Bronsted acidic ionic liquids catalyze the high-yield production of diphenolic acid/esters from renewable levulinic acid

Article abstract of DOI:10.1039/c2gc36630d

The high-yielding synthesis of diphenolic acid (DPA) using Bronsted acidic ionic liquids (BAILs) has been demonstrated in this study. Importantly, p,p′-DPA was obtained as the preferential product over o,p′-DPA with an isomer ratio over 100. Moreover, diphenolic esters were also prepared in high yield through a one-pot method.

Full text of DOI:10.1039/c2gc36630d

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Brønsted acidic ionic liquids catalyze the high-yield
production of diphenolic acid/esters from renewable
levulinic acid†
Cite this: Green Chem., 2013, 15, 81
Received 16th October 2012,
Accepted 29th October 2012
Hai-Feng Liu,a,b Fan-Xin Zeng,a,b Li Deng,*a Bing Liao,a Hao Panga and
Qing-Xiang Guo*c
DOI: 10.1039/c2gc36630d
www.rsc.org/greenchem
The high-yielding synthesis of diphenolic acid (DPA) using
Brønsted acidic ionic liquids (BAILs) has been demonstrated
in this study. Importantly, p,p′-DPA was obtained as the prefer-
ential product over o,p′-DPA with an isomer ratio over 100.
Moreover, diphenolic esters were also prepared in high yield
through a one-pot method.
With the aim of achieving a sustainable future, growing efforts
have been devoted to replacing fossil resources with renewable
biomass. In this context, several bio-based platform chemicals
and various routes based on these chemicals have been pro-
posed.1 Among these routes, the condensation of levulinic
acid (a versatile compound derived from carbohydrates) with
phenols is of special interest because the product diphenolic
acid (γ,γ-bishydroxyphenyl valeric acid, DPA) is not only an
alternative for bisphenol A (BPA), which is widely used in the
polycarbonate and epoxy resin industry, but also allows chemi-
cal modification on its intrinsic carboxylic group via esterifica-
tion, reduction and amidation, potentially offering much more
functionalized materials (Scheme 1).2
Fig. 1 BAILs examined in this work.
re-examined extensively with more robust and greener cata-
lysts, i.e., solid acids instead of HCl or H2SO4, due to the attrac-
tive merits of levulinic acid and DPA. Clark and Guo et al.
conducted the pioneering investigation on the synthesis of
DPA using mesoporous silica-supported heteropolyacid cata-
lysts and an excellent catalytic activity (TOF over 50 h−1) was
achieved at 100 °C.4 To the same end, sulfonated hyper-
branched polymer catalysts were prepared for this reaction by
Sels et al.5 More importantly, they demonstrated the promot-
ing effect of thiol compounds on the yield of DPA. Similar
results can also be found in the study of BPA synthesis. Davis
et al. systematically studied the enhanced catalytic activity
caused by the combination of immobilized thiol groups and
acid sites on the surface of solid acids and a possible mechan-
ism for the role of the thiol group in the condensation was
also proposed.6
The synthesis of DPA is a traditional reaction, in which HCl
and H2SO4 were usually employed as catalysts.3 Recently, this
reaction has regained interest researchers and has been
Despite the recent advances in high TOFs and the role of
thiol, the existing solid catalysts are only able to convert levuli-
nic acid into DPA with a middle yield according to the
literature.3–5 Therefore, an effective catalytic system for the
high-yield production of DPA should be developed. Here, we
prepared a series of BAILs (Fig. 1) for this condensation. BAILs
are commonly recognized as robust, designable and eco-
friendly solvents and catalysts for various organic reactions,
especially for reactions involving bio-based chemicals.7 In
terms of their application for DPA synthesis, they possess at
least three advantages: (1) designable structure which enables
the preparation of task-specific catalysts containing both acid
sites and thiols, (2) low solubility in some organic extracts
Scheme 1 BAILs catalyzed condensation of levulinic acid and phenol.
aKey Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute
of Chemistry, Chinese Academy of Sciences, Guangzhou, 510650, P. R. China.
E-mail: dengli@gic.ac.cn; Fax: (+86) 2085232917; Tel: (+86) 2085232917
bUniversity of Chinese Academy of Sciences, Beijing, 100039, P. R. China
cDepartment of Chemistry, University of Science and Technology of China,
Hefei 230026, P. R. China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2gc36630d
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Table 1 Synthesis of DPA in the presence of 1aa
Entry Loading of 1ab (mol%) Temp. (°C) Time (h) Yield of DPAb (mol%) Conversion of levulinic acid (mol%) Ratio of p,p′ : o,p′
1
2c
3
4
5
6
7
50
50
50
50
100
50
0
60
60
24
24
48
48
48
48
48
33
20
55
54
50
38
0
40
24
70
82
74
56
3
17
16
16
12
20
40
—
60
70
60
50
100
a Reaction conditions: levulinic acid (0.46 g, 4 mmol) and phenol (1.5 g, 16 mmol). b Based on the loading of levulinic acid. c Levulinic
acid (0.46 g, 4 mmol) and phenol (1.13 g, 12 mmol).
allowing easy recycling and (3) catalytic activity to catalyze
esterification of DPA with alcohols into diphenolic esters. To
our delight, the yield of DPA can be improved over 90 mol% in
the presence of BAILs and diphenolic esters were also prepared
with high yields in a one-pot method. Interestingly, an un-
expected discrimination between the p,p′-DPA isomer and
o,p′-DPA isomer, in the presence of BAILs, was observed. The
ratio of p,p′-DPA to o,p′-DPA exceeds 100, which is the highest
value reported to date.
Table 1 shows the results of the ionic liquid 1a catalyzed
condensation of levulinic acid with excessive phenol under
different conditions. With 50 mol% 1a, the yield of DPA from
levulinic acid and phenol (molar ratio = 1 : 4) reached 33 mol%
after 24 h at 60 °C, and extending the reaction time to 48 h
afforded 55 mol% DPA and a p,p′-DPA to o,p′-DPA ratio of 16.
Increasing the ratio of levulinic acid to phenol (1 : 3) gave
lower yield of DPA (Table 1, entry 2). With higher loadings of
1a, the DPA yield was not improved but the isomer ratio rose
to 20. Moreover, elevating the reaction temperature did not
obviously increase the yield either (Table 1, entry 4) though
higher TOFs could be achieved (Table S2†). A room tempera-
ture reaction could not provide DPA as a product, and only
38 mol% DPA was obtained at 50 °C. However, it should be
noted that the low temperature is essential for the isomer
ratio. Compared to the value at 60 or 70 °C, a higher ratio up
to 40 was achieved at 50 °C. This also agrees with the exper-
iments carried out at 80, 90 and 100 °C with much a lower
ionic liquid loading (Table S2†), indicating the isomer ratio
could be well tuned by the temperature together with the cata-
Table 2 Transformation of levulinic acid and phenol into DPA in the presence
of BAILs at 60 °Ca
Yield of DPA Ratio of
Entry Catalysts
Additives
(mol%)
p,p′ : o,p′
1
1c
—
—
—
—
—
—
—
—
35
13
50
44
50
75
48
47
64
45
80
—
32
30
21
33
32
50
33
33
24
25
30
—
2
1d
3
2
4
3
5
4ab
6
4bb
7
5b
8
6b
9
H2SO4
—
—
—
10
11
12
13
14
15
16
17
18
19
20
21
22
CF3SO3H
HS(CH2)3SO3H
—
HSCH2COOH
1a
HSCH2COOH 79
HSCH2COOH 80
HSCH2COOH 87
HSCH2COOH 88
HSCH2COOH 79
HSCH2COOH 81
HSCH2COOH 76
9
2
9
3
9
4ab
5b
20
13
9
6b
H2SO4
4ac
4ac
4ae
9
1b
4b
4b
85
91
59
>100d
>100d
40
a Reaction conditions: levulinic acid (0.46 g, 4 mmol), phenol (1.5 g,
16 mmol), catalysts (2 mmol) and additives (0.2 mmol), 60 °C for
48 h. b 1 mmol germinal dicationic BAILs. c 0.9 mmol 4a,
0.1 mmol 4b. d Only trace amount of o,p′-DPA was detected.
e Without Ar protection.
lysts. Besides, the control experiment without catalyst also BAILs were stable at the temperature applied in this work
reveals that the reaction could not be auto-catalyzed by the car- (Fig. S2†). All the BAILs containing a triflic anion gave middle
boxylic group of levulinic acid.
yields of DPA, between 44–55 mol%, which are lower than
Next, another nine BAILs, which were readily prepared by those catalyzed by the thiol-containing ionic liquid 4b (75 mol
adding acids to zwitterionic precursors (see ESI†), were exam- %, Table 2, entry 6). By comparison, H2SO4, triflic acid and
ined in this reaction. Before the catalytic tests, their acidic 3-mercaptopropyl sulfonic acid were also tested and gave DPA
strength was determined by UV-visible spectroscopy using in a yield of 64 mol%, 45 mol% and 80 mol%, respectively.
nitroaniline as an indicator in methanol (Fig. S1 and The promoting effect of the thiol groups was confirmed again,
Table S1†).8 However, no clear relationship could be found which was in agreement with the results reported by Sels
between the catalytic performance and acidic strength. The et al.5
catalytic activities of the BAILs decreased in the order of 4b >
It is noteworthy that the isomer ratio rose up to 50 in the
1a > 4a = 2 > 5 > 6 > 3 > 1c > 1d (Table 2, entries 1–8 and presence of 4b, revealing the advantage of combining thiol
Table 1, entry 3). The thermal gravity analysis showed that the groups and an acidic ionic liquid. The ratio could also be
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ratio significantly (Table 2, entry 22). A possible reason may be
that the thiol groups were oxidized under air, although 4b is of
good thermal stability according to the thermal gravity analysis
(TGA, Fig. S2†).
Table 3 Preparation of diphenolic esters via levulinates or esterification of
DPAa
DPA
Diphenolic Total
ester yield yield
yield
Entry Substrates Additives
(mol%) (mol%)
(mol%)
On the basis of the above results, we attempted to study the
preparation of diphenolic esters since esters are more con-
venient than carboxylic acids for further transformation in
principle, such as amidation and hydrogenation. Moreover,
the esterification can also facilitate the extraction of the
product from BAILs by organic solvents due to the higher oleo-
philicity compared to DPA. Before performing this conversion,
we proposed two strategies: (1) condensation of levulinates
with phenol and (2) esterification of DPA after condensation.
Both strategies can be realized through a one-pot reaction
because the BAILs are good catalysts for esterification. The
results reveal that by replacing ethyl levulinate for levulinic
acid, the main product was ethyl diphenolic ester with middle
yields, and DPA was formed as the co-product due to hydrolysis
during the reaction. The total yield of condensation products
was about 80 mol% (Table 3, entries 1–3). Similar results were
obtained by using methyl levulinate and n-butyl levulinate,
which afforded diphenolic esters with yields of 57 mol% and
65 mol%, respectively. On the other hand, the yields of diphen-
olic esters could be improved effectively and most DPA was
transformed into the corresponding esters without obvious
decomposition of DPA by adding alcohols for esterification at
40 °C for another 8 h after the condensation of levulinic acid
with phenol. Based on this sequence (i.e., condensation fol-
lowed by esterification), over 83 mol% diphenolic esters were
obtained and the total yield of diphenolic esters and DPA was
close to that of DPA before esterification (Table 3, entries 6–8).
Finally, the BAILs recycling experiment was also carried out
by extraction of DPA and diphenolic esters, as well as phenol
with ethyl acetate (see ESI†). After extraction with 25 mL ethyl
acetate 3 times, the recovery rate of the ethyl diphenolic ester
was 82%, which was close to the above mentioned yield of
ethyl diphenolic ester (Table 3, entry 7), indicating that most
of the ethyl diphenolic ester was transferred into the organic
phase from the BAILs catalytic system. By contrast, the recovery
rate of DPA was only 65% under the same extraction con-
ditions. This result evidently showed that the partition of ethyl
diphenolic ester in the extraction could be enhanced effec-
tively. Importantly, the regenerated BAILs also provided high
catalytic performance for both the condensation and esterifica-
tion reactions. Only a slight decrease in the yield of the ethyl
diphenolic ester occurred after the first run and the following
recycle catalytic tests afforded ethyl diphenolic ester with
yields between 74–78 mol% (Fig. 2).
1
2
3
4
5
6
7
8
Ethyl
HSCH2COOH 19
54
56
58
57
65
87
83
86
73
levulinate
Ethyl
1b
4b
4b
4b
4b
4b
4b
21
22
22
16
2
77
levulinate
Ethyl
80
levulinate
Methyl
levulinate
n-Butyl
levulinate
Levulinic
acid
79
81
89
Levulinic
acid
Levulinic
acid
6
89
4
90
a Reaction conditions for entries 1–5: levulinates (4 mmol), phenol
(1.5 g, 16 mmol), 4a (0.9 mmol) and additives (0.2 mol or
0.1 mmol 4b) at 60 °C for 48 h. Reaction conditions for entries 6–8:
levulinic acid (0.46 g, 4 mmol), phenol (1.5 g, 16 mmol), 4a
(0.9 mmol) and 4b (0.1 mol) at 60 °C for 48 h and 2 mL alcohol
(methanol for entry 6, ethanol for entry 7 and n-butanol for entry
8) at 40 °C for 8 h.
increased to more than 30 by using 1c, 1d, 3 and geminal dica-
tionic type BAILs (4a, 5 and 6).
Considering the promoting effect of thiol groups, mercap-
toacetic acid was employed as an additive in order to achieve
higher yields. The results (Table 2, entries 12–19) clearly show
that the yields of DPA increased to 76–88 mol% with the
addition of mercaptoacetic acid in the presence of BAILs as
well as H2SO4, though mercaptoacetic acid alone exhibited no
catalytic activity due to the low acidic strength of the carboxylic
group. Nevertheless, it resulted in an undesired decline in the
isomer ratio to a different extent. This is quite different from
the former reported promotive performance of thiol com-
pounds in the presence of solid acids.5,6 Although the exact
reason for this was not clear, the BAIL catalytic system might
be responsible for the unusual results.
Surprisingly, with 4a and 4b (molar ratio = 9 : 1), this con-
densation could not only be improved with a DPA yield of
91%, but also offered a high selectivity for p,p′-DPA. Only a
trace amount of o,p′-DPA was detected by high-performance
liquid chromatography (HPLC, Fig. S3†). The results are better
than those of 4b and 3-mercaptopropyl sulfonic acid used
solely (Table 2, entries 6 and 11). Similar promoting effects of
1b were also found, revealing that the thiol-containing anion
in BAILs played a key role in the high-yield and selective syn-
thesis of DPA. Although a reasonable mechanism has not been
obtained, the preference for p,p′-DPA should be attributed to
the cooperation of thiol groups and cationic counterparts in
the ionic liquid. Besides, it should be not ed that the reaction
involving thiols should be conducted carefully under an inert
atmosphere because the air could reduce the yield and isomer
Conclusions
In summary, this communication has demonstrated the viabi-
lity of the BAILs catalyzed condensation of renewable levulinic
acid with phenol. Note that the addition of thiol-containing
BAILs offers both a high yield of DPA (over 90 mol%) and a
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Fig. 2 Yield of ethyl diphenolic ester in recycled catalytic runs. Reaction con-
ditions: 4a (4.5 mmol), 4b (0.5 mmol), levulinic acid (2.32 g, 20 mmol) and
phenol (7.52 g, 80 mmol) reacted at 60 °C for 48 h and ethanol (10 mL) at
40 °C for another 8 h.
unique preference for the p,p′-DPA isomer. The successive
esterification of DPA provides distinct advantages in terms of
chemical modification and product separation. The recycling
of BAILs has also been carried out in this study and the results
suggest that the BAILs retain a high catalytic performance
after 5 runs. Moreover, to extend the scope of the feedstocks
and to explore the value of this catalytic system, further
studies on the synthesis of other diphenolic compounds from
levulinic acid, acetone, aldehydes and renewable phenols
derived from lignin9 is in progress.
Acknowledgements
The authors are grateful to the NFSC-Guangdong Joint Fund
(U0834005), Youth Innovation Promotion Association, CAS and
Foshan Centre for Functional Polymer Materials, CAS for
financial support.
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