Gas phase hydrogenation of levulinic acid to γ-valerolactone over supported Ni catalysts with formic acid as hydrogen source

Article abstract of DOI:10.1039/c5nj02655e

The catalytic continuous vapor phase hydrogenation of levulinic acid (P = 1 atm; T = 250°C) in the absence of external hydrogen was investigated over inorganic oxide (Al₂O₃, MgO and hydrotalcite) supported Ni (30 wt%) catalysts. The present protocol enables the utilization of the unavoidable co-product (i.e. formic acid) formed during the production of levulinic acid as a hydrogen source. Among the tested catalysts, the Ni/Al₂O₃ catalyst was an efficient catalyst for the production of γ-valerolactone through hydrogen-independent hydrogenation. A significant decrease in catalytic performance of Ni/MgO and Ni/hydrotalcite catalysts was observed during the time-on-stream; while a gradual decrease was noted in the catalytic performance of the Ni/Al₂O₃ catalyst. The considerable decline in catalytic performance of Ni/MgO and Ni/hydrotalcite catalysts was attributed to water generation during the course of the reaction, rather than the coking of reaction intermediates (angelica lactone). Furthermore, the co-feeding of water and formic acid with levulinic acid was performed and a significant decrease was noted in the catalytic performance of Ni/MgO and Ni/hydrotalcite catalysts compared to the Ni/Al₂O₃ catalyst. The results evidently signify the role of water in the activity of Ni/MgO and Ni/hydrotalcite catalysts, which could be ascribed to brucite-periclase transition of MgO with the water, which was formed during the hydrogenation of levulinic acid.

Full text of DOI:10.1039/c5nj02655e

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Gas phase hydrogenation of levulinic acid to γꢀvalerolactone over
supported Ni catalysts with formic acid as hydrogen source
1
Varkolu Mohan *, Velpula Venkateshwarlu, Burri David Raju, Kamaraju Seetha Rama Rao *
*Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology,
Hyderabadꢀ500607, India. Tel.: +91 40 27191712; Fax: +91 40 27160921,
Eꢀmail:ksramarao@iict.res.in.
1
Present address: School of Chemistry & Physics, University of KwaZuluꢀNatal, Westville
Campus, Chiltern Hills, Durbanꢀ4000, South Africa. Eꢀmail:mohan.iict@gmail.com,
Varkolum@ukzn.ac.za.
Abstract
The catalytic continuous vapor phase hydrogenation of levulinic acid (P = 1 atm; T =
2
50 °C) in the absence of external hydrogen was investigated over inorganic oxide (Al O ,
2 3
MgO and Hydrotalcite) supported Ni (30 wt%) catalysts. The present protocol enables the
utilization of unavoidable coꢀproduct (i.e. formic acid) formed during the production of
2 3
levulinic acid as a hydrogen source. Among the tested catalysts, Ni/Al O catalyst to be an
efficient catalyst for the production of γ–valerlactone through the hydrogen independent
hydrogenation. The significant decrease in catalytic performance of Ni/MgO and
Ni/Hydrotalcite catalysts were observed during the time on stream; while a gradual decrease
2 3
was noticed in the catalytic performance of Ni/Al O catalyst. The considerable decline in
catalytic performance of Ni/MgO and Ni/Hydrotalcite catalysts were attributed to the water
generation during the course of the reaction rather than the coking of reaction intermediates
(
angelica lactone). Furthermore, the coꢀfeeding of water and formic acid with levulinic acid
was performed and noticed the significant decrease in the catalytic performance of Ni/MgO
and Ni/Hydrotalcite catalysts compare to Ni/Al catalyst. The results evidently signify the
2
O
3
role of water on the activity of Ni/MgO and Ni/Hydrotalcite catalysts which could be
ascribed to brucite – periclase transition of MgO with the water, which formed during the
hydrogenation of levulinic acid.
2 3
Keywords: vapour phase, hydrogenation, levulinic acid, formic acid, Ni/Al O
1

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Introduction
With the growing anxiety about environmental pollution and ever rising rigorous
energy demand. A sustainable progress is necessary owing to the energy exhaust and global
warming. The imbalance was created due to rapid depletion of exhaustible fuels through the
anthropogenic actions than its generation. Furthermore, utilizations of these limited fuels
2
discharge carbon dioxide (CO ) to the environment. To tackle the problems associated with
the exhaustible fuels, we need to explore for an alternative source. In this scenario, leading
researchers from the globe have predicted the “Roadmap for Biomass Technologies” due to
its vast availability and expected to produce 20% of fuels and 25% of chemicals.
Additionally, biomass is the fourth most abundant source available on the globe after coal,
natural gas, and crude oil. Currently, 14% of biomass being used for energy supply and rest
of the biomass is useless [1]. So, the utilization of biomass and/or biomassꢀderived platform
molecules to fuels, fuel additives, and chemicals are crucial.
Consequently, utilization of biomass burgeoning lot of interest due to deteriorating of
fossil fuels and a global warming effect on the environment. Production of fuels and
chemicals from the biomass composed of two steps: 1) production of small intermediates 2)
further to fuels and chemicals from the intermediate. One such intermediate is the levulinic
acid which is generally obtained by the hydrolysis of cellulose, furfuryl alcohol and also a byꢀ
product of the paper industry [2]. Hydrogenation of levulinic acid, which consists of both ꢀ
C=O and –COOH can give γꢀvalerolactone (GVL), 2ꢀmethyl tetrahydrofuran (2MeTHF), and
1, 4ꢀPentanediol respectively from the subsequent hydrogenation. In particular, GVL has a lot
of applications such as the solvent for lacquers, insecticides, and adhesives, in cutting oil,
brake fluid and coupling agent in dye bath [3], along with these applications, we can also
produce fuels and chemicals from this chemical such as diesel fuel and jet fuels [4ꢀ6].
The production of GVL from the hydrogenation of LA can be done by both
homogeneous and heterogeneous catalytic systems [7]. In the literature, it is reported that the
hydrogenation of levulinic acid can be done in the batch as well as the continuous process
[
7]. The continuous process has several advantages like ease of product separation, solventꢀ
free conditions, etc. In turn, hydrogenation is possible in both the ways either by using
molecular hydrogen or utilization of formic acid as a sole hydrogen source. Formic acid
comprised of 4.4 wt % hydrogen storage, in addition, it can release stored hydrogen by
heterogeneous catalysts and also nowadays its utilization as an immense interest because of
2

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its stateꢀofꢀtheꢀart of storage capacity and also its production through simple biomass
processing. In our previous works, we performed the levulinic acid hydrogenation using
molecular hydrogen over Niꢀbased catalysts [2, 8 ꢀ12]. Besides, we also shown the influence
of impurities like formic acid and water on the hydrogenation activity [8, 10]. The earlier
works made us to show interest in the hydrogenation of levulinic acid in the absence of an
external hydrogen (H
mention that the Niꢀbased catalysts are efficient for the dehydrogenation of formic acid [13ꢀ
6]. Furthermore, the Shell oil company patented the hydrogenation of levulinic acid and its
2
) or use of formic acid as a hydrogen source. It is noteworthy to
1
esters using formic acid as a sole hydrogen source [17]. Few researchers reported the
hydrogen independent hydrogenation of levulinic acid [18, 19]. Recently, few studies
suggested the hydrogenation of levulinic acid using formic acid as hydrogen source [12, 20].
In particular, formic acid has diverse benefits, for example, renewable nature, inexpensive
and safer than gaseous hydrogen derived from petroleum derivatives. On the other hand,
2 2
formic acid decomposes to H and CO2, the in situ liberated H more reactive than the ex situ
2
H .
Hence, the present work focused on the preparation of Niꢀbased catalysts and their
evaluation for levulinic acid hydrogenation in the absence of an external hydrogen (H ) or
2
utilization of formic acid as a sole hydrogen source. The extensive physicochemical
characterization of the catalysts were done by means of XRD, SEM, TEMꢀSAED, HRTEM,
N
2
physisorption analysis, H
2
ꢀTPR and H
2
pulse chemisorption techniques (see
, Ni/MgO, and Ni/HT
supplementary information). The prepared catalysts, namely, Ni/Al
2
O
3
were designated as NA, NM and NHT respectively hereafter and applied to gas phase
hydrogenation of biomassꢀderived levulinic acid to γꢀ valerolactone with formic acid as sole
hydrogen source at 250 °C at atmospheric pressure.
Experimental
Materials and catalysts preparation
Materials Ni(NO ) 6H O (M/s. LOBA Chemie, India, 98%); commercial supports
3
2
2
2 3
such as Al O (M/s. SudꢀChemie, India), MgO (M/s.Sd fine India Ltd, India) were purchased
and used without any further purification. The MgꢀAl hydrotalcite (Mg/Al=2) support was
prepared by precipitation method reported elsewhere [21, 22]. In a typical synthesis, nitrate
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precursors of the Mg and Al were dissolved in deionized water and precipitated by 5% NaOH
and 5% Na CO mixed solution by maintaining the pH in a range of 9ꢀ12. The resulting
2
3
precipitate was filtered and washed thoroughly with deionized water several times till the
excess base was removed. Subsequently, the filtered mass was dried overnight in an air oven
and then calcined at 450 °C for 18 h. Al O , MgO and Hydrotalcite supported Ni catalysts
2
3
were prepared by conventional wet impregnation method by adding the inorganic oxide
support to a Ni(NO ・6H O (M/s. Sd fine India Ltd, India) aqueous solutions followed by
3
)
2
2
drying at 80 °C until dryness, the amount of Ni loading was fixed at 30 wt %. The resultant
solids were dried at 100 °C for 12 h and, then, calcined in air at 450 °C for 5 h. The prepared
catalysts Ni/ Al
respectively.
2 3 2 3
O , Ni/MgO and Ni/ MgOꢀAl O are designated as NA, NM and NHT
Catalyst Characterization:
XRD patterns of the catalysts were registered using a Rigaku UltimaꢀIV (M/s Rigaku
Corporation, Japan) Xꢀray diffractometer having Niꢀfiltered Cu Kα radiation (λ= 1.5406 Å)
−1
with a scan speed of 4° min and a scan range of 2 – 80° at 40 kV and 20 mA.
The BET surface area of all the catalysts was determined by N physisorption at liquid N
2
2
temperature i.e. ꢀ196 °C (ASAP 2020 Adsorption unit, M/s. Micromeritics, USA). Before the
physisorption analysis, the catalyst samples were degassed under vacuum at 250 °C for 1 h to
remove the physisorbed moisture.
Temperature programmed reduction (TPR) of the fresh and spent catalysts were performed
on homemade reactor setup. About 50 mg of catalyst was placed in a quartz reactor and preꢀ
3
ꢀ1
2
treated in Ar flow 100 °C for 2 h. A flow of 5% H –Ar mixture gas (60 cm min ) with a
ꢀ1
temperature ramping of 10 °C min was maintained. The hydrogen consumption was
monitored by using a thermal conductivity detector (TCD).
4

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A JEOL JEM 2000EXII transmission electron microscope, operating at 160 and 180 kV used
to investigate structural features of catalysts. The specimens were prepared by dispersing the
samples in acetone using an ultrasonic bath and evaporating a drop of resultant suspension
onto the lacey carbon support grid.
H – Chemisorption using pulse (100ꢁL) titration procedure was carried out at 40 °C on a
2
AUTOSORBꢀiQ, automated gas sorption analyser (M/s. Quantachrome Instruments, USA) to
know the dispersion and metal particle size, metal surface area of the catalyst. Prior to the
experiment, the catalyst was reduced at 500 °C for 2 hrs followed by evacuation for 2 hrs.
Catalysis procedure
Catalytic tests were performed at atmospheric pressure in a glass down flow fixed bed
reactor (14 mm inner diameter, 200 mm long ) loaded with 1 g of catalysts mixed with 1 g of
quartz beads. Before the catalytic performance, the catalysts were reduced at 500 °C online
for 4 h in H flow (30 ml/min). The feed solution comprising of required molar ratio of
2
levulinic acid and formic acid, was fed at a flow rate of 1 ml/h through the micro purfusor
feed pump in a stream of N
outlet of the reactor and analyzed by a gas chromatograph equipped with a flameꢀionization
detector. In particular, formic acid decomposition to H and CO was analyzed by a gas
2
flow (30 ml/min). The product samples were collected at an
2
2
chromatograph equipped with a thermal conductivity detector. Furthermore, the conversion
of formic acid is not considered because formic acid used as a hydrogen source.
Results and discussion
Temperature programmed reduction studies
Metalꢀsupport interactions and reducing behavior of supported metal particles were
inferred from TPR results (Figure ꢀ1), the NA and NHT catalysts showed three reduction
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peaks in their TPR profiles. The first reduction peak appeared at lower temperature (i.e. < 400
C) is due to the reduction of bulk NiO [23], second peak found at above 500 °C is attributed
to the reduction of weakly interacted NiO species with the Al support and third reduction
peak noticed at Tmax centered at around 820 °C is due to the reduction of NiAl O spinel
°
2
O
3
2
4
species whose formation is apparent as a result of the strong metalꢀsupport interaction [24] It
.
is known from the literature that the NiMgO
2
solid solution is reduced at the higher
solid
o
temperature at around 800 C [25]. So one cannot ignore the formation of NiMgO
2
solution in NHT catalyst. Hence, the major reduction peak found in the NHT belongs to the
reduction of NiMgO species. As it can be seen from the Figureꢀ1, the contribution of
2
NiMgO is very high compared to NiAl O . The TPR results suggested that the weakly
2
2
4
interacted NiO species are the foremost species in the NA catalyst whereas strongly
interacted species i.e., NiMgO are the major species in the NHT catalyst. On the contrary,
2
two broad reduction peaks were observed in the NM catalyst. The first broad reduction peak
noticed at around 500 °C is ascribed to weakly interacted NiO with MgO support [25, 26].
While the second broad peak found at the higher temperature around 800 °C is attributed to
2
the reduction of NiO–MgO (MgNiO ) solid solution [27]. The formation of NiOꢀMgO solid
solution is conceivable in NM and NHT catalyst due to the presence of defect sites in MgO
and similar sizes of NiO and MgO [28]. Furthermore, the strong reduction peak at the higher
temperature in the case of NHT might be attributed to the substitution of Ni in place of Mg in
hydrotalcite. The higher temperature reduction peak could be ascribed to substituted Ni in
hydrotalcite structure [29].
6

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NA
NM
NHT
200
400
600
800
0
Temperature ( C)
Figure – 1. TPR profiles of all the Niꢀbased catalysts.
Xꢀray diffraction studies
Figureꢀ2 represents the reduced XRD patterns of NA, NM and NHT catalysts. The
NA catalyst showed the 2θ values at 44.49°, 51.85° and 76.38° which corresponds to the
metallic Ni with face centered cubic geometry of the space group Fm3m (225) with cell
parameter 3.523 A° (ICDD.No.87ꢀ0712). In addition to these peaks, the catalysts, NM and
NHT are also showed some 2θ values at 37.04, 62.49, and 78.88º which belong to NiMgO
2
(ICDD No.24ꢀ0712) or MgO. It is renowned that the XRD patterns of the (Mg, Ni)O (i.e.
0
2
NiMgO ) phase cubic system, space group Fm3m and parameter a = 4.1922 A , has similar
structural integrity as that off MgO (cubic system, spatial group Fm3m, parameter a = 4.209
A °) phase [25] so it’s hard to assign those peaks belongs to any one of the single phase i.e.
2
NiMgO or MgO. The peaks corresponding to hydrotalcite structure was not found in both
(calcined and reduced NHT catalysts) catalysts. This could be due to the loss of interlayer
anions during the calcination and reduction processes.
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NA
NM
NHT
10
20
30
40
50
60
70
80
2
Theta (deg)
Figure –2. XRD patterns of all the Niꢀbased catalysts.
Scanning electron microscopy studies
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2 3
Figure – 3. SEM patterns of all the Niꢀbased catalysts (A) Ni/Al O (B) Ni/MgO and (C)
Ni/MgOꢀAl O .
2
3
Figureꢀ 3(A) clearly shows the presence of Ni (small whitish particles) on Al O surface. It
2
3
can be seen from the Figureꢀ3(B) that Ni particles are uniformly distributed on/in MgO
surface by forming a solid solution and hence there is no clear distinction between Ni and
MgO. Similar kind of information is noticed in the case of NHT, Figureꢀ3(C), which might be
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due to the predominant formation of NiMgO solid solution. The results are in line with the
2
TPR results as evidenced from the Figureꢀ 1. Elemental mapping investigations were
performed by EDS analysis from various points of the NA, NM and NHT catalysts revealed
the respective elements.
Figure – 4. TEM, HRTEM image, SAED pattern and particle size distribution of NA
catalyst.
Besides, from the TEM image of NA, one can conclude that the Ni particles are well
dispersed and in a range of 5ꢀ15 nm which is fine harmonized with the H chemisorption
2
studies (Tableꢀ1). It can be inferred from the HRTEM (Figureꢀ4) that the interplanar distance
between the Ni crystals is found to be 0.21 nm which is concomitant with the previous
reports [30]. Besides, the SAED pattern of NA catalyst clearly justifies the crystalline nature
of Ni as evidenced from the XRD studies (Figureꢀ2).
1
0

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100
8
0
0
0
0
6
4
2
Conversion of LA
Yield of GVL
1:1
1:3
1:5
1:7
Molar ratio
Figureꢀ 5. Influence of LA and FA molar ratio on the LA hydrogenation activity over NA
catalyst. Reaction conditions: Weight of the catalyst =1 g, Temperature=250 °C, Pressure=1
ꢀ1
atm, carrier gas (N ) = 1800 ml h .
2
Surprising results were found when we carried out the hydrogenation of levulinic acid using
formic acid as a sole hydrogen source. It can be inferred from the Figureꢀ 5 that the influence
of LA and FA molar ration on the LA hydrogenation over NA catalyst. The results suggest
that the conversion of LA increases as the molar ratio of FA increases. The conversion of
levulinic acid is very low (26%) when the molar ratio of LA: FA is 1:1 and it is found to be
increased to 96% when the molar ratio of LA: FA is 1:3 and further it reached to 99% when
the molar ratio of LA: FA is 1:5. No significant variation in the conversion of LA to GVL is
noticed beyond the molar ratio of 1:5. Hence, this optimum parameter (i.e. LA: FA=1:5) is
used for the further studies such as catalyst optimization and time on stream. From the
Figureꢀ6, it is clear that the NA catalyst showed highest catalytic activity compared to other
two catalysts. Lowest conversion is noticed over NHT due to the substitution of Ni in place
of Mg as inferred from the TPR profiles (Figure ꢀ 1). Furthermore, NM suggesting high
catalytic performance similar to NA might be due to the formation of NiMgO
which results in promoting the enhanced performance. The highest catalytic activity of NA
can be attributed to the largely available weakly interacted NiO species with the Al
support which upon reduction yield the high number of Ni metallic species. The catalytic
2
solid solution
2
O
3
1
1

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activity results suggest that the NA is an efficient catalyst for the hydrogenation of LA.
Additionally, as it can be inferred from the XRD patterns of reduced catalysts (Figureꢀ 2), the
differentiation of Ni in NM and NHT is not possible might be due to the formation of
2
NiMgO solid solution (Figureꢀ 2). Furthermore, the XRD patterns and SEM results are in
good agreement with the TPR patterns (Figureꢀ 3).
Moreover, we performed the reaction over high surface area materials like zeolite and silica
supported Ni catalysts at an optimized reaction conditions. Around 91.5% conversion of
levulinic acid was noticed with 67.1% selectivity to GVL over Ni/HZSMꢀ5 catalyst.
Interestingly, 25.8% of pentanoic acid was observed which could be due to the cleavage of
formed GVL i.e. well know Bronsted acidity of zeolite). In addition, 7% of angelica lactone
intermediate was also noticed. On the other hand, 98.2% levulinic acid conversion and
2
77.1% selectivity to GVL was observed over Ni/SiO catalyst. For the comparison, we also
performed the hydrogenation of levulinic acid with external hydrogen and noticed 98.6%
levulinic acid conversion and 84% selectivity to GVL over NA catalyst.
100
90
80
70
60
50
40
30
20
10
0
Conversion of LA
Yield of GVL
NA
NM
NHT
Catalysts
Figureꢀ 6. Variation in the conversion of LA and yield of GVL against all the Niꢀbased
catalysts. Reaction conditions: Weight of the catalyst=1 g, Temperature=250 °C, Pressure=1
ꢀ1
ꢀ1
2
atm, carrier gas (N ) = 1800 ml h , FA/LA molar ratio = 5, Feed flow = 1 ml h .
1
2

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To further verify the reasons for the differences in the catalytic activity of all the
catalysts, we performed the H pulse chemisorption studies (Tableꢀ1) to know the metal
2
2 3
surface area, particle size and Ni dispersion on various carriers such as MgO, Al O , and HT,
which influence a lot on the catalytic activity. The chemisorption results indicate that the NA
catalyst has a high active metal (Ni) surface area along with lower particle size and high
dispersion than the other catalysts.
Catalyst SA
Nm
AMSA
d
D
2
a
b
2
b
b
b
(
m /g)
(µmoles/g)
165.9
(m /g )
(nm)
15.6
25.4
36.9
(%)
6.5
4.0
2.7
NA
134
15
13.0
8.0
NM
NHT
101.9
18
70.12
5.49
a
Tableꢀ1. Physicoꢀchemical properties of all the nickelꢀbased catalysts. BET surface area
b
determined from N gas adsorption, Calculated by H ꢀpulse chemisorption.
2
2
As evidenced from the H
2
pulse chemisorption and TEM, the greater number of
surface Ni species with lower particle size and high dispersion of Ni stands the NA as a
promising catalyst for hydrogenation of LA among the other catalytic systems. A large
number of Ni surface species with smaller particle size can be conceivable due to the high
2 3
content of weakly interacted NiO species with Al O in the NA catalyst found in its TPR
profile, and all these species are reducible in the catalyst reduction temperature i.e. 500 °C
used in the present investigation.
As aforementioned that the all the three catalysts are efficient for the hydrogenation of
levulinic acid in the absence of an external hydrogen. In addition, to find out the stability of
all the catalysts, time on stream analysis was carried out for a period of 10 h at an optimized
ꢀ1
reaction conditions (FA/LA molar ratio = 5, Feed flow = 1 ml h , Temperature 250 °C, 1
atmosphere). The outcome of the results is illustrated in Figureꢀ 7A. Although, all the
catalysts showed decreased trend in the formation of γꢀ valerolactone, the drastic decline in
the yield of GVL is seen in NM and NHT catalysts. Whereas the gradual decline in the
yield of GVL can be observed during the time on stream analysis over NA catalysts; this
might be due to an accumulation of carbon species on the surface of the catalyst. While, the
presence of the MgO in NM and NHT might be the responsible for the fast catalyst
1
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deactivation during the time on stream analysis over this catalysts, which can be due to the
possible transition of a bruciteꢀpericlase structure of MgO which could occur whenever water
as a one of the byꢀproduct in the reaction [31]. To acquire apparent distinction of the activity
during the time on stream, we further performed the coꢀfeeding of water along with Levulinic
acid, and formic acid over all the catalytic systems. The results (Figureꢀ7B) obtained from the
2
coꢀfeeding of water along with levulinic acid and formic acid (1:1:5 molar ratio of H O:
FA:LA) substantiates a decrease in the catalytic performance. The phenomenon of drastic
decrease of catalytic performance during the time on stream over NM and NHT catalysts
presumably due to poisoning effect of water rather than the coke (as evidenced by the TPR
patterns of spent catalysts). Although the coke formation is common in all the catalytic
systems (as evidenced from the TPR of spent catalysts), the drastic decrease of catalytic
performance certainly due to water poisoning. The selectivity to GVL over NA catalyst was a
bit lower (94%) up to 3 h after that the selectivity remains complete, i.e., 100%. While in the
case of NM and NHT catalysts the selectivity always cent percentage. Though the selectivity
is almost cent percentage but the drastic decrease in conversion was noticed over NM and
NHT catalysts presumably due to the poisoning of water.
(A)
100
Conversion of LA over NA
Conversion of LA over NM
Conversion of LA over NHT
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Time (h)
1
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100
(B)
Conversion of LA over NA
Conversion of LA over NM
Conversion of LA over NHT
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Time (h)
Figureꢀ 7. Influence of timeꢀonꢀstream over various catalysts at 250 °C. Reaction Conditions:
Weight of the catalyst = 1 g, Temperature = 250 °C, Pressure = 1 atm, carrier gas (N ) = 1800
ml h , HCOOH/LA molar ratio = 5, Feed flow = 1 ml h . (A) without coꢀfeeding of water
2
ꢀ1
ꢀ1
(B) with coꢀfeeding of water.
From the timeꢀonꢀstream studies, one can understand that the catalyst deactivation can
be occurred either by the accumulation of carbon species on the catalyst surface or water
liberated [32] during the course of the reaction. It is well known that the accumulation of
carbon species occurs at the catalyst surface through the condensation of reaction
intermediates [2, 25].
In order to find out reasons for catalyst deactivation, we have
performed the temperature programmed reduction (TPR) studies for spent catalysts and
profiles shown in Figureꢀ8. It is well known in the literature [25, 33] that the accumulated
carbon species on the catalyst surface undergo hydrogenation and form methane with the
negative signals in the TPR studies of spent catalysts. In the present work, we found the
negative signals for all the three catalysts. As a result of the hydrogenation of condensed
reaction intermediate such as angelica lactone, on the catalyst surface during the time onꢀ
stream analysis. These results suggest that the catalysts deactivation is majorly due to the
condensation of reaction intermediates on the catalyst surface. But one cannot ignore the
negative effect of water whenever the presence of MgO as a support [25]. the coꢀfeeding of
water along with the levulinic acid and formic acid studies showed the drastic decrease in
catalytic performance of NM and NHT catalysts. Hence, the drastic decrease in activity
during the time on stream over Ni/MgO and Ni/Hydrotalcite catalysts were attributed both
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the water produced during the reaction and the coke formation through the condensation of
reaction intermediates.
From the bibliographic standpoint, the levulinic acid hydrogenation has been operated
in two pathways [2]. In pathꢀ1, LA undergoes dehydration over Bronsted acid sites yield
angelica lactone (α, β) which upon undergo hydrogenation over metallic sites to form γꢀ
valerolactone [2]. In pathꢀ2, hydrogenation of levulinic acid occur over metallic sites
produce 4ꢀhydroxy levulinic acid which upon inevitably undergoes dehydration over
Bronsted acid sites to form γꢀvalerolactone [6]. In the present investigation, we observed the
formation of angelica lactone as an intermediate in our reaction mixture (as evidenced from
GCꢀMS). It suggests that the hydrogenation of LA with formic acid proceeds through pathꢀ1.
It indicates that the formic acid not only act as a hydrogen source it can also acts as an acid
catalyst which is well established in the earlier literature [31].
UNA
UNM
UNHT
100 200 300 400 500 600 700 800 900 1000
0
Temparature ( C)
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Figureꢀ 8. TPR patterns of spent catalysts.
Conclusions
In summary, we have developed a catalytic system for the efficient conversion of biomassꢀ
derived levulinic acid to γꢀ valerolactone in the absence of external hydrogen source. The
2 3
generation of largely available weakly interacted NiO species with Al O support results the
higher number of Ni metallic species with smaller sizes having high dispersion makes the
NA as a promising catalytic system for the hydrogenation of levulinic acid with formic acid
as hydrogen source than the other catalytic systems. NA catalyst deactivates slowly compared
to NM and NHT. Whereas, the presence of MgO in NM and NHT may undergo brucite –
periclase transition with the water formed during the hydrogenation of levulinic acid is the
major reason for drastic deactivation of these catalytic systems along with coke formation
clearly noticed in the study of coꢀfeeding effect of water. Therefore, one can understand that
the catalytic systems having MgO as one of the components is not suitable for the reactions in
which one of the products is water.
Acknowledgments
One of the authors, VM is grateful to the Council of Scientific and Industrial Research, New
Delhi, India for the facilities and financial support.
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Graphical abstract
Efficient route for the transformation of levulinic acid without external hydrogen
source in continuous process over supported Ni catalysts.
2
0