CATALYSIS ON FUEL CHEMISTRY BASIC INFORMATION AND TUTORIALS

CATALYSIS AND ITS APPLICATION IN FUEL CHEMISTRY
What is Catalysis?

The topic of catalysis recurs throughout fuel chemistry. A catalyst increases the rate of a chemical reaction without itself being permanently altered by the reaction, or appearing among the products. The key word is rate. Catalysts affect reaction kinetics.

A catalyst affects reaction rate by providing a different mechanism for the reaction, usually one that has a markedly lower activation energy than that of the non-catalyzed reaction. Catalysts do not change reaction thermodynamics; they do not alter the position of equilibrium [A], but they can help reach equilibrium much more quickly. And, they cannot cause a thermodynamically unfavorable reaction to occur.

Catalysts can be classified as homogeneous, in the same phase as the reactants and products, and heterogeneous, in a separate phase. Homogeneous catalysts mix intimately with the reactants. This good mixing often leads to enormous rate enhancements, in some cases by more than eight orders of magnitude.

But, because they are in the same phase as the reactants and products, industrial use would require a separation operation for catalyst recovery downstream of the reaction, unless one were willing to throw away the catalyst (possibly allowing it to contaminate the products) as it passes through the reactor. For many catalytic processes, the catalyst costs much more than the reactants do, so loss of the catalyst would result in a significant economic penalty.

Usually, heterogeneous catalysts have no major separation problems, thanks to their being in a separate phase from reactants and products. However, because of their being in a separate phase, mass-transfer limitations can hold up access of the reactants to the catalyst, or hold up departure of products. Heterogeneous catalysis can also be affected by various problems at the catalyst surface.

Large-scale industrial processing almost always favors use of heterogeneous catalysts, to avoid possibly difficult downstream separation issues. Nevertheless, steady progress is being made in finding ways to overcome separation problems with homogeneous catalysts, including, as examples, membrane separation, selective crystallization, and use of supercritical solvents.

While, by definition, a catalyst remains unchanged at the end of a reaction, it can, and often does, change during a reaction. Mechanisms of many catalytic reactions often involve many steps, which collectively comprise the catalytic cycle. The catalyst might undergo change during one or more of the elementary reaction steps of the mechanism, but at the end, when its action is complete, the catalyst must emerge in its original form, ready for another catalytic cycle.

Most homogeneous catalytic reactions occur in the liquid phase. Some reactions can be catalyzed in the gas phase by homogeneous catalysts (which, because they are homogeneous, must be gases themselves). Probably the most important example of homogeneous catalysis in the gas phase is the chlorine-catalyzed decomposition of ozone, the reaction responsible for the so-called ozone hole in the atmosphere [B].

Various parameters can be used to describe quantitatively the quality or “goodness” of a catalyst. Turnover number, and the related turnover frequency, compare the efficiency of different catalysts. Turnover number indicates the number of molecules of reactant that one molecule of catalyst can convert into product.

The term “turnover” comes from the notion that the catalytic conversion is “turning over” reactant molecules into product molecules. Turnover frequency is the turnover number expressed per unit time. Selectivity expresses the fraction of the desired product, usually in weight percent or mole percent, among all products of the reaction. Ideally, selectivity should be as close to 1, or 100%, as possible. Catalyst activity can be broadly defined in terms of rate of consumption of the reactant(s) or rate of formation of products.