During the refining of petroleum, large hydrocarbon molecules are broken down into smaller ones in a process known as cracking. The amount of gasoline that can ultimately be produced from a barrel of oil depends on how efficiently cracking is performed. If carried out incorrectly, cracking can lead to the formation of gases such as methane and ethane, and high-molecular-weight components called residue, which cannot be used to make gasoline or other transportation fuels. Today, through the use of highly optimized catalysts, more than 70% of the cracked products end up as transportation fuels.

The story began in 1936 when acid-washed natural clays were first employed as catalysts. Subsequent research revealed that higher cracking efficiencies could be achieved by using amorphous silica-alumina. In the late 1950s and early 1960s, significantly greater efficiencies were found to be possible by using cracking catalysts based on zeolites. These materials are crystalline solids containing pores and cavities of molecular dimensions. The interior surfaces of the zeolite contain highly acidic centers that serve as the active sites for cracking petroleum (Figure 2.1). Mobil Corp. introduced the first zeolite-containing cracking catalyst in the early 1960s, and today virtually every refinery in the world uses catalysts containing Y-zeolite and, in some instances, ZSM-5.

The use of zeolite catalysts has greatly benefited the U.S. balance of payments, because the improved efficiency in cracking has permitted a savings of more than 400 million barrels of oil per year, or more than $8 billion a year at $20 a barrel, and the story continues with new catalysts showing promise of even greater selectivity. For example, a mere 1% shift in product selectivity to gasoline allows a reduction in oil imports by more than 22 million barrels of crude per year or more than $400 million in the U.S. balance of payments.

Looking into the future, one can see many exciting challenges and opportunities for developing totally new catalytic technologies and for further improving existing ones. Increasing public concern with the effects of chemicals and industrial emissions on the environment calls for the discovery and development of processes that eliminate, or at least minimize, the use and release of hazardous materials. Concern with the environment and the supply of raw materials is also focusing attention on the opportunities for recycling. Of particular interest for the chemical industry is the prospect of producing polymers that are readily recyclable. Although the world supply of petroleum is adequate for current demand, there is a need to continue the search for technologies that will permit the conversion of

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