Conversion of D-Fructose Into Ethyl Lactate Over a Supported Sno2–Zno/Al2o3 Catalyst

colloids and interfaces

Article Conversion of D-Fructose into Ethyl Lactate Over a Supported SnO2–ZnO/Al2O3 Catalyst

Svitlana V. Prudius *, Nataliia L. Hes and Volodymyr V. Brei

Institute for Sorption and Problems of Endoecology of National Academy of Sciences of Ukraine, 13 General Naumov Str., Kyiv 03164, Ukraine; [email protected] (N.L.H.); [email protected] (V.V.B.) * Correspondence: [email protected]; Tel.: +38-067-7678855

Received: 17 December 2018; Accepted: 22 January 2019; Published: 25 January 2019

Abstract: The study is directed to the search of a simple effective catalyst for ethyl lactate obtained from fructose as renewable raw material. A series of SnO2-containing oxides prepared by impregnation of alumina were characterized by several techniques in order to determine their textural and acid-base properties. The transformation of a 13% fructose solution in 98% ethanol over ◦ SnO2/Al2O3 catalysts using autoclave rotated with 60 rpm at 160 C for 3 h was studied. It was found that doping SnO2/Al2O3 samples with ZnO improves selectivity towards ethyl lactate. The supported SnO2-ZnO/Al2O3 catalyst provides 100% fructose conversion with 55% yield of ethyl lactate at 160 ◦C. A possible scheme of fructose transformation into ethyl lactate on L-acid IVSn4+ sites is discussed.

Keywords: fructose; ethyl lactate; SnO2/Al2O3 catalyst

1. Introduction Ethyl lactate obtained from lactic acid and ethanol is used mainly as green solvent, as well as in the food, cosmetic and pharmaceutical industries [1–3]. At present, ethyl lactate is considered as material for synthesis of monomeric lactide and ethyl acrylate [3]. Therefore, considerable attention is paid to the search for alternative methods for its preparation [4–8]. Some progress was made in the synthesis of alkyl lactates from dihydroxyacetone [4–6] and glycerol [7]. In a particular, the direct ◦ synthesis of ethyl lactate from glycerol solution in ethanol on CeO2/Al2O3 catalyst at 230 C was realized [7]. Recently, many works have been devoted to studying the activity of Sn-containing oxides, such as mesoporous MCM-41 [9,10], SBA-15 [10] materials and MFI [10], BEA [10], MWW [11] zeolites as catalysts for conversion of carbohydrates into alkyl lactates. Holm with co-workers [8] found in 2010 that Sn-Beta zeolite directly catalyzes the transformation of mono- and disaccharides in methanol (0.1 mmol/L) into methyl lactate at 160 ◦C. Also, authors [12] showed that Zn-Sn-Beta converts sucrose into lactic acid with 54% yield at 190 ◦C after 2 h. Numerous studies with different zeolites and their acidity showed that a combination of Lewis and Bronsted acidity catalyzes the selective conversion of monosaccharides to lactates [8,9]. It is known that Brønsted strong acid sites catalyze alternative reactions that lead to the formation of undesirable by–products [8–11]. Recent studies have shown that the addition of alkali ions to the reaction mixture or using dual metal tin-containing zeolite system as a catalyst increases the conversion of sucrose to lactic acid or lactate [12,13]. However, the preparation of Sn-zeolites is rather complicated procedure. In this work, we have used several Sn-containing oxides obtained by a simple impregnation method as catalysts of one-pot conversion of fructose into ethyl lactate.

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2. Materials and Methods ◦ Pure SnO2 was prepared by precipitation from solution of SnCl4·5H2O with urea at 90 C. After washing and drying at 110 ◦C, the precipitate was granulated and calcined at 550 ◦C in flowing air for 2 h. Alumina-supported Sn-catalysts containing 10–25 wt.% SnO2 were prepared by incipient wetness impregnation of commercial γ-Al2O3 (Alvigo) with calculated aqueous solutions of SnCl4·5H2O ◦ (Aldrich, 98%). Before the impregnation procedures, γ-Al2O3 was treated at 250 C in flowing air for 3 h. The samples were denoted as zSnO2/Al2O3, where z is the SnO2 content expressed in wt.%. ◦ The supported SnO2/Al2O3 precursors were calcined at 550 C for 2 h. Sample 10SnO2/Al2O3 was additionally impregnated with an aqueous solution of Zn(CH3COO)2 by incipient wetness impregnation. After thermal treatment, the zinc oxide content was 5 wt.%. The sample was denoted as 10SnO2–5ZnO/Al2O3. Thermal analysis studies were carried out using a Perkin Elmer thermogravimetric analyser (TGA). Samples were purged with either nitrogen or oxygen and heated from room temperature to 800 ◦C at a heating rate of 10 ◦C/min. XRD patterns of samples were recorded with DRON-4-07 diffractometer (CuKα radiation). Diffraction patterns were identified by comparing with those from the JCPDS (Joint Committee of Powder Diffraction Standards) database. The textural parameters of the samples were calculated from the adsorption–desorption isotherms of nitrogen using the BET method (Quantachrome Nova 2200e Surface Area and Pore Size Analyser). The computational error for Ssp determination is ≤2%. UV-Vis diffuse reflectans spectra were recorded using a Perkin Elmer Lambda 40 spectrophotometer equipped with a diffuse reflectance chamber and integrating sphere (Labsphere RSA-PE-20). Prior to each experiment, samples were compacted in a sample holder to obtain a sample thickness of ≈2 mm. In order to determine the forbidden gap energy E0, the reflectance spectra were 2 1/2 recalculated to absorption spectra using Kubelka–Munk formula, F = (hν(1 − R) /2R) .E0 values were determined from the nearly linear long-wave segment of absorption band plot extrapolated to interception with abscissa [14]. The material used as a reference was MgO. Total number of acid or base sites was determined by reverse titration using n-butylamine or 2,4-dinitrophenol solution in cyclohexane, respectively, with bromthymol blue as an indicator. The highest acid or base strength was examined by the Hammett indicators method [15] using 0.1% solution of the corresponding indicators in cyclohexane. For catalytic experiments, 13 wt.% solution of D-fructose (N98%, Merck) in anhydrous ethanol was used as a reaction mixture. The experiments were carried out in a rotated autoclave (60 rpm) at 160 ◦C for 3 h. Usually, 1.5 g of fructose, 10 g of anhydrous ethanol and 0.68 g (6 wt.%) of a catalyst were placed into a 25-mL Teflon can. The formation of insoluble products at these conditions was not observed. The reaction products were analyzed using 13C NMR spectroscopy (Bruker Avance 400, Karlsruhe, Germany). The conversion values of fructose (X) and selectivity of products (S, mol%) were calculated from 13C NMR spectra. Yields were calculated as Y = S·X.

3. Results and Discussion All synthesized samples were analyzed by several techniques in order to study their chemical, structural, textural and acid properties. Textural and acidic properties of synthesized SnO2/Al2O3 oxides are summarized in Table1. Colloids Interfaces2018, 2, x FOR PEER REVIEW 3 of 8

Colloids InterfacesTable2019 1. Textural, 3, 16 parameters and total acidity of synthesized SnO2/Al2O3 samples. 3 of 8 Specific Pore Average Pore Highest Acid Total Content of

Sample Table 1. TexturalSurface parametersVolume, and total acidityDiameter, of synthesized nm Strength, SnO2/Al H0max2O3 samples.Acid Sites, Area,m2/g cm3/g mmol/g Speciﬁc Highest Acid Total Content Pore Volume, Average Pore AlSample2O3 Surface290 Area, 0.82 3 10.7 +3.3Strength, of1.2 Acid Sites, 2 cm /g Diameter, nm m /g H0max mmol/g SnO2 39 0.07 7.2 +1.5 0.4 Al2O3 290 0.82 10.7 +3.3 1.2 10SnO2/Al2O3 250 0.67 11.4 +1.5 1.3 SnO2 39 0.07 7.2 +1.5 0.4 20SnO10SnO2/Al2/Al2O3 2O3 228250 0.62 0.67 10.8 11.4 +1.5 +1.5 1.5 1.3 20SnO2/Al2O3 228 0.62 10.8 +1.5 1.5 25SnO2/Al2O3 242 0.62 10.2 +1.5 1.8 25SnO2/Al2O3 242 0.62 10.2 +1.5 1.8 10SnO10SnO2–5ZnO/Al2–5ZnO/Al2O3 2O3 232232 0.65 0.65 10.2 10.2 +3.3 +3.3 1.3 1.3

According to the thermogravimetric analysis of as-calcined 20SnO2/Al2O3 sample (Figure 1), the According to the thermogravimetric analysis of as-calcined 20SnO2/Al2O3 sample (Figure1), condensationthe condensation of structural of structural OH-groups OH-groups with water with waterrelease release is observed is observed at 300 at °C, 300 and◦C, andcrystallites crystallites of of SnO2 are formed at 450–550 °C. ◦ SnO2 are formed at 450–550 C.

DTA

10 Weight loss, % DTG

40 100 200 300 400 500 600 700 800 900 1000 T, 癈

Figure 1. TG/DTA curves of as-synthesized 20SnO2/Al2O3 sample.

The small decreaseFigure1. (10–18%)TG/DTA curves in the of surface as-synthesized area of alumina20SnO2/Al support2O3 sample. occurs upon the deposition of tin oxide what probably explained by blocking of the aluminum pores with the precipitated oxide (TableThe small1). As decrease seen from (10–18%) the pore in the size surface distribution area of curvesalumina (Figure support2), occurs derived upon from the the deposition desorption of tinbranches oxide what of isotherms probably usingexplained the DFTby blocking method, of thethe depositionaluminum ofpores SnO with2 on the alumina precipitated surface oxide leads to (Tabledecrease 1). As inseen pore from content the withpore rsize≤ 3 distributi nm for 10SnOon curves2/Al2 O(Figure3 and 20SnO2), derived2/Al2 Ofrom3 samples. the desorption At the same time, for 25SnO2/Al2O3 sample, a decrease in pore content with r~4–5 nm is observed. Obviously, branches of isotherms using the DFT method, the deposition of SnO2 on alumina surface leads to the SnO2 crystallites with sizes of more than 3 nm are formed at increase in the content of supported decreasetin dioxide. in pore content with r ≤ 3 nm for 10SnO2/Al2O3 and 20SnO2/Al2O3 samples. At the same time, for The25SnO structural2/Al2O3 sample, analysis aby decrease XRD shows in pore that content at loadings with r~4–5≤20 nm wt.% is SnOobserved.2 no characteristic Obviously, the peaks SnOassigned2 crystallites to tin with oxide sizes were of observedmore than (Figure 3 nm 3are). Itfo indicatesrmed at increase high dispersion in the content of tin oxide of supported on the alumina tin ◦ ◦ ◦ ◦ dioxide.surface. Small peaks appeared at 2θ = 26.6 ;33.9 ;38.0 and 51.8 corresponding to tetragonal SnO2 for 25SnO2/Al2O3 sample were detected (Figure3).

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0.030 γ-Al O 2 3 0.025 10SnO /Al O 2 2 3 20SnO /Al O 0.020 2 2 3 Colloids Interfaces2018, 2, x FOR PEER REVIEW 25SnO /Al O 4 of 8 Colloids Interfaces 2019, 3, 16 2 2 3 4 of 8 dV(r) 10SnO -5ZnO/Al O 0.015 2 2 3

0.030 0.010 γ-Al O 2 3 0.025 10SnO /Al O 0.005 2 2 3 20SnO /Al O 0.020 2 2 3 0.000 25SnO /Al O 23456789102 2 3 dV(r) 0.015 10SnO -5ZnO/Al O r , nm 2 2 3 pore

0.010 Figure 2. DFT pore-size distributions of samples calcined at 550 °C. 0.005 The structural analysis by XRD shows that at loadings ≤20 wt.% SnO2 no characteristic peaks assigned to tin oxide0.000 were observed (Figure 3). It indicates high dispersion of tin oxide on the alumina surface. Small2345678910 peaks appeared at 2θ = 26.6°;33.9°;38.0° and 51.8°corresponding to tetragonal

SnO2 for 25SnO2/Al2O3 sample were detected (Figure 3).r , nm pore

◦ Figure 2. DFT pore-size distributions of samples calcined at 550 C. Figure25SnO 2. DFT/Al Opore-size distributions of samples calcined at 550 °C. 2 2 3 ∗ ∗ 2 The structural analysis by XRD shows∗ that at loadings ≤20 wt.% SnO no characteristic peaks assigned to tin oxide were observed (Figure 3). It indicates high dispersion of tin oxide on the 20SnO /Al O 2 2 3 alumina surface. Small peaks appeared at 2θ = 26.6°;33.9°;38.0° and 51.8°corresponding to tetragonal SnO2 for 25SnO2/Al2O3 sample were detected (Figure 3). ∗

γ−Al O 2 3 25SnO /Al O 2 2 3 ∗ ∗ ∗ 20SnO /Al O 202 2 303 40 50 60 70 2θ, ο

∗ ◦ Figure 3. XRD patterns of zSnO2/Al2O3 samples after calcinations at 550 C for 2 h.

AccordingFigure to the3. γ−XRD titrationAl patternsO results, of zSnOγ2/Al-Al2O2O3 samples3 is weakly after calcinations acid oxide at with 550 °C H for0 ≤ 2 h.+3.3 (Table1). 2 3 The addition of SnO2 to alumina surface increases the strength of acid sites of supported SnO2/Al2O3 2 3 0 samplesAccording to H0 ≤ to+1.5. the Also,titration at increasingresults, γ-Al SnOO2 content is weakly from acid 10 tooxide 25 wt.%, with theH concentration≤ +3.3 (Table 1). of acidThe additionsites raises of fromSnO2 1.2 toto alumina 1.8 mmol/g surface (Table increases1). the strength of acid sites of supported SnO2/Al2O3 IV 4+ VI 4+ According to the UV-Vis data, Sn or Sn ions with different coordination are present in studied samples. So, UV20−Vis spectrum 30 of 25SnO 40/Al 50O sample 60 shows a broad 70 line around 260 nm, 2 ο 2 3 VI 4+ 2θ, which is attributed to octahedral Sn ions in SnO2 phase (Figure4a). However, for 10SnO 2/Al2O3 and 20SnO2/Al2O3 samples the maximum intensity is observed at 200 nm that attributed to isolated

tetrahedral IVSn4+ ions [14] (Figure4a).

Figure 3. XRD patterns of zSnO2/Al2O3 samples after calcinations at 550 °C for 2 h.

According to the titration results, γ-Al2O3 is weakly acid oxide with H0 ≤ +3.3 (Table 1). The addition of SnO2 to alumina surface increases the strength of acid sites of supported SnO2/Al2O3

Colloids Interfaces2018, 2, x FOR PEER REVIEW 5 of 8 samples to H0 ≤ +1.5. Also, at increasing SnO2 content from 10 to 25 wt.%, the concentration of acid sites raises from 1.2 to 1.8 mmol/g (Table 1). According to the UV-Vis data, IVSn4+ or VISn4+ ions with different coordination are present in studied samples. So, UV−Vis spectrum of 25SnO2/Al2O3 sample shows a broad line around 260 nm, which is attributed to octahedral VISn4+ ions in SnO2 phase (Figure 4a). However, for 10SnO2/Al2O3 Colloidsand 20SnO Interfaces2/Al20192O3, 3samples, 16 the maximum intensity is observed at 200 nm that attributed to isolated5 of 8 tetrahedral IVSn4+ ions [14] (Figure 4a).

0,30 3 3

0,25 γ-Al O 2 3 0,20

0,15 Kubelka Munk Units 2 0,10 2

0,05

0,00 25SnO /Al O 200 250 300 350 400 450 500 2 2 3 Kubelka Munk Units Munk Kubelka Wavelength, nm Kubelka Munk Units Munk Kubelka 1 25SnO /Al O 1 2 2 3 10SnO /Al O 10SnO /Al O 2 2 3 2 2 3 20SnO /Al O 20SnO /Al O 2 2 3 2 2 3

0 0 200 225 250 275 300 325 3,7 464,8 4,9 E, eV Wavelength, nm

(a) (b)

Figure 4. UV-Vis diffusediffuse reﬂectancereflectance spectraspectra ofofz SnOzSnO22/Al/Al2OO33 samplessamples with with different different Sn Sn loading loading ( (aa);); optical energy band gap (b).

Figure 44bb illustratesillustrates thethe opticaloptical bandband gapgap (E(Egg) for these samples. It is known [[14]14] thatthat EEgg value for massive SnO2 is near 3.55 eV. As stated in [5,16,17], the larger SnO2 particle size, the lower Eg for massive SnO2 is near 3.55 eV. As stated in [5,16,17], the larger SnO2 particle size, the lower Eg values. The Eg values determined for studied samples are in the interval of 3.7–4.9 eV (Figure4b) what values. The Eg values determined for studied samples are in the interval of 3.7−4.9 eV (Figure 4b) coincide with similar data in [5,17]. Eg value for γ-Al2O3 is ~5.25 eV. Thus, for 20SnO2/Al2O3 sample, what coincide with similar data in [5,17]. Eg value for γ-Al2O3 is IV~5.254+ eV. Thus, for 20SnO2/Al2O3 the shift of Eg to 4.8 eV indicates the nanosized SnO2 species with Sn ions that absorb UV light at sample,200 nm (Figurethe shift4). of Eg to 4.8 eV indicates the nanosized SnO2 species with IVSn4+ ions that absorb UV light Synthesizedat 200 nm (Figure Sn-containing 4). oxides were tested in the conversion of 13 wt.% fructose-ethanol solution into ethyl lactate. The results are summarized in Table2. Without a catalyst, the slight Synthesized Sn-containing oxides were tested in the conversion of 13 wt.% fructose–ethanol dehydration of fructose to 5-hydroxymethylfurfural (5-HMF) is observed at 27% fructose conversion solution into ethyl lactate. The results are summarized in Table 2. Without a catalyst, the slight (Table2). dehydration of fructose to 5-hydroxymethylfurfural (5-HMF) is observed at 27% fructose conversion 1 (Table 2). Table 2. Product content of fructose conversion.

Conversion Content of Products, mol% Yield of Catalyst Table 2. Product content of fructose conversion1. of Fructose, % Fructose EL 5-HMF Others 3 EL, % 1 - 27 73Content - of Products, 4 mol% 23 - 2Catalyst SnO2 Conversion100 of Fructose, % - 2 85 13Yield of 2 EL, % Fructose EL 5-HMF Others3 3 10SnO2/Al2O3 95 5 34 37 24 32 1 4- 20SnO 2/Al2O3 2797 3 73 51- 374 23 9 49- 5 25SnO2/Al2O3 95 5 34 58 3 32 2 2 6 SnO20SnO2 2/Al2O3 100 -- 612 85 34 13 5 612 7 10SnO -5ZnO/Al O 100 - 56 14 30 56 3 10SnO2/Al2O2 3 2 3 95 5 34 37 24 32 EL—ethyllactate, 5-HMF—5-hydroxymethylfurfural; 1 Reaction condition: 1,5 g fructose, 0,68 g catalyst, 10 g 98% 4 20SnO2/Al◦ 2O3 2 97 3 3 51 37 9 49 ethanol, 160 C, 3 h; 3 mg K2CO3 was added to 10 g 98% ethanol; Others: β-D-Fructopyranose, benzoic acid, 5 5-methyl-2-furaldegyde,25SnO2/Al2O3 levulinic and formic95 acids and its esters. 5 34 58 3 32

6 20SnO2/Al2O32 100 - 61 34 5 61 ◦ On pure SnO2 at 160 C, 100% conversion of fructose occurs with the formation of acid 7 10SnO2-5ZnO/Al2O3 100 - 56 14 30 56 dehydration product—5-hydroxymethylfurfural (Table2). Levulinic and formic acids and their esters 1 are alsoEL—ethyllactate, formed, as it was5-HMF—5-hydroxymethylfurfural; in [18]. Reaction condition: 1,5 g fructose, 0,68 g 2 3 catalyst,Target ethyl 10 g lactate98% ethanol, is formed 160° withС, 3 34–53%h; 3 mg selectivity K2CO3 was at 95–100%added to fructose10 g 98% conversion ethanol; onOthers: prepared β-D-Fructopyranose, benzoic acid, 5-methyl-2-furaldegyde, levulinic and formic acids and its esters. SnO2/Al2O3 samples (Table2). However, the signiﬁcant amount of 5-hydroxymethylfurfural is also 13 ◦ formed (Table2). For instance, in C NMRspectrum of products obtained on 20SnO2/Al2O3at 160 C, the signals were observed from target ethyl lactate (175.8; 66,9; 61.5; 20.4; 14.2 ppm) and by-products: 5-hydroxymethylfurfural (178.0; 161.4; 152.1; 123.9; 110.1; 57.2 ppm), benzoic acid (172.8; 113.8; 130; 129.4; 128.5 ppm) and 5-methyl-2-furaldegyde (176.8; 159.8; 152; 124; 109.6; 14 ppm) (Figure5). The intensive ethanol signals at 56; 18.5 ppm and weak lines of unreacted fructose (104; 101.9; 82.8; 81.8; 80.8; 75.7; 75.6; 75.2; 63.6; 62.8; 60.9 ppm) were registered also (Figure5). The ratio of signal areas at Colloids Interfaces2018, 2, x FOR PEER REVIEW 6 of 8

On pure SnO2 at 160 °C, 100% conversion of fructose occurs with the formation of acid dehydration product—5-hydroxymethylfurfural (Table 2). Levulinic and formic acids and their esters are also formed, as it was in [18]. Target ethyl lactate is formed with 34–53% selectivity at 95–100% fructose conversion on prepared SnO2/Al2O3 samples (Table 2). However, the significant amount of 5-hydroxymethylfurfural is also formed (Table 2). For instance, in 13C NMRspectrum of products obtained on 20SnO2/Al2O3at 160°С, the signals were observed from target ethyl lactate (175.8; 66,9; 61.5; 20.4; 14.2 ppm) and by-products: 5-hydroxymethylfurfural (178.0; 161.4; 152.1; 123.9; 110.1; 57.2 ppm), benzoic acid (172.8; 113.8; 130; 129.4; 128.5 ppm) and 5-methyl-2-furaldegyde (176.8; 159.8; 152; 124; 109.6; 14 ppm) (Figure 5). The intensive ethanol signals at 56; 18.5 ppm and weak lines of Colloids Interfaces 2019, 3, 16 6 of 8 unreacted fructose (104; 101.9; 82.8; 81.8; 80.8; 75.7; 75.6; 75.2; 63.6; 62.8; 60.9 ppm) were registered also (Figure 5). The ratio of signal areas at 175.8; 178.0; 172.8; 176.8 ppm allows estimate selectivity of ethyl175.8; lactate 178.0; 172.8; formation. 176.8 ppm At allowsthat, preliminary estimate selectivity the calibration of ethyl lactate13C NMR formation. spectra At of that, ethyl preliminary lactate: 5-hydroxymethylfurfural:the calibration 13C NMR spectraethanol ofmixtures ethyl lactate: with molar 5-hydroxymethylfurfural: ratios of 0.5:1:10; 1:1:10; ethanol 1:2:10 mixtures have been with recorded.molar ratios of 0.5:1:10; 1:1:10; 1:2:10 have been recorded.

1313 FigureFigure 5. 5. CC NMR NMR spectrum spectrum of of fructose fructose conversion products onon 20SnO20SnO22/Al22OO33 (13%(13% fructose fructose solution solution inin ethanol, ethanol, 160 160 °◦СC,, 3 3 h; h; 100 100 MHz, MHz, NS=256, NS = 256, D0=4 D0 =s,4 D1=4 s, D1 μ =s 4(30°)).µs (30 ◦)).

TheThe addition ofof aa small small amount amount (0.03 (0.03 wt.%) wt.%) of potassiumof potassium carbonate carbonate to the to fructose–ethanol the fructose–ethanol initial initialmixture mixture raises raises the pH the value pH ofvalue a reaction of a reaction solution solu fromtion 6 from to 10. 6 Asto 10. a result, As a result, the yield the of yield ethyl of lactate ethyl lactateincreased increased due to due inhibition to inhibition of hexose of hexose dehydration dehydration (Table (Table2). The 2). same The same effect effect was observedwas observed in [ 13 in]. [13].Ethyl Ethyl lactate lactate selectivity selectivity increased increased from 53% from to61% 53% at to 100% 61% conversion at 100% ofconversion fructose whileof fructose simultaneous while simultaneousslight decrease slight in the decrease amount in of the dehydration amount of dehydration products on 20SnOproducts2/Al on2 O20SnO3 catalyst2/Al2O (Table3 catalyst2). Further (Table 2).increase Further in addedincrease potassium in added carbonate potassium up carbonate to 0.1 wt.% up leads to 0.1 to wt.% decrease leads of ethylto decrease lactate of selectivity ethyl lactate from selectivity61% to 49%. from 61% to 49%. WeWe have doped SnO2/Al/Al2O2O3 3withwith ZnO ZnO oxide oxide for for decreasing decreasing acidity acidity of of the the catalyst. catalyst. As As a a result, result, ethylethyl lactate lactate yield yield increased increased to to 56% 56% with significant signiﬁcant decrease decrease of 5-HMF content (Table 22).). AsAs seenseen fromfrom Table Table1 1,, thethe highest highest acid acid strength strength (H (H0max,0max,)) of of this this catalyst catalyst decreases decreases fr fromom +1.5 to to +3.3. At At that, that, in in comparisoncomparison with 10SnO22/Al22OO3,3 ,for for 10SnO 10SnO2–5ZnO/Al2–5ZnO/Al2O2O3 sample3 sample the the weak weak base base sites sites with with H H0=+7.20 = +7.2 at totalat total content—0.6mmol/g content—0.6mmol/g were were detected. detected. The The decreasing decreasing ofof 5-HMF 5-HMF formation formation was was observed observed on on the the 2+ Sn-Sn-ββ zeolitezeolite doped doped with with Zn Zn2+ ionsions also also in in [12]. [12]. ItIt should should be be noted noted that that in in the the experiments experiments we we ha haveve used used a 13% a 13% fructose fructose solution solution in inethanol ethanol at massat mass ratio ratio of offructose/catalyst fructose/catalyst = 2.4, = 2.4, whereas whereas usually usually the the weaker weaker concentrations concentrations of of 1–3% 1–3% with with fructose/catalystfructose/catalyst = = 1.4 1.4 were were used used [8–13]. [8–13]. The principal scheme of fructose–ethanol transformation into ethyl lactate is

known [8–10,13,15,16]. The ﬁrst step of the reaction is the aldol decondensation of fructose to glyceraldehyde and dihydroxyacetone. In acidic medium, the equilibrium shifts to gliceral formation. Further, gliceral dehydration occurs with the formation of pyruval that easily reacts with ethanol to form hemiacetal. Then, there is a subsequent isomerization of hemiacetal into ethyl lactate. The obtained results show that acid IVSn4+ L-sites catalyze aldol decondensation of fructose as the ﬁrst stage of the ethyl lactate formation (Scheme1a) as well as initiate the isomerization of hemiacetal into ethyl lactate (Scheme1b). Colloids Interfaces2018, 2, x FOR PEER REVIEW 7 of 8

The principal scheme of fructose–ethanol transformation into ethyl lactate is known [8–10,13,15,16]. The first step of the reaction is the aldol decondensation of fructose to glyceraldehyde and dihydroxyacetone. In acidic medium, the equilibrium shifts to gliceral formation. Further, gliceral dehydration occurs with the formation of pyruval that easily reacts with ethanol to form hemiacetal. Then, there is a subsequent isomerization of hemiacetal into ethyl lactate. The obtained results show that acid IVSn4+ L-sites catalyze aldol decondensation of fructose as Colloids Interfaces 2019, 3, 16 7 of 8 the first stage of the ethyl lactate formation (Scheme [a]) as well as initiate the isomerization of hemiacetal into ethyl lactate (Scheme [b]).

(a)

(b)

SchemeScheme. 1. PossiblePossible schemes schemes of of fructose fructose decondensation decondensation ( (aa)) and and the the rearrangement rearrangement of hemiacetal intointo ethylethyl lactatelactate ((bb)) onon acidacid IVIVSn4+ L-sites.L-sites.

4.4. ConclusionsConclusion EthylEthyl lactatelactate with 56% yield has been obtained fromfrom concentrated 13% fructose solutions in 98% ◦ ethanolethanol onon supportedsupported SnO–ZnO/AlSnO–ZnO/Al22OO33 catalystcatalyst at at 160 160 °C.C. At that, 100% fructose conversionconversion waswas IV 4+ observed.observed. ItIt isis supposedsupposed thatthat L-acidL-acid IVSn4+ sitessites provide provide both both C3–C4 C3–C4 aldol aldol decondensation of fructose andand subsequentsubsequent isomerisationisomerisation ofof formedformed hemiacetalhemiacetal ofof pyruvalpyruval intointo ethylethyl lactate.lactate.

AuthorAuthor Contributions:Contributions: Conceptualization,Conceptualization, V.V.B.;V.B.; investigation,investigation, S.V.P.S.P. andand N.L.H.;N.H.; data curation,curation, S.V.P.;S.P.; writing—originalwriting—original draftdraft preparation,preparation, S.V.P.;S.P.; writing—re writing—reviewview and and editing, editing, V.B.; V.V.B.; project project administration, administration, V.B. V.V.B.

Funding:Funding: This research receivedreceived nono externalexternal fundingfunding Conﬂicts of Interest: The authors declare no conﬂicts of interest. Conflicts of Interest: The authors declare no conflicts of interest.

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