Synthesis and performance of piperidinium-based ionic liquids as catalyst for alkylation of p-xylene with 1-hexadecene

Article abstract of DOI:10.1002/aoc.6147

Piperidinium-based ionic liquids (ILs) were synthesized under mild conditions by coordinating N-alkyl-N-methylpiperidinium bromide ([Pip_1,x]Br) with anhydrous AlCl₃. The weak basicity of N-methylpiperidine had little impact on downstream problems from proton impurities and catalyst poisoning. The results of Fourier transform infrared (FT-IR), electrospray ionization mass spectrometry (ESI-MS), and nuclear magnetic resonance (²⁷Al NMR) confirmed that the active components in [Pip_1,x]Br-2AlCl₃ (χ = 0.67) were [Al₃Cl₉Br]^? and [Al₂Cl₆Br]^?. Besides, the reaction performance of [Pip_1,8]Br-2AlCl₃ under different conditions was carried out by single-factor experiment and further confirmed by orthogonal experimental method. Among these reaction conditions, the optimal combination was the reaction temperature of 40°C, ILs/O molar ratio of 0.2:1, px/O molar ratio of 12:1, and the reaction time of 30 min. Furthermore, [Pip_1,8]Br-2AlCl₃ had excellent stability and can be recycled 14 times. It was the high-efficiency catalytic activity and prominent recyclability that made continuous industrial production possible.

Full text of DOI:10.1002/aoc.6147

Received: 24 September 2020
DOI: 10.1002/aoc.6147
Revised: 27 November 2020
Accepted: 15 December 2020
F U L L P A P E R
Synthesis and performance of piperidinium-based ionic
liquids as catalyst for alkylation of p-xylene with
1-hexadecene
Sujuan Ge
| Yuming Zhou
| Xiaoli Sheng
| Chunfeng Mao |
Lina Zhao | Jianxing Xu | Lei Qiu
School of Chemistry and Chemical
Piperidinium-based ionic liquids (ILs) were synthesized under mild
conditions by coordinating N-alkyl-N-methylpiperidinium bromide ([Pip1,x
Engineering, Jiangsu Optoelectronic
Functional Materials and Engineering
Laboratory, Southeast University,
Nanjing, 211189, China
]
Br) with anhydrous AlCl3. The weak basicity of N-methylpiperidine had
little impact on downstream problems from proton impurities and catalyst
poisoning. The results of Fourier transform infrared (FT-IR), electrospray
ionization mass spectrometry (ESI-MS), and nuclear magnetic resonance
(27Al NMR) confirmed that the active components in [Pip1,x]Br-2AlCl3
(χ = 0.67) were [Al3Cl9Br]− and [Al2Cl6Br]−. Besides, the reaction perfor-
mance of [Pip1,8]Br-2AlCl3 under different conditions was carried out by
single-factor experiment and further confirmed by orthogonal experimental
method. Among these reaction conditions, the optimal combination was the
reaction temperature of 40ꢀC, ILs/O molar ratio of 0.2:1, px/O molar ratio of
12:1, and the reaction time of 30 min. Furthermore, [Pip1,8]Br-2AlCl3 had
excellent stability and can be recycled 14 times. It was the high-efficiency
catalytic activity and prominent recyclability that made continuous industrial
production possible.
Correspondence
Yuming Zhou, School of Chemistry and
Chemical Engineering, Jiangsu
Optoelectronic Functional Materials and
Engineering Laboratory, Southeast
University. Nanjing 211189, China.
Email: ymzhou@seu.edu.cn
Funding information
Fundamental Research Funds for the
Central Universities, Grant/Award
Numbers: 2242019k30042, 3207042001C3;
Fund Project for Transformation of
Scientific and Technological
Achievements of Jiangsu Province of
China, Grant/Award Numbers:
BA2018045, BA2019054; Natural Science
Foundation of Jiangsu Province, Grant/
Award Numbers: BK20171357,
K E Y W O R D S
alkylation, piperidinium-based ionic liquids, reaction performance, recyclability
BK20180366; Priority Academic Program
Development of Jiangsu Higher Education
Institutions (PADA), Grant/Award
Number: 1107047002; Postgraduate
Research & Practice Innovation Program
of Jiangsu Province, Grant/Award
Numbers: KYCX19_0103, KYCX20_0125;
National Nature Science Foundation of
China, Grant/Award Number: 51673040
1 | INTRODUCTION
surfactants, have attracted more attention and investi-
gated extensively. Generally, LABs are formed by the
Alkyl benzene sulfonate (ABS) is one of the most gener-
ally consumed anionic surfactants owing to its excellent
processing performance, broad serviceability, and
substantial economic benefits.[1,2] Therefore, linear alkyl
benzenes (LABs), as core intermediates of the ABS
Friedel–Crafts reaction of aromatic hydrocarbons with
1-halogenated hydrocarbon or 1-olefin.[3,4] The Friedel–
Crafts alkylation is an electrophilic alkylation in which
carbocation is produced by acid catalysts and serves as an
electrophilic group. In general, traditional LABs synthetic
Appl Organomet Chem. 2021;35:e6147.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 14
https://doi.org/10.1002/aoc.6147

2 of 14
GE ET AL.
processes often use toxic and corrosive catalysts, such as
HF, H2SO4, and AlCl3.[5,6] However, there are some com-
mon problems in each process, such as serious environ-
mental pollution, complicated product recovery, and
purification, moreover, hardly recyclability because of no
recrystallization of catalyst.[7] In order to solve the inher-
ent disadvantages of traditional catalysts, ILs have been
widely studied and considered as promising alternatives
due to their distinct physicochemical properties including
high thermal stability, low melting point, low vapor pres-
sure, and potential recyclability.
with 1-hexadecene, and a catalyst mechanism was
proposed. Subsequently, the reaction performance under
different conditions was investigated and optimized by
single-factor and orthogonal experimental method. More-
over, the cycle stability of [Pip1,x]Br-2AlCl3 catalyst was
verified. We believe that the high-efficiency catalytic
activity and excellent recyclability of piperidinium-based
ILs provide a possibility to replace the traditional cata-
lysts in industrial applications.
In recent years, ionic liquids (ILs) have been exten-
sively used in synthesis,[8,9] electrochemistry,[10] liquid–
liquid extraction,[11] absorbents,[12] and catalysis.[13–20]
In particular, IL-catalyzed Friedel–Crafts alkylation
in the homogenous biphasic regime arouses much
attention.[21–27] Qiao et al.[28] investigated the alkylation
of benzene with 1-dodecene, and the [BMIM][AlCl4] cat-
alyst obtained a higher efficiency and a longer stable run-
ning time in the continuous flow reactor. Xin et al.[29]
synthesized alkylation catalysts [Rmim]+Al2Cl6X− ILs
(R = butyl, octyl, and dodecyl; X = Cl, Br, and I), of
which [BMIM]Al2Cl6Br had the best catalytic perfor-
mance, with a conversion of 91.8% and a selectivity of
38.0% to 2-dodecylbenzene. In addition, a review wrote
by Qiao et al.[30] especially highlighted the applications of
chloroaluminate ILs for alkylation of benzene with long-
chain olefins in petrochemical and detergent industry.
Besides, Xiao and Malhotra[31] researched that the
pyridinium-based ILs were suitable media for benzene
alkylation with high reaction rate and high product con-
version rate. However, the unreacted residue in the ILs of
1-methylimidazole as a kind of Lewis base left underlying
downstream problems from proton impurities and cata-
lyst poisoning, although the raw material pyridine was
toxic and posed a hazard to human health. A method to
determine the unreacted 1-methylimidazole was
reported by Holbrey et al.,[32] whereas it was difficult to
remove from ILs at a later stage because of the tendency
2 | EXPERIMENTAL
Characterization of catalysts and the evaluation of cata-
lysts performance for alkylation are listed in Supporting
Information.
2.1 | Materials
All chemicals and solvents were commercially available
and used as received. 1-bromobutane (99.0%, AR),
1-bromine hexane (99%, AR), 1-bromine octane (99%,
AR), and anhydrous aluminum chloride (99%, AR) were
all purchased from Shanghai Macklin Biochemical Co.,
Ltd. China. 1-Methylpiperidine (97%, AR) was purchased
from Aladdin Industrial Corporation. 1-Hexadecene was
obtained from the chemical market.
2.2 | Synthesis of [Pip1,x]Br
The synthesis conditions of [Pip1,x]Br were optimized on
the basis of the literatures.[34,35] 1-Bromobutane
(0.10 mol) and 1-methylpiperidine (0.08 mol) were placed
in the three-neck round-bottom flask equipped with a
thermometer, a magnetic stirrer bar, and a cooler, and
20 ml of ethyl alcohol was added as a solvent. The reac-
tion was stirred for 12 h in a nitrogen atmosphere at
ꢀ
to
coordinate.
To
avoid
the
disadvantages
60 C, then cooled to room temperature. Ethyl alcohol
mentioned above, piperidinium-based ILs were selected
as the research object. On the one hand, compared
with 1-methylimidazole, the weak basicity of
N-methylpiperidine had little impact on subsequent
catalysts. Moreover, the low toxicity and lower density
than pyridinium-based ILs made piperidinium-based ILs
useful in many industrial processes.[33]
was removed from the mixture using a rotary evaporator.
After that, a large amount of solid was separated through
extracting and washing with ethyl acetate. At last, the
pale yellow or white solid was dried under vacuum. In
the same way, the precursors of [Pip1,6]Br and [Pip1,8]Br
were composited.
In this work, a series of [Pip1,x]Br-nAlCl3 (x = 4,
6, and 8; n = 1, 2, and 3) catalysts were prepared by mild
conditions and raw materials with low toxicity. With the
aim of exploring the impact of piperidinium-based ILs as
sustainable and low-toxicity Lewis acid catalysts, herein
we investigated the Friedel–Crafts alkylation of p-xylene
2.3 | Synthesis of piperidinium-based ILs
[Pip1,x]Br-nAlCl3 is composed of [Pip1,x]Br and AlCl3,
where the amount of substance of [Pip1,x]Br is assumed
to be 1.0, and the amount of substance AlCl3 is n. As n is

GE ET AL.
3 of 14
greater than, less than or equal to 1.0, these systems
exhibit acidity, basicity, or neutrality. The molar fraction
χ of AlCl3 is calculated by
separated with a separation funnel and stored in a
desiccator.
n
1 + n
2.4 | The formulas involved in this
article
χ =
:
When χ = 0.50, 0.67, and 0.75, the bromide alumi-
num chloride anions AlCl3Br−, Al2Cl6Br−, and Al3Cl9Br−
are formed in the equations:
2.4.1 | The conversion of the
1-hexadecene, yield, and selectivity is
defined as follows
ꢀ
ꢁ
ꢀ
ꢁ
Pip1,x Br + AlCl3⇌ Pip1,x AlCl3Br,
Nh,in −Nh,out
1−hexadecene conversion : Xhð%Þ =
1−hexadecene yield : Yhð%Þ =
2−LAB selectivity : S2−LABð%Þ =
× 100%,
Nh,in
ꢀ
ꢁ
ꢀ
ꢁ
Pip1,x AlCl3Br + AlCl3⇌ Pip1,x Al2Cl6Br,
NLAB
ꢀ
ꢁ
ꢀ
ꢁ
× 100%,
Nh,in
Pip1,x Al2Cl6Br + AlCl3⇌ Pip1,x Al3Cl9Br:
N2−LAB
NLAB
Take χ = 0.67 as an example, and operate according
to Scheme 1. 0.025 mol [Pip1,4]Br was added to a three-
neck flask equipped with 20 ml of cyclohexane. Anhy-
drous AlCl3 (0.05 mol) was appended to a three-neck
flask in batches, stirred for 30 min until a new liquid
phase was formed, then heated to 80ꢀC, and stirred for
4 h. After the reaction, it was cooled to room tempera-
ture, and the lower [Pip1,4]Br-2AlCl3 catalyst was
× 100%,
where Nh,in and Nh,out are the initial and final
mole number of the 1-hexadecene. NLAB is the final mole
number of the 2-alkyl-1,4-dimethylbenzene. N2-LAB is
the final mole number of the 1,4-dimethyl-2-(1-methyl-
pentadecyl)benzene abbreviated as 2-LAB.
SCHEME 1 The formation of [Pip1,4
]
Br-2AlCl3

4 of 14
GE ET AL.
2.4.2 | The bromine number of product
is calculated as follows
3 | RESULTS AND DISCUSSION
3.1 | Characterization of catalysts
3.1.1 | FT-IR of ILs and catalysts
ðV1 −V2Þ × 0:008
Χ =
× 100,
5
Three precursors were synthesized and characterized by
Fourier transform infrared (FT-IR) spectra: (a) [Pip1,4]Br,
(b) [Pip1,6]Br, (c) [Pip1,8]Br, and catalysts (d) [Pip1,x]Br-
2AlCl3. It can be observed from Figure 1a–c that the peak
at 2930 5 cm−1 is antisymmetric stretching vibrations
of CH2, whereas the peak located at 2850
is assigned to the symmetrical stretching vibration
of CH2. The characteristic absorption peak at
where V1 and V2 are on behalf of the Na2S2O3 volume
consumed by the blank experiment and products, respec-
tively, and 0.008 is equivalent to the amount of bromine
in 1 ml sodium thiosulfate standard solution.
10 cm−1
2.4.3 | Al leaching loss is defined as
follows
1460 10 cm−1 is attributed to antisymmetric deforma-
tion of CH3 or deformation of CH2. The peak of
(CH2)n (n > 4) is always at 720 cm−1. Due to the
strong moisture absorption of [Pip1,x]Br, sometimes
ωAl,fre −ωAl,re
Al leaching loss ð%Þ =
× 100%,
ωAl,fre
hydroxyl peak in water is observed at about 3400 cm−1
.
Besides, acetonitrile (ACN) and [Pip1,x]Br-2AlCl3 cat-
alyst were mixed uniformly in a volume ratio of 1:3,
coated on a KBr window, and the infrared absorption
spectrum was determined by the liquid film method.[36]
where ωAl,fre is the Al content of the fresh catalyst and
ωAl,re is the Al content of catalyst after recycling ten
times.
FIGURE 1 Fourier transform infrared (FT-IR) spectra of (a) [Pip1,4]Br, (b) [Pip1,6]Br, (c) [Pip1,8]Br, and (d) [Pip1,x]Br-2AlCl3

GE ET AL.
5 of 14
As can be seen from Figure 1d and Table S1, the charac-
concluded that as the alkyl chain length increases, the
thermal stability of the [Pip1,x]Br is gradually declining.
teristic absorption peaks of ACN are 2291 and 2253 cm−1
.
The complex formed between CN and Lewis acid center
has a new peak between 2200 and 2400 cm−1. As the
acidity of Lewis increases, the absorption peak at this
place shifts blue. Therefore, it is rather obvious that the
3.1.3 | ESI-MS spectrum of ILs cations
and anions
sequence of Lewis acid is [Pip1,8]Br-2AlCl3 ≈ [Pip1,6
]
Br-2AlCl3 > [Pip1,4]Br-2AlCl3.
The structure of a mass peak can be seen when the sample
is covalently bonded in a positive ion model of mass spec-
trometry. The cation of [Pip1,x]+ is detected and depicted
in Figure 3. The molecular ion [Pip1,4]+ is located at m/z
157.2. And the fragment ion [Pip1-C4H8]+ cleaved of
C4H9 from [Pip1,4]+ corresponds to m/z 156.2. Similarly,
the peak of [Pip1,6]+ is at m/z 185.2, and m/z 184.2 ascribes
to the cleavage of C6H13 from [Pip1,6]+ resulted in frag-
ment ion of [Pip1-C6H12]+. Moreover, the peak at m/z
213.2 is associated with [Pip1,8]+, whereas the peak at m/z
212.2 is corresponded to [Pip1-C8H16]+.
In this work, the anionic species of [Pip1,8]Br-2AlCl3
were characterized by electrospray ionization mass spec-
trometry (ESI-MS)[9,37] as illustrated in Figure 4. Within
the m/z range of 150–450, there exist plentiful binuclear
and polynuclear anion species. The most intense peak
presented at m/z 308.2 is related to the [Al2Cl4BrO2H]−
3.1.2 | TG of [Pip1,x]Br
Generally speaking, the ionic bond in [Pip1,x]Br becomes
weaker as the cation radius increases, resulting in a
decrease in the thermal stability. This can be confirmed
by Figure 2. We can have a clear recognition that [Pip1,x
]
Br has no evident weight loss before 200ꢀC.
Significant weight loss occurs in the temperature range of
200–300ꢀC, and the mass loss of the sample is more
than 93%. It is speculated the carbonization decomposi-
tion process of the [Pip1,x]Br. The Derivative ther-
mogravimetric analysis (DTG) results show that [Pip1,4
Br, [Pip1,6]Br, and [Pip1,8]Br decompose most violently at
293.09ꢀC, 279.55ꢀC, and 262.29ꢀC, respectively. It can be
]
FIGURE 2 Thermogravimetric and Derivative thermogravimetric (TG-DTG) results (10ꢀC/min) in nitrogen for (a) [Pip1,4]Br, (b) [Pip1,6
Br, and (c) [Pip1,8]Br
]

6 of 14
GE ET AL.
+
FIGURE 3 The electrospray ionization mass spectrometry (ESI-MS) of (a) [Pip1,4]+, (b) [Pip1,6]+, and (c) [Pip1,8
]
3.1.4 | 27Al NMR
27Al NMR spectra are effective method to characterize
the Al species of ILs catalyst,[38,39] as revealed in
Figure 5. For the 1.0[Pip1,8]Br-1.0AlCl3 (χ = 0.50), there
is only one peak in position 104.05 ppm, which is the
resonance signal of the [AlCl3Br]−. As for 1.0[Pip1,8
]
Br-2.0AlCl3 (χ = 0.67), three peaks corresponding to
three Al species are observed, in which [AlCl3Br]− is at
104.04 ppm, [Al2Cl6Br]− is located in 100.35 ppm, and
[Al3Cl9Br]− situates is in 95.35 ppm. The formation of
these bromochloroaluminate anion species can be
described by the equations below.
½Al2Cl6ꢁ + Br− ! ½Al2Cl6Brꢁ− ,
2½AlCl3Brꢁ− ⇌½Al2Cl6Brꢁ− + Br− ,
FIGURE 4 The electrospray ionization mass spectrometry
(ESI-MS) of anion of [Pip1,8]Br-2AlCl3
ion. [Al2Cl6Br]−, and [Al3Cl9Br]− ions theoretically
exist in the system when χ = 0.67; however, they are
liable to fragmentation or reactions because of sensitivity
to moisture or oxygen, resulting in extremely low inten-
sity. In consequence, oxybromochloroaluminate anion
species such as [Al2Cl3BrO]−, [Al2Cl6O]−, [Al3Cl5O2]−,
[Al3Cl5BrO]−, [Al3Cl8OH]−, and [Al3Cl6BrO]−, and frag-
ment ion peaks of [Al2Cl4]−, [Al3Cl5Br]−, and [Al3Cl8]−
are observed.
2½Al2Cl6Brꢁ− ⇌½Al3Cl9Brꢁ− + ½AlCl3Brꢁ− :
Similarly, three peaks appear in Figure 5c, among
which the peak at 95.54 ppm is linked to [Al3Cl9Br]− ion,
and the chemical shift of 75.68 ppm is in connection with
[Al(OH)4]− on account of the sensitivity to moisture,
which is consistent with Figure 5d. The third peak at

GE ET AL.
7 of 14
FIGURE 5 Nuclear magnetic resonance (27Al NMR) spectra of [Pip1,8]Br-nAlCl3 (a) χ = 0.5, (b) χ = 0.67, (c) χ = 0.75, and (d) [Al
(H2O)6]3+ in D2O
79.68 ppm may be assigned to the precipitated AlCl3
solid, because the catalyst tends to solidification due to
Lewis acidity increases with the elongation of cation
chain length, which is more conductive to alkylation.
Based on this, the effects of different cations on the con-
version rate and selectivity of the reaction were studied,
and the results are listed in Table 1. The reaction condi-
tions are as follows, reaction time of 60 min, reaction
temperature of 50ꢀC, the p-xylene/1-hexadecene molar
ratio of 10:1, and the 1-hexadecene/catalyst molar ratio
of 0.3:1. It is obvious from Table 1 that [Pip1,8]Br-2AlCl3
has the best catalytic effect, the bromine value is as low
as 0.0192 g/100 ml, the 1-hexadecene conversion reaches
99.70%, and the selectivity of 2-LAB is as high as 37.51%.
the strengthen force between [Al3Cl9Br]− anion and
+
[Pip1,x
]
cation. As a consequence, when χ = 0.50, neu-
tral ion [AlCl3Br]− cannot catalyze alkylation; when
χ = 0.75, the catalyst has a tendency to solidify so that
the catalytic effect is not theoretically good; however,
when χ = 0.67, [Pip1,x]Br-2AlCl3 contains a variety of
active components with good stability, thus it is selected
as the catalyst for alkylation reaction.
3.2 | Effect of cations on the alkylation
This is attributed to the weakened force between
+
Al2Cl6Br− anion and [Pip1,x
]
cation as the volume and
The acidity of the catalyst has been discussed in the
section on catalyst characterization. Generally speaking,
structure asymmetry of cation further increase, resulting
in improvement of catalyst activity.
TABLE 1 Catalytic performance of
[Pip1,x]Br-2AlCl3 on alkylation
Catalyst
Selectivity (%)
X (g/100 ml)
0.0448
Xh (%)
97.21
98.10
99.70
Yh (%)
96.05
97.50
99.34
2-LAB
32.87
34.05
37.51
3-LAB
20.35
16.71
14.62
4-LAB
22.64
24.06
23.49
[Pip1,4]Br-2AlCl3
[Pip1,6]Br-2AlCl3
[Pip1,8]Br-2AlCl3
0.0304
0.0192
Notes. n(catalyst) = 0.003 mol, n(1-hexadecene) = 0.01 mol, n(p-xylene) = 0.1 mol, T = 50ꢀC, t = 60 min,
V1 = 10.02 ml.

8 of 14
GE ET AL.
FIGURE 6 Fourier transform infrared
(FT-IR) spectra of alkylation products catalyzed
by [Pip1,x]Br-2AlCl3
In addition, the FT-IR analysis of the three catalytic
can be seen from Figure 7a, when the catalyst is suffi-
cient, the reaction can reach a higher conversion rate
within 30 min, and extending reaction time has little
effect on the bromine value.
products verified that the reaction was proceeding in the
direction as expected. As described in Figure 6, the peaks
with wavenumber of 1460, 2850, and 2930 cm−1 are the
bending and stretching vibration peaks of CH3 and/or
CH2, and the peaks with wavenumber of 1500 and
1600 cm−1 are the C C skeleton vibration peaks of an
aromatic ring. Besides, the peaks at 720 cm−1 are always
associated with (CH2)n , (n > 4), which demonstrates
the addition of 1-hexadecene. Moreover, through par-
tially magnifying the over frequency absorption peak in
the range of 2000–1650 cm−1, it can be seen that the
obtained product spectrum is consistent with the stan-
dard spectrum of 1,2,4-trisubstituted benzene rings.
3.3.2 | Reaction temperature
The temperature is a vital factor affecting the alkylation
reaction. Some side reactions become active accompanied
with accessorial energy consumption when the tempera-
ture rises. In this experiment, the minimum temperature
was selected at 30ꢀC, and the range of temperature was
30–70ꢀC. The reaction was implemented under the fol-
lowing conditions: reaction time of 30 min, the Cat/H
molar ratio of 0.3:1, the px/H molar ratio of 8:1. The
results are exhibited in Figure 7b, at the initial stage, the
bromine value declines with the ascent of temperature,
indicating the augment of the olefin conversion rate.
Nevertheless, after a short plateau, the bromine value
reveals an upward trend because the degree of side effects
gradually intensifies when the temperature exceeds 60ꢀC.
As a consequence, the appropriate temperature for the
alkylation reaction is set at 40ꢀC.
3.3 | Single-factor experiment results
After selecting [Pip1,8]Br-2AlCl3 as the alkylation catalyst,
the impact of reaction conditions on the alkylation per-
formance were conducted. Bromine value is used as the
evaluation standard of reaction conversion rate.
3.3.1 | Reaction time
The alkylation was implemented under the following
conditions: reaction temperature of 50ꢀC, the molar ratio
of [Pip1,8]Br-2AlCl3 catalyst to 1-hexadecene (Cat/H
molar ratio) of 0.3:1, the molar ratio of p-xylene
to1-hexadecene (px/H molar ratio) of 8:1. The time was
taken as 10, 20, 30, 40, 50, and 60 min, respectively. As
3.3.3 | The Cat/H molar ratio
Response time is 30 min, the reaction temperature in
the selected is 40ꢀC, and the px/H molar ratio was equal
to 8:1. When the dosage of the catalyst is relatively
small, almost all the active centers of the catalyst are

GE ET AL.
9 of 14
FIGURE 7 Effect of (a) reaction time, (b) reaction temperature, (c) Cat/H molar ratio, and (d) px/H molar ratio
occupied, which seriously impacts the catalytic effi-
ciency. Therefore, there is an abrupt fall when the Cat/H
molar ratio gradually climbs to 0.1 (Figure 7c). The
Cat/H molar ratio continues to increase until the active
center is sufficient, and the curve tends to be smooth
after greater than 0.2. Accordingly, an appropriate Cat/H
molar ratio is selected as 0.2 to achieve a higher olefin
conversion rate.
3.4 | Orthogonal test for the alkylation
process
Based on single-factor experiment, a three-level orthogo-
nal experiment was designed to further explore the influ-
ence of reaction temperature, Cat/H molar ratio, px/H
molar ratio, and reaction time on the alkylation reaction,
and these four factors were recorded as A, B, C, and
D. Suppose that these four factors are independent of
each other and noninteractive. In light of the orthogonal
experimental results, the optimum technological condi-
tions are determined.
3.3.4 | The px/H molar ratio
At this point, three conditions have been defined,
namely, the reaction time of 30 min, the reaction temper-
ature of 40ꢀC, and the Cat/H molar ratio of 0.2. The
impact of the px/H molar ratio on the alkylation reaction
was studied according to the above conditions. In
Figure 7d, there is a smooth trend of decreasing, indicat-
ing that the olefin conversion rate gradually raises as the
px/H molar ratio ascends. When the molar ratio climbs
to 12:1, the bromine value is as low as 0.016 g ml−1. How-
ever, there is no obvious fluctuation in bromine value
with the px/H molar ratio growing, but it leads to an
expensive cost of p-xylene recycling. In conclusion, the
molar ratio of p-xylene to olefin as high as 12:1 is
beneficial.
3.4.1 | Experimental factors and levels
Three levels are set for each factor, and the orthogonal
test tables is shown in Table 2.
3.4.2 | Orthogonal experiment and
results
According to the above factors and levels, the orthogonal
design (Table 3) of L9 (34) is listed with bromine value as
the investigation index. Then, range analysis is carried out,

10 of 14
GE ET AL.
TABLE 2 Factor levels of orthogonal experiment
1-hexadecene. Furthermore, the reaction time ranks
fourth possibly because a higher conversion rate can be
achieved in a shorter time. Another important informa-
tion available from Table 4 is that the optimum reaction
conditions of [Pip1,8]Br-2AlCl3 catalytic alkylation are
A2B3C3D2, in which A2 represents reaction time of 40ꢀC,
B3 indicates the Cat/H molar ratio of 0.2:1, C3 is on
behalf of px/H molar ratio of 12:1, and D2 stands for the
reaction time of 30 min.
Levels
Factors
A: T/(ꢀC)
B: Cat/H
C: px/H
1
2
3
30
40
50
0.1:1
8:1
20
0.15:1
10:1
30
0.2:1
12:1
40
D: t/(min)
TABLE 3 L9 (34) Orthogonal experiments and results
3.5 | Cyclic performance of catalysts
Factors/levels
Bromine value/
(g/100 ml)
Excellent cyclic stability is the most direct criterion to
determine whether catalysts have industrial application
value. The [Pip1,x]Br-2AlCl3 catalysts in the lower layer
were recycled under the following reaction conditions:
reaction time of 20 min, reaction temperature of 40ꢀC,
the px/H molar ratio of 10:1, and the Cat/H molar ratio
of 0.2:1. Figure 8 shows the bromine value of the alkyl-
ation of 1-hexadecene and p-xylene during the catalyst
cycle. It can be obviously seen that the bromine number
of [Pip1,4]Br-2AlCl3 is not low enough. The catalytic abil-
ity improved slightly at first, which may be related to the
gradual exposure of the active center of catalyst, and then
the bromine number increased. Unfortunately, the bro-
mine number exceeded what was required by the stan-
dard and became inactivation after being recycled
10 times. In contrast, the cycle capacity of [Pip1,6]Br-
2AlCl3 has increased three times to 13, but the bromine
number is not as low as [Pip1,8]Br-2AlCl3. From the per-
spective of Figure 8, compared with the other two cata-
lysts, [Pip1,8]Br-2AlCl3 has the best cycle stability, and
the maximum number of cycles can reach 14 times. This
phenomenon can be explained by the influence of the
alkyl chain length of the cationic structure on the perfor-
No
1
A
1
1
1
2
2
2
3
3
3
B
1
2
3
1
2
3
1
2
3
C
1
2
3
2
3
1
3
1
2
D
1
2
3
3
1
2
2
3
1
0.0704
0.0576
0.0288
0.0352
0.0464
0.0144
0.0208
0.0688
0.0224
2
3
4
5
6
7
8
9
and the results are shown in Table 4. The bromine values
of the designed experiments all meet the requirements.
Based on the results in Table 4, the range order of
four factors is B > A > C > D. In general, when the
amount of the catalyst is insufficient, almost all the active
centers of the catalyst are occupied, leading to low effi-
ciency. The result shows that the Cat/H molar ratio is the
most fluctuating factor among the four reaction condi-
tions. Also, temperature is a vital factor affecting the
alkylation reaction. Some side reactions become active
when the temperature rises, whereas an appropriate
px/H molar ratio could reduce the side-reactions of
mance of catalyst. On the one hand, the interaction
+
between Al2Cl6Br− anion and [Pip1,x
]
cation weakens
TABLE 4 Orthogonal experimental
Factors
A: T/(ꢀC)
0.1568
B: Cat/H
0.1264
C: px/H
0.1536
D:t/(min)
0.1392
calculation structure (range analysis)
K1
K2
0.0960
0.1728
0.1152
0.0928
K3
0.1120
0.0656
0.0960
0.1328
k1
0.05227
0.03200
0.03733
0.02027
B > A > C > D
A2
0.04213
0.05760
0.02187
0.03573
0.05120
0.03840
0.03200
0.01920
0.04640
0.03093
0.04427
0.01547
k2
k3
Range (R)
Priority
Optimal level
Optimal combination
B3
C3
D2
A2B3C3D2

GE ET AL.
11 of 14
when the mole fraction of AlCl3 is 0.67, the catalytic
activity is the highest. This is closely related to the stron-
ger catalytic substances produced by homobimetallic
association.[41] Besides, Quarmby et al.[42] observed that
some buffered melts exhibit a “latent acidity” and could
form AlCl3 adducts B: AlCl3 with weak Lewis bases. Qi
et al.[43] reported that alkylation was induced by the
Lewis acid AlCl3, which was obtained by the dissociation
of [Al2Cl7]−. On the side, AlCl3 is a kind of hard acid in
the alkylation reaction, and the complexes with X− has a
stability sequence of F− > Cl− > Br− > I− based on
Hard and Soft Acids and Bases (HSAB) theory.[44,45]
Hence, AlCl3 is coordinated with Cl− rather than Br−,
and there is an equilibrium of anion reaction
[Al2Cl6Br]− ⇌ [AlCl3Br]−+AlCl3 as shown in Scheme 2.
FIGURE 8 Cyclic performance of [Pip1,x]Br-2AlCl3 catalysts,
where the line 1 represents the maximum bromine value that meets
the requirements
AlCl3 induces and polarizes 1-hexadecene to form carbo-
L
cation (AlCl3:CH2 CH
C14H29) and then attacks
p-xylene to generate an unstable σ complex. The reaction
rate of forming σ complex is relatively slow, which is a
step to determine the whole reaction rate. Since H+ is
more electrophilic than AlCl3, the H+ attached to the car-
bocyclic ring will be transferred to the electronegative
1-carbon of the side chain to substitute for AlCl3. The σ
complex that has lost a proton is restored to a stable ben-
zene ring, resulting in 2-LAB, namely, 1,4-dimethyl-
2-(1-methyl-pentadecyl)benzene.
with the augment of volume and structure asymmetry of
cation. As a result, AlCl3 is easier to dissociate from
Al2Cl6Br−, which is the cause for improvement of catalyst
activity. On the other hand, as the cationic alkyl chain
increases, the hydrophilicity of cation decreases. It is well
known that the [Pip1,8]Br-2AlCl3 ILs are sensitive to
moisture, so a longer hydrophobic alkyl chain is condu-
cive to protecting the active sites of the catalyst,[40]
thereby making the catalyst more stable in the reaction
system. Furthermore, a comparison was conducted and
exhibited in Table 5 between the [Pip1,8]Br-2AlCl3 syn-
thesized in this work with some other reported halide-
aluminum IL catalysts, and the results further illustrate
that the [Pip1,8]Br-2AlCl3 catalyst has good catalytic
activity as well as excellent recyclability, which makes
continuous industrial production possible.
The distribution of 2-alkyl-1,4-dimethylbenzene is
formed by the reaction of p-xylene with other carbocation
L
derived from the migration of AlCl3:CH2 CH
C14H29
carbocation. It is desirable to limit the migration of car-
bocation, because the products required are the most bio-
degradable 2-LAB.[23]
3.7 | Deactivation of the catalysts
The Lewis acidity changes of the catalysts were investi-
gated via FT-IR using ACN as the probe. The results in
Figure 9a indicate that after recycling ten times the peak
symbolized Lewis acidity between 2200 and 2400 cm−1
takes place obvious variation compared with new cata-
lysts. To be specific, the disappearance of the peak at
3.6 | Catalytic mechanism of Lewis acid
in the alkylation
In the optimization experiment of the catalytic alkylation
reaction of p-xylene and 1-hexadecene, we found that
TABLE 5 The comparison of alkylation properties of [Pip1,8]Br-2AlCl3 with some reported halide–aluminum ionic liquid (IL) catalysts
Catalyst
Xh (%)
99.85
100
S2-LAB (%)
32.99
/
Cycle number
Reaction type
Reference
Liu et al.[6]
Zhao et al.[7]
Xin et al.[29]
Ge et al.[40]
This work a
[HMIM][Cl]-2AlCl3
BuPyBr- AlCl3
[bmim]Al2Cl6Br
[C6Et3N]-AlCl3
[Pip1,8]Br-2AlCl3
10
3
Toluene with 1-octadecene
α-Methylnaphthalene with long-chain alkenes
Benzene with 1-dodecene
91.8
38
2
99.92
99.70
32.99
37.51
24
14
Toluene with 1-octadecene
p-Xylene with 1-hexadecene
aConditions: n(catalyst) = 0.003 mol, n(1-hexadecene) = 0.01 mol, n(p-xylene) = 0.1 mol, T = 50ꢀC, t = 60 min.

12 of 14
GE ET AL.
SCHEME 2 Catalytic mechanism of Lewis acid
in alkylation
FIGURE 9 (a) Fourier transform infrared (FT-IR) of the [Pip1,x]Br-2AlCl3 after recycling 10 times (1#: [Pip1,4] Br-2AlCl3; 2#: [Pip1,6
]
Br-2AlCl3; 3#: [Pip1,8] Br-2AlCl3) and (b) Nuclear magnetic resonance (27Al NMR) spectra of devitalized [Pip1,4]Br-2AlCl3
2332 cm−1 of [Pip1,4]Br-2AlCl3 demonstrates that the loss
of the acid site, and the catalyst is inactivated. The red-
shift of the characteristic peaks occurs simultaneously in
[Pip1,6]Br-2AlCl3 and [Pip1,8]Br-2AlCl3 reflecting the
decrease of acidity and catalytic performance. Differently,
the peak of [Pip1,8]Br-2AlCl3 is more pronounced and
can continue to be used without deactivation after
recycling ten times.
under the same reaction conditions were measured.
It can be seen from Table 6 that the Al leaching loss
is in the order of [Pip1,4]Br-2AlCl3 [Pip1,6
>
]
Br-2AlCl3 > [Pip1,8]Br-2AlCl3. Among them, the Al
leaching loss of [Pip1,4]Br-2AlCl3 is severe after
10 cycles, and the catalytic ability is reduced or lost. In
comparison, the Al leaching loss of [Pip1,6]Br-2AlCl3
and [Pip1,8]Br-2AlCl3 are 24.61% and 22.02%, respec-
tively, indicating that they are relatively stable.
According to the above analysis, it can be concluded
that the main reason for catalyst deactivation is that
the action of trace moisture in the cycling process with
AlCl3 to release HCl gas, and Al3+ ion loses its coordi-
nation Cl− ion to generate inactive Al(OH)3. Moreover,
a part of Al3+ ion were dissolved in the reaction system
and lost as the cycle experiment progresses.
In addition, the deactivated catalyst [Pip1,4
]
Br-2AlCl3 was taken to 27Al NMR testing and ICP-OES.
The peak of neutral ion [AlCl3Br]− can be observed
at 104 ppm in Figure 9b, whereas the peaks of
[Al2Cl6Br]− and [Al3Cl9Br]− as the active component
with Lewis acidity disappear. Furthermore, the peak at
63.35 ppm possibly corresponds to Al(OH)3. At the
same time, the Al leaching loss of the three catalysts

GE ET AL.
13 of 14
TABLE 6 The Al leaching loss
during the test of reusability of [Pip1,x
Br-2AlCl3
Catalyst
ωAl,fre (wt%)a
10.63
ωAl,re (wt%)b
6.92
Al leaching loss (%)
]
[Pip1,4]Br-2AlCl3
[Pip1,6]Br-2AlCl3
[Pip1,8]Br-2AlCl3
34.90
24.61
22.02
9.59
7.23
9.47
7.29
Notes: n(catalyst) = 0.002 mol, n(1-hexadecene) = 0.01 mol, n(p-xylene) = 0.1 mol, T = 40ꢀC, t = 20 min,
cycle number = 10.
aThe Al content of the fresh catalysts is measured by ICP-OES.
bThe Al content of catalysts after recycling 10 times is measured by ICP-OES.
4 | CONCLUSION
BK20180366), and Fund Project for Transformation of
Scientific and Technological Achievements of Jiangsu
[Pip1,x]Br-nAlCl3 ionic liquids were prepared as efficient
Lewis acid catalyst for alkylation of p-xylene with
1-hexadecene under mild conditions with low tempera-
ture and shorter reaction time. The precursors were syn-
thesized at a reaction temperature of 60ꢀC and a
reaction time of 12 h. Subsequently, the catalysts syn-
thesis temperature was 80ꢀC, and the reaction time was
4 h. The EIS-MS and 27Al NMR spectra confirmed that
the active components with Lewis acidity were
[Al2Cl6Br]− and [Al3Cl9Br]−. Using [Pip1,8]Br-2AlCl3 as
the catalyst, the highest conversion of 1-hexadecene and
selectivity of 2-LAB were 99.70% and 37.51%, respec-
tively. In addition, compared with the other two cata-
lysts, [Pip1,8]Br-2AlCl3 had the best cycle stability, with
the maximum number of cycles up to 14 times. Further-
more, the reaction properties under different conditions
were investigated and optimized by single-factor
experiment, and the results were as follows, the reaction
temperature of 40ꢀC, ILs/O molar ratio of 0.2:1, px/O
molar ratio of 12:1, and the reaction time of 30 min.
Among the four factors, the order of influence on the
catalyst effect was ILs/O molar ratio > experimental
temperature > px/O molar ratio > reaction time.
Moreover, the catalytic mechanism of Lewis acid was
Province of China (BA2019054, BA2018045).
AUTHOR CONTRIBUTIONS
Sujuan Ge: Conceptualization; data curation; formal
analysis; funding acquisition; investigation; methodology;
project administration; resources; software; supervision;
validation; visualization. Yuming Zhou: Conceptualiza-
tion; data curation; formal analysis; funding acquisition;
investigation; supervision; validation; visualization.
Xiaoli Sheng: Data curation; funding acquisition; inves-
tigation; methodology; project administration; validation.
Chun feng Mao: Data curation; investigation; methodol-
ogy; software; supervision. Lina Zhao: Data curation;
formal analysis; funding acquisition; investigation; soft-
ware; supervision. Jianxing Xu: Funding acquisition;
investigation; project administration; resources; supervi-
sion; visualization. Lei Qiu: Conceptualization; data
curation; investigation; validation; visualization.
CONFLICT OF INTEREST
There are no conflicts to declare.
DATA AVAILABILITY STATEMENT
Data available on request from the authors.
proposed. The easier dissociation of AlCl3 from [Pip1,8
]
Br-2AlCl3 was the cause of improvement of catalyst
activity. In conclusion, the cycling stability and low-
temperature activity of piperidinium-based ILs show
potential application prospects in energy saving and cost
reduction.
ORCID
Sujuan Ge https://orcid.org/0000-0002-8113-6596
Yuming Zhou https://orcid.org/0000-0002-3506-3517
Xiaoli Sheng https://orcid.org/0000-0003-3837-2232
REFERENCES
ACKNOWLEDGMENTS
[1] H. Y. Cai, Y. Zhang, Z. Y. Liu, J. G. Li, Q. T. Gong, Q. Liao, L.
Zhang, S. Zhao, J. Mol. Liq. 2018, 266, 82.
[2] M. T. García, E. Campos, I. Ribosa, A. Latorre, J. Sánchez-leal,
Chemosphere 2005, 60, 1636.
[3] R. P. Eganhouse, J. Pontolillo, Environ. Sci. Technol. 2008, 42,
6361.
[4] E. D. Ivanchina, E. N. Ivashkina, I. O. Dolganova, V. V.
Platonov, Petroleum Chem. 2014, 54, 445.
The authors are grateful to the financial support of the
National Nature Science Foundation of China
(51673040), the Fundamental Research Funds for the
Central Universities (2242019k30042, 3207042001C3),
Postgraduate Research & Practice Innovation Program of
Jiangsu Province (KYCX19_0103, KYCX20_0125), Priority
Academic Program Development of Jiangsu Higher
Education Institutions (PADA) (1107047002), Natural
Science Foundation of Jiangsu Province (BK20171357,
[5] A. S. Amarasekara, Chem. Rev. 2016, 116, 6133.
[6] Y. H. Liu, Y. M. Zhou, X. L. Sheng, B. B. Wang, Z. Y. Zhu,
Q. L. Nan, Energy Fuel 2018, 32, 9763.

14 of 14
GE ET AL.
[7] Z. K. Zhao, W. Q. Qiao, G. H. Wang, Z. S. Li, L. B. Cheng,
J. Mol. Catal. A: Chem. 2005, 231, 137.
[30] C. Z. Qiao, Y. H. Cai, Q. H. Guo, Front. Chem. Eng. China
2008, 2, 346.
[8] M. L. Guo, J. Fang, H. K. Xu, W. Li, X. H. Lu, C. H. Lan, K. Y.
Li, J. Membr. Sci. 2010, 362, 97.
[9] M. Antonietti, D. B. Kuang, B. Smarsly, Y. Zhou, Am. Ethnol.
2004, 43, 4988.
[10] L. Mayrand-Provencher, S. X. Lin, D. Lazzerini, D. Rochefort,
J. Power Sources 2010, 195, 5114.
[11] N. M. Kanaan, C. Hamel, T. Grabinski, B. Combs, Nat.
Commun. 2020, 11, 1.
[12] S. J. Zeng, H. S. Gao, X. C. Zhang, H. F. Dong, X. P. Zhang,
S. J. Zhang, Chem. Eng. J. 2014, 251, 248.
[13] D. Betz, P. Altmann, M. Cokoja, W. A. Herrmann, F. E. Kühn,
Coord. Chem. Rev. 2011, 255, 1518.
[14] W. Q. Su, C. Yang, D. Z. Xu, Cat. Com. 2017, 100, 38.
[15] H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catal. Gen.
2009, 373, 1.
[31] Y. Xiao, S. V. Malhotra, J. Mol. Catal. A: Chem. 2005, 230, 129.
[32] J. D. Holbrey, K. R. Seddon, R. Wareing, Green Chem. 2001,
3, 33.
[33] U. Domañska, M. Królikowska, K. Paduszyñski, Fluid Phase
Equilib. 2011, 303, 1.
[34] Y. Shimizu, K. Fujii, M. Imanari, K. Nishikawa, J. Phys. Chem.
B 2015, 119, 12552.
[35] A. Triolo, O. Russina, B. Fazio, G. B. Appetecchi, M.
Carewska, S. Passerini, J. Chem. Phys. 2009, 130, 164521.
[36] Y. L. Yang, Y. Kou, Chem. Commun. 2004, 226.
[37] S. P. Wicelinski, R. A. Laine, Anal. Chem. 1988, 60, 2228.
[38] S. J. Nara, J. R. Harjani, M. M. Salunkje, J. Org. Chem. 2001,
66, 8616.
[39] Y. Liu, R. S. Hu, C. M. Xu, H. Q. Su, Appl. Catal. Gen. 2008,
346, 189.
[16] Q. H. Zhang, S. G. Zhang, Y. Q. Deng, Green Chem. 2011, 13,
2619.
[17] T. Welton, Coord. Chem. Rev. 2004, 248, 2459.
[18] D. B. Zhao, M. Wu, Y. Kou, E. Z. Min, Catal. Today 2002,
74, 157.
[40] G. Y. Ge, Y. M. Zhou, X. L. Sheng, Y. H. Liu, B. B. Wang, H. Y.
Yang, X. Sha, Appl. Organomet. Chem. 2020, 34, 5878.
[41] H. Albright, P. S. Riehl, C. C. McAtee, J. P. Reid, J. R. Ludwig,
L. A. Karp, P. M. Zimmerman, M. S. Sigman, C. S. Schindle,
J. Am. Chem. Soc. 2019, 141, 1690.
[19] H. Amirfirouzkouhi, A. N. Kharat, Sep. Purif. Technol. 2018,
196, 132.
[42] I. C. Quarmby, R. A. Mantz, L. M. Goldenberg, R. A.
Osteryoung, Anal. Chem. 1994, 66, 3558.
[20] H. Li, S. Saravanamurugan, S. Yang, A. Riisager, ACS Sustain-
able Chem. Eng. 2015, 3, 3274.
[43] G. P. Qi, F. Jiang, X. W. Sun, S. Q. Zhao, Sci. China: Chem.
2010, 53, 1102.
[21] S. Zhang, L. Ma, R. Dong, C. Y. Zhang, W. J. Sun, M. J. Fan,
D. S. Yang, F. Zhou, W. M. Liu, RSC Adv. 2017, 7, 11030.
[22] J. Zhang, C. P. Huang, B. H. Chen, P. J. Ren, M. Pu, J. Catal.
2007, 249, 261.
[44] P. W. Ayers, R. P. Parr, R. G. Pearson, J. Chem. Phys. 2006,
124, 194107.
[45] P. A. Gauden, M. Wisniewski, Catal. Today 2010, 150, 147.
[23] K. Matuszek, A. Chrobok, J. M. Hogg, F. Coleman, S.
Swadzba-kwasny, Green Chem. 2015, 17, 4255.
[24] S. W. Tang, A. M. Scurto, B. Subramaniam, J. Catal. 2009,
268, 243.
[25] B. Thi Le Thuy, K. Wolfgang, A. Stephan, J. Andreas, Green
Chem. 2009, 11, 1961.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
[26] X. J. Cui, S. G. Zhang, F. Shi, Q. H. Zhang, L. Y. Ma, L. J. Lu,
Y. Q. Deng, ChemSusChem 2010, 3, 1043.
[27] X. Q. Xing, G. Y. Zhao, J. Z. Cui, Sci. China Chem. 2012, 55,
1542.
[28] C. Z. Qiao, Y. F. Zhang, J. C. Zhang, C. Y. Li, Appl. Catal. Gen.
2004, 276, 61.
[29] H. L. Xin, Q. Wu, M. H. Han, D. Z. Wang, Y. Jin, Appl. Catal.
Gen. 2005, 292, 354.
How to cite this article: Ge S, Zhou Y, Sheng X,
et al. Synthesis and performance of piperidinium-
based ionic liquids as catalyst for alkylation of p-
xylene with 1-hexadecene. Appl Organomet Chem.
2021;35:e6147. https://doi.org/10.1002/aoc.6147