Water Coordination, Proton Mobility, and Lewis Acidity in HY Nanozeolites: A High-Temperature 1 H and 27 Al NMR Study Marios S. Katsiotis,* ,† Michael Fardis, ‡ Yasser Al Wahedi, † Samuel Stephen, † Vasilios Tzitzios, ‡ Nikolaos Boukos, ‡ Hae Jin Kim, § Saeed M. Alhassan, † and Georgios Papavassiliou* ,‡ † Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates ‡ Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research “Demokritos”, Aghia Paraskevi, Attiki 153 10, Greece § Division of Material Science, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-go, Daejeon 305-806, Republic of Korea ABSTRACT: A nanosized HY zeolite was synthesized and studied by means of 1 H and 27 Al NMR during thermal dehydration in the temperature range 20-600 °C. The nanozeolite is comprised of a mixture of well-crystallized ultrathin platelets and octahedral nanocrystals, dressed with pentacoordinated extraframework Al (V) . 1 H NMR spin-lattice (T 1 ) and spin-spin (T 2 ) relaxation measurements in combination with 27 Al 3Q-MAS NMR reveal two different interaction paths between water molecules and the nanozeolite solid matrix: (i) water molecules strongly interacting with Al (V) cations, indicated by the high T 1 /T 2 ratio, and (ii) water molecules with amply smaller T 1 /T 2 ratio, interacting moderately with Al (IV) and Al (VI) cations. Relevant measurements on bulk HY rich in extraframework Al (VI) show the presence of the second relaxation channel only, indicating that the enhanced water adsorption observed for the nanozeolite originates partly from its extended surface and partly from the Al (V) decoration. Al (IV) sites in the nanozeolite appear to be highly resilient during heating, even while the framework starts to collapse and Al (VI) transforms to Al (V) . Finally, 1 H NMR shows that water protons interact particularly strongly with the Al sites in the nanozeolite at temperatures as high as 500 °C, unveiling the important role of the Al (V) decoration on this nanocatalyst. ■ INTRODUCTION It is well established that acidity in zeolites depends on the coordination of aluminum and the chemical nature of its neighbors. 1 Active sites are primarily framework bridging hydroxyl protons, known as Brønsted acid sites (BAS), which are compensating for the excess of negative charge created by Al atoms in the zeolite framework. The concentration and strength of BAS influences strongly the catalytic reactivity of zeolites and particularly of zeolite Y. 1-7 In its as-synthesized form with framework Si/Al ratios in the range 2.4-2.9, zeolite Y exhibits only weak acidity and low hydrothermal stability. 8,9 Both properties can greatly be improved by steam calcination, which results in partial removal of framework aluminum (FAL) and the formation of ultrastable Y zeolite (USY). Dehydroxylation, steaming, or dealumination of acidic zeolites remove aluminum from the lattice, transforming it into extraframework aluminum (EFAL) which can enhance catalytic activity remarkably. Various types of EFAL have been proposed in the literature, such as AlO + , Al(OH) 2 + , and Al(OH) 2+ cations or neutral species such as AlOOH, Al(OH) 3 , and Al 2 O 3 . 10,11 In addition, experiments and calculations on dealuminated zeolites (including USY) imply that EFAL may be rich in pentacoordinated aluminum containing Lewis acid sites (LAS). 12,13 BAS/LAS synergy can result in enhanced catalytic perform- ance of dealuminated HY zeolites. 14,15 Mirodatos et al. suggested that the superacid sites in dealuminated zeolites were produced by the interactions between protonic sites and polymeric oxoaluminum deposited into the zeolite voids, while Guisnet et al. proposed that the inductive influence of the LAS on the protonic sites of the zeolite was responsible for the promoting effect on the rates of isomerization, cracking, and hydrogen transfer in dealuminated HY zeolites. 16,17 In this context an important role is played by water confined into the zeolite cages. 1 H broad-line NMR experiments at 4 K and 1 H MAS NMR at 300 K have shown that BAS/LAS synergy in dealuminated HY zeolites, mediated through adsorbed water molecules, is responsible for the increase in the number of hydroxonium ions. 14 The link between the Brønsted and the Lewis acidic sites is established by binding FAL (BAS) and EFAL (LAS) sites through hydrogen bonds between adjacent water molecules. Such hydrogen bonds can include both proton donor and proton acceptor roles for adjacent water molecules. Evidently, the formation of this network weakens the ZO-H bonds, thus making ionization Received: December 31, 2014 Revised: January 21, 2015 Published: January 22, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 3428 DOI: 10.1021/jp513030w J. Phys. Chem. C 2015, 119, 3428-3438