Hydrogen-lean two-stage upgrading of highly acidic palm acid oil via decoupled acid reduction and deoxygenation

Upgrading highly acidic waste lipids into liquid hydrocarbons under limited hydrogen availability remains challenging due to catalyst deactivation, excessive cracking, and poor process stability. Palm acid oil (PAO), characterized by extremely high free fatty acid content, represents a particularly demanding feedstock. This study proposes a hydrogen-lean two-stage upgrading strategy that decouples acid value (AV) reduction from hydrocarbon formation through staged reactor operation. In the first stage, batch pretreatment under low hydrogen pressure (initial 0.1 MPa) enabled rapid apparent AV reduction. However, increasing temperature promoted thermally driven degradation, highlighting intrinsic limitations of single-stage severity intensification. Methanol-assisted pretreatment further decreased AV mainly through esterification and formation of oxygenated intermediates. In the second stage, hydrogen-free fixed-bed upgrading over oxide-based catalysts exhibited a distinct operating window near 365 °C, where stable condensed liquid recoveries of 50–60 wt% were obtained. Deoxygenation proceeded predominantly via decarbonylation/decarboxylation pathways. Among the catalysts investigated, ZrO₂–Co showed superior stability and lower residual AV. Overall, reactor staging enabled AV reductions exceeding 90% within 1.5–2 h while maintaining stable liquid recovery, demonstrating an effective upgrading strategy for highly acidic waste lipids under hydrogen-lean conditions.