As-synthesized TEA-BEA zeolite: Effect of Si/Al ratio on the Knoevenagel condensation

• BEA zeolite with the highest ratio Si/Al have more [SiO−][TEA+] species.
• Si/Al ratio is greater near the surface than in the bulk.
• As-synthesized zeolite BEA was an efficient catalyst for Knoevenagel condensation.
• Catalytic activity indicate that only a fraction of the [SiO−][TEA+] species is on the external surface.

The zeolite with BEA structure has been obtained using different Al amounts in the presence of tetraethylammonium as a structure directing agent. The effect of the Si/Al molar ratio on the crystal size, morphology, yield, chemical composition and catalytic properties was investigated. The materials have been characterized towards zeolitic properties by XRD, SEM, EDS, TGA, 29Si MAS-NMR, XPS and TEM. The chemical composition of the crystals depends on the concentration of the initial reaction mixture and that a decrease in the Al content is associated with an increase in particle size. The characterization by TGA/DTA of as-synthesized zeolites with BEA structure showed that an increase of Si content resulted in the formation of [SiO−][TEA+] species, this was supported by 29Si MAS-NMR. The XPS results also showed that the chemical composition of the surface differed from that of the bulk material, this was supported by TEM/EDS results. The results of catalytic activity tests indicated that a fraction of the [SiO−][TEA+] species was present on the external surfaces of the crystals, although the largest fraction appeared to be in the interior of the particles. As-synthesized BEA zeolite showed good catalytic activity and suggests that this material can be applied in other reactions involving basic sites.

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