(93e) Ni-Au Single Atom Alloys for the Selective Oxidation of Methacrolein with Methanol to Methyl Methacrylate

Direct oxidative esterification of methacrolein with methanol has been introduced as a more environmentally benign alternative to the traditional acetone cyanohydrin method for the production of methyl methacrylate (MMA). Gold has been considered as a potential catalyst for this reaction owing to its high selectivity to MMA, and appreciable yields can be achieved when deposited on basic supports. Industrially, core-shell structures with Au in the core and a shell of NiO_x on a basic support (SiO₂-MgO-Al₂O₃) exhibit high MMA yields but the reaction mechanism and role of Ni have not been investigated in detail¹, while high methanol concentrations were used to counter the stronger adsorption of methacrolein on Au surface.

In the present work, we aim to address these issues by studying NiAu single-atom alloy (SAA) nanoparticles supported on silica, which are promising for the non-oxidative ethanol dehydrogenation². The catalytic structure is followed by CO-DRIFTS, EXAFS and STEM, while catalytic tests in the liquid phase on NiAu SAAs and bare Au NPs show approximately 100% selectivity to methyl methacrylate on both systems and a considerable increase in the reactivity by the addition of Ni atoms, which is comparable to that of the commercial catalysts³.

In kinetic measurements on Au and NiAu SAAs similar apparent activation energies were found, and the combined spectroscopic characterization of the used catalysts has identified a common active site for the reaction, [Au-O_x]-. The formation of these species is facilitated by the presence of Ni atoms in the Au surface and their higher concentration resulted in proportionately increased MMA production rates.

Extensive batch reactor studies at various methanol to methacrolein ratios coupled with in operando spectroscopic measurements, have elucidated the reaction mechanism and captured the formation of hemiacetal as the reaction intermediate. Interestingly, NiAu SAAs can operate with almost equal amounts of the reactants without deactivation, while maintaining their high selectivity. This result, shown for the first time, indicates that isolated Ni atoms act as binding sites for the adsorption of methanol, allowing for lower methanol recycle, and thus provide a pathway to a more environmentally friendly process overall.

References

(1) Suzuki, K.; Yamaguchi, T.; Matsushita, K.; Iitsuka, C.; Miura, J.; Akaogi, T.; Ishida, H. Aerobic Oxidative Esterification of Aldehydes with Alcohols by Gold-Nickel Oxide Nanoparticle Catalysts with a Core-Shell Structure. ACS Catal. 2013, 3 (8), 1845–1849.

(2) Giannakakis, G.; Trimpalis, A.; Shan, J.; Qi, Z.; Cao, S.; Liu, J.; Ye, J.; Biener, J.; Flytzani-Stephanopoulos, M. NiAu Single Atom Alloys for the Non-Oxidative Dehydrogenation of Ethanol to Acetaldehyde and Hydrogen. Top. Catal. 2018, 61 (5–6), 475–486.

(3) Trimpalis, A.; Giannakakis, G.; Cao, S.; Flytzani-Stephanopoulos, M. NiAu Single Atom Alloys for the Selective Oxidation of Methacrolein with Methanol to MMA. Catal. Today 2019.