Selective hydrogenation of levulinic acid to γ-valerolactone over a Ru/Mg-LaO catalyst

Article abstract of DOI:10.1039/c4ra16557h

Ruthenium supported on magnesium-lanthanum mixed oxide (Ru/Mg-LaO) obtained from Mg-La hydrotalcite (a base support) catalyzes the hydrogenation of biomass derived levulinic acid (LA) to γ-valerolactone (GVL). The conversion of LA to GVL in toluene was found to be 92% with the selectivity >99% at 80°C and 0.5 MPa hydrogen pressure in 4 h. It is the first example on the basic support under mild reaction conditions. The Ru nanoparticles on the Mg-LaO support exhibited excellent GVL yields for 5 cycles, making it a sustainable process.

Full text of DOI:10.1039/c4ra16557h

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Selective hydrogenation of levulinic acid to g-
valerolactone over a Ru/Mg–LaO catalyst†
Cite this: RSC Adv., 2015, 5, 9044
*
V. Swarna Jaya, M. Sudhakar, S. Naveen Kumar and A. Venugopal
Received 17th December 2014
Accepted 23rd December 2014
DOI: 10.1039/c4ra16557h
www.rsc.org/advances
Ruthenium supported on magnesium–lanthanum mixed oxide (Ru/ complex afforded high yields of GVL by hydrogenation of LA at
Mg–LaO) obtained from Mg–La hydrotalcite (a base support) catalyzes 100 ꢀC and 10 MPa of hydrogen pressure in ethanol, but
the hydrogenation of biomass derived levulinic acid (LA) to g-valer- external bases were used.10
olactone (GVL). The conversion of LA to GVL in toluene was found to
However, it should be noted that the use of heterogeneous
be 92% with the selectivity >99% at 80 ꢀC and 0.5 MPa hydrogen catalysts is much more desirable than those of using
pressure in 4 h. It is the first example on the basic support under mild complexes under homogeneous conditions; they offer easy
reaction conditions. The Ru nanoparticles on the Mg–LaO support separation and multiple reaction cycles without signicant
exhibited excellent GVL yields for 5 cycles, making it a sustainable drop in both yield and catalytic activity of the reaction. Many
process.
heterogeneous catalysts were applied for this hydrogenation
reaction in batch reactor, vapor phase and in supercritical
conditions. Christian et al. reported 94% of GVL from LA at
220 ꢀC and 4.82 MPa H2 using RANEY®Ni as catalyst. They also
examined the copper–chromium oxide as a catalyst and
obtained 11% of GVL at 245 ꢀC and 20 MPa.11 Fu and co-
workers used RANEY®Ni in the catalytic transfer hydrogena-
tion of ethyl levulinate to yield 99% GVL, where isopropanol is
used as hydrogen source at room temperature in Ar atmo-
sphere for 9 h.12 Broadbent et al. showed 71% of GVL yield on
Fossil fuel is a main source for the production of energy and
carbon based chemical products. A rapid depletion of fossil fuel
resources has driven researchers to develop sustainable alter-
natives to renewable resources.1,2 Of the many, natural
resources for energy production, biomass is the only renewable
resource which could produce carbon based intermediates or
building blocks of valuable chemicals and synthetic fuels.3
Levulinic acid (LA) a keto-carboxylic acid, is one of the biomass-
derived platform chemical that is obtained by acid hydrolysis of
carbohydrates. The active functional groups of LA (keto and
carboxylic acid) help in producing several derivatives which can
be used in the synthesis of materials that are bio-degradable
with good strength, and can be solvents and fuel additives.4
Some of the important chemicals such as lactones can be
synthesized by hydrogenation of LA. GVL is a water soluble,
saturated lactone which is used as food and fuel additives,5 as a
solvent,6 plastics7 and ne chemicals.8 Therefore, synthesis of
GVL has received more attention in recent times. Several
homogeneous systems such as complexes of RuCl3 or Ru(acac)3
with BINAP, PPh3, sulfonated phosphine ligands or combina-
tion of PPh3 and TPPTs were explored in the synthesis of GVL in
excellent yields (>99%).9 The homogeneous iridium pincer
rhenium heptoxide at 100 C and 15 MPa pressure.13 Cao and
ꢀ
his group developed gold supported on zirconia catalyst for
the hydrogenation of LA using formic acid as the hydrogen
ꢀ
source and achieved 99% of GVL at 150 C in nitrogen atmo-
sphere.14 Z.-P. Yan et al. evaluated 5% of Ru/C catalyst that
gave 99% selectivity for GVL with 92% conversion of LA at 130
ꢀC and 1.16 MPa hydrogen pressure.15 It is noteworthy to
report 1 wt% Ru/TiO2 as highly active catalyst for hydrogena-
tion of LA under neat reaction conditions at 40 bar (4 MPa)
hydrogen pressure and 200 C in 4 h.16 Many Ru based cata-
ꢀ
lysts were reported in the presence of water soluble phosphine
ligands,17 co-catalysts18 and ionic liquid.19 Using Pd nano-
particles on g-Al2O3, Yan and his group reported 63%
conversion of LA with 96% selectivity towards GVL in water at
160 ꢀC, 4.5 MPa hydrogen pressure in 6 h.20 Recently, Rode and
Hengne reported Cu/ZrO2 for the conversion of LA to GVL in
methanol at 200 ꢀC and 3.4 MPa of hydrogen pressure with
90% GVL selectivity.21 Bimetallic catalysts such as Ru–Re/C,
Ru–Sn, Cu–Cr were tested for the hydrogenation of LA at
high temperatures. However, development of catalyst system
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad-500 007, India. E-mail: swarnajv@gmail.com; Fax: +91-40-
27160921; Tel: +91-40-27193510
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra16557h
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which is lucrative under mild conditions is still a challenge as
most of these reports adopted severe and harsh experimental
conditions in order to obtain higher GVL yields. In recent
years, Mg–La mixed oxide derived from hydrotalcite has
received much attention as support and base catalyst in many
organic transformations.22,23
In the present study, we report an efficient Ru(0) nano-
particles supported on Mg–LaO mixed oxide as a recyclable
heterogeneous catalyst for the hydrogenation of LA under mild
experimental conditions (80 ꢀC and 0.5 MPa H2) with consistent
activity and selectivity towards the desired product. In addition,
the method is base, additive or phosphine free.
Fig. 2 TEM images of (a) reduced Ru/Mg–LaO (50 nm), (b) particle size
analysis and (c) used Ru/Mg–LaO (200 nm).
The TPR prole of the calcined Ru impregnated Mg–LaO
catalyst (Fig. 3) shows two reduction peaks at about 224 ꢀC and
376 ꢀC. The low temperature peak is related to Ru cations that
The powder XRD patterns of Mg–La catalysts were depicted
in Fig. 1. As prepared (oven dried) Mg–La-HT (Fig. 1a) shows
well resolved XRD reections due to lanthanum carbonate
hydroxide hydrate (JCPDS-46-0368, 2q ¼ 30.19, 24.18, 20.65,
38.30, 16.02, 33.82) and magnesium hydroxide (JCPDS-75-0447)
phases, indicating a biphasic system as reported in the litera-
ture.22 Upon calcination at 650 ꢀC, the HT is transformed to
form mixed oxides along with lanthanum hydroxide is pre-
sented in Fig. 1b, and the reections are related to lanthanum
oxide (JCPDS-74-1144, hexagonal symmetry),24 lanthanum
hydroxide (JCPDS-36-1481, hexagonal)24 and magnesium oxide
(JCPDS-75-0447, cubic with face-centered)25 phases. Fig. 1c
depicts the XRD pattern of the reduced Ru/Mg–LaO sample. The
pattern shows the presence of Ru(0) in two phases, the peaks
that are indexed as (101), (100) and (102) planes (marked red)
corresponds to hexagonal (JCPDS-06-0663)26 and the diffraction
lines that are indexed as (111) and (200) to face centered cubic
(FCC) phase (JCPDS-88-2333).26 The other peaks represent the
metal phases as in Fig. 1b and lanthanum oxide carbonate. The
TEM image of reduced Ru/Mg–LaO (Fig. 2a and b) displays the
Ru nanoparticles an average size of <10 nm is dispersed on the
support. The SEM-EDX studies of reduced (Ru/Mg–LaO) catalyst
revealed the presence of Ru, Mg, La, Cl and O and are found to
be 23.53% atomic oxygen, 4.92% atomic ruthenium, 12.82%
atomic magnesium, 57.45% of atomic lanthanum and 1.27%
atomic chlorine.
ꢀ
are well dispersed on the surface; the peak at about 376 C is
assigned to the reduction of RuO2 and Ru-oxychloride.27 The
XPS analysis of reduced catalyst shows (Fig. 4a) the charac-
teristic binding energy (BE) for Ru 3d5/2 at 280.2 eV, which is in
the overlapped portion with the C 1s (carbon) 285 eV. The Ru
3p3/2 signal at 464.15 eV and Ru 3p1/2 signal at 485.61 eV
clearly indicate the presence of surface Ru(0) phase
(Fig. 4b).28–30
The Ru/MgLaO allowed the hydrogenation of LA in a batch
reactor (Scheme 1), the hydrogen pressure was maintained
manually. The samples were collected periodically and analysed
by GC and the product components were identied by GC-MS-
QP-5050 (M/s. Shimadzu Instruments, Japan).
From the activity data (Table 1) it is understood that support
seems to play an important role in the hydrogenation of LA to
GVL. All the catalysts were prepared by a similar procedure as is
adopted for Ru/Mg–LaO. The catalysts Ru/MgO and Ru/Mg–AlO
(Table 1, entries 3 & 4) show low LA conversions than Ru/Mg–
LaO although with similar ruthenium content, metal disper-
sion, metal surface area (obtained from CO chemisorption) and
particle size (see ESI†) is being used. Interestingly, when
Mg–LaO (Table 1, entry 2) support alone was used, about 4% LA
conversion with 99% selectivity for GVL is obtained. In
controlled experiment (Table 1, entry 1) the conversion
was found to be 3% in 16 h but the selectivity for GVL
remained >99%.
In order to optimize the reaction conditions (i.e. solvent,
pressure, temperature and weight of the catalyst), a series of
experiments were carried out using levulinic acid (2.0 g, 17.24
mmol), solvent (18.0 ml) and Ru/Mg–LaO catalyst. Initially,
catalytic runs were conducted at 130 ꢀC and 1.2 MPa of
Fig. 1 XRD pattern of (a) Mg–La-HT, (b) Mg–LaO (calcined at 650 ꢀC)
and (c) Ru/Mg–LaO (reduced at 450 ꢀC).
Fig. 3 TPR profile of Ru(II)/Mg–LaO calcined.
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9) was used as base, the LA conversion was found to be
moderate (68%) but the selectivity for GVL was 99%, and with
KOH (2 mmol) (Table 2, entry 8) the conversion was 93% and
GVL selectivity was 99%. But the solid catalyst could not be
recovered quantitatively (20 mg). For the complete recovery of
the catalyst, stoichiometric amount of KOH was used it shows a
decrease in selectivity towards GVL, indicating that formed
product is reacting with KOH.5,32
Fig. 4 XPS spectra of Ru/Mg–LaO (a) Ru 5d and (b) Ru 3p.
The results on solvent screening have shown toluene to be
the best solvent, this could be due to its non-polar nature, it will
be unable to solvate the reaction intermediates, rather the
intermediates are stabilized by the surface of the catalyst and
enables them to come in close proximity to each other and
facilitates reaction of the unsolvated species.33 Further
screening at different temperatures, pressures and catalyst
weight were performed in toluene solvent. It seems temperature
has sturdy affect on the reaction, by reducing the temperature
Scheme 1 Hydrogenation of levulinic acid.
ꢀ
from 80 to 50 C, there is a decrease in the conversion of LA
hydrogen pressure in different solvents (Table 2). Using polar
solvents such as methanol and ethanol, good conversions were
obtained but the selectivity towards the GVL is decreased due to
the formation of LA-esters (Table 2, entries 1, 5 & 6). The
formation of levulinate esters by transesterication as byprod-
from 92 to 48% (Table 2, entries 10–13). Similarly, when the
reaction was conducted at different pressures i.e. 0.5, 0.3 and
0.1 MPa at 80 ꢀC, the conversion of LA is lowered from 92, 65 to
21% respectively (Table 2, entries 10, 14 and 15). Upon
ucts ruled out reforming of ethanol. In case of water (Table 2, decreasing the catalyst loading from 5 to 3.65 wt%, the
entry 2) the conversion (99%) and selectivity (>99%) towards conversion of LA is dramatically fallen to 35% (Table 2, entry
desired product was excellent but the catalyst could not be 10e) which is in consistent with literature.15 From the above
results, the derived optimum parameters for 5% Ru/Mg–LaO
are 80 ꢀC, 0.5 MPa of H2 pressure, 5 wt% of 5 wt% Ru/Mg–LaO
catalyst (with respect to LA) in toluene. At these conditions the
recovered due to lower pH. When dioxane (a polar aprotic
solvent) was used, the reaction was very sluggish; with 50%
conversion and 45% GVL selectivity even aer prolonged reac-
tion times (overnight) (Table 2, entry 3). Interestingly, toluene (a
non-polar aromatic solvent) was proved to be the most prom-
ising one that displayed excellent selectivity for GVL (>99%)
maximum normalized reaction rate achieved for the Ru/MgLaO
ꢁ1
was 13.345 mol hꢁ1 (molRu
)
(see ESI†).
The recyclability results on Ru/Mg–LaO are reported in Fig. 5.
(Table 2, entry 4). However at this temperature catalyst recovery For the reusability studies, aer completion of reaction the
became difficult due to formation of insoluble organic matter mixture was centrifuged, the solid was washed with ethyl
acetate to remove organic substances from the surface, later
oven dried. To this recovered catalyst, fresh aliquots of reactants
were added and reused for 4 cycles with consistent activity and
selectivity. A small decrease in activity was observed in 5th cycle.
The TEM image (Fig. 2c) of used catalyst shows some carbon
called humins on the surface of the catalyst. In order to prevent
formation of humins Dumesic et al.31 used biphasic system with
GVL as one of the solvent. In our case for the complete recovery
of the catalyst and for the prevention of formation of humins,
we have tried to decrease the reaction temperature, pressures
and by neutralization of the reaction medium with bases such deposition. The XPS of the used catalyst shows two broad peaks
as KOH and NEt3.
at 465.23 eV and 485.64 eV, which are characteristic of Ru 3p
On the contrary at 80 ꢀC good conversion with excellent (Fig. 6).28–30 The CHNS elemental analysis of used catalyst shows
the presence of 1.15% carbon on the catalyst and it could be one
of the reasons for lower activity.
selectivity was obtained in different solvents (Table 2, entries 6–
10). The reaction in water (Table 2, entry 7) gave good results
hence; we tried to neutralize the water medium using bases, in
order to recover the catalyst. When NEt3 (2 mmol) (Table 2, entry
Table 1 Hydrogenation of levulinic acid catalyzed by different catalystsa
ꢁ1
Entry
Catalyst
Temp. (ꢀC)
Time (h)
Conv.b (%)
Selec.b (%)
Ratec mol hꢁ1 (molRu
)
1
2
3
4
5
—
MgLaO
Ru(0)/MgO
Ru(0)/MgAlO
Ru(0)/MgLaO
80
80
80
80
80
16
4
4
4
4
3
4
62
68
92
>99
>99
>99
>99
>99
—
—
9.00
9.86
13.345
a
Reaction conditions: levulinic acid 2.0 g (17.24 mmol), 5% of 5 wt% catalyst (0.100 g), toluene + levulinic acid (20.0 ml), hydrogen pressure 0.5 MPa
H2. b The conversion and selectivity were determined by GC. Rate is calculated as per normalized rate equation (see ESI section).
c
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Table 2 Hydrogenation of levulinic acid catalyzed by Ru/Mg–LaO in different solvents at different temperatures and pressuresa
Selec.b (%)
Entry
Solvent
Temp. (ꢀC)
Pres. (MPa)
Conv.b (%)
GVL
LA ester
28
Others AL
55
1
2
3
4
5
6
7
8
Methanol
H2O
130
130
130
130
130
80
80
80
80
80
1.2
1.2
1.2
1.2
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.3
0.1
92
99
50
98
84
92
92
93
72
>99
45
>99
76
86
>99
98
Dioxanec
Toluene
EtOH
24
14
EtOH + H2O (1 : 1)
H2O
KOH + water
Et3N + water
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
9
68
99
10
11
12
13
14
15
92, 90d, 35e
>99, 98, >99
99
99
99
>99
>99
70
60
50
80
90
81
48
65
21
80
a
b
Reaction conditions: levulinic acid 2.0 g (17.24 mmol), 5% of 5 wt% Ru/Mg–LaO (0.100 g), solvent + levulinic acid 20.0 ml, time 4 h. The
conversion and selectivity were determined by GC. Overnight (16 h). 5th cycle. e 3.65% of catalyst.
c
d
a TECNAI 12 FEI TEM instrument. The samples were suspended
in ethanol, treated with ultrasound, and was applied to a carbon
carrier foil (LaB6, KO-AP3, D ¼ 50 mm, single tilt holder). XPS
measurements were conducted with Kratos XPS Axis 165 spec-
trometer, equipped with a hemispherical energy analyzer. The
non-monochromatized Mg Ka X-ray source (hn ¼ 1253.6 eV) was
operated at 5 kV and 15 mA, with pass energy of 80 eV and a step
of 0.1 eV. The samples were degassed for several hours in the
XPS chamber to minimize air contamination to sample surface.
In order to overcome the charging problem, charge neutralizer
of 2 eV was applied and the binding energy of C 1s core level (BE
¼ 284.6 eV) of adventitious hydrocarbon is taken as standard.
The TPR prole was recorded on Micromeritics (Auto Chem
2910) using 0.05 g of catalyst sample. In a typical method the
catalyst was loaded in an isothermal zone of a quartz reactor (i.d
¼ 6 mm, length ¼ 300 mm) heated by an electric furnace at a
rate of 10 ꢀC minꢁ1 to 573 ꢀC minꢁ1 in owing helium gas at a
rate of 30 ml minꢁ1, which facilitates desorption of the physi-
cally adsorbed water. Aer degassing, the sample was cooled to
room temperature and the helium gas was switched to 36 ml
minꢁ1 reducing gas of 5% H2 in argon and the temperature was
increased to 725 ꢀC at a ramping rate of 10 ꢀC minꢁ1. Hydrogen
consumption is measured by analyzing effluent gas by means of
thermal conductivity detector. The consumption of hydrogen
was calibrated measuring the TPR of Ag2O (20 mg), with the
same protocol. GC analysis was done by GC-2010, gas chro-
matograph Shimadzu, with ZB5 capillary column, internal
diameter 0.53 mm, lm thickness 1.50 mm, length 30 m, initial
temperature 80 ꢀC (hold 10 min), ramp 7 ꢀC minꢁ1 – 279 ꢀC
(hold 7 min), injection temperature 250 ꢀC and detector
temperature 280 ꢀC (FID). CO chemisorption measurements
were carried out on AutoChem 2910 (Micromeritics, USA)
instrument. Prior to adsorption measurements, ca. 100 mg of
the sample was reduced in a ow of hydrogen (50 ml minꢁ1) at
Fig. 5 Recyclability studies on Ru/Mg–LaO for hydrogenation of LA at
80 ꢀC, 0.5 MPa H2 pressure.
Fig. 6 XPS spectra of Ru 3p3/2 of used Ru/Mg–LaO.
Experimental section
Materials & methods
Levulinic acid (LA, 98%), g-valerolactone (GVL, 98%), ruthe-
nium trichloride anhydride were purchased from Aldrich.
Lanthanum nitrate hexahydrate, aluminum nitrate non-
ahydrate, magnesium nitrate hexahydrate, sodium hydroxide,
sodium carbonate, magnesium oxide and solvents were
purchased from SD Fine chemicals Ltd., India. All of them were
used without any further purication. The powder X-ray powder
diffraction data were collected on a Siemens/(D5000) diffrac-
˚
tometer using Cu Ka radiation (l ¼ 1.5406 A) with a scan speed
ꢀ
of 2q ¼ 0.045/0.5 s in the range of 2–80ꢀ. TEM was performed on
250 C for 3 h and ushed out subsequently in a pure helium
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ꢀ
gas ow for an hour at 250 C. The sample was subsequently
cooled to ambient temperature in the same He stream. CO
uptake was determined by injecting pulses of 9.96% CO
balanced helium from a calibrated on-line sampling valve into
the helium stream passing over the reduced samples at 250 ꢀC.
Ruthenium surface area, percentage dispersion and Ru average
particle size were calculated assuming the stoichiometric factor
(CO/Ru) as 1. Adsorption was deemed to be complete aer three
successive runs showed similar peak areas.
Acknowledgements
V. Swarna Jaya thanks Department of Science and Technology
(DST), New Delhi, India, for the award of Women Scientist
Scheme. M. Sudhakar thanks Council of Scientic and Indus-
trial Research (CSIR), New Delhi, India, for the award of Senior
Research Fellowship (SRF). All the authors thank Dr M.
Lakshmi Kantam, Director, IICT for her constant help and
encouragement. VSJ thanks Dr B. Sreedhar for recording TEM
images and constant support.
Catalyst preparation
References
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Hydrogenation of LA
Hydrogenation of levulinic acid was carried out in a 100 ml
mechanically stirred Parr autoclave equipped with a PID
controller 4848. In a typical experiment about 2.0 g (17.3 mmol)
of LA and 18.0 g of solvent were added to 0.10 g of 5 wt% Ru/Mg–
LaO and stirred at 1000 rpm by applying 80 ꢀC and 0.5 MPa H2
pressure. The products were periodically analyzed during the
course of the reaction by using gas chromatography (GC)
equipped with ZB5 capillary column and ame ionization
detector (FID) and GC-MS.
Conclusions
In conclusion, Ru-heterogenized on Mg–LaO, a hydrotalcite
derivative has been synthesized, characterized by an array of
sophisticated analytical techniques. The catalytic activity has 17 C. Delhomme, L. A. Schaper, M. Z. Prebe, G. R. Sieber,
been tested for the hydrogenation of LA to GVL. It is D. W. Botz and F. E. Kuhn, J. Org. Chem., 2013, 724, 297–299.
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