Glycerol upgrading over zeolites by batch-reactor liquid-phase oligomerization: Heterogeneous versus homogeneous reaction

Glycerol upgrading to diglycerols in the presence of basic (Na⁺ or Cs⁺) ion-exchanged (FAU or BEA) zeolite catalysts was studied in a liquid-phase batch rector at 260 °C under normal pressure. Homogeneous NaHCO₃ and CsHCO₃ catalysts were studied for comparison. All the catalysts, including NaHCO₃ and CsHCO₃, displayed the same conversion-selectivity relationship. The selectivity to linear diglycerols decreased at higher conversions/reaction times owing to the consecutive formation of higher oligomers, with preferential further conversion of a,a'-diglycerol. The maximum yield of linear diglycerols was limited to about 30 %. The activities of the zeolites followed the order X>Y>Beta, independent of the alkali ion present. Catalysis by the zeolites starts with an induction period attributed to a slow leaching of alkaline cations from the zeolite. Thereafter, the reaction is characterized by a progressive loss of the microporous structure of the zeolite and increasing overlap of heterogeneous and homogeneous catalysis, where, primarily, the activity depends on the cation content of the zeolite.