A Green Cold Precipitation Route for Asphaltenes Using D-Limonene: Selective Fractionation and Molecular Characterization

Asphaltenes are the most polar and refractory fraction of crude oil, and are typically isolated using petroleum-derived precipitants (e.g., n-hexane, n-heptane) and then dissolved in aromatic solvents such as toluene, which raises safety and sustainability concerns. Here we evaluate D-limonene, a renewable terpene, as a green, room-temperature precipitant for asphaltene fractionation and benchmark it against n-alkanes and the ASTM D-6560 workflow. Multi-technique characterization (ATR-FTIR/NIR, TGA, CHN, EDS, LDI(+) FT-ICR MS, and 1H/13C NMR) shows that D-limonene yields a lower mass of precipitate yet a fraction with reduced thermal refractoriness (lowest TGA residue, broader/attenuated DTG peak). Molecular readouts indicate lower aromatic condensation/cross-linking in the precipitated subpopulation—narrower DBE envelopes by FT-ICR MS and lower aromatic carbon indices (Car_tot, Car-b, Car-j) by 13C NMR—consistent with a mechanism in which π–π/dispersion interactions retain highly condensed multi-ring aggregates in solution under cold, static conditions. These results establish D-limonene as a selective green precipitant for asphaltenes, offering immediate analytical benefits (cleaner, safer fractionation for molecular studies) and a sustainable basis for pretreatments of heavy fractions.