High-Capacity, Cooperative CO2 Capture in a Diamine-Appended Metal–Organic Framework through a Combined Chemisorptive and Physisorptive Mechanism

Diamine-appended Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) metal–organic frameworks are promising candidates for carbon capture that exhibit exceptional selectivities and high capacities for CO2. To date, CO2 uptake in these materials has been shown to occur predominantly via a chemisorption mechanism involving CO2 insertion at the amine-appended metal sites, a mechanism that limits the capacity of the material to ∼1 equiv of CO2 per diamine. Herein, we report a new framework, pip2–Mg2(dobpdc) (pip2 = 1-(2-aminoethyl)piperidine), that exhibits two-step CO2 uptake and achieves an unusually high CO2 capacity approaching 1.5 CO2 per diamine at saturation. Analysis of variable-pressure CO2 uptake in the material using solid-state nuclear magnetic resonance (NMR) spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals that pip2–Mg2(dobpdc) captures CO2 via an unprecedented mechanism involving the initial insertion of CO2 to form ammonium carbamate chains at half of the sites in the material, followed by tandem cooperative chemisorption and physisorption. Powder X-ray diffraction analysis, supported by van der Waals-corrected density functional theory, reveals that physisorbed CO2 occupies a pocket formed by adjacent ammonium carbamate chains and the linker. Based on breakthrough and extended cycling experiments, pip2–Mg2(dobpdc) exhibits exceptional performance for CO2 capture under conditions relevant to the separation of CO2 from landfill gas. More broadly, these results highlight new opportunities for the fundamental design of diamine–Mg2(dobpdc) materials with even higher capacities than those predicted based on CO2 chemisorption alone.


■ INTRODUCTION
Rising atmospheric CO 2 levels are a leading cause of deleterious climate change, and carbon capture from point sources in the power-generation and industrial sectors is being intensively investigated as one of several key mitigation strategies. 1,2−24 Polyamine-appended frameworks of the type amine− Mg 2 (dobpdc) (dobpdc 4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), 25−35 and recently amine−Mg 2 (olz) (olz 4− = (E)-5,5′-(diazene-1,2-diyl)bis(2-oxidobenzoate)) materials, have shown particular promise for CO 2 capture, 36 and some of these are being developed for testing at the pilot scale. 37The vast majority of these materials selectively capture CO 2 via a cooperative mechanism involving CO 2 insertion into the metal−amine bonds to form ammonium carbamate chains. 26A hallmark of this chemisorption mechanism is step-shaped CO 2 adsorption, where CO 2 uptake occurs within a narrow temperature or pressure window, and as a result, relatively small temperature or pressure swings can be used for material regeneration.Depending on the choice of parent framework and appended amine, these materials can exhibit very high CO 2 capacities exceeding 3 mmol/g at a range of pressures relevant to the capture of CO 2 from diverse flue streams.Fundamentally, the discovery of related materials exhibiting even higher CO 2 adsorption capacities represents an important advance for the field.However, to date, the uptake of CO 2 in diamine−Mg 2 (dobpdc) and diamine−Mg 2 (olz) materials has consistently been limited to one molecule of CO 2 per diamine (Figure 1a), with small amounts of additional CO 2 uptake due to physisorption.
−32 Together with gas sorption analysis, solid-state nuclear magnetic resonance (NMR) spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data support a mechanism involving initial uptake of CO 2 as ammonium carbamate, followed by simultaneous chemisorption and physisorption of CO 2 .Powder X-ray diffraction data supported by van der Waals (vdW)-corrected density functional theory (DFT) calculations indicate that CO 2 is physisorbed in a highly ordered fashion, occupying a pocket created by ammonium carbamate chains and the framework linker.Under conditions relevant to landfill CO 2 capture, pip2−Mg 2 (dobpdc) adsorbs nearly 5 mmol CO 2 per gram, and extended CO 2 cycling and breakthrough studies under simulated landfill gas 38 reveal that this mixed adsorption mechanism is highly robust.These results highlight an opportunity to design a new family of amine-appended MOFs exhibiting enhanced CO 2 capacities via tandem and cooperative chemisorption and physisorption for a range of capture applications.
■ RESULTS AND DISCUSSION Single-Component Gas Adsorption Experiments.−32 Briefly, methanol-solvated Mg 2 (dobpdc) (Figures S1−S3) was soaked in a toluene solution of pip2 for several hours, and then the resulting solid was isolated and activated at 130 °C under flowing N 2 for 1 h (see Experimental Section for details and Figures S4−S6).Based on solution-phase 1 H NMR spectroscopy analysis of a digested sample of pip2−Mg 2 (dobpdc), diamine loading in the material is quantitative (∼100%).From N 2 adsorption data collected for activated pip2−Mg 2 (dobpdc) at 77 K (Figure S5), we calculated a Langmuir surface area of 570 m 2 /g (Brunauer−Emmett−Teller surface area = 490 m 2 /g), consistent with surface areas reported for other diamine− Mg 2 (dobpdc) variants appended with bulky diamines. 34s an initial assessment of the CO 2 adsorption properties of pip2−Mg 2 (dobpdc), we collected adsorption and desorption isobars between 120 and 30 °C under 1 bar of CO 2 .The material exhibits double-step CO 2 adsorption behavior with steps located at ∼55 and 40 °C (defined as the midpoint of the step region; Figure 2a, blue trace).Cooperative two-step CO 2 uptake has previously been reported for several diamine− Mg 2 (dobpdc) materials featuring bulky 1°,2°-diamines, 33,34 and it was ascribed to steric interactions between resulting ammonium carbamate chains. 34However, for the latter materials, each step is associated with uptake of ∼0.5 equiv of CO 2 per diamine, and the total CO 2 uptake in these materials has not been reported to exceed ∼1.2 CO 2 per diamine, with the additional uptake beyond 1 CO 2 per diamine corresponding to physisorption in the poststep uptake region. 33,34,38For pip2−Mg 2 (dobpdc), the first step in the adsorption isobar is also associated with uptake of close to 0.5 equiv of CO 2 per diamine, although in contrast to previously studied materials, nearly double this quantity is taken up in the second step, resulting in an overall capacity of 1.4 CO 2 per diamine, or 5.1 mmol/g, at 30 °C.Isobaric CO 2 desorption from pip2−Mg 2 (dobpdc) also occurs in a two-step fashion, with moderate desorption hysteresis leading to desorption steps at ∼55 and 70 °C.
To further investigate the CO 2 adsorption properties of pip2−Mg 2 (dobpdc), we collected the CO 2 adsorption isotherms at 25, 40, and 50 °C (Figure 2b).The material exhibits double-step profiles at all temperatures, consistent with the isobaric experiments.At 25 and 40 °C, the first adsorption step occurs at 50 and 150 mbar, respectively, and is associated with uptake of 1.3 and 1.5 mmol of CO 2 /g; the uptake in the second step (at 200 and 500 mbar, respectively) is nearly double that of the first step (3.3 and 3.1 mmol/g, respectively), giving rise to total capacities of 4.9 and 4.6 mmol/g, respectively.This same trend is observed at 50 °C, although the uptake after each step (∼1.0 and 2.0 mmol/g, respectively) is less than at lower temperatures, consistent with the temperature-dependent uptake characterized in the adsorption isobar data.Notably, the high CO 2 uptake achieved at 300 mbar and 25 °C (4.4 mmol/g) suggests that pip2− Mg 2 (dobpdc) may be a promising candidate for CO 2 capture from landfill gas, which is composed of 40−60% CO 2 , 40−60% CH 4 , and 2−5% N 2 , at 25 °C (see further discussion below). 39−41 Indeed, single-component CH 4 and N 2 adsorption isotherms collected for pip2−Mg 2 (dobpdc) at 25 °C suggest that the material is highly selective for CO 2 over these other gases (Figure S7).
The CO 2 adsorption isotherms at all temperatures were fit using linear interpolation, and the resulting fit data (pressures and loadings) were used with the Clausius−Clapeyron equation to calculate the differential enthalpy (Δh ads ) and entropy (Δs ads ) of CO 2 adsorption as a function of loading (see Experimental Section and Figures S8−S10).From these data, we determined Δh ads values of −59(2) and −53(1) kJ/ mol for the first and second steps, respectively, corresponding to the loading values associated with the midpoint of each step (0.5 and 2.5 mmol/g).Using the reversible heat capacity (1.29 J/g•°C) of pip2−Mg 2 (dobpdc) measured by differential scanning calorimetry (Figure S11) and the operating temperature range (adsorption at 25 °C and desorption at 80 °C), we further estimated an approximate regeneration energy of 1.58 MJ/kg CO 2 for a temperature swing adsorption process, which is approximately one-third of the regeneration energy for a monoethanolamine solution used for landfill CO 2 capture (∼4.5 MJ/kg CO 2 ). 42,43pectroscopic Investigation of CO 2 Uptake.To identify the adsorbed species formed upon CO 2 uptake in pip2−Mg 2 (dobpdc), we collected in situ DRIFTS data for a sample of the material dosed with CO 2 at room temperature and pressures of 100 and 300 mbar, corresponding to immediately after the first and second adsorption steps in the 25 °C isotherm.In separate experiments, the activated framework was first dosed with 100 mbar of natural-abundance CO 2 (∼99% 12 CO 2 ) or 13 CO 2 and allowed to equilibrate before the spectra were collected (5 h in each case).Two additional experiments were carried out by dosing the activated framework with 300 mbar of CO 2 or 13 CO 2 , and spectra were collected at regular intervals until equilibration occurred after 13 h.Isotopic difference spectra were generated from both sets of data by subtracting the equilibrated 13 CO 2 spectrum obtained at 100 or 300 mbar from the corresponding equilibrated CO 2 spectrum.As shown in Figure 3a, the two difference spectra both feature characteristic bands associated with the C�O and C−N stretches of carbamate at 1644 and 1325 cm −1 , respectively, and the intensity of both bands increased at higher dosing pressure.These data are consistent with a mechanism involving ammonium carbamate formation upon the CO 2 uptake.
We also generated difference spectra by subtracting the spectrum of pristine pip2−Mg 2 (dobpdc) from time-resolved spectra collected for the framework after dosing with 300 mbar of CO 2 .These spectra are plotted in Figure 3b (pale to dark green) along with a difference spectrum obtained from subtracting the spectrum for the pristine framework from the equilibrated spectrum collected after dosing with 100 mbar of CO 2 (gray data).In the 100 mbar difference spectrum, negative peaks were apparent between 3400 and 3200 cm −1 , along with a large positive feature at 3394 cm −1 , consistent with the conversion of N−H vibrations associated with primary amines to those associated with secondary amines as carbamate is formed upon CO 2 uptake.At 100 mbar, no peak was apparent for physisorbed CO 2 (expected at ∼3696 cm −1 ; see Figure 3b, gray trace), consistent with only chemisorption occurring up to adsorption of ∼0.5 equiv of CO 2 per diamine.However, in the time-resolved difference spectra generated after dosing with 300 mbar of CO 2 , new peaks were apparent at 3696 and 3587 cm −1 , corresponding to the combination bands of physisorbed CO 2 . 44Notably, these peaks were visible within 15 min and grew concomitant with an increase in the intensity of the carbamate peak at 3394 cm −1 , indicating that well before equilibration at this pressure, physisorption is occurring simultaneously with ammonium carbamate formation.Both peaks associated with physisorbed CO 2 grew in intensity over the course of several hours (Figure 3b and inset) along with the carbamate peak.These data strongly suggest that CO 2 physisorption is occurring simultaneously with chemisorption in pip2−Mg 2 (dobpdc) as the material adsorbs CO 2 beyond 0.5 equiv per diamine.To gain additional insight into the adsorbed species formed upon CO 2 uptake in pip2−Mg 2 (dobpdc), we collected solidstate magic angle spinning (MAS) 15 N and 13 C NMR spectra after dosing the framework with 13 CO 2 .The 1 H → 15 N crosspolarization spectrum of pip2−Mg 2 (dobpdc) dosed with 1 bar of 13 CO 2 features resonances at ∼85 and ∼51 ppm (Figure 4a).Both peaks are consistent with resonances characterized previously for carbamate and ammonium generated upon CO 2 adsorption in various diamine−Mg 2 (dobpdc) materials. 38In addition, a two-dimensional 1 H → 13 C HETCOR spectrum collected for the same sample features a strong correlation at 4.9 ppm ( 1 H) and 163.2 ppm ( 13 C), consistent with the formation of ammonium carbamate species (Figure 4b,c). 38olid-state MAS 13 C NMR spectra were additionally collected for activated samples of pip2−Mg 2 (dobpdc) dosed and equilibrated at room temperature with 13 CO 2 pressures ranging from 0 to 300 mbar (see Experimental Section for details).After dosing with 100 mbar of 13 CO 2 , a clear resonance was apparent at 162.7 ppm, which we assign to carbamate based on 13 C NMR spectra reported for other diamine−Mg 2 (dobpdc) variants dosed with CO 2 . 38Upon increasing the 13 CO 2 pressure to 200 mbar, a new resonance became apparent at 125.1 ppm, which was assigned to physisorbed CO 2 . 38Both resonances persisted upon dosing with 300 mbar of 13 CO 2 , consistent with the DRIFTS data.
Notably, the presence of chemisorbed and physisorbed CO 2 at 200 mbar�a pressure within the second step of the 25 °C CO 2 isotherm�is further evidence that physisorption indeed occurs simultaneously with chemisorption in pip2− Mg 2 (dobpdc) after adsorption of an initial ∼0.5 equiv of CO 2 .Finally, integration of a 13 C NMR spectrum collected after dosing pip2−Mg 2 (dobpdc) with 1 bar of 13 CO 2 (Figure S12) yielded a chemisorbed:physisorbed CO 2 ratio of 1:0.43.Assuming saturation of the material with CO 2 under the experimental conditions, this ratio is consistent with the total uptake of ∼1.4 equiv of CO 2 per diamine in pip2− Mg 2 (dobpdc) (Figure 2b), significantly more than the 10− 15% physisorbed CO 2 that has been reported for other diamine-appended frameworks. 38Altogether, these results indicate that the unusually high CO 2 capacity of pip2− Mg 2 (dobpdc) arises due to a unique cooperative mechanism involving the simultaneous physisorption of CO 2 with CO 2 chemisorption at pressures corresponding to the second isothermal adsorption step.In contrast, for other diamineappended frameworks, chemisorption alone is associated with the step regions of the isotherm, whereas physisorption is predominantly a poststep phenomenon. 33,34tructural and Computational Investigation of CO 2 Binding.To elucidate the structure resulting upon CO 2 uptake in pip2−Mg 2 (dobpdc) and identify the location of the physisorbed CO 2 , we collected in situ powder X-ray diffraction data for a sample of the framework before and after dosing with 1 bar of CO 2 at 298 K (see Experimental Section for details).The diffraction pattern of pip2−Mg 2 (dobpdc) features more peaks than that of Mg 2 (dobpdc) (Figure S1), for example, three low intensity peaks at low 2θ values, while there are more differences at higher angles.Interestingly, the diffraction pattern collected for the CO 2 -dosed sample does not feature these additional peaks (Figure S6), and it is more consistent with typical diffraction patterns of diamineappended Mg 2 (dobpdc) with and without CO 2 . 33,34The disappearance of peaks after CO 2 dosing indicates that CO 2 adsorption gives rise to a more ordered and higher-symmetry space group for the resulting structure.Additionally, the diffraction peaks for the CO 2 -dosed sample appear at 2θ values lower than those for pip2−Mg 2 (dobpdc), indicating an expansion of the crystal lattice to accommodate CO 2 .
Single-crystal and powder X-ray diffraction analysis of other diamine-appended Mg 2 (dobpdc) and Mg 2 (olz) frameworks has revealed that these structures adopt various trigonal space groups, 34,36 although the diffraction pattern collected for pip2−Mg 2 (dobpdc) could not be indexed to any trigonal space group due to the presence of the additional peaks noted above.Attempts to index the pattern of activated pip2−Mg 2 (dobpdc) with lower symmetry space groups (either monoclinic or triclinic) were unsuccessful, although some extra peaks that could not be indexed with trigonal space groups were indexed by lower symmetry space groups (Figure S13 and Tables S1  and S2).The difficulties encountered with indexing the diffraction pattern to a single space group could be indicative of the presence of two distinct phases of pip2−Mg 2 (dobpdc) due to different orientations of the bulky diamine in the framework pores.
In contrast, the diffraction pattern collected for pip2− Mg 2 (dobpdc) dosed with CO 2 could be successfully indexed to the space group P3 2 21 (Table S2), consistent with the space group assignment for other diamine-appended Mg 2 (dobpdc) and Mg 2 (olz) frameworks dosed with CO 2 . 33,34To locate physisorbed CO 2 in the structure, a calculated structure for pip2−Mg 2 (dobpdc) featuring one CO 2 per diamine in the form of ammonium carbamate chains was generated using DFT with vdW corrections (see Experimental Section for details). 38We then introduced one additional CO 2 molecule for every two metal sites in the unit cell of the structure to simulate the uptake of half an equivalent of CO 2 per diamine and performed geometry optimization to allow the CO 2 molecule to rearrange in the pore.Repeating this calculation with different starting coordinates for CO 2 yielded three  candidate structures that were used for Rietveld refinement of the diffraction pattern for the CO 2 -dosed framework.In the first of these structures, the physisorbed CO 2 is located near one of two adjacent pip2 moieties in the ab plane (structure A, Figure S14).In the second structure, CO 2 is located in the center of the pore (structure B, Figure S16), and in the third structure, CO 2 is located in a pocket formed by two carbamates and the linker (structure C, Figure S18).
When the occupancy of the extra CO 2 molecule was freely refined in the candidate structures, structures A and B gave physically unreasonable negative occupancies, while structure C yielded a positive occupancy for extra CO 2 after the initial refinement.Further refinement of the atomic coordinates of structure C yielded a final structure for the CO 2 -dosed pip2− Mg 2 (dobpdc) (Figure 5, Figures S20 and S21, and Tables S2  and S3).Moreover, our DFT calculations with vdW corrections show that structure C is more stable than structures A and B by 10.1 and 8.0 kJ/mol, respectively.As shown in Figure 5, the physisorbed CO 2 molecules run along the framework channels and are located in the region between adjacent ammonium carbamate chains.The distance between the carbon of the CO 2 molecule and the oxygen atom of each carbamate is 3.4(2) Å (see Table S3).Based on the van der Waals radii of carbon and oxygen (1.70 and 1.52 Å, respectively), 45 this distance suggests that the physisorbed CO 2 molecules are engaged in stabilizing interactions with the ammonium carbamate chains.The physisorbed CO 2 was ultimately refined to an occupancy of 59(2)%; because it is located at a special position (Wyckoff position 3b) of the P3 2 21 space group, this occupancy translates to an uptake of approximately 0.3 equiv per carbamate, consistent with the CO 2 uptake determined from isobaric and isothermal analyses.
To further support our assignment of the position of physisorbed CO 2 , we used vdW-corrected DFT to simulate geometry-optimized structures for pip2−Mg 2 (dobpdc), pip2− Mg 2 (dobpdc) loaded with 0.5 equiv of CO 2 per diamine, and pip2−Mg 2 (dobpdc) loaded with 1.5 equiv of CO 2 , the latter based on the final structure discussed above.Using these optimized structures, we calculated the CO 2 binding energies and NMR chemical shifts associated with species formed after loading of 0.5 and 1.5 equiv of CO 2 per diamine (see Tables S4  and S5).For pip2−Mg 2 (dobpdc) loaded with 1.5 CO 2 per diamine, the computed binding energy is within 5 kJ/mol of the corresponding experimental Δh ads value (−48.6 versus −53(1) kJ/mol, respectively), and the simulated carbamate 13 C NMR shift of 165.9 ppm is comparable to the experimental peak position at 162.5 ppm.Taken together, the spectroscopic and computational results clearly support a mechanism of CO 2 uptake in pip2−Mg 2 (dobpdc) involving initial chemisorption of CO 2 at half of the diamine sites, followed by a dual chemisorptive/physisorptive process wherein binding of CO 2 at the remaining diamine sites is accompanied by uptake of an additional half equivalent of physisorbed CO 2 .
Adsorption Performance under Simulated Landfill Gas.−41 Both methane and CO 2 are potent greenhouse gases, and methane has a global warming potential nearly 30 times that of CO 2 . 46Capturing and refining methane from landfill gases have the potential to significantly reduce greenhouse gas emissions and also provide a valuable energy source.The generation of purified methane from landfill gas involves the removal of moisture and impurities, followed by removal of CO 2 . 46To evaluate the potential of pip2− Mg 2 (dobpdc) for this separation, we carried out breakthrough experiments using a custom-built breakthrough apparatus and binder-free pellets of the material (see Experimental Section for details).For safety reasons, a blend of CO 2 in N 2 was employed for the experiments instead of the use of CO 2 and CH 4 .A stream of dry 60% CO 2 in N 2 at 25 °C and ambient pressure was passed through the breakthrough column, and the outlet gas composition and flow rate were monitored over time.As shown in Figure 6a, a sharp breakthrough of CO 2 occurred after 15 min, and the CO 2 uptake capacity of the material under these conditions was determined to be 5.1 mmol/g.This high capacity remained unchanged over the course of four consecutive desorption (80 °C under flowing He) and breakthrough cycles (Figure 6a), and 1 H NMR spectroscopy analysis of a digested sample of the framework following this cycling experiment revealed that diamine loading remained at approximately 100%.
The framework also exhibits excellent stability to long-term TGA adsorption/desorption cycling (Figure 6b; adsorption: 15 min, dry 60% CO 2 in N 2 at 1 atm (Figure S24); desorption: 1 min, dry CO 2 at 1 atm and 80 °C).Over the course of 500 Journal of the American Chemical Society cycles, pip2−Mg2(dobpdc) exhibited a stable capacity of ∼16.9 g/100 g (or 3.83 mmol/g; note that this capacity is slightly lower than that determined from the breakthrough experiments as a result of the slightly higher adsorption temperature of 30 °C).Powder X-ray diffraction analysis of the material after the cycling experiment revealed that it remained highly crystalline (Figure S25), and 1 H NMR spectroscopy analysis of a digested framework sample revealed that diamine loading remained at ∼100%.These results highlight the excellent stability of pip2−Mg 2 (dobpdc) to long-term cycling under conditions relevant to CO 2 capture from landfill gas.

■ CONCLUSIONS
We have discovered a new diamine-appended metal−organic framework, pip2−Mg 2 (dobpdc), that captures CO 2 via an unprecedented mixed chemisorption/physisorption mechanism that endows it with a capacity of nearly 1.5 equiv of CO 2 per diamine, significantly more than previously reported materials in this class (typically <1.2 equiv per diamine).This behavior is associated with two-step CO 2 uptake in isothermal and isobaric adsorption data, involving sequential capture of ∼0.5 equiv of CO 2 per diamine in the first step, followed by ∼1 equiv of CO 2 per diamine in the second step.In situ DRIFTS and solid-state magic angle spinning NMR spectroscopy data support a mechanism in which uptake in the first step is associated with the formation of ammonium carbamate chains at half of the diamine sites in the material, followed by uptake of additional CO 2 via chemisorption at the remaining diamine sites and physisorption of CO 2 between resulting adjacent ammonium carbamate chains.Importantly, pip2− Mg 2 (dobpdc) retains its high capacity and exhibits exceptional stability over the course of breakthrough and extended cycling experiments using a simulated landfill gas.Ultimately, these results suggest that, with judicious choice of diamine, it may be possible to design a new class of amine-appended Mg 2 (dobpdc) (and Mg 2 (olz) 36 materials with significantly enhanced capacities as a result of a mixed chemisorption and physisorption mechanism, as demonstrated here.Additional studies are ongoing to understand the role of the diamine structure and pore environment in facilitating this adsorption mechanism.

■ EXPERIMENTAL SECTION
General Procedures.All of the experiments are carried out in the air, unless noted otherwise.All reagents and solvents were purchased from Sigma-Aldrich at reagent-grade purity or higher and used without further purification.The ligand H 4 dobpdc was purchased from Hangzhou Trylead Chemical Technology Co. Ultrahigh purity (>99.998%)gases used for all adsorption measurements were purchased from Praxair, as well as the custom gas blends of 60% CO 2 in N 2 .The solution-phase 1 H nuclear magnetic resonance (NMR) spectra were collected on a Bruker AMX 400 MHz NMR spectrometer for digested framework samples, which were referenced to residual dimethyl sulfoxide (δ = 2.50 ppm).

Synthesis of Mg 2 (dobpdc).
−32 Generally, Mg(NO 3 ) 2 •6H 2 O (11.6 g, 45.2 mmol) and H 4 (dobpdc) (9.75 g, 35.6 mmol) were dissolved in a 55:45 (v:v) mixture of methanol and N,N-dimethylformamide (DMF) (total volume of 200 mL) with sonication.The resulting solution was filtered to remove undissolved particles and transferred to a 350 mL high-pressure round-bottom flask equipped with a stir bar.The flask was sealed with a Teflon cap and then heated at 120 °C in an oil bath for 20 h with stirring (300 r.p.m.).After this time, an insoluble white solid had formed.The mixture was allowed to cool to room temperature, and the solid product was filtered using a Buchner funnel.The solid was then soaked in 300 mL of fresh DMF at 80 °C for 6 h and then isolated again via filtration.This process was repeated two more times, and the resulting solid was then soaked in 300 mL of fresh methanol at 60 °C for 6 h and then isolated via filtration; this process was repeated two more times.The resulting solid Mg 2 (dobpdc) was stored in fresh methanol at room temperature.−32 Synthesis of pip2−Mg 2 (dobpdc).Methanol-solvated Mg 2 (dobpdc) (150 mg) was dispersed in a 5 mL solution of 20% pip2 in toluene.The powder was soaked for 18 h and then filtered and washed with fresh toluene in a Buchner funnel (3 × 20 mL) at room temperature to remove as much residual diamine as possible prior to activation.The diamine-appended MOF was then activated at 130 °C for 1 h under flowing N 2 in a Schlenk flask equipped with a rubber septum and venting needle (the activation temperature was determined by the thermogravimetric decomposition analysis; see Figure S5).Diamine loading was determined by solution-phase 1 H NMR spectroscopy analysis of a digested sample of the material.Briefly, ∼2 mg of material was suspended in 0.5 mL of dimethyl sulfoxide-d 6 and 100 μL of DCl solution (35 g/100 g in D 2 O, ≥ 99 atom % D) was added to dissolve the sample. 32Surface areas, powder X-ray diffraction patterns, and decomposition profiles are presented in Figures S4−S6.
Thermogravimetric Analysis.Thermogravimetric analysis (TGA) data were collected by using a TA Instruments Discovery thermogravimetric analyzer.Thermogravimetric decomposition experiments were carried out under 100% N 2 with a temperature ramping rate of 2 °C/min from 30 to 600 °C with a gas flow rate of 25 mL/min.Masses were not corrected for buoyancy effects.Isobaric data under pure CO 2 were collected at ambient pressure using a gas flow rate of 25 mL/min.Prior to isobar collection, pip2− Mg 2 (dobpdc) was first activated by heating at 120 °C for 30 min under flowing N 2 .The inlet gas was then switched to 100% CO 2 , and the sample was held isothermally at 120 °C under flowing 100% CO 2 for 30 min to completely purge the system of N 2 .Adsorption isobar data were obtained while slowly cooling the sample to 30 °C with a ramping rate of 1 °C/min, and desorption data were collected upon then heating the sample to 130 °C at the same ramping rate.The reported two step temperatures from the adsorption isobar were determined using the inflection points of the adsorption steps, and the desorption temperature was determined at the point of closure of the hysteresis loop.
For analysis of adsorption kinetics (see Figure S24), the sample was first activated at 120 °C for 30 min under a 100% dry N 2 stream to remove remaining unreacted amine in the framework pores and then cooled to 30 °C prior to the kinetics experiments.The inlet gas was then switched to 60% CO 2 in N 2 at 1 bar, and the sample was held isothermally for 60 min to study the adsorption kinetics under conditions intended to simulate exposure to landfill gas (Figure S21).
For the cycling experiments, the conditions were based on the adsorption/desorption kinetics results.The materials were first activated at 120 °C for 30 min under a 100% dry N 2 stream to remove the extra diamine in the framework pores and then ramping to 30 °C prior the cycling experiments.The inlet gas was then switched to dry 60% CO 2 in N 2 (∼1 bar) at 30 °C, and the sample was held isothermally for 15 min to adsorb CO 2 ; the furnace was then ramped to 80 °C and held isothermally for 1 min as 100% dry CO 2 (∼1 bar) was flowed over the sample to desorb CO 2 .A total of 500 adsorption/ desorption cycles were performed.
Breakthrough Measurements.Breakthrough experiments were conducted using pip2−Mg 2 (dobpdc) to gauge the separation performance of the material under a multicomponent dynamic stream intended to simulate landfill gas.Experiments were carried out using a custom-built breakthrough apparatus consisting of Swagelok fittings and 1/8″ copper tubing connecting gas flow to the sample holder or bypassing the sample holder.Cylinders of CO 2 and N 2 were connected to the breakthrough manifold by using Alicat mass flow controllers.Gas flow was controlled to achieve 60% CO 2 and 40% N 2 gas streams with a flow rate of 5 sccm.In particular, prior to flowing the mixture through the pip2−Mg 2 (dobpdc) sample, the gas mixture was equilibrated by flowing through the breakthrough manifold with the sample column closed off.The outlet flow rate was verified using an Agilent ADM Flow Meter, and the outlet gas stream composition was verified using an SRI Instruments 8610 C GC equipped with a 6′ Haysep D column maintained at 110 °C.This purging of the manifold leads to the initial nonzero flow rate in the first few minutes of the breakthrough data in Figure 6.
The pip2−Mg 2 (dobpdc) sample was pelletized using a pellet die with 1 in.diameter and separated with a 20−40 mesh sieve.A sample of pellets (460 mg) was loaded into the sample holder and activated at 80 °C and under flowing He at 5 sccm for 20 min.Then, the sample was cooled to 25 °C for the breakthrough experiments.The outflow composition and flow rate throughout the breakthrough experiment were analyzed using the same flowmeter and GC specified above.Once CO 2 had broken through the packed pip2− Mg 2 (dobpdc) bed, the stream was switched to He gas at a flow rate of 5 sccm, and the sample holder was heated to 80 °C to fully desorb CO 2 from the column prior to subsequent breakthrough measurements.
Gas Adsorption Isotherms.Carbon dioxide adsorption isotherms were collected using a Micromeritics 3Flex gas adsorption analyzer, and 77 K N 2 adsorption isotherms were collected on a Micromeritics ASAP 2420 instrument.All gases were 99.998% pure or higher.The temperature was controlled by an oil bath during CO 2 adsorption isotherm collection and controlled by liquid nitrogen when collecting N 2 adsorption isotherms.Approximately 50 mg of activated pip2−Mg 2 (dobpdc) powder was transferred to a glass adsorption tube with a Micromeritics Transeal.Samples were regenerated at 100 °C under dynamic vacuum (<10 μbar) for overnight between isotherms.The isotherm data points were considered equilibrated if the pressure change is <0.01%after 11 consecutive equilibration time intervals (15  s).
Calculation of Differential Enthalpies and Entropies for Adsorption.The differential enthalpy (Δh ads ) and entropy (Δs ads ) of CO 2 adsorption for pip2−Mg 2 (dobpdc) were calculated using the Clausius−Clapeyron equation: From the isotherm fits, the exact pressure (p q ) corresponding to CO 2 loading (q) was determined at different temperatures (T) by plotting ln(p q ) versus 1/T at constant values of q.The y intercepts of these linear trendlines are equal to − Δs ads /R at each loading, and the slopes are equal to − Δh ads /R.Further details are shown in Figure S8−S10.
Differential Scanning Calorimetry.Differential scanning calorimetry (DSC) experiments were carried out using a TA Instruments Q1000/RCS90 DSC instrument.A sample of activated pip2−Mg 2 (dobpdc) was characterized under an ambient pressure of N 2 using a gas flow rate of 50 mL/min.The temperature was first ramped to 120 °C and held for 30 min to reactivate the material.Then, the material was cooled to 0 °C and heated to 120 °C (ramp rate of 2 °C/min).DSC data were collected between 25 and 80 °C (Figure S11).
Solid-State NMR Spectroscopy.All solid-state NMR spectra were collected at 11.7 T using a 4.0 mm Bruker MAS probe with a MAS rate of 10 kHz, except for the 13 C{ 1 H} 2D HETCOR spectrum, which was acquired at 16.5 T using a 3.2 mm Bruker MAS probe with a magic-angle spinning rate of 15 kHz.For data collection, activated pip2−Mg 2 (dobpdc) was packed into a 4.0 mm zirconia NMR rotor inside an argon-filled glovebox before being transferred with a rotor cap (to prevent air exposure) to a home-built gas manifold.For the 13 CO 2 dosing experiment, the rotor was transferred to a home-built gas manifold and degassed for 15 min.Subsequently, variable pressures (100, 200, 300, and 1000 mbar) of 13 CO 2 gas (Sigma-Aldrich, 99 atom % 13 C, <3 atom % 18 O) were dosed into the sample at room temperature and allowed to equilibrate over the course of at least 12 h before capping the rotor for data collection.In particular, after the material was initially dosed at each pressure, the pressure was monitored over time and additional 13 CO 2 was dosed into the system until the final pressure was the same as the desired dosing pressure (see our previous study for details on the gas manifold, which allows samples to be closed inside rotors at controlled CO 2 pressures). 38olid-state 13 C{ 1 H} and 15 N{ 1 H} cross-polarization (CP) NMR spectra were acquired by using an optimal contact time in the range of 1−5 ms with proton Spinal64 decoupling at a B1 field of 57.1 kHz during acquisition.Solid-state MAS 13 C NMR spectra (11.7 T) for a sample of pip2−Mg 2 (dobpdc) dosed at different 13 CO 2 pressures were acquired by a direct pulse with recycle delays (∼100−120 s) and with two-pulse phase modulation 1 H decoupling at 32 kHz.The quantitative 13 C NMR spectrum acquired after dosing pip2− Mg 2 (dobpdc) with 1 bar of 13 CO 2 was acquired by applying a long recycle delay (1000 s) with continuous-wave 1 H decoupling (MAS rate of 15 kHz).The 13 C{ 1 H} 2D HETCOR experiments also employed magnetization transfer by cross-polarization with a short contact time of 100 μs, used to selectively show short-range correlations.The 1 H, 15 N, and 13 C chemical shifts were referenced to 1.85 ppm (adamantane), 33.4 ppm (glycine), and 38.48 ppm (adamantane, tertiary carbon�left-hand resonance), respectively.
In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS).In situ DRIFTS data were collected using a Bruker Vertex 70 spectrometer equipped with a glowbar source, KBr beamsplitter, and liquid nitrogen cooled mercury−cadmium−telluride detector.A custom-built diffuse reflectance system with an IRaccessible gas dosing cell was used for all of the measurements.The cell is equipped with a heater controlled by a thermocouple in direct contact with the sample, and the sample atmosphere was controlled by a Micromeritics ASAP 2020Plus gas adsorption analyzer.In a typical experiment, a sample of the activated framework was dispersed in diamond powder (10 wt %) and evacuated at 120 °C before dosing.Known pressures of CO 2 (99.998%) or 13 CO 2 (Sigma-Aldrich, 99 atom % 13 C, <3 atom % 18 O) were dosed into the sample using a Micromeritics ASAP 2020Plus gas sorption analyzer.Spectra at 4 cm −1 resolution were generated from 128 scans collected over the course of approximately 35 s and collected at 1 min intervals until no further changes were observed.All spectra were processed in pseudoabsorbance units.Difference DRIFTS spectra were generated by subtracting the spectrum of the activated framework from the framework under various dosing conditions, and isotopic difference DRIFTS spectra were generated by subtracting the spectrum of the framework under 13 CO 2 from the spectrum of the framework under the same pressure of natural abundance CO 2 .
Powder X-ray Diffraction Data.Laboratory powder X-ray diffraction data were collected on a Bruker AXS D8 Advance diffractometer with Cu Kα radiation (λ = 1.5418Å) with sample powders placed on an open-air sample holder.Synchrotron powder Xray diffraction data were collected at Beamline17-BM-B at the Advanced Photon Source at Argonne National Laboratory using an average wavelength of λ = 0.45399 Å. Activated samples were packed in borosilicate glass capillaries (1.0 mm in diameter) under an N 2 atmosphere before being attached to a custom-designed gas-dosing cell equipped with a gas valve. 47These cells were then mounted on the goniometer head and connected to a gas-dosing manifold for in situ diffraction measurements.The sample temperature was controlled using an Oxford CryoSystems Cryostream800.A diffraction pattern was first collected for the activated sample at room temperature, and then the sample was briefly heated to 120 °C under dynamic vacuum before 1 bar of CO 2 was dosed to the framework.The sample was then cooled to 298 K under a CO 2 atmosphere.Diffraction patterns were recorded using a PerkinElmera-Si FlatPanel detector and monitored to confirm that the materials had reached equilibrium under gas-dosing conditions.Diffraction patterns were analyzed using TOPAS−Academic v6.1. 48nit cell parameters for CO 2 -dosed pip2−Mg 2 (dobpdc) were obtained by structureless Pawley refinement using TOPAS−Academic v6.1. 48The backgrounds of the pattern were modeled with Chebyshev polynomial functions.Peak shapes were described with the fundamental parameter approach.Using the parameters obtained by the Pawley refinement, Rietveld refinement was performed with the structural models of CO 2 -dosed pip2−Mg 2 (dobpdc) obtained from DFT calculations, as discussed (see Figures S13, S15, and S17; see DFT Calculations below for details of the geometry optimization).
The occupancy of CO 2 was first freely refined by using these models, and it was found (as discussed above) that only structure C gave a positive occupancy.For Rietveld refinement of structure C, the positions and atomic displacement parameters of the Mg atom and the atoms of the linker were refined with restraints.The positions and atomic displacement parameters of the C, O, and N atoms of the carbamate group attached to Mg 2+ were also refined with restraints.For the other atoms of the pip2 moiety, only atomic displacement parameters were refined, while the positions were not refined.The positions were fixed based on the DFT structure, the alkyl chain and the six-membered ring of pip2 are expected to be flexible at the analyzed temperature (298 K), and it may not be reasonable to determine the conformation of pip2 from these data.The position, atomic displacement parameters, and occupancy of physisorbed CO 2 were refined with restraints on the C−O bond and O−C−O angle, while the central carbon was freely refined at a Wyckoff position 3b between carbamates.The hydrogen atoms were added to the dobpdc 4− linker, ammonium carbamate, and pip2 after refinement with a C−H distance of 1.09 Å and a N−H distance of 1.02 Å using the structure edit tool of Mercury.Although the conformation of the ethylene moiety and the six-membered ring of pip2 was not refined owing to their flexibility, the refinement of the Mg 2 (dobpdc) framework and the physisorbed CO 2 gave a low R wp value (6.58%).This analysis supports the idea that the refined structure is a reasonable model for the approximate position of the physisorbed CO 2 .Note, however, that the bond lengths, bond angles, and atomic displacement parameters of the pip2 moiety are not captured in this model.
DFT Calculations.First-principles density functional theory (DFT) calculations were performed using GBRV pseudopotentials 49 and the revised Perdew−Burke−Ernzerhof (RPBE) 50 exchangecorrelation functional with the Quantum ESPRESSO plane wave DFT code. 51To include the effect of the van der Waals (vdW) dispersive interactions on energetics, we performed structural relaxations with Grimme's D3 correction for all calculations, as implemented in Quantum ESPRESSO. 52Additionally, for all calculations, we used (i) a 1 × 1 × 3 k-point grid sampling, (ii) a 60 Ry plane-wave cutoff energy, and (iii) a 600 Ry charge-density cutoff energy.We explicitly treated 10 valence electrons for Mg (2s 2 2p 6 3s 2 ), 6 for O (2s 2 2p 4 ), 5 for N (2s 2 2p 3 ), 4 for C (2s 2 2p 2 ), and 1 for H (1s 1 ).Using the above input parameters and initial structures obtained from the Rietveld refinement, we fully relaxed both lattice parameters and internal coordinates.The ions were relaxed until the force is less than 1 × 10 −4 Ry/Bohr.
Isotropic NMR chemical shielding values (δ iso ) were computed using the linear response method of Yates et al. 53 implemented in the Vienna ab initio Simulation Package (VASP) 54−57 according to the equation δ iso = (−δ iso-DFT − δ ref-DFT ), where δ ref-DFT is a reference value from the 13 C and 15 H chemical shifts of adamantane and glycine, respectively, from prior work. 38o compute CO 2 binding energies, we optimized three different structures: (1) a structure of pip2−Mg 2 (dobpdc) prior to CO 2 adsorption (E MOF ), (2) a structure of the framework interacting with CO 2 in the gas phase (E COd 2 ) within a 15 Å × 15 Å × 15 Å cubic supercell, and (3) a structure of pip2−Mg 2 (dobpdc) with adsorbed CO 2 (E COd 2 −MOF ).The binding energy (E B ) was then obtained by using the following equation:

Figure 1 .
Figure 1.(a) Depiction of cooperative CO 2 insertion into diamine− Mg 2 (dobpdc) to form chains of ammonium carbamate.(b) (Left) DFT-simulated structure of pip2−Mg 2 (dobpdc) and (right) detailed structure of first coordination sphere of a Mg II site in this structure.Green, blue, red, gray, and white spheres represent Mg, N, O, C, and H atoms, respectively.(c) Structure of linker H 4 dobpdc (left) and pip2 (right).

Figure 3 .
Figure 3. (a) Isotopic difference spectra generated by subtracting equilibrated DRIFTS data collected for pip2−Mg 2 (dobpdc) dosed with 12 CO 2 and 13 CO 2 at 100 mbar (gray trace) and 300 mbar (green trace).Blue bands highlight peaks associated with carbamate.(b) Difference spectra generated by subtracting DRIFTS data for bare pip2−Mg 2 (dobpdc) from spectra obtained for pip2−Mg 2 (dobpdc)dosed with 100 mbar of CO 2 after 5 h and time-resolved spectra for pip2−Mg 2 (dobpdc) dosed with 300 mbar of CO 2 .Dosing with 300 mbar of CO 2 results in the appearance of peaks at 3696 and 3587 cm −1 assigned to physisorbed CO 2 , concomitant with an increase in the intensity of a peak at 3394 cm −1 associated with carbamate formation (light to dark green).

Figure 5 .
Figure 5. Views of the structure of (CO 2 ) 1.5 −pip2−Mg 2 (dobpdc) determined from Rietveld refinement (left) along the c axis and (right) in the ab plane.Green, red, blue, gray, and white spheres represent Mg, O, N, C, and H, respectively.

Figure 6 .
Figure 6.(a) Breakthrough data for pip2−Mg 2 (dobpdc) exposed to flowing dry 60% CO 2 in N 2 (∼1 bar, 5 sscm) at 40 °C.The material adsorbed 5.1 ± 0.2 mmol of CO 2 per gram under these conditions.Note that the nonzero flow detected at the earliest time points is due to residual gas in the tubing downstream from the breakthrough column, resulting from purging the manifold with the gas mixture before the start of each run (see Experimental section for details).This initial flow does not affect the breakthrough capacity calculation.(b) Thermogravimetric temperature−swing cycling data collected for pip2−Mg 2 (dobpdc) at atmospheric pressure; adsorption: 30 °C, 60% CO 2 in N 2 , 15 min; desorption: 80 °C, CO 2 , 1 min.