Synergism studies on alumina-supported copper-nickel catalysts towards furfural and 5-hydroxymethylfurfural hydrogenation

Article abstract of DOI:10.1016/j.molcata.2016.11.023

The structure of alumina-supported copper-nickel catalysts prepared by impregnation method was studied by using a combination of various characterization techniques including XRD, N₂-sorption, TEM, H₂-TPR, NH₃-TPD, XANES/EXAFS and CHNS methods. A series of characterizations revealed that dispersion of copper increases with an increase in nickel loading on ?-Al₂O₃ due to strong interaction between nickel and copper oxide particles. This also led to the formation of mixed copper-nickel oxides in calcined catalysts at the highest loading of nickel and copper (i.e. Cu/Ni = 1). In catalytic activity, both the monometallic Cu/?-Al₂O₃ and Ni/?-Al₂O₃ showed lower activity and selectivity towards hydrogenation of both Furfural and 5-Hydroxymethylfurfural to 2-methylfuran (2-MF) and 2, 5-dimethylfuran (DMF), respectively. However, with increase in nickel loading, the activity and the selectivity of Cu/?-Al₂O₃ drastically increased for both the cases and Cu-Ni/?-Al₂O₃ (Cu/Ni = 1) showed the highest catalytic activity. Furthermore, combination of Cu/Ni ratios and temperature plays a significant role in the product distribution, as in the case of furfural hydrogenation, at a lower temperature, furfuryl alcohol (FOL) appears as the main product while at a higher temperature, 2-methylfuran (2-MF) is found to be the dominant product over Cu-Ni/?-Al₂O₃ (Cu/Ni = 1) catalysts. Similarly, 2,5-bishydroxymethylfuran (BHF) is found to be the major product at a lower temperature and 2,5-dimethylfuran (DMF) is selectively produced at a higher temperature in the HMF hydrogenation. Furthermore, reaction pathways are discussed for both the reactions.

Full text of DOI:10.1016/j.molcata.2016.11.023

Accepted Manuscript
Title: Synergism studies on alumina-supported copper-nickel
catalysts towards furfural and 5-hydroxymethylfurfural
hydrogenation
Author: Sanjay Srivastava G.C. Jadeja Jigisha Parikh
PII:
S1381-1169(16)30499-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.11.023
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16-11-2016
16-11-2016
Please cite this article as: Sanjay Srivastava, G.C.Jadeja, Jigisha Parikh, Synergism
studies on alumina-supported copper-nickel catalysts towards furfural and 5-
hydroxymethylfurfural hydrogenation, Journal of Molecular Catalysis A: Chemical
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Synergism studies on alumina-supported copper-nickel catalysts towards furfural and 5-
hydroxymethylfurfural hydrogenation
Sanjay Srivastava, G. C. Jadeja, Jigisha Parikh*
Department of Chemical Engineering, Sardar Vallabhbhai National Institute of Technology,
Surat-395007, Gujarat, India.
*
Corresponding author, Tele: + 91261-2251689
E-mail: jk_parikh@yahoo.co.in, Jigisha K Parikh
1

Graphical abstract
Present contribution focuses on the structural and performance studies on alumina-supported
copper-nickel catalysts for hydrogenation of furfural and 5-hydroxymethylfurfural towards 2-
methylfuran and 2, 5-dimethylfuran which are used as biofuels additives.
Highlights



Synergy between Cu and Ni results in highly dispersed Cu particles on to alumina
surface.
Cr-free, noble metal free bimetallic alumina supported Cu-Ni catalysts are found to be
active for hydrogenation of both FAL and HMF.
Temperature played a significant role in driving the selectivity towards 2-MF and DMF
via hydrogenolysis.
2

Abstract
The structure of alumina-supported copper-nickel catalysts prepared by impregnation method was
studied by using a combination of various characterization techniques including XRD, N
sorption, TEM, H -TPR, NH -TPD, XANES/EXAFS and CHNS methods. A series of
characterizations revealed that dispersion of copper increases with an increase in nickel loading
on ɣ-Al due to strong interaction between nickel and copper oxide particles. This also led to
the formation of mixed copper-nickel oxides in calcined catalysts at the highest loading of nickel
and copper (i.e. Cu/Ni=1). In catalytic activity, both the monometallic Cu/ɣ-Al andNi/ɣ-Al
2
-
2
3
2
O
3
2
O
3
2
O
3
showed lower activity and selectivity towards hydrogenation of both Furfural and 5-
Hydroxymethylfurfural to 2-methylfuran (2-MF) and 2, 5-dimethylfuran (DMF), respectively.
However, with increase in nickel loading, the activity and the selectivity of Cu/ɣ-Al
2
3
O drastically
increased for both the cases and Cu-Ni/ɣ-Al (Cu/Ni=1) showed the highest catalytic activity.
2
O
3
Furthermore, combination of Cu/Ni ratios and temperature plays a significant role in the product
distribution, as in the case of furfural hydrogenation, at a lower temperature, furfuryl alcohol
(
FOL) appears as the main product while at a higher temperature, 2-methylfuran (2-MF) is found
to be the dominant product over Cu-Ni/ɣ-Al (Cu/Ni=1) catalysts. Similarly, 2,5-
2
O
3
bishydroxymethylfuran (BHF) is found to be the major product at a lower temperature and 2,5-
dimethylfuran (DMF) is selectively produced at a higher temperature in the HMF hydrogenation.
Furthermore, reaction pathways are discussed for both the reactions.
Keywords: Furfural, 5-hydromethylfurfural, Cu-Ni, 2-methylfuran, 2,5-dimethylfuran
3

1
. Introduction
Over the years, valorization of abundantly accessible biomass derived commodities such as
furfural (FAL) and 5-hydroxymethylfurfural (HMF) to fuel and other value-added chemicals has
been the focus of academic research in lieu of the reduction of our dependence on fossil resources
and to minimize the environmental pollution caused by the increasing level of CO and CO in the
2
atmosphere [1-2]. FAL and HMF are the platform chemicals which can be obtained by acid
hydrolysis of C-5 and C-6 sugars such as xylose and cellulose [3-4]. FAL and HMF can be
upgraded to value added chemicals and fuel additives via different processes such as
hydrogenation, hydrogenolysis, etherification, aldol-condensation and acetallization [5-8].
Hydrogenation followed by hydrogenolysis of FAL and HMF is the most studied process for the
conversion of FAL and HMF towards bio-fuel additives such as 2-methylfuran (2-MF), 2-
methyltetrahydofuran (MTHF), Pentane and 2,5-dimethylfuran (DMF), respectively[9-16].
Hydrogenation of FAL gives a variety of products which are used as important industrial chemicals
and as gasoline additives such as FOL, 2-MF, MTHF, tetrahydrofurfuryl alcohol (THFOL),
cyclopentanone, furfurylamine, 1,5-pentanediol, and so on [Scheme.1]. Amongst them, the
synthesis of FOL and 2-MF has been receiving attention owing to their wide range of applications.
FOL is widely used in the resin industries for the synthesis of epoxy resin, liquid resin, and as a
modifier for phenolic and urea resins. It is also used in the making of acid resistant bricks,
corrosion proof fibre glass, anti-corrosion fibres concrete, lysine, vitamin C, lubricants, and
plasticizers [17-19]. 2-MF has been recently used as a gasoline blend in standard vehicles, as
perfume intermediates, chloroquine lateral chains in medical intermediates, and as a raw material
for the production of crysanthemate pesticides [12-13].
4

Similar to FAL, HMF is also known to be a resourceful chemical which is used to form a variety
of products [Scheme.2]. However, it is particularly known for the synthesis of DMF which is used
-3
as a gasoline blend due to its high energy density (30 kJcm ) and research octane number (RON)
of 119 [14].
Over the last two decades, various chromium-free and noble-metal free catalysts have been
developed for the conversion of both FAL and HMF [12, 13, 16-23]. Normally, noble metal
catalysts are found to be highly active even at mild reaction conditions. However, due to FAL and
2
HMF being adsorbed and activated on the surface of noble metal through the model of η (C,O)-
coordination, the very high activity of these catalysts usually lead to high selectivity for some
undesirable side reaction such as C=C hydrogenation [5]. In this context, Cu-catalysts exhibited
highly efficient hydrogenation of biomass-derived FAL and HMF on which adsorption proceeded
1
1
via η (O)- coordination model wherein, η (O)-surface species is generated to bound the active
metal, leading to high hydrogenation activity towards C=O bond and therefore, further
hydrogenolysis of C-O bond [12, 13, 21]. The deactivation of Cu-based catalysts is associated with
the sintering and leaching of copper species. The addition of a second metal (Co, Ni, or, Fe)
improves the activity, selectivity and the stability of copper-based catalysts for liquid-phase
catalytic reactions. In our previous work, we have successfully demonstrated the role of Co in
enhancing the dispersion of Cu over supported Cu catalysts for hydrogenolysis of FAL to MF [12].
The present work has been undertaken based on the above background. Herein, we aimed to
explore chromium-free and noble metal-free alumina supported Cu–Ni catalysts for the
hydrogenation of furfural and 5-hydroxymethylfural. The structure of Cu-Ni catalysts was studied
by using a series of characterization techniques such as XRD, BET, TEM, NH
3
-TPD, H -TPR,
2
XANES/EXAFS and CHNS analysis. The Cu–Ni/ɣ-Al catalysts were found to be efficient for
2
O
3
5

the hydrogenation of biomass derived FAL and HMF, which respectively led to the selective
production of 2-MF (84% yield) and DMF (56% yield) under mild conditions.
2
. Experimental
.1 Materials
Metal salts such as copper nitrate (Cu(NO
2
3
)
2
.3H
2
O) and nickel nitrate (Ni(NO
3
)
2
.6H O), chemicals
2
such as 5-hydroxymethylfurfural, 5-methylfurfuryl alcohol, furfural, furfuryl alcohol, 2-methyl
furan, 2-methyltetrahydrofuran, 2-pentanone, 2-pentanol, 2, 5-dimethylfuran, and tetrahydrofuran
were of GC grade, purchased from Sigma-Aldrich Mumbai, India. Support ɣ-Al
was purchased from a Sterling Chemical Pvt. Ltd. Surat India.
.2 Catalysts synthesis and characterization
The bi-metallic catalysts were prepared by impregnation method by using respective metal salts.
The Cu–Ni/ ɣ-Al catalysts containing fixed (Cu =10 wt%) and varying Ni loadings of 10, 5,
and 2.5 wt%, respectively, are denoted as Cu-Ni/ɣ-Al (Cu/Ni=1), Cu-Ni/ɣ-Al (Cu/Ni=2)
and Cu-Ni/ɣ-Al (Cu/Ni=4). In order to study the textural and structural properties of prepared
2
3
O (99% pure)
2
2
O
3
2
O
3
2
O
3
2
O
3
catalysts, all the samples were characterized by X-ray diffraction (XRD), BET-surface area, H
temperature-programmed reduction (TPR), transmission electron microscopy (TEM), NH
2
-
-
3
temperature-programmed desorption (TPD), X-ray absorption spectroscopy (XANES/EXAFS),
and CHNS analysis. Detailed procedure regarding the synthesis and characterization of supported
bimetallic copper-nickel catalysts is provided in the supplementary information.
2
2
.3 Catalytic Activity
.3.1 Hydrogenation of furfural
The catalytic hydrogenation of FAL was carried out in 100 mL stainless steel autoclave with a
magnetic stirrer, a pressure gauge and automatic temperature control. In a typical experiment, 1.0g
o
of catalyst was reduced by passing H
2
at 300±2 C and 0.1MPa for 3 h. The reactor was cooled
6

down to room temperature and flushed with N . Then 4.32g of FAL in 20 mL of 2-propanol was
2
added and charged into the reactor. The catalytic activity of prepared catalysts was investigated at
o
1
30-220 C and 4.0 MPa with stirring speed at 1000 rpm. Product samples were analyzed using a
Sigma GC system with FID detector. AB-5 capillary column with dimension of 30 m x 0.25 mm
o
x 0.5μm was used. In a typical procedure, the initial oven temperature was held at 80 C for 4 min
o
o
and increased up to 220 C with the ramp of 20 C /min and hold for 4 min. one μL of liquid
product was injected into the capillary column by using the split ratio of 20:1, where nitrogen was
used as a carrier gas.
2
.3.2 Hydrogenation of 5-hydroxymethylfurfural
Same apparatus was used for the catalytic hydrogenation of HMF. Prior to reaction, 0.5g of catalyst
o
was reduced by passing H
2
at 300±2 C and 0.1MPa for 3 h. Then calculated amount of substrate
(
0.2 g of HMF in 20 mL of THF) was added and charged into the reactor. The catalytic activity of
o
prepared catalysts was investigated at 130 and 200 C, and 3.0 MPa with stirring speed at 1000
rpm. Each experimental run was repeated twice for ensuring reproducibility. The product samples
were analyzed using Sigma GC system with FID detector. AB-5 capillary column with dimension
of 30 m x 0.25 mm x 0.5μm was used. In a typical procedure, the initial oven temperature was held
o
o
o
o
at 50 C and increased up to 180 C with the ramp of 10 C/min, and further increased up to 280 C
o
with the ramp of 5 C/min and hold for 2 min. one μL of liquid product injected into the capillary
column by using the split ratio of 20:1, where nitrogen was used as a carrier gas.
3
. Result and discussion
3
3
.1 Structure Evaluation of Catalysts
.1.1 X-ray diffraction
The supported bi-metallic Cu-Ni/ɣ-Al
2
3
O (Cu/Ni=1, 2 and 4) catalysts were characterized using
XRD to recognize the crystalline phases. The XRD spectra of calcined and reduced catalysts are
7

illustrated in Figures. 1 & 2 respectively. In case of calcined samples, the broad peak observed at
θ= 67.2° for all the catalysts is corresponding to a reflection of ɣ-Al (JCPDS: 86-1410) [12].
2
2
O
3
All three catalysts displayed sharp diffraction peaks at 2θ = 35.6° and 38.7°, corresponding to the
planes (111) and (111) of monoclinic copper oxide (JCPDS 80-1917, 45-0937) [17]. The two
diffraction peaks at 2θ = 37.3° and 43.3°, corresponding NiO (JCPDS 78-0643) [24]. The average
crystallite size of NiO was calculated by line broadening of (100) of NiO at about 2θ = 43.3° using
Scherrer’s equation and found to be <15 nm. Likewise, the average crystallite size of CuO was
calculated by line broadening of (111) of CuO (JCPDS 80-1917) at about 2θ = 35.6°. The size of
CuO was found to be 35 nm for Cu-Ni/ ɣ-Al
Cu-Ni/ ɣ-Al (Cu/Ni=1). This shows that the adding up of Ni precursor promoted the dispersion
of CuO species and reduced the CuO crystallite size drastically from 35 nm to a size of (< 20 nm).
The crystal phase evolution of reduced Cu-Ni/ɣ-Al catalysts with different Cu/Ni ratios was
2
O
3
(Cu/Ni=4) catalyst which decreased to 20 nm for
2
O
3
2
O
3
also observed in the wide angle domain to further look at the structure of bimetallic catalysts. The
catalysts were activated by adopting the same procedure as reported in our previous work [12]. In
o
brief, all the samples were reduced in an autoclave by using external hydrogen at 300 C and 0.1
MPa for 3h. The reduced samples were kept in a vacuum desiccator till further analysis with XRD.
The recorded XRD spectra are shown in Figure. 2. It can be deduced that copper oxide is almost
o
reduced to copper metal in presence of Ni at 300 C. The featured diffraction lines of Cu metal, i.e.,
(
111), (200), and (220) (FCC structure), were noticeable at 2θ = 43.6°, 50.8°, and 74.4°,
respectively in all the three catalysts (JCPDS 04-0836) [17]. Furthermore, the reduction of CuO to
Cu metal increased with an increase in Ni content from 2.5 to 10 wt% which was further justified
o
by XANES/EXAFS analysis. Other than metallic Cu, two noticeable diffraction spectra at 37 and
o
o
4
4.3 ascribed to NiO and Ni or Cu, correspondingly. It is noteworthy that the 2θ values of Cu
o
and Ni are almost similar; consequently it is not easy to make a distinction among them at 2θ =
8

o
4
4.3 [24]. Moreover, mixed CuNiO solution might be shaped owing to the similar FCC structures
with similar lattice parameters [29]. Thereby, it can be figured out that the reduced samples are
having NiO and metallic Ni or Cu species. These observations are consistent with the previous
reports on bimetallic Cu-Ni catalysts [24-32]. Furthermore, in order to get clarity in the case of the
bimetallic samples, a progressive shift of the metallic phase peak (the position of the Cu (220)
reflection) is observed with an increase in the Ni loading (Figure.2) which suggests the formation
of Cu-Ni alloy due to synergy between Cu and Ni metals. Notably, a similar trend was previously
reported in the literature which revealed the formation of bimetallic alloy by varying the
composition of Cu/Ni [28-29].
3
.1.2 N
2
-Sorption analysis
Textural properties of all the synthesized catalysts are shown in the Table.1. The support ɣ-Al
2
O
3
2
featured a BET of 161 m /g and a total pore volume of 0.54 mL/g [12]. The surface area and pore
volume of the impregnated catalysts decreased with respect to bare ɣ-Al , attributed to the
relatively high metal loading (10 wt.%), wherein, Ni/Cu particles might block the pores of ɣ-Al
2
O
3
2
O
3
during metal impregnation which in turn decreased the pore volume of catalysts. Furthermore,
when all three bimetallic catalysts were compared, it was observed that the surface area, the pore
volume of bi-metallic catalysts increased with increasing amounts of Ni and Cu-Ni/ɣ-Al
2
O
3
(
Cu/Ni=1) showed the maximum values (ref. Table.1). The strong interaction between Cu and Ni
might be facilitating the dispersion of Cu. The BET results are consistent with the XRD results
which revealed that Cu dispersion is enhanced with increase in Ni loading. On considering the
XRD results, it could be postulated that Ni loading has a strong influence on the stabilization of
the copper nanoparticles because any apparent diffraction peaks allocated to CuO could not be
observed, which showed that the incorporation of nickel species could prevent the agglomeration
of copper species, leading to an increase in surface area with an increase in Ni loading [33-34]. It
9

was further justified from TEM analysis that after activation in hydrogen, the metal species
dispersed evenly on the surface of the support when Ni was at its maximal loading.
3
.1.2 Transmission electron microscopy
Figure.3 demonstrates the TEM micrograph of alumina supported Cu-Ni/ɣ-Al
2
O
3
catalysts with
at 300 C. From a pictorial view, it was
observed that the dispersion of metal particles increased over alumina support with adding up Ni
species and aggregation of metal particles appeared to be minimum in Cu-Ni/ɣ-Al (Cu/Ni=1).
o
various (Cu/Ni=1, 2 and 4) ratios reduced under H
2
2
O
3
It was interesting to see that Ni modified (Cu/Ni=4) catalyst contained two kinds of metal species,
highly dispersed metal particles having smaller size and segregated clusters (Figure. 3f). With
further increased nickel content, an optimized dispersion of copper species could be observed
(
Figure. 3b). Moreover, Cu-Ni/ɣ-Al
the range of 25-35 nm. However, as Ni loading increases from (2.5 wt%) to (10 wt%) i.e.,
Cu/Ni=4) to (Cu/Ni=1) the size of crystallites found to be decreased from 25-35 nm to 10-15 nm
2
O
3
(Cu/Ni=4) revealed the formation of bigger crystallites in
(
with uniform distribution of metal nanoparticles (Figure. 3 & 4). This is consistent with the XRD
results. The diffraction peaks of Cu (FCC) decreased with Ni loading and Ni peaks were very
weak, indicating the presence of smaller particles. These observations are well matched with
reported literature on bi-metallic Cu-Ni catalysts [35]. To further estimate the composition of
doped metals, energy dispersive scattering analysis was carried out. The results showed the
presence of Ni and Cu in appropriate proportions. The corresponding metal loadings are presented
in Table 1. The results were in agreement with the actual loading amounts used in the synthesis
procedure within acceptable errors.
3
.1.4 H -temperature program reduction
2
Figure.5 shows the reduction patterns of monometallic Cu, Ni and bi-metallic Cu-Ni (Cu/Ni=1, 2
o
and 4) catalysts supported on γ-Al
2
O
3
. The low temperature reduction (150-280 C) profile of CuO
10

on ɣ-Al
2
O
3
was observed. This reduction pattern might either follow a one-step reduction 퐶푢2+
표
+
→
퐶푢 mechanism or proceed through an intermediate 퐶푢 state. The peak located at a lower
o
temperature (190 C) is attributed to the reduction of highly dispersed CuO on γ-Al
2
3
O , while the
o
peak at a higher temperature (260 C) represents the reduction of bulk CuO particles. The Ni/ γ-
Al showed broad multiple reduction peaks. This might be attributed to the presence of mixed
NiOx species, resulting via distinct interaction of NiO with γ-Al . Furthermore, the different
2
O
3
2
O
3
reduction temperatures could be partially ascribed to a different size of NiO particles. In general,
on Ni-catalysts, the low temperature peaks are attributed to the reduction of the NiO particles
weakly interacting with the support, while the high temperature peaks are confined to the reduction
of NiO species in intimate contact with the Al
2
O
3
and/or attributed to the new structure of NiAl
2
O
4
[
36].
The reduction profiles of bimetallic Cu-Ni catalysts are found to be much different from their
monometallic counterparts; exhibited reduction peaks between them which clearly suggest the
synergy between CuO and NiO oxides. It is interesting to see that the H
2
consumption peak
corresponding to NiO reduction shifted to a lower temperature, and that of CuO shifted to a higher
2
+
temperature as illustrated in Figure.5. It was reported that the ultimate integration of Ni into the
2
+
CuO framework results in a slight narrowing of the lattice owing to the smaller radius of Ni than
2
+
that of Cu [33]. Naghash et al. reported that the reduction temperature of NiO greatly reduces
o
(
[
<300 C) in presence of Cu and the reduction of NiO was more likely catalyzed by metallic copper
o
37], as pure nickel oxide is typically reduced at above 450 C. On the other hand, the maxima
o
related to copper oxide reduction was progressively shifted to higher temperature (275 to 295 C)
on increase in Ni loading. This can be explained by the two possible reasons; (1) the positive
influence of NiO addition on the CuO dispersion by the dilution effect [38-39] and/or (2) the
11

synergistic effect between Ni and Cu [40]. These observations are consistent with XRD, BET,
TEM and further XANES/EXAFS analyses.
3
.1.5 NH
The surface acidity of supported bi-metallic Cu–Ni/ɣ-Al
and 4) ratios was determined by NH -TPD experiments. Generally, catalysts show three types of
surface acidity which has been classified as weak (<523K), medium (523-623K) and strong
>623K) acidity based on desorption of ammonia with respect to temperature. In the present work,
we have used commercial ɣ-Al as a support to bimetallic Cu-Ni catalysts having the total
acidity of 0.28 mmol of NH /g of catalyst [12].. It is worth noting that presence of five different
types of OH groups on the surface of ɣ-Al ; exhibits a distinct “net electric charge” (σ),
3
-Temperature programmed desorption
2
3
O catalysts with various (Cu/Ni = 1, 2
3
(
2
O
3
3
2
O
3
depending on the number of Al neighbours and on Al coordination [41-42]. The removal of OH
groups during high temperature treatment (calcination) creates co-ordinatively unsaturated surface
(
cus) cations where tetrahedral (Al IV) and octahedral (Al VI) aluminium co-ordinations are the
most widely accepted [43-45]. Therefore, both type of acidity such as Lewis and Bronsted may
exist on ɣ-Al depending on the degree of dehydration. The quantity of these acid sites may vary
with the loading of Cu and Ni. The ammonia desorption profiles for Cu–Ni/ɣ-Al (Cu/Ni = 1,
and 4) were found to be very broad that can be de-convoluted to many peaks (Figure.6). Cu-
2
O
3
2
O
3
2
o
o
Ni/Al
2
O
3
(Cu/Ni = 4) showed broad NH -desorption peak starting from 150 C to 600 C with
3
o
maxima at 380 C, being de-convoluted to many desorption peaks attributed to weak, medium and
strong acidity. Although, both Cu-Ni/ɣ-Al
2
O
3
(Cu/Ni = 2) and Cu-Ni/Al
2
3
O (Cu/Ni = 1) showed
o
o
broad humps starting from 140 C to 730 C having many shoulder peaks ascribed to all three
acidic sites (weak, medium and strong). However, the intensity of maxima desorption increases
with increase in Ni loading resulted in the highest acidity (weak, medium and strong) in Cu-Ni/ ɣ-
Al
2
3
O (Cu/Ni = 1) catalyst. The increase in acidity with increase in Ni loading may be attributed
12

to the (1) synergy between Cu and Ni, (2) synergy between NiO and support material; increased
with increase in Ni loading due to poor reduction of NiO at present conditions. The weak and
medium acid sites which are increased with Ni loading act as active sites and would contribute to
hydrogenolysis of the saturated C-O band. Table.2 shows the total acidic sites present in all the
three catalysts. As discussed above, the total acidity of Cu-Ni/ Al
Cu/Ni ratio as Cu-Ni/ɣ-Al (Cu/Ni = 4) contained 0.89 (mmol/g cat), Cu-Ni/ ɣ-Al
) contained 1.95 (mmol/g cat) and Cu-Ni/ ɣ-Al (Cu/Ni = 1) contained the highest 3.44
mmol/g cat).
.1.6 X-ray absorption spectroscopy
The Cu-K XANES and derivative XANES spectra of alumina supported Cu-Ni/ɣ-Al
2
O
3
catalysts increased with
2
O
3
2
O
3
(Cu/Ni =
2
2
O
3
(
3
2
O
3
catalysts
calcined at 450 C and reduced at 300 C) with various Cu/Ni (1, 2 and 4) ratios were compared
with CuO and Cu-foil (Figure.7). The XANES results for calcined catalysts point out the presence
o
o
(
2
+
of Cu in the bulk of CuO which is characterized by the edge position located at near 8990 eV.
2
+
9
The XANES spectrum of CuO displayed two characteristic features of Cu (3d ) compounds with
a pre-edge absorption at 8979 eV, and shoulder at 8986 eV that determines 1s→4p ‘shake-down’
transition in copper (ref. Figure.7b) [12]. On a closer scrutiny, relative to crystalline CuO, the
shape of all three bi-metallic calcined catalysts is found to be different from that of CuO. The
intensity of first peak decreases with an increase in Ni loading (Figure. 7a), inferring a change in
the coordination environment, i.e., bimetallic Cu-Ni nanoparticle formation. These results are in
good agreement to the results obtained by XRD and H -TPR.
2
The XANES results of reduced catalysts showed the presence of metallic Cu, and have the similar
absorption energy pattern as that of the copper edge of a copper foil (8979 eV) (ref. Figure.7 c). It
was clearly observed that the intensity of a first peak in XANES derivative (ref. Figure.7d)
increases with Ni loading due to the enhanced dispersion of Cu and the formation of bi-metallic
13

Cu-Ni alloy. The quantity of CuO and Cu (0) can be estimated from the XANES spectrum by
Linear Combination Fitting in the Athena program [46]. Cu foil and CuO are used as reference
materials to fit the data of reduced catalysts. The LCF results are summarized in the Table.3. It
was observed that incorporation of Ni metal improves the reducibility of bi-metallic catalysts as
Cu-Ni/ɣ-Al
of fresh calcined and reduced Cu-Ni/ɣ-Al
so as to determine the reduction of NiO (Figure.8). The fresh calcined Cu-Ni/ɣ-Al
catalyst showed similar spectra which is characterized by NiO spectrum. However, XANES
spectra of reduced Cu-Ni/ɣ-Al (Cu/Ni=1) was slightly shifted towards lower energy level, and
2
O
3
(Cu/Ni=1) has maximal Cu (0). Nevertheless, we have studied Ni-K edge XANES
(Cu/Ni=1), and were compared with NiO and Ni-foil
(Cu/Ni=1)
2
O
3
2
O
3
2
O
3
its pre-edge appeared probably near the Ni-foil. Thereby, Ni-K edge XANES spectra indicated the
presence of NiO in fresh calcined catalyst and a partial reduction of NiO to Ni metal in reduced
catalyst. These results are consistent with the XRD, H
2
-TPR results which evident for the presence
of Ni metal.
Furthermore, the Cu-Kedge EXAFS analyses of CuO and bi-metallic Cu-Ni/ɣ-Al
2
3
O having the
various Cu/Ni ratios of 1, 2 and 4 were carried out in order to confirm the formation of a bimetallic
Cu-Ni alloy in calcined catalysts which are presented in Figure S1. Lower intense radial allocation
peaks for the second and third Cu-Cu coordination shells at 2.54 and 3.01 Å in the calcined
catalysts indicate smaller sizes of CuO particles and higher Debye–Waller factor [47]. Note that
the Fourier transform modulus of the Cu-Ni/ɣ-Al
content is rather different from the EXAFS Fourier transform modulus of CuO and other two
samples Cu-Ni/ɣ-Al (Cu/Ni=2) and Cu-Ni/ɣ-Al (Cu/Ni=4). In addition, first peak assigned
to the Cu-O co-ordination was shifted to higher radial distance might be correspond to
2
O
3
(Cu/Ni=1) catalyst with smaller copper
2
O
3
2
O
3
2
+
incorporation of Ni into the CuO framework that might be resulted the formation of Cu-Ni alloy
at the highest loading (Cu/Ni=1)[33].
14

3
.2 Catalytic Activity
.2.1 Hydrogenation of Furfural
Hydrogenation of FAL was extensively studied over supported and un-supported Cu catalysts [10-
9]. Nagaraja et al. [48] proposed that the presence of more Cu metal and Cu+, as opposed to CuO,
3
1
promoted high selectivity towards FOL. Therefore, present work focuses on the synergy between
Ni and Cu in order to determine the dispersion of Cu and enhanced Cu active sites for FAL
hydrogenation. In this work, Cu-Ni/ɣ-Al
2
3
O catalysts with different (Cu/Ni=1, 2 and 4) ratio are
o
o
examined at two different temperatures, viz. 130 C and 200 C at 4.0 MPa. The results were
compared with monometallic Cu/ɣ-Al
2
O
3
(10 wt%) and Ni/ɣ-Al
2
O
3
(10 wt%) (Table.4). On
o
monometallic Ni/ɣ-Al
2
O
3
(10 wt%), the conversion was very low (22% at 130 C and 32% at 200
oC) (Table.4, Entries 9&10). These results are very much expected due to poor reduction of NiO
o
o
to Ni at present conditions as NiO reduces at a temperature >400 C. On the other hand, at 130 C
on monometallic Cu, the conversion of FAL was recorded nearly 64% (Table.4, Entry.1). This
FAL conversion increases with Ni loading and reached (92%) (Table.4, Entry.3). However, at a
o
high temperature of 200 C, the conversion of FAL increases from 64% to 94% over monometallic
Cu and accordingly it reached 100% with the lowest amount of Ni (i.e. Cu/Ni=4) and further
remained constant for all Cu/Ni ratios which have been used in the present study. Although, at a
o
higher temperature (200 C) further increase in Ni loading was found to be insignificant towards
FAL conversion, yet, to a certain extent, these observations suggested that both metal loading and
temperature could be the important factors in enhancing the activity of Cu-catalysts for FAL
hydrogenation. The hydrogenation on Cu is expected to be less due to the much lower affinity of
the Cu surface for H dissociation [49]. Increase in the activity for bi-metallic Cu-Ni catalysts may
2
be attributed to a hydrogen spill-over effect by Ni to Cu sites where the aldehyde hydrogenation
reaction takes place. Hydrogen spill-over has been studied in a number of bimetallic systems where
15

a more active metal is used to activate molecular hydrogen; the atomic hydrogen then diffuses to
the less active metal sites. This less active site can then be the site of reaction [49]. Recently, Pang
et al. also reported synergistic effect in Cu-Ni catalysts in hydrogenation reactions due to a
hydrogen spillover effect from Ni to Cu [50]. This argument regarding hydrogen spillover is also
consistent with a recent report published over bimetallic Cu-Ni catalysts [51]. Also, it is well
known that temperature accelerates the rate of reaction towards the forward direction and at a
higher temperature; the conversion was close to 100% that cannot be further increased. Hence, it
is necessary to recognize the individual role of both the Ni loading (synergy between Ni and Cu)
and temperature that might play an important role in FAL hydrogenation [12]. Moreover, to get
clarity about synergy between temperature and metal active sites, the activity was compared in the
kinetic regimes for different catalysts for the hydrogenation of both FAL and FOL (Fig.S4). As
o
observed, at a constant temperature (200 C), the conversion of FAL increased rapidly at the start
of reaction and reached about 55% in 15 min over Cu-Ni/ɣ-Al
increases linearly with time (ref. Fig.S4, a). Additionally, it can be seen that the conversion of FAL
was increased with Ni loading at the respective time i.e., 35% over Cu-Ni/ɣ-Al (Cu/Ni=4) to
5% over Cu-Ni/ɣ-Al (Cu/Ni=1). On contrary, FOL converts slowly from the start of reaction
2
O
3
(Cu/Ni=1) which further
2
O
3
5
2
O
3
and followed the similar trend for Cu/Ni ratios as observed in FAL hydrogenation (ref. Fig.S4, c).
Nevertheless, the distinct conversion of FAL and FOL over all three catalysts suggested that the
conversion was influenced by the synergy between temperature and metal active sites. These
observations regarding FAL and FOL conversion via C=O and C-O bond scission are well matched
with the previous reports available for the hydrogenation of FAL [10, 18-19, 48].
However, As far as selectivity is concerned, two major products, viz. FOL and 2-MF were
observed with a small amount of by-products like MTHF, THFOL, Furan etc,. Herein, similar to
FAL conversion, product selectivity was also altered with increase in both the Ni loading and the
16

o
temperature. At a lower temperature (130 C), FOL was the main product with 72% selectivity
over monometallic Cu. This selectivity towards FOL was further increased with an increase in Ni
loading and reached 93% over Cu-Ni/ɣ-Al
2
O
3
(Cu/Ni=1). However, at a high temperature of (200
oC), the selectivity towards FOL significantly dropped and the formation of 2-MF increases from
8
% to 54% over monometallic Cu and further reached to 81% over Cu-Ni/ɣ-Al
2
3
O (Cu/Ni=1)
(
Table.4, Entry 6). The above results were compared with monometallic Ni, it was observed that
o
it did not produce even traces of 2-MF at 130 C but at a higher temperature nearly 5% MF was
obtained. Thereby, it can be concluded that Ni catalysed the FAL towards selective production of
FOL and little of THFOL, Furan, etc. for both the temperatures. In order to look at the influence
of Cu/Ni ratio on the selectivity of 2-MF on various catalysts, the 2-MF selectivity from both FAL
and FOL was plotted at the kinetic regimes (Fig.S.4, b&d). In case of FAL, it was observed that
initially the selectivity towards 2-MF was low due to the presence of intermediate FOL which was
further converted to 2-MF as reaction prolonged. Also, the selectivity towards 2-MF increased
with Cu/Ni ratio and maximal incremental 2-MF formation was observed at Cu/Ni=1. However,
for FOL hydrogenolysis, initially 2-MF was selectively formed over both mono and bimetallic
catalysts wherein, Cu-Ni/ɣ-Al
2
3
O (Cu/Ni=1) showed the maximum selectivity of 98% within 15
min which was slightly dropped 88% at 60 min (ref. Fig.S.4, d). Additionally, for 2-MF selectivity,
similar trends were clearly observed for different Cu/Ni ratios. In order to validate the argument
discussed above about the synergy between Cu and Ni; the most influential parameter involved in
-1
our bimetallic catalytic system, the TOF (turnover frequency, S ) values for the formation of FOL
and 2-MF via FAL and FOL hydrogenation respectively, were also evaluated (ref. Table S1). TOF
values for hydrogenation of FAL to FOL are apparently larger than that of FOL hydrogenolysis to
2
-MF in all the catalysts, which suggests that the second reaction may be the rate-determining step
for the conversion of FAL to 2-MF in the present catalytic system. In particular, TOF values for
17

both the reactions over bimetallic catalysts are noticeably greater than that of monometallic
counter-parts, which proposed the synergy between Cu and Ni that improved the catalytic activity.
The drastic change in the selectivity towards various products can be explained by considering the
role of both the temperature and the structural changes occurred in the catalysts after being loaded
with Ni. A series of characterization techniques such as XRD, BET, H -TPR and XANES study
2
revealed that with an increase in Ni loading, the dispersion of Cu increases and ultimately reduces
the size of Cu grain which is an important factor for C-O hydrogenolysis [13]. In addition, at the
highest loading of Ni, mixed Cu-Ni alloy was also formed which was responsible for this enhanced
activity. It is well known and reported many times in the literature that synergy between transition
metals led to enhance the dispersion and/ or structural properties of active species and ultimately
altered the activity and selectivity. Also, sometimes it was evident that there is an alloy formation
within the bimetallic catalyst system that may be responsible for the improved activity of catalysts
compared with their monometallic counterparts. It was reported that the formation of Ni–Cu alloy
is able to enhance the mobility of surface lattice hydrogen at high temperature and become a more
activated step in hydrogenation [52]. According to d-band vacancies theory the binding energies
of Ni and Cu changes during alloy formation [53]. This is also supported by s-p-d re-hybridization
in Cu-Ni catalysts wherein, both Ni and Cu gain an assessable amount of d charge in alloy
formation [54]. This helps in filling of the un-occupied d-band of Ni by Cu after complete filling
of d band of Cu. It has been found in the literature that a combination of bimetallic composition,
the binding energy, and the bonding morphologies in the bimetallic surface structure significantly
altered the hydrogenation activity in selectively tailored hydrogenation reactions including C=O
and C=C reduction [55-56].
Furthermore, in combination with surface acidity (ref. Fig.6, Table.2), it was evident that the weak
sites as well as total acidity increases with Ni loading. Thereby, combining with structural
18

properties and surface acidity, it can be deduced that at a lower temperature mostly metal particles
might be playing the role which are known to break the C=O bond present in FAL and resulting
in FOL as a major product. However, at higher temperature C-O bond is polarized due to high
o
activation energy as well as due to activation of the weak acid site at 200 C. These weak acid sites
are accountable to promote C-O bond hydrogenolysis owing to the strong oxophilic nature [13].
In addition, with an increase dose of Ni, a small amount of furan was also observed. These
observations are in consistent with previous reports for FAL hydrogenation [12, 13, 16, 19, 57].
3
.2.1.1 Effect of temperature on the activity of Cu-Ni/ɣ-Al
hydrogenation
Influence of the reaction temperature for FAL hydrogenation over Cu-Ni/ɣ-Al
2
O
3
(Cu/Ni=1) for FAL
2
O (Cu/Ni=1) is
3
shown in Fig.9. It was interesting to see that the reaction temperature plays an important role in
the product distribution. The general trend observed for liquid phase hydrogenation involving
furfural and H
2
using Cu-Ni catalysts is that hydrogenation dominates at lower reaction
o
temperatures (<170 C), giving FOL as the major product. However, at a high temperature
o
(
>200 C) further conversion of FOL to MF, MTHF or furan takes place. As usual, FAL conversion
increased from 92% (130 °C) to 100 % (200 °C) and remained constant with any further increase
in temperature. In fact, it was observed that the selectivity towards 2-MF at a lower temperature
(
130 °C) was very low, due to the presence of the intermediate FOL. It is worth noting that higher
temperature promotes the C-O hydrogenolysis for the cleavage of the carbonyl group due to the
higher activation of the C-O bond [57-58]. Therefore, the rise in temperature is much more
prominent for the -CH
2
-OH hydrogenolysis of the intermediate FOL.
3
.2.1.2 Reaction pathway of furfural hydrogenation
The hydrogenation of FAL to FOL has been studied extensively, however, only limited reports are
available on underlying the reaction pathway, especially for the hydrogenolysis of FOL to 2-MF.
19

In our previous work [12], we have briefly discussed that over Cu-catalysts C=O bond
preferentially hydrogenated giving FOL which is further converted into 2-MF by hydrogenolysis.
In this work, we have studied hydrogenation of FAL and FOL in the temperature range of 130-
o
2
30 C at 4.0 MPa in order to look in to the reaction pathways using Cu-Ni/ɣ-Al
2
3
O (Cu/Ni=1)
(
ref. Figs. 9&10). The results of both the FAL and FOL hydrogenation revealed a similar trend for
the selectivity towards 2-MF. The selectivity of 2-MF increased with increasing temperature and
reached the maximum of 84%. Afterwards, based on the products that were detected in GC spectra,
a catalytic hydrogenation pathway has been proposed in Scheme.1. It was found that FAL was
initially hydrogenated to form the intermediate product FOL via preferential adsorption of FAL
on to the Cu(111) surface, which subsequently underwent the hydrogenolysis to 2-MF as reported
earlier [57 b]. However, a small amount of furan was also observed at higher temperature, which
might be due to the presence of Ni metal via decarbonylation of FAL [57 a]. Considering the above
observations, it is proposed that hydrogen molecule undergoes dissociative adsorption as hydrogen
atoms on the catalyst surface (Cu and Ni). It is assumed that in a first step, the acidity of the catalyst
attracts the lone pair of aldehyde oxygen that helps in the adsorption of FAL on the Cu (111)
catalyst surface in a preferred orientation [57 b]. 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 FOL. Further,–CH -OH oxygen helps in adsorption of this intermediate FOL
2
on the catalyst surface in a preferred orientation similar to the case of FAL adsorption The
formation of 2-methylfuran occurred through hydroxyl alkyl intermediate followed by removal of
water molecule (leaving group).
3
.2.2 Hydrogenation of HMF
In order to illustrate the role of Cu/Ni ratio on the catalytic activity in the liquid-phase
hydrogenation of HMF, bi-metallic Cu-Ni/ɣ-Al with (Cu/Ni=1, 2, and 4) were screened over
2
O
3
20

o
o
two different reaction temperatures such as 130 C and 200 C and 3.0 MPa. The results were
compared with their monometallic counterparts Cu/ɣ-Al
2
O
3
(10 wt %) and Ni/ɣ-Al
2
3
O (10 wt %)
o
which are presented in Figs. 11 & 12. At a lower temperature (130 C), HMF conversion of ~56.2%
was obtained over monometallic Cu/ɣ-Al
increases and reached the maximum value of 70.6% over Cu-Ni/ɣ-Al
far as selectivity is concerned, BHF was found to be a dominant product and its selectivity
increases with increase in Ni loading and it reached a maximum of 88.4% over Cu-Ni/ɣ-Al
Cu/Ni=1) catalyst. These results can be attributed to the increased number of metallic Cu sites
2
O
3
. With an increase in Ni loading, HMF conversion
2
O
3
(Cu/Ni=1) catalyst. As
2
O
3
(
which are increased with increasing Ni loading (ref. Table. 3). Furthermore, these experimental
findings revealed the incapability of Cu-Ni catalysts for hydrogenolysis of C-O and O-H bond at
o
a lower temperature of 130 C. However, Cu-Ni catalysts showed slightly decarbonylation ability
and produced FOL. Literature disclosed that along with hydrogenation and hydrogenolysis,
decarbonylation followed by ring opening products were also obtained especially on Ni and Pt
metals. First, FOL was obtained via decarbonylation of HMF which was further converted into C4
&
C5 ring opening products as reported in the furfural hydrogenation over the same metals [59-
6
0]. In addition, over bifunctional Ni and Pt catalysts, C6 ring opening products were also reported.
In the present work, we observed the formation of 2-MF; via decarbonylation of HMF to FOL
o
followed by C-O hydrogenolysis of FOL to 2-MF. Furthermore, at higher temperature of 200 C,
the conversion of HMF was increased from 56.2 to 92.7% over Cu/ɣ-Al
2
3
O and all three bimetallic
Cu-Ni catalysts showed about 99% conversion of HMF. These results might be attributing to the
increase in the reaction rate, facilitated by the breaking of C=O bond over metallic Cu at higher
temperature [59-63]. It was reported that the bond energy of C=O, and the bond energy of C=C
−
1
−1
are 715 kJ·mol and 615 kJ·mol , respectively, making the hydrogenation of the C=C bond much
easier than the C=O bond [63]. However, the presence of a conjugated furan ring makes the
21

hydrogenation of the C=O bond relatively easier than the C=C bond in the selective hydrogenation
of HMF into DMF which is promoted by higher temperature. However, it was observed that further
loading of Ni was insignificant to the conversion of HMF as all the catalysts showed similar
activity. Moreover, Ni/ɣ-Al
too, wherein, it displayed very low conversion (nearly 14% at 130 C and 19 % at 200 C) (Fig.11
12). A series of characterization presented in this work well supported these observations which
2
O
3
(10 wt %) was found to be very less active for HMF hydrogenation
o
o
&
were already discussed in previous section. Similar to FAL hydrogenation, role about synergy
between temperature and metal active sites was also explored for HMF hydrogenation at the
kinetic regime for different catalysts (Fig.S5,a). In contrast to FAL hydrogenation, initial HMF
conversion was found to be slow, which speeds up further with time. However, conversion trend
observed was found to be almost similar to FAL hydrogenation.
o
In context to product distribution at 200 C, hydrogenation of the C=O bond followed by
hydrogenolysis of C-O and O-H in HMF was favored, resulting in DMF as a major product and
BHF, 5-MF, MFOL as minor products over bi-metallic Cu-Ni catalysts [62]. However, BHF
(
45.4% and 71% selectivity) still appeared as a major product over both the monometallic Cu and
Ni, respectively. It was reported that BHF is the major intermediate over all the metals and
especially over transition metals which was further converted into DMF via intermediate MFOL
at higher temperature via subsequent hydrogenation and hydrogenolysis reactions [20-21, 64-65].
According to literature, this could not be possible over only Cu metal which required either the
acidity or early transition metal oxides [10-21]. Therefore, it could be suggested that the synergy
between Cu-Ni sites as evident in XRD, BET, TPR and XANES/EXAFS results in the present bi-
metallic Cu-Ni catalysts and temperature can lead to hydrogenation/ hydrogenolysis reaction
towards HMF to DMF. In order to validate the argument, selectivity towards DMF was plotted at
the kinetic regimes. It was observed that selectivity towards DMF was increased with increase in
22

Cu/Ni ratio with respect to time (Fig.S5.b). Interestingly, two different reaction pathways were
observed, one is the hydrogenation of C=O bond and the other is decarbonylation of C=C. The
hydrogenation of C=O bond followed by hydrogenolysis of C-O and O-H leading to the formation
of DMF via intermediate BHF, 5-MF, MFOL. The DMF selectivity increased with Ni loading
from 46.2 to 56.6 % from Cu-Ni (Cu/Ni=4) to Cu-Ni (Cu/Ni=1). The selectivity of FOL also
increased from 12.2 to 17.4 %, resulting in lowered selectivity towards DMF. These observations
are well matched with the previous reports, wherein partially reduced NiO acts as active sites for
decorbonylation reaction [65].
3
.2.2.1 Reaction pathway of HMF hydrogenation
Scheme.2 displays the possible reaction pathways for the HMF conversion which was drawn
considering the GC spectra obtained over the present catalytic system. As HMF contains three
functional groups in its structure, namely C=O, O-H, and a furan ring (refer. Scheme.2), and
therefore, hydrogenation of HMF is found to be a very complex reaction. The product distribution
depends on the temperature, H -pressure, catalysts and solvent [14, 66-67]. In the present work,
2
we observed that HMF first undergoes hydrogenation of C=O bond and produced a diol
intermediate such as BHF. Furthermore, hydrogenolysis of BHF leads to the formation of DMF
via intermediate 5-MFOL. These observations are in consistent with the previous reports [20-23].
On the contrary, a small amount of 5-MF was produced via parallel hydrogenolysis reaction
probably on acidic sites. Literature revealed that initial hydrogenolysis in HMF only occurs over
bifunctional catalysts or in the presence of strong acid such as H
2
SO , HCl, formic acid etc [15,
4
6
4]. It was observed that the reduction of C-O and O-H is difficult over C=O on Cu, and Ni.
However, along with hydrogenation /hydrogenolysis, decarbonylation followed by ring opening
products were also observed. These observations are well matched with previous reports published
especially on Ni and Pt metals [20-23]. Wherein, FOL was first produced via decarbonylation
23

which was further converted into 2-MF via hydrogenolysis of a C-O bond as reported in the FAL
hydrogenation over the same metals. In addition, C6 ring opening products were also reported,
which might be due to the presence of Ni.
3
.2.3 Stability and reusability of the catalyst
Cu-Ni/ɣ-Al (Cu/Ni=1) catalyst that exhibited the best catalytic efficiency was recycled three
2
O
3
times to evaluate its stability for hydrogenation of FAL to 2-MF. The used catalyst without
o
calcinations (3nd Cycle, dried at 100 C in an oven) is characterized using TEM, XRD, and CHNS
analysis in order to examine the characteristic of used catalysts (refer Fig. S3 & S2, and Table S2).
The XRD pattern of used catalyst somewhat resembles the characteristic features of Cu–Ni/ɣ--
Al
2
3
O (Cu/Ni =1) catalyst; displayed the similar peaks assigned to most stable Cu metal species in
the spent catalyst, as indicated by the diffraction peaks at 2θ = 43.6°, 50.8°, and 74.4°, which were
respectively indexed as (111), (200), and (220) (FCC structure) [17] no peak corresponding to
carbon was observed (Fig. S2). However, the XRD peaks related to Ni, NiO and CuO were also
appeared may be due to surface segregation of metallic particles. The XRD result of used catalyst
is well supported by TEM and CHNS analyses, which revealed the absence or little presence of
carbon (Fig. S3, Table S2). Besides the Cu characteristic features, however, TEM and XRD
revealed that the size of particles slightly increased after 3nd cycle which may be the reason for a
drop in the selectivity of catalysts. In order to explore the recyclability of Cu–Ni/ɣ--Al
2
O
3
(Cu/Ni
o
=
1) catalyst, the used catalyst was separated from a reaction mixture and dried at 100 C for
overnight in the oven. The hydrogenation of furfural was then performed over the used catalyst
without calcination (Fig.13). A slight decrease in conversion of furfural (nearly 5-6% up to 3rd
cycle) was observed. However, the selectivity towards MF was found to be decreased to nearly
3
0% might be due to increase in the size of metal nanoparticles which favored the formation of
FOL and have poor hydrogenolysis ability [13]. Also, it was reported that small amount of Ni
24

addition prevent Cu catalysts from coke at the mild conditions [29]. Thus, it could be concluded
that catalyst is active and stable during the hydrogenation of furfural with little expense of 2-MF
selectivity. Therefore, it is suggested that the agglomeration of metal particles should be prevented
during reaction and considered as a future task in synthesis bimetallic Cu-Ni catalysts for FAL
hydrogenation.
4
. Conclusion
In the present work, the performance of Cu-Ni/ɣ-Al
2
3
O bimetallic catalysts on the furfural and 5-
hydroxymethylfurfural under mild operating conditions was studied. A series of characterization
techniques offered a comprehensible portrait of the origin of active sites in alumina supported Cu-
Ni catalysts to be synthesized. XRD, TPR and XANES/EXAFS indicated a rise in dispersion of
copper in the presence of nickel over alumina support due to strong interaction between nickel and
copper oxide particles. Further, TPR measurements showed a much easier reduction of copper in
the presence of nickel. The catalytic system was found to be capable towards 100 % conversion of
the FAL and HMF under mild conditions and also furnished almost 84% and 56% selectivity
towards the production of biofuel additives such as 2-MF and DMF, respectively. As far as Cu/Ni
ratio concerned, the superior catalytic performance of Cu-Ni/ɣ-Al
2
3
O (Cu/Ni=1) catalysts could
be attributed to the highly dispersed Cu species. In addition, the temperature played a significant
role in driving the selectivity towards a desired product. At lower temperature, only FOL and BHF
appeared as a major product due to hydrogenation of C=O bond present in FAL and HMF,
respectively. However, at a high temperature further hydrogenolysis of both the intermediates lead
to the formation of desired products such as 2-MF and DMF, respectively. Eventually, it can be
concluded that appropriate combinations of transition metal and temperature can lead to the
selective hydrogenation of FAL and HMF to 2-MF and DMF which is a valuable step in search of
precious metal free and environmentally benign catalysts for the development of biofuels
25

production process. Ultimately, it could be concluded that catalyst is active and stable during the
hydrogenation reactions only if the growth of metal particles size could be prevented during
reaction that may be considered as a future task in the synthesis of bimetallic Cu-Ni catalysts for
hydrogenation reactions.
Acknowledgement
We gratefully acknowledge Dr. S. N. Jha for help with XANES experiments which were
performed using beam line-8 at the Raja Ramanna Centre for Advanced Technology, Indore, India.
We also acknowledge SAIF and Department of Metallurgical Engineering and Materials Science,
IIT Mumbai for TEM and XRD analyses; Department of Chemistry, IIT Madras for TPD and TPR
analyses.
26

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