Within the last decade, interest in using biphasic systems for producing furans from biomass has grown significantly. Biphasic systems continuously extract furans into the organic phase, which prevents degradation reactions and potentially allows for easier separations of the products. Several heterogeneous catalyst types, including zeolites, ion exchange resins, niobium‐based, and others, have been used with various organic solvents to increase furan yields from sugar dehydration reactions. In this minireview, we summarized the use of heterogeneous catalysts in biphasic systems for furfural and 5‐hydroxymethylfurfural production from the past five years, highlighting trends in chemical and physical properties that effect catalytic activity. Additionally, the selection of an organic solvent for a biphasic system is extremely important and we review and discuss properties of the most commonly used organic solvents.
Autocatalytic dehydration of xylose to furfural was studied in pure aqueous and monophasic organic/water mixtures to determine the effect reaction media and conditions have on conversion and yield. This study identified that the severity (R o ) of the reaction and polarity, as determined by the Hansen Solubility Parameter, δ P , strongly correlate with xylose conversion and furfural yield. Increasing the R o and δ P increased both conversion and yield in pure aqueous and organic/water mixtures of sulfolane, γ-butyrolactone, γ-valerolactone, γ-hexalactone, and tetrahydrofuran. Additionally, it was found that at a specified R o and δ P , similar conversions and yields were achieved using different combinations of time, temperature, and solvent mixture. Using principal component analysis and projection to latent structures, a semi-empirical model was developed that provided estimates of xylose conversion and furfural yield over a range of experimental R o and δ P values.Furfural has been recognized as an important building block for fuels, solvents, and other value-added chemicals, [1] and numerous recent publications have focused on optimizing furfural production from model sugars and lignocellulosic biomass. The use of organic solvents for xylose dehydration reactions has recently gained interest due to faster reaction rates [2] and the higher furfural yields obtained, which have been reported as high as 80 % in biphasic systems. [3] Solvent selection has been identified as an important reaction variable with mentions of solvent polarity [4] and the number of oxygen containing groups [5] being potential explanations as to why solvents improve furan yields, but currently no systematic method for solvent selection exists. Researchers have previously used Kamlet-Taft theory [6] or Hansen Solubility Parameters (HSPs) [7] to explain differences in selectivities, conversions, and yields; however, the HSP parameters (dispersion (δ D ), polarity (δ P ), and hydrogen bonding (δ H )) are functions of both solvent mixture and temperature, [8] which often times are not considered.This work aims to bridge the gap between xylose conversion, furfural yield, and solvent properties to guide solvent selection in monophasic, autocatalytic systems. Previous research has explored acid-catalyzed reactions in organic solvent mixtures for both monophasic and biphasic systems. [3,9] In biphasic systems using aluminium sulfate as the catalyst, Yang et al. hypothesized that as the polarity of a solvent increases, furfural yields increase in γ-valerolactone (GVL), methyl isobutyl ketone, tetrahydrofuran (THF), and 2-methyltetrahydrofuran; [10] however, no direct measure of solvent polarity was provided.Although acid catalysts increase the rate of reaction, furans can be produced through an autocatalytic process (Scheme 1), with the mechanism initiated by the formation of hydronium ions in high temperature water. [11] Additionally, formic acid, a degradation product, has been shown to contribute to the reaction activity. [12] Previous pu...
SAPO-34 zeolite crystals were grown on zeolite 5A beads, characterized, and then used to produce furfural from xylose and 5-hydroxymethylfurfural (HMF) from glucose. The SAPO-34/5A bead catalysts resulted in moderate furfural and HMF yields of 45% from xylose and 20% from glucose (463 K; 3 h) and were easier to recover than the SAPO-34 powder catalyst. At 463 K, the SAPO-34/5A beads were more selective than 0.02 M sulfuric acid for producing HMF and, unlike the sulfuric acid system, no levulinic acid was formed. The SAPO-34/5A bead catalysts had no significant loss in activity after three rounds of recycle when water washed or heated overnight between reactions; however, the heat-treated beads did show signs of thermal stress after the second reuse. The SAPO-34/5A bead catalysts show promise for dehydration reactions to produce furfural and HMF from xylose and glucose, respectively, and tailoring the catalyst and the support bead could lead to even higher selectivities and yields.
Zeolites are known to be effective catalysts in biomass converting processes. Understanding the mesoporous structure and dynamics within it during such reactions is important in effectively utilizing them. Nuclear magnetic resonance (NMR) T2 relaxation and diffusion measurements, using a high-power radio frequency probe, are shown to characterize the dynamics of water in mesoporous commercially made 5A zeolite beads before and after the introduction of xylose. Xylose is the starting point in the dehydration into furfural. The results indicate xylose slightly enhances rotational mobility while it decreases translational motion through altering the permeability, K, throughout the porous structure. The measurements show xylose inhibits pure water from relocating into larger pores within the zeolite beads where it eventually is expelled from the bead itself.
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