Acetyl Acetone Covalent Triazine Framework: An Efficient Carbon Capture and Storage Material and a Highly Stable Heterogeneous Catalyst

Article abstract of DOI:10.1021/acs.chemmater.8b01409

We present, for the first time, Covalent Triazine Frameworks functionalized with acetyl acetonate group (acac-CTFs). They are obtained from the polymerization of 4,4'-malonyldibenzonitrile under ionothermal conditions and exhibit BET surface areas up to 1626 m2/g. The materials show excellent CO2 uptake (3.30 mmol/g at 273 K and 1 bar), H2 storage capacity (1.53 wt% at 77 K and 1 bar) and a good CO2/N2 selectivity (up to 46 at 298 K). The enhanced CO2 uptake value and good selectivity are due to the presence of dual polar sites (N and O) throughout the material. In addition, acac-CTF was used to anchor VO(acac)2 as a heterogeneous catalyst. The V@acacCTF showed outstanding reactivity and reusability for the modified Mannich-type reaction with a higher turnover number than the homogeneous catalyst. The higher reactivity and reusability of the catalyst comes from the coordination of the vanadyl ions to the acetyl acetonate groups present in the material. The strong metalation is confirmed from Fourier Transform Infrared analysis, 13C MAS NMR spectral analysis and X-ray photoelectron spectroscopy measurement. Detailed characterization of the V@acac-CTF reveals that electron donation from O^O of the acetyl acetonate group to VO(acac)2, combined with the very high surface area of acac-CTF, is responsible for the stabilization of the catalyst. Overall, this contribution highlights the necessity of stable catalytic binding sites on heterogeneous supports to fabricate greener catalysts for sustainable chemistry.

Full text of DOI:10.1021/acs.chemmater.8b01409

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Article
Acetyl Acetone Covalent Triazine Framework: An Efficient Carbon
Capture and Storage Material and a Highly Stable Heterogeneous Catalyst
Himanshu Sekhar Jena, Chidharth Krishnaraj, Guangbo Wang, Karen
Leus, Johannes Schmidt, Nicolas Chaoui, and Pascal Van Der Voort
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01409 • Publication Date (Web): 03 Jun 2018
Downloaded from http://pubs.acs.org on June 3, 2018
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Chemistry of Materials
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Acetyl Acetone Covalent Triazine Framework: An Efficient Carbon
Capture and Storage Material and a Highly Stable Heterogeneous
Catalyst
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Himanshu Sekhar Jena,a Chidharth Krishnaraj,a Guangbo Wang,a Karen Leus,a Johannes Schmidt,b
Nicolas Chaoui,b Pascal Van Der Voorta *ꢀ
aCenter for Ordered Materials, Organometallics and Catalysis (COMOC), Department of Chemistry, Ghent University,
Krijgslaan 281 (S3), 9000 Ghent, Belgium
bTechnische Universität Berlin, Institut für Chemie – Funktionsmaterialien, Hardenbergstraße 40, 10623 Berlin,
Germany
ABSTRACT: We present, for the first time, Covalent Triazine Frameworks functionalized with acetyl acetonate group (acac-
CTFs). They are obtained from the polymerization of 4,4'-malonyldibenzonitrile under ionothermal conditions and exhibit
BET surface areas up to 1626 m2/g. The materials show excellent CO2 uptake (3.30 mmol/g at 273 K and 1 bar), H2 storage
capacity (1.53 wt% at 77 K and 1 bar) and a good CO2/N2 selectivity (up to 46 at 298 K). The enhanced CO2 uptake value and
good selectivity are due to the presence of dual polar sites (N and O) throughout the material. In addition, acac-CTF was
used to anchor VO(acac)2 as a heterogeneous catalyst. The V@acacCTF showed outstanding reactivity and reusability for the
modified Mannich-type reaction with a higher turnover number than the homogeneous catalyst. The higher reactivity and
reusability of the catalyst comes from the coordination of the vanadyl ions to the acetyl acetonate groups present in the
material. The strong metalation is confirmed from Fourier Transform Infrared analysis, 13C MAS NMR spectral analysis and
X-ray photoelectron spectroscopy measurement. Detailed characterization of the V@acac-CTF reveals that electron dona-
tion from O^O of the acetyl acetonate group to VO(acac)2, combined with the very high surface area of acac-CTF, is respon-
sible for the stabilization of the catalyst. Overall, this contribution highlights the necessity of stable catalytic binding sites on
heterogeneous supports to fabricate greener catalysts for sustainable chemistry.
for carbon capture and storage as well as heterogeneous
catalysis.21-26 Among CTFs, hexaazatriphenylene-based
conjugated CTF25 (HAT CTF 450/600) showed the highest
CO2 uptake value of 6.3 mmol/g at 273 K and 1 bar and
bipyridine-CTF23 synthesized at 600°C showed the highest
H2 storage of 2.12 wt% at 77 K and 1 bar.
INTRODUCTION
Engineering new porous materials with exceptional poros-
ities and active binding sites has been one of the most
exciting research domains in chemistry.1,2 This research
area is still growing rapidly, aiming at the optimization of
their primary applications in gas storage/separation, ca-
talysis, sensing, drug delivery, and energy conversions.3-11
Metal Organic Frameworks (MOFs) and Porous Organic
Frameworks/Polymers (POFs/POPs) are attracting partic-
ular attention because of their high porosity, comprised of
a uniform and tunable pore size, easy incorporation of
diverse guest molecules and introduction of a wide range
of chemical functionalities by post-synthesis treatment. 3-11
Furthermore, designing MOFs and Covalent Organic
Frameworks (COFs) with active homogeneous catalytic
In heterogeneous catalysis, CTFs have been used as solid
support for oxidation,27-30 hydrogenation,31-34 hydrolysis of
ammonia borane,35 carbonylation36, etc. Moreover, design-
ing new CTFs with active binding sites (N^N, N^O, O^O,
etc) can produce efficient heterogeneous supports for
more sustainable and environmentally friendly catalytic
reactions. In this direction, CTFs containing pyridine, 2,2’-
bipyridine and N-Heterocyclic Carbene (NHC) based func-
tional ligands have been developed and used as catalytic
supports for transition and noble metal complexes.27-38
sites would offer new opportunities in the field of hetero-
12-15
geneous catalysis.
Among COFs, Covalent Triazine
Frameworks (CTFs)16-20 are high performance materials
with intrinsic repetitive triazine moieties, high surface
areas and importantly, low framework densities. In gen-
eral, CTFs are synthesized from nitrile containing mono-
mers via either the ionothermal route using molten
17,18
ZnCl2
or by CF3SO3H catalyzed19,20 trimerization reac-
tions. These materials have been used as smart candidates
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Scheme 1. Schematic representation of synthesis of acac-CTF supported VO(acac)2 complex, V@acac-CTF.
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Apart from these, acetyl acetone (acac) based homogene-
ous transition metal complexes have shown significant
catalytic performance in a wide variety of organic trans-
formation as well.39-41 Notably, only few acac-
functionalized heterogeneous systems like silica and peri-
odic mesoporous organosilica (PMO) have shown signifi-
cant reactivity and reusability for the cross coupling reac-
tion, hydroxylation, click reaction, Glaser-type coupling
and alcohol oxidation.42-45
a Quantachrome iSorb-HP gas sorption analyzer. X-ray
powder diffraction (XRPD) patterns were collected on a
Thermo Scientific ARL X’Tra diffractometer, operated at 40
kV, 30 mA using Cu-Kα radiation (λ = 1.5406 Å). Thermo-
gravimetric analysis (TGA) was performed on a Netzsch
STA-449 F3 Jupiter-simultaneous TG-DSC analyzer within
a temperature range of 20 – 800 ºC, under a N2 atmos-
phere and heating rate of 2 ºC/min.13C MAS NMR spectra
were acquired at 125.69 MHz using a 4 mm MAS NMR
probe with a spinning rate of 8 kHz and pulse width of 2.5
μs for a π/4 pulse. 1800-2700 scans were further accumu-
lated with a 4s recycle delay. The X-ray photoelectron
spectroscopy (XPS) measurements were performed on a
Thermo Fisher Scientific Escalab 250 Xi. The vanadium
loading was determined by an Inductively Coupled Plasma
(ICP) measurement Optima 8000 atomic Emission Spec-
trometer (ICP-OES). Bright-field Scanning Transmission
Electron Microscopy (BF-STEM) was performed on a JEOL
JEM-2200FS high resolution scanning transmission elec-
tron microscope with a spatial resolution of 0.13 nm, an
image lens spherical aberration corrector, an electron
energy loss spectrometer (filter) and an emission field gun
(FEG) operating at 200 KeV.
However, none of the POFs/POPs have been functionalized
with acac groups yet. Therefore, we aimed to functionalize
acac group into a CTF backbone in order to design a robust
acac-CTF supported transition metal catalyst for sustaina-
ble catalysis.
Following this rationale, acetyl acetone functionalized
CTFs from 4,4'-malonyldibenzonitrile46 were synthesized.
As a proof of concept, the acac-CTF supported VO(acac)2
complex (V@acac-CTF; Scheme 1) efficiently catalyzed the
modified Mannich reaction with a higher turnover number
(TON) than Uang et al.,40 protocol where VO(acac)2 was
used as homogenous catalyst. The catalyst is reusable for
at least four reaction cycles without any loss in reactivity.
To the best of our knowledge, this is the first example of an
acac-functionalized POF employed as carbon capture and
storage material and a catalytic support for transition-
metal complex to perform an organic transformation.
RESULTS AND DISCUSSION
Synthesis and Characterization of acac-CTFs:
For synthesis of the targeted acac-CTF materials, the nitrile
linker, 4,4'-malonyldibenzonitrile, was synthesized by
following the procedure reported by Carlucci et al.46 To
obtain a family of acac-functionalized CTF materials with
different structural properties such as specific surface
area, total pore volume, and pore structure, synthesis was
undertaken at 400 °C and 500 °C using 1:5 and 1:10 molar
ratio of linker to ZnCl2. All CTFs were synthesized in mol-
ten ZnCl2 serving as both solvent and Lewis acid catalyst
during the reaction. For the synthesis, an ampoule was
charged with the monomer (500 mg, 1.8 mmol) and 5/10
equivalents of anhydrous ZnCl2 in a glovebox (Table S1).
Subsequently, respective ampoules were evacuated, flame-
sealed, and slowly heated to the desired temperatures
(400/500 °C) with the heating step of 1 °C/min for 48 h in
a Nabertherm furnace oven. After the oven had cooled to
room temperature, the ampoules were opened and the
EXPERIMENTAL SECTION
Materials and Methods: The required chemicals were
purchased from Sigma-Aldrich and used without further
1
purifications. The H and 13C NMR analysis of the nitrile
linker and all Mannich reaction products were performed
on a Bruker Advance 300 MHz spectrometer using mesity-
lene as internal standard. Elemental analyses (C, H, N and
O) were carried out on a Thermo Scientific Flash 2000
CHNS-O analyzer equipped with a TCD detector. Fourier
Transform Infrared Spectroscopy (FT-IR) in the region of
4000-650 cm-1 was performed with a Thermo Nicolet 6700
FT-IR spectrometer equipped with a nitrogen-cooled MCT
detector and a KBr beam splitter. Dinitrogen (N2) adsorp-
tion isotherms were obtained using a Belsorp Mini appa-
ratus measured at 77 K. Carbon dioxide (CO2) and dihy-
drogen (H2) adsorption measurements were carried out on
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crude material was grounded and refluxed in water for 6 h
zation.17,18 As expected, there might be mixture of keto-
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to remove unreacted ZnCl2, as well as filtered and thor-
oughly washed using distilled water. Afterwards, to re-
move the unreacted linker and other organic impurities
from the obtained materials, the black powder was stirred
in HCl (1 M) overnight followed by successive washing
with water until neutral pH was reached. A final wash with
tetrahydrofuran was necessary to remove the water. The
materials were dried and activated at 150 °C under vacu-
um overnight prior to further analysis. For more simplicity,
the obtained acac functionalized CTFs were denoted as
acac-CTF-5-400, acac-CTF-10-400, acac-CTF-5-500 and
acac-CTF-10-500 (acac-CTF-molar equivalent of ZnCl2-
synthesis temperature) (Table S1). Among them, acac-CTF-
10-500 showed highest Brunauer-Emmett-Teller (BET)
surface area of 1626 m2/g (Table 1). To ensure the pres-
ence of an acac functional group even after the harsh CTF
synthesis, FT-IR measurements were performed on all four
acac-CTF materials. Notably, a sharp band at 1710 cm-1 (-
C(O)) and 3060 cm-1 (=C-H) in acac-CTF-5-400 and acac-
CTF-10-400 confirmed the presence of acac functional
group, even after CTF synthesis (Figure 1).39-42 However, in
case of acac-CTF-5-500 and acac-CTF-10-500, broad bands
were observed at those same places. This occurs due to the
materials’ carbonization at high synthesis temperatures as
noted in several cases of CTFs.15,16 Additionally, the ab-
sence of -CN band (2226 cm-1) and presence 1590 cm-1 (-
C=N; triazine) confirmed the complete trimerization of
nitrile group to triazine ring ensuring the successful CTF
formations.
enol tautomer formation during the CTF synthesis and of
the material undergoes partial carbonization due to rela-
tively unstable enol isomers resulting in only 50% ideal
structure (Scheme S1). Notably, in case of acac-CTF-10-
400, a higher C/N ratio was observed, confirming the high-
er nitrile cleavages and triazine ring decomposition during
the trimerization process.17,18 Additionally, the observed
lower C/O ratio in acac-CTF-10-400 suggests lower de-
composition of acac functional groups and the correspond-
ing presence of more acac functional groups in the final
product.47 The physiochemical stability of the obtained
CTFs was confirmed by TGA under N2 atmosphere (Figure
S2). All the CTFs are stable up to 500 °C and start to de-
compose at higher temperatures.
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Gas Sorption Properties: The permanent porosity and
surface area properties of all the obtained acac-CTFs were
explored using N2 sorption measurements at 77 K (Figure
2). Prior to the measurement, all materials were activated
at 150 °C overnight and under vacuum. The obtained plot
consists of two types of isotherms (type-I, (P/Po; <0.5 bar)
and type-II (P/Po; >0.5 bar)) like most of the reported
CTFs.17,18, 48, 49 The steep increase observed of N2 uptake
from a region of low relative pressure confirms the mate-
rials’ microporous nature whereas the presence of hyste-
resis from P/Po > 0.5 bar indicates mesopores. The ob-
served mesopores are due to the presence of relatively
large pores of size 2.5 nm (Scheme S2). Besides, at 500 °C
corrosive fragmentation of the walls on top of the mi-
cropores cannot be ruled out which might be another rea-
son for observed mesopores. As expected, with a tempera-
ture increase and additional ZnCl2, the BET surface areas,
total pore volumes (Vtot) and micropore volumes (Vmicro) of
the materials were increased (Table 1). Hence, acac-CTF-
10-500 showed the highest BET surface area and pore
volume which is possibly due to more defects in the struc-
ture.
Figure 1. FT-IR spectral comparison between acac-CTFs
obtained at different reaction conditions with respect to
the monomer.
To study the materials’ phase and nature, powder X-ray
diffraction (PXRD) measurement was undertaken (Figure
S1). Based on these measurements, all the acac-CTF mate-
rials were found to be amorphous with a broad peak at 2θ
= 24°, indicating graphitic like two-dimensional layer for-
mation.15,16 This is common for CTFs where two-
dimensional sheets are stacked on top of each other at an
average d-spacing of 3.7 Å. Elemental analysis was carried
out to confirm the percentage of C, N, H and O in the mate-
rials. The obtained results indicate a lower H, N and O
content and a higher C content than the theoretical values
(Table S2), probably due to the materials’ partial carboni-
Figure 2. Nitrogen adsoption/desorption isotherms of
acac-CTFs measured at 77 K, filled and empty symbols
represent adsorption and desorption, respectively.
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Table 1. BET Surface Areas, Pore Volumes, CO2 and H2 Adsorption Uptakes, CO2 Isosteric Heat of adsorption (Qst),
and CO2/N2 selectivity of the presented CTFs.
a
b
c
e
f
CTF
SABET
Vtot
Vmicro
Vmicro/Vtot
N
(%)d
O
(%)d
CO2 (mmol/g)
273 K 298 K
QST
CO2/N2
H2
uptake
(wt%)
(m2/g) (cm3/g) (cm3/g)
(kJ/mol)
acac-CTF-5-400
1131
1150
0.90
0.95
0.77
0.77
85
80
5.442 3.225 2.87
3.908 4.929 3.06
1.89
1.96
23.0
25.5
38
20
1.51
1.18
acac-CTF-10-
400
9
acac-CTF-5-500
1556
1626
1.20
1.60
1.10
1.23
91
77
5.070 5.021 3.30
4.227 4.395 3.16
1.97
1.91
28.6
23.6
46(50g) 1.53
21 1.44
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acac-CTF-10-
500
b
a BET surface area was calculated over the relative pressure range of 0.05-0.3 at 77 K. Vtot, total pore volume was calculated at
P/P0 = 0.98. c Vmicro, micropore volume was calculated by N2 adsorption isotherms using the t-plot method. d Percentage of nitrogen
content calculated from elemental analysis, e Calculated isosteric heat of adsorption of CO2 using Clausius-Clapeyron equation. f The
selectivity of material for CO2 adsorption over N2 at 298 K was calculated by the Henry model, g Selectivity at 273 K.
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acac-CTF-5-400
acac-CTF-10-400
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1.5
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1
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298 K
0.5
0
273 K 298 K
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Pressure (bar)
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298 K
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Figure 3. CO2 adsorption (closed symbols) and desorption (open symbols) isotherms of the studied acac-CTFs measured up
to 1 bar at 273 K and 298 K.
Notably, acac-CTF-5-500 exhibited the highest number of
micropores (Vmicro/Vtot) with respect to the material’s total
pore. Additionally, the materials’ polar functional groups
(N-/O-), high surface area as well as their high mi-
croporous to mesoporous nature prompted us to further
explore their gas sorption/storage properties at atmos-
pheric pressure (1 bar) and their selectivity for CO2 over
N2.
To explore the gas sorption properties of these materials,
CO2 adsorption/desorption isotherms were recorded at
273 K and 298 K, both at 1 bar (Figure 3). The uptake val-
ues at both temperatures are included in Table 1. On com-
paring these values with respect to the synthesis tempera-
tures, acac-CTFs obtained at 500 °C shows more CO2 up-
take than 400 °C.
However, the amount of ZnCl2 used was not linearly relat-
ed with the uptake amount. The acac-CTF-5-500 showed
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the best uptake value (3.30 mmol/g at 273 K and 1.97 to store more H2. The hydrogen uptake of acac-CTF-5-
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mmol/g at 298 K). This might be due to a higher amount of
dual polar sites (N and O; and higher number of Vmicro/Vtot
present in the materials (Table 1).25 Although the obtained
best value is lower than HAT CTF (450/500)25, it is higher
than several CTFs measured at 273K [CTF-1 (2.47
mmol/g),23 CTF-DCN-500 (2.7 mmol/g)50 and etc] includ-
400/500 is comparable to several other CTF materials and
higher than the reported O-containing CTFs.50-53 However,
the value is slightly smaller compared to that of fl-CTF-400
(1.95 wt%)24 and caCTF-1-700 (2.26 wt %)57 at 77 K and 1
bar of H2. This allows us to conclude that the H2 storage
ability of CTFs does not necessarily only depend on the
surface area but also on the N content.
ing other O-containing CTFs51-54
.
Furthermore, to solely encourage the higher interaction of
CO2 with acac-CTF-5-500 and thus exclude the other acac-
CTFs, the isosteric adsorption heat (Qst) of CO2 was calcu-
lated using the Clausius-Clapeyron equation, considering
CO2 adsorption isotherms at 273 K and 298 K. The ob-
tained result was plotted as Qst vs adsorbed amount of CO2
(Figure S3). The exact values are included in Table 1. When
comparing the acac-CTFs, it was observed that acac-CTF-5-
500 shows the largest Qst value (28.6 kJ/mol). This sup-
ports our statement that the acac-CTF-5-500 CO2 uptake
ranks first indeed.
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0.8
acac-CTF-5-400
acac-CTF-10-400
acac-CTF-5-500
0.4
acac-CTF-10-500
Moreover, all calculated acac-CTFs Qst values fall within a
range of 23-28 kJ/mol, a range much higher than the lique-
faction heat of bulk CO2 (17 kJ/ mol)55 and activated car-
bon at low CO2 pressure (17.8 kJ/mol)56 and are also com-
parable with O-containing CTFs (16 – 36 kJ/mol)51-54
0
0
0.2
0.4
0.6
0.8
1
1.2
Pressure (bar)
Figure 4. H2 adsorption (closed symbols) and desorption
(open symbols) isotherms for acac-CTFs measured at 77 K.
In addition to adsorption, the materials’ selectivity to CO2
over other gases makes up a crucial factor for carbon cap-
ture and storage as well. It has been reported that a higher
reaction temperature affords a more defective CTF struc-
ture with less N content and hence lower selectivity.
Therefore, to explore the impact of N content on the selec-
tivity behavior of acac-CTFs, CO2/N2 selectivity was esti-
mated using the slopes ratio in the Henry region of the CO2
(< 0.05 bar) and N2 (< 0.1 bar) adsorption isotherms at 298
K (Figure S4). The calculated values (Table 1) represent
moderate to good CO2 selectivity and are in accordance
with the N/O content of the respective materials. The acac-
CTF-5-400 (38) / acac-CTF-5-500 (46) showed higher
selectivity than the other two. This is because higher O
content and higher number ratio of Vmicro/Vtot pores are
less favorable for nitrogen adsorption.51
Catalytic Properties:
To explore these materials as a heterogeneous support,
VO(acac)2 was anchored onto the material, as the latter
was previously used as suitable homogenous catalyst in
the modified Mannich reaction.40 However, it was reported
that even after long reaction times, VO(acac)2 showed low
reactivity for substrates like 1-naphthol, phenol and ester
substituted 2-naphthol. This might be due to catalyst deac-
tivation during the reaction. In addition, the separation of
soluble catalyst from the reaction mixture and the produc-
tion of vanadium-based waste, limits the protocol. In the
present case, a strong metalation between the CTF’s acac
functional group and VO(acac)2 is expected because of
their strong cooperativity.42-45 This can not only reduce the
chance of catalyst deactivation during the reaction but also
facilitates easy separation, increases catalyst reusability
and minimizes waste formation.
From this study, it can be concluded that not only high
temperatures but also larger amounts of ZnCl2 generate
more defective acac-CTF structures with low N content
and hence low CO2 uptake as well as lower CO2/N2 selectiv-
ity.
As inferred from the elemental analysis (Table S2), the
oxygen content of acac-CTF-10-400 and acac-CTF-5-500 is
similar and higher than in the other two acac-CTFs. The
strong carbonyl stretching band of the acac functional
group (Figure 1) combined with the lowest C/O ratio in
acac-CTF-10-400 results in the largest number of available
acac-functional sites when compared to the other three.
Moreover, the pore size and surface area of acac-CTF-10-
400 is sufficient to efficiently diffuse 2-naphthol, NMO and
the target Mannich product during the catalytic reactions.
Therefore, acac-CTF-10-400 was chosen to act as catalytic
support to anchor VO(acac)2 and explore the latter’s cata-
lytic activities for the targeted Mannich reaction. Prior to
these catalytic reactions, the stability of acac-CTF was
determined and found stable in air, moisture and insoluble
in common organic solvents such as dichloromethane,
After the above study concerning CO2 adsorption and se-
lectivity (over N2) study, acac-CTFs materials were further
tested on their H2 storage properties. Like electricity, H2 is
considered as a potential clean energy carrier. However,
storing large amounts of H2 in a safe manner remains chal-
lenging. Anticipating better storage capacities, MOFs and
COFs have recently been explored as H2 storage materi-
als.12-15 Therefore, to explore the H2 storage properties of
acac-CTF materials, adsorption measurements were taken
at 77 K and 1 bar (Figure 4) and included in Table 1. Nota-
bly, acac-CTF-10-400/500 showed lower storage proper-
ties than acac-CTF-5-400/500. Since acac-CTF-5-400 and
acac-CTF-5-500 have a comparably high N content, they
show similar amount of H2 storage of 1.51 wt% and 1.53
wt% respectively. This result signifies that materials with
high N content interacts more frequently and shows ability
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tetrahydrofuran, acetone, ethanol, methanol, diethyl ether, acac-CTF-10-500 was chosen because it has a relatively
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chloroform, dimethylformamide and dimethyl sulfoxide.
low zinc content (Table S2) and it exhibits mesopore (Fig-
ure 1). It has been reported that mesopores might speed
up the diffusion of the reactants to the catalytic center and
hence better reactivity can be expected.26 Therefore,
V@acac-CTF-10-500 was synthesized using a similar
method and used as catalyst for the Mannich reaction un-
The V@acac-CTF catalyst was synthesized via the post-
synthetic metalation method, where VO(acac)2 was used as
the metal source and toluene as solvent. In a typical exper-
iment, 144 mg (0.54 mmol) of VO(acac)2 was dissolved in
50 mL of anhydrous toluene to which 500 mg (1.8 mmol)
of acac-CTF was added under argon. The mixture was
stirred under reflux overnight, filtered under vacuum and
dried. Further, the catalyst was first washed thoroughly
using Soxhlet extraction with toluene to remove weakly
bound VO(acac)2, then filtered and dried under vacuum.
Finally, the catalyst was dried under vacuum at 120 °C for
12 h to remove the solvent and used for further experi-
ments. From the CHNO analysis of V@acac-CTF, the de-
crease of the C content and increase of the O content con-
firmed the incorporation of VO(acac)2 in the framework
(Table S2). To determine the exact amount of metal ions
present in fresh V@acac-CTF catalyst, an ICP-OES meas-
urement was undertaken and the sample contained 0.306
mmol/g (1.6%) of vanadium ions. The absence of zinc ion
confirmed that all the accessible residual zinc was replaced
by vanadium ion. However, the remaining residual amount
can be assigned to the trapped metal salts or anions in the
pores that are impossible to remove.17, 23, 49
1
der similar conditions. From H NMR analysis of the reac-
tion mixture, it was observed that the reaction was com-
pleted after only 8h with similar reactivity as the V@acac-
CTF-10-400. This further confirmed that residual zinc has
no interference on the reactivity and the surface area as
well as pore volume of acac-CTFs are sufficient to allow
easy substrate diffusion to the vanadium center.
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Further, to explore the probability of vanadium ions leach-
ing from the support, hot filtration techniques were per-
formed. The catalyst was removed from the mother liquor
after 4 h and the reaction allowed to proceed for further 8
h (Figure S6). Notably, the reaction did not proceed at all
which rules out leaching. In addition, ICP-OES measure-
ment of recovered catalyst confirmed no change in vanadi-
um content after first cycle of catalytic reaction. After the
first cycle, the turn over number (TON = 213) of the cata-
lyst was calculated and found to be 23 times greater than
the homogeneous catalyst. This large TON combined with
its recyclability adds great value to the V@acac-CTF and
hence proves that acac-CTF is likely to be used as a catalyt-
ic support for fabrication of greener heterogeneous cata-
lysts. Furthermore, to avoid using excess oxidant (three
equivalents of NMO40), the reactions were repeated using
two equivalents (Table 2, entry 3) and one equivalent
(Table 2, entry 4) of NMO under the same reaction condi-
tions. To our delight, 95% of 3a was isolated in both cases.
This further added to the novelty of V@acac-CTF over
homogeneous catalysts. Additionally, the reusability of the
V@acac-CTF was explored using the same catalyst for
successive reactions. The catalyst retains similar reactivity
over at least four reaction cycles.
In a preliminary screening, V@acac-CTF was used as cata-
lyst for the Mannich reaction between 2-naphthol (1a) and
three equivalents of N-methylmorpholine N-oxide (NMO)
(2a) in dichloromethane (DCM) at 40 °C for 24 h (Table 2,
entry 2). The reaction mixture was analyzed at different
time intervals using 1H NMR-spectroscopy. 15% of the
target product 1-(morpholinomethyl)naphthalen-2-ol (3a)
was obtained within 30 minutes (Figure S5). The yield
increased up to 95% after 8 h of reaction time. However,
even after 24 h, the reaction yield no longer improved.
Table 2. Catalytic activity of V@catalysts for Mannich
reaction.
Entry
1b
2c
3c
4c
Catalyst
1a:2a
1:3
3a (%)a
100 (92)
100 (95)
100 (95)
100 (95)
VO(acac)2
V@acac-CTF
V@acac-CTF
V@acac-CTF
1:3
1:2
Figure 5. FT-IR spectra comparison between pristine acac-
1:1
CTF (a), V@acac-CTF (b).
1
aConversion (yield) were calculated from H NMR analysis
To understand the coordinative interaction, an FT-IR anal-
ysis of V@acac-CTF was performed and compared with the
pristine acac-CTF (Figure 5). The observed shifting of C=O
stretching frequency of the acac group from 1710 cm-1 to
1700 cm-1 confirmed the coordination of vanadium ions
towards O^O binding sites. The observed no changes at
1590 cm-1 ruled out the possible coordination of vanadium
ions to the triazine-Ns. Thus, a strong coordination of the
vanadium species on the acac-CTF resulted in a greater
b
c
using mesitylene as internal standard, 10 mol%, 0.306
mmol/g of vanadium ion calculated from ICP-OES analysis.
To eliminate the role of residual zinc, blank catalytic tests
were performed using pristine acac-CTFs. From the 1H
NMR analysis, 3a was not detected in any cases, which
ruled out the interference of zinc in the catalytic reaction.
To further explore the performance of other acac-CTFs,
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Chemistry of Materials
reactivity of V@acac-CTF compared to its homogenous
counterpart.
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Figure 6. Deconvoluted XPS N 1s spectra (left) and O 1s spectra (right) of (a) pristine acac-CTF (b) V@acac-CTF.
Additionally, to support the coordinative interaction be-
tween VO(acac)2 and pristine acac-CTF, 13C-CP-MAS-ss-
NMR spectroscopic analyses were carried out on both the
pristine CTF and V@acac-CTF catalyst and compared. As
shown in Figure S7, the peak at 124 ppm corresponds to
the aromatic C. In addition, the small peak at 185 ppm
indicates the presence of a C=O entity in the acac group.
As expected, no changes were observed in fresh V@acac-
CTF (Figure 6(b), left) emphasizing the absence of any sort
of coordination towards triazine-N.58 The XPS O 1s spectra
of acac-CTF showed two peaks at 533 and 531 eV which
can be assigned to C=O and -C-OH due to tautomerization
of acac group in both keto and enol form (Figure 6(a),
right). Moreover, O 1s spectra of V@acac-CTF showed an
additional peak at 529 eV which can be assigned to the
V=O bond present in the catalyst (Figure 6(b), right).
The slight decrease in intensity of the C=O bond in the acac
group and shift in aromatic carbon towards a lower chemi-
cal shift (123 ppm) further support the metalation. Sur-
prisingly, peaks corresponding to triazine-C and CH2 were
not observed which might be due to their relative lower
concentration. A similar observation was also noticed
recently.49
In addition, to confirm the oxidation state of vanadium
ions, the XPS spectrum of V@acac-CTF in the V2p region
(Figure 7) was measured. The observed peak at binding
energy of vanadium ion 516 eV can be assigned to the
characteristic V 2p3/2 peak, as the previously reported
VO(acac)2, SB-V and VO(acac)2 anchored on PMO as well.44
The, binding energy at 523 eV can be assigned to V 2p1/2
peak. This clearly indicates the presence of V(IV) in the
V@acac-CTF.
To explore the surface area properties after anchoring of
VO(acac)2, the N2 adsorption isotherm of V@acac-CTF
catalyst was measured and compared with the pristine
acac-CTF (Figure S8). As expected, both the BET surface
area (900 m2/g) and pore volume (0.65 cm3/g) ranks low-
er when compared to pristine acac-CTF corroborating to
the successful coordination. Bright-field scanning trans-
mission electron microscopy (BF-STEM; Figure S9-10)
analysis shows homogeneous distribution of C, N, O and
vanadium elements in acac-CTF as well as in V@acac-CTF.
Furthermore, to explore the elements’ nature and their
binding energy as well as the nature of catalysts and the
amount of catalytic active center, X-ray photoelectron
spectroscopy (XPS) analysis was performed. To study the
coordinative interaction, XPS spectra of pristine acac-CTF
and V@acac-CTF were analyzed by fitting the respective
atoms to their binding energies. The observed XPS C 1s
(deconvoluted) peaks at 284, 285 and 288 eV can be as-
signed to C-C/C-H, C-N and C=O species respectively pre-
sent in pristine acac-CTF (Figure S11). A similar observa-
tion was obtained for V@acac-CTF, apart from the de-
crease of C=O intensity and peak broadening due to the
coordinative interaction of acac group with the vanadium
center. The N 1s spectrum of acac-CTF showed three peaks
at 398, 400 and 403 eV (Figure 6(a), left) which can re-
spectively be assigned to the phenyl functionalized triazine
unit, the non-functionalized triazine and the oxidized N-O
species.58
Figure 7. Deconvoluted XPS spectra of vanadium in the V
2p region for V@acac-CTF catalyst.
The above experimental evidences and discussion con-
firmed that the strong interaction between acac-CTF and
V(IV) ion leads to the excellent reactivity of the catalyst.
After complete characterization of the catalyst, a plausible
mechanism was proposed which involves similar steps as
reported by Uang et al.40 (Scheme S3). In brief, the coordi-
nation of NMO to the V(IV) center followed by intramo-
lecular elimination facilitates a six-membered transition
state (A) which results in the formation of iminium ion (B).
The abstraction of a proton from 2-naphthol by [V(OH)(O-
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)] generates a lone pair electron on the oxygen which mi- V(IV) center by O^O coordination sites and minimized
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grates to the aromatic ring to generate acidic C that subse-
quently attacks the iminium ion to form the target Mannich
base (3a). Further [V(OH)2] releases one water molecule to
generate the catalyst. Since the surface area of the catalyst
is very high, many substrates diffuse into the pores of the
materials and have more access to the catalytic reactive
center. This increases the interaction of the naphtholate
ion with the iminium ion and hence shows higher reactivi-
ty. This further supported the importance of the materials
for the in-situ formation of iminium ion from amine-oxide
catalyzed heterogeneous catalysts which have never been
reported yet. In addition, the robustness of the support
increases the stability of the catalyst for multiple reaction
cycles.
catalytic deactivation.
Conclusion:
In conclusion, for the first time, a family of acetyl acetone
functionalized Covalent Triazine Framework materials
were synthesized under ionothermal conditions. The ob-
tained CTF materials showed tunable surface areas and
porosities based on variations in the synthesis tempera-
ture and molar equivalents of ZnCl2 used. Among them, the
acac-CTF synthesized using 5 equivalents of ZnCl2 at 500
°C showed both the best CO2 uptake at 273 K (3.30
mmol/g) and 298 K (1.97 mmol/g) at 1 bar as well as the
highest H2 storage uptake (1.53 wt%) at 77 K and 1 bar. In
addition, a heterogeneous catalyst (V@acac-CTF) was
prepared by anchoring VO(acac)2 on pristine acac-CTF that
showed excellent reactivity in a modified Mannich reaction
with a TON of 213 and a wide substrate scope. Significant-
ly, the heterogeneous catalyst showed excellent activity
with the substrate where the homogenous VO(acac)2 cata-
lyst failed instead. Characterization of V@acac-CTF re-
vealed that coordinative interaction between V(IV) with
the O^O site of the acetyl acetone group, high surface area,
robustness of the material and efficient diffusion of sub-
strate and products caused such high reactivity. V@acac-
CTF not only showed excellent reactivity, but also allowed
reusability, making the reaction more sustainable. Fur-
thermore, the expected in situ formation of iminium ions
from tertiary amine oxides in presence of V(IV) anchored
on acetyl acetone functionalized CTF facilitates the poten-
tial application in modified Mannich type reactions. The
above results signify the importance of catalytic binding
site on the porous supports for efficient coordinative in-
teraction and hence greater reactivity. Creating heteroge-
neity showed promising results in the catalytic reactivity
compared to the homogeneous catalyst. These results
show the potential of customized-CTFs with in-built cata-
lytic active sites for sustainable chemistry.
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Table 3. Substrate scope
ASSOCIATED CONTENT
Supporting Information
The Supporting Information contains detailed experi-
mental procedure, elementary analysis data, powder X-ray
diffraction data, thermogravimetric analysis, gas adsorp-
tion data, time dependent reaction profile, leaching test,
13C CP-MAS ssNMR spectral analysis, bright-field scanning
transmission electron microscopy images, XPS spectra,
schemes, proposed mechanism, general catalytic proce-
dure, NMR data and is available free of charge on the ACS
Publication website.
The reactivity of V@acac-CTF with different substituted 2-
naphthol, 1-naphthol, phenol derivatives was also exam-
ined (Table 3) and compared to the homogeneous reaction.
Surprisingly, bromo- (3b), methoxy- (3c), ester-
substituted 2-naphthol (3d) showed above 90% of yield in
8 h, whereas in the homogenous reaction system, <90% of
3b and 3c and only 61% of 3d was reported even after 72
h of reaction. In addition, 1-naphthol, phenol and phenol
derivate showed moderate to good yield under optimized
reaction conditions. The excellent reactivity of V@acac-
CTF compared to that of VO(acac)2 can be attributed to the
robustness of the CTF support, its large surface area, suffi-
cient pore volume that facilitate efficient substrate and
product diffusion and importantly, the presence of a cata-
lytic binding site. Therefore, upon acac functionalization,
the CTF offers a strong coordinative interaction to the
AUTHOR INFORMATION
Corresponding Author
* Pascal.VanDerVoort@UGent.be
Author Contributions
The manuscript was written through contributions of all au-
thors. All authors have given approval to the final version of
the manuscript.
Notes: The authors declare no competing financial interest.
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ACKNOWLEDGMENT
Chemistry of Materials
Ultrahigh Surface Area: Dynamic Reorganization of Porous Poly-
mer Networks, J. Am. Chem. Soc. 2008, 130, 13333-13337.
(19) ꢀRen, S.; Bojdys, M. J.; Dawson, R.; Laybourn, A.; Khim-
yak, Y. Z.; Adams, D. J.; Cooper, A. I. Porous, Fluorescent, Covalent
Triazine-Based Frameworks via Room-Temperature and Micro-
wave-Assisted Synthesis, Adv. Mat. 2012, 24, 2357-2361.
1
H.S.J. thanks FWO [PEGASUS]2 Marie Sklodowska-Curie grant
agreement No 665501 for Incoming postdoctoral fellowship.
C. K. acknowledges the Research Board of Ghent University
(GOA010-17, BOF GOA2017000303) for the PhD funding. G.-B.
Wang thanks the China Scholarship Council (CSC) and UGENT
BOF grant for PhD Funding. K. L. acknowledges the financial
support from Ghent University. The authors thank Funda Aliç
for Elementary analysis and Katrien Haustraete for STEM
measurement. The authors thank the Max Planck Institute of
Colloids and Interfaces for the fruitful discussions
2
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