Surface Engineered N/S Co-Doped Porous Carbons Derived from L-Cysteine and Calcium Citrate for the Metal-free Chemoselective Reduction of p-Chloronitrobenzene

The development of sustainable and efficient metal-free catalysts for the selective reduction of halogenated nitroarenes remains a significant challenge. Herein, N/S co-doped porous carbons were synthesized via direct pyrolysis of L-cysteine and calcium citrate, systematically varying precursor ratio and temperature (750–850 °C) to tune dopant speciation, defect density, and pore architecture. Comprehensive characterization (TGA, CHNS, XPS, Raman, and N 2 /CO 2 physisorption) reveals that pyrolysis conditions govern the stabilization of catalytically relevant species, particularly pyridinic nitrogen and thiophenic sulfur, as well as the development of hierarchical porosity. The resulting materials exhibit high activity and outstanding chemoselectivity (>99%) in the hydrazine-mediated reduction of p-chloronitrobenzene to p-chloroaniline under mild conditions, without hydrodechlorination. The optimal sample (CysCit 1:2 P850) achieves the best balance between surface functionality, defect structure, and pore accessibility, leading to the highest apparent turnover frequency and improved stability. Post-reaction XPS analysis demonstrates that catalyst deactivation is primarily governed by surface-related phenomena, including partial site blockage and oxidation (N–O x and S–O x species), rather than by structural degradation of the carbon framework. These findings highlight that catalytic performance is controlled by the accessibility and local environment of active sites rather than their absolute concentration, providing key insights for the rational design of sustainable metal-free carbon catalysts for selective transformations of functionalized aromatic compounds. Schematic representation of the synthesis of N/S co-doped porous carbons from L-cysteine and calcium citrate, followed by their application in the selective reduction of p-chloronitrobenzene to p-chloroaniline. • Sustainable synthesis of N/S co-doped porous carbons from amino acid and citrate precursors. • Pyrolysis temperature and precursor ratio govern dopant speciation, defects and porosity. • Metal-free catalysts achieve highly selective hydrazine-mediated reduction of p-chloronitrobenzene. • Optimized sample (1:2, 850 °C) combines high TOF with excellent structural stability. • Mechanistic analysis suggests a combined effect of N and S active sites in selective nitro group reduction.