Conversion of D-Fructose into Ethyl Lactate Over a Supported SnO 2 – ZnO / Al 2 O 3 Catalyst

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.


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][2][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][5][6][7][8].Some progress was made in the synthesis of alkyl lactates from dihydroxyacetone [4][5][6] and glycerol [7].In a particular, the direct synthesis of ethyl lactate from glycerol solution in ethanol on CeO 2 /Al 2 O 3 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][9][10][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.

Materials and Methods
Pure SnO 2 was prepared by precipitation from solution of SnCl  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 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 S sp 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 E 0 , the reflectance spectra were recalculated to absorption spectra using Kubelka-Munk formula, F = (hν(1 − R) 2 /2R) 1/2 .E 0 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 13 C NMR spectroscopy (Bruker Avance 400, Karlsruhe, Germany).The conversion values of fructose (X) and selectivity of products (S, mol%) were calculated from 13 C NMR spectra.Yields were calculated as Y = S•X.

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 SnO 2 /Al 2 O 3 oxides are summarized in Table 1.According to the thermogravimetric analysis of as-calcined 20SnO 2 /Al 2 O 3 sample (Figure 1), the condensation of structural OH-groups with water release is observed at 300 According to the thermogravimetric analysis of as-calcined 20SnO2/Al2O3 sample (Figure 1), the condensation of structural OH-groups with water release is observed at 300 °C, and crystallites of SnO2 are formed at 450-550 °C.The small decrease (10-18%) in the surface area of alumina support occurs upon the deposition of tin oxide what probably explained by blocking of the aluminum pores with the precipitated oxide (Table 1).As seen from the pore size distribution curves (Figure 2), derived from the desorption branches of isotherms using the DFT method, the deposition of SnO2 on alumina surface leads to decrease in pore content with r ≤ 3 nm for 10SnO2/Al2O3 and 20SnO2/Al2O3 samples.At the same time, for 25SnO2/Al2O3 sample, a decrease in pore content with r~4-5 nm is observed.Obviously, the SnO2 crystallites with sizes of more than 3 nm are formed at increase in the content of supported tin dioxide.The small decrease (10-18%) in the surface area of alumina support occurs upon the deposition of tin oxide what probably explained by blocking of the aluminum pores with the precipitated oxide (Table 1).As seen from the pore size distribution curves (Figure 2), derived from the desorption branches of isotherms using the DFT method, the deposition of SnO 2 on alumina surface leads to decrease in pore content with r ≤ 3 nm for 10SnO 2 /Al 2 O 3 and 20SnO 2 /Al 2 O 3 samples.At the same time, for 25SnO 2 /Al 2 O 3 sample, a decrease in pore content with r~4-5 nm is observed.Obviously, the SnO 2 crystallites with sizes of more than 3 nm are formed at increase in the content of supported tin dioxide.
The structural analysis by XRD shows that at loadings ≤20 wt.% SnO 2 no characteristic peaks assigned to tin oxide were observed (Figure 3).It indicates high dispersion of tin oxide on the alumina surface.Small peaks appeared at 2θ = 26.6 • ;33.9 • ;38.0 • and 51.8 • corresponding to tetragonal SnO 2 for 25SnO 2 /Al 2 O 3 sample were detected (Figure 3).The structural analysis by XRD shows that at loadings ≤20 wt.% SnO2 no characteristic peaks assigned to tin oxide were observed (Figure 3).It indicates high dispersion of tin oxide on the 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 3).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  The structural analysis by XRD shows that at loadings ≤20 wt.% SnO2 no characteristic peaks assigned to tin oxide were observed (Figure 3).It indicates high dispersion of tin oxide on the 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).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 According to the titration results, γ-Al 2 O 3 is weakly acid oxide with H 0 ≤ +3.3 (Table 1).The addition of SnO 2 to alumina surface increases the strength of acid sites of supported SnO 2 /Al 2 O 3 samples to H 0 ≤ +1.5.Also, at increasing SnO 2 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, IV Sn 4+ or VI Sn 4+ ions with different coordination are present in studied samples.So, UV−Vis spectrum of 25SnO 2 /Al 2 O 3 sample shows a broad line around 260 nm, which is attributed to octahedral VI Sn 4+ ions in SnO 2 phase (Figure 4a).However, for 10SnO 2 /Al 2 O 3 and 20SnO 2 /Al 2 O 3 samples the maximum intensity is observed at 200 nm that attributed to isolated tetrahedral IV Sn 4+ ions [14] (Figure 4a).
According to the UV-Vis data, IV Sn 4+ or VI Sn 4+ 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 VI Sn 4+ ions in SnO2 phase (Figure 4a).However, for 10SnO2/Al2O3 and 20SnO2/Al2O3 samples the maximum intensity is observed at 200 nm that attributed to isolated tetrahedral IV Sn 4+ ions [14] (Figure 4a).  Figure 4b illustrates the optical band gap (Eg) for these samples.It is known [14] that Eg value 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 3.7−4.9eV (Figure 4b) what coincide with similar data in [5,17].Eg value for γ-Al2O3 is ~5.25 eV.Thus, for 20SnO2/Al2O3 sample, the shift of Eg to 4.8 eV indicates the nanosized SnO2 species with IV Sn 4+ ions that absorb UV light at 200 nm (Figure 4).
On pure SnO 2 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].
We have doped SnO2/Al2O3 with ZnO oxide for decreasing acidity of the catalyst.As a result, ethyl lactate yield increased to 56% with significant decrease of 5-HMF content (Table 2).As seen from Table 1, the highest acid strength (H0max,) of this catalyst decreases from +1.5 to +3.3.At that, in comparison with 10SnO2/Al2O3, for 10SnO2-5ZnO/Al2O3 sample the weak base sites with H0=+7.2 at total content-0.6mmol/gwere detected.The decreasing of 5-HMF formation was observed on the Sn-β zeolite doped with Zn 2+ ions also in [12].
It should be noted that in the experiments we have used a 13% fructose solution in ethanol at mass ratio of fructose/catalyst = 2.4, whereas usually the weaker concentrations of 1-3% with fructose/catalyst = 1.4 were used [8-13].The addition of a small amount (0.03 wt.%) of potassium carbonate to the fructose-ethanol initial mixture raises the pH value of a reaction solution from 6 to 10.As a result, the yield of ethyl lactate increased due to inhibition of hexose dehydration (Table 2).The same effect was observed in [13].
Ethyl lactate selectivity increased from 53% to 61% at 100% conversion of fructose while simultaneous slight decrease in the amount of dehydration products on 20SnO 2 /Al 2 O 3 catalyst (Table 2).Further increase in added potassium carbonate up to 0.1 wt.% leads to decrease of ethyl lactate selectivity from 61% to 49%.
We have doped SnO 2 /Al 2 O 3 with ZnO oxide for decreasing acidity of the catalyst.As a result, ethyl lactate yield increased to 56% with significant decrease of 5-HMF content (Table 2).As seen from Table 1, the highest acid strength (H 0max, ) of this catalyst decreases from +1.5 to +3.3.At that, in comparison with 10SnO 2 /Al 2 O 3 , for 10SnO 2 -5ZnO/Al 2 O 3 sample the weak base sites with H 0 = +7.2 at total content-0.6mmol/gwere detected.The decreasing of 5-HMF formation was observed on the Sn-β zeolite doped with Zn 2+ ions also in [12].
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 IV Sn 4+ L-sites catalyze aldol decondensation of fructose as the first stage of the ethyl lactate formation (Scheme 1a) as well as initiate the isomerization of hemiacetal into ethyl lactate (Scheme 1b).
Diffraction patterns were identified by comparing with those from the JCPDS (Joint Committee of Powder Diffraction Standards) database.

Table 1 .
Textural parameters and total acidity of synthesized SnO 2 /Al 2 O 3 samples.

Table 2 .
Product content of fructose conversion1.