Enhanced in-situ pyrolysis of oil shale by supercritical CO₂ coupled with nanocatalysts: Mechanisms and product evolution

To address the bottlenecks of high temperatures, prolonged reaction durations, and low yields during the in-situ conversion of oil shale, a multi-metal embedded nano-catalyst was synthesized via a hydrothermal method in this study. The prepared catalyst effectively lowered the pyrolysis temperature of oil shale and enhanced hydrocarbon recovery. Comprehensive characterization confirmed that the synthesized catalyst exhibited a well-defined framework, which was consistent with the design intent, with uniform crystal sizes of 10–30 nm and excellent thermal stability. At 350 °C, the total acidity of the prepared catalyst reached 203.97 μmol·g⁻¹ . Pyrolysis experiments demonstrated that under a supercritical CO₂-assisted catalytic system, a hydrocarbon yield of 48% was achieved at 350 °C, significantly reducing the required pyrolysis temperature compared with 39% at 450 °C under N₂. Product analyses by GC–MS and GC revealed a clear shift toward lighter fractions and increased olefin production, with olefin content rising by 17.4% and heavy fractions being suppressed, thereby improving oil and gas quality. Two-dimensional NMR spectra, density distribution profiles, and molecular dynamics simulations of density distributions and interaction energies provided further evidence that the synergistic interaction between supercritical CO₂ and the multi-metal composite catalyst facilitates kerogen decomposition, promotes its conversion into mobile oil, weakens the adsorption of hydrocarbon products on the shale, and enhances the migration of hydrocarbons. These results demonstrate that the supercritical CO₂–nanocatalyst strategy provides an efficient pathway to improve yield and product quality at lower effective temperature, offering practical implications for cleaner and more efficient in-situ upgrading of oil shale.