A liquid-liquid process for production of 2,4-diphenyl-4-methyl-1-pentene by dimerization of α-methylstyrene

Article abstract of DOI:10.1021/op980023m

A liquid-liquid process for the dimerization of α-methylstyrene (AMS) in order to produce selectively the commercially useful 2,4-diphenyl-4-methyl-l-pentene has been developed. The aqueous phase contained sulfuric acid as a catalyst and methanol as an additive, and the organic phase was 97% α-methylstyrene (AMS). An AMS conversion of 55% was achieved in 7 h when the dimerization was conducted at 80 °C. The product dimer contained 93% 2,4-diphenyl-4-methyl-1-pentene and only 0.06% 1,1,3-triniethyl-3-phenylindan, the saturated dimer. The liquid-liquid process can be used commercially to produce selectively 2,4-diphenyl-4-methyl-1-pentene at locations where cation-exchange resins are not readily available.

Full text of DOI:10.1021/op980023m

Organic Process Research & Development 1999, 3, 220−223
Technical Notes
A Liquid-Liquid Process for Production of 2,4-Diphenyl-4-methyl-1-pentene by
Dimerization of r-Methylstyrene
Basab Chaudhuri
Department of Chemical Engineering, Calcutta UniVersity, 92 Acharya Prafulla Chandra Road, Calcutta 700009, India
Abstract:
A liquid-liquid process for the dimerization of r-methylstyrene
(AMS) in order to produce selectively the commercially useful
2,4-diphenyl-4-methyl-1-pentene has been developed. The aque-
ous phase contained sulfuric acid as a catalyst and methanol
as an additive, and the organic phase was 97% r-methylstyrene
(AMS). An AMS conversion of 55% was achieved in 7 h when
the dimerization was conducted at 80 °C. The product dimer
contained 93% 2,4-diphenyl-4-methyl-1-pentene and only 0.06%
1,1,3-trimethyl-3-phenylindan, the saturated dimer. The liquid-
liquid process can be used commercially to produce selectively
2,4-diphenyl-4-methyl-1-pentene at locations where cation-
exchange resins are not readily available.
Introduction
Figure 1. Dimerization of AMS: reaction network/mechanism.
The acid-catalyzed dimerization of R -methylstyrene
(AMS) is an organic reaction of commercial importance and
academic interest. Chaudhuri and Sharma (1989) have
reported a comprehensive study on this system. A number
of products are formed in the acid-catalyzed dimerization
of AMS. These products include 2,4-diphenyl-4-methyl-1-
pentene (UD1), 2,4-diphenyl-4-methyl-2-pentene (UD2), and
1,1,3-trimethyl-3-phenylindan, also called saturated dimer
(SD). Under certain conditions, one more saturated dimer,
cis- and/or trans-1,3-dimethyl-1,3-diphenylcyclobutane (CB)
may also be formed. Figure 1 gives the reaction scheme of
the dimerization of AMS. The unsaturated dimers of AMS
are useful as chain-transfer agents or molecular weight
regulators in the production of polymers such as polystyrene,
SAN resin, ABS resin, SBR, and the like.1 Of the unsaturated
dimers, UD2 is not preferred as a molecular weight regulator
since it hinders the initiating reaction and accordingly
requires an undesirably longer induction period. Furthermore,
the saturated indanic dimer, SD, and higher oligomers, e.g.,
trimers and tetramers, are of no value as molecular weight
regulators, and their presence in the product dimer unneces-
sarily reduces the effective concentration of UD1. In order
that the unsaturated dimers of AMS can function effectively
as molecular weight regulators, the purity of UD1 in the
product dimer should be as high as possible, preferably above
93%.
I, r-methylstyrene; II and III, carbocations; IV, 1,1,3-trimethyl-
3-phenylindan (SD); V, 2,4-diphenyl-4-methyl-1-pentene (UD1);
and VI, 2,4-diphenyl-4-methyl-2-pentene.
In their paper Chaudhuri and Sharma2 have described a
method of producing commercially UD1 of purity greater
than 92% by conducting the dimerization in the presence of
solid acid catalysts such as ion-exchange resins under
carefully controlled conditions and also with aqueous sulfuric
acid as a catalyst in a heterogeneous liquid-liquid (L-L)
system. The problem with the heterogeneous L-L system
with aqueous sulfuric acid as a catalyst was that, at low levels
of conversion of AMS, the purity of UD1 in the product
was 92% as desired, but as the fractional conversion of AMS
increased beyond 20%, the selectivity for UD1 decreased,
thus rendering the L-L operation unsuitable for production
of UD1 of 92+% purity at high conversion level of AMS.
The end users of AMS dimer now prefer UD1 purity of 93%
(minimum) in the product. The objective of the present
investigation was to explore an aqueous medium with sulfuric
acid as the acidic material that would give a product dimer
having UD1 at a concentration higher than 93% at high levels
of conversion of AMS. It was also thought that the L-L
operation would allow the aqueous phase to be reused again
and again in the process. The use of such an aqueous medium
(2) Chaudhuri, B.; Sharma, M. M. Some Novel Aspects of the Dimerization
of R-Methylstyrene with Acidic Ion Exchange Resins, Clays and Other
Acidic Materials as Catalysts. Ind. Eng. Chem. Res. 1989, 28, 1757-1763.
(1) Encyclopedia of Chemical Technology; Kirk and Othmer, Wiley-Inter-
science: New York, 1978; Vol. 21, p 819.
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10.1021/op980023m CCC: $18.00 © 1999 American Chemical Society and Royal Society of Chemistry
Published on Web 04/17/1999

Table 1. Product Distribution in the Dimerization of AMS in L-L Mode (Aqueous Sulfuric Acid Catalyst)a
product distribution (GC area %)
H2SO4 concentration (% w/v)
in aqueous phase
AMS conversion
(%)
time (h)
UD1
UD2
SD
PDMC
UD1:UD2 ratio
65
49
0.58
1.5
2.0
1.16
2.16
4.32
10.0
2.0
4.75
8.0
25
55
65
7.4
14.0
25.7
52.0
3.6
8.0
13.0
18.0
20.9
47.3
53.9
4.3
5.5
12.2
14.4
0.63
1.6
3.6
7.4
0.15
0.32
0.80
1.01
2.0
4.4
5.8
0.75
0.68
0.42
1.8
1.8
1.7
2.0
1.8
3.5
4.2
79.1:20.9
79.4:20.6
78.9:21.1
87.2:12.8
86.0:14.0
85.5:14.5
84.6:15.4
92.3:7.7
92.4:7.6
91.8:8.2
91.9:8.1
0.35
0.25
0.35
0.61
0.08
0.09
0.07
0.05
9.9
21.3
40.7
1.8
3.9
9.0
36.7
12.0
11.4
5.6
a Conditions: aqueous phase, 300 mL; AMS, 345 mL; temperature, 80 °C; speed of agitation, 2000 rpm.
for dimerization of AMS will obviate the need to restrict
the conversion level of AMS to within 10-12% in a process
employing only aqueous sulfuric acid as a catalyst. There
will also be a substantial saving on separation as it will no
longer be necessary to distill out a large quantity of unreacted
AMS from the organic phase and recycle it back into the
process. The results of the investigation are reported in this
paper.
AMS, that is, at conversions of AMS of the order of 55-
60%. Such a method is not reported in the open literature.
The present technique will be an addition to the existing
methods for production of 93+% pure UD1 by the dimer-
ization of AMS.
Results and Discussion
Dimerization with Aqueous Sulfuric Acid. The rates
of dimerization of AMS and the product selectivities were
determined by conducting the dimerization reaction in L-L
mode with aqueous H2SO4 solutions containing different
quantities of H2SO4 in them, and the results are given in
Table 1. With a catalyst concentration of 65% (w/v) H2SO4
in the aqueous phase, the rate of dimerization was quite
fast: an AMS conversion of 65% was achieved in only 2 h.
The product dimer did not meet the quality specification of
a chain terminator, as the ratio of UD1:UD2 was only 79:
21 and the concentration of SD in the product was also
substantially high. At an intermediate catalyst concentration
of 49% (w/v) H2SO4 in the aqueous phase, moderately high
rates of dimerization were realized, and the selectivity of
UD1 in the product dimer was around 85% at an AMS
conversion level of 52%. When the catalyst concentration
in the aqueous phase was 36.7% (w/v) H2SO4, the rate of
dimerization was slow: only 18% conversion of AMS was
realized in 12 h. The product dimer was rich in UD1, the
ratio of UD1:UD2 being around 92:8. The concentration of
SD in the product dimer was the lowest with the catalyst
concentration of 36.7% (w/v). The results delineated above
brought out two important points. (1) The higher the
concentration of H2SO4 in the aqueous phase, the higher was
the rate of dimerization. This was because of the high acidity
of the aqueous phase with increased H2SO4 concentration.
(2) The larger the concentration of water in the aqueous
phase, the smaller was the concentration of SD in the product
dimer. Other than the dimers, phenyl dimethyl carbinol
(PDMC) was the byproduct formed by the acid-catalyzed
hydration of AMS. In the range of H2SO4 concentrations
(in the aqueous phase) investigated, the formation of PDMC
was favoured when the aqueous phase contained the largest
amount of water.
Previous Studies
It has been stated in the Introduction that Chaudhuri and
Sharma2 (1989) have reported a detailed study on the
dimerization of AMS. Various aspects of the reaction, such
as kinetics of the dimerization and selectivities for the
different products in the presence of cation-exchange resins
Amberlyst 15, K2661, Lewasorb AC 10 FT, etc., were
investigated in the work. A method to produce selectively
UD1 of purity greater than 92% was presented by Chaudhuri
and Sharma.2 Recently, Sun et al.3 reported the dimerization
of AMS on Amberlyst 15, and also on Nafion perfluoro
sulfonic acid resin in three different microstructures: gel type
Nafion NR50, a carbon-supported Nafion resin, and a Nafion
resin/Silica composite material. Supported Nafion on carbon
and Amberlyst 15 were found to be more active than pure
Nafion. To compare the efficiencies of different catalysts in
nonpolar media such as cumene and polar media such as
p-cresol, Sun et al. used techniques such as temperature-
programmed desorption and thermogravimetric analysis.
We find that practically no attempt has been made so far
to find an aqueous medium having H2SO4 as a catalyst that
can be used in L-L mode to dimerize AMS to obtain 93%
pure UD1 at a reasonably high conversion of AMS. Our
focus on H2SO4 is due to the fact that the material is readily
available almost everywhere in the world. Secondly, the L-L
mode of operation does not allow run-away reaction to occur,
as experienced for the dimerization of 99% pure AMS in
the presence of solid acid catalysts. In this paper, we present
a methodology for producing 93+% UD1 by dimerizing
AMS in the L-L mode at a reasonably high conversion of
(3) Sun, Q.; Farneth, W. E.; Harmer, M. A. Dimerization of R-Methylstyrene
(AMS) Catalyzed by Sulfonic Acid Resins: A Quantitative Kinetic Study.
J. Catal. 1996, 164, 62-69.
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Table 2. Product Distribution in the Dimerization of AMS in L-L Mode (Aqueous Sulfuric Acid Catalyst and Methanol)a
product distribution (GC area %)
quantity of methanol (mL)
added to 300 mL of aqueous phase
AMS conversion
(%)
time (h)
UD1
UD2
SD
CME
PDMC
UD1:UD2 ratio
75
1
2.5
7
1
2.66
7
10
1
2.6
7
9.8
18.9
45.0
19
28
55
67
22
33
64
4.0
13.5
37.4
3.35
13.67
43.74
57.0
4.4
0.3
1.1
3.0
0.21
0.92
3.27
4.6
0.3
1.5
0.04
0.04
0.05
0.03
0.03
0.03
0.03
0.05
0.05
0.05
4.1
3.8
3.3
12.9
11.3
7.2
1.5
1.6
1.2
2.6
2.6
1.7
1.4
2.5
2.5
2.5
93.0:7.0
92.4:7.6
92.6:7.4
94.0:6.0
93.6:6.4
93.0:7.0
92.5:7.5
93.6:6.4
92.3:7.7
92.6:7.4
(20% v/v)
100
(25% v/v)
6.9
125
14.7
13.9
13.8
(29.4% v/v)
18.1
47.8
3.8
a Conditions: aqueous phase, 49% (w/v) H2SO4, 300 mL; methanol, as given in the table; AMS, 345 mL; temperature, 80 °C; speed of agitation, 2000 rpm.
Effect of Addition of Alcohol on the Dimerization of
AMS in L-L Mode. Chaudhuri and Sharma2 have reported
that alcohols such as methanol and tert-butyl alcohol acted
as inhibitors for the dimerization of AMS in the presence of
cation-exchange resin catalysts. To examine the role of
alcohol in the L-L mode of operation, the dimerization of
AMS was conducted in the presence of methanol. In all
experiments, 300 mL of 49% (w/v) aqueous H2SO4 solution
and 345 mL of AMS were contacted, with varying quantities
of methanol added to the two-phase system. The quantity of
methanol was varied from 75 to 125 mL, and its addition
into the aqueous phase enhanced the volume of the aqueous
phase and, therefore, reduced the catalyst concentration.
Results obtained on addition of methanol into the two-phase
system are given in Table 2. When 100 mL of methanol
was used along with 300 mL of 49% (w/v) aqueous H2SO4
solution, the effective concentration of H2SO4 was around
36.7% (w/v). On comparison of results given in Table 1 for
the sulfuric acid concentration of 36.7% (w/v) and those
given in Table 2 for 100 mL of methanol added to the
aqueous phase, it is found that the presence of methanol
improved the selectivity for the desired dimer UD1, and this
improvement was the highest when the aqueous phase
contained 25% (v/v) methanol. Under otherwise uniform
conditions, a higher or lower dosage of methanol gave poorer
selectivity for the desired UD1 in the product dimer.
There was practically no effect of the speed of agitation
in the range of 1000-3000 rpm on the rates of dimerization
in L-L mode (with or without methanol). We concluded,
therefore, that the dimerization was kinetically controlled.
For a L-L reaction for which the locale of the reaction is
the aqueous phase, an increase in the volume of the aqueous
phase would proportionately increase the rate of reaction in
moles per hour. In Table 1, results on the dimerization of
AMS are reported for 36.7% (w/v) H2SO4, and for this case
an aqueous phase volume of 300 mL was used. For an
aqueous phase volume of 400 mL, the rate would be 400/
300, that is, 1.33 times the rate for the 300-mL aqueous
phase. This would mean an AMS conversion of around 25%
in 12 h for an aqueous phase volume of 400 mL of 36.7%
(w/v) H2SO4. Results given in Table 2 for 400 mL of aqueous
phase [consisting of 300 mL of 49% (w/v) aqueous H2SO4
+ 100 mL of methanol] show that an AMS conversion of
67% was attained in 10 h. Therefore, the addition of methanol
in the aqueous phase enhanced the rate of dimerization at
the same effective H2SO4 concentration in addition to the
improvement in selectivity with respect to UD1. The
enhancement in rate in the presence of methanol in the
aqueous phase was probably due to the enhanced solubility
of AMS in the aqueous medium.
Other than the dimers UD1, UD2, and SD, the products
formed in the dimerization of AMS with aqueous H2SO4 and
methanol were phenyldimethyl carbinol (PDMC), formed by
the hydration of AMS, and methyl cumyl ether (CME),
formed by the reaction of methanol with AMS. These
byproducts are very useful. PDMC is an important perfumery
compound and has a potential of replacing phenylethyl
alcohol as a perfumery material. CME has properties of
enhancing octane number of gasoline like methyl tert-butyl
ether (MTBE) and ethyl tert-butyl ether (ETBE). It is
interesting to note that, for 25% (v/v) methanol used (Table
2), the concentration of CME in the product increased to as
high as 12.9% in a reaction time of 1 h; CME then gradually
decomposed as the reaction proceeded. Similarly, the con-
centration of PDMC decreased from 2.6% to 1.4% as the
reaction continued from 1 to 10 h. These are some notable
features of the dimerization of AMS with aqueous H2SO4
and methanol in L-L mode of operation.
We also checked the reusability of the aqueous phase [300
mL of 49% (w/v) H2SO4 + 100 mL of methanol] by
conducting the dimerization of AMS with the same aqueous
medium (that is, by separating the aqueous phase by simple
phase separation and carrying out the dimerization in the
medium) five times, and the same results as those reported
in Table 2 were obtained in each experiment.
The UD1:UD2 ratio obtained on the dimerization of AMS
with 300 mL of 49% (w/v) H2SO4 and 100 mL of methanol
are reported in Table 2. At an AMS conversion of 19%, the
ratio of UD1:UD2 was found to be 94:6, and this ratio
reduced to 93:7 at an AMS conversion of 55%. The ratio
fell below 93:7 as the AMS conversion increased beyond
55%. It is remarkable that the concentration of SD in the
product dimer was as low as 0.06%. To the best of our
knowledge, no report published in the open literature has
claimed such a low concentration of SD in the product dimer;
the large quantity of water present in the system probably
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suppressed the cyclization reaction, leading to the formation
of SD.
demonstrates that it is, indeed, possible to produce 93% UD1
with H2SO4 as a catalyst in L-L mode of operation at a
reasonably high conversion level of AMS.
In a typical experiment, the product mixture (organic
phase) was subjected to distillation under reduced pressure
(15-20 mm) after the organic phase was washed with an
aqueous alkali solution. Unreacted AMS, PDMC, and CME
were flashed off, and they were then recycled in the
dimerization reaction. Recycling of the byproducts did not
give any distortion in the selectivity of UD1, and practically
no difference in the rates of dimerization was observed.
Chaudhuri and Sharma2 reported that they produced UD1
of purity of 93+% by conducting the dimerization of AMS
in the presence of cation-exchange resin as a catalyst and
also 4% (w/w) of isopropyl alcohol as an additive (with
AMS). It was thought that the alcohol became adsorbed on
the surface of the cation-exchange resin, thereby deactivating
some of the acid sites of the catalyst and reducing its activity.
It is interesting to note that the aqueous medium consisting
of 300 mL of 49% (w/v) H2SO4 and 100 mL of methanol
used in this work gave 93+% UD1 in the product, compa-
rable with that reported by Chaudhuri and Sharma.2 This
similarity in product distribution between solid-liquid and
liquid-liquid heterogeneous reactions leads us to believe that
the role of alcohol in controlling the rates of dimerization
and in improving the selectivity of UD1 is more fundamental.
Probably the mechanism of the reactions leading to the
formation of the different dimers becomes affected in the
presence of alcohol. It is also probable that the microenvi-
ronment created in the resin bead in the presence of isopropyl
alcohol is the same as that created in the aqueous phase
containing H2SO4 and methanol, as used in this work. For
both cases, therefore, the selectivities for UD1 are identical
at values of around 93%. More work is necessary to justify
these similarities between solid-liquid and liquid-liquid
heterogeneous reactions beyond any shadow of doubt.
Scale-Up Study. To check whether the dimerization of
AMS with 300 mL of 49% (w/v) H2SO4 and 100 mL of
methanol scaled up, an experiment was performed in which
1035 mL of AMS was contacted with 900 mL of 49% (w/
v) H2SO4 and 300 mL of methanol at 80 °C in a 4-L fully
baffled reactor. The reaction was conducted for 7 h, and an
AMS conversion of 54% was achieved. This was comparable
with the results reported in Table 2 for 300 mL of 49% (w/
v) H2SO4 and 100 mL of methanol. The product distribution
was the same as that reported in Table 2, and the UD1:UD2
ratio was 93:7. The experiments reported in this paper could,
therefore, be scaled up without difficulty.
Experimental Section
Experiments were conducted in a 0.10-m-i.d. fully baffled
mechanically agitated contactor. A six-bladed glass-disk
turbine impeller was used for agitation. All the experiments
were carried out at the desired temperature by placing the
reactor in a constant-temperature bath.
At first, a known volume of the aqueous phase was placed
in the reactor and was slowly heated to the reaction
temperature. For reactions where methanol was used, the
mixture of aqueous acid phase and methanol was placed in
the reactor. The organic phase, which was basically AMS,
was heated to the same temperature in a separate vessel.
Subsequently, the organic phase was carefully transferred
with the help of a long funnel into the reactor, and the
agitation of the liquid-liquid system was started. This was
taken as the starting time of the reaction. Actual quantities
of aqueous and organic phases used are given in Tables 1
and 2.
The liquid-liquid system readily separated into two
distinct phases as soon as the agitation was stopped. Samples
(2 mL) from the organic phase were collected at definite
time intervals and analyzed on a Chemito model 9A gas
chromatograph. A 4-m-long column, OV-17 on chromosorb
L, was used for analysis. Typical conditions employed for
analysis were as follow: injector and detector temperatures,
250 °C each; oven temperature, 100 °C (zero isothermal
time), raised to 250 °C with a ramp rate of 4 °C/min; carrier
gas, N2 gas;and rate of flow of N2, approximately 20 mL/
min.
AMS of purity greater than 99% was procured from
Lancaster Synthesis Ltd., U.K. A commercial variety having
97% AMS and cumene and tert-butylbenzene as impurities,
manufactured by Herdillia Chemicals Ltd., Navi Mumbai,
India, was procured through a local agent. In most experi-
ments reported in this paper, this commercial grade AMS
was used. It was confirmed that rates of the dimerization
and product selectivities were practically the same for both
grades of AMS. Sulfuric acid and methanol, both of AR
grade, were procured from S.D. Fine. Chem. Pvt. Ltd.,
Mumbai, India. Aqueous acidic solutions were made of
distilled water; it was confirmed, however, that the dimer-
ization reactions conducted with aqueous acidic solutions
prepared from tap water gave the same rates and product
selectivities as those conducted with aqueous acidic solutions
prepared from distilled water.
Conclusions
A L-L process using H2SO4 as a catalyst to produce 93%
2,4-diphenyl-4-methyl-1-pentene by the dimerization of AMS
was developed. The concentration of UD1 in the product
dimer was as high as 93% at an AMS conversion of 55%
achieved in 7 h; the concentration of SD in the product dimer
was only 0.06%. The aqueous phase could be repeatedly used
for the dimerization. The present process can replace the
process based on ion-exchange resins, particularly for those
cases where the resin is not locally available and the supply
of the resin does not occur regularly. The present work also
Acknowledgment
The help of Ms. M. Chaudhuri in preparation of the
manuscript is gratefully acknowledged.
Received for review March 11, 1998.
OP980023M
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