Fluorinated solvent-assisted photocatalytic aerobic oxidative amidation of alcoholsviavisible-light-mediated HKUST-1/Cs-POMoW catalysis

Article abstract of DOI:10.1039/d1nj02401a

Considering the irreplaceable importance of photocatalytic functionalization reactions and the widespread attention paid to the use of metal-organic frameworks, especially their modified variants, for this purpose in recent years, different types of HKUST-1/POMoW composites were prepared through the immobilization of a series of Keggin-type polyoxometalates (POMs; POW = H₃PW₁₂O₄₀, POMo = H₃PMo₁₂O₄₀, and POMoW = H₃PMo₆W₆O₄₀) on HKUST-1 as a metal-organic framework (HKUST-1; Cu₃(1,3,5-benzenetricarboxilicacid)₂). Then, to produce HKUST-1/Cs-POM, the substitution of H⁺cations with Cs⁺ones as counter cations was carried out. The prepared composites were fully characterized with the PXRD (powder X-ray diffraction), FT-IR (Fourier transform infrared spectroscopy), BET and BJH (sorption of N₂), TGA (thermo-gravimetric analysis), SEM (scanning electron microscopy), EDX (energy dispersive X-ray), TEM (transmission electron microscopy), UV-vis DRS (diffuse reflectance UV-vis spectroscopy), photoluminescence (PL) spectroscopy and ICP-AES (inductively coupled plasma atomic emission spectroscopy) techniques. The great importance?of the amide functional group and the attractiveness of photocatalytic oxidative functionalization?reactions led us to study the formation of this functional group using the prepared catalytic system in line with our previous research in this field. The HKUST-1/Cs-POMoW composite showed a raised photocatalytic performance compared to the discrete components, HKUST-1 and Cs-POMs, in aerobic oxidative amidation of alcohols under illumination with visible light, owing to the presence of catalytically active Cs-POMs deposited on the MOF particles. Besides, the combination of composite components mitigated the recombination rate of the electron-hole pairs, raising its photocatalytic activity. The attractiveness of fluorine solvents for oxidation reactions has led to the study of their role in the efficiency of oxidative amidation of alcohols and their significant effect on the efficiency of the process has been confirmed. The Cu-MOF/POM catalyst showed excellent stability during the reaction, and no significant decrease in its ability was observed during five consecutive cycles.

Full text of DOI:10.1039/d1nj02401a

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Fluorinated solvent-assisted photocatalytic
aerobic oxidative amidation of alcohols via
visible-light-mediated HKUST-1/Cs-POMoW
catalysis†
Cite this: New J. Chem., 2021,
45, 14024
Meghdad Karimi, ‡a Samira Sadeghi,‡a Haleh Mohebali,‡a Zahra Azarkhosh,b
c
a
Vahid Safarifard,
Alireza Mahjoub
and Akbar Heydari*a
Considering the irreplaceable importance of photocatalytic functionalization reactions and the widespread
attention paid to the use of metal–organic frameworks, especially their modified variants, for this purpose in
recent years, different types of HKUST-1/POMoW composites were prepared through the immobilization of
a series of Keggin-type polyoxometalates (POMs; POW = H3PW12O40, POMo = H3PMo12O40, and POMoW =
H3PMo6W6O40) on HKUST-1 as a metal–organic framework (HKUST-1; Cu3(1,3,5-benzenetricarboxilicacid)2).
Then, to produce HKUST-1/Cs-POM, the substitution of H+ cations with Cs+ ones as counter cations was
carried out. The prepared composites were fully characterized with the PXRD (powder X-ray diffraction),
FT-IR (Fourier transform infrared spectroscopy), BET and BJH (sorption of N2), TGA (thermo-gravimetric
analysis), SEM (scanning electron microscopy), EDX (energy dispersive X-ray), TEM (transmission electron
microscopy), UV-vis DRS (diffuse reflectance UV-vis spectroscopy), photoluminescence (PL) spectro-
scopy and ICP-AES (inductively coupled plasma atomic emission spectroscopy) techniques. The great
importance of the amide functional group and the attractiveness of photocatalytic oxidative functionali-
zation reactions led us to study the formation of this functional group using the prepared catalytic system in
line with our previous research in this field. The HKUST-1/Cs-POMoW composite showed a raised
photocatalytic performance compared to the discrete components, HKUST-1 and Cs-POMs, in aerobic
oxidative amidation of alcohols under illumination with visible light, owing to the presence of catalytically
active Cs-POMs deposited on the MOF particles. Besides, the combination of composite components
mitigated the recombination rate of the electron–hole pairs, raising its photocatalytic activity. The attractive-
ness of fluorine solvents for oxidation reactions has led to the study of their role in the efficiency of oxidative
amidation of alcohols and their significant effect on the efficiency of the process has been confirmed. The
Cu-MOF/POM catalyst showed excellent stability during the reaction, and no significant decrease in its
ability was observed during five consecutive cycles.
Received 15th May 2021,
Accepted 15th June 2021
DOI: 10.1039/d1nj02401a
rsc.li/njc
materials to moisture, solvents, and temperature, providing active
agents in stoichiometric amounts, and quite significantly, creating a
1. Introduction
Since amide bonding has extensive use in natural materials, tremendous amount of unfortunate by-products.1 Therefore, after
biological compounds, and supramolecules, such as peptides Nakagawa innovated in 1995 on oxidative amidation, researchers
and proteins, numerous methods have been developed to form have considered various oxidative strategies.2 Following these minus
such compounds. However, conventional methods via an acti- points, researchers have made efforts to develop catalytic methods
vated carboxylic acid are associated with some drawbacks, instead of traditional ones by using different precursors such as
including low economic efficiency, the sensitivity of the raw ammonia,3 primary, secondary and tertiary amines,4 aldehydes,5
alcohols6–10 and carboxylic acids,10,11 as well as compounds contain-
ing carbon–carbon12 and carbon–nitrogen multiple bonds.13
In this context, a great deal of attention is paid to creating
appropriate catalytic systems for performing oxidation reactions
efficiently. These catalytic systems have mainly been utilized under
wet oxidation environments using different oxidants such as H2O2
and TBHP. However, some issues arise from toxic oxidants that
have occupied chemists’ minds.14–17
a Chemistry Department, Tarbiat Modares University, P.O. Box 14155-4838, Tehran,
Iran. E-mail: heydar_a@modares.ac.ir; Tel: +98 21 82883444
b Chemistry Department, Payam-e-Noor University, Golpayegan, Isfahan, Iran
c Department of Chemistry, Iran University of Science and Technology,
Tehran 16846-13114, Iran
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj02401a
‡ These authors contributed equally.
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Recently, much research has been performed on the design enhancing charge carrier extraction and thus decreasing the recom-
and development of photocatalysts for selective and efficient bination rate during photoexcitation. Moreover, perfluorinated sol-
oxidation reactions.18,19 In heterogeneous photocatalysts used vents used as the reaction medium had a significant effect on the
in chemical transformations, some popular semiconductors photocatalytic activity. In the case of stability, the photocatalytic
such as TiO2, WO3, Ta2O5, ZnS, CdS, and In2O3 are used system was also stable with good recycling.
accompanied by some other specific and complex materials
in their preparation process.20,21
As for some of such tough compounds, polyoxometalates
(POMs) have been used quite often as they have some compel-
2. Experimental section
2.1. Materials and instrumentation
ling features which are the same as the properties of semicon-
All materials were purchased from Merck and used without
ductor photocatalysts, such as fully-specified HOMO–LUMO
further purification. Infrared (IR) analyses were performed
band gaps, hampering the recombination of photogenerated
electron–hole pairs in mild conditions.22–25 Accordingly, they
using a Shimadzu FTIR-8400S spectrometer using KBr pellets
in the range of 400–4000 cmÀ1. Thermogravimetric analysis was
have been utilized in various chemical transformations, includ-
performed with a thermal analyzer with a heating rate of
ing alcohol oxidation, activation of alkenes, and degradation of
20 1C minÀ1 in the temperature range T = 25–700 1C under
pollutants in aqueous environments.26
flowing nitrogen at 50 mL minÀ1. Powder X-ray diffraction
Among assorted POMs, the ones with cesium salts as acidic
(PXRD) measurements were performed using a Philips X’pert
sites are the most attractive candidates because of their excel-
diffractometer with monochromated Cu Ka radiation. Investi-
lent shape selectivity, insolubility in different solvents, and
high catalytic activity;27 however, due to their low surface area,
gation of the morphology and surface properties was performed
using SEM on a Zeiss-Sigma VP 500 and transmission electron
chemists usually prepare the composite form of POMs, espe-
cially with metal–organic frameworks (MOFs).28,29 MOFs are a
microscopy (TEM) at 120 kV (Philips, model CM120). Nitrogen
adsorption–desorption analysis of the prepared catalysts was
new category of porous crystalline materials consisting of metal
performed using a BEL: PHS 1020. A Shimadzu-UV-2550 spec-
ions or clusters and organic ligands with a variety of functional
groups29 which endow them with favorable properties such as
trophotometer was employed to observe the UV-vis diffuse
reflectance spectrum (DRS). Room temperature photolumines-
ultrahigh porosity and specific surface area, variability of the
pore size, regular cavities,30 open metal sites (Lewis acid),31 the
cence spectra (PL) were detected with an RF6000 Shimadzu
possibility of p–p stacking and hydrogen bonding,32
fluorescence spectrophotometer. ICP analysis was performed
stability,33,34 ligand-based designable structure and interac-
using a VISTA-PRO CCD simultaneous ICP system. Thermal
tions with reactive species.35
stability of composite was analysed and sketched on a plot
through a gradual heating rate of 20 1C minÀ1 over a tempera-
These features pave the way for MOFs to be used in applica-
tions such as drug delivery,36 energy storage,37 molecular
ture range of 25–1000 1C under a stream of purified N2. Primary
sensing,38 environmental remediation39 and catalysis.40–43
results for the reaction progress were obtained by gas chroma-
tography (GC) (Yonglin 6100; BP-5; 30 m  0.25 mm  0.25 mm)
MOFs can act as nanoreactors as the presence of large pore
with toluene as a general solvent for injection and nitrogen as
volumes and open-metal sites enables them to control and
an inert carrier gas. All yields refer to isolated products.
adjust the transport of reactive species and products through
Lewis acid–base interactions.44 Besides, since there are rapid
2.2. Catalyst preparation
metal-to-ligand and ligand-to-metal charge transfer within
MOFs, they can facilitate the spatial separation of photo-
2.2.1. Synthesis of HKUST-1. HKUST-1 was synthesized
induced charge carriers (electrons and holes) and suppress based on Abedi et al.’s procedure.51 Typically, 3.36 mmol
recombination, which is considered as an inhibitor of photocata- (0.71 g) of benzene-1,3,5-tricarboxylic acid (H3BTC) and 6 mmol
lytic processes.45,46 Therefore, it is concluded that the synergistic (1.4 g) of copper nitrate trihydrate were added to 40 mL DMF
effect between the superior properties of MOFs and POMs estab- and stirred for almost 2 h at ambient temperature. Then, the
lishes a highly efficient photocatalytic system.47–49
prepared mixture was transferred into a glass vial and kept at
Following our previous work,7,9,50 herein, HKUST-1 was used as a 80 1C in an oven for 24 h (Scheme 1A). The obtained precipitate
host material for the composite of cesium salt-loaded tungsten- was filtered, rinsed with EtOH and DMF, dried at 80 1C, and
substituted molybdophosphoric acids (Cs-POMs; POW = H3PW12 washed completely with hot EtOH/DMF. Eventually, a blue
O40, POMo = H3PMo12O40, and POMoW = H3PMo6W6O40). All the solid, which is HKUST-1, was obtained and dried in air at
composites denoted as HKUST-1@Cs-POMoW were used for photo- 110 1C overnight.
catalytic oxidative amidation of alcohols. Fortunately, because of a
2.2.2. Synthesis of HPOMoW. Tungsten-substituted molyb-
good match between the band gaps of HKUST-1 and Cs-POMoW, dophosphoric acid with a general formula of H3PMo6W6O40
the prepared composite showed supreme photocatalytic activity for was prepared based on a stepwise procedure reported by
aerobic oxidative amidation of benzyl alcohol under visible light Huixiong et al.52 Initially, a 12.5 mL solution of 178.96 g per
irradiation. The generated heterojunction between HKUST-1 and L Na2HPO4 in distilled water was mixed with a 25 mL solution
Cs-POMoW was a direct Z-scheme type, in which the photo-induced of 362.8 g per L of sodium molybdate in distilled water to
electrons in HKUST-1 combined with the holes in Cs-POMoW, reach a homogeneous form. Then, this solution was warmed
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Scheme 1 Schematic diagram of HKUST-1/Cs-POMoW synthesis.
for 30 min at 90 1C under magnetic stirring. After that, a constant via a cooling fan under visible light illumination from
12.5 mL solution of 984 g per L Na2WO4Á2H2O in distilled a 500 W Hg lamp with high pressure. After such a period, the
water was added to the previously prepared solution. Then, to amine (1.2 mmol) was also added, and the reaction advance-
give the obtained solution a pH value of 1.5–2, concentrated ment was checked via TLC after 5 h. After assuring reaction
sulfuric acid was added to it dropwise. After the adjustment of completion, the dilution of the mixture was accomplished with
the pH value, the mixture was allowed to stir at 90 1C for 8 h, ethyl acetate, and the solid catalyst was also centrifuged and
and then the resultant heteropolyacid was extracted using rinsed four times with ethanol. The final mixture was extracted
diethyl ether and subsequently was re-crystallized (Scheme 1B). by adding water, and the organic layer was separated by column
2.2.3. Synthesis of HKUST-1@Cs-POMo6W6. HKUST-1@ chromatography. Then, all compounds were characterized
Cs-POMo6W6 composites were prepared according to the using melting point, FT-IR, 1H-NMR, and 13C-NMR methods.
method clarified as follows: 0.1 g HKUST-1 was dispersed in The obtained spectral data of the relevant compounds were
10 ml ethanol with ultrasonic irradiation to create a uniform compared with those reported in previous literature.
dispersion. Separately, specific amounts of HPOMo6W6 equal
to 10, 20, 30, and 40 wt% of the HKUST-1 weight were dissolved
3. Characterization of the HKUST-1/
Cs-POMoW catalyst
in 5 mL of ethanol. Next, the prepared HPOMo6W6 solutions
were steadily added to the former solution, and the mixture was
stirred overnight (Scheme 1C). Afterward, to introduce the Cs+
3.1. FT-IR (Fourier-transform infrared spectroscopy)
ions to the composite, a 5 mL solution of 0.6 mol LÀ1 of cesium
FT-IR spectra were used to investigate the functional groups in the
HKUST-1/Cs-POMoW structure to prove the existence of Cs-
POMoW in the catalyst structure. The FT-IR spectra of HKUST-1,
HPOMoW, Cs-POMoW, and HKUST-1/Cs-POMoW are illustrated
in Fig. 1a. There is a broad band located at the 3100–3600 cmÀ1
region in all the spectra, indicating the O–H stretching vibration
of water molecules (H2O) in all the structures. In the case of the
HKUST-1 spectrum, the two characteristic absorption bands in
the 1300–1500 cmÀ1 and 1500–1700 cmÀ1 range are ascribed to
carbonate in ethanol was poured dropwise into the mixture and
stirred at ambient temperature overnight. Finally, the obtained
mixture was filtered and rinsed many times with ethanol and
subsequently with deionized water and dried at 80 1C for 12 h
(Scheme 1D).
2.3. General method for the photocatalytic aerobic oxidative
amidation of benzyl alcohol
Oxidative amidation of benzyl alcohol was performed according stretching modes of sym(C–O2) and asym(C–O2), respectively,
to the following procedure: benzyl alcohol (1 mmol), 0.5 mL indicating the deprotonation of the BTC ligand. Also, the char-
and 2 mL of HFB (hexafluorobenzene) and acetonitrile, and the acteristic absorption band at 725 cmÀ1 is assigned to the Cu–O
catalyst (15 mg) were mixed together and stirred for 2 h at room stretching vibration. Also, there is no band related to DMF
temperature using an air balloon and under visible light molecules, showing the exchange of DMF with EtOH through
illumination. During the reaction, the temperature was kept the washing and preparation steps. There are two bands around
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Fig. 1 (a) FT-IR, (b) PXRD, (c) BET-BJH, (d) TGA, (e) UV-vis DRS, (f) Kubelka–Munk plot, and (g) PL spectroscopy of the prepared compounds.
1450 cmÀ1 and 700 cmÀ1, where the former is related to the vibration bands of Cs-POMoW in the composite compared with
combination of benzene ring stretching and deformation modes, Cs-POMoW.53 As for the carbonyl stretch vibrations of the
and the latter is due to the n(C–H) bending mode.10
carboxylate, they remain intact in the HKUST-1/Cs-POMoW
Regarding the spectrum of Cs-POMoW, some different composite; therefore, the interaction between the surface Cs-
peaks appeared, including those at 700–1100 cmÀ1, which POMoW oxygen atoms and the open copper metal sites is
can be related to the structure of cesium partially exchanged evident.
salts of POMoW. Besides, the absorption bands observed at
1060, 964, 879, and 780 cmÀ1 are allocated to P–Oa (central
3.2. PXRD (powder X-ray diffraction) pattern
tetrahedral PO4), M–Od (terminal bonds MQOd), M2013Ob XRD analysis was used to investigate the identity of the
(oxygen atom belonging to two different M3O13 groups) and designed HKUST-1/Cs-POMoW. As displayed in Fig. 1b, the
M–Oc (oxygen atom belonging to the same M3O13 group), XRD pattern of HKUST-1 showed intense diffraction peaks at
respectively. By comparing the spectra of Cs-POMoW and 2y = 6.51, 9.31, 11.51, 13.31, 17.51, 19.11, 20.01, 24.31, 29.31, 35.41
HPOMoW, it is observed that their prominent characteristic and 39.21 related to the control Cu–BTC, which was in agree-
peaks (i.e., P–O, M–Oa, M–Ob, and M–Oc vibrational bands) ment with previous XRD data for HKUST-1.54–57 When the
pretty well match. When it comes to the FT-IR spectrum of the HKUST-1/Cs-POMoW composite was prepared, the prominent
HKUST-1/Cs-POMoW composite, all the characteristic peaks diffraction peaks including 2y = 6.51, 9.31, 11.51, 17.51, etc.
related to its discrete components are pretty obvious. Due to remained almost constant, indicating preservation of the struc-
the interaction between the open copper metal sites and the ture of HKUST-1. It is noteworthy that there were also diffrac-
oxygen atoms of M–Ob and M–Oc in Cs-POMoW, a bright blue tion peaks at 2y = 5.71, 6.51–9.31 and 15.61 of which the ones
shift is observed in all of the prominent unique peaks, espe- around 6.51–9.31 were only in the XRD pattern of Cs-POMoW,
cially the M–Ob and M–Oc (956 and 870 cmÀ1 respectively) suggesting the deposition of Cs-POMoW on HKUST-1. However,
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the new peaks at two theta values of 5.71 and 15.61 do not of 2–4% was related to eliminating solvents and small mole-
appear in the XRD pattern of POMoW; maybe, based on Fig. 1b, cules coordinated to copper trimers or the degradation of Cs-
they are due to chemical bonding between POMoW acid and PMoW units. Finally, in the 250–500 1C range, a weight loss of
Cu2+ in the HKUST-1 structure, leading to the crystal deforma- 23–25% was achieved, relating to BTC decomposition, which
tion and reconstruction of HKUST-1 and creating a new will lead to the collapse of the HKUST-1 framework. The
structure.58,59 These changes of composite structure based on presence of a weight loss difference between HKUST-1/Cs-
XRD are in accordance with the conversion of morphology PMoW and HKUST-1 can be assigned to the decomposition of
based on SEM.
Cs-PMoW within the pores. The results indicate the presence of
0.15 mmol of CsPOMo6W6 per gram of the prepared catalyst.
Moreover, TG analysis also revealed that Cs-PMoW species were
indecomposable at the temperature of the reaction medium
(room temperature).
3.3. Gas adsorption
The N2 adsorption–desorption isotherms of HKUST-1 and Cs-
POMoW modified HKUST-1 were measured at 77 K, and their
related curves are illustrated in Fig. 1c. As illustrated in Fig. 1c,
isotherm curves with similar shapes were obtained for HKUST-
1 and HKUST-1/CsPOMoW. Adsorption equilibria were quickly
obtained in the low relative pressure region, and no hysteresis
loop was observed in both isotherms at high relative pressure,
indicating the mesoporous structure of the prepared
compounds.60,61 Moreover, according to the IUPAC (Interna-
tional Centre for Theoretical and Applied Chemistry) classifica-
tion, both compounds have a Type-I isotherm, which shows
their microporous (o2 nm) structure.62 Some specific para-
meters such as specific surface area (SBET) and pore volume
(Vmicro) were calculated by this method and are listed in Table 1.
3.5. UV-vis DRS (UV-vis diffuse reflectance spectroscopy)
The photocatalytic activity of a photocatalyst depends on its
optical absorption features. Therefore, DRS analyses of the as-
prepared HKUST-1, Cs-POMoW, and HKUST-1/Cs-POMoW
composites were carried out to characterize their optical prop-
erties (Fig. 1e). Before 500 nm, pure Cs-POMoW exhibits
stronger visible light absorption than pure HKUST-1; whereas,
this trend becomes inverse after such a wavelength. However,
during the whole wavelength range, the light absorption of the
HKUST-1/Cs-POMoW composite is placed between the pure
materials. In the DRS of pure Cs-POMoW, the main absorption
band in the UV-vis range centered around 420 nm can be
The BET surface area of control HKUST-1 was 1135.07 m2 gÀ1
while that of HKUST-1/Cs-POMoW(2) was obtained as
412.07 m2 À1. It is apparent that, with the introduction of
,
ascribed to ligand–metal charge transfer (LMCT): O2–Mo6+
.
Cs-POMoW presents the LMCT band at 240 and 330 nm with
a shoulder at 360 nm.64 The pristine HKUST-1 shows an
absorption edge around 320 nm related to an LMCT transition
of charge carriers from oxygen to copper (O - Cu) atoms and
the absorption band of the d–d transition related to Cu(II)
species in an octahedral geometry around 700 nm. According
to the Kubelka–Munk plots, as shown in Fig. 1f,65 to obtain the
value of the bandgap of the prepared samples, the following
equation is used: ahn = A(hn À Eg)n/2, in which a, A, hn, and Eg
are the absorption coefficient, proportionality constant, photon
energy (eV), and bandgap energy, respectively.50 To determine
the value of ‘‘n’’, the characteristics of the transition in a
semiconductor should be considered, and commonly n = 1 is
for a direct transition and n = 4 is for an indirect transition. In
this way, the plot of ln(ahn) versus ln(hn À Eg) is drawn, and, by
taking an approximate value of Eg with the slope of the straight
line near the band edge, the value of ‘‘n’’ is determined.66 It is
noted that the value of n for all the samples is almost 1,
indicating a direct transition. The absorption coefficient’s
square versus energy plot is linear for a direct optical transition
in the absorption edge region. The band-gap was also calcu-
lated approximately through the Kubelka–Munk method. As
mentioned, we plot (ahn)2 versus photon energy. By extrapolat-
ing from the high slope region of the spectra, the value of the
bandgap is estimated via the intercept of the extrapolated line
to the hn axis.67 As presented in Fig. 1f, the bandgaps of
Cs-POMoW, HKUST-1, and HKUST-1/Cs-POMoW were obtained
as 2.9, 3.4, and 3.1 eV, respectively. According to the DRS
spectra results, the composite absorbance was higher than that
of the pristine HKUST-1 in the range of 330–480 nm. The higher
g
Cs-POMoW into HKUST-1, the BET surface area decreased. As
an example from a similar study, the BET levels decreased with
Ag loading on HKUST-1 as reported by Conde-Gonzalez et al.,63
suggesting that the adsorbed particles by covering the surface
and filling the pores of HKUST-1 reduce the BET surface area.
Therefore, when the Cs-POMoW particles are adsorbed on
HKUST-1 to form the HKUST-1/Cs-POMoW(2) composite, the
micropore and total pore volume are diminished. These results
indicate the successful deposition of Cs-POMoW molecules on
the HKUST-1 system.
3.4. TGA (thermo-gravimetric analysis)
Thermo-gravimetric analysis was used to examine the thermal
stability of HKUST-1 and HKUST-1/Cs-PMoW as well as to
measure the amount of composite components. The TG curves
of HKUST-1 and HKUST-1/Cs-PMoW are shown in Fig. 1d. The
TG curve of HKUST-1 showed a mass loss of B8% in the first
stage (o100 1C), which was related to the loss of adsorbed
solvents in the pores; whereas, in the case of HKUST-1/Cs-
PMoW, the loss of mass was lower (B5%), maybe due to the
occupation of HKUST-1 pores with Cs-PMoW particles and its
higher integrity. In the second stage (100–250 1C), the mass loss
Table 1 Specific surface areas and micropore volumes of HKUST-1 and
HKUST-1/Cs-POMoW
Name of samples
SBET [m2 gÀ1
]
Vmicro [cm3 gÀ1
]
HKUST-1
HKUST-1/Cs-POMoW
1135.07
412.07
0.48
0.27
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absorbance makes it possible to use HKUST-1/Cs-POMoW as a through surface interactions. In the case of this composite,
photo-responsive catalyst under visible-light irradiation in the since surface bond formation has not been observed as a
photocatalytic process.
necessary factor to increase d–d, the second factor seems to
An interesting increase in the d–d transition band can be be more important and its compatibility with our experimental
attributed to several factors that can directly or indirectly have findings is more accurate than the first. Therefore, the charge
an effect: (1) mixing d–p through surface interactions between transfer within the composite, due to the creation of surface
HKUST-1 and CsPoMoW and (2) increasing charge transfer interactions and consequently new electron paths provided by
Fig. 2 SEM images of HKUST-1 (a) and HKUST-1/Cs-POMoW (b), EDS elemental mapping of the HKUST-1/Cs-POMoW nanocomposite (c and d) and
TEM images of HKUST-1 (e) and the HKUST-1/Cs-POMoW (f) nanocomposite.
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the addition of CsPoMoW, can well explain the increase in d–d 3.8. EDX (energy dispersive X-ray)
as well as the improvement of the photocatalytic activity.68
Energy-dispersive X-ray spectroscopy (EDS) and EDS elemental
mapping analyses were performed to obtain information about
the composition of HKUST-1/Cs-POMoW more precisely. As
Fig. 2c and d illustrate, all the familiar elements exist in the
structure of the developed composite, including C, O, Cu, P, Cs,
Mo, and W, with a weight percent of 20.8%, 24.5%, 10.4%,
10.5%, 1.5%, 12.5%, and 19.8%, respectively, indicating their
uniform distribution. Among the common elements, Cs had
the lowest content, even less than the detection limit, making it
difficult to detect. Moreover, the associated EDS elemental
mapping confirmed all the mentioned elements in the compo-
site structure. According to these results, it was found that the
Cs-POMoW particles were loaded on the porous surface of
HKUST-1 successfully, producing HKUST-1/Cs-POMoW, which
acts as a microreactor for interactions between reactants and
active sites.
3.6. PL spectroscopy
To prepare the most efficient photocatalysts, their separation
efficiency of photo-generated electron–hole pairs is quite essen-
tial. Therefore, photoluminescence (PL) analysis was used to
investigate such parameters in the prepared samples. Indeed,
there is a direct relationship between the PL emission inten-
sities and the recombination rate.69 As the PL intensity
increases, the electron–hole recombination rate increases,
whereas the photocatalytic activity decreases.70 The PL spectra
of the pure HKUST-1, Cs-POMoW, and HKUST-1/Cs-POMoW
composites are shown in Fig. 1g. HKUST-1 showed the highest
PL emission intensity among these samples, indicating the
highest recombination rate of the photo-generated electron–
hole pairs in its structure. However, after loading Cs-POMoW,
the PL intensity of the as-synthesized HKUST-1/Cs-POMoW
composite plummeted compared to HKUST-1, indicating the
decrease of the recombination rate in the composite structure.
Hence, it is concluded that the introduction of Cs-POMoW into
the HKUST-1 structure enhances the separation efficiency of
charge carriers and thus increases the photocatalytic activity.
3.9. TEM (transmission electron microscopy)
The TEM images of HKUST-1 and the POMoW/HKUST-1 nano-
composite are shown in Fig. 2e and f. The dark spots are related
to Cs-POMoW nanoparticles distributed on the smooth and
bright surface of HKUST-1 with an octahedral structure,
demonstrating the decoration of HKUST-1 with Cs-POMoW
particles. Besides, the average size of octahedral particles
obtained from TEM is in agreement with the SEM images.
3.7. SEM (scanning electron microscopy)
The SEM images of HKUST-1 and HKUST-1/Cs-POMoW are
displayed in Fig. 2a and b. The SEM image of HKUST-1 shows
an octahedral structure with a regular morphology and smooth
surfaces, and an average particle size of B20 mm. The presence
of microscopic piles of crystals inside the pores, including
copper metal centers and organic bonds, has caused non-
uniformity in the resulting particle size. In other words, the
crystallization phenomenon occurs through the coordination
of carboxylic acid groups of H3BTC with the copper metal
clusters, forming a 3-D framework. Regarding the SEM image
of the HKUST-1/Cs-POMoW composite, Cs-POMoW nano-
particles have spread on the octahedral facets of HKUST-1
particles with a cumulative spherical morphology (Fig. 2b).
After introducing Cs-POMoW, no perceptible change in the
HKUST-1/Cs-POMoW composite particle size was observed;
however, the degree of crystallinity declined slightly. Moreover,
it can be seen that the surfaces of the HKUST-1/Cs-POMoW
catalyst have become rougher, and there is a disconnection in
some parts of it, indicating the successful formation of the
composite. The construction of Cs-POMoW with spherical
morphology can be described using two mechanisms of Ost-
wald ripening and aggregation. The theory of Ostwald ripening
is based on the dissolution of small nanocrystals and their re-
growth on more massive particle surfaces.71 However, in sphe-
rical structures, primary crystals are self-assembled and form
secondary accumulated structures based on aggregation and
interconnections. Therefore, small Cs-POMoW nanoparticles
accumulate, followed by self-assembly on the surface of the
HKUST-1 particles, to form larger nanoparticles.
3.10. Inductively coupled plasma atomic emission
spectroscopy (ICP-AES)
The amount of Mo and W within the structure of HKUST-1/
CsPOMo6W6 was obtained by ICP-AES analysis. In this way,
1 g of the catalyst was dissolved in a mixture containing
hydrochloric acid and nitric acid with a volume ratio of 3 to
1. Next, the remaining solid catalyst was centrifuged. The
obtained solution containing Mo and W was analyzed via
ICP. Based on the obtained results, the Mo and W content
were 0.083 and 0.79 mmol gÀ1, respectively, which matched
with the ones reported by TGA analysis.
4. Catalyst study
4.1. Photocatalytic experiments
To test the photocatalytic efficiency of HKUST-1/Cs-POMoW, we
performed the aerobic oxidative amidation of benzyl alcohol
under visible light (l Z 400 nm) irradiation as a model reaction
(Scheme 2) with regard to various catalysts, solvents, catalyst
loading and time (Fig. 3).
The model reaction was performed according to these con-
ditions: benzyl alcohol (1 mmol), morpholine (1 mmol), catalyst
(5 mg), 10 mL of solvent, under visible light irradiation and
using an air-balloon for 7 hours at room temperature. The
completion of the reaction was determined by TLC and con-
trolled by GC-FID spectroscopy with an internal standard. One
of the main aims in this step was to find the best catalyst
among the disparate prepared composites. Therefore, despite
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was improved, indicating the critical role of this weight ratio in the
catalyst efficiency. From then on, all reactions were performed with
a 20% ratio of Cs-POMoW to HKUST-1 (namely HKUST-1/
Cs-POMoW(2)). Although the porosity and BET surface area of
HKUST-1 were diminished after the addition of Cs-POMoW, its
catalytic performance in aerobic oxidative amidation of alcohols
was much higher than its discrete components, pure HKUST-1 and
Cs-POMoW (Fig. 1c). In other words, merely 10% of progress in
aerobic oxidative amidation was observed using HKUST-1 but 28%
using Cs-POMoW modified HKUST-1. Hence, the study of HKUST-1
functionalized by Cs-POMoW structurally and its application in
aerobic oxidative amidation of alcohols would be worth it. There-
fore, the HKUST-1/Cs-POMoW composite was selected as the most
suitable photocatalyst. Indeed, some features have made this
composite quite desirable for oxidative amidation of alcohols.
Firstly, the presence of open metal sites within the structure of
HKUST-1 could facilitate the way the reactive species such as
alcohol, aldehyde, amine and imine as a byproduct are trapped.72
Secondly, Cs-POMoW with open metal sites can also act as a Lewis
acid. Thirdly, there is a significant synergistic effect between the
Scheme 2 The model reaction for photocatalytic oxidative amidation.
the usage of HKUST-1@Cs-POMoW composites, the catalytic cap-
ability of some other composite components such as HKUST-1,
H-POMo, H-POW, Cs-POMo, Cs-POW, and Cs-POMoW was also
screened (Fig. 3a). As Fig. 3a shows, when HKUST-1 was used as a
photocatalyst solely in the model reaction under light illumination,
a meaningful efficiency was not obtained. Moreover, by replacing
the visible light source with ultraviolet, the associated efficacy
declined sharply. Among the POM salts, the one with the molyb-
denum–tungsten composition led to the higher efficiency. Thus, in
subsequent runs, the Cs-POMoW compound was used. Besides, by
increasing the mass ratio of Cs-POMoW to HKUST-1 in the
developed HKUST-1/Cs-POMoW composite, the reaction efficacy
Fig. 3 Optimization of the catalysts, solvents, catalyst loading and time for aerobic oxidative amidation of benzyl alcohol.
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Table 2 Preparation of various amides in the presence of HKUST-1/Cs-
electronic properties of HKUST-1 and Cs-POMoW. As is clear from
the results of photoluminescence spectroscopy, the composite
formation has reduced the intensity of photoluminescence, which
means a reduction in recombination. All in all, these advantages
guarantee the usage of the HKUST-1/Cs-POMoW composite as a
promising catalyst for performing the aerobic oxidative amidation
of alcohols. In the next step, the effect of various solvents on the
efficiency of the aerobic photocatalytic oxidative amidation reaction
was tested (Fig. 3b). The investigations demonstrated that acetoni-
trile showed a more substantial effect on the progress of the
reaction. However, based on some results, it was inferred that
using some solvents or additives can increase the reaction efficacy
more. For instance, the usage of perfluorinated solvents in aerobic
oxidation reactions has been reported significantly.73–75 Nonethe-
less, in this work, mixtures of polyfluorinated compounds and
other classic solvents in different ratios were utilized. Among them,
the mixture of hexafluorobenzene and acetonitrile with a mass
ratio of 1 : 1 was chosen as the optimum solvent, yielding the
highest amount of product and, more importantly, the highest
efficiency (Fig. 3b). It is worth mentioning that by using non-polar
and polar aprotic solvents, the model reaction was facilitated.76–78
This could be attributed to the generation of non-selective oxidant
species such as ꢀOH in protic solvents (Fig. 3b).79 After identifying
the type of optimal catalyst, the catalyst loading was also examined.
In this way, the effect of various amounts of HKUST-1/Cs-POMoW
composite on the reaction efficiency was screened, and 15 mg was
POMoWa
a
Reaction conditions: catalyst (15 mg), benzyl alcohol (1 mmol), amine
(1.5 equiv.), 7 h at room temperature in CH3CN/HFBz (2 mL, in a ratio
of 1/1) as a solvent, using an air-balloon and under visible-light
chosen as the optimum and suitable amount (Fig. 3c). Finally, the irradiation. Yields refer to the isolated pure product.
time period of the reaction as another important factor for the
reaction efficacy using HKUST-1/Cs-POMoW under the optimized
formation stage, electron-withdrawing groups also react much
conditions was evaluated. As seen in Fig. 3d, the reaction efficiency
is time-dependent. But, over time, due to the saturation or aggrega-
tion of the composite, little progress is made in the process.
Therefore, 420 min was considered the optimal reaction time.
After identifying the optimization factors, we decided to
expand our study to other benzyl alcohol and amine derivatives.
As illustrated in Table 2, generally, benzyl alcohol derivatives
led to more amide products than non-benzylic aliphatic alco-
better. More importantly, there is a p–p-stacking interaction
between benzyl alcohol aromatic rings and MOF ligands,
especially in electron-withdrawing groups, as they lead to
greater uptake of reactive species into MOF cavities, and thus
enhance the catalytic performance of HKUST-1/Cs-POMoW.
4.2. Photocatalytic aerobic oxidative amidation mechanism
hols. The benzylic hydrogen is separated radically from its To predict the mechanism of aerobic oxidative amidation of
relevant benzyl alcohol more easily than those of non- various alcohols catalyzed by HKUST-1/Cs-POMoW more
benzylic ones. Two significant factors influence the efficiency precisely, some radical quenchers such as ammonium oxalate
of the oxidative amidation reaction: steric and electronic ele- (AO), tert-butanol (TBA), potassium persulfate (K2S2O8), benzoqui-
ments. Regarding steric effects, there are some functional none (BQ), and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) were
groups in amine derivatives that act as steric hindrances and employed to quench holes (h+), hydroxyl radicals (ꢀOH), electrons
lead to lower product yields (Table 2, entries 1i, 1k and 1l). In (eÀ), superoxide free radicals (O2 À) and all radicals, respectively.
ꢀ
some amine derivatives, especially cyclic amines, the Such scavengers were used to identify which radical is mainly
steric hindrance can be mitigated through torsional strain responsible for performing the oxidative amidation reaction. As
(Table 2, entry 1l). To further check the catalytic system Fig. 4a shows, by adding benzoquinone to the reaction mixture,
capability, an amino acid ester was used, and a high efficacy the reaction yield decreased drastically, indicating the significant
À
ꢀ
was obtained, indicating the successful derivability of this role of O2 in the photocatalytic aerobic oxidative amidation
catalytic system (Table 2, entry 1d). In terms of electron- process. Next in rank, h+ or ꢀOH species were also standing in
donating (EDG) and electron-withdrawing substituents (EWG) second and third places to perform the mentioned reaction as the
existing on benzyl alcohol, the ones with electron-withdrawing associated scavengers, namely ammonium oxalate (AO) and tert-
groups act more efficiently than the electron-donating ones butanol (TBA), reduced the efficiency of the reaction, respec-
because the EWG can stabilize the generated radicals through tively. The band edge levels for the Cs-POW, Cs-POMo, and
their p electrons, and the unpaired electron can take advantage Cs-POMoW compounds were obtained based on the following
of this (Table 2, entry 1m–o).80 Moreover, during the imine equations:81
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Fig. 4 (a) Reaction efficiency in the presence of different quenchers. (b) Proposed mechanism for photocatalyzed aerobic oxidative amidation of
alcohols.
ECB = w À Ee À 0.5Eg
EVB = ECB + Eg
In these formulas, ECB and EVB are the conduction band edge
potential and valence band edge potential, respectively; w is
considered the absolute electronegativity of the semiconductor,
which was obtained as 6.53, 6.09 and 6.25 eV for Cs-POW,
CsPOMo, and Cs-POMoW, respectively; Ee is the energy of free
electrons on the hydrogen scale, which was about 4.5 eV, and
finally, the band-gap energy of the semiconductor is denoted as
Eg. In terms of ECB and EVB, they were estimated to be 0.55,
À0.42 and À0.22 eV for Cs-POW, CsPOMo, and Cs-POMoW and
2.86, 2.10 and 2.36 eV, respectively. Concerning HKUST-1, ECB
and EVB were À0.18 eV and 2.97 eV, respectively.82 The band
edge positions of Cs-POMoW, HKUST-1, and the HKUST-1/Cs-
POMoW composite and the photocatalytic reaction mechanism
are illustrated in Fig. 4b. The VB and CB levels of CsPOMo are
Fig. 5 Hot filtration test (a), catalyst reusability (b), and FT-IR (c) and PXRD
(d) of the recovered heterogeneous catalyst after the 5th run.
less positive than those of Cs-POW, Cs-POMoW, and HKUST-1;
however, among them, the VB and CB levels of Cs-POMoW are
closer to HKUST-1, which facilitates the transfer of photo-
a hemiaminal, followed by an amide. If the catalyst was
removed at this stage, the reaction continued for only 5%.
Therefore, the catalyst also acts as an essential factor.8,21
induced electrons in the CB of Cs-POMoW to the CB of
HKUST-1, and, in an inverse trend, photo-induced holes in
the VB of the MOF migrate to the VB of Cs-POMoW. Therefore,
the created pathway for conducting charge separation at the
interface of HKUST-1/Cs-POMoW can diminish the recombina-
4.5. Hot filtration test
tion rate of photo-induced electron–hole pairs, increasing the Since HKUST-1/Cs-POMoW(2) is a heterogeneous composite, a
photocatalytic performance of the HKUST-1/Cs-POMoW com- heterogeneity test called the hot filtration test was carried out to
posite. To explain the mechanism of aerobic oxidative amida- assess the degree of heterogeneity of it in the reaction medium.
tion of alcohols, the trapped electrons in the CB of HKUST-1 In this way, first, the reaction was allowed to continue in the
can react with the adsorbed O2 molecules on the surface of the presence of the developed composite, and then, after 100 min,
À
ꢀ
photocatalyst and form active O2 radicals. Based on Fig. 4b, the reaction was performed without the catalyst, which was
first of all, the active sites of the catalyst converted the alcohol removed from the reaction mixture by centrifugation. As seen
to an aldehyde by producing superoxide free radicals (O2 À). in Fig. 5a, no progress of the reaction was observed in the
ꢀ
The aldehyde was apparently identified as a mediator in this absence of the composite, demonstrating no leakage of the
reaction. According to the previously suggested mechanism in solid catalyst into the reaction mixture. Hence, HKUST-1/Cs-
the presence of copper, a hemiaminal was produced as a POMoW(2) is heterogeneous and is necessary to perform the
mediator.6,7 The aldehyde reacted with the amine to produce photocatalytic aerobic oxidative amidation of alcohols.
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4.6. Catalyst reusability
The catalyst was eliminated from the concerned reaction by
centrifugation, to understand how much the prepared HKUST-
1/Cs-POMoW(2) composite is stable and reusable in the aerobic
oxidative amidation of alcohols. After washing the catalyst with
ethanol and drying it at 80 1C, the retrieved catalyst was used
again in the new reaction with new precursors. As illustrated in
Fig. 5b, by using this catalyst for up to 5 runs, a high yield of the
amide product was obtained without a significant decrease in
the catalytic activity. FT-IR and PXRD analyses of the recovered
catalyst after the 5th run were also performed. According to
Fig. 5c and d, the matching of the FT-IR and XRD patterns
obtained after the 5th run with the intact catalyst was con-
firmation of the maintenance of the catalyst structure.
11 D. Saberi, S. Mahdudi, S. Cheraghi and A. Heydari,
J. Organomet. Chem., 2014, 772–773, 222–228.
12 Q. Qin, Y.-Y. Han, Y.-Y. Jiao, Y. He and S. Yu, Org. Lett., 2017,
19, 2909–2912.
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ACS Sustainable Chem. Eng., 2017, 5, 8439–8446.
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Green Chem., 2019, 21, 962–967.
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Sci. China: Chem., 2018, 61, 660–663.
16 K. Azizi, N. Esfandiary, M. Karimi, M. Kazemi and
A. Heydari, Appl. Organomet. Chem., 2017, 31, e3658.
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A. Heydari, RSC Adv., 2016, 6, 89313–89321.
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19 S. Sadeghi, M. Karimi, I. Radfar, R. G. Gavinehroudi,
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5. Conclusion
This work presented the successful synthesis and characteriza-
tion of an HKUST-1/Cs-POMoW binary composite, which was
employed in the photocatalytic aerobic oxidative amidation
reaction of alcohols under light illumination in the visible
region. Characterizations confirmed the structure of the pre-
pared catalyst. The combination of HKUST-1 with Cs-POMoW
to form a composite leads to synergy between their desirable
features, such as the high surface area of the MOF and the
optical properties of Cs-POMoW and its high stability and
reusability as well. Cs-POM as a part of this catalytic system
was a suitable compound to improve the catalytic efficiency.
Thus, Cs-POMoW exhibited much better performance than
Cs-POW and Cs-POMo due to the proximity of its energy levels
to HKUST-1, facilitating electron transfer. Moreover, the per-
fluorinated solvents were another critical factor in improving
the advancement of the concerned reaction because of their
higher capability to dissolve more oxygen molecules. Besides,
based on trapping experiments, the primary role of photogen-
À
ꢀ
erated holes and O2 radicals in photocatalytic aerobic oxida-
tive amidation of alcohols was confirmed.
Conflicts of interest
24 M. Zhang, H. Li, J. Zhang, H. Lv and G.-Y. Yang,
Chin. J. Catal., 2021, 42, 855–871.
There are no conflicts to declare.
25 Y. Dong, Q. Han, Q. Hu, C. Xu, C. Dong, Y. Peng, Y. Ding
and Y. Lan, Appl. Catal., B, 2021, 293, 120214.
26 M. Misono, Chem. Commun., 2001, 1141–1152.
27 M. Bonchio, M. Carraro, G. Scorrano, E. Fontananova and
E. Drioli, Adv. Synth. Catal., 2003, 345, 1119–1126.
28 K. Suzuki, N. Mizuno and K. Yamaguchi, ACS Catal., 2018, 8,
10809–10825.
Acknowledgements
This work was supported by Tarbiat Modares University of Iran.
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