Shape-selective isopropylation of aromatic hydrocarbons over h-mordenite in supercritical carbon dioxide medium

Article abstract of DOI:10.1246/bcsj.20110214

The isopropylation of aromatic hydrocarbons isobutylbenzene (IBB), naphthalene (NP), and biphenyl (BP) was examined over H-mordenite (MOR), H-β (BEA), and H-Y (FAU) zeolites in supercritical carbon dioxide (sc-CO2) medium. MOR was only selective for the formation of the least bulky 4-isobutylcumene (4-IBC) in the isopropylation of IBB. In particular, the catalytic activity and selectivity for 4-IBC were enhanced by the dealumination of MOR; MOR with 110 of SiO2/Al2O3 ratio rendered the highest performance; however, the catalytic activity was decreased by further dealumination. Thermogravimetric analyses confirmed the reduction of coke formation on the catalysts in sc-CO2 medium, preventing the deactivation of MOR. Shape-selective formation of the least bulky isomers, 2,6- diisopropylnaphthalene (2,6-DIPN) and 4,4′-diisopropylbiphenyl (4,4′-DIPB), was also observed in the isopropylation of NP and BP over MOR in sc-CO2. sc-CO2 works as an efficient medium to access and/or replace substrates and their products to/from acidic sites in the MOR channels. In particular, the removal of coke precursors from acidic sites on the zeolite is enhanced by the sc-CO2 medium, resulting in decreased coke formation.

Full text of DOI:10.1246/bcsj.20110214

©
2011 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 84, No. 11, 1283­1290 (2011) 1283
Shape-Selective Isopropylation of Aromatic Hydrocarbons over
H-Mordenite in Supercritical Carbon Dioxide Medium
³
Subhash Chandra Laha, Hiroaki Naiki, Kenichi Komura, and Yoshihiro Sugi*
Department of Materials Science and Technology, Faculty of Engineering, Gifu University, Gifu 501-1193
Received July 15, 2011; E-mail: ysugi@gifu-u.ac.jp
The isopropylation of aromatic hydrocarbons isobutylbenzene (IBB), naphthalene (NP), and biphenyl (BP) was
examined over H-mordenite (MOR), H-¢ (BEA), and H-Y (FAU) zeolites in supercritical carbon dioxide (sc-CO2)
medium. MOR was only selective for the formation of the least bulky 4-isobutylcumene (4-IBC) in the isopropylation of
IBB. In particular, the catalytic activity and selectivity for 4-IBC were enhanced by the dealumination of MOR; MOR
with 110 of SiO2/Al2O3 ratio rendered the highest performance; however, the catalytic activity was decreased by
further dealumination. Thermogravimetric analyses confirmed the reduction of coke formation on the catalysts in
sc-CO2 medium, preventing the deactivation of MOR. Shape-selective formation of the least bulky isomers, 2,6-
diisopropylnaphthalene (2,6-DIPN) and 4,4¤-diisopropylbiphenyl (4,4¤-DIPB), was also observed in the isopropylation of
NP and BP over MOR in sc-CO2. sc-CO2 works as an efficient medium to access and/or replace substrates and their
products to/from acidic sites in the MOR channels. In particular, the removal of coke precursors from acidic sites on the
zeolite is enhanced by the sc-CO2 medium, resulting in decreased coke formation.
Zeolite-catalyzed shape-selective catalysis is a promising
other fine chemicals.24­27 In particular, we emphasize our
results on dealuminated H-mordenite (MOR) catalysts and the
way to synthesize symmetrically substituted isomers for
industrial chemicals and advanced materials.1 Shape-selective
catalysis occurs to form the least bulky isomer because the
zeolite pores prevent the formation of bulkier isomers.1 Types
of zeolite, adjustment of pore radii and acidity, and/or
deactivation of external acid sites are all key factors for the
enhancement of shape-selective catalysis.1
­9
impact of sc-CO medium for achieving higher conversion,
2
and, more importantly, higher selectivity for 4-IBC. We also
studied the isopropylation of polynuclear aromatic hydro-
carbons, naphthalene (NP) and biphenyl (BP) to elucidate
­6
shape-selective catalysis in sc-CO medium over MOR.
2
,2,4­6
Experimental
In general, solid acid catalysts such as zeolites with confined
micropores suffer deactivation due to coke formation on the
Zeolites. H-Mordenites (MOR(10), MOR(26), MOR(73),
MOR(110), MOR(128), and MOR(230)) and H-ZSM-5
active sites of the catalysts.1
0­12
Therefore, one of the most
28
important challenges for green catalytic processes is to
minimize coke formation by controlling diffusion of reac-
tant(s), product(s), and coke-precursors from active sites.
Supercritical fluids, such as supercritical CO (sc-CO ), have
become more attractive media for chemical reactions. They
offer enhanced solubility of reactants, products, and coke-
precursors, faster heat transfer from catalysts to dense gases,
(MFI(54)) obtained as the NH -form) were obtained from
4
TOSOH Corporation, Tokyo, Japan. H-Y (FAU(30) and
FAU(80)) and H-¢ (BEA(26)) zeolites were obtained from
Zeolyst International, PA, USA. ZSM-11 (MEL(49)) zeolite
was obtained from N. E. CHEMCAT Corporation, Tokyo,
2
2
2
9
Japan. BEA(100) was synthesized according to the literature.
All zeolites were calcined at 550 °C in air for 6 h prior to use.
and easier product separation from the catalysts, by tuning
Isopropylation in sc-CO . The isopropylation was carried
2
pressure and temperature.1
3­15
Moreover, higher molecular
weight carbon-rich compounds such as polynuclear aromatic
hydrocarbons are soluble in the reaction conditions due to
out in a 100 mL SUS-316 autoclave (equipped with an agitator)
using dry ice as the source of CO2. Typical reaction conditions
for the isopropylation of IBB were 3.9 g of IBB (25 mmol),
1.5 g of 2-propanol (25 mmol), and 0.125 g of the catalyst, with
the high density and low viscosity of sc-CO , resulting in
2
minimizing catalyst deactivation. All of these reasons have
8­12 MPa CO pressure at 130­170 °C for 12 h. Dry ice, an
2
established sc-CO as an attractive choice for some reactions,
amount calculated for target pressure, was placed with IBB,
2-propanol, and the catalyst in an autoclave before heating.
After reaching the reaction temperature, the reaction was
started by the agitation, and the pressure and the temperature
were kept constant throughout the reaction. The pressures at
reaction temperatures were almost the same as target pressures
by calculating from the weight of dry ice. After the reaction,
the reactor was depressurized very slowly after cooling in an
ice bath. The contents of the reactor were extracted with
2
such as solid acid catalysis.1
6­23
In this paper, we report the isopropylation of isobutylben-
zene (IBB) to isobutylcumene (IBC) isomers in sc-CO2
medium, where 3- and 4-IBCs are important precursors for
the synthesis of antioxidants, pharmaceuticals, fragrances, and
³
Present address: SABIC Research and Technology Pvt. Ltd.,
Manjusar, Vadodara 391775, Gujarat, India
Published on the web November 9, 2011; doi:10.1246/bcsj.20110214

1
284 Bull. Chem. Soc. Jpn. Vol. 84, No. 11 (2011)
Alkylation of Aromatic Hydrocarbons in sc-CO2
100
5
Table 1. Isopropylation of IBB over Typical Zeolite
Catalysts
a)
Selectivity/mol %
8
6
0
0
4
Conv.
mol % 2-IBC 3-IBC 4-IBC DIPIBB
Catalysts
4-/3-IBC
4-IBC
/
MOR(26) 18.5
®
14.5
11.7
®
22.9
40.7
13.0
®
77.1
20.8
31.8
®
®
4.2
34.3
®
3.4
0.51
2.4
®
3
BEA(26)
FAU(30)
MFI(54)
MEL(49)
40.8
40.0
®
Conversion
2
40
®
®
®
®
®
®
a) Reaction conditions: IBB, 3.35 g (25 mmol); 2-propanol,
.50 g (25 mmol); catalyst, 0.50 g; CO2 pressure, 11 MPa;
temperature, 150 °C; period, 12 h.
20
1
1
3-IBC
0
0
0
50
100
150
200
250
acetone and analyzed with a Shimadzu GC-14C or GC-18A gas
chromatograph equipped with an Ultra-1 capillary column
SiO
2
/Al
2
O
3
(
25 m © 0.2 mm; film thickness: 0.33 ¯m; Agilent Techno-
Figure 1. Influence of SiO2/Al2O3 ratio of MOR on the
isopropylation of IBB. Reaction conditions: IBB, 3.35 g
logies) for the products from IBB and BP, and with a TC-17
column (25 m © 0.25 mm; film thickness: 0.25 ¯m; GL Sci-
ences, Tokyo) for the products from NP. The products were also
confirmed by GC-MS (Shimadzu GC-MS5000). Thermogravi-
metric analyses (TGA) were conducted on a Shimadzu DTG-50
analyzer with a temperature-programmed rate of 10 °C min in
an air stream, where the catalyst used for the reaction was
separated by filtration and dried overnight at 40 °C.
(25 mmol); 2-propanol, 1.50 g (25 mmol); catalyst, MOR
(SiO /Al O = 9­230) 0.50 g; CO pressure, 11 MPa;
2
2
3
2
temperature, 150 °C; period, 12 h.
¹
1
100
SiO
2
/Al
2
O
3
98
Results and Discussion
230
9
9
9
9
8
6
4
2
0
8
The Isopropylation of Isobutylbenzene (IBB). Table 1
shows the isopropylation of IBB over typical zeolites at 150 °C
110
under 11 MPa of CO pressure. 2-Propanol was used as the
73
2
precursor of alkylating agents because it converted to propene
under the reaction conditions. MOR(26), which has 12-
membered ring (12-MR) pore entrances, gave a high selectivity
for isobutylcumene (4-IBC) (77.1%), although IBB conversion
was only 18.5%. There were no 2-IBC and diisopropylisobutyl-
2
1
6
0
benzene (DIPIBB) isomers formed. Here, CO pressure was
2
25
200
400
600
800
adjusted by the amount of dry ice, where the pressure was the
same as calculated. We believe the water adhered to the dry ice
and formed by the dehydration did not much influenced the
catalysis judging from our previous results.1
High selectivity for 4-IBC was not achieved over BEA(26)
and FAU(30) zeolites with three-dimensional 12-MR pore
entrances, although moderate IBB conversion was recorded
Temperature /°C
Figure 2. Influence of SiO2/Al2O3 ratio of MOR on the TG
profiles of the catalysts after the isopropylation of IBB.
The catalysts were recovered by filtration and dried at
40 °C overnight.
8­20
(
µ40%). MFI(54) and MEL(49), which have smaller 10-MR
of IBCs resulted in an overall increase of the 4-IBC/3-IBC
ratio, which is the index of preferred recognition of 4- and
3-IBC by zeolites.
pore entrances, showed no catalytic activity. These selectivity
patterns were in accordance with the pore diameter and
dimensionality of these zeolites and implied low shape
selectivity for BEA and FAU zeolites. MFI and MEL have
smaller pore diameters for the isopropylation of IBB.
Figure 2 shows the TGA profiles of the MOR catalysts after
the reaction. The weight losses were strongly dependent on the
SiO /Al O ratio. In particular, the weight losses between
2
2
3
Figure 1 summarizes the influence of the SiO /Al O ratio
of MOR on the isopropylation of IBB at 150 °C under 11 MPa
350­650 °C, which were due to coke formation during the
reaction, decreased rapidly with increased ratios. SiO /Al O
3
2
2
3
2
2
of CO pressure. The catalytic activity increased rapidly with
ratios of 10, 26, 73, 110, and 230 resulted in weight losses of
2.2, 3.5, 2.0, 1.0, and 1.2%, respectively. These results
signified that decreased coke formation enhanced catalytic
activity although the dealumination decreased the number of
acidic sites on MOR. Therefore, a reduced number of catalytic
sites is not always the main reason for decreased catalytic
activity. In retrospect, the reduction in coke formation in zeolite
was more important for stable and improved catalytic perform-
2
increases in the SiO /Al O ratio, and the selectivity for
2
2
3
4
-IBC also improved slightly with the dealumination. In
addition, MOR(110) showed a remarkable increase in IBB
conversion (µ70%), along with a very high selectivity for
4
-IBC (µ81%). However, further dealumination resulted in
decreased catalytic activity with slight improvement in the
selectivity for 4-IBC. These changes in the selectivity patterns

S. C. Laha et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 11 (2011) 1285
1
00
4.5
Table 2. Influences of SiO2/Al2O3 Ratio of FAU and BEA
Zeolites on the Isopropylation of IBB
Conversion
a)
8
0
Conv.
mol % 2-IBC 3-IBC 4-IBC DIPIBB
Selectivity/mol %
Catalysts
4-/3-IBC
4
-IBC
/
BEA(26)
BEA(100)
FAU(30)
FAU(80)
40.8
9.7
40.0
2.4
14.5
7.3
11.7
28.7
40.7
40.0
13.0
25.8
20.8
52.7
31.8
45.5
4.2
®
0.51
1.3
2.4
60
4
34.3
®
4
0
1.8
a) Reaction conditions: IBB, 3.35 g (25 mmol); 2-propanol,
.50 g (25 mmol); catalyst, 0.50 g; CO2 pressure, 11 MPa;
1
20
temperature, 150 °C; period, 12 h.
3
-IBC
0
3.5
ance because deposited coke chokes pores and channels, and
6
9
12
15
18
3
0
retard the catalysis. However, deep dealumination of MOR to
30 of SiO /Al O ratio decreased IBB conversion, suggesting
Reaction period /h
2
2
2
3
that catalytic activity declined due to lower acid density even
with the decrease in choking pores and channels. It must be
Figure 3. Influence of reaction time on the isopropylation
of IBB over MOR(110). Reaction conditions: IBB, 3.35 g
(25 mmol); 2-propanol, 1.50 g (25 mmol); catalyst:
MOR(110), 0.50 g; CO2 pressure, 11 MPa; temperature,
noted that coke formation in sc-CO medium was much lower
2
than in conventional solvent-free or organic solvent.
1
50 °C; period, 6, 12, and 18 h. Legends: see Figure 1.
Similar influences of the SiO /Al O ratio of MOR were
2
2
3
3
1,32
observed in liquid-phase isopropylation of NP and BP.
In
100
5
both cases, the catalytic performances were enhanced by deep
dealumination, with the highest catalytic activity and selectiv-
ity for 2,6-diisopropylnaphthalene (2,6-DIPN) and 4,4¤-diiso-
Conversion
4-IBC
80
propylbiphenyl (4,4¤-DIPB) observed over MOR with a
SiO /Al O ratio of around 200.4
­6,31,32
4.5
These differences
2
2
3
60
among IBB, NP, and BP were due to differences in reaction
atmosphere (see below and our previous results1 ) and to the
coke formation from them and their products. It was also noted
that IBB could more easily diffuse through MOR channels than
BP and NP, resulting in decreased catalytic performance in
MOR with deep dealumination.
8­20
40
4
20
Table 2 summarizes the influences of SiO /Al O ratio of
2
2
3
3-IBC
BEA and FAU in the isopropylation of IBB. The catalytic
activities decreased with increasing the SiO /Al O ratios of
0
3.5
130
140
150
160
170
2
2
3
Reaction temperature /°C
BEA and FAU, particularly for FAU. The formation of DIPIBB
was not observed over BEA and FAU with the high ratio.
However, the formation of IBCs also decreased; however, the
selective formation of 4-IBC was not observed because of the
simultaneous formation of 2- and 3-IBCs.
Figure 4. Influence of reaction temperature on the isopro-
pylation of IBB over MOR. Reaction conditions: IBB,
.35 g (25 mmol); 2-propanol, 1.50 g (25 mmol); catalyst:
MOR(110), 0.50 g; CO2 pressure, 11 MPa; temperature,
3
The coke formation also decreased with increasing SiO2/
Al O ratio of BEA and FAU (data not shown). These results
130­170 °C; period, 12 h. Legends: see Figure 1.
2
3
are also quite different from the results of MOR. The decreases
in catalytic activities of FAU and BEA by the increase in SiO2/
Al O ratio are ascribed to the decrease in acid sites although
of coke formed on the catalysts during the isopropylation
reduced the catalytic activity, but its effect was most
significant in the case of one-dimensional MOR. Based on
the above discussions, MOR(110) was selected for further
studies.
2
3
these zeolites are not easily deactivated by the coke formation
in them because of their three-dimensional pore system.
However, the pores of one-dimensional zeolites like MOR
were prone to the deactivation due to coke formation. These
results indicate that the decrease in coke formation does not
always result in improved catalytic performance of the zeolites.
These differences were principally due to the pore structures of
the zeolites, MOR, BEA, and FAU.
Figure 3 shows the influence of reaction time on the
isopropylation of IBB at 150 °C over MOR(110). The IBB
conversion increased with reaction time under sc-CO medium,
2
however, the selectivity for 4-IBC decreased slightly with
reaction time. Thus, the ratio of 4-/3-IBC gradually decreased
from 4.2 to 4.0, although 2-IBC was not detected even after
18 h. The continuation of the reaction resulted in saturation of
the IBB conversion, but prolonged reaction times also triggered
the formation of a small amount of DIPIBB (1.2%). Thus, it is
obvious that external acid sites catalyzed the formation of
3-IBC and DIPIBB isomers.
The existence of dense and strongly acidic sites on the
zeolites also triggered the formation of 2-IBC, DIPIBB, and
other products in a higher proportion. Furthermore, dense acid
sites in aluminum-rich catalysts caused coke formation in
higher amounts on the catalysts.1
0,12,30
Clearly, higher amounts

1
286 Bull. Chem. Soc. Jpn. Vol. 84, No. 11 (2011)
Alkylation of Aromatic Hydrocarbons in sc-CO2
1
00
100
100
80
5
4
-IBC
8
6
4
2
0
4
3
2
1
0
9
8
7
0
0
0
0
0
0
60
4
2
0
0
0
3-IBC
0
60
hexane
sc-CO
2
without solvent
8
9
10
11
12
CO
2
pressure /MPa
Figure 6. Isopropylation of IBB over MOR(110) in differ-
ent reaction media. Reaction conditions: IBB, 3.35 g
Figure 5. Influence of CO2 pressure on the isopropylation
of IBB over MOR(110). Reaction conditions: IBB, 3.35 g
25 mmol); 2-propanol, 1.50 g (25 mmol); catalyst:
MOR(110), 0.50 g; temperature: 150 °C; CO2 pressure,
­12 MPa; period, 12 h. Legends: see Figure 1.
(
25 mmol); 2-propanol, 1.50 g (25 mmol); catalyst:
MOR(110), 0.50 g; CO2 pressure, 11 MPa; temperature,
50 °C; period, 12 h or as stated in the footnote. A
(
1
conventional reaction was carried out in hexane under
autogeneous pressure. The reaction without solvent was
carried out at 11 MPa of N2 pressure. Legends:
: Conversion; : 4-IBC; : 3-IBC; : 4-/3-IBC.
8
Figure 4 shows the influence of reaction temperature on
the isopropylation of IBB under 11 MPa of CO2 pressure
over MOR(110). As expected, IBB conversion increased
with temperature, while the selectivity for 4-IBC gradually
decreased from 82.3% at 130 °C to 79.6% at 170 °C. On the
contrary, the formation of 3-IBC and DIPIBB (0.5% at 170 °C)
increased slightly with temperature, accompanying a decrease
in 4-/3-IBC ratio. Nevertheless, 2-IBC was not formed even at
indicate that sc-CO was an excellent reaction medium for the
isopropylation of IBB. As expected, the reaction carried out
with neat reactants under autogeneous pressure also produced
2
similar results to that observed under 11 MPa of N pressure,
2
indicating that the inert N gas had no significant influences on
2
the promotion of the isopropylation of IBB. The faster diffusion
of reactants/products and efficient removal of coke precursors
1
70 °C.
Figure 5 shows the influence of CO2 pressure on the
from the zeolitic channels promoted by sc-CO medium is
2
isopropylation of IBB at 150 °C over MOR(110). The IBB
conversion increased almost linearly up to 11 MPa of CO2
responsible for the higher IBB conversion and higher selectivity
for 4-IBC in comparison with those in conventional reaction
systems in hexane solvent or without solvent.
pressure. However, further increases in CO pressure slightly
2
decreased the IBB conversion due to higher CO density. The
Coke formation over acid sites is considered one of the
principal reasons for rapid deactivation of the catalysts.
2
higher CO pressures may decrease the access of the reactant
2
molecules to the catalytic sites because of filling the channels
It is important, however, to note
that the selectivity for 4-IBC, represented as the 4-/3-IBC ratio,
Enhanced catalyst performances in sc-CO medium were thus
achieved due to lower coke formation. This was visually
observed based on the different colors of the catalysts used: off-
2
with highly dense CO2 in.1
3­15
remained almost constant with varied CO pressure. Never-
white in the sc-CO medium, and dark yellow in hexane and
2
2
theless, higher CO concentrations reduced coke formation on
without solvent under N pressure. In addition, TGA profiles of
2
2
the catalysts, which likely allowed the formation of 3-IBC in a
slightly higher fraction.
the used catalysts revealed that soft coke (µ1 wt %; weight loss
in the range of 350­650 °C) was predominantly found in the
Figure 6 shows the influence of reaction atmosphere of
the isopropylation of IBB at 150 °C over MOR(110). The
reaction performed in sc-CO2 medium showed very high
conversion (µ80%) as well as high selectivity for 4-IBC
catalyst used under sc-CO medium as shown in Figure 7.
2
However, after the reactions in hexane and without solvent, the
catalysts contained much higher amounts of coke: 10 wt % in
hexane and 12­14 wt % without solvent. Thus, the coke in
(80%). On the other hand, conventional liquid-phase reaction
sc-CO formed was much less than those in hexane and without
2
under autogeneous pressure in hexane (used as solvent) showed
lower conversions. The conversion of the isopropylation with-
solvent. These results unambiguously demonstrated the supe-
riority of sc-CO medium for chemical transformations that
2
out solvent by using neat substrate under 11 MPa of N pressure
are prone to the decrease in coke formation. Similar decreases
2
was lower conversion than that in sc-CO medium, but higher
in coke formation in sc-CO medium were also found in the
tert-butylation of phenol and p-cresol.
2
2
1
8
19
than that in hexane. In addition, the selectivity for 4-IBC in
hexane was much less (µ67%) than that in sc-CO2 medium.
However, the selectivity without solvent was almost the same
Table 3 shows the recycle and regeneration of MOR(110) in
the isopropylation of IBB. The results showed the highest
selectivity for 4-IBC. Nevertheless, the IBB conversion over
MOR(110) reused after filtration and subsequent drying
dropped considerably during the 1st run. In fact, the coke
as under the sc-CO atmosphere. In all cases, there was no
2
formation of 2-IBC, and only small amounts of DIPIBB isomers
(1.0­1.2%) were identified in the product mixture. These results

S. C. Laha et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 11 (2011) 1287
100
100
40
β,β-DIPN
8
6
4
2
0
9
9
8
8
5
0
5
0
30
2
,6-DIPN
0
0
0
2
1
0
0
0
sc-CO
2
α,β-DIPN
α,α-DIPN
2,7-DIPN
without solvent
Hexane
25
200
400
600
800
0
1
30
140
150
160
170
Temperature /°C
Reaction temperature /°C
Figure 7. Influence of reaction media on the TG profiles
of the catalysts after isopropylation of IBB. Reaction
conditions: see Figure 6. The catalysts were recovered by
filtration and dried at 40 °C overnight.
Figure 8. Influence of reaction temperature on the isopro-
pylation of NP over MOR(128). Reaction conditions: NP,
6
.25 mmol (800 mg); 2-propanol, 6.25 mmol (1.11 g);
catalyst: MOR(128), 125 mg; temperature, 130­170 °C;
period, 12 h; CO2 pressure, 11 MPa.
Table 3. Recycle and Regeneration of MOR Used in the
a)
Isopropylation of IBB
100
60
40
20
0
Conv.
mol % 2-IBC 3-IBC 4-IBC DIPIBB
Selectivity/mol %
Catalysts
4-/3-IBC
β,β-DIPN
/
8
0
MOR(110)
MOR(110)
MOR(110)
85.6
47.3
79.7
®
®
®
19.6 80.4
17.8 82.2
19.4 80.6
®
®
®
4.1
4.6
4.2
b)
c)
60
40
2,6-DIPN
a) Reaction conditions: IBB, 3.35 g (25 mmol); 2-propanol,
.50 g (25 mmol); catalyst: MOR(110), 0.50 g; CO2 pressure,
1 MPa; temperature, 150 °C; period, 12 h. b) Second run for
MOR(110) after filtration and drying of the catalyst. c) After
calcination of MOR(110) used for second run.
1
1
2
,7-DIPN
20
α,β-DIPN
α,α-DIPN
could cover the active sites, and reduce the channel size,
resulting in decrease in catalytic activity and increase in the
selectivity for 4-IBC. Regeneration of MOR(110) used for a
0
8
9
10
11
12
CO
2
pressure /MPa
2
nd run by calcination at 500 °C confirmed that the catalysts
Figure 9. Influence of CO2 pressure on the isopropylation
of NP over MOR(128). Reaction conditions: BP, 6.25
mmol (800 mg); 2-propanol, 6.25 mmol (1.11 g); catalyst:
MOR(128), 125 mg; temperature, 150 °C; period, 12 h;
CO2 pressure, 8­12 MPa.
retained activity and similar product selectivity with respect to
the fresh catalyst.
Isopropylation of Naphthalene (NP). The isopropylation
of NP with 2-propanol over MOR(128) in sc-CO medium
2
yielded isopropylnaphthalene (IPN) and diisopropylnaphtha-
lene (DIPN) isomers (Figure S1 in Supporting Information).
Yields of these alkylates increased with the increases in
reaction temperature. The formation of alkylates occurred with
the aid of sc-CO2 medium. The sc-CO2 medium enhanced the
diffusion of NP and its products in and out of the MOR
channels because of the high solubility of organic compounds
in it, resulting in efficient catalysis even at low amounts of
alkylating agent in our work.
in the ¢,¢-DIPN isomers. This difference was caused by steric
interaction with the MOR channels in the transition states
because 2,6-DIPN had less steric hindrance than 2,7-DIPN in
the MOR channels.
There was no significant decrease in the selectivities for
¢,¢- and 2,6-DIPN observed from increasing the reaction
4­6
temperature in the isopropylation of NP in sc-CO medium.
2
Figure 8 shows the influences of reaction temperature on the
This observation was due to lower reaction temperature, which
prevented the isomerization of 2,6-DIPN at external acid sites
compared to the results in conventional conditions.
The influences of CO2 pressure on the yields of IPN and
DIPN isomers are summarized in Figure S2 in Supporting
Information. The catalytic activity increased with increasing
isopropylation of NP over MOR(128) in sc-CO medium. The
2
selectivity for ¢,¢-DIPN (2,6- and 2,7-DIPN) was around 90%
at all temperatures because ¢,¢-DIPN was the least bulky
among the possible isomers. The selectivity for ¢,¢-DIPN
remained almost constant over the range of 140­170 °C. These
results meant that the less bulky ¢,¢-DIPN predominated over
the other bulky isomers, and 2,6-DIPN favored over 2,7-DIPN
CO pressure, resulting in increased yields of IPN and DIPN
2
isomers.

1
288 Bull. Chem. Soc. Jpn. Vol. 84, No. 11 (2011)
Alkylation of Aromatic Hydrocarbons in sc-CO2
1
00
30
100
80
50
8
6
4
2
0
0
0
0
0
4,4'-DIPB
40
30
20
10
0
2
1
0
0
0
6
4
2
0
0
0
2,x'-DIPB
3,4'-DIPB
160
0
sc-CO
2
without solvent
hexane
1
30
140
150
170
Reaction temperature /°C
Figure 10. Isopropylation of NP over MOR(128) in differ-
ent reaction media. Reaction conditions: NP, 6.25 mmol
Figure 11. Influence of reaction temperature on the iso-
propylation of BP over MOR(128). Reaction conditions:
BP, 6.25 mmol (964 mg); 2-propanol, 6.25 mmol (1.11 g);
catalyst: MOR(128), 125 mg; temperature, 130­170 °C;
period, 12 h; CO2 pressure, 11 MPa.
(
800 mg); 2-propanol, 6.25 mmol (1.11 g); catalyst:
MOR(128), 125 mg; temperature, 130­170 °C; period,
2 h; CO2 pressure, 11 MPa. A conventional reaction was
1
carried out in hexane under autogeneous pressure. The
reaction without solvent was carried out at 11 MPa of
N2 pressure. Legends: : ¢,¢-DIPN, : ¡,¢-DIPN;
:
¡,¡-DIPN; : 2,6-DIPN; : 2,7-DIPN; : Conversion.
temperature, as also observed in the isopropylation of NP. The
formation of alkylates was also enhanced with the use of sc-CO2
medium. Thus, the diffusion of BP and its products in and out of
Figure 9 shows the influences of CO2 pressure on the
selectivities for DIPN isomers in the isopropylation of NP
the channels was enhanced by sc-CO medium, resulting in
2
over MOR(128) in sc-CO medium. Apparently, there was no
efficient catalysis even at lower amounts of alkylating agent.
Figure 11 summarizes the influence of reaction temperature
on the isopropylation of BP over MOR(128) in sc-CO2
medium. 4,4¤-DIPB was preferentially obtained with a selec-
tivity around 90% in the range of 130­170 °C. These results
suggested that the increase in reaction temperature also
2
significant change in the selectivities for ¢,¢- and 2,6-DIPN
in spite of the increased catalytic activity with increased
CO pressure. Because increased CO pressure enhanced the
2
2
solubility of organic compounds, NP, its alkylated products,
and propene, could easily diffuse in and out of MOR channels
in dense sc-CO , which resulted in enhanced catalytic activity.
enhanced the reaction in sc-CO medium, and that the shape-
2
2
However, the higher diffusivity of NP and its products under
selective formation of 4,4¤-DIPB occurred inside the MOR
channels through a restricted transition state mechanism.
4
,6,32
increased CO pressure did not influence product selectivity.
2
These observations indicate that isopropylation occurred in
the MOR channels and that the bulky transition states were
excluded by the channels, resulting in the preferential formation
of the less bulky ¢,¢- and 2,6-DIPN isomers. Furthermore,
the differences between NP and IBB observed in this study are
due to the difference in solubility of both compounds, because
solubility of nonpolar organic compounds in nonpolar sc-CO2
medium are generally related to molecular weight of the
Increased CO pressure also increased the formation of IPBP
2
and DIPB in the isopropylation of BP over MOR(128) for
the same reasons discussed for NP (Figure S4 in Supporting
Information). These results also indicate sc-CO2 enhanced the
alkylation of BP because increased solubility of BP and its
products accelerate the access to the acid sites in the MOR
channels.
Figure 12 summarizes the influence of the CO pressure on
the selectivity for DIPB isomers in the isopropylation of BP
over MOR(128) in sc-CO medium. Increased CO pressure
1
3,33
compounds.
IBB is more easily dissolved in the sc-CO2
medium even at lower CO pressure in comparison with NP
2
2
2
and BP. These indicate that the isopropylation of IBB was less
affected by CO2 pressure than that those of NP and BP.
Figure 10 shows the isopropylation of NP over MOR(128)
remarkably enhanced the formation of 4,4¤-DIPB, and the
selectivity for 4,4¤-DIPB was as high as 90% even under
increased CO2 pressures. These results indicate that the
formation of 4,4¤-DIPB occurred inside the channels by a
restricted transition state mechanism because the diffusion
of BP and its products into the channels was enhanced by
increased pressure.
in different reaction media. sc-CO medium showed higher NP
2
conversion than other conventional reaction media. However,
there was no difference in the selectivity for ¢,¢- and 2,6-
DIPN. It was envisaged that the higher activities were due to
rapid removal of coke precursors, such as alkylated NP and
propene oligomers from catalytic sites.
sc-CO Medium for the Alkylation of Aromatic Hydro-
2
carbons. Coke is inevitably formed during acid catalysis over
solid acid catalysts under liquid and vapor phase conditions.
Coke forms on acidic sites, and causes the deactivation of
catalytic sites. Coke formed in zeolite chokes their channels
and pores, resulting in significant reductions in catalytic
performance. For these reasons, the prevention of coke
Isopropylation of Biphenyl (BP). The isopropylation of
BP with 2-propanol over MOR(128) in sc-CO medium yielded
2
isopropylbiphenyl (IPBP) and diisopropylbiphenyl (DIPB)
isomers, as shown in Figure S3 in Supporting Information.
The yields of IPBP and DIPB increased with increasing reaction

S. C. Laha et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 11 (2011) 1289
1
00
The results in this work demonstrated that sc-CO medium is
2
3
2
1
0
0
0
0
a potential system for the prevention of catalyst deactivation by
severe coking and for improvement of activity in solid acid
catalysis, resulting in the establishment of efficient green
chemical catalysis with low environmental burden. Further-
4
,4'-DIPB
8
6
4
2
0
0
0
0
more, the catalyses in sc-CO medium can also get rid of huge
2
quantities of organic solvent waste and simplify post-reaction
separation processes.
Conclusion
Supercritical CO (sc-CO ) as reaction medium can remark-
2
2
3
,4'-DIPB
2
,x'-DIPB
8
ably enhance the catalytic activity and selectivity for 4-
isobutylcumene (4-IBC) in the isopropylation of isobutylben-
zene (IBB) over H-mordenite (MOR), particularly MOR(110)
with high dealumination. Similarly, highly shape-selective
formation of the least bulky isomers, 2,6-diisopropylnaphtha-
lene (2,6-DIPN) and 4,4¤-diisopropylbiphenyl (4,4¤-DIPB) was
0
10
12
CO2 pressure /MPa
Figure 12. Influence of CO2 pressure on the isopropylation
of BP over MOR(128). Reaction conditions: BP, 6.25
mmol (964 mg); 2-propanol, 6.25 mmol (1.11 g); catalyst:
MOR(128), 125 mg; temperature, 150 °C; period, 12 h;
CO2 pressure, 8­12 MPa.
observed in sc-CO medium in the isopropylation of naph-
2
thalene (NP) and biphenyl (BP), respectively, over dealumi-
nated MOR(128) at 140­170 °C and 14 MPa. sc-CO medium
2
renders active acid sites free from coke formation through
faster diffusion of reactants and products in zeolite catalyst and
also through efficient dissolution of coke precursors. Thus,
deactivation of the catalyst is reduced substantially without
losing shape-selectivity.
formation in zeolite catalysis is the most urgent issue for the
establishment of green chemical processes.
Solid acid catalysis in sc-CO medium is well known for the
2
8­21
reduction of coke formation.1
In particular, coke precursors,
These results demonstrate that sc-CO medium can prevent
2
probably alkylated polynuclear aromatics formed from the
reactants/products, can easily diffuse from active sites to the
bulk phase because of high solubility in sc-CO2 medium,
resulting in easy removal of these precursors. Therefore, the
active sites are not deactivated during catalysis, resulting in
the significant reduction of coke formation. However, coke
precursors in conventional reactions with or without organic
solvent mostly remain at the acidic sites and hardly diffuse to
bulk phase during the catalysis, preventing the cleaning of the
deactivation of catalyst by severe coking and improve the
selectivity of the desired product(s). The results in this work
represent a new path for green catalysis. Further investigations
on these aspects are currently ongoing and will be reported in
due course.
A part of this work was financially supported by a Grant-in-
Aid for Scientific Research 16310056 (B), 19310060 (B), and
21510098 (C) of the Japan Society for the Promotion of
Science (JSPS). S. C. Laha gratefully acknowledges JSPS for
the postdoctoral fellowship. Y.S. also appreciates Prof. A. Vinu
of Australian Institute for Bioengineering and Nanotechnology,
The University of Queensland, Australia for helpful discussion,
and the Department of Research, Nagoya Industrial Science
Research Institute for their continuing cooperation.
catalytic surfaces. These properties of sc-CO medium enhance
2
catalysis at clean active sites.
As shown in Figure 7, coke formation in sc-CO medium
2
is much lower than in conventional systems using hexane
as solvent or without solvent under N2 atmosphere in the
isopropylation of IBB over MOR(110). Thus, low coke
formation in the sc-CO medium could enhance the catalytic
performance of MOR, resulting in high catalytic activities with
highly selective formation of 4-IBC.
2
Supporting Information
Figure S1: Influences of reaction temperature on the iso-
Catalytic activities were increased by increasing the CO2
pressure in sc-CO2 medium in the isopropylation of NP
and BP (Figures 9 and 12). These phenomena were due to the
maintenance of clean active sites by the diffusion of coke
precursors. Moreover, the reactants and products more easily
diffused in and out of MOR channels in sc-CO2 medium
because of their increased solubility with increasing pressure.
Thus, the isopropylation of NP and BP occurred effectively
in sc-CO2 medium, resulting in high conversion and high
selectivities for 2,6-DIPN and 4,4¤-DIPB at relatively low
temperatures (below 170 °C). The isopropylation reactions in
this work were carried out by using limited amounts of
alkylating agent. We can expect much higher conversion with
high formation of 2,6-DIPN and 4,4¤-DIPB in the presence of
sufficient amounts of alkylating agent.
propylation of NP, Figure S2: Influences of CO pressure on
the isopropylation of NP, Figure S3: Influences of reaction
temperature on the isopropylation of BP, and Figure S4:
2
Influences of CO pressure on the isopropylation of BP. These
2
materials are available free of charge on the Web at http://
www.csj.jp/journals/bcsj/.
References
1
2
3
S. M. Csicsery, Zeolites 1984, 4, 202.
P. B. Venuto, Microporous Mater. 1994, 2, 297.
N. Y. Chen, W. E. Garwood, F. G. Dwyer, Shape Selective
Catalysis in Industrial Applications, 2nd ed., Marcel Dekker Inc.,
New York, NY, 1996.
4
Y. Sugi, Y. Kubota, in Catalysis, ed. by J. J. Spivey, Royal
Society of Chemistry, 1997, Vol. 13, pp. 55­84.

1
290 Bull. Chem. Soc. Jpn. Vol. 84, No. 11 (2011)
Alkylation of Aromatic Hydrocarbons in sc-CO2
5
6
7
8
Y. Sugi, J. Chin. Chem. Soc. 2010, 57, 1.
Y. Sugi, J. Jpn. Pet. Inst. 2010, 53, 263.
J. Čejka, B. Wichterlová, Catal. Rev. 2002, 44, 375.
P. R. Pujadó, J. A. Rabó, G. J. Antos, S. A. Gembicki,
2008, 47, 2538.
22 R. Amandi, P. Licence, S. K. Ross, O. Aaltonen, M.
Poliakoff, Org. Process Res. Dev. 2005, 9, 451.
23 R. Gläser, J. Weitkamp, Ind. Eng. Chem. Res. 2003, 42,
6294.
Catal. Today 1992, 13, 113.
9
0
C. Perego, P. Ingallina, Catal. Today 2002, 73, 3.
24 H. G. Franck, J. W. Stadelhofer, Industrial Aromatic
Chemistry, Springer-Verlag, Berlin, 1988.
25 P. J. Harrington, E. Lodewijk, Org. Process Res. Dev. 1997,
1, 72.
1
M. Guisnet, P. Magnoux, Appl. Catal. 1989, 54, 1.
G. Perot, M. Guisnet, J. Mol. Catal. 1990, 61, 173.
D. M. Bibby, R. F. Howe, G. D. McLellan, Appl. Catal., A
1
1
1
2
1
992, 93, 1.
A. Clifford, Fundamentals of Supercritical Fluids, Oxford
University Press, Oxford, 1998.
26 R. A. Sheldon, Chem. Ind. 1992, 7, 903.
27 C. Venkatesan, M. Chidambaram, K. R. Kamble, A. P.
Singh, Catal. Lett. 2003, 85, 171.
1
3
1
1
1
4
5
6
A. Baiker, Chem. Rev. 1999, 99, 453.
R. Noyori, Chem. Commun. 2005, 1807.
M. G. Hitzler, F. R. Smail, M. Poliakoff, M. G. Hitzler,
28 The number in parenthesis expresses SiO2/Al2O3 ratio of
the zeolite.
29 M. Matsukata, M. Ogura, T. Osaki, P. R. H. P. Rao, M.
Nomura, E. Kikuchi, Top. Catal. 1999, 9, 77.
30 H. G. Karge, J. Weitkamp, Chem. Ing. Tech. 1986, 58, 946.
31 J.-H. Kim, Y. Sugi, T. Matsuzaki, T. Hanaoka, Y. Kubota,
X. Tu, M. Matsumoto, Microporous Mater. 1995, 5, 113.
32 Y. Sugi, S. Tawada, T. Sugimura, Y. Kubota, T. Hanaoka, T.
Matsuzaki, K. Nakajima, K. Kunimori, Appl. Catal., A 1999, 189,
251.
S. K. Ross, Chem. Commun. 1998, 359.
L. M. Petkovic, D. M. Ginosar, K. C. Burch, J. Catal. 2005,
34, 328.
G. Kamalakar, K. Komura, Y. Sugi, Ind. Eng. Chem. Res.
006, 45, 6118.
G. Kamalakar, K. Komura, Y. Sugi, Appl. Catal., A 2006,
10, 155.
A. Sakthivel, R. Nakamura, K. Komura, Y. Sugi,
J. Supercrit. Fluids 2007, 42, 219.
A. Sakthivel, K. Komura, Y. Sugi, Ind. Eng. Chem. Res.
1
7
2
2
3
1
8
1
9
2
0
33 D. Tomida, T. Konishi, T. Ebina, T. Nishino, C. Yokoyama,
Netsu Bussei 2009, 23, 27.
2
1