Dimerization of α-methylstyrene (AMS) catalyzed by sulfonic acid resins: A quantitative kinetic study

Article abstract of DOI:10.1006/jcat.1996.0363

The dimerization of α-methylstyrene was examined at 50°C and 1 atm in nonpolar cumene and polar p-cresol solvents, using both soluble liquid acids and resin-based solid acids as catalysts. In particular, the activities of a representative macroporous sulfonic acid resin, Amberlyst-15, and Nafion perfluorosulfonic acid resin in three different microstructures, gel-type Nafion NR-50 resin, a carbon-supported Nafion resin, and a Nafion resin/silica composite material, have been compared. The Nafion resin/silica composites showed, by far, the highest activity. Supported Nafion resin on carbon and Amberlyst-15 are effective catalysts as well and are all more active than pure Nafion resin based on catalyst weight. None of the small molecule acids were sufficiently soluble to show significant catalytic action in the nonpolar solvent cumene. On the other hand, in the polar p-cresol media, the homogeneous acids showed relative activities in the same order as their pK_as; and the strongest aqueous acid, triflic acid was more active than any of the resin catalysts on an acid equivalent basis. Among the resin catalysts, only the Nafion NR50 gel showed significant activity enhancement in p-cresol. In order to gain insight into these relative catalytic activities, the acid capacity and accessibility of acid groups in these solid acid catalysts were studied using temperature programmed desorption/thermogravimetric analysis (TPD/TGA) techniques with isopropanol as probe molecule. The TPD/TGA studies show that the sulfonic acid groups are readily accessible to 2-propanol vapor in the 13 wt% Nafion resin/SiO₂ composite but not in gel-type Nafion resin and Amberlyst-15.

Full text of DOI:10.1006/jcat.1996.0363

JOURNAL OF CATALYSIS 164, 62–69 (1996)
ARTICLE NO. 0363
Dimerization of ꢀ-Methylstyrene (AMS) Catalyzed by Sulfonic Acid
Resins: A Quantitative Kinetic Study
ꢀ
Qun Sun, William E. Farneth, and Mark A. Harmer
Experimental Station, The DuPont Company, Wilmington, Delaware 19880-0356
Received February 21, 1996; revised July 23, 1996; accepted July 25, 1996
acids in reactions like hydration, etherification, and esterifi-
The dimerization of ꢀ-methylstyrene was examined at 50ꢁC and
1 atm in nonpolar cumene and polar p-cresol solvents, using both
soluble liquid acids and resin-based solid acids as catalysts. In par-
ticular, the activities of a representative macroporous sulfonic acid
resin, Amberlyst-15®, and Nafion® perfluorosulfonic acid resin in
three different microstructures, gel-type Nafion® NR-50 resin, a
carbon-supported Nafion® resin, and a Nafion® resin/silica compos-
ite material, have been compared. The Nafion® resin/silica compos-
ites showed, by far, the highest activity. Supported Nafion® resin on
carbon and Amberlyst-15® are effective catalysts as well and are all
more active than pure Nafion® resin based on catalyst weight. None
of the small molecule acids were sufficiently soluble to show signif-
icant catalytic action in the nonpolar solvent cumene. On the other
hand, in the polar p-cresol media, the homogeneous acids showed
relative activities in the same order as their pKas; and the strongest
aqueous acid, triflic acid was more active than any of the resin cata-
lysts on an acid equivalent basis. Among the resin catalysts, only
the Nafion® NR50 gel showed significant activity enhancement in
p-cresol. In order to gain insight into these relative catalytic activi-
ties, the acid capacity and accessibility of acid groups in these solid
acid catalysts were studied using temperature programmed desorp-
tion/thermogravimetric analysis (TPD/TGA) techniques with iso-
propanol as probe molecule. The TPD/TGA studies show that the
sulfonic acid groups are readily accessible to 2-propanol vapor in the
13 wt% Nafion® resin/SiO2 composite but not in gel-type Nafion®
c
cation of olefins, and condensations of carbonyl compounds
(1–2). Another type of cation exchange resin, perfluori-
®
nated sulfonic acid resin (e.g., Nafion ), can also be used
to catalyze many of these transformations, as has been well
documented by Olah’s group and other researchers (1, 3–5).
®
Nafion resin’s advantages include: apparent stronger
acidity, and higher thermal and chemical stability relative
to macroporous sulfonic acid resins, but its disadvantages,
including high cost and a low surface area gel-type struc-
®
ture, have prevented widespread adoption of Nafion
resin as a catalytic material. There have been several
®
attempts to disperse Nafion resin on higher surface area
supports to increase the catalytic activity in nonswelling
®
media (4, 6). These supported Nafion resin catalysts
showed improved performance in comparison with the
corresponding bulk materials for hydrocarbon conversion
processes such as alkylation of aromatics or isoparaffins
with olefins, oligomerization or hydration of olefins, and
so on; however, only modest dispersion of the acid resin
on these supports could be obtained, and the long term
stability of these catalysts has not been documented.
®
Potential drawbacks include washing out of the Nafion
resin from the support by the solvent and reactant mixture
when employed in the liquid phase.
resin and Amberlyst-15®.
ꢂ 1996 Academic Press, Inc.
We have recently described the preparation of a novel
®
high surface area Nafion resin/silica composite catalyst
INTRODUCTION
®
(7). In this new class of composite materials, Nafion resin
particles are encapsulated within a porous silica network,
leading to both high Bro¨nsted acid site densities and high
surface areas. In this study we have examined the activity
of this composite catalyst relative to a variety of other acid
catalysts, both heterogeneous and homogeneous, for the ꢀ-
methylstyrene dimerization. We have explored the chem-
istry in both polar and nonpolar solvents. We demonstrate
that under some conditions, the sulfonic acid groups in the
There are many commercial processes that are currently
catalyzed by concentrated homogeneous mineral acids.
These reaction media are highly corrosive, and the pro-
cesses often produce large volumes of chemically reactive
waste streams. As a result, a clear opportunity exists for
new technologies that can replace these processes with
more environmentally friendly solid acids. Besides various
zeolite catalysts, sulfonic acid cation exchange resins such
®
®
Nafion -H resin/silica composite are much more readily
as Amberlyst-15 have found increasing use in industrial
accessible than they are in other catalyst microstructures,
and this leads to significant activity enhancements.
applications of this kind, as solid acid substitutes for mineral
ꢀ E-mail: sunq@A1.ESVAX.UMC.DUPONT.COM.
Nafion is a registered trademark of The DuPont Company.
Most of the published work on AMS dimerization is
in the patent literature (8). Four dimerization products,
®
62
0021-9517/96 $18.00
c
Copyright ꢂ 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

ꢀ-METHYLSTYRENE DIMERIZATION
63
®
®
solution of Nafion resin (150 ml 0.4M NaOH added to
300 ml 5% Nafion resin by weight, perfluorinated ion-
exchange polymer available from Aldrich Chemical Com-
pany). The base induces rapid condensation of the silica
and a hard gel is formed in seconds. The gel is then dried
(at 90ꢁC) and re-acidified with nitric acid (stir with 500 ml of
25% nitric for 1 h, filtered and washed with distilled water
and repeated a total of four times). The resulting material
is most accurately described as a nanocomposite wherein
®
the Nafion resin is distributed at the nanometer level (in
the 20–60 nm range) within the porous silica network. The
®
Nafion resin loading in the composite material that we
have used throughout this work is about 13 wt% ; however,
if desired this can be varied (up to about 80 wt% ), by in-
®
creasing the amount of Nafion resin solution used in the
general procedure outlined above.
®
Nafion NR50resin wasobtained from the DuPont Com-
pany. The proton form has an acid capacity ꢃ0.89 meq/g
REACTION SCHEME 1
®
(5). It was used without further treatment. Amberlyst-15
was purchased from Rohm and Haas. This resin contains
a small amount of free acid which could result in a much
higher activity in the polar solvent p-cresol. This effect has
also been noticed by Chaudhuri and Sharma (8). Therefore,
2,4-diphenyl-4-methyl-1-pentene (I), 2,4-diphenyl-4-me-
thyl-2-pentene (II), 1,1,3-trimethyl-3-phenylindan (III),
and cis- and trans-1,3-dimethyl-1,3-diphenylcyclobutane
(IV) have been reported. We observed only I–III under
our reaction conditions (Reaction Scheme 1).
®
the Amberlyst-15 catalyst was washed with an acetone
and water mixture several times until a neutral solution
was obtained. The washed catalyst has an acid capacity of
The unsaturated dimers of AMS, particularly 2,4-diph-
enyl-4-methyl-1-pentene (I), are industrially important and
are useful as chain-transfer agents or molecular weight
regulators in the production of polymers. Derivatives of
the saturated dimer, 1,1,3-trimethyl-3-phenylindan (III),
are used in the electronic industry as a liquid vehicle for
inorganic particles. Dimers are produced by using min-
eral acid catalyst AlCl3 (9). A BASF process that converts
styrene to 1-methyl-3-phenylindane for the production of
anthraquinone was catalyzed by mineral acids (10). Chaud-
huri and Sharma have recently described the use of this re-
action to compare the catalytic activity for a number of solid
®
4.3 meq/g. The supported Nafion resin employed here is
®
Nafion resin on shot coke which was prepared accord-
ing to an earlier DuPont patent (6). The catalyst contains
®
ꢃ2 wt% Nafion resin. Before testing, this catalyst was re-
activated by stirring in a 25 wt% HNO3 acid solution at 80ꢁC
overnight, followed by several washings with distilled wa-
ter. Reactant AMS, solvents cumene and p-cresol, p-TSA
and triflic acid were all purchased from Aldrich of the high-
est purity available. Dry molecular sieves were added to
AMS and cumene to eliminate water in these samples.
®
acids (8). When Nafion resin was used, a strong solvent de-
Characterization of Solid Catalysts
pendency of the dimerization rate constant was observed.
We have extended the work of Chaudhuri and Sharma and
®
The Nafion resin/silica composite is a hard, porous,
®
glass-like material. Individual particles are a few millime-
ters in size, with values of the BET surface area, pore diam-
included in the study the new Nafion -H resin/silica com-
posites. We have observed that the solvent effect is com-
pletely absent for these materials.
eter, and pore volume of 344 m2 gꢄ1, 9.8 nm, and 0.85 ccgꢄ1
,
respectively. The material could be crushed and sieved to
desirable sizes for particular applications. The surface area
is approximately 20,000 times greater than that of the pure
EXPERIMENTAL
®
polymer (Nafion NR50 resin, with a surface area of about
Catalysts and Materials
0.02 m2 gꢄ1). The 2 wt% Nafion resin on shot coke has a
®
®
The Nafion resin/silica composite was prepared using
BET surface area ꢃ1 m2 gꢄ1 and a mean pore diameter of
an in situ sol–gel technique. The synthesis and detailed
characterization of these materials have been described
elsewhere and only a brief summary will be presented
here (7). In a typical synthesis, a solution of partially hy-
drolyzed tetramethylorthosilicate (204 g of Si(OMe)4, 33 g
100 nm.
TGA/TPD Measurement
Chemisorption and temperature programmed desorp-
of H2O, and 3 g of 0.04MHCl) is rapidly mixed with a basic tion of 2-propanol on the solid acids was studied using

64
SUN, FARNETH, AND HARMER
a combined vacuum microbalance/mass spectrometer sys-
tem. Samples are loaded onto the microbalance pan (typ-
ically 10 mg) and the system is pumped down to back-
ground pressures of <10ꢄ7 torr with a turbomolecular
pump. Gases are admitted into the chamber from an at-
tached manifold. Pressures are measured with a Baratron
capacitance manometer. Uptake is followed gravimetri-
cally. Following adsorption, the system is reevacuated to
background pressures. Residual strongly bound material is
reflected in the mass increase. The temperature is ramped
at 5ꢁC/min to 400ꢁC. Mass spectral profiles of the evolv-
ing gases are followed with a UTI 100C quadrupole mass
spectrometer. Details of this apparatus have been described
previously (11).
Catalytic Reactions
The AMS dimerization reactions were carried out in a
two-neck flask with magnetic stirring in solvents cumene or
p-cresol at 50ꢁC. For a typical run, 6 g of AMS was added
to 54 g of solvent. Usually, between 0.1 to 1.0 g of solid
acid catalyst was used for the reactions. Prior to reaction,
all the solid acid catalysts were dried in a vacuum oven at
FIG. 1. AMS concentration (% ) as a function of reaction time (min)
when AMS (6 g) dimerization was carried out in cumene (54 g) at 50ꢁC
®
with 13 wt% Nafion resin/silica composite catalyst (0.5 g).
the reaction was carried out at 0ꢁC, a selectivity to dimer
(I) of 92.4% at 93.0% AMS conversions can be obtained.
Product (III) appears to be the thermodynamically favored
product. At higher temperatures given enough reaction
time, products (I) and (II) could both be converted to 1,1,3-
trimethyl-3-phenylindan (III). At 50ꢁC, Fig. 1 is very repre-
sentative of most of the reaction systems we have examined.
Figure 2 is a plot of ln([AMS]t/[AMS]0) versus time for
ꢁ
®
150 C overnight, except Amberlyst-15 which was dried in
a vacuum oven at 110ꢁC. Flasks used for carrying out the
AMS dimerizations were also dried in a vacuum oven. Due
to their extraordinarily high activities in p-cresol, the two
soluble acids p-TSA and triflic acid were first dissolved in
the solvent and then the acidic solutions were added to the
reaction mixture. During the reaction, aliquots (ꢃ0.2 ml)
were taken at certain time intervals and analyzed by GC
and GC/MS for product quantification and identification,
respectively. External standard was used for the GC anal-
ysis and good mass balances (better than 90% ) were ob-
tained by monitoring both the AMS converted and the
dimers formed in the reaction mixtures. In p-cresol solu-
tion, besides the three dimers, a small amount of alkylation
product formed by addition of AMS to p-cresol was also
observed. In most cases this product amounted to less than
5% of the total product mixture. On the other hand, in
cumene solution no significant amount of by-product was
ever detected.
®
the same catalyst, 13 wt% Nafion resin/silica composite
(0.2 g), as in Fig. 1 but from a different experiment, where
[AMS]t and [AMS]0 are the AMS concentrations at time t
and time zero, respectively. This plot shows excellent linear-
ity. Reaction is clearly first order in AMS under these condi-
tions as also observed by Sharma et al. (8). First-order rate
RESULTS
Reaction Kinetics
Figure 1 is a plot of AMS and product concentration
®
versus time for the 13 wt% Nafion resin/silica composite
(0.5 g) catalyzed dimerization of AMS at 50ꢁC in cumene.
It appears that the unsaturated 2,4-diphenyl-4-methyl-1-
pentene (I) isthe kineticallyfavored product. It wasthe pre-
dominant product until AMS conversion reached ꢃ90% in
this case. The product distribution also depends on the cata-
lyst, solvent, and reaction temperature employed. Low tem-
FIG. 2. Plot of ln([AMS]t/[AMS]0) versus reaction time (min) for
®
AMS (6 g) dimerization in cumene at 50ꢁC with 13 wt% Nafion resin/
perature favors the formation of product (I). In fact, when silica composite catalyst (0.2 g).

ꢀ-METHYLSTYRENE DIMERIZATION
65
®
constants are calculated from data of this kind using AMS
The 13 wt% Nafion resin/silica composite is the most
conversions from 0–30% . For all catalysts, the reaction rate active catalyst among the heterogeneous catalysts tested.
®
was independent of the stirring rate (200–1000 rpm). In ad- Nafion NR50 resin is not an efficient solid acid cata-
®
dition, for the 13 wt% Nafion resin/silica composite cata- lyst in the nonswelling solvent cumene for AMS dimeriza-
lyst no particle size effect (9 to >80 mesh) was observed. tion. However, its activity is enhanced by over an order
These observations imply that diffusion limitations were of magnitude in p-cresol. In contrast, the performance of
®
not present under the conditions employed. When AMS the composite, the supported Nafion resin catalyst, and
®
dimerization was carried out with plain silica support, un- Amberlyst-15 did not depend on the solvents used. Even
®
der the same reaction conditions, no AMS conversion was though Amberlyst-15 has the highest acid capacity and a
detected.
high BET surface area, it is not very active for the dimer-
All of the first-order rate constants for AMS dimeriza- ization of AMS under the reaction conditions employed
®
tion measured over various catalysts and in both cumene here. For Nafion NR50 resin and Amberlyst-15 catalyzed
and p-cresol solutions are listed in Table 1. The acid capaci- AMS dimerization, the rate constants that we obtained are
ties of the solid acids listed in Table 1 were determined consistent with those reported by Chaudhuri et al. (8) at
by simple acid/base titrations, for which solid acid was first higher temperatures. In particular, the relative rate con-
®
ion exchanged with large excess of sodium chloride (100/1 stants for Amberlyst-15 (in cumene), NR50 (in cumene),
for Na+ to H+) and then the aqueous solution was titrated and NR50 (in p-cresol) are very similar, (1 : 0.06 : 3.6) versus
with volumetric NaOH standard after filtering out the solid (1 : 0.16 : 3.8).
catalyst and extensive washing with deionized water. The
We have also benchmarked the heterogeneous catalyst
pKa values of small organic acids listed here (12) are quite performance against several soluble acids. p-TSA does not
different from that of the Hammett acidity function (ꢄH0) dissolve in cumene and no AMS conversion was observed.
values for these acids commonly cited in the literature (13). However, it does dissolve in p-cresol and very high activity
We have chosen to list these values because all of them were was observed in that solvent. With triflic acid, AMS dimeri-
available from a single, self-consistent source. In the discus- zation can be observed in cumene, but due to its very low
sion that follows (vide infra) we will be mainly concerned solubility the reaction rate could not be quantified. Triflic
with the relative proton donor abilities of these acids in acid is an even more active catalyst than p-TSA in p-cresol.
polar media, and we feel that the values listed in Table 1 The weaker acid trifluoroacetic acid can also catalyze the
adequately capture this trend.
dimerization in p-cresol, although it is considerably less
active than p-TSA. Glacial acetic acid is not catalytically
active. Table 1 lists estimated pKa values for these acids.
Although the Bro¨nsted acid strengths of these molecules in
p-cresol may not be the same as in water, the trend in activi-
ties follows pKa, suggesting that the homogeneous catalysts
act as specific acid catalysts in p-cresol. The pKa of proto-
nated p-cresol is about the same as that of the p-TSA (13).
So one might expect nearly complete proton transfer from
p-TSA or triflic acid to solvent in p-cresol. The lower activ-
ity of the weaker acids may reflect the lower concentrations
of free protons.
TABLE 1
First-Order Rate Constants for the Dimerization of ꢀ-Methyl-
styrene (AMS, 6 g in 54 g Solvent) over Solid and Homogeneous
Acid Catalysts in Organic Solvents at 50ꢁC and Properties of Acids
Solvent: Cumene p-Cresol
(1/meq H+ ꢅ h)
Acid properties
Surface area pKa
[H+]
a
Catalyst
NR-50
0.1
110.1
2.3 0.89 meq/g
106.8 0.14
0.02 m2/g
350
Composite
13% Nafion
resin/SiO2
®
®
Amberlyst-15
0.6
3.9
0.3 4.30
4.9 0.02
45
ꢃ1
DISCUSSION
®
2% Nafion resin
on Shot Coke
Glacial acetic acid
Trifloroacetic acid
p-Toluene sulfonic
acid (p-TSA)
A simplified version of Reaction Scheme 1 suitable for
kinetic analysis is Scheme 2:
b
b
b
ꢃ0.0 6.67
1.0 8.77
1,200 5.26
4.8
ꢄ0.6
ꢄ2.7
1. AH + AMSꢄ↔ (HAMS)+Aꢄ
2. (HAMS)+A + AMS → [H(AMS)2]+Aꢄ
3. [H(AMS)2]+Aꢄ → prod. + AH.
c
Triflic acid
15,000 6.67
ꢄ5.5
CF3SO3H
Based on this reaction scheme, assuming k3 is fast and ap-
plying a steady state approximation for (HAMS)+Aꢄ, one
can derive the following expression for the rate of product
appearance:
a pKa values for these acids have also often been estimated using Ham-
mett acidity function methods. The trends in relative acidity seem to be
the same, although H0 values are typically more negative.
b
These acids are insoluble in cumene solution and no activity were
measured.
c Triflic acid is very active, but itssolubility is verylow in cumene solution
and therefore the activity could not be quantified.
Rate (R) = k1k2[AMS]2[AH]/(k2[AMS] + kꢄ1).
[1]

66
SUN, FARNETH, AND HARMER
This expression suggests two limiting kinetic regimes. When materials since the proton donor in all cases is a perfluoro-
k2[AMS] ꢆ kꢄ1, then the rate expression reduces to
R = k1[AMS][AH].
sulfonic acid residue. The relative rates then might be re-
lated to different numbers of accessible acid sites within the
different microstructures. Under nonswelling conditions,
we estimate the accessible acid site density from the num-
ber of sulfonic acid residues on the exterior surface of the
[2]
The rate should appear first order in AMS concentration,
and the observed rate constant is the product of the ele-
mentary rate constant for formation of the carbenium ion,
k1, and the effective acid catalyst concentration. On the
other hand, when kꢄ1 ꢆ k2[AMS] then the rate expression
reduces to
®
Nafion resin particulates. The ratio of the BET surface
areas in 1.0/50/17500. Of course for the composite, most of
the BET surface area is the noncatalytic, silica surface. Mi-
croscopy data suggests that in the Nafion/silica composite,
®
the Nafion resin is uniformly dispersed as 20–60 nm par-
ticulates within the silica matrix. Therefore if we presume
that the Nafion resin surface area in the composite is given
R = Kk2[AMS]2[AH],
[3]
®
where K is the equilibrium constant for proton transfer be-
tween AMS and whatever the effective acid AH is. The
first-order behavior of all of the acid catalysts (vide infra)
suggests that we can rationalize their relative catalytic ac-
tivities within the framework of Eq. [2].
by the weight fraction times the BET surface area, then the
®
Nafion resin surface area ratios would be 1.0/50/2280. For
®
the supported catalyst this presumes that the Nafion resin
is well dispersed over the support. This ratio is very close
to the relative rate ratio in cumene, although both the sup-
ported and composite materials appear to be a factor of 2
or so less active than this rough model suggests. It is quite
In the case of the homogeneous acids it seems likely that
the reaction is specific acid-catalyzed and that the proton
donor is a solvated proton. This appears to be implicitly
assumed by Beltrame et al. (14). It is suggested by the clear
trend in rates with pKa that we observe and is also consistent
with the modest activity of oxalic acid ( pKa ꢃ 1) reported
by Chaudhuri et al. in phenol (8). Since the effective pro-
ton donor is the protonated solvent, SH+, for all homoge-
neous acids, k1, kꢄ1, and k2 in Eq. [1] will be independent of
the nature of the added acid. Relative rates are then deter-
mined bythe effective concentrationsofSH+, fixed bya pre-
equilibrium between the added Bronsted acid and p-cresol,
i.e., CF3COOH + S ↔ CF3COOꢄ + ꢄSH+, where S repre-
sents a solvent molecule. Assuming complete dissociation
for the strongest acid, triflic acid, this interpretation implies
that the equilibrium constant for the dissociation of p-TSA
is about 0.2, and for TFA is about 4 ꢇ 10ꢄ3 in this medium.
Furthermore, the entry in Table 1 for triflic acid represents
the turnover frequency for the solvated proton within this
set of assumptions, since [AH] = [SH+] = [TFA]0. This can
be a benchmark for other proton sources.
®
likely that the Nafion resin is not uniformly dispersed on
the supported catalyst and that the weight fraction overes-
®
timates the Nafion resin surface area fraction in the com-
posite (15).
We can pursue this picture a little further by estimat-
ing the absolute number of acid residues on the external
®
surface of Nafion NR-50 resin. Assuming a bulk den-
sity of 2 g/cc, and a BET surface area of 0.02 m2/g we
calculate that the fraction of sulfonic acid residues within
the average acid–acid site separation distance of the sur-
face is 4.0 ꢇ 10ꢄ5, the so-called noninteraction accessibil-
ity suggested by Buttersack [16]. This implies that the ac-
tivity/accessible acid site, or turnover frequency, for aꢄs5ul-
®
fonic acid residue in Nafion NR-50 resin is 0.1/4.0 ꢇ 10
=
2500 hꢄ1. This is about a factor of 6 lower than we esti-
mated above for the solvated proton in homogeneous me-
®
dia and suggests that the sulfonic acid residues in Nafion
resin have a high inherent activity even in nonsolvating me-
dia. This is consistent with previous work comparing struc-
turally identical sulfonic acid sites in heterogeneous and
homogeneous acids which shows similar but slightly lower
enthalpies for proton transfer from the solid acid systems
to a variety of bases (17).
It is clear from Table 1 that the same interpretation can-
not be applied to the resin acids. The reaction cannot be spe-
cific acid catalyzed. The absence of a solvent effect for the
®
supported catalyst, the Nafion resin/silica composite and
®
Amberlyst-15 , suggests that these processes must be gen-
From this point of view, we may look at the composite as a
way of increasing the accessible fraction of acid residues by
eral acid catalyzed, with proton transfer occurring directly
from the resin acid to AMS. Relative rates for different
catalysts can then be described within the context of Eq. [2],
where both k1, and the nature and concentration of the ef-
fective proton donor, AH will differ for each solid acid.
®
stabilizing Nafion resin spheres of approximately 30-nm
radius. Since the specific surface area of a collection of uni-
form spheres is given by S = 3/rp, where r is the radius and
p is the bulk density, then we would expect that decreasing
a sphere radius from 75 ꢁm in NR-50 resin to 30 nm in the
composite, the number of surface accessible sites would in-
crease by about 2.5 ꢇ 103. We observe a rate increase of
about 1100 in cumene. Again, the estimated increase in
accessible sites is likely to be high, since the spheres are
®
Let us consider first, the three Nafion resin-based cata-
lysts. The ratio of the observed first-order rate constants
per acid equivalent in the nonpolar solvent, cumene, is
1.0/39/1100 for NR-50/supported/composite. We might be-
gin by assuming that k1 will be the same for these three

ꢀ-METHYLSTYRENE DIMERIZATION
67
embedded in the silica matrix and the surfaces cannot be posite (12% , based on uniform 30 nm spheres), p-cresol
fully available to the substrate. This argument also does not acts principally to stabilize the undissociated active sites
consider the possibility that the AMS concentration in con- through hydrogen bonding and thereby reduces the overall
®
tact with the Nafion surface within the composite might catalytic activity.
be different from the solution concentration. However, be-
We have carried out another type of experiment that
cause of the structural similarity of AMS and cumene we tries to tie these swelling and acid site accessibility argu-
would not expect the partitioning to be large. On balance, ments together. The basic experiment is described in the
the simple surface-area based accessibility arguments ap- Experimental section. Briefly, 5–50 mg of catalyst is placed
pear to give a reasonable semiquantitative rationalization in the pan of a microbalance enclosed within a high vac-
of the relative activities of this set of catalysts.
uum chamber. The sample is dehydrated by mild heating in
®
We can apply the same analysis to Amberlyst-15 cata- a vacuum and stabilized in a vacuum at background pres-
lyst. Using the BET surface area (45 m2/g) and the assump- sures of <10ꢄ7 torr. The sample is then exposed to adsor-
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tions applied to Nafion NR-50 resin above, we calculate bate vapor at room temperature and the uptake is followed
that the fraction of acid sites accessible from the external continuously as the sample weight increases. After satura-
medium should be 3.9 ꢇ 10ꢄ2 in Amberlyst-15 (16). This tion is reached, the chamber is evacuated and a residual
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implies an activity/external acid site in cumene of 15. This strongly bound mass is retained on the catalyst. Finally, the
is a factor of 266 lower than our estimated turnover num- system is heated and the mass falls as chemisorbed mate-
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ber for Nafion resin. Taken at face value, as the ratio of rial is desorbed. The partial pressures of desorbing products
k1 values for the two types of solid acid sites present in are followed by mass spectroscopy during this phase. A typ-
these resins, it could be interpreted as reflecting differences ical TGA profile and the changes in mass spectral intensi-
in the acid strength between the benzene sulfonic acid-like ties for selected masses during TPD are shown in Fig. 3 for
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residues in Amberlyst-15 and the perfluoroalkanesulfonic 2-propanol as adsorbate on the 13 wt% Nafion resin/silica
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acid-like residues in Nafion resin. However, we are uncer- composite catalyst. Notice the rapid uptake rates, the stable
tain that the BET surface area has much relevance to the residual mass, and the clean desorption profiles for m/e 41
actual surface area presented to the medium by the macro- and m/e 18. M/e 41and 18are used to track the desorption of
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porous Amberlyst-15 resin in any solvent. So while we propene and water, respectively. For this catalyst, all of the
believe that this analysis may offer some insights into the mass loss during temperature programmed desorption can
factors affecting activity for a given resin in different mi- be accounted for as propene and water. Results of this ex-
crostructures, as for the case with Nafion resin prepara- periment (10 torr 2-propanol; 23ꢁC) carried out on all four
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tions above, we are reluctant to attach much quantitative of the heterogeneous acid catalysts are compared in Table 2.
significance to this type of comparison when it is extended
We use time to half-saturation to characterize the adsorp-
to different resins where the interactions with solvent might tion kinetics, although the uptake process is not strictly first
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be very different.
order. The pure resin catalysts, both Nafion NR-50 resin
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Solvent swelling of resin catalysts is another means of and Amberlyst-15 , are much slower to reach saturation
increasing the effective number of acid sites. In the polar than the supported or composite catalysts. The saturation
solvent, p-cresol, the rate with NR-50 resin increases by a uptakes also vary widely. It is interesting that with all of
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factor of 23, the rate for the supported catalysts increases the Nafion resin catalysts, the saturation uptakes exceed
by 20% and the rate with the composite is essentially un- what would be required for a stoichiometry of one adsorbed
changed. Perhaps most surprising, the rate for Amberlyst-
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15 is more or less unchanged. Although we have not mea-
TABLE 2
sured the equilibrium swelling of these catalysts in p-cresol,
it would be hard to rationalize these results if the only role
Summary of TGA/TPD Data
of the solvent were to increase the number of acid sites that
Uptake Uptake Residual
are available to AMS. For example, we observe slight re-
t(1/2)
(min)
sat.
mass
[H+]
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TPDa
ductions in activity for Amberlyst-15 and the composite,
Sample
(g/g) (mmol/g) (meq/g)
implying that solvents can also affect the proton transfer
rate constant, k1. This would not be unexpected, since a
solvent molecule on either side of the equilibrium in the
first step of Scheme 2 will surely change the overall ther-
mochemical driving force for proton transfer (17). Perhaps
when the noninteraction accessibility is low, as in NR-50
resin (0.0025% ) the enhancement in the number of accessi-
NR-50
13% Nafion
15
2
0.73
0.99
0.80
0.09
0.89
0.14
69, 57, 43, 41, 18
41, 18
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resin/silica
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2% Nafion
1
0.0051
0.01
0.02
57, 43, 41, 18
resin on
shot coke
Amberlyst-15
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22
0.154
0.26
4.30
64, 48, 41, 18
a Principal m/e values displaying significant intensity in the desorption
ble sites is the dominant factor, but when the noninteraction
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accessibility is high as in Amberlyst-15 (4% ) or the com- spectrum.

68
SUN, FARNETH, AND HARMER
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FIG. 3. (a) Weight changes during adsorption/temperature programmed desorption of 2-propanol on 13% Nafion resin/silica catalyst (dashed
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line) and Nafion NR50 resin beads (solid line). (b) Mass spectral profiles for m/e 41, 18, and 69, during TPD for 13% Nafion resin/silica composite.
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2-propanol/acid equivalent, but with Amberlyst-15 it is sumably due to oligomerization reactions of propene dur-
only about 0.5 of this value. On reevacuation of the ing the slow diffusion out of the catalyst bulk. Amberlyst-
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TGA chamber, the residual masses of 2-propanol, the 15 yields nearly complete conversion to propene and wa-
chemisorbed amount, is close to the expected acid site ter, but above 125ꢁC significant amounts of SO and SO2 are
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concentration for the Nafion -based catalysts, but only also evolved.
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about 5% of that amount for Amberlyst-15 . Finally, on
These results clearly illustrate the differences in acces-
temperature-programmed desorption to 150ꢁC, while the sibility of chemisorption sites, presumably Bro¨nsted acid
composite catalyst yields only propene and water, the prod- sites, among these materials at least for adsorption from the
ucts of unimolecular dehydration (Fig. 3b), the NR-50 resin gas phase at low temperature and pressure. The easy access
shows significant amounts of higher mass fragments pre- to strongly binding active sites in the composite catalysts

ꢀ-METHYLSTYRENE DIMERIZATION
69
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correlates well with the high solvent-independent activity thermal stability, chemical inertness), make the Nafion -H
of this catalyst in the AMS dimerization. It illustrates the resin/silica composites very attractive materials for a vari-
promise of this family of catalysts for Bro¨nsted acid cata- ety of solid acid catalyzed processes.
lyzed chemistry in nonpolar and gas phase environments.
ACKNOWLEDGMENTS
CONCLUSIONS
We thank Ms. S. Prilutski, X. Yang, Mr. S. Winchester, and W. Dolinger
for technical assistance and Dr. E. I. Baucom and M. Deschere for helpful
comments.
First-order rate constants for the AMS dimerization
catalyzed by heterogeneous solid acids as well as solu-
ble organic acids were determined in the liquid phase.
REFERENCES
Very high activity was obtained from the highly porous
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13 wt% Nafion -H resin/silica composite catalyst. It ex-
hibited more than two orders of magnitude higher activity
1. Chakrabarti, A., and Sharma, M. M., Reactive Polymers 20, 1
(1993).
2. Neier, W., in “Ion Exchangers” (K. Dorfner, Ed.), Chap. 2, p. 981.
de Gruyter, Berlin/New York, 1991.
3. Olah, G. A., Iyer, P. S., and Prakash, G. K. S., Synthesis 513 (1986).
4. McClure, J. D., and Brandenberger, S. G., U.S. Patent 4,052,475
(1977).
5. Waller, F. J., and Van Scoyoc, R., CHEMTECH 17, 438 (1987).
6. Butt, M. H., and Waller, F. J., U.S. Patent 5,094,995 (1992).
7. Harmer, M. A., Farneth, W. E., and Sun, Q., J. Am. Chem. Soc., 118,
7708 (1996).
8. Chaudhuri, B., and Sharma, M. M., Ind. Eng. Chem. Res. 28, 1757
(1989).
9. Wilczek, L., and O’Neil, J. W., U.S. Patent 5,344,592 (1994).
10. BASF, DE 2,064,099, 1970.
11. Farneth, W. E., Ohuchi, F., Staley, R. H., Chowdhury, U., and Sleight,
A. W., J. Phys. Chem. 89, 2493 (1985).
12. Stewart, R., “The Proton: Applications to Organic Chemistry,”
Chap. 2. Academic Press, London, 1985.
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than the bulk Nafion -H resin. The 2 wt% Nafion resin
on shot coke was significantly more active than the plain
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®
Nafion resin catalyst as well. Amberlyst 15 was roughly
a factor of 200 less active than the composite on an acid
equivalent basis, about a factor of 7 less active on a weight
basis.
A strong correlation is found between the catalytic ac-
tivity of these heterogeneous catalysts for the AMS dimer-
ization and the accessibility of their Bro¨nsted acid groups.
®
For example, Nafion NR50 resin show very low activity in
the nonpolar media and enhanced activity was observed in
polar solvent which is a result of framework swelling by the
solvent and increased acid accessibility. This enhancement
in accessibility can be made solvent independent by incor-
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13. March, J., “Advanced Organic Chemistry,” 4th ed., Chap. 8. Wiley,
New York, 1992.
porating the Nafion resin into a silica matrix. Because of
the importance of solvent effects, BET surface area alone
may not be a good indicator of accessibility. In particular,
14. Beltrame, P. L., Ind. Eng. Chem. Res. 27, 4 (1988).
®
15. A 60 nm diameter sphere of Nafion resin with a density of 2.0 has
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a specific surface area of 50 m2/g. Since the composite as a whole has
a BET surface area of 350 m2/g, using a weight fraction to determine
even though the BET surface area of the Amberlyst-15
is very high (45 m2/g), both catalytic and TGA data indi-
cated that the accessibility of the acid group is significantly
lower than that in the Nafion resin/silica composite mate-
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the Nafion “effective” surface area is probably an overestimate.
16. Buttersack, C., React. Polym. 10, 143(1989). ꢂ = Sꢃ2/3/(CNA)1/3, where
ꢂ = the number of accessible sites, S = specific surface area, ꢃ = bulk
density, C = meq/g, and NA is Avogadro’s constant.
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rial. We believe that the enhancement in accessibility, com-
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bined with the inherent advantages of Nafion perfluoro-
17. Arnett, E. M., Haaksma, R. A., Chawla, B., and Healy, M. H., J. Am.
Chem. Soc. 108, 4888 (1986).
sulfonic acid resins as a solid acid source(high acid strength,