Catalysis and CO2 Capture by Palladium-Incorporated Covalent Organic Frameworks

Article abstract of DOI:10.1002/cplu.201700342

Triazine-based imine and β-ketoenamine linked covalent organic frameworks (COFs), TAT-DHBD (1) and TAT-TFP (2), have been synthesized from 1,3,5-tris-(4′-aminophenyl)triazine (TAT) and 2,5-dihydroxybenzene-1,4-dicarboxaldehyde (DHBD) or 1,3,5-triformylphloroglucinol (TFP) under solvothermal conditions in dioxane/mesitylene mixture. These COFs exhibit significant surface areas owing to their meso- and micropores. The presence of basic nitrogen sites offers excellent affinity towards Pd nanoparticles and carbon dioxide. Post treatment of COFs 1 and 2 with palladium acetate gives Pd^II/TAT-DHBD (3) and Pd^II/TAT-TFP (4), which on reduction by NaBH₄ yields Pd⁰/TAT-DHBD (5) and Pd⁰/TAT-TFP (6), respectively. The new COFs have been characterized by FTIR, solid-state ¹³C NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), SEM, TEM, and Brunauer–Emmett–Teller (BET) surface area measurements. TEM studies corroborated the uniform distribution of Pd^II and Pd⁰ sites in the COFs. Compounds 3–6 are active towards the Suzuki–Miyaura cross-coupling of arenes with meager catalyst leaching even after five cycles. In addition, 4 exhibits CO₂ uptake of 11 wt % and 7.5 wt % at 273 and 298 K, respectively, at 1 bar.

Full text of DOI:10.1002/cplu.201700342

Accepted Article
Title: Catalysis and CO₂ capture by palladium incorporated covalent
organic frameworks
Authors: Dhananjayan Kaleeswaran, Rajendran Antony, Abhishek
Sharma, Ateeque Malani, and Ramaswamy Murugavel
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Catalysis and CO2 capture by palladium incorporated covalent organic
frameworks
Dhananjayan Kaleeswaran,[a] Rajendran Antony,[a] Abhishek Sharma,[b,c,d] Ateeque Malani[b]
and Ramaswamy Murugavel*[a]
[a]
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076.
Maharashtra.
[b]
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai
400076, India.
[c] IITB-Monash Research Academy, Indian Institute of Technology Bombay, Mumbai 400076, India.
[d]
Commonwealth Scientific and Industrial Research Organisation (CSIRO) Manufacturing, Clayton,
Victoria 3169, Australia.
Abstract
Triazine based imine and β-ketoenamine linked COFs, TAT- DHBD (1) and TAT-TFP (2)
have been synthesized from 1,3,5-tris-(4-aminophenyl)triazine (TAT) and 2,5-
dihydroxybenzene-1,4-dicarboxaldehyde (DHBD) or 1,3,5-triformylphloroglucinol (TFP)
under solvothermal conditions in dioxane/mesitylene mixture. These COFs exhibit significant
surface area due to their meso- and micropores. The presence of basic nitrogen sites offers
excellent affinity towards Pd nanoparticles and carbon dioxide. Post treatment of COFs 1 and
2 with palladium acetate gives Pd(II)/TAT-DHBD (3) and Pd(II)/TAT-TFP (4), which on
reduction by NaBH4 yields Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6), respectively. The
new COFs have been characterized by FT-IR, solid-state 13C-NMR spectroscopy, XPS, SEM,
TEM and BET surface area measurements. TEM studies corroborated uniform distribution of
Pd(II) and Pd(0) sites in the COFs. 3-6 are active towards the Suzuki-Miyaura cross coupling
of arenes with meager catalyst leaching even after five cycles. Besides, 4 exhibits CO2 uptake
of 11 wt % and 7.5 wt% at 273 and 298 K, respectively, at 1 bar.
1
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Introduction
Covalent organic frameworks (COFs) are evolving as a special class of thermally stable
crystalline porous polymers. Composed of light atoms such as B, C, N, O and H and linked via
strong covalent bonds, enable them to function as low density materials.[1] Judicious selection
of organic building units and the reversible dynamic bond formation process paves way for
fine tuning surface area, crystallinity, pore size and structural topology of these
materials.[1b,c,e,f,i] In the case of two-dimensional (2D) COFs, the organic building units
integrate into a 2D-layered sheet which are stacked together with the help of π-π interactions
and form periodic columnar π-arrangements with oriented one-dimensional (1D) channels
ranging from a few angstroms to a few nanometers.[2] The ordered π-arrangements and the
accessible channels in the COFs offer opportunities for potential applications in the field of gas
storage and separation,[3] drug delivery,[4] catalysis,[5] energy storage and conversion,[6]
environmental remediation,[7] membrane separation[8] and proton conduction.[9] Further, COFs
decorated with fluorescent organic building units find applications in the area of sensing,[10]
optoelectronics,[11] semiconductive and photoconductive devices.[12]
The excellent stability and the availability of open cavities present in these COFs render
them as outstanding supports for anchoring catalytically important metal nanoparticles.[5f],[13]
Further, the ordered and oriented 1D channels and inherent covalent bond design in the COFs
make them better scaffolds than porous organic polymers (POPs) and metal organic
frameworks (MOFs) for immobilizing the nanoparticles for efficient heterogeneous
catalysis.[14] Ultra-small metal nanoparticles can be introduced within the COF cavities through
a confined synthesis approach.[5f-h],[13k],[15] Remarkably, the COF scaffolds maintain framework
integrity and crystallinity during the treatment with harsh reducing agents such as NaBH4,
LiAlH4 or hydrazine to immobilize metal nanoparticles.[5i-k],[13k] Among various transition
metal nanoparticles loaded COFs that are explored as heterogeneous catalysts, palladium
loaded COFs are particularly in high demand and are probed because of the tendency of Pd to
make strong bonds with carbon.[16,17]
Imine linked COFs have served as excellent supports for the immobilization of metal
nanoparticles. For example, Ding and others have demonstrated various C-C bond forming
reactions and tandam reactions with Pd incorporated COFs.[5f-k],[13k] One of the advantages of
imine linked COFs is their stability towards water, acids and bases, apart from the strong
interaction of Pd with imine nitrogen atom.[6c,e],[18] Because of this interaction, there has been
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a lot of interest to investigate N-rich COFs which contain a triazine core. Pd loaded N-rich
COFs prevent leaching of Pd from the COF materials and increase the re-usability of catalyst
without compromising on the conversion and product yield.[13k],[19]
In addition to the catalysis, carbon dioxide capture and sequestration (CCS) is another
important area of interest for both academicians and industrial communities.[20],[21] Current
technologies for CCS implement aqueous amine solutions such as monoethanolamine,
methyldiethanolamine and piperazine as liquid adsorbents.[22] The disadvantages of this
method is the consumption of high energy during regeneration of trapped CO2, as the CO2
chemically interacts with amine solutions, apart from the toxicity, high volatility and
corrosivity of the amine solutions.[23] In order to overcome this, new alternatives have been
suggested for CO2 capture, e.g. various functionalized porous materials such zeolites,[24]
carbon-based sorbents,[25] metal–organic frameworks (MOFs),[26] covalent organic frameworks
(COFs),[3] mesoporous materials and porous organic frameworks (POFs).[27] Among these
materials, COFs have been extensively utilized for the CO2 capture as they are highly stable
(chemically and thermally). On the other hand, the building units of the COFs have been
designed rationally with CO2-philic nitrogen atoms. The ordered cavities can also be post
modified with suitable functional groups to enhance the CO2 capture.[3d,g,l] Recent work from
our group has highlighted the importance of imine and aminal functional groups for CO2
capture.[10b] In the present investigation, we have examined imine and β-ketoenamine based
triazine functionalized COFs TAT-DHBD (1) and TAT-TFP (2) as suitable scaffolds for
incorporating Pd nanoparticles, and have further studied them as catalysts for C-C coupling
reactions. We also demonstrate that these nitrogen rich COFs can be further used as CO2
absorbing materials.
Results and Discussion
As shown in Scheme 1, the imine and β-ketoenamine linked COFs, namely TAT-
DHBD (1) and TAT-TFP (2), were synthesized through condensing the amine (TAT) with
respective aldehydes (DHBD and TFA) under solvothermal conditions using
dioxane/mesitylene (1:1) mixture as solvent and aqueous acetic acid (6M) as catalyst in sealed
glass tubes for three days at 120 °C. The COFs were washed with copious amount of acetone,
THF and dichloromethane to ensure the complete removal of soluble impurities. Further, the
COFs were soaked in acetone (2 days) followed by THF (2 days) for solvent exchange. During
the course of the solvent exchange, the respective solvent was replaced after every 6 h.
Subsequently each of the sample was heated at 120 °C under vacuum. The resultant COFs were
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characterized by elemental analysis, FT-IR and cross polarization magic angle spinning (CP-
MAS) 13C NMR spectroscopy, thermal gravimetric analysis, powder X-ray diffraction and N2
and CO2 adsorption measurements.
The formation of new bonds and the structural integrity of the COFs have been
investigated by FT-IR spectroscopy. TAT-DHBD (1) shows characteristic absorptions for
azomethine (C=N) linkage at 1623 cm-1 and hydroxyl (-OH) stretching at 3352 cm-1, whereas
TAT-TFP (2) exhibits two bands at 1620 and 1572 cm-1 corresponding to carbonyl (C=O) and
–C=C- stretching vibrations arising out of keto-enol tautomerism between the hydroxyl group
of phloroglucinol and the azomethine group.[18],[6c,e] In addition, both the COFs exhibit C-N
stretching vibration at 1293 and 1294 cm-1, the aromatic C=C stretching at 1432 and 1455 cm-
1
and the aromatic C-H stretching bands around 3050 cm-1, respectively. The stretching
frequencies of COFs have been compared, in Figure 1, with the model Schiff base, TAT-Sal
(7), which also exhibits similar vibrations. Besides, the attenuated bands around 3400-3300
and 1700 cm-1 are attributed to terminal amine and aldehyde groups.[28]
Figure 1. FT-IR spectra of TAT-Sal (7) (see ESI), TAT-TFP (2) and TAT-DHBD (3).
To further confirm the formation of imine and β-ketoenamine linkages, and the integrity
of framework structure, CP-MAS 13C NMR spectra of the solid samples have been investigated
(Figure 2). The azomethine (C=N) carbon atoms in 1 resonate at 145 ppm while the aminal
(CH-NH) carbons in 2 appear at 150 ppm. The resonance at 158 ppm is attributed to the carbon
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atom attached to the hydroxyl group in TAT-DHBD (1) whereas the peak at 183 ppm
corresponds to the carbonyl carbon atom of the TAT-TFP (2) due to keto-enol tautomerism.[18]
Both the COFs exhibit a resonance at 167 ppm corresponding to the carbon atom of the triazine
core. The peaks in the range between 137 and 105 ppm are attributed to aromatic carbon atoms
of the COFs (Figure 2).
Figure 2. CP-MAS 13C NMR spectra of TAT-DHBD (1) and TAT-TFP (2).
Thermogravimetric analysis of the new COFs were investigated by heating the
activated samples under continuous flow of nitrogen at the heating rate of 10 °C/minute (Figure
S3, SI). TAT-DHBD (1) is found to be stable up to 250 °C whereas TAT-TFP (2) exhibits an
extended stability up to 400 °C. To confirm the hydrolytic and chemical stability, the COFs are
dispersed in water, acid (1M) and base (9M) for seven days and characterized by the IR
spectroscopy. The IR spectra of COFs exhibit similar spectral features of as-synthesized COFs,
confirming that the structural integrity of the COFs is maintained even after soaking them in
water, acid or base (Figure S4-S5, SI). The crystallinity of the pristine COFs were examined
by PXRD measurements after treating them with water, HCl and NaOH (Figure S6, SI). While
both COFs have been found to be perfectly stable in water. Further, while COF 2 exhibits no
loss, a partial loss of crystallinity was observed for COF 1 when treated with HCl or NaOH.
The crystalline nature of the COFs were investigated by PXRD studies. The PXRD
pattern reveals that the as synthesised COFs are highly crystalline with periodic hexagonal
pores (Figure 3a and 3d). TAT-DHBD (1) exhibits peaks at 2 = 2.8, 4.9, 5.6, 7.4, 9.8 and
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26.3° corresponding to 100, 110, 200, 120, 220 and 001 planes, respectively. TAT-TFP (2)
similarly exhibits peaks at 2 = 5.8, 10.0, 15.1 and 26.5° for the 100, 110, 210 and 001 planes,
respectively.[29] The π-π interaction between the two sheets are derived from the d spacing of
the 001 plane (26.5°) and it is calculated to be ~3.4 Å for both the COFs.
In order to glean more information from the above experimental PXRD results and
determine the type of stacking, molecular dynamics (MD) simulations were performed to
obtain relaxed configuration of both the COFs using the Forcite module of Materials Studio
(see SI).[30] We used various configurations obtained during the last 1 ns of the MD simulation
and calculated the theoretical XRD patterns of the COFs using the Reflex module, resulting in
good agreement with the experimental PXRD patterns (Figure 3). The combined PXRD and
MD analysis establish slipped arrangement of both the COFs with stacking fault (Figure 3b, c
and 3e, f), as COFs are energetically stable in slipped configurations.[31]
Figure 3. Comparison of the experimental and simulated PXRD pattern of (a) TAT-DHBD (1)
and (d) TAT-TFP (2). Packing diagrams (b) and (c) show two perpendicular views of a minor
slipped stacking of COF layers in 1. Similarly (e) and (f) exhibit the slipped arrangement of 2
in perpendicular directions.
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Figure 4. SEM images of (a) TAT-DHBD (1) and (b) TAT-TFP (2) and TEM images
of (c) TAT-DHBD (1) and (b) TAT-TFP (2).
The morphology of the as-synthesized COFs were probed by field emission gun-
scanning electron microscopy (FEG-SEM). Both the COFs exhibit similar morphology, where
smaller spherical particles are connected to each other to form several hundred nanometer sized
noodle like structures (Figure 4a and 4b). Transmission electron microscopy images of both
the COFs exhibit layered structure which further confirms the formation of 2D- sheets with π-
π stacking (Figure 4c and 4d).
The permanent porosity and pore size distribution of the COFs have been investigated
by measuring N2 adsorption isotherms at 77 K (Figure 5a). The nitrogen isotherm of TAT-TFP
(2) exhibits classical Type I isotherm with increased nitrogen uptake at low partial pressures
(P/Po = 0.0 to 0.1 bar), indicating the presence of permanent microporosity with reversible
adsorption-desorption processes. On the other hand, the N2 adsorption isotherm of TAT-DHBD
(1) exhibits typical Type IV isotherm indicating the presence of mesopores in the later. In
addition, the isotherm exhibits a hysteresis in the low pressure region (P/Po < 0.30), suggesting
a swelling effect under liquid nitrogen temperature [32] and significant interaction between the
COF and the nitrogen gas. At higher pressures (P/Po > 0.10), nitrogen uptake increases due to
capillary condensation.[1a] The Brunauer−Emmett−Teller (BET) surface areas of TAT-DHBD
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(1) and TAT-TFP (2) were estimated to be 750 m2g-1 and 646 m2g-1. The surface area of these
COFs are high and are comparable to other reported structurally similar COFs (CCOF-1, 266
m2g–1 and CCOF-2, 335 m2g–1;[5e] COF-LZU1, 410 m2g–1;[5f] 2,3-DmaTph, 668 m2g–1;[5g]
TpPa-1, 484 m2g–1;[5i] trzn-COF, 408 m2g–1;[13k] TRIPTA, 609 m2g−1;[29] ). The pore size
distribution (PSD) of the COFs has been evaluated from Non-Local Density Functional Theory
(NLDFT) by fitting to the adsorption branch of nitrogen isotherm. The PSD study reveals the
pore diameter of TAT-TFP (2) to be 11.7 Å whereas TAT-DHBD (1) exhibits two pore
diameters of 26.1 and 37.9 Å, consistent with the Type I and IV isotherms exhibited by these
two COFs, respectively (Figure 5b). The pore diameters obtained from the NLDFT method
compare well with the values of 11.8 and 25.6 Å obtained for COFs 2 and 1 from MD simulated
structures. The other pore diameter of 37.9 Å obtained from NLDFT presumably arises due to
swelling effect at 77 K, which is seen from the hysteresis at low pressure region. The pore
volumes of TAT-TFP (2) and TAT-DHBD (1) were calculated to be (from the nitrogen gas
adsorbed at P/P0 = 0.99) 0.36 and 0.44 cc g-1, respectively. These results suggest that the pores
in the TAT-TFP (2) and TAT-DHBD (1) would be accessible for hosting various gases such
as CO2 etc.
Figure 5. (a) Nitrogen sorption isotherm and (b) pore size distribution for TAT-DHBD (1)
and TAT-TFP (2).
The high surface area, pore diameter, pore volume and the presence of nitrogen atoms
in the form of imine and triazine functionalities prompted us to incorporate palladium metal
ion or palladium nanoparticles in the COF materials and utilize the later in the catalysis. The
Pd2+ incorporated COFs Pd(II)/TAT-DHBD (3) and Pd(II)/TAT-TFP (4) have been prepared
by stirring TAT-DHBD (1) and TAT-TFP (2) with Pd(OAc)2 at room temperature for 24 h.
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The palladium nanoparticles embedded COFs Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6)
have been synthesized by stirring a respective COFs in a methonalic solution of Pd(OAc)2
followed by addition of excess amount of NaBH4. The Pd loaded COFs have been characterized
13
by elemental analysis, FT-IR, CPMAS C-NMR, TGA, PXRD, FE-SEM, EDAX analysis,
HR-TEM and nitrogen sorption experiments.
FT-IR spectra of the pristine COFs 1 and 2 and Pd incorporated COFs 3-6 exhibit
similar vibrational bands that correspond to the triazine core and imine functionalities (Figure
6), suggesting that the COF structures are preserved even after incorporation of Pd(II) or Pd(0)
13
in their framework. The C CP/MAS NMR spectra of Pd(II)/TAT-DHBD (3), Pd(II)/ TAT-
TFP (4), Pd(0)/ TAT-DHBD (5) and Pd(0)/ TAT-TFP (6) (Figure 7) are almost identical to
those of the pristine COFs 1-2 in the phenyl region, also confirming the preservation of pristine
COF structures after the metal incorporation. In the case of Pd(II)/ TAT- DHBD (3) and Pd(II)/
TAT-TFP (4), two additional minor peaks appear at 179 and 23 ppm, corresponding to the
carbonyl and methyl groups of the acetate ligand in Pd(OAc)2. Free Pd(OAc)2 exhibits carbonyl
carbon resonance at 190 ppm.[5f] Thus, the observed upfield shift of 11 ppm confirms the strong
binding of Pd(OAc)2 with COFs. Thermogravimetric analysis of Pd incorporated COFs exhibit
gradual and continuous weight-loss above 140 °C under nitrogen atmosphere, similar to the
thermal behaviour of other Pd loaded COFs described earlier (Figure S15, SI).[5f,i],[13k]
Figure 6. FT-IR spectra of a) TAT-DHBD (1), Pd(II)/TAT-DHBD (3) and Pd(0)/TAT-DHBD
(5); b) TAT-TFP (2), Pd(II)/TAT- TFP (4) and Pd(0)/TAT- TFP (6).
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Figure 7. CP-MAS 13C NMR spectra of Pd(II)/TAT-DHBD (3), Pd(II)/TAT-TFP (4),
Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6) (* represents CH3 resonances at 23 ppm and #
represents C=O resonance 179 ppm).
The observed PXRD pattern of each of the Pd loaded COFs is almost identical to the
corresponding pristine COFs, suggesting only a minor loss of crystallinity of the COFs after
Pd incorporation (Figures 3 and Figure S17, SI). In the case of Pd(0)/ TAT-DHBD (5) and
Pd(0)/ TAT-TFP (6), additional peaks appear at 2 = 40° and 47.5°, due to the presence of
embedded metallic Pd nanoparticles in the COF structures.[13k] Since Pd nanoparticles are
incorporated between the interlayers of 2D sheets, it affects the π-π stacking, which is evident
from the slight broadening of the (001 plane) peaks at 2 = 26.5°.[5i]
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Figure 8. SEM images of Pd(II)/TAT-DHBD (3), Pd(II)/TAT-TFP (4), Pd(0)/TAT-DHBD (5)
and Pd(0)/TAT-TFP (6) ((a), (c), (e), (g)) and their respective EDS for Pd ((b), (d), (f), (h))
Figure 9. TEM images of (a) Pd(II)/TAT-DHBD (3), (b) Pd(II)/TAT-TFP (4), (c) Pd(0)/TAT-
DHBD (5) and (d) Pd(0)/TAT-TFP (6).
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FE-SEM analysis indicate that the morphology of the Pd incorporated COFs are also
identical to those of the corresponding pristine COFs (Figure 8, left column), further suggesting
the structural integrity of COFs even after treatment with Pd(OAc)2. Energy dispersive X-ray
analysis (EDX)-elemental mapping of Pd incorporated COFs shows that the Pd nanoparticles
are homogeneously dispersed over the frameworks (Figure 8, right column). The Pd loadings
in the COFs has also been evaluated from the EDX analysis.[33] It is found that 10.5 and 14.9
wt % of Pd is loaded in Pd(II)/TAT-DHBD (3) and Pd(II)/TAT-TFP (4), while 11.4 and 10.6
wt % of Pd is loaded in the case of Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6). Similar
results were found from the TGA traces of Pd incorporated COF under air (3, 7.5 wt %; 4,
18.3 wt %; 5, 15.7 wt % and 6, 9.9 wt %) (Figure S16, SI). FEG-TEM studies reveal that the
layered-structure of pristine COFs, which once again remained unchanged even after Pd
nanoparticles incorporation. Besides, Pd(0) nanoparticles (< 5 nm) are uniformly dispersed in
the pores of the COFs (Figure 9 and Figure S18, SI). The TEM analyses of Pd(II) incorporated
COFs 3 and 4 exhibit aggregation of bigger nanoparticles. It appears that, due to high
acceleration potential of 300 kV used for TEM studies, conversion of Pd(II) to Pd(0) takes
place (Figure 9a and 9b). Similar reduction behaviour under TEM beam has been observed in
other Pd-COFs earlier in the literature.[5i]
To further investigate the incorporation of Pd nanoparticles into the COFs and as well
as to establish the oxidation state of Pd species in the framework, X-ray photoelectron
spectroscopy (XPS) measurements have been carried out. The binding energies obtained are
presented in Table 1. Pd(II)/TAT-DHBD (3) and Pd(II)/TAT-TFP (4) exhibit characteristic
peaks for Pd 3d5/2 at 337.9 and 337.86 eV, respectively, while the peaks for Pd 3d3/2 are
observed at 343.2 and 343.1 eV, respectively (Figure 10a and 10c). This confirms the presence
of Pd in +2 oxidation state. The observed binding energy values are negatively shifted by ~0.5
eV in comparison to free Pd(OAc)2 (338.4 eV), suggesting strong bonding of Pd(OAc)2 with
the framework.[5f,i] Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6) further exhibit
characteristic peaks for Pd 3d5/2 at 335.5 and 335.8 eV, whereas the peaks for Pd 3d3/2 are
observed at 340.7 and 341.1 eV, respectively, confirming that Pd is present in the zero
oxidation state.[5i],[13k] Besides, slender humps are observed at 337.0 and 343.1 eV due to the
incomplete reduction of Pd(II) by NaBH4 for both 5 and 6.
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Figure 10. XPS spectra of (a) Pd(II)/TAT-DHBD (3) and Pd(0)/TAT-DHBD (5) in Pd 3d
region; (b) TAT-DHBD (1), Pd(II)/TAT-DHBD (3) and Pd(0)/TAT-DHBD (5) in N 1s region;
(c) Pd(II)/TAT-TFP (4) and Pd(0)/TAT-TFP (6) in Pd 3d region and (d) TAT-TFP (2),
Pd(II)/TAT-TFP (4) and Pd(0)/TAT-TFP (6) in N 1s region.
Table 1. Binding energies of the Pd 3d3/2, Pd 3d5/2, N 1s and O 1s from the XPS
COFs/Elements
Pd 3d3/2
-
Pd 3d5/2
-
N 1s
O 1s
398.6
532.2
532.1
532.1
531.9
531.9
532.4
TAT-DHBD (1)
343.2
340.7
-
337.9
335.5
-
398.6, 400.0
398.6, 399.9
398.5, 399.7
398.7, 400.1
398.7, 400.1
Pd(II)/TAT-DHBD (3)
Pd(0)/TAT-DHBD (5)
TAT-TFP (2)
343.1
341.1
337.8
335.8
Pd(II)/TAT-TFP (4)
Pd(0)/TAT-TFP (6)
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N 1s XPS spectrum of TAT-DHBD (1) exhibits only one peak at 398.6 eV, which is
assigned to the triazine and imine nitrogen (both the nitrogen atoms are chemically equivalent
(C=N)), while TAT-TFP (2) exhibits two peaks at 399.7 (aminal nitrogen) and 398.5 eV
(triazine core). After the treatment of Pd(OAc)2 the peaks for the N 1s in Pd(II)/TAT-DHBD
(3) and Pd(II)/TAT-TFP (4) are shifted to 399.9 and 400.1 eV, respectively (Figure 10b and
10d). This observation confirms the coordination of imine and aminal nitrogen to Pd. The non-
participation of phenolic group in coordination has been confirmed from the O 1s XPS spectra
of TAT-DHBD (1), Pd(II)/TAT-DHBD (3) and Pd(0)/TAT-DHBD (5) since all the three COFs
exhibit a peak at ~532.2 eV, with no observable shifts (Figure S20b and S21b, SI).
The permanent porosity and pore size distribution of the Pd incorporated COFs have
been evaluated by nitrogen sorption measurements at 77 K and 1 bar (Figure 11). The BET
surface area of the COFs has significantly reduced on metal loading, suggesting that the Pd
nanoparticles are presumably embedded within the pores of the COFs (Table 2). Pore size
distribution of the Pd loaded COFs have also been obtained from NLDFT calculations by fitting
the adsorption branch of the nitrogen isotherm. From the above studies (such as SEM, TEM,
XPS and surface area analysis), it is clear that Pd(II) and Pd(0) are finely dispersed inside the
pore surface and also in between the layers of the COF matrix.
Figure 11. (a) Nitrogen sorption isotherms and (b) Pore size distribution of Pd(II)/TAT-DHBD
(3), Pd(II)/TAT-TFP (4), Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6).
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Table 2. Nitrogen sorption data at 273 K and 1 bar.
COFs
SBET (m2 g-1)
139.0
SLang (m2 g-1) Pore volume (cc g-1)
331.1
388.0
76.0
0.143
0.145
0.074
0.094
Pd(II)/TAT-DHBD (3)
Pd(II)/TAT-TFP (4)
Pd(0)/TAT-DHBD (5)
Pd(0)/TAT-TFP (6)
292.0
21.5
52.2
159.4
Catalytic studies
The catalytic activity of Pd(II)/TAT-DHBD (3), Pd(II)/TAT-TFP (4), Pd(0)/TAT-
DHBD (5) and Pd(0)/TAT-TFP (6) has been examined for the carbon-carbon bond forming
reactions through Suzuki-Miyaura coupling reaction. This reaction is mostly performed under
homogeneous medium to achieve effective C-C bond formation.[17a-b] However, the inability
to recycle the catalyst has remained a major disadvantage when this reaction is carried out
under homogeneous reaction conditions. To obviate this shortcoming, there have been reports
in recent times where the catalyst has been embedded within solid matrices such MOFs and
COFs.[5],[14] Hence, the Pd incorporated COFs 3-6 have been utilized as active catalysts for C-
C bond formation reactions in the present study.
The initially model reaction was performed with phenyl boronic acid (1.5 mmol) and
bromobenzene (1 mmol) using K2CO3 (2 mmol) as a base and the COF catalyst (10 mg) in
dimethylformamide (4 mL) at 100 °C for 24 h. The conversion of the reaction was monitored
with the help of GC-MS and it was found that complete conversion was achieved within 24 h
(Table 3). The catalytic activity of Pd-COFs was initially tested with unsubstituted bromo
benzene, for which only moderate catalytic activity was observed. To check the selectivity in
terms of homo/hetero coupling, substituted bromobenzenes were then employed. More than 80
% hetero-coupling products were identified in all the cases, revealing that the present catalysts
are selective towards the desired heterocoupled products. The effect of substituents was also
explored and it was established that the catalysts are more active when electron withdrawing
substituent (-CN) was present on the aryl ring. Significantly, complete conversion (100 %) has
been achieved within 4 h when 4-cyano bromobenzene was used (Table 3).
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Table 3: Catalytic activity of Pd(0)/Pd(II)-COFs in Suzuki-Miyaura coupling reaction
Selectivity (%)
Conversion
(%)
Catalyst
R
a
b
H
56
-
CH3
OCH3
CN
34
24
15
24
-
76
85
76
-
Pd(II)/TAT-DHBD (3)
59
100
80
H
CH3
OCH3
CN
60
25
19
-
75
81
100
Pd(II)/TAT-TFP (4)
Pd(0)/TAT-DHBD (5)
Pd(0)/TAT-TFP (6)
67
100
62
H
-
CH3
-OCH3
-CN
-H
49
20
11
3
80
89
97
64
100
83
-
-CH3
-OCH3
-CN
69
30
16
1
70
84
99
73
100
Reaction conditions: COF catalyst (10 mg), boronic acid (1.5 mmol), bromo compound (1
mmol), K2CO3 (2 mmol), dimethylformamide (4 ml), temperature (100 oC) and time (24 h).
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These Pd-COFs have been investigated for their reusability for five successive
(repeated) catalytic runs (Figure 12). To perform the reusability study, Pd(II)/TAT-TFP (4) and
Pd(0)/TAT-TFP (6) catalysts have been chosen as they exhibit better catalytic activity
compared to Pd(II)/TAT-DHBD (3) and Pd(0)/TAT-DHBD (5). Thus, at the end of each run
the catalyst was repeatedly washed with water, ether and methanol and dried at 110 oC prior to
every successive catalytic run. From this study, it emerges that these catalysts (4 and 6) are
indeed reusable even after the fifth catalytic run (note that after the second run, the conversion
more or less stabilizes to about 85-90%).
Figure 12: Reusability of Pd(II)/TAT-TFP (4) and Pd(0)/TAT-TFP (6) in C-C coupling
reaction between phenyl boronic acid and 4-cyano bromo benzene.
Figure 13. FT-IR spectra of Pd(II)/TAT-DHBD (3), Pd(II)/TAT-TFP (4), Pd(0)/TAT-DHBD
(5) and Pd(0)/TAT-TFP (6) after the catalysis.
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To evaluate the amount of palladium leaching after the third cycle, suitably washed and
dried catalyst (Pd(II)/TAT-TFP (4) or Pd(0)/TAT-TFP (6)) was treated with K2CO3 in
dimethylformamide at 100 oC for 4 h under constant stirring. The catalyst was then filtered and
the solution collected was subjected to Inductively Coupled Plasma - Atomic Emission
Spectroscopy (ICP-AES). These solutions exhibited no evidence for the presence of palladium
species in the solution (< 0.25 ppm). This clearly confirms that there is no leaching of
Pd(0)/Pd(II) from the catalysts that have been already subjected to three runs.
To further ascertain that these solutions contained no palladium, substrates 4-
cyanaobromo benzene and phenylboronic acid were added to the above solution and stirred at
100 oC for 4 h, exhibiting no reaction. The IR spectra of the catalysts have also been compared
before and after the catalytic reactions and found to be similar (Figure 13).
Figure 14. SEM images of Pd(II)/TAT-DHBD (3), Pd(II)/TAT-TFP (4), Pd(0)/TAT-DHBD
(5) and Pd(0)/TAT-TFP (6) ((a), (c), (e), (g)) and their respective EDS for Pd ((b), (d), (f),
(h)) after the catalysis
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Figure 15. TEM images of (a) Pd(II)/TAT-DHBD (3), (b) Pd(II)/TAT-TFP (4), (c)
Pd(0)/TAT-DHBD (5) and (d) Pd(0)/TAT-TFP (6) after three catalytic runs
Figure 16. XPS spectra of (a) Pd(II)/TAT-DHBD (3) and (b) Pd(II)/TAT-TFP (4) in Pd 3d
region after the catalysis.
The FE-SEM analysis shows that the morphology of the Pd loaded COFs remains
unaltered after the catalytic runs. The EDX-elemental mapping analysis reveals that the Pd
nanoparticles are well-dispersed on the COF material and not agglomerated after the reaction
(Figure 14). This has further been confirmed by the TEM analysis. TEM images of the Pd
incorporated COFs after the reaction shows homogeneously dispersed nano particles and the
2D sheets of COFs remain unaltered (Figure 15). In order to evaluate the oxidation state of Pd
in 3 and 4 after three catalytic runs, XPS spectra have been recorded (Figure 16). The results
obtained shows that the oxidation state of a small fraction of Pd centres is converted from +2
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to 0 after catalysis, as has been previously reported for other Pd(II) incorporated polymeric
materials.[34]
CO2 uptake studies
Nitrogen enriched pristine COFs and their Pd immobilized variants are expected to be
CO2-philic. Thus, these COFs has been utilized for the capture of CO2. The Pd loaded polymers
are mostly investigated for the hydrogen storage.[15],[35] Barring a few,[36] less attention has been
given for CO2 uptake. In order to evaluate the CO2 uptake, the measurements were carried out
at two different temperatures (273 and 298 K) and up to 1 bar (Figure 17). The results obtained
are listed in Table 4. TAT-TFP (2) exhibited the highest CO2 uptake of 17.1 wt% and 10.6 wt%
at 273 and 298 K, respectively, at 1 bar, similar to a recent literature report.[29a] In contrast,
TAT-DHBD (1) showed low CO2 uptake of 5.98 wt % and 3.5 wt % at 273 and 298 K,
respectively, at 1 bar. Interestingly, even after immobilizing the Pd metal particles, Pd(II)/TAT-
TFP (4) showed 11.0 and 7.5 wt % of CO2 uptake while Pd(0)/TAT-TFP (6) exhibited 5.8 and
3.8 wt% of CO2 uptake at 273 and 298 K, respectively. This CO2 uptake is higher than that
reported for Pd loaded polymer Azo-CPP-1-Pd (9.17 wt % at 273 K and 1 bar).[35] It can be
thus clearly established that Pd(II) is coordinated to only imine group and not to triazine
functionality, which is still available for the CO2 uptake. In case of Pd(0)/TAT-TFP (6), the
nanoparticles are spread homogeneously all over the framework, masking the available
nitrogen sites for interaction with CO2. The CO2 isotherms are completely reversible for all the
COFs, which indicates that the interaction between CO2 and the COFs is weak and the material
can be easily regenerated without any external stimuli. The isosteric heats of adsorption (Qst)
calculated from Classius-Clayeperon equation is found to vary between 27.2 and 38.4 kJ mol-
1, as derived from the adsorption data collected at 273 and 298 K (Figure 18).
Table 4. CO2 sorption data for TAT-DHBD (1), TAT-TFP (2), Pd(II)/TAT-DHBD (3),
Pd(II)/TAT-TFP (4), Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6)at 1 bar.
COF
CO2 uptake at
273 K (wt%)
5.98
CO2 uptake at
298 K (wt%)
3.46
Qst (kJ/mol)
TAT-DHBD (1)
TAT-TFP (2)
Pd(II)/TAT-DHBD (3)
Pd(II)/TAT-TFP (4)
Pd(0)/TAT-DHBD (5)
Pd(0)/TAT-TFP (6)
27.24
29.96
38.40
29.96
35.29
32.32
17.11
4.55
11.01
4.79
10.66
3.20
7.50
3.09
5.83
3.84
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Figure 17. CO2 adsorption isotherms at (a) 273 K and (b) 298 K for TAT-DHBD (1), TAT-
TFP (2), Pd(II)/TAT-DHBD (3), Pd(II)/TAT-TFP (4), Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-
TFP (6) at 1 bar.
Figure 18. Isosteric heat of adsorption for TAT-DHBD (1), TAT-TFP (2), Pd(II)/TAT-DHBD
(3), Pd(II)/TAT-TFP (4), Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6) at 1 bar.
Conclusions
Dihydroxybenzene-1,4-dicarboxaldehyde (DHBD) and triformylphloroglucinol (TFP) have
been independently combined with tris(aminophenyl)triazine (TAT) to yield either imine or β-
ketoenamine based mesoporous and microporous covalent organic frameworks, TAT-DHBD
(1) and TAT-TFP (2), respectively, exhibiting high surface areas. The presence of abundant
number of pyridinic and imine nitrogen centers in both these COFs apart from a number of
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phenolic –OH groups, render them highly suitable for metal ion / metal nanoparticle
incorporation. Choosing palladium as an example, it has been shown that these COFs are
indeed excellent supports for Pd(II) complexes and Pd(0)-nanoparticles. These Pd loaded COFs
are useful heterogeneous catalysts for the C-C bond forming Suzuki-Miyaura coupling
reaction. It is particularly interesting to note that Pd(0)/TAT-TFP exhibits 100 % substrate
conversion for the cross coupling of 4-cyano-bromobenzene within 4 hours. Besides, the utility
of these Pd loaded COFs for CO2 capture has also been explored.
It appears that the pristine frameworks 1 and 2 are very robust and can host other types
of metal ions through N,O chelation (which has not been realized yet) with metal ions such as
Mn, Co, Ru, etc. We are presently investigating these aspects with the objective of realizing
newer heterogeneous catalysts for other organic transformation.
Experimental section
Methods, Materials and Instruments. See Supporting Information (S1)
Synthesis of TAT-Sal (7). 1,3,5-Tris(4'-aminophenyl)triazine (TAT) (0.050 g, 0.10 mmol) was
dissolved in dry MeOH (20 mL). To this solution, a methanolic solution (in 5.0 mL) of
salicylaldehyde (0.037 g, 0.035 mmol) and 2-3 drops of formic acid (cat.) were added. A
precipitate started to appear upon heating the reaction mixture at reflux conditions for 30
minutes. The heating was continued for another 10 h. The crystalline product formed was
collected by filtration and washed well with MeOH (30 mL x 3). Finally it was dried under
vacuum to obtain off-white crystalline powder (0.04 g, 69 %) suitable for analysis. Mp.; 242
°C. Anal. Calcd for C42H30N6O3: C, 74.45 (75.66); H, 3.86 (4.54); N, 12.94 (12.60). 1H NMR
(400 MHz, CDCl3): δ 13.1 (s, 3H, -OH), 8.86 (d, 6H, ArH), 8.73 (s, 3H, CH=N), 7.48 (d, 9H,
ArH), 7.43 (t, 3H, ArH), 7.07 (d, 3H, ArH), 6.98 (t, 3H, ArH) ppm. 13C{1H} NMR: δ 171.7,
163.9, 161.5, 152.4, 134.8, 133.9, 132.8, 130.6, 121.7, 119.5 and 117.6. FT-IR (KBr diluted
+
disc); 3027, 2923, 2854, 1627(s), 1598, 1504, 1024, 829 cm-1. ESI-MS calcd for C45H33N3O3
[M]+ 666.23, found 667.54 [M+1]+.
Synthesis of TAT- DHBD (1). A mixture of 1,3,5-tris(4'-aminophenyl)triazine (TAT) (0.100
g, 0.28 mmol), DHBD (0.070 g, 0.42 mmol) , 6 M aq. acetic acid (0.3 mL) and dry 1,4-dioxane
(1.5 mL) and mesitylene (1.5 mL) was taken in a glass tube of dimension 10 × 8 mm (o.d. ×
i. d.). The mixture was sonicated for 10 mins to get a clear suspension. The tube was flash
frozen at 77 K (liquid nitrogen bath) by three freeze-pump-thaw cycles and flame sealed under
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vacuum. The sealed tube was heated at 120 ºC for 3 days yielding orange precipitate. The
precipitate was isolated by filtration and washed THF, acetone and dried under high vacuum
for 7 h at 120 ºC to afford TAT-DHBD. Yield - 0.070 g, (64 %). Anal. calcd for C33H21N6O3;
13
found (Calcd): C, 58.07 (72.12); H, 3.47 (3.85); N, 13.79 (15.29). CP-MAS C{1H} NMR
(295 K): δ 160.7, 149.0, 146.0, 139.5, 137.1, 131.7, 128.1, 125.8, 121.3, 27.3, 22.5 ppm. FT-
IR (KBr diluted disc); 3357, 2959, 2923, 2869, 1634, 1598, 1462, 1410, 1168 cm-1.
Synthesis of TAT-TFP (2). This compound was prepared by following procedure described
above. TAT (0.100 g, 0.28 mmol), 1,3,5-triformylphluoroglucinol (0.063 g, 0.28 mmol), 6 M
aq. acetic acid (0.3 mL) and dry 1,4-dioxane (1.5 mL)/mesitylene (1.5 mL). Yield 0.060 g
(43%). Anal. calcd for C30H19N6O3; found (calcd): C, 65.25 (70.44); H, 3.89 (3.74); N, 14.81
13
(16.43). CP-MAS C{1H} NMR (295 K): δ 184.1, 146.4, 137.6, 137.2, 127.3, 120.3, 119.6,
115.5, 106.1 ppm. FT-IR (KBr diluted disc); 2924, 2852, 1619, 1593, 1574, 1455, 1293, 1262
cm-1.
General procedure for Pd(II) incorporated COFs
The COFs 1 and 2 (100 mg) were dispersed in 50 mL of DCM in a 100 mL round
bottomed flask. To this, Pd(OAc)2 (30 mg) was added and stirred in room temperature for 24
hours. The Pd(II) incorporated COFs 3 and 5 were filtered, washed with DCM followed by
acetone and dried in vacuum for 6 h at 120 °C.
Pd(II)/TAT-DHBD (3): Yield-0.115 g (93 %). Anal. calcd for C78H60N12O18Pd3; found
(calcd): C, 46.59 (52.85); H, 2.55 (3.41); N, 11.64 (9.48). CP-MAS 13C{1H} NMR (295 K): δ
179.0, 167.8, 158.3, 150.7, 127.7, 120.1, 117.1, 114.7, 114.2 and 23.1 ppm. FT-IR (KBr diluted
disc); 3366, 2923, 2852, 1740, 1662, 1579, 1505, 1411, 1362, 1176, 1144 and 814 cm-1.
Pd(II)/TAT-TFP (4): Yield-0.120 g (95 %). Anal. calcd for C72H56N12O18Pd3; found (calcd):
C, 47.87 (50.97); H, 3.79 (3.33); N, 11.09 (9.91). CP-MAS 13C{1H} NMR (295 K): δ 182.8,
182.1, 167.8, 145.7, 139.5, 128.6, 118.6, 112.5, 105.1 and 22.3 ppm. FT-IR (KBr diluted disc);
3368, 2922, 1623, 1574, 1508, 1454, 1367, 1286, 1257, 1179, 1145 and 811 cm-1.
General procedure for Pd(0) incorporated COFs
The COFs 1 and 2 (100 mg) were dispersed in 50 mL of methanol in a 100 mL round
bottomed flask. To this, Pd(OAc)2 (30 mg) was added and stirred in room temperature for 1h.
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Then NaBH4 (100 mg) was added by small portion under 0 °C for 30 mins. The mixture was
stirred in room temperature for 24 hours. The Pd(0) incorporated COFs were filtered, washed
with copious amount of water followed by acetone and dried in vacuum for 6 h at 120 °C.
Pd(0)/TAT-DHBD (5): Yield-0.092 g (86 %). Anal. calcd for C33H21N6O3Pd; found (calcd):
C, 54.14 (60.42); H, 3.69 (3.23); N, 13.24 (12.81). CP-MAS 13C{1H} NMR (295 K): δ 167.8,
158.3, 150.7, 127.7, 120.1, 117.1, 114.7 and 114.2 ppm. FT-IR (KBr diluted disc); 3366, 2922,
2852, 1662, 1580, 1505, 1411, 1363, 1177, 1145 and 813 cm-1.
Pd(0)/TAT-TFP (6): Yield-0.090 g (85 %). Anal. calcd for C30H19N6O3Pd; found (calcd): C,
13
52.15 (58.31); H, 4.09 (3.10); N, 11.98 (13.60). CP-MAS C{1H} NMR (295 K): δ 184.0,
167.9, 145.9, 139.5, 128.7, 118.5, 112.0 and 104.9 ppm. FT-IR (KBr diluted disc); 3220, 2920,
1622, 1574, 1558, 1505, 1454, 1367, 1284, 1256, 1178 and 811 cm-1.
Supporting Information
1
Methods, materials, instruments, H and 13C NMR spectra of model compound, FT-IR spectra,
TGA curves, PXRD, EDS mapping, HR-TEM and XPS spectra of all the COFs.
Acknowledgement
This work was supported by (1) SERB, New Delhi (SB/S1/IC-48/2013) and (2) IIT-Bombay
Bridge Funding. R. M. thanks SERB, New Delhi for J. C. Bose Fellowship (SB/S2/JCB-
85/2014). D. K. thanks UGC, New Delhi, and R. A. thanks SERB for NPDF fellowship
(PDF/2017/00037). We thank the Pawsey supercomputing facility of CSIRO for the computing
support.
Keywords: COFs, Palladium, Suzuki-Miyaura coupling, C-C bond formation, CO2 uptake.
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