The influence of the acidity of ionic liquids on catalysis

Article abstract of DOI:10.1002/cssc.201000075

Reactions performed in ionic liquids with BF₄^- as anion progress under strongly acidic conditions. The acidity of some air- and moisture-stable ionic liquids is explored and its effect on catalytic reactions is studied. The function of these ionic liquids in some traditional acid-catalyzed reactions is also tested, and the results merit a reconsideration of their influence on catalytic reactions and use in other applications.

Full text of DOI:10.1002/cssc.201000075

DOI: 10.1002/cssc.201000075
The Influence of the Acidity of Ionic Liquids on Catalysis
[
a, b]
[a, b]
[a]
[a]
[a]
[a]
Xinjiang Cui,
Youquan Deng*
Shiguo Zhang,
Feng Shi,* Qinghua Zhang, Xiangyuan Ma, Liujin Lu, and
[
a]
In the past 20 years, the concept of ionic liquids (ILs) have
been extensively applied in the fields of chemistry, materials,
[
1]
and life sciences. Undoubtedly, the ionic liquids composed of
À
À
quaternary ammonium cations and anions, such as BF , PF6
,
4
À
À
Cl , and NTf2 , have been the backbone of this area since im-
midazolium cation ionic liquids were synthesized by Zaworot-
ko et al. and were brought into catalysis and synthesis by
[
2]
Seddon, Rogers, Welton, Wasserscheid, and others. Thou-
sands of reactions have been performed in these ionic liquids
and many of them exhibited better behavior than organic sol-
[
3]
vents. Normally, the fine performance of these ionic liquids
was attributed to the specific ionic environment of the ionic
liquid. Nevertheless, the acidity of the air- and moisture-stable
ionic liquids and its effect on catalysis is an interesting topic.
As it is well known, a large amount of organic reactions can be
[
4]
catalyzed or promoted by an acid environment. During our
investigation of the function of ionic liquids in catalysis, espe-
cially air- and moisture-stable ones, we found that these ILs
normally exhibit weak acidity in the presence of a small
amount of water. That means the interpretations about the
function of air- and moisture-stable ionic liquids in catalytic re-
actions are possibly wrong because the presence of trace
amount of water is not avoidable in reality. Herein, we present
our results on the study of acidity of air- and moisture-stable
ionic liquids and their activity in some known acid-catalyzed
reactions. We hope these results could be helpful for research-
ers in this area to reconsider the influence of the acidity of air-
and moisture-stable ionic liquids on catalysis and also in other
fields.
At the initial stage, the acidity of ionic liquids–water with dif-
ferent cations and anions were measured with a pH meter. The
concentration of ionic liquid in water was 0.1m. The operation
was performed with methods given in the Annual Book of
American Society for Testing and Materials Standards (ASTM)
4À
Figure 1. pH values of the aqueous phase of ionic liquids with BF as anion
0.1m). The numbers in the parentheses are the values of the standard devi-
ation.
(
[
5]
with slight modification.
As shown in Figure 1, ionic liquid–water mixtures with BF4
À
respectively. BMImBF ionic liquids purchased from Merck (lot
4
anions were all acidic. Interestingly, the acidity of the ionic liq-
uids could be tuned via substituted alkyl variation. The pH
code: S5204049909) and Sigma–Aldrich (lot code: 0001415814)
were also measured for comparison. Under the same condi-
tion, their pH values were 4.70(0.05) and 4.30(0.09), respective-
ly, which are exactly the same as the acidity of the ionic liquids
that we synthesized. The incorporation of an hydroxyl group
value of EMImBF ionic liquid reached 3.44(0.03) but the pH
4
value of BMImBF and HMImBF were 4.27(0.14) and 6.61(0.03)
4
4
À
would further enhance the acidity of ionic liquids with BF4
[
a] X. Cui, S. Zhang, Dr. F. Shi, Dr. Q. Zhang, X. Ma, L. Lu, Prof. Y. Deng
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences, Lanzhou, 730000 (PR China)
Fax: (+86)9314968116
anion. The pH value reached 3.12(0.01) and 3.11(0.02) with hy-
droxyethyl or hydroxypropyl groups. The substitution of the
C2 position with a methyl group weakens the acidity of this
Homepage: www.licp.cas.cn
E-mail: fshi@licp.cas.cn
kind of ionic liquid. For the ionic liquid BMMImBF , the pH
4
value was 6.51(0.04). This was almost the same as that with
the ionic liquid from Merck, that is, 6.46(0.23) (lot code:
EQ005416). For ionic liquids with tetrabutyl ammonium and
tetrabutyl phosphonium cations, the IL solutions were close to
ydeng@licp.cas.cn
[
b] X. Cui, S. Zhang
Graduate School of Chinese Academy of Sciences
Beijing, 100049 (PR China)
ChemSusChem 2010, 3, 1043 – 1047
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1043

neutral. The pH values were 6.90(0.09) and 6.56(0.04). Similar
acidity was observed for an ionic liquid with butyl pyridinium
cation, that is, 4.25(0.01). Therefore, the acidity of ionic liquids
were performed using the ionic liquid BMImBF . The variation
4
of the pH value versus the ionic liquid concentration is shown
in Figure 3. The pH value changed remarkably when its con-
tent was lower than 10%, and then a platform appeared at
À
with BF4 anion could be finely tuned via cation variation in
the pH value range of 3.0–7.0.
For another commonly employed ionic liquid, BMImPF , an
6
interesting result was also obtained (Figure 2). The pH value of
its solution was 7.16(0.04), although it is normally considered
4
Figure 3. Variation of pH values with the concentration of BMImBF in water.
10–80 wt%. Surprisingly, the acidity of the ionic liquid solution
again decreased dramatically when its content was higher
than 80 wt%. The pH value of samples that contained 99 wt%
ionic liquid (1 wt% water in the ionic liquid) sharply decreased
to 1.05(0.43). Although this result is not as accurate as the
samples contain more water, it does suggest the formation of
a strong acid environment in the air- and water-stable ionic
liquid. Under real reaction conditions, the presence of water
completely can never be completely excluded, and thus all the
Figure 2. pH values of the aqueous phase of ionic liquids with 1-butyl-3-
methyl imidazolium as cation (0.1m).
À
to be more easily hydrolyzed than BMImBF . Similar results
reactions performed in ionic liquids with BF4 anions progress
4
were obtained when applying BMImPF , purchased from
in a strongly acidic environment. This phenomenon was also
observed for ionic liquids BPyBF and BMImPF . The pH value
6
Merck, that is, 6.30(0.10) (Lot code: S9587950917) and Sigma–
Aldrich, that is, 6.55(0.08) (lot and filling code: 1393737 and
4
6
of these two ionic liquids containing 1 wt% H O decreased to
2
3
0909232). Other ionic liquid solutions with varied anions such
4.34 and 1.40. However, the pH value of an ionic liquid con-
À
as BMImCl, BMImN(CN) , BMImOTf, and BMImClO were close
taining stable ClO4 anion was the same as the corresponding
2
4
to neutral except for the weak acidity exhibited by the ionic
0.1 m aqueous solution (i.e., 6.38).
liquid BMImNO , that is, 6.11(0.01). In the end, ionic liquids
A similar phenomenon was observed when using acetic acid
3
À
À
with HSO4 and H PO4 anions were also measured. The pH
instead of the ionic liquid BMImBF (Figure 4). Although not as
2
4
À
values for ionic liquids with HSO4 anion were 1.45(0.01) and
obvious as the system with BMImBF , the two turning points
4
À
1
.37(0.05) (Merck EQ412630928), and for the H PO4 anion the
also appeared with 10 wt% and 80 wt% acetic acid. However,
the pH value was <0 when the acetic acid concentration was
>90% and was not detectable by the pH meter. Therefore,
the acidity of acetic acid above 90% was not measured. These
2
value was 3.00(0.01). At the same time, the pH value of ionic
À
liquid with protonized imidazole and BF4 anion was
2.65(0.01).
By comparing the pH values of ionic liquid solutions dis-
results indicate that the ionic liquid BMImBF behaves similarly
4
cussed above, it could be concluded that the acidity of ionic
to a weak acid in water.
À
liquids with BF4 anions is comparable with the acid-function-
However, it is also possible that this critical point appears
due to the inability of the pH meter to operate in highly con-
centrated ionic liquid–water systems. Therefore, a simple but
effective method is the use of a pH indicator to check the
changes in acidity of the ionic liquid–water mixture. Here,
thymol blue was chosen because of its suitable pH transition
alized ionic liquids BMImH PO and MImHBF . Conversely, ionic
2
4
4
À
liquids with PF6 are more stable and result in neutral aqueous
À
solutions. Therefore, ionic liquids with PF6 anions are more
stable in water at room temperature.
More importantly, our further explorations suggested an in-
teresting rule for the acidity of ionic liquids containing differ-
ent amounts of water (i.e., 1%–99%). Detailed experiments
[6]
interval (pH 1.2–2.8, pK =1.65). A color transition point was
a
observed at exactly >80 wt% ionic liquid (Figure 5).
1
044
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ChemSusChem 2010, 3, 1043 – 1047

À
À
HOBF3 , F , BMImBF , BMImHOBF , BMImF, HBF , H(HOBF ),
4
3
4
3
and HF provides a buffer function for the ionic liquid–water
system.
Based on the results discussed above, a reconsideration of
the effect of these ionic liquids on catalytic reactions is neces-
sary. Three typical acid-catalyzed reactions were chosen as
model reactions to compare the catalytic activity of some typi-
cal ionic liquids. The first reaction is the acetalization reaction
[8]
of p-chloride-benzaldehyde and propylene glycol (Table 1).
Table 1. Acetalization of p-chloride-benzaldehyde and propylene glycol
catalyzed by ionic liquids.
Figure 4. Variation of pH values with the concentration of acetic acid in
water.
[
b]
[c]
Entry
Ionic liquid
OHEMImBF
Conv. [%]
Sel. [%]
1
2
3
4
4
>99
95
9
>99
>99
>99
>99
BMImBF
BMImPF
BMImCl
4
6
5
[
a] Reaction conditions: p-chloride-benzaldehyde (2 mmol, 280 mg), pro-
pylene glycol (10 mmol, 760 mg), ionic liquid (1 mL), 40 mL pressure tube
Sigma–Aldrich), 508C, 4 h, Ar. [b] Conversion of p-chloride-benzaldehyde,
(
obtained by GC–MS analysis. [c] Selectivity to 2-phenyl-1,3-dioxane, ob-
tained by GC–MS analysis.
The results are in complete agreement with the acidity order
of the ionic liquids. For the acidic ionic liquids (OHE)MImBF4
and BMImBF the conversions were 99% and 95%, respective-
4
ly, with 99% selectivity. When the ionic liquids BMImPF and
6
BMImCl were applied, the conversions were <10%.
Another acid-catalyzed reaction, the etherization of benzyl
4 2
Figure 5. UV/Vis absorbance of thymol blue–BMImBF –H O.
[9]
alcohol and tert-butyl alcohol, was performed (Table 2). The
conversion with ionic liquid OHEMImBF was 60%, with 95%
4
The main absorption peak of thymol blue is at approximate-
selectivity. When the reaction was carried out in the ionic
ly 435 nm in pure water and BMImBF /water solutions with up
liquid BMImBF , the conversion was 23% and the selectivity
4
4
to 80 wt% ionic liquid. If the BMImBF concentration is above
was >99%. When the reaction was performed in the ionic
4
80 wt%, an obvious blue-shift of the UV absorbance is ob-
liquid BMImPF , no desired product was detected, and the
6
served. The absorption wavelength for an BMImBF ionic liquid
benzyl alcohol was completely converted into dibenzyl ether
4
containing 1 wt% water shifted to 404 nm. This trend agrees
with the results obtained by using the pH meter. It strongly in-
dicates the existence of an exact critical point in the pH value/
hydrogen ion activity.
and the tert-butyl alcohol was converted into olefins of varied
[a]
Table 2. Etherization of benzyl alcohol and tert-butyl alcohol.
À
The acidity of the ionic liquids with BF4 should be due to
+
the hydrolysis of the anion, which can be hydrolyzed into H
À
[7]
and HOBF3 (Scheme 1). Other species produced through
À
deep hydrolysis are less prevalent; HOBF is the dominating
[b]
[c]
3
Entry
Ionic liquid
OHEMImBF
Conv. [%]
Sel. [%]
[
7a]
+
À
species.
The presence of species such as BMIm , BF4 ,
1
2
3
4
4
60
23
>99
0
95
>99
0
BMImBF
BMImPF
BMImCl
4
6
0
[
a] Reaction conditions: benzyl alcohol (2 mmol, 216 mg), tert-butyl alco-
hol (10 mmol, 740 mg), ionic liquid (1 mL), 40 mL pressure tube (Sigma–
Aldrich), 1208C, 10 h, Ar. [b] Conversion of benzyl alcohol, obtained by
GC–MS. [c] Selectivity to benzyl tert-butyl ether, obtained by GC–MS.
Scheme 1. Possible species in BMImBF
ChemSusChem 2010, 3, 1043 – 1047
4 2
–H O.
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
1045

structure. No reaction occurred in the presence of BMImCl. The
Experimental Section
peculiar activity of the ionic liquid BMImPF is due to the insta-
6
À
General Information: All ionic liquids used were synthesized in our
laboratory or purchased from Sigma–Aldrich or Merck. The HNMR
purity of all ionic liquids was >99%. All the ionic liquids were
bility of PF6 at elevated temperature. After the reaction, a hy-
1
drofluoric acid mist was observed and the pH value of the re-
action system was 0.72 with the addition of 5 mL water. The
À
treated under vacuum at 808C for 8 h before use. The halide (Cl
À
reaction performed in the ionic liquid BMImPF was really cata-
or Br ) contents of the ionic liquids were all <500 ppm, measured
6
lyzed by hydrofluoric acid, emitted by the hydrolysis of
BMImPF . For the ionic liquid BMImBF , the pH value of the re-
by Mettler Toledo Seven Multi instrument, except BMImH PO
2
4
(1.5 wt%). The water contents were measured by Karl Fischer coul-
ometer (Metro 831 KF coulometer). Normally, the water content
was <300 ppm, but <700 ppm was measured for hydroxyl-func-
6
4
action mixture was 2.12 when the same method as with
BMImPF was used, which was not far from the acidity mea-
6
tionalized ionic liquids, 27819 ppm for BMImHSO , and 2282 ppm
4
sured at room temperature (i.e., 3.51). These results suggest
for BMImH PO . The pH value was measured with a pHS-25 pH
À
2
4
that an ionic liquid with BF4 is stable enough at 1208C. More
meter (ShangHai Precision & Scientific Instrument). The pH meter
was calibrated by a standard buffer solution with pH value 4.00
before use. UV/Vis measurements were performed with an Agilent
8453 instrument. The catalytic reactions were analyzed by GC–MS
(6890–5973).
importantly, the acidity exhibited by BMImBF could be regard-
4
À
ed as one of the properties of an ionic liquid with BF4 anion,
which is a balanced and stable system.
The condensation of benzaldehyde and acetophenone to
[
10]
chalcone was also used as a test reaction with the same
ionic liquids (Table 3). All of the ionic liquids behaved similarly
as in the etherization reaction. The conversions with ionic liq-
General procedure for the measurement of pH values of ionic
liquid–water: Ionic liquid (2 mmol) and distilled water (20 mL) were
added to a glass vessel (30 mL). After being shaken for 2 min and
placed for 30 min, the pH value of the aqueous phase was mea-
sured. All the measurements were repeated three times and the
average pH value and standard deviation were given.
Table 3. Condensation reaction between benzaldehyde and acetopheno-
[
a]
ne.
General procedure for pH value measurement of BMImBF4 or
AcOH/H O with varied concentrations: For the pH value of
2
BMImBF –H O, a series of samples including 0.1 wt%, 0.5 wt%,
4
2
1
.0 wt%, 2.0 wt%, 8.0 wt%, 10 wt%, 15 wt%, 95 wt%, and 99 wt%
BMImBF4 were prepared and measured in triplicate. For other
points, the pH was measured continuously by the addition of dif-
ferent amounts of water into 15 wt% sample. The operation was
also performed in triplicate. A similar operation was performed for
[
b]
[c]
Entry
Ionic liquid
OHEMImBF
Conv. [%]
Sel. [%]
1
2
3
4
4
>99
95
>99
0
>99
>99
>99
0
BMImBF
BMImPF
BMImCl
4
6
the pH value measurement of AcOH/H O solution. However, a suit-
2
able amount of acetic acid was added into water for the continu-
ous measurement of samples containing 15–90 wt% acetic acid
[
(
a] Reaction conditions: acetophenone (2 mmol, 240 mg), benzaldehyde
10 mmol, 1060 mg), ionic liquid (1 mL), 40 mL pressure tube (Sigma–Al-
because the pH value of AcOH–H O solution was <0 when the
2
drich), 1208C, 10 h, Ar. [b] Conversion of acetophenone, obtained by GC–
concentration of acetic acid was >90 wt%, which could not be
MS. [c] Selectivity to chalcone, obtained by GC–MS.
measured by the pH meter.
General procedure for the acetalization of p-chloride-benzaldehyde
and propylene glycol: p-Chloride-benzaldehyde (2 mmol, 280 mg),
propylene glycol (10 mmol, 760 mg), and ionic liquid (1 mL) were
added into a 40 mL pressure tube, which was flushed with Ar for
À
uids containing BF4 anion were >99% and 95%, with >99%
5
min after which time the reaction was performed at 508C for 4 h
selectivity, but no conversion was detected when using
under magnetic stirring.
BMImCl. When using BMImPF the conversion and selectivity
6
General procedure for the etherization of benzyl alcohol and tert-
butyl alcohol: benzyl alcohol (2 mmol, 216 mg), tert-butyl alcohol
were also >99%, but it is difficult to define this as a BMImPF6-
catalyzed reaction because of the decomposition into hydro-
fluoric acid, as discussed earlier in the etherization reaction.
In conclusion, the acidity of some air- and moisture-stable
ionic liquids was explored and a preliminarily study of its effect
(
10 mmol, 740 mg), and ionic liquid (1 mL) were added to a 40 mL
pressure tube, which was flushed with Ar for 5 min and reacted at
208C for 10 h under magnetic stirring.
1
À
General procedure for the condensation reaction between benzal-
dehyde and acetophenone: Acetophenone (2 mmol, 240 mg), ben-
zaldehyde (10 mmol, 1060 mg), and ionic liquid (1 mL) were added
to a 40 mL pressure tube, which was flushed with Ar for 5 min and
reacted at 1208C for 10 h under magnetic stirring.
on catalytic reactions was performed. Ionic liquids with BF4 as
anion can be defined as a system that behaves as a weak acid
and maintains stability up to 1208C involving water. Ionic liq-
À
uids with PF6 as anion are more stable than an IL containing
À
BF4 anions at room temperature, but decompose remarkably
at 1208C. The activity of these ionic liquids in some traditional
acid-catalyzed reactions is also in good agreement with the
acidity order obtained in this work. Therefore, the influence of
air- and moisture-stable ionic liquids on catalytic reactions, and
also on their applications in other fields, should be reconsid-
ered.
Acknowledgements
This work was supported by the “Hundred Talents Program” of
the Chinese Academy of Sciences (20533080).
1
046
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ChemSusChem 2010, 3, 1043 – 1047

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Keywords: anions · acidity · catalysis · cations · ionic liquids
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1047