Promotion effect of Mg on a post-synthesized Sn-Beta zeolite for the conversion of glucose to methyl lactate

Abstract

Mg-Sn-Beta zeolites with different Mg/Sn molar ratios were prepared from the parent deAl-Beta by a coimpregnation method. The samples were characterized by powder X-ray diffraction (XRD), N₂ physisorption, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy of the –OH region and probe molecule (pyridine, CD₃CN and CHCl₃) adsorption. The results indicate that Mg²⁺ and Sn⁴⁺ can react with silanol nests and incorporate into the framework of Beta zeolite. Moreover, it is relatively facile to incorporate Sn⁴⁺ into the framework. The framework metal sites generate Lewis (L) acid sites; however, the L acid strength of the framework Mg²⁺ is weaker than that of framework Sn⁴⁺. A small amount of Mg²⁺ interacts with isolated silanols to form extraframework species. Therefore, the introduction of Mg²⁺ results in a significant reduction in silanol defects and an increase in L acidity/basicity of Mg-Sn-Beta. Mg-Sn-Beta with Mg/Sn molar ratio of 1 has the least silanol defects. In the conversion of glucose to methyl lactate (MLA), Mg-Beta is less active than Sn-Beta due to its weaker L acidity. The TOF value for MLA formation increased in the order of Sn-Beta < 0.25Mg-Sn-Beta ≈ 4Mg-Sn-Beta < 1Mg-Sn-Beta, which is closely related to the amount of silanol defects in the catalysts. A kinetic study indicates that the apparent activation energy of the retro-aldol of fructose to MLA, which is the rate-determining step of glucose conversion to MLA, decreases over Mg-Sn-Beta compared to that over Sn-Beta and thus, the formation of MLA was promoted.