Zirconium-Beta zeolite as a robust catalyst for the transformation of levulinic acid to γ-valerolactone via Meerwein-Ponndorf-Verley reduction

Article abstract of DOI:10.1039/c4ra01120a

Zr-Beta zeolite is a robust and active catalyst for the Meerwein-Ponndorf-Verley reduction of levulinic acid to γ-valerolactone, a versatile intermediate for bio-fuels and chemicals. In a batch reactor, γ-valerolactone was formed with a selectivity of >96%. In a continuous flow reactor, >99% yield of γ-valerolactone was obtained with a steady space-time-yield of 0.46 mol_GVLg_Zr^-1 h ^-1 within 87 h, on a par with that of noble metal based catalysts. The high activity of this catalyst was attributed to the presence of Lewis acidic sites with moderate strength. Due to the relatively few basic sites, it is not poisoned by the acidic reactant. Its robustness in liquid and gas phase reactants coupled with good thermal stability makes Zr-Beta a green regenerable catalyst that can be used directly on levulinic acid without the need for derivatization. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01120a

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Zirconium–Beta zeolite as a robust catalyst for the
transformation of levulinic acid to g-valerolactone
via Meerwein–Ponndorf–Verley reduction†
Cite this: RSC Adv., 2014, 4, 13481
Jie Wang, Stephan Jaenicke and Gaik-Khuan Chuah*
Zr–Beta zeolite is a robust and active catalyst for the Meerwein–Ponndorf–Verley reduction of levulinic acid
to g-valerolactone, a versatile intermediate for bio-fuels and chemicals. In a batch reactor, g-valerolactone
was formed with a selectivity of >96%. In a continuous flow reactor, >99% yield of g-valerolactone was
Zrꢀ
1
ꢀ1
obtained with a steady space-time-yield of 0.46 molGVL
g
h
within 87 h, on a par with that of noble
metal based catalysts. The high activity of this catalyst was attributed to the presence of Lewis acidic
sites with moderate strength. Due to the relatively few basic sites, it is not poisoned by the acidic
reactant. Its robustness in liquid and gas phase reactants coupled with good thermal stability makes Zr–
Beta a green regenerable catalyst that can be used directly on levulinic acid without the need for
derivatization.
Received 8th February 2014
Accepted 26th February 2014
DOI: 10.1039/c4ra01120a
www.rsc.org/advances
19–23
24
based chemicals.
Recently, the group of Dumesic devel-
1
Introduction
oped a route to convert g-valerolactone into branched alkanes
The threat of future shortage and ultimately of the depletion of with molecular weights appropriate for liquid transportation
fossil fuels has made the utilization of biomass especially fuels. Esterication of pentenoic acid derived from g-valer-
1–5
attractive to researchers and manufacturers. In a bio-renery, olactone gives “valeric biofuels” which are potential substitutes
25
bio-fuels and value-added chemicals are produced from for gasoline and diesel components. Useful chemicals such as
6–8
renewable bio-feedstocks. The US Department of Energy has 1,4-pentanediol or 2-methyltetrahydrofuran can be obtained by
1
3,26,27
identied several biomass-derived compounds as platform chemoselective hydrogenolysis of g-valerolactone.
9,10
molecules on which to focus future research endeavors.
As biomass derivatives have high oxygen content, an oxygen
Levulinic acid is one of these platform molecules; it can be removal step is essential for the upgrading of biomass feedstock
10,28
obtained through hydrolysis/dehydration of hexoses such as to biofuels and chemicals.
glucose and fructose, or hexose-containing polymers like starch
incurring drawbacks such as the need for pressure equip-
The utilization of nonedible ment, the loss of the petroleum-derived H in the form of water,
lignocelluloses is particularly attractive as it avoids any poten- and the use of noble metal-based homogeneous or heteroge-
This normally requires external
H
2
11–15
and cellulose (Scheme 1).
2
16–18
29–34
tial competition with food supplies.
neous catalysts.
Instead of gaseous H , the catalytic transfer
2
The hydrogenation of levulinic acid gives g-valerolactone hydrogenation (CTH) process offers an alternative approach by
(
GVL), which is a sustainable liquid for energy and carbon- using hydrogen donors such as formic acid, formate salts, sec-
35
alcohols, cyclohexene and hydrazine. The use of formic acid
for reduction is especially attractive since an equimolar amount
of formic acid is formed during the production of levulinic acid
36–39
from carbohydrates.
sive noble metal catalysts
more, the noble metals catalyze the decomposition of formic
acid so that the reduction seems to proceed via in situ formed H
However, the reaction requires expen-
36–38
39,40
or harsh conditions.
Further-
2
rather than transfer hydrogenation, necessitating the use of a
closed system.
Scheme 1 Catalytic conversion of hexoses into levulinic acid.
36–38
Instead of formic acid, the carbonyl group in levulinic acid
can be reduced by secondary alcohols via the Meerwein–Ponn-
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Kent
Ridge, 117543, Singapore. E-mail: chmcgk@nus.edu.sg; Fax: +65 6779 1691; Tel:
41–43
dorf–Verley (MPV)
reduction (Scheme 2). Wise and Wil-
44
liams showed that although the carbonyl group of levulinic
acid is not chemically labile, the reaction is favored due to
lactonisation of 4-hydroxypentanoic acid (4-HPA) to
+
65 6516 2839
†
Electronic supplementary information (ESI) available: Fig. S1–S10, Tables S1
and S2. See DOI: 10.1039/c4ra01120a
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ꢂ
Teon-lined stainless steel autoclave at 140 C for 20 days. The
solid product obtained was ltered, washed with deionized water,
ꢂ
ꢂ
dried at 100 C and calcined at 580 C for 10 h. ZrAl–Beta zeolites
were synthesized following the above procedure with 0.161 g
ZrOCl $8H O (Si/Zr 100) and varying amounts of Al(NO ) $9H O.
2 2 3 3 2
Scheme 2 MPV reduction of levulinic acid to g-valerolactone.
The samples are denoted as Zr-Beta-x or ZrAl-Beta-y, where x and
y stands for the Si/Zr and Si/Al ratio, respectively.
Synthesis of hydrous zirconia and zirconium oxide. A 10 wt%
g-valerolactone. However, little attention was paid to this aqueous solution of ZrCl was added dropwise into an excess of
4
45
strategy, until Chia and Dumesic recently screened different 5 M ammonium hydroxide solution. Aer aging for 24 h, the
metal oxides for this reaction and found that zirconium oxide suspension was digested in a Teon round-bottom ask at
ꢂ
was the most active. In the ow sheet of the bio-renery, the 100 C for another 48 h, ltered and the precipitate washed free
biomass is primarily transferred into 5-hydromethylfurfural of chloride. The as-synthesized hydrous zirconia, ZrO(OH) -100,
n
46
ꢂ
(
HMF) which further hydrolyses to levulinic acid. Following was dried overnight at 100 C. Hydrous zirconia calcined at
this, a biphasic system could be employed to extract the levu- different temperatures for 2 h are designated as ZrO(OH) -T,
n
47,48
ꢂ
If sec-alcohols are where T stands for the temperature of calcination ( C).
linic acid from the aqueous phase.
employed as the extraction reagent, they can form alternative
hydrogen donors other than formic acid. The MPV reaction has
the advantage that inexpensive non-noble metal catalysts can be
2.2 Characterization
used. Whilst the traditional MPV catalyst was aluminium The surface area and porosity of the samples were determined by
alkoxide, number of heterogeneous catalysts such as nitrogen adsorption (Micromeritics Tristar 3000). Prior to each
a
4
9–52
53,54
zeolites,
ides,
mesoporous materials,
metal oxides or hydrox- measurement, the sample was thoroughly degassed under a
5
5–62
63,64
ꢂ
hydrotalcites
3
and K PO
4
(ref. 65) have been reported nitrogen ow for 4 h. The degassing temperature was 100 C for
ꢂ
ꢂ
in recent times. The conditions for liquid phase MPV reduction ZrO(OH) calcined below 300 C, and 300 C for the other cata-
n
are usually mild, taking place under ambient pressure at the lysts. The crystalline phase was determined by powder X-ray
boiling point of the secondary alcohol.
diffraction (Siemens D5005 equipped with Cu anode and vari-
Our previous work has shown that zirconium-based cata- able slits). The diffractograms were measured at a step size of
ꢂ
lysts, especially Zr–Beta zeolites, are highly active for the MPV 0.02 and a dwell time of 1 s. The elemental composition was
reduction of substituted cyclohexanones and a,b-unsaturated determined by inductively coupled plasma-atomic emission
52,66,67
aldehydes.
This motivated us to investigate the use of Zr- spectroscopy (ICP-AES) aer dissolving the sample in HF.
Beta in the MPV reduction of levulinic acid to g-valerolactone. Infrared spectra of the samples in KBr were recorded in the range
ꢀ
1
By incorporating very small amounts of zirconium (Si/Zr ꢁ75– of 4000–400 cm using a Bio-Rad Excalibur FT-IR spectrometer
ꢀ
1
2
00) into the zeolite framework, isolated zirconium atoms with with a resolution of 4 cm . Thermogravimetric analyses were
Lewis acidic property are formed. Computational studies using performed on a Dupont SDT 2960 apparatus to determine the
density functional theory show that the zirconium ions are water content in the hydrous zirconia. The sample was kept at
ꢂ
located at specic crystallographic positions of the zeolite 100 C for 0.5 h to remove physically absorbed water, and then
ꢂ
ꢂ
ꢀ1
framework and play an important role in adsorption and acti- heated to 800 C at 10 C min . The degree of hydroxylation was
68
vation of reactants. The ability of the catalyst to function calculated from the weight loss. The desorption of residual
under acidic conditions is important for the title reaction. organics on used Zr-Beta as well as the catalyst with adsorbed
n 2
Hydrous zirconia (ZrO(OH) ) and zirconia (ZrO ) have been levulinic acid or g-valerolactone was measured by thermog-
compared. The activities were tested in a batch reactor as well as ravimetry combined with mass spectrometry (Mettler-Toledo
e
in a continuous ow reactor for potential industrial utilization. TGA/DSC Star with PfeifferThermostar mass spectrometer). The
acidic and basic nature of the samples was quantied by
temperature programmed desorption of NH
3 2
and CO , respec-
2
Experimental
tively. The sample was pretreated at its calcined temperature for
2.1 Catalyst preparation
ꢀ1
ꢂ
2
h in a ow of helium (50 ml min ). Aer cooling to 100 C, NH3
Synthesis of Zr–Beta and ZrAl–Beta zeolites. Zr–Beta zeolites or CO gas was introduced for 15 min. The sample was ushed
2
ꢂ
ꢀ1
(
Si/Zr 75, 100, 150, 200) were synthesized in uoride medium with helium for another 2 h before heating at 10 C min . The
52
following the procedure reported previously. Briey, 10.42 g desorption of NH
TEOS was mixed with 10.31 g tetraethyl-ammonium hydroxide spectrometer (Balzers Prisma 200). Lewis and Brønsted acidity
TEAOH, 40 wt% solution) and hydrolyzed under stirring. Aer was investigated by FT-IR spectra of adsorbed pyridine. A self-
3 2
or CO was monitored by a quadrupole mass
(
2
h, 1.55 g of an aqueous solution containing the required supporting sample disc (10–20 mg) was pretreated in an evacu-
ꢂ
amount of ZrOCl
2
$8H
2
O was added dropwise. The mixture was ated (100 Pa) glass cell at 300 C for 2 h. Aer cooling to room
stirred for another 7–8 h until the ethanol formed upon temperature, a background spectrum was recorded using a Bio-
hydrolysis of TEOS was evaporated. Finally, 1.215 ml of HF (40% Rad Excalibur FT-IR spectrometer. The sample was exposed to
ꢂ
solution) and 0.105 g pure silica zeolite Beta seeds in 1 g of water pyridine for 15 min and re-evacuated at 100–300 C for 1 h before
were sequentially added. The crystallization was carried out in a measuring the IR spectra.
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Table 1 Textural properties of (Zr, Al)–Beta zeolites
2.3 Catalytic activity evaluation
Batch reaction. Typically, a reaction mixture containing
mmol levulinic acid and 5 ml sec-alcohol as solvent and Sample
BET surf. area
Pore vol.
(cm g
a
a
2
ꢀ1
3
ꢀ1
1
Si/Zr
Si/Al
(m g
)
)
hydrogen donor was placed in a 25 ml round-bottomed ask
equipped with a septum port, reux condenser and a guard
tube. N-dodecane was added as internal standard to monitor
the mass balance during the reaction. Aer heating to the
desired temperature, 100 or 200 mg of catalyst was added to the ZrAl-Beta-25
reaction mixture. Aliquots were removed at different reaction
Zr-Beta-75
Zr-Beta-100
Zr-Beta-150
Zr-Beta-200
78
107
163
209
105
110
—
—
—
—
27
104
503
474
439
436
490
454
0.29
0.27
0.27
0.26
0.27
0.26
ZrAl-Beta-100
a
times and the products were analyzed by gas chromatography
GC) and gas chromatography mass spectrometry (GCMS). For
Determined by ICP analyses.
(
temperatures higher than the boiling point of the alcohol, the
reaction was carried out under He (5 bar) in a 50 ml Teon-lined
autoclave (Parr) equipped with magnetic stirring.
Continuous ow reaction. The catalyst bed of Zr-Beta-100 m g . The ZrAl-Beta samples showed similar textural proper-
catalyst (200 or 500 mg depending on the required space ties as the Zr–Beta zeolites (Table 1). All the samples were largely
velocity) and glass beads (701-1, 180 microns, Sigma-Aldrich) microporous with total pore volumes of 0.26–0.29 cm g . The
was placed on a fritted disk in a quartz reactor (i.d. 10 mm). The ICP analyses conrmed that both aluminium and zirconium
temperature of the reactor was controlled by a Eurotherm were successfully incorporated into the samples, although the
temperature controller. The liquid feed containing 5 wt% lev- Si/Al and Si/Zr ratios in the zeolite were slightly higher than in
(Fig. 1). The samples had high surface areas between 436–503
2
ꢀ1
3
ꢀ1
ulinic acid in 2-propanol was introduced into the reactor with the initial synthesis gel.
ꢂ
co-feeding of He in a downow conguration. The liquid feed
The as-synthesized hydrous zirconia dried at 100 C had a
2
ꢀ1
rate was varied using a syringe pump (New Era, NE-1000). The high surface area of 400 m
g
(Table 2) and contained
ꢀ
1
He ow rate of 20 ml min was controlled by a mass ow micropores as well as mesopores up to 12 nm (Fig. S1†). Aer
ꢂ
2
controller (Brooks 5850). The eluent was condensed in an ice- calcination at 300 C, the surface area was reduced to 271 m
ꢀ1
cooled trap and samples were taken periodically for GC
g
. Although there was a reduction in the micropore volume,
analysis. For regeneration, the catalyst was calcined in situ at the pore size distribution of the mesopores remained similar to
ꢂ
ꢀ1
5
00 C under a ow of air (50 ml min ) for 3 h.
that of the as-synthesized sample. However, aer calcination at
ꢂ
2
ꢀ1
Sample analysis. The collected samples were analyzed on a 400 and 600 C, the surface area decreased to 141 and 51 m g
,
ꢂ
gas chromatograph (HP 6890) equipped with a HP-FFAP capil- respectively. The samples calcined above 400 C contained
lary column (250 mm ꢃ 0.25 mm ꢃ 30 m) and a ame ionization bigger mesopores in the range of 8–20 nm. XRD showed that the
detector. The products were veried either by comparing the hydrous zirconia samples were amorphous, but aer calcina-
ꢂ
retention times with authentic samples or by GCMS analysis. tion at 400 C, crystalline zirconia with predominantly mono-
Mass balances closed within 5% for all data points.
clinic phase was formed (Fig. S2†). The percentage of
monoclinic phase increased from 77 to 83% as the calcination
ꢂ
temperature was raised from 400 to 600 C. The transformation
3
Results and discussion
to zirconia is accompanied by condensation of the hydroxyl
groups in hydrous zirconia and the weight loss was monitored
by thermogravimetry. Samples calcined at higher temperatures
had smaller weight loss.
The amount of water, n, associated with the hydrous
zirconia, ZrO $nH O, decreased from 0.92 in the as-synthesized
3.1 Materials
The X-ray diffractograms of Zr-Beta samples (Si/Zr 75–200)
showed the characteristic peaks of the zeolite Beta phase
2
2
Table 2 Textural properties of hydrous zirconia and zirconia
Surf.
area
(m g
Mean
Pore
Monoclinic Weight
a
b
pore f vol.3
phase
(%)
loss
(%)
2
ꢀ1
ꢀ1
Sample
)
(nm)
(cm g )
ZrO(OH)
ZrO(OH)
ZrO(OH)
ZrO(OH)
ZrO(OH)
ZrO(OH)
n
n
n
n
n
n
-100 400
5.3
5.3
5.5
8.4
12.5
17.0
0.56
0.47
0.42
0.35
0.32
0.26
Amorp.
Amorp.
Amorp.
77
81
83
11.8
8.3
7.1
4.2
3.8
3.6
-200 326
-300 271
-400 141
-500
-600
84
51
a
Pore size distributions were calculated from the desorption branch of
b
Fig. 1 X-ray diffraction patterns of (a) Zr-Beta-75 (b) Zr-Beta-100 (c) the isotherms by the BJH method. From TGA results in the range of
ꢂ
1
00 to 800 C.
Zr-Beta-150 (d) Zr-Beta-200 and (e) ZrAl-Beta-100.
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Table 3 Acid–base properties of the catalysts
which is in the temperature range intermediate between the
low and temperature peaks observed for hydrous zirconia.
Furthermore, the density of acidic sites, 45 mmol g , was lower
a
ꢀ1
Acidity (mmol g
)
ꢀ1
Basicity
(mmol g
b
ꢀ1
Sample
Weak
Strong
L/B ratio
)
than that for the zirconia samples.
The nature of the acid sites on the catalysts was determined
by FT-IR measurements aer adsorption of pyridine (Fig. S4†
and Table 3). The bands at 1446 and 1540 cm are assigned to
ZrO(OH)
ZrO(OH)
ZrO(OH)
ZrO(OH)
Zr-Beta-100
ZrAl-Beta-25
n
n
n
n
-300
-400
-500
-600
137
99
25
12
45 (In total)
n.d.
—
6.7
2.1
n.d
n.d
8.4
132
112
53
25
3.8
n.d
204
146
35
ꢀ1
pyridine adsorbed at Lewis and Brønsted acid sites, respec-
71
n
tively. On ZrO(OH) -300 and Zr-Beta-100, Lewis acid sites
0.85
dominated, with Lewis/Brønsted ratio of 6.7 and 8.4, respec-
a
ꢂ
tively. Only weak Brønsted acid sites were found for these two
Integrated from the NH
3
TPD peak areas with cutoff at ꢁ300 C.
b
ꢂ
ꢀ1
Lewis/Brønsted ratio from pyridine IR aer heating at 100 C for 1 h. samples as the intensity of the 1540 cm band decreased
ꢂ
signicantly aer heating at 100 C. On the other hand,
n
ZrO(OH) -400 and ZrAl-Beta-25 showed stronger Brønsted acid
ꢀ1
sites as the 1540 cm band was still present even aer heating
to 200 C. These samples had a higher density of Brønsted acid
sites than Zr-Beta-100.
ꢂ
sample to 0.30 aer calcination at 400 C. These results corre-
ꢂ
late well with that from IR spectroscopy (Fig. S3†). Hydrous
zirconia has absorption bands at ca. 3400 cm due to O–H
stretching, ca. 1620 cm due to the scissor bending mode of
coordinated molecular water and ca. 1390 cm due to atmo-
spheric CO adsorbed on the sample forming a bicarbonate-like
ꢀ
1
The presence of basic sites was probed by CO TPD. There
2
ꢀ1
was hardly any desorption of CO2 from Zr-Beta-100 showing
that it has very few basic sites. In contrast, the amount of CO2
ꢀ
1
2
desorbed from hydrous zirconia/zirconia was in the range of
69,70
ꢀ1
species.
All these bands decreased in intensity as the sample
2
5 to 132 mmol g , reecting the amphoteric nature of these
was heated to higher temperatures.
samples. With increase of calcination temperature from 300 to
The overall acidity and basicity of the samples was deter-
ꢂ
6
00 C, the CO
2
desorption was shied to higher temperatures,
mined by TPD of NH
3
and CO , respectively (Table 3). For
-300, desorption of NH was observed from 120 to
00 C with a maximum at 200 C (Fig. 2). When hydrous
zirconia had been calcined to 400 C and higher, the NH
desorption curve became bimodal with maxima at around
ꢂ ꢂ ꢂ
00 C and 420 C. The lower temperature peak below 300 C
2
indicating an increase in the basicity of the samples (Fig. S5†).
ZrO(OH)
3
n
3
ꢂ
ꢂ
ꢂ
3.2 Effect of sec-alcohol and reaction temperature
3
The MPV reduction of levulinic acid was tested using a number
of different secondary alcohols. The alcohols were used in
excess, acting both as solvent and as hydrogen donor. Normally,
2
was assigned to desorption from weak acid sites while NH3
52,65
desorption above this temperature was taken to be indicative of
strong acidic sites. Aer calcination at 400 C, the amorphous
2-propanol is the best reducing agent.
However, in the liquid
ꢂ
phase reduction of levulinic acid with 2-propanol, only 5.6%
conversion was obtained aer 18 h despite using 200 mg of Zr-
Beta-100 (Table 4). The low activity could be due to the aliphatic
nature of levulinic acid as Zr-Beta had previously been shown to
be a highly efficient catalyst for the MPV reduction of cyclo-
hydrous zirconia transformed to crystalline zirconia. The
results show that the resulting oxide has weak as well as strong
acid sites, with the latter being predominant. The density of
both weak and strong acid sites decreased with higher calci-
nation temperature, due to a loss in surface area. In contrast to
52
hexanones and a,b-unsaturated aldehydes. The formation of
g-valerolactone involves the tandem reduction of levulinic acid
to 4-hydroxypentanoic acid followed by cyclization and loss of
one molecule of water. By using 2-propanol as the hydrogen
hydrous zirconia, Zr-Beta-100 has acid sites of moderate
strength. The NH desorption occurred between 220 to 390 C,
ꢂ
3
ꢂ
donor and solvent, the reaction temperature is limited to 82 C
Table 4 MPV reduction of levulinic acid using different sec-alcohols
a
over Zr-Beta-100
Temp.
( C)
Time
(h)
LA conv.
(%)
GVL sel.
(%)
GVL yield
(%)
ꢂ
Alcohol
2
2
2
2
2
-propanol
-butanol
-pentanol
-pentanol
-pentanol
82
100
100
118
118
150
18
18
18
10
22
6
5.6
77
58
72
93
95
96
95
82
4.0
72
55
96
83
100
88
100
b
c
Cyclohexanol
82
a
Reaction condition: 200 mg Zr-Beta-100, 1 mmol LA, 5 ml sec-alcohol.
100 mg Zr-Beta-100 as the catalyst. By-products from cyclohexanol
b
c
Fig. 2 NH
c) ZrO(OH)
3
desorption profiles of (a) Zr-Beta-100 (b) ZrO(OH)
-400 (d) ZrO(OH) -500 and (e) ZrO(OH) -600.
n
-300
(
n
n
n
were formed.
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concentrations of levulinic acid, the selectivity to g-valer-
olactone was lower, showing that the esterication rate
increased more than the MPV reduction. Use of 2 ml of 2-pen-
tanol (7.2 wt% levulinic acid) resulted in only 75% selectivity to
g-valerolactone at full conversion whereas with 5 ml of 2-pen-
tanol (2.9 wt% levulinic acid), 98% selectivity with 46%
conversion was observed aer 6 h (Table S1†). Even without
adding any catalyst, the yield of the ester increased with higher
levulinic acid concentration (Table S2†). Hence, a dilute system
of 1 mmol (2.9 wt%) levulinic acid in 5 ml solvent was employed
in the liquid phase to eliminate to reduce the rate of esteri-
cation and improve the selectivity to g-valerolactone.
Scheme 3 Transformation of levulinic acid to g-valerolactone and
ester.
in open system which may be too low to remove the water
formed. To increase the reaction temperature, aliphatic
3.4 Catalyst screening for liquid phase transformation
secondary alcohols with higher boiling points like 2-butanol Comparing the Zr–Beta zeolites with different Si/Zr ratios, Zr-
and 2-pentanol were employed.
Beta-100 showed the best activity with 100% conversion and a
The conversion increased signicantly to 77% when using 2- very high selectivity of 96% to g-valerolactone (Table 5). The
ꢂ
butanol at 100 C. Although the reducing capability of 2-pen- only by-product was sec-pentyl levulinate due to esterication.
tanol is not as good as that of 2-butanol, giving only 58% Over Zr-Beta-150 and Zr-Beta-200, the conversion was lower,
ꢂ
conversion of levulinic acid at 100 C, its higher boiling point of 71% and 27%, respectively. This may be attributed to the lower
ꢂ
1
18 C enabled a faster MPV reduction in liquid phase. Hence, zirconium content in these samples. Despite their lower activity,
aer 10 h, full conversion was achieved. For all these linear the selectivity to g-valerolactone was high, between 97–99%. On
alcohols, the only by-product was due to esterication between the other hand, Zr-Beta-75 contained more zirconium than Zr-
levulinic acid and the corresponding alcohol. Cyclohexanol has Beta-100 but this catalyst was less active and selective, sug-
an even higher boiling point, but by-products such as cyclo- gesting that an optimum number of isolated zirconium sites is
ꢂ
hexene were formed when the reaction was conducted at 150 C. essential for reaction. Based on the levulinic acid converted per
g of Zr, the initial rates of the Zr-Beta catalysts with Si/Zr of 75–
ꢀ
1
ꢀ1
2
00 was between 16–31 mmolGVL
indicates the difficulty to reduce the aliphatic carbonyl group in
The sec-alcohol was used in excess (46 equiv.) and served both levulinic acid. In comparison, the initial rate for the MPV
g
Zr
h . The low initial rate
3.3 Effect of substrate concentration
52
as the solvent and reducing agent. As levulinic acid has both reduction of cyclohexanone and a,b-unsaturated aldehydes, is
carbonyl and carboxyl groups, two competing reactions, MPV generally 20–30 times larger even at a lower temperature of
reduction and esterication, can occur simultaneously 82 C. To check for any leaching, the Zr-Beta-100 was hot ltered
ꢂ
(Scheme 3). An increase in the concentration of the limiting from the reaction medium aer the conversion had reached
substrate accelerates both reactions. However, at higher 40%. No further increase in conversion was observed in the
a
Table 5 MPV reduction of levulinic acid over zirconium-based catalysts
c
Zrꢀ1 ꢀ1
h )
Catalyst
Time (h)
LA conv. (%)
GVL sel. (%)
GVL yield (%)
Initial rate (mmolGVL
g
Zr-Beta-75
10
10
10
10
6
88
100
71
27
100
91
11
13
16
37
93
96
97
>99
71
79
74
67
67
62
33
45
92
56
46
36
82
96
69
27
71
72
8.3
8.8
11
23
11
5.4
92
38
41
16
19
30
31
16
—
—
0.071
0.075
0.096
0.22
0.16
0.076
—
—
—
—
Zr-Beta-100
Zr-Beta-150
Zr-Beta-200
ZrAl-Beta-25
ZrAl-Beta-100
6
ZrO(OH)
ZrO(OH)
ZrO(OH)
ZrO(OH)
ZrO(OH)
ZrO(OH)
n
-100
n
-200
n
-300
n
-400
n
-500
n
-600
24
24
24
24
24
24
6
16
16
16
33
12
b
Zr-Beta-100
100b
ZrO(OH)
ZrO(OH)
ZrO(OH)
n
-200
n
-400
n
-600
68
b
89
b
45
a
ꢂ
b
Reaction conditions: 200 mg catalyst, 1 mmol LA, 5 ml 2-pentanol, 118 C, reux. Reaction conditions: 100 mg catalyst, 2 mmol LA, 5 ml
ꢂ
c
2
-butanol, 150 C and autogenous pressure plus 5 bar He in autoclave. Calculated based on the conversion aer rst 2 h.
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catalyst-free reaction mixture, showing that the MPV reduction
of levulinic acid is heterogeneously catalyzed by Zr-Beta.
The incorporation of Al into Zr-Beta led to Brønsted acidity
which promoted the conversion of levulinic acid. Over ZrAl-
Beta-25, the time for full conversion was reduced from 10 h to
6
h. However, the presence of strong Brønsted acid sites accel-
erated the esterication to sec-pentyl levulinate so that at 100%
conversion, the selectivity to g-valerolactone was only 71%
(Scheme 3, Route a). Furthermore, the Brønsted acid sites
catalyzed other side reactions of pentanol leading to hemi-
acetals, pentene and its isomers. Hence, the MPV reduction of
the carbonyl group in levulinic acid is best catalyzed by a Lewis
acid catalyst as the presence of Brønsted acid sites will reduce
the overall selectivity to g-valerolactone.
Fig. 3 MPV conversion after 2 h starting from methyl levulinate (,) and
levulinic acid (-) over Zr-Beta-100 and ZrO(OH)
n
-T. Reaction condi-
ꢂ
tions: 1 mmol substrate, 5 ml 2-pentanol, 200 mg catalyst, 118 C.
n
Compared to Zr-Beta zeolites, the ZrO(OH) -T samples were
less active and selective. Aer 24 h, the conversion was only
between 11 to 37% (Table 5). The selectivity to g-valerolactone
decreased from 74 to 45% with increasing calcination temper-
ature of the hydrous zirconia. The best catalyst among the
activity. As Zr-Beta has very few basic sites, such inhibitory
effects cannot occur. Furthermore, the microporous structure of
the zeolite coupled with the absence of strong acid sites hinder
the competing esterication reaction, so that a very high g-
valerolactone selectivity of 93–99% could be obtained.
hydrous zirconia was ZrO(OH) -400 with 23% yield of g-valer-
n
olactone. The dependence of the catalytic activity and selectivity
on the calcination temperature may be attributed to its effect on
the density of surface hydroxyl groups and the acidity of the
samples. Surface hydroxyl groups are important in the MPV
reaction as they react with the reducing alcohol to form the
alkoxide intermediate. With increase of calcination tempera-
ture, the density of hydroxyl groups decreased as they
condensed to form the oxide. On the other hand, NH3 TPD
results show that stronger acid sites are formed that would favor
the cyclization step. Hence a balance in the density of surface
hydroxyls and acid strength is important for activity of the
hydrous zirconia.
The inuence of the acid group in levulinic acid was inves-
tigated by using methyl levulinate as substrate. Over Zr-Beta-
100, the conversion aer 2 h was about twice that of levulinic
acid (Fig. 3). The higher rate of reaction may be explained by the
higher hydrophobicity of methyl levulinate which leads to
improved adsorption at the surface of the hydrophobic Zr-Beta.
The effect was more pronounced over hydrous zirconia where
the 2 h-conversion of methyl levulinate was 4.5–11 times higher
than levulinic acid. As with levulinic acid, the most active
55
hydrous zirconia sample was ZrO(OH) -400 with 100% conver-
n
sion of methyl levulinate aer only 6 h.
The activity of hydrous zirconia and Zr-Beta-100 were next
tested under conditions similar to those reported by Chia and
3
.5 Continuous ow reactions over Zr–Beta zeolite
45
Dumesic. The authors used a closed system at a higher Zr-Beta-100 catalyst was also used in a continuous ow reactor
ꢂ
temperature of 150 C with 2-butanol as the reducing alcohol to explore its stability, reusability and potential for industrial
(
Table 5). It was found that a high g-valerolactone yield of 92% application. The feed was 5 wt% levulinic acid in 2-propanol.
ꢂ
could be achieved based on the zirconium oxide catalyst. Initial tests showed that at a reaction temperature of 150 C with
ꢀ1
However, an extremely dilute system (1 wt% levulinic acid) was a weight hourly space velocity (WHSV) of 0.16 h , full conver-
necessary. When the concentration was increased to 5 wt%, the sion was sustained for around 10 h (Fig. S6†). Due to the high
selectivity was only 42% at 51% conversion, giving a g-valero-
lactone yield of 22% These results are comparable with this
work where 16% yield of g-valerolactone was obtained over
ZrO(OH) -600 (Table 5). We observed that hydrous zirconia
n
ꢂ
ꢂ
calcined at a lower temperature of 400 C instead of 600 C,
ZrO(OH) -400, was more active with 89% conversion and 46%
yield of g-valerolactone. Compared to hydrous zirconia, Zr-Beta-
00 was able to maintain a very high g-valerolactone selectivity
n
1
of 92% at full conversion even at 5 wt% levulinic acid (Table 5).
The better activity of Zr–Beta zeolite for transforming levu-
linic acid to g-valerolactone can be attributed to its Lewis acidity
of moderate strength and its lack of basic sites which otherwise
would react with the carboxylic acid functional group in the
45
substrate. Chia and Dumesic found that levulinic acid was less
readily reduced than levulinate esters over zirconia. They sug-
gested that this was due to strong binding of levulinic acid to
the basic sites on zirconia, leading to blockage and loss of WHSV ¼ 0.64 h . Catalyst was regenerated at 110 h and 218 h.
Fig. 4 GVL yield as a function of time on stream for the MPV of lev-
ꢂ
ulinic acid over Zr-Beta-100 at 250 C in a continuous flow reactor of
ꢀ1
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Table 6 Comparison of activity for vapour phase GVL production over different catalysts
ꢂ
a
metalꢀ1 ꢀ1
Productivity (molGVLg h )
Catalyst
T ( C)
P (bar)
H
2
Source
GVL yield (%)
Stability (h)
Ref.
Zr-Beta
Ru/C
RuSn/C
Pd/C + Ru/C
250
265
220
170
265
150
1
1
35
35
10
20
2-PrOH
>99
99
93
95
>99
20–40
0.46
0.09
0.36
0.11
0.09
87
50
<100
400
100
150
—
74
75
76
26
45
H
H
2
2
HCOOBu
Cu/SiO
2
H
2
ZrO
2
2-BuOH
<0.001
a
2
-PrOH: 2-propanol; HCOOBu: butyl formate; 2-BuOH: 2-butanol.
ꢂ
boiling points of levulinic acid (245–246 C) and g-valerolactone
(
4
Conclusions
ꢂ
207–208 C), it is highly probable that at this low temperature,
these molecules remained signicantly adsorbed at the surface Using sec-alcohol as the hydrogen source, biomass-derived
of the catalyst. Thermogravimetric measurements of Zr-Beta- levulinic acid was converted to g-valerolactone via a tandem
1
00 that had been used in the reaction for 34 h showed a weight MPV reduction and cyclisation/lactonization. Zr–Beta zeolite
ꢂ
loss of 9.7% aer heating in air to 500 C (Fig. S7†). The weight (Si/Zr 100) is highly active and selective for the g-valerolactone
loss prole of the used Zr-Beta was similar to that of the catalyst production, both in liquid phase batch reactions and in a gas
adsorbed with levulinic acid with a steep weight loss between phase continuous ow system. Quantitative conversion with
ꢂ
2
00 to 350 C (Fig. S8†). The hydrogen/carbon (H/C) ratio of the >99% yield of g-valerolactone was obtained with a steady
ꢀ
1
ꢀ1
used Zr-Beta was 1.57, which is close to the H/C ratio of levulinic generation rate of 0.46 molGVLgZr
h
during a single run in
acid and g-valerolactone rather than that of coke in zeolites the ow reactor. The presence of Lewis acid sites with moderate
72,73
(H/C around 1.25).
The catalytic activity was recovered aer strength and only relatively few basic sites are key factors for its
calcination which also shows that the deactivation was mainly good activity and lack of poisoning by the acidic substrate. Due
due to the adsorption of substrate and product molecules rather to its high thermal stability, Zr-Beta could be easily recalcined
than by heavy coke formation.
and reused. In comparison, hydrous zirconia and zirconia were
To promote the desorption of molecules, the reaction less active for the MPV reduction of levulinic acid with the most
ꢂ
temperature was raised to 250 C, which is higher than the active hydrous zirconia catalyst being that obtained aer calci-
ꢂ
boiling points, and the space velocity was also increased. At nation at 400 C.
ꢀ1
WHSV of 0.64 h , the g-valerolactone yield remained at > 99%
for as long as 87 h (Fig. 4). Aer this time, the conversion
dropped and increasing amounts of propyl levulinate were
Acknowledgements
formed. However, the activity was fully regained by recalcining Financial support for this work from the National University of
ꢂ
the catalyst in air at 500 C. Full conversion could again be Singapore grants R-143-000-476-112 and R-143-000-550-112 is
maintained for prolonged periods of 72 and 78 h in the second gratefully acknowledged.
and third run, respectively. The stability of the catalyst is of
signicant importance for industrial applications. The
Notes and references
productivity of >99% g-valerolactone yield under WHSV of
ꢀ
1
ꢀ1 ꢀ1
0
.64 h for the long period works out to 0.46 molGVL
g
Zr
h
.
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