Selective oxidation of cyclooctene over copper-containing metal-organic frameworks

Abstract

The selective oxidation of cyclooctene with tert-butylhydroperoxide (TBHP) was studied over Cu-containing MOFs at 348 K in liquid toluene. With Cu3(BTC)2, complete conversion was achieved after 120 h. The epoxide selectivity steadily increases to >80%. The conversion is accelerated by a radical initiator and decelerated by a radical inhibitor. A radical-based mechanism is proposed with alkoxy- or polyperoxides being formed as intermediates to the epoxide. The comparison of the cyclooctene conversion with TBHP over Cu3(BTC)2 with FeBTC, the ion-exchanged zeolites Na,Cu-Y and Na,Fe-Y and TS-1 proves that Cu3(BTC)2 is active for radicals generation from TBHP, but less suitable for catalyzing alkene epoxidation. The two recently reported Cu-MOFs [Cu(Me-4py-trz-ia)]∞3 and [(Cu4(μ4-O)(μ2-OH)2(Me2trz-pba)4]∞3 are up to four times more active for cyclooctene oxidation than Cu3(BTC)2. This was attributed to the specific coordination geometry of the catalytically active Cu sites within these materials. Moreover, the liquid-phase conversion of cyclooctene with TBHP is shown to be a sensitive test reaction to evaluate the potential of new solid oxidation catalysts, especially of transition metal-containing MOFs.