Phase transformations in γ-Al(OH)3 under heat treatment in air and water vapor

Lopushan, V. I.; Kuznetsov, G. F.; Pletnev, R. N.; Kleshchev, D. G.

doi:10.3103/s1062873807080394

Bull. Russ. Acad. Sci. Phys.

2007

DOI: 10.3103/s1062873807080394

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Phase transformations in γ-Al(OH)3 under heat treatment in air and water vapor

V. I. Lopushan

G. F. Kuznetsov

R. N. Pletnev

et al.

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Cited by 7 publications

(3 citation statements)

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“…Boehmite with V p = 0.20-0.32 cm 3 /g, S p = 100-120 m 2 /g and pore diameter 11.4-15.1 nm [15][16][17] is obtained from amorphous precipitates of aluminum hydroxides under hydrothermal conditions. A large-crystalline nonporous boehmite with S p = 3.1 m 2 /g and a pore diameter of 24.6 nm is obtained from gibbsite under the hydrothermal conditions at a temperature C 150°C [18,19].There are a few types of pores: open, deadlock and closed pores. The parameters of the former two are important in the development and synthesis of catalysts, catalyst carriers, adsorbents from hydroxides and aluminum oxides.…”

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Formation of closed mesopores in boehmite during the phase transformation of gibbsite under hydrothermal conditions

Egorova

Mukhamed’yarova

Kurbangaleeva

et al. 2018

Reac Kinet Mech Cat

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We report a study of the phase transformation of gibbsite-boehmite under hydrothermal treatment in aqueous suspension and under the ''dry steam'' conditions at T = 180-210°C and P = 1.0-1.9 MPa for 30-540 min. It is the first time that the closed mesopores formed during the splicing of large crystals in which water is encapsulated in an amount of up to 1.8% of the masses are found in boehmite. The spread of closed mesopores by the diameters depends on the conditions of hydrothermal treatment: in the aqueous suspension the mesopores are of the diameters less than 4 nm, under conditions of ''dry steam''-in the range of 4-6 nm and * 10-200 nm. With those closed mesopores, it possible to use boehmite in various fields of chemistry and technology for the encapsulation and transport of solutions containing the active components and connections. ().,-volV) (0123456789().,-volV) Boehmite (c-AlOOH) is an important mesoporous [1] material; due to the high specific surface, pore volume and presence of mesopores it is used as adsorbent [2], catalyst carrier [3], and intermediate phase in the production of alumina carriers, petrochemical and petroleum processing catalysts [4], membranes and filters [5], a component of toothpastes, enveloping and adsorbing agents in medicine. The parameters of the mesoporous system determine the adsorptive capacity of boehmite with respect to toxic organic and inorganic compounds in the wastewater treatment and in solving environmental problems [6].The dimensions of the crystals and the way they are packaged are very important for the functional materials obtained from boehmite; they affect its porous system, the temperatures of phase transitions into aluminum oxides. There are several types of boehmite: pseudoboehmite of the composition c-AlOOHÁnH 2 O (n = 0.3-1.0) with crystallite sizes of 2.5-3.5 nm (the specific surface area S p up to 450 m 2 /g and the pore volume V p up to 0.80 cm 3 /g) [7]; fine crystalline boehmite of the composition c-AlOOHÁnH 2 O (n = 0.0-0.2) with crystallite sizes of 11-13 nm (S p = 220-250 m 2 /g, V p = 0.58-0.60 cm 3 /g) [8,9]; well crystallized boehmite of the composition c-AlOOHÁnH 2 O (n = 0.0-0.2) with crystal sizes up to 80-140 nm (S p \ 150 m 2 /g, V p \ 0.15 cm 3 /g) [10]. Parameters of the boehmite porous system are varied by the method and conditions of its production.Precipitation from acidic or alkaline solutions of aluminum compounds [11,12], hydrolysis of aluminum alkoxides [13,14] produce the mesoporous pseudoboehmite with V p = 0.26-1.00 cm 3 /g, S p = 102-354 m 2 /g and pore diameters of 5.3-15.2 nm. Boehmite with V p = 0.20-0.32 cm 3 /g, S p = 100-120 m 2 /g and pore diameter 11.4-15.1 nm [15][16][17] is obtained from amorphous precipitates of aluminum hydroxides under hydrothermal conditions. A large-crystalline nonporous boehmite with S p = 3.1 m 2 /g and a pore diameter of 24.6 nm is obtained from gibbsite under the hydrothermal conditions at a temperature C 150°C [18,19].There are a few types of pores: open, deadlock and closed pores. The par...

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confidence: 99%

Formation of closed mesopores in boehmite during the phase transformation of gibbsite under hydrothermal conditions

Egorova

Mukhamed’yarova

Kurbangaleeva

et al. 2018

Reac Kinet Mech Cat

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“…Exothermic peak at 297.7°C refers to intensive synthesis of MgA1 2 O 4 , which according to a curve of mass loss proceeds until 800°C. Endothermic peak at 369.3°C refers to decomposition of the residual magnesium hydroxide with formation of MgO; endothermic peak at 458.4°C, of γ-AlOOH with formation of γ-Al 2 O 3 [7]. A broad endothermic peak in the range of 600-850°C is associated, apparently, with an interaction of residual MgO and γ-Al 2 O 3 and transition of MgA1 2 O 4 from amorphous to crystalline state.…”

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“…Our studies demonstrate that the two-step reverse precipitation (fi rst precipitation of Mg 2+ ions and afterward of the Al 3+ ions) with ammonium carbonate followed by calcination of produced precursor at 800°C for 1 h is the best way to get MgA1 2 O 4 . Consumption of (NH 4 ) 2 CO 3 is 10.8 M to form a mole of MgA1 2 O 4 that is 2 times less than in [7].…”

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Synthesis of magnesium aluminum spinel from carbonate hydroxyde precursors

et al. 2012

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Three ways to synthesize МgА1 2 О 4 precursor were studied with use of a two-step precipitation at the beginning of ions Mg 2+ and then Al 3+ with ammonium hydroxide, ammonium carbonate, and mixture thereof. Effi ciency of these ways was evaluated with the aid of a residual amount of cations Mg 2+ and Al 3+ in the mother liquor.Magnesium aluminum spinel MgA1 2 O 4 due to the set of physical chemical properties and above all, to the chemical resistance and high temperature strength is a promising compound for the production of ceramic materials. Spinel ceramics are widely used in the manufacture of protective covers of thermocouples, crucibles for melting metals, alloys of turbine blades. It is widely employed in the manufacture of high-temperature sensors in the aircraft gas turbine engines [1]. Transparent ceramics based on MgA1 2 O 4 is a good alternative to expensive and hard-to-produce a single-crystal spinel MgA1 2 O 4 . Optical applications of the transparent ceramic include infrared laser windows and inert valves [2]. It is known that the good properties of ceramics are the result of the correct choice of the way of synthesis and sintering conditions, as well as of the favorable properties of the original powder. It is impossible to get good, uniform, porous ceramics, suitable for optical applications, without using a highly active, pure, and non-sintered powder.Traditionally, the alumina magnesium spinel was produced by two-step technology: a synthesis of spinel from MgO and A1 2 O 3 (1300-1500°C) followed by molding and sintering them at a temperature of 1600-1700°C. According to data in [3], the spinel formation begins at 600-900°C and proceeds more rapidly at 1300-1500°C. Nevertheless, the degree of conversion of MgA1 2 O 4 is at 1500°C only 80%; at 1750°C, 86%; at 1850°C, 88%.Synthesis of spinel by co-precipitation of magnesium and aluminum hydroxide and subsequent calcination is of great interest [4]. Using this method makes it possible to reduce the synthesis temperature to 200-400°C compared with the traditional ceramic technology. However, production of a stoichiometric spinel fails: [always there occurs alumina and(or) magnesium oxide in the free state] [3]. It is established that the complete precipitation of the ions Mg 2+ and Al 3+ depends on the pH of the mother liquor therewith at pH ≤ 12 incomplete precipitaition of the ions Mg 2+ occurs; at pH ≥ 10.5, of ions Al 3+ [5].Co-precipitaition of the ions Mg 2+ and Al 3+ using ammonium carbonate is also interesting [6]. A solution of stoichiometric amounts of aluminum and magnesium nitrates was added dropwise to a solution of ammonium carbonate at 50°C. To increase the pH of the mother liquor to 9.8 we added ammonium hydroxide. Consumption of (NH 4 ) 2 CO 3 was 20 M to form 1 M of MgA1 2 O 4 . We found that initially only ions Al 3+ are precipitated in the form of NH 4 A1(OH) 2 CO 3 ·H 2 O (ammonium davsonite) and Mg 2+ ions are precipitated after suspension aging (over 17 h) at pH 9.8 and T = 50°C in the form of Mg 6 A1 2 (CO 3 ) (OH) 16 ·4H 2 O...

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Examination of dehydration and dehydroxylation of synthetic layered (oxy)hydroxides through thermal analysis (TG-DSC-EGA-MS) and a discussion to the second Pauling’s rule

Madej

2018

Inorganica Chimica Acta

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Phase transformations in γ-Al(OH)3 under heat treatment in air and water vapor

Cited by 7 publications

References 2 publications

Formation of closed mesopores in boehmite during the phase transformation of gibbsite under hydrothermal conditions

Formation of closed mesopores in boehmite during the phase transformation of gibbsite under hydrothermal conditions

Synthesis of magnesium aluminum spinel from carbonate hydroxyde precursors

Examination of dehydration and dehydroxylation of synthetic layered (oxy)hydroxides through thermal analysis (TG-DSC-EGA-MS) and a discussion to the second Pauling’s rule

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