05 March 2015

Which catalyst is the best?!

Catalysis is an outstanding chemical phenomenon that increases the rate of chemical reaction by spending only a tiny amount of additional substance, called a catalyst. The impact of catalysis on our world is remarkable – around 90% of all commercially manufactured products involve chemical substances processed in catalytic procedures at some stage. Whatever we are surrounded by in our everyday life was produced with the help of chemical catalysis – materials, dyes, textile, fuels, paper, devices, cars' and aircrafts' parts, …, food, pharmaceuticals, cosmetics and many other.

Due to the fact that only a small amount of catalyst is required to drive a production of chemical substances, the catalytic process can be tuned to high cost-efficiency. For instance, polymerization is an excellent representative example of catalytic efficiency. The manufactured products – polyethylene, plastics, etc. – are produced at million tones scale every year and are consumed in all countries. Another point of catalysis is well reflected by preparation of pharmaceuticals and drugs. Being made at much smaller scale, these unique chemicals require exceptional structural diversity. It is the area, where an outstanding potential of catalysis is now comprehensively explored.

Such increasing growth of industrial production requires more and more catalysts to be developed. The price of metal catalysts has significantly increased and even a tiny amount of catalyst may now cost a lot. For example, the Nasdaq price for widely used catalytic metal – Palladium – increased more than 4 times during the last 6 years. Production of catalysts is the main area of consumption of palladium with many demanding applications.

Nowadays the tendency is clearly shifting towards replacing expansive metals by much cheaper analogs. A brief comparison of catalyst prices shows that such a simple metal like Nickel is superior to Pd, Pt, Au, Rh (Figure 1). On the moment these noble metals are ubiquitously used in the catalytic applications.

Figure 1. Estimates of the costs of Ni, Pd, Pt, Au and Rh catalyst precursors.

So, is it possible to replace noble metals by nickel in the catalytic applications? The desired goal appears to be rather difficult to achieve. Each metal has a unique set of chemical and physical properties that furnish a particular catalytic transformation. Replacing one catalytic metal for another is a challenging task.

Of particular interest is the observation of nickel catalyst made about a century ago by the famous chemist and Nobel Laureate Paul Sabatier: "It can be compared to a spirited horse: delicate, difficult to control, and incapable of sustainable work" (Sabatier, P. Catalysis in Organic Chemistry, NY, 1922, p. 15.). Thus, it is not a simple task, but rather a long standing question. Can the chemists force nickel to do a desired synthetic work? Although there are many successful nickel catalysts on the market, the researchers are still facing the challenge.

Figure 2. Mechanistic findings suggest that cheap and easily available nickel species can be much more chemically reactive compared to expansive palladium and platinum species. Chemists explore opportunities how to utilize this advantage in practice.

After multiple unsuccessful direct forcing attempts, chemists have made impressive progress in understanding the mechanisms of nickel-catalyzed reactions. The research provides the opportunity to understand the "spirit" of the metal and to make use of its advantages.

A recent article published in the ACS Catalysis journal highlights state-of-the-art in this field and emphasizes that we have reached a turning point in the studies of nickel catalysis. As it is discussed in the article, fundamental knowledge on the reaction mechanisms can fully uncover the power of nickel catalysts and minimize plausible drawbacks for the practical applications (Figure 2). The key advantages of the nickel catalysts, summarized in the article, include:

1) High performance in those reactions, where other metals were not efficient;

2) Large variability of electronic states – Ni(0)/Ni(I)/Ni(II)/Ni(III);

3) New reactions and transformation beyond the known limits of other metals;

4) Facile activation and transformation of chemically less-reactive molecules;

5) Excellent opportunities in photocatalytic and hybrid catalytic cycles.

Will a new Ni hero appear soon in the catalysis team? Let's give it some time.

The article "Nickel: The "Spirited Horse" of Transition Metal Catalysis" by Valentine P. Ananikov was published in ASC Catalysis (The Journal of the American Chemical Society).

Reference: ACS Catal., 2015, 5, pp. 1964 – 1971; DOI: 10.1021/acscatal.5b00072

Online link: http://dx.doi.org/10.1021/acscatal.5b00072