Abstract
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► Basic sulfur poisons interact with Brønsted acid sites forming oligomeric carbocations. ► Oligomeric carbocations are coke intermediates. ► Coking depends on zeolite structure and nature of thiophenes. ► Confocal fluorescence microscopy reveals the location of sulfur poisons within FCC particles.
The effect and localization of thiophene-like poisons were studied on fluid catalytic cracking (FCC) catalyst at the individual particle level. The thiophene-like poisons interact on the Brønsted acid sites of the catalytic materials, forming oligomeric carbocations and coke species, which absorb and emit light in the visible region. The matrix components are not active in the formation of those light absorbing species. In contrast, zeolite Y and ZSM-5 were very active in inducing oligomer formation and the product distribution was different depending on the zeolite pore structure. Comparison of thiophene results with alkane and alkene catalytic cracking studies reveal that FCC particles have more affinity to react with thiophene molecules compared to n-hexane, but 1-hexene may compete with thiophene in the formation of carbocationic species on Brønsted acid sites. Moreover, a different reactivity was observed in thiophenes with distinct electron withdrawing/releasing substituents and molecular sizes. Our results demonstrate that the carbocations are coke intermediates, and the FCC particles containing zeolite Y promote to a higher extent coke formation: the large supercages allow the accommodation of more bulky coke species. On the other hand, FCC particles containing ZSM-5 stabilize the carbocations within the narrower cylindrical pores, diminishing coke formation. Confocal fluorescence microscopy can resolve the location of sulfur components at the single particle level with submicron resolution. Fluorescence microscopy images reveal heterogeneous domains with highly bright fluorescence across the FCC particles, which are attributed to the selective formation of oligomeric carbocations and coke species on the zeolitic material. The presence of thiophenes with different substituents and sizes was also studied by this approach. This demonstrates the potential of confocal fluorescence microscopy to identify reactivity differences of thiophene-like molecules on FCC catalyst particles in a spatially-resolved manner.