I Heterogeneous Catalysis

---

Alberto Nieto Loborotorio de Fisicoquimico FOCUI~O de~ Quimico Montevideo, Uruguoy

I

Heterogeneous Catalysis Resin-cak~l~zed saponification o f methyl acetate

This experiment was designed to illustrate a heterogeneous catalyzed reaction that may also be carried out with soluble acid (homogeneous catalysis). The reaction studied is: CH3C02CH3 Hz0 CHBOH + CH3COzH. Methyl acetate hydrolysis is followed measuring the acid by titration. The experimental procedure is a simplified version of that described by Haskell ( 1 ) and may be finished in 3 hr by students. Castellan ( 2 ) and Barrow (3)may be used as introduction to heterogeneous catalysis and its reaction mechanisms. Additional data on ion-exchange resins and their use as catalyzers may he extracted from Kunin ( 4 ) , Samuelson (5), Bemhard et al. (6, 7), and Reisz (8). All the experiments were performed at 37 + 1'C, and special care was taken to avoid temperature variations in the solutions. Sampling was carried out and the flasks were reelased as fast as possible to avoid volatile compound evaporation. The composition of the ester solution is: acetone, 70 ml; methyl acetate, 2 ml; and distilled water up to 100 ml. Three Erlenmeyer flasks, each containing 100 ml of ester solution, were stoppered and placed in the thermostatic bath. Three samples of Dowex 50W-X8 were weighed; one of 10 g, another of 15 g, and another of 20 g. Each of them was added to one of the thermostated flasks, taking this moment as zero time for the reaction. The flasks are stirred manually at 1-min interva!~. Samples are withdrawn by decanting the resin and pipetting 1.0 ml of reaction solution to a titration vessel containing 20 ml of distilled water and 2 drops of 0.5% phenolphthalein, at 20, 40, 60, 80, 100, 120, and 140 mi" of reaction. Then each sample was titrated with 0.01 NNaOH, and the volume of NaOH used was called x. If a represents the volume of 0.01 N NaOH equivalent to an acetic acid solution of the same molar concentration of the initial estei solution, the integrated first-order law may be written

+

=

2.303 log ( a / ( a- x)) = k't When log (a/(a - x ) ) is plotted against t, k' is obtained from the slope of this graph. These plots were constructed by the least squares method for the three resin concentrations; those from student I are shown in the figure. The correlation coefficient r and the probability p of obtaining the given value of r by chance were computed for each plot. Since the reaction volume was constant, the resin weight is proportional to acid (catalyzer) concentration. If k' is proportional to acid concentration: k' = k (resin weight); and k must be constant for a given resin under fixed reaction conditions (temperature and reaction volume). The table shows the k', k and p values from seven students. The values of p for the 21 plots reflect the coincidence of experimental data with the first-order law. The Chauvenet criterion was used to select the data from the 21 determinations of k, and none of them must he excluded. The average value of k and its mean deviation found from these data are 6.6 0.3 X sec-l g-l.

*

846 /Journal of Chemical Education

Plot 01 the kinetic data at three different resin concentrations (from student I ) .

The statistical methods used proved that, from the experiment carried out by students, constant and reproducible values of k were obtained and that very good correlation between the experimental results and the first-order law was attained despite the simplicity of its design. Finally it may be noted that by working a t constant resin concentration and different temperatures, activation energy may he measured with the same experimental procedure. Literature Cited (1) HaskeI1.V. C..andHamrnet. L. P . . J Amer Chsm Soc.. 71.12&1(1949l. (2) Castellan. G. W.. "Physbal Chemistry" (2nd d l . Addison-WPP~~Y Publishing Ca. Inc.. 1967, p. M9. (a) Barrow, G . M.. "Quimiea Fiaica" (2nd ed.). Editorial h e r @ 1968, p. RII. (41 Kunin. R.. "Ion Exchanee Resins" (2nd d l , John Wiley & Sons. Inc., New Yoxk. 1958. p.249. (51 Samuelson. 0.. "Ion Exchanger8 in Analytical Chemistry," John Wiley & Sons. h e . . NewYork. 1953. p. 18. (6) Bernhard. S. A . sndHsmmet,L.P..J.Ampr C k m . Soe., 75,1789(19531. (7) Bemhard. S. A,. Garfield. E.. and Hammet. L. P.. J. Amsr. Cham. Soc.. 16. 992 ,,OCA>

Ex~erimentalData Obtained bv a Groue of Seven Studentsa k = k'jrerin Student

Resin Weight tdl

k' x l(r

weight (~103

Probability inf 0.001 inf. 0.00, inf. 0.001 0.01,s

QUO7 0"ll 0.002 info.Ou1 "O"2 ini. Onnl i"L0OOl in1 0.001 in1 0.0ill i"f.0.ll0l i"f.O."Oi 0.001 0.003 OUUL inf. 0.001 inf. 0.001 i"f.O.UO1