DOI: 10.1002/cctc.201501229
Communications
Ultrasmall Platinum Nanoparticles Supported Inside the
Nanospaces of Periodic Mesoporous Organosilica with an
Imidazolium Network: An Efficient Catalyst for the Aerobic
Oxidation of Unactivated Alcohols in Water
Babak Karimi,*[a] Zahra Naderi,[a] Mojtaba Khorasani,[a] Hamid M. Mirzaei,[a] and
Hojatollah Vali[b]
The imidazolium group inside the wall of a periodic mesopo-
rous organosilica provides an excellent environment for the
stabilization of ultrasmall Pt nanoparticles (<1.5 nm), and this
allowed the formation of a catalyst system (i.e., Pt(NP)@PMO-IL)
with significant activity and recyclability in the selective aero-
bic oxidation of various alcohols in water at ambient pressure
of oxygen. In particular, the catalyst exhibited high activity in
the oxidation of unactivated primary alcohols and sterically en-
cumbered secondary aliphatic alcohols, which remain challeng-
ing substrates for many catalytic aerobic protocols.
Among them, platinum nanoparticles have shown excellent
catalytic activity owing to their high oxidizing ability (Pt/Pt2+
Eox = +1.12 V).[5d] In this respect, various platinum-based cata-
lysts either in the homogeneous or supported form such as
soluble Pt nanoparticles (PtNPs), Pt/SBA-15, Pt/Al2O3, Pt/C, Pt/
polymer, Pt/Bi2O3, Pt/hC (hollow porous carbon shell), and Pt-
Bi/C have been used in the aqueous aerobic oxidation of alco-
hols.[6] Whereas these methods provided interesting results, it
is evident that many of them suffer from high catalyst loading
(up to 10 mol%), poor reusability, limited substrate scope, low
activity, low selectivity in the case of unactivated alcohols, as
well as catalyst agglomeration under the reaction conditions.
In addition, two important issues may limit the widespread ap-
plication of platinum nanoparticles: catalyst poisoning and de-
activation through product adsorption and chemisorption of
oxygen on the catalyst surface to generate inactive Pt oxide
species.[5] Therefore, despite their high catalytic activity it
seems that the development of highly stable and durable cata-
lytic systems based on PtNPs is still hindered by many chal-
lenges. Ionic liquids (ILs) have been found to be appropriate
media to generate and stabilize highly dispersed metallic
nanoparticles without extra stabilizing molecules.[7] However,
despite the successful examples of catalysis in ILs through bi-
phasic homogeneous/semi-heterogeneous modes, their appli-
cation in process chemistry is still hampered because many of
them are very expensive and are a cause of toxicological con-
cern.[8] Moreover, catalyst recycling and separation in homoge-
neous IL-based systems are not always satisfactory. To partly
overcome these issues, the concept of supported ionic liquid
phase (SILP) catalysis has emerged, which combines the advan-
tages of ionic liquids (e.g., stabilizing role for metal NPs) with
those of heterogeneous systems (e.g., easy recyclability).[9]
However, it has been shown that in many cases the ionic-
liquid layer, which serves as the reaction phase, can be partially
dissolved in the reaction phase, which thus restricts catalyst re-
covery. To address these issues, we recently introduced several
periodic mesoporous organosilica (PMO) with extraordinary
stable ionic liquid frameworks, so called PMO-IL. These materi-
als show superior capabilities as efficient supports for the im-
mobilization and stabilization of PdNPs, RuNPs, and AuNPs in
CÀC bond-forming reactions, the aerobic oxidation of alcohols,
and the synthesis of propargylamines through A3 coupling re-
actions.[10]
The selective oxidation of alcohols to carbonyl compounds is
an important research area owing to the widespread utility of
the products.[1] This transformation is traditionally performed
by using hazardous oxidants in non-green organic solvents
under harsh reaction conditions.[2] To address these shortcom-
ings, many researches have recently focused on the develop-
ment of ecofriendly and recoverable catalytic systems based
on transition-metal species to achieve effective aqueous-phase
oxidation of alcohols by using O2 as the final oxidant.[3] Howev-
er, successful examples of the direct and selective catalytic
aerobic oxidation of primary alcohols to their corresponding
carboxylic acids in water are rare.[4] The use of water as a reac-
tion medium offers some advantages, because it is a cheap,
odorless, nontoxic, and readily available solvent. However, the
performance of the catalyst might be lower than expected be-
cause of poor solubility of the organic substrate and/or metal
catalyst deactivation in the aqueous medium.
On the other hand, nanosized metal catalysts, mainly plati-
num, gold, and palladium nanoparticles, have been found to
be promising catalysts for this interesting transformation.[5]
[a] Prof. Dr. B. Karimi, Z. Naderi, Dr. M. Khorasani, Dr. H. M. Mirzaei
Department of Chemistry
Institute for Advanced Studies in Basic Sciences (IASBS)
P.O. Box 45137-66731, Gava Zang, Zanjan (Iran)
Fax: (+98)24-33153225
[b] Prof. H. Vali
Department of Anatomy and Cell Biology and
Facility for Electron Microscopy Research
McGill University
Montreal, Quebec, H3A 2A7 (Canada)
Supporting Information for this article is available on the WWW under
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