Mechanochemical defect engineering of HKUST-1 and impact of the resulting defects on carbon dioxide sorption and catalytic cyclopropanation

Article abstract of DOI:10.1039/c9ra10412g

Metal-organic frameworks (MOFs) are recognized as ideal candidates for many applications such as gas sorption and catalysis. For a long time the properties of these materials were thought to essentially arise from their well-defined crystal structures. It is only recently that the importance of structural defects for the properties of MOFs has been evidenced. In this work, salt-assisted and liquid-assisted grinding were used to introduce defects in a copper-based MOF, namely HKUST-1. Different milling times and post-synthetic treatments with alcohols allow introduction of defects in the form of free carboxylic acid groups or reduced copper(i) sites. The nature and the amount of defects were evaluated by spectroscopic methods (FTIR, XPS) as well as TGA and NH₃temperature-programmed desorption experiments. The negative impact of free -COOH groups on the catalytic cyclopropanation reaction of ethyl diazoacetate with styrene, as well as on the gravimetric CO₂sorption capacities of the materials, was demonstrated. The improvement of the catalytic activity of carboxylic acid containing materials by the presence of Cu^Isites was also evidenced.

Full text of DOI:10.1039/c9ra10412g

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PAPER
Mechanochemical defect engineering of HKUST-1
and impact of the resulting defects on carbon
dioxide sorption and catalytic cyclopropanation†
Cite this: RSC Adv., 2020, 10, 19822
Timothy Steenhaut, * Nicolas Gregoire,
´
Yaroslav Filinchuk * and Sophie Hermans
Gabriella Barozzino-Consiglio,
*
Metal–organic frameworks (MOFs) are recognized as ideal candidates for many applications such as gas
sorption and catalysis. For a long time the properties of these materials were thought to essentially arise
from their well-defined crystal structures. It is only recently that the importance of structural defects for
the properties of MOFs has been evidenced. In this work, salt-assisted and liquid-assisted grinding were
used to introduce defects in a copper-based MOF, namely HKUST-1. Different milling times and post-
synthetic treatments with alcohols allow introduction of defects in the form of free carboxylic acid
groups or reduced copper(I) sites. The nature and the amount of defects were evaluated by
spectroscopic methods (FTIR, XPS) as well as TGA and NH
experiments. The negative impact of free –COOH groups on the catalytic cyclopropanation reaction of
ethyl diazoacetate with styrene, as well as on the gravimetric CO sorption capacities of the materials,
3
temperature-programmed desorption
Received 11th December 2019
Accepted 6th May 2020
2
DOI: 10.1039/c9ra10412g
was demonstrated. The improvement of the catalytic activity of carboxylic acid containing materials by
I
rsc.li/rsc-advances
the presence of Cu sites was also evidenced.
from missing linkers and/or metal nodes, are at work in those
fascinating materials. Sometimes those defects are responsible
Introduction
28–33
Among the wide variety of known porous materials, the quite for dramatic changes in the MOFs' properties.
However,
recently discovered metal–organic frameworks (MOFs) show there is still a lot to be discovered concerning the formation and
1
–5
34
good potential for applications in gas sorption and catal- mode of action of those defects in both catalysis and gas
6–12
35–38
ysis. MOFs are made up of metal centres (or clusters) that are sorption applications.
connected together by organic bridging ligands. These highly
We here focus on the introduction of defects in one of the
crystalline materials possess very high surface areas, up to 7000 most studied MOFs, namely HKUST-1. This material is based
39
2
ꢀ1 13
m
g
,
exceeding those of other porous compounds like on copper(II) paddlewheel units that are connected together by
3
ꢀ
14
15–24
zeolites or carbon derivatives.
nature, the properties and performance of MOFs have been deprotonated form of trimesic acid (H
Because of their crystalline the benzene-1,3,5-tricarboxylate ligand (BTC ), which is the
40
BTC) (Fig. 1). Being one
3
mainly explained by the presence of well-dened active sites of the most popular MOFs covered in the literature over the past
arising from the ordered arrangement of ligands and metals few years, HKUST-1 has been extensively studied and some
into the framework. In particular, the metal centres are oen aspects dealing with defects inside its structure, either formed
41
coordinated to solvent molecules, present as synthesis residues. during the synthesis or upon exposure to moisture or solvents,
In some cases, those can be removed by simple thermal treat- have already been addressed. For example, the formation of
ment, in a process commonly called ‘activation’, leading to defects in solvothermally obtained HKUST-1 by adding iso-
42
25
coordinatively unsaturated sites (CUS). Those CUS are ideal phthalate as defective linker was studied by Zhang et al.
26
for applications in catalysis and can also serve as preferential Investigation through microscopic techniques were also per-
27
adsorption sites for a large range of gasses.
However, recent work demonstrated that in fact much more MOF.
complex species, present as structural defects mainly resulting defects in HKUST-1 were also described.
formed to identify and localize inherent defect sites in this
43,44
Strategies to overcome and reconstruct structural
45–47
In this work, we aim at introducing defects into the structure
of HKUST-1 without addition of defective linkers, but by
modifying the synthesis conditions (Fig. 2). The defects will be
characterized and their impact on the gas sorption and catalytic
properties of the material evaluated. For this purpose, we used
recently developed mechanochemical synthesis strategies,
Universit ´e catholique de Louvain, IMCN, Place Louis Pasteur 1/L4.01.03, 1348
Louvain-la-Neuve, Belgium. E-mail: timothy.steenhaut@uclouvain.be; yaroslav.
linchuk@uclouvain.be; sophie.hermans@uclouvain.be
†
Electronic supplementary information (ESI) available. See DOI:
1
0.1039/c9ra10412g
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nature of these defects to be free carboxylic acid groups of the
partially protonated ligand and reduced copper(I) sites,
respectively. We systematically evaluated the carbon dioxide
sorption properties of the materials to determine the inuence
of the defects on the interaction with this gas. We also tested all
the obtained MOFs in the cyclopropanation of styrene with ethyl
diazoacetate, as it was previously shown that HKUST-1 is able to
51
efficiently catalyse this reaction.
Materials and methods
Instrumental
Fig. 1 3D structural model of HKUST-1 (left) and zoom on Cu–Cu
paddlewheel unit (right). Copper atoms are depicted in blue, oxygen in
red and carbon in black. Reproduced from ref. 31 with permission from
the Royal Society of Chemistry.
Powder X-ray diffraction patterns were measured using a MAR345
diffractometer with X-rays generated by a Rigaku ultraX 18S X-ray
˚
generator (molybdenum anode, 0.71073 A), with a mono-
namely liquid-assisted grinding (LAG) using a small amount of
chromated beam (Xenocs FOX 3D mirror). The samples were
prepared in glass capillaries (diameter: 0.7 mm). The raw data
was integrated through the FIT2D soware, using LaB
measured in a 0.1 mm diameter capillary) as a reference sample.
Nitrogen sorption isotherms were measured at 77 K using
a Micromeritics ASAP2020 equipment. All samples were activated
at 150 C under dynamic vacuum for 10 h prior to analysis.
Infrared spectra were recorded on a Bruker Alpha spectrom-
eter equipped with a Platinum ATR module (diamond crystal)
housed in an argon-lled glovebox in the 4000–370 cm range
48
49
ethanol and salt-assisted grinding (SAG). Mechanochemical
synthesis of MOFs gives high yields of material and is therefore
considered as a possible production method at industrial
6
(
50
scale. The latter technique, SAG, was recently shown to be
a convenient way to introduce mesoporosity into HKUST-1,
leading to hierarchized solids. This approach consists in
milling the precursors of the MOF, namely copper(II) acetate
monohydrate and trimesic acid, in the presence of a given
amount of salt, giving rise to the inclusion of very small salt
particles, which are removable from the MOFs by simple
washing. We used post-synthetic treatment procedures
ꢁ
ꢀ1
ꢀ1
with a resolution of 4 cm
.
TGA measurements were performed on a Mettler Toledo
TGA/DSC system equipped with a sample robot. The used air
ow was set at 100 ml min . An initial isotherm at 27 C was
applied during 15 minutes before heating the sample to 900 C
49
different from those described earlier. Our procedures consist
in treating the MOFs with alcohols, which did not yield hier-
archized porosity but allowed us to introduce in a controllable
manner two types of structural defects. We determined the
3+
ꢀ1
ꢁ

ꢁ
ꢁ
ꢀ1
at a rate of 10 C min
.
Fig. 2 Scheme of the synthesis of HKUST-1 samples via the different preparation routes envisaged in this work: (A) solvothermal procedure to
obtain single crystals, (B) liquid-assisted grinding (LAG) and salt assisted-grinding (SAG) with post-synthetic treatments using (C) methanol and (D)
ethanol.
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3
NH –TPD was used to evaluate the acidity of the HKUST SAG
H
2
O # 3 ppm, O
2
# 2 ppm, C
n
H
m
# 0.5 ppm) were supplied
EtOH materials. A Hiden Catlab reactor combined with a QGA from Air Liquide.
Hiden quadrupole mass spectrometer was used to perform these
Optical microscopy characterization was performed using
a Euromex IS.1053-PLPOLRi polarizing microscope equipped
ꢀ
1
experiments. Samples were pretreated under Ar (40 ml min
,
ꢁ
5
.0 Air Liquide) at 200 C during 90 minutes. NH
3
adsorption was with a Euromex VC 3036 camera. Calibration was realized using
ꢁ
performed at 70 C during 1 hour by owing a mixture of Argon a 1 mm/100 (10 mm/division) calibration slide and images were
ꢀ
1
ꢀ1
(
20 ml min ) and 5% NH
sample. The MOF was then ushed in Ar (40 ml min ) during 90
minutes and then the NH desorption measurement was per-
3
in He (20 ml min ) through the collected using the ImageFocus 4 soware.
ꢀ1
3
ꢀ
Chemicals
1
ꢁ
formed under Ar (40 ml min ) from 70 to 300 C (heating rate of
Trimesic acid (98%) was purchased from Acros Organics. The
used sodium chloride was food grade. Copper(II) acetate mon-
ohydrate (>99%, AnalaR) was purchased from the British Drug
Houses Ltd. Denaturated ethanol (Technisolv, 99%), methanol
ꢁ
ꢀ1
10 C min ).
XPS analyses were carried out with a SSI-X-probe (SSX
00/206) photoelectron spectrometer from Surface Science
1
Instruments, equipped with a monochromatized microfocus Al
X-ray source. The samples were prepared by sticking onto
a double-face adhesive tape xed onto small brass sample
holders that were placed on an insulating ceramic carousel
(HiPerSolv, CHROMANORM), dimethylformamide (HiPerSolv,
CHROMANORM), dichloromethane (HiPerSolv, CHROMA-
NORM) and HCl 37% (AnalaR, NORMAPUR) were purchased
from VWR Chemicals. Copper(II) chloride hexahydrate (min.
(
Macor). An electron ood gun (8 keV) combined with a nickel
grid were used to avoid charge effects. The CasaXPS soware
Casa Soware Ltd.) was used to perform the data treatment,
using a Shirley type baseline and xing the value of C 1s peak to
84.6 eV. The samples obtained through ball-milling were
analyzed as obtained, whereas the single crystals were ground
9
9%) was purchased from Riedel-de Ha ¨e n. Ethyl diazoacetate
contains $13% dichloromethane) was purchased from Sigma-
Aldrich. Styrene (99.5%) and trimethoxybenzene (99%) were
purchased from Acros Organics. CDCl (D > 99.8%, H O <
.01%) was purchased from Euriso-top. Styrene was distilled
(
(
3
2
2
0
under reduced pressure to eliminate stabilizers before use. All
other chemicals were used as received.
ꢀ
8
prior to analysis. The high vacuum (ꢂ10 Torr) used in the
analysis chamber made the samples change colour from light to
dark blue, indicating in situ activation.
Syntheses
GC analyses were performed under helium ow (38.9
ꢀ
1
ml min ) using a SHIMADZU GC2010 equipped with a Varian
Single crystals. Single crystals of HKUST-1 were obtained by
FactorFour VF-5ms (30 M ꢃ 0.25 MM ID DF ¼ 0.25, part number adding a clear solution containing 1.47 g of copper(II) chloride
CP8944) capillary column. The initial temperature of the oven hexahydrate and 2.33 ml of N,N-dimethylformamide in 200 ml of
ꢁ
was 50.0 C (0.2 min equilibration time), which was raised at deionized water to a clear solution containing 2.10 g of trimesic
ꢁ
ꢀ1
ꢁ
5
.0 C min to 100.0 C for elution (the column was then heated acid and 0.21 ml HCl 37% in 200 ml of denaturated ethanol. The
ꢁ
ꢁ
ꢀ1
to 300.0 C at 50.0 C min to ensure complete removal of any obtained clear mixture was then introduced in a 500 ml glass
residue in the column). The temperature of the injection port bottle closed with a screw cap and was le to crystallize in a labo-
ꢁ
ꢁ
was set at 250.0 C.
ratory oven at 80 C for 5 days. The obtained crystals were recov-
1
H-NMR spectra were recorded at room temperature (296 K) ered by ltration from the still warm solution and were soaked 2
on a Bruker Avance II 300 spectrometer operating at 300.1 MHz times in a 50 : 50 volume mixture of ethanol and water for 2 days,
1
for H. Experiments were run under TopSpin program (3.2 then were soaked one more day in ethanol. The crystals were then
1
version, Bruker) using a BBFO { H, X} probehead equipped with ltered out from the washing solution and allowed to dry at room
1
a z-gradient coil. H chemical shis were referenced to the temperature for 30 min before being stored in a closed glass vial.
signal of the aromatic H of 1,3,5-trimethylbenzene (6.07 ppm)
Ball-milling. Ball-milling was performed with a Fritsch Pul-
that was used as internal standard. For a correct quantication, verisette 7 premium line apparatus. All experiments were
T1 of styrene was determined by means of the inversion recovery carried out in stainless-steel milling bowls of 45 ml (Fristch™
sequence (see ESI†) and the recycle delay (d1) time was set at 50.9750.00), and three stainless-steel balls of 10 mm (Fritsch™
3
0 s (T
1
¼ 5 ꢃ d1) for the measurements to ensure that inte- 55.0100.09) were used for each synthesis. Millings were carried
gration of the peaks yielded suitable quantitative values (with out at room temperature (ꢂ20 ꢁC), the local heating during
an error of ꢄ1%).
milling was not monitored in situ, but milling pauses (breaks)
Gravimetric carbon dioxide sorption experiments were real- were introduced to limit the potential overheating.
3
+
ized using a Mettler Toledo TGA/DSC system equipped with
Washings. Washings by centrifugation were performed in
a sample robot. The samples were pretreated under He atmo- 50 ml plastic centrifugation tubes using a Heraeus Biofuge
ꢀ
1
ꢁ
ꢁ
ꢀ1
ꢀ1
sphere (100 ml min ), by heating to 200 C (10 K min ), fol- Stratos instrument running at 5500 rpm for a duration of 3
lowed by a 2 h isotherm and cooling to 27 C (10 K min ). The minutes.
samples were then further purged with He for 30 min at 27 C,
ꢁ
LAG sample. 0.59 g (3 eq.) of copper(II) acetate monohydrate
before being subjected to 3 adsorption desorption cycles with and 0.41 g (2 eq.) of trimesic acid were introduced into a milling
ꢀ
1
ꢀ1
CO
desorption). Carbon dioxide (N27; H
000 ppm, N
# 4000 ppm) and helium (Alphagaz 1, $99.999%; pause). The sample was washed with 3 ꢃ 45 ml H
2
(100 ml min , on adsorption) and He (100 ml min , on jar together with 1 ml of ethanol. The mixture was subjected to
O # 10 ppm, O milling at 300 rpm for 20 min (4 cycles of 5 min with 2 minutes
O and 3 ꢃ
2
2
#
1
2
2
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ꢁ
4
5 ml EtOH and was then dried under vacuum at 70 C using obtained compounds show powder patterns revealing the
a rotary evaporator, followed by cooling under dynamic vacuum presence of HKUST-1 as single phase (Fig. S1†). Some peak
using a Schlenk line. broadening as well as slightly bumpy background features can
SAG samples. A rst mixture of 0.59 g (3 eq.) of copper(II) be observed for the materials synthesized by the SAG method
acetate monohydrate and 0.2 g of NaCl and a second mixture compared to the LAG and single crystal samples. This, as well as
ꢁ
0
.41 g (2 eq.) of trimesic acid and 0.2 g of NaCl were pre-milled differences in relative intensities of the peaks at 4.38 and
ꢁ
52
at 500 rpm for 1 min in two individual milling jars. Then, the 5.32 , is indicative of the presence of a signicant amount of
two mixtures were gathered together and further milled at structural defects in those materials.
300 rpm for a given amount of time (5, 10 or 20 minutes
Nitrogen sorption isotherms were measured for all the
depending on the sample) in cycles of 5 min with 2 min pause. synthesized compounds. The single crystal sample shows a well-
The ethanol-treated samples were washed with 3 ꢃ 45 ml H O dened I (a) type isotherm, characteristic of microporous
2
ꢁ
53
and 3 ꢃ 45 ml EtOH and were then dried under vacuum at 70 C compounds, which is expected based on the crystal structure
using a rotary evaporator, followed by cooling under dynamic of HKUST-1 (Fig. 3a). The materials synthesized by the mecha-
vacuum using a Schlenk line. The methanol-treated samples nochemical approaches also show a steep uptake at very low
were washed with 3 ꢃ 45 ml H
2
O and 3 ꢃ 45 ml MeOH followed P/P
0
values, typical of microporous solids. However, for those
materials, hysteresis loops of the H4 type are also observed,
ꢁ
by drying in an oven at 45 C and ambient pressure for 18 h.
The MOF samples obtained by SAG presented a much lower which can be ascribed to the presence of intergrain spaces
yield, whatever the nature of the solvents used for performing formed by the aggregation of small crystallites (Fig. 3a–c).
the washings, than when we used the LAG procedure (addition
of some ethanol allows the reaction to be more complete).
Importantly, the calculated Brunauer–Emmett–Teller (BET)
surface areas of the samples vary largely according to the
parameters used for the synthesis as well as the applied washing
and drying procedures (Fig. 3d–f). The highest surface area is
Catalytic tests
2
ꢀ1
obtained for the HKUST-1 single crystals (1881 m g ), directly
Note. Stirring and heating of the reaction mixtures for the
catalytic tests was achieved by using a Heidolph Hei-Tec magnetic
stirrer. The rotation speed accuracy was of ꢄ2%. The stirrer was
equipped with an external Pt 1000 temperature probe ensuring
2
ꢀ1
followed by the sample made by LAG (1739 m g ). The surface
areas of the samples obtained by the SAG procedure decreases
with increasing milling time when the post-synthetic treatment
with ethanol is applied, which could be explained by degrada-
tion of the crystal structure under prolonged high-energy
milling. However, the samples series washed with methanol
and dried in an oven show the opposite tendency: the surface
area increases with increasing milling time. This indicates that
the structure rearranges upon removal of the sodium chloride
particles trapped in the cavities formed during the mechano-
chemical synthesis, and that this rearrangement is dependent
on the applied post-synthetic treatment.
ꢁ
a temperature accuracy of ꢄ1 C. Heating was achieved by using
a stirred oil bath coated with aluminium foil to insulate the bath,
ꢁ
allowing to easily attain the activation temperature of 150 C.
Cyclopropanation reactions were carried-out in a 50 ml three-
neck round-bottom ask equipped with a condenser and a glass
stopcock connected to a Schlenk line. An ellipsoid shaped
magnetic stirring bar was introduced into the ask (all the
experiments were run at 300 rpm). 36 mg of the MOF sample was
ꢁ
introduced in the experimental setup and heating to 150 C under
dynamic vacuum for 1 h was applied to activate the catalyst. The
ꢁ

ask was allowed to cool down to room temperature (25 C) and Identication of defect sites
the volume was relled with argon. 224 mg of 1,3,5-trimethox-
The FTIR spectra of the materials all show the typical vibration
frequencies expected for the chemical bonds in HKUST-1 as
described in earlier reports, the main features being nas and nsym
ybenzene (internal standard, 1.33 mmol) in 3 ml of CDCl was
3
then introduced into the setup. 0.46 ml (4 mmol) of styrene was
added to the stirred mixture. The reaction was initiated by the
dropwise addition of EDA (containing $13% dichloromethane)
ꢀ1
of carboxylates that are present at 1649 and 1451 cm respec-
46,54–56
tively.
In the as-synthesized samples, nC–O bands
diluted in 7 ml of CDCl
3
over two hours using a syringe pump.
ꢀ1
assigned to primary alcohols also appear at 1106 and 1042 cm
Aer complete addition of the EDA, the reaction mixture was
allowed to react for another 1 h. 1 ml was then sampled from the
reaction mixture and the catalyst was separated by ltration on
custom-made lter columns made from Pasteur pipettes lled
with silica (ꢄ20 mm) that was retained through a small cotton
(
Fig. S2†). Those disappear aer thermal activation of the
ꢁ
samples under vacuum at 150 C, showing that this procedure is
effective to desorb residual coordinated methanol and ethanol
molecules. Furthermore, differences in the IR spectra appear
between the various series of samples, indicating the formation
of different types of defects, depending on the used synthetic
procedure and applied post-synthetic treatment.
3
wool plug. The lter was eluted by adding 1 ml of CDCl and
1
0
.5 ml of the ltrate was used for H-NMR analysis.
An absorption band characteristic of C]O stretching of free
ꢀ
1
carboxylic acids is observed at around 1710 cm both for the
MOFs obtained by SAG and in the form of single crystals,
whereas this feature is not present in the one synthesized by
Results and discussion
Crystal structure and textural properties
The crystal structure of all the synthesized MOF materials was LAG (Fig. 4). Interestingly, the intensity of this signal is quite
veried by PXRD on the as-synthesized powders. All the weak for the samples treated with MeOH as well as for the single
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Fig. 3 Nitrogen sorption isotherms of (a) single crystals and LAG samples, (b) SAG samples washed with MeOH, (c) SAG samples washed with
EtOH and (d–f) corresponding calculated Brunauer–Emmett–Teller surface areas.
crystals. On the opposite, the samples that underwent the EtOH
The presence of free carboxylic acid moieties in the EtOH
post-synthetic treatment show a signicantly more intense washed samples, especially for the 20 min EtOH material, is
absorption band that increases with increasing milling time further supported by the weight losses observed in the TGA
used for the synthesis of the materials. The presence of –COOH curves (Fig. S4†), which show an excess weight loss during the
functions in those samples is further conrmed by the presence decomposition (61.2%, 61.5% and 68.8% for 5, 10 and 20 min
ꢀ
1
of absorption bands around 1193 cm (C–O stretching) and milled samples respectively) compared to the value calculated
ꢀ
1
57
1232 cm
(OH bending), typical of carboxylic acids.
3 2
for a perfect Cu BTC formula without any defects (60.5%),
Furthermore, a shi of the nas band towards lower wave- considering a CuO residue. This means that the organic linker/
ꢀ
1
ꢀ1
numbers, from 1649 cm to 1647, 1645 and 1643 cm for the metal ratio is higher than the one expected from the 2 : 3 stoi-
, 10 and 20 min milled samples, respectively, accompanies the chiometric ratio. This observed excess of organic linker is in
increase of signals related to carboxylic acids. perfect agreement with the presence of free –COOH moieties.
5
The acidity of the EtOH treated samples obtained by SAG was Moreover, the thermal stability of the EtOH treated SAG
investigated by ammonia temperature-programmed desorption samples increases with the amount of present carboxylic acid
(
NH –TPD) experiments (Fig. S3†). The obtained curves show sites (Fig. S4†). The thermal stability of the HKUST-1 materials
3
the presence of two overlapped desorption peaks, one at about was compared to starting copper acetate and trimesic acid and
ꢁ
ꢁ
1
50 C and the other, less intense, at ꢂ200 C. The same showed that the acetate has a lower decomposition temperature
ꢁ
ꢁ
experiment was performed on the sample obtained by LAG (289 C) than HKUST-1 (between 300 and 350 C, depending on
ꢁ
which showed only one desorption peak at 150 C. The peak at the sample) which itself is less stable than trimesic acid (Tdec
¼
ꢁ
ꢁ
1
50 C can be assigned to desorption of NH coordinated to the 408 C). This indicates that the observed increase of thermal
3
copper sites of the MOF. The presence of a second peak at stability is indeed due to the presence of excess, partially
higher temperatures for the EtOH treated samples means that protonated, carboxylate ligand in the materials.
a second, more acidic, site is present, which is in this case
For the MeOH treated SAG samples, as well as for the single
attributed to the presence of –COOH moieties in the materials. crystals, the FTIR spectra show the appearance of an absorption
ꢀ
1
This peak is especially visible for the sample obtained aer band at 1616 cm . This quite intense band does not appear in
0 min of milling, which correlates with the most intense the spectra of the EtOH treated SAG samples and is of negligible
2
–
COOH bands in the FTIR spectra.
intensity in the case of the LAG sample. Based on its position,
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II
I
reduction of Cu into Cu by methanol during the prolonged
ꢁ
drying at 45 C used for the post-synthetic treatment. Further-
more, this ratio increases with increasing milling time
I
(
Fig. S5†). From this, it is concluded that Cu , associated with
a change in coordination of the carboxylate moieties, is the
main type of defect present in the MeOH treated materials and
that the amount of such defects increases with the milling time
used during the mechanochemical synthesis. From the FTIR
spectra, it is expected that the same type of defects are present
in the single crystals. However, those were ground prior to XPS
analysis, exposing the bulk composition, whereas the XPS
spectra of all other samples are representative of the external
surface of the as-obtained powders.
I
The presence of Cu defect sites in the MeOH treated and
single crystal materials is further conrmed by the weight losses
observed during their thermal decomposition (Fig. S4†), which,
on the contrary to the EtOH treated samples, are lower than the
expected value for Cu BTC . This observation means that an
3
2
excess of Cu is present in the materials, which is in agreement
I
with the existence of Cu . Furthermore, this weight loss
I
decreases with increasing milling time and associated Cu
content revealed by XPS. Interestingly, the loss observed during
ꢁ
the decomposition of the single crystals (58.4 C) indicate that
I
they contain an amount of Cu that is intermediate between that
ꢁ
ꢁ
of the 10 min (ꢂ60.6 C) and 20 min (57.5 C) milled samples.
The thermal stabilities of the MeOH treated and single crystal
samples, similar to that of the MOF obtained by LAG, are lower
than for the EtOH treated SAG materials. It should be noted that
from TGA analysis, the as obtained single crystals have an
ꢁ
apparent higher thermal stability (360 C), which is due to the
large size of the crystallites (ꢂ100–400 mm, see optical micros-
copy characterization in the ESI†). This thermal stability is
greatly lowered to 320 C by reducing the particle size by gentle
grinding in an agate mortar prior to analysis (Fig. S4†), enabling
a more accurate comparison with the small sized particles
Fig. 4 FTIR spectra of the HKUST-1 materials obtained through the
different synthesis pathways: (a) single crystals and LAG samples, (b)
SAG samples washed with MeOH, (c) SAG samples washed with EtOH.
ꢁ
The zones highlighted in colour indicate the absorption bands that are
I
the most affected by the presence of Cu (green) and –COOH (red) resulting from the mechanochemical syntheses.
defects.
Finally, although possessing the second highest surface area
of all the materials, the MOF obtained through the LAG proce-
dure seems to be the one that possesses the least amount of
this absorption band can be ascribed to a nas of carboxylate. structural defects. The atness of the background of the PXRD
However, as stated previously, the asymmetric stretching band pattern, the absence of absorption bands associated to –COOH
I
of the carboxylates in typical HKUST-1 materials is located at in the FTIR spectrum or Cu defects detected by XPS, its weight
ꢀ1
1
649 cm . It has been established since a long time that the loss during decomposition that is close to the expected value for
positions of the symmetric and asymmetric stretching bands of Cu BTC as well as its decomposition temperature are all
3
2
carboxylates in inorganic compounds depend on the nature of features that support the low defect content of this MOF.
ꢀ
58
the coordination mode of the –COO moiety. Thus, from the Furthermore, the XPS data show that the surface composition of
FTIR spectra, it seems that a different type of metal-to-ligand this sample is nicely located in between that of the two SAG
coordination is present in the MeOH treated samples.
XPS measurements were performed to further investigate the
defects present within the different samples. It was observed
sample series (Fig. S5†).
Carbon dioxide sorption properties
that they all contain copper in the +I and +II oxidation state at
II
the surface, see Fig. S5† for more details. Although Cu readily Carbon dioxide sorption capacities of the different materials
I
59
reduces into Cu during XPS measurements, the analysis of all were evaluated by using a thermogravimetric analyser. This type
the samples under strictly identical conditions allows for of measurement allows obtaining an evaluation of the gravi-
a reasonable comparison between the different materials. It was metric sorption by using a minimum amount of sample. The
ꢁ
observed that the MeOH treated samples display a much higher samples were activated thermally in situ at 200 C under
I
II
Cu /Cu ratio than all other samples. This can be explained by a helium ow before three CO adsorption/desorption cycles
2
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Fig. 5 Gravimetric CO
2
sorption cycles realized using TGA with (a) single crystals and LAG material, (b) SAG samples washed with methanol and
ꢁ
dried at 45 C and (c) SAG samples washed with ethanol and dried under vacuum. Errors on the measurements are estimated to be ꢄ1 wt% (see
ESI† for details on corrections and reproducibility).
ꢁ
that were performed at 27 C. Appropriate corrections were Catalytic cyclopropanation
applied for each measurement (see details in the ESI†) and the
obtained results are presented in Fig. 5.
The materials were tested for their ability to catalyse the cyclo-
propanation reaction of styrene with ethyl diazoacetate (EDA)
The CO
rst loading is not completely released aer 30 min of purging
with He: about 2 to 3% weight CO remains adsorbed on the
2
that is initially adsorbed by the materials during the
(Scheme 1) to evaluate the impact of the number and type of

defects on the catalytic activity. The LAG sample was used as
reference catalyst and kinetic follow-up by gas chromatography
2
samples. This can be expected since the desorption kinetics is
slower than the adsorption kinetics. Furthermore, the adsorp-
tion curves display two distinct regions, the rst one showing
a very steep uptake, likely related to the lling of a preferential
(GC) of the reaction in dichloromethane showed that EDA was
totally converted in less than 30 minutes (Fig. S6†). The GC
experiment also showed the formation of signicant amounts
of side products: diethyl fumarate and diethyl maleate. Further
experiments were therefore run by adding the EDA dropwise
over 2 hours to a solution of styrene containing the catalyst by
using a syringe pump. In this way, the concentration of EDA in
the reaction mixture was kept as low as possible, lowering the
formation of unwanted side products. The solvent was shied
CO sorption site (probably the desolvated copper sites). The
2
second adsorption step is much slower and is related to occu-
pation of less favourable adsorption sites, likely in the cavities
of the MOF's pores.
The best performing material for gas sorption are the single
crystals, which are able to adsorb about 17% weight of CO
2
. This
from dichloromethane to CDCl , which has similar properties
3
is much higher than the values reported previously under
to CH Cl , and 1,3,5-trimethoxybenzene was added as internal
2
2
60
similar conditions for HKUST-1 materials, and even in
comparison with other porous materials measured under
similar conditions (see Table 1, ESI†). Only MOFs of the MOF-74
type and MOFs functionalized with amines, which are known to
1
standard to enable reaction follow-up by H-NMR. All the
reactions were stopped one hour aer complete addition of the
EDA to the reaction mixture. The catalysts were all activated
ꢁ
under vacuum at 150 C prior to the catalytic tests, although this
exhibit very good affinity for CO
Surprisingly, the LAG material, having a surface area only about
.5% smaller than the single crystals, showed an adsorption of
2
, possess higher uptake values.
7
only 10% weight of CO . The adsorption capacity of the 5 min
2
MeOH SAG sample is very similar to that observed for the LAG
material, although its surface area is much lower. However, the
1
0 and 20 min MeOH samples adsorb a lower amount of CO
whereas they possess higher surface areas.
Importantly, the presence of –COOH defects in the EtOH
treated samples has a very negative effect on the CO sorption of
the materials, allowing the sorption of only 5% weight CO . This
2
2
2
can be attributed to the Brønsted acidity of these MOFs, which
is not favourable for the adsorption of carbon dioxide that is an
acidic gas. The obtained results demonstrate that the CO2
sorption capacity of HKUST-1 is largely governed by the type and
amount of defects in the MOF rather than by its surface area.
Scheme 1 Cyclopropanation reaction of styrene with EDA catalysed
by HKUST-1.
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step could have been skipped as exposure to small halogen-
oalkanes, like CH Cl and CDCl , can achieve activation of
2
2
3
61
HKUST-1 in situ.
The EDA conversion and selectivity of all the catalysts are
summarized in Fig. 6, 7 and Table S2.† A blank test was run
without catalyst and showed no conversion of the reactants aer
3
hours. On the contrary, the LAG sample showed 100% EDA
conversion. The EtOH treated SAG samples, on the one hand,
showed very poor (5 min milled sample) or no conversion at all.
This indicates that the presence of acidic defects in the MOF
drastically inhibits the catalytic activity of HKUST-1 towards
cyclopropanation of styrene with EDA. On the other hand, the
methanol treated samples showed better EDA conversion,
I
which increases with the amount of Cu defects at the surface of
the sample, reaching 100% conversion for the 20 min milled
sample. The conversion observed when using the single crystals
as catalyst (92.6%) is comprised between that of the MeOH
treated SAG samples that were obtained by milling for 10 min
Fig. 7 Selectivity of the cyclopropanation reaction of EDA with styrene
obtained by using the different HKUST-1 materials as catalysts.
(15.2%) and 20 min (100%). This is in striking agreement with
the previous conclusion that the single crystals possess the
same type of defects as the MeOH treated samples and that the
amount of those defects is comprised between that of the 10
and 20 min milled materials.
On the contrary, the 5 min EtOH and 20 min MeOH materials
have a surface composition largely differing from that of the
LAG MOF but are both the best performing catalysts of their
series. This observation shows that the course of the catalytic
reaction is not governed by the surface of the MOF but rather by
its bulk composition.
Furthermore, the selectivity of the single crystal and meth-
anol washed catalysts show that the formation of cyclopropanes
I
is favoured when the amount of Cu defects increases, whereas
a low amount of defects, such as in the 5 min MeOH sample
favour the formation of dimethylfumarate and – maleate side-
products. The LAG sample, which presents the least defects,
shows similar conversion and selectivity than the 20 min MeOH
material, which is the best performing one in its category.
Interestingly, the XPS analyses (Fig. S5†) indicate that the
Overall, the outcome of the experiments show that acidic
defects in the form of free –COOH functions in HKUST-1 poison
the catalytic activity towards cyclopropanation. On the other
hand, the activity of the MeOH treated SAG samples and the
single crystals, also showing the presence of small amounts of
20 min EtOH and 5 min MeOH treated SAG samples, which are
I
free –COOH functions, increases with increasing Cu concen-
both the worst performing materials of their series, possess
a surface composition quite similar to that of the material ob-
tained by LAG, which is the best performing of all tested MOFs.
I
tration. This indicates that Cu defects can counteract the
poisoning effect of carboxylic acid. The LAG sample, which
shows nearly no defects, seems to possess the best activity and
conversion toward cyclopropanes, although being close to the
I
ones obtained for the Cu rich 20 min MeOH sample in the
conditions used for the experiments. In this perspective, it
II
seems that the defectless Cu sites are the most active towards
the cyclopropanation reaction.
Conclusions
We demonstrated that mechanochemical synthesis of HKUST-1
by salt-assisted grinding induces the formation of high
amounts of defects in the structure of this MOF. Importantly,
the type and amount of defects is dependent on the applied
post-synthetic treatment and the duration of the milling. We
showed that washing with water and ethanol followed by drying
ꢁ
under vacuum at 70 C mainly results in the presence of free
–
COOH non-coordinated functions in the MOF, whereas treat-
ment with water and methanol followed by gentle heating at
ꢁ
I
II
4
5 C for 18 h results in partial reduction of the Cu sites into
Cu . We also evidenced that large (ꢂ100–400 mm) single crystals
Fig. 6 Ethyl diazoacetate conversions after the reaction with styrene
in the presence of the different HKUST-1 materials as catalysts.
of HKUST-1 obtained in this work also possess a large amount
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I
of similar Cu defects. Surprisingly, liquid-assisted grinding
9 M. Liu, J. Wu and H. Hou, Chem.–Eur. J., 2019, 25, 2935–
(LAG) appears to be the strategy resulting in the material with
2948.
the lowest amount of structural defects. This encourages the 10 V. R. Remya and M. Kurian, Int. Nano Lett., 2019, 9, 17–29.
use of LAG for making HKUST-1, as it also results in high 11 L. Zhu, X. Q. Liu, H. L. Jiang and L. B. Sun, Chem. Rev., 2017,
synthetic yields. Those conclusions were drawn through thor-
ough characterization of the materials by vibrational spectros- 12 C. Wang, B. An and W. Lin, ACS Catal., 2019, 9, 130–146.
copy (FTIR), XPS, TGA and NH temperature-programmed 13 O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser,
desorption experiments. We also showed that CO adsorption C. E. Wilmer, A. A. Sarjeant, R. Q. Snurr, S. T. Nguyen,
117, 8129–8176.
3
2
¨
by HKUST-1 seems to be the most efficient when using mate-
rials with low amounts of structural defects. Acidic –COOH sites
A. O. Yazaydin and J. T. Hupp, J. Am. Chem. Soc., 2012,
134, 15016–15021.
in the materials result in poor adsorption of CO but strongly 14 E. G. Fawaz, D. A. Salam, H. Nouali, I. Deroche, S. Rigolet,
2
increases affinity of the materials for NH . However, even in the
presence of some Cu defects, HKUST-1 in the form of large 15 Z. Yang, Y. Xia and R. Mokaya, J. Am. Chem. Soc., 2007, 129,
B. Lebeau and T. Jean Daou, Molecules, 2018, 23, 1–13.
3
I
single crystals show the highest gravimetric carbon dioxide
1673–1679.
uptake, evidencing an inuence of the texture and size of the 16 Z. Heidarinejad, M. H. Dehghani, M. Heidari, G. Javedan,
MOF particles on its performance. Finally, we showed that the
I. Ali and M. Sillanp ¨a ¨a , Environ. Chem. Lett., 2020, 18, 393–
presence of –COOH defect sites in HKUST-1, despite increasing
415.
the thermal stability of the material, strongly poisons the 17 N. Mohamad Nor, L. C. Lau, K. T. Lee and A. R. Mohamed, J.
catalytic activity of this MOF towards the cyclopropanation of Environ. Chem. Eng., 2013, 1, 658–666.
styrene with ethyl diazoacetate. The presence of high amounts 18 B. Fidalgo and J. A. Menendez, Cuihua Xuebao, 2011, 32, 207–
´
´
I
of Cu seems to partly counteract this poisoning effect. However,
the absence of carboxylic acid sites in the LAG-obtained mate- 19 A. Jain, R. Balasubramanian and M. P. Srinivasan, Chem.
rial leads to the highest conversion as well as the best selectivity Eng. J., 2016, 283, 789–805.
towards the desired cyclopropanes, even with low amounts of 20 A. G. Pandolfo and A. F. Hollenkamp, J. Power Sources, 2006,
216.
I
II
Cu , indicating that the pristine Cu sites are the most catalyt-
ically active ones.
157, 11–27.
21 L. J. Konwar, J. Boro and D. Deka, Renewable Sustainable
Energy Rev., 2014, 29, 546–564.
2
2 M. Ahmad, A. U. Rajapaksha, J. E. Lim, M. Zhang, N. Bolan,
D. Mohan, M. Vithanage, S. S. Lee and Y. S. Ok, Chemosphere,
2014, 99, 19–33.
Conflicts of interest
There are no conicts to declare.
2
3 M. Inagaki, F. Kang, M. Toyoda and H. Konno, Advanced
Materials Science and Engineering of Carbon, Elsevier, 1st
edn, 2013.
Acknowledgements
The authors acknowledge nancial support through the FRIA 24 H. Marsh and F. R. Reinoso, Activated Carbon, Elsevier, 1st
funding of F.R.S./FNRS for T. S. We thank Dr François Devred
edn, 2006.
for his help with NH –TPD measurements, Dr Pierre Eloy for 25 M. Dincǎ and J. R. Long, Angew. Chem., Int. Ed., 2008, 47,
3
help with the XPS measurements and interpretation of the data.
6766–6779.
We also acknowledge Laurent Collard for his help with CG 26 J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang and C. Y. Su, Chem.
measurements and Jean-François Statsyns for his help with the
TGA measurements.
Soc. Rev., 2014, 43, 6011–6061.
27 L. Chen, C. A. Morrison and T. D u¨ ren, J. Phys. Chem. C, 2012,
1
16, 18899–18909.
8 J. Canivet, M. Vandichel and D. Farrusseng, Dalton Trans.,
016, 45, 4090–4099.
2
Notes and references
2
1
2
S. Kazemi and V. Safarifard, Nanochem. Res., 2018, 3, 62–78. 29 S. Dissegna, K. Epp, W. R. Heinz, G. Kieslich and
K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, R. A. Fischer, Adv. Mater., 2018, 30, 1–23.
E. D. Bloch, Z. R. Herm, T. H. Bae and J. R. Long, Chem. 30 Z. Fang, B. Bueken, D. E. De Vos and R. A. Fischer, Angew.
Rev., 2012, 112, 724–781. Chem., Int. Ed., 2015, 54, 7234–7254.
Z. Hu, Y. Wang, B. B. Shah and D. Zhao, Adv. Sustainable 31 D. S. Sholl and R. P. Lively, J. Phys. Chem. Lett., 2015, 6, 3437–
3
Syst., 2019, 3, 1800080.
3444.
4
5
A. Uzun and S. Keskin, Prog. Surf. Sci., 2014, 89, 56–79.
H. Li, K. Wang, Y. Sun, C. T. Lollar, J. Li and H. C. Zhou,
Mater. Today, 2018, 21, 108–121.
32 A. K. Cheetham, T. D. Bennett, F. X. Coudert and
A. L. Goodwin, Dalton Trans., 2016, 45, 4113–4126.
33 A. Dhakshinamoorthy, Z. Li and H. Garcia, Chem. Soc. Rev.,
2018, 47, 8134–8172.
6
7
L. Chen and Q. Xu, Matter, 2019, 1, 57–89.
S. Sabale, J. Zheng, R. S. Vemuri, X.-Y. Yu, B. P. McGrail and 34 D. Yang and B. C. Gates, ACS Catal., 2019, 9, 1779–1798.
R. K. Motkuri, Synth. Catal.: Open Access, 2016, 1, 1–8.
W. G. Cui, G. Y. Zhang, T. L. Hu and X. H. Bu, Coord. Chem.
Rev., 2019, 387, 79–120.
35 P. Iacomi, F. Formalik, J. Marreiros, J. Shang, J. Rogacka,
A. Mohmeyer, P. Behrens, R. Ameloot, B. Kuchta and
P. L. Llewellyn, Chem. Mater., 2019, 31, 8413–8423.
8
19830 | RSC Adv., 2020, 10, 19822–19831
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
3
3
6 S. Chong, G. Thiele and J. Kim, Nat. Commun., 2017, 8, 49 J. Yang, X. Feng, G. Lu, Y. Li, C. Mao, Z. Wen and W. Yuan,
539. Dalton Trans., 2018, 47, 5065–5071.
7 L. M. Rodr ´ı guez-Albelo, E. L ´o pez-Maya, S. Hamad, A. R. Ruiz- 50 M. Klimakow, P. Klobes, A. F. Th u¨ nemann, K. Rademann
1
Salvador, S. Calero and J. A. R. Navarro, Nat. Commun., 2017,
, 14457.
8 X. Zhang, Y. Yang, L. Song, J. Chen, Y. Yang and Y. Wang, J.
Hazard. Mater., 2019, 365, 597–605.
9 S. S. Chui, S. M. Lo, J. P. H. Charmant, A. G. Orpen and
I. D. Williams, Science, 1999, 283, 1148–1151.
0 C. H. Hendon and A. Walsh, Chem. Sci., 2015, 6, 3674.
1 M. Todaro, G. Buscarino, L. Sciortino, A. Alessi, F. Messina,
and F. Emmerling, Chem. Mater., 2010, 22, 5216–5221.
51 A. Corma, M. Iglesias, F. X. Llabr ´e s i Xamena and F. S ´a nchez,
Chem.–Eur. J., 2010, 16, 9789–9795.
52 H. V. Doan, A. Sartbaeva, J. C. Eloi, S. A. Davis and V. P. Ting,
Sci. Rep., 2019, 9, 1–9.
53 M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier,
F. Rodriguez-Reinoso, J. Rouquerol and K. S. W. Sing, Pure
Appl. Chem., 2015, 87, 1051–1069.
8
3
3
4
4
M. Taddei, M. Ranocchiari, M. Cannas and F. M. Gelardi, J. 54 S. Loera-Serna, L. L. N u´ n˜ ez, J. Flores, R. L ´o pez-Simeon and
Phys. Chem. C, 2016, 120, 12879–12889.
H. I. Beltr ´a n, RSC Adv., 2013, 3, 10962–10972.
2 W. Zhang, M. Kauer, P. Guo, S. Kunze, S. Cwik, M. Muhler, 55 J. B. DeCoste, G. W. Peterson, B. J. Schindler, K. L. Killops,
4
Y. Wang, K. Epp, G. Kieslich and R. A. Fischer, Eur. J.
Inorg. Chem., 2017, 2017, 925–931.
M. A. Broweb and J. J. Mahle, J. Mater. Chem. A, 2013, 1,
11922–11932.
4
4
3 M. Sh ¨o ˆa e `e , J. R. Agger, M. W. Anderson and M. P. Atteld, 56 T. Toyao, K. Liang, K. Okada, R. Ricco, M. J. Styles,
CrystEngComm, 2008, 10, 646–648.
Y. Tokudome, Y. Horiuchi, A. J. Hill, M. Takahashi,
M. Matsuoka and P. Falcaro, Inorg. Chem. Front., 2015, 2,
434–441.
4 R. Ameloot, F. Vermoortele, J. Hoens, F. C. De Schryver,
D. E. De Vos and M. B. J. Roeffaers, Angew. Chem., Int. Ed.,
2
013, 52, 401–405.
57 Y. K. Seo, G. Hundal, I. T. Jang, Y. K. Hwang, C. H. Jun and
J. S. Chang, Microporous Mesoporous Mater., 2009, 119, 331–337.
4
5 G. Majano, O. Martin, M. Hammes, S. Smeets, C. Baerlocher
and J. P ´e rez-Ram ´ı rez, Adv. Funct. Mater., 2014, 24, 3855– 58 G. B. Deacon and R. J. Phillips, Coord. Chem. Rev., 1980, 33,
865. 227–250.
6 K. M u¨ ller, N. Vankova, L. Sch o¨ ttner, T. Heine and L. Heinke, 59 Y. Liu, P. Bailey, T. C. Q. Noakes, G. E. Thompson,
3
4
4
4
Chem. Sci., 2019, 10, 153–160.
7 T. Wang, H. Zhu, Q. Zeng and D. Liu, Adv. Mater. Interfaces,
P. Skeleton and M. R. Alexander, Surf. Interface Anal., 2004,
36, 339–346.
2
019, 6, 1–27.
60 Y. Chen, X. Mu, E. Lester and T. Wu, Prog. Nat. Sci.: Mater.
Int., 2018, 28, 584–589.
ˇ
´
ˇˇ ´
8 T. Stolar, L. Batzdorf, S. Lukin, D. Zilic, C. Motillo, T. Friscic,
F. Emmerling, I. Halasz and K. U ˇz arevi ´c , Inorg. Chem., 2017, 61 H. K. Kim, W. S. Yun, M. Kim, J. Y. Kim, Y. Bae and J. Lee, J.
6, 6599–6608. Am. Chem. Soc., 2015, 137, 10009–10015.
5
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