Metals intercalated in graphite. IV. Intercalation from CCl, solution and extraction of intercalated species

JEAN MARC LALANCETTE, LINDA ROY, AND JEANNINE LAFONTAINE Departement de chimie, Universiti de Sherbrooke, Sherbrooke (Qui . ) , Canada J l K 2Rl

Received March 15, 1976

JEAN MARC LALANCETTE, LINDA ROY, AND JEANNINE LAFONTAINE. Can. J. Chem. 54, 2505 (1976).

Some chlorides, mainly of transition elements, can be intercalated in graphite from a solution of carbon tetrachloride if their solubility is in the range of 1 to 4 mg/ml in CCL, at reflux tempera- ture, in the presence of chlorine.

The rate of extraction of intercalated salts with solvents has been compared to the rate of solution of these salts in the same solvents.

JEAN MARC LALANCETTE, LINDA ROY et JEANNINE LAFONTAINE. Can. J. Chem. 54, 2505 (1976).

Certains chlorures derives principalement des metaux de transition ont pu &tre inserts dans le graphite, en presence du chlore et a la temperature de reflux, a partir de leurs solutions dans le tetrachlorure de carbone lorsque leurs solubilites se situaient dans la region 1 a 4 mg/ml de CCI,.

La vitesse &extraction des sels inserks par certains solvants a 6te comparte a la vitesse de dissolution de ces sels dans les m&mes solvants.

A great variety of elements or compounds can be intercalated in graphite (1). These inter- calated species can be held by different types of bonding within the lattice of graphite. For example, fluorine, under appropriate experi- mental conditions, will react with graphitic carbon to give a compound of formula (C,F),. In this case, there is a covalent bond formed between graphite and fluorine, the combination is irreversible, and the fluorinated graphite shows no fluorine vapor pressure until the decomposition temperature is reached (2). In other instances, with certain transition metal salts there may be a true coordination bonding between the intercalated salt and the n-electron system of graphite, as shown recently by Volpin (3). In some cases, the bonding between the intercalated species and graphite can be very weak. This situation is well illustrated by the case of bromine intercalates. Bromine can be intercalated in graphite up to a composition of C,Br. But the larger part of the intercalated bromine can be removed simply by aeration at room temperature (4).

In fact, it can be said that, providing a weak type of bonding between an intercalated salt and graphite and appropriate volatility of the intercalated species exist, there may be an equilibrium between the intercalated state and

the vapor state of the intercalated substrate. A similar situation will prevail if the substrate is in solution, as shown by the use of solvent techniques for intercalation (5). This paper, as a part of a study on the chemistry of inter- calated metals (6), describes intercalations achieved by means of carbon tetrachloride solutions and indicates interesting aspects of equilibrium between a solvent and an inter- calated species.

We have investigated the behavior of several chlorides, known to be intercalated by the standard method of heating a mixture of salt and graphite in an atmosphere of chlorine in order to see if these salts could be intercalated when dissolved in carbon tetrachloride. Results are presented in Table 1. Intercalation can be done with solutions of appropriate salts in CCl, if an atmosphere of chlorine is present. Otherwise the intercalation is much slower or absent (7). We interpret this observation as an indication that carbon tetrachloride is not able to promote the opening of the lattice and that the small amount of chlorine present in the refluxing carbon tetrachloride, by itself or com- bined with the salt, is sufficient to allow the intercalation to proceed.

In this study, we have used a graphite that was 20 mesh or finer. Hooley (5) has shown the

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TABLE 1 . Intercalation of chlorides in graphite from carbon tetrachloride solutions

Analysis of intercalates

Starting chloride

Duration Solubilityb Intercalation %by weight %by weight of reflux (mg/ml) (T "C) (%I of metalc of chlorine

AlCl, AlC13a FeCI, NiCI, PtCl', PdC1, CuCl, RhC1, UCb MoCl, w c l , FeCl, and AlC1,

FeCl, and AlCl,

40 < 1 20 24.5 14

3.7 22

1 .2 10

< 1 < 1

9 .5 (A1C13)d 1 1 (FeCl,) 9 .4 (A1C13)d

12 (FeCl,)

"Absence of chlorine. bSolubility of the salt at temperature (7') in CC14 saturated with CI,. 'Iron and aluminum have been determined by gravimetry. All the other metals have been determined by atomic absorption,

using a Varian instrument, model AA-6. *Determined by the ratio of metals in the intercalate.

relation between the size of particles and inter- calation phenomena with graphite. With this size of particles, we have observed intercalation with either standard methods or the solvent method.

We have noted that a very important factor concerning the ability of a carbon tetrachloride solution to promote intercalation is the solu- bility of the salt in this solvent saturated with chlorine, at reflux temperature. The intercala- tion will proceed if the salt has a low solubility in the halogenated solvent. With the salts that we have tried, too high a solubility or no solubility led to no intercalation. With the experimental conditions that we have used, intercalation proceeded well with solubilities in the range of 1 to 5 mg/ml. With higher values there was very little intercalation. It is also to be noted that with the mixture AlC1,-FeCl, dissolved in carbon tetrachloride, both com- ponents of the mixture were intercalated simul- taneously.

The equilibrium of an intercalated salt which is not very strongly bonded to graphite with a solvent can lead to the complete extraction of the salt from graphite, if the salt is soluble in the solvent and if the solvent is not saturated with the intercalated salt. However, under similar conditions of temperature and con-

centration, the rate of extraction of an inter- calated salt from the lattice of graphite is much slower than the rate of solution of the same salt in a free state, present on the surface of graphite for example. This difference in the rate of extraction us. rate of solution can be used to remove excess nonintercalated salt from a re-

V o l u m e ( m l )

FIG. 1. Weight of AlCl, eluted by anhydrous tetra- hydrofuran from a 10% intercalate of AlCl, in graphite (A) and from a 10% mixture of AlCl, with graphite (0).

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LALANCETTE ET AL.

V o l u m e ( m l )

FIG. 2. Weight of FeCl, eluted by 1% hydrochloric acid from a 15% intercalate of FeCI, in graphite (A) and from a 15% mixture of FeCI, with graphite (0).

action mixture if the rate of extraction of a given intercalated salt with a given solvent is known. In fact, by measuring the rate of extraction, it is possible to estimate, in a first approximation, if a salt is intercalated or not. Such a technique is definitely not a substitute to formal X-ray diffraction or DTA analysis, but offers interesting possibilities for a rapid preliminary evaluation of a given intercalate. Typical curves of extractions and solutions are presented in Fig. 1 and Fig. 2 for aluminum chloride and ferric chloride.

Experimental ( I ) Intercalation of AICI, from a CCI, Solution

In a three-necked flask of 500 ml, 20.0 g of well-dried graphite was suspended in 250 ml of CCI, dried over alumina (Woehm, grade I). Graphite was of spectro- scopic grade and ground to pass through a 20 mesh sieve. Then 8.0 g of anhydrous aluminum chloride (Fisher, Reagent grade) was added and the reaction mixture, under good stirring, was heated to reflux temperature, a slow stream of chlorine being bubbled through the boiling CCI, (5 mllmin). Chlorine was Matheson Research grade 99.96% C12 and was used without purification. After a reflux of 3 h, the system was filtered and the residue washed with 500 ml of boiling CCh. Any non- intercalated AlCl, was then removed by heating the inter- calate to constant weight under 1.0 torr at 198 OC for 18 h. Under such conditions, any nonintercalated alumi- num chloride was eliminated by sublimation. The inter- calated aluminum chloride was analyzed as reported before (6b), for aluminum and chlorine.

A similar technique was used for other intercalations reported in Table 1. With nonvolatile salts, the amount of salt under experiment was adjusted to remain below the maximum concentration that could be dissolved in the CCI, present. The presence of intercalation was verified by differential thermal analysis and comparison of these measurements with a similar analysis on samples pre- pared by heating the salts and graphite in an atmosphere of chlorine. For the determination of the intercalated metals, the intercalates were digested in concentrated sulfuric acid, followed by treatment of the filtrate by aqua regia and determination of the concentration of metals by atomic absorption. The results of these analyses are presented in Table 1.

( 2 ) Rate of Extraction of Intercalated Salts and Salts Mixed with Graphite

A 10 g sample of a 10% intercalate of aluminum chloride in graphite was placed in a 100 ml burette used as a column. Anhydrous tetrahydrofuran was flowed through the sample at a rate of 30 ml/h. Aliquots were taken at every 10 ml. A plot was made of the weight of AlCI, extracted against volume of solvent. This gave the curve indicated by the triangles in Fig. 1. A similar experiment was made using a mixture of AICI,, and graphite instead of intercalate, the concentration of AlCl, in the mixture being the same as the concentration of AICl, in the intercalate. This led to the curve bearing the circular points in Fig. 1. A similar pattern was observed using other concentrations of AlCl, and FeCI, inter- calates or mixtures, the extraction from the mixture being much more rapid than the extraction from the intercalate. Elutions with ethyl ether, or in the case of FeC1, with 1% HCl, also gave a similar pattern as shown in Fig. 2. In the case reported in Fig. 1, AlC13 was re- covered as an etherate of aluminum chloride. The amount of aluminum recovered was determined as aluminum

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2508 CAN. J . CHEM. VOL. 54, 1976

oxide by hydrolysis and calcination at high temperature. A similar pattern was observed with FeC13 (Fig. 2), the etherate being hydrolyzed and the amount of metal recovered determined as Fe,O,.

Acknowledgment The authors thank the National Research

Council of Canada for financial assistance.

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