Temperature Effect on 1H Chemical Shift of Hydroxyl Groups in

Article pubs.acs.org/JPCC

Temperature Effect on 1H Chemical Shift of Hydroxyl Groups in Zeolites and Their Catalytic Activities as Solid Acids Hajime Munakata,† To-ru Koyama,† Tatsuaki Yashima,‡ Naoki Asakawa,§ Toshinori O-Nuki,† Ken Motokura,† Akimitsu Miyaji,† and Toshihide Baba*,† †

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, Gobanchou 12-5, Chiyoda-ku, Tokyo 102-8251, Japan § Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan ABSTRACT: The influence of temperature on the 1H chemical shift due to hydroxyl groups in H+-exchanged any kind of zeolites (ZSM-5 type zeolites including silicalite, mordenite and Y zeolites) was examined by variable temperature 1H magic-angle spinning nuclear magnetic resonance (1H MAS NMR) spectroscopy measured from 298 to 673 K. The 1H chemical shifts due to bridging hydroxyl groups (Si−(OH)−Al, Brönsted acid sites), which have a lower activation energy for proton jumping, terminal silanol groups, and nest silanol groups increased with temperature. The temperature dependence of the 1H chemical shift due to the bridging hydroxyl groups was more pronounced than that due to the terminal silanol groups and also the nest silanol groups. The relationship between the 1H chemical shift and the acid strength of the hydroxyl groups in zeolites was discussed on the basis of the variable temperature 1H MAS NMR measurements of zeolites, previously reported thermal analysis data, such as the heat of NH3 adsorption measured by NH3 temperature-programmed desorption, and also the catalytic activity for the conversion of ethanol and 1-hexene. It was found that the nest silanol groups of silicalite functioned as Brönsted acid sites and could catalyze the conversion of ethanol and 1-hexene with the same activity as the bridging hydroxyl groups in B-ZSM-5 at 673 K.

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INTRODUCTION Zeolites have been widely used as solid acid catalysts in the oil refining and petrochemical industries. The acidity of zeolites (type of acid sites, their strength, and concentrations) is one of the most important properties with respect to their activity for acid-catalyzed reactions.1 The bridging hydroxyl groups (Si− (OH)−Al) in zeolites act as proton donor sites (Brönsted acid sites) and are responsible for the ability of zeolites to catalyze many reactions, such as the cracking of paraffins. Many experimental methods have been developed to determine the acidic properties of the hydroxyl groups in zeolites.1 For example, infrared (IR) spectroscopy has been widely used to study the bridging hydroxyl groups in zeolite. Ammonia temperature-programmed desorption (NH3 TPD) is also typically used to determine the amounts and strength of the acid sites in zeolites. Among many IR spectroscopy studies on ZO-H, Ward examined the influence of temperature on the intensity and frequency of the absorption IR bands due to the stretching vibration of the bridging hydroxyl groups in Y zeolite to reveal the delocalization of protons.2 IR spectra were measured over the temperature range from 298 to 703 K, and two significant phenomena were observed in the H-Y zeolite:2 © 2012 American Chemical Society

(1) The integrated intensity of the bridging hydroxyl groups decreased at elevated temperatures. (2) The frequency of the band near 3640 cm−1 due to acidic protons present in a supercage structure decreased with increasing in temperature. Ward interpreted result (1) as being due to the delocalization of acidic protons. He also proposed that the frequency shift (result (2)) indicated a change in the interaction between neighboring atoms in the structure, particularly oxygen and aluminum.2 This meant that the chemical properties of acidic protons were possibly changed, which prompted our investigation by measurement of the variable-temperature 1H MAS NMR spectra in various H+-exchanged zeolites. In our previous study, we have reported that the delocalization of acidic protons in ZSM-5 zeolites was observed by 1H MAS NMR around 400 K.3,4 The dynamic process involves a proton jump between neighboring oxygen atoms on the AlO4 tetrahedron, as shown below: Received: May 7, 2012 Revised: June 5, 2012 Published: June 5, 2012 14551

dx.doi.org/10.1021/jp3043945 | J. Phys. Chem. C 2012, 116, 14551−14560

The Journal of Physical Chemistry C

Article

procedure was repeated three times. After the treatment, the sample was washed with a large amount of ion-exchanged water until the pH of the filtrate was 7.0. The sample was then dried at 383 K for 10 h. This sample was denoted as silicalite (final). (2). Identification of Zeolites. Powder X-ray diffraction (XRD) patterns of all of zeolites used in this work were obtained to confirm the structures of the synthesized materials using a diffract meter (Rigaku MultiFlex) with Cu Kα radiation at 40 kV and 40 mA, and at a scan rate of 0.5° min−1 in 2θ. (3). Elemental Analysis of Materials. The amounts of Si and Al in zeolites were determined by inductively coupled plasma atomic emission spectroscopy. The Si/Al ratio in the framework was also determined using 29Si MAS NMR measurements. The amount of residual Na+ in zeolites was measured by atomic absorption spectroscopy. (4). Preparation of NH4+-Exchanged Zeolites. Each zeolite, except silicalite (as-prepared) and silicalite (final), was heated in dry air at 703 K for 40 h and then subjected to ionexchange in a NaCl solution before heating in dry air at 723 K for 20 h to remove residual organic materials. The zeolite was then converted to the ammonium form using an ion-exchange method in NH4Cl solution at 353 K. Atomic absorption spectra indicated that no Na+ ions were present the specimens, and thus, the degree of NH4+ ion exchange was 100% in both the Al-ZSM-5 and B-ZSM-5, while that was 76% in the mordenite. Na-Y was obtained from Toso Co. Ltd. NH4-Y was prepared from Na-Y using a conventional ion-exchange method in an aqueous NH4Cl solution at 353 K. The degree of NH4+ exchange was determined to be 8.0% by atomic absorption analysis of the residual Na+ in NH4-Y. (5). Preparation of Samples for 1H MAS NMR Measurements. Each 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.2,3 Details of the measurements have been previously reported in refs 2 and 3. The glass tube, into which the zeolite was packed, can be heated to convert the NH4+-type zeolite to the H+-exchanged form. Thus, the zeolite was heated in a stream of dry air flowing at a 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. The sample was further heated to 723 K at a constant rate of 0.5 K min−1 and held at 723 K for 4 h. The sample was then evacuated and held at 723 K for 3 h. The resultant zeolite was transferred into a glass capsule under vacuum to completely and evenly fill the capsule. The neck of the capsule was sealed with a microtorch, while the sample temperature was maintained at 77 K. (6). 1H MAS NMR Measurements. 1H MAS NMR spectra were measured using sealed glass capsules to avoid the influence of humidity.2,3 1H MAS NMR spectra were recorded on a Bruker Avance III spectrometer operating at 400 MHz, equipped with a 4 mm CRAMPS probe. A sample sealed in a glass tube was inserted into the zirconia rotor, and 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 1H 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. (7). Thermal Gravimetric Differential Thermal Analysis and Surface Area Measurements. Thermal gravimetric differential thermal analysis (TG-DTA) was performed using a

This proton jump was revealed by examining the temperature dependence of the line width of a peak measured by 1H MAS NMR. Sarv et al. also proposed the delocalization of acid protons after analyzing the spinning sidebands due to acidic protons in H+-exchanged zeolites, such as ZSM-5.5 In the present study, we examine the temperature dependence of 1H chemical shift due to hydroxyl groups in H+exchanged zeolites (ZSM-5-type zeolites including silicalite, mordenite, and Y zeolites) by measuring variable temperature 1 H MAS NMR spectra in the wide range of temperatures from 298 to 673 K to investigate the physicochemical and catalytic properties of hydroxyl groups in zeolites. The relationship between the 1H chemical shifts due to hydroxyl groups and their acid strength around 673 K is discussed on the basis of the temperature dependence, catalytic activity for the dehydration of ethanol, and the isomerization of 1-hexene conversion at 673 K and also previously reported data, such as the heat of NH3 adsorption measured by NH3 TPD. The temperature of 673 K, at which the Brönsted acid sites (bridging hydroxyl groups) on the zeolites catalyze many reactions of hydrocarbons through the carbenium ion mechanism, such as the isomerization of m-xylene,6 is much higher than that typically used for the measurement of 1H MAS NMR spectra, i.e., room temperature. Thus, it is important to investigate the physicochemical properties of hydroxyl groups in zeolites at or near the temperatures required for acidcatalyzed reactions.

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EXPERIMENTAL SECTION (1). Synthesis of Zeolite Materials. ZSM-5-type zeolites containing Al3+ or B3+ ions in their lattice (Al-ZSM-5 and BZSM-5, respectively) and mordenite were synthesized using conventional hydrothermal methods. Silicalite with ZSM-5 structure was synthesized hydrothermally using tetraethyl orthosilicate (TEOS), aqueous solution of tetrapropylammonium hydroxide (TPAOH), tetrapropylammonium bromide (TPABr), and KOH aqueous solution, based on the methods reported by Kitamura et al.7 TPABr was added to an aqueous solution of KOH, and the aqueous solution of TPAOH was introduced slowly into this solution. TEOS was then added dropwise to the mixed solution of TPABr, TPAOH and KOH. To hydrolyze TEOS, the resulting mixture was stirred vigorously at 298 K for 24 h and then stirred at 303 K under a nitrogen atmosphere for 5 h. The solution was then transferred into a Teflon-lined stainless-steel autoclave and crystallized by thermal treatment under autogenous pressure and static conditions at 378 K for 48 h. The white solid product was centrifuged and washed with ion-exchanged water until the pH was ca. 8 and then dried at 383 K for 16 h and calcined at 803 K for 1 h. This resultant sample was denoted as silicalite (as-prepared). The silicalite (as-prepared) sample was then treated with a basic solution as follows. A total of 10 g of silicalite (asprepared) was charged in an autoclave. A mixture of aqueous ammonium nitrate solution (7.5 wt%, 110 g) and ammonia aqueous solution (25 wt%, 168 g) was then added to the autoclave. The mixture was stirred at 363 K for 1 h and the solid product was separated from the solution by filtration. This 14552



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Shimadzu DTG-60 instrument. The temperature of the samples was increased from 298 to 1073 K at a heating rate of 5 K min−1 under a high-purity helium flow at 100 mL min−1. The surface areas of the zeolites, from which organic materials such as structure-directing agents (SDA) were removed by calcination and were obtained using a dynamic N2 adsorption technique with an automatic gas adsorption apparatus (BELSORP-mini). Nitrogen adsorption isotherms were measured at 77 K using the same instrument. (8). Measurements of Scanning Electron Microscopy. The microscopic features of the synthesized zeolites were examined by scanning electron microscopy (SEM). Image of the samples containing no metal were obtained using a Keyence VE-9800 instrument. The mean particle diameters of the materials were estimated by measuring the size of 150 particles from SEM images. (9). Procedure for 1-Hexene and Ethanol Conversion. A zeolite sample was pressed, crushed, and sorted into grains using 16-32 meshes. The grains were packed into a reactor of silica tubing (10 mm or 6 mm i.d.) in a vertical furnace and heated in an air stream at a heating rate of 0.5 K min−1 from room temperature to 723 K. The catalyst was then calcined at 723 K for 3 h. The NH4+-exchanged zeolites were converted into the proton form by this procedure. The resulting 1H MAS NMR spectra showed no peaks attributable to ammonium cations. After calcination of the catalyst, 1-hexene or ethanol conversion was conducted in a continuous flow reactor at atmospheric pressure at 673 K. 1-Hexene or ethanol was delivered by a motor-driven syringe to be vaporized in a preheating zone of the reactor containing quartz chips. Helium served as the carrier gas. The pressure of ethanol was 33.3 kPa, whereas that of 1-hexene was 3.3 kPa. The effluent gas was withdrawn periodically from the outlet of the reactor and analyzed by gas chromatography with a flame ionization detector. In the products, the amounts of aliphatic hydrocarbons with less than three carbon atoms were determined using a Porapak Q column, and those with more than three carbon atoms were determined using an OV-101 column. Analysis for butenes, such as 2-methyl propene, was performed using a Unicarbon A-400 column. The hydrocarbon distributions were expressed on a carbon-number basis, excluding the coke remaining in the reactor.

Table 1. Physicochemical Properties of Zeolite Samples. ratio of Si/M (M = Al or B)

degree of NH4+ exchange/%

surface area/ m2 g−1

crystal shape average size/ μm

Al-ZSM-5

Si/Al = 19.5a

100

385

Al-ZSM-5

Si/Al = 67a

100

392

B-ZSM-5

Si/B = 110a

100

380

silicalite (asprepared) silicalite (final) mordenite Y

Si/Al = 5700b

c

390

plate-like 2.5 × 1.0 overlapped rectangulrar 0.5 overlapped rectangulrar 1.0 cuboid 0.12

Si/Al = 3600b

c

406

cuboid 0.10

Si/Al = 5.1a Si/Al = 2.9a

76 8

516 840

spherulite 0.20 octahedron 0.5−2

zeolite

a

Determined by 29Si MAS NMR measurement. bICP analysis. cNa+ was not detected.

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RESULTS (1). Synthesized Zeolites and Their Physicochemical Properties. The X-ray diffraction patterns of the synthesized zeolites were in good agreement with previously reported diffraction data. Analytical data for the zeolites used in this work are summarized in Table 1. Their surface areas, shapes and mean particle diameters were also given in Table 1. (2). MAS NMR Spectra of ZSM-5-Type Zeolites. (a). 1H MAS NMR Spectra of Al-ZSM-5 with an Si/Al Ratio of 19.5. Figure 1 shows the temperature dependence of 1H MAS NMR spectra for Al-ZSM-5 with an Si/Al ratio of 19.5. A peak was observed at 4.1 ppm at 298 K (Figure 1a). The peak at 4.1 ppm was attributed to the bridging hydroxyl groups, Si−(OH)−Al, which are Brönsted acid sites.8,9 The peak around 1.9 ppm was attributed to terminal silanol groups, which are nonacidic.8,9 The amount of terminal silanol groups in this sample was very small. SEM photographs of the Al-ZSM-5 crystals indicated a single phase consisting of large crystals (ca. 2.5 × 1.0 μm).

Figure 1. Temperature dependence of the 1H MAS NMR spectra for Al-ZSM-5 (Si/Al = 19.5) measured at (a) 298, (b) 353, (c) 393, (d) 433, (e) 473, (f) 523, (g) 573, (h) 623, and (i) 673 K, and (j) after the sample was cooled to 298 K. The spinning frequency of the sample was 5 kHz.

The 1H chemical shift due to Si−(OH)−Al increased from 4.1 to 4.7 ppm when the temperature was increased from 298 to 673 K (Figures 1a−i). The original spectrum was restored after lowering the temperature from 673 to 298 K (Figure 1j). 14553



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The 1H chemical shift due to Si−(OH)−Al is plotted against temperature in Figure 2.

Figure 3. 1H chemical shift due to nonacidic protons as a function of temperature. □, silanol of Al-ZSM-5 (Si/Al = 67); Δ, silicalite (final) (Si/Al = 3600)

Figure 2. 1H chemical shift due to acidic protons as a function of temperature. ○, Al-ZSM-5 (Si/Al = 19.5); □, Al-ZSM-5 (Si/Al = 67); ●, mordenite (Si/Al = 5.1); ◑, H(8%)-Y.

The line width, defined as the half line width of the peak due to 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, which is caused by the delocalization of protons.3,4 In Al-ZSM-5 with a Si/Al ratio of 19.5, the intensity of the peak around 1.9 ppm due to terminal silanol groups was very weak, and the change in the chemical shift with temperature was not clear. (b). 1H MAS NMR Spectra of Al-ZSM-5 with a Si/Al Ratio of 67. 1H MAS NMR spectra of Al-ZSM-5 with a Si/Al ratio of 67 were also recorded with increasing temperature. SEM micrographs of this sample revealed a morphology composed of aggregates of ca. 500 nm particles, which indicates that the sample would have a large number of defects and/or hydroxyl nests. Two signals, one due to Si−(OH)−Al and the other due to terminal silanol groups, were clearly observed in the spectrum obtained at 298 K at 4.0 and 1.9 ppm, respectively. The chemical shift of the peaks due to both Si−(OH)−Al and terminal silanol groups was dependent on the temperature. The temperature dependence of the chemical shift for Si− (OH)−Al is shown in Figure 2. The variation in the chemical shift of Al-ZSM-5 (Si/Al ratio 67) was almost the same as that of Al-ZSM-5 (Si/Al ratio 19.5). The 1H chemical shift due to Si−(OH)−Al in both Al-ZSM-5 zeolites was changed by 0.6 ppm by an increase in the temperature from 298 to 673 K. Thus, the effect of temperature on the 1H chemical shift due to Si−(OH)−Al in Al-ZSM-5 was not dependent on the Si/Al ratio. The peak observed at 1.9 ppm in Al-ZSM-5 with a Si/Al ratio of 67, due to nonacidic terminal silanol groups,8,9 increased to 2.1 ppm with increasing temperature from 298 to 673 K, as shown in Figure 3. The extent of the shift (0.2 ppm) was much smaller than that of 0.6 ppm for the Si−(OH)−Al peak, which indicates that the variation of the 1H chemical shift due to Brönsted acid sites was larger than that due to the terminal silanol groups. (c). 29Si MAS NMR Spectra of Silicalite. The 29Si MAS NMR spectrum of silicalite (as-prepared) with a Si/Al ratio of 5700 was measured at 298 K and is shown in Figure 4a. A main peak was observed at −114 ppm, and small peaks were observed at

Figure 4. 29Si MAS NMR spectra of (a) silicalite (as-prepared) (Si/Al = 5700), (b) silicalite (final) (Si/Al = 3600), and (c) CP-MAS NMR of silicalite (final) (Si/Al = 3600) measured at 298 K.

−110, −117, and −118 ppm, all of which were attributed to Si(OSi)4, (Q4 peak).10,11 A peak around −103 ppm was due to Si(OH)(OSi)3 (Q3 peak).10,11 Quantification of the 29Si signals due to Q3 and Q4 peaks revealed that 2.3% of the Si was present as Si(OH). After treatment of silicalite (as-prepared) with alkaline solution, the resultant Si/Al ratio of silicalite (final) was decreased from 5700 to 3600, which confirmed that desilylation was successful. A 29Si MAS NMR spectrum of silicalite (final) measured at 298 K is shown in Figure 4b. The peaks observed at −103 and −105 ppm were attributed to Si(OH)(OSi)3, (Q3 peak) and those at −113 and −115 ppm were due to Si(OSi)4, (Q4 peak). Other Si species, such as Si(OH)2(OSi)2 were not 14554



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from 298 to 673 K, as observed for Al-ZSM-5 with a Si/Al ratio of 67. On the other hand, the 1H chemical shifts due to silanol groups other than terminal silanol groups increased from 2.2 and 2.1 ppm to 2.5 and 2.4 ppm, respectively, by increasing the temperature from 298 to 673 K. The original spectrum was restored (Figure 5j) with lowering the temperature from 673 to 298 K. The temperature dependence of the 1H MAS NMR spectra for silicalite (final) is shown in Figure 6. The only peak

observed. The areas of the Q3 and Q4 peaks indicate that 14% of the Si was present as Si(OH) in silicalite (final). A 1H-29Si cross-polarization (cp) spectrum of silicalite (final) is shown in Figure 4c. The short contact time preferentially enhances the signals of 29Si nuclei close to protons. The dominance of the signals around −103 and −105 ppm confirms the assignment as Si(OH)(OSi)3 due to the presence of protons near Si(OSi)4. (d). 1H MAS NMR of Silicalite. The temperature dependence of the 1H MAS NMR spectra of silicalite (as-prepared) is shown in Figure 5. At 298 K, the main peak was observed at 1.9

Figure 6. Temperature dependence of the 1H MAS NMR spectra for silicalite (final) (Si/Al = 3600) measured at (a) 298, (b) 353, (c) 393, (d) 433, (e) 473, (f) 523, (g) 573, (h) 623, and (i) 673 K, and (j) after the sample was cooled to 298 K. The spinning frequency of the sample was 5 kHz.

Figure 5. Temperature dependence of the 1H MAS NMR spectra for silicalite (as-prepared) (Si/Al = 5700) measured at (a) 298, (b) 353, (c) 393, (d) 433, (e) 473, (f) 523, (g) 573, (h) 623, and (i) 673 K, and (j) after the sample was cooled to 298 K. The spinning frequency of the sample was 5 kHz.

ppm (Figure 5a), whereas the shoulder peaks were located at 2.2 and 2.1 ppm. The peak at 1.9 ppm was also observed for AlZSM-5 with a Si/Al ratio of 67; therefore, the peak at 1.9 ppm for silicalite (as-prepared) is attributed to terminal silanol groups, which is different from that for silanol groups observed at 2.2 and 2.1 ppm. Gil et al. assigned the peak at 2.3 ppm to silanols inside zeolitic faults or to strings of silanols interacting with each other.12 A broad peak was observed around 4 ppm; however, the intensity of this peak was too weak to determine the precise chemical shift or make a peak assignment. This broad peak was observed for all of the zeolites examined in this work, which suggested that it was due to background, possibly from the MAS probe of the NMR instrument. The temperature dependence of the 1H chemical shifts due to silanol groups in silicalite (as-prepared) are shown in Figures 5a−i. The chemical shift due to terminal silanol groups increased from 1.9 to 2.1 ppm with increasing temperature

observed was at 2.2 ppm, which was also observed for silicalite (as-prepared), while the no peaks for terminal silanol groups at 1.9 ppm were evident. These results strongly indicated that the peak at 2.2 ppm was clearly different from the peak due to terminal silanol groups. This peak at 2.2 ppm could be assigned to nest silanol groups. The Si/Al ratio of silicalite (final) was decreased from 5700 to 3600 by the treatment of silicalite (as-prepared) with alkaline solution. This treatment generates nest silanol groups by the desilylation of silicalite (as-prepared). Kitamura and Ichihashi reported that a broad IR adsorption band was generated around 3500 cm−1 by treatment of the high silica ZSM-5 zeolite with alkaline solution.13,14 Thus, the very broad IR adsorption band around 3500 cm−1 has been assigned to nest silanol groups.13−15 The same method was applied to the preparation of silicalite (final) from silicalite (as-prepared) in this work. 14555



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The temperature dependence of the 1H MAS NMR spectra of silicalite (final) was shown in Figure 6a−i. The chemical shift increased from 2.2 to 2.5 ppm with increase in the temperature from 298 to 673 K. This temperature dependence for the nest silanol groups was slightly larger than that for terminal silanol groups. The original spectrum was obtained (Figure 6j) by lowering the temperature from 673 to 298 K. The 1H MAS NMR spectra clearly distinguished the nest silanol groups from the terminal silanol groups, and the nest silanol groups from bridging hydroxyl groups, (Si−(OH)−Al). Thus, 1H MAS NMR measurement is a useful experimental tool to distinguish various hydroxyl groups in zeolites. On the other hand, IR signals due to nest silanol groups were observed as very broad peaks around 3500 cm−1 and those due to terminal silanol groups in ZSM-5 type zeolites including silicate were typically observed around 3740 cm−1.13−15 The peak due to Si−(OH)−Al in Al-ZSM-5 was observed around 3610 cm−1;16 however, the broad peak due to nest silanol groups overlapped the peaks due to bridging Si−(OH)−Al groups. (3). 1H MAS NMR Spectra of B-ZSM-5 with a Si/B Ratio of 110. 1H MAS NMR spectra of B-ZSM-5 with a Si/B ratio of 110 were measured with increasing temperature. At 298 K, partially overlapped peaks were observed. Using Lorentzian lines, these peaks were deconvoluted into four signals at 2.7, 2.4, 2.2, and 1.9 ppm (Figure 7a). The peaks at 2.4 and 2.2 ppm

could not be distinguished clearly at 298 K. In addition, the intensity of the peak at 2.7 ppm was lower than those of the peaks at 2.4 and 2.2 ppm. The peaks at 2.7, 2.4, and 2.2 ppm were attributed to the bridging OH group, Si−OH−B,17 and the peak at 1.9 ppm was attributed to terminal silanol groups. The ratio of the peak area at 2.4 ppm to that at 2.2 ppm was estimated as approximately 40:60. The temperature dependence of the intensity and chemical shift of the three peaks was examined. Figures 7a−h, measured from 298 to 673 K, showed that the peak area ratio of the 2.4 to 2.2 ppm peaks was independent of temperature, which indicated that no proton exchange reaction occurred between these two types of protons with increasing temperature up to 673 K. In contrast, the 1H chemical shifts of these peaks were dependent on the temperature. The peak at 2.4 ppm shown in Figure 7a was a shoulder peak that was shifted to 2.8 ppm at 673 K (Figure 7h). The peaks observed at 2.2 and 2.7 ppm at 298 K were shifted to 2.6 and 3.1 ppm at 673 K. In addition, the original spectrum was restored (Figure 7i) by cooling from 673 to 298 K. The temperature dependence of the 1H chemical shifts due to Si−(OH)−B in B-ZSM-5 was smaller than that for Al-ZSM5, whereas the 1H chemical shifts due to terminal silanol groups was the same as that for Al-ZSM-5 with a Si/Al ratio of 67. (4). 1H MAS NMR Spectra of Mordenite with a Si/Al Ratio of 5.1. The 1H MAS NMR spectrum of mordenite with a Si/Al ratio of 5.1 was measured at 298 K. A peak attributed to Si−(OH)−Al was observed at 4.0 ppm,18 while the peak due to terminal silanol groups was observed at 1.9 ppm. The 1H chemical shift due to Si−(OH)−Al was changed from 4.0 to 4.6 ppm by increasing the temperature from 298 to 673 K (see Figure 2), whereas the 1H chemical shift due to the terminal silanol groups changed from 1.9 to 2.1 ppm in the same temperature range. (5). 1H MAS NMR Spectra of Y Zeolite with a Si/Al Ratio of 2.9. To avoid interaction between bridging hydroxyl groups in a supercage and those in a sodalitecage of Y zeolite, Y zeolite with 8% H+-exchange was prepared (H(8%)-Y). The 1H MAS NMR spectrum of the H(8%)-Y was measured at 298 K. The 1H chemical shift due to bridging hydroxyl groups (Si− (OH)−Al) located in a supercage close to the face of a sixmembered oxygen ring was observed at 3.9 ppm,18 as shown in Figure 8a, and a very small peak due to terminal silanol groups was observed around 1.8 ppm. The 1H chemical shift due to 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 8a−i, although the peak was broadened above 523 K. Cooling from 673 to 298 K resulted in the original spectrum being restored (Figure 8j). (6). Definition of Acid Strength in This Work. The definition of acid strength must be clarified to discuss the experimental results of this work. The acid strength of an acid site on the solid has been defined as the ability of the surface to neutralize an adsorbed alkali into the conjugated acid, as proposed by Tanabe et al.19 The elementary step for catalysis by Brönsted acid sites in zeolites is proton transfer from the bridging hydroxyl groups (ZO-H) to the adsorbed molecule, M

Figure 7. Temperature dependence of the 1H MAS NMR spectra for B-ZSM-5 (Si/B = 110) measured at (a) 298, (b) 353, (c) 393, (d) 473, (e) 523, (f) 573, (g) 623, and (h) 673 K, and (i) after the sample was cooled to 298 K. The spinning frequency of the sample was 5 kHz.

ZO‐H + M → ZO− + MH+

(1)

On the basis of the definition of acid strength, if M is NH3, then MH+ is NH4+. 14556



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The ratio of B/Si was determined by measuring 29Si MAS NMR spectrum and the amounts of Si and B were measured by ICP analysis. The amount of protons in B-ZSM-5 was determined to be 1.5 × 10−1 mmol g−1, which was almost equal to the amount of nest silanol groups in silicalite (final). Silicalite (as-prepared) could catalyze only the dehydration of ethanol to produce diethyl ether and ethylene at 673 K, as shown in Table 3. The conversion of ethanol was 42.9%, and the distributions of diethyl ether and ethylene were 31.6% and 68.4%, respectively. When silicalite (final) was used as a catalyst, ethanol was completely converted into hydrocarbons, with ethylene as the main dehydration product. Moreover, butenes, pentenes, and hexenes were also produced in very small yield. B-ZSM-5 showed almost same catalytic activity and selectivity as silicalite (final). Silicalite (as-prepared) and silicalite (final) also exhibited catalytic activity for the conversion of 1-hexene, as shown in Table 4. The conversion of 1-hexene was defined as the number of moles of 1-hexene converted into hydrocarbons other than 1-hexene. The conversion of 1-hexene over silicalite (asprepared) was 52.1%; the double bond isomerization of hexenes was dominant, whereas the skeletal isomerization of hexenes proceeded to a much lesser extent. The conversion of 1-hexene was significantly increased from 52.1% for silicalite (as-prepared) to 87.9% for silicalite (final) under the same reaction conditions. The distributions of products formed over silicalite (final) were very different from those formed over silicalite (as-prepared); not only the double bond isomerization products but also the skeletal isomerization products were formed. The products distribution by the skeletal isomerization was 44.5%, whereas those by the double bond isomerization were 53.9%. Furthermore, a small amount of cracking products from hexenes were also observed. It was also shown that the conversion of 1-hexene and the products distribution over BZSM-5 were nearly the same to those over silicalite (final).

Figure 8. Temperature dependence of the 1H MAS NMR spectra for H(8%)-Y (Si/Al = 2.9) measured at (a) 298, (b) 353, (c) 393, (d) 433, (e) 473, (f) 523, (g) 573, (h) 623, and (i) 673 K, and (j) after the sample was cooled to 298 K. The spinning frequency of the sample was 5 kHz.

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DISCUSSION (1). 1H chemical Shift as an Index of Acid Strength. Pfeifer has reported the theoretical reasons to show that the 1H chemical shift is related to the acid strength. Namely, increasing acid strength corresponds to a decreasing value for the deprotonation energy.21 This fact was shown by quantum chemical calculation to increasing values for the net atomic charge of the hydrogen atom.22,23 Ernst has also theoretically shown that an increasing net atomic charge of the hydrogen atom leads to a decrease of the shielding of an external magnetic field and hence to an increase of the chemical shift of the protons.24 This means that the higher 1H chemical shift of hydroxyl groups over zeolite should be the higher acid strength, since the hydroxyl groups are isolated over zeolites. Infrared spectroscopy (IR) is one of the efficient spectroscopic methods to examine the physicochemical properties of the hydroxyl groups (O-H) over various zeolites. Pfeifer has reported that the wavenumber of the O-H stretching vibration band due to the hydroxyl groups over various zeolites has a linear relationship with their 1H chemical shifts.8,12 This means that some 1H MAS NMR spectra due to the protons of the hydroxyl groups, such as the bridging hydroxyl group (Si− (OH)−Al) over zeolites, correspond to the typical IR spectra and this spectral specificity is observed on the zeolite materials with nearly the same structure. Under this restriction, the 1H chemical shift reflects a characteristic of both Si−OH and Al− OH bonds in the bridging hydroxyl group (Si−(OH)−Al) over

With respect to the acid strength, which is not dependent on the particular molecule expressed as M in eq 1, Bartmess et al. reported that the hypothetical decomposition process in eq 1 led to a definition of the acid and base strength of gas phase molecules as the standard Gibbs free energy change for reactions 2 and 3, respectively.20 ZO‐H → ZO− + H+

(2)

H+ + M → MH+

(3)

On the basis of this idea, the deprotonation energy, i.e., the energy difference between ZO− and ZO-H, could be a measure of the acid strength of the zeolite. (7). Conversion of Ethanol and 1-Hexene over Silicalites and B-ZSM-5. Conversion of both ethanol and 1hexene were conducted at 673 K to compare the catalytic activities of silicalite (as-prepared) and silicalite (final) with that of B-ZSM-5, and the results at 1 h of time on stream are summarized in Tables 3 and 4, respectively. In both reactions, the deactivation and the change of products distribution with time on stream over all of zeolites catalysts were not observed up to 5 h of time on stream. The total amounts of silanol groups in silicalite (as-prepared) and silicalite (final) were determined by TG-DTA analysis. The amount of nest silanol groups and terminal silanol groups were estimated by comparing the area of the peaks at 2.2 and 2.1 ppm with that at 1.9 ppm. 14557



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Table 2. Physicochemical Properties of Protons in Zeolites. zeolite

Al-ZSM-5

Si/Al or Si/B ratio

Si/Al = 19.5

proton type

Si− (OH)− Al

Si/Al = 67 Si− (OH)− Al

Si−OH (terminal silanol groups)

mordenite

H(8%)-Y

B-ZSM-5

silicalite (final)

Si/Al = 5.1

Si/Al = 2.9

Si/B = 110

Si/Al = 3600a

Si− (OH)− Al

Si−OH (terminal silanol groups)

Si−(OH)− Al

Si−(OH) (terminal silanol groups)

Si−(OH)−B

Si−OH (nest silanol groups)

1

H MAS NMR chemical shift/(ppm) 298 K 673 K Δδ (difference)b IR O-H stretching bands/cm−1 proton jump activation energy/kJ mol−1

NH3 microcalorimetry/kJ mol−1 Heat of NH3 adsorption, as measured by NH3 TPD/kJ mol−1

deprotonation energy (−ΔH)/kJ mol−1 a

4.1 4.7 0.6 3610 ref

4.0 4.6 0.6 16

1.9 2.1 0.2 3720 ref 16

45 (Si/Al = 35), ref 5 24 (Si/Al = 50), ref 26 20 (Si/Al = 53), ref 4 18 (Si/Al = 35), ref 27 17 (Si/Al = 12), ref 4 11 (Si/Al = 21), ref 3 140∼150 (Si/Al > 27), refs 29 and 30 150 (Si/Al = 15), ref 32 137 (Si/Al = 12), ref 33 134 (Si/Al = 23), ref 33 1200 ref 36

4.0 1.9 4.6 2.1 0.6 0.2 3610 3743 ref 24 ref 24 78 (Si/Al = 2.4), ref 27 54 (Si/Al = 7), ref 5 28 (Si/Al = 53), ref 28

3.9 4.3 0.4 3640 ref 2

2.7 3.1 0.4

2.4 2.8 0.4 3725 ref 16

2.2 2.6 0.4

1.9 2.1 0.2 3740 ref 16

2.2 2.5 0.3 ∼3500 refs 13−15 immeasurable from spectra

61 (Si/Al = 3) ref 5

immeasurable from spectra



150∼142 (Si/Al = 7.5), ref 33 115∼117 (Si/Al = 12∼39), ref 34

115∼130 (Si/Al = 2.8), ref 31 119 (Si/Al = 2.6), ref 33 104 (Si/Al = 2.4), ref 35



1195 ref 36

1171 ref 36

not calculated

not calculated

160 (Si/Al = 20∼46), ref 29

ICP analysis. bΔδ = (1H chemical shift at 673 K) − (1H chemical shift at 298 K).

zeolite. According to ref 8, a 0.1 ppm difference in chemical shift is equivalent to a wavenumber of 10 cm−1. For example, the 0.6 ppm difference in the chemical shift of Al-ZSM-5 between 298 and 673 K would be equivalent to the shift of 60 cm−1 in wavenumber. On the other hand, such linear relationship between the 1H chemical shift and the shift of IR wavenumber was not applied to other materials, such as Ca(OH)2, which have different structures from zeolite.8 (2). Temperature Dependence of 1H Chemical Shift Due to Hydroxyl Groups. The temperature dependence of 1 H chemical shift due to hydroxyl groups in zeolites was observed by measuring variable-temperature 1H MAS NMR spectra. In all cases, the chemical shift increased with increasing temperature. The larger 1H chemical shift is caused by the larger reduced shielding of the external magnetic field and thus leads to a higher net atomic charge for the hydrogen atom of a bridging hydroxyl group in zeolites (Si−(OH)−Al); the higher net atomic charge has the lower O-H bond energy. This is consistent with the temperature dependence of O-H stretching bands due to bridging hydroxyl groups in the IR spectra of zeolites. Thus, the wavenumbers of the O-H stretching vibration bands for H-Y2 and H-mordenite25 decrease with increasing temperature. The difference in chemical shift due to the bridging hydroxyl groups between 298 and 673 K depended on the zeolite, as tabulated in Table 2. The chemical shift value decreases in the following order

This order is about the same as the decrease in the acid strength estimated by any experimental data measured with various methods, such as IR spectroscopy and NH3 adsorption, as tabulated in Table 2. As mentioned before, the acid strength is defined as the deprotonation energy, which is independent of the temperature. However 1H chemical shift values due to the hydroxyl groups depended on temperature, indicating that the 1H chemical shift does not directly mean the deprotonation energy. Furthermore the protons due to the hydroxyl group with the stronger acid strength should show the smaller temperature dependence of 1H chemical shift. On the other hand, the experimental results in Table 2 and Figure 2 is conflicted with the definition of the acid strength given by the deprotonation energy. The equilibrium constant, Keq, of the dissociation of hydroxyl group as expressed by eq 4 depends on the temperature.

The acid strength of the hydroxyl group would be possibly expressed as a function of combination of the temperature and the deprotonation energy. (3). Nest Silanol Groups as Brö nsted Acid Sites at 673 K. The 1H chemical shift due to nest silanol groups was increased from 2.2 to 2.5 ppm with increasing temperature from 298 to 673 K. This 1H chemical shift to 2.5 ppm was almost equal to the chemical shift due to Si−(OH)−B in BZSM-5 at 298 K. It was suggested that the nest silanol groups in silicalite could possibly act as Brönsted acid sites at high

Al‐ZSM‐5 ≈ mordenite > H‐Y > B‐ZSM‐5 ≈ silicalite (nest silanols) 14558



Article

Table 3. Conversion of Ethanol at 673 Ka

a

zeolite

silicalite (as-prepared)

silicalite (final)

B-ZSM-5

amount of nest silanol groups or Si−(OH)−B/mmol g−1 conversion of C2H5OH/% distribution of products/% C2H5-O-C2H5 C2H4 C4H8 (butenes) C6H12 (hexenes) others

7.9 × 10−2 42.9

1.7 × 10−1 100

1.5 × 10−1 100

31.6 68.4 0 0 0

0 97.9 1.4 0.3 0.3

0 99.3 0.1 0.3 0.3

Pressure of C2H5OH: 33.3 kPa, W/F = 6.0 g h mol−1.

Table 4. Conversion of 1-Hexene at 673 Ka

a

zeolite

silicalite (as-prepared)

silicalite (final)

B-ZSM-5

amount of nest silanol groups or Si−(OH)−B/mmol g−1 conversion of 1-hexene/% distribution of products/% C2H4 C3H6 C4H8 (butenes) C5H10 (pentenes) n-hexenes except 1-hexene monomethylpentenes dimethylbutenes others

7.9 × 10−2 52.1

1.7 × 10−1 87.9

1.5 × 10−1 87.0

0 0 0 0 94.2 5.3 0.3 0.2

0.1 0.3 0.5 0.6 53.9 42.1 2.4 0.1

0.1 0.5 0.2 0.2 55.6 41.5 1.8 0.1

Pressure of 1-hexene: 3.3 kPa, W/F = 2.9 × 10−2 g h mol−1.

temperatures, such as 673 K. Kitamura and Ichihashi,13,14 and Heitmann et al.15 reported that the nest silanol groups in silicalite would catalyze the Beckman rearrangement of cyclohexanone oxime to ε-caprolactam around 623 K. Table 3 showed that the amount of nest silanol groups in silicalite (final) was larger than that in silicalite (as-prepared), whereas no terminal silanol groups were present in silicalite (final), as shown in Figure 6. Therefore, the nest silanol groups in silicalite could act as Brönsted acid sites at 673 K. In the conversion of ethanol over B-ZSM-5, ethanol was completely converted to hydrocarbons and the distributions of hydrocarbons were almost the same as that for silicalite (final), as shown in Table 3. And also, in the case of 1-hexene conversion, B-ZSM-5 showed the nearly same activity and selectivity to silicalite (final) as shown in Table 4. These results indicate that the acid strength of silicalite, i.e., that of the nest silanol groups, would be almost the same strength as the bridging hydroxyl groups in B-ZSM-5 at 673 K. (4). Relationship between Chemical Shift and Acid Strength Estimated with Any Experimental Parameters. The order of 1H chemical shift due to Si−(OH)−Al, δH (AlZSM-5 ≈ mordenite > H-Y) is distinct from that of the proton jump activation energy (Al-ZSM-5 < mordenite < H-Y), as summarized in Table 2. The order for the proton jump activation energy is the same as that of the deprotonation energy (Al-ZSM-5 < mordenite < H-Y). Thus, zeolites with a lower proton jump activation energy have a lower deprotonation energy (O-H bond energy). The order of the chemical shift due to Si−(OH)−Al, (Al-ZSM-5 ≈ mordenite > H-Y) can be explained by the relative magnitude of the proton jump activation energy, although the 1H chemical shift for Al-ZSM-5 was almost equal to that for mordenite. The heat of NH3 adsorption measured by NH3 TPD and by NH3 microcalorimetry are typically used to determine acid

strength. The former mode of measurement revealed the order Al-ZSM-5 ≈ mordenite > H-Y, which was the same as that for δH (Al-ZSM-5 ≈ mordenite > H-Y), as summarized in Table 2. However the NH3 microcalorimetry yielded the order mordenite > Al-ZSM-5 > H-Y, which was not correspondent with the 1H chemical shift order. The acid strength of silicalite (final) and B-ZSM-5 was too weak to measure experimentally from the heat of NH3 adsorption, as shown in Table 2. The IR bands due to the bridging hydroxyl groups are one of the criteria of acid strength. The order of the IR bands from Table 2 is summarized as follows; nest silanol groups in silicalite < Al-ZSM-5 ≈ mordenite < Y < B-ZSM-5 ≈ terminal silanol groups in zeolites. This order does not always correspond with the order of 1H chemical shift. The variable temperature 1H MAS NMR measurements and the previously reported results presented in Table 2 suggest that some methods are not always suitable to evaluate the acid strength. Thus, acid strength remains an elusive topic in quantitative discussion, even though it has been correlated with several factors, such as the 1H chemical shifts of acidic protons. Therefore, it is important to use appropriate methods for specific research objectives.

■

CONCLUSION We have measured 1H MAS NMR spectra of H+-exchanged zeolites at high temperature up to 673 K. The 1H chemical shift due to the hydroxyl groups depended on the temperature. The difference in the chemical shift depended on the zeolite. The hydroxyl group with the stronger acid strength showed the larger chemical shift change. 1 H MAS NMR is an excellent tool to clearly distinguish nest silanol groups from terminal silanol groups and bridging hydroxyl groups. The variable temperature 1H MAS NMR measurements presented in this study revealed that the nest 14559



Article

(34) Karge, H. G.; Dondor, V. J. Phys. Chem. 1990, 94, 765. (35) Karge, H. G.; Dondor, V.; Weitkamp, J. J. Phys. Chem. 1991, 95, 283. (36) Bändle, M.; Sauer, J. J. Am. Chem. Soc. 1998, 120, 156.

silanol groups in silicalite can act as Brönsted acid sites and have the same strength of the acid sites of B-ZSM-5 (Si− (OH)−B) at higher temperature. These results were supported by the conversion of ethanol and 1-hexene at 673 K.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

REFERENCES

(1) For example: Corma, A Chem. Rev. 1995, 95, 559. (2) Ward, J. W. J. Catal. 1970, 16, 386. (3) Baba, T.; Inoue, Y.; Shoji, H.; Uematsu, T.; Ono, Y. Microporous Mater. 1995, 3, 647. (4) Baba, T.; Komatsu, N.; Ono, Y.; Sugisawa, H. J. Phys. Chem. B 1998, 102, 804. (5) Sarv, P.; Tuherm, T.; Lippmaa, E. J. Phys. Chem. 1995, 99, 13763. (6) Tukun, N. M.; Al-Khattaf, S. Chem Eng. J. 2011, 166, 348. (7) Kitamura, M.; Ichihashi, H.; Tojima, H. U.S. Patent 5,212,302, 1991. (8) Brunner, E.; Karge, H. G.; Pfeifer, H. Z. Z. Phys. Chem. Neue Folge 1992, 176, 173. (9) Pfeifer, H.; Freude, D.; Hunger, M. Zeolites 1985, 5, 274. (10) Woolery, G. L.; Alemany, L. B.; Dessau, R. M.; Chester, A. W. Zeolites 1986, 6, 14. (11) Chezeau, J. M.; Delmotte, D.; Guth, J. L.; Gabelica, Z. Zeolites 1991, 11, 598. (12) Gil, B.; Zones, S. I.; Hwang, S-J.; Bejblova, M.; Cejka, J. J. Phys. Chem. C 2008, 112, 2997. (13) Kitamura, M.; Ichihashi, H. Stud. Surf. Sci. Catal. 1994, 90, 67. (14) Ichihashi, H.; Kitamura, M. Catal. Today 2002, 73, 23. (15) Heitmann, G. P.; Dahlhoff, G.; Hölderich, W. F. J. Catal. 1999, 186, 12. (16) Chu, C. T-W; Chang, C. D. J. Phys. Chem. 1985, 89, 1569. (17) Fild, C.; Shauts, D. F.; Lobo, R. F.; Koller, H. Phys. Chem. Chem. Phys. 2000, 2, 3091. (18) Pfeifer, H. NMR Basic Principles and Progress; Springer: Berlin, 1994; Vol. 31, pp 31−90. (19) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. Stud. Surf. Sci. Catal. 1989, 51, 1. (20) Bartmess, J. E.; Scott, J. A.; McIver, R. T., Jr. J. Am. Chem. Soc. 1979, 101, 6046. (21) Pfeifer, H. J. Chem. Soc. Faraday Trans. 1 1988, 84, 3777. (22) Datka, J.; Geelings, P.; Mortier, J.; Jacobs, P. J. Phys. Chem. 1985, 89, 3488. (23) Saur, J. J. Phys. Chem. 1987, 91, 2315. (24) Ernst, H. Z. Phys. Chem. (Leipzig) 1987, 268, 405. (25) Shen, J.-P.; Sun, T.; Yang, X.-W.; Jiang, D.-Z.; Min, E.-Z. J. Phys. Chem. 1995, 99, 12332. (26) Lee, B.; Kondo, J. N.; Wakabayashi, F.; Domen, K. Bull. Chem. Soc. Jpn. 1998, 71, 2149. (27) Ernst, H.; Freude, D.; Milder, T.; Pfeifer, H. In Proceedings of the 12th International Zeolite Conference; Baltimore, Maryland, July 5−10, 1998; Treacy, M. M. J., Malcus, B. K., Bisher, M. E., Higgins, J. B., Eds.; Materials Research Society: Warrendale, PA, 1999; Vol. 4, p 2955. (28) Baba, T.; Ono, Y. Annu. Rep. NMR Spectrosc.; Webb, G. A., Ed:, Academic Press: New York, 1999; Vol. 38, p 356. (29) Parrillo, D. J.; Gorte, R. J.; Farneth, W. E. J. Am. Chem. Soc. 1993, 115, 12441. (30) Chen, D. T.; Zhang, L.; Dumesic, C. Y. J. J. Catal. 1994, 146, 257. (31) Lohse, U.; Parlitz, B.; Patzelova, V. J. Phys. Chem. 1989, 93, 3677. (32) Niwa, M.; Katada, N.; Sawa, N.; Murakami, Y. Stud. Surf. Sci. Catal. 1995, 98, 101. (33) Suzuki, K.; Noda, T.; Katada, N.; Niwa, M. J. Catal. 2007, 250, 151. 14560


Temperature Effect on 1H Chemical Shift of Hydroxyl Groups in

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