Molecular Insights into Carbon Dioxide Sorption in Hydrazone-Based Covalent Organic Frameworks with Tertiary Amine Moieties

Tailorable sorption properties at the molecular level are key for efficient carbon capture and storage and a hallmark of covalent organic frameworks (COFs). Although amine functional groups are known to facilitate CO2 uptake, atomistic insights into CO2 sorption by COFs modified with amine-bearing functional groups are scarce. Herein, we present a detailed study of the interactions of carbon dioxide and water with two isostructural hydrazone-linked COFs with different polarities based on the 2,5-diethoxyterephthalohydrazide linker. Varying amounts of tertiary amines were introduced in the COF backbones by means of a copolymerization approach using 2,5-bis(2-(dimethylamino)ethoxy)terephthalohydrazide in different amounts ranging from 25 to 100% substitution of the original DETH linker. The interactions of the frameworks with CO2 and H2O were comprehensively studied by means of sorption analysis, solid-state NMR spectroscopy, and quantum-chemical calculations. We show that the addition of the tertiary amine linker increases the overall CO2 sorption capacity normalized by the surface area and of the heat of adsorption, whereas surface areas and pore size diameters decrease. The formation of ammonium bicarbonate species in the COF pores is shown to occur, revealing the contributing role of water for CO2 uptake by amine-modified porous frameworks.


SEM
SEM measurements were performed on a Zeiss Merlin or a VEGA TS 51300MM (TESCAN). TEM TEM was performed on a Philips CM30ST (300 kV, LaB6 cathode) with a CMOSS camera F216 (TVIPS). Samples were suspended in butanol and drop-cast onto a lacey carbon film (Plano). PXRD PXRD patterns were recorded at room temperature on a Bruker D8 Discovery with Ni -filtered CuKαradiation (1.5406 Å) and a position-sensitive detector (Lynxeye). IR Fourier-transform infrared spectra were measured on a Jasco FT/IR-4100 or a Perkin Elmer Spectrum NX FT-IR System. Structural models Structural models were obtained with Materials Studio v6.0.0 Copyright © 2011 Accelrys Software using the Forcite Geometry optimization with Ewald electrostatic and van der Waals summation methods. Sorption Sorption measurements were performed on a Quantachrome Instruments Autosorb iQ MP with Argon at 87 K or with CO 2 at 273, 288 or 298 K. Weight percentage was calculated by referencing to sorbent weight.

COF synthesis
All products were obtained as fluffy solids. To remove residual starting materials, powders were washed intensely with DMF, THF and dichloromethane and subsequently dried in a vacuum desiccator overnight.

Heats of adsorption
Heats of Adsorption were calculated from Henry's law. At low surface excess concentration, the dilute adsorbate phase is treated as a two dimensional ideal gas. The relation is given by = Where is the Henry's law constant and n represents the specific surface excess amount. By modeling adsorption in the low-pressure region via a virial-type equation, Henry's law constants can be calculated.
With CO 2 adsorption measurements at 273, 287, and 295 K, Henry's law constants for each temperature were identified. The differential enthalpy of adsorption at zero coverage ∆ℎ 0 was then calculated from the Van't Hoff equation.
By plotting ln[ ] versus 1 and linearly fitting, the zero coverage enthalpy is equal to the slope of the fit multiplied by the ideal gas constant R. As shown by the 1D 13 C{ 1 H} DNP-CP MAS spectra in Fig. S9(a,b), the 13 C amide signal (orange band) has stronger intensity for the longest CP contact time of 5 ms. Although 13 C-depleted glycerol was used in the DNP solvent formulation, there is a small intensity shoulder ranging from 65 to 80 ppm from dilute amounts of 13 Ccontaining glycerol. By comparison, the 1D 13 C{ 1 H} LTMAS-CP MAS spectra in Fig. S9(c,d) were acquired on vacuum-dried 100%-amine-coCOF-H upon exposure to dry 100% 13 C-enriched CO 2 , before and after a subsequent degassing step. As discussed in the materials section, these materials were characterized without DNP to minimally influence adsorbed CO 2 . Under otherwise identical conditions, there is significantly more 13 C signal at 160 ppm for the material exposed to 13 C-enriched CO 2 . In the 1D spectra the adsorbed bicarbonate (red band) and amide have overlapping signal intensity at 160 ppm, however for short contact times (500 µs) signal contributions from the amide are partially reduced.
Similarly, the 2D 13 C{ 1 H} DNP-HETCOR presented in Fig. S10a shows only weak correlated intensity from correlations between the 13 C amide signal (ca. 159 ppm) and 1 H aromatic signals (7.0 -8.0 ppm). DNP-NMR can improve signal sensitivity by up to γe/γ 1H = 658 or γe/γ 15N = 6,500 for 1 H and 15 N nuclei respectively. Compared to Figure 6b in the main text, the 2D 13 C{ 1 H} DNP-HETCOR spectra depicted in Figure S10a has significantly improved signal-to-noise and resolves two additional 13 C signals at 21 and 32 ppm from residual ethoxy linker moieties. The 13 C signal at 159 ppm from framework amide moieties in both Figure S10a and Figure 6b exhibits weak 13 C{ 1 H}correlated intensities due to the short CP contact time (500 µs). Similarly, in Figure 6a in the main text, the absence of correlated 2D intensity associated with bicarbonate 1 H species and 13 C moieties (1.1% natural abundance) in the 100%-amine-coCOF-H framework is explained by the low absolute quantity of these dipolar-coupled spin pairs in comparison to the quantity of spin pairs arising from 13 C moieties in 13 C-CO 2 and 1H species in the COF framework. Importantly, as depicted in Figure S10b, DNP-NMR enables the acquisition of 1D and 2D 15 N{ 1 H} natural abundance spectra in low-N containing COF materials which would otherwise be infeasible. In Figure S10b, the tertiary amine linker moieties with 15 N signals at 24 and 36 ppm and the framework amide 15 N signal at 181 ppm are strongly correlated to 1 H signals at ca. 4.0 ppm, which arise from H 2 O adsorbed in the COF pore or introduced by the DNP solvent. All five 15 N signals have correlated intensity with aromatic or hydrazone 1 H moieties ranging from 7 -8 ppm. Lastly, weakly correlated 15 N{ 1 H} intensity between the amide 15 N signal at 181 ppm and a 1 H signal at 11.7 ppm is consistent with the 1 H chemical shift measured for compound (6). Overall, the DNP-enhanced 2D 13 C{ 1 H} and 15 N{ 1 H} HETCOR spectra confirm that the local structure of the COF framework is retained on addition of the tertiary amine linker moieties, consistent with the analyses presented in the main text.