Oct., 1958
STUDIESOF SILICATE MINERALS
1217
STUDIES OF SILICATE MINERALS. V. A QUANTITATIVE DETERMINATION OF THE ACID STRENGTH OF EXCHANGE SITES ON ATTAPULGITE BY J. J. CHESSICK AND A. C . ZETTLEMOYER A Contribution from the Surface Chemistry Laboratory, Chemistry Department, Lehigh University, Bethlehem, Penna. Received March 8, 1068
A quantitative determination of the acid strength of exchange sites on attapulgite clay was carried out. Samples first activated under vacuum at 400" were exposed to saturated butylamine vapor at 25". The total amount of amine chemisorbed and amounts desorbed at temperatures between 25 and 400" were determined. With this information heats of immersional wetting in butylamine were then measured as a function of surface concentration. Differential heat and site energy distribution curves were calculated from the integral heat of immersion data. Thus, the spectrum of acid strengths for the exchange sites on attapulgite was developed.
Introduction The surface acidity of solids is important in many technological applications including the use of ion exchange resins,l in drier metal loss to pigments in paint and ink systems on aging,2and in deciding the activity of cracking catalysts.s Indeed, most of the current interest in measuring surface acidity has been promoted by petroleum technology where the strongly acid sites on catalysts promote a carbonium ion mechanism. The question as to whether the sites are of a Bronsted or of a Lewis type has not been completely resolved, but the evidence is that even at cracking temperatures they are at least partly Bronsted in nature. Numerous investigations of the strength of these active sites on cracking catalysts has proceeded from early aqueous titration^,^ t o non-aqueous titrations to avoid the effect of water,4 and more recently to gas titrations5 with a base. From the fraction of base which desorbs slowly, the latter method allows some estimation of the number of effective acid sites and the average acid strength of the sites as measured by activation energies for desorption. Acid strength measurements are also provided by the simple indicator method of Walling6 which has been extended by Benesi'; from the color changes induced in adsorbed indicators, the acidity of at least portions of some surfaces have been found t o be tremendously strong. A rapid method for estimating the number of active sites is also provided by differential thermal analysis.* All of these methods, however, are incapable of providing a topographical picture of the number of acid sites and their individual strengths, that is, a site energy distribution. From heat of immersion measurements, just such a distribution can be derived. Immersion of the solid in a simple organic base, for example, can (1) "Technique of Organic Chemistry, Volume X, Fundamentals of Chromatography," Intersoience Publishers, Inc., New York, N. Y., 1957, p. 293. (2) (a) A. C. Zettlemoyer and D. M. Nace, Ind. Enw. Chem., 42, 491 (1950): (b) D. M. Nace and W. C. Walker, ibid., 46, 769 (1954). (3) (a) C. L. Thomas, ibid., 41, 2573 (1949); (b) A. Grenall, ibid., 41, 1485 (1949). (4) (a) M. W. Tamele, Disc. Faraday Soc., 8, 270 (1950); (b) 0. Johnson, THISJOURNAL, 89, 827 (1955). (5) (a) G . A. Mills, E. R. Boedeker and A. G. Oblad, J . A m . Chem. SOC.,7 2 , 1559 (1950); (b) R. L. Richardson and 8. W. Benson, THIS JOURNAL, 61, 405 (1957). (6) C. Walling, J . A m . Chsm. &e., 72, 1164 (1950). (7) H. A. Benesi. ibid., 78, 5490 (1956). (8) R. L. Stone and H. F. Rase, Anal. Chem., 29, 1273 (1957).
be followed by successive experiments with samples on which increasing amounts of the base are preadsorbed. From the gradual decrease in heat liberated a distribution curve may be estimated. Extension to bases of different strengths would allow a topographical map of surface acid strengths to be developed. Various aspects of the method have been demonstrated here with attapulgite clay since in its applications as an adsorbent the acid sites are of considerable interest. These initial studies were restricted to its interaction with n-butylamine. Further studies with other bases and other solids are in progress. Experimental Attapulgite clay is an acicular, hydrated magnesium silicate. The primary clay fibers contain channels which run parallel to the crystal length and which are ermeable only to small polar molecules such as NHa and &O. The needle-like particles are in loose, somewhat parallel aggregation. The sample was a carefully selected mine sample specially treated to make it grit free.9 The sample was washed well with distilled water to remove soluble materials, dried a t 50" in air and then crushed in an agate mortar. Certain characteristics of this sample of attapulgite will be compared briefly with those of other samples studied earlier .lo+ Such comparisons are of considerable interest beyond the present work because it appears likely that little work has been done with a really pure attapulgite. Careful X-ray work with the sample used here led to the conclusion that it contained as much as 10-15% montmorillonite. Ammonia sorption, especially on samples outgassed between 350 and 400", provides a simple technique for measuring purity; in this case it is higher than for the previous sample, thus suggesting the montmorillonite impurity since ammonia is readily imbibed between the montmorillonite.Ia The channels in the attapulgite outgassed a t such hi h temperatures are excluded. !because of the large water content of attapulgite and its variation with changes in relative humidity during storage it was necessary to establish that the initial water content of all the samples waa the same. A standard initial state was obtained by storing the clay in a desiccator a t 25" and 20% R.H. before use. Weight losses on evacuation were large and are expressed as per cent. dry weight after a 25" evacuation to constant weight under an ultimate vacuum to a previously adopted convena t 25" for 2 d
activated under two conditions, 4 hr., were determined. Irrered after evacuation at the upper
(9) Obtained from Minerals and Chemicals Corporation of America, Menlo Park, N. J. (10) J. J. Chessick and A. C. Zettlemoyer, THISJOURNAL, 60, 1181 (1956). (11) R. M. Barrer and N. Mackenzie, ibid., 88, 560 (1954). (12) A. C. Zettlemoyer, G. J. Young and J. J. Chessick, ibid.. 89, 962 (1955).
1218
J. J. CHESSICK AND A. C. ZETTLEMOYER
Vol. 62
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Fig. 1.-Surface
200
300
400
500
areas of attapulgite samples as a function of activation temperature.
- 8.0 - 7.0 c - 6.0
loo zoo aoo 400 soo Fig. 3.-Heats of immersion of clean and butylamine covered samples in butylamine (average of three runs) as a function of evacuation temperature. f i s t at 25" for 2 days, then at 400" for 4 hr. on the adsorption aDparatus; ( 2 ) the evacuated sample was exposed to saturated butylamine vapor a t 25" for 8 hr.; (3) the sample was then evacuated at 25" to constant weight. The gain in weight of the attapulgite sample after this cycle of treatment was attributed to chemisorbed butylamine. The amount of amine remaining on treated samples after evacuation a t 100, 200, 300 and M O O , respectively, was then determined. When the amount of amine chemisorbed on attapulgite was known as a function of activation temperature, the heats of immersional wetting of clean and amine covered samples in butylamine were measured. Calorimetric techniques and the method of purification of butylamine were previously described.Ia
Results and Discussion Surface areas as determined by nitrogen adsorption are plotted in Fig. 1 for the attapulgite used here and for two samples studied earlier.lO~ll The sample investigated by Barrer showed an abrupt decrease in surface area at evacuation temperatures near 105", associated, according to Barrer, with changes in the surface corrugations. The less abrupt decreases near the transition region recorded in the case of the two other samples may simply reflect the fewer measurements made. Nevertheless, the shapes of the curves are similar and differences most likely reflect differences in impurity content and initial particle size. The amount of amine remaining on 400" actiFig. 2.-The desor tion of butylamine on 400" activated vated samples after exposure to saturated butylamine vapor at 25" are plotted as a function of desamples, 88 a $notion of evacuation temperature. sorption temperature in Fig. 2. Temperatures up temperature. Most of the water lost was desorbed irreversibly and came from within the intercrystalline channels."J t o 400" are necessary to remove all the amine adThe entrances to these channels became blocked during sorbed at 25", Le., the butylamine adsorbs on activation due to structural changes of the sample. There- heterogeneous sites with a variety of adsorption after, water or other molecules were taken up principally on energies. The possibility exists that some of the the external surface so that the activation decreased the external area by nearly 50%. Activation for 2 days at 25" and amine remaining after evacuation at 25" is physically adsorbed. The criterion of irreversible ad4 hr. at 400" yielded a weight loss of 20.3y0and a nitrogen surface areaof 120 m.2/g. compared to 11.5% and225 m.9/g. sorption at 25" for chemisorption is somewhat for evacuation at 25". arbitrary. Results from the heat of immersion The surface characteristics of samples on which known amounts of butylamine were adsorbed, were investigated. The procedure was as follows: ( 1 ) the sLtmple was evacuated
(13) F. H. Healey, J. J. Chessick. A. C. Zettlemoyer and G. J. Young, THISJOURNAL,68, 887 (1854).
Oct., 1958
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STUDIES OF SILICATE MINERALS
1219
amine for a bare sample and for samples partially precoated with amine are plotted in Fig. 3. The fact that the heat of immersion value for bare samples activated at 400" agrees with the value for amine treated samples activated at 400" indi cates that all chemisorbed amine is removed by this outgassing treatment. From thesc -.sults in Fig. 3, the heat of adsorption of butylamine onto attapulgite can be calculated directly. The integral heat of adsorption from the gaseous state at 25" is the difference between the heat of immersion of the bare sample, ~ I ( s L ) ,and the heat of immersion of a sample covered with N , adsorbed molecules, ~ I ( s , L ) . Differential heats of adsorption, A H d , from the gaseous state onto attapulgite at 25' were obtained from a differentiation of the integral heat curve. These are plotted in Fig. 4 against the moles of adsorbed amine. The differential heats are analogous to isosteric heat values, although the heat of liquefaction has not been added to A H d . The heterogeneous nature of the strengths of the acid sites is reflected in this differential heat curve. A distribution function, g(r), can be defined so that the sites with energy between E and E de can accommodate g(e)de moles at STP of adsorbed gas.14 If the fall in heat of adsorption with increasing coverage is due to non-uniformity alone (a reasonable assumption in this case where the total number of active sites is only a fraction of all the sites), the function can be obtained from a slope of a plot of A H d , against moles adsorbed, N , where A H d o is the differential heat at absolute zero. Thus
+
'
BUTYL 2 AMINE 3ADSORBED 4 (MOLE 5S / C * C L A 6Y ~ I ~ L
Fig. $.-Differential heat of adsorption for butylamine adsorbed onto attapulgite from the liquid state at 25".
d N l d ( A H d = g(AHdo)
Localized adsorption is assumed and this condition is satisfied where chemisorption occurs. Such a distribution function for butylamine chemisorbed on this attapulgite is shown plotted in Fig. 5. Since differential heat values were not available at absolute zero, A H d values at 25" were used. The distribut,ion function, therefore, is not the precise operative distribution function since vibrational motions of adsorbed molecules are included in the energy terms. Furthermore, it should be appreciated that the double differentiation necessary to obtain A H d and g(E) is not sufficiently accurate E KCAL./YOLE. to show up minor variations. Nevertheless, both Fig. 5.-The distribution of energy of adsorption sites. the differential heat and distribution function measurements discussed below permit a more defi- curves serve to give a representative picture of the nite decision. energy distribution of acid sites. These curves The amount of amine retained by attapulgite are necessarily derived from the integral heat curve after outgassing at 25" amounted to about 70 which itself is the quantitative measure of the millimoles per 100 grams of clay. This value strength of the acid sites on this particular sample, seems high, particularly when compared to ionThe lowest heats of adsorption on the site energy exchange capacities. There is no doubt that the distribution curve, Fig. 5, are almost double the major adsorption of butylamine occurs on acid sites heat of liquefaction of butylamine. Therefore, it on the external surface of the attapulgite fibers. appears safe to conclude that the amount remaining Any excess amine uptake could be accounted for upon evacuation at 25" is chemisorbed. A sizable by strong sorption between platelets of mont- fraction was removed between 100 and 200°, but a morillonite particles present as an impurity, plus few very high energy sites do exist. any physically adsorbed amine. It should be reAcknowledgment.-This work was supported emphasized, however, that the primary aim here is by the Minerals and Chemicals Corporation of to illustrate a general method for obtaining a America, and wish to thank them for making the quantitative estimate of energy distribution for acid X-ray examination of the attapulgite sample. exchange sites on any high surface area solid. (14) L. E. Drain and J. A. Morrison, Trans. FaradaV Soc., 48, 316 Heat of immersional wetting values into butyl- (1952).
