ION EXCHANGE RESIN-CATALYZED ESTERIFICATION OF SALICYLIC ACID WITH
M ETHAN0 L M A R T I N B A R R Y BOCHNER A N D S A M U E L M. GERBER Organic Chemicals Diviszon, American Cyanamid Co., Bound Brook, .V. J .
WOLF R . V l E T H A N D ALAN J. RODGER Department of Chemical Engineering, Massachusetts Institute of Technology. Cambridge 39, M a s s . An experimental study was carried out to evaluate the performance of a cation exchange resin in the acid form as a catalyst for the esterification of salicylic acid with methanol. From the results of experiments in which the agitation rate was varied, it was determined that bulk diffusion is not a limiting step in the reaction over the range of experimental conditions. Rates of conversion were found to be insensitive to catalyst particle size in the range studied, indicating that neither bulk diffusion nor pore diffusion is rate-limiting, and the extent of the reverse reaction was insignificant.
However, an increase in initial water concentration
markedly decreased the rate of formation of methyl salicylate. To explain these results, a kinetic expression based on a Langmuir-Hinshelwood model has been developed which provides a satisfactory fit of the data over the range of conversion.
reports of esterification of acids with alcohols using strong acid ion exchange resins as catalysts have appeared, which in many cases include kinetic studies ( 7 , 2, 73, 74, 16, 7 7 ) . T h e esterification of salicylic acid with methanol catalyzed by a strong acid ion exchange resin has been reported in an East German patent (78). Starting at this point, the esterification of salicylic acid with methanol using the acid form of a sulfonated polystyrenedivinylbenzene ion exchange resin. Dowex 50-W (X-8), as a catalyst was studied. Application of several kinetic approaches, wherein linear relationships are assumed between reactant concentrations in the external solution and those at the reactive sites in the resin, failed to yield a model which fitted the data over the entire range of conversions. In contrast, the existence of a nonlinear relationship to describe molecular sorption on ion exchange resins is suggested from the data of Bafna and Govindan ( 3 ) ,who found that Freundlich expressions fit their data for sorption of low molecular weight aliphatic and aromatic acids on cation exchange resins in the acid form. Considering that a linear distribution law may not be sufficient to describe the system fully, it became apparent that an approach involving application of the Langmuir isotherm might be suitable (72). Basically, such an approach assumes that a specific type of sorption takes place-namely, that association of one or more types of reactant or product molecules with the hydrogen counterions which are essentially fixed in place at sites near the resin skeleton causes local concentrations of the sorbed species to be different from their values in the pore liquid, even though a linear distribution law may correctly relate concentrations in the bulk liquid and pore liquid. Prior to this investigation, the Langmuir approach had been applied to ion exchange of alkali metal ions by Boyd, Schubert, and Adamson ( 4 ) .while during this investigation its application to vapor phase reactions catalyzed by a cation exchanger was reported by Kabel and Johanson ( 7 7 ) , who studied the acid-catalyzed dehydration of ethanol. More recently, Frilette. Mower, and Rubin ( 6 ) have successfully formulated UMEROUS
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FUNDAMENTALS
a Langmuir-Hinshelwood model for the dehydration of tertbutyl alcohol (2-methyl-2-propanol) on an ion exchange resin in the acid form. T h e model chosen was one in which competitive chemisorption of water and alcohol is postulated. A similar model appears to be obeyed in the system investigated in this study, in that competitive chemisorption of water and salicylic acid at the proton sites can successfully explain the data, the existence of the protonated acid as the reactive intermediate being supported by the mass of evidence accumulated from studies of homogeneous esterification reactions ( 9 ) . In this paper, experiments are described which delineate the effects of catalyst size, agitation, initial water concentration, and reverse reaction in the esterification of salicylic acid with methanol. After testing the data with several Langmuir models, one was selected and is discussed in detail. It fits the data over the range of conversion from 0 to 84.5%. Thus, it is believed that some insight into the mechanism of the catalytic reaction has been obtained. Experimental
Analytical, Methyl salicylate, methanol, and water were determined by vapor phase chromatography using an F 8L M Model 500 instrument. An internal standard of 10 weight % dimethylformamide (DMF) was used. The instrument was operated as follows: Column. 10-foot 10% Carbowax 1500 (polyethylene glycol, molecular weight 1500) on Fluoropak 80 (polymeric fluorocarbon) Injcction port temperature. 250' C. Block temperature. 300' C. Helium flow rate. 60 ml. per minute Column temperature. Programmed from 100' to 180' C. at 1I C. per minute Sample size. 2 PI. Elution time, Methanol, 1.5 minutes; water, 2.5 to 2.7 minutes; dimethyl formamide, 6.5 minutes; methyl salicylate. 13 to 13.5 minutes Weight per cent compositions were obtained by determining ratios of areas of peaks to that of the D M F peak and comparing these to the calibration curves. The estimated precision of the
data for methyl salicylate and water is 1 3 to 5y0,while for methanol it is = l o % . I n some cases, the water analyses were checked by titration \vith Karl Fischer reagent by standard procedures, while salicylic acid \vas deterinined by titration \vith 0 . 1 s sodium h)-droxide to a phenolphthalein end point of a methanol solution of the sample. Estimated precision is 3 to 5Yc. Materials. Methanol used was Fischer analyzed reagent. Salicylic acid was Heyden-Newport U.S.P. crystals and methyl salicylate was obtained from Matheson Coleman 8r Bell. T h e cation exchange resin, Dowex 50-kV (X-8), was obtained from the J. T. Ba.ker Chemical Co, as Baker's analyzed product in the acid form and \vet Lvith water. T h e catalyst was then treated to remove as much of the water as possible. A iveighed quantily of catalyst \vas charged to a roundbottomed flask and covered with methanol. T h e mixture was heated to the boil and airter a short Lvhile filtered on a sintered glass funnel. After \vashing with dry methanol, the resin was recharged to the flask and the procedure repeated tmice again. After the third treatment, the catalyst \vas left in methanol overnight and \vas filtered and \vashed with dry methanol just prior to charging. Then the catalyst \vas either air-dried for several minutes or placed in an oven a t 85' C . for 15 minutes. P r o c e d u r e . Runs 1 to 8 were made with 3.2 gram-moles of salicylic acid using a 3-liter, three-necked round-bottomed flask, while runs 9 and 10 were made with 7 moles of salicylic acid in 5-liter flasks. 'l'he catalyst and a portion of the methanol were charged to the flask and heated to 63-65' C. with stirring a t the desired rate. X solution of the salicylic acid in the remainder of the methanol, and in the case of run 9, also the methyl salicylate, heated to .55' to 65' C. was then added rapidly to the stirred catalyst. After approximately 1 minute's stirring, to allow for complete mixing, the zero time sample was taken. T h e reaction mixtures were heated rapidly to reflux (68-69' C., 2 to 5 minutes required:) and stirred a t reflux for the duration of the experiment. Samples were taken in a 20-ml. pipet (for runs 9 and 10, 25-n1.1. samples were taken), transferred to bottles, rapidly cooled in an ice bath, and then held in a freezer until they were analyzed. I n general, five samples were taken in the first 8 hours and then two each on the second and third days and, \vhere applicable, one each day thereafter.
*
Reaction conditions are summarized in Table I. In all runs, the weight of resin used was such that 268 meq. of hydrogen ion per mole of salicylic acid were obtained; the initial concentration of methyl salicylate was zero except for run 9, where it \vas 0.21 mole per liter. Results
Conversion of salicylic acid to methyl salicylate as a function of time is shown in Figures 1 through 4. Figure 1 presents the effect of stirring rate on the rate of formation of methyl
Table 1.
Stirrer Speed, R.P.M. 200 175 200 250 250 250 250 250 300 300
Run ~VO.
1 2 3 4 5 6
7 8 9 10
Reaction Conditions
(MeOH)i (SA)i 8.27 9 0 9 23 7 26 9 24 9 61 9.5 9.55 8.96 9.13
(HzOIi, Mole/ Liter
Catalyst
0.68
50-1 00
0 53 0 68 0 54 0 02 0 04 0.20 0.07 0.36 0.31
50-100 50-100 50-100 100-200 200-400 50-1 00 50-100 50-100 50-100
Size, Mesh
salicylate. Figure 2 illustrates the effect of particle size, while Figure 3 illustrates the effect of the reverse reaction. I n Figure 4,the effect of initial water content is presented. Discussion of Results
I n runs 1 through 4 (Figure l ) , the rate of agitation varied over approximately a 1.5-fold range, while the average initial water concentration remained constant at approximately 0.6 mole per liter. T h e reaction rate was found to be insensitive to stirring rate in the range studied; hence, it appears that bulk diffusion from the external solution to the catalyst surface is not the rate-limiting step in the reaction. With this background, the next variable explored was catalyst particle size, which ranged from 50- to 100-mesh to 200- to 400-mesh. Figure 2 shows the percentage conversion of methyl salicylate as a function of time for runs 5 , 6, and 8. The average initial water content was approximately 0.04 mole per liter for these runs. The results show that rate of conversion of salicylic acid to methyl salicylate is virtually unaffected by catalyst particle size, supporting the finding that bulk diffusion is not rate-limiting and further indicating that intraparticle diffusion is also not rate-limiting ( 8 ) . I n runs 9 and 10, the effect of the reverse reaction was explored (Figure 3). T h e initial water concentration was approximately 0.3 mole per liter in each r u n ; in run 9, the ratio of methyl salicylate to salicylic acid initially was 0.1, while for r u n 10 it was zero. Inspection of the graph reveals that the effect of preaddition of methyl salicylate is negligibleLe., the rate of the reverse reaction appears to be low in the catalytic esterification.
I
RUN # 0
v
d I
5
IO
15
Figure 1.
Run 2 Run 3 Run 4
I I 20 25 30 TIME (HOURS)
175 200 250
I 35
Effect of agitation
PARTICLE S I Z E 5 0 - 100 100 -200 2 0 0 -400
( H 2 0 ) i a 0.04 mol / I .
I
I
40
45
50
A
1'0
Figure 2.
1'5
20
3b (HOURS)
25
TIME
25
I
1
40
45
56
Effect of catalyst particle size VOL. 4
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Figure
410
3.
50 60 70 80 TIME (HOURS)
90
IO0
110
120'
The effect of the initial water concentration on the rate of reaction is demonstrated in Figure 4 for a series of runs in which (HrO) a n d that other species are less strongly associative, the expression would reduce, in the limit. to Equation 4. (-4 number of other plausible models. deriving from Equation 3, were also tested, but none fit the data ivell! leading to the trial of Equation 4.) Y
=
k b l (SA)(MeOH)/[ 1 $L b 3 ( H r O ) ]
(4)
or r = k'(S24)(MeOK)/[1
+
b3
(H20)]
(5)
To test this model, polynominal curve fits of the data for runs 9 and 10 were obtained with the aid of a computer. Next, the model itself was tested on the computer, which obtained the best match of the computed and experimental data.
reaction rate \\ith increasing concentration of water. Finally, a kinetic expression based on a Langmuir-Hinshelwood model has been developed which fits the d a t a over the entire range of conversions. Acknowledgment
RUN 9 o EXPT ‘L x THEORY
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.
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50 75 TIME (HOURS)
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5.
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O u r appreciation is extended to the American Cyanamid Co. for its support of this investigation. T h e authors also thank P. .4.Ruziska. P. Hetrakul. E. Elices. and G. de Vitry. former M.I.T. Practice School students. for their assistance in the laboratory and Joseph Morath for his aid in establishing vapor phase chromatography methods. H. Alcalay furnished many helpful suggestions and our gratitude is expressed to E. H . Keefer and R . G. BrL iner for their aid in the computational xvork. literature Cited
Test of model
From this procedure the appropriate values of k‘ and 6 3 were determined as 3.11 x liter gram mole minute and 2.02 liters per gram mole. The former result seems reasonable for a reaction of this type \\.hen compared with the results of other resin-catalyzed esterifications (73). Furthermore. as may be seen from Figure 5, the agreement between the theoretical model and the actual data is satisfactory. T h e results lend support to the hypothesis that the rate-limiting step in this system is the reaction between protonated salicylic acid and methanol in solution, T:;ith the reaction product. Lvater: thence competing with salicylic acid for the remaining catalytic sites and forming hydrated protons which are inactive u i t h respect to the catalysis of the er1:erification reaction. Summary
Bulk diffusion is not a limiting step in the reaction over the range of experimental conditions. Intraparticle diffusion is not rate-limiting I t is suggested that the reaction product, water. forms hydrated ions M ith the hydrogen counterions of the catalyst which are inactive for the esterification of salicylic acid with methanol, accounting for depression of the observed
(1) Andreas, F., Chem. Tech. (Berlin) 11, 2408 (1959). (2) Andriano\-a: T. I.: Bruns, B. P., Kznetika i K a t a l i ~ 1 , 440 (1960). (3) Bafna, S. L.: Govindan? K. P., Ind. Eng. Chem. 48, 310 (1956). (4) Boyd. G. E.: Schubert, J., Adamson, \V.A , J . A m . Chem. Soc. 69, 2818 (1947). (5) Davies, C. iV.>Owen, B. D. K., J: Chem. Soc. 1956, pp. 1676, 1681. (6) Frilette, V. J.: Mower, E. B., Rubin; M . K., J . Catalysis 3, 25 (1964). (7) Glueckauf, E., Kitt, G. P., Proc. Ro). Soc. 228a, 322 (1955). (8) Helfferich, F., “Ion Exchange,’‘ pp. 50iff. ,McGraw-Hili, New York. 1962. (9) Hine, J.. “Physical Organic Chemistry,” 2nd ed., pp. 275-80, 285-7, McGraw-Hill, S e w York, 1962. (10) Hinshelivood, C. iY.,“Kinetics of Chemical Change,” Oxford University Press, Xew York, 1940. (11) Kabel: K. L.: Johanson, L. N., 4 . I. Ch. E. J . 8, 621 (1962). (12) Langtriuir! I., J . Am. Chem. Soc. 38, 2221 (1916). (13) Levesque. C. L., Craig, A. M., Ing. Eng. Chem. 40, 96 (1948). (14) Nicolescu, I. \’., Suceveanu, A., Angelescu, E., Acad. Rep. Populur Romine Sludii Cerceturi Ckem. 7, 621-30 (1959). (15) Reichenberg, D., il’all, \V. F., J . Chem. Soc. 1956, 3364. (16) Saletan, D. I., iyhite, R. R., Chem. Eng. P r o ~ r .Symp. Ser. 48, NO. 4: 59-74 (1952). (17) Sussman, S., Ind. Eng. Chem. 38, 1228 (1946). (18) Vasilescu, V., Ger. (East) Patent 10,808 (Nov. 28. 1955). RECEIVED for review April 14, 1964 ACCEPTED .4pril 19. 1965 Division of Physical Chemistry, 146th Meeting, ACS, Denver, Colo., January 1964.
SIMULTANEOUS ABSORPTION OF HYDROGEN SULFIDE AND CARBON DIOXIDE IN AQUEOUS HYDROXIDE SOLUTIONS G I A NN I A STA R I TA A N D
FR A N
c0
G I 0 IA
absorption of hydrogen sulfide and carbon dioxide is commonly carried out in industry. T h e two gases have analogous chemical properties, so that simultaneous chemical absorption is easy. Selectivity for hydrogen sulfide mav be of interest, because often H I S is actually the only component Irhich needs to be eliminated from the gas phase. This paper discusses the phenomenon of simultaneous absorption in aqueous hydroxide solutions. 1 Present address, Chemical Engineering Department, University of Kansas, Lawrence, Kan. IMCLTANEOUS
,’
Istituto di Chimica Industriale, Uniuersity o j A’uples: Naples, Italy
Theory
Chemistry of the Process. When COPand H 2 S are being absorbed in a hydroxide solution, the following reactions should be considered :
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