(560aj) HMF Synthesis from Glucose By Reaction-Extraction System Using a Microreactor

1. Introduction

5-Hydroxymethylfurfural (HMF) is one of the valuable chemicals obtainable via biomass thermochemical conversion. Basically glucose, one of the main components of lignocellulosic biomass, converts into HMF under an acidic aqueous condition. However, under such a condition, HMF further hydrolyze to levulinic acid. Suppression of this hydrolysis is essential for an effective HMF production. As one of the suppression techniques, biphasic reaction system was proposed by Román-Leshkov et al.[1] The system consists of an acidic aqueous phase as the reaction phase and an organic phase as the extraction phase. Because HMF is lipophilic, the product HMF via reaction in an acidic aqueous phase immediately transfers into the organic phase, which prevents further conversion. After the biphasic system was introduced, the average yield of HMF from glucose improved to about 60 mol% [2,3]. Our previous study employed a segmented flow with the biphasic system, and succeeded in producing HMF at the yield of 76 mol% [4]. A segmented flow is a unique flow which is realizable using a microreactor. When more than two kinds of immiscible fluids flow in a thin tube, each fluid flows alternately as segment. In each segment an internal circulation occurs, which promotes mass transfers between phases by reducing boundary layers’ thicknesses and also by preventing accumulations of transferred components near boundaries. In this report, the improvement of HMF yield and recovery of HMF from the extraction phase were examined.

2. Experimental method

As a material, glucose (FUJIFILM Wako Pure Chemical, Japan) or fructose (KISHIDA CHEMICAL, Japan) was solubilized into phosphate buffer saline (PB). PB was prepared by solubilizing NaH₂PO₄ 2H₂O (Nacalai tesque, Japan) into dilute phosphoric acid. AlCl₃ 6H₂O (Nacalai tesque) was also dissolved in PB when catalytic conversion was examined. As an extraction phase, methyisobutyl ketone (MIBK) (FUJIFILM Wako Pure Chemical) or 2-sec butylphenol (SBP) (Tokyo Chemical Industry, Japan) was used. All the chemicals were used as purchased without any further purification.

2.1. HMF synthesis using a microreactor

The experimental apparatus consisted of two high pressure pumps, SUS tubes, a PTFE tube, hastelloy tubes, a union tee, and a back pressure regulator. The sugar solution in PB and the organic phase (MIBK or SBP) were fed by the high pressure pump, respectively, and collided at the T-mixer. After the T-mixer, the fluids flowed through the PTFE tube, the hastelloy tube, and the SUS tube. The PTFE tube was installed to visualize the flow state. The hastelloy tube with the inner diameter of 2 mm and the length of 1.6 m or 19.7 m was for the reactor section, which was soaked in the oil bath. The SUS tube with the inner diameter of 2 mm and the length of 1 m was for the cooling section, which was soaked in the ice bath. All the channels from the high pressure pumps to the cooling section were pressurized at 3.45 MPa by the back pressure regulator installed after the cooling section. The depressurized product was collected at the ambient temperature and pressure in the glass bottle. The reaction temperature, residence time, sugar type, sugar concentration, pH of PB, and organic phase/aqueous phase volumetric ratio (O/A) were changed to evaluate the effects on the HMF yield. In addition, the effect of Lewis acidic catalyst was examined using the sugar solution prepared with AlCl₃ 6H₂O (5 wt% of sugar). The product was analyzed by the high performance liquid chromatography (HPLC).

2.2 HMF back extraction from organic phase to water phase

Double pipe extractor was constructed for the HMF extraction from product solution. The outer tube was PFA tube with the outer diameter of 1.6 mm and the inner diameter of 1.0 mm, whereas the inner tube was the porous PP tube with the outer diameter of 0.63 mm, the inner diameter of 0.33 mm, and the pore diameter of 0.2 µm. The product extraction phase of MIBK from synthesis part (1 wt% glucose, 16.8 min, 180°C, O/A = 2) flowed in the outer channel, whereas deionized water flowed in the inner channel at the same volumetric flow rates. The obtained extract and raffinate were analyzed by HPLC.

3. Results and discussion

3.1 HMF synthesis using a microreactor

Fructose was converted into HMF by the prepared system. The concentration of fructose, reaction temperature, residence time, and O/A were 1 wt%, 180 °C, 5 min, and 2, respectively. The HMF yield was about 1.8 fold when SBP was used as an extraction phase (SBP: 76.8 %, MIBK: 43.4 %). After the reactions, 97 % of HMF was in the extraction phase when SBP was used while only 75 % was in the extraction phase when MIBK was used. This result indicated that the extraction ability (or capacity) had significant effect on HMF yield. This was confirmed by increasing O/A, which resulted in the HMF yield increase.

Because Lewis acid is known to promote an isomerization of glucose to fructose, the use of Lewis acidic catalyst of AlCl₃ was examined. The reaction condition was determined according to our previous study [4], where the concentration of glucose, reaction temperature, residence time, and O/A were 1 wt%, 180 °C, 47 min, and 3, respectively, with the use of SBP as the extraction phase. The HMF yield reached 84.9 mol% as the result, which was much higher comparing with the one without the catalyst (76 mol%). Thus, the benefit of catalyst use on HMF conversion was confirmed.

3.2 HMF extraction from organic phase

The synthesis experiments revealed the extraction ability of MIBK was inferior to that of SBP. However, the inferior extraction ability might be the advantage when whole the process was taken into account. Because HMF is not tolerant to heat, the distillation is not suitable for the purified HMF recovery. This means the recovery requires the operations such as back extraction, crystallization, or drying. The inferior extraction ability works as the advantage in these processes. Therefore, the back extraction of the product HMF in MIBK after the synthesis reaction was examined using a constructed microextractor. After the examination, the chromatogram of the obtained MIBK decreased the intensity of HMF only. In addition, the obtained water phase only delivered the peak of HMF. Thus, using the constructed microextraction apparatus, HMF was selectively extracted from product MIBK to deionized water. When the extraction was conducted by counter-current, 19 min of residence time enabled the extraction to the equilibrium, which extracted 74 % of HMF into the water phase.

4. Conclusions

HMF synthesis by biphasic system was examined using a microreactor. Two kinds of extraction agents were examined, and the extraction ability was clarified as the important factor for high HMF yield. However, HMF was selectively recovered from the extraction phase when MIBK was used. When SBP was used, by adding the Lewis acidic catalyst of AlCl₃ for the isomerization promotion, the HMF yield successfully reached as high as 84.9 mol%.

References

[1] Yuriy Román-Leshkov et al., Science, 312 (2006)

[2] Vinit Choudhary et al., JACS, 135 (2013)

[3] Yomaira J. Pagan-Torres et al., ACS Catal., 2 (2012)

[4] Yosuke Muranaka et al., I&EC Res., 56 (2017)