Ind. Eng. Chem. Res. 1994,33, 2215-2219
2215
Hydrolysis of Methyl Acetate and Sucrose in S03H-Group-Containing Grafted Polymer Chains Prepared by Radiation-Induced Graft Polymerization Tomotoshi Mizota, Satoshi Tsuneda, and Kyoichi Saito' Department of Chemical Engineering, Faculty of Engineering, University of Tokyo, Hongo, Tokyo 113, Japan
Takanobu Sugo Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma 370-12; Japan
The sulfonic acid (S03H)group was attached to polyethylene by radiation-induced graft polymerization of sodium p-styrenesulfonate and compared with the S03H group introduced into the polystyrene chain cross-linked with divinylbenzene. The catalytic activities of the S03H group were measured in a batch mode for the hydrolysis of methyl acetate and sucrose. The temperature dependence of the reaction rate constants for the hydrolysis was determined. For the hydrolysis of methyl acetate, the S03H group anchored to the grafted polymer chain had the same activity as the S03H group introduced into the cross-linked polymer chain. On the other hand, the graft-type acid catalyst had about 10 times the hydrolytic activity for sucrose as the cross-linked acid catalyst.
Introduction A strongly acidic cation-exchange resin containing a sulfonic acid (SO3H) group is usually used as an acid catalyst for hydrolysis. Conventional sulfonic-acid-type resin is prepared by sulfonation of the styrene (St)divinylbenzene (DVB) copolymer, where the polystyrene chains were cross-linked with DVB. The cross-linked polymer network containing the S03H group will retard the access of reactants with sizes larger than the interstices of the polymer chains. On the other hand, the S03Hgroup-containingpolymer chains grafted onto an insoluble support will readily accept reactants. Figure 1 shows a comparison of polymer structures immobilizing the S03H group between the cross-linked acid catalyst and the grafttype acid catalyst. Yoshioka and Shimamura (1984)prepared a fibrous acid catalyst by introducing S03H groups into a lightly crosslinked polystyrene/polypropylenecomposite fiber. When the reactant was a smaller molecule such as methyl acetate, this fiber showed the same catalytic activity as St-DVBbased acid catalysts. On the other hand, the catalytic activity of the fiber for the hydrolysis of a larger molecule such as sucrose was higher than that of the St-DVB-based acid catalysts. They demonstrated that the diffusion of the sucrose into the fiber did not govern the overall reaction rate because methylene bonds cross-linked the polymer network more loosely than the DVB-cross-linked bonds. Their results indicate that absence of cross-linking leads to more effective catalytic performance. The S03Hgroup on the grafted polymer chain prepared by graft polymerization will be most effective in hydrolyzing larger molecules due to the absence of cross-linking. Table 1summarizes previous studies on hydrolysis using catalysts based on the grafted polymer structure. Higher catalytic activity was observed for the SO3H-groupcontaining polymer chains grafted onto an insoluble support. Arai (1989) prepared polystyrene-grafted crosslinked polystyrene beads by UV-light-inducedgraft polymerization, and then the grafted polymer chain was subsequently sulfonated with 1,Cdioxane-sulfur trioxide adduct. Polednick et al. (1989) grafted styrene or vinylbenzyl chloride onto previously hydroperoxided polypropylene beads, followed by appropriate sulfonations. These
polystyrene chain
(a)
grafJed chain
(b)
Figure 1. Comparisonof polymer chain structure containing sulfonic acid groups. (a) Cross-linked polymer chain; (b) grafted polymer chain.
studies verified a distinct effectiveness of the S03Hgroup on the grafted polymer chains as an acid catalyst at a prescribed reaction temperature. However, the acid catalyst suggested in the previous studies had the following problems: (1) severe reaction conditions of sulfonation, which induced side reactions such as oxidation, and (2) restricted shape of beads, whose periphery was exclusively occupied by the grafted polymer chains. Recently, we have reported a simple introduction of the S03H group onto various shapes of existing trunk polymers, such as polyethylene microporous hollow fiber, polytetrafluoroethylene film,and polypropylene nonwoven fabric, simply by immersingthe electron-beam-irradiated trunk polymer in sodium p-styrenesulfonate (SSS, CH2=CHCeH4S03Na) solution (Tsuneda et al., 1993; Sugiyama et al., 1993). A sufficient density of the S03H group was attached by cografting of SSS with hydrophilic monomers such as acrvlic acid IAAc. CHv=CHCOOH) and 2-hydroxyethylInithacrylate (HEMA, CH2=CCH3: COOCH2CHzOH). In the present work, we prepared the polymer chain grafted onto the micropores of a polyethylenehollow fiber by radiation-induced graft polymerization (RIGP) of a vinyl monomer originally containing the SO3H group, i.e., SSS, followed by conversion of the Na form into the H form. Reactants of two different molecular sizes, i.e.,
o a s a - ~ a a ~ i 9 ~ i 2 ~ 3 3 - 2 2 ~ ~ ~ 0 40. 51994 0 / 0 American Chemical Society
2216 Ind. Eng. Chem. Res., Vol. 33, No.9, 1994 Table 1. Previous Studies on Hydrolysis Using the Catalyst Based on Grafted Polymer Structure trunk functionalgroup polymee source graft monomeP reactanti PNPA, ANBAs Overberger and Dixon (1971) chemicals L-histidine imidazole/amine PEI Pavlisko and Overberger (1981a) imidazole PEI chemicals BMIHB p-nitrophenyl esters Pavlisko and Overberger (1981b) PEI chemicals BMIHB ANBAs imidazole Tomko and Overberger (1985) PVAm PNPA, ANBAs chemicals imidazolylalkanoicacid imidazole -pray st dextrin Aiba et al. (1986) sulfonicacid PTFE dextrin Arai (1988) sulfonic acid St/DVB nhotob St sucrw Arai (1989) sulfonic acid St/DVB photob St sulfonic acid sucrw Polednick et al. (1989) PP ozone St, vinylbenzylchloride PNPA Kubota (1992) imidazole photoc N-vinylimidazolel PE MAA, AA, MMA, GMA etc. pyridine PE phot@ 4-vinylpyridine PNPA Kubota (1993) sulfonicacid PE EBd SSS/HEMA sucrose, methyl acetata this study Abbreviationsof trunk polymer: PEI = poly(ethylenimine),PVAm = poly(vinylamine),PTFE = poly(tetmfinorDsthylene),St = styrene, DVB = divinylbenzene,PP =polypropylene,PE = polyethylene. UV light. Hg lamp. Electron heam. *Abbreviationsof graft monomer: BMIHB = 4(5)-bmmometbylimidazolehydrobromide, MAA = methacrylic acid, AA = acrylic acid, MMA = methyl methacrylate, GMA = glycidyl methacrylate, SSS = sodium p-styrenesulfonate, HEMA = 2-hydroxyethyl methacrylate. f Abbreviations of reactant: PNPA = p-nitrophenyl acetate, ANBAs = 4-alkanoyloxy-3-nitrobenzoicacids.
PE
SSS/HEMA-T fiber plyethylene SSS : sodium pslyrenesulfonate HEMA : 2-hydroxyelhyl methacrylate EB : electron beam Fi(ture 2. Preparation echeme showing the introduction of sulfonic acid group into polyethylene substrate. PE :
methyl acetate and sucrose, were adopted to characterize areaction field for hydrolysis. Determinationoftheoverall activation energy for the hydrolysis of reactants with different sizes will provide a definite advantage of the graft-typeacidcatalyst over thecross-linked acid catalyst. The objective of this study is to verify the higher catalytic activity of the S03H groupanchored to theflexiblegrafted polymer chains from the determination of overall activation energy of hydrolysis. Experimental Section Materials. Polyethylene microporous hollow fiber was used asa trunk polymer forgrafting. Thisoriginal hollow fiber has inner and outer diameters of 1.9 and 3.2 mm, respectively, porosity of 71'6, average pore diameter of 0.34 pm, and specific surface area of 14 m2/g. Sodium p-styrenesulfonate (SSS)was purchased from Tosoh Co. and purified by washing with methanol prior to grafting. 2-Hydroxyethyl methacrylate (HEMA) was purchased from Kanto Chemical Co., Inc. and used without further purification. Methyl acetateand sucroseof reagent grade were purchased from WakoPweChemical Industries,Ltd. A commercially available SOJH-type ion-exchange bead, Amberlite IR-120B (Rohm and Haas Co.), was used for the comparison of catalytic activity. Preparation of SOsH-Croup-Containing Catalysts. Figure 2 shows an introduction scheme of the sulfonic acid (S03H)group onto the polyethylene by the preirradiation technique of radiation-induced graft polymerization (RICP). After irradiation of the electron beam onto
Table 2. Reaction Conditions of Hydrolysis of Methyl
3 mol/& initial reactant mncn 3m o l mole of SOfi liquid weight mo g molar ratio of S W to reactantb 0.01 temperature 313-328 K a Kilogram of water. At initial time.
0.3 mol/kg 3 mmol 100 E 0.1
329343 K
the trunk polymer, SSS and HEMA were cografted by immersing the trunk polymer in an SSS/HEMAmonomer mixture. Experimental procedures and reaction conditions have been detailed in a previous publication (Tsuneda et al., 1993). The total amount of SWHEMA grafted polymer chains was calculated from the weight gain after grafting, and the SOaH group density was determined by measurement of the salt-splitting capacity. The resulting hollow fiber will be referred to as an SSS/HEMA-Tfiber, where T denotes tubular. The SSS/HEMA-T fiber was converted into the H form before use by regeneration with 1 mol/L hydrochloric acid, and was repeatedly washed with deionized water. The ion-exchange bead (Amberlite IR-120B) was also converted into the H form. Hydrolysis of Methyl Acetate and Sucrose. The hydrolysis of methyl acetate and sucrose using the SSS/ HEMA-T fiber as an acid catalyst was evaluated in a batch mode. Reaction conditions are summarized in Table 2. A 150-mL three-necked flask equipped with a condenser was fixed in a water bath maintained at a prescribed temperature. About 5-mm-long cut fibers were immersed in
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2217 Table 3. Properties of 801H-Group-ContainingCatalysts. SSS/HEMA-T fiber
PE
matrix shape size'
SO3H group density water content HEMA density
hollow fiber i.d. = 3.8 mm 0.d. = 6.3mm 2.8 mol/kgd 10.5 mol/kge
St, DVBb bead diam = 0.45-0.60 mm
4.4 mol/ kpd 44-48 %
81%
kat = -( W/@ln(C/Co)
1.9 mollkgd 7.3 mol/kge
OAbbreviations: P E = polyethylene, St = styrene, DVB = divinylbenzene, i.d. = inner diameter, 0.d. = outer diameter, HEMA = 2-hydroxyethyl methacrylate. Degree of cross-linking = 8%.c In a wet state. Kilogram of dry product of H form. e Kilogram of trunk polymer.
100 g of a reactant-and-water mixture. The mixture was stirred by magnetic stirrer. For comparison, similar hydrolysisof methyl acetate was performed using the beads (Amberlite IR-12OB) with the same amount of the S03H group as the SSS/HEMA-T fibers. At prescribed intervals, 0.1 mL of liquid was withdrawn from the flask using a syringe. The total amount withdrawn was negligible compared to the total amount of the liquid. Methyl acetate, water, acetic acid, and methanol were determined by gas chromatography (Hitachi 263-30; column, GL Sciences Gaskuropack-54). Sucrose, D-glucose, and Dfructose were determined by liquid chromatography (RI detector, Tosoh RI-8010; column, Tosoh TSK-gel Amide80). First, the hydrolysis of methyl acetate obeys a secondorder reaction (first order in methyl acetate and first order in concentration of acid) in excess water (Long and Paul, 1957). The apparent reaction rate constant k, can be expressed by
where W, q, and t are the weight of the reaction mixture, the mole number of the sulfonic acid group, and the reaction time, respectively. C and C, are the concentrations of acetic acid at time t and in equilibrium, respectively. Second, sucrose is hydrolyzed into D-glucose and Dfructose according to the second-order reaction (first order in sucrose and first order in concentration of acid).
where COis the concentration of sucrose at initial time.
Results and Discussion Propertiesof SOsH-Group-ContainingFiber. Table 3 summarizes the physical properties of the resultant sulfonic acid (SO3H)-group-containingion exchanger,SSS/ HEMA-T fiber. The matrix of the commercial ion exchanger is styrene-divinylbenzene copolymer whose degree of cross-linking is 8 ?G ,and the S03Hgroup density is 4.4 mol/kg of dry product of the H form. In comparison, the S03H group density of the SSS/HEMA-T fiber was 2.8 mol/kg of dry product of the H form. Hydrolysis of Methyl Acetate and Sucrose. Time courses of the concentrations of reactants and products for the hydrolysis of methyl acetate and sucrose are shown in Figure 3a and Figure b, respectively. The decrease in the concentrations of methyl acetate and water equaled the increase in the concentrations of acetic acid and methanol; finally the reaction reached equilibrium. The equilibrium constant for the hydrolysis using the SSS/ HEMA-T fiber as a catalyst calculated here agreed well with that determined in homogeneous sulfuric acid. Also, the hydrolysis of sucrose satisfied the mass balance: the decrease of sucrose was almost equivalent to the amounts of D-glucose and D-fructose produced. Although the acid hydrolysis of sucrose is accompanied by the formation of di-D-fructosedianhydrides as a side reaction (Rearick and Olmstead, 19931, its amount was negligible in this study, compared to the amounts of D-glucose and D-fructose. Figure 4 shows examples of the analysis of kinetic data. The ordinates of the figures correspond to the right-hand sides of eqs 1and 2. The slope of the resultant straight line is the apparent reaction rate constant. Comparison of Catalytic Activity between GraftType and Cross-LinkedCatalysts. Apparent reaction rate constants for the hydrolysis of methyl acetate and sucrose were determined at temperatures ranging from 313 to 328 K and from 328 to 343 K, respectively. Figure 5aand Figure b show the Arrhenius plots for the hydrolysis of methyl acetate and sucrose, respectively. For the hydrolysis of methyl acetate, the k , values of the SSS/ HEMA-T fiber were consistent with those of Amberlite IR-12OB. As a result, the S03H group attached to the flexible grafted polymer chains exhibited the same catalytic 0.5
I45
I 0 methyl acetate 1
Z
[I 0 sucrose 1
I
Reaction temp. : 333 K
w.. 0 40 0 ~
0
10
20
(2)
Amberlite IR-12OB
30
Reaction time [h]
40
5
I
a ,
10
Reaction time [h]
Figure 3. Time courses of concentrations of reactants and products upon hydrolysis. (a) Methyl acetate: (b) sucrose.
15
2218 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994
300 0 SSSIHEMA-T fiber
0 SSSIHEMA-T fiber I Amberlite IR-1208
=
z 9 Y
I
200
018 C
q I w 100 0
-4
Reaction temp. : 333 K
5
0
Reaction time [h]
10
15
Reaction time [h]
(b)
(a) Figure 4. Examples of analysis of kinetic data in hydrolysis. (a) Methyl acetate; (b)sucrose.
Reaction temp. : 313-328 K
,
cn
Reaction temp. :
v \
328-343 K
m
Y
AE=73 kJ/mol
m
Y
10-3
3.1 3.2 1TT X103 [K-’1
:
3.3
2.9
3.0
3.1
1TT X103 [K-’]
Figure 5. Dependence of k, on reaction temperature. (a) Methyl acetate; (b) sucrose.
activity as the S03Hgroup introduced into the cross-linked polymer chains. The activation energy for the hydrolysis of methyl acetate in hydrochloric acid is reported to be 72 kJ/mol (Herbert and Arthur, 1941). The same activation energy of 73 kJ/mol calculated for two different catalysts demonstrates that the hydrolysis of methyl acetate is governed by an intrinsic reaction; a small reactant (methyl acetate, molecular size = 0.22 nm at 298 K estimated by the Stokes-Einstein equation) cannot distinguish between grafted polymer chain and cross-linked polymer chain during its approach to the S03H group. On the other hand, Figure 5b shows a distinct difference between graft-type and cross-linked acid catalysts. The SSS/HEMA-T fiber showed a higher catalytic activity with a higher activation energy than the cross-linked catalyst. The lower catalytic activity of the cross-linked catalyst is caused by the decrease of accessible S03H groups for sucrose, and the presence of the diffusional mass-transfer resistance of sucrose into the cross-linkedpolymer network contributes to the lower activation energy; thus, a large reactant (sucrose, molecular size = 0.46 nm at 298 K) has far more difficult access to the S03H group during the restricted diffusion in the interstices of the cross-linked polymer chains. The activation energy of 95 kJ/mol for the hydrolysis of sucrose using the SSS/HEMA-T fiber
was consistent with the intrinsic activation energy of 95 kJ/mol determined in a homogeneous medium (HC1) (Rhim et al., 1989). A definite characterization of the poly-SSS chain grafted onto polyethylene microporous hollow fiber is difficult at present because the graft polymer chain could not be isolated from the chemically stable polyethylene. Alternatively, the permeability of pure water through the micropores where the grafted polymer chain was immobilized can provide a semiquantitative image of the grafted polymer chain. The pure water flux of the original hollow fiber was 0.7 m/h at a permeation pressure of 0.1 MPa, while the SSS/HEMA-cografted hollow fiber had a negligibly low pure water flux. This drastic decrease indicates that micropores of about 0.3-pm diameter with 71% porosity was filled with the SO3H grafted polymer chain stretching from the micropore wall; the length of the S03H-group-containing grafted polymer chain was, at least, on the order of submicrons, providing a reaction field favorable for larger reactants.
Conclusions The sulfonic acid (SO3H) group on the flexible polymer chain was attached to a polyethylene microporous hollow fiber by radiation-induced graft polymerization. The
Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 2219 catalytic activity of the SO3Hgroup anchored to the grafted polymer chain for the hydrolysis of methyl acetate and sucrose was determined by varying the reaction temperature to obtain the overall activation energy, and was compared to that of the SO3H group immobilized on the cross-linked polymer chain. A larger reactant (sucrose) has far more difficult access to the S03H group of the cross-linked catalyst. On the other hand, the catalyst attached to the grafted polymer chain provides access to both methyl acetate and sucrose due to its flexible polymer structure. Since the preparation method of the acid catalyst suggested in this study is applicable to various shapes of substrates such as nonwoven fabric and fiber, we can design a novel shape of packing suitable for a reactive-distillation column.
Acknowledgment Helpful discussions with Makoto Chiwa of Organo Co., Ltd., Japan, and Minoru Seki of the Department of Chemical Engineering, University of Tokyo, are gratefully acknowledged. The authors also wish to thank Yoshizumi Inai of the Industrial Membrane Division of Asahi Chemical Industry Co., Ltd., Japan, for his help in providing the starting hollow fiber.
Nomenclature C = reactant concentration Co = reactant concentration at initial time C, = reactant concentration in equilibrium k, = apparent reaction rate constant for hydrolysis q = mole number of sulfonic acid group t = reaction time W = weight of reaction mixture DVB = divinylbenzne HEMA = 2-hydroxyethyl methacrylate PE = polyethylene SSS = sodium p-styrenesulfonate St = styrene Literature Cited Aiba, S.; Hiratani, K.; Nakagawa, T. Preparation of Poly(styrenesulfonic acid)-Grafted Microporous Polytetrafluoroethylene Membranes and Their Activity as Hydrolysis Catalysts. Makromol. Chem., Rapid Commun. 1986, 7,91. Arai, K. Hydrolysis of Dextrin in the Presence of Sulfonated CrossLinked Polystyrene Beads with Grafted Chain as Spacer. Sen’i Cakkaishi 1988,44, 558.
Arai, K. Grafted Chain as Spacer for an Insoluble Polymer Catalyst. J. Appl. Polym. Sci. 1989, 38, 969. Herbert, S.H.; Arthur, M. R. J. The Acid Hydrolysisof Methyl Acetate in Dioxane-Water Mixtures. J. Am. Chem. SOC. 1941,63,1993. Kubota, H. Catalytic Activity of N-Vinylimidazole-GraftedPolyethylene: Effect of Method of Introduction of Grafted Chains by Means of Photografting. Eur. Polym. J. 1992,223, 267. Kubota, H. Catalytic Activity of 4-Vinylpyridine-Grafted Polyethylene Prepared by Photografting: Effect of Grafting Conditions. Eur. Polym. J. 1993,29, 551. Long, F. A.; Paul, M. A. Application of the Ho Acidity Function to Kinetics and Mechanisms of Acid Catalysis. Chem. Rev. 1967,57, 935. Overberger, C. G.; Dixon, K. W. Hydrolysis of Activated Esters Catalyzed by L-Histidine Graft Copolymers. J.Polym. Sci.,Polym. Chem. Ed. 1977,15, 1863. Pavlisko, J. A.; Overberger, C. G. Polyethylenimine Catalysts Containing an Isolated Apolar Binding Site: Solvolysis of p Nitrophenyl Esters. J.Polym Sci., Polym. Chem. Ed. 1981a, 19, 1621. Pavlisko, J. A.; Overberger, C. G. Solvolysis of 4-Acyloxy-3-Nitrobenzoic Acid Substrates Catalyzed by Polyethylenimine Derivatives. J. Polym. Sci., Polym. Chem. Ed. 1981b, 19, 1757. Polednick, C.; Yaacoub, E.; Widdecke, H.; Guyot, A. New Pellicular Ion Exchaners by Grafting onto Polypropylene Beads Part 11. Catalysis of the Sucrose Inversion. J. Mol. Catal. 1989,56,351. Rearick, D. E.; Olmstead,L. J. SucroseInversionProducts: Analytical Inconsistencies due to Di-D-fructose Dianhydride Formation. Proc. Sugar Process. Res. Conf. 1993,97. Rhim, J. W.; Nunes, R. V.; Jones, V. A,; Swartzel, K. R. Appearance of a Kinetic Compensation Effect in the Acid-CatalyzedHydrolysis of Disaccharides. J. Food Sci. 1989,54, 222. Sugiyama,S.;Tsuneda, S.;Saito,K.; Furuaaki, S.; Sugo,T.; Makuuchi, K. Attachment of Sulfonic Acid Group to Various Shapes of Polyethylene, Polypropylene and Polytetrafluoroethylene by Radiation-Induced Graft Polymerization. React. Polym. 1993, 21, 187. Tomko,R.; Overberger,C. G. PolymericCatalysis: ImidazolesGrafted ontoPoly(vinylamine). 11. Kinetics of the Esterolysis of Activated Phenyl Esters. J. Polym. Sci., Polym. Chem. Ed. 1986,23, 279. Tsuneda, S.; Saito, K.; Furusaki, S.; Sugo, T.; Makuuchi, K. Simple Introduction of Sulfonic Acid Group onto Polyethylene by Radiation-Induced Cografting of Sodium Styrenesulfonate with Hydrophilic Monomers. Znd. Eng. Chem. Res. 1993,32, 1464. Yoshioka, T.; Shimamura, M. Studies of Polystyrene-Based Ion Exchanger Fiber. 11. A Novel Fiber-Form Catalyst for Sucrose Inversion and Methyl Acetate Hydrolysis. Bull. Chem. SOC.Jpn. 1984, 57, 334.
Received for review December 15, 1993 Revised manuscript received May 13, 1994 Accepted June 16,19940 @
Abstract published in Advance ACS Abstracts, July 15,1994.
