Inorganic Chemistry
Article
their dual acid/base character and cooperative work of Lewis
and Brønsted sites.27,33−35
EXPERIMENTAL SECTION
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Instrumentation. GC analyses were performed using a gas
chromatograph Chromos GC-1000 equipped with a flame ionization
detector and a quartz capillary column BPX5 (30 m × 0.25 mm). GC−
MS analyses were carried out using an Agilent 7000B system with the
triple-quadrupole mass-selective detector Agilent 7000 (ZB-Wax, 25 m
× 0.25 mm × 0.25 μm). HPLC measurements were performed using
HPLC Agilent Technologies 1220 Infinity LC using ZORBAX Eclipse
Plus C-18 column (4.6 × 150 mm, 5-Micron, H2O−iPrOH = 40:60 (for
MPS conversion) or H2O−MeCN = 40:60 (for MPSO2 yield), 1 mL/
To date, the catalytic performance of Zr-MOFs, including
UiO-66 (without any MOF modification with other transition
metals), in oxidation catalysis has been much less explored,36−42
although UiO-66 demonstrated fairly good stability in the
presence of aqueous H2O2.16 The majority of publications deal
with oxidations of S-compounds, notably oxidative desulfuriza-
tion of thiophenes36−38 with aqueous H2O2 as oxidant. It was
proposed that the catalytic activity of UiO-66 in the oxidation of
S-compounds correlates with the number of missing-linker
defects36,38 and accessible “open” sites39 in the crystalline
structure. Importantly, it was also demonstrated that the H2O2-
based oxidation catalysis over UiO-66 has a truly heterogeneous
nature, that is, the reaction occurs on the catalyst surface and is
not due to Zr species leached into solution.36
Very few attempts were undertaken to unravel the
mechanisms of H2O2 activation and oxygenation reactions
over Zr-MOFs. Nguyen and co-workers investigated the
oxidation of methyl phenyl sulfide (MPS) by kinetic and
computational tools and suggested that both MPS and
corresponding sulfoxide involves Zr-μ1-OOH active intermedi-
ates formed at defect open sites of UiO-66.39 A predomination
of sulfone over sulfoxide, observed in MeCN and CH2Cl2
solvents, was rationalized in the framework of a model where
sulfoxide product binds to a site adjacent to the active Zr-μ1-
OOH species, resulting in a higher degree of over-oxidation
through increased local concentration.39 However, Zheng et al.
suggested that homolytic decomposition of H2O2 producing
OH radicals is responsible for the oxidative desulfurization of
dibenzothiophene over Zr-MOFs.41
1
min, 25 °C). H NMR spectra were recorded at 400.130 MHz on a
Bruker AVANCE-400 spectrometer. X-ray diffraction patterns were
̈
collected on the Siemens D500 instrument using Cu Kα radiation.
Infrared spectra of 0.5−2.0 wt % samples in KBr pellets were recorded
on an Agilent Cary 660 FTIR spectrometer. Infrared spectra of
adsorbed CO and CDCl3 were recorded on a Shimadzu IRTracer-100
spectrometer. The Raman spectrometer T64000 (Horiba Jobin Yvon)
with the micro-Raman setup was used to measure the Raman spectra.
All experimental spectra were collected in the backscattering geometry
using the 514.5 nm line of an Ar+ laser. The spectral resolution was not
worse than 1.5 cm−1. The detector was a silicon-based CCD matrix and
cooled with liquid nitrogen. The power of the laser beam reaching the
sample was 2 mW. The band at 520.5 cm−1 of Si single crystal was used
to calibrate the spectrometer. Scanning electron microscopy (SEM)
images were acquired by means of a JEOL JSM-6460 LV microscope.
Thermogravimetric analysis (TGA) was carried out in airflow (30 mL/
min) using a NETZSCH STA 449C instrument. The sample weight
was 10 mg in all experiments and the heating rate in TG experiment was
5 °C/min.
Zr-MOF Synthesis and Characterization. UiO-66 was synthe-
sized from zirconyl chloride and H2BDC by a solvothermal method
following the procedure reported by Ragon et al.43 with some
was prepared from zirconium chloride and 4,4′-biphenyldicarboxylic
acid (H2BPDC) according to the protocol described by Katz et al.44
The structure of UiO-66 and UiO-67 was confirmed by XRD (Figures
revealed that the UiO-66 solid consisted of very small particles (ca. 20
nm), while UiO-67 had larger particles (ca. 200 nm). According to
TGA measurements (Figure S5), the structure of UiO-66 and UiO-67
samples comprised missing linker defects (10.8 BDC and 10.0 BPDC,
respectively, instead of 12 in the ideal structure). Cluster
[Zr6O4(OH)4(OAc)12]2 (Zr6)2 was synthesized by reacting ZrOCl2·
8H2O with acetic acid under solvothermal conditions.45
Catalytic Oxidation of S-Compounds. Catalytic reactions were
performed under vigorous stirring (500 rpm) in thermostat-equipped
glass vessels. Each experiment was reproduced 2−3 times. Reactions of
MPS oxidation were initiated by addition of H2O2 (0.1 mmol) to a
solution of MPS (0.1 M) in 1 mL of solvent (typically, MeCN) at 27 °C
containing 2 mg of UiO-66 or 2.4 mg of UiO-67 (7 μmol Zr). The
oxidation products were identified by the comparison of GC retention
time with the retention time of the authentic samples and by GC-MS
analysis. MPS conversion and product yields were quantified by GC
using biphenyl as internal standard.
Oxidation of thianthrene oxide (SSO) was performed at 27 °C by
addition of H2O2 (0.05 mmol) to a mixture of SSO (0.05 mmol), UiO-
66 (1 mg), and MeCN (2 mL). After 24 h, the catalyst was filtered off,
the solvent was evaporated, and CDCl3 was added to the solid to
dissolve the probe for 1H NMR analysis of the reaction products. SSO
conversion and product yields were calculated by the integration of
corresponding signals in the 1H NMR spectra, considering that the total
amount of all products is 100%.
Catalytic Oxidation of α,β-Unsaturated Ketones. Oxidation of
unsaturated carbonyl compounds was carried out at 70 °C. H2O2 (0.8
mmol) was added to a mixture containing substrate (0.1 mmol), UiO-
66(67) (5 mg), and 1 mL of solvent (MeCN or EtOAc). The reaction
products were identified by GC-MS. Substrate conversions and product
yields were determined by GC using biphenyl as internal standard.
Recently, we have found a remarkable effect of acid additives
on the selectivity of H2O2-based oxidation of cyclohexene over
Zr-MOFs (UiO-66, UiO-67, and MOF-801), which is not
accompanied by alterations in the MOF structure and formation
of new defects.42 It was demonstrated that protons facilitate
heterolytic activation of the oxidant and prevent its
unproductive degradation on Zr-MOFs, thereby leading to
selective oxygenation of the CC bond without oxidation of
the allylic C−H bonds via a homolytic mechanism.
So far, the question about the nature of the reactivity and
electronic character of the active peroxo species operating in Zr-
MOFs was not addressed, though understanding this is the key
to understanding the origin of selectivity. Motivated by this
knowledge gap, we investigated the mechanism of the oxidation
of S-compounds using a methodology that involves competitive
sulfide−sulfoxide oxidation experiments, product analysis on the
probe substrate thianthrene 5-oxide, and Hammett correlations,
along with kinetic, isotopic, and spectroscopic tools. The results
acquired in this work unambiguously indicate a predominantly
nucleophilic character of the oxidizing species formed upon the
interaction of H2O2 and Zr-MOF in nonprotic solvents and
responsible for the highly selective formation of sulfones. A
capability of the catalyst system UiO-66(67)/H2O2/MeCN to
perform epoxidation of electron-deficient CC bonds in
unsaturated ketones also supports this suggestion. FTIR
spectroscopic techniques with CDCl3 as a probe molecule and
adsorption of isobutyric acid were employed to evaluate the
presence of basic sites in UiO-66 that might be responsible for
the nucleophilic activation of hydrogen peroxide. To the best of
our knowledge, this is the first report that describes the
capability of MOFs for the formation of a nucleophilic oxidant
from H2O2.
B
Inorg. Chem. XXXX, XXX, XXX−XXX
Chesalov, Yuriy A.
