Toward Catalytic Ketonization of Volatile Fatty Acids Extracted from Fermented Wastewater by Adsorption

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

Toward Catalytic Ketonization of Volatile Fatty Acids Extracted from Fermented Wastewater by Adsorption

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

Tandem catalytic aromatization of volatile fatty acids

The transition towards a circular economy requires closing the carbon loop, e.g. by the development of new synthesis routes to valuable intermediates and products from organic-rich waste streams. Volatile fatty acids (VFA) can be fermentatively produced from wastewater and serve as circular platform chemicals. We show that these VFA can be catalytically upgraded to light aromatics (i.e., benzene, toluene, ethylbenzene and xylenes, BTEX) via a tandem catalytic reaction involving TiO2-catalyzed ketonization and zeolite ZSM-5 catalyzed aromatization. Including this intermediate ketonization step is demonstrated to be much more efficient than direct VFA aromatization, as direct acid conversion mainly gave rise to short-chain olefins by decarboxylation and low BTEX yields of 1%. A one-reactor, tandem catalytic conversion instead significantly improved the yield to 45% when zeolite Ga/ZSM-5 was used. Furthermore, the effect of VFA-derived ketone composition, a process parameter set by the fermentation process, on aromatics production efficiency and product distribution was found to be very pronounced for zeolite Ga/ZSM-5, but not for non-promoted zeolite HZSM-5. This suggests a different reaction mechanism to dominate on zeolite Ga/ZSM-5, involving dehydration on the Brønsted acid sites and cyclization/aromatization on the Ga sites. Finally, water, expected to be present in the feed during VFA upgrading, caused the activity of zeolite Ga/ZSM-5 to drop reversibly, but also led to lower coke buildup. Analysis of the spent catalyst with solid-state 27Al nuclear magnetic resonance spectroscopy and temperature-programmed reduction with H2 showed that the catalyst structure remained intact, also with water present in the feed. Together, the results demonstrate that catalytic ketonization/aromatization is an attractive circular approach for converting waste-derived carboxylic acids into renewable aromatics

Crystal Phase Effects on the Gas-Phase Ketonization of Small Carboxylic Acids over TiO2 Catalysts

Invited for this month′s cover is the group of Pieter Bruijnincx from Utrecht University. The image shows an imaginary police line-up of two molecules suspected to be involved as intermediates in the catalytic ketonization reaction. Based on the evidence collected, depicted on the pinboard on the wall, the scientist discusses the impact of all this with somebody interested in catalysis that converts waste, wastewater-derived volatile fatty acids in this case, to value-added circular chemicals. The Full Paper itself is available at 10.1002/cssc.202100721

Tandem catalytic aromatization of volatile fatty acids

The transition towards a circular economy requires closing the carbon loop, e.g. by the development of new synthesis routes to valuable intermediates and products from organic-rich waste streams. Volatile fatty acids (VFA) can be fermentatively produced from wastewater and serve as circular platform chemicals. We show that these VFA can be catalytically upgraded to light aromatics (i.e., benzene, toluene, ethylbenzene and xylenes, BTEX) via a tandem catalytic reaction involving TiO2-catalyzed ketonization and zeolite ZSM-5 catalyzed aromatization. Including this intermediate ketonization step is demonstrated to be much more efficient than direct VFA aromatization, as direct acid conversion mainly gave rise to short-chain olefins by decarboxylation and low BTEX yields of 1%. A one-reactor, tandem catalytic conversion instead significantly improved the yield to 45% when zeolite Ga/ZSM-5 was used. Furthermore, the effect of VFA-derived ketone composition, a process parameter set by the fermentation process, on aromatics production efficiency and product distribution was found to be very pronounced for zeolite Ga/ZSM-5, but not for non-promoted zeolite HZSM-5. This suggests a different reaction mechanism to dominate on zeolite Ga/ZSM-5, involving dehydration on the Brønsted acid sites and cyclization/aromatization on the Ga sites. Finally, water, expected to be present in the feed during VFA upgrading, caused the activity of zeolite Ga/ZSM-5 to drop reversibly, but also led to lower coke buildup. Analysis of the spent catalyst with solid-state 27Al nuclear magnetic resonance spectroscopy and temperature-programmed reduction with H2 showed that the catalyst structure remained intact, also with water present in the feed. Together, the results demonstrate that catalytic ketonization/aromatization is an attractive circular approach for converting waste-derived carboxylic acids into renewable aromatics

Toward Catalytic Ketonization of Volatile Fatty Acids Extracted from Fermented Wastewater by Adsorption

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

Crystal Phase Effects on the Gas-Phase Ketonization of Small Carboxylic Acids over TiO2 Catalysts

Invited for this month′s cover is the group of Pieter Bruijnincx from Utrecht University. The image shows an imaginary police line-up of two molecules suspected to be involved as intermediates in the catalytic ketonization reaction. Based on the evidence collected, depicted on the pinboard on the wall, the scientist discusses the impact of all this with somebody interested in catalysis that converts waste, wastewater-derived volatile fatty acids in this case, to value-added circular chemicals. The Full Paper itself is available at 10.1002/cssc.202100721