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ditions are usually fulfilled over ZSM-5 zeolite, particularly after
surface modification.
amorphous silica-alumina and H-ZSM-5 in the gas phase and inves-
tigated the effect of pore mouth narrowing. They concluded that
paring reaction played a decisive role during the conversion over
Kareem et al. [10] reported on toluene disproportionation over
modified ZSM-5 zeolite ion-exchanged with Ni, Cr, Mg, Bi and Zn
in a down-flow continuous fixed-bed reactor. The results indicate
an increased toluene conversion, as well as a higher p-xylene yield
beyond the equilibrium predicted level on Ni-exchanged catalyst.
On the other hand, Mg- and Cr-ZSM-5 provided the lowest yields.
Coke particles are responsible for pore-mouth reduction allowing
smaller isomers to escape easily from the channel system. However,
the effect of coke on selectivity is still rather controversial [11,12].
Formation of mesopores in MCM-22 zeolite by desilication resulted
also in increased toluene conversion [13].
m-Xylene isomerization (I) and disproportionation (D) have
been used for characterization of acidic zeolites [14–17]. m-Xylene
isomerizes to the para and ortho isomers and can disproportion-
ate into trimethylbenzenes (TMBs) and toluene as illustrated in
Fig. 1B. Both isomerization and disproportionation reactions are
catalyzed by Brønsted acid sites. As the disproportionation is a
bimolecular reaction, it requires a higher concentration (density)
of acid sites [18]. While interconnected 12-ring channels allow the
formation of bimolecular transitions states, 10-ring zeolites prefer
monomolecular mechanism from steric reasons [14]. Larger values
of the para/ortho (P/O) ratio of the products correspond to medium-
pore ZSM-5. In zeolites with 12-rings, Martens et al. [19] showed
that zeolites with adjacent cages favor the formation of the bulky
◦
H-ZSM-5 at elevated temperature (450 C). Over both catalysts, iso-
merization of 1,2,4-TMB to 1,2,3- and 1,3,5-TMBs proceeded with
about 90% selectivity at low conversion. The 1,2-methyl-shift took
place on the external surface of the ZSM-5 zeolite crystals. The
TeMBs reinforced the diffusional resistances by pore mouth nar-
rowing and favored the paring reaction inside the zeolite crystals.
Atias et al. [30] developed a heterogeneous kinetic model for the
conversion of 1,2,4-trimethylbenzene under relevant FCC operat-
ing conditions for USY zeolite catalysts having different crystallite
sizes (0.4 and 0.9 m). The determined intrinsic kinetic parameters
for both isomerization and disproportionation reactions showed
that 1,2,4-TMB underwent both isomerization and disproportion-
ation. Molecular mechanistic study showed a slow formation of
1,3,5-TMB in mordenite, although its diffusion in mordenite 12-
ˇ
ring channels is possible [31]. Cejka et al. [32] studied the effect of
the structure of large pore zeolites on the activity, selectivity and
time-on-stream (T-O-S) in trimethyl benzene disproportionation.
Zeolites Y and beta (providing optimum reaction space) exhibited
higher conversions of TMBs and high selectivity to xylenes and
◦
TeMBs at 400 C when compared with mordenite and zeolites L.
They also measured the diffusion coefficients of 1,2,4- and 1,3,5-
◦
TMBs and 1,2,3,5-TeMBs at 25 and 100 C over zeolites Y, beta/1
and mordenite. The diffusion coefficients followed the decreasing
order of m-xylene ꢀ 1,2,4-TMB > 1,3,5-TMB ≈ TeMB. Further on, it
was shown that TMB disproportionation plays a dominant role in
transalkylation of TMB with toluene [33–35].
1
,3,5-trimethylbenzene isomer while zeolites with straight chan-
nels and side pockets at regular distances, such as mordenite, are
favorable for the formation of the 1,2,3-trimethylbenzene isomer.
Since disproportionation necessarily requires a bimolecular reac-
tion, larger pore systems being able to accommodate the required
transition state provide higher rates of disproportionation [20]. In
contrast, xylene isomerization can also proceed via monomolecu-
lar mechanism as evidenced by elegant catalytic experiments using
a mixture of deuterated and undeuterated p-xylene by Corma and
Sastre [16]. Later on, Mirth et al. confirmed the monomolecular
mechanism by combining in situ FTIR studies with GC analysis [17].
Iliyas and Al-Khattaf [21] carried out a systematic study on the
influence of reaction conditions (temperature, time, and reactant
type) on the selectivity of xylene transformation over USY zeo-
lite. Initial product selectivity revealed that both isomerization and
disproportionation of xylenes are primary reactions. Higher con-
version was observed with p-xylene reactant as compared with
m- and o-xylene most probably due to easier desorption of the
smallest xylene isomer to the channel system. Furthermore, a
comprehensive kinetic model for xylenes isomerization and dispro-
portionation has been reported by Iliyas and Al-Khattaf [22]. With
respect to that, accessibility and acidity of active sites play the crit-
ical role in transformations of aromatics catalyzed by zeolites [23].
Shape selective conversion of xylenes over ZSM-5 has been
investigated in detail by several workers as reviewed in Ref.
Although abundant literature has been published on alkyl ben-
zenes transformation, in most instances, fixed-bed reactor was
utilized for the reaction. Thus, the present study is aimed at inves-
tigating alkyl benzenes transformation (toluene, m-xylene, and
1,2,4-trimethyl benzene) over medium-pore (ZSM-5 or TNU-9)
and large-pore (mordenite, SSZ-33) zeolite based catalysts in a
fluidized-bed reactor. TUN is a novel high-silica zeolite with a
monoclinic unit cell. Its framework contains two distinct straight
10-ring channels, the size of which is slightly larger than of
ZSM-5, which makes it of a particular interest (0.52 × 0.60 and
0.51 × 0.55 nm). TUN crystallizes under hydrothermal conditions
in a rather narrow range of Si/Al and NaOH/SiO2 ratios. The
study focuses on the effect of catalyst structure and reaction
conditions (time, temperature, and conversion) on the ratios of
disproportionation to isomerization products (D/I), distribution of
trimethylbenzene (TMB) isomers and values of p-xylene/o-xylene
(P/O) ratios. SSZ-33 zeolite is novel large-pore zeolite with 12-12-
10 ring channel system and its acidic and catalytic properties were
recently described in Refs. [36,37].
2. Experimental procedure
[
5]. ZSM-5 zeolite is shape selective catalyst due to the geom-
2.1. Materials
etry and architecture of its channel systems. Modification of
ZSM-5 using various organic and inorganic compounds enhanced
p-xylene selectivity in xylene isomerization process. Similarly,
para-selectivity was improved by the selective coking of the exter-
nal surface of ZSM-5 [24–26].
Trimethylbenzene transformations (Fig. 1C) have been inves-
tigated over medium-pore zeolites like ZSM-5 and large-pore
zeolites such as Y, Beta, and also mordenite. Collins et al. [27] inves-
tigated isomerization and disproportionation of trimethylbenzenes
The mordenite zeolites with Si/Al ratios 9.5 (mordenite/A) and
100 (mordenite/B) were obtained from Tosoh Company, Japan. The
ZSM-5 zeolite was obtained from Catal UK. The TNU-9 was syn-
thesized using 1,4-methylpyrrolidinium bromide as a template.
In a typical synthesis, the final gel contained distilled water, 1,4-
MPB, Al(NO ) ·9H O (p.a., Lach-Ner), NaOH (98%, Lach-Ner) and
3
3
2
fumed silica (Aldrich) with the molar gel composition 4.5(1,4-
MPB)–11Na O–0.6Al O –30SiO ·–1200H O. The gel was stirred
2
2
3
2
2
(
TMBs) over LaY catalysts. The authors reported that disproportion-
ation appeared to be linearly related to the total conversion for both
,2,4- and 1,3,5-TMB, whereas for 1,2,3-TMB, isomerization was
at room temperature for 1 day and then transferred to Teflon-
◦
lined 90 ml autoclaves. The synthesis proceeded at 160 C for 12
1
days under agitation and autogenous pressure. The SSZ-33 zeolite
was synthesized according to the patent [38] using the template
molecule derived from 8-keto tricyclo[5.2.1.0]decane. Details of the
much more favored than disproportionation. Roger et al. [28,29]
studied the conversion of 1,2,4-trimethylbenzene (1,2,4-TMB) over
Al-Khattaf
