I n d . Eng. Chem. Res. 1994,33,1454-1459
1454
Kinetic Study of the Carboxymethylation of Cellulose Tapio Salmi,' Daniel Valtakari, and Erkki Paaterot Department of Industrial Chemistry, Abo Akademi, FIN-20500 Turku, Finland
Bjarne Holmbom Department of Forest Products Chemistry, Abo Akademi, FIN-20500 Turku, Finland
Rainer Sjoholm Department of Organic Chemistry, Abo Akademi, FIN-20500 Turku, Finland
The kinetics of carboxymethylation of mercerized Na-cellulose was studied in a laboratory-scale batch reactor operating at atmospheric pressure and a t temperatures in the range 30-80 O C . Based on the experimental data a bimolecular kinetic model for carboxymethylation was developed. The model takes into account the decline of the intrinsic activities of the hydroxyl groups of cellulose as the substitution proceeds. The parameters of the kinetic model were determined by regression analysis, and the model described well the experimentally determined carboxymethylation kinetics. The distribution of mono-, di-, and trisubstituted anhydroglucose units in (carboxymethy1)cellulose was simulated using the parameters of the kinetic model.
Introduction (Carboxymethy1)cellulose (CMC) is produced in an alkaline milieu through a nucleophilic substitution reaction between sodium cellulose and monochloroacetate: cell-0-
+ CH2'CleC00-
-
cell-O-CH,COO-
+ C1-
(1)
where cell-0- denotes the ionized hydroxyl group (HO-2, HO-3, or HO-6) in the anhydroglucose unit of cellulose. The reaction occurs in an alcohol solvent (ethanol or 2-propanol). In principle all hydroxyl groups (HO-2, HO3, and HO-6) in the anhydroglucose unit of cellulose can be substituted, the maximum degree of substitution (DS) being 3. In commercial CMC the degree of substitution is typically much lower, commonly 0.4-1.4 (Sjostrom, 1993). The reactivities of the different hydroxyl groups have been determined by GC-MS analysis of CMC hydrolysates (Niemela and Sjostrom, 1989). According to the GC-MS studies the reactivities of the hydroxyl groups decrease in the order HO-2 = HO-6 > HO-3; the corresponding ratio between the rate constants was determined to be k2:ks:ks = 3:1:2-3 for hardwood-CMC hydrolysates (Niemela and Sjostrom, 1989). In spite of the large industrial production of CMC the investigations of the substitution kinetics are rather sparse. In the pioneering work of Time11 (1950) some substitution reactions of cellulose were studied, and a monomolecular kinetic model was proposed. Recently Xiquan and coworkers (1990) studied the kinetics of carboxymethylation of cellulose in 2-propanol at 45-65 "C using a conductometric method for the determination of the DS. The results were described by a monomolecular kinetic model. The study was limited to the initial stage of carboxymethylation, and the DS was always less than 0.85 in the experiments. The goal of the present work is to determine the kinetics of carboxymethylationfrom the beginning of the reaction
* Author to whom correspondence should be addressed. E-mail:
[email protected]. + Present address: Lappeenranta University of Technology, FIN-53851 Lappeenranta, Finland.
toa high degree of substitution and to obtain aquantitative description of the kinetic data.
Experimental Section Chemicals. The chemicals used in the carboxymethylation were cellulose (Metsa-Serla Chemicals; hardwood, birch; dry contents 91 5% ;DP (SCAN,from viscosity) 1500; diffuse blue reflectance factor (ISO) 88%; particle size 0.35 mm; extractives (dichloromethane) < 0.4%), NaOH (Merck), and 2-propanol (10076, Suomen Plastkem Oy). The NaOH pellets were dissolved in distilled water, and a 50 wt % solution was prepared. 2-Propanol was used as the solvent in most carboxymethylation experiments. Monochloroacetic acid was obtained from Fluka AG (puriss. p.a. > 99%1. Ethanol of technical grade (Oy Alko Ab, 96.4%) was used to precipitate the reaction product. In the pretreatment of the reaction product, the following chemicals were used: 1M HC1 (min 37 7% ,Riedel-deHaen AG) was used to interrupt the reaction, 2 M NaOH prepared from pellets was used to neutralize the solution, and phenolphthalein was used as a pH indicator during the neutralization. In the analysis of the degree of substitution (DS) the following chemicals were used: sulfuric acid (0.05M, Titrisol, Merck) was used in titration of NaOH in the product ash; the indicator was methyl red (0.25% 1. In the determination of the NaCl contents of the product ash AgNO3 (0.1 M) was used with KMn04 (10 % ) as an indicator. The chemicals were used as received. Apparatus. The reactor was a 700 mL glass vessel equipped with a heating jacket and a Methrohm cap. In the heating jacket, water was circulated from a reservoir using a heating thermostat. On the Methrohm lock of the glass reactor a cooling condenser was placed to reflux the solvent vapors. A monoethylene glycol-water mixture was used as a cooling medium in the condenser. The cooling medium temperature was -22 "C, and it was circulated by a cryothermostat. The reaction mixture was stirred by a magnetic stirrer (LR20,Framo Geratetechnik) equipped with a two-blade propeller. The maximum stirring rate was 1800 rpm; a typical stirring rate in the experiments was 800-1200 rpm. ExperimentalProcedure. The reactor was filled with 12.00 g of pure cellulose, 500 mL of 2-propanol was added
0888-588519412633-1454$04.50/0 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 6,1994 1455 to the reactor vessel, which was closed tightly, and the stirrer was turned on. During this premixing of the suspension a 50 wt % NaOH solution was prepared and added over a period of 20 min to the suspension of cellulose and 2-propanol. The temperature was kept at 20 "C, and the data acquisition was started. The mercerization with NaOH was continued for 120 min. After the mercerization the monochloroacetic acid was added to the suspension and the temperature was raised-partially utilizing the heat of neutralization-to the desired reaction temperature. The reaction temperatures were 30, 40, 60, and 80 "C. All experiments were performed at atmospheric pressure. Monochloroacetic acid (mp 63 "C) was preheated and dissolved in 2-propanol before it was added to the suspension. The reaction time was usually 120 min. Degree of Substitution. Samples were usually taken from the reaction mixture every 20 min using a Pasteur pipette. A typical sample size was 40 mL. The sample was treated with 0.1 M HC1to interrupt the reaction. After 10 min the sample was neutralized with NaOH. Phenolphthalein was used as an indicator. The reaction product was precipitated by washing it at least 4-5 times with ethanol. The volume each time was approximately 380450 mL. A sample (0.7-1.0 g) of the solid product was dried at 70-80 "C in an oven for 8 h. The dried sample was cooled in a desiccator for 2 h until the temperature was 40 "C. Half of the sample (0.35-0.55 g) was weighed in a ceramic crucible, and it was pyrolyzed in an oven at maximum temperature of 600 "C. The oven temperature was increased from 25 to 550 "C in 20 min and from 550 to 600 "C in 30 min. The sample was kept at 600 "C in 240 min. The sample was allowed to cool to room temperature by switching off the own. The cwoling time was approximately 4 h. The ash containing NazO (and possibly traces of NaC1) was dissolved in hot (80 "C) distilled water. The methyl red indicator was added to the solution, which became yellow. The solution was titrated with 0.1 N (0.05 M) H2SO4 until a red color appeared. The reddish solution was heated to remove dissolved COz, until the yellow color was regained. A second titration was performed with HzS04, and the total consumption of the acid was registered (bt,,t, in mL). A typical acid consumption was approximately 20 mL for a DS 1. If the weight of the unburnt cellulose sample is denoted by ~ C M (in C g) and the consumption of HzS04 is denoted by btot, we can define a ratio B (Wilson, 1960): B = 0 . l b d ~ C M C where , 0.1 is the normality of the solution. The degree of substitution (DS) was obtained from the formula: DS = 0.162B/(1-0.08B),where0.162isthemolar mass of the glucose unit (in kg/mol) and 0.08 is the molar mass (in kg/mol) of the group (CHCOO-Na+)substituted on cellulose. The sample weight (MCMC) is the corrected sample weight: the weight of NaCl in the original sample was subtracted from the original sample weight. The amount of NaCl was determined by argentometric titration with AgNOs using KMn04 as the indicator. In most cases the amount of NaCl was negligible.
-
Results and Discussion Carboxymethylation of Cellulose. The effect of the reaction temperature on the kinetics of carboxymethylation was investigated. The experimental results are depicted in Figure 1. In carboxymethylation it is possible to achieve degrees of substitution clearly above 1, since all the hydroxyl groups (HO-2, HO-3, and HO-6) are reactive with monochloroacetic acid. The final degree of substitu-
OS
0.5
0
ti. 0
20
40
60
80
100
120
140
Figure 1. Kinetics of carboxymethylation at different temperatures: 30 "C (a), 40 OC ( O ) , 60 "C (E), and 80 O C (m). DS
1.5
1
0.5
H ffmin 0
20
40
60
80
100
120
140
Figure 2. Kinetics of carboxymethylation of cellulosein 2-propanol at 80 "C using instantaneous (M) and stepwise (0)addition of monochloroacetic acid.
tion was higher than 1.6 at temperatures in the range 6080 "C, and even at the two lowest temperatures a rapid substitution occurred: after 100 min the degree of substitution was about 0.7 and 1.3 at 30 and 40 O C , respectively. A t reaction times longer than 100 min the reaction rate, however, decreased considerably, and DS seemed to approach a limiting value. The rate increased with increasing temperature (Figure 11, but the difference between the rates at 30 and 40 "C is higher than, e.g., the difference between the rates at 60 and 80 "C. This indicates that the substitution rate became retarded by diffusion of monochloroacetate in the cellulose particles. To investigate the effect of the monochloroacetic acid concentration on the reaction kinetics, an experiment was performed where the acid was added to the reaction mixture at 80 "C in six aliquots. The final molar ratios cellu1ose:acid:NaOH were 1:4:8. The result is shown in Figure 2. The degree of substitution was 0.9 after 120 min, which is clearly less than the degree of substitution DS = 1.6 obtained in the corresponding experiment with an instantaneous addition of the acid (Figure 1). The result shows that the destruction of monochloroacetate through side reactions is not the limiting factor in carboxymethylation. The positions of the substituents (HO-2, HO-3, or HO6) were not determined here. However, recent studies suggestthat the reactivities of the hydroxylgroups decrease in the order HO-2 1 HO-6 > HO-3 in carboxymethylation, the hydroxyl group HO-3 being much less reactive than the groups HO-2 and HO-6. The highest reactivity of HO-2
1456 Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994
in carboxymethylation is explained by the fact that it has the most acidic proton among the hydroxyl groups (Sjostrom, 1993). Reaction Kinetics. In the work of Xiguan et al. (1990) it was assumed that the substitution rate is dependent on the monochloroacetate concentration only, which implies a monomolecular nucleophilic substitution mechanism. It is, however,generally agreed upon that the nucleophilic substitution between an alkyl halide acid and an alkoxy group is bimolecular. Thus, we assume that the substitution reaction proceeds through the bimolecular nucleophilic substitution mechanism, which implies that the reaction rate depends on the concentrations of the monochloroacetate ion and the cellulose hydroxyl group. The balance equations of the unsubstituted hydroxyl groups (HO-2,HO-3, and HO-6) in cellulose can be written as
dCoi - -k’,CoiCHA --
Tt0C 30 40 60 80
ko 0.00367 0.00481 0.0263 0.0135
st% 39 22 52 117
a0
3.62 0.763 1.433 1.002
st% 32 40 35 119
WSRS 0.0214 0.0601 0.112 0.401
MRS 0.00357 0.00858 0.0186 0.100
of substituted groups, Le., to the DS. Thus the infinitesimal decrease, Ak‘, is given by Ak’ = -aok’ADS
(10)
where a0 is a constant. Letting Ak’ and ADS approach zero the differential equation is obtained
‘_ : - -aodDS
(11)
which after integration (k’ = ko at DS = 0) gives an exponential relationship between k’ and DS:
dt
where Coi and CHAdenote the dimensionless concentration of the unsubstituted hydroxyl group (i = 2,3, or 6) and the monochloroacetate ion, respectively. The balance equations of the substituted hydroxyl groups are dCi -dt - k’iCOiCHA
Table 1. Kinetic Parameters of Carboxymethylation
k’ = k0eaS
(12)
The balance equations 6 and 7 can now be written as dCi
dt = koe“ODSai(l- Ci)CHA (3)
where Ci denotes the dimensionless concentration of a substituted hydroxyl group. The balance equation of the monochloroacetate ion is
where DS is given by eq 8. The temperature dependence of the rate parameter ko is obtained from the Arrhenius equation
(4)
ko = Afe-Ea/RT The concentrations of the unsubstituted hydroxyl groups are obtained from the total balance equation
coi+ ci = 1
(5)
which is inserted in eqs 3 and 4: dCi - k’i(1_
Ci)CHA
dt
(7) Equations 5-7 describe the kinetics of all functional groups. The DS is obtained from the sum of the substituted groups: DS = x C i
(15)
where A‘ and E, denote the frequency factor and the activation energy, respectively. Parameter Estimation. In the estimation of the kinetic parameters from the experimental time-DS data the minimum of the sum of the residual squares was searched using the Levenberg-Marquardt method (Marquardt, 1963)as implemented in the nonlinear regression program package Reproche (Vajda and Valkb, 1985). A t every iteration cycle of the Levenberg-Marquardt method the differential equations were solved using a semiimplicit Runge-Kutta method, a fourth-order Rosenbrock-Wanner (ROW) (Gottwald and Wanner, 1981) method suitable for stiff differential equations. The sum of residual squares (8)was defined as
(8)
i
The decline of the reactivities of the hydroxyl groups is probably attributed to the decrease of the chemical reactivity and to diffusional limitations. The decrease of the reactivity during the course of the substitution is described by a simple phenomenological approach. The rate constants for substitution of sites HO-2, HO-3, and HO-6 (k’i) can be written as k’i = aik’, i = 2, 3,6
(9)
where ai is a proportionality coefficient. The parameter k‘ is supposed to decline as the substitution proceeds. The decrease of k’ is assumed to be proportional to the number
where DSjpp is the experimentally observed degree of substitution at time ti and DSj,dc is the degree of substitution calculated from the model equation (8). The kinetic parameters of the model, estimated separately at each experimental temperature, are listed in Table 1. It can be seen from Table 1that the overall fit of the kinetic model is good, the deviation between the experimental and calculated DS values was typically less than 5%. The experimental and calculated DS values at 30, 40, and 60 “Care compared in Figure 3. The primary rate parameter (ko)has an increasing tendency with temperature (Table l ) , whereas the exponent of the activity decay parameter (ao) varies randomly with temperature. This is due to the mutual correlation of the parameters. To
Ind. Eng. Chem. Res., Vol. 33, No. 6,1994 1457 DS
k3
Figure 5. Reaction scheme for the formation of substituted anhydroglucose units in carboxymethylation. Notation: (0) unsubstituted anhydroglucose unit; (2), (3), (6) monosubstituted anhydroglucose units (substituted at hydroxyl groups 2, 3, and 6); (23), (26),(36) disubstitutedanhydrogluccseunits (substitutedathydroxyl groups 2 and 3, 2 and 6, and 3 and 6); (236) trisubstituted anhydroglucose units.
a 20 40 60 80 io0 120 140 Figure 3. Kinetics of carboxymethylation at 30 “C (a),40 “C ( O ) , and 60 O C (0)according to the model calculated with individual parameters.
The reactions are illustrated in Figure 5. According to the reaction scheme given in Figure 5, the generation rates of the unsubstituted ( f g ) , monosubstituted ( f z , r‘3, and P ’ ~ ) , disubstituted ( ~ ’ 2 3 ,f 2 6 , and f 3 6 ) and trisubstituted
DS
1
0.5
0
20
0
40
60
80
100
120
140
Figure 4. Kinetics of carboxymethylation at 30 “C ( O ) ,40 “C ( O ) , and 60 O C (0) according to the model calculated with the parameters of the Arrhenius law. Table 2. Temperature Dependence of the Kinetic Parameters of Carboxymethylationa ~
T/”C 30 40 50 60 80 4
a0
= ko 0.00237 0.00635 0.0160 0.0382 0.188
hs.0
kDs-1.0
0.000343 0.000920 0.00232 0.00554 0.0272
kDS-3
7.21 X 1.93 X 4.88 X 1.16 X 5.72 X
le 106 106 106 lp
= 1.93, s = 25%;In A’ = 24.82, s = 16%;E J R = 9353 K, s =
where C’O,CHA,C’2, ..., c’236, and C H Adenote dimensionless concentrations of the glucose units and the chlorocarboxylic acid. The rate parameters k‘z, k‘3, and k‘6 are defined by eq 25:
17%.
test the thermodynamic consistency of the model, the temperature dependence of the parameter ko was determined by a simultaneous estimation of the frequency factor and activation energy from the DS data obtained at 30, 40, and 60 “C. The activity decay parameter (ao) was assumed to be temperature independent. The estimated parameters are listed in Table 2, and the fit of the model to the experimental data is shown in Figure 4. The fit is reasonable: the prediction of DS at 30 and 60 “C can be regarded as correct, whereas slightly too low values of DS are predicted at 40 “C. However, within the experimental accuracy, the rate parameter follows the Arrhenius law. Product Distribution in Carboxymethylation. The distribution of the substitutions can be calculated using the kinetic constants k’z, k’3, and k’6. The unsubstituted glucose unit forms three types of monosubstituted units (2,3, and 6), and each monosubstituted unit forms three types of disubstituted units (23,26, and 36). The reaction of a disubstituted unit gives the trisubstituted unit (236).
The consumption rate of the monochloroacetate ion is given by
The unknown concentrations of the different glucose units and that of the acid can be solved by integration of the balance equations
dC’,
-- r’i, dt
i = 0, 2, 3,6, 23, 26,36, 236, HA
(27)
using the initial conditions CI0 = 1, C H A= COHA, C’Z= C ’ 3 = C ’ 6 = c ’ 2 3 = c ’ 2 6 = c ’ 3 6 = c’236 = 0, at t = 0. The
1458 Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 C
C 1C
0
I
20
40
60
80
100
120
140
1
1
0
Figure 6. Concentrations of unsubstituted (O), mono- (I), di- (II), and trisubstituted (111)anhydroglucose units in carboxymethylation at 30 "C.
20
40
60
80
100
120
140
Figure 8. Concentrations of unsubstituted (O), mono- (I), di- (II), and trisubstituted (111)anhydroglucose units in carboxymethylation at 60 "C. C
C
t\
3
20
40
80
60
100
120
0
140
0.5
1
1.5
2
Figure 7. Concentrations of unsubstituted (O), mono- (I), di- (II), and trisubstituted (111)anhydroglucose units in carboxymethylation at 40 " C .
Figure 9. Distribution of the unsubstituted, mono-, di-, and trisubstituted anhydroglucoseunits in carboxymethylation. The ratio of the rate constants was k'2:k'S:k'e = 1:0.3:1.
concentrations of the mono- (CI), di- (CII),and trisubstituted (CIII)units are then obtained from
of the substituted groups HO-2 and HO-6 were equal, since kz was assumed to be equal to k6 (k2:k3:k6 = 1:0.3:1). The substitutions to HO-2 and HO-6 were dominating according to the calculation, whereas the substitution to HO-3 is of minor importance. The interesting factor is the distribution between mono-, di-, and trisubstituted units at 30, 40, and 60 "C. The developments of the concentrations of mono- (CI),di- (CII), and trisubstituted (CIII) units are shown in Figures 6-8. The calculations suggest that monosubstituted units are dominating a t the lowest temperatures (30 and 40 "C), but the concentration of disubstituted units exceeds that of the monosubstituted units a t 60 " C at our maximal reaction times (120 min). The concentration of trisubstituted units is low in all cases. This is due to the stagnation of the reaction rate as the degree of substitution increases (Table 2). A t 30 "C the concentration of monosubstituted units increases monotonically during the reaction (Figure 6), whereas the maximum of the monosubstituted units is passed at 40 and 60 " C during the reaction time (120 min) (Figures 7 and 8). Since the activation energies of the rate constants (ko) were assumed to be equal (eq 151, the distribution of the substituted units is independent of the temperature at a fixed degree of substitution. The simulated distribution between mono-, di-, and trisubstituted units as a function of DS is shown in Figure 9. The simulation suggests that the concentration of monosubstituted units has its maximum a t DS = 1, and the concentration of disubstituted units has a maximum a t DS i= 2. The calculation indicates
CI =
c', + c'3 + c'6
(28)
(30) and DS is calculated from
The concentrations of the substituted sites HO-2, HO-3, and HO-6 (Ci)are obtained by addition of the concentrations of the corresponding glucose units
(34) The differential equations (27) were solved by the Rosenbrock-Wanner method (Gottwald and Wanner, 1981) using a simulation program. The model was applied to the carboxymethylation kinetics a t 30,40, and 60 OC. The kinetic parameters listed in Table 2 were used, and the concentrations of the mono-, di-, and trisubstituted anhydroglucose units were simulated as a function of the reaction time. The concentrations
Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 1459 that monosubstituted units dominate in carboxymethylation at 30 and 40 "C at DS = l,whereas the amounts of mono- and disubstituted units are almost equal in the experiment a t 60 O C at DS = 1.5.
r = reaction rate r' = transformed generation rate, r' = r/cO s = standard deviation of a parameter ( % ) t = time T = temperature
Conclusions The kinetics of carboxymethylation of cellulose was studied by measuring the degree of substitution as a function of the reaction time in a batch reactor. In the carboxymethylation experiments a t 30-80 "C the reaction rate increased with temperature, and it was possible to achieve degrees of substitution (DS) between 1.5 and 2 within 2 h at the highest reaction temperatures (60-80
Greek L e t t e r s ai = proportionality coefficient
"C). The kinetics of carboxymethylation was described with a bimolecular nucleophilic substitution scheme. The decrease of the reaction rate during the substitution was caused by the decline of the reactivity of the hydroxyl groups of cellulose. In the kinetic model for the carboxymethylation the decline of the reactivity was expressed as an exponential decrease of the kinetic constants. The kinetic parameters of the rate model were estimated by nonlinear regression analysis. A comparison of the experimental and the calculated degrees of substitution revealed that the kinetic model provided a good fit to the experimental data thus supporting the validity of the kinetic model.
Nomenclature A' = frequency factor a0 = activity decay factor btot = consumption of H z S 0 4 B = ratio, used in the determination of DS c = concentration co = amount of cellulose (anhydroglucose units) per liquid volume C = dimensionless concentration, C = C/CO C' = dimensionless concentration of an anhydroglucose unit DS = degree of substitution E , = activation energy ki = rate constant of hydroxyl group HO-i, i = 2, 3, or 6 ko = rate parameter k' = rate constant, k'i = Qik' ~ C M = C mass of the cellulose sample Q = objective function, weighted sum of residual squares (WSRS)
Subscripts a n d Superscripts
i = index of the substituted hydroxyl group HO-i, i = 2, 3, or 6 j = experiment index in regression analysis 0 = initial value Oi = index of the unsubstituted hydroxyl group HO-i, i = 2, 3, or 6 Abbreviations
CMC = (carboxymethy1)cellulose H A = monochloroacetic acid, monochloroacetate MRS = mean residual square WSRS = weighted sum of residual squares (8)
Literature Cited Gottwald, B. A.; Wanner, G. A. A Reliable Rosenbrock Integrator for Stiff Differential Equations. Computing 1981,26, 355-360. Marquardt, D. W.An Algorithm for Least-squares Estimation of Nonlinear Parameters. J.Soc.Zndust. Appl. Math. 1963,11,431441. Niemela, K.; Sjostrom, E. Characterization of Hardwood-derived Carboxymethylcelluloseby Gas-liquid Chromatography and Mass Spectrometry. Polym. Commun. 1989,30, 254-256. Sjostrom, E. Wood Chemistry Fundamentals and Applications; 2nd ed.; Academic Press: San Diego, 1993;Chapter 9,pp 204-222. Timell, T. Studies on Cellulose Reactions. Ph.D. Thesis, Royal Institute of Technology, Esselte, Stockholm, 1950,275 pp. Vajda, S.; Valko, P. Reproche-Regression Program for Chemical Engineers, Manual; European Committee for Computers in Chemical Engineering Education: Budapest, 1985;36 pp. Wilson, K.A Modified Method for Determination of Active Agent and Degree of Substitution in Carboxymethyl Cellulose (CMC). Sven. Papperstidn. 1960,63,714-715. Xiquan, L.;Tingzhu, Q.; Shaoqui, Q. Kinetics of Carboxymethylation of Cellulose in the Isopropyl Alcohol System. Acta Polym. 1990, 41, 220-222.
Received for review November 15, 1993 Revised manuscript received March 17, 1994 Accepted March 28, 1994'
Abstract published in Advance ACS Abstracts, May 1,1994.
