Sulfonated Polymers as Alkylation Catalysts John M. Kapural and Bruce C. Gates* Department of Chemical Engineering, University of Delaware, Newark, Del. 19711
Rates of alkylation of benzene with propylene catalyzed b y a series of sulfonated polymers were measured with a differential flow reactor operating with vapor-phase reactants at atmospheric pressure and temperatures from 135" to 250°C. Rates of reaction were greatest for a fluorocarbon polymer containing catalytically active -CFLF2S03H groups. The electron-attracting fluorine atoms increased the acidity and catalytic activity of the acid groups. Catalysts lost activity in operation as they were desulfonated and poisoned by polyolefinic products. Four sulfonated polymers (fluorocarbon, polyphenylene oxide, polyphenyl, and cross-linked polystyrene) suffered rapid loss of acidity and catalytic activity at temperatures above 150°C. The observed lack of stability of sulfonated polyphenyl contradicted results in the literature, casting doubt on the possibility of synthesizing strong-acid organic ion exchangers stable at high temperatures.
E a s e of separation of fluid-phase reaction products from solid catalysts accounts for the predominance of heterogeneous catalysis in large-scale chemical processing. Yet, homogeneous catalysis is becoming increasingly important, offering the advantages of highly dispersed cat'alysts and reaction chemistry more readily understood and predicted than that involving surface intermediates. Some advantages of both heterogeneous and homogeneous catalysis can be realized in applications of insoluble organic polymers. Such polymeric catalysts include ion exchangers with easily accessible acidic or basic functional groups and resins bound as ligands to active transition metal complexes. Enzymes as well are macromolecular catalysts (some are found in cytoplasmic solution, and some bound within cellular membranes) which have active sites involving acidic arid basic functional groups and coordinated metal ions. The activity and specificity of enzymes indicate the potential for improvement of synthetic macromolecular catalysts. Matrix-bound transition metal complexes have only receiit'ly begun to find applicat'ion as catalysts (Kohler and I)atvans, 1972). Ion-exchange resins have been used since they became commercially available more t'haii 20 years ago. Interest has centered on resins containing strongly acidic groups. Sulfonated styrene-dirinylbenzene copolymer, the most widely applied of these catalysts, is active for such reactions as alkylation of aromatics, hydration of olefins, and esterification. The thorough literature compilations of Polyanskii (1962, 19iO) indicate much consideration has been given to this resin as an iiidust'rial catalyst,, though few large-scale processes are cited in the open literahre. The literature is characterized by fragmentary information, failing to clearly define the practical limitations of polymeric acid catalysts. An objective of t'his research was to measure catalytic activit'ies and stabilities of several new acidic polymers in a n industrially important aromatic alkylation reaction. Severe operatiiig conditions were chosen with vapor-phase reactants at atmospheric pressure to provide data for evaluation of the limits of applicability of this class of catalysts. The choice of catalysts was influenced by recent work of Manassen aiid Khalif (1967) and Paushkin and Logashin (1970) indicating potential for application of sulfonated polymers as Present address, Dow Chemical Co., Midland, illich. 48640. 62 Ind.
Eng. Chem. Prod. Res. Develop., Vol. 12, No. 1, 1973
catalysts a t temperatures as high as several hundred degrees centigrade. Experimental
Catalyst Preparation and Characterization. Ainberlyst 15, a commercially available catalyst (Rohm and Haas),
is a sulfonated. macroporous copolymer of styrene and divinylbenzene (DVB). A sample was obtained as 16-20 mesh spherical particles containing 5.4 ineq - SOjH groups/g in a n air-dried state. With a n internal surface area of abo2t 40 m2/g and a n average pore diameter of about 300 A (Kunin et al., 1962), this catalyst offers a n array of -SO,H groups easily accessible to fluid-phase reactants, even when they do not swell the polymer. It is presumed to contain about 20% DVB cross-linking agent, based on its swelling characteristics and on the effect of DVl3 on catalyst performance cited in a patent (Bortnick. 1962). The ion-exchange resin was washed with methanol and water and cycled between the sodium and hydrogen forms before use as a catalyst. A sulfonated fluorocarbon vinyl ether polymer (Du Pont XR resin) containing 1.09 nieq -%&E groups/g was obtained as irregularly shaped particles with a n average dimension of 300 p . The resin is one of a series of fluorocarbon polymers and copolymers reported in a patent (Connolly and Gresham, 1966). The catalytically active groups in the polymer are -CF2CF2S03H groups (hnderson, 1972). The structure of the catalyst, which has been inferred from the patent, is shown in the summary of Table I. This copolymer is a gel. lacking true internal surface. It is a p m e d to have a n effective pore diameter of the order of 10 A, which is typical of cross-linked polystyrene gels (Downing and Hetherington, 1963). Poly(2,6-dimethyl-l,4-phenyleneoside) (PPO) was obtained from the General Electric Co. Chlorosulfonic acid was added to a solution of PPO in chloroform, and the resulting sulfonated polymer formed a precipitate (Fos and Shenian, 1966) which contained 6.4 ineq -S03H groups/g after drying for 24 hr under vacuum at 12OOC. Water was not completely removed from the gel by this drying procedure. The product x a s used as 32-65 mesh particles. Polyphenyl was synthesized, and fractions of the product were sulfonated and phosphonated accordillg t o the methods of Manassen and Khalif (1967), who used these highly
t
Benzene
Table 1. Catalyst Structures STRUCTURE
CATP, LY S T
Syringe Pump
Mixing Chcmber
Amberlyst
15
0
CH2 -CH-CH2 -CH -CH2--.
1
S03H I
$ F2 CF2 CF3 F ( oc F~c F ),OC F ~ F~ CS O ~ H
$
Fluorocarbon Resin
y 2
-
I
n =l,2, or 3
Flex1ble Metal Hose
ProDvlene
' - 1 1
u
I I
u
Nitrogen
Figure 1. Schematic diagram of reactor system
0010
S u l f o n a t e d Polyphenyl 0
t 0008 U 0
S u l f o n a t e d PPO
- 0006 n e %
;; 0004
9
crystalline polymers as catalysts for alcohol dehydration and olefin isomerization. The catalysts were obtained as 220-p particles containing 1.8 meq -s03H groups/g and 1.5 meq ---P03H2groupslg, respectively. Measurement of Catalyst Activity and Stability. T h e packed-bed flow-reactor system depicted in Figure 1 was used to determine catalyst activities and stabilities. Iteact,ants, propylene and benzene vapor, were mised aiid passed downward through the reactor, a heated exit line, :md a heated gas sampling valve, froin which the product : h e a m was intercepted for analysis by gas chromatography. 'The lines were stainless steel and glass, and the reactor was a glass tube 2.5 cm in diameter and 30 cm in length. Catalyst particles mixed with 0.4-cm diameter glass beads were contained in a 10-cm length of reactor, with glass beads filling the upstream and dcwnstream sections. -4glass well extended to the center of the reactor, \There temperatures were measured with a chromel-alumel thermocouple. The reactor was mounted in a. Lindberg Hevi-Duty electiical combustion furnace equipped with an on-off controller which maintained temperatures within 1 2 ' C . In a typical experiment, fresh catalyst in the reactor was brought to temperature in flowing nitrogen in a period of 40 min. Benzene and then propylene flon-s were started and maintained a t constant rates, and the product stream was sampled and analyzed atJabout IO-min intervals for a period of as much as several hours. React'or products were analyzed with a Hewlett-Packard model 700 gas chromatograph employing a 14-ft long, '/&i. diameter column containing Carbowax 2031 on Chromosorb W. The column was maintained a t 150°C,, and the helium carrier gas-flow rate was 50 ml/min. Products were eluted in
;0002 0 000
1
I
I
05
10
I 5
10'4 x I n v e r s e (Moles PropylenelHr
Space
; 0
Velocity
E q u i v a l e n t -S03H
Groups).'
Figure 2. Demonstration of differential reactor operation: alkylation of benzene with propylene in 5/1 mole ratio at 170°C catalyzed by fluorocarbon resin
the folloiTiiig order: propylene, benzene, cumene, 1,3-diisopropylbenzene, 1,4-diisopropylbenzene, and triisopropylbenzene. The following ranges of variables were investigated: temperature, 135-250OC; pressure 1 a t m ; and space velocity, 90-12,400 (moles propylene fed):'(hr equivalent of catalyst --S03H groups). The mole ratio of benzene to propylene was 5,'l ill most runs; occasionally it was 0.511. Data were obtained for the catalysts listed in Table I. Measurement of Catalyst Desulfonation Rates. .liter runs of varying lengths, catalyst charges were remoi-ed from the reactor and titrated against standard S a O H solution to establish catalyst acidity loss in operation. T o determine aciditj- loss in t h e absence of reactaiits, similar experiments were performed with nitrogen flow through t h e therinostated reactor. Details of the procedure are given b y 1ial)ura (1972). Results
Catalyst Activity. T h e esperimeiit was designed t o determine rates of t h e alkylation of benzene with propylene in Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 1, 1973
63
II. Summary of Rates of Catalyst Desulfonation a n d Activity Loss
Table
Catalyst
Fractional rate of acidity loss, hr-1 Reaction Nitrogen
Temp, 'C
Amberlyst 15
135 158 170 175 170 140 175 220 230 240 320 135 170 180 190 170 195
Sulfonated PPO Sulfonated polyphenyl
Fluorocarbon resin
Initial rate of activity loss in reaction, mol propylene reacted/ hr2 equiv -SOsH groups
0.01 ...
... 0.04 0.95
