Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides Part II. Al(ClO4)3·9H2O

Aluminium hydroxide was precipitated during a hydrolysis of aluminium perchlorate in ammonia medium. The materials were studied with the following methods: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Freshly precipitated boehmite had a high value of S_BET=211 m² g^–1 determined from nitrogen adsorption, good sorption capacity for benzene vapours, developed mesoporous structure and hydrophobic character. After prolonged refluxing at elevated temperature its crystallinity increased which was accompanied by an increase of specific surface determined from nitrogen adsorption up to 262m²g^–1 , decrease of sorption capacity for benzene vapours and stronger hydrophobic character. The calcinations of all boehmites at temperature up to 1200°C resulted in formation of à-Al₂O₃ via transition form of γ-, δ- and θ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C were characterised with high values of specific surface area of 205–220 and 138–153 m² g^–1 , respectively. The SBET values calculated for the oxide samples derived from aged hydroxides and calcined at 1200°C are higher than for the analogous sample prepared without the ageing step. It was concluded that the process of ageing at elevated temperature developed thermal stability of aluminium oxides.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

The paper concerns aluminium hydroxides precipitated during hydrolysis of aluminium acetate in ammonia medium, as well as aluminium oxides obtained through their calcination at 550, 900 or 1200°C for 2 h. The following techniques were used for analysing of obtained materials: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption-desorption of benzene vapours and scanning electron microscopy. Freshly precipitated boehmite/pseudoboehmite had high value of S _BET, very good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. After prolonged refluxing at elevated temperature its crystallinity increased which was accompanied by a decrease of specific surface determined from nitrogen adsorption, decrease of sorption capacity for benzene vapours and weakening of the hydrophilic character. Calcination of all hydroxides at the temperature up to 1200°C resulted in the formation of α-Al₂O₃ via transition forms of γ-, δ-and θ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C were characterised with high values of specific surface area and displayed quite high heat resistance, probably due to a specific morphology of starting hydroxides. The process of ageing at elevated temperature developed thermal stability of aluminium oxides.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

The process of hydrolysis of aqueous aluminium sulfate was carried on in ammonia medium at 100°C and for different time intervals (0, 20, 39 or 59 h). The products thus obtained were calcined at 550, 900 or 1200°C for 2 h with the aim to obtain aluminium oxides. The materials were studied with the following methods: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption–desorption of benzene vapours and scanning electron microscopy. Freshly precipitated material was an amorphous basic aluminium sulfate which after prolonged refluxing at elevated temperature in a mother liquor underwent a phase transformation into highly crystalline NH₄Al₁₃(SO₄)₂(OH)₆ containing tridecameric unit Al₁₃. It was accompanied by a decrease of specific surface area and the formation of a porous structure less accessible for benzene molecules. Regardless of the duration of the hydrolysis process, all products were characterised with poorly developed porous structure and hydrophilic character. Their calcination at the temperature up to 1200°C resulted in the formation of α-Al₂O₃ via transition forms of γ/η- and δ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C had higher values of specific surface area than starting materials due to processes of dehydroxylation and desulfurization. The process of calcination up to 900°C was reflected in developing of not only porous structure but also hydrophobic character of prepared materials. The S _BET values calculated for the oxide samples obtained from aged products of hydrolysis at 1200°C were lower than for the analogous sample prepared without the ageing step. It was concluded that prolonged refluxing at elevated temperature of the products of hydrolysis of aluminium sulfate decreased thermal stability of final aluminium oxides.     

Thermal transformations of the products of partial hydrolysis of hydrous aluminium nitrate

Thermal analysis, mass spectrometry, infrared spectrophotometry, X-ray phase analysis, scanning electron microscopy, and sorption methods were used in the study of thermal transformations of the products of partial hydrolysis of hydrous aluminium nitrate in ammonia medium. It has been found that the process of aluminium nitrate hydrolysis under the conditions applied and in the presence of ammonia gives boehmite as the main product, with some admixture of a basic salt.Aluminium oxide obtained from the products of partial hydrolysis of hydrous aluminium nitrate in ammonia medium at 550°C has a crystalline -Al₂O₃ structure. Its specific surface, as determined by low-temperature adsorption of nitrogen, exceeds 200 m²g^–1. Features of the products are well developed mesopore structure and considerable ability of benzene adsorption. Calcination of the obtained aluminium oxide for 2 h at 900°C reduces its specific surface to about 110 m² g^–1.This revised version was published online in November 2005 with corrections to the Cover Date.     

A study of the influence of the conditions of preparation of γ-aluminum oxide as a carrier for reforming catalysts on its physicochemical properties

The formation of highly imperfect γ-Al₂O₃ oxide prepared by calcining pseuodoboehmite and plasticized by organic acids was studied. The nature of the organic acid-aluminum hydroxide plasticizer was found to substantially influence the degree of γ-Al₂O₃ structure imperfection estimated qualitatively as the difference between the X-ray structural density and effective density with respect to helium and aluminum oxide. A high degree of imperfection caused an increase in the intensity of the absorption band at 3775 cm⁻¹ corresponding to OH groups localized on five-coordinate Al³⁺ and the concentration of Lewis acid centers. The adsorption and catalytic properties of systems based on these carriers were studied.     

Effect of pH, ionic strength, foreign ions, fulvic acid and temperature on 109Cd(II) sorption to γ-Al2O3

The sorption of Cd(II) from aqueous solution on γ-Al₂O₃ was investigated under ambient conditions. Experiments were carried out as a function of contact time, solid content, pH, ionic strength, foreign ions, fulvic acid and temperature. The results indicated that the sorption of Cd(II) was strongly dependent on pH and ionic strength. At low pH, the sorption of Cd(II) was dominated by outer-sphere surface complexation and ion exchange with Na⁺/H⁺ on γ-Al₂O₃ surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) models were used to simulate the sorption isotherms at three different temperatures. The thermodynamic data (ΔG ⁰, ΔS ⁰, ΔH ⁰) calculated from the temperature dependent sorption isotherms suggested that the sorption of Cd(II) on γ-Al₂O₃ was an spontaneous and endothermic process.     

Studies of the roles of Sn or Fe on γ-Al2O3-supported Pt catalysts by CO adsorption microcalorimetry and dehydrogenation reaction of C4 alkanes

CO adsorption microcalorimetry was employed in the study of γ-Al₂O₃-supported Pt, Pt-Sn and Pt-Fe catalysts. The results indicated that the initial differential heat of CO adsorption of the Pt/γ-Al₂O₃ catalyst was 125 kJ/mol. As CO coverage increased, the differential heat of adsorption decreased. At higher coverages, the differential heat of adsorption decreased significantly. 60% of the differential heat of CO adsorption on the Pt/γ-N₂O₃ catalyst was higher than 100 kJ/mol. No significant effect on the initial differential heat was found after adding Sn and Fe to the Pt/γ-Al₂O₃ catalyst. The amount of strong CO adsorption sites decreased, while the portion of CO adsorption sites with differential heat of 60–110 kJ/mol increased after increasing the Sn or Fe content. This indicates that the surface adsorption energy was changed by adding Sn or Fe to Pt/γ-N₂O₃. The distribution of differential heat of CO adsorption on the Pt-Sn(C)/γ-Al₂O₃ catalyst was broad and homogeneous. Comparison of the dehydrogenation performance of C₄ alkanes with the number of CO adsorption sites with differential heat of 60–110 kJ/mol showed a good correlation. These results indicate that the surface Pt centers with differential heats of 60–110 kJ/mol for CO adsorption possess superior activity for the dehydrogenation of alkanes. Project supported by FORD and the National Natural Science Foundation of China (Grant No. 09412302) and the Transcentury Training Program Foundation for the Talents by The State Education Commission of China.     

Infrared absorption spectra of Er3+-doped Al2O3 nanopowders by the sol-gel method

The Er³⁺-doped Al₂O₃ nanopowders have been prepared by the sol-gel method, using the aluminium isopropoxide [Al(OC₃H₇)₃]-derived γ-AlOOH sols with addition of the erbium nitrate [Er(NO₃)₃·5H₂O]. The five phases of γ-(Al,Er)₂O₃, θ-(Al,Er)₂O₃, α-(Al,Er)₂O₃, ErAlO₃, and Al₁₀Er₆O₂₄ were detected with the 0–20 mol% Er³⁺-doped Al₂O₃ nanopowders at the different sintering temperature of 600–1200°C. The average grain size was increased from about 5 to 62 nm for phase transformation of undoped γ-Al₂O₃→α-Al₂O₃ at the sintering temperature from 600 to 1200°C. At the same sintering temperature, average grain size was decreased with increase of the Er³⁺ doping concentration. Infrared absorption spectra of γ-Al₂O₃ and θ-Al₂O₃ nanopowders showed the two broad bands of 830–870 and 550–600 cm⁻¹, the three broad bands of 830–870, 750–760, and 550–600 cm⁻¹, respectively. The infrared absorption spectra for the α-Al₂O₃ nanopowder showed three characteristic bands, 640, 602, and 453 cm⁻¹. The two characteristic bands of 669 and 418 cm⁻¹ for Er₂O₃ clusters were observed for the Er³⁺-doped Al₂O₃ nanopowders when Er³⁺ doping concentration was increased up to 2 mol%. The 796, 788, 725, 692, 688, 669, 586, 509, 459, and 418 cm⁻¹ are the characteristic bands of Al₁₀Er₆O₂₄ phase.     

Sorption behavior of Co(II) on γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> in the presence of humic acid

The fate and transport of toxic metal ions and radionuclides in the environment is generally controlled by sorption reactions. The extent of sorption of divalent metal cations is controlled by a number of factors including cosorbing or complexing. In this work, the effects of pH, humic acid HA/Co(II) addition orders, ionic strength, concentration of HA, and foreign cations on the Co(II) sorption on γ-Al₂O₃ in the presence of HA were investigated. The sorption isotherms of Co(II) on γ-Al₂O₃ in the absence and presence HA were also studied and described by using S-type sorption model. The experimental results showed that the Co(II) sorption is strongly dependent on the pH values, concentration of HA, but independent of HA/Co(II) addition orders, ionic strength, and foreign cations in the presence of HA under our experimental conditions. The results also indicated that HA enhanced the Co(II) sorption at low pH, but reduced the Co(II) sorption at high pH. It was hypothesized that the significantly positive influence of HA at low pH on the Co(II) sorption on γ-Al₂O₃ was attributed to strong surface binding of HA on γ-Al₂O₃ and subsequently the formation of ternary surface complexes such as ≡S-OOC-R-(COO⁻) _x Co^2−x . Chemi-complexation may be the main mechanism of the Co(II) sorption on γ-Al₂O₃ in the presence of HA.     

ETAAS determination of aluminium and copper in dialysis concentrates after microcolumn chelating ion-exchange preconcentration

A procedure was developed for the preconcentration and determination of aluminium and copper in dialysis concentrates at the ng cm^–3 level. The preconcentration was achieved on microcolumns filled with Chelex-100 resin adjusted to a pH of 4.0. Five repetitive cycles of the sample through the column ensured a sufficient contact time for quantitative retention of aluminium and copper ions. The retained ions were eluted with HNO₃ (0.5 mol dm^–3). Aluminium and copper were determined in the eluate by Zeeman ETAAS using the standard addition technique. The procedure was performed under clean room conditions (class 10,000), The reliability of the results was evaluated by recovery tests, using dialysis concentrates spiked with aluminium and copper. The recoveries obtained ranged from 86 to 106% for aluminium and from 92 to 97% for copper. Using the recommended procedure, the LOD of aluminium and copper in dialysis concentrates (preconcentration factor 2) was found to be 0.5 ng cm^–3 and 0.2 ng cm^–3, respectively. Received: 19 December 1997 / Revised: 10 March 1998 / Accepted: 28 March 1998     

Catalytic synthesis of indole from tetrahydroindole on Pd-and Cr-containing catalysts

In the presence of Pd-and Cr-containing catalysts applied to γ-Al₂O₃ or sibunite 4,5,6,7-tetrahydroindole is converted into indole. Indole was obtained in quantitative yield on sulfided 0.15–0.5% Pd/γ-Al₂O₃ catalyst at 360°C and on catalysts containing 5% Cr₂O₃, 5% La₂O₃ (or 5% polirit), 1% K₂O/89% γ-Al₂O₃ at 475–480°C. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1176–1178, August, 2006.     

On the molecular models of lewis acid sites on the surface of γ-Al2O3 and in zeolites: a density functional study of CO adsorption

Adsorption complexes of CO-Lewis acid sites with 3- and 5-coordinated Al³⁺ are modeled by the density functional method using the cluster approach. Cluster models of the site with 4-coordinated Al³⁺ on the surface of γ-Al₂O₃ and in zeolites are suggested. For these models of adsorption complexes, C-O vibration frequencies are calculated and the energetics of CO adsorption is evaluated. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 5, pp. 834–839, September–October, 1997.     

ETAAS determination of aluminium and copper in dialysis concentrates after microcolumn chelating ion-exchange preconcentration

A procedure was developed for the preconcentration and determination of aluminium and copper in dialysis concentrates at the ng cm^–3 level. The preconcentration was achieved on microcolumns filled with Chelex-100 resin adjusted to a pH of 4.0. Five repetitive cycles of the sample through the column ensured a sufficient contact time for quantitative retention of aluminium and copper ions. The retained ions were eluted with HNO₃ (0.5 mol dm^–3). Aluminium and copper were determined in the eluate by Zeeman ETAAS using the standard addition technique. The procedure was performed under clean room conditions (class 10,000), The reliability of the results was evaluated by recovery tests, using dialysis concentrates spiked with aluminium and copper. The recoveries obtained ranged from 86 to 106% for aluminium and from 92 to 97% for copper. Using the recommended procedure, the LOD of aluminium and copper in dialysis concentrates (preconcentration factor 2) was found to be 0.5 ng cm^–3 and 0.2 ng cm^–3, respectively. Received: 19 December 1997 / Revised: 10 March 1998 / Accepted: 28 March 1998     

A catalytic process for preparation of thiophene from furan and hydrogen sulfide

Thiophene formation under various conditions from furan and H₂S in the presence of γ-Al₂O₃, both unpromoted and promoted with transition metal oxides, was examined. The conditions enabling preparation of thiophene in a 95–98 mol% yield were determined.     

The dehydroxylation of aluminium hydroxides and the kinetics ofα-Al2O3 formation

Studies were made of the dehydroxylation of several aluminium hydroxide modifications and the kinetics of-Al₂O₃ formation. The investigated samples differed in both mineral composition and the level of alkali admixtures. It was found that the rate of formation and the quantity of-Al₂O₃ depend mainly on the purity of the aluminium hydroxides, while the transition forms of alumina depend on the initial type of the aluminium hydroxide.

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Zusammenfassung Es wurde die Dehydroxylierung verschiedener Aluminiumhydroxidmodifizierungen sowie die kinetische Untersuchung der Bildung von-Al₂O₃ durchgeführt. Die untersuchten Proben unterschieden sich sowohl in ihrer Mineralienzusammensetzung als auch im Grad der Verschmutzung mit Alkalien. Man stellte fest, daß sowohl Geschwindigkeit als auch Menge von-Al₂O₃ hauptsächlich von der Reinheit des Aluminiumhydroxides abhängen und daß die Übergangsformen von Aluminiumoxid von ihrem ursprünglichen Typ von Aluminiumhydroxid abhängen.

     

Application of FTIR spectra for evaluating interfacial reactions in metal matrix composites

Manufacturing of Saffil/MgLi metal matrix composites by the melt infiltration process is accompanied by extensive interfacial redox reaction between δ-Al₂O₃ fibers (Saffil) and lithium. The present paper deals with the Fourier transform infrared spectroscopy examination of Saffil fibers isolated from Mg–8 wt% Li alloy by the bromine/methylacetate agent focusing on the insertion of Li⁺ ions into δ-Al₂O₃ and their influence on water adsorption. Insertion of Li⁺ into δ-Al₂O₃ is monitored by gradual change of Al–O stretching bands (400–900 cm⁻¹) towards more simple patterns of a spinel-like product assigned as δ(Li) which transforms to LiAl₅O₈ during subsequent annealing. Rapid increase in the water adsorption with increase in Li content, indicated by the changes in H–O–H bending (about 1,650 cm⁻¹) and O–H stretching (about 3,500 cm⁻¹), is connected with the ionicity of the δ(Li) phase, which attracts polar water molecules.     

Influence of pH,humic acid,ionic strength,foreign ions,and temperature on <Superscript>60</Superscript>Co(II) sorption onto γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The sorption of ⁶⁰Co(II) on γ-Al₂O₃ was conducted under various conditions, i.e., contact time, adsorbent content, pH, ionic strength, foreign ions, humic acid (HA), and temperature. Results of sorption data analysis indicated that the sorption of ⁶⁰Co(II) on γ-Al₂O₃ was strongly dependent on pH and ionic strength. At low pH the sorption was dominated by outer-sphere surface complexation or ion exchange, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The presence of different cation ions influenced ⁶⁰Co(II) sorption, while the presence of different anion ions had no obvious influences on ⁶⁰Co(II) sorption. The presence of HA decreased the sorption of ⁶⁰Co(II) on γ-Al₂O₃. The sorption isotherms were simulated well with the Langmuir model. The thermodynamic parameters (ΔH ⁰ , ΔS ⁰ and ΔG ⁰ ) calculated from the temperature-dependent sorption isotherms indicated that the sorption of ⁶⁰Co(II) on γ-Al₂O₃ was an endothermic and spontaneous process. Experimental results indicated that the low cost material was a suitable material in the preconcentration of ⁶⁰Co(II) from large volumes of aqueous solutions.     

FTIR study of CO adsorption on molybdena-alumina catalysts for surface characterization

CO adsorption at low temperature has been used to probe Lewis acid sites created upon dehydroxylation of γ-Al₂O₃ and reduction of Mo/Al₂O₃ catalysts, using Fourier Transform Infrared spectroscopy (FTIR). Carbon-monoxide adsorption on γ-Al₂O₃ and Mo/Al₂O₃ catalysts dehydroxylated and reduced at different temperatures was studied at 78 K by IR spectroscopy. However, our results indicate that there is an approximately linear correlation between the increase either of dehydroxylation or the extent of reduction of the catalysts and the increasing absorbance of CO due to CO adsorption on Lewis acid sites created upon dehydroxylation of γ-Al₂O₃ and reduction of Mo/Al₂O₃.     

Effect of phenyl group on the structure and formation of transitional alumina from Al (OPh)<Subscript>3</Subscript>

The structure of freshly prepared Al(OPh)₃, its decomposition product, the hydrolyzed products and their structural evolution were investigated employing ²⁷Al MAS NMR spectroscopy, PXRD, TGA/DTA/DSC/FTIR techniques. In the ²⁷Al MAS NMR spectrum of the aluminium phenoxide, three signals with the chemical shift at 3.78, 21 and 45 ppm were observed. The chemical shift at 3.78 and 45 ppm revealed the presence of four and sixfold coordinated aluminum. The signal at 21 ppm corresponded to fivefold coordinated aluminium. When the aluminium phenoxide was directly decomposed in air at 600 °C, it resulted in amorphous product as evidenced from the PXRD pattern. The observed signals with chemical shifts at 10.1, 42, 73.6 ppm in the ²⁷Al MAS NMR spectrum indicated the presence of 6, 5 and 4 coordination for the aluminium atoms suggesting disordered transitional γ-alumina to be the product. The hydrolysis studies of Al(OPh)₃ with excess of water at 70 °C yielded bohemite (γ-AlOOH). The alumina obtained after dehydration at 600 °C was X-ray amorphous. The dehydrated product at 600 °C showed the presence of four and six coordinated aluminium atoms in the ²⁷Al MAS NMR spectrum confirming it to be ordered γ-Al₂O₃. Crystalline γ-Al₂O₃ was obtained on further heating at 800 °C.     

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of ¹³C¹⁸O+¹²C¹⁶O mixtures on the Pt(2.9%)/γ-Al₂O₃, (Pt(2.6%)+Cu(2.7%))/γ-Al₂O₃, and (Pt(2.6%)+Cu(5.1%))/γ-Al₂O₃ catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated ¹³C¹⁸O molecules adsorbed on Pt are ∼1940 and ∼1970 cm⁻¹, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed ¹³C¹⁸O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 831–836, May, 2007.