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  • Print publication year: 2013
  • Online publication date: February 2013

13 - Heterogeneous catalysis

Summary

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].

Recommended reading
Bowker, Michael.The Basis and Applications of Heterogeneous Catalysis. Oxford University Press: Oxford, 1998. This short monograph on heterogeneous catalysis provides a very useful introduction to the essential principles of the field.
Bruch, L.W., Cole, Milton W., and Zaremba, Eugene.Physical Adsorption. Dover Publications: Mineola, NY, 1997. A detailed discussion of the physics of adsorption of gases on surfaces, with extensive theoretical discussions.
Gates, Bruce C. Catalytic Chemistry. Wiley: New York, 1992. An excellent textbook intended to cover most of the field of catalysis, by a world-class expert in the field. Chapter 6 relates particularly to the present chapter.
Kolasinski, Kurt W. Surface Science. Wiley: Chichester, 2008. This book provides a detailed discussion of dynamics of adsorption and desorption, surface structures, and catalysis. Chapter 6 is particularly relevant to the present chapter.
Le Page, J.F.Applied Heterogeneous Catalysis. Éditions Technip: Paris, 1987. Detailed treatment of using catalysts, beginning with selection, through preparation and properties measurement, to designing catalytic reactors.
Rothenberg, Gadi. Catalysis: Concepts and Green Applications. Wiley-VCH: Weinheim, 2008. A useful monograph that covers both homogeneous and heterogeneous catalysis. Chapters 2 and 4 are specifically relevant here.
Vannice, M. Albert. Kinetics of Catalytic Reactions. Springer: New York, 2005. As the title implies, the principal focus is on acquiring reaction rate data, kinetic analysis, and modeling reactions on surfaces.