Comparison between the different properties of the different MOFs.
Molecular-level characterization of interaction between small gases and metal organic frameworks (MOFs) is crucial to elucidate the adsorption mechanism and establish the relationship between the structure and chemical features of MOFs with observed adsorptive properties, which ultimately guide the new structure design and synthesis for enhanced functional performance. Among different techniques, vibrational spectroscopy (infrared and Raman), which provides fingerprint of chemical bonds by their vibrational spectra, is one of the most powerful tools to study adsorbate-adsorbent interaction and give rich detailed information for molecular behaviors inside MOFs pores. This chapter reviews a number of exemplary works utilizing vibrational spectroscopy to study the interaction of small molecules with metal organic frameworks.
- small molecules
- metal organic frameworks (MOFs)
- infrared (IR)
- Raman spectroscopy
- vdW-DF calculation
In the past decade, metal organic frameworks (MOFs) have become one of the fastest growing new fields in chemistry. Tremendous advances has been made in synthesis for new structures (>20,000 reported), structure determination or postmodification, and exploration of potential application in different fields such as gas storage and separation (H2, CO2, N2, and CO2), drug delivery, sensing, luminescence, and catalysis based on adsorption. The mechanistic understanding of the interaction of the molecules with MOFs is critical for the rational design of new MOFs with desired properties and accurate assessment of functional performance in practical applications. Traditional characterization methods for MOFs materials have relied mainly on physical measurements, such as X-ray diffraction, thermogravimetric, gas adsorption isotherm, and breakthrough analysis. These techniques are powerful in deriving some critical parameters, such as crystal structures, chemical composition, thermal stability, adsorptive uptake, enthalpy, and selectivity, for assessing adsorption properties; however, mechanistic information about the local bonding sites, adsorption geometry, and guest-host, guest-guest cooperative, or competitive interaction is particularly difficult to derive. Experimental methods currently employed by the community to analyze how the molecules interact with a framework include infrared and Raman spectroscopy, X-ray (neutron) diffraction, and inelastic neutron scattering. While all these techniques have been shown to be useful to identify the binding sites of the MOFs toward the small molecules, vibration spectroscopy, i.e., infrared and Raman spectroscopy is particularly sensitive to probe the local interaction between guest molecules and the surface of metal organic frameworks. These two spectroscopic techniques provide complementary information about the nature of interaction, bonding configurations, intermolecular attraction, or repulsion through their vibrational spectra. Furthermore, they require lower capital cost and have greater accessibility of the instrumentation, which is easily modified for in situ measurements in a wide range of temperatures and pressures.
In this chapter, the recent progress of infrared and Raman spectroscopy studies on the underlying interactions that govern adsorption behaviors of small molecules, i.e., H2, CO2, H2O, O2, CO, NO, H2S, SO2, in different MOFs materials is discussed and summarized. In most cases for nonreactive molecules, such as H2 and CO2, van der Waals forces dominate the interaction between the guest molecules and the building units of the MOFs. In some cases, chemical reaction involving electron transfer occurs upon adsorption of reactive molecules, e.g., H2O, leading to a significant modification of MOFs crystalline structure. Combined with calculation, especially the recent successful effort to include van der Waals forces, self-consistently in DFT(Density functional theory) in the form of a van der Waals density functional, molecular weak physical interactions within MOFs materials are accurately described and experimental data can be well interpreted and rationalized.
2. Interaction of small molecules with MOFs
2.1. Energy carrier H2
H2 molecule is IR inactive as other homonuclear diatomic molecules due to their lack of dipole moment; however, once the molecule interacts with the MOF, it undergoes a perturbation that polarizes the originally symmetric molecule and makes it weakly IR active. This perturbation is usually accompanied by red shift of the H─H stretching modes, located at 4161 and 4155 cm−1 for para and ortho H2, respectively. Nijem’s measurement on different type of prototypical MOFs suggest that magnitudes of the H2 stretching frequency shifts, intensities, and line widths contains important information about the nature of the H2 interaction in the pores and depend on the structure and chemical nature of the MOF hosts .
By examining several prototypes of metal organic framework materials such as M(bdc)(ted)0.5 (bdc = 1, 4-benzenedicarboxylate; ted = triethylenediamine), M(bodc)(ted)0.5 (M = Ni, Co; bodc = bicyclo[2. 2. 2]octane-1, 4-dicarboxylate), M3(HCOO)6 (M = Ni, Mn, Co, HCOO = formate), and Zn2(bpdc)2(bpee), where bpdc = 4,4′-biphenyl dicarboxylate and bpee = 1,2-bis(4-pyridyl)ethylene , it is concluded (see Table 1) that IR shifts are dominated by the environment (organic ligand, metal center, and structure) rather than the strength of the interaction. For instance, the organic ligands with π = electrons such as benzene rings cause the frequency of H2 shift more than the ligand without π = electrons, e.g., H2 bands are more shifted (~−3 cm−1) for Ni(bdc)(ted)0.5 than Ni(bodc)(ted)0.5 even though Ni(bodc)(ted)0.5 has a higher binding energy. The same observation was also found by comparing the IR shift (−38 cm−1) of hydrogen molecules in Zn(bdc)(ted)0.5 with that (−30 cm−1) in MOF-74 (also called M2(dobdc), M = metal ions; dobdc4−= 2,5-dioxidobenzene-1,4-dicarboxylate), a structure containing unsaturated metal center Zn2+ with a higher binding energy (10 kJ/mol).
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 corresponding IR cross-section is higher . 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 . 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 adsorption energy .
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 .
Splitting of bending mode β(CO2) due to the removal of degeneracy of the in-plane and out-of-plane bending is commonly observed in the case of electron-donor-acceptor (EDA) complexes of CO2 with basic functional groups in polymer such as ─NH2, ─OH via carbon of CO2 as an electron acceptor . 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.
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 . 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 CO2 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 CO2 adsorption isotherms that shift markedly with temperature . 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, characteristic features of ammonium.
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 . 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 CO2 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) . 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.
2.3. Reactive gas molecules
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 , MOF-177 , HKUST-1 , and MOF-74  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 representative 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+] 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 hydrolysis 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 frequency 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.
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 . 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 back-donation 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.
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 interaction, with an irreversible capacity of 9.3 wt%, corresponding to the adsorption of one O2 molecule per two iron centers . 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 . 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 , M(bdc)(ted)0.5 , FMOF-2 , and NOTT-300 ). 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 . 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 unperturbed 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.
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 measurement. 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 . 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.
Compared to the extensive studies that focus on the single component adsorption, the coadsorption of muticomponents remains scarcely investigated due in part to experimental difficulties, for instance, the isotherm of muticomponent adsorption, the composition of each species adsorbed can only be derived indirectly by measuring variation of gas-phase composition. Methods, such as mass spectrometry, have to be incorporated with isotherm measurements. 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 . 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. Surprisingly, 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 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 highlight 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 measurements, 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.
Ab initio simulations was performed to calculate the exchange barrier (activation energy) and study the reaction pathway for the H2O → CO2 and SO2 → CO2 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.
Vibrational spectroscopy has been proved to be the very informative technique to investigate the interaction of small gas molecules with metal organic frameworks. By examining subtle changes in the spectra of both adsorbate and adsorbent, insightful details regarding the adsorption mechanism are revealed. With the help of theoretical calculation, which provides direct access to many properties of the system, the experimental models are validated and a complete understanding of the adsorption behaviors can be derived.
For H2, although free molecule is IR inactive, the stretching mode is activated and becomes observable once the molecule is polarized by binding to the surface. A wealth of information for the interaction details, i.e., binding site and geometry, interaction potential can be extracted by analyzing the peak position, intensity, and width.
For CO2 molecules, both the perturbation of stretching and bending mode convey important information for the nature of interaction. For physical adsorption with lower binding energy (<50–60 kJ/mol), the stretching mode suffers a small shift (<15 cm−1) compare to gas phase value and the bending mode is spitted due to the loss of degeneracy. If the molecules are chemically adsorbed with a high adsorption heat over 60–70 kJ/mol, IR adsorption features of new species such as carbamate can be observed. The structural modifications for functional groups are reflected by tracking the spectroscopic signatures.
For the reactive molecules such as H2O, O2, H2S, SO, and NO adsorbing into MOFs, the crystalline structure is strongly modified and even become degraded. By examining the difference spectra before and after adsorption, the weak point of the complicated MOFs structure can be identified and reaction pathway can be also unveiled, which is crucial to design robust structure.
Finally, infrared spectroscopy provides an unique advantage to study the adsorption behaviors of mixture components since the vibrational modes of different molecules usually can be well distinguished in the infrared spectra. The occupation of actual adsorption sites for mixtures can be measured as a function of parameters such as time, temperature, and partial pressure. Recent works in measuring CO2 competition with a variety of molecules, e.g., H2O, NH3, SO2, NO, and NO2 in MOF-74 show kinetics for exchange process is an important parameter which needs to be taken into account for coadsorption and separation process. It also underscores the need of combined studies, using spectroscopic methods and ab initio simulations to uncover the atomistic interactions of small molecules in MOFs that directly influence coadsorption.