(49e) Textural and Catalytic Properties of Mo Loaded Meso-/Microporous Layered Zeolites for Direct Methane Conversion
The hierarchical meso-/microporous lamellar zeolites consist of a zeolitic layer structure, with microporosity and mesoporosity within the layers and between the layers, respectively. The consequences of lamellar meso-/microporous structures on distribution and activity of active sites (Brønsted acid and metal sites) in molybdenum (Mo) impregnated MFI were investigated by textural and acidity property characterizations and direct methane aromatization (DMA) reactions. In comparison with their microporous analogies, which are Mo impregnated conventional microporous MFI with different crystallite sizes, the Mo/lamellar MFI zeolites showed lower number of free Brønsted acid sites and lower number of near surface Mo species. The presence of mesoporosity in the Mo/lamellar zeolites facilitates the dispersion of Mo in zeolite micro-channels to form interacting Mo-Brønsted acid site species. The catalytic DMA reactions showed that the Mo loaded meso-/microporous lamellar zeolites enabled efficient methane conversion, aromatic product formation, and coke accumulation in the initial stage of reaction, while their performances are similar to those of Mo loaded conventional microporous zeolite catalysts in the long term run. A plausible explanation to this scenario is that the active sites surrounded by mesopores account for initial fast reaction rate and deactivation rates of Mo/ lamellar zeolite catalysts in DMA reactions; while the active sites in microporous channels are responsible for the long term catalyst activity due to the shape selectivity of the zeolite micropores.