Reflections on Catalytic Selective Oxidation: Opportunities and Challenges

Currently, and looking forward, there is an ever increasing demand to perform chemical transformations with optimized atom and energy efficiency. In parallel, there is also growing interest in diversification of chemical feedstocks from more traditional ones. For example, effective and economic transformation of biorenewable chemical feedstocks and shale-gas, are just two that can be highlighted. These targets are demanding and challenging, and the required shift in industrial production of energy and chemicals will not be possible without advances in catalysis. This Special Issue focuses on selective catalytic oxidation, as it offers significant potential as an approach to achieving green and efficient chemical transformations of a broad range of substrates. Selective oxidation offers many challenges, not least of which is the control of product selectivity to the required products. Control of selectivity can be particularly challenging, as the desired products are less thermodynamically favoured compared to carbon oxides. This can be a particular problem if high temperatures are required to activate stable molecules, as these conditions can result in significant sequential over-oxidation of the desired products. A number of strategies have been developed to minimise such affects, and perhaps one of the most common is to utilise oxidants which are more active than molecular oxygen. A number of oxidants can be employed for selective catalytic oxidation, and depending on scale, reaction efficiency and relative cost of the oxidant and product, specific oxidants can be economic to use (e.g., hydrogen peroxide). However, the use of molecular oxygen, preferably directly from air, to achieve high yields of target products selectively must be the ultimate aim. There are some large scale selective catalytic oxidation processes that operate commercially. Formaldehyde is a bulk commodity chemical with a large worldwide market, it has many important uses, with major applications for thermosetting resins, manufacture of polyurethane foam, thermoplastics and adhesives. The two major industrial processes for production of formaldehyde are from the catalytic oxidation of methanol. One method is via an oxidation-dehydrogenation reaction using a silver-based catalyst. The alternative is direct oxidation of methanol to formaldehyde by oxygen using a mixed metal oxide iron molybdate catalyst [1]. The iron molybdate catalysed process is becoming increasingly prevalent, since it operates at a lower temperature than the silver catalysed process, and the catalyst is more robust. These advantages mean that the cost per tonne of formaldehyde production using iron molybdate catalysts is lower and yields a higher return on capital investment [2]. The industrial iron molybdate catalyst consists of a mixed metal oxide phase, Fe2(MoO4)3, together with excess MoO3. Fe2(MoO4)3 is considered to be the active/selective phase, with MoO3 being selective but with low activity, and Fe2O3 unselective forming CO2. It has been reported that the catalyst must contain both Fe2(MoO4)3 and excess MoO3 in order to achieve high activity, selectivity and life-time. Catalyst deactivation is due to loss of molybdenum, and it is proposed that the excess MoO3 ensures that no iron rich phase is formed during the catalyst lifetime. Hence, although the iron molybdate catalyst is used industrially, and has been for many years, scope still exists to develop improved catalysts and improve our understanding of the process.