Role of group V elements on the hydrogenation activity of Ni/TiO2 catalyst for the vapour phase conversion of levulinic acid to Γ-valerolactone

Article abstract of DOI:10.1016/j.cattod.2018.07.019

Influence of group V elements such as Ta, Nb and V on the product distribution in the vapour phase hydrogenation of levulinic acid (LA) over Ni/TiO₂ catalyst was examined at ambient pressure. The Nb promoted Ni/TiO₂ demonstrated a high selectivity towards γ-valerolactone (GVL) compared to other catalysts at 275 °C. The TPR results showed a lower H₂ uptake over Ta and V modified Ni/TiO₂ which was explained due to a strong interaction between these oxide species with nickel. Presence of a high ratio of ionic nickel (Ni²⁺) on Ta and V modified catalyst could be a possible reason for the formation of valeric acid (VA) through the ring opening of GVL. The high GVL selectivity over the Ni-Nb/TiO₂ catalyst attributed to the presence of a high proportion of Lewis acid sites in conjunction with finely dispersed Ni species on the catalyst surface. This however, is accomplished by the pyridine adsorbed diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and CO-chemisorption results.

Full text of DOI:10.1016/j.cattod.2018.07.019

Accepted Manuscript
Title: Role of group V elements on the hydrogenation activity
of Ni/TiO catalyst for the vapour phase conversion of
2
levulinic acid to ␥-valerolactone
Authors: Ganga Bhavani Peddakasu, Vijay Kumar Velisoju,
Manasa Kandula, Naresh Gutta, Komandur VR Chary,
Venugopal Akula
PII:
S0920-5861(18)30926-X
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.07.019
CATTOD 11564
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
21-3-2018
26-6-2018
4-7-2018
Please cite this article as: Bhavani Peddakasu G, Kumar Velisoju V, Kandula M, Gutta
N, VR Chary K, Akula V, Role of group V elements on the hydrogenation activity of
Ni/TiO catalyst for the vapour phase conversion of levulinic acid to ␥-valerolactone,
2
Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.07.019
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Role of group V elements on the hydrogenation activity of Ni/TiO catalyst for the vapour
2
phase conversion of levulinic acid to γ-valerolactone
Ganga Bhavani Peddakasu, Vijay Kumar Velisoju*, Manasa Kandula, Naresh Gutta,
Komandur VR Chary, Venugopal Akula*
Catalysis and Fine Chemicals Division, CSIR-Indian Institute of Chemical Technology, Uppal
Road, Tarnaka, Hyderabad, Telangana State - 500 007 India.
*
Corresponding author’s E-mail: velisoju.sva@gmail.com (Vijay Kumar Velisoju) and
akula.iict@gov.in (Venugopal Akula); IICT Communication number: IICT/Pubs./2018/206.
Tel: +91-40-27191720/3165; Fax: +91-40-27160921.
Graphical abstract
Highlights


Nb modified Ni/TiO showed a high ratio of surface metallic Ni and Lewis acid sites
2
2
+
Presence of Ni and Brønsted acid sites leads to formation of valeric acid, MTHF
1



-valerolactone rate was significantly improved upon modification of Ni/TiO
2
with Nb
Surface mechanism explained by using pyridine-DRIFTS and CO chemisorption data
Abstract
Influence of group V elements such as Ta, Nb and V on the product distribution in the
vapour phase hydrogenation of levulinic acid (LA) over Ni/TiO catalyst was examined at
ambient pressure. The Nb promoted Ni/TiO demonstrated a high selectivity towards γ-
valerolactone (GVL) compared to other catalysts at 275 °C. The TPR results showed a lower
uptake over Ta and V modified Ni/TiO which was explained due to a strong interaction
2
2
H
2
2
2
+
between these oxide species with nickel. Presence of a high ratio of ionic nickel (Ni ) on Ta
and V modified catalyst could be a possible reason for the formation of valeric acid (VA)
through the ring opening of GVL. The high GVL selectivity over the Ni-Nb/TiO catalyst
2
attributed to the presence of a high proportion of Lewis acid sites in conjunction with finely
dispersed Ni species on the catalyst surface. This however, is accomplished by the pyridine
adsorbed diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and CO-
chemisorption results.
Key words: Levulinic acid, γ-Valerolactone, Ni-Nb-TiO , Pyridine-IR, Biomass
2
1
. Introduction
The conversion of biomass derived levulinic acid (LA) to various fuels and fuel
additives is an important process in bio-refinery [1-2]. One such conversion is its
hydrogenation to γ-valerolactone (GVL; a fuel additive and green solvent) [3]. Although
promising results were obtained in the liquid phase conversion of LA to GVL, the vapour phase
conversion has also been considered due to additional benefits. These benefits include no
2

requirement for high pressure conditions, purification, space-to-time yield productivity,
reduced likelihood of catalyst deactivation, efficiency, safety and waste emission [4-11]. Very
few reports are available in the open domain on LA conversion to GVL in the vapour phase
[
12-15]. Dumesic’s group reported the use of the bimetallic Ru-Sn supported on carbon catalyst
for GVL synthesis from LA under continuous flow conditions with a turnover frequency of
-1
0
.051 s under 35 bar H pressure and 180 °C reaction temperature [16]. Upare and co-workers
2
studied the vapour phase synthesis of GVL from LA over Ni modified copper-silica catalysts
using formic acid as the hydrogen source [17-21]. Although the earlier reports showed some
promising results for GVL production from LA, there is still significant room for the
development of better conversion strategies that use stable and active non-noble metal catalysts
under mild reaction conditions. In this context, few reports have appeared on the vapour phase
LA conversion to GVL which showed that supported Ni and/or Cu are suitable catalysts for
GVL formation. These studies claimed that the catalysts with only surface Lewis acidity and
metal sites are responsible for GVL selectivity and the presence of other surface sites such as
Brønsted acid sites and basic sites will lead to the formation of products such as valeric acid
(VA) and pentane diols through ring opening of GVL [11,13]. Although esters of VA has
potential applications particularly in the transportation sector (diesel and gasoline blenders),
special attention is required for tuning the surface acid-base properties of the catalysts for the
selective one-pot conversion of LA to GVL and/or VA esters [11]. Although few reports
appeared in the vapour phase conversion for LA to GVL over the supported non-noble metal
catalysts, the studies carried out were not more focussed at tuning the surface properties of the
catalysts [11, 13, 19]. In this attempt, our group has investigated the promoted Ni catalysts for
the vapour phase LA conversion and in our recent reports it was concluded that the Ni/TiO is
2
selective towards GVL and the addition of promoter changed the product distribution,
depending on the type of promoter (acid/base) [22].
3

Therefore, based on the previous reports on the vapour phase LA conversion to GVL,
catalyst (catalyst with promising activity and stability towards GVL) is chosen in this
Ni/TiO
2
study by modifying its surface using high valence metal oxides such as V, Nb and Ta to
compare activity and selectivity. The group five elements (V, Nb and Ta) are known to exhibit
different redox and acid properties, the Ni/TiO
examined for the LA conversion in the vapour phase at ambient pressure [13, 23-27]. The
results of these investigations revealed that the Nb promoted Ni/TiO was found to be selective
towards GVL, whereas, the Ta and V modified Ni/TiO catalysts showed significant amount
of valeric acid, at constant LA conversion. To examine the dramatic change in product
distribution and the unusual behaviour of Nb modified Ni/TiO relative to other catalysts, the
catalysts were characterized by BET-SA, H -TPR, CO-pulse chemisorption, XPS and pyridine
adsorbed IR spectroscopy in order to understand the surface characteristics of the catalysts.
2
has been modified using V, Nb and Ta and
2
2
2
2
2
. Experimental Section
2
.1. Chemicals and materials
Levulinic acid (LA; 98%), RuCl
3
.xH O (99.9%), %), α-angelica lactone (AL; 98%), γ-
2
valerolactone (GVL; 99%), acetone (>99%), methanol (>99%), anisole (99.7%) and pyridine
(
99.8%) obtained from Sigma-Aldrich (AR grade), NH
Nb .H O), Ta , oxalic acid (Merck, >99%) used without further purification. De-ionised
(Degussa-P-25;
4
VO
3
(Fluka,>99%), niobic acid
(
2
O
5
2
2
O
5
water was used for the dilution of substrate and catalyst preparation. The TiO
2
2
-1
BET-surface area (SBET) = 56 m g ) obtained was used as a support for the preparation of
supported Ni catalysts.
2
.2. Catalyst preparation
The V, Nb and Ta modified Ni supported on TiO
2
catalysts were prepared by co-
impregnation method [22]. In a typical procedure, required amounts of aqueous Ni(NO
3 2
)
4

.
9H O (equivalent to 10wt% of Ni) and precursors of V, Nb and Ta (dissolved in oxalic acid)
2
corresponding to give 2wt% was instantaneously mixed with a known amount of solid support
under continuous stirring at 100 °C until the excess water was evaporated. The obtained powder
samples were dried at 120 °C for overnight and calcined in static air at 450 °C for 4 h.
2
.3. Catalyst characterization
The calcined samples were reduced at 450 °C in 5%H
2
/Ar for 3 h and were
-TPD, CO-pulse
O and/or pyridine adsorbed diffuse reflectance
infrared Fourier transform (DRIFT) spectroscopy. The hydrogenation activities were carried
out in a continuous flow fixed bed quartz reactor under sub-ambient H pressure. The
characterized by powder XRD, BET-surface area, H
2
-TPR, NH
3
chemisorption, CHNS, TEM, XPS and H
2
2
experimental conditions and the methods adopted in the present study are identical to the
experimental conditions used in our earlier studies and the complete details are given in ESI
[
27,28].
3
3
. Results and discussion
.1. Powder X-ray diffraction (XRD)
The XRD patterns of Ni loaded titania (reduced at 450 °C for 3 h) samples are shown
in Fig. 1. The diffraction lines ascribed to metallic Ni are clearly observed in all Ni/TiO
catalysts with reflections at 2θ = 44.3, 52.0 and 76.5 corresponding to Ni (1 1 1), (2 0 0) and
2 2 0) planes respectively (ICDD #: 04-0850). The remaining diffraction lines are attributed
to anatase and rutile TiO phases (ICDD #: 84-1286 and 88-1175 respectively). Although the
XRD patterns showed the diffraction signals due to TiO and metallic Ni; no diffraction signal
is observed due to promoters (V, Nb and Ta) indicating their fine dispersion on the surface or
because of low content as compared to Ni and TiO . In addition, a small shift in 2 θ position of
2
(
2
2
2
Ni is however observed in the promoted catalysts which are presumably due to an interaction
5

between promoter and Ni species (Fig. S1). The BET-surface area of the modified (V, Nb and
Ta) Ni/TiO samples are marginally decreased compared to Ni/TiO sample (Table 1).
.2. Physicochemical properties of the catalysts
The surface and bulk properties of catalysts are investigated using powder XRD, N
sorption, H -TPR, CO-pulse chemisorption, and pyridine adsorbed DRIFTS analysis (Table 1).
No significant changes are observed in the surface area of the catalysts after addition promoters.
This result also found consistent with the H -TPR analysis of the catalysts where no significant
changes are observed in H -consumption values obtained from TPR experiments. This is
however attributed to the presence of same loading of Ni in all the catalysts. From CO-pulse
chemisorption experiments, a higher CO uptake observed for Nb promoted Ni/TiO
consequently a relatively higher Ni surface area is found while compared to other catalysts
Table 2). The TPD of NH results revealed that the V and Nb promoted Ni/TiO catalysts
exhibited slightly higher acidity compared to the un-promoted and Ta promoted Ni/TiO
2
2
3
2
-
2
2
2
2
(
3
2
2
catalysts (Table 1 and Fig. S2). This increase in V and Nb promoted catalysts may be due to
the different acidic properties of V and Nb species which is further supported by the pyridine
adsorbed DRIFTS analysis of these catalysts which showed improved surface Brønsted and
Lewis acidity respectively. Finally, it is also interesting to note that the Ni crystallite size is
slightly decreased upon modification of Ni/TiO
observed in case of Ni-Ta/TiO
2
with V and Nb whilst a small increase is
2
.
3
.3. X-ray photoelectron spectroscopy (XPS)
The Ni species at the near surface region is investigated using XPS analysis and the
0
2+
spectra showed the presence of both metallic (Ni ) and ionic Ni (Ni ) species (Fig. 2). The
XPS signal at a binding energy of 852.5 eV confirms the presence of metallic Ni and the second
2
+
one centred at around 855.6 eV is assigned to Ni with a satellite peak at 860.9 eV. These
6

2
+
results are in good correlation with the reported studies [28,29]. The ionic Ni (Ni ) species
observed on the surface of these catalysts is probably due to a strong interaction between nickel
and the promoter or partly ascribed to the surface oxidation by air during the sample transfer
for XPS analysis [30]. In addition, a small shift in binding energy of Ni 2p position is observed
over Nb and Ta modified Ni/TiO
2
and no significant changes were seen in case of Ni-V/TiO
2
sample. The surface composition obtained for these catalysts showed a high proportion of ionic
2
+
Ni (Ni ) species on the surface of Ni-Ta/TiO
obtained for Nb promoted one which showed relatively a high metallic Ni species on its surface
Table S1).
2
catalyst followed by Ni/TiO and the lowest is
2
(
3
.4. H
2
-temperature programmed reduction (TPR)
The reduction behaviour of the catalysts is investigated by H -TPR analysis (Fig. 3)
2
and the results of this analysis indicated one major signal centred around 400 °C which is
explained due to the reduction of NiO species [31]. In the comparative analysis, no significant
change in the Tmax is observed after V addition to Ni/TiO
in Tmax towards lower temperature is found with Nb modified Ni/TiO
35 °C. In case of Ta promoted Ni/TiO catalyst, although Tmax appeared in the same position,
2
. On the other hand a dramatic shift
2
with a shoulder peak at
2
2
the reduction signal is found to be broad mostly likely due to the presence of large size clusters
of nickel species which is also in well correlation with the XRD data where the crystallite size
of nickel is higher (Table 1).
3
.5. Pyridine adsorbed DRIFTS analysis
The nature of acid sites present on the catalyst surface is investigated by using pyridine
as a basic probe molecule with FT-IR spectroscopy (Fig. 4). The spectra show a high ratio of
-1
Lewis acid sites (LAS; co-ordinated pyridine: ~1450 cm ) compared to Brønsted acid sites
-1
(
BAS; protonated pyridine: ~1540 cm ) [32,33]. Although Ni/TiO
2
and Ta modified Ni/TiO
2
7

-1
catalysts showed some Brønsted acidity, the vibrational band (around 1540 cm )
corresponding to the Brønsted acid sites is found more evident in case of vanadium modified
Ni/TiO
2
catalyst (Fig. 4). In case of Nb promoted Ni/TiO catalyst, a significant improvement
2
in the Lewis acidity is observed with negligible Brønsted acidity on the surface of the catalyst.
In the comparative analysis, the relative ratios calculated from the normalized DRIFT spectra
indicated a high BAS/LAS ratio for the Ni-V/TiO
2
catalyst and the lowest ratio are observed
for Nb promoted Ni/TiO sample (Table 2 and Fig. S3).
2
The activity measurements of vapour phase LA hydrogenation to GVL is correlated
with the physicochemical properties of the catalysts and the results revealed a higher GVL rate
over Nb promoted Ni/TiO
GVL rate was observed due to the formation of VA at the cost of GVL over Ni/TiO
Particularly in case of V and Ta modified Ni/TiO catalysts, more amount of VA is observed
2
compared to the other catalysts (Table 2). On the contrary, a lower
2
.
2
which is explained by the ring opening of GVL (Scheme 1). Therefore, it can be seen that the
higher GVL rate over the Nb promoted catalyst can be ascribed to the presence of more surface
metallic Ni species (Table S1) along with a high ratio of Lewis acid sites. Presence of surface
2
+
Brønsted acidity along with high proportion of ionic Ni (Ni ) species on the catalyst surface
led to the formation of VA and other products which is good correlation with the earlier reports
and surface characteristics of the catalysts investigated in this study (Table 2 and Table S1)[34].
In further studies, the reaction temperature is optimised for Nb promoted Ni/TiO
2
catalysts and the results of this analysis revealed 275 °C is the optimum reaction temperature
for required GVL selectivity (Fig. 5 and Fig. S4). Although slightly high conversion is obtained
at high temperatures, a decrease in GVL selectivity is observed at the cost of other products
formation particularly valeric acid. The formation of valeric acid at high reaction temperatures
8

is however attributed to the ring opening of GVL to form pentenoic acid which will be
subsequently hydrogenated to VA over the metallic Ni sites.
flow: 20 cc/min; GHSV = 9.74 mL s gcat-1; Feed: 10wt% LA in H
O;
-
1
Reaction conditions: H
2
2
Data collected after 6 h of continuous operation.
Based on the product distribution and surface characteristics of the catalysts, a reaction
mechanism is also proposed (Scheme 1) in this study which is in good correlation with the
reported studies on the LA hydrogenation to GVL [35]. It is proposed that, at 275 °C reaction
temperature, first LA (enolic form) will undergo dehydration to form an intermediate called
angelica lactone preferably over the mild acid sites (Lewis acid sites in this case) as it is
evidenced from pyridine adsorbed DRIFTS. The formed angelica lactone is then subsequently
hydrogenated to GVL over the metallic sites (Ni in this case). As mentioned, in case if there is
any surface Brønsted acidity, the ring opening of GVL will take place that leads to the
formation of valeric acid.
In addition to the optimization of reaction parameters, long-term stability of the best
performance catalyst i.e. Nb promoted Ni/TiO
2
catalyst is also tested for 30 h of reaction time
pressure. The results of this
(Table S2) at 275 °C reaction temperature and atmospheric H
2
analysis showed no significant changes in the rate or TOF (both LA conversion and GVL
selectivity) of GVL even after 30 h of continuous operation. This indicates the robustness of
the catalyst system under the experimental conditions used.
Finally, the used catalyst is investigated using TPD of NH
3
, CHNS and XRD analyses
results
and the results are also compared with that of fresh catalyst (Table 3). The TPD of NH
3
indicated a slight decrease in activity which is attributed to the adsorption of water molecules
during the course of the reaction and the small amount of carbon deposition is due to the
9

irreversible chemisorption of intermediates or reactant molecules such as AL and LA
respectively [11,36]. From XRD analysis of used (after 30 h) Ni-Nb/TiO catalyst, no
2
significant change in the Ni particle size is observed between the fresh and used forms.
Conclusions
Influence of promoters such as V, Ta and Nb on the modification of surface Lewis,
Brønsted acid sites on Ni/TiO
phase at atmospheric H pressure. It is observed that Nb promoted catalyst is found to be the
promising catalyst in terms of activity and stability compared to the other catalysts. The high
activity of Ni-Nb/TiO catalyst is attributed to the presence of highly populated Lewis acid
2
was examined for the LA hydrogenation to GVL in vapour
2
2
sites in parenthesis to the Ni metal surface area of the catalyst compared to the other catalysts
which were rationalized by pyridine adsorbed DRIFTS analysis in conjunction with CO-
chemisorption experiments respectively. The detailed investigations of Ni-Nb/TiO catalyst in
2
the vapour phase LA conversion to GVL revealed that the catalyst is not only active and
selective towards GVL but also stable under the optimised experimental conditions. Further
studies are in progress on the exploration of HCOOH as an in-situ H
2
source for the
hydrogenation of LA at low temperatures.
Acknowledgements
All the authors thank DST New Delhi for funding under India-Australia (AISRF:
DST/INT/AUS/P-64/2015) program. PGB thank DST-Inspire for the award of research
fellowship. VVK and KVRC thank CSIR New Delhi for research funding through Emeritus
scientist scheme.
1
0

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3

Fig. 1. Powder XRD patterns of reduced (a) 10Ni/TiO
and (d) 10Ni-2Ta/TiO samples.
2
(b) 10Ni-2V/TiO
2
(c) 10Ni-2Nb/TiO
2
2
1
4

Fig. 2. Ni 2p spectra of reduced (a) 10Ni/TiO
0Ni-2Ta/TiO samples.
2
(b) 10Ni-2V/TiO
2
(c) 10Ni-2Nb/TiO and (d)
2
1
2
1
5

Fig. 3. H
2
-TPR profiles of (a) 10Ni/TiO
2
(b) 10Ni-2V/TiO
2
(c) 10Ni-2Nb/TiO and (d) 10Ni-
2
2
Ta/TiO
2
samples.
1
6

Fig. 4. Pyridine adsorbed DRIFT spectra of (a) 10Ni/TiO
2
(b) 10Ni-2V/TiO (c) 10Ni-
2
2
Nb/TiO and (d) 10Ni-2Ta/TiO samples.
2
2
1
7

Fig. 5. Influence of reaction temperature over Nb promoted Ni/TiO catalyst.
2
1
8

Scheme 1. Plausible surface reaction mechanism for the GVL production from LA.
1
9

Table 1. Physico chemical properties of the catalysts.
cH
-uptake dNH
-
eCrystallite
size
(nm)
29.6
22.8
23.5
a
Surface area bCO-uptake
2
3
Catalyst
10wt%Ni)
(
µmol/gcat) uptake
2
(
(m /g)
(µmol/gcat)
(µmol/gcat
)
Ni/TiO
Ni-2wt%V/TiO
Ni-2wt%Nb/TiO
Ni-2wt%Ta/TiO
2
48
46
43
40
33.5
29.6
42.2
26.3
1037
1019
1061
1011
40.8
55.4
53.8
41.2
2
2
2
32.1
a
b
c
d
e
Determined from BET surface area analysis;
Obtained from CO-pulse chemisorption experiments;
Obtained from H -TPR analysis;
Determined from NH -TPD analysis;
Calculated by Scherrer equation w.r.t. Ni(111) plane.
2
3
2
0

Table 2. Activity measurements of the catalysts. Reaction conditions: H
GHSV = 9.74 mL s gcat ; Reaction temperature: 275 °C; Feed: 10wt% LA in H
collected after 6 h of continuous operation.
2
flow: 20 cc/min;
O; Data
-1
-1
2
LA conversion Selectivity (%)
%)
GVL AL VA
RateGVL
bSNi
(m /gNi
cBAS/LAS
Catalyst
Othersa (µmol gcat s )
-1
-1
2
)
ratio
0.18
0.62
0.05
0.26
(
1
1
1
1
0wt%Ni/TiO
0wt%Ni-2wt%V/TiO
0wt%Ni-2wt%Nb/TiO
0wt%Ni-2wt%Ta/TiO
2
17.3
19.2
25.3
15.5
78.3
54.3
99.0
60.9
7.2 5.9
25 16.5
0.0 0.0
12 11.5
8.6
4.2
1.0
15.6
0.77
0.38
1.21
0.46
13.1
11.6
16.5
10.3
2
2
2
a
b
c
Others include: C
Calculated using CO-uptakes;
Calculated using pyridine adsorbed DRIFT spectra.
4 5
,C -alcohols and MTHF;
2
1

Table 3. Characterization of the used catalyst.
Catalyst NH uptake
10% Ni-2%Nb)
3
Carbon
(wt%)
-
Particle size
(nm)
23.5
-1
(
(µmol gcat
53.8
)
Fresh
Used
48.2
0.96
24.1
2
2