Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 139

Two different research groups recently employed magnesium oxide as a
supporting material for gold in the selective oxidation of alcohols under
solvent-free conditions, thus showing its versatility. 56,57 Choudhary and
Dumbre 56 prepared an MgO-supported nano-gold catalyst by the deposition-
precipitation technique and tested it in the oxidation of different primary
alcohols (Ph-CH 2 OH, NO 2 -Ph-CH 2 OH, MeO-Ph-CH 2 OH, Ph-CH 2 -CH 2 OH)
and aldehydes to the corresponding aldehydes and carboxylic acids, re-
spectively, by molecular oxygen (under atmospheric pressure) in the absence
of any solvent. This led to high activity/selectivity and excellent reusability.
The influence of the catalyst calcination temperature (400-900 1C), reaction
temperature (50-120 1C) and different solvents (toluene, p-xylene, DMF or
DMSO) on the oxidation reaction was also studied. The catalyst displayed
inferior performance in the presence of different solvents, whereas in all
cases the catalyst could be easily removed from the reaction mixture and
reused several times without any significant decrease in activity. One year
later, Gusevskaya and co-workers 57 showed gold nanoparticles supported on
magnesium oxide to be ecient heterogeneous catalysts for the liquid-phase
oxidation of another wide range of alcohols using molecular oxygen as a sole
oxidant, employing toluene or methanol as the solvent but in the absence
of co-catalysts or additives. The Au/MgO catalyst was also prepared through
the deposition-precipitation method and characterized by XRD, XPS, XAS,
HRTEM, UV-Vis spectroscopy and N 2 adsorption techniques. Various car-
bonylic monoterpenoids important for the fragrance and pharmaceutical in-
dustries were obtained in good to excellent yields starting from biomass-based
monoterpenic alcohols, such as isoborneol, perillyl alcohol and carveol.
Moreover, the Au/MgO catalyst allowed the selective one-pot oxidative ester-
ification of benzyl alcohol, in alkali-free methanol solutions and in the ab-
sence of any additive, to give methyl benzoate in virtually quantitative yield. 57
A novel frontier of supported gold catalysts for the selective oxidation of
alcohols is the use of carbon nanotubes (CNTs) as the supporting material.
Doris and coworkers prepared and then assessed the catalytic activity of
CNT-gold nanohybrids in the aerobic oxidation of alcohols under mild
conditions (i.e. room temperature, air), first taking benzyl alcohol as a model
substrate, and subsequently screening variously substituted primary and
secondary alcohols. 59 They found the CNT-supported gold nanohybrid to be
active for the aerobic oxidation of diverse alcohols without any added O 2 and
no heating. While alcohols and aldehydes were chemoselectively oxidized to
the corresponding acids, under anhydrous conditions, primary alcohols
could also be converted to the corresponding aldehydes. Finally, they elu-
cidated the role of water in the oxidation process by labeling water with 18 O.
This CNT-based system compares favorably with other supported gold
nanoparticle catalysts in terms of recyclability, catalytic activity, selectivity
and mildness of the operating conditions, and with the advantage of no
added O 2 .
Gold nanoparticles were supported on multi-walled carbon nanotubes
(MWCNTs) following different methods by ´ rf˜o's group. 66 The catalytic

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