Controlling the Surface Hydroxyl Concentration by Thermal Treatment of Layered Double Hydroxides

Layered double hydroxides (LDHs) are important materials in the field of catalyst supports, and their surface hydroxyl functionality makes them interesting candidates for supporting well-defined single-site catalysts. Here, we report that the surface hydroxyl concentration can be controlled by thermal treatment of these materials under vacuum, leading to hydroxyl numbers (α_OH) similar to those of dehydroxylated silica, alumina, and magnesium hydroxide. Thermal treatment of [Mg_0.74Al_0.26(OH)₂](SO₄)_0.1(CO₃)_0.03·0.62(H₂O)·0.04(acetone) prepared by the aqueous miscible organic solvent treatment method (Mg_2.84Al-SO₄-A AMO-LDH) is shown to yield a mixed metal oxide above 300 °C by a combination of thermogravimetric analysis, powder X-ray diffraction (PXRD), BET surface area analysis, and FTIR spectroscopy. PXRD shows the disappearance of the characteristic LDH 00l peaks at 300 °C indicative of decomposition to the layered structure, coupled with a large increase in the BET surface area (95 vs 158 m² g^-1 from treatment at 275 and 300 °C, respectively). Titration of the surface hydroxyls with Mg(CH₂Ph)₂(THF)₂ indicates that the hydroxyl number is independent of surface area for a given treatment temperature. Treatment at 450 °C under vacuum produces a mixed metal oxide material with a surface hydroxyl concentration (α_OH) of 2.14 OH nm^-2 similar to the hydroxyl number (α_OH) of 1.80 OH nm^-2 for a sample of SiO₂ dehydroxylated at 500 °C. These materials appear to be suitable candidates for use as single-site organometallic catalyst supports.