Synthesis of a Double Alkoxide Precursor to Spinel (MgAl₂O₄) Directly from Al(OH)₃, MgO, and Triethanolamine and Its Pyrolytic Transformation to Spinel

Abstract

Reaction of Al(OH) 3 and MgO or Mg(OH) 2 with triethanolamine [TEA, N(CH 2 CH 2 OH) 3 ] in ethylene glycol (EG) provides, in one step, access to a polymer-like precursor to spinel. On the basis of high-resolution mass spectroscopy, chemical analysis and 27 Al solution NMR, the precursor appears to be a trimetallic double alkoxide consisting of two four-coordinate TEAA1 (alumatrane) moieties linked via a bridging TEA group that enfolds the Mg cation. The 27 Al NMR shows only tetracoordinate Al centers (64.8 ppm). The same compound can be prepared stepwise by reaction of tetrameric alumatrane, (TEAAl) 4 , with Mg and TEA. Product evolution upon pyrolysis was followed as a function of temperature using TGA, DTA, XRD, and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrolysis at 700 °C for 2 h in air appears to produce a γ-Al 2 O 3 -MgAl 2 O 4 solid solution and a small amount (<5 wt %, by TGA) of X-ray amorphous MgCO 3 . Transformation of the solid solution to pure spinel is a function of pyrolysis temperature and time, with only spinel evident in the XRD data, after pyrolysis at 1200 °C for 2 h. BET analysis of the pyrolysis products gave surface areas as high as 160 m 2 /g (500 °C/2 h/air). Porosimetry reveals a bimodal distribution of micropores centered around 10 and 60 Å, accounting for the majority of the surface area. The powder morphology was briefly examined using SEM.

Influence of calcination temperature on textural and structural properties, reducibility, and catalytic behavior of mesoporous γ-alumina-supported Ni–Mg oxides by one-pot template-free route

“…As the XRD patterns of NiAl 2 O 4 and MgAl 2 O 4 spinels are similar to that of c-Al 2 O 3 , their diffraction peaks are indistinguishable from each other due to the broadening and overlapping [44]. However, the formation of NiAl 2 O 4 and MgAl 2 O 4 phases can be identified with the variation in the relative intensities and the slight shifts of the diffraction peaks for spinel around 2h = 37.0°, 45.5°, and 67.0° [44,45].…”

“…However, the formation of NiAl 2 O 4 and MgAl 2 O 4 phases can be identified with the variation in the relative intensities and the slight shifts of the diffraction peaks for spinel around 2h = 37.0°, 45.5°, and 67.0° [44,45]. For example, for NiAl 2 O 4 (PDF no.…”

“…It could be seen that for all the reduced catalysts, there were two new diffraction peaks at 51.9°and 76.4°corresponding to (2 0 0) and (2 2 0) reflections, respectively, and a diffraction peak at ca. 44 the Ni (1 1 1) plane, which was overlapped with the diffraction peak at ca. 45°corresponding to the (4 0 0) plane for spinel.…”

“…On the other hand, since the ionic radii of Ni (0.069 nm) and Mg (0.072 nm) are larger than that (0.057 nm) of aluminum, the incorporation of Ni and/or Mg ions expands the lattice; in other words, the NiAl 2 O 4 and MgAl 2 O 4 spinels have larger lattice parameters than crystalline c-Al 2 O 3 , resulting in the shifts of the diffraction peaks to smaller angles[29,44]. As illustrated inFig.…”

Surfactant-Assisted Synthesis of Mesoporous Alumina Showing Continuously Adjustable Pore Sizes

“…In this way, elimination of water from the Al/TEA/water mixture by gentle heating allows the formation of a quite stable TEA-Al 3+ complex. [19] Then, when water is again added, hydrolysis reactions must proceed through a substitution mechanism and the hydrolysis rate is lowered enough to allow the partially hydrolyzed inorganic moieties to be organized together with the organic surfactant molecules. When the amount of water is increased, the balance between the different processes shifts towards the formation of lamellar moieties, which gives rise to the increase in the pore size; the theoretical extrapolation of this tendency would be the formation of a fully lamellar compound, which would correspond to an infinite pore'.…”