S. Nashed

Thermal analyses of mono- and divalent montmorillonite cationic derivatives

Abstract DTA and TG analyses were carried out for mono- and divalent cationic derivatives of Jelsovy-Potok montmorillonite. A certain relation between the charge density and the highest dehydration DTA peak was established for both series, since the two variables are a function of the magnitude of binding the hydrated water molecules. According to the charge density levels the cationic derivatives can be classified into three categories: (i) the single peak group falls in the range of charge density lower than 1 C m −2 ; (ii) the double peak group falls between 1 and about 2 C m −2 ; and (iii) for the higher charge densities three peaks are observed. The Ba derivative presents the only exception in this series. As the charge density increases, not only the number of dehydration peaks increases, but also the isolation between them becomes better, i.e., the overlapping of these peaks decreases in the order Ca > Li > Mg. The slopes of the TG curves in the temperature range 200–500°C increase with increase in charge density on the cationic derivative. This indicates that the rate of dehydration differs by changing the exchangeable cation of the surface, whereas the rate of dehydroxylation for all derivatives is the same since they all have the same origin.

Thermal dehydration of mono- and di-valent montmorillonite cationic derivatives

Abstract The kinetics of the isothermal dehydration of seven derivatives of Jelsovy-Potok montmorillonite comprising two series of mono- and di-valent cations were studied together with TG and DTA of these compounds. A certain correlation was established between the charge density and (1) the highest dehydration DTA peak, (2) the first order rate constant for the dehydration reaction. A diffusion controlled mechanism is suggested for the kinetics of theisothermal dehydration. This is supported by (1) the dependence of the first order rate constant on the cationic radius, (2) the relatively large values of the first order rate constants for the monovalent cations compared with those for the di-valent cations, and (3) the values of the energy of activation (≈ 23 kJ mol −1 ) for all the derivatives studied. Values of about 220 J K −1 were obtained for the entropy of activation supporting the hypothesis of a two dimensional diffusion reaction.

Structure and phase changes in thermally treated mixed hydroxides of Mg and Al-effect of compaction

Abstract Mechanically mixed (I) and coprecipitated (II), hydroxides of magnesium and aluminium were investigated in both powder and compact forms in the temperature range 200–1000°C using DTG, DTA and X-ray techniques. Upon thermal treatment of the mechanically mixed hydroxide, the solid material gives rise to products which at first become poorly crystalline and then gradually pass into the crystalline state indicating a mixture of MgO, θ, δ and α—alumina together with a new spinel. This new spinel is stable in the temperature range 400–800°C—three of its distinct d -spacings are at 4.87, 3.86 and 3.74 A. Coprecipitated hydroxides (II) proved to constitute a new species—probably a hydrated spinel. The spinel MgAl 2 O 4 commences to form at a temperature as low as 200°C. Compaction with either 10 or 30 tons in. −2 decreases the crystallinity of products from (I) dehydrated ⩽500°C, whereas it favours crystallization for the products from II. Above ∼500°C, compaction has no appreciable effect on crystallization.

Effects of thermal treatment on the surface properties of a wide pore silica gel

Abstract A mesoporous silica gel Davidson 59 was thermally treated in vacuo, in the temperature range 20–1000°C. Effects of thermal treatment on the water contents, nitrogen surface areas, pore structure and heats of immersion in water were investigated and discussed. The temperatures selected were 20, 110, 200, 290, 380, 480, 510 and 1000°C. These temperatures were found to cover all the various textural changes resulting from the heat effect. It could be shown that the heats of immersion in water depend primarily on the water content of the sample and are proportional, at least qualitatively to the number of hydroxyl groups on the surface and their availability for interaction with liquid water. The interesting result obtained is that a second factors is involved, namely the pore structure of the adsorbent. A qualitative parallelism exists between the normalized heat of immersion per unit area, and the average pore radius. Apparently the packing of water molecules in narrow pores leads to a decrease in the heat of immersion due to repulsion between the permanent dipoles of the molecules. In narrower pores, the heat of immersion in water is smaller than in wide pores.

Mechanism of dehydration of magnesium oxalate dihydrate

Abstract The isothermal dehydration of magnesium oxalate dihydrate has been studied at various temperatures between 190 and 260°C in the presence of air. The isothermal dehydration curves show the usual sigmoidal character and display an induction period which is highest at 190°C. The dehydration velocity constant ( k ) values (obtained by the application of Mampels equation) plotted vs. 1/ T according to the Arrhenius equation gave a plot in which two linear sections intersect at ∼215°C with activation energies of 12.3 and 18.3 kcal mole −1 for the lower- and higher-temperature sections, respectively. This behaviour is tentatively explained in terms of a change in the mechanism of dehydration, and not the formation of some new phase other than the dihydrate and the anhydrous oxalate phases present in both the original crystalline oxalate and the sample heated at 200°C for 120 min. X-Ray patterns of the heated oxalate sample left for a few days at room temperature showed a marked sensitivity for rehydration of the anhydrous oxalate phase.

Nitrogen and water vapour adsorption on monovalent and divalent montmorillonite derivatives and their heats of immersion in polar liquid

Abstract Nitrogen and water vapour adsorption measurements were carried out at -195 °C and 35 °C respectively on some alkali metal and alkaline earth derivatives of Jelsovy Potok montmorillonite; the derivatives contained lithium, sodium, caesium, magnesium, calcium, strontium or barium. The nitrogen area was unaffected by the exchangeable ion and can be considered as a measure of the external surface area of montmorillonite. The water area was found to increase with decrease in the ionic radius of the exchangeable ions owing to their capacity for hydration. The crystallographic area provided information on the number of layers per particle and on the change in cross-sectional area of the water molecule whose packing factor was varied by changing the exchangeable ion.

Surface and structural properties of 3, 5 and 10A synthetic zeolites

Abstract The surface and structural properties of synthetic zeolites thermally treated between 100 adn 1000°C were investigated by X-ray, DTA, TG and nitrogen adsorption at −195°C. The zeolites under investigation possess a crystalline structure similar to nepheline; their diffraction patterns completely disappeared on thermal treatment at 800°C due to destruction of the zeolitic lattice. Differential thermal analysis exhibited an endothermic effect centered at about 225°C which may be attributed to dehydration of loosely bound zeolitic water, and two successive exothermic effects; the former was small, centered at 855°C, the latter, which was sharper, appeared at about 920°C. These may be due to evolution of structural water followed by destruction of the zeolite lattice, respectively. Thermogravimetric analysis indicated that the water loss in air exceeded that in vacuum, an effect which is indicative of contraction of the zeolite structure by the action of a high vacuum procedure at elevated temperature. Nitrogen adsorption studies at −195°C were used as a probe to follow the textural and structural changes due to thermal treatment between 100 and 1000°C. Evolution of the monolayer equivalent using the method of Dubinin Radushkevich gave higher estimated values than those calculated by the BET method. These higher values can be attributed to the influence of the micropore filling of nitrogen in the low pressure region which led to distortion in the shape of the isotherms in the initial pressure region, and hence the evaluation of V m by the BET method may be considered unreliable in this connection. In general, monolayer equivalents increase with increase in the pretreatment temperature up to 400°C followed by a gradual decrease as the pretreatment temperature increases. The increase in monolayer equivalents may be correlated with the removal of loosely bound water in the intracrystalline cavities or pores, an effect which leads to an increase in the accessibility of the nitrogen molecule. The decrease in nitrogen uptake on samples thermally treated at temperatures higher than 400°C can be attributed to contraction of the zeolite pores by heating in vacuum: an effect which precedes partial collapse of the structure. The marked decreased obtained on thermal treatment at 800°C may be correlated with destruction of the zeolite lattice producing a mixture of different sintered oxides.

The heats of wetting of carbons in relation to their specific surface areas

Abstract No simple correlation was found between the heats of wetting in methanol of a number of carbons, prepared from corn cobs and date pits under varying conditions, and the specific surface areas of the carbons calculated from the adsorption isotherms of methanol vapour. Actually, the heat of wetting per unit area varied several folds as a result of changes in the nature and energetics of the surface produced by thermal treatment and activation. It is concluded that the use of the heat of wetting as a measure of the specific surface area of carbons has to be adopted with much reserve, and is only warranted if there is independent proof that the heat of wetting per unit area is the same for the various carbons.

Characterization of some Egyptian clays to be used as bleaching agents

Abstract Two Egyptian clays, namely Maadi clay C(M) and Alexandria-Cairo road clay C(A), were selected as possible substitutes for an Indian clay and a Russian clay used industrially as bleaching agents for mineral lubricating oils. In view of the operational conditions in industry, assessment of the selected clays in comparison with the imported Indian and Russian clays as references was carried out. This comprised textural and structural studies involving different techniques, i.e. adsorption of nitrogen at −196 °C, X-ray analysis and thermogravimetric and differential thermal analyses. For complete identification of the clay constituents, X-ray techniques and thermal analysis appear to play a complementary role. C(M) clay was shown to contain more montmorillonite, a desirable clay constituent, than C(A) clay. The surface characteristics of the four tested clays versus the temperature of treatment were obtained from analysis of the nitrogen isotherms by the Brunauer-Emmett-Teller and α s methods, and the results are discussed in correlation with the structural changes. The bleaching efficiencies of the selected local clays were tested on samples of crude lubricating oils, in comparison with the reference imported clays using the EEL colorimeter. The results obtained indicate a satisfactory bleaching efficiency for the local clays as possible substitutes for the imported clays after the local clays have received a suitable activation treatment; an extended study is now in progress.

Structures and phase changes of three mixed hydroxide systems and their thermal dehydration products: Effect of compaction

Abstract Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were carried out for various tested samples, namely the parent hydroxides of aluminium, magnesium, calcium and cadmium, the mechanically mixed powders of Al-Mg, Al-Ca and Al-Cd hydroxides, and the compacted form of the mixed powders. X-ray analysis was carried out on the samples and on their dehydration products between 200 and 800 °C. From DTA and TGA it is noticed that the peaks of the Al-Mg and Al-Cd hydroxides correspond to the differential peaks of the parent hydroxides. By contrast, Al-Ca mixed hydroxide exhibits two peaks at 245 °C and 280 °C for aluminium and calcium hydroxides respectively whereas parent Al(OH)3 and Ca(OH)2 exhibit peaks at 200 °C and 450 °C respectively. This indicates that the mass loss observed from TGA data is not due to dehydration of Ca(OH)2 but due to dehydration of another entity formed from the mixing of Al(OH)3 and Ca(OH)2. X-ray analysis indicates that a spinel species of chemical formula Ca3Al2(OH)12 formed even at 110 °C. Therefore the differential effect centred at 285 °C may be correlated with the dehydration of this spinel. The parent hydroxides of aluminium, magnesium, calcium and cadmium reveal the main characteristic diffraction lines of β-Al2O3, brucite, portlandite and cadmium hydroxide respectively. Thermal treatment led to partial disappearance of the main diffraction pattern of the hydroxides and at the same time the appearance of other lines corresponding to the formation of both oxides and spinels except for Ca(OH)2 with which no characteristic lines were observed for the corresponding oxide up to 600 °C. Compaction before thermal treatment led to a marked decrease in the relative intensities of the X-ray diffraction lines of individual hydroxides, oxides and spinels of Al-Mg, and the spinel of Al-Ca which formed up to 200 °C, compared with the loose powders. By contrast, the crystallization of the Al-Ca spinel that formed at 400 °C was enhanced markedly and also, slightly, was that of Al-Cd spinel. The dimunution in the relative intensities of the X-ray diffraction lines by compaction may be tentatively correlated with a lower density of the crystalline forms than of the amorphous forms in the solid matrix.