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Isomerization of M-xylene Abstract:
In this study, gas-phase isomerization reaction of
m-xylene was studied over various catalystsincluding Ni, Sn, Pt, Ga, Re or Zr
over alumina and zeolite carriers. The catalysts prepared over ZSM-5 carriersand
consisted of Ga andlor Pt as active components showed high activity and
selectivity on p-xylene compared to o-xylene. Kinetics of the reaction was
studied over these selective catalysts and it was found that experimental data
were pretty well suited to the kinetic model developed. Key
words: Isomerization, m-xylene, o-xylene, ZSM-5
zeolite, disproportionation Xylenes are
C$ benzene homologues
with the molecular formula C$H。as seen in
Fig. 1 .The term mixed xylenes describes a mixture of ethyl benzene and the
three xylene isomers, namely p-, m-and o-xylene. They are important chemicals
and find wide
and various applications in Indus仰.O-xylene is
a reactant for the production of phythalic anhydride. M-xylene is used in manufacture of plasticizers, azo dyes, wood
preservers, etc. P-xylene upon oxidation yields
terephthalic acid used in the manufacture of synthetic textile
fibers and poly (p-xylene)
which is produced
by p-xylene has
had economic importance since 1960. Mixed xylenes are largely
produced from petroleum
such as catalytic reformat,
pyrolysis of gasoline,
toluene disproportionation and coke-oven light oil.
However, as a result of important applications to which
the individual xylene isomers are subjected, it is often
very important to have high concentrations of a particular
xylene.
This can be
accomplished converting a non-equilibrium mixture of the xylene
isomers, which is low in the desired xylene isomer, to a mixture
which approaches
equilibrium concentrations. Numerous catalysts
have been proposed
for use in
xylene isomerization processes. More recently, a number of studies
have disclosed the
use of crystalline aluminosilicate zeolite
containing catalysts for isomerization and conversion of C$
alkylaromatics}'}. Most of山e studies isomerization of xylenes are mixed-xylenes. Only a few
m the literature
for the related to the isomerization of are about a pure material such as isomerization of m-xylene and its
kinetics. The reaction of m-xylene isomerization is of great interest for
organic chemical industry where the demand for the
para and ortho isomers exceeds the supply. The good
activity of zeolite
catalysts was proved
in the gaseous ase isomerization of m-xylene but with respect
to kinetics relatively few catalysts were investigated.
In spite of the considerable industrial interest on the
isomerization and disproportionation of xylenes, especially
catalysed饰 zeolites, no satisfactory correlations have
been indicated between catalytic activity and the solid
state properties as well.
Giordono et al.}Z} investigated in particular the interaction m ordenites of
m-xylene with various
dealuminated The
conversion of m-xylene
over dealuminated mordenites was found to be a
function of the acid strength rather than the number of
the acidic sites, as the SilAl ratio was directly
proportional to acid strength. Chatterjee and Ganguli}3}
have investigated the isomerization and associated reactions of
o-xylene over 20 catalysts based on a large variety of
multicationic (mono to trivalent) zeolite Y samples. All
the catalysts showed very high selectivity for m-xylene,
but relative improvement in p-xylene yield was obtained
with catalysts containing La" and Ni". Isuchiya et al.}，一，was
calculated that A1Br31Gra ite and A1Br31MSC-showed structure selectivity in the isomerization of xylenes
in which 1, 2 methyl shift mechanism has been considered
and the isomerization of xylenes was almost first
order with respect
to the pressures
of the reactants. Mavradinova et crl.}s} have compared the
catalytic action of a series of pentasil type zeolites with almost
the same SilAl ratio synthesized by different
methods and they have reported that pentasil
type zeolites with
close chemical composition exhibited pronounced
differences in crystal morphology, acid function (active site number and
strength distributions), catalytic
activity and selectivity depending on the method of
preparation. It was observed that the disproportionation of
m-xylene was more sensitive to the acidic function of
the catalysts, whereas their activity and selectivity with
respect to the yield of xylene isomers were more
consistent with the presence of differences in the size and morphology of the zeolite crystallites. In a U. S Patent}fi},
it is reported that a xylene isomer mixture is treated with场drogen in
the presence of a cracking catalyst
comprising a zeolite selected from ZSM-5, ZSM-11, ZSM-12, ZSM-34
and ZSM-48 series at least 20% of its canon
site being occupied by a canon of a metal selected
from alkali metals and
alkaline earth metals
such as lithium,
sodium and
strontium and a
refractory inorganic oxide (gamma-alumina) having platinum (in a
weight percent of 0.005 to 5% based on the weight of the
carrier) supported thereon to suppress side reactions
involving a loss of xylenes such as disproportionation and场drogenation. Dumitriu‘al.}'} was
found that HZSM-5 had the best selectivity for both the isomerization
reaction and p-xylene
formation. Acid zeolites
catalyse the isomerization of xylenes and also their
disproportionation into three methylbenzene
and toluene. Two
kinetic models, the
first taking into account only
m-xylene isomerization to
p-and o-xylene and
the second considering
the isomerization and
disproportionation reaction of xylene, were adopted. In a
triangular model (Fig.
2) involving simultaneous
isomerization and disproportionation was also tested and the
rate constants were calculated. The calculations of the
rate constants indicated that HZ SM-5 was a proper
catalyst for m-xylene isomerization though its activity was
smaller than that of zeolites exchanged with rare earth metals,
the selectivity of HZSM-5 was undoubtedly higher. Acid
properties of HNaZSMS [SilAl=47 andNalAl=0.85] zeolites
ofvarious exchange degrees (0-98%) were studied}8}.
Isomerization of o-xylene and alkylation of toluene with
methanol were chosen as the test reactions, both of them
known as proceeding on the Bronsted acid sites, to
investigate the catalytic activity of zeolite. The strength of the acid centres and their concentrations were
studied卜IR spectroscopy. In the zeolites of exchange
degrees higher than 11%，the conversion increased linearly with the number of OH groups of 3609 cm-' which were
strong Bronsted acid sites. Conversion of m-xylene
was also studied on H-fa灼asite and on
several acid zeolites with ten-or twelve-membered rings}9'口}. It was
reported that in the m-xylene conversion, activity of
H-faujasite depended strongly
on its degree
of dealumination and
plo selectivity was always high for
ten-membered ring zeolites with crystals bigger than 1 pm although
twelve-membered rings showed much more disproportionation.
Leu and Chao studied"} the properties of
crystalline microporous molecular sieve A1P04 modified场incorporating
B, Si, Fe, Ca,
Mg or Zn
element into its
framework. The incorporation of magnesium in the A1P04
structure gives high preference of p-xylene, whereas the
incorporation of silicon and ferric canons results in a value of p-lo-xylene ratio less than unity. The incorporation of
cobalt into A1P0。一:leads to the
o-xylenel p-xylene yield in a state of equilibrium. Sastre et crl.}'Z}
investigated the conversion of m-xylene over offretites with different
levels of ion exchange. The main conclusion of that study
is that the catalytic activity increases monotonically
upon exchange of
K' canons. By
removal of these
canons, the concentration of protons increases as the
accessibility to the internal porosity of the zeolite with
the result that the selectivity for isomerization is promoted
at the expense of disproportionation. Pere'Z
Parientence}'3}, studied the relationship between the pore structure and
SilAl ratio of ze olitep，which is
twelve-membered ring zeolite, with its catalytic activity and selectivity in the
conversion of m- xylene
was elaborated. The
isomerization and disproportionation activity of zeolite p
depend strongly on the aluminum content, decreasing with
increasing framework aluminum content. Maximum
activity is found for
SilAl ratios between 14 and 15,
while the plo selectivity remains constant. It was seen
that the plo selectivity
in m-xylene isomerization
is ca. 1.2, independent of the SilAl ratio of zeolite p
and of the conversion. The studies of Shashikala et
al.}'4} showed that conversion increased with temperature,
but para selectivity
decreased and much
higher m-xylene conversion, as well as higher stability of
the catalyst is achieved饰using platinum-exchanged HZSM-35 (O.OSwt.% platinum). Li et al.}'S} used a
pulse microreactor- chromatography technique
to study the
xylene isomerization reaction on HZSMS zeolite
catalyst. The kinetic
parameters of the reaction network proposed, (Fig. 3), were calculated. The catalytic properties of zeolites ZSM-5 dealuminated饰HCl were
investigated for isomerization of m-xylene}'6}. Besides the
increase of the SilAl ratio, the dealumination leads to an
inversion of the SilAl concentration gradient in
crystallites, to a shift of acidity
spectrum and consequently
to a change
in catalytic activity and selectivity. A
theoretical model modified场Wugeng et
crl.}"} was used to explain and predict
the high para-selectivities in
toluene disproportionation and xylene
isomerization. The reaction of m-xylene is a useful tool for
characterising zeolite structures especially for all range of pore
sizes}'$}. The selectivity of these reactions gives
information allowing one to estimate pore diameter and
architecture for medium through extra-large pore zeolites. As
one-dimensional large and extra large pore zeolites give a
plo ratio<1， likely
due to a
significant occurrence of
the bimolecular isomerization mechanism,
multi-dimensional large
pore zeolites such
as USY and
beta and the low-silica，二一dimensional
zeolite LTL give a plo ratio>1. The
straight channel uni-dimensional zeolites SSZ- 24，-31，SITS, UTD-1，Z SM-12，-48 have the
lowest到。 ratios over the range of pore size studied.
Morin et crl.}'9} examined influence of coke deposits on the
selectivity of m-xylene transformation on the
isomerization mechanism over HFAU zeolites and found the stronger
the acid sites the faster their deactivation by coke
deposits.
As understood from the given literature, much attention
has been focused
on the shape selective catalysts,
especially ZSM-5 type
zeolites, for the isomerization of m-xylene. On the other han氏the kinetics of xylene isomerization over some zeolite
based catalysts has been studied by several researchers. To
understand the kinetic behaviour of xylenes in
isomerization reaction on
the catalyst, the
knowledge of detailed
kinetic parameters
must be available.
It means that
more theoretical and experimental study is
needed. The aim of this
work is to
investigate kinetics of
m-xylene isomerization over a selective catalyst
determined after a selectivity screening-test, to
calculate the kinetic parameters of the model and to compare them
with the values in the previous studies. Catalyst preparation:
Eighteen different catalysts depending
on the information about the catalysts
in literature for the isomerization of
m-xylene were prepared. Alumina, natural zeolite (rich in
clinoptilolite) and several synthetic
zeolites such as
MEM5766, MEM1510, ZSMS困a) and
ZSMS(H)-Pentasil were employed as carriers and aqueous solutions of salts of
metals such as Ni, Sn, Re, Pt, Ga, Zr, Sn were used to add
the active components to the carriers. Preparation steps of the catalysts are given in Fig. 4. Origins of
materials used in the catalyst preparation and some physical
properties of the catalyst were presented in Table 1. The
chemical compositions of the
catalysts prepared were determined by gravimetric and
volumetric (atomic absorption spectrometer) methods
and were given in
Table 2. Nitrogen
adsorption experiments were performed at 77K using a static volumetric
apparatus (Coulter Omnisorp 100 CX) up to PIPo}0.95.
The catalyst samples were previously degassed at 4000C
for 4 h before nitrogen adsorption measurements. Specific
total surface areas and monolayer pore volumes were
calculated using Langmuir equation and
specific pore volumes
were estimated from the nitrogen uptake. Table 3
shows the results
of Nz adsorption
measurements of ZSMS, ZSMSPtGa and ZSMSPt catalysts. An attempt
was made to apply the Dubinin-Astakhov (D-A), Langmuir and BET adsorption
models for the
characterization of the catalysts. The Dubinin-Astakhov equation is
expressed as: Where,
W is the
amount adsorbed at the
relative pressure, Wo is the total micropore
adsorption capacity, k is a constant, n is the structural
parameter. The value of the
adsorption capacity, Wo,
can be obtained from the intercept of the D业plot (Log W)
vs. (Log PIPo)". Tahle 3 shows that surface area. of 7,}M5
decreases with impregnation of Ga and/or Pt. In the Table 3, A}},g, AeET and AD.A are surface areas, Vm is monolayer
capacity, b and c are constants,戈。二is half
width of pores and Vm} is the maximum amount adsorbed. Impregnation
with Ga andlor Pt causes a decrease in the pore
volume of original ZSMS, resulting in the increase of half
width of pores. Experimental set-up: Set-up is given in
Fig. 5. Nitrogen was used as the场drocarbon
carrier. It was cleaned from dust, COZ and moisture饰passing
through a series of glass wool, KOH and silica gel,
respectively and was metered with a rotameter before fed to the
reactor. The 场drocarbon stream from the saturator was fed to the reactor through a Pyrex tube that was kept
over 1400C in order to prevent xylene condensation.
M-xylene was kept in the saturator located in a constant
temperature oven and the gas flow leaving the system was
also measured with a flow meter. The hydrocarbon heated
at constant temperature was carried by the nitrogen
flow to the reactor. The amount of场drocarbon
stream carried is dependent on both the velocity of the
carrier gas and the temperature of the saturator. During the
experiments the temperature
of the saturator
was kept constant
at 700C, but the gas feed rate was changed
between 1 and 2.5 mL sec-' to carry场drocarbon in
different amounts to the reactor.
The amount of场drocarbon carried to the reactor at each experiment was sensitively measured by
weighing the saturator at the beginning and at the
end of the experiment.
The tubular reactor
employed in the experiments was in 26 mm in diameter, made
of Pyrex glass. It was located in an electrical oven
and heated by radiation. A Fe-constant an thermo element
was located in
the middle of
the catalyst bed to
measure the temperature of the catalyst and the temperature
was recorded continuously during the experiment
and it was kept constant. The
product stream from the reactor passed through two cold traps was connected
in series. The cold traps were kept in salt-ice
mixture and liquid nitrogen, respectively. The products and unreacted m- xylene collected in two traps were analyzed卜a gas chromatograp琢(GC) Hewlett Packard
589012; on a capillary column of HHP-FFAP using flame
ionization detector (FID). Selectivity studies: These experiments were
carried out with a molar flow rate of m-xylene varied
0.00392 and 0.08 mole h-', at a temperature range of
270-3800C, with a constant fresh catalyst amount (1 or 4 g.)
for a reaction time of 4 h. At the end of each experiment,
used catalyst was regenerated under nitrogen flow at
3800C for 4 h (Table 4).
Selectivity studies showed
that the catalysts numbered from 1 to 12 are almost inactive
catalysts in isomerization of m-xylene.
Disproportionation products such as toluene (TOL) and threemethlbenzene
(TMB) are not present in the reaction mixture over
these catalysts even for severer reaction conditions, when
compared with catalysts 13-18, higher temperature, higher
m-xylene feed and four-fold catalyst amount. Catalysts prepared on ZSMS(Na), numbered from 13 to 18, represent
better activity than the above ones. ZSMSPt
(numbered 15) is the most active catalyst under the reaction
conditions studied. In the reaction mixture various
TMB isomers, toluene
are formed as
disproportionation products. ZSMSPt-Ga, ZSMSPt-Zr and ZSMSPt exhibit
higher selectivity on p-xylene compared to
o-xylene, SP,o, than the others. These catalysts possess the
catalytic activity in the order: 28M5Pt>又8M5Pt.C"ra>又8M5Pt7,r
Catalysts ZSMSPt and ZSMSPtGa, which are the ones composed of 1.8*10-3 (in w%) Pt and
3.04*10-3 (in w%) Ga, 1.8*10-3 (in w%) Pt over Z8M5 support,
respectively, were chosen for kinetic analysis, due to
the their higher a.ctmt}es Kinetic study: Kinetic runs were performed
under the similar conditions to those in selectivity
studies. A kinetic run took 8 h. One gram of catalyst (except
one group of experiments, which were carried out to find
the effect of catalyst amount on reaction rate) was
placed to the reactor. During the runs with different
flow rates of m- xylene, the change in the feed rate of
m-xylene was obtained by varying the flow rate of
nitrogen (between 1 and 2.5 mL sec-') which is used to carry琢drocarbon to the reactor.
After each run, the reaction mixture collected in the traps was analysed by the gas
chromatography. In the reaction mixture p-, o-xylene as
isomerization products,截止到241
various
trimethylbenzene (TMB) isomers,
toluene were formed as disproportionation products.
Unreacted m-xylene was measured but no benzene was
observed in the reaction mixture. In the Table 5, the
total conversion of m-xylene, conversions to p-, o-xylene
and conversions to toluene and TMB are represented. The
conversions on Table 5 were obtained from mole fractions of related components in the reaction mixture. The
reaction network which was used in kinetic modeling, is
presented in Fig. 6.
This model takes into account m-xylene isomerization to
p-, o-xylene, disproportination and
demethylation reactions of m-xylene, The reactions
between o-xylene and p-xylene and between methyl groups were neglected, Assuming that the reactions are first
order, the rate expressions were given in Fig. 7. Xylene
concentrations in the rate equations were expressed in
terms of related conversions
which were calculated
from the mole fractions in the reaction mixture}z0}.
Because of the fact that total conversions achieved in all the
experiments are greater than >_ 10%, the reactor is
treated as an integral reactor, By using the continuity equation
in a tubular reactor
for plug flow
behaviour, the ratio
of tube diameterlparticle diameter should be >_
10, which is 19.5 to conclude plug flow behaviour}Z'} reaction rates were obtained饰differentiating the
X versus WIFE curves drawn for all runs for each product where X
is the conversion to the product, such as
o-xylene, p-xylene, toluene and TMB}z口〕.First of
all, the best curve fitting to the X; versus WIFA0 data, making it pass
through the origin is founc，For this
purpose, several polynomials with different orders are tested for the
goodness of fit of the polynomial with experiment, the best fit is determined
and the slope of the curve at any point gives
the reaction rate at that point. Table 6 presents the
correlation ofXversus W/F} data, Table 7 represents reaction rate
constants calculated using the experimental data in
Table 5. Finally, the activation energies and frequency
factors of the rate constants corresponding reactions were estimated by means of Atrhenius equation. Table 8
represents these results with regression coefficients.
DICSSION DISCUSSION
Partial and total conversions of m-xylene vs, space time (WIFE) plots were presented in Fig. 8
for ZSMSPtGa and ZSMSPt catalysts at different reaction
temperatures. It is obvious that there is an appreciable
increase in conversion to each product as the
temperature increases from
270 to 3400C, Above 3400C,
such a significant change couldn't be observed in the partial
and total conversions of m-xylene. Figure 9 illustrates the effect of temperature
on the selectivities to
several products calculated from data in Table 5. Important
increases (almost four fol由in the
selectivities sP、口一and sP一二二were obtained
over ZSMSPtGa catalyst at
3800C and addition
of Ga to
ZSMSPt catalyst improved
the selectivity to p-xylene.
P-lo-selectivities remained almost unchanged as the total conversions increased. The Fig. 9
presents p-lo- selectivities at all the temperature
studied over the two catalysts and for all the temperatures
studied. It was observed that in general p-lo-selectivity
has not been affected significantly with increasing
temperature from 270 and 3400C over ZSMSPtGa and ZSMSPt catalysts. It was observed that there was no a significant
difference in selectivities over ZSMSPtGa and ZSMSPt
catalysts in the temperature
range of 270
and 3400C. The
kinetic model taking into account m-xylene
isomerization to p-and o-xylene, disproportionation and demethylation
reactions of m-xylene fitted the experimental data
fairly well. First order reaction mechanism was used and
isomerization was taken
reversible, but disproportionation anc demethylation irreversible}z0}. By
use of standard procedures}ZZ} it was shown that both
external and internal diffusion
do not influence the reaction
rate, External diffusion effects were calculated by use of
the generalised JD factor and it was found that (Cb-Cs)ICe was equal to 3.5* 10} for ZSMSPtGa and 2.1 *10} for
ZSMSPt catalyst at the maximum reaction temperature of
3800C, so it was assumed that Cb=--Cs, To calculate the
internal diffusion effects, the generalised Thiele modulus
based on the reaction rate was determine氏For this
purpose, Weisz's criterion,
rkZ(r ,}甲乓)} X1,0, where, r:声ameter of catalyst, n,t:density of catalyst, De: effective diffusion coefficient, Cs:concentration of reactant
on the catalyst surface, rHC: reaction rate, was tested for
the experiments, The ratio mentioned above was found to be
0.1396 and 0.1351
for ZSMSPtGa and
for ZSMSPt catalysts, respectively, It means that
Weisz's criterion was satisfied, Hence the effectiveness factor can be taken
equal to unity which means that internal diffusion
resistance can be neglected.
The kinetic parameters calculated in this study were compared with the ones obtained by Dimitriu
et crl.}7} and Li et al.}'S}. Dimitriu et al.}'} tested
the zeolites SK-500, HndY
and HZSMS in
the reaction of
m-xylene isomerization, It was found that HZSMS had
the best selectivity for p-xylene formation, Three
kinetic models including isomerization and
disproportionation reactions were employed to explain the kinetic data,
It was observed that as the temperature was increase氏the rate constants were also increased, as expected, Li et
al.}'S} used a pulse micro reactor chromatograp场technique to
study the xylene isomerization reaction on HZSMS
zeolite catalyst, A mathematical model including diffusion,
adsorption and reaction steps were develope氏They also
declared that the reaction rate constants increased with
increasing temperature, They observed that m-xylene
and o-xylene couldn't produce any toluene, i.e.玫and k} in
Fig. 3 had to be small, because m-xylene and o-xylene
were strongly adsorbed and dealkylation was very
difficult under the experimental conditions used场them, As
seen from Table 7 clearly, rate constants calculated
in this stu即 also increase with increasing temperature,
which is the right
trend for the
constants, Activation energies obtained in this study are very low with
respect to the activation energies given by Li et al.}'S}.
But there is a good fitness between the values of the
reverse reaction rate constants of the isomerization
reaction calculated at 628 K in this study by using Arrhenius
equation obtained from experimental data (Table 9) over
ZSMSPtGa catalyst and by Li et al.}'S}, over HZSMS catalyst.
It is obvious that the numerical values of the kinetic
parameters given by Dumitriu et al.}"} are very different
from those given by this stu即and饰Li et
al.}'S}. It should be kept in mind that the catalyst, the experimental
conditions, the kinetic models
used in the studies
mentioned above have significant differences which cause
important deviations in the numerical values of kinetic
parameters.
An experimental run took 8 h and no significant catalyst deactivation was noted. Just to
have an idea about the magnitude of the deactivation,
isomerization of m-xylene was carried out over ZSMSPt and
ZSMSPtGa catalysts for durations longer than the
reaction time used in kinetic analysis (8 h) at the same
temperature (3400C) and at a constant feed rate of m-xylene. As
the reaction time increased from 8 to 40 h, a
significant decrease in total conversion of m-xylene (over ZSMSPtGa
from 40 to 20% and over ZSMSPt from 42 to 18%) was
observed}z0}, but up to a reaction time of 8玖an
appreciable decrease in total conversion was not measured and even
a significant change
in color was not noted
over two catalysts employed. However, it was clearly seen that
addition of Ga to catalyst lengthened its life. C'ONC'T,TJSTONS
In this study,
gas一户ase isomerization reaction of m-xylene was studied over various catalysts
including Ni, Sn, Pt, Ga, Re or Zr in different
compositions over alumina and
zeolite catalysts. The
catalyst composed of 1,8*10-3% Pt and the one composed of
1.8*10-3% Pt and 3,04 * 10-3%Ga, over Z SMS support were
found to be the selective ones to p-xylene, The ZSMS catalysts
including Pt and Zr and the HZSMS type of zeolite
have also given rather good results when compared to the
other ones prepare氏The kinetic model
with first order mechanism taking
into account disproportionation and m-xylene
isomerization, demethylation fitted
the experimental data fairly well |
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