Synthesis of Hollow Zn/ZSM-5 Nanosheets via Different Alkali Treatments with ZIF-8 as a Zn Source for Efficient Aromatization of Methanol

Diffusion limitations and monofunctional acidity of ZSM-5 molecular sieves affect the catalyst stability and aromatic yield in the reaction of methanol to aromatics (MTA). In this study, based on ZSM-5 nanosheets as parent molecular sieves, Zn-modified hollow ZSM-5 nanosheets were obtained after hydrothermal treatment by adding ZIF-8 or zinc nitrate as a source of Zn while treating with different types and concentrations of alkali solutions. The physical and chemical properties of the fabricated samples and their catalytic performance of methanol aromatization were systematically investigated by a combination of XRD, TEM, N₂ adsorption-desorption, NH₃-TPD, Py-IR, ²⁷Al MAS NMR, ²⁹Si MAS NMR, XPS, and TG characterization analyses and MTA experimental evaluation. The results indicated that a hollow structure emerged in the samples after alkaline treatment, with a significant increase in the proportion of mesopores, which further increased with the concentration of the alkaline solution. Both alkaline treatment and the introduction of Zn led to changes in the acidity of the catalyst. The increase in tetrahedrally coordinated aluminum during the alkaline treatment resulted in a higher content of B acid sites, while the introduction of Zn formed Zn-Lewis acid sites. Among the prepared samples, the catalyst obtained using ZIF-8 as the Zn source and treated with a mixed alkaline solution of 0.15 M sodium hydroxide and tetrapropylammonium hydroxide (Zn_Z8(N+T)/Z5) exhibited higher relative crystallinity, more appropriate micromesopore ratio, a greater amount of Zn(OH)⁺, and a suitable B/L ratio (0.74). Under the same conditions (450 °C, atmospheric pressure, weight hourly space velocity (WHSV) = 5 h^-1), the aromatic product yield of Zn_Z8(N+T)/Z5 reached 26.2%, which is 14 percentage points higher than that of the parent ZSM-5 and 4 percentage points higher than that of Zn_ZN(N+T)/Z5 modified with zinc nitrate as the Zn source. After 8 h of reaction, the aromatic yield over Zn_Z8(N+T)/Z5 could still be maintained above 18.5% with good catalyst stability.