Synthesis and ion-exchange properties of Na-4-mica Kevin R. Franklin* and Elizabeth Lee Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, UK L63 3JW The preparation of the high-charge-density sodium fluorophlogopite mica Na-4-mica has been investigated with respect to developing a simplified procedure, and controlling crystal size. Na-4-mica can be readily prepared using solid state synthesis procedures with reaction temperatures between 700 and 1100 "C. A simple 'dry mix and calcine' method (all-in-one method) has been found to yield essentially the same product as that obtained by the more complex multi-step procedure described previously in the literature. The level of potassium which can be exchanged into the Na-4-mica was found not to be critically dependent on the ion exchange capacity of the clay, as determined by alkylammonium ion exchange, but rather on the initial hydration state of the Na-4-mica.Studies on the ion exchange, adsorption and catalytic proper- ties of clay minerals have for many years been restricted to low-charge-density materials, such as the smectite and kaolinite types, and their synthetic analogues, e.g. laponite.lv2 These materials typically have ion exchange capacities of <100 milli- equivalents (meq) per 100g of dry clay. In recent years, however, synthetic clays with ion-exchange capacities in the order of 200 to 250 meq (100 g)-l have been prepared and are now the subject of considerable interest, particularly with respect to their catalytic and adsorption In many cases solid state synthesis procedures have been used to prepare these materials, and fluoride ions have been employed to aid minerali~ation.~-' The preparation in the last few years of Na-4-mica, a very highly charged sodium fluorophlogopite mica of chemical composition Na4Mg,A14Si4020F4 xH20 and theoretical ion-exchange capacity of 468 meq (100 g)-' of anhydrous clay, has extended the range of interesting clay minerals still further.1°-12 Large crystals (0.1-2 mm) of Na-4- mica were first prepared as one of a heterogeneous mixture of crystalline phases by Gregorkiewitz and Rausell-Colomlo by heating a mixture of ground augite, sodium fluoride and magnesium fluoride at 1080°C in an open furnace and then cooling slowly to 850 "C.Paulus et ul." later showed that pure phase materials of much smaller crystal size, could be prepared using an elaborate multi-step process. They further showed that the materials so produced had high ion-exchange selec- tivities for many divalent transition-metal ions and for stron- tium and barium, but not for other alkali-metal ions, magnesium and calcium." The exchange of strontium ions was accompanied by the collapse of the clays interlayer region leading to trapping of the strontium ions, a process considered highly desirable for the removal of radioactive "Sr from contaminated solutions." We now report the development of a much simplified process for the preparation of pure phase Na-4-mica, and show how the crystal size may be readily controlled.The effect of synthesis conditions on the hydration state of the Na-4-mica, and its subsequent effect on the ion exchange properties of the clay, are also discussed. Experimental All reagents used in this study were analytical grade and were purchased from Fluka, B. D. H. and J. T. Baker. Na-4-mica was prepared by a number of methods developed sequentially from that described previously by Paulus et uL1' Following the original procedure, 82.05 g magnesium nitrate hexahydrate, and 80 g aluminium nitrate nonahydrate were dissolved in 342 cm3 ethanol in a 500 cm3 glass bottle. After stirring for 1 h, 44.43 g tetraethylorthosilicate was added and the mixture stirred for a further 3 h. The bottle was then capped and heated at 60°C for 3 days to form a gel.The bottle was then uncapped and the gel heated at 100°C until dry. The dry gel was broken up, transferred to a silica crucible and then calcined at 475°C for 18 h. The resulting solid was ground in a ball mill until it passed through a 325 mesh sieve. 80g of the powder produced by the above method (about 2 batches worth) was mixed thoroughly with 80 g of sieved (325 mesh) sodium fluoride. The mixture was transferred to a 500 cm3 platinum dish and calcined for 18 h at a temperature between 700 and 1100°C. The resulting solid was ground up, dispersed in water and then thoroughly washed with saturated boric acid solution to remove fluoride impurities. The product was washed with water and either freeze dried or air dried at room temperature.The Na-4-mica was finally stored in a desiccator over saturated sodium chloride solution (~~0.75) for 2 days. To establish the importance of some of the steps in this rather long process, a series of modifications were carried out. (i) The same basic procedure was employed but without any sieving of the precursor gel or the sodium fluoride. (ii) The procedure was modified as (i), but the sol-gel method for preparing the precursor gel was replaced; the magnesium nitrate and aluminium nitrate were dissolved in a small amount of water and then 12.80g of fumed silica (CAB-0-SIL M-5) was stirred in to make a thick paste. This was dried and then ground up before calcining at 475°C.(iii) As (ii), but the fumed silica was dry-mixed with the magnesium nitrate and aluminium nitrate. (iv) The magnesium nitrate, aluminium nitrate, fumed silica and sodium fluoride were ground together for 5 min using a pestle and mortar, and then calcined directly at between 700 and 1100 "C. The ion exchange of sodium for potassium was investigated at room temperature and at 90°C under forcing conditions. 5 g of Na-4-mica were weighed into a 1 dm3 plastic bottle. 25.3 g of potassium nitrate dissolved in 250 cm3 water were then added and the dispersion thoroughly shaken. After 4 days the solution phase was filtered off and replaced by a fresh potassium nitrate solution of the same strength. After a further 3 days the mica was filtered off, washed with water and then freeze dried.The composition of the solid was subsequently determined after the mica had been contacted with water vapour as described above. Power X-ray diffraction (XRD) was carried out to determine the basal spacings of the Na-4-mica and to monitor crystallinity using a Siemens D5000 diffractometer fitted with variable J. Muter. Chem., 1996, 6(l), 109-115 109 divergence and anti-scatter slits. Crystal shape and size were determined with a Cambridge Instruments Stereoscan 360 SEM apparatus. Sodium, potassium, magnesium, aluminium and silicon contents were determined from X-ray fluorescence (XRF) analysis using a Philips XRFS PW1404 instrument. Fluoride was determined by ion-selective electrode analysis.Water contents were determined by thermal analysis using a Perkin-Elmer TGA7 thermogravimetric analyser. Maximal ion-exchange capacities were determined by contacting the Na-4-mica with a large excess of octadecylammonium chloride solution at 60 "C overnight. The resulting alkylammonium- exchanged mica was filtered off, washed with large quantities of ethanol and then freeze-dried. The amount of alkyl-ammonium present in the mica was obtained from CHN microanalysis. Results and Discussion Synthetic procedures Employing the procedure of Paulus et u2.l' and a crystallisation temperature of between 700 and 1100°C gave crystalline materials with XRD patterns which were consistent with natural mica-type structures13 and with the limited powder XRD data reported previously for Na-4-mica." Specific characteristic features were the strong reflections at 1.54, 2.63, I-* 2 I 3.25 and 4.20A.Peaks were also observed at about 9.8, 12.1 and 13.2A which correspond to the basal spacing of the anhydrous material, and the first and second interlayer hydrates, respectively." The relative peak intensities and the peak definition were, however, found to vary with the crystallis- ation temperature as illustrated in Fig. 1. This is probably a function of changes in crystal shape and size, and the crystal- linity of the materials. The change in crystal shape and size is clearly seen in Fig. 2; as the final calcination temperature was increased the crystals became larger, more 'book-like', and had more clearly distinguishable edges (hexagonal in shape).Repeating the experiments at 900 and 1000°C, but without sieving the calcined Mg-Al-Si precursor gel and the NaF before combining them, had no adverse effect on the Na-4- mica produced. When the silica source was changed from tetraethylorthosilicate to fumed silica and the ethanolic sol-gel process replaced by a procedure in which the silica was simply combined with a solution of magnesium nitrate and aluminium nitrate, the products again remained essentially unaltered. Furthermore, when the water was removed and the silica, aluminium nitrate and magnesium nitrate were simply dry- mixed and calcined at 475 "C to prepare the precursor gel, the final product was again found to be Na-4-mica.In a final permutation, the sodium fluoride was dry-mixed with the other solid reagents at the start and the mixture calcined directly at I I I I I I 4 I I 5 10 15 20 25 30 35 40 45 50 55 60 I I ---T+ 5 2e /degrees Fig. 1 XRD patterns for samples of Na-4-mica prepared by the method of Paulus et al." Samples prepared at (a) 800 and (b) 1000°C. &Spacings are given above the major peaks. 110 J. Muter. Chem., 1996, 6(1), 109-115 Fig. 2 Electron micrographs of Na-Cmica prepared by the method of Paulus et al." Samples prepared at (a) 700, (b) 800, (c) 900, (d) 1O00, and (e) 1100"C.Size bars =1pm. the final crystallisation temperature. Again, all the products obtained at between 700 and 1100°C were found to be Na-4-mica (Fig.3). Comparison of the XRD patterns for the products obtained by this 'all-in-one' procedure and by the method reported by Paulus et al." (Fig. 1 and 3) shows very good agreement for the higher temperature products. With the lower reaction temperatures agreement is less good; the 'all-in-one' materials exhibit more XRD peaks and in general show greater similarity with the patterns obtained for the higher temperature products. SEM images of the products from the 'all-in-one' method (Fig. 4) show the same effects of crystallisation temperature as observed with the procedure of Paulus et al." It is, however, also noticeable that crystals produced by the 'all-in-one' method show a wider size distribution. The effect of crystallisation time on the literature route and the 'all-in-one' route products was also investigated.With both methods, powder XRD showed that Na-4-mica was produced in all reactions carried out with crystallisation temperatures of 900 and lOOO"C, and crystallisation times of between 2 h and 5 days. SEM images showed that crystal size increased with time. At times >18 h, however, the images showed clear degradation of some of the crystals. This was particularly noticeable with the high temperature reactions (Fig. 5). Chemical composition and ion exchange capacity The molar chemical compositions, standardised with respect to the silicon content, of the Na-4-micas prepared by the literature route of Paulus et al." and by the 'all-in-one' method, are given in Table 1.The compositions are broadly in line with the proposed ideal composition for Na-4-mica (see introduc- tion), although the Mg: Si ratios were generally higher. The sodium contents were also consistently higher than expected with the highest temperature preparations. The hydration water contents of the Na-4-micas were all lower than the 0.5 water molecules per :odium reported previously by Paulus et ul." for the 12.2 A hydrated material, and the 2.7 water molecules per sodium suggested by Gregorkiewitz and Rausell- Colom'' for the same phase. This is, however, in line with the XRD patterns obtained for these materials,o which in many cases showed the presence of anhydrous 9.8 A phase material.The effective ion-exchange capacity (IEC) of a range of the Na-4-micas, as determined by ion exchange with octadecyl- ammonium ions, are given in Table 2. Clays have very high selectivities for such alkylammonium ions and their replace- ment of exchangeable sodium ions can generally be carried out quantitatively.' In all cases the 900 "C products were found to have the highest IECs, and to be close to the theoretical J. Muter. Chem., 1996,6(1), 109-115 111 .-0 5 10 15 20 25 30 35 40 45 50 55 60 I 5 10 15 20 25 30 35 40 45 50 55 60 Fig. 3 XRD patterns for samples of Na-4-mica prepared by the 'all-in-one' method. Samples prepared at (a)800 and (b) 1000"C. d-spacings are given above the major peaks. IEC based on the ideal chemical composition [468 meq (lOOg)-l], while those obtained for the 700 and 1100°C products were generally the lowest.While there are some quite large individual variations between materials prepared at the same temperature, in general all methods appear to give very similar products in terms of IEC. The measured IECs were found to vary with reaction time as well as reaction temperature as shown in Table 3. In general the IECs increased with reaction time up to about 18 h and then declined again at longer reaction times. This decline was most pronounced with the higher temperature reactions where crystal degradation was observed (see earlier). Loss of IEC could also have resulted from glass formation at the extended reaction times, however, no evidence was obtained to suggest that glass formation occurred.Characterisation of octadecylammonium exchanged Na-Cmica XRD patterns obtained for the octadecylammonium-exchanged fluoromica! showed the expansion of the mica basal spacing to about 48 A. This was the case irrespective of the method and temperature employed to prepare the Na-4-mica. Strong diffraction peaks corresponding to d/l to d/5 were visible (Fig. 6), indicating a high degree of ordering. From simple molecular-size calculations, it is probable that the alkylammonium ions are packed in bilayers between the clay plates, with the alkyl chains slightly tilted from the perpendicu- lar. Peaks corresponding to the basal spacing of the original Na-4-mica, whether due to anhydrous or hydrated interlayers, were absent in all cases from the alkylammonium clays, suggesting that the hydration state of the sodium clay had no effect on the ability of the Na-4-mica to undergo exchange.The interlayer expansion which accompanied octadecyl- ammonium-ion exchange of the fluoromica had a visible effect on the crystal shape; the crystals became far more block-shaped rather than plate-like. The layer striations also became more evident (Fig. 7). Effect of hydration state on potassium-ion exchange While alkylammonium-ion exchange in clays occurs readily, the exchange of sodium ions with other inorganic cations is often more difficult (especially with highly charged clays). Previous work suggests that Na-4-mica shows little selectivity for potassium ions.12 It was therefore thought that the Na-K system may provide a useful method of probing the effects of hydration and interlayer swelling on the ion-exchange reaction.The levels of potassium exchange achieved at room tempera- ture and at 90 "C for 7 days are given in Table 4. The first point of interest is the relatively low levels of potassium exchange in all cases, despite the use of highly forcing con- ditions. There also seems little correlation between the levels of potassium exchange and the IECs measured by alkyl- ammonium exchange (Table 2). Indeed, the material with the 112 J. Muter. Chem., 1996, 6(1), 109-115 Fig. 4 Electron micrographs of Na-4-mica prepared by the 'all-in-one' method.Samples prepared at (a) 700 and (b) 900 "C. Size bars = 1 pm. Fig. 5 Electron micrograph of Na-4-mica prepared by the 'all-in-one' method at 1000 "C for 5 days. Size bar = 1 pm. Table 1 Chemical composition of some Na-4-mica samples Preparative method", temperaturePC compositionb A, 700 Na4.1 Mg6.4 si4 A14.1 O20.20 F4.8 * 1.20H20 A, 800 Na3.8 Mg6.3 si4A14.0 020.40 F3.6 *0.58H20 A, 900 Na3.8 Mg6.3 si4 A14.0 020.25 F3.9*0-37HzO A, lo00 Na5.2 Mg6.6 si4 A14.3 O21.05 F5.2 1.28H20 A, 1100 Na5.0 Mg6.5 si4 A14.2 O20.90 F4.8 1.07HZ0 B, 700 Na5.4 Mg7.0 si4 A14.3 021.05 '6.2 1.40H20 B, 800 Na4.1 Mg6.9 si4A14.4 021.45 F4.2 * 1*88H20 B, 900 Na4.0 Mg7.2 si4 A14.6 O22.15 F3.9. 1-43H~O B, lo00 Na5.3 Mg7.z si4 Al4.7 022.65 F4.5 * 1.49H2O B, 1100 Na4.5 Mg6.7 si4 A14.3 021.95 F2.9 '1.40H20 a A =ref. 11; B =all-in-one.The oxygen value was not determined, but was rather inferred from electrical neutrality considerations. highest IEC (Paulus et al." at 900 "C)gave the lowest level of potassium exchange. In fact, it appears that the level of potassium exchange is controlled by the water content (hydration state) of the sodium form starting material. The effect was most clear with the 'Paulus route' (ref. 11) 800 and Table 2 Ion-exchange capacities" of Na-4-micas as determined by octadecylammonium ion exchange reaction literature method aqueous gel two-step dry mix all-in-one tempPC (ref. 11) methodb method' method 700 192 356 282 243 800 381 415 266 289 900 470 433 45 1 439 lo00 413 341 321 310 1100 213 275 262 318 a In meq (100 g)-' of dry sodium clay.Aqueous solution of Mg and A1 nitrates combined with fumed silica, calcined at 450 "C, mixed with NaF, and calcined at reaction temperature given in the table. 'As for aqueous gel method, but Mg and A1 nitrates dry-mixed with the silica. Table3 Effect of reaction time on the ion exchange capacities of Na-Cmica prepared by the all-in-one method. ion-exchange capacity reaction time/h at 900 "C at 1o00"C 2 323 264 6 404 178 18 439 310 48 302 222 300 303 146 900°C materials which had very low water contents an! showed low levels of potassium exchange. Major 9.8 A anhydrous sodium-phase basal-spacing peaks were present in the XRD patterns of the starting sodium forms and in the potassium-exchanged forms of these materials.Conversely, the 'all-in-one' route 800 "C material, which gave much greater potassium exchang?, had a much higher water content and a predominant 12.1 A hydrated-phase basal-spacing peck in its XRD pattern. Upon potassium exchange the 12.1 A phas? peak completely disappeared and was replaced by a 12.8 A peak, which is consistent with a hydrated potassium phase," and !n unassigned (possible anhydrous potassium phase) 10.4 A peak. It is concluded, therefore, that formation of the hydrated sodium-ion phase is a prerequisite for potassium-ion exchange to occur. The reduction in the total cation content (Na+K) of the exchanged materials relative to the starting material (Table 4) is most probably due to the dissolution of impurities during the ion exchange process.Since hydration of the interlayer region of Na-4-mica appears to be so important for effective potassium exchange, attempts were made to hydrate the Na-4-mica prior to exchange. Storing the materials in a humid atmosphere for 6 weeks had virtually no effect on the hydration state. Further contacting the mate- rials with water at room temperature and at 90°C for periods up to 1 month followed by air-drying, resulted in no change in the hydration state. Potassium exchange carried out sub-sequently likewise showed that the material treated in this way had no greater tendency to undergo exchange than the starting materials. Thus it appears that, contrary to previous once dehydrated, Na-4-mica is not readily reh ydrated.In all of the studies reported above, the materials used had been freeze-dried after synthesis. To investigate whether this extreme form of drying was the cause of irreversible dehy- dration, some materials were prepared as previously, but were air-dried. All materials so prepared, however, had very similar levels of hydration to those prepared using freeze-drying. Once again, the 'Paulus et al. (ref. 11) route at 900°C' material had a particularly low level of hydration. Likewise, potassium exchange of the air-dried materials occurred to essentially the same level as in the corresponding freeze-dried materials. J. Mater. Chem., 1996,6(l), 109-115 113 I I I I I I I I mvi nIL10 15 10 15 20 25 30 35 40 45 50 55 60 Fig 6 XRD patterns of octadecylammonium exchanged Na-4-mica. Na-4-mica prepared at 800°C using (a) the method of Paulus et al.” and (b)the ‘all-in-one’method.D-spacings are given above the major peaks. Fig 7 Electron micrograph of octadecylammonium-exchanged Na-4-mica. Size bar = 1 lm. The possibility that the difficulty in hydrating the Na-4-mica was due to the presence of an impurity of a lower-charge-density non-swelling clay similar to natural micas’ was also considered. To be responsible for the observed hydration effects, it would require a large amount of such a phase to be present. The presence of a large amount of a lower-charge-density phase is, however, inconsistent with the measured chemical compositions and IECs, and this possibility can therefore be discounted.Thus, it is still unclear why some 114 J. Muter. Chem., 1996, 6(1), 109-115 Table 4 Composition of potassium-exchanged micas room starting temperature exchanged 90 “C exchanged preparative method: material - product product temperature/”C Na:Si K:Si Na:Si K:Si Na:Si A, 700 1.03 0.19 0.69 0.55 0.28 A, 800 0.95 0.08 0.84 0.38 0.55 A, 900 0.95 0.07 0.88 0.13 0.78 A, 1000 1.30 0.35 0.76 0.53 0.33 A, 1100 1.25 0.46 0.67 0.63 0.30 B, 700 1.35 0.18 0.89 0.38 0.43 B, 800 1.03 0.33 0.58 0.50 0.28 B, 900 1.00 0.38 0.53 0.45 0.35 B, 1000 1.33 0.35 0.88 0.43 0.45 B, 1100 1.13 0.38 0.60 0.43 0.45 A =ref.11; B =all-in-one. samples of Na-4-mica appear to undergo irreversible dehydration. Conclusions Na-4-mica may be prepared readily using solid-state synthesis procedures with reaction temperatures between 700 and 1100"C.The multi-step procedure used to combine the reac- tants described previously by Paulus et a!." is, however, complex, and a much simpler 'dry mix and calcine' method ('all-in-one' method) has been shown to yield essentially the same product. The crystallisation temperature is very import- ant in controlling the crystallinity and crystal size of Na-4- fluoromica; both increase with temperature. Reaction time is also important, particularly with high temperature (21000 "C), as with long reactions (>18 h) crystal degradation can occur.The ion-exchange capacity (IEC), as measured by alkylam- monium-ion exchange, varies with reaction temperature and time; the highest values [close to the theoretical maximum, 468 meq (100g)-'] were obtained with 900 "C reactions. The level of potassium-ion exchange that could be achieved under forcing conditions was not, however, a function of the IEC, but was rather controlled by the hydration state of the interlayer region of the Na-4-mica. Ion exchange of sodium ions out of anhydrous layers proved very difficult. Despite earlier reports to the contrary, hydration of anhydrous phase Na-4-mica did not occur readily, and thus, avoiding the formation of this phase during the synthesis of the material is highly important if it is to be ion exchanged with inorganic cations.The authors would like to thank the following people for their contribution to the characterisation of the Na-4-micas; Ms. Sharon Evans and Mr. Ian Tucker (XRD), Mr. Jaz Lechy, Ms. Helen Bills and Ms. Jane Munro-Brown (SEM), and Ms. Di Savage (XRF, fluoride analysis). All are employees of Unilever Research Port Sunlight Laboratory. References 1 H. van Olphen, An Introduction to Clay Colloid Chemistry, 2nd edn., Wiley, New York, 1977. 2 B. K. G. 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Con$ Ion Exchange, 1991,p. 51. 13 R. E. Grim, Clay Mineralogy, McGraw-Hill, New York, 1953, pp. 93-95. Paper 5103989D; Received 20th June, 1994 J. Muter. Chem., 1996, 6(1), 109-115 115