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The role of charcoal on DTA curves of organo-clay complexes: an overview
Applied Clay Science 24 (2004) 225 – 236 www.elsevier.com/locate/clay
The role of charcoal on DTA curves of organo-clay complexes: an overview Shmuel Yariv * Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received 25 December 2002; received in revised form 24 March 2003; accepted 8 April 2003
Abstract DTA of organo-clay complexes supplemented by other thermal analysis methods supplies information on the thermal reactions, properties and stability of the complex, the amount and properties of the adsorbed water in the organo-clay and on the bonding between the organic species and the clay. It is used to identify the mineral to differentiate between various complexes composed of the same clay and the same organic ligand and to establish their composition. During the gradual heating in oxidizing atmospheres the adsorbed organic material is oxidized, giving rise to significant exothermic peaks. DTA curves of organo-clays are divided into three regions: (1) the dehydration of the clay, (2) the thermal oxidation of the organic material and (3) the dehydroxylation of the clay. The exothermic oxidation reaction occurring during the gradual heating of the sample takes place in two steps, in the range 200 – 500 jC, oxidation of organic hydrogen and formation of water and charcoal, and 400 – 750 jC, oxidation of charcoal and formation of CO2. The exothermic peak temperatures depend on the mineral and on the organic compound and on the types of bonding between these two components of the organo-clay complex. The present communication concentrates in the role of the combustion of charcoal in the study of the fine structure of the complex and of the type of associations between the organic compound and the clay. D 2003 Elsevier B.V. All rights reserved. Keywords: Charcoal; DTA; Organo-clay complexes; Palygorskite; Sepiolite; Smectite; Thermal analysis; Water evolution curves
1. Introduction 1.1. Organo-clay complexes Adsorption of organic matter by clay minerals is one of the most widespread reactions in and on the earth, in the environment, agriculture and in various industrial processes. It includes (i) exchange of initially present inorganic ions by organic ions and (ii)
* Tel.: +972-2-6585-617; fax: +972-2-6585-319. E-mail address: [email protected] (S. Yariv). 0169-1317/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2003.04.002
accumulation of polar and nonpolar molecules on the surface of the mineral. Organic cations, anions or molecules are transferred from solutions, liquid or gaseous states to the clay surface. During adsorption, physical or chemical bonds (long- or short-range interactions, respectively) are formed between the mineral and the organic matter. The adsorption products are defined as organo-clay complexes. The adsorption of organic materials by clay minerals and the different adsorption mechanisms have been widely investigated during the last century and have been reviewed (see e.g., Weiss, 1969; Mortland, 1970; Theng, 1974, 1979; Yariv and Cross, 1979; Lagaly,
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1984, 1993; Rausell-Colom and Serratosa, 1987; Yariv, 2001a). 1.2. Thermal-analysis of organo-clay complexes Following the recommendation of ICTAC (International Confederation for Thermal Analysis and Calorimetry), the definition of the various techniques comprising thermal analysis is as follows: ‘‘Thermal analysis is a group of techniques in which a property of the sample is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed. The program may involve heating or cooling at a fixed rate of temperature change, or holding the temperature constant, or any sequences of these’’ (Hill, 1991). The most applied thermal analysis methods in the study of organo-clay complexes are: thermo-IR spectroscopy analysis (Yariv, 1996, 1999, 2000, 2001b); thermoXRD analysis (Yermiyahu et al., 2002); evolved-gas analysis (EGA) (Mu¨ller-Vonmoos et al., 1977; Morgan et al., 1988; Paulik et al., 1989; Yariv, 1990); thermogravimetry (TG); differential-thermal analysis (DTA) (Paulik, 1995) and differential-scanning calorimetry (DSC). Recently emanation thermal analysis (ETA) showed its usefulness in the study of thermal changes of the microstructure of organo-clay complexes (Balek et al., 2002). During the thermal treatment of organo-clays in the temperature range 250 –300 jC the adsorbed organic compound is transformed into charcoal. The features of the charcoal depend very much on the nature of the precursor organo-clay. Consequently, the study of the charcoal contributes to the interpretation of the results of the different thermal analysis methods. In the thermo-IR spectroscopy analysis and thermoXRD analysis the sample is heated to different temperatures and its IR spectrum or X-ray diffractogram, respectively, are recorded after each thermal treatment. From the different spectra or diffractograms, one gets information about reversible and irreversible changes due to temperature. In the evolved-gas analysis the evolved gases during the thermal treatment pass through either a mass spectrophotometer, through an infrared spectrometer or through gastitrimeter, where the evovled gases and their relative evolution with temperature are determined. In the thermogravimetry the mass of the sample is continuously monitored
during a programmed heating and a curve describing mass-loss against time or temperature is recorded. In order to enhance the steps in the thermogravimetric curve, the derivative-thermogravimetric (DTG) trace is frequently used. This is the plot of the rate of mass change with time. In the DTA technique the test sample in one holder and an inert reference material (such as calcined kaolinite or alumina) in another holder are gradually heated side-by-side from the same source. A thermocouple is positioned in the test sample and in the reference material. A thermal reaction in the test sample, whether exothermic or endothermic, gives rise to difference in temperatures between the two holders, measured by the thermocouple. If the difference in temperature (DT) between the test sample and the reference sample is plotted against the temperature of the latter (T) a DTA curve is obtained. A straight line parallel to the T axis is recorded as long as no thermal reaction occurs in the holder of the sample. When a thermal reaction occurs, a peak develops on the DTA curve. This peak can be either exothermic or endothermic and its area is proportional to the magnitude of the evolved or absorbed energy, respectively. In recent years there has been much progress in the accuracy of the measurement of energy evolved or absorbed by the analyzed sample during the DTA run. Differential scanning calorimetry (DSC) is now used in many applications where usually DTA had been used. In this technique the difference in heat flow (power) to a sample and to a reference is monitored against temperature while the temperature of the sample is programmed. DSC instruments were able to heat the samples up to 700 jC. Only in recent years some DSC instruments were constructed to heat the samples up to 1600 jC. But to the best of our knowledge until recently they were not used for organo-clay complexes. The temperature at which a reaction starts is defined as the ‘‘onset temperature’’. The peak maximum (or minimum) appears where the rate of heat evolution (or absorption) by the reaction is equal to the difference between the rate of supply of heat to the specimen and to the inert material. The reaction is not complete at this maximum. In modern instruments DTA is recorded simultaneously with TG. By using simultaneous DTA-TG one can differentiate between peaks associated with
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weight loss and those associated with phase transition. DTA has been used very widely in the study of thermal reactions of clay minerals. These reactions include dehydration of adsorbed water, dehydroxylation (evolution of water from hydroxyls belonging to the clay skeleton) followed by the transformation of the clay to a meta-phase and recrystallization of the meta-phase into a crystalline phase. The first two reactions are endothermic whereas the recrystallization of the meta-phase is exothermic (Smykatz-Kloss, 1974; Langier-Kuzniarowa, 1989). The DTA peak temperatures are characteristic for each mineral and DTA curves are applicable for the identification and determination of many clays (Mackenzie, 1957, 1970). Simultaneous DTA-TG is used to differentiate between swelling and non-swelling clay minerals. The intense endothermic water evolution peak at about 100 jC, accompanied by considerable amounts of weight-loss, is characteristic for swelling clays. In case of non-swelling clays this peak may appear but is very weak and is accompanied by a small weight loss. The appearance of this peak in the DTA – DTG curves of non-swelling clays is due to the evaporation of external water. It may be avoided by drying the sample for example in a vacuum cell at 80 jC, before the thermal analysis run.
2. DTA study of organo-clay complexes A comprehensive review of DTA of organo-clays has recently been published by Langier-Kuzniarowa (2001). In previous literature the DTA investigations of organo-clays were carried out with the following objectives: to study thermal reactions of organo-clay complexes (Yariv, 1985); to study the thermal properties and stability of the adsorbed organic matter (Greene-Kelly, 1957); to establish whether organoclay complexes are formed or whether the clay and the organic material are present merely as mixtures (Heller-Kallai et al., 1986); to study the effect of adsorbed organic material on the amount and properties of the adsorbed water (Jordan, 1949; Yariv et al., 1992); to identify and differentiate between different clay minerals (Allaway, 1949; Carthew, 1955; Greene-Kelly, 1957; Ramachandran et al., 1961a, b); to identify and differentiate between various organoclay complexes composed of the same clay and the
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same organic ligand (Bodenheimer et al., 1963a, b, c, 1966); to establish the type of association between organic species and exchangeable metallic cations, water molecules or silicate layers. In the present communication we concentrate mainly on the last objective and on the role of the combustion of charcoal in the DTA study of the fine structure of the complex and of the type of association between the organic compound and the clay mineral. 2.1. DTA curves of organic substances and of organoclay complexes DTA curves of organic substances recorded in oxidizing and inert environments show diagnostic exothermic and endothermic peaks, respectively, associated with combustion, decomposition, dehydration, fusion, vaporization, sublimation and solid-state transitions (Mitchell and Birnie, 1970). Some peaks disappear, the temperatures of other peaks shift and their relative intensities change after adsorption by the clay. DTA of organic material or of organo-clay complexes is carried out either in an oxidizing environment (in air or under a flow of oxygen) or under a flow of an inert gas (such as nitrogen or argon). In air or under oxygen, the adsorbed organic material is oxidized, giving rise to significant exothermic peaks. Under an inert atmosphere, weak endothermic peaks attributed to desorption and pyrolysis of the organic material are obtained. Since the endothermic peaks are very weak, most DTA studies of organo-clay complexes were carried out in air or oxygen atmospheres. Nevertheless the use of an inert atmosphere provides some interesting and valuable information, if complemented by EGA results. An oxidizing atmosphere, however, is much more useful because of the richer data in the form of many sharp exothermic DTA effects, which can characterize the samples investigated (see e.g. Cebulak et al., 2002). Differentiation between various clayorganic complexes is possible because in air each variety gives rise to characteristic exothermic peak temperatures. 2.2. Simultaneous DTA-EGA study of organo-clay complexes A simultaneous DTA-EGA was applied in the study of organo-clay complexes by several investi-
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gators (see e.g. Yariv et al., 1988, 1989a,b,c; Breen et al., 1993; Breen, 1994). In this technique the evolved gases (vaporization, combustion or thermal decomposition products) were analyzed by mass-spectrometry simultaneously with DTA and an unequivocal interpretation of the thermal reactions was obtained. A gas evolution curve is the plot of the counts of collisions of ionized gas molecules having a certain mass on the detector of the spectrometer against the temperature of the reference. Information on the combination of carbon or nitrogen of the adsorbed organic compounds with oxygen (oxidation) is directly obtained from CO2 and NO2 evolution curves, respectively. The information on the combination of hydrogen from adsorbed organic source with airoxygen is more complicated, since water is evolved, in part due to the oxidation of organic matter and in part due to the dehydration and dehydroxylation of the clay. Data on the combination of hydrogens originating from the organic compound with oxygens is obtained by subtracting the H2O evolution curve of the organo-clay complex recorded under N2 from that recorded in air. The difference between the two curves gives information on the oxidation of the hydrogen originating from organic matter. Thermal analysis curves of butylamine- and pyridine-sepiolite are shown in Figs. 1 and 2, respecitvely. The H2O evolution curve obtained in air is called the ‘‘total water evolution curve’’ whereas that obtained under N2 is called the ‘‘inorganic water evolution curve’’. The calculated (subtraction product) curve is called the ‘‘organic water evolution curve’’. Sepiolite from Vallecas, Spain and palygorskite from Quincy, FL, USA, were loaded with butylamine and pyridine representing aliphatic and aromatic amines, respectively. DTA, TG and EGA curves of amine treated clays were recorded in air and under nitrogen (Shuali et al., 1990, 1991). The CO2 evolution curves of butylamine treated clays differ from those of pyridine treated clays. The aliphatic amine shows a plateau between 360 and 725 jC with a small peak at 600 jC in sepiolite (Fig. 1b) and between 390 and 585 jC in palygorskite. The aromatic amine shows a single peak at 540 jC and shoulders at 395 and 720 jC in sepiolite (Fig. 2b) and a single peak at 650 jC in palygorskite. The organic water evolution curves of butylamine-clays differ from those of pyridine-clays. The aliphatic amine shows one continu-
Fig. 1. Thermal analysis of butylamine-treated sepiolite under flow of air. (A) DTG and DTA curves. (B) EGA-MS evolution curves of different masses (H2O evolution curves in air and under nitrogen are shown). (C) EGA-MS calculated evolution curves of ‘‘organic water’’. Heating rate 10 jC/min. Reproduced with permission of the Mineralogical Society of Great Britain and Ireland, from Shuali et al., 1990.
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Fig. 2. Thermal analysis of pyridine-treated sepiolite under flow of air. (A) DTG and DTA curves. (B) EGA-MS evolution curves of different masses (H2O evolution curves in air and under nitrogen are shown). (C) EGA-MS calculated evolution curves of ‘‘organic water’’. Heating rate 10 jC/min. Reproduced with permission of the Mineralogical Society of Great Britain and Ireland, from Shuali et al., 1991.
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high rate of oxidation before the latter. For clay treated with aliphatic amines, due to the fact that the organic molecule has a high H to C atomic ratio, relative to the aromatics, the DTA curve has a profile similar to that of the ‘‘organic water evolution curve’’ (H oxidation). However, the temperatures of the principal exothermic peaks of the DTA curves are determined by the rate of C oxidation, whereas temperatures of exothermic shoulders are determined by the rate of H oxidation. For clays treated with aromatic amines, due to the fact that the organic molecule has a low H to C atomic ratio, the DTA curve has a profile similar to that of CO2 evolution curve and the peak maxima are, in principal, those of the CO2 evolution curve. Peak maxima of the ‘‘organic H2O evolution curve’’ appear as shoulders in the DTA curve (Yariv, 1990). Mass-spectrometer analysis of evolved gases (EGA) during DTA of sepiolite and palygorskite treated with butylamine showed the following gases evolving under nitrogen in temperatures above 200 jC: butylamine (by desorption), NH3 and CH4 (by pyrolysis and cracking), H2 (due to condensation of C to charcoal), traces of CO2 (by thermal hydrolysis) and very small amounts of propane, propanol, ethanol, acetic acid and butene. In the DTA of sepiolite treated with pyridine recorded under nitrogen thermal desorption of pyridine was observed at 260 and 650 jC, whereas in DTA of palygorskite only traces of pyridine were detected among the evolved gases. In addition to desorption, reactions of pyrolysis and condensation to coke were detected by the evolution of NH3, CH4 and H2, and the blackening of the samples. However, the resolution of the DTA endothermic peaks was very poor and it was impossible to relate the endothermic peaks to certain reactions (Shuali et al., 1990, 1991). 2.3. The use of DTA curves as finger-prints in the study of organo-clay complexes
ous step of hydrogen oxidation, whereas the aromatic amine shows two stages, the first at 260– 275 jC (relatively sharp peak) and the second is a broad peak extending between 425 – 765 and 530 – 650 jC in sepiolite and palygorskite, respectively. The profiles of the exothermic region in the DTA curves are governed by the combination of both principal elements of the organic compounds, hydrogen and carbon with oxygen, the former reaches a
DTA curves of montmorillonites treated with ethylenediamine (En) are described here as representative examples for their use as finger prints in the study of organo-clay complexes. When ethylenediamine is adsorbed by montmorillonite from organic or aqueous solutions, neutral amine molecules as well as ammonium cations are detected by IR spectroscopy in the interlayer space (Yariv, 2001a, b). Repeated washings
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with water can leach the former, but not the latter out. In Fig. 3 curves a and b DTA curves of montmorillonite saturated with ethylenediamine unwashed and washed respectively, carried out in air, are presented. Exothermic peaks are obtained, in addition to the large endothermic peaks A and B, which are diagnostic for dehydration and dehydroxylation of montmorillonite. Considerable differences appear between washed and unwashed samples. The DTA curve of the unwashed sample shows a large endothermic peak at 165 jC, attributed to the evolution of free En in addition to the dehydration of the ammonium-clay. The DTA curve of a washed sample shows three exothermic peaks, assigned in the figure by D, E and F, located at 290, 480 and 550 jC, respectively. In the DTA curve of the unwashed sample the exothermic reaction starts at a lower temperature. An exothermic peak at 220 jC (peak C) is attributed to the combustion of adsorbed molecular amine. Other exothermic peaks are broad and not well defined, but they seem to be similar to those of the washed sample (Bodenheimer et al., 1963c). When an aqueous solution of ethylenediamine was added to an aqueous suspension of Cu-montmorillonite the amine was totally adsorbed by the clay up to amine/metal molar ratio of 2.0. The color of the pale blue copper clay changes first to deep blue and then to violet due to the formation of the coordination d-complex cations [CuEn]2 + and [CuEn2]2 +, respectively. With excess amine the following cation exchange occurred to a small extent: ½CuEn2 MontðsÞ þ EnH2þ 2 ðaqÞ 2þ ! EnH2þ 2 MontðsÞ þ ½CuEn2 ðaqÞ
Fig. 3 curves c and d show DTA curves of CuEnmontmorillonite and CuEn2-montmorillonite. Endothermic peaks A and B at 130 and 700 jC, respectively, represent the dehydration and dehydroxylation
Fig. 3. DTA curves of Wyoming bentonite saturated with ethylenediamine; (a) unwashed (ethylenediammonium-ethylenediamine-montmorillonite), (b) washed (ethylenediammonium-montmorillonite); DTA curves of Cu-montmorillonite equilibrated with dilute aqueous solutions of ethylenediamine (c) 20 mmol, (d) 60 mmol and (e) 100 mmol amine per 100 g clay in the equilibrium system; recorded in air. CEC of Cu-montmorillonite is 38 mmol/ 100 g clay.
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of the clay. Exothermic peaks K and M appear at 320 and 500 jC, the latter increases with the amount of amine. They are attributed to amine coordinated to copper. When the molar ratio amine/Cu is above 2.0, additional exothermic peaks D and E appear at 280 and 480 jC, respectively, attributed to EnH22 +-montmorillonite (Fig. 3e). No additional peaks are observed when the amine/Cu ratio is above 3.0. The additional peaks D and E in the latter DTA curve is due to the presence of small amounts of the ammonium cation (Bodenheimer et al., 1963a).
3. The nature of the DTA curves of organo-clay complexes in oxidizing environment 3.1. The shape of DTA curves of organo-clay complexes DTA curves of most organo-clay complexes, recorded either in oxidizing or inert atmospheres, can be divided into three regions (Yariv, 1991): 1. The region of the dehydration of the clay (below 200 or 250 C). In this region endothermic peaks appear which characterize dehydration of the clay (evolution of interlayer or external water), melting, boiling and evaporation of non- or weakly adsorbed organic molecules. 2. The region of the thermal reactions of the organic material. In this region the type of the peaks, whether exothermic or endothermic, depends on the reaction atmosphere. In oxidizing atmosphere exothermic peaks appear characterizing oxidation of organic matter (H and C) and the formation of charcoal. In inert atmosphere endothermic peaks appear characterizing evaporation and decomposition of organic compounds. 3. The region of the dehydroxylation of the clay and recrystallization of the meta-clay (above 500 or 600C). In inert atmosphere endothermic peaks appear characterizing clay dehydroxylation. Exothermic peaks appear at higher temperatures characterizing recrystallization of meta-phases of the clay. In oxidizing atmosphere exothermic peaks appear characterizing oxidation of organic matter, whose oxidation has not been completed in the second region.
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The shape of the first region does not depend on whether the DTA is recorded in an oxidizing or an inert atmosphere. The thermal dehydration of clay usually occurs in the same temperature range in which clay samples, not treated with organic material lose their adsorbed water. Since some of the water is replaced by the adsorbed organic matter, the size of this peak, in the DTA curve of the organo-clay complex, is smaller than that in the DTA curve of the untreated clay. The presence of organic species in the interlayer space, breaks the water-structure and, consequently, the onset and the peak in the curve of the organo-clay appear at lower temperatures compared to the untreated clay. In this region melting and boiling of non-adsorbed organic compounds are identified by their endothermic peaks. This is a way to determine the presence of non-adsorbed organic matter in the mixture. If the adsorbed organic matter has a high vapor pressure (and a low boiling or sublimation point) part of it is evolved in the temperature range of this region, giving rise to endothermic peaks (Eltantawy, 1974; Ovadyahu et al., 1998). 3.2. The exothermic peaks Simultaneous DTA-EGA study of organo-smectite complexes showed that the exothermic oxidation reaction occurring during the gradual heating of the sample takes place in two steps, in the range 200 – 500 and 400 –750 jC (Yariv, 1990). The two-step mechanism was proved by data collected by Yariv (1991) from the literature on DTA curves of different organo-clay complexes. Each step is represented by one or more exothermic peaks. The peak temperature of the exotherm depends on the type of clay mineral and on the organic compound. The two steps of the oxidation of the adsorbed organic matter are as follows: First step = combination of organic hydrogen with oxygen and formation of water and charcoal. Combustion of the organic matter commences at a temperature which is independent on the amount of material present, but is dependent on the activation energy of the combustion reaction. This is the ‘‘onset temperature’’ of the oxidation. The substrate clay mineral serves as a catalyst for the combustion reaction and the onset and peak temperatures differ from
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the temperatures recorded in the DTA curves of the neat organic matter. The nature of the thermal reaction depends on the loading of the clay with the organic matter. If the total amount of the organic material in the DTA cell is small (a few mmol organic matter per 100 g clay) oxidation of H and C will be completed at a relatively low temperature (Bodenheimer et al., 1966). If this amount is high (tens of mmol organic matter per 100 g clay), the available oxygen in the system is insufficient for complete burning. In the latter case, in the first step of the exothermic reactions most of the hydrogen but only part of the carbon are oxidized to H2O and CO2. The other part of carbon forms a black residue which is sometimes defined as ‘‘petroleum coke’’ and sometimes as ‘‘charcoal’’ (Bradley and Grim, 1948; Allaway, 1949). Second step = oxidation of charcoal and formation of CO2 and NO2. Oxidation of the ‘‘black residue’’ is the second step occurring at higher temperatures. The onset temperature of the combustion of charcoal depends on the degree of cross-linking of this complex material and on the size of the polymeric species from which it is composed. These are affected by (a) the rate of the oxygen flow; (b) the composition, size and shape of the parent organic molecule; (c) the type of clay mineral, whether it is a swelling or a nonswelling clay; (d) the surface acidity of the clay and (e) the bonding between the clay and the organic compound. Fig. 4 shows the effect of dilution on the shape of the DTA curves of cyclohexylammonium-montmorillonite diluted with calcined alumina. At low clay concentrations (below 5% of organo-clay) the exothermic peaks E and F corresponding to the second oxidation step are less intense than peak D corresponding to the first oxidation step. They increase with increasing clay concentration until they become the dominant exothermic peaks (curve d). The reactivity of molecules inside the interlayer space during their transformation into charcoal is influenced by the stereoselectivity limitations that the interlayer space imposes on the orientation and packing arrangement of the molecules. The charcoal in the interlayer space is composed of monolayers of carbon atoms the structure of which is imposed by the parallel alumino-silicate layers. On the external surface randomness prevails on the structure of the coke.
Fig. 4. DTA curves of (a – d) cyclohexylammonium montmorillonite (Camp Berteau) diluted with calcined alumina to different concentrations as indicated in the figure.
The completion of the oxidation of the charcoal and the temperature of the last exothermic peak depend on (a) the mineral, (b) the precursor organic compound and (c) the type of bonds between the charcoal and the clay mineral. (a) Non-expanding clay minerals adsorb only small amounts of organic matter onto the external surface (broken bonds) and charcoal is not formed, or, is formed with a small degree of cross-linking. The oxidation of these complexes is complete below 500 jC. Expanding clay minerals adsorb considerable amounts of organic matter into the interlayer space and charcoal is formed during the first step of the oxidation stage of the DTA run with a high degree of cross-linking, and the last step of oxidation occurs at temperatures above 500 jC. (b) In Wyoming bentonite loaded with aliphatic ammonium cations, an increase in the ratio C/N leads to a higher peak temperature. With C/N z 4, this peak appeared with the dehydroxylation of the clay at 650 jC or above this temperature (compare DTA curves of ethylenediammonium-montmorillonite, Fig. 3 with curves of cyclohexylammonium-montmorillonite, Fig. 4).
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(c) A spectrophotometric study revealed that in montmorillonite, but not in Laponite, there are p interactions between the aromatic rings of the cationic dyes and the oxygen plane (Yariv, 2001c). A combined DTA-EGA study of montmorillonite and Laponite complexes of the cationic dyes, crystalviolet and acridine-orange, showed that k interactions between aromatic organic matter (the precursor) and the oxygen plane of the silicate layer increased the thermal stability of the charcoal formed during the first step of the oxidation reaction. The last exothermic peak and the CO2 evolution in the thermal curves of montmorillonite-dye complexes occurred at higher temperatures than in Laponite (Yariv et al., 1988, 1989a, b, c). Simultaneous DTA-TG curves of Na-, Cs-, Mg-, Al- and Fe-montmorillonite treated with the anionic dye congo-red were studied by Yermiyahu et al. (2003). Being negatively charged, this dye does not form k bonds with the oxygen plane of the clay. Inside the interlayer space of montmorillonite some of the adsorbed dye is protonated and the aromatic rings become positively charged. The positive variety of congo-red is able to form k bonds with the clay oxygen plane. Electronic spectroscopy study showed that the protonation of congo-red depends on the acid-strength of the exchangeable cation. It is very high in Fe- or Al-clay and only a small fraction of the adsorbed dye is protonated in Na- or Cs-clay (Fig. 5). From the relative areas and temperatures of the exothermic peaks and the relative weight loss in each stage of the thermal-analysis the authors concluded that two different types of charcoal were obtained, low- and high-temperature-stable charcoal, giving peaks in the second and third regions of the thermal analysis curve, respectively. The high-temperaturestable charcoal was obtained from the protonated positively charged precursor, which formed k bonds with the oxygen plane whereas the low-temperaturestable charcoal was obtained from the negatively charged precursor, which did not form these k bonds. Based on this study the authors concluded that the second step of the oxidation of the adsorbed organic matter could be sub-divided into two sub-steps (a and b) as follows: Second step (a)=(400 –600 jC) oxidation of low temperature stable charcoal and formation of CO2 and NO2 (second region of the DTA curve).
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Fig. 5. DTA curves of congo-red treated (a) Na-montmorillonite, (b) Fe-montmorillonite. The base lines were constructed in such a way that the dehydroxylation endothermic peak was included in the exothermic area.
Second step (b)=(550– 800 jC) oxidation of high temperature stable charcoal and formation of CO2 and NO2 (third region of the DTA curve). A combined DTA-EGA study of montmorillonite and Laponite complexes of the cationic dye rhodamine 6G, was carried out by Yariv et al. (1988). Due to steric hindrance, interactions between the oxygen plane of montmorillonite and rhodamine 6G occur to a small extent only; in this molecule phenyl is sterically constrained to be roughly perpendicular to the planar xanthene group. The DTA curves of laponite and
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montmorillonite treated with rhodamine 6G are similar and most of the charcoal is oxidized before the dehydroxylation of these clays. A very small peak appears in the CO2 evolution curves of montmorillonite but not of laponite, indicating the presence of small amounts of thermally stable charcoal in the interlayer space of montmorillonite, persisting above 600 jC. DTA-TG curves of rhodamine 6G-montmorillonite obtained by mechanochemical adsorption in a dry system did not show any exothermic reaction above 600 jC, suggesting that the dye was located on the external surfaces of the clay (Landau et al., 2002). DTA-TG curves of crystal-violet-montmorillonite complex obtained by a dry mechanochemical adsorption showed a very small exothermic peak at 620 jC, together with the dehydroxylation of the clay, associated with small amounts of weight loss up to 670 jC, suggesting that the dye was located on the external surfaces of the clay. After washing the ground complex with water the DTA-TG curve showed a significant exothermic peak at 670 jC, associated with considerable amounts of weight loss up to 770 jC, suggesting that the dye was located inside the interlayer space of the clay. This curve was similar to the DTA-TG curve of crystal-violet-montmorillonite complex obtained in an aqueous suspension. Thermo-XRD-analysis of unwashed and washed ground samples confirmed these observations. It appears that water is essential for the penetration of cationic dyes into the interlayer space of montmorillonite (Lapides et al., 2002). 3.3. The nature of the charcoal More moles of H2O than CO2 are evolved in the first step of the exothermic reactions, indicating that only part of the equivalent carbon is oxidized to CO2. The rest of the carbon forms petroleum coke or charcoal. The high temperature peaks in CO2 evolution curves represent the oxidation of the charcoal. The evolution of ‘‘organic H2O’’ continues almost up to the last stages of CO2 evolution proving that the initially formed charcoal is composed largely of carbon with some residual hydrogen. Carbonization of organic matter in the interlayer space of montmorillonite was investigated by graphitizing polymers between the lamellae of this clay. The black residue obtained at 700 jC was a film shape highly oriented graphite and highly stacked structure
with small interplanar spacing of 0.337 nm. The formation of such a unique coke is attributed to the peculiar carbonization method where the two dimensional space between the lamellae serves as a unique field for carbonization. Garfinkel-Shweky and Yariv (1997) studied by thermo-XRD-analysis the thermal transformation of vermiculite, montmorillonite, saponite, laponite and beidellite loaded with different amounts of the cationic dye acridine-orange to the charcoal-clay complex. At 300 jC charcoal was formed inside the interlayer space. The basal spacings of 1.26 –1.38 nm obtained after 300 jC were those of the clay complex with the interlayer charcoal. This basal spacing is characteristic for a carbon monolayer. Yermiyahu et al. (2002) studied by thermo-XRDanalysis the thermal products of various montmorillonites treated with the anionic dye congo-red at 360 jC. Some of the charcoal was obtained as monolayers with basal-spacings of 1.10 – 1.33 nm. But some charcoal was obtained as multilayers with basal spacings of 1.61– 2.10 nm. The differences between the charcoal obtained from an anionic dye and that obtained from a cationic dye are due to differences in the orientation of the negative and positive ions in the interlayer space before the thermal treatment.
4. Conclusions The present communication shows that the temperatures of the exothermic peaks in DTA curves of organo-clay complexes are dependent on the nature of the charcoal which is formed during the gradual heating of the clay complex. Since the nature of the charcoal is dependent on the bonding between the clay and the precursor organic compound it seems that after a better understanding of this relationship, DTA will become a useful tool for the study of the type of interactions between the organic compound and the adsorption sites on clay surface in organo-clay complexes.
References Allaway, W.H., 1949. Differential thermal analysis of clays treated with organic cations as an aid in the study of soil colloids. Proc. Soil Sci. Soc. Am. 13, 183 – 188.
S. Yariv / Applied Clay Science 24 (2004) 225–236 Balek, V., Malek, Z., Ehrlicher, U., Gyo¨ryova´, K., Matuschek, G., Yariv, S., 2002. Emanation thermal analysis of TIXOTON (activated bentonite) treated with organic compounds. Appl. Clay Sci. 21, 295 – 302. Bodenheimer, W., Kirson, B., Yariv, S., 1963a. Organometallic clay complexes. Part I. Isr. J. Chem. 1, 69 – 78. Bodenheimer, W., Heller, L., Kirson, B., Yariv, S., 1963b. Organometallic clay complexes. Part III. Rosenqvist, I.Th. (Ed.), Proc. 1st Intern. Clay Conf., Stockholm, 1963, vol. 2. Pergamon, Oxford, pp. 351 – 363. Bodenheimer, W., Heller, L., Kirson, B., Yariv, S., 1963c. Organometallic clay complexes. Part IV. Nickel and mercury aliphatic polyamines. Isr. J. Chem. 1, 391 – 403. Bodenheimer, W., Heller, L., Yariv, S., 1966. Organo-metallic clay complexes: VII. Thermal analysis of montmorillonite-diamine and glycol complexes. Clay Miner. 6, 167 – 177. Bradley, W.F., Grim, R.E., 1948. Colloid properties of layer silicates. J. Phys. Chem. 52, 1404 – 1413. Breen, C., Flynn, J.J., Parkes, G.M.B., 1993. Thermogravimetric, infrared and mass-spectroscopic analysis of the desorption of methanol, propan-1-ol, propan-2-ol and 2-methylpropan-2-ol from montmorillonite. Clay Miner. 28, 123 – 137. Breen, C., 1994. Thermogravimetric, infrared and mass spectroscopic analysis of the desorption of tetrahydropyran, tetrahydrofuran and 1,4-dioxan from montmorillonite. Clay Miner. 29, 115 – 121. Carthew, A.R., 1955. Use of piperdine saturation in the identification of clay minerals by differential thermal analysis. Soil Sci. 80, 337 – 347. Cebulak, S., Langier-Kuzniarowa, A., Grotek, I., 2002. The application of the OTA of kerogen to the evaluation of the rockmass heating. J. Therm. Anal. Calorim. 69, 227 – 235. Eltantawy, I.M., 1974. Differential thermal micro-analysis (DTMA) of clay-organic molecule complexes. Bull. Groupe Fr. Argiles 26, 211 – 218. Garfinkel-Shweky, D., Yariv, S., 1997. The determination of surface basicity of the oxygen planes of expanding clay minerals by acridine orange. J. Colloid Interface Sci. 188, 168 – 175. Greene-Kelly, R., 1957. The montmorillonite minerals (smectites). In: Mackenzie, R.C. (Ed.), The Differential Thermal Investigation of Clays. Mineralogical Society, London, pp. 140 – 164. Heller-Kallai, L., Yariv, S., Friedman, I., 1986. Thermal analysis of the interaction between stearic acid and pyrophyllite or talc. IR and DTA studies. J. Therm. Anal. 31, 95 – 106. Hill, J.O. (Ed.), 1991. For Better Thermal Analysis and Calorimetry, 3rd ed. International Confederation for Thermal Analysis, Bundoora, Australia. Jordan, J.W., 1949. Alteration of the properties of bentonite by reaction with amines. Mineral. Mag. 28, 598 – 605. Lagaly, G., 1984. Clay organic reactions. Philos. Trans. R. Soc. Lond. A311, 315 – 332. Lagaly, G., 1993. Reaktionen der tonmineral. In: Jasmund, K., Lagaly, G. (Eds.), Tonminerale und Tone. Steinkopff Verlag, Darmstadt, pp. 89 – 167. Landau, A., Zaban, A., Lapides, I., Yariv, S., 2002. Simultaneous DTA-TG study of montmorillonite mechanochemically treated with Rhodamine-6G. J. Therm. Anal. Calorim. 70, 103 – 113.
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Langier-Kuzniarowa, A., 1989. The present state of thermal investigations of clays. Thermochim. Acta 148, 413 – 420. Langier-Kuzniarowa, A., 2001. Thermal analysis of Organo-clay complexes. In: Yariv, S., Cross, H. (Eds.), Organo Clay Complexes and Interactions. Marcel Dekker, New York, pp. 273 – 344. Lapides, I., Yariv, S., Golodnitsky, D., 2002. Simultaneous DTATG styudy of montmorillonite mechanochemically treated with crystal-violet. J. Therm. Anal. Calorim. 67, 99 – 112. Mackenzie, R.C. (Ed.), 1957. The Differential Thermal Investigation of Clays. Mineralogical Society (Clay Minerals Group), London. Mackenzie, R.C., 1970. Simple phyllosilicates based on gibbsiteand brucite-like sheets. Mackenzie, R.C. (Ed.), Differential Thermal Analysis. 1. Academic Press, London, pp. 497 – 537. Mitchell, B.D., Birnie, A.C., 1970. Organic compounds. Mackenzie, R.C. (Ed.), Differential Thermal Analysis 1. Academic Press, London, pp. 611 – 641. Morgan, D.J., Warrington, S.B., Warne, S.St.J., 1988. Earth sciences applications of evolved gas analysis: a review. Thermochim. Acta 135, 207 – 212. Mortland, M.M., 1970. Clay organic complexes and interactions. Adv. Agron. 22, 75 – 117. Mu¨ller-Vonmoos, M., Kahr, G., Rub, A., 1977. DTA-TG-MS in the investigation of clays. Quantitative determination of water, carbon monoxide and carbon dioxide by evolved gas analysis with mass spectrometer. Thermochim. Acta 20, 386 – 393. Ovadyahu, D., Yariv, S., Lapides, I., Deutsch, Y., 1998. Mechanochemical adsorption of phenol by TOT swelling clay minerals: II. Simultaneous DTA and TG study. J. Therm. Anal. 51, 431 – 447. Paulik, F., 1995. Special Trends in Thermal Analysis. Wiley, Chichester. Paulik, F., Paulik, J., Arnold, M., Inczedy, J., Kristof, J., LangierKuzniarowa, A., 1989. Simultaneous TG, DTG, DTA and EGA examination of argillaceous rocks. Part I. J. Therm. Anal. 25, 1849 – 1860. Ramachandran, V.S., Garg, S.P., Kacker, K.P., 1961a. Mechanism of thermal decomposition of organo-montmorillonites. Chem. Ind., 790 – 792. Ramachandran, V.S., Kacker, K.P., Patwardhan, N.K., 1961b. Basic dyestuffs in clay mineralogy. Nature 191, 696. Rausell-Colom, J.A., Serratosa, J.M., 1987. Reactions of clays with organic substances. In: Newman, A.C.D. (Ed.), Chemistry of Clays and Clay Minerals. The Mineralogical Society, London, pp. 371 – 422. Shuali, U., Yariv, S., Steinberg, M., Mu¨ller-Vonmoos, M., Kahr, G., Rub, A., 1990. Thermal analysis of sepiolite and palygorskite treated with butylamine. Clay Miner. 25, 107 – 119. Shuali, U., Yariv, S., Steinberg, M., Mu¨ller-Vonmoos, M., Kahr, G., Rub, A., 1991. Thermal analysis of pyridine-treated sepiolite and palygorskite. Clay Miner. 26, 497 – 506. Smykatz-Kloss, W., 1974. Differntial Thermal Analysis. Application and Results in Mineralogy, Springer, Berlin. Theng, B.K.G., 1974. The Chemistry of Clay-Organic Reactions Adam Hilger, London. Theng, B.K.G., 1979. Formation and Properties of Clay-Polymer Complexes. Elsevier, Amsterdam.
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Weiss, A., 1969. Organic derivatives of clay minerals. In: Eglinton, G., Murphy, M.T.J. (Eds.), Organic Geochemistry. Springer, Berlin, pp. 737 – 781. Yariv, S., 1985. Study of the adsorption of organic molecules on clay minerals by differential thermal analysis. Thermochim. Acta 88, 49 – 68. Yariv, S., 1990. Combined DTA-mass spectroscopy of organo-clay complexes. J. Therm. Anal. 36, 1953 – 1961. Yariv, S., 1991. Differential thermal analysis (DTA) of organo-clay complexes. In: Smykatz-Kloss, W., Warne, S.St.J. (Eds.), Thermal Analysis in the Geosciences. Lecture Notes in Earth Sciences, vol. 38. Springer, Berlin, pp. 328 – 351. Yariv, S., 1996. R-spectroscopy analysis of the interactions between organic pollutants and clay minerals. Thermochim. Acta 274, 1 – 35. Yariv, S., 2000. The use of infrared spectroscopy in the study of the interaction of organic compounds and smectite clay minerals. Gomes, S.S.F. (Ed.), Proc. 1st. Latin-American Clay Conf. Funchal, 2000. University of Aveiro, Aveiro, Portugal, pp. 187 – 212. Yariv, S., 2001a. Introduction to organo-clay complexes and interactions. In: Yariv, S., Cross, H. (Eds.), Organo-Clay Complexes and Interactions. Marcel Dekker, New York, pp. 39 – 111. Yariv, S., 2001b. IR spectroscopy and thermo-IR-spectroscopy in the study of the fine structure of organo-clay complexes. In: Yariv, S., Cross, H. (Eds.), Organo-Clay Complexes and Interactions. Marcel Dekker, New York, pp. 463 – 566. Yariv, S., 2001c. Staining or clay minerals and visible absorption
spectroscopy of dye-clay complexes. In: Yariv, S., Cross, H. (Eds.), Organo-Clay Complexes and Interactions. Marcel Dekker, New York, pp. 463 – 566. Yariv, S., Cross, H., 1979. Geochemistry of Colloid Systems Springer, Berlin. Yariv, S., Kahr, G., Rub, A., 1988. Thermal analysis of the adsorption of rhodamine 6G by smectite minerals. Thermochim. Acta 135, 299 – 306. Yariv, S., Mu¨ller-Vonmoos, M., Kahr, G., Rub, A., 1989a. Thermal analytic study of the adsorption of crystal violet by montmorillonite. Thermochim. Acta 148, 457 – 466. Yariv, S., Mu¨ller-Vonmoos, M., Kahr, G., Rub, A., 1989b. Thermal analytic study of the adsorption of crystal violet by laponite. J. Therm. Anal. 35, 1941 – 1952. Yariv, S., Mu¨ller-Vonmoos, M., Kahr, G., Rub, A., 1989c. Thermal analytic study of the adsorption of acridine orange by smectite minerals. J. Therm. Anal. 35, 1997 – 2008. Yariv, S., Ovadyahu, D., Nasser, A., Shuali, U., Lahav, N., 1992. Thermal analysis study of heat of dehydration of tributylammonium smectites. Thermochim. Acta 207, 103 – 113. Yermiyahu, Z., Lapides, I., Yariv, S., 2002. Thermo-XRD analysis of the adsorption of congo-red by montmorillonite saturated with different cations. J. Therm. Anal. Calorim. 69, 317 – 332. Yermiyahu, Z., Landau, A., Zaban, A., Lapides, I., Yariv, S., 2003. Monoionic montmorillonites treated with Congo-Red. Differential thermal analysis study. J. Therm. Anal. Calorim. 72, 431 – 441.
The role of charcoal on DTA curves of organo-clay complexes: an overview Shmuel Yariv * Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received 25 December 2002; received in revised form 24 March 2003; accepted 8 April 2003
Abstract DTA of organo-clay complexes supplemented by other thermal analysis methods supplies information on the thermal reactions, properties and stability of the complex, the amount and properties of the adsorbed water in the organo-clay and on the bonding between the organic species and the clay. It is used to identify the mineral to differentiate between various complexes composed of the same clay and the same organic ligand and to establish their composition. During the gradual heating in oxidizing atmospheres the adsorbed organic material is oxidized, giving rise to significant exothermic peaks. DTA curves of organo-clays are divided into three regions: (1) the dehydration of the clay, (2) the thermal oxidation of the organic material and (3) the dehydroxylation of the clay. The exothermic oxidation reaction occurring during the gradual heating of the sample takes place in two steps, in the range 200 – 500 jC, oxidation of organic hydrogen and formation of water and charcoal, and 400 – 750 jC, oxidation of charcoal and formation of CO2. The exothermic peak temperatures depend on the mineral and on the organic compound and on the types of bonding between these two components of the organo-clay complex. The present communication concentrates in the role of the combustion of charcoal in the study of the fine structure of the complex and of the type of associations between the organic compound and the clay. D 2003 Elsevier B.V. All rights reserved. Keywords: Charcoal; DTA; Organo-clay complexes; Palygorskite; Sepiolite; Smectite; Thermal analysis; Water evolution curves
1. Introduction 1.1. Organo-clay complexes Adsorption of organic matter by clay minerals is one of the most widespread reactions in and on the earth, in the environment, agriculture and in various industrial processes. It includes (i) exchange of initially present inorganic ions by organic ions and (ii)
* Tel.: +972-2-6585-617; fax: +972-2-6585-319. E-mail address: [email protected] (S. Yariv). 0169-1317/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2003.04.002
accumulation of polar and nonpolar molecules on the surface of the mineral. Organic cations, anions or molecules are transferred from solutions, liquid or gaseous states to the clay surface. During adsorption, physical or chemical bonds (long- or short-range interactions, respectively) are formed between the mineral and the organic matter. The adsorption products are defined as organo-clay complexes. The adsorption of organic materials by clay minerals and the different adsorption mechanisms have been widely investigated during the last century and have been reviewed (see e.g., Weiss, 1969; Mortland, 1970; Theng, 1974, 1979; Yariv and Cross, 1979; Lagaly,
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1984, 1993; Rausell-Colom and Serratosa, 1987; Yariv, 2001a). 1.2. Thermal-analysis of organo-clay complexes Following the recommendation of ICTAC (International Confederation for Thermal Analysis and Calorimetry), the definition of the various techniques comprising thermal analysis is as follows: ‘‘Thermal analysis is a group of techniques in which a property of the sample is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed. The program may involve heating or cooling at a fixed rate of temperature change, or holding the temperature constant, or any sequences of these’’ (Hill, 1991). The most applied thermal analysis methods in the study of organo-clay complexes are: thermo-IR spectroscopy analysis (Yariv, 1996, 1999, 2000, 2001b); thermoXRD analysis (Yermiyahu et al., 2002); evolved-gas analysis (EGA) (Mu¨ller-Vonmoos et al., 1977; Morgan et al., 1988; Paulik et al., 1989; Yariv, 1990); thermogravimetry (TG); differential-thermal analysis (DTA) (Paulik, 1995) and differential-scanning calorimetry (DSC). Recently emanation thermal analysis (ETA) showed its usefulness in the study of thermal changes of the microstructure of organo-clay complexes (Balek et al., 2002). During the thermal treatment of organo-clays in the temperature range 250 –300 jC the adsorbed organic compound is transformed into charcoal. The features of the charcoal depend very much on the nature of the precursor organo-clay. Consequently, the study of the charcoal contributes to the interpretation of the results of the different thermal analysis methods. In the thermo-IR spectroscopy analysis and thermoXRD analysis the sample is heated to different temperatures and its IR spectrum or X-ray diffractogram, respectively, are recorded after each thermal treatment. From the different spectra or diffractograms, one gets information about reversible and irreversible changes due to temperature. In the evolved-gas analysis the evolved gases during the thermal treatment pass through either a mass spectrophotometer, through an infrared spectrometer or through gastitrimeter, where the evovled gases and their relative evolution with temperature are determined. In the thermogravimetry the mass of the sample is continuously monitored
during a programmed heating and a curve describing mass-loss against time or temperature is recorded. In order to enhance the steps in the thermogravimetric curve, the derivative-thermogravimetric (DTG) trace is frequently used. This is the plot of the rate of mass change with time. In the DTA technique the test sample in one holder and an inert reference material (such as calcined kaolinite or alumina) in another holder are gradually heated side-by-side from the same source. A thermocouple is positioned in the test sample and in the reference material. A thermal reaction in the test sample, whether exothermic or endothermic, gives rise to difference in temperatures between the two holders, measured by the thermocouple. If the difference in temperature (DT) between the test sample and the reference sample is plotted against the temperature of the latter (T) a DTA curve is obtained. A straight line parallel to the T axis is recorded as long as no thermal reaction occurs in the holder of the sample. When a thermal reaction occurs, a peak develops on the DTA curve. This peak can be either exothermic or endothermic and its area is proportional to the magnitude of the evolved or absorbed energy, respectively. In recent years there has been much progress in the accuracy of the measurement of energy evolved or absorbed by the analyzed sample during the DTA run. Differential scanning calorimetry (DSC) is now used in many applications where usually DTA had been used. In this technique the difference in heat flow (power) to a sample and to a reference is monitored against temperature while the temperature of the sample is programmed. DSC instruments were able to heat the samples up to 700 jC. Only in recent years some DSC instruments were constructed to heat the samples up to 1600 jC. But to the best of our knowledge until recently they were not used for organo-clay complexes. The temperature at which a reaction starts is defined as the ‘‘onset temperature’’. The peak maximum (or minimum) appears where the rate of heat evolution (or absorption) by the reaction is equal to the difference between the rate of supply of heat to the specimen and to the inert material. The reaction is not complete at this maximum. In modern instruments DTA is recorded simultaneously with TG. By using simultaneous DTA-TG one can differentiate between peaks associated with
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weight loss and those associated with phase transition. DTA has been used very widely in the study of thermal reactions of clay minerals. These reactions include dehydration of adsorbed water, dehydroxylation (evolution of water from hydroxyls belonging to the clay skeleton) followed by the transformation of the clay to a meta-phase and recrystallization of the meta-phase into a crystalline phase. The first two reactions are endothermic whereas the recrystallization of the meta-phase is exothermic (Smykatz-Kloss, 1974; Langier-Kuzniarowa, 1989). The DTA peak temperatures are characteristic for each mineral and DTA curves are applicable for the identification and determination of many clays (Mackenzie, 1957, 1970). Simultaneous DTA-TG is used to differentiate between swelling and non-swelling clay minerals. The intense endothermic water evolution peak at about 100 jC, accompanied by considerable amounts of weight-loss, is characteristic for swelling clays. In case of non-swelling clays this peak may appear but is very weak and is accompanied by a small weight loss. The appearance of this peak in the DTA – DTG curves of non-swelling clays is due to the evaporation of external water. It may be avoided by drying the sample for example in a vacuum cell at 80 jC, before the thermal analysis run.
2. DTA study of organo-clay complexes A comprehensive review of DTA of organo-clays has recently been published by Langier-Kuzniarowa (2001). In previous literature the DTA investigations of organo-clays were carried out with the following objectives: to study thermal reactions of organo-clay complexes (Yariv, 1985); to study the thermal properties and stability of the adsorbed organic matter (Greene-Kelly, 1957); to establish whether organoclay complexes are formed or whether the clay and the organic material are present merely as mixtures (Heller-Kallai et al., 1986); to study the effect of adsorbed organic material on the amount and properties of the adsorbed water (Jordan, 1949; Yariv et al., 1992); to identify and differentiate between different clay minerals (Allaway, 1949; Carthew, 1955; Greene-Kelly, 1957; Ramachandran et al., 1961a, b); to identify and differentiate between various organoclay complexes composed of the same clay and the
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same organic ligand (Bodenheimer et al., 1963a, b, c, 1966); to establish the type of association between organic species and exchangeable metallic cations, water molecules or silicate layers. In the present communication we concentrate mainly on the last objective and on the role of the combustion of charcoal in the DTA study of the fine structure of the complex and of the type of association between the organic compound and the clay mineral. 2.1. DTA curves of organic substances and of organoclay complexes DTA curves of organic substances recorded in oxidizing and inert environments show diagnostic exothermic and endothermic peaks, respectively, associated with combustion, decomposition, dehydration, fusion, vaporization, sublimation and solid-state transitions (Mitchell and Birnie, 1970). Some peaks disappear, the temperatures of other peaks shift and their relative intensities change after adsorption by the clay. DTA of organic material or of organo-clay complexes is carried out either in an oxidizing environment (in air or under a flow of oxygen) or under a flow of an inert gas (such as nitrogen or argon). In air or under oxygen, the adsorbed organic material is oxidized, giving rise to significant exothermic peaks. Under an inert atmosphere, weak endothermic peaks attributed to desorption and pyrolysis of the organic material are obtained. Since the endothermic peaks are very weak, most DTA studies of organo-clay complexes were carried out in air or oxygen atmospheres. Nevertheless the use of an inert atmosphere provides some interesting and valuable information, if complemented by EGA results. An oxidizing atmosphere, however, is much more useful because of the richer data in the form of many sharp exothermic DTA effects, which can characterize the samples investigated (see e.g. Cebulak et al., 2002). Differentiation between various clayorganic complexes is possible because in air each variety gives rise to characteristic exothermic peak temperatures. 2.2. Simultaneous DTA-EGA study of organo-clay complexes A simultaneous DTA-EGA was applied in the study of organo-clay complexes by several investi-
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gators (see e.g. Yariv et al., 1988, 1989a,b,c; Breen et al., 1993; Breen, 1994). In this technique the evolved gases (vaporization, combustion or thermal decomposition products) were analyzed by mass-spectrometry simultaneously with DTA and an unequivocal interpretation of the thermal reactions was obtained. A gas evolution curve is the plot of the counts of collisions of ionized gas molecules having a certain mass on the detector of the spectrometer against the temperature of the reference. Information on the combination of carbon or nitrogen of the adsorbed organic compounds with oxygen (oxidation) is directly obtained from CO2 and NO2 evolution curves, respectively. The information on the combination of hydrogen from adsorbed organic source with airoxygen is more complicated, since water is evolved, in part due to the oxidation of organic matter and in part due to the dehydration and dehydroxylation of the clay. Data on the combination of hydrogens originating from the organic compound with oxygens is obtained by subtracting the H2O evolution curve of the organo-clay complex recorded under N2 from that recorded in air. The difference between the two curves gives information on the oxidation of the hydrogen originating from organic matter. Thermal analysis curves of butylamine- and pyridine-sepiolite are shown in Figs. 1 and 2, respecitvely. The H2O evolution curve obtained in air is called the ‘‘total water evolution curve’’ whereas that obtained under N2 is called the ‘‘inorganic water evolution curve’’. The calculated (subtraction product) curve is called the ‘‘organic water evolution curve’’. Sepiolite from Vallecas, Spain and palygorskite from Quincy, FL, USA, were loaded with butylamine and pyridine representing aliphatic and aromatic amines, respectively. DTA, TG and EGA curves of amine treated clays were recorded in air and under nitrogen (Shuali et al., 1990, 1991). The CO2 evolution curves of butylamine treated clays differ from those of pyridine treated clays. The aliphatic amine shows a plateau between 360 and 725 jC with a small peak at 600 jC in sepiolite (Fig. 1b) and between 390 and 585 jC in palygorskite. The aromatic amine shows a single peak at 540 jC and shoulders at 395 and 720 jC in sepiolite (Fig. 2b) and a single peak at 650 jC in palygorskite. The organic water evolution curves of butylamine-clays differ from those of pyridine-clays. The aliphatic amine shows one continu-
Fig. 1. Thermal analysis of butylamine-treated sepiolite under flow of air. (A) DTG and DTA curves. (B) EGA-MS evolution curves of different masses (H2O evolution curves in air and under nitrogen are shown). (C) EGA-MS calculated evolution curves of ‘‘organic water’’. Heating rate 10 jC/min. Reproduced with permission of the Mineralogical Society of Great Britain and Ireland, from Shuali et al., 1990.
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Fig. 2. Thermal analysis of pyridine-treated sepiolite under flow of air. (A) DTG and DTA curves. (B) EGA-MS evolution curves of different masses (H2O evolution curves in air and under nitrogen are shown). (C) EGA-MS calculated evolution curves of ‘‘organic water’’. Heating rate 10 jC/min. Reproduced with permission of the Mineralogical Society of Great Britain and Ireland, from Shuali et al., 1991.
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high rate of oxidation before the latter. For clay treated with aliphatic amines, due to the fact that the organic molecule has a high H to C atomic ratio, relative to the aromatics, the DTA curve has a profile similar to that of the ‘‘organic water evolution curve’’ (H oxidation). However, the temperatures of the principal exothermic peaks of the DTA curves are determined by the rate of C oxidation, whereas temperatures of exothermic shoulders are determined by the rate of H oxidation. For clays treated with aromatic amines, due to the fact that the organic molecule has a low H to C atomic ratio, the DTA curve has a profile similar to that of CO2 evolution curve and the peak maxima are, in principal, those of the CO2 evolution curve. Peak maxima of the ‘‘organic H2O evolution curve’’ appear as shoulders in the DTA curve (Yariv, 1990). Mass-spectrometer analysis of evolved gases (EGA) during DTA of sepiolite and palygorskite treated with butylamine showed the following gases evolving under nitrogen in temperatures above 200 jC: butylamine (by desorption), NH3 and CH4 (by pyrolysis and cracking), H2 (due to condensation of C to charcoal), traces of CO2 (by thermal hydrolysis) and very small amounts of propane, propanol, ethanol, acetic acid and butene. In the DTA of sepiolite treated with pyridine recorded under nitrogen thermal desorption of pyridine was observed at 260 and 650 jC, whereas in DTA of palygorskite only traces of pyridine were detected among the evolved gases. In addition to desorption, reactions of pyrolysis and condensation to coke were detected by the evolution of NH3, CH4 and H2, and the blackening of the samples. However, the resolution of the DTA endothermic peaks was very poor and it was impossible to relate the endothermic peaks to certain reactions (Shuali et al., 1990, 1991). 2.3. The use of DTA curves as finger-prints in the study of organo-clay complexes
ous step of hydrogen oxidation, whereas the aromatic amine shows two stages, the first at 260– 275 jC (relatively sharp peak) and the second is a broad peak extending between 425 – 765 and 530 – 650 jC in sepiolite and palygorskite, respectively. The profiles of the exothermic region in the DTA curves are governed by the combination of both principal elements of the organic compounds, hydrogen and carbon with oxygen, the former reaches a
DTA curves of montmorillonites treated with ethylenediamine (En) are described here as representative examples for their use as finger prints in the study of organo-clay complexes. When ethylenediamine is adsorbed by montmorillonite from organic or aqueous solutions, neutral amine molecules as well as ammonium cations are detected by IR spectroscopy in the interlayer space (Yariv, 2001a, b). Repeated washings
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with water can leach the former, but not the latter out. In Fig. 3 curves a and b DTA curves of montmorillonite saturated with ethylenediamine unwashed and washed respectively, carried out in air, are presented. Exothermic peaks are obtained, in addition to the large endothermic peaks A and B, which are diagnostic for dehydration and dehydroxylation of montmorillonite. Considerable differences appear between washed and unwashed samples. The DTA curve of the unwashed sample shows a large endothermic peak at 165 jC, attributed to the evolution of free En in addition to the dehydration of the ammonium-clay. The DTA curve of a washed sample shows three exothermic peaks, assigned in the figure by D, E and F, located at 290, 480 and 550 jC, respectively. In the DTA curve of the unwashed sample the exothermic reaction starts at a lower temperature. An exothermic peak at 220 jC (peak C) is attributed to the combustion of adsorbed molecular amine. Other exothermic peaks are broad and not well defined, but they seem to be similar to those of the washed sample (Bodenheimer et al., 1963c). When an aqueous solution of ethylenediamine was added to an aqueous suspension of Cu-montmorillonite the amine was totally adsorbed by the clay up to amine/metal molar ratio of 2.0. The color of the pale blue copper clay changes first to deep blue and then to violet due to the formation of the coordination d-complex cations [CuEn]2 + and [CuEn2]2 +, respectively. With excess amine the following cation exchange occurred to a small extent: ½CuEn2 MontðsÞ þ EnH2þ 2 ðaqÞ 2þ ! EnH2þ 2 MontðsÞ þ ½CuEn2 ðaqÞ
Fig. 3 curves c and d show DTA curves of CuEnmontmorillonite and CuEn2-montmorillonite. Endothermic peaks A and B at 130 and 700 jC, respectively, represent the dehydration and dehydroxylation
Fig. 3. DTA curves of Wyoming bentonite saturated with ethylenediamine; (a) unwashed (ethylenediammonium-ethylenediamine-montmorillonite), (b) washed (ethylenediammonium-montmorillonite); DTA curves of Cu-montmorillonite equilibrated with dilute aqueous solutions of ethylenediamine (c) 20 mmol, (d) 60 mmol and (e) 100 mmol amine per 100 g clay in the equilibrium system; recorded in air. CEC of Cu-montmorillonite is 38 mmol/ 100 g clay.
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of the clay. Exothermic peaks K and M appear at 320 and 500 jC, the latter increases with the amount of amine. They are attributed to amine coordinated to copper. When the molar ratio amine/Cu is above 2.0, additional exothermic peaks D and E appear at 280 and 480 jC, respectively, attributed to EnH22 +-montmorillonite (Fig. 3e). No additional peaks are observed when the amine/Cu ratio is above 3.0. The additional peaks D and E in the latter DTA curve is due to the presence of small amounts of the ammonium cation (Bodenheimer et al., 1963a).
3. The nature of the DTA curves of organo-clay complexes in oxidizing environment 3.1. The shape of DTA curves of organo-clay complexes DTA curves of most organo-clay complexes, recorded either in oxidizing or inert atmospheres, can be divided into three regions (Yariv, 1991): 1. The region of the dehydration of the clay (below 200 or 250 C). In this region endothermic peaks appear which characterize dehydration of the clay (evolution of interlayer or external water), melting, boiling and evaporation of non- or weakly adsorbed organic molecules. 2. The region of the thermal reactions of the organic material. In this region the type of the peaks, whether exothermic or endothermic, depends on the reaction atmosphere. In oxidizing atmosphere exothermic peaks appear characterizing oxidation of organic matter (H and C) and the formation of charcoal. In inert atmosphere endothermic peaks appear characterizing evaporation and decomposition of organic compounds. 3. The region of the dehydroxylation of the clay and recrystallization of the meta-clay (above 500 or 600C). In inert atmosphere endothermic peaks appear characterizing clay dehydroxylation. Exothermic peaks appear at higher temperatures characterizing recrystallization of meta-phases of the clay. In oxidizing atmosphere exothermic peaks appear characterizing oxidation of organic matter, whose oxidation has not been completed in the second region.
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The shape of the first region does not depend on whether the DTA is recorded in an oxidizing or an inert atmosphere. The thermal dehydration of clay usually occurs in the same temperature range in which clay samples, not treated with organic material lose their adsorbed water. Since some of the water is replaced by the adsorbed organic matter, the size of this peak, in the DTA curve of the organo-clay complex, is smaller than that in the DTA curve of the untreated clay. The presence of organic species in the interlayer space, breaks the water-structure and, consequently, the onset and the peak in the curve of the organo-clay appear at lower temperatures compared to the untreated clay. In this region melting and boiling of non-adsorbed organic compounds are identified by their endothermic peaks. This is a way to determine the presence of non-adsorbed organic matter in the mixture. If the adsorbed organic matter has a high vapor pressure (and a low boiling or sublimation point) part of it is evolved in the temperature range of this region, giving rise to endothermic peaks (Eltantawy, 1974; Ovadyahu et al., 1998). 3.2. The exothermic peaks Simultaneous DTA-EGA study of organo-smectite complexes showed that the exothermic oxidation reaction occurring during the gradual heating of the sample takes place in two steps, in the range 200 – 500 and 400 –750 jC (Yariv, 1990). The two-step mechanism was proved by data collected by Yariv (1991) from the literature on DTA curves of different organo-clay complexes. Each step is represented by one or more exothermic peaks. The peak temperature of the exotherm depends on the type of clay mineral and on the organic compound. The two steps of the oxidation of the adsorbed organic matter are as follows: First step = combination of organic hydrogen with oxygen and formation of water and charcoal. Combustion of the organic matter commences at a temperature which is independent on the amount of material present, but is dependent on the activation energy of the combustion reaction. This is the ‘‘onset temperature’’ of the oxidation. The substrate clay mineral serves as a catalyst for the combustion reaction and the onset and peak temperatures differ from
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the temperatures recorded in the DTA curves of the neat organic matter. The nature of the thermal reaction depends on the loading of the clay with the organic matter. If the total amount of the organic material in the DTA cell is small (a few mmol organic matter per 100 g clay) oxidation of H and C will be completed at a relatively low temperature (Bodenheimer et al., 1966). If this amount is high (tens of mmol organic matter per 100 g clay), the available oxygen in the system is insufficient for complete burning. In the latter case, in the first step of the exothermic reactions most of the hydrogen but only part of the carbon are oxidized to H2O and CO2. The other part of carbon forms a black residue which is sometimes defined as ‘‘petroleum coke’’ and sometimes as ‘‘charcoal’’ (Bradley and Grim, 1948; Allaway, 1949). Second step = oxidation of charcoal and formation of CO2 and NO2. Oxidation of the ‘‘black residue’’ is the second step occurring at higher temperatures. The onset temperature of the combustion of charcoal depends on the degree of cross-linking of this complex material and on the size of the polymeric species from which it is composed. These are affected by (a) the rate of the oxygen flow; (b) the composition, size and shape of the parent organic molecule; (c) the type of clay mineral, whether it is a swelling or a nonswelling clay; (d) the surface acidity of the clay and (e) the bonding between the clay and the organic compound. Fig. 4 shows the effect of dilution on the shape of the DTA curves of cyclohexylammonium-montmorillonite diluted with calcined alumina. At low clay concentrations (below 5% of organo-clay) the exothermic peaks E and F corresponding to the second oxidation step are less intense than peak D corresponding to the first oxidation step. They increase with increasing clay concentration until they become the dominant exothermic peaks (curve d). The reactivity of molecules inside the interlayer space during their transformation into charcoal is influenced by the stereoselectivity limitations that the interlayer space imposes on the orientation and packing arrangement of the molecules. The charcoal in the interlayer space is composed of monolayers of carbon atoms the structure of which is imposed by the parallel alumino-silicate layers. On the external surface randomness prevails on the structure of the coke.
Fig. 4. DTA curves of (a – d) cyclohexylammonium montmorillonite (Camp Berteau) diluted with calcined alumina to different concentrations as indicated in the figure.
The completion of the oxidation of the charcoal and the temperature of the last exothermic peak depend on (a) the mineral, (b) the precursor organic compound and (c) the type of bonds between the charcoal and the clay mineral. (a) Non-expanding clay minerals adsorb only small amounts of organic matter onto the external surface (broken bonds) and charcoal is not formed, or, is formed with a small degree of cross-linking. The oxidation of these complexes is complete below 500 jC. Expanding clay minerals adsorb considerable amounts of organic matter into the interlayer space and charcoal is formed during the first step of the oxidation stage of the DTA run with a high degree of cross-linking, and the last step of oxidation occurs at temperatures above 500 jC. (b) In Wyoming bentonite loaded with aliphatic ammonium cations, an increase in the ratio C/N leads to a higher peak temperature. With C/N z 4, this peak appeared with the dehydroxylation of the clay at 650 jC or above this temperature (compare DTA curves of ethylenediammonium-montmorillonite, Fig. 3 with curves of cyclohexylammonium-montmorillonite, Fig. 4).
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(c) A spectrophotometric study revealed that in montmorillonite, but not in Laponite, there are p interactions between the aromatic rings of the cationic dyes and the oxygen plane (Yariv, 2001c). A combined DTA-EGA study of montmorillonite and Laponite complexes of the cationic dyes, crystalviolet and acridine-orange, showed that k interactions between aromatic organic matter (the precursor) and the oxygen plane of the silicate layer increased the thermal stability of the charcoal formed during the first step of the oxidation reaction. The last exothermic peak and the CO2 evolution in the thermal curves of montmorillonite-dye complexes occurred at higher temperatures than in Laponite (Yariv et al., 1988, 1989a, b, c). Simultaneous DTA-TG curves of Na-, Cs-, Mg-, Al- and Fe-montmorillonite treated with the anionic dye congo-red were studied by Yermiyahu et al. (2003). Being negatively charged, this dye does not form k bonds with the oxygen plane of the clay. Inside the interlayer space of montmorillonite some of the adsorbed dye is protonated and the aromatic rings become positively charged. The positive variety of congo-red is able to form k bonds with the clay oxygen plane. Electronic spectroscopy study showed that the protonation of congo-red depends on the acid-strength of the exchangeable cation. It is very high in Fe- or Al-clay and only a small fraction of the adsorbed dye is protonated in Na- or Cs-clay (Fig. 5). From the relative areas and temperatures of the exothermic peaks and the relative weight loss in each stage of the thermal-analysis the authors concluded that two different types of charcoal were obtained, low- and high-temperature-stable charcoal, giving peaks in the second and third regions of the thermal analysis curve, respectively. The high-temperaturestable charcoal was obtained from the protonated positively charged precursor, which formed k bonds with the oxygen plane whereas the low-temperaturestable charcoal was obtained from the negatively charged precursor, which did not form these k bonds. Based on this study the authors concluded that the second step of the oxidation of the adsorbed organic matter could be sub-divided into two sub-steps (a and b) as follows: Second step (a)=(400 –600 jC) oxidation of low temperature stable charcoal and formation of CO2 and NO2 (second region of the DTA curve).
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Fig. 5. DTA curves of congo-red treated (a) Na-montmorillonite, (b) Fe-montmorillonite. The base lines were constructed in such a way that the dehydroxylation endothermic peak was included in the exothermic area.
Second step (b)=(550– 800 jC) oxidation of high temperature stable charcoal and formation of CO2 and NO2 (third region of the DTA curve). A combined DTA-EGA study of montmorillonite and Laponite complexes of the cationic dye rhodamine 6G, was carried out by Yariv et al. (1988). Due to steric hindrance, interactions between the oxygen plane of montmorillonite and rhodamine 6G occur to a small extent only; in this molecule phenyl is sterically constrained to be roughly perpendicular to the planar xanthene group. The DTA curves of laponite and
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montmorillonite treated with rhodamine 6G are similar and most of the charcoal is oxidized before the dehydroxylation of these clays. A very small peak appears in the CO2 evolution curves of montmorillonite but not of laponite, indicating the presence of small amounts of thermally stable charcoal in the interlayer space of montmorillonite, persisting above 600 jC. DTA-TG curves of rhodamine 6G-montmorillonite obtained by mechanochemical adsorption in a dry system did not show any exothermic reaction above 600 jC, suggesting that the dye was located on the external surfaces of the clay (Landau et al., 2002). DTA-TG curves of crystal-violet-montmorillonite complex obtained by a dry mechanochemical adsorption showed a very small exothermic peak at 620 jC, together with the dehydroxylation of the clay, associated with small amounts of weight loss up to 670 jC, suggesting that the dye was located on the external surfaces of the clay. After washing the ground complex with water the DTA-TG curve showed a significant exothermic peak at 670 jC, associated with considerable amounts of weight loss up to 770 jC, suggesting that the dye was located inside the interlayer space of the clay. This curve was similar to the DTA-TG curve of crystal-violet-montmorillonite complex obtained in an aqueous suspension. Thermo-XRD-analysis of unwashed and washed ground samples confirmed these observations. It appears that water is essential for the penetration of cationic dyes into the interlayer space of montmorillonite (Lapides et al., 2002). 3.3. The nature of the charcoal More moles of H2O than CO2 are evolved in the first step of the exothermic reactions, indicating that only part of the equivalent carbon is oxidized to CO2. The rest of the carbon forms petroleum coke or charcoal. The high temperature peaks in CO2 evolution curves represent the oxidation of the charcoal. The evolution of ‘‘organic H2O’’ continues almost up to the last stages of CO2 evolution proving that the initially formed charcoal is composed largely of carbon with some residual hydrogen. Carbonization of organic matter in the interlayer space of montmorillonite was investigated by graphitizing polymers between the lamellae of this clay. The black residue obtained at 700 jC was a film shape highly oriented graphite and highly stacked structure
with small interplanar spacing of 0.337 nm. The formation of such a unique coke is attributed to the peculiar carbonization method where the two dimensional space between the lamellae serves as a unique field for carbonization. Garfinkel-Shweky and Yariv (1997) studied by thermo-XRD-analysis the thermal transformation of vermiculite, montmorillonite, saponite, laponite and beidellite loaded with different amounts of the cationic dye acridine-orange to the charcoal-clay complex. At 300 jC charcoal was formed inside the interlayer space. The basal spacings of 1.26 –1.38 nm obtained after 300 jC were those of the clay complex with the interlayer charcoal. This basal spacing is characteristic for a carbon monolayer. Yermiyahu et al. (2002) studied by thermo-XRDanalysis the thermal products of various montmorillonites treated with the anionic dye congo-red at 360 jC. Some of the charcoal was obtained as monolayers with basal-spacings of 1.10 – 1.33 nm. But some charcoal was obtained as multilayers with basal spacings of 1.61– 2.10 nm. The differences between the charcoal obtained from an anionic dye and that obtained from a cationic dye are due to differences in the orientation of the negative and positive ions in the interlayer space before the thermal treatment.
4. Conclusions The present communication shows that the temperatures of the exothermic peaks in DTA curves of organo-clay complexes are dependent on the nature of the charcoal which is formed during the gradual heating of the clay complex. Since the nature of the charcoal is dependent on the bonding between the clay and the precursor organic compound it seems that after a better understanding of this relationship, DTA will become a useful tool for the study of the type of interactions between the organic compound and the adsorption sites on clay surface in organo-clay complexes.
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