(198b) Synthesis of 5-Hydroxymethylfurfural from Renewable Resources Using Microreaction Technology

Introduction

The interest in renewable resources as chemical feedstocks is growing considerably fast. For example, furan derivatives obtained from renewable carbohydrates such as fructose have a huge potential to become sustainable substitutes for petroleum-derived chemical components [1, 2]. For example, 5-hydroxymethylfurfural (5-HMF) is a key intermediate for the synthesis of 2,5-furandicarboxylic acid, furandiamine and other potential building blocks for polymers [1-5].

Today, the synthesis of 5-HMF from renewable resources is usually based on the catalytic dehydration of mono- and polysaccharides using organic acids (e.g. oxalic acid), inorganic acids (e.g. H₃PO₄, HCl), zeolithes or salts (MgCl₂). However, all these batch processes show considerable limitations in performance. When dehydration is conducted in pure water at high temperatures only low selectivities and a wide variety of by-products are obtained. On the other hand, processing at low temperature give certainly fair selectivities (>80%) but only poor conversion of < 50% can be achieved. To overcome restrictions in conversion and selectivity dehydration processes have also been conducted in biphasic systems based on water and an organic co-solvent such as methyl-isobutyl-ketone (MIBK), 2-butanol or dimethylsulfoxide (DMSO) [1], but also in sub- and supercritical mixtures of acetone/water, in sub- and supercritical methanol as well as in subcritical acetic acid [3]. Although conversion and HMF selectivity could be improved in comparison to conventional HMF syntheses the major drawback of these processes is their negative impact on the sustainability of the process since organic solvents are used that are partly hazardous, harmful to the environment and raise process costs significantly.

In recent years microfluidic structures have received an increasing interest as reactors that allow applying new process windows to chemical reactions as a result of improved mass and heat transport characteristics. Here, we report on the microreactor-based green synthesis of 5-hydroxymethylfurfural in water avoiding the use of any organic co-solvent. The process conditions were deliberately shifted towards high temperature and pressure regimes in combination with short residence times to significantly increase space/time yields and achieve satisfying selectivities.

Experimental

10% aqueous solutions of fructose were processed with diluted hydrochloric acid in various microreactors providing different mixing efficienies. Syringe pumps (Sykam S1610) were used for continuous pulsation-free flow of the reactants and back pressure valves were installed to avoid inhomogeneous flow behaviour. At elevated temperatures cartridge check valves were used to keep pressure above the vapour pressure of water (15-20 bars). The reaction was thermally quenched at the reactor outlet by rapid cooling in capillaries (800 µm diameter). Quenched reaction mixtures were analyzed quantitatively by HPLC (Agilent 1200) including potential by-products such as levulinic acid and formic acid.

The dehydration process was carried out under systematic variation of process temperature (100°C - 200°C) and residence time (1 min - 3 min) applying different flow rate ratios (fructose _aq : HCl _aq = 1:1 ? 1:5) and hydrochloric acid concentrations (0.1 mol/L and 0.5 mol/L).

Results and Discussion

Table 1 shows some exemplary results obtained for the dehydration of fructose in microrectors at a residence time of one minute. The best selectivity for 5-HMF (75%) was obtained at 185°C employing an HCl concentration of 0.1 mol/L. The conversion of fructose can be increased, as expected, by raising the temperature. However, at temperatures above 190°C insoluble humins and polymers are formed causing a blockage of the reactors. Higher concentrations and flow rates of hydrochloric acid lead to higher conversion but diminish 5-HMF selectivity due to an increase in byproduct formation.

Tab. 1:      Exemplary results for fructose dehydration in a microreactor within one minute residence time.

* flow rate ratio fructose:HCl = 1:5

Usually, the batch synthesis of 5-HMF in pure aqueous solution leads to conversion and selectivity values in the range of 50% or less [1]. Due to the intensified mass and heat transport attainable in continuously operated microreactors the dehydration process can be performed under conditions of significantly increased temperature and reduced residence time. Consequently, the dehydration of fructose could be optimized providing 20% higher conversion and 25% higher 5-HMF selectivity (Fig. 1). Moreover, the space/time yield could be significantly improved in comparison to the batch process.

To improve conversion and selectivity in batch processes organic co-solvents like MIBK and 2-butanol have been used to extract HMF from the aqueous phase equilibrium [1]. In addition, DMSO has been used to stabilize the more reactive furanose form of fructose [1]. In Fig. 1 the values of fructose conversion and 5-HMF selectivity obtained in batch processes using different combinations of co-solvents are compared with the results obtained from the microreaction process. The data show that 5-HMF selectivity obtained in the microreaction process is significantly higher than in most of the biphasic batch processes. Only if a combination of three different organic co-solvents (namely DMSO, MIBK and 2-butanole) is used a slightly better conversion (75% vs. 71%) and 5-HMF selectivity (80% vs. 75%) were obtained than achievable in the microprocess. However, the microreaction process offers a much more faster, cheaper and environmental friendly way to synthesize 5-HMF from fructose.

Conclusion

The green synthesis of 5-Hydroxymethylfurfural (5-HMF) from aqueous solutions of fructose was successfully conducted in a continuous microreaction process without using any organic solvent. By shifting reaction parameters such as temperature, pressure and residence time towards new process windows 5-HMF was obtained with increased selectivity of 75% at high space/time yields.

The subsequent catalytic oxidation of 5-HMF to 2,5-furandicarboxylic acid (FDCA) in microreactors is currently under investigation.

Fig. 1:       Comparison of 5-HMF selectivity (black bars) and fructose conversion (grey bars) obtainend in continuous microreactor and in batch [1] processes.

Acknowledgement

Financial support by the Deutsche Bundesstiftung Umwelt (German Environmental Foundation) is gratefully acknowledged.

References

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