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J. Phys. Chem. B 2006, 110, 13812-13818
Effects of Adsorbate Molecules on the Quadrupolar Interaction of Framework Aluminum Atoms in Dehydrated Zeolite H,Na-Y Jian Jiao,† Johanna Kanellopoulos,‡ Babita Behera,§ Yijiao Jiang,† Jun Huang,† V. R. Reddy Marthala,† Siddharth S. Ray,§ Wei Wang,† and Michael Hunger*,† Institute of Chemical Technology, UniVersity of Stuttgart, 70550 Stuttgart, Germany, Abteilung Grenzfla¨chenphysik, UniVersita¨t Leipzig, 04103 Leipzig, Germany, and Indian Institute of Petroleum, Dehardun, 248005, India ReceiVed: February 28, 2006; In Final Form: May 17, 2006
The effect of adsorbate molecules on the quadrupolar interaction of framework aluminum atoms with the electric field gradient in dehydrated zeolite H,Na-Y has been studied by 27Al MAS NMR and 27Al MQMAS NMR spectroscopy at magnetic fields of 9.4 and 17.6 T. Upon adsorption of molecules interacting with bridging OH groups by hydrogen bonds (acetonitrile and acetone), the quadrupole coupling constant of framework aluminum atoms was found to decrease from 16.0 MHz (unloaded zeolite) to 9.4 MHz. Adsorption of molecules, which cause a proton transfer from the zeolite framework to the adsorbates (ammonia and pyridine), reduces the quadrupole coupling constant to 3.8 MHz for coverages of 0.5-2 molecules per bridging OH group. The experiments indicate that the quadrupole coupling constant of framework aluminum atoms in dehydrated zeolite H,Na-Y reflects the chemical state of adsorbate complexes formed at bridging OH groups. In agreement with earlier investigations it was found that a proton affinity of the adsorbate molecules of PA ) 812-854 kJ/mol is necessary to induce a proton transfer from the zeolite framework to the adsorbed compounds. This proton transfer is accompanied by a strong improvement of the tetrahedral symmetry of zeolitic framework AlO4 tetrahedra and a decrease of the electric field gradient.
Introduction An important topic of research in the field of catalysis on solid acids is the chemical state of reactants adsorbed at Brønsted acidic surface sites of these materials. In a number of studies, FTIR and solid-state NMR spectroscopy were applied to investigate the formation of hydrogen bonds or the proton transfer from the catalyst framework to probe molecules.1-8 Band positions or chemical shifts indicate whether the proton affinity of the probe molecule and the acid strength of the surface site lead to a proton transfer between the involved compounds. In the 1980s, 29Si MAS NMR spectroscopy of dehydrated zeolites loaded with different probe molecules evidenced that the zeolite structure is affected by guest molecules.8 However, the study of the influence of adsorbates on framework aluminum atoms in dehydrated zeolites was hindered due to strong quadrupolar interactions of these nuclei and limitations in the experimental technique at this time. In the case of zeolites applied as acidic catalysts, bridging OH groups (SiOHAl) on oxygen bridges between framework silicon and aluminum atoms act as Brønsted acid sites. Already in early solid-state 27Al NMR investigations, performed by application of the spin-echo technique, it was found that the oxygen coordination and local symmetry of framework AlO4 tetrahedra in dehydrated H-form zeolites depend strongly on the interaction of neighboring bridging OH groups with adsorbate molecules.7,9,10 For 27Al nuclei with spin I ) 5/2, the * To whom correspondence should be addressed. Fax: +49 711 68564081. E-mail:
[email protected]. † University of Stuttgart. ‡ Universita ¨ t Leipzig. § Indian Institute of Petroleum.
oxygen coordination and local symmetry is reflected by their quadrupolar interaction. An important parameter describing the strength of this quadrupolar interaction is the quadrupole coupling constant CQCC ) e2qQ/h. Here, eQ corresponds to the electric quadrupolar moment of the nucleus, eq is the zcomponent of the electric field gradient at the position of the nucleus, and h denotes Planck’s constant.11,12 The evaluation of MQMAS spectra yields the second-order quadrupolar effect parameter SOQE ) CQCC(1 + η2/3)1/2, where η is the asymmetry parameter of the electric field gradient tensor. Generally, the asymmetry parameter covers a range of 0 e η e 1, but often has values of η ) 0.3-0.6 in the case of framework aluminum atoms in dehydrated zeolites.11 Hence, the values of SOQE and CQCC deviate by a maximum of 10%, which is often in the order of the experimental accuracy. Framework aluminum atoms in hydrated zeolites are tetrahedrally coordinated and give rise to an 27Al MAS NMR signal at an isotropic chemical shift of ca. 60 ppm with a quadrupole coupling constant CQCC of ca. 2 MHz.13-17 The recently introduced 27Al DFS MQMAS NMR method makes the investigation of aluminum atoms in dehydrated zeolites feasible.18 Dehydration of H-form zeolites is accompanied by a strong increase of the quadrupole coupling constant of framework aluminum atoms to a value of 16 MHz, which leads to a strong broadening of the corresponding solid-state 27Al NMR signals, making them invisible in MAS NMR spectra recorded at moderate magnetic fields (B0 ca. 9.4 T).18-21 On the other hand, 27Al spin-echo NMR studies of dehydrated zeolites H,Na-Y and H-ZSM-5 loaded with ammonia and pyridine showed that a strong decrease of the quadrupole coupling constant of framework aluminum atoms to CQCC values of ca. 5 MHz occurs for these materials.7,9 Quantum-chemical inves-
10.1021/jp0612533 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/27/2006
Framework Al Atoms in Dehydrated Zeolite H,Na-Y tigations of the effect of probe molecules interacting with clusters describing the local structure of bridging OH groups in acidic zeolites supported the above-mentioned experimental results.22 In the present work, the effect of acetonitrile (CH3CN), acetone (CH3COCH3), ammonia (NH3), and pyridine (C5H5N) on the state of framework aluminum atoms in dehydrated zeolites H,Na-Y has been experimentally studied for the first time in a systematic manner by 27Al MAS NMR and MQMAS NMR spectroscopy at magnetic fields of B0 ) 9.4 and 17.6 T. The adsorption process was performed by application of an in situ injection technique.23,24 Depending on the base strength of the probe molecules, the formation of hydrogen bonds or a proton transfer from the catalyst framework to the probe molecules occurred. The base strength of probe molecules is described by the proton affinity, which amounts to PA ) 779, 812, 854, and 930 kJ/mol for acetonitrile, acetone, ammonia, and pyridine, respectively.25,26 It is demonstrated that the chemical state of surface complexes formed by adsorption of probe molecules at bridging OH groups in dehydrated zeolite H,Na-Y is reflected by the strength of the quadrupolar interaction of neighboring framework aluminum atoms. In studies of heterogeneously catalyzed reactions on acidic zeolites, the above-mentioned interdependence could be utilized to investigate the chemical state of reactants adsorbed at Brønsted acid sites under reaction conditions. Experimental Section 1. Materials. Dehydrated zeolite H,Na-Y was prepared via the following procedure: Zeolite Na-Y (nSi/nAl ) 2.7) delivered by Degussa AG, Hanau, Germany, was 6-fold exchanged in a 1.0 M aqueous solution of NH4NO3 at 353 K for 12 h. The obtained zeolite NH4,Na-Y was washed by demineralized water until no nitrate ions could be detected in the solution. Subsequently, the powder material was dried in air at 353 K for 12 h. After this treatment, a cation-exchange degree of 93.3% was reached as determined by inductively coupled plasma atomic emission spectroscopy (AES-ICP, Perkin-Elmer Plasma 400). X-ray diffraction (Siemens D5000, Cu KR radiation) and solidstate NMR spectroscopy were applied to exclude the presence of framework defects and extraframework aluminum in zeolite NH4,Na-Y. Before NMR investigations of dehydrated samples were performed, the material was evacuated in a vacuum of p e 1.5 Pa and at a temperature of 723 K for 12 h. In situ NMR experiments including adsorption of acetonitrile, acetone, ammonia, and pyridine were carried out utilizing the injection equipment described elsewhere.23,24 Before these investigations, the dehydrated catalyst was filled into a 4 mm rotor under dry nitrogen in a glovebox and carefully shaped to a cylindrical catalyst bed by a special tool. The rotor was closed by a cap with hole, which was blocked by a plug. Upon transfer of the rotor into the turbine and starting of the purging gas at the top of the turbine, the plug on the cap was removed and the injection tube was inserted into the rotor. The amounts of adsorbate molecules were controlled by the flow of carrier gas (nitrogen) or ammonia via a mass flow controller and the flow duration. The equivalent number “equiV” used to describe the adsorbate coverage is the ratio of the number of adsorbed molecules, nad, and the number of bridging OH groups (nOH), both determined by quantitative 1H MAS NMR spectroscopy. For the NMR measurements performed at the Bruker AVANCE 750 spectrometer, the dehydrated samples were loaded with adsorbate molecules (up to 2 equiV) using a vacuum line and filled into a 2.5 mm rotor under dry nitrogen in a glovebox.
J. Phys. Chem. B, Vol. 110, No. 28, 2006 13813 2. Spectroscopic Characterization. In situ NMR experiments upon adsorption of probe molecules were performed using a Bruker MSL 400 spectrometer at room temperature. 1H and 27Al MAS NMR spectra were recorded at resonance frequencies of 400.1 and 104.3 MHz, with the sample spinning rate of ca. 9 kHz, after single-pulse excitations of 2.1 µs (π/2) and 0.61 µs (
