Continuous acylation of anisole by acetic anhydride in mesoporous solid acid catalysts: Reaction media effects on catalyst deactivation

The acylation of anisole with acetic anhydride was carried out in a continuous slurry reactor over mesoporous supported Nafion® (SAC-13) and heteropolyacid (HPA) catalysts. At 70 °C, using an anisole-rich feed molar ratio of 5:1 and a space velocity of 1.6 gacetic anhydride g−1cat. h−1, acetic anhydride conversions of 40–50% with excellent selectivity (>95%) toward the primary product, p-methoxyacetophenone (p-MOAP), were observed at time on stream (TOS) of a few hours. However, all the catalysts deactivated completely during liquid-phase operation in less than 24 h. It was observed that the Keggin ions from the supported HPA-based catalyst (70% HPA/SiO2) leached out into the solution, as confirmed by elemental analysis. The 50% Cs2.5-HPA/SiO2 catalyst, on the other hand, was more leach-resistant, yet deactivated rapidly during liquid-phase operation. SAC-13-type catalysts, which displayed the best combination of stability and leach resistance during liquid-phase operation, were evaluated in CO2-expanded liquids (CXLs) to better enhance the transport properties and potentially mitigate deactivation. It is observed that the CXL media gave lower conversion and surprisingly, faster deactivation compared with liquid-phase operation, indicating that CO2 had a detrimental effect despite the use of polar cosolvents like nitromethane. The spent catalysts were subjected to Soxhlet extraction with polar solvents like nitromethane. Such treatment did not restore catalyst activity. BET surface area, pore volume of the fresh and spent catalysts, GC/MS analysis of the Soxhlet extract, and IR analysis of the spent catalyst (before and after Soxhlet extraction) indicate that the deactivation could be caused by the primary product, p-MOAP and/or multiply acetylated products in the micropores of Nafion® catalyst aggregates. Treating the spent catalyst with boiling HNO3 solution restored complete activity of the SAC-13-type catalysts. The high TON (∼400) achieved with these catalysts before deactivation and their ability to regain complete activity for acylation reactions indicate that Nafion® catalysts are promising alternatives to the conventional homogeneous Lewis acids like AlCl3.