Communications
dizing agents such as MnO2 and CrO3, or harsh mineral acids
like H2SO4 and HNO3, which result in hazardous, corrosive and
toxic operating conditions as well as the generation of large
quantities of polluting waste by-products.[3] Due to the height-
ened demand for the chemical and related industries to
reduce their negative impact on the environment, there is
a significant drive to find alternative economic and sustainable
oxidation reagents and synthetic protocols.[3] The abundance
of molecular oxygen, its high atom-economy in oxidation reac-
tions, low cost and mild reaction conditions make it an appeal-
ing candidate as an oxidant,[4] although high-pressure and rela-
tively high reaction temperatures are often required, since the
reaction occurs in the gas phase. Similarly, simple peroxides
such as H2O2 and tert-butyl hydroperoxide (TBHP) are often
considered as alternative green oxidants.[4] Although, their use
often leads to the generation of larger quantities of by-prod-
ucts in comparison to reactions with molecular oxygen, these
oxidants are slightly more reactive due to the lower bond dis-
sociation energy (BDE) of the single OÀO bond (49.7 kcalmolÀ1
for H2O2 and 45.0 kcalmolÀ1 for TBHP)[5] relative to the O=O
bond (118 kcalmolÀ1),[6] with the added potential to operate
under comparatively mild conditions. Nonetheless, suitable cat-
alysts are often still required to activate the oxidants in order
to achieve satisfactory activity and selectivity within reasonable
reaction conditions.
By using platinum nanoparticle catalysts that are generated in
situ by extrusion from a porous copper chlorophosphate
framework, the role of oxidants in the selective oxidation of
benzyl alcohol to benzaldehyde was evaluated, with a view to
establishing structure–property relationships. With a detailed
study of the kinetic properties of the oxidation reaction, it has
been determined that the aerobic oxidation pathways progress
with lower levels of product selectivity and higher activation
energies (72.4 kJmolÀ1) than the peroxide-based ones
(23.6 kJmolÀ1); affording valuable insights in the design of
solid catalysts for selective oxidation reactions. Furthermore,
through the use of X-ray absorption spectroscopy, the effect of
calcination temperature on the degree of extrusion and its in-
fluence on nanoparticle formation have been evaluated, lead-
ing to the establishment of structure–activity correlations be-
tween the observed activation energies and the proportion of
nanoparticle species generated.
Oxidation reactions are ubiquitous in the pharmaceutical and
fine-chemical industries, and are fundamentally important for
introducing oxygen-rich functional groups to organic mole-
cules. For example, aldehyde- and ester-containing molecules
are heavily employed as flavouring and fragrance agents
owing to their volatility and distinctly aromatic nature they are
often pleasing to smell and taste.[1] Benzaldehyde is a key com-
ponent for most almond-based flavourings, although it finds
other uses as a precursor for plastic additives and as an inter-
mediate in the synthesis of antibiotic drugs such as chloram-
phenicol and ampicillin, as well as stimulants like ephedrine.[2]
Industrially, it is synthesized by the hydrolysis of benzal chlo-
ride; however, the dehydrogenative oxidation of benzyl alcohol
is a viable, chloride-free and environmentally benign alterna-
tive.[2]
In recent years, supported noble-metal nanoparticle (NP) cat-
alysts have shown great promise in the activation of both mo-
lecular oxygen[7] and simple peroxides[8] for the selective oxida-
tion of alcohols, and more recently, the activation of CÀH
bonds in hydrocarbon substrates such as toluene.[8c,9] We re-
cently reported a new method for the synthesis of highly
active, uniform and site-isolated metal NPs in situ, by extrusion
of chlorometalate precursor anions from the one-dimensional
channels of a porous copper chlorophosphate (CuClP) frame-
work, for the activation of O2 and base-free oxidation of alco-
hols.[10] It was demonstrated that calcination of the as-synthes-
ised materials in air could generate fractions of supported,
active NP catalysts for the aerobic oxidations, with Pt/CuClP
showing the best activity and selectivity. The development of
sustainable oxidation catalysts relies on a detailed understand-
ing of the nature of the active sites at a molecular level, which
requires a multidisciplinary design-application approach, draw-
ing on strengths of materials chemistry and spectroscopic
characterization. By gaining in-depth knowledge of the local
structural environment of the active centres within catalytic
nanostructures (through x-ray absorption spectroscopy), we
expect that precise structure–property correlations can be es-
tablished, with a view to adopting a more generalized ap-
proach for the predictive design of single-site heterogeneous
catalysts in industrially significant, sustainable oxidation reac-
tions.
Traditionally, industrial oxidation reactions are based on the
use of stoichiometric quantities of inorganic metal-based oxi-
[a] C. S. Hinde, A. M. Gill, Dr. R. Raja
University of Southampton
Southampton SO17 1BJ (United Kingdom)
[b] Dr. P. P. Wells
UK Catalysis Hub
Oxon OX11 0FA (United Kingdom)
[c] Dr. P. P. Wells
University College London
London WC1H 0AJ (United Kingdom)
[d] Prof. T. S. A. Hor
Institute of Materials Research and Engineering (IMRE)
Agency for Science, Technology & Research (A*STAR)
3 Research Link, Singapore 117602 (Singapore)
[e] Prof. T. S. A. Hor
Department of Chemistry
Herein we evaluate the propensity of Pt/CuClP catalysts
toward the activation of O2 and TBHP for the selective oxida-
tion of benzyl alcohol to benzaldehyde under batch condi-
tions. Through detailed kinetic studies, activation energies are
calculated in order to compare the ability of the catalysts to
activate both oxidants, with the selectivity profiles which were
National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
Supporting information for this article is available on the WWW under
Part of a Special Issue to celebrate Singapore’s Golden Jubilee. To view
ChemPlusChem 2015, 80, 1226 – 1230
1227
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim