Isopropylation of xylenes catalyzed by ultrastable zeolite Y (USY) and some other solid acid catalysts

Article abstract of DOI:10.1006/jcat.2002.3776

The isopropylation of all three xylene isomers was carried out over ultrastable zeolite Y (USY) catalyst to give corresponding dimethyl (1-methylethyl) benzenes, or in other words dimethyl cumenes (DMCs), using isopropanol as alkylating agent. The effect of reaction temperature, space velocity, substrate-to-alkylating-agent molar ratio, and time-on-stream on conversion of xylene isomers and selectivity to dimethyl cumene was studied. Isopropylation of xylenes over USY gives quite high (80 to 95%) DMC selectivity among the dimethyl cumenes, along with a 70-90% yield of DMCs in total products with respect to limiting reagents, i.e., isopropylating agents at relatively low reaction temperatures (423 ± 10 K) and at quite high xylene conversions (85-97% of theoretical maximum value). The solid acid catalysts zeolites H-Y, H-beta, H-mordenite, as well as silica-alumina and sulfated zirconia, were included for comparative studies in the isopropylation of m-xylene.

Full text of DOI:10.1006/jcat.2002.3776

Journal of Catalysis 212, 216–224 (2002)
doi:10.1006/jcat.2002.3776
Isopropylation of Xylenes Catalyzed by Ultrastable Zeolite Y (USY)
and Some Other Solid Acid Catalysts
Chitta Ranjan Patra and Rajiv Kumar1
Catalysis Division, National Chemical Laboratory, Pune 411 008, India
Received May 22, 2002; revised July 31, 2002; accepted July 31, 2002
production of xylenols (20). Needless to say, xylenols are
The isopropylation of all three xylene isomers was carried out important in the production of pesticides, perfumes, phar-
over ultrastable zeolite Y (USY) catalyst to give corresponding maceuticals (21), heat-transfer media (22), polymers and
dimethyl (1-methylethyl) benzenes, or in other words dimethyl
cumenes (DMCs), using isopropanol as alkylating agent. The ef-
fect of reaction temperature, space velocity, substrate-to-alkylating-
agent molar ratio, and time-on-stream on conversion of xylene
isomers and selectivity to dimethyl cumene was studied. Isopropy-
that the isopropylation of o-xylene over 0.91 NdNa–Y
lation of xylenes over USY gives quite high (80 to 95%) DMC se-
lectivity among the dimethyl cumenes, along with a 70–90% yield
special solvents (23), phenolic resins (24), and electronic
equipment, automobiles (polyethers, etc.), and polycarbo-
nates (25).
In brief communications, Isakov et al. (15, 16) reported
zeolite yields a mixture of dimethyl-isopropyl-benzenes
(
or dimthyl cumenes (DMCs)) containing 70–97% 1,2-
of DMCs in total products with respect to limiting reagents, i.e.,
dimethyl-4-isopropyl benzene (3,4-DMC). Although in
isopropylating agentsatrelativelylowreaction temperatures(423 ±
1
0 K) and at quite high xylene conversions (85–97% of theoret- these studies the main emphasis was on tert-butylation
ical maximum value). The solid acid catalysts zeolites H–Y, H- of xylenes, isopropylation of xylenes was also mentioned
beta, H–mordenite, as well as silica–alumina and sulfated zirco- briefly. Further, it was also reported by the authors that iso-
nia, were included for comparative studies in the isopropylation of
m-xylene. ꢀc 2002 Elsevier Science (USA)
Key Words: isopropylation; xylenes; solid acid catalysts; zeolites;
USY; formation of dimethyl (1-methyl ethyl) benzenes; dimethyl
propylation of p-xylene could not take place on the same
catalyst. These results are in contrast to our earlier results
on zeolite beta (19) as well as to the results reported in
the present communication, where significant conversion of
cumenes.
all three xylenes was obtained during isopropylation over
USY catalyst. This article aims to report detailed studies on
the effects of various reaction parameters, such as reaction
temperature, space velocity, xylene-to-isopropanol molar
INTRODUCTION
ratio, and time-on-stream (TOS) on xylene conversion and
product distribution, particularly dimethyl cumene(s) se-
lectivity, in the isopropylation of xylene isomers catalyzed
by ultrastable zeolite Y (USY), one of the most open micro-
porous zeolitic structures, using isopropanol as alkylating
agent. Additionally, five other solid acid catalysts, zeolites
H–Y, H-beta, and H–mordenite and silica–alumina and sul-
fated zirconia, were included for comparative studies in the
isopropylation of m-xylene.
Alkylation of aromatic substrates catalyzed by solid
acids such as zeolites constitutes a class of reactions of both
academic and industrial importance. Among alkylation
reactions, isopropylation of aromatic compounds has
attracted considerable attention. The isopropylation of
benzene (1–7) and toluene (8–10) catalyzed by zeolites to
give cumene and cymenes, respectively, has been studied
extensively, as these isopropylated aromatics are converted
to corresponding phenolic compounds. Methylation (11,
1
2) and ethylation of xylenes (13, 14) over solid catalysts
are relatively well-studied reactions compared with the
isopropylation of xylenes, particularly over solid or zeolite
catalysts. In fact, there are only a few reports available in
the literature on the isopropylation of xylene isomer(s) (15–
EXPERIMENTAL
Catalyst Preparation and Characterization
Zeolites beta (26, 27), mordenite (28), and Na–Y (29)
and sulfated zirconia (SZ) (30) were synthesized hydrother-
mally using methods described in the literature. The silica–
alumina (SAL) was supplied by M/S Joseph Crossfield
and the ultrastable zeolite Y (USY) was supplied by
Union Carbide (batch number 966184061053-S-1). The
1
9) to produce corresponding dimethyl (1-methylethyl)
benzene (simply denoted hereafter as dimethyl cumene(s)
(
DMC)), which could be used as raw material for the
1
To whom correspondence should be addressed. Fax: +91-20-589
3
761/589 3355. E-mail: rajiv@cata.ncl.res.in.
216
0
ꢀ
021-9517/02 $35.00
c 2002 Elsevier Science (USA)
All rights reserved.

ISOPROPYLATION OF XYLENES OVER ZEOLITE USY
217
synthesized zeolites were filtered, washed with deionized the center of the reactor in such a way that the catalyst bed
water, and dried at 373 K for 2 h followed by calcination was sandwiched between inert porcelain beads. The reactor
at 813 K for 12 h in a flow of air. The protonic form of was placed in a double-zone furnace. It was equipped with
the zeolite was obtained by repeated (thrice) ammonium a thermocouple in a thermowell for sensing the reaction
exchange with a NH4NO3 solution (1 M) for 8 h at 353 K temperature. The catalyst was activated at 673 K for 8 h in
to obtain the ammonium form. The samples were then fil- a flow of dry air followed by dry nitrogen before the reac-
tered, washed, and dried at 393 K overnight and further tions were conducted. A mixture of xylene and isopropanol
calcined at 813 K for 12 h in flowing air. The structure and (IPA) with the desired xylene-to-IPA molar ratio was in-
crystallinity and phase purity of the samples were charac- troduced into the reactor using a syringe pump (Sage
terized by X-ray (powder) diffraction (Rigaku, D/MAX III Instruments, Model 352, USA) at a particular feed rate and
VC, Japan) using CuKα radiation (λ = 1.5404). The crys- particular temperature. Nitrogen was used as a carrier gas
tal sizes were determined by scanning electron micrograph with a flow rate of 35 ml/min. Although fresh catalysts were
(
SEM; Jeol, JSM-5200), the surface area was measured by used every time for different measurements, the reusability
N2 adsorption and calculated using the BET equation, and of the catalyst was also studied, by regenerating the catalyst
the chemical composition of the samples was analyzed by by thermal treatment in the presence of air at 773 K and
atomic absorption spectroscopy.
reusing it.
The products were chilled, collected, and analyzed by gas
chromatograph (Agilent 6890 series, GC system) using a
Catalytic Measurements
The vapor-phase catalytic isopropylation of xylene iso- flame ionization detector and SUPELCO BETA-DEX 110
mers (ACROS, USA, 99+%) with isopropanol (S. d. fine- high-resolution (10% permethylated B-cyclodextrin) cap-
chem. Ltd., Bombay, 99+%) was carried out at moderate illary column (30 m × 0.320 mm × 0.25 µm). Nitrogen was
temperature (393–453 K) and at atmospheric pressure us- used as carrier gas (2 ml/min) in the following GC temp-
ingafixed-bedverticaldownflowglassreactorwitha15-mm erature program: 333 K (5 min), 4 K/min to 423 K (5 min),
internal diameter. The required amount of catalyst powder 10 K/min to 493 K (10 min). The six isomers can be iden-
was pressed (under 10 tons of pressure), palletized, crushed, tified in this temperature program. The order of elution of
and sieved to obtain 30- to 40-mesh-size (ASTM, USA) par- the six isomers matches that of the published literature (18,
ticles. The catalyst (0.5 g), diluted with a fourfold amount of 31). CarefulmeasurementoftheretentiontimesofallDMC
porcelain beads (weight basis) of equal size, was loaded at isomers (as shown in Fig. 1), in the GC column, showed the
FIG. 1. Chromatogram for the six DMC isomers with retention times (min) where only that portion of the chromatogram in which the all DMC
isomers are found is displayed. The products were analyzed by gas chromatograph (Agilent 6890 series, GC system) using a flame ionization detector
and SUPELCO BETA-DEX 110 high-resolution (10% permethylated B-cyclodextrin) capillary column (30 m × 0.320 mm × 0.25 µm). Nitrogen was
used as carrier gas (2 ml/min) in the following GC temperature program: 333 K (5 min), 4 K min to 423 K (5 min), 10 K/min to 493 K (10 min).

2
18
PATRA AND KUMAR
+
H
-
+
-
+
-
following order of elution: 3,5-DMC (21.24min)<2,5-DMC
22.11 min)<2,4-DMC(22.46min)<3,4-DMC (22.87 min)<
,6-DMC (23.76 min) < 2,3-DMC (24.28 min), where the
CH CH(OH)CH
+
Zeol
CH CH(OH ) Zeol
(CH )
+
H
O + Zeol
2
3
3
CH
3
2
3 2
(
2
retention times are given in parentheses. These retention
times, with one exception, also follow the order of boiling
points: 3,5-DMC (468.5 K) < 2,5-DMC (469.2 K) < 2,4-
DMC (472 K) < 3,4-DMC (474.8 K) < 2,6-DMC (473 K) <
+
(
CH ) CH
+
3
2
-
H Zeol
p-xylene
2,5 DMC
2
,3-DMC (475.6 K). The products were also identified
by gas chromatography–mass spectrometry (Shimadzu,
GCMS-QP2000A)andgaschromatography–infraredspec-
trometry (Perkin–Elmer, GC-IR 2000).
+
(
CH )
+
3
CH
2
-
+
H Zeol
RESULTS AND DISCUSSION
o-xylene
3,4 DMC
2,3 DMC
The X-ray powder diffraction pattern of all samples
clearly showed that except for silica–alumina, the sam-
ples were fully crystalline with no identifiable impurities.
The scanning electron microscopy of the samples showed
an absence of amorphous matter external to the zeolite
pore system. The sizes of all samples used in this work
are given in Table 1, along with other physicochemical
characteristics.
+
(
CH )
+
3
CH
+
2
+
-
H Zeol
m-xylene
3,5 DMC
2,4 DMC
2,6 DMC
SCHEME 1
A total of six DMC isomers can be formed during iso-
propylation of xylene isomers (Scheme 1). While p-xylene
yields 2,5-DMC (all regio positions are the same), m-xylene DMC. However, in order to get the complete picture of
gives 2,4- and 2,6-DMC as primary products (alkylation be- the relative stability of all DMC isomers, we contacted
ing an ortho–para directing electrophilic reaction) and 3,5- Rutkowska–Zbik and Witko (33), who kindly provided us
DMC as a secondary product, and o-xylene gives 2,3- and with the relative energies of the DMC isomers, calculated
3
,4-DMC as primary products.
by using the generalized gradient approximation (GGA)
Although the thermodynamic equilibrium values for based on the nonlocal revised Perdew–Burke–Ernzerhof
all DMC isomers are not readily available in the liter- potential (RPBE) (34, 35) functional for optimized ge-
ature, we tried to calculate the thermodynamic equilib- ometries. The stability of all six DMC isomers follows the
◦
rium constants (Ke) from the Gibbs free energy (ꢀG ) order 3,5-DMC > 3,4-DMC > 2,5-DMC ≥ 2,4-DMC ꢁ 2,3-
of formation for 2,3-, 2,4-, 2,5-, and 2,6-DMC isomers DMC ꢁ 2,6-DMC.
(
Table 2) at the different temperatures available in the
The conversion of isopropanol (IPA) was complete in
literature (32). Similar values for the other two isomers, most of the experiments. However, no aromatic compound
namely 3,5-DMC and 3,4-DMC, were not available (32)
and therefore Ke values for these isomers could not be
calculated. The relative stability of these four isomers
TABLE 2
follows the order 2,5-DMC ≈ 2,4-DMC ꢁ 2,3-DMC ꢁ 2,6-
Equilibrium Constants (Ke) and Standard Gibbs Free Energy
of Formation for DMC Isomers at Different Temperatures
TABLE 1
300 K
400 K
500 K
◦
◦
◦
Si/Al Molar Ratio, Crystallite Size, and BET Surface Area of the
Different Catalysts Used for the Isopropylation of m-Xylene
ꢀG
Ke
ꢀG
Ke
ꢀG
Ke
−27
−28
)
Isomer
(KJ/mol) (10−24) (KJ/mol) (10
)
(KJ/mol) (10
2
,3-DMC
136.2
131.8
131.8
137.9
1.945
11.347
11.347
0.984
203.1
198.1
198.1
205.4
3.032
13.631
13.631
1.518
272.5
267.1
267.1
275.4
0.344
1.26
1.26
Si/Al molar
ratio
Crystal size
Surface area
(m g )
2
−1
2,4-DMC
Sample
H-beta
H–mordenite
H–Y
USY
Silica–alumina
Sulfated zirconia
(µm)
2,5-DMC
17.5
5.9
2.9
5.6
2.6
0.2–0.3
0.3–0.6
1–1.5
0.4–0.6
—
609
519
700
655
105
100
2,6-DMC
0.171
◦
◦
Note. Ke were calculated using ꢀG = −RT ln Ke (where ꢀG is in
joules per mole, gas constant R = 8.314, and temperature T is in degrees
Kelvin). Similar data for the remaining 3,4- and 3,5-DMC isomers could
not be obtained and hence the percentage equilibrium distribution of all
3 wt% sulfur
0.2
◦
the isomersisnotgiven. The ꢀG KJ/mol values weretaken fromRef. (32).

ISOPROPYLATION OF XYLENES OVER ZEOLITE USY
219
was obtained when pure IPA was fed as reactant (in the ab-
sence of xylene) under the same reaction conditions. Only
TABLE 3
Effect of Temperature on Conversion (Conv.) and Product Se-
propylene was formed. Similarly, blank experiments where lectivity (Sel.) in the Isopropylation of Xylene Isomers over Zeolite
USY
pure xylene isomers were fed as reactant (in the absence
of IPA) under otherwise the same reaction conditions were
also carried out. Although little or no isomerization and
disproportionation products were formed at lower temper-
Reaction temperature (K)
393 403 413 423 433 443
Conv. or Sel.
(mol%)
Xyl.
atures (393–423 K), small amounts of isomerized and dis- p-Xyl. Conv.
6.9
27.6 36.0 72.8 76.0 90.4 96.8
93.4 92.8 92 85.3 78.6 66.7
25.8 33.4 67.0 64.8 71.1 64.6
9.0 18.2 19.0 22.6 24.2
Conv. (theo. max.)a
proportionated products were obtained at slightly higher
temperatures (433 K and above). The mass balance, car-
ried out for various experiments, ranged around 96 ± 2%.
At lower xylene conversions unreacted propylene was ob-
served in the products (mainly as a gaseous stream). Al-
though not given in the product distribution, it was included
in the computing of the mass balances.
Sel.DMCs
DMC yieldb
Sel.ISO (m- + o-xylene)
0.0
0.0
6.6
0.0
0.9
6.2
0.0 1.5 2.3 4.4
2.0 5.2 10.9 21.1
6.0 8.0 8.2 7.8
c
Sel.DISP
d
e
Sel.DIPT + Sel.DIPX
Sel. 2,5-DMC
100.0 100.0 100.0 97.3 95.0 93.6
0.0 0.0 0.0 2.7 5.0 6.4
Sel.Other(2,4-DMC)
m-Xyl. Conv.
9.3 12.3 17.5 21.8 23.8 24.9
37.2 49.2 70.0 87.2 95.2 99.6
95.3 94.8 91.6 90.2 88.5 73.8
34.7 45.7 64.1 78.7 84.3 73.5
Conv. (theo. max.)a
Sel.DMCs
Influence of Reaction Temperature
DMC yieldb
Asexpected, catalyticactivityandselectivityfordimethyl
cumene isomers in the isopropylation of all three xylene
isomers over USY are highly influenced by temperature
Sel.ISO (p- + o-xylene)
0.0
0.0
4.7
0.9
0.3
4.0
1.4 1.9 2.2 5.7
3.1 4.0 5.4 15.6
3.9 3.9 3.9 4.9
Sel.DISPc
Sel.DIPTd + Sel.DIPX
e
(
Table 3). With increasing temperature, the conversion of
Sel.3,5-DMC
58.8 59.5 68.0 69.3 74.3 77.0
40.7 39.5 29.6 28.0 22.8 19.5
Sel.2,4-DMC
xylenesincreases, almostreachingthetheoreticalmaximum
value (according to the xylene-to-isopropanol molar ratio
taken). However, with an increase in the reaction tempera-
ture there is an increase in some side reactions, such as dis-
proportionation (formation of toluene and trimethylben-
zenes) and isomerization (formation of xylenes other than
the substrate one) of the substrate xylene isomer. There is
a marginal increase in the dialkylated fraction (diisopropyl
toluene (DIPT) + diisopropylxylenes (DIPX)) with an in-
crease in temperature, as expected.
Sel.2,6-DMC
0.5
0.0
1.0
0.0
1.5 1.6 1.8 2.1
0.9 1.1 1.1 1.4
Sel.Other(2,5-DMC)
o-Xyl. Conv.
6.3 12.9 17.0 19.9 20.7 21.4
25.2 51.6 68.0 79.6 82.8 85.6
92.7 90.7 85.3 84.2 82.1 79.1
23.4 46.8 58.0 67.0 68.0 67.7
Conv. (theo. max.)a
Sel.DMCs
DMC yieldb
Sel.ISO (m- + p-xylene)
0.0
0.0
7.3
0.9
0.8
7.6
1.1 1.2 1.5 1.9
4.2 6.3 9.2 11.5
9.4 8.3 7.2 7.5
Sel.DISPc
Sel.DIPTd + Sel.DIPX
Sel.3,4-DMC
e
72.3 82.4 92.6 94.6 95.0 95.1
Sel.2,3-DMC
Sel.
Other(2,4-DMC)
26.5 16.1
1.2 1.5
6.4 3.1 2.9 2.7
1.0 2.3 2.1 2.2
In the case of p-xylene isopropylation, the selectivity to-
ward 2,5-DMC (Sel.2,5-DMC) also decreases slightly with in-
creasing temperature (from ca. 100% at 393 K to 93.6% at
Note. Reaction conditions: xylene-to-isopropanol molar ratio = 4 : 1;
−
1
WHSV = 6.5 h ; TOS = 1 h.
4
2
43 K). This could be possible (i) due to isomerization of
,5-DMC to 3,5-DMC and 2,4-DMC via a 1,2-methyl shift
a
b
Conversion of theoretical maximum.
DMC yield = (% Conv. (theo. max.) × % Sel./100); Sel.
DMC
= DMCs
or (ii) by the isopropylation of other xylene isomers (m- in total products; isopropanol conv. = 100%. At low xylene conversion
propylene was present in gaseous stream. Observed mass balance = 95 ±
and o-xylene) formed via isomerization of p-xylene.
In the case of m-xylene isopropylation, by increasing the
temperature, the selectivity for 2,4-DMC (Sel.2,4-DMC) de-
creases at the cost of the energetically most stable isomer,
3
% in all cases.
c
Toluene + trimethyl benzenes.
d
e
Diisopropyl toluenes.
Diisopropyl xylenes.
3
,5-DMC (Sel.3,5-DMC). 2,6-DMC was formed only in small
quantities (<2%), with the 2,4-DMC and 3,5-DMC isomers
In the case of o-xylene isopropylation, the selectivity of
being predominant. Hence, the energetically most favored 2,3-DMC (Sel.2,3-DMC) decreases from ca. 26% at 393 K
isomer, 3,5-DMC, can easily be formed through isomeriza- to ca. 3% at 443 K, with a consequent increase (from ca.
tion of 2,4-DMC via a 1,2-propyl shift, as evidenced by the 72% at 393 K to ca. 95% at 443 K) in the selectivity for
fact that Sel.2,4-DMC decreases with a consequent increase in 3,4-DMC (Sel.3,4-DMC), indicating that the energetically less
Sel.3,5-DMC with increasing temperature, whereas Sel.2,6-DMC favored isomer, 2,3-DMC, decreases with an increase in re-
remains almost unchanged. However, the formation of 3,5- action temperature, as expected. Here it may be pertinent
DMC can also be partly due to bimolecular isomerization to mention that Isakov et al. (15, 16) also reported the for-
of 2,4-DMC to 3,5-DMC in the case of USY, where there mation of a mixture of DMCs with 70–97% selectivity for
may be enough space to accommodate the bulky bimolec- 3,4-DMCduringisopropylationofo-xyleneovertheNdNa–
ular intermediate, along the lines similar to that shown by Y-type zeolite, with the remaining isomer being 2,3-DMC.
Corma and Sastre (36) for xylene isomerization.
It may be recalled that only o-xylene could be alkylated,

2
20
PATRA AND KUMAR
while no alkylation could be obtained when p-xylene was contact time of reactants with catalyst) and the Sel.DMCs
used as substrate (15, 16). These results are in contrast to increases during isopropylation of all xylene isomers. The
the present data, where significant conversion of all xylene enhancement of Sel.DMCs among products with increasing
isomers was obtained during isopropylation over USY cata- WHSV is due to a consequent decrease in the side reaction
lyst. Although an unambiguous explanation for such a dif- of xylene disproportionation.
ference in the reported and present results needs further
The selectivity for primarily formed DMC isomer(s) also
focused studies, it may be likely that the stronger acidity of increases with an increase in feed rate (decreasing resi-
zeolite USY compared with that of zeolite Y in general and dence time) (Table 4). In the case of p-xylene isopropy-
NdNa–Y in particular may be one of the main reasons.
lation, Sel.2,5-DMC increases marginally, from 98 to 100%.
For m-xylene isopropylation, Sel.3,5-DMC decreases from
ca. 75 to 53% while Sel.2,4-DMC increases correspondingly
and Sel.2,6-DMC does not change significantly with increas-
ing WHSV. This may be due to the fact that at high feed
rate (low contact times), alkylation reactions are favored
over the isomerization of primarily formed 2,4-DMC into
3,5-DMC. Similarly in the case of o-xylene isopropyla-
tion, with increasing WHSV and a consequent decrease in
conversion, the Sel.3,4-DMC decreases from ca. 95 to 81%,
with a consequent increase in the Sel.2,3-DMC, corroborat-
ing the results obtained using varying reaction tempera-
tures, as mentioned above. In both cases, viz. m-xylene and
o-xylene, the selectivity for thermodynamically more sta-
ble products such as 3,5-DMC (m-xylene) and 3,4-DMC
Influence of Feed Rate
The space velocity of the feed, i.e., WHSV (weight hourly
space velocity), is another parameter which influences con-
version and product selectivity (Table 4). As expected, the
conversion of xylene isomers decreases sharply with in-
−1
creasing WHSV, from 3.25 to 13 h (i.e., a decrease in
TABLE 4
Effect of Space Velocity on Conversion and Product Selectivity
in the Isopropylation of Xylene Isomers over Zeolite USY
WHSV (h 1
−
)
Xylene
isomer
Conv. or Sel.
(mol%)
3.25
6.50
13.00
(
o-xylene) increases with increasing conversion. Conse-
quently, at lower reaction temperatures and shorter contact
times, the selectivity for 2,4-DMC and 2,3-DMC (o-xylene)
93.3 is higher, as expected, because as the severity of the re-
p-Xyl.
Conv.
24.3
97.2
90.5
88.0
1.3
3.7
4.5
98.0
2.0
18.2
72.8
92
67.0
0.0
2.0
6.0
100
0.0
11.8
47.2
Conv. (theo. max.)a
Sel.DMCs
DMC yieldb
44.0
0.0
0.0
6.7
100
0.0
action increases, the isomer distribution tends to shift to-
ward the thermodynamic equilibrium values of the DMC
isomers.
Sel.ISO (m- + o-xylene)
c
Sel.DISP
Sel.DIPTd + Sel.DIPXe
Sel.2,5-DMC
Sel.Other(2,4-DMC)
Influence of Xylene/IPA Molar Ratio
m-Xyl.
Conv.
20.6
82.4
90.7
74.7
1.8
4.3
3.2
74.8
21.9
2.3
17.5
70.0
91.6
64.1
1.4
3.1
3.9
68.0
29.6
1.5
10.7
42.8
Conv. (theo. max.)a
The results of the influence of the xylene-to-isopropanol
93.1 molar ratio on catalytic activity and product distribution
Sel.DMCs
DMC yieldb
39.8
0.5
1.9
4.5
are presented in Table 5. With an increase in the xylene-to-
isopropanol molar ratio, the efficiency for the utilization of
the limiting reagent isopropanol for an isopropylation re-
action increased significantly in the case of all three xylene
Sel.ISO (m- + p-xylene)
c
Sel.DISP
Sel.DIPTd + Sel.DIPXe
Sel.3,5-DMC
52.8
Sel.2,4-DMC
45.0 isomers used as feed. Although absolute xylene conversion
Sel.2,6-DMC
1.5
0.7
decreased with an increase in the xylene-to-isopropanol
ratio, as expected, the conversion based on theoretical max-
imum values increased in all cases. At the lower xylene-
to-isopropanol molar ratio of 2, the selectivity of the
Sel.Other(2,5-DMC)
1.0
0.9
o-Xyl.
Conv.
24.3
97.2
80.0
77.8
1.7
11.3
7.0
94.6
4.2
17.0
68.0
85.3
58.0
1.1
4.2
9.4
92.6
6.4
1.0
9.4
37.6
86.4
32.5
Conv. (theo. max.)a
Sel.DMCs
DMC yieldb
dialkylated fraction (DIPT + DIPX) was significantly
Sel.ISO (m- + p-xylene)
0.0 higher (22% for p- and m-xylene and ca. 16% for o-xylene),
c
Sel.DISP
2.9
10.7
81.3
18.0
mainly at the cost of DMC, clearly indicating enhanced fur-
ther isopropylation of DMC to the dialkylated fraction.
However, with an increase in the xylene-to-isopropanol
molar ratio, the Sel.DMCs increased significantly because the
limited isopropanol was totally reacted with xylene to pro-
duce DMCs, resulting in less of a chance there would be
formation of a dialkylated fraction.
With an increase in the xylene-to-isopropanol molar
ratio from 2 : 1 through 10 : 1, Sel.2,5-DMC, in the case of
Sel.DIPTd + Sel.DIPX
e
Sel.3,4-DMC
Sel.2,3-DMC
Sel.Other(2,4-DMC)
1.2
0.7
Note. Reaction conditions: xylene-to-isopropanol molar ratio = 4 : 1;
T = 413 K; TOS = 1 h; isopropanol conv. = 100%. At low xylene conver-
−1
sion (WHSV = 13 h ), remaining unreacted propylene was present in
gaseous stream.
a–e
As in Table 3.

ISOPROPYLATION OF XYLENES OVER ZEOLITE USY
221
TABLE 5
tion does not seem to occur. In the case of o-xylene iso-
propylation, where the Sel.2,3-DMC decreased from ca. 12%
Effect of Xylene-to-Isopropanol Molar Ratio on Conversion and
Product Selectivity in the Isopropylation of Xylene Isomers over
Zeolite USY
(
(
at a xylene-to-isopropanol molar ratio of 2 : 1) to ca. 4%
at a xylene-to-isopropanol molar ratio of 10 : 1), the cor-
responding increase in Sel3,4-DMC was from ca. 87 to 96%
due to isomerization of the less stable 2,3-DMC to the more
stable 3,4-DMC.
Xylene-to-isopropanol
molar ratio
Xylene
isomer
Conv./Sel.
(mol%)
2 : 1
4 : 1
8 : 1
10 : 1
Influence of Time-on-Stream (TOS)
p-Xyl.
Conv.
34.4
68.8
76.8
52.8
0.0
18.2
72.8
92
67.0
0.0
2.0
6.0
100
0.0
9.7
77.6
97.6
75.7
0.0
0.4
2.0
100
0.0
8.9
89.0
100
89.0
0.0
a
Conv. (theo. max)
Sel.DMCs
Figure 2 exhibits the effect of TOS on conversion and
selectivity in the isopropylation of all three xylenes. With
increasing TOS the conversion of all xylenes (curve a in
Figs. 2A–C for p-, m-, and o-xylene, respectively) remained
DMC yieldb
Sel.ISO (m- + o-xylene)
c
Sel.DISP
1.6
0.0
d
e
Sel.DIPT + Sel.DIPX
21.6
97.3
2.7
0.0 almost the same for the first 2 h and then decreased, rather
Sel.2,5-DMC
100
0.0
sharply in the beginning, from ca. 95 to 75% xylene con-
version (of the theoretically maximum value of 25 mol%),
before stabilizing.
The product distribution is plotted in Figs. 2A–2F. With
TOS, the overall selectivity for alkylated products (SALK)
Sel.Other(2,4-DMC)
m-Xyl.
Conv.
Conv. (theo.max.)
22.2
44.4
74.6
33.1
1.0
17.5
70.0
91.6
64.1
1.4
3.1
3.9
68.0
29.6
1.5
10.3
82.4
98.0
80.8
0.0
0.6
1.4
70.6
27.8
1.1
8.8
88.0
99.0
87.1
a
Sel.DMCs
DMC yieldb
Sel.ISO (o- +p-xylene)
0.0 (curve b in Figs. 2A–2C), containing both mono- (DMCs)
0.2 and diisopropylated products (DIPT + DIPX), remained
c
Sel.DISP
1.8
Sel.DIPTd + Sel.DIPX
e
22.6
53.1
44.1
1.7
0.8
71.5
27.1
0.9
0.5
Sel.3,5-DMC
Sel.2,4-DMC
Sel.2,6-DMC
Sel.Other(2,5-DMC)
1.1
0.9
0.5
o-Xyl.
Conv.
Conv. (theo. max.)
21.4
42.8
80.4
34.4
0.3
17.0
68.0
85.3
58.0
1.1
4.2
9.4
92.6
6.4
1.0
10.7
85.6
85.9
73.5
1.2
5.5
7.4
94.4
4.8
0.8
9.4
94.0
88.0
82.7
1.3
5.4
5.3
95.6
3.7
0.7
a
Sel.DMCs
DMC yieldb
Sel.ISO (m- +p-xylene)
c
Sel.DISP
3.1
Sel.DIPTd + Sel.DIPX
e
16.2
86.5
12.1
1.4
Sel.3,4-DMC
Sel.2,3-DMC
Sel.Other(2,4-DMC)
Note. Reaction conditions: WHSV = 6.5 h−1; T = 413 K; TOS = 1 h;
isopropanol conv. = 100%. At xylene/IPA ratio = 2, remaining unreacted
propylene was present in gaseous stream. Theoretical maximum conver-
sions: 2 : 1 = 50.0; 4 : 1 = 25.0; 8 : 1 = 12.5; 10 : 1 = 10.0.
a–e
As in Table 3.
p-xylene isopropylation, increases from ca. 97 to 100%. In
the case of m-xylene, Sel.3,5-DMC increases from ca. 53 to
7
4
2.0% and consequently the Sel.2,4-DMC decreases, from ca.
4 to 27% and Sel.2,6-DMC is almost unchanged (ca. 1.5%).
These results indicate that with an increase in the xylene-
to-isopropanol molar ratio, the isomerization of 2,4-DMC
to 3,5-DMC (via a 1,2-propyl shift) is favored, once again
corroborating the selectivity trend influenced by tempera-
ture and WHSV. It may be recalled here that the formation (DMC + DIPT + DIPX); Sel.ISO, selectivity of xylene isomerized prod-
of 3,5-DMC can also be, at least partly, due to bimolecular
isomerization of 2,4-DMC to 3,5-DMC in the case of USY
with a large void space, where a bulky bimolecular interme-
diate can be accommodated. However, in the case of other
zeolites (mordenite and beta), such bimolecular isomeriza- isopropanol (IPA) molar ratio, 4 : 1; WHSV, 3.25 h−1
FIG. 2. (A–C) Conversion versus time-on-stream (h) (curve a),
Sel.ALK (b), Sel.ISO (c), and Sel.DISP (d); (D–F) Sel.DMC (e), Sel.2,5-DMC
(
f), Sel.3,5-DMC (g), Sel.2,4-DMC (h), Sel.2,6-DMC (i), Sel.3,4-DMC (j),
and Sel.2,3-DMC (k). SALK, Selectivity of total alkylated product
ucts other than the substrate one; Sel.DISP, selectivity of diproportionated
products; Sel.DMC, selectivity of dimethyl cumenes among all products. For
Sel.2,5-DMC, Sel.3,5-DMC, Sel.2,4-DMC, Sel.2,6-DMC, Sel.2,3-DMC, and Sel.3,4-DMC
isomers, the number in the subscript indicates the corresponding dimethyl
cumene. Reaction conditions: Temperature, 413 K; substrate (xylene)-to-
.

2
22
PATRA AND KUMAR
constant or increased marginally. The selectivities for iso-
merized (Sel.ISO, curve c) and disproportionated (Sel.DISP,
TABLE 6
Effect of TOS on Conversion and Product Selectivity in the Iso-
curve d) products also did not vary significantly and ranged propylation of Equilibriuam Mixtures of Xylenes over Zeolite USY
between 0 and 5% in the case of p- and m-xylenes. How-
ever, in the case of o-xylene, while Sel.ISO was nearly zero,
the Sel.DISP ranged between 10 and 7%. The selectivity of
dimethyl cumenes (Sel.DMCs) (curve e, Figs. 2D–2F) also re-
mained constant over time, at ca. 90% for p- and m-xylenes Conv. (theo. max.)
and ca. 80–85% for o-xylene.
Regarding DMC isomer distribution, there was no sig-
nificant change over TOS in the case of the 2,5-DMC
Time-on-stream
Conv. or Sel. (mol%)
Conversion
1
2
3
4
5
23.7
94.8
95.8
3.3
23.6
94.4
96.2
2.8
22.1
88.4
96.4
2.0
21.7
86.8
96.8
1.6
21.1
84.4
97.0
1.0
Sel.DMCs in total products
Sel.DISP in total products
Sel.( in total products
0.9
1.0
1.6
1.6
2.0
DIPT+DIPX)
DMC isomer distribution
Sel.3,5-DMC
(
Sel.2,5-DMC) formed from p-xylene isopropylation (curve f,
41.1
30.6
16.1
10.5
1.2
39.6
30.4
17.3
11.0
1.2
39.6
29.1
18.2
11.8
0.9
37.6
28.8
19.4
13.0
0.8
36.7
28.8
19.9
13.4
0.8
Fig. 2D), with ca. 100% selectivity among DMCs. In the
case of m-xylene, the distribution of DMC isomers was in-
fluenced significantly, while the selectivity for the energet-
ically less favored 2,4-DMC (curve h, Fig. 2E) increased
from ca. 22 to 30% at the cost of the energetically more fa-
vored 3,5-DMC (curve g, Fig. 2E). Selectivity of 2,6-DMC
Sel.3,4-DMC
Sel.2,5-DMC
Sel.2,4-DMC
Sel.2,3-DMC
Sel.2,6-DMC
0.5
0.5
0.4
0.4
0.4
Note. Reaction conditions: temperature = 413 K; xylene mixture
(Sel.2,6-DMC; curve i, Fig. 2E) remained almost unchanged (24% p-xylene, 54% m-xylene, and 22% o-xylene)/isopropanol molar
1
(ca. 2%) with increasing TOS.
−
ratio = 4 : 1; WHSV = 3.25 h
.
A decrease in selectivity for 3,5-DMC (Sel.3,5-DMC) along
with a decrease in conversion with TOS may be ascribed to
(
i) selective poisoning of stronger acid sites, which are re- tally obtained selectivity order of all the DMC isomers
sponsible for isomerization of 2,4-DMC to 3,5-DMC (while follows the order 3,5-DMC > 3,4-DMC > 2,5-DMC > 2,4-
the isopropylation (primary reaction) reaction leading to DMC > 2,3-DMC > 2,6-DMC. It may be recalled that the
2
(
,4-DMC may take place on the less strong acid sites), and thermodynamic stability of the DMC isomers also follows
ii) narrowing of the void space due to coking. In other the same order. Formation of the energetically most stable
words, the diffusion of 3,5-DMC (which requires more isomer, 3,5-DMC, can be accounted for by the isomeriza-
space than the other isomers) becomes slower than that tion of mainly 2,4-DMC (as mentioned above). The total
of 2,4-DMC and hence selectivity of 2,4-DMC is increased DMC selectivity ranged between 95.8 and 97.0 mol%, with
at the cost of 3,5-DMC with increasing TOS. This observed remaining 3.0–4.2 mol% being the disproportionated and
selectivity trend of 3,5- and 2,4-DMC isomers also corrobo- dialkylated products.
rates the results reported in Tables 3–5 and discussed above.
However, the isomer distribution in the case of o-xylene Comparison of USY with Different Catalysts
isopropylation (Fig. 2F) did not change with time, with the
selectivity of 2,3-DMC (Sel.2,3-DMC) (curve k, Fig. 2F) and
selectivity of 3,4-DMC (Sel.3,4-DMC) (curve j, Fig. 2F) re-
maining almost the same, at ca. 4 and 94%, respectively;
the remaining 2% was 2,4-DMC.
in m-Xylene Isopropylation
From the above studies it is clear that while in the iso-
propylation of p- and o-xylene, the product distribution,
particularly DMC isomer distribution, did not change signi-
ficantly with various reaction parameters; the DMC isomer
distribution in the case of m-xylene (2,4-DMC versus 3,5-
DMC) was significantly influenced by different reaction pa-
rameters. Hence, it was thought worthwhile to investigate
Isopropylation of Equilibrium Mixture of Xylenes
over Zeolite USY
In order to study the overall selectivity pattern of all the influence of difference pore geometries using different
DMC isomers, an equilibrium mixture of xylenes (24% p- solid acid catalysts. Therefore, the isopropylation of m-
xylene, 54% m-xylene, and 22% o-xylene) was isopropy- xylenewasstudiedasafunctionoftime-on-streamusingdif-
lated over the USY catalyst as a function of TOS between ferent catalysts, such as large-pore zeolites (H–mordenite,
1
and 5 h (Table 6). The overall xylene conversion ranged H-beta, and H–Y), amorphous silica–alumina, and sulfated
between 23.7 and 21.1 mol% (94.8–84.4% of the theoretical zirconia (Fig. 3). It is quite interesting that all the cata-
maximum of 25 mol%, based on a 4 : 1 xylene : isopropanol lysts, except USY, gave 2,4-DMC as the predominant iso-
molar ratio). The relative distribution of the six isomers mer (ranging between 60 and 85%) where the Sel.3,5-DMC
(
3
Table 6) in their order of elution from GC (Fig. 1) was ranged between 10 and 35%. In contrast, the correspond-
,5-DMC (41.1 to 36.7%), 2,5-DMC (16.1 to 19.9%), 2,4- ing isomer distribution over USY was just the opposite
DMC (10.5 to 13.4%), 3, 4-DMC (30.6 to 28.8%), 2,6-DMC where Sel.2,4-DMC ranged between 20 and 30%, compared
0.5 to 0.4%), and 2,3-DMC (1.2 to 0.8%). The experimen- with Sel.3,5-DMC of ca. 65–75%. As mentioned earlier, this
(

ISOPROPYLATION OF XYLENES OVER ZEOLITE USY
223
1
00
100
C
F
E
D
8
0
0
0
0
80
2
, 4 DMC
6
4
2
Conv.
60
40
20
0
2, 4 DMC
Conv.
3, 5 DMC
3, 5 DMC
0
0
2
4
6
8
0
2
4
4
4
6
8
1
00
100
80
60
40
20
0
B
8
0
0
0
0
2, 4 DMC
2
, 4 DMC
6
4
2
Conv.
4
Conv.
3, 5 DMC
3, 5 DMC
2
0
0
2
6
8
0
6
8
1
00
100
80
60
40
20
0
A
8
0
0
0
0
2, 4 DMC
3
, 5 DMC
6
4
2
Conv.
Conv.
8
3, 5 DMC
2
2, 4 DMC
0
0
2
4
6
8
0
6
Time on Stream, h
FIG. 3. Conversion versus time-on-stream (TOS; h), Sel.3,5-DMC, and Sel.2,4-DMC versus TOS for the isopropylation m-xylenes over different
catalysts (A, USY; B, H–mordenite; C, silica–alumina; D, H–Y; E, H-beta; and F, sulfated zirconia). Reaction conditions: Temperature, 413 K; m-xylene-
−1
to-isopropanol (IPA) molar ratio, 4 : 1; WHSV, 3.25 h
.
may also be due to the fact that isomerization of 2,4-DMC H–mordenite and H-beta, the formation of 3,5-DMC might
to 3,5-DMC or 2,3-DMC to 3,4-DMC may occur, at least have even been suppressed due to geometric constraints
partly, through a bimolecular mechanism in USY, which has also. However, in H–Y zeolite, which also exhibited only
larger cavities compared with other zeolites (beta, morden- ca. 10–12% 3,5-DMC, such constraints may be less pro-
ite), as the intermediate of this bimolecular reaction can be nounced. Hence, it is clear that both the acidity (mainly
too bulky to be accommodated within the void space of the the strength of Brønsted acid sites) and the geometric con-
BEA or MOR structures.
straints of the catalysts significantly influence the product
Even at comparable conversion levels, for example, the distribution. These results provide leads for further studies,
DMC isomer distribution over silica–alumna (Fig. 3C) and particularly on the isopropylation of m-xylene using differ-
sulfated zirconia (Fig. 3F) is completely different from that ent catalysts with the same structure and varying acidity.
obtained over USY (Fig. 3A). These results clearly indicate
Figure 3 also shows the deactivation nature of the differ-
that the isomer distribution, particularly that of 2,4-DMC ent catalysts with time. While all the catalysts were deac-
and 3,5-DMC in m-xylene isopropylation, could be con- tivated with time, H–mordenite was deactivated very fast,
trolled by various factors, such as (i) the ability of the cata- probably due to rapid formation of coke. It seems that both
lysts in the isomerization vis- a` -vis alkylation of 2,4-DMC, the acid characteristics and the pore structure of the zeolites
(
ii) the selective poisoning of the stronger acid sites (re- play a significant role during coking, which is responsible
sponsible for isomerization) in certain zeolites and other for catalyst deactivation.
catalysts, (iii) the partial contribution of bimolecular iso-
merization in the case of USY, and (iv) the partial blockage
of channels due to coking. While amorphous silica–alumina
and sulfated zirconia exhibited very good alkylation
CONCLUSIONS
The isopropylation of all three xylene isomers is readily
activity in m-xylene isopropylation, the isomerization of catalyzed by zeolite USY. Detailed studies on the influ-
,4-DMC to 3,5-DMC was rather suppressed. In the case of ence of different parameters (such as temperature, space
2

2
24
PATRA AND KUMAR
velocity, xylene-to-isopropanol molar ratio, and time-on- 13. Barile, G. C., and Kaeding, W. W., Eur. Patent 0 021 600 (1981).
1
1
1
1
4. Babin, E. P., Lozovoi, V. I., Krasnoshchek, A. P., Kurilo, L. I., and
Begunov, N. A., Vopr. Khim. Tekhnol. 54, 36 (1979) (Russian).
5. Isakov, Y. I., Minachev, K. M., Kalinin, V. P., and Isakova, T. A., Dokl.
Akad. Nauk 335, 322 (1994) (Russian).
6. Isakov, Y. I., Minachev, K. M., Kalinin, V. P., and Isakova, T. A., Izv.
Akad. Nauk Ser. Khim. 12, 2912 (1996) (Russian).
stream) governing conversion and selectivity demonstrated
that zeolite USY is a quite active catalyst for the isopropyla-
tion of all three xylene isomers. A quite moderate reaction
temperature range (413–423 K) is suitable for high activ-
ity and DMC selectivity. The isopropylation of m-xylene
was also carried out over five other solid acid catalysts, ze-
olites H–Y, H-beta, and H–mordenite and silica–alumina
and sulphated zirconia, for comparative studies. It is inter-
esting that while over USY the Sel.3,5-DMC was quite high
7. Sabu, K. R., Rao, K. V. C., and Nair, C. G. R., Indian J. Chem. Sect. B
33B(11), 1053 (1994).
1
1
8. Novak, M., and Heinrich, J., J. Chem. Educ. 70, A150 (1993).
9. Patra, C. R., Kartikeyan, S., and Kumar, R., Stud. Surf. Sci. Catal. 135,
283 (2001).
(
70–80%), all other solid acid catalysts, including the zeo- 20. Ito, K., Hydrocarbon Process. 50, 89 (1973).
2
2
2
2
2
2
1. Block, S. S., in “Kirk–Othmer Encyclopedia of Chemical Technology”
J. I. Kroschwitz and M. Howe-Grant, Eds.), Vol. 8. p. 237. Wiley–
lites, exhibited high selectivity for 2,4-DMC. Among all the
catalysts studied, USY was most resistant toward deactiva-
tion and H–mordenite deactivated the fastest.
(
Interscience, New York, 1992.
2. Derfer, J. M., and Derfer, M. M., in “Kirk–Othmer Encyclopedia of
Chemical Technology” (M. Grayson and D. Eckroth, Eds.), Vol. 22.
p. 709. Wiley–Interscience, New York, 1978.
3. Welsted, W. J., in “Kirk–Othmer Encyclopedia of Chemical Tech-
nology” (M. Grayson and D. Eckroth, Eds.), Vol. 9. p. 542. Wiley–
Interscience, New York, 1978.
4. Brode, G. L., in “Kirk–Othmer Encyclopedia of Chemical Technol-
ogy” (M. Grayson and D. Eckroth, Eds.), Vol. 17. p. 384. Wiley–
Interscience, New York, 1978.
5. Lorenc, J. F., Lambeth, G., and Scheffer, W., in “Kirk–Othmer Ency-
clopedia of Chemical Technology” (J. I. Kroschwitz and M. Howe–
Grant, Eds.), Vol. 2. p. 113. Wiley–Interscience, New York, 1992.
6. Ratnasamy, P., Bhat, R. N., Pokhriyal, S. K., Hegde, S. G., and Kumar,
R., J. Catal. 119, 65 (1989).
ACKNOWLEDGMENTS
C. R. Patra is thankful to CSIR, New Delhi, India, for providing a re-
search fellowship and to Mr. N. R. Shiju for helpful discussions. We are
also thankful to Dr. D. Rutkowska-Zbik and Prof. M. Witko, Institute of
Catalysis and Surface Chemistry, PAS, Poland, for providing the relative
energy of all six DMC isomers.
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