Catalytic fast pyrolysis (CFP) is an efficient two-stage process for upgrading biomass into biofuels. However, a number of volatile oxygenates, including acetaldehyde, acetone, and butanone, are produced in relatively high yields (7-10% of initial biomass carbon) and are not captured by the primary liquid bio-oil product. The unique functionality associated with these three compounds – the carbonyl group – provides the opportunity for catalytic valorization to liquid fuels through aldol condensation (AC). The AC reaction is a C-C bond forming reaction that may be utilized to couple carbonyl-containing oxygenates into heavier, fuel-range molecules. However, AC of mixed-component feed streams, particularly under CFP-relevant conditions, has not been a focus in the literature. The goal for this work was to explore the AC reaction as a strategy for combining mixtures of light oxygenates over a powder TiO2 catalyst and understand the effect of competitive adsorption and reaction phenomena on trends in catalytic activity and selectivity. Mixtures of acetaldehyde, acetone, and butanone at CFP-relevant feed ratios were converted to heavier C5+ compounds at high conversion and selectivity over a TiO2 catalyst. Preliminary reaction results indicated that these three compounds could be converted to fuel range products at up to 90% C yield. The product stream contained a complex mixture of self and cross condensation products as well as multiple unidentified reaction products. Single molecule reactions as well as competitive, binary reactions revealed that acetaldehyde tends to exhibit the highest conversion of the three molecules, while butanone exhibits the lowest. Differences in competitive reaction conversion for the three carbonyls were hypothesized to be due to their molecular structure. Specifically, the molecular structure of the carbonyls is expected to influence the stability of the enolate intermediate as well as the steric accessibility of the carbonyl. The stability of TiO2 catalysts during AC was also investigated by monitoring trends in conversion and reaction selectivity during mixed-feed stream reactions at various reaction temperatures. Catalyst deactivation tended to occur rapidly over the first few hours on stream while selectivity changed continuously over the course of the entire reaction. Selectivity to single-stage condensation products, including C5 and C6 oxygenates, tended to increase at longer times on stream.

Aldol condensation over TiO2 for upgrading light oxygenates

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.