Mesoporous BiCl₃ immobilized silicic acid as a stabilized Lewis acid catalyst for the selective synthesis of 4-nitro-o-xylene from the nitration of o-xylene with NO₂ under mild conditions

A solvent-free, acid-free, and efficient strategy was developed for the selective preparation of 4-nitro-o-xylene (4-NOX) from the catalytic nitration of o-xylene with NO₂ mediated O₂ over BiCl₃ immobilized silicic acid catalyst (BiCl₃-SA). The results indicated that the Lewis acid BiCl₃-SA conjoined NO₂–O₂ as a composite system synergistically promotes o-xylene conversion and 4-NOX selectivity. Under optimal conditions, 52.4% of o-xylene conversion with 68.4% 4-NOX selectivity was obtained at 35 °C. The characterization demonstrated that the highly dispersed metal Bi species form stable chemical bonds with the SA surface, which can effectively inhibit the loss of metal species and generate abundant acid sites. The developed catalyst is not only inexpensive but also has excellent stability and catalytic performance. Furthermore, a plausible mechanism for the catalytic nitration of o-xylene using NO₂ as a nitration agent over BiCl₃-SA was proposed. This work provides an eco-friendly and practical protocol for improving desirable 4-NOX selectivity and reducing the discharge of acidic wastewater, with potential application prospects.