Greener, Recyclable, and Reusable Ruthenium(III) Chloride/Polyethylene Glycol/Water System for the Selective Hydrogenation of Biomass-Derived Levulinic Acid to γ-Valerolactone

Article abstract of DOI:10.1002/cctc.201600872

A versatile and efficient catalyst for the hydrogenation of biomass-derived levulinic acid to γ-valerolactone (GVL) by using a RuCl₃/polyethylene glycol (PEG-400)/H₂O catalyst system was developed. The catalysis system was also found to be active for the hydrogenation of levulinate ester. The present protocol proceeded with 99 % conversion and 99 % selectivity towards GVL without the formation of any side products. Remarkably, the reaction did not require sensitive phosphine ligands, a base, additives, or a co-catalyst to promote the reaction. This method involved the use of environmentally benign PEG-400 and water as a solvent, which is nontoxic, inexpensive, commercially available, nonflammable, and easy to handle. The RuCl₃/PEG-400/H₂O catalyst system could be easily recycled and reused six times with high activity and selectivity.

Full text of DOI:10.1002/cctc.201600872

Accepted Article
Title: Greener, Recyclable and Reusable RuCl3/PEG-400/H2O System
for the Selective Hydrogenation of Biomass Derived Levulinic acid
to γ-valerolactone
Authors: Nilesh M Patil and Bhalchandra Mahadeo Bhanage
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To be cited as: ChemCatChem 10.1002/cctc.201600872
Link to VoR: http://dx.doi.org/10.1002/cctc.201600872
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Greener, Recyclable and Reusable RuCl3/PEG-400/H2O System for
the Selective Hydrogenation of Biomass Derived Levulinic acid to
γ-valerolactone
Nilesh M. Patil[a] and Bhalchandra M. Bhanage*[a]
Abstract: This work reports a versatile and an efficient catalyst for
the hydrogenation of biomass derived levulinic acid to γ-
valerolactone (GVL) using RuCl3/PEG-400/H2O catalyst system. The
developed catalytic system is also found to be active for levulinate
ester hydrogenation. The present protocol furnished 99% conversion
with 99% selectivity towards γ-valerolactone (GVL) without formation
any side products. Remarkably, reaction does not require sensitive
phosphine ligands, base and additives or co-catalyst to promote the
reaction. This methodology uses environmentally benign poly-
mixture with GVL better the combustion because of its lower
vapor pressure. Even though GVL has not been used as a clean
fuel, still GVL and ethanol have similar combustion energy (35
MJ L−1) and a higher energy density. Hence, in future GVL can
be use as an alternative liquid fuel. [13]
Catalytic hydrogenation of levulinic acid (LA) to form 4-
hydroxyvaleric acid (4-HVA) then followed by dehydration of 4-
HVA produces γ-valerolactone (GVL) as shown in Scheme 1. To
date, numerous heterogeneous and homogenous metal catalytic
(ethylene glycol) (PEG-400) and water as a solvent which is nontoxic, systems have been developed for hydrogenation of LA to GVL
inexpensive, commercially available, nonflammable and easy to
handle. RuCl3/PEG-400/H2O catalyst system could be easily recycle
and reused six times with high activity and selectivity.
using molecular hydrogen.[14] Among these catalysts, ruthenium
catalysts are found to be the best catalyst providing excellent
conversion and selectivity towards the GVL.[15] Yan et al. studied
5% of Ru/C catalyzed hydrogenation of LA to GVL and this
system show that excellent conversion (92%) and selectivity
(99%) towards GVL at 130 oC and 1.16 MPa hydrogen
pressures.[16] Of note, highly active 1 wt% Ru/TiO2 catalyst was
studied for reduction of LA at 200 oC and 40 bar under neat
reaction conditions for 4h.[17] Further, hydrogenation of LA has
also been carried out in presence of bimetallic catalysts such as
Ru–Sn and Ru–Re/C at high temperatures.[18]
Introduction
Due to the increasing demand of energy and
environmental problem the utilization of biomass into value
added chemical or fuel has gained considerable attention.[1-2]
Acid hydrolysis of an inexpensive and abundantly available
biomass can be converted into carbohydrate such as glucose,
fructose, and sucrose. Furthermore acidic dehydration of these
carbohydrates gives 5-hydroxymethylfurfural (HMF) and it
further converted Levulinic acid (LA) via hydrolysis.[3-4] LA is an
important class of compound having a ketone carbonyl group
and an acidic carboxyl group and is wildly used for production of
fuel additives and also used for synthesis of polymer, resin,
organic chemical, flavor substances with numerous applications
in many industries. [5]
γ-valerolactone (GVL) is an essential renewable compound
as it is useful for the synthesis of various range of biofuels and
wide range of fine chemical compounds.[6-7] Since, GVL is highly
water soluble, its biodegradation and bioremediation can be
easily achieved. GVL is a key intermediate for synthesis of
MeTHF, 1,4-pentanediols, alkyl-4-alkoxyvalerates, mixtures of
alkanes, butenes, adipic acid 4-hydroxypentane amides via
pentenoic acids.[8-12] GVL is not only applicable for chemical
industries but also used in petroleum industries. Non toxic GVL
has unique physical and chemical properties and also have
been found to be present in fruits, thereby making it useful in
food industries and pharmacy industries. Nowadays, GVL is
extensively used as a fuels additive derived from petroleum
industries in comparison with ethanol. Recently, Horvath et al.
studied comparison of GVL and ethanol, and results reveals that
mixture of 90 vol% gasoline with 10 vol% ethanol or 10 vol%
GVL has similar octane number and also observed that the
Scheme 1. Hydrogenation of LA into GVL.
In the last decades, homogenous recyclable or biphasic
catalytic systems have been of paramount interest to
researchers for LA hydrogenation.[19] In this regard, several
homogenous methods such as RuCl3 or Ru(acac)3 with
phosphine or sulfonated phosphine ligands have been
demonstrated for GVL synthesis in excellent yields.[20] The
aqueous biphasic Ru/TPPTS system for conversion of LA to
GVL has also been explored with 95% yields.[21] Heeres et al.
showed Ru with modified TPPTS catalyst in CH2Cl2/H2O
biphasic system for the hydrogenation of LA.[22] The selective
synthesis of GVL, 1,4-pentanol, and 2-methyl-tetrahydrofuran
(2-MTHF) have been achieved by hydrogenation of LA using
Ru/1,1,1-tris-(diphenylphoshinomethyl)ethane (Ru/TriPhos) as a
[23]
catalyst as well.
The homogeneous system of iridium pincer
complex provides good yield of GVL by reduction of LA at 100
oC and 10 MPa of hydrogen pressure in ethanol solvent in
presence of base.[24] Recently, Beller and coworkers have
shown RuCl2(p-cymene)]2 with various mono and bidentate-
diNHC ligands in dioxane solution for the reduction of LA.[25]
Although these catalytic systems provide high yield of GVL, the
use of phosphine or nitrogen containing ligands, flammable
organic solvents, non-recyclable catalytic system and harsh
reaction conditions limits its applicability. Therefore, the
exploration of greener and recyclable catalytic system for
reduction of LA is highly desirable.
[a]
Department of Chemistry, Institute of Chemical Technology,
Matunga, Mumbai-400019. India. E-mail: bm.bhanage@gmail.com,
bm.bhanage@ictmumbai.edu.in; Tel.: +91 2233612603; Fax: +91
2233611020.
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Nowadays, the use of poly(ethylene glycols) (PEGs), as a
solvent has captured considerable attention due to its non
toxicity, negligible vapor pressure, commercially available,
inexpensive, recoverable and thermally stable. Recently, eco-
friendly PEGs have been used in many organic transformations.
was employed as a solvent 69% conversion was obtained.
(Table 1, entry 5). The use of PEG-400 and water mixture in the
ratio of 7:3 was found to be very effective for LA hydrogenation
as this system gave 99% conversion and excellent selectivity of
99% for GVL. No formation any side products were noted (Table
1, entry 6). The use of PEG 400/H2O solvent system makes this
protocol greener, recyclable and reused of precious metal.
Hence, this PEG 400/H2O solvent system was chosen for further
investigations. Low conversion of LA was noted when the
reaction was carried out under neat condition (Table 1, entry 7).
Increasing the temperature up to 130 oC led to the full
conversion of LA within 1 h (Table 1, entry 8). There was no
formation of any side product when reaction was performed at
high pressure (up to 40 bar) at 130 oC for 12 h (Table 1, entry 9).
At room temperature, conversion was also very low (Table 1,
entry 10).
[26]
In continuation of our ongoing research toward the
development of environmentally benign, greener, recyclable
[27]
catalyzed hydrogenation,
herein, we showed RuCl3/PEG-
400/H2O as a versatile catalytic system for hydrogenation of LA
to GVL using molecular hydrogen without using any additives
(Scheme 2).
Table 1. Hydrogenation of LA to GVL in different solvents at different temperatures
and pressures.[a]
Temp
(oC)
Conv./selec.
(%)b
Scheme 2. RuCl3/PEG-400/H2O catalyzed hydrogenation of LA into GVL.
Entry
Solvent
H2 (bar)
Time (h)
1
2
3
4
5
Toluene
Dioxane
Ethanol
Water
20
20
20
20
20
110
110
110
110
110
4
4
4
4
4
52/99
10/99
99/42
99/99
64/99
Experimental Section
Materials
Levulinic acid (LA, 98%) and ethyl levulinate (EL, 98%)
were purchased from Alfa Aesar. Ruthenium chloride and
methyl levulinate (EL, 98%) were obtained from Aldrich. PEG
400 was procured from Spectrochem (India). All the chemicals
and solvents were purchased from reputed firm and used
without further purification. The products are well-known in the
literature and were confirmed by GCMS (Shimadzu GC-MS QP
2010).
PEG 400
PEG 400/H2O
(7:3)
6
7
8
20
20
20
110
110
130
4
4
1
99/99
51/99
99/99
Neat
Results and Discussion
PEG 400/H2O
(7:3)
Optimization study for selective hydrogenation of LA to
GVL.
PEG 400/H2O
(7:3)
9
40
20
130
RT
12
12
99/99
15/99
The reaction conditions (i.e. solvent, pressure, temperature
and time) were optimized and for which preliminary studies were
conducted using levulinic acid (5 mmol), solvent (10 ml) and
RuCl3 as a catalyst without using any additives, base and ligand
(Table 1). At first, we employed non polar solvents such as
toluene and dioxane and both solvents gave low conversion of
LA (Table 1, entries 1,2 ). In polar solvent like ethanol,
conversion (96%) was good but it gave poor selectivity (42%)
due to the formation of ethyl levulinate (Table 1, entry 3). The
transesterification of LA acid with ethanol forms ethyl levulinate
as a by-product. Generally, water favors the LA hydrogenation,
hence, we employed water as a solvent and it was noticed that
water furnished highest conversion (99%) with 99% selectivity
towards desired product (Table 1, entry 4). When only PEG-400
PEG 400/H2O
(7:3)
10
[a] Reaction conditions: LA (5 mmol), RuCl3 (2.5 mol%), solvent (10 mL), molecular
hydrogen, temperature and time.
[b] Conversion and selectivity based on GC-MS analysis.
In the beginning, the reaction was carried out in the
absence of transition metal catalyst, but the reaction did not
proceed which indicated the necessity of catalyst (Table 2, entry
1). The activity of RuCl3 catalysed hydrogenation of LA in
PEG400/H2O was compared with other transition metal catalysts
such as PdCl2, CoCl2•6H2O and NiCl2•6H2O (Table 2, entries 2-
4). Among them RuCl3 catalyst was found to be the best catalyst
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COMMUNICATION
furnishing highest 99% conversion and excellent selectivity
toward the desired product (Table 2, entry 5).
Table 2. Hydrogenation of LA to GVL with different catalyst.[a]
Entry
1
Catalyst
Catalyst loading
(mol %)
Conversion
(%)b
Selectivity
(%)b
No catalyst
-
00
00
2
3
4
5
PdCl2
CoCl2·6H2O
NiCl2·6H2O
RuCl3
2.5
2.5
2.5
2.5
00
00
00
99
00
00
00
99
[a] Reaction conditions: LA (5 mmol), 10 mL of PEG400/H2O (v/v=7/3) (10
mL), H2 (20 bar), temperature (110 oC) and time (4h).
[b] Conversion and selectivity based on GC-MS analysis.
Effect of temperature and hydrogen pressure on LA
hydrogenation.
To investigate the effects of the reaction temperature and
hydrogen pressure a series of hydrogenation experiments were
conducted over RuCl3 as the catalyst in PEG400/H2O system
and the results are shown in Figure 1. The increase in
Figure 1. a) effect of temperature b) effect of pressure
o
temperature from 50 to 110 C led to the significant increase in
the conversion along with maximum 99% selectivity for this
reaction (Figure 1a). A rapid decrease in the conversion of LA
was found when pressure decrease from 20 to 5 bar. No effect
on selectivity was noted when hydrogen pressure was
decreased (Figure 1b). These results clearly indicate that the
hydrogenation reaction is very sensitive to the temperature and
pressure.
Effect of time and catalyst loading on LA hydrogenation
Next, the effect of reaction duration on conversion was
explored and it was found that the conversion of LA increases
during the course of reaction from 1 to 4h. Thus, minimum 4h
duration is needed to complete the conversion (Figure 2a.) In
transition metal-catalyzed LA hydrogenation reactions, the use
of minimum amount of expensive and precious transition metal
catalyst is an important aspect. Considering this aspect in mind,
we determined the optimum concentration of the catalyst by
varying the catalyst loading ranging from 1 to 2.5 mol%. It was
observed that 2.5 mol% catalysts required to complete
conversion of LA to GVL (Figure 2b).
Hydrogenation of biomass derived levulinate ester to
GVL.
Encouraged by these results, the reduction of Levulinate Ester
into GVL was investigated under the same optimized reaction
conditions. At first, we check our present catalyst system for
Figure 2. a) effect of time b) effect of catalyst loading.
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reaction mixture. The collected Ru-NPs were confirmed using
hydrogenation of methyl levulinate (ML) and 99% conversion of
ML was observed with excellent 99 % selectivity of GVL (Table 3,
entry 1).Next, we employed EL hydrogenation under optimized
condition but only 48% conversion was obtained (Table 3, entry
2). In both the cases, no formation of any side products was
observed during the course of reaction.
Firstly, Fu and co-workers used homogenous iron complex
with phosphine ligands for ethyl levulinate hydrogenation.[28]
Thereafter, various groups attempted for the reduction of ethyl
levulinate (EL) to GVL. Rinaldi and co-workers studied Raney
nickel catalyst for reduction of alkyl levulinates using 2-propanol
as hydrogen source.[29] Nadgeri et al. reported liquid-phase
hydrogenation of methyl levulinate [ML] using ruthenium support
on graphite- and zeolite in water.[30] Guan and co-workers
reported ethyl levulinate hydrogenation using Pd–AC doped with
Nb2O5 as a catalyst.[31] Although, these reports are highly
efficient but there was a need to develop a simple environment
friendly protocol for the reduction of levulinate esters to GVL
under mild reaction conditions.
transmission
electron
microscopy
(TEM)
and
X-ray
photoelectron spectroscopy (XPS) techniques. The TEM image
shows the generation of spherical Ru-NPs having size range
100 nm to 200 nm (Figure 5). The XPS spectrum (Figure 6) of
Ru-NPs showed the peaks at 284.84 eV correspond to Ru 3d3/2
and 280.34 eV correspond to Ru 3d5/2 respectively which similar
to the literature.[33] The XPS spectrum confirms that the Ru is in
zero state.
Table 3. Hydrogenation of biomass derived levulinate ester to GVL.[a]
Entry
Substrate
Catalyst loading
(mol %)
Conversion
(%)b
Selectivity
(%)b
Methyl
1
2
levulinate
2.5
2.5
99
48
99
99
Ethyl
levulinate
[a] Reaction conditions: levulinate ester (3 mmol), RuCl3 (2.5 mol%), 10 mL of
PEG400/H2O (v/v=7/3), H2 (20 bar) temperature (110 oC) and time (4h).
[b] Conversion and selectivity based on GC-MS analysis.
Figure 3. Diagrammatic representation for catalyst and product separation
and recycle study.
Recycle study
In an effort to make the present catalytic system more
economically and ecofriendly, the recycling of precious metal is
definitely important tasks in the catalytic reactions. Hence, we
tested the recyclability and the stability of RuCl3/PEG-400/H2O
for LA hydrogenation under identical conditions as shown in
figure 3. After the completion of reaction, GVL was extracted
using diethyl ether from RuCl3/PEG-400/H2O system before it
was subjected to the next run. The current catalytic system
could be reused effectively for six successive recycles without
loss in its activity and selectively (Figure 4).
In situ nano particles formations
Literature study reveals that several methods have been
explored for the synthesis of metal nanoparticles from the
corresponding metal salts in polyethylene glycol.[32] Recently,
Tay et al. report reduction of LA to GVL in presence of Ru-NPs
which is in situ generated from homogenous Ru-NHC
complexes.
We observed that in situ formation of ruthenium
nanoparticles (Ru-NPs) during the reaction. The nanoparticles
were isolated by centrifugation at 8000 rpm for 20 min from the
Figure 4. Recycle study.
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Notably, the reaction works smoothly under base free, ligand
free and additives free conditions in 99% conversion with
excellent 99% selectivity towards γ-valerolactone (GVL) without
formation of any side products. The present protocol uses a
stable homogenous recyclable catalyst, easily available
molecular hydrogen and eco-friendly PEG 400/water as a
solvent, thus contribute towards development of greener
methodologies in hydrogenation reaction.
Figure 5. TEM images of Ru-NPs.
Acknowledgements
Author Nilesh M. Patil is thanking to the University Grant
Commission (UGC-SAP), India for providing the Senior
Research Fellowship (SRF).
Keywords: biomass • levulinic acid • hydrogenation • greener •
reusable.
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10.1002/cctc.201600872
ChemCatChem
COMMUNICATION
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COMMUNICATION
Highly
efficient
and
reusable
Nilesh M. Patil[a] and Bhalchandra M.
Bhanage*[a]
RuCl3/PEG-400/H2O catalyst system
for selective hydrogenation of biomass
derived levulinic acid and levulinate
ester has been developed. The
reaction works smoothly under base
free, ligand free and additives free
conditions in 99% conversion with
excellent 99% selectivity towards γ-
valerolactone (GVL) without formation
of any side products. The present
protocol reused the catalyst as well as
solvent system.
Page No. – Page No.
Greener, Recyclable and Reusable
RuCl3/PEG-400/H2O System for the
Selective Hydrogenation of Biomass
Derived Levulinic acid to γ-valerolactone
This article is protected by copyright. All rights reserved.