ORIGINAL RESEARCH published: 28 May 2021 doi: 10.3389/fenrg.2021.679709 Catalytic Conversion of Starch to 5-Hydroxymethylfurfural by Tin Phosphotungstate Jiawen Hao 1, Xueping Song 1,2, Shaowu Jia 1, Wei Mao 1, Yuxiao Yan 3* and Jinghong Zhou 1,2* 1College of Light Industry and Food Engineering, Guangxi University, Nanning, China, 2Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Guangxi University, Nanning, China, 3College of Life Science and Technology, Guangxi University, Nanning, China Heteropoly acids containing Brønsted and Lewis acids show excellent catalytic activity. Brønsted acids promote the depolymerization of polysaccharides (such as starch and cellulose) into glucose, while Lewis acids catalyze the conversion of glucose to 5- hydroxymethylfurfural (HMF). Designing stable Brønsted-Lewis acid-containing bifunctional heterogeneous catalysts is crucial for the efficient catalytic conversion of polysaccharides to HMF. In this study, a series of Brønsted -Lewis acid bifunctional catalysts (SnxPW, X 0.10–0.75) were investigated for the conversion of cassava starch to Edited by: HMF. The structure of the catalysts was characterized by X-ray diffraction, Fourier Cameron M. Moore, transform infrared spectroscopy, Pyridine Fourier transform infrared spectroscopy, and Los Alamos National Laboratory (DOE), United States X-ray photoelectron spectroscopy. The acid strength and acid capacity were also Reviewed by: investigated. The effects of reaction time, temperature, catalyst concentration, and Jacob Kruger, cassava starch concentration on the selectivity, conversion rate, and yield were National Renewable Energy Laboratory (DOE), United States examined. The results showed that, among the analyzed catalysts, Sn0.1PW presented Pedro Maireles-Torres, the best ability under the test conditions for catalyzing the conversion of starch to HMF. At University of Malaga, Spain the optimized conditions of a reaction temperature of 160°C, a catalyst dosage of *Correspondence: 0.50 mmol/gstarch, and a 1 h reaction time, the starch conversion rate was 90.61%, Yuxiao Yan [email protected] and the selectivity and yield of HMF were 59.77 and 54.12%, respectively. Our Jinghong Zhou findings contribute to the development of HMF production by the dehydration of [email protected] carbohydrates. Specialty section: Keywords: starch, heteropoly, phosphotungstate, catalytic reaction, 5-hydroxymethylfurfural This article was submitted to Bioenergy and Biofuels, a section of the journal INTRODUCTION Frontiers in Energy Research Received: 12 March 2021 Abundant renewable biomass is an ideal and promising alternative for the production of valuable Accepted: 12 May 2021 chemicals (Perea-Moreno et al., 2019). Among various compounds derived from biomass, 5- Published: 28 May 2021 hydroxymethylfurfural (HMF) is an important and versatile platform intermediate for the Citation: production of fuels and chemicals such as 2,5-furandicarboxilic acid (FDCA), 2,5- Hao J, Song X, Jia S, Mao W, Yan Y dhydroxymethylfuran (2,5-DMF), 5-ethoxymethylfurfural (5-EMF), and levulinic acid (LA). and Zhou J (2021) Catalytic Among these chemicals, 2,5-DMF and 5-EMF have attracted much attention because of their Conversion of Starch to 5- excellent biofuel characteristics. 5-EMF can be prepared without a cumbersome hydrogenation step, Hydroxymethylfurfural by fi Tin Phosphotungstate. requiring only HMF and ethanol etheri cation (Mascal and Nikitin, 2008). At present, multiple Front. Energy Res. 9:679709. catalysts have been reported for the catalytic conversion of HMF to 2.5-DMF (Huang et al., 2014; doi: 10.3389/fenrg.2021.679709 Wang et al., 2014). 5-EMF is considered a very good fuel additive. Its energy density is close to that of Frontiers in Energy Research | www.frontiersin.org 1 May 2021 | Volume 9 | Article 679709 Hao et al. Catalytic Synthesis of HMF gasoline (8.8 kwh/L) and diesel (8.7 kwh/L), and higher than that linked through α-1,4 and α-1,6-glycosidic bonds. The glycosidic of fuel ethanol (6.1 kwh/L; Mascal and Nikitin, 2008). FDCA may bonds are unstable in acid solution and can be easily hydrolyzed replace terephthalic acid, the main component of polyethylene to glucose. Chheda et al. (2007) used hydrochloric acid as catalyst terephthalate. Based on the wide application of FDCA as a to prepare HMF from starch in a water-MIBK-sec-butyl alcohol platform chemical, FDCA is listed as one of the 12 important solvent mixture, with a yield of 43%. Wu et al. (2018) reported glycosyl platform chemicals for the production of bio-based that homogeneous Fenton reagent was able to promote starch chemicals and materials. dehydration, and a 46.5% yield of HMF was achieved. Nikolla Various carbohydrates can be used to synthesize HMF, e.g., et al. (2011) obtained a 51.75% yield of HMF using β-zeolite fructose (Akien et al., 2012), glucose (Chheda et al., 2007), sucrose containing tin (Sn) and hydrochloric acid in a water/ (Yu et al., 2017), starch (Chun et al., 2010), and cellulose (Fang tetrahydrofuran two-phase system. However, because the yield et al., 2020). Generally, there are two pathways for the conversion of HMF prepared from starch is generally low, it is important to of glucose to HMF. One is the direct dehydration of glucose to develop an efficient catalyst for the effective utilization of starch. HMF, and the other includes two steps: glucose isomerization to In this paper, we report a catalytic SnxP system with a fructose, followed by the dehydration of fructose to HMF controllable amount of Brønsted and Lewis acids. Different fi (Choudhary et al., 2013; Zhao et al., 2018). The rst pathway amounts of SnCl4 were introduced into phosphotungstic acid is usually more difficult than the second one because of the to prepare catalysts with different amounts of Brønsted and Lewis different homomorphic conformation distributions of glucose acids for the production of HMF from starch. Here, Sn provides and fructose in solution. While the isomerization of glucose into Lewis acid sites that promote the isomerization of glucose to fructose is usually completed in an alkaline reaction system fructose, while phosphotungstic acid (HPW) provides Brønsted (Binder and Raines, 2009; Su et al., 2009), the dehydration of acid sites that promote starch hydrolysis to glucose and fructose fructose to HMF requires an acidic environment (Nie et al., 2020). and then dehydration to HMF. We found a significant increase in The introduction of metal ions through Brønsted acidification of HMF yield when moving from HPW to Sn-doped HPW, without heteropoly acids (HPAs) can form a range of insoluble solid any significant structural changes to the catalyst, which we Lewis-Brønsted acidic catalysts that are effective in the attribute to the introduction of Lewis acid sites to the catalyst. conversion of glucose and sucrose to HMF (Carniti et al., The effects of reaction temperature, reaction time, catalyst, and 2011; Deng et al., 2012). These heterogeneous acid catalysts substrate concentration on the conversion of cassava starch, HMF possess Lewis (metal sites) and Brønsted acidic sites. Zhao yield, and selectivity were studied. et al. (2011) reported that the catalytic conversion of fructose to HMF, with solid HPA and Cs2.5H0.5PW12O40 as catalysts, presented excellent selectivity (94.7%) and a relatively high yield MATERIALS AND METHODS (74.0%) after 60 min at 388 K. This new catalyst is tolerant to high feedstock concentrations, and it can be recycled. Hu et al. (2009) Materials revealed that Sn4+ can efficiently catalyze the conversion of Cassava starch was obtained from Hongfeng Starch Co., Ltd. sucrose to HMF in an ionic liquid, with a maximum yield of (Guangxi, China). Methanol and glacial acetic acid were of 65%. The Sn atoms act as isolated Lewis acid centers and are chromatographic grades, and all other reagents were of responsible for the isomerization of glucose to fructose. The analytical grade and used without further purification. bifunctional PTSA-POM/AlCl3.6H2O (60.7%HMF yield) Phosphotungstic acid hydrate, Sn chloride (99.998%), 5- catalyst showed a higher activity than the Lewis acid catalyst hydroxymethyl-2-furadehyde (99%), dimethyl sulfoxide AlCl3.6H2O (50.8% HMF yield) and the Brønsted acid catalyst (anhydrous solvent grade), and tetrahydrofuran (anhydrous PTSA-POM (9.4% HMF yield), and directly transformed glucose solvent grade) were obtained from Shanghai Aladdin to HMF owing to the balanced Lewis and Brønsted acid sites, with Industrial Inc. (China). a 60.7% HMF yield (Xin et al., 2017). These results indicated that a bifunctional acid catalyst with balanced Brønsted and Lewis Catalyst Preparation – acid sites can obtain a high yield of HMF from hexose. In general, Sn phosphotungstate catalysts SnxPW (x 0.10 0.75) were a high HMF yield can be obtained from monosaccharides, such as synthesized as reported in the literature (Okuhara et al., 2000). fructose and glucose (Nie et al., 2020). SnCl4 is a common Lewis A series of SnxPWs were prepared by titration, with the Sn acid, being cost-effective, highly active, and having a low toxicity, content ranging from 0.1 to 0.75 mmol. Six parts of which is often used in glucose conversion to HMF (Hu et al., phosphotungstic acid (1 mmol) were completely dissolved in 2009; Qiu et al., 2020). However, fructose and glucose are not 100 ml of distilled water, into which SnCl4 was slowly added, abundant, and thus, are expensive, thereby limiting the large- respectively. These mixtures were maintained under magnetic scale sustainable production of HMF. Therefore, an efficient stirring at 60°C for 6 h until the solution was clear and process must be developed to produce HMF using starch, transparent. Subsequently, the prepared solution was slowly cellulose, and other cheap polysaccharides as raw materials. dried at 60°C until the water completely evaporated and a Starch is one of the most abundant carbohydrates on earth white solid appeared. After drying, the white precipitate was and is widely explored in the chemical industry (Hornung calcined in a tubular furnace at 300°C for 2 h under a nitrogen et al., 2016).