Catalysis and CO2 Capture by Palladium-Incorporated Covalent Organic Frameworks

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 CO₂capture 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 NaBH₄yields 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 CO₂uptake

of 11 wt % and 7.5 wt% at 273 and 298 K, respectively, at 1 bar.

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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,

LiAlH₄or 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 CO₂capture, 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 CO₂capture 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 CO₂capture.^[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 CO₂adsorption 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^-1and hydroxyl (-OH) stretching at 3352 cm^-1, whereas

TAT-TFP (2) exhibits two bands at 1620 and 1572 cm^-1corresponding 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^-1are 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 N₂adsorption 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 N₂adsorption 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^-1and 646 m²g^-1. The surface area of these

COFs are high and are comparable to other reported structurally similar COFs (CCOF-1, 266

m²g^–1and 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 CO₂etc.

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)₂at 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

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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)

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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)₂exhibits carbonyl

carbon resonance at 190 ppm.^[5f]Thus, the observed upfield shift of 11 ppm confirms the strong

binding of Pd(OAc)₂with 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 CH₃resonances 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 3d_5/2at 337.9 and 337.86 eV, respectively, while the peaks for Pd 3d_3/2are

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)₂with

the framework.^[5f,i]Pd(0)/TAT-DHBD (5) and Pd(0)/TAT-TFP (6) further exhibit

characteristic peaks for Pd 3d_5/2at 335.5 and 335.8 eV, whereas the peaks for Pd 3d_3/2are

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 NaBH₄for 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 3d_3/2, Pd 3d_5/2, N 1s and O 1s from the XPS

COFs/Elements

Pd 3d_3/2

-

Pd 3d_5/2

-

N 1s

O 1s

398.6

532.2

532.1

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

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)₂the 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

S_BET(m²g^-1)

139.0

S_Lang(m²g^-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 K₂CO₃in

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]

CO₂uptake 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 CO₂uptake. In order to evaluate the CO₂uptake, 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 CO₂uptake 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 CO₂uptake 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 CO₂uptake while Pd(0)/TAT-TFP (6) exhibited 5.8 and

3.8 wt% of CO₂uptake at 273 and 298 K, respectively. This CO₂uptake 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 CO₂uptake. 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 CO₂isotherms are completely reversible for all the

COFs, which indicates that the interaction between CO₂and the COFs is weak and the material

can be easily regenerated without any external stimuli. The isosteric heats of adsorption (Q_st)

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. CO₂sorption 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

CO₂uptake at

273 K (wt%)

5.98

CO₂uptake at

298 K (wt%)

3.46

Q_st(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. CO₂adsorption 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 CO₂capture 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, CO₂uptake.

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Catalysis and CO2 Capture by Palladium-Incorporated Covalent Organic Frameworks

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