A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of Levulinic Acid into Γ-Valerolactone Using Water-Soluble Platinum Catalysts Modified with Nitrogen-Containing Ligands

Article abstract of DOI:10.1007/s10562-019-02707-9

Abstract: The catalytic performance of novel water-soluble platinum catalysts modified with various nitrogen-containing and phosphine ligands in the hydrogenation reaction of levulinic acid (LA) into γ-valerolactone (GVL) has been studied in environmentally attractive, green, aqueous monophasic systems. The presence of the Lewis acid aluminum enormously increases the catalytic activity of water-soluble platinum catalysts modified with nitrogen-containing ligands in the LA hydrogenation reaction and high catalytic activities up to 3540 TOF’s per hour with a quantitative selectivity towards GVL have been achieved using Na₂PtCl₆·6H₂O precursors modified with the bidentate bathophenanthrolinedisulfonic acid disodium salt (BPhDS) ligand and low amounts of AlCl₃·6H₂O promotors (molar ratio of AlCl₃·6H₂O/Pt = 0.17) in aqueous media. This unprecedented increase in catalytic activity with aluminum promotors for water-soluble transition metal catalytic systems in aqueous-phase hydrogenation reactions has not been described until now in the literature. The apparent activation energy of platinum catalyst modified with the monodentate nitrilotriacetic acid trisodium salt ligand in aqueous medium was calculated and amounts to a relative low value of 73.04?kJ?mol^?1 when one considers that in the LA hydrogenation reaction this catalyst reduces a less reactive keto group into alcohol functionality. A recycling experiment of the Pt/BPhDS/Al catalyst from the aqueous monophasic LA hydrogenation reaction mixture followed by biphasic recovery of the catalyst in active form from organic reaction products by extraction and simple phase separation of an aqueous/organic two-phase system formed after addition of diethyl ether has shown that the Pt/BPhDS/Al catalyst is stable without loss of activity and selectivity in a consecutive run. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02707-9

Catalysis Letters
https://doi.org/10.1007/s10562-019-02707-9
A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-
Phase Hydrogenation of Levulinic Acid into γ-Valerolactone Using
Water-Soluble Platinum Catalysts Modified with Nitrogen-Containing
Ligands
Kalliopi Krommyda1 · Christina Panopoulou1 · Chrysavgi Moustani1 · Eleni Anagnostopoulou1 ·
Konstantina Makripidi1 · Georgios Papadogianakis1
Received: 3 December 2018 / Accepted: 3 February 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The catalytic performance of novel water-soluble platinum catalysts modified with various nitrogen-containing and phosphine
ligands in the hydrogenation reaction of levulinic acid (LA) into γ-valerolactone (GVL) has been studied in environmentally
attractive, green, aqueous monophasic systems. The presence of the Lewis acid aluminum enormously increases the catalytic
activity of water-soluble platinum catalysts modified with nitrogen-containing ligands in the LA hydrogenation reaction and
high catalytic activities up to 3540 TOF’s per hour with a quantitative selectivity towards GVL have been achieved using
Na2PtCl6·6H2O precursors modified with the bidentate bathophenanthrolinedisulfonic acid disodium salt (BPhDS) ligand
and low amounts of AlCl3·6H2O promotors (molar ratio of AlCl3·6H2O/Pt=0.17) in aqueous media. This unprecedented
increase in catalytic activity with aluminum promotors for water-soluble transition metal catalytic systems in aqueous-phase
hydrogenation reactions has not been described until now in the literature. The apparent activation energy of platinum catalyst
modified with the monodentate nitrilotriacetic acid trisodium salt ligand in aqueous medium was calculated and amounts
to a relative low value of 73.04 kJ mol−1 when one considers that in the LA hydrogenation reaction this catalyst reduces a
less reactive keto group into alcohol functionality. A recycling experiment of the Pt/BPhDS/Al catalyst from the aqueous
monophasic LA hydrogenation reaction mixture followed by biphasic recovery of the catalyst in active form from organic
reaction products by extraction and simple phase separation of an aqueous/organic two-phase system formed after addition of
diethyl ether has shown that the Pt/BPhDS/Al catalyst is stable without loss of activity and selectivity in a consecutive run.
Graphical Abstract
SO3Na
NaO3S
(TOF = 3540 per hour)
N
N
Pt
O
H2O
H2O
O
O
OH
H2
H2O
+
+
[molar ratio AlCl
3
·6H O/Pt= 0.17]
2
O
[H2O]
Levulinic acid (LA)
γ-Valerolactone (GVL)
Keywords Aqueous-phase hydrogenation · Levulinic acid · Platinum · Aluminum · Water-soluble ligands
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

K. Krommyda et al.
systems [1, 2]. The use of water circumvents the need for
organic solvents and facilitates recovery and recycling
of the catalyst which can provide substantial economic
and environmental benefits and water is a non-toxic, non-
inflammable, safe, inexpensive, abundantly available and
a green solvent [1–5]. Τhe polarity of water makes it an
ideal solvent to convert polar biomass raw materials with
high oxygen content such as hydrophilic carbohydrates and
their derivatives by water-soluble transition metal cata-
lytic complexes in aqueous monophasic systems followed
1 Introduction
Because of the broad range of potential industrial appli-
cations there is increasing interest in catalysis in aque-
ous media [1–5]. Water-soluble rhodium catalytic com-
plexes modified with the sodium salt of trisulfonated
triphenylphosphine [TPPTS, P(C6H4-m-SO3Na)3, Fig. 1],
for example, is applied industrially in the hydroformyla-
tion of propene employing aqueous/organic two-phase
SO3Na
SO3Na
NaO3S
HO2C
CO2H
CO2Na
NaO2C
N
P
NaO3S
NaO2C
KO2C
N
N
N
N
NaO2C
SO3Na
NTA.Na3
TPPTS
BPyDCA
BPhDS
CO2H
CO2Na
N
SO3H
CO2H
N
CO2Na
N
HO2C
N
CO2H
N
NaO2C
EDTA.Na4
3-PSA
2,6-PyDCA
3,4-PyDCA
SO3Na
NaO3S
CO2K
H2N
H2N
SO3Na
SO3H
N
N
N
N
CH3
H3C
Taurine
SANa
BQC
BCDS
P
CO2Na
N CO2Na
CO2Na
N
N
(HO)2OP
PO(OH)2
N
NaO2C
N
N
P
N
N
N
N
CO2Na
DTPA.Na5
(HO)2OP
NTPA
PTA
T2PyP
Fig. 1 Structures of the water-soluble ligands triphenylphosphinetri-
sulfonic acid trisodium salt (TPPTS), nitrilotriacetic acid trisodium
salt (NTA·Na3), 2,2′-bipyridine-4,4′-dicarboxylic acid (BPyDCA),
bathophenanthrolinedisulfonic acid disodium salt (BPhDS), ethylen-
ediaminetetraacetic acid tetrasodium salt (EDTA·Na4), 2,6-pyridin-
edicarboxylic acid (2,6-PyDCA), 3-pyridinesulfonic acid (3-PSA),
3,4-pyridinedicarboxylic acid (3,4-PyDCA), 2,2′-biquinoline-
4,4′dicarboxylic acid dipotassium salt (BQC), 2-aminoethanesulfonic
acid (Taurine), bathocuproinedisulfonic acid disodium salt (BCDS),
sulfanilic acid sodium salt (SANa), diethylenetriaminepentaacetic
acid pentasodium salt (DTPA·Na5), nitrilotris(methylphosphonic
acid) (NTPA), tris(2-pyridyl)phosphine (T2PyP) and 1,3,5-triaza-
7-phosphaadamantane (PTΑ)
1 3

A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of…
by biphasic recovery of the catalyst in active form from
organic reaction products by simple phase separation of an
aqueous/organic two-phase system formed after external
addition of an organic solvent such as diethyl ether.
Nowadays, there is growing interest in the use of hydro-
philic carbohydrates and their platform chemical derivatives
from renewable biomass because of their far reaching appli-
cations as attractive industrial feedstocks for biorefineries to
produce biofuels and bio-based chemicals according to the
principles of Green—Sustainable Chemistry and because
they contribute to lower greenhouse gas emissions [6–14].
Today, levulinic acid (LA, Scheme 1) is considered as a
versatile and key platform chemical of central relevance in
the development of biorefineries of the future with many
potential industrial applications. LA is readily available
from both C6 and C5 carbohydrates and the most famous
industrial continuous process for the production of LA is
the Biofine process which was developed by BioMetics
Inc. in the earlies 1990s and proceeds with sulfuric acid
catalyzed hydrolysis of e.g lignocellulose [8, 10, 15]. The
GFBiochemicals company after revision and redesign of the
process engineering of Biofine has developed on commer-
cial scale a process for the production of LA directly from
biomass with the capacity 10,000 MT/a of LA [12, 16, 17].
Other companies producing LA are Golden Race Group [16]
and DuPont [18] and is expected that the higher production
capacity will cause a reduction in the LA price from 5 to 8
US$ per Kg which is currently, to lower than 1 US$ per Kg
[18]. The last years, the hydrogenation reaction of LA into
γ-valerolactone (GVL, Scheme 1) is a very active research
field and various heterogeneous catalysts [11, 14, 18–24],
catalytic nanoparticles [25–27] and conventional apolar
homogeneous transition metal catalytic complexes have been
developed for this reaction in the absence or presence of
organic or aqueous solvents [28–33]. GVL is an important
C5 platform chemical for the production of advanced bio-
fuels, bio-based fine chemicals and polymers and could be
also used as an additive for gasoline, as a sustainable polar
aprotic solvent and as a food additive [10–12, 14, 16, 19, 20,
23, 26, 34, 35].
[TPPMS = PPh2(C6H4-m-SO3Na)]. Horváth et al. [37, 38]
used water-soluble Ru(acac)3/TPPTS catalysts in the aque-
ous monophasic hydrogenation of LA to yield 95% GVL
which was isolated after extraction by means of a biphasic
system formed with addition of ethyl acetate. Heeres et al.
[39, 40] investigated the biphasic catalytic hydrogenation of
LA into GVL by water-soluble RuCl3·3H2O/TPPTS systems
to observe a conversion of 82% of LA in a dichloromethane/
water two-phase system. In a recycling experiment, however,
the RuCl3·3H2O/TPPTS catalyst was partially deactivated
because in the first run the conversion of LA was 81% and
in the followed catalyst recycling experiment only 55% of
LA were converted [39]. Delhomme et al. [41] described the
hydrogenation of LA into GVL catalyzed by RuCl3·3H2O or
Ru(acac)3 precusrors modified with water-soluble phosphine
ligands such as TPPTS, TPPMS, PTA (Fig. 1) in aqueous
monophasic systems to achieve a maximum catalytic activ-
ity of TOF = 210 h−1 with RuCl3·3H2O/TPPTS catalysts
and an activity of TOF= 202 h−1 using Ru(acac)3/TPPTS
with a conversion of LA of 99% and selectivity towards
GVL of 97%. Fu et al. [42, 43] disclosed the hydrogena-
tion of LA to GVL using pentamethylcyclopentadienylirid
ium(III) catalysts modified with water-soluble ligands such
as 4,4′-dicarboxy-2,2′-bipyridine and 4,4′-dihydroxy-2,2′-
bipyridine and found high catalytic activities up to 12,200
TOFs per hour in aqueous monophasic systems.
Recently, we reported [44] the aqueous-phase hydrogena-
tion of LA into GVL employing water-soluble ruthenium
catalysts modified with various nitrogen-containing ligands
and the recovery of such catalysts in a biphasic system
formed after addition of diethyl ether. We now report the
novel and efficient aqueous-phase hydrogenation of LA into
GVL using water-soluble platinum catalysts modified with
nitrogen-containing and phosphine ligands and an enormous
increase in catalytic activity observed in the presence of
the Lewis acid aluminum in the aqueous reaction mixture
as well as the recovery of the catalyst in active form from
organic reaction products by extraction and a simple phase
separation of the biphasic system formed after addition of
diethyl ether without any loss of the catalytic activity and
selectivity. To our knowledge, this is the first example of a
hydrogenation reaction of LA into GVL using water-sol-
uble platinum catalysts in aqueous media and the unprec-
edented increase in catalytic activity with aluminum pro-
motors for water-soluble transition metal catalytic systems
The aqueous-phase catalytic hydrogenation reaction of
LA into GVL employing water-soluble transition metal
catalytic complexes has been described by several research
groups started with the pioneer work of Joó et al. [36]
in the presence of water-soluble Ru/TPPMS complexes
Scheme 1 Selective hydro-
genation of levulinic acid (LA)
into γ-valerolactone (GVL)
catalyzed by water-soluble plati-
num catalysts modified with
nitrogen-containing ligands in
aqueous media
O
[Pt catalyst]
O
O
[H2O]
OH
H2
H2O
+
+
O
Levulinic acid (LA)
γ-Valerolactone (GVL)
1 3

K. Krommyda et al.
in aqueous-phase hydrogenation reactions has not been
described until now in the literature.
that no memory effects of the autoclave regarding previous
transition metal catalytic systems are still operative. After
this procedure, the hydrogenation reactions of LA were car-
ried out in the presence of water-soluble platinum catalysts
modified with nitrogen-containing ligands and phosphines
in aqueous monophasic systems. The catalytic hydrogen-
ation experiment of LA of entry 4/2 of Table 4 was car-
ried out with the following procedure. The water-soluble
Na2PtCl6·6H2O/BPhDS/AlCl3·6H2O catalyst precursor was
first synthesized by dissolving under stirring of 2.81 mg
(0.005 mmol) Na2PtCl6·6H2O, 2.86 mg (0.005 mmol)
BPhDS (molar ratio BPhDS/Pt = 1) and 0.21 mg
(0.00085 mmol) AlCl3·6H2O (molar ratio AlCl3·6H2O/
Pt = 0.17) under argon in 20 ml deaerated demineralized
water within 10 min of time at room temperature. After
addition of 2.322 g (20 mmol) LA to the aqueous catalyst
solution under stirring for further 5 min the reaction mixture
at room temperature, having a platinum concentration of
44 ppm, was charged into an Autoclave Engineers autoclave
of a nominal volume of 100 ml which was previously evacu-
ated and filled with argon. In the reaction mixture the molar
ratio of LA/Pt was 4000. After a number of pressurising-
depressurising cycles with hydrogen to remove last traces of
air oxygen, the autoclave was pressured and contents were
heated with stirring (stirring rate =880 rpm). The heating
time to reach a reaction temperature of 150 °C was 20 min.
At the reaction temperature of 150 °C the hydrogen partial
pressure was 60 bar and the reaction time 1 h. After the
reaction the autoclave was cooled (within 20 min) to room
temperature, vented of hydrogen and the aqueous reaction
mixture having a pH value of 2.91 was removed. The prod-
uct mixture was analyzed by gas chromatography (GC) after
addition of N,N-dimethylformamide as standard and the
obtained results are given in entry 4/2 of Table 4. Emphasis
has been placed in all reaction described in Tables 1, 2, 3
and 4 in order to obtain reproducible results.
2 Experimental
2.1 Materials
Hydrogen (quality 5.0) was purchased from Air Liquide Hel-
las A.E.B.A. (Athens) and used without further purification.
Demineralized water was deoxygenated in an ultrasound
bath under high vacuum for 2 h. During the deoxygenation
the flask was disconnected from the vacuum, and the aque-
ous solvent was saturated with argon; this procedure was
repeated several times. Levulinic acid (LA) with a purity of
98% was purchased from Alfa Aesar and used without any
further purification. γ-valerolactone (GVL) and valeric acid
(VA) were obtained from Alfa Aesar. 1,4-Pentanediol (1,4-
PDO) and N,N-dimethylformamide (DMF) were purchased
from Aldrich. Na2PtCl6·6H2O, PtCl2, Pt(NH3)4Cl2·H2O,
PtO2·H2O and Pt(acac)2 were purchased from Alfa Aesar.
TPPTS was prepared according to literature procedures
[45–52] and isolated with purity higher than 98%. Tris(2-
pyridyl)phosphine (T2PyP) was prepared according to the
procedure of the group of Wilkinson [53] with purity higher
than 99%. Nitrilotriacetic acid trisodium salt (NTA·Na3),
bathocuproinedisulfonic acid disodium salt (BCDS), eth-
ylenediaminetetraacetic acid tetrasodium salt (EDTA·Na4)
and 2,2′-bipyridine-4,4′-dicarboxylic acid (BPyDCA) were
purchased from Alfa Aesar; bathophenanthrolinedisulfonic
acid disodium salt (BPhDS), 2-aminoethanesulfonic acid
(Taurine), 3-pyridinesulfonic acid (3-PSA), 2,6-pyridinedi-
carboxylic acid (2,6-PyDCA) and 3,4-pyridinedicarboxylic
acid (3,4-PyDCA) were obtained from Acros Organics;
nitrilotris(methylphosphonic acid) (NTPA) and diethylen-
etriaminepentaacetic acid pentasodium salt (DTPA·Na5)
were obtained from Fluka; 2,2′-biquinoline-4,4′dicarboxylic
acid dipotassium salt (BQC), sulfanilic acid sodium salt
(SANa) and 1,3,5-triaza-7-phosphaadamantane (PTΑ) were
purchased from Aldrich. All nitrogen containing ligands
mentioned, were used without any further purification. NaI
and NaCl were purchased from Alfa Aesar; AlCl3·6H2O
was obtained from Sigma Aldrich; NaH2PO4·H2O and
MgCl2·6H2O were purchased from Merck.
2.3 Analyses
The purity of the TPPTS ligand was determined by
quantitative 31P{1H}NMR analysis in D2O at 25 °C. δ
TPPTS = − 5.4 ppm. 31P{1H}NMR spectra (121 MHz,
referenced to external 85% H3PO4) were recordered on a
Varian Unity Plus 300/54 spectrometer. The hydrogena-
tion product γ-valerolactone (GVL) and the side products
1,4-pentanediol (1,4-PDO) and valeric acid (VA) were
identified by comparison of GC and gas chromatography/
mass spectrometry (GC/MS) analytic data with data for
authentic samples. GC/MS was measured on an Agilent
6890N GC coupled with an Agilent 5975B MS. The GC
was equipped with a HP-5MS column (30 m × 0.25 mm
i.d. × 0.25 µm film thickness). Carrier gas was He with
2 ml min−1. Injector system: pulsed splitless. The injector
2.2 Procedure of a Typical Catalytic Hydrogenation
Experiment
First, the autoclave was thoroughly cleaned and followed
by numerous series of treatment of the autoclave at elevated
temperatures (150 °C) and pressures (60 bar of H2) within
1 h each time using an aqueous solution of BPhDS (Fig. 1)
in the absence of any transition metals in order to be sure
1 3

A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of…
Table 1 Effect of reaction time, platinum precursor, molar ratio of the ligand to platinum and of the nature of the ligand on the platinum-cata-
lyzed hydrogenation of levulinic acid (LA) into γ-valerolactone (GVL) in completely aqueous medium
Entry
Catalyst precursor
t (h)
L/Pt molar Conversion
Selectivity
TOFa (h−1
)
ratio
LA (mol%)
GVL (mol%)
1,4-PDO
(mol%)
VA (mol%)
1/1b,c
1/2b,c
1/3b
Na2PtCl6·6H2O/BPhDS
Na2PtCl6·6H2O/BPhDS
Na2PtCl6·6H2O/BPhDS
Na2PtCl6·6H2O/BPhDS
Na2PtCl6·6H2O/BPhDS
PtCl2/BPhDS
Pt(NH3)4Cl2·H2O/BPhDS
PtO2·H2O/BPhDS
Pt(acac)2/BPhDS
Na2PtCl6·6H2O/BPhDS
Na2PtCl6·6H2O/BPhDS
Na2PtCl6·6H2O/BPhDS
Na2PtCl6·6H2O/NTA·Na3
Na2PtCl6·6H2O/BPyDCA
Na2PtCl6·6H2O/BPhDS
Na2PtCl6·6H2O/EDTA·Na4
Na2PtCl6·6H2O/2,6-PyDCA
Na2PtCl6·6H2O/3-PSA
Na2PtCl6·6H2O/3,4-PyDCA
Na2PtCl6·6H2O/BQC
Na2PtCl6·6H2O/Taurine
Na2PtCl6·6H2O/BCDS
Na2PtCl6·6H2O/SANa
Na2PtCl6·6H2O/DTPA·Na5
Na2PtCl6·6H2O/TPPTS
Na2PtCl6·6H2O/NTPA
Na2PtCl6·6H2O/T2PyP
Na2PtCl6·6H2O/PTA
1
1.6
3
6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
3
4
2
1
1
1
2
2
2
1
2
1
2
1
2
2
1
2
50
75
100
100
13
13
9
8
3
7
5
99.6
99.5
99.3
99.2
99.2
98.0
96.5
97.4
92.4
95.5
96.0
94.2
97.4
99.6
98.5
98.07
98.2
98.92
98.7
96.54
98.6
98.5
96.1
98.3
94.2
94.7
94.9
95.5
0.4
0.5
0.7
0.8
0.8
1.0
3.5
1.3
3.8
3.0
2.0
2.9
2.6
0.4
1.5
0.87
1.8
0.48
0.7
1.11
1.4
1.0
1.8
1.0
3.2
2.6
2.5
1.5
–
–
–
–
–
1.0
–
1.3
3.8
1.5
2.0
2.9
–
151
136
100
50
63
52
44
38
13
33
1/4b,d
1/5c
1/6c
1/7c
1/8c
1/9c
1/10c
1/11c
1/12c
1/13e
1/14c,e
1/15c,e
1/16c,e
1/17e
1/18e
1/19e
1/20c,e
1/21e
1/22c,e
1/23e
1/24e
1/25e
1/26e
1/27e
1/28 c,e
25
17
3
96
92
91
76
74
56
55
54
51
28
22
20
10
3
287
277
273
227
222
167
163
162
154
83
67
61
31
8
–
–
1.06
–
0.6
0.6
2.35
–
0.5
2.1
0.7
2.6
2.7
2.6
3
3
2
8
5
Reaction conditions: T=140 °C; PH =50 bar; 2.81 mg (0.005 mmol) Na2PtCl6·6H2O; 0.2905 g (2.5 mmol) of LA (molar ratio of LA/Pt=500);
2
20 ml of deairated demineralised water; pH of the aqueous catalyst phase after the reaction=2.72–3.80; [Pt]=48 ppm; black precipitate pre-
sumed to be metallic platinum; stirring rate=880 rpm
aDefined as mole of GVL, 1,4-PDO and VA produced per mole of platinum per hour
b5.62 mg (0.01 mmol) Na2PtCl6·6H2O; 0.348 g (3 mmol) of LA (molar ratio of LA/Pt=300); [Pt]=96 ppm.
cMinor amount of metallic platinum
dPH =60 bar
2
eT=150 °C; PH =60 bar; 2.81 mg (0.005 mmol) Na2PtCl6·6H2O; 0.1742 g (1.5 mmol) of LA (molar ratio of LA/Pt=300)
2
and detector temperatures were set at 300 °C and 250 °C,
respectively. Ms quad: 150 °C, MS source: 230 °C. The
oven temperature was initially at 60 °C for 0 min and
then increased to 240 °C with a rate of 10 °C min−1. The
GC analyses were run on a Shimadzu GC-14B equipped
with a FID detector and a HP-Innowax capillary col-
umn (30 m × 0.251 mm i.d. × 0.50 µm film thickness)
which was purchased from Agilent Technologies. Car-
rier gas was N2 at 100 kPa. The oven temperature was
initially at 120 °C for 0 min and then increased to 240 °C
at 10 °C min−1. The injector and detector temperatures
both were set at 240 °C.
1 3

K. Krommyda et al.
Table 2 Effect of temperature, dihydrogen pressure and of the molar ratio of C=O units/Pt on the hydrogenation of levulinic acid (LA) into
γ-valerolactone (GVL) in completely aqueous medium
Entry Catalyst precursor
T (°C) PH (bar) LA/Pt molarratio Conversion Selectivity
TOFa (h−1
)
2
LA (mol%) GVL (mol%) 1,4-PDO VA (mol%)
(mol%)
2/1b
2/2b
2/3b
2/4
2/5
2/6
2/7
2/8
2/9
2/10
2/11
2/12
2/13
2/14
Na2PtCl6·6H2O/NTA·Na3 110
50
50
50
50
50
20
30
40
50
50
50
50
50
50
500
500
500
500
500
900
900
900
900
12
20
57
67
100
44
46
77
97
100
75
98.4
98.2
97.3
98.4
99.2
98.7
98.0
97.1
96.9
98.0
98.7
98.6
96.8
98.5
0.8
0.7
1.2
0.7
–
–
–
–
–
–
–
0.8
1.1
1.5
0.9
0.8
1.3
2.0
2.9
3.1
2.0
1.3
1.4
3.2
1.5
62
99
Na2PtCl6·6H2O/NTA·Na3 120
Na2PtCl6·6H2O/NTA·Na3 130
Na2PtCl6·6H2O/NTA·Na3 140
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
Na2PtCl6·6H2O/NTA·Na3 150
285
337
500
399
410
689
870
800
750
686
679
612
800
1000
1100
1200
1500
62
57
41
–
–
–
Reaction conditions: t=1 h; 2.81 mg (0.005 mmol) Na2PtCl6·6H2O; 2.74 mg (0.01 mmol) NTA·Na3 (molar ratio NTA·Na3/Pt=2); 0.2905 g
(2.5 mmol) of LA (molar ratio of LA/Pt=500); 20 ml of deaerated demineralised water; pH of the aqueous catalyst phase after the reac-
tion=2.94–3.86; [Pt]=48 ppm; black precipitate presumed to be metallic platinum; stirring rate=880 rpm
aDefined as mole of GVL, 1,4-PDO and VA produced per mole of platinum per hour
bMinor amount of metallic platinum
THF and water have shown that the presence of H-bonded
water molecules dramatically reduces the energetic span of
the reaction pathway thus enhancing the catalytic activity in
the aqueous medium [54]. Another study emphasized that
addition of water accelerated the rates in Ru/C-catalyzed
hydrogenation reactions of LA to GVL [94]. Moreover, with
ruthenium(0) nanoparticle catalysts the highest conversions
in LA hydrogenation reactions to GVL were observed in
the presence of the aqueous solvent whereas in organic
solvents such as 1.4-dioxane, THF, ethanol and methanol
significantly lower yields were obtained in this reaction
[27, 56]. Heterogeneous platinum based catalytic systems
have been usually applied under forcing conditions in the
hydrogenation of LA beyond GLV to produce valeric acid
(VA, Scheme 2) which in the presence of alcohols under
acidic conditions yields alkyl valerates well known as valeric
biofuels [57–60]. Lange et al. [57, 58] hydrogenated in the
gas-phase LA into GVL over Pt/TiO2 catalysts with a pro-
ductivity of 10 gGVL gcat−1 h−1 and selectivity to GVL over
95 mol% with negligible deactivation over 100 h at 200 °C
and 40 bar hydrogen. In the next gas-phase transformation
step GVL was hydrogenated over Pt/H-ZSM-5 catalysts into
3 Results and Discussion
3.1 Hydrogenation of LA into GVL Using
Water‑Soluble Platinum Catalysts Modified
with Nitrogen‑Containing Ligands in Aqueous
Media
Homogeneous transition metal catalytic systems applied
in the hydrogenation of LA which contains two functional
groups (–C=O and –CO2H) under mild reaction condi-
tions typical for homogeneous catalysis usually are capable
to hydrogenate selectively the more reactive functionality
among them which is the carbonyl moiety into the alco-
hol group to form the intermediate 4-hydroxyvaleric acid
which subsequently readily undergoes dehydration to yield
by highly favorable intramolecular esterification the C5
cyclic ester product namely GVL (Scheme 1). Due to the
high oxygen content and, therefore, the hydrophilicity of
levulinic acid the hydrogenation on its C=O group should be
carried out in aqueous media. In various studies [27, 54–56]
it is proved that the presence of the aqueous solvent has a
beneficial effect in LA hydrogenation reactions to GVL cata-
lyzed by heterogeneous and nanoparticles ruthenium based
catalytic systems. A joint experimental and theoretical study
with DFT calculations for the hydrogenation reaction of LA
to GVL catalyzed by heterogeneous ruthenium catalysts in
−1
VA with a productivity of 2 gVA gcat h−1 and selectivity
towards VA over 90 mol% at 250 °C and 10 bar H2 pres-
sure. This performance could be kept for more than 1500 h
with catalyst regeneration at 400 °C and 10 bar hydrogen
1 3

A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of…
1 3

K. Krommyda et al.
Table 4 Effect of molar ratio of AlCl3·6H2O/Pt on the
Na2PtCl6·6H2O/BPhDS/AlCl3·6H2O-catalyzed hydrogenation reac-
tion of levulinic acid (LA) into γ-valerolactone (GVL) in completely
aqueous medium and recycling experiments of the water-soluble cata-
lyst in aqueous/organic two-phase systems
Entry Catalyst precursor
L/Pt
LA/Pt
molar
ratio
Al/Pt
molar
ratio
Conversion Selectivity
TOFa (h−1
)
molar
ratio
LA (mol%) GVL (mol%) LA (mol%) GVL (mol%)
4/1
4/2
4/3
Na2PtCl6·6H2O/BPhDS/
1
1
2
3
3
3
4000
4000
4000
4000
1000
1000
17
23
89
99.1
100
0.9
–
–
910
3540
1670
1040
1000
1000
AlCl3·6H2O
Na2PtCl6·6H2O/BPhDS/
0.17
0.17
0.17
0.17
0.17
–
AlCl3·6H2O
Na2PtCl6·6H2O/BPhDS/
AlCl3·6H2O
42
98.6
99.7
100
0.9
0.3
–
0.5
–
4/4b Na2PtCl6·6H2O/BPhDS/
AlCl3·6H2O
26
4/5b Na2PtCl6·6H2O/BPhDS/
AlCl3·6H2O
100
100
–
4/6b,c Recycled catalyst
99.2
0.8
–
Reaction conditions: T=150 °C; PH =60 bar; t=1 h; 2.81 mg (0.005 mmol) Na2PtCl6·6H2O; 20 ml of deairated demineralised water; pH
2
of the aqueous catalyst phase after the reaction=2.59–3.0; [Pt]=44–47 ppm; black precipitate presumed to be metallic platinum; stirring
rate=880 rpm
aDefined as mole of GVL, 1,4-PDO and VA produced per mole of platinum per hour
bMinor amount of metallic platinum
cThe aqueous catalyst layer (20 ml) of entry 4/4, after extraction with 5 ml of diethyl ether and separation of the upper organic phase, was re-
used with addition of a new portion of 0.581 g (5 mmol C=O units) of LA
OH
OH
O
[Pt catalyst]
[H2O]
[Pt catalyst]
[H2O]
[Pt catalyst]
[H2O]
O
O
OH
O
O
OH
OH
H
+
H
+
H
+
2
2
2
O
.
H O
2
-
γ-Valerolactone (GVL)
1,4-Pentanediol (1,4-PDO)
Levulinic acid (LA)
H+
[H2O]
Η
O
[Pt catalyst]
[H2O]
O
H+
OH
OH
OH
H
2
+
O
O
O
Pentenoic acid
Valeric acid (VA)
Scheme 2 Hydrogenation of LA beyond GLV to produce 1,4-pentanediol (1,4-PDO) and valeric acid (VA) catalyzed by water-soluble platinum
catalysts modified with nitrogen-containing ligands in aqueous media
pressure. The final step is esterification of VA with alco-
hols to valeric acid esters by an acidic ion-exchange resin
catalyst. Lange et al. [57, 58] further reported the one-step
conversion of GVL into valeric esters over Pt or Pd/TiO2
catalysts with a selectivity of 20–50 mol% pentyl valerate
at 275–300 °C. The group of Shimizu [59] described the
one-pot hydrogenation of LA catalyzed by Pt/H-ZSM-5 at
200 °C and 8 bar H2 pressure to yield under solvent-free
conditions up to 99 mol% of VA and with methanol as sol-
vent changed the selectivity resulting to 87 mol% of methyl
valerate. Gu et al. [60] also synthesized Pt/H-ZSM-5 cata-
lyst and applied in the aqueous-phase hydrogenation of
LA at 200 °C and 10 bar hydrogen pressure to yield up to
91.4 mol% of VA.
We chose water-soluble nitrogen-containing ligands to
modify platinum catalysts and applied in the LA hydrogena-
tion reaction in aqueous media because nitrogen-containing
ligands have the great advantage that they are low price com-
pounds and because such catalytic systems have not been
applied until now in aqueous-phase catalytic conversions at
1 3

A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of…
all and because of the superior activity of palladium cata-
lysts modified with nitrogen-containing ligands compared
with TPPTS benchmark modifier in the hydrogenation reac-
tion of renewable polyunsaturated methyl esters of soybean
oil into their monounsaturated counterparts in aqueous/
organic two-phase micellar systems [61] and also because
of the higher catalytic activity exhibited ruthenium catalysts
modified with water-soluble nitrogen-containing ligands in
the aqueous-phase hydrogenation reaction of LA into GVL
compared with ruthenium catalysts modified with water-
soluble phosphines such as TPPTS and T2PyP (Fig. 1) in
aqueous media [44]. To our knowledge, the only example
of an aqueous-phase catalytic hydrogenation reaction using
water-soluble Pt/TPPTS catalysts is the selective hydrogena-
tion of polyunsaturated methyl esters of vegetable oils into
their monounsaturated counterparts with low formation
of undesired trans-isomers in aqueous/organic two-phase
systems [62]. Moreover, we chose water-soluble nitrogen-
containing ligands such as BPhDS, taurine etc. (Fig. 1) to
modify platinum catalysts for the hydrogenation of LA to
GVL because such catalytic systems are not susceptible to
air oxygen and palladium catalysts modified with nitrogen-
containing ligands such as BPhDS and taurine have been
applied even in aerobic oxidation reactions of terminal ole-
fins to methyl ketones in aqueous media [63, 64].
obtain reproducible results. All the following experiments
were carried out systematically without a prior prereduction
of the Pt/BPhDS catalyst. The conversion of LA increases
with increasing reaction time from 1 up to 6 h to give values
from 50 up to 100 mol% with very high selectivities towards
GVL from 99.2 up to 99.6 mol% in aqueous media (Table 1,
entries 1/1–1/4). Even at a longer reaction time of 6 h the
selectivity of GVL remained high (99.2 mol%) with a small
formation of the side product 1,4-pentanediol (1,4-PDO)
of only 0.8 mol% (entry 1/4) which is obtained from Pt/
BPhDS-catalyzed hydrogenation of GVL (Scheme 2). The
reaction pathway for the formation of 1,4-PDO from GVL
hydrogenation has been described by Geilen et al. [33] and
involves hydrogenation of the C=O bond of GVL to form
the cyclic hemiacetal compound which is in equilibrium
with the open 4-hydroxypentanal form followed by further
hydrogenation of the carbonyl functional group to yield 1,4-
PDO (Scheme 2). After the hydrogenation reaction with a
duration of 1 and 1.6 h a minor amount of metallic platinum
was observed indicating to a minor extent decomposition of
the Pt/BPhDS catalyst (entries 1/1, 1/2) whereas at longer
reaction times from 3 up to 6 h larger amounts of metallic
platinum were formed at the end of the reaction (entries
1/3, 1/4).
We have studied the influence of operating reaction
parameters such as the reaction time, nature of platinum
precursors, molar ratio of ligand/platinum, nature of ligands,
temperature, hydrogen pressure, the molar ratio of LA/Pt,
added aqueous solvent, salts, the pH value and the molar
ratio of AlCl3·6H2O/Pt on the hydrogenation reaction of LA
in order to achieve high catalytic activities and selectivi-
ties to GVL and performed recycling experiments to prove
the stability of the water-soluble platinum catalyst modified
with nitrogen-containing ligands and aluminum in aqueous
media.
3.1.2 Effect of Platinum Precursor and of BPhDS/Pt Molar
Ratio
The water-soluble Na2PtCl6·6H2O/BPhDS and
Pt(NH3)4Cl2·H2O/BPhDS catalyst precursors were syn-
thesized by dissolving under stirring Na2PtCl6·6H2O
or Pt(NH3)4Cl2·H2O with BPhDS under argon in 20 ml
deairated demineralized water within 10 min of time at room
temperature. After addition of LA to the aqueous catalyst
solution under stirring for further 5 min at room tempera-
ture the reaction mixture was than charged into an Autoclave
Engineers autoclave and was further proceeded as described
in Sect. 2.2. The synthesis of the catalyst precursors PtCl2/
BPhDS, PtO2·H2O/BPhDS and Pt(acac)2/BPhDS was car-
ried out with a different procedure due to the low solubility
of these platinum precursors in water at room temperature.
The autoclave was direct loaded under argon with the plati-
num precursor, BPhDS, deairated demineralized water and
LA and subsequently was closed. After a number of pres-
surising–depressurising cycles with hydrogen to remove last
traces of air oxygen under stirring, the autoclave was pres-
sured and contents were heated with stirring. The synthesis
of these water-soluble platinum catalyst precursors takes
place under the reaction conditions namely a temperature
of 140 °C and a hydrogen pressure of 50 bar. In the aqueous-
phase hydrogenation reaction of LA in the presence of water-
soluble platinum catalysts modified with BPhDS ligands
the catalytic activity of platinum precursors decreased in
3.1.1 Effect of Reaction Time
First, we investigated the influence of the reaction dura-
tion on the hydrogenation of LA catalyzed by water-soluble
Na2PtCl6·6H2O catalysts precursors modified with the nitro-
gen-containing ligand bathophenanthrolinedisulfonic acid
disodium salt (BPhDS, Fig. 1) at molar ratios of LA/Pt=300
and of ligand/platinum=1, a reaction temperature of 140 °C,
50 bar H2 pressure and addition of 20 ml of deairated dem-
ineralized aqueous solvent with a platinum concentration of
96 ppm in water in the absence of any added organic solvent
(Table 1, entries 1/1–1/4). The hydrogenation of LA started
immediately with hydrogen consumption without an induc-
tion period of the reaction. Therefore, no prereduction of
the Pt/BPhDS catalyst was carried out and only made sure
that the reaction conditions were well chosen in order to
1 3

K. Krommyda et al.
the order: Na2PtCl6·6H2O > PtCl2 > Pt(NH3)4Cl2·H2O > Pt
O2·H2O > Pt(acac)2 to give TOF values of 63 h−1, 52 h−1,
44 h−1, 38 h−1 and 13 h−1, respectively, at molar ratios of
LA/Pt= 500 and of ligand/platinum precursor= 1, a reac-
tion temperature of 140 °C, 50 bar H2 pressure and addition
of 20 ml of deairated demineralized aqueous solvent with
a platinum concentration of 48 ppm in water within 1 h of
reaction time (Table 1, entries 1/5–1/9). With the water-sol-
uble platinum catalysts precursors Na2PtCl6·6H2O/BPhDS
and Pt(NH3)4Cl2·H2O/BPhDS the selectivity towards GVL
was 99.2 and 96.5 mol%, respectively, with a formation of
only one side product namely 1,4-PDO with a selectivity of
0.8 mol% and 3.5 mol%, respectively (entries 1/5,1/7). In
contrast, using the water-soluble PtCl2/BPhDS, PtO2·H2O/
BPhDS and Pt(acac)2/BPhDS catalyst precursors the selec-
tivity of GVL was 92.4–98.0 mol% and of 1,4-PDO from
1.0 up to 3.8 mol% with, however, formation of a second
side product namely valeric acid (VA, Scheme 2) which was
obtained with a selectivity from 1.0 up to 3.8 mol% (entries
1/6, 1/8 and 1/9). According to the mechanism proposed by
the group of Dumesic [65, 66] protonation of GVL leads to
ring-opening of GVL through intermedites bearing carbe-
nium ions to produce pentenoic acid which is subsequently
hydrogenated to yield VA (Scheme 2). After the hydrogena-
tion reactions with all applied water-soluble platinum cata-
lyst precursors a minor amount of metallic platinum was
observed indicating to a minor extent decomposition of the
Pt/BPhDS catalysts (entries 1/5–1/9).
the best results were also obtained at a BPhDS/Pd molar
ratio of 1. The Pt/BPhDS-catalyzed hydrogenation reaction
of LA is highly selective to give at the low L/Pt molar ratio
of 1 a selectivity towards GVL of 99.2 mol% (entry 1/5) and
at the higher molar ratio of BPhDS/Pt=4 the selectivity to
GVL remained relative high (94.2 mol%) with a formation
of 2.9 mol% of both side products 1,4-PDO and VA (entry
1/12). After the hydrogenation reactions a minor amount of
metallic platinum formation was observed at the molar ratios
of BPhDS/Pt=1–4 indicating to a minor extent decomposi-
tion of the Pt/BPhDS catalysts (entries 1/5 and 1/10–1/12).
3.1.3 Effect of the Nature of Ligands
Table 1, entries 1/13–1/28 show the activity and selectiv-
ity of in-situ formed water-soluble platinum catalysts in the
hydrogenation of LA as a function of different phosphines,
P⁀N chelating and various nitrogen-containing ligands
(Fig. 1) in aqueous media. The highest catalytic activity
(TOF=287 h−1) with a conversion of LA of 96.0 mol% and
selectivities towards GVL of 97.4 mol% and 1,4-PDO of
2.6 mol% in the hydrogenation reaction of LA was achieved
with Na2PtCl6·6H2O precursors modified with water-solu-
ble nitrilotriacetic acid trisodium salt (NTA·Na3) ligands
(Fig. 1) at molar ratios of LA/Pt=300 and NTA·Na3/Pt=2,
a reaction temperature of 150 °C under 60 bar hydrogen
pressure within 1 h of reaction time and addition of 20 ml
of deairated demineralized water with a platinum concentra-
tion of 48 ppm in the aqueous phase (Table 1, entry 1/13). In
contrast, the catalytic activity exhibited the Na2PtCl6·6H2O
precursor modified with the TPPTS benchmark ligand
(Fig. 1) was much lower (TOF=31 h−1) with a conversion
of LA of 10.0 mol% and selectivity to GVL of 94.2 mol%,
to 1,4-PDO of 3.2 mol% and to VA of 2.6 mol% under the
same conditions in this hydrogenation reaction (Table 1,
entry 1/25). Catalytic activities in the range from 277 to
222 TOF’s per hour with high selectivities towards GVL
of 99.6–98.07 mol% were obtained with Na2PtCl6·6H2O
precursors modified with water-soluble 2,2′-bipyridine-
4,4′-dicarboxylic acid (BPyDCA, Fig. 1), bathophenan-
throlinedisulfonic acid disodium salt (BPhDS), ethylen-
ediaminetetraacetic acid tetrasodium salt (EDTA·Na4) and
2,6-pyridinedicarboxylic acid (2,6-PyDCA) ligands in the
hydrogenation of LA in aqueous medium (Table 1, entries
1/14–1/17). The catalytic hydrogenation activity exhib-
ited Na2PtCl6·6H2O precursors modified with the water-
soluble nitrogen-containing ligands 3-pyridinesulfonic
acid (3-PSA), 3,4-pyridinedicarboxylic acid (3,4-PyDCA),
2,2′-biquinoline-4,4′dicarboxylic acid dipotassium salt
(BQC), 2-aminoethanesulfonic acid (Taurine), bathocu-
proinedisulfonic acid disodium salt (BCDS), sulfanilic
acid sodium salt (SANa) and diethylenetriaminepentaacetic
acid pentasodium salt (DTPA·Na5) were in the range of
We further investigated the influence of the molar ratio
of L/Pt on the hydrogenation of LA catalyzed by water-sol-
uble Na2PtCl6·6H2O precursors modified with the nitrogen-
containing ligand BPhDS at a molar ratio of LA/Pt=500,
a reaction temperature of 140 °C, 50 bar H2 pressure and
addition of 20 ml of deairated demineralized aqueous solvent
with a platinum concentration of 48 ppm in water within
1 h of reaction time (Table 1, entries 1/5 and 1/10–1/12).
As expected, the highest catalytic activity with a turnover
frequency (TOF) of 63 h−1 was obtained at a low BPhDS/
Pt molar ratio of 1 (entry 1/5) whereas at increasing ligand/
platinum molar ratios the catalytic activity decreases to
give at a molar ratio of BPhDS/Pt=4 a TOF value of 17 h−1
(entry 1/12). This lower catalytic activity at higher BPhDS/
Pt molar ratios could probably be rationalized by assum-
ing that a competition between the free BPhDS ligand and
the C=O units of LA for a coordination site on platinum
takes place which may lead to a retardation in the activa-
tion of the LA hydrogenation reaction. Similarly, the group
of Joó [67] also observed a retarding effect of excess of
water-soluble TPPMS ligand added to ruthenium precursor
in Ru/TPPMS-catalyzed hydrogenation reactions in aque-
ous media. Moreover, the group of Sheldon [63, 64] has
shown in Pd/BPhDS-catalyzed aerobic oxidation reactions
of terminal olefins to methyl ketones in aqueous media that
1 3

A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of…
167 and 61 TOF’s per hour and the selectivities to GVL of
98.92–96.1 mol% (Table 1, entries 1/18–1/24) which are still
higher than the catalytic activity (TOF =31 h−1) exhibited
the Na2PtCl6·6H2O/TPPTS catalyst (Table 1, entry 1/25).
Using the water-soluble ligands nitrilotris(methylphosphonic
acid) (NTPA), tris(2-pyridyl)phosphine (T2PyP) and
1,3,5-triaza-7-phosphaadamantane (PTΑ) as modifiers of
Na2PtCl6·6H2O precursors the catalytic LA hydrogenation
activities obtained were very low from 8 to 5 TOF’s per
hour (Table 1, entries 1/26–1/28). After the hydrogena-
tion reaction of LA into GVL catalyzed by Na2PtCl6·6H2O
precursors modified with water-soluble BPyDCA, BPhDS,
EDTA·Na4, BQC, BCDS and PTA ligands a formation of a
minor amount of metallic platinum was observed indicat-
ing to a minor extent decomposition of the catalyst (entries
1/14–1/16, 1/20, 1/22 and 1/25) whereas in the presence
of the other water-soluble ligands more metallic platinum
was formed after the reaction. In general, in the aqueous-
phase hydrogenation reaction of LA the catalytic activities
exhibited Na2PtCl6·6H2O precursors modified with water-
soluble phosphines such as TPPTS and PTA were much
lower with 31 and 5 TOF’s per hour, respectively (Table 1,
entries 1/25 and 1/28) compared with the catalytic activities
exhibited with nitrogen-containing ligands such as NTA·Na3,
BPyDCA and BPhDS with 287, 277 and 273 TOF’s per
hour, respectively (entries 1/13–1/15). In order to explain
the higher catalytic activities exhibited water-soluble plati-
num catalysts modified with nitrogen-containing ligands
compared with their platinum phosphine counterparts in
the aqueous-phase hydrogenation reaction of LA to GVL
we consider the mechanistic study by Geilen et al. [68] for
the homogeneous LA hydrogenation reaction catalyzed by
[Ru(TriPhos)H]+ in ionic liquids. In this reaction mechanism
[68] hydride transfer from the Ru-H group of the catalyst to
the carbon atom of coordinated C=O functionality of the
starting material LA takes place followed by protonation of
the resulting Ru-OR moiety via σ-bond metathesis from a
nonclassical coordinated dihydrogen molecule, η2-Η2, regen-
erating at the same time the catalytically active classical
ruthenium hydride, namely the Ru-H functionality. Geilen
et al [68] mentioned that the heterolytic cleavage of the non-
classical coordinated dihydrogen molecule η2-Η2 and the
associated protonation of the Ru-OR group may be assisted
by basic centers or solvents which may reduce the energy
barrier for both the cleavage and protonation steps even fur-
ther. According to this mechanism [68] the results obtained
in the aqueous-phase platinum-catalyzed LA hydrogenation
reaction were rationalized by assuming that in the presence
of nitrogen-containing compounds which act both as ligands
to platinum and as bases their N-atoms facilitates both steps
the heterolytic cleavage of coordinated dihydrogen mol-
ecule η2-Η2 and the associated protonation of the Pt-OR
group to obtain GVL with higher reaction rates compared
with platinum catalysts modified with phosphines because
phosphines are weaker bases compared with their nitrogen-
containing counterparts. Moreover, in the aqueous-phase
platinum-catalyzed hydrogenation reaction of LA the nature
of the highly polar aqueous medium could also facilitate
both steps the heterolytic cleavage of coordinated dihydro-
gen molecule and the associated protonation of the Pt-OR
moiety to obtain GVL as described in the above mentioned
reaction mechanism [68].
3.1.4 Effect of Temperature, Hydrogen Pressure and Molar
Ratio LA/Pt
The catalytic activity in the Na2PtCl6·6H2O/NTA·Na3-cata-
lyzed hydrogenation of LA to GVL increases with increasing
temperature from 110 °C up to 150 °C to give TOFs from
62 up to 500 h−1 with conversions of LA from 12 up to
100 mol%, respectively, and high selectivities to GVL from
97.3 up to 99.2 mol% with 1,4-PDO formations of 0.7 up to
1.2 mol% and VA formations from 0.8 up to 1.5 mol% at a
molar ratio of LA/Pt=500 and NTA·Na3/Pt=2 under 50 bar
H2 pressure and a platinum concentration of 48 ppm in
water within 1 h reaction time in aqueous medium (Table 2,
entries 2/1–2/5). Using the Na2PtCl6·6H2O/NTA·Na3 cata-
lytic system after the hydrogenation reaction a formation
of a minor amount of metallic platinum was observed at
reaction temperatures from 110 up to 130 °C indicating to a
minor extent decomposition of the catalyst (entries 2/1–2/3)
whereas at higher temperatures of 140 °C and 150 °C more
metallic platinum was formed after the reaction (entries
2/4 and 2/5). The apparent Arrhenius parameter of the
activation energy of the Pt/NTA·Na3-catalyzed hydrogena-
tion reaction of LA in aqueous media was calculated from
results obtained from hydrogenation reactions carried out at
temperatures 110–150 °C and are summarized in Table 2,
entries 2/1–2/5. The apparent activation energy which was
calculated with these data (Fig. 2) amounts to a relative low
value of 73.04 kJ/mol when one considers that in the LA
hydrogenation reaction this catalyst reduces a less reactive
keto group into alcohol functionality.
The hydrogen pressure has a pronounced effect on the
activity in the Na2PtCl6·6H2O/NTA·Na3-catalyzed hydro-
genation reaction of LA and is shown in Table 2, entries
2/6–2/9. The catalytic activity increased from TOF=399 h−1
up to TOF = 870 h−1 and the selectivity towards GVL
decreased from 98.7 mol% down to 96.9 mol% with increas-
ing hydrogen partial pressure from 20 to 50 bar at a reaction
temperature of 150 °C, molar ratios of LA/Pt=500 and of
NTA·Na3/Pt=2 and a platinum concentration of 48 ppm in
water within 1 h reaction time in the aqueous monophasic
system. At the low hydrogen pressure of 20 bar the selectiv-
ity to VA was 1.3 mol% and with increasing pressure the
1 3

K. Krommyda et al.
Fig. 2 Calculation of the
apparent Arrhenius parameter
of the activation energy of the
hydrogenation reaction of LA
catalyzed by water-soluble Pt/
NTA·Na3 catalysts in aqueous
media
-1.8
0.00235
0.00245
0.00255
0.00265
-2.3
-2.8
-3.3
-3.8
-4.3
y = -8785.2x + 18.92
R² = 0.9481
1/T
selectivity to VA increased to give under 50 bar hydrogen
pressure 3.1 mol% of VA (entries 2/6–2/9).
intermediates and the associated protonation of the Pt-OR
moiety to obtain GVL which were suggested according to
the reaction mechanism of Geilen et al. [68] for the Ru-
catalyzed LA hydrogenation and were used to explain our
results and mentioned in detail in part Sect. 3.1.3.
The effect of LA/Pt molar ratio in the Na2PtCl6·6H2O/
NTA·Na3-catalyzed hydrogenation of LA to GVL in aqueous
media is shown in Table 2, entries 2/9–2/14. The catalytic
activity increased from TOF = 800 h−1 to TOF = 870 h−1
while the selectivity to GVL decreased from 98.0 mol% to
96.9 mol% with increasing molar ratios of LA/Pt from 800
to 900 at a reaction temperature of 150 °C, 50 bar hydrogen
pressure, a molar ratio of NTA·Na3/Pt=2 at a platinum con-
centration of 48 ppm in water within 1 h in the hydrogena-
tion of LA in aqueous media (Table 2, entries 2/10 and 2/9).
Raising the LA/Pt molar ratio higher has a negative effect on
the reaction rate in the aqueous-phase hydrogenation of LA.
Thus, the catalytic activity decreased from TOF=750 h−1
down to TOF=612 h−1 with increasing LA/Pt molar ratios
from 1000 up to 1500 (entries 2/11–2/14).
Addition of a salt with a strongly coordinating anion such
as NaI has a detrimental effect on the catalytic activity of
the Na2PtCl6·6H2O/NTA·Na3 system to obtain a TOF value
of 80 h−1 (Table 3, entry 3/3) compared with the high activ-
ity of TOF = 1338 h−1 exhibited by the Na2PtCl6·6H2O/
NTA·Na3 in the absence of the salt (entry 3/2) in the hydro-
genation reaction of LA at a reaction temperature of 150 °C
under 50 bar of hydrogen pressure and molar ratios of LA/
Pt = 2000, NTA·Na3/Pt = 2 and NaI/Pt = 100, addition of
40 ml of deairated demineralized water with a platinum con-
centration of only 12 ppm in water within 1 h of reaction
time. In the presence of NaCl, containing the same cation
but a less strongly coordinating anion compared to NaI, the
catalytic activity increased to obtain 528 TOF’s per hour in
the Na2PtCl6·6H2O/NTA·Na3-catalyzed hydrogenation of
LA in aqueous medium (entry 3/4). Compared to the low
catalytic activities (TOF values 80 and 528 h−1) exhibited
Na2PtCl6·6H2O/NTA·Na3 catalysts in the LA hydrogena-
tion in the presence of monovalent alkali metal salts NaI
and NaCl at molar ratios of salt/Pt= 100 (entries 3/3 and
3/4) the addition of the bivalent alkaline earth metal salt,
MgCl2·6H2O, at a molar ratio of MgCl2·6H2O/Pt=50, gives
rise to an increase in catalytic activity to reach 1036 TOF’s
per hour (entry 3/5).
3.1.5 Effect of Added Aqueous Solvent, Salts and of the pH
Value
The amount of added aqueous solvent has a pronounced
effect on the activity in the Na2PtCl6·6H2O/NTA·Na3-cata-
lyzed hydrogenation reaction of LA and is shown in Table 3,
entries 3/1 and 3/2. The catalytic activity increased from
TOF=816 h−1 to TOF=1338 h−1 with increasing amount
of added water as a solvent from 20 to 40 ml at a low plati-
num concentration of 24 and 12 ppm, respectively, a reac-
tion temperature of 150 °C, 50 bar hydrogen pressure, molar
ratios of LA/Pt = 2000 and of NTA·Na3/Pt = 2 within 1 h
reaction time in the aqueous monophasic system (entries
3/1 and 3/2). The effect of higher catalytic activity at higher
amount of added water could probably be explained by the
nature of the highly polar aqueous medium which could
facilitate both steps the heterolytic cleavage of coordinated
dihydrogen molecule on catalytically active platinum key
The hydrogenation of LA catalyzed by Na2PtCl6·6H2O/
NTA·Na3/AlCl3·6H2O systems is influenced by the pH value
of the aqueous phase (Table 3, entries 3/8–3/11). The high-
est catalytic activity (TOF=2007 h−1) was achieved under
acidic conditions at pH 2.52 in the absence of any added
buffer (entry 3/8) whereas with addition of NaH2PO4·H2O at
pH 2.70 the activity drops to 57 TOFs per hour (entry 3/9).
1 3

A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of…
This high decrease in catalytic activity from 2007 down to
57 TOF’s per hour could probably be explained with the
increased ionic strength of the solution due to the presence
of added NaH2PO4·H2O salt at a molar ratio of salt/Pt=100.
It is well known in the field of aqueous-phase catalysis that
addition of different salts as a means to increase the solu-
tion ionic strength gives rise to a dramatic drop in cata-
lytic activity of e.g. water-soluble Rh/TPPTS catalysts in
aqueous-phase hydroformylation reaction of olefins [69],
water-soluble Pd/TPPTS catalysts in aqueous-phase car-
bonylations reactions of 5-hydroxymethylfurfural (HMF)
[70–72] and of 1-(4-isobutylphenyl)ethanol to yield ibu-
profen [73, 74] and of water-soluble Pt/TPPTS, Ru/TPPTS
and Rh/TPPTS catalysts in aqueous-phase hydrogenation
reactions of polyunsaturated methyl esters of vegetable oils
into their monounsaturated counterparts and of benzene
into cyclohexane [62, 75–80]. The hydrogenation reaction
of LA catalyzed by Na2PtCl6·6H2O/NTA·Na3/AlCl3·6H2O
systems was carried out with addition of NaH2PO4·H2O salt
under acidic conditions at pH 2.70 (entry 3/9) in order to
be able to study the effect of pH value under similar condi-
tions because for the adjustment of pH 7.03 it is necessary
to add the NaH2PO4·H2O salt to create the NaH2PO4/NaOH-
buffer. The catalytic activity was slightly lower under neutral
conditions at pH 7.03 adjusted by a NaH2PO4/NaOH-buffer
(TOF=15 h−1, entry 3/10) and slightly increased under basic
conditions at pH 10.40 to give a TOF of 33 h−1 (entry 3/11).
in the aqueous medium (Table 4, entries 4/1 and 4/2). In
the Na2PtCl6·6H2O/BPhDS/AlCl3·6H2O-catalyzed hydro-
genation reaction of LA at 150 °C under 60 bar of hydro-
gen and molar ratios of LA/Pt = 4000, BPhDS/Pt = 1 and
AlCl3·6H2O/Pt=17, addition of 20 ml of deairated demin-
eralized water with a platinum concentration of 44 ppm in
water within 1 h of reaction time the catalytic activity was
TOF=910 h−1 (entry 4/1) whereas at the lower molar ratio
of AlCl3·6H2O/Pt = 0.17 an exceptionally high catalytic
activity was achieved which was as high as 3540 TOF’s
per hour (entry 4/2).
This remarkable effect of the enormous increase in cata-
lytic activity in water in the presence of aluminum in the
Na2PtCl6·6H2O/BPhDS-catalyzed LA hydrogenation could
probably be rationalized by assuming that the Lewis acid
aluminum strongly influences platinum catalytic active key
intermediates species in the way that it facilitates the step of
heterolytic cleavage of coordinated dihydrogen molecule, η2-
Η2, on platinum and also the protonation step of the Pt-OR
moiety to yield GVL in the way that aluminum may facili-
tates the formation of the classical platinum hydride, Pt-H,
functionality. These platinum catalytically active key inter-
mediates were suggested according to the reaction mecha-
nism of Geilen et al. [68] for the Ru-catalyzed LA hydro-
genation and were used to explain our results as mentioned
in detail in part Sect. 3.1.3. It may also be possible that
aluminum salts of levulinic acid are formed in the highly
polar aqueous medium which promotes their coordination
on platinum and also the formation of the above mentioned
platinum catalytic active key intermediate species and in
this way increase the rates in the LA hydrogenation reac-
tion. We cannot exclude, however, an alternative mechanism
involving the ability of aluminum to abstract an anionic
ligand such as Cl− from coordination sphere of platinum
complex rendering that complex cationic with formation of
a corresponding non-coordinating aluminate counter anion
and in this way enormously increases the rates in the LA
homogeneous hydrogenation reaction. It is well known in
the literature that the presence of the Lewis acid aluminum
could facilitate the abstraction of an anionic ligand from
coordination sphere of a transition metal catalytic complex
rendering that complex cationic with formation of a cor-
responding non-coordinating aluminate counter anion and
in this way considerably increases the rates in the homoge-
neous catalytic reaction. For example, Williams et al. [81]
applied highly efficient homogeneous Pd(PPh3)2(OAc)2
catalysts with Al(OTf)3 promotors instead of the classical
Brønsted acid cocatalysts in the methoxycarbonylation of
styrene and explained the high catalytic activity by assuming
that the Lewis acid Al(OTf)3 dissociates the acetate anionic
ligand from the coordination sphere of the palladium com-
plex making that complex cationic with formation of a non-
coordinating aluminate [(AcO)Al(OTf)3]− counter anion.
3.1.6 Effect of AlCl3·6H2O Addition and of Molar Ratio Al/Pt
on LA Hydrogenation Reaction
In the presence of the trivalent salt AlCl3·6H2O at a molar
ratio of AlCl3·6H2O/Pt = 33, the catalytic activity consid-
erably increased in the Na2PtCl6·6H2O/NTA·Na3-catalyzed
hydrogenation reaction of LA in water to reach a value of
1682 TOF’s per hour (entry 3/6). At a lower molar ratio
of AlCl3·6H2O/Pt = 17 the activity in the Na2PtCl6·6H2O/
NTA·Na3-catalyzed LA hydrogenation reaction further
increased to achieve a value of TOF=2000 h−1 (entry 3/7)
and at the higher molar ratio of LA/Pt = 3000 the activ-
ity in Na2PtCl6·6H2O/NTA·Na3/AlCl3·6H2O-catalyzed LA
hydrogenation reaction could be slightly increased up to
TOF = 2007 h−1 in the aqueous medium (entry 3/8). In
the presence of Na2PtCl6·6H2O/BPhDS catalysts after the
hydrogenation reaction of LA a minor amount of metal-
lic platinum was observed indicating to a minor extent
decomposition of the catalyst (entry 1/15) whereas with
Na2PtCl6·6H2O/NTA·Na3 catalysts more metallic platinum
was formed after the reaction indicating to a higher extent
decomposition of this catalyst (entry 1/13). Therefore, we
selected the more stable Na2PtCl6·6H2O/BPhDS catalyst
to investigate further the influence of the molar ratio of
AlCl3·6H2O/Pt at the higher molar ratio of LA/Pt = 4000
1 3

K. Krommyda et al.
It should be also mentioned that, the higher catalytic
activity exhibited Na2PtCl6·6H2O/NTA·Na3 catalysts in
the LA hydrogenation to GVL in water in the presence of
AlCl3·6H2O with TOF=1682 h−1 (entry 3/6) compared with
the activity of TOF=1036 h−1 (entry 3/5) in the presence
of MgCl2·6H2O could probably be explained because alu-
minum is a stronger Lewis acid compared to magnesium.
a reaction temperature of 150 °C was 20 min. At the reac-
tion temperature of 150 °C the hydrogen partial pressure
was 60 bar and the reaction time 1 h. After the reaction the
autoclave was cooled (within 20 min) to room temperature,
vented of hydrogen and the aqueous reaction mixture was
removed. A sample of 100 mg was taken from the aque-
ous reaction mixture and was analyzed by gas chromatog-
raphy (GC) after addition of N,N-dimethylformamide as
standard (entry 4/5). Subsequently, 5 ml of diethyl ether
were added to the 20 ml of the aqueous reaction mixture
to create a biphasic system. The Pt/BPhDS/Al catalyst
was easily recovered from reaction products by intensive
extraction using a separatory funnel being shaken very
well by hands for 5 min and a simple phase separation
of the lower aqueous red coloured layer from the upper
organic product uncoloured layer. The lower aqueous layer
containing the Pt/BPhDS/Al catalyst after addition of a
new portion of 0.581 g (5 mmol) of LA (molar ratio of
LA/Pt = 1000) was re-used under the same hydrogena-
tion reaction conditions. After the recycling hydrogena-
tion experiment a minor amount of metallic platinum was
observed indicating to a minor extent decomposition of
the catalyst and the pH value of the aqueous catalyst solu-
tion was 3.0. The catalytic activity remained unchanged
high (TOF = 1000 h−1) using the recycled Pt/BPhDS/Al
catalyst in the aqueous-phase hydrogenation experiment of
entry 4/6 compared with the first hydrogenation reaction
of LA of entry 4/5 catalyzed by the same Pt/BPhDS/Al
system (TOF = 1000 h−1). These results have shown that
the platinum catalyst modified with the nitrogen-contain-
ing bidentate ligand BPhDS with aluminium promotors
is stable without loss of activity and selectivity which is
remarkable when one considers that reaction parameters
such as the temperature of 150 °C and the acidic aqueous
medium (pH = 3.0) are rather demanding conditions for a
homogeneous catalytic hydrogenation reaction.
3.1.7 Recycling Experiments of the Pt/BPhDS/Al Catalyst
In the aqueous-phase catalytic hydrogenation reaction of
LA in the presence of Na2PtCl6·6H2O/BPhDS/AlCl3·6H2O
catalysts at the molar ratio of BPhDS/Pt=1 after the reac-
tion an amount of metallic platinum was formed indicat-
ing to an extent decomposition of the catalyst (entry 4/2).
Therefore, we carried out hydrogenation reactions of LA
at higher BPhDS/Pt molar ratios in order to increase the
stability of the Na2PtCl6·6H2O/BPhDS/AlCl3·6H2O catalyst
in the polar aqueous medium. At a molar ratio of BPhDS/
Pt=2 the catalytic activity was TOF=1670 h−1 with some
decomposition of the catalyst after the reaction (entry 4/3)
whereas at the higher molar ratio of BPhDS/Pt=3 the cata-
lytic activity was lower (TOF = 1040 h−1) with, however,
a minor amount of metallic platinum formation after the
reaction indicating a higher stability of the catalyst under
these reaction conditions in the aqueous reaction mixture
(entry 4/4). Therefore, the recycling experiments of the
Na2PtCl6·6H2O/BPhDS/AlCl3·6H2O catalyst were carried
out at a molar ratio of BPhDS/Pt=3 due to the higher stabil-
ity of the in-situ formed water-soluble platinum/bidentate
ligand/aluminum catalyst in the aqueous medium.
A recycling experiment with a consecutive run were
carried out in the hydrogenation of LA in order to get
more information for the stability of Pt/BPhDS/Al cat-
alysts in the aqueous medium (Table 4, entries 4/5 and
4/6). 2.81 mg (0.005 mmol) Na2PtCl6·6H2O, 8.58 mg
(0.015 mmol) BPhDS (molar ratio BPhDS/Pt = 3) and
0.21 mg (0.00085 mmol) AlCl3·6H2O (molar ratio
AlCl3·6H2O/Pt = 0.17) were dissolved under argon in
20 ml of deairated demineralized H2O and the mixture
was stirred for about 10 min at room temperature to give
a clear, red coloured, solution. After addition of 0.581 g
(5 mmol) LA to the aqueous catalyst solution under stir-
ring for further 5 min the aqueous reaction mixture, having
a ruthenium concentration of 47 ppm, was charged into
an Autoclave Engineers autoclave of a nominal volume
of 100 ml which was previously evacuated and filled with
argon. In the reaction mixture the molar ratio of LA/Pt
was 1000. After a number of pressurizing-depressurizing
cycles with hydrogen to remove last traces of air oxygen,
the autoclave was pressured and contents were heated with
stirring (stirring rate=880 rpm). The heating time to reach
It is relevant to point out, that after the hydrogenation
reaction it is necessary to add low amounts such as 5 ml
of diethyl ether to 20 ml of the aqueous catalytic reaction
mixture to create a biphasic system in order to extract the
reaction products in the upper organic layer from the lower
aqueous catalyst layer for the recovery and recycling of the
Pt/BPhDS/Al catalyst. If higher amounts such as 10 ml of
diethyl ether were added for the extraction of 20 ml aque-
ous reaction mixture the catalytic activity dramatically
decreases in the consecutive LA hydrogenation run mainly
due to the diethyl ether and to a lesser extent of the GVL
and 1,4-PDO products [LA was not present because of the
quantitative conversion in the first run (entry 4/5)] which
are inevitably dissolved in small amounts in the highly
polar aqueous catalyst solution which gives rise to a drop
in the polarity of the LA hydrogenation reaction mixture.
1 3

A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of…
11. Serrano-Ruiz JC, Luque R, Sepúlveda-Escribano A (2011)
4 Conclusions
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15. Bozell JJ (2010) Science 329:522
The work presented in this paper demonstrated again the
high potential of green aqueous-phase catalysis through a
new application in the field of utilization of levulinic acid
(LA) a renewable biomass based polar platform chemical
of central relevance in the development of biorefineries of
the future in a sustainable way. We have disclosed a novel
hydrogenation reaction of LA into γ-valerolactone (GVL)
catalyzed by water-soluble platinum catalysts modified
with various nitrogen-containing and phosphine ligands in
environmentally attractive, green, aqueous monophasic sys-
tems. Interestingly, the presence of the Lewis acid aluminum
enormously increases the catalytic activity of water-soluble
platinum catalysts modified with nitrogen-containing ligands
in the LA hydrogenation reaction and high catalytic activi-
ties up to 3540 TOF’s per hour with a quantitative selectiv-
ity towards GVL have been achieved using Na2PtCl6·6H2O
catalysts precursors modified with the bidentate bathophen-
anthrolinedisulfonic acid disodium salt (BPhDS) ligand
and low amounts of AlCl3·6H2O promotors (molar ratio
of AlCl3·6H2O/Pt = 0.17) in aqueous media. A recycling
experiment of the Pt/BPhDS/Al catalyst from the aqueous
monophasic LA hydrogenation reaction mixture followed by
biphasic recovery of the catalyst in active form from organic
reaction products by extraction and simple phase separation
of an aqueous/organic two-phase system formed after addi-
tion of diethyl ether has shown that the catalyst is stable
without loss of activity and selectivity in a consecutive run.
Hence, we are currently investigating the scope of this useful
catalytic hydrogenation reaction.
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Acknowledgements Financial support of this research by the Post-
graduate Studies Programme on “Catalysis and its Applications in the
Industry” of Greek Ministry of Education and by the Special Account
for Research Grants of the Research Committee of the National and
Kapodistrian University of Athens under contract 70/4/7568 are grate-
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Affiliations
Kalliopi Krommyda1 · Christina Panopoulou1 · Chrysavgi Moustani1 · Eleni Anagnostopoulou1 ·
Konstantina Makripidi1 · Georgios Papadogianakis1
1
* Georgios Papadogianakis
Industrial Chemistry Laboratory, Department of Chemistry,
papadogianakis@chem.uoa.gr
National and Kapodistrian University of Athens,
Panepistimiopolis-Zografou, 157 71 Athens, Greece
1 3