1
Introduction

This chapter outlines the principles of green chemistry, and explains the connection between catalysis and sustainable development. It covers the concepts of environmental impact, atom economy, and life-cycle analysis, with hands-on examples. Then it introduces the reader to heterogeneous catalysis, homogeneous catalysis, and biocatalysis, explaining what catalysis is and why it is important. The last two sections give an overview of the tools used in catalysis research, and a list of recommended books on specialized subjects in catalysis.

1.1 Green Chemistry and Sustainable Development

In the 20th century, chemistry changed our lives. It has shaped our modern technological society by supplying us with energy, medicines, crop protection, foodstuffs, and new materials worldwide. Unfortunately, even though chemistry is the science with the highest impact on our everyday lives, chemicals and the chemical industry have a poor public image. This is partly due to misconceptions and media scares, but there is also a valid reason: the traditional chemical industry, certainly until the 1980s, was a hazardous and polluting one. It generated stoichiometric amounts of waste, causing much pollution of both air and water. A number of major chemical accidents have reinforced this image in recent decades [1,2]. The most infamous are the Bhopal catastrophe in 1984, where 3000 people were killed and more than 40 000 injured [1], and the grounding of the Exxon Valdez [3] in the Prince William Sound in Alaska in 1989, that still affects the marine ecosystem nearly 20 years later [4].

Apart from the immediate health and environmental hazards, there is also the problem of resource management. The chemical industry during the past 200 years drew heavily on resources. Today, the escalating costs of petrochemicals, and the increasing energy and raw material demands in Asia's emerging markets, are forcing a change. Two popular terms associated with this change are sustainability, or sustainable development. A sustainable society is one that "meets the needs of the current generation without sacrificing the ability to meet the needs of future generations." Sustainable development is a strategic goal. It can be reached using various approaches, and this is where green chemistry comes in. Figure 1.1 shows the relationship between the strategic goals, the practical approaches, and the operational and monitoring tools. Thus, green chemistry is just one step (albeit an important one) along the road to sustainability.

Figure 1.1 The strategic goal of sustainable development relies on practical approaches such as green chemistry, industrial ecology, and green engineering. These approaches use various operational tools (e.g., catalysis), and monitoring tools (e.g., life-cycle assessment).

1.1.1 What is "Green Chemistry"?

In the 1990s, the concept of "green chemistry" was initiated in both the US and Europe, and has since been adopted widely by the chemical industry [5]. Green chemistry deals with designing chemical products and processes that generate and use fewer (or preferably no) hazardous substances. By applying the principles of green chemistry, companies embrace cleaner and more efficient technologies, with an a priori commitment to a cleaner and healthier environment. The green chemistry message is simple: "Seek prevention, not cure." In 1998, Anastas and Warner formulated the following 12 principles of green chemistry [6,7] (I have rephrased these in the active voice, in keeping with the spirit of this book):

• - Prevent waste instead of treating it.
• - Design atom-efficient synthetic methods.
• - Choose synthetic routes using nontoxic compounds where possible.
• - Design new products that preserve functionality while reducing toxicity.
• - Minimize the use of auxiliary reagents and solvents.
• - Design processes with minimal energy requirements.
• - Preferably use renewable raw materials.
• - Avoid unnecessary derivatization.
• - Replace stoichiometric reagents with catalytic cycles.
• - Design new products with biodegradable capabilities.
• - Develop real-time and on-line process analysis and monitoring methods.
• - Choose feedstocks and design processes that minimize the chance of accidents.

Green chemistry offers an alternative to the traditional environmental protection agenda, mainly because it deals with avoiding hazards, rather than treating and solving exposure problems. Three forces drive the green chemistry initiative [8]: Government legislation, societal pressure, and economic benefit (Figure 1.2). The new 800-page EU Directive on the registration, evaluation, and assessment of chemicals (REACH) [9] is changing the chemical industry across Europe. Similar regulations are expected worldwide in the coming decade. But legislation is just one of the drivers. Societal pressure is also important: the public favors industrial green chemistry initiatives, as they lead to safer and eco-friendly products and processes.

Figure 1.2 Green chemistry initiatives are driven by government legislation, societal pressure, and economic benefits.

This creates goodwill for the company, something that is difficult to quantify but undoubtedly important.

The third driver is bona fide economic benefit. Applying the principles of green chemistry decreases both capital investment and operating costs. If you use less (or no) solvent, and replace stoichiometric reagents with catalytic cycles, your reactor space-time yields go up. Similarly, processes running at ambient temperatures are less energy-intensive. This means doing the same work using smaller and safer equipment. Eliminating waste also eliminates the need for waste treatment and disposal. Replacing toxic reagents with benign ones saves on safety costs during transportation and storage. Thus, more and more companies are adopting green chemistry because it simply improves their bottom-line performance.

1.1.2 Quantifying Environmental Impact: Efficiency, E-factors, and Atom Economy

Everyone agrees that green chemistry and "green manufacture" are good things. The websites and pamphlets of all the major chemical companies emphasize their concern for the environment. They all say that their processes and products are "efficient," "green," and "environmentally friendly." But how should we compare these processes? How should we judge such claims?

First, let us define some key terms. One method for quantifying a reaction's efficiency is by examining the reactant conversion, the product selectivity, and the product yield over time. The reactant conversion is the fraction of reactant molecules that have transformed to product molecules (regardless of which product it is). The selectivity to product P is the fraction (or percentage) of the converted reactant that has turned into this specific product P. The yield of P is simply conversion × selectivity. High conversions in short time spans mean smaller and safer reactors. Similarly, high selectivity means less waste, and simpler and cheaper separation units. Thus, conversion, selectivity, and yield are all measures of the reaction efficiency.

In addition, there are specific rulers for measuring the "greenness" or "eco-friendliness" of processes and products. One such measure is the E-factor, introduced by Roger Sheldon in 1994 [10,11]. A reaction's E-factor is the quotient kgwaste/kgproduct (here "waste" is everything formed in the reaction except the desired product). The waste can be gases such as CO2 or NOx, water, common inorganic salts (e.g., NaCl, Na2SO4, or (NH4)2SO4), heavy metal salts, and/or organic compounds. Table 1.1 compares the production tonnage and E-factors of various industrial sectors. Note that the petrochemicals and the bulk chemicals sectors are the least polluting. This is surprising, as we are used to thinking of such chemicals as pollutants. In fact, E-factors increase substantially when going from bulk chemicals to fine chemicals and specialties. This is partly because fine-chemicals production often involves multistep syntheses, and partly because stoichiometric reagents are more often used for producing fine chemicals and pharmaceuticals.

Table 1.1 Annual production and E-factors in the chemical industry.

The concept of atom economy, introduced by Barry Trost in 1991, is similar to that of the E-factor [12]. Here one considers how many and which atoms of the reactants are incorporated into the products. With these two concepts, we can evaluate chemical reactions to get a quantitative...