For more than a decade, a core set of chemists and chemical engineers has been developing novel reaction systems that combine the best features of homogeneous and heterogeneous catalysis. The goal is to come up with greener industrial processes that couple homogeneous conditions for improved reaction rates and selectivity with heterogeneous conditions for ease of product isolation and catalyst recycling.
In recent weeks, two research groups have reported important contributions to help advance the field. They describe two different processes utilizing supercritical carbon dioxide as the solvent to aid the efficiency of product separation.
The unifying theme of these examples is that they operate at milder pressures than is typical for supercritical CO2 processes. Under the milder conditions, the researchers can manipulate the physical and chemical properties of the systems, such as solvent-phase behavior. Such control makes the processes potentially more viable for industrial applications where they could help reduce the loss of catalyst by leaching, reduce the use of conventional organic solvents, and reduce the energy needed for separations.
In one case, chemistry professor Martyn Poliakoff and coworkers at the University of Nottingham, in England, have devised a continuous process for hydrogenating levulinic acid to γ-valerolactone using water and supercritical CO2 as the solvent system (Chem. Commun. 2007, 4632). By carefully controlling pressure in a separation chamber, the team can separate pure γ-valerolactone with reduced energy requirements relative to conventional separation processing, such as distillation.
In a second case, chemistry professor David J. Cole-Hamilton and coworkers at the University of St. Andrews, in Scotland, report a method that permits hydroformylation of medium-chain-length alkenes to aldehydes in a continuous process while maintaining the homogeneous catalyst in the reactor (Dalton Trans., DOI: 10.1039/b712683b). The researchers extract the aldehyde with supercritical CO2 as it is being formed, which is effective because the catalyst isn't soluble in CO2.
Supercritical CO2 and supercritical or near-critical water have been shown to be effective media for industrially important continuous catalytic reactions, including alkylations, hydroformylations, and hydrogenations. But there are still some limitations, Poliakoff points out.
One problem is the poor ability of nonpolar CO2 to dissolve polar compounds. Another is the need to pump solid substrates into the reactor in some cases, which means a polar cosolvent is often necessary. At the end of the reaction, the cosolvent has to be separated from the product, which reduces process efficiency. Poliakoff and coworkers devised a new approach that works around these problems to integrate the reaction and separation into a single continuous process.
Levulinic acid is a promising biomass-derived feedstock obtained by acid-catalyzed dehydration of hexose sugars, Poliakoff explains. Conversion of the linear molecule to γ-valerolactone proceeds via hydrogenation and subsequent intramolecular cyclization with elimination of a water molecule. As for γ-valerolactone, it's a candidate to make renewable transportation fuels and to serve as a commodity chemical building block (C&EN, Aug. 21, 2006, page 47).
A patented continuous process for making γ-valerolactone already exists, the researchers point out. But because levulinic acid is solid at room temperature, it has to be dissolved in dioxane in order to pump it into the reactor. In the end, γ-valerolactone has to be separated from the dioxane, water by-product, and unreacted levulinic acid. Poliakoff's group discovered that the separation step can be eliminated simply by replacing the dioxane with water.
To test the idea, the team used a concentrated mixture of levulinic acid in water, which can be readily pumped into a continuous reactor and mixed under pressure with CO2 and H2. Using a ruthenium catalyst immobilized on silica, which is packed into the reactor, the reaction provides essentially 100% yield of γ-valerolactone.
The product, which is miscible with water, is transported into a separation chamber as it forms. This is the point where CO2 proves advantageous, Poliakoff notes. By reducing the temperature to subcritical conditions in the separation chamber, it's possible to effect a liquid-liquid separation of γ-valerolactone from water and unreacted levulinic acid, he says. As the conditions are manipulated, γ-valerolactone partitions into the CO2, leaving behind water and unreacted levulinic acid, which can be drained off and recycled to the front of the reactor.
Asked to comment on the work, Philip G. Jessop, a chemistry professor at Queen's University, Kingston, Ontario, retorts: "How can you use water to deliver the reagents into a flow reactor but not have water come out with the product?" This would seem quite difficult, especially if the product is water-soluble, he says.
"But Poliakoff and coworkers have shown that CO2 is the answer," Jessop continues. This solution is based on previous phase-behavior observations by other researchers showing that CO2 can force organic liquids out of water, he says.
"The work by Poliakoff's group represents a very clever demonstration of how CO2 and H2O, two green solvents, may be elegantly exploited to combine reaction, separation, and recycle steps," adds Bala Subramaniam, a chemical engineering professor at the University of Kansas, Lawrence. "The key to this demonstration is the recognition of the ideal phase behavior exhibited by this system for separation."
Overall, the Nottingham separation process eliminates the need to distill γ-valerolactone and does so without any additional energy requirements, Poliakoff points out. Supercritical CO2 reactions typically are more energy-intensive than conventional reactions because of the high pressures needed, he says. So if the separation step can be integrated, the high-pressure process becomes considerably more attractive from an energy point of view. The researchers believe their strategy could be applied to a range of reactions in which water is a by-product.
Hydroformylation of alkenes, the target reaction of Cole-Hamilton's group, is an important industrial reaction for production of aldehydes and alcohols that are used to make soaps, detergents, and plasticizers. Supercritical CO2 is already being used as a solvent for these reactions, but one issue for homogeneous catalytic processes is that the catalysts tend not to be soluble in CO2. Attempts to work around this problem have included using catalysts immobilized on solid supports and using cosolvents to create biphasic solvent systems.
Subramaniam's group previously has shown that the medium-chain-length olefin starting material can dissolve hydroformylation catalysts for CO2-mediated processes without the need for a cosolvent. Cole-Hamilton's group, in turn, has discovered that their hydroformylation catalysts are soluble in the aldehyde products.
Cole-Hamilton and coworkers reasoned that they could use an aldehyde to initially help solubilize the homogeneous catalyst and then, in a novel twist, remove the accumulating product aldehyde from the reactor by using supercritical CO2. By balancing the rate of addition of fresh olefin substrate with the removal of the aldehyde product, the catalyst remains immobilized in the steady-state reaction mixture. The team has been working during the past couple of years to build a new reactor system to develop and refine the process.
The researchers now report their success with the octene-to-nonanal system. The team first mixed octene with nonanal and the catalyst precursors in the reaction chamber. The soluble rhodium catalyst forms in situ while the reactor is being pressurized with H2, CO, and CO2. Once the reaction begins, supercritical CO2 continuously transports the reactants into the reactor. And because nonanal has a preference for CO2 over the reaction mixture, it can be whisked out of the reactor as it forms and separated from the CO2 gas.
The reaction conditions are optimized for very high conversion of octene, which means the researchers obtain nearly pure aldehyde and very little unreacted starting material. They are now achieving results approaching that of commercial hydroformylations, Cole-Hamilton notes, and they plan to begin extending the methodology to a variety of other reactions.
Because homogeneous catalysts are difficult to separate from products and even more difficult to use in continuous-flow processes, researchers have resorted to inventing techniques for "heterogenizing" homogeneous catalysts, Jessop observes. Attaching the catalyst to a solid support is one way, but doing so without changing catalyst performance is very difficult, he says.
"Cole-Hamilton and coworkers have come up with a brilliant and counterintuitive twist on the concept—dissolve the catalyst in the alkene-aldehyde mix and use supercritical CO2 to pull out the product," Jessop says. "Their catalyst remains in the reaction mixture because the catalyst was designed to not be soluble in CO2."
The researchers need no solvent other than the CO2, he adds. That could be fortuitous for the future of CO2-based processes since the gas is relatively inexpensive and has the added benefit of being available as a recycled material from electric power plants, Jessop notes.