Effects of Adsorbate Molecules on the Quadrupolar Interaction of Framework Aluminum Atoms in Dehydrated Zeolite H,Na-Y Jian Jiao, ² Johanna Kanellopoulos, ‡ Babita Behera, § Yijiao Jiang, ² Jun Huang, ² V. R. Reddy Marthala, ² Siddharth S. Ray, § Wei Wang, ² and Michael Hunger* ,² 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 27 Al MAS NMR and 27 Al 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 AlO 4 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, 29 Si 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 27 Al NMR investigations, performed by application of the spin-echo technique, it was found that the oxygen coordination and local symmetry of framework AlO 4 tetrahedra in dehydrated H-form zeolites depend strongly on the interaction of neighboring bridging OH groups with adsorbate molecules. 7,9,10 For 27 Al nuclei with spin I ) 5 / 2 , the 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 C QCC ) e 2 qQ/h. Here, eQ corresponds to the electric quadrupolar moment of the nucleus, eq is the z- component 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 ) C QCC (1 + η 2 /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 C QCC deviate by a maximum of 10%, which is often in the order of the experimental accuracy. Framework aluminum atoms in hydrated zeolites are tetra- hedrally coordinated and give rise to an 27 Al MAS NMR signal at an isotropic chemical shift of ca. 60 ppm with a quadrupole coupling constant C QCC of ca. 2 MHz. 13-17 The recently introduced 27 Al DFS MQMAS NMR method makes the investigation of aluminum atoms in dehydrated zeolites fea- sible. 18 Dehydration of H-form zeolites is accompanied by a strong increase of the quadrupole coupling constant of frame- work aluminum atoms to a value of 16 MHz, which leads to a strong broadening of the corresponding solid-state 27 Al NMR signals, making them invisible in MAS NMR spectra recorded at moderate magnetic fields (B 0 ca. 9.4 T). 18-21 On the other hand, 27 Al 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 C QCC values of ca. 5 MHz occurs for these materials. 7,9 Quantum-chemical inves- * To whom correspondence should be addressed. Fax: +49 711 68564081. E-mail: michael.hunger@itc.uni-stuttgart.de. ² University of Stuttgart. ‡ Universita ¨t Leipzig. § Indian Institute of Petroleum. 13812 J. Phys. Chem. B 2006, 110, 13812-13818 10.1021/jp0612533 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/27/2006