tert-Butylation of Phenol over Ordered Solid Acid Catalysts in Supercritical Carbon Dioxide:  Efficient Synthesis of 2,4-Di-tert-butylphenol and 2,4,6-Tri-tert-butylphenol

The tert-butylation of phenol was carried out over ordered solid acid catalysts (zeolites and MCM-41-supported heteropolyacids and rare-earth-metal triflates) in supercritical CO2. The catalytic performances in supercritical CO2 were superior to those in other reaction media such as liquid-phase and N2 atmosphere conditions. The tert-butylation of phenol gave 2,4-di-tert-butylphenol (2,4-DTBP) in 65% yield over H−Y zeolites and 2,4,6-tri-tert-butylphenol (2,4,6-TTBP) in 40% yield over Sc(OTf)3 supported on MCM-41 [Sc(OTf)3/MCM-41]. The coke formation in supercritical CO2 was much lower than that in other reaction media. These high performances are due to the high solubilities of reactants and products and to the minimal coke formation: supercritical CO2 works as the reaction medium; removes coke precursors, which are heavy aromatics from phenol and/or isobutene oligomers, before they convert to coke materials; and keeps the catalytic sites clean during the catalysis. The spatial requirement for catalytic active sites is also important for the selective synthesis of 2,4-DTBP and 2,4,6-TTBP. H−Y zeolites can afford the reaction environment to allow the formation of 2,4-DTBP, but not 2,4,6-TTBP. 2,4-DTBP can be easily formed inside H−Y zeolites and diffuse out from their 12-membered pores. Sc(OTf)3/MCM-41 can allow the formation of 2,4,6-TTBP and subsequent diffusion of the products from the channels. The yields of 2,4-DTBP and 2,4,6-TTBP were significantly influenced by reaction conditions, particularly CO2 pressure. The optimum CO2 pressure was 10 MPa at 130 °C. However, a further increase of CO2 pressure rapidly decreased the formation of 2,4-DTBP and 2,4,6-TTBP. The highly dense CO2 might decelerate the catalysis by preventing access of the reactants to the catalytic active sites. The catalysts can be recycled without significant loss of the activity and were stable after three cycles.