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(CH2CH2OH)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 27Al 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 27Al 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-Al2O3-MgAl2O4 solid solution and a small amount (<5 wt %, by TGA) of X-ray amorphous MgCO3. 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 m2/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.