Nickel-promoted copper-silica nanocomposite catalysts for hydrogenation of levulinic acid to lactones using formic acid as a hydrogen feeder

Article abstract of DOI:10.1016/j.apcata.2014.12.007

Highly active, thermally stable nickel-promoted copper-silica nanocomposite catalysts were prepared via a deposition-precipitation method and used for hydrogenation of levulinic acid (LA) using formic acid (FA) as H₂ feeder. Ni(20)Cu(60)-SiO₂ (3:1 weight ratio of Cu to Ni, 80wt% metal content) showed better activity for vapor-phase formation of γ-valerolactone (GVL) from LA with FA as a hydrogen source. The catalyst selectively converts 99% of LA into 96% of GVL; the remaining 4% is angelica-lactone (AL). The effect of different concentrations of Ni promoted on Cu-silica and different LA to FA molar ratios on the catalyst activity affecting the hydrogen-free hydrogenation of LA was studied. The catalyst Ni(20)Cu(60)-SiO₂ exhibited long-term stability (200 h) without loss in activity. Characterization using TEM, XPS, TPR, XRD and N2O titration was performed to find the most active phase for LA hydrogenation to GVL and the reason for the long-term stability. It was found that Ni-promoted well-dispersed metallic Cu species were the most active phases in hydrogenation, and the nanocomposite nature of the catalyst helped in providing long-term stability to the active phase.

Full text of DOI:10.1016/j.apcata.2014.12.007

Accepted Manuscript
Title: Nickel-Promoted Copper-Silica Nanocomposite
Catalysts for Hydrogenation of Levulinic Acid to Lactones
using formic acid as a hydrogen feeder
Author: Pravin P. Upare Myung-Geun Jeong Young Kyu
Hwang Dae Han Kim Young Dok Kim Dong Won Hwang
U.-Hwang Lee Jong-San Chang
PII:
S0926-860X(14)00758-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.12.007
APCATA 15145
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
16-5-2014
28-11-2014
2-12-2014
Please cite this article as: P.P. Upare, M.-G. Jeong, Y.K. Hwang, D.H. Kim,
Y.D. Kim, D.W. Hwang, U.-H. Lee, J.-S. Chang, Nickel-Promoted Copper-
Silica Nanocomposite Catalysts for Hydrogenation of Levulinic Acid to Lactones
using formic acid as a hydrogen feeder, Applied Catalysis A, General (2014),
http://dx.doi.org/10.1016/j.apcata.2014.12.007
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Nickel-Promoted Copper-Silica Nanocomposite Catalysts for
Hydrogenation of Levulinic Acid to Lactones using formic acid as
a hydrogen feeder
[
a]
[a,b]
*[a,c]
[b]
Pravin P. Upare, Myung-Geun Jeong,
Young Kyu Hwang,
Dae Han Kim, Young
[a,b]
[a,c]
[a]
*[a,b]
Dok Kim,
Dong Won Hwang,
U-Hwang Lee, and Jong-San Chang
a
Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical
Technology (KRICT), 141 Gajeong-Ro, Yuesong, Daejeon 305-600, Korea, Fax: (+) 82-
42-860-7676,
E-mail: ykhwang@krict.re.kr and jschang@krict.re.kr
Department of Chemistry, Sungkyunkwan University, Suwon 440-476, Korea E-mail:
jschang020@skku.edu
b
c
Department of Green Chemistry, University of Science and Technology (UST),217 Gajeong-
Ro, Yuseong, Daejeon 305-350, Korea
Abstract
Highly active, thermally stable nickel-promoted copper-silica nanocomposite catalysts were
prepared via a deposition-precipitation method and used for hydrogenation of levulinic acid
(
LA) using formic acid (FA) as H
0 wt% metal content) showed better activity for vapor-phase formation of γ-valerolactone
GVL) from LA with FA as a hydrogen source. The catalyst selectively converts 99% of LA
2 2
feeder. Ni(20)Cu(60)-SiO (3:1 weight ratio of Cu to Ni,
8
(
into 96% of GVL; the remaining 4% is angelica-lactone (AL). The effect of different
concentrations of Ni promoted on Cu-silica and different LA to FA molar ratios on the
catalyst activity affecting the hydrogen-free hydrogenation of LA was studied. The catalyst
Ni(20)Cu(60)-SiO
2
exhibited long-term stability (200 h) without loss in activity.
O titration was performed to find the
Characterization using TEM, XPS, TPR, XRD, and N
2
most active phase for LA hydrogenation to GVL and the reason for the long-term stability. It
was found that Ni-promoted well-dispersed metallic Cu species were the most active phases
in hydrogenation, and the nanocomposite nature of the catalyst helped in providing long-term
stability to the active phase.
Keywords: γ-valerolactone • Levulinic acid • Hydrogenation • Formic acid • Nickel-
promoted copper on silica
1
Page 1 of 32

1
. Introduction
For over half a century, most chemical industries have depended on fossil resources for
major feed stocks; however, most of the fossil-fuel reservoirs are in the mature stages of their
economic life spans. Hence, it is crucial to develop integrated systems that can produce
valuable chemicals from alternative biomass [1-4]. Safe and economic production of bio-
based chemicals and biofuels is a major challenge of today [1-4]. Among the bio-based
chemicals, levulinic acid (LA) is a well-known product developed by the hydrolysis of
hexoses (six-carbon sugars, C6 sugars), and it can be obtained inexpensively from the
decomposition of cellulosic materials [3]. Alternatively, formic acid (FA) may be coproduced
with LA via acid-catalyzed conversion of C6 sugars [4]. Very recently, Upare et al.
successfully demonstrated the Br nsted-acid-catalyzed chemical conversion of C6 sugars and
Ø
cellulose to LA [4a]. Consequently, LA is an attractive starting material for the production of
many useful C5-based compounds such as γ-valerolactone (GVL), 2-methyltetrahydrofuran
(MTHF), and other derivatives. GVL can be utilized as a versatile platform chemical for
various valuable products [5]; it is also useful in the industry as a solvent for lacquers,
insecticides, and adhesives. It also has some uses in cutting oil, brake fluid, and as a coupling
agent in dye baths [5].
Recently, Dumesic et al. reported an integrated process for the production of liquid
alkenes from GVL and suggested an inexpensive method to produce GVL from biomass [6].
Catalytic approaches to the hydrogenation of LA to form GVL have been reported in the
literature [3, 7, 8]. Most researchers used batch-type reactors and high-pressure hydrogen [3,
7
] or supercritical CO
viable hydrogen-free (H
a batch-type reactor over ruthenium-based homogenous and heterogeneous catalytic systems
9]. They also reported the selective synthesis of LA and FA in a molar ratio of
approximately 1:1.
There are a few recent reports involving the use of heterogeneous catalysts for the
2
for their studies [8]. Recently, Fu et al. reported an economically
2
-free) synthesis of GVL from LA, using FA as a hydrogen source, in
[
direct synthesis of GVL from LA and FA as a hydrogen source using gold [10] and
ruthenium-based catalysts [9-11]. Very recently, Dumesic et al. reported the continuous
2 4
production of GVL from LA and FA using H SO over a noble-metal-supported catalyst at 35
bar [12]. It is worthwhile to mention that continuous vapor-phase processes have advantages
over batch-type processes, such as easy recovery of both the catalyst and products. As
reported elsewhere, the noble metals Ru, Pt, and Pd are frequently considered good
2
Page 2 of 32

hydrogenation catalysts at high temperatures [7]. Transitions metals can also be utilized as
catalysts for hydrogenation [13]. However, copper and nickel are rarely considered effective
hydrogenation catalysts at high temperatures because of their leaching in the liquid phase and
sintering of copper particles at high temperatures [14]. Recently, Upare et al. reported which
provide a sustainable catalyst life in a continuous hydrocyclization of biomass-derived
carboxylic acids to corresponding hydrofurans and lactones under vapor-phase conditions in
hydrogen [15]. The nanocomposite nature of copper-silica catalysts helps in preventing
metallic sintering and in avoiding significant deactivation. Therefore, it seemed worthwhile
to investigate the evolution of the active metal surface in catalysts. Herein, we report a
continuous process for selective synthesis of GVL from LA using FA as a hydrogen source
2
over an inexpensive highly stable Ni-promoted Cu-SiO nanocomposite catalyst system under
atmospheric pressure. Scheme 1 represents the continuous production of GVL from biomass
via hydrogenation of LA using FA as a hydrogen source.
HO
O
HOH2C
O
Formic acid
OH
O
HO
HO
O
Cat.
HO
O
O
OH
GVL
OH
O
HOH2C
O
Levulinic acid
Hexose Sugars
H2
O
MTHF
Scheme 1. Synthesis of lactones from biomass-derived levulinic acid and formic acid.
2
2
. Experimental Section
.1. Materials
Cu(NO
was obtained from local suppliers at Daejeon, South Korea, and were used as received. The
SiO source was Ludox SM-30 colloidal silica supplied by Aldrich. It had a surface area of
45 m g and was used as received. LA (98%, Alfa Aser), FA (99%, Aldrich), and 1,4-
dioxane (99.5%, Alfa Aser) were used for hydrogenation of LA to GVL.
3 2 2 3 2 2
) ·3H O and Ni(NO ) ·6H O were supplied by Wako, and NaOH (AR grade)
2
2
-1
3
3
Page 3 of 32

2.2. Catalyst preparation
3 2 2
In a typical catalyst-preparation procedure, known amounts of Cu(NO ) ·3H O and
3 2 2
Ni(NO ) ·6H O are dissolved in doubled-distilled water, and the desired amount of silica
2
-1
solution (Ludox SM-30, SBET = 345 m g ) comprising 7-nm silica nanoparticles was added
dropwise to the water at 4 °C. For precipitation, a solution of 0.1-N NaOH was added to the
above suspension until the pH became 9.2. The suspension was then stirred for 12 h at room
temperature, followed by stirring at 85 C for 5 h. The resulting suspension was filtered and
washed with distilled water repeatedly until sodium was no longer detected in the filtrate. The
solid was then dried in air at 120 C for 12 h, pressed into pellets, crushed, sieved (No. 20–40
mesh), and finally calcined in air at 600 C for 8 h. Before the reaction, catalyst samples were
reduced at 290 C with a mixture of 5% H
2
in N
2
(30 ml/min) for 2 h. The metal composition
, x + y = 80 wt% based on metal
with silica will be described hereinafter as Ni(x)Cu(y)-SiO
2
oxides, NiO and CuO. Prior to the characterization of catalysts, the reduced and used
o
materials were stabilized in N
2
atmosphere after their reduction at 290 C or after reaction at
o
2
65 C, by which we could somehow able to prevent the significant oxidation of reduced
metallic species before characterizations of catalysts.
2.3. Catalyst characterization
The structure and crystallinity of the catalyst samples were determined by X-ray
diffraction analysis on Rigaku Ultima IV Diffractometer (40KV, 40mA), which is equipped
with Cu tube in Graphite-Monochromatic for Cu Kα radiation. XRD results were recorded
o
o
o
by using PDXL software program in the 2 theta range between 5 to 80 using slower 1 per
min scanning rate. H -TPR experiments were carried out in Micromeritics model Pulse
Chemisorb 2705 equipped with a thermal conductivity detector (TCD) to monitor H
2
2
o
o
consumption is in the temperature range of 100 C to 800 C in 5%H
2
/He. XPS analysis of
the catalysts was carried out to identify the chemical state of the surfaces before and after
reduction. The XPS system consisted of a preparation and main chamber, connected via a
o
gate valve. Reduced catalysts were treated under 1 atm of 5% H
2
/N
2
at 350 C for 4h. The
reduction step was advanced into the preparation chamber, and then XPS spectra of each
-10
catalyst were obtained in the main chamber (base pressure ~3.0  10 Torr) without
exposing the samples to air. XPS spectra were collected using Mg K (1253.6 eV) as a
monochromatic energy source and with a concentric hemispherical analyzer (CHA,
4
Page 4 of 32

PHOIBOS-Has 2500, SPECS). Charging effects were calibrated with Ag 3d5/2 (368.3 eV)
and 3d3/2 (374.3 eV) spectra. For detailed analysis, Cu and Ni 2p3/2 spectra were
deconvoluted using CASA-XPS software. Background subtraction was processed by
Shireley method. Each spectrum was fitted with a linearly combined Gaussian/Lorenzian
functions. Specific surface areas of catalysts were measured by N
nitrogen temperature using Micromeritics Tristar 3000 surface area analyzer and standard
multipoint BET analysis method. Samples were degassed in flowing N for 12 h at 200 C
before N physisorption measurements. The specific surface areas were evaluated using the
Brunauer-Emmett-Teller (BET) method in the p/p range of 0.05-0.2. The particle
2
physisorption at liquid
2
2
0
morphology and crystal size of catalysts were examined using a transmission electron
microscope (TEM; Technai G2 Retrofit). Samples for TEM analysis were prepared by
suspending the catalyst particles in ethyl alcohol and mounting them on carbon-filmed
copper grids.
2.4. Levulinic acid hydrogenation
The hydrogenation of LA with FA was performed in a conventional stainless-steel fixed-
bed reactor (internal diameter 1 cm and length 300 mm), in which 1 g of catalyst was packed.
Vaporized mixtures of LA and FA dissolved in 1,4-dioxane (10 wt.% = 7.2g of LA and 2.85
g of FA in 90 g of 1,4-dioxane) were introduced into the reactor via a syringe pump with 20
mL/min of nitrogen flow through a preheated line. In order to study the effect of FA
concentration, we keep constant weight of LA (7.2 g) and concentration of FA was varied
accordingly. The liquid products were analyzed using a gas chromatograph (DONAM
DS6200) equipped with a flame ionization detector and capillary column (CycloSil-B). Only
the molar conversion of LA was considered for this study because FA was used as a
2 2
hydrogen source. The decomposition of FA to form H and CO was confirmed using GC
equipped with a TCD detector with a Carbosphere column.
3
3
3
. Results and Discussion
.1. Catalyst characterization results
.1.1. XRD analysis of catalysts
The XRD patterns of the NiCu-SiO
2
catalysts with different metallic loading are shown
in Figure 1. The peak position on XRD patterns of the catalysts were conformed from the
reference JCPDS cards (NiO from no. 47-1049, CuO from no. 41-0254 and Cu
2
O from no.
5
Page 5 of 32

71-3645). On the XRD pattern of calcined catalysts, we could clearly see the oxide phases of
o
o
o
o
o
o
nickel (NiO 2θ at 37.25 , 43.29 and 62.86 ) and copper (CuO at 35.44 , 38.73 , 48.73 ,
o
o
o
5
2
3.47 , 61.55 and Cu O at 36.43 , respectively). Presence of CuO and NiO peaks at their
reference position without any shifting on the XRD pattern of calcined catalysts (Figure 1)
clearly indicates the absence of solid solution between CuO and NiO in catalysts [17]. The
XRD peaks of the NiO were also observed in the case of reduced and used Cu-Ni catalysts
(Figure 2 and 3).
The diffraction peaks for metallic Cu were observed in the XRD patterns of both
reduced and used catalysts (Figure 2 and 3). The diffraction peaks at 2θ of 43.30°, 50.44°,
and 74.13° (JCPDS card no.4-0836) are the characteristic peaks corresponding to the (111),
(200), and (220) planes of metallic Cu, and the peaks at 2θ of 44.51°, 51.85°, and 76.37°
(JCPDS card no.4-0850) are the characteristic peaks for metallic Ni corresponding to the
same (111), (200), and (220) lattice planes of metallic Cu, respectively [15,16]. However, the
characteristic diffraction peaks related to nickel phases were difficult to observe, presumably
because of the high dispersion of metallic nanoparticles and the similarity of their 2θ peak
2
positions with that of metallic Cu in the XRD profiles of the NiCu-SiO catalysts. Even for
the 20% to 40% Ni-loaded catalysts, the XRD peak for Ni was not seen clearly because of the
o
high resistance of nickel oxide for sintering under reduction condition at at 290 C for 2h [13].
This also means that Ni-promoted catalysts existed with some metallic Ni moieties along
with NiO in the fixed bed reactor during hydrogenation, which can also help to enhance the
ability of copper for selective hydrogenation of LA to GVL and also improved the stability of
o
the catalysts. When the Ni(80)/SiO
2
catalyst was reduced at 290 C, we could see the reduced
Ni species. However, all catalysts showed only a characteristic metallic Cu peak in the XRD
patterns, which suggests that there was less possibility to form solid solution between
metallic Cu and Ni [16]. As per XRD patterns and recent reference [17], this is the not an
indication for existences of Cu-Ni solid solution in the synthesized materials.
The metallic particles are finely dispersed on the silica and played an important role in
enhancing the catalytic activity in the vapor-phase hydrogenation of biomass-derived LA.
The average particle sizes of the metallic Cu particles were calculated by using Scherrer
equation using FWHM values. Particle size of Cu dramatically decreased with the increasing
of nickel amount in the Cu-Ni catalyst, and it is estimated to be around 23 nm, 19 nm, 7nm, 6
nm and 4.3 nm for the nickel with 0 to 40% loading, respectively. Very fine particles of NiO
(1.7 to 2 nm) were observed in the case of Ni with 80% of loading (Ni(80)/SiO
2
). After LA
6
Page 6 of 32

hydrogenation, the diffraction peaks of the metallic Cu remained almost the same for the
Nickel with 8% and 20% loading, even after 200 h of hydrogenation. This is consistent well
with the TEM results.
3.1.2. TEM analysis of catalysts
The TEM and HR-TEM images for the reduced and used catalysts are presented in
Figure 4. The HR-TEM images of the Ni(8)Cu(72)-SiO
Figure 4) show that metallic nanoparticles exist in the ellipsoid shapes of (111) or (100)
faces [18], and it is clearly shown in the HR-TEM image (Figure 4e) of Ni(8)Cu(72)-SiO
catalyst. Homogenous dispersion of metallic nanoparticles is clearly seen in the TEM images
of both catalysts; these results correlate with the H -chemisorption results (Table 1). The
2 2
and Ni(20)Cu(60)-SiO catalysts
(
2
2
average size of the metal particles was determined by measuring the projected areas of
individual particles in the TEM images and calculating the equivalent diameter. This
corresponds to the diameter of a circle with the same area and was found to be 17–19 nm for
2 2
Ni(8)Cu(72)-SiO and 5–9 nm for Ni(20)Cu(60)-SiO reduced catalysts. The particle size
increased to 20–22 nm in the used catalysts (after 200 h of reaction time) because of slight
sintering of metallic nanoparticles at higher reaction temperatures. However, this did not
significantly reduce the activity of the catalysts. This was proven with the time-on-stream
results. More interestingly, even the specific morphology of both catalysts did not change
much, even after a long reaction time (Figure 4). This means that these catalysts exhibit long-
term stability and can be reused several times for hydrogenation reactions. It was very
difficult to determine the state of existence of the nickel in the catalyst after reduction by
TEM analysis because of its electromagnetic nature. Although we are not sure about the
nickel species (in terms of nickel oxide or copper-nickel alloy), data from other
2
characterization techniques (i.e., TPR, H -chemisorption, and XPS) clearly showed that the
nickel species are highly dispersed on the support, and they interact strongly with the support
and Cu. The TEM-EDS mapping images for each metal in a selected area of the
Ni(20)Cu(60)-SiO
the homogenous dispersion of metallic Cu and Ni nanoparticles on the support. The EDS of
Ni(20)Cu(60)-SiO catalyst are shown in Figure 5. From these, the amount of atomic loading
2
catalyst are presented in Figure 5. The TEM-EDS analysis clearly shows
2
and their distribution were clearly obtained. The metallic loading was also confirmed through
the EDS analysis, and these results matched those from the ICP analysis. The TEM and
TEM-EDS mapping analyses confirmed that the copper and nickel species are homogenously
7
Page 7 of 32

dispersed in the catalyst. These results also indicate that silica nanoparticles in the low silica-
loaded catalysts can be used as an inorganic matrix to enhance homogeneous dispersion of
high content metals instead of catalyst support. No evidence was obtained for the presence of
copper-nickel alloy in the catalyst.
3.1.3. XPS and TPR analyses of catalysts
Tables 2 and 3 present the atomic content of the bulk and surfaces of various catalysts,
as determined by ICP and XPS, respectively. The ICP data (Table 2) indicate that the Cu ratio
decreased with increased Ni loading, whereas the Si content was nearly constant as a function
of Ni loading. In contrast, the XPS data (Table 3) show that increasing the Ni ratio decreased
the Si ratio, whereas the Cu ratio on the catalyst surfaces was almost constant. No significant
change was observed in the atomic ratios on the catalyst surfaces before and after reduction.
Figure 6 shows the Cu 2p and Ni 2p XPS spectra of the calcined and reduced catalysts.
In Figure 6 (a), the Cu 2p3/2 states of the calcined catalysts show two peaks centered at 934.1
eV and 935 eV, which can be attributed to CuO and Cu(OH)
2
, respectively [17, 19, 20]. In
addition, the satellite peaks were observed at higher binding energy of ~ 9 eV than that of Cu
2+
2p3/2 and 2p1/2. According to the references, the satellite of Cu 2p was typically in Cu
2+
species, which is well-known shake-up satellite peak, not Cu and metallic Cu species [17,
9, 20]. The shape of Cu 2p for calcined catalysts was shown similar regardless of the Ni-
1
loading, indicating that Ni content didn’t affect almost states of Cu species before reduction.
The Ni 2p3/2 spectra presents two Ni states at 855.5 and 856.5 eV, which correspond to
2+
2 3
binding energy of Ni oxides species such as Ni(OH) and nickel silicates (NiSiO ). Also,
the presence of shake-up satellites, which were attributed to a multiple electron excitation,
could be evidence supporting to that the oxidation states of Ni in calcined catalysts is +2 [20-
22].
2
For a reduced Cu-SiO catalyst (Figure 6 (c) and (d)), the Cu 2p3/2 state show a
pronounced peak at 932.6 eV. It is ambiguous to distinguish oxidation state of Cu species
2+
according to binding energy [17, 19, 20]. Cu could be characterized from metallic copper
1+
and Cu by satellite peak. Comparing before reduction, the peak for satellite of Cu 2p was
almost disappeared, even if the intensity in Ni(20)Cu(60)-SiO sample remained slightly (as
2
shown in inset). Also, the Auger parameter, which is defined as sum of the kinetic energy of
Cu LMM transition and the binding energy of Cu 2p3/2 can provide clue to distinguish
1+
0
between Cu and Cu [17, 19, 20]. Auger parameter for three catalysts is equal to 1851.4 eV,
8
Page 8 of 32

which is typical value of metallic copper. Thus, we can infer that the main state of copper in
0
reduced catalysts is Cu . Moreover, a little intensity of satellite in Ni(20)Cu(60)-SiO
2
means
2
+
existence of Cu state, indicating the presence of Ni suppressed the reduction of Cu(OH)
and CuO to metallic Cu upon thermal treatment under H . This result is in line with the TPR
data shown in Figure 5. Here, the reduction temperature of Cu increased with increasing Ni
content. From the H -chemisorption data, it was observed that H uptake increased with
nickel loading (Table 1). A significant difference was observed in H uptake with and without
nickel catalyst: higher H uptake occurred with the Ni-loaded catalysts, particularly much
higher at Ni(40)Cu(40)-SiO . Ni 2p3/2 spectra of reduced catalysts appeared additional peak
2
2
2
2
2
2
2
centered at 852.7 eV, which assigned to metallic nickel. This result is accordance with XRD
pattern, though there are some difference in relative ratio of metallic Ni between bulk and
surface. However, the main component of nickel was still silicate form. In Si 2p spectra of
2
Ni(20)Cu(60)-SiO , the presence of shoulder at lower binding energy than main peak (104.5
eV) is in accordance with the result of Ni spectra [22] (SI supporting).
The TPR profile calcined catalysts are presented in Figure 7. The catalyst Cu(80)-SiO
2
showed the reduction peak for copper centered at 245 °C, and which gets shifted toward the
higher temperature with the addition of nickel, which might result from strong interaction
between copper and nickel [13, 23]. The reduction peaks of 20% loading to 40% loading are
o
much shifted to the 269 C and they are broader as compare to the nickel with zero and 8%
loading. These boarder reduction peaks are seems to be merging of two reduction peaks due
to reduction of CuO and bulk or surface NiO species. As per literature, lower temperature
reduction of NiO is possible if they are free or weekly associated with the supports [13, 24].
Yin et al. [13] have also observed significant decreased the reduction temperature (lower than
o
2
97 C) of nickel species due to the existence of copper. This result is also in agreement with
o
the reported result [25]. The reduction peaks for nickel has been observed at 490 C and 592
o
C (as shown in inset of Figure 7) because nickel oxide was well dispersed and strongly
interacted with Cu-silica [13, 17, 23, 24]. For comparison, we have also showed the reduction
profile of Ni(80)/SiO
2
material in Figure 7(f). On which, three reduction peaks are centered at
o
o
o
3
14 C, 490 C and 295 C are displayed. Lower reductions peaks is due to the reduction of
bulk or surface nanosized NiO species and higher temperature reduction because of its well
dispersion and strong interaction of NiO with support [24]. TPR results represent that Cu and
Ni species are well dispersed on the silica support with their strong interaction with each
9
Page 9 of 32

other. This is consistence with that the TEM data (Figure 2) clearly show that the Ni and Cu
species are homogenously dispersed with silica nanoparticles.
3
3
.2 Catalytic results of LA hydrogenation
.2.1. Effect of variation in Ni and Cu loading
2
The Ni-promoted nanocomposite Cu-SiO catalysts were prepared by deposition co-
precipitation method. Details of catalyst preparation are given in the experimental section and
in another paper [15]. The same procedure has been used for the preparation of 10Cu- and
2 2
Cu(80)-SiO . Initially, we performed vapor-phase hydrogenation of LA over Cu(10)-SiO at
265°C at atmospheric pressure and in a nitrogen flow with 1:1 molar ratio of LA and FA. The
catalytic results for LA hydrogenation by silica catalysts with different Cu and Ni
concentrations are presented in Table 4. Only 10% of the Cu on the silica converts 66% of
LA into 55% angelica-lactone (AL) and 45% GVL. To increase the LA conversion,
hydrogenation with a higher loading of Cu on silica (i.e., 80 wt.% Cu-SiO
The Cu(80)-SiO catalyst produced 81% GVL and 19% AL with 83% conversion of LA. To
enhance the LA conversion and GVL selectivity, we introduced a Ni-promoted Cu-SiO
nanocomposite catalyst in the reactor. The use of Ni has been well promoted for
hydrogenation and hydrolysis [26]. In the LA hydrogenation, the Ni-promoted Cu-SiO
catalyst increased the LA conversion as well as GVL selectivity. Recently, we reported a
higher activity of the NiCu-SiO nanocomposite catalyst for direct hydrogenation of LA with
to form 2-methyltetrahydrofuran (MTHF), wherein the catalyst was very active and stable
for 320 h without significant loss in its activity [15].
Ni and Cu loadings on SiO catalyst at different concentrations were tested. A marginal
difference in the LA conversion and GVL selectivity was observed for Ni(4)Cu(76)-SiO
The Ni(4)Cu(76)-SiO catalyst converted 85% of LA into 82% GVL and 18% AL remained.
This means that 4% Ni was not very effective for increasing the GVL selectivity in LA
hydrogenation of LA. In contrast, for 8% Ni loading Ni(8)Cu(72)-SiO , the LA conversion
88%) and GVL selectivity (88%) were higher and 12% AL remained. The Ni(20)Cu(60)-
2
) was performed.
2
2
2
2
H
2
2
2
.
2
2
(
2
SiO catalyst provides excellent catalytic activity and longer stability for hydrogenation of
LA and has a high selectively for GVL. The maximum LA conversion of 98% and GVL
selectivity of 92% were obtained with 20% nickel under the reaction conditions presented in
2
Table 4. The higher nickel concentration above 20% of Ni (i.e., Ni(40)Cu(40)-SiO ) did not
1
0
Page 10 of 32

improve the GVL selectivity (71%), though it did produce additional hydrogenation products,
e.g., 20% 1,4-PDO and 8–9% pentane.
3.2.2. Effect of FA concentration on LA hydrogenation
The FA concentration used to selectively obtain GVL was changed by changing the LA to
2
FA molar ratio in the feed over Ni(8)Cu(72)-SiO catalyst (Table 5). The LA to FA molar
ratios of 1:1, 1:2, and 1:3 were tested in the comparison study. LA conversion and GVL
selectivity increased with increasing FA concentration in the feed. The highest LA
conversion of 97% and GVL selectivity of 98% were obtained with 1:3 LA to FA molar ratio
(Table 5). We also performed the experiment using a higher concentration of FA (i.e., 1:10)
to check further possibilities. Not surprisingly, the GVL selectivity (79%) decreased with
higher FA concentration, and additional hydrogenation products were created (15% 1, 4-PDO
and 3% MTHF). For comparison, we conducted synthesis of GVL over Ni(20)Cu(60)-SiO
2
catalyst using 1:0.5 LA to FA molar ratio in the feed. This combination afforded poor LA
conversion and poor GVL selectivity compared to 1:1 molar ratio of LA to FA. In this case,
AL was the major product instead of GVL. However, the highest GVL yield from LA (98%)
2
was obtained using Ni(20)Cu(60)-SiO and 1:2 LA to FA molar ratio in the feed.
3
.2.3. Effect of reaction temperature on LA hydrogenation
From the study above, the molar ratios of 1:3 and 1:2 (of LA to FA) are optimum to
obtain GVL selectively under the reaction conditions given in Table 6 for Ni(8)Cu(72)-SiO
and Ni(20)Cu(60)-SiO catalysts, respectively. However, it should be noted that LA and FA
2
2
are always obtained in the 1:1 (molar) proportion because of the decomposition of hexose
sugar [4]. By considering this, a further study was carried out using 1:1 molar ratio of
LA/FA in the feed. The effect of different reaction temperatures on the activity of the
catalyst is presented in Table 6. At the lower reaction temperature of 240 °C, the LA
conversion and GVL selectivity were lower compared to those obtained with higher
reaction temperatures. The selective conversion of 73% of the LA to 67% GVL was
2
obtained using Ni(8)Cu(72)-SiO catalyst. It also provided an intermediate A-lactone
selectivity of 31% at 240 °C. At the same temperature, the catalyst [Ni(20)Cu(60)-SiO
2
]
yielded 78% GVL and 21% A-lactone with 90% conversion of LA. The catalytic activities
of both the catalysts were greater at higher reaction temperatures. The highest LA
conversion (99%) and GVL selectivity (96%) were obtained at the reaction temperature of
1
1
Page 11 of 32

2
8
9
85 °C with the Ni(20)Cu(60)-SiO
% of nickel Ni(8)Cu(72)-SiO catalyst at 285 °C, and this combination yielded converted
2% LA into 88% GVL. However, this temperature is very close to the Tamman
2
catalyst. The second best results were obtained using
2
temperature of copper, which is considered the point at which sintering begins in the metal
species. A marginal difference in activity was observed for the top two catalysts at 265°C
compared to that obtained at 285°C for hydrogenation of LA.
3
.2.4. Long-term activities of Ni-promoted Cu-SiO
To check the stability of the Ni-promoted Cu-SiO
conducted catalytic measurements as a function of reaction time. The time-on-stream profiles
for the Ni(8)Cu(72)-SiO and Ni(20)Cu(60)-SiO catalysts are presented in Figure 8. In the
2
catalysts
2
nanocomposite catalysts, we
2
2
case of the former catalyst, the initial LA conversion was found to be 87–90% with 86% GVL,
after up to 100 h of reaction. After 100 h of reaction, a marginal decrease in the LA
conversion (83%) and GVL selectivity (82%) was observed; 15–19% selectivity for AL was
2
also observed (Figure 6). It is clear that the Ni(20)Cu(60)-SiO catalyst exhibited excellent
performance and longer stability (200 h) without deactivation. Furthermore, it yielded 92%
GVL selectively with 98% conversion of LA in LA hydrogenation using formic acid as a
hydrogen source.
4. Conclusions
2
The highly dispersed Ni-promoted Cu-SiO nanocomposite catalysts provide excellent
catalytic activity for hydrogenation of biomass-derived LA (using FA as the hydrogen
source) to selectively produces GVL. The addition of Ni to Cu-loaded catalysts enhances the
o
thermal stability, conforming through the excellent catalytic results obtained at 285 C
without any deactivation, and more importantly, the nanocomposite nature of catalysts could
also helps to prevents Cu sintering even after long reaction time after 200 h of reaction time.
The current catalyst system has shown its efficient and steady workability for the carboxylic
2
acid hydrogenation continuously in vapor phase. Therefore, Ni-promoted Cu-SiO catalysts
would be considered a promising candidate of catalysts for hydrogenation of LA to GVL
without metal leaching. Hence, the current finding seems to be more affordable and allows us
to think about the synthesis of value added chemicals via hydrogenation of biomass derived
platform chemicals without any external hydrogen
1
2
Page 12 of 32

Acknowledgements
This work was supported by the Institutional Research Program (KK-1401-F0) and partly
by from the Korea Research Council for Industrial Science and Technology (ISTK, SK-1411).
The authors thank Mr. K.-H. Cho and Ms. S.-K. Lee for their beneficial contributions.
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Page 15 of 32

Table 1. H
2
chemisorptions of Cu-SiO
2
and Ni-promoted Cu-SiO
2
catalysts
Metallic Surface
Total Dispersion
(%)
Area
H Uptake
2
2
Catalyst
(
m /g)
(mmol/g)
Cu
Ni
0
Cu
Ni
Cu(80)-SiO
2
4.26
0.030
0.035
0.039
0.130
0.82
0
Ni(8) Cu(72)-SiO2
3.05
2.10
6.56
1.06
1.51
5.30
0.58
0.55
2.04
0.41
0.45
2.11
Ni(20)Cu(60)-SiO
2
Ni(40)Cu(40)-SiO2
1
6
Page 16 of 32

2 2
Table 2. ICP analysis of Cu-SiO and Ni-promoted Cu-SiO catalysts.
SiO
2
CuO
NiO
Catalyst
Atomic %
88.25
26.1
Atomic %
Atomic %
Cu(10)/SiO
2
9.62
73.9
-
-
Cu(80)-SiO
2
Ni(4)Cu(76)/SiO
2
2
26.4
71.36
3.14
7
.0
Ni(8)Cu(72)-SiO
26.8
24.4
66.2
58.8
Ni(20)Cu(60)-SiO
2
16.8
Ni(30)Cu(50)-SiO
2
2
21.18
22.3
47.8
36.1
30.03
38.2
Ni(40)Cu(40)-SiO
1
7
Page 17 of 32

Table 3. Atomic ratio of Cu-SiO
2
and Ni-promoted Cu-SiO
2
catalysts from XPS analysis.
Ni
Si Cu
Atomic %
Reduction
Calcination)
Atomic %
Reduction
(Calcination)
Atomic %
Reduction
(Calcination)
Catalyst
(
Cu(80)-SiO
2
54.1(53.8)
44.8(44.3)
40.3(41.1)
45.9(46.2)
43.4(44.2)
44.0(43.5)
-
Ni(8)Cu(72)-SiO
2
11.8(11.5)
14.7(15.4)
Ni(20)Cu(60)-SiO
2
1
8
Page 18 of 32

Table 4. Catalytic activities for hydrogenation of levulinic acid using FA as H
2
source.
TOF
a
Selectivity (%)
Catalyst
S
m /g)
BET
2
Conv.
(%)
(
AL
55
GVL
1
0
0
.72
.28
.29
Cu(10)-SiO
2
2
291
66
83
85
88
40
81
82
88
Cu(80)-SiO
142
160
166
19
18
12
Ni(4)Cu(76)-SiO2
0
.30
Ni(8) Cu(72)-SiO
2
Ni(20)Cu(60)-SiO
2
214
214
202
193
98
96
8
11
0
92
89
81
71
0.32
0.32
0.31
0.32
b
Ni(20)Cu(60)-SiO2
c
Ni(30)-Cu(50)-SiO
2
d
100
100
Ni(40)Cu(40)-SiO
2
0
o
*
Reaction conditions: Temp. of 265 C, LA: FA of 1:1 (molar ratio), Feed 10% in 1,4-
-1
dioxane, Nitrogen flow of 20ml/min, Catalyst wt. of 1gm, Acid WHSV of 0.512 h , Pressure
of 1 atm., TOS of 100 h. TOF= (Reacted mole of LA)/Total mole of metals (Cu-Ni) per hour
Abbreviation: AL, angelica lactone; GVL, -valerolactone.
a
b
Reaction was carried out using mixture of H
i.e. LA: hydrogen =1:1 molar ratio).
2 2
and CO as a hydrogen source instead of FA,
(
c
Other products include 1,4-PDO (13-14%) and pentane (4-5%).
d
Other products include 1,4-PDO (20%) and pentane (8-9%).
*
1
9
Page 19 of 32

Table 5. Effect of different molar ratio of LA/FA on catalytic activity.
Selectivity (%)
A- lactone
Molar ratio
of LA/FA
Conversion
(%)
Catalyst
GVL
86
1
90
93
97
12
3
Ni(8)Cu(72)-SiO
2
0.5
96
0
.33
2
98
a
0
.1
100
87
-
47
8
79
45
92
98
2
Ni(20) Cu(60)-SiO
2
1
98
0
.5
100
2
o
Reaction conditions: Temperature 265 C, pressure = 1 bar, Nitrogen flow 20 ml/min,
-1
Catalyst wt. = 1.0 gm, Acid WHSV = 0.512 h , pressure = 1 atm, TOS = 100h
a
Other products include 1,4-pentanediol (15%), MTHF (3%), hydrocarbons (3%) such as
2
pentane, CO and CO . *The concentration of LA (7.2 g) was kept constant and concentration
of formic acid was varied accordingly
2
0
Page 20 of 32

Table 6. Effect of reaction temperature on catalytic activity.
Catalyst
Temp.
Conv.
(%)
Selectivity (%)
AL
o
(
C)
GVL
67
2
40
73
31
Ni(8)Cu(72)-SiO
2
2
2
65
85
90
92
12
9
86
88
2
40
90
21
78
Ni(20)Cu(60)-SiO
2
2
2
65
85
98
99
8
3
92
96
Reaction conditions: LA: FA= 1:1 (molar ratio), N
2
flow 20 ml/min, catalyst wt. = 1.0 gm,
-1
Acid WHSV = 0.512 h , pressure = 1 atm, TOS-100 h.
2
1
Page 21 of 32

Figure captions
Figure 1. XRD patterns of calcined Cu-Ni catalysts at 550 C for 8h. (a) Cu(80)-SiO
o
2
, (b)
Ni(8)Cu(72)-SiO
2
, (c) Ni(20)Cu(60)-SiO
2
, (d) Ni(30)Cu(60)-SiO , and (e) Ni(40)Cu(40)-
2
SiO2.
o
Figure 2. XRD patterns of reduced Cu-Ni catalysts at 290 C with a mixture of 5% H
2
in N
2
for 2h. (a) Cu(80)-SiO2, (b) Ni(8)Cu(72)-SiO
2
, (c) Ni(20)Cu(60)-SiO
2
, (d) Ni(30)Cu(50)-
SiO , (e) Ni(40)Cu(40)-SiO , and (f) Ni(80)/SiO
2
2
2
Figure 3. XRD patterns of used Cu-Ni catalysts after hydrogenation of 200 h. (a) Cu(80)-
SiO , (b) Ni(8)Cu(72)-SiO , (c) Ni(20)Cu(60)-SiO , (d) Ni(30)Cu(60)-SiO , and (e)
Ni(40)Cu(40)-SiO
Figure 4. TEM images of catalysts: (a) Ni(8)Cu(72)-SiO
2
2
2
2
2
.
2
(reduced), (b) Ni(8)Cu(72)-SiO
2
(
used), (c) Ni(20)Cu(60)-SiO
2
(reduced), (d) Ni(20)Cu(60)-SiO
2
(used) and (e) HR-TEM of
o
reduced Ni(8)Cu(72)-SiO
2
catalyst. The calcined catalysts were reduced at 290 C for 2 h
under 5% H
Figure 4. TEM-EDS mapping images of the reduced Ni(20)Cu(60)-SiO
Figure 5. X-ray photoelectron spectra of calcined and reduced catalysts for Cu-SiO
NiCu-SiO
2
in inert gas, and then were used for the hydrogenation for 200 h.
2
catalyst.
2
and
2
: (a) Cu 2p and (b) Ni 2p spectra of the calcined catalysts. (c) Cu 2p and (d) Ni 2p
spectra of the reduced catalysts, respectively.
Figure 6. X-ray photoelectron spectra of calcined and reduced catalysts for Cu-SiO
2
and
NiCu-SiO
spectra of the reduced catalysts, respectively.
Figure 7. TPD profiles of Cu-SiO and NiCu-SiO
Ni(8)Cu(72)-SiO , and (c) Ni(20)Cu(60)-SiO
2
: (a) Cu 2p and (b) Ni 2p spectra of the calcined catalysts. (c) Cu 2p and (d) Ni 2p
2
2 2
catalysts: (a) Cu(80)-SiO , (b)
2
2
.
Figure 8. Catalytic activities on stream for hydrogenation of levulinic acid: (a) Ni(8)Cu(72)-
o
SiO
2
and (b) Ni(20)Cu(60)-SiO
2
catalysts. Reaction conditions: Temperature of 265 C, LA:
FA of 1:1 (molar ratio), Nitrogen flow of 20ml/min, Feed of 10 % in 1, 4-dioxane, Catalyst
-1
weight of 1g, Acid WHSV of 0.512 h , and pressure of 1 atm.
2
2
Page 22 of 32






CuO
 u2O
NiO

C









(a)
b)
(
(
c)
d)




(


(e)
30
40
50
60
70
80
2 / degree
o
Figure 1. XRD patterns of calcined Cu-Ni catalysts at 550 C for 8h. (a) Cu(80)-SiO
2
, (b)
Ni(8)Cu(72)-SiO
SiO2.
2 2 2
, (c) Ni(20)Cu(60)-SiO , (d) Ni(30)Cu(60)-SiO , and (e) Ni(40)Cu(40)-
2
3
Page 23 of 32





Cu
u2O
Ni

C


NiO



(a)
(
b)


(c)
(
(
d)
e)



(f)

30
40
50
60
70
80
2
/ degree
o
Figure 2. XRD patterns of reduced Cu-Ni catalysts at 290 C with a mixture of 5% H
for 2h. (a) Cu(80)-SiO2, (b) Ni(8)Cu(72)-SiO , (c) Ni(20)Cu(60)-SiO , (d) Ni(30)Cu(50)-
SiO , (e) Ni(40)Cu(40)-SiO , and (f) Ni(80)/SiO
2 2
in N
2
2
2
2
2
.
2
4
Page 24 of 32





Cu
u2O
Ni

C


NiO


(
a)
b)

(


(c)
(
d)




(e)

30
40
50
60
70
80
2 / degree
Figure 3. XRD patterns of used Cu-Ni catalysts after hydrogenation of 200 h. (a) Cu(80)-SiO
b) Ni(8)Cu(72)-SiO , (c) Ni(20)Cu(60)-SiO , (d) Ni(30)Cu(60)-SiO , and (e) Ni(40)Cu(40)-
SiO
2
,
(
2
2
2
2
.
2
5
Page 25 of 32

(a)
(b)
(c)
(d)
(e)
(f)
Figure 4. TEM images of catalysts: (a) Ni(8)Cu(72)-SiO
used), (c) Ni(20)Cu(60)-SiO (reduced), (d) Ni(20)Cu(60)-SiO
of reduced Ni(8)Cu(72)-SiO
2
(reduced), (b) Ni(8)Cu(72)-SiO
2
(
2
2
(used), (e) and (f) HR-TEM
2
((100) and (111) plane of Cu nanoparticles, respectively). The
o
calcined catalysts were reduced at 290 C for 2 h under 5% H
for the hydrogenation for 200 h.
2
in inert gas, and then were used
2
6
Page 26 of 32

2
00 nm
IMG1
200 nm
Si K
2
00 nm
Ni K
200 nm
Cu K
2
Figure 5. TEM-EDS mapping images of the reduced Ni(20)Cu(60)-SiO catalyst.
2
7
Page 27 of 32

Calcined
934.1 eV (a)
Calcined
(b)
6
0Cu,20Ni/SiO2
935 eV
8
56.5 eV
855.5 eV
6
0Cu,20Ni/SiO2
2Cu,8Ni/SiO2
8
62.2 eV
72Cu,8Ni/SiO2
7
80Cu/SiO2
80Cu/SiO2
970
960
950
940
930
890
880
870
860
850
Binding Energy (eV)
Binding Energy (eV)
Reduced
0Cu,20Ni/SiO2
9
34.1 eV 932.6 eV
(d)
Reduced
6
856.5 eV
55.5 eV
9
35 eV
8
8
62.2 eV
950
945
940
Binding Energy (eV)
72Cu,8Ni/SiO2
852.7 eV
60Cu,20Ni/SiO2
80Cu/SiO2
72Cu,8Ni/SiO2
80Cu/SiO2
970
960
950
940
930
890
880
870
860
850
Binding Energy (eV)
Binding Energy (eV)
Figure 6. X-ray photoelectron spectra of calcined and reduced catalysts for Cu-SiO
NiCu-SiO : (a) Cu 2p and (b) Ni 2p spectra of the calcined catalysts. (c) Cu 2p and (d) Ni 2p
spectra of the reduced catalysts, respectively.
2
and
2
2
8
Page 28 of 32

o
2
45 C
o
592 C
o
89 C
4
o
(
a)
269 C
(
(
(
b)
c)
d)
(
e)
o
2
97 C
(f)
100
200
300
400
500
600
700
800
o
Temperature ( C)
(
a)
b)
c)
d)
e)
f)
(
(
(
(
(
100
200
300
400
500
600
700
800
o
Temperature ( C)
Figure 7. TPR profiles of Cu-SiO
Ni(8)Cu(72)-SiO , (c) Ni(20)Cu(60)-SiO
and (f) Ni(80)/SiO . * TPR of Ni(80)/SiO
2
and NiCu-SiO
2
catalysts: (a) Cu(80)-SiO
2
, (b)
2
2
, (d) Ni(30)Cu(50)/SiO
2
, (e) Ni(40)Cu(40)/SiO ,
2
2
2
is used here as a references
2
9
Page 29 of 32