A dramatic effect of the ionic liquid structure in esterification reactions in protic ionic media

Article abstract of DOI:10.1039/c2gc35941c

Bronsted acidic ionic liquids (ILs) have been synthesized and investigated as catalysts in esterification and transesterification reactions: simple and non-corrosive salts characterized by aliphatic cations associated with an "acidic" anion (in particular, [HSO₄]^-) gave the higher yields. A quite good correlation between IL acidity (H ₀) and catalytic ability was found although also the hydrophilic nature of the ionic medium probably affects the process efficiency.

Full text of DOI:10.1039/c2gc35941c

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A dramatic effect of the ionic liquid
structure in esterification reactions in
protic ionic media
Cinzia Chiappe, Sunita Rajamani and Felicia
D’Andrea
Brønsted acidic ionic liquids (ILs) have been
synthesized and investigated as catalysts in
esterification and transesterification
reactions

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Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
A dramatic effect of the ionic liquid structure in esterification reactions
in protic ionic media
Cinzia Chiappe,*a Sunita Rajamania and Felicia D’Andreab
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Brønsted acidic ionic liquids (ILs) have been synthesized and investigated as catalysts in esterification
and transesterification reactions: simple and nonꢀcorrosive salts characterized by aliphatic cations
associated with an “acidic” anion (in particular, [HSO4]ꢀ) gave the higher yields. A quite good correlation
between IL acidity (H0) and catalytic ability was found although also the hydrophilic nature of the ionic
10 medium probably affects the process efficiency.
Introduction
salts simply by addition of an equimolar amount of a strong
inorganic acid (HCl, H2SO4, HNO3, CF3COOH and H3PO4) to a
The possibility to use Brønsted acidic ionic liquids (ILs) as 50 commercial Nꢀbase, like morpholine, pyrrolidine, piperidine,
catalysts and solvents in various reactions, instead of the
15 corrosive acids, represents an attractive research area.1 In this
contest, the Fischer esterification of carboxylic acids with
alcohols has been object of extensive investigation.1,2 This
equilibrium reaction, of industrial importance,3 generally requires
an acid catalyst and an excess of one of the reagents or a
20 continuous removing of the formed water to shift the equilibrium
towards the ester; water scavengers and/or the use of an
additional solvent have been largely employed in the past to carry
over the water in the form of azeotrope. However, such
operations require large energy input to recycle the solvents
25 and/remove the excess of reactants. Furthermore, the loss of
volatile organic solvents to the atmosphere results into increase in
the cost of production and also damage to the environment.4
betaine and imidazole.
Cation:
H
N
O
N
N
N
N
H3C
H
H3C
H
H3C
H
CH3
[HPip]+
[HPyrr]+
[HMor]+
[Hmim]+
H
N
H3C
H3C
N
COOH
CH3
N
[Hdabco]+
[Bet]+
Starting from 2002, Brønsted acidic ionic liquids have been
proposed as catalysts and/or media to improve the Fischer
30 esterification of organic substrates. Initially, Lewis acidic ILs (1ꢀ
butylpyridinium chloride + aluminium(III) chloride) have been
tested5 but rapidly the interest has moved towards more the
manageable Brønsted acidic salts and both ILs bearing a
protonated acidic group on the cation alkyl chain (such as −SO3H
35 and −CO2H) or a proton on the quaternary nitrogen atom of
cation, as well as ILs having an available proton on anion
-
-
-
-
-
Anion = Cl- ; HSO4 ; NO3 ; H2PO4 ; SO42-; CF3CO2 ; BF4
Scheme 1. Protic cations used in combination with the reported anions
55
to obtained Brønsted acidic ILs.
Here, we report on the application of these salts (some of them
have melting points higher than 100 °C and, consequently, cannot
be defined ILs) in esterification and transesterification reactions
proving a simple, sustainable and effective method to prepare and
60 isolate esters. In particular, they have been tested in esterification
of acetic acid with butanol, octanol and methylꢀβꢀ
glucopyranoside. In all the investigated reactions, [HPyrr][HSO4]
(followed [HPip][HSO4] and [Hmim][HSO4]) was found to be the
best catalytic environment suggesting that cation structure and
65 counteranion acidity have an important role in the process. A
quite good correlation between IL acidity (H0) and catalytic
ability has been found although the capability of these media to
bind water and give a biphasic system with the formed ester
surely favour the shift of the esterification reaction towards
(HSO4 , H2PO4 ) have been screened.6 Despite the variety of
cations and anions which can give Brønsted acidic ionic liquids,
the majority of the ILs (not necessarily Brønsted acidic ILs)
40 nowadays reported for esterification are imidazole and pyridine
−
−
D
ꢀ
derivatives7
([(C2H5)3NH][HSO4],
although
simple
triethylammonium
salts
and
[(C2H5)3NH][H2PO4]
[(C2H5)3NH][BF4]) have used with success in esterification of
some carboxylic acids with primary alcohols.8
45 With the aim to develop new classes of “greener” ILs (i.e. salts
prepared through more simple procedures and having a lower
environmental impact9), we have prepared some Brønsted acidic
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ppm) 2.85 (s, 3H, N+CH3); 3.20 (m, 2H, HCN+); 3.46 (m, 2H,
same acidic salts were also tested in the transesterification of 60 HCN+); 3.80 (m, 2H, HCO); 4.09 (m, 2H, HCO).13C NMR (63
products, contemporaneously favouring ester recover. Finally, the
ethyl transꢀcinnamate with methanol and octanol. Also in this
case, the nonꢀcorrosive [HPyrr][HSO4] gave the best results
among all.
MHz, D2O, δ ppm) δ: 42.66; 52.60; 63.31.
Nꢀmethylmorpholinium sulfate [HMor]2[SO4], deliquescent
white solid: 1H NMR (250 MHz, D2O, δ ppm) 2.90 (s, 3H,
N+CH3); 3.20 (m, 2H, HCN+); 3.53 (m, 2H, HCN+); 3.80 (m, 2H,
65 HCO); 4.09 (m, 2H, HCO).13C NMR (63 MHz, D2O, δ, ppm) δ:
42.60; 52.70; 63.42.
Nꢀmethylmorpholinium chloride [HMor]Cl, deliquescent crystals
: 1H NMR (250 MHz, D2O, δ ppm) 2.93 (s, 3H, N+CH3); 3.20 (m,
2H, HCN+); 3.50 (m, 2H, HCN+); 3.80 (m, 2H, HCO); 4.09 (m,
70 2H, HCO).13C NMR (63 MHz, D2O, δ, ppm) δ: 42.66; 52.70;
63.38.
5
Experimental
Materials and Methods
Chemicals, including Nꢀmethylimidazole, Nꢀmethylmorpholine,
Nꢀmethylpyrrolidine, Nꢀmethylpiperidine, DABCO and betaine
10 were purchased from SigmaꢀAldrich with purities > 98%. Nꢀ
Methylimidazolium hydrogen sulfate [Hmim][HSO4], Nꢀ
methylimidazolium sulfate [Hmim]2[SO4], Nꢀmethylimidazolium
dihydrogen phosphate [Hmim][H2PO4], Nꢀmethylimidazolium
trifluoroacetate [Hmim][CF3CO2], Nꢀmethylimidazolium nitrate
15 [Hmim][NO3], Nꢀmethylimidazolium chloride [Hmim]Cl, Nꢀ
methylmorpholinium hydrogen sulfate [HMor][HSO4], Nꢀ
Nꢀmethylpiperidinium hydrogen sulfate [HPip][HSO4], m.p. 32
1
°C: H NMR (250 MHz, D2O δ ppm): 1.2ꢀ1.7 (m, 6H); 2.74 (s,
3H, N+CH3); 2.85 (m, 2H); 3.38 (m, 2H).
75 Nꢀmethylpiperidinium sulfate [HPip]2[SO4], hygroscopic white
solid: 1H NMR (250 MHz, D2O δ ppm): 1.2ꢀ1.7 (m, 6H); 2.74 (s,
3H, N+CH3); 2.85 (m, 2H); 3.38 (m, 2H).
methylmorpholinium
methylmorpholinium chloride [HMor]Cl, Nꢀmethylpiperidinium
hydrogen sulfate [HPip][HSO4], Nꢀmethylpiperidinium sulfate
sulfate
[HMor]2[SO4],
Nꢀ
Nꢀmethylpyrrolidinium hydrogen sulfate [HPyrr][HSO4], m.p. 34
°C: 1H NMR (250 MHz, D2O, ppm): 3.36 (t, 2H, NCH2), 2.76 (t,
80 2H, NCH2), 2.62 (s, 3H, NCH3), 1.85–1.73 (m, 4H, NCH2CH2).
20 [HPip]2[SO4],
Nꢀmethylpyrrolidinium
hydrogen
sulfate
[HPyrr][HSO4], Nꢀmethylpyrrolidinium chloride [HPyrr]Cl,
betaine chloride ([Bet]Cl), betaine nitrate ([Bet][NO3]), betaine
hydrogensulfate ([Bet][HSO4]) and betaine tetrafluoroborate
([Bet][BF4]) were prepared by addition dropwise of an equimolar
25 amount of the proper acid to the selected base or zwitterion. The
mixtures were stirred for 4 to 5 h at 60°C to ensure that all of the
bases are reacted. Then, water was removed at reduced pressure
to obtain a colorless viscous liquid or a solid.
Nꢀmethylimidazolium hydrogen sulfate [Hmim][HSO4] m.p. 39
30 °C: 1H NMR (250 MHz, D2O δ, ppm) 3.21 (s, 3H, CH3Nꢀ); 6.77
(s, 2H, aromatic); 7.97 (s, 1H, aromatic). 13C NMR (63 MHz,
DMSOꢀd6, δ ppm) 34.27, 118.18, 121.83, 134.18.
Nꢀmethylimidazolium sulfate [Hmim]2[SO4], hygroscopic white
solid, m.p. 62 °C: 1H NMR (250 MHz, DMSOꢀd6, δ ppm) 3.77 (s,
35 3H, CH3N); 7.22 (s, 1H, aromatic); 7.39 (s, 1H, aromatic); 8.33
(s, 1H, aromatic) 7.20 (br, 1H, N+ꢀH). 13C NMR (63 MHz,
DMSOꢀd6, δ ppm) 34.02, 121.76, 123.67, 136.69.
1
Nꢀmethylpyrrolidinium chloride [HPyrr]Cl, m.p. 158ꢀ160 °C: H
NMR (250 MHz, D2O, ppm): 3.30 (t, 2H, NCH2), 2.75 (t, 2H,
NCH2), 2.60 (s, 3H, NCH3), 1.85–1.73 (m, 4H, NCH2CH2).
1
Betaine hydrogen chloride ([Bet]Cl), m.p. 240 °C: H NMR (250
85 MHz, D2O, δ ppm): 3.28 (s, 9H, (CH3)3N+); 4.26 (s, 1H, COOH);
4.85 (s, 2H CH2). 13C NMR (63 MHz, D2O, δ ppm): 54.5, 64.0,
168.3.
Betaine nitrate ([Bet][NO3]), highly hygroscopic white solid, m.p
128 °C: 1H NMR (250 MHz, D2O, δ ppm): 3.28 (s, 3H,
90 (CH3)3N+); 4.25 (s, 1H, COOH); 4.85 (s, 2H, CH2). 13C NMR
(63 MHz, D2O, δ ppm): 54.5, 64.2, 171.2.
1
Betaine hydrogen sulfate ([Bet][HSO4]) m.p. 148 °C: H NMR
(250 MHz, D2O, δ ppm): 3.28 (s, 3H, (CH3)3N+); 4.25 (s, 1H, ꢀ
COOH); 4.85 (s, 2H, CH2). 13C NMR (63 MHz, D2O, δ ppm):
95 54.5, 64.0, 171.3.
Betaine tetrafluoroborate ([Bet][BF4]), m.p. 200 °C: 1H NMR
(250 MHz, DMSOꢀd6, δ ppm): 3.19 (s, 3H, (CH3)3N+); 4.29 (s,
2H, CH2). 13C NMR (63 MHz, D2O, δ ppm): 53.5, 63.0, 166.3.
Nꢀmethylimidazolium dihydrogen phosphate [Hmim][H2PO4]
m.p. 137 °C: H NMR (250 MHz, CDCl3, δ ppm) 3.7 (s, 3H,
1
40 NCH3); 6.88 (s, 1H, aromatic), 7.35 (s, 1H, aromatic); 8.92 (s,
1H, aromatic; 9.87 (N+ꢀH).
100 Procedure for esterification of acetic acid with alcool (butanol
or octanol)
Nꢀmethylimidazolium
trifluoroacetate
[Hmim][CF3CO2],
hygroscopic white solid, mp 55 °C: 1H NMR (250 MHz, CDCl3,
δ, ppm) 3.93 (s, 3H, NCH3); 7.14 (s, 1H, aromatic), 7.35 (s, 1H,
45 aromatic); 8.92 (s, 1H, aromatic); 9.87 (br N+ꢀH). 13C NMR (63
MHz, CDCl3, δ ppm): 35.8, 121.3, 122.1, 136.3; 162.1.
Nꢀmethylimidazolium nitrate [Hmim][NO3], hygroscopic white
solid, mp 67 °C: 1H NMR (250 MHz, D2O δ, ppm) 3.75 (s, 3H,
NCH3); 7.33 (s, 2H, aromatic), 8.46 (s, 1H, aromatic). 13C NMR
50 (63 MHz, coaxial tube at 70 °C, δ ppm): 35.2, 118.3, 122.1,
134.3.
Generally, to 1 g of the selected Brønsted acidic IL, 2 ml of the
selected alcool (butanol or octanol) and 1 equivalent mole of
acetic acid (Carlo Erba, 99.9%) were added. In the case of
105 octanol, the reaction mixtures were heated for 4 h at 110°C. Then,
products were separated just by decanting, when two layers were
formed, or by extraction with ethyl ether. The crude reaction
mixtures were analyzed by NMR and GCꢀMS. With some
selected ILs, reactions were carried out also modifying
110 temperature and time. Results are shown in Table 1 and 3.
Nꢀmethylimidazolium chloride [Hmim]Cl, hygroscopic white
1
solid, mp 78 °C: H NMR (250 MHz, CDCl3, δ ppm) 3.5 (s, 3H,
Procedure for esterification of acetic acid with β-methyl-D-
glucopyranoside
To 1.5 g of the selected Brønsted acidic ILs ([Hmim]Cl or
115 [Hmim][NO3] or [Hmim][HSO4] or [Hmim][H2PO4]) 0.5 g
NCH3); 6.82 (s, 1H, aromatic), 6.87 (s, 1H, aromatic); 8.54 (s,
55 1H, aromatic; 10.7 (N+ꢀH). 13C NMR (63 MHz, coaxial tube, δ
ppm): 35.2, 118.3, 122.1, 134.3.
Nꢀmethylmorpholinium hydrogen sulfate [HMor][HSO4],
hygroscopic white solid, mp 32 °C: H NMR (250 MHz, D2O, δ
1
(0.00258 moles) of βꢀmethylꢀDꢀglucopyranoside and 1.1
2
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equivalent mole of acetic acid (Carlo Erba, 99.9%) were added.
The reaction mixtures were heated for 4 h at 60°C. Then,
products were extracted with ethyl ether and, after solvent
evaporation at reduced pressure, the crude reaction mixtures were
analyzed by NMR. Results are reported in Table 4.
and the exposing surface area. The results of corrosion tests are
summarized in Table 2.
60
Results and Discussion
5
Several imidazolium, morpholinium, pyrrolidinium, piperidinium
dabco and betaine based Brønsted acidic ILs were used as
Procedure for transesterification of ethyl trans-cinnamate.
65 solvents and catalysts in esterification and transesterification
reactions (Scheme 1, not all of them were used in each reaction).
Transesterification using methanol
10 To 1 g (0.00567 moles) of ethyl transꢀcinnamate 3 equivalents of
methanol and an equimolar amount of the selected Brønsted
acidic IL ([HPyrr][HSO4], [Hmim][HSO4], [Bet][H2SO4],
[HMor]Cl and [HPip][HSO4]) were added and the reaction
mixtures were heated at 90°C for 30 h into a reactor fitted with a
15 reflux condenser. After stopping, the products were extracted
with ethyl ether and, after solvent evaporation at reduced
pressure, the crude reaction mixtures were analyzed by NMR.
Results are reported in Table 5.
Scheme 2. Fischer esterification of acetic acid.
Preliminary experiments were carried out at 110°C, working with
70 a 1:1 molar ratio of octanol and acetic acid (Scheme 1) and using
ca. 1 g of the selected IL, the solventꢀcatalyst loading being
around 60% w/w (based on the mass of octanol) corresponding to
the IL/octanol molar ratios % reported in Table 1. To assess the
catalytic ability of the selected ILs conversions were evaluated by
75 NMR. Data are reported in Table 1. Experiments carried out at
shorter reaction times showed that 4 hours assured equilibrium
conditions, at least in the case of the more efficient media.
20 Transesterification using octanol
To 1 g (0.00567 moles) of ethyl transꢀcinnamate equimolar
amounts of octanol as well as equimolar amounts of the selected
Brønsted
acidic
ILs
([Hmim][HSO4](1:1)
and
[Hmim][HSO4](1:1.1)) were added. After stopping the reaction,
25 products were extracted with ethyl ether and, after solvent
evaporation at reduced pressure, the crude reaction mixtures were
analyzed by NMR. Results are reported in Table 6.
Table 1. Esterification of acetic acid with octanol, conversions and yields
80 after 4 h at 110°C
Determination of IL Brønsted acidity
30
Entry
IL
IL/alcool
(%)
Ester : alcohola
Recovered
material
(%)
83
The Brønsted acidity of some selected ILs was evaluated from
the determination of the Hammett functions (H0) by UVꢀvisible
spectroscopy. Standard solutions of ILs (200 mmol/L) and 4ꢀ
nitroaniline (0.2 mmol/L) in water were prepared and the UVꢀvis
35 spectra were recorded at 25 °C both in the absence (1.5 ml of dye
solution mixed with 1.5 ml water) and in the presence of IL (1.5
ml of dye solution mixed with 1.5 ml of IL solution). The values
of Ho have been calculated based on the intrinsic pKa of the
indicator (4ꢀnitroaniline) in water and the molar ratio of
40 protonated and unprotonated dye [I]/[IH+] using the following
equation:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
[Hmim][HSO4]b
[Hmim][HSO4]
[Hmim][HSO4]
[HMor][HSO4]
[HPyrr][HSO4]
[HPip][HSO4]
[HPip]2[SO4]
[HMor]2[SO4]
[Hmim]2[SO4]
[Hmim][H2PO4]
[Hmim][NO3]
[HPyrr]Cl
46
46
25
42
45
42
30
28
32
46
48
60
55
60
42
54
76:24
82:18
80:20
80:20
92: 8
88:12
55:45
41:59
20:80
21:79
62:38
24:76
20:80
34:66
43:57
85:15
85
82
85
95
87
63
68
63
83
65
81
75
55
85
85
[Hdabco]Cl
[HMor]Cl
[Hmim][CF3CO2]
[HBet]Cl
Ho = pK(I)a + log ([I]/[IH+])
45 According to Lambert–Beer’s Law, the value of [I] and [HI+]
were calculated from the UV–vis spectra, on the basis of the
absorbance values at 380 nm (absorption maximum).
a Determined by NMR. bReaction carried out at 75 °C.
No byꢀproduct arising from side reactions has been detected in all
the examined reactions. In the case of [Hmim][HSO4],
85 experiments were also carried out at 75 °C and varying IL loading
from 60% w/w to 30% w/w (Table 1, entries 1ꢀ3). A moderate
effect of the temperature and an extremely low effect of IL
loading was observed. Similar results were also obtained in the
case of [HPyrr][HSO4] and [HPip][HSO4] showing that, at least
90 with these highly active systems, lower amounts of IL can be
sufficient.
Corrosion test
50
A stainless steel plate with 46 mm length, 12 mm width and 0.5
mm thickness (previously polished, cleaned and dried to constant
weight) was completely immersed in the IL (ca. 15 g) contained
in a suitable tube and maintained at 85 °C for 24 h. At the end of
55 the process, the stainless steel sheets were took out of the IL,
washed in water, dried and weighted. The corrosiveness of
stainless steel plates by the ILs was calculated from the mass loss
ꢀ
ꢀ
It is noteworthy that for all ILs having HSO4 , SO42ꢀ, NO3 and
H2PO4ꢀ as counteranion, after the stopping of the reactions, when
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the temperature was brought to room conditions, two phases were
lower conversions; the ester percentages ranged from 20 to 34%.
The stronger interactions between cations and anions in ILs
bearing highly coordinating anions (such as chloride and
formed (Figure 1); the lower phase was constituted by the IL and
water and the upper layer by the formed ester and unreacted
alcohol. This latter could be easily separated from the IL by 55 trifluoroacetate)10 probably reduce the ability of IL cation to act
5
simply decanting into another flask. A product recover (ester and
eventually unreacted alcohol) higher than 80% characterized ILs
as a catalyst although the role of other factors (such as water
binding ability) cannot be excluded.
An evaluation of the role of the IL acidity in this reaction was
performed determining the Hammett acidity functions (Ho) of
−
having as counteranion HSO4− and H2PO4 . Since this procedure
2ꢀ
ꢀ
gave lower recovers in the case of SO4 and NO3 based ILs
(ranging from 50ꢀ60%) for these latter catalytic systems product 60 seven ILs which give significantly different conversions under
10 extraction with ethyl ether was also applied, a moderate increase
in product recover was observed. In all examined cases, the IL
(lower layer) could be dried under reduced pressure to remove the
water formed during the esterification process and eventually
recycled.
15 In the case of [Hdabco]Cl and [HPyrr]Cl, since at room
temperature the reaction mixture was practically an unique solid
phase enclosing the reaction product, ethyl ether was used to
recover the product. On the other hand, another situation
characterized the same process in [HMor]Cl; at room temperature
20 the IL solidified and formed the lower layer whereas the ester
constituted the upper liquid layer. Hence, the solventꢀfree
separation could be applied also in this case.
comparable conditions.
Table 2. Acidity (Ho) and corrosiveness of stainless steel by Brønsted
acidic ILs
ILs
Ho
Corrosivenessa
(mg/cm2)
0.08
[Hmim][HSO4]
[HPip][HSO4]
[Hmim]2[SO4]
[Hmim][H2PO4]
[Hmim][NO3]
[Hmim][CF3CO2]
[Bet]Cl
1.93
1.58
2.87
2.55
2.22
~3b
0.05
0.07b
0.03
0.06
0.01
ND
1.47
aThe stainless steel sheets were polished, cleaned and dried to constant
65 weight, then fully immersed in the same amount of IL (15g). After 24 h at
85 °C, the stainless steel sheets were took out of the IL, washed in water,
dried and weighted. Being this IL a solid, having a high melting point,
the experiment was carried out in the presence of 5% of water. bThe
absorbance change was too small to obtain a significant value.
70
The Ho values, reported in Table 2, were measured
spectrophotometrically determining the protonation of an
uncharged indicator (4ꢀnitroaniline) in water in terms of the
measurable ratio of [I]/[IH+]. From the data reported in Table 2
75 the acidity decreases in the following order, [HBet]Cl
25
Figure 1. Situation at the end of reaction. Left: Two different layers
are formed. Top layer (ester + unreacted alcohol) lower layer (IL +
water formed during the reaction). Right (reaction in [Hdabco]Cl: just
one solid layer was formed.
> [HPip][HSO4]
>
[Hmim][HSO4] [Hmim][NO3]
>
>
[Hmim][CF3CO2]
>
[Hmim]2[SO4] > [Hmim][H2PO4]. The
strongest acids ([HBet]Cl, [HPip][HSO4], [Hmim][HSO4]) gave
the highest yields, whereas considerably lower conversionswere
80 achieved using ([Hmim]2[SO4] and [Hmim][H2PO4]), which are
the lowest acids.
30 On the basis of the data reported in Table 1 it is possible to
establish that the catalytic activity of the investigated Brønsted
acidic ILs follows the order: [HPyrr][HSO4] > [HPip][HSO4] >
[HBet]Cl > [Hmim][HSO4] > [HMor][HSO4] > [Hmim][NO3] >
[HPip]2[SO4] > [Hmim][CF3COO] > [HMor]2[SO4] > [HMor]Cl
35 > [HPyrr]Cl > [Hmim][H2PO4] >[Hmim]2[SO4] = [Hdabco]Cl.
The Brønsted acidic ILs having hydrogen sulfate as the
counteranion give the best results: [HPyrr][HSO4] is able to
assure up to 92% of conversion in four hours at 110 °C with a
product recover higher than 95%. On the other hand, Brønsted
40 acidic ILs having the unprotonated sulfate as anion give
significantly lower conversions. This behavior suggests that the
presence of a sufficiently acidic proton on the anionic component
of the IL plays an important role in the activation of the reaction.
About this, it is noteworthy that when [Hmim][H2PO4] was used
45 as catalyst and solvent only very low conversions were obtained
(21%) suggesting a lower acidity of the [H2PO4]ꢀ protons with
respect the acidic proton of [HSO4]ꢀ also in the non aqueous
environment charactering neat ILs.
100
80
60
40
20
0
1
2
3
4
H0
85 Figure 2. Relation between IL acidity (Ho) and conversion of acetic acid
esterification with octanol
As previously observed,11 the acidity of Brønsted acidic ILs in
water, as well as in other polar solvents, depends on anion and
On the other hand, only a moderate conversion was observed in
50 [Hmim][NO3] and [Hmim][CF3CO2], 62% and 43% respectively,
whereas the Brønsted acidic ILs having Cl− as counteranion gave
4
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cation nature: a fairly good correlation between the strength of
the acid and the conversion (%) can be evidenced (Figure 2).
However, the deviations from a linear behavior characterizing the
plot of Figure 2 suggest that the acidity of these media is not the
the end of the reaction also in the case of butyl acetate synthesis
two phases were obtained and product was recovered simply by
upper phase separation.
Once again, [HPyrr][HSO4] assured the higher conversion
5
sole factor affecting the efficiency of the process. Probably, as 55 although comparable values were also obtained in
previously suggested6h in a recent paper, when the IL is used as in
this study in a relevant amount its ability to bind the formed water
and contemporaneously to give a biphasic system with the
formed ester can shift the equilibrium towards products,
[Hmim][HSO4] and [HBet]Cl. Nevertheless, recycling
experiments confirmed the possibility to carry out at least three
consecutive processes in these ILs without significant reduction
in conversion.
10 increasing the process efficiency.
60 The selectivity of the esterification reaction was checked using a
Finally, it is to note that when a taskꢀspecific ammoniumꢀbased
salt, [HBet]Cl, bearing a carboxy group on the alkyl chain has
been used, despite the presence of chloride as counteranion, a
conversion comparable to that found in [HSO4]− based ILs was
more complex alcohol, methylꢀβꢀDꢀglucopyranoside, a glucoꢀ
derivative characterized by a primary hydroxyl group and three
secondary equatorial hydroxyl groups. As a consequence of the
low solubility of the substrate in most of the synthesized acidic
15 obtained; the presence of the acidic group (–COOH) on cation is 65 ILs the reactions were conducted at a lower reagent concentration
able to assure the catalytic effect. Although in principle the
carboxylic group on IL could be esterified, probably the presence
of the adjacent positive charge reduce this ability: no
transformation on IL has been detected by NMR analysis. It is
(around 1.5 M) only in the ILs reported in Table 4. The behavior
of the reaction was checked stopping the esterification at prefixed
times (1, 2 and 4 h and eventually longer). The reaction mixtures
were extracted with ethyl ether (extraction yield higher than 75%)
20 also to note that, since this IL is a high melting point solid and the 70 and product distribution was analyzed by NMR.
product is not soluble in this medium it has been possible to
recover the formed ester through a simple decanting procedure.
[HBet]Cl may therefore represent a valid alternative to the other
protic Brønsted acidic ILs.
25 On the other hand, since the best results were obtained using
[HPyrr][HSO4], this IL has been selected for recycling
experiments. After three recycles, conversion decreased from
92% to 85%, although the recovery of the product by simple
decantation of the upper phase becomes less efficient (from 85%
30 to 70%).
[HPyrr][HSO4] was also used as test solvent and catalyst for the
esterification of acetic acid with butanol. Considering the lower
boiling point of butanol, the reaction was initially carried out at
four different temperatures ranging from 85 ꢀ 100 °C by working
35 with a 1:1 molar ratio of butanol ꢀ acetic acid and an IL loading
of 60% w/w (based on the mass of butanol). To evaluate the
catalytic ability of the different ILs, reactions were stopped after
2ꢀ4 h and the conversions were evaluated by NMR. On the basis
of these data, we decided that a temperature between 80ꢀ90°C
40 and 2 h of reflux might represent the best conditions to evaluate
the efficiency of acidic ILs in this reaction having practically
reached the equilibrium.
Scheme 3. Esterification between methylꢀβꢀDꢀglucopyranoside with
acetic acid
In all investigated ILs, the reaction occurred with a high
75 regioselectivity: only the acetyl derivative arising from
esterification of primary hydroxyl group on C(6) was detected
besides the unreacted product, conversions at different reaction
times are reported in Table 4.
Table 3. Esterification of acetic acid with butanol, conversions and yields
after 2 h at 85°C a
Entry
IL
IL/alcool
(%)
24
Ester : alcoholb
80 Figure 3. 13C NMR of the crude reaction mixture arising from
1
2
3
4
5
6
7
[HPyrr][HSO4]
[HPip][HSO4]
[Hmim][HSO4]
[HBet]Cl
[HBet][HSO4]
[HBet][NO3]
[HBet][BF4]
90:10
75:25
88:12
85:15
79:21
67:33
65:35
acetylation of methylꢀβꢀ
Dꢀglucopyranoside.
23
25
29
21
25
22
Table 4. Esterification of acetic acid with methylꢀ
β
ꢀD
ꢀglucopyranosidea
ILs
1 h
NR
2 h
NR
4 h
NR
[Hmim]Cl
45 a Yields of the recovered product (ester + unreacted alcohol) by
decantation ranged around 85ꢀ90%. bDetermined by NMR.
[Hmim][NO3]
[Hmim][HSO4]
[Hmim][H2PO4]
36%
ꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀ
50%
60%
40%
30%
70%
50%
Therefore, the same conditions were applied to test the Brønsted
acidic ILs reported in Table 3, which have been found to give the
50 higher conversions in octyl acetate synthesis. In these media, at
a Conversions were determined by NMR.
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It is to note that in the case of [Hmim][NO3] a decrease in
increase in conversion was obtained working in the presence of a
small excess of H2SO4, probably as a consequence of the
superacidic behavior of species of the type, AꢀꢀꢀHꢀA−, which
should be present in the reaction mixture.
conversion on increasing the reaction time from 2 to 4 h was
observed. This peculiar behavior has to be attributed to the nature
of [Hmim][NO3] which, probably less than [Hmim][HSO4], is
5
able to “capture” the produced water. It is to note that in this case 50 When the transesterification process of ethyl transꢀcinnamate in
the product is soluble in the ionic liquid and the presence of
increasing amounts of freeꢀwater after a given conversion (the
water coordination ability of the nitrate anion is not infinite!),
may favor the hydrolysis process and shift the equilibrium
[Hmim][HSO4] was carried out using octanol, and an 1:1 molar
ratio ester : alcohol, lower conversions were obtained under
comparable conditions. However, the presence of a small excess
of inorganic acid (H2SO4) significantly increased the conversion
10 position towards reagents. It is noteworthy that also in this 55 also in this case (Table 6).
reaction the higher conversion was obtained in the IL having
hydrogensulfate as counteranion, [Hmim][HSO4]. Finally,
Table 6. Transesterification of transꢀethyl cinnamate with octanol in
various Brønsted acidic ILs a
attempts to increase the conversion prolonging the reaction
beyond 4 h failed due to the formation of relevant amounts of
15 polyacetylation products.
In this study also the possibility to use Brønsted acidic ILs in
transesterification processes was checked investigating the
reaction of ethyl transꢀcinnamate with a small alkylꢀchained
alcohol (methanol) and with a longer alkylꢀchained alcohol
20 (octanol). Initially, the reaction with methanol was investigated in
[HPip][HSO4] and temperature and reaction time were varied to
find the best conditions.
Entry
Time (h)
Temp.
(°C)
80ꢀ90
75
90
90
IL
Conversion
%
NR
35
25
52
1
2
3
4
5
[Hmim][HSO4]
[Hmim][HSO4]
[Hmim][HSO4]
[Hmim][HSO4] b
38
30
30
a The molar ratio of the ester and alcohol was 1:3. b In this case 1.1 eq of
60 acid was used to prepare [HPyrr][HSO4]. NR=no reaction
Finally, corrosive tests were carried out with six ILs using
stainless steel plates and determining the weight loss after
complete immersion in the selected ILs for 24 h at 85°C. The
65 results (Table 2) showed that all the investigated ILs are
practically noncorrosive media, in agreement with recent studies
performed on analogous ILs.6h,12
Scheme 4. Transesterification of ethyl transꢀcinnamate
25
Practically, no reaction occurred, even after 24 h, until the
temperature was raised to 90°C. Since, at this temperature we
could not exclude that the moderate conversion (50%) was at
least partially due to the volatility of methanol the reaction was
30 carried out also using an excess (3 equivalents) of this reagent:
conversion increased at 73%. The efficiency of other acidic ILs
was also tested at 90 °C. Results are reported in Table 5.
Conclusions
70
In conclusion, these data show that the water scavenging nature
of many Brønsted acidic ILs and the insolubility of many esters
in these media strongly affect the catalytic efficiency of these
salts in esterification and transesterification reactions. In both
75 cases, protic ILs bearing Brønsted acidic onium (pyrrolidinium)
or imidazolium cations, associated with the “acidic”
[HSO4]− anion, gave the highest yields. In the case of
esterification of acetic acid with octanol and butanol a high
conversion was also obtained using a taskꢀspecific ammoniumꢀ
80 based IL, [HBet]Cl, bearing an acidic group (–COOH) on the
alkyl chain. Nevertheless, acetylation of a relatively complex
Table 5. Transesterification of ethyl transꢀcinnamate with methanol in
35 various ILsa
Entry
IL
Conversion %
Recovered
material
%
0
0
1b
2c
3b
4c
5b
6b
7b
[HMor]Cl
[HMor]Cl
NR
NR
NR
95
37
75
alcohol, such as methylꢀβꢀDꢀglucopyranoside, in Brønsted acidic
ILs occurs with high regioselectivity: only the acetyl derivative
arising from esterification of the primary hydroxyl group on C(6)
85 was detected, beside the unreacted product (30%), when the
reaction was performed in [Hmim][HSO4]. Finally, the improved
catalytic ability of the 1:0.1 [Hmim][HSO4]ꢀH2SO4 mixture in the
transesterification reaction, probably as a consequence of the
superacidic behavior of species of the type, AꢀꢀꢀHꢀA−, has been
90 evidenced.
[HBet][HSO4]
[HPyrr][HSO4]
[Hmim][HSO4]
[Hmim][HSO4]b
[HPip][HSO4]
0
95
87
92
90
73
a All the reactions were carried out at 90°C for 28 h. b In this experiment
1.1 equivalent of acid with respect to the nitrogen base was used to
prepare [HPyrr][HSO4].
40 Whereas no reaction was observed in chloride based ILs, also
working with an excess of alcohol, conversions ranging from 40
to 95% characterized reactions performed in hydrogensulfateꢀ
based ILs. The best results were obtained, also for the
transesterification process, in [HPyrr][HSO4]. It is also
45 noteworthy that, in particular in [Hmim][HSO4] a significant
Acknowledgement. We thank University of Pisa and Fondazione
CariPisa for the financial support.
Notes and references
95 aDipartimento di Chimica e Chimica Industriale, via Bonanno 33, 56126
Pisa, Italy. Eꢀmail:cinziac@farm.unipi.it.
6
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