Efficient formation of angelica lactones in a vapor-phase conversion of levulinic acid

Article abstract of DOI:10.1016/j.apcata.2016.07.025

Vapor-phase lactonization of levulinic acid to produce angelica lactones, which include α-, β- and γ-form isomers, was performed in fixed-bed down-flow glass reactors over various oxide catalysts. SiO₂ and SiO₂-Al₂O₃ showed relatively high catalytic activity. The lactonization of levulinic acid to angelica lactones was found to be an endothermic equilibrium reaction, and the pressure equilibrium constant was calculated to be 0.2?atm at 275?°C. High temperatures and reduced pressures were effective for shifting the equilibrium from levulinic acid to angelica lactones, while the suitable reaction temperature was estimated to be 275?°C because temperatures higher than 275?°C decreased the selectivity to angelica lactones. The highest angelica lactones yield of 87.5% was achieved at a levulinic acid conversion of 95.3% over SiO₂ under reduced pressure conditions of ca. 5?kPa at 275?°C. IR, NH₃-TPD and TG analyses were performed for characterizing the catalysts used after the reactions together with a silylated SiO₂ prepared for studying the active species on SiO₂. The silanol groups of SiO₂ with weak acidity were proposed to be the active species.

Full text of DOI:10.1016/j.apcata.2016.07.025

Accepted Manuscript
Title: Efficient formation of angelica lactones in a vapor-phase
conversion of levulinic acid
Author: Daolai Sun Yuta Takahashi Yasuhiro Yamada Satoshi
Sato
PII:
S0926-860X(16)30394-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.07.025
APCATA 15962
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
24-6-2016
26-7-2016
30-7-2016
Please cite this article as: Daolai Sun, Yuta Takahashi, Yasuhiro Yamada, Satoshi Sato,
Efficient formation of angelica lactones in a vapor-phase conversion of levulinic acid,
Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.07.025
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A manuscript for the original paper submitted to the journal of Applied Catalysis A
Efficient formation of angelica lactones in a vapor-phase conversion of levulinic acid
Daolai Sun, Yuta Takahashi, Yasuhiro Yamada, Satoshi Sato*
Graduate School of Engineering, Chiba University, Chiba, 263-8522, Japan
*
Corresponding author. Tel. +81 43 290 3376; Fax: +81 43 290 3401
E-mail address: satoshi@faculty.chiba-u.jp (S. Sato)
1

Graphic abstract
1
00
8
6
4
2
0
0
0
0
Reduced pressure
Atmospheric pressure
O
O
SiO2
OH
O
H O
+
o
2
2
75 C
O
Levulinic acid (LA)
Angelica lactones (ALs)
0
5
10
15
W /F / h
cat. LA
2

Highlights
Vapor-phase lactonization of levulinic acid into angelica lactones was investigated.
SiO
2
is one of the most efficient catalysts for the formation of angelica lactones from levulinic acid.
are the active species for the lactonization of levulinic acid.
Silanol groups on SiO
2
Reduced pressure conditions are effective for achieving the high yield of angelica lactones.
o
The pressure equilibrium constant in the reaction is calculated to be 0.2 atm at 275 C.
Abstract:
Vapor-phase lactonization of levulinic acid to produce angelica lactones, which include α-, β- and γ-
form isomers, was performed in fixed-bed down-flow glass reactors over various oxide catalysts. SiO
2
and
SiO -Al showed relatively high catalytic activity. The lactonization of levulinic acid to angelica lactones
2
O
2 3
was found to be an endothermic equilibrium reaction, and the pressure equilibrium constant was calculated to
o
be 0.2 atm at 275 C. High temperatures and reduced pressures were efficient for shifting the equilibrium
from levulinic acid to angelica lactones, while the suitable reaction temperature was estimated to be 275 oC
o
because temperatures higher than 275 C decreased the selectivity to angelica lactones. The highest angelica
lactones yield of 87.5% was achieved at a levulinic acid conversion of 95.3% over SiO
2
under reduced
-TPD and TG analyses were performed for characterizing
the catalysts used after the reactions together with a silylated SiO prepared for studying the active species on
SiO . The silanol groups of SiO with weak acidity were proposed to be the active species.
o
pressure conditions of ca. 5 kPa at 275 C. IR, NH
3
2
2
2
Keywords: levulinic acid, angelica lactone, silanol, vapor-phase lactonization, reaction equilibrium.
3

1
. Introduction
Applications of renewable biomass provide facile routes to alleviate the shortage of fossil fuels as
well as to reduce CO emission. Biomass represents an abundant carbon-neutral renewable resource for the
2
production of bioenergy and biochemicals, which can replace the energy and the materials produced from
fossil resource [1]. Shifting society’s dependence away from petroleum to renewable biomass resources is
generally viewed as an important contributor to the development of a sustainable industrial society and
effective management of greenhouse gas emissions [1-3]. Levulinic acid (LA) is an attractive chemical,
which can be produced from cellulose [2,4] and hemicellulose [2,5-7] as well as sugars [8,9] presented in
agricultural and forest residues. LA is used as a food flavoring agent and also a starting material for the
preparation of a variety of industrial and pharmaceutical compounds [10,11]. Many useful C5 chemicals,
such as angelica lactones (ALs), γ-valerolactone (GVL), 2-methyltetrahydrofuran, and 1,4-pentanediol, can
be derived from LA [3,12]. Among these chemicals, attention has been highly focused on the production of
GVL because of the important use of GVL as a solvent and an intermediate in the chemical industry for
energy storage [13,14]. Although ALs have important uses in various fields, selective conversion of LA to
ALs has rarely been reported.
ALs include three isomers, α-, β- and γ-angelica lactone [15], as shown in Scheme 1. α-Angelica
lactone (α-AL) has been widely used as a flavoring essence in the food and tobacco industry for its pleasant
aromas [16]. In the application of ALs, ALs can be used for producing degradable polymers [17], alkyl
levulinates [18], and γ-valerolactone [15,19]. Recently, the conversion of ALs to C6-C13 gasoline-like
hydrocarbons has attracted much attention [20,21]. In this process, dimerization and trimerization of ALs are
efficiently catalyzed by K
2
CO , and C6-C13 hydrocarbons can be selectively produced by the further
3
hydrogenation of the corresponding dimers and trimers of ALs. Thus, ALs are expected to be novel
feedstocks for the production of chemicals and transportation fuels. In some previous patents, production of
ALs from LAwas performed under reaction distillation conditions, and ALs yields higher than 90% could be
achieved in the presence of sulfuric acid, phosphoric acid, sulfated ZrO
2
, or SiO
2
-Al
2
O [3,22-25]. Mascal et
3
al. performed LAlactonization over montmorillonite clay catalyst in a distillation flask, and 92% yield of ALs
o
was achieved at 165 C under a reduced pressure of 0.07 MPa [20]. Xin et al. also performed the same
4

reaction under reaction distillation conditions, and ALs yield reached to 97% in the presence of H-ZSM5 at
o
1
30 C under a reduced pressure of 0.01 MPa [21].
In this study, we firstly report a vapor-phase lactonization of LA to ALs over solid catalysts under
continuous flow conditions using fixed-bed down flow reactors. The reaction equilibrium was investigated
and the suitable reaction conditions for ALs formation from LA were studied. The active species for the ALs
formation from LAwere also discussed and a detailed formation route for the lactonization of LAto ALs was
proposed.
2
. Experimental
.1 Samples
LA was purchased from Tokyo Chemical Industries Co., Ltd., and it was used for the catalytic
reaction without further purification. Three different silicas (CARiACT Q3, Q6 and Q10 with mean pore
diameters of 3, 6 and 10 nm, respectively) were supplied by Fuji Silycia Chemical Ltd. Al (N611) and
2
O
2 3
SiO
active carbon (A-BAC-LP) were supplied by Daiichi Kigenso Kagaku Kogyo Co. Ltd., Japan and Kureha
Corporation, respectively. Rutile TiO (JRC-TIO3) was supplied by Catalyst Society of Japan. MFI and H-
2
-Al
2
O
3
(N631L) were purchased from Nikki Chemical Co., Ltd. Monoclinic ZrO
2
(RSC-100) and
2
Y zeolites were supplied by Clariant Catalysts Industries Co., Ltd. and Catalysts & Chemicals Industries Co.,
Ltd., respectively.
A silylated SiO
2
2
catalyst was prepared as the following procedures: 3 g of SiO -Q3 and 3 g of
chlorotrimethylsilane were stirred in a solvent of n-hexane of 30 g for 12 h, and the silylated SiO
2
was
o
obtained after a separation followed by drying at 110 C for 12 h.
.2 Catalytic reaction
Typically, at an ambient pressure, vapor-phase lactonization of LA was performed in a fixed-bed
2
down-flow glass reactor with an inner diameter of 17 mm. Prior to the reaction, a catalyst of 0.5 g was placed
in the catalyst bed and heated at a prescribed temperature for 1 h to stabilize the temperature of the catalyst bed.
After that, either pure LA or its aqueous solution was fed through the top of the reactor at a liquid feed rate of
-1
3
-1
1
.2 g h together with an N
2
flow of 30 cm min . In the reaction under reduced pressure conditions at ca. 5.3
5

-1
kPa, LA was feed at a liquid feed rate of 2.9 g h over 2.0 g of SiO -Q3 placed in the catalyst bed, and a
2
vacuum pump was connected with the bottom of the reactor without using a carrier gas to reduce the pressure
o
inside the reactor. The hydration of ALs to LA was performed over 4.0 g of SiO
2
-Q3 at 275 C using α-AL as
-1
the reactant, while α-AL and H
2
O were individually fed at the same feed rate of 1.3 g h . The liquid effluents
o
collected in a dry ice-acetone trap (-78 C) every hour were analyzed by a FID-GC (GC-2014, Shimadzu)
with a 30-m capillary column of RtxR-Wax (GL-Science, Japan). A GC-MS (QP5050A, Shimadzu) was
used for identification of the products in the effluent. The details for the products separation and identification
are shown in the supplemental file. γ-Butyrolactone was used as the internal standard. The conversion and
selectivity were calculated as follows:
ꢅꢆꢇꢈꢉꢀꢆꢊꢀꢀꢋꢈꢌꢆꢍꢈꢋꢈꢎꢀꢋꢈꢏꢌꢐꢏꢑꢐ
Conversionꢀꢁ%ꢂ ꢃ ꢁ1 ꢄ
Selectivityꢀꢁ%ꢂ ꢃ
ꢂ ꢔ 100
(eq. 1)
(eq. 2)
ꢉꢒꢅꢀꢆꢊꢀꢅꢆꢇꢈꢉꢀꢆꢊꢀꢐꢓꢈꢀꢋꢈꢏꢌꢐꢏꢑꢐ
ꢅꢆꢇꢈꢉꢀꢆꢊꢀꢉꢕꢈꢌꢖꢊꢖꢌꢀꢕꢋꢆꢎꢒꢌꢐ
ꢔ 100
ꢉꢒꢅꢀꢆꢊꢀꢅꢆꢇꢈꢉꢀꢆꢊꢀꢐꢓꢈꢀꢋꢈꢏꢌꢐꢏꢑꢐꢗꢅꢆꢇꢈꢉꢀꢆꢊꢀꢋꢈꢌꢆꢍꢈꢋꢈꢎꢀꢋꢈꢏꢌꢐꢏꢑꢐ
2.3 Characterization of the catalysts
The specific surface area of the catalysts was calculated by the BET method using N
2
isotherm at -
o
1
96 C. The temperature-programmed desorption (TPD) of adsorbed NH
titration using an electric conductivity cell immersed in an aqueous solution of H
desorbed NH , which was bubbled into the solution, was monitored by the change in the conductivity of the
solution. A cumulative amount of desorbed NH was obtained as a function of the desorbed temperature and
3
was measured by neutralization
2
SO . The amount of
4
3
3
then differentiated to give a TPD profile as an acid strength distribution. The details of TPD experiment are
described in our previous studies [26]. The thermogravimetry (TG) analysis was performed using
Thermoplus 8120E2 (Rigaku) under the conditions: sample weight, ca. 10 mg; the rate of the increased
o
-1
o
temperature, 5 C min ; heating range, from the room temperature to 900 C. The diffuse reflectance infrared
Fourier-transform (DRIFT) spectra of catalysts were recorded on a spectroscopy using FT/IR-4200
o
(
JASCO) under vacuum conditions, and the catalysts were pretreated at 150 C in vacuum for 1 h in order to
remove the H O adsorbed on the catalyst surface.
2
6

3. Results
3.1. Lactonization of LA to ALs over various catalysts.
Table 1 shows the reaction results of lactonization of LA to ALs over various solid catalysts. The
o
reactions were performed in N
2
flow at 250 C with a contact time of W/FLA of 0.8 h, where W and FLA were
-1
the catalyst weight (g) and the feed rate of LA (g h ), respectively, using 50 wt.% LA aqueous solution as the
reactant. The main products were α-, β- and γ-AL in all the reactions. The total selectivity to ALs exceeded
5
0% over each catalyst, and the residual selectivity was assigned to the undetectable products. Among the
three AL isomers, α- and β-AL were mainly generated with a similar amount. The reaction did not proceed
in the absence of a catalyst. SiO catalysts showed relatively high selectivities to ALs, and SiO -Q3 gave the
2
2
2
highest ALs selectivity of 95.0% among all the tested catalysts. SiO -Q3 had a higher surface area, and gave
a high LA conversion of 19.6% comparing with those of SiO -Q6 and Q10. On the other hand, SiO
2
2
-Al
2 3
O
showed the highest LA conversion of 25.0% and the highest ALs yield of 21.8% among all the tested
catalysts. TiO was not so active, and the conversion of LA was as low as 8.6%. MFI and HY zeolites had
relative high surface areas and also gave relative high conversions of LA, whereas the selectivities to ALs
were lower than 80%. ZrO and γ-Al showed poor activity for ALs formation, and the selectivity to ALs
2
2
O
2 3
was even lower than 60%. The catalytic activity of active carbon and SiO
2
-Q3 was compared at a
temperature of 275 C. The active carbon with a higher surface area gave a lower conversion of LA than that
of SiO -Q3 as well as a lower selectivity to ALs.
o
2
o
At 250 C, SiO
2
-Al
2
O
3
showed the highest LA conversion and SiO -Q3 gave the highest selectivity
2
to ALs (Table 1). Thus, they were selected for the further study of LA lactonization to ALs. Fig. 1 shows the
o
changes in activity of SiO
the reactant. Over SiO -Al
selectivity to ALs decreased at W/FLA higher than 0.8 h. Over SiO
2
-Al
2
O
3
and SiO
2
-Q3 with W/FLA at 275 C using 50 wt.% LA aqueous solution as
2
2
O
3
, the conversion of LA increased with increasing the W/FLA, whereas the
2
-Q3, on the other hand, the conversion of
LA increased monotonically with increasing the W/FLA, and kept a constant value of ca. 47% at W/FLA
higher than 3.5 h. The selectivities to ALs kept at ca. 92% even at high W/FLA values. As a result, the yields
of ALs kept at ca. 43% at W/FLA higher than 3.5 h.
7

3.2. Lactonization of LA to ALs under different reaction conditions.
2
Figure 2 shows the changes in catalytic activity of SiO -Q3 with W/FLA at different reaction
temperatures. At each temperature, the conversion increased with increasing the W/FLA, and trended to reach
a constant value when the W/FLA was higher than 1.6 h. These results indicate that the lactonization of LA to
ALs is an equilibrium reaction, and that the reactions performed at high W/FLA values have achieved the
equilibrium. In Fig. 2, the conversion at the same W/FLA increased with increasing the temperature, whereas
the selectivity to ALs at the same W/FLA decreased. The selectivity to ALs also slightly decreased with
increasing the W/FLA at each temperature.
In order to study the reaction equilibrium in LA lactonization to ALs, reactions were performed at
o
2
75 C using aqueous LA solutions with different LA concentrations (Table 2 and Fig. 3). The catalyst
weight was changed with the concentration of LA in order to achieve the same constant W/FLA of 3.6 h. The
conversion of LA and the yield of ALs increased with increasing the concentration of LA, whereas the
selectivity to ALs slightly decreased.
The lactonization of LA to ALs was also performed under reduced pressure conditions. Fig. 4
compares the changes in catalytic activity with W/FLA over SiO
pressure conditions. Because the catalytic deactivation was observed at W/FLA values lower than 1.6 h, the
initial activity of SiO -Q3 was used for comparison. The conversion increased with increasing the W/FLA at
2
-Q3 under atmospheric and reduced
2
low W/FLA values, and trended to reach to a constant value at high W/FLA values under both atmospheric
and reduced pressure conditions. On the other hand, at a same W/FLA, the conversion under reduced pressure
conditions was much higher than that under atmospheric pressure conditions. Although the selectivity to
ALs decreased with increasing the W/FLA under both atmospheric and reduced pressure conditions, the
selectivity to ALs under reduced pressure conditions was higher than that under atmospheric conditions at a
same W/FLA. A high ALs yield of 87.5% with a LA conversion of 95.3% was achieved at a W/FLA of 14.6 h.
3.3. Catalyst stability and regeneration
2
Since the catalytic activity of SiO -Q3 was stable in the initial 5 h at W/FLA higher than 1.6 h, we
performed the reaction at a low W/FLA of 0.4 h in order to evaluate the catalytic stability (Table 3 and Fig. 5).
8

The conversion of LA decreased from 51.6% to 44.9% during the initial 5 h, which indicated that SiO -Q3
2
deactivated with time on stream. TG analyses of the as-received and used catalysts were performed to
evaluate the amount of carbon accumulated on the catalysts after the reaction: the accumulated carbon was
calculated to be 13.4 wt.% of the used catalyst.
The reaction over a reactivated SiO
2
-Q3 catalyst, which was prepared by calcination of a used
catalyst in air at 500 C for 6 h, was performed, and the catalytic activity of the reactivated SiO -Q3 was
compared with that of the as-received SiO -Q3 (Table 3). The reactivated SiO -Q3 showed a similar
catalytic activity to that of the as-received SiO -Q3, and only slight decrease in ALs yield was observed.
o
2
2
2
2
3
.4. Hydration of ALs to LA over SiO
In order to study the reaction equilibrium in the lactonization of LA to ALs, we performed the
reverse reaction, the hydration of α-AL to LA, over SiO -Q3 (Table 4). The reaction was performed at 275
oC under atmospheric pressure conditions. Under such reaction conditions, the conversion of α-AL, the
selectivity to LA, and the yield of LA were 60.6, 82.4, and 49.9%, respectively. It is reasonable that SiO -Q3
works as an acid catalyst in the hydration of α-AL to LA.
2
-Q3
2
2
3
.5. Lactonization of LA to ALs over silylated SiO
The silylated SiO was tested in order to understand the active species in SiO
compares the changes in activity with time on stream over a pure SiO -Q3 and a silylated SiO
lactonization of LA to ALs. The silylated SiO -Q3 gave a much lower initial conversion of LA than that of
as-received SiO -Q3, whereas it increased along with the time on stream up to 4 h. At a time on stream of 4 h,
the silylated SiO -Q3 showed a conversion close to that over the as-received SiO -Q3, and then it decreased
with increasing the time on stream. The selectivity to ALs was stable at ca. 98% over both silylated SiO -Q3
2
-Q3.
2
2
-Q3. Figure 5 also
2
2
-Q3 in the
2
2
2
2
2
o
and as-received SiO
silanol and hexamethyl disiloxane, were detected in the reaction effluents during the reaction when the
silylated SiO -Q3 was used as a catalyst. The amount of silicon-contained compounds decreased with
increasing the time on stream, as shown in Fig. 5.
2
-Q3 at 275 C. On the other hand, silicon-contained compounds, such as trimethyl
2
9

3.6. Characterization of the catalysts
Figure 6 shows the NH -TPD profiles of SiO
3
2
-Al
2
O
3
, SiO
2
-Q3 and SiO
2
-Q10 samples. SiO
2
-Al
2
O
3
o
showed two desorption peaks at 283 and 373 C. SiO
2
-Q3, SiO
2
-Q10 and the used SiO
2
-Q3 showed
o
desorption peaks at 211, 188 and 167 C, respectively, whereas no peak was observed in the profile of the
silylated SiO
2.2, 19.0 and 27.3 μmol g , respectively. The order of the acid strength were SiO
Q10> used SiO -Q3> silylated SiO -Q3.
2
-Q3. The acid amount of SiO
2
-Al
2
O
3
, SiO
2
-Q3, SiO
2
-Q10 and the used SiO
2
-Q3 were 524.8,
-1
9
2
-Al > SiO
2
O
3
2
-Q3> SiO
2
-
2
2
Figure 7 shows the DRIFT spectra of as-received SiO
2
-Q3, used SiO
2
-Q3 and silylated SiO -Q3.
2
-1
The peak at 1090 cm , which was assigned to the symmetric vibration of Si-O-Si [27,28], was observed in
-
1
all the silica catalysts. The peak at 840 cm , which was attributed to the deformation vibration of Si-CH
was only observed in the spectra of silylated SiO -Q3. This indicates that the silanol group is exchanged with
a silyl group through the silylation of SiO -Q3, and that trimethylsilyl groups are successfully introduced on
the catalyst surface. In the spectra of the silylated SiO -Q3 and used SiO -Q3, the peaks assigned to the
stretching vibration of C-H in CH and CH were observed at 2854 and 2924 cm , respectively [29]. The
peak at 1745 in the spectra of the used SiO -Q3 was attributed to the stretching vibration of a C=O group
3
[28],
2
2
2
2
-1
2
3
2
[
30]. The existence of CH
2
, CH
3
2
and C=O in the used SiO -Q3 clarified the formation of carbonaceous
-1
compounds on catalytic surface during the reaction. The peak observed at 803 cm , which is attributed to the
-1
asymmetric vibration of Si-OH [27,28], and the broad peak observed at 3400 cm , which is assigned to
hydrogen bonded -OH groups [27], could be observed in all the catalysts, while the as-received SiO -Q3
showed the highest intensity.
. Discussion
.1. Effect of catalyst properties on the formation of ALs.
As shown in Table 1, the catalytic activity of the tested catalysts is much different. Although the
2
4
4
residual products besides ALs were unidentified, they are probably the oligomers of ALs because it has been
known that oligomerization of ALs easily proceeds over a base catalyst or a catalyst with strong acidity
[
20,21]. ZrO
2
and γ-Al
2
O are widely used as acid-base catalysts [29,31-32], and we have also confirmed
3
1
0

their acid-base properties by NH
3
- and CO
2
-TPD analysis in our previous study [33]. Thus, it is reasonable
that the base sites in ZrO and γ-Al
2
2 3
O
catalyze the further oligomerization of ALs, and that results in the low
selectivity to ALs. The MFI and HY zeolites have been used for catalyzing the dehydration of 1,2-
propanediol to propanal [34] and the cyclodehydration of diethylene glycol to 1,4-dioxane [29], respectively,
and the oligomerization of propanal and diethylene glycol occurs in the reactions because of their strong
acidity. In this study, MFI and HY zeolites show relatively low ALs selectivities, which is also proposed to
attribute to the further oligomerization of ALs over the strong acid sites in zeolites. Because of the low
surface area of TiO , it shows the lowest activity for LA conversion.
2
It has been reported that the lactonization of LA to ALs can be efficiently catalyzed by sulfuric acid,
phosphoric acid under reaction distillation conditions [3], which indicates that Brønsted acid is effective for
the reaction. SiO
activity. Among the tested SiO
largest amount of active sites among the SiO
SiO -Q6 and Q10. SiO -Al reduced the selectivity to ALs because of the oligomerization of ALs (Fig. 1)
whereas it also gives a higher ALs yield than that of SiO -Q3 with high surface area (Table 1). In contrast to
strong acidic SiO -Al , SiO -Q3 can be used at large W/FLA values without obvious decrease in ALs
selectivity (Fig. 1). This would be attributed to the weak acidity of SiO -Q3 (Fig. 6), which accelerates no
further oligomerization of ALs. Thus, a weak acidic SiO catalyst would be suitable in such a vapor-phase
2
-Al
2
O
3
and SiO
2
are known as solid Brønsted acid catalysts, which show the relatively high
catalysts, SiO -Q3 has the highest surface areas, which means it has the
catalysts. Thus, SiO -Q3 is more active than the other silicas,
2
2
2
2
2
2
O
2 3
2
2
2
O
3
2
2
2
reaction, because the selectivity and the stability of a catalyst are highly required especially in an industrial
application.
4.2. Study on the reaction equilibrium of the LA lactonization
In the reaction results shown in Fig. 1b, in which the conversion of LA keeps at a constant value at
high W/FLA values. Thus, it is suggested that the lactonization of LA to ALs and water is an equilibrium
reaction (Scheme 2). Decreasing the water vapor pressure in the reaction system is proved to be effective for
shifting the equilibrium from LA to ALs (Table 2 and Fig. 3), which is consistent with Le Chatelier's
principle. In order to confirm the reaction equilibrium in the conversion of LA to ALs and water, we
1
1

p
calculated the pressure equilibrium constant, K , in the reactions using aqueous LA solutions with different
LA concentrations. The experimental K is estimated using the following equation:
p
PALs×P
PLA
H O
2
K =
p
(eq. 3)
where P
x
is the partial pressure of the reactant and products, and is calculated by the molar fraction of the
o
reaction mixture. The calculation results are shown in the last column of Table 2 and Fig. 3. At 275 C, the
values of pressure equilibrium constant at different W/FLA are close to 0.2 atm, which indicates that the
equilibrium was achieved in the reactions at a W/FLA of 3.6 h, and the pressure equilibrium constant is 0.2
o
atm in the lactonization of LA to ALs and water at 275 C.
In order to further confirm the above-mentioned pressure equilibrium constant, we performed the
o
reverse reaction, the hydration of ALs, at 275 C and calculated its pressure equilibrium constant, K
p
'. (Table
4
). The K ' is estimated using the following equation:
p
PLA
K ′ ꢃ
p
(eq. 4)
PALs×P
H O
2
The reciprocal of K
p
' is 0.34 atm, which is a little higher than 0.2 atm of K . The small difference is supposed
p
to be attributed to the purity of the reactant. Nevertheless, this result demonstrates that the calculated K
p
would be reliable.
For an equilibrium reaction, the equilibrium is dependent on the reaction temperature. Since the
conversion increases with increasing the temperature (Fig. 2), the lactonization of LA to ALs is an
endothermic reaction. However, the selectivity to ALs is low at high temperatures, which is because the
oligomerization of ALs prefers high temperatures. Thus, an efficient production of ALs from LA cannot be
achieved at high temperatures. On the other hand, based on Le Chatelier's principle, the equilibrium is also
dependent upon the pressure in a molecule number-changed reaction. The conversion of LA to ALs and
water is a reaction with increased molecule number, so that reduced pressure conditions would be effective
for shifting the equilibrium from LA to ALs and H O. Furthermore, a reduced pressure could induce the
2
1
2

desorption of ALs from SiO surface, and this would inhibit the further oligomerization of ALs. Therefore,
2
reduced pressures promote the conversion of LA, and also improve the selectivity to ALs (Fig. 4).
There is an essential issue for the separation of AL isomers. Although the maximum selectivity
towards a particular compound is lower than 50%, separation of the ALs is not necessary in some
applications of ALs, such as the hydrogenation of ALs to γ-valerolactone [15,19] and the conversion of ALs
to hydrocarbons [20,21], because a mixture of ALs can be used as the reactant.
4
.3. Mechanistic consideration of lactonization of LA to ALs over SiO .
2
SiO
reactions such as the dehydration of aldoximes to nitriles [35]. Additionally, silanol groups are generally
considered to be the active sites in SiO [35]. In order to confirm the active species for the lactonization of
2
has been usually used as a catalyst support and it also could be used as a catalyst in some
2
LA to ALs, the silylation of silanol of SiO
groups are expected to be partially exchanged by the trimethylsilyl groups through such an operation, as
shown in Scheme 2, and the existence of trimethylsilyl groups in a silylated SiO -Q3 is confirmed by DRIFT
2
-Q3 with chlorotrimethylsilane was conducted. The silanol
2
(
Fig. 7). The disappearance of the NH
supports that the surface of the SiO
conversion of LA over the silylated SiO
trimethylsilyl groups on the SiO surface, and the silanol groups are the active species for catalyzing the
3
-desoption peak in the NH
-Q3 is covered with trimethylsilyl groups (Fig. 6). The low initial
-Q3 indicates that lactonization of LA (Fig. 5) is inhibited by the
3
-TPD profile of the silylated SiO
2
-Q3 also
2
2
2
lactonization of LA. It is interesting that the conversion of LA increases along with the time on stream up to 4
h. Because the generation of silicon contained products are detected in the reaction effluent in each hour of
the reaction, it is reasonable that the silylated SiO -Q3 reacts with water, which is generated together with
2
ALs from LA, and the silanol groups are regenerated during the reaction, as shown in Scheme 3. Thus, the
increased conversion of LA is corresponded with the increasing amount of the regenerated silanol groups,
which demonstrates again that the surface silanol groups of SiO
2
are the active species for the lactonization of
LA to ALs.
1
3

The decrease in the conversion of LA from a time on stream from 4 to 5 h over a silylated SiO
is considered to be caused by the catalytic deactivation (Fig. 5), which is also observed in the reaction over
the as-received SiO -Q3 at a low W/FLA of 0.4 h. Based on the TG and DRIFT analyses of the used SiO -Q3
Fig. 7), the deactivation is supposed to be caused by the accumulation of carbonaceous compounds during
the reaction. Although the carbonaceous compounds were not identified, based on the absorption region of
C=O, they are proposed to have lactone structures [30]. Comparing with the NH -TPD profile of the as-
2
-Q3
2
2
(
3
received SiO
2
-Q3, the acid amount in the used SiO -Q3 is small (Fig. 6), which is attributed to the poisoning
2
of the silanol active sites by the carbonaceous compounds. In another word, the accumulation of
carbonaceous compounds on catalyst surface results in the catalytic deactivation. On the other hand, the used
catalysts can be regenerated by calcination without obvious decrease in the catalytic activity (Table 3).
In the previous studies, pseudo-LA was proposed to be the intermediate in the lactonization of LA to
ALs over a Brønsted acid [36,37], whereas detailed formation routes were not discussed. We proposed a
formation route of LA lactonization to ALs, as shown in Scheme 4. The reaction starts from the attacking of
a proton to the carbonyl at position 4 of LA, which activates the carbon in the carbonyl. After that,
cyclization proceeds through the connection of the carbon activated with the oxygen atom in the carbonyl of
the carboxyl groups, and then a proton is released, followed by the formation of pseudo-LA. The
intermediate, pseudo-LA, is easily activated by a proton, and it dehydrates to α- and γ-AL through two
different ways. The generated α-AL further isomerizes to β-AL, which is also catalyzed by the Brønsted acid.
5
. Conclusions
Vapor-phase lactonization of LA to ALs was performed in fixed-bed down-flow glass reactors over
various solid catalysts. SiO -Q3 showed the highest catalytic performance, especially at high W/FLA values.
2
The weak acidity of SiO
therefore, increases the selectivity to ALs. In the study of trimethylsilylation of surface silanol of SiO
catalyst, we proposed that the silanol groups of SiO are the active species. Slight catalytic deactivation of
2
, which is proposed to inhibit the further oligomerization of generated ALs,
2
2
SiO
2
was observed at a low W/FLA of 0.4 h, which was attributed to the coke accumulation on the catalyst
o
surface, whereas the used catalyst can be regenerated by calcination at 500 C without obvious decrease in
1
4

the catalytic activity. Lactonization of LA to ALs and water is an equilibrium reaction, and the pressure
o
equilibrium constant is calculated to be ca. 0.2 atm at 275 C. Although high reaction temperatures can
improve the conversion of LA, they decrease the selectivity to ALs. Therefore, the suitable temperature for
o
lactonization of LA to AL is estimated to be 275 C. In addition, reduced pressure conditions are efficient for
shifting the equilibrium form LA to ALs, and an ALs yield of 87.5% with a LA conversion of 95.3% and an
o
ALs selectivity of 91.8% was achieved over SiO
2
-Q3 at 275 C.
Acknowledgements
This research was partly supported by JST, Strategic International Collaborative Research Program, SICORP.
1
5

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5
[
1
7

(a)
(b)
1
00
100
80
60
40
20
ALs selectivity
ALs selectivity
8
6
4
2
0
0
0
0
LA conversion
ALs yield
LA conversion
ALs yield
0
1
2
3
4
0
1 2 3 4 5 6 7 8
W/F / h
LA
W/F / h
LA
Figure 1 Changes in catalytic activity of SiO
2
-Al
2
O
3
2
(a) and SiO -Q3 (b) with W/FLA.
o
Reaction conditions: reactant, 50 wt.% LA aqueous solution; reaction temperature, 275 C; N
2
gas flow rate,
3
-1
3
0 cm min .
100
ALs selectivity
80
60
40
20
LA conversion
o
3
2
2
00 C
o
75 C
o
50 C
0
1
2
3
4
W/F / h
LA
2
Figure 2 Changes in catalytic activity of SiO -Q3 with W/FLA at different reaction temperatures.
3
-1
Reaction conditions: reactant, pure LA; N gas flow rate, 30 cm min .
2
1
8

1
00
3
2
1
0
ALs selectivity
LA conversion
8
6
4
2
0
0
0
0
ALs yield
Kp
0
20 40 60 80 100
LA concentration/ mol%
Figure 3 Changes in catalytic activity of SiO
Reaction conditions: catalyst weight, 0.98-4.27 g; reaction temperature, 275 C; N
1;
W/ FLA, 3.6 h.
2
-Q3 with the concentration of aqueous LA solution.
o
gas flow rate, 30 cm min-
3
2
100
80
60
40
20
Reduced pressure
Atmospheric pressure
0
5
10
15
W/F / h
LA
Figure. 4 Changes in catalytic activity with W/FLA over SiO -Q3 under atmospheric and reduced pressure
2
conditions. Open symbols, conversion of LA; Closed symbols, selectivity to ALs.
o
Reaction conditions: reactant, pure LA; reaction temperature, 275 C.
The values present the initial conversions for 1 h. No carrier gas was used under reduced pressure conditions,
3
-1
and the flow rate of N was 30 cm min under atmospheric pressure conditions.
2
1
9

1
00
0.5
0.4
0.3
0.2
0.1
0
ALs selectivity
8
6
4
2
0
0
0
0
SiO -Q3
2
Silylated SiO -Q3
2
LA conversion
Silicon contained compounds
0
1
2
3
4
5
Time on stream/ h
Figure. 5 Changes in activity with time on stream over SiO
2
-Q3 and silylated SiO
2
-Q3.
o
Reaction conditions: catalyst weight, 0.5 g; reactant, pure LA; reaction temperature, 275 C; N
2
gas flow rate,
3
-1
3
0 cm min ; W/ FLA, 0.4 h.
SiO -Al O
3
2
2
SiO -Q3
2
SiO -Q10
2
Used SiO -Q3
2
Silylated SiO -Q3
2
0
200
400
600
800
o
Temperature/ C
Figure 6 NH
3
-TPD profiles of SiO
2
-Al
2
O
3
, SiO
2
-Q3, SiO
2
-Q10, used SiO
2
-Q3 and silylated SiO
2
-Q3.
2
The reaction conditions for the used SiO -Q3 were as follows: reactant, pure LA; reaction temperature, 275
oC; W/FLA, 0.8 h; N
gas flow rate, 30 cm min ; reaction time, 10 h.
3
-1
2
2
0

8
40
1745
1090
2924
2854
803
3400
Silylated SiO -Q3
2
Used SiO -Q3
2
SiO -Q3
2
1
000
2000
3000
4000
-
1
Wavenumber (cm )
Figure 7 DRIFT spectra of SiO
2
-Q3, used SiO
2
-Q3 and silylated SiO
2
-Q3.
2
The reaction conditions for the used SiO -Q3 were as follows: reactant, pure LA; reaction temperature, 275
oC; W/ FLA, 0.8 h; N
gas flow rate, 30 cm min ; reaction time, 10 h.
3
-1
2
2
1

O
-
H O
O
O
O
2
O
O
O
OH
O



Levulinic acid
Angelica lactones
Scheme 1 Lactonization of levulinic acid to angelica lactones.
O
O
OH
O
+
H O
2
O
Levulinic acid
Angelica lactones
Scheme 2 Equilibrium between LA and ALs with water.
Si
Si
OH
Si
HO
O
Cl
Si
OH
OH
O
HO
OH
Si
O
O
Si
+ HCl
Si
+
Si
Si
Si
Si
Si
Si
Si
O
Si
Si
O
Si
Silylated SiO surface
Silica surface
Chlorotrimethylsilane
2
Scheme 2 Silylation of the SiO surface
2
Si
Si
OH
Si
OH
Si
HO
O
OH
O
OH
O
HO
OH
O
O
Si
H O
+
2
+
Si
Si
Si
Si
Si
Si
Si
Si
Si
O
Si
Si
Si
SiO surface
Trimethyl silanol
Silylated SiO surface
2
2
Scheme 3 Regeneration of the SiO surface
2
2
2

+
H
+
O
.
.
.
.
O
.
OH
OH
+
+
-
H
OH
OH
OH
OH
+
+
H
O
. .
O
. .
O
O
LA
+
O
+
O
+ H
- H
O
O
O
..
..
H
O
O
O
+
O
HO
HO
+
HO
H
..
+
+
HO
-
H
+ H
+
H
pseudo-LA
-
H
H O H O
2
2
+
O
+
O
+ H
O
-
H
+
O
O
O
O
O
+
+
- H
+
+
+
H
H
H
H

-AL
+
+
-
H
+ H
+
+
-
H
+ H
O
O
O

-AL
-AL
Scheme 4 Proposed reaction mechanism in the lactonization of LA to ALs.
2
3

Table 1. Lactonization of LA to ALs over various catalysts.a
Selectivity/ mol % b
S
m g
BET
2
Conversion b
/ mol %
ALs yield b
/ mol %
Catalyst
-1
/
α-AL β-AL γ-AL Total of ALs
SiO
SiO
SiO -Q10
SiO -Al
TiO
2
-Q3
705
451
295
440
40
19.6
13.6
13.0
25.0
8.0
45.3 44.8
43.2 42.9
42.5 43.6
45.2 38.3
27.0 25.5
34.3 35.6
38.6 38.7
26.4 26.2
25.3 24.6
4.9
6.7
6.7
3.8
8.6
2.4
2.4
5.1
5.7
0
95.0
92.8
92.8
87.2
61.1
72.3
79.8
57.8
55.5
0
18.6
12.6
12.1
21.8
4.9
2
-Q6
2
2
O
2 3
2
MFI zeolite
HY zeolite
431
540
106
166
-
20.1
16.0
14.1
10.3
0
14.5
12.8
8.1
ZrO
γ-Al
None
SiO
-Q3 c
2
2
O
3
5.7
0
0
0
2
705
33.5
19.5
43.5 43.4
38.0 38.1
5.2
4.8
92.1
80.9
30.8
15.8
Active carbon c 1100-1300
a
o
Reaction conditions: reactant, 50 wt.% LA aqueous solution; reaction temperature, 250 C; N gas flow
2
3
-1
b
c
o
rate, 30 cm min ; W/FLA, 0.8 h; Averaged activity in the initial 3 h. Reaction temperature, 275 C.
Table 2. Lactonization of LA to ALs over SiO
-Q3 at different concentrations of LA aqueous solution. a
2
LA wt.% LA feed rate Catalyst W/ FLA Conversion b ALs selectivity b ALs yield b
K
c
p
(
mol%)
5 (4.9)
0 (13.4)
5 (31.8)
5 (74.7)
00 (100)
/ g h-1
weight / g
/ h
/ mol %
/ mol %
/ mol %
/ atm
-1
2
5
7
9
0.28
0.56
0.91
1.14
1.20
0.98
2.00
3.26
4.07
4.27
43.0
47.0
52.5
68.5
73.7
95.6
92.4
91.5
89.4
89.2
41.1
43.4
48.0
61.3
65.8
0.28
0.24
0.20
0.20
0.19
3.6
1
a
o
3
-1 b
Reaction conditions: reaction temperature, 275 C; N
2
gas flow rate, 30 cm min . Averaged activity in the
c
initial 3 h. K
p
, pressure equilibrium constant of LA lactonization to ALs.
2
4

Table 3. Comparison of the catalytic activity between reactivated SiO
2
-Q3 and as-received SiO
-Q3. a
2
b
b
Selectivity / mol %
b
Conversion
ALs yield
/ mol %
Catalyst
/
mol %
α-AL
45.9
45.7
β-AL
γ-AL
6.2
Total of ALs
98.3
As-received
48.5
47.3
46.2
44.4
47.7
45.7
c
Reactivated
6.6
96.7
a
o
3
-1
Reaction conditions: reactant, pure LA; reaction temperature, 275 C; N gas flow rate, 30 cm min ;
2
b
c
o
W/FLA, 0.4 h. Averaged activity in the initial 5 h. The catalyst was reactivated by the calcination at 500 C
for 6 h.
Table 4. Hydration of ALs to LA over SiO
2
-Q3. a
O feed rate ALs conversion b LA selectivity b LA yield b
K
/ atm
p
' c
1/ K '
p
ALs feed rate
H
2
-
1
g h-
1
-1
/ atm
/
/ g h
/ mol %
60.6
/ mol %
82.4
/ mol %
1.3
1.3
49.9
2.9
0.34
a
o
3
-1
Reaction conditions: reaction temperature, 275 C; N
2
gas flow rate, 30 cm min ; catalyst weight, 4 g.
b
c
Averaged activity in the initial 3 h. K ', pressure equilibrium constant of ALs hydration to LA.
p
2
5