Oxidative Dehydrogenation of Propane in the Realm of Metal–Organic Frameworks

Propylene (propene) is one of the most important feedstocks in the chemical industry. It is a starting material for the synthesis of a wide variety of commodity chemicals ranging from small molecules (cumene, isopropanol, acrylonitrile, acrylic acid, propylene oxide, and butyraldehyde) to polymers (most notably, polypropylene). In 2013, approximately 85 million tons of propylene were processed worldwide14 million in the United States alone. Traditionally, propylene is produced along with other light olefins by steam cracking or fluid catalytic cracking of higher hydrocarbons. These methods are not ideal as the costs of the starting materials escalate and due to poor selectivity increasing production, purification, and energetic costs. An intriguing alternative is to develop and utilize “onpurpose” methods, in which propylene is the intended end product and is produced with high selectivity (as opposed to a range of light olefins). One of the most promising processes in this regard is propane dehydrogenation (PDH), which ideally produces just propylene and hydrogen. Although some industrial processes such as Catofin and Oleflex already take advantage of this transformation, they satisfy only a small percentage of the propylene demand currently. As we excavate more shale gas and shift from naphtha to ethane steam cracking, a favorable price difference between propane and propene emerges. In addition, as the demand for propylene increases, the role of PDH in propene production will continue to gain importance; however, the classical PDH process still faces several challenges. Most importantly, the formation of propylene and hydrogen is endothermic. Reasonable conversions thus require high temperature, which also leads to significant coking and gradual catalyst deactivation. One exergonic alternative to PDH is the oxidative dehydrogenation (ODH) of propane, where addition of O2 to the propane feed ideally produces propene and water. Here, the challenge is to avoid further oxidation of propylene and formation of CO2, which is favorable thermodynamically and often drastically reduces overall selectivity for propene. Typical catalysts for ODH are supported vanadium, molybdenum, and chromium oxides. They still require quite high operating temperatures in the 300−650 °C range. Because the identity and morphology of the support and the active species are obviously key in defining catalyst performance, improvements in ODH catalysts are largely empirical. The ability to control these variables is thus attractive for generating new catalysts. In their recent report, Li et al. introduce the ODH of propane into the realm of metal−organic frameworks (MOFs) (Figure 1). These already well-established materials built from