Metal ion exchange in Prussian blue analogues: Cu(ii)-exchanged Zn-Co PBAs as highly selective catalysts for A3 coupling

Article abstract of DOI:10.1039/c9dt00388f

The occurrence of metal ion exchange in Zn₃[Co(CN)₆]₂ and Cu₃[Co(CN)₆]₂ Prussian blue analogues (Zn-Co and Cu-Co PBAs) was demonstrated for the first time. While Cu(ii) ion exchange easily occurs in Zn-Co PBA, the exchange of Cu(ii) atoms in Cu-Co PBA by Zn(ii) proved to be more difficult. At low to medium Cu(ii) loadings, the catalytic activity of the exchanged PBAs for the A³ coupling reaction of benzaldehyde, piperidine and phenylacetylene was higher than that of the bimetallic PBAs and that of multi metal PBAs of similar composition prepared by co-precipitation. This result showcases the benefits of the ion exchange process as a preparation method of PBA catalysts, since it is believed to lead to the incorporation of the desired metal in a more accessible position for reactant molecules. At higher Cu(ii) loadings, ion exchange with Cu(CH₃COO)₂·H₂O also resulted in co-incorporation of CH₃COO^-. This incorporation considerably boosted the catalytic activity of the PBAs by providing a basic function that facilitates the C-H activation of phenylacetylene. The most active of the studied PBAs, catallytically outperforms other Cu(ii) based A³ coupling catalysts and completely suppresses the activity for the homocoupling of phenylacetylene, even under oxidative conditions. Furthermore, the basicity of the PBAs was investigated in the nitroaldol (Henry) reaction, where a clear effect of the presence of CH₃COO^- was observed. The CH₃COO^- containing PBAs exhibited an activity three times higher than the rest of the PBAs. The presence of the basic CH₃COO^- groups represents the first case of basic functionalization of PBAs.

Full text of DOI:10.1039/c9dt00388f

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Metal ion exchange in Prussian blue analogues:
Cu(II)-exchanged Zn–Co PBAs as highly selective
catalysts for A3 coupling†
Cite this: DOI: 10.1039/c9dt00388f
Carlos Marquez,
Cédric Van Goethem,
Trees De Baerdemaeker
Francisco G. Cirujano,
Simon Smolders,
Dirk De Vos* and
Ivo Vankelecom,
*
The occurrence of metal ion exchange in Zn3[Co(CN)6]2 and Cu3[Co(CN)6]2 Prussian blue analogues
(Zn–Co and Cu–Co PBAs) was demonstrated for the first time. While Cu(II) ion exchange easily occurs in
Zn–Co PBA, the exchange of Cu(II) atoms in Cu–Co PBA by Zn(II) proved to be more difficult. At low to
medium Cu(II) loadings, the catalytic activity of the exchanged PBAs for the A3 coupling reaction of benz-
aldehyde, piperidine and phenylacetylene was higher than that of the bimetallic PBAs and that of multi
metal PBAs of similar composition prepared by co–precipitation. This result showcases the benefits of the
ion exchange process as a preparation method of PBA catalysts, since it is believed to lead to the in-
corporation of the desired metal in a more accessible position for reactant molecules. At higher Cu(II)
loadings, ion exchange with Cu(CH3COO)2·H2O also resulted in co-incorporation of CH3COO−. This
incorporation considerably boosted the catalytic activity of the PBAs by providing a basic function that
facilitates the C–H activation of phenylacetylene. The most active of the studied PBAs, catallytically out-
performs other Cu(II) based A3 coupling catalysts and completely suppresses the activity for the homo-
coupling of phenylacetylene, even under oxidative conditions. Furthermore, the basicity of the PBAs was
investigated in the nitroaldol (Henry) reaction, where a clear effect of the presence of CH3COO− was
observed. The CH3COO− containing PBAs exhibited an activity three times higher than the rest of the
PBAs. The presence of the basic CH3COO− groups represents the first case of basic functionalization
of PBAs.
Received 28th January 2019,
Accepted 8th February 2019
DOI: 10.1039/c9dt00388f
rsc.li/dalton
use as dyes and pigments dates back to the 18th century.1–3
Since then, they have received substantial attention due to
Introduction
Prussian blue analogues (PBAs) are porous coordination poly- their interesting magnetic,4–6 textural,7–9 electrochemical,10–12
mers with a structural formula of Mu1[M2(CN)6]v (M1-M2 PBAs). ion exchange13–15 and catalytic properties.1,16–19 PBAs can be
M1 can be, for example, a divalent metal, such as Zn(II), Fe(II), easily synthesized by a fast precipitation reaction using
Cu(II), Co(II), and M2 a trivalent metal such as Co(III), Fe(III) or aqueous solutions of a M1 salt (M1Zv, where Z is usually Cl−,
Cr(III). The structure of PBAs typically consists of a rock salt NO3 or CH3COO−) and a M2 cyanide salt (Yu[M2(CN)n], where
−
type arrangement of [M1]v+ and [M2(CN)6]u−. For v = 2 and u = Y is usually K+ or Na+). The stoichiometry of the final product
3, one out of three [M2(CN)6]3− sites is vacant to obtain charge depends on the oxidation states of the metals and the possible
neutrality. PBAs – and especially Prussian blue – are among incorporation of other cations.20 One of the advantages of
the oldest known synthetic coordination compounds, as their PBAs is their high tunability due to the numerous potential
variations in M1. This leads to the preparation of complexes
with different compositions and properties, even allowing the
incorporation of two different divalent metals on the M1 posi-
Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F,
3001 Leuven, Belgium. E-mail: dirk.devos@kuleuven.be,
tion in the PBA.21 For example, Liu et al.14 prepared a series of
FexZn1−x–Co PBAs for the capture of Cs+ that showed an
improved performance as the Zn/Fe ratio increased. Likewise,
the group of Reguera studied mixed composition PBAs and
metal nitroprussides for H2 storage and found that the adsorp-
tion potential can be modulated by combining different
trees.debaerdemaeker@kuleuven.be
†Electronic supplementary information (ESI) available: Elemental analysis of the
Zn(II) exchanged samples, synthesis procedure of multi metal PBAs (co–precipi-
tation) and Cu(II)-supported materials, experimental details of nitroaldol reac-
tion, additional characterization (PXRD, TGA-MS, N2 sorption, FTIR spectra) and
additional catalytic data. See DOI: 10.1039/c9dt00388f
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metals.21,22 For electrochemical applications, varying the com-
position of PBA electrode materials by element doping can
enhance their electrochemical performance, as shown by
studies of NixFe1−x–Fe, NixMn1−x–Fe, NixCo1−x–Fe, NixCu1−x–Fe
Experimental
Catalysts preparation
Synthesis of parent and reference PBAs. The parent (Zn–Co
and CoxMn1−x–Fe PBAs as cathode materials.11,23–26 In catalysis, PBA) and the reference PBAs (CuCl2–Co and Cu(OAc)2–Co PBA)
García-Ortiz et al.27 employed FexCu1−x–Co PBAs as solid cata- were synthesized by modifying previously reported pro-
lysts for the aerobic oxidation of oximes to the corresponding cedures.34 Solution A was prepared by dissolving 29.35 mmol
ketone and recently, our group reported the use of a series of a M1 salt (ZnCl2, CuCl2·2H2O or Cu(CH3COO)2·H2O) in
of CuxZn1−x–Co PBAs as heterogeneous catalyst for the A3 coup- 35 mL of water. Solution
ling of phenylacetylene, benzaldehyde and piperidine.28
13.43 mmol of K3[Co(CN)6] to 36 mL of water. Solution B was
B was prepared by adding
However, in all these examples, the respective multi metal then added dropwise to solution A under vigorous stirring.
PBA was synthesized by the standard co-precipitation pro- The mixture was subsequently stirred for 2 h at room tempera-
cedure, and while this is an easy approach, the final solid ture. The obtained solids were recovered by centrifugation and
most likely consists of a random arrangement of the different washed three times with deionized water, followed by over-
divalent metals on the M1 position all throughout the crys- night drying at 60 °C.
tals.21 For some applications, especially in catalysis on the
Post-synthetic metal ion exchange procedure. A schematic
outer surface and where two different M1 cooperate in the cata- representation of the ion exchange procedure is presented in
lytic cycle, there is no need to introduce a second M1 in the Scheme 1. The parent material (Zn–Co PBA) was dispersed
bulk of the crystals. Therefore, the selective incorporation (5 g L−1) in aqueous solutions containing different concen-
of the second M1 closer to the external surface would be trations of Cu(II) salts, such as CuCl2·2H2O, Cu(ClO4)2·6H2O or
beneficial for the catalytic activity of PBAs. Since M1 atoms at Cu(CH3COO)2·H2O for 4, 8 or 16 h at room temperature. The
the outer surface are, on average, coordinated to fewer N solids were then centrifuged, washed 3 times with distilled water
atoms, they are more likely to be released from the structure and dried at 60 °C overnight to obtain
a series of
first,14 and post-synthetic metal ion exchange (PSE) of these [Cu(Z)v]x@Zn1−x–Co PBAs, where Z corresponds to the copper
atoms represents an interesting preparation method of cata- salt anion and x to Cu/(Cu + Zn). To study if Cu(II) atoms in
lytically active multi metal PBAs. PSE of metal ions has already Cu–Co PBAs could also be exchanged with Zn(II), the procedure
been observed for several complexes, like CdSe ionic nanocrys- was repeated by dispersing the reference CuCl2–Co PBA in
tals,29 octanuclear Cu(II) wheels30 and MOFs31–33 but to the aqueous solutions (5 g L−1) containing different concentrations of
best of our knowledge, observation of such phenomena in Zn(CH3COO)2·2H2O or ZnCl2 for 16 h at room temperature unless
PBAs has not been reported.
otherwise specified, to obtain the solids [Zn(Z)v]x@Cu1−x–Co
Herein, PSE of metal ions in Zn–Co and Cu–Co PBAs is PBAs. The detailed ion exchange conditions are presented in
studied for the first time based on the quantitative correlation Tables 1 and S1.† Ion exchange degree for the Cu(II) exchange
between the concentration of metal ions released from the (IEDCu) and Zn(II) exchange (IEDZn) were calculated as follows:
ðatoms of Zn in ZnꢀCo PBAÞ ꢀ ðatoms of Zn in exchanged sampleÞ
IEDCu
¼
ꢁ 100
atoms of Zn in ZnꢀCo PBA
ðatoms of Cu in CuCl2ꢀCo PBAÞ ꢀ ðatoms of Cu in exchanged sampleÞ
atoms of Cu in CuCl2ꢀCo PBA
IEDZn
¼
ꢁ 100:
PBA framework and the metal loading in the solid. We chose Characterization
to work with these two PBAs because of the reported high
activity of CuxZn1−x–Co PBA catalysts for A3 coupling reactions The metal content of the PBAs was determined by ICP-OES ana-
by combining the high conversion obtained with Cu–Co PBA lysis using a Varian 720-ES equipped with a double-pass glass
with the excellent selectivity obtained with Zn–Co PBA.28 The cyclonic spray chamber, a Sea Spray concentric glass nebulizer
ion exchanged PBAs were characterized by powder X–ray diffr- and a high solids torch. The digestion of the samples was done
action (PXRD), thermogravimetric analysis coupled with mass following a previously reported procedure.35 PXRD patterns
spectroscopy (TGA–MS), N2 physisorption, transmission elec- were collected on a Malvern PANalytical Empyrean diffrac-
tron microscopy (TEM), Fourier transform infrared spec- tometer (in transmission mode) over a 1.3–50° 2θ range, using a
troscopy (FTIR) and FTIR with pyridine as probe molecule. PIXcel3D solid state detector and Cu anode (Cu Kα1: 1.5406 Å;
Their catalytic performance for the A3 coupling of phenyl- Cu Kα2: 1.5444 Å). Lattice parameters were refined with
acetylene, benzaldehyde and piperidine was evaluated. TOPAS-Academic V5 using the Le Bail method in space group
36,37
ˉ
ˉ
Furthermore, the basic character of selected samples was Fm3m (or Pm3m for CuCl2–Co PBA).
The textural properties
studied for the nitroaldol (Henry) reaction of nitromethane of selected samples were studied by N2 physisorption. The N2
and benzaldehyde.
isotherms were collected at −196.15 °C on a Micromeritics 3Flex
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Scheme 1 Post-synthetic metal ion exchange procedure.
Table 1 Elemental analysis of Cu exchanged samples and the remaining supernatant after the exchange process for selected samples
b
c
Concentration
(mM)
Time
(h)
Cu
(wt%)
Zn
(wt%)
Co
(wt%)
Znloss
(wt%)
IEDCu
(%)
Entry
PBA
Cu(II) salt
M1/Coa
1
2
3
4
5
6
7
8
Zn–Co
—
—
5
25
2.5
2.5
5
5
5
7
12.5
12.5
25
25
50
50
5
50
200
300
—
—
—
16
16
8
16
4
16
8
16
8
16
8
16
4
16
16
8
16
16
—
—
—
32.0
28.2
20.4
28.1
28.3
27.0
26.1
25.4
25.6
18.5
16.1
11.8
10.7
7.56
3.23
26.1
20.5
14.6
5.70
—
18.7
18.7
18.9
18.5
18.5
18.5
18.4
18.5
18.3
18.0
18.0
17.5
17.6
18.4
18.1
18.4
17.7
18.3
18.2
17.6
17.0
1.54
1.55
1.50
1.58
1.60
1.59
1.62
1.60
1.65
1.74
1.75
1.89
1.87
1.65
1.72
1.62
1.80
1.66
1.71
1.83
2.01
—
3.03
10.2
—
[Cu(ClO4)2]0.10@Zn0.90–Co
[Cu(ClO4)2]0.33@Zn0.67–Co
[Cu(OAc)2]0.11@Zn0.89–Co(a)
[Cu(OAc)2]0.11@Zn0.89–Co(b)
[Cu(OAc)2]0.15@Zn0.85–Co
[Cu(OAc)2]0.19@Zn0.81–Co
[Cu(OAc)2]0.20@Zn0.80–Co
[Cu(OAc)2]0.21@Zn0.79–Co
[Cu(OAc)2]0.45@Zn0.55–Co
[Cu(OAc)2]0.53@Zn0.47–Co
[Cu(OAc)2]0.67@Zn0.33–Co
[Cu(OAc)2]0.70@Zn0.30–Co
[Cu(OAc)2]0.77@Zn0.23–Co
[Cu(OAc)2]0.90@Zn0.10–Co
[CuCl2]0.20@Zn0.80–Co
[CuCl2]0.42@Zn0.58–Co
[CuCl2]0.55@Zn0.45–Co
[CuCl2]0.83@Zn0.17–Co
CuCl2–Cod
Cu(ClO4)2
Cu(ClO4)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CuCl2
3.02
10.2
3.45
3.51
4.69
6.05
6.46
6.90
15.3
17.8
24.0
24.8
25.2
30.5
6.67
14.9
18.1
18.2
35.8
38.0
11.9
36.3
9.69
9.06
13.4
14.4
18.4
20.0
40.6
48.4
62.2
65.9
75.8
89.9
18.4
35.9
54.4
82.2
—
4.18
6.63
9
10
11
12
13
14
15
16
17
18
19
20
21
24.1
29.5
5.79
11.2
CuCl2
CuCl2
CuCl2
CuCl2
—
—
Cu(OAc)2–Coe
Cu(OAc)2
—
—
a (Cu + Zn)/Co molar ratio in the final PBA multi metal complex. b Zn wt% loss of the sample, based on the concentration of Zn released into solu-
tion during the exchange process. c Atomic ion exchange degree defined as atoms of Zn exchanged per atoms of Zn present in Zn–Co PBA. d This
sample was synthesized by mixing of an aqueous solution of K3[Co(CN)6] and an aqueous solution of CuCl2·2H2O. e This sample was synthesized
by mixing of an aqueous solution of K3[Co(CN)6] and an aqueous solution of Cu(CH3COO)2·H2O.
Surface Analyzer. The specific surface area (SBET) was obtained before reheating to 150 °C to record the IR spectrum. The Lewis
using the BET method (0.05–0.3 p/p0 range), and the micropore acid site density was calculated from the area of the absorption
volume (Vmicro) and external surface area (Sext) were determined band at 1450 cm−1 in the difference spectrum using the inte-
using t-plot analysis. Before analyses, the samples were evacu- grated molar extinction coefficient from Emeis.38 High angle
ated at 110 °C for 16 h. FTIR spectra of KBr wafers (containing annular dark field (HAADF) images and EDX maps were
∼1 wt% of sample) were collected on a Bruker IFS 66 v/S obtained in a JEOL ARM-200F TEM with a probe Cs corrector
Vacuum FTIR spectrometer. The acid nature and acid site operated at 200 kV. Prior to imaging, the samples were sus-
density were determined by pyridine adsorption followed by pended in ethanol and dropped onto a Cu grid (300 Mesh,
FTIR spectroscopy using a Nicolet 6700 FTIR spectrometer. For Pacific Grid Tech, USA) coated with a Lacey carbon layer.
this, a self-supported wafer (∼10 mg cm−2) was placed in a cell Thermogravimetric analysis (TGA) measurements were carried
under vacuum and activated at 250 °C for 1 h. After this, a refer- out on a TGA Q500 of TA Instruments with a heating rate of
ence spectrum was recorded at 150 °C. The cell was then cooled 10 °C min−1 under compressed air atmosphere. Additional
down further and pyridine (25 mbar) was adsorbed onto the TGA–MS analyses were performed under similar conditions on
wafer at 50 °C until the sample was saturated. The weakly co- a NETZSCH STA 449 F3 Jupiter® thermal analyser coupled with
ordinated pyridine was removed by evacuation for 30 min a Hiden HPR-20 EGA gas analysis system.
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A3 coupling
The catalysts (10 mg) were activated at 80 °C under vacuum for
16 h and then loaded into glass reaction vials with phenyl-
acetylene (0.05 mmol), piperidine (0.1 mmol), benzaldehyde
(0.1 mmol), 2-butanol (0.5 mL, solvent) and dodecane
(0.1 mmol, internal standard). The vials were then placed in a
preheated aluminum block at 110 °C and stirred at 300 rpm
using a magnetic stirring bar. After reaction, the catalyst was
removed by centrifugation and the liquid supernatant was ana-
lyzed by GC (Shimadzu 2014 GC equipped with a FID detector
and a CP-Sil 5 CB column) and GC-MS (Agilent 6890 gas
chromatograph, equipped with a HP-5MS column, coupled to
a 5973 MSD mass spectrometer). Recycling test reactions were
carried out after recovery and re-activation of the sample
before each run.
Fig. 1 Effect of time of the exchange in the final Cu(II) loading in
[Cu(OAc)2]x@Zn1−x–Co PBAs as a function of the solution concentration.
Results and discussion
Ion exchange
The amount of Cu(II) incorporated in the Zn–Co PBA via ion the prime focus of several studies in diverse fields, like energy
exchange was quantified by ICP-OES (Table 1). The Cu(II) storage10–12,23–26,42–45 and radionuclide sorption.13–15,46–48 In
loading was found to be dependent on the metal salt used in electrochemical insertions – charging and discharging of
the ion exchange process. The lowest level of exchange was cations – it is believed that the intercalation of these cations in
achieved with Cu(ClO4)2·6H2O (ion exchange degree, IEDCu
12%; entry 2) in comparison to the other salts (IEDCu = 14% reaction of the central anionic group.43,44,49 However, as no
for Cu(CH3COO)2·H2O, entry 7; and IEDCu 18% for reduction of Co(III) takes place during the ion exchange
,
the so-called A sites occurs concomitantly with the redox
=
CuCl2·2H2O, entry 16). At higher Cu concentrations an IEDCu process (Fig. S2†), a mechanism similar to that observed in
of 90% could be achieved when Cu(CH3COO)2·H2O was used electrochemical insertions is, in all likelihood, not the main
as copper source, with Cu constituting 30.5 wt% of the PBA pathway of the Cu(II) exchange. On the other hand, the mecha-
structure (entry 15). The Cu(II) salt anion may influence the nism of radionuclide sorption in PB and PBAs is still under
exchange process in several ways: changes in the pH of the ion debate. Cs+ is widely believed to adsorb on PBAs by K+ or H+
exchange solution (Fig. S1†) and the stability constants of the exchange.15,48 Of these two possible processes, only proton
Zn salts resulting from the exchange process,39,40 may account exchange is relevant in this research, as elemental analysis
for the notable differences in IEDCu. Furthermore, there are (ICP) of the parent Zn–Co PBA material showed no presence of
clear differences between these salts regarding dissociation: K in its structure. In order to study the possibility of a H+
while dissociation is complete for Cu(ClO4)2·6H2O, exchange mechanism, we investigated the pH of the super-
Cu(CH3COO)2·H2O is known to produce [Cu(CH3COO)]+ in natant solutions during the ion exchange process. The negli-
solution.41 This suggests that the lattice-terminating Zn(II) gible changes in pH observed during the ion exchange process
cation is more readily replaced by a soft [Cu(CH3COO)]+ cation (Fig. S1†) prove that the Cu(II) loading does not induce dis-
with charge +1, than by a fully hydrated cation with charge +2.
sociation of adsorbed water molecules and subsequent release
The effect of time on the ion exchange process is presented of protons into solution. Additionally, the amount of Zn
in Fig. 1. The Cu(II) content increases with the Cu(II) salt released into the solution (Znloss) corresponds with the
concentration in the solution. Nevertheless, for the amount of copper introduced in the sample after ion exchange
[Cu(OAc)2]x@Zn1−x–Co series, increasing ion exchange time (Cu wt%) and with the decrease in Zn content compared to the
from 8 to 16 h hardly affected the final Cu loading, especially parent Zn–Co PBA (Table 1). This implies that the Zn(II) atoms
at low Cu(CH3COO)2·H2O concentrations (Fig. 1 and Table 1, in the PBA framework have solely been replaced by Cu(II)
entries 4 and 5, and entries 7 and 8). High solid yields (>90%, atoms.
mass based) were obtained at all studied exchange conditions
To study if Cu(II) atoms in Cu–Co PBAs could also be
and no Co was detected in solution after exchange, confirming exchanged with Zn(II), CuCl2–Co PBA was dispersed in aqueous
that Zn is the only metal ion extracted from the structure and solutions of Zn(CH3COO)2·2H2O or ZnCl2 (Table S1†). In this
that there is no further dissolution of the PBA framework case, the metal ion exchange process was less effective, leading
during the post synthetic exchange treatment (Co loss <1 wt% only to an IEDZn of ∼47% at high salt concentrations, which
leached from the PBA).
suggests that Cu(II) ions bind preferentially — in comparison
The insertion of monovalent, divalent and even trivalent to Zn(II) — to the cyanide group. This has also been observed
cations into the interstitial sites of PBA frameworks has been in the synthesis of multi metal CuxZn1−x–Co PBAs by
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co-precipitation.28 However, when the ion exchange was
carried out at higher temperatures, a greater Zn loading
could also be achieved (IEDZn
= 79 for the sample
[Zn(OAc)2]0.78@Cu0.22–Co PBA, prepared at 80 °C).
Characterization
The crystallinity of the samples was confirmed by PXRD
(Fig. 2, S3, S4†). The patterns of the [Cu(Z)v]x@Zn1−x–Co PBAs
show no prominent differences when compared to that of
Zn–Co PBA, with all samples exhibiting reflections corre-
50–52
ˉ
sponding to the cubic space group Fm3m, typical of PBAs.
However, the peak positions are slightly shifted to higher angles
as the Cu content is increased, which is expected given the
smaller ionic radius of Cu(II) compared to that of Zn(II).53
Moreover, using Le Bail refinements, the cell parameter a was
refined for the exchanged samples. A linear correlation between
the value of a and the Cu/(Cu + Zn) ratio was obtained (Fig. 3),
which is in accordance with Vegard’s law and again implies
partial replacement of Zn(II) by Cu(II) in the framework.21,54–56
Furthermore, from HAADF-STEM images (Fig. 4) no segregated
Cu-rich or Zn-rich phases could be observed.
Fig. 3 Variation of the cell parameter (a) with the Cu(II) content for
○
[Cu(OAc)2]x@Zn1−x–Co PBAs. Empty circles ( ) correspond to a obtained
from the Le Bail refinement of the PXRD data.
The physicochemical properties of the ion exchanged
samples, namely textural properties, Lewis acid site density
and thermal stability, were found to be intermediate between
those of the bimetallic PBAs (Zn–Co, CuCl2–Co and Cu(OAc)2–Co).
For all studied samples, a type I isotherm was obtained,
characteristic of microporous PBAs (Fig. S5†). Furthermore,
there was no decrease in the SBET and Vmicro of the Cu(II)
exchanged samples, compared to the SBET and Vmicro of Zn–Co
PBA (Table S2†). This indicates that the Cu(II) atoms were
incorporated into the PBA framework and that no Cu-rich par-
ticles are blocking the pores. The difference FTIR spectra of
adsorbed pyridine obtained for selected PBA samples are pre-
sented in Fig. S6.† The amount of pyridine adsorbed on Lewis
acid sites per mass of PBA decreases in the order Zn–Co PBA
(0.10 mmol g−1) > [Cu(OAc)2]0.90@Zn0.10–Co PBA (0.072 mmol
g−1) > Cu–Co PBA (0.044 mmol g−1). The TGA profiles of all
Fig. 4 HAADF-STEM images and EDX composition mapping for
Cu and Zn of the samples: (a) [Cu(OAc)2]0.45@Zn0.55–Co PBA,
(b) [Cu(OAc)2]0.67@Zn0.33–Co PBA and (c) [Cu(OAc)2]0.90@Zn0.10–Co
PBA. Scale bar: 200 nm.
studied samples (Fig. S7†) exhibit a first mass decay before
100 °C attributed to the loss of water and a sharp mass decay
between 300 and 370 °C, corresponding to the decomposition
of the framework. However, the TGA profile of sample
[Cu(OAc)2]0.90@Zn0.10–Co PBA presents an additional mass
loss between 200 and 300 °C. In order to identify the com-
pounds removed at certain temperatures, TGA-MS studies were
performed. Results show (Fig. S8,† m/z 60 curve) that this
additional, intermediate step in the TGA curve corresponds to
the release of acetic acid from the PBA structure, which was
incorporated during ion exchange with Cu(CH3COO)2·H2O.
Fig. 2 XRD patterns of selected [Cu(OAc)2]x@Zn1−x–Co PBAs.
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Fig. 5 CuN stretching region of the FTIR spectra (left) of
[Cu(OAc)2]x@Zn1−x–Co PBAs starting from the parent material (bottom
spectrum) to the reference PBA (top spectrum) and variation of the
ν(CuN) with the Cu(II) content (right).
Fig. 6 Yield of A3 product after 6 h reaction time for the coupling of
phenylacetylene (0.05 mmol), piperidine (0.1 mmol) and benzaldehyde
(0.1 mmol) at 383
Cu(OAc)2–Co ( ), [Cu(OAc)2]x@Zn1−x–Co ( ), [CuCl2]x@Zn1−x–Co (
K
over 10 mg of Zn–Co
(
), CuCl2–Co
(
),
)
□
and CuxZn1−x–Co PBAs ( ) with different Cu/(Cu + Zn).
FTIR spectra of selected samples (Fig. S9†) show all the
typical bands of PBAs: a band around 1600 cm−1 corre-
sponding to the bending vibration of water molecules, a wide
band observed around 3500 cm−1, due to the symmetric and
asymmetric vibration of water molecules, a band around
470 cm−1 attributed to the bending vibrations of the Co–CuN
chain and a band at ∼2190 cm−1 ascribed to the stretching
vibrations of the CuN bond.57 However, the FTIR spectra
of [Cu(OAc)2]0.70@Zn0.30–Co, [Cu(OAc)2]0.77@Zn0.23–Co and
[Cu(OAc)2]0.90@Zn0.10–Co present additional absorption bands
in the range 1500–1300 cm−1, which corresponds to the
–COO− stretching region.58–60 Such bands were also observed
in the FTIR spectrum of Cu(CH3COO)2·H2O. This observation
is in agreement with the TGA-MS analyses that revealed the
presence of acetate in the structure of [Cu(OAc)2]0.90@Zn0.10–Co
PBA. In contrast, these bands were not observed in the FTIR
spectra of the sample Cu(OAc)2–Co PBA. Moreover, the fre-
quency of the CuN stretching vibration allows a qualitative
characterization of the distribution of Cu–N and Zn–N
bonds in the samples. Indeed, a closer inspection of the
CuN stretching region in the FTIR spectra of the
[Cu(OAc)2]x@Zn1−x–Co PBAs (Fig. 5) reveals a continuous vari-
ation in the ν(CuN) as a function of the Cu(II) content.
the Cu(II) atoms are necessary for the rapid activation of
phenylacetylene via generation of metal acetylide
a
intermediate.65–67 Furthermore, a higher selectivity to the A3
product was obtained when the reaction was catalyzed by
multi metal PBAs, in comparison to that obtained with bi-
metallic Cu–Co PBAs. Zn(II) atoms can facilitate the formation
of the iminium ion intermediate and aid in the coupling
between the iminium ion and the metal acetylide,28,68 thus
increasing the yield of the A3 product over phenylacetylene-
derived side products.
At low to medium Cu(II) loadings (up to Cu/(Cu + Zn) 0.67),
the [Cu(Z)v]x@Zn1−x–Co PBAs exhibit a higher activity than
PBAs of similar composition synthesized by co-precipitation
(CuxZn1−x–Co PBAs, ESI†). Although no Cu(II) concentration
gradient could be observed in the EDX composition mapping
of the [Cu(Z)v]x@Zn1−x–Co PBAs (Fig. 4), it is likely that the
Cu(II) content at the crystal outer edges is higher – and closer
to an optimal Zn(II)/Cu(II) ratio – than in the bulk. Based on Zn
K–edge EXAFS measurements, Liu et al.14 concluded that Zn(II)
atoms coordinated by fewer N atoms – for instance, at defects
or surrounded by a higher than average number of [Co(CN)6]3−
vacancies, but also at the crystal outer surface – are more likely
to be released from the structure. Consequently, these highly
A3 coupling
The catalytic activity of the [Cu(Z)v]x@Zn1−x–Co PBAs was accessible sites would be occupied more easily by Cu(II) atoms
investigated in the synthesis of propargylamines via the A3 as a result of the ion exchange procedure. In contrast, via a co–
coupling of phenylacetylene, benzaldehyde and piperidine. precipitation procedure both divalent metals (Zn and Cu) are
Propargylamines are versatile intermediates in the production arbitrarily arranged in the M1 position, so a higher bulk Cu(II)
of N-containing compounds, and their motif is part of content is required for an optimal Zn(II)/Cu(II) ratio near the
pharmaceutical products used for treatment of chronic neuro- accessible outer surface.21 Remarkably, at high Cu(II) loading
degenerative diseases.61–64 The yield of A3 product obtained and especially with the catalysts prepared using copper acetate
with the studied samples is presented in Fig. 6 and Table S3.† for PSE ([Cu(OAc)2]x@Zn1−x–Co series), the yield of A3
The incorporation of Cu into the structure increases the cata- product is even more increased. In fact, with the sample
lytic activity of the material in comparison to Zn–Co PBA, as [Cu(OAc)2]0.77@Zn0.23–Co PBA, full conversion is achieved after
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only 6 h reaction time. Furthermore the turnover frequency coupling of phenylacetylene) was detected in the product
(TOF), determined at initial reaction rates, was calculated to be mixture. Considering that Zn has been found to increase the
6.48 h−1 (compared to TOF = 2.1 h−1 with Cu0.86Zn0.14–Co selectivity to the A3 product,28 the reaction was repeated using
PBA). The series [Cu(OAc)2]x@Zn1−x–Co not only exhibits a a mixture of ZnCl2 and CuCl2·2H2O (entry 5), and ZnCl2 and
higher activity than that of CuxZn1−x–Co PBAs, but is also Cu(CH3COO)2·H2O (entry 6) in the same molar Cu : Zn ratio of
more active than the [CuCl2]x@Zn1−x–Co PBAs of similar com- the most active PBA (Cu : Zn 77 : 23). Once again, high phenyl-
position. This outstanding activity is attributed to the acetylene conversions are achieved after 6 h reaction time.
presence of CH3COO− in the PBA framework, as evidenced by However, in this case, the presence of Zn as a homogenous
FTIR and TGA-MS. The acetate ion has been found to salt in the reaction mixture has a negligible effect on the
participate in other coupling reactions;69,70 it can provide a selectivity to the A3 product. This indicates that the coordi-
basic function to the PBA, facilitating the C–H activation nation of Cu and Zn in the PBA framework is effective in sup-
of phenylacetylene (generation of the metal acetylide pressing the homocoupling of phenylacetylene, even in the
intermediate).71–74
presence of air, by stabilization of the oxidation state of
To test the basicity of the CH3COO− containing PBAs, Cu(II).28,66 Additionally, we compared the activity of the ion
selected samples were studied as catalysts for the nitroaldol or exchanged PBAs with that of other heterogeneous Cu(II) cata-
Henry reaction of nitromethane and benzaldehyde. This lysts (ESI†), namely Cu(II)/MCM-41 (Cu wt% = 19.4), Cu(II)/BEA
organic transformation is an interesting route towards the pro- (Cu wt% = 23.6), Cu(II)/SiO2 (Cu wt% = 19.8) and Cu(II)-BTC
duction of nitroaldol intermediates.75,76 Since this reaction (Cu wt% = 26.2%). These materials also exhibited a lower
usually requires the presence of a base to proceed, it can serve phenylacetylene conversion and selectivity to the A3 product
as a tool to probe the basic nature of the samples.77–79 As can than [Cu(OAc)2]0.77@Zn0.23–Co PBA (Table 2, entries 7–10),
be observed in Fig. S10,† the benzaldehyde conversion which highlights the advantages of the ion exchanged PBAs
obtained with the sample [Cu(OAc)2]0.90@Zn0.10–Co PBA was over other heterogeneous A3 coupling catalysts. Moreover, the
more than three times higher than the one obtained with the heterogeneity of the PBA catalyst was studied by a hot filtration
rest of the PBAs. This is a clear effect of the presence of test (Fig. S11†). No appreciable activity is observed after
CH3COO− moieties in the structure of [Cu(OAc)2]0.90@Zn0.10–Co removal of the PBA, indicating that no active species leach
PBA, which provide the basic function to facilitate the nitroaldol from the solid. Finally, recycling tests show that the sample
reaction. This represents, to the best of our knowledge, the first [Cu(OAc)2]0.90@Zn0.10–Co PBA maintains its activity after
report of basic functionalization of PBAs.
10 runs (Fig. S12†), with no notable loss of crystallinity or
The catalytic activity for the A3 coupling reaction of phase change observed after reaction (Fig. S13†).
[Cu(OAc)2]0.77@Zn0.23–Co PBA was further compared to that of
homogeneous catalysts, like CuCl2·2H2O, Cu(ClO4)2·6H2O or
Cu(CH3COO)2·H2O (Table 2). Although a high phenylacetylene
conversion is obtained with all the copper salts (entries 2–4),
Conclusions
only 1,4–diphenylbuta-1,3-diyne (formed by oxidative homo- We have demonstrated the occurrence of PSE of metal ions in
PBAs for the first time, based on the quantification of the
amount of atoms released from the structure and the amount
incorporated in the PBA, as well as, on the characterization of
the PBA after the PSE. While high temperatures were
necessary to achieve a high Zn(II) ion exchange degree (IEDZn),
Table 2 Comparison of [Cu(OAc)2]0.77@Zn0.23–Co PBA with homo-
geneous catalysts and Cu(II)-supported materials
the metathesis between Zn(II) and Cu(II) in Zn–Co PBA
Entry
Catalyst
Xa (%)
Sb (%)
Y (%)
occurred easily at room temperature. At low to medium Cu(II)
loadings, the Cu(II) exchanged PBAs exhibited a higher activity
in the A3 coupling reaction than the bimetallic PBAs and the
multi metal samples prepared by co-precipitation. At high
Cu(II) loadings, the presence of the CH3COO− (confirmed by
FTIR and TGA-MS) provided a basic function resulting in a
superior catalytic activity for the series [Cu(OAc)2]x@Zn1−x–Co.
The sample [Cu(OAc)2]0.77@Zn0.23–Co was also more active
than other commonly used A3 coupling catalysts, effectively
suppressing the activity of the oxidative homocoupling of
phenylacetylene, even in the presence of air. The basicity of
PBAs was further studied in the nitroaldol (Henry) reaction,
where a higher conversion was obtained with the CH3COO−
containing PBAs. All in all, we believe that these findings can
have several implications in the way these materials are syn-
thesized, as the ion exchange procedure allows the incor-
1
2
3
4
5
6
7
8
9
10
[Cu(OAc)2]0.77@Zn0.23–Co
Cu(OAc)2
Cu(ClO4)2
>99
97
69
94
90
95
63
42
61
87
98c
4e
98
4
<1
<1
<1
7
43
31
42
66
<1e
<1e
<1e
7e
CuCl2
(CuCl2 + ZnCl2)d
(Cu(OAc)2 + ZnCl2)d
Cu(II)/BEA
68 f
73 f
69 f
76 f
Cu(II)/MCM-41
Cu(II)/SiO2
Cu(II)-BTC
a Conversion of phenylacetylene. b Selectivity to the A3 product based
on phenylacetylene. c Acetophenone was the only phenylacetylene-
derived side-product detected. d Mixture of salts in
a Cu : Zn
77 : 23 molar ratio. e 1,4-Diphenylbuta-1,3-diyne (from the homocou-
pling of phenylacetylene) was the only phenylacetylene-derived side-
product detected. f Both acetophenone and 1,4-diphenylbuta-1,3-diyne
were detected. Reaction conditions: Phenylacetylene (0.05 mmol),
piperidine (0.1 mmol) and benzaldehyde (0.1 mmol) at 383 K over
10 mg of catalyst for 6 h.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This project has received funding from the European Union’s 16 X. Li, J. Liu, A. I. Rykov, H. Han, C. Jin, X. Liu and J. Wang,
Horizon 2020 research and innovation programme under the Appl. Catal., B, 2015, 179, 196–205.
Marie Sklodowska-Curie grant agreement No 641887 (project 17 A. Peeters, P. Valvekens, R. Ameloot, G. Sankar,
acronym: DEFNET). F. G. C. would like to thank the European
Union’s Horizon 2020 research and innovation programme
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under the Marie Sklodowska-Curie grant agreement 18 S. Lee, S. T. Baek, K. Anas, C.-S. Ha, D.-W. Park, J. W. Lee
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support. F. W. O. Vlaanderen (Research Foundation Flanders) 19 C. Marquez, M. Rivera-Torrente, P. P. Paalanen,
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