UCLA

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.