Ni/H-ZSM-5 as a promising catalyst for vapour phase hydrogenation of levulinic acid at atmospheric pressure

Article abstract of DOI:10.1039/c3ra46485g

Among Ni supported on H-ZSM-5 catalysts with various loadings of Ni, a catalyst with 30 weight% Ni has been identified as an effective catalyst for the hydrogenation of levulinic acid to γ-valerolactone in vapour phase at atmospheric pressure. The catalysts have been characterized by different techniques such as XRD, BET surface area, pore size distribution, TPR, AAS, pulse chemisorption, FE-SEM-EDS, TEM, XPS and pyridine-adsorbed IR. XRD patterns suggest that the structure of H-ZSM-5 is intact even after incorporation of Ni. Pyridine-adsorbed IR patterns reveal the presence of both Lewis and Bronsted acid sites, which are responsible for the dehydration and cleavage of γ-valerolactone. The superior activity exhibited by the 30 wt% Ni/H-ZSM-5 catalyst (demonstrating the highest productivity of 0.9090 kg _GVL kg_catalyst^-1 h^-1 at 250 °C) is due to the presence of a greater number of surface Ni species. Non-noble metal Ni-based catalysts offer advantages over the present processes involving Ru-based catalysts, which have certain constraints, some of which are unavoidable such as cost ineffectiveness, the use of volatile organic solvents and high pressure operation. The 30 wt% Ni/H-ZSM-5 catalyst can be seen as an alternative and promising catalyst which could be of great importance to the chemical industry.

Full text of DOI:10.1039/c3ra46485g

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Ni/H-ZSM-5 as a promising catalyst for vapour
phase hydrogenation of levulinic acid at
atmospheric pressure†
Cite this: RSC Adv., 2014, 4, 9660
Varkolu Mohan, Chakali Raghavendra, Chodimella Venkata Pramod, Burri David Raju
and Kamaraju Seetha Rama Rao*
Among Ni supported on H-ZSM-5 catalysts with various loadings of Ni, a catalyst with 30 weight% Ni has
been identified as an effective catalyst for the hydrogenation of levulinic acid to g-valerolactone in
vapour phase at atmospheric pressure. The catalysts have been characterized by different techniques
such as XRD, BET surface area, pore size distribution, TPR, AAS, pulse chemisorption, FE-SEM-EDS, TEM,
XPS and pyridine-adsorbed IR. XRD patterns suggest that the structure of H-ZSM-5 is intact even after
incorporation of Ni. Pyridine-adsorbed IR patterns reveal the presence of both Lewis and Brønsted acid
sites, which are responsible for the dehydration and cleavage of g-valerolactone. The superior activity
exhibited by the 30 wt% Ni/H-ZSM-5 catalyst (demonstrating the highest productivity of 0.9090 kgGVL
catalystꢀ
1
ꢀ1
ꢁ
kg
h
at 250 C) is due to the presence of a greater number of surface Ni species. Non-noble
Received 8th November 2013
Accepted 11th December 2013
metal Ni-based catalysts offer advantages over the present processes involving Ru-based catalysts,
which have certain constraints, some of which are unavoidable such as cost ineffectiveness, the use of
volatile organic solvents and high pressure operation. The 30 wt% Ni/H-ZSM-5 catalyst can be seen as
an alternative and promising catalyst which could be of great importance to the chemical industry.
DOI: 10.1039/c3ra46485g
www.rsc.org/advances
12–14
GVL has several uses in solvents, insecticides, adhesives etc.
Furthermore, GVL is a potential candidate for the production of
1
. Introduction
12,15–21
Due to a decline in the reserves of non-renewable fuels, biomass ne chemicals, fuels and fuel additives.
A key step in the
is regarded as one of the renewable sources for the production production of levulinic acid is the acid hydrolysis of furfuryl
of fuels. In this regard, researchers have focused their attention alcohol, which is obtained from an abundant source of biomass
towards the production of fuels from biomass and/or biomass and also from the extraction of black liquor in the paper-making
22
derivatives. In this regard, the U.S. and European Union process. Few researchers have studied the direct conversion of
2
3,24
governments are planning to produce 20% of fuels and 25% of biomass and biomass-derived platform molecules to GVL.
1–3
chemicals from renewable sources by 2030. Biomass is the GVL serves as an intermediate for the production of diesel fuel
fourth largest source of energy in the world aer coal, petro- and the production of branched alkanes with molecular weights
leum oil and natural gas and provides about 14% of the world's appropriate for jet fuel. Approximately 50 kg of diesel/fuel can
4
energy requirements. It is also considered an alternative feed- be formed per 100 kg of GVL. Alternatively, GVL can undergo
stock source for the production of commodity chemicals. catalytic conversion to produce butane and CO
According to the U.S. Department of Energy, levulinic acid (one with butene oligomerization to form C –C20 alkenes. It can also
of the top 12 biomass-derived compounds) is one of the most undergo ring-opening and reduction steps to form pentanoic
2
, combining
8
5,6
promising candidates for the production of chemicals.
Cellulosic biomass (a non-food renewable source) normally completed by hydrodeoxygenation to form nonane.
acid, followed by ketonization to form 5-nonanone, and nally
consists of biopolymeric species such as cellulose (35–50%),
Christian et al. studied the liquid phase hydrogenation of LA
ꢁ
hemicellulose (15–25%) and lignin (15–30%). Acidic hydrolysis under batch conditions using a Raney® Ni catalyst at 220 C and
ꢁ
of cellulosic biomass gives levulinic acid, formic acid, furfural 48 bar hydrogen pressure and over copper chromite at 250 C
7–11
25
and char. Therefore, biomass-derived levulinic acid (LA) is an and 220 bar hydrogen pressure. Schutte et al. reported LA
effective feedstock for the production of g-valerolactone (GVL). hydrogenation over a platinum oxide catalyst in various organic
26
solvents at 3 bar hydrogen pressure at room temperature.
A
patent led by Manzer discloses the hydrogenation of LA in
Indian Institute of Chemical Technology, Hyderabad, India. E-mail: ksramarao@iict.
res.in
supercritical CO over Pt, Pd, Ru and Re catalysts supported over
2
ꢁ
various oxides at 200 C and 200 bar hydrogen pressure in 1,4-
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ra46485g
27
dioxane solvent. Similarly, Bourne et al. utilised supercritical
1
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ꢁ
2 2
for LA hydrogenation over Ru/SiO at 200 C in 100 bar the catalysts was also estimated by dissolving a known amount
CO
hydrogen pressure. Yan et al. used Ru/C in a batch reactor at of catalyst in aqua regia followed by dilution with water and by
28
ꢁ
30 C and 12 bar hydrogen pressure to yield GVL. Pt/TiO or using atomic absorption spectroscopy (AAS) (A-300, Perki-
2
29
1
Pt/ZrO catalysts have also been reported for the hydrogenation nElmer, Germany). For the quantication of Ni, a 10 ppm Ni
2
ꢁ
of levulinic acid at 200 C, 40 bar H
5
2
with a H
2
/LA molar ratio of solution was used. FTIR spectra were recorded on a GX spec-
8
:1, to obtain a greater amount of GVL. Dumesic et al. investi- trometer (PerkinElmer, Germany) with a scan range of 4000–
gated LA hydrogenation over Ru/C and Ru–Sn/C at 180 C and 35 400 cm . The samples were nely ground in the ratio 1 : 10
bar hydrogen pressure.
ꢁ
ꢀ1
3
0,31
Recently, Galletti et al. reported a (sample : KBr) and pelletized then measured in the spectral
ꢁ
ꢀ1
ꢀ1
higher yield of GVL at 70 C and 30 bar hydrogen pressure over a range of 400–4000 cm with a resolution of 4 cm , and 10
Ru/C catalyst using an ion exchange resin (Amberlyst A70) as a scans were recorded for each spectrum. For pyridine-adsorbed
3
2
ꢁ
co-catalyst. Hengue et al. reported using Cu on various FTIR analysis, the sample was oven dried at 100 C for 1 h. To
ꢁ
33
supports to obtain GVL at 200 C and 35 bar pressure.
the oven dried sample (50 mg), 0.1 ml of pyridine was admixed
It is interesting to note that most of them reported the use of directly. To remove the physisorbed pyridine present in the
batch conditions at very high H
2
pressures. Only a few reports sample, the admixed sample tube was then kept in a vacuum
ꢁ
have been published describing experiments in the vapour oven at 120 C for 1 h. Aer ice cooling to room temperature, the
34,35
ꢀ1
phase at atmospheric pressure.
However, 1,4-dioxane is used spectrum was recorded with a nominal resolution of 4 cm in
ꢀ1
as a feed additive in all of these works.
the spectral range of 1400–1700 cm using a KBr background
36
The present investigation highlights the application of and 10 scans were recorded for each spectrum.
zeolite (H-ZSM-5) supported Ni catalysts to obtain GVL with The textural characteristics of the catalysts were obtained
high productivity via LA hydrogenation in the vapour phase using a JEOL JEM 2000EXII transmission electron microscope,
without any feed additive at atmospheric pressure.
operating between 160 and 180 kV. The specimens were
prepared by dispersing the samples in methanol using an
ultrasonic bath and evaporating a drop of the resultant
suspension onto a lacey carbon support grid.
2. Experimental
2
.1. Catalyst preparation
Temperature programmed reduction (TPR) of the catalysts
was performed on an AUTOSORB-iQ automated gas sorption
analyser (Quantacrome instruments, USA). About 50 mg of the
catalyst was placed in a quartz reactor and pre-treated in a He
H-ZSM-5 (Si/Al ¼ 40) was purchased from S u¨ d Chemie India Pvt.
Limited. Ni/H-ZSM-5 catalysts with various Ni loadings were
prepared by a wet impregnation method. An aqueous solution
containing the requisite amount of Ni(NO ) $6H O salt (AR
3 2 2
grade obtained from Loba Chemie, India) was added to the
support particles in a beaker, which was then placed on a hot
plate with stirring at 80 C to remove the excess water. The
prepared solid mass was dried for 12 h at 100 C and then
calcined at 450 C for 5 h in air. The catalysts so prepared were
designated as xNHZ where x represents the loading of Ni in wt%.

2
ow at 393 K for 1 h. Later, the catalyst was treated with 5% H –
3
ꢀ1
ꢁ
Ar mixture gas (3600 cm h ) with a temperature ramp of 20 C
min . The hydrogen consumption was monitored using a
thermal conductivity detector (TCD).
ꢀ
1
ꢁ
ꢁ
H
2
-chemisorption using a pulse (100 mL) titration procedure
ꢁ
ꢁ
was carried out at 40 C on an Autosorb-iQ automated gas
sorption analyser (Quantachrome Instruments, USA) to deter-
mine the dispersion and metal particle size, and metal surface
area of the catalyst. Prior to the experiment, the catalyst was
2.2. Catalyst characterisation
ꢁ
reduced at 500 C for 2 h followed by evacuation for 2 h. The
XRD patterns of the catalysts were recorded with an Ultima-IV monolayer uptake, active metal surface area, metal dispersion,
3
7–39
X-ray diffractometer (Rigaku Corporation, Japan) using Ni and active crystallite size were calculated.
˚
ꢁ

ltered Cu K
a
radiation (l¼ 1.5406 A) with a scan speed of 4
Monolayer uptake of hydrogen in micromoles per gram was
ꢀ
1
ꢁ
min and a scan range of 2–80 at 40 kV and 20 mA. Surface calculated by
area, pore volume and pore size of all the catalysts were
N
m
¼ 44.61V
m
obtained from N
2
adsorption–desorption isotherms measured
ꢁ
at ꢀ196 C (ASAP 2020, Micromeritics, USA). The surface area of
the catalysts was determined by the Brunauer, Emmett, and
Teller (BET) equation whereas the pore size was measured by
the Barret–Joyner–Halenda (BJH) method. Prior to the adsorp-
tion–desorption isotherms, the catalysts were degassed under
ꢀ
1
where N
m
is in mmol g and V
m
is the volume required to ll a
3
ꢀ1
monolayer of adsorbed gas in cm g (STP). Assuming a stoi-
chiometry of 2 between Ni and H , the active metal surface area
AMSA) was calculated by:
2
(
ꢀ6
ꢁ
vacuum (10 torr) at 150 C for 3 h to remove the physisorbed
moisture. The total pore volume (V ) was determined from the
isotherms at a relative pressure (P/P ) of 0.98.
AMSA ¼ N
m
m
SA /166
p
0
where S is the adsorption coefficient and Am is the cross-
sectional area occupied by each active surface atom.
Metal dispersion (D) was calculated by:
Field Emission Scanning Electron Microscopy (FE-SEM)
S-4800, Hitachi Company, Japan) along with Energy Dispersive
(
X-ray analysis (EDS) (LINK ISIS-300, Oxford Instruments) was
used to study the morphology as well as the composition of
H-ZSM-5 supported Ni catalysts, respectively. The Ni content of
D ¼ N
m
SM/100L
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where M and L are the molecular weight and percentage loading Table 1 Physicochemical properties of H-ZSM-5 and various loadings
of Ni on H-ZSM-5
of the supported metal catalyst.
Active crystallite size was obtained by:
EDS
AAS
S
BET
V
p
D
p
d
composition composition
t (nm) ¼ 100Lf/(AMSA ꢂ Z)
where Z is the density of the metal and f is the particle shape HZ
2
ꢀ1
3
ꢀ1
( A˚ ) (nm) (wt%)
Catalyst (m g
)
(cm g
)
(wt%)
305.6
292.8
0NHZ 274.2
0NHZ 215.9
0NHZ 209.1
0NHZ 173.5
0.21
0.20
0.16
0.16
0.15
0.11
0.12
27.3
—
—
—
correction factor (6 for spherical particles).
5NHZ
26.8 9.82 4.8
23.1 10.09 8.3
28.7 10.39 15.5
28.2 11.03 28.9
25.8 11.39 33.0
30.7 12.17 36.1
4.1
8.7
16.7
29.6
32.4
38.5
1
2
3
4
The XPS analysis was performed using a KRATOS AXIS 165
apparatus equipped with a dual anode (Mg and Al) using a Mg
K
a
source. The non-monochromatized Al K
a
X-ray source
(
hn ¼ 1486.6 eV) was operated at 12.5 kV and 16 mA. Analysis 50NHZ 161.5
was done at room temperature and prior to analysis the samples
were maintained under a rigorous vacuum, typically in the
ꢀ8
order of 10 Pa to avoid the presence of contaminates. All
binding energies measured were within an accuracy of ꢃ 0.2 eV.
For energy calibration, the carbon 1s binding energy was taken
as a reference value of 284.6 eV .
ꢁ
3.9 and 24.4 ) correspond to HZ in the NHZ catalysts indicating
the intactness of the zeolite structure even aer the loading of
metal. All the reduced catalysts showed XRD reections at 44.49
111), 51.85 (200) and 76.38 (220) corresponding to metallic Ni
2
ꢁ
ꢁ ꢁ
(
with a face-centred cubic geometry of the space group Fm3m
2
.3. Catalytic activity
˚
(
225) with a cell parameter of 3.523 A (ICDD. No. 87–0712). The
1
g of the catalyst particles was sandwiched between the two
intensity of these peaks increased as the Ni loading in the
catalyst increased. The crystallite sizes (d) of a series of catalysts
were calculated using the Debye–Scherrer formula from their full
width at half maximum (FWHM) values. The crystallite sizes of
all the Ni catalysts are presented in Table 1.
quartz wool plugs placed at the middle of a xed bed tubular
reactor made of glass (250 mm long and 12 mm i.d.). Prior to the
reaction, the catalyst was reduced for 4 h at 773 K. The reaction
was carried out in the temperature range of 523–568 K with a
liquid feed ow rate of 1 cm h using H
5
3
ꢀ1
2
as a carrier (H
2
/LA ¼
2
Fig. 2 depicts the N adsorption–desorption isotherms of H-
.5). The reaction products were collected at regular intervals of
ZSM-5 with various loadings of Ni supported on the H-ZSM-5.
The nitrogen adsorption isotherms of the catalysts show the
presence of a hysteresis loop, associated with capillary
condensation, which indicates the formation of a mesoporous
temperature in an ice cooled trap and analyzed by a Flame
Ionization Detector (FID) equipped gas chromatograph, GC-17A
(
(
(
Shimadzu Instruments, Japan) with an OV-1 capillary column
30 m length, 0.53 mm i.d.) and conrmed by GC-MS, QP 5050A
Shimadzu Instruments, Japan).
40
structure (secondary pores). The surface area (SBET), pore
volume (V
p p
) and average pore size (D ) of the samples are given
2
ꢀ1
in Table 1. The surface area of H-ZSM-5 is 305.6 m g and the
pore volume is 0.21 cm
decrease in surface area from 292.8 to 161.5 m g and in the
pore volume from 0.20 to 0.11 cm
3
ꢀ1
g . However, there is a marginal
3
. Results and discussion
2
ꢀ1
3
ꢀ1
XRD patterns of the H-ZSM-5 (HZ) and reduced NHZ catalysts
g
as the Ni loading
are shown in Fig. 1. The reections (2q ¼ 7.9, 8.8, 23.0, 23.2, 23.6,
Fig. 2
Fig. 1 XRD profiles of reduced Ni/H-ZSM-5 catalysts (a) HZ, (b) 5NHZ, HZ, (b) 5NHZ, (c) 10NHZ, (d) 20NHZ, (e) 30NHZ, (f) 40NHZ and (g)
c) 10NHZ, (d) 20NHZ, (e) 30NHZ, (f) 40NHZ and (g) 50NHZ. 50NHZ.
2
N adsorption–desorption profiles of Ni/H-ZSM-5 catalysts (a)
(
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FE-SEM pictures (depicted in Fig. S1†) show a cubic shaped
morphology. The composition of Ni obtained from EDS
patterns (depicted in Fig. S2†) and AAS is very close to the
theoretical value up to 30 wt% Ni (shown in Table 1). In the
impregnation method, the deposition of metal/metal oxide on a
support is generally limited beyond a certain amount. In the
present case, 30 wt% appears to be a limiting value. It depends
on the nature of the metal and the porosity of the support.
Transmission electron microscopy (TEM) and selected area
electron diffraction (SAED) images of the reduced and used
catalysts of 30NHZ (depicted in Fig. S3†) indicate that the
average particle size of the reduced and used catalysts calcu-
lated by Image-J soware were 10.98 nm and 11.74 nm,
respectively, which is close to the particle size obtained from
pulse chemisorption (9.22 nm) and also from the XRD (11.03
nm). The SAED pattern of Ni indicates the presence of regular
spots and also fringes attributed to the crystalline Ni with fcc
47
structure as reported by Wu et al. (conrmed from the XRD).
Fig. 3 TPR profiles of Ni/H-ZSM-5 catalysts (a) 5NHZ, (b) 10NHZ, (c)
The XPS spectra of Ni species in the reduced and used
catalysts are shown in Fig. 4. In the case of the reduced catalyst,
two major peaks are observed with binding energies (BE) of
20NHZ, (d) 30NHZ, (e) 40NHZ and (f) 50NHZ.
increases from 5 to 50 wt% due to pore blockage by Ni parti- 852.5 eV (69.4%) and 869.7 eV (30.6%), corresponding to the
cles. The blockage of pores by Ni creates a strain on the core level Ni 2p3/2 and Ni 2p1/2 transitions, while in the case of
structure of the zeolite that results in changes to the the used catalyst, two major peaks are observed with BEs of
4
1
isotherms. In addition to the blockage of H-ZSM-5 pores by 852.3 eV (70.7%) and 870.3 eV (29.3%), respectively, indicating
48
NiO particles, extra porosity due to NiO particles might have the presence of nickel in the metallic state. The BE values are
49
resulted. Because of this, the average pore diameter remains consistent with the literature reported by Xiao et al. No satel-
the same with loading. As can be seen from the XRD results, lite peak was observed with either of the catalysts. The differ-
there is no signicant change in the structural properties of ence between the two peaks (17.2 eV for the reduced and 18 eV
49
the zeolite.
The TPR proles of the catalysts are shown in Fig. 3. The
peak observed at higher temperature for the 5NHZ sample Ni were impregnated on the HZ. The signicant broad bands at
for the used) is very close to that reported by Xiao et al.
Fig. 5 illustrates the FTIR patterns when various loadings of
ꢀ
1
ꢀ1
ꢀ1
indicates the presence of strong metal–support interactions, 1230 cm , 1100 cm and 796 cm are attributed to the Si–O–
and the intensity of this phenomenon decreases as the metal Si asymmetric and symmetric stretching frequencies as well as
loading increases for subsequent catalysts of the series. The internal stretching frequencies of the zeolite. The band at
ꢁ
ꢀ1
peak at 500 C can be ascribed to the reduction of smaller NiO 1637 cm is associated with H–O–H for all the catalysts. The
ꢀ
1
particles and the other (low temperature signal at a T
of 400 absorption bands around 543 and 450 cm are characteristic of
C) due to bulk NiO. Maia et al. compared the reduction the ZSM-5 crystalline structure.
max
ꢁ
42
50
patterns of Ni/H-ZSM-5 prepared by different methods such as
impregnation and ion exchange method and concluded that if
+
2+
any H is exchanged by the incorporated Ni in the zeolite, this
could be reduced at a higher temperature than that located
43
outside the zeolite. In addition, Pawelec et al. anticipated that
nickel distributed in the zeolite was reduced above 630 C, at
ꢁ
the same time as the reduction of nickel oxide started around
ꢁ
44,45
5
00 C.
These results suggest that in all our prepared cata-
lysts, NiO is located predominantly outside the zeolite, which
was clearly indicated by the XRD patterns (preservation of
zeolite structure in all the catalysts). To the best of our knowl-
edge, in previous studies, various domains were suggested such
ꢁ
ꢁ
ꢁ
ꢁ
as <500 C, 500–630 C and >630 C. Among which, <500 C
ꢁ
could be assigned to the reduction of NiO, the 500–630 C zone
corresponds not only to smaller Ni particles but also to Ni
ꢁ
2+
oligomeric species and >630 C might be due to Ni in the
46
zeolite. In the present case, no such peak was observed at
higher temperature, which clearly demonstrates that Ni resides
predominantly outside the zeolite.
Fig. 4 XPS patterns of 30NHZ (a) reduced and (b) used.
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Table 2
H
2
uptake, active metal surface area, Ni particle size and
dispersion of Ni/H-ZSM-5 catalysts
ꢀ1
2
Niꢀ1
Catalyst
N
m
(mmol g
)
AMSA (m g
)
t (nm)
D (%)
5
1
2
3
4
5
NHZ
74.79
137.80
218.00
280.30
231.76
190.68
117.01
107.79
85.27
73.09
45.32
29.83
5.76
6.25
7.90
9.22
14.87
22.59
17.56
16.17
12.79
10.97
6.80
0NHZ
0NHZ
0NHZ
0NHZ
0NHZ
4.48
lactone (obtained from the dehydration of LA over HZM-5) over
Ni and observed the formation of GVL and minor quantities of
valeric acid. Hence, one can assume that Ni is responsible for
GVL formation by the hydrogenation of an intermediate,
angelica lactone.
Fig. 5 FT-IR profiles of Ni/H-ZSM-5 catalysts (a) HZ, (b) 5NHZ, (c)
Pulse chemisorption studies were conducted for all the
catalysts and the data compiled in Table 2. We observed that the
hydrogen uptake increases correspondingly with the Ni loading
up to 30NHZ. Thereaer, the uptake value decreased due to a
decrease in the number of surface Ni species which form bulk
Ni. The average particle size and dispersion are in good agree-
ment with each other.
1
0NHZ, (d) 20NHZ, (e) 30NHZ, (f) 40NHZ and (g) 50NHZ.
Fig. 6 shows Py-IR spectra of the catalysts at room tempera-
ture, used to investigate the acidity of the catalysts. The bands at
1
pyridine coordinated on Lewis acid sites created by metallic Ni
as well as the support.
ꢀ1
445, 1595 and 1612 cm are attributed to the adsorption of
51–54
ꢀ1
The band appearing at 1490 cm is
ascribed to the vibration frequency of the pyridine ring on 3.1. Catalytic activity
5
1,52
Brønsted and Lewis acid sites
created by both Ni and the
56–59
According to the literature,
the formation of GVL follows
ꢀ
1
support. In addition, IR bands near 1545 and 1637 cm are due
to pyridinium ions formed by the abstraction of a proton from
either of the two mechanisms (path I and II) shown in the
scheme in Fig. 7. The selection of Ni (non-noble metal) sup-
ported on H-ZSM-5 (acid support) is because of the fact that the
reaction proceeds through dehydration–cyclisation followed by
hydrogenation on acidic and metallic sites, respectively.
To gain insight into the reaction pathway, we have carried
out the reaction over H-ZSM-5 (51% conversion of LA and
selectivity to alpha angelica lactone (A) is 71% and to beta
angelica lactone (B) is 29%) that contains Brønsted as well as
Lewis acid sites conrmed by pyridine adsorption studies,
obtained angelica lactone (A) which isomerised to angelica
lactone (B), which on further hydrogenation over Ni gave the
target molecule. It is reported that the formation of angelica
51,52
the catalyst support.
The above observations reveal that both
Brønsted and Lewis acid sites are present on these catalysts in
which one or both may be responsible for the formation of GVL.
It is also a well-known fact that Ni favours the hydrogenation
reaction. In previous reports, Ni (111) was demonstrated to be
active for the hydrogenation reaction. To further clarify the
formation of GVL, we carried out the reaction of angelica
55
Fig. 6 Py-IR profiles of Ni/H-ZSM-5 catalysts (a) HZ, (b) 5NHZ, (c)
56–59
1
0NHZ, (d) 20NHZ, (e) 30NHZ, (f) 40NHZ and (g) 50NHZ.
Fig. 7 Reported LA hydrogenation reaction pathways.
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Fig. 9 Effect of the H /LA molar ratio on levulinic acid hydrogenation
over 30NHZ catalyst.
(Table 2) which are responsible for the hydrogenation of the
levulinic acid. However, the decrease in conversion with an
increase in loading of Ni beyond 30 wt% is ascribed to the
formation of larger Ni species (conrmed from the pulse
chemisorption results). Similarly, the yield also follows the
same trend as conversion which clearly indicates that surface Ni
species are playing a pivotal role in catalytic performance. The
30NHZ catalyst provided the optimum performance.
3
.3. Effect of H /LA molar ratio
2
Fig. 8 Effect of temperature and Ni loading on levulinic acid hydro-
genation with various NHZ catalysts.
Having optimized the catalyst composition, the effect of the H
to LA molar ratio in the feed was studied (Fig. 9). As the molar
2
ratio increases from 5.5 to 8, the conversion increases from 90–
1
00%. Furthermore, the yield of the g-valerolactone increases
6
0,61
lactone has been observed over acid catalysts.
Hence, we
from 75.3 to 92.2%. Further increases in the molar ratio to 11
and 14 result in a decline in the conversion and yield. This is not
only due to a decrease in residence time but also due to cleavage
of g-valerolactone to give pentanoic acid (Fig. 10). These prod-
ucts were conrmed by GC-MS. Similar results were included in
a patent application. Similarly, W. Luo et al. have reported the
inuence of the support by comparing H-ZSM-5, H-b, TiO and
ZrO supported Ru catalysts for levulinic acid hydrogenation
believe that the reaction proceeds via path I as shown in the
scheme. In a similar manner, Galletti et al. studied the role of
the acid as a co-catalyst to understand the reaction pathway. In
our case, we monitored the reaction with GC-MS to conrm the
reaction intermediates by carrying out the reaction on the
support only, as well as on a Ni-incorporated support.
The presence of angelica lactone (A & B) conrms the
pathway of the reaction and GVL formation conrms the further
hydrogenation of angelica lactone to yield the desired product.
Very recently, W. Luo et al. reported the inuence of the support
by comparing the acidity of H-ZSM-5, H-b, TiO and ZrO for the
32
63
2
62
2
2
2
62
levulinic acid hydrogenation.
3.2. Effect of temperature
In order to determine the optimum loading of Ni and also the
inuence of the temperature on the H-ZSM-5 support, the
hydrogenation of biomass-derived levulinic acid to g-valer-
olactone was carried out with all the prepared catalysts (5NHZ,
1
0NHZ, 20NHZ, 30NHZ, 40NHZ and 50NHZ) with the molar
ꢁ
ratio 5.5 (H /LA) in the vapour phase in the range of 250–295 C
2
and the data is shown in Fig. 8. The results show an increase in
conversion of LA from 70 to 90% with an increase in Ni loading
Fig. 10 Proposed mechanism for the hydrogenation of levulinic acid
from 5 to 30 wt% due to an increase in the surface Ni species over NHZ catalyst.
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reaction may also contribute to the deactivation. Further
work is needed to overcome deactivation during the time on
stream.
3.5. Productivity
The productivity was calculated by
Productivity ¼
Weight of LA pumped per hour ꢂ fractional yield of GVL :
Weight of catalyst
The productivity of other reported catalysts were also calcu-
lated and compared with our results and summarized in Table
Fig. 11 Effect of time-on-stream on levulinic acid hydrogenation over
ꢁ
3
0NHZ catalyst at 250 C and a ratio of H
2
/LA ¼ 8.
3. Upare et al. reported a 98.6% yield of GVL over 5% Ru/C while
the yield of GVL was 90 and 30% over 5%Pd/C and 5%Pt/C,
34
respectively, at atmospheric pressure. Though the yields are
good over Ru/C, the feed was diluted (10% LA in 1,4-dioxane)
with a solvent (1,4-dioxane). As well as being a highly cost
effective non-noble metal catalyst, the productivity of our cata-
lyst is twice that of Ru/C catalyst, whereas the productivity of
where they observed the maximum yield of pentanoic acid over
H-ZSM-5 and H-b supported Ru catalysts.
To gain further insight into the mechanism, the reaction
was carried out by pumping a larger quantity of reactants
3
ꢀ1
(
liquid feed (LA) ¼ 5 cm h , H /LA ¼ 5.5) over 1 g of catalyst
2
other catalysts follows the order 5% Cu/SiO > 5%Pd/C > 5%Pt/
2
and 85% conversion of LA was observed along with a 54% yield
of GVL, a 16.9% yield of angelica lactone (A), a 7.9% yield of
angelica lactone (B) and minor quantities of valeric acid.
Hence, we believe that the reaction is propagating in a
sequence of steps. So, we proposed the mechanism in our own
way as shown in Fig. 10.
C. The same group observed a 99.9% yield of GVL over Cu/SiO2
at 10 bar pressure and also with the same feed ratio. Signi-
cantly, we are getting a 92.2% yield of GVL with a productivity of
ꢀ1
ꢀ1
0.9090 kgGVL kgcatalyst
h
without adding any solvent at
atmospheric pressure.
The turnover frequency (TOF) of LA on each catalyst is
dened as the number moles of LA converted per one surface Ni
atom per second. It is calculated by
Furthermore, we carried out the reaction over Ni/SiO
Ni/Al (prepared by a wet impregnation method) which
resulted in lower yields than with the Ni/HZM-5 catalyst at a H
2
and
2 3
O
2
/
TOF ¼
LA molar ratio of 8. The yields over Ni/SiO and Ni/Al O were 87
2
2 3
and 83% with complete conversion, respectively.
No: of moles of LA pumped per second ꢂ fractional conversion :
No: of surface Ni sites
3.4. Effect of time-on-stream
Time-on-stream studies were conducted for 5 h over the
Effect of TOF on the loading of Ni is displayed in Fig. 12. The
ꢁ
3
0NHZ catalyst at 250 C and a ratio of H
2
/LA ¼ 8. The results number of surface Ni sites are assumed to be equal to half the
are shown in Fig. 11. From the 3rd hour, there is a slight number of H
2
moles adsorbed on the catalyst per gram. The
decrease in the activity which may be due to catalyst deacti- TOF was found to decrease up to a 20 wt% Ni loading, beyond
vation by water produced during the reaction or by fouling which the TOF was consistent. This clearly indicates that LA
6
4
65,66
agents like angelica lactone formed as an intermediate.
hydrogenation is structure-sensitive
up to 20 wt%, above
There is also a chance that the coke (ꢄ2%) formed during the which it is structure-insensitive.
Table 3 Productivity values of our catalyst compared with other reported catalysts
Catalyst source/
preparation method
Reaction
LA
GVL Productivity
ꢁ
catalystꢀ1 ꢀ1
h ]
Catalyst
Reaction conditions
temperature ( C) conversion yield [kgGVL kg
Ref.
3
5
5
5
5
0NHZ
Wet impregnation method Vapour phase (1 bar)
00% LA
Vapour phase (1 bar)
0% LA in 1,4-dioxane
Vapour phase (1 bar)
0% LA in 1,4-dioxane
Vapour phase (1 bar)
250
265
265
265
100
100
100
100
100
92.2 0.9090
98.6 0.4545
Present
study
34
1
%Ru/C
%Pd/C
%Pt/C
Sigma-Aldrich
Sigma-Aldrich
Sigma-Aldrich
1
90
30
0.4040
0.1515
34
34
35
1
10% LA in 1,4-dioxane
%Cu/SiO
2
Precipitation–
Vapour phase, H
2
(10 bar), 265
99.9 0.4444
deposition method
10% LA in 1,4-dioxane
9666 | RSC Adv., 2014, 4, 9660–9668
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8
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