Formation of Strong Brønsted Acid Sites on Aluminosilicate Surfaces during Catalytic Cracking

Abstract Amorphous aluminosilicates are essential components of fluid catalytic cracking (FCC) catalysts, where they provide structural support, hierarchical porosity, and acid functionality within the mesoporous matrix. A molecular-level description of the active acid site ensemble remains challenging because these materials are compositionally heterogeneous and structurally disordered. Most experimental and theoretical studies have focused on Si-doped γ–Al2O3 that serves as a convenient model system. Here, we move beyond this simplified representation toward more realistic and chemically complex aluminosilicate structures. We combine density functional theory (DFT), equivariant machine-learning interatomic potentials (MLIPs), and grand-canonical basin hopping (GCBH) to map structure-acidity relationships for aluminosilicate compositions that arise under FCC-relevant hydrothermal conditions (T = 550 °C and PH2O = 1.2 bar), representative of the reactor inlet. We study a family of mullite surfaces across Al2O3:SiO2 ratios (denoted n:m) and associated oxygen-vacancy contents, and we explicitly model additional SiO2 deposition to emulate silica redistribution during phase evolution from kaolin through spinel intermediates to mullite. Machine-learning potentials trained on structurally related sillimanite surfaces accurately describe Al–Si–O–H chemistry and transfer to vacancy-rich and silica-modified mullite phases. Using NH3 binding as a descriptor of Brønsted acidity, we find that mullite can host acid sites with strengths reaching ΔENH3 ≈ –140 kJ mol−1 in 3:2 mullite, which is comparable to external zeolitic acid sites and substantially stronger than those reported on Si-doped γ–Al2O3. Silica grafting onto otherwise weakly acidic mullite terminations further generates an ensemble of strong bridging and pseudo-bridging Brønsted acid sites. We attribute this enhanced acidity to the local emergence of tetrahedrally coordinated AlIV environments that strongly polarize Al–OH–Si linkages. These results identify defect-rich as well as silica-decorated mullite surfaces as a realistic and potentially dominant source of strong matrix acidity in FCC catalysts under operating conditions.