Continuous Synthesis of 5-Hydroxymethylfurfural from Glucose Using a Combination of AlCl3 and HCl as Catalyst in a Biphasic Slug Flow Capillary Microreactor

5-Hydroxymethylfurfural (HMF) was synthesized from glucose in a slug flow capillary microreactor, using a combination of AlCl3 and HCl as the homogeneous catalyst in the aqueous phase and methyl isobutyl ketone as the organic phase for in-situ HMF extraction. After optimization, an HMF yield of 53% was obtained at a pH of 1.5, 160 °C and a residence time of 16 min, and it could be further increased to 66.2% by adding 20 wt% NaCl in the aqueous phase. Slug flow operation in the microreactor greatly promoted mixing/reaction in the aqueous droplet and facilitated HMF extraction to the organic slug, enabling the reaction to run (largely) under kinetic control and an enhanced HMF yield by suppressing its further rehydration, degradation and/or polymerization. Confining reaction in the aqueous droplet prevented humin deposition on the microreactor wall. In line with the literature, [Al(OH)2]+ was confirmed by ESI-MS as the catalytically active species, and is responsible for the glucose isomerization to fructose under various pH values. The ratio between AlCl3 and HCl was optimized for the highest HMF yield and the best results were obtained with 40 mM AlCl3 and 40 mM HCl. Compared with batch results, a higher HMF yield was obtained in the microreactor at the same reaction time mainly due to a higher heating rate therein. The aqueous catalyst was recycled and reused three times without a noticeable performance loss. Thus, the present recyclable and stable homogenous catalyst system, combined with biphasic microreactor operation, is an attractive concept for the glucose conversion to HMF.</p

Insights into the reaction network and kinetics of xylose conversion over combined Lewis/Brønsted acid catalysts in a flow microreactor

The catalytic effect of Lewis acid on xylose conversion to furfural has been widely reported, while the underlying reaction network and kinetics are not fully elucidated. This work presents experimental and kinetic modelling studies on xylose conversion to furfural, using AlCl3/HCl as combined Lewis/Brønsted acid catalysts in monophasic water or a biphasic water–methyl isobutyl ketone system in a flow microreactor. The reaction network and kinetics were developed, where Al(OH)2+ was identified as the catalytically active species for isomerization between xylose, lyxose and xylulose, while both Al(OH)2+ and H+ catalyze the sugar dehydration to furfural and side reactions leading to humins (e.g., sugar condensation and furfural degradation). The promoting effect of AlCl3 is attributed to not only the tandem catalysis through xylulose, but also the parallel Al(OH)2+-catalyzed sugar dehydration. Within the studied range (120 to 180 °C, 0.05–0.4 M HCl, 0.04–0.12 M AlCl3), the furfural selectivity relied mainly on the concentration ratio of HCl to AlCl3 rather than their individual concentrations or the temperature. The volcano-like evolution of the maximum furfural yield with increasing HCl/AlCl3 ratio is related to (i) the lower reaction orders in Al(OH)2+ for reactions forming furfural than for side reactions; and (ii) a gradual shift of the dominant reaction pathway from the Al(OH)2+-catalyzed one towards H+-catalyzed one. An optimized furfural yield up to 90% was achieved with 40 mM AlCl3 and 100 mM HCl at 160 °C from 1 M xylose in a continuous slug flow microreactor. The catalytic aqueous phase could be recycled and reused four times without performance loss

Efficient synthesis of furfural from xylose over HCl catalyst in slug flow microreactors promoted by NaCl addition

Efficient synthesis of furfural from xylose over the HCl catalyst in a water-methyl isobutyl ketone biphasic system was achieved in slug flow microreactors, using NaCl as a promotor which facilitates xylose dehydration and suppresses xylose condensation. An optimized furfural yield of 93% was obtained from 1 M xylose over 0.2 M HCl with 10 wt% NaCl at 180 °C within 4 min. A comprehensive kinetic model was developed from monophasic experiments in water in microreactors, by incorporating the acidity in water and kinetic constants as a function of the chloride ion concentration. The coupling of kinetic model with furfural extraction, with consideration of phase volume change as a function of temperature and partial phase miscibility, enables to predict the results of biphasic experiments in microreactors where mass transfer limitation was eliminated. The aqueous phase containing HCl and NaCl could be readily recycled and reused multiple times without noticeable performance loss

Selective fructose dehydration to 5-hydroxymethylfurfural from a fructose-glucose mixture over a sulfuric acid catalyst in a biphasic system: Experimental study and kinetic modelling

A two-step process combining the (equilibrium) glucose isomerization to fructose with selective dehydration of fructose in the obtained sugar mixture to 5-hydroxymethylfurfural (HMF), where glucose is largely unconverted and recycled, represents an attractive concept to increase the overall efficiency for HMF synthesis. This work presents experimental and modelling studies on the conversion of such fructose-glucose mixture to HMF using the sulfuric acid catalyst in a water-methyl isobutyl ketone biphasic system under a wide range of conditions (e.g., temperature, catalyst and sugar concentrations). Through detailed product analyses and ESI-MS spectroscopy, the excess formation of formic acid (together with humins) by the direct sugar/HMF degradation was confirmed and included in the reaction network (neglected in most literatures). The kinetic modelling based on batch experiments in monophasic water well describes the measurements thereof, whereas distinct deviations were found in the prediction of typical literature kinetic models. The incorporation of HMF equilibrium extraction into the developed kinetic model, with consideration of phase volume change as a function of temperature and partial phase miscibility, enables to predict reaction results in the biphasic system in batch. This kinetic model allows to optimize conditions for HMF synthesis that are favored in continuous reactors with minimized back mixing. Based on the model implications, the biphasic system was optimized with slug flow microreactors to better address process intensification and scale-up aspects. Using a simulated fructose-glucose mixture feedstock to represent commercially available high fructose corn syrups, a maximum HMF yield of 81% was obtained at 155 °C over 0.05 M H2SO4 at a residence time of 16 min in the microreactor, with 96% fructose conversion and over 95% of glucose remaining unconverted

Titanium Phosphate Grafted on Mesoporous SBA-15 Silica as a Solid Acid Catalyst for the Synthesis of 5-Hydroxymethylfurfural from Glucose

The grafting of titania on SBA-15 followed by its phosphation was presented to prepare a mesoporous Lewis–Brønsted bifunctional solid acid catalyst for the tandem conversion of glucose via fructose to 5-hydroxymethylfurfural (HMF). Titania was dispersed on SBA-15 as an amorphous surface layer containing abundant coordinatively unsaturated tetrahedral Ti ions, which was reactive and readily transformed upon phosphation into a new titanium phosphate phase with the chemical formula identified as Ti2O3(H2PO4)2·2H2O. The ordered mesoporous structure was well maintained after three modification cycles, affording a desirable surface area of over 300 m2/g. The SBA-15-supported titanium phosphate layer affords higher overall acidity and Brønsted to Lewis acid ratio, compared with the conventional post-phosphated bulk anatase titania. The tetrahedral Ti ions and the adjacent protonated phosphate groups on the titanium phosphate layer could form Lewis–Brønsted acid pairs at molecular level proximity, which largely enhanced the selective tandem catalysis for glucose conversion via fructose to HMF. An optimized HMF yield of 71% was achieved at 160 °C in a water–methyltetrahydrofuran biphasic system over the SBA-15-supported titanium phosphate catalyst. The catalyst exhibited good hydrothermal stability with a rather limited silicon and phosphate leaching, and no distinct pore collapse or performance loss over three sequential reaction runs

Selective tandem catalysis for the synthesis of 5-hydroxymethylfurfural from glucose over in-situ phosphated titania catalysts: Insights into structure, bi-functionality and performance in flow microreactors

5-Hydroxymethylfurfural (HMF) synthesis from glucose over in-situ phosphated titania catalysts is presented. Phosphates were incorporated into titania framework forming a titanium phosphate surface layer, where the coordinatively unsaturated tetrahedral TiO4 units act as water-tolerant Lewis acid site (LAS) and the adjacent protonated phosphate as Brønsted acid site (BAS), together forming Lewis-Brønsted acid pairs at molecular-level proximity. Glucose turnover and HMF selectivity were enhanced due to the rapid fructose transfer from LAS to the adjacent BAS for its dehydration to HMF, facilitating LAS liberation for another glucose turnover. Reactions in a water-2-methyltetrahydrofuran biphasic system in packed-bed microreactors gave 66% HMF yield (from 1 M glucose at 150 °C), where the HMF space time yield is about two orders of magnitude higher than that in batch and the literature work. Phosphate leaching from the catalyst is rather limited, whereas the catalyst deactivated mainly by humin deposition and could be regenerated by calcination

High-Yield 5-Hydroxymethylfurfural Synthesis from Crude Sugar Beet Juice in a Biphasic Microreactor

5-Hydroxymethylfurfural (HMF) is an important biobased platform chemical obtainable in high selectivity by the hydrolysis of fructose (FRC). However, FRC is expensive, making the production of HMF at a competitive market price highly challenging. Here, it is shown that sugar beet thick juice, a crude, sucrose-rich intermediate in sugar refining, is an excellent feedstock for HMF synthesis. Unprecedented high selectivities and yields of '90 % for HMF were achieved in a biphasic reactor setup at 150 °C using salted diluted thick juice with H2SO4 as catalyst and 2-methyltetrahydrofuran as a bioderived extraction solvent. The conversion of glucose, obtained by sucrose inversion, could be limited to '10 mol %, allowing its recovery for further use. Interestingly, purified sucrose led to significantly lower HMF selectivity and yields, showing advantages from both an economic and chemical selectivity perspective. This opens new avenues for more cost-effective HMF production