Effect of relative percentage of acid and base sites on the side-chain alkylation of toluene with methanol

Article abstract of DOI:10.1039/d1ra00263e

K₃PO₄/NaX catalysts were prepared by loading potassium phosphate on NaX zeolite, and the catalytic performance was studied for the side-chain alkylation of toluene with methanol to styrene and ethylbenzene. Combined with the characte

Full text of DOI:10.1039/d1ra00263e

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Effect of relative percentage of acid and base sites
on the side-chain alkylation of toluene with
methanol
Cite this: RSC Adv., 2021, 11, 12703
b
Huijun Li,†a Bin Wang, †a Yueli Wen,
Chunyao Hao,a Yuhua Liua
*
ac
*
and Wei Huang
K3PO4/NaX catalysts were prepared by loading potassium phosphate on NaX zeolite, and the catalytic performance
was studied for the side-chain alkylation of toluene with methanol to styrene and ethylbenzene. Combined with the
characterization of XRD, TPD-NH3, TPD-CO2, and FTIR and the determination of phosphorus content, it is revealed
that firstly, the P element loaded on the catalyst surface can prominently promote the percentage of middle base sites,
which will accordingly enhance the selectivity of the products (styrene and ethylbenzene) of side-chain alkylation of
toluene; secondly, if there are enough middle base sites distributed on the catalysts, the higher percentage of strong
base sites benefit the selectivity of styrene, which is confirmed by the performance of the two-stage catalysts.
Received 12th January 2021
Accepted 19th March 2021
DOI: 10.1039/d1ra00263e
rsc.li/rsc-advances
For the SCAT reaction, it is worth noting that both the base
and acid sites are necessary. The base sites are in charge of
1 Introduction
absorbing and stabilizing toluene, while the acid sites are
responsible for activating the C–H bonds on the side chain of
toluene and dehydrogenation of methanol to formaldehyde.5,6
Otherwise, in this process, the formation of xylene by dispro-
portionation of toluene and the benzene-ring alkylation of
toluene will occur on acid sites.7 In addition, strong basic sites
are required for the abstraction of H+ from toluene. However,
very strong basic sites would promote the formation of form-
aldehyde from methanol, and formaldehyde would further
decompose into carbon monoxide and hydrogen.8 Further-
more weak basic sites could only promote the dehydrogena-
tion of methanol to formaldehyde without abstracting H+ from
toluene. So it is very meaningful to discover the optimal acid–
base property and amount of acid and base sites for side-chain
alkylation of toluene with methanol.
In 1967, Sidorenko et al. rst reported that side-chain alkylation
of toluene with methanol could form styrene and ethylbenzene
on an alkali-metal modied X-zeolite catalyst,1 and thereaer
researchers around the world followed this discovery. Recently,
extensive efforts have been made on studying the reaction
mechanism of the side-chain alkylation of toluene. It is well
known that side-chain alkylation of toluene with methanol
(SCAT) is an acid–base synergistic catalysis process.2–4 During the
toluene side-chain alkylation process, the main reaction
networks occur as follows:
In the study of catalysts for this reaction, most of the
research has concentrated on the modication of various
zeolites with alkali metals, alkaline earth metals and other
promoters to adjust the acidity and alkalinity of the catalysts.
Quite a lot of types of catalysts have been investigated for SCAT
so far, like X zeolite catalysts modied with different compos-
ites (such as alkali earth metal,9,10 rare earth metal,11 transition
metal,12–15 boron16,17 and phosphorus18–20), metal oxide cata-
lysts,21,22 layered double hydroxides (LDHs),23,24 N-doped cata-
lysts25,26 and other alkaline catalysts, in which X zeolite27–29
modied with alkaline metal showed the superior catalytic
performance. Our group has prepared K3PO4/CsX catalysts and
used for SCAT, and it was found that the moderate addition of
K3PO4 could decrease the amount of weak acid sites and
increase the amount of middle base sites. Herein, in order to
discover how the acid/base sites affect the catalytic performance
aKey Laboratory of Coal Science and Technology of Ministry of Education and Shanxi
Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China. E-mail:
huangwei@tyut.edu.cn
bCollege of Environmental Science and Engineering, Taiyuan University of Technology,
Taiyuan, Shanxi 030024, China. E-mail: wenyueli@tyut.edu.cn
cCoal Conversion Technology
& Engineering Co., Ltd., Taiyuan University of
Technology, Taiyuan, 030024, Shanxi, China
† Huijun Li and Bin Wang are co-rst authors who have equally contributed to
this work.
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for SCAT, a series of K3PO4/NaX catalysts were prepared and the ultraviolet-visible (UV-vis) spectrophotometer. Fourier trans-
effect of the relative percentage of acid/base sites on this cata- form infrared spectroscopy (FTIR) were recorded on a Bio-Rad
lytic reaction were investigated in this work.
FTS-60A spectrometer with the KBr wafer technique.
2 Experimental
2.4 Catalysis reaction and products analysis
2.1 Materials
The evaluation of catalysis activity for side-chain alkylation of
toluene with methanol was carried out at atmospheric pressure
in a stainless steel xed-bed reactor. In order to prevent
blockage happening in the reactor, the catalysts were granu-
lated to 40–60 mesh. The catalyst sample (0.9 g) was rstly
All reagents including methanol, methylbenzene, potassium
hydroxide, potassium phosphate, anhydrous ethanol were all of
analytical grade (purchased from Tianjin Kemiou Chemical
Reagent Co. Ltd of China) and were used without further puri-
cation. NaX zeolite (Si/Al ¼ 1.25) were purchased from Tianjin
Nankai University. Distilled water was used in all experiments
without further treatment.
ꢀ
activated at 450 C for 2 h under a ow of N2 and then cooled
ꢀ
down to 425 C, the desired reaction temperature. The liquid
mixture of toluene and methanol with a molar ratio of 5 : 1 was
pumped into the reactor using a peristaltic pump at a rate of
0.6 mL hꢁ1, the vaporized reactants were diluted by owing
nitrogen stream (3 mL minꢁ1). Aer the catalysts reached steady
state, the reaction products were analyzed by an on-line gas
chromatograph (GC950, China Shanghai HaiXin Chromato-
graphic Instruments Co. Ltd) with a ame ionization detector
(FID) using an FFAP capillary column (0.53 mm ꢂ 50 m).
The conversion of methanol, selectivity and yield of the
products (SX and YX) were dened in the following equations:
2.2 Catalyst preparation
2.2.1 Preparation of pro-catalyst. A series of catalysts were
prepared by a repeated impregnation–washing method. The
specic process was described as follows: 10 g NaX zeolite was
ion-exchanged in different concentrations of K3PO4 solution
(solid/liquid ratio, 10 g/100 mL) three times at 80 ꢀC for 2 h.
Aer separated from the slurry by using Buchner funnel and
washed twice with a 0.01 mol Lꢁ1 solution of K3PO4, the ob-
ꢀ
ꢁ
methanol outlet
methanol inlet
ꢀ
tained solid was dried at 80 C overnight, and then calcined at
CMeth
¼
1 ꢁ
ꢂ 100%
ꢀ
500 C for 3 h. Finally, the collected white powder, as the pro-
catalyst, was named as Cat-n, n is the concentration of K3PO4
solution.
ꢀ
ꢁ
X
2.2.2 Preparation of co-catalyst. 10 g NaX zeolite was
modied with KOH solutions of different concentrations by
incipient wetness impregnation. The loading amount of
potassium hydroxide was controlled by the concentration of
potassium hydroxide aqueous solution. The obtained solid were
dried at 80 ꢀC overnight, and then calcined at 500 ꢀC for 3 h. The
co-catalysts were named as m%KOH/NaX, m represented the
weight percentage of KOH.
SX
¼
ꢂ 100%
methanol inlet ꢁ methanol outlet
YX ¼ SX ꢂ CMeth
In order to discover how the strong base sites affect the side-
chain alkylation of toluene with methanol to styrene, the co-
catalyst was placed in front of pro-catalyst, separated by
quartz wool.
3 Results and discussion
3.1 Catalyst characterization
3.1.1 XRD analysis. The XRD patterns of different catalysts
are exhibited in Fig. 1. All the samples showed sharp XRD peaks
at 6.2ꢀ, 23.4ꢀ, 26.7ꢀ and 31.0ꢀ, which has almost the same
2.3 Catalyst characterization
The powder X-ray diffraction (XRD) patterns were collected on diffraction pattern with NaX without ion-exchange (Cat-0) and
a Rigaku D/Max-2500 (l ¼ 0.1542 nm) diffractometer with a Ni- can be attributed to the characteristic peaks of faujasite zeolite
ltered Cu Ka radiation source operating at 40 kV and 100 mA. structure. In addition, the characteristic diffraction peaks of the
The angle 2q were recorded from 5ꢀ to 85ꢀ with a scanning rate zeolite aer ion-exchange are weak overall as compared to the
of 8 minꢁ1. Temperature-programmed desorption (TPD) was NaX (Cat-0). A possible reason is that part of the zeolite struc-
carried out using CO2 or NH3 as probe molecules. The proce- ture was slightly damaged during the ion-exchange process,
dures were as follows: 100 mg of fresh catalyst was preprocessed which has been discovered by many researchers. It is worth
at 450 ꢀC under He stream for 30 min and then cooled down to mentioning that the diffraction peak intensity at 31.8ꢀ, which
50 ꢀC. CO2 or NH3 was infused into the stream at 50 ꢀC for can be assigned to K3PO4 phase (PDF no. 20-0921), increases
30 min. The physically adsorbed CO2 or NH3 was removed by He when the concentration of K3PO4 aqueous solution is more than
stream at 50 ꢀC for 1 h. Then the catalyst was heated from 50 ꢀC 0.25 mol Lꢁ1. This phenomenon may be due to the surface
to 850 ꢀC in He stream at a heating rate of 10 ꢀC minꢁ1, and the deposition of K3PO4 with well-structured crystal on the NaX
desorbed gas was detected by a thermal conductivity detector at zeolite when aqueous solution concentration is relatively large.
the same time. Determination of P elements content in the When the concentration of K3PO4 solution is less than
catalysts was based on Chinese Standard GB175-1999 by using 0.25 mol Lꢁ1, the K3PO4 species can be highly dispersed and
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Fig. 1 XRD patterns of the catalysts NaX (Cat-0), Cat-0.1, Cat-0.25,
Cat-0.5, Cat-0.75 and Cat-1.
interact well with NaX zeolite, in which K3PO4 doesn't exhibit
good crystal structure.
3.1.2 FTIR analysis. In order to discover the functional
group and the compositions that presence in Cat-n catalysts, the
Fourier transform infrared spectroscopy (FTIR) was used. As
shown in Fig. 2, the strong absorption peaks at 3468 cmꢁ1 and
1645 cmꢁ1 can be observed, which are attributed to the O–H
bending vibrations of absorbed water. The absorption peaks at
968 cmꢁ1, 741 cmꢁ1 and 670 cmꢁ1 are assigned to the asym-
metric and symmetric stretching vibrations of tetrahedrons in
NaX zeolite structure. Among them, the main peak appearing at
968 cmꢁ1 belongs to the asymmetric telescopic vibration peak,
and the other two absorption peaks at 741 cmꢁ1 and 670 cmꢁ1
are weaker, which correspond to the symmetrical telescopic
vibration peak. The spectra of the catalysts are almost the same
as the characteristic absorption spectra of NaX zeolite.30 The
stretching vibration peak of P]O was found at 1371 cmꢁ1 in the
catalyst obtained by ion exchange with the K3PO4 solution
compared to NaX. When the concentration of K3PO4 solution is
larger than 0.1 mol Lꢁ1, a weak absorption peak appears at
Fig. 3 Profiles of NH3-TPD for the catalysts Cat-0, Cat-0.1, Cat-0.25,
Cat-0.5, Cat-0.75 and Cat-1.
1081 cmꢁ1, which belongs to the P–O vibration of K3PO4. This
indicated that when K3PO4 with higher concentrations were
used as an exchange solution, some of the PO43ꢁ ions could be
loaded on the catalyst.
3.1.3 TPD analysis. The NH3-TPD curves of the catalysts are
showed in Fig. 3. All the samples have two desorption peaks at
the range of 50–200 ꢀC and 200–500 ꢀC, which correspond to the
weak and middle acid sites respectively. Furthermore, the areas
of desorption peaks were integrated, the relative percentage of
the weak and middle acid sites were calculated by the integral
areas shown in Table 1. It is found that the presence of K3PO4
makes the percentage of middle acid sites decreased obviously
on the catalysts, but the concentration of K3PO4 solution has
less effect on the distribution of different types of acid sites on
the catalyst.
The CO2-TPD curves of Cat-n catalysts aer the ion-
exchanged are shown in Fig. 4. It can be seen that all the
samples have three isolated desorption peaks zones: the rst
ꢀ
one in the range of 50–250 C belongs to weak base sites, the
second one in 250–500 ꢀC can be attributed to middle base sites,
Table 1 Distribution of different types of acid sites on the catalyst
Middle/all
Catalyst
Weak/all (%)
(%)
Cat-0
67
97
95
95
93
97
33
3
5
5
7
Cat-0.1
Cat-0.25
Cat-0.5
Cat-0.75
Cat-1
Fig. 2 FTIR spectra of the catalysts NaX (Cat-0), Cat-0.1, Cat-0.25,
Cat-0.5, Cat-0.75 and Cat-1.
3
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the catalyst is enhanced by increasing the concentration of
K3PO4 solution. The peak area of TPD curve is a function of the
amount of acid/base sites.31 By integrating the peak areas of
different types of base sites distributed in the three regions
(weak base, middle base and strong base) in each catalyst
respectively, and calculating the percentage of the amount of
weak, middle or strong base sites in total base sites, the nal
results are summarized in Table 2. It is found that when the
concentration of the K3PO4 solution were 0.1 mol Lꢁ1 and
0.25 mol Lꢁ1, the middle base sites and strong base sites
account for the largest proportion in total base sites respec-
tively. With the concentration of K3PO4 solution increasing, the
percentage of the amount of the middle base sites decreased.
That might because the lower concentration of K3PO4 solution
could combine better with NaX to improve alkalinity and alkali
content of the catalyst signicantly, which is consistent with the
XRD analysis.
3.1.4 Determination of phosphorus content. The content
of phosphorus in all the catalysts was determined by Water
Quality – Determination of Phosphorus – Ammonium Molyb-
date Spectrometric Method (ISO 6878-2004) as shown in Table
3. By comparing all the catalysts, it is found that when the
concentration of K3PO4 solution was 0.1 mol Lꢁ1, the phos-
phorus content of in the catalyst was signicantly higher than
the others, which suggests that the lower K3PO4 concentration
Fig. 4 Profiles of CO2-TPD for catalysts Cat-0, Cat-0.1, Cat-0.25,
Cat-0.5, Cat-0.75 and Cat-1.
Table 2 Distribution of different types of base sites on the catalyst
Middle/all
(%)
Strong/all
(%)
3ꢁ
environment is benecial to the diffusion of PO4 in NaX.
Catalyst
Weak/all
Cat-0
21
22
21
33
32
32
15
62
25
19
13
12
64
16
54
48
55
56
Cat-0.1
Cat-0.25
Cat-0.5
Cat-0.75
Cat-1
3.2 Catalytic reaction
A series of catalysts Cat-n were evaluated on x-bed reactor for
the side-chain alkylation of toluene with methanol. The cata-
lytic reaction results were shown in Table 4, from which it could
be seen that ethylbenzene and styrene are the main products,
and formaldehyde is an intermediate product, accompanied by
some by-products such as xylene and methane. The selectivities
and yields of the main products are signicantly improved over
the catalysts loaded with K3PO4 via ion-exchange method. When
the concentration of K3PO4 solution is 0.1 mol Lꢁ1, the selec-
tivity of ethylbenzene reaches the highest value (82.9%); in
addition, when the K3PO4 solution concentration is
0.25 mol Lꢁ1, the selectivity of styrene is the highest (18.1%).
But when the K3PO4 solution concentration is more than
0.5 mol Lꢁ1, the selectivity and yield of the main products
(ethylbenzene and styrene) begin to decrease, the selectivity of
xylene and methane start to increase at the same time.
Table 3 Phosphorus content in Cat-n catalysts
Catalyst
Cat-0.1
14.82
Cat-0.25
10.87
Cat-0.5
10.26
Cat-0.75
10.30
Cat-1
10.22
CP (mg Lꢁ1
)
and the last one located around 500–700 ꢀC can be attributed to
strong base sites. With the increase of the concentration of
K3PO4 solution, the middle base sites shi toward higher
temperature, which indicates that the middle base strength of
Table 4 Catalytic performance for the side-chain alkylation of toluene with methanol over Cat-n series of catalysts
Selectivity of products (%) Yield of products (%)
Catalyst
MET conv%
SEB
SSTY
SCH
SXY
SHCHO
YEB
YSTY
Y(EB+STY)
4
Cat-0
99.9
99.9
99.9
99.8
99.7
99.8
0.0
0.0
8.0
18.1
6.3
5.7
10.4
16.9
5.4
8.2
25.2
20.3
19.2
83.1
3.7
3.2
9.3
11.9
19.4
0.0
0.0
0.0
5.9
5.0
9.5
0.0
0.0
8.0
18.1
6.3
5.7
10.4
0.0
90.8
88.5
59.6
62.6
51.8
Cat-0.1
Cat-0.25
Cat-0.5
Cat-0.75
Cat-1
82.9
70.4
53.4
57.1
41.4
82.8
70.4
53.3
56.9
41.4
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In order to gure out why the selectivities of ethylbenzene anion. Therefore it indicates that the middle base is a key
and styrene were much different for the same series of catalysts, requirement for the SCAT reaction. Benzyl anion will be
the correlation analysis between the distribution of the acid and consumed along two pathways. Part of benzyl anions will
base sites and the catalytic performance was carried out as combine with methanol (raw material) to form ethylbenzene,
shown in Fig. 5. Herein three relative percentages of acid and the other part of benzyl anions will bond with formaldehyde
base sites are introduced to depict the formation probability of (derived from methanol on strong base sites) to form styrene.
phenyl cation, benzyl anion and formaldehyde. From Fig. 5a The more strong base sites appear on the surface of catalyst, the
and b, the selectivity of (ethylbenzene + styrene) shows the same more formaldehyde will be produced, the relative percentage of
tendency with relative percentage of (middle base–middle acid). the strong base can be used to describe the formation proba-
In contrast, the relative percentage of (middle acid–middle bility of formaldehyde. In brief, the distribution of [(middle
base) exhibits the same tendency with the selectivity of base–middle acid) ꢂ strong base] shows the same tendency
(methane + xylene). Furthermore, the percentage of [(middle
base–middle acid) ꢂ strong base] has the same tendency with
the selectivity of styrene as shown in Fig. 5c.
According to the above results, the mechanism for SCAT
reaction was proposed as shown in Fig. 6. Toluene and meth-
anol are sensitive to the base and acid property of catalyst. A
pair of competitive activations of toluene take place on middle
acid and middle base sites respectively, that is, toluene converts
to phenyl cation (activation of the benzene ring of toluene) over
middle acid sites while it tends to become benzyl anion (acti-
vation of the side chain of toluene) over middle base sites. The
more middle base sites and the less middle acid sites present on
the surface of the catalyst, the more benzyl anion will be
formed. Thus the relative percentage of (middle base–middle
acid) can be used to depict the formation probability of benzyl
Fig. 6 The proposed reaction mechanism for SCAT reaction on the
acid and base sites.
Fig. 5 Correlation analysis between relative percentages of acid and base sites and catalytic activities for Cat-n series of catalysts. (a) (middle
acid–middle base)% vs. selectivity of (CH4+XY); (b) (middle base–middle acid)% vs. selectivity of (EB+STY); (c) (middle acid– middle base)* strong
base % ) vs. selectivity of STY).
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Table 5 Catalytic performances of Cat-0.1 and two-stage catalysts for the SCAT reaction
Selectivity of products (%)
Yield of products (%)
Catalyst
MET conv%
SEB
SSTY
SCH
SXY
3.7
19.0
11.5
SHCHO
YEB
YSTY
Y(EB+STY)
4
Cat-0.1
3%KOH/Cat-0.1
5%KOH/Cat-0.1
99.9
99.7
99.9
82.9
60.0
45.3
8.0
17.8
42.4
5.4
3.1
0.9
0.0
0.0
0.0
82.8
59.9
45.2
8.0
17.8
42.4
90.8
77.6
87.6
with the styrene selectivity changing for the Cat-n series of
catalysts as shown in Fig. 5c.
References
In order to verify the strong base sites that could promote the
side-chain alkylation of toluene with methanol to form styrene,
a series of co-catalysts m%KOH/NaX were prepared and placed
in front of Cat-0.1, where there are abundant middle base sites,
and separated by quartz wool. Under the same catalytic reaction
condition the catalytic performance for the SCAT reaction are
showed in Table 5. Compared to Cat-0.1, the selectivity of
styrene is enhanced signicantly aer introducing co-catalysts
in. When the co-catalyst is replaced by 5 wt% KOH/Cat-0.1,
the selectivity of styrene reaches 42.4%, which suggests that
the sufficient middle and strong base sites are the key factors
for the SCAT reaction to styrene.
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potassium phosphate solution can obviously improve the cata-
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the SCAT reaction. Toluene and methanol are very sensitive to
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Acknowledgements
The authors are grateful to the nancial support from the
National Key Technology Research & Development Program
(Grant No. 2013BAC14B04), the National Natural Science
Foundation of China (Grant No. 21336006), the Shanxi Province
Key Research & Development Program (International Coopera-
tion, Grant No. 201803D421099), Research Project supported by
Shanxi Scholarship Council of China (Grant No. 2017-035).
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© 2021 The Author(s). Published by the Royal Society of Chemistry
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