Serendipity in Catalysis Research: Boron Based Materials for Alkane Oxidative Dehydrogenation.
Light olefins such as ethylene and propylene form the foundation of the modern chemical industry, with yearly production volumes well into the hundreds of millions of metric tons. Currently, these light olefins are mainly produced via energy intensive steam cracking. Alternatively, oxidative dehydrogenation (ODH) of light alkanes to produce olefins allows for lower operation temperatures and extended catalyst lifetimes, potentially leading to valuable process efficiencies. The potential benefits of this route have led to significant research interest due to the wide availability of natural gas from shale deposits. Advances in this area have still not yielded catalysts that are sufficiently selective to olefins for industrial implementation, and ODH still remains a holy grail of selective alkane oxidation research. The main challenge in selective oxidation lies in preventing the overoxidation of the desired product, such as propylene during propane oxidation, to CO and CO2. Research into selective heterogeneous catalysts for the oxidative dehydrogenation of propane has led to the extensive use of vanadium oxide based catalysts, and studies on the surface mechanism involved have been used to improve the catalytic activity of the material. Despite decades of research, however, selectivity toward propylene has not proven satisfactory at industrially relevant conversions. It is imperative for new catalytic systems that minimize product overoxidation to be developed for future applications of oxidative dehydrogenation processes. While rational catalyst design has been successful in developing homogeneous catalyst systems, its practical use in heterogeneous catalyst development remains modest. The complexity of surfaces with a variety of terminations and bulk structures, let alone their modification by the chemical potential of a reaction mixture, makes heterogeneous catalyst discovery serendipitous in many cases. The catalyst family presented in this Account is no exception. The importance of catalysis research lies in exploring the science behind serendipity. In this Account, we will first present our initial discovery of boron nitride (BN) as an unexpected catalyst for the oxidative dehydrogenation of light alkanes. Beyond its surprising activity, BN also drew interest due to its low selectivity to carbon oxides. This observation made BN distinct from previously studied metal oxide catalysts for selective alkane oxidation. We narrowed down its unique reactivity to the oxygen functionalization of the catalyst surface, particularly the formation of B O species as probed by various spectroscopic techniques. In investigating the critical role of each of the structural elements during ODH, we discovered that not only BN but an entire class of boron containing compounds are active and selective for the formation of propylene from propane. All these materials form a complex oxidized surface with a distribution of BO x surface sites. This discovery opens the doors to a new field of boron based oxidation chemistry that currently has more questions than answers. We aim to make this Account a starting point for the research community to explore these new materials to understand their surface mechanisms and the surface species that offer a unique selectivity toward olefinic products. Effective use of these materials may lead to novel processes for efficient use of abundant light alkane resources by oxidation chemistry.
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