Increase in selectivity of toluene disproportionation and in p-xylene yield by optimization of zeolite microporous structure

Article abstract of DOI:10.1007/s11167-005-0621-4

The possibility of increasing the yield of target products and para-selectivity of toluene disproportionation on modified zeolites by optimization of their microporous structure was studied. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11167-005-0621-4

Russian Journal of Applied Chemistry, Vol. 78, No. 11, 2005, pp. 1854 1858. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 11,
2005, pp. 1886 1890.
Original Russian Text Copyright
2005 by Agaeva, Dadashev, Tagiev, Abasov.
CATALYSIS
Increase in Selectivity of Toluene Disproportionation
and in p-Xylene Yield by Optimization
of Zeolite Microporous Structure
S. B. Agaeva, B. A. Dadashev, D. B. Tagiev, and S. I. Abasov
Mamedaliev Institute of Petrochemical Processes, National Academy of Sciences of Azerbaijan,
Baku, Azerbaijan
Received September 20, 2004
Abstract The possibility of increasing the yield of target products and para-selectivity of toluene dispropor-
tionation on modified zeolites by optimization of their microporous structure was studied.
Benzene and xylenes (especially p-xylene) are
widely used in industry. The main method of the syn-
thesis of the above compounds is alkene dehydrocy-
clization. In the process, benzene (BZ) and xylenes
(XLs) are formed together with large amounts of
toluene (TL). Disproportionation of TL allows an in-
crease in the production of BZ and XLs.
for the catalyst synthesis. The samples were syn-
thesized by decationization of the initial zeolites by
ion exchange with 2 M NH Cl followed by calcina-
4
tions at 823 K and dealumination with 2.5 M HCl [7].
After these procedures, the SiO /Al O ratio for M
2
2 3
was determined by the method suggested in [8]. Then,
the zeolites obtained were modified with a rare-earth
element (REE) (Gd) by the ion-exchange procedure
(0.5 mol % Gd O ) and with a transition metal (Co)
The TL disproportionation (TD) is an acid-type
reaction and is catalyzed by H forms of zeolites [1, 2].
A study of TD on Y and M type zeolites and on pen-
tasils showed that TD proceeds with the highest effi-
ciency but with low selectivity on HY and on cata-
lysts based on Y, with rapid deactivation of the cata-
lysts. The catalysts based on mordenite are less active
but more selective and stable than Y-based catalysts.
The pentasil-based catalysts (ZSM-5) are less active
but very selective and stable. They are also character-
ized by high selectivity for p-xylene (p-XL).
2
3
by ion-exchange impregnation (2.5 wt %). The modi-
fied zeolites were molded with a binding agent, alu-
mina gel, taken in the amount of 25 g of Al O per
2
3
100 g of dry catalyst.
The catalytic conversion of TL was studied at at-
mospheric pressure in a flow-type catalytic apparatus
equipped with an adiabatic reactor. Experiments were
performed within the 673 748 K range at the space
1
velocity of 1 h and H /toluene ratio of 3.
2
Thus, development of new zeolite catalysts of TD
should involve either Y- and M-based zeolite catalysts
or enhancing the activity of ZSM-5. Taking into ac-
count that mordenite catalysts are highly active, selec-
tive [3 5], have commercial application [6], and are
more stable with respect to modification than Y, we
believe that the synthesis of more selective and active
M-based catalysts for TD is possible.
The products of TL conversion were analyzed chro-
matographically on a capillary column (50 0.25 mm)
coated with liquid-crystalline phase, methoxyethoxy-
azobenzene (T = 360 K; carrier gas argon).
Adsorption properties of the catalysts were studied
gravimetrically on a vacuum device equipped with
a McBain quartz balance. Benzene was used as an
adsorbate. Experiments were performed at 293 K
In this study, we examined the effects of modifying
synthetic mordenite on the selectivity of TD and p-XL
yield.
within the range of relative pressures p/p 0 0.3. Iso-
0
therms of benzene adsorption on zeolites used as
catalysts are related to type I isotherms in accordance
with Brunauer classification and are well described by
the Dubinin Radushkevich equation [9]
EXPERIMENTAL
K[RTln(p0 /p)]2,
Na-exchanged zeolites Y (Y), synthetic mordenite
M (M), ZSM-5, and natural mordenite (NM) from
Chananab deposit (Azerbaijan) were initial materials
lna = lnW0
where a is the amount of the adsorbate sorbed
1070-4272/05/7811-1854 2005 Pleiades Publishing, Inc.

INCREASE IN SELECTIVITY OF TOLUENE DISPROPORTIONATION
1855
Table 1. Distribution of products of TL conversion on modified zeolite catalysts
Product composition,* wt %
Catalyst,
SiO2/Al2O3
T, K
Co/zeolite
BZ
TL
o-XL
m-XL
p-XL
EBZ
PMBZ
AN
Co/HM10
Co/HM25
Co/HM30
Co/HM10
Co/HM25
Co/HM30
Gd/HM25
GdCo/HM25
GdCo/HZSM-5
GdCo/HNM
GdCo/HY
10
25
30
10
25
30
25
25
61
10
5
673
18.2
21.3
22.3
20.9
22.4
23.7
22.7
21.5
19.5
17.4
24.1
60.8
54.7
52.5
56.2
52.2
51.3
52.8
54.0
59.3
65.0
49.3
2.1
2.8
3.2
2.2
3.0
3.2
2.3
2.8
2.0
4.0
4.6
9.4
11.5
13.2
10.6
12.0
12.7
13.0
6.7
6.0
6.2
5.4
6.6
6.5
5.4
4.8
11.2
8.7
4.3
4.9
0.3
0.4
0.3
0.4
0.3
0.2
0.3
0.4
0.4
0.2
0.7
2.3
2.1
2.2
2.5
2.9
3.1
3.4
2.9
2.5
1.0
5.7
0.9
1.0
0.9
0.6
0.5
0.4
0.7
0.5
0.6
0.5
0.8
723
723
723
748
723
673
7.0
7.6
9.9
* (EBZ) Ethylbenzene, (PMBZ) polymethylbenzene, and (AN) alkanes C C .
1
2
1
3
(mg g ); , adsorbate density (g cm ); and W , the
namely, for BZ and XL sum, and for TD. Taking also
into account that p-XL is of particular significance
among XLs [10], the selectivity for p-XL should be
considered separately. The experimental yields of par-
ticular products are listed in Table 1, and quantities
reflecting the selectivity for target products (BZ +
XL/TL conversion), for TD (molar ratio XL/BZ),
and for p-XL ( = p-XL/ XL), in Table 2.
0
3
1
volume of adsorption area (cm g ), which is found
by extrapolation of the isotherms obtained to p /p = 1.
0
In accordance with the theory of micropore volume
filling TMVF [9], the parameter K related to the cross
section of zeolite micropore inlets is calculated from
the shapes of the Dubinin Radushkevich isotherms
obtained. The results listed in Table 1 show that BZ
and XL are the main products of TD.
As follows from Table 2, the activity of cobalt
mordenite (CoM) catalysts weakly depends on the
zeolite dealumination (5 8%). The same conclusion
follows from the comparison of the selectivities of
these catalysts. Such a behavior of Co M systems is
caused by the diffusion control of the TL conversion.
This is confirmed by the independence of the catalyst
activity in TD from temperature within the 673 748 K
range. Hence, the TL conversion and product yields
are, to a first approximation, limited by thermodynam-
ic features of TL transformations and by the probable
subsequent transformations of the molecules formed.
The process control by diffusion of the reactant to
active centers and of the reaction products from the
reaction zone can be associated with molecular-sieve
properties of zeolites. Such a dependence is known
for H forms of the zeolites used [11].
We also detected among the products ethylbenzene
(EBZ), polymethylbenzene (PMBZ), and alkanes C
C (AN). Formation and distribution of these products
1
2
indicate that toluene transforms along different path-
ways. Large amounts of BZ and XL formed show that
TD makes the prevalent contribution to TL transfor-
mations; other arenes and AN are formed by side reac-
tions: XL disproportionation, realkylation, hydrode-
alkylation, etc., which are also acid-catalyzed reac-
tions. As a result, the selectivity is an ambiguous
quantity. For instance, when determining the selectiv-
ity for TD products found as the weight ratio of BZ +
XL to converted TL, the fraction of benzene formed
by TD and the conversion of TL are not quite clear
because of the occurrence of XL disproportionation
and hydrodemethylation, which is confirmed by the
presence of the above-mentioned compounds among
the conversion products. However, taking into account
small amounts of trimethylbenzenes formed, whose
participation in BZ and TL transalkylation is insig-
nificant, we can consider the XL mole fraction in the
product mixture as the initial value characterizing
the TD selectivity.
It follows from Table 2 that the mole ratio of XL
and BZ formed at 673 K is practically independent of
the SiO /Al O ratio in mordenite catalysts. As tem-
2
2 3
perature is increased from 673 to 723 K,
=
XL/BZ
corresponding to HM and HM samples decreases,
10
30
whereas for HM it remains constant. It is interesting
25
that characterizing para-selectivity for all these sam-
ples remains unchanged. Thus, in TL conversion, the
distribution of disproportionation products weakly
Thus, the yields of toluene conversion products can
be characterized by selectivities for target products,
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1856
AGAEVA et al.
Table 2. Adsorption and catalytic parameters of Co-containing zeolite catalysts of TD
Co/HM25 Co/HM25
Parameter
Co/HM10
Co/HM25
Co/HM30
HY
673
50.7
86
HZM HNM
(I)
(II)
T, K
,* %
S,** %
XL/BZ, M
= p-XL/ XL
673
723
43.8
92
0.68
0.34
673
723
47.8
92
0.71
0.30
673
723
48.7
92
0.66
0.25
723
723
748
748
35.0
95
0.67
0.27
39.2
91
45.3
92
47.5
93
47.2
92
46.0
92
40.7
91
=
0.71
0.34
0.71
0.30
0.72
0.25
0.71
0.24
0.82
0.54
0.59
0.25
0.67
0.49
Adsorption parameters of Dubinin Radushkevich equation
1
W0, cm3 g
K
0.0747
0.0255
0.0903
0.0295
0.0895
0.0305
0.0995
0.0250
0.1005
0.0270
0.1650 0.0786 0.0210
0.0310 0.0260
* ( ) Toluene conversion. ** (S) Selectivity to BZ and XL.
depends on the temperature. Modifying HM with
the micropore inlet cross sections for various types of
zeolites identically modified with Co and REE. It
25
REE [M (I)] does not affect the TD selectivity. The
25
TD selectivity increases by 10 11% only when HM
should be noted that, in the series HY, HM (II),
25
25
is modified with Co and REE. At similar modifica-
tions of different zeolites, the TD selectivity depends
on their nature (Table 2), similarly to unmodified zeo-
lites [11]. These results demonstrate the influence of
molecular-sieve properties of zeolites studied on the
selectivity of TL conversion. However, whereas in the
series of unmodified zeolites ZSM-5, HM, HY ZSM-5
is the most selective, after modifying with Co and
REE the selectivity of this zeolite decreases and that
HZSM-5, and HNM, the cross sections of the inlets to
wide micropores of zeolites decrease. The comparison
of the catalytic properties of samples prepared from
these zeolites shows that only their activity in TL
conversion varies in the same order. The other catalyt-
ic parameters listed in Table 2, in contrast to [11],
show different trends. Thus, modifying with Co and
REE substantially promotes only HM .
25
Previously Gnep et al. [13] suggested to use the
sensitivity of XL transformation to zeolite micropor-
ous structure for identification of the nonmodified
zeolite type. As mentioned above, the major pathway
of TL transformation is disproportionation compli-
cated by the further transformations of XLs formed.
Therefore, ambiguous influence of the modification
on the selectivity of TL conversion may be due to
changes in microporous structure of the zeolites con-
sidered.
of HM (II) increases. Then, the selectivity series
25
becomes as follows: HM (II) > ZSM > HY.
25
The most substantial changes are observed for .
As seen, the p-XL yield decreases with dealumination
of the synthetic mordenite with other catalyst param-
eters remaining practically constant. Modifying of
HM with REE also decreases p-XL yield. The
25
modifications performed do not affect the activity of
M catalysts, but favor attaining equilibrium distribu-
tion of isomeric XLs in TD [12]. Taking into account
that enrichment of TD products with the narrowest
XL isomer (i.e., p-XL) is associated with a decrease in
size of zeolite micropores [11], the above-mentioned
decrease in in the series of M catalysts can be attri-
buted to an increase in the cross section of micropore
To estimate the effect of modification on the zeo-
lite microporous structure, we studied the adsorption
properties of the TD catalysts synthesized for BZ.
The isotherms measured are well described by the
Dubinin Radushkevich two-parameter adsorption
equation. The values of parameters are listed in
Table 2 and can be interpreted in terms of TMVF [9].
In accordance with these data, only within the range
of SiO /Al O ratios from 10 to 25 the volume of ad-
inlets. In this case, modification of HM with Co and
25
REE simultaneously gives an opposite effect. Hence,
the substantial increase in the selectivity for TD and
p-XL is associated with a decrease in the inlet cross
2
2 3
sorption space W noticeable changes, with an in-
0
section in modified HM as compared to the other
25
crease in the catalyst activity by 4%. The further de-
alumination of HM to SiO /Al O = 30 and modi-
HMs.
25
2
2 3
Table 2 allows comparison of the catalytic proper-
ties of zeolite samples in TL conversion in relation to
fication with REE or REE + Co affects W insignif-
0
icantly; however, it should be noted that the catalyst
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INCREASE IN SELECTIVITY OF TOLUENE DISPROPORTIONATION
1857
activities and W vary similarly. Such a sensitivity of
0
TL conversion to W is an evidence of reactant inter-
0
action with active centers located in micropores. At
the same time, whereas the selectivities S for the
target products remains practically constant, i.e., in-
dependent of W , the HM (II) sample, in contrast to
0
25
the other catalysts of HM series, exhibits higher selec-
tivity for TD. The elevated selectivity for TD of this
sample cannot by the result of somewhat elevated W
(Table 2).
0
In contrast to the quantities characterizing the
selectivity of TD, the - selectivity for p-XL is more
para-Selectivity of toluene transformation as a function
of the parameter K of Dubinin Radushkevich adsorption
sensitive to modification. High sensitivity of
to
modification is attributable to substantial difference
between the kinetic diameters of p-XL (0.66 nm) and
o-XL and m-XL (0.76 nm) [14]. Taking into account
closeness of the kinetic diameters of BZ, TL, and
p-XL, we can suggest that the diffusion of the reactant
to active centers and of products (p-XL and BZ) from
the reaction zone follows the same relationships,
whereas large kinetic diameters of o-XL and m-XL
can complicate removal of these products from the
reaction zone. Hence, the yield of products of TL
transformations on zeolite catalysts depends on the
cross section of zeolite micropore inlets. Also, ap-
parently, depending on inlet cross sections, the cases
are possible when the internal space of micropores is
inaccessible to reactant (HNM), or, vice versa, o-XL
and m-XL will freely escape from the reaction zone
(zeolite of HY type).
equation.
the above effect of the loss of molecular-sieve proper-
ties takes place because of o-XL and m-XL free diffu-
sion from the reaction zone (for HY) or, vice versa,
inaccessibility of the inner pore space to TL (for
HNM). On the one hand, due to this effect the HY ac-
tivity is the highest and that of HNM is the lowest.
On the other hand, the para-selectivity of HNM is
close to that of HY, and selectivity for TD, to that of
HZSM. The above-mentioned coincidence of HNM an
HZSM properties in TD and the difference in the be-
havior of the HY sample may be due to features of the
subsequent XL transformations on these catalysts. As
shown in [15 17], TD proceeds on HNM and HZSM
in the surface layer and on HY, inside micropores.
In the series of catalysts prepared from various
types of zeolites, an HM (II) sample stands out by
Table 2 shows that modification of HMs affects the
adsorption parameter K. This parameter, related to the
diameter of zeolite micropore and channel inlets, in-
creases with dealumination and decreases with intro-
duction of REE. At the same time, introduction of Co
into the sample containing REE increases K. However,
in this case, K (0.027) is less than in the absence of
REE (0.029). The comparison of the adsorption pa-
25
higher selectivity in TD and p-XL formation, and in
the activity it is inferior only to HY. In view of the
above reasonings, high activity of this sample can be
associated with accessibility of the inner volume of
mordenite micropores. It is seen from the comparison
of W values that these pores are larger than the pores
0
of HZSM and smaller than the pores of HY. Probably,
the structure of HM pores is such that side trans-
25
rameter K with the catalytic parameter
shows
formations of TL are sterically hindered and diffusion
of BZ and p-XL is easier compared to HZSM. Indeed,
the adsorption parameter K of of Dubinin Radushke-
vich equation occupies an intermediate position bet-
ween the values of this parameter for HY and HZSM
(Table 2). Thus, the selectivity in p-XL formation is
governed by the size of inlets to wide pores of zeolites
and thus can be characterized by the parameter K of
Dubinin Radushkevich adsorption equation. The de-
pendence of the parameter on K is shown in the fig-
ure. As seen, the K curve consists of two branches.
The left branch corresponds to worsening of molecu-
lar-sieve properties due to a decrease in accessibility
of zeolite micropores to the reactant, and the right
that K corresponding to maximal has an intermediate
value in the series of modified HMs studied. This in-
dicates that there is some optimal diameter of micro-
pore inlet, favoring selective TD and elevated yield
of p-XL.
To comment upon this suggestion, let us consider
the results of TL transformation on a series of cata-
lysts prepared from various types of zeolites. In the
series HY, HZSM, and HNM, the highest para-selec-
tivity is shown by HZSM. At the same time, wide-
3
1
1
pore zeolite HY (W = 0.165 cm g ) and narrow-
0
3
pore HNM (W = 0.021 cm g ) exhibit approxi-
0
mately equal para-selectivity. Obviously, in this case,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 11 2005

1858
AGAEVA et al.
branch corresponds to worsening of molecular-sieve
properties because of easier diffusion of large mole-
cules of products (o-XL and m-XL) from the reaction
zone in micropores. Intersection of these branches cor-
responds to the maximum possible para-selectivity
and hence to the optimal microstructure of zeolite
catalyst for TD.
5. Agaeva, S.B., Dadashev, B.A., Tagiev, D.B., et al.,
Neftekhimiya, 2004, vol. 44, no. 1, pp. 42 46.
6. ACS Symp. Ser., 1984, no. 248, p. 15.
7. Tsybulevskii, A.M., Klyachko, A.L., Brueva, T.R.,
et al., Izv. Akad. Nauk SSSR, Ser. Khim., 1980, no. 12,
pp. 2690 2694.
8. Ponomarev, A.I., Metody khimicheskogo analiza sili-
katnykh i karbonatnykh gornykh porod (Methods of
Chemical Analysis of Silicate and Carbonate Rocks),
Moscow: Akad. Nauk SSSR, 1961, pp. 18 19.
CONCLUSIONS
(1) The selectivity of toluene conversion is gov-
erned by two cross sections of micropore inlets in
zeolite catalysts: the lower limit is dictated by diffu-
sion of the reactant to active centers located in micro-
pores, and the upper limit, by removal of larger prod-
ucts from the reaction zone.
9. Belen’kaya, I.M., Dubinin, M.M., and Kristofori, I.I.,
Izv. Akad. Nauk SSSR, Ser. Khim., 1971, no. 7,
pp. 1391 1397.
10. Neft’, Gaz Neftekhim. Rubezhom, 1989, no. 11,
pp. 100 101.
11. Penchev, V., Mavrodinova, V., and Minchev, Khr.,
Izv. Akad. Nauk SSSR, Ser. Khim., 1982, no. 5,
pp. 1168 1171.
12. Zhorov, Yu.M., in Termodinamika khimicheskikh pro-
tsessov (Thermodynamics of Chemical Processes),
Moscow: Khimiya, 1985, p. 223.
(2) The yield of the target products of toluene
conversion and selectivity of TD on modified zeolites
can be increased by optimization of their microporous
structure.
13. Gnep, N.S., Tejada, J., and Guisnet, M., Bull. Soc.
Chim. Fr., Part 1, nos. 1 2, pp. 5 8.
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