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Part 2:

The Pros and Cons of Cheap Metals
In addition to the large price advantage that comes with substituting a prevalent, cheap metal for a rare, expensive metal, cheap metals are often environmentally more benign. Losses of metal are more easily tolerated in an industrial process, which can reduce or eliminate the recycling steps that are almost mandatory with expensive metal catalysts. In the pharmaceutical industry, the Food and Drug Administration may or may not allow trace levels of residual catalyst in a final drug product. As Bullock stated, “How much palladium can you have in a pharmaceutical body compared to how much iron?”

The reasons that more cheap metal catalysts are not widely used today are many, and Bullock listed several of them. One reason is that reactions catalyzed by cheap metals have not been widely studied to date, though they are receiving more attention now. Another reason is that the selectivity of cheap metal catalysts is not as good as is obtained with palladium catalysts, and the scope of the reactions is not as broad. Boosting the activity of cheap metal catalysts can mean using more expensive ligands; for example, catalysts based on aryl iodides are more reactive, but more expensive, than aryl chlorides.

Cheap metal catalysts are often less tolerant of functional groups on the reactants. A reaction that works with an ester moiety present may not work when an alcohol or carboxylic acid functional group is present. In contrast, palladium-based catalysts often work with a wide range of modified starting materials. In addition, cheap metals may require a higher catalyst loading than when palladium is used, negating some of the cost advantage. Bullock added, though, that this may be a result of the fact that cheap metal catalysis has not been studied as exhaustively as has palladium-based catalysis, and that additional research is likely to make headway on this problem.

The final problem facing cheap metal catalysts is one of motivation. For a pharmaceutical company making a high-value-added drug at small scale, and for which catalyst cost is not a major factor in the final price of the drug, there is often little motivation to expend research dollars solving a relatively small problem.

National Research Council (US) Chemical Sciences Roundtable.
Washington (DC): National Academies Press (US); 2012.

Part 1:

Replacing critical materials with abundant materials, particularly in applications that use large amounts of catalysts, would have many benefits. Abundant materials are cheaper, less susceptible to supply fluctuations, and more environmentally benign. Cheap and abundant metals also can be less selective, less tolerant of functional groups, and use more expensive ligands than rare and expensive metals, but research gradually is reducing these shortcomings.

A particular application discussed in this chapter is the use of precious metals in automotive catalytic converters. The automotive industry is a major user of platinum, palladium, and rhodium in catalytic converters, which has spurred research on the use of other types of materials as catalysts. Although no good alternatives to the use of these materials yet exist, promising approaches are being investigated.
In many important processes powered by a homogeneous catalyst, the cost of the catalyst’s metal component is a small part of the overall expense. Nonetheless, chemists are developing novel reaction schemes that use homogeneous catalysts made with “cheap metals,” said Morris Bullock, Laboratory Fellow and Director of the Center for Molecular Electrocatalysis at the Pacific Northwest National Laboratory (PNNL). These efforts are centered on using abundant, inexpensive metals—mostly first-row metals, but also molybdenum and tungsten—to replace precious metals.

Even in cases where an expensive metal is a fraction of a catalyst’s total cost, creating efficient catalysts from inexpensive metals is likely to produce significant savings, said Bullock. Platinum, on a per mole basis, is approximately 4,000 times more expensive than nickel and 10,000 times more expensive than iron. Similarly, palladium is 3,000 times more expensive than copper, while ruthenium is 2,000 times more expensive than iron.

Palladium-based homogenous catalysis, in particular, is of critical importance in the pharmaceutical and agricultural industries for forming carbon-carbon bonds. The 2010 Nobel Prize in Chemistry was awarded for palladium-catalyzed cross-coupling reactions, which can be used to make virtually any type of carbon-carbon bond needed. The powerful Buckwald-Hartwig carbon-nitrogen bond-forming reactions are another class of palladium-catalyzed chemistries used widely in the pharmaceutical and agricultural industries (Hartwig, 1998; Wolfe et al., 1998). This latter set of reactions, Bullock noted, uses palladium loadings as low as 10 parts per million (ppm), so the expense of the precious metal in this case is not a significant factor.

By Mark Lessard
06.12.2018
Thermo Fisher Scientific

Catalytic converters are pollution control devices coated with chemicals and a combination of the platinum group metals (PGM) platinum (Pt), rhodium (Rh) and/or palladium (Pd). The PGMs are responsible for the conversion reactions that turn pollutants into harmless gases. Most present-day vehicles that run on gasoline, including more than 98% of new cars sold worldwide each year, as well as trucks, buses, trains, motorcycles, and planes have exhaust systems with a catalytic converter.

Given the high level of demand and the scarcity of supply, efforts to recover PGMs from spent catalytic converters and other industrial products have been underway for several years. Recycling efforts must be combined with careful elemental analysis of the recovered metal to determine its exact chemical composition and to ensure the metal is free from contaminants or hazardous materials. X-ray fluorescence (XRF), a non-destructive analytical technique used to determine qualitative and quantitative analysis of materials, is a widely-used technology for this type of analysis.

An alternative to recycling expensive PGMs from catalytic converters is to use less of the metals. Researchers from Washington State University and Tufts University have made progress on this front by demonstrating that a single metal atom can act as a catalyst in converting carbon monoxide into carbon dioxide, the chemical reaction performed by catalytic converters to remove harmful gases from car exhaust.

According to a press release on the Washington State University web site, car companies have struggled to meet strict emissions standards because as engines have become more efficient, their combustion temperature has become lower, making it harder for catalytic converters to work and creating even more harmful emissions. While studying low-temperature catalysts, the researchers tested the ability of single metal atoms to act as catalysts at lower temperatures and demonstrated that the reaction can work with single platinum atoms on a copper oxide support near room temperature. The single platinum atom holds the carbon monoxide in place while the copper oxide supplies the oxygen to convert it into carbon dioxide. Other cheaper metals such as copper will be studied next. The research was published in the journal Nature Catalysis.

https://www.thermofisher.com/blog/metals/new-reduced-platinum-catalyst-for-catalytic-converters/