Production of formic acid. In this process, carbon dioxide is not only a starting material; it also acts—in a supercritical state—as the solvent for separation of the product. This integrated approach makes it possible to directly obtain free formic acid as the product in a single step for the first time. A paper on the work is published in the

Formic acid is an important product in the chemical industry and has many applications, including agriculture, food technology, and the leather industry. It is also being contemplated as a potential hydrogen-storage material—vehicles powered by fuel cells could fill up with formic acid, from which the hydrogen could then be produced catalytically.

The hydrogenation of CO₂ to formic acid (HCO₂H) is a subject of intensive research because it offers direct access to chemical products based on waste products from the use of fossil fuels for energy.

Homogeneous catalysts for the production of formic acid from CO₂ have been investigated since the mid 1970s. However, this process involves an equilibrium reaction for which the equilibrium heavily favors the reactants. In order to suppress the constantly occurring back-reaction, the formic acid must be removed—in the form of a salt, adduct, or derivative—to take it out of the equilibrium. To obtain the desired free formic acid in the end, additional separation steps are thus required to separate the adducts from the catalyst and finally to release and isolate the formic acid.

Prof. Leitner and his colleagues use a two-phase reaction system that employs supercritical CO₂ as the mobile phase and a liquid salt—an ionic liquid—as the stationary phase. The catalyst and the base used to stabilize the formic acid are both dissolved in the ionic liquid, which holds them both in the reactor.

The CO₂ flows through the reactor at pressures and temperatures above the critical values (74 bar, 31 °C) and selectively removes the formic acid from the mixture. The dual role played by CO₂ as both reactant and extractive phase has significant advantages.

The product is continuously extracted and flushed from the reactor, which causes the equilibrium to readjust constantly. Once out of the reactor, the free formic acid can be obtained with high purity by decompression or washing. Ionic liquids do not dissolve in supercritical CO₂, nor do the catalyst and base, so these do not contaminate the product. The process can run continuously. In laboratory experiments, stable operation was demonstrated for over 200 hours.

Our results with formic acid demonstrate that the systematic implementation of modern solvent techniques in continuous reactor equipment makes it possible to perform conversions that cannot be achieved under conventional conditions. Naturally we can’t ‘defeat’ thermodynamics in this way—but there are many possibilities for the integration of reactions and materials separation that may open new routes for more efficient and sustainable processes.

—Prof. Leitner

Production of methanol. Methanol and its products can not only be used as a fuel or for driving fuel cells, they are also a versatile feedstock for chemical industry. The conventional industrial process for the production of methanol starts with syngas, a mixture of hydrogen and carbon monoxide obtained from fossil resources. The process requires extremely high pressures and temperatures, involving a heterogeneous catalyst, which is a solid and therefore in a different phase than the gaseous or liquid educts and products.

A number of approaches for converting CO₂ to methanol (CH₃OH) have been developed. The big challenge for catalytic researchers is not only to activate the very stable CO₂ molecule but also to catalyze the multistep conversion to methanol. Tailored catalysts are the key to enable the activation of this poorly reactive C₁ building block.

The RWTH Aachen team pursued a new approach to obtain methanol by the hydrogenation of CO2 with elemental hydrogen in an homogeneous process—i.e., the catalyst and the reactants are in the same phase, a solution. Homogeneous catalysis often require milder reaction conditions and the targeted development of the catalyst often enables a better selectivity. However, a homogeneous metal complex that is able to catalyze the multistep conversion of CO₂ and hydrogen into methanol has not yet been reported.

The special ruthenium phosphine complex catalyst developed by the team lead by Klankermayer and Leitner is dissolved in a solvent, in the simplest case in methanol itself, and put under pressure together with CO₂ and hydrogen in an autoclave. It subsequently connects a molecule of CO₂ in a stepwise fashion with three molecules of hydrogen to produce methanol and water.

This is the first example of the hydrogenation of CO₂ to methanol by use of a molecularly defined catalyst under relatively mild reaction conditions. We are now investigating in detail how the reaction works in order to develop our catalyst further.

—Leitner and Klankermayer

Resources

• Wesselbaum, S., Hintermair, U. and Leitner, W. (2012) Continuous-Flow Hydrogenation of Carbon Dioxide to Pure Formic Acid using an Integrated scCO₂ Process with Immobilized Catalyst and Base. Angewandte Chemie International Edition, doi: 10.1002/anie.201203185

• Wesselbaum, S., vom Stein, T., Klankermayer, J. and Leitner, W. (2012) Hydrogenation of Carbon Dioxide to Methanol by Using a Homogeneous Ruthenium–Phosphine Catalyst. Angew. Chem. Int. Ed., 51: 7499–7502. doi: 10.1002/anie.201202320