Supercritical carbon dioxide: Forming formic acid

Solvent as reactant

 

Carbon dioxide can be both starting material for the production of formic acid through hydrogenation and also the solvent for the reaction if held in the supercritical; the usual complexities of extraction of product from solvent essentially removed as the solvent can be evaporated by simply releasing the pressure from the system and trapping the carbon dioxide gas released. NMR spectroscopy allows the German inventors of the process to keep track of reactants and products.

Walter Leitner of RWTH Aachen University, Germany, and colleagues there and at Yale University explain how exploiting the greenhouse gas, carbon dioxide, as both solvent and reactant in an integrated approach has allowed them to obtain free formic acid as a product of catalytic hydrogenation in a single step for the first time.

"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," explains Leitner. "Naturally we cannot '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."

Chemists have investigated the potential of homogeneous catalysis in the production of formic acid, a key industrial feedstock chemical, from carbon dioxide since at least the middle of the 1970s. Of course, fundamentally, the conversion of the gas to the acid is an equilibrium reaction that is mostly biased to the carbon dioxide side of the chemical equation rather than the useful product. The back reaction must therefore be constantly stifled if the equilibrium is to be pushed towards the right and this could involve using a continuous flow system that pulls out the formic acid almost as soon as it forms, perhaps as a salt adduct or other temporary precipitate. Such precipitates then require further, often costly and time-consuming extractions and separations.

Leitner and colleagues have turned to supercritical fluids, a concept pioneered in several industrial schemes by Nottingham University's Martin Poliakoff since the 1990s. The big advantage of SCFs is that the processes can not only avoid toxic organic solvents they can also be far more amenable to a simple extraction of the product from the solvent. Leitner have taken this a step further by exploiting their supercritical solvent as the starting material at the same time. The reaction and separation steps can therefore be integrated into a single processing unit, the team explains.

Multitasking carbon dioxide

The success of the team's approach hinges on a two-phase reaction system that employs supercritical carbon dioxide as the mobile phase and a liquid salt - an ionic liquid - as the stationary phase. The catalyst (in situ ruthenium catalyst) and the base (aqueous ammonia) used to stabilize the formic acid are both dissolved in the ionic liquid, which holds them both in the reactor. The carbon dioxide then flows through the reactor at pressures and temperatures above the critical values (7.4 MPa, 31 Celsius), the system selectively removes the formic acid from the mixture pushing the equilibrium to the right all the while. Importantly, ionic liquids are entirely immiscible with supercritical carbon dioxide. Moreover, the catalyst and base used are only soluble in supercritical carbon dioxide and not the ionic liquid. This precludes product contamination with these auxiliary substances. The team explains that the process can be run continuously and in their laboratory experiments the researchers have demonstrated stable operation for a time period of almost ten days. Indeed, the proton NMR spectrum of the optimal ionic liquid recovered from the reactor after 211 hours on-stream was virtually unchanged, the team says.

The hydrogenation of carbon dioxide 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. Formic acid is used in the manufacture of chemicals for agriculture, food technology, and the leather industry.