Catalytic oxidations are an important backbone of the chemical industry and can also play a significant role in addressing some of society’s most pressing issues such as climate change, poor air quality, and plastic waste. The latter is of increasing scientific interest as it promises to give spent plastics a new life, helping establish a Circular Economy. However, plastic upcycling generates substantial amounts of small alkanes (C1-C6). Small alkanes, such as methane, are important greenhouse gases contributing to climate change. Waste-methane (and other small alkane) mitigation is also necessary to reduce large-scale flaring and biogas leaking from municipal waste. Selective oxidation promises a path forward.
Mastery of selective methane oxidation using man-made catalysts – materials capable of performing desirable molecule transformations without being consumed or changed themselves – requires the efficient isolation of the initial product (e.g. methanol). A breakthrough came in 2005 employing an inorganic, crystalline and porous aluminosilicate – a zeolite. These materials are commonly used in a large number of industrial processes, due to their acidity, thermal stability and molecular sieving capability. There are more than 250 different zeolite structures known to date, materials with a variety of pore architectures, differing pore apertures (typically a pore diameter 0.4-1.0 nm) and accessibility.
However, there is a gap in our knowledge regarding the structural motif of the active oxidation site related to the zeolite framework which KeyMAT attempts to address. The synergy between redox sites and acid sites will be explored with the goal of being optimized. This will be accomplished by achieving atomistic changes in the zeolite framework initiated during the material synthesis. The success of KeyMAT will bring important insight on redox reactions not only over zeolites but also over supported and bulk metal oxide catalysis
Selective oxidation of alkanes such as methane promises to be an efficient waste mitigation strategy, provided superior and scalable catalysts can be designed, exhibiting both high activity and selectivity. The challenge is in the strong C-H bond of methane with a bond dissociation energy (BDE) of 435 kJ/mol posing a large energy barrier for H-abstraction, requiring large-scale, energy intensive steam reforming facilities. Here the molecule is completely disassembled. Yet, the remote and distributed location of the gaseous waste-methane feedstock precludes the use of this centralized chemical industry. Thus, ground-breaking selective oxidation technology is crucial.
KeyMAT will tackle this challenge with an evolutionary catalyst synthesis approach, inspired by enzymatic complexity, tuning acid sites to unlock the most efficient redox sites. Methane consuming enzymes are able to use molecular oxygen to catalytically functionalize the methane molecule without its disassembly. The enzyme's success is precipitated on a variety of complex facets such as metal-oxo clusters coordinated by histidine ligands, co-enzymes, and an intricate gating mechanism preventing the overoxidation of the methanol product to carbon dioxide. However, their implementation in chemical processing plants is limited by low turnovers, stability and scale-up issues. Translating enzymatic specificity to inorganic bio mimics such as zeolites has yet to be successfully reported. Cu-zeolites can oxidize methane with molecular oxygen albeit it only in a non-catalytic fashion. This is owed to strong retention of intermediates and insufficient substrate/product discrimination. KeyMAT's approach aims to resolve this by introducing more complexity into the zeolite host structure. This will allow control over active site location and coordination, and substrate/product discrimination paving the way for the next generation of selective oxidation catalysts for small alkanes.