**2.2. Greenhouse emission CO2**

CO2 is a linear molecule and has large quadrupole moment and high polarizability. The symmetric stretching mode (ν1, 1342 cm−1) is Raman active but not IR active, whereas the antisymmetric modes (β, 667 cm−1 and ν3, 2349 cm−1) are IR active. By interaction with adsorbent surface, the frequency position is perturbed and the adsorption band is affected.

Upon adsorption of CO2 onto the active binding site of open metal ions within M-MOF-74 systems (M = Mg2+, Zn2+, Ni2+, Co2+, Mn2+), the induced frequency shift of antisymmetric mode ν3 is highly dependent on the nature of metal ions: it undergoes a blue shift in Mg2+ MOF while red shifts in Zn2+ MOF and other transitional metal analogs as shown in **Figure 2** [4, 5]. *Ab initio* calculations were performed in Mg2+ and Zn2+ MOFs utilizing vdW-DF, finding three factors contributing to this shift, i.e., namely, (i) the change in the molecule length, CO2 molecules in Zn-MOF-74 experiences a larger elongation (0.0009 Å) than in Mg-MOF-74 (0.0009 Å), (ii) the asymmetric distortion of the CO2 molecule, the asymmetric distortion in Mg-MOF-74 is stronger than that in Zn-MOF-74, which is consistent with a larger blue shift effect in the CO2 ν3 frequency, and (iii) the direct influence of the metal center. Specifically, the presence of the d orbitals in Zn2+ prevents the charge density transfer within the adsorbed CO2 molecules, leading to small charge transfer compared to Mg2+ case [4].

Splitting of bending mode β(CO2) due to the removal of degeneracy of the in-plane and outof-plane bending is commonly observed in the case of electron-donor-acceptor (EDA) com‐ plexes of CO2 with basic functional groups in polymer such as ─NH2, ─OH via carbon of CO2 as an electron acceptor [7]. The earliest spectroscopic evidences for the formation of an electron-donor-acceptor complex between CO2 and functional groups of MOFs was reported in a MOF of type MIL-53 (MIL stands for Materials Institute Lavoisier) and amino-based MOF Zn4O (NH2-BDC)3 (IR-MOF-3) and (NH2-MIL-53(Al) [6, 8]. Two bands at 669 and 653 cm−1 in **Figure 3**, corresponding bending mode of CO2 was observed, indicating a lowering of symmetry upon adsorption. Moreover, ν(OH) and δ(OH) modes are red shifted by −19 and −30 cm−1, respectively, suggesting that oxygen atoms of hydroxyl group act as the electron donor.

Integrated intensity of the H2 stretching modes is a sensitive measure of the number and symmetry of the sites and local interaction between H2 and organic ligand. Asymmetric site with multiple interaction points produced larger induced dipole moment and the correspond‐ ing IR cross-section is higher [1]. The symmetric sites lead to reduced dynamic dipole moment and lower the IR band intensity. **Figure 1** compares the adsorption site of H2 in M(bdc) (ted)0.5 and Zn2(bpdc)2(bpee): H2 interacts with several benzene rings in Zn2(bpdc)2(bpee) and the adsorption site is very asymmetric. Furthermore, the delocalized electrons in the double benzene rings in Zn2(bpdc)2(bpee) is more easily polarized by adsorbed H2 than that in the single benzene ring in Zn(bdc)(ted)0.5. All these factors cause IR intensity of H2 adsorbed in Zn2(bpdc)2(bpee) almost a magnitude higher than that of H2 adsorbed in M(bdc)(ted)0.5.

It is worth to note that IR is not only sensitive to host-gust interaction but also capable to detect molecular interactions within confined nanopores. In MOF-74 with unsaturated metal center, a small shift (−30 cm−1 with respect to the unperturbed molecules) is observed in the low loading regime when H2 is dominantly adsorbed on the metal site [2]. Additional ~−32 cm−1 IR shift and a large variation in dipole moment are observed once the neighboring oxygen site was occupied with H2 molecule to form a "pair" with H2 molecules on the metal site. Since large variation of dynamic dipole moment take place as a function of loading, due to the interaction among the adsorbed molecules and therefore the integrated areas of IR bands do not always correlate with the amount of molecules adsorbed. Cautions must be taken when using variable temperature IR to measure the absorbance of molecular hydrogen bands and estimate the

CO2 is a linear molecule and has large quadrupole moment and high polarizability. The symmetric stretching mode (ν1, 1342 cm−1) is Raman active but not IR active, whereas the antisymmetric modes (β, 667 cm−1 and ν3, 2349 cm−1) are IR active. By interaction with adsorbent

Upon adsorption of CO2 onto the active binding site of open metal ions within M-MOF-74 systems (M = Mg2+, Zn2+, Ni2+, Co2+, Mn2+), the induced frequency shift of antisymmetric mode ν3 is highly dependent on the nature of metal ions: it undergoes a blue shift in Mg2+ MOF while red shifts in Zn2+ MOF and other transitional metal analogs as shown in **Figure 2** [4, 5]. *Ab initio* calculations were performed in Mg2+ and Zn2+ MOFs utilizing vdW-DF, finding three factors contributing to this shift, i.e., namely, (i) the change in the molecule length, CO2 molecules in Zn-MOF-74 experiences a larger elongation (0.0009 Å) than in Mg-MOF-74 (0.0009 Å), (ii) the asymmetric distortion of the CO2 molecule, the asymmetric distortion in Mg-MOF-74 is stronger than that in Zn-MOF-74, which is consistent with a larger blue shift effect in the CO2 ν3 frequency, and (iii) the direct influence of the metal center. Specifically, the presence of the d orbitals in Zn2+ prevents the charge density transfer within the adsorbed

Splitting of bending mode β(CO2) due to the removal of degeneracy of the in-plane and outof-plane bending is commonly observed in the case of electron-donor-acceptor (EDA) com‐

surface, the frequency position is perturbed and the adsorption band is affected.

CO2 molecules, leading to small charge transfer compared to Mg2+ case [4].

adsorption energy [3].

224 Metal-Organic Frameworks

**2.2. Greenhouse emission CO2**

**Figure 2.** (Top) Illustration of CO2 absorbed in Zn, Mg-MOF-74. (Bottom) IR absorption spectra of CO2 absorbed into Mg-MOF-74 (bottom left) and Zn-MOF-74 (bottom right) at changing CO2 pressure (1–6 Torr). Adapted with permis‐ sion from [4]. Copyrighted by the American Physical Society.

**Figure 3.** CO2 introduction on MIL-53 (Cr) activated at 473 K. Spectra of activated MIL-53(Cr) deposited on silicon wa‐ fer (dotted lines) in three regions and then after introduction of increasing CO<sup>2</sup> equilibrium pressures into the cell (full lines): (a) 1066 Pa, (b) 2400 Pa, (c) 3850 Pa, (d) 5000 Pa, (e) 5850 Pa. Inset: perturbation of the δ(OH) mode upon CO<sup>2</sup> adsorption. Reproduced from [6] with permission of The Royal Society of Chemistry.

MOF H3[(Cu4Cl)3(BTTri)8](H3BTTri = 1,3,5-tri(1H- 1,2,3-triazol-4-yl)benzene) functionalized with N, N′-dimethylethylenediamine (mmen) shows significantly enhanced CO2 adsorption with exceptional large isosteric heat of CO2 around 96 kJ/mol [9]. The strong interaction of amine group of mmen with CO2 molecules was directly proved by diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) measurements. Upon dosing CO<sup>2</sup> to the sample with increasing pressure, the intensity of ν(N─H) band at 3283 cm−1 is significantly weakened and a new band at 1669 cm−1 appears, suggesting the formation of zwitterionic carbamates. N─H stretching band returns to back by regeneration of the solid under vacuum and heating to 60°C.

A very recent work shows that diamine-appended metal-organic frameworks M2(dobpdc) (dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), an expanded variant of the well-studied metal-organic framework MOF-74, with N,N′-dimethylethylenediamine (mmen) can behave as "phase change" adsorbents, with unusual step-shaped CO<sup>2</sup> adsorption isotherms that shift markedly with temperature [10]. In the unprecedented cooperative process it was found that, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing the reorganization of ammine into well-ordered chains of ammonium carbamate. **Figure 4** shows the insertion mechanism for CO2 adsorption and spectral evolution upon cooling process. The formation of ammonium carbamate was confirmed by detecting the ν(C═O) mode at 1690 cm−1, ν(C─N) at 1334 cm−1, and broadening of ν(N─H) band, charac‐ teristic features of ammonium.

**Figure 4.** (Left) A cooperative insertion mechanism for CO<sup>2</sup> adsorption. (Right) Infrared spectra on dosing an activated sample of mmen-Mg2(dobpdc) (black) with CO<sup>2</sup> and cooling from 150°C to 30°C (red to blue) under 5% CO2 in N2. The three different regions show bands corresponding to N─H (left), C─O (middle), and C─N (right) stretching vibra‐ tions. Reprinted by permission from Macmillan Publishers Ltd: [Nature] [10], copyright (2015).

CO2 molecules are also stimuli to induce the structural transformation of some flexible MOFs. One of the earliest reports was CO2 adsorption in chromium terephthalate MIL-53 [11]. CO2 isotherm exhibits two-step adsorption and combined study of *in situ* diffraction and IR measurement provides clear explanation to this observation: after the first portion of CO<sup>2</sup> is absorbed into degassed solid, the host-guest interactions force the framework to close and cell to shrink. A further adsorption of CO2 between 5 and 10 bar, reopens the framework while accepting additional CO2 molecules into a newly formed channel. The strong interaction of CO2 with the frameworks involves the formation of electron-donor-acceptor complexes between C atom of molecules and electron-donor center OH group of the framework, which was deduced from bending mode β(CO2) splitting and ν(OH) mode red shift by *in situ* IR spectroscopy. The pore opening phenomenon was also observed in CO2 adsorption in Zn2(bpdc)2(bpee) [12]. Raman and IR spectroscopy are combined together to investigate the CO2 molecules interaction with Zn2(bpdc)2(bpee) and found that interaction of CO2 with the framework weakens the C─C inter-ring of the bpdc ligand, allowing it to rotate slightly around the monodentate connectivity C─O─Zn node. This rotation causes a series of changes resulting in pore opening, which is responsible for the preferential adsorption of CO2 over N2. Raman spectra in **Figure 5** shows the spectroscopic evidences for these changes: that the C─C inter-ring stretching in the bpdc ligand at 1296 cm−1 undergoes a red shift of ~−3 cm−1 and coordinated C─O symmetric stretching at 1355 cm−1 blue shift by ~+10 cm−1, A ~+4 cm−1 blue shift of the band at 1644 cm−1 corresponding to the C═C stretching in the ethylene of the bpee ligand, is also observed. DFT calculations provide the support for the qualitative picture derived from the experimental analysis that more energetically favorable positions for the CO2 molecules are closer to the C─C bond of the bpee and the C─C bond of the bpdc instead of their benzene and pyridine rings.

**Figure 5.** (Left) Raman spectra of activated Zn2(bpdc)2(bpee) in a vacuum and after introduction of 1 atm of CO2 at room temperature; (right) adsorption sites of CO2 in complete occupation in Zn2(bpdc)2(bpee) determined by vdW-DFT, where each pore within a unit cell is occupied by four CO2 molecules. Reprinted with permission from [12]. Copyright (2011) American Chemical Society.

#### **2.3. Reactive gas molecules**

MOF H3[(Cu4Cl)3(BTTri)8](H3BTTri = 1,3,5-tri(1H- 1,2,3-triazol-4-yl)benzene) functionalized with N, N′-dimethylethylenediamine (mmen) shows significantly enhanced CO2 adsorption with exceptional large isosteric heat of CO2 around 96 kJ/mol [9]. The strong interaction of amine group of mmen with CO2 molecules was directly proved by diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) measurements. Upon dosing CO<sup>2</sup> to the sample with increasing pressure, the intensity of ν(N─H) band at 3283 cm−1 is significantly weakened and a new band at 1669 cm−1 appears, suggesting the formation of zwitterionic carbamates. N─H stretching band returns to back by regeneration of the solid under vacuum and heating to 60°C.

A very recent work shows that diamine-appended metal-organic frameworks M2(dobpdc) (dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), an expanded variant of the well-studied metal-organic framework MOF-74, with N,N′-dimethylethylenediamine (mmen) can behave as "phase change" adsorbents, with unusual step-shaped CO<sup>2</sup> adsorption isotherms that shift markedly with temperature [10]. In the unprecedented cooperative process it was found that, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing the reorganization of ammine into well-ordered chains of ammonium carbamate. **Figure 4** shows the insertion mechanism for CO2 adsorption and spectral evolution upon cooling process. The formation of ammonium carbamate was confirmed by detecting the ν(C═O) mode at 1690 cm−1, ν(C─N) at 1334 cm−1, and broadening of ν(N─H) band, charac‐

**Figure 4.** (Left) A cooperative insertion mechanism for CO<sup>2</sup> adsorption. (Right) Infrared spectra on dosing an activated sample of mmen-Mg2(dobpdc) (black) with CO<sup>2</sup> and cooling from 150°C to 30°C (red to blue) under 5% CO2 in N2. The three different regions show bands corresponding to N─H (left), C─O (middle), and C─N (right) stretching vibra‐

CO2 molecules are also stimuli to induce the structural transformation of some flexible MOFs. One of the earliest reports was CO2 adsorption in chromium terephthalate MIL-53 [11]. CO2 isotherm exhibits two-step adsorption and combined study of *in situ* diffraction and IR measurement provides clear explanation to this observation: after the first portion of CO<sup>2</sup> is absorbed into degassed solid, the host-guest interactions force the framework to close and cell to shrink. A further adsorption of CO2 between 5 and 10 bar, reopens the framework while accepting additional CO2 molecules into a newly formed channel. The strong interaction of CO2 with the frameworks involves the formation of electron-donor-acceptor complexes between C atom of molecules and electron-donor center OH group of the framework, which was deduced from bending mode β(CO2) splitting and ν(OH) mode red shift by *in situ* IR

tions. Reprinted by permission from Macmillan Publishers Ltd: [Nature] [10], copyright (2015).

teristic features of ammonium.

246 Metal-Organic Frameworks

Water stability is a main concern for any potential applications of MOFs in industrial settings because moisture is ubiquitous in the environment, i.e., complete removal of H2O from gas sources is difficult. Many widely investigated MOFs, particularly built by carboxylate acid ligand such as MOF-5 [13], MOF-177 [14], HKUST-1 [15], and MOF-74 [16] are susceptible to reaction with moisture. Understanding the degradation mechanism is a complex problem because there are a variety of independent factors that play a critical role in the stability of MOFs. However, the metal-ligand bond is regarded as the weakest point of a MOF structure. To decouple the effects of metal-ligand bond from other factors such as topology, porosity, and surface areas on the structural stability of MOFs, two types of prototypical and represen‐ tative isostructure MOFs with different metal centers: (1) MOFs M(bdc)(ted)0.5 [M = Cu2+, Zn2+, Ni2+, Co2+] with saturated metal centers and (2) MOF-74 [M2(dobdc), M = Mg2+, Zn2+, Ni2+, Co2+]

**Figure 6.** Schematic illustration of decomposition pathway of M(bdc)(ted)0.5 [M = Cu, Zn, Ni, Co] reaction with D2O molecules. Reprinted with permission from [17]. Copyright (2012) American Chemical Society.

with unsaturated metal centers were chosen for study [17, 18]. The former involves a secondary building unit (metal center configuration) that is very common to MOFs. The latter, referred to as MOF-74, is one of the most characterized materials for single gas adsorption as it is one of the best carbon capture materials. Combining spectroscopy methods (*in situ* infrared absorption and Raman) and powder X-ray diffraction, it was found that the stability of MOF in the presence of water vapor critically depends on their structure and the specific metal cation in the building units; and that water condensation inside pores, which is highly dependent on external vapor pressure and temperature, is the critical phenomenon that induces the hydrol‐ ysis reaction. In the case of M(bdc)(ted)0.5, the metal-bdc bond is the most vulnerable for Cu(bdc)(ted)0.5, while the metal-ted bond is first attacked for the Zn and Co analogs. In contrast, Ni(bdc)(ted)0.5 remains stable under conditions where all other M(bdc)(ted)0.5 materials are chemically attacked as shown in **Figure 6**. In the case of M2(dobdc), or MOF-74, the weak link is the phenolate-metal bond. At room temperature, water is molecularly adsorbed inside the MOF channel. Above 150°C, the water molecule is dissociatively adsorbed at the metal-oxygen group with OH adsorption directly on the metal center and H adsorption on the bridging O of the phenolate group. Interestingly, the latter O─H bond is only detected when D2O is used due to the strong vibrational coupling of the O─H bending vibration to the dobdc linker vibrations (see **Figure 7**). In contrast, the O─D bending vibration is fully decoupled from the linker vibrations (i.e., behaves as a local vibrational mode) leading to a strong, sharp, and detectable absorption band. Due to the passivation of open metal sites by hydroxyl group, the MOFs compounds lose a substantial fraction of their original gas uptake capacity.

NO adsorption has been studied before in Ni, Co-MOF-74 by isotherm, X-ray diffraction infrared and Raman spectroscopy, showing that NO interacts strongly with metal centers, forming NO coordination adduct with a binding energy of 90–92 kJ/mol [19, 20]. Infrared spectra shows that the stretching band ν(NO) of adsorbed NO molecules appears at a fre‐ quency between 1845 and 1838 cm−1 as coverage increases (see **Figure 8**). The red shift from the gas phase value at 1876 cm−1 indicates the interaction between NO and Ni-MOF-74 involves back donation of d electrons from the metal to the antibonding orbital of nitrosyl group, therefore weakening N─O bond. The coverage dependent of ν(NO) stretching frequency suggest dipole-dipole interaction of adsorbed NO molecules. By gradually dosing the water vapor on the NO loaded sample, infrared spectra of **Figure 8** shows that NO is gradually removed. This ability of water displacing preadsorbed NO in slow kinetics make this materials a promising candidate for NO delivery in biological tissues.

with unsaturated metal centers were chosen for study [17, 18]. The former involves a secondary building unit (metal center configuration) that is very common to MOFs. The latter, referred to as MOF-74, is one of the most characterized materials for single gas adsorption as it is one of the best carbon capture materials. Combining spectroscopy methods (*in situ* infrared absorption and Raman) and powder X-ray diffraction, it was found that the stability of MOF in the presence of water vapor critically depends on their structure and the specific metal cation in the building units; and that water condensation inside pores, which is highly dependent on external vapor pressure and temperature, is the critical phenomenon that induces the hydrol‐ ysis reaction. In the case of M(bdc)(ted)0.5, the metal-bdc bond is the most vulnerable for Cu(bdc)(ted)0.5, while the metal-ted bond is first attacked for the Zn and Co analogs. In contrast, Ni(bdc)(ted)0.5 remains stable under conditions where all other M(bdc)(ted)0.5 materials are chemically attacked as shown in **Figure 6**. In the case of M2(dobdc), or MOF-74, the weak link is the phenolate-metal bond. At room temperature, water is molecularly adsorbed inside the MOF channel. Above 150°C, the water molecule is dissociatively adsorbed at the metal-oxygen group with OH adsorption directly on the metal center and H adsorption on the bridging O of the phenolate group. Interestingly, the latter O─H bond is only detected when D2O is used due to the strong vibrational coupling of the O─H bending vibration to the dobdc linker vibrations (see **Figure 7**). In contrast, the O─D bending vibration is fully decoupled from the linker vibrations (i.e., behaves as a local vibrational mode) leading to a strong, sharp, and detectable absorption band. Due to the passivation of open metal sites by hydroxyl group, the

**Figure 6.** Schematic illustration of decomposition pathway of M(bdc)(ted)0.5 [M = Cu, Zn, Ni, Co] reaction with D2O

molecules. Reprinted with permission from [17]. Copyright (2012) American Chemical Society.

268 Metal-Organic Frameworks

MOFs compounds lose a substantial fraction of their original gas uptake capacity.

NO adsorption has been studied before in Ni, Co-MOF-74 by isotherm, X-ray diffraction infrared and Raman spectroscopy, showing that NO interacts strongly with metal centers, forming NO coordination adduct with a binding energy of 90–92 kJ/mol [19, 20]. Infrared spectra shows that the stretching band ν(NO) of adsorbed NO molecules appears at a fre‐ quency between 1845 and 1838 cm−1 as coverage increases (see **Figure 8**). The red shift from the gas phase value at 1876 cm−1 indicates the interaction between NO and Ni-MOF-74 involves

**Figure 7.** (Left) IR spectra of Zn-MOF-74 exposed to H2O (blue) and D2O (red), referenced to the activated MOF spec‐ tra, compared with IR absorption spectra of activated MOF-74, referenced to the KBr pellet; (right) calculated vibra‐ tional modes of (a) Zn-MOF-74 with dissociated D2O (D + OD) at 950 cm−1, (b) Zn-MOF-74 with dissociated H2O (H + OH) at 1317 cm−1. Reprinted with permission from [18]. Copyright (2014) American Chemical Society.

**Figure 8.** (Bottom) IR spectra of increasing NO equilibrium pressures dosed at RT on Ni-MOF-74 (equilibrium *p*max = 0.1 mbar). Bold gray curve, vertically translated for clarity, reports the spectrum collected before NO dosage. (Middle) Spectra obtained upon successive progressive outgassing of loaded NO. (Top) Effect of water dosage on the irreversi‐ ble NO (last spectrum of the middle part). Black spectra report the effect of 0.1 mbar NO dosage. Gray curves show the further interaction of water, at the vapor pressure as a function of contact times (up to 10 min). Reprinted with permis‐ sion from [19]. Copyright (2008) American Chemical Society.

CO is demonstrated to be reversibly adsorbed in MOF-74 (M = Mg, Mn, Fe, Co, Ni, Zn) analogies with the binding strength following by the order of Ni > Co > Fe > Mg > Mn > Zn [21]. This sequence is in distinct contrast to that observed for CO2 adsorption in these materials. The molecular adsorption configurations are shown in **Figure 9**. While CO2 interaction with metal ions of MOF-74 frameworks is predominately electrostatic, CO coordination also involves σ and π orbital interactions, as being probed by infrared spectroscopy. For instance, CO exhibits a small blue shift in Fe, and Co-MOF-74 compared to Mg and Zn-MOF-74 since π backdonation in Fe and Co weaken the C-O bond, however, Mg2+ ions lack d electrons and unable to back-donate into the empty CO orbitals and Zn2+ ions has full occupied 3d orbital and are not available to accept σ charge donation from CO.

**Figure 9.** (Upper left) Structures from powder neutron diffraction. A view down a channel (along the *c* axis) in the structure of Fe2(dobdc)·1.5CO, as determined by Rietveld analysis of powder neutron diffraction data and (upper right) coordination environment for a single Fe2+ site in Fe2(dobdc)·1.5CO; (bottom) first coordination sphere for the M2+ ions in M2(dobdc)·1.5CO, with M─CO distances and M─C─O angles indicated; (Right) Background subtracted FTIR spectra of M2(dobdc) collected at 77 K in the presence of CO. Light to dark lines represent increasing CO cover‐ age on samples. Reprinted with permission from [21]. Copyright (2014) American Chemical Society.

O2 is IR inactive and nonreactive to many MOFs materials, however, gas adsorption isotherms at 298 K indicate that Fe2(dobdc) binds O2 preferentially over N2 via electron transfer interac‐ tion, with an irreversible capacity of 9.3 wt%, corresponding to the adsorption of one O2 molecule per two iron centers [22]. Infrared spectra show that upon oxygenation at low temperature. A partially reduced (near superoxo) O2 species coordinated to FeII/III sites was observed at 1129 cm−1, assigned to ν(O─O). The formation of iron(III)-peroxide species at 790 cm−1 was detected at room temperature. The charge-transfer interaction was also found in adsorption of O2 in a Zn-MOF containing 7, 7, 8, 8-tetracyano-p-quinodimethane (TCNQ) ligand where organic linker is the active adsorption site. The large red shift of the sharp ν(O═O) band by 100 cm−1 was observed in IR spectra upon loading O2 at 90 K [23]. This frequency shift is too large to be induced by the confinement effect alone. It suggests that O2 molecules accommodated in TCNQ MOF have a partial negative charge, donated by electron rich organic linker.

SO2 adsorption has been studied in MOFs materials such as IR-MOFs (which have the same underlying topology as MOF-5 [24], M(bdc)(ted)0.5 [25], FMOF-2 [26], and NOTT-300 [27]). Among all studied structures, the uptake of SO2 in M(bdc)(ted)0.5 at room temperature is highest, 9.97 mol/kg at 1.13 bar [25]. The adsorption mechanism of SO2 within this class of MOFs is further explored by *in situ* IR spectroscopy, finding that the major adsorbed SO2 molecules contributing to the isotherm measurements are characterized by stretching bands at 1326 and 1144 cm−1, −36 and −7 cm−1 (see **Figure 10**), red shift respectively from the unper‐ turbed values of 1362 and 1151 cm−1 of gas phase. In addition, the IR spectra reveal the presence of another minor species at 1242 and 1105 cm−1 that is more strongly bound, requiring a higher temperature (~150°C) to remove. SO2 adsorption also induces significant changes to the frameworks vibrational modes (see **Figure 10**): (1) a blue shift of the νas(COO) mode (Δν = +56 cm−1) and of the νs(COO) mode (Δν = +39 cm−1), and (2) a decrease in intensity of νas(CH2), νs(CH2), and ν(CH) modes at 2874, 2938, and 3076 cm−1, respectively. Furthermore, the CH2 rocking mode and benzene ring deformation mode σ12 are red shifted by −8 and −12 cm−1. Calculations based on vdW-DF codes suggest that two adsorption configurations are possible for these SO2 molecules. One geometry involves an SO2 molecule bonded through its sulfur atom to the oxygen atom of the paddlewheel building unit and its two oxygen atoms to the C─H groups of the organic linkers by formation of hydrogen bonds. Such a configuration results in a distortion of the benzene rings, which is consistent with the experimentally observed shift of the ring deformation mode.

CO is demonstrated to be reversibly adsorbed in MOF-74 (M = Mg, Mn, Fe, Co, Ni, Zn) analogies with the binding strength following by the order of Ni > Co > Fe > Mg > Mn > Zn [21]. This sequence is in distinct contrast to that observed for CO2 adsorption in these materials. The molecular adsorption configurations are shown in **Figure 9**. While CO2 interaction with metal ions of MOF-74 frameworks is predominately electrostatic, CO coordination also involves σ and π orbital interactions, as being probed by infrared spectroscopy. For instance, CO exhibits a small blue shift in Fe, and Co-MOF-74 compared to Mg and Zn-MOF-74 since π backdonation in Fe and Co weaken the C-O bond, however, Mg2+ ions lack d electrons and unable to back-donate into the empty CO orbitals and Zn2+ ions has full occupied 3d orbital and are

**Figure 9.** (Upper left) Structures from powder neutron diffraction. A view down a channel (along the *c* axis) in the structure of Fe2(dobdc)·1.5CO, as determined by Rietveld analysis of powder neutron diffraction data and (upper right) coordination environment for a single Fe2+ site in Fe2(dobdc)·1.5CO; (bottom) first coordination sphere for the M2+ ions in M2(dobdc)·1.5CO, with M─CO distances and M─C─O angles indicated; (Right) Background subtracted FTIR spectra of M2(dobdc) collected at 77 K in the presence of CO. Light to dark lines represent increasing CO cover‐

O2 is IR inactive and nonreactive to many MOFs materials, however, gas adsorption isotherms at 298 K indicate that Fe2(dobdc) binds O2 preferentially over N2 via electron transfer interac‐ tion, with an irreversible capacity of 9.3 wt%, corresponding to the adsorption of one O2 molecule per two iron centers [22]. Infrared spectra show that upon oxygenation at low temperature. A partially reduced (near superoxo) O2 species coordinated to FeII/III sites was observed at 1129 cm−1, assigned to ν(O─O). The formation of iron(III)-peroxide species at 790 cm−1 was detected at room temperature. The charge-transfer interaction was also found in adsorption of O2 in a Zn-MOF containing 7, 7, 8, 8-tetracyano-p-quinodimethane (TCNQ) ligand where organic linker is the active adsorption site. The large red shift of the sharp ν(O═O) band by 100 cm−1 was observed in IR spectra upon loading O2 at 90 K [23]. This frequency shift is too large to be induced by the confinement effect alone. It suggests that O2 molecules accommodated in TCNQ MOF have a partial negative charge, donated by electron

age on samples. Reprinted with permission from [21]. Copyright (2014) American Chemical Society.

not available to accept σ charge donation from CO.

2810 Metal-Organic Frameworks

rich organic linker.

**Figure 10.** Snapshot of initial adsorption configuration models of SO2 in Zn(bdc)(ted)0.5 and IR absorption spectra after loading SO2 at 256 Torr, recorded immediately after evacuation of gas phase (within 16 s). Reprinted with permission from [25]. Copyright (2013) American Chemical Society.

MOFs such as MIL-47 (V), MIL-53(Al, Cr) with saturated metal center show weak interaction with H2S and adsorption/desorption behavior is completely reversible in isotherm measure‐ ment. For some MOFs structures MIL-101(Cr), HKUST-1 with unsaturated metal sites [28, 29], the adsorption of H2S is quite reactive, leading to release of BTC ligand from coordination with copper metal center and the formation of carboxlyate C═O group as indicated by the appearing of a band at 1710 cm−1. MOF-74 with nickel center shows a strong binding strength toward H2S with an adsorption heat ΔHads of ~57 kJ/mol [30]. The PXRD pattern shows that the structure itself is stable under H2S exposure, which is consistent with the observation from IR spectra that most MOFs phonon modes are slightly affected. The molecular state of H2S in Ni-MOF-74 is characterized by its clear IR features at 2626, 2614, and 1182 cm−1, corresponding to asymmetric, symmetric stretching, and bending mode.

#### **2.4. Coadsorption**

Compared to the extensive studies that focus on the single component adsorption, the coad‐ sorption of muticomponents remains scarcely investigated due in part to experimental diffi‐ culties, for instance, the isotherm of muticomponent adsorption, the composition of each species adsorbed can only be derived indirectly by measuring variation of gas-phase com‐ position. Methods, such as mass spectrometry, have to be incorporated with isotherm meas‐ urements. *In situ* IR spectroscopy in conjunction with *ab initio* modeling can provide information on local bonding sites and exchange mechanisms, as demonstrated by recent work in prototypical structure MOF-74 [31]. Using CO2 as a probe molecule, competitive coadsorption of CO2 with a variety of small molecules (H2O, NH3, SO2, NO, NO2, N2, O2, and CH4) in M-MOF-74 (M = Mg, Co, Ni) was investigated by infrared spectroscopy. Sur‐ prisingly, the displacement of CO2 adsorbed at the metal center by other molecules such as H2O, NH3, SO2, NO, NO2, N2, O2, and CH4 is mainly observed for H2O and NH3, even though SO2, NO, and NO2 have higher binding energies (~70–90 kJ/mol) to metal sites (Mg2+, Ni2+, Co2+) than that of CO2 (~38–48 kJ/mol) and slightly higher than that of water (~60–80 kJ/ mol) as shown in **Figure 11**. The exchange process is differentiated by plotting the intensity of H2O band ν(OH) ~above 2600 cm−1 and CO2 band νas at 2341 cm−1 (see **Figure 12**). The uptake of water is dominated by transport; therefore, taking its complement (red curve in **Figure 12**) for the CO2 release (gray curve) simulates what is expected if exchange kinetics

**Figure 11.** (Left, a) IR absorption spectra of 8 Torr H2O (broad peak above ~2600 cmȒ1) adsorbed into Co-MOF-74 with preloaded CO2 (peak at 2341 cm−1) as a function of time. All referenced to the activated MOF under vacuum (<20 mTorr). The black line in the bottom shows the absorption spectra of CO2 before loading water. (Right, b) IR absorp‐ tion spectra of activated and CO2-preloaded Co-MOF-74 exposed to ~150 Torr SO2. The bottom spectrum shows the adsorbed CO2 in Co-MOF-74 under vacuum (<20 mTorr). The middle part shows the time-dependent spectrum during SO2 exposure and the spectrum after evacuation of gas phase SO2. The differential spectrum on the top right of (b) shows the spectrum recorded after loading SO2 for 10 min and evacuation of gas phase, referenced to the spectrum before loading SO2 as shown in the bottom black spectrum. Reprinted with permission from [31]. Copyright (2015) American Chemical Society.

were much faster than diffusion. It represents the amount of CO2 displaced by water as it arrives at the CO2 site. This simulated CO2 concentration was specifically plotted to high‐ light that the measured CO2 concentration (black curve in **Figure 12**) evolved more slowly (additional time is required for exchange since the barrier is not negligible). This simulated curve (based on the water intake) is then used to normalize the CO2 absorption measure‐ ments, i.e., remove the transport part of the time evolution by subtracting the time due to transport (diffusion of water molecules to inner pores). The net result is shown in the inset of **Figure 12** that now represents only the exchange kinetics.

IR spectra that most MOFs phonon modes are slightly affected. The molecular state of H2S in Ni-MOF-74 is characterized by its clear IR features at 2626, 2614, and 1182 cm−1, corresponding

Compared to the extensive studies that focus on the single component adsorption, the coad‐ sorption of muticomponents remains scarcely investigated due in part to experimental diffi‐ culties, for instance, the isotherm of muticomponent adsorption, the composition of each species adsorbed can only be derived indirectly by measuring variation of gas-phase com‐ position. Methods, such as mass spectrometry, have to be incorporated with isotherm meas‐ urements. *In situ* IR spectroscopy in conjunction with *ab initio* modeling can provide information on local bonding sites and exchange mechanisms, as demonstrated by recent work in prototypical structure MOF-74 [31]. Using CO2 as a probe molecule, competitive coadsorption of CO2 with a variety of small molecules (H2O, NH3, SO2, NO, NO2, N2, O2, and CH4) in M-MOF-74 (M = Mg, Co, Ni) was investigated by infrared spectroscopy. Sur‐ prisingly, the displacement of CO2 adsorbed at the metal center by other molecules such as H2O, NH3, SO2, NO, NO2, N2, O2, and CH4 is mainly observed for H2O and NH3, even though SO2, NO, and NO2 have higher binding energies (~70–90 kJ/mol) to metal sites (Mg2+, Ni2+, Co2+) than that of CO2 (~38–48 kJ/mol) and slightly higher than that of water (~60–80 kJ/ mol) as shown in **Figure 11**. The exchange process is differentiated by plotting the intensity of H2O band ν(OH) ~above 2600 cm−1 and CO2 band νas at 2341 cm−1 (see **Figure 12**). The uptake of water is dominated by transport; therefore, taking its complement (red curve in **Figure 12**) for the CO2 release (gray curve) simulates what is expected if exchange kinetics

**Figure 11.** (Left, a) IR absorption spectra of 8 Torr H2O (broad peak above ~2600 cmȒ1) adsorbed into Co-MOF-74 with preloaded CO2 (peak at 2341 cm−1) as a function of time. All referenced to the activated MOF under vacuum (<20 mTorr). The black line in the bottom shows the absorption spectra of CO2 before loading water. (Right, b) IR absorp‐ tion spectra of activated and CO2-preloaded Co-MOF-74 exposed to ~150 Torr SO2. The bottom spectrum shows the adsorbed CO2 in Co-MOF-74 under vacuum (<20 mTorr). The middle part shows the time-dependent spectrum during SO2 exposure and the spectrum after evacuation of gas phase SO2. The differential spectrum on the top right of (b) shows the spectrum recorded after loading SO2 for 10 min and evacuation of gas phase, referenced to the spectrum before loading SO2 as shown in the bottom black spectrum. Reprinted with permission from [31]. Copyright (2015)

to asymmetric, symmetric stretching, and bending mode.

**2.4. Coadsorption**

3012 Metal-Organic Frameworks

American Chemical Society.

**Figure 12.** Evolution of integrated areas of the IR bands of water ν(OH) (red) and CO2 νas(CO2) (black) in Co-MOF-74 at 24°C. The inset purple bar curve shows the νas(CO2) band evolution corrected by removing the water transport time (taken from the gray curve) and hence provides the intensity evolution solely controlled by exchange kinetics. Reprint‐ ed with permission from [31]. Copyright (2015) American Chemical Society.

**Figure 13.** Adsorption configuration of the H2O (top), NH3 (middle), SO2 (bottom) molecule on Mg-MOF-74 fully load‐ ed with CO2 molecules at the metal centers. Red, brown, gray, blue, and orange spheres denote O, C H, N, and Mg atoms, respectively. Parts of the MOF have been removed for visualization purposes. Reprinted with permission from [31]. Copyright (2015) American Chemical Society.

*Ab initio* simulations was performed to calculate the exchange barrier (activation energy) and study the reaction pathway for the H2O → CO2 and SO2 → CO<sup>2</sup> exchange processes at the primate adsorption site of the open metal center. It was found that hydrogen bonding of H2O or NH3 molecules with the nearby oxygen of the organic linker facilitates the positioning of the H2O oxygen atom toward the metal center and displacing the preadsorbed CO2 molecule as shown in **Figure 13**. However, SO2 (and other molecules without H atoms) are not able to do so and remain bound at a distant site of carbon ring from metal center. In order to displace the CO2 molecules at metal site, SO2 needs to break away from the attractive force of the initial adsorption sites to move to the meter center and overcomes a high energy barrier (~20 kJ/mol) than water molecules (~13 kJ/mol) to remove the preoccupied CO2 molecules.

This important scientific finding revolutionized the understanding of MOF coadsorption by establishing that the displacement of one molecule by another within porous materials is a complex process that the energetics consideration alone cannot successfully predict. In other words, the binding energy at the most favorable adsorption site is not a sufficient indicator of the molecular stability in MOFs and kinetics of exchange process must be considered.
