Clay Minerals as Solid Acids and Their Catalytic Properties A DemonstrationTest With Montmorillonite J. Heisen K. U. Leuven, Dept. Metaalkunde, de Croylaan 2, 8-3030, Leuven, Belgium
In a recent issue of THIS JOURNAL White and Campbell ( I ) stressed the fundamental importance of surfaces and their role in heterogeneous catalysis. The feasibility of a great number of industrial chemical processes depends upon the catalytic properties of a series of inorganic solids as metals, oxides, or silicates. As an example, White and Campbell described an apparently simple process namely the reaction of carbon monoxide and oxygen, an infinitely slow reaction hut easily triggered by metal catalysts such as Pt, Pd, Rh, or 11. An abundance of examples of reactions catalyzed by solid catalysts is available. One type of surface, however, is particularly intriguing because it may be of fundamental importance in the understanding of the origin of life on earth. Clay minerals may have interfered in an important way in the prebiotic world as an adsorbent for the simple organic molecules formed in that period and subsequently as catalysts in the formation of oligomers out of these simple molecules. Lahav, e t al., (2)report experiments with two typical clay minerals, kaolinite, and bentonite, on peptide formation in what they consider as typical effective prebiotic conditions. The clays were subjected to subsequent wetting and drying and to temperature fluctuations in the range of 25 to 94%. The wetting was done using a solution 23 mM in glycine during 2 to 3 days. Drying was done a t 60°C for 1or 2 days. After 21 wetting-drying cycles an amount of 2.0nmolediglycine per milligram clay, 0.4 nmole triglycine and 0.3 nmole tetraglycine could he desorbed from kaolinite. These figures were, respectively, 8.0, 0.6, and 0.0 mmolelmg clay for bentonite. The duration of the experiment was, of course, out of comparison with geological periods but exactly that is making the results even brighter. The concentration of oligomers obtained after such a short time is certainly aremarkable result. A survey paper on this subject was published in 1976by Lahav and Chang (3)and is recommended for further reading. I t is clear that much of the information drawn from the experimental material is highly speculative but the data a t least prove the possibility that clays may have been active participants in chemical processes of the prebiotic world. Another example of work with a similar aim is Nobel prize winner Steven Weinberg's book "The First Three Minutes," published by Basic Books, Inc., New York, 1977. In this paper we will give more detail on the basis of the catalytic power of that very common material "clay" with fascinating properties governing our every day lives by its relation to plant growth, its importance in the retention of fertilizer and organic matter, and its use for brickmaking, etc.
thorough scientific thought. The acidity scale was determined from the color chanee of a series of indicators belonging to the nitranilines and bei;aving as uncharged ~ r ~ s t e d l b a i e s : with B the neutral base and BH+ its conjugate acid. The acidity function, determined by Hammett and Deyrup, is defined by the equation:
where KRH-denotes the thermodynamic ionization constant of theconiueate acid.. IBI . . and lBH+I the concentration of the indicator and itsconjugate a c i d , o ~the + proton activity, and f the activitv coefficient. Accordina to Devrur, and Hammett iloc. cit., p. i723) "the ualue of ~ o i s d e f i n i t eand independent, of the oarticular indicator used to measure i t to the extent that o h fundamenrol ossurnption that the ratio fdfnlc+ is the some fur different bases in a riven aolution is exact" and in that way a unique acidity scale% obtained. The appearan'ce of the color of the conjugate acid indicates Ho to be equal to or lower than the pK, of the indicator. The nitranilines interact with Bransted acids as well as with Lewis acids. The indicators are chosen so that their pK's cover the whole acidity range. The Hammett acidity function has been reyiewed by Paul and Long (5). Triphenylmethanol was rejected ai an indicator in the original work of Hammett because cryoscopic measurements had indicated a behavior different from the reaction scheme given above. Gold and Hawes (6).however, derived and tested expressions for the relation between degree of ionization of triarylmethanols and acidity:
.-
ROH+H+*Rt+HzO
where ROH denotes (Phenvh-COH . " and R+ the carbonium ion of triphenylmethanol. The thermodynamic ionization constant K of this reaction may be written as follows:
[ ] are the analytical concentrations, a is the activity, and [ROH]/[R+] is the experimentally observable concentration ratio of the indicator. After taking the -log of eqn. ( 2 ) and rearranging, we find: -pK
+ log- [ROW = -log an+ + loga40 [R+1
= Ho
The Acidity Function
As we will demonstrate below, the catalytic properties of clays are attributed to the acidity of the surface. In order to understand how solid acids are described, we will look first a t how concentrated solutions of strong acids are approached. For the description of strong acids in solutions, Hammett and Deyrup expanded in 1932 (4) the acidity scale from dilute solutions up to the acidity of 100% H2S04 Incidentally, Hammett's paper may stand as an example of clarity and
+ log anZO= JO
(3)
Ry eqn. (3) n new function ia defined which is equal to the Hammett acidity function plus the log of the activity of water. 'ihe ratio of the activity coefficientn of [ROHI and [Rt] is also suo~osedto be unitv. This deduction is simolified. For a more ri&rous deduction; the paper of Gold and Hawes (loc. cit.) should he consulted. The use of triphenylmethanol allows the discrimination between Brdnsted and Lewis acids hecause of its exrlusive reaction with Bransted acids. Volume 59
Number 12
December 1982
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Solid Adds In 1950 Cheves Walling (7) extended the acidity concept of Hammet and Deyrup t o solid surfaces. The acidity of solid surfaces is described as the ability of the surfaces to convert an absorhed neutral base to its conjugate acid. As for solutions, the strength is measurable hy observing the color change of a series of indicators, i.e., the appearance of the color of the conjugate acid indicates that Ho of the surface is lower than the pK, of the indicator used. Although subject to some restrictions, this method has been employed and proved to be useful on a large scale in studies of the relation between acidity and catalytic activity. Discrimination between Lewis and Bransted acids poses the same problem as for solutions. A concise text about solid acids and bases is found in the book of Kozo Tanabe (8). Clava and -.-, - and -..- Solid - ...- Acids ... .-- -. .- Catalvsts - -.-.,. .. Most of the exveriments with resved to catalvtic . properties . of clays have been executed on kaoiinite and montmoAlonite whirh are typical representatives of, respectively, 1:l and 2 1 type phyllosilicates. A 1:l type mineral means that the individual clay vlatelew consist of a network of tetrahedra with silicon in [hk center and oxygen at the edges and a network of octahedra with aluminium in the center and oxygen and hydroxyl groups at the edges. Both "layers" are linked together by sharing oxygen atoms a t the edges. A 2 1 type mineral has a similar structure, but the octahedral layer is linked on both sides to a tetrahedral layer. The basal spacing (001-reflection) is about 7.1 .&forkaolinite and 9.6 .& for dmmontmoriuonite. It is evident that such dimensions are ahre to result in very high soecific surface areas. For montmorillonite it adds uo to 8 6 milg! Details about their structures can be found i n t h e clay mineral literature (e.g., references (9)and (10)). In what follows, the treatment is limited to a few examples in which the acid or catalytic properties of clays are well established. Only montmorillonites will be considered. Solid bases will not be discussed. The catalytic properties of clays are generally ascribed to the Bransted and Lewis acidity of the surface. Bransted acidity is the ability to donate protons, and Lewis acidity is the ability to share lone electron pairs. Lewis acidity of clay minerals is due to the presence of threefold coordinated aluminium ions exposed at the edges of the platelets, but this type of acidity will not be discussed in this paper. A great number of clay minerals contain isomorphons substitutions in their lattice. In montmorillonite AP+ may be substituted for Mgz+ in the octahedral layer. This gives rise t o an unbalanced neeative charge which is com~ensatedfor by adsorption on the surface of monovalent or polyvalent cations. These cations are exchangeable and are coordinated to water. The water molecules exhibit the peculiar property of being strongly dissociated, a t least when the amount of adsorbed water is at the double layer level or less. The enhanced dissociation is ascribed to the polarization effect of the cations to which they are coodinated. So, the surface concentration of mobile protons is increased which is responsible for the Bransted acidity. This type of acidity is dominating and the acid strength is increased on dehydrating; for a given degree of hydration it will depend upon the polarization effect of the exchangeable cations (11-15). I t can be demonstrated by many experiments. A first example is the decompositon of cobalt(II1)hexammine ions on montmorillonite studied by Chaussidon et al. (14) and Fripiat and Helsen (15). The proposed reaction is ~~
~~~
61C0(NH&~+l.d. + 12H20 * 6(Co(OH)2+).dS + 181NHl+l.*
+ 16NHa + N2
The reaction is the same as in solution and is acid catalyzed in solution as well as on the surface. Another example is the study of Mortland and Raman (13; 1064
Journal of Chemical Education
fimoleR+/rng clay ionic~adius(A)
25
24
23
40
28
0.68
1.47
1.67
0.66
0.99
23
19
1.12 i.34
16, p. 344 and ff.) of the acidity of montmorillonite, not by adsorption of indicators but through the ability of the surface to convert ammonia into ammonium according to the equilibrium:
+ INHalgaa* IM(HzO)~-~OH+"-~ + NH4'I.d. IM(H20)x+"l~d~ These authors could also state the following points. The exchangeable cations have a significant rffect on the formation of NH4-, and the degree of hydration of clay has a signillrant effect on the protonation of ammonia. In both examples, I R spectroscopywas used as the analytical tool. A last example is the study done by adsorption of triphenylmethanol. This dye, as already stated, only reacts with Bransted acids and has been applied as such for the determination of the aciditv streneth of montmorillonites. In their work on the interaction montmorillonite-triphenylmethanol. Helsen t I 1 ) and Helsen et al. i12) use the J Junction, introduced by dold and Hawes (6) for the quantitkve study of the surface aciditv. The maximum amounts of R+, the carbonium ion of triphe&nerhanol, formed on the surface of montmorillwites, heing homoionic in a series of alkaline or alkaline earth metal ions, are summarized in the table. The ionic radii of the exchangeable cations are also given. As the radius increases, the charge density and the polarizing action on the coordinated water molecules decreases, which in turn is reflected in the amounts of R+ formed. With the help of the functions determined by Gold and Hawes and the ratio [ROH]/[R+] determined spectrophotometrically, the acidity of the surface was compared with the acidity of solutions of sulfuric acid-triphenylmethanol giving the same concentration ratio [ROH]/[R+]. I t was found that, for example, Li-montmorillonite at 36% with a monolayer of water has an acidity comparable to a solution of 1%H2S04 in acetic acid. As can easily be seen in the table, the acidity is decreasing with increasing ionic radius. Na+ and K+ are dropped from the homologous series hecause their hehavior is more complex due to some particularities of the clay surface, which are beyond the scope of this ----..-:--A:-..
IjuIIIIIIUJllcaWuII.
Demonstration Test According to the work of Helsen (11) and Helsen et al. (12), a qualititative demonstration test of the formation of triphenylcarbonium ions on montmorillonites a t different hydration levels can be introduced easily. For that purpose, a few milligrams of montmorillonite or bentonite are saturated with Li+, Na+, or Ca2+by suspending the clay in a solution of the corresponding metal chloride. After washing by centrifugation with distilled water and drying (the clay is easier resuspended when freeze-dried, hut it is not strictly necessary), the whole amount of clay is treated with 2 ml of an ethanolic solution of 5 mg triphenylmethanol per 100 ml. The suspension is poured on a glass object and allowed to dry. At room temperature a white layer is obtained. When heated to approximately 10O0Cthe color will turn to yellow, the typical color of the carhonium ion (a doublet absorption peak centered around 420 nm with a molar extinction coefficient of 38,000). The color disappears on cooling and rehydrating. The wetting-dryingsequsnce described may be compared to what has been said in the introduction about the prebiotic processes. - -
~
~~~
Concluding Remarks
This DaDer . . is limited to montmorillonites. We consider this a fair restriction because it is one of the most intensively studied clay minerals. Our concern is to hrine to the attention of the reader an aspect of solution chemistry which is also relevant to the study of solids as well. Particular attention was paid to those solids witha vast influence on our everyday life namely the solid catalysts. The formation of carbonium ions on montmorillonite may be used as a demonstration of the presence of surface acidity, of the enhanced dissociation of water molecules when polarized by cations, and of the way the surface can interact with oreanic substances. Carhonium ions are well recoenized intermediates in many reactions (17). The future deielopment of the study of solids from this point of view will certainly provide more evidence for the intensive interaction of the clay with its surroundines. Exam~lesto he found in the current literatureabout thecatalysis by montmorilloniteand kaolinice of the conversion of DDT to other ornanic derivatives or the decomposition of pesticides suppo& this viewpoint (10, 18). In modern research, however, it must be recognized that the indicator method is gradually overwhelmed by methods such as IR, EPR, and related spectroscopic techniques. Nevertheless, it remains important to include the indicator method in introductory courses about acidity, the role of solid acids
and bases, and the related catalytic properties not only for historic but also for conceptual reasons. Acknowledgment .
Prof. P. J. Slootmaekers of the Dept. of Chemistry is gratefully acknowledged for the critical reading of the text.
Literature Cited (1) White, J. M. andCampbel1,C. T..J. C-.EDuc., 51,471L414(1984). (2) Lahav,N.,White, D., and Chang, S., Scienn,M1,6749(1978). (3) Lahau, N. aod Chang, S.,J. M d E~I.,8,357380 (1976). (4) Hmett, L. P. and Dey~p,A. J., J Arne,. Chom. Sm.,54,2721-27.39(1932). ( 5 ) Paul, M. A. and Long, F. A,, ChernieolReuiem.57,145 (1957). (6) Gold,V.aodHavw,B.W.V.. J. ChemSoc.,210%2111(1951). (7) Walling, C.,J Amer. Chem Sac,12,116L116811950). 18) . . Tsnabe. Koeo. "Solid Acids and Bases." Kodanahs.. T o h Academic Press. New ~ ~ ~ k 19:70.h ~ d ~ ~ ~ (9) Bmm, G., (Editor), 'TbeX-Ray IdentitieationandCmMStmdlusaof Clay Minerals: Mineralogical Soeiety,London, 1912. (10) Then& 8. K. G.,"The Cherniatrvof Cl8y.OnanieReaetions,"AdamHilre.London,
..
(13) Martland. M. M andRaman,K. V.. Cloya and Clny Mi~mla, 16,39&38S (1%). (14) Chaussidon, J., Calvd, R., Hehen, J., and Fripiat. J. J.. Nature, 196. 161-162 (1962). 4thNat Canf. on Clay. (15) Fripist,J. J. and Helaen, J., Cleys ond Cloy Minemls,h. and Clay Minerals. Bukeley, CA, Pergamon Preaa, Oxford & Nw Ymk, 1966. 163-179. (16) Little. L. H., "Infrared SpeeVa of Adsodsobe3Swiss:' Academic b a a . h d o n aod New Vork. 1965. (17) Olsh. G. A. and Von R S$hleyet,Paul, '"CarboniumIona:' Volume I. Inteneienee Publiakrs,New York, London, Sydney, 1968. (U)Frenkel. M. and Solomon,D. H., Clay8 and Cloy Minemlr, 25.463464 (1977).
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