coatings Article Influence of the Obtaining Method on the Properties of Amorphous Aluminum Compounds Aliya N. Mukhamed’yarova *, Oksana V. Nesterova, Kirill S. Boretsky, Juliya D. Skibina, Avgustina V. Boretskaya, Svetlana R. Egorova and Alexander A. Lamberov Alexander Butlerov Institute of Chemistry, Kazan Federal University, Kazan 420008, Russia; [email protected] (O.V.N.); [email protected] (K.S.B.); [email protected] (J.D.S.); [email protected] (A.V.B.); [email protected] (S.R.E.); [email protected] (A.A.L.) * Correspondence: [email protected]; Tel.: +7-960-051-3178 Received: 1 December 2018; Accepted: 10 January 2019; Published: 14 January 2019 Abstract: 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. Keywords: amorphous aluminum compound; alumina catalyst; alumina; pseudoboehmite; precipitation; thermal decomposition 1. Introduction Gamma-alumina (γ-Al2O3) is widely used in the petrochemical industry as a catalyst, catalyst carrier, and adsorbent. γ-Al2O3 is formed during the thermal decomposition of the boehmite [1], obtained from the sol-gel method from bayerite [2], bauxite [3], salts and salt-like aluminum compound [4–6], amorphous aluminum oxide, and hydroxide [5,7]. Depending on the nature and characteristics of the precursor, it is possible to regulate the physicochemical properties of γ-alumina. One of the most important characteristics of the catalyst precursor is the properties of its surface, such as morphology, porous system, and acidity. On an industrial scale, γ-Al2O3 is predominantly produced by the thermal decomposition of pseudoboehmite (PB). There are many methods for the synthesis of PB, for example, a hydrothermal treatment of gibbsite or alumina, a precipitation from solution salts under hydrothermal conditions [8,9], and others. However, the most common methods of PB obtaining are the hydrolysis of aluminum alkoholates [10] and the precipitation of aluminum hydroxide from Al-containing acidic solutions by a base [11]. These methods allow for controlling the phase composition and physicochemical properties of the obtained aluminum hydroxides. In both cases, the formation of Coatings 2019, 9, 41; doi:10.3390/coatings9010041 www.mdpi.com/journal/coatings CoatingsCoatings2019 2018, ,9 7,, 41 x FOR PEER REVIEW 22 ofof 14 13 Coatings 2018, 7, x FOR PEER REVIEW 2 of 13 properties of the obtained aluminum hydroxides. In both cases, the formation of amorphous amorphouspropertiescompounds of compounds is theobserved obtained is in observed addition aluminum into additionthe hydroxides target to pr theoduct. targetIn dueboth product to cases, the forming due the to formation the of formingincomplete of of amorphous incomplete hydrolysis hydrolysiscompoundsproducts in products isthe observed first incase the in and addition first the case organic to and the the targetresidues organic pr oductinresidues the duesecond to in the thecase, forming second in accordance of case, incomplete inaccordance with Schemehydrolysis with 1. Schemeproducts1. in the first case and the organic residues in the second case, in accordance with Scheme 1. SchemeScheme 1.1.Forming Forming ofof X-rayX-ray amorphousamorphous aluminumaluminum compoundscompounds byby the the precipitation precipitation and and hydrolysis. hydrolysis. Scheme 1. Forming of X-ray amorphous aluminum compounds by the precipitation and hydrolysis. 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