Sept., 1955
ACIDITYAND POLYMERIZATION ACTIVITY OF SOLIDACIDCATALYSTS
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ACIDITY AND POLYMERIZATION ACTIVITY OF SOLID ACID CATALYSTS BY 0. JOHNSON Shell Development Company, Erneryville, California Received February 96,1066
A method has been developed for the determination of the acidity of solid surfaces which consists of the titration of the solid suspended in benzene with n-butylamine, using p-dimethylaminoazobenzene as the indicator. This basic indicator forms a red color on acidic solids, and the end- oint of the titration is the disappearance of the red color. It is shown by comparison with solid acids whose acid strengtg is known in aqueous solution that the amine titration method determines acids comparable in strength to those with a dissociation constant of 10-6 or higher in an aqueous solution. Propylene polymerization by acidic catalysts was used to study the correlation of acidity and catalytic activity. The propylene polymerization reaction a t 200" was found to follow the equation dx/dt = k(a - z)/(1 bz), corres onding to a firstcorder reaction retarded by products (a = initial pressure of propylene, z = pressure drop in time t ) . %he catalysts used were a series of alumina-silica catalysts prepared by the interaction of one silica gel with measured amounts of alumina sol. There was found to be a linear relation between the acidity of these alumina-silica catalysts and the propylene polymerization rate. It is concluded that the amine titration adequately measures acid sites on the solid catalyst which are important for pol mcrization of propylene a t 200". It is also shown that for a given number of acid sites the polymerization activity marked& increases with acid strength.
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The understanding of the acidic nature of a solid surface is of great importance in the study of acidic catalysts for cracking, polymerization, alkylation and isomerization reactions. It is generally agreed that catalysts for these reactions are solid acids; it remains to be shown both the total number of acid sites on the solid surface and their acid strength. Several methods for the determination of number of acid sites have been used for alumina-silica catalysts. 1 They include titration in aqueous solution with standard alkali, evolution of COZ from Na2C03solution, hydrolysis of sucrose, evolution of CHI from Zerewitinoff reagent, adsorption of quinoline vapor2 and adsorption of NHs a t elevated temperatures. The acidity determinations which are carried out in aqueous solution give acidity values which probably include all the surface hydroxyl groups. In principle the adsorption of basic gases at elevated temperatures should give accurate values for numbers of acid sites; actually such measurements are difficult to interpret because of very strong physical adsorption by solids such as alumina-silica catalysts. The method which will be presented here falls between the extremes of aqueous titration and gas adsorption. It involves the titration of the solid acid with an organic base with benzene as the solvent and p-dimethylaminoazobenzene as the indicator to determine the end-point. Acidity of Solid Surfaces by Amine Titration.G. N. Lewis first reported the titration of a solid acid using an indicator dye to determine the endpoint. He showed that AlCll suspended in CC1, could be titrated with an amine solution using crystal violet as an indicator. This suggested that solid acid catalysts could be titrated in the same way. Crystal violet did not give an acidic color on alumina-silica catalyst so other basic indicators were tried. Indicators with low pK values were chosen since it is desired to titrate strong acids on the solid surface. In the titration of a solid acid the basic indicator forms a colored compound with the acid sites on (1) See the review articles by M. W. Tarnele, Faraday S O C .Discs., 8 , 270 (1950); T. Milliken, G..A. Mills and A. G. Oblad, ibid., 8, 279 (1950). (2) G. A. Mills, E. R. Boedeker and A. G. Oblad, J . Am. Cham. SOC.,72, 1559 (1950).
the surface; upon addition of the titrating base, which must be a stronger base than the indicator, the indicator base is displaced by the titrating base, and the color disappears or changes. Since the p K of the indicator is the factor determining the level of acid strength3 of the acid sites which are titrated, all of the indicators in Table I were tried. TABLE I DATAO N INDICATORS USEDFOR AMINETITRATION OF SOLID ACIDS Indicator
pX
p-Ethoxychrysoidin Aminoazoxylene p-Dimethylaminoazobenzene 2-Amino-6azotoluene Benzeneazodiphenylamine 4-Dimethylamino-azo-1-naphthalene Crystal violet
5 3.5 3.3 2.0 1.5 1. 2 0.8
Color change (base-acid)
Yellow-red Yellow-red Yellow-red Yellow-red Yellow-purple Yellow-purple Blue-yellow
All the indicators except crystal violet gave acid colors when added to alumina-silica and to acid salts such as KHS04, but did not give acid colors on pure silica gel or pure alumina. Thus, it appeared probable that the indicators were in the proper p K range for titration of acid catalysts. Since no appreciable difference in total acidities was found when different indicators were used, pdimethylaminoazobenzene (butter-yellow) was chosen for further study because it gave the most distinct end-point. Benzene, CCla and isooctane all appeared satisfactory as solvents for the titration; benzene was chosen because it is readily available in dry anhydrous form. Basic solvents such as ether, acetone or alkyl acetates are not suitable. Several organic bases of differing basic strength were tested for the titration of solid acids. Trimethylamine (k = 7.4 X gave satisfactory titrations, but the titrating solution required frequent standardization due to volatility of triand methylamine. Aniline (k = 4.6 X quinoline (k = 1 X both gave dark products on some catalysts which interfered with the observation of a color change; they also required use of the indicators which are weaker bases than (3) C. Walling, ibid.. 72, 1164 (1950).
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butter-yellow. Benzylamine (IC = 2 X and n-butylamine (IC = 4 X gave completely satisfactory results. n-Butylamine was chosen for use as the titrating base and was used in all subsequent experiments. The following titration technique wns developed and has been used in carrying out the titrations of a large number of solid acids. The catalyst sample is 0.1 g. of 100-200 mesh powder which has been dried for 2 hours a t 500". It is placed in a 10-ml. stoppered bottle, and 5 ml. of dry benzene containing 0.2 mg. of p-dimethylaminoazobenzeneper 100 ml. is added. The yellow dye reacts with the solid to form the red acidic form of the indicator. Then the base, 0.1 N n-butylamine in benzene, is added dropwise from a calibrated pipet. The neutralization is followed by noting the gradual disappearance of the red color of the solid particle. The end-point, which is quite sharp, is taken as the point a t which all the red color disappears. A 2-3 day period is allowed for the titration since gradual addition of amine will minimize adsorption of the amine on non-acidic portions of the surface. I n order to ensure that no excess amine is present on the solid at the end-point, a drop of 0.05 N trichloroacetic acid in benzene is added. A reappearance of the red color indicates that no appreciable excess of amine was present. Small overtitrations can be corrected for by subtracting the amount of trichloroacetic acid required to restore the red color. This correction can only be used if small amounts of trichloroacetic acid (
