Micropore formation due to thermal decomposition of hydroxide layer of Mg-chlorites: interactions with water

Applied Clay Science, 8 (1993) 147-168 Elsevier Science Publishers B.V., Amsterdam

147

Micropore formation due to thermal decomposition of hydroxide layer of Mg-chlorites: interactions with water Fr6d6ric Villi6rasa,l, Jacques Yvon a, Mich~le Franqoisa, Jean Maurice Cases a, Franqois Lhote b and Jean-Pierre U r i o t b aLaboratoire Environnement et Minkralurgie, UA 235 du CNRS, rue du Doyen Marcel Roubault, BP 40, 54 501 Vandoeuvre les Nancy cedex, France bCentre de Recherches Pktrographiques et Gkochimiques, CNRS, 15 rue Notre Dame des Pauvres, BP 20, 54 501 Vandoeuvre les Nancy cedex, France (Received December 1, 1992; accepted after revision February 1, 1993 )

ABSTRACT The first stage of dehydroxylation of magnesian chlorites involves the dehydroxylation of the brucite-like layer which removed water from the structure. This reaction provokes the modification of basal reflection intensities and the development of long basal spacings. Infrared spectroscopy as well as thermogravimetry and water vapour adsorption reveal the formation of structural micropores filled with molecular atmospheric water once the samples are cooled down. A high temperature treatment is needed to release the different phases condensed in these micropores. A heterogeneous dehydroxylation mechanism is proposed involving magnesium and oxygen concentration in acceptor regions and micropores in donor regions. This leads to a structure where micropores and enriched oxide interlayers alternate along the z-axis of the mineral which generates long-basal spacings. According to this model theoretical calculation shows that only a part of the microporous volume is accessible to water vapour.

INTRODUCTION

Due to the presence of two distinct hydrous octahedral layers the thermal transformation of chlorites occurs in two steps. In the case of magnesian chlorites, dehydroxylation of the hydroxyl interlayer occurs around 550°C and dehydroxylation of the 2:1 layer around 800°C. This latter reaction is immediately followed by recrystallization of high temperature species which are Mg-spinel, forsterite and enstatite. Papers about structural changes of heated chlorites are rare (Brindley and All, 1950; Caill~re and H6nin, 1951; Weiss and Rowland, 1956; Brindley and Chang, 1974). In these papers, only X-ray diffraction has been performed. These authors showed that the dehydroxyla1To whom correspondence should be sent.

0169-1317/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

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tion of the hydroxyl layer of chlorites is accompanied by an increase in intensity of the (001) basal reflection and a collapse of the (002) and (003) reflections. The remaining reflections, i.e. (001), (004) and (005), are shifted towards smaller spacings during the dehydroxylation reaction. These thermal properties were used to detect chlorite in samples containing both chlorite and vermiculite. After brucitic dehydroxylation, about two-third of the octahedral cations have moved from a center position in the octahedral plane to sites previously occupied by hydroxyls (Brindley and Ali, 1950). Concomittantly, superlattice reflections of about 2.7 nm appear (Brindley and Chang, 1974 ). They are interpreted as resulting from the short range migration of cations in the hydroxide interlayers into two different alternating arrangements. Surprisingly, other spectroscopic methods such as infrared spectroscopy have not been used for studying the thermal modifications of chlorites. The interest of infrared spectroscopy lies in its potentialities for the direct observation of adsorbed water, hydroxyl groups and cation surrounding and in its sensitivity to short range disorder (Farmer, 1974; Bacchiorini et al., 1986; Delmastro et al., 1989). For instance, infrared studies of the thermal transformation of dioctahedral phyllosilicates have revealed the presence of pentacoordinated aluminium (Heller et al., 1962; Farmer, 1974; Delmastro et al., 1989). Thermal decomposition mechanisms can also be studied by characterizing morphological changes of the mineral particles. Adsorption isotherm can then be used to elucidate the textural and surface properties of the decomposition products. In the case of simple oxi-hydroxides minerals, i.e. magnesian, iron, cobalt and aluminium oxi-hydroxides, adsorption methods show that slitshaped micropores (0.4 to 1 nm) are progressively opened during the course ofdehydroxylation reaction (Rouquerol et al., 1979; Naono et al., 1982, 1987; Naono 1989; Kittaka et al., 1989; Riebeiro Carrott et al., 1991 ). The restructuring of the microcristallites at higher temperatures leads to: - the closing of micropores (i.e. in the case of brucite to periclase transform a t i o n - Naono et al., 1989; Riebeiro Carrott et al., 1991 ); - the formation of fine mesopores formed by accumulation of the micropores (i.e. in the case ofgoethite to hematite transformation - - Naono et al., 1987 ). On the basis of these textural changes, structural mechanisms of thermal decompositions were then proposed. In the case of phyllosilicates, such textural studies have rarely been carried out. Brown and Gregg ( 1952 ) and Sennett (1990) reported that the specific surface area of heated kaolinite presents only slight variations in comparison with those obtained with hydroxides. The purpose of the present work is to study the structural and textural changes due to the interlayer dehydroxylation of Mg-chlorites using thermogravimetric analysis (TGA), X-ray diffraction (XRD), infrared spectroscopy (IR), immersion calorimetry and water vapour adsorption. On the basis

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of the structural modifications, the formation of a microporous texture and the mechanism of thermal decomposition will be considered. EXPERIMENTAL

Materials The magnesian chlorite originates from the orebody of Trimouns (Pyrenees, France) and was provided by Talc de Luzenac S.A. This chlorite arises from the alteration of micaschists through a hydrothermal process. It contains about 20% of talc and various accessory minerals such as zircon, titanite, graphite. The chemical and mineralogical compositions are presented in Tables 1 and 2. This chlorite can be mineralogically defined as a clinochlore. The iron content (Table 1 ) is not due to the presence of accessory minerals but to substitutions by octahedral (Fe 2÷ ) and tetrahedral (Fe 3÷ ) cations (De Parseval et al., 1991 ). Minerals were dry ground down to 300/tm. The dso value is 28/tm (Mastersizer, Malvern Instruments) and the argon specific surface area is 2.3 m2/g.

Thermal analysis Combined thermogravimetric and thermodifferential analyses were carried out using an Ugine-Eyraud B70 balance (Setaram) equipped with an TABLE 1

Chemical composition of chlorite (in wt%) SiO2 A1203 Fe203 MgO CaO TiO2 P205 LOI

35.89 16.33 2.20 32.9 0.30 0.80 0.30 11.41

LOI = Loss on ignition. TABLE 2

Mineralogical composition of chlorite (in wt%) Chlorite Talc Dolomite Apatite TiO2

80.3 + 1 17.7 +_ 1 0.1 + 0.5 0.6 + 0.5 0.8 + 0.3

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F. VILLIERAS ET AL.

universal TG-TD head. A sample weight of 0.2 g was heated in a platinum crucible up to 1030°C in self generated atmosphere using a heating rate of 2°C/min. Temperature, weight and differential temperature were simultaneously recorded on a PC computer. Using controlled rate thermal analyses (CRTA), the vapour flow from the sample and the vapour pressure above it are carefully controlled, in order to maintain the temperature and pressure gradient in the powder at a very low level (Rouquerol, 1970, 1987). With this method, the studied sample is kept under quasi-equilibrium dehydration conditions. The experimental conditions were a sample mass of about 0.15 g, a residual pressure of 0.26 Pa over the sample and a dehydration rate of 10-3 g/h. Isothermal pyrolysis (refered as classical thermal treatments) of 30 g of powder were carried out, in air, in a muffle furnace (Prolabo) during two hours. Calcinations were performed in a rectangular silica holder. The samples were heated for two hours from 400 to 1000°C at intervals of 50°C.

X-ray diffraction XRD patterns of calcined powders were obtained on a Jobin-Yvon SIGMA 2080 diffractometer using Cu Koq radiation and working by reflection. Spectra were recorded at a rotation rate of 0.5 °/min. For fine spacing determination, the spectra were recorded at 0.25 °/min. In situ analyses were performed from room temperature to 700 °C using a home built vacuum and temperature controlled system (Berend, 1991 ). X-rays were produced by a monochromated Co emission tube. Detection is performed using a curved INEL detector and the diffraction chamber was provided by INEL allowing dynamic observation and direct comparison of intensities at the transformation temperatures.

Infrared spectroscopy IR spectra were recorded on an IFS 88 (Bruker) Fourier transformed infrared spectrometer. Two types of IR techniques were used: (1) conventional transmission spectroscopy using pressed KBr pellets (0.002 g of sample diluted in 0.148 g of KBr) and (2) diffuse reflectance spectroscopy where the diffuse part is collected by a Harrick accessory (0.07 g sample diluted in 0.37 g of KBr). This last technique enhances the bands of weak intensities and, in particular, the absorption bands corresponding to surface species (Griffiths and Haseth, 1986 ).

Water vapour adsorption Adsorption gravimetry of water vapour were carried out at 303 K with the experimental apparatus described by Rouquerol and Davy ( 1978 ), Poirier et

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al. (1987) and Cases et al. (1992). It was built around a Setaram MTB 10-8 symmetrical microbalance. Water vapour supplied from a source kept at 314 K is introduced at a slow flow rate to ensure quasi-equilibrium conditions. Specific surface areas were calculated according to the BET equation and assuming a cross-sectional area of 0.148 n m 2 for the adsorbed molecule (Hagymassy et al., 1969).

Immersion microcalorimetry in water Immersion microcalorimetry was used to determine the enthalpy of immersion in water of the calcined samples with or without precoverage with water vapour. The curve of enthalpy versus precoverage equilibrium relative pressure, P/Po, allows to study the heterogeneity of the sample (Zettlemoyer, 1965 ) and to derive the external surface area without cross-sectional area hypothesis by applying a modified Harkins and Jura method (Partika et al., 1979; Cases and Francois, 1982; Fripiat et al., 1982). The sample, placed in a glass bulb with a brittle end, was successively outgassed, pre-equilibrated with vapour pressure at the desired conditions, sealed, and finally introduced into the experimental cell (half filled with water) of a conventional Tian-Calvet microcalorimeter (Setaram, 100 cm 3 cells). After thermal equilibrium at 303 K the brittle end was broken and the liquid entered the bulb and wetted the sample. The resulting heat flow was recorded as a function of time.

Stability in water In order to study the stability in water, 0.2 g of each calcined sample was immersed in 50 cm 3 of demineralized and distilled water and stirred during 30 days at 30°C. The suspension was subsequently filtered and the aluminium and magnesium concentrations in the liquid were determined by atomic absorption. RESULTS

Thermal analysis Figure 1 shows the curves of thermal analysis obtained by conventional thermogravimetry (curve a ), controlled rate analysis (curve b ) classical thermal treatments (curve c) and thermodifferential analysis (curve d). Thermogravimetric and thermodifferential curves are typical of powdered Mgchlorites (Mackenzie, 1957 ). The DTA curves exhibit two endothermic peaks and a sharp exothermic one. The release of the water contained in the brucitic layer leads to the first endothermic peak, around 600 ° C. Water from the 2: 1

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Fig. 1. Thermal analyses of Mg-chlorite obtained by (a) conventional TGA, (b) CRTA, (c) calcination in furnace and (d) thermodifferentialanalysis. layer is eliminated around 800 °C, giving rise to the second endothermic peak, immediately followed by the exothermic peak, around 830 °C corresponding to Mg-spinel crystallization. Topotactic rearrangement in enstatite and forsterite arises without energetic consumption. The curves obtained by controlled rate thermal analysis and furnace experiments, show, as well known, that these equilibrium methods lead to lower transformation temparatures than with the 2 °C constant heating rate procedure of TGA. For the different reactions, the differences of temperatures range between 70 and 100°C. Dehydroxylation of the brucitic and 2:1 layers starts between 450 and 500°C and, 700 and 800°C, respectively.

X-ray diffraction X R D patterns of the calcined samples are similar whether the samples are heated in the furnace or in the vacuum and temperature controlled chamber

153

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Fig. 2. X R D p a t t e r n s of ( a ) initial chlorite a n d chlorite heated at 600 ° C recorded at ( b ) 600 ° C a n d (c) after 2 hours heating a n d cooling at room temperature. *: artifacts due to defocalisations. Arrows refer to the large spacings a r o u n d 2.7 nm.

TABLE 3 Intensities of basal reflections normalized to the intensity of (001) reflection of the unheated sample. Values obtained on the sample heated in the vacuum and temperature controlled chamber Calcination temp. ( ° C ) Unhealed 320 360 400 440 480 520 560 600 640 680

I(00,1/2)

I(001)

1(002)

I(003)

I(004)

I(005)

4.33 4.22 3.31 4.13 3.18 3.47 0.22

2.73 2.80 2.80 2.84 2.70 2.22

0.32 0.22 0.41 0.42 0.22

1.00 1.37 1.44 1.46 1.48 2.23 9.35 9.91 10.18 10.39 10.36

3.38 3.24 3.21 3.33 3.42 3.44 1.07 1.00 0.85 0.77 0.84

1.56 1.37 1.40 1.35 1,36 1.51 1.38 1.23 1.44 1.53 1.41

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TABLE 4 Spacings measured on basal reflections. Values obtained on the sample heated in the vacuum and temperature controlled chamber

Calcination temp. (°C)

d(001 ) (nm)

d(004) (nm)

d(005 ) (nm)

Unheated 320 360 400 440 480 520 560 600 640 680

1.401 1.397 1.399 1.399 1.399 1.397 1.391 1.390 1.389 1.389 1.385

0.3557 0.3568 0.3563 0.3557 0.3541 0.3538 0.3489 0.3479 0.3472 0.3459 0.3453

0.2828 0.2837 0.2832 0.2828 0.2824 0.2813 0.2787 0.2771 0.2765 0.2762 0.2755

(Fig. 2). The first crystallographic transformations occur between 450 and 500°C and are completed between 500 and 550°C. This new structure is stable up to 700 ° C. The intensity of the (001) reflections is reinforced, the (002) and (003) reflections collapse, the intensity of the (004) reflection decreases and the (005) reflection is not affected (Table 3 ). New broad reflections appear at 2.6-2.7 nm and 0.94 nm. These changes are accompanied by a decrease of the basal spacings (Table 4). At 750°C, the structure begins to collapse and recrystallization into forsterite, enstatite and spinel occurs between 850 and 1000°C. These results are coherent with the observations reported by Brindley and Ali (1950), Caill6re and H6nin (1951), Weiss and Rowland (1956) and Brindley and Chang ( 1974 ). Infrared spectroscopy

The evolution of the IR spectra of the two chlorites upon heating was observed using both transmission (Fig. 3 ) and diffuse reflectance (Fig. 4 ). 4000-1500 c m - 1 range

Changes in the IR spectra appear at 500 ° C (spectra b in Figs. 3 and 4 ). The intensities of the 3580 and 3428 c m - l stretching bands of the brucitic hydroxyls (Shirozu, 1980, 1985 ) decrease and shift to 3593 and 3447 cm-~. At this stage, molecular water can be observed by two bending modes at 1660 and 1610 c m - ', particularly noticeable in the diffuse reflectance spectra ( Fig. 4b). This pattern is typical of microporous modulated clay minerals such as sepiolite or palygorskite (Hayashi et al., 1969). The 1660 cm-1 band can be

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Fig. 3. Transmission infrared spectra of chlorite (a) unheated, (b) heated at 500°C, (c) heated at 650°C and (d) heated at 750°C.

assigned to zeolitic water and the 1610 c m - ' band to water bound to octahedral Mg at the edge of the structure. This water is still present in samples heated to 700 ° C (Fig. 4c ) and disappears at 750 ° C (Fig. 4d), together with a part of the stretching vibration of the 2 : 1 hydroxyls at 3670 c m - ]. 1500-400 cm- 1 range

Dehydroxylation of the brucitic OH groups provokes some changes in the Si-O stretching bands in the 1100-900 cm - ' range which transform into one single broad band. This band broadens between 500 and 750°C as evidenced by the transmission spectra (Fig. 3, curves b, c and d). This could be indicative of changes in the short range ordering of the tetrahedral sheet of the 2: 1 layer (Bacchiorini et al., 1986; Delmastro et al., 1989 ). However, this broadening could also be related to the appearance of shoulders near 900 and 800 c m - ' . The 827 and 750 cm-~ vibrations disappear, the 670 cm-~ one decrease and the 450-470 c m - ~is practically not affected.

F. VILLIERASET AL.

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Fig. 4. Diffuse reflectance infrared spectra of chlorite (a) unheated, (b) heated at 500 ° C, (c) heated at 650°C and (d) heated at 750°C.

Interaction with water Outgassing conditions Prior to any adsorption experiment, it is necessary to study the thermal properties of the samples in order to determine suitable outgassing conditions. Figure 5 shows conventional and controlled rate thermal analyses of the sample heated at 600 ° C. These curves indicate that an extra weight loss appears between 450 and 520°C with conventional TGA and between 370 and 400°C with CRTA. The weight loss measured by TGA on the samples calcined between 500 and 750°C are reported in Table 5 (column 2). The m a x i m u m weight loss is recorded at 550 °C and decreases from 550 to 700 ° C. This p h e n o m e n o n disappears at 750°C. These weight losses are well correlated to the area of the bending bands of water calculated between 1700 and 1600 cm-1 and normalized to the surface of the 1000 c m - I Si-O-Si band wich is nearly stable between 500 and 750°C (Fig. 6). The correlation is excellent with the areas obtained by transmission and less good with those ob-

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Fig. 5. Thermal analysis of chlorite calcined at 600 °C obtained by (a) conventional TGA and (b) controlled rate thermal analysis.

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Fig. 6. Relationship between weight loss measured between 450 and 520 °C on samples calcined between 450 and 750 °C and the area of IR water bending bands obtained by (a) transmission and (b) diffuse reflectance. tained by diffuse reflectance. Therefore, the weight loss observed by T G A between 450 and 520 °C can be assigned to water trapped in micropores. However, T G A and CRTA experiments did not exhibit any discrimination between the two types o f water.

158

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Water vapour adsorption and desorption Adsorption and desorption isotherms obtained on the samples calcined at different temperatures are presented in Figs. 7 and 8. Outgassing conditions and numerical results are given in Table 5 (columns 3 to 9). Water adsorption on the non heated sample outgassed at 200 °C is reversible (Fig. 7a). The low pressure adsorption branches feature two knees at P/Po values of 0.1 and 0.22, respectively. These features characterize a swelling behaviour which can be assigned to the presence of some vermiculite layers interstratified with chlorite as shown by Yvon (1984) for Trimouns chlorites. Adsorption isotherms of the samples heated between 500 and 700°C and

30

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Fig. 7. Adsorption-desorption of water vapour isotherms obtained on (a) uncalcined chlorite outgassed at 200°C, (b) chlorite calcined at 450°C and outgassed at 400°C, (c) chlorite heated at 500°C and outgassed at 490°C and (d) chlorite heated at 550°C and outgassed at 490°C.

159

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Fig. 8. Adsorption-desorption of water vapour isotherms obtained on (a) chlorite heated at 600°C and outgassed at 200°C, (b) chlorite calcined at 600°C and outgassed at 400°C, (c) chlorite heated at 700°C and outgassed at 490°C and (d) chlorite heated at 750°C and outgassed at 490 ° C.

outgassed at 400 or 490 °C (Figs. 7c, 7d, 8b, 8c) present a vertical branch at very low relative pressure. This shape is typical of high energetic interactions as it is the case for ultramicroporous solids (Type I isotherm, Sing et al., 1985 ). The equivalent specific surface area reaches a maximum at 500°C and decreases from 500 to 750°C. If the sample is outgassed at 200°C (Fig. 8a), these micropores are not detected. These isotherms are not reversible and all desorption isotherms present an important hysteresis at zero relative pressure (Table 5, column 9). The undesorbed quantities are always higher than the monolayer quantities measured on adsorption branches. All the desorption

160

F. VILLII~RASET AL.

TABLE 5 Weight loss and parameters obtained by TGA and adsorption-desorption experiments. All the values are normalized to the stoichiometric composition of the initial chlorite Calcination Am Outgassing Outgassing C Monolayer Specific temp. TGA temp. weight loss const, capacity surface area (°C) (mg/g) (°C) (mg/g) (cm3/g (m2/g) STP )

t-plot volume (cm3/g STP )

Hysteresis volume (cm3/g STP )

Unheated 450 500 550 600 600 600 650 700 750

0.1 1.8 11.5 6.8 0.0 4.6 5.5 3.3 1.1 0.1

0.0 6.1 16.5 12.7 1.3 10.2 11.9 10.0 8.3 3.4

8.0 8.4 6.8 6.8 6.8 6.2 4.4 0.0

200 490 490 490 200 400 490 490 490 490

2.9 29.5 23.8 11.9 1.1 9.4 10.2 8.2 5.3 1.3

100 340 09 09 72 1240 3000 09 320 72

1.1 2.2 12.4 8.4 0.60 6.6 6.8 4.8 2.9 0.54

3.2 8.8 49.4 33.4 2.4 26.3 27.1 19.1 11.5 2.1

isotherms have the same feature, without swelling contribution, and correspond to a BET specific surface area around 2.3 ma/g (when undesorbed quantities at P/Po= 0 are subtracted) with an energetic constant C ranging between 20 and 26. The isotherm obtained on a sample heated at 450 °C and outgassed at 400 °C is intermediate between those obtained with the non-calcined chlorite and the 500°C sample (Fig. 7b). This isotherm presents some adsorption in micropores at low relative pressure, a desorption hysteresis at P/Po= 0 but the shape of the desorption branch is still typical of the swelling properties of the pristine mineral. At 750°C, heated chlorite is not microporous anymore (Fig. 8d). Adsorption and desorption isotherms are strictly parallel. However, hysteresis is still important. It does not correspond to micropores. Table 6 reports the magnesium quantities solubilized when the heated samples are immersed during 30 days. The obtained values indicates that solubilization reaches an important maximum for the sample heated at 750°C. This behaviour is characteristic of highly disordered samples as shown by Bacchiorrini and Murat ( 1986 ) for kaolinite. It is worth noting that the maximum of solubility, i.e. the maximum of disorder, is observed at the same temperature (750°C) for kaolinite and chlorite. The desorption hysteresis could then be assigned to magnesium dissolution and complexation at high water coverage. Micropores quantities were determined using the t-plot curves published by Hagymassy et al. (1969). These curves were established taking into account the energetic constant C obtained for desorption on the external sur-

MICROPORE FORMATION

161

TABLE 6 Magnesium concentration of water equilibrated at 30°C during 30 days with heated chlorite. Aluminium concentration is always lower than the detection limit Calcination temp. (°C)

Mg2+ conc. (rag/l)

Calcination temp. (°C)

Mg2+ conc. (mg/l)

Unheated 400 450 500 550 600 650

0.2 0.2 0.2 0.5 0.2 0.3 0.4

700 750 800 850 900 950 1000

0.7 9.1 0.8 0.6 0.5 0.4 0.4

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750

Temperature (°C) Fig. 9. Comparison between liquid microporous volumes obtained by (a) the t-plot, (b) the monolayer capacities (corrected of external surfaces contributions), (c) the hysteresis of desorption, the weight losses (d) determined during the outgassing procedure and (e) obtained between 450 and 520°C by TGA experiments. All the values are normalized to the stoichiometric composition of the initial chlorite. All the samples are outgassed at 490 ° C.

faces, i.e. C= 23. The results are reported in Table 5, column 8. Surprisingly, the obtained values are much lower than the monolayer capacities. Transformation of gaseous quantities into liquid quantities allows to determine the total geometric volume of accessible micropores which can then be

162

F. VILLII~RAS ET AL.

compared to the weight loss. Liquid volumes corresponding to the values obtained by the t-plot (Fig. 9, curve a ) , the monolayer capacities (corrected of external surfaces contributions) (Fig. 9, curve b ) and the desorption hysteresis (Fig. 9, curve c) are compared with weight losses determined during the outgassing procedure (Fig. 9, curve d) and weight losses obtained between 450 and 520°C by TGA experiments (Fig. 9, curve e). This figure shows that micropore quantities obtained by the t-plot procedure (curve a) are lower than TGA weight losses (curve e) except for the sample calcined at 500°C. In this case, the outgassing at 490 °C provokes supplementary dehydroxylation. Microporous volumes estimated at the monolayer (curve b) are still lower. Curves a and b exhibit a parallel evolution. Curve c obtained from the undesorbed quantities is parallel to the two precedent ones in the case of the samples heated between 500 and 700°C and deviates at 750°C. The abstracted quantities (curve c ) range between the weight loss measured by TGA (curve e ) and outgassing procedure (curve d ) except for the samples calcined at 700 and 750°C. For the 700°C sample, this deviation is probably due to magnesium dissolution and complexation as it is the case for the 750°C sample.

Immersion microcalorimetry Figure l 0 shows immersion enthalpies of the uncalcined chlorite outgassed at 100, 200, 300, and 400°C together with the values obtained for the calcined samples outgassed at 400 ° C. Immersion enthalpies increase sharply be30 25 O) ,,.j

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!

P/Po

Fig. 11. Chlorite heated at 600°C. Immersion enthalpy of the sample outgassed at 400°C versus precoverage relative pressure (dots) and corresponding water vapour adsorption isotherms (dashed lines) obtained after outgassing at (a) 400°C and (b) 200°C.

tween 450 and 550°C, i.e. when micropores are developed. The values are maximal at 550 and 600°C and decrease between 600 and 750°C. The evolution of immersion enthalpy with precoverage relative pressure was determined for the sample calcined at 600°C (Fig. 11 ). Enthalpy decreases rapidly at low precoverage pressure corresponding to the filling of the micropores. For precoverage relative pressures ranging between 0.1 and 0.7 the decrease is slow. For P/Po> 0.7, the immersion enthalpy reaches a constant value. This value P/Po = 0.7 corresponds to the completion of the second water layer on the external surfaces as evidenced by the water adsorption isotherm (Fig. 11 ) obtained after outgassing this sample at 200°C. The plateau value, for P/Po > 0.7, was then used to derive the external specific surface area, according to the Harkins and Jura method and using a value of 119.5 m J / m 2 for the internal energy of water surface at 303 K (Cases and Frangois, 1982). The value of 2 m2/g is the same as the Harkins and Jura surface area obtained for the non-calcined sample and is very close to the surface areas measured on all desorption isotherms and on adsorption isotherms of the sample calcined at 600 °C and outgassed at 200 ° C. DISCUSSION

The first structural transformations of Mg-chlorite occur between 450 and 500°C. The new phase is stable from 500 to 700°C. From these samples, correlations between ( 1 ) weight loss between 450 and 520°C, (2) area of the IR water bending band, ( 3 ) quantity of adsorbed water vapour, and (4) immersion enthalpy at zero relative pressure show that the structural changes lead

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to the formation of micropores. The volume of accessible micropores is maximal between 500 and 550 ° C. The microporosity then decreases from 600 to 700°C and disappears at 750°C. Two hypotheses could be proposed regarding the origin of this water: structural trapped water or atmospheric condensed water vapour. Experimental observations seem to favour the second hypothesis: this water is released at temperatures lower than the dehydroxylation temperature of the brucite-like layer and is still present in samples which have been heated 200 °C above the dehydroxylation temperature. The temperature required to drain the micropores, immersion enthalpy data at zero coverage as well as the shape of adsorption isotherms indicate that the condensed water is strongly trapped in micropores. Micropores filling involves diffusion processes as the values derived from the t-plot and the BET procedures are lower than the total accessible microporous volume. This total filling is reached at high relative pressure. The parallel behaviour of curves corresponding to the t-plot volume, BET derived volume and undesorbed volume indicates that the shape and the size distribution of micropores is constant between 500 and 700°C, even if the accessible volumes change. Such evolutions of specific surface area, and then of microporous volume, with heating temperatures were often reported in the case of hydroxide minerals. The adsorbed quantities increase linearly with the degree of decomposition (Rouquerol et al., 1979; Naono, 1989; Ribeiro-Carott, 1991 ) and generally decrease during recrystallisation processes (Naono, 1989, RibeiroCarott, 1991 ). However, the maximum microporous volume measured is always lower than the theoretical calculated volume. Micropores smaller than the size of the adsorbate are then invoked in order to explain this discrepancy. In our case, the studied samples are dehydroxylated but XRD patterns and IR spectra show that no important structural changes occur between 550 and 700 ° C. Therefore, only local organization due to the difference of calcination temperature could be invoked for explaining the evolution of microporous volumes. The development of micropores can not been explained by the dehydroxylation mechanism proposed by Brindley and Ali (1974). Their mechanism involves only short range migrations so that the two different oxide sheets are filled by Mg and O atoms whereas micropores development involves large scale migrations of atoms inside the solid as it is the case in a heterogeneous mechanism (Ball and Taylor, 1961 ). In the case of heterogeneous dehydroxylation, the solid is divided into two different regions: acceptor regions which gain cations coming from donor regions. In the donor regions hydroxyls combine to protons coming from acceptor regions to form water molecules which diffuse towards the surface and are removed from the solid. Then, in this mechanism, the donor regions are transformed into micropores. Using this heterogeneous dehydroxylation model, it is now possible to ex-

MICROPOREFORMATION

165

plain the different phenomena involved in the brucitic dehydroxylation of chlorites. The different steps involved in this mechanism can be presented as follows: Donor region Initial state

Acceptor region

MgzAI(OH)6

Mg2AI(OH )6 2Mg2+-+

At 500°C

MgzAI(OH)2(0- )4

( [[]2- )2AI(OH) 6 ,-4H +

Final state

AIO+ (5H20)?

Mg4AIOs + (H20)t

In this model, A1 cations were kept at their original position in order to balance the negative electrostatic charge defect due to tetrahedral substitutions in the 2: 1 layer. After thermal reaction, the dehydroxylated brucite-like layer is divided into areas concentrated in MgO (acceptor regions) and porous areas with a deficit in MgO (donor regions). Micropores are subsequently formed in these donor regions. Upon cooling, water adsorbs in these micropores where it can interact strongly with magnesium atoms located at the acceptor-donor frontier as evidenced by the IR band at 1610 c m - i. Along the z axis, interlayer donor and acceptor regions alternate in an irregular way. This leads to the broad long spacings reflections at 2.6-2.7 nm observed in the XRD patterns of dehydroxylated chlorites. Further evidence can be obtained by simulating, along the z axis, the XRD patterns of the interstratified dehydroxylated structure. The simulation was carried out using the method proposed by Plan~on and Tchoubar (1976). The results are shown in Fig. 12. The simulation according to the heterogeneous dehydroxylation mechanism was carried out in two cases: ( 1 ) Assuming a regular interstratification (Fig. 12, curve a). In this case, the simulated pattern exhibits a very intense peak around 2.7 nm which is more intense than the peak at 1.4 nm. (2) Assuming pAB=pBA=0.8, where Pij represents the conditional probability of a layer o f t y p e j following a layer of type i in the stacking. In this case (Fig. 12, curve b ), the simulated pattern exhibits nearly all the features of the experimental diffractogram (Fig. 2 ). These results validate the heterogeneous dehydroxylation model. Furthermore, they suggest a tendency towards an ordered structure. Thus, it seems, as stated by Brindley and Chang ( 1974 ), that the process of dehydroxylation of one brucitic layer influences the process in adjacent interlayers. According to this model, it is possible to derive a theoretical size and quantity of the micropores resulting from the heterogeneous dehydroxylation of the brucite-like layer of Mg-chlorites. If one assumes that the mica-like layer

166

F. VILLII~RASET AL.

J

I

0

i

i

^

I

1

i

i

i

I

L

2

,

A

i

^

I 3

i

L

L

t

J

I

4

-1

1/d (nm ) Fig. 12. Simulated XRD of dehydroxylated Mg-chlorites according to a heterogeneous dehydroxylation mechanism, (a) with pAB= 1 and (b) with pAB=0.8.

remains totally unaffected by the reaction, the width of the micropores must be equal to 0.45 nm. The micropore capacity must be roughly equal to 0.02 cm3/g. This means that, in all cases, a maximum of 50% of the total microporosity is accessible to water molecules. Similar conclusions were obtained in the case of nitrogen adsorption on dehydroxylated brucite (Naono, 1989; Ribeiro-Carott, 1991 ) and gibbsite (Rouquerol et al., 1979 ). CONCLUSIONS

Thermal transformations of Mg-chlorites, in the range of brucitic layer dehydroxylation, lead to the formation ofa microporous structure with a 26-27 /~ spacing. This structure can be described using a heterogeneous dehydroxylation mechanism. According to this mechanism, the dehydroxylated brucitic layer is divided into areas enriched in MgO (acceptor regions) and microporous regions (donor regions). Only a part of the microporosity (50% maximum) is filled by water when the mineral cools down. This filling is very energetic and involves diffusive processes. Furthermore, some water molecules interact strongly with the MgO walls located at donor-acceptor frontiers. Therefore, high temperature conditions are needed to empty the micropores.

MICROPOREFORMATION

167

ACKNOWLEDGEMENTS

This research was supported by the European Economic Community: Program Raw Materials, grant Ma 2M CT90 0036 DTEE and the French "Minist6re de la Recherche et de la Technologie", Program PHYGIS. We acknowledge Y. Grillet for the realization of controlled rate thermal analyses.

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