Synthetic strategy for Fe-MCM-41 catalyst: a key factor for homogeneous or heterogeneous phenol oxidation

Iron(III) was introduced into the mesoporous molecular sieve MCM-41 using two protocols: (i) iron nitrate was added in situ in the course of MCM-41 synthesis; (ii) a nitrate or triiron oxoacetate complex was loaded into the as-synthesized MCM-41 sieve by using incipient wetness impregnation method. In both cases the iron content was in the range of 1-11 wt%. The prepared materials were characterized by N-2-BET, XRD, H-2-TPR, NH3-TPD, ESR and Mossbauer spectroscopy techniques. All the catalysts were found to exhibit high enough activity in the liquid phase oxidation of phenol by hydrogen peroxide. At 25 degrees C the oxidation of phenol occurs in homogeneous phase and is mediated only by the Fe3+ ions leached from the solid, while at 80 degrees C the framework iron atoms mainly contribute into the total activity so that the mechanism of true heterogeneous catalysis is operative. For iron content of 0.25 wt% and less, the framework iron is fairly stable and highly resistant to the acidic conditions of liquid-phase reactions.