Direct Solid-State NMR Observation of Tetrahedral Aluminum

Mar 3, 2007 - However, no direct spectroscopic evidence was provided to verify this ... a need to supply new NMR data to facilitate the assignment of ...
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4495

2007, 111, 4495-4498 Published on Web 03/03/2007

Direct Solid-State NMR Observation of Tetrahedral Aluminum Fluorides in Zeolite HY Fluorinated by Ammonium Fluoride Hsien-Ming Kao* and Yi-Chen Liao Department of Chemistry, National Central UniVersity, Chung-Li, Taiwan 32054, R.O.C ReceiVed: January 28, 2007; In Final Form: February 21, 2007

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F to 27Al CPMAS and 27Al{19F} 2D HETCOR NMR experiments provide direct evidence of the formation of tetrahedral Al-F species in zeolite HY fluorinated by an aqueous solution of ammonium fluoride at 80 °C. On the basis of these NMR observations, a NMR peak assignment for the 19F signals at -173 and -182 ppm to the tetrahedral Al-F species corresponding to an 27Al signal at 50 ppm is made for the first time.

Fluorine, the most electronegative element, is often used as a component or modifier of oxide catalysts such as alumina and zeolites to improve their catalytic activity. Different fluorinating agents and fluorine contents have been studied widely and found to have an important impact on the activity of the fluorinated catalysts.1-6 For example, Becker and Kowalak7 have observed a significant increase in the acidity and catalytic activity of zeolites mordenite, ZSM-5, and Y upon fluorine treatment using an aqueous solution of ammonium fluoride. More recently, Xu et al. used the same fluorination method to improve the acidity of mesoporous material Al-MCM41.8 It is essential to take into account the detailed fluorination procedures and the effect on the structure and acidity of fluorinated catalysts. According to the previous IR, XPS, and 27Al MAS NMR results,4 the enhanced activity was attributed mainly to the formation of Si-F or Al-F species, which, acting as strong acidic sites, interact with acidic OH groups, as illustrated in Scheme 1. However, no direct spectroscopic evidence was provided to verify this fluorination mechanism. The structure of the active species in fluorinated oxide catalysts plays a crucial role in determining their catalytic performance. Therefore, identification of aluminum fluoride species from the fluorinated zeolites is critical to the understanding of the fluorination mechanism and its relation to the catalytic properties. It has been reported that the change in the coordination environment of aluminum upon fluorination is responsible for the observed modification of the acidity of these materials. Although this is generally admitted, definite description and precise identification of these aluminum fluoride species is somehow difficult. Few studies have concentrated on the structure of fluoride and how fluoride interacts with the acid sites in zeolites. This is partially due to the lack of a precise identification method for these fluorinated species in fluorinated zeolites, although earlier IR observation has shown that fluorine can substitute for hydroxyl groups in the acid sites. Moreover, structural identification is often complicated by the rich structural chemistry of the aluminum oxy/hydroxyfluorides because some of these phases are structurally closely related. Therefore, it is * Corresponding author. E-mail: [email protected]. Fax: +8863-4227664. Phone: +886-3-4275054.

10.1021/jp070739w CCC: $37.00

SCHEME 1: Two Possible Reaction Pathways for Fluorination of Zeolites

of great importance to have a powerful tool to explore the exact nature of these fluorinated species and to correlate their structure with the role played in the catalytic activity. With the advent of a fast magic angle spinning (MAS) probe, 19F MAS NMR has become a direct and sensitive method for distinguishing different fluorine species in a variety of materials. However, past NMR studies of fluorinated amorphous aluminum oxides have remained inconclusive.9,10 A full characterization of the state and exact nature of the aluminum fluoride species formed during the fluorination process still remains to be determined. Recently, three different fluorine sites corresponding to the three aluminum local environments AlO6-xFx (x ) 1-3) in a fluorinated alumina, prepared through aqueous impregnation with NH4F, have been identified.11 Although some effort has been made on comprehensive correlations between local environments of 27Al and 19F chemical shifts,12 there is still a need to supply new NMR data to facilitate the assignment of different 19F resonances, particularly for heterogeneous aluminum fluoride phases. This prompted us to conduct a study of zeolite HY fluorinated with an aqueous solution of ammonium fluoride by solid-state 19F and 27Al MAS, 19F to 27Al CP (cross-polarization), and corresponding 2D HETCOR (heteronuclear correlation) NMR measurements. © 2007 American Chemical Society

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Figure 1. (A) 27Al and (B) 19F MAS NMR spectra of F-HY/x, where x ) (a) 0.5, (b) 3, and (c) 4. 27Al NMR spectrum was obtained with a small flip angle of approximately 15° (0.6 µs) and with a repetition time of 2 s. The spinning speed for 27Al NMR spectra is 24 kHz (a) and 20 kHz (b,c), respectively. 19F NMR spectra were acquired with a rotor synchronized spin-echo sequence with an echo time of 41.7 µs (νr ) 24 kHz) and a repetition time of 15 s. Asterisks denote spinning sidebands. The 27Al and 19F chemical shifts are referenced to 1.0 M aqueous aluminum sulfate and CFCl3 as external standards at 0.0 ppm.

The parent zeolite HY was obtained from Zeolyst (CBV600, Si/Al ) 2.6). Fluorination of zeolite HY was performed by preheating HY (1 g) at 80 °C in 50 mL of distilled water, followed by the dropwise addition of 10 mL of aqueous NH4F solution of appropriate concentrations over a period of 1 h. The solution was then maintained at 80 °C with constant stirring for another 3 h. The resulting mixture was then cooled to room temperature, filtered, and dried at 60 °C overnight. The fluorinated samples obtained were not washed by water in order to reserve all of the fluorine-containing species for characterization. These fluorinated samples were designated as F-HY/x, where x denotes the molar ratio of NH4F to the total Al content of the parent zeolite. Figure 1 shows the 27Al and 19F MAS NMR spectra, acquired on a Varian Infinityplus-500 spectrometer equipped with a Chemagnetics HFXY 3.2 mm quadruple-resonance probe, of HY treated with various NH4F contents. At low NH4F loading (x ) 0.5, the F/Al ratio), three signals centered at 60, 30, and 0 ppm, attributed to four-coordinate, five-coordinate, and octahedral aluminum, respectively, were observed. The corresponding 19F NMR spectrum (part a of Figure 1B, collected using a Hahn echo sequence) shows a major peak at -143 ppm and small peaks at -123, -173, and -182 ppm. The same spectral feature was observed for the samples with an F/Al ratio up to 1. With further increasing of the F content (x ) 3), one major 27Al resonance at 60 ppm and a smaller peak at around 0 ppm were observed. The corresponding 19F NMR spectrum

shows a predominant peak at -173 ppm associated with a shoulder at -182 ppm and a small but broad peak at -150 ppm were observed. With a higher level of fluorination (x ) 4), a broad resonance spreading from 0 to -90 ppm and two signals at -151 and -166 ppm were observed in the 27Al and 19F MAS NMR spectra, respectively. These signals correspond to extraframework NH4AlF4 crystalline phase as identified in our previous study.13 Alternatively, the peaks at -1 ppm in the 27Al NMR spectrum and at -141 ppm in the 19F NMR spectrum are due to the 27Al and 19F resonances in (NH4)3AlF6(s).13 The sharp peak at -123 ppm observed for all of the samples is due to the presence of the F- counterion of NH4+ or H+ occurring in the channels of the zeolites.14 The 19F resonances in the range of -170 to -200 ppm are the chemical shifts generally observed for fluorine atoms coordinated to tetrahedral and octahedral Al atoms.10 For example, the 19F resonances observed in this chemical shift range have been assigned to F- ions in sodalite cages and to Al-F bonds in aluminum species in the channels of zeolites.14 In the study of fluorinated alumina, DeCanio et al. have assigned the chemical shifts at -173 and -183 ppm to hydrated and dehydrated AlF3 phases, respectively.15 It should be noted that the bulk AlF3 phase has a corresponding 27Al resonance at -16 ppm. Moreover, the 19F resonance at -180 ppm has been assigned to fluoride ions associated with a hydrated zeolite framework and the resonance at -172 ppm to octahedral Al-F species in the basic faujasite zeolites treated with fluorocar-

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Figure 2. 19F{27Al} TRAPDOR NMR spectra of F-HY/3 (a) without and (b) with 27Al on-resonance irradiation (r.f. field strength ) 85 kHz, τ ) 41.7 µs, spinning speed ) 24 kHz). The difference spectrum is shown in c.

bons.16 From solution NMR, the 19F resonances at -187 ppm have been observed for the AlF4- species.17 In a recent study of fluorinated aluminas, Fischer et al. have observed broad but resolved peaks at -173, -154, -143, and -130 ppm and assigned these peaks to partially hydrated AlF3 and to (AlVIO5F), (AlVIO4F2), and (AlVIO3F3), respectively.11 In contrast to the assignment, Prins et al. concluded that the latter two aluminum fluoride species were unlikely present in their fluorinated alumina samples based on the change in the intensity of 19F peaks upon fluorine loadings.18 The inconclusive results from the past NMR studies of fluorinated alumina and zeolites might suggest that there is a possibility of different fluorine-containing species resonating at the same chemical shift range. Although the 19F peaks at -143, -173, and -182 ppm have been observed in the F-HY/0.5 and F-HY/3 samples, their exact nature deserves further detailed investigation. 19F{27Al} TRAPDOR (transfer population in double resonance) NMR experiments are suitable to investigate the dipolar coupling between 19F nuclei (I ) 1/2) and quadrupolar nuclei such as 27Al nuclei (S ) 5/2).19 Therefore, the 19F{27Al} TRAPDOR NMR experiment was performed on F-HY/3 (Figure 2) to explore differences in the F-Al dipolar couplings, and thus the connectivities, of the different fluorine sites. The experiment was performed using fast spinning speeds to obtain the highest resolution possible. The 19F signals at -173 and -182 ppm gave TRAPDOR fractions of 36% and 28%, respectively, clearly indicating that the TRAPDOR experiment is still feasible under such fast MAS conditions. It is clear from the significant TRAPDOR fractions (Figure 2c, the difference spectrum) that all of the fluorine sites are located in close proximity (i.e., directly bound) to aluminum atoms. Cross-polarization from 19F to 27Al was also performed, and surprisingly a resonance at 50 ppm was detected in the 27Al CP spectrum of F-HY/3 (Figure 3a). This resonance must arise from the 27Al atom in close proximity to a 19F nucleus. Because this signal is slightly shifted from the resonance from framework aluminum atoms (at 60 ppm, see Figure 1) in the 27Al MAS NMR spectrum of F-HY/3 and is also close to that observed in the solution NMR spectrum of AlF4- (49.2 ppm),17 we therefore assign this resonance to a tetrahedral aluminum fluoride species. An 27Al signal at 47 ppm has been observed and identified as tetrahedral Al-F species in a previous study of basic faujasite zeolites treated with fluorocarbons.16

Figure 3. (a) 19F to 27Al CPMAS and (b) 27Al{19F} 2D HETCOR spectra, where the nuclei in the brackets are detected in the t1 dimension, of the F-HY/3 sample, acquired with a spinning speed of 20 kHz and a short contact time of 0.1 ms. Asterisks denote spinning sidebands. The Hartmann-Hahn conditions were determined with anhydrous aluminum fluoride. The typical contact time for 19F to 27Al CP was 0.1 ms, and a repetition time of 1 s was used.

The 27Al{19F} 2D HETCOR NMR spectrum, where the 27Al signals are detected in the t1 dimension, for the F-HY/3 sample is shown in Figure 3b and provides a more direct method of establishing the correlation between the different 19F chemical shifts and the local environments of the Al atoms. A short contact time of 0.1 ms was used to ensure that the major peaks in the spectrum correspond to directly bonded Al-F species. The advantage of using this type of experiment becomes more apparent when a comparison is made between the 27Al dimension and the 27Al one-pulse spectrum of the same sample shown in Figure 1. It also provides additional information over that obtained from the CP experiment. The signal from aluminum atoms in close proximity (i.e., directly bonded) to fluorine is selected in the 27Al dimension. As shown in Figure 3b, both 19F signals at -173 and -182 ppm are correlated to the 27Al signal at 50 ppm, which is not clearly resolved in the 27Al onepulse spectrum. This observation suggests that both fluorinated species are tetrahedral Al-F species. Further discrimination between these two species is not feasible at the present time. Nevertheless, 27Al{19F} HETCOR NMR is a useful means to make comprehensive correlations between local environments of 27Al and 19F chemical shifts for the tetrahedral Al-F species observed. The 27Al{19F} HETCOR NMR spectrum (Supporting information, Figure S1) of the F-HY/0.5 sample shows that the 19F signal at -143 ppm is correlated to the 27Al signal centered at

4498 J. Phys. Chem. C, Vol. 111, No. 12, 2007 0 ppm, indicating that the fluorinated species is an octahedral Al-F species as identified previously in the literature.11 Both 19F NMR signals at -151 and -166 ppm are correlated with the broad pattern spreading from 0 to -90 ppm in the 27Al NMR spectrum, as evident from the 27Al{19F} HETCOR NMR spectrum of the F-HY/4 sample (Supporting information, Figure S2). These signals correspond to the extraframework NH4AlF4 crystalline phase.13 In general, the 19F chemical shifts become more negative as the aluminum chemical shifts move toward more negative values. For example, it has been reported that the fluorine chemical shifts of -150 and -160 ppm correlate with aluminum chemical shifts of -5 and -15 ppm, respectively.12 The 19F and 27Al chemical shifts for bulk AlF3 have been reported to be -172 and -16 ppm, respectively.12 Although our observed 19F signals are very close to that of previously reported AlF 3 phase, we did not observe any corresponding 27Al signal at around -16 ppm due to the octahedral Al-F species. In contrast, the present 19F to 27Al CP and 27Al{19F} 2D HETCOR NMR results provide direct evidence for the formation of tetrahedral Al-F species in fluorinated HY, as illustrated in Scheme 1b. 19F to 29Si CP techniques were also employed in order to reveal the presence of Si-F species in our fluorinated HY samples. No 29Si signal from the 19F magnetization transfer was observed in F-HY/3 (Supporting information, Figure S3), indicating that the possibility for the formation of Si-F species (Scheme 1a) in fluorinated HY can be excluded. The major process that appears to occur during fluorination was believed to be the reaction of the coordinatively unsaturated aluminum sites (i.e., five-coordinate or highly four coordinate sites) to form octahedral aluminum oxy-fluoride species. In contrast to previous observation, our results show that the formation of tetrahedral Al-F species is also possible for fluorinated HY under the present experimental conditions. Further investigation of the fluorination mechanism in zeolites is currently in progress. In conclusion, the utility of 19F to 27Al CP and 27Al{19F} 2D HETCOR NMR experiments allows us to provide direct evidence of the formation of tetrahedral Al-F species in zeolite HY fluorinated by ammonium fluoride at 80 °C. On the basis of these NMR observations, a new NMR peak assignment for

Letters the 19F signals at -173 and -182 ppm to the tetrahedral Al-F species corresponding to an 27Al signal at 50 ppm is made for the first time. Acknowledgment. We thank the National Science Council of Taiwan for financial support. Supporting Information Available: 19F to 27Al CP and 2D HETCOR NMR spectra of F-HY/0.5 and F-HY/4 (Figures S1 and S2) and 19F to 29Si CP NMR spectra of F-HY/3 (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

27Al{19F}

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