Abstract
The deactivation of Sn-Beta zeolite catalyst during retro-aldolization and isomerization of glucose is investigated. Confocal fluorescence microscopy reveals that retro-aldolization of glucose in CH3OH at 160 °C is accompanied with the build-up of insoluble oligomeric deposits in the micropores, resulting in a rapid catalyst deactivation. These deposits accumulate predominantly in the outer regions of the zeolite crystals, which points to mass transport limitations. Glucose isomerization in water is not only accompanied by the formation of insoluble deposits in the micropores, but also by the structural degradation of the zeolite due to desilication and destannation. Enhanced and sustained catalytic performance can be achieved by using ethanol/water mixtures as the reaction solvent instead of water.
Highlights
Efficient conversion processes of renewable feedstocks into fuels and chemicals is pivotal to a more sustainable business model for the chemical industry
Confocal fluorescence microscopy reveals that retro-aldolization of glucose in CH3OH at 160 °C is accompanied with the build-up of insoluble oligomeric deposits in the micropores, resulting in a rapid catalyst deactivation
Zeolites functionalized with Lewis acid sites such as Sn-Beta are active catalysts for the isomerization of glucose to fructose, [1,2,3,4,5,6] which can be combined with acid-catalyzed dehydration of fructose to 5-HMF [7,8] and the retro-aldolization of C6 sugars and the conversion into lactic acid and derivatives [3,9,10,11,12,13]
Summary
Efficient conversion processes of renewable feedstocks into fuels and chemicals is pivotal to a more sustainable business model for the chemical industry. The carbohydrate fraction of lignocellulosic biomass is considered a potential feedstock for future biorefinery processes, from which specific platform chemicals with a wide range of downstream applications can be produced (Fig. 1). This motivates researchers in industry and academia to search for new catalysts that can selectively promote the cascade reactions from cellulose via glucose towards platform molecules, such as 5-hydroxymethylfurfural (5-HMF) and lactic acid. Alkali promoters modify the acidity towards a lower rate of deactivation [11] Another approach is to synthesize more active catalysts that by operating at lower temperature produce less by-products [13]. As catalyst deactivation due to heavy product deposition is a general problem in zeolite catalysis [15,16,17,18], locating these species by spectroscopic methods has been pursued [19]
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