Synthesis of mesoporous alumina with CO2 expanded carbonation and its catalytic oxidation of cyclohexanone

•CO2 expanded carbonation technique used to synthesize mesoporous alumina.•Alumina particles were flower-like and honey-comb-like due to different volume expansions.•Efficient catalytic oxidation of cyclohexanone to ɛ-caprolactone.•Hydrolysis of ɛ-caprolactone could be avoided.

A CO2 expanded carbonation technique is proposed for direct synthesis of alumina powders that does not require structure directing substances or templates. Mesoporous amorphous flower-like alumina was synthesized at relatively low volume expansions (lower ethanol to water volume ratio), whereas mesoporous crystalline honey-comb-like alumina was synthesized at high volume expansions. The alumina powders exhibited high surface area and pore size with small crystallite sizes. The alumina structures were stable from 400 to 800 °C. Experimental tests showed that the alumina powders could catalytically convert cyclohexanone to ɛ-caprolactone efficiently. The use of the calcined catalysts (at 400 and 800 °C; flower-like alumina) at equal ethanol to water volume ratio avoids the usual and inevitable hydrolysis of ɛ-caprolactone to ɛ-hydroxyhexanoic acid. The catalyst was recyclable and stable for up to five reaction cycles.

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