In situ generated Ni(0)@boehmite from NiAl-LDH: An efficient catalyst for selective hydrogenation of biomass derived levulinic acid to Γ-valerolactone

Article abstract of DOI:10.1016/j.catcom.2017.08.020

We report in situ generation of Ni(0)@boehmite from NiAl-layered double hydroxide (LDH) under reaction conditions which showed superior catalytic activity in the hydrogenation of levulinic acid (LA) to γ-valerolactone (Gvl) with quantitative yield in aqueous medium. Formation of Ni(0)@boehmite in course of the reaction was confirmed by PXRD, TEM, TGA and FT-IR. This catalyst shows superior conversion compared to a series of simple Ni based homogeneous as well as heterogeneous catalysts. The Ni(0)@boehmite catalyst is recyclable, and active even at higher concentrations of LA (25 wt%).

Full text of DOI:10.1016/j.catcom.2017.08.020

Accepted Manuscript
In situ generated Ni(0)@boehmite from NiAl-LDH: An efficient
catalyst for selective hydrogenation of biomass derived levulinic
acid to γ-valerolactone
Sreedhar Gundekari, Kannan Srinivasan
PII:
DOI:
Reference:
S1566-7367(17)30351-5
doi: 10.1016/j.catcom.2017.08.020
CATCOM 5169
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Catalysis Communications
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Revised date:
Accepted date:
12 June 2017
8 August 2017
14 August 2017
Please cite this article as: Sreedhar Gundekari, Kannan Srinivasan , In situ generated
Ni(0)@boehmite from NiAl-LDH: An efficient catalyst for selective hydrogenation of
biomass derived levulinic acid to γ-valerolactone. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
Catcom(2017), doi: 10.1016/j.catcom.2017.08.020
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ACCEPTED MANUSCRIPT
In situ generated Ni(0)@boehmite from NiAl-LDH: An efficient catalyst for selective
hydrogenation of biomass derived levulinic acid to γ-valerolactone
Sreedhar Gundekari,a,b and Kannan Srinivasana,b*
aInorganic Materials and Catalysis Division, CSIR-Central Salt and Marine Chemicals
Research Institute, Council of Scientific and Industrial Research (CSIR), GB Marg,
Bhavnagar - 364 002, India
bAcademy of Scientific and Innovative Research, CSIR-Central Salt and Marine Chemicals
Research Institute, GB Marg, Bhavnagar-364 002, India
*Manuscript Correspondence:
Prof. Dr. Kannan Srinivasan
Inorganic Materials and Catalysis Division
CSIR-Central Salt and Marine Chemicals Research Institute
Council of Scientific and Industrial Research (CSIR)
Bhavnagar - 364 002
India
Tel:
+91-278-2567760 Ext. 7030
FAX: +91-278-2567562
E-mail: skannan@csmcri.res.in ; kanhem1@yahoo.com

ACCEPTED MANUSCRIPT
Abstract
We report in situ generation of Ni(0)@boehmite from NiAl-layered double hydroxide
(LDH) under reaction conditions which showed superior catalytic activity in the
hydrogenation of levulinic acid (LA) to γ-valerolactone (Gvl) with quantitative yield in
aqueous medium. Formation of Ni(0)@boehmite in course of the reaction was confirmed by
PXRD, TEM, TGA and FT-IR. This catalyst shows superior conversion compared to a series
of simple Ni based homogeneous as well as heterogeneous catalysts. The Ni(0)@boehmite
catalyst is recyclable, and active even at higher concentrations of LA (25 wt%).
Keywords
Levulinic acid; γ-valerolactone; Hydrogenation; Nickel; Boehmite; Layered double
hydroxides

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1. Introduction
The consumption of energy is increasing with an increase in world population. Fossil
fuels constitute the main source of this energy whose cost is largely governed by the
availability as its reserves are diminishing. Another concern is the increase in the emission of
greenhouse gases (GHG). A viable and sustainable alternative to the use of these fossil fuels
is biomass as its usage would maintain the CO2 level in the atmosphere [1-4]. Lignocellulosic
biomass is one of the major categories of biomass which comprises carbohydrates that are
present in the plant cell wall [5]. Several research groups across the globe have been working
to obtain chemical building blocks from carbohydrate biomass [6]. Levulinic acid (LA), one
of the biomass derived platform chemicals, is easily obtained from C6/C5 sugars by acid
catalysed transformations. LA is a precursor for the production of biofuels, polymers, and
various chemical intermediates [7-12]. The hydrogenation of LA forms three different
products namely γ-valerolactone (Gvl), 1-4-pentanediol, and 2-methyltetrahydrofuran. All the
three products are industrially relevant, particularly Gvl, a sustainable feedstock for energy
and carbon-based chemicals [13,14]. Gvl has diverse applications such as production of
biofuels in the form of valeric esters, aromatic hydrocarbons and liquid alkanes [15-17],
polymer intermediates such as α-methylene-γ-valerolactone, 1,4-pentanediol and adipic acid
[18-21]. Gvl itself acts as a solvent for many organic transformations and as a green reaction
medium in biomass conversion [22,23].
Several research groups are working on the effective production of Gvl by selective
hydrogenation of LA, and for this transformation various homogeneous and heterogeneous
catalysts are reported using diverse hydrogen sources such as H2, formic acid, and alcohols.
The Ru and Ir metal containing phosphine based homogeneous complexes gave good LA
conversion with Gvl selectivity but have a serious drawback of separation of catalysts from
the reaction mixture, besides being energy intensive and expensive. On the other hand,
different heterogeneous catalysts are also explored for this reaction. The precious metal
catalysts like Ru, Pt, Pd and Au on different supports like carbon, TiO2, Al2O3, SiO2, H-
ZSM5, H-β, Nb2O5, polyethersulfone, DOWEX resin, HMS, MCM-41, polyethersulfone, and
hydroxyapatite are used for the reaction. Few reported studies used different co-catalysts, for
example Amberlyst-70, Sn, Re etc., for efficient hydrogenation [24,25]. Although, these
catalysts gave good conversion and selectivity for this reaction, their high cost and sparse
availability preclude their use for bulk scale production of Gvl.

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Recently few research groups have reported non-precious metal catalysts comprising
Ni based catalysts such as Raney Ni, Ni supported on MoOX/C, SiO2, H-ZSM-5, mesoporous
silica, Al2O3, TiO2, Cu based catalysts supported on ZrO2, SiO2, Al2O3, ZnO, ZrO2, Co-
containing catalysts namely Co/Al2O3, reduced Co3O4 and mixed metal derived from
hydrotalcites like Ni4.59Cu1Mg1.58Al1.96Fe0.70, Cu–Al, Cu–Cr, and Cu–Fe in catalysing LA to
Gvl conversion [12, 26-30]. In most of the cases, high energy was needed for the synthesis of
catalysts (reduction of the catalyst, calcination etc.) that are often more intensive than that
employed for the catalytic reaction. Another drawback being the use of organic medium that
was not suitable for bulk scale manufacture of Gvl. To overcome these problems, we
demonstrate here in situ generated Ni(0)@boehmite from NiAl-LDH (layered double
hydroxide or otherwise referred as hydrotalcite-like compounds) as an efficient and
recyclable catalyst for the selective preparation of Gvl from LA in aqueous medium under
milder reaction conditions (SI. Table 1 in the ESI).
2. Experimental
2.1 Materials
Levulinic acid (98%), γ-valerolactone (99%), methyl levulinate (≥98%) were
purchased from Sigma-Aldrich. Metal salts NiCl2, Ni(NO3)2, NiSO4, NiO, Ni(0) and Al(0)
metal powders were purchased from s. d. fine Chemicals Limited, Mumbai, India.
2.2 Catalyst precursor (NiAl-LDH) preparation
Catalyst precursor NiAl-LDH was prepared by co-precipitation method under low
supersaturation. The desired amount of metal nitrate solutions (Ni to Al atomic ratio 3:1)
were mixed with solution of precipitating agents (i.e., NaOH and Na2CO3) slowly and
simultaneously in a Schott auto-titrator assembly (Titroline, Schott, Germany) while
maintaining pH at 9.5 ± 0.1 under stirring at room temperature. The precipitated solution was
aged at 65 °C for 18 h in an oil bath, filtered, washed with deionized water until neutral pH
and dried at 100 °C for 12 h.
2.3 Hydrogenation experiments and product analysis
A detailed description of the hydrogenation procedure and product analysis are described in
SI. 1 in the ESI.

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3. Results and discussion
3.1. Catalyst screening
Initially various commercial and synthesized Ni containing homogeneous and
heterogeneous catalysts were used for the selective hydrogenation of LA to Gvl at 2.5 wt%
LA , 10 wt% of Ni catalyst with respect to LA, 200 °C, 40 bar H2 for 6 h. A blank reaction
was carried out without using any catalyst which gave only 4% yield of Gvl with 5%
conversion of LA (Table 1, entry 1), The commercial homogeneous catalysts like NiCl2,
Ni(SO4)2, Ni(NO3)2 gave low to moderate LA conversion and yields of Gvl (Table 1, entry 2-
4). Commercially available heterogeneous catalysts like NiO and Ni(0) showed almost
similar LA conversion of ~ 35%, and Gvl yields (~ 31%) (Table 1, entry 5-6). In literature, a
hydrotalcite catalyst (Cu-Fe) was reported with 90% selectivity of Gvl with >98% of LA
conversion at 200 °C and 70 bar H2 in 10 h [31]. From this observation, here we have
explored NiAl-LDH as catalyst for the reaction, and observed 100% conversion of LA with
quantitative production of Gvl (Table 1, entry 7). Further the catalytic activity was compared
with other Ni based catalysts prepared such as Ni(OH)2 which showed lesser activity
compared to NiAl-LDH (Table 1, entry 8). Further, comparison with premixed Ni(70)Al(30)
catalyst (as metal powders similar to Ni/Al atomic ratio in NiAl-LDH) which also showed
very low activity of this catalyst (Table 1, entry 9). Thus, it can be concluded that NiAl-LDH
is a superior catalyst for this conversion compared to other Ni-based reported systems. The
optimization of reaction conditions was then carried out using NiAl-LDH catalyst.
3.2. Optimization of reaction conditions
Parameter variation studies were carried out for optimizing reaction conditions using
NiAl-LDH catalyst. Initially, the temperature was decreased from 200 °C to 150 °C which
drastically decreased the conversion of LA from 100% to 20% but almost maintained the
selectivity of Gvl (Table 2, entry 1-2). Then the pressure was decreased from 40 to 30 bar H2
keeping other conditions constant. 100% LA conversion with 100% yield of Gvl was
observed while a further decrease in the pressure from 30 to 20 bar H2 resulted in a decrease
in the conversion of LA from 100 to 91% while maintaining selectivity (Table 2, entry 1 and
3-4). Then for further studies, the pressure was fixed at 30 bar H2 and temperature at 200 °C.
Further, wt% of Ni in the catalyst with respect to LA was varied from 10 to 4 at the above
optimized conditions which showed that 7 wt% of Ni in NiAl-LDH catalyst is sufficient for

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the complete conversion of LA to Gvl (Table 2, entry 1 and 5-8). While maintaining 7 wt%
of Ni in the catalyst, the reaction time was reduced from 6 to 4 h which decreased LA
conversion to 60% (Table 2, entry 8-9). From the above studies, the optimized reaction
conditions were: 200 °C, 30 bar H2 pressure, 7 wt% of Ni and 6 h for complete conversion of
LA (1g) to Gvl (0.86) (Table 2, entry 8).
3.3. In situ formation of Ni(0)@boehmite from NiAl-LDH during the reaction
After the reaction under optimized condition, the recovered catalyst was analysed
using powder X-ray diffraction (PXRD), transmission electron microscopy (TEM),
thermogravimetric analysis (TGA) and Fourier-transformed infra-red (FT-IR) spectroscopy.
The PXRD of synthesized NiAl-LDH showed 003, 006, 012, 015, 018, 110 and 113
reflections that are characteristic of hydrotalcite shown in Fig.1, which completely converted
to 020, 120, 140, and 051 reflections for boehmite, and 111 with 200 for Ni (0) that matched
well with the JCPDF file of boehmite (JCPDS PDF Card No. 01-074-290 and literature [32])
and Ni (JCPDS PDF Card No.: 00-004-0850). TEM images of precursor NiAl-LDH showed
platelet-like morphology, which changed completely after the reaction due to the formation
of boehmite along with Ni(0) particles. The in situ formed boehmite during the reaction
(Table 2, entry 8) showed well-defined hexagonal long rod-like morphology with nearly
spherical embedded Ni (0) particles with the size of less than 20 nm as shown in Fig. 2. The
morphology of this boehmite phase matched well with the reported morphology in literature
[33]. The in situ formation of Ni(0)@boehmite was further confirmed by using FT-IR (Fig.
-1
-1
-1
3); NiAl-LDH showed characteristic bands at 3450 cm , 1640 cm , and 1380 cm ,
correspondingly attributed to OH stretching of hydroxyl groups from the layers and interlayer
water molecules, bending mode of water molecules in the interlayer space and asymmetric
stretching vibrations of carbonate that matched well with reported literature [34]. After the
reaction, FT-IR of the used catalyst matched well with reported boehmite [35]. Two strong
-1
-1
-1
bands observed at 3309 cm and 3095 cm corresponds to Al-OH, and the band at 1639 cm
-1
corresponds to adsorbed water in the crystal of boehmite, while the bands at 1158 cm and
-1
1070 cm are ascribed to the symmetrical bending vibrations of hydrogen bonds of Al-
-1
-1
-1
OH….OH-Al and the bands at 750 cm , 627 cm and 474 cm refers to vibration modes of
Al-O and Al-O-Al. Furthermore, TGA (Fig.4) also supported the formation of boehmite
wherein generally NiAl-LDH exhibits a sharp weight loss between 200-350 °C corresponding
to removal interlayer water molecules along with partial dehydroxylation of the material that

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was not observed for the material after the reaction. Instead, a weight loss was observed
(between 370-550 °C), which may be attributed to the removal of strongly held hydroxyl
groups of boehmite. To surmise, PXRD, TEM, TGA and FT-IR analysis corroborated the
transformation of NiAl-LDH to Ni(0)@boehmite under the reaction conditions. Thus, the
high selectivity observed with in situ generated Ni(0)@boehmite catalyst is due to uniform
dispersion of Ni nanoparticles on boehmite support (as witnessed from TEM) which probably
were not observed for the other Ni-based catalysts (Table 1, entry 2-6 and 8-9).
Mechanistically, partial decomposition of the layered lattice of NiAl-LDH under
higher reaction temperature and hydrogen environment renders
a
mixed metal
oxide/oxyhydroxide of NiO and AlOOH (boehmite) wherein the former undergo
hydrogenation to Ni(0) resulting in Ni(0)@boehmite catalyst [SI.2 in the ESI]. This was also
witnessed by the colour change from green of NiAl-LDH to grey due to the formation of
Ni(0) (black), and boehmite (white). In addition, the absence of peaks at positions at 2θ=37.1,
43.3, and 62.8o in PXRD of the used catalyst suggests the absence of NiO.
3.4. Recyclability of catalyst
Assessment of recyclability of the catalyst is beneficial for industrial applications.
After the reaction, the catalyst was removed from the product mixture by simple
centrifugation, washed with deionised water and further used for 2nd cycle. Interestingly, it
showed good activity similar to first cycle indicating that the in situ generated
Ni(0)@boehmite is the active species for the reaction. The recyclability studies were
conducted for up to four cycles and observed some decrease in the conversion of LA in each
cycle while the selectivity of Gvl was maintained as illustrated in Table 3. The decrease in
the conversion of LA is due to the decrease in the amount of Ni with respect to LA as it
leached into solution in each cycle as elucidated from ICP. The ICP of the product mixture
showed Ni leaching in the range of 0.1 to 0.4 wt% of Ni (with respect to LA) in every cycle.
3.5. Scale-up study
The catalytic activity was successfully demonstrated at 10 g scale of LA and achieved
100% of Gvl yield under similar reaction conditions which was used for 1g (Table 2, entry
10). After the reaction, Gvl was extracted from aqueous product mixture using ethyl acetate
which was later evaporated resulting in a maximum isolated yield of 99% for Gvl (8.5 g). The
evaporated solvent was further used for next cycles and it has good recyclability. The

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1
structural integrity of the recovered Gvl was confirmed by using H-NMR (SI.Fig.1 in the
ESI), 13C-NMR (SI.Fig.2 in the ESI), FT-IR (SI.Fig.3 in the ESI), and whose purity was
~100% as elucidated from UHPLC (SI.Fig.4 in the ESI) and HR-MS (SI.Fig.5 in the ESI)
studies.
4. Conclusion
We report in situ generated Ni(0)@boehmite from NiAl-LDH as an active catalyst for
the selective hydrogenation of LA to Gvl. Nearly stoichiometric conversion of LA with 100%
selectivity of Gvl was observed at 200 °C, 30 bar H2 pressure in aqueous medium in 6 h. The
formation of Ni(0)@boehmite was ascertained by various physicochemical techniques
namely PXRD, FT-IR, TEM and TGA. The active catalyst (Ni(0)@boehmite) was recyclable
up to four cycles with slight decrease in the conversion of LA in each cycle, however,
maintained the Gvl selectivity. Further, the catalyst was successfully demonstrated at 10 g
scale of LA and achieved nearly 100% Gvl yield under similar reaction condition as that of
small scale (1g). The structural and functional characteristics of the isolated Gvl was
ascertained by both spectroscopic and chromatographic techniques whose purity was 100%.
Acknowledgements
CSIR-CSMCRI Communication No. 081/2017. S.G. thanks CSIR, New Delhi, for a
Senior Research Fellowship. The authors thank Analytical Division & Centralized
Instrumental Facilities of this institute for analytical support.
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Table 1 Screening of nickel catalysts for LA hydrogenation to Gvl
S.No Catalyst
LA conv. (%)
Gvl yield (%)
1
2
3
4
5
6
7
8
9
Blank
5
4
NiCl2
23
22
44
35
34
100
75
44
16
17
37
31
32
100
75
38
Ni(SO4)2
Ni(NO3)2
NiO
Ni(0) metal
NiAl-LDH
Ni(OH)2
Ni(70)Al(30)
Reaction conditions: LA 2.5 wt% (0.0086 moles (1g)) in
aqueous medium (40 ml), 10 wt% of Ni with respect to LA,
200 °C, 40 bar H2, 6 h.
Table 2 Reaction conditions optimization studies using NiAl-LDH catalyst precursor
S.No Temp. (°C) Pressure (H2) Catalyst
Time (h)
LA conv.
(%)
100
20
Gvl yield
(%)
100
20
in bar
40
wt(%)
1
200
150
200
200
200
200
200
200
200
200
10
10
10
10
8
6
6
6
6
6
6
6
6
4
6
2
40
3
30
100
91
100
90
4
20
5
30
100
90
99
6
30
6
89
7
30
4
44
43
8
30
7
100
60
100
60
9
30
7
10b
50
7
100
99
Reaction condition: LA 2.5 wt% (0.0086 moles (1g)) in aqueous medium (40 ml), 4-10
wt% of Ni with respect to LA, 150-200 °C, 20-40 bar H2. bLA 25 wt% (0.086 moles (10
g)) in aqueous medium (40 ml).

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Table 3 Recycle studies with Ni(0)@boehmite catalyst
S.No
Catalyst
Cycle-1
Cycle-2
Cycle-3
Cycle-4
LA conv. (%) Gvl yield (%)
1
2
3
4
100
96
100
95
87
84
71
70
Reaction conditions: LA 2.5 wt% (0.0086 moles (1g))
in aqueous medium (40 ml), 7 wt% of Ni (161 mg of
NiAl-LDH) with respect to LA, 200 °C, 30 bar H2, 6 h.
NiAl-LDH
Ni(0)@boehmite
JCPDS-004-0850
Nickel
JCPDS-01-074-2901
Boehmite
10
20
30
40
50
60
70
2 theta degree
Fig. 1 Powder XRD pattern for NiAl-LDH (catalyst precursor) and Ni(0)@boehmite (catalyst
obtained after the reaction - Table 2 Entry 8). The XRD pattern for Ni(0)@boehmite matched
well with nickel (JCPDS card no: 004-0850; stick pattern included) and boehmite (JCPDS
card no: 01-074-290).

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NiAl-LDH
0.1 µm
Ni@boehmite
0.1 µm
Ni(0)@boehmite
20 nm
Insitu generated
NiAl-LDH before reaction
Fig. 2 TEM images showing the morphology and crystallinity of NiAl-LDH catalyst
precursor and Ni(0)@boehmite catalyst at different magnification (catalyst obtained after the
reaction - Table 2 Entry 8).
Ni(0)@boehmite
1158
1639
1384
3095
627
3309
NiAl-LDH
750
474
1070
560
600
500
1640
3450
3000
1380
1000
4000
3500
2500
2000
1500
-1
Wave number (cm )
Fig. 3 The FT-IR spectra of NiAl-LDH catalyst precursor and Ni(0)@boehmite catalyst
(catalyst obtained after the reaction - Table 2 Entry 8)

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Ni(0)@boehmite
NiAl-LDH
100
200
300
400
500
600
700
800
900
Temperature (C)
Fig. 4 TGA of NiAl-LDH catalyst precursor and in situ derived Ni(0)@boehmite catalyst
(catalyst obtained after the reaction - Table 2 Entry 8)

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Graphical abstract
NiAl-LDH
10
20
30
40
50
60
70
2 theta degree
H2
Ni@boehmite
-H2O
γ-valerolactone
Levulinic acid
10
20
30
40
50
60
70
2 theta degree


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Highlights
 NiAl-LDH is reported for the first time for the hydrogenation of levulinic acid to γ-
valerolactone
 Stoichiometric conversion with 100% selectivity is obtained under optimized
conditions
 During hydrogenation NiAl-LDH converts in situ into active catalyst
Ni(0)@boehmite
 Formation of Ni(0)@boehmite is confirmed by PXRD, TEM, FT-IR and TGA
 The catalyst is recyclable, active at high LA concentration (25 wt%) and at larger
scale