(243e) The Catalytic Effect of H-Beta, Ce- and Ln-Doped Beta Zeolite on the Hydrothermal Transformation of Glycerol
The growth of the biodiesel industry over the last decade has resulted in a price drop of glycerol, making it an attractive feedstock for transformation into value-added chemicals . The three hydroxyl groups of glycerol are highly reactive but their similar activation energies complicate their selective conversions. Among the processes tested for glycerol transformations, “hydrothermal technology” has gained attention due to its potential to overcome the difficulties associated with other methods such as long reaction times of fermentation . High T/P water acts as a potent acid-base medium enabling short reaction times [2, 3] while the addition of a suitable catalyst may increase the selectivity to desired products .
Lewis acidic Sn-, Zr-, Ti- doped Beta zeolites exhibit high selectivity to the formation of lactic acid from trioses in both methanol and water at 80 °C and 125 °C, respectively . In this study, the incorporation of even stronger Lewis acidic Ce and Ln into the BEA framework of zeolites was carried out using two different procedures: i) solid state ion exchange (SSIE) between H-Beta zeolite and a metal nitrate at 550 °C and ii) dealumination of H-Beta zeolite by strong mineral acid without collapsing the zeolite structures followed by SSIE .
Transformation of glycerol was investigated under hydrothermal conditions (270-360 °C, 55-186 bar, 5-180 min) using H-Beta, Ce- and Ln-doped Beta zeolites in batch tubular reactors. The catalysed glycerol transformation followed mainly two concomitant pathways in all systems: dehydration and/or oxidation. The former took place at faster reaction rates yielding ~17 %C yield of acrolein within the first 10 min. The latter primarily produced lactic acid having yield increased with reaction time; max ~20 %C. The metal-doped Beta zeolites prepared by simple SSIE showed a ~30% increase in selectivity towards lactic acid as compared to H-Beta zeolite. The incorporation of the metals resulted in lower Brønsted acidity of the zeolite surface but higher Lewis acidity which may play an important role in the transformation of pyruvaldehyde (PAL), the final intermediate product which transforms into lactic acid via Cannizzaro reaction as proposed by Bicker et al.. The selectivity to lactic acid obtained was slightly higher in the reactions catalysed by Ce- (~30 %C) than those catalysed by Ln-doped Beta zeolite (~25 %C).
Herein, we report for the first time high yield of lactic acid (~ 20 %C, ~ 80 %mol glycerol conversion) achieved directly from glycerol under base-free hydrothermal conditions using high Brønsted-acidic H-Beta and Lewis acidic Ce- and Ln-doped Beta zeolites. The lactic acid yield achieved was competitive to those obtained in the reactions catalysed by Pd/TiO2, Pt/TiO2 or Au/TiO2 at 90 °C in basic environment  and greater than in WO3/TiO2catalysed reactions under supercritical water conditions .
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