|Abstract: ||High availability and low price of crude glycerol, the main by-product of the biodiesel industry, make it an attractive feedstock for transformations into value-added chemicals. The aim of this thesis was to improve our understanding of the hydrothermal conversion of glycerol facilitated by zeolite-based catalysts.
A range of Lewis acidic Ce-, La-, Sn- and Zn-doped ZSM-5 and Beta zeolites were prepared by solid state ion exchange reaction. The effect of those catalysts as well as their parent NH4- and H-forms on the dehydration of glycerol was investigated under hydrothermal conditions (270-360 °C, 55-186 bar, 5-300 min) in batch tubular reactors. Several reaction products were detected, of which acrolein was the main liquid product with the highest selectivity of ~38 mol% achieved within the first 5 min at 330 °C on H-Beta zeolite. At longer reaction times acrolein decomposed and acetaldehyde became the main product (max. selectivity ~26 mol% in 30 min). The addition of metal-doped zeolites did not increase the degree of glycerol conversion but increased the total selectivity towards the liquid products. 4.8 wt% La-doped NH4-Beta zeolite resulted in a 56 mol% glycerol conversion with a 36 mol% selectivity towards acetaldehyde.
The oxidation of glycerol with H2O2 in subcritical water was investigated in a continuous fixed bed reactor at 125-175 °C, 35 bar, 60-300 s using H-Beta zeolite, 2.5 wt%- and 4.8 wt% Cu-doped H-Beta zeolite extruded with γ-Al2O3. As compared to non-catalysed oxidation, the addition of Cu-doped H-Beta zeolites did not increase the degree of conversion but promoted the conversion rate of glycerol as well as the selectivity towards liquid products. The liquid products detected included dihydroxyacetone (DHA), formic acid (FA), acetic acid, glycolic acid, pyruvaldehyde and lactic acid (LA). The distribution of these products varies with the temperature, residence time and the type of catalyst. The top-three main products obtained were DHA, FA and LA. The highest yield of DHA (~8 mol%) was achieved with 4.8 wt% Cu/H-Beta/γ-Al2O3 (MC) at 150 °C, 60 s. The same catalyst also provided LA with the highest yield of 11.5 mol% at 175 °C, 240 s. FA was detected with the highest yield of ~9 mol% at 175 °C, 60 s on H-Beta/γ-Al2O3 (MC).|