The development of lower-cost catalysts that meet critical performance characteristics could have significant economic benefits. However, the cost of the metal is not the only factor that must be considered. A catalyst that uses an inexpensive metal but expensive ligands could end up costing the same. Stevens elaborated to say that the current platinum-based catalyst is used because it has the desirable cure kinetics that enable the polymer to remain liquid for the time necessary to fill a mold. It also produces regioselectivity and chemoselectivity, resulting in high-molecular-weight polymers, which is a critical parameter.
Potential approaches to meeting these requirements with a lower-cost catalyst include identifying new silane and olefin activation chemistries and new reaction mechanisms that do not require the two-electron redox processes catalyzed so effectively by platinum. The application of new high-throughput catalyst discovery methodologies holds promise for making such advances.
Another promising area is the development of new catalysts for acetic acid production, the second largest use of homogeneous catalysis. In the 1960s, BASF launched an acetic acid process that used a cobalt catalyst, but this was replaced in 1970 with Monsanto’s greatly improved rhodium-based catalyst. The new catalyst was more highly selective and required lower pressures, reducing overall costs per pound of acetic acid. Rhodium recovery was also very high.
This catalyst, however, was replaced in the 1990s by an iridium-based catalyst developed by BP. This catalyst, which could be used in the same plant as Monsanto’s rhodium-based catalyst, had higher selectivity still and afforded better water use, thereby lowering the capital costs associated with drying columns. The lower capital costs more than made up for the increased cost of the metal used. Iridium, unlike rhodium, is not at the moment considered a critical material because it is not used in significant quantities today. If demand for iridium increases significantly, it could be considered a critical element.
The latest version of this catalyst, introduced a decade ago by Celanese, uses rhodium again but offers better iodine and water management and allows for larger plant construction. This, in turn, lowered the cost per pound of the final product. Though rhodium is a critical material, the amount used by the chemical industry to manufacture acetic acid is relatively small.
Emissions catalysts, particularly for use in cleaning up diesel exhaust, are significant users of platinum-group metals. Diesel is growing as a percentage of vehicle production worldwide, largely because of the higher fuel efficiency of diesel engines. However, diesel exhaust also has significant emissions problems. Removing NOx from diesel exhaust requires a reduction catalyst and is difficult using existing catalyst technology, whereas removal of carbon monoxide and unburned hydrocarbons requires an oxidation catalyst (Figure 3-2). Particulates must also be removed from diesel exhaust, and filters contain metal catalysts.
Today, emissions catalysts account for 81 percent of U.S. platinum-group metal imports. Emissions catalysts also use significant quantities of cerium, which acts as an oxygen buffer in NOx reduction. Looking ahead, new filter structures will require new catalysts, and there are opportunities to develop catalysts that do not use platinum-group metals. Nickel-based catalysts may prove useful, as may copper-based catalysts now that concern over the potential to produce dioxins as a byproduct has been alleviated.
Hydroformylation and Enantioselective Catalysis
The biggest application of homogeneous catalysis, in terms of the amount of product made, is in hydroformylation. The first catalysts were rolled out in the 1940s and used cobalt, a first-row transition element. Cobalt catalysis suffered from the need for high pressures and temperatures,
FIGURE 3-2 Removing pollutants from diesel emissions requires several catalysts and filters. SCR: selective catalyst reduction; LNT: lean NOx trap.
SOURCE: Stevens (2011).