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

Abstract

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) similar to those of dehydroxylated silica, alumina, and magnesium hydroxide. Thermal treatment of [MgAl(OH)](SO)(CO)·0.62(HO)·0.04(acetone) prepared by the aqueous miscible organic solvent treatment method (MgAl-SO-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 m g from treatment at 275 and 300 °C, respectively). Titration of the surface hydroxyls with Mg(CHPh)(THF) 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) of 2.14 OH nm similar to the hydroxyl number (α) of 1.80 OH nm for a sample of SiO dehydroxylated at 500 °C. These materials appear to be suitable candidates for use as single-site organometallic catalyst supports.

EPR Spectroscopy of Iron- and Nickel-Doped [ZnAl]-Layered Double Hydroxides: Modeling Active Sites in Heterogeneous Water Oxidation Catalysts

“…These edge, corner, and defect sites must be coordinated by labile waters or hydroxides, and ligand losses likely will occur upon 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 20 heating. 40,41,39 Five-coordinate (square pyramidal and trigonal bipyramidal) high-spin…”

Synthesis, characterisation and slurry phase ethylene polymerisation of rac-(PhBBI*)ZrCl2 immobilised on modified layered double hydroxides

“…150 °C (Mg3Al-CO3-A) or 200 °C as these temperatures have been shown to maximise the polymerisation activity of catalytic systems. 28,46 The PXRD patterns of all AMO-and AIM-Mg3Al-CO3-y LDHs (before thermal treatment; Figure 2) are similar to that of Mg3Al-CO3-H2O (d003=7.63 Å). 47 However, the interlayer separations (given by d003) slightly increased in both the AMO-and AIM-Mg3Al-CO3-y LDHs, this slight increase in the interlayer separation appears to be independent of the dispersing solvent and may arise from less intra-layer hydrogen bonding.…”

“…The lattice constants indicate that the brucite-type layer structure is retained up to 300 °C, a small decrease in the interlayer separation may indicate loss of some intercalated water or AMO/AIM solvent. 46 The peak widths at half maximum of the 003 Bragg reflections can be used to study the variation in crystallinity of the LDHs. Table S4 shows the crystalline domain lengths (CDLs) for a variety of AMO-and AIM-LDHs.…”

“…observed for AMO-LDHs. 46 Below the 1 st minimum in the derivative (T1; 210 and 146 °C respectively) there is a loss of coordinated surface water and AMO/AIM solvent molecules, which is followed by the formation of layered double oxides (LDOs) below the 2 nd minimum in the derivative (T2; 400 and 335 °C respectively). It can be seen that in Mg2.73Al-CO3-1H, these mass loss events occur, with T1 at 150 °C and T2 at 350 °C, however there is an additional low temperature minimum in the first derivative at 85 °C.…”

Structural Analysis of Magnesium-Aluminium Hydrotalcites Modified with Iron III Obtained by Hydroxide Precipitation Method

“…The third thermal event is the layer dehydroxylation process through the water loss originated from the condensation of hydroxyl groups, which originate metallic oxide mixture in material thermally treated above that temperature range [36] [37]. In spite of the weight loss not to be noticed dependence with iron (III) content among the samples, the final temperature and the DTG peak for that process are displaced to lower temperatures, as shown in the third column of the Table 1.…”