Confinement effects facilitate low-concentration carbon dioxide capture with zeolites

Significance Direct air capture (DAC) of CO2 from the atmosphere is being pursued to aid in mitigating global CO2 amounts and possibly reaching net negative emissions by 2050. We report that a type of commercialized zeolite, mordenite (MOR)-type zeolite, is a promising adsorbent for DAC because of its high CO2 capacity, high selectivity, fast kinetics, low isosteric heat of adsorption, and high stability under simulated DAC conditions. We demonstrate that the primary site for CO2 adsorption in the MOR-type zeolite is located at the side-pocket and that its size (i.e., the confinement effect) is the key to the performance by comparing its adsorption behavior to those obtained from a number of other zeolites with varying pore space sizes.

Calcination of zeolites. After the synthesis was completed, the resulting solids were washed three times with distilled water. The materials synthesized with OSDAs were further washed by acetone. To remove the OSDAs from zeolites, the obtained solids were then dried at 80 ºC before calcining in a flow air furnace at 580 ºC for 8 h, with a ramp rate of 1.0 ºC/min. Crystallinity of the materials was examined using lab based X-ray diffraction (XRD).
Aqueous-phase Ion-Exchange of Zeolites. Na-type zeolites were prepared by ion exchange of calcined or as received zeolites with 1M aqueous NaNO3 solution. Typically, 600 mg of zeolites were added to 30 mL of salt solutions, which were then stirred at 80 ºC for 24 h. The exchanged crystals were dried at 100 ºC in ambient air in a free convention oven overnight.
A similar ion exchange procedure was used to prepare samples for the preparation of MOR-type zeolites with varying Na + loading. The MOR6 zeolites (CBV10A, Zeolyst, 4# in Table 1) were first fully converted to ammonium form after two times (48 h in total) ion exchange using 1 M NH4NO3 aqueous solution at 80 ºC. Depending on the targeting Na + exchange level, 30 mL of 0.002 M to 5 M aqueous NaNO3 solution were used as the precursor. The materials were recovered by centrifugation and washed 6 times with copious amount of distilled water. The exchanged crystals were dried at 100 ºC overnight. The Naand H-density were measured using energy-dispersive X-ray spectroscopy (EDS) and solid state 1 H nuclear magnetic resonance (NMR) spectroscopy, respectively.
Pyridine-modification of zeolites. A previously reported method was followed for pyridine modification of the MOR-type zeolites, denoted as py-MOR. Briefly, Na-MOR was dehydrated at 170 ºC under vacuum (0.1 mbar) overnight. The sample was cooled to room temperature and flushed with ultra-high purity Ar before dosing pyridine into MOR zeolites with an amount of ca. 0.5 mL pyridine/mg zeolites. The py-MOR zeolites were heated to 300 ºC and kept for 19 h to remove the water and excess pyridine prior to the CO2 adsorption measurements at 30 ºC in the dynamic column fixed bed system.

S3. Characterizations
X-ray diffraction. The crystallinity of the materials was examined using powder X-ray diffraction (XRD). The XRD patterns were collected using a Rigaku Miniflex II desktop instrument with a Cu radiation source, Kα = 1.5418 Å.
Scanning electron microscopy. The morphology of the materials was measured using scanning electron microscopy (SEM, ZEISS 1550 VP FESEM). The SEM was equipped with an Oxford X-Max SDD. Energy dispersive X-ray spectroscopy (EDS) used for determining the element contents of each sample.
Solid-state, magic-angle spinning nuclear magnetic resonance. Solid-state, magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were obtained on a Bruker AVANCE 500 MHz (11.2 T) spectrometer using a 4 mm zirconia rotor with a Kel-F cap. 1 H MAS NMR spectroscopy was measured to quantify the residue H-density of Na + exchanged MOR-type zeolites as a function of Na + loading. NH4-form materials were used to avoid the dealumination through ion exchange and dehydration. The samples were loaded in the rotor and dehydrated under vacuum (10-2 Torr) at 400 °C for 12 h in a Schlenk manifold. The spectra were acquired at 500.1 MHz with a 90° pulse length of 4 μs and a spinning rate of 12 kHz, and then were deconvoluted using DMFit software. The number of framework NH4 site (mmol/g) was measured by referencing the signal intensities to hexamethyl benzene and normalizing by the sample mass, followed by dividing by 4, that is the number of protons per NH4 + . The residual NH4 site density of Na-exchanged MOR zeolites was then quantified with the same procedure as a function of Na loading. To examine the state of aluminum in Na-exchanged MOR zeolites, the 27 Al MAS NMR spectra were also recorded on the same samples without undergoing dehydration at 130.2 MHz with a 10°pulse length of 0.5 μs, a cycle delay time of 0.5 s, and a spin rate of 12 kHz.
Fourier transform infrared spectroscopy. Fourier transform infrared (FTIR) spectra were collected on zeolite samples using a Nexus 470 FTIR spectrometer equipped with a deuterated, L-alanine doped triglycine sulfate (DTGS) detector. Catalyst samples (ca.10-12 mg) were pressed into a self-supporting wafer (ca. 1.2 cm in diameter) and placed in a custom-built FTIR cell. The wafers were treated in flowing dry air at 723 K for 120 min, and then cooled to RT for CO2 adsorption under flowing dry air for 30 min. Spectra were collected with a resolution of 4 cm -1 and averaged over 64 scans.
Single-crystal X-ray diffraction. Single-crystal X-ray diffraction was performed at 100 K on a Bruker D8Venture CCD diffractometer using Cu Kα radiation (λ = 1.5406 Å). Crystal was mounted on Mitegen MicroMount. A multi-scan absorption correction was applied to the collected reflections. The structure was solved with the ShelXS structure solution program using Direct Methods and refined with the ShelXL refinement package using Least Squares minimization. All non-hydrogen atoms were refined anisotropically. The figures were exported using Olex2.

CO2 adsorption kinetics
The adsorption kinetics of MOR-type zeolites for CO2 adsorption was examined using TGA (Perkin Elmer STA6000). For the TGA measurements, 49.54 mg 13X and 57.19 mg MOR-type zeolites were used to obtain comparable volumes (ca. 0.10 mL). The loaded sample was activated at 600 ºC for 20 h with a 70 mL/min He flow. After the temperature was cooled to 30 ºC, the gas was switched to a 70 mL/min 400ppm CO2/He flow. Desorption was performed with a ramp rate of 10 ºC/min to 600 ºC with a 70 mL/min He flow. The weight of the sample was recorded as a function of time on stream.

Isosteric heat of adsorption
The isotherms measured at different temperatures were first fitted using the Langmuir-Freundlich (LF) isotherm model: where "a" is the saturation uptake (mmol/g), "b" is the corresponding adsorption equilibrium constant showing the affinity coefficient, "n" is the deviation from the ideal homogeneous surface, "p" is the equilibrium pressure. The heat of adsorption was calculated from the Clausius-Clapeyron equation: where "p" is the pressure (bar), "ΔH" is the changing in enthalpy (kJ/mol), "R" is the universal gas constant (8.314 J/K/mol), and "T" is the temperature (K). The function can be simplified to: ln = − ( 1 ) + (3) for the calculation of ΔH (isosteric heat of adsorption).
Bar graphs comparison of the capacity for 400 ppm CO2 over MOR-type zeolites, including commercial samples, zeolites synthesized with and without organic-structure directing agents (OSDAs). The data are obtained from 0.0004 bar from isotherms measured up to 1 bar at 25 ºC. Sample codes are listed in Table S1.   Table S2.

Fig. S7
. CO2 isotherms of the organic structure-directing agents free MOR-type zeolites measured at 25 ºC. 14*# is the MOR-type zeolites (14#) after Na + ion exchange. The results show that high CO2 adsorption capacity can be obtained directly from the as-prepared MOR-type zeolites without further ion exchange. Moreover, the CO2 adsorption capacity can be adjusted by varying the Al content through OSDA-free synthesis. The OSDA-free MOR-type zeolites (14#) shows a capacity of 1.14 mmol/g, comparable to that of OSDA-directed material (7#). This would lead to a cost-efficient, environmentally friendly preparation of MOR-type zeolites with high capacity in large industrial scale, offering opportunities for lowering the cost of the DAC technique.          S17. FTIR spectra of the OH stretching region for the MOR-type (4# in NH4 + form) samples with various Na + loading. The spectra were deconvoluted using a Voigt function, with contributions from Gaussian and Lorentz functions. Note that the spectra were not normalized based on weight, thus the area only qualitatively reflect the consumption of BAS with the loading of Na + cations.   S19. 27 Al NMR spectra of the MOR-type (4# in NH4 + form) with various Na + loading. The results show that all materials were free of extraframework Al 3+ , although small OH stretching bands attributed to extraframework Al 3+ were observed for some samples from FTIR (Fig. S17).    S22. Visualization of the sittings of Na + cations in Na-MOR-type zeolites as a function of Na + loadings. a) Four distinct extraframework cations sites, namely N1, N2, N3, N4, were found in the MOR-type framework. Oxy represents the oxygen atom connected to Tx and Ty sites. The extrafremwork cations in the N1 and N3 sites are located at the center of the 8MRs in the side-pocket and in the intersection between the side-pocket and 12MR channel, respectively. Those in the N2 site are located at the 8MR side-pocket balancing the O33. Those in the N4 site are located at the 12MR channel balancing the O44. Na&N represents the site partially exchanged by Na + . b-e) Results from samples (A-D) highlighted in Fig. 2c, respectively. Panel b) shows that sample A had Na + cations in the N1 site. Panel c) shows that sample B had Na + cations in the N1 and N2 sites. Panel d) shows that sample C had both N1 and N2 sites fully occupied by Na + cations, while the N3 site was partially occupied by Na + cations. Panel e) shows that the occupancy of sample D is similar to that of sample C, with extra Na + cations added to N4 site. The details of the occupancy for each site are summarized in Tables S7-S10.  It has been demonstrated in the present work that the side-pockets of LTL-, MAZ-, MOR-type frameworks are responsible for the adsorption of 400 ppm CO2. The longest dimension of the side pockets of these zeolites were used to define the confined space. Similar concept was used for the FER-type framework due to the structural similarity between the MOR-and FER-type frameworks. The sizes are measured via VESTA, and the values are 5.2, 6.7, 6.3, and 9.7 Å for the LEL-, MAZ-, MOR-, and FER-type zeolites, respectively. It should be noted that a larger size was obtained for the FER-type zeolites than that for the MOR-type zeolites. This is due to the "antenna" structure as well as the larger size of the swelling shape of the side-pocket of FER-type framework, compared to the side-pocket of MOR-type framework. For Na-MFItype zeolites, it was shown by simulation that CO2 molecules preferentially adsorbs in the intersection of the two channels.(9) Thus, the size of the intersection of MFI-type framework was applied for defining its confined space. Similar concept was used for MEL-type framework due to the similarity between MFI-and MEL-type frameworks.    The results show that the amount of CO2 adsorbed in the MAZ and py-MAZ materials are comparable. As the pyridine moleculs selectively shield the 12MR channels (10), the 8MR side-pocket is the only accessible channel in the py-MAZ material. Therefore, these data suggest that the 8MR side-pocket is responsible for the adsorption of 400 ppm CO2 in MAZ-type zeolites. Supplementary Information Tables S1-S12 b. Values were determined from the deconvolution of the OH stretching from the FTIR spectra. 1.00 4.66 Note: a. The CO2 capacity obtained at 0.0004 bar from the isotherms measured at 30 °C.
b. The number of Na + cations in the 8MR side-pocket for each MOR unit cell was calculated from the FTIR results in the OH stretching region and the elemental analysis from EDS.   Note: The compositions were measured using EDS.   a. Elemental analysis was performed using EDS.