Reflections on Catalytic Selective Oxidation: Opportunities and Challenges

Abstract

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, Fe 2 (MoO 4 ) 3 , together with excess MoO 3 . Fe 2 (MoO 4 ) 3 is considered to be the active/selective phase, with MoO 3 being selective but with low activity, and Fe 2 O 3 unselective forming CO 2 . It has been reported...

Selective oxidation conversion of methanol/dimethyl ether

“…However, using this kind of catalyst, the high operating temperatures cause the sintering and aggregation of silver particles, thus negatively affecting the formaldehyde yield. 29 In 1931, Adkin et al reported a new iron-based catalyst (Fe–Mo) to directly oxidize methanol into formaldehyde with an impressive catalytic performance. 30,31 The Fe–Mo-based approach for the production of FA proceeds according to eqn (2).CH 3 OH + 1/2 O 2 → CH 2 O + H 2 O Δ H = −159 kJ mol −1 …”

Chemical looping methanol oxidation using supported vanadium phosphorous oxide carriers for formaldehyde production

“…In the case of the silver process, the catalyst undergoes agglomeration due to prolonged operation at high temperatures and needs to be replaced every 3 to 12 months. [14][15][16] The Formox process with a Fe-Mo catalyst also faces the problem of catalyst deactivation, which can be attributed to the volatilization of MoO 3 , formed due to the reaction of formaldehyde with the catalyst. [17][18][19][20] Thus, there has been a signicant research thrust in the synthesis of a new catalyst formulation that is low-cost, stable, and sustainable.…”