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.