Host–Guest Interactions in Dealuminated HY Zeolite Probed by 13C

Letter pubs.acs.org/JPCL

Host−Guest Interactions in Dealuminated HY Zeolite Probed by 13 C−27Al Solid-State NMR Spectroscopy Shenhui Li,†,# Frédérique Pourpoint,‡,# Julien Trébosc,‡ Lei Zhou,† Olivier Lafon,‡ Ming Shen,‡,§ Anmin Zheng,† Qiang Wang,† Jean-Paul Amoureux,*,‡,§ and Feng Deng*,† †

State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, The Chinese Academy of Sciences, Wuhan 430071, China ‡ Unit of Catalysis and Chemistry of Solids (UCCS), ENSCL, CNRS UMR-8181, University Lille North of France, University of Lille 1, 59652 Villeneuve d’Ascq, France § Physics Department and Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, Shanghai 200062, China S Supporting Information *

ABSTRACT: Host−guest interactions in dealuminated HY zeolite have been investigated by advanced 13C−27Al solid-state NMR experiments. This analysis allows us to report new insights into the adsorption geometry of acetone and its interaction with acid sites in the zeolite channels.

SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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essential to probe the interactions between the zeolite and the acetone (and its reaction products). Indeed, the formation of (i) the hydrogen bond between the proton of the Brønsted acid site and carbonyl oxygen of acetone or (ii) the dipolar bond between the aluminum atom of the Lewis acid site and the carbonyl oxygen increases the isotropic chemical shift of the carbonyl 13C nucleus of adsorbed acetone. 13 C NMR of adsorbed acetone as well as DFT calculations demonstrated that the Brønsted/Lewis acid synergy considerably enhances the Brønsted acid strength of dealuminated HY zeolite.11 Furthermore, this strong acidity results in spontaneous aldol addition and condensation of acetone adsorbed in dealuminated HY zeolite at room temperature (Figures S1 and S2, Supporting Information). In a previous work, geometries for the acetone adsorption complexes on the Brønsted and Lewis acids have been calculated by DFT.11 However, the isotropic chemical shift of the carbonyl 13C site was the only experimental constraint to validate the DFT geometries. Here, 13 C−27Al NMR experiments on 2-13C-acetone adsorbed into dealuminated HY zeolite provide novel experimental restraints about the structure of these complexes. Nevertheless, double-resonance 13C and 27Al NMR experiments are challenging due to their very close Larmor frequencies (about 3.6 MHz difference at 9.4 T). As far as

olid acid catalysts, such as zeolites and metal oxides, represent key materials in the catalytic cracking, hydrocracking, and isomerization reactions in the petrochemical industry. Their widespread application is mainly attributed to their high acid-catalyzed activity.1 However, there are still unsettled questions about the catalytic reaction process, including, (i) the characterization of detailed acidic properties and (ii) the elucidation of the catalytic reaction mechanism.2,3 Host−guest interactions between adsorbed molecules and acid sites strongly influence the performances (activity and selectivity) of solid acid catalysts because they play essential roles in (i) the adsorption, (ii) the desorption, and (iii) the catalytic reaction process.4 Consequently, numerous efforts have been made to investigate the location of guest molecules inside of the zeolite cages by combining various spectroscopies (magnetic resonance, infrared, Raman, X-ray absorption, ...) and density functional theory (DFT) calculations.5,6 Nevertheless, direct experimental evidence for the exact interaction modes is scarce, owing to the lack of suitable characterization techniques. Solid-state nuclear magnetic resonance (NMR) is a wellestablished technique that allows characterization of the acidity property as well as the catalytic reactions over solid acid catalysts. 13C MAS NMR of adsorbed the 2-13C-acetone molecule7,8 has been frequently used as a probe technique for characterizing the acidic properties of Brønsted and Lewis acid sites, which are associated in zeolites with framework (FAL) and extra-framework (EFAL) Al species, respectively.9,10 In such systems, knowing carbon−aluminum connectivities is © 2014 American Chemical Society

Received: July 3, 2014 Accepted: August 21, 2014 Published: August 21, 2014 3068

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dipolar dephasing (the pulse sequence used is shown in Figure S4, Supporting Information). This comparison permits the identification of carbon sites of the guest molecule, which are close to aluminum nuclei of acid sites in the zeolite. It is noteworthy that all of the signals in the chemical shift range of 228−240 ppm are subject to a strong 13C−27Al dipolar dephasing (Figure 1), which could be ascribed to either the hydrogen-bonding interaction between the carbonyl oxygen of acetone and the Brønsted acidic proton or to acetone directly bound to the Al atom of the Lewis acid site. The smaller dephasing for the carbonyl group (214 ppm) can probably be ascribed to a relatively greater size of diacetone alcohol/mesityl oxide, resulting in steric hindrance effects inside of the zeolite channel. The vinyl groups of mesityl oxide associated with signals within the 188−199 ppm range are more distant from the carbonyl group and hence from 27Al nuclei, which results in smaller 13C−27Al dipolar dephasing. It must be noted that the quaternary carbon at 75 ppm is submitted to an intense 13 C−27Al dipolar dephasing. This is possibly due to the strong hydrogen bond interaction between the neighboring OH group in diacetone alcohol and the Brønsted acid site, rendering this quaternary carbon in close proximity to the reaction center. More detailed analysis can be performed in order to directly estimate the 13C−27Al distances. S-RESPDOR dephasing curves are shown in Figure 2, versus the recoupling time, for the

we know, no solid-state NMR experiment combining both nuclei has been performed hitherto to observe 13C−27Al distances in zeolites. In fact, simultaneous observation of 13C and 27Al nuclei is precluded with usual NMR probes. However, with the help of a frequency splitter and advanced new NMR methods, carbon−aluminum dipolar couplings, and hence distances, can be evaluated.12−16 We show here how these advanced NMR methods provide new insights into the host−guest interactions in heterogeneous catalysis systems. One-dimensional (1D) 13C−{27Al} symmetry-based rotational-echo saturation-pulse double-resonance (SRESPDOR) experiments12,17,18 were performed to measure quantitatively the carbon−aluminum distances, and these values determined with NMR agree with those calculated by DFT. Furthermore, the 27Al−{13C} dipolar-mediated heteronuclear multiple quantum correlation (D-HMQC)14,19 2D spectrum permits us to confirm the nature of the 27Al sites near the carbonyl sites. The 13C CP-MAS spectrum (see Figure S1, Supporting Information) of 2-13C-acetone adsorbed on dealuminated HY zeolite exhibits several distinct signals resonating at 29, 75, 188, 199, 214, 220, 228, 234, and 240 ppm. All of these 13C resonances can be assigned according to the literature (Figure S2, Supporting Information).8,11 The resonance at 240 ppm corresponds to 2-13C-acetone adsorbed on Lewis acid sites, and the two signals at 228 and 234 ppm are associated with 2-13Cacetone adsorbed on two Brønsted acid sites having Lewis acid sites in close proximity. The small shoulder peak at 220 ppm is assigned to 2-13C-acetone directly adsorbed on the Brønsted acid sites. Signals resonating at 188 and 199 ppm are associated with the products formed through the aldol condensation of acetone.8 The carbonyl group from diacetone alcohol/mesityl oxide species resonates at 214 ppm. At 75 ppm resonates the quaternary carbon in diacetone alcohol. The signal at 29 ppm is assigned to the methyl group. This signal disappears in the SRESPDOR experiment (Figure 1) most probably due to a short

Figure 1. 13C MAS NMR spectra for 2-13C-acetone loaded on dealuminated HY zeolite acquired at 10 kHz MAS. The blue and red lines represent the spectra observed with (S) and without (S0) 13C− {27Al} S-RESPDOR dipolar dephasing accumulated with a recoupling period of τ = 4 ms.

Figure 2. ΔS/S0 experimental 13C−{27Al} S-RESPDOR fractions and their error bars for the signals at (a) 234 and (b) 240 ppm. The continuous curves shown in (a) and (b) have been calculated with analytical expression given in ref 12 and the best-fit 13C−27Al dipolar coupling constants of 205 and 315 Hz, respectively.

T2′ value and the long recoupling times used. Only one broad peak with different contributions can be observed in the 27Al MAS spectrum of dealuminated HY loaded with acetone, and the signals of the various environments are not resolved (Figure S3, Supporting Information).20 13 C−{27Al} S-RESPDOR experiments can easily be performed at relatively low field (e.g., 9.4 T) as there is no detection of broad 27Al signals, and 13C line widths are dominated by a distribution of chemical shifts. Figure 1 compares 13C MAS spectra of 2-13C-acetone adsorbed on dealuminated HY zeolite, acquired with and without 13C−27Al

carbons resonating at 234 and 240 ppm. A fit of these experimental fractions12 yields the 13C−27Al dipolar couplings and hence the carbon−aluminum distances between the adsorbed 2-13C-acetone (guest molecules) and Brønsted and Lewis acid sites (host zeolite). As shown in Figure 2, the 13 C−27Al dipolar coupling constants between acetone and the Brønsted acid with the Lewis acid site being in close proximity (234 ppm) and that between acetone and Lewis acid (240 ppm) sites were determined to be 205 and 315 Hz, respectively. 3069

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distorted four-coordinated FAL (B), five-coordinated EFAL (C), and six-coordinated EFAL (D). This attribution is in full agreement with the literature.9,23,24 Isotropic chemical shifts δcs and second-order quadrupolar interaction parameters PQ are summarized in Table S2 (Supporting Information). Meanwhile, the interaction between acetone and FAL/EFAL (or Brønsted/ Lewis acid, respectively) species could be clearly observed in the 27Al−{13C} D-HMQC spectrum (Figure 3b). The appearance of correlation peak pairs at (228, 53) and (229, 22) ppm suggests that the acetone molecule is in spatial proximity to both FAL and EFAL species, which agrees with the previous experimental and computational results, confirming the existence of Brønsted/Lewis acid synergy and FAL− EFAL in close spatial proximity in the dealuminated HY zeolite.11,25,26 Similar results can be found on the basis of appearance of the correlations at (234, 48) and (234, 28) ppm. In addition, the correlation peak at (240, 29) ppm justifies the proposed model of acetone bounded with EFAL species (Lewis acid site). Interestingly, it can be concluded that acetone is possibly coordinated with the distorted four-coordinated FAL or five-coordinated EFAL rather than with six-coordinated EFAL. Moreover, the strong correlations between 13C signals at 183 and 214 ppm and 27Al signals at 26−60 ppm are clearly observable, suggesting the spatial proximity of acetic acid and carbonyl sites of aldol reaction products to the FAL and/or five-coordinated EFAL. The acetic acid is readily formed upon a mild reaction treatment, and hence, the intensity of the peak at 183 ppm strongly increases in the 13C spectrum of acetone reacted on dealuminated HY zeolite at 300 °C for 3 h (Figure S8, Supporting Information). Furthermore, this signal exhibits significant 13C−27Al dipolar dephasing in the 13C−{27Al} SRESPDOR spectrum, which confirms the strong binding of acetic acid to the acid sites of the zeolite. Conversely, as observed in Figure 1, the distance between the vinyl group of aldol condensation reaction products (188−197 ppm) and acid sites must be long, which results in small 13C−27Al dipolar coupling and weak dephasing. Consequently, this 13C−27Al correlation is hardly observable in the 2D 27Al−{13C} DHMQC spectrum acquired with a recoupling period of 1 ms. The host−guest interactions between reactants and reaction active centers play essential roles in the catalytic process and can strongly affect the catalysis performance. Several types of reaction intermediates such as carbocations27 and methoxy species28 have been experimentally observed using in situ MAS NMR techniques. Even though the host−guest interactions between reactants and the acid sites could be easily demonstrated from the DFT theoretical calculations,29 the experimental evidence to describe this kind of interaction is still lacking so far. 13C−27Al solid-state NMR techniques provide experimental confirmation of the interaction models between acetone and Brønsted and Lewis acid sites in dealuminated HY zeolite, which were proposed earlier using DFT calculations. By utilizing 13C−{27Al} S-RESPDOR experiments, the distances between the adsorbed acetone and reaction active centers can be measured. Particularly, we found that a significant 13C−27Al dephasing effect could be detected for acetone and acetic acid tightly adsorbed on the Brønsted and/or Lewis acid sites in dealuminated HY zeolite, thus proving a close proximity between these two groups. Furthermore, the spatial interaction between the adsorbed reactants with different active centers (Brønsted and Lewis acids) was also clearly manifested from the 2D 27Al−{13C} D-HMQC experiment. Our current work

Accordingly, the distances between the carbonyl carbon of acetone and the nearby FAL and EFAL species were estimated to be 3.4 and 2.9 Å, respectively. These distances are in reasonable agreement with those derived from the optimized geometries of acetone tightly adsorbed on Brønsted (4.1 Å) and Lewis (2.9 Å) acid sites, as shown in Figure S5 (Supporting Information). The smaller distance of the former measured by S-RESPDOR compared to the optimized geometry could be ascribed to multispin systems due to the presence of Brønsted/ Lewis acid synergy in dealuminated HY zeolite.11 Because acetone as a probe molecule can interact with both Brønsted and Lewis acid sites in zeolites, we also performed NMR experiments on a high-field 18.8 T spectrometer to distinguish the signals of 27Al sites of Brønsted and Lewis acid sites. Figure 3 shows the 27Al MQMAS21,22 and 27Al−{13C} D-

Figure 3. (a) 27Al 3QMAS and (b) 27Al−{13C} D-HMQC spectra of 2-13C-acetone loaded on dealuminated HY zeolite acquired at 18.8 T under 20 kHz MAS. The asterisk denotes spinning side bands.

HMQC 2D spectra of 2-13C-acetone adsorbed on dealuminated HY recorded to qualitatively probe the distances between aluminum and carbon sites. The 27Al−{13C} D-HMQC 2D experiment is conducted using the pulse sequence as shown in Figure S6 (Supporting Information). With the high resolution of the sequence and the high magnetic field, four 27Al signals were observable in the 27Al 3QMAS spectrum (Figure 3a), while only three were distinguished in the 1D spectrum (Figure S7, Supporting Information). Assignment of the four signals can unambiguously be made to four-coordinated FAL (A), 3070

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strength for the 13C π/2 pulses was set to 96 kHz. The pulse lengths for the CT-selective pulses on the 27Al channel using an rf field strength of 8.3 kHz were fixed to 10 and 20 μs. SFAM1 max recoupling on the 13C channel was used with (Δνmax ref,13C,νnut,13C) = (70, 118) kHz and a recoupling time of 1.15 ms. Continuouswave 1H decoupling, with an amplitude of 90 kHz, was used during SFAM1, but no decoupling was applied during acquisition. The increment interval in the indirect dimension was set to 63 μs. Typically, 5120 scans were acquired for each t1 increment, and two-dimensional data sets consisted of 50t1 × 3736t2. The recycle delay was set to 1.0 s. The 27Al 3QMAS spectrum was recorded with the pulse sequence proposed by Amoureux et al.22 For the 3QMAS experiment, the pulses used for the excitation and the reconversion of the triple-quantum coherences lasted 4.5 and 1.4 μs, respectively, and employed an rf field strength of about 150 kHz. The length of the CTselective pulse in the 3QMAS experiment was 11 μs. A total of 1128 scans and 100 increments with an interval of 63 μs were accumulated, and the recycling delay was set to 0.5 s.

presents experimentally novel evidence for the interaction between the probe molecule and the acid sites in heterogeneous zeolite catalysts. The combination of in situ MAS NMR and 27Al−{13C} SRESPDOR experiments paves avenues for monitoring different reaction processes and elucidating the reaction mechanism in heterogeneous catalysis. The investigation of host−guest interactions will clarify the structure−property relationship during heterogeneously catalyzed reaction processes.

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EXPERIMENTAL METHODS Zeolite Na−Y (nSi/nAl = 2.8) was exchanged in a 1.0 mol L−1 aqueous solution of NH4Cl at 80 °C for 4 h. The process was repeated four times. The obtained zeolite NH4Y was washed with distilled water until it became chloride-free. Subsequently, the powder material was dried in air at 80 °C for 12 h. The obtained NH4Y sample was placed in a quartz crucible in a tube furnace and calcined at 500 °C in air for 4 h (the temperature was raised from room temperature to 500 °C at a rate of 3 °C/ min). The Si/Al ratios, determined by 29Si MAS NMR (not shown here), are 2.8 and 3.5 for the obtained HY and dealuminated HY zeolites, respectively. Prior to NMR experiments, the dealuminated HY sample was placed in a glass tube and dehydrated at 400 °C for 12 h on a vacuum line. After the dehydrated sample was cooled down to room temperature, a known amount (∼5 molecule/u.c.) of 2-13C-acetone was introduced into the catalysts and frozen by liquid N2 separately. Then, the sample was flame-sealed and transferred into an NMR rotor under a dry argon atmosphere in a glovebox. Three-channel HXY probes were used in double-resonance mode, added with a frequency splitter, which enabled tuning and matching to both 13C and 27Al Larmor frequencies.12 Adamantane was used as an external reference for 13C by setting the CH2 resonance to 38.5 ppm, and 27Al NMR spectra were referenced with respect to Al(NO3)3 1 mol·L−1 at 0 ppm. For the 1H → 13C cross-polarization transfer, performed at 9.4 T with 10 kHz MAS, 1H and 13C rf field amplitudes were set to 55 and 45 kHz, respectively. The 13C CP/MAS spectrum was acquired with a contact time of 1.9 ms. The 13C−{27Al} S-RESPDOR experiments were conducted on a Bruker 400 MHz Avance-II spectrometer using a 4 mm probe under 10 kHz MAS. The pulse sequence for acquiring the 13 C−{ 27 Al} S-RESPDOR is shown in Figure S4 (Supporting Information). SFAM1 dipolar recoupling was max used on the 13C channel with (Δνmax ref,13C,νnut,13C) = (60, 50) 30,31 1 kHz. Continuous-wave H decoupling with an amplitude of 80 kHz was used during SFAM1, while a SPINAL-64 (smallphase incremental alternation with 64 steps) decoupling with an amplitude of 55 kHz was used during acquisition. A π pulse length of 12 μs was used on the 1H channel. Two saturation pulses with an amplitude of 71 kHz and length of 75 μs = 0.75 TR each were utilized to transfer the 13C−27Al dipolar interaction (see Figure S4, Supporting Information). The recycle delay was set to 2 s. Typically, 512−4096 scans were accumulated. Experimental S-RESPDOR fractions were fitted with the previously reported analytical expression using the Mathemetica software package.12 The error bars in Figure 2 were calculated on the basis of the signal-to-noise ratios in SRESPDOR spectra (S and S0). The 27Al−{13C} D-HMQC experiment,14,19 illustrated in Figure S6 (Supporting Information), was carried out on a Bruker-800 MHz spectrometer equipped with an Avance-III console under 20 kHz MAS, using a 3.2 mm probe. The rf field

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ASSOCIATED CONTENT

* Supporting Information S

Setting details and computational results for DFT calculations, 1D 13C and 27Al MAS NMR spectra for acetone loaded on dealuminated HY zeolite, chemical shift assignments for the 13C CP/MAS NMR spectra of 2-13C-acetone adsorbed on dealuminated HY zeolite, pulse sequences for 13C−{27Al} SRESPDOR and 27Al−{13C} D-HMQC, and 13C MAS NMR spectra for 2-13C-acetone reacted at 300 °C for 3 h over dealuminated HY zeolite. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.D.). *E-mail: [email protected] (J.-P.A.). Author Contributions #

S.L. and F.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (21210005, 21221064, 21373265, and 21120102038) and from the Region Nord/Pas de Calais, Europe (FEDER), CNRS, ANR-2010-jcjc-0811-01, and Infrastructure of Research in Nuclear Magnetic Resonance (IR-NMR). S.L. is grateful for the financial support to visit the University Lille North of France from the Chinese Academy of Sciences.

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REFERENCES

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