REACTIONS CATALYSED BY MINERALS

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1 Clay Minerals (1968) 7, 389. REACTIONS CATALYSED BY MINERALS IV. THE MECHANISM OF THE BENZIDINE BLUE REACTION ON SILICATE MINERALS D. H. SOLOMON, B. C. LOFT AND JEAN D. SWIFT Division o[ Applied Mineralogy, C.S.I.R.O., Melbourne, Australia (Received 6 May 1968) ABSTRACT: The oxidation of benzidine to benzidine-blue on silicate minerals is shown to occur at aluminium atoms exposed at crystal edges and at transition metal atoms in the higher valency state present in the silicate lattices. The transition metals in the silicate lattice undergo chemical oxidation-reduction reactions. Changes in the colour of the benzidine-blue radical cation with the conditions of reaction are discussed in terms of the solvent used for the reaction and its influence on the ph of the mineral surface. INTRODUCTION The interaction of organic entities with mineral surfaces is of considerable theoretical and practical interest. For example, in the paint, plastic and rubber industries, the interaction of organic polymers with the minerals used as fillers has a direct bearing on the properties of the product. Similarly, in the petroleum industry the cracking or isomerization of hydrocarbons involves interaction between organic compounds and catalysts, some of which are derived from, or are closely related to, the clay minerals. Furthermore, field geologists and soil scientists have used the conversion of specific organic compounds to coloured derivatives as a quick test for identifying certain minerals. In this context, the conversion of benzidine to benzidine-blue by montmorillonite is well known, although considerable controversy exists as to the mechanism and specificity of this reaction (e.g. Page, 1941; Hauser & Leggett, 1940; Hauser, Le Beau & Pevear, 1951). Recent studies on the influence of minerals on the polymerization of organic monomers (Solomon & Rosser, 1965; Solomon & Swift, 1967; Solomon & Loft, 1968) and on cross-linking reactions between polymers have further emphasized the need for a better understanding of mineral-organic interactions, particularly B

2 390 D. H. Solomon, B. C. Loft and Jean D. Swift the nature and location of the active sites of a mineral surface, and the manner in which the sites activate the organic monomers. This paper is concerned with the mechanisms by which colour-producing reactions, particularly the benzidine to benzidine-blue reaction, can occur on a mineral and with the nature and location of the active sites on the mineral surface. A subsequent paper will be directed at applying the model developed for the benzidine-blue transformation to other colour-producing reactions and to relating the reactions of other organic entities to the colour-reactions. RESULTS AND DISCUSSION Benzidine (1) can be oxidized to benzidine-blue (2) by simple organic or inorganic oxidants (Page, 1941). Benzidine-blue is a radical-cation and the blue colour indicates that both aromatic rings are involved in the resonance structure (Weiss, 1938). IH 2 NH 2 (1) --e 9 +NH 2 (2) All montmorillonites give a blue colour when treated with aqueous solutions of benzidine but the intensity of the colour varies widely from light to dark blue (Table 1). Montmorillonites which give the more intense colour have ferric ion present in the silicate lattice, although other transition metals in the higher valency state would be expected to behave similarly. Pale blue colours are given by montmorillonites which do not contain significant amounts of iron, or by montm~rillonites in which the iron is in the ferrous form. The oxidation state of the iron can be changed readily by classical methods. Heating the mineral in the air will oxidize it and increase the intensity of the colour given when benzidine reagent is added to the mineral On the other hand a montmorillonite after hydrazine or electrochemical reduction gives only a very pale blue colour with benzidine. The change in the oxidation state of the iron is shown by an increase in the signal at g=4.3 (ferric ion) in the ESR spectra (Castner et al., 1960) when montmorillonites are oxidized, by cation exchange capacity measurements which give higher values for the reduced form of the montmorillonite, and by the increased ferrous content found by chemical analysis of the reduced clay. Farmer & Russell (1967) have shown by infrared studies that octahedral ferric ions can be reduced by hydrazine.

3 Reactions catalysed by minerals~ IV 391 It is unlikely that all the iron atoms present in the silicate lattice are involved in the oxidation-reduction reactions. Consideration of the size of the ferrous and ferric atoms and the dimensions of the silicate lattice suggests that possibly only some of the octahedral sites undergo a change in valency state. The analytical figures found for the total iron and for the ferrous iron in the oxidized and reduced clays indicate that only about 25% of the total iron undergoes valency changes under the conditions used. The calculated difference in exchange capacity of an oxidized and a reduced clay, based on the ferrous iron analysis figures, equates with the experimental values. Reduced montmorillonites after treatment with a polyphosphate do not give any colour with the benzidine reagent (Table 1). Since polyphosphate is adsorbed specifically at the crystal edges (Michaels, 1958), it can be concluded that these are oxidizing sites. TABLE 1. Reaction of montmorillonite with aqueous benzidine solutions* Montmorillonite source Volclayt Wyoming bentonite Otay, California bentonite$ Fe20a content = 3;54 Fe~0a content =0"97 ~ Colour produced on mineral in benzidine solution Mineral (as obtained) Oxidized mineralw Reduced mineralw Polyphosphate-treated oxidized mineral Polyphosphate-treated reduced mineral ii Dark blue Very dark blue Very pale blue Medium blue No colour Light blue Dark blue Very pale blue Very pale blue No colour * 15 montorillonites were examined. The results reported are typical of those obtained with the other montmorillonites. t Registered trademark 0fAmerican Colloid Co. ++ A sample was chosen which appeared to contain very little iron. w ESR spectra showed a strong band at g=4"3 which has been assigned by Castner et al (1960) to ferric iron. The oxidized minerals showed an increase in the g = 4-3 signal as compared with the reduced mineral. II With some samples a trace of blue colour developed after a few minutes.

4 392 D. H. Solomon, B. C. Loft and Jean D. Swift Oxidized montmorillonites have their oxidizing power decreased by po]yphosphate treatment; at low iron contents, the loss of the crystal edge sites has a significant effect on the colour with benzidine but at high iron contents, the change is small (Table 1). Thus, it can be concluded that montmorillonites oxidize benzidine at two distinct sites; at iron atoms within the silicate lattice, and at aluminium atoms at crystal edges. For the sake of brevity these sites will be referred to as planar surface and edge sites. The edge sites are at the surface of the mineral and the planar sites are located within the silicate lattice probably in the octahedral layer. The relative importance of these two sites depends on the montmorillonite and on its history. For example, in some natural montmorillonites, the iron is predominantly in the oxidized form (Table 1), whereas in others, the metal atom is in the lower valency state. The exchangeable cations have an influence on the rate at which montmorillonite can oxidize aqueous solutions of benzidine. This rate effect is related to the ease with which the clay swells in water and hence, the rate at which benzidine can penetrate to the inter-layer regions. With sodium montmorillonites, the final colour intensity is developed rapidly, whereas with calcium or cobalt montmorillonites some minutes are required for the same colour intensity as the sodium clay to develop. No other influence of the exchangeable cation was observed and in this connection it should be noted that previous reports on the enhanced oxidizing power of montmorillonites which had transition metal cations in the exchange sites (Solomon & Rosser, 1965), are better e.xplained by the oxidation of the mineral during preparation of the homoionic clay. Sodium and cobalt montmorillonites prepared from oxidized Volclay montmorillonites, eventually give similar colours with benzidine. Other silicate minerals are able to oxidize benzidine to benzidine-blue. The intensity of the colour produced can be related to the number of layer and edge sites available. Table 2 indicates the location of the active sites on the various minerals as determined by selective masking of the edge sites by polyphosphate treatment and oxidation or reduction of the transition metal sites. The active sites on kaolinite and pyrophyllite are almost exclusively at the crystal edges while with hectorite and nontronite, the edge sites are only a small percentage of the total and sites in the planar surfaces are of prime importance. Solvent has a marked influence on the benzidine-mineral interaction and this is most apparent when non-aqueous solvents such as benzene are used. When a solution of benzidine in benzene is added to the dried minerals, the results are in marked contrast to those obtained in aqueous suspension. Firstly, the differences in the colours produced by the minerals are relatively small, and secondly, the colour is shifted towards the yellow. Under anhydrous conditions, benzidene gives a yellow colour on oxidized montmorillonite, and in the presence of small amounts of water, a yellow green colour is produced. The yellow or yellow-green colour is the result of the low ph of the dried mineral surface. Fripiat (1963), Tahoun & Mortland (1965), Benesi (1956, 1957) and Walling (1950), have all shown that a

5 Reactions catalysed by minerals. IV 393 dried mineral surface is very acidic and can be comparable to a ph as low as 0"8. At low ph values the benzidine radical-cation takes up a proton on the electron pair of the nitrogen atom (Dodd & Ray, 1960) and this limits the number of resonance structures possible. Consequently, the colour is shifted towards the yellow. The number of molecules oxidized apparently decreases as a result of the loss of resonance energy since ESR failed to show the presence of radical species in the yellow complex. Non-polar solvents such as benzene besides influencing the ph of the mineral surface limit the access of benzidine molecules to the inter-layer sites and consequently the initial reaction is mainly at crystal edges and at the outer planar surfaces; interlayer complex formation from benzene solution is slow compared with oxidation. The acidity of the dried minerals varies with structure (Benesi, 1957) and the kaolinite surface is more acidic than that of a montmorillonite. This could account in part for some difference in colour of the oxidized benzidine species on the two surfaces. In general, the kaolinite shifted the colour of the benzidine cation more towards yellow than did montmorillonite. TABLE 2. The reaction of minerals with aqueous benzidine solution and the location of the reactive sites Mineral Location of majority* of active sites Colour of mineral after 1 rain in benzidine solution Talc No active sites No colour Pyrophyllite (Robins, North Carolina, U.S.A.). Edge sites Pale blue Hectorite Planar sites Very pale blue Muscovite Edge sites Very pale blue Nontronite Planar sites Very pale blue Illite (Fithian, Illinois, U.S.A.) Edge sites Medium blue Attapulgite Edge and planar sites Medium blue Kaolinite Edge sites Very light blue (Mt Egerton, Australia) * The location of active sites was determined by comparison of the reactivity of the oxidized and reduced minerals and polyphosphate-treated oxidized and reduced minerals.

6 394 D. H. Solomon, B. C. Loft and Jean D. Swift The yellow benzidine mineral complex can be converted to the blue form by the addition of water and conversely, the benzidine-blue complex becomes yellow when strong acid is added or when water is removed, Other solvents examined (acetone, ethanol and hexane) behave in a similar manner to benzene provided that care is taken in removing traces of water from the solvent. These results imply that benzidine competes successfully with the solvent for the active sites on the mineral. The colourless to yellow complexes formed in these dry solvents are converted readily to the characteristic benzidine blue species by the addition of water. The presence of oxygen increases the ease with which a mineral can oxidize benzidine to benzidine-blue and this observation is indirect support for an electron transfer reaction; previously it has been noted that oxygen catalyses electron transfer in alumino-silicate cracking catalysts (Hirschler, 1966). Where crystal edge sites are involved the catalytic action of oxygen can be compared with its ability to promote the oxidation of organic compounds with stronger Lewis acids; Sato (1965) has noted that oxygen favours the reaction of dimethyl aniline with aluminium chloride. The expianations developed in this paper for the manner in which minerals oxidize benzidine to benzidine-blue allow a rationalization of much of the confusion that surrounds the benzidine-blue reaction. The introduction of the concepts of the crystal edge as an oxidizing site, and of variations in the valency state of the lattice transition metal atoms in natural and treated minerals are novel additions to existing theories. However, it should be noted that the theory developed herein does not conflict in any major way with existing postulates most of which are consistent with the theory proposed and can be incorporated into it. For example Weil-Malherbe & Weiss (1948) and Block, Charbonelle & Kayser (1953) found that lattice iron undergoes oxidation-reduction reactions when benzidine is added to montmorillonite, but they did not take into account the effect of crystal edge sites. Similarly the observations of Dodd & Ray (1960) and of Hauser & Leggett (1940) on the influence of ph and solvent on the colour formed from benzidine-montmorillonite complexes are readily interpreted in terms of the lack of availability of the interlayer sites in non-polar solvents and of the highly acidic nature of the mineral surface. The increased oxidizing power of fresh montmorillonite samples after grinding, noted by Keller (1955) and attributed by him to oxidation of octahedrally co-ordinated ferrous iron is also explained by the present theory which suggests that in addition to oxidation more crystal edges would be exposed by grinding and these would contribute also to the enhanced oxidation of the mineral. The significance of grinding in exposing additional edge sites is clearly demonstrated by grinding a pre-oxidized polyphospfiate-coated montmorillonite. The mineral then shows enhanced ability to oxidize benzidine. The addition of polyphosphate to mask the new edges reduces the oxidizing power of the mineral to that observed for the polyphosphated oxidized mineral before grinding. The lack of specificity of the benzidine colour reaction as a means of identifying montmorillonite (Page, 1941) is readiiy understood in view of the proposed theory.

Materials Reactions catalysed by minerals. IV 395 EXPERIMENTAL Solvents used were purified and dried by standard techniques (Vogel, 1951). Benzidine (B.D.H.

7 Materials Reactions catalysed by minerals. IV 395 EXPERIMENTAL Solvents used were purified and dried by standard techniques (Vogel, 1951). Benzidine (B.D.H. extra pure for blood testing), was recrystallized from water and had a m.p. of 128 ~ C. It was used as: (i) a saturated solution of the hydrochloride in water; (ii) as a M solution in benzene. Minerals The minerals were ground and micronized if necessary to give a particle size of approximately 2 ~. The minerals were purified by sedimentation and then by dialysis. Oxidized minerals were prepared by heating the mineral at 130 ~ C for 3 hr. Reduced minerals were prepared by: A. Treating the mineral (10 g) with a 5% aqueous solution of hydrazine (2 20 ml far 2 hr). The clay was washed with water until the washings were free of hydrazine. The hydrazine reduced clays were shown to be free of occluded hydrazine by treatment with sodium hydroxide solution and then steam distillation. B. Montmorillonite was also reduced using the electro-chemical method of Leach et al. (1965). The electrolyte in the cathode compartment of the electrolysis cell was a suspension of mineral (0"2 g) in 0"1 M NaC1 with M 2-mercapto ethanol added as the regenerable reducing agent. The cathode potential was maintained at --1"2 V (vs S.C.E.) during the period of the electrolysis (24 hr). The reduced clay was washed three times in 0"1 M NaCI to remove mercapto ethanol then washed five times with distilled water. The clay was dried under reduced pressure. Polyphesphate modified minerals were prepared by adding the mineral (10 g previously oxidized or reduced) to water (20 ml) in which was dissolved a sodium polyphosphate (0"40 g, Calgon marketed by ICIANZ Ltd). The treated mineral was separated by centrifuging the solution. This process was then repeated and the modified mineral washed with water (5 10 ml), then dried at room temperature under reduced pressure. Homoionic minerals (cobalt, calcium and sodium), were prepared by treating previously oxidized or reduced minerals with a 1 S solution of the appropriate chloride until saturation was achieved then washing with water until chloride free. Where applicable polyphosphate treatment was carried out prior to cation exchange. The minerals were dried at room temperature under reduced pressure. The exclusion of oxygen during the drying of the reduced minerals was particularly important since re-oxidation is very facile. The exchange "capacities were measured by a modification of the ammonium acetate method described by Mackenzie (1951). Calcium oxidized montmoriuonite was found to have an exchange capacity of 81.6 m-eq/100 g while that of calcium reduced montmorillonite was found to be 89-1 m-eq/100 g. The total iron content

8 396 D. H. Solomon, B. C. Loft and Jean D. Swift was 3"54% Fe20~ as determined by the colourimetric method described by Sandell (1959). The ferrous iron was determined by the method described by Groves (1951); reduced calcium montmorillonite analysed for 0.85% FeO and the oxidized for 0"10% FeO. Reaction o] benzidine with minerals In water. The mineral (0"2 g) was added to an aqueous benzidine solution (1 ml) and the colour noted after 1 min. Typical results for montmorillonite are reported in Table 1, and for other minerals in Table 2. In benzene. All benzene solutions were degassed by the freeze-thaw method and the minerals were dried at 10 -~ mm pressure (3 hr). The clay sample and the benzidine-benzene solution, still under reduced pressure, were mixed and the colour noted after 15 min. Either dry oxygen or deoxygenated water was added to samples prepared and reacted as above. Sodium montmorillonite after 2 hr standing in a benzene solution of benzidine gave an interlayer spacing corresponding to the montmorillonite. However after 10 hr standing the interlayer spacing corresponded to a benzidine/montmorillonite complex. In acetone, ethanol and hexane. The mineral samples and the solvents were prepared as for the benzene experiments. Regeneration of benzidine [rom benzidine-blue-mineral complex Kaolin (10 g, dried 150 ~ C for 2 hr) was added to a solution of benzidine (0"120 g) in ethanol/water (50 ml/5 ml). After shaking for 1 hr the blue clay was filtered off and washed with the solvent (2 x 20 ml). Visible spectra of this blue complex corresponded to that of benzidine-blue obtained by Kotov & Terenin (1959). The mineral was then extracted with ethanol (24 hr) and the ethanol extract was evaporated to dryness (0"084 g). Treatment Of this residue with ethanol/water gave benzidine as crystals (0-051 g), m.p. and mixed m.p. with pure benzidine ~ C. The infrared spectrum corresponded to that of benzidine. Further standing of the ethanol/water solution yielded a product (0-033 g) which melted above 170 ~ C and over a wide temperature range. Polyphosphate-treated oxidized montmorillonite was treated in a similar manner. Visible spectra of the blue complex formed corresponded to benzidine-blue. The ethanol extraction product was shown by m.p., mixed m.p. and infrared to be benzidine. No other product was obtained. ACKNOWLEDGMENTS The authors would like to thank Dr G. F. Walker for valuable comments and suggestions, Miss B. C. Terrell for the clay analyses and Mr F. D. Looney for ESR spectra.

9 Reactions catalysed by minerals. IV 397 REFERENCES BENESI H.A. (1956) J. Am. chem. Soc, 78, BENES~ H.A. (1957) J. phys. Chem. 61, 970. BLOCrI J.M., CHARBONNELLE J. & KAVSER F. (1953) C.r. hebd. Sdanc. Aead. Sci. Paris 237, 57. CASTNER T., NEWELL G.S., HOLTON W.C. & SLICHTER C.P. (1960) J. chem. Phys. 32, 668. DODD C.G. & RAY S. (1960) Clays Clay Miner. 8, 237. FARMER V.C. & RUSSELL J.D. (1967) Clays Clay Miner. 15, 121. FRrPIAT J.J. (1963) Clays Clay Miner. 12, 327. GROVES A.W. (1951) Silicate Analysis 2nd edn, p. 90. George Allen and Unwin, London. /-/AUSER E.A., LE BEAU D.S. & PEVEAR P.P. (1951) J. phys. colloid. Chem. gs, 68. HAUSER E.A. & LEG6ETT M.B. (1940) J. Am. chem. Soc. 62, HmSCrlLER A.E. (1966) J. Catalysis 5, 196. KELLER W.D. (1955) Am. Miner. 40, 348. KOTOV E.I. & TERENIN A.N. (1959) Proc. Acad. Sci. USSR, Phys. Chem. Sect. (English Translation), LEACH S.J., MESCHERS A. & SWANEVOEL O.A. (1965) Biochemistry, Easton 4, 23. MACKENZIE R.C. (1951) J. colloid. Sci. 6, 219. MICHAELS A.S. (1958) Ind, engng Chem. 50, 951. PAGE J.B. (1941) Soil Sci. 51, 133. SANDELL E.B. (1959) Colout4metric Determination of Traces of Metals, p Interscience, New York. SATO H. (1965) Bull. chem. Soc. Japan 38, SOLOMON D.H, & LOFT B.C. (1968) J. Appl. polym. Sci. 12, SOLOMON D.H. & ROSSER M.J. (1965) J. Appl. polym. ScL 9, SOLOMON D.H. & SWIFT JEAN D. (1967) J. Appl. polym. Sci. 11, TAHOUN S.A. & MORTLAND M.M. (1966) Soil Sci. 102, 248. VOGEL A.I. (1951)Practical Organic Chemistry. Longmans, London. WALLING C. (1950) J. Am. chem. Soc. 72, WEIL-MALHERBE H. & WEISS J. (1948) J. chem. Soc., WEISS J. (1938) Chemy Ind. 1938, 517.

THE USE OF PIPERIDINE AS AN AID TO CLAY-MINERAL IDENTIFICATION

THE USE OF PIPERIDINE AS AN AID TO CLAY-MINERAL IDENTIFICATION By J. M. OADES* and W. N. TOWNSEND Department of Agriculture, The University of Leeds. [Received 30th August, 1962] ABSTRACT It is suggested