Influence of the Obtaining Method on the Properties of Amorphous

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. Currently,Currently, thethe amorphousamorphous aluminumaluminum compoundscompounds havehave beenbeen studiedstudied poorlypoorly andand theythey areare describeddescribedCurrently, inin aa small smallthe amountamorphousamount ofof research researchaluminum works. works. compounds However,However, have thesethese been compoundscompounds studied affect affectpoorly thethe and propertiesproperties they are ofof γdescribedγ-Al-Al22OO33 catalysts. in a small amount of research works. However, these compounds affect the properties of γ-Al2OnOOn3 catalysts. thethe oneone hand,hand, the amorphous component component presence presence as as a a part part of of alumina alumina catalyst catalyst precursor precursor in invarious variousOn thepetrochemical petrochemical one hand, the processes processes amorphous is isunfavorable. unfavorable.component Th presence Thee amorphous amorphous as a part compound compoundof alumina is a iscatalyst gel a gelthat that precursor reduced reduced the in thevariousfilterability ﬁlterability petrochemical of the of theprecipitation precipitation processes product is unfavorable. product and and complica Th complicatede amorphousted the thewashing compound washing of PB of isfrom PB a gel from impurity that impurity reduced ions ions(Na, the (Na,filterabilityFe, etc.) Fe, etc.)that of are thatthe catalytic precipitation are catalytic poisons. poisons.product In addition, and In addition, complica the amorphous theted the amorphous washing component componentof PB has from a lowimpurity has specific a low ions speciﬁc surface (Na, surfaceFe,area etc.) [12]. areathat Therefore, [are12]. catalytic Therefore, the amountpoisons. the amountof In the addition, amorphous of the the amorphous amorphous component component component in the PB in is has thereduced a PB low is specificat reduced the synthesis surface at the synthesisareaprocess. [12]. process.Therefore, the amount of the amorphous component in the PB is reduced at the synthesis process.OnOn the the other other hand, hand, the amorphousthe amorphous component component has a positivehas a effectpositive on theeffect catalyst. on the An catalyst. amorphous An baseamorphousOn aluminum the baseother salts aluminum hand, provide the salts theamorphous provide plastic propertiesthecomponent plastic ofproperties ha thes a catalyst positive of the precursor catalysteffect on duringprecursor the catalyst. a molding.during An a Theamorphousmolding. salts decomposeThe base salts aluminum decompose with the salts with formation provide the formation athe new plastic binding a new properties binding phase of afterphase the thecatalyst after calcination the precursor calcination [13]. during In[13]. our Ina previousmolding.our previous works The saltsworks [14], decompose we [14], have we already havewith thealready shown formation thatshown it isa possiblethnewat itbinding is topossible improve phase to theafter improve properties the calcination the ofproperties catalyst [13]. for Inof skeletalourcatalyst previous isomerization for skeletalworks [14],isomerization of n-butenes we have to alreadyof isobutylene n-butene showns byto th transformingatisobutylene it is possible theby amorphoustransformingto improve componentthe the properties amorphous using of hydrothermalcatalystcomponent for usingskeletal treatment. hydrothermal isomerization treatment. of n-butene s to isobutylene by transforming the amorphous componentThereThere areusingare severalseveral hydrothermal waysways toto obtaintreatment.obtain the the amorphous amorphous aluminum aluminumcompounds: compounds: aa calcinationcalcination ofof thethe aluminumaluminumThere saltsaresalts several [[7],7], the ways hydrolysis hydrolysis to obtain of of the the aluminum aluminumamorphou alcoholatess alcoholatesaluminum [15], [compounds:15 ],and and the the precipitation a precipitation calcination from of from theAl- Al-containingaluminumcontaining salts solutions solutions [7], the by byhydrolysisthe the base base [7,16]. of [7, 16the As]. aluminum Asa resu a result,lt, various alcoholates various non-crystalline non-crystalline [15], and thealuminumaluminum precipitation compounds compounds from Al- are arecontainingformed formed differing differingsolutions in in aby phase a the phase base composition composition [7,16]. As anda and resu the thelt, physicochemical various physicochemical non-crystalline properties aluminum of surface.surface. compounds High-purityHigh-purity are amorphousformedamorphous differing aluminumaluminum in a phase hydroxidehydroxide composition containingcontaining and the thethe physicochemical organicorganic residuesresidues properties isis obtainedobtained of bysurface.by thethe hydrolysishydrolysis High-purity ofof organoaluminumamorphousorganoaluminum aluminum compound.compound. hydroxide The The containing residues residues of ofthe salts salts orga pr precursornicecursor residues are are alsois also obtained presented presented by inthe in the thehydrolysis structure structure ofof oforganoaluminumamorphous amorphous compounds compounds compound. that that Theare are formedresidues formed by of by thesalts the precipitation precipitationprecursor are and also the presented thermalthermal in decompositiondecomposition the structure ofofof aluminumamorphousaluminum salts, salts,compounds for for example, example, that nitrate, nitrate,are formed according according by the to to the precipitationthe Scheme Scheme2 .2. and the thermal decomposition of aluminum salts, for example, nitrate, according to the Scheme 2.

Scheme 2. Forming of X-ray amorphous aluminum compound by the thermal decomposition. Scheme 2. Forming of X-ray amorphous aluminum compound by the thermal decomposition. TheseScheme schemes 2. Forming are debatable, of X-ray amorphous and further aluminum research co ismpound needed by to the clarify thermal them. decomposition. Therefore, the purpose of our research was a comparative analysis of physicochemical surface properties of the amorphous aluminum compounds that were obtained by the methods, such as the

Coatings 2019, 9, 41 3 of 14 precipitation method, the thermal decomposition of aluminum nitrate, and alcoxide hydrolysis product. We also propose a new method for calculating the phase composition of the amorphous aluminum compounds using a group of XPRD, TG-DSC, mass spectrometry (MS), and CHN-analysis methods.

2. Materials and Methods Three samples of the amorphous aluminum compounds were obtained by the various methods. Sample 1 (S1) was carried out by the precipitation from 0.25 g/mL Al(NO3)3·9H2O (analytically pure, according to the State standard GOST 3757-75 [17] contains <0.004 wt % Fe, <0.05 wt % K + Na) solution by ammonia water (chemically pure, according to the State standard GOST 3760-79 [17] 2− contains <0.0001 wt % Fe, <0.0001 wt % Mg + Ca, <0.001 wt % CO3 ) at pH = 6.0 and the room temperature without the stabilization and aging stages. The obtained gel-like product was centrifuged 20 min in 700 mL of H2O 4 times to remove the ammonium nitrate formed during the precipitation. Subsequently, sample S1 was drying during 2 h at the 100–105 ◦C. A precipitate with the maximum amount of basic salt was obtained. Sample 2 (S2) was synthesized by heat treatment of Al(NO3)3·9H2O (analytically pure, the State standard GOST 3757-75) at 350 ◦C during 1 h at an atmospheric pressure. Sample 3 (S3) was obtained by heat treatment at 550 ◦C during 2 h at the atmospheric pressure from non-crystalline aluminum hydroxide (impurities: 0.52 wt % C, <0.001 wt % Na, Mg and Fe) that was obtained by the method of hydrolysis of aluminum isopropoxide (chemically pure, <0.0001 wt % Na, Mg, Fe) by water vapor. The hydrolysis of aluminum isopropoxide with water vapor was carried out liquid vapor solid vapor by the following equation: Al(i-C3H7O)3 + 3H2O = Al(OH)3 + 3C3H7OH solid The resulting aluminum hydroxide (Al(OH)3 ) (Figure S1 in the Supplementary Material) is an amorphous compound with organic residues, which was heat treated at 550 ◦C/2 h up to their complete removal. The element composition of the samples was made using a system for the CHNS/O analysis of PE 2400-II (Perkin Elmer, Waltham, MA, USA) and an iCAP Q inductively coupled plasma mass spectrometer (ICP-MS) (Thermo Fisher Scientific, Waltham, MA, USA). The phase composition of samples was determined using X-ray powder diffraction (XRPD) by a MiniFlex 600 diffractometer (Rigaku, Tokyo, Japan) equipped with a D/teX Ultra detector. In this experiment, Cu Kα radiation (40 kV, 15 mA) was used and data was collected at room temperature in the range of 2θ from 2◦ to 100◦ with a step of 0.02◦ and exposure time at each point of 0.24 s without sample rotation. The phase concentrations were determined using the thermal analysis (TA; Netzsch STA–449C Jupiter, Selb, Germany). The TA was carried out in a way that is capable of recording the thermogravimetric (TG), derivative thermogravimetric (DTG), and differential thermal analysis (DTA) curves simultaneously. The samples were heated in the temperature range of 30–1000 ◦C at the uniform heating rate of 10 ◦C/min in an argon flow. Concentrations of aluminum hydroxides were calculated from the amount of the water that was released during the aluminum hydroxides dehydration and dehydroxilation. Mass spectrometry (MS) analysis was carried out at the heating of samples at the same rate in He current using of a ThermoStar GSD 320 T gas analyzer (Pfeiffer Vacuum, Nashua, NH, USA). The measurements by scanning electron microscopy (SEM) were carried out with an EVO 50XVP electron microscope combined with an INCA 350 energy-dispersive spectrometer (Carl Zeiss, Upper Cohen, Germany). The spectrometer resolution was 130 eV. The analysis was carried out at an acceleration voltage of 20 kV and flange back distance of 8 mm. The specific surface area SBET (calculated with the Brunauer–Emmett–Teller method [18]) and the pore volume V (calculated by the last point of isotherm [18]) were determined using a multipurpose Autosorb-iQ analyser (Quantachrome Instruments, Boynton Beach, FL, USA). Adsorption isotherms were obtained at −196 ◦C (77 K) after the degassing of samples at 150 ◦C under the residual pressure of 0.013 Pa. The pore volumes’ distributions over the pore diameters were calculated from the curve of desorption isotherm by using the standard Barrett–Joyner–Halenda mechanism (VBJH)[18]. Coatings 2019, 9, 41 4 of 14

The surface acidity of S3 sample and alumina derived from the S1 and S2 samples was analyzed by the method of temperature-programmed desorption of ammonia (TPD-NH3) on a flow-type instrument with a thermal conductivity detector ChemBet Pulsar (Quantachrome Instruments, Boynton Beach, FL, USA). Before ammonia adsorption, the sample was degassed by heating up to 550 ◦C in a helium flow. The adsorption step was carried out in a stream of ammonia for 30 min at a temperature of 100 ◦C. The physically adsorbed ammonia was removed by helium flow at 100 ◦C for 30 min. After analysis, the sample was cooled to room temperature in a helium flow. Temperature programmed desorption of ammonia was carried out from room temperature to 700 ◦C at a rate of 10 ◦C/min. Calculations of TPD-NH3 data on the distribution of acid sites were performed according to the method that is given in the research [19].

3. Results and Discussion All the samples of amorphous compounds obtained by different methods contain 95–99 wt % of the basic elements of Al, H, O, and differ from the impurity composition (Table1). The most amount of impurities is identiﬁed in sample S2 obtained by heat treatment of aluminum nitrate Al(NO3)3·9H2O, which consisted of the nitrate according to the State standard (GOST 3757-75). Sample S3 was synthesized from the high-purity product of the hydrolysis of aluminum alcoxide. Therefore, after calcining, S3 contains the minimum amount of Li, C, N, Na, and Mg impurities. Samples S1 3− and S2 include a considerable amount of nitrogen in the form of NO3 group (Table1). S1 is the product by the precipitation from the solution of aluminum nitrate with ammonia where a nitrogen in its structure remains due to incomplete hydrolysis. S2 is the product of thermal decomposition of aluminum nitrate, the heat treatment leads to the incomplete release of nitrogen oxides from the nitrate when residual water molecules are still present [6].

Table 1. Impurity content of samples S1–S3.

Content (wt %) Sample Obtaining Method Conditions Na Mg Li C N Fe S1 Precipitation pH = 6.00 <0.009 <0.001 0.000 0.41 4.57 <0.009 Heat treatment of 350 ◦C, S2 0.017 0.015 0.005 0.21 1.71 0.011 aluminum nitrate 1 h Heat treatment of 550 ◦C, S3 0.007 0.011 0.000 ~0.05 0.00 0.022 hydrolysis product 2 h

Samples S1–S3 are identiﬁed on the diffractograms as the X-ray amorphous aluminum compounds. A wide halo effect at the range of 20◦–40◦ of the S2 and S3 and of S1 are shown in Figure1. ◦ ◦ The diffractogram of S1 is characterized by the diffuse diffraction lines of PB at 20 –70 with d/I(120) = 0.316 nm, d/I(031) = 0.235 nm, d/I(200) = 0.177 nm, d/I(002) = 0.143 nm (JCPDS Card 00-021-1307), and a ◦ diffraction line at 8 with the d/I(100) = 1.203 nm of the aluminum complex Alx(OH)y(NO3)z(NH4) (JCPDS Card 01-080-7534) (Figure1). The presence of PB of the sample S1 is also conﬁrmed by TG-DSC and MS analyses. According to the TG-DSC data, PB is exhibited by a shoulder of the II endothermic effect on the DSC curve of sample S1 at the temperature of 397–570 ◦C with a mass loss (∆m) of 3.63 wt %, which is accompanied by the release of H2O and NO (Figure2a,b):

400−550 ◦C Al2O3·nH2O −−−−−−−→γ − Al2O3 + nH2O ↑

On the DSC curves of all samples, two endothermic effects and the exothermic effect (Figure2a–f, Table2) are observed due to the following transformations [20,21]:

◦ amorph 200−300 C amorph I endothermic effect : Al2O3·3H2O −−−−−−−→Al2O3 + 3H2O ↑ Coatings 2019, 9, 41 5 of 14

300−400 ◦C II endothermic effect : Al O ·xH O·yNOamorph −−−−−−−→Al O amorph + xH O ↑ +yNO ↑ Coatings 2018, 7, x FOR PEER REVIEW2 3 2 2 3 2 5 of 13

14000 13000 o 12000 11000 10000 9000 + + +

8000 + 7000 S1 6000

5000Intensity 4000 S2 3000 2000 S3 1000 0 410 14243444546474 20 30 40 50 60 70 80 2 θ ° (WL=1.54187)

Figure 1. Diffractograms of samples S1–S3 (+, PB; o, aluminum complex).

The I endothermic effect corresponds to the release of water from the porous system of samples at The I endothermic effect◦ corresponds to the release of water from the porous system of samples atthe the temperature temperature up toup 200 to C200 and °C during and during thermal thermal decomposition decomposition of amorphous of amorphous aluminum aluminum hydroxide amorph ◦ ◦ Al2O3·xH2O above 200 C. The II endothermic effect at the 321–495 C is identiﬁed as the hydroxide Al2O3·xH2O amorph above 200 °C. The II endothermic effect at the 321–495 °C is identified as amorph thermal decomposition of the basic aluminum salts Al2O3·xH2O·yNO amorphdue to the release of H2O the thermal decomposition of the basic aluminum salts Al2O3·xH2O·yNO due to the release of and NO (Table2, Figure2a–d), which corresponds to m/z=18 and 30, respectively, on the mass spectra. H2O and NO (Table 2, Figure 2a–d), which corresponds to m/z=18 and 30, respectively, on the mass spectra.The exothermic effect on the DSC curves is due to phase transformation low-temperature Al2O3 to high-temperature oxide without changing the mass (Figure2a–f, Table2), according to the following The exothermic effect on the DSC curves is due to phase transformation low-temperature Al2O3 amorph → γ → θ → α amorph toscheme high-temperature [20]: Al2O3 oxide withou-Al2Ot3 changing-Al2O 3the mass-Al2 O(Figure3. The amorphous2a–f, Table alumina 2), according (Al2O3 to the) is formed due to thermal decomposition of X-ray amorphous aluminum hydroxide and basic aluminum following scheme [20]: Al2O3amorph → γ-Al2O3 → θ-Al2O3 → α-Al2O3. The amorphous alumina salts [7]. The obtained from PB γ-Al O is transformed to high-temperature alumina without a visible (Al2O3amorph) is formed due to thermal2 decomposition3 of X-ray amorphous aluminum hydroxide and exothermic effect on the S1 DSC curve [1,4,7,12,22] (Figure2a,b). basic aluminum salts [7]. The obtained from PB γ-Al2O3 is transformed to high-temperature alumina without a visible exothermic effect on the S1 DSC curve [1,4,7,12,22] (Figure 2a,b). Table 2. TG-DSC and mass spectrometry (MS) analyses results of samples S1–S3.

Table 2. TG-DSC and mass spectrometryEndothermic (MS) analyses Effects results of samples S1-S3. Exothermic Effect Obtaining Sample Conditions I Effect II Effect Method Endothermic Effects Tp ∆m Tp ∆m ◦ Exothermic Effect◦ ◦ I Effect ◦ II Effect Tp, C Tb–Tf, C Obtaining (Tb–Tf*) ( C) (wt %) (Tb–Tf)( C) (wt %) Sample Conditions Тp Method pH = 6.00 258 Тp Δm 354 Δm Тp, S1 Precipitation ◦ 14.59 (Тb–Тf) 23.48 863 814–907Тb–Тf, °С T = 25 C (104–276)(Тb–Тf*) (°С) (wt %)(321–397) (wt %) °С (°С) Heat treatment 350 ◦C, 198 399 S2 of aluminum pH = 6.00 258 1.53 354 8.89 884 855–911 S1 Precipitation 1 h (129–278) 14.59 (339–495) 23.48 863 814–907 nitrate T = 25 °C (104–276) (321–397) HeatHeat treatment treatment 550 ◦C,350 °C, 102 198 399 S3S2 of hydrolysisof aluminum 10.69 1.53 − − 8.89 799884 683–882855–911 2 h 1 h (30–140)(129–278) (339–495) productnitrate Heat* T , treatment peak temperature; T , temperature of effect beginning; T , temperature of effect ﬁnish. p 550 b°C, 102 f S3 of hydrolysis 10.69 − − 799 683–882 2 h (30–140) Hereby, theproduct method of the precipitation from a solution of aluminum nitrate with ammonia at pH = 6.0 without stabilization and aging (sample S1) allows for obtaining a complex amorphous * Tp, peak temperature; Tb, temperature of effect beginning; Tf, temperature of effect finish. compound in the form of a phase mixture that consists of amorphous aluminum hydroxide, basic aluminum salt, and poorly crystallized pseudoboehmite.

Coatings 2018, 7, x FOR PEER REVIEW 6 of 13

Hereby, the method of the precipitation from a solution of aluminum nitrate with ammonia at pH = 6.0 without stabilization and aging (sample S1) allows for obtaining a complex amorphous compoundCoatings 2019 ,in9, 41the form of a phase mixture that consists of amorphous aluminum hydroxide, basic6 of 14 aluminum salt, and poorly crystallized pseudoboehmite.

9 8 m/z=18 / А 7

-10 6 10 ⋅ 5 4 3 2 Ion current Ion 1 m/z=30 0 0 100 200 300 400 500 600 ° Temperature / С (a) (b) 18 16 / А 14 -10 12 10

⋅ m/z=18 10 8 6 4 Ion current Ion 2 m/z=30 0 0 100 200 300 400 500 600 Temperature / °С (c) (d) 16 14 / А 12 -10

10 10

⋅ m/z=18 8 6 4 Ion current Ion 2 m/z=44 0 0 100 200 300 400 500 600 ° Temperature / С (e) (f)

Figure 2. TG-DSC and MS analyses results of samples: S1 (a,b), S2 (c,d), S3 (e,f). Figure 2. TG-DSC and MS analyses results of samples: S1 (a,b), S2 (c,d), S3 (e,f). The quantitative phase composition of the complex precipitation product (S1) was calculated fromThe the quantitative mass loss of thephase corresponding composition endothermic of the complex effects precipitation on the DSC product curve and (S1) the was areas calculated of the NO frompeak the in themass mass loss spectrum of the corresponding (Figure2a,b), endothermi since otherc nitrogen effects on oxides the DSC are almostcurve and not observed.the areas of Based the NOon thepeak total in the nitrogen mass contentspectrum in the(Figure sample 2a,b), (Table sinc1e), other the total nitrogen amount oxides of NO are released almost fromnot observed. S1 during Basedthermal on decompositionthe total nitrogen to 650content◦C is in 9.79 the wtsample %. NO (Table is exhibited 1), the total by two amount peaks of on NO S1 released mass spectrum from S1 in duringthe temperature thermal decomposition range of II endothermic to 650 °C effectis 9.79 and wt its%. shoulderNO is exhibited on the DSC by two curve peaks of the on sample S1 mass in a spectrumratio of 28.6:1.0 in the temperature (Figure2a,b). range In this of way, II endothermic almost all nitrogen effect and is releasedits shoulder from on the the basic DSC aluminum curve of the salt samplestructure in a (only ratio II of endothermic 28.6:1.0 (Figure effect). 2a,b). There In this is a way, possibility almost that all nitrogen the release is released of NO 0.32 from wt the % duringbasic aluminumthermal decomposition salt structure of(only PB isII identiﬁedendothermic by theeffect). shoulder There on is DSCa possibility curve is that due tothe the release rearrangement of NO 0.32 of wtthe % hydroxide during thermal lattice todecomposition oxide (PB → γof-Al PB2O is3). identified 3.63 wt % by H2 theO is shoulder also released on DSC at the curve temperature. is due to Sincethe rearrangementPB in the sample of isthe poorly hydroxide crystallized, lattice asto evidencedoxide (PB by→ diffuseγ-Al2O diffraction3). 3.63 wt lines% H2 (FigureO is also1), thereleased value at of interlayer water n in its structure was taken as the maximum, i.e., n = 2.0 [23]. In this case, the amount of PB in the sample S1 is 12.69 wt %. Coatings 2019, 9, 41 7 of 14

Based on the total content of NO, 9.47 wt % NO and 14.01 wt % H2O are released during thermal decomposition of the basic aluminum salt at the II endothermic effect, i.e., 0.32 and 0.78 mol, respectively. Therefore, the empirical salt formula is Al2O3·0.78H2O·0.32NO. It was reported in the following scientiﬁc works [7,21,22] that the empirical formula amorphous aluminum hydroxide that was obtained by the precipitation of the aluminum salts by the base is ◦ Al2O3·3H2O. Its thermal decomposition begins at the temperature more than 200 C. Hence, the mass loss of I endothermic effect on the S1 DSC curve was divided into two parts, 8.18 wt % H2O, of which ◦ is physically adsorbed water (<200 C) and 6.41 wt % H2O releases during the thermal decomposition of amorphous hydroxide. The calculated amount of basic aluminum salt and amorphous Al2O3·3H2O in the sample S1 are 18.52 wt % and 60.61 wt %, respectively. According to the results of element, XRPD, TG-DSC, and MS analyses, the phase composition of the precipitation product (S1) calculated without taking into account physically adsorbed water is presented in Table3.

Table 3. The phase composition of amorphous S1–S3 compounds.

Sample Obtaining Method Conditions Empirical Formula Amount, wt % Al O ·3H O (amorphous hydroxide) 20.2 pH = 6.00, 2 3 2 S1 Precipitation Al O ·0.78H O·0.32NO (basic salt) 66.0 T = 25 ◦C 2 3 2 Al2O3·2H2O (PB) 13.8 Heat treatment of 350 ◦C, Al O ·3H O (amorphous hydroxide) 16.3 S2 2 3 2 aluminum nitrate 1 h Al2O3·0,29H2O·0,12NO (basic salt) 83.7 Heat treatment of 550 ◦C, S3 Al O (amorphous oxide) 100.0 hydrolysis product 2 h 2 3

◦ The method of thermal decomposition of aluminum nitrate Al(NO3)3·9H2O at 350 C for 1 h allows for obtaining a complex X-ray amorphous compound in the form of the phase mixture of aluminum hydroxide and basic salt in the amount of, respectively, 16.3 and 83.7 wt % (Table3). These phases are exhibited by two corresponding endothermic effects on the S2 DSC curve and the release of H2O and NO on the MS spectrum (Table2, Figure2c,d). The quantitative S2 composition was calculated, as in case of Sample S1, by the mass loss of endothermic effects on the S2 DSC curve (Table2, Figure2c,d). Based on the total nitrogen content in the sample (Table1), the total amount of released NO is 3.66 wt %. The amount of amorphous aluminum hydroxide (Al2O3·3H2O) was calculated based on mass loss on the S2 DSC curve more than 200 ◦C and equaled 16.27 wt %. Based on the total content of NO (3.66 wt %) and the mass spectrum of sample S2 (Figure2d), the release of NO occurs only during the thermal decomposition of salt (II endothermic effect) and 5.23 wt % H2O is also released, i.e., respectively 0.12 and 0.29 mol. Consequently, the empirical formula of the basic aluminum salt is Al2O3·0.29H2O·0.12NO. The thermal decomposition method of the product of aluminum alcoxide hydrolysis at 550 ◦C for 2 h (sample S3) allows for obtaining a phase-homogeneous X-ray amorphous alumina, which has a small amount of metal impurities (Table1). Residual organic compounds are exhibited by a pronounced shoulder of the I endothermic effect in the low-temperature region on the S3 DSC curve and the presence in the mass spectrum of peaks, corresponding to the release of CO2 and H2O (Figure2e,f). The precipitation method (pH = 6.0) allows for obtaining the mixture of particles (~100 µm), the surface of which consists of thin plates of size to 1 µm layering on each other closely, with extended cracks resulting from drying the gel-like precipitate [7,21] (Figure3a,b). It is impossible to identify the porous system of S1 in the SEM images. Indeed, according to the data of low-temperature nitrogen adsorption, sample S1 has a speciﬁc surface area <1 m2/g and a pore volume <0.01 cm3/g (Table4). Coatings 2019, 9, 41 8 of 14 Coatings 2018, 7, x FOR PEER REVIEW 8 of 13

(a) (b)

(e) (f)

Figure 3. Scanning electron microscopy (SEM) images of external and internal surface of samples S1 (a,b), S2 (c,d), S3 (e,,f).).

The heat-treated product ofof Al(NOAl(NO33)3·9H·9H22OO (sample (sample S2) S2) is is a a mesoporous compound (as shown in the FigureFigure S2S2 inin thethe SupplementarySupplementary Material),Material), whichwhich hashas SS andand VV of of 29 29 m m2/g2/g andand 0.040.04 cmcm33/g,/g, respectively (Table4 4,, Figure Figure4 a).4a). S2 S2 particles particles are are irregularly irregularly shaped shaped particles particles of of > >100 100 µmµm (Figure (Figure 33c).c). The externalexternal surfacesurface is is smooth, smooth, it hasit has a size a size of 50–100 of 50–100µm, andµm, it and contains it contains small fragmentssmall fragments with cavities. with Thecavities. internal The surfaceinternal of surface S2 particles of S2 particles includes includes large cavities largein cavities the form in the of hemispheres form of hemispheres with 5–10 withµm 5–10diameters, µm diameters, sponge-like sponge-like structure structure with cavity with sizes cavi ofty 50–850 sizes of nm, 50–850 and itnm, accretes and it plate-likeaccretes plate-like particles particlesbetween whichbetween conical which spaces conical with spaces a base with diameter a base lessdiameter than 530 less nm than are 530 formed nm are (Figure formed3d). (Figure As shown 3d). Ason theshown SEM on images the SEM (Figure images3c,d), (Figure sample 3c,d), S2 sample has the cavitiesS2 has the formed, cavities probably formed,at probably the intense at the releasing intense releasingof water andof water nitrogen and oxide nitrogen molecules oxide duringmolecules the du thermalring the decomposition thermal decomposition of nitrate. The of nitrate. cavities The are cavitiesdistributed are throughoutdistributed thethroughout sample. Therefore,the sample. almost Therefore, uniform almost distribution uniform ofdistribution VBJH pore diametersof VBJH pore is diametersobserved (Tableis observed4). (Table 4). Heat treatment at 550 ◦C of the hydrolysis product makes it possible to obtain a highly porous sample with a high acidity of the surface (Table2; Table4, Figure4b). Sample S3 particles are the

Coatings 2018, 7, x FOR PEER REVIEW 9 of 13

CoatingsHeat2019, treatment9, 41 at 550 °C of the hydrolysis product makes it possible to obtain a highly porous9 of 14 sample with a high acidity of the surface (Table 2; Table 4, Figure 4b). Sample S3 particles are the large particles of 3–133 µm, which consist of aggregates of irregularly shaped plates (Figure 3e). The largeaggregates particles are of formed 3–133 µbym, whichsmall particles consist of of aggregates 10–30 nm. of There irregularly are narrow shaped and plates large (Figure cavities,3e). Therespectively, aggregates of are<9 nm formed and 0.3–2.5 by small µm particles between ofthem. 10–30 nm. There are narrow and large cavities, respectively, of <9 nm and 0.3–2.5 µm between them. Table 4. Porous system parameters of amorphous compounds S1–S3. Table 4. Porous system parameters of amorphous compounds S1–S3. Distribution of VBJН Dmax, (nm) Obtaining S, V, VBJН, Distributionover Dp, of cmVBJH3/g (%) Dmax, (nm) Sample Obtaining Conditions S, V, VBJH, 3 dV/dD, Sample Method Conditions (m2/g) (cm3/g) (cm3/g) over<3D p, cm3–10/g (%) >10 dV/dD, Method (m2/g) (cm3/g) (cm3/g) (cm3/g⋅nm) <3 nmnm 3–10 nmnm >10 nmnm (cm3/g·nm) pHpH = 6.00,= 6.00, S1S1 Precipitation <1<1 <0.01<0.01 <0.01<0.01 −− −− −− − − T =T25 = 25◦C °C HeatHeat treatment 350350◦C, °C, 0.02 0.020.02 0.020.01 0.01 S2S2 of aluminum 2929 0.040.04 0.050.05 3.7/0.023.7/0.02 1 h1 h (40) (40)(40) (40)(20) (20) nitrate Heat treatment Heat treatment 550 ◦C, 0.01 0.56 0.38 S3 of hydrolysis 550 °C, 344 0.90 0.95 0.01 0.56 0.38 4.5/0.19 S3 of hydrolysis 2 h 344 0.90 0.95 (1) (59) (40) 4.5/0.19 product 2 h (1) (59) (40) product

InIn the the crack crack of of aggregate, aggregate, parallel parallel channelschannels withwith aa diameterdiameter ofof 280–360280–360 nmnm areare formedformed (Figure(Figure3 f),3f), thethe entranceentrance toto whichwhich isis thethe largerlarger wellswells ofof ~400 ~400 nmnm diameterdiameter onon thethe externalexternal surfacesurface ofof particle.particle. 2 DespiteDespite the the absence absence of of micropores, micropores, sample sample S3 S3 has has high high S S (344 (344 m m/g),2/g), whichwhich includesincludes59% 59% pores pores with with DDpp == 3–10 nm (as (as shown shown in in the the Figure Figure S2 S2 in in the the Supplementary Supplementary Material Material and and Table Table 4).4 ).Probably, Probably, the thenarrow narrow pores pores with with Dp =D 4.5p = nm 4.5 of nm the of sample the sample are the are spaces the spacesbetween between the primary the primary particles particles of alumina. of alumina.The cavities The are cavities pores are with pores Dp > with 10 nm,Dp >which 10 nm, consist which of consist 40 % of of the 40 S3 % ofporous the S3 system. porous system.

30 600 /g 3 /g

3 25 500 20 400 15 300 10 200 Volume (V), cm

Volume (V), cm 5 100 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Relative pressure, P/P0 Relative pressure, P/P0 (a) (b)

Figure 4. Isotherms of samples S2 (a) and S3 (b). Figure 4. Isotherms of samples S2 (a) and S3 (b). Acid and base sites possessing various structure and strength are presented on the surface of aluminaAcid and obtained base sites by possessing the heat treatmentvarious structur (HT)e ofand the strength amorphous are presented aluminum on the compounds. surface of Thealumina distribution obtained of sitesby the is mainlyheat treatment caused by(HT) the of phase the compositionamorphous ofaluminum a compound. compounds. Acid sites The aredistribution Brønsted-type of sites (terminal is mainly or bridge caused surface by the hydroxyl phase groups)composition and Lewis of a (coordination-unsaturatedcompound. Acid sites are aluminumBrønsted-type cations) (terminal centers or [24 bridge,25]. Both surface types hydrox of acidyl sitesgroups) are identifiedand Lewis by (c variousoordination-unsaturated methods one of whichaluminum is the cations) TPD of NHcenters3 method. [24,25]. It Both allows types for determiningof acid sites theare totalidentified acidity by of various the surface. methods However, one of thewhich proton is the affinity TPD energyof NH3 ofmethod. the strongest It allows Brønsted for determining sites of alumina the total is not acidity sufficient of the for surface. the protonation However, ofthe ammonia proton moleculesaffinity energy [26]. of the strongest Brønsted sites of alumina is not sufficient for the protonationAfter dehydration, of ammonia the molecules Lewis acid [26]. and basic sites are simultaneously formed on the surface of the alumina.After Lewis dehydration, acid sites the transform Lewis acid into Brønstedand basic sitessites in are the simultaneously presence of water formed molecules on the [ 27surface]: of the alumina. Lewis acid sites transform into Brønsted sites in the presence of water molecules [27]:

Coatings 2019, 9, 41 10 of 14

Coatings 2018, 7, x FOR PEER REVIEW 10 of 13 Coatings 2018, 7, x FOR PEER REVIEW 10 of 13

Interacting thethe NHNH3 molecule with Lewis acid sites forms a coordination (donor-acceptor) bond Interacting the NH3 molecule with Lewis acid sites forms a coordination (donor-acceptor) bond due to the free electron pairpair ofof thethe nitrogennitrogen atom:atom: due to the free electron pair of the nitrogen atom: ≡Al …NH ≡Al …NH For studying the samples’ acid properties,≡ Al S1 . . .an NHd S23 were previously heat treated (HT) at the For studying the samples’ acid properties, S1 and S2 were previously heat treated (HT) at the 550 °C to remove excess water in the form of hydroxyl groups from the surfaces. Sample S3 was not 550For °C studyingto remove the excess samples’ water acid in the properties, form of hydroxyl S1 and S2 groups were previouslyfrom the surfaces. heat treated Sample (HT) S3 atwas the not additionally treated because it was obtained by heating at 550 °C. The TPD curves of NH3 samples additionally◦ treated because it was obtained by heating at 550 °C. The TPD curves of NH3 samples 550are presentedC to remove in Figure excess 5. water The indistribution the form of of hydroxyl acid sites groups (a.s.) according from the surfaces.to the ammonia Sample S3desorption was not additionallyare presented treated in Figure because 5. itThe was distribution obtained by of heating acid sites at 550 (a.s.)◦C. according The TPD curvesto the ofammonia NH samples desorption are energy (Ed) for the obtained samples is given in Table 5. The a.s. with Ed <110 kJ/mol,3 which energy (Ed) for the obtained samples is given in Table 5. The a.s. with Ed <110 kJ/mol, which presentedcorresponds in Figureto the 5desorption. The distribution temperature of acid up sites to 210 (a.s.) °C, according were attributed to the ammonia to the weak desorption strength energy sites. (Ecorresponds) for the obtained to the samplesdesorption is given temperature in Table 5up. The to 210 a.s. °C, with wereE <110attributed kJ/mol, to the which weak corresponds strength sites. to Thed a.s. with Ed to 110 to 142 kJ/mol and Ed >142 kJ/mol are, drespectively, taken for the medium The a.s. with Ed to 110 to 142 kJ/mol◦ and Ed >142 kJ/mol are, respectively, taken for the medium the(desorption desorption temperature temperature of up210–350 to 210 °C)C, and were the attributed strong strength to the weak sites strength(desorption sites. temperature The a.s. with is moreEd to (desorption temperature of 210–350 °C) and the strong strength sites (desorption temperature is more 110than to 350 142 ° kJ/molС). and Ed >142 kJ/mol are, respectively, taken for the medium (desorption temperature ofthan 210–350 350 ◦°C)С). and the strong strength sites (desorption temperature is more than 350 ◦C).

0.007 0.007

0.006 /(g·s)

3 0.006

/(g·s) S1 (HT) 3 S1 (HT) S2 (HT) 0.005 S2 (HT) 0.005 S3 S3 0.004 0.004

0.003 0.003

0.002 0.002 desorption rate, µmol NH 3 desorption rate, µmol NH

0.0013 0.001 NH NH 0 0 0 50 100 150 200 250 300 350 400 450 500 550 600 0 50 100 150 200 250 300 350 400 450 500 550 600 Temperature, °С Temperature, °С Figure 5. Curves of the temperature-programmed desorption (TPD) of ammonia of samples (HT-heat FigureFigure 5. 5. Curves of of the the temperature-programmed temperature-programmed desorption desorption (TPD) (TPD) of ammonia of ammonia of samples of samples (HT-heat treated). (HT-heattreated). treated).

TPD-NH3 curves of all samples begin from the temperature of ~75 ◦°C (Figure 5). Diffuse peaks TPD-NHTPD-NH3 curves3 curves of of all all samples samples begin begin from from the the temperature temperature of of ~75 ~75C °C (Figure (Figure5). 5). Diffuse Diffuse peaks peaks are noted to the dependences of the ammonia desorption rate on the temperature of S1HTHT and S2HTHT areare noted noted to to the the dependences dependences of of the the ammonia ammonia desorption desorption rate rate on on the the temperature temperature of S1of S1HTand and S2 S2HT samples in the region of ~140–190 °C.◦ An intensive peak is noted on the curve of S3. These peaks are samplessamples in in the the region region of of ~140–190 ~140–190 °C.C. AnAn intensiveintensive peakpeak is noted on the the curve curve of of S3. S3. These These peaks peaks are characterized by weak a.s. In the area of medium a.s. of S1HTHT and S2HT samples, the ammonia arecharacterized characterized by by weak weak a.s. a.s. In In the area ofof mediummediuma.s. a.s. of of S1 S1HTand and S2 S2HTsamples, samples, the the ammonia ammonia desorption rate with increasing temperature decreases slowly, while the desorption rate on the S3 desorptiondesorption rate rate with with increasing increasing temperature temperature decreases decreas slowly,es slowly, while while the desorption the desorption rate on rate the S3on curve the S3 curve increases with a peak at the temperature of 235–250◦ °C. In the field of strong a.s., an equable increasescurve increases with a peak with at a the peak temperature at the temperature of 235–250 of C.235–250 In the °C. ﬁeld In of the strong field a.s., of strong an equable a.s., an decrease equable decrease in the desorption rate to 400, ◦500, and 540 °C occurs HTfor samplesHT S1HT, S2HT, and S3, indecrease the desorption in the rate desorption to 400, 500, rate and to 540400, C500, occurs and for 540 samples °C occurs S1 ,for S2 samples, and S3, S1 respectively.HT, S2HT, and S3, respectively. respectively.

Coatings 2019, 9, 41 11 of 14 Coatings 2018, 7, x FOR PEER REVIEW 11 of 13

HTHT HTHT TableTable 5. 5.Ammonia Ammonia (NH (NH3)3) TPD TPD acid acid characteristics characteristics of of samples samples S1 S1 ,, S2S2 ,, and and S3. S3.

NumberNumber of a.s., ofµ a.s.,mol μ NHmol3/g NH3/g Sample Obtaining Method Conditions 110 < Ed< Sample Obtaining Method ConditionsEd < 110,Ed< 110,110 < Ed< Ed >E 142,d > 142, Coatings 2018, 7, x FOR PEER REVIEW 142, 11 TotalofTotal 13 (kJ/mol)(kJ/mol)142, (kJ/mol) (kJ/mol)(kJ/mol) (kJ/mol) HT Table 5. Ammonia (NH3pH) TPD =6.00, acid characteristics of samples S1HT, S2HT, and S3. S1 Precipitation ◦ pH=6.00,99.1 77.8 23.3 200.2 S1HT Precipitation T = 25 C 99.1 77.8 23.3 200.2 ◦ T = 25 °C Number of a.s., μmol NH3/g HT Heat treatment of 350 C, S2 350 °C,45.6 63.2110 < Ed< 7.1 115.9 S2HT Sample Heat treatmentaluminumObtaining of nitrate aluminum Method nitrate 1 h Conditions Ed<45.6 110, 63.2 Ed > 142, 7.1 115.9 1 h 142, Total Heat treatment of 550 ◦C, (kJ/mol) (kJ/mol) S3 Heat treatment of hydrolysis 550 °C,150.9 427.6(kJ/mol) 73.7 652.2 S3 hydrolysis product 2 h pH=6.00, 150.9 427.6 73.7 652.2 S1HT productPrecipitation 2 h 99.1 77.8 23.3 200.2 T = 25 °C HT 350 °C, In theS2HT case of Heat a precipitationtreatment of aluminum product nitrate S1 HT , the intermediate45.6 value of63.2 acidity in comparison 7.1 115.9 with In the case of a precipitation product S1 , the 1intermediate h value of acidity in comparison with HT S2S2HT and S3 samples Heat is due treatment to the of content hydrolysis of Li, Na, 550 Mg °C, and Fe impurity ions (Table1 1).). Also, Also, after after the the S3 150.9 427.6 73.7 652.2 heatheat treatmenttreatment ofof basicbasic aluminumaluminumproduct salts,salts, aa smallsmall amountamount2 h ofof a.s.a.s. onon thethe surfacesurface ofof thethe amorphousamorphous oxideoxide isis probablyprobably duedue toto thethe formationformation ofof thethe basebase sitessites duringduring thethe removingremoving ofof NONO andand HH22OO moleculesmolecules toto In the case of a precipitation product S1HT, the intermediate value of acidity in comparison with thethe followingfollowing scheme:scheme: S2HT and S3 samples is due to the content of Li, Na, Mg and Fe impurity ions (Table 1). Also, after the heat treatment of basic aluminum salts, a small amount of a.s. on the surface of the amorphous oxide is probably due to the formation of the base sites during the removing of NO and H2O molecules to the following scheme:

γ-Al2O3 also contributes to the total a.s. quantity [25]. Indeed, S1HTHT contains the strong a.s. and γ-Al2O3 also contributes to the total a.s. quantity [25]. Indeed, S1 contains the strong a.s. and medium a.s. in amounts of 12% and 38%, respectively (Table 5). medium a.s. in amounts of 12% and 38%, respectively (Table5). After heat2 treatment3 of the sample S2, amorphous hydroxide HTand basic aluminum salt form Afterγ heat-Al O treatment also contributes of the to sample the tota S2,l a.s. amorphous quantity [25]. hydroxide Indeed, S1 and contains basic aluminumthe strong a.s. salt and form amorphousmedium alumina a.s. in amounts that possesses of 12% and a 38%,small respectively amount of (Table a.s., 5).96% of which are weak and medium amorphous alumina that possesses a small amount of a.s., 96% of which are weak and medium strength (TableAfter heat5, Figure treatment 5). As of thepreviously sample S2,noted, amorphous this effect hydroxide is observed and basic due aluminum to the high salt content form of strength (Table5, Figure5). As previously noted, this effect is observed due to the high content Na, Mg,amorphous and Fe impurity alumina ionsthat (Tablepossesses 1). aAlso, small the amount small amountof a.s., 96% of a.s. of whichis probably are weak due andto the medium formation of Na,strength Mg, and (Table Fe impurity5, Figure 5). ions As (Tablepreviously1). Also, noted, the this small effect amountis observed of a.s.due to is the probably high content due to of the of base sites during the removal of NO and H2O molecules after the thermal decomposition of formationNa, Mg, of base and Fe sites impurity during ions the (Table removal 1). Also, of NO the and sma Hll amount2O molecules of a.s. is after probably the thermal due to the decomposition formation nitrogen-containing basic aluminum salts. of nitrogen-containingof base sites during basic the aluminumremoval of salts.NO and H2O molecules after the thermal decomposition of Amorphous alumina obtained by thermal decomposition of the alcoxide hydrolysis product has Amorphousnitrogen-containing alumina basic obtained aluminum by thermal salts. decomposition of the alcoxide hydrolysis product has a large number of a.s. on the surface, 65% of which is medium strength (Table 4). S3 consists of the a large numberAmorphous of a.s. alumina on the surface,obtained by 65% thermal of which decomposi is mediumtion of strengththe alcoxide (Table hydrolysis4). S3 consistsproduct has of the small amount of Na (Table 1) and the impurities do not block a.s. its surface as against the S1HT and small amounta large number of Na (Tableof a.s. on1) andthe surface, the impurities 65% of which do not is medium block a.s. strength its surface (Table as 4). against S3 consists the S1 of HTthe and S2HT. In addition, during the heat treatment of the hydrolysis product, thermal decompositionHT of S2HT.small In addition, amount of during Na (Table the heat1) and treatment the impurities of the do hydrolysisnot block a.s. product, its surface thermal as against decomposition the S1 and of - underhydrolyzedS2HT. In addition, alcoholate during occursthe heat with treatment the formation of the hydrolysis of CO2 molecules,product, thermal the O -decomposition atom of which of was underhydrolyzed alcoholate occurs with the formation of CO2 molecules, the O atom of which was probablyunderhydrolyzed bended with alcoholate Al. Hence, occurs the Lewis with the a.s. formation is formed of accordingCO2 molecules, to the the following O- atom of scheme: which was probably bended with Al. Hence, the Lewis a.s. is formed according to the following scheme: probably bended with Al. Hence, the Lewis a.s. is formed according to the following scheme:

Coatings 2019, 9, 41 12 of 14

4. Conclusions The comparative analysis of amorphous aluminum compounds obtained by the precipitation method, the thermal decomposition of aluminum nitrate and alcoxide hydrolysis product, and their physicochemical properties were studied. For the first time a new method for calculating quantitative phase composition of amorphous aluminum compounds using a group of methods as XPRD, TG-DSC, MS, and CHN-analysis was described. The method of precipitation of 0.25 g/mL of aluminum nitrate by ammonia solution at pH = 6.0 without aging and stabilization stages allows for obtaining a complex amorphous aluminum compound, which includes amorphous hydroxide (Al2O3·3H2O), basic salt (Al2O3·0.78H2O·0.32NO), and pseudoboehmite (Al2O3·2H2O). This compound is non-porous, its particles has large cracks. For the heat-treated precipitation product, which is a mixture of amorphous alumina and γ-alumina, is characterized by the total number of a.s. of 200.2 µmol/g containing 12% strong and 50% weak acid sites on its surface due to the presence in the sample of Li, Na, Mg, and Fe impurity ions. The method of thermal decomposition of aluminum nitrate at 350 ◦C for 1 h allows for obtaining a complex amorphous aluminum compound, which includes amorphous hydroxide (Al2O3·3H2O) and basic salt (Al2O3·0.29H2O·0.12NO). This compound is mesoporous with the low specific surface area and pore volume, a maximum on the differential distribution curve of pore volumes over the pore diameter is 3.7 nm. The product of thermal decomposition of nitrate is a monolithic irregularly shaped particle with cavities that were evenly distributed throughout the sample. The thermal decomposition of the sample leads to amorphous oxide, which is accompanied by the formation of a small quantity of acid sites on its surface, which contains 6% strong and 55% medium sites due to the high content of Na, Mg, and Fe impurities in the sample. The method of heat treatment at 550 ◦C for 2 h of the hydrolysis product of the alcoxide allows for obtaining a monophasic amorphous alumina. It is highly porous with high specific surface area and pore volume, a maximum on the distribution curve of pore volumes over diameters is 4.5 nm. Amorphous alumina is a large aggregates formed by small particles, between which are narrow and large cavities. It has a large quantity of acid sites on the surface, which is 65% medium and 12% strong sites due to the small amount of impurities in the sample.

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-6412/9/1/41/s1, Figure S1. Diffractogram and thermal curves of amorphous aluminum hydroxide obtained by the hydrolysis (S3 sample precursor); Figure S2. The curves of distribution of VBJH with Dp of samples S2 (a) and S3 (b). Author Contributions: Conceptualization, A.N.M. and S.R.E.; Methodology, S.R.E. and A.A.L.; Validation, A.N.M., S.R.E. and A.A.L.; Formal Analysis, O.V.N., K.S.B., A.V.B. and J.S.; Investigation, A.N.M. and A.V.B.; Resources, A.A.L.; Data Curation, A.N.M.; Writing-Original Draft Preparation, A.N.M.; Writing-Review & Editing, A.N.M.; Visualization, A.N.M.; Supervision, S.R.E. and A.A.L.; Project Administration, A.N.M. and S.R.E.; Funding Acquisition, A.N.M. and A.A.L. Funding: The reported study of samples S1 and S2 (products of precipitation aluminum nitrate by ammonia solution and thermal decomposition of aluminum nitrate) was funded by RFBR according to the research project No. 18-33-00559. The study of sample S3 (product of thermal decomposition of aluminum hydroxide obtained by the organoaluminum compounds hydrolysis) was performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. Acknowledgments: The thermal analysis of samples was carried out at the Federal Center for Collective Use of the Kazan Federal University with the support of the Russian Agency for Science and Innovation A.V. Gerasimov. The scanning electron microscopy measurements were performed at the Interdisciplinary Center “Analytical Microscopy” of the Kazan Federal University V.V. Vorobiev and V.G. Evtyugin respectively. Conﬂicts of Interest: The authors declare no conﬂict of interest. Coatings 2019, 9, 41 13 of 14

References

1. Ivanova, A.S.; Litvak, G.S.; Kryukova, G.N.; Tsybulya, S.V.; Paukshtis, E.A. Real structure of metastable forms of aluminum oxide. Kinet. Catal. 2000, 41, 122–126. [CrossRef] 2. Macêdo, M.I.F.; Osawa, C.C.; Bertran, C.A. Sol-gel synthesis of transparent alumina gel and pure gamma alumina by urea hydrolysis of aluminum nitrate. J. Sol-Gel Sci. Technol. 2004, 30, 135–140. [CrossRef] 3. Garbarino, G.; Travi, I.; Pani, M.; Carnasciali, M.M.; Busca, G. Pure vs ultra-pure γ-alumina: A spectroscopic study and catalysis of ethanol conversion. Catal. Commun. 2015, 70, 77–81. [CrossRef] 4. Novak, C.; Pokol, G.; Izvekov, V.; Gal, T. Studies on the reactions of aluminium oxides and hydroxides. J. Therm. Anal. 1990, 36, 1895–1909. [CrossRef] 5. Nayar, P.; Khanna, A.; Kabiraj, D.; Abhilash, S.R.; Beake, B.D.; Losset, Y.; Chen, B. Structural, optical and mechanical properties of amorphous and crystalline alumina thin films. Thin Solid Films 2014, 568, 19–24. [CrossRef] 6. Pacewska, B.; Keshr, M. Thermal transformations of the products of partial hydrolysis of hydrous aluminium nitrate. J. Therm. Anal. Calorim. 2004, 75, 113–123. [CrossRef] 7. Landau, M.V. Sol-Gel Processing. In Synthesis of Solid Catalysts; de Jong, K.P., Ed.; Wiley-VCH: Weinheim, German, 2009; pp. 83–109. 8. Oh, C.J.; Yi, Y.K.; Kim, S.J.; Tran, T.; Kim, M.J. Production of micro-crystalline boehmite from hydrothermal processing of Bayer plant alumina tri-hydrate. Powder Technol. 2013, 235, 556–562. [CrossRef] 9. Tsuchida, T. Hydrothermal synthesis of submicrometer crystals of boehmite. J. Eur. Ceram. Soc. 2000, 20, 1759–1764. [CrossRef] 10. Yoldas, B.E. Hydrolysis of aluminium alkoxides and bayerite conversion. J. Appl. Chem. Biotechnol. 1973, 23, 803–809. [CrossRef] 11. Hwang, K.T.; Lee, H.S.; Lee, S.H.; Chung, K.C.; Park, S.S.; Lee, J.H. Synthesis of aluminium hydrates by a precipitation method and their use in coatings for ceramic membranes. J. Eur. Ceram. Soc. 2001, 21, 375–380. [CrossRef] 12. Lippens, B.C.; de Boer, J.H. Studies on pore systems in catalysts: V. The t method. J. Catal. 1965, 4, 319–323. [CrossRef] 13. Ma, C.C.; Zhou, X.X.; Xu, X.; Zhu, T. Synthesis and thermal decomposition of ammonium aluminum carbonate hydroxide (AACH). Mater. Chem. Phys. 2001, 72, 374–379. [CrossRef] 14. Mukhambetov, I.N.; Egorova, S.R.; Mukhamed’yarova, A.N.; Lamberov, A.A. Hydrothermal modification of the alumina catalyst for the skeletal isomerization of n-butenes. Appl. Catal. A 2018, 554, 64–70. [CrossRef] 15. Lehmkuhl, H. Organoaluminum compounds. Ann. New York Acad. Sci. 1965, 125, 124–130. [CrossRef] 16. Hayrapetyan, S.S.; Mangasaryan, L.G.; Tovmasyan, M.R.; Khachatryan, H.G. Precipitation of aluminum hydroxide from sodium aluminate, by treatment with formalin, and preparation of aluminum oxide. Acta Chromatogr. 2006, 16, 192. [CrossRef] 17. GOST 3757-75 Reaktivy Alyuminij azotno-kislyj 9-vodnyj. Tekhnicheskie usloviya; Standartinform Publ.: Moscow, Russia, 1993. (In Russian) 18. Gregg, S.J. Adsorption, Surface Area and Porosity; Academic Press: London, UK, 1982. 19. Bronsted, J.N. Acid and Basic Catalysis. Chem. Rev. 1928, 5, 231–338. [CrossRef] 20. Von Hehl, A.; Krug, P. Aluminum and Aluminum Alloys. In Structural Materials and Processes in Transportation; Lehmhus, D., Busse, M., Herrmann, A.S., Kayvantash, K., Eds.; Wiley-VCH: Weinheim, German, 2013. 21. Du, X.; Wang, Y.; Su, X.; Li, J. Influences of pH value on the microstructure and phase transformation of aluminum hydroxide. Powder Technol. 2009, 192, 40–46. [CrossRef] 22. Zhang, X.; Honkanen, M.; Levänen, E.; Mäntylä, T. Transition alumina nanoparticles and nanorods from boehmite nanoflakes. J. Cryst. Growth 2008, 310, 3674–3679. [CrossRef] 23. de Boer, J.H.; Linsen, B.G.; Osinga, T.J. Studies on pore systems in catalysts: VI. The universal t curve. J. Catal. 1965, 4, 643–648. [CrossRef] 24. Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. Hydroxyl groups on γ-alumina surfaces: A DFT study. J. Catal. 2002, 211, 1–5. [CrossRef] 25. Tago, T.; Okubo, Y.; Mukai, S.R.; Tanaka, T.; Masuda, T. Simultaneous characterization of acidic and basic properties of solid catalysts by a new TPD method and their correlation to reaction rates. Appl. Catal. A 2005, 290, 54–64. [CrossRef] Coatings 2019, 9, 41 14 of 14

26. El-Nadjar, W.; Bonne, M.; Trela, E.; Rouleau, L.; Mino, A.; Hocine, S.; Payen, E.; Lancelot, C.; Lamonier, C.; Blanchard, P.; Courtois, X.; Can, F.; Duprez, D.; Royer, S. Infrared investigation on surface properties of alumina obtained using recent templating routes. Microporous Mesoporous Mater. 2012, 158, 88–98. [CrossRef] 27. Phung, T.K.; Proietti Hernández, L.; Lagazzo, A.; Busca, G. Dehydration of ethanol over zeolites, silica alumina and alumina: Lewis acidity, Brønsted acidity and conﬁnement effects. Appl. Catal. A 2015, 493, 77–89. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).