Reactions in Confined Spaces
Chemical reactions are crucial for our daily life. They play an essential role in our cellular processes, thereby creating the essential pathways for the synthesis of essential building blocks, such as amino acids and sugars, including their oligomers and polymers. Enzymes, the catalysts in nature, are responsible for the production of these building blocks, while health issues are often associated with the improper functioning of the metabolic pathways catalyzed by enzymes. All reactions between the molecules of life are taking place in confined spaces, such as cells and organelles. An interesting example of this is the mitochondrion; the energy factory of the cell. When the space surrounding molecules becomes restricted their reactivity and related behavior will be altered, and the role of water as solvent, including effects of local pH and ionic strength gradients, becomes dominant. Such chemical processes are, for example, critical in the so-called key-lock mechanism of enzymes, where hydrogen bonding as well as other chemical and physical interactions become a crucial component of both catalytic activity and selectivity.
Similar confinement effects are encountered when we consider the working principles of porous solids, like zeolites and metal-organic-frameworks. This themed issue in ChemPhysChem comprises a selection of 22 articles devoted to the topic of “Reactions in Confined Spaces” where there is a clear emphasis on the physical chemistry aspects of the interaction between molecules and the local environment within porous solids. The variety of articles strives to represent the diverse research activities that are currently focusing on chemical reactivity and catalysis as well as molecular diffusion and mobility. It is clear that the organic/inorganic host influences by confinement the size and type of organic molecules formed during catalysis as well as synthesis.
An example being the confinement effects taking place when hydrocarbon pool molecules are formed within zeolite cages and channels during the methanol-to-olefins process. Here, the subtle, but important distinction has to be made between active species, for generating, for example, ethylene and propylene, and deactivating species, allowed to be formed within the pore space of the zeolite materials. Another example of such confinement is shown in Figure 1, where a Wheland reaction intermediate is formed within the restricted space of the microporous channels of zeolite ZSM-5 during the reaction of benzene with ethanol. Clearly, advanced characterization tools have helped us to better understand these effects. Often, inspiration for these tools has been drawn from methods already fully exploited in life sciences (e.g., fluorescence microscopy). Another important theme examined is the influence of confined molecules, including solvent molecules, on the reactivity of the porous solid or even, in a more direct manner, on the type of confined space that is formed around a specific guest molecule. An example of the latter is the role that template molecules (or structure directing agents) play during the synthesis of microporous solids. Here, some similar properties as for the key-lock principle with enzymes can be envisaged (e.g., different template conformers and influence of pH on the protonation of the template molecule). Of course, articles on metal organic frameworks (MOFs) are also included, as they provide us with an expanding list of well-designed porous structures and new opportunities for in-depth understanding of interactions of guest molecules and pore surface. It is these small, but important subtleties that physical chemists have to fully appreciate and understand in order to make use and arrive at so-called designer solids for specific applications, including gas separation, gas storage, catalysis, and light harvesting.
An example of confinement effects taking place within the micropores of a porous material is the spotlighted reaction intermediate, also known as the Wehland complex. This carbocationic σ-complex can be formed during Friedel-Crafts alkylation of benzene with ethanol within the micropores of zeolite ZSM-5. Operando UV/Vis spectroscopy is able to catch this intermediate during catalysis.
We would like to take the opportunity to thank all the authors and referees for their valuable time in composing and reviewing this excellent compilation of articles giving the readers different perspectives on this blossoming field of research. Our gratitude also goes to the editor Dr. Greta Heydenrych and her team for the hard work and dedication to make this Special Issue of ChemPhysChem reality.
Biographical Information
Bert Weckhuysen received his master and Ph.D. degree from Leuven University (Belgium) and has worked as a postdoc at Lehigh University (USA) and at Texas A&M University (USA). In 2000 Weckhuysen was appointed Full Professor at Utrecht University (The Netherlands) and is currently Distinguished University Professor at the same institute. He received several awards, including the Emmett Award, the Bourke Award, the Spinoza Award and the Tanabe Prize. Weckhuysen is a member of the Royal Dutch Academy of Sciences, the Royal Flemish Academy of Belgium for Sciences and Arts, and the European Academy of Science.
Biographical Information
Susumu Kitagawa received his Ph.D. from Kyoto University and is now Distinguished Professor of Kyoto University Institute for Advanced Study (KUIAS) and Director of Institute for Integrated Cell-Material Sciences (WPI-iCeMS) at Kyoto University. His main research field is coordination chemistry, and his current research interests are centered on synthesis and properties of porous coordination polymers (PCPs)/metal-organic frameworks (MOFs) towards gas science and technology for energy and environmental issues and contributing to human welfare.
Biographical Information
Michael Tsapatsis joined the Department of Chemical Engineering and Materials Science at the University of Minnesota in September 2003 as a professor. He currently holds the Amundson Chair and the McKnight Presidential Endowed Chair. He received an Engineering Diploma (1988) from The University of Patras, Greece, and MS and Ph.D. degrees from the California Institute of Technology (Caltech) working with G. R. Gavalas. He was a post-doctoral fellow with M. E. Davis at Caltech. Before joining the University of Minnesota, he was a faculty member in the Chemical Engineering Department at the University of Massachusetts Amherst (1994-2003). With his research group and collaborators, he has published ∼250 papers in the areas of membrane separations and catalysis. He is the inventor/co-inventor of 10 issued patents and ∼10 patent applications. He has supervised/co-supervised to completion the PhD thesis of 34 graduate students and advised 25 former postdoctoral fellows. He is a member of the US National Academy of Engineering.
