(723d) Br?nsted and Lewis Acid Catalyzed Conversion of Cellobiose into 5-Hydroxymethylfurfural

In recent years, a significant attention has been directed towards the conversion of lignocellulosic biomass (LCB) into high value-added chemicals and fuels. Among these chemicals, 5-hydroxymethylfurfural (5-HMF) has emerged as a key intermediate with versatile applications in various industries such as pharmaceuticals, polymers, paint, and food/flavor industry [1]. It can be produced through the dehydration of LCB derived C6 sugars i.e. cellulose, cellobiose, glucose and fructose. The conversion of cellulose to HMF involves a series of complex chemical reactions (Figure 1) including hydrolysis of cellulose to glucose, isomerization of glucose to fructose followed by dehydration of fructose to 5-HMF in the presence of BrÓ§nsted (B) and Lewis (L) acid catalysts [2]. These reactions are highly influenced by various factors such as feedstock composition, reaction conditions, and catalysts. Therefore, understanding and optimizing these parameters are crucial for achieving the highest possible yield and selectivity to 5-HMF. Hence, this study is targeted at developing sulfonic acid functionalized activated carbon, optimizing the reaction conditions, and most importantly, B/L ratio towards 5-HMF production from cellobiose, used as a soluble model of raw cellulose.

Figure 1: Typical reaction pathway of cellobiose hydrolysis to 5-HMF production. Cellobiose is the simplest disaccharide used as soluble model for cellulose, which is a polymer of glucose units.

Methods

The activated carbon (SX+) functionalized with sulfonic acid groups was prepared by following the diazonium coupling technique as reported by us [3]. The grafted carbon material is labelled as SO₃H/SX+. The synthesized material was characterized by XPS, N₂-phsisorption and Boehm titration. The catalytic activity of SO₃H/SX+ was evaluated in a high-pressure autoclave stainless steel reactor (300 mL) in a water (milliQ pure water, MQ) and biphasic medium i.e. MQ/THF, starting from cellobiose as model soluble compound. Moreover, it was performed with the addition of AlCl₃.6H₂O as a source of Lewis acidity. The reaction mixture after removal of the catalysts by filtration, was analyzed by high-pressure liquid chromatography (HPLC, WATERS, USA) equipped with an Aminex HPX87-H+ column (Bio-Rad, USA) with RI detector (2414 WATERS, USA), using 5mM H₂SO₄ as eluent.

Results

The XPS spectra of functionalized SO₃H/SX+ carbon sample are presented in Figure 2(b) and 2(c). XPS results confirmed the successful grafting of sulfonic acid groups on the surface of SX+. The surface S/C atomic ratio was determined as 0.05 in line with our previous results [3]. The total acidity of SO₃H/SX+ was 133 mmol/100 g, as determined by Bohem titration, in significant increase compared to the starting pure activated carbon (4 mmol/100 g). The BET surface area was found to be reduced to ~124 m²/g from ~915 m²/g after the SO₃H group functionalization on SX+.

Evaluation of the catalytic performance was carried out first by varying the molar ratio of SO₃H vs. Al to obtain 5-HMF as final product. The results obtained are presented in Figure 2(c). During the reaction, the major products, glucose (GLU), fructose (FRU) and 5-HMF, were observed and quantified by HPLC. The production of fructose and 5-HMF was not observed in the presence of pure SO₃H/SX+, as expected, due to the lack of Lewis acidic sites in the catalyst. The cellobiose conversion was increased from ~80% to ~98% together with a higher glucose yield of ~87% after addition of AlCl₃ in the reaction mixture. Traces of 5-HMF were observed at the SO₃H/Al mol ratio of 1:4 and found to be increased together with a decreasing yield of glucose to ~23% at SO₃H/Al mol ratio of 1:15. These results indicate that on increasing the amounts of Al³⁺, the reaction proceeds in the forward direction and the formation of fructose and 5-HMF was started. Therefore, to obtain higher yields of 5-HMF, the effect of various reaction media such as MQ, MQ/THF (1:3 vol.%), MQ-NaCl/THF (1:3 vol. %) at lower SO₃H/Al ratio (1:4) was studied as shown in Figure 2 (d). The different reaction media gave different product distributions. Among these solvent systems, MQ/THF gave the highest yield of glucose (~53%), fructose (~28%), 5-HMF (~10%) with almost complete conversion of cellobiose due to favorable properties for cellobiose hydrolysis. These results indicated that the product distribution obtained from cellobiose hydrolysis is influenced not only by the selected solid acid catalyst but also by the synergistic effect of the solvent. By further optimizing the reaction conditions, a higher yield of 5-HMF (48-50%) could be obtained at 150°C in 4 h at the SO₃H/SX+: Al ratio of 1:10 in MQ/THF media.

Figure 2. XPS spectra in the regions of (a) carbon (C1s) and (b) sulfur (S2p) for the functionalized SO₃H/SX+ carbon catalyst, (c) Effect of SO₃H to Al mol ratio on catalytic HMF production from cellobiose (d) Effect of reaction solvents on HMF production (140°C, 4 h, 1 g cellobiose, 300 mL high pressure reactor, MQ water, N₂ environment).

Implications

The sulfonic acid groups grafted onto activated carbon were successfully synthesized with high amounts of surface groups, resulting in high total acidity. Using this catalyst, a high yield (of approximately 50% in 5-HMF) was obtained directly in one pot from the disaccharide cellobiose, with an optimal ratio of Brønsted to Lewis acidity (1:10) in a MQ/THF medium. The B/L ratio and choice of reaction solvent play important roles in determining the selective product distribution, enabling the upgrading of cellulose into a significant platform chemical.

References

1. B. Saha, M.M. Abu-Omar, Green. Chem., 2014, 16, 24-38.

2. Zhe Tang, Jianhui Su, Carbohydr. Res., 201, 481, 52–59.

3. S. Carlier, S. Hermans, Frontiers in Chemistry, 2020, 8, 347.