Probing coke formation during the methanol-to-hydrocarbon reaction on zeolite ZSM-5 catalyst at the nanoscale using tip-enhanced fluorescence microscopy

The deactivation mechanism of the widely used zeolite ZSM-5 catalysts remains unclear to date due to the lack of analytical techniques with sufficient sensitivity and/or spatial resolution. Herein, a combination of hyperspectral confocal fluorescence microscopy (CFM) and tip-enhanced fluorescence (TEFL) microscopy is used to study the formation of different coke (precursor) species involved in the deactivation of zeolite ZSM-5 during the methanol-to-hydrocarbon (MTH) reaction. CFM submicron-scale imaging shows a preferential formation of graphite-like coke species at the edges of zeolite ZSM-5 crystals within 10 min of the MTH reaction (i.e., working catalyst), whilst the amount of graphite-like coke species uniformly increased over the entire zeolite ZSM-5 surface after 90 min (i.e., deactivated catalyst). Furthermore, TEFL nanoscale imaging with ∼35 nm spatial resolution revealed that formation of coke species on the zeolite ZSM-5 surface is non-uniform and a relatively larger amount of coke is formed at the crystal steps, indicating a higher initial catalytic activity.