Investigation of the correlation of modified Al/B ratio with structural distortion and Ni2+ incorporation in ZSM-5 Zeolites

Abstract

ZSM-5 zeolite, an aluminosilicate having high silica content, is used in catalysis and adsorption. The framework is formed by connecting TO4 tetrahedra (T = Si, Al), making intersected pores, with ten-member ring openings. B/ZSM-5 zeolites contain boron (B) atoms within the framework. Though the individual effects of Al/ZSM-5 and B/ZSM-5 zeolites have been investigated, the systematic influence of Al/B ratio on framework distortion, Bronsted acidity, and efficiency of Ni incorporation has not been comprehensively examined. This study aimed to synthesise Al/B modified ZSM-5 zeolites and determine the effect of compositional differences on structural order, Bronsted acidity, and Ni2+ addition. Five samples of ZSM-5 zeolites with varying Al/B ratios - 100% Al; 75% Al, 25% B; 50% Al, 50% B; 25% Al, 75% B; and 100% B were synthesised using a hydrothermal method with NaAlO2, NaOH, tetraethylammonium bromide, colloidal silica, and H3BO4. The catalysts were made by impregnating each composition with nickel using [Ni(H2O)6](NO3)2. The characteristic ZSM-5 peak in Raman spectroscopy was located at 385 cm–1; therefore, all the compositions exhibit a sharp peak around this value, indicating the presence of zeolite crystals. As the B content increased, this peak became sharper, suggesting a reduction of framework distortion. B reduces the lattice strain through its smaller ionic size, thereby minimising vibrational dephasing and inhomogeneous broadening. Other prominent peaks could be observed at 845 cm–1 and 1320 cm–1, corresponding to the symmetric stretching of T-O vibrations within the framework. UV-visible spectroscopy was employed to confirm the incorporation of Ni. The absorbance values for Ni decreased gradually when the B content was increased. The incorporation of Ni2+ diminished with the increasing B content. This study demonstrates that the nature of the Bronsted acid sites within the crystal framework influences Ni2+ incorporation. Al-rich samples possess Al-OH-Si linkages within the framework, featuring highly acidic and polarizable O-H bonds that govern effective ion exchange. In contrast, B-substituted zeolites have weaker Bronsted acid sites, resulting in reduced ion exchange capacity and inefficient Ni incorporation.