Construction of b-Axis-Oriented ZSM-5 Nanosheets for Improved Catalytic Upgrading of Lignite Pyrolysis Volatiles to Light Aromatic Hydrocarbons: Synergy and Balance between Acidity and Diffusivity

Constructing ZSM-5 nanosheets is an effective strategy to alleviate diffusion limitations and improve reaction efficiency. Several b-axis-oriented HZ5-Sx nanosheets (x represents average thicknesses from 200 to 24 nm) were prepared for the catalytic upgrading of coal pyrolysis volatiles to light aromatic hydrocarbons (LAHs). HZ5-S55 achieved the highest LAH yield, up to 40.0 mg g^–1, significantly surpassing conventional ZSM-5 and other HZ5-Sx nanosheets (<32.3 mg g^–1). Compared to conventional ZSM-5, HZ5-S55 exhibited an increased pore volume (from 0.18 to 0.26 cm³ g^–1), specific surface area (from 369 to 417 m² g^–1), acid content (from 534.85 to 566.69 mmol g^–1), and diffusivity (increased by 3.55 times). These enhancements significantly improved intermediate and product diffusion, acid site accessibility, and the transfer of active hydrogen and radicals. Compared to HZ5-S55, HZ5-S200 and HZ5-S95 showed significantly lower LAH yields (<32.3 mg g^–1), primarily attributed to their lower diffusivities (<3.86 × 10^–9 m² s^–1) than HZ5-S55 (4.05 × 10^–9 m² s^–1). Moreover, HZ5-S40 and HZ5-S24 showed decreased LAH yields (<31.0 mg g^–1) relative to HZ5-S55, despite their higher diffusion coefficients (>4.30 × 10^–9 m² s^–1). This decline was primarily due to severe acidity loss (decreases of 136.37 mmol g^–1 for HZ5-S40 and 172.85 mmol g^–1 for HZ5-S24) compared to that for HZ5-S55, resulting from excessive b-axial growth obstruction. As the b-axial thickness reduced from 200 to <55 nm, the thinner HZ5-S55, HZ5-S40, and HZ5-S24 nanosheets exhibited enhanced resistance to carbon deposition, forming higher proportions (>45.00 wt %) of soft coke that is readily removable, in contrast to the hard carbon observed in thicker HZ5-S200 and HZ5-S95 nanosheets. Reducing the b-axis length from 200 to 24 nm enhanced diffusivity, mitigating carbon deposition by improving molecular transfer and inhibiting hard coke formation by relocating coking sites to the catalyst surface. Furthermore, HZ5-S55 demonstrated excellent regenerability via carbon deposition combustion. Therefore, precisely constructing b-axis-oriented ZSM-5 nanosheets with well-balanced acidity and diffusivity can significantly enhance the synergistic effect on the catalytic upgrading efficiency.