Introductory Remarks

1. Introduction

The production of valuable chemicals such as 5-hydroxymethylfurfural (5-HMF), formic acid (FA), levulinic acid (LA) from lignocellulosic biomass (LCB) has gained significant attention due to the environmental concerns with fossil resources. LCB consists of three main compounds such as cellulose (35–50%), hemicellulose (25–30%), and lignin (25–30%) [1]. The biomass-derived cellulose can be a potential source of D-glucose because it consists of a linear chain of several hundred to thousands of linked D-glucose units. Glucose is the most abundant and environmentally friendly hexose that exists in nature. It can be a promising alternative for the production of 5-HMF, lactic acid, LA, FA, etc [2]. Among all these chemicals, 5-HMF has been considered as the best 10 platform chemical worldwide, and it can be used as a feed-stocks for the production of many important chemicals such as 2,5-furandicarboxylic acid (2,5 FDCA), 2,5-diformylfuran etc. The conversion of glucose to 5-HMF follows a two-step mechanism, i.e., glucose isomerization to fructose followed by the dehydration of fructose to 5-HMF. Various recent literature discussed the performance of different heterogeneous catalysts with Lewis and Bronsted acidities such as zeolites, metal sulfates and phosphates, transition metal oxides, and ionic liquids for the dehydration of glucose [3], in various reaction mediums such as aqueous, organic (DMSO, IPA, MIBK), biphasic (organic: H₂O), and an ionic liquids, respectively [4]. However, the production cost, products separation inefficiency, and undesired side product formations have been identified as the major issues. Therefore, the development of a low energy input and the selective process is extremely necessary for the production of 5-HMF from lignocellulosic biomass. The present study discussed the formation of various value-added chemicals including fructose, 5-HMF, FA, and LA via the dehydration of glucose in the presence of various solid acid catalysts in DSMO medium.

2. Experimental

The solid acid catalysts were synthesized by sol-gel followed by wet impregnation method and characterized by X-ray diffraction (XRD), N₂ adsorption-desorption, NH₃ temperature-programmed desorption (NH₃-TPD), and field-scanning electron microscope (FE-SEM), etc. The glucose dehydration reaction was carried out at 160°C for 6 h in a 100 mL glass reactor placed in an oil bath. The temperature of the oil bath was controlled with the help of a temperature controller. . After the reaction, the reaction mixture was quenched to room temperature and the catalyst was separated in a centrifuge at 8000 rpm. Further, the reaction mixture was analyzed in high-pressure liquid chromatography (HPLC, WATERS, USA) equipped with an Aminex HPX87-H+ column (Bio-Rad, USA) with RI detector (2414 WATERS, USA).

3. Results and discussion

The BET surface area obtained from N₂ adsorption-desorption analysis of all the synthesized catalysts followed the order as TiO₂ > 0.5MSO₄^2-/TiO₂ > ZnO > 1.5MSO₄^2-/ZnO. The NH₃-TPD curves of all the catalysts are shown in Fig. 1(a). The total acidity of the catalyst was calculated based on the amount of ammonia desorbed in the TPD analysis. TPD results suggested that, among all others, The sulphated-TiO₂ catalyst demonstrated the highest total acidity of (0.57 mmol g^-1), and, the total acidity of the catalyst followed the order as 0.5MSO₄^2-/TiO₂ > 1.5MSO₄^2-/ZnO >ZnO >TiO₂.

The catalytic activity and product yield obtained over all catalysts at the standard reaction condition is compared in Fig. 1(b). The primary observed reaction products were fructose, 5-HMF, LA, and FA, respectively. Results depicted that the 0.5MSO₄^2-/TiO₂ catalyst was the most active which demonstrated the maximum glucose conversion of 98.5% with the 5-HMF, LA, FA yield of 36.5%, 2.7% and 13.1%, respectively, at the optimum reaction condition obtained. It has been observed that the total acidity, as well as the ratio of Lewis to Bronsted acidity in the catalyst, played a significant role in dehydration activity as well as products yields.

4. Conclusions

The catalytic activity of various solid acid catalysts was evaluated and compared for the dehydration glucose to valuable chemicals in an organic DMSO medium. The solid acid catalysts were prepared, and the sulfate group (SO₄^2-) was impregnated on the metal oxide to improve the acidity. NH₃-TPD results confirmed the enhancement of the acidity of the sulfate incorporated catalyst. Among all others, sulfate impregnated TiO₂ catalyst demonstrated the highest dehydration activity, and 4-HMF, LA, and FA was identified as major reaction products.

Keywords: glucose, dehydration, solid acid catalyst, organic medium

References

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2. Corma Canos, A., Iborra, S. and Velty, A. (2007) 'Chemical routes for the transformation of biomass into chemicals', Chemical Reviews, 107(6), pp. 2411–2502. doi: 10.1021/cr050989d.
3. Nakajima, K. et al. (2014) 'Selective glucose transformation by titania as a heterogeneous Lewis acid catalyst', Journal of Molecular Catalysis A: Chemical. Elsevier BV, 388–389, pp. 100–105. doi: 10.1016/j.molcata.2013.09.012.
4. Yan, H. et al. (2009) 'Catalytic conversion of glucose to 5-hydroxymethylfurfural over SO₄^2−/ZrO₂ and SO₄^2−/ZrO₂–Al₂O₃ solid acid catalysts', Catalysis Communications, 10(11), pp. 1558–1563. doi: 10.1016/j.catcom.2009.04.020

Fig. 1 (a) NH₃-TPD pattern of catalyst, (b) Catalytic activity at 160°C after 6 h at the glucose to catalyst ratio of 0.5 in DMSO medium.