CO Guest Interactions in SDB-Based MetalâOrganic Frameworks: A

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CO Guest Interactions in SDB-based Metal-Organic Frameworks – A Solid-State Nuclear Magnetic Resonance Investigation Ying-Tung Angel Wong, Troy K Babcock, Shoushun Chen, Bryan E.G. Lucier, and Yining Huang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02205 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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CO Guest Interactions in SDB-based Metal-Organic Frameworks – A Solid-State Nuclear Magnetic Resonance Investigation Y. T. Angel Wong,† Troy K. Babcock,† Shoushun Chen,† Bryan E. G. Lucier,† Yining Huang*,† †

Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, Ontario,

Canada, N6A 5B7

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Abstract Metal-organic frameworks (MOFs) are promising materials for greener carbon monoxide (CO) capture and separation processes. SDB-based (SDB = 4,4’-sulfonyldibenzoate) MOFs are particularly attractive due to their remarkable gas adsorption capacity under humid conditions. However, to the best of our knowledge, their CO adsorption abilities have yet to be investigated. In this report, CO-loaded PbSDB and CdSDB were characterized using variable temperature (VT)

13

C solid-state nuclear magnetic resonance (SSNMR) spectroscopy. These

MOFs readily captured CO, with the adsorbed CO exhibiting dynamics as indicated by the temperature-dependent changes in the SSNMR spectra. Spectral simulations revealed that the CO simultaneously undergoes a localized wobbling about the adsorption site and a nonlocalized hopping between adjacent adsorption sites. The wobbling and hopping angles were also found to be temperature-dependent. From the appearance of the VT spectra and the extracted motional data, the CO adsorption mechanism was concluded to be analogous to that of CO2. In order to gain a better understanding on the gas adsorption properties of these MOFs and their CO capture abilities, the motional data were subsequently compared to those reported for CO2 in SDB-based MOFs and CO in MOF-74, respectively. A significant contrast in adsorption strength was observed in both cases due to the different physical properties of the guests (i.e., CO vs. CO2) and the MOF frameworks (i.e., SDB-based MOFs vs. MOFs with open metal sites). Our results demonstrate that SSNMR spectroscopy can be employed to probe variations in binding behavior.

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Introduction Carbon monoxide (CO) consists of great industrial importance as it is a common precursor for various chemical commodities. For instance, it is employed for the production of simple organic compounds as well as various oxo-alcohols.1 Since CO is generally produced as a gas mixture (e.g., CO2, H2, N2, etc.), separation must first be performed in order for CO to be utilized as a feedstock.2 Cryogenic distillation is commonly employed for CO purification; however, it is a highly energy demanding process. In comparison to distillation, CO adsorption via metal-organic frameworks (MOFs) is much more energy efficient and is therefore an attractive alternative for CO separation.3 MOFs are crystalline materials constructed from metal ions/clusters connected by organic linkers.4, 5 They possess high porosity and internal surface area, making them highly suitable for gas adsorption.4-8 Furthermore, by using different combinations of metal centres and linkers, MOFs can be easily altered to adsorb specific gas molecules (e.g., CO) with a desired interaction strength.3,

6-8

As such, they are a promising class of materials for gas

separation and capture. Many of the MOF studies on CO capture are conducted using MOFs with open metal sites (OMSs).9-29 While these classes of MOFs have shown notable CO adsorption abilities, many of them degrade upon exposure to water vapor30-32 and are therefore not suitable for usage under ambient environments.6 In order to rationally design and optimize MOFs for practical purposes, systems with alternative guest adsorption mechanisms should be investigated. In the past few years, a series of MOFs have been constructed with SDB ligands (SDB = 4,4’sulfonyldibenzoate; Figure 1).33-46 These MOFs demonstrated remarkable abilities for gas capture and separation despite the lack of OMSs.33,

35-46

For instance, strong carbon dioxide


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(CO2) affinity and selectivity were observed for CaSDB41,

45

and CdSDB37 even under humid

conditions. Crystallographic studies revealed that the CO2 molecules reside in the V-shaped “πpockets” formed by the phenyl rings of the linkers,37, 44-45 and CO2 binding was concluded to result from the interaction between the CO2 quadrupole moment and the delocalized π electrons of the phenyl rings.45 Moreover, these MOFs are promising materials for CO capture. Firstly, the pore sizes of these MOFs (e.g., ca. 13 Å x 11 Å for CdSDB37, 44 and ca. 10 Å x 10 Å for PbSDB35) are larger than the kinetic diameter of CO (3.76 Å47). Secondly, CO possesses a nonnegligible quadrupole moment (2.50 x 10-26 esu∙cm2)48, and a recent computational analysis showed that CO interacts with aromatic π-electrons primarily through quadrupole driven electrostatic interactions.49 As such, CO can be expected to bind with the SDB-based MOFs and these MOFs can potentially be employed for CO adsorption. Nevertheless, to the best of our knowledge, CO adsorption studies using SDB-based MOFs have yet to be reported and the corresponding interaction has yet to be investigated. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is a powerful method for MOF characterization as it can provide detailed knowledge on the framework structures,50-52 location of the adsorption sites,53,

54

guest dynamics,14,

29, 44, 55-59

and host-guest interaction

strengths.14, 44, 54, 56, 58, 60 For instance, the type of motion the guest molecules display is closely associated with the corresponding adsorption mechanism.14,

44, 54, 56, 60

Even though guest

dynamics can be difficult to study using alternative experimental techniques (e.g., single crystal X-ray diffraction), it can easily be investigated via SSNMR spectroscopy by the use of static variable temperature (VT)

13

C experiments.14,

29, 44, 53-55, 58, 59

In contrast to the sharp peaks

detected in solution phase NMR spectra, broad powder patterns are generally observed for 13C


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in the solid state due to the presence of chemical shift anisotropy (CSA). The appearance of the powder pattern is governed by the local environment of the nuclei as well as the molecular orientations. Consequently, molecular motions can result in predictable changes in the spectral appearance and dynamic information (i.e., the type of motion exhibited by the guest molecules and the corresponding rates and angles) can be extracted via line shape simulations. Using this method, CO2 dynamics in various types of MOFs,44,

53-55, 59, 60

including that of SDB-based

MOFs,44 as well as CO motions in different kinds of MOF-74 have been successfully studied.14 Moreover, knowledge on the guest motions can provide a better understanding on the corresponding adsorption mechanism. Previous studies have demonstrated that guest dynamics can be employed to assess binding strengths since molecules that are bound tighter can be expected to be less dynamic.14, 54, 56, 60 For guest molecules that undergo wobbling upon a localized site, a smaller wobbling angle (α) signifies a stronger binding.14, 54, 60 In the case of CO, α has been measured for MOF-74 systems.14 The extracted trends were found to be in great agreement with the heat of adsorption values, demonstrating that CO binding strengths can be evaluated based on motional angles. Here, we investigated the host-guest interaction between CO and two SDB-based MOFs, PbSDB and CdSDB, using 13C SSNMR spectroscopy. VT spectra were recorded from 173 to 353 K for PbSDB and 153 to 373 K for CdSDB. Analytical simulations were performed as to extract the corresponding

13

C chemical shift (CS) tensor parameters, while dynamic simulations were

conducted to gain a better understanding on the CO motions. To assess the CO binding abilities of SDB-based MOFs, the extracted motional data were compared with those reported for CO adsorbed in MOF-74. The effect of guest identity on the adsorption strength of SDB-based


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MOFs was also investigated by contrasting the CO dynamics in PbSDB and CdSDB with those reported for CO2.

Experimental Section Sample Preparation PbSDB was synthesized using previously published procedures.35 Briefly, 0.165 g of Pb(NO3)2 (BDH Chemicals, 99.5%), 0.306 g of H2SDB (Sigma-Aldrich, 97%) and 0.123 g of LiNO3 (BDH, 99.5%) were dissolved in 10 ml of a 1:1 dimethylformamide (DMF, Caledon Laboratories, 99.8%)/methanol (MeOH, Fisher Chemical, 99.9%) solution. The resulting mixture was placed in a Teflon chamber within a Teflon-lined stainless-steel autoclave and sealed. The sealed autoclave was first heated at 160 oC for 2 days using an oven, then cooled at room temperature for 5 days without agitation. Colorless needle-shaped crystals were acquired after 5 days, and the crystals were separated via vacuum filtration and three MeOH washes were performed. CdSDB was synthesized according to literature,37 but with minor modifications. A mixture of 0.109 g Cd(NO3)2∙4H2O (Strem Chemicals, 98%) and 0.104 g of H2SDB (Sigma-Aldrich, 97%) was dissolved using 7 mL of ethanol (EtOH, Fischer Chemical, 95%) and 3 mL of deionized H2O as solvent. The resulting mixture was then placed in a Teflon chamber with a Teflon-lined stainless-steel autoclave. The sealed autoclave was then oven-heated at 180 oC for 2 days and subsequently cooled at room temperature for 48 hours without agitation. Colorless rectangular crystals were obtained, which were then separated using vacuum filtration and washed with EtOH three times. Powder X-ray diffraction was performed on the as-made PbSDB and as-made CdSDB. The experimental powder patterns were in good agreement with the corresponding calculated patterns, both of which are provided in the Supporting Information. 6 ACS Paragon Plus Environment

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In order to activate the MOFs and to load them with 13CO, the MOFs were first packed into the bottom of an L-shaped glass tube and a small amount of glass wool was employed to hold the sample in place. The glass tube was then attached to a vacuum line and heated under vacuum at a temperature of 180oC and a pressure of ≤ 1 mbar for approximately 12 hours. Once activated, the MOFs were loaded with an excess amount of 13CO (Sigma-Aldrich, 99% 13C isotope enriched). This was accomplished by first introducing the 13CO gas into the vacuum line, then submerging the glass tube in liquid nitrogen in order to condense and trap the 13CO. Lastly, the glass tube was removed from the vacuum line by flame-sealing. SSNMR Spectroscopy All SSNMR spectra were recorded using a Varian Infinity Plus wide-bore NMR spectrometer equipped with an Oxford wide-bore magnet (B0 = 9.4 T, ν0(13C) = 100.5 MHz) and a 5 mm HX static Varian Chemagnetics probe. Dry nitrogen gas and a Varian VT control unit were also employed. Temperature calibration was executed using the

207

Pb chemical shift of Pb(NO3)2,61

with an error of ±2 K. Chemical shift referencing was performed using the methylene carbon signal from ethanol (δiso = 58.05 ppm with respect to tetramethylsilane (TMS))62. The DEPTHecho pulse sequence was employed for all experiments in order to suppress probe background signal.54, 63 For the CO-loaded PbSDB samples, a π/2 pulse length of 2.25 μs was employed and a total of 480000 scans were executed for each spectrum. For the CO-loaded CdSDB samples, the π/2 length was set to 2.7 μs and a total of 10000000 scans were acquired at each temperature in order to obtain an optimal signal to noise ratio. For both samples, the recycle delay was 3 s and the echo delay was 40 μs. All data were processed using MNova and the recorded FID were left-shifted to the echo maxima. Analytical simulations were performed using WSolids64.


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Motional simulations were conducted by modelling the CSA line shapes at 9.4 T using EXPRESS65. The 13C NMR parameters of rigid CO66 were used as input parameters. The spectral width was set to 100 kHz and powder averaging was accomplished using a minimum of 1000 powder increments and the ZCW method. For the PbSDB simulations, the π/2 pulse width was 2.25 μs and 1 to 2 k points were used in the FID. For the CdSDB simulations, the π/2 pulse width was 2.7 μs and 512 points were used in the FID. The motional types, angles and rates were systematically varied until an agreement was reached between the simulated and experimental spectra.

Results and Discussion VT 13C SSNMR Spectroscopy Results A series of static VT 13C SSNMR experiments were performed for CO-loaded PbSDB (T = 173 to 353 K) and CO-loaded CdSDB (T = 153 to 373 K), and the corresponding spectra are shown in Figure 2a and 3a, respectively. As 99% isotopically-labelled

13

CO gas was employed for all

experiments, only the guest molecules contribute significantly to the observed NMR signals. For CO in PbSDB, two overlapping resonances can be seen in the NMR spectra via visual inspection – a sharp peak at ca. 184 ppm and a broad powder pattern with significant CSA influence (Figure 2a). The isotropic nature of the narrow resonance indicates that the CO molecules are undergoing rapid reorientation and therefore concluded to originate from gaseous, free CO species. This assignment is also in agreement with the previously reported isotropic chemical shift (δiso) value (ca. 182 ppm) for rapidly tumbling CO molecules.66 On the other hand, CO species adsorbed in the MOF framework can be expected to undergo restricted motion and the corresponding NMR resonance will be dominated by CSA.14, 8 ACS Paragon Plus Environment

29

Therefore, the broad powder

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pattern was assigned to the adsorbed CO molecules. Furthermore, temperature-dependent changes in the relative intensities of the signals can be seen (Figure 2a). At higher temperatures, the isotropic signal dictates the spectra, while at colder temperatures, the spectra are predominated by the anisotropic component. Similar behavior has also been reported for CO in Mg-MOF-74,14 Zn-MOF-7414 and Cu3-xZnx(btc)229, and can be attributed to an increase in CO adsorption at lower temperatures.14 Likewise, for the CdSDB sample, visual inspection reveals that the spectra can be deconvoluted into contributions from gaseous and adsorbed CO molecules (Figure 3a), and the relative intensities of the signals were found to be temperature dependent. However, the spectral feature arising from the adsorbed CO molecules deviates from that of an ideal powder pattern, hinting towards the presence of more than one resonance. Nevertheless, it can be concluded from this quick analysis that SDB-based MOFs are capable of CO adsorption. To further interpret the VT experiment results, analytical simulations of the SSNMR spectra were performed for both samples (Figure 2b for CO-loaded PbSDB and Figure 3b for CO-loaded CdSDB). For CO in PbSDB, the results confirm the existence of a single powder pattern (Figure 2b) and the corresponding 13C CS tensor parameters are summarized in Table 1. Since the number of NMR signals is directly related to the number of crystallographically inequivalent sites, the detection of a single powder pattern indicates that there is only one crystallographically distinct CO adsorption site in PbSDB. Moreover, the apparent CS tensor span, Ω, was found to be ca. 45 ppm in the studied temperature range (T = 353 to 173 K), which is significantly less than that of rigid CO (Ω = 353 ppm)66. The observed skew, κ, was also found to increase from 0.30 to 0.86 when the temperature was raised. Since rapid molecular motions


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can result in the averaging of

13

C CS tensors,67,

68

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the observed behaviors of Ω and κ both

suggest that the adsorbed CO experiences dynamic motion. For the CdSDB sample, analytical simulation uncovered the presence of two powder patterns (site 1 and site 2; Figure 3b), and thus, two crystallographically inequivalent CO adsorption sites. The corresponding 13C CS tensor parameters are given in Table 2. Previous Xray diffraction studies on CdSDB showed that some of the linkers in the pore channel consist of intrinsically disordered phenyl rings,37, 44 which allows for an unique guest adsorption site that is absent in PdSDB (Figure 4).44 Consequently, the presence of two crystallographically distinct CO sites can be expected. A comparison between the δiso values showed that site 2 likely corresponds to the site that is unobserved for PbSDB as the corresponding δiso values were drastically different (δiso,CdSDB

site 2

≈ 145 ppm vs. δiso,

PbSDB

≈ 182 ppm), while the chemical

environment of CO in site 1 resembled that of PbSDB as the corresponding δiso values were similar (δiso,CdSDB site 1 ≈ 178 ppm; δiso, PbSDB ≈ 182 ppm). Dynamics were once again detected for the adsorbed CO as the apparent 13C CS tensor parameters for the CdSDB samples were temperature dependent (Table 2) and the measured Ω was smaller than that of rigid CO at any given temperature (Ω = 55 to 110 ppm for site 1 and 110 to 150 ppm for site 2 at 373 to 153 K; Ω = 353 ppm for rigid CO66). In comparison to PbSDB, where only the κ was found to be temperature dependent, both the Ω and κ of CO in CdSDB were influenced by changes in temperature. For site 1, the apparent Ω reduces from 110 to 55 ppm and the apparent κ decreases from 0.75 to 0.50 as the temperature was raised from 153 to 373 K. Similarly for site 2, an increase in temperature from 153 to 373 K resulted in a decrease in the observed Ω (150 to 110 ppm). However, the κ was found to increase from 0.20


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to 0.50 in this temperature range, in sharp contrast to that observed for CO in site 1 (0.75 at 153 K to 0.50 at 373 K). Since temperature-induced changes in the apparent CS tensor parameters can result from molecular motions, the differences in the trends for Ω and κ imply that temperature has a unique influence on the dynamics of CO in PbSDB and both sites of CdSDB. Differences in adsorption strength and site accessibility were also detected for the two CdSDB sites. Firstly, the Ω observed for CO in site 2 was found to be larger than that of site 1 and PbSDB at any given temperature, with ΩCdSDB site 2 = 110 ppm, ΩCdSDB site 1 = 67 ppm and ΩPbSDB = 42 ppm at 293 K (Table 1 and 2). This indicates a decrease in molecular motion and therefore an increase in interaction strength for CO in site 2 as an increase in dynamics can result in a reduction in the apparent Ω.56, 67, 69 The relative intensities of site 1 and 2 were also detected to be temperature dependent, with site 2 being more populated at the highest temperature (373 K) and site 1 being more populated at lower temperatures (Figure 3b). Since site 2 consists of a stronger CO interaction as signified by the larger Ω, it can be expected to have a higher occupancy at greater temperatures. The higher CO population in site 1 at lower temperatures suggests that site 1 is more easily accessible to the guest molecules compared to site 2. Dynamics of CO in PbSDB and CdSDB The SSNMR data presented thus far closely resembles those reported for CO2 in PbSDB and CdSDB. For instance, the number of inequivalent adsorption sites were found to be 1 and 2 for CO2 in PbSDB and CdSDB, respectively, with one of the CdSDB sites being distinct from that of PbSDB.44 The CdSDB site that is similar to that of PbSDB was also found to be more populated at


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lower temperatures (and therefore more easily accessible by CO2) and consists of a weaker CO2 interaction compared to the other CdSDB site.44 These parallels strongly imply that CO binds to these MOFs at the same sites as CO2. Furthermore, since CO2 absorbs to these MOFs via quadrupole-π interactions with the SDB linkers45 and CO has been shown to primarily interact with delocalized aromatic electrons in an analogous fashion,49 the CO molecules likely bind to these MOFs via the same mechanism as CO2. Nevertheless, to further investigate the CO binding mechanism, motional simulations were conducted to qualitatively and quantitatively assess the CO dynamics. The simulated spectra are given in Figure 5 and 6 for CO-loaded PbSDB and CO-loaded CdSDB, respectively, and the extracted motional angles and rates are summarized in Table 1 for PbSDB and Table 2 for CdSDB. For both samples, simulations revealed that the adsorbed CO simultaneously exhibits a 3-fold (C3) rotation as governed by α, and a 2-fold (C2) jump as defined by the hopping angle β (Figure 7a). It should be noted that identical powder patterns were obtained for motions with rotational symmetries ≥ C3. The rotation and jump rates were found to be ≥ 107 Hz in the studied temperature range, which is considered to be fast on the NMR timescale given that the breadth of the powder pattern for rigid CO is in the order of 104 Hz at 9.4 T. The same types of motions were also previously reported for CO2 in PbSDB and CdSDB,44 further supporting the notion that CO and CO2 undergo the same adsorption mechanism. As such, we propose that the CO dynamics can be described using an equivalent motional model as the one suggested for CO2.44 In this model, the guest molecules undergo a localized wobbling (as portrayed by the C3 rotation) upon the adsorption site (Figure 7b) and a


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non-localized hopping (as depicted by the C2 jump) between adjacent adsorption sites located along the longitudinal direction of the MOF channel (Figure 7c). Motional angles can be expected to relate to the

13

C CS tensor parameters, where

changes in α and β can be reflected by the magnitudes of Ω and κ, respectively.54 In the case of PbSDB and CdSDB, CO molecules with a larger wobbling gave rise to smaller Ω, while ones with a larger β resulted in a smaller κ (Table 1 and 2, and Figure S2 and S3 of Supporting Information). Similar trends have also been reported for CO2 adsorbed in various types of MOFs (e.g., MIL-53 and NH2-MIL-53).54 Moreover, since the motional angles are closely related to the 13

C CS tensor parameters, temperature dependent changes in α and β were also detected. For

CO in PdSDB, α remained relatively constant (ca. 50o) from 173 to 353 K while β dropped from 32o to 9o across the same temperature range. On the other hand, α climbed with temperature for CO residing in site 1 (41o to 47o) and site 2 (31o to 40o) of CdSDB, whereas β increased for CO in site 1 (19o to 26o) and decreased for CO in site 2 (32o to 26o) when temperature was raised. These trends mimic that of the 13C CS tensor parameters – Ω remains uniform for CO adsorbed in PbSDB and changes with temperature for those in CdSDB, whereas κ changes with temperature for CO in both MOFs. However, κ for CO in PbSDB and site 2 of CdSDB were observed to behave differently as compared to those in site 1 of CdSDB (Table 1 and 2, and Figure S2 and S3 of Supporting Information). Thus, temperature has a different influence on the dynamics (and therefore CS tensor parameters) of CO that are absorbed in PbSDB and both sites of CdSDB, likely arising from variations in MOF topology and local adsorption site geometry. Comparison of CO dynamics in SDB-based MOFs and MOFs with OMSs


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From the motional simulations, we obtained the wobbling angles for CO in PbSDB and CdSDB. We can now compare our data with those reported for M-MOF-74 (M = Mg, Zn)14 in order to evaluate the adsorption strength between SDB-based MOFs and MOFs with OMSs. For both Mg- and Zn-MOF-74, CO adsorption occurs at the OMS through an interaction between the carbon and the metal centre.9 Since Mg2+ and Zn2+ ions in these MOFs cannot participate in M CO π back-donation, the metal-CO interaction is primarily electrostatic in nature and is driven by ion-induced dipole interactions. On the other hand, for SDB-based MOFs, the CO adsorption sites are likely located at the V-shaped “π-pockets” and theoretical results showed that CO interacts with aromatic π-electrons via the positive regions around the C–O bond.49 The corresponding stabilization primarily results from molecular quadrupole – π electron interactions. Since the modes of binding are distinctly different between the two types of MOFs, variations in binding strength can also be expected.

For CO in Mg- and Zn- MOF-74, the CO molecules undergo localized wobbling and a nonlocalized hopping in the temperature range of 173 to 373 K at a rate ≥ 107 Hz, and α was observed to increase from 15o to 27o for Mg-MOF-74 and 17o to 32o for Zn-MOF-74.14 As compared to CO in SDB-based MOFs, these angles are noticeably smaller at any given temperature (e.g., at 293 K, α = 22o for Mg-MOF-7414 and 27o for Zn-MOF-7414 vs. α = 51o for PbSDB, 46o for site 1 of CdSDB and 40o for site 2 of CdSDB). The smaller α values indicate that the CO motions are more restricted in MOF-74, signifying an increase in CO affinity as compared to SDB-based MOFs. Even though the magnitude of α can be expected to relate to Ω, similar comparisons could not be made based on the apparent Ω(13C) values since the CO hopping motions are modelled differently (see Supporting Information for a more detailed 14 ACS Paragon Plus Environment

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discussion). The difference in affinity is perhaps unsurprising as stronger guest adsorption has been observed for MOFs with OMSs when compared to those without OMSs.33, 70 Our results are also consistent with a previous theoretical report, in which a greater interaction was observed between CO and a cation (interaction energy, Ee, = -39 kJ/mol) than CO and aromatic π-electrons (Ee ≈ -7 kJ/mol).49 Nevertheless, this comparison demonstrates that differences in adsorption mechanisms can result in a noticeable change in the interaction strength, which is then strongly reflected by the SSNMR results. Comparison between CO and CO2 dynamics in PbSDB and CdSDB We also compared our motional data with those reported for CO2 in PbSDB44 and CdSDB44 in order to evaluate the effect of guest identity on the corresponding binding strengths. Even though CO and CO2 can be expected to display the same adsorption mechanism and motional model, some of the associated SSNMR data were found to be distinctly different. Firstly, the apparent Ω(13C) values were measured to be reduced to different extents, with a more drastic decrease observed for CO (Table 3). For CO in PbSDB, the Ω at 293 K was detected to be ca. 12% of that measured for rigid CO. On the other hand, the Ω measured for CO2 at 293 K was found to be ca. 34% of the static value. Similar observations were also made for CO and CO2 in CdSDB – apparent Ω values for CO in site 1 and 2 at room temperature were calculated to be ca. 19 % and 31 % of the static value, while the experimental Ω values for CO2 in site 1 and site 2 at room temperature were found to be ca. 33% and 43% of that of rigid CO2. Furthermore, α was also found to be larger for CO at a given temperature (Table 4), with αCO = 51o and αCO2 = 38o44 for the PbSDB samples at 293 K, and αCO = 46o and αCO2 = 37o44 for site 1 of the CdSDB samples at 293 K. Similar comparisons could not be made for CO and CO2 adsorbed in site 2 of CdSDB as


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the corresponding motional data for CO2 were not reported by previous studies. The greater reduction in Ω and the larger space sampled by CO can perhaps be rationalized by a decrease in steric hinderance, which would then lead to an increase in motion; however, this seems unlikely as CO consists of a larger kinetic diameter as compared to CO2 (3.76 vs. 3.30 Å)47. A more probable explanation stems from the magnitude of the quadrupole moments. CO has a smaller quadrupole moment than CO2 (2.50 vs. 4.30 x 10-26 esu∙cm2)48 and can therefore be expected to have a weaker affinity with the phenyl rings of the linkers. As such, CO is bound less tightly to the MOF framework, leading to a larger value of α and a greater reduction in the apparent Ω.

Conclusions VT 13C SSNMR studies were performed on CO-loaded SDB-based MOFs (i.e., PbSDB and CdSDB) and valuable information regarding the host-guest interaction was attained. Our results revealed the presence of a non-negligible CO-MOF interaction, showing that these types of MOFs can readily bind with CO despite the lack of OMSs. Analytical simulation of the SSNMR spectra provided the 13C CS tensor parameters, while motional simulations offered quantitative insights into the CO dynamics (i.e., motional rates and angles). Both sets of data revealed temperature-dependent changes and comparison of these data showed that the motional angles are closely linked with the apparent

13

C CS tensor parameters. The SSNMR spectral

features and the extracted motional data of the adsorbed CO are also reminiscent of those reported for CO2. This strongly suggests that CO and CO2 adsorption occur via analogous mechanisms (i.e., electrostatic interaction due to the quadrupole moment of the guest molecules interacting with the delocalized aromatic π clouds). Moreover, CO was observed to 16 ACS Paragon Plus Environment

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sample a significantly greater wobbling angle when adsorbed in PbSDB and CdSDB as compared to MOF-74, indicating that the use of SDB-based MOFs can result in a weaker CO binding interaction. This can be advantageous for practical applications since weaker interactions can provide a more energy efficient regeneration. Similar comparisons were also made between CO and CO2 adsorbed in PbSDB and CdSDB, and a noticeable difference in absorption strength was observed. This suggests that CO and CO2 separation can potentially be executed using SDBbased MOFs, thereby providing a possibly greener avenue for industrial CO production.

Supporting Information Experimental and simulated powder x-ray diffraction patterns of as-made CdSDB and as-made PbSDB (Figure S1);

13

C NMR parameters and motional angles of CO in CdSDB plotted as a

function of temperature (Figure S2); 13C NMR parameters and motional angles of CO in PbSDB plotted as a function of temperature (Figure S3); illustrations of the CO motions for CO in CdSDB (Figure S4); influence of a C3 rotation, C6 rotation, C2 jump, C6 jump, C6 rotation and C6 jump, and C3 rotation and C2 jump on the 13CO powder pattern (Figure S5)

Author Information Corresponding Author *

Email: [email protected]

ORCID Y. T. Angel Wong: 0000-0003-0118-1440 Bryan E. G. Lucier: 0000-0002-9682-4324 Yining Huang: 0000-0001-9265-5896

Acknowledgments 17 ACS Paragon Plus Environment

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Y. H. thanks the Natural Science and Engineering Research Council (NSERC) of Canada for a Discovery grant and a Discovery Accelerator Supplements Award.


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Figure 1 The extended crystal structures of two SDB-based MOFs.35, 44 (a) CdSDB as viewed from the crystallographic c axis and (b) PbSDB as viewed from the crystallographic a axis. Carbon is light grey, sulfur is yellow, oxygen is red, cadmium is blue, and lead is dark grey. Hydrogen atoms and disorder sites are omitted for clarity.


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Figure 2 Experimental (a) and simulated (b) 13C SSNMR spectra of 13CO-loaded PbSDB recorded for a static sample at temperatures ranging from 173 to 353 K (B0 = 9.4 T). Asterisk (*) denotes resonance arising from gaseous, unadsorbed CO molecules. At temperatures below 233 K, the gaseous CO peak is no longer clearly visible and therefore was not marked with an asterisk; however, its presence can still be detected via spectral simulations. The simulated spectra are given as a summation (black dotted trace) of the resonances resulting from the adsorbed (red solid trace) and the gaseous CO molecules (green solid trace). The 13C CS tensor parameters of the adsorbed CO molecules as obtained from the simulations are provided in Table 1. 29 ACS Paragon Plus Environment

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Figure 3 (a) Experimental 13C SSNMR spectra of 13CO-loaded CdSDB. The spectra were measured at 9.4 T using a static sample with temperatures ranging from 153 to 373 K. Signals due to gaseous, free CO molecules are marked by an asterisk (*). (b) The corresponding simulated spectra given as a summation (black dotted trace) of the resonances and also deconvoluted into signals arising from CO adsorbed at site 1 (red solid trace), CO adsorbed at site 2 (blue solid trace) and gaseous, free CO (green solid trace). The experimental 13C CS tensor parameters for the CO molecules adsorbed in site 1 and 2 are provided in Table 2.


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Figure 4 (a) Schematic illustrating the two unique guest adsorption sites (green spheres labelled 1 and 2, respectively) in CdSDB. Site 1 is similar to the site in PbSDB (b, green sphere), while site 2 is absent in PbSDB. Light grey, red, blue, yellow and dark grey spheres represent carbon, oxygen, cadmium, sulfur, and lead. The other possible locations for the disordered phenyl carbons in CdSDB are given by the violet spheres. Hydrogens are omitted for clarity.


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Figure 5 13C VT SSNMR spectra of 13CO-loaded PbSDB (left) simulated using a C3 wobbling and a C2 hopping motional model for the CO dynamics (right). Both motions occur at a rate ≥ 107 Hz and the motions are illustrated in Figure 7. Only the powder pattern arising from the adsorbed CO was simulated, and the corresponding motional parameters can be found in Table 1. The isotropic signal arises from gaseous, unbound CO molecules. All experimental spectra were recorded at 9.4 T using a stationary sample.


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Figure 6 Analytical simulations of the resonances arising from CO adsorbed in site 1 (blue solid trace) and in site 2 (red solid trace) of CdSDB shown together with the corresponding motional simulation (black dotted trace). The analytical simulations were obtained via spectral fitting of the VT 13C SSNMR spectra as shown in Figure 3, while the motional simulations were obtained


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using a model that consists of a C3 rotation and a C2 jump (as illustrated in Figure 7). Both processes occur at a rate ≥ 107 Hz and the associated motional angles can be found in Table 2.


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Figure 7 Illustrations of the motions experienced by the CO molecules adsorbed in PbSDB. The CO molecules simultaneously undergo a C3 rotation with an angle of α and a C2 jump with an angle of β (a). Based on the motional model purposed for CO2 and the crystal structure of CO2loaded PbSDB,44 the C3 rotation describes a localized wobbling upon the adsorption site (b) and the C2 jump describes a non-localized hopping between adjacent adsorption sites located along the longitudinal direction of the pore channel (c). Metal centres and hydrogen atoms are omitted in (b) and (c) for clarity. Oxygen, carbon and sulfur atoms are shown in red, grey, and yellow, respectively. An analogous motional model is expected for CO in CdSDB and is depicted in Figure S4 of Supporting Information.


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Table 1 Experimental 13C CS tensor parameters (δiso, Ω, κ) and the motional angles (α and β) of CO adsorbed within PbSDB, as obtained via analytical simulations and motional simulations, respectively. The wobbling (a C3 rotation) angles are given by α and the hopping (a C2 jump) angles are described by β.a All motions occur at rates ≥ 107 Hz. MOF PbSDB

a b

Temperature [K] 353 333 313 293 273 253 233 213 193 173

δiso [ppm] 183 ± 1 183 ± 3 183 ± 1 183 ± 1 182 ± 2 181 ± 2 180 ± 2 180 ± 2 181 ± 1 180 ± 2

Ω [ppm] 45 ± 5 45 ± 5 46 ± 3 42 ± 4 46 ± 4 48 ± 3 42 ± 2 42 ± 3 46 ± 4 48 ± 4

κ

α [o]

β [o]

0.86 ± 0.12 0.93 ± 0.07 0.90 ± 0.10 0.92 ± 0.08 0.75 ± 0.10 0.58 ± 0.10 0.49 ± 0.03 0.45 ± 0.03 0.37 ± 0.03 0.30 ± 0.10

51 ± 1 51 ± 1 50 ± 1 51 ± 1 50 ± 1 50 ± 1 48 ± 1 49 ± 1 49 ± 1 49 ± 1

9 + 1b 9 + 1b 9 + 1b 9 + 1b 15 ± 1 25 ± 2 30 ± 1 30 ± 1 31 ± 1 32 ± 1

Molecular motions are graphically depicted in Figure 7. Identical powder patterns were obtained for β ≤ 9o.


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Table 2 Experimental 13C CS tensor parameters (δiso, Ω, κ) of CO molecules adsorbed in CdSDB, as obtained via analytical simulations and the corresponding motional angles (α and β) as acquired via motional simulations. α describes the wobbling (a C3 rotation) angle and β describes the hopping (a C2 jump) angle.a All motions occur at rates ≥ 107 Hz. MOF CdSDB (Site 1)

CdSDB (Site 2)

a

Temperature [K] 373 353 333 313 293 273 253 233 213 193 173 153 373 353 333 313 293 273 253 233 213 193 173 153

δiso [ppm] 177 ± 5 177 ± 1 177 ± 1 177 ± 1 178 ± 1 178 ± 1 177 ± 1 177 ± 1 177 ± 1 178 ± 1 177 ± 1 177 ± 1 150 ± 3 150 ± 3 150 ± 3 150 ± 3 150 ± 3 145 ± 3 140 ± 5 136 ± 4 140 ± 3 145 ±5 135 ± 5 145 ± 3

Ω [ppm] 55 ± 4 60 ± 3 65 ± 2 65 ± 2 67 ± 3 72 ± 2 77 ± 3 78 ± 3 80 ± 3 85 ± 2 90 ± 3 110 ± 2 110 ± 4 110 ± 5 110 ± 5 110 ± 5 110 ± 5 115 ± 5 133 ± 5 140 ± 5 145 ± 5 150 ± 5 150 ± 5 150 ± 5

κ

α [o]

β [o]

0.50 ± 0.10 0.50 ± 0.05 0.50 ± 0.03 0.50 ± 0.03 0.50 ± 0.04 0.48 ± 0.03 0.50 ± 0.03 0.54 ± 0.04 0.54 ±0.06 0.60 ± 0.03 0.65 ±0.05 0.75 ± 0.03 0.50 ± 0.10 0.50 ± 0.10 0.50 ± 0.10 0.50 ± 0.10 0.40 ± 0.10 0.40 ± 0.10 0.30 ± 0.03 0.20 ± 0.05 0.20 ± 0.06 0.20 ± 0.10 0.20 ± 0.10 0.20 ± 0.10

47 ± 1 46 ± 1 46 ± 1 46 ± 1 46 ± 1 46 ± 1 46 ± 1 44 ± 1 45 ± 1 44 ± 1 43 ± 1 41 ± 1 40 ± 1 40 ± 1 40 ± 1 40 ± 1 40 ± 1 40 ± 1 38 ± 1 36 ± 1 33 ± 1 31 ± 1 31 ± 1 31 ± 1

26 ± 1 26 ± 1 26 ± 1 26 ± 1 26 ± 1 26 ± 1 26 ± 1 26 ± 1 25 ± 1 24 ± 1 24 ± 1 19 ± 1 26 ± 1 26 ± 1 26 ± 1 26 ± 1 28 ± 1 28 ± 1 30 ± 1 32 ± 1 32 ± 1 32 ± 1 32 ± 1 32 ± 1

The molecular motions are graphically depicted in Figure S4 of Supporting Information.


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Table 3 The apparent Ω(13C) of CO (ΩCO) and CO2 (ΩCO2) adsorbed in PbSDB and CdSDB at different temperatures. ΩCO and ΩCO2 are expressed as percentages of the static Ω(13C).a,b Temperature [K] 413 383 373 353 333 323 313 293 273 253 243 233 213 193 183 173 153

PbSDB ΩCO [%]

ΩCO2 [%] 30 ± 1 31 ± 1

13 ± 1 13 ± 1

31 ± 1

CdSDB (Site 1)

CdSDB (Site 2)

ΩCO [%]

ΩCO [%]

16 ± 1 17 ± 1 18 ± 1

33 ± 1 13 ± 2 12 ± 1 13 ± 1 14 ± 1

34 ± 1

35 ± 1

18 ± 1 19 ± 1 20 ± 1 22 ± 1

31 ± 1 31 ± 1 31 ± 1

33 ± 0

22 ± 1 23 ± 1 24 ± 1

34 ± 0

31 ± 1 31 ± 1 33 ± 1 38 ± 1

39 ± 1

43 ± 1

47 ± 1 40 ± 1 41 ± 1 42 ± 1

35 ± 0 25 ± 1 31 ± 1

ΩCO2 [%] 35 ± 1 37 ± 1

41 ± 1

34 ± 0

37 ± 1 14 ± 1

31 ± 1 32 ± 1

34 ± 1 12 ± 1 12 ± 1 13 ± 1

ΩCO2 [%] 27 ± 1 30 ± 1

50 ± 1 52 ± 1

42 ± 1 42 ± 1

The Ω(13C) values of CO2 in the SDB-based MOFs were obtained from reference 44 and converted to percentages using the Ω(13C) of static CO2 provided by reference 66. The values reported for CO in the SDB-based MOFs were calculated using the apparent Ω(13C) given in Table 1 and 2, and the Ω(13C) of static CO provided by reference 66 b The Ω(13C) of static CO = 353 ppm; the Ω(13C) of static CO2 = 335 ppm.66 a


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Table 4 Comparison between the wobbling angles of CO (αCO) and CO2 (αCO2) in PbSDB and site 1 of CdSDB at various temperatures.a All motions occur at rates ≥ 107 Hz. Temperature [K] 413 383 373 353 333 323 313 293 273 253 243 233 213 193 183 173 153 a

CdSDB (Site 1)

PbSDB αCO [o]

αCO2 [o]

αCO [o]

39 ± 1 51 ± 1 51 ± 1

47 ± 1 46 ± 1 46 ± 1

39 ± 1 38 ± 1

50 ± 1 51 ± 1 50 ± 1 50 ± 1

38 ± 1 37 ± 1

46 ± 1 46 ± 1 46 ± 1 46 ± 1

38 ± 1

36 ± 1 48 ± 1 49 ± 1 49 ± 1

αCO2 [o] 48 ± 4 40 ± 2

37 ± 1

35 ± 1 44 ± 1 45 ± 1 44 ± 1

36 ± 1 35 ± 1

33 ± 1 32± 1

49 ± 1

43 ± 1 41 ± 1

The CO2 data was acquired from reference 44.


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TOC Graphics


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CO Guest Interactions in SDB-Based MetalâOrganic Frameworks: A

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