Chapter 1
Chemical Industry and Homogeneous Catalysis

1. 1.1 Feedstocks, Fuels, and Catalysts
2. 1.2 Crude Oil to Gasoline and Basic Building Blocks by Heterogeneous Catalysts
   1. 1.2.1 Cracking Reactions
   2. 1.2.2 Hydrodesulfurization Reactions
3. 1.3 Basic Building Blocks to Downstream Products by Homogeneous Catalysis
4. 1.4 Comparison among Different Types of Catalysis
5. 1.5 Catalyst Recovery
6. 1.6 Environmental Issues
   1. 1.6.1 Background
   2. 1.6.2 Biofuel, Ethanol, and Glycerol
   3. 1.6.3 Biodegradable Plastics
   4. 1.6.4 Hydrogen and Carbon Dioxide
7. Problems
8. Bibliography

The chemical industry manufactures a very large number of products for different uses. In industrial parlance, the products are often classified under different categories such as polymers and resins, fine chemicals, flavors and fragrances, and pharmaceutical intermediates. Some of these such as plastics are produced in millions of tons, while some others less than a few tons per year. As we will see, homogeneous catalysis plays an important role at both these extremes.

It is estimated that the chemical industry contributes about 10% to the world's total trade and about 5% to the total income. It employs about 10 million employees and generates a combined turnover of more than 3 trillion dollars including from pharmaceuticals. The manufacturing processes of many of the products mentioned are critically dependent on the use of catalysts. In recent years, catalytic research has gained additional momentum for two main reasons.

First, many existing chemical products and processes have been found to have adverse effects on the environment and this has spurred search for alternatives that are more environment friendly. In this approach, catalysis plays a pivotal role. Second, catalysts help to save energy and to avoid the formation of unwanted products. Thus the use of catalysts for new chemical processes makes them commercially attractive.

1.1 Feedstocks, Fuels, and Catalysts

The manufacture of all organic chemicals and carbon-based polymers requires a carbon-containing precursor, i.e., a feedstock. The main feedstocks of the chemical industry are crude oil, other oils that are difficult to process, coal, and natural gas. These feedstocks are also used to meet much of today's worldwide energy requirements. To emphasize their geological origin and finite availability, crude oil, coal, etc., are referred to as fossil fuels.

Crude oil is currently the main feedstock used by the oil industry to manufacture processed petroleum products such as petrol, diesel, kerosene, and aviation fuel. Of the total amount of available crude oil, only about 10% is used for the manufacture of chemicals and the rest are used as fuels. Basically, crude oil is a complex physical mixture of many hydrocarbons where the number of carbon atoms per molecule could be as high as 60 or more.

The phenomenological definition of a catalyst is a substance that accelerates a chemical reaction but in the process does not undergo any chemical change itself. Catalysis plays a critical role not just in the oil and chemical industries but also in the manufacture of many inorganic chemicals, pollution abatement, and fuel cells. At a rough estimate, more than 75% of all existing industrial chemical transformations and 90% of newly developed processes involve the use of catalysts.

In most of these applications, the catalysts are insoluble solids and are called heterogeneous catalysts. In this book we deal almost exclusively with homogeneous catalytic processes. These are processes in which soluble catalysts are used and the catalytic reactions take place in the liquid phase. However, both heterogeneous and homogeneous catalysts operate by reducing the energy required to bring about the reorganization and changes of molecular structures of the reactants.

At a molecular level, most homogeneous catalysts are well characterized in terms of their chemical composition and structure. As all the molecules of a given homogeneous catalyst have the same structure, they facilitate breaking, forming, and reorganization of chemical bonds of the reactants in an identical manner. In contrast, in heterogeneous catalytic processes the molecules of the gaseous or liquid reactants are adsorbed on the surfaces of the solid catalysts. Unlike homogeneous catalysts, solid surfaces consist of an infinite array of ions or atoms with different types of local structures, i.e., potential reaction sites. To emphasize the homogeneity at a molecular level, some homogeneous catalysts are also called single site catalysts.

1.2 Crude Oil to Gasoline and Basic Building Blocks by Heterogeneous Catalysts

To put the importance of homogeneous catalysis in perspective, we first present a very brief summary of the basic processes of the petrochemical industry. Most of these processes are catalytic, and the goal is to convert crude oil to gasoline, other fuels, and basic building blocks for downstream chemicals.

Crude oil is composed of many hydrocarbons that differ in the number of carbon atoms per molecule. As the number of carbon atoms increases, the boiling point (BP) also increases. The BP and the number of carbon atoms per molecule in crude oil typically range from <30°C to >610°C and 1 to >60, respectively. By subjecting crude oil to fractional distillation, the major components such as crude gasoline (~5–12), naphtha (~8–12), kerosene (~11–13), and diesel (~13–17) are separated. The approximate number of carbon atoms of the hydrocarbons present in these components is given in the parentheses.

As shown in Figure 1.1, in the oil and petrochemical industry, the catalytic conversion of crude oil to hydrogen, usable grades of fuel, and small organic molecules is very important. Hydrogen is produced by a reaction called steam reformation (see Section 1.6.4) and the mixture of CO and H2 is called synthesis gas. Small molecules such as ethylene, propylene, and benzene are produced by subjecting naphtha to catalytic cracking.

Figure 1.1 Conversion of crude oil to gasoline, other fuels, and basic building blocks for most chemicals.

The small organic molecules, aromatics, and the mixture of CO and hydrogen, or synthesis gas, are the base chemicals or basic building blocks for most chemicals. The production of high octane gasoline and/or diesel with low sulfur involves distillation followed by two heterogeneous catalytic reactions: hydrodesulfurization (HDS) and reformation. Because of their enormous importance in the chemical industry, brief descriptions of cracking and HDS reactions are given.

1.2.1 Cracking Reactions

As the name suggests, in cracking, high molecular weight organic molecules are broken down into molecules of low molecular weights. Cracking could be induced thermally, but when catalysts are used the amount of gasoline produced increases significantly.

Fluid catalytic cracking or FCC is a widely used technology where the reactor is a vertical or upward sloped pipe. In the reactor, finely powdered heterogeneous catalyst particles are brought into contact with crude oil for a short time and at a high temperature (>650°C). This is achieved by spraying the crude oil upward through the catalyst bed. On contact with the hot catalyst particles, the oil vaporizes and the cracking reactions start.

For cracking reactions, combinations of zeolites, alumina, clay, and silica are used as the catalyst. These acidic materials, which contain both BrØnstead and Lewis acidic sites, initiate a complex set of carbonium- and carbenium ion–based reactions. Note that carbonium ions are protonated alkyl groups (e.g., C2H6+), while carbenium ions refer to alkyl cations (e.g., C2H5+). To enhance the acidic properties, rare earth ions such as La3+ and Ce3+ are often incorporated in the zeolites by ion exchange. FCC catalysts must have high acidity, and their bulk density, particle size distribution, porosity, structural strength, etc., must meet stringent specifications.

During the cracking reaction, coke, which is basically a complex mixture of hydrocarbons with very high carbon content, is deposited on the catalyst particles. This reduces the activity and selectivity of the catalyst very significantly. The catalyst is regenerated by burning the deposited coke with oxygen or air. As cracking is an endothermic reaction, the energy obtained by burning coke is used to supply the heat for the cracking reaction. A variant of catalytic cracking is hydrocracking where hydrogen is mixed with crude oil.

1.2.2 Hydrodesulfurization Reactions

In crude oil, along with the hydrocarbons, small amounts of sulfur- and nitrogen-containing organic compounds are also present. Typically, the nitrogen and sulfur contents in terms of elemental composition range from ~0.2–3% to 0.5–6%, respectively.

Hydrodesulfurization belongs to the general catalytic process called hydrotreatment where heteroatoms are removed from the hydrocarbons of crude oil by reaction with hydrogen. Removal of sulfur from crude oil and its cracked products is extremely important for two reasons. First, on burning all...