"Super" Fluid Eases Catalysis

Sensitive detector reveals fluid properties and reactions that could solve separations problems, reduce toxicity

Many refinery and petrochemical operations hinge on processes that use soluble catalysts dissolved in a fluid. Substituting "supercritical" fluids in place of conventional solvents in these processes could reduce energy use and benefit the environment. For example, an energy-intensive step could be eliminated during use of the hydroformylation process to convert olefins into aldehydes, which are widely used to make detergents, plastics, and agricultural products.

Argonne's toroid cavity detector supports high-resolution, in-situ chemical analysis of high-pressure, high-temperature industrial catalysis processes. It measures the identity of chemical species under conditions up to about 76 MPa (750 atmospheres) and 250 degrees C (480 degrees F).

The solvent that looks most promising at present is carbon dioxide. At high pressures, it becomes dense like a liquid, yet flows as freely as a gas -- thus, a supercritical fluid. The high pressure, however, makes chemical analysis difficult with conventional equipment.

To solve this problem, Argonne has invented nuclear magnetic resonance (NMR) devices for measuring kinetic and thermodynamic properties during catalytic reactions at high temperatures and pressures. The detailed chemical data obtained show how catalytic processes can be modified to take advantage of the unique properties of supercritical fluids. One of Argonne's family of NMR devices, a toroid cavity imager, won an R&D 100 Award in 1994.

In addition to displaying both gas-like and liquid-like properties, supercritical fluids also change sharply in density with small changes in pressure. As a result, it is possible to fine-tune the solubilities of dissolved species. These properties can improve catalysis because they lead to:

More efficient product/catalyst separation. Such separations are a problem in homogeneous catalysis and currently require energy-intensive distillations. More complete mixing. The dual nature of supercritical carbon dioxide allows better liquid-gas mixing, which could improve reactivity, product selectivity, and catalyst stability. Lower toxicity. Supercritical carbon dioxide offers an environmentally benign alternative to the use, storage, and disposal of the organic solvents that are the mainstay of conventional catalytic processes. In research funded by the U.S. Department of Energy's Office of Basic Energy Sciences, Division of Chemical Sciences, Argonne researchers are using NMR devices to find ways to employ super-critical carbon dioxide as a nontoxic substitute for organic solvents in the oxo process for hydroformylation of olefins. The oxo process produces aldehydes and alcohols.

In its work on the oxo process, Argonne has conducted in-situ NMR experiments involving propylene hydroformylation with Co₂ (CO)₈ as the catalyst. High-pressure thermochemical and kinetic measurements show that the key parameters of the reaction in carbon dioxide are comparable with those measured in conventional liquid media. The techniques used also reveal some previously unsuspected intermediate reactions. In another study, these techniques have revealed important details of hydroformy-lation with a phosphine-modified catalyst. The high sensitivity and resolution of the Argonne NMR devices, which provide a combination of capabilities available from no other instrument in the world, have made possible this new level of detail.

The laboratory is interested in collaborative work with industry to examine the feasibility of using supercritical fluids in other homogeneous catalytic processes, such as alkylations, carbonylation, hydrogenation, polymerization, oxidations, and metatheses.

For more information, contact Jerry Rathke (630-252-4549, fax 630-252-9373, rathke@cmt.anl.gov).