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DETECTION OF BRØNSTED ACID SITES IN ZEOLITE L: A COMBINED NEUTRON AND SYNCHROTRON DIFFRACTION STUDY

By G. Beltrami, E. Rodeghero, S. Dolabella, L. Pasti and A. & Cruciani G. Martucci

Abstract

Due to their catalytic activity, excellent chemical and thermal stability and their reaction selectivity, acidic zeolites\ud are preferred as catalysts in petroleum and chemical industries (Corma, 1995). Synthetic L zeolite (empirical formula\ud K9[Na3Al9Si27]O72 21H2O) is a large pore material with LTL framework topology and hexagonal symmetry (P6/mmm\ud space group). Its framework is built from columns of cancrinite cages stacked with double six membered rings (D6R)\ud along the c axis. This peculiar arrangement creates large undulating 12-ring (12MR) channels, which are separated from\ud each other by elliptical 8-membered ring channels, parallel to the original columns along the c axis. Its acidic and\ud structural features are suitable for technological applications, such as innovative optical devices (e.g., luminescent\ud probes, optical lens and filters) and photonic antenna systems based on dye–zeolite host–guest materials (Albuquerque\ud & Calzaferri, 2007). The aim of the present study is: firstly, to determine the number and location of Brønsted sites in\ud zeolite L and secondly, to study the interaction of water with hydroxyl groups in the rehydrated D-form. These\ud informations provide a complete understanding of the hydrogen-bonding network and allow speculations on the degree\ud of confinement of zeolitic water in LTL. A sample of untreated L zeolite (LTL), purchased by Tosoh Corporation\ud (500KOA code, SiO2/Al2O3 ratio = 6.1, Na2O = 0.25 wt.%, K2O = 16.8 wt.% and BET = 290 m2/g), was used in this\ud study. The acidic form (D-LTL) was prepared as follows: 1) exchanging in a 1 M solution of ND4Cl on D2O for 192 h\ud at 40°C (ND4-LTL); 2) washing with D2O and drying overnight at 96ºC; 3) and then calcining at 550ºC for 2 hours\ud under a flux of air in order to obtain the acidic form. The D-LTL powder pattern was measured at 2.25 K at the D2B\ud line (ILL, Grenoble). Thereafter, the rehydrated D-form (RD-ILL) was collected at the high resolution ID22 (ESRF)\ud Beam Lines, as a function of temperature, from RT to 800°C (heating rate of 8°C/min). Rietveld refinements revealed\ud the presence of two Brønsted acid sites (corresponding to 7.6 hydroxyl groups). The first one was on framework oxygen\ud O5, not far from the center of the 8-ring of the cancrinite cage. The second one was on the framework oxygen O1,\ud heading towards the center of the 12 MR channel. The RD-LTL refinement highlighted the presence of six additional\ud extraframework sites attributed to re-adsorbed H2O molecules. Very interestingly, these last form water-chains\ud interacting strongly with the framework oxygens. These results are of paramount importance as the presence of water\ud molecules, even in undetectable amounts, is crucial in the zeolitic catalyst in catalytic processes.\ud Albuquerque, R.Q. & Calzaferri, G. (2007): Proton activity inside the channels of zeolite L. Chem. Eur. J., 13, 8939-8952.\ud Corma, A. (1995): Inorganic solid acids and their use in acid-catalysed hydrocarbon reactions. Chem. Rev., 95, 559-614

Publisher: Litotipografia Alcione, Lavis (TN)
Year: 2017
DOI identifier: 10.3301/ABSGI.2017.01.
OAI identifier: oai:iris.unife.it:11392/2383438
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