Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in combustion and catalysis, forming during incomplete fuel combustion and hydrocarbon/biomass hydrolysis, leading to detrimental soot formation. PAH growth in zeolite-catalyzed reactions is analogous to soot formation, involving the extension and clustering of aromatic molecules. However, the nano-sized pores of zeolites influence PAH dimensions and structures. PAH formation is unavoidable in catalysis, particularly in petrochemical processes (catalytic cracking, isomerization, transalkylation) and coal-based processes (MTO, syngas conversion). PAH deposition causes catalyst coking and deactivation, requiring regeneration and impacting process design. Understanding PAH structures at a molecular level is crucial for preventing catalyst deactivation and improving industrial processes. While methods like Guisnet's method (inorganic acid dissolution, solvent extraction, GC-MS) and various spectroscopic techniques (IR, UV-Raman, UV-Vis, NMR) have been employed, they are limited in resolving the complex structures of larger PAHs, especially the insoluble fraction of coke. Microscopic techniques like CFM and APT provide spatial distribution information but lack molecular fingerprints. Sophisticated mass spectrometry techniques (tandem MS, HPLC/UV/MS, synchrotron vacuum ultraviolet photoionization MS) have also been used, but identifying precise molecular structures is challenging due to isomerism. MALDI FT-ICR MS has emerged as a powerful tool for coke identification due to its soft ionization, but alone it struggles with isomer-specific measurements. This study aims to overcome these limitations by coupling MALDI FT-ICR MS with isotope labeling to comprehensively characterize PAHs in the industrially important SAPO-34-catalyzed MTO reaction, elucidating their growth mechanism.

Existing methods for identifying PAHs in catalytic reactions have limitations. Guisnet's method, while effective for smaller molecules, struggles with larger PAHs due to decreased solubility. Spectroscopic techniques like IR, UV-Raman, UV-Vis, and NMR can distinguish between soluble and insoluble PAHs but cannot determine their precise molecular structures. Microscopy (CFM, APT) provides spatial information but lacks molecular-level detail. Mass spectrometry techniques offer some structural information, but isomeric complexity remains a significant challenge. While previous studies have utilized MALDI FT-ICR MS for coke identification, this work enhances its capabilities by incorporating isotope labeling, which enables a more detailed characterization of the PAHs. Several prior studies speculated about the nature of insoluble coke in SAPO-34, with some suggesting external surface formation, but lacked conclusive experimental evidence. This research addresses the need for a comprehensive approach to precisely identify and characterize the heavier PAHs contributing to catalyst deactivation.

This study used the industrially relevant SAPO-34-catalyzed MTO reaction as a model system. The catalyst bed was divided into four layers to minimize the effects of inhomogeneous axial deactivation. The catalytic performance was monitored by analyzing product selectivities and methanol conversion over time on stream (TOS). Carbonaceous deposits were extracted using a modified HF dissolution-CCl4 extraction method. The soluble fraction was analyzed by GC-MS, while the insoluble fraction was analyzed by MALDI FT-ICR MS. DFT calculations were used to estimate the maximum size of aromatic molecules that could be accommodated within a single SAPO-34 cage. Isotope labeling (using 13C-methanol and D-methanol) was combined with MALDI FT-ICR MS to determine the precise chemical compositions (CxHy) of the larger PAHs. 13C solid-state NMR spectroscopy was employed to characterize the structure of the isolated insoluble coke. The generalizability of the findings was tested by analyzing coke species from other cage-structured molecular sieves (SAPO-35, SAPO-18, DNL-6).

The study demonstrated that the majority of PAHs responsible for deactivation in SAPO-34 reside within the pores rather than on the external surface. MALDI FT-ICR MS revealed a unique mass distribution pattern for the insoluble coke, characterized by discrete, grouped mass peaks with regular intervals corresponding to the mass of three- to four-ring aromatic species. This suggested a 'cage-passing' growth mechanism, where PAHs initially formed in separate cages subsequently cross-link through the 8-membered ring windows, leading to the formation of cross-linked multi-core PAHs with a graphene-like structure. Isotope labeling experiments confirmed this hypothesis, enabling the determination of the chemical formulas and potential structures of the larger PAHs. The 13C ssNMR analysis supported the aromatic and graphene-like nature of the coke. These findings were validated in other cage-structured molecular sieves. The size and connectivity of the cages influenced the type of PAHs formed; for example, SAPO-35 with smaller cages yielded biphenyl-coupled PAHs, while SAPO-18 with larger cages yielded pyrene-based PAHs. DNL-6, with differently connected cages, did not exhibit cage-passing growth and yielded coronene-based PAHs.

The findings of this study provide a novel understanding of PAH growth mechanisms in confined spaces, particularly within the pores of zeolites. The cage-passing growth mechanism explains the formation of large, cross-linked PAHs that contribute significantly to catalyst deactivation. The results highlight the importance of considering the interplay between the zeolite's pore structure and the chemical evolution of PAHs. The proposed deactivation model, incorporating the spatial distribution of coke and the identified cage-passing PAHs, provides a more complete understanding of catalyst deactivation in MTO reactions and offers new avenues for improving industrial processes. The identified structural features of the PAHs also have implications for other carbon-rich materials, such as petroleum asphaltene, coal, and soot.

This study successfully elucidated a novel cage-passing growth mechanism for PAHs in confined spaces using a combination of advanced techniques. The findings are important for understanding catalyst deactivation and offer potential avenues for optimizing industrial processes. Future research could explore the generality of this mechanism across a broader range of catalysts and reaction conditions, and further investigation of the detailed structural features of the larger PAHs would be beneficial. In addition, further exploration of the interplay between the zeolite topology and the cage-passing mechanism would provide a more refined understanding.

While the study successfully identified the cage-passing mechanism, the precise structures of the larger PAHs could not be completely determined due to the isomeric complexity. The study focused on specific zeolites, and further research is needed to confirm the generality of the findings across a wider range of zeolite structures and reaction conditions. The DFT calculations relied on approximations, and more sophisticated calculations may provide further insights.