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Amorphous aluminum compounds are formed during the synthesis of the γ-Al2O3 catalyst precursor. Amorphous compounds influence on the alumina catalyst variously due to different physicochemical properties, which depend on the method of their preparation. In this research, the comparative analysis of physicochemical properties of amorphous aluminum compounds that were obtained by the precipitation method, the thermal decomposition of aluminum nitrate, and alcoxide hydrolysis product were studied. It is the first time that a new method for calculating of quantitative phase composition of amorphous aluminum compounds using the X-ray powder diffraction, thermogravimetric and differential scanning calorimetry analysis, mass-spectrometry, and CHN-analysis was described. Properties of obtaining samples were studied using scanning electron microscopy, low-temperature nitrogen adsorption, and temperature programmed desorption of ammonium analyses. The methods of precipitation and thermal decomposition of aluminum nitrate allows for obtaining non-porous samples consisting of a mixture of amorphous phases (hydroxide and basic salt) that contain the metals impurities and have low acidity of the oxides obtained from them. The highly porous amorphous alumina formed by the thermal decomposition of the alcoxide hydrolysis product with the least amount of impurities and a high acidity of the surface was observed.

Abstract:The presence of χ-Al 2 O 3 resulting from thermal decomposition gibbsite as part of alumina catalysts is unfavorable because of its acid characteristics. One of the available techniques of χ-Al 2 O 3 removal is crystallization under hydrothermal conditions into boehmite, which is a main precursor of active γ-Al 2 O 3 . The influence of products of the hydrothermal treatment of χ-Al 2 O 3 obtaining in result of thermal decomposition gibbsite under T = 150-200 • C, P = 0.5-1.5 MPa and pH = 4.0-9.2 were studied. The hydrothermal treatment products in these conditions are gibbsite and boehmite phases which are formed coincidently by parallel ways. In the alkaline medium at pH = 8.0-9.2 three-dimensional parallelepiped boehmite crystals with the edge length > 200 nm are formed, at pH = 4.0 two-dimensional rhombic-shaped plates with thickness 20-100 nm and with the edge length~80-500 nm are formed. Crystallization of coarse boehmite particles promotes the formation of large and closed mesoporous.

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...