Influence of the channel size of isostructural 3d-4f MOFs on the catalytic aerobic oxidation of cycloalkenes

Article abstract of DOI:10.1039/c9nj02091h

The present work reports a new group of heterogeneous catalysts with a 3D structure, CuLnIDA, {[Cu₃Ln₂(IDA)₆]·8H₂O} (Ln: La^III, Gd^III or Yb^III), with an organic linker (H₂IDA: iminodiacetic acid). Different sets of O₂ pressure and time were used in order to obtain the optimal reaction conditions at 75 °C. The reaction was found to depend on the [aldehyde]/[substrate] ratio. The best results, with a conversion of 73% for CuLaIDA as the catalyst, were obtained for the smallest ratio of 0.2. Finally, the importance of the pore size was analysed by comparing the catalytic activity of the as formed catalyst with that of the thermally activated one. The conversion increased ca. 26-35% for the different catalysts when they were previously activated. In addition, the selectivity increased towards cyclohexenone. The use of molecular oxygen as the oxidizing agent in a system where an auxiliary solvent is not used, as the cyclohexene substrate and products play the role of a solvent, permitted us to generate a more friendly environmental system for the oxidation of cycloalkenes under mild conditions.

Full text of DOI:10.1039/c9nj02091h

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Santibañez , C. Stevens, P. Fuentealba, N. Audebrand, D. Aravena, J. Torres, S. Martínez, C. Kremer and E.
Spodine, New J. Chem., 2019, DOI: 10.1039/C9NJ02091H.
Volume 40 Number
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January 2016 Pages 1–846
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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Influence of the channel size of isostructural 3d-4f
MOFs on the catalytic aerobic oxidation of
cycloalkenes.
a,b
a
a
a,b
d
e
P. Cancino , L. Santibañez , C. Stevens , P. Fuentealba , N. Audebrand , D. Aravena , J.
c
c
c
*a,b
Torres , S. Martinez , C. Kremer , E. Spodine
The present work reports a new group of heterogeneous catalysts with a 3D structure, CuLnIDA, {[Cu
(Ln: La , Gd or Yb ) with an organic linker (H IDA: iminodiacetic acid). Different sets of O
2 2
3
Ln
pressure and time were used
in order to obtain the optimal reaction conditions at 75◦C. The reaction was found to depend on the
aldehyde]/[susbtrate] ratio. The best results, with a conversion of 73% for CuLaIDA as catalyst, were obtained for the
2 6 2
(IDA) ] • 8H O}
III
III
III
[
smallest ratio of 0.2. Finally, the importance of the pore size was analysed by comparing the catalytic activity of the as
formed catalyst with that of the thermally activated one. The conversion increased ca. 26-35% for the different catalysts
when these were previously activated. In addition, the selectivity increased towards cyclohexenone. The use of molecular
oxygen, as the oxidizing agent in a system where an auxiliary solvent is not used, as cyclohexene substrate and products
play a role of solvent, permitted to generate a more friendly environmental system for the oxidation of cycloalkenes in
mild conditions.
8–
inorganic polymers has been reported in the oxidation of cycloalkenes,
15
proving their low solubility and stability under the reaction
Introduction
16
conditions . Some examples related to catalytic systems based on
1
7–
homonuclear metal-organic frameworks (MOFs) based on copper (II),
The oxidation of olefins has become a topic of interest among many
1
9
cobalt (II),20,21 vanadium (IV),22,23 manganese (II, III) or iron (III)24 ions
1
research groups . The oxidation of cycloalkenes is particularly
have been reported.25–27
investigated, due to the products that this reaction yields which are used
in the agrochemical and pharmaceutical industry, as well as in the
manufacture of perfumes, glues, among others . Thus, the efforts aim reactions such as, hydrogenation of small olefins using Rh polymers
to find new efficient, economical and clean catalytic routes to obtain containing Cu , Ni and Pd metalloporfirins. Degradation of methyl
these oxidation products. In some of the catalytic routes, the use of a blue and rhodamine B can be produced using heterometallic uranium
Heterometallic polymers have been also used as catalysts in
II
2
–5
II II
II
28
II
solvent implies an environmental impact. A solvent-free oxidation based polymers; with Ni as secondary ion for methyl blue
reaction decreases the effect on the environment. Besides, the high cost degradation29 and Ag for the rhodamine B one . In the area of
of the mostly used environmentally “green” oxidants, such as hydrogen oxidation reactions, heterometallic MOFs show catalytic activity for
peroxide and organic hydroperoxides, makes molecular oxygen a perfect differentkindsofsubstrates. The studied catalytic systems were based
,
I
30
and optimal oxidant to be considered in the oxidation reactions of on the use of the existing empty spaces in the MOFs to incorporate
6
cycloalkenes .
metallic nanoparticles, which can cooperate in the reaction
In catalytic systems, the use of homogeneous and supported mechanism, together with the original active metal sites existing in the
7
31
II
catalysts presents well known problems: the inability of recovering the structure. For example, porous Cu MOFs, based on a pyridyl
0
catalyst in the homogeneous systems, and leaching or decomposition substituted diketonate ligand, with adsorbed Au0 32 or Pd
33
of supported catalysts.
nanoparticles have been used in the oxidation of benzylic alcohols.
0 0
In the heterogeneous catalysis area, the use of homometallic Furthermore, bimetallic catalysts of Pd -Au
nanoparticles
III
encapsulated in an Al MOF were studied in the aerobic oxidative
3
4
carbonylation of amines. Besides, Sun et al. report the catalytic
a Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Santiago,
Chile espodine@ciq.uchile.cl
b Centro para el Desarrollo de la Nanociencia y la Nanotecnología, CEDENNA,
Santiago, Chile
c Departamento Estrella Campos, Facultad de Química, Universidad de la República,
Montevideo, Uruguay.
d Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226,
F-35000 Rennes, France.
II II
performance of a Co /Ni MOF-74, which corresponds to a partially
Co-substituted Ni-MOF-74. The authors informed that the catalytic
II
activity increased as the amount of incorporated Co increased,
suggesting that this ion was acting as the active site for the oxidation of
3
5,36
cyclohexene.
On the other hand, only a few 3d-4f heterometallic
e Departamento de Química y Biología, Universidad de Santiago de Chile, Santiago,
Chile.
polymers have also been reported as catalytically active systems in the
oxidation of alkenes and benzylic substrates.
37–39
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However, despite the fact of the numerous catalysts available for X-ray data collection and refinement
olefin oxidation, for many of them the reaction still does not occur
A summary of crystal data, data collection, structure determination
and refinement for CuYbIDA is given in Table S1-S6.
without the assistance of an activator or a secondary oxidation agent.
Depending on the mechanism of the reaction, these activators perform
in diverse ways, proving themselves helpful as they allow the
performance of reactions under less aggressive conditions, which is
always a plus when trying to find a new catalytic route.
Diffraction data were collected at 100 K using a Bruker D8 Venture
single crystal diffractometer equipped with a CuK sealed tube X-ray
α
source operating at 50 kV/30 mA and a PHOTON 100 area detector
operating in shutterless mode. Unit cell determination, data collection
and processing were performed using Bruker APEX3 software suite.
Structure determination and refinement was performed with SHELXT and
SHELXL (with OLEX2 graphical user interface) programs respectively.
Crystallization water molecules could not be refined adequately because
of disorder. Toavoid this problem solvent masking procedure was applied
to disordered electron density in the cavities using OLEX2 program.
Some examples can be found in literature such as the use of
44
4
0
isobutyraldehyde as activator in the epoxidation of olefins, or the use
of 2,2,6,6-tetramethyl-1- piperidinyloxy radical (TEMPO) as activator for
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45,46
4
1
the oxidation of benzylic alcohols. Another widely used initiator is tert-
butylhydroperoxide (TBHP), also reported for the oxidation of
4
2,43
alkenes.
47
The present work reports the importance of the channel size Hydrogen atoms on C1, C3 and N1 were located geometrically and refined
existing in the structure of a series of isostructural heterogeneous allowing to ride on their parent atoms Uiso(H) 1.2Ueq(C) and Uiso(H)
III III III
=
=
catalysts, CuLnMOFs, {[Cu
3
Ln
2 6
(IDA) ]
2
·8H O}
n
(Ln: La , Gd or Yb 1.2Ueq(N) respectively. Supplementary crystallographic information,
(H
2
IDA: iminodiacetic acid) on the observed activity in the oxidation bond lengths, bond angles and torsion angles can be found in Tables S1-
of cycloalkenes in mild conditions.
S6. Crystallographic data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/structures, CCDC 1878174.
Materials and methods
III
III
III
Synthesis of [Cu
3 2 6 2
Ln (IDA) ]·8H O (CuLnIDA) (Ln = La , Gd , Yb )
Powder X-ray diffraction data, used to check the catalyst stability after
the catalytic reaction, were collected with a Bruker D8 Advance
diffractometer, operating with a Cu Kα1 monochromatic radiation,
equipped with a LynxEye detector, in the range of 5to 120° 2. The whole
pattern matching was done using the FullProf software in the WinPLOTR
interface (see supplementary material).
Complexes were prepared by direct reaction of iminodiacetic acid
with the stoichiometric amounts of Cu and Ln salts following the
II
III
2
previously reported procedures. CuCl
2
•6H
2
O (0.13 g, 0.75 mmole) and
2
O (0.5 mmole) were dissolved in 5mL of water. Iminodiacetic
IDA) (0.20 g, 1.5 mmole) was dissolved in 5 mL of water and the
3
LnCl •6H
acid (H
2
pH value was adjusted to 9.0 with diluted ammonia. Both solutions were
mixed and a blue solution wasobtained. pH value was readjusted to 4.8–
.1 with diluted ammonia. After 3-5 days, blue crystals were obtained,
Catalytic reactions
Three different systems were used for the catalytic reactions.
5
separated by filtration and washed with water. Yield 50-60%. Anal. for
CuLaIDA: Calc. N, 6.0; C, 20.7; H, 2.9. Found:N, 5.9; C, 20.1; H, 3.8%. Anal.
for CuGdIDA: Calc. N, 5.9; C, 20.2; H, 2.8. Found: N, 5.8; C, 19.8;
H, 3.6%. Anal. for CuYbIDA: Calc. N, 5.8; C, 19.7; H, 2.8. Found:N, 5.9;
C, 19.8; H, 3.4%. IR peaks associated to the IDAligand appear at ca.:
OH
O
O
O
2 (variable pressure)
75°C
()n
(
)n
()n
()n
isobutirylaldehyde (20%mole)
3
230 (N–H), 2930 (C–H), 1620 and 1407 (COO), 1450 (C–N), 950 (C–
(
a)
(b)
(c)
(d)
-1
C), 1115 (C-N-C) cm . TGA diagrams show a single weight loss between
n = 1, 2, or 3
◦
room temperature and 60 C, corresponding to crystallization water
Scheme 1. Principal products obtained from the oxidation of cycloalkenes
(a): epoxide (b); conjugated alcohol (c); conjugated ketone (d). 1,2-
cyclohexanediol was detected as minor product in specific reactions
conditions.
molecules. Calc. for CuLaIDA: 10.3, Found: 10,0%. Calc. for CuGdIDA:
1
0.0, Found: 9.8%. Calc. for CuYbIDA: 9.9, Found: 9.5 %. Decomposition
◦
point, ca. 290 C. It was possible to obtain single crystals of CuYbIDA by
this procedure, which was characterized by X-ray diffraction.
In the first system, 20 mmole of substrate, 4 mmole of
Chemicals and equipment
isobutyraldehyde
[Cu Ln (IDA) 8H
as
O}
activator
and
0.0035
mmole
of
All laboratory chemicals were of reagent grade, purchased from
commercial sources and used without further purification. The infrared
spectra (as KBr pellets) were obtained with a Bomen MB 102 FTIR
spectrophotometer. Elemental analysis (C, H, N) was performed on a
Flash 2000 instrument. Thermal analyses were carried out on aShimadzu
TGA-50 instrument with a TA 50I interface, using a platinum cell and
nitrogen atmosphere. Experimental conditions were 0.5 C min
temperature ramp rate from 20°C until 700 C and 50 mL min nitrogen
flow rate.
{
3
2
6
]
·
2
n
were all placed together in a high pressure
resistant round bottom flask of 25 ml capacity and the mixture was
◦
stirred at 75 C for 24 hours, with a charge of 1 or 5 atm of oxygen.
For the second system, 40 mmole of substrate (C
: Cycloheptene or C : Cyclooctene), 8 mmole (20% in mole) of
-1 isobutyraldehyde as activator and 0.007 mmole {[Cu Ln (IDA) ]
6
: Cyclohexene,
C
7
8
◦
3
2
6
III
III
III
◦
-1
·8H O} (Ln: La , Gd or Yb ) were all placed together in a double
2
n
neck round bottom flask of 25 ml of capacity. The mixture was stirred
◦
at 75 C under a continuous flow of 1 atm of O
2
for24 hours.
ICP spectroscopy
For the third system the catalytic conditions were the same used
in the second system, the only difference was that the catalysts were
previously thermally activated. The activation of the catalysts was
The solutions obtained after catalytic reactions were analyzed by
optical ICP spectroscopy, using a Perkin Elmer Optima 2000 DV model
spectrometer. The free copper concentrations were determined using
standards of different concentrations.
This journal is © The Royal Society of Chemistry [year]
◦
done using a vacuum oven at 100 C for two hours. Following this
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procedure, the samples were cooled in a desiccator for fifteen Solvent accessible void was calculated using PLATON program,60 being
minutes, and used in the catalytic reactions. 26.5 % of the total volume cell.
Samples of all the catalytic reactions were characterized and Catalytic Results.
quantified by gas chromatography using a 5890 model SERIES II Hewllet
3 2 6 2 n
The three isostructural catalysts {[Cu Ln (IDA) ]·8H O} (CuLaIDA,
Packard gas chromatograph. Nitrobenzene was used as external
standard in the final stage, just before the injection of the sample. The
amount was 20 mg for 1 mL of sample. The column was a capillary non-
polar Equity-1.
CuGdIDA and CuYbIDA) were used to study the oxidation reaction of
◦
cyclohexene in a system where an auxiliary solvent is not used at 75 C,
with molecular oxygen as oxidant and isobutyraldehyde as activator. The
effect of the amount of molecular oxygen was tested. In the closed
“batch” system a first set of reactions were carried out, loading 1 or 5 atm
of molecular oxygen in the batch reactor, and permitting the catalytic
system to react for 24 h.
Computational Details
0
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0
II
Discrete models of the Cu site for CuLaIDA, CuGdIDA and CuYbIDA
48–50
were constructed from their crystallographic structures.
Toaccount
for the chemical environment of the copper centre, the complete
coordination sphere of the central Cu and its four neighbour Cu
In preliminary studies, the reactions were carried out in the absence of
isobutyraldehyde, and the results after 24h showed a null conversion,
II II
positions was kept intact from the crystal structure. Thefour lanthanides thus demonstrating the importance of the co-oxidant for reactions done
connecting the copper centres were also included. Dangling IDA groups under mild conditions. Moreover, it is well known that oxygen activation
at the end of the model were replaced by H
2
O ligands. Peripheral copper occurs by the direct electron transfer with the metal, or can also be
II
positions were replaced with Zn atoms to force the reduction process achieved by using aldehydes in combination with metal ions, as was done
II
61
to happen at the central Cu site. The structural model contains 201 in our work. The mechanism to activate molecular oxygen using
61
atoms, charge -2, multiplicity 2. In the first step, hydrogen positions isobutyraldehyde has been well described in the literature , and
were relaxed using the XTB program (GFN1 model, solvent water).51 corresponds to the initial attack of the activator on the copper(II) centre
Vertical electron affinities were calculated as the difference between the in a redox reaction. After that, the next step involves the interaction of
II
oxidized and reduced species using the ORCA 4.0.1.2 software molecular oxygen with the species generated from the reduction of Cu
52
53,54
package. Two density functionals were considered (BP86
and and the activator to produce the copper-acylperoxy intermediate. This
55,56
B3LYP
), calculations were performed in gas phase and under a intermediate finally reacts with the substrate to generate the products by
61
continuum solvation model (CPCM, water). Two different basis sets a series of radical chain reactions and regenerate the catalyst (Scheme
57,58
were tested (def2-SVP and def2-TZVP
), both cases include a 28- S1).
59
electron ECP for the lanthanide atom. In all cases, the sametrendin
electronaffinitywasobservedacrosstheseries. Results for Def2-TZVP
in water are presented.
Table 1 summarizes the obtained results for 24 h of reaction with an
initial pressure of 1 atm. It is possible to observe that the conversion, in
the presence of the activator, is similar for the three catalytic systems,
with a low value of ca. 25%. However, it is evident that the catalyst
modulates the selectivity. Thus, the percentage of cyclohexenone,
corresponding to the product of allylic oxidation, is greater for CuLaIDA
than for CuGdIDA and CuYbIDA, while the percentage of cyclohexene
oxide is the lowest when CuLaIDA is used as catalyst. Due to the low
conversion obtained after 24 h of reaction with an initial pressure of 1
atm, a second set of reactions was carried out, using a pressure of 5 atm
of oxidant under the same reaction conditions as for the first set. These
reactions were stopped after 3 and 24 hours, in order to study the effect
of time on the conversion and selectivity.
Results and discussion
3 2 6 2 n
Description of the structure of {[Cu Yb (IDA) ]·8H O}
Complex {[Cu Yb (IDA) 8H O} (CuYbIDA) crystallizes in the trigonal
3
2
6
]
·
2
n
crystal system, space group P-3c1. The structure can be described as
III
[
Cu(IDA)
2
II
] units which link Yb ions in a neutral 3D structure (Fig. 1a).
Each Cu ion is coordinated by two deprotonated tridentate IDA ligands.
The geometry of the Cu ion can be described as distorted octahedral
II
with the two elongated axial bonds (Cu1–O3 distance, 2.429(3) Å),
resulting from the Jahn–Teller effect (Fig. 1b). This is reflected in the
bond distances around this metal ion and crystallographic data, as
Both systems gave approximately the same conversion of ca. 70%
(Table 2). The percentage of cyclohexenol remained similar for both
reactions (3 and 24 h), regardless of the nature of the used catalyst.
However, while the selectivity of cyclohexene oxide was similar for the
three catalysts for the 3h reaction, a slight decrease in the amount of this
product was detected for that of 24 h, as the catalyst was changed from
depicted in Table S1-S6. Each [Cu(IDA)
2
] unit has four oxygen atoms not
III
coordinated to copper (O1 and O4), which are used to bind to Yb ions.
The Ln ion is nine-coordinated in a distorted tricapped trigonal prism
geometry, surrounded by nine carboxylic oxygen atoms from six
neighboring IDA ligands (Fig. 1c).
The links are supported by two kinds of bridges: three 
2
anti–anti
III
III
the La to Yb based catalyst. Besides, a new product was detected after
III
carboxylate bridges per Ln ion (O1-C2-O2), and three 
2
oxygen (O3)
2
4 h of reaction for all three catalysts; this was identified as
bridges. Hexagonal channels are present in the structure, running along
the crystallographic c axis (Fig. 2). The channels are delimited by six
cyclohexanediol.
In order to have a more complete idea of the influence of the
concentration of molecular oxygen on the conversion and selectivity of
CuO
is approximately 8.32(1) Å. A comparison with the average channel size
along the series [Cu Ln (IDA) ] (Fig S1) shows that CuYbIDA presents the
4 2 9
N polyhedra and six LnO polyhedra. The diameter of the channel
3
2
6
narrowest channel among the reported structures.
The lattice water molecules are situated inside the channels. The TGA
profile shows a single weight loss at 50–90°C (Fig S2), which
corresponds to the eight crystallization water molecules. This fact
suggests that water molecules are only weakly bound to the network.
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Fig. 1 (a) Unit cell; (b) coordination environment around CuII (c) coordination environment of YbIII with labeling scheme. Thermal
ellipsoids are shown at 50% probability. H atoms and crystallization water molecules are omitted for clarity. Color code: YbIII (dark
orange) ; CuII (cyan) ; O (red); N ( blue).
Fig. 2 Packing of CuYbIDA along the crystallographic c axis, showing the channels. Color code: YbIII , dark orange; Cu , cyan.
II
The oxidation products, a continuous flow of 1 atm of oxygen was interaction of the activator and oxygen with the metal centres less
used during 24 h of reaction. These systems presented a conversion feasible. The catalyst with the biggest pore size is CuLaIDA and presents
between 39 and 70% depending on the used catalyst, as shown in the the better catalytic results (conversion: 73%; selectivity: 66% to ketone).
Table 3. As compared with the “batch” experiments the continuous flow Using CuLaIDA, the reusability of this heterometallic polymer in the
systems showed a dramatic dependence with the nature of the catalyst. oxidation reaction was studied in order to determine if it could be
considered as a good heterogeneous catalyst. After three catalytic cycles,
A high catalytic activity is appreciated for the CuLaIDA catalyst (73%)
with a selectivity favoring the formation of the cyclohexenone (66%). In
the activity decreased ca. 15%. ICP analyses were done after each cycle
II
to determine the amount of Cu in solution. After the first cycle the
contrast, as the size of the lanthanide ion becomes smaller, the
conversion decreases showing the lowest conversion for the CuYbIDA
catalyst (39%). The selectivity is also altered, as the main product for both
CuGdIDA and CuYbIDA catalysts was the conjugated alcohol (52 and 46%
respectively). Thus, CuLaIDA is the best catalyst among the studied MOFs
II
amount of Cu in solution was less than 0.13%, for the second cycle 0.10%
and for the third cycle 0.20%. With these results it is possible to infer that
the catalytic reactions proceed by a heterogeneous mechanism.
Powder X-ray diffractograms were done using pristine ground single
for the attainment of the conjugated ketone. These results can be crystals, and after the first and the third catalytic cycle (Fig S3). The
explained by considering the size of the pores present in the studied diffractograms of the pristine catalyst and the recycled catalyst, after the
catalysts, since along the c axis the catalysts present channels. As first cycle, were indexed with unit cell parameters closely related to the
6
2
reported by Kremer et al. the size of the pore depends of the ones obtained from single crystal data (table S7, Fig S4, S5), proving the
secondary metal ion present in the network (Fig S1). Thus when the stability of the catalyst.
lanthanide ion radius decreases the pore size decreases, and makes the
4
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Journal Name, [year], [vol.]
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Page 5 of 10
1
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02091H
Table 1 Oxidation of cyclohexene under an initial pressure of 1 atm of molecular oxygen in a “batch” system.
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Catalyst
Conversion
%)
Products
Selectivity
(%)
TOF
(h )
230
-1
(
CuLaIDA
25
25
25
5
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
◦
26
35
38
20
30
50
18
39
43
18
35
47
CuGdIDA
CuYbIDA
Blank
230
230
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Reaction conditions: Temperature (75 C), Substrate (20 mmole), Isobutyraldehyde (4 mmole), Catalyst (0.0035 mmole),
II
Cu per mole of catalyst (0.0005 mmole), Reaction time (24h), S/C ratio (2870/1 based on catalyst; 22077/1 based on
II
II
Cu ), [aldehyde]/[substrate]
= 0.2. TOF values are calculated as moles of product per mole of active Cu centre.
However, after the third cycle the diffractogram of the reused reactivity of the double bond, favouring the epoxidation for larger rings
catalyst showed additional and broader peaks, indicating a modification such as C
of the crystal structure. Furthermore, the FTIR spectra of the solid
obtained after the third cycle also showed differences, as compared with
the one of the single crystals of the pristine catalyst and the one after the
first cycle (Fig S6). Vibration bands due to the ligand (IDA) indicated a
modification of the catalyst.
2
7,63
8 7 6
, and the allylic oxidation for smaller rings like C or C .
Previous reports have shown an interesting dependence of the
conversion and the selectivity with the [aldehyde]/[substrate] ratio used
The authors reported that when the ratio
is greater than two, the epoxidation is first-order dependent on the
aldehyde, and conversion and selectivity to epoxide is increased. In order
61,63–65
in the catalytic reactions.
When comparing the “batch” results at 1 atm with those obtained for to corroborate this trend, experiments using [aldehyde]/[cyclohexene]
the constant flow of molecular oxygen at 1 atm, the first conclusion is ratios from 0.2 to 3.0 were done (Fig 3).
that the amount of oxygen is determinant for the conversion and the
selectivity percentages obtained with the different catalysts. For
example, for CuLaIDA the conversion increased from 24 to 73%, while the
percentage of cyclohexenone from 23 to 66%. On the other hand, the
percentage of cyclohexenol remains invariable, while that of
cyclohexene oxide decreased dramatically from 38 to 4 %. For CuYbIDA
the effect is less evident, since the conversion increases only from 26 to
8
0
70
60
50
40
30
20
3
9%. In addition, the selectivity to cyclohexenone also increases from 18
to 31%, while that of cyclohexenol from 39 to 46%, and cyclohexene
oxide decreases from 43 to 24%. Thus, the results of the “batch” and flow
systems are affected by the way that the oxidant is provided to the
reacting mixture, the effect being more evident for CuLaIDA than for
CuYbIDA, the latter with the smallest channels.
1
0
Analogous reactions for 24 h were studied in order to determine the
influence of the nature of the substrate on the conversion and selectivity
0.0 0.5 1.0 1.5 2.0 2.5 3.0
aldehyde]/[cyclohexene]
using CuLaIDA as the catalyst. The results are summarized in Table S8. A Fig. 3 Dependence of the conversion with the [aldehyde]/[cyclohexene]
ratio. Reaction conditions: Temperature (75◦C), Isobutyraldehyde
variable), Catalyst (0.007 mmole), Cu II per mole of catalyst (0.13),
of the studied system. For this purpose, cycloheptene (C
7
) and
[
cyclooctene (C ) were also oxidized under a continuous oxygen flow,
8
similar conversion value ca. 50% was observed for both substrates;
however, a difference in the selectivities is well defined. For C the
presence of “over-oxidation” products was detected, unlike for C where
(
7
Reaction time (24h), S/C ratio (5740/1 based on catalyst; 44000/1 based
on CuII).
8
these products were not observed. Nevertheless, the high selectivity for
cyclooctene epoxide (90%) proved the influence of the ring size over the
4
| 1–9
Journal Name, [year], [vol.]
This journal is © The Royal Society of Chemistry [year]

New Journal of Chemistry
Page 6 of 10
View Article Online
DOI: 10.1039/C9NJ02091H
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2 Oxidation of cyclohexene under an initial pressure of 5 atm of molecular oxygen in a “batch” system.
Product
Conversion
%)
Selectivity
(%)
TOF
(h )
Catalyst
-
1
(
3
h
24h
73
3h
24h
3h
24h
667
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
29
44
27
-
34
38
28
-
36
37
27
-
25
30
45
-
24
41
23
12
25
42
20
13
31
40
16
13
28
35
37
.
CuLaIDA
70
70
70
7727
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
,2-Ciclohexanediol
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
CuGdIDA
CuYbIDA
73
73
7727
7727
667
667
1
,2-Ciclohexanediol
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
1,2-Ciclohexanediol
Cyclohexenone
Cyclohexenol
Blank
20
Cyclohexene oxide
1
,2-Ciclohexanediol
◦
II
Reaction conditions: Temperature (75 C), Substrate (20 mmole), Isobutyraldehyde (4 mmole), Catalyst (0.0035 mmole), Cu per
II
mole of catalyst (0.0005 mmole), Reaction time (24h), S/C ratio (2870/1 based on catalyst; 22077/1 based on Cu ),
II
[
aldehyde]/[substrate]
=
0.2. TOF are calculated as moles of product per mole of Cu active centre.
The obtained results were opposite to those reported in literature; ratio increased, the selectivity to epoxide was enhanced. Thus, when the
the conversion decreased as the [aldehyde]/[substrate] ratio increased. ratio used was three, the selectivity to cyclohexene oxide reached 60%
The results have been rationalized by assuming that, as the concentration (Table S9). However, when the ratio was less than one, the main product
of aldehyde in the reacting mixture increased the pores were occupied was different from epoxide. The oxidation occurred preferably over the
by the activator making the diffusion of the substrate into the pores more allylic positon, being the main product cyclohexenol (41%, for a ratio of
difficult. In this way the activation process would be slowed down.
0.5), and cyclohexanone (66%, for a ratio of 0.2).
The selectivity was the expected, as when the [aldehyde]/[substrate]
Table 3 Oxidation of cyclohexene under a continuous flow of 1 atm of molecular oxygen
-1
Catalyst
Conversion (%)
Product
Selectivity (%)
TOF (h )
Cyclohexenone
Cyclohexenol
66
30
4
CuLaIDA
73
1343
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
26
52
22
30
46
24
17
33
50
CuGdIDA
CuYbIDA
Blank
45
39
15
819
718
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
II
Reaction conditions: Temperature (75°C), Substrate (40 mmole), Isobutyraldehyde (8 mmol), Catalyst (0.007 mmol), Cu per mol of catalyst
II
(
0.0009 mmole), Reaction time (24h), S/C ratio (5740/1 based on catalyst; 44000/1 based on Cu ). [aldehyde]/[substrate] = 0.2. TOF values are
II
calculated as moles of product per mole of active Cu centre.
6
| 1–9
Journal Name, [year], [vol.]
This journal is © The Royal Society of Chemistry [year]

Page 7 of 10
1
New Journal of Chemistry
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DOI: 10.1039/C9NJ02091H
Table 4 Oxidation of cyclohexene under a continuous flow of 1 atm of molecular oxygen using the activated catalysts
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Catalyst
Conversion
%)
Selectivity
TOF
-
1
(
(%)
76
18
0
(h )
CuLaIDA
99
77
74
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
1821
1
,2-cyclohexanediol
6
CuGdIDA
CuYbIDA
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
43
37
15
5
37
40
18
5
1417
1361
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
,2-cyclohexanediol
Cyclohexenone
Cyclohexenol
Cyclohexene oxide
1
,2-cyclohexanediol
II
Reaction conditions: Temperature (75◦C), Substrate (40 mmole), Isobutyraldehyde (8 mmole), Catalyst (0.007 mmole), Cu per mole of catalyst
0.0009 mmole), Reaction time (24h), S/C ratio (5740/1 based on catalyst; 44000/1 based on Cu ). [aldehyde]/[substrate] = 0.2. TOF values are
calculated as moles of product per mole of active Cu centre.
II
(
II
Since the TGA analyses showed that the water molecules present in Table 5 DFT calculated vertical electron affinities (eV) for CuLaIDA,
the channels were lost at ca. 60◦C (Fig S2), and the reactions were done CuGdIDA and CuYbIDA models. Sign convention for electron affinity is (Eox
at 75◦C, it was assumed that the water molecules present inside the – Ered). Results were calculated using the B3LYP and BP86 density
channels were partially removed.
functional in conjunction with the def2-TZVP basis set (CPCM solvation
model for water).
To determine the relevance of the pore size, the reactions were also
done with catalysts having a previous thermal treatment at 100 C to
ensure that the water molecules present inside the porous were
completely removed before the catalytic reaction started. The obtained
results are summarized in Table 4.
◦
DFT Functional
B3LYP
2.00
3.49
3.66
BP
CuLaIDA
CuGdIDA
CuYbIDA
2.44
4.64
5.13
The systems maintained the tendency observed for the reactions
without the activation of the catalysts. However, the conversion
increased ca. 26-35% for the different catalysts, when these were
previously thermally activated.
After geometry optimization of the hydrogen positions, the vertical
electron affinity was calculated using one GGA and one hybrid density
functional (BP86 and B3LYP, respectively). In all cases, the reduced state
associated with the formation of copper(I) stabilizes when advancing from
La to Yb (Table 5). In this way, the higher catalytic activity of CuLaIDA
cannot be related with changes in redox properties of the copper centre.
Thus, pore size appears as the key factor determining the differences in
reactivity observed for CuLaIDA; CuGdIDA and CuYbIDA.
Besides all the studied systems became slightly more oxidant as
compared with the one done with the catalyst without thermal
treatment, evidenced by the appearance of a small amount of 1,2-
cyclohexanediol among the oxidation products, and also by the increase
of the selectivity of cyclohexenone. The catalyst CuLaIDA remains the
most active catalyst for the family with a conversion of 99% after 24h of
reaction, and a selectivity of 76% towards the generation of
cyclohexenone. These results confirm that the pore size is determinant
in the activity of this family of catalysts, and that the partial or total
occupancy of the pores by the water molecules affects the catalytic
results.
III
III
Conclusions
For the studied catalysts, the amount of oxygen is determinant when
the reactions were done in a batch reactor. On the other hand, the nature
of the catalyst was important when the reactions were done with a
constant oxygen flow of 1 atm.
Electronic Structure Calculations
Besides the inorganic polymer pore size, there could be another
structural and/or electronic factor related with the higher catalytic
The [aldehyde]/[substrate] ratio was shown to control the catalytic
III
III
activity of CuLaIDA in comparison to the Gd and Yb derivatives. To parameters. When the reaction was done using a ratio of less than 1, the
check this point, electronic structure calculations were performed for conversion increased and the main product was cyclohexenone or
II
the three structures. Considering the Cu centre as the active site, cyclohexenol. On the other hand, when the ratio was higher than 1, the
models describing the internal surface of the channels for CuLaIDA, conversion decreased and the main product was the epoxide.
II
CuGdIDA and CuYbIDA were built. The model considers a central Cu ion
The pore size was found to be the main factor to explain the different
III
II
surrounded by four Ln centres and four distant Cu positions, which
results. DFT calculations showed that the higher catalytic activity of
CuLaIDA could not be related with changes in redox properties of the
copper centre. Besides, when the reactions were done using thermally
activated catalysts, the conversion increased. Thus, when the water
molecules present in the channels were eliminated by the thermal
treatment, the catalytic reactions were enhanced.
II
were replaced by redox inactive Zn ion. In this way, calculations for the
II
reduction process of Cu focus on the central ion of the model (technical
details are presented in computational details section and the model
structure is depicted in (Fig. S8).
6
| 1–9
Journal Name, [year], [vol.]
This journal is © The Royal Society of Chemistry [year]

New Journal of Chemistry
Page 8 of 10
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DOI: 10.1039/C9NJ02091H
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Chem. Commun., 2017, 86, 112–117.
Acknowledgements
The authors thank Proyecto Anillo ACT 1404, and Proyecto Basal
CEDENNA FB0807, LIA 3M-CNRS 1207 grant. C. K. thanks CSIC
2
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K. Leus, M. Vandichel, Y. Y. Liu, I. Muylaert, J. Musschoot,
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(Uruguay) for financial support through Programa de Apoyo a Grupos de
Speybroeck and P. Van Der Voort, J. Catal., 2012, 285,
Investigación. Powered@NLHPC: This research was partially supported
by the supercomputing infrastructure of the NLHPC (ECM-02).
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53x41mm (96 x 96 DPI)