Proton Exchange Reaction between Hydroxyl Groups in the

Article pubs.acs.org/JPCC

Proton Exchange Reaction between Hydroxyl Groups in the Supercage and Those in the Sodalitecage of Y Zeolite As Studied by Variable Temperature 1H MAS NMR Naoki Asakawa,† Ken Motokura,‡ Tatsuaki Yashima,§ To-ru Koyama,‡ Toshinori O-nuki,‡ Akimitsu Miyaji,‡ and Toshihide Baba*,‡ †

Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan § Advanced Research Institute for the Sciences and Humanities, Nihon University, 12-5 Gobanchou, Chiyoda-ku, Tokyo 102-8251, Japan ‡

ABSTRACT: Proton-exchanged Y zeolite (H−Y zeolite) has two types of bridging hydroxyl groups; one exists in the supercage and the other exists in the sodalitecage. The dynamic property of protons due to these hydroxyl groups was investigated by variable temperature 1H magic-angle spinning nuclear magnetic resonance (1H MAS NMR). Proton exchange between two types due to these hydroxyl groups proceeded at temperatures higher than ca. 500 K. The temperature dependence of the spectra was explained using the McCornell equation. The activation energy of the proton exchange was estimated to be 50 kJ mol−1.

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intensity of these hydroxyl stretching bands in H−Y zeolite.5 The H−Y zeolite was prepared by outgassing NH4−Y overnight under high vacuum at ∼523 K. The gases (N2, O2, CH4, Ar, and Kr) were then contacted with the H−F band at 203 K. These gases were found to perturb the bridging hydroxyl groups, giving rise to the H−F band, whereas the L−F band was unaffected at 203 K. On the basis of these results, they concluded that the hydroxyl groups observed around 3640 and 3540 cm−1 were located in the supercage and sodalitecage of Y zeolite, respectively. As previously mentioned, small molecules, such as CH4, could not enter the sodalitecage. It was suggested that NH4+ cations were too large to enter the sodalitecage. However, the bridging hydroxyl groups were observed around 3640 and 3540 cm−1, when NH4−Y zeolite, in which more than about 30% of the Na+ cations was exchanged with NH4+ cations, was heated under vacuum for 6 h at 753 K.6 Thus, the intensity of the H−F band increased almost linearly with increasing effect of the exchange. The L−F band increased only slightly until about 30% of the Na+ cations was exchanged with NH4+ cations. However, the contradicted results were also reported. For example, NH4−Y zeolite was further exchanged with Cs+ cations in cesium nitrate solution at room temperature. The sample contained 31.5% cesium cations (38 atoms per unite

INTRODUCTION Proton-exchanged Y zeolite (H−Y zeolite) has been used as a solid catalyst in many reactions, including cracking and isomerization of paraffins, alkylation, and dehydration of alcohols. These reactions took place on the Brönsted acid sites. In the case of H−Y zeolite, the Brönsted acid sites (bridging hydroxyl groups) were formed by decomposition of NH4+-exchanged Y zeolite (NH4−Y zeolite) around 550 K, as shown in Scheme 1.1,2 Scheme 1. Decomposition of NH4+-Exchanged Y Zeolite to H−Y Zeolite

On the basis of previous infrared spectroscopic studies, the hydroxyl stretching bands due to the bridging hydroxyl groups (Si-(OH)-Al) shown in Scheme 1 were observed around 3640 and 3540 cm−1.3 Ward has revealed that the hydroxyl groups due to these bands around 3640 and 3540 cm−1 are located in supercage and sodalitecage, respectively.4 These two bands were expressed as H−F and L−F bands, respectively, to distinguish two types of bridging hydroxyl groups in this section. White et al. studied the effect of physically adsorbed gases (N2, O2, CH4, Ar, and Kr) at 203 K on the position and © 2012 American Chemical Society

Received: June 19, 2012 Revised: July 27, 2012 Published: July 27, 2012 17734

dx.doi.org/10.1021/jp306004x | J. Phys. Chem. C 2012, 116, 17734−17738

The Journal of Physical Chemistry C

Article

dynamical modes such as exchange, rotation, translational diffusion couple with molecular reorientation, and so on.12 In our previous study, we have reported that the delocalization of acidic protons in ZSM-5 and mordenite zeolites was observed by 1H MAS NMR around 400 K.13,14 The dynamic process involved a proton jump between neighboring oxygen atoms on the AlO4 tetrahedron, as shown in Scheme 2.

cell). This cesium ammonium Y zeolite sample was heated under vacuum at 623 K.4 The H−F band was completely eliminated, whereas the L−F band was virtually uninfluenced in the cesium ammonium Y zeolite sample.4 On the basis of these results, Ward concluded that Cs+ cations exchange could occur at the ion exchange sites in only supercage, not in sodalitecage. Cesium cation (ionic radius: 0.169 nm) is too large to enter the hexagonal prism, whereas sodalitecage has the 0.22 to 0.25 nm diameter windows constructed with a six-oxygen-membered ring. Polar basic molecules, piperidine and pyridine, which are too large to enter the sodalitecage, interacted with protons due to the bridging hydroxyl groups located in the both supercage and sodalitecage.2,7 For example, piperidine was contacted with H− Y zeolite at 308 K, after the calcination of NH4−Y zeolite at 573 K, and the sample was then evacuated at room temperature. The intensities of both H−F and L−F bands could not be measured.7 This results shows that the protons due to the bridging hydroxyl groups in both supercage and sodalitecage reacted with piperidine to produce piperidinium ion at 308 K. This sample was further evacuated at 473 K and the L−F band was partially restored, whereas both H−F and L−F bands were completely restored upon degassing at 673 K. When pyridine was contacted with H−Y zeolite at 383 K after calcination of NH4−Y zeolite at 723 K, the H−F band was completely removed to produce pyridinium ion, whereas the L−F band was considerably reduced in intensity. After evacuation this sample at 423 K, the L−F band was restore to its original intensity, whereas the H−F band was not restored.7 Ammonia1 and cumene8 molecules also interact with the protons, giving the L−F band due to hydroxyl groups in sodalitecage. Considering the pKa values of the bases (piperidine, 11.2; pyridine, 5.2; NH3, 9.3), limits seem to be set on the acid strength of the bridging hydroxyl groups. The bridging hydroxyl groups in sodalitecage reacted more easily with base molecules having the larger pKa values, whereas they did not interact with Cs+ cations at 623 K, as previously mentioned. This apparent contradiction is presumably rationalized in terms of proton mobility. Hence, the protons due to the bridging hydroxyl in both the supercage and sodalitecage possibly undergo proton exchange, which should be dependent on the temperature. We focus to investigate the dynamic property of protons in H−Y zeolite. To examine physicochemical properties of protons in zeolites, the measurements of protons in zeolite have been successfully studied by 1H MAS NMR technique. Two kinds of protons in H−Y zeolite were also detected by this method. The recent advances in solid-state NMR spectroscopy are notable; in particular, double-quantum (DQ) 1H NMR in solids is quite informative to structural elucidation for various compounds.9 For example, Deng and coworkers applied the DQ NMR to investigate the static structure of H−Y zeolites.10,11 Therefore, the important information on the influence of Lewis acid sites based on aluminum leaved from zeolite lattice to Brönsted acid sites in dealuminated H−Y zeolite have been found by the DQ spin dynamics 1H MAS NMR method. In this work, the proton exchange between acidic O−H groups (acidic protons) in supercage and those in the sodalitecage was investigated by using variable temperature 1 H MAS NMR method. From the viewpoint of the investigation of dynamic behavior, DQ NMR method is now a developing stage particularly for dynamic systems with several

Scheme 2. Proton Jump between Neighboring Oxygen Atoms on the AlO4 Tetrahedron

This proton jump was revealed by examining the temperature dependence of the line width of a peak due to the protons of the bridging hydroxyl groups in proton-exchanged ZSM-5 (H-ZSM-5 zeolite) and mordenite (H-mordenite zeolite) measured by 1H MAS NMR. However, according to our previous study, in the case of H− Y zeolite, the variations of the line widths of the peaks of 1H MAS NMR spectra attributed to the protons due to the bridging hydroxyl groups in both the supercage and sodalitecage did not change up to 473 K,13 which was the highest measurable temperature about 20 years ago. This encouraged us to measure the variable temperature 1H MAS NMR spectra of H−Y zeolite. In this study, we examine the dynamic properties of the bridging hydroxyl groups as Brönsted acid sites in both the supercage and sodalitecage measured by 1H MAS NMR from 298 to 673 K.

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EXPERIMENTAL SECTION 1. Preparation of Y-Type zeolite materials. Na−Y zeolite with a Si/Al ratio of 2.9 was obtained from Toso. NH4− Y zeolite was prepared from Na−Y zeolite using a conventional ion-exchange method in an aqueous NH4Cl solution at 353 K. The percentage of NH4+ exchange was determined to be 8.0 and 68% by atomic absorption analysis of the residual Na+ in NH4−Y zeolite. 2. Preparation of Samples for 1H MAS NMR Measurements. NH4−Y zeolite (ca. 0.20 g) was packed into a glass tube with side arms, each of which was connected to a glass capsule used for 1H MAS NMR measurements. The details of the measurements have been previously reported.13,14 The glass tube was heated to convert NH4−Y zeolite to H−Y zeolite in a stream of dry air at a flow rate of 500 cm3 min−1. The sample temperature was increased from room temperature to 393 K at a constant rate of 1 K min−1 and held at 393 K for 2 h. It was further heated to 723 K at a constant rate of 0.5 K min−1 and held at 723 K for 4 h. Finally, the sample was evacuated and held at 723 K for 3 h. The resultant zeolite was transferred into a glass capsule under vacuum to fill the capsule completely and evenly. The neck of the capsule was sealed with a microtorch, whereas the sample temperature was maintained at 77 K. 3. 1H MAS NMR Measurements. 1H MAS NMR spectra were measured using sealed glass capsules to avoid the influence of humidity.13,14 The 1H MAS NMR spectra were recorded on a Bruker Avance III spectrometer operated at 400 MHz and equipped with a 4 mm CRAMPS probe. A sample sealed in a glass tube was inserted into the zirconia rotor, and 17735



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fore, only one peak was observed at 3.9 ppm when the spectrum was measured at 298 K. This peak was assigned to the Si-(OH)-Al located in the supercage,19,20 as shown in Figure 1a,

the spectra were recorded while raising the sample temperature stepwise from 298 to 673 K. The rotation frequency of the glass capsule was 5.0 kHz at each prescribed temperature. To reduce background signals from the probe material, the DEPTH2 pulse sequence was used with a π/2 pulse width of 3.6 μs and a recycle delay of 60 s. 4. Simulation of 1H MAS NMR Spectra. Numerical simulation using the McCornell equation15 was carried out to extract information about the proton exchange rate with respect to temperature, which gave the activation energy of the exchange reaction. Here we give a brief description on the details of the numerical simulation. The McCornell equation is as follows ⎡ ⎤ 1 i Ω1 − k − k ⎢ ⎥⎡ ⎤ T21 d ⎡ M1 ⎤ ⎢ ⎥⎢ M1 ⎥ ⎢ ⎥= 1 ⎥⎣ M 2 ⎦ dt ⎣ M 2 ⎦ ⎢ k iΩ2 − k − ⎢ ⎥ T22 ⎦ ⎣

(1)

where M1 and M2 are the proton magnetizations at two exchange sites, Ω1 and Ω2 are the chemical shifts for nuclei at the sites, k(Hz) is the exchange rate, and T21 and T22 are the transverse relaxation times for the two sites. Equation 1 is also re-expressed as eq 2 d M = UM dt

(2)

where ⎡ ⎤ 1 i Ω1 − k − k ⎢ ⎥ ⎡ M1 ⎤ T21 ⎥ M ≡ ⎢ ⎥ and U ≡ ⎢⎢ ⎥ 1 M ⎣ 2⎦ k iΩ2 − k − ⎢ ⎥ T22 ⎦ ⎣

Figure 1. Temperature dependence of the 1H MAS NMR spectra for H(8%)-Y zeolite (Si/Al = 2.8) measured at (a) 298, (b) 353, (c) 393, (d) 433, (e) 473, (f) 523, (g) 573, (h) 623, and (i) 673 K as well as (j) after the sample was cooled to 298 K. The spinning frequency of the sample was 5 kHz.

(3)

Using exponential matrix formalism, the formal solution of eq 1 is expressed as

M(t ) = eUt M(0)

(4)

and a very small peak due to terminal silanol groups was observed around 1.8 ppm.18 The 1H chemical shift due to the Si-(OH)-Al increased from 3.9 to ca. 4.3 ppm by stepwise increase of the temperature from 298 to 673 K, as shown in Figure 1a−i, although the peak was broadened above 523 K. Here the line width, which was defined as the half width of the peak, was widened. Therefore, the line width of the peak due to the Si-(OH)-Al, was dependent on the temperature. We have previously reported the variation in the line width of the peak due to Si-(OH)-Al in Al-ZSM-5 with increasing temperature up to 473 K, caused by the delocalization of protons.14 The intensity of the peak at around 1.8 ppm due to terminal silanol groups was very weak, and the change in chemical shift with temperature was not clear. Cooling from 673 to 298 K resulted in restoration of the original spectrum (Figure 1j). 2. Proton-Exchange Reaction of Bridging Hydroxyl Groups. NH4−Y zeolite, whose the rate of NH4+ exchange was 68%, was calcined at 723 K and then evacuated at the same temperature to measured 1H MAS NMR spectra from 298 to 673 K. As mentioned in the Introduction, the NH4−Y zeolite calcined at 723 K was converted to H−Y (H(68%)-Y zeolite), in which the hydroxyl groups in both supercage and sodalitecage.

where M(0) denotes the initial magnetization after a π/2 pulse. It should be noted that we disregarded the time dependence of the 1H chemical shift due to chemical shift anisotropy, which could be modulated by magic-angle sample spinning and residual 1H−1H magnetic dipolar coupling, assuming that these effects were negligible in our case. Therefore, we did not take into acount the conventional Dyson’s time ordering operator16,17 but instead assumed the operator U as timeindependent. The obtained M(t) was fast Fourier transformed to a numerical spectrum, which was then used to fit the exterimental results at various temperatures. The variables for the simulation were the chemical shifts of the two sites (Ω1 (= Ω − δ Ω) and Ω2 (= Ω + δ Ω)), the line width (2π/T21 and 2π/T22), and the proton exchange rate (k).

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RESULTS AND DISCUSSION 1. 1H MAS NMR Spectra of Y Zeolite with 8% Proton Exchange. To avoid interactions between the bridging hydroxyl groups (Si-(OH)-Al) in the supercage and those in the sodalitecage of Y zeolite, Y zeolite with a low proton exchange rate was prepared. We have already reported the variable-temperature 1H MAS NMR spectra of H−Y zeolite with 8% proton-exchanged rate (H(8%)-Y zeolite).18 There17736



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When 1H MAS NMR spectrum of the H(68%)-Y zeolite was measured at 298 K, two peaks attributed to acidic protons were observed as an overlapping peak. These peaks were deconvoluted into two peaks, with chemical shifts of 4.7 and 4.0 ppm. It could be suggested that the protons shifted at 4.7 ppm were in the sodalitecage,19,20 whereas the protons shifted at 4.0 ppm were in the supercage. The former protons caused a considerable increase in chemical shift (4.7 ppm) without a corresponding increase in acid strength by forming a hydrogen bond with the zeolite framework.19 When the temperature was increased to 433 K, the two peaks observed at 298 K merged into a single peak, indicating that proton exchange between two types of protons due to the bridging hydroxyl groups occurred. The chemical shift of the peak was 4.7 ppm, and a half width of the peak was 831 Hz. The chemical shift was gradually decreased from 4.7 to 4.4 ppm with increasing temperature from 433 to 673 K. The half width of the peak increased and reached a maximum, being 892 Hz at 573 K, whereas it decreased to 587 Hz at 673 K. Cooling from 673 to 298 K resulted in restoration of the original spectrum (Figure 2j). The simulated spectra for the corresponding temperature are also shown in Figure 2. The temperature dependence of the

spectral shapes was well-reproduced by the simulation, indicating that the proton exchange proceeded between two kinds of protons due to the bridging hydroxyl groups in supercage and in sodalitecage at temperatures higher than ca. 500 K. The temperature dependence of the spectra was explained using the McCornell equation. The parameters for the best fitted spectra for corresponding temperature are tabulated in Table 1. The activation energy for the proton Table 1. List of Variable Parameters of the McCornell Equation Used for Numerical Simulations temp/K

Ω0/ppm

δΩ/ppm

T21−1/Hz

T22−1/Hz

298 353 393 433 473 523 573 623 673

4.335 4.335 4.335 4.335 4.335 4.335 4.335 4.335 4.335

0.42 0.42 0.42 0.42 0.42 0.42 0.42 0.42 0.42

220 440 560 600 560 590 590 510 400

250 290 300 320 320 320 320 270 190

k/Hz 1.0 1.0 1.0 1.0 1.0 1.0 2.0 4.0 9.0

× × × × × × × × ×

10−1 10−1 10−1 10−1 101 102 102 102 102

exchange was estimated as 50 kJ mol−1 from the Arrhenius plot shown in Figure 3, where the clear Arrhenius behavior was visible over the temperature range higher than ca. 500 K. The pre-exponential factor is ∼7.2 MHz.

Figure 3. Arrhenius plot of the proton exchange rate in H(68%)-Y zeolite.

Sarv and coworkers reported that the activation energy for the delocalization of protons due to the bridging hydroxyl groups over H−Y zeolite was estimated to be 61 kJ mol−1,21 which was larger than that for the proton exchange. This result would show that the proton exchange over H−Y zeolite proceeded more easily than the delocalization of protons.

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CONCLUSIONS In this article, we attempted to obtain the evidence of exchange of protons due to the hydroxyl groups between in the supercage and in the sodalitecage and successfully evaluated the dynamic parameters of their protons in H−Y zeolite by using variable temperature 1H MAS NMR method. Therefore, the temperature dependence spectra were simulated using the McCornell equation. It was found that such proton exchange in H−Y zeolite between in the supercage and in the sodalite cage proceeded at higher than 500 K. The most chemical reactions

Figure 2. Temperature dependence of the 1H MAS NMR spectra for H(68%)-Y zeolite (Si/Al = 2.8) measured at (a) 298, (b) 353, (c) 393, (d) 433, (e) 473, (f) 523, (g) 573, (h) 623, and (i) 673 K as well as (j) after the sample was cooled to 298 K. The spinning frequency of the sample was 5 kHz. The spectra of (a′)−(j′) were simulated. 17737



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catalyzed by the acidic sites of H−Y zeolite, proceed at higher than 500 K. So, in the reactions, both protons in the supercage and in the sodalite cage could work effectively as the active sites. The activation energy of the proton exchange was estimated to be 50 kJ mol−1, and the pre-exponential factor is ∼7.2 MHz.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by a Grant-in-Aid for challenging Exploratory Research (no. 24656486) and Scientific Research (A) (no. 21246120).

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REFERENCES

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Proton Exchange Reaction between Hydroxyl Groups in the

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