Delaminated layered double hydroxides as catalysts for the Meerwein–Ponndorf–Verley reaction

• Hydrotalcites containing DBS are effectively delaminated in 1-butanol.
• Exfoliated solid was ion-exchanged to obtain discrete LDH nanolayers.
• Calcining delaminated LDH at 450 °C produced a solid highly basic.
• The calcined solids exhibited excellent conversion in MPV reaction.

The Meerwein–Ponndorf–Verley (MPV) reaction involves the hydrogen-transfer reduction of a carbonyl compound by an alcohol used as hydrogen donor. The process is catalysed by both homogeneous and heterogeneous catalysts. The latter can be of the acid or basic type. In this work, we used delamination and ion-exchange in combination to obtain delaminated solids from layered double hydroxides (LDHs). Three Mg/Al LDHs (Mg/Al ratio = 2) containing carbonate, hydroxyl or dodecylbenzenesulphonate (DBS) as interlayer anion were prepared using a coprecipitation method or by rehydration of the solid obtained by calcinations of carbonate containing LDH. The X-ray diffraction patterns for LDHs exhibited the typical lines for LDHs intercalated with carbonate, hydroxyl or DBS ions. The DBS-containing LDH was delaminated by sonicating at 60 °C a suspension of this solid in 1-butanol. The DBS ions in the delaminated LDH could be exchanged by nitrate or hydroxyl ions by stirring a suspension of this delaminated LDH in 1-butanol containing an appropriate amount of Ca(NO3)3 of Ca(OH)2, respectively. The resulting delaminated solids consisted of brucite-like nanolayers excess charge in which was countered by dodecylbenzenesulphonate, nitrate or hydroxyl ions. Some of these solids exhibited moderate catalytic activity in the MPV reaction of benzaldehyde with 2-propanol. The reaction was conducted at 82 °C (0.06 mol of 2-propanol was treated with 0.003 mol of benzaldehyde in the presence of 1 g of catalyst). Calcining at 450 °C the delaminated LDHs, however, provided solids with a high surface activity that made them highly effective catalysts for the reaction. The catalyst obtained by calcining delaminated LDH containing hydroxyl ions provided the best catalyst (100% conversion after 2 h of reaction) and has been used in the MPV reaction of other aldehydes with excellent results.

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