Hydrogenative cyclization of levulinic acid into γ-valerolactone by photocatalytic intermolecular hydrogen transfer

Article abstract of DOI:10.1039/c5gc02971f

The hydrogenative cyclization of levulinic acid (LA) into γ-valerolactone (GVL) is an attractive route toward the use of renewable bio-sources but it normally suffers from the consumption of H₂. In this study, we report that an intermolecular hydrogen transfer from isopropanol to LA can be realized efficiently under photocatalytic conditions over gold-loaded TiO₂ catalysts. In this manner, isopropanol is dehydrogenated as acetone and pinacol with the total selectivity of >99%, whereas LA is hydrogenated and cyclized as GVL with the selectivity of up to 85%. In this reaction process, the production of GVL is mediated with hydrogenated dehydration of LA into an acetyl propionyl radical, which is further hydrogenated and cyclized as GVL. This hydrogenation-dehydrogenation coupling process provides an atom-economical green way for the conversion of LA into GVL.

Full text of DOI:10.1039/c5gc02971f

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Hydrogenative cyclization of levulinic acid into
γ-valerolactone by photocatalytic intermolecular
hydrogen transfer†
Cite this: DOI: 10.1039/c5gc02971f
Received 13th December 2015,
Accepted 12th January 2016
Hongxia Zhang,*a,b Min Zhao,a,b Tianjian Zhao,a,b Li Lib and Zhenping Zhu*b
DOI: 10.1039/c5gc02971f
www.rsc.org/greenchem
The hydrogenative cyclization of levulinic acid (LA) into γ-valero- direct hydrogenative cyclization under noble-metal-based
lactone (GVL) is an attractive route toward the use of renewable thermo-catalysis conditions with a H2 molecule as the hydro-
bio-sources but it normally suffers from the consumption of H2. In gen source.1,13–21 Although H2-fed catalytic conversion is
this study, we report that an intermolecular hydrogen transfer highly efficient and able to obtain high GVL yield and selecti-
from isopropanol to LA can be realized efficiently under photo- vity, it requires significant H2 consumption. Given that H2
catalytic conditions over gold-loaded TiO2 catalysts. In this generation relies on fossil fuels, particularly coal, H2 consump-
manner, isopropanol is dehydrogenated as acetone and pinacol tion will make the thermo-catalytic process unsustainable.22
with the total selectivity of >99%, whereas LA is hydrogenated and Therefore, alternative routes should be explored for the green
cyclized as GVL with the selectivity of up to 85%. In this reaction conversion of LA into GVL.23,24
process, the production of GVL is mediated with hydrogenated
Photocatalysis has been extensively used in diverse appli-
dehydration of LA into an acetyl propionyl radical, which is further cations, such as water splitting, organic synthesis and polluted
hydrogenated and cyclized as GVL. This hydrogenation–dehydro- effluent recovery25–27 and some important progress has been
genation coupling process provides an atom-economical green achieved in chemical synthesis.28–31 Numerous studies have
way for the conversion of LA into GVL.
proven that various organic reactions can selectively operate
under light irradiation with help from semiconductor or mole-
cular photocatalysts; this finding shows the great potential of
using renewable solar energy to drive chemical synthesis.28–33
We have recently found that under NaTaO3-supported photo-
catalysis conditions, intermolecular hydrogen transfer from iso-
propanol to acetone can be achieved efficiently, thus leading
to selective production of pinacol.34 In this process, both the
hole-induced oxidation half-reaction and electron-induced
reduction half-reaction are effectively utilized, thus increasing
the efficiency and atomic-economy of the total reaction.
Inspired by this strategy, we attempted to convert LA to GVL by
Levulinic acid (LA) is one of the most important biomass plat-
form molecules because it can be readily obtained from ligno-
cellulosic biomass by simple hydrolysis processes.1–3 The
variety of functional groups in LA endows this molecule with
versatile chemical properties. Thus, the rational conversion of
LA to downstream chemical chains has drawn significant
attention for future sustainable energy and chemical indus-
tries.1,2,4 For example, the selective hydrogenation of LA can
generate a variety of value-added chemicals, such as γ-valero-
lactone (GVL), diphenolic acid, 2-methyl-tetrahydrofuran, and
1,4-pentanediol.1,5,6 As a stable and liquid chemical, GVL exhi-
bits remarkable potential to act as a green solvent, food addi-
tive and an organic intermediate in the synthesis of fine
chemicals.7–9 GVL can also be used as a promising fuel addi-
tive to replace ethanol in gasoline–ethanol blends or as a pre-
cursor for diesel production.10–12
a
light-driven dehydrogenation–hydrogenation coupling
process without external hydrogen consumption. In this
process, the hydrogen required for LA hydrogenation is directly
fed from isopropanol dehydrogenation, which simultaneously
produces usable acetone and pinacol selectively (eqn (1) and
Scheme 1).
The conversion of LA into GVL has been extensively studied
in recent years.1–3,13,14 This process is normally realized by
ð1Þ
aInstitute of Application Chemistry, Shanxi University, Taiyuan 030006, China.
E-mail: hxzhang@sxu.edu.cn
bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, China. E-mail: zpzhu@sxicc.ac.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5gc02971f
The experiments for the coupling of LA hydrogenative cycli-
zation and isopropanol dehydrogenation were performed in a
tubular quartz reactor (20 ml) and under an argon atmo-
sphere, irradiated by using a high-pressure Hg-lamp (300 W) at
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Scheme 1 Schematic illustration of the TiO2-photocatalytic hydrogen-
ation–dehydrogenation coupling reaction for LA/isopropanol.
room temperature. First, we irradiated a mixture of LA and
isopropanol (1.0 mol L−1 for LA), without any catalyst. After
9 h irradiation, only a small amount (3.6%) of LA was
converted (Table 1, entry 1). Product analysis showed the
production of GVL but in a trace quantity (approximately 0.5%
selectivity). LA dominantly behaves in a photolysis process and
generates propionic acid (PA) and acetic acid (HOAc).35
Furthermore, isopropyl levulinate (IL) was also detected in
small amounts. In the presence of a platinum-loaded TiO2
photocatalyst (Pt/P25-TiO2) prepared from commercial TiO2
(Degussa P25) and an in situ platinum deposition,26 LA conver-
sion sharply increases to 54% and GVL is readily produced at a
selectivity of 55% (Table 1, entry 2) with low formation of
photolysis products. Considering the failed reaction under
dark conditions (Table 1, entry 3), this result suggests that Pt/
P25-TiO2 has a significant photocatalytic function in the
hydrogenative cyclization of LA. In this case, one new product
is formed from LA. As analyzed by gas chromatography-mass
spectrometry (GC-MS) and related ion fragments (Fig. 1a), this
new product can be well assigned to 6-hydroxy-6-methyl-
heptane-2,5-dione (HMHD) (for assignment details see the
ESI†). HMHD can be viewed as the geometric isomer of IL and
is formed from a unique LA-isopropanol intermolecular de-
hydration process, wherein the α-H in isopropanol participates
in the dehydration and leads to C–C construction (unlike in esteri-
zation reaction). This dehydration process seems to be more
Fig. 1 (a) Mass spectrum of HMHD. (b) Product generation during the
photocatalytic process in the LA/isopropanol. Reaction conditions: solu-
tion, 20 ml 1 M LA in isopropanol; catalyst, 0.05 g P25-TiO2 loaded
0.5 wt% platinum; atmosphere, argon; temperature, 20 °C; light source,
300 W high-pressure Hg-lamp.
favorable than the esterization reaction because HMHD forms at
a considerably high selectivity (Table 1, entry 2, Fig. 1b), as esti-
mated from the peak-area analysis of gas chromatography (GC).
During the hydrogenative cyclization of LA, isopropanol is
majorly dehydrogenated as acetone with a selectivity of
Table 1 Experimental data of the TiO2-photocatalytic hydrogenated cyclization for LA into GVLa
Sel. (%) of LA
Sel. (%) of isopropanol
H2 amount
(mmol)
Entry
Catalyst
LA Conv. (%)
GVL
HMHDb
ILc
PA + HOAc
Pinacol
Acetone
1
2
3
4
5
6
7
8
9
None
3.6
54
0.0
47
56
69
71
79
5.6
0.5
55.2
ND
48.1
59.2
69.5
76.4
85.3
2.5
NDd
33.7
ND
40.8
31.4
21.9
17.4
8.2
1.8
2.3
ND
2.1
2.3
2.0
1.9
2.0
2.0
97.0
6.8
ND
7.1
6.4
4.5
4.5
3.1
89.2
ND
10.8
ND
11.6
13.2
11.4
12.9
10.3
9.8
ND
88.2
ND
87.3
86.8
88.6
87.1
89.7
90.2
ND
10.3
ND
2.6
2.9
10.9
9.8
Pt/P25-TiO2e
Pt/P25-TiO2
Pt/NT-TiO2
Pt/R-TiO2
Pt/RA-TiO2
Pd/RA-TiO2
Au/RA-TiO2
RA-TiO2
4.6
0.1
ND
a Reaction conditions: LA, 0.02 mol; isopropanol, 20 ml; TiO2, 0.05 g; noble metal co-catalyst, 0.5 wt%; atmosphere, argon; temperature, 20 °C;
light source, 300 W high-pressure Hg-lamp; irradiation time, 9 h. b HMHD is 6-hydroxy-6-methylheptane-2,5-dione. c IL is isopropyl levulinate.
d ND, not detected. e Without light irradiation.
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approximately 88% (Table 1, entry 2, Fig. 1b). Isopropanol also was prepared by a hydrothermal process by using P25-TiO2 as
behaves as a dehydrogenation C–C coupling process34 and pro- the starting material.36 The TEM observations (Fig. S4†)
duces pinacol with a selectivity of approximately 10%, follow- showed that the particle size is approximately 20–30 nm for
ing a mechanism mediated with the formation of a hydroxyl RA-TiO2 and approximately 100 nm for R-TiO2. The diameter
isopropyl ((CH3)2COH) radical (see the ESI†). These data indi- of NT-TiO2 is approximately 8 nm and the length is approxi-
cate that intermolecular hydrogen transfer is successfully rea- mately 150 nm. Catalysis tests showed that different phase
lized in the photocatalytic function of Pt/P25-TiO2. In this structure catalysts exhibited different activities (Fig. 2b and
process, a part of the hydrogen atoms derived from iso- Table 1, entries 4–6). Pure phase NT-TiO2 and R-TiO2 catalysts
propanol dehydrogenation is converted into molecular H2 and show conversion and selectivity lower than P25-TiO2, whereas
it competes with the hydrogenative cyclization of LA.
the mixed structure Pt/RA-TiO2 exhibits better LA conversion
To investigate the effect of the photocatalyst structure on (69%) and GVL selectivity (66%) than P25-TiO2 because of the
the hydrogenation–dehydrogenation coupling process, we facilitation of the mixed phase-resulted heterojunctions for
modulated the TiO2 structure and inspected the effect of the electron and hole separation.26
TiO2 structure on catalyst performance. A commercial TiO2
A co-catalyst is necessary to promote LA hydrogenative cycli-
powder (Degussa P25) was calcined at 600 °C to obtain a zation, as witnessed by the performance of bare RA-TiO2,
mixed rutile and anatase structure TiO2 (RA-TiO2), and at which exhibits low activity (Table 1, entry 9). Although plati-
800 °C to obtain a rutile phase structure TiO2 (R-TiO2) num displays a significant catalytic activity for LA hydrogena-
(Fig. 2a). A TiO2 nanotube (NT-TiO2) with the anatase phase tive cyclization, it has a high work function and can greatly
facilitate H2 generation from H+,26,37 which is theoretically
competitive to LA hydrogenation. To suppress H2 generation,
we inspected the performance of palladium and gold co-cata-
lysts that exhibit low work functions.26,37 The results showed
that such catalysts can significantly reduce H2 generation
while improving LA hydrogenative cyclization (Table 1, entries
6–8, Fig. 3a). Au exhibited the highest LA conversion of 79%
and GVL selectivity of 85%. The calculated rate of GVL pro-
duction is 29.8 mmol GVL per gram catalyst per hour, which is
comparable to the rates obtained from the previous H2-fed
15
thermocatalysis with Cu/ZrO2
and Ru/C catalysts18,19 (see
details in Table S1†). If only active metal (Au) is considered,
the specific rate of GVL production is 5968.9 mmol g−1 h−1
,
about 5 and 120 times higher than Ru and Cu, respectively. In
addition, compared to the previous thermocatalytic process
that was operated at high temperature (>100 °C) and high
pressure, the present photocatalytic process runs at normal
pressure and temperature, which is beneficial to a system
operation.
On the basis of the molar quantity of H2 and GVL obtained
from RA-TiO2-based catalysts, we inspected the effect of the
metal co-catalyst on the efficiency (η) of hydrogen transfer
from isopropanol to LA by using the following equation: η =
{MGVL/(MH2 + MGVL)} × 100%. η is 46%, 52%, and 74% for Pt,
Pd, and Au, respectively. The results show that gold is a more
suitable co-catalyst for LA hydrogenative cyclization, benefiting
from its low work function and catalytic activity for the un-
desired H+-to-H2 reduction. This characteristic of gold allows
for suppression of the H+-to-H2 reaction and consequently
encourages the interaction between H+ and LA and thus the
photoexcited electron-induced hydrogenative cyclization of LA.
As described above, LA also exhibits a photolysis behavior
and produces PA and HOAc, likely following a water-facilitated
mechanim (see the ESI†) because the hydrogenative cyclization
of LA can by-produce water stoichiometrically. This reaction is
theoretically competitive to the hydrogenative cyclization of
LA. On the basis of all data obtained, we observed that with
the increasing LA conversion rate, GVL production steadily
Fig. 2 (a) XRD pattern of catalysts; (b) catalyst effect on hydrogenated
product GVL during the photocatalytic process in the LA/isopropanol.
Conditions: solution, 20 ml 1 M LA in isopropanol; 0.05 g catalyst loaded
0.5 wt% platinum; atmosphere, argon; temperature, 20 °C; light source,
300 W high-pressure Hg-lamp.
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(approximately 2% selectivity). This situation is likely due to
the subsequent hydrogenation conversion of IL. This result
was verified by employing ethyl levulinate as the starting
reactant to perform the photocatalytic hydrogen transfer reac-
tion (Fig. S5†). Ethyl levulinate can be readily hydrogenated
and cyclized into GVL with a selectivity of approximately 75%.
To understand the photocatalytic reaction mechanism, we
monitored the radicals formed during photocatalytic processes
via a radical capture technique and in situ analysis by the elec-
tron paramagnetic resonance spectrum (EPR). Fig. 4a shows
the EPR spectrum obtained from the Pt/RA-TiO2-catalytic de-
hydrogenation of isopropanol in the absence of LA by employing
5,5-dimethyl-1-pyrroline N-oxide (DMPO, 0.05 M) as a radical
capture molecule. A simulated analysis of the spectrum by
using Easyspin software38 reveals that both isopropoxy
(CH3)2CHO and hydroxyl isopropyl (CH3)2COH radicals are
produced as intermediates with relative contents of approxi-
mately 70% and 30%; this analysis agrees with previous
EPR observations for the photocatalytic alcohol reaction over
TiO2-based catalysts.39–41 Acetone formation is believed to
originate from the further dehydrogenation of radicals
whereas pinacol formation is caused by the C–C coupling
reaction of (CH3)2COH radicals.34
Fig. 4b shows the EPR spectrum obtained from the LA-iso-
propanol hydrogen transfer reaction. This EPR spectrum is
different from the spectrum for the sole isopropanol system,
with a significant shift in some resonance signals, thus
suggesting a change in the electron spin coupling constant
and the formation of new carbon-related radical intermediates.
Although the identification of the new radicals is difficult, they
Fig. 3 (a) H2 (hollow) and GVL (solid) generation with different co-cata-
lysts. (b) The changes of GVL, HMHD and LA generation with the reac-
tion rate of LA during the photocatalytic process in LA/isopropanol.
Conditions: solution, 20 ml 1 M LA/ethyl levulinate in isopropanol;
0.05 g RA-TiO2 loaded 0.5 wt% co-catalyst; atmosphere, argon; temp-
erature, 20 °C; light source, 300 W high-pressure Hg-lamp.
increases and photolysis products nearly remain a constant
quantity (Fig. 3b). This result suggests that although photolysis
is difficult to avoid, it can be suppressed to a relatively low
level by improving the hydrogen transfer efficiency. Further-
more, the intermolecular dehydration between LA and iso-
propanol to produce IL and HMHD also seems to be
competitive with target hydrogenative cyclization. For HMHD,
its formation linearly decreases with the increasing LA conver-
sion rate (Fig. 3b). HMHD selectivity can be greatly reduced
from 40.8% for Pt/NT-TiO2 and 21.9% for Pt/RA-TiO2 to 8% for
Au/RA-TiO2 (Table 1), thus suggesting that an efficient hydro-
gen transfer can effectively suppress HMHD formation. For IL
formation, it should theoretically be favorable in the present
photocatalysis environment because H+ ions that are intrinsi-
cally formed from hole-induced dehydrogenation of iso-
propanol32 will act as an acid catalyst for an esterization
reaction. However, IL was actually detected in small quantities
Fig. 4 (a) Measured EPR spectrum in neat isopropanol and its simulated
EPR data with 30% [DMPO-(CH3)2C(OH)]•(aN = 15.03, aH = 22.67, g =
2.0057) marked as * and 70% [DMPO-(CH3)2CHO]• (aN = 13.2, aH = 8.4,
○
aH = 0.8, g = 2.0057) marked as ; (b) EPR spectrum in LA + isopropanol
(0.1 M LA). EPR are obtained upon irradiation of Pt/RA-TiO2 suspensions
in the presence of a spin trapping agent DMPO (DMPO = 0.05 M).
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are assumed to originate from the LA reaction. According to erature. The resulting precipitate was washed with deionized
the formation of HMHD in a considerable quantity, HMHD is water, and ion exchanged with 0.1 mol L−1 hydrochloric acid
formed via a C–C coupling reaction between a (CH3)2COH until a pH value near 8 was reached, and then treated at
radical and an acetyl propionyl radical (following eqn (2)):
300 °C for 5 h to obtain TiO2 nanotubes (NT-TiO2). Noble
metal co-catalysts were loaded onto the catalyst by in situ
photodeposition.26 H2PtCl6·6H2O, PdCl2 and HAuCl4·3H2O
were used as precursors.
ð2Þ
The phase structure was characterized by X-ray diffraction
on a D8 Advance power X-ray diffractometer with Cu Kα (λ =
0.15406 nm) radiation. The morphology and microstructure of
samples were examined using a transmission electron micro-
scope (JEM-2010).
The EPR studies were carried out in a flat quartz cell, and
the spectra were recorded on a Bruker EMXPlus-10/12 EPR
spectrometer. Two samples were prepared: isopropanol solu-
Acetyl propionyl radicals can be formed by the hydrogen-
ation dehydration of LA. On the basis of these observations
and deductions, we propose that the photocatalytic hydrogena-
tive cyclization of LA is mediated with the formation of acetyl
propionyl radicals, which is further hydrogenated and cyclized
as GVL, as expressed in eqn (3):
tion containing 0.05
M 5,5-dimethyl-1-pyrroline N-oxide
(DMPO) and 2.5 mg ml−1 TiO2 with 0.5 wt% platinum; isopro-
panol solution containing 1 M LA, 0.05 M DMPO and 2.5 mg
ml−1 TiO2 with 0.5 wt% platinum. The samples were firstly
sonicated for 5 min in an ultrasonic bath, and then were care-
fully deoxygenated by a stream of argon, subsequently filled in
the quartz flat cell. During the EPR photochemical experi-
ments the samples were irradiated at 298 K directly in the EPR
resonator, and the EPR spectra were recorded in situ during a
continuous photoexcitation. The operating conditions were as
follows: microwave frequency ν = 9.86 GHz, power of microwave
W = 10.02 mW, and scanning range 3416–3616 G. The
irradiation of the sample in the EPR cavity was carried out
using a 100 W UV lamp. The g-values ( 0.0001) were determined
using a built-in magnetometer. The EPR spectra so obtained
were analyzed and simulated using the Easyspin software.38
ð3Þ
This proposal can well explain the other data observed,
such as the profiles of GVL and HMHD production (Fig. 3b).
The GVL and HMHD formations are intrinsically competitive
because both of them are derived from the same intermediate;
thus, HMHD formation must be suppressed when GVL for-
mation is promoted by a facilitated hydrogen transfer.
Conclusions
This study is the first to report that an intermolecular hydro-
gen transfer from isopropanol to LA can be realized efficiently
under photocatalytic conditions over gold-loaded TiO2 cata-
lysts. In this manner, isopropanol is dehydrogenated as
acetone and pinacol with the total selectivity of >99%, whereas
LA is hydrogenated and cyclized as GVL with the selectivity of
up to 85% and the hydrogen transfer efficiency (η) up to 74%.
This hydrogenation–dehydrogenation coupling process provides
an atom-economical green way for the conversion of LA into
GVL. Further improvement of the reaction efficiency and GVL
selectivity is possible via the modification of the photocatalyst
composition and structure or the development of new catalysts.
In
a general photocatalytic synthetic procedure, LA
(20 mmol) was dissolved in 20 ml isopropanol and 0.05 g TiO2
catalyst was added (loading 0.5 wt% noble metal). All reactions
were performed in a tubular quartz photoreactor with pure Ar
continuously bubbling. A 300 W high-pressure Hg lamp was
used as the light source and was cooled by 20 °C water circula-
tion. Liquid products were analyzed by using a gas-chromato-
graph (GC-950 equipped with a flame ionization detector (FID)
and Rtx-Wax column from Restek) and a gas chromatograph-
mass spectrometer (Shimadzu GCMS-QP2010 with a mass
spectrometer and DB-5 ms column from Alltech). The sample
analysis was confirmed by comparing the mass spectrum and
retention time of pure products. A GC-9790 (equipped with a
Experimental section
All chemicals were analytical-grade reagents and used without thermal conductivity detector (TCD) and a flame ionization
further purification. RA-TiO2 and R-TiO2 were prepared using a detector (FID)) was used to detect CH4, CO, CO2 and H2.
calcination method with commercial TiO2 powder (P25,
Degussa) as a starting material. A typical calcination was
carried out in a muffle furnace in air at 600 °C for 60 h
(RA-TiO2) and 800 °C for 8 h (R-TiO2). TiO2 nanotube
Acknowledgements
(NT-TiO2) photocatalysts were prepared by a hydrothermal This work was supported by the National Natural Science
process.36 Titania powder (1 g) and an aqueous solution of Foundation of China (21173250 and 21573137), the Scientific
NaOH (10 M, 100 ml) were mixed under magnetic stirring. Research Foundation for the Returned Overseas Chinese Scho-
After stirring for 30 min, the resulting milk-like suspension lars, the Natural Science Foundation of Shanxi (2014011014-3),
was transferred to a Teflon-lined autoclave, maintained at and the Foundation of State Key Laboratory of Coal Conversion
150 °C for 48 h, and then allowed to cool down to room temp- (J13-14-913).
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