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

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

doi:10.3390/molecules25153434

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.

Highlights

Zeolite as heterogeneous catalysts are connected with the outstanding level of development in refining and petrochemistry in the second half of 20th century [1], and, they are considered in several new highly required applications as deNOx processes, utilization of biomass and renewables or methane and carbon dioxide [2,3,4]
Molecules 2020, 25, 3434 of Al atoms to the silicate framework introduce a negative charge of the zeolite framework, which has to be balanced by extra-framework cationic species: H+ role of Brønsted acidic and transition metal cations role of redox catalytic centers with well-defined but tunable properties and located in the confined space of the zeolite channel system
The synthetic approach based on the utilization of amorphous aluminosilicate precursors prevents the formation of technologically disadvantageous gel during the synthesis and maximize synthesis yields

Summary

Introduction

Zeolite as heterogeneous catalysts are connected with the outstanding level of development in refining and petrochemistry in the second half of 20th century [1], and, they are considered in several new highly required applications as deNOx processes, utilization of biomass and renewables or methane and carbon dioxide [2,3,4]. Corner-sharing TO4 (T = Si, Al) tetrahedra are building mechanically and chemically stable 3D frameworks of nearly 250 structural types of zeolites containing regular channel/cavity systems of molecular size with large surface area ranging hundreds of square meters [5,6]. This well-defined and tunable (by the selection of the zeolite topology) channel systems provides shape selectivity (substrate, product, transition state) for catalytic applications. Molecules 2020, 25, 3434 of Al atoms to the silicate framework introduce a negative charge of the zeolite framework, which has to be balanced by extra-framework cationic species: H+ role of Brønsted acidic and transition metal cations role of redox catalytic centers with well-defined but tunable properties and located in the confined space of the zeolite channel system.

Methods

Discussion

Conclusion