Brønsted acid sites location and thermal stability of NH4-exchanged omega zeolite

Due to their structural features, e.g., the pore system which highly affects their selectivity, acid zeolites are considered of great importance as catalysts in cracking processes of oil refining (isomerization and hydrocarbons cracking). Catalytic activity of acid zeolites depends on the presence of the so-called Brønsted acid sites, i.e., protons bonded to framework oxygen atoms. In order to evaluate their catalytic efficiency, the determination of nature, density, strength and location of Brønsted sites is of particular relevance. Since hydrocarbons catalysts are employed at high temperature, and their molecular sieve features as well as sorptive and catalytic properties are enhanced by heating, the thermal stability of those compounds must be accurately investigated. Zeolitic catalysts characterization through X-ray and neutron diffraction at non-ambient conditions is one of the best analytic tool to prove both their efficiency and stability and characterize their shape selectivity. In this contribution, a NH4-form omega zeolite was characterized to disclose the presence of acid sites and investigate temperature induced modifications. The as-synthesized omega zeolite (a mazzite analogue with formula Na6.6TMA1.8(H2O)22.2[Al8.4Si27.6O72]-MAZ) was previously studied by Martucci et al., 2003. Its NH4-exchanged form (Na2.4TMA0.9(H2O)4.2(NH4)20 [Al8.4Si27.6O72] was obtained through cationic exchange at Room Temperature (RT) and at 90°C, 3-times each. Powder patterns were collected at the GILDA-BM8 Beamline (ESRF) from RT to 900 °C (heating rate: 5°C/min) and at the D2B Beamline (ILL) at 4 K, and Rietveld refinements were performed through the GSAS-EXPGUI package. Obtained results clearly show: 1) a progressive TMA template and NH4 release induced by the heating process; 2) a NH4 migration highlighted by O–O shortening and T–O–T variations within 6MR, 8MR units and the gmelinite cage which progressively become more distorted on heating. Such structural deformation is particularly highlighted by 1) the variation of T–O2–T angles (i.e., progressive shift of O2 framework oxygen towards the centre of gmelinite cage; and 2) a decreasing of O1–O2 bond distances (due to Brønsted acid sites formation on O2 oxygen atom). Moreover, neutron refinements revealed a disordered Si–Al distribution within tetrahedral sites with a preferential occupation of Al3+ for the T2 site with respect to T1. Although non-reversible framework modifications occur, all the information here gained reveal the omega zeolite as performing hydrocarbons catalyst even at high temperature: indeed, it is stable up to 700°C. Martucci, A., Alberti, A., de Lourdes Guzman-Castillo, M., Di Renzo, F., & Fajula, F. 2003. Crystal structure of zeolite omega, the synthetic counterpart of the natural zeolite mazzite. Microporous and mesoporous materials, 63(1-3), 33-42