Abstract
The synthesis of oxygenate products, including cyclic ketones and phenol, from carbon dioxide and water in the absence of gas‐phase hydrogen has been demonstrated. The reaction takes place in subcritical conditions at 300 °C and pressure at room temperature of 25 barg. This is the first observation of the production of cyclic ketones by this route and represents a step towards the synthesis of valuable intermediates and products, including methanol, without relying on fossil sources or hydrogen, which carries a high carbon footprint in its production by conventional methods. Inspiration for these studies was taken directly from natural processes occurring in hydrothermal environments around ocean vents. Bulk iron and iron oxides were investigated to provide a benchmark for further studies, whereas reactions over alumina and zeolite‐based catalysts were employed to demonstrate, for the first time, the ability to use catalyst properties such as acidity and pore size to direct the reaction towards specific products. Bulk iron and iron oxides produced methanol as the major product in concentrations of approximately 2–3 mmol L−1. By limiting the hydrogen availability through increasing the initial CO2/H2O ratio the reaction could be directed to yield phenol. Alumina and zeolites were both observed to enhance the production of longer‐chained species (up to C8), likely owing to the role of acid sites in catalysing rapid oligomerisation reactions. Notably, zeolite‐based catalysts promoted the formation of cyclic ketones. These proof‐of‐concept studies show the potential of this process to contribute to sustainable development through either targeting methanol production as part of a “methanol economy” or longer‐chained species including phenol and cyclic ketones.
Highlights
10’s bar Industrial chemicals and fuels are ubiquitous in modern society. Their production from hydrocarbons, oxygenates, cyclic and aromatic compounds is essential for the global economy, an overwhelming proportion are produced from unsustainable sources
There are relatively few previous studies on the hydrothermal conversion of CO2 in the absence of an additional hydrogen source[7,17,23,27,28,40]. The majority of these have focussed on investigating the role of such reactions in the production of molecules necessary for the evolution of life on Earth and have principally considered the use of bulk iron powder and/or iron oxides as reductants or catalysts
Comparing the production of longer-chained products over zeolite-based catalysts to that over the bulk iron powder and bulk iron oxides (Figure 2), a clear difference is the presence of cyclic ketones with five to seven carbons in significant concentrations, e.g. eighteen-times more cyclopentanol is produced over HY than over Fe2O3
Summary
Industrial chemicals and fuels are ubiquitous in modern society. Their production from hydrocarbons, oxygenates, cyclic and aromatic compounds is essential for the global economy, an overwhelming proportion are produced from unsustainable sources. Chen et al demonstrated that in the presence of magnetite, Fe3O4, aromatic compounds are observed at 300 °C[27], while at higher temperatures of up to 350 °C organic acids are obtained[28] In these cases, the role of iron is to promote the reduction of carbon dioxide. The use of bifunctional catalysts containing both a metallic and an acidic zeolite component has previously been proposed in other systems as a means of producing longer chain hydrocarbons in the gasoline range from gaseous CO2 and H2[38], and may present a route to the formation of larger species in hydrothermal media. The primary aim is to develop a nature-inspired, H2-free route to synthesise value-added organic species from carbon dioxide and water with the aid of iron-containing catalysts
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