The choice of TiO 2 crystal phase (i. e., anatase, rutile, or brookite) greatly influences catalyst performance in the gas‐phase ketonization of small volatile fatty acids, such as acetic acid and propionic acid. Rutile TiO 2 was found to perform best, combining superior activity, as exemplified by an exceptional reaction rate of 141.8 mmol h −1 g cat −1 (at 425 °C and 24 h −1 ) with excellent ketone selectivity when propionic acid was used. Brookite, to the best of our knowledge never reported before as a viable ketonization catalyst, was found to outperform the well‐studied anatase phase, but not rutile. Operando Fourier‐transform IR spectroscopy measurements combined with on‐line mass spectrometry showed that bidentate carboxylates were the most abundant surface species on the rutile and brookite surfaces, while on anatase both monodentate and bidentate carboxylates co‐existed. The bidendate carboxylates were thought to be precursors to the active ketonization species, likely monodentate intermediates more prone to C−C coupling. Ketonization activity did not directly correlate with acidity; the observed, strong crystal phase effect did suggest that ketonization activity is influenced strongly by geometrical factors that determine the ease of formation of the relevant surface intermediates.

A method was developed to recover and convert a carboxylic acid from a phosphonium phosphinate ionic liquid.

Volatile fatty acids (VFA) produced by fermentation of organic-rich wastewater streams can, after efficient recovery from the dilute fermentation broth, serve as a circular source of carbon and be catalytically upgraded into various valuable platform molecules. Waste-derived VFA, that is, a mixture of acetic, propionic, and butyric acids, can thus be converted into mixed ketones, which in turn are valuable intermediates for light aromatics synthesis. Here, an integrated process is presented for the recovery and in-line catalytic conversion of VFA extracted from a fermentation broth by adsorption on a nonfunctionalized resin adsorbent. Gas-phase ketonization of the VFA was studied with and without co-fed water, which is inevitably coextracted from the broth, over TiO2 anatase catalysts to assess catalyst performance, including stability as a function of time on stream. While VFA conversion over bare TiO2 at 375 °C proceeded at 90% conversion with 100% selectivity to ketones, the presence of water in the feed resulted in an activity drop to 40%. Catalyst stability toward water could be greatly improved by dispersing the titania on a hydrophobic carbon support. The carbon-supported catalyst showed superior performance in the presence of excess water, providing a quantitative yield toward ketones at 400 °C. The approach thus allows coupling of VFA recovery from a fermentation broth with successful catalytic upgrading to mixed ketones, thus providing a novel route for the production of value-added products from waste streams.