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Synthesis of Mg–Al Mixed Oxides with Markedly High Surface Areas from Layered Double Hydroxides with Organic Sulfonates
journal contribution
posted on 2018-12-07, 19:03 authored by Rei Tanaka, Isao Ogino, Shin R. MukaiMg–Al
mixed oxides with record-high surface areas and basic
site concentrations were synthesized from Mg–Al layered double
hydroxides with interlayer isethionate (Ise) or 3-hydroxy-1-propanesulfonate
(HPS). Anion exchange of interlayer CO32– in synthetic hydrotalcites with the organic sulfonates induces disorders
in layer stacking as characterized by powder X-ray diffraction and
enables facile delamination in water. Thermal treatment of materials
anion-exchanged by Ise (MgAl–Ise) and HPS (MgAl–HPS)
in N2 and H2 resulted in the formation of Mg–Al
mixed oxides with marked enhancement in Brunauer–Emmett–Teller
(BET) surface area relative to those treated in air. Treatment in
a flow of H2 is particularly effective, doubling the surface
area of mixed oxides derived from MgAl–Ise relative to those
obtained in a flow of N2. A higher degree of disorder in
layer stacking in MgAl–HPS than MgAl–Ise resulted in
the formation of Mg–Al mixed oxides with higher surface areas
than those from MgAl–Ise. As a result, thermal activation of
MgAl–HPS in a flow of H2 yielded Mg–Al mixed
oxides with the highest BET surface area (410 m2 g–1) and CO2 uptake (1.6 mmol g–1 at 25 °C and 100 kPa) in all samples. These values are significantly
higher than those obtained from the initial hydrotalcites as well
as those reported in the literature with similar Mg–Al ratios.
Investigation of the thermal activation steps by thermogravimetric
analysis and mass spectrometry indicates that the key factors to achieve
high surface area and CO2 uptake are to weaken interactions
between layers by inducing stacking disorders and to facilitate the
removal of interlayer sulfonates by preventing the formation of sulfates
from them via thermal activation under a reducing environment.