Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 2
cyclohexane to K/A oil under UV light irradiation at room temperature [30]. Finally, a short report
emphasizing the importance of reliable procedures and very worth reading within cyclohexane
oxidation with Cu catalysts is ref. [179].
2.3.5 Hydroxylation of aromatics
The oxidation of benzene or other aromatics to phenol or hydroxylated aromatics with hydrogen
peroxide is a reaction which has experienced considerable attention with heterogeneous Cu
catalysts. Apparently, two different classes of Cu catalysts exist: those which require acetonitrile as a
solvent and those preferring other reaction media; the former will first be described.
2.3.5.1 Hydroxylation in acetonitrile
Acetonitrile is in some cases assumed to participate directly in the catalytic oxidation [180] therefore
being consumed. The oxidation of benzene also proceeds in other solvents but often either highly
unselectively or with low rates [31, 45, 180]. Reactions are typically carried out at temperatures
around 60 °C; higher reaction temperatures cause H2O2 decomposition. When comparing Cu with
oxidic V and Fe incorporated in SiO2‐based amorphous microporous mixed oxides, the latter two
metals gave higher yields [180]. TOFs for Cu were around 2 h ‐1 at 60 °C (Table 2‐6, entry 1).
Increasing the hydrophobicity of the silica matrix by end‐capping with methyl groups had a negative
effect though the surface areas were nearly unaffected. Leaching was experienced with every
catalyst though doping with Al helped in the case of Fe. The effect of Al‐doping on Cu leaching was
not investigated. In another study, co‐impregnation of Cu and Al indeed had a beneficial effect on the
catalytic activity [31]. Cu(II)‐doped materials (AlPO4‐5 and MCM‐41) gave higher phenol yields than
impregnated catalysts with TOFs around 30‐40 h ‐1 (entry 2). Catalysts featuring isolated Cu sites are
often superior to supported CuxO particles especially where leaching is a major problem.
Interestingly, incorporation of Cu in MCM‐41 did not have the often observed negative effect on the
silica material when Al was co‐doped. Benzene could be hydroxylated to phenol with nearly 100 %
selectivity. Alkylated aromatics underwent both ring and side‐chain hydroxylation though usually
with good to moderate yields to the ring hydroxylation product. The oxidation of 2,3,5‐
trimethylphenol (TMP) to 2,3,5‐trimethylbenzoquinone (TMBQ) also proceeded in benzaldehyde as
the solvent using molecular oxygen as the oxidant. These conditions are typical for epoxidation
reactions. Consequently, Cu‐AlPO4‐5 and Cu‐Al‐ MCM‐41 might also be active in epoxidation
reactions (note however that steel is already capable of activating aldehydes for epoxidation
reactions [189]). Detailed studies on TMP oxidation with Cu‐Al‐MCM‐41 were reported in ref. [38]
(entry 3). TOFs around 8 h ‐1 were measured for the hydroxylation of benzene with Cu‐AlPO4‐5 and
Cu‐AlPO4‐11 at slightly higher reaction temperatures (entry 4) [41]. Contrary to the previous paper,
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