Solid-state NMR Studies of Phenethyl Sulfonic Acid-functionalized MCM-41

Abstract

A sulfonic acid-water-silanol system in SO₃H-functionalized MCM-41 was investigated using solid-state nuclear magnetic resonance techniques. The proton exchange rate between a water molecule and a silanol group in the S-PE-MCM-41 was determined by analyzing the 1D proton spectra, the proton EXSY spectrum, and ²H spin-lattice relaxation data under various hydration levels. Two kinds of water-bounding sites were found in the S-PE-MCM-41: weakly and strongly bound sites. Over several hours, water molecules bound to the weakly bound sites at the low hydration level migrated to the strongly bound sites. At high temperature, the S-PE-MCM-41 easily lost water molecules weakly bound to the silanol, while the strongly bound water molecules survived. Water molecules that participated in the hydration of the phenethyl sulfonate were involved in the hydrogenbonded silanol mechanism of proton conductivity. This phenomenon contributes higher proton conductivity to the S-PE-MCM-41 by the cooperation of sulfonyl and silanol groups in the proton transfer process, even at higher temperature.

INTRODUCTION

The recent energy problem relating to global warming has brought into focus energy generating devices, such as the fuel cell. Among the various fuel cells, high-temperature operating fuel cells have many advantages, such as fast electrode kinetics. The polymer composite membrane fuel cell offers promise; it can be effectively operated at high temperature, and its mechanical instability can be enhanced by implementing an inorganic filler, such as a mesoporous silicate MCM−41.¹ In addition, the mesoporous structure of MCM−41 made of silica with mesostructured hexagonal framework will efficiently function for proton conductivity at the high operational temperature of the fuel cell. The higher proton conductivity by implementing the mesoporous additives results from enhancement of the water adsorption capacity of the membranes.²

The characteristics of water confined in mesoporous material were found to be an Arrhenius type of temperature-dependence, unlike that of the bulk water, by measuring the residence time for translational diffusion and the relaxation time for rotational diffusion. From the results, the amount of water molecules neighboring the silica surface was also determined.³ The translational diffusion coefficient of the confined water measured by quasielastic neutron scattering and the solid structure information obtained by XRD indicated the strong interaction of water molecules with the surface hydroxyls and the distortion of the hydrogen bond network.⁴ More disordered hydrogen-bonded structure of the confined water was also comprehended by measuring the ideal glass transition temperature of confined water.⁵ In the MCM−41 case, it was found that there are three sites for water adsorption on MCM−41, namely, a slow site, a fast site I, and a fast site II.⁶

After sulfonic acid functionalized zeolite BEA nanocrystals were synthesized and characterized,⁷ the effects of both acid strength and pore size on the proton conductivity of the acid functionalized mesoporous material MCM−41 were invesigated.⁸ Modification of MCM−41 with acid was effective for the water storage increment, and then proton conductivity.⁹ The results of the impedance spectroscopic experiments with different sulfonic acid contents in the mesoporous silica showed that proton transfer was obviously associated with the strongly acidic groups grafted within the pores.¹⁰ Hydration of MCM−41 allows the bridging of acid sites, assisting ion hopping, and providing additional charge carriers. To investigate the size effect on proton conductivity, the mesoporous SO₃H-functionalized nanoparticles of Si-MCM−41 were synthesized, and the proton conductivity was compared to that of the micrometer-sized Si-MCM−41.⁹ The formation of a higher number of grain boundaries of the nanoparticles might also be responsible for the strong increase in proton conductivity of the mesoporous SO₃H-functionalized nanoparticles of Si-MCM−41, due to the shorter diffusion paths inside the silica particles.

Although various modifications of MCM−41 have been found to be effective for increment of the proton conductivity of MCM−41, the information at the molecular level of the proton conduction mechanism is still limited. In this study, the structural and dynamic characteristics of the sulfonic acid–water–silanol system in the SO₃H-functionalized MCM−41 was investigated by using multinuclear and multidimensional solid-state nuclear magnetic resonance techniques. The dynamic information of proton behavior in the system at different temperatures was obtained by varying the water contents.

EXPERIMENTAL

Pure silica MCM−41 was purchased from Aldrich. S−PE−MCM−41 sample was made by grafting the surface of MCM−41 with phenethyltrimethoxy silane, and by following the sulfonation of phenethyl group via oleum treatment, as described in a previous paper.⁹ The purchased MCM−41 sample was vacuum-dried at the pressure of 5 × 10⁻² torr, and at the temperature of 120 ℃. The powder mixture was loaded into a 4 mm ZrO₂ rotor.

Solid-state ¹H and ²H MAS NMR spectra were recorded using a Bruker Avance 500 MHz spectrometer with a 11.7 T magnet and a Bruker 4 mm MAS probe. The Larmor frequencies of ¹H and ²H nuclei were 500.23 and 76.79 MHz, respectively. 90° pulses for ¹H and ²H were 4.8 and 2.0 µs, respectively.

After ¹H T₁ experiment, the vacuum-dried MCM−41 sample was hydrated in a D₂O chamber, followed by vacuum drying at 120 ℃. The D₂O hydration and vacuum drying treatment were performed once more, to confirm the complete deuteration, and then ²H T₁ experiment was conducted.

The as-made S−PE−MCM−41 sample was dried at vacuum of 5 × 10⁻² torr at 50 ℃, and then at 120 ℃, respectively, and ¹H MAS NMR experiments were conducted to determine the hydration state. The vacuum-dried S−PE−MCM−41 sample was rehydrated in the air for 15 min, and ¹H MAS NMR experiments were performed at 20 and 50 ℃. The air-exposed S−PE−MCM−41 sample for 15 min was re-exposed to the air for another 30 min for further hydration, and ¹H MAS NMR experiments were performed at 20 and 50 ℃, respectively. The hydrated S−PE−MCM−41 sample was incubated for 15 h, and the ¹H MAS NMR spectra were obtained at 20 and 50 ℃, respectively. ¹H 2D-EXSY experiment of 15 h-incubated S−PE−MCM−41 sample was also performed at 20 ℃ under the MAS rate of 13 kHz.

For deuterium NMR experiments, the MCM−41 and S−PE−MCM−41 samples were treated as follows. The MCM−41 and the as-made S−PE−MCM−41 samples were solvated by adding 60 uL of D₂O, and incubated overnight. The samples were vacuum-dried for 30 min at 120 ℃. The ²H MAS NMR spectra were obtained under MAS rate of 6 kHz. To investigate the hydration effect, the vacuum-dried sample of S−PE−MCM−41 was rehydrated in a D₂O chamber for 1, 10, and 60 min in consecutive order. The ¹H and ²H MAS NMR spectra were obtained under the MAS rates of 12 and 6 kHz, respectively. The ²H T₁ experiments were also performed for the samples rehydrated with different times of (1, 10, 60) min under the MAS rate of 6 kHz.

RESULTS AND DISCUSSION

Water Molecules in the S−PE−MCM−41 During the Hydration Process

To understand the roles of SO₃H and SiOH functional groups and water molecules in the proton conductivity increase of S−PE−MCM−41, the ¹H MAS NMR spectra were measured at various hydration conditions. Fig. 1(a) shows that there are four characteristic groups at 1.7, (2−4), (5.8 − 6.2), and (7.2−8.5) ppm. The signals at 1.7, (2−4), (5.8−6.2), and (7.2−8.5) ppm were assigned to the isolated SiOH,¹¹ ethyl protons in phenethyl group, hydrogen-bonded water-sulfonyl-silanol protons, and phenethyl protons in phenethyl group, respectively. An isolated sulfonyl proton might be overlapped in the (7.2−8.5) ppm region.¹¹

Figure 1. (a) The ¹H MAS NMR spectrum of S−PE−MCM−41 rehydrated for 45 min in the air, and measured at 20 ℃ under the MAS rate of 13 kHz, and (b) the changes of ¹H MAS NMR spectra (the MAS rate of 12 kHz) during the hydration and dehydration procedure.

Fig. 1(b) shows that after vacuum drying at 50 ℃, the proton signal of water that appeared at 5.78 ppm under the fully hydrated condition shifted to 6.22 ppm. After vacuum drying at 120℃, this 6.22 ppm water signal was also removed, and as a result, the silanol signal at 1.7 ppm appeared more clearly. In general, waters hydrated in the surface of MCM−41 have been known to be almost removed at the condition of temperature of over 120 ℃. After complete dehydration, 15 min rehydration in the air condition nearly did not hydrate the S−PE−MCM−41, while 45 min rehydration gave the hydrated water signal at 5.78 ppm when measured at the temperatures of 20 and 50 ℃, which might come from the hydrogen-bonded water. After 15 h incubation, the proton signal at 5.78 ppm shifted to 6.22 ppm.

As the temperature increases from −30 to 50 ℃, the water peak shifts from 7.27 toward 5.80 ppm. The silanol peak shifts from 1.75 ppm toward downfield (1.90 ppm), and is broadened, as shown in Fig. 2. This phenomenon might be related to the proton exchange between water and silanol groups. As the temperature increases, the water molecules strongly hydrogen-bonded to the sulfonyl group make a hydrogen-bonded network with a silanol, and chemically exchange its proton with a silanol group. This network gives S−PE−MCM−41 the higher proton conductivity, even at higher temperature.⁹

Figure 2. Temperature-dependent changes of (a) the ¹H MAS NMR spectra under the MAS rate of 12 kHz, and (b) chemical shifts of water and silanol protons of the partially hydrated, and then 15 h-incubated S−PE−MCM−41.

Fig. 3 compares two samples to determine the status of water molecules, sulfonyl, and silanol groups. The ¹H MAS NMR spectrum of the 45 min-hydrated sample measured at 50 ℃ shows the simultaneous decrement of signal intensities at 7.7 and 1.7 ppm, along with the decrement of signal intensity at 5.78 ppm, compared to the spectrum measured at 20 ℃. The ¹H MAS NMR spectrum of the 15 h-incubated sample measured at 50 ℃ shows the signal shift of 6.22 to 5.78 ppm and decrement of only the 1.75 ppm signal, compared to the spectrum measured at 20 ℃. This phenomenon can be interpreted as follows:

Figure 3. ¹H MAS NMR spectra with the MAS rate of 12 kHz (a) measured at 50 ℃ (red) and 20 ℃ (black) after 45 min rehydration, (b) measured at 50 ℃ (red) and 20 ℃ (black) after 15 h incubation.

During the first 45 min-hydration process, water molecules hydrate the surfaces remote from the silanol and sulfonyl groups. Most sulfonyl and silanol protons are nearly isolated from the water molecules, and give signals at around (7.2−8.5) and 1.75 ppm, respectively. At the higher temperature of 50 ℃, effective exchange occurs among the water, SO₃H, and SiOH, and then the signals of both (7.2−8.5) and 1.75 ppm decrease. After 15 h incubation process, most of the hydrated water molecules migrate into the strongly bounded sulfonyl sites (6.22 ppm). The 6.22 ppm signal at the low temperature of 20 ℃ is proton signal from the hydrogen-bonded water–sulfonyl network. As the temperature increases, the hydrogen-bonded network expands to the silanol group, and the exchange between the hydrogen-bonded silanol and an isolated silanol occurs. This decreases the signal (1.75 ppm) of the isolated silanol group, and shifts the water-related proton signal from 6.22 to 5.78 ppm, including the contribution of the signals from the hydrogen-bonded silanol.

Proton Exchange in the S−PE−MCM−41

In the 2D EXSY spectrum of Fig. 4, cross peaks among the four types of protons were measured with increasing mixing times. A cross peak at (1.75, 6.22) ppm comes from the chemical exchange of SiOH between two states of an isolated silanol and a water hydrogen-bonded silanol. This means that a strongly bound water to the sulfonyl group can hydrogen-bond to the nearby silanol. This structure can make the proton conductivity of S−PE−MCM−41 high. Cross peaks at ((1.75, 7.2) and (1.75, 7.7)) ppm that originate from the nuclear Overhauser effect (NOE) between the silanol and phenyl protons show that the silanol group is very close to the phenethyl group. A cross peak at (6.22, 7.2) that originates from the NOE between a water molecule and phenyl protons shows that the water molecule resides around the phenethyl group. The water molecule strongly bound to the sulfonyl might reside closely around the phenyl group of phenethyl sulfonate. A cross peak at (7.2, 7.7) comes from the NOE between the two phenyl protons in phenethyl sulfonate.

Figure 4. (a) Experimental, and (b) simulated 2D EXSY spectra of the 15 h-incubated S−PE−MCM−41 measured with various mixing times of 5, 10, 30, 60, and 100 ms.

McKeen et al. described that S−PE−MCM−41 has many dangling hydroxyl groups, and is hydrophilic. The presence of hydroxyl groups is pivotal to achieving high proton conductivity, presumably due to the formation of a more complete hydrogen-bonded water network in the materials with many dangling −OH, and better water saturation inside the molecular sieves.¹² Marschall et al. also observed that an increase in temperature strongly affects both processes of water diffusion and proton hopping. The diffusion is faster, and since the anchored SO₃H-modified propyl chains vibrate and rotate faster around their anchoring point, the SO₃H groups can more easily come into close contact to the next neighboring groups, enhancing the proton hopping.⁹ Our 2D EXSY measurement provides the spectroscopic evidence to support those descriptive and quantitative results. From the 2D EXSY measurement, the chemical exchange rates and cross relaxation rates among the protons in the chemical species in S−PE−MCM−41 were determined by simulating the experimental 2D EXSY spectra (Fig. 4 and Table 1). The simulation was performed by using matlab program coded by author. In simulation program four proton sites were considered. The chemical shifts, relaxation rates, line widths, and intensities of four peaks were adjusted during the simulation for the best fitting. The spectral width for the simulation was 5000 Hz and the number of data points for t₁ and t₂ domain of 2D EXSY were 256, respectively.

Table 1. Simulation results of the 2D EXSY spectra for the exchange rates and cross relaxation rates among the water, sulfonyl, and silanol protons

The cross-relaxation rate via dipole–dipole interaction is dependent on the sixth power of the internuclear distance. The distance between two phenyl protons is 2.5 Å. Not considering the motional effect, the distance between an isolated silanol and phenyl protons was determined to be about 3.5 Å, by comparing the two cross relaxation rates of 3.5 and 0.5. A phenyl group grafted into the MCM−41 surface is in very close contact with silanol in the MCM−41 channel. Based on the same logic, the water proton strongly bound to the sulfonyl is close to one of the phenyl protons. From the fact that the cross-relaxation rate at the (6.2, 7.7) cross peak is 3.5, the same as the cross-relaxation rate of 3.5 at the (7.2, 7.7) cross peak, the hydrated silanol protons were found to be much closer to the phenethyl group at a distance of about 2.5 Å. Of course, this value is not exact, because other motional factors, except for the geometrical factor, were not considered. The exchange rate between water and the silanol group was 0.5 s⁻¹.

Deuterium Dynamics in the S−PE−MCM−41 During the Hydration Process

Fig. 5 shows the ²H MAS and the inversion recovery spectra of the vacuum-dried MCM−41 and S−PE−MCM−41. From the inversion recovery spectra, at least two kinds of deuterium seemed to exist in the MCM−41 and S−PE−MCM−41. Two and three kinds of deuterium were fitted by using dmfit program, and the quadrupolar NMR parameters were obtained, along with the population of each deuterium (Fig. 6 and Table 2). In dmfit program quad 1^st model was applied and anisotropic chemical shifts were not considered.

Figure 5. (a) The ²H MAS NMR, and (b) inversion recovery, spectra of the MCM−41 and S−PE−MCM−41, vacuum-dried for 30 min at 120 ℃. The spectra were measured with the MAS rate of 6 kHz.

Figure 6. Experimental and simulated ²H MAS NMR spectra of the (a) MCM−41, and (b) S−PE−MCM−41.

Table 2. Populations and quadrupolar NMR parameters of different deuterium sites of the MCM−41 and S−PE−MCM−41 (deuterated, vacuum-evacuated at 120 ?/30 min, and then deuterated again).

The deuterium with a smaller QCC value (C_Q) of 34.7 kHz in MCM−41 can be assigned to the one in the hydrogen-bonded silanol group. The small C_Q might result from the dynamical jumping motion of deuterium among the neighboring silanol sites, which was expected, along with a large asymmetry factor η_Q.^13,14 The other one with large C_Q of 107.9 kHz might be an isolated silanol deuterium, even though its value is rather greater than the value of 67 kHz reported in the previous article.¹³ The faster relaxation of an isolated silanol deuterium than that of the hydrogen-bonded silanol, as shown in Fig. 5b, might be a larger quadrupolar relaxation contribution due to the larger QCC values. In general, the quadrupolar relaxation contribution depends on the square of C_Q.

Similar to MCM−41, S−PE−MCM−41 has three types of deuterium with C_Q of 35.5, 104.0, and 192.7 kHz. The 35.5 and 104.0 kHz signals should come from the silanol deuterium, while the 192.7 kHz signal evidently comes from sulfonyl deuterium. This larger C_Q value in sulfonyl deuterium than in silanol deuterium may result from two sources: the O−D bond structure, and the motion of the deuterium atom. The smaller C_Q values of silanol and sulfonyl groups than the 320 kHz of solid water result mainly from the motional average of the O−D bond around the axis of the Si–O and S−O bond, respectively. From this view, the bond angle of S−O−D might be greater than the 105° of Si–O−D. Larger values than the rotationally averaged C_Q values (67 kHz for the freely rotated silanol deuterium) might indicate that the rotational motion of the O−D bond around the Si–O or S−O bond axes was restricted by structural hindrance. But the precise reason is not yet obvious.

To understand the hydration state of the S−PE−MCM−41, the ¹H and ²H spectra and the ²H spin–lattice relaxation data were measured in various hydration conditions. Until 10 min hydration, there was no significant change of the deuterium spectrum, but after 1 h hydration, the deuterium spectrum changes dramatically, as shown in Fig. 7. In the evacuated S−PE−MCM−41 sample, silanol deuterium with smaller QCC value of ~40 kHz has longer spin–lattice relaxation time (2.5 s) than that (200 ms) of sulfonyl deuterium with larger QCC value of ~130 kHz (Fig. 8 and Table 3). In the dehydrated sample, sulfonyl deuterium with larger QCC value and more protons around the sulfonyl group will be expected to relax faster than silanol, due to the larger quadrupolar and dipolar interaction mechanism. After hydration, all deuterium, both in the silanol and sulfonyl groups, relaxed more rapidly, due to the dipolar relaxation mechanism of the water protons. Hydration process occurred at both silanol and sulfonyl sites, even though the degrees of hydration varied a little from each other. After 1 min hydration, the increment amount of the relaxation rate of a silanol deuterium was much greater than that of a sulfonyl deuterium, which means that early hydration proceeded more effectively around silanol groups. This understanding is consistent with the results of Fig. 3. After 1 h hydration, the water content was much more than 10 times that of the silanols, which could be probed from the ¹H spectrum. After 1 h hydration, all types of deuterium signals show a similar relaxation rate of ~ 32 ms, due to the dominant contribution of the dipolar mechanism of water protons.

Figure 7. (a) The ¹H MAS (12 kHz) NMR spectra, and (b) the ²H MAS (6 kHz) NMR spectra of S−PE−MCM−41 after evacuation (120 ℃/30 min), and then deuterium hydration.

Figure 8. The ²H inversion-recovery experiment of S−PE−MCM−41 (a) after evacuation at 120 ℃/30 min, and (b) after 1 h deuterium hydration.

Table 3. The ²H T₁ values of S−PE−MCM−41 determined from the inversion-recovery experiment shown in Fig. 8

CONCLUSION

The proton exchange rate between a water molecule and a silanol group in S−PE−MCM−41 was determined by analyzing both the 1D proton spectra and the 2D proton EXSY spectrum. When the status of water molecules bound to the surface of S−PE−MCM−41 was analyzed using the proton spectra and ²H spin-lattice relaxation data under the various hydration levels, there were at least two kinds of water-bounding sites in S−PE−MCM−41, strong and weak sites, in terms of their evacuation property at 50 and 120 ℃, respectively. In the early hydration process, water molecules bound to the weakly bound sites, and then migrated to the strongly bound sites over several hours. This migration made the water molecules reside tightly in the S−PE−MCM−41. Water molecules that participated in the hydration of the phenethyl sulfonate may be involved in the hydrogen-bonded silanol mechanism of the proton conductivity, which was evidenced by the 2D EXSY measurement. Even though the S−PE−MCM−41 easily lost water molecules weakly bound to the silanol at high temperature, the strongly bound water molecules survived. This phenomenon contributes to the higher proton conductivity of the S−PE−MCM−41 with the cooperation of the sulfonyl and silanol groups in the proton transfer process.⁹