Selective Chlorination of Toluene and Halobenzenes Using Modified BaSO4-Supported Catalysts: A Sustainable Approach with (NH4)2S2O8 and H2O2 as Oxidants

In this study, nine barium sulfate (BaSO4)-supported heterogeneous catalysts (C1–C9) were synthesized via three modification strategies: stearic acid coating (C1–C3), physical doping (C4, C6, and C8), and sol-gel processing with calcination (C5, C7, and C9). Their comprehensive characterization revealed that sol-gel-synthesized C7 exhibited the most favorable surface properties, including well-dispersed Al–O–Si species, tetrahedrally coordinated Al3+, and abundant Brønsted acid sites. Their catalytic performance was evaluated in the chlorination of toluene, fluorobenzene, bromobenzene, and iodobenzene, using hydrochloric acid (HCl) as the chlorine source and either hydrogen peroxide (H2O2) or ammonium persulfate ((NH4)2S2O8) as the oxidant. C7 achieved complete toluene conversion (100%) at 60 °C under optimized conditions and exhibited high conversions of fluorobenzene (55%), bromobenzene (76%), and iodobenzene (46%). Notably, ammonium persulfate enabled a unique in situ halogen exchange pathway, yielding chlorobenzene as the exclusive product from bromobenzene and iodobenzene. XRD and XPS analysis of crystalline by-products confirmed the formation of NH4HSO4, BaSO4, and NH4Cl, providing evidence for the persulfate-driven radical mechanism. Iodine detection in upper-layer crystals confirmed iodobenzene products, while the absence of chlorine signals in the upper layer confirmed separation of organic and inorganic species. The detection of barium sulfate peaks confirms that the catalyst support retains its structural integrity under harsh reaction conditions, demonstrating chemical stability and reusability potential. Collectively, these findings establish a clear structure–activity relationship and demonstrate that the synergy between modified BaSO4 surfaces and persulfate-generated radicals provides an efficient, sustainable platform for aromatic chlorination, offering significant potential for pharmaceutical, agrochemical, and fine chemical manufacturing applications.