Using dialkyl amide: Via forming hydrophobic deep eutectic solvents to separate citric acid from fermentation broth

Article abstract of DOI:10.1039/c9gc04401a

Nowadays, developing appropriate technology is one of the biggest challenges for society to reduce environmental impact. In this research, to avoid the traditional calcium salt method which produces a large amount of waste gypsum residue, a new way of separating citric acid from fermentation broth was developed by forming hydrophobic deep eutectic solvents (DESs), in which amide and citric acid were used as the hydrogen bond acceptor and donor respectively when amide was in contact with the fermentation broth containing citric acid. Among these amides, C₁₀H₂₁NO was found to be an efficient hydrogen bond acceptor forming hydrophobic DESs with the citric acid based on the molecular size and shape and has the largest hydrophobic equilibrium constant of 3.14. The hydrophobic DES formation mechanism was studied by analyzing the chemical bonds using FT-IR and quantum chemical (QC) calculations. C₁₀H₂₁NO was regenerated by elevating the temperature of the hydrophobic DESs. The regenerated C₁₀H₂₁NO exhibited good recycling properties with no obvious reduction of the ability to form hydrophobic DESs. This effective way of obtaining high-quality citric acids provides new ideas for the separation of other carboxylic acids.

Full text of DOI:10.1039/c9gc04401a

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DOI: 10.1039/C9GC04401A
Green Chemistry
Paper
Using dialkyl amide via forming hydrophobic deep eutectic
solvents to separate citric acid from fermentation broth
Received 00th January 20xx,
Accepted 00th January 20xx
Lijuan Liu, a, b Qifeng Wei, *a, b Yong Zhou and Xiulian Ren *
c
a, b
DOI: 10.1039/x0xx00000x
Nowadays, developing appropriate technology is one of the biggest challenges for society to reduce environmental
impact. In this research, to avoid the traditional calcium salt method producing a large amount of waste gypsum residue, a
new way of separating citric acid from fermentation broth was developed by forming hydrophobic deep eutectic solvents
(DESs), in which amide and citric acid was used as hydrogen bond acceptor and donor respectively when amide was in
contact with fermentation broth containing citric acid. Among these amides, C10 21NO was found as an efficient hydrogen
H
bond acceptor forming hydrophobic DESs with the citric acid based on the molecular size and shape which has the largest
hydrophobic equilibrium constant 3.14. The hydrophobic DESs formation mechanism was studied by analyzing the
chemical bonds using FT-IR and quantum chemical (QC) calculations. C10
temperature of the hydrophobic DESs. The regenerated C10 21NO exhibited good recycling properties with no obvious
reduction of the ability to form hydrophobic DESs. This effective way obtaining high-quality citric acids provides new
ideas for other carboxylic acids separation.
H21NO was regenerated by elevating the
H
the sustainability of the process, which is expensive and
2
,3,7,10
Introduction
unfriendly to environment.
Therefore, the development
of a new, efficient and competitive recovery method of citric
acid product separation is a critical step that determines the
bio-based production of carboxylic acids. Liquid-liquid
extraction with aliphatic amines and quaternary amines
dissolved in organic diluents for the carboxylic acids recovery
was extensively reported in the literature in the 1990s.
Some traditional extractant are flammable, volatile, or toxic,
which significantly limits their applicability in practice, so it is
necessary to develop a new generation of solvents that can be
easily recovered, more environmentally friendly and safer than
conventional solvents. Currently, two major classes of solvent
Citric acid is one of the most important commercially valuable
products due to its wide spread use in food, pharmaceuticals,
cosmetics, metal cleaning, environmental protection and other
industries.1 Currently, the demand for citric acid is reported
to be increasing at a rapid rate of 5% per year along with
advanced applications of carboxylic acids such as drug delivery
6
-3
1
1-13
2
,4
and tissue engineering for culturing a variety of cells. It is the
most demanding organic acid in the world at present. Studies
have shown that the value of citric acid in the global market
4
will reach $3.6 billion by 2020. Nowdays, citric acid is mainly
produced from the biological transformation of biorefinery
sugars via cassava or corn fermentation using Aspergillus niger
and is often the most expensive bio-based intermediates to
recover.2 The downstream recovery process for citric acid
1
4-17
18-21
ionic liquids (ILs)
and eutectic mixtures
are being
developed by scholars for bioactive compounds separation.
However, the relatively complicated synthesis process and
expensive raw chemicals make ILs still face challenges in large-
,5
4
,6,7
accounts for 30–40% of the production cost.
society and the industry are facing major challenges to
incorporate chemical products into the biorefinery’s portfolio
Both the
1
8-19,21
scale industrial applications.
Conversely, the deep
eutectic solvents (DESs) have been widely concerned due to
their biodegradability, easier preparation, lower cost and more
8
,9
regarding social responsibility and environmental concerns.
2
2-26
25
environmentally benign.
In 2003, Abbott and colleagues
The conventional method using calcium hydroxide
precipitation requires a lot of lime and sulfuric acid, and also
generates solid waste in the form of calcium sulfate, reducing
first coined the term “deep eutectic solvents”. After that, a
large number of literatures on DES, particularly on
fundamental studies such as physicochemical properties
2
1,28-30
density and viscosity has been published.
In 2015,
a. School of Chemistry and Chemical Engineering, Harbin Institute of Technology,
hydrophobic DESs were presented for the first time which
were used to the recovery of volatile fatty acids (VAFs) from
diluted aqueous solutions successfully.
Citric acid containing both hydroxyl and carboxyl groups in
its molecule has high a solubility in water. However, there is no
Harbin 150001, China, E-mail: renxiulian@126.com, weiqifeng163@163.com
School of Marine Science and Technology, Harbin Institute of Technology at
b.
3
1
Weihai, Weihai Wenhua West Road 2, Weihai, Shandong 264209, China.
COFCO Biotechnology Co., Ltd., No. 1 COFCO Avenue, Bengbu City, Anhui
c.
Province, E-mail: zy197579@163.com
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See report about hydroxycarboxylic acid seperation using
DOI: 10.1039/x0xx00000x
hydrophobic DESs due to their strong hydrophilicity and there
1
| J. Name., 2012, 00, 1-3
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Paper
is also no report about equilibrium and thermodynamic study DESs is as follows: n-C10
Green Chemistry
H
H
21NO>n-C14H29NO> n-C18H37NO>i-
View Article Online
DOI: 10.1039/C9GC04401A
on citric acid with amide to form hydrophobic DESs in
literature till date. In this paper, dialkyl amides were
C
C
18
H
37NO> n-C12
H
25NO>i-C12
25NO>n-C15
H
31NO>n-C16H33NO>i-
16
H33NO> n-C18
H N O . It can be found that Kex increases as
36 2 2
developed as biodegradable hydrogen bond acceptors (BHBAs) BHBA van der Waals volume decreases for acetamide
via forming hydrophobic DESs to separate citric acid from containing two symme-
cassava fermentation broth, similar to molecularly imprinted trical branches (n-C10H21NO> n-C14H29NO> n-C18H37NO). And
polymers (MIPs) with specific molecular recognition properties the Kex decreases as BHBA van der Waals volume size increases
3
2,33
for target molecules in solid-phase extraction.
and the for amide with constant alkyl chain length (n-C14
H29NO>n-
separation mechanism was also discussed assisted with
quantum chemistry (QC) calculations.
15
C H31NO>n-C16H
33NO). The reason may be that as the length
2
6
of the alkyl chain increases so does the volume of alkyl amides,
which results in an increase in the space steric, consequently
reduces their interactions. The McGowan volumes of citric acid
Results and discussion
3
-1
and C10H21NO are 161.83 and 163.31 cm mol respectively.
The BHBA equivalent to the citric acid in size has better
bonding effect on citric acid. The closer the BHBA volume to
the HBD, the greater the equilibrium constant of the
hydrophobic DESs, so the more thorough the citric acid is
separated from the fermentation broth. However, different
effects on the separation of citric acid (n and i derivatives of
HBAs) are realized for HBAs of the same volume. That is
because the steric hindrance of i derivatives of HBAs is greater
than n derivatives of HBAs in the same carbon number.
Effect of BHBAs shape and size on the formation of
hydropho-
bic DESs. Unlike traditional method of obtaining DESs, in this
†
work, BHBAs with different alkyl chains (Table S1 ESI for the
details structure) were used as extractants to separate citric
acid from fermentation broth, and hydrophobic DESs were
obtained simultaneously. The distribution ratios of citric acid
are used to evaluate the separation ability of BHBAs, and the
equilibrium constant are used to evaluate the ability to form
hydrophobic DESs. Table 1 shows the distribution ratios of
citric acid between hydrophobic DESs and fermentation broth
at the initial citric acid concentration of 12 wt% and the
different volume fraction of BHBA in isostearyl alcohol solvent,
respectively. It can be found that the distribution ratio of citric
acid decreases as the alkyl chain on the BHBA increases for
C
18
H
36
N
2
O
2
has the largest steric hindrance, and the
equilibrium constant of hydrophobic DESs formed by
and citric acid is the lowest 0.0386 in all BHBAs.
18 36 2 2
C H N O
According to Eq. (2), E is only 3.72 %. It can be seen that the
spatial structure and molecular size of alkyl amides have an
important influence on the the equilibrium constant of
hydrophobic DESs. C10H21NO achieved the highest
equilibrium constant within our limited range of experiments
to form the
acetamide containing two symmetrical branches (n-C10
H21NO>
n-C14H29NO> n-C18H37NO). When the length of the symmetrical
branch chain is kept constant, the distribution ratio of citric
acid is as follows: the distribution ratio containing acetyl
group> propionyl group> butyryl group such as n-C14
H29NO>n-
15
C H31NO>n-C16H33NO. For BHBAs with the same molecular
formula and different structures (Isomers), the higher the
degree of branching of the alkylamide, the smaller the
distribution ratio (eg: n-C18H37NO>i-C18H37NO, n-C12H25NO>i-
C H25NO, n-C16H33NO>i-C16H33NO). The same phenomenon
12
can be concluded for the different volume fraction of BHBA in
isostearyl alcohol solvent. To gain further insight into the
properties of forming the hydrophobic DESs used for the citric
acid seperation, the equilibrium constant of forming the
hydrophobic DESs were calculated according to the S(1) - S(5)
†
and Fig. S4(ESI ). The BHBAs’ molecular structure was
4
0,41
optimized, its van der Waals volume
and McGowan
3
4
volume were cal-
†
culated respectively (see the ESI Table S3 for the details data).
The variety trend using McGowan volume is consistent with
computer simulations according to Fig. S3. From Fig.1 it can be
seen that forming the equilibrium constant of hydrophobic
Fig. 1 The equilibrium constant of Hydrophobic DES based on
different alkylamide as a BHBA and citric acid as a HBD, the BHBAs
molecular structure is optimized using the Gaussian 09 software at
B3LYP/6-31G* level and calculated its van der Waals volume.
Table 1 The distribution ratios of citric acid (HBD) between hydrophobic DESs and fermentation broth
a
V
(HBA)
%
iC18H37NO
nC18H37NO
nC14H29NO
nC10H21NO
36 2
nC18H N O
2
nC15H31NO
iC12H25NO
nC12H25NO
iC16H33NO
nC16H33NO
1
8
6
4
00
0.3029
0.2285
0.1621
0.1315
0.4178
0.3085
0.2148
0.1728
1.5202
0.9326
0.4637
0.2870
3.0551
1.6767
0.8005
0.4659
0.0386
0.0307
0.0228
0.0189
0.1861
0.1435
0.1039
0.0851
0.2647
0.2011
0.1436
0.1169
0.3641
0.2715
0.1907
0.1540
0.0972
0.0763
0.0561
0.0464
0.1387
0.1080
0.0788
0.0648
0
0
0
2
| J. Name., 2012, 00, 1-3
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Green Chemistry
Paper
2
0
0.1025
0.1336
0.1713
0.2315
0.0151
0.0669
0.0957
0.1195
0.0367
0.0512
View Article Online
DOI: 10.1039/C9GC04401A
a
The volume fraction of BHBA in isostearyl alcohol solvent (V %). Extraction conditions: mass ratio of BHBAs to feed is 1:1, temperature is 303.15 K, and the contact
time is 20 min.
hydrophobic DESs, and can reach up to 3.14 under preliminary
conditions, and E is 75.34 %. Therefore, by controlling the
Table 2 Thermodynamic parameters of forming hydrophobic DESs
length of the amide alkyl chain and the arrangement of the
alkyl chain, the selectivity of the citric acid is improved, and
BHBA
ΔG
kJ mol
-2.45
ΔH
kJ mol
ΔS
J mol K
-85.75
Vmin
a
−1
−1
−1 −1
−1
kJ mol
finally C10H21NO is selected as the BHBA based on the
nC10
nC14
nC18
H
H
H
21NO
-28.48
-31.04
-35.72
-36.89
-37.56
-47.66
-60.67
-78.00
-89.57
-92.63
-245.14
-245.60
-245.81
-245.56
-243.47
-243.05
-243.34
-243.97
-243.72
-197.40
molecular size and shape. The results showed that seperation
of citric acid from fermentation broth via forming hydrophobic
DESs was greatly influenced by HBAs. The size of the hydrogen
bond acceptor should be moderate, and the hydrogen bonding
force should be moderate between the hydrogen bond donor
and the hydrogen bond acceptor.
29NO
37NO
37NO
-0.895
2.24
3.04
2.58
3.40
4.25
4.99
5.88
8.20
-99.45
-125.21
-131.70
-132.40
-168.42
-214.22
-273.77
-314.87
-332.61
iC18H
nC12H25NO
iC12
nC15
nC16
H
H
H
25NO
31NO
33NO
Thermodynamic Behavior of forming hydrophobic DESs. The
effect of temperatures was studied on the forming
hydrophobic DESs at 25, 30, 35, 40, and 45°C while keeping all
other experimental conditions constant. The change of
distribution of citric acid in hydrophobic DESs and
fermentation broth with temperature is shown in Table S4. By
keeping [B] constant in the Supporting Information S(5), the
following equation was derived:
iC16H33NO
36 2 2
nC18H N O
a
The minima of BHBAs electrostatic potential value on the vdW surface.
The plot of lnD is a function of 1/T, which yielded a straight
line as depicted in Fig. S5. According to the above derivation,
ther-
modynamic parameters of forming hydrophobic DESs is
obtain-
ned in Table 2. Using C10H21NO as a BHBA forming hydrophobic
DES, the Gibbs free energy change is -2.45 kJ/mol which is the
smallest of the hydrogen bond receptors studied. This is in
∂
푙푛 퐷
∂푙푛 퐾푒푥
=
(3)
[
1
]
[
1
]
∂
( )
∂
( )
푇
푇
푃
푃
The temperature dependency of the equilibrium constants in
the system are converted in the thermodynamic properties
consistent with C10
constant within the limited BHBAs. The Gibbs free energy
change for C18 is 8.20 kJ/mol, which can be interpreted
H21NO achieving the largest equilibrium
enthalpy and entropy using the Van 't Hoff equation:
36 2 2
H N O
휃
∂
푙푛 퐾푒푥
― 훥푟퐻푚
푅
by quantitative analysis of molecular surface electrostatic
potential via the Gaussian 09 program and VMD 1.9.3
program. The lower the surface electrostatic potential value of
=
(4)
[
1
]
∂
( )
푇
푃
From (3) and (4), Eq.(5) and (6) was arrived to obtain the the amide as BHBAs, the easier it is to form DES with citric
enthalpy change:
18 36 2 2
acid, the easier it is to react spontaneously. The C H N O
― 훥푟퐻휃푚
푅
∂
푙푛 퐷
surface electrosta-
tic potential is -197.40 kJ/mol while the value of C H NO is
=
(5)
[
1
]
∂
( )
푇
푃
10 21
-
245.14 kJ/mol, therefore, C18
H
36
N
2
O
2
is more difficult to form
21NO. The procedure is
휃
―
훥푟퐻푚 1
hydrophobic DES compared with C10
H
푙푛퐷 =
·푇 +퐶
(6)
푅
exothermic. The differences in enthalpy formation
hydrophobic DESs indicate that different hydrogen bond
acceptors interact with citric acid to form DESs with different
hydrogen bonding force strengths. The enthalpy change for
The gibbs free energy change and entropy change were calcul-
ated by the following equation:
휃
휃
휃
휃
10
C H21N is -28.48 kJ/mol, this means that C10H21NO can absorb
―
푅푇푙푛 퐾 = 훥 퐺 = 훥 퐻 ―푇훥 푆
(7)
푟
푚
푟
푚
푟 푚
citric acid from the fermentation broth to form hydrophobic
DES. This is due to the high enthalpy of formation of the
eutectic mixture and hydrogen bond formation between
carboxyl group on citric acid and carbonyl group on C10H21NO.
And the high entropy changes arising from forming a DES
indicate that these weaker Interac-
tions are favourable for citric acid seperation.
Hydrophobic DESs formation mechanism. According to the
slope method used in Fig.S4 (ESI†), coupling coefficient of
10
C H21NO and citric is 2.8 indicating that there are 3:1
structure
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Green Chemistry
C4H9
C H
9
View Article Online
CH3
H C
4
3
N
CH
O
CH
O
N
DOI: 10.1039/C9GC04401A
C4H9
4 9
C H
OH
HO
OH
O
O
O
O
C4H9
OH
C
N
3：1
CH3
C4H9
BHBA C4H9
Hydrophobic DESs
H3C
N
C
O
C4H9
K4
HO
O
K1
O
O
H3C
C4H9
C4H9
3
OH
C
N
2
·
HO
OH aqueous phase
O
O
C H
9
+
Ka1
4
CH
N
OH
HO
OH
H
+
+
H3C
C4H9
K2
O
O
O
O
O
O
O
O
OH C H9
OH
+
4
K3
C4H9
CH
N
N
C
Fig. 4 FT-IR spectra of HBA and hydrophobic DES.
HO
OH
H3C
CH3
OH
C4H9
C4H9
in the forming of hydrophobic DESs. The citric acid is partially
in
the form of free molecules and partially in the form of
carboxylic
Fig. 2 The formation mechanism of hydrophobic DESs
(b)
acid groups in the fermentation broth. Due to the electron-
withdrawing effect of the hydroxyl group, the dissociation
reaction of citric acid occurs preferentially at the carboxyl
group at the 3 position closest to the hydroxyl group. The
amide com-
(a)
bines with hydrogen ions to form oxonium. The oxonium is
unstable and combines with citrateions to form an ion associa-
tion. The 1st and 5th positions of the carboxylic acid group
farther from the hydroxyl group are mainly in the form of a
carboxylic acid molecule, and undergo a hydrogen bond
(c)
(d)
association reaction with the C10H21NO. The formation mecha-
nism of hydrophobic DESs has been described in detail in Fig. 2.
The ionic association bond and hydrogen bond are found in
the hydrophobic DESs which combined with HBAs and HBDs,
which is confirmed in the FT-IR part.In order to justify
hydrogen bond sites, we plot electrostatic potential colored
molecular van der Waals surface by the VMD 1.9.3 program.
36 2 2
In Fig. 3 (b) and (c) the nitrogen of C10H21NO or C18H N O is
(e)
the positions of the minima and is clearly HBA sites. while the
carboxyl hydrogens of citric acids stand out in Fig. 3 (a), and
these are surface local maxima of electrostatic potential as
available for hydrogen bon- ding. However, in Fig. 3 (c)
C H N O
18 36 2 2
structure exhibits large
steric hindrance, and this is in accordance with the aforementi-
Fig. 3 Electrostatic potential distribution colored on the molecular van der Waals oned minimum equilibrium constant of the hydrophobic DES
surface. (a) citric acid, (b) C10H21NO, (c) C18H N O , (d) hydrophobic DES penetration
36 2 2
map of vdW surface of monomers in the molar ratio 2: 1, (e) hydrophobic DES
penetration map of vdW surface of monomers in the molar ratio 3: 1. The red and blue
spheres correspond to the positions of maxima and minima of electrostatic potential
on the van der Waals surface, respectively.
4
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View Article Online
DOI: 10.1039/C9GC04401A
Fig. 5 Recycling the BHBA Conditions: mass ratio of BHBAs to
feed is 1:1, temperature is 303.15 K, and the contact time is 20
min.
formed by C18
atic potential colored penetration map of van der Waals
surface of monomers citric acid and C10 21NO in Fig. 3 (d) and
e). From the map, the interpenetration between the van der
36 2 2
H N O and citric acid. Finally, plotting electrost-
H
(
Waals surfaces of the C10H21NO and citric acid due to
formation of hydrogen bonds can be clearly seen, and which
reveal the electrostatic nature of the hydrogen bonds.
FT-IR spectra analysis of hydrophobic DESs. In order to further
explore formation process of hydrophobic DESs, IR spectra
were also investigated which was an attractive method to
3
5
obtain hydrogen bonding information. Fig. 4 shows the FT-IR
spectra of HBA C10H21NO, hydrophobic DESs C10H21NO-citric
acid compl-
exes. In citric acid, the electron cloud density of the carbonyl
oxygen atom moves to the carbon direction due to the
induction of electrons by the hydroxyl oxygen atom connected
to the carbonyl. The carbonyl stretching vibration frequency of
Fig. 6 Schematic flow sheet for citric acid seperation from a
fermentation borth with C10 21NO as BHBA via forming hydrophobic
-1 36
citric acid is located at 1746, 1756 cm . The conjugation
effect of the nitrogen atom in the tertiary amide weakens the
amide carbonyl double bond characteristics, and the carbonyl
H
-1 36
stretching vibration frequency of the amide is at 1639 cm .
Which also be confirmed by plotting the theoretical infrared
spectra of C10
the infrared spectra measured by hydrophobic DESs and the
HBA 21NO, the most obvious difference is the
H21NO and citric acid in Fig. S9. Comparison of
10
C H
-1
characteristic absorption peak at 1732 cm , which is the
stretching vibration
peak of the carbonyl group in the carboxylic acid.37 The DESs based on alkyl amide as a HBA with citric acid as a HBD
formation of a hydrogen bond between the C=O group of involves two mechanisms of hydrogen bonding and ion
10
C H21NO and the proton of the COOH group in the carboxylic association.
acid lead to downward shift to lower wave numbers from 1746 Regenerating and recycling the hydrogen bond acceptor.
−
1
−1
cm to 1732 cm . Similarly, the characteristic peak of The procedure is a cyclic process in which the citric acid as HBD
-1
-1
10
C H21NO moves from 1639 cm to 1622 cm , the reason is via forming hydrophobic DESs to achieve separation from
that the formation of hydrogen bonds averages the electron impurities in the fermentation broth is eventually returned to
cloud density so that the stretching vibration frequency is the aqueous phase, HBAs should be recycled to continue to
lowered and moved to a low wave number. The complex combine with citric acid in the fermentation broth to form
formation was based on the Lewis acid–base mechanism. And hydrophobic DESs. The temperature of the hydrophobic DESs
the observed significant indicated the presence of an was 75°C, which was raised to break intermolecular hydrogen
interaction of carbonyl oxygen of the amide with COOH group. bonds. BHBA was regenerated. Fig. 5 shows the equilibrium
-
1
At the same time, the absorption peak appears at 1490 cm
constant of the regenerated BHBA forming hydrophobic DES.
-1
and 1604 cm , these bands are characteristic of carboxylate As we can see recycling properties of regeneration HBAs is
stretching vibration. Once again, it is shown that hydrophobic better than that of HBAs in the cycles 1 and 3, the reason is
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Green Chemistry
that in the amide synthesis process (ESI Fig. S1) acetic acid as a
by-product combined with the HBA. As the number of uses
increases, the acetic acid is replaced and the effect is better.
Experimental
Chemicals and materials.
BHBAs had been synthesized by one pot method. The details
can be found in Fig. S1, Fig. S2 and Table S1 (ESI ). The fermen-
tation broth was kindly provided by our cooperation company
COFCO Biotechnology Co., Ltd. Anhui, China. Acetic anhydride (
View Article Online
DOI: 10.1039/C9GC04401A
3
9
The regenerated C10H21NO exhibited good recycling properties
†
with no obvious reduction of the ability of forming
hydrophobic DESs over five cycles, compared with imprinted
3
2
polymerization self-assembly system and activated carbon
≥
99.0%), propionic anhydride (≥99.0%), n-butyric anhydride
3
8
adsorption which reproducibi-
(
9
9
≥ 99.0%), di-n-butylamine ( ≥ 99.0%), dihexylamine ( ≥
8.0%), dioctylamine ( ≥ 98.0%), bis(2-ethylhexyl) amine ( ≥
9.0%), oxalyl chloride ( ≥ 98.0%) were purchased from
lity and stability decreased 9.7% after four cycles due to the
slight swelling of the adsorbent. This indicates that the
developed system is efficient and sustainable for the
separation
of citric acids. The process flow diagram for citric acid seperati-
on from fermentation broth is presented in Fig. 6. The straight
lines represent the streamline of the control flow, explaining
the substances involved in the entire cycle process and using
arrows to indicate the process of its next participation. The
rectangles represent processing functions, performing four
specific operations obtain high-quality citric acid. The product
purity analysis are shown in Fig. S6-S8. The HPLC analysis
indica-
ted that the separated high-purity citric acid is 99.8%, which is
higher than the purity of the product obtained by the
traditional extraction agent TOA. Prior to this, our team has
successfully applied the craft containing the centrifugal
contactor separator device for separation of lactic acids with
annual output of 100,000 tons. It was found that the
continuous dynamic separation process present low energy
consumption, ease of scale-up showed advantages in terms of
cost effectiveness and simplicity.
Sinopharm Chemical Reagent Co. Ltd., China. All chemicals
were used as received without further purification. Double-
distilled water was obtained using two distillation purification
apparatus (Yarong Biochemical Instrument Co., Ltd., Shanghai).
BHBAs molecular structure is optimized using the Gaussian 09
software at B3LYP/ 6-31G* level and calculate its van der
Waals volume.
4
0,41
To obtained the electrostatic potential distribution, the
4
2
wave function analyses were carried out using Multiwfn 3.6.
And the color mapped isosurface graphs of the electrostatic
potential were rendered by the VMD 1.9.3 program.4
3
Procedures and analytical methods for forming
hydrophobic DESs experiments
Gas chromatography-mass spectrometry (GC-MS) analysis.
BHBAs contents were measured with Agilent 7890GC/5975MS
using a DB-FFAP column. The column has temperature limits
between 40 °C and 250 °C. The inlet temperature and the
detector temperature were 250 °C and 240 °C. Helium
(
99.999%) was used as the carrier gas with a flow rate of 2
Conclusions
4
4,45
mL/min.
The BHBAs was dissolved with ethanol and filtered
The seperation of citric acid from fermentation broth with 10 through a 0.45µm hydrophobic polyethersulfone membrane.
kinds of amides synthesized as HBAs was studied. The The GC oven temperature program is as following: start at 30
equilibrium constant of forming hydrophobic DESs are °C, hold for 10 min; raise from 30°C to 220°C at a heating rate
significantly affected by the structure of alkyl amides. of 5°C/min, hold for 10 min. Mass spectral library was provided
Increased alkyl amide carbon chain or increased alkyl by NIST (China National Institute of Standards and
branching will reduce the equilibrium constant of forming Technology).
hydrophobic DESs. The analysed result shows that the HPLC conditions. The citric acid concentration in the aqueous
equilibrium constant of combination with citric acid is affected solutions was determined with high performance liquid
by the spatial structure and molecular size of alkyl amide. chromatography (HPLC) system Agilent 1200 series. The
Comparison with other HBAs shows that C10H21NO has column used was an Agilent ZORBAX SB-Aq column (250 mm ×
superior properties for citric acid as the HBD to forming 4.6 mm, 5 μm) and a UV spectrophotometer (Two-channel UV
hydrophobic DESs and has stable recycling properties. FTIR and detector). The detection wavelength was 210 nm. The mobile
thermodynamic studies provides insight into modes of forming phase was an aqueous mixture containing 5 % of methanol
hydrophobic DESs and further clarifications emanating from and 3.90 g/L of NaH PO with pH adjusted to 2.50 using
2
4
theoretical calculations by quantitative analysis of molecular phosphoric acid. The flow rate was 0.6 mL/min and the column
surface electrostatic potential. The simple and effective temperature was 30.0± 0.3 °C.44,45 The liquid sample was
recycling of the HBA confirms the method in a green and prepared and filtered through a 0.45 μm hydrophilic
sustainable manner. The seperation processes presented in polyethersulfone membrane. Each of the experiments was
this paper serve as model systems for the recovery of the main repeated 3 times, and the relative deviation did not exceed
hydroxycarboxylic acid from fermentation broth. It is in line 0.5%. The content of citric acids was calculated by means of a
with the "green, environmentally friendly, energy-saving" calibration curve established with the regression equation y
production concept advocated by the world today.
=0.03548 x +0.00463 (R
2
= 0.9998, Fig. S2, ESI ). Y is the citric
†
6
| J. Name., 2012, 00, 1-3
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Green Chemistry
Paper
acid concentration and x is the peak height. The standard
solutions of citric acid were diluted to 0.1-1.0 g/L.
Fourier Transform Infrared Spectroscopy (FT-IR)
Acknowledgements
The authors gratefully acknowledge the Department of Science
and Technology of Shandong Province China (2016ZDJS04B06)
for financial support of this project.
View Article Online
DOI: 10.1039/C9GC04401A
FT-IR characterization was performed on a Fourier transform
infrared spectrophotometry (Nicolet 380, Thermo Electron cor-
poration, USA) with the scanning wavenumber ranged from
-
1
-1
4
000 to 500 cm at a data acquisition rate of 2 cm per point References
at 25 °C. FTIR analysis of C10 21NO, hydrophobic DESs (the
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BHBAs biodegradable hydrogen bond acceptors
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View Article Online
DOI: 10.1039/C9GC04401A
The separation of citric acids was realized by utilizing hydrophobic deep eutectic
solvents (DESs), which was formed by N, N-dibutylacetamide as hydrogen acceptor
and citric acid as hydrogen bond.