Electrocatalytic valorization into H2 and hydrocarbons of an aqueous stream derived from hydrothermal liquefaction

Abstract

Electrocatalytic oxidation is an attractive process for valorizing the organic compounds and removing the nitrogen in aqueous waste streams at ambient conditions. We evaluated the electrocatalytic oxidation reaction as a function of applied potential over Pt electrodes of an aqueous stream generated via hydrothermal liquefaction. We quantified the conversion of particular organic compounds (e.g., carboxylic acids, alpha hydroxyacids, alcohols, ketones, and amides) and the removal of carbon, nitrogen, sulfur, and chemical oxygen demand. Organic nitrogen and sulfur were oxidized to nitrates and sulfates. The main reaction products from the electrocatalytic oxidation at the anode were short chain volatile hydrocarbons (i.e., olefins, and paraffins) and CO2, while H2 was generated at the cathode. Unidentified compounds were converted to short chain carboxylic acids, alcohol and ketones, while ammonia was oxidized into N2. Studies with model compounds showed that amides and alpha hydroxyacids yielded carboxylic acids that convert further via (non-)Kolbe chemistry. Simultaneous denitrification, valorization of organic compounds, and H2 generation from the aqueous stream can potentially simplify the unit operations currently used in a hydrothermal process. The cost of the electricity required to drive the electrocatalytic operation can be partially mitigated by selling the excess H2 produced.