T.M. El-Akkad

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.

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 effect of the addition of a water-soluble resin on the porosity and adsorption properties of alumina

Abstract The surface properties of pure and resin-containing alumina samples were investigated by nitrogen and cyclohexane adsorption. The structural and phase changes were studied by X-ray diffractometry and differential thermal analysis. The addition of resin led to the disappearance of β- Al 2 O 3 · 3H 2 O from samples dried at 110 °C and to an enhancement of the X-ray diffraction patterns characteristic of γ-Al 2 O 3 in samples heated at 550 °C. An increase in the pore volume of the alumina due to widening of the pore radii was also observed. However, the marked increase in the total pore volume was not accompanied by an equivalent change in the surface area. Thermal treatment at 550 °C produced no significant changes in the porous character of alumina samples with low resin contents, whereas the pore radii of samples with a high resin content were substantially reduced by calcination.

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.

The effect of compaction on the surface characteristics of some synthetic zeolites: Compaction versus porosity

Abstract The surface characteristics of loose and compacted 3A, 5A and 10A zeolite samples were investigated by nitrogen adsorption at — 195 °C. Three series of samples were examined: (i) untreated A zeolites; (ii) acid-activated zeolites; (iii) zeolites which had been deactivated by presoaking in water-soluble polymers. Compaction at 5 and 10 tonf in -2 produced changes in the shapes of the isotherms from type I to type II of the Brunauer-Emmett-Teller classification and high pressure hysteresis loops were formed. Activation of the parent zeolites with a 1:1 hydrochloric acid solution produced an increase in the porosity, whereas presoaking in polyglycol solution led to deactivation of the zeolites due to blockage of the pores with the long-chain polymer molecules. Compaction of the parent zeolites at 5 tonf in -2 produced a more noticeable decrease in their nitrogen adsorption capacities over the monolayer and multilayer regions owing to the destruction of micropores that were originally accessible for nitrogen adsorption in the loose powder. An increase in the compaction to 10 tonf in -2 resulted in the isotherm being moved upwards. However, it still lay below the isotherm for the loose powder, and this effect is correlated with the occurence of plastic deformation that partially compensates the adhesion effects that exist at the lower compaction pressure of 5 tonf in -2. Compaction of the acid-activated zeolite samples led to downward shift of the isotherm to below that of the loose zeolite samples; this is due to the effect of the solid-solid interface formed by compaction. Compaction of deactivated zeolite samples at a pressure of 5 tonf in -2 invariably produced a marked upward shift in the nitrogen isotherm above that for the corresponding loose powder; this effect is assumed to indicate the occurrence of a controlling plastic deformation effect. However, increasing the compaction pressure to 10 tonf in -2 produced a downward shift in the nitrogen isotherm to below that obtained for the sample compacted at 5 tonf in -2 but still above that obtained for the loose sample. This observation indicates the influence of adhesion effects that partially compensate the plastic deformation effects. Thus the effect of compaction on the surface characteristics of zeolites is different in the untreated, acid-activated and deactivated series of zeolites and depends on the porosity of the original loose powder. In view of the complications arising from the adhesion and fragmentation of compacted particles, the present results indicate that a systematic and comparative study is required to determine the limits on the various effects involved in the compaction of solids.

Correlation of the heats of immersion of 3A, 5A and 10A zeolites with their adsorption characteristics in polar and non-polar liquids

Abstract The adsorption of water and cyclohexane vapours at 35 °C by 3A, 5A and 10A zeolites (where 3A, 5A and 10A denote pore dimensions of 3 A, 5A and 10 A respectively) and by their thermal dehydration products obtained between 110 and 800 °C was investigated. The samples were precharacterized by nitrogen adsorption. The cyclohexane uptake of the 10A zeolites was much greater than that of the 3A and 5A zeolites. This is due to the large size of the cyclohexane molecule which limits its accessibility to narrow pores. In contrast, the water uptake of the 5A and 10A zeolites was less than the nitrogen uptake. This is because water interacts with the zeolite surface via specific forces only, whereas in the case of nitrogen the enhanced adsorption potential in the micropores and its effect on the polarizability of the nitrogen molecule enhance both its specific quadrupole-dipole interaction and its non-specific interaction with the zeolite surface. The integral heats ΔHi of immersion of the 3A, 5A and 10A zeolite samples and of their thermal dehydration products obtained between 110 and 800 °C were measured at 35 °C in both water and cyclohexane liquids. The integral heats of immersion in both liquids increased with increasing dehydration temperature up to 400 °C and thereafter decreased. The heats of immersion in cyclohexane were much lower than those in water, perhaps because of the inaccessibility of the bulky cyclohexane molecule to a large fraction of the internal pores. The values of ΔHicyc were in the order 10A zeolite > 5A zeolite > 3A zeolite, whereas the ΔHi values for water were in the order 3A zeolite > 5A zeolite > 10A zeolite. This behaviour demonstrates the correlation between pore narrowing in zeolites and the increased energetic interaction in polar wetting liquids.

Effect of the addition of water-soluble resin to magnesia, magnesia lithia and magnesia alumina gels

Abstract The surface properties of resin-free and resin-containing magnesia, magnesia lithia and magnesia alumina gels were investigated by nitrogen adsorption at - 195 °C. The addition of the resin during precipitation led to an increase in the surface area and in the pore volume of the gels. The highest surface areas were obtained in magnesia alumina followed by magnesia and magnesia lithia. This indicates that the presence of higher valency ions enhances the increase in the surface area caused by the addition of resin during the precipitation. A sharp increase in the surface area of the resin-free gels was observed after thermal treatment at 450 °C. This increase may be due to perforation of the gel during the dehydration process. In contrast, resin-containing samples exhibited a decrease in the surface area that can be attributed to the strengthening of the skeletal walls of the gels by addition of resin during the precipitation so that they were able to withstand perforation effects due to the dehydration process. A pronounced decrease in the surface area was observed when aluminium was present in the gel.

The effect of the impregnation of zeolites of different porosities with aqueous solutions of polyglycol 400 and polyglycol 4000

Abstract The impregnation of zeolites with dilute solutions of polyglycol 400 and polyglycol 4000 acts to seal pores of sizes ranging between 3 and 10 A. The sealing action of polyglycol 4000 is rather better than that of polyglycol 400. Impregnation with concentrated solutions leads to undesirable results.

The effect on the surface properties of eponit charcoal of impregnation with K2Cr2O7 solution at various pH values

Abstract The effect of the impregnation of Eponit charcoal with K2Cr2O7 solution at different pH values was studied by polarographic and surface techniques. In acid media, where dichromate ions predominate, the Eponit charcoal was oxidized. The oxidation reaction was found to be first order with respect to the dichromate, and an increase in the concentration of H+ ions accelerated the reaction rate. Although the cation exchange capacity of the Epoint charcoal had increased markedly after oxidation, the surface area for gas adsorption was decreased. This decrease was interpreted as being due to blockage of the narrow pores by the Cr3+ ions. In alkaline media, where chromate ions predominate, no oxidation of the Eponit charcoal took place but preferential adsorption of OH- groups on the Eponit surface enhanced solution occurred. Adsorption of OH- groups on the Eponit surface enhanced the dipole-quadrupole interaction with nitrogen, and hence there was a slight variation between monolayer and multilayer adsorption.