Alkylation of Toluene with tert-Butyl Alcohol over Different Zeolites with the Same Si/Al Ratio

Article abstract of DOI:10.1134/S1070427220070071

Three zeolites (Beta, Mordenite and ZSM-5), with different pore channels but the same Si/Al ratio (25), were applied as catalysts to evaluate the effects of acidity and channel structure on the catalytic activity for toluene butylation at 180°C in an automated high-pressure stainless steel reactor. The same Si/Al ratio of the three zeolites resulted to different catalytic activity, since both acidity and channel structures of zeolites influenced catalytic activity. Zeolite Beta possessing a three-dimensional, 12-ring channel system, the smallest crystal size and larger amount of B acid sites demonstrated the highest toluene conversion (54percent). Mordenite, with 32.7percent toluene conversion, exhibited higher para-selectivity than Beta, which could be explained by the shape-selective catalysis. ZSM-5 with the largest amount of B acid sites presented the lowest catalytic activity, since alkylation can occur only on the surface active sites.

Full text of DOI:10.1134/S1070427220070071

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2020, Vol. 93, No. 7, pp. 991–997. © Pleiades Publishing, Ltd., 2020.
CATALYSIS
Alkylation of Toluene with tert-Butyl Alcohol over Different
Zeolites with the Same Si/Al Ratio
Yuanyuan Wanga,*, Hua Songa, Yixian Hana, Xinglong Sunb,
Jiaojing Zhanga, Wenyi Wanga, and Xueqin Wanga
a Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering,
Northeast Petroleum University, Daqing, Heilongjing, 163318 China
b Daqing Petrochemical Engineering Co., Ltd., Daqing, Heilongjing, 163714 China
*e-mail: wangyuanyuan2016@126.com
Received December 27, 2019; revised April 13, 2020; accepted May 18, 2020
Abstract—Three zeolites (Beta, Mordenite and ZSM-5), which present different pore channels but the same Si/Al
ratio (25), were used as catalysts to investigate the effects of acidity and channel structure on the catalytic activity for
toluene butylation at 180°C in an automated high-pressure stainless steel reactor. The same Si/Al ratio of the three
zeolites did not result in a similar catalytic activity, since both acidity and channel structures of zeolites can affect
catalytic activity. Zeolite Beta possessing a 3D 12-ring channel system, the smallest crystal size and larger amount of
B acid sites exhibited the highest toluene conversion (54.0%). Mordenite, with 32.7% toluene conversion, presented
higher para-selectivity than Beta, which can be explained by the shape-selective catalysis. ZSM-5 with the largest
amount of B acid sites showed the lowest catalytic activity, since alkylation can occur only on the surface active sites.
Keywords: toluene, alkylation, same Si/Al ratio, shape-selective catalysis, reaction mechanism
DOI: 10.1134/S1070427220070071
INTRODUCTION
selectivity and environment friendliness in catalytic reac-
tions, owing to their regular micropore structures and ad-
justable acidity [11, 12]. Because of the defined micropore
structures of zeolites, the reactants can be selected to enter
the micropores acting as real molecular dimension, which
is called “shape-selective catalysis” [13, 14]. Moreover,
the composition and acidic properties of zeolites can be
controlled, which will benefit carbocation-mediated reac-
tions. Although zeolites are widely used as catalysts in
petrochemical and refining processes [15], there is still
a keen interest in understanding the effects of channel
structures and acidity on the catalytic activity and selec-
tivity of zeolites. It is known that the Si/Al ratio of zeolite
affects the density and strength of the acid. However, the
existing studies focus on the different catalytic activities
of the same zeolite with varying Si/Al ratios, but ignore
the different catalytic activities of different zeolites with
the same Si/Al ratio.
Tert-Butyl toluene (TBT) is an important monomer
used in the petrochemical and chemical industries [1].
In particular, p-TBT can be used to prepare p-tert-butyl
benzaldehyde and p-tert-butyl benzoic acid, which can be
used into synthesis of resins, fragrances, agrochemicals,
and pharmaceuticals [2]. The existing methods for
synthesis of p-TBT include (i) alkylation of isobutene
with toluene catalyzed by H2SO4 or Al2O3 + SiO2 + Ni;
(ii) alkylation of t-butyl alcohol (TBA) with toluene
catalyzed by H2SO4, TeCl4, BF3-polyphosphoric acid or
HF/FSO3H; (iii) alkylation of 2-chloro-2-methylpropane
with toluene catalyzed by anhydrous AlCl3, Mo(CO)6
or NaCl + AlCl3. However, these catalysts have many
shortcomings (e.g., environmental pollution [3–5],
corrosion of equipment, difficulty of recovery [6–8]) and
thus, many attempts have been devoted to developing
environmental-friendly zeolite catalysts [9, 10].
The pore systems differ among different zeolites. For
instance, HBEA is a large-pore zeolite with a complex
Zeolites have been widely used in the petrochemical
industry and the refining processes and demonstrate high
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YUANYUAN WANG et al.
Fig. 1. The skeleton structures of zeolites.
structure, in which two mutually perpendicular straight
channels with 12-T rings intersect with a sinusoidal
channel to form large internal cavities (Fig. 1a) [16].
HMOR is also a large-pore zeolite and contains two
types of one-dimensional channels, one with 12-T rings
and the other with 8-T rings (Fig. 1b). HZSM-5 is a
medium-pore zeolite with a 10-T ring that leads to oval
pores (Fig. 1c). ZSM-5 contains straight and sinusoidal
channels with cross-sections around 5.1–5.6 Å. The
structural characteristics of the zeolites [17, 18] under
study are listed in Table 1.
X-ray diffractometer. Scanning electron microscopy
(SEM) images were obtained on a field-emission scanning
electron microscopy (Zeiss-ΣIGMA). The specific
surface area was based on the BET analysis method using
Quantachrome micromeritics adsorption equipment of
NOVA2000e. Samples acidity was measured by NH3
temperature-programmed desorption (NH3-TPD) using a
Quantachrome Chembet-3000 Characterization System.
IR spectra with pyridine adsorption (Py-IR) of samples
were recorded using a Bruker FT-IR spectrometer
(Tensor 27) together with a high-temperature vacuum cell.
In this study, three zeolites with different porous
structures but the same Si/Al ratio (25), including BEA,
MOR, and ZSM-5, were investigated in the alkylation
of toluene with TBA at 180 °C using an automated high-
pressure stainless steel reactor. The aim was to study
the effects of pore structure and acid properties on the
catalytic activity and para-selectivity. In addition, the
mechanisms about toluene alkylation with TBAand about
the isomerization of p-TBT to m-TBT were investigated.
Catalytic reaction. The liquid-phase alkylation of tolu-
ene with tert-butyl alcohol (TBA) was carried out in a 300
mLautoclave reactor at 180°C.Atypical batch consisted of
100 mmol of toluene, 300 mmol of TBA, 1.0 g of catalyst
(1.30 wt %), and 60 mL of cyclohexane (used as solvent).
The liquid phase mixture was analyzed on the GC-14C gas
chromatograph equipped with a FID detector and SE-30
capillary column (50 m × 0.25 mm × 0.25 μm).
RESULTS AND DISCUSSION
EXPERIMENTAL
Materials. The HBEA, HMOR, and HZSM-5 (Si/
Al = 25) zeolites were purchased from Zeolyst Int.,
China. Toluene, tert-butyl alcohol, and cyclohexane (all
analytical grade) were bought from ACROS Organics
and used without further purification.
Characterization of the catalysts. HBEA, HMOR,
and HZSM-5 with the same Si/Al ratio of 25 were selected
to diminish the effects of framework compositions
(Table 1). Results show the same Si/Al ratio of three
zeolites does not result in a similar catalytic activity, since
channel structures and acidity can both affect toluene
conversion and p-TBT selectivity of zeolites [19].
Catalyst characterization. Powder X-ray diffraction
(XRD) patterns were carried out on a D/max-2200PC-
Table 1. List of zeolites under study and their structural characteristics
Property
IZA code
Beta
BEA
Mordenite
MOR
HZSM-5
MFI
Si/Al ratio
25.0
25.0
25.0
Channel dimensions, Å
Pore topology
6.6 × 6.7 (12), 5.6 × 5.6 (12)
3D, 12-T rings
6.5 × 7.0 (12), 2.6 × 5.7 (8)
2D, 12- and 8-T rings
5.4 × 5.6 (10), 5.5 × 5.1 (10)
3D, 10-T rings
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ALKYLATION OF TOLUENE WITH tert-BUTYL ALCOHOL
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500 nm
500 nm
500 nm
Fig. 2. SEM images of zeolite samples under investigation.
Fig. 3. XRD patterns of zeolite samples under investigation.
The SEM images of zeolite catalysts indicate a rather
homogeneous distribution of shapes and particle size
(Fig. 2). HBEA presents its small cubic morphology and
particle size in the range of 0.2–0.3 μm. HMOR has a
prism-like structure, and the overall size is about 0.3 ×
4 μm. HZSM-5 displays large cubic crystals.
different. HBEA possesses the largest specific surface
area (495 m2/g), pore volume (0.48 cm3/g) and pore size
(3.90 nm), which are much larger compared with HMOR
and HZSM-5. HBEAhas a large specific surface area, and
its external surface makes a large contribution (366 m2/g),
which is due to its much smaller particle size (Fig. 2) [24].
As for other samples, HMOR larger a slightly higher
Brunauer–Emmett–Teller (BET) surface area and pore
volume compared to HZSM-5.
XRD patterns of HBEA, HMOR and HZSM-5 are
provided in Fig. 3. The XRD patterns clearly evidence
the high phase purity and crystallinity of the three studied
samples and are consistent with some reported data
[20, 21], without additional crystalline phases. These
results well agree with the adsorption data (Fig. 3),
proving the high micropore volumes of all samples.
The NH3-temperature-programmed desorption (TPD)
profiles of the different zeolites are presented in Fig. 5.
HBEA and HZSM-5 both exhibit two desorption peaks
around 250 and 450°C, which are related to the weak
acid sites and the strong acid sites respectively. On the
The N2 adsorption-desorption isotherms of the zeo-
lites are depicted in Fig. 4. All samples show type-I of
IUPAC isotherms with H4 adsorption hysteresis loops.
The presence of a hysteresis loop at higher P/P0 (close
to 1) evidences the simultaneous existence of micropores
and mesopores [22]. HBEA has the largest nitrogen ad-
sorption since its larger void volume, and the increase
in nitrogen adsorption at higher P/P0 can be attributed
to the small particles composed of mesopores in its ag-
gregates [23].
P/P0
Table 2 summarizes the textural properties of all
zeolites. Though the Si to Al ratio is 25 in all these
three zeolites, their textural properties are largely
Fig. 4. The N2 adsorption–desorption isotherms of zeolite
samples.
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YUANYUAN WANG et al.
Table 2. The structural parameters of zeolite samples
Si/Al BET surface Micropore External sur- Pore vol- Pore size, B acidc,
Zeolites
L acidd,
μmol/g
47.2
12.7
3.6
Total acid,
μmol/g
131.4
44.9
ratio
25
areaa, m2/g area, m2/g face area, m2/g umeb, cm3/g
nm
μmol/g
84.2
HBEA
495
339
301
366
280
246
126
59
0.48
0.22
0.20
3.90
2.61
2.72
HMOR
25
32.2
HZSM-5
25
55
100.2
103.8
a
Specific surface area calculated by the BET method. b Total pore volume determined at P/P0. c Deduced from the intensity of the band
located at around 1540 cm–1. d Deduced from the intensity of the band located at around 1450 cm–1.
contrary, HMOR displays only the desorption peak at
low temperature.Among the three zeolites, HBEAshows
the highest acidity in ammonia desorption, and the order
of total acidity is HMOR < HZSM-5 < HBEA. The
difference in acidity may be attributed to the different
structural characteristics of these zeolites [25].
HBEA possessing the smallest particle size, a 3D 12-
ring channel system, and the highest amount of acidity
exhibits the highest catalytic activity, followed by HMOR
and HZSM-5 (Table 3). Toluene butylation is reportedly
an acid-catalyzed process and the acidic protons are
recognized as the activities. The high toluene conversion
over HBEA is logical given its high amount of B acid
sites. However, the order of toluene conversion does not
match with the amount of B acid sites (HMOR < HBEA<
HZSM-5). Despite the highest amount of B acid sites,
HZSM-5 only has a 4.5% toluene conversion, which
confirms that acidity alone is a non-sufficient factor for
catalytic activity.
Figure 6 displays the Py-IR spectra of the three
zeolites with the same Si/Al ratio at 150°C. Clearly, the
absorption peaks at 1450 and 1540 cm–1 are attributed to
L acid and B acid respectively, while the absorption peak
at 1490 cm–1 is ascribed to both B and L acids. Despite
the same Si/Al ratio, the total acidity differs among the
three zeolites and ranks as HMOR < HZSM-5 < HBEA,
which well agrees with the results of NH3-TPD. The order
of B acidity is HMOR < HBEA < HZSM-5, which can
be explained by the framework- and extra-framework-
aluminum species [18].
The channel structures and types of individual zeolite
channels also play important roles in the catalytic activity.
The large-pore HBEA and HMOR both show higher
toluene conversion (54.0 and 32.7%, respectively)
than the medium-pore HZSM-5 (4.5%). The shortest
dimensions of 10-ring portals in HZSM-5 (5.1 or 5.3 Å)
are fairly smaller than those of the analogous portals
in HBEA and HMOR (6.6 and 6.5 Å) (Table 1). Thus,
the diffusive toluene molecules with a kinetic diameter
of 5.9 Å cannot enter and react in the internal pores of
HZSM-5. Though HZSM-5 has the highest amount of B
acid sites, alkylation only occurs on the surface active
sites as a consequence of its low toluene conversion.
HMOR with 10- and 8-ring intersecting channels is
Catalytic activity. The results of toluene alkylation
with TBA over HBEA, HMOR and HZSM-5 are shown
in Table 3. The order of toluene conversion at 180°C under
the studied reaction conditions is HBEA(54.0%) > HMOR
(32.7%) > HZSM-5 (4.5%). The results show no straight
forward relationships between toluene conversion or acid-
ity of zeolites and pore sizes or channel connectivity.
The alkylation of toluene with TBA provides insight
into the roles of acidity, structure, and type of zeolites.
Fig. 5. the NH3-TPD pattern of zeolite samples.
Fig. 6. Py-IR pattern of zeolite samples.
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ALKYLATION OF TOLUENE WITH tert-BUTYL ALCOHOL
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The mechanism for toluene tert-butylation.
Toluene alkylation with TBA catalyzed by zeolites is
an electrophilic substitution on the aromatic ring and
is commonly considered to proceed via the carbenium
ion mechanism (Scheme 1). Under a typical reaction
condition, toluene mainly undergoes not only alkylation
but also isomerization. Because most of the acid sites of
zeolites are on the internal surface, the alkylation reaction
mainly occurs on the internal surface of zeolites. On the
contrary, the isomerization reaction runs more easily on
the external surface, because of less sterical hindrance.
Hence, we use two different mechanisms to describe the
reaction processes over the internal and external surfaces.
considered as a 1D material. Since the alkylation of
toluene over HMOR occurs in elliptical straight channels
of 12-rings pore entrances (6.5 × 7.0 Å), the diagonal
side pockets of 8-ring pore entrances (2.6 × 5.7 Å) are
not concerned in the catalysis because toluene (5.9 Å)
cannot diffuse through them. HBEA has 12 × 12 × 12
pore entrances, two mutually perpendicular straight
channels (each with a cross-section of 6.7 × 6.6 Å) and
one sinusoidal channel of 5.6 × 5.6 Å. Toluene can easily
enter and react in the internal pores of HBEA. Hence,
the special channel structure of HBEA contributes to the
higher toluene conversion than the other two zeolites.
HMOR shows the best para-selectivity (near 80%;
Table 2). HBEA is not as selective as HMOR and the
para-selectivity is only around 70%. The selectivity to
p-TBT over HBEA decreases due to two factors: higher
acidity and larger external surface. Both the acidic
strength and acid amount of HBEAare much higher than
those of HMOR, and this higher acidity may accelerate
the isomerization of p-TBT that is formed in the first
instance as a result of kinetic considerations. The particle
size of HBEAis about 0.2 × 0.2 μm, while that of MOR is
0.3 × 4 μm. HBEAhas a much larger external surface than
HMOR (Table 2), and the larger external surface leads to
an increase in the isomerization rate of p-TBT to m-TBT.
The TBA molecules are strongly adsorbed over
the internal acidic active sites of zeolites through the
formation of an O(alcohol)–H(zeolite) hydrogen bond. After
adsorption, the C–O bond in the adsorbed TBAis cleaved
to form a tert-butyl carbenium ion by releasing a water
molecule [26]. The tert-butyl carbenium ion can react
either with toluene to form p-TBT and m-TBT or with
deprotonate to form isobutene. The former route can occur
by three steps: (1) toluene is attacked by the tert-butyl
carbenium ion, (2) toluene moleculars are adsorbed on
zeolites via σ–π interaction between the aromatic ring
electrons of toluene and the tert-butyl carbenium ion
(p-TBT transition state: int-p-TBT; m-TBT transition
state: int-m-TBT), and (3) p-TBT and m-TBT are finally
formed by deprotonation of transition states, and the acid
site of HBEA is regenerated too.
In all cases, toluene alkylation with TBA leads to
two monoalkylated products, including the main product
para-isomer (p-TBT) and the minor product m-TBT
(meta-isomer). O-TBT (ortho-isomer) was not detected in
the reaction products. That p-TBT is the main product is
logical, since the para-position is favored by the influence
of steric hindrance from the methyl group and tert-butyl
group. Though m-TBT with larger diameter (0.65 nm)
has a lower diffusion rate than p-TBT (0.58 nm), p-TBT
will be gradually transformed to m-TBT, since m-TBT is
the most thermodynamically favored. This isomerization
runs easily on the external surface of zeolites, where there
is no sterical hinderance as in the pores of the zeolites.
In addition, a trace amount of 3,5-di-tert-butyltoluene
(3,5-DTBT) was also detected.
Since m-TBT is the most thermodynamically favored
isomer, p-TBT will gradually transform to m-TBT. The
isomerization consists of five steps: (1) adsorption of
p-TBT molecules onto the acidic active sites of zeolites
through the formation of an πphenyl ring–Hzeolite bond,
(2) formation of π nature intermediate (int-p-TBT) by
protonation of adsorbed p-TBT, (3) isomerization of this
linear int-p-TBT into the branched π nature intermediate
(int-m-TBT), (4) decomposition of int-m-TBT into
adsorbed m-TBT, and (5) desorption of adsorbed m-TBT
to form m-TBT.
Table 3. Alkylation of toluene with TBA over various zeolites with same Si/Al ratioa
Distribution of products, %
Catalyst
Toluene conversion, %
p-TBT selectivity, %
p-TBT
37.6
26.0
3.2
m-TBT
14.5
6.1
others
1.9
HBEA
54.0
32.7
4.5
69.5
79.7
71.1
HMOR
0.6
HZSM-5
1.1
0.2
a Reaction conditions : catalyst 1.0 g, n(TBA) : n(toluene) = 3 : 1, temperature 180°C , time 4 h, cyclohexane 60 mL.
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Scheme 1. The mechanism for alkylation of toluene with TBA.
CONCLUSIONS
12-ring channel system, and the higher amount of B acid
sites exhibited the highest toluene conversion (54.0%).
HMOR, a 1D 12 MR zeolite (6.5 × 7.0 Å), showed high
toluene conversion (32.7%) with the highest p-TBT
selectivity (79.7%). HZSM-5 with the highest amount
of B acid sites showed the lowest catalytic activity since
the alkylation can only react on the surface active sites.
In addition, the mechanisms of alkylation of toluene
with TBA over internal active sites of zeolites and the
p-TBT isomerization over external active sites on the
surface were investigated.
Alkylation of toluene with tert-butyl alcohol (TBA)
was studied over zeolites HBEA, HMOR and HZSM-5
(with the same Si/Al ratio=25) to investigate the effects
of acidity and channel structure on the catalytic activity.
XRD, SEM, N2 adsorption-desorption, TPD-NH3 and
Py-IR were used to characterize these catalysts. The
same Si/Al ratio among the three zeolites did not result
in the similar toluene conversion, since both acidity and
channel structures can affect the catalytic activity of
zeolites. HBEApossessing the smallest crystal size, a 3D
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ACKNOWLEDGMENTS
https://doi.org/10.1016/j.catcom.2019.01.006
12. Naranov, E.R., Sadovnikov, A.A., Maximov, A.L., and
Karakhanov, E.A., Micropor. Mesopor. Mat., 2018,
vol. 263. p. 150.
We are also grateful for the measurement assistants from
Analysis & Testing Center of Northeast Petroleum University.
FUNDING
https://doi.org/10.1016/j.micromeso.2017.12.021
This work was supported by the Natural Science Foundation
of Heilongjiang Province of China (QC2017005), the Youth
Fund of Northeast Petroleum University (2018QNL-26), the
Fund of Characteristic & Preponderant Discipline of Chemical
Engineering &Technology of Northeast Petroleum University
(15041260273), the Fund of Scientific Research Foundation for
the Introduction of Talent of Northeast Petroleum University
(1305021821).
13. Portilla, M.T., Llopis, F.J., Martinez, C., Valencia, S., and
Corma, A., Appl. Catal. A-Gen., 2011, vol. 393, p. 257.
https://doi.org/10.1016/j.apcata.2010.12.009
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https://doi.org/10.1039/D0QI00012D
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Tang, Q H., Catal. Commun., 2019, vol.119, p. 96.
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CONFLICT OF INTEREST
16. Wang, Y.Y., Song, H., and Sun, X.L., RSC Adv., 2016,
vol. 6, p. 107239.
The authors declared no conflict of interest.
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