Catalytic upgrading of α-angelica lactone to levulinic acid esters under mild conditions over heterogeneous catalysts

Article abstract of DOI:10.1039/c5cy00446b

Butyl levulinate was prepared starting from α-angelica lactone and butanol over Amberlyst 36. Different reaction conditions were optimized, which resulted in full conversion and 94% selectivity toward the ester at 75°C. A reaction network analysis reveals pseudo-butyl levulinate and levulinic acid as intermediates in the preparation of butyl levulinate. The mild protocol was successfully applied for different alcohols and compared with the esterification of levulinic acid. Overall, this study identifies α-angelica lactone as a better candidate than levulinic acid for the heterogeneously catalysed preparation of levulinic acid esters. A catalyst screening shows that also zeolites and zirconia-based catalysts are able to catalyse the reaction. However, the transformation of the intermediate pseudo-butyl levulinate into butyl levulinate requires acid sites of sufficient strength to proceed.

Full text of DOI:10.1039/c5cy00446b

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Catalytic upgrading of α-angelica lactone to
levulinic acid esters under mild conditions over
Cite this: DOI: 10.1039/c5cy00446b
heterogeneous catalysts†
Mohammad G. Al-Shaal, Wirawan Ciptonugroho, Fabian J. Holzhäuser,
Joel B. Mensah, Peter J. C. Hausoul and Regina Palkovits*
Butyl levulinate was prepared starting from α-angelica lactone and butanol over Amberlyst® 36. Different
reaction conditions were optimized, which resulted in full conversion and 94% selectivity toward the ester
at 75 °C. A reaction network analysis reveals pseudo-butyl levulinate and levulinic acid as intermediates in
the preparation of butyl levulinate. The mild protocol was successfully applied for different alcohols and
compared with the esterification of levulinic acid. Overall, this study identifies α-angelica lactone as a
better candidate than levulinic acid for the heterogeneously catalysed preparation of levulinic acid esters. A
catalyst screening shows that also zeolites and zirconia-based catalysts are able to catalyse the reaction.
However, the transformation of the intermediate pseudo-butyl levulinate into butyl levulinate requires acid
sites of sufficient strength to proceed.
Received 25th March 2015,
Accepted 10th August 2015
DOI: 10.1039/c5cy00446b
www.rsc.org/catalysis
2 3
For example, using TiO –SO H as a catalyst, 80% yield of ML
Introduction
1
5
was obtained after 1 h at 175 °C in methanol. Ethyl
levulinate (EL) has been prepared from fructose with yields
up to 84% at 120 °C for 24 h over sulfonated carbon nano-
As a result of the massive exploitation of petroleum and its
foreseeable depletion, the search for renewable resources is
becoming increasingly urgent. Biomass presents a promising
1
16
tubes. LAE have also been prepared from furfuryl alcohol
example of a sustainable source of energy since it can be uti-
lized to produce liquid fuels or fuel additives. Particularly,
(FA). EL and butyl levulinate (BL) were prepared in high yields
from FA in alcoholic medium over different heterogeneous
catalysts at temperatures between 110 and 140 °C. However,
2
1
9
levulinic acid esters (LAE) are a group of compounds with
3
–6
high potential for use, not only, in the fuel sector, but also
considering that biorefinery processes likely use water as sol-
7–9
10
11
20
for polymers,
green solvents, and fragrances. LAE can
vent or generate water during the processes, LA and FA
2
1,22
be produced through a catalytic conversion of various
become the final products of the hydrolysis of HMF.
Many studies indicate that the esterification reaction of
4
lignocellulosic-derived intermediates (Scheme 1). The direct
23–25
conversion of polysaccharides in a one-pot reaction to LAE,
pure LA in alcoholic medium delivers LAE in high yields.
Nevertheless, the formation of water during esterification
1
2
although highly attractive, is severely limited by the harsh
conditions (e.g. 175 °C for 20 h) required to achieve this reac-
1
3,14
tion.
The one-pot conversion of cellulose to methyl
1
5
levulinate (ML) is the only example thus far. Other studies
have focused on mono- and disaccharides as starting mate-
1
5,16
rial.
These sugars can be obtained through hydrolysis of
1
7
polysaccharides and less by-products and milder conditions
are expected. LAE can be prepared from glucose through acid-
catalysed dehydration via 5-hydroxymethylfurfural (HMF) and
1
8
levulinic acid (LA) and esterification in alcoholic medium.
Institut für Technische und Makromolekulare Chemie, RWTH Aachen University,
Worringerweg 2, D-52074, Aachen, Germany.
E-mail: palkovits@itmc.rwth.aachen.de; Fax: +49 (0)241 80 22177;
Tel: +49 (0)241 80 26497
†
Electronic supplementary information (ESI) available: Textural properties of
the catalysts and kinetic studies of the addition reaction. See DOI: 10.1039/
c5cy00446b
Scheme
lignocellulosic compounds.
1 Possible routes for the preparation of LAE using
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prohibits complete conversion. For an efficient esterification
of LA obtained from biomass streams, the removal of water
is required both before and after the reaction, respectively.
This stresses the need for new protocols in which LAE can be
prepared in high yield without costly separation steps. In this
regard, the conversion of α-angelica lactone (α-AL) to LAE via
6
,26–28
the addition of alcohols is a promising alternative.
α-AL
can be readily generated from biomass-derived LA via an acid
2
7
catalysed dehydration reaction (Scheme 1). The synthesis of
LAE from α-AL and alcohols has been reported as early as
2
9,30
1
948.
Using HCl, Langlois et al. observed pseudo-LAE in
high yield before obtaining LAE as a final product. Patents
awarded to Du Pont report the use of solid acid catalysts for
6
,26–28
this reaction.
Different LAE were efficiently prepared in
2
8
high selectivity at 100 °C over Amberlyst® 15 (A15).
Nevertheless, only little is known about the influence of
the alcohol, the acidic catalyst or the reaction mechanism.
Recently, we have introduced α-AL as an alternative to LA for
3
1–33
the preparation of γ-valerolactone (γ-VL).
Herein we study
the preparation of LAE starting from α-AL and LA using dif-
ferent alcohols. The influence of temperature and reaction
time were investigated for different catalysts and alcohols.
The data reveal a strong dependency of the catalytic activity
and selectivity on number and nature of available acid sites.
Scheme 2 Proposed network for the reaction of α-AL and alcohols
over solid acid catalysts.
obtain similar results using the homogenous catalyst
p-toluenesulfonic acid (see Fig. S1†).
Results and discussion
A similar behaviour was observed at 75 °C (93.1% yield
after 120 min) and 100 °C (98.4% yield after 60 min) (Table 1,
entries 2 and 3). In all cases, minor amounts of LA are
formed during the reaction. Interestingly the maximum
amount of LA decreases from 10.5% at 50 °C to 2.8% at 100
In a first set of experiments, the influence of the temperature
was investigated over time. The preparation of BL was
conducted using α-AL and butanol (BuOH) over Amberlyst®
3
6 (A36). Fig. 1 shows that at 50 °C, initially pseudo-butyl
levulinate (pseudo-BL) (identified by GC-MS, Fig. S2†) is the
main product of the reaction. Over time a trans-esterification
of pseudo-BL to BL takes place (Scheme 2, pathway 1). 330
min were required for full conversion and a final yield of
°
C. Reference experiments in the presence of 5 and 10 mol%
water relative to α-AL confirm the significant effect of water
causing LA formation and decreased LAE production (ESI:†
Fig. S8). To exclude impurities of water in the reactants or
the reaction vessel, all materials were dried prior to the reac-
tion and reactions conducted under inert gas and water-free
conditions. Despite of this, LA persisted in the reaction. In
addition, GC-analysis also revealed the presence of trace
8
9.3% BL (Table 1, entry 1), while 100 min were required to
Table 1 A36-catalyzed addition of BuOH to α-ALa
b
α-AL b
conv.
[%]
Yields [%]
T
t
Entry
1
[C°]
[min]
BL
Pseudo-BL
LA
4.8
50
75
30
99.4
100
100
99.7
100
100
99.7
100
100
18.1
62.0
89.3
42.1
79.8
94.0
68.5
91.4
98.4
67.2
27.8
1.8
50.1
10.0
1.0
30.4
5.3
0.1
1
3
20
30
10
10.5
9.5
6.9
9.3
5.2
2.2
2.8
2.1
2
3
60
240
100
10
3
6
0
0
a
Fig. 1 A36-catalyzed addition of BuOH to α-AL. Conditions: 50 °C;
α-AL (5.00 g, 51.00 mmol); BuOH (3.78 g, 51.00 mmol); A36 (250 mg).
Conditions: α-AL (5.00 g, 51.00 mmol); BuOH (3.78 g, 51.00 mmol)
b
A36 (250 mg). Determined by GC.
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Table 2 Addition of different alcohols to α-AL over A36a
amounts of β-angelica lactone (β-AL), γ-methylene-γ-
butyrolactone (γ-BL), and 2,4-pentadienoic acid in the reaction
b
α-AL b
Yields [%]
3
2,34,35
mixtures.
of acidic catalyst, a blank reaction was performed with A36
Fig. 2). Interestingly, 45.3% conversion of α-AL was observed
To study the stability of α-AL in the presence
t
conv.
Entry
1
ROH
[min]
[%]
LAE
Pseudo-LAE
LA
(
EtOH
10
180
60
99.1
99.2
99.8
100
98.9
99.3
43.0
91.3
33.1
93.8
40.5
75.8
45.0
11.1
7.8
13.9
5.0
11.6
19.7
after 180 min at 75 °C. While the yield of α-AL isomers and
LA only reaches 7.5%, a browning of the reaction mixture
and formation of precipitates occurred suggesting the conver-
sion to polymeric species. The formation of LA only reaches a
maximum yield of 3.1% (Fig. 2). It is reasonable to argue that
the remaining water formation and hydrolysis process are
related to the isomerization of α-AL. In presence of an acidic
catalyst β-AL can suffer ring opening to yield 2,4-pentadienoic
acid, which in the presence of BuOH undergoes esterification
with concomitant water formation (Scheme 2, pathway 3).
The presence of LA can then be explained by hydrolysis of
α-AL, BL, or pseudo-BL, respectively (Scheme 2, pathway 2). A
test using neat BL and A36 showed that BL remains stable
under these conditions. Alternatively, the hydrolysis of
pseudo-BL can yield the acetal form of LA which after elimi-
0.1
52.9
0.5
40.5
3.4
2
3
OcOH
3
00
40
20
i
PrOH
1
a
Conditions: 75 °C; α-AL (5.00 g, 51.00 mmol); alcohol (51.00 mmol);
b
A36 (250 mg). Determined by GC.
40 min are required for reaching 99.4% conversion of α-AL.
These differences show that the steric properties of the alkyl
3
7
chain influence the reaction.
Nevertheless, good to near full conversion to LAE is possi-
ble in all cases. For comparison, we studied the esterification
of LA under the same conditions (Fig. 3 and Table 3). In case
of EL, the yield only increases to 66.1%. This suggests that
the reaction moves to equilibrium. BuOH and OcOH show a
similar behaviour (Table 3, entries 2 and 3). Esterification of
BuOH reaches equilibrium after 240 min with 58.2% yield of
BL after 300 min. In case of OcOH, the equilibrium is
3
6
nation of the alcohol ultimately yields LA. Similarly, addi-
tion of water to α-AL may yield pseudo-LA which can readily
convert back to LA. Nevertheless, the results show, that in the
presence of BuOH, increasing the temperature results in a
decrease of LA formation. Thus, the more rapid conversion of
α-AL to pseudo-BL prevents isomerization and ring-opening
thereby minimizing water formation.
i
reached after 210 min with 41.3% yield of OL. Finally, PrOH
facilitates the poorest conversion and only 8.8% of the ester
is obtained (Table 3, entry 4). These data clearly demonstrate
the superiority of the addition reaction in comparison to the
esterification of LA.
Focusing on BL formation, we studied the activity and
selectivity of different classes of commercially available solid
acid catalysts under the same reaction conditions (Fig. 4).
Their specific surface area and pore volume are summarized
in Table S1.† Amberlyst® catalysts, A36 and A15 exhibit
almost identical behaviour toward the formation of BL over
time. Both catalysts enable full conversion after 10 min and
above 90% yield of BL after 120 min. This observation is
comparable to the results of Manzer who reported full con-
The reactions of ethanol (EtOH), octanol (OcOH), and
i
2
-propanol ( PrOH) with α-AL were tested to gain more
insight into the role of the alkyl group on the addition reac-
tion. The data (Table 2) confirm a clear structure activity
trend for linear alkyl chains following the order: EtOH >
BuOH > OcOH. In case of EtOH, up to 91.3% EL is obtained
after 180 min whereas for BuOH and OcOH, 240 and 300
min are required for 94.0% BL and 93.8% octyl levulinate
i
(
OL), respectively (Table 2, entries 1 and 2). In case of PrOH
the reaction proceeds significantly slower than for BuOH, as
version of α-AL and 99% selectivity toward the ester after 1 h
6
at 100 °C. ZrO
2
-based catalysts exhibit high thermal and
mechanical stability and have been successfully applied as
3
8–42
solid acid catalysts for various reactions.
However, one
Fig. 3 Addition of EtOH to α-AL (left) and esterification of LA (right).
Conditions: 75 °C; α-AL or LA (51.00 mmol); EtOH (2.34 g, 51.00
mmol); A36 (250 mg).
Fig. 2 Isomerization and polymerization of α-AL. Conditions: 75 °C;
α-AL (5.00 g, 51.00 mmol); A36 (250 mg).
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Table 3 Esterification of LA with different alcohols over A36a
both a higher concentration of acid sites as well as more acid
sites of high strength facilitating α-AL conversion. Interest-
ingly, WZ does not enable BL production and even in the
presence of SZ pseudo-BL presents the dominant product.
Additionally, two zeolites were investigated as catalysts for
b
LA
conv.
[%]
Yields [%]
b
t
Entry
ROH
[min]
LAE
Pseudo-LAE
1
2
3
3
EtOH
BuOH
OcOH
180
240
300
120
66.1
56.3
42.1
8.8
65.8
56.1
41.3
8.6
0.1
0.2
0.7
0.2
4
4,45
the synthesis of LAE.
The commercial zeolites, HBEA-25
and 150 (Si/Al ratio = 25 and 150) were employed and gave
full conversion of α-AL after 100 min. Over HBEA-150, 93%
yield of BL were achieved after 360 min, whereas for HBEA-
i
PrOH
a
Conditions: 75 °C; LA (5.92 g, 51.00 mmol); alcohol (51.00 mmol);
b
A36 (250 mg). Determined by GC.
25, 63% yield were obtained. As can be seen, only trace
amounts of pseudo-BL (0.5%) remained after 360 min when
HBEA-150 was applied. On the other hand, for HBEA-25 still
3
1% pseudo-BL are present. NH -TPD confirms a higher con-
3
centration of acid sites in case of HBEA-25 in agreement with
the higher aluminium content of this material. It would be
expected that this leads to higher activity; however, our study
emphasizes that the activity of both catalysts is quite compa-
rable. In fact HBEA-150 facilitates higher yields of LAE than
HBEA-25. A potential explanation relates to higher acidity of
the individual acid sites for lower aluminium contents in the
zeolite framework. Future studies will aim at a comprehen-
sive analysis of the influence of concentration and nature of
acid sites on the addition reaction with special emphasis on
the impact of acidity and the difference between Lewis and
Broensted acid sites.
Overall, the data above suggest that strong acidic sites are
required for trans-esterification of pseudo-LAE. Moreover,
these results also suggest that the trans-esterification of the
pseudo-ester is the rate limiting step of the reaction. The pro-
posed mechanism for the conversion of α-AL to LAE via
pseudo-LAE is shown in Scheme 3. Protonation of α-AL leads
to a stabilized cation which can undergo nucleophilic addi-
tion of an alcohol to give pseudo-LAE as product. Protonation
of the ester functionality of pseudo-LAE in turn allows
another nucleophilic addition and ring opening occurs
resulting in the hemi-acetal of LAE. Elimination of the alco-
hol yields LAE.
Fig. 4 Synthesis of BL over different solid acid catalysts. Conditions:
7
5 °C; α-AL (5.00 g, 51.00 mmol); BuOH (3.78 g, 51.00 mmol); catalyst
(
250 mg).
Finally, kinetic studies at different reaction temperatures
were carried out (ESI:† Fig. S4–6, Table S3) to obtain the
has to note that the acid site density of inorganic materials
usually is one order of magnitude lower compared to ion
exchange resins such as Amberlyst (ESI:† Table S2). The activ-
2−
3 2 4 2
ity of WO /ZrO (WZ) and SO /ZrO (SZ) was tested for the
addition reaction of α-AL. For comparison a bare monoclinic
ZrO2 was included in the study. After 360 min, ZrO2 gave
1
6.2% conversion of α-AL whereas WZ gave 85.1% conver-
sion. Interestingly, in both cases pseudo-BL is obtained as the
main product, together with small amounts of LA and BL. In
contrast, in the presence of SZ, α-AL is already fully converted
within 80 min accompanied by a higher yield of 11.5% BL
and a comparable yield of 3.6% LA. Hence, the different
activity of SZ is likely caused by its increased number of acid
4
3
sites and acid strength compared to WZ. The results of
NH -TPD are displayed in Fig. S3 and Table S2† summarizes
the quantitative analysis of these data. Indeed, SZ exhibits
3
Scheme 3 Proposed mechanism for the conversion of α-AL into LAE
via pseudo-LAE.
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activation energy for a consecutive formation of pseudo-LAE
and LAE. Data fitting was performed using pseudo-BL and BL
as model compounds.
Sample analysis
The identity of reaction products (e.g. pseudo-alkyl
levulinates, β-angelica lactone, γ-methylene-γ-butyrolactone,
From the estimation, the activation energy for the forma-
2,4-pentadienoic acid, and levulinic acid) was confirmed by
−
1
tion of pseudo-BL and BL are 3 and 50 kJ mol respectively
ESI:† Fig. S7 and Table S3). The data confirm that the pro-
GC-MS analysis and compared with authentic samples. Quan-
titative analysis of samples of the addition reactions was
performed by GC-FID using a CP-Wax-52 column (60 m × 250
μm × 0.25 μm). Qualitative analysis was performed by GC-MS
on a Trace GC chromatograph 1310 equipped with a Restek
Rxi-1 MS column (60 m × 250 μm × 0.5 μm) and a Thermo
(
tonation of α-AL and addition of the alcohol are an almost
barrier-less process. Consequently, weak acidic sites should
be sufficient to catalyse this transformation. In contrast, the
barrier for trans-esterification of pseudo-LAE is considerably
higher. The step requires activation of an ester toward nucle-
+
Scientific ISQ mass spectrometer (EI , 70 eV, 250 °C).
ophilic addition and the data show that the acidity of ZrO
2
and WZ are not suitable for this reaction. On the other hand,
SZ, Amberlyst® and the studied zeolites possess strong acidic
sites enabling a smooth conversion of pseudo-LAE to LAE. In
addition, both HBEA zeolites and zirconia-based catalysts
give rise to much lower amounts of LA compared to the
Amberlyst® materials. It appears therefore, that these two
classes of catalysts possess a lower ability to promote the
isomerization of α-AL to β-AL.
In summary, transformation routes via α-AL bear the
potential to facilitate e.g. γ-VL as well as LAE formation over-
coming water release during these reactions. Consequently,
α-AL has to be considered as important platform chemical in
the frame of efficient future biorefinery concepts.
Catalysts characterization
N physisorption measurement was performed using Micro-
2
meritic ASAP 2000. Before being measured, the samples were
evacuated at 120 °C overnight and measurements were
conducted at −196 °C (Table S1†). NH -TPD was measured by
ChemBET Pulsar TPR/TPD Automated Chemisorption
3
Analyser. Approximately 40–70 mg sample was firstly
−
1
subjected to 20 mL min of He flow for 30 min at 400 °C to
remove any contaminant on its surface. The preheated sam-
ple was further cooled down to room temperature. Subse-
quently, the adsorption of NH was carried out for 20 minute
at 100 °C by passing a 50%NH
3
3
–50%Ar gas mixture. The
physisorbed NH molecules were evacuated by switching the
3
flow to He and held for 20 min. Then the chemisorbed NH3
was gradually desorbed by passing He gas with a ramping
rate 20 °C min .
Conclusions
−
1
In conclusion, we were able to demonstrate the successful
conversion of α-AL to LAE over solid acids. Full conversion
and high yields of different LAE were achieved under rela-
tively mild conditions. Comparing the addition of alcohols to
α-AL and the esterification of LA shows the superiority of the
addition reactions under mild conditions. A kinetic investiga-
tion enables a better understanding of the reaction network
and the stability of intermediates and products. Finally, a cat-
alytic screening shows a dependency of the reactivity and the
selectivity toward LAE on the acidity and acid strength of the
catalyst. While α-AL conversion and the formation of
pseudo-LAE proceeds rapidly over all investigated catalysts,
trans-esterification of pseudo-LAE to LAE appears to require
acid sites of sufficient acid strength.
Acknowledgements
We wish to thank the Higher Institute of Applied Science and
Technology of Syria for the studentship of M. G. Al-Shaal and
the support of Indonesian Ministry of National Education
Scholarship for W. Ciptonugroho. We acknowledge financial
support by the Robert Bosch Foundation. Part of this work
was performed within the Cluster of Excellence “Tailor-Made
Fuels from Biomass” funded by the Excellence Initiative by
the German federal and state governments to promote sci-
ence and research at German universities.
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General procedure
A dried 50 mL three-neck round flask, fitted with a condenser
was charged with α-angelica lactone or levulinic acid (51.00
mmol), alcohol (51.00 mmol), and the catalyst (250 mg)
under inert gas. The reactor was heated using an oil bath
and agitation was provided by magnetic stirrer at 350 rpm.
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syringe and centrifuged. The resulting clear samples were
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