Tailoring yolk-shell thickness unlocks efficient desulfurization of bulky sulfur compounds

Driven by increasingly strict environmental regulations limiting sulfur content to below 10 ppm, ultra-deep hydrodesulfurization (HDS) is crucial for clean fuel production. Conventional NiMo catalysts loaded on γ-alumina perform well for simple aliphatic sulfides but struggle with large-molecule aromatic sulfides like 4,6-DMDBT. This is because the methyl groups at the 4- and 6-positions create severe steric hindrance that hinders the direct desulfurization pathway, while alumina's weak acidity is not conducive to isomerization pathways that can alleviate this hindrance.

To address this, researchers at the China University of Petroleum (Beijing), PetroChina, and Changzhou University synthesized yolk-shell structured TS-1-SiO₂ composites (denoted as TYₓSᵧ). TS-1 zeolite provides Brønsted acid sites to promote isomerization, while the yolk-shell architecture offers open multi-level pores and a hollow cavity to facilitate diffusion of bulky reactants and products. The team used TMB as a swelling agent to precisely control the shell thickness and yolk size. As TMB dosage increased, both the shell thickness and yolk size gradually decreased.

The researchers found a clear trade-off: thinner yolk-shell structures facilitated diffusion and better exposed active sites, but overly thin shells compromised mechanical and hydrothermal stability. Thicker shells enhanced stability but restricted mass transfer. The optimal catalyst, NiMo/TY₁₀S₁₃, balanced these conflicting requirements.

At a weight hourly space velocity of 10 h⁻¹, NiMo/TY₁₀S₁₃ achieved desulfurization efficiencies of 98.0% for DBT and 93.3% for 4,6-DMDBT, compared with 88.5% and 75.0% for the catalyst with the thickest shell (NiMo/TY₅₀S₂₆). It exhibited the highest turnover frequency (7.5 × 10⁻⁴ s⁻¹ for DBT and 6.1 × 10⁻⁴ s⁻¹ for 4,6-DMDBT) and apparent HDS rate constants (2.6 × 10⁻⁷ mol·g⁻¹·s⁻¹ for DBT and 2.1 × 10⁻⁷ mol·g⁻¹·s⁻¹ for 4,6-DMDBT). The catalyst favored the direct desulfurization pathway for DBT and the isomerization pathway for 4,6-DMDBT, with the isomerization contribution reaching 19.0%, correlating well with its highest Brønsted acid content (15.1 μmol·g⁻¹) and B/L ratio (2.01).

A 120-hour stability test at 340 °C and 4.0 MPa confirmed that the tailored yolk-shell architecture preserved the integrity of the active phases, maintaining steady-state efficiencies of 98.0% (DBT) and 93.3% (4,6-DMDBT).

This work demonstrates that precisely tuning the yolk-shell thickness is an effective strategy for designing high-performance catalysts for the deep removal of large molecular sulfur compounds, supporting the production of cleaner fuels.