(241h) Hydrothermal Water a Solvent, Reactant and Catalyst for Sustainable Catalytic Engineering

The use of hydrothermal media (high temperature water, HTW) presents an opportunity to harness the complex chemistry of water for catalytic transformations. HTW describes water between 180 °C and 374 °C and at a sufficiently high pressure to ensure that it remains in the liquid-phase. When compared with ambient water, HTW has a lower dielectric constant and fewer and weaker hydrogen bonds. The use of HTW offers an exciting possibility to conduct hydrogenations without the need for gas-phase H₂. Herein we demonstrate the use of HTW in, firstly, the direct conversion of CO₂; and secondly, in the production of renewable carbonaceous catalysts and sorbents.

The direct conversion of CO₂ in HTW has been carried out at 300 °C and an initial CO₂ pressure before heating of 25 bar (CO₂:H₂O = 0.26). Iron catalysts were employed to facilitate CO₂ hydrogenation. The reaction products vary with conditions, however in all cases methanol is produced alongside more complex species including heptanal, 2-octanonone and phenol (Figure 1).

The conversion of biomass feedstocks including avocado pits, brewers’ spent grain and bread waste to hydrochars in hydrothermal media has also been studied. These hydrochars have applications as sorbents, catalysts and catalyst supports. Using binary mixtures of alcohols and water has a direct impact on the physical characteristics of the hydrochar produced. Carbon-supported iron catalysts have also been produced through conducting the synthesis in the presence of iron sulfate. This one-pot synthesis represents a sustainable route to the production of such functional materials.

This work demonstrates the potential use of water in future biorefinery applications and related processes. Notably, the ability to use –OH groups as hydrogen donors in place of gas-phase hydrogen and to use the innate structuring of water and water/alcohol mixtures to control the products of catalytic and catalyst synthesis are demonstrated.