Inorganic Chemistry
Article
We used the sequential one-pot oxidation reactions of
alcohols to carboxylic acids in a method controlling the presence
of MOF-TEMPO (TEMPO = (2,2,6,6-tetramethylpiperidin-1-
yl)oxyl) (the MOF in-and-out method). The MOF-catalyzed
aerobic oxidation of alcohols to corresponding aldehydes is well-
known chemistry and is widely studied. A variety of strategies
have been incorporated into MOFs to carry out aerobic
oxidation, and they could be generally categorized as amino-
oxy-radical-containing MOFs,26−32 metal-cation-incorporated
MOFs,33−38 and metal-nanoparticle-embedded MOFs.39−42 In
particular, a zirconium-based UiO series of MOFs (UiO =
University of Oslo) with TEMPO have been extensively studied
with respect to aerobic oxidation.30,31 Zhaung and co-workers
successfully developed an efficient aerobic oxidation of alcohols
to aldehydes with MOF-TEMPO in the UiO-68 platform in the
presence of O2, and the synergic effect between Zr nodes of
MOF and TEMPO was investigated.30
application of this study. Moreover, our MOF in-and-out
method was demonstrated to cover a wide range of substrates
producing products in relatively good yields as well as being
recycled for up to eight cycles (for the step with the MOF).
RESULTS AND DISCUSSION
■
Screening of Catalytic Conditions and Control Experi-
ments. We rationally selected a well-known, simple, stable
zirconium-based UiO-68-TEMPO (MOF-TEMPO, Scheme 2)
Scheme 2. Synthetic Scheme of MOF-TEMPO
Molecular dioxygen (O2) is commonly utilized as the final
oxidant for all the three types of MOF catalysts (Scheme 1), and
water is produced as the only byproduct. However, the oxidation
of alcohols to aldehydes has not been successful with most of the
functionalized MOFs. Until now, the overoxidation of aldehydes
to carboxylic acids by employing such MOF-based catalysts has
not been reported. This is because the second step in oxidation,
autoxidation, is inhibited by radicals such as TEMPO.43−46
Therefore, neither aerobic oxidation nor autoxidation can occur
sequentially from alcohols to carboxylic acids in the same flask or
in a single MOF pore. In the homogeneous system, the
dehydrogenation and oxidation processes have been developed
to convert alcohols to carboxylic acids with transition-metal
systems47−50 and metal-free systems.51−53 It should be noted
that the amino-oxy-radical-catalyzed aerobic oxidation of
alcohols to carboxylic acids through iron-catalyzed reactions
[the Fe(NO3)3/TEMPO system and TEMPO-consumption
process] and the electrochemical methods (with amine-
functionalized TEMPO) has been recently illustrated in the
case of homogeneous catalysis.54−62 Finally, a TEMPO-
functionalized polymer was also utilized for the oxidation of
alcohols to carboxylic acids in mechanochemical systems.63 This
polymer-based catalyst, however, is the combinatorial system
between TEMPO and another oxidant: Oxone. TEMPO is used
in the aerobic oxidation of alcohols to aldehydes followed by the
Oxone-catalyzed second oxidation to carboxylic acids. The
catalysis is not able to be recycled without refilling Oxone.
To achieve selective transformation from alcohols to
carboxylic acids using heterogeneous, well-known, TEMPO-
functionalized MOFs, our MOF in-and-out method was
thoroughly investigated via optimization and control experi-
ments. Research on the MOF-catalyzed oxidation of alcohols to
carboxylic acids has been sluggish, relative to that of other
available catalytic systems, due to multiple limitations, including
the incorporation of the produced carboxylic acids into the
structures of MOF catalysts,27 the difficulty of controlling the
selectivity,64,65 the inhibition of overoxidation by amino-oxy
radicals,26−32 the detachment of oxy species from MOFs
through N−O cleavage (by Fe catalysts),66,67 the accumulation
of salts in MOFs (by excess amounts of OCl sources),60−62 and
the narrow scope of aliphatic substrates.68,69 Indeed, we
confirmed the facile recoverability of the MOF used for this
study, which is a significant advantage of MOFs and has not
received as much attention as their reusability in the literature.
The deterrence of MOF-TEMPO to autoxidation was also
verified to efficiently stabilize benzaldehyde in air as a practical
as a TEMPO-functionalized MOF catalyst and tert-butyl nitrite
(TBN) to design our method for continuous two-step oxidation
processes from alcohols to carboxylic acids.30,31 The main
ligand, TPDC-TEMPO (TPDC = p-terphenyl-4,4″-dicarboxylic
acid), and MOF-TEMPO (from the reaction with ZrCl4) were
prepared by previously reported procedures with minor
modifications, as illustrated in Scheme 2.30,31 MOF-TEMPO
as the main MOF catalyst was obtained by isolating pale-pink
octahedral crystals. The structure of MOF-TEMPO was
confirmed by powder X-ray diffraction (PXRD) patterns (Figure
S1). The TEMPO functionality in MOF was characterized by
1H nuclear magnetic resonance (NMR) spectroscopy after acid
digestion (Figure S2). Finally, N2 adsorption/desorption
experiments were conducted to determine the porosity and
surface area of MOF-TEMPO (Figure S3). The BET
(Brunauer−Emmett−Teller) surface area was calculated to be
871 m2/g, and the pore size distribution analysis through the
NLDFT (nonlock density functional theory)70 model displayed
12 and 15 Å for the main contributions (Figure S3). Since the
TEMPO moiety connected to an amide group is quite large, the
surface area and the pore size were decreased, compared to those
of pristine UiO-68 (2143 m2/g for the BET surface area and 16.4
to 17.7 Å for the pore size).71,72
As the first step, the aerobic oxidation of benzyl alcohol (1a)
to benzaldehyde (2a) was first optimized through a combination
of MOF-TEMPO and TBN, as summarized in Table 1 (entries
1−3). No significant conversion of 1a to 2a was observed by
MOF-TEMPO or TBN alone in acetonitrile (CH3CN) under an
O2 atmosphere. When both MOF-TEMPO and TBN were
introduced into the reaction solution (entry 4), 1a was nearly
fully oxidized to 2a at room temperature within 2 h. Notably, in
the presence of MOF-TEMPO, the overoxidation of 2a to
benzoic acid (3a) was not indicated under any conditions, as
confirmed by TLC (thin-layer chromatography), GC (gas
chromatography), and 1H NMR spectroscopy (Table 1, entry 4
and Table S1, entry 5). It should be noted that 3a was not
produced in the previous study for the aerobic oxidation of 1a to
2a by elevating the temperature to 80 °C under the same
catalytic condition (UiO-68-TEMPO and TBN).30 As men-
tioned earlier, the optimized conditions involve two steps: (i) 2a
C
Inorg. Chem. XXXX, XXX, XXX−XXX