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Jun 16, 2017 - Solid-state NMR Studies of Host−Guest Interaction between UiO-67 and Light Alkane at Room Temperature. Jing Li,. †,‡. Shenhui Li,...
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Solid-state NMR Studies of Host−Guest Interaction between UiO-67 and Light Alkane at Room Temperature Jing Li,†,‡ Shenhui Li,*,† Anmin Zheng,† Xiaolong Liu,† Ningya Yu,‡ and Feng Deng*,† †

State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China ‡ Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province, Hunan Normal University, Changsha 410081, China S Supporting Information *

ABSTRACT: The host−guest interaction between adsorbent and adsorbate plays essential roles in the gaseous storage and chemical separation using metal−organic frameworks (MOFs). Solid-state NMR spectroscopy was employed to explore the interactions between light alkanes including methane, ethane, propane and a representative MOF, UiO-67 at room temperature. The existence of host−guest interaction between light alkanes and UiO-67 framework is clearly evidenced from twodimensional 1H−1H spin diffusion homonuclear correlation and 1 H−13C HETCOR with spin diffusion experiments. By fitting the spin diffusion buildup curves, it is found that methane is more readily to diffuse to the UiO-67 framework compared to ethane and propane. Moreover, the spin diffusion MAS NMR results reveal that methane is mainly adsorbed neighboring the metal cluster, whereas propane and ethane are preferentially present nearby the site away from the metal cluster due to the steric hindrance effect. The results presented herein would provide a better understanding of the structure−property relationship of MOFs in the alkane storage.

1. INTRODUCTION Metal−organic frameworks (MOFs) are attractive porous materials having wide applications in gas storage, separation, catalysis, and many other important fields, due to their excellent porous structures and high specific surface area.1−6 Methane is a desirable fuel because it has a higher hydrogen to carbon ratio than any other hydrocarbon fuel and burns more cleanly than gasoline. MOFs can efficiently increase the volumetric density of stored methane. Tremendous efforts have been made to design new MOF materials to improve the methane storage capacity.7−11 The fundamental issue regarding the methane storage in various MOFs attracts broad interests toward understanding the interaction between methane and these types of functional materials. The primary methane adsorption sites over some representative MOFs have been comprehensively studied by advanced spectroscopic techniques as well as theoretical calculations.12−18 Savage and co-workers18 provided direct observation and quantification of the location, binding, and rotational modes of adsorbed methane within MFM300(In) using neutron diffraction coupled with density functional theory (DFT) modeling. Additionally, Wu and coworkers13 proposed that the binding energies of CH4 on the open metal sites in MOF compounds M2(dhtp) were significantly higher than those on other adsorption sites on the basis of initial first-principles calculations, and primary CH4 adsorption occurred directly on the open metal sites in MOF © 2017 American Chemical Society

compound M2(dhtp) as revealed from the neutron diffraction experiments conducted at temperature below 80 K. However, direct experimental evidence for the interaction between the gaseous methane and MOFs at room temperature is still hard to obtain due to its motional dynamics and highly disorder in the complex system. This work aims to explore the host−guest interactions between light alkanes and MOFs at ambient temperature using solid-state NMR. Solid-state NMR spectroscopy has been widely applied to investigate the detailed structure and characterize the dynamic behavior of functional materials including MOFs.19,20 Huang and co-workers explored the adsorption models of guest molecules confined in MOFs using high-field 67Zn, 91Zr, 17O and 25Mg solid-state NMR techniques.11,21−25 The CO2 dynamics in MOFs such as Mg-2(dobdc) and CPO-27 with open metal sites and MIL-53 without open metal sites were clearly demonstrated using variable temperature 13C and 17O solid-state NMR.26−28 The distribution of functional groups in a series of multivariate metal−organic frameworks was correlated from the spatial proximity among various linkers via distance measurements.29 Received: May 13, 2017 Revised: June 16, 2017 Published: June 16, 2017 14261

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from alkane to the UiO-67 framework. The cross-polarization contact time is fixed to 0.3 ms. During the T2 filter period of 1− 2 ms, the signal from UiO-67 framework can be largely suppressed whereas the signal from light alkanes can be mostly maintained. A spin diffusion period (tm) of 36 ms can allow the polarization transfer from alkanes to the UiO-67 framework. 1H and 13C chemical shifts were externally referenced with respect to tetramethylsilane (TMS).

In this work, the adsorbent−adsorbate interaction between methane and a representative metal−organic framework UiO67, which has two distinct proton sites in the organic linker, was comprehensively studied using spin diffusion solid-state NMR techniques. For comparison, the adsorption behavior of ethane and propane inside UiO-67 pore was also explored on the basis of solid-state NMR observation.

2. METHODS AND MATERIALS 2.1. Sample Preparation. UiO-67 was synthesized according to the procedure as described elsewhere.30 Here, 85 mg (0.35 mmol) of 4,4′-biphenyldicarboxylic acid (H2BPDC), 1.28 g (10.5 mmol) of benzoic acid, and 82 mg (0.35 mmol) of ZrCl4 were mixed and dissolved in 20 mL of DMF in a vial. The vial was capped and heated at 120 °C for 48 h for crystallizations. Subsequently, the obtained samples were washed using DMF to remove the unwanted species. Finally, the products were washed with 20 mL of acetone five times and dried in vacuo overnight. Prior to solid-state NMR experiments, 60 mg of the UiO-67 was packed into a glass tube, which was then connected to a vacuum line. The temperature was gradually increased from room temperature to 473 K at a rate of 1 K/min, and kept at this temperature and a pressure below 2 Pa for 12 h to dehydrate the sample. After the samples were cooled to room temperature, a certain amount of 13C isotope-enriched methane, ethane and 1,3-13C-propane were separately introduced to UiO-67 and frozen by liquid N2 in the glass tube. Subsequently, the glass tubes were flame-sealed. The sealed sample was frozen and quickly transferred into a 4 mm NMR rotor. 2.2. Solid-State NMR Spectroscopy. Solid-state NMR experiments were carried out on a Varian Infinityplus-300 spectrometer with a 4 mm double-resonance MAS probe and a Bruker Avance III 500 spectrometer with a 4 mm tripleresonance MAS probe. The Larmor frequencies are 299.8 and 75.4 MHz for 1H and 13C nuclei, respectively, on the 300 MHz spectrometer, and are 500.6 and 125.9 MHz for 1H and 13C nuclei, respectively, on the 500 MHz spectrometer. 1H MAS NMR spectra were acquired with a π/2 pulse length of 5.0 μs and a recycle delay of 5 s. 1H 2D spin diffusion homonuclear correlation NMR31 experiments were carried out under 10 kHz MAS. The increment interval in the indirect dimension was set to 100 μs. Typically, 8 scans were acquired for each t1 increment and two-dimensional data sets consisted of 512 t1 × 5120 t2. The 13C CP/MAS NMR experiments were performed with a contact time of 3.0 ms, a recycle delay of 2 s. A total of 3200−6400 scans was accumulated to acquire the 13 C CP/MAS NMR spectra. The 13C NMR spectra of methane adsorbed on UiO-67 was recorded using the pulse sequence as shown in Figure S1, in which both direct polarization and crosspolarization signals were simultaneously acquired. The 1H−13C HETCOR experiments32 were conducted with a CP contact time of 0.3 ms at a spinning speed of 10 kHz. On the other hand, the 1H−13C HETCOR spectra with 1H−1H spin diffusion was recorded as illustrated in Figure S2, in which 36 ms spin diffusion mixing time was incorporated prior to 1 H−13C cross-polarization. The increment interval in the indirect dimension was set to 100 μs. Typically, 320 scans were acquired for each t1 increment. One-dimensional spin diffusion NMR experiment with 13C detection, as shown in Figure S3, was employed to detect the polarization transfer

3. RESULTS AND DISCUSSION 3.1. 1H and 13C NMR. 1H and 13C MAS NMR is frequently used to analyze the chemical composition of various materials. In order to investigate the influence of adsorption of light alkane to the UiO-67 framework, 1H and 13C NMR experiments were conducted to monitor the chemical shift

Figure 1. 1H MAS NMR spectra of (a) UiO-67 with adsorption of a certain amount of (b) methane, (c) ethane, and (d) propane.

variation of UiO-67 upon alkane loading. Figure 1 shows the 1 H MAS NMR spectra of the UiO-67 upon loading a certain amount of light alkanes, including methane, ethane, and propane. As shown in Figure 1a, two main resonance peaks at 7.6 and 6.8 ppm due to two distinct types of aromatic protons, namely H1 and H2 (illustrated in Figure 2), respectively, can be clearly observed. The signal at around ∼2 ppm is due to Zr3(μ−OH) according to the literatures.33,34 In addition, the resonance at ca. 12.4 ppm is assigned to residual benzoic acid, which is consistent with the previous observation by Ko and co-workers.30 Upon introducing the light alkane, the resonances in range of −0.4 ∼ −0.8 ppm ascribed to light alkane are observable. The characteristic multipeaks are arising from either distinct protons sites or J coupling splitting with expected value (J1H‑13C 120−150 Hz) in Figure 1b−d. The adsorption amount of methane, ethane, and propane can be roughly estimated to be 0.24, 0.26, and 0.17 mol/kg, corresponding to 0.44, 0.47, and 0.32 molecule in each unit cell, respectively, according to the 1H MAS NMR spectra as shown in Figure S4. Figure 3 shows the 13C MAS NMR spectra of the UiO-67 upon adsorption of methane, ethane, and propane. The 13C CP MAS NMR spectra of UiO-67 as shown in Figure 3a consists of signals at 124−143 ppm arising from aromatic carbons and a 14262

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between UiO-67 and light alkane through solid-state NMR. 2D H−1H spin diffusion homonuclear correlation NMR experiment has been proved to be a simple but effective approach to provide direct evidence of spatial proximities among various protons in MOFs.37,38 Figure 4 shows the 2D 1H−1H spin diffusion homonuclear correlation NMR spectra of UiO-67 with adsorbed methane at 1

Figure 2. Schematic illustration of UiO-67. Different proton and carbon sites are labeled and indicated in the figure.

Figure 4. 2D 1H−1H spin diffusion homonuclear correlation NMR spectra of UiO-67 upon methane adsorption with a spin diffusion mixing time of (a) 1, (b) 36, and (c) 121 ms.

Figure 3. 13C NMR spectra of (a) UiO-67 with introduction a certain amount of (b) methane, (c) ethane, and (d) propane.

signal at 171 ppm associated with carboxylate carbons. Upon light alkane adsorption, three additional signals at −11, 5, and 15 ppm arising from methane, ethane, and propane, respectively, can be well resolved (Figure 3b−d). It should be noted that the 1H and 13C chemical shifts of aromatic framework remain almost unchanged after introducing the light alkanes, suggesting that the interaction between these adsorbates and adsorbent has negligible influence on the topology of UiO-67 framework. Differently, the 1H and 13C NMR chemical shifts of organic framework could change significantly due to breathing phenomenon in several typical MOFs such as MIL-53.35,36 3.2. 2D 1H Spin Diffusion Homonuclear Correlation NMR. To investigate the host−guest interaction between MOFs and adsorbates, it is desirable to establish the correlation

different spinning diffusion mixing time. As shown in Figure 4a, the correlation peaks between methane and organic framework is almost invisible at a mixing time of 1 ms. As spin diffusion mixing time increases to 36 ms, the appearance of cross-peaks at (7.7, −0.4) and (7.0, −0.4) ppm indicates the presence of host−guest interaction between UiO-67 framework and methane. (Figure 4b) Additionally, the correlation between Zr3(μ−OH) and UiO-67 aromatic protons can also be discerned, which is evidenced from the correlations at (7.7, 2.1) and (7.0, 2.1) ppm. The appearance of the signals at (7.7, 12.5) and (7.0, 12.5) ppm suggests that the residual benzoic acid is present in the UiO-67 pore. Note that no correlations at (−0.4, 12.5) and (12.5, −0.4) ppm are visible, indicating that there is no direct interaction between residual benzoic acid and 14263

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roughly analyzed from the spin diffusion buildup curves extracted from a series of 2D 1 H−1 H spin diffusion homonuclear correlation experiments. During the mixing period, magnetization transfer via 1H−1H spin diffusion occurs, resulting in an increase of correlation peak intensity. Accordingly, the diffusion coefficient can be estimated by measuring the correlation peak intensity as a function of mixing time. Figure 6 shows the 1H−1H spin diffusion buildup curves

gaseous methane. This result indicates that UiO-67 instead of benzoic acid enables the storage of methane at the relatively lower loading, which is in consistence with the methane adsorption isotherms.39−41 Meanwhile, the correlation peaks arising from the interaction between the UiO-67 framework and methane become pronounced upon increasing the spin diffusion mixing time to 121 ms (Figure 4c). Similar phenomenon can also be observed in cases of ethane and propane adsorption on UiO-67 (see Figures S5 and S6). It can be concluded that UiO-67 can be utilized to storage light alkanes due to the presence of van der Waals interaction. 3.3. 1H−13C HETCOR. 1H−13C HETCOR NMR experiment is frequently used to provide chemical shift assignments and explore structural information on various functional materials. Figure 5a shows the normal 1H−13C HETCOR

Figure 6. Spin diffusion buildup curves for the correlations from (a) H1-methane, (b) H2-methane, (c) H1-ethane, (d) H2-ethane, (e) H1propane, and (f) H2-propane extracted from 2D 1H−1H spin diffusion homonuclear correlation experiments. Figure 5. 1H−13C HETCOR NMR spectra of UiO-67 upon methane adsorption with spin diffusion mixing time of (a) 0 ms and (b) 36 ms.

for the correlations between UiO-67 aromatic protons and light alkanes. The intensity of correlation peaks as a function of square root of the mixing time (t1/2) is displayed in Figure 6, from which the equilibrium t1/2 of 7, 11, and 14 ms1/2 can be directly read off for the H1-methane, H1-ethane and H1propane pair, respectively. The equilibration of magnetization always occurs fast in case of large spin diffusion coefficient assuming the distribution of methane, ethane and propane inside the UiO-67 pore is similar. The diffusion behavior of the light alkanes derived from spin diffusion MAS NMR is in general consistence with the molecular dynamics simulations results.42,43 The dynamic size of methane, ethane and propane was estimated to be 3.8, 4.3, and 4.5 Å, respectively, which played key roles in the diffusion of light alkanes inside MOFs.44 Additionally, the preferential adsorption sites for the light alkanes can also be explored from the spin diffusion buildup curves. It is clearly that the magnetization for H1-methane reaches equilibrium relatively faster compared to H2-methane site. This suggests that the H1 site, which is approaching the metal cluster, has priority for methane adsorption, in consistence with the experimental observations and theoretical calculations.13,14,45−47 Whereas in case of propane adsorption as shown in parts e and f of Figure 6, the magnetization reaches equilibrium slowly for the H1 site in comparison with the H2

spectrum of UiO-67 upon methane adsorption, in which only short-range 1H−13C polarization transfer can be allowed. As shown in Figure 5a, the main correlation peak is ascribed to an aromatic ring in the UiO-67 framework. The correlation peak arising from methane does not show up, probably due to the low cross-polarization transfer efficiency in gaseous phase. Figure 5b manifests the 1H−13C HETCOR spectrum with 1 H−1H spin diffusion, in which a mixing time of 36 ms is used prior to 1H−13C cross-polarization. As shown in Figure 5b, the proton signals from both methane (−0.4 ppm) and Zr3(μ− OH) (2.1 ppm) are correlated to the UiO-67 aromatic framework carbon. The appearance of the cross peak at (130, −0.4) ppm further confirms the presence of host−guest interaction between UiO-67 framework and methane. Similar results are observable in the 1H−13C HETCOR spectra of UiO67 upon ethane and propane adsorption (see Figures S7 and S8). The 1H−13C HETCOR spectra with 1H−1H spin diffusion provide another experimental evidence for the interaction between UiO-67 and various light alkanes. 3.4. Spin Diffusion Buildup Curves. The host−guest interaction between UiO-67 framework and light alkanes can be 14264

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The Journal of Physical Chemistry C site. It can be concluded that at relatively low loading, methane is readily to adsorb on the site neighboring the metal cluster, whereas propane has priority to be present near the H2 site, which could be resulted from the steric hinder effect for relatively larger molecular size of propane. Similar to propane adsorption, ethane is likely more closer to H2 site compared with H1 site according to the spin diffusion buildup curves as manifested in parts c and d of Figure 6. The preferred adsorption sites for methane, ethane and propane are related to the possible distributions of the adsorbed molecules confined inside the UiO-67 pores, which are strongly dependent on the adsorbent−adsorbate interaction. 3.5. 13C-Detection Spin Diffusion NMR. In order to further determine the primary adsorption sites of the light alkanes in UiO-67, 13C-detection spin diffusion NMR experiment was conducted. In the experiment (the pulse sequence is shown in Figure S3), a T2 filter time of 2 ms was used, during which the rigid component arising from the UiO-67 aromatic protons could be largely suppressed while the mobile methane could be mostly remained. A spin diffusion time of 36 ms allows the polarization transfer from light alkane to the UiO-67 organic framework. Figure 7 shows the 13C-detection spin

Figure 8. 13C-detection spin diffusion NMR spectra of UiO-67 upon propane adsorption. T2 filter period was fixed to 1 ms. Spin diffusion mixing time was set to (a) 0 and (b) 36 ms.

As shown in Figure 8a, the 13C signals from organic framework as well as propane are clearly distinguishable after a T2 filter period of 1 ms prior to cross-polarization. When a spin diffusion period of 36 ms is included, the signals at 130 and 125 ppm from C3 and C2 sites increase largely compared to those of C1 and C4 sites. This indicates that the preferred adsorption sites for propane might be close to the H2 site, which could be probably resulted from the steric hindrance effect. The 13Cdetection spin diffusion NMR data is in good consistence with the above-mentioned 2D 1H−1H spin diffusion homonuclear correlation experimental observations. Similar experimental results can be obtained for ethane adsorption as manifested in Figure S9. Our solid-state NMR observation provides experimental evidence on the presence of host−guest interaction between light alkanes and UiO-67. It is found that the diffusion of methane is faster than that of ethane and propane confined inside UiO-67 according to analysis of the spin diffusion MAS NMR experiments. Meanwhile, the primary adsorption site for methane is approaching the metal clusters, whereas that the preferential adsorption site of propane and ethane is nearby the H2 site due to steric hindrance effect. The adsorption model for methane, ethane and propane inside UiO-67 pore is schematically summarized in Figure 9. MOFs have high potentials to be applied in various industrially important chemical processes including clean energy storage, greenhouse gas adsorption and chemical separation.1,9,48−50 The performances of MOFs are strongly relied to their structures, dynamic characteristics as well as host−guest interactions. The investigation of host−guest interaction mechanism is of great importance toward understanding the structure−property relationship for rational design and modification of MOFs. The host−guest interactions between adsorbent and adsorbate and adsorption behaviors are strongly dependent on the alkane loading amount as well as the experimental temperature. One the basis of in situ smallangle X-ray scattering experimental observation conducted by Cho et al.,51 it was concluded that the adsorbate−adsorbent interaction, adsorbate−adsorbate interaction within an individual pore and across adjacent pores gradually appear as the increase of adsorption pressure. Additionally, the primary adsorption models for the methane in several representative MOFs have been proposed via neutron diffraction and singlecrystal X-ray diffraction.52−54 Our current work provides experimental evidence for the adsorption behavior of light

Figure 7. 13C-detection spin diffusion NMR spectra of UiO-67 upon methane adsorption. T2 filter period was fixed to 2 ms. Spin diffusion mixing time was set to (a) 0 and (b) 36 ms.

diffusion NMR spectra of UiO-67 upon methane adsorption in absence and presence of spin diffusion. As shown in Figure 7a, the aromatic carbon signals at 124, 130, 135, and 143 ppm and carbonyl group resonance at 171 ppm become relatively weak due to the employed 2 ms T2 filter suppression. The chemical shifts for these sites can be assigned according the work conducted by Larabi and co-workers.34 Upon inclusion of a spin diffusion mixing time of 36 ms, the signals at 135 and 143 ppm arising from C1 and C4 sites, respectively, increase significantly compared to that of C3 and C2 sites. The result suggests that adsorbed methane is preferentially approaching the H1 site with respect to the H2 site, which agrees well with the experimental analysis from 1H−1H spin diffusion buildup curves. It is notable that we only compare with the peak intensity from the same carbon sites as a function of spin diffusion periods, which means that the influence from crosspolarization kinetics can be ignored. On the basis of our solidstate NMR observation, the position in close proximity to the metal clusters should be a preferred adsorption site for methane. For propane adsorption, different result is found in the 13Cdetection spin diffusion NMR experiment. Figure 8 shows the 13 C-detection spin diffusion NMR spectra of UiO-67 upon propane adsorption in absence and presence of spin diffusion. 14265

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Figure 9. Schematic model for (a) methane, (b) ethane, and (c) propane adsorption inside UiO-67.

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alkanes at small loading amount and room temperature. All these findings are important toward understanding the role of UiO-67 for alkane adsorption in highly dynamic states. The interaction between light alkanes and UiO-67 is some kind of van der Waals interaction, which is relatively weaker than other interactions such as H-bonding interaction, covalent interaction, π−π stacking, and cation-π interaction. The spin diffusion MAS NMR techniques enable us to investigate adsorbent−adsorbate interaction in long-rang disordered system, which is of great significance to explore the adsorption model and separation mechanism in MOFs. The results presented herein would provide a deeper understanding of the structure−property relationship of MOFs in the alkane storage.

*(S.L.) E-mail: [email protected]. Fax: +86-27-87199291. *(F.D.) E-mail: [email protected]. ORCID

Anmin Zheng: 0000-0001-7115-6510 Xiaolong Liu: 0000-0002-7346-7846 Feng Deng: 0000-0002-6461-7152 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21373265, 21210005, and 21473246).

4. CONCLUSION

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The interaction mechanism between light alkane and MOFs are crucial toward understanding the fundamental issues regarding gaseous storage and chemical separation. In this work, solidstate NMR results provide direct experimental evidence for the host−guest interaction between light alkanes and UiO-67 at small loading amount and room temperature. The adsorption behavior of the light alkanes including methane, ethane, and propane can be deduced from the analysis of spin diffusion buildup curves. The experimental observation reveals that methane has the priority to adsorb on the site neighboring the metal clusters, whereas propane and ethane are preferential to be present near the H2 site away from the metal clusters due to the steric hindrance effect. The findings presented herein provide insights into understanding the adsorption mechanism of gaseous light alkane inside MOF pore.

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AUTHOR INFORMATION

Corresponding Authors

REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04611. Solid state NMR pulse sequences applied in this work and additional 1H−1H 2D spin diffusion homonuclear correlation, 1H−13C HETCOR, and 13C-detection spin diffusion solid state NMR spectra (PDF) 14266

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