Next Article in Journal
Technology and Properties of Peripheral Laser-Welded Micro-Joints
Previous Article in Journal
Comparison of the Performances of Different Computational Methods to Calculate the Electrochemical Stability of Selected Ionic Liquids
Previous Article in Special Issue
Study on Porosity in Zinc Oxide Ultrathin Films from Three-Step MLD Zn-Hybrid Polymers
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Materials
Volume 14
Issue 12
10.3390/ma14123214
materials-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Covalent Triazine Frameworks Based on the First Pseudo-Octahedral Hexanitrile Monomer via Nitrile Trimerization: Synthesis, Porosity, and CO2 Gas Sorption Properties

by
Isabelle D. Wessely
1,
Alexandra M. Schade
1,2,
Subarna Dey
3,
Asamanjoy Bhunia
4,
Alexander Nuhnen
3,
Christoph Janiak
3 and
Stefan Bräse
1,5,*
1
Institute of Organic Chemistry (IOC), Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, D-76131 Karlsruhe, Germany
2
Herbstreith & Fox GmbH & Co. KG Pektin-Fabriken, D-75305 Neuenbürg, Germany
3
Institute of Inorganic and Structural Chemistry, Heinrich-Heine-University Düsseldorf, D-40204 Düsseldorf, Germany
4
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Jadavpur, Kolkata 700032, India
5
Institute of Biological and Chemical Systems (IBCS-FMS), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
*
Author to whom correspondence should be addressed.
Materials 2021, 14(12), 3214; https://doi.org/10.3390/ma14123214
Submission received: 17 May 2021 / Revised: 7 June 2021 / Accepted: 7 June 2021 / Published: 10 June 2021
(This article belongs to the Special Issue Advances in Microporous and Mesoporous Materials)
Browse Figures
Versions Notes

Abstract

:
Herein, we report the first synthesis of covalent triazine-based frameworks (CTFs) based on a hexanitrile monomer, namely the novel pseudo-octahedral hexanitrile 1,4-bis(tris(4′-cyano-phenyl)methyl)benzene 1 using both ionothermal reaction conditions with ZnCl2 at 400 °C and the milder reaction conditions with the strong Brønsted acid trifluoromethanesulfonic acid (TFMS) at room temperature. Additionally, the hexanitrile was combined with different di-, tri-, and tetranitriles as a second linker based on recent work of mixed-linker CTFs, which showed enhanced carbon dioxide captures. The obtained framework structures were characterized via infrared (IR) spectroscopy, elemental analysis, scanning electron microscopy (SEM), and gas sorption measurements. Nitrogen adsorption measurements were performed at 77 K to determine the Brunauer-Emmett-Teller (BET) surface areas range from 493 m2/g to 1728 m2/g (p/p0 = 0.01–0.05). As expected, the framework CTF-hex6 synthesized from 1 with ZnCl2 possesses the highest surface area for nitrogen adsorption. On the other hand, the mixed framework structure CTF-hex4 formed from the hexanitrile 1 and 1,3,5 tricyanobenzene (4) shows the highest uptake of carbon dioxide and methane of 76.4 cm3/g and 26.6 cm3/g, respectively, at 273 K.

1. Introduction

Porous solids such as metal-organic frameworks (MOFs) [1,2,3,4], covalent organic frameworks (COFs) [5,6,7,8,9], porous organic polymers (POPs), and microporous organic polymers (MOPs) with adsorption properties due to a high surface area are widely used for gas separation and storage [10,11,12,13]. Especially, porous organic polymers are excellent candidates because of their high thermal and chemical stability, wide synthetic diversity as well as stability against water and acidic conditions [5,14]. A range of MOPs and POPs, which are often differentiated according to their tectons, have been developed, such as, hyper-crosslinked polymers (HCPs) [15,16], polymers of intrinsic microporosity (PIMs) [17,18], porous aromatic frameworks (PAFs) [14,19], conjugated microporous polymers (CMPs) [20,21], porous polymer networks (PPNs) [22,23] or porous covalent triazine-based frameworks (CTFs) [24,25,26,27,28]. Since their first synthesis by Kuhn et al. in 2008 [24,29,30], CTFs have received considerable attention for CO2 adsorption [27,31,32,33,34,35,36,37,38,39,40,41]. Post-combustion capture of CO2 is of great interest since CO2 is one of the main components influencing global warming [42,43]. To improve the CO2 uptake in porous polymers, π-systems and nitrogen atoms have been incorporated to achieve strong electrostatic interactions between the quadrupole moment of CO2 molecules and the heteroatoms or π-clouds of the pore walls also at low pressures [26,31,44].
Kuhn et al. developed an ionothermal synthesis method for trimerizing aromatic nitriles to triazine-based framework structures with permanent porosity and high thermal and chemical stabilities using anhydrous ZnCl2 at high temperatures [24,29,30]. Molten ZnCl2 acts as a solvent for the aromatic nitriles, as a Lewis acid catalyst, and as a pore-forming solvent and, therefore, as a templating agent for the polymerization [24,26,45,46]. Reaction temperatures of around 400 °C lead to lower BET surface areas (<2000 m2/g) than reaction temperatures of around 600 °C (>3000 m2/g possible) [29]. However, decomposition and condensation reactions such as C–H bond cleavage and carbonization occur, leading to a nitrogen deficiency in the elemental composition compared to their idealized structure [29,47].
Cooper et al. developed a method using the strong Brønsted acid trifluoromethanesulfonic acid (TFMS) at room temperature or under microwave conditions to avoid these decomposition reactions [25]. Besides the mild reaction conditions, the CTF synthesis with TFMS exhibits more advantages, such as short reaction times and the absence of ZnCl2 contaminations [48]. In contrast to CTFs formed via ionothermal conditions, the TFMS method provides lower surface areas and reduced nitrogen adsorption [25,29]. CTFs can also be synthesized by Friedel–Crafts reaction, e.g., from cyanuric chloride and aromatic hydrocarbons in the presence of AlCl3 [48,49,50,51,52,53,54,55]. Further, mechanochemical synthesis is a solvent-free alternative for CTF synthesis using the Friedel-Crafts route [56]. Highly crystalline CTFs were obtained due to the control of the nucleation by in situ formation of aldehyde monomers through the controlled oxidation of alcohols. The aromatic dialdehyde is then reacted with terephthalimidamide in a polycondensation reaction in DMSO in the presence of Cs2CO3 under air to form the triazine units. The BET surface areas of the CTFs from this synthetic approach were, however, relatively low (<600 m2/g) [57,58]. Higher surface areas with crystalline CTFs were reported from the condensation of aromatic diamides with P4O10 at 200 °C [59].
To conclude, the preliminary works of Kuhn et al. [24,29,30] and Cooper et al. [25,57] prompted us to investigate these strategies to novel core structures such as the HPX systems introduced by us [60].

2. Materials and Methods

2.1. General Remarks

Solvents, reagents, and chemicals were purchased from Sigma-Aldrich, ABCR, Acros Organics, and Fisher Scientific. All solvents, reagents, and chemicals were used as purchased unless stated otherwise. Absolute solvents were purchased from commercial suppliers (abs. DMF (Acros Organics, Fair Lawn, NJ, USA, <50 ppm water), absolute chloroform (Fischer Scientific GmbH, Nidderau, Germany, extra dry over molecular sieves), abs. NMP (N-methyl-2-pyrrolidinone, Fischer Scientific GmbH, Nidderau, Germany, <50 ppm water)). Reactions with air- or water-sensitive reagents were performed under Argon using standard Schlenk techniques. The synthesis of triazine frameworks under ionothermal conditions was performed in a tube furnace LOBA-1200-50-400-1-OW from HTM Reetz GmbH. The syntheses of 1,4-bis(tris(4′-cyanophenyl)methyl)benzene (1), 4,4′-dicyano-1,1′-biphenyl (3), 1,3,5-tricyanobenzene (4), and tetrakis(4-cyanophenyl)methane (5) are given in the Supplementary Information.

2.2. Gas Adsorption

Nitrogen sorption isotherms for CTF-hex2 to CTF-hex5 at 77 K were obtained using a NOVA-4000e instrument and a Thermo Scientific gas-adsorption-porosimeter for CTF-hex1. DFT calculations for the pore size distribution curves were done with the native ASWin 2.03 software from Quantachrome Instruments using the ‘N2 at 77 K on carbon, slit pore, nonlinear density functional theory (NLDFT) equilibrium’ model as well as the ‘N2 at 77 K on carbon, slit pore, quenched solid density functional theory (QSDFT) adsorption branch and equilibrium’ model, which is favorable for disordered micro/mesoporous carbon materials. CO2 and CH4 (and N2 for CTF-hex6) sorption isotherms were measured with a Micromeritics ASAP 2020 automatic gas sorption analyzer. The instrument is equipped with oil-free vacuum pumps, which deliver an ultimate vacuum of less than 10−8 mbar) and valves to allow contamination-free measurements. All gases (H2, He, N2, CO2, and CH4) were of ultrahigh purity (UHP, grade 5.0, 99.999%), and the standard temperature and pressure (STP) gas uptake volumes are reported in line with the NIST standards, which are at 293.15 K and 101.325 kPa. N2 sorption isotherms were recorded at 77 K (liquid nitrogen cooling). CO2 and CH4 sorption isotherms were measured at 293 ± 1 K and 273.15 K with the temperature set by a passive thermostat and an ice/deionized water bath, respectively. The density functional theory (DFT) pore size distributions from CO2 were based on the ‘NLDFT slit pore’ model using the ASAP 2020 v3.05 software.

2.3. Synthesis of CTF-hex1–6

General procedure with trifluoromethanesulfonic acid:
Under an argon atmosphere in a closed 20 mL vial, trifluoromethanesulfonic acid and chloroform (3.0 mL) were cooled to 0 °C. At this temperature, BTB-nitrile 1 (1.00 eq) and the respective aryl nitrile linker (3.00 eq for di-, 2.00 eq for tri-, and 0.60 eq for tetratopic tectones) dissolved in 10 mL chloroform were added over 30 min. The mixture was stirred for another 2 h at 0 °C and afterward at room temperature overnight. Then, the reaction mixture was poured on a water/NH3(aq)-mixture (100 mL, 20:1) and stirred at room temperature for an additional 2 h. The precipitate was filtered off, washed with distilled water (3 × 10 mL), ethanol (3 × 10 mL), acetone (3 × 10 mL), and chloroform (3 × 10 mL), and dried under high vacuum at 120 °C for 2 d to yield pale, light-yellow powders. For further details and analytical data, see in the Supplementary Information.

Ionothermal Synthesis

A mixture of 88.0 mg (123 μmol, 1.00 eq.) BTB-nitrile 1 and 168 mg (1.23 mmol, 10.0 eq.) dry ZnCl2 were heated in an oven up to 400 °C in a Pyrex® ampule (3 mm × 120 mm) for 42 h. After cooling to room temperature, the ampule was opened carefully. The solid residue was washed with water (200 mL), stirred in dilute HCl (15 mL) overnight, and filtered as well as washed with water (3 × 10 mL) and tetrahydrofuran (3 × 10 mL). The obtained black solid was dried under a high vacuum (150 °C and 10−6 mbar); for analytical data, see in the Supplementary Information.

3. Results

In previous work, we investigated the synthesis of CTFs with various linker systems such as, for example, di-, tri-, and tetra-substituted adamantane derivatives [61] or tetra(4-cyanophenyl)ethylene [45,61]. In the latter case, ionothermal [62] and strong Brønsted [29] reaction conditions were used, respectively, for the framework synthesis. In dependence on earlier literature results, the nitrogen BET surface areas for the frameworks synthesized with TFMS were much lower. On the other hand, the CO2 and CH4 uptakes are in similar ranges [45,63]. While a mixed-linker assembly strategy is already widely applied to metal-ligand coordination polymers [64,65] and is also known, for example, for imine-based COFs [62], to the best of our knowledge, mixed-linker CTFs were only recently reported [66]. Recently, we could show that combining two nitrile linkers positively influences the framework structures and properties [46,66].
The use of tetrahedral adamantane derivatives as well as the successful mixed-building block approach motivated us to transfer this approach on another multi-nitrile linker structure, the pseudo-octahedral 1,4-bis(tris(4′-cyanophenyl)methyl)benzene (BTB-nitrile, 1), which is, to the best of our knowledge, the first hexanitrile used in CTF preparation [60]. We combined this hexanitrile 1 with different planar dinitriles 2 and 3 and a trinitrile 4 and a tetrahedral tetraphenylmethane base nitrile 5 (reaction Scheme 1, Table 1).

3.1. Synthesis of Covalent Triazine Frameworks CTF-hex1–6

As described before, ionothermal reaction conditions are optimal for synthesizing triazine-based covalent organic frameworks with very high surface areas [13,24,25,26,27,28]; we used the novel pseudo-octahedral hexanitrile 1,4-bis(tris(4′-cyanophenyl)methyl)benzene 1 as tectone and investigated the framework formation with dry ZnCl2 under vacuum at 400 °C. According to previous work [46,63], a molar ratio of monomer to ZnCl2 of 1:10 leads to a higher surface area [14]. Therefore, this ratio was also used in the present work. A black solid in moderate to good yield was obtained (Table 1, entry 6).
Because of the instability of some organic molecules under ionothermal reaction conditions, the milder conditions from Cooper et al. [25] with TFMS at room temperature were used to synthesize triazine-based frameworks with two building blocks in which pseudo-octahedral hexanitrile 1 was always used as a tectone (Table 1, entries 2–5). The two different linkers were used in an equimolar ratio for the nitrile moieties. To better compare with the triazine-based framework CTF-hex6, the hexanitrile 1 was first reacted with itself using TFMS (Table 1, entry 1). All triazine frameworks CTF-hex1–5 synthesized with TFMS were isolated as slightly yellow powders.
The produced framework structures CTF-hex1–6 were characterized via IR spectroscopy, elemental analysis, scanning electron microscopy (SEM), and gas sorption measurements. The elemental analyses show deviations from the calculated values for a hypothetical full-conversion (Table S2, Supplementary Information). Such deviations are reported in the literature due to incomplete conversion, adsorption of water or other molecules, and decomposition during the reaction [25,26,46,47,66,67]. The decreased amount of nitrogen, e.g., for CTF-hex1 calculated 11.79%, found 9.27%, indicates the elimination of nitrogen species [29,46,66]. As expected, the percentage of nitrogen of the triazine framework CTF-hex6 synthesized under ionothermal reaction conditions is the lowest compared to the synthesized frameworks CTF-hex1–5 due to more defects and more significant decomposition at higher temperatures [25,29,67]. The structure of CTFs from ionothermal reactions with ZnCl2 approaches those of porous carbon materials, especially at temperatures above 400 °C, where a significant amount of nitrogen is lost, such that these CTFs may be better described as nitrogen-doped porous carbon [14,29,34,68]. On the other hand, TFMS-catalysed CTFs usually approach the idealized structure [29]. A clear indication is given by elemental analysis with the significantly higher nitrogen content, i.e., lower nitrogen loss through C–H bond cleavage and carbonization under Brønsted-acid synthesis conditions (Table S2) [25].
IR spectroscopic investigations of all frameworks CTF-hex1–6 show a significant amount of water, as seen at the large IR band for water between 2900 and 3600 cm−1 (Figure 1 for CTF-hex6 and Supplementary Information Figures S1–S3 for CTF-hex1–5, blue). This supports the assumption that one reason for the deviations of the elemental analyses is adsorbed water molecules in the microporous networks during sample preparation. The differences for CTF-hex6 are probably due to the additional zinc species from the ZnCl2 catalyst and porogen. Additionally, the IR spectra show the characteristic C–N stretching and breathing modes for triazine units at around 1500 and 1360 cm−1 as well as the breathing modes for the triazine unit at around 810 cm−1 (Figure 1 for CTF-hex6 and Supplementary Information Figures S1–S3 for CTF-hex1–5, green). Simultaneously, the intense IR bands for the nitrile group at around 2230 cm−1 decreased significantly compared to the starting material (Figure 1 for CTF-hex6 and Supplementary Information Figures S1–S3 for CTF-hex1–5, red) [27,43,46,68]. These observations prove a successful polymerization, but the presence of the nitrile signal indicates an incomplete conversion and supports the results of the elemental analysis again.
Morphologies of all CTFs were studied by scanning electron microscopy (Figure 2 and Figure S4, Supplementary Information). CTF-hex1 exhibits a combination of aggregation of spherical particles as well as irregular lumps with different sizes. However, CTF-hex2–5 show the general morphology of aggregates of irregular lumps with different sizes, whereas CTF-hex6 shows sheet-like morphology.
Powder X-ray diffractograms in Figure S14, Supplementary Information illustrate the expected largely amorphous nature of the CTF-hex materials. The diffractograms for the mixed-linker compounds exhibit three broad bands around 17°, 27°, and 41° 2-theta. For the mono-linker CTF-hex1 and CTF-hex6, the first band occurs already at 2θ ≈ 15° and 11°, respectively, and the band at 17° is not well developed. Noteworthy, the mixed-linker CTF-hex2–5 also features a comparatively sharp peak at 2θ = 17°, which in other mixed-linker work (prepared by the ionothermal route) was assigned to the (111) plane reflection from ZnCl2 [46,66]. Obviously, this assignment was not correct, in view of the synthesis of mixed-linker CTF-hex2–5 with the Brønsted acid route by using only trifluoromethanesulfonic acid.

3.2. Gas Sorption Studies

The porosities of the six synthesized triazine frameworks were characterized by N2 sorption measurements at 77 K. Figure 3 shows exemplarily the N2 sorption isotherms for CTF-hex6 (Figure 3a). The N2 sorption isotherms of the triazine frameworks CTF-hex1–5 are shown in the Supplementary Information. The Brunauer–Emmett–Teller (BET) surface areas were found to be in the range of 493 m2/g to 639 m2/g for the CTFs-hex1–5 and 1728 m2/g (p/p0 = 0.01–0.05) for the framework CTF-hex6 via ionothermal reaction conditions (Scheme 1, Table 2). As described before, triazine-based frameworks via ionothermal reaction conditions achieve much higher BET surface areas. One possible explanation for the larger surface area is the decomposition of the triazine moieties because of the high temperature leading to larger pores due to the loss of triazine knots or expanding the structure through the gas formation [14].
The surface area of 1728 m2/g for CTF-hex6 is still at the high end for surface areas for CTFs, which were synthesized at 400 °C under ionothermal conditions. The surface area of CTFs increases with temperature, so CTFs synthesized at, e.g., 600 °C will have higher surface areas [66]. Examples of CTFs with higher surface areas (Table S3, Supplementary Information) are fl-CF-400 to -600 (2862–2113 m2/g from 9H-fluorene-2,7-dicarbonitrile) [27], PCTF-1 (2235 m2/g, from tetrakis(4-cyanophenyl)ethylene) [61,63] or mixed-linker MM1 and MM3 (1800 and 1884 m2/g, from the tetranitrile tetrakis(4-cyanophenyl)ethylene (M) with terephthalonitrile (M1), and 4,4′-biphenyldicarbonitrile (M3), respectively) [66].
The isotherm of CTF-hex6 can be classified as a type Ib isotherm that indicates the framework’s microporous nature (Figure 3a) [69]. The isotherms of CTF-hex1–5 synthesized with TFMS show different behavior than the isotherm of CTF-hex6 but are similar among each other and can be classified as a combination of isotherm type IV in the lower pressure region and type II at higher relative pressure (Figure S5, Supplementary Information). Mesoporous adsorbents give type IV isotherms due to adsorbent-adsorptive interactions and the interactions between the molecules in the condensed state. Type II isotherms result from unrestricted monolayer multilayer adsorption observed for nonporous or macroporous materials [69]. The adsorption isotherms of CTF-hex2–5 also exhibit hysteresis loops (Figure S5, Supplementary Information). The hysteresis of CTF-hex2 can be classified as an H3 type of hysteresis, which is generally observed for non-rigid aggregates of plate-like particles and macropores not wholly filled with pore condensate. The triazine frameworks CTF-hex3–5 exhibit H4 type of hysteresis associated with narrow slit-like pores as shown in the Supplementary Information (Figure S5) [69]. The isotherm of CTF-hex1 is only of type II with H4 type hysteresis.
Pore size distributions were derived by the density functional theory (DFT) with the ‘carbon slit pore’ model (Section 2.2, Figure S6, Supplementary Information). The micropore (V0.1) and total pore volume (Vtot) was calculated from the N2 adsorption isotherms at 77 K. The ratio of V0.1/Vtot represents the degree of microporosity (Table 2). All CTFs show V0.1/Vtot values in the range of 0.7–0.8; CTF-hex2 possesses the highest microporosity with 82%, followed by CTF-hex5 with 81% (Table 2).
Typically, the ultramicropores (pores of width < 7 Å) are favorable for CO2 capture because small pore size could contribute to a deep overlap of potential and thus strong interaction. Therefore, we calculated ultramicropores from CO2 uptake at 273 K (Table 2) as the diffusion of N2 molecules at 77 K into ultramicropores is relatively slow (Figure S4, Supplementary Information) [70]. Using CO2 to determine ultramicropores ensures a faster equilibration and a slight extension towards the analysis of smaller pores [46,61].
Nitrogen-containing framework structures are known for their excellent CO2 uptake capacity because of the quadrupole moment of CO2 and the Lewis-basic properties of nitrogen atoms [13,26,46,71,72]. Therefore, we determined the gas uptakes of triazine frameworks CTF-hex1–6 obtained from the respective adsorption isotherms (Figure S4, Supplementary Information) for CO2 at 1 bar, as summarized in Table 2. The CO2 adsorption of the CTF networks CTF-hex1–6 at 273 K, and 1 bar is in the range of 62–76 cm3/g and shows complete reversibility, i.e., a coincidence of the adsorption and desorption branches as shown exemplarily for framework CTF-hex6 in Figure 3 right (for CTF-hex1–5 see Figure S4, Supplementary Information). This complete reversibility indicates that CO2 sorption occurs through unhindered physisorption in predominantly microporous materials.
Among all six CTFs, the mixed-linker triazine framework CTF-hex4 exhibits the highest CO2 adsorption of 76 cm3/g (at 273 K) and 48 cm3/g (at 293 K) at 1 bar (Table 2, entry 4). This value is higher than that for the pure hexanitrile CTF-hex1 with 76 cm3/g (at 273 K and 1 bar, Table 2, entry 1). The CO2 sorption values of CTF-hex4 are highly comparable to previously reported uptake capacities of CTFs (Table S3, Supplementary Information) [3,25,27,46,64].
In contrast to the nitrogen sorption isotherms, the CO2 uptake of framework CTF-hex6 synthesized via ionothermal reaction is not exceptionally higher than the other frameworks CTF-hex1–5. It is relatively comparable to the other CO2 uptakes. Compared to the pure hexanitrile framework CTF-hex1 synthesized with TFMS, the framework CTF-hex6 has a slightly higher CO2 uptake of 70 cm3/g at 273 K and a slightly lower uptake of 35 cm3/g at 293 K.
The isosteric heat of CO2 adsorption, Qst, was calculated over the whole adsorption range from the 273 K and 293 K isotherms for CO2 in CTF-hex1–6 using the Virial method (Figure 4, see Supplementary Information for details) [73,74]. The different behaviors and physical properties of CTF-hex6 versus CTF-hex1 can be explained by a different activation of the nitrile and hence a different number of side-products, unreacted end groups, and remaining reagent traces.

4. Discussion

The heat of adsorption at zero loadings, Q0st is expectedly very similar and between 23 kJ/mol for CTF-hex5 and 37 kJ/mol for CTF-hex3. These values are within the observed range for many CTF materials (Table S2, Supplementary Information). Still, the heats of CO2 adsorption remain mostly larger than 25 kJ/mol and, thereby, stay well above the heat of liquefaction of CO2 with 17 kJ/mol [36]. A meaningful characterization of porous materials should consider the heat of adsorption over the entire adsorption range (not just at zero coverage). Adsorption usually starts at the sites of the highest binding energy, that is, the heat of adsorption. With the saturation of these sites, then the heat of adsorption decreases. At low coverage, the value of Qst is determined mainly by the interaction with the strongest binding sites. Hence, CO2-interacting functionalities or highly polarising adsorption sites will give the highest Qst values.
Minor deviations in Qst are within the error margin of at least ±3 kJ/mol, which can be assumed on average for Qst data points [75,76]. Consequently, Q0st values should not be reported or discussed with decimal digits. The calculated increase in Qst with CO2 uptake can be a simultaneous, exothermic process such as the rearrangement of already adsorbed CO2 molecules towards a closer, energetically more favorable configuration phase transition material. Binding sites in small channels can cooperatively bind CO2 molecules and lead to a slipped-parallel arrangement of CO2 molecules by CTF:CO2:CO2:CTF binding, which gives an extra gain of attraction of about 3 kJ mol−1 [71,77]. The CTF:CO2:CO2:CTF binding interaction correlates with the significant increase in CO2 adsorption enthalpy with increasing CO2 uptake for CTF-hex2, -hex3, and -hex5. The more typical decrease in Qst with increased loading of CO2 is only seen for CTF-hex6. Here, the occupation of binding sites in the order of decreasing binding energies takes place and, at the same time, also indicates an adsorbent with different sites. The different, albeit more typical, Qst behavior can be explained by the significantly lower ultamicropore volume (Vmicro(CO2) in Table 2) together with also a much higher fraction of pores above 20 Å than the CTF-hex1-5 materials (Figure S6a Supplementary Information). In larger pores, the above-noted CTF:CO2:CO2:CTF binding interactions cannot take place.
In the case of the adsorption and desorption of CH4 of the networks CTF-hex1 (Figure S4, Supplementary Information) and CTF-hex6 (Figure 3, right), the pure hexanitrile CTF-hex6 synthesized with ZnCl2 has higher uptake capacities of 20 cm3/g at 273 K as well as 12.3 cm3/g at 293 K and 1 bar (Table 2, entry 6) than framework CTF-hex1 (18.1 cm3/g at 273 K and 10.2 cm3/g at 293 K, Table 2, entry 1). All in all, the CH4 uptake capacities were found to be in the range of 17.6–27 cm3/g at 273 K and 1 bar. Again, the mixed-linker framework CTF-hex4 has the highest CH4 uptake capacity. Within CTF materials, the reported CO2 and CH4 uptake capacities correspond to frequently reported values (Table S3, Supplementary Information).

5. Conclusions

In summary, we presented the extended, pseudo-octahedral 1,4-bis(tris(4′-cyanophenyl)methyl)benzene (BTB-nitrile, 1) as a new tectone for the synthesis of covalent triazine-based frameworks CTFs. Trimerization reactions among the BTB-nitrile 1 were performed under ionothermal reaction conditions with ZnCl2 at 400 °C and under strong Brønsted acid conditions with trifluoromethanesulfonic acid (TFMS) at room temperature. As expected, the framework CTF-hex6 synthesized via ionothermal reaction conditions exhibited a high BET surface area of 1728 m2/g compared to the triazine framework CTF-hex1 with 557 m2/g using milder Brønsted acid conditions. In contrast, the uptake of CO2 at 293 K was higher for the structure CTF-hex1 than for CTF-hex6.
Depending on previous work in our group, we performed a mixed-linker approach combining BTB-nitrile 1 with various linkers using TFMS as Brønsted acid. This approach resulted in higher BET surface areas of around 620 m2/g for nitrogen adsorption than the pure BTB-based framework CTF-hex1. Only the surface area of the triazine framework CTF-hex3 is in the same range. A possible explanation could be the interpenetration of the framework structure. However, the combinations between BTB-nitrile 1 and 1,3,5-tricyanobenzene (4) and tetrakis(4-cyanophenyl)methane (5), respectively, yielded framework structures with good CO2 and CH4 uptake capacities at 273 K. Because of their high stability, the six triazine framework structures CTF-hex1–6 synthesized by exploring the extended, pseudo-octahedral nitrile 1 are promising materials mainly for the storage of CO2 and CH4.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/ma14123214/s1.

Author Contributions

Conceptualization, S.B. and C.J.; methodology, A.M.S.; validation, A.M.S., S.D. and A.B.; formal analysis, A.M.S. and S.D.; virial analysis, A.N.; heat of adsorption, A.N.; investigation, S.B., A.B. and S.D.; resources, S.B.; data curation, S.B.; writing—original draft preparation, A.M.S. and I.D.W.; writing—review and editing, S.B.; visualization, I.D.W.; supervision, S.B. and C.J.; project administration, S.B.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript.

Funding

We acknowledge the SFB 1176 funded by the German Research Council (DFG) in the context of projects B2 and C6, as well as the Carl-Zeiss-Stiftung for funding. A.B. gratefully acknowledges the DST-Inspire Faculty fellowship program. The cluster “3D Matter Made to Order”, all funded under Germany’s Excellence Strategy 2082/1-390761711, is acknowledged for financial contributions.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The molecular data are deposited on the chemotion depository.

Acknowledgments

We thank Claus Feldmann for the help with the ionothermal experiments and Stefanie Bügel for help with the revision.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

  1. Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O.M. Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage. Science 2002, 295, 469–472. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  2. Rosi, N.L.; Eckert, J.; Eddaoudi, M.; Vodak, D.T.; Kim, J.; O’Keeffe, M.; Yaghi, O.M. Hydrogen Storage in Microporous Metal-Organic Frameworks. Science 2003, 300, 1127–1130. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  3. Wang, H.; Zhu, Q.-L.; Zou, R.; Xu, Q. Metal-Organic Frameworks for Energy Applications. Chemistry 2017, 2, 52–80. [Google Scholar] [CrossRef] [Green Version]
  4. Pettinari, C.; Marchetti, F.; Mosca, N.; Tosi, G.; Drozdov, A. Application of metal-organic frameworks. Polym. Int. 2017, 66, 731–744. [Google Scholar] [CrossRef]
  5. Ding, S.-Y.; Wang, W. Covalent organic frameworks (COFs). From design to applications. Chem. Soc. Rev. 2013, 42, 548–568. [Google Scholar] [CrossRef]
  6. Feng, X.; Ding, X.; Jiang, D. Covalent organic frameworks. Chem. Soc. Rev. 2012, 41, 6010–6022. [Google Scholar] [CrossRef]
  7. Wu, M.-X.; Yang, Y.-W. Applications of covalent organic frameworks (COFs): From gas storage and separation to drug delivery. Chin. Chem. Lett. 2017, 28, 1135–1143. [Google Scholar] [CrossRef]
  8. Liu, J.; Zou, R.; Zhao, Y. Recent developments in porous materials for H2 and CH4 storage. Tetrahedron Lett. 2016, 57, 4873–4881. [Google Scholar] [CrossRef]
  9. Furukawa, H.; Yaghi, O.M. Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications. J. Am. Chem. Soc. 2009, 131, 8875–8883. [Google Scholar] [CrossRef]
  10. Muller, T.; Bräse, S. Tetrahedral organic molecules as components in supramolecular architectures and in covalent assemblies, networks and polymers. RSC Adv. 2014, 4, 6886–6907. [Google Scholar] [CrossRef]
  11. Das, S.; Ben, T.; Qiu, S.; Heasman, P. Porous Organic Materials: Strategic Design and Structure-Function Correlation. Chem. Rev. 2017, 117, 1515–1563. [Google Scholar] [CrossRef]
  12. Dawson, R.; Adams, D.J.; Cooper, A.I. Chemical tuning of CO2 sorption in robust nanoporous organic polymers. Chem. Sci. 2011, 2, 1173–1177. [Google Scholar] [CrossRef]
  13. DeBlase, C.R.; Dichtel, W.R. Moving Beyond Boron: The Emergence of New Linkage Chemistries in Covalent Organic Frameworks. Macromolecules 2016, 49, 5297–5305. [Google Scholar] [CrossRef]
  14. Ben, T.; Qiu, S. Porous aromatic frameworks: Synthesis, structure and functions. CrystEngComm 2013, 15, 17–26. [Google Scholar] [CrossRef]
  15. Plietzsch, O.; Schilling, C.I.; Grab, T.; Grage, S.L.; Ulrich, A.S.; Comotti, A.; Sozzani, P.; Muller, T.; Bräse, S. Click chemistry produces hyper-cross-linked polymers with tetrahedral cores. New J. Chem. 2011, 35, 1577–1581. [Google Scholar] [CrossRef]
  16. Martin, C.F.; Stockel, E.; Clowes, R.; Adams, D.J.; Cooper, A.I.; Pis, J.J.; Rubiera, F.; Pevida, C. Hypercrosslinked organic polymer networks as potential adsorbents for pre-combustion CO2 capture. J. Mater. Chem. 2011, 21, 5475–5483. [Google Scholar] [CrossRef] [Green Version]
  17. McKeown, N.B.; Budd, P.M. Exploitation of Intrinsic Microporosity in Polymer-Based Materials. Macromolecules 2010, 43, 5163–5176. [Google Scholar] [CrossRef]
  18. McKeown, N.B.; Gahnem, B.; Msayib, K.J.; Budd, P.M.; Tattershall, C.E.; Mahmood, K.; Tan, S.; Book, D.; Langmi, H.W.; Walton, A. Towards polymer-based hydrogen storage materials: Engineering ultramicroporous cavities within polymers of intrinsic microporosity. Angew. Chem. Int. Ed. 2006, 45, 1804–1807. [Google Scholar] [CrossRef] [Green Version]
  19. Ben, T.; Pei, C.; Zhang, D.; Xu, J.; Deng, F.; Jing, X.; Qiu, S. Gas storage in porous aromatic frameworks (PAFs). Energy Environ. Sci. 2011, 4, 3991–3999. [Google Scholar] [CrossRef]
  20. Dawson, R.; Laybourn, A.; Clowes, R.; Khimyak, Y.Z.; Adams, D.J.; Cooper, A.I. Functionalized Conjugated Microporous Polymers. Macromolecules 2009, 42, 8809–8816. [Google Scholar] [CrossRef]
  21. Cooper, A.I. Conjugated microporous polymers. Adv. Mater. 2009, 21, 1291–1295. [Google Scholar] [CrossRef]
  22. Lu, W.; Yuan, D.; Zhao, D.; Schilling, C.I.; Plietzsch, O.; Muller, T.; Bräse, S.; Guenther, J.; Blumel, J.; Krishna, R.; et al. Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/Separation. Chem. Mater. 2010, 22, 5964–5972. [Google Scholar] [CrossRef]
  23. Yuan, D.; Lu, W.; Zhao, D.; Zhou, H.-C. Highly stable porous polymer networks with exceptionally high gas-uptake capacities. Adv. Mater. 2011, 23, 3723–3725. [Google Scholar] [CrossRef] [PubMed]
  24. Kuhn, P.; Antonietti, M.; Thomas, A. Porous, covalent triazine-based frameworks prepared by ionothermal synthesis. Angew. Chem. Int. Ed. 2008, 47, 3450–3453. [Google Scholar] [CrossRef]
  25. Ren, S.; Bojdys, M.J.; Dawson, R.; Laybourn, A.; Khimyak, Y.Z.; Adams, D.J.; Cooper, A.I. Porous, Fluorescent, Covalent Triazine-Based Frameworks Via Room-Temperature and Microwave-Assisted Synthesis. Adv. Mater. 2012, 24, 2357–2361. [Google Scholar] [CrossRef]
  26. Hug, S.; Stegbauer, L.; Oh, H.; Hirscher, M.; Lotsch, B.V. Nitrogen-Rich Covalent Triazine Frameworks as High-Performance Platforms for Selective Carbon Capture and Storage. Chem. Mater. 2015, 27, 8001–8010. [Google Scholar] [CrossRef]
  27. Hug, S.; Mesch, M.B.; Oh, H.; Popp, N.; Hirscher, M.; Senker, J.; Lotsch, B.V. A fluorene based covalent triazine framework with high CO2 and H2 capture and storage capacities. J. Mater. Chem. A 2014, 2, 5928–5936. [Google Scholar] [CrossRef] [Green Version]
  28. Hu, X.-M.; Chen, Q.; Zhao, Y.-C.; Laursen, B.W.; Han, B.-H. Straightforward synthesis of a triazine-based porous carbon with high gas-uptake capacities. J. Mater. Chem. A 2014, 2, 14201–14208. [Google Scholar] [CrossRef]
  29. Kuhn, P.; Forget, A.; Su, D.; Thomas, A.; Antonietti, M. From Microporous Regular Frameworks to Mesoporous Materials with Ultrahigh Surface Area: Dynamic Reorganization of Porous Polymer Networks. J. Am. Chem. Soc. 2008, 130, 13333–13337. [Google Scholar] [CrossRef]
  30. Kuhn, P.; Thomas, A.; Antonietti, M. Toward Tailorable Porous Organic Polymer Networks: A High-Temperature Dynamic Polymerization Scheme Based on Aromatic Nitriles. Macromolecules 2009, 42, 319–326. [Google Scholar] [CrossRef]
  31. Zhao, Y.; Yao, K.X.; Teng, B.; Zhang, T.; Han, Y. A perfluorinated covalent triazine-based framework for highly selective and water-tolerant CO2 capture. Energy Environ. Sci. 2013, 6, 3684–3692. [Google Scholar] [CrossRef]
  32. Gunasekar, G.H.; Park, K.; Ganesan, V.; Lee, K.; Kim, N.-K.; Jung, K.-D.; Yoon, S. A Covalent Triazine Framework, Functionalized with Ir/N-Heterocyclic Carbene Sites, for the Efficient Hydrogenation of CO2 to Formate. Chem. Mater. 2017, 29, 6740–6748. [Google Scholar] [CrossRef]
  33. Mukherjee, S.; Zeng, Z.; Shirolkar, M.M.; Samanta, P.; Chaudhari, A.K.; Tan, J.-C.; Ghosh, S.K. Self-Assembled, Fluorine-Rich Porous Organic Polymers: A Class of Mechanically Stiff and Hydrophobic Materials. Chem. Eur. J. 2018, 24, 11771–11778. [Google Scholar] [CrossRef]
  34. See, K.A.; Hug, S.; Schwinghammer, K.; Lumley, M.A.; Zheng, Y.; Nolt, J.M.; Stucky, G.D.; Wudl, F.; Lotsch, B.V.; Seshadri, R. Lithium Charge Storage Mechanisms of Cross-Linked Triazine Networks and Their Porous Carbon Derivatives. Chem. Mater. 2015, 27, 3821–3829. [Google Scholar] [CrossRef] [Green Version]
  35. Taeuber, K.; Dani, A.; Yuan, J. Covalent Cross-Linking of Porous Poly(ionic liquid) Membrane via a Triazine Network. ACS Macro Lett. 2017, 6, 1–5. [Google Scholar] [CrossRef] [Green Version]
  36. Keskin, S.; van Heest, T.M.; Sholl, D.S. Can Metal-Organic Framework Materials Play a Useful Role in Large-Scale Carbon Dioxide Separations? ChemSusChem 2010, 3, 879–891. [Google Scholar] [CrossRef]
  37. Buyukcakir, O.; Je, S.H.; Talapaneni, S.N.; Kim, D.; Coskun, A. Charged Covalent Triazine Frameworks for CO2 Capture and Conversion. ACS Appl. Mater. Interfaces 2017, 9, 7209–7216. [Google Scholar] [CrossRef]
  38. Jia, J.; Chen, Z.; Belmabkhout, Y.; Adil, K.; Bhatt, P.M.; Solovyeva, V.A.; Shekhah, O.; Eddaoudi, M. Carbonization of covalent triazine-based frameworks via ionic liquid induction. J. Mater. Chem. A 2018, 6, 15564–15568. [Google Scholar] [CrossRef] [Green Version]
  39. Janeta, M.; Bury, W.; Szafert, S. Porous silsesquioxane-imine frameworks as highly efficient adsorbents for cooperative iodine capture. ACS Appl. Mater. Interfaces 2018, 10, 19964–19973. [Google Scholar] [CrossRef]
  40. Lyu, J.; Zhang, X.; Otake, K.-I.; Wang, X.; Li, P.; Li, Z.; Chen, Z.; Zhang, Y.; Wasson, M.C.; Yang, Y.; et al. Topology and porosity control of metal-organic frameworks through linker functionalization. Chem. Sci. 2019, 10, 1186–1192. [Google Scholar] [CrossRef] [Green Version]
  41. Flaig, R.W.; Osborn Popp, T.M.; Fracaroli, A.M.; Kapustin, E.A.; Kalmutzki, M.J.; Altamimi, R.M.; Fathieh, F.; Reimer, J.A.; Yaghi, O.M. The Chemistry of CO2 Capture in an Amine-Functionalized Metal-Organic Framework under Dry and Humid Conditions. J. Am. Chem. Soc. 2017, 139, 12125–12128. [Google Scholar] [CrossRef] [Green Version]
  42. Dawson, R.; Cooper, A.I.; Adams, D.J. Chemical functionalization strategies for carbon dioxide capture in microporous organic polymers. Polym. Int. 2013, 62, 345–352. [Google Scholar] [CrossRef]
  43. Patel, H.A.; Karadas, F.; Canlier, A.; Park, J.; Deniz, E.; Jung, Y.; Atilhan, M.; Yavuz, C.T. High capacity carbon dioxide adsorption by inexpensive covalent organic polymers. J. Mater. Chem. 2012, 22, 8431–8437. [Google Scholar] [CrossRef]
  44. Ahmed, D.S.; El-Hiti, G.A.; Yousif, E.; Ali, A.A.; Hameed, A.S. Design and synthesis of porous polymeric materials and their applications in gas capture and storage: A review. J. Polym. Res. 2018, 25, 1–21. [Google Scholar] [CrossRef]
  45. Bhunia, A.; Esquivel, D.; Dey, S.; Fernández-Terán, R.; Goto, Y.; Inagaki, S.; Van Der Voort, P.; Janiak, C. A photoluminescent covalent triazine framework: CO2 adsorption, light-driven hydrogen evolution and sensing of nitroaromatics. J. Mater. Chem. A 2016, 4, 13450–13457. [Google Scholar] [CrossRef] [Green Version]
  46. Dey, S.; Bhunia, A.; Boldog, I.; Janiak, C. A mixed-linker approach towards improving covalent triazine-based frameworks for CO2 capture and separation. Microporous Mesoporous Mater. 2017, 241, 303–315. [Google Scholar] [CrossRef]
  47. Kuecken, S.; Schmidt, J.; Zhi, L.; Thomas, A. Conversion of amorphous polymer networks to covalent organic frameworks under ionothermal conditions: A facile synthesis route for covalent triazine frameworks. J. Mater. Chem. A 2015, 3, 24422–24427. [Google Scholar] [CrossRef] [Green Version]
  48. Puthiaraj, P.; Cho, S.-M.; Lee, Y.-R.; Ahn, W.-S. Microporous covalent triazine polymers: Efficient Friedel-Crafts synthesis and adsorption/storage of CO2 and CH4. J. Mater. Chem. A 2015, 3, 6792–6797. [Google Scholar] [CrossRef]
  49. Lim, H.; Cha, M.C.; Chang, J.Y. Preparation of Microporous Polymers Based on 1,3,5-Triazine Units Showing High CO2 Adsorption Capacity. Macromol. Chem. Phys. 2012, 213, 1385–1390. [Google Scholar] [CrossRef]
  50. Rengaraj, A.; Puthiaraj, P.; Haldorai, Y.; Heo, N.S.; Hwang, S.-K.; Han, Y.-K.; Kwon, S.; Ahn, W.-S.; Huh, Y.S. Porous Covalent Triazine Polymer as a Potential Nanocargo for Cancer Therapy and Imaging. ACS Appl. Mater. Interfaces 2016, 8, 8947–8955. [Google Scholar] [CrossRef] [PubMed]
  51. Puthiaraj, P.; Kim, S.-S.; Ahn, W.-S. Covalent triazine polymers using a cyanuric chloride precursor via Friedel-Crafts reaction for CO2 adsorption/separation. Chem. Eng. J. 2016, 283, 184–192. [Google Scholar] [CrossRef]
  52. Dey, S.; Bhunia, A.; Esquivel, D.; Janiak, C. Covalent triazine-based frameworks (CTFs) from triptycene and fluorene motifs for CO2 adsorption. J. Mater. Chem. A 2016, 4, 6259–6263. [Google Scholar] [CrossRef] [Green Version]
  53. Xiang, L.; Zhu, Y.; Gu, S.; Chen, D.; Fu, X.; Zhang, Y.; Yu, G.; Pan, C.; Hu, Y. A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions. Macromol. Rapid Commun. 2015, 36, 1566–1571. [Google Scholar] [CrossRef]
  54. Zhu, X.; Mahurin, S.M.; An, S.-H.; Do-Thanh, C.-L.; Tian, C.; Li, Y.; Gill, L.W.; Hagaman, E.W.; Bian, Z.; Zhou, J.-H.; et al. Efficient CO2 capture by a task-specific porous organic polymer bifunctionalized with carbazole and triazine groups. Chem. Commun. 2014, 50, 7933–7936. [Google Scholar] [CrossRef]
  55. Buegel, S.; Spiess, A.; Janiak, C. Covalent triazine framework CTF-fluorene as porous filler material in mixed matrix membranes for CO2/CH4 separation. Microporous Mesoporous Mater. 2021, 316, 110941. [Google Scholar]
  56. Troschke, E.; Graetz, S.; Luebken, T.; Borchardt, L. Mechanochemical Friedel-Crafts Alkylation-A Sustainable Pathway Towards Porous Organic Polymers. Angew. Chem. Int. Ed. 2017, 56, 6859–6863. [Google Scholar] [CrossRef]
  57. Wang, K.; Yang, L.-M.; Wang, X.; Guo, L.; Cheng, G.; Zhang, C.; Jin, S.; Tan, B.; Cooper, A. Covalent Triazine Frameworks via a Low-Temperature Polycondensation Approach. Angew. Chem. Int. Ed. 2017, 56, 14149–14153. [Google Scholar] [CrossRef] [Green Version]
  58. Liu, M.; Huang, Q.; Wang, S.; Li, Z.; Li, B.; Jin, S.; Tan, B. Crystalline Covalent Triazine Frameworks by In Situ Oxidation of Alcohols to Aldehyde Monomers. Angew. Chem. Int. Ed. 2018, 57, 11968–11972. [Google Scholar] [CrossRef]
  59. Yu, S.-Y.; Mahmood, J.; Noh, H.-J.; Seo, J.-M.; Jung, S.-M.; Shin, S.-H.; Im, Y.-K.; Jeon, I.-Y.; Baek, J.-B. Direct Synthesis of a Covalent Triazine-Based Framework from Aromatic Amides. Angew. Chem. Int. Ed. 2018, 57, 8438–8442. [Google Scholar] [CrossRef]
  60. Plietzsch, O.; Schade, A.; Hafner, A.; Huuskonen, J.; Rissanen, K.; Nieger, M.; Muller, T.; Bräse, S.; Braese, S. Synthesis and Topological Determination of Hexakis-Substituted 1,4-Ditritylbenzene and Nonakis-Substituted 1,3,5-Tritritylbenzene Derivatives: Building Blocks for Higher Supramolecular Assemblies. Eur. J. Org. Chem. 2012, 2013, 283–299. [Google Scholar] [CrossRef]
  61. Bhunia, A.; Boldog, I.; Moeller, A.; Janiak, C. Highly stable nanoporous covalent triazine-based frameworks with an adamantane core for carbon dioxide sorption and separation. J. Mater. Chem. A 2013, 1, 14990–14999. [Google Scholar] [CrossRef]
  62. Pang, Z.-F.; Xu, S.-Q.; Zhou, T.-Y.; Liang, R.-R.; Zhan, T.-G.; Zhao, X. Construction of Covalent Organic Frameworks Bearing Three Different Kinds of Pores through the Heterostructural Mixed Linker Strategy. J. Am. Chem. Soc. 2016, 138, 4710–4713. [Google Scholar] [CrossRef] [PubMed]
  63. Bhunia, A.; Vasylyeva, V.; Janiak, C. From a supramolecular tetranitrile to a porous covalent triazine-based framework with high gas uptake capacities. Chem. Commun. 2013, 49, 3961. [Google Scholar] [CrossRef] [PubMed]
  64. Du, M.; Li, C.-P.; Liu, C.-S.; Fang, S.-M. Design and construction of coordination polymers with mixed-ligand synthetic strategy. Coord. Chem. Rev. 2013, 257, 1282–1305. [Google Scholar] [CrossRef]
  65. Chen, D.-M.; Xu, N.; Qiu, X.-H.; Cheng, P. Functionalization of Metal-Organic Framework via Mixed-Ligand Strategy for Selective CO2 Sorption at Ambient Conditions. Cryst. Growth Des. 2015, 15, 961–965. [Google Scholar] [CrossRef]
  66. Dey, S.; Bhunia, A.; Breitzke, H.; Groszewicz, P.B.; Buntkowsky, G.; Janiak, C. Two linkers are better than one: Enhancing CO2 capture and separation with porous covalent triazine-based frameworks from mixed nitrile linkers. J. Mater. Chem. A 2017, 5, 3609–3620. [Google Scholar] [CrossRef]
  67. Katekomol, P.; Roeser, J.; Bojdys, M.; Weber, J.; Thomas, A. Covalent Triazine Frameworks Prepared from 1,3,5-Tricyanobenzene. Chem. Mater. 2013, 25, 1542–1548. [Google Scholar] [CrossRef]
  68. Öztürk, S.; Xiao, Y.-X.; Dietrich, D.; Giesen, B.; Barthel, J.; Ying, J.; Yang, X.-Y.; Janiak, C. Nickel nanoparticles supported on a covalent triazine framework as electrocatalyst for oxygen evolution reaction and oxygen reduction reactions. Beilstein J. Nanotechnol. 2020, 11, 770–781. [Google Scholar] [CrossRef]
  69. Thommes, M.; Kaneko, K.; Neimark, A.V.; Olivier, J.P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K.S.W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1051–1069. [Google Scholar] [CrossRef] [Green Version]
  70. Salinas-Martinez de Lecea, C.; Linares-Solano, A.; Rodriguez-Reinoso, F.; Sepulveda-Escribano, A. Carbon dioxide subtraction (CDS) method applied to a wide range of porous carbons. Stud. Surf. Sci. Catal. 1988, 39, 173–182. [Google Scholar]
  71. Lee, C.-H.; Wu, J.-Y.; Lee, G.-H.; Peng, S.-M.; Jiang, J.-C.; Lu, K.-L. Amide-containing zinc(II) metal-organic layered networks: A structure-CO2 capture relationship. Inorg. Chem. Front. 2015, 2, 477–484. [Google Scholar] [CrossRef]
  72. Wang, B.; Côté, A.P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O.M. Colossal cages in zeolitic imidazolate frameworks as selective carbon dioxide reservoirs. Nature 2008, 453, 207–211. [Google Scholar] [CrossRef]
  73. Zheng, X.; Huang, Y.; Duan, J.; Wang, C.; Wen, L.; Zhao, J.; Li, D. A microporous Zn(II)-MOF with open metal sites: Structure and selective adsorption properties. Dalton Trans. 2014, 43, 8311–8317. [Google Scholar] [CrossRef]
  74. Nuhnen, A.; Janiak, C. A practical guide to calculate the isosteric heat/enthalpy of adsorption via adsorption isotherms in metal-organic frameworks, MOFs. Dalton Trans. 2020, 49, 10295–10307. [Google Scholar] [CrossRef]
  75. Jeremias, F.; Khutia, A.; Henninger, S.K.; Janiak, C. MIL-100 (Al, Fe) as water adsorbents for heat transformation purposes—A promising application. J. Mater. Chem. 2012, 22, 10148–10151. [Google Scholar] [CrossRef]
  76. Jeremias, F.; Lozan, V.; Henninger, S.K.; Janiak, C. Programming MOFs for water sorption: Amino-functionalized MIL-125 and UiO-66 for heat transformation and heat storage applications. Dalton Trans. 2013, 42, 15967–15973. [Google Scholar] [CrossRef]
  77. Lee, C.-H.; Huang, H.-Y.; Liu, Y.-H.; Luo, T.-T.; Lee, G.-H.; Peng, S.-M.; Jiang, J.-C.; Chao, I.; Lu, K.-L. Cooperative Effect of Unsheltered Amide Groups on CO2 Adsorption Inside Open-Ended Channels of a Zinc(II)−Organic Framework. Inorg. Chem. 2013, 52, 3962–3968. [Google Scholar] [CrossRef]
Materials 14 03214 sch001 550
Scheme 1. Synthesized triazine-based frameworks CTF-hex1–6 with monomers 1–5 via nitrile trimerization with the strong Brønsted acid trifluoromethanesulfonic acid (method a) or via ionothermal reaction conditions (method b). The latter was only done with monomer 1.
Scheme 1. Synthesized triazine-based frameworks CTF-hex1–6 with monomers 1–5 via nitrile trimerization with the strong Brønsted acid trifluoromethanesulfonic acid (method a) or via ionothermal reaction conditions (method b). The latter was only done with monomer 1.
Materials 14 03214 sch001
Materials 14 03214 g001 550
Figure 1. IR spectrum of the synthesized triazine-based framework CTF-hex6. In green, at around 1500, 1360, and 810 cm−1, the IR bands for triazine units are shown; in red, at around 2200 cm−1, there is a small IR band for the nitrile moiety in all spectra, and between 2900 and 3600 cm−1, a significant signal for water (in red) is observed.
Figure 1. IR spectrum of the synthesized triazine-based framework CTF-hex6. In green, at around 1500, 1360, and 810 cm−1, the IR bands for triazine units are shown; in red, at around 2200 cm−1, there is a small IR band for the nitrile moiety in all spectra, and between 2900 and 3600 cm−1, a significant signal for water (in red) is observed.
Materials 14 03214 g001
Materials 14 03214 g002 550
Figure 2. SEM images of the covalent triazine-based frameworks CTF-hex5 and -hex6. The SEM images of the triazine frameworks CTF-hex1–4 are shown in the Supporting Information.
Figure 2. SEM images of the covalent triazine-based frameworks CTF-hex5 and -hex6. The SEM images of the triazine frameworks CTF-hex1–4 are shown in the Supporting Information.
Materials 14 03214 g002
Materials 14 03214 g003 550
Figure 3. Nitrogen adsorption–desorption isotherms at 77 K (a), as well as carbon dioxide and methane uptake at 1 bar (b) for triazine framework CTF-hex6, are shown exemplarily (closed symbols for adsorption and open symbols for desorption). The other isotherms are shown in Figures S5 and S7 in the Supporting Information.
Figure 3. Nitrogen adsorption–desorption isotherms at 77 K (a), as well as carbon dioxide and methane uptake at 1 bar (b) for triazine framework CTF-hex6, are shown exemplarily (closed symbols for adsorption and open symbols for desorption). The other isotherms are shown in Figures S5 and S7 in the Supporting Information.
Materials 14 03214 g003
Materials 14 03214 g004 550
Figure 4. The variation of isosteric heat of adsorption (Qst) with the amount of CO2 adsorbed for CTF-hex1–6, calculated from a pair of adsorption isotherms measured at 273 K and 293 K using the Virial method [74].
Figure 4. The variation of isosteric heat of adsorption (Qst) with the amount of CO2 adsorbed for CTF-hex1–6, calculated from a pair of adsorption isotherms measured at 273 K and 293 K using the Virial method [74].
Materials 14 03214 g004
Table
Table 1. Monomers and ratio, synthesis method and yields for triazine-based frameworks CTF-hex1–6.
Table 1. Monomers and ratio, synthesis method and yields for triazine-based frameworks CTF-hex1–6.
EntryMonomer (Molar Ratio)FrameworkMethod aYield b
11CTF-hex1a84%
21 with 2 (1:3)CTF-hex2a46%
31 with 3 (1:3)CTF-hex3a50%
41 with 4 (1:2)CTF-hex4a65%
51 with 5 (1:0.6)CTF-hex5a52%
61CTF-hex6b68%
a CTF-hex1–hex5 was synthesized using TFMS, whereas CTF-hex6 was synthesized by using ZnCl2. b The calculation of the yield is based on hypothetical 100% polymerization. A hybrid linker approach using TFMS as Brønsted acid was carried out for the first time. The yields were not optimized.
Table
Table 2. Porosity data for the covalent triazine-based frameworks CTF-hex1–5 synthesized with TFMS and CTF-hex6 synthesized via ionothermal reaction conditions and carbon dioxide and methane uptake capacities; the corresponding adsorption–desorption isotherms are shown in the Supplementary Information a.
Table 2. Porosity data for the covalent triazine-based frameworks CTF-hex1–5 synthesized with TFMS and CTF-hex6 synthesized via ionothermal reaction conditions and carbon dioxide and methane uptake capacities; the corresponding adsorption–desorption isotherms are shown in the Supplementary Information a.
EntryCTF-hexSBET b
[m2/g]		SLang c
[m2/g]		V0.1 d
[cm3/g]		Vtot e
[cm3/g]		V0.1/VtotVmicro(CO2) f
[cm3/g]		CO2 (273 K)CO2 (293 K)Q0st (CO2)CH4 [cm3/g]
[cm3/g][mmol/g] g[cm3/g][mmol/g] h[kJ/mol] i273 K293 K
11557 j669 j– k0.246 j– k0.091642.8401.653318.110.2
226206800.230.280.820.111582.5331.342320– l
334936260.190.240.790.091562.5261.053217.6– l
446097590.230.310.740.118763.4481.962927– l
556387900.250.310.810.101622.7331.352924– l
66172821230.660.870.760.068703.1351.46372012.3
a Values were rounded according to the estimated measurement uncertainties. For gas uptakes, this uncertainty is ±5%. The N2 gas uptakes are the basis for BET and Langmuir surface areas as well as pore volumes for this then also uncertainties of ±5% can be assumed. This gives, for example, an uncertainty of ±25 m2 g−1 for surface areas of 500 m2 g−1 and ±50 m2 g−1 for surface areas of 1000 m2 g−1. We note, however, that in the literature, CO2 and other gas uptakes are typically given with one decimal digit in the unit cm3/g and with two decimal digits in the unit mmol/g, which is more than the underlying measurement accuracy would allow. b BET surface area derived from the N2 adsorption isotherm at 77 K over the relative pressure range p/p0 = 0.01–0.05. c Langmuir surface area calculated over the ‘extended’ p/p0 range of 0–0.15 for improved averaging and agreement between data. d Pore volume from N2 adsorption isotherm at p/p0 = 0.1 for pores ≤2 nm (20 Å) (micropore volume). e Total pore volume at p/p0 = 0.95 for pores ≤20 nm. f Pore volume from the CO2 NLDFT model at 273 K for pores with diameters smaller than 1 nm (10 Å) (ultramicropore volume) (cf. Figure S6, Supplementary Information). g Transformation from cm3/g into mmol/g at 273 K: value in [cm3/g]: (22.711 cm3/mmol) = value in [mmol/g] (22.711 L is the molar volume at 1 bar and 273 K for an ideal gas). h Transformation from cm3/g into mmol/g at 293 K: value in [cm3/g]: (24.375 cm3/mmol) = value in [mmol/g] (24.375 L is the molar volume at 1 bar and 293 K for an ideal gas). i The heat of adsorption for CO2 at zero loadings (p/p0 = 0.0078) from CO2 adsorption isotherms acquired at 273 and 293 K and calculated via the Virial method (see Supplementary Information for details). j Surface areas were determined several times and obtained BET surface areas depended strongly on preparation and were found to be in the range of 0–557 m2/g. k Because of the results from BET surface determination, no micropore volume was calculated. l not measured.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Wessely, I.D.; Schade, A.M.; Dey, S.; Bhunia, A.; Nuhnen, A.; Janiak, C.; Bräse, S. Covalent Triazine Frameworks Based on the First Pseudo-Octahedral Hexanitrile Monomer via Nitrile Trimerization: Synthesis, Porosity, and CO2 Gas Sorption Properties. Materials 2021, 14, 3214. https://doi.org/10.3390/ma14123214

AMA Style

Wessely ID, Schade AM, Dey S, Bhunia A, Nuhnen A, Janiak C, Bräse S. Covalent Triazine Frameworks Based on the First Pseudo-Octahedral Hexanitrile Monomer via Nitrile Trimerization: Synthesis, Porosity, and CO2 Gas Sorption Properties. Materials. 2021; 14(12):3214. https://doi.org/10.3390/ma14123214

Chicago/Turabian Style

Wessely, Isabelle D., Alexandra M. Schade, Subarna Dey, Asamanjoy Bhunia, Alexander Nuhnen, Christoph Janiak, and Stefan Bräse. 2021. "Covalent Triazine Frameworks Based on the First Pseudo-Octahedral Hexanitrile Monomer via Nitrile Trimerization: Synthesis, Porosity, and CO2 Gas Sorption Properties" Materials 14, no. 12: 3214. https://doi.org/10.3390/ma14123214

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop