Abstract
Layered double hydroxides (LDHs) have been widely used as an important subset of solid base catalysts. However, developing low-cost, small-sized LDH nanoparticles with enhanced surface catalytic sites remains a challenge. In this work, silica aerogel (SA)-supported, small-sized Mg–Al LDH nanosheets were successfully prepared by one-pot coprecipitation of Mg and Al ions in an alkaline suspension of crushed silica aerogel. The supported LDH nanosheets were uniformly dispersed in the SA substrate with the smallest average radial diameter of 19.2 nm and the thinnest average thickness of 3.2 nm, both dimensions being significantly less than those of the vast majority of LDH nanoparticles reported. The SA/LDH composites also showed large pore volume (up to 1.3 cm3·g) and pore diameter (>9 nm), and therefore allow efficient access of reactants to the edge catalytic sites of LDH nanosheets. In a base-catalyzed Henry reaction of benzaldehyde with nitromethane, the SA/LDH catalysts showed high reactant conversions and favorable stability in 6 successive cycles of reactions. The low cost of the SA carrier and LDH precursors, easy preparation method, and excellent catalytic properties make these SA/LDH composites a competitive example of solid-base catalysts.
Highlights
Layered double hydroxides (LDHs) are a class of two-dimensional anionic clays composed of positively-charged layers of mixed bivalent and trivalent metal hydroxides with charge-balancing anions and hydrogen-bonded water molecules in the interlayer [1,2,3,4]
There is still a lack of extensive research of mesoporous silica-supported LDH systems where mSiO2 can provide more surface area, far more than that of silica nanosphere, for the depositing of LDH nanoparticles. This may be attributed to two reasons: (1) In the mSiO2-supported HT structure, the supported LDH nanoparticles located at the inner surface of mSiO2 would block the pore channels, limiting the accessibility of active sites of the confined LDH; (2) The higher cost and more complex preparation of mSiO2 compared with the SiO2 sphere usually synthesized by a simple Stöber method
The morphology, textural properties, and base site strength of a series of silica aerogel (SA)/LDH composites synthesized at different Mg/Al ratios and hydrothermal temperatures were characterized by Transmission Electron Microscope (TEM), X-ray diffraction (XRD), EDS, BET, and CO2-TPD
Summary
Layered double hydroxides (LDHs) are a class of two-dimensional anionic clays composed of positively-charged layers of mixed bivalent and trivalent metal hydroxides with charge-balancing anions and hydrogen-bonded water molecules in the interlayer [1,2,3,4]. The obtained SA/LDH composites showed large average pore diameters (>9 nm), large pore volume (up to 1.3 cm3·g), and high surface area (up to 587.4 m2·g) and could provide easy access of reactants to the edge catalytic sites of the incorporated LDH nanosheets. In this context, the morphology, textural properties, and base site strength of a series of SA/LDH composites synthesized at different Mg/Al ratios and hydrothermal temperatures were characterized by TEM, XRD, EDS, BET, and CO2-TPD. A base-catalytic Henry reaction of benzaldehyde with nitromethane was employed as a model reaction to evaluate the catalytic properties of the supported LDHs using the unsupported LDH nanosheets and bare SA as control samples
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