Conversion of Furfuryl Alcohol Into Ethyl Levulinate Over Glucose-Derived Carbon-Based Solid Acid in Ethanol

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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 has and several convenient advantages, strategy including that has lower several reaction advantages, temperature, including cheaper lower raw reaction 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, can to note be extremely that these expensivecatalysts, except due to for their sulfuric complex acid, preparationcan be processes.extremely Furthermore, expensive sulfuric due to their acid complex and sulfonic preparation acid functionalized processes. Furthermore, ILs are homogeneous, sulfuric acid and creating serioussulfonic drawbacks acid in functionalized terms of separation, ILs are recycling, homogeneous, and equipment creating 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 potentiallymay 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 aninexpensive inexpensive and and robust robust carbon-based carbon-based solid solid acid acid catalyst catalyst was was successfully successfully synthesizedsynthesized and and evaluated evaluated for for the the conversion conversion of of 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 of 50°, two which samples, can bewhich assigned exhibit to two C (002) 2θ◦ weak and 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 be Molecules 2019, 24, x FOR PEER REVIEW 3 of 9

Molecules 2019, 24, 1881 3 of 9

assignedMolecules to C-OH2019, 24 stretching, x FOR PEER and REVIEW -OH bending vibrations, implying that hydroxyl groups exist on the3 of 9 prepared carbon materials [30,31].

Figure 1. XRD patterns of unsulfonated glucose-derived carbon (UGC) and glucose-derived carbonaceous catalyst (GCC).

The UGC and GCC were further characterized by FT-IR, and the results are shown in Figure 2. The vibration bands at 1384, 1172, and 1035 cm−1 (SO3 stretching) on GCC are evidence that -SO3H groups were successfully incorporated into the UGC. In addition, the absorption bands at 1715 and 1620 cm−1 correspond to -C=O (carbonyl) and -OH (hydroxyl) bending vibrations, implying that carboxyl groups exist on the prepared carbon materials. The bands at 3424 and 1620 cm−1 can be assigned to C-OH stretching and -OH bending vibrations, implying that hydroxyl groups exist on

the Figureprepared 1. XRD carbon patterns material of unsulfonateds [30,31].glucose-derived carbon (UGC) and glucose-derived carbonaceous catalystFigure (GCC). 1. XRD patterns of unsulfonated glucose-derived carbon (UGC) and glucose-derived carbonaceous catalyst (GCC).

Figure 2. FT-IR spectra of UGC and GCC. Figure 2. FT-IR spectra of UGC and GCC. 2.2. Identiﬁcation of the Product 2.2. Identification of the Product The mechanism responsible for the conversion of FA to EL is complex. Additionally, it is also possibleThe mechanism for FA to responsible react with ethanol for the atconversion higher temperatures of FA to EL tois formcomplex. ether, Additionally, poly FA, and it otheris also by-productspossible for [ 24 FA,27 to]. In react an attempt with ethanol to identify at higher the product, temperatures the liquid to form phase ether, of the poly reaction FA, and mixture other obtainedby-products from [24,27] the conversion. In an attempt of FA in to the identify presence the ofproduct, GCC (at the 423 liq Kuid for phase 60 min) of wasthe analyzedreaction mixture using GC-MS.obtained The from GC-MS the conversion spectra are of given FA in in the Figures presence S1 and of GCC S2, located (at 423 inK for the 60 Supplementary min) was analyzed Materials using section.GC-MS. GC-MS The GC analysis-MS spectra of the are reaction given in mixture Figures showsS1 and theS2, located presence in ofthe EL Supplementary at a retention Materials time of 10.54section. min. GC The-MS mass analysis spectrum of the of reaction the product mixture exhibits shows molecular the presence ionization of EL atat ma /retentionz 144, corresponding time of 10.54 tomin. the molecularThe mass formulaspectrum of of EL the (C 7productHFigure12O3). 2.exhibits Furthermore,FT-IR spectra molecular of the UGC baseionization and peak GCC fragmentation .at m/z 144, corresponding of the ester is to relatedthe molecular to CH3CO formula (m/z = 43). of EL Two (C other7H12O important3). Furthermore, ion peaks, the located base peak at m fragmentation/z 99 and 129, are of due the esterto the is formation2.2. Identification of the ion acyliumof the Product RCO+ , resulting from the loss of either the alkoxy or methyl group from the ester. Additional bond cleavages occurred through some pathways and created fragment ions at The mechanism responsible for the conversion of FA to EL is complex. Additionally, it is also 40, 55, 74, and 101 m/z [10]. possible for FA to react with ethanol at higher temperatures to form ether, poly FA, and other by-products [24,27]. In an attempt to identify the product, the liquid phase of the reaction mixture obtained from the conversion of FA in the presence of GCC (at 423 K for 60 min) was analyzed using GC-MS. The GC-MS spectra are given in Figures S1 and S2, located in the Supplementary Materials section. GC-MS analysis of the reaction mixture shows the presence of EL at a retention time of 10.54 min. The mass spectrum of the product exhibits molecular ionization at m/z 144, corresponding to the molecular formula of EL (C7H12O3). Furthermore, the base peak fragmentation of the ester is

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related to CH3CO (m/z = 43). Two other important ion peaks, located at m/z 99 and 129, are due to the formation of the ion acylium RCO+, resulting from the loss of either the alkoxy or methyl group from the ester. Additional bond cleavages occurred through some pathways and created fragment ions at Molecules40, 55,2019 74,, and24, 1881 101 m/z [10]. 4 of 9

2.3. Effect of Reaction Parameters 2.3. Effect of Reaction Parameters The catalytic conversions of FA were conducted with temperatures ranging from 373 to 423 K, usingThe differing catalytic time conversions intervals, of to FA deduce were t conductedhe optimum with conditions temperatures for increasing ranging the from EL 373yield. to 423As seen K, usingin Figure differing 3, the time reaction intervals, temperature to deduce and the time optimum have conditionsa great influence for increasing on the overall the EL EL yield. yield. As Under seen inlower Figure temperature3, the reaction condition, temperature only and35.5% time EL haveyield awas great achieved influence after on the the reaction overall ELtime yield. of 15 Under0 min at lower373 Ktemperature. Elevating condition,the reaction only temperature 35.5% EL yield to 423 was K achieved increased after the the yield reaction of EL time rapidly of 150 with min at the 373extension K. Elevating of the the reaction reaction time, temperature and the equilibrium to 423 K increased point (67.1%) the yield was of ELreached rapidly by with 60 min. the extensionMoreover, ofat the 398 reaction and 423 time, K, when and the EL equilibriumyields reached point their (67.1%) peak wasvalues, reached longer by reaction 60 min. time Moreover,s resulted at 398 in lower and 423yield K, whens of EL. EL The yields low reached yield of their EL indicates peak values, that longerthe substance reaction might times decompose resulted in lowerto some yields extent, of EL.and Themore low undesired yield of EL byproducts indicates that were the formed substance under might higher decompose temperature to somes [32] extent,. and more undesired byproducts were formed under higher temperatures [32].

Figure 3. Eﬀect of reaction temperature on EL yield. Reaction conditions: furfuryl alcohol (FA) 1 g, Figure 3. Effect of reaction temperature on EL yield. Reaction conditions: furfuryl alcohol (FA) 1 g, ethanol 19 g, GCC 0.5 g. ethanol 19 g, GCC 0.5 g. Subsequently, the eﬀect of the catalyst loading dosage on the conversion of FA into EL was studied, and theSubsequently, results are presented the effect below of the in Figurecatalyst4. loading When the dosage reaction on timethe conversion was 60 min of and FA 1 wt.%into EL GCC was wasstudied, used, theand EL the yield results was are only presented 26.5%. Increasing below in the Fig GCCure 4 dosage. When to the 2.5 reaction wt.% resulted time was in a 60 remarkable min and 1 increasewt.% GCC in EL was yield used, to 67.1%.the EL yield However, was only when 26.5%. the catalyst Increasing loading the GCC dosage dosage was doubledto 2.5 wt.% to 5.0resulted wt.%, in thea remarkable yield of EL increase increased in by EL only yield 1.3%. to 67.1%. Moreover, However, after when 60 min, the thecatalyst EL yield loading decreased dosage faster was doubled when 5.0to wt.% 5.0 wt.%, GCC the was y used,ield of which EL increased could be by attributed only 1.3%. to theMoreover, increased after availability 60 min, the of acidEL yield sites, decreased derived faster when 5.0 wt.% GCC was used, which could be attributed to the increased availability of acid fromMolecules excessive 2019, 24 GCC,, x FOR facilitatingPEER REVIEW the decomposition of EL. Thus, it was determined that a 2.5 wt.%5 of 9 catalystsites, derived loading from dosage excessive was optimal. GCC, facilitating the decomposition of EL. Thus, it was determined that a 2.5 wt.% catalyst loading dosage was optimal.

Figure 4. Effect of catalyst dosage on EL yield. Reaction conditions: FA 1 g, ethanol 19 g, temperature 423K. Figure 4. Effect of catalyst dosage on EL yield. Reaction conditions: FA 1 g, ethanol 19 g, temperature 423K.

2.4. Recyclability of GCC The reusability and long-term stability of a given catalyst are extremely important factors in reducing production cost for practical biomass transformation. After the reaction was completed, the GCC was separated from the reaction mixture by filtration, washed 10 times with 10 mL of ethanol, and dried at 378 K for 24 h in a vacuum oven. Then, the GCC was used in the next reaction experiment under the same reaction conditions. As shown in Figure 5, GCC maintained good catalytic activity after four reactions, and an EL yield of 57.3% was still achieved. Subsequently, the four time-recycled GCC was regenerated by simple sulfonation at 453 K for 1 h using a solid/concentrated sulfuric acid ratio of 1:10 under a nitrogen atmosphere. The yield of the regenerated EL was restored to 66.4%, which was comparable to that of the fresh catalyst (67.1%).

Figure 5. Reuse of the GCC catalyst. Reaction conditions: FA 1 g, ethanol 19 g, GCC 0.5 g temperature 423 K, reaction time 60 min.

Under the same experimental conditions (at 423 K for 60 min), the catalytic activity of UGC (EL yield 11.6%) is much lower than that of GCC (EL yield 67.1%). The remarkable catalytic activity of GCC may be due to the high acidity of the sulfonated carbon catalyst, as well as the presence of -SO3H groups on the amorphous carbon catalyst. The acid density and elemental content of the reclaimed GCC are summarized in Table 1. The acid density and the sulfuric content of GCC decreased significantly with the increasing reaction cycles. On the other hand, the carbon content of GCC increased from 52.2 to 60.6 wt.% throughout the course of the four reaction runs. The results indicated that the slight decrease in EL yield was possible due to the leaching of the -SO3H group coupled with the deposition of humins on the surface of the GCC studied. In addition, the acid

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Figure 4. Effect of catalyst dosage on EL yield. Reaction conditions: FA 1 g, ethanol 19 g, temperature Molecules423K.2019 , 24, 1881 5 of 9

2.4. Recyclability of GCC 2.4. Recyclability of GCC The reusability and long-term stability of a given catalyst are extremely important factors in reducingThe reusability production and cost long-term for practical stability biomass of atransformation. given catalyst After are extremely the reaction important was completed, factors inthe reducingGCC was production separated costfrom for the practical reaction biomassmixture by transformation. filtration, washed After 10 the times reaction with 10 was mL completed, of ethanol, theand GCC dried was at separated 378 K for from 24 the h inreaction a vacuum mixture oven. by ﬁltration, Then, the washed GCC 10 was times used with in 10 the mL next of ethanol, reaction andexperiment dried at 378 under K for the 24 h same in a vacuum reaction oven. conditions. Then, the As GCC shown was inused Fig inure the 5 next, GCC reaction maintained experiment good undercatalytic the activity same reaction after four conditions. reactions, As and shown an EL in yield Figure of5 57.3%, GCC was maintained still achiev gooded. catalyticSubsequently, activity the afterfour four time reactions,-recycled and GCC an EL was yield regenerated of 57.3% was by still simple achieved. sulfonation Subsequently, at 453 the K four for time-recycled 1 h using a GCCsolid/concentrated was regenerated sulfuric by simple acid sulfonation ratio of at1:10 453 under K for 1 a h nitrogen using a solid atmosphere./concentrated The sulfuric yield of acid the ratioregenerated of 1:10 under EL was a nitrogenrestored to atmosphere. 66.4%, which The was yield compara of theble regenerated to that of the EL fresh was restoredcatalyst (67.1%). to 66.4%, which was comparable to that of the fresh catalyst (67.1%).

Figure 5. Reuse of the GCC catalyst. Reaction conditions: FA 1 g, ethanol 19 g, GCC 0.5 g temperature Figure 5. Reuse of the GCC catalyst. Reaction conditions: FA 1 g, ethanol 19 g, GCC 0.5 g 423 K, reaction time 60 min. temperature 423 K, reaction time 60 min. Under the same experimental conditions (at 423 K for 60 min), the catalytic activity of UGC Under the same experimental conditions (at 423 K for 60 min), the catalytic activity of UGC (EL (EL yield 11.6%) is much lower than that of GCC (EL yield 67.1%). The remarkable catalytic activity of yield 11.6%) is much lower than that of GCC (EL yield 67.1%). The remarkable catalytic activity of GCC may be due to the high acidity of the sulfonated carbon catalyst, as well as the presence of -SO3H GCC may be due to the high acidity of the sulfonated carbon catalyst, as well as the presence of groups on the amorphous carbon catalyst. The acid density and elemental content of the reclaimed GCC -SO3H groups on the amorphous carbon catalyst. The acid density and elemental content of the are summarized in Table1. The acid density and the sulfuric content of GCC decreased signiﬁcantly reclaimed GCC are summarized in Table 1. The acid density and the sulfuric content of GCC with the increasing reaction cycles. On the other hand, the carbon content of GCC increased from decreased significantly with the increasing reaction cycles. On the other hand, the carbon content of 52.2 to 60.6 wt.% throughout the course of the four reaction runs. The results indicated that the slight GCC increased from 52.2 to 60.6 wt.% throughout the course of the four reaction runs. The results decrease in EL yield was possible due to the leaching of the -SO3H group coupled with the deposition indicated that the slight decrease in EL yield was possible due to the leaching of the -SO3H group of humins on the surface of the GCC studied. In addition, the acid density and elemental content of the coupled with the deposition of humins on the surface of the GCC studied. In addition, the acid regenerated GCC were nearly identical to the fresh catalyst, indicating that the regeneration process not only recovered the catalytic activity of GCC, but also utilized the humins deposited on GCC.

Table 1. Total acid density and elemental content of the reclaimed GCC.

Element Content (wt.%) b 1 a Run Total Acid Density (mmol g− ) · Carbon Sulfur 1 2.6 52.2 1.5 2 1.9 57.9 1.1 3 1.7 59.2 1.0 4 1.4 60.6 0.9 5 c 2.5 53.1 1.5 a Determined by acid-base titration. b Obtained by elemental analysis. c The regenerated GCC.

The comparative SEM images of the fresh and four time-recycled GCC are shown in Figure6. It is clear that they both exhibited an intertwined structure and consisted of irregular particles with a size of several micrometers. However, the particles of the four time-recycled GCC were larger, and their Molecules 2019, 24, x FOR PEER REVIEW 6 of 9 density and elemental content of the regenerated GCC were nearly identical to the fresh catalyst, indicating that the regeneration process not only recovered the catalytic activity of GCC, but also utilized the humins deposited on GCC.

Table 1. Total acid density and elemental content of the reclaimed GCC.

Element Content (wt.%) b Run Total Acid Density (mmol·g−1) a Carbon Sulfur

1 2.6 52.2 1.5

2 1.9 57.9 1.1

3 1.7 59.2 1.0

4 1.4 60.6 0.9

5 c 2.5 53.1 1.5 a Determined by acid-base titration. b Obtained by elemental analysis. c The regenerated GCC.

MoleculesThe2019 comparative, 24, 1881 SEM images of the fresh and four time-recycled GCC are shown in Figure 66. of It 9 is clear that they both exhibited an intertwined structure and consisted of irregular particles with a size of several micrometers. However, the particles of the four time-recycled GCC were larger, and theirsurfaces surfaces were were smoother, smoother, indicating indicating substantial substantial humin humin deposition deposition on the on surface the surface of GCC of GCC after after four fourreaction reaction runs. runs.

Figure 6. SEMSEM images images for for fresh fresh catalyst catalyst ( (A)) and and recycled recycled catalyst after f fourour runs ( B).

3. Materials Materials and and Methods Methods

3.1. Materials Materials Ethyl levulinate levulinate and and furfuryl furfuryl alcohol alcohol were were purchased purchased from Aladdin from Aladdin Chemical Chemical Reagent (Shanghai, Reagent (China).Shanghai, Other China reagents). Other werereagents obtained were obtained from Sinopharm from Sinopharm Chemical Chemical Reagent Reagent (Shanghai, (Shanghai, China). ChinaAll reagents). All reagents and chemicals and chemicals were analytical were analytical grade and grade used and without used further without puriﬁcation further purification or treatment. or treatment. 3.2. Catalyst Preparation 3.2. CatalystUsing the Preparation typical preparation method, glucose (30 g) was dissolved in 50 mL of distilled water in a cylindrical stainless steel pressurized reactor with 100 mL total volume (PARR, Moline, IL, USA). Using the typical preparation method, glucose (30 g) was dissolved in 50 mL of distilled water The reactor was sealed and heated to 473 K with agitation rate of 500 rpm for 6 h. After the hydrothermal in a cylindrical stainless steel pressurized reactor with 100 mL total volume (PARR, Moline, IL, carbonization process was completed, the solid carbon was separated by ﬁltration, washed with USA). The reactor was sealed and heated to 473 K with agitation rate of 500 rpm for 6 h. After the distilled water, and subsequently dried in an oven at 383 K for 24 h. The resulting solid carbon (5 g) hydrothermal carbonization process was completed, the solid carbon was separated by filtration, was then ground to a powder and heated at 453 K for 5 h in 50 mL of concentrated sulfuric acid washed with distilled water, and subsequently dried in an oven at 383 K for 24 h. The resulting solid (98%) under a nitrogen-rich atmosphere to introduce -SO H groups. The sulfonated mixture was then carbon (5 g) was then ground to a powder and heated3 at 453 K for 5 h in 50 mL of concentrated diluted with 1000 mL of deionized water to form a black precipitate, which was subsequently washed sulfuric acid (98%) under a nitrogen-rich atmosphere to introduce -SO3H groups. The sulfonated repeatedly in hot distilled water (>353 K) until the sulfate ions were no longer detected in the residual mixture was then diluted with 1000 mL of deionized water to form a black precipitate, which was washing water. Finally, the resulting glucose-derived carbonaceous catalyst (GCC) was dried in an oven at 383 K for 24 h.

3.3. Catalyst Characterization The prepared GCC was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diﬀraction spectroscopy (XRD), scanning electron microscopy (SEM), elemental analysis, and acid-base 1 titration. The FT-IR spectrum was recorded over the range of 400 to 4000 cm− on a Nicolet iS 50 FT-IR spectrometer, made by Thermo Fisher Scientiﬁc, Waltham, MA, USA. The XRD pattern of the GCC was recorded by an X’ Pert PRO powder XRD spectrometer (Panalytical, Almelo, The Netherlands) using a Cu Kα radiation source (λ = 0.154 nm). The morphology of the GCC was observed using a S-4800 scanning electron microscope (Hitachi, Tokyo, Japan) at 30 kV. The elemental content of the GCC sample was determined using a Vario EL III elemental analyzer, made by Elementar, Hanau, Germany. The acid density of the GCC was determined via acid-base titration.

3.4. General Procedure for the Synthesis of EL The synthesis of EL from FA were conducted in a 50-mL cylindrical, stainless steel, pressurized reactor (PARR, Moline, IL, USA). The reactor was heated in an adjustable electric stove. In a typical experiment, FA (1 g), ethanol (19 g), and a given amount of GCC were introduced into the reactor. Molecules 2019, 24, 1881 7 of 9

After the reactor was sealed, the above mixture was heated to the desired temperature and stirred magnetically at 500 rpm. At the end of the reaction, the resulting mixture was cooled to room temperature. The liquid products and solid acid catalyst were separated through centrifugation at 10,000 rpm for 3 min. After the process was completed, the contents were analyzed.

3.5. Analysis of Products The qualitative analysis of the sample was conducted with a Shimadzu 2010A gas chromatograph (GC) system, coupled with a Shimadzu QP2010 mass spectrometer (GC-MS). The amount of product was analyzed by GC on an Agilent 7890 instrument, equipped with a HP-5 capillary column and a ﬂame ionization detector (FID). The injection port temperature was 523 K, and the detector temperature was 543 K. The column temperature was maintained at 313 K for 4 min, then raised to 523 K with a ramp rate of 15 K/min. The amount of EL present at the end of the process was calculated using an external standard. The yield of EL on a molar basis was calculated as follows:

EL (mol) EL yield(%) = 100% (1) FA (mol) ×

4. Conclusions An environmentally benign and inexpensive solid acid catalyst GCC was demonstrated to be highly active for use in the conversion of FA into EL in ethanol. A good EL yield of 67.1% was obtained under moderate temperature and short reaction times (423 K, 60 min). GCC could be recycled four times, with little deactivation, and could be easily regenerated with a simple sulfonation process.

Supplementary Materials: The following are available online. Figure S1: GC spectra of the product of the conversion of FA into EL., Figure S2: MS spectra of the product of the conversion of FA into EL. Author Contributions: G.Z. conceived and designed the experiments; M.L. and X.X. performed the experiments; L.L. analyzed the data; B.X. contributed reagents and analysis tools; and G.Z. wrote the paper. Funding: This work was financially supported by the National Natural Science Foundation of China (21606188), the Foundation of Henan Educational Committee (15A530011), the Nanhu Scholars Program for Young Scholars of XYNU, and the Doctoral Research Foundation of Xinyang Normal University. Acknowledgments: This research was supported by the Analysis & Testing Center of Xinyang Normal University. We would like to thank Liu Peifang and Zhang Zongwen for the assistant and technique support in SEM. Conflicts of Interest: The authors declare no conflict of interest.

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