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. 2020 Jul 28;25(15):3434.
doi: 10.3390/molecules25153434.

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

Kinga Mlekodaj  1 , Joanna E Olszowka  1   2 , Venceslava Tokarova  3 , Edyta Tabor  1 , Ales Kasparek  3 , Jana Novakova  3 , Gabriela Stavova  3 , Olga Gonsiorova  3 , Lenka Peliskova  3 , Jiri Brus  4 , Radim Pilar  1 , Petr Klein  1 , Jiri Dedecek  1
Affiliations

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

Kinga Mlekodaj et al. Molecules. .

Abstract

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 27Al and 29Si MAS NMR, together with FTIR adsorption of d3-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)3AlOH 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.

Keywords: Brønsted acid sites; acid sites; air calcination; alkali-free synthesis; ammonia calcination; beta zeolite; de-templating; extra-framework Al-Lewis sites; framework Al-Lewis sites.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
X-ray diffraction patterns of as-synthesized B1–B3 samples together with *BEA polymorphs A and B (a) and 27Al MAS NMR spectra of as-synthesized B1-B3 samples (b).
Figure 2
Figure 2
X-ray diffraction patterns of B1–B3 samples calcined in ammonia (a) and in air (b).
Figure 3
Figure 3
SEM images of B1, B2, and B3 samples as-synthesized (a–c), calcined in ammonia (d–f) and in air (g–i).
Figure 4
Figure 4
29Si MAS NMR spectra of B1-B3 samples calcined in ammonia (a) and in air (b) solid lines, together with spectra simulation dashed and gray lines.
Figure 5
Figure 5
27Al MAS NMR spectra of hydrated B1-B3 samples calcined in ammonia (a) and in air (b). 27Al MAS NMR and 27Al CP MAS NMR spectra of B2-NH3 sample black solid lines, together with spectra simulation (c) gray and red lines.
Figure 6
Figure 6
Low-temperature N2 adsorption (full symbols) and desorption (empty symbols) isotherm registered for B1 (black), B2 (blue) and B3 (green) samples calcined in ammonia (a) and in air (b).
Figure 7
Figure 7
The effect of d3-acetonitrile (c,d) and pyridine (e,f) adsorption in the region of OH vibrations of B1–B3 evacuated samples calcined in ammonia (a) and in air (b).
Figure 8
Figure 8
FTIR spectra of adsorbed d3-acetonitrile for dehydrated H-forms of calcined in ammonia (a) and air (b) B2 sample together with deconvolution of the spectra (gray lines) and after adsorption of pyridine (c,d).
Figure 9
Figure 9
Comparison of the Brønsted and Lewis acid sites concentration (Lewistotal derived from FTIR of acetonitrile adsorption; Brønsted and chequered LewisPYR derived from pyridine adsorption) in mmol/g (a) and as percentage value (b) in B1–B2 beta zeolites.
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