Abstract
Glycerol solutions were vaporized and reacted over ceria catalysts with different morphologies to investigate the relationship of product distribution to the surface facets exposed, particularly, the yield of bio-renewable methanol. Ceria was prepared with cubic, rodlike, and polyhedral morphologies via hydrothermal synthesis by altering the concentration of the precipitating agent or synthesis temperature. Glycerol conversion was found to be low over the ceria with a cubic morphology, and this was ascribed to both a low surface area and relatively high acidity. Density functional theory calculations also showed that the (100) surface is likely to be hydroxylated under reaction conditions which could limit the availability of basic sites. Methanol space-time-yields over the polyhedral ceria samples were more than four times that for the cubic material at 400 °C, where 201 g of methanol was produced per hour per kilogram of the catalyst. Under comparable glycerol conversions, we show that the rodlike and polyhedral catalysts produce a major intermediate to methanol, hydroxyacetone (HA), with a selectivity of ca. 45%, but that over the cubic sample, this was found to be 15%. This equates to a 13-fold increase in the space-time-yield of HA over the polyhedral samples compared to the cubes at 320 °C. The implications of this difference are discussed with respect to the reaction mechanism, suggesting that a different mechanism dominates over the cubic catalysts to that for rodlike and polyhedral catalysts. The strong association between exposed surface facets of ceria to high methanol yields is an important consideration for future catalyst design in this area.
Highlights
Increased concerns regarding rising CO2 levels and the associated environmental consequences have resulted in increased demands for sustainable liquid biofuels
We have previously reported on the effect of ceria calcination temperatures and the subsequent physicochemical properties of ceria on the reaction of glycerol, which revealed that there is no clear relationship between the density of defect sites and the reactivity of glycerol or its intermediate products, when samples are compared at a constant space velocity and activity is normalized to catalyst surface area.[26]
Sharper reflections were observed for Ce−C compared with Ce−R and Ce−P, indicating a higher level of crystallinity for the cubic material than for the rods or polyhedral, which probably arises from the harsher synthesis conditions required to form the cubic morphology, using both concentrated base and a higher reaction temperature of 180 °C
Summary
Increased concerns regarding rising CO2 levels and the associated environmental consequences have resulted in increased demands for sustainable liquid biofuels. The valorization of glycerol is not a new concept, with several reviews detailing the progress made in glycerol dehydration,[6,7] hydrogenolysis,[8−11] oxidation,[12,13] gasification,[14,15] esterification,[16] etherification,[17] oligomerization,[18] acetylation, and carboxylation.[19] The conversion of glycerol into lower alcohols provides an attractive route for glycerol valorization due to their industrial applicability and potential for fuel blends.[20] van Ryneveld et al reported alcohol selectivities exceeding 68% (methanol, ethanol, and propanol combined) over Ni/SiO2 catalysts, at a reaction temperature of 320 °C and 60 bar H2.21 A similar study by Friedrich and coworkers showed that Mo and W catalysts supported on alumina and silica could be used to convert glycerol to lower alcohols,[22] with a total mono-alcohol (methanol, ethanol, 1-propanol, and 2-propanol) selectivity of >85% at 325 °C and 60 bar H2.
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