Chemical Kinetics of the Alkylation of Xylenol for the Separation of Their Close-Boiling Isomers from Coal Tar

Article abstract of DOI:10.1134/S0965544120110031

Abstract: To support the industrial design and process development for the separation of xylenol isomers from coal tar, the present work was focused on 2,4/2,5-xylenol mixture as 2,4-xylenol and 2,5-xylenol have very close boiling points. Specifically, th

Full text of DOI:10.1134/S0965544120110031

ISSN 0965-5441, Petroleum Chemistry, 2020, Vol. 60, No. 11, pp. 1291–1299. © Pleiades Publishing, Ltd., 2020.
Published in Russian in Neftekhimiya, 2020, Vol. 60, No. 6, pp. 834–843.
Chemical Kinetics of the Alkylation of Xylenol for the Separation
of Their Close-Boiling Isomers from Coal Tar
Cong-Yu Kea, Guo-Min Lua, Ying-Lin Weia, Xiao-Xia Zhanga, Wu-Juan Suna, Xuan Tanga,
Qun-Zheng Zhanga, and Xun-Li Zhanga, *
aCollege of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an, 710065 China
*e-mail: xlzhang@xsyu.edu.cn
Received January 29, 2020; revised May 18, 2020; accepted July 10, 2020
Abstract—To support the industrial design and process development for the separation of xylenol isomers
from coal tar, the present work was focused on 2,4/2,5-xylenol mixture as 2,4-xylenol and 2,5-xylenol have
very close boiling points. Specifically, the kinetics of the alkylation of 2,4/2,5-xylenol mixture was investi-
gated that formed the first step of the alkylation-distillation-dealkylation separation strategy. By quantifying
the influence of various key factors on the reaction rate, the rate equations of 2,4-xylenol and 2,5-xylenol
r2,4 = 1538e−39360 RT
c
P1.0 c0.4
r2,5 = 516e−32241 RT
c ,
P0.8c0.7
1.1
1.0
alkylation were determined as
and
2,4 PIB PTSA
2,5 PIB PTSA
respectively. The alkylation rate of 2,4-xylenol was notably higher than that of 2,5-xylenol. The influence of
temperature, xylenol concentration and isobutylene partial pressure on the reaction rate of 2,4-xylenol was
also greater than that of 2,5-xylenol, whilst the influence of catalyst dosage on the alkylation rate of 2,5-xyle-
nol was found to be greater than that of 2,4-xylenol. The results can be potentially used for reactor design and
process development for the separation of 2,4/2,5-xylenol mixture from coal tar.
Keywords: 2,4-xylenol, 2,5-xylenol, alkylation, reaction kinetics, separation
DOI: 10.1134/S0965544120110031
INTRODUCTION
based chemical industry, such as limited feedstock and
increasing environmental concerns, more research
efforts have been directed to either partial or complete
replacement of chem-stocks of petroleum-based
industries [2]. For example, coal- including coal tar-
based chemicals are of particular relevance in China,
mainly due to the significant coal reserves and pro-
duction [14–16].
Coal tar is a byproduct in the production of coke or
coal gas from coal, typically in form of brown/black
viscous liquid. When obtained through the process in
the low-medium temperature range (typically, 500–
900°C), the coal tar consists of mainly polyalkylene
aromatic hydrocarbon, aliphatic chain alkane, alkene
and phenols [17–19]. The content of phenols is about
20–30 wt %, while 2,4-xylenol and 2,5-xylenol
account for about 13–15% of the total phenol content
(for the tar produced in Shaanxi Province, China) [14,
20, 21]. The annual production of coal tar in this
region is estimated to be over 3 million tonnes, thus
regarded as a promising alternative source of 2,4-xyle-
nol and 2,5-xylenol [15, 22].
Xylenol (or hydroxyxylene) generally refers to any
of
the
six
isomers
of
dimethylphenol
[(CH3)2C6H3OH] or to combinations thereof.
Xylenols are naturally-occurring phenolic com-
pounds, which are present in various sources such as
coal, petroleum and tobacco [1, 2]. Individual xylenol
isomers may be produced through separation, and/or
synthetic processes. When extracted from coal tar, the
products are typically in the form of mixtures of more
than one isomers, which normally need further sepa-
ration if pure xylenol isomers are required [3, 4].
Owing to their unique reactivity and solvency proper-
ties, xylenols are regarded as key raw materials in many
products manufacturing, e.g. resins and polymers,
antioxidants in fuels and plastics, and reactive solvents
in applying insulation to electrical wires/cables [5, 6].
Overall, the technological development trend with
time in the production of xylenols has followed that in
the general chemical industry, showing heavy depen-
dence on sources of raw materials [2, 7]. The interest
in using coal tar as raw materials for xylenols produc-
tion dates back to the 1930s [8–10], that was largely fine chemical products, the market demand for single
replaced by the natural gas- and oil-based resources component 2,4-xylenol and 2,5-xylenol has also been
for chemical synthesis of xylenols [11–13]. However, growing steadily especially in the Chinese manufac-
with the growing challenges faced by the petroleum- ture sector, while the price is rising. It was estimated in
With the rapid increase of demand for downstream
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1292
CONG-YU KE et al.
2019 that the market price of mixed 2,4/2,5-xylenol ture raised slowly to the specified temperature,
was 2200 USD/ton, while single component 2,4-xyle- 2.5 wt % catalyst PTSA was added in after the solid
nol was 6100 USD/ton, and 2,5-xylenol was phase of 2,4/2,5-xylenol mixture turned to liquid.
8400 USD/ton [23]. Therefore, the development of Then gas isobutylene was introduced into the reaction
deep separation technology of 2,4/2,5-mixed xylenol vessel at the bottom where the gas bubbles were broken
in coal tar can not only meet the market demand, but up by agitation. Both flow rates of the inlet and outlet
also increase the added value of coal tar and bring huge tail gases were measured, respectively, by two gas flow
benefits economically [20–22]. However, no indus- meters (50–500 mL/min, Ningbo Dongchi Measure-
trial process is currently available for the production of ment and Control Technology Co., Ltd). When no
their individual isomers through separation, while measureable difference between and inlet and outlet
they are mostly produced and sold in the form of gas flow rates was observed, the reaction was stopped.
2,4/2,5-xylenol mixtures.
During the reaction, samples were taken at regular
intervals and analyzed immediately by gas chromatog-
raphy. All experiments were repeated in triplicate, and
the average values are reported.
The key challenge in 2,4/2,5-xylenol separation is
associated with their very close boiling points at 211.0
and 211.2°C, respectively, under normal pressure.
That makes the conventional distillation economically
unviable. To address this issue, the three-step process
Products analysis. The alkylation products were
analyzed by gas chromatography (GC6890, Agilent)
with an FID detector, and a column (KB-CRESOL,
50 m × 0.25 mm × 0.2 μm) supplied by Beijing Keri-
mai Technology Co., Ltd. The GC operated at a
detector temperature of 250°C, and column box tem-
perature of 140°C. The temperature rising procedure
included; first to keep column temperature at 60°C for
3 min, then to raise temperature to 100°C at 5°C/min
rate, and finally to continue to raise temperature to
180°C at 10°C /min for 3 min. The split ratio was 50 :
1, while the carrier gas flow rate was set at 1 mL/min,
the airflow rate at 400 mL/min, and the hydrogen
flow rate at 50 mL/min. Standard control and nor-
malization methods were used for qualitative and
quantitative analysis.
of alkylation-distillation-dealkylation provides
a
promising route to separation, based on the extensive
studies on the separation of close-boiling cresols [3, 4,
24–29]. Furthermore, even without the last step of
dealkylation, the alkylation product itself, e.g. 6-tert-
butyl-2,4-xylenol (or TBX) has been widely used as an
antioxidant agent [30, 31].
With an ultimate goal to provide fundamental
understanding and data for industrial design and pro-
cess development, the present work was focused on
the kinetic and mechanistic studies on alkylation of
xylenols, the first step of the process for close-boiling
xylenols separation. Isobutylene was selected as
alkylating agent which has been widely used for the
alkylation of cresols [32–34]. p-Toluenesulfonic acid
was used as catalyst mainly due to its capacity for high
yield and good selectivity [35–37]. The reaction rate,
the product yield and selectivity were investigated by
varying the reactant concentration, partial pressure,
catalyst amount and gas-liquid contact time. Based on
the experimental results, a reaction kinetic model was
established and the reaction orders were obtained with
rate equations. Furthermore, with the rate constants
determined at different temperatures, the activation
energies and pre-exponential factors were calculated.
RESULTS AND DISCUSSION
Reaction mechanism of alkylation of 2,4/2,5-xyle-
nol mixture. The alkylation of dimethylphenol with
isobutylene is an electrophilic substitution reaction on
benzene ring, a classical Friedel-Crafts reaction. In a
generally understood mechanism, the alkylation
reagent isobutylene produces alkylcarbocation
[+C(CH3)3] under the action of acid catalyst PTSA,
which attacks benzene ring as electrophilic reagent to
replace the hydrogen above, and then deprotons to
generate alkylaromatics [38, 39]. Theoretically, for
2,4/2,5-xylenol, the hydroxyl group is an either ortho-
or para-orientation group, and the alkylation products
may include 6-tert-butyl-2,4-xylenol, 5-tert-butyl-
2,4-xylenol and 3-tert-butyl-2,4-xylenol from 2,4-
xylenol, whilst 4-tert-butyl-2,5-xylenol, 6-tert-butyl-
2,5-xylenol and 3-tert-butyl-2,5-xylenol from 2,5-
xylenol, as illustrated in Fig. 1. As expected, the main
alkylation products were 6-tert-butyl-2,4-xylenol and
4-tert-butyl-2,5-xylenol, with high selectivities of
98.75% and 93.91%, respectively. The GC-MS analy-
sis results are summarized in Table 1, which are in line
with the previous studies [38, 39].
EXPERIMENTAL
Chemicals. 2,4-Xylenol and 2,5-xylenol (Analyti-
cal grade) were obtained from Juye Runjia Chemical
Co., Ltd) and isobutene (Analytical grade) was sup-
plied by Shaanxi Baotashan New Material Co., Ltd.
p-Toluenesulfonic acid, or PTSA (Analytical grade)
was purchased from Nanjing Datang Chemical Co.,
Ltd. All chemicals were used as received without any
further purification.
Experimental methods. Alkylation of 2,4/2,5-xyle-
nol mixture. 50.0 g 2,4-Xylenol and 50.0 g 2,5-xylenol
mixture were weighed into 250 mL 3-neck round-bot-
tom glass flask that was placed in a water bath. With
Effect of reaction time on alkylation of 2,4/2,5-
the mechanical agitator set at 800 rpm, and tempera- xylenol mixture. The alkylation took place with 100.0 g
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CONG-YU KE et al.
Table 1. Selectivity of alkylation products
xylenol at different reaction times. The production of
6-tert-butyl-2,4-xylenol increased rapidly within the
first 120 min approximately in linear, and then at a
slower rate of increase before reaching a yield of 97.3%
when the reaction time was 360 min.
Materials
Alkylation products
Selectivity, %
98.75
1.25
6-tert-butyl-2,4-xylenol
5-tert-butyl-2,4-xylenol
4-tert-butyl-2,5-xylenol
6-tert-butyl-2,5-xylenol
Overall, the yield of 4-tert-butyl-2,5-xylenol was
lower (15–30%) than that of 6-tert-butyl-2,4-xylenol
at any given time (Fig. 2b). The yield also increased
rapidly before reaching a maximum of 82.3% at reac-
tion time 240 min, which was followed by a slight
decline. Further experiments confirmed the partial
decomposition of 4-tert-butyl-2,5-xylenol (to 2,5-
xylenol and isobutene) at higher reaction temperature
(above 65°C) in the presence of catalyst PTSA (data
not shown). This suggested the reverse decomposition
reaction to be largely accountable for the slight reduc-
tion in the product yield after 240 min.
Kinetic model development for the alkylation of
2,4/2,5-xylenol. In view of providing a chemical
kinetic model for reactor design and process develop-
ment, it is essential to quantify the influence of various
operating factors, leading to the establishment of
mathematical description of the course of the reaction
for each reaction step as a function of components in
the system [40]. The key factors include reactant con-
centration, reaction time, reaction temperature, and
catalyst amount used.
2,4-Xylenol
93.91
6.09
2,5-Xylenol
of 2,4/2,5-xylenol mixture (1 : 1 wt) and 2.5 wt % of
catalyst PTSA, at a stirring speed of 800 rpm, tem-
perature 65°C, under atmospheric pressure, with an
isobutylene flow rate of 100 mL/min, the composition
and content of products were analyzed with GC at dif-
ferent reaction stages. The reaction percentage yield
(%) was calculated by dividing the amount of the
product actually produced by the theoretical amount
of product expected. The yields (%) of both products
and byproduct as a function of reaction time are plot-
ted in Fig. 2.
As can be seen in Fig. 2a, the total yield of the target
products of 6-tert-butyl-2,4-xylenol and 4-tert-butyl-
2,5-xylenol began to rise with the increase of reaction
time. At reaction time about 240 min, the total yield
reached a maximum of 89.5%, which was followed by
a slight decline when the reaction continued. It was
also observed that the yield of the byproduct increased
with time in an approximately linear way, up to 10%
when the reaction was stooped after 360 min. It was
found that the byproduct consisted of mainly light
components through isobutylene oligomerization,
including dimers and trimers [33].
According to the reaction scheme (Fig. 1), the
reaction rates for the alkylation of 2,4/2,5-xylenol can
be generally expressed by Eqs. (1) and (2).
dc2,4
a
b
c1
1
1
(1)
r2,4 = −
= k1c2,4 pPIBcPTSA
,
dt
dc2,5
dt
r2,5 = −
= k2c2a,25 pPb2IBcPc2TSA
,
(2)
where r2,4 and r2,5 represent the reaction rates of 2,4-
xylenol and 2,5-xylenol, respectively. k1 and k2 are
their reaction rate constants. c2,4 and c2,5 represent the
Figure 2b compares the yields of two target prod- molar concentrations of 2,4-xylenol and 2,5-xylenol,
ucts of 6-tert-butyl-2,4-xylenol and 4-tert-butyl-2,5- respectively. pPIB and cPTSA are the partial pressure of
(a)
100
80
60
40
20
(b)
100
80
60
40
20
6-2,4-/4-2,5-xylenol
Light components
6-Tert-butyl-2,4-xylenol
4-Tert-butyl-2,5-xylenol
80 120 160 200 240 280 320 360
80 120 160 200 240 280 320 360
0
40
0
40
T, min
T, min
Fig. 2. Yields of (a) product mixture and byproduct (light components), and (b) two individual products as a function of time.
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CHEMICAL KINETICS OF THE ALKYLATION
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(a)
lnr2,5
–4.0
(b)
lnr2,4
–2.0
y = 1.1453x – 4.1712
R2 = 0.9993
y = 0.9978x – 5.0963
R2 = 0.9928
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
5.5
–4.5
–5.0
–5.5
–6.0
–6.5
7.0
–1.0
–0.5
0
0.5
1.0
lnc2,5
–0.5
0
0.5
1.0
1.5 –1.5
–1.0
lnc2,4
Fig. 3. Dependences of concentration of 2,4-xylenol (a) and 2,5-xylenol (b) on alkylation reaction rate. Experimental conditions:
2,4/2,5-xylenol concentration, 2.73–8.20 mol/L; catalyst PTSA concentration, 0.145 mol/L; stirring speed, 800 rpm; tempera-
ture, 65°C; isobutylene flow rate, 100 mL/min with partial pressure 101.32 kPa; reaction time, 30 min.
isobutylene and the molar concentrations of catalyst
PTSA, respectively. a1, b1 and c1 represent the reaction
orders of 2,4-xylenol, isobutylene and PTSA during
the alkylation of 2,4-xylenol, and a2, b2 and c2 , the
reaction orders of 2,5-xylenol, isobutylene and PTSA,
respectively.
In the reaction system of 2,4/2,5-xylenol alkyla-
tion, there were four main components at the start, i.e.
2,4-xylenol, 2,5-xylenol, isobutylene and PTSA. In
order to determine the reaction order for a specific
reactant, the concentrations of other components
were set to be greatly excessive, reasonably regarded as
a constant. Therefore, the reaction order of the tested
component was obtained through the c ~ t relation-
ship.
dc2,4
dt
dc2,5
dt
a
1
(3)
(4)
(5)
r2,4 = −
r2,5 = −
= k1c2,4,
= k2c2a,25,
dc



2,4 

lnr2,4 = ln
lnr2,5 = ln
= a lnc2,4 + lnk1,
1
dt

dc
2,5 

(6)
= a2lnc2,5 + lnk2.



dt

The initial reaction rates, obtained for five different
xylenol concentrations, are plotted against xylenol
concentration on a log-log scale (Fig. 3). By best-fit-
ting straight lines to the experimental data, the reac-
tion orders (a1 and a2) and the reaction rate constants
(k1 and k2) were determined.
As can be seen in Fig. 3, with increase in the con-
centration of 2,4-xylenol and 2,5-xylenol, the reaction
rate increased correspondingly with a good linear rela-
tionship. With the equation of the line of best fit, the
reaction orders for 2,4-xylenol (a1) and 2,5-xylenol
(a2) were found to be 1.1 and 1.0, respectively.
Determination of alkylation reaction order for 2,4-
xylenol and 2,5-xylenol. To determine the reaction
order, the reaction initial rate is normally measured in
order to eliminate the interference of the product.
Based on the relationship between the yield and reac-
tion time for the alkylation of 2,4/2,5-xylenol (Fig. 2),
at the early stage of the reaction, the side reactions
were insignificant along with the higher rate of alkyla-
tion main reaction, whilst the yield changed appropri-
ately in linear with time. Therefore, the initial reaction
Determination of reaction order for isobutylene. Sim-
time of 30 min was selected for characterizing reaction ilarly, the effect of partial pressure (p/p0) of isobuty-
kinetics.
For the alkylation of 2,4/2,5-xylenol mixture,
lene was quantified by diluting isobutylene with nitro-
gen, with excessive amounts of 2,4/2,5-xylenol mix-
ture and catalyst. The corresponding reaction rates,
measured at a temperature of 65°C, are plotted against
partial pressure on a log-log scale, as illustrated in
Fig. 4.
10 times more isobutylene and catalyst were added,
respectively, in the reaction vessel. The start concen-
tration of xylenol was adjusted by adding nitroben-
zene, and the reaction rates of 2,4-xylenol and 2,5-
xylenol were determined at different start concentra-
Both reaction rates of 2,4-xylenol and 2,5-xylenol
tions. Since the concentration of isobutylene and had a good linear relationship with the change of
PTSA can be considered unchanged during the pro- isobutylene partial pressure on a log-log scale. The
cess, the reaction rate Eqs. (1) and (2) can be simpli- reaction rate increased with the rising of isobutylene
fied as Eqs. (3) and (4), and then Eqs. (5) and (6).
partial pressure. Using the equation for a straight line
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CONG-YU KE et al.
lnr2,4
–3.0
(a)
lnr2,5
–4.6
(b)
y = 0.9624x – 3.0863
R2 = 0.9988
y = –0.7591x – 4.8475
R2 = 0.9761
–3.3
–3.6
–3.9
4.2
–4.9
–5.2
–5.5
5.8
–0.9
–0.6
–0.3
0
–0.9
–0.6
–0.3
0
–1.2
–1.2
lnp/p0
lnp/p0
Fig. 4. Dependence of partial pressure of 2,4-xylenol (a) and 2,5-xylenol (b) on alkylation reaction rate. Experimental condi-
tions: 2,4-/2,5-xylenol concentration, 8.20 mol/L; isobutylene flow rate, 100 mL/min with partial pressure form 25.43 kPa to
101.32 kPa; other conditions, same as those shown in Fig. 3.
lnr2,4
(a)
lnr2,5
(b)
–5.2
–5.3
–5.4
–5.5
–5.6
5.7
–6.3
y = 0.4163x – 4.6013
R2 = 0.9939
y = 0.6513x – 5.3472
R2 = 0.9932
–6.5
–6.7
–6.9
7.1
–2.3
–2.1
–1.9
–1.7
–1.5
–2.3
–2.1
–1.9
–1.7
–1.5
–2.5
–2.5
ln[PTSA]
ln[PTSA]
Fig. 5. Dependence of the alkylation reaction rate of 2,4-xylenol (a) and 2,5-xylenol (b) from the amount of catalyst. Experi-
mental conditions: catalyst PTSA concentration, 0.087–0.203 mol/L; concentration of 2,4-/2,5-xylenol, 8.20 mol/L; other con-
ditions, same as those shown in Fig. 3.
of best fit, it was found that for the alkylation of 2,4- tration of catalyst PTSA, as expected. From the equa-
xylenol, the reaction order of isobutylene was b1 = 1.0,
while for the alkylation of 2,5-xylenol, the reaction
order of isobutylene was b2 = 0.8.
tion for a straight line of best fit, it was determined
that, for the alkylation of 2,4-xylenol and 2,5-xylenol,
the reaction orders for catalyst PTSA were c1 = 0.4 and
c2 = 0.7, respectively.
Determination of reaction order for catalyst. The
reaction rates of 2,4-xylenol and 2,5-xylenol were also
determined by varying the amount of catalyst PTSA,
with excessive amounts of 2,4/2,5-xylenol mixture
and isobutylene at a reaction temperature of 65oC
within 30 min of reaction time. The relationships of
lnr2,4 with ln(PTSA), and lnr2,5 with ln(PTSA) are
depicted in Fig. 5.
It should be noted that the reaction order n of ele-
mentary reactions and simple reactions may be integer
one, two and three (though three applies to only a few
reactions), while the reaction order n of complex reac-
tions is generally non-integer. The reaction order
obtained in the present study is not an integer, indicat-
ing complex reactions occurring.
It can be seen from Fig. 5 that, within the range of
Estimation of activation energy and Arrhenius con-
catalyst amount examined, the alkylation rate of
2,4/2,5-xylenol increased with the increasing concen- stant A. The temperature (T) dependence of the reac-
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lnk2,4
–3.8
(a)
(b)
lnk2,5
–4.7
y = –4.7342x + 9.6414
R2 = 0.9994
y = –3.8779x + 6.2470
R2 = 0.9994
–4.0
–4.2
–4.4
–4.6
4.8
–4.9
–5.1
–5.3
5.5
2.85
2.90
2.95
3.00
3.05
2.90
2.95
3.00
3.05
2.80
2.85
1/T, 10–3 K–1
1/T, 10–3 K–1
Fig. 6. Dependence of the reaction rate constants of the alkylation of 2,4-xylenol (a) and 2,5-xylenol (b) on temperature in Arrhe-
nius coordinates. Experimental conditions: concentration of 2,4/2,5-xylenol, 8.20 mol/L; temperature, 60–75°C; other condi-
tions, same as those shown in Fig. 3.
tion rate constants k is defined according to the Arrhe- tion energy for the alkylation of both 2,4-xylenol and
nius approach given in Eq. (7).
2,5-xylenol, demonstrating the effectiveness of the
catalyst used for these two reactions, generally by pro-
viding an alternative pathway with lower activation
energy requirements [39, 41].
−Ea
k = AeRT
,
(7)
where the kinetic parameters A and Ea describe the
pre-exponential factor (collision factor) and the acti-
vation energy of the reaction, respectively, and R is the
ideal gas constant. Then logarithm of both sides of
Eq. (7) is expressed by Eq. (8).
Kinetic equations of the alkylation of 2,4/2,5-xyle-
nol. As analyzed above, in the alkylation process of
2,4-xylenol and 2,5-xylenol, the reaction orders of
2,4-xylenol, isobutylene and PTSA were 1.1, 1.0 and
0.4, respectively, with a total reaction order of 2.5; the
activation energy was 39.36 kJ/mol, and the Arrhenius
constant, 15389 mol–0.5 L0.5 min–1. For the alkylation
of 2,5-xylenol, the reaction orders of 2,5-xylenol,
isobutylene, and PTSA were 1.0, 0.8 and 0.7, respec-
tively, with a total reaction order of 2.5, and the activa-
tion energy was 32.24 kJ/mol, and the Arrhenius con-
stant, 516 mol–0.7 L0.7 min–1. By inserting these values
into the reaction rate Eqs. (1) and (2), the kinetic
Eqs. (9) and (10) for the alkylation of 2,4/2,5-xylenol
can be obtained.
−Eα
RT
(8)
lnk =
+ lnA.
By measuring the reaction rate constants of 2,4-
xylenol and 2,5-xylenol at different temperatures, and
then plotting lnk against 1/T, the values of Ea and A
were obtained from the equation for a straight line of
best fit, as illustrated in Fig. 6.
Figure 6 shows a good linear relationship was
observed between lnk and 1/T in the temperature
range investigated for the alkylation of both 2,4-xyle-
nol and 2,5-xylenol. From these plots and the best-fit-
ting equations, the activation energy Ea1 and Arrhenius
constant A1 for 2,4-xylenol alkylation were found to be
r2,4 = k1c P1.0c0.4
1.1
2,4 PIB PTSA
(9)
= 1538e−39360 RT c1.1P1.0
c
,
0.4
2,4 PIB PTSA
39.36 kJ/mol and 15389 mol–0.5 L0.5 min–1, respec-
tively, while the activation energy Ea1 and Arrhenius
constant A2 for 2,5-xylenol alkylation were
r2,5 = k2c P0.8c0.7
1.0
2,5 PIB PTSA
(10)
= 516e−32241 RT c1.0P0.8
c
.
0.7
32.24 kJ/mol and 516 mol–0.7 L0.7 min–1, respectively.
To the best of our knowledge, there is no data reported
in the literature on the activation energy for the xylenol
alkylation reactions. For the alkylation of p-cresol
with isobutylene catalyzed by sulfated zirconia the
activation energy was found to be 95.73 kJ/mol [39],
indicating the comparison of the kinetic data obtained
2,5 PIB PTSA
It is understood that the rate constant k is indepen-
dent of concentration, whose value directly reflects
the reaction rate. For example, at 65°C, the rate con-
stants k1 = 1.2 × 10–2 mol–0.5 L0.5 min–1, and k2 = 5.4 ×
10–3 mol–0.7 L0.7 min–1, thus k1/k2 = 2.22, indicating
in this study. The results indicated a very low activa- that the reaction rate of 2,4-xylenol alkylation was
PETROLEUM CHEMISTRY Vol. 60 No. 11 2020

1298
CONG-YU KE et al.
FUNDING
2.2 times that of 2,5-xylenol. This was in good agree-
ment with the experimental observation (Fig. 2).
This work was supported by the National Natural Sci-
ence Foundation of China (21676215), Key Research and
Development Program of Shaanxi Province (2018ZDXM-
GY-159), Postgraduate Innovation and Practical Ability
Training Plan of Xi’an Shiyou University (YCS18211013),
and Key Scientific Research Plan of the Shaanxi Provincial
Department of Education (20JY057).
As can be seen from Eqs. (9) and (10), the level of
the activation energy also indicated the influence of
temperature on reaction rate. Compared to 2,5-xyle-
nol, 2,4-xylenol had a higher activation energy (39.36
kJ/mol vs 32.24 kJ/mol), suggesting that the reaction
of 2,4-xylenol was more “sensitive” to temperature
changes than that of 2,5-xylenol [38, 39]. It was in line
with the experimental observation; the increase in
temperature was more favorable for the alkylation of
2,4-xylenol, though both reaction rates increased with
rising temperature.
CONFLICTS OF INTEREST
There are no conflicts to declare.
SUPPLEMENTARY MATERIALS
As also seen from reaction rate Eqs. (9) and (10),
the higher the reaction order, the greater the influence
Supplementary materials are available for this article at
of concentration on the reaction rate [42]. Therefore, doi: 10.1134/S0965544120110031 and are accessible for
the influence of xylenol concentration and isobuty- authorized users.
lene partial pressure on the alkylation rate of 2,4-xyle-
nol was greater than that of 2,5-xylenol (1.1 vs 1.0 for
xylenol concentration, and 1.0 vs 0.8 for isobutylene
ADDITIONAL INFORMATION
partial pressure). As a result, the overall reaction rate
and final yield of the 2,4-xylenol reaction was notably
higher than that of 2,5-xylenol, though the effect of
catalyst amount underwent an opposite way which,
however, appeared to be insignificant.
Cong-Yu Ke, ORCID: 0000-0002-9625-7024
Wu-Juan Sun, ORCID: 0000-0002-6465-2468
Qun-Zheng Zhang, ORCID: 0000-0001-5901-654X
Xun-Li Zhang, ORCID: 0000-0002-0174-2159
For the alkylation of 2,5-xylenol, although the
yield can be improved by optimizing the reactant ratio
and reaction temperature, from the perspective of
industrial production, the rate of chemical reaction
can also be accelerated by optimizing the selection of
catalyst, so as to improve the yield in a certain period
of time.
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