ChemInform Abstract: THERMAL DECOMPOSITION OF GIBBSITE UNDER LOW PRESSURES. II. FORMATION OF MICROPOROUS ALUMINA

Rouquérol, J.; Rouquérol, F.; Ganteaume, Max

doi:10.1002/chin.197926030

Cited by 9 publications

(14 citation statements)

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“…This is because the dehydration is pseudomorphic. If crystallite size is large (small specific surface area), when water is expelled, space will be created in the particle, which thus becomes microporous because the external volume of particles does not change very much [9][10][11]. Conversely, in the case of nanocrystalline boehmite, the crystallites are so small that the collapse of the structure during the expulsion of the water leads to particle shrinkage and does not create significant internal porosity.…”

Section: Introductionmentioning

confidence: 93%

Surface and porosity of nanocrystalline boehmite xerogels

Alphonse

Courty

2005

Journal of Colloid and Interface Science

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Boehmite xerogels are prepared by hydrolysis of Al(OC4H9)3 followed by peptization with HNO3 (H+/Al = 0, 0.07, 0.2). XRD and TEM show that these gels are made of nanosized crystals (5-9 nm in width and 3 nm thick). According to the amount of acid, no significant differences are found in size and shape, but only in the spatial arrangement of the crystallites. Nitrogen adsorption-desorption isotherms of nonpeptized gels are of type IV, whereas isotherms of peptized gels are of type I. These isotherms are analyzed by the t-plot method. The majority of pore volume results from intercrystalline mesopores, but the peptized gels also contain intercrystalline micropores. The particle packing is very dense for the gel peptized with H+/Al = 0.2 (porosity = 0.26), but it is less dense in non-peptized gel (porosity = 0.44). Heating these gels under vacuum creates, from 250 degrees C onwards, an intracrystalline microporosity resulting from the conversion of boehmite into transition alumina. But heating also causes intercrystalline micropores collapsing. The specific surface area increases up to a limit temperature (300 degrees C for nonpeptized gels and 400 degrees C for peptized) beyond which sintering of the particles begins and the surface decreases. The PSD are calculated assuming a cylindrical pore geometry and using the corrected Kelvin equation proposed by Kruk et al. Peptized xerogels give a monomodal distribution with a maximum near 2 nm and no pores are larger than 6 nm. Nonpeptized gels have a bimodal distribution with a narrow peak near to 2 nm and a broad unsymmetrical peak with a maximum at 4 nm. Heating in air above 400 degrees C has a strong effect on the porosity. As the temperature increases, there is a broadening of the distribution and a marked decrease of small pores (below 3 nm). However, even after treatment at 800 degrees C, micropores are still present.

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Section: Introductionmentioning

confidence: 93%

Surface and porosity of nanocrystalline boehmite xerogels

Alphonse

Courty

2005

Journal of Colloid and Interface Science

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“…This gives gibbsite a monoclinic symmetry (space group P2 1 /n) with a hexagonal close packing of the hydroxyl groups. Upon heating, the hydroxyls are progressively lost through the structural channels formed in the gibbsite lattice by the formation of water by proton exchange between neighbouring hydroxyls (7)(8)(9). This is accompanied by the structural rearrangement of the crystal lattice, a pseudomorphic transformation (i.e., the original morphology of the parent gibbsite is retained) with little change in the external dimensions, and as a consequence, a large internal porosity develops.…”

Section: Introductionmentioning

confidence: 97%

“…The deformed spinel structures (usually represented as Al8 ᮀ O 12 , where ᮀ represents a vacant Al), either with octahedral or tetrahedral cation vacancies, gives a theoretical fraction of octahedral aluminum of 0.63 and 0.75, for octahedral or tetrahedral vacancies, respectively. Based on the results in this…”

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confidence: 99%

Characterization of metallurgical-grade aluminas and their precursors by ²⁷Al NMR and XRD

Perander¹,

Žujović²,

Groutso³

et al. 2007

Can. J. Chem.

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The structure of metallurgical-or smelter-grade aluminas (MGAs) is complex and poorly understood. Ultrahigh-field solid-state 27 Al NMR results on industrial as well as on laboratory-prepared aluminas are discussed in relation to XRD results. It is demonstrated that high-field NMR can effectively be used to quantify the proportion of the thermodynamically stable alpha-alumina phase in these materials. The results demonstrate that 27 Al NMR is a vital adjunct to XRD methods to quantify the transition alumina phases that invariably dominate the MGAs. The nature of the disorder in these materials, determined by 27 Al NMR, is also compared with literature data, such as XANES and EXAFS studies, on typical laboratory-prepared materials. The utility of 27 Al NMR studies to provide new insight into the structural complexity of metallurgical aluminas is shown.Résumé : La structure des alumines de qualité métallurgique (AQM) ou pour électrolyse est complexe et mal comprise. Opérant à champ élevé et à l'état solide, on a déterminé les spectres RMN du 27 Al d'alumines industrielles et d'autres préparées en laboratoire et on discute de ces spectres en relation avec les résultats obtenus par diffraction des rayons X (DRX). On a démontré que la RMN à champ élevé peut effectivement être utilisée pour quantifier la proportion de la phase thermodynamiquement stable d'alumine alpha qui est présente dans ces matériaux. Les résultats démontrent que la RMN du 27 Al est une addition absolument nécessaire aux méthodes de DRX pour quantifier les phases d'alumines de transition qui dominent invariablement les AQM. On a aussi comparé la nature du désordre dans ces matériaux, telle que déterminée par RMN du 27 Al, avec les résultats déjà publiés dans la littérature et obtenus par des études à l'aide de la spectroscopie d'absorption X de type XANES ou EXAFS sur des matériaux typiques préparés en laboratoire. On démontre donc l'utilité de la RMN du 27 Al à fournir de nouvelles indications sur la complexité structurale des alumines métallurgiques.

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“…This approach is the most convenient under medium and high pressures, but in the low pressure range the gas flow control is more sensitive Figure 2 shows two TG curves obtained for identical amounts of gibbsite, AI(OH)3, but under different "vacuum" conditions [8]. The lower curve, obtained by conventional TG, shows that the main part of the dehydration takes place from 150 to 300L The upper curve, obtained by the method described above, shows a nearly vertical fall, the dehydration taking place from 170 to only 200 ~ The latter curve clearly indicates a nearly zero-order reaction, which is explained by the advance of a constant-area reaction interface, parallel to itself and to the larger side of the flat, hexagonal, gibbsite elementary crystals, themselves conglomerated into larger grains [11 ]. Because of the uncontrolled residual pressure and temperature gradients, the conventional TG curve is completely misleading: these gradients drown any mechanism at the elementary particle scale.…”

Section: Methodsmentioning

confidence: 96%

Thermolysis under vacuum: Essential influence of the residual pressure on thermoanalytical curves and the reaction products

Rouquérol¹,

Ganteaume²

1977

Journal of Thermal Analysis

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This paper examines the influence of residual pressures in the range from 10-~to 5 torr on the course of thermal analysis. With the help of examples concerning in particular the thermolysis of gibbsite, AI(OH)3 , it is shown that a) the control of residual pressure is of virtually no use unless the rate of decomposition is also controlled (otherwise, the TG curves represent a composite phenomenon, which is practically unintelligible); b) the influence of residual pressure may be unexpectedly high both on the shape of the TG curves (and therefore on the apparent kinetic parameters) and on the nature (porosity, structure) of the products.The influence of the gas atmosphere on a thermal decomposition of the type solid A ~ solid B + gas is widely accepted. In 1951, Rowland and Lewis [1] in this way explained the usual DTA trace for calcite thermolysis, where the onset of decomposition may be observed as soon as 650 ~ (low partial pressure of CO~) and where "the reaction progressively becomes more vigourous until a peak is reached at about 925 ~ the temperature at which the reaction would begin if the furnace had been filled with COz at one atmosphere pressure. In 1960, Garn suggested simplification of the situation by operating under self-generated atmospheres [2], and several devices were proposed and used in order to control the overall pressure, e.g. the "cold sink", especially convenient in the case of water evolution [3], or the discontinuous and automatic gas extractor [4]. Nevertheless, "vacuum" was often considered as one pressure at which it may be worthwhile to carry out one experiment in order to complement a set of data obtained under higher pressures. Indeed, various authors, such as Eyraud et al. at the time of the first vacuum TG experiments, pointed out the "simplifying" part played by vacuum-operation, which was supposed to eliminate disturbing phenomena such as adsorption or diffusion processes [5] or secondary reactions [6] and to give rise to the simplest mechanism with higher decomposition rate [7]. Now pressures lower than I torr, which are usually referred to as "vacuum", actually cover a wide experimental range, down to 10 -~ torr, whereas the range of medium and *

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ChemInform Abstract: THERMAL DECOMPOSITION OF GIBBSITE UNDER LOW PRESSURES. II. FORMATION OF MICROPOROUS ALUMINA

Cited by 9 publications

References 1 publication

Surface and porosity of nanocrystalline boehmite xerogels

Surface and porosity of nanocrystalline boehmite xerogels

Characterization of metallurgical-grade aluminas and their precursors by ²⁷Al NMR and XRD

Thermolysis under vacuum: Essential influence of the residual pressure on thermoanalytical curves and the reaction products

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ChemInform Abstract: THERMAL DECOMPOSITION OF GIBBSITE UNDER LOW PRESSURES. II. FORMATION OF MICROPOROUS ALUMINA

Cited by 9 publications

References 1 publication

Surface and porosity of nanocrystalline boehmite xerogels

Surface and porosity of nanocrystalline boehmite xerogels

Characterization of metallurgical-grade aluminas and their precursors by 27Al NMR and XRD

Thermolysis under vacuum: Essential influence of the residual pressure on thermoanalytical curves and the reaction products

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Characterization of metallurgical-grade aluminas and their precursors by ²⁷Al NMR and XRD