Unveiling coke formation mechanism in MFI zeolites during methanol-to-hydrocarbons conversion

Coke formation over MFI zeolites of different crystallite sizes was rigorously investigated in methanol-to-hydrocarbon conversion (MTH). The key experimental idea was based on the fact that soft coke in the zeolite micropores (internal coke) could be selectively removed by thermal treatment under H2, while heavy coke at the zeolite external surface (external coke) remained intact. This enabled us to analyze the amount, composition, chemical nature, and deactivating effects of internal and external cokes, separately. As the mass transfer of hydrocarbons was retarded with increasing zeolite crystallite size, an aromatic-based catalytic cycle became dominant in MTH compared to an olefin-based cycle. Consequently, methylated benzene intermediates were accumulated within the zeolite micropores. These methylated benzenes could also act as coke precursors and polymerize to form internal coke within the micropores. Aromatic products that diffused out of zeolite micropores could also be condensed at the external surface of the zeolite crystallites. Once the carbon deposits were formed at the external zeolite surface, the external coke continued to grow even non-catalytically by the thermal reaction with the methylated benzenes. Different zeolite catalysts and reaction times affected only the relative amounts of internal and external cokes, but not their respective chemical natures. The internal coke appeared to have the H/C ratio of 1.26 and a density of 1.0 g cm−3, while the external coke had much lower H/C ratio (0.28) and higher density (1.5 g cm−3). We propose that internal coke is likely to have the polymeric structures of the methylated acenes (i.e., linearly fused aromatic rings such as benzene, naphthalene, and anthracene) connected via methylene bridges. On the other hand, the external coke is highly polyaromatic with many fused rings. In terms of catalyst deactivation, the internal coke proved to be much more detrimental than the external coke.