An investigation on the influence of support type for Ni catalysed vapour phase hydrogenation of aqueous levulinic acid to γ-valerolactone

Article abstract of DOI:10.1039/c5ra24199e

Ni (20 wt%) supported on SiO₂, γ-Al₂O₃ and ZrO₂ catalysts was examined for hydrogenation of aqueous levulinic acid (LA) to γ-valerolactone (GVL) at 270°C and ambient pressure. The band intensities of Br?nsted (BAS: 1540 cm^-1) and Lewis acid sites (LAS: 1450 cm^-1) estimated by pyridine adsorbed DRIFT spectra revealed a lower ratio of BAS/LAS over the Ni/SiO₂ catalyst than over the Ni/ZrO₂ and Ni/γ-Al₂O₃ catalysts. The rate of angelica lactone (AL) formation was lower than the rate of AL hydrogenation over the Ni/SiO₂ catalyst. The poisoning and regeneration of the Ni/SiO₂ catalyst using pyridine and 2,6-dimethylpyridine demonstrated that Lewis acid sites influenced the conversion of LA to AL and subsequent hydrogenation of AL to GVL occurred on surface Ni sites. In contrast, Br?nsted acid sites were responsible for the ring opening of GVL to valeric acid (VA). Kinetic data emphasized that the hydrogenation activity and product distribution were dependent on the type of acid site, and the Ni sites in close proximity to Br?nsted acid sites are prone to hydrogenolysis of GVL to valeric acid and hydrocarbons.

Full text of DOI:10.1039/c5ra24199e

RSC Advances
View Article Online
View Journal | View Issue
PAPER
An investigation on the influence of support type
for Ni catalysed vapour phase hydrogenation of
aqueous levulinic acid to g-valerolactone†
Cite this: RSC Adv., 2016, 6, 9872
ab
ab
a
a
Velisoju Vijay Kumar, Gutta Naresh, Medak Sudhakar, Chatla Anjaneyulu,
b
b
c
Suresh Kumar Bhargava, James Tardio, Vanga Karnakar Reddy,
c
a
Aytam Hari Padmasri and Akula Venugopal*
Ni (20 wt%) supported on SiO , g-Al O
2 2 3
and ZrO
2
catalysts was examined for hydrogenation of aqueous
ꢀ
levulinic acid (LA) to g-valerolactone (GVL) at 270 C and ambient pressure. The band intensities of
ꢁ1
ꢁ1
Brønsted (BAS: 1540 cm ) and Lewis acid sites (LAS: 1450 cm ) estimated by pyridine adsorbed DRIFT
spectra revealed a lower ratio of BAS/LAS over the Ni/SiO catalyst than over the Ni/ZrO and Ni/g-Al
catalysts. The rate of angelica lactone (AL) formation was lower than the rate of AL hydrogenation over
the Ni/SiO catalyst. The poisoning and regeneration of the Ni/SiO catalyst using pyridine and 2,6-
2
2
2 3
O
2
2
dimethylpyridine demonstrated that Lewis acid sites influenced the conversion of LA to AL and
subsequent hydrogenation of AL to GVL occurred on surface Ni sites. In contrast, Brønsted acid sites were
responsible for the ring opening of GVL to valeric acid (VA). Kinetic data emphasized that the
hydrogenation activity and product distribution were dependent on the type of acid site, and the Ni sites
in close proximity to Brønsted acid sites are prone to hydrogenolysis of GVL to valeric acid and hydrocarbons.
Received 16th November 2015
Accepted 7th January 2016
DOI: 10.1039/c5ra24199e
www.rsc.org/advances
ꢀ
2
00 C and 70 bar H pressure over Cu-based catalysts derived
2
1
. Introduction
16
from hydrotalcite precursors during the LA hydrogenation.
The conversion of levulinic acid (LA), a compound that is ob-
The vapour phase conversion of LA to GVL does however
tained from inedible, woody biomass feedstock, namely ligno- have a number of advantages with respect to the liquid phase
cellulose, to fuels and ne chemicals is a topic of increasing process, such as having no requirement for high pressure
1,2
interest. In order to achieve the conversion of LA to fuels, the conditions, purication, space-to-time yield productivity, envi-
rst step required is to convert LA to g-valerolactone (GVL). This ronmental impact and generally a reduced likelihood of catalyst
can be done via catalytic dehydration of LA to angelica lactone deactivation, as well as additional benets in terms of effi-

17
(
AL) followed by subsequent catalytic hydrogenation of AL, ciency, safety and waste emission. Dumesic et al. investigated
giving rise to GVL. The GVL produced can then be converted to the vapour phase conversion of LA to GVL using a carbon sup-
a range of valuable compounds (Scheme 1).
ported bimetallic Ru–Sn catalyst with a turnover rate of 0.051
ꢁ
1
ꢀ
Numerous studies have been conducted on the catalytic
hydrogenation of LA to GVL and also the catalytic conversion of
s
H
using 2-sec-butyl-phenol as the solvent at 180 C and 35 bar
18
2
pressure. Upare et al. studied vapour phase LA hydroge-
3–7
ꢀ
GVL to valuable products. The majority of the aforemen- nation at ambient pressure and 265 C over a commercial Ru/C
tioned studies have been reported in the liquid phase with very catalyst and achieved 100% LA conversion with 100% GVL
8
–14
19
few studies being examined in the vapour phase.
Zhang et al. selectivity. Earlier, we reported LA hydrogenation over
reported the selective hydrogenation of LA to GVL in methanol hydroxyapatite (HAP) supported metal (Ru, Pt, Pd, Ni, Cu)
ꢀ
over magnetic Ni4.59Cu
1
15
Mg1.58Al1.96Fe0.70 catalyst with a GVL catalysts in the vapour phase at ambient H
2
pressure and 275 C
ꢀ
yield of 98.1% at 142 C. Yan et al. reported a 91% GVL yield at with 92% conversion and 99.8% GVL selectivity over the Ru/
20
HAP catalyst.
a
Although the studies on Ru based catalysts have led to
Catalysis Laboratory,
I & PC Division, CSIR – Indian Institute of Chemical
Technology, Uppal Road, Hyderabad, Telangana-500 007, India. E-mail: akula@iict. promising results, there is scope for development of cheaper
res.in; Fax: +91-40-27160921; Tel: +91-40-27193165
catalysts, such as those based on transition metals. Upare et al.
investigated the conversion of aqueous LA to GVL using a Ni
promoted copper–silica nano composite catalyst in the pres-
b
Centre for Advanced Materials & Industrial Chemistry (CAMIC), School of Applied
Sciences, RMIT University, Melbourne-3001, Australia
c
Department of Chemistry, University College for Women, Osmania University, Koti,
21
ence of formic acid. Mohan et al. examined the Ni/H-ZSM-5
Hyderabad-500 095, Telangana, India
ꢀ
catalyst for LA to GVL at 250 C and 1 bar H pressure with
1
2
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra24199e
22
00% LA conversion and 92.2% selectivity to GVL. The
1
9872 | RSC Adv., 2016, 6, 9872–9879
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Scheme 1 Possible products in the catalytic hydrogenation of levulinic acid.
mechanism of hydrogenation of LA to GVL occurs via either incipient wet impregnation method. Briey, the required
dehydration to angelica lactone followed by its reduction or via amount of solid support (e.g. SiO , g-Al O , ZrO ), and in some
2
2
3
2
reduction of LA to 4-hydroxyvaleric acid and subsequent dehy- cases a-Al O , was added to an aqueous solution of
2
3
dration. For both of these routes, an active metal combined with Ni(NO ) $6H O (an amount required to obtain a 20 wt% Ni
3
2
2
ꢀ
an acid site should be present on the catalyst surface. It has loading) and the suspension was then stirred at 100 C until the
been reported that Brønsted acid sites are more prone to ring water had evaporated. The recovered solid was then oven dried
ꢀ
ꢀ
opening of GVL to valeric acid (VA: another fuel additive) and at 120 C overnight and calcined in static air at 500 C for 5 h.
hydrocarbons.
23,24
However, studies pertaining to the support effect for the
vapour phase hydrogenation of LA to GVL have not been
2.2 Characterization of the catalysts
investigated in detail except for recent interesting reports from The powder XRD analyses of the catalysts were recorded with an
the Weckhuysen group over supported Ru catalysts at high Ultima-IV X-ray diffractometer (M/s. Rigaku Corporation, Japan)
2
5,26
pressures.
Particularly, the inuence of acid–base charac- using a Ni-ltered Cu Ka (l ¼ 0.15418 nm) radiation source and
teristics of the various conventional and non-conventional a scintillation counter detector. The diffraction patterns were
supports, such as g-Al
ꢀ
ꢁ1
ꢀ
2
O
3
, SiO
2
, ZrO
2
, TiO
2
, MgO, ZnO, La
2
O
3
,
recorded with a scan rate of 5 min in the 2q range of 10–70
hydroxyapatite and carbon, as supports for either noble metals at 40 kV and 20 mA. The average crystallite size (D) of the
or base metals have not been explored for the vapour phase catalysts was determined by applying the Debye–Scherrer
conversion of LA to GVL. In our continued activities on the equation with respect to the Ni(111) plane. The surface areas of
selective hydrogenation of LA to GVL over Ru based catalysts, we the reduced samples were measured by N adsorption at
2
ꢀ
have examined several support materials for Ru; for brevity their ꢁ196 C (Micromeritics ASAP 2010 surface area analyzer). The
10,20
catalytic activities are given in Table S1 (ESI†).
Hydrogena- nature of the acid sites of the catalysts was examined by pyri-
tion of levulinic acid was also carried out at high H
over various noble and non-noble metal catalysts.
been reported that the nature and type of (Brønsted and/or with a resolution of 2 cm with 64 scans for each spectra
2
2
pressures dine adsorbed FT-IR spectroscopy (Carry 660, Agilent Technol-
7–30
ꢁ1
It has ogies). Spectra were obtained in the range of 1400–1700 cm
ꢁ
1
Lewis) acid site would inuence the product distribution collection. The experiments were performed in situ using
24
during the hydrogenation of levulinic acid.
a purpose-made IR cell connected to a vacuum adsorption
The present investigation is focussed on the characteristics setup. In a typical method, the reduced samples were pressed
ꢁ
3
of Ni supported on g-Al O , SiO , and ZrO catalysts; the role of into self-supporting wafers (density ꢂ 40 mg cm ) at a pressure
2
3
2
2
5
the metal site and/or the acid sites would inuence the product of 10 Pa. Subsequently, the wafers were transferred into the IR
distribution. The catalysts were characterized by DRIFT analysis cell and were pre-treated in N ow by heating at a rate of 10 C
and the role of Brønsted and Lewis acid sites is rationalized by min up to 400 C for 1 h. Aer cooling down to 150 C, the
poisoning of the catalysts using pyridine and 2,6-dimethylpyr- spectrum was collected in the dri mode. The sample was then
idine during the course of the reaction. The catalytic activities exposed to pyridine until surface saturation in successive pulse
ꢀ
2
ꢁ
1
ꢀ
ꢀ
ꢀ
were evaluated strictly under a kinetic regime. Some of the injections at 150 C and subsequently the sample was purged
samples were examined by BET-SA, TEM, and H -TPR, and the for 30 min in N ow before recording the spectrum. The dri
2
2
used catalysts were analysed by CHNS to investigate potential spectra aer pyridine adsorption were subtracted from the
coking.
spectra of the untreated catalyst to obtain the vibrational bands
owing to pyridine acid site interaction. Finally, the spectra were
quantied with the Kubelka–Munk (K–M) function and the
2. Experimental

tted curves were used to measure the relative ratios of
2.1 Preparation of catalysts
Brønsted (BAS) and Lewis acid sites (LAS) for the corresponding
spectral lines at full width at half maximum (FWHM). The
H -TPR analysis was carried out in a quartz micro-reactor
2
2
ꢁ1
2
ꢁ1
The SiO
2
(BET-SA: 395.0 m g ), g-Al
2
O
3
(BET-SA: 230.5 m g
)
2
ꢁ1
ZrO
2
(BET-SA: 52.8 m g ) Ni(NO
3
)
2
$6H
2
O, levulinic acid (98%),
g-valerolactone (99%), angelica lactone (98%), valeric acid, and interfaced to a gas chromatograph (GC) equipped with
methyl tetrahydrofuran (MTHF) were purchased from Sigma- a thermal conductivity detector (TCD) unit. Prior to TPR anal-
ꢀ
Aldrich and used as received. The a-Al
2
O
3
was received from ysis, the catalyst was degassed at 300 C in helium gas for
NORPRO-R97655. All the catalyst samples were prepared by 30 min and then cooled to room temperature. The helium gas
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 9872–9879 | 9873

View Article Online
RSC Advances
Paper
ꢂ
ꢃ
was switched to 4.97% H
2
in argon with a ow rate of 30 mL
LAc ꢁ LAc
in
out
ꢁ
1
ꢀ
Conversion of LA ðXLAÞ ¼
ꢃ 100
min and the temperature was increased to 900 C at a ramp-
LAcin
ꢀ
ꢁ1
ing rate of 5 C min . The hydrogen uptakes of the samples
ꢂ
ꢃ
were measured using a calibration curve of Ag O TPR under
SGVL
2
Selectivity of GVLðSGVLÞ ¼
ꢃ 100
a similar protocol. Calibrated mass ow controllers (Alicat
Scientic, USA) were used to regulate the ow rates for all the
gases used. The carbon contents in the used samples were
measured using a VARIO EL, CHNS analyzer. The elemental
analysis of the fresh and used samples was carried out by
atomic absorption spectroscopy (AAS) PerkinElmer, Analyst-
SGVL þ SAL þ Sothers
ꢄ
ꢅ
P
i
Selectivity ¼ P
ꢃ 100 ðwhere i ¼ GVL; AL and othersÞ
P
i
ꢁ
1
GVLyield ꢃ LAflow rateðmol s
cat
Þ
r
GVL
¼
300. The AAS analysis of the fresh and used samples indicated
weight of the catalystðg Þ
no leaching of metal during the course of the reaction (Table
S2†). The CO pulse chemisorption experiments were carried out
using a pulse titration procedure at 40 C on an AUTOSORB-iQ
automated gas sorption analyzer (M/s. Quantachrome Instru-
ꢀ
ꢁ
ꢁ
1
ꢁ1
rate mol s gcat
TOF ¼
ꢀ
ꢁ
ꢀ
ꢁ1
CO uptake mol g
cat
ments, USA). In a typical method, the sample was reductively
ꢀ
pre-treated at 450 C for 2 h in 4.97% H
2
, balance Ar, then the 3. Results
ꢀ
sample was ushed in helium gas for 30 min at 450 C followed
by titration with 5.02% CO, balance He, at 40 C. The Ni
dispersion, Ni metal surface area and Ni particle size were
calculated using the following equations:
3
.1 X-ray diffraction and BET surface area analysis
ꢀ
The XRD patterns of Ni supported on SiO , g-Al O , and ZrO
2
2 3 2
samples are shown in Fig. 1. Two main diffraction lines at 2q ¼
ꢀ
4
4.5 corresponding to Ni (111) plane (ICDD #: 04-0850) and 2q
ꢀ
ꢁ
ꢁ
1
CO uptake mmol g
ꢀ
cat
¼ 52.0 (Ni (110); ICDD #: 04-0850) are ascribed to the metallic
Ni phase present in all the samples. The other diffraction lines
present in the XRD patterns were owing to their respective
Dispersionð%Þ ¼
ꢀ
ꢁ ꢃ 100
ꢁ1
total metal mmol g
cat
supports, as conrmed by the ZrO
2
phase (ICDD # 37-1484). The
Metal area ¼ metal cross sectional area
ꢃ no. of metal atoms on surface (i.e. CO uptake)
2
0 wt% Ni/SiO catalyst displayed only a broad peak centred
2
ꢀ
around 2q ¼ 23 showing the micro-crystalline nature of SiO
2
.
From the previous reports, it is clear that because of the close
proximity between the Ni, NiAl O and g-Al O phases, it is
2
4
2 3
6
000
Particle sizeðnmÞ ¼
r : metal density
ꢀ
ꢁ1ꢁ
;
difficult to discriminate the isolated phases. However, the
formation of the NiAl phase was justied by H -TPR analysis
of the Ni/Al catalyst (Fig. 2). The BET surface area and Ni
domain size of 20 wt% Ni supported on SiO , g-Al , and ZrO
catalysts are reported in Table 1. The 20 wt% Ni/SiO and 20
wt% Ni/g-Al samples showed signicantly higher surface
areas than that of the 20 wt% Ni/ZrO catalyst. The XRD
patterns (Fig. 1) indicated a higher Ni crystallite size on the 20
wt% Ni/ZrO (37.8 nm), while the 20 wt% Ni/SiO and 20 wt%
Ni/g-Al
catalysts both had a domain size of Ni of ꢂ21 nm.
2
Ni metal surface area m gcat
ꢃ r
2
O
4
2
2 3
O
2
2
O
3
2
2
2 3
O
2
.3 Activity measurements
2
The vapour phase hydrogenation of aqueous levulinic acid was
2
2
carried out in a xed bed quartz reactor (i.d. ¼ 10 mm, length ¼
2 3
O
4
20 mm) in down ow mode. The experimental conditions and
product analysis details were similar to those reported by us
20
earlier. The carbon mass balance in all the experiments was
99% unless otherwise stated. The inuence of both Brønsted and
>
Lewis acid sites in the LA conversion was examined by carrying
out two independent experiments: (1) by using 2,6-dimethylpyr-
idine (2,6-lutidine) as a selective Brønsted acid site blocker and (2)
pyridine (Py) as both a Brønsted and Lewis acid site blocker.
In a typical method, about 12.4 mmol of probe was
injected successively in 4 pulses (3.1 mmol each) into the
aqueous LA stream. Aer each pulse, the samples were
collected and analyzed by GC-MS. Aer the dosage, the
ꢀ
catalyst was regenerated at 450 C in owing air and the
ꢀ
catalysts were subsequently reduced at 450 C with 4.97% H
2
,
balance Ar, before the aqueous LA was subjected to the
catalyst. The conversion, selectivity, rates and turnover
frequency (TOF) on product formation were calculated using
the equations given below:
Fig. 1 XRD patterns of 20 wt% Ni supported on (a) SiO
2
, (b) g-Al
2
O
3
ꢀ
and (c) ZrO
2
samples reduced at 450 C for 3 h.
9874 | RSC Adv., 2016, 6, 9872–9879
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
3.3 Pyridine adsorption studies
To determine the proportion of BAS to LAS on the catalyst
surface, pyridine adsorption was used in conjunction with in
situ DRIFTS analysis (Fig. 3) and the ratios of Brønsted (BAS) to
Lewis acid sites (LAS) are reported in Table 1. The relative ratios
ꢁ
1
of pyridine adsorbed spectral bands at 1450 cm (Lewis acid
ꢁ
1
site) and ꢂ1540 cm (Brønsted acid site) emphasized that all
36
the catalysts had a higher number of LAS compared to BAS.
The proportion of BAS to LAS (0.38) was signicantly higher on
the 20 wt% Ni/ZrO catalyst compared to the 20 wt% Ni/g-Al O
2
2 3
2
(0.27) and 20 wt% Ni/SiO (0.05) catalysts (Table 1).
Fig. 2 The H
2
-TPR profiles of 20 wt% Ni supported on (a) SiO
2
, (b)
2 2 3
ZrO and (c) g-Al O catalysts.
3.4 Catalytic hydrogenation of levulinic acid
The vapour phase hydrogenation of aqueous levulinic acid over
The crystallite size obtained for the respective catalysts indi- SiO , g-Al and ZrO supported 20 wt% Ni catalysts is re-
cates that the lower the surface area of the support, the higher ported in Table 2. It can be seen that the 20 wt% Ni/SiO catalyst
the extent of Ni agglomeration, as a result large sized Ni parti- exhibited better LA conversion and selectivity of GVL compared
cles are formed during the preparation (Table 1).
to that of the 20 wt% Ni/g-Al and 20 wt% Ni/ZrO catalysts.
The low conversion of LA over the 20 wt% Ni/ZrO catalyst may
2
2
O
3
2
2
2
O
3
2
2
have been partly owing to larger Ni particles and also owing to
lower Ni dispersion of the catalyst.
3
.2 H -temperature programmed reduction (TPR)
2
The 20 wt% Ni/SiO catalyst demonstrated very good selec-
2
The H
2 2 2
-TPR proles of 20 wt% Ni supported on SiO , ZrO and
2 3
tivity of GVL (90.6%) while the 20 wt% Ni/g-Al O and 20 wt%
g-Al O catalysts are presented in Fig. 2 and their H uptakes are
2
3
2
Ni/ZrO catalysts showed moderate selectivity of 60% and 72%,
2
reported in Table 1. The reduction maxima (Tmax) were different
for all the catalysts, although the samples have constant Ni
loading. This suggests the variation in the Ni interaction with
varied supports. The reduction signals appearing in the TPR
respectively. In addition to AL, valeric acid (VA), methylte-
trahydrofuran (MTHF), 1,4-pentanediol and hydrocarbons such
as butane and pentane are observed. Formation of AL is likely
owing to the rate at which AL is formed being faster than the
rate at which it is hydrogenated to GVL. The hydrogenation
reactivity of these samples is related to the surface Ni metal area
0
patterns are associated with NiO reduction to Ni . For the
ꢀ
2
0 wt% Ni/SiO
2
sample, Tmax found at 322 C is an indication
of dispersed NiO particles. These ndings are in good
agreement with the results reported by Mile et al.
3
1,32
In the
case of the 20 wt% Ni/ZrO catalyst, the reduction maximum
2
ꢀ
occurred at 420 C, which is assigned to relatively large size
1
3,33
NiO particles.
The reduction prole of the 20 wt% Ni/g-
Al O sample indicated very high temperature signals; the
2
3
one at Tmax ꢂ 550 ꢀC is attributed to NiO interacting with
ꢀ
g-Al
2
O
3
and the high temperature peak (Tmax > 800 C) is
possibly owing to an NiAl
temperature for its reduction.
2
O
4
spinel, which requires high
However, XRD analysis did
3
4,35
not conrm the reections owing to the NiAl O phase
2
4
because of the superimposition of diffraction lines with that
of the g-Al phase. The TPR results revealed that the
2
O
3
reduction behaviour of NiO particles was strongly inuenced
Fig. 3 Pyridine adsorbed drift spectra of 20 wt% Ni supported on (a)
by the type of support.
2 2 3 2
ZrO , (b) g-Al O and (c) SiO catalysts.
Table 1 Physicochemical characteristics of the 20 wt% Ni supported on SiO
2 2
, g-Al O
3
and ZrO
2
samples
2
0 wt%
2
ꢁ1
a
b
catꢁ1
c
Ni supported on
BET surface area (m g
)
Ni crystallite size (nm)
H
2
uptake (mmol g
)
BAS/LAS ratio
SiO
g-Al
2
155.5
108.8
23.5
20.5
21.4
37.8
1237
1308
1204
0.05
0.27
0.38
2 3
O
ZrO
2
a
b
c
Calculated from XRD spectra using Scherrer equation w.r.t Ni(111) plane. Obtained from H
ratio obtained from pyridine adsorbed FT-IR spectra.
2
-TPR analysis; calibrated with Ag
2
O TPR. BAS/LAS
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 9872–9879 | 9875

View Article Online
RSC Advances
Paper
Table 2 Product distribution in the hydrogenation of LA over 20 wt% Ni supported on SiO
2
, ZrO
2
and g-Al
2
O
3
catalysts. Reaction conditions: 10
ꢀ
3
ꢁ1
ꢁ1
ꢁ1
wt% LA in H
2
O, 270 C, catalyst wt ꢂ 0.05 g, H
2
¼ 20 cm min , GHSV ¼ 19.5 mL s
gcat , activity data measured after 6 h on stream
%
selectivity
a
b
catꢁ1
c
2
Niꢁ1
d
ꢁ1
e
2
0 wt% Ni on % XLA
GVL AL
VA
MTHF Others
CO uptake (mmol g
)
S
Ni (m g
)
TOFGVL (s
)
Carbon (%)
SiO
ZrO
g-Al
2
15.7
3.5
10.7
90.6
60.2
71.9
1.0
0.4 3.9
2.9
4.7
9.0
74.8
39.3
56.5
14.6
7.7
11.1
1.82
0.52
1.32
0.52
2.57
2.28
2
16.9 15.8 2.4
4.2
2
O
3
6.6 8.2
a
b
c
1
,4-Pentanediol, pentane, butane, CO
2
.
CO uptakes measured from pulse chemisorption. Ni metal surface areas calculated from CO uptakes.
d
e
Rate/CO uptake. Obtained from CHNS analysis of catalysts recovered aer 15 h on stream.
(
Table 2). The presence of VA and MTHF, however, indicates
4. Discussion
that either GVL had reacted further and/or AL had undergone
ring opening (Scheme 2). Higher selectivity of AL over the 20 The product distribution is strongly inuenced by the type of
wt% Ni/ZrO catalyst is most likely owing to this intermediate acid sites (Brønsted and/or Lewis) as well as their strength on
2
not undergoing further reaction to GVL where this conversion the catalyst surface. A detailed investigation is carried out using
would rely on the hydrogenation activity of the surface Ni. One two different probe molecules while co-feeding along with the
possible reason is that the rate of hydrogenation of the double aqueous LA, such as pyridine (both Brønsted and Lewis acid site
bond (in AL) on the metal site (Ni) seems to be very high on the blocker) and 2,6-dimethylpyridine (as a selective Brønsted acid
20 wt% Ni/SiO
2
catalyst compared to the Ni on ZrO
2
and g-Al
2
O
3
site blocker; Scheme 3).
supports. Consequently, AL undergoes ring opening to produce
An equimolar concentration of both pyridine (12.4 mmol)
VA, MTHF and HCs. Another reason may presumably be owing and 2,6-dimethylpyridine (12.45 mmol) was introduced in the
to the conversion of GVL to VA, MTHF and hydrocarbons, since reaction stream in (4 equal) successive pulses in separate
both the 20 wt% Ni/g-Al O and 20 wt% Ni/ZrO2 catalysts experiments. The activity data obtained aer the addition of
2
3
exhibited higher numbers of Brønsted acid sites.
pyridine and/or 2,6-dimethylpyridine are given in Table 3. It can
The difference in selectivity of GVL, VA and MTHF is be seen that a signicantly lower conversion was obtained over
explained in terms of the variation in the nature and type of the 20 wt% Ni/SiO catalyst when pyridine was fed along with
2
(
Brønsted and/or Lewis) acid sites present on the catalyst aqueous LA. The conversion of LA was decreased to 9.7% from
surface. The sample (20 wt% Ni/SiO
) with a lower ratio of 29.9% (activity loss of ꢂ60%); however, the GVL selectivity was
Brønsted to Lewis acid sites (Table 1) demonstrated lower marginally decreased when pyridine was co-fed over the 20 wt%
selectivity for VA and MTHF, while both the 20 wt% Ni/g-Al O
Ni/SiO catalyst (Table 3). From this, it can be inferred that the
2
2
3
2
and 20 wt% Ni/ZrO2 catalysts had relatively higher BAS/LAS active Ni sites are not affected by the addition of pyridine. Since
ratios and exhibited high selectivity for GVL ring opening the BAS/LAS ratio on the 20 wt% Ni/SiO catalyst is less than 0.1,
2
2
4
products. The cyclisation of LA to AL is initiated by Lewis acid it is therefore concluded that Lewis acid sites are responsible for
sites present on the catalyst surface. In the presence of the dehydration of LA. According to the reaction mechanism,
23
st
a signicantly high ratio of BAS/LAS on the 20 wt% Ni/ZrO
2
and the acid sites are crucial for the dehydration of LA to AL (1 step
2
0 wt% Ni/g-Al catalysts, the rate of ring opening of either AL in Scheme 2). The poisoned catalyst was then regenerated by
2 3
O
ꢁ
1
and/or GVL to form VA is higher; as a result, a decrease in GVL replacing the aqueous LA with air ow (ꢂ20 mL min ) at 450
ꢀ
selectivity was observed over both the catalysts (Table 2).
C for 3 h and subsequently reducing the catalyst in H stream
2
ꢀ
ꢁ1
at 450 C for 30 min . Aer regeneration, the sample was
Scheme 2 Reaction pathways during LA hydrogenation in the vapour phase.
9876 | RSC Adv., 2016, 6, 9872–9879
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Scheme 3 Surface poisoning of Brønsted and Lewis acid sites using pyridine (both Brønsted and Lewis site blocker) and 2,6-dimethylpyridine (a
selective Brønsted acid site blocker) as probes.
Table 3 Influence of probe addition on aqueous LA hydrogenation be concluded that a metal site (Ni) in close proximity to a Lewis
over 20 wt% Ni/SiO
2
catalyst. Reaction conditions: 10 wt% aqueous LA,
acid site is active for the selective conversion of LA to GVL.
These results suggest modication of surface properties can
alter the product distribution thus the desired product selec-
tivity can be ne-tuned.
ꢀ
3
ꢁ1
ꢁ1
2
g
70 C; catalyst wt: ꢂ0.075 g, H
2
¼ 20 cm min , GHSV ¼ 9.74 mL s
ꢁ1
cat
LA hydrogenation reaction
condition and analysis
X
LA
S
GVL
S
others
The time on stream (TOS: 15 h) analysis emphasized the
2
long-term stability of the 20 wt% Ni/SiO catalyst (Table S3†).
The CHNS analysis of the used catalysts revealed that there was
a
Without probe addition
29.9
9.70
27.8
35.3
25.2
30.7
90.5
81.1
88.8
92.5
91.5
89.6
10.4
18.9
11.2
7.45
8.50
10.4
b
Aer pyridine addition
c
a very low amount of carbon deposition over the 20 wt% Ni/SiO2
Aer regeneration
a
Without probe addition
catalyst compared to that over the other catalysts (Table 2). The
d
Aer 2,6-dimethylpyridine addition
lower activity of the 20 wt% Ni/g-Al O and 20 wt% Ni/ZrO2
2
3
e
Aer regeneration
catalysts may also possibly be owing to the deactivation of the
a
b
Activities over 20 wt% Ni/SiO
2
.
Activities aer the addition of 12.4 ꢃ catalysts owing to fouling agents, like angelica lactone formed
ꢁ3
c
1
0
moles of pyridine. Activities aer regeneration of catalyst.
as an intermediate, which leads to coke deposition (ꢂ2%) on
d
ꢁ3
Activities aer ethe addition of 12.45
ꢃ
10
moles of 2,6-
19
the catalyst surface.
dimethylpyridine. Activities aer regeneration of catalyst.
To understand the nature of bulk NiO, samples using an
inert support, i.e. a-Al O and 20 wt% Ni/a-Al O , were exam-
2
3
2 3
ꢁ
1
ꢁ1
ined for LA hydrogenation at a GHSV of 1.95 mL gcat
s
and
examined for LA hydrogenation under similar experimental
conditions (Table 3). Although the catalyst suffered a loss in LA
conversion aer probe addition, the catalytic activity was
regained aer the successive treatments at 450 C in owing air
2
and in diluted H . In contrast to this, no signicant change in
the results are reported in Table 4. The a-Al O (known as an
inert material possessing neither acid nor basic sites) showed
minute activity at 325 C with 100% selectivity towards AL, while
the bulk NiO demonstrated about 9.5% LA conversion with
8.8% selectivity towards GVL. On the other hand, the 20 wt%
Ni/a-Al catalyst exhibited 11.2% LA conversion and 93.6%
GVL selectivity (Table 4). To further conrm the inuence of
2
3
ꢀ
ꢀ
9
LA conversion and GVL selectivity was observed when 2,6-
dimethylpyridine was co-fed with aqueous LA. Pyridine would
block both the Brønsted and Lewis acid sites when the catalyst
2 3
O
acidity type and the role of metal in this hydrogenation, CeO (a
2
ꢀ
is exposed at 270 C (Scheme 3). A drastic reduction in LA
37
Lewis acidic material) was taken and tested in the LA hydro-
genation reaction, which showed about 4.4% conversion of LA
conversion indicates the importance of Lewis sites for LA
conversion to AL. On the other hand, when 2,6-dimethylpyr-
idine was added, a marginal decrease in LA conversion implies
the existence of available Lewis acid sites since the 2,6-dime-
Table 4 LA hydrogenation activities over bulk NiO, a-Al
2
O
3
and 20 wt%
ꢀ
ꢁ1
thylpyridine is a selective Brønsted acid site blocker (Table 3). Ni/a-Al O catalysts; reaction conditions: 10 wt% aqueous LA, 325 C;
2
3
3
ꢁ1
ꢁ1
catalyst wt: ꢂ0.5 g, H
2
¼ 20 cm min , GHSV ¼ 1.95 mL s
g
cat
Therefore, it can be concluded that Brønsted acid sites play
a vital role in the ring opening of GVL, which is further sup-
plemented by the pyridine adsorbed FT-IR analysis. This could
be explained owing to the presence of strong acid sites
ꢁ9
ꢁ9
r
AL/10
rGVL/10
catꢁ1 ꢁ1
catꢁ1 ꢁ1
s )
Catalyst
X
LA
S
AL
S
GVL
(mol g
s
)
(mol g
(
Brønsted); LA chemisorption occurs strong enough to cause a-Al
NiO
0 wt%
2 3
Ni/a-Al O
2
O
3
0.2 100
0.0 0.98
0.0
46
133
24
9.5
1.7 98.8 0.79
6.4 93.6 8.60
the ring opening of AL. It is thus suggested that the 20 wt% Ni/
g-Al and 20 wt% Ni/ZrO catalysts possess weaker hydroge-
nation activity than the 20 wt% Ni/SiO catalyst. Finally, it can
2
11.2
2
O
3
2
2
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 9872–9879 | 9877

View Article Online
RSC Advances
Paper
with 100% selectivity of angelica lactone. This result thus
emphasizes the role of Ni and the support material that is being
used to disperse the active metal. We believe that the trans-
formation of LA is occurring at the interface of the catalyst.
Therefore, we can conclude that the LA conversion is initi-
ated by the Lewis acid sites present on the catalyst surface. A
combination of surface Ni metal with adjacent Brønsted acid
sites on the catalyst surface leads to ring opening of either GVL
and/or AL to form VA and MTHF along with hydrocarbons. As
aforementioned, a Lewis site combined with nickel is suitable
for the selective hydrogenation of LA to GVL. Finally, it can be
5 P. P. Upare, J. M. Lee, Y. K. Hwang, D. W. Hwang, J. H. Lee,
S. B. Halligudi, J. S. Hwang and J. S. Chang, ChemSusChem,
2011, 4, 1749.
6 P. P. Upare, Y. K. Hwang, J. M. Lee, D. W. Hwang and
J. S. Chang, ChemSusChem, 2015, 8, 2345.
7 R. A. Bourne, J. G. Stevens, J. Ke and M. Poliakoff, Chem.
Commun., 2007, 4632.
8 W. Luo, P. C. A. Bruijnincx and B. M. Weckhuysen, J. Catal.,
2014, 320, 33.
9 B. P. Kumar, N. Nagaraju, V. P. Kumar and K. V. R. Chary,
Catal. Today, 2015, 250, 209.
concluded that Lewis acid sites in close proximity to Ni sites (i.e. 10 M. Sudhakar, M. L. Kantam, V. S. Jaya, R. Kishore,
at interface) are responsible for dehydrocyclization of LA to
form AL, followed by its hydrogenation, thus producing GVL. In
K. V. Ramanujachary and A. Venugopal, Catal. Commun.,
2014, 50, 101.
contrast to this, although Ni supported on the a-Al
strated lower LA conversion, the reaction proceeds to cyclisation
followed by hydrogenation to GVL. The high intrinsic activity of 12 J. M. Tukacs, R. V. Jones, F. Darvas, G. Dibo, G. Lezsak and
the 20 wt% Ni/SiO2 catalyst may be attributed to the higher L. T. Mika, RSC Adv., 2013, 3, 16283.
proportion of Lewis acid sites than Brønsted acid sites. We also 13 V. Mohan, V. Venkateshwarlu, C. V. Pramod, B. D. Raju and
believe that a uniform distribution of Ni particles on the 20 wt% K. S. R. Rao, Catal. Sci. Technol., 2014, 4, 1253.
Ni/SiO catalyst may be another reason for selective conversion 14 J. Zhang, S. Wu, B. Li and H. D. Zhang, ChemCatChem, 2012,
of LA to GVL (Fig. S1†).
2
O
3
demon- 11 W. R. H. Wright and R. Palkovits, ChemSusChem, 2012, 5,
1657.
2
4, 1230.
15 J. Zhang, J. Chen, Y. Guo and L. Chen, ACS Sustainable Chem.
Eng., 2015, 3, 1708.
5. Conclusions
16 K. Yan, J. Liao, X. Wu and X. Xie, RSC Adv., 2013, 3, 3853.
The inuence of supports was assessed for Ni in the LA 17 C. M. Marrodan and P. Barbaro, Green Chem., 2014, 16, 3434.
hydrogenation by means of different conventional supports. It 18 S. G. Wettstein, J. Q. Bond, D. M. Alonso, H. N. Pham,
was found that SiO support is suitable for Ni for high selectivity
of g-valerolactone. In the case of other supports, such as ZrO2
A. K. Datye and J. A. Dumesic, Appl. Catal., B, 2012, 117–
118, 321.
2
and g-Al
cost of GVL formation. The 20 wt% Ni/ZrO
Al
consequently high selectivity of angelica lactone is observed. A
high ratio of Brønsted acid sites on the 20 wt% Ni/ZrO and 20
2 3
O , the selectivity of VA and MTHF were detected at the 19 P. P. Upare, J. M. Lee, D. W. Hwang, S. B. Halligudi,
2
and 20 wt% Ni/g-
Y. K. Hwang and J. S. Chang, J. Ind. Eng. Chem., 2011, 17, 287.
2 3
O catalysts demonstrated mild hydrogenation activity and 20 M. Sudhakar, V. V. Kumar, G. Naresh, M. L. Kantam,
S. K. Bhargava and A. Venugopal, Appl. Catal., B, 2016, 180,
113.
2
wt% Ni/g-Al O catalysts resulted in ring opening of GVL. 21 P. P. Upare, M. G. Jeong, Y. K. Hwang, D. H. Kim, Y. D. Kim,
2
3
Finally, it can be concluded that the product distribution can be
tuned to get the desired fuel additive (GVL or VA) as the strength
D. W. Hwang, U. H. Lee and J. S. Chang, Appl. Catal., A, 2015,
491, 127.
of the surface acid–base sites regulates the selectivity in the 22 V. Mohan, C. Raghavendra, C. V. Pramod, B. D. Raju and
hydrogenation of levulinic acid.
K. S. R. Rao, RSC Adv., 2014, 4, 9660.
2
3 V. V. Kumar, G. Naresh, M. Sudhakar, J. Tardio, S. K. Bhargava
and A. Venugopal, Appl. Catal., A, 2015, 505, 217.
4 K. Kon, W. Onodera and K. I. Shimizu, Catal. Sci. Technol.,
2014, 4, 3227.
Acknowledgements
2
VVK and GN thank RMIT, Australia for the award of fellowships;
MS, CA and VK thank CSIR New Delhi for the award of fellow- 25 W. Luo, U. Deka, A. M. Beale, E. R. H. V. Eck,
ships. All authors acknowledge Frank Antolasic, RMMF staff,
RMIT, Australia for the support during characterization.
P. C. A. Bruijnincx and B. M. Weckhuysen, J. Catal., 2013,
301, 175.
26 W. Luo, P. C. A. Bruijnincx and B. M. Weckhuysen, J. Catal.,
2
014, 320, 33.
References
2
7 A. M. Ruppert, J. Grams, M. Jedrzejczyk, J. Matrs-Michalska,
N. Keller, K. Ostojska and P. Sautet, ChemSusChem, 2015, 8,
1538.
1
2
3
4
M. J. Climent, A. Corma and S. Iborra, Green Chem., 2014, 16,
16.
5
J. C. S. Ruiz, D. J. Braden, R. M. West and J. A. Dumesic, Appl. 28 J. Lv, Z. Rong, Y. Wang, J. Xiu, Y. Wang and J. Qu, RSC Adv.,
Catal., B, 2010, 100, 184. 2015, 5, 72037.
J. M. Bermudez, J. A. Men ´e ndez, A. A. Romero, E. Serrano, 29 K. Shimizhu, S. Kanno and K. Kon, Green Chem., 2014, 16,
J. G. Martinez and R. Luque, Green Chem., 2015, 15, 2786. 3899.
J. Xin, D. Yan, O. Ayodele, Z. Zhang, X. Lu and S. Zhang, 30 R. Rodiansono, M. D. Astuti, A. Ghofur and K. C. Sembiring,
Green Chem., 2015, 17, 1065. Bull. Chem. React. Eng. Catal., 2015, 10(2), 192.
9878 | RSC Adv., 2016, 6, 9872–9879
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
31 B. Mile, D. Stirling, M. Zammitt, A. Lovell and M. Webb, J. 35 F. Marino, E. Cerrella, S. Duhalde, M. Jobbagy and
Catal., 1988, 114, 217. M. Laborde, Int. J. Hydrogen Energy, 1988, 23, 1095.
3
3
2 J. Xiong, J. Chen and J. Zhang, Catal. Commun., 2007, 8, 345. 36 H. Kn ¨o zinger and P. Ratnasamy, Catal. Rev., 1978, 17, 31.
3 H. S. Roh, K. Y. Koo, J. H. Jeong, Y. T. Seo, D. J. Seo, Y.-S. Seo, 37 Y. Wang, F. Wang, Q. Song, Q. Xin, S. Xu and J. Xu, J. Am.
W. L. Yoon and S. B. Park, Catal. Lett., 2007, 117, 85.
Chem. Soc., 2013, 135, 1506.
34 A. J. Akande, R. O. Idem and A. K. Dalai, Appl. Catal., 2005,
287, 159.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 9872–9879 | 9879