In practical industrial processing, reactions must take place on time scales reasonably short from a human perspective – ideally in units of hours, at the most. Compared to natural geological processes, reaction times need to be reduced by up to ten orders of magnitude. Two approaches can do this. One is to increase reaction severity, usually increasing temperature. As a rough rule, reaction rate doubles for every 10 K increase in temperature. The highest temperature encountered in fuel formation is ≈225 °C, the closing of the gas window or the fourth coalification jump. Temperatures of fuel processing are often much higher, and reaction rates are correspondingly higher. The second approach is to use a catalyst to enhance reaction rate. Of course, in many situations both strategies are used together.
A catalyst changes the rate, outcome, or both, of a reaction without appearing in the net equation for the reaction (i.e. without being consumed in the reaction, or being permanently altered by the reaction). Although catalysts often find use to enhance rate, sometimes they are used to arrive at a different set of products. This is very important in, e.g., the production of high-quality gasoline (Chapter 14). As materials, catalysts are of extreme importance. Virtually all biochemical processes in living organisms are catalyzed by enzymes. About 90% of the fuels, synthetic chemicals, and plastics produced by the chemical industry have benefited from a catalyst in at least one of their processing steps.
Chapter 2 introduced the concept of catalysis, and focused on homogeneous catalysis. For large-scale production of commodities such as fuels, a homogeneous catalyst requires separation and recovery steps downstream of the reactor, unless the catalyst either is thrown away or is allowed to dilute or contaminate the product. This adds to the complexity and expense of a process. Heterogeneous catalysts are favored by industry, especially for production of commodities. In part, this derives from a very easy, even non-existent, separation from the process stream. Many heterogeneous catalysts can withstand more severe conditions of temperature and pressure than homogeneous catalysts, especially enzymes. Heterogeneous catalysts work well for gas-phase reactions, where it might be difficult to select a homogeneous catalyst [A].