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    Temperature-induced modifications in natural zeolite clinoptilolite: effects on acidity and catalytic acetalization

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    This study delves into the acid modification of natural zeolite clinoptilolite, focusing on the identification of acid site types and their catalytic activity in the Brønsted acid-catalyzed acetalization of benzaldehyde with 1,3-butanediol. Following calcination, the samples underwent acidification via ammonium-ion exchange, resulting in approximately 45 % of the clinoptilolite cations being exchanged with ammonium ions. The investigation evaluates the structural, morphological, and textural alterations induced by this modification using XRD, FTIR, and nitrogen adsorption-desorption measurements. Ammonia-temperature-programmed desorption (NH3-TPD) analysis confirms the presence of medium to strong acidic protons, highlighting the acidity of the modified samples. Employing 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy elucidated changes in the state and coordination of aluminum post-sample activation. Specifically, the 27Al MAS NMR spectra indicate a partial dealumination, evidenced by the emergence of 5 and 6-fold coordinated aluminum. Moreover, 29Si MAS NMR measurements tracked variations in the Si/Al ratio. The study probes the nature of these sites, their influence on catalytic activity, and the synergistic interplay between Brønsted acid sites and 5-fold coordinated aluminum. The results showcase that the prepared acidic natural clinoptilolite catalysts augment acidity and porosity, fostering promising implications for catalytic applications.<br/
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