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

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.