced catalysts are given in Fig. S2
(ESI). For all the catalysts, diffraction patterns are
predominantly attributed to the reflections of γ-Al2O3.No
discernible peaks wereobserved for both Fe and Cu metallic
features as in the catalysts are corroborated that the highly
dispersed nature of both species. For bimetallic FC-12.5
catalyst, a peak appeared at 30.3° and 35.6°was attributed to
the (200) and (311) planes of Fe3O4 (JCPDS #75-0033),
respectively.34However, a small reflection at 44.6° is the
indicative of the presence of iron metallic species. In all
Fig.
3 XRD patterns of the FC-10 catalyst during in-situ reduction with different
temperatures at 40 mL min-1 flow of H2.
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Morphological aspects of the reduced monometallic C-10, particles were observed in both cases, which proved the
F-10 and bimetallic FC-10 catalysts were studied by TEM amorphous or nanocrystalline behavior. This aspect can be
analysis and the results are shown in Fig. 4. For C-10 catalyst further confirmed by the SAED patterns (Fig. 4 insets). The
(Fig. 4a), uniform needles like morphology with spherical reductionbehavior of FC-10 catalyst is distinctly differs from
particles was noticed, which is usually observed for γ-alumina the monometallic catalysts. As shown in Fig. 4d-f, TEM images
supported materials.28However, the clear interface between of FC-10 clearly exhibited thatbimetallic FeCu nanoclusters
the copper phase and the support is difficult to be identified, (Fig. 4f) without agglomeration of particles are highly
due to poor contrast between the metal and the support.30 dispersed on the support. The average particle size of Cu in FC-
TEM image of F-10 (Fig. 4b) also shows good dispersion with 10 catalyst is 8.2 nm (Fig. 4f).The observed interlayer
dark particles. The calculated interlayer distances 0.251 and distances hows dominant oxide features (Fe3O4 and Cu2O) as
0.294 nmisdue to the (311) and (220) phases of Fe2O3, which evidenced from XRD of reduced catalysts.
are in good accordance with XRD results. No discrete Cu or Fe
Fig. 4 TEM images of reduced samples(a) C-10,(b, c)F-10 and (d-e) FC-10. (f) Particle size distribution of the FC-10 catalyst.
To understand the more insightsinto the surface chemical Clearly, Cu2+ was reduced to Cu0, such a binding energy value is
states of Fe and Cu in the FC-10 catalyst during catalysis, the in slightly deviates apart from those reported in the literature,
situ XPS analysis was performed. The catalyst was placed indicates that the chemical environment of the Cu changes,
inside the XPS chamber and heated to different temperatures presumably owing to the interaction of Cu with Fe species
in the presence of 0.1 mbar H2 pressure. The core level during the activating process. Meanwhile, the appearance of
spectrums were recorded at Ultra High Vacuum (UHV) Cu 2p satellite peaks implying the existence of both Cu+/Cu0
followed by room temperature (RT), 250and 400°C under 0.1 species. Furthermore, the recorded spectra at 400 °C clearly
mbar H2 pressure; the temperatures were kept constant for 2 proved that the complete reduction of CuO to metallic Cu (B.E
hours prior to each recording. The photoemission features of = 932.3 eV).41 Further analysis using the Auger parameter
both Fe and Cu 2p core levels are shown in Fig. 5a and 5b, confirmed that the presence of metallic Cu (567.9 eV) instead
respectively. For the survey scan of Fe (2p3/2) presented in Fig. of Cu+ after reduction at 400 °C (Fig. S3).The observed binding
5a, the binding energy observed at 711.4 and 724.2 eV and energy values for Oxygen1s core levels were also in great
their satellitefeatures evidenced that the surface phase at UHV agreement with the above conclusions.
and RT is Fe2O3.36 No peaks were seen for reduced Fe species
on treatment at 250 °C, which is might be due to the
insufficient reduction temperature. Upon reduction at 400 °C,
the binding energy values at 707.6 and 720.8 eV corroborated
that Fe2O3 was reduced to Fe3O4and further to metallic Fe.36,37
The spectra recorded at UHV and RT (Fig. 5b) shows
predominant spin orbit peaks for Cu2p3/2 and Cu 2p1/2 binding
energy values at 934.8±0.2and 955 eV, respectively, which are
in good accordance with the reported literature.38-40Moreover,
the presence of Cu2+ state is confirmed by the observation of
satellite features around 942 eV. Upon treatment at 250 °C in
0.1 mbar H2, the position of Cu 2p3/2 down-shifted to 932.6eV.
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]
Table 2. Product distributions during FAL hydrogenation over different catalysts.[a
Entry
Catalyst
FAL Conversion
(mol %)
Selectivity (mol %)
FOL
2-MF
Furan
1
2
3
4
5
6
7
8
γ-Al2O3
C-10
3.6
---
---
Trace
6.4
4.8
4.2
3.3
0.6
0.5
---
62.0
66.2
72.1
81.0
93.4
84.6
23.2
87.8
90.7
91.2
92.3
98.6
91.3
12.4
5.8
4.5
4.1
3.4
1.3
7.4
---
FC-2.5
FC-5
FC-7.5
FC-10
FC-12.5
F-10
[a] Activity results were at steady state under the optimized reaction
condition, i.e. 175 °C, LHSV = 1 h−1 with respect to FAL and GHSV = 1200 h−1
with respect to H2, 1 atm. pressure and 1 mL catalyst.
addition of Fe presented a promoting effect on the activity of
C-10. As the Fe content increased, the conversion exhibited a
slight volcano curve and reached the maximum at the 10 wt%
of Fe. In clearer, the conversion increased from 62 to 93.4% at
175 °C and from 90 to 100% at 250 °C with Fe loading
increased from 0 to 10 wt%. Among all the catalysts tested, FC-
10 gave an excellent conversion in all reaction temperatures.
Clearly, an optimum Fe loading is needed for the enhancing
the catalytic activity of the monometallic C-10 catalyst.
Fig. 6b shows the product distribution of FAL
hydrogenation with varied Fe loadings. As shown in Fig. 6b, the
dominant hydrogenated product on C-10 and FC-X catalyst is
FOL. Furthermore, the addition of Fe to C-10 significantly
changed the product distributions. As Fe loading increased, the
selectivity of FOL is increased, whereas the selectivity of 2-MF
and Furan were decreased. In clearer, C-10 catalyst showed
87.8, 5.8 and 6.4% selectivity to FOL, 2-MF and Furan,
respectively at 175 °C. However, the selectivity of FOL
decreased to 50% and selectivity of Furan increased to 29% at
250 °C. With increasing the reaction temperature the
decarbonylation (C-C cleavage) of the FAL is more favourable
overC-10 catalyst. For bimetallic FC-X catalysts, the selectivity
to decarbonylation product is very less (<10%) at all
temperatures. It is concluded that the addition of Fe to C-10
diminished the selectivity to Furan but enhanced the
selectivity to FOL. Among all, catalyst FC-10 gave better
selectivity to FOL at all temperatures employed. Furthermore,
selectivity of FOL for all FC-X catalysts were almost above 90%
at 175 °C, which implies that the selectivity mainly depends on
Cu, whereas, FAL conversion depends on the cooperative
interaction between Fe and Cu. The evaluated results were
concluded that the introduction ofFe into the Cu-based
catalyst can promote the catalytic activity and selectivity. It has
allowed us to suggest that this promoting effect results from
the synergistic interaction between Cu and Fe.36Furthermore,
the appropriate Fe/Cu ratio helps to achieve an excellent yield
of FOL over others. From the results FC-10 is optimized as the
best catalyst and it is used for further studies.
Fig. 5In situ XPS spectra recorded for a) Fe 2p and b) Cu 2p core level of
bimetallic FC-10 catalyst at different temperatures under UHV and 0.1 mbar H2.
Catalytic performance
We began our studies with the effect of different metal
loadings and followed to test onvarious reaction parameters
on the conversion of FAL to FOL. Under the studied reaction
conditions the main products were: FOL, 2-MF and Furan. The
results are given in Table 2. The conversion of FAL was <90% of
all samples except for entry 6(93%), whereas the selectivity of
FOL was >90% for all catalysts except for F-10 (entry 2 and 8).
This dissimilar was extensively discussed by studying the
selective hydrogenation of FAL by using various Fe content and
by keeping the Cu content same. We have also investigated
the influence of experimental conditions on the product
distribution over bimetallic FC-10 and monometallic F-10 and
C-10 catalysts for comparison. A control experiment, loading
the reactor tube with γ-alumina was conducted under the
optimized reaction conditions which exhibited scarce
conversion of FAL.
Effect of Fe loading
Fig. 6 displays the effect of the Fe wt% loading on the catalyst
activity.Fig. 6 (a) indicates that C-10 had a much higher conversion
for hydrogenation of FAL than F-10.However, the
As mentioned above, the selectivity to C-C the cleavage
product was remarkably reduced by the addition of Fe to C-10.
This is also related to the formation of FeCu bimetallic
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Fig. 6Performance of selective hydrogenation of FAL as
a function of reaction
interaction in the catalytic system. However, the formation of
FeCu bimetallic synergism with Fe enriched surface leads to
the interference of the C–C hydrogenolysis at higher
temperatures. Besides, Cu has also been known for decreasing
the C–C hydrogenolysis activity due to its strong repulsion
against furan ring.41 It is found that the decarbonylation and
hydrogenation are not strongly dependent on conversion but
mainly depend on reaction temperature.42
temperature and Fe loadings.a) FAL conversion and b) Selectivity of FOLand side
products include 2-MF and Furan. Reaction conditions: LHSV=1 h−1 with respect to
FAL and GHSV=1200 h−1 with respect to H2,
1 atm. pressure and 1 mL
catalyst.(S=Selectivity).
Since FC-10 exhibited a superiorselective hydrogenation
performance, the effects of LHSV (Liquid Hourly Space
Velocity) and GHSV (Gas Hourly Space Velocity) on product
distributions during hydrogenation of FAL were discussed. Fig.
7 shows the effect of LHSV of FAL on the performance of the
FC-10 catalyst. As the LHSV of FAL rose from 1 to 2.5 h-1 both
the conversion and FOL selectivity decreased from 93.4 to
74.6% and from 98.6 to 82.5%, respectively, while the
selectivity of 2-MF and Furan increased slightly, indicating the
selective hydrogenation performance is decreased with
increased LHSV. The decreased contact time with increased
LSHV also account for the decrease in selectivity. Furthermore,
the increased flow of FAL enhanced the coke formation might
be due to the formation of polymeric species on the catalyst
surface.
Effect of reaction conditions
Fig. 6 also shows the effect of reaction temperature on the
performance of different catalysts. In all cases, increasing
temperature promoted the conversion of FAL due to the
increased reaction rate. As the temperature increased from
175 to 250 °C, the selectivity to FOL significantly decreased and
selectivity to 2-MF and Furan increased consequently because
high temperature favouredthe hydrodeoxygenation and
decarbonylationof FAL, respectively. At higher temperatures,
the decreased selectivity to FOL is ascribed to the enhanced
cracking reaction. It is remarkable that the effect of reaction
temperature on the selectivity of cracked product is more
conclusive for C-10 as compared to bimetallic FC-X catalysts.
This further substantiates that the formation of FeCu
bimetallic interaction is significant for the superior
performance of bimetallic FC-X catalytic system.
The effect of GHSV of hydrogen on the performance FC-10
catalyst is shown in Fig. 8. Forincreased GHSV from 600 to
3000 h-1the conversion and the selectivity to FOL tended to
decrease slightly, while the selectivity to 2-MF increased
slightly with the rise of GHSV. This seems to indicate that the
increase of GHSV favours the C-O hydrogenolysis, which is
plausibly due to the increased hydrogen concentration on the
catalyst surface.No considerable variation in activity results at
all GHSV shows thus the hydrogenation of FAL follows pseudo
first order kinetics.43
Fig. 7 Conversion of FAL and selectivity of FOL at various LHSV of FAL over bimetallic FC-
10 catalyst. Reaction conditions: 175 °C, GHSV=1200 h−1 with respect to H2,1 atm.
pressure and 1 mL catalyst. (S=Selectivity)
Stability of Catalyst
To underpin the stability of the monometallic C-10 and
bimetallic FC-10 catalyst, hydrogenation of FAL reaction was
carried out for a longer time on stream (TOS, 24 hours) under
the optimized reaction conditions. Fig. 9 shows the conversion
and selectivity to FOL. As shown in Fig. 10, C-10 catalyst shows
a maximum conversion (62%) and FOL selectivity (87.8%),
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while both conversion and selectivity decreased with surface area from 202.6 to 172.4 m2 g-1(Table 1) for C-10 spent
prolonging the reaction time. It is noteworthy that the C-10 catalyst also accounts for this decline in activity.The ICP-OES
has less stability, indicating the agglomeration of Cu particle analysis of the spent catalysts and the product mixtures shows
after certain period. In contrast with C-10 activity, bimetallic that there is no leaching of metal. The observed results
FC-10 catalyst exhibited the steady state catalytic activity even conclude that the developed catalyst system is robust and
up to 24hours. Almost 94% conversion and >98% selectivity displayed an admirablecatalytic activity for the selective
were achieved with a marginal decline of catalytic activity. This hydrogenation of FAL to FOL and it can be applied to the
kind of stable activity was rarely reported due to the coke hydrogenation of various biomass derived substrates to obtain
deposition during the by-product formation in the conversion value added chemicals and fuels.
of biomass derived substrates. Generally, stability of the
catalyst is mainly depends on the coke deposition during
reaction course, herein C-C cleavage is the strong possibility
for the coke formation.It is also evidence that the addition of
Fe preventing the sintering of Cu and also inhibits the Cu
agglomeration on surfaces during the reaction. Besides, we
concluded that the addition of Fe depleting the C-C cleavage
through synergistic FeCu bimetallic interaction, which might be
the reason for less or trivial by-products (<5%) and longer
stability.
To perceive the properties of catalystafter TOS, both C-10
and FC-10 spent catalysts were characterized and shown in Fig.
10. We exempted XRD analysis since all catalysts exhibited
high dispersion. TEM image of FC-10 spent catalyst clearly
depicts that there is no morphological change even after 24
hours of reaction time, whereas the C-10 catalyst showed
Fig. 9 Conversion of FAL and selectivity of FOL for 24 hours TOS over C-10 and FC-10
agglomeration.From the Cu particle sizes after reaction for 24
catalyst. Reaction conditions: 175°C, 1 atm. LHSV=1 h−1 with respect to FAL and
hours, it was found that increase in particle size for C-10 (11.7
GHSV=1200 h−1 with respect to H2, 1 atm. pressure and 1 mL catalyst. (S=Selectivity)
nm) as compared to FC-10 (9.1 nm) implies that the sintering
Plausible mechanism
The exact mechanistic pathway for the reduction of C=O group
of FAL to FOL on catalytic surface is not fully understood.
Although the kinetics and mechanism for FAL hydrogenation
activity were discussed by Resasco et al. and other groups,41,42
the origin of the activity on bimetallic catalyst systems are still
unclear. The observed results from in situ XRD and XPS
investigations provide some new and critical information
about the reducibility of both Fe and Cu. Reduction treatment
provided reduced Cu as well as Fe particles and those are in
close proximity. In mechanistic view, adsorption of C=O and
C=C of FAL on FeCu/γ-Al2O3 surface are the two prospects of
adsorption modes for the formation of FOL.There are
considerable reports were available for the mechanism of both
possibilities.42,46Sincemetallic Cu surfaces have a very weak
Fig. 8 Conversion of FAL and selectivity of FOL at various GHSV of hydrogen over
affinityto C=C bonds, and α,β-unsaturated aldehydes,including
FAL,24implied that the adsorption of FAL on Cu results in an
ɳ1(O)-surface species, in which the carbonyl group is bound
bimetallic FC-10 catalyst. Reaction conditions: 175°C, LHSV=1 h−1 with respect to FAL, 1
atm. pressure and 1 mL catalyst. (S=Selectivity)
tothe metal through the oxygen lone pair while the rest of the
of metal particles on C-10 was more serious than that on FC-10
molecule is pushed away from the surface due to a net
(Table 1, entry 10, 11).It is evidence that the addition of Fe
repulsion betweenC=C and Cu. The preferred adsorption in the
ɳ1(O)-mode is responsible for the high hydrogenation
inhibited growth of Cu particles. The change in the morphology
of the monometallic C-10 catalyst as compared with bimetallic
selectivity to FOL, typically observed on Cu-based catalysts.In
FC-10 attributed to the decline in the activity. The recorded
our studies, no ring hydrogenated products were observed at
XPS spectrum of spent catalyst proves the presence of Cu0 (B.E
any temperature, LHSV as well as GHSV (Fig. S4 and S5, GC-MS
value of 933.3 eV) and Cu2+ (B.E value of 935.3 eV), the
data, ESI), which indicatesthe Cu surface is strongly interacting
availability of Cu0 species are the most probable active centers
with FAL via C=O and strongly repulsing furan ring (Scheme
2).42
for the prolonged catalytic activity. Moreover, the decrease in
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Scheme 2.Schematic illustration of possible reaction pathway of FAL hydrogenation
over monometallic Cu/γ-Al2O3 and bimetallic Fe-Cu/γ-Al2O3catalysts.
Conclusions
Herein, wereported the synthesis of γ-Al2O3 supported FeCu
based heterobimetallic system by simple co-impregnation
method. The characterization results confirmed that the
formation of FeCu bimetallic species during pre-treatment at
400 °C under H2 flow. The conversion of FAL on FeCu/γ-Al2O3
yields mainly FOL. The better catalytic activity with >98% FOL
selectivity was achieved over a FC-10 catalyst under optimized
reaction conditions. It was found that the addition of Fe
enhances the activity that results in higher FAL conversion,
whereas the selectivity was driven mostly by Cu. We discussed
that the product distributions over various reaction
parameters. The prolonged catalytic activity (up to 24 hours
TOS) without any significant loss in activity was observed,
proved the robustness of the catalyst. The analysis of spent
catalysts also evidences that there was no morphology change
and no significant increase in Cu particle size after the
reaction. The plausible mechanism was proposed on the basis
of products formed and the catalyst characterization results.
The adsorption behavior of both Fe and Cu were also
discussed. Further, the cooperative activity of FeCu system
was extensively explained. Based on the studies we strongly
believethat our presented work may induce to carving out Cr-
free noble-metalfree innovative catalysts for the selective
conversion of biomass derived compounds to fuels and
chemicals.
Fig. 10(a&b) are the TEM images of C-10 and FC-10 spent catalysts, respectively. c) XPS
core level spectra of Cu for calcined and spent FC-10 catalyst.
It is also known that Cu has higher H2 adsorption and
activation abilities but a weaker oxygen affinity than Fe.
According to the DFT calculations,42 it has been found that the
metal–oxygen affinity of Fe is higher than FeCu bimetallic and
Cu. For monometallic F-10 and C-10 catalysts, the oxygen
vacancies on the iron oxide and Lewis acidic sites of alumina
can bind oxygen atom of C=O group through the donation of
the lone pair of electrons and subsequently the C=O bond is
activated followed by hydrogenation.45,46For bimetallic FC-X
catalysts, carbonyl group oxygen is preferentially adsorbed on
either Fe metallic or Fe oxides than metallic Cu sites and both
Fe and Cu sites could be synergistically promoting the
conversion of FAL. In other words, H2 is easily dissociated on
Cu sites and spills over to the neighboring Fe sites that is
having adsorbed oxygen atom of a C=O group of FAL, where
the hydrogenation takes place leading to the hydrogenation
product.This phenomenon can be witnessed from Fig. 6b, i.e.,
the selectivity to FOL increased and decarbonylation
suppressed with the Fe loading. This is obviously determinable
to the promoting role of Fe for the selective hydrogenation as
well as for reducing C-C hydrogenolysis. This promotional
effect on the selective hydrogenation/hydrogenolysis owing to
the increase in oxygen affinity of metal sites has also been
reported overFeNi/SiO2catalysts.42
Experimental section
Synthesis of catalysts
The FeCu/γ-Al2O3catalysts with different Fe loadings were
prepared by incipient wetness impregnation method, using an
aqueous solution containing both metal nitrate precursors
Cu(NO3)2.6H2O and Fe(NO3)3.9H2O (98%, Merck). Boehmite
derived γ-Al2O3 having a surface area of 182 m2/g was used as
support, which was dried at 110 °C prior to impregnation.
Then, aqueous solution containing the desired amount of
metals was directly impregnated to the powdered γ-
Al2O3support. In all cases the content of Cu was kept constant
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at 10 wt% and Fe wt% was varied from 2.5 to 12.5 wt%. After The products were condensed using chiller and collected by
impregnation, samples were first dried at room temperature every hour and the same were analyzed by Gas
for overnight and subsequently at 110 °C for 12 hours in oven. Chromatography (Varian-CP3800) with FID having HP-5
The obtained samples were calcined at 500 °C for 4 hours. For capillary column (30 m × 0.32 mm × 0.25µm). The carbon
comparison, γ-Al2O3 supported Fe and Cu (10 wt%, balances were checked in every run and found to be ranged
respectively) were prepared with the procedures as same to between 98 and 101% and all data points were obtained in
those of FeCu/γ-Al2O3 catalysts. Synthesized catalysts are duplicate with an error of ±2%. The FAL conversion, FOL
labelled as FC-X, whereas X indicates the amount of Fe loading selectivity and yield were calculated and defined as follows:
and FC stands for Fe and Cu, respectively. Where, F-10 and C-
10 are the γ-Al2O3supported monometallic catalysts containing
10 wt% of Fe and 10 wt% of Cu, respectively. The bulk
compositions of catalysts were analyzed by ICP-OES, which
were in good agreement with the theoretical values. All
prepared catalysts were directly used after calcination.
Characterization techniques
Powder X-ray diffraction (XRD) data of synthesized materials
were collected from PAN analytical X’pert Pro dual goniometer
diffractometer. The chemical compositions of the synthesized
catalysts were estimated by Inductively Coupled Plasma-
Acknowledgements
Optical Emission Spectrometry (ICP-OES, Spectro Arcos, FHS-
TR thanks the Department of Science and Technology, India-SERB
12).Temperature programmed reduction (TPR) and H2-N2O
(no. SR/S1/PC-17/2011) and CSC 0125 12 FYP for funding and MM
thanks DST for fellowship.
titration studies were carried out on Micrometrics 2920
instrument. Specific surface area, pore volume and average
pore diameter of the materials were determined by nitrogen
adsorption–desorption analysis at liquid nitrogen temperature
(−196°C) using AutosorbIQ Quantachrome, USA. The
Brunauer–Emmett–Teller (BET) equation was used to calculate
the surface area from the adsorption branch at a pressure
range of (p/po= 0.05 to 0.3). Pore volume and pore size
measurements were calculated using Barret, Joyner and
Halenda (BJH) methodat a relative pressure of 0.99. A FEI
TECNAI F20 electron microscope operating at 200 kV was used
for recording high resolution transmission electron microscopy
(TEM) of all materials. Iron and copper core levels were
studied with X-ray photoelectron spectrometer (XPS) system
from Prevac and equipped with VG Scienta monochromator
(MX650) using AlKα anode (1486.6 eV).
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