Silica-supported sulfonic acids as recyclable catalyst for esterification of levulinic acid with stoichiometric amounts of alcohols

Article abstract of DOI:10.3762/bjoc.12.207

Converting biomass into value-added chemicals holds the key to sustainable long-term carbon resource management. In this context, levulinic acid, which is easily obtained from cellulose, is valuable since it can be transformed into a variety of industrially relevant fine chemicals. Here we present a simple protocol for the selective esterification of levulinic acid using solid acid catalysts. Silica supported sulfonic acid catalysts operate under mild conditions and give good conversion and selectivity with stoichiometric amounts of alcohols. The sulfonic acid groups are tethered to the support using organic tethers. These tethers may help in preventing the deactivation of the active sites in the presence of water.

Full text of DOI:10.3762/bjoc.12.207

Silica-supported sulfonic acids as recyclable catalyst for
esterification of levulinic acid with stoichiometric
amounts of alcohols
Raimondo Maggi*1, N. Raveendran Shiju*2, Veronica Santacroce1,2, Giovanni Maestri1,
Franca Bigi1,3 and Gadi Rothenberg2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 2173–2180.
1Clean Synthetic Methodology Group, Dipartimento di Chimica,
Università di Parma, Parco Area delle Scienze 17A, I-43124 Parma,
Italy, 2Van ’t Hoff Institute for Molecular Sciences, University of
Amsterdam, Science Park 904, 1098 XH, Amsterdam, The
Netherlands. Tel: +31-20-5256515 and 3Istituto IMEM-CNR, Parco
Area delle Scienze 37/A, I-43124 Parma, Italy
doi:10.3762/bjoc.12.207
Received: 27 July 2016
Accepted: 22 September 2016
Published: 12 October 2016
This article is part of the Thematic Series "Green chemistry".
Guest Editor: L. Vaccaro
Email:
Raimondo Maggi* - raimondo.maggi@unipr.it; N. Raveendran Shiju* -
n.r.shiju@uva.nl
© 2016 Maggi et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
esterification; heterogeneous catalysis; renewable feedstocks;
supported organic catalysts; sustainable chemistry
Abstract
Converting biomass into value-added chemicals holds the key to sustainable long-term carbon resource management. In this
context, levulinic acid, which is easily obtained from cellulose, is valuable since it can be transformed into a variety of industrially
relevant fine chemicals. Here we present a simple protocol for the selective esterification of levulinic acid using solid acid catalysts.
Silica supported sulfonic acid catalysts operate under mild conditions and give good conversion and selectivity with stoichiometric
amounts of alcohols. The sulfonic acid groups are tethered to the support using organic tethers. These tethers may help in
preventing the deactivation of the active sites in the presence of water.
Introduction
Vegetal biomass is mankind’s only source of renewable carbon for covering the worldwide demand for non-fuel chemicals
on a human timescale. It is abundantly available, with the [1-4]. Currently, the main research thrust is directed at lignocel-
potential of replacing fossil-based carbon on a scale sufficient lulose, the most abundant fraction of biomass. The mass com-
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Beilstein J. Org. Chem. 2016, 12, 2173–2180.
position of lignocellulose could be roughly represented by a grant to levulinates a novel route of application. By tailoring
5/3/2 ratio of cellulose, hemicellulose and lignin, respectively. their physicochemical properties they could become comple-
All of these polymers are the subject of many studies [5-11].
mentary to common esters and other solvents, which might be
more harmful for both humans and the environment [16]. It
Levulinic acid (LA) is one of the most important platform should be also noted that ethyl levulinate could shrink the emis-
chemicals as it is a versatile building block for a variety of sion of nitrogen oxides from exhausts of diesel engines when
value-added agrochemicals, fine chemicals and pharmaceutical used as additive [17,18].
intermediates [12,13] (Scheme 1, bottom). Moreover, it can be
obtained from cellulose with relative ease and high selectivity Due to their importance, new strategies have been developed for
(see Scheme 1, top) [14].
the production of levulinic esters [19-22]. Homogeneous
Brønsted acids could catalyse the esterification of levulinic acid
in the presence of alcohols and reports on this reactivity date
back to the nineties [23]. Although this route could ensure high
chemical yields, it still presents a series of drawbacks. In partic-
ular, issues with catalyst recycling and product separation limits
the environmental viability of this strategy. As a result, it
remains of high interest to develop alternatives to trigger this
reaction, which are more sustainable, for instance through the
design of suitable and recyclable solid acid catalysts. In the lit-
erature, methods that use solid heteropolyacids, such as ammo-
nium or mixed ammonium and silver-doped phosphotungstic
acid, sulfated metal oxides (such as sulfated titania, sulfated
zirconia), zeolites and hydrotalcites have been reported [24-30].
These solid catalysts share several advantages, including high
activity and an easy recovery, which might provide a real basis
for future application in commercial processes. Nevertheless,
they require high temperatures (usually above 100 °C) and long
reaction times [24-30]. Furthermore, they often share another
common pitfall, namely the use of large molar excess of
alcohol, either for practical convenience [31] or to minimise
ester hydrolysis. As meaningful examples, it has been recently
reported that acid ZSM-5 zeolites, with encapsulated
maghemite particles to allow magnetic catalyst recover, could
be used to directly convert furfuryl alchol into an alkyl levuli-
nate upon warming at 130 °C for 8 hours in the presence of a
large excess of alchol as solvent/reagent (100 equiv) [32]. Al-
though the behaviour of many metal oxides has been investigat-
ed, reports featuring the activity of supported organic Brønsted
acids are very few. In particular, Tejero reported that sulfonic
acid supported on polymeric resins could catalyse the esterifica-
tion of LA, providing conversions up to 94% upon warming at
80 °C for 8 hours in the presence of 3 equiv of n-butanol [33].
Melero described the synthesis of mesostructured silica frame-
works featuring pending organosulfonic arms. The best catalyst
provided quantitative conversion of LA upon warming of the
reaction mixture at 130 °C for 2 hours in the presence of a five-
Scheme 1: Synthesis of levulinic acid from ligno-cellulosic feedstocks
and its principal uses to access fine chemicals.
Levulinic acid esters are of particular interest for the chemical fold molar excess of ethanol, used as solvent/reagent [34].
industry [12,13]. Their main current market is represented by
the formulation of flavours and fragrances [15], although the Here we present an alternative strategy in which a heterogen-
scale of these preparations did not boosted demand yet. Howev- eous catalyst triggers the selective esterification of levulinic
er, the seek to develop more eco-compatible solvents might acid with a stoichiometric amount of alcohol.
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In the last years, many methods have been developed for the in the literature we did not retrieve any catalytic method for the
transformation of homogeneous catalysts into recyclable hetero- esterification of biomass-derived acids that operates with a
geneous ones. To prevent leaching, a common strategy is teth- lower molar excess of alcohol [24-34]. 1-Pentanol was selected
ering the active species with the support via covalent bonds as model substrate in order to work over an ample range of
[35]. This approach increases the stability of the catalyst itself operating temperatures. Thus, in a typical experiment, 10 mmol
compared to impregnation (Figure 1). Furthermore, the activity of levulinic acid were stirred at 100 °C in a sealed tube for 2 h
of the catalyst can be tuned through adoption of a suitable under air in the presence of the amount of a solid catalyst neces-
linker.
sary to have 1 mol % of acid sites. The results are reported in
Table 1.
All of the prepared silica-supported sulfonic acids showed very
good catalytic activity for the esterification of levulinic acid
(Table 1, entries 1–4). Materials with an arylsulfonic moiety
were initially investigated (Table 1, entries 1 and 2). They
present a comparable loading of Brønsted sites (0.73 and
0.65 mmol/g respectively) and ensured conversion of 1 above
90% within two hours (94 and 92%). The catalyst without any
alkyl tether was more selective, ultimately delivering the
desired product 3a in 90% yield. Silica-supported propyl
sulfonic acid provided slightly better results (Table 1, entries 3
Figure 1: Anchoring methodologies: a) impregnation; b) covalent
binding.
Results and Discussion
As part of our interest in acid catalysis [36-38], we prepared a and 4). The material presented a lower density of Brønsted sites
set of solid materials for the esterification of levulinic acid. (0.51 mmol/g), but delivered almost complete conversion of 1
Upon preliminary screening, supported sulfonic acids seemed within 2 h (>95%), affording 3a in 93% yield. We then repeated
promising candidates. They were prepared following a reported the experiment adding activated molecular sieve in the reaction
procedure by the tethering method [39], which consists of the flask (Table 1, entry 4) to check whether water coproduced by
immobilisation of a functional moiety on an inorganic support the reaction could cause any harm. The outcome paralleled the
via covalent bonds ensured by a suitable linker [35].
standard procedure, conversion and yield being 96% and 94%,
respectively. This result shows that the presence of water is
In preliminary experiments reactions were carried out with a tolerated by the catalytic system, which in turn did not easily
five-fold molar excess of alcohol. We started from this ratio as trigger the hydrolysis of levulinates under these conditions.
Table 1: Screening of different solid acids in the esterification reaction of levulinic acid with 1-pentanol.
Entry
Sulfonated catalyst
Catalyst acidity (mmol H+/g) Conversion of 1 (%) Yield of 3a (%) Selectivity of 3a (%)
1
2
SiO2-(CH2)3-O-C6H4-SO3H
SiO2-C6H4-SO3H
SiO2-(CH2)3-SO3H
SiO2-(CH2)3-SO3H
Amberlyst 15
0.73
0.65
0.51
0.51
4.70
0.80
0.12
94
92
95
96
52
72
84
57
84
90
93
94
31
68
80
55
89
98
98
98
60
94
95
96
3
4a
5
6
Nafion®
7
Aquivion®
8
H2SO4
aWith the addition of 4 Å molecular sieves. Values by GC upon calculation of response factors for 1 and 3a from pure samples over the concentration
interval of the reaction; the selectivity has been calculated as the ration between yield of 3a and conversion of 1.
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Remarkably, the presence of water on the catalyst surface can water coproduced by the esterification did not cause any signifi-
inhibit the catalytic sites of inorganic materials instead [40]. cant hydrolysis of the desired ester 3a. Furthermore, the almost
Acidic and/or hydrophilic metal oxides and sulfates easily complete conversion of 1 with a stoichiometric amount of 2a
adsorb water on their surface, which is the coproduct of the allows minimising the consumption of reagents and therefore
esterification. This can severely reduce the activity of the cata- the overall costs of the transformation. To the best of our know-
lyst. Considering our supported sulfonic acids, we speculate that ledge, using stoichiometric amounts of alcohol has not been re-
their organic tethers could smooth the hydrophilic character of ported previously. In the present case, this can be possible as we
their Brønsted sites and thus prevent the deactivation due to could show that a relatively high concentration of water did not
water.
hinder the reaction. Catalysts more prone to deactivation might
require a larger molar excess of alcohols to prevent water-
We then tested a selection of commercial catalysts (Table 1, poisoning.
entries 5–7). Despite encouraging literature precedents [22,33],
Amberlyst 15 gave only 52% conversion of 1 within 2 h The reaction conditions were further optimized studying the
(Table 1, entry 5, 31% yield). We then switched to perfluori- effect of the temperature. So, a series of experiments were
nated resins. Nafion® and Aquivion® showed an interesting carried out using the sulfonated catalyst (1%), an equimolecu-
selectivity towards 3a, but conversion of 1 proved once again lar amount of reagents under solvent free conditions, and
below that observed with supported sulfonic acids (72% and varying the temperature between 50–100 °C. Results are re-
84%, respectively). Finally, a common homogeneous acid was ported in Table 3.
used for comparison. The use of 1 mol % of H2SO4 (Table 1,
entry 8) delivered 3a in 55% yield only. Furthermore, conver- In all cases, the selectivity towards the esterification product 3a
sion of 1 remained stuck at 57% even prolonging the reaction remained complete. A comparable yield of 3a was recovered
time for up to 24 h. This result shows that heterogeneous reducing the temperature from 100 to 75 °C (Table 3, entry 2,
sulfonic acids outperform their homogeneous peer under these 93%). By reducing the temperature to 50 °C (Table 3, entry 3),
conditions.
longer reaction times became necessary. At 50 °C, conversion
peaked at 79% upon 7 h and no longer improved even by
Upon identification of silica-supported sulfonic acids as cheap keeping the mixture for further 24 h. Reasoning on the practical
and promising candidates for the selective esterification of LA, viability of the method, we therefore continued our study fixing
the reaction parameters were then optimized in order to the temperature at 75 °C. We then evaluated the amount of cata-
maximise the environmental viability of the method. We thus lyst (Table 4).
tried to shelve the molar excess of the alcohol (Table 2).
Surprisingly, an increase of the catalyst amount to 5 mol %
Reactions were carried out at 100 °C and regularly monitored resulted in lower selectivity towards 3a, which has been
for 2 h. To our delight, varying the amount of alcohol did not retrieved in 85% yield together with traces of one unidentified
hamper neither conversion nor selectivity. Indeed, 3a was byproduct (Table 4, entry 1). On the other hand, reduction of
recovered in 93% yield using a two-fold molar excess of 2 the catalyst loading to 0.1 mol % slows down the process,
(Table 2, entry 2). A comparable result was achieved with a conversion being just 36% upon 2 h (Table 4, entry 3). Further
stoichiometric amount of pentanol (Table 2, entry 3, 94% reduction to 0.01 mol % confirmed this trend and delivered 3a
yield). It is remarkable that even in this case the amount of in 14% yield (Table 4, entry 4). Even by prolonging the reac-
Table 2: Variation of the acid/alcohol ratio.
Entry
Acid:alcohol ratio
Conversion of 1 (%)
Yield of 3a (%)
Selectivity of 3a (%)
1
2
3
1:5
1:2
1:1
96
95
96
94
93
94
98
98
98
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Table 3: Variation of the reaction temperature.
Entry
Temperature (°C)
Conversion of 1 (%)
Yield of 3a (%)
Selectivity of 3a (%)
Time (h)
1
2
3
100
75
96
95
79
94
93
77
98
98
97
2
2
7
50
Table 4: Effect of the amount of catalyst.
Entry
Catalyst amount (%)
Conversion of 1 (%)
Yield of 3a (%)
Selectivity of 3a (%)
1
2
3
4
5
1
95
95
36
15
85
93
35
14
89
98
97
93
0.1
0.01
tion time to 24 h, conversion did not reach completion and the
ester was isolated in 58 and 38% yield with 0.1 and 0.01 mol %
of catalyst, respectively. In any case, the selectivity remained
almost complete (>98% by GC).
We then ensured that the catalyst acts as a heterogeneous
species by performing a filtration test. In agreement with the
hypothesis, we monitored no further conversion on the filtrate
[41], proving that no leaching occurred. The recyclability of the
catalyst was then evaluated. The catalyst was recovered by
filtration, washed with ethyl acetate (10 mL), dried and reused
for a further esterification. The results are shown in Figure 2.
The catalyst can be recovered and reused for 6 cycles at least,
fully preserving its activity and selectivity. For instance,
conversion of 1 and yield of 3a were 94 and 92%, respectively,
upon the fifth re-cycle.
Figure 2: Activity of the supported sulfonic acid catalyst within the first
six cycles. Reaction conditions: 1 mol % cat., acid:alcohol ratio = 1:1,
solvent free, 75 °C, 2 h.
Finally, with optimized conditions in our hands, we checked the method could be extended to secondary alcohols as isopropanol
scope of this catalytic methodology (Table 5).
and L-menthol. Despite their increased steric hindrance, very
good results were obtained with a selectivity towards 3 >99%
As expected, the best performances were obtained with primary (Table 5; entries 3 and 4, 59 and 76% yield). In particular, it is
alcohols (Table 5, entries 1 and 2), which afforded the desired important to underline that a single diastereomer of product 3d
ester in 93 and 79% yield, respectively. Gratifyingly, the was formed (Table 4, entry 4). This implies that the present
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Beilstein J. Org. Chem. 2016, 12, 2173–2180.
Table 5: Esterification of levulinic acid with different alcohols.
Entry
1
Alcohol
Conversion of 1 (%)
Yield of 3 (%)a
Selectivity of 3 (%)
95
93
79
98
2a
2b
3
80
60
99
98
2b
2c
59
76
4
77
0
99
–
2d
2e
5c
0
aIsolated yields upon chromatography; bby warming for 5 h; cby warming for 24 h.
method, likely thanks to its mild conditions, allows to preserve any molar excess of reagents, these features highlight the prac-
chiral information present on substrates and could thus effi- tical and environmental viability of this catalytic method.
ciently transfer it on the products.
Experimental
On the other hand, no conversion of 1 was observed using Catalysts preparation
tertiary alcohols, as witnessed by entry 5. Probably, their steric SiO2-(CH2)3-SO3H [35]: Amorphous silica (8.0 g) has been re-
hindrance quenches any reactivity.
fluxed under stirring for 24 h with (3-mercaptopropyl)tri-
methoxysilane (MPTS) (1.15 mL; 6.1 mmol) in toluene
(120 mL) and the resulting supported propylmercaptane has
Conclusion
Silica-supported sulfonic acids proved very active heterogen- been oxidized to propanesulfonic acid by treatment with 30%
eous catalysts for the selective esterification of levulinic acid aq H2O2 (100 mL; 1 mol) for 24 h under stirring at rt, adding a
with stoichiometric amounts of primary alcohols. The esterifica- few drops of concentrated sulfuric acid after 12 h. Acidity has
tion can be carried out under mild conditions (75 °C, 2 h) and been measured via the titration method [35] (0.51 mmol H+/g).
provides good to excellent yields with various primary and sec-
ondary alcohols. The selectivity towards desired products SiO2-C6H4-SO3H [35]: Amorphous silica (8.0 g) has been re-
remained complete in all cases. The coproduct of the reaction, fluxed in toluene (120 mL) with phenyltriethoxysilane (2.0 mL,
namely water, did not hamper the efficiency of this solvent-free 8.3 mmol) under stirring for 24 hours. The resulting solid was
process.
then filtered off and washed with toluene (3 × 20 mL). The sup-
ported phenyl group was then sulfonated by refluxing in 1,2-
The selected catalyst is cheap, can be easily prepared from com- dichloroethane (60 mL) the functionalized material with
mercial reagents and proved very robust. It is very active and cholorosulfonic acid (10 mL, 150 mmol) under stirring for
selective, water-tolerant and recyclable. It represents therefore 4 hours. The solid was then recovered by filtration and
an interesting and complementary alternative to existing esteri- washed with 1,2-dichloroethane (3 × 20 mL), acetone
fication catalysts. Together with the absence of solvents and of (3 × 20 mL) and water (3 × 50 mL) to deliver the title com-
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