Chapter 1
Introduction

Leon Lefferts1, Ulf Hanefeld2 and Harry Bitter3

1Science and Technology, Universiteit Twente, Langezijds Bldg., 7500 AE Enschede, The Netherlands

2Technische Universiteit Delft, Gebouw 58, Van der Maasweg 9, 2629 HZ Delft, The Netherlands

3Biobased Chemistry and Technology, Wageningen University & Research, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands

1.1 A Few Words at the Beginning

Catalysis is at the very basis of life. The fact that this sentence could be read is due to the catalytic processes proceeding in our body, in this case biocatalytic processes. At the same time a heterogeneous catalytic process, the Haber-Bosch synthesis of ammonia, made it possible that the world has its current size of population. Without it insufficient fertilizer would exist and only half as many humans could live and they would have significantly less meat to eat. Catalysis is thus an essential science that finds its way into every aspect of our lives. It is commonly divided into three different disciplines, namely, homogeneous catalysis, heterogeneous catalysis, and biocatalysis. All these disciplines work according to the same underlying principles. This book therefore aims at explaining these principles while at the same time teaching each of the three disciplines as well as the engineering that is necessary to bring catalysis into industrial action.

1.2 Catalysis in a Nutshell

Catalysis is purely a kinetic and not a thermodynamic phenomenon. It is about speeding up reactions and lowering activation barriers, not about changing equilibria. Catalysis is a cycle in which reacting molecules bind to the catalyst, where they react to a product that subsequently desorbs and leaves the catalyst available for the next reaction sequence. Catalysis is thus a cycle of stoichiometric elementary reactions. Neither of these steps can be called catalytic in itself. It is the cyclic combination of events from which the catalyst emerges unchanged that makes the sequence catalytic. When different types of catalysts are used, these steps can be very different even when the starting material and the product are the same. Indeed, some processes have been industrialized with heterogeneous catalysis and then switched to biocatalysis or homogeneous catalysis and vice versa.

A reaction in heterogeneous catalysis starts with the adsorption of the reactants onto the surface. For example, let us consider the catalytic oxidation of CO on a noble metal such as platinum (Pt) ( Figure 1.1). Homogeneous catalysis as well as biocatalysis operates in the same manner; however, nomenclature can be rather different as will be discussed in Section 1.4. Carbon monoxide adsorbs molecularly, while dissociates. Adsorption is an exothermic process that decreases the potential energy. Next, the adsorbed CO and O react on the surface to produce , which is weakly bound and desorbs into the gas phase, leaving the surface free for the next reaction event.

Figure 1.1 Potential energy scheme of a heterogeneously catalyzed reaction: the CO oxidation.

Figure 1.1 shows how the catalyst offers an alternative pathway for the reaction, with a mechanism that is more complex than the direct gas-phase reaction, but which leads to a lower overall activation energy for the reaction. Generally according to the Arrhenius Law, the rate constant, k, of an elementary reaction depends exponentially on the activation energy

where v is the pre-exponential factor, R is the gas constant, and T is the temperature. Hence, from Figure 1.1, one realizes immediately that a decreased activation energy accelerates the reaction tremendously.

Note that the overall change in free energy is determined by the reactants and products and in no way by the catalyst. Hence, if the conversion of a gas-phase reaction under certain conditions of temperature and pressure is limited to the equilibrium concentrations of products and reactants, then a catalyst cannot alter this. A catalyst affects the kinetics of a reaction, but NOT the thermodynamics.

Another important point to note is that the catalyst offers an energetically favorable pathway not only for the forward reaction but also for the reverse one. Hence an effective catalyst used for the formation of from CO and would also be a good choice for the reverse reaction. For example,

The forward reaction is known as the steam reforming of methane to produce synthesis gas , which is an endothermic reaction carried out with nickel catalysts at high temperature. The reverse reaction is the (exothermic) methanation reaction that is applied to purify from traces of CO or even to produce substitute natural gas from coal or biomass. This reaction also utilizes nickel catalysts but at temperatures much lower than the steam reforming.

Expertise in catalysis is extremely important in the chemical industry for two main reasons. By definition, catalysts enable reactions to proceed faster so that smaller reactors as well as milder conditions, that is, temperatures and pressures, can be employed. Thus, costs to achieve the chemical conversion can be reduced. However, even more important is the fact that catalysts allow enhancement of the desired conversion, without increasing the rate of formation of undesired products. In other words, selectivity of the conversion can be improved. Therefore, more useful product can be produced per amount of feedstock, limiting the production of waste as well as the need to separate targeted products from waste produced by unselective reactions. The result is that costs can be reduced, in terms of both investment and operating costs.

1.3 History of Catalysis

Without catalysis, there would be no life. All organisms in nature, including ourselves, exist by the grace of enzymes, biocatalysts, steering the chemical processes in the organisms that allow them to act at all, including self-repair and reproduction. Obviously, mankind using catalysis to make a product is much more recent than the origin of life. Still it is a long time ago and occurred without any notion of the concept of catalysis. Enzymes in yeast have been used for 8000 years or so to convert sugars, producing products such as wine, beer, and bread.

Catalysis, more specifically biocatalysis, thus has allowed us to form societies and live as civilization. Once humans started to live closely together in large groups, safe supply of water and food and the storage thereof became very important. Usage of yeast to produce alcoholic beverages and at the same time suppressing all other pathogenic microorganisms ensured relatively safe drinking supplies. Equally, bread made with yeast allowed the production of storable produce. Lactobacillus and similar organisms that outgrow pathogens while creating acidic media and thus suppressing all other microorganisms were and are another common application of catalysis. Yoghurt, cheese, and countless other dairy products to preserve milk and sauerkraut, salami, and many other forms of food rely on this type of catalytic formation of lactic acid. Meat was made tender by wrapping it into papaya leaves, in this case proteases from the leaves digested the meat a little making it more suitable for human consumption. All of these were performed without any knowledge of the term and the meaning of catalysis.

The concept of catalysis started to develop in the nineteenth century [1]. It was observed that ethanol could either be decomposed to acetaldehyde with a pungent smell or to ethylene, depending on which solid was added. We now know that metals catalyze dehydrogenation, whereas oxides with acid functionality induce dehydration. A first human-designed application, again without knowing the concept at that time, was actually a sensor that is able to detect explosive gases. In 1817 Davy, assisted by a young Michael Faraday, discovered that a heated Pt wire would detect H2, CO, and CH4 in air as oxidation of these gases would produce so much heat that the wire would light up. This provided a way to detect these gases in coal mines before they would increase to levels within the explosion limits. Based on this the Miner's lamp was developed, an important safety device in coal mines at that stage. It was JJ Berzelius in 1835 who for the first time suggested a definition for the underlying mechanism and proposed the term catalysis, inspired by the Greek word for "loosen." It was proposed that the catalyst was able to induce decomposition of bodies like the components in mine gas. The concept was not essentially different from the concept of dissociative adsorption in surfaces as we know it now.

1.3.1 Industrial Catalysis

Despite the lack of understanding, the first industrial application, albeit on a very small scale as compared to today's standards, was developed in the early nineteenth century. In 1831, the production of sulfuric acid was patented, in which oxidation of SO2 to SO3 using finely divided Pt was a slow critical step. Much later, Pt was replaced by the much cheaper vanadium. In 1838, it was discovered that ammonia could be oxidized over Pt to produce nitric acid, which is essentially the foundation of the current process for production of nitric acid via ammonia oxidation over Pt-Rh gauzes. At that time, natural nitrate in the form of Chile saltpeter was much cheaper.

By the turn of the nineteenth century those natural nitrate resources soon ran out and N-containing salts were in high...