Cu2+-doped zeolitic imidazolate frameworks (ZIF-8): Efficient and stable catalysts for cycloadditions and condensation reactions

Article abstract of DOI:10.1039/c4cy01505c

Cu²⁺-doped zeolitic imidazolate framework (ZIF) crystals were efficiently prepared by reaction of Cu(NO₃)₂, Zn(NO₃)₂, and 2-methylimidazole in methanol at room temperature. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction showed that the Cu/ZIF-8 particles were nanosized (between ca. 120 and 170 nm) and that the body-centered cubic crystal lattice of the parent ZIF-8 framework is continuously maintained, regardless of the doping percentage. Moreover, thermogravimetric analyses and specific BET surface area measurements demonstrated that the doping does not alter the high stability of ZIF-8 crystals and that the porosity only decreases at a high doping percentage (25% in Cu²⁺). The Cu/ZIF-8 material showed excellent catalytic activity in the [3 + 2] cycloaddition of organic azides with alkynes and in Friedl?nder and Combes condensations due to the high catalyst surface area and the high dispersion of Cu/ZIF-8 particles. Notably, the Cu/ZIF-8 particles not only exhibit excellent performance but also show great stability in the reaction, allowing their reuse up to ten times in condensation reactions. Our findings explored a simple and powerful way to incorporate metal ions into the backbones of open framework materials without losing their properties.

Full text of DOI:10.1039/c4cy01505c

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DOI: 10.1039/C4CY01505C
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ARTICLE TYPE
Cu2+-doped zeolitic imidazolate frameworks (ZIF-8): Efficient and
stable catalysts for cycloadditions and condensation reactions
Aleksandra Schejn,a Abdelhay Aboulaich,b Lavinia Balan,c Véronique Falk,a Jacques Lalevée,c Ghouti
Medjahdi,d Lionel Aranda,d Kevin Mozet,a Raphaël Schneider*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Cu2+ꢀdoped zeolitic imidazolate frameworks (ZIFs) crystals were efficiently prepared by reaction of
Cu(NO3)2, Zn(NO3)2, and 2ꢀmethylimidazole in methanol at room temperature. Scanning electron
10 microscopy, transmission electron microscopy and Xꢀray diffraction showed that the Cu/ZIFꢀ8 particles
were nanosized (between ca. 120 and 170 nm) and that the bodyꢀcentered cubic crystal lattice of the
parent ZIFꢀ8 framework is continuously maintained, regardless of the doping percentage. Moreover,
thermogravimetric analyses and specific BET surface area measurements demonstrated that the doping
doesn’t alter the high stability of ZIFꢀ8 crystals and that the porosity only decreases at a high doping
15 percentage (25% in Cu2+). The Cu/ZIFꢀ8 material showed excellent catalytic activity in the [3+2]
cycloaddition of organic azides with alkynes and in Friedländer and Combes condensations due to the
high catalyst surface area and the high dispersion of Cu/ZIFꢀ8 particles. Notably, the Cu/ZIFꢀ8 particles
not only exhibit excellent performances but also had a great stability in the reaction, allowing their reuse
up to ten times in condensation reactions. Our findings explored a simple and powerful way to
20 incorporate metal ions into the backbones of open framework materials without losing their properties.
displaying sorptive, magnetic and/or catalytic properties, we
sought to investigate the consequences of Cu2+ doping on the
50 ZIFꢀ8 framework. ZIFꢀ8 is a wide bandgap material (bandgap
energy Eg = 4.9 eV) and adsorbs only in the UV region.26 Doping
copper into ZIFꢀ8 should not only allow to engineer its bandgap
and displace its absorption into the visibleꢀlight area, but also to
develop new nanomaterials combining both the catalytic and
55 magnetic properties of Cu2+ and the high thermal and chemical
stability of ZIFꢀ8 crystals. For example, the presence of Cu2+ ions
in the ZIFꢀ8 structure should enable their use as reusable catalysts
for coupling reactions and oxidationꢀreduction reactions, as
recently described for Cu(im)2,27,28 CuBTC29 (BasoliteTM C300,
Introduction
Metalꢀorganic frameworks (MOFs), which are porous crystalline
materials consisting of metal ions or metal containing clusters
coordinated to rigid organic molecules to form oneꢀ, twoꢀ, or
25 threeꢀdimensional networks, have attracted great attention due to
their exceptional high surface area, high porosity, low density,
and chemically tunable structures.1ꢀ8 Among MOFs, the soꢀcalled
zeolitic imidazolate frameworks (ZIFs) are extremely promising
materials because they combine the pore size tunability of MOFs
30 and the thermal stability of zeolites making them ideal candidates
for applications such as gas storage,9,10 gas separation,6,11
catalysis12 or drug delivery.13 ZIFs are constructed from
tetrahedral units, in which each metal atom such as Zn2+ or Co2+
connects four imidazolate (imꢀ) ligands to form neutral
35 frameworks.14ꢀ20 ZIFꢀ8 (Zn(mim)2, mimꢀ = 2ꢀmethylimidazolate),
with the sodalite (SOD) topology, crystallizes in the cubic space
60 BTC
=
1,3,5ꢀbenzene tricarboxylate) and Fe(BTC).30 The
association of Cu2+ ions with imidazole or its derivatives as
ligands/linkers to construct MOFs27,28,31ꢀ46 or coordination
polymers47,48 is wellꢀestablished. The doping in copper of ZIFꢀ67
(Co(im)2) and the photocatalytic properties of the Cu/ZIFꢀ67
65 crystals under visible light irradiation have recently been
reported.49 Very recently, Li et al. reported also the successful
incorporation of sixꢀcoordinated nickel(+2) clusters into ZIFꢀ8
crystals cavities.50,51 The materials prepared found applications in
alcohol sensing or carbon dioxide capture and separation after
70 pyrolysis. So far, Cu2+ꢀdoping into Zn(im2)2 (ZIFꢀ1) has only
been described one time,52 and Cu2+ꢀdoping into ZIFꢀ8 has never
been explored.
group I 4 3m with a lattice constant of 16.992 Å and contains
276 atoms in the unit cell (Zn12N48C96H120). The sodalite cages
possess a pore diameter of 11.6 Å and the aperture between two
40 cages is 3.4 Å. ZIFꢀ8 crystals are typical porous MOFs with
unusual high thermal and chemical stability, as well as tunable
zeotype topologies.2,14ꢀ19
While the incorporation of metal or semiconductor nanoparticles
into the ZIF framework is now wellꢀdocumented,20ꢀ25
45 substitutional introduction of metal cations in the crystalline
lattice has far less been studied. In the course of our studies
related to the construction of ZIFꢀ8 derived hybrid materials
In this work, we report the successful preparation of Cuꢀdoped
ZIFꢀ8 crystals and the application of these substitutionally doped
75 particles as efficient heterogeneous catalysts for copperꢀcatalyzed
Huisgen 1,3ꢀdipolar cycloadditions and for Friedländer and
Combes condensations. These catalytic activities were gained
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without alteration of the stability of ZIFꢀ8 crystals and with only
a slight decrease of the specific area and pore size at high dopant
percentage (25% Cu relative to Zn). The Cu/ZIFꢀ8 crystals could
be separated from the reaction mixture by simple centrifugation
and could be reused up to ten times without a significant
alteration in catalytic activity.
55 with 8 mol% of the Cu/ZIFꢀ8 catalyst at 100°C for 3 h. Then, the
catalyst was separated by centrifugation (4000 rpm, 15 min),
washed three times with toluene and dried. The reaction mixture
was centrifuged and the toluene phase concentrated. The product
5
1
was purified by flash chromatography and analyzed with H and
60 13C NMR.
Huisgen’s dipolar cycloaddition
Experimental
Click reactions were conducted according to a slightly modified
literature procedure.27 The general protocol for 1,3ꢀdipolar
65 cycloaddition of the azides to the alkynes was as follows. The
Cu/ZIFꢀ8 catalyst (5 mol% of catalyst) in ethanol (6 ml) was
placed inside a round bottom flask connected to a reflux
condenser and the temperature was set to 70 °C under argon.
Azide (1 mmol) and alkyne (1.2 mmol) were then added and the
70 mixture was stirred during 3 h. Then the catalyst was isolated by
centrifugation, washed with ethanol and dried. The product was
Materials
10 Zinc nitrate hexahydrate (98%, Aldrich), copper nitrate trihydrate
(99.5%, Merck), 2ꢀmethylimidazole Hmim (99%, Aldrich),
acetylacetone (99%, Aldrich), 2ꢀaminobenzophenone (98%,
Aldrich), aniline (99.5%, Aldrich), benzyl bromide (≥ 98%,
Fluka), sodium azide (≥ 99.5%, Aldrich), phenyl acetylene (˃
15 98%, Alfa Aesar), ethynyltrimethylsilane (98%, Aldrich), 1ꢀoctyl
bromide (99%, Aldrich), methyl 5ꢀbromovalerate (97%, Aldrich),
sodium chloride (99ꢀ100.5%, Carlo Erba), sodium sulphate (≥
99%, Carlo Erba), DMSO (≥ 99.7%, Fisher), ethanol (≥ 99.8%,
Aldrich), methanol (≥ 99,9%, Aldrich), toluene (≥ 99%, Prolabo),
20 diethyl ether (≥ 99.5%, Aldrich) were used as received without
any further purification. DI water (18.2 Mꢁ·cm) was used in all
experiments. Benzyl azide, 1ꢀoctyl azide and methyl 5ꢀ
azidovalerate were prepared according to the synthetic protocol
described in reference 53.
1
purified by flash chromatography and analyzed with H and 13C
NMR.
75 Characterization
Transmission electron microscopy (TEM) images were taken by
placing a drop of the ZIFꢀ8 particles in methanol onto a carbon
filmꢀsupported copper grid. Samples were studied using a Philips
CM20 instrument operating at 200 kV. Scanning electron
80 microscopy (SEM) pictures were prepared using JEOL Scanning
Electron Microscope JSMꢀ6490 LV. The Xꢀray powder
diffraction data were collected from an X'Pert MPD
diffractometer (Panalytical AXS) with a goniometer radius 240
mm, fixed divergence slit module (1/2° divergence slit, 0.04 rd
85 Sollers slits) and an X'Celerator as a detector. The powder
samples were placed on a silicon zeroꢀbackground sample holder
and the XRD patterns were recorded at room temperature using
Cu Kα radiation (λ = 0.15418 nm). The textural properties of the
materials were investigated with a Micromeritics ASAP 2420
90 instrument using liquid nitrogen (ꢀ196 °C). Prior to the analyses,
the samples were outꢀgassed overnight in vacuum at 40°C on the
degassing port followed by 4 h outꢀgassing on the analyse port.
The resulting isotherms were analysed using the BET (Brunauerꢀ
EmmettꢀTeller) method while the micropore volume (Vmicro) was
95 determined using the HorvathꢀKawazoe (HK) equation. The
surface areas, pore volumes and pore sizes determined (see Table
1) are the average of two independent measurements.
25
Synthesis of Cu-doped ZIF-8
A solution of the Zn(NO3)2 and Cu(NO3)2 (total 1 mmol) in 11.3
mL methanol and Hmim (660 mg, 8 mmol) in the same volume
of methanol were prepared separately. Then, in a threeꢀneckꢀ
30 flask, the two solutions were mixed by dropwise addition of the
Hmim solution to the Zn2+ and Cu2+ solution. The synthesis was
conducted under nitrogen flow at room temperature with stirring
for 1 h. The Cuꢀdoped ZIFꢀ8 crystals were separated by
centrifugation (4000 rpm, 15 min) and washed with methanol (3
35 × 30 mL). The material was dried at room conditions overnight
before analysis. To use the material in catalytic reactions, the
Cu/ZIFꢀ8 samples were activated by treatment of the powders at
200°C in a programmable oven for 6 h, and then cooled to room
temperature naturally. The vials containing the Cu/ZIFꢀ8 powders
40 were tightly capped and stored at room temperature before use.
Friedländer reaction
Thermogravimetric measurements were performed on
a
2ꢀAminobenzophenone (82.1 mg, 0.42 mmol) and ethyl
acetoacetate (81.3 mg, 0.62 mmol, 1.48 equiv.) were added to
45 Cu/ZIFꢀ8 crystals (8 mol% of catalyst) dispersed in 2 mL of
toluene. The mixture was stirred at 90°C and the progress of the
reaction was monitored by TLC. After cooling, the reaction
mixture was centrifuged and the toluene phase concentrated. The
product was purified by flash chromatography and analyzed with
50 1H and 13C NMR.
SETARAM Setsys Evolution thermoanalyzer coupled with an
100 Omnistar GSD301CꢀPfeiffer Vacuum mass spectrometer.
Samples were filled into platinum crucibles and heated in a flow
air with a ramp of 5°C.minꢀ1 from room temperature up to 800°C.
1H NMR spectra were recorded in CDCl3 using a 300 MHz
spectrometer (Avance 300, Bruker, Bremen, Germany). Diffuse
105 reflectance UVꢀvis spectra of the samples were measured using a
Shimadzu UVꢀ2101 PC spectrophotometer. BaSO4 is used as a
standard for baseline measurements and spectra are recorded in a
range of 200ꢀ1400 nm. A VARIAN 720ꢀES Inductively Coupled
PlasmaꢀOptical Emission Spectrometer (ICPꢀOES) was used for
110 multiꢀelemental analyses. ESR experiments were carried out
using a XꢀBand EMXꢀplus spectrometer (Bruker Biospin). The
Combes reaction
The catalysis was conducted in a solventꢀfree environment.
Briefly, aniline (1 mmol) and acetylacetone (5 mmol) were mixed
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samples were investigated at room temperature. The double
integration of the ESR spectrum is proportional to the
concentration in paramagnetic centers.54 A calibration was first
done using 2,2,6,6ꢀ
5
Scheme 1 Schematic representation of the synthesis of Cuꢀdoped ZIFꢀ8 crystals.
Fig. 1 Photographs showing the color change of Cu/ZIFꢀ8 crystals upon increasing the doping percentage in copper. (a) ZIFꢀ8 as reference, (b), (c), (d)
10
and (e) are ZIFꢀ8 crystals doped with 1, 5, 10 and 25% Cu, respectively.
tetramethylpiperidineꢀ1ꢀoxyl (TEMPO) to ensure that this
procedure was usable in solid state. TEMPO was also used as
standard for calibration (g = 2.0061).
the color changed from white to brown beige.
35
Cu-doped ZIF-8 nanocrystals characterization
15
When preparing pure ZIFꢀ8 crystals from Zn(NO3)2, the solution
became cloudy within 2 min after the addition of Hmim and a
dense precipitate was obtained after 1 h of reaction. With the
Results and discussion
Cu-doped ZIF-8 crystals synthesis
40 increase of the copper content in the reaction medium, the
precipitate became less dense, thus indicating that the number of
nuclei generated by the complex formation decreased. This was
confirmed by TEM and SEM experiments (Fig. 2 and 3 and Fig.
S1 for size distributions). With increasing the Cu2+ doping
45 percentage in the nanocrystals, the average diameters were found
to increase from ca. 120 to 170 nm. This confirms that less nuclei
were formed upon addition of Hmim and that Cuꢀdoped ZIFꢀ8
crystals initially formed grow through further addition of single
The synthesis of Cu/ZIFꢀ8 crystals was performed under
solvothermal conditions by mixing Zn(NO3)2, Cu(NO3)2 and
20 Hmim in ethanol at room temperature for 1 h (Scheme 1).19,55
Molar percentages of 1, 5, 10 and 25% of Cu(NO3)2 to Zn(NO3)2
were used in these experiments. The crystals prepared will be
noted Cu1%/ZIFꢀ8, Cu5%/ZIFꢀ8, Cu10%/ZIFꢀ8, and Cu25%/ZIFꢀ8
thereafter. The incorporation of Cu2+ ions in the crystal lattice
25 weakly slows down the kinetics of ZIFꢀ8 particles precipitation
(vide infra). It is also worth to mention that a complete collapse
of ZIFꢀ8 structure was observed at higher doping percentage
(50%). The crystals obtained after reaction were recovered by
centrifugation, washed and dried before characterization. A
30 significant observation is that the color intensity of the samples
and the strength of their main absorption bands (vide infra)
correlate qualitatively with their Cu2+ content. As shown in Fig.
1, upon increasing the doping percentage in Cu2+ from 1 to 25%,
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ICPꢀOES
Fig. 3 TEM images of ZIFꢀ8 crystals doped with (a) 1, (b) 5, (c) 10 and
15 (d) 25% Cu2+, respectively.
Fig. 2 SEM images of ZIFꢀ8 crystals doped with (a) 1, (b) 5, (c) 10 and
(d) 25% Cu2+, respectively.
reveals that the Cu/Zn ratios for all samples are lower than Cu/Zn
ratios used for the synthesis, irrespective for the dopant
percentage (the Cu2+ doping percentage in the crystals are 0.6,
2.1, 3.9, to 8.7% for reactions conducted with 1, 5, 10, and 25%
20 Cu2+, respectively).
Powder Xꢀray diffraction (XRD) patterns of ZIFꢀ8 and Cu/ZIFꢀ8
crystals are shown in Fig. 4. For all the samples, the Cu/ZIFꢀ8
crystals show a bodyꢀcentered cubic crystal lattice group I 4 3m
with a cell parameter a varying between 17.015 et 17.025 Å,
monomeric mimꢀ and solvated Zn2+ and Cu2+ species until the
framework is formed. Regardless of the doping percentage, TEM
and SEM images showed rather small and monodispersed
5
nanoparticles with
a
wellꢀdefined truncated rhombic
dodecahedron structure, which is the typical ZIFꢀ8 morphology.
Inductively coupled plasmaꢀoptical emission spectrometry (ICPꢀ
10 OES) was used to determine the final copper content of Cu/ZIFꢀ8
crystals for further characterization and catalytic experiments.
4
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which is in good accordance with the results reported for ZIFꢀ8 in 45 percentage. ZIFꢀ8 is a wide band gap material with a band gap
the literature.14,56 A slight decrease of the fullꢀwidth at halfꢀ
maximum
energy equal to 4.9 eV.26 The band gap energies of the Cu/ZIFꢀ8
samples have been determined from the absorbance [F(R)]
spectra using the KubelkaꢀMunk formalism and Tauc plot. For
indirect
5
Fig. 4 XRD patterns of Cu/ZIFꢀ8 crystals prepared with different doping
percentages in Cu2+.
of the (011) peak intensity was also observed when increasing the
Cu doping percentage, which is consistent with the crystals size
increase observed by TEM and SEM. Niꢀsubstituted ZIFꢀ8
10 crystals prepared via mechanochemical synthesis exhibited extra
peaks in XRD patterns that were assigned to the clusters formed
between Ni(+2) ions and Hmim that are trapped in ZIFꢀ8
cavities.50,51 These extra peaks were not observed in our solution
phase synthesis, thus indicating that Cu2+ ions are probably
15 substitutionally doped in ZIFꢀ8 crystals.
Results depicted in Fig. 4 demonstrate that the presence of Cu2+
in the reaction medium during the growth of ZIFꢀ8 crystals did
not cause any structure alteration and that ZIFꢀ8 materials still
retained crystalline integrity after being doped with Cu2+. It is
20 likely that the doping with Cu2+ does not distort the lattice
structure of ZIFꢀ8 crystals since the ionic size of Cu2+ (0.71 Å) is
only slightly smaller than that of Zn2+ (0.74 Å) in tetrahedral
coordination. Finally, it is worth to mention that Cu2+ ions have
been demonstrated to damage the crystallinity of ZIFꢀ8 once
25 added to these nanocrystals in aqueous solution due to
interactions between Cu2+ ions and the mimꢀ linkers.57 Such
alterations were not observed under our experimental conditions.
The presence of Cu2+ in the samples was further confirmed by
UVꢀvisible spectroscopy (Fig. 5). From the diffuse reflectance
30 spectra and the corresponding absorption spectra of pure ZIFꢀ8
and Cuꢀdoped ZIFꢀ8 crystals (Fig. 5a and 5b), it can be seen that
doping shifts the adsorption edge of ZIFꢀ8 crystals from the UV
to the visible region. The absorption spectra of Cuꢀdoped ZIFꢀ8
crystals exhibit a first wellꢀdefined band located at ca. 490 nm
35 and a second broad one starting at 600 nm and going to the
infrared with a shoulder at ca. 850 nm. These bands can both be
attributed to dꢀd transitions of Cu(+2) ions and confirm the
heterobimetallic nature of the ZIF materials produced. The slight
differences observed in the absorption spectra compared to
40 Cu(+2)/imidazolate complexes described in the literature may
originate from different coordination geometries around Cu(+2)
ions.58,59 Additionally, reflectance and absorption spectra of all
Cu/ZIFꢀ8 crystals exhibit the same bands, demonstrating that the
structures of these materials are the same regardless of the doping
50
Fig. 5 (a) Diffuse reflectance and (b) absorption spectra of ZIFꢀ8 and Cuꢀ
doped ZIFꢀ8 crystals, and (c) [F(R)hν]1/2 vs hν curves for ZIFꢀ8 and
Cu/ZIFꢀ8 crystals.
band gap semiconductors, the energy band gap can be estimated
55 from the tangent line in the plot of the square root of Kubelkaꢀ
Munk functions F(R) against photon energy, as shown in Fig. 5c.
The tangent lines, which are extrapolated to (F(R))1/2 = 0, indicate
that the band gaps of Cu1%/ZIFꢀ8, Cu5%/ZIFꢀ8, Cu10%/ZIFꢀ8, and
Cu25%/ZIFꢀ8 are 5.09, 5.07, 5.04, and 4.92 eV, respectively, while
60 the band gap of undoped ZIFꢀ8 is equal to 5.10 eV. These results
are in agreement with the emergence of strong absorption bands
within ZIFꢀ8 after Cu2+ doping. The results are also in line with
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the stronger absorption intensity observed with increasing Cu2+
doping as shown in Fig. 5b.
The ESR spectra of the Cu/ZIFꢀ8 crystals are shown in Fig. 6a.
The spectra exhibit one intense signal in the high field and four
1).14,19 The BET surface areas and pore volumes of nanocrystals
prepared with 25% Cu doping were found to be lower than those
measured with weaker doping percentage in Cu. Finally, pore
size distribution (calculated by the HorvathꢀKawazoe method) are
5
wellꢀresolved peaks in the low field region, which corroborate the 35 centered around 1.2 nm and perfectly agree with that from the
presence of Cu(+2) in the materials. Interestingly, the double
ZIFꢀ8 structural model.
Table 1 Comparison of BET, pore volume, and pore size for ZIFꢀ8 and
Cuꢀdoped ZIFꢀ8 crystals
Material
BETa,b
(m2gꢀ1)
Pore volumeb
(cm3gꢀ1)
Pore sizeb
(nm)
ZIFꢀ8
1700 ± 30
1541 ± 40
1736 ± 59
1639 ± 56
1205 ± 47
0.662
0.518
0.601
0.571
0.440
1.2922
1.2214
1.2179
1.2169
1.2190
Cu1%/ZIFꢀ8
Cu5%/ZIFꢀ8
Cu10%/ZIFꢀ8
Cu25%/ZIFꢀ8
40 a BET surface area. b Values are the average of two experiments.
Fig. 6 (a) ESR spectra for the different Cu/ZIFꢀ8 samples and (b) double
integration of the ESR spectrum vs. Cu doping.
10 integration of the ESR spectra increases linearly with the Cu
doping (Fig. 6b). The Cu/Zn ratios estimated by ESR (1.0, 1.87,
3.81 and 8.54% Cu2+ for Cu1%/ZIFꢀ8, Cu5%/ZIFꢀ8, Cu10%/ZIFꢀ8,
and Cu25%/ZIFꢀ8 crystals, respectively) are in good agreement
with values determined by ICPꢀOES (vide supra).
15 Porosity and stability of Cu/ZIF-8 crystals
After outꢀgassing the samples overnight in vacuum at 40°C,
permanent porosities of Cu/ZIFꢀ8 materials were demonstrated
by N2 adsorption at 77K (Fig. 7). Type I isotherms were obtained
for the four Cu/ZIF crystals with BET surface areas of 1541,
20 1736, 1639, and 1205 m2.gꢀ1 for 1, 5, 10 and 25% Cu doping,
respectively, indicating microporous structure (Table 1). This
structure was further confirmed by the increase in the volume
adsorbed at low relative pressures (P/P0 < 0.08). The adsorptionꢀ
desorption hysteresis loop of N2 near P/P0 = 1, which originates
25 from interparticle mesopores, is consistent with the interparticle
voids and further confirms the dual microꢀ and mesoporosity of
Cu/ZIF crystals.60 Until 10% of doping in Cu2+, the crystals
properties were only slightly affected by the Cu2+ substitution.
Surface areas and micropore volumes correspond well with those
30 of pure ZIFꢀ8 (SBET = 1700 m2gꢀ1 and Vmicro = 0.63 cm3gꢀ1) (Table
Fig. 7 N2 adsorption/desorption curves at 77K for Cu/ZIF materials doped
with (a) 25% Cu, (b) 10% Cu, (c) 5% Cu, and (d) 1% Cu, giving surface
45 areas of 1205, 1639, 1736, and 1541 m2 gꢀ1, respectively. Black and red
data correspond to the adsorption and desorption branches, respectively.
The thermogravimetric analyses coupled to mass spectrometry
(TGA/MS) of ZIFꢀ8 and Cu/ZIFꢀ8 samples only differ slightly
(Fig. S2). From 20 to 200°C, the TGA curves exhibited only a
50 very weak weight loss of less than 0.5%, corresponding to the
removal of guest molecules (methanol or Hmim and/or CO2 from
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the cavities as indicated by MS analyses). A long plateau was
then observed after the formation of the guestꢀfree Cu/Zn(mim)2 55 to reactants. With the increase of Cu2+ doping, more of these ions
and that Cu2+ ions located in the pores are probably not accessible
crystals until 350°C, indicating good thermal stability of the
threeꢀdimensional network for all samples, which is comparable
to the literature.14,19 All curves exhibit one step which can be
assigned to the decomposition of the mimꢀ linker above 350°C
should be present at the surface of the Cu/ZIFꢀ8 catalyst and its
activity increased. To confirm the key role of Cu2+ species in the
Friedländer reaction, we carried out the condensation of 2ꢀ
aminobenzophenone with ethyl acetoacetate in the presence of
5
(onset of the exothermic decomposition). The sharp weight loss 60 100 equivalents of pyridine relative to the Cu/ZIFꢀ8 catalyst. In
of ca. 63ꢀ64% observed upon increasing the temperature from
400 to 500°C is in good agreement with the theoretical weight
the presence of the pyridine catalyst poison, the quinoline 2 was
not detected, thus indicating that pyridine strongly bounds and
completely blocks catalytically active Cu2+ sites. Finally, to
demonstrate that the Cu/ZIFꢀ8 is a real heterogeneous catalyst
10 loss of 64%. The residue is wurtzite Cuꢀdoped ZnO as proven by
XRD measurements. It is also worth to mention that Cu/ZIFꢀ8
crystals from this work exhibit higher thermal stability than Cuꢀ 65 and that catalytically active Cu2+ ions are not leaked in the
based MOFs like Cu(BDC) used for catalytic applications, which
already decompose at 200°C.27
15 Finally, the chemical stability of Cu/ZIFꢀ8 crystals was evaluated
by dispersing the samples in boiling water, 1M NaOH, toluene or
medium during the course of the reaction, a centrifugation was
carried out after 15 min of reaction and the catalyst separated. No
further conversion of starting materials was observed upon
further heating the supernatant at 100°C, thus confirming that the
methanol, conditions that reflect extreme operational parameters 70 condensation is mediated by a heterogeneous catalyst.
of typical industrial chemical process. Powder XRD patterns
recorded for each sample after one week heating indicate that the
20 topology of the ZIFꢀ8 framework is preserved in 1M NaOH,
toluene or methanol. Only the sample heated in water at neutral
pH gradually decomposed into ZnO (see Fig. S3).
We also investigated different catalyst loadings in the course of
the reaction leading to quinoline 2 and found that 8 mol%
Cu5%/ZIFꢀ8 afforded the best results. Decreasing the amount of
Cu5%/ZIFꢀ8 led to lower yields of 1 and 2, therefore the optimum
75 catalyst amount was kept at 8 mol%.
We next studied the Combes condensation using Cu/ZIFꢀ8 as
catalyst of aniline with acetylacetone leading to 2,4ꢀ
dimethylquinoline 3 (Scheme 3). Since both starting materials are
liquid, the reaction was carried out under solventꢀfree conditions
80 using an excess of acetylacetone (5 equiv.). Quinoline 3 was
isolated in 96% yield after column chromatography using the
Cu5%/ZIFꢀ8 catalyst for 5 h at 100°C. As previously observed
during the Friedländer condensations, product 3 was not detected
in the absence of catalyst or with ZIFꢀ8.
Cu/ZIF-8 particles as catalysts for Friedländer and Combes
25 condensations
The catalytic activity of Cu/ZIFꢀ8 crystals was first evaluated in
the copperꢀcatalyzed synthesis of quinoline derivatives,
heterocycles of high interest due to their broad range of
pharmaceutical and biological activities.61ꢀ64 We carried out the
30 condensation of 2ꢀaminobenzophenone with acetylacetone or
ethyl acetoacetate in the presence of Cu/ZIFꢀ8 crystals (Scheme
2).
85
Scheme 3 Cu/ZIFꢀ8ꢀcatalyzed Combes reaction between aniline and
acetylacetone.
Scheme 2 Cu/ZIFꢀ8 catalyzed Friedländer reactions between 2ꢀ
35 aminobenzophenone and active methylene compounds.
Finally, we checked the reusability of Cu/ZIFꢀ8 as heterogeneous
catalyst in Friedländer and Combes reactions yielding products 2
90 and 3, respectively. After the reaction, the catalyst was recovered
by centrifugation, washed with ethanol, dried at 70°C and used
for another consecutive run without further treatment. In both
cases, the catalyst exhibited no decrease in activity after five runs.
As shown in Fig. 8, the shape and the crystallinity of the particles
95 after the five cycles remained unchanged, thus showing its
stability and recyclability. A strong decrease in catalytic activity
was only observed after the 10th run (44%). After five recyclings
in the Combes reaction, the textural properties of the Cu5%/ZIFꢀ8
material were evaluated by N2 adsorptionꢀdesorption isotherm at
100 77K. The curve obtained for the catalyst remained a typical typeꢀI
isotherm (see Fig. S4), indicating that the particles maintained
their micropores. The value of SBET decreased from 1639 to 1030
m2.gꢀ1 after the 5th run, probably due to the irreversible adsorption
of reagents and/or byꢀproducts at the surface of the catalyst.
105
The
reactions
were
conducted
using
a
2ꢀ
aminobenzophenone/active methylene compound molar ratio of
1/1.5 and in toluene at 100°C. Poor conversion of 2ꢀ
40 aminobenzophenone (less than 10%) was observed with the
Cu1%/ZIFꢀ8 material but the reaction proceeded efficiently in 8 h
with Cu5%/ZIFꢀ8 crystals yielding compounds 1 and 2 in
quantitative yields. In addition, although the final yields of 1 and
2
45 when operating with catalysts containing a higher copper
percentage (10 or 25%) were almost the same, the reaction times
were reduced to ca. 5 h. It is also worth to mention that products
1 and 2 of the Friedländer condensation were not obtained in
control experiments carried out either without catalyst or with
50 ZIFꢀ8 crystals. Cu2+ ions, acting as Lewis acids in the Friedländer
reaction, are randomly located in the Cu/ZIFꢀ8 crystals. Poor
results obtained with the Cu1%/ZIFꢀ8 material seem to indicate
that the reaction proceeds on the external surface of the catalyst
Cu/ZIF-8 crystals in 1,3-dipolar cycloadditions
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35 Cu25%/ZIFꢀ8 in toluene at 100°C. As can be seen in Table 3,
various combinations of organic azides and alkynes were
efficiently converted into the corresponding disubstituted
triazoles without any additive. Not only benzylic but also
aliphatic azides gave excellent yields. Aliphatic alkynes worked
Huisgen’s dipolar cycloaddition (click reaction) between organic
azides and alkynes is the most important synthetic route to 1,2,3ꢀ
triazoles.65 The discovery that copper(+1) efficiently catalyses
this cycloaddition providing triazoles under mild conditions and
5
the 1,4ꢀisomer with high regioselectivity was
a welcome 40 also well as reaction partners of organic azides. The
advance.66,67 In recent years, click reactions have found many
applications in chemistry, biology and materials science.68ꢀ70
Cu(+2) in the absence of reducing agent like ascorbate can also
regioselectivity was good, varying from 78/22 to 95/5. After
reaction, the Cu25%/ZIFꢀ8 particles were separated through
centrifugation and used in click reactions between
phenylacetylene and octylazide without loss of catalytic activity
45 during five cycles.
10 catalyse the
Table 2 Influence of the Cuꢀdoping on the regioselectivity of
cycloadditions catalysed by Cu/ZIFꢀ8 crystals.
Catalyst
1,4ꢀregioisomera
1,5ꢀregioisomera
Yieldb (%)
Cu1%/ZIFꢀ8
Cu5%/ZIFꢀ8
Cu10%/ZIFꢀ8
Cu25%/ZIFꢀ8
45
50
62
92
55
50
38
8
79
98
99
99
50 a Percentages determined by 1H NMR. b Isolated yields after column
chromatography.
Table 3 Synthesis of 1,2,3ꢀtriazoles using Cu/ZIFꢀ8 particles.
Fig. 8. (a) SEM image of ZIFꢀ8 crystals after five reuses in the Combes
condensation and (b) XRD patterns of asꢀsynthesized ZIFꢀ8 (red) and
ZIFꢀ8 crystals after five reuses (black) in the Combes condensation.
15
55
R1
R2
1,4/1,5a
Yieldb (%)
cycloaddition of azides and alkynes but this reaction generally
suffers from drawbacks like high catalyst loading and low yield
compared to Cu(+1)ꢀcatalyzed reactions.71ꢀ75 In these screens, a
mixture of benzylazide and phenylacetylene (1:1.2 equiv,
20 respectively) was heated in toluene at 100°C for 3 h in the
presence of 5 mol% of the Cu/ZIF catalyst. The resulting mixture
Bn
Octyl
Octyl
Ph
Ph
SiMe3
92/8
79/21
95/5
99
90
84
O
Ph
78/22
97
H3CO
1
was next analyzed by H NMR. As shown in Table 2, we were
a Percentages determined by 1H NMR. b Isolated yields after column
chromatography.
pleased to find out that Cu/ZIFꢀ8 crystals catalyse the formation
of 1,2,3ꢀtriazoles with catalytic activity and regioselectivity being
25 dependent on the Cu dopant percentage. With the Cu25%/ZIFꢀ8
catalyst, the consumption of benzylazide was complete and the
1,4ꢀisomer was obtained with a high regioselectivity (92/8).
Under similar experimental conditions, undoped ZIFꢀ8 crystals
were completely ineffective in catalysing the cycloaddition and
30 no trace
Conclusions
60 In summary, we have demonstrated that ZIFꢀ8 crystals could be
easily doped with Cu2+ ions (between 1 and 25%) without
alteration of ZIFꢀ8 thermostability (up to 350°C in air) and
crystallinity. An increase of ZIFꢀ8 particles from ca. 120 to 170
nm was observed when increasing the dopant percentage from 1
65 to 25%. Until 10% doping in copper, the permanent porosities of
Cu/ZIFꢀ8 particles were not altered but decreased slightly from
of triazole was detected even after 24 h heating in toluene at
100°C.
We next evaluated the scope of this new Cu/ZIFꢀ8 catalyzed
process. All reactions were performed with
5 mol% of
8
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DOI: 10.1039/C4CY01505C
ca. 1600 to 1200 m2.gꢀ1 when 25% Cu2+ relative to Zn2+ was used.
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Importantly, Cu/ZIFꢀ8 particles were proven to be efficient and
reusable catalysts for the [3+2] cycloaddition of organic azides
with alkynes and for Friedländer and Combes condensations. The
Cu5%/ZIFꢀ8 crystals showed high catalytic activity in the
synthesis of quinolines using 2ꢀaminobenzophenone as starting
material. 1,4ꢀDisubstituted triazoles were obtained with excellent
yields and good regioselectivity using the Cu25%/ZIFꢀ8 material.
These results combined with the high stability and the ease of
65
70
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This work was supported by ICCEL and MICA Carnot Institutes.
We thank Hervé Marmier (LIEC, UMR CNRS 7360, Université
de Lorraine) for ICPꢀOES measurements.
85
Notes and references
20 a Laboratoire Réaction et Génie des Procédés (LRGP), UMR CNRS 7274,
Université de Lorraine, 1 rue Grandville 54001 Nancy, France. Tel: +33
3 83 17 50 53; E-mail: raphael.schneider@univ-lorraine.fr
b Clermont Université, Université Blaise Pascal, Institut de Chimie de
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† Electronic Supplementary Information (ESI) available: Size
distributions of Cu/ZIFꢀ8 crystals and XRD patterns of crystals after
chemical treatment. See DOI: 10.1039/b000000x/
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