Enhancing Post-Synthesis Demetalation in Zeolites by Heteroatom Selection

The ability to control the composition and defect density of zeolites is gaining attention based on their impact in a wide range of catalysis, adsorption, and separation processes. This study focuses on the use of demetalation as a strategic approach for defect engineering and tailoring acid strength wherein we venture beyond conventional aluminosilicate zeolites to assess the impact of heteroatom substitution. We selected three commercially relevant zeolite structures (CHA, MFI, beta) with distinct pore networks and prepared isostructures by replacing aluminum with boron, gallium, and zinc. Demetalation was carried out using either deionized water or mild acid treatment to effectively remove metals without using toxic chemicals, expensive reagents, or harsh conditions. A combination of experiments and density functional theory (DFT) calculations confirm the efficiency of demetalation follows the order B > Zn > Ga > Al, with beta exhibiting the highest degree of metal removal. The judicious selection of heteroatom leads to distinct defect sites and silicon-to-metal gradients in demetalated zeolite crystals, dictating the extent to which crystallinity is retained. DFT calculations reveal that boron removal is the only thermodynamically favorable process under both pH-neutral and acidic conditions, which allows for almost complete removal of metal from zeolites without appreciably compromising their structural integrity. Catalytic tests using methanol-to-hydrocarbons (MTH) as a benchmark reaction reveal several benefits of demetalation over one-pot syntheses. Dealumination of CHA leads to the introduction of mesopores, which enhances mass transport, leading to prolonged catalyst lifetime and increased cumulative methanol turnover. We also show that the same process for zeolite MFI tailors its acidity in ways that cannot be emulated by direct synthesis of catalysts with equivalent Si/Al composition. The collective findings from this study provide valuable insights into the efficiency and extent of demetalation for a broad class of heteroatom-substituted zeolites; and also highlight demetalation as an alternative to conventional techniques for effectively tuning the physicochemical properties of zeolites.