Abstract
• Alkaline-treatment of H-Y resulted in mesopores and tetrahedral extra-framework lewis acidic al. • Alkaline-treated H-Y yielded high isomerization selectivity and low side product formation. • Na-Y unaffected by alkaline-treatment maintaining low isomerization performance despite a high number of acidic sites. Isomerization of glucose to fructose is a key reaction step in the efficient production of valuable renewable chemical intermediates from biomass. Zeolites have been widely used to promote glucose isomerization, but only limited structure-activity relationship has been established and applied to optimize the reaction. In this work, commercial Y zeolites were modified by alkaline-treatment and their physicochemical properties and structures were correlated in order to optimize the catalytic performance for glucose isomerization. For H-Y, the alkaline-treatment developed new mesoporous structures with larger pore, modified the Si-O(H)-Al bonds and extracted silica from framework structures creating more tetrahedral extra-framework Lewis acidic aluminum. This increased the isomerization selectivity towards fructose more than twelve times and decreased significantly acetalization/ketalization side reactions compared to the pristine zeolite. In contrast, Na-Y contained mainly micropores and had less tetrahedral non-framework acidic Al-species, resulting in low glucose conversion and low fructose yield due to limited substrate accessibility to the pores, although the number of acid sites was tenfold higher than in H-Y. The study demonstrates how commercial Y zeolites can be designed by alkaline-treatment to comprise mesopores and weak acid sites, which greatly improve the catalytic performance for glucose isomerization to fructose.
Highlights
Lignocellulosic biomass is an abundant carbon-neutral resource, and its conversion to renewable fuels and chemicals through green catalytic processes has become a promising strategy to mitigate energy shortage and CO2 emission [1,2,3,4]
The study demonstrates how commercial Y zeolites can be designed by alkalinetreatment to comprise mesopores and weak acid sites, which greatly improve the catalytic performance for glucose isomerization to fructose
The peak intensity of the Na-Y zeolites remained nearly unaffected by the alkaline-treatment revealing an intact structure (Fig. 1b), though a shift of the (533) peak (2θ = 24.7◦) to lower angle indicated some exchange of protons to Na+ in the structure and a corresponding increase in unit cell volume and interplanar spacing [34, 37]
Summary
Lignocellulosic biomass is an abundant carbon-neutral resource, and its conversion to renewable fuels and chemicals through green catalytic processes has become a promising strategy to mitigate energy shortage and CO2 emission [1,2,3,4]. The materials can accommodate a wide variety of metal cations or cationic metal complexes, which modify the acidity and basicity of the material This makes zeolites attractive solid catalysts for sugar transformations, including the isomerization of glucose to fructose [8, 9]. The studies reported in the literature on glucose isomerization with zeolites or zeotype materials comprise many different types of zeolites with the materials being used as an acidic or basic catalyst or as catalyst support [21, 28, 29], but few studies have investigated the relationship between physicochemical properties and structures of the zeolites and their catalytic performance [30, 31]. Based on the obtained results, we have attempted to establish the structure-activity relationship to optimize the catalytic performance in the isomerization of glucose to fructose, and provide a rational design of an improved zeolite catalyst for the conversion of sugar
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