Esterification of alkyl acids with alkanols over MCM-41 molecular sieves: Influence of hydrophobic surface on condensation reaction

The Al-MCM-41 mesoporous materials with Si/Al ratios 29, 52, 74 and 110 were synthesized by hydrothermal method and characterized by powder X-ray diffraction (XRD), nitrogen adsorption–desorption measurements (BET), 29Si-MAS-NMR, 27Al-MAS-NMR, DRIFT, SEM and TGA–DTG techniques. The XRD studies indicated ordered hexagonal structure for all the samples. The specific surface area, pore volume, and pore size distribution of the samples were comparable to mesoporous molecular sieves. The catalytic activity of these materials was tested for the esterification of acetic acid (AA) with 1-hexanol (HeOH), 2-ethyl-1-hexanol (2E-1HOH), isoamyl alcohol (IAOH), and propionic acid (PA) with isoamyl alcohol under autogenous conditions. The effects of temperature from 75 to 150 °C, initial molar feed ratios, acid:alcohol, 1:1, 1:2, 2:1, 3:2 and 5:2, amount of catalyst and the reaction time were also investigated. The yield of the product ester was observed with higher for the feed ratio 5:2. Under the experimental conditions, Al-MCM-41 (110) a hydrophobic material showed higher alcohol conversion with acetic acid at 125 °C than the other catalysts. The conversion with respect to the alcohols followed the order HeOH (88.4%) > IAOH (87.7%) > 2E-1HOH (84.4%). All the product esters showed selectivity close to 100%. The hydrophobic properties of the catalysts were suggested to play important role in the esterification reaction.

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