Hybrid Metal Oxide Cluster@MOF catalysts: design, synthesis by atomic layer deposition, and catalytic methane oxidation

Abstract

According to a recent perspective on the direct catalytic valorization of methane using heterogeneous catalysis, MOFs offer the desired structural versatility to afford a finetuning of their chemical properties for mimicking active sites and surroundings of oxidation enzymes (methanemonooxygenases), which are the most selective catalysts for the direct transformation of methane into methanol. This holistic approach, however, will not be void of challenges, especially when also considering the oxidant activation. It will most likely requirean oxidant activation mechanism where the key factor will be in controlling the electrophilic character of the activating O atom and directing it toward better abstraction of the hydrogen atom from CH4, while being able to regenerate these sites using inexpensive oxidizing agents. Selectivity of these catalysts will have to be controlled by the surroundings of the active site. For example, copper oxide clusters synthesized via Atomic Layer Deposition (ALD) on the nodes of the MOF NU-1000 are active for the oxidation of methane to methanol undermild reaction conditions. The obtained products, methanol, dimethyl ether, and CO2, can beeasily desorbed with the passage of 10% water/He at 135°C, giving a carbon selectivity formethane to methanol of 4560%. However, the obtained activity is not high (typically 50mmol of oxygenated products/g MOF), suggesting that other more efficient and tailored MOF-based catalysts should be developed and applied for the direct oxidation of methane to methanol. Thus, in this sub-project, we propose the design, synthesis by atomic layer deposition and catalytic evaluation of novel hybrid metal oxide cluster @MOF materials. Synthesis and full characterization of new hybrid catalysts composed of metal oxides or clusters (based on Cu, Fe, V, Al, Ga) deposited on the nodes of model MOFs such as NU-1000 will be performed. Other promising MOFs incorporating Cu or Fe (or heterometallic) nodes will also be explored as support matrices. The obtained hybrid catalysts will be tested in the selective oxidation of methane to methanol, using air/O2 and/or H2O2 as oxidants. A library of organic ligands (imadazole, pyiridine, pyrazine and carboxylic acid derivatives) will be explored as potential ligands (with a recognized promoting catalytic function) for the incorporation into the structure of a MOF material. The most promising catalysts and catalytic systems will be fully optimized. (AU)