Mechanisms of montmorillonite-mediated high-temperature catalysis of organic matter under variable hydration states and implications for petroleum geology

Clay minerals play a crucial role in catalyzing kerogen to form hydrocarbons, significantly influencing petroleum system evolution. Montmorillonite (MT) dehydrates upon heating; however, the catalytic mechanisms governing the thermal decomposition of organic matter at different dehydration levels remain unclear. Additionally, the mechanism by which external water suppresses MT’s catalytic efficiency remains debated. To resolve this issue, this study conducted pyrolysis simulation experiments (340 °C, 10 days) with various combinations of octadecanoic acid (OA), water, hydrochloric acid (HCl) solution, MT, dehydrated-MT, and illite. We integrated mineral/organic transformation analyses (XRD/FTIR/SSNMR/SEM), temperature-dependent characterization of solid acid sites (NH3-TPD/FTIR), and quantitative product measurements (GC/GC-MS) to elucidate how clay-bound water and external water differentially regulate organic-matter cracking pathways. The results suggest that clay-bound water controls reaction pathways by tuning both the type (Brønsted vs. Lewis) and density of solid acid sites. External water inhibits catalytic efficiency by reducing direct contact between organic matter and solid acid sites. Compared with untreated MT, 150 °C-dehydrated MT-OA system exhibited strong interlayer water polarization, which increased Brønsted acid site density and enhanced the carbonium-ion mechanism, thereby promoting isoalkane production. In contrast, 250 °C-dehydrated MT-OA system, where interlayer water was nearly eliminated, had fewer Brønsted acid sites but greater exposure of Lewis acid sites, facilitating decarboxylation and increasing CO2 production. In hydrous systems, the addition of HCl solution did not enhance the carbonium-ion mechanism compared to the hydrous system with only water, indicating that only protons bound to solid acid sites, rather than liquid H+ in water, can trigger the carbonium-ion reaction. This shows that the catalysis of organic-matter cracking by clay minerals such as MT is fundamentally an interfacial chemical process that requires direct mineral-organic contact; the presence of external water reduces catalytic efficiency primarily by physically separating OA from MT and hindering that contact. This study elucidates the controlling mechanisms of MT-catalyzed thermal cracking under different water conditions and deepens our understanding of hydrocarbon-generation pathways during kerogen maturation in sedimentary basins.