Hydrogenation without Gases

By simultaneously generating both hydrogen and supercritical carbon dioxide, a new continuous hydrogenation process avoids the problems of handling gases under pressure.

Developed by chemists at the University of Nottingham, in England, the gasless laboratory technology relies on the decomposition of liquid formic acid, HCO₂H, over a heated platinum or palladium catalyst at 450 °C in a miniature reactor. The resulting H₂ and supercritical CO₂ are mixed with the material to be hydrogenated, then passed over a noble-metal catalyst in a second reactor.

"The gasless equipment is simple to use and eliminates the need for high-pressure gas cylinders," says chemistry professor Martyn Poliakoff, who developed the process with postdoc Jason R. Hyde [Chem. Commun., published online May 27, http://www.rsc.org/is/journals/current/chemcomm/ccadvarts.htm].

Decomposition of HCO₂H yields H₂ and CO₂ in a 1:1 ratio. Feeding liquid ethyl formate, HCO₂C₂H₅, into the first reactor, where it decomposes to CO₂ and ethane, can lower the H₂ concentration. The parallel decomposition of HCO₂H and HCO₂C₂H₅ allows control of the H₂ concentration in the supercritical CO₂/C₂H₆ mixture.

Decomposition of HCO₂C₂H₅ in the absence of HCO₂H opens up possibilities for carrying out other supercriticial fluid reactions. Hyde and Poliakoff have demonstrated, for example, that the equipment can be used for acid-catalyzed Friedel-Craft alkylations.

The Nottingham team members have been collaborating with HEL, a company based in Hertfordshire, England, that produces research-scale automated equipment, to develop the technology. HEL will launch a commercial version of the equipment next month.

"Our gasless technology unit eliminates the need to store, meter, and control gases," says HEL Managing Director Jasbir Singh. "The unit contains the electronics, the pumps, the pressure and temperature controls, and even an embedded computer with preloaded software."

The equipment can be used to automatically vary the process parameters for hydrogenations, polymerizations, and for other processes, he points out.

David J. Cole-Hamilton, a chemistry professor at the University of St. Andrews, in Scotland, and an expert on supercritical chemistry, believes the technology provides an attractive new route into supercritical reactions.

"Although industry has learned to handle pressurized gases, they represent a considerable hazard," he says. "Any method for reducing their inventory holds significant safety advantages. The Chemical Communications paper presents an elegant method for doing this."

Problems still need to be overcome, however. For example, HCO₂H decomposition leads to the generation of small quantities of carbon monoxide and water. If this decomposition is not controlled, "the concentration of CO in the gases generated will rise," the authors note. "Currently, we are investigating how the presence of CO in the H₂/CO₂ mixture can be exploited for gasless supercritical hydroformylation."