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Functional silica supported polymers IV. Synthesis and catalytic activity of silica-grafted sulfonated polyphenylsilsesquioxane
15
Reactiee Polymers, 21 (1993) 15-25
Elsevier Science Publishers B.V., Amsterdam
Functional silica supported polymers IV. Synthesis and catalytic activity of silica-grafted sulfonated polyphenylsilsesquioxane E. Carlier a, A. Revillon a,., A. Guyot ~ and P. Baumgartner b CNRS-LMO, Lyon-Solaize, BP 24, F-69390 Vernaison, France b IFP-CEDI, Solaize, BP 6, F-69390 Vernaison, France
(Received January 21, 1993; accepted in revised form April 2, 1993)
Abstract The ladder polymer, polyphenylsilsesquioxane, has been grafted onto porous silica and sulfonated to obtain catalysts of high stability with e n h a n c e d site accessibility and increased n u m b e r of sites, as well as high acidity level. The catalytic behaviour, even at a low level of capacity, has shown excellent esterification and phenol alkylation activity, with, in the latter case, excellent selectivity, as compared to classical ion exchangers or highly acidic Nation material. In batch reaction, hydration of the support leads to partial deactivation; activity may be partly recovered and the rate constant is still higher than that obtained with the reference catalyst. Keywords: functional silica; solid acid catalysts; supported reagents; siloxane polymers;
sulfonated polymers
Introduction Acidic polystyrene resins have been used for many years by industry in a large n u m b e r of applications [1,2]. Despite their success, they suffer from some drawbacks, mainly due to the lack of thermal stability. This precludes their use for reactions at temperatures > 120°C. They also show poor mechan-
* Corresponding author.
ical properties. Moreover, due to their limited acidity, they cannot be used to replace strong mineral acids in some reactions, where a high acidity level is required, such as in aliphatic alkylation or benzene nitration in the vapour phase [3]. Additionally, for environmental reasons (acid slurry pollution) the use of these acids will become more restricted in the forthcoming years. At the same time, industry is demanding supported acid catalysts with improved thermal and mechanical properties and, if possible, higher acidity
0923-1137/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
E. Carlier et al. /React. Polym. 21 (1993) 15-25
16
than the existing commercial sulfonic acid resins. So, several attempts have been made to improve these three properties. We would like to mention here the interesting approach of Widdecke [1] to obtain perfluorinated resins from classical sulfonic resins. The target with an organic support is to reach the acidity of Nation. This has been shown to be one of the best acid catalysts ever used for a large number of reactions. However, it is not easy to handle and very expensive. Other interesting routes to improved acid catalysts have been explored by different authors using porous silica as the matrix [4]. The two major conclusions that can be drawn from the earlier works are that the capacity of the supports is rather low and that the acidity of the sulfonated silanes grafted on the silica surface is higher than the acidity of commercial sulfonic resins [5]. Attempts to form a polysiloxane network on silica surfaces have been described by Sander and Wise [6]. A more recent work, published by Suzuki et al. [7], shows very promising properties as compared to the reference acid catalyst Amberlyst 15. Their approach to increase the capacity of silica-based acid catalyst is to use a sol-gel process using Si(OEt) 4 and a functional silane, to form a silica network thus allowing after sulfonation a cata-
lyst to be obtained with an acid capacity of 4 meq/g.
Results and discussion
In our extensive effort to improve both thermal and mechanical stability, as well as accessibility of supported reagents, our latest approach has been to graft functional polymers on silica surfaces by various procedures [8]. Since thermal stability is a part of our goal, we have chosen to graft a thermostable polymer on various silica gels having different characteristics, reported in Table 1: mean volume particle diameter, D(v,0.5); pore diameter, %; total pore volume in cm3/g, lip; specific surface area in m2/g, S (from BET isotherms and porosimetry); and number of surface silanol groups (determined previously by measurement of alkane formation after reaction with an alkylaluminium). The polymer, polyphenylsilsesquioxane (PPSS), has a ladder structure, with a thermal stability of over 400°C [9]. It bears phenyl rings capable of being derivatized and also suitable endgroups for grafting onto the silica surface by coupling with silanol groups. After grafting onto the support, the polymer is sulfonated (SPPSS) and the acid catalyst obtained has been tested in two acid-catalyzed reactions:
TABLE 1 Chemical and structural characteristics of silica supports Silica a A B C D E F
Particle d i a m e t e r D(v,0.5) (/xm)
SBET
Sporo
Vp
(me/g)
(m2/g)
(cm3/g)
rp (nm)
[SiOH] ( / x m o l / m 2)
142 63 1500 28 5.5
18 29 78 89 175 400
12 76 86 200
1 1.1 0.98 0.9 0.8 1.8
300 155 55 70 22 18
4.8 5.3 3.3
a A = XOC005 from R h S n e - P o u l e n c ; B = F1000 from Merck; C, from Shell; D = RP1 and E = RP3 from R h 6 n e Poulenc; F = E D 3 from Grace.
E. Carlier et al./React. Polym. 21 (1993) 15-25
17
esterification of acetic acid, and alkylation of phenol by 1-dodecene. Catalyst synthesis and physical characterization
The synthesis of linear ladder polyphenylsilsesquioxane through C13SIC6 H 5 hydrolysis in various solvents has been described by several authors [10-12]. The synthesis is a two-step process, illustrated in Scheme 1. The first one is the grafting of polyphenylsilsesquioxane onto the silica surface, in toluene medium. Under the conditions used in our approach (see experimental part), it can be shown that a larger amount of phenyl groups are grafted than by silane hydrolysis in the presence of the silica support. Unger [4] reported that direct hydrolysis of C 1 3 S i C 6 H 5 leads to a maximal amount of 4 /xmol/m 2, for a silica with 8 ~ m o l / m 2 of silanol groups. For a silica support of 100
.20 l~
c1 -CI
(3
/•li-O"H /.
.o:i_%.
/
Ph I
\
Ph l
2 n C1503H I
.SO3H
Ph I
Si-O.-Si-O--Si--&-Si--&--Si
dx
'o t
'o t
"oI
Si--O-Si--O-Si--OH Si--O--Si
Ox
-si-o,
Ph
Ph
o I--I ~i-o, o H ~i.,,£1I O/ "CH
sO3H
Scheme 1. Hydrolysis of phenyltrichlorochlorosilane, condensation with silanols of silica and sulfonation.
b
c a
- l~2e
- IL4.O
- t~6B
- 1'86
-2~88
PPM
04
O4
O
o.S O,
o.S,.o o
o~Si"O "0
I
(a)-91.7 ppm
I
(b)-I01 ppm
I
(c)-111pprn
Fig. 1.29Si C P / M A S NMR solid state spectrum of silica, with assignment of bands to silanol surface groups of silica.
E. Carlier et al. /React. Polym. 21 (1993) 15-25
18
Ph Ph Ph i i I Si--O-Si--O--Si--O--Si--O--Si d, \ \O L , \ 0 H2 Si--O-Si--O--Si-Cl-I Si--O--Si I \
A=-91 ppm B = -101 p p m C = -111 p p m
b
H= -68 ppm I= - 7 7 p p m
C B Si--O~ 0 H A!i~O(-I 0
hi
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i
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,~
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.
.
.
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.
a
.
-Io 'L~o
.
. -go
.
.
' " -.o"" -1'1o
s .... -J20
J .... J ...... -130 -|40 -1-0
Fig. 2. 295i C P / M A S NMR solid state spectrum of silica grafted with (a) PPSS and (b) SPPSS, with assignment of bands to modified silanoi surface groups.
E. Carlier et al./React. Polym. 21 (1993) 15-25
19
m2/g, this represents a potential capacity of phenyl rings of 0.4 m e q / g (and, after sulfonation, 0.3 meq of acid/g). Tables 2 and 3
show the amount of PPSS grafted on different silica supports, which is about twice as high as the above value. The specific amount
Si
/ 3
4
tO0
t60
t40
Si
120
tO0
O0
60
40
20
ts4 SO:H
f 3
r - ~
1-r-r"~r~'t
ZIO
'"
;ZOO
I . . . .
tgo
I ' ~" 190
~"~ . . . . LTO
I .... t60
I ....
t~O
i''
140
' J .... t30 Ppl4
J .... l=O
! .... ttO
I . . . . tOO
I . . . . 9Q
I .... 90
I .... 70
I .... 60
! .... 50
I' 40
Fig. 3. 13C C P / M A S NMR solid state spectrum of silica grafted with (a) PPSS and (b) SPPSS.
0
-20
E. Carlier et aL / React. Polym. 21 (1993) 15-25
20 TABLE 2
%
Grafted PPSS on different silica supports
E+A"
Silica initial
aBE T
(m2/g)
/zmol PPSS/m 2
~mol PPSS/g
A B C D E F
18 29 78 89 175 400
0.49 0.45 0.38 0.40 0.27 0.22
8.8 13.0 29.6 35.6 47.2 88.0
of PPSS per m e decreases when the specific surface area increases (decrease of average pore diameter, effect of steric hindrance), but the total amount of polymer increases. The use of 29Si C P / M A S NMR solid state spectroscopy has allowed us to demonstrate the effectiveness of the grafting. Fig. 1 represents the spectrum of an untreated silica surface, with peak assignment to different silanol groups. Fig. 2a represents the spectrum of this silica after treatment with a linear polyphenylsilsesquioxane. The two signals at - 6 8 and - 7 7 ppm correspond to the structure indicated. The polymer molecular weight is 1500 Dalton, as determined by size exclusion chromatography or by 1H NMR, after the end-groups have been silylated with hexamethyldisilazane (HMDS) in CHC13. If TABLE 3 Characteristics of silica-grafted sulfonated PPSS Silica initial
g Polymer/g SiO 2
Theoretical capacity (meq/g)
Effective c capacity (meq/g)
Sulfonated groups (%)
A B D E C C
0.015 0.030 0.117 0.178 0.153 0.153
0.14 0.14 0.56 0.8 0.7 0.7
0.06 a 0.05 ~ 0.2 a 0.24 a 0.28 a 0.26 b
43 36 36 30 40 37
Sulfonation by HC1SO 3 in CHCI 3. b Sulfonation by SO 3 in Forane 113 (1,1,2-trifluoro-trichloroethane). c Conductimetric determination of acid functions.
+
-
~,
+ H+A -
Scheme 2. Silicon-aromatic C bond cleavage mechanism by electrophiles.
we assume the ladder structure to be perfect, the extent of modified silanol groups is 1520%. 13C C P / M A S NMR also gives evidence for grafting of polymer; the signal assignment attribution is Cipso = Cpara = 130 ppm, f o r t h o = 144 ppm and C m e t a ~- 127 ppm (Fig. 3a). The second step is the sulfonation of the grafted PPSS. This step is certainly more critical, because of the sensitivity of the Si-C aromatic bond towards strong electrophiles (Scheme 2). Ipso substitution of Si atom attached to an aromatic ring is a well-known reaction in organic synthesis. Si replacement has also been observed [13,14] with the following electrophiles: HX, H2SO4, HNO3, A1CI 3. Among different sulfonating agents [15], we have chosen C1SO3H, in CHCI 3. The sulfonation reaction has not been optimized but unlike the results reported by Suzuki et al. [16], aromatic ring cleavage was observed. As a consequence, the experimental acid capacities of the catalyst were somewhat lower than expected for a complete sulfonation without phenyl ring scission. Catalysts with capacities varying between 0.1 and 0.4 m e q / g were obtained, instead of the theoretical value up to 0.8, as indicated in Table 3. From the elemental analysis results, we can conclude that sulfonation of the aromatic ring is complete (1 SO3H/1 ring), but only a part of the phenyl rings remain attached to the siloxane skeleton of PPSS ( ~ 30-40%). This is confirmed by 29Si NMR solid state analysis (Fig. 2b), where the signal at - 6 8 and - 7 7 ppm have decreased strongly, in comparison with those of Fig. 2a. In that sample, aromatic groups cleavage has been estimated to
21
E. Carlier et al./React. Polym. 21 (1993) 15-25
be 77%. The use of 13C C P / M A S N M R also gives evidence for sulfonation. The signal at 142 ppm is attributed to the substituted meta C, since ortho and para positions are deactivated (Fig. 3b). The Hammett acidity function H 0 of these new catalysts has been measured using a differential calorimetric method developed in the IFP laboratory [17]. The method is based on the determination of the heat of hydrolysis of dried catalysts. First, we measured the hydration heat of H 2 S O 4 at various concentrations, from 20 to 90% by weight. Plotting these heat values versus H 0 values for these concentrations as reported by Hammett and coworkers [18,19] and obtained with a colorimetric method, gives a linear calibration curve. Then, simple measurement of hydration heats of various catalysts and non-sulfonated supports, leads to an assignment of their H 0. H 0 has been estimated to be - 4 . 7 for SPPSS (hydration heat 3 cal/meq), in comparison with - 2 . 3 for Amberlyst 15 (hydration heat 1.38 cal/meq), this value being in agreement with that given by colorimetric methods [20,21]. Amberlyst 15 is an ion-exchange resin from Rohm and Haas (4.3 meg acidity/g, dry resin basis). Another silica supported polymer has been prepared from styrene sulfonic potassium salt by copolymerization with trimethoxysilylpropyl methacrylate as the silica grafted coupling agent; this was carried out in an aqueous medium, with potassium persulfate as initiator, at 60°C, using the technology described in a previous paper [22]. The amount of grafted polymer is 0.35 g / g silica, the acid capacity being 1.5 m e q / g . Nation 501 is a Du Pont commercial copolymer bearing perfluorosulphonic acid groups (capacity 0.5 meq/g). H 0 is above - 1 0 , a value which has been confirmed by our thermal method of determination (hydration heat 8.7 cal/meq).
100 -
80' ¢:
.o
>=
60
¢0
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20 a O ~
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.
.
.
!
.
.
.
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.
|
0
50
100
150
0
50
100
150
rain
100
8O
"i
60
8
4O
20
0 rain
Fig. 4. (a) Acetic acid esterification by isopropanol in presence of Amberlyst 15 ([]), SPPSS-C (zx), and SSH-C ([]). (b) Catalytic run of acetic acid esterification by isopropanol, first run, SPPSS-silica ( • ) ; second run, in presence of Amberlyst 15 ([]) and SPPSS-C with ( + ) intermediate reactivation and without ([]).
Catalytic tests
The potential value of these new catalysts has been estimated in several test-reactions, with reference to the commercial catalysts indicated above. We will mention here only the results obtained in the esteritication of acetic acid by isopropanol, and the alkylation of phenols by 1-dodecene, in the presence of catalysts with a reasonable capacity.
22
E. Carlier et al. /React. Polym. 21 (1993) 15-25
Esterification of acetic acid by isopropanol
Alkylation of phenols by 1-dodecene
As shown by the kinetic curves obtained with three catalysts, in Fig. 4a, the rate and the limit of conversion using silica-grafted sulfonated PPSS and silica-grafted styrene sulfonic acid, are higher than that using Amberlyst 15. We must also remember the large difference in capacity between the three catalysts. We attempted to give a kinetic interpretation in terms of apparent rate constant k. This gives a relative k scale: 1.5, 30 and 50 (in min -1 eq -1 H+), respectively, for Amberlyst, SSH and SPPSS. In our opinion, this behaviour may have three origins: the higher acidity of the catalyst, the enhanced accessibility in the porous support and the highly polar reaction medium. Unfortunately, the catalyst shows a decreasing activity after the first run, the origin being the water adsorption on the polar surface. This lowers the acidity function of the aryl sulfonic acid (Fig. 4b). The activity is partially recovered by treatment with dilute H 2 5 0 4 , followed by washing with methanol and drying. With the new catalyst the specific activity per m e q / g is still higher than that of the Amberlyst. Other tests would be necessary to assess the long term stability.
The thermal stability of these catalysts has been tested in phenol and o-cresol alkylation with 1-dodecene at 120°C, which is the optimal temperature for that reaction. An original method for determining the conversion rate is the measurement of the heat evolved versus time, with a Thermokinegraph, followed by data treatment for conversion in rate constant, by assuming a second-order reaction (example in Fig. 5). This was successful for the Amberlyst 15 catalyst, but an exothermic side reaction takes place when an olefin is contacted with silica, so that this method could not be used with our catalysts. Consequently, the conversion was followed by chromatographic analysis, which gives additional information on the nature of the reaction products. As shown in Fig. 6a, the activity of the silica-based acid catalyst obtained by grafting potassium styrene sulfonate through radical polymerization is excellent, and similar to that of Amberlyst 15. Nation, however, remains the most effective catalyst, despite its lower capacity. At the same capacity level, the activity of the new sulfonic siloxane catalyst is very interesting, mainly if we consider the high selectivity in
1
i
i
Aide A=68sgs,~
i ,
I
! t
.~. ~
z
___
1
I B
I
J I I
JI
J S
ii
01
t
!
i
i
I
,
!
i
'
r
i
-,
~.....
,
it
Fig. 5. Thermokinetic analysis of o-cresol alkylation, in presence of Amberlyst 15: (A) thermal exchange; (B) conversion.
23
E. Carlier et al./React. Polym. 21 (1993) 15-25
80
TABLE 4
t
Selectivity in phenol monoalkylation
Catalyst
Reaction time (min)
Olefin conversion (%)
Monoalkylation (%)
Amberlyst 15 SPPSS-C SSH-D
30 180 15
64 53 54
14 82 55
60 ~ 4O 8 ~
2O o 0
100
200
rain
100
~
80
i
60
~ 8
40'
~
20. OT o
b !
!
100
200
rain
Fig. 6. (a) Phenol alkylation by 1-dodecene in presence of Amberlyst 15 (fi3), Nation (0), SPPSS-C (D), and SSH-D (×). (b) Phenol alkylation by 1-dodecene in presence of Amberlyst 15 ([]) and SPPSS-C ([]), first run, and after thermal treatment at 180°C, respectively ( • ) and (~) for these catalysts.
mono-alkylation. A relative scale of apparent rate constant is 9, 25 and 38 min-1 eq-1 H + for Amberlyst, SSH and SPPSS, respectively. Nation does not obey the same law, and the reaction is to fast to be studied in these conditions. Improved thermal stability was demonstrated through the following test: 1.4 g of the catalyst was heated in phenol at 180°C for 24 h; then the mixture was cooled to 120°C, and dodecene added. The kinetics of the alkylation were recorded by gas phase chromatography analysis, after addition of a
proper amount of octadecane, as internal standard. As it can be seen in Fig. 6b, such thermal treatment affects negatively the activity of Amberlyst 15, possibly through a desulfonation mechanism and a subsequent loss of catalytic sites [1]. On the contrary, the activity of silica-grafted sulfonated PPSS is increased after 2-h reaction, instead of levelling off. It is also interesting to note that selectivity in mono-alkylation products is influenced by the type of catalyst. The best selectivities are achieved with inorganic sulfonic acids, as shown in Table 4. This may be explained by better diffusion through the inorganic support.
Conclusion We have shown that our approach to improve simultaneously the thermal stability, the accessibility and the acidity of supported acid catalysts, by use of sulfonated polystyrene or polyphenylsilsesquioxane grafted onto silica support, is very promising. Both sulfonated polymers grafted on silica have similar activity in esterification, far higher than that of Amberlyst 15. This could be due to the effect of the silica support (favorably hydrophilic for esterification) and the morphology of the polymer. In a second test, a decrease of activity is observed, due to hydration of the catalyst, yet the rate constant remains higher than that of Amberlyst 15. Attempts to obtain higher capacity with a fully sulfonated PPSS without any loss of
24
aromatic rings is under investigation. Testing of this catalyst in a reaction where Amberlyst 15 was ineffective, should allow the classification of this type of catalyst between commercial sulfonic resins and solid superacid catalysts, like Nation or zirconium oxide treated with n 2 s o 4.
Experimental Synthesis Polyphenylsilsesquioxane synthesis: 30 cm 3 of CC13C6H 5 and 100 cm 3 of C2HsOC2H5 were poured into a stirred reaction vessel, and deionized water added dropwise until the solution was slightly turbid. Reaction was allowed to continue for 2 h; then, the solution was washed three times with 100 cm 3 of water and dried over NazSO 4. The solvent was removed under vacuum and heat. The polymer was dried under vacuum (1 mbar) at 50°C to yield a white crystalline powder. Grafting of polyphenylsilsesquioxane onto silica: 10 g of silica dried for 4 h under vacuum (1 mbar) at 120°C were contacted with 5 g of PPSS in 50 cm 3 of dry toluene, under reflux for 2 h. After reaction, the silica beads were filtered and dried under vacuum (1 mbar) at 50°C. Later, the ungrafted polymer was extracted with toluene in a Soxhlet, apparatus.
E. Carlier et al. / React. Polym. 21 (1993) 15-25
Acetic acid esterification with isopropanol: 25 cm 3 of acetic acid were reacted with 2 cm 3 of isopropanol in the presence of 0.3 g of catalyst, at 60°C. This corresponds to a 20 times molar excess of acid. The kinetics were determined by vapour phase chromatography (VPC) analysis. Phenol alkylation with 1-dodecene: 9.4 g of phenol, in presence of 1.4 g of catalyst and 0.1 cm 3 of octadecane as internal standard for VPC analysis were heated at 120°C, 6.3 cm 3 of 1-dodecene were then added. The conversion and selectivity were determined from VPC analysis with a SE 30 column. The same procedure was used with o-cresol.
Characterization 295i C P / M A S NMR spectra were recorded using a Bruker AC 200 apparatus, working at 39.76 and 50.3 MHz, respectively. About 300 fid's were accumulated; the chemical shifts are referenced to tetramethylsilane. The differential Thermokinegraph has been developed by the IFP Laboratory and is manufactured by Technochim Engineering (F-60230 Chambly). Heat is continuously measured, relative to an inert reference. Corrected and smoothed values are transformed as a measure of conversion. For one hour, there is a good fit with a second-order reaction, allowing the rate constant in min-1 to be determined.
Reactions Sulfonation of silica grafted polymer: 10 g of silica grafted polymer were contacted with 50 cm 3 of dry CHC13; 10 cm 3 of HC1SO 3 were added dropwise; then the reaction vessel was heated for 150 min at 50°C. Excess of HC1SO 3 was destroyed by addition of CH3OH (10 cm3). The silica was filtered, washed successively with methanol and water, and methanol again, and dried under vacuum for 2 h.
References 1 H. Widdecke, Design and industrial application of polymeric acid catalysts, in D.C. Sherrington and P. Hodge (Eds.), Synthesis and Separation using Functional Polymers, Wiley, Chichester, 1988, pp. 149179. 2 K. Dorfner (Ed.), Ion Exchangers, W. de Gruyter, Berlin, 1990. 3 S. Suzuki, K. Tohmori and Y. Ono, Preparation of sulfonated polyorganosiloxanes and their acid catalysis, J. Mol. Catal., 43 (1987) 41.
E. Carlier et al./React. Polym. 21 (1993) 15-25 4 K.K. Unger, Porous Silica, Elsevier, Amsterdam, 1979. 5 A. Saus and E. Schmid, Benzyl sulfonic acid siloxane as a catalyst: oligomerization of isobutene, J. Catal., 94 (1985) 187. 6 L.C. Sander and S.A. Wise, Synthesis and characterization of polymeric C18 stationary phase for liquid chromatography, Anal Chem., 56 (1984) 504. 7 S. Suzuki, K. Tohmori and Y. Ono, Vapor-phase nitration of benzene over polyorganosiloxanes bearing sulfo groups, Chem. Lett. (1986) 747. 8 E. Carlier, A. Guyot and A. Revillon, Functional polymers supported on porous silica. II. Radical polymerization of vinylbenzyl chloride from grafted precursors, React. Polym., 16 (1991/1992) 115. 9 Z. Xinsheng, S. Lianghe, L. Shuquing and L. Yizhen, Thermal stability and kinetics of decomposition of polyphenylsilsesquioxanes and some related polymers, Polym. Degrad. Stability, 20 (1988) 157. 10 J.F. Brown Jr, L.H. Vogt Jr and P.I. Prescott, Preparation and characterization of the lower equilibrated phenylsilsesquioxanes, J. Am. Chem. Soc., 86 (1964) 1120. 11 K.A. Andrianov and N.N. Makarova, Polymerization of hydrolysis products of organotrichlorosilanes, Vysokomol. Soyed. Ser. A., 12 (1970) 663; Polym. Sci. USSR Ser. A., 12 (1970) 747. 12 C. Janin and A. Guyot, Preparation du polyph6nylsiloxane. I. Polymdrisation ?~ partir des produits d'hydrolyse du phdnyltrichlorosilane, J. Phys. Chem. (1972) 1120. 13 V. Bazant, V. Chvalovski and J. Rathousky,
25
14
15
16
17
Organosilicon Compounds, Publishing House, Prague, 1965. C. Eaborn, Cleavages of aryl-silicon and related bonds by electrophiles, J. Organomet. Chem., 100 (1975) 45. J.P. Planche, A. Revillon and A. Guyot, Chemical modification of polynorbornene. II. Extended sulfonation processes, J. Polym. Sci., Polym. Chem., 28 (1990) 1377. S. Suzuki, Y. Ono, S. Nakata and S. Asaoka, Structures and thermal and hydrothermal stabilities of sulfonated poly(organosiloxanes) by 29Si and 13C C P / M A S NMR, J. Phys. Chem., 91 (1987) 1659. P. Baumgartner and P. Duhaut, Application of differential thermal measurements to the study of reaction kinetics, Bull. Soc. Chim. Ft., 6 (1960) 1187.
18 L.P. Hammett and A.J. Derup, A series of simple basic indicators. I. The acidity functions of mixtures of sulfuric and perchloric acids with water, J. Am. Chem. Soc., 54 (1932) 2721. 19 L.P. Hammett and A.M. Paul, A series of simple basic indicators. III. The zero point of the acidity function scale, J. Am. Chem. Soc., 56 (1934) 827. 20 P. Rys and W.J. Steinegger, Acidity function of solid-bound acids, J. Am. Chem. Soc.. 101 (1979) 4801. 21 F.J. Waller and R.W. Van Scoyoc, Catalysis with Nation, Chemtech, (1987) 438. 22 S.J. Sondheimer, N.J. Bunge and C.A. Fyfe, Structure and chemistry of Nation-H: a fluorinated sulfonic acid polymer, J. Macromol. Sci. Ret'. Macrotool. Chem. Phys. C, 26 (1986) 353.
Reactiee Polymers, 21 (1993) 15-25
Elsevier Science Publishers B.V., Amsterdam
Functional silica supported polymers IV. Synthesis and catalytic activity of silica-grafted sulfonated polyphenylsilsesquioxane E. Carlier a, A. Revillon a,., A. Guyot ~ and P. Baumgartner b CNRS-LMO, Lyon-Solaize, BP 24, F-69390 Vernaison, France b IFP-CEDI, Solaize, BP 6, F-69390 Vernaison, France
(Received January 21, 1993; accepted in revised form April 2, 1993)
Abstract The ladder polymer, polyphenylsilsesquioxane, has been grafted onto porous silica and sulfonated to obtain catalysts of high stability with e n h a n c e d site accessibility and increased n u m b e r of sites, as well as high acidity level. The catalytic behaviour, even at a low level of capacity, has shown excellent esterification and phenol alkylation activity, with, in the latter case, excellent selectivity, as compared to classical ion exchangers or highly acidic Nation material. In batch reaction, hydration of the support leads to partial deactivation; activity may be partly recovered and the rate constant is still higher than that obtained with the reference catalyst. Keywords: functional silica; solid acid catalysts; supported reagents; siloxane polymers;
sulfonated polymers
Introduction Acidic polystyrene resins have been used for many years by industry in a large n u m b e r of applications [1,2]. Despite their success, they suffer from some drawbacks, mainly due to the lack of thermal stability. This precludes their use for reactions at temperatures > 120°C. They also show poor mechan-
* Corresponding author.
ical properties. Moreover, due to their limited acidity, they cannot be used to replace strong mineral acids in some reactions, where a high acidity level is required, such as in aliphatic alkylation or benzene nitration in the vapour phase [3]. Additionally, for environmental reasons (acid slurry pollution) the use of these acids will become more restricted in the forthcoming years. At the same time, industry is demanding supported acid catalysts with improved thermal and mechanical properties and, if possible, higher acidity
0923-1137/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
E. Carlier et al. /React. Polym. 21 (1993) 15-25
16
than the existing commercial sulfonic acid resins. So, several attempts have been made to improve these three properties. We would like to mention here the interesting approach of Widdecke [1] to obtain perfluorinated resins from classical sulfonic resins. The target with an organic support is to reach the acidity of Nation. This has been shown to be one of the best acid catalysts ever used for a large number of reactions. However, it is not easy to handle and very expensive. Other interesting routes to improved acid catalysts have been explored by different authors using porous silica as the matrix [4]. The two major conclusions that can be drawn from the earlier works are that the capacity of the supports is rather low and that the acidity of the sulfonated silanes grafted on the silica surface is higher than the acidity of commercial sulfonic resins [5]. Attempts to form a polysiloxane network on silica surfaces have been described by Sander and Wise [6]. A more recent work, published by Suzuki et al. [7], shows very promising properties as compared to the reference acid catalyst Amberlyst 15. Their approach to increase the capacity of silica-based acid catalyst is to use a sol-gel process using Si(OEt) 4 and a functional silane, to form a silica network thus allowing after sulfonation a cata-
lyst to be obtained with an acid capacity of 4 meq/g.
Results and discussion
In our extensive effort to improve both thermal and mechanical stability, as well as accessibility of supported reagents, our latest approach has been to graft functional polymers on silica surfaces by various procedures [8]. Since thermal stability is a part of our goal, we have chosen to graft a thermostable polymer on various silica gels having different characteristics, reported in Table 1: mean volume particle diameter, D(v,0.5); pore diameter, %; total pore volume in cm3/g, lip; specific surface area in m2/g, S (from BET isotherms and porosimetry); and number of surface silanol groups (determined previously by measurement of alkane formation after reaction with an alkylaluminium). The polymer, polyphenylsilsesquioxane (PPSS), has a ladder structure, with a thermal stability of over 400°C [9]. It bears phenyl rings capable of being derivatized and also suitable endgroups for grafting onto the silica surface by coupling with silanol groups. After grafting onto the support, the polymer is sulfonated (SPPSS) and the acid catalyst obtained has been tested in two acid-catalyzed reactions:
TABLE 1 Chemical and structural characteristics of silica supports Silica a A B C D E F
Particle d i a m e t e r D(v,0.5) (/xm)
SBET
Sporo
Vp
(me/g)
(m2/g)
(cm3/g)
rp (nm)
[SiOH] ( / x m o l / m 2)
142 63 1500 28 5.5
18 29 78 89 175 400
12 76 86 200
1 1.1 0.98 0.9 0.8 1.8
300 155 55 70 22 18
4.8 5.3 3.3
a A = XOC005 from R h S n e - P o u l e n c ; B = F1000 from Merck; C, from Shell; D = RP1 and E = RP3 from R h 6 n e Poulenc; F = E D 3 from Grace.
E. Carlier et al./React. Polym. 21 (1993) 15-25
17
esterification of acetic acid, and alkylation of phenol by 1-dodecene. Catalyst synthesis and physical characterization
The synthesis of linear ladder polyphenylsilsesquioxane through C13SIC6 H 5 hydrolysis in various solvents has been described by several authors [10-12]. The synthesis is a two-step process, illustrated in Scheme 1. The first one is the grafting of polyphenylsilsesquioxane onto the silica surface, in toluene medium. Under the conditions used in our approach (see experimental part), it can be shown that a larger amount of phenyl groups are grafted than by silane hydrolysis in the presence of the silica support. Unger [4] reported that direct hydrolysis of C 1 3 S i C 6 H 5 leads to a maximal amount of 4 /xmol/m 2, for a silica with 8 ~ m o l / m 2 of silanol groups. For a silica support of 100
.20 l~
c1 -CI
(3
/•li-O"H /.
.o:i_%.
/
Ph I
\
Ph l
2 n C1503H I
.SO3H
Ph I
Si-O.-Si-O--Si--&-Si--&--Si
dx
'o t
'o t
"oI
Si--O-Si--O-Si--OH Si--O--Si
Ox
-si-o,
Ph
Ph
o I--I ~i-o, o H ~i.,,£1I O/ "CH
sO3H
Scheme 1. Hydrolysis of phenyltrichlorochlorosilane, condensation with silanols of silica and sulfonation.
b
c a
- l~2e
- IL4.O
- t~6B
- 1'86
-2~88
PPM
04
O4
O
o.S O,
o.S,.o o
o~Si"O "0
I
(a)-91.7 ppm
I
(b)-I01 ppm
I
(c)-111pprn
Fig. 1.29Si C P / M A S NMR solid state spectrum of silica, with assignment of bands to silanol surface groups of silica.
E. Carlier et al. /React. Polym. 21 (1993) 15-25
18
Ph Ph Ph i i I Si--O-Si--O--Si--O--Si--O--Si d, \ \O L , \ 0 H2 Si--O-Si--O--Si-Cl-I Si--O--Si I \
A=-91 ppm B = -101 p p m C = -111 p p m
b
H= -68 ppm I= - 7 7 p p m
C B Si--O~ 0 H A!i~O(-I 0
hi
OH
PPM
b
i
r,.
.
.
2~
.
.
.
,~
.
.
.
';'
.
.
.
.
.
.
.
.
.
.
.
.
.
.
-;o " -"2 0 .... - 3 0' .... - 4 '0 ..... Ao
.
.
.
40
PPM
.
.
.
.
.
a
.
-Io 'L~o
.
. -go
.
.
' " -.o"" -1'1o
s .... -J20
J .... J ...... -130 -|40 -1-0
Fig. 2. 295i C P / M A S NMR solid state spectrum of silica grafted with (a) PPSS and (b) SPPSS, with assignment of bands to modified silanoi surface groups.
E. Carlier et al./React. Polym. 21 (1993) 15-25
19
m2/g, this represents a potential capacity of phenyl rings of 0.4 m e q / g (and, after sulfonation, 0.3 meq of acid/g). Tables 2 and 3
show the amount of PPSS grafted on different silica supports, which is about twice as high as the above value. The specific amount
Si
/ 3
4
tO0
t60
t40
Si
120
tO0
O0
60
40
20
ts4 SO:H
f 3
r - ~
1-r-r"~r~'t
ZIO
'"
;ZOO
I . . . .
tgo
I ' ~" 190
~"~ . . . . LTO
I .... t60
I ....
t~O
i''
140
' J .... t30 Ppl4
J .... l=O
! .... ttO
I . . . . tOO
I . . . . 9Q
I .... 90
I .... 70
I .... 60
! .... 50
I' 40
Fig. 3. 13C C P / M A S NMR solid state spectrum of silica grafted with (a) PPSS and (b) SPPSS.
0
-20
E. Carlier et aL / React. Polym. 21 (1993) 15-25
20 TABLE 2
%
Grafted PPSS on different silica supports
E+A"
Silica initial
aBE T
(m2/g)
/zmol PPSS/m 2
~mol PPSS/g
A B C D E F
18 29 78 89 175 400
0.49 0.45 0.38 0.40 0.27 0.22
8.8 13.0 29.6 35.6 47.2 88.0
of PPSS per m e decreases when the specific surface area increases (decrease of average pore diameter, effect of steric hindrance), but the total amount of polymer increases. The use of 29Si C P / M A S NMR solid state spectroscopy has allowed us to demonstrate the effectiveness of the grafting. Fig. 1 represents the spectrum of an untreated silica surface, with peak assignment to different silanol groups. Fig. 2a represents the spectrum of this silica after treatment with a linear polyphenylsilsesquioxane. The two signals at - 6 8 and - 7 7 ppm correspond to the structure indicated. The polymer molecular weight is 1500 Dalton, as determined by size exclusion chromatography or by 1H NMR, after the end-groups have been silylated with hexamethyldisilazane (HMDS) in CHC13. If TABLE 3 Characteristics of silica-grafted sulfonated PPSS Silica initial
g Polymer/g SiO 2
Theoretical capacity (meq/g)
Effective c capacity (meq/g)
Sulfonated groups (%)
A B D E C C
0.015 0.030 0.117 0.178 0.153 0.153
0.14 0.14 0.56 0.8 0.7 0.7
0.06 a 0.05 ~ 0.2 a 0.24 a 0.28 a 0.26 b
43 36 36 30 40 37
Sulfonation by HC1SO 3 in CHCI 3. b Sulfonation by SO 3 in Forane 113 (1,1,2-trifluoro-trichloroethane). c Conductimetric determination of acid functions.
+
-
~,
+ H+A -
Scheme 2. Silicon-aromatic C bond cleavage mechanism by electrophiles.
we assume the ladder structure to be perfect, the extent of modified silanol groups is 1520%. 13C C P / M A S NMR also gives evidence for grafting of polymer; the signal assignment attribution is Cipso = Cpara = 130 ppm, f o r t h o = 144 ppm and C m e t a ~- 127 ppm (Fig. 3a). The second step is the sulfonation of the grafted PPSS. This step is certainly more critical, because of the sensitivity of the Si-C aromatic bond towards strong electrophiles (Scheme 2). Ipso substitution of Si atom attached to an aromatic ring is a well-known reaction in organic synthesis. Si replacement has also been observed [13,14] with the following electrophiles: HX, H2SO4, HNO3, A1CI 3. Among different sulfonating agents [15], we have chosen C1SO3H, in CHCI 3. The sulfonation reaction has not been optimized but unlike the results reported by Suzuki et al. [16], aromatic ring cleavage was observed. As a consequence, the experimental acid capacities of the catalyst were somewhat lower than expected for a complete sulfonation without phenyl ring scission. Catalysts with capacities varying between 0.1 and 0.4 m e q / g were obtained, instead of the theoretical value up to 0.8, as indicated in Table 3. From the elemental analysis results, we can conclude that sulfonation of the aromatic ring is complete (1 SO3H/1 ring), but only a part of the phenyl rings remain attached to the siloxane skeleton of PPSS ( ~ 30-40%). This is confirmed by 29Si NMR solid state analysis (Fig. 2b), where the signal at - 6 8 and - 7 7 ppm have decreased strongly, in comparison with those of Fig. 2a. In that sample, aromatic groups cleavage has been estimated to
21
E. Carlier et al./React. Polym. 21 (1993) 15-25
be 77%. The use of 13C C P / M A S N M R also gives evidence for sulfonation. The signal at 142 ppm is attributed to the substituted meta C, since ortho and para positions are deactivated (Fig. 3b). The Hammett acidity function H 0 of these new catalysts has been measured using a differential calorimetric method developed in the IFP laboratory [17]. The method is based on the determination of the heat of hydrolysis of dried catalysts. First, we measured the hydration heat of H 2 S O 4 at various concentrations, from 20 to 90% by weight. Plotting these heat values versus H 0 values for these concentrations as reported by Hammett and coworkers [18,19] and obtained with a colorimetric method, gives a linear calibration curve. Then, simple measurement of hydration heats of various catalysts and non-sulfonated supports, leads to an assignment of their H 0. H 0 has been estimated to be - 4 . 7 for SPPSS (hydration heat 3 cal/meq), in comparison with - 2 . 3 for Amberlyst 15 (hydration heat 1.38 cal/meq), this value being in agreement with that given by colorimetric methods [20,21]. Amberlyst 15 is an ion-exchange resin from Rohm and Haas (4.3 meg acidity/g, dry resin basis). Another silica supported polymer has been prepared from styrene sulfonic potassium salt by copolymerization with trimethoxysilylpropyl methacrylate as the silica grafted coupling agent; this was carried out in an aqueous medium, with potassium persulfate as initiator, at 60°C, using the technology described in a previous paper [22]. The amount of grafted polymer is 0.35 g / g silica, the acid capacity being 1.5 m e q / g . Nation 501 is a Du Pont commercial copolymer bearing perfluorosulphonic acid groups (capacity 0.5 meq/g). H 0 is above - 1 0 , a value which has been confirmed by our thermal method of determination (hydration heat 8.7 cal/meq).
100 -
80' ¢:
.o
>=
60
¢0
40'
20 a O ~
.
.
.
.
!
.
.
.
.
i
.
.
.
.
|
0
50
100
150
0
50
100
150
rain
100
8O
"i
60
8
4O
20
0 rain
Fig. 4. (a) Acetic acid esterification by isopropanol in presence of Amberlyst 15 ([]), SPPSS-C (zx), and SSH-C ([]). (b) Catalytic run of acetic acid esterification by isopropanol, first run, SPPSS-silica ( • ) ; second run, in presence of Amberlyst 15 ([]) and SPPSS-C with ( + ) intermediate reactivation and without ([]).
Catalytic tests
The potential value of these new catalysts has been estimated in several test-reactions, with reference to the commercial catalysts indicated above. We will mention here only the results obtained in the esteritication of acetic acid by isopropanol, and the alkylation of phenols by 1-dodecene, in the presence of catalysts with a reasonable capacity.
22
E. Carlier et al. /React. Polym. 21 (1993) 15-25
Esterification of acetic acid by isopropanol
Alkylation of phenols by 1-dodecene
As shown by the kinetic curves obtained with three catalysts, in Fig. 4a, the rate and the limit of conversion using silica-grafted sulfonated PPSS and silica-grafted styrene sulfonic acid, are higher than that using Amberlyst 15. We must also remember the large difference in capacity between the three catalysts. We attempted to give a kinetic interpretation in terms of apparent rate constant k. This gives a relative k scale: 1.5, 30 and 50 (in min -1 eq -1 H+), respectively, for Amberlyst, SSH and SPPSS. In our opinion, this behaviour may have three origins: the higher acidity of the catalyst, the enhanced accessibility in the porous support and the highly polar reaction medium. Unfortunately, the catalyst shows a decreasing activity after the first run, the origin being the water adsorption on the polar surface. This lowers the acidity function of the aryl sulfonic acid (Fig. 4b). The activity is partially recovered by treatment with dilute H 2 5 0 4 , followed by washing with methanol and drying. With the new catalyst the specific activity per m e q / g is still higher than that of the Amberlyst. Other tests would be necessary to assess the long term stability.
The thermal stability of these catalysts has been tested in phenol and o-cresol alkylation with 1-dodecene at 120°C, which is the optimal temperature for that reaction. An original method for determining the conversion rate is the measurement of the heat evolved versus time, with a Thermokinegraph, followed by data treatment for conversion in rate constant, by assuming a second-order reaction (example in Fig. 5). This was successful for the Amberlyst 15 catalyst, but an exothermic side reaction takes place when an olefin is contacted with silica, so that this method could not be used with our catalysts. Consequently, the conversion was followed by chromatographic analysis, which gives additional information on the nature of the reaction products. As shown in Fig. 6a, the activity of the silica-based acid catalyst obtained by grafting potassium styrene sulfonate through radical polymerization is excellent, and similar to that of Amberlyst 15. Nation, however, remains the most effective catalyst, despite its lower capacity. At the same capacity level, the activity of the new sulfonic siloxane catalyst is very interesting, mainly if we consider the high selectivity in
1
i
i
Aide A=68sgs,~
i ,
I
! t
.~. ~
z
___
1
I B
I
J I I
JI
J S
ii
01
t
!
i
i
I
,
!
i
'
r
i
-,
~.....
,
it
Fig. 5. Thermokinetic analysis of o-cresol alkylation, in presence of Amberlyst 15: (A) thermal exchange; (B) conversion.
23
E. Carlier et al./React. Polym. 21 (1993) 15-25
80
TABLE 4
t
Selectivity in phenol monoalkylation
Catalyst
Reaction time (min)
Olefin conversion (%)
Monoalkylation (%)
Amberlyst 15 SPPSS-C SSH-D
30 180 15
64 53 54
14 82 55
60 ~ 4O 8 ~
2O o 0
100
200
rain
100
~
80
i
60
~ 8
40'
~
20. OT o
b !
!
100
200
rain
Fig. 6. (a) Phenol alkylation by 1-dodecene in presence of Amberlyst 15 (fi3), Nation (0), SPPSS-C (D), and SSH-D (×). (b) Phenol alkylation by 1-dodecene in presence of Amberlyst 15 ([]) and SPPSS-C ([]), first run, and after thermal treatment at 180°C, respectively ( • ) and (~) for these catalysts.
mono-alkylation. A relative scale of apparent rate constant is 9, 25 and 38 min-1 eq-1 H + for Amberlyst, SSH and SPPSS, respectively. Nation does not obey the same law, and the reaction is to fast to be studied in these conditions. Improved thermal stability was demonstrated through the following test: 1.4 g of the catalyst was heated in phenol at 180°C for 24 h; then the mixture was cooled to 120°C, and dodecene added. The kinetics of the alkylation were recorded by gas phase chromatography analysis, after addition of a
proper amount of octadecane, as internal standard. As it can be seen in Fig. 6b, such thermal treatment affects negatively the activity of Amberlyst 15, possibly through a desulfonation mechanism and a subsequent loss of catalytic sites [1]. On the contrary, the activity of silica-grafted sulfonated PPSS is increased after 2-h reaction, instead of levelling off. It is also interesting to note that selectivity in mono-alkylation products is influenced by the type of catalyst. The best selectivities are achieved with inorganic sulfonic acids, as shown in Table 4. This may be explained by better diffusion through the inorganic support.
Conclusion We have shown that our approach to improve simultaneously the thermal stability, the accessibility and the acidity of supported acid catalysts, by use of sulfonated polystyrene or polyphenylsilsesquioxane grafted onto silica support, is very promising. Both sulfonated polymers grafted on silica have similar activity in esterification, far higher than that of Amberlyst 15. This could be due to the effect of the silica support (favorably hydrophilic for esterification) and the morphology of the polymer. In a second test, a decrease of activity is observed, due to hydration of the catalyst, yet the rate constant remains higher than that of Amberlyst 15. Attempts to obtain higher capacity with a fully sulfonated PPSS without any loss of
24
aromatic rings is under investigation. Testing of this catalyst in a reaction where Amberlyst 15 was ineffective, should allow the classification of this type of catalyst between commercial sulfonic resins and solid superacid catalysts, like Nation or zirconium oxide treated with n 2 s o 4.
Experimental Synthesis Polyphenylsilsesquioxane synthesis: 30 cm 3 of CC13C6H 5 and 100 cm 3 of C2HsOC2H5 were poured into a stirred reaction vessel, and deionized water added dropwise until the solution was slightly turbid. Reaction was allowed to continue for 2 h; then, the solution was washed three times with 100 cm 3 of water and dried over NazSO 4. The solvent was removed under vacuum and heat. The polymer was dried under vacuum (1 mbar) at 50°C to yield a white crystalline powder. Grafting of polyphenylsilsesquioxane onto silica: 10 g of silica dried for 4 h under vacuum (1 mbar) at 120°C were contacted with 5 g of PPSS in 50 cm 3 of dry toluene, under reflux for 2 h. After reaction, the silica beads were filtered and dried under vacuum (1 mbar) at 50°C. Later, the ungrafted polymer was extracted with toluene in a Soxhlet, apparatus.
E. Carlier et al. / React. Polym. 21 (1993) 15-25
Acetic acid esterification with isopropanol: 25 cm 3 of acetic acid were reacted with 2 cm 3 of isopropanol in the presence of 0.3 g of catalyst, at 60°C. This corresponds to a 20 times molar excess of acid. The kinetics were determined by vapour phase chromatography (VPC) analysis. Phenol alkylation with 1-dodecene: 9.4 g of phenol, in presence of 1.4 g of catalyst and 0.1 cm 3 of octadecane as internal standard for VPC analysis were heated at 120°C, 6.3 cm 3 of 1-dodecene were then added. The conversion and selectivity were determined from VPC analysis with a SE 30 column. The same procedure was used with o-cresol.
Characterization 295i C P / M A S NMR spectra were recorded using a Bruker AC 200 apparatus, working at 39.76 and 50.3 MHz, respectively. About 300 fid's were accumulated; the chemical shifts are referenced to tetramethylsilane. The differential Thermokinegraph has been developed by the IFP Laboratory and is manufactured by Technochim Engineering (F-60230 Chambly). Heat is continuously measured, relative to an inert reference. Corrected and smoothed values are transformed as a measure of conversion. For one hour, there is a good fit with a second-order reaction, allowing the rate constant in min-1 to be determined.
Reactions Sulfonation of silica grafted polymer: 10 g of silica grafted polymer were contacted with 50 cm 3 of dry CHC13; 10 cm 3 of HC1SO 3 were added dropwise; then the reaction vessel was heated for 150 min at 50°C. Excess of HC1SO 3 was destroyed by addition of CH3OH (10 cm3). The silica was filtered, washed successively with methanol and water, and methanol again, and dried under vacuum for 2 h.
References 1 H. Widdecke, Design and industrial application of polymeric acid catalysts, in D.C. Sherrington and P. Hodge (Eds.), Synthesis and Separation using Functional Polymers, Wiley, Chichester, 1988, pp. 149179. 2 K. Dorfner (Ed.), Ion Exchangers, W. de Gruyter, Berlin, 1990. 3 S. Suzuki, K. Tohmori and Y. Ono, Preparation of sulfonated polyorganosiloxanes and their acid catalysis, J. Mol. Catal., 43 (1987) 41.
E. Carlier et al./React. Polym. 21 (1993) 15-25 4 K.K. Unger, Porous Silica, Elsevier, Amsterdam, 1979. 5 A. Saus and E. Schmid, Benzyl sulfonic acid siloxane as a catalyst: oligomerization of isobutene, J. Catal., 94 (1985) 187. 6 L.C. Sander and S.A. Wise, Synthesis and characterization of polymeric C18 stationary phase for liquid chromatography, Anal Chem., 56 (1984) 504. 7 S. Suzuki, K. Tohmori and Y. Ono, Vapor-phase nitration of benzene over polyorganosiloxanes bearing sulfo groups, Chem. Lett. (1986) 747. 8 E. Carlier, A. Guyot and A. Revillon, Functional polymers supported on porous silica. II. Radical polymerization of vinylbenzyl chloride from grafted precursors, React. Polym., 16 (1991/1992) 115. 9 Z. Xinsheng, S. Lianghe, L. Shuquing and L. Yizhen, Thermal stability and kinetics of decomposition of polyphenylsilsesquioxanes and some related polymers, Polym. Degrad. Stability, 20 (1988) 157. 10 J.F. Brown Jr, L.H. Vogt Jr and P.I. Prescott, Preparation and characterization of the lower equilibrated phenylsilsesquioxanes, J. Am. Chem. Soc., 86 (1964) 1120. 11 K.A. Andrianov and N.N. Makarova, Polymerization of hydrolysis products of organotrichlorosilanes, Vysokomol. Soyed. Ser. A., 12 (1970) 663; Polym. Sci. USSR Ser. A., 12 (1970) 747. 12 C. Janin and A. Guyot, Preparation du polyph6nylsiloxane. I. Polymdrisation ?~ partir des produits d'hydrolyse du phdnyltrichlorosilane, J. Phys. Chem. (1972) 1120. 13 V. Bazant, V. Chvalovski and J. Rathousky,
25
14
15
16
17
Organosilicon Compounds, Publishing House, Prague, 1965. C. Eaborn, Cleavages of aryl-silicon and related bonds by electrophiles, J. Organomet. Chem., 100 (1975) 45. J.P. Planche, A. Revillon and A. Guyot, Chemical modification of polynorbornene. II. Extended sulfonation processes, J. Polym. Sci., Polym. Chem., 28 (1990) 1377. S. Suzuki, Y. Ono, S. Nakata and S. Asaoka, Structures and thermal and hydrothermal stabilities of sulfonated poly(organosiloxanes) by 29Si and 13C C P / M A S NMR, J. Phys. Chem., 91 (1987) 1659. P. Baumgartner and P. Duhaut, Application of differential thermal measurements to the study of reaction kinetics, Bull. Soc. Chim. Ft., 6 (1960) 1187.
18 L.P. Hammett and A.J. Derup, A series of simple basic indicators. I. The acidity functions of mixtures of sulfuric and perchloric acids with water, J. Am. Chem. Soc., 54 (1932) 2721. 19 L.P. Hammett and A.M. Paul, A series of simple basic indicators. III. The zero point of the acidity function scale, J. Am. Chem. Soc., 56 (1934) 827. 20 P. Rys and W.J. Steinegger, Acidity function of solid-bound acids, J. Am. Chem. Soc.. 101 (1979) 4801. 21 F.J. Waller and R.W. Van Scoyoc, Catalysis with Nation, Chemtech, (1987) 438. 22 S.J. Sondheimer, N.J. Bunge and C.A. Fyfe, Structure and chemistry of Nation-H: a fluorinated sulfonic acid polymer, J. Macromol. Sci. Ret'. Macrotool. Chem. Phys. C, 26 (1986) 353.