Effect of Alkali-Free Synthesis and Post-Synthetic Treatment on Acid Sites in Beta Zeolites

• Published in: Molecules (Basel, Switzerland) Vol. 25; no. 15; p. 3434
• Main Authors: Mlekodaj, Kinga, Olszowka, Joanna E, Tokarova, Venceslava, Tabor, Edyta, Kasparek, Ales, Novakova, Jana, Stavova, Gabriela, Gonsiorova, Olga, Peliskova, Lenka, Brus, Jiri, Pilar, Radim, Klein, Petr, Dedecek, Jiri
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 28.07.2020; MDPI
• Subjects: Acetonitrile; acid sites; Acids; alkali-free synthesis; Alkalies - chemistry; Aluminosilicates; Aluminum silicates; Ammonia; beta zeolite; Brønsted acid sites; Electrolytes; extra-framework Al-Lewis sites; framework Al-Lewis sites; Lewis acid; Magnetic Resonance Spectroscopy; NMR spectroscopy; Precursors; Pyridines; Roasting; Zeolites; Zeolites - chemical synthesis; Zeolites - chemistry; Brønsted acid sites; ammonia calcination; de-templating; air calcination; beta zeolite; framework Al-Lewis sites; extra-framework Al-Lewis sites; alkali-free synthesis; acid sites
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules25153434

Beta zeolites with Si/Al around 14 were prepared using three new alkali-free synthesis methods based on the application of amorphous aluminosilicate precursor and calcined in ammonia or air. All samples exhibit structural and textural properties of standard beta zeolite. Comprehensive study by Al and Si MAS NMR, together with FTIR adsorption of d -acetonitrile and pyridine were used to characterize the influence of both the synthesis and calcination procedure on the framework Al atoms and related Brønsted and Lewis acid sites. While calcination in ammonia preserves all framework Al atoms, calcination in air results in 15% release of framework Al, but without restrictions of the accessibility of the beta zeolite channel system for bulky pyridine molecules. Terminal (SiO) AlOH groups present in the hydrated zeolites were suggested as a precursor of framework Al-Lewis sites. Surprisingly, the mild dealumination of the air-calcined zeolites result in an increase of the concentration of Brønsted acid sites and a decrease of the total concentration of Lewis sites with the formation of the extra-framework ones.