Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering

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mation very generally here to mean a change in composition, state, or organization of matter, or movement or rearrangement of material by flow, heat or diffusion, which describes the very essence of chemical science and technology. We need to understand the mechanisms of chemical reactions that we invent, as well as the transformations that occur in living systems and in the environment. Such an understanding will aid in the design of new transformations.

In this domain of transformations of matter, the full scope of the chemical sciences—from basic to applied to technological innovation—is easily seen. For example, it is a fundamental challenge to understand how efficient catalysts, such as nature’s enzymes, are able to perform transformations with very high speeds and selectivities. Such insight would be useful in applied chemistry to design catalysts or conditions for the transformations. Catalysis has been key to a large number of new chemical products and processes in the last half century. Catalytic chemical reactors are the central elements in chemical process systems in which thermodynamic phase behavior, flow, heat transfer, and diffusion all play key roles in performance. Many processes arising in chemical science and technology involve physical transformations in addition to, or even without, chemical reactions, such as phase change, polymer and particulate processing flows, or diffusion in liquids, solids, and membranes. This chapter provides an overview of chemical and physical transformations of matter that are the heart of the chemical science enterprise.

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A chemical transformation can occur when molecules collide with sufficient energy. Typically, reaction happens when the molecules are heated or irradiated to provide the energy necessary to overcome the activation barrier that separates reactants from products. A catalyst reduces the magnitude of the activation barrier by changing the pathway of the reaction. As described in Chapters 3, 7, and 10, catalysis is fundamental to biology, to synthesis, to manufacturing, and to energy saving and generation.

Many intermediates along the path of a reaction cannot be directly observed with current instrumental technology, making this objective one of the long-standing goals of the chemical sciences. In 1999, Ahmed Zewail received the most recent of the several Nobel Prizes recognizing developments of methods to follow fast reactions. He was able to witness the bond-breaking and bond-making process on the time scale of 10⁻¹⁵ seconds, which some say is the limiting time scale for chemical reactions. In 1994, George A Olah received a Nobel prize for work establishing the properties of reactive carbon cations, which are intermediates in many important transformations.

Whereas the detailed molecular structures involved throughout the entire procession from starting materials through the transition states to products in a chemical reaction are so far not directly observable, there are certain classical ways in

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