Synthesis and Surface Acid/Base Properties of Magnesium-Aluminum Mixed Oxides Obtained from Hydrotalcites

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

“…Further spectroscopic characterization was carried out in order to understand the local structure of aluminum and magnesium present on the hydrotalcite samples pretreated at various temperatures. As seen in Figure 3a, one-dimensional (1D) 27 Al MAS NMR of as-synthesized hydrotalcite (Mg/Al = 3) shows the presence of a single, sharp 27 Al signal at 9 ppm characteristic of octahedral (O h ) AlO 6 coordination. 30 This result is consistent with previous reports that as-synthesized hydrotalcite consists of positively charged brucite-like layers due to the presence of trivalent cation such as Al 3+ in octahedral units between the lamellae.…”

“…0.05 g of catalyst was pressed into a 20 mm-diameter pellet (<1 mm thick) and placed into a custom-built transmission cell equipped with CaF 2 windows, a K-type thermocouple for temperature control, and resistive cartridge heaters. 27 Al signals further, two-dimensional multiple quantum MAS (MQMAS) experiments were performed using the soft pulse added mixing (SPAM) technique for sensitivity enhancement and with rotor synchronization or with half-rotor synchronization depending on the required spectral window in the F1 dimension. The pulses used for excitation and conversion were optimized individually for each sample.…”

“…31 To resolve the Al signal further and to clearly understand the local structure of the Al 3+ cations, the samples were analyzed by two-dimensional 27 Al multiple quantum-magic angle spinning (MQ-MAS) NMR spectroscopy ( Figure 4). The MQMAS experiment for half integer quadrupolar nuclei is a powerful technique to separate anisotropic interactions from isotropic interactions.…”

“…16,17,20−24 The catalytic competence of hydrotalcites after calcination is a result of the high surface area and moderate acidity/basicity of the resultant Mg− Al mixed oxides. 27 However, calcined hydrotalcites are known to be quite susceptible to deactivation by water. 17 Although acid−base character is essential for the catalytic activity of hydrotalcite-type catalysts, the relationship between catalytic activity, chemical composition, and the structural stability issues of calcined hydrotalcites in the presence of water remain elusive in the catalytic upgrading reaction of bioderived methyl ketones.…”

“…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.…”