Efficient catalytic hydrogenation of levulinic acid: A key step in biomass conversion

Article abstract of DOI:10.1039/c2gc35503e

γ-Valerolactone (GVL) has been proposed as a sustainable liquid, and could be used for the production of hydrocarbons by using both homogeneous and heterogeneous catalytic systems. The selective reduction of levulinic acid (LA) to GVL is a key transformation for biorefinery concepts based on platform molecules. We report a detailed investigation of the conversion of LA to GVL using molecular hydrogen in the presence of a catalyst in situ generated from Ru(acac)₃, and electronically and sterically characterized alkyl-bis(m-sulfonated-phenyl)- and dialkyl-(m-sulfonated-phenyl)phosphine (R_nP(C₆H₄-m-SO₃Na)_3-n (n = 1 or 2; R = Me, Pr, iPr, Bu, Cp) ligands. The hydrogenation experiments were performed in the range of 5-100 bar H₂ at 140 °C using 0.016 mol% catalyst and 5-20 eqv. of ligand. The effects of hydrogen pressure and Ru/ligand ratio on the LA conversion were determined. The nBuP(C ₆H₄-m-SO₃Na)₂ (χ = 12.5, _Tol = 153°) showed the highest activity achieving turnover numbers up to 6200 with a yield and selectivity higher than 99% in a solvent, chlorine and promoter free reaction mixture. The catalyst was successfully recycled for six consecutive runs without loss of activity. The characterization of sulfonated and non-sulfonated phosphines indicated that the sulfonation had no significant effect on the steric and electronic properties of the ligands. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2gc35503e

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Efficient catalytic hydrogenation of levulinic acid: a key step in biomass
conversion
a
a
a
a
a
József M. Tukacs, Dávid Király, Andrea Strádi , Gyula Novodarszki, Zsuzsanna Eke,
a
b
,a
Gábor Dibó, Tamás Kégl , and László T. Mika*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Gammaꢀvalerolactone (GVL) has been proposed as a sustainable liquid, and could be used for the producꢀ
tion of hydrocarbons by using both homogeneous and heterogeneous catalytic systems. The selective
reduction of levulinic acid (LA) to GVL is a key transformation for biorefinery concepts based on platꢀ
form molecules. We report a detailed investigation of the conversion of LA to GVL using molecular hyꢀ
1
1
2
0
5
0
drogen in the presence of a catalyst in situ generated from Ru(acac) , and electronically and sterically
3
characterized alkylꢀbis(mꢀsulfonatedꢀphenyl)ꢀ and dialkylꢀ(mꢀsulfonatedꢀphenyl)phosphine
(
R P(C H ꢀmꢀSO Na) (n = 1 or 2; R = Me, Pr, iPr, Bu, Cp) ligands. The hydrogenation experiments
n 6 4 3 3ꢀn
were performed in the range of 5–100 bar H at 140°C using 0.016 mol% catalyst and 5–20 eqv of ligand.
2
The effects of hydrogen pressure and Ru/ligand ratio on the LA conversion were determined. The
χ
nBuP(C H ꢀmꢀSO Na) ( = 12.5, θTol = 153°) showed the highest activity achieving turnover numbers up
6
4
3
2
to 6200 with a yield and selectivity higher than 99% in a solvent, chlorine and promoter free reaction
mixture. The catalyst was successfully recycled for six consecutive runs without loss of activity. The
characterization of sulfonated and nonꢀsulfonated phosphines indicated that the sulfonation had no signifꢀ
icant effect on the steric and electronic properties of the ligands.
Introduction
The global energy production and the carbon based products of
the chemical industry are almost exclusively produced from fossil
1
carbon resources. Although, it is very difficult to predict the
2,3
Scheme 1
2
3
3
4
4
5
0
5
0
5
exact date of the depletion of crude oil and natural gas, some
4
alternative solutions have been proposed including carbon
waterꢀsoluble homogeneous HRuCl(TPPMS)3 catalyst for the
reduction of oxoꢀ and keto acids was reported by Joó et al. years
before others. However, the catalytic activity was low (TOF =
5
6
dioxide and methanol economy. Since the conversion of carbon
dioxide to carbonꢀbased building blocks and key intermediates is
17
ꢀ
1
slow to emerge, Nature can help by converting carbon dioxide to 50 13 h ) at pH = 1. It was shown that LA could be converted to
biomass, which could be used as sustainable feedstock for chemiꢀ
cal industry. New and innovative approaches have been develꢀ
GVL by the use of Ru/TPPTS catalyst in aqueous solution with
95% yield. The application of TPPTS modified Ru catalyst in
7
18
oped for the green production of fuels, platform molecules and
biphasic hydrogenation of LA in CH Cl /H O was reported by
2
2
2
8
19
valueꢀadded chemicals, which are important components of the
Heeres et al. resulting in excellent yields under mild conditions.
biorefinery.
55 It has been demonstrated that the activity of the ruthenium cataꢀ
lyst can be improved by the use of more electronꢀdonating phosꢀ
Gammaꢀvalerolactone (GVL), one of the possible platform
molecules has been proposed as a sustainable liquid and used as
18,20
phine ligands such as PBu3.
A very active chlorine free
9
10
fuel, solvent, and feedstock for production of alkenes and
Ru/1,1,1ꢀtrisꢀ(diphenylphoshinomethyl)ethane (Ru/TriPhos) cataꢀ
1
1
transportation fuels. GVL can be easily produced from levulinic
lysts have been reported for the reduction of LA resulting in
2
1
acid (LA) by catalytic hydrogenation via 4ꢀhydroxyvaleric acid 60 GVL, 1,4ꢀpentanediol and methylꢀtetrahydrofurans. It is imꢀ
(4ꢀHVA) (Scheme 1.). The reduction can be carried out under
portant to note, that some oxygenates such as 2ꢀMeꢀ
tetrahydrofuran readily form peroxide, which must be a major
heterogeneous conditions resulting in GVL in good to excellent
12
22
yields. By using 5% Ru/SiO or Ru/C catalyst, the complete
safety concern. Although, the beneficial application of alꢀ
2
conversion of LA with higher than 97% GVL selectivity should
kylphosphines in transition metal catalysis could result in higher
13
be mentioned. Ruthenium based homogeneous catalyst systems 65 activity, catalyst stability etc., they are toxic, volatile, could have
1
4
15,16
using either molecular hydrogen or formic acid
as hydrogen
unpleasant smell, and usually oxygen and water sensitive. The
source have also shown high activity. The first application of
incorporation of ionic fragment(s) such as –SO Na into the ligand
3
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structure can minimize their vapor pressure and increase their 35 L10 is limited because the sulfonated species are insoluble in
solubility and stability in polar reaction environment.
We report the electronic and steric characterization of novel
electronꢀdonating sulfonated phosphines and the detailed investiꢀ
gation of their application in the Ru catalyzed reduction of LA
including in situ IR and NMR studies.
CH Cl . Hanson et al. has proposed an alternative method for the
2 2
characterization of waterꢀsoluble phosphine ligands allowing
25
χ
the direct comparison of
values of sulfonated and nonꢀ
5
sulfonated ligand pairs. We have found that L1 ꢀ L10 can be disꢀ
40 solved in THF/H O (5:1). Consequently, the Ni(CO) (PR ) comꢀ
2
3
3
plexes of nonꢀsulfonated phosphines P1ꢀP10 (Table 1.) and L1ꢀ
L10 were prepared by published method in the appropriate mediꢀ
um at room temperature.
Results and discussion
25
The hydrogenation of LA was performed in the presence of
electronꢀdonating alkylꢀbis(mꢀsulfonatedꢀphenyl)ꢀ ((C H ꢀmꢀ
The Ni tricarbonyl monophosphine complexes were identified
by their characteristic carbonyl stretching modes in the IR specꢀ
6
4
4
5
5
6
5
0
5
0
10
SO Na) PR; R = Me, Pr, iPr, nBu, Cp) and dialkylꢀ(mꢀsulfonatedꢀ
31
3
2
tra. The PꢀNMR spectrum of each reaction mixture also indicatꢀ
phenyl)ꢀ ((C H ꢀmꢀSO Na)PR ; R = Me, Pr, iPr, nBu, Cp) phosꢀ
6
4
3
2
ed the presence of the tricarbonyl species. The IR spectral data
1
8
phines (Scheme 2. L1 - L10) at 140 °C, in the range of 5–100
bar hydrogen pressure using 0.016 mol% catalyst and 5–20
χ
and values for all characterized phosphines as well as for TPP
and TPPTS, as a reference pair, are presented in Table 1.
It is evident, that the σꢀdonating and πꢀaccepting characterisꢀ
tics of the phosphorus atom can easily be tuned by the incorporaꢀ
tion of one or two alkyl substituents and maintain the solubility in
ꢀ
protic reaction media by the –SO functional group on the phenyl
3
ring. The dialkyl derivatives (L2, L4, L6, L8, L10) are more
electronꢀdonating than the corresponding monoalkyl species (L1,
L3, L5, L7, L9), as expected. According to Tolman’s observaꢀ
tion, the influence of the different alkyl groups on the electron
density of the phosphorus atom diminishes with increasing length
23
of alkyl chain(s). It appears that the branched (iPr) and cyclic
(Cp) disubstituted L8 and L10 phosphines are significantly more
electronꢀdonating then their monosubstituted L7 and L9 anaꢀ
χ
logues. No significant effects of the sulfonato groups on the
value of these phosphines were detected, similarly to the series of
previously reported P[(CH ) C H ꢀpꢀC H SO Na] (x = 1, 2, 3,
2
x
6
4
6
4
3
3
18
16
14
12
10
8
6
4
2
0
2
Scheme 2
R =0.994
equivalents of ligand. For better understanding of the effect of
ligands on the hydrogenation of LA, the electronic and steric
properties of L1 - L10 were determined.
15
Characterization of the Sulfonated Phosphines
6
7
8
9
10
11
12
13
14
Phosphines (PR ) are primarily electronꢀdonating ligands with
3
χ
non-sulfonated ligands
donation occurring through the lone pair of the phosphorus atom.
Besides the direct electronic interactions, steric factors also play
χ
Fig 1. Correlation of electronic values ( ) of sulfonated and nonꢀ
20
25
30
sufonated phosphines.
23
an important role in the coordination chemistry of phosphines.
Consequently, their characterization is one of the most important
parts of catalyst design.
The electronic parameters of PR3 ligands have been deterꢀ
2
5
6
7
7
5
0
5
and 6) types, as it can be shown by the excellent linear correlaꢀ
χ
tion of values of the sulfonated and the nonꢀsulfonated species
(Fig. 1.).
mined by IR spectroscopy using the ν(CO)A stretching frequenꢀ
1
2
4
cies of the nickel carbonyl complexes Ni(CO) (PR) . Tolman
3
3
The steric bulkiness of the phoshine ligands is typically exꢀ
pressed in the terms of cone angles, usually the Tolman cone
angle (θTol) derived from the space filling modell or the Musco
cone angle (θMus) derived from Xꢀray diffraction data. An emꢀ
pirical linear correlation between the PꢀNMR chemical shift of
the transꢀPdCl (PR ) complexes and θTol has been used to deꢀ
termine the cone angle without Xꢀray structure determination.
In order to determine the bulkiness of the phosphines (L1 ꢀ
χ
has defined an electronic parameter , as
23
χ
PR
3
=
ν
(CO)(A1(PR
3
Ni(CO)
3
)
) ꢀ
ν
(CO)(A1(P(tBu)
3
Ni(CO)
3
)
)
26
31
23
where the A stretching frequencies are measured in CH Cl . By
1
2
2
2
3 2
χ
this relationship, value decreases as the electronꢀdonating charꢀ
2
7,28
acter of the phosphine approaches the properties of P(tBu) . The
3
determination of the ν(CO)A of Ni(CO) (PR ) complexes of L1ꢀ
1
3
3
II
L10) the transꢀPd complexes of L1 ꢀ L10 were prepared by the
2
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Table 1. Electronic and Steric Parameters of the Nonꢀsulfonated and Sulfonated Phosphines
Entry Nonꢀsulfonated
Phosphine
νA1
χ
θTol
Sulfonated phosphines
νA1
χ
νA1 calc. θTol
θTol calc.
ꢀ1
ꢀ1
ꢀ1
a
(cm ) (deg)
ꢀ1
b
(cm )
ꢀ1
(deg)
(deg)
(
cm )
(P1) 2068.2
) (P2) 2066.6
(cm )
(cm )
1
2
3
4
5
6
7
8
9
1
1
MeP(C
Me P(C
PrP(C
Pr P(C
BuP(C
Bu P(C
iPrP(C
iPr P(C
CpP(C
Cp P(C
PPh
6
H
5
)
5
2
12.1
10.5
11.2
8.8
11.0
8.5
11.1
7.6
10.5
7.1
138
112
144
130
147
132
179
193
171
165c
145
MeP(C
Me P(C
PrP(C
Pr P(C
BuP(C
Bu P(C
iPrP(C
iPr P(C
CpP(C
Cp P(C
TPPTS
6
H
4
ꢀmꢀSO
ꢀmꢀSO
ꢀmꢀSO Na)
ꢀmꢀSO Na) (L4)
ꢀmꢀSO Na) (L5)
Na) (L6)
3
Na)
Na) (L2)
(L3)
2
(L1)
2067.9
2065.2
2066.8
2063.4
2066.9
2063.2
2066.4
2061.9
2065.9
13.5
10.8
12.1
8.6
12.5
8.1
11.9
7.2
11.3
6.7
2059.7
2055.8
2055.5
2051.5
2055.2
2051.6
2056.1
2051.5
2055.5
2048.5
2062.7
140
127
151
144
153
133
183
196
172
177
173
173
143
177
137
177
136
178
179
172
174
167
2
6
H
2
6
H
4
3
6
H
5
)
2
(P3)
) (P4)
(P5)
2067.3
2064.9
2067.1
6
H
4
3
2
2
6
H
5
2
6
H
4
3
6
H
5
)
2
6
H
4
3
2
2
6
H
5
) (P6) 2064.6
(P7) 2067.2
) (P8) 2063.7
(P9) 2066.6
2
6
H
4
4
ꢀmꢀSO
3
6
H
5
)
5
2
6
H
ꢀmꢀSO
3
Na)
2
(L7)
2
6
H
2
6
H
4
4
ꢀmꢀSO
3
Na) (L8)
6
H
5
)
2
6
H
ꢀmꢀSO
3
Na)
2
(L9)
0
1
2
6
H
5
) (P10) 2063.2
2
6
H
4
ꢀmꢀSO
3
Na) (L10) 2061.5
3
2069.4
13.3
2070.1
15.3
a
b
c
Spectra recorded in CH
Spectra recorded in THF/H
From reference 27.
2
Cl
2
in CaF
O (5:1) in CaF plates
2
plates
2
2
reaction of cisꢀ/transꢀ PdCl (PhCN) with 2 equiv of phosphine in
2
2
2
1
1
1
00
80
60
40
CH Cl or CH Cl /H O/MeOH biphasic mixture for nonꢀ
2
2
2
2
2
2
9
sulfonated and sulfonated phosphines, respectively (eq. 4.).
First, we determined again the Tolman cone angles ( Tol
θ
) for
2
R =0.968
5
nonꢀsulfonated phosphines (P1ꢀP10) and TPPTS. While the reꢀ
sults were similar to those published earlier for nonꢀsulfonated
2
7
species, a significant difference between our experimental data
3
1
120
00
(
26.2 ppm) and the previously reported PꢀNMR chemical shift
25
at 34.3 ppm for transꢀPdCl (TPPTS) was observed. According
2
2
θ
1
1
0
to Bartik et al., a sharp singlet at 34.8 ppm could be detected in
25
8
0
1
H O. If the reaction was performed in H O/MeOH (1:1) a sinꢀ
2
2
00
120
140
θ
Tol non-sulfonated ligands
160
180
200
ο
glet peak at 26.2 ppm indicated the formation of other isomer,
similarly to the value assigned to the transꢀspecies by Stelzer et
al. It was established that the formation of cis- and transꢀ
30
Fig 2. Correlation of steric properties (
θTol) of sulfonated and nonꢀ
sufonated phosphines.
1
5
isomers strongly depends on the solvent, and the resonance of
31
determination of ν(CO)A stretching frequency of complexes
cisꢀisomer is generally downfield from that of the transꢀisomer.
1
Ni(CO )(PtBu ) and Ni(CO )(PPh ) and a close agreement with
The θTol value obtained here by spectroscopic method is in good
agreement with the value calculated by Darensbourg et al.
3
3
3
3
ꢀ
1
ꢀ1
4
4
5
5
6
0
5
0
5
0
the experimental ν(CO) = 2056.1 cm and 2069.4 cm was
ꢀ
1
ꢀ1
32
found with computed values of 2042.9 cm and 2057.6 cm ,
respectively. For the sulfonated ligands, the computed Tolman
electronic parameters are closely in agreement the experimental
values. The closest match was obtained for ligands L3ꢀL6 conꢀ
taining nꢀalkyl substituents. For TPPTS, however, noticeable
(170°). Since the modified correlation was successfully used for
ꢀ
1
χ
difference was observed with a computed of 19.8 cm , as opꢀ
ꢀ
1
posed to 15.3 cm , obtained experimentally. The θTol values were
determined by full geometry optimization of the sulfonated ligand
L1ꢀL10. The computed cone angles showed reasonable agreeꢀ
ment with the experimentally determined θTol values. However,
for the nꢀalkylꢀdiaryl ligands L1, L3, and L5 significant deviaꢀ
tions were obtained as the calculated θTol values were over 170°,
whereas the experimentally determined cone angles remained
between 140° (for L1) and 153° (for L5). The large difference
might be attributed to the facile conformational changes of the
aryl rings in solution. Including implicit solvation, however, it did
not bring any significant change in the calculated cone angles.
For the more bulky substituents, such as isopropyl or especialꢀ
2
2
3
3
0
5
0
5
the nonꢀsulfonated species and TPPTS, it was employed for the
31
characterization of new phosphines in H O/MeOH (1:1). The Pꢀ
2
NMR spectra were recorded directly without isolation using
H PO as external reference. Each spectrum has shown the presꢀ
3
4
ence of both cisꢀ and transꢀisomers and a small amount of phosꢀ
phine oxide. The derived steric parameters are collected in Table
1
, which indicates that the sulfonated species are only slightly
larger than the neutral one. A good linear correlation between the
θTol value of sulfonated and nonꢀsulfonated phosphines can be
established (Fig. 2.), and the difference becomes negligible in the
case of more bulky species, for example iPr derivatives verifying
3
3
ly cyclopentyl, the computed θTol parameters fall closer to the
experimentally determined ones. Close agreement was found also
for TPPTS, where the deviation was only 6°.
Horváth’s original observation.
In order to verify the steric and electronic properties, theoretiꢀ
cal calculations were performed within the framework of density
functional theory using the PBEPBE gradientꢀcorrected functionꢀ
al in combination with the 6ꢀ31G(d,p) basis set. For nickel the
SBKJC basis set was employed. This model chemistry (among
other DFT functional/basis set combinations) was tested on the
Hydrogenation of levulinic acid to γ-valerolactone
The selective dehydration of carbohydrates to levulinic and
This journal is © The Royal Society of Chemistry [year]
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homogeneous Ruꢀcatalyst showed saturation kinetics for hydroꢀ
formic acids is a particularly attractive approach for biomass
based production of γꢀvalerolactone. Since the formic acid can be 45 gen pressure. Subsequently, the effect of the hydrogen pressure
38
decomposed to CO and H in the presence of palladium, silver,
on the GVL formation was determined in the range of 5 ꢀ 100 bar
using Ru/BuꢀDPPDS (L5) system, which was the most active
catalyst at 100 bar (Table 1. Entry 1.). The results for the converꢀ
sion of LA in 110 minutes at different pressures are shown on Fig
2
2
3
4
copper or even more efficiently by the use of tris[(2ꢀ
3
5
5
0
5
0
5
0
5
diphenylphosphino)ethyl]phosphine modified iron catalyst, the
latter can also be used for the hydrogenation of levulinic acid to
GVL. It was established that LA (33 mmol) can be converted to 50 3. The reduction of LA slightly depended on the hydrogen presꢀ
GVL with a yield of 95% by using waterꢀsoluble ruthenium cataꢀ
sure above 20 bar, and become independent over 50 bar. These
results are in good agreement with recently reported kinetics for
hydrogenation of LA in CH Cl /H O biphasic system using
Ru/TPPTS (1:1) catalyst, and allow to reduce the system presꢀ
55 sure without significant decrease in the activity. On the other
hand, the mass transfer limitation on the hydrogen concentration
cannot be excluded at higher pressures.
lyst prepared in situ from Ru(acac) (0.055 mmol) and 10 eqv. of
3
1
1
2
2
3
3
TPPTS in water (50 mL) or by reducing neat LA (588 mmol) in
the presence of Ru(acac) (0.37 mmol), PBu (3.7 mmol) and
2
2
2
1
9
3
3
15
NH PF (1.8 mmol).
4
6
According to previous papers, the slightly acidic NH PF could
4
6
1
8,21a
be eliminated from the reaction mixture.
We have confirmed
that the levulinic acid (293.02 mmol) can be quantitatively transꢀ
formed to GVL (TON = 6370) without solvents and any additive
in the presence of Ru(acac) (0.046 mmol) and PBu (0.46 mmol)
1
00
90
80
70
60
50
40
3
3
under 100 bar of H by heating at 140 °C for 45 mins. It should
2
be noted that the reduction is about 10 times faster in the presence
of PBu than that of TPPTS under identical conditions.
3
In order to approach the activity of the trialkylphosphine modiꢀ
fied system, the influence of the electronꢀdonating waterꢀsoluble
3
2
1
0
0
0
0
alkylꢀbis(mꢀsulfonatedꢀphenyl)ꢀ
sulfonatedphenyl)phosphines (L5, L6), and TPPTS on the hyꢀ
drogenation of LA was investigated under 100 bar H . (Table 2.
(L1,
L2),
dialkylꢀ(mꢀ
3
6
2
Entry 1ꢀ5.). It was clearly demonstrated that the catalytic activity
can be considerably enhanced by the incorporation of one or two
alkyl functional group(s) into the phophorus ligands. While the
complete conversion of LA to GVL was achieved by using either
methyl or butyl derivatives, TPPTS exhibited only 35% converꢀ
sion for 4 hours. The accelerating effect of more the electronꢀ
donating dialkyl substituted phosphines (L1, L5) was lower than
that of the corresponding monoalkyl substituted phosphine (L2,
L6), which already converted LA to GVL in less than 2 hours
under identical conditions. It could be explained that the catalytiꢀ
0
10 20 30 40 50 60 70 80 90 100
Pressure (bar)
Fig 3. Hydrogen pressure effect on the hydrogenation of LA.
Reaction conditions: [Ru] = 1.55ꢁ10 mol/dm (0.016 mol%), : [L5] =
ꢀ3
3
ꢀ2
3
1
2
.55ꢁ10 mol/dm (0.16 mol%), 10 bar H , 140 °C, t = 110 min.
To better understand the influence of the electronic and steric
properties of the ligands on the hydrogenation, a series of sulꢀ
fonated phosphines were tested for the GVL production under 10
6
6
7
0
5
0
bar of H and 140°C (Table 2. Entry 6ꢀ15.). According to pubꢀ
2
3
7
cally active species formed by ligand dissociation.
36
lished methods the novel sulfonated L3, L4, L7, L8, L9, and
Table 2 Hydrogenation of levulinic acid in the presence of electronꢀ
donating waterꢀsoluble phosphine ligands.
L10 have been prepared and characterized (Table 1.). Although,
the alkyl substituent(s) increase(s) the electron density of the
phosphorus ligands, the comparison of the activity of monoꢀ and
dialkyl phosphine modified systems revealed higher activity of
monoalkyl species for each pair. Higher than 98% yield of GVL
was achieved in the case of a linear alkyl substituent(s) containꢀ
ing PrꢀDPPDS (L3) ( = 12.1, θTol = 151°) and BuꢀDPPDS (L5)
( = 12.5, θTol = 153°). Similarly to previous data, the complete
conversion of LA was obtained by the use of the catalyst formed
in situ by reaction of Ru(acac) and BuꢀDPPDS (L5) (Table 2.
Entry Ligand
Pressure Time
Yield GVL
(%)
TON
(bar)
(h)
1.8
1.8
1.8
1.8
1.8
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
BuꢀDPPDS (L5)
MeꢀDPPDS (L1) 100
100
>99.9
96.2
91.8
75.5
23
26.9
69.5
98.9
90.7
>99.9
79.2
49.9
45.9
38.8
42.1
6370
6128
5847
4809
1465
1714
4427
6230
5778
6370
5045
3179
2924
2472
2681
Bu
Me
2
ꢀMPPMS (L6) 100
ꢀMPPMS (L2) 100
χ
2
χ
TPPTS
TPPTS
100
10
MeꢀDPPDS (L1) 10
PrꢀDPPDS (L3) 10
Pr ꢀMPPMS (L4) 10
BuꢀDPPDS (L5) 10
Bu ꢀMPPMS (L6) 10
iPrꢀDPPDS (L7) 10
iPr ꢀMPPMS (L8) 10
CpꢀDPPDS (L9) 10
Cp ꢀMPPMS (L10) 10
3
χ
Entry 10.) exceeding the activity of MeꢀDPPDS (L1) ( = 13.5,
θTol = 140°). While branched substituent(s) containing ligands
2
0
1
2
3
4
5
2
75 were found to be more electronꢀdonating, e.g. Cp
2
ꢀMPPMS (L10)
= 6.7), the accelerating effect of these more bulky phosphines
L7 ꢀ L10) with θTol > 172° is significantly lower (Table 2. Entry
2ꢀ15.) than that of species in the range of 153° > θTol > 133°
Table 2. Entry 7ꢀ11.). A very slow reaction could be observed in
χ
(
(
2
1
2
(
Reaction conditions: 34.02 g (293.02 mmol) LA, Ru(acac)
mmol, [Ru] = 1.55ꢁ10 mol/dm (0.016 mol%), Ligand/Ru = 10,
3
= 0.046
χ
ꢀ3
3
80 the presence of less electronꢀdonating TPPTS ( = 15.3,
θ =
Tol
40
1
73°) at 100 bar and 10 bar of H , respectively (Entry 5ꢀ6.). It
T = 140°C TON defined as mol of substrates converted per mol of Ru.
It has been reported that related hydrogenation reactions using
2
was demonstrated that while the activity strongly depends on the
electronic and steric properties, the selectivity was found to be
independent from the ligand structure.
4
| Journal Name, [year], [vol], 00–00
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O
2500
2000
1500
1000
O
COOH
O
10
15
20
7.5
5
500
0
0
.005 0.010 0.015 0.020 0.025 0.030 0.035
3
L5 concentration (mol/dm )
1
Fig 4. Turnover frequencies (TOF = mole of LA/mole of Ru/hour at
0% conversion) at different BuꢀDPPDS (L5) concentrations. Reaction
Fig 7. HꢀNMR spectra of hydrogenation of LA (293.02 mmol) in the
presence of catalyst in situ formed from Ru(acac) = 0.046 mmol and
O at different δ (ppm)
5
3
ꢀ3
3
conditions: [Ru] = 1.55ꢁ10 mol/dm (0.016 mol%), 10 bar H
2
, 140 °C.
2 2
L5 (0.46 mmol) under 10 bar H at 140 °C. *: H
The L5/Ru ratios are also indicated at the corresponding concentrations.
depending on the conversion.
Ru/BuꢀDPPDS (L5) catalyst in the concentration range of [P] =
3
ꢀ3
3
0
.0077 – 0.03 mol/dm using [Ru] = 1.55ꢁ10 mol/dm . As exꢀ
pected, the ligand concentration has a strong influence on the
GVL formation as can be seen in Fig 4. The concentration deꢀ
1
1
2
2
0
5
0
5
pendence of the reaction rate goes through a maximum at [P] =
3
0
.015 mol/dm . This is consistent with the observation of the
ligand excess favored reduction of carbonyl group in the presence
39a
of Ru/TPPMS.
The reproducibility of the catalytic reaction was confirmed by
repeating the experiment under 10 bar of H using BuꢀDPPDS
2
(
L5) at 140°C. Complete conversion was achieved in both cases
after 4.5 hours.
Highꢀpressure in situ IR studies were performed to obtain addiꢀ
tional insights into the LA conversion and the catalyst activity.
18
Fig 5. . In situ IR spectra of the reaction mixture during the reduction of
0 mL + 6x10 mL LA (586.4 mmol + 6x98.7 mmol) to GVL in the
presence Ru(acac) (0.753 mmol), L5 (7.5 mmol) under 100 bar H at
40 °C. 1., 2., 3, … 6.: Addition of 10 ml of LA.
Similarly to the PBu containing system, quantitative formation
3
6
of GVL could be achieved in 5 hours, evidenced by the decreasꢀ
3
2
ꢀ
1
ing carbonyl stretching frequency of LA at 1710 cm and the
1
ꢀ
1
appearance of a band at 1768 cm assigned to the GVL (Fig 5.,
6.). To check the activity and stability of the L5 modified Ru
catalyst, 10 mL (98.7 mmol) of levulinic acid was added to the
reaction mixture by using 25 bar hydrogen pressure after the
complete conversion (Fig. 5., 6. [1.]). The reaction profiles (Fig.
Abs
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
O
O
6
.) indicated the decreasing concentration of LA and the forꢀ
30 mation of GVL achieving full conversion. This sequence was
repeated for six times (Fig 5. and Fig 6.: [1…6.]). It was clearly
established that the catalyst was still active and completely conꢀ
verted the LA to GVL after six consecutive LA additions. The
31
PꢀNMR spectrum of the final reaction mixture did not indicate
3
4
4
5
0
5
any decomposition of the ligands including the formation of
phosphine oxide. The catalyst is very stable in the presence of the
electron donating L5. It is also important to note that the forꢀ
mation of sideꢀproducts was not observed under these reaction
conditions. The reduction of LA to GVL was also monitored by
high pressure HꢀNMR (Fig. 7.). The disappearance of the singlet
of the methyl group of LA at
the methyl group of GVL at
chemical shift of the side product water has changed according to
its concentration in the reaction mixture. Similarly to the results
obtained by highꢀpressure IR, the side product formation can be
excluded. The highꢀpressure NMR measurement of the mixture
of Ru(acac) (0.046 mmol) and BuꢀDPPDS (L5) (0.46 mmol) in
1
0
5
10
15
20
25
Time (h)
Fig 6. . Reaction profiles of the selected bands of the reaction mixture
during the reduction of 60 mL + 6x10 mL LA (586.4 mmol + 6x98.7
mmol) to GVL in the presence Ru(acac)
under 100 bar H at 140 °C. 1., 2., 3, … 6.: Addition of 10 ml of LA.
1
3
(0.753 mmol), L5 (7.5 mmol)
δ
2.5 ppm and the appearance of
2
δ
1.6 ppm were easily followed. The
ꢀ1
ꢀ1
LA: 1710 cm (ꢀ), GVL: 1768 cm (○)
Another crucial factor for high rate of hydrogenation reaction
is the phosphine content of the reaction mixture. Although it is
customary in the literature to report kinetic and selectivity data as
a function of P/Ru ratio , the ligand concentration was found to
be a most important parameter of homogeneous catalysis.
3
9
3
40
5
.5 mL LA under 100 bar H at 23°C indicated the formation of a
2
Therefore, we investigated the LA reduction using
monoꢀnuclear ruthenium hydride species by a singlet at
δ ꢀ5.1
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |
5

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ꢀL of reaction mixture was dissolved in 1 mL of methylene chloꢀ
ride followed by the addition of 10 L toluene as internal standꢀ
ard. Infrared spectra were recorded with ReactIR 1000 Reacꢀ
ppm, which disappeared by heating the reaction mixture up to
40°C.
The feasibility of the catalyst recycling was determined under
1
ꢀ
TM
batch condition by performing the experiment using [Ru] = 45 tion Analysis System attached with highꢀpressure silicon
ꢀ
2
3
3 18,21
TM
5
0
5
0
1.55ꢁ10 mol/dm (0.16 mol%), [P] = 0.15 mol/dm ,
100 bar
(SiComp ) probe head. In situ highꢀpressure IR experiments
of H at T = 140°C. The full conversion of LA to GVL was
achived after 1.8 hours as initially determined in Table 2. Entry 1.
The reaction mixture was transferred into a Schlenkꢀflask under
nitrogen. The volatile compounds were removed by vacuum 50 performed in a highꢀpressure 120 mL Parr HastelloyꢀC reactor
transfer resulting in a clear solution as distillate. The reddishꢀ
orange gluey residue can be completely reꢀdissolved in 34.02 g
293.02 mmol) levulinic acid. The light orange solution was
loaded into the autoclave and the reduction was started by presꢀ
surizing the reactor to 100 bar H and heating it up to 140°C. The
start of the reduction was indicated by the significant initial presꢀ
sure drop and resulted in full conversion of LA again. The quantiꢀ
tative reduction of LA was achieved for six consecutive runs
without significantly decreasing the activity of the catalyst
Fig.8). These results indicated, that the Ru based catalyst formed
in situ from Ru(acac) and the electron donating BuP(C H ꢀmꢀ 60 reaction mixture was pressurized and heated up to adjust to the
were carried out in a customꢀmanufactured 300 mL stainless steel
reactor equipped with highꢀpressure silicon (SiComp ) probe
head and highꢀpressure gas system. The catalytic reactions were
2
TM
42
1
1
2
equipped with a safety relief, manometer, temperature controller
(Parr 4842 PID), sampling line, went line and connection to the
gas system.
(
Catalytic Hydrogenation of Levulinic acid
2
55
In a typical experiment, the highꢀpressure reactor was charged
with 34.02 g (30 mL, 293.02 mmol) levulinic acid, 0.0186 g
(0.0466 mmol) ruthenium(III)ꢀacetylacetonate and 10 eqv. (0.466
mmol) of corresponding phosphine ligand resulting in a light red
ꢀ
3
3
(
solution with [Ru] = 1.55ꢁ10 mol/dm as base condition. The
3
6
4
SO Na) (L5) ligand was stable enough for successfully catalyst
desired values. Samples were collected for offꢀline GC analysis
by taking a small amount of sample with a dipꢀleg into a sample
holder, cooled with dry ice. After a given reaction time, the autoꢀ
clave was cooled to ambient temperature and the stirring was
stopped.
3
2
recycling by the removal of the volatile compounds by distillaꢀ
2
1a
tion, as proposed recently.
6
5
4000
3500
3000
2500
2000
1500
1000
Preparation and Characterization of Phosphine Ligands
The preparation of P1, P2, P5, P6, L1, L2, L5, L6 ligands
36
were performed according to published methods. The purity
100
1
31
70 was checked by H and PꢀNMR. The experimental data for the
preparation of P3, P4, P7, P8, P9, and P10 collected in supporting
information.
General procedure for the sulfonation of mono- and dia-
rylphosphines
The sulfonation of the freshly distilled corresponding alkylꢀ
500
0
75
1
2
3
4
5
6
Cycle
diphenylphosphine (P3, P7, P9) and dialkylꢀphenylphosphine
3
6
Fig 8. Turnover Numbers and conversions in the investigation of the cataꢀ
lyst recycling.
(P4, P8, P10) was based on previously published methods. The
1
31
purity was checked by H and PꢀNMR.
Disodium Salt of Propyl-bis(m-sulfonatedphenyl)phosphine
(L3)
80
Starting materials: propylꢀdiphenylphosphine (10.26 g, 45
mmol), oleum (20%): 100 mL, water for dilution: 250 mL, triizoꢀ
octylamine: 30 mL (38.4 mmol), toluene: 150 mL. PꢀNMR of
25
Experimental
31
Cyclopentyl chloride, isopropyl chloride, propyl bromide,
choloroꢀdiphenylphosphine, dichlorophenylphosphine, triisoꢀ
the protonated form: δ 9.6 (d, J =525 Hz). pH values for the
PꢀH
1
2
3
octylamine, bisꢀbenzonitrileꢀPd(II)ꢀchloride, levulinic acid, 85 phase separation: pH : 5.3, pH : 8.3, pH : 8.7. The product was
1
gammaꢀvalerolactone and oleum (20% SO ) were purchased from
isolated as a white powder (14.6 g, 79%). H–NMR (250 MHz,
3
30
35
40
Aldrich Chemical and used as received without further purificaꢀ
tion. Toluene, sodium hydroxide and diethyl ether were obtained
D O): δ 0.83 (t, 3H), 1.20 (m, 2H), 1.98 (t, 2H) 7.30 ꢀ 7.80 (m,
2
31
13
8H). P–NMR (100 MHz, D O): δ ꢀ14.9 (s). CꢀNMR (62.5
2
from Molar Ltd. All preparations were carried out under N atꢀ
MHz, D O): δ 14.8 (d, J = 13.7 Hz), 18.6 (d, JPꢀC = 14.6 Hz),
2
2
PꢀC
mosphere using standard Schlenk techniques. The solvents were 90 28.3 (d, JPꢀC = 7.5 Hz), 126.0 (s), 129.71 (d, JPꢀC = 6.4 Hz), 129.3
deoxygenated by vacuum and bubbling nitrogen for at least 30
minutes. The pH was measured by OPꢀ264/1 pH meter with a
Radelkis standard glass electrode. All preparations were carried
out in a wellꢀventilated hood. The highꢀpressure NMR experiꢀ
(d, JPꢀC = 7.3 Hz), 135.4 (d, JPꢀC = 16.2 Hz), 138.6 (d, JPꢀC = 11.1
Hz), 142.7 (d, JPC = 6.7 Hz)
Sodium Salt of Dipropyl(m-sulfonatedphenyl)phosphine (L4)
Starting materials: dipropylꢀphenylphosphine (8.7 g, 45
ments were performed in a 10 mm singe crystal sapphire tube 95 mmol), oleum (20%): 100 mL, water for dilution: 250 mL, triisoꢀ
41
with titanium head. GC analyses were performed on an Agilent
octylamine: 30 mL (38.4 mmol), toluene: 150 mL. pH values for
1
2
3
1
6890N instrument with HPꢀInnowax capillary column (15 m x
the phase separation: pH : 3.0, pH : 4.6, pH : 6.6. H–NMR (250
0
.25 ꢀm x 0.25 ꢂm) using He as carrier gas. For the analysis 10
| Journal Name, [year], [vol], 00–00
MHz, D O): δ 0.81 (t, 6H), 1.25 (m, 4H), 1.53 (t, 4H) 7.20 ꢀ 7.98
2
6
This journal is © The Royal Society of Chemistry [year]

Page 7 of 9
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phosphines were prepared by the reaction of cisꢀ/transꢀ
MHz, D O): δ 15.4 (d, J = 12.8 Hz), 19.1 (d, JPꢀC = 13.1 Hz), 60 PdCl (PhCN) with 2 equiv of phosphine in CH Cl at room temꢀ
31
13
(
m, 4H). P–NMR (100 MHz, D O): δ ꢀ24.5 (s). CꢀNMR (62.5
2
2
PꢀC
2
2
2
2
2
(
9.4 (d, JPꢀC = 8.9 Hz), 126.1 (s), 128.8 (d, JPꢀC = 5.4 Hz), 129.3
d, JPꢀC = 22.1 Hz), 133.9 (d, JPꢀC = 14.0 Hz), 139.5 (d, JPꢀC = 13.8
Hz), 143.1 (d, JPC = 8.2 Hz)
Disodium Salt of
phosphine (L7)
perature. The NMR spectra were recorded directly without isolaꢀ
tion.
5
The bisꢀphosphine palladium complexes of sulfonated ligands
(L1 – L10 and TPPTS) were prepared in situ in a biphasic reacꢀ
65 tion mixture. In a typical experiment 15.2 g (0.04 mmol) of
PdCl (PhCN) was dissolved in 0.25 mL CH Cl resulting in light
Isopropyl-bis(m-sulfonatedphenyl)-
Starting materials: isopropylꢀdiphenylphosphine (7.06 g, 30.97
mmol), oleum (20%): 71 mL, water for dilution: 200 mL, triisoꢀ
octylamine: 36 mL (46.1 mmol), toluene: 128 mL. PꢀNMR of
2
2
2
2
yellow solution. A 1 mL MeOH/H O (1:1 v%) solution of 1
2
31
1
1
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
5
0
5
mmol of the desired sulfonated phosphine was then added and
the protonated form: δ 22.6 (d, J =512 Hz). pH values for the
stirred for ~ 30 min. The colorless organic phase was separated
PꢀH
1
2
3
31
phase separation: pH : 2.8, pH : 4.8, pH : 7.0. The product was 70 and discarded, the PꢀNMR spectra were recorded directly from
1
31
isolated as a white powder (10.64 g, 79%). H–NMR (in D O): δ
the aqueous solution using H PO4 as reference. P NMR (δ,
2
3
3
1
1
.02 (d, 6H), 2.59 (m, 1H), 7.45 ꢀ 7.95 (m, 8H) P–NMR (D O):
1.83 (s). C–NMR (D O): δ 18.9 (d, JPꢀC=17.01 Hz), 24.3 (d, JPꢀC
MeOH/H O=1:1, ppm) data of the Pd complexes are collected in
2
2
13
supporting information.
2
=
4.14 Hz), 126.6 (s), 129.7 (d, JPꢀC=6.89 Hz), 130.3 (d, JPꢀ
The Ni(CO) (PL ) complexes of nonꢀsulfonated ligands (P1 –
3
3
C=20.22 Hz), 136.7 (d, JPꢀC=17.01 Hz), 137.6 (d, JPꢀC=11.95 Hz), 75 P10 and PPh ) were prepared in situ use of published protocol in
3
3
1
25
31
1
43.0 (d, JPꢀC=7.45 Hz). Oxide: P–NMR (D O): 46.5 (s)
CH Cl and THF:H O = 5:1, respectively. The IR and PꢀNMR
2 2 2
2
Sodium salt of Diisopropylphenylphosphine-3-sulfonate (L8)
Starting materials: diisopropylꢀphenylphosphine (2.07 g, 10.67
mmol), oleum (20%): 16 mL, water for dilution: 70 mL, triisoꢀ
(ref. H PO ) spectrum was recorded directly without isolation of
3 4
the tricarbonyl compounds. The spectroscopy data are presented
in the supporting information.
3
1
octylamine: 5 mL (6.4 mmol), toluene: 40 mL. PꢀNMR of the
80
The Ni(CO) (PL ) complexes of the sulfonated ligands (L1 –
3
3
protonated form: δ 40.3 (d, J =485 Hz). pH values for the phase
L10 and TPPTS) were prepared in situ in a mixture of tetrahydroꢀ
PꢀH
1
2
3
25
separation: pH : 3.8, pH : 5.8, pH : 8.6. The product was isolated
as a white powder (1.32 g, 41%). H–NMR (in D O): δ 0.56 –
.10 (m, 12H), 2.1 (m, 2H), 7.40 ꢀ 7.80 (m, 4H)
furan and water (5:1) using literature procedure. The IR and
1
31
31
PꢀNMR spectra were recorded directly from this solution. Pꢀ
NMR (δ, THF:H O=5:1, ppm, ref. H PO ) and IR spectra
2
31
1
(
P–NMR
2
3
4
13
ꢀ1
D O): 13.5 (s). C–NMR (D O): δ 19.3 (d, JPꢀC=16.0 Hz), 22.1 85 (THF:H O=5:1,
ν
(CO), cm ) are presented in the supporting
2
2
2
(d, JPꢀC = 5.52 Hz), 126.5 (s), 129.1 (d, JPꢀC=7.81 Hz), 131.1 (d,
information.
JPꢀC=16.08 Hz), 137.8 (d, JPꢀC=18.40 Hz), 138.6 (d, JPꢀC=11.95
Theoretical methods. All of the minima and transition states
were computed within the framework of density functional theory
employing the gradientꢀcorrected exchange functional developed
90 by Perdew, Burke and Ernzerhof in combination with a
3
1
Hz), 142.5 (d, JPꢀC=6.43 Hz). Oxide: P–NMR (D O): 62.7 (s)
2
Disodium salt of Cyclopentylphenylphosphine-3-sulfonate
(
L9)
Starting materials: cyclopentylꢀphenylphosphine (10.16g, 40.0
mmol), oleum (20%): 100 mL, water for dilution: 300 mL, triisoꢀ
4
3
correlation functional also developed by the same authors and
denoted as PBEPBE. For nickel atoms the SBKJC basis set,
which is valence tripleꢀzeta for transition metals, with the
31
octylamine: 40 mL (92.3 mmol), toluene: 150 mL. PꢀNMR of
44
the protonated form: δ 23.7 (d, J =512 Hz). pH values for the
corresponding smallꢀcore pseudopotential was utilized. For the
PꢀH
1
2
3
45
phase separations: pH :3.0, pH :5.0, pH :6.9. The product was 95 remaining atoms the 6ꢀ31G(d,p) basis set was used. The calcuꢀ
1
isolated as a white powder (14.6 g, 79%). H–NMR (250 MHz,
lations were carried out using the Firefly 7.1.G (formerly known
3
1
46
D O): δ 1.00ꢀ1.70 (m, 9H), 7.20 ꢀ 7.90 (m, 8H). P–NMR (100
as PCꢀGAMESS) software . Optimized geometries were identiꢀ
2
1
3
MHz, D O): δ ꢀ2.1 (s). CꢀNMR (62.5 MHz, D O): δ 26.6 (d, J
Pꢀ
fied by the absence of negative eigenvalues in the vibrational
frequency analysis. In order to determine the Tolman’s cone anꢀ
2
2
=
7.8 Hz), 30.8 (d, JPꢀC = 18.8 Hz), 34.7 (d, JPꢀC = 4,6 Hz), 126.6
C
47
(s), 129.7 (d, JPꢀC = 6.4 Hz), 129.8 (d, JPꢀC = 7,3 Hz), 136.2 (d, JPꢀC 100 gles the STERIC program was used.
=
17.0 Hz), 139.1 (d, JPꢀC = 12.4 Hz), 143.3 (d, JPC = 6.9 Hz)
Sodium salt of Dicyclopentylphenylphosphine-3-sulfonate
(L10)
Conclusions
Starting materials: dicyclopentylꢀphenylphosphine (8.92g, 36.28
mmol), oleum (20%): 60 mL, water for dilution: 250 mL, triisoꢀ
We have demonstrated that the biomass derived levulinic acid
could be reduced effectively to γꢀvalerolactone in the presence of
31
octylamine: 20 mL (46.1 mmol), toluene: 150 mL. PꢀNMR of
1
1
1
05 a catalyst in situ generated from Ru(acac) and different sulfonatꢀ
3
the protonated form: δ 32.1 (d, J =478 Hz). pH values for the
PꢀH
1
2
3
ed phoshine R P(C H ꢀmꢀSO Na)3ꢀn (n = 1,2; R = Me, Pr, iPr,
n 6 4 3
phase separations: pH : 3.0, pH : 5.5, pH : 9.5. The product was
isolated as a white powder (9.16 g, 72%). H–NMR (in D O): δ
.60ꢀ2.15 (m, 18H), 7.20 ꢀ 7.90 (m, 4H). P–NMR: δ 3.1 (s).
CꢀNMR: δ 26.0 (d, JPꢀC = 6.4 Hz), 26.8 (d, JPꢀC = 7.8 Hz), 31.0
d, JPꢀC = 17.4 Hz), 31.3 (d, JPꢀC = 20.2 Hz), 36.9 (d, JPꢀC = 7.8
1
nBu, Cp) ligands in a solventꢀ and promoterꢀfree reaction enviꢀ
ronment. The characterization of sulfonated and nonꢀsulfonated
ligands showed that the incorporation of sulfonato group into the
10 ligand structure has no significant influence on the electronic and
steric properties, respectively. It was established that the activity
of the electronꢀdonating sulfonated phosphine modified catalysts
significantly exceeded the activity of the Ru/TPPTS system
achieving turnover number up to 6370 and kept the activity after
15 full conversion of LA to GVL. The influence of the system presꢀ
2
3
1
0
1
3
(
Hz), 126.5 (s), 129.6 (d, JPꢀC = 4,6 Hz), 129.8 (d, JPꢀC = 27,1 Hz),
1
9
35.5 (d, JPꢀC = 10.5 Hz), 139.2 (d, JPꢀC = 15.6 Hz), 143.3 (d, JPC
.2 Hz).
=
The L PdCl complexes (L = P1 – P10) of the nonꢀsulfonated
2
2
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sure and the ligand concentration on the activity was also deterꢀ
J. Am. Chem. Soc., 1947, 69, 1961. (c) T. Osawa, E. Mieno, T.
Harada and O. Takayasu, J. Mol. Catal. A. Chem., 2003, 200, 315.
(d) P. J. van den Brink, K. L. von Hebel, J.ꢀP. Lange and L. Petrus,
U. S. Patent Appl. 0162239 (2006)
3
mined with an optimum at [BuꢀDPPDS] = 0.015 mol/dm by
3
using constant [Ru] = 0.0015 mol/dm at 10 bar. The in situ IR
and NMR studies did not show any sign of byꢀproduct formation.
The catalyst recovery was successfully demonstrated. The
volatile compound can be removed by vacuum, the remain cataꢀ
lyst compounds can be dissolved in levulinic acid and exhibited
similar activity than before distillation.
1
3 (a) L. E. Manzer Appl. Catal. A. Gen., 2004, 272, 249.; (b) R. A.
Bourne, J. G. Stevens, J. Ke and M. Poliakoff, Chem. Commun.,
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3, 3853.
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5 Horváth, I. T.; Mehdi, H.; Fábos, V. and Kaposy, N., Hung. Patent.
Appl. HU 08 00662 (2008).
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Acknowledgment
10
This work was supported by the Hungarian National Scientific
Research Fund (Grant No. OTKAꢀCNKꢀ78065). The European
Union and the European Social Fund have provided financial
support to the project under the grant agreement no. TÁMOP
1
4
.2.1./Bꢀ09/KMRꢀ2010ꢀ0003. The authors thank Prof. István T.
1
9 M. Chalid, A. A. Broekhius and H. J. Heeres, J. Mol. Catal. A. Chem.,
2011, 341, 14.
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Horváth (City University of Hong Kong) for continuous and
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