Structural and Surface Acid/Base Properties of Hydrotalcite-Derived MgAlO Oxides Calcined at Varying Temperatures

Shen, Jianyi; Tu, Mai; Hu, Chen

doi:10.1006/jssc.1997.7739

Cited by 105 publications

(56 citation statements)

References 29 publications

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“…Acid-base sites can be active sites for many reactions including Meerwein-Ponndorf-Verley reactions, [160,161] cyclo additions of carbon dioxide to epoxides, [162] and aldol condensations to form 2-nonenal. [163] The acid-base properties of Mg-Al mixed oxides are governed by the Mg : Al molar ratio, [29,146,147] calcination temperature [164] and preparation conditions. [29,38,105,145] The mixed oxides derived from thermal treatment of Mg-Al HT are among the solids that have the potential to complement homogeneous catalysts with considerable success since they are inexpensive and non-toxic and their basic properties can be tailored to the process of interest.…”

Section: Hts As Base Catalystsmentioning

confidence: 99%

Synthetic hydrotalcites from different routes and their application as catalysts and gas adsorbents: a review

Othman

Helwani

Martunus

et al. 2009

Applied Organom Chemis

195

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In this paper, widely accepted methods of hydrotalcite preparation such as co-precipitation, urea hydrolysis, hydrothermal, sol-gel, microwave irradiation, steam activation and solvothermal have been selected and reviewed. Our review indicates that the nature of the divalent cations, the synthesis method, the calcination temperature and the nature of the interlayer species are determinant factors in shaping the surface properties of the layered double hydoxides. The basic strength of the surface base site and structural changes produced in the mixed oxides can be adjusted conveniently by varying the Al content during the synthesis. The combination of sol-gel with microwave irradiation during the gelling and crystallization steps has also been found to increase the surface area of the hydrotalcite-like compound.

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Section: Hts As Base Catalystsmentioning

confidence: 99%

Synthetic hydrotalcites from different routes and their application as catalysts and gas adsorbents: a review

Othman

Helwani

Martunus

et al. 2009

Applied Organom Chemis

195

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“…The tetrahedral aluminum peak at 80 ppm evidently has a higher electron density and thereby can be assigned to Al T −O bonded to Mg 2+ (Al T −O−Mg), while the 60 ppm peak can be assigned to Al T −O bonded to Al (Al T −O−Al). 34 and O h sites produces the defect sites as cationic vacancies in the MgO framework are needed to compensate the positive change generated. As a consequence, the O 2− ions adjacent to the Mg 2+ or Al 3+ become coordinatively unsaturated, leading to the formation of new basic sites.…”

Section: Industrial and Engineering Chemistry Researchmentioning

confidence: 99%

Effects of Composition and Structure of Mg/Al Oxides on Their Activity and Selectivity for the Condensation of Methyl Ketones

Shylesh¹,

Kim²,

Gokhale

et al. 2016

Ind. Eng. Chem. Res.

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The effects of chemical composition and pretreatment on Mg−Al hydrotalcites and alumina-supported MgO were evaluated for the gas-phase, self-condensation reaction of C 3 −C 5 biomass-derived methyl ketones. We show that the selectivity toward the acyclic dimer enone and the cyclic enone trimer can be tuned by controlling the temperature of hydrotalcite calcination. Methyl ketone cyclization is promoted by Lewis acidic sites present on the hydrotalcite catalysts. XRD and thermal decomposition analysis reveal that the formation of periclase MgO starts above 623 K accompanied by complete disappearance of the hydrotalcite structure and is accompanied by an increase in hydroxyl condensation as the formation of well-crystallized periclase. 27 Al MQMAS and 25 Mg MAS NMR show that at progressively higher temperatures, Al 3+ cations diffuses out of the octahedral brucite layers and incorporate into the tetrahedral and octahedral sites of the MgO matrix thereby creating defects to compensate the excess positive charge generated. The oxygen anions adjacent to the Mg 2+ /Al 3+ defects become coordinatively unsaturated, leading to the formation of new basic sites. A kinetic isotope effect, k H /k D = 0.96, is observed at 473 K for the reaction of (CH 3 ) 2 CO versus (CD 3 ) 2 CO, which suggests that carbon−carbon bond formation leading to the dimer aldol product is the ratedetermining step in the condensation reaction of methyl ketones. We also show that acid−base catalysts having similar reactivity and higher hydrothermal stability to that of calcined hydrotalcites can be achieved by creating defects in MgO crystallites supported alumina as a consequence of the diffusion of Al 3+ cations into MgO. The physical properties of these materials are shown to be very similar to those of hydrotalcite calcined at 823 K.

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“…[35][36][37] To establish a correlation between the catalytic activity and the textural and basic properties, experiments were carried out over both rehydrated samples. From the TPD of CO 2 in Figure 10, two peaks were identified indicating that two types of basic sites can be distinguished in the rehydrated Mg-Al mixed oxides.…”

Section: Nuclear Magnetic Resonance Of 27mentioning

confidence: 99%

Aldol Condensations Over Reconstructed Mg–Al Hydrotalcites: Structure–Activity Relationships Related to the Rehydration Method

Abelló

Medina

Tichit

et al. 2004

Chemistry A European J

229

188

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Two different rehydration procedures in the liquid or gas phase have been applied to reconstruct mixed oxides derived from calcined hydrotalcite-like materials to be used as catalysts for aldol condensation reactions. The as-synthesized hydrotalcite, its decomposition product, as well as the reconstructed solids upon rehydration were characterized by XRD, N(2) adsorption, He pycnometry, FTIR, SEM, TEM, (27)Al MAS-NMR and CO(2)-TPD (TPD=temperature-programmed desorption). Compared to the Mg-Al mixed oxide rehydrated in the gas phase (HT-rg), that rehydrated in the liquid phase (HT-rl) exhibits a superior catalytic performance with respect to the aldol condensation of citral with ketones to yield pseudoionones and in the self-aldolization of acetone. The textural properties of HT-rl and HT-rg differ strongly and determine the catalytic behavior. A memory effect led to a higher degree of reconstruction of the lamellar structure when the mixed oxide was rehydrated in the gas phase rather than in the liquid phase, although liquid-phase rehydration under fast stirring produced a surface area that was 26 times greater. This contrasts to typical statements in the literature claiming a higher degree of reconstruction in the presence of large amounts of water in the medium. CO(2)-TPD shows that the number of OH(-) groups and their nature are very similar in HT-rg and HT-rl, and cannot explain the markedly different catalytic behavior. Accordingly, only a small fraction of the available basic sites in the rehydrated samples is active in liquid-phase aldol condensations. Our results support the model in which only basic sites near the edges of the hydrotalcite platelets are partaking in aldol reactions. Based on this, reconstructed materials with small crystallites (produced by exfoliation during mechanical stirring), that is, possessing a high external surface area, are beneficial in the reactions compared to larger crystals with a high degree of intraplatelet porosity.

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Structural and Surface Acid/Base Properties of Hydrotalcite-Derived MgAlO Oxides Calcined at Varying Temperatures

Cited by 105 publications

References 29 publications

Synthetic hydrotalcites from different routes and their application as catalysts and gas adsorbents: a review

Synthetic hydrotalcites from different routes and their application as catalysts and gas adsorbents: a review

Effects of Composition and Structure of Mg/Al Oxides on Their Activity and Selectivity for the Condensation of Methyl Ketones

Aldol Condensations Over Reconstructed Mg–Al Hydrotalcites: Structure–Activity Relationships Related to the Rehydration Method

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