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...