Synthesis and characterization of a new acid molten salt and the study of its thermal behavior and catalytic activity in Fischer esterification

Article abstract of DOI:10.1039/d0nj06273a

A new acid molten salt was prepared and its structure elucidation was conducted by FTIR, 1D NMR, 2D NMR, and mass spectrometry. Further support to elucidate the chemical structure of the 1H,4H-piperazine-N,N′-diium ring of the new acid molten salt was achieved by¹H and¹³C NMR, and COSY analyses of 1H,4H-piperazine-N,N′-diium dibromide, which is synthesized and characterized for the first time in the current work. The analysis of FTIR and NMR spectra as well as pH and titrimetric analysis excluded the formation of [SO₄]^2?and the presence of an excess of H₂SO₄. Moreover, no distinguishing peak was detected for the acid proton of [HSO₄]^?in DMSO-d₆. The thermal phase transition and thermal stability of the acid molten salt were also recorded, which approved the strong interaction between a dication and hydrogen sulfate anions. According to the acidity of the new molten salt, we encourage the study of its catalytic activity for the acetylation ofn-pentanol using glacial acetic acid. Pentyl acetate was obtained in 89.0% conversion and 78.0% isolated yield. The¹H NMR spectrum of the residue showed an excess of HOAc together with molten salt, whereas the¹H NMR spectrum of the upper phase exhibited pure pentyl acetate. After separation of the upper phase, the residue was concentrated and used in the next run without further purification. No significant changes in the chemical structure and catalytic activity of the new molten salt were observed even after the 5th run. Two chiral alcohols, including (?)-menthol and (+)-borneol, as well as α-tocopherol (α-TCP) were also acetylated with acetic acid in the presence of the new acid molten salt under optimized reaction conditions, which afforded the desired acetates in high yields.

Full text of DOI:10.1039/d0nj06273a

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Synthesis and characterization of a new acid
molten salt and the study of its thermal behavior
Cite this: New J. Chem., 2021,
4
5, 7081
and catalytic activity in Fischer esterification†
a
a
a
Lia Zaharani, Nader Ghaffari Khaligh, * Mohd Rafie Johan and
b
Hayedeh Gorjian
A new acid molten salt was prepared and its structure elucidation was conducted by FTIR, 1D NMR, 2D
NMR, and mass spectrometry. Further support to elucidate the chemical structure of the 1H,4H-
0
1
13
piperazine-N,N -diium ring of the new acid molten salt was achieved by H and C NMR, and COSY
0
analyses of 1H,4H-piperazine-N,N -diium dibromide, which is synthesized and characterized for the first
time in the current work. The analysis of FTIR and NMR spectra as well as pH and titrimetric analysis
2
ꢀ
excluded the formation of [SO
4
]
2 4
and the presence of an excess of H SO . Moreover, no distinguishing
ꢀ
peak was detected for the acid proton of [HSO
4
]
6
in DMSO-d . The thermal phase transition and
thermal stability of the acid molten salt were also recorded, which approved the strong interaction
between a dication and hydrogen sulfate anions. According to the acidity of the new molten salt, we
encourage the study of its catalytic activity for the acetylation of n-pentanol using glacial acetic acid.
1
Pentyl acetate was obtained in 89.0% conversion and 78.0% isolated yield. The H NMR spectrum of the
1
residue showed an excess of HOAc together with molten salt, whereas the H NMR spectrum of the
upper phase exhibited pure pentyl acetate. After separation of the upper phase, the residue was
concentrated and used in the next run without further purification. No significant changes in the
chemical structure and catalytic activity of the new molten salt were observed even after the 5th run.
Two chiral alcohols, including (ꢀ)-menthol and (+)-borneol, as well as a-tocopherol (a-TCP) were also
acetylated with acetic acid in the presence of the new acid molten salt under optimized reaction
conditions, which afforded the desired acetates in high yields.
Received 27th December 2020,
Accepted 16th March 2021
DOI: 10.1039/d0nj06273a
rsc.li/njc
can be indistinguishable due to many parameters, for example,
the poor solubility of the sample or the presence of water in the
Introduction
The structure elucidation of new compounds is the most deuterated solvent, the rapid rate of proton exchange, the use of
1
important part of chemistry and spectroscopy analyses are the protic or aprotic deuterated solvent, H-bonding, etc. Regarding
ꢀ
2ꢀ
4 d
or [SO ] , C3v and T ,
first tool used for the characterization of chemical structures the different symmetry of [HSO
4
]
including IR, Raman, NMR, Mass, UV-Vis spectroscopy, etc. respectively, the characteristic vibrations have been displayed
2
–4
Hydrogen bonding (H-bonding) has been extensively studied at different frequencies of IR spectra.
using IR and NMR spectroscopy. The treatment of organic and Ionic liquids (ILs)/molten salts are well-known as liquids or
inorganic bases by sulfuric acid (SA) can give acid or neutral salts solids containing cationic and anionic organic components.
ꢀ
2ꢀ
containing [HSO
4
] or [SO
4
]
based on various parameters such The physical and chemical properties of ILs/molten salts
1
as the mole ratio and basicity of base. H NMR spectroscopy depend on the nature, size, and charge of cations and anions,
cannot be a key technique for differentiating species [HSO
ꢀ
4
]
or therefore, they are tunable chemicals and can be modulated to
2ꢀ
ꢀ
4
[SO ]
, because the chemical shift of the acid proton of [HSO
4
]
acquire the appropriate properties. Although they were initially
used as eco-friendly solvents in chemical reactions, ILs/molten
salts are now employed in many fields including catalysis,
a
Nanotechnology and Catalysis Research Centre, Institute for Advanced Studies
IAS), University of Malaya, 50603 Kuala Lumpur, Malaysia.
5
(
electrochemistry, spectroscopy, and materials science.
E-mail: ngkhaligh@gmail.com, ngkhaligh@um.edu.my
Department of Food Science and Technology, Sari Agricultural Sciences and
Natural Resources University, Sari, Iran
Pentyl acetate is a well-known ester in the flavoring industry
b
6
due to its scent similar to those of bananas and apples. It is
1
13
also used in the coating industry for lacquers or paints, and as
fuel in the Hefner lamp for photometry. Furthermore, it is a
†
Electronic supplementary information (ESI) available: FTIR, H NMR, C NMR,
DSC, and TGA/DTA copies. See DOI: 10.1039/d0nj06273a
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0
0
useful extractant in the preparation of penicillin, and a good demonstrates a lack of a well-defined AA BB pattern (see the
7
solvent for synthetic and natural resins. Carboxylic acid esters ESI,† Fig. S2).
can be synthesized by using different catalysts in various
Two different products can be formed when two equivalents
8
reaction media. Fischer esterification is the conventional of SA (98%) are added to PZ. The possible products contain the
9
0
1H,4H-piperazine-N,N -diium dication together with different
and most common process for the esterification reaction.
Although numerous methods have been presented for the counterions, [HSO
ꢀ
2ꢀ
4
] , as shown in Scheme 1.
4
]
or [SO
1
0–18
esterification reaction,
the development of practical and
After stirring the reaction mixture in an aprotic solvent
greener methods for the preparation of carboxylic acid esters overnight, a white solid was isolated in high yield. The structure
using carboxylic acids instead of acid anhydride is still one of elucidation of the product was conducted by different spectro-
the challenging research topics. Therefore, developing an scopy analyses. In order to determine the acidity, complementary
efficient and greener methodology to overcome some draw- analyses including the triplet determination of pH and acid–base
backs and achieve a suitable balance between the economic titration were also performed. Finally, the thermal phase
and ecological aspects of the process is required.
transition and thermal stability of the white solid were recorded
In continuation of our interest in the synthesis of new ionic by DSC and TGA/DTA analyses, respectively, which demonstrated
liquids/molten salts and investigation of their catalytic the strong electrostatic interaction and H-bond formation
1
9–22
ꢀ
activity,
reaction of piperazine (PZ) and SA in CH
structure was demonstrated by FTIR, 1D NMR, 2D NMR, and single CH
a new acid molten salt was fabricated via the between protonated PZ and [HSO
4
] .
The H NMR consists of a single {NH
at 3.33 ppm in DMSO-d . The chemical shift
1
+
2
Cl
2
. Its chemical
2
peak at 8.96 and a
2
+
6
1
13
mass analysis. A comparison of the H and C NMR spectra of of {NH2 implied the presence of weak intramolecular
two new ionic liquids i.e., 1H,4H-piperazine-N,N -diium hydrogen H-bonding in DMSO-d (see the ESI,† Fig. S3). The sharp singlet
sulfate {(PZ–SA (1 : 2)} and 1H,4H-piperazine-N,N -diium dibro- at 3.33 ppm is at a higher chemical shift than PZ (Dd H =
2 2
mide ([PZH ] -2Br ), proves the chemical structure of protonated 0.72 ppm), probably due to the deshielding effect of {NH
PZ as a six-member ring bearing two {NH
proved that only complementary spectral data and physical protons (see the ESI,† Fig. S3). The C NMR showed a single
properties need to be considered for the structure elucidation of peak at 40.63 ppm, which is lower than that of PZ (Dd C =
the organic salts containing sulfate or hydrogen sulfate anions. 6.29 ppm). This demonstrated the protonation of PZ as
The thermal phase transition and thermal stability of a new acid reported in the literature (see the ESI,† Fig. S4).
0
6
0
1
2+
ꢀ
+
+
2
groups. Our results moieties and a decrease of the electron density around the
1
3
1
3
2
3
1
13
molten salt were also recorded and described. Finally, the
The H and C NMR spectrum of the white solid was
esterification of n-pentanol, two chiral alcohols, including recorded in D O (see the ESI,† Fig. S5 and S6). The disappearance
2
+
(
ꢀ)-menthol and (+)-borneol, as well as a-tocopherol (a-TCP) with of the peak at 8.96 ppm demonstrated that the {NH
glacial acetic acid, was studied using the new molten salt as a are exchangeable and labile in D O.
The H, H-COSY spectrum exhibited the correlation
demonstrated the catalytic efficiency and high recyclability of between peaks at 3.33 and 8.96 ppm. This fact supported the
2
protons
2
1
1
Br o¨ nsted acid catalyst under solvent-free conditions, which
the new molten salt.
formation of protonated PZ ring (see the ESI,† Fig. S7).
0
Furthermore,
1H,4H-piperazine-N,N -diium
dibromide
2
+
ꢀ
(
[PZH ] 2Br ) was synthesized and characterized for further
2
support of the chemical structure of the new acid molten salt.
The PZ was dissolved in CH Cl and two equivalents of HBr
ACS reagent, Sigma Aldrich 48 wt% in water) were added
Results and discussion
Structure elucidation of PZ–SA (1 : 2)
2
2
(
Due to rapid interconversion of the PZ ring at room temperature, dropwise into the solution in an ice bath (Scheme 2).
1
the H NMR spectrum of PZ showed only a singlet at 2.59 ppm
After overnight stirring at room temperature, the solvent was
for the axial and equatorial protons of methylene groups. Also, removed by a rotary evaporator. The white waxy product was
an extensive broadening of the N–H peak, centered at 2.04 ppm, isolated during washing with CH Cl . The color changed to
2
2
was observed probably owing to H-bonding N–Hꢁ ꢁ ꢁN and rapid brown for a few hours. The brown waxy product was washed
PZ ring/nitrogen inversion (see the ESI,† Fig. S1). The formation several times with CH Cl , which gave a pale brown waxy product
2
2
1
13
1
1
of H-bonding between N–H groups of PZ and DMSO-d
6
, a good (see the ESI,† Fig. S8). The H, C NMR, and H, H-COSY spectra
] 2Br exhibited a similar NMR pattern to those of the
peak was detected at 46.92 ppm in the C NMR spectrum which new molten salt (see the ESI,† Fig. S9–S11) (Table 1).
2
+
ꢀ
H-bond acceptor, resulted in the high field of N–H peak. One of [PZH
2
1
3
Scheme 1 The procedure, conditions, and two possible chemical structures of the reaction two equivalents of SA with PZ.
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Scheme 2 Reaction conditions for the synthesis of 1H,4H-piperazine-N,N -diium dibromide.
1
13
ꢀ1
Table 1 The collection of H and C NMR data of piperazine (PZ), new vibrations were displayed at 663 and 602 cm . The peaks at
2+
ꢀ
ꢀ1
molten salt, and [PZH
2
]
2Br (600 MHz, DMSO-d
6
)
1
324 and 1270 cm were attributed to the C–N stretching
0
vibration, and the asymmetric and symmetric C–C stretching
CH-A, CH-A ,
ꢀ
1 25
0
0
CH-B and CH-B
{N–H
C-A, C-A , C-B, vibrations of PZ were observed at 1128 and 1088 m
.
Two
ꢀ
1
(
ax and eq)
(ax and eq)
and C-B’
(d C ppm)
peaks at 3444 and 3390 cm
stretching vibration of protonated piperazine ring. The broad
were assigned to the N–H
1
1
13
Compound
(d H ppm)
(d H ppm)
26
ꢀ1
PZ
2.59
3.33
3.36
2.04
8.96
9.25
46.92
40.36
40.25
and medium intensity bands at 2481 and 2622 and 2790 cm
are due to strong intermolecular hydrogen bonding of –NH and
New molten salt
2+
ꢀ
[
2
PZH ] 2Br
2
7
–
SOH groups in the new molten salt. The asymmetric and
symmetric stretches of methylene groups were observed at
ꢀ
1
2
997 and 2981 cm . Two medium intensity peaks at 1614
ꢀ
1
and 1455 cm were assigned to the deformation vibration modes
of {NH groups. A strong band at 995 cm with a shoulder at
2
68 cm is assigned to the SO
high-intensity bands at 1132 and 1101 cm are assigned to the
SO asymmetric stretching mode. The sharp band at 870 cm
and the strong bands at 577 and 525 cm are attributed to the
deformation and bending modes of [HSO ] .
In the presence of bromide anion, the methylene peak does
+
28
ꢀ1
1
+
not shift (Dd H = 0.03 ppm), but the {NH
8
2
protons shift from
ꢀ1
1
9
2
symmetric stretch vibrations. Two
.96 ppm to 9.25 ppm (Dd H = 0.29 ppm). This fact is probably
ꢀ
1
due to stronger interactions between the dication and two
bromide anions.
ꢀ
1
2
ꢀ
1
Further study was performed through recording and analysis
ꢀ
26,29,30
1
of H NMR of neat sulfuric acid (98%) and a sample of the new
4
It is worth mentioning that the neat FTIR spectrum of
molten-salt displays strong hydrogen bonds, while these hydrogen
bonds are weakened due to the formation of hydrogen bonds with
molecules DMSO-d as a well-known H-bonding acceptor.
6
The LC-ESI-MS analysis of the new molten salt was performed
at positive and negative modes. In positive ion mode, the m/z of
molten salt containing 1.0 mmol excess of SA (98%), which
showed a sharp peak at 7.13 ppm and a multiplet peak in the
range of 9.33–9.32 ppm, respectively (see the ESI,† Fig. S12 and
S13). Based on the lack of sharp or broadened peaks in the
1
above-mentioned range of the H NMR spectrum of new molten
2 4
salt, the presence of H SO was expelled.
1
63.1083, 607.3755, 585.3944, and 563.3252 for cation species of
An aqueous solution of the new molten salt (50 mg into
+
[(PZ + CH
3
OH + CO
Na] , [(4PZ + 3CH OH + 2CO
) + H] were observed, respectively; as well as a dication
2
) + H] , [(4PZ + 3CH
3
+
OH + 2CO
2
+ 2CO) +
2
5 mL deionized water) displayed a pH of 2.03 ꢂ 0.01, whereas
+
3
2
+ 2CO) + H] , and [(3PZ + 4CH OH
3
a pH of 1.69 ꢂ 0.01 was detected for the same concentration of
SA (98%). Also, the acid–base titration was performed using
NaOH as a titrant (see the ESI,† Fig. S14). The titration result
exhibited that the new acid molten salt is diprotic acid in the
aqueous solution (50 mg in 25 mL) and the second acid
dissociation is much weaker than the first one. Regarding the
+
+
4CO
2
2
+
species of [(4PZ + 4CH
negative mode, the anion species of [PZ + HSO
HSO
3
OH + 4CO
2
) + 2H] at m/z of 650.4174. In
ꢀ
4
]
and [PZ +
ꢀ
4
3
+ CH OH] were detected at m/z 183.0436 and 215.0698.
The LC-MS results showed the presence of solvent and back-
+
ꢃ
+
ꢃ
2
ꢀ
ground gases, such as m/z 32 = [CH OH] , m/z 28 = [CO] , and
above results, the formation of [SO
4
]
was also ruled out.
3
+
ꢃ
m/z 44 = [CO
2
]
which exist often inside the mass spectrometer
The absence of a sharp or broaden characterized peak for
ꢀ
1
together with the anion and cation peaks.
the acid proton of [HSO4] in the H NMR spectrum of the new
acid molten salt could be due to various reasons, including
Regarding the above complementary results, the structure (I)
was approved for the new acid molten salt and it is being
referred to as ‘‘PZ–SA (1 : 2)’’.
H-bonding between DMSO-d
6
and the –NH and –SOH groups in
the new acid molten salt or rapid proton exchange between
ꢀ
4 6
] and residue water in DMSO-d .
24
Ionic salts are named based on their melting points 4100 1C
[HSO
3
1
and o100 1C as molten salts or ionic liquids, respectively, and
the melting point of PZ–SA (1: 2) was in the range of 180–181 1C.
Also, the PZ–SA (1: 2) water content at relative humidity and the
room temperature was 2.10 ꢂ 0.05 wt%.
The FTIR spectra of PZ and the new molten salt are depicted
in Fig. 1. The new molten salt displayed a completely different
IR spectrum compared with PZ. The PZ has a sharp strong peak
ꢀ
1
at 3272 cm
due to the N–H stretching vibration of two
secondary amine functional groups, and this peak disappeared
in the spectra of the new molten salt. The C–H stretching
Thermogravimetric analysis of PZ-SA (1 : 2)
vibrations are assigned to peaks at 2958, 2953, 2834, and Fig. 2 shows TGA/DTA curves recorded for PZ–SA (1 : 2) in a
ꢀ
1
ꢀ1
ꢀ1
2
752 cm . The peaks at 1459 and 1417 cm were assigned flow of nitrogen (20 mL min ) at a constant heating rate of
ꢀ1
to CNH deformation vibration, and the CCN deformation 10 1C min in the temperature range of 25–800 1C. A small
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Fig. 1 FTIR spectra of piperazine (PZ) and the new acid molten salt.
mass loss (ca. 4.0 wt%) was observed below 150 1C which can takes place within the temperature range of 290–360 1C in
indicate the amount of the physically adsorbed water and two steps (ca. 80 wt%). Finally, the decomposition is completed
solvent in PZ–SA (1 : 2). There is no significant mass loss till at 720 1C.
ca. 296 1C. Above this temperature, PZ–SA (1 : 2) begins to
2
It is indicated that the molecules of SO3(g) and H O(g) are
decompose. The main stage of the thermal decomposition released in the first stage of the thermal decomposition, which
2
Fig. 2 TGA/DTA of PZ–SA (1 : 2) under a N atmosphere.
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occurs within the temperature range 296–310 1C, and 1H,4H- highest yield was observed in 100 mg of PZ–SA (1 : 2) (Table 2,
0
piperazine-N,N -diium sulfate is formed. This compound starts entry 4). Also, the most effective mole ratio of n-pentanol to HOAc
to lose the next sulfuric acid molecule as SO3(g) and H O up to was 1.5 at 60 1C within 4 h (Table 2, entry 8).
2
(g)
3
60 1C. Above 360 1C a moderate decomposition of created
Although a significant improvement was obtained in the
conversion when the model reaction was carried out at higher
temperatures up to 80 1C; no improvement was observed at
higher temperatures (Table 2, entries 12 and 13). Due to the
carbon nitride takes place.
DSC analysis of PZ–SA (1 : 2)
In order to study the thermal phase transition of PZ–SA (1 : 2), reversibility of the esterification process, increasing or decreasing
differential scanning calorimetry (DSC) was obtained in a flow of reaction time caused a lower conversion (Table 2, entries 14–16).
ꢀ1
ꢀ1
nitrogen (20 mL min ) at a constant heating rate of 10 1C min
Based on the results in Table 2, entry 12 was indicated as the best
under a nitrogen atmosphere. The measurements were recorded experimental condition.
in three sequential cycles with different initial and final heating
Moreover, enantioselectivity of the current protocol was
and cooling temperatures (Fig. 3). The first cycle of heating/ studied through the acetylation of (ꢀ)-menthol ([a] = ꢀ50 ꢂ
D
cooling, in the range of 25–150 1C, has not shown any significant 11, c = 10 in ethanol, Z99% e.e.) and (+)-borneol ([a]
D
= +37 ꢂ
changes in the heat flux curve. One distinct anomaly was 21, c = 5 in ethanol, 99% e.e.). The (ꢀ)-menthyl acetate ([a]
displayed on the second cycle of heating/cooling, at a range of ꢀ801, in benzene, 97% e.e.) and (+)-bornyl acetate ([a] = +411,
5–200 1C, which can be due to the degassing and dehydration. neat, 96% e.e.) were obtained in 68 and 71% isolated yield,
The sharp exothermic peak was observed at 131.7 1C with the respectively, with retention of the configuration at the chiral
D
=
D
2
ꢀ1
estimated enthalpy change ꢀ60.9 J g . Due to the lack of mass center (the optical rotation was recorded and reported based on
loss in TGA around this temperature, this enthalpy was indicated the samples from Sigma-Aldrich) (Scheme 3). The esterification
as the crystallization point of PZ–SA (1: 2). Also, the endothermic of a-tocopherol (a-TCP) was also carried out to investigate the
peak in the range of 174–182 1C was indicated as the melting potential of the current protocol for scale-up in industrial
point of PZ–SA (1 : 2) with the estimated enthalpy change of applications. Owing to the light and air sensitivity of tocopherols
ꢀ
1
+
57.4 J g . Some weak endothermic peaks were observed that (TCPs) and the reduction of their antioxidant activity, they are
32,33
can be attributed to the degassing process and dehydration.
often stabilized by the esterification reaction.
of the a-TCP (8.61 g, B20 mmol) and glacial acetic acid (1.71 mL,
0 mmol) was conducted in the presence of PZ–SA (1 : 2) (0.4 g,
.4 mmol, 7.0 mol%) and was conducted at 80 1C. After four hours
The esterification
Using the PZ–SA (1 : 2) as an acid catalyst for the esterification
of n-pentanol with glacial acetic acid
3
1
The esterification of n-pentanol and acetic acid (HOAc) was of incubation in the dark under the nitrogen atmosphere, the
carried out at different conditions and four parameters, including O-acetyl-a-tocopherol (Ac-TCP) was detected in 89.2% yield by
catalyst type and loading, the mole ratio of n-pentanol to HOAc, LC-MS (8.18 g).
temperature, and reaction time was varied to find the optimal
The reusability of PZ–SA (1: 2) was studied for the preparation
conditions. As the results showed (Table 2), the esterification of n-pentyl acetate under optimized reaction conditions. The
conversion increased in the presence of the catalyst, and the PZ–SA (1: 2), water, and unreacted HOAc form a combined
2
Fig. 3 DSC profile of PZ–SA (1 : 2) under a N atmosphere.
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Table 2 Optimization of the esterification of n-pentanol and HOAc
a
Entry
Catalyst
Catalyst loading (mol%)
A mole ratio of HOAc to n-pentanol
Temp. (1C)
Reaction time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
—
—
1.0 : 1.0
1.0 : 1.0
1.0 : 1.0
1.0 : 1.0
1.0 : 1.0
1.2 : 1.0
1.4 : 1.0
1.5 : 1.0
1.8 : 1.0
1.5 : 1.0
1.5 : 1.0
1.5 : 1.0
1.5 : 1.0
1.5 : 1.0
1.5 : 1.0
1.5 : 1.0
r.t.
r.t.
60
60
60
60
60
60
60
70
75
80
90
80
80
80
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
2.0
6.0
8.0
0
8
PZ–SA (1 : 1)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
PZ–SA (1 : 2)
3.5
3.5
7.0
10.5
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
34
42
42
52
58
68
68
72
81
89
89
54
81
75
0
1
2
3
4
5
6
b
a
b
GC-MS conversion. Optimized reaction conditions.
Scheme 3 Acetylation of various alcohols using PZ–SA (1 : 2) under optimal reaction conditions.
3
bottom phase, while n-pentyl acetate forms the upper phase. The carried out using ionic liquids of PZ–HBr (1: 2), TMDP-ClSO H
2
1
35
PZ–SA (1 : 2) is not soluble in n-pentyl acetate, and the product can (1: 2), PZ–ClSO H(1 : 2),34 Im-1,4-butane sultone (1: 1), and
3
3
6
be separated from the bottom phase by simple decantation. More- Im-1,3-propane sultone (1: 1) that afford n-pentyl acetate in 24,
over, the formation of two phases facilitates the equilibrium to shift 61, 59, 69, and 71% yields, respectively (monitored by GC-MS),
to the right toward a higher concentration of the product, which in while PZ–SA (1: 2) gave 89% yield under similar conditions. It is
turn improves the conversion in the esterification reaction. The worth mentioning that 1-bromopentane and acetyl bromide were
desired product was detected in 89–82% yield during five runs.
also detected by GC-MS when PZ–HBr (1: 2) was employed as a
In order to verify any advantage of using the new molten salt, a catalyst under optimized reaction conditions.
comparison of the catalytic activity of PZ–SA (1 : 2) and a few ionic
Also, the catalytic efficiency of the previous reported ionic
liquids was performed for the model reaction. All mentioned ionic liquids bearing hydrogen sulfate anion, including pyridinium
+
ꢀ
liquids were prepared according to previous reports in the literature. hydrogen sulfate ([HPy] [HSO ] ), N-methyl imidazolium hydrogen
Then, the conversion of n-pentanol to n-pentyl acetate was sulfate ([HMIm] [HSO
4
+
ꢀ
4
] ), triethylammonium hydrogen sulfate
Table 3 Compare the catalytic efficiency of PZ–SA (1 : 2) and some previously reported ionic liquids
b
Entry
Catalyst/amount
Temp. (1C)
Reaction time (h)
Conversion (%)
Ref.
a
+
ꢀ
1
[HPy] [HSO
4
] /25 wt% (based on the mass of HOAc)
85
2
2
2
2
1.5
1.5
4
71
55
50
39.5
46
52.3
89
37
38
38
38
39
39
b
+
ꢀ
2
[HMIm] [HSO
NH] [HSO
[N2224] [HSO
[HMIm] [HSO
[Py-(CH
PZ–SA (1 : 2)/22.2 wt% (based on the mass of HOAc)
4
] /25 wt% (based on the mass of HOAc)
85 (reflux)
85 (reflux)
85 (reflux)
80
80
80
b
+
ꢀ
3
[Et
3
4
] /25 wt% (based on the mass of HOAc)
b
+
ꢀ
4
4
] /25 wt% (based on the mass of HOAc)
c
+
ꢀ
5
4
] /25 wt% (based on the mass of HOAc)
c
2+
ꢀ
6
2
)
6
-Py] 2[HSO
4
] /25 wt% (based on the mass of HOAc)
8
This work
a
b
c
Reaction conditions: acetic acid (0.20 mol), n-pentanol (0.20 mol). Reaction conditions: acetic acid (0.02 mol), n-pentanol (0.02 mol). Reaction
conditions: acetic acid (0.25 mol), n-pentanol (0.25 mol).
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a
Table 4 Corrosive study of stainless steel by PZ–SA (1 : 2) and SA (98%)
PZ–SA (1 : 2) and unreacted HOAc, was isolated by simple
decantation or separating funnel (scale-up). To determine the
conversion in the optimization step, the aliquot of the upper
Weight of stainless steel (g)
The duration time The solution of PZ–SA The solution of SA
phase in n-hexane was analyzed using a GC-MS instrument. The
Entry (h)
(1 : 2)
(98%)
1
purity of isolated ethyl pentanoate was analyzed using
H
1
2
3
4
—
2
8
3.9942
3.4942
3.3638
3.2926
3.9917
2.8012
1.3425
—
13
and C NMR spectra (see the ESI,† Fig. S15 and S16).
The reusability of PZ–SA (1 : 2) was investigated in the same
procedure. After the concentration and removal of water, the
residue was directly used in the next run.
24
a
Two stainless steel sheets were polished, cleaned, and dried to
constant weight, then placed into the solution of PZ–SA (1 : 2) and SA,
respectively. After a mentioned time, two stainless steel sheets were
pulled out carefully, washed in water, and dried and weighed.
Conclusions
In conclusion, a new acid molten salt was synthesized and its
chemical structure was characterized by FTIR, 1D NMR, and 2D
NMR, mass spectra analyses. Moreover, 1H,4H-piperazine-N,N -
+
ꢀ
4
(
(
[Et NH] [HSO ] ),
butyltriethylammonium
hydrogensulfate
3
ꢀ
0
[N2224] + [HSO ] ), and 1,1 -hexamethylene bis(pyridinium)
hydrogen sulfate ([Py-(CH ) -Py] 2[HSO ] )
0
4
2
+
ꢀ 36–38
are compared
2
6
4
diium bromide was synthesized and characterized to support
the chemical structure of new acid molten salt. The pH and
titrimetric results showed that the PZ–SA (1 : 2) is a diprotic acid
salt. The thermal phase transition and thermal stability of new
acid molten salt were also recorded which revealed high
thermal stability of PZ–SA (1 : 2) as was expected due to the
strong coulombic and electrostatic attractions along with
with PZ–SA (1 : 2) in Table 3. The results revealed that the nature
and number of the cations and anions play a critical role to
improve the conversion as previously reported. A key feature of
PZ–SA (1 : 2) is that it contains two hydrogen sulfate anions which
lead to a stronger acidity.
The corrosive property of catalysts is a crucial factor for
acid catalytic experiments, and the reduction or elimination of
corrosion plays a vital role in industrial processes. Therefore, a
corrosive test was conducted with PZ–SA (1 : 2) and SA (98%) for a
16L austenitic stainless steel with 50, 20, and 0.5 mm in length,
width, and thickness, respectively. The same amount of PZ–SA
1 : 2) and SA (98%) (100 mg) was added into 1 mL of deionized
water, and the stainless steel plates were put into the solution. As
one can see in Table 4, PZ–SA (1 :2) showed a much weaker
corrosion property compared with SA (98%), and the surface of
stainless steel was intact even after 24 h. It seems that the
formation of H-bonding between PZ and SA can lead to less
corrosive acid salt.
2+
ꢀ
H-bonding between the [PZH
2
]
dication and [HSO ] . Despite
4
the pH of ca. 2.0, the corrosion property of PZ–SA (1 : 2) was
much less than the same amount of sulfuric acid (98%), thus,
PZ–SA (1 : 2) can be a safe and greener alternative for H SO .
3
2
4
The PZ–SA (1 : 2) was employed to promote the esterification
(
process of several alcohols such as n-pentanol, (ꢀ)-menthol,
(+)-borneol, and a-tocopherol with glacial acetic acid. Some
advantages can be mentioned for the acetylation of alcohols
using the new acid molten salt; for example, practical and
simple procedure, good yield of the desired products, easy
workup, high reusability of catalyst, avoiding catalysts containing
heavy-metal, and minimizing hazardous waste generation. Our
group is investigating other organic transformations in the
presence of PZ–SA (1 : 2) with promising results.
Experimental
The synthesis of PZ–SA (1 : 2)
s
Piperazine (PZ) (ReagentPlus , Z99%, Merck, 1.723 g,
Conflicts of interest
2 2
20 mmol) was dissolved in dry CH Cl (ACS grade, Z99.5%,
There are no conflicts to declare.
Sigma Aldrich, 20 mL) and stirred for 30 min. Then, sulfuric
acid (SA) (98%, 2.18 mL, ca. 40 mmol) was added dropwise into
the solution in an ice-bath. The solution was mixed by a stirrer Acknowledgements
at room temperature overnight. The upper layer was decanted
and the bottom solid phase was washed several times with
The authors used a Research Grant (RU001-2020) to conduct
this work from the Universiti Malaya, Malaysia. The authors are
also grateful to all staff members in the Analytical and Testing
Centre of Nanotechnology & Catalysis Research Centre for their
partial support.
CH Cl (3 ꢄ 10 mL). A white solid was isolated in 94% yield
2
2
(
3.463 g) after removing the solvent and drying under vacuum.
The esterification of n-pentanol and HOAc
Glacial acetic acid (RA grade, 100%, Friendemann Schmidt
chemicals, 429 mL, 7.5 mmol), and PZ–SA (1 : 2) (100 mg,
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