Synthesis of alcohol ester 12 in 1, 8-diazabicyclo [5.4.0] undec-7-ene (DBU)-based Self-separation catalytic system

Article abstract of DOI:10.1002/aoc.5145

The synthesis of alcohol ester 12 is one of the valuable industrial processes, but it was impeded by poor separating property and recycling ability of the catalytic systems. Herein, four novel DBU-based basic ionic liquids (DBILs) of [BDBU]IM, [BDBU]OH, [ODBU]IM, [[ODBU]OH were synthesized successfully by introducing the alkyl chains of 1-bromobutane or 1-bromooctane to 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU), and then, employing imidazole (IM^?) or hydroxide (OH^?) as counter ions. The above obtained four ionic liquids were applied in the synthesis of alcohol ester 12 in isobutyraldehyde (IBD)/aqueous media for the first time. Interestingly, after reaction, production of alcohol ester 12 can be self-separated from ionic liquids/water (ILs/W) catalytic system automatically. Furthermore, the self-separated ILs/W can be recycled and used in next catalytic reaction for at least 5 times without obvious loss of catalytic performance. In this work, the structure, purity, thermal stability and alkalinity of DBILs were characterized systematically. [BDBU]IM shows high alkalinity and thus enhances yield of 66.17%. From thermo gravimetric analyzer (TGA), [BDBU]IM also exhibits excellent thermal stability. So [BDBU]IM was chosen for the further studying. Furthermore, quantum chemistry is applied to calculate the interaction forces and electron energies of reactants by DFT, and the calculation results illustrate the feasibility of synthetic process of DBILs. The self-separation strategy of DBILS in this work may open up a new avenue for the clean synthesis of other industrial products.

Full text of DOI:10.1002/aoc.5145

Received: 17 May 2019
DOI: 10.1002/aoc.5145
Revised: 4 July 2019
Accepted: 5 July 2019
F U L L P A P E R
Synthesis of alcohol ester 12 in 1, 8‐diazabicyclo [5.4.0]
undec‐7‐ene (DBU)‐based Self‐separation catalytic system
Zhiguo Lv1
| Shuying Zhang1 | Zhenmei Guo2 | Xi Cheng1 | Jiaomei Wang1 |
Chao Zhang1
1 State Key Laboratory Base for Eco‐
chemical Engineering, School of Chemical
Engineering, Qingdao University of
Science and Technology, Qingdao 266042,
China
2 School of Marine Science and Biological
Engineering, Qingdao University of
Science and Technology, Qingdao 266042,
China
The synthesis of alcohol ester 12 is one of the valuable industrial processes, but
it was impeded by poor separating property and recycling ability of the cata-
lytic systems. Herein, four novel DBU‐based basic ionic liquids (DBILs) of
[BDBU]IM, [BDBU]OH, [ODBU]IM, [[ODBU]OH were synthesized success-
fully by introducing the alkyl chains of 1‐bromobutane or 1‐bromooctane to
1,8‐diazabicyclo [5.4.0] undec‐7‐ene (DBU), and then, employing imidazole
(IM−) or hydroxide (OH−) as counter ions. The above obtained four ionic liq-
uids were applied in the synthesis of alcohol ester 12 in isobutyraldehyde
(IBD)/aqueous media for the first time. Interestingly, after reaction, production
of alcohol ester 12 can be self‐separated from ionic liquids/water (ILs/W) cata-
lytic system automatically. Furthermore, the self‐separated ILs/W can be
recycled and used in next catalytic reaction for at least 5 times without obvious
loss of catalytic performance. In this work, the structure, purity, thermal stabil-
ity and alkalinity of DBILs were characterized systematically. [BDBU]IM
shows high alkalinity and thus enhances yield of 66.17%. From thermo gravi-
metric analyzer (TGA), [BDBU]IM also exhibits excellent thermal stability.
So [BDBU]IM was chosen for the further studying. Furthermore, quantum
chemistry is applied to calculate the interaction forces and electron energies
of reactants by DFT, and the calculation results illustrate the feasibility of syn-
thetic process of DBILs. The self‐separation strategy of DBILS in this work may
open up a new avenue for the clean synthesis of other industrial products.
Correspondence
Chao Zhang, State Key Laboratory Base
for Eco‐chemical Engineering, School of
Chemical Engineering, Qingdao
University of Science and Technology.
Qingdao 266042, China.
Email: chaozhangchem@qust.edu.cn
Funding information
doctor foundation of Shandong province,
Grant/Award Number: ZR2019BB010;
Natural Science Foundation of National,
Grant/Award Number: NSFC21376128;
Natural Science Foundation of Shandong
Province, Grant/Award Number:
ZR2017ZC0632; Shandong Key Laboratory
of Reactions and Isolations of Muti‐phase
Liquid, Grant/Award Number:
2019MFRSE‐B03
KEYWORDS
alcohol ester 12, functionalized ionic liquid, recycling of catalyst; environmental friendly synthesis,
self‐separation
1 | INTRODUCTION
alcohol ester 12 has entered the application stage in
China. In the domestic, it has been monopolized by East-
Alcohol ester 12, a high‐boiling water‐insoluble colorless
transparent liquid, is an environmental friendly green
additive that has already been industrialized and widely
used in electrochemistry, physical chemistry, biological
applications, chemical engineering, etc.[1] In the mid‐
1980s, with the rapid development of coatings industry,
man Company of the United States with market volume
as high as 80%. Through the efforts of Chinese, at the
end of the twentieth century, the process of alcohol ester
12 is becoming more and more perfect, and the produc-
tion scale and product quality are constantly improving,
gradually embarking on large‐scale production. However,
Appl Organometal Chem. 2019;e5145.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5145

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LV ET AL.
there are some disadvantages in traditional industrial
preparation process of alcohol ester 12, e.g., poor recycla-
bility, corrosivity for equipment and wastewater treat-
ment.[2] Finding an novel catalyst is one of the
promising strategies to address these problems.
By reading a large amount of literatures, it is found
that ionic liquids (ILs) present sever unique
advantages, e.g., self‐separation ability, recyclability, easily
accessibility and simple reaction process.[3] Besides, ILs are
considered as “designable solvents” since they are com-
posed of abundant anions and cations.[4] Nowadays,
various enormous ILs have been synthesized and the most
Through single factor inquiry experiment, an appropriate
condition for [BDBU]IM is determined (7 wt%, 60 °C,
3 h). After reaction, the ILs/W‐alcohol ester 12 form a
heterogeneous system, which helps to separate prod-
ucts.[9] Thus, the self‐separated ILs/W can be recycled
in next catalytic reaction for at least 5 times with no obvi-
ously decreased in yield.
2 | EXPERIMENTAL
2.1 | Chemicals and apparatus
normally
studied
cations
are
nitrogen‐based,
1,8‐diazabicyclo [5.4.0] undec‐7‐ene (DBU) (purity >99%),
1‐bromobutane (purity >98%), 1‐brooctane (purity >98%)
were purchased from Shanghai Aladdin Biochemical
Technology Co. Ltd., Isobutyraldehyde (IBD) (purity
>98.5%), sodium hydroxide (purity >96%), potassium
hydroxide (purity >98%), imidazole (purity >99%), were
purchased from Tianjin Bodi Chemical Co. Ltd.. The
DBU‐based ILs were characterized on a Bruker TENSOR
27 FT‐IR Fourier transfer infrared spectrometer, Structure
of DBU‐based ILs was identified by 1H NMR using Bruker
AV500 NMR spectrometer at room temperature operating
at 500 MHZ, The thermal stability of four DBU‐based ILs
in the range of 25–500 °C was tested by TGA4000 thermo
gravimetric analyzer in N2 environment at a heating rate
of 10 °C/min, The C, H, and N elements of DBU‐based
ILs were analyzed using a Vario EL elemental analyzer,
the pH values were measured three times by a PHS‐25
pH meter; Quantitative analysis of DBU‐based ILs by SP‐
6800A gas chromatography with polar.
namely, imidazolium‐, pyrrolidinium‐, pyridinium‐
piperidinium‐, and quaternary‐ammonium. Also,
chloride (Cl−), bromide (Br−), acetate ([C1CO2]−),
hexafluorophosphate ([PF6]−) and tetrafluoroborate
([BF4]−) are widely used as anions in ILs.[5] At present,
most of the ILs used in catalytic reactions are based on
imidazole,[6] but it has been impeded seriously due to the
use of additives (such as, acids, bases and catalyst ligands
etc.).[7] How to enhance the performance of alkaline ILs
has attracted more and more interest of researchers. It is
well known that DBU shows large amount of binding site
to anion, because of its ring structure. Furthermore, its
strong nucleophilicity makes it easier to attack the posi-
tively α‐H on IBD, promoting affinity reaction of two mol-
ecules of IBD. In addition, alkyl chain groups exhibit
excellent electrophilic performance. Introducing alkyl
chain groups to 1,8‐diazabicycloundec‐7‐ene (DBU)ring
can increase the potential Lewis alkaline of cation. Thence,
1,8‐diazabicyclo [5.4.0] undec‐7‐ene‐based basic ionic liq-
uids (D BILs) may overcome the above mentioned short-
coming. All these above features inspired us to apply
DBILs in the synthesis of alcohol ester 12.
2.2 | Synthesis of the DBILs
To date, almost none reports about synthesis of alcohol
ester 12 based on DBILs have been found. It is difficult to
rationally design DBILs due to complex interactions.[8]
Considering the base‐catalyzed reaction, in this work,
imidazolium and hydroxide are selected as anions, intro-
ducing the alkyl chains of 1‐bromobutane or 1‐brooctane
in DBU to synthesize four different kinds of [BDBU]IM,
[BDBU]OH, [ODBU]IM and [ODBU]OH, respectively.
DBILs are characterized by infrared (IR) and nuclear
magnetic (NMR) to study its chemical structure. The
results demonstrate the successful synthesis of four
DBILs. They were tested by thermo gravimetric analyzer
(TGA), elemental analyzer (EA) and pH value to deter-
mine its thermal properties, purity, and alkalinity. The
results demonstrate that all four DBU‐based ILs have
high purity. Besides, [BDBU]IM exhibits high thermal
stability and alkalinity. Among the four DBILs, it is found
that [BDBU]IM shows the highest yield of 66.17%.
2.2.1 | Synthesis of 8‐butyl‐1,8‐diazabicyclo
[5.4.0] undec‐7‐enium bromide ([BDBU]Br)
0.11 mol n‐bromobutane was dropwise added into a mix-
ture of 0.1 mol DBU and 20 ml acetone in a flask of
100 ml under vigorous stirring at 343.15 K for 6 hr. After
reaction, [BDBU]Br was obtained by vacuum‐rotary evap-
oration, washed with diethyl ether and dried at 343.15 K
under vacuum for 12 hr. About 25.84 g of colorless yellow
liquid product was obtained with a yield of 89.31%.
2.2.2 | Synthesis of 8‐butyl‐1,8‐diazabicyclo
[5.4.0] undec‐7‐enium hydroxide ([BDBU]
OH)
0.5 mol [BDBU]Br was slowly added into the mixture of
0.5 mol KOH and 15 ml absolute methanol in a flask of

LV ET AL.
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100 ml under vigorous stirring at room temperature for
12 hr. After reaction, the resulting white precipitate was
filtered off. [BDBU]OH was obtained by vacuum‐rotary
evaporation, washed with diethyl ether and dried at
343.15 K under vacuum for 24 hr. About 20.30 g of
slightly yellow liquid product was obtained with a yield
of 88.45%.
SCHEME 1 Synthesis process and molecular structure of
[BDBU]Br
2.2.3 | Synthesis of 8‐butyl‐1,8‐diazabicyclo
[5.4.0] undec‐7‐enium imidazolide ([BDBU]
IM)
SCHEME 2 Synthesis process and molecular structure of
[BDBU]OH
0.5 mol imidazole was added into a mixture of 0.5 mol
NaOH and 15 ml methanol in a flask of 100 ml under
vigorous stirring at room temperature for 0.5 hr. After
reaction, imidazole sodium (NaIM) was obtained by
vacuum‐rotary evaporation. Immediately after, 0.5 mol
[BDBU]Br was slowly added into the mixture of 0.5 mol
NaIM and 15 ml absolute methanol in a flask of 100 ml
under vigorous stirring at room temperature for 20 hr.
After reaction, the resulting white precipitate was filtered
off. [BDBU]IM was obtained by vacuum‐rotary evapora-
tion, washed with diethyl ether and dried at 343.15 K
under vacuum for 24 hr. About 25.05 g of slightly yellow
liquid product was obtained with a yield of 89.31%.
The synthesis of 8‐octyl‐1,8‐diazabicyclo [5.4.0] undec‐
7‐enium bromide ([ODBU]Br), 8‐octyl‐1,8‐diazabicyclo
[5.4.0] undec‐7‐enium hydroxide ([ODBU]OH) and 8‐
octyl‐1,8‐diazabicyclo [5.4.0] undec‐7‐enium imidazolide
([ODBU]IM) is similar to that of DBILs with n‐butyl car-
bon chains. The yield was 87%, 90.12%, 91.32%,
respectively.
SCHEME 3 Synthesis process and molecular structure of
[BDBU]IM
[BDBU]Br and NaIM reacted through ion exchange reac-
tion (e.g., IM−substitutes Br−). The synthetic process of
Scheme S1, S2 and S3 were same as that of Scheme 1, 2
and 3, respectively.
3.2 | Chemical structure of DBILs
Figure 1 shows the FT‐IR spectrum of DBILs in the range
400–4000 cm−1.From Figure 1, there was an obvious peak
around 1628 cm−1 in the spectrum of all DBILs, due to typ-
ical characteristic peak of C=N species in DBU ring. The
peak in the range of 2857–2292 cm−1 was ascribed to the
2.2.4 | Synthesis of alcohol ester 12 in
DBU‐based ILs catalytic system
IBD was dropwise added into a mixture of DBU‐based
ILs/Water in a flask of 100 ml under magnetic stirring.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis process of the DBILs
Four DBILs were synthesized through the facile two‐step
ion‐exchange method. Firstly, via quaterisation, bromide
with target cation ([BDBU]Br) was obtained successfully
(Scheme 1). Then, from Scheme 2, [BDBU]Br and KOH
reacted through ion exchange reaction (e.g., OH−substi-
itutes Br−). As shown in Scheme 3, firstly, sodium imidaz-
ole (NaIM) was obtained through neutralization reaction
of imidazole and NaOH (OH− reacts with N‐H), then,
FIGURE 1 FT‐IR spectra of DBU‐based ILs ([BDBU]IM, [BDBU]
OH, [ODBU]IM and [BDBU]OH)

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LV ET AL.
stretching vibration of saturated C‐H in alkyl chains.[10]
The peak in the range of 2853–2930 cm−1 was related to
the stretching vibration of C‐H in methylene. The broad
peak absorption band at ca. 3400 cm−1 (Figure 1a and 1b)
proved the existence of hydroxyl groups.[11] There were
some obvious peaks at 3100, 1572 and 1169 cm−1
(Figure 1c and 1d). It was the typical characteristic peaks
of unsaturated C‐H, skeleton vibration and symmetric
stretching vibration in imidazole units, respectively. The
characteristic peaks of C=C, C=N stretching vibration in
the imidazole ring were found at 1428 to 1465 cm−1, 1337
to 1428 cm−1, respectively. These above FI‐IR results dem-
onstrated the successful synthesis of DBU‐based ILs.
[BDBU]OH (Figure 2a), 1H NMR (Brucker DMX‐
500 MHz, CDCl3): d 0.89 (t, 3H, CH3(CH2)3N), 1.32
(m, 2H, CH3‐CH2(CH2)2N), 1.43 (m, 2H, BDBU C4),
1.61–1.70 (m, 8H, BDBU C3, C5, and C10,
CH3CH2CH2CH2N), 2.47 (t, 2H, BDBU C6), 2.54
(m, 4H, BDBU C2 and C11), 3.31 (t, 2H, BDBU C9),
3.42 (t, 2H, CH3(‐CH2)2CH2N); [ODBU]OH (Figure 2b),
1H NMR (Brucker DMX‐500 MHz, CDCl3): d 0.84
(t, 3H, CH3(CH2)3N), 1.2532 (m, 2H, CH3‐CH2(CH2)2N),
1.37 (m, 2H, BDBU C4), 1.57–1.66 (m, 8H, BDBU C3,
C5, and C10, CH3CH2CH2CH2N), 2.44 (t, 2H, BDBU
C6), 2.49 (m, 4H, BDBU C2 and C11), 3.26 (t, 2H, BDBU
C9), 3.35 (t, 2H, CH3(‐CH2)4CH2N); [BDBU]IM (Figure 2
c), 1H NMR (Brucker DMX‐500 MHz, CDCl3): d 0.85
(t, 3H, CH3(CH2)3N), 1.29 (m, 2H, CH3‐CH2(CH2)2N),
1.41 (m, 2H, BDBU C4), 1.59–1.67 (m, 8H, BDBU C3,
C5, and C10, CH3CH2CH2CH2N), 2.46 (t, 2H, BDBU
C6), 2.52 (m, 4H, BDBU C2 and C11), 3.28 (t, 2H, BDBU
C9), 3.39 (t, 2H, CH3(‐CH2)2CH2N), 6.95 (s, 2H, IM C4
and C5), 7.26 (s, 1H, IM C2); [ODBU]IM (Figure 2d),
1H NMR (Brucker DMX‐500 MHz, CDCl3): d 0.84 (t,
3H, CH3(CH2)3N), 1.26 (m, 2H, CH3‐CH2(CH2)2N), 1.38
(m, 2H, BDBU C4), 1.57–1.68 (m, 8H, BDBU C3, C5,
and C10, CH3CH2CH2CH2N), 2.40 (t, 2H, BDBU C6),
2.49 (m, 4H, BDBU C2 and C11), 3.24 (t, 2H, BDBU
C9), 3.33 (t, 2H, CH3(‐CH2)4CH2N), 7.04 (s, 2H, IM C4
and C5), 7.25 (s, 1H, IM C2).
On the basis of the results of FT‐IR and 1H NMR spec-
tra, it could be found that DBILs were synthesized and
grafted with alkyl groups successfully.
3.3 | Thermal stability of DBILs
From Figure 3, when anions are same, [ODBU]OH and
[BDBU]OH decomposed at ca. 110 °C, by contrast,
[ODBU]IM and [BDBU]IM could withstand high tempera-
ture of 134 °C and 145 °C, respectively, which is consistent
with the feature that the thermal stability decreases signif-
icantly with increasing carbon chain.[12] When there are
same cations, the thermal stability of [BDBU]IM
([ODBU]IM) is higher than [BDBU]OH ([ODBU]OH).
The enhanced thermal stability is due to the large volume
of imidazole. Based on the results of the thermal analysis,
it was proved that [BDBU]IM presented the highest ther-
mal stability among the DBILs. The characteristic may
FIGURE 2 NMR spectra of DBU‐based
ILs

LV ET AL.
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Conservation of quality:
CB ¼ ½B−ꢀ þ ½HBꢀ ¼ Aþ
‐
Conservation of charge:
½Aþꢀ þ ½Hþꢀ ¼ ½B−ꢀ þ ½OH−ꢀ
Get:
Kb
þ ½Hþꢀ ¼ ½OH−ꢀ
−
CB
⋅
½OH−ꢀ þ Kb
B−: anions. CB−: concentration of DBILs. Kb: dissocia-
tion constant. A+: concentration of cations. The above
units are all in molar concentration.
From the above formula, the pKb values could be cal-
culated facilely. Firstly, DBILs were configured an aque-
ous solution with a concentration of 0.11 mol/l. Then,
the pH values were measured three times by a PHS‐25
pH meter. The molar concentration of [H+], [OH−] and
Kb values can be obtained by the above formula. The
smallest pKb showed the strongest alkaline.[13]
The order of the alkalinity is [BDBU]IM > [BDBU]
OH > [ODBU]IM > [ODBU]OH (Table 2). When having
the same cations, the alkalinity decreases with increasing
carbon chain. In theory, long carbon chains lead to high
alkalinity.[14] This phenomenon is due to the interaction
between net residual charge in entire DBU ring and elec-
tron effect of the carbon chains. With increasing carbon
chains, the positive charge on the surface of the cations
enhances. Accordingly, the stability of DBILs decreases,
and the electrostatic attraction between ILs and OH−
enhances sequentially. Thereby inhibiting the ionization
of anion and decreasing its alkalinity. With same cations
(e.g., [BDBU]IM and [BDBU]OH, or [ODBU]IM and
[ODBU]OH), the alkalinity of ILs with IM− is stronger
than its counterpart with OH− because of the interaction
of OH− and electron‐rich aromatic ring of imidazolium.
FIGURE 3 TG curves of DBU‐based ILs
contribute to the excellent recyclability in the synthesis of
alcohol ester 12.
3.4 | Purity of DBILs
Table 1 shows that the elemental analysis measured
values are in close proximity to the calculated values,
illustrating that high purity DBILs were synthesized
successfully.
3.5 | Alkaline of DBILs
Alkaline ionic liquids in aqueous solution, the anion will
be hydrolyzed.
B− þ H2O↔HB þ OH−
Defined by Kb:
½HBꢀ½OH−ꢀ
Kb ¼
½B−ꢀ
TABLE 1 Elemental analysis data of DBU‐based ILs
Measured value (calculated value)/%
Molecular
ILs
formula
C
H
N
TABLE 2 pKb value of DBU‐based ILs
[BDBU]
OH
C13H26N2O 68.92(68.80) 11.49(11.32) 12.37(12.21)
ILs
concentration
pH
pKb
[BDBU]OH
[ODBU]OH
[BDBU]IM
[ODBU]IM
0.01
0.01
0.01
0.01
10.66
10.47
11.31
10.58
4.66
5.05
3.28
4.82
[BDBU]IM C12H27N4 51.92(51.78) 9.74(9.78) 20.19(20.02)
[ODBU]
OH
C17H34N2O 72.25(72.10) 12.04(11.85) 9.92(9.87)
[ODBU]IM C16H35N4 57.60(57.44) 10.50(10.33) 16.80(16.75)

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LV ET AL.
TABLE 4 Yield of alcohol ester 12 in various solvents
3.6 | Solubility of DBILs
Solvent
solvent‐free
MeOH
CH3Cl
H2O
Solubility of DBILs was studied and listed in Table 3. It
was found that four DBILs have different solubility in
various solvents, e.g., [BDBU]OH could be easily dis-
solved in water, hardly dissolved in C7H8; CH2Cl2 could
dissolved [ODBUOH well but [ODBU]IM poorly due to
the different polarity of the solvent and ILs.[15] Water sol-
ubility of ILs with long carbon chains ([ODBU]OH and
[ODBU]IM) was lower than that with short carbon
chains ([BDBU]OH and [BDBU]IM). Which can be
attributed to the decreased polarity of [ODBU]OH and
[ODBU]IM.
Yield/%
10.23
12.15
11.45
61.14
Reaction condition:60 °C, 2 hr, 500 r/min, 10 g Isobutyraldehyde, 2 g solvent.
TABLE 5 Effect of DBU‐based ILs on conversion, selectivity and
yield
ILs
Conversion rate/% Selectivity/% Yield/%
[ODBU]OH 80.52
73.58
86.27
79.70
84.11
59.25
59.63
66.17
62.10
[ODBU]IM
[BDBU]IM
[BDBU]OH
69.12
83.03
73.83
Reaction condition:60 °C, 2 hr, 500 r/min, 10 g Isobutyraldehyde, 2 g water,
6 wt% DBILs.
3.7 | Catalytic performance of [BDBU]IM
in various solvents
3.9 | Single factor exploration experiment
The yields in different solvents under the identical condi-
tions are showed in Table 4. It could be found that the
synthesis of alcohol ester 12 could be performed in
solvent‐free, MeOH, CH3Cl. But compared with water,
the yields are rather low, which might due to the below
two factors: (i) the hydrophobic effect, which leads to
high negative volume of activation and greater stabiliza-
tion of activated complexes than hydrophobic reactants
in the reaction; (ii) the polarity effect, where the high
polarity of water results in more polar translated states
than initial states.
3.9.1 | Effect of mass fraction of [BDBU]
IM on reaction
The effect of mass fraction of [BDBU]IM on conversion,
selectivity and yield was studied. From Figure 5, in mass
fraction range of 4–7 wt%, conversion rate and selectivity
were gradually increased. Yield reached a maximum of
64.75% at 7 wt% (Table 6). Then, with mass fraction
increasing from 7 to 8 wt%, the conversion rate and selec-
tivity decreased limearly. Due to the excessive mass frac-
tion, which inhibition reaction and lead to the occurrence
of side reactions. The incomplete conversion of IBD pro-
duced a large amount of isobutyric acid in the second
step, impeding the progress of the aldol condensation
and Cannizzaro reaction. Therefore, 7 wt% of [BDBU]
IM was selected to be the optimum model catalyst.
3.8 | Catalytic performance of DBILs
Different DBILs show diverse catalytic ability (Table 5
and Figure 4). In this work, [BDBU]IM shows the highest
yield of 66.17%, which is 1.12 times, 1.11 times, 1.07 times
higher than [ODBU]OH, [ODBU]IM and [BDBU]OH,
respectively. Considering the yield rested with selectivity
and conversion, [BDBU]IM was selected as a model to
investigate its properties in the system of synthesis of
alcohol ester 12.
3.9.2 | Effect of reaction temperature on
reaction
The effect of reaction temperature on conversion, selec-
tivity and yield was researched. From Figure 6, within
temperature range of 50–75 °C, the conversion rate and
TABLE 3 Solubility of DBU‐based ILs in common organic solvents
ILs
H2O
MeOH
EtOH
C6H12
C7H8
CH2Cl2
[BDBU]OH
[BDBU]IM
[ODBU]OH
[ODBUIM
×
×
×
×
×
×
×
×
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
×
√: mutual dissolution. ×: immiscible.

LV ET AL.
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FIGURE
selectivity and yield
6
Effect of reaction temperature on conversion,
FIGURE
selectivity and yield
4
Effect of four DBU‐based ILs on conversion,
selectivity extended a relatively high level. Before 65 °C,
conversion rate and selectivity increased at practically
parallel rate. In temperature range of 65–70 °C, the
increased rate of selective enhanced greater than that of
conversion rate. Moreover, yield reached a maximum of
69.02% at 65 °C (Table 7). After 70 °C, selectivity
decreased sharply, while, conversion rate decreased
mildly. Low temperature is conducive to the reaction
of aldol condensation. However, too low temperature
(< 50 °C) would hinder reaction rate and prolong reac-
tion time. And, accordingly, high temperature is favor-
able for the Cannizzaro reaction. Nevertheless, too high
temperature (> 70 °C) would result in reverse hydrolysis
of alcohol ester 12. The harsh reaction environment could
decrease the yield of alcohol ester 12. Therefore, 70 °C
was selected to be optimum reaction temperature.
FIGURE 5 Effect of mass fraction of [BDBU]IM on conversion,
selectivity and yield
3.9.3 | Effect of reaction time on reaction
The reaction time was explored as well except for mass
fraction and reaction temperature. From Figure 7, in
TABLE 6 Effect of mass fraction of [BDBU]IM on conversion,
selectivity and yield
TABLE 7 Effect of reaction temperature on conversion, selectiv-
ity and yield
Mass fraction/
wt%
Conversion
rate/%
Selectivity/%
Yield/%
Temperature/°C Conversion rate/% Selectivity/% Yield/%
4
5
6
7
8
75.37
78.90
80.25
84.84
76.62
69.32
71.16
73.25
76.32
66.53
52.24
56.14
58.78
64.75
50.98
50
60
65
70
75
74.15
77.97
80.86
81.11
76.83
75.38
78.63
81.27
85.10
74.12
55.89
61.31
65.71
69.02
56.95
Reaction condition: 70 °C, 2 hr, 10 g Isobutyraldehyde, 2 g water, mass frac-
tion of [BDBU]IM: 4, 5, 6, 7 and 8 wt%, respectively.
Reaction condition: 7 wt%, 2 hr, 10 g Isobutyraldehyde, 2 g water, reaction
temperature: 50, 60, 65, 70, and 75 °C, respectively.

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LV ET AL.
The strict reaction environment could decrease the yield
of alcohol ester 12. Considering comprehensively, 3 hr
was chosen as optimal reaction time.
The suitable reaction condition for synthesis of alcohol
ester 12 in [BDBU]IM.
catalytic system was finally determined (7 wt%,
70 °C, 3 hr).
4 | PRODUCTION SEPARATION
AND PURIFICATION
4.1 | Separation of alcohol ester 12
After reaction, the production of alcohol ester 12 could be
self‐separated from ionic liquids/water (ILs/W) catalytic
system automatically (Figure 8 and S1). This phenomenon
is due to hydrogen‐bonding effect (Figure S2, there is a
strong hydrogen bond between hydrogen (14H) and ter-
tiary nitrogen (2 N) in the DBU ring). When the system
of IBD and ILs/W is heated, the hydrogen bond could be
destroyed. After reaction, due to the restored hydrogen
bond at room temperature, alcohol ester 12 is separated
from ILs/W. Then, ILs/W and crude product could be sep-
arated by simple phase separation easily. The separated
ILs/W was reused in next catalytic reaction up to 5
times without any significant decrease in yield (Figure 9).
The water‐catalyst organic two‐phase catalytic
systems,[16] such as water/nanometer Rhodium‐1‐butanol
two‐nanometer Rhodium system and water/(p‐
C6H4(C2H4)5OH)3‐styrene two‐phase system, can be con-
trolled by temperature. That is, “Low temperature phase
separation, high temperature homogeneity”. In essence,
it is thermoregulated transfer catalysis that is the reverse
temperature‐water solubility property of the nonionic sur-
factant (cloud point), which form a hydrogen bond with
water at a low temperature and break hydrogen bond at
high temperature. DBU‐based ILs are amphiphilic and
FIGURE 7 Effect of reaction time on conversion, selectivity and
yield
reaction time range of 1–3 hr, conversion rate and selec-
tivity were gradually increased. Yield reached a maxi-
mum of 70.78% at 3 hr (Table 8). Following, with time
increasing from 3 to 3.5 hr, the conversion rate and selec-
tivity decreased sharply. Too short reaction time (< 1 hr)
caused incomplete conversion of IBD, and, too long reac-
tion time (> 3 hr) lead to hydrolyzing of alcohol ester 12.
TABLE 8 Effect of reaction time on conversion, selectivity and
yield
Time/hr
Conversion rate/%
Selectivity/%
Yield/%
1
77.06
79.18
81.40
83.18
78.20
76.04
78.74
82.74
85.10
79.70
58.60
62.34
62.35
70.78
62.33
2
2.5
3
3.5
Reaction condition: 7 wt%, 70 °C, 10 g Isobutyraldehyde, 2 g water, reaction
time: 1, 2, 2.5, 3 and 3.5 hr, respectively.
FIGURE 8 Process simulation of
synthesis of alcohol 12 in DBU‐based ILs
catalytic system

LV ET AL.
8 of 11
shows that the characteristic peak of molecular fragment
is completely consistent with the standard spectrum, its
molecular formula is C12H24O3 and molecular mass is 216
(Figure 10), confirming the successful synthesis of alcohol
ester 12.
The mechanism of the synthesis of alcohol 12 in
[BDBU]IM/IBD/aqueous media catalytic system involves
three steps of aldol condensation, cannizzaro reaction,
esterification reaction (Figure 11 and Scheme S4). Partic-
ularly, [BDBU]IM as model catalyst acted as both base
catalyst and phase transfer catalyst in the reaction. In
addition, [BDBU]IM together with aqueous media pre-
sents good recyclability. This unique feature of [BDBU]
IM would imply its significant ascendancy in industrial
processes.
FIGURE 9 The reusability of [BDBU]IM
4.3 | Computation
easy to form hydrogen bonds with water. Thus, their cloud
points are about 40 °C. Solubility and thermoregulated per-
formance of DBU‐based ILs/W was shown in Figure S3.
At high temperature, the hydrogen bond is destroyed,
and DBU‐based ILs enter the organic phase of
isobutyraldehyde to react. After the reaction, cooling to
room temperature, the hydrogen bond is restored, and
the product is separated from DBU‐based ILs‐water phase.
The mechanism of DBU‐based ILs/W‐Isobutyraldehyde
two phase system is the same as mentioned above. Thus,
the slight decreased in yield result from the slight loss of
catalyst during separation process. The recyclability of
[BDBU]OH, [ODBU]OH and [ODBU]IM was as good as
[BDBU]IM (Figure S4). The purification of separated crude
product: firstly, the crude product was washed with water
after sitting in ice bath for 24 hr, then, the aqueous phase
containing by‐products was separated and the oil phase
was treated with reduced pressure distillation, collecting
sterling alcohol ester 12 at 144–146 °C.
Energy calculations and structure optimization of DBU‐
based were calculated by density functional theory
(DFT) in the Gaussian 09 program. All‐electron basis of
Pople was 6‐31G (+, d, p). Via the electrostatic potential,
otherwise complex number of different atomic and
molecular properties (such as, site of chemical potential,
intermolecular force, etc.) can be converted into a visible
figure. Electrostatic potential map shows that electrostatic
potential is diverse at different sites. The blue and red
colors show the positive regions and negative respec-
tively. Negative electrostatic potential is vulnerable to
electrophilic reagthents attack, while positive electrostatic
potential is vulnerable to nucleophilic reagents attack.[17]
We choose [BDBU]IM as a model catalyst for further
computing.
There is strong negative potential of [BDBU]+, which
was vulnerable to attack groups with positive potential
(Figure 12 and 13). In addition, the force was directly
related to the electrostatic potential. Through the com-
parison of the electrostatic potential map of four kinds
of ILs, the intermolecular interaction force between IM−
and [BDBU]+ was concluded to be strong. For all the
optimization objects, the harmonic vibration frequency
4.2 | Product analysis
The product of alcohol ester 12 shows a clear transparent
appearance. The result of mass spectrum (MS) analysis
FIGURE 10 Mass spectrum of alcohol
ester 12

9 of 11
LV ET AL.
FIGURE 11 The 3D structure of mechanism for the synthesis of alcohol 12
FIGURE 12 The optimized geometries with corresponding
FIGURE 13 The optimized geometries with corresponding
electrostatic potential at the isodensity contour (0.004 electrons/
bohr3) surface of the reactants, and in color ranges of ‐5.86 (red)
through 0.0 (green) to 5.86 (blue) (unit: eV). The position of Is, min
for an DBU canion group is denoted by a cyan hemisphere. The
value of Is, min for [BDBU]+ and IM‐ is shown in the column graph,
in kJ/mol.
electrostatic potential at the isodensity contour (0.004 electrons/
bohr3) surface of the reactants, and in color ranges of ‐157 (red)
through 0.0 (green) to 157 (blue) (unit: kJ/mol). The position of Vs,
min for an DBU canion group is denoted by a cyan hemisphere. The
value of Vs, min for [BDBU]+ and IM‐ is shown in the column graph,
in kJ/mol.
and total energy were calculated, and the total energy
through the zero point correct. The overall research
object is the interaction energy, which is calculated as fol-
lows:
between 0.8 and 7.7 eV (1 eV = 96.5 kJ/mol), manifest-
ing that there is strong coordination bonds
formed between cations and anions, thereby verifying
the rationality of the mechanism for the synthesis of
DBU‐base ILs.
△EðeVÞ ¼ 27:21 × ½EABðauÞ − ðEAðauÞ − EBðauÞÞꢀ
5 | CONCLUSION
The EAB is the total energy; EA and EB are the energy
of cations and anions. Figure 12 and 13 showed that the
intermolecular interaction energy of the complexes was
In this study, four DBILs (e.g., [BDBU]IM, [BDBU]OH,
[ODBU]IM and [ODBU]OH) were synthesized for the

LV ET AL.
10 of 11
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ACKNOWLEDGMENTS
This work was supported by grant from Natural Science
Foundation of Shandong Province (ZR2017ZC0632),
Natural Science Foundation of National (NSFC21376128)
and doctor foundation of Shandong province
(ZR2019BB010), Qingdao Applied Basic Research Pro-
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SUPPORTING INFORMATION
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