The production of organic chemicals via heterogeneously catalyzed selective oxidations is one of the most important segments in modern chemical industry. Important products include acrylic acid, acrylonitrile, ethylene oxide, formaldehyde, maleic anhydride, methacrylic acid, and phthalic anhydride.¹  Table 1.1 lists major heterogeneously catalyzed selective oxidation processes discussed within the scope of this book. Due to the high level of empirical development of these processes, further improvements represent a tremendous challenge, which will largely benefit from a mechanistic understanding of selective oxidation catalysts. However, despite extensive research activities over the last decades, still very little is known about the mode of operation of selective oxidation reactions on an atomistic level.^1–3 

A working catalyst requires an interplay of processes over multiple length- and timescales. With respect to time these range from elementary steps such as the breaking of bonds in the substrate and active site (∼100 fs) to transport phenomena as well as solid-state transformations of the catalyst (up to years). Simultaneously, in the course of these processes lengthscales from subnanometers up to meters are covered.

An important aspect of the rational development of more efficient selective oxidation processes is the ability to control the catalyst structure and particle size on the nanometer scale, strongly linking research in heterogeneous catalysis with material science. Such nanostructured catalysts are naturally divided into supported and bulk systems. In general, supported catalysts consist of an oxide support such as Al₂O₃, SiO₂ or TiO₂ onto which either metal nanoparticles are deposited or metal-oxide aggregates are grafted forming monolayer-type systems. If reduced to a small number of atoms, such systems may be designed as “single-site” catalysts, which allow for molecular control of the active site and its surrounding environment. Besides, with recent progress in the development of nanostructured (mesoporous) materials, designed regular-pore systems are now available that can serve as support for the anchoring of active sites. The ultimate goal in rational catalyst synthesis is the preparation of catalysts on the basis of identified active-site structures. The synthesis of bulk systems can then be envisioned as assembly of these sites into nanostructured inorganic solids with high surface area, similar to the synthesis of polymers starting from basic building blocks.⁴  A special case of bulk systems are heteropoly compounds, which are built on nanoclusters of a central heteroatom caged by oxygen-linked MO₆ octahedrons.

While methane and ethylbenzene can be considered as limiting cases of low and high reactivity, respectively, the C2–C4 substrates ethane, propane, propene, butane, isobutene and isobutene (see Table 1.1) due to their similar reactivity behavior in oxidative dehydrogenation and oxidative functionalization form a suitable platform for a discussion of general principles. The type of catalysts used for these reactions are in general vanadium and/or molybdenum containing bulk oxide materials including vanadium phosphorus oxides (VPO), heteropoly compounds (HPC) or mixed-metal oxides (MMO) such as MoVTeNb oxide. Many supported systems also constitute efficient catalysts for the above processes. However, with the exception of titania supported vanadium oxide (commercially used for benzene/naphthalene to phthalic anhydride conversion) bulk systems give higher yields as compared to supported systems and are therefore the focus of industrial research. Nevertheless, due to their “simplicity” supported systems can give valuable insights into the operation of selective oxidation reactions, as will be shown in detail below.

There exist various reviews^2–7  and books^1,8–13  covering heterogeneously catalyzed selective oxidation reactions. However, the high level of empirical development of many of the above processes strongly contrasts our current level of scientific understanding. It is probably fair to say that the current development of selective oxidation catalysts is largely based on phenomenological concepts (among which the principle of site isolation and the principle of phase cooperation are fundamental) rather than a profound understanding of their mode of operation. To this end, the purpose of this book is to bring together the current state of knowledge on selective oxidation reactions and, by combination with previous findings, to develop a consistent picture of the working principle of selective oxidation catalysts.

Commercially important classes of selective oxidation reactions are the oxidative dehydrogenation of methanol and the epoxidation of ethylene. The epoxidation of propylene has the potential to be commercialized. For these reactions mainly catalysts based on coin metals (Cu, Ag, Au) are used. Ag is a particularly interesting material as it can serve as a catalyst for two completely different processes, methanol oxidation to formaldehyde and epoxidation of ethylene to ethylene oxide. For formaldehyde production besides silver, iron molybdate catalysts are used.

A detailed discussion of all aspects related to selective oxidation catalysts is outside the scope of this book. For example, an important aspect of selective oxidation reactions that has barely been addressed in the literature is the influence of steam on the catalyst structure and dispersion.^14,15  In general, water is a product of selective oxidation reactions. In addition, water vapor is often added to the feed to improve the catalyst performance. It should be mentioned that under the conditions of operation hydrothermal reactions involving oxolation and olation processes may lead to polymerization/depolymerization of an initial M_xO_y condensate, which sets high standards for catalyst stability towards sintering. Another example is the role of carbon deposits on catalytic performance in selective oxidation reactions, which represents a largely unexplored research area.¹⁶