How Micropore Topology Influences the Structure and Location of Coke in Zeolite Catalysts

Abstract

Zeolite catalysts exhibit microporous structures, akin to the pockets in naturally occurring enzyme catalysts, which enable the confinement of reaction intermediates, thus facilitating chemical transformations. Nonetheless, the micropores also influence the formation of coke species, which is the main source of catalytic activity loss. Unveiling the relationships between the micropore topology and the internal structure and location of deactivating coke compounds is of high relevance for comprehending the deactivation mechanisms. In this study, we used an approach exploiting powder neutron diffraction to assess the location of coke and determine the dominating structures in the topologically distinct ZSM-5 (MFI topology), ZSM-35 (FER), and SSZ-13 (CHA) zeolite catalysts deactivated in industrially relevant methanol-to-hydrocarbon (MTH) conversion. In ZSM-5 and ZSM-35 catalysts, coke resides along the straight 10-membered ring (MR) channels and exhibits the highest concentration in their intersecting regions with sinusoidal 10 MR and straight 8 MR pores, respectively. In the SSZ-13 catalyst, coke is not only located in cages but also protrudes through their 8 MR windows, suggesting the interconnectivity of coke molecules between the large cavities. Notably, the coke-associated signals in the ZSM-5 and ZSM-35 catalysts show a strong planar arrangement that can be fitted by polycyclic and monocyclic arene structures, respectively. These averaged coke structures are consistent with the composition of coke assessed by gas chromatography−mass spectrometry, 13 C and two-dimensional 1 H double-quantum single-quantum magic-angle spinning nuclear magnetic resonance, and operando diffuse reflectance ultraviolet−visible spectroscopic analysis. The findings evidence that the pore topology directs the confinement and structure of coke, wherein the largest void zones of the micropore space are the most susceptible to coking.

Structural Changes of ZSM-5 Catalysts during Methanol-to-Hydrocarbons Conversion Processes

“…Although all framework aluminum sites are susceptible to dealumination, the 27 Al MAS NMR spectra indicate a higher dealumination propensity of the sites residing in channels with respect to those facing the channel intersections. The lower propensity of the Al F sites in the channel intersections may be related to the preferential accumulation of HP active and coke species in this region of the crystal, which reduce their interaction with steam. , Moreover, the smaller size of channel with respect to the intersection voids may increase the local concentration of steam in this region of the micropore space, thus promoting the dealumination rate . Notably, a significant portion of generated Al FA or Al EF sites remains in close proximity of the framework aluminum sites, as corroborated by the 27 Al– 27 Al DQ–SQ MAS NMR experiment (Figure b).…”