molecules Article Conversion of Furfuryl Alcohol into Ethyl Levulinate over Glucose-Derived Carbon-Based Solid Acid in Ethanol Geng Zhao 1,*, Ming Liu 1, Xinkui Xia 2, Li Li 1 and Bayin Xu 1 1 Analysis and Testing Center, College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, Henan, China; [email protected] (M.L.); [email protected] (L.L.); [email protected] (B.X.) 2 College of Food Science, Xinyang Agriculture and Forestry University, Xinyang 464000, Henan, China; [email protected] * Correspondence: [email protected]; Tel.: +86-376-639-3155 Received: 27 March 2019; Accepted: 15 May 2019; Published: 16 May 2019 Abstract: In this study, a carbon-based solid acid was created through the sulfonation of carbon obtained from the hydrothermal pretreatment of glucose. Additionally, ethyl levulinate, a viable liquid biofuel, was produced from furfuryl alcohol using the environmentally benign and low-cost catalyst in ethanol. Studies for optimizing the reaction conditions, such as reaction time, temperature, and catalyst loading, were performed. Under the optimal conditions, a maximum ethyl levulinate yield of 67.1% was obtained. The recovered catalyst activity (Ethyl levulinate yield 57.3%) remained high after being used four times, and it was easily regenerated with a simple sulfonation process. Moreover, the catalyst was characterized using FT-IR, XRD, SEM, elemental analysis, and acid-base titration techniques. Keywords: furfuryl alcohol; ethanol; ethyl levulinate; carbon-based solid acid 1. Introduction Because of the recent diminishment of fossil fuel resources, as well as environmental degradation resulting from greenhouse gas emissions, significant effort has been devoted to converting renewable biomass into liquid fuels, fuel additives, and organic bulk chemicals [1–3]. Ethyl levulinate (EL) is considered to be a potential liquid biofuel for the future [4]. Ethyl levulinate is a short chain fatty ester, with properties similar to the fatty acid methyl ester of biodiesel. It has many excellent benefits, such as high lubricity, non-toxicity, flashpoint stability, and good flow properties under cold conditions [5]. Moreover, EL is of particular interest due to its extensive applications in the flavoring, solvent, and plasticizer sectors [6]. Additionally, it has found applications in the area of organic chemistry for the synthesis of the viable biofuel γ-valerolactone [7,8]. Currently, acid-catalyzed esterification of levulinic acid (LA) is the most efficient method for the synthesis of EL, with high yields regularly achieved [9–12]. However, LA is an expensive raw material, because it needs to be prepared from carbohydrates or biomass firstly, and then needs to be purified for synthesis of ethyl levulinate [13,14]. On the other hand, an increasing number of studies have focused on the direct production of EL from biomass in ethanol, such as hexose [15–17], cellulose [18], wood, bagasse [19], and wheat straw [20]. As a direct feedstock, raw biomass is abundant and inexpensive; however, the highest yield (55%) was achieved starting from the aforementioned feedstock (hexose). As can be seen from Scheme1, furfuryl alcohol (FA) is produced industrially via the hydrogenation of furfural [21,22]. It should be noted that furfural can be derived from the hydrolysis and dehydration of xylan contained in hemicellulose-rich biomass, including corncobs, corn stock, rice hulls, and olive Molecules 2019, 24, 1881; doi:10.3390/molecules24101881 www.mdpi.com/journal/molecules Molecules 2019, 24, x FOR PEER REVIEW 2 of 9 Molecules 2019, 24, 1881 2 of 9 As can be seen from Scheme 1, furfuryl alcohol (FA) is produced industrially via the hydrogenation of furfural [21,22]. It should be noted that furfural can be derived from the hydrolysis stones.and Presently, dehydration there of arexylan more contained than 400 in hemicellulose factories producing-rich biomass, furfural including in China, corncob and thes, corn annual stock, global rice hulls, and olive stones. Presently, there are more than 400 factories producing furfural in China, production of furfural exceeds 700,000 tons [23,24]. Therefore, furfural and FA are not fully utilized in and the annual global production of furfural exceeds 700,000 tons [23,24]. Therefore, furfural and FA the chemical market [25]. The conversion of FA into EL is regarded as an economic and convenient are not fully utilized in the chemical market [25]. The conversion of FA into EL is regarded as an strategyeconomic that hasand several convenient advantages, strategy including that has lowerseveral reaction advantages, temperature, including cheaper lower rawreaction material, and highertemperature, product cheaper yield. raw However, material, littleand higher effort hasproduct been yield. made However, to employ little this effort approach has been to made produce EL. Someto employ researchers this approac haveh obtainedto produce EL EL. from Some FA researchers catalyzed have by sulfuric obtained acid EL from [24], acidicFA catalyzed ion-exchange by resinssulfuric [26], sulfonic acid [24], acid acidic functionalized ion-exchange ILs [resins27], and [26] aluminosilicates, sulfonic acid functionalized [28]. It is important ILs [27] to, noteand that thesealuminosilicates catalysts, except [28] for. It sulfuric is important acid, canto note be extremelythat these expensivecatalysts, except due to for their sulfuric complex acid, preparationcan be processes.extremely Furthermore, expensive sulfuricdue to their acid complex and sulfonic preparation acid functionalized processes. Furthermore, ILs are homogeneous, sulfuric acid and creating serioussulfonic drawbacks acid infunctionalized terms of separation, ILs are recycling,homogeneous, and equipmentcreating serious corrosion. drawbacks Therefore, in itterms is necessary of separation, recycling, and equipment corrosion. Therefore, it is necessary to develop an to develop an environmentally benign and low-cost solid acid catalyst for the conversion of FA into environmentally benign and low-cost solid acid catalyst for the conversion of FA into EL. In the EL. In the resulting studies, high EL yields were achieved, but under longer reaction time (24 h) resulting studies, high EL yields were achieved, but under longer reaction time (24 h) or lower or lowersubstrate substrate concentration concentration (1.0 wt. (1.0%), wt.%), which which may potentially may potentially be hurdles be hurdles in the in process the process of the of the commercializationcommercialization of EL. of EL. SchemeScheme 1. The1. The route route of of synthesizing synthesizing levulinate esters esters from from hemicellulose hemicellulose.. In thisIn this study, study, an an inexpensive inexpensive and and robust robust carbon-basedcarbon-based solid solid acid acid catalyst catalyst was was successfully successfully synthesizedsynthesized and and evaluated evaluated for for the the conversion conversion ofof FA into EL EL in in ethanol. ethanol. A maximum A maximum EL yield EL yield of of 67.1%67.1% was achievedwas achieved under under the the optimal optimal conditions, conditions, and and allall thethe conversionconversion reactio reactionsns were were performed performed in triplicate.in triplicate The recovered. The recovered catalyst catalyst possessed possessed good good catalytic catalytic activity activity (EL (EL yield yield 57.3%), 57.3%), eveneven afterafter four four cycles, and it was easily regenerated by a simple sulfonation process. Moreover, the catalyst cycles, and it was easily regenerated by a simple sulfonation process. Moreover, the catalyst was was characterized using FT-IR, XRD, SEM, elemental analysis, and acid-base titration techniques. characterized using FT-IR, XRD, SEM, elemental analysis, and acid-base titration techniques. 2. Results and Discussion 2. Results and Discussion 2.1. Catalyst Characterizations 2.1. Catalyst Characterizations Structural information about unsulfonated glucose-derived carbon (UGC) and glucose-derived Structuralcarbonaceous information catalyst (GCC about) sample unsulfonateds were obtaine glucose-derivedd by FT-IR and carbon XRD. (UGC)Figure 1 and shows glucose-derived the XRD carbonaceouspatterns of catalyst two sample (GCC)s, which samples exhibit were two obtained2θ° weak diffraction by FT-IR andpeaks XRD. between Figure 10 and1 shows 30° and the 35 XRD patternsand of50°, two which samples, can be which assigned exhibit to twoC (002) 2θ◦ weakand C di (101),ffraction respectively. peaks between The results 10 and indicate 30◦ and that 35 and 50◦, whichamorphous can becarbon, assigned composed to C (002)of aromatic and C carbon (101), sheets, respectively. is orientedThe in results a random indicate fashion that [29] amorphous. Two carbon,samples composed have si ofmilar aromatic XRD carbonpatterns, sheets, indicating is oriented that the in sulfonation a random process fashion had [29 ].no Two effect samples on the have similarmorphology XRD patterns, of carbon indicating. that the sulfonation process had no effect on the morphology of carbon. The UGC and GCC were further characterized by FT-IR, and the results are shown in Figure2. 1 The vibration bands at 1384, 1172, and 1035 cm− (SO3 stretching) on GCC are evidence that -SO3H groups were successfully incorporated into the UGC. In addition, the absorption bands at 1715 and 1 1620 cm− correspond to -C=O (carbonyl) and -OH (hydroxyl) bending vibrations, implying that 1 carboxyl groups exist on the prepared carbon materials. The bands at 3424 and 1620 cm− can