Synthesis of Ethyl Levulinate as a Fuel Bioadditive by a Novel

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Synthesis of Ethyl Levulinate as a Fuel Bioadditive by a Novel Catalytically Active Pervaporation Membrane Derya Unlu and Nilufer Durmaz Hilmioglu* Department of Chemical Engineering, Kocaeli University, 41380 Iż mit, Kocaeli, Turkey ABSTRACT: The pervaporation catalytic membrane reactor (PVCMR) is an innovative process where the reactor is combined with a separation unit. The PVCMR consumes less energy than conventional processes and is an environmentally friendly process. The catalytic composite membrane is used in which catalysts are embedded onto the permselective membrane. The production process of ethyl levulinate is a progressive, green, energy-efficient technology by the PVCMR. In present paper, a catalytically active membrane is prepared and used in the synthesis of ethyl levulinate. Ethyl levulinate is one of the fuel bioadditives. A heterogeneous catalyst (silicotungstic acid) was embedded in the membrane. A hydrophilic hydroxyethyl cellulose membrane was prepared. Reaction parameters, such as the temperature, catalyst concentration in the membrane, and initial molar ratio of reactants, were investigated on the conversion of levulinic acid. The reaction performance of the PVCMR and batch reactor was compared, and it was found that the PVCMR showed higher performance with regard to the batch reactor.

1. INTRODUCTION As a result of unavoidable consumption of petroleum-based fuels, biofuels have become important in recent years. Different from other bio-based chemicals, ethyl levulinate (EL) has received a lot of attention. EL is synthesized by esterification of ethyl alcohol and levulinic acid (LA).1−3 Ethanol and LA are biomass-derived chemicals, and for this reason, the production of EL is known as a “environmental green process”. EL is used in the flavoring, aroma, and fuel sectors. With the exception of the superior properties of biodiesel, it has some drawbacks, such as weak oxidation stability and high cloud point and pour point. Also, biodiesel produces higher NOx emissions. The using of EL as a blending component to biodiesel causes the modifications in fuel properties. The addition of EL improves the cold flow properties, oxidation stability, and emission values of biodiesel. EL is a diesel-miscible biofuel, which can be directly used up to 5 wt % in diesel car engines.4−6 Synthesis of EL is generally carried out using homogeneous catalysts in a conventional batch reactor. Because of corrosive and toxic properties, the homogeneous catalyst has environmental problems.7 The pervaporation catalytic membrane reactor (PVCMR) is a potential alternative process to the batch reactor. The PVCMR is an innovative, green process for the production of EL. Furthermore, the synthesis of EL is carried out by a reversible esterification reaction. The conventional reaction system has lower conversion with respect to the thermodynamic limitations. More reactants should be used or more products should be removed for high conversion/ yield in the reaction. To improve the reaction conversion, reactive distillation can be used to remove the water. Large amounts of reactants and energy are required to reach high purities and yields in the reactive distillation.8 PVCMR is more economic than reactive distillation. The integration of reaction and separation units in one system is called the PVCMR.9 Composite membranes have double layers: the catalytic layer and the separation layer. While the reaction is carried out on the catalytically active surface of © XXXX American Chemical Society

the membrane, separation occurs on the separative surface of the membrane. PVCMR can overcome the equilibrium limitations for reversible reactions as a result of removal of the byproduct from the reaction mixture continuously.10 An appropriate polymeric medium can adjust the selective sorption of reactants, enhancing catalytic activity. The presence of spaces between the polymer molecules in the catalytic layer seems to lead to an increment of diffusivity of reactants through the membrane by a concentration gradient, and reactants reach the catalytic active sites without any resistance. Reaction occurs at the reaction mixture−membrane interface, and high conversions are displayed by the catalytic membrane. Pervaporation is a promising membrane process for separating components of liquid−liquid mixtures. In pervaporation, liquid feed contacts the surface of the membrane. The membrane allows for permeation of one or more of the components. A vacuum pump is used to create a driving force at the permeate side. The membrane has a higher affinity to one of the components in the feed mixture. This component selectively is absorbed into the membrane, diffuses across the membrane, and desorbs into vapor at the downstream side. Related component permates through the membrane from the feed side to the permeate side by the partial pressure difference.11,12 The PVCMR may reduce production costs in four ways: higher yield, faster reaction, purer products, and less energy.13−15 Hydroxyethyl cellulose (HEC) was chosen as the membrane material. HEC is a natural cellulose ether, and it forms compatible blends with polymers, such as water-soluble and derived from natural sources.16,17 The catalytic membrane was prepared using silicotungstic acid as a catalyst. Silicotungstic acid is a heteropolyacid catalyst Received: December 14, 2015 Revised: March 10, 2016

A

DOI: 10.1021/acs.energyfuels.5b02911 Energy Fuels XXXX, XXX, XXX−XXX

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60, and 70 °C), different molar feed ratios of reactants (only at the batch reactor) (M = ethanol/LA; M = 1:1, 2:1, and 3:1), and different catalyst concentrations (Ccat = ratio of weight of catalyst/polymeric membrane solution volume, g/L; Ccat = 2, 5, and 8 g/L) were examined. The activity of the catalytic membrane was measured in a stirred batch reactor fitted with a reflux condenser and magnetic stirrer with heating. Reactants were heated to reaction temperature separately. The catalytic membrane was cut into tiny equal pieces and added to the reaction mixture as a catalyst. Samples from the reaction mixture were taken periodically and analyzed by titration to calculate conversion values of LA. Phenolphthalein was used as an indicator. The sample was titrated with 0.1 M NaOH solution. Conversion values of LA were determined using the following equation (eq 1): a − at x= i ai (1)

in Keggin series. It presents high catalytic activity and stability. It has strong Brönsted acid sites. Silicotungstic acid is an efficient, environmentally friendly, and recyclable catalyst for the synthesis of EL.18,19 In this study, the silicotungstic-acid-loaded catalytic HEC membranes were prepared and used for the synthesis of EL. The esterification reaction of ethanol with LA occurred in a batch reactor and in a PVCMR. Experiments were carried out at the same reaction conditions, and the results are compared between two reactors. The effects of the reaction temperature (T), catalyst concentration of the membrane (Ccat), and initial molar ratio of reactants (M) on conversion of LA and separation performance were examined and interpreted. To date, there have been no studies on the synthesis of EL by the catalytic membrane in the membrane reactor found in the literature.

where ai is the initial acidity and at is the acidity at time t.21 The PVCMR experiment system is shown in Figure 2.22 The catalytic membrane was placed in the glass membrane cell. Before the

2. EXPERIMENTAL SECTION 2.1. Materials. HEC as the membrane material was purchased from Sigma-Aldrich. LA, ethanol, phosphoric acid, and sodium hydroxide were obtained from Merck Chemicals. The catalytic material silicotungstic acid and solvent isopropanol were supplied by Sigma-Aldrich. 2.2. Preparation of the Catalytic Membrane. The catalytic composite membrane was prepared from casting solutions with HEC and the coating technique. The composite membrane consisted of silicotungstic acid loading HEC top layer on HEC separation layer. HEC (5 wt %) was dissolved in distilled water at 25 °C, then the membrane solution was poured into a clean poly(methyl methacrylate) area at room temperature for preparation of the separation layer. Silicotungstic acid was used as a catalyst for the catalytic layer of the membrane. The HEC−silicotungstic acid catalytic membranes were prepared by a dissolution procedure using 2.5 wt % HEC and different concentrations of silicotungstic acid. It was then added to the mixture and agitated at 25 °C for 3 h. The catalytically active layer was cast on the separation layer. After the casting of the catalytic layer on the separation HEC layer, the obtained catalytic composite membrane was dried and cross-linked by immersing the membrane in a bath containing an isopropanol/water mixture (90%, v/v) with 3.5 vol % H3PO4 as the cross-linking agent at room temperature for 3 h.20 Figure 1 shows the structure of the HEC and also the HEC crosslinked with phosphoric acid. After the cross-linking bath, the

Figure 2. PVCMR configuration: (1) thermocouple, (2) agitator, (3) reflux condenser, (4) reactor, (5) membrane, (6−8) Dewar flasks, and (9) vacuum pump.

reaction starts, a heater was used to bring the reactants up to the reaction temperature separately. After heating of the reactants, the feed pump was started and the reaction mixture was fed into the membrane chamber. The effective membrane area is 28.26 cm2. The vacuum pump was used for diffusion of components through the membrane. Downstream pressure was reduced to 20 mbar using a vacuum pump. The catalytic composite HEC membrane has two layers. Synthesis of EL was carried out on the catalytic layer of the composite HEC membrane. A separation layer of the composite HEC membrane is capable of selectively permeating water, because HEC is a well-known hydrophilic polymer. The reaction and separation mechanism of the catalytic composite HEC membrane can be explained as follows. LA and ethanol were fed at the top of the membrane. Reactants are converted to products in the catalytic surface of the membrane. The products were EL and water. Because the catalytic layer has no permselective property, only water permeates through the separation layer. While water diffused through the membrane, EL diffused backward into the reaction medium. Water was permeated through the permselective membrane as the vapor phase as a result of the reduced pressure in the downstream of the membrane. Water was collected in the liquid phase using liquid nitrogen traps. Titration and gas chromatography methods were used for determination of the concentration of the samples and conversion of LA hourly. Conversion was calculated as written before in eq 1. The mass of the collected permeates was determined to use the calculation of total flux. Total flux was defined as eq 2

Figure 1. Structural representation of HEC cross-linked with phosphoric acid. membrane was placed into an oven for drying at 100 °C to remove the residuals. 2.3. Membrane Characterization. Characteristic properties of the catalytic composite membranes were determined by measuring the swelling degree and scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analyses. 2.4. Catalytic Experiments of Synthesis of Fuel Bioadditive EL. Synthesis of EL was carried out in a stirred batch reactor and in a PVCMR using the catalytic membranes. Different temperatures (50, B

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Figure 3. FTIR spectra of (a) uncross-linked pristine HEC, (b) cross-linked pristine HEC, (c) uncross-linked catalytic HEC, and (d) cross-linked catalytic HEC.

J=

wp St

(2)

where wp denotes the mass of the permeate, S is the membrane area, and t is the time. Total flux means all components include products and reactants, but water has the highest flux value, while EL and LA have much lower values. The separation ability of the membrane was determined by the water selectivity (α) calculated as follows in eq 3: α=

Pa /Pb Fa /Fb

(3)

where F and P represent the mass fractions of components in the feed side and permeate side and a and b symbolize the component of water and the other components in the sample.23 At the beginning of the reaction (t = 0), there is no water in the feed side. The concentration of water in the feed side (eq 3) means the concentration of water at any time. After reaction starts, flux and selectivity values are calculated hourly, and once reaction starts, ester and water form. Selectivity is determined using concentrations measured of water and other components. The separation ability of the membrane for water over other components is stated as water selectivity.

Figure 4. SEM images of the catalytic composite membrane: (a) surface and (b) cross-section.

The interaction of the separation layer and catalytic layer is seen in the pictures of the cross-sectional structure of the composite HEC membranes. The surface appearances of the membranes demonstrate that silicotungstic acid is dispersed homogeneously in the catalytic layer of the membrane. 3.3. Determination of the Best Membrane CrossLinking Method by the Sorption Experiment. The catalytic composite HEC membrane was cross-linked by different formulas. The influence of different cross-linking treatments on the sorption properties of the membranes was investigated using sorption experiments. The HEC membrane was cross-linked using glutaraldehyde (GA) and phosphoric acid (H3PO4).16,20 The sorption experiments were carried out at 25 °C for 48 h. The sorption degree of the membrane is calculated by eq 4 m − md sorption degree (%) = s × 100 md (4)

3. RESULTS AND DISCUSSION 3.1. FTIR. The FTIR spectra of the pristine HEC membrane, catalytic HEC membrane, and cross-linked pristine and catalytic membranes were scanned using a PerkinElmer Spectrum 100 FTIR spectrophotometer. Figure 3 shows the FTIR spectra of the pristine and catalytic HEC membranes. Spectra a and b of Figure 3 represent the uncross-linked pristine and cross-linked pristine HEC membranes, respectively. The characteristic O−H peak in HEC can be seen at 2900 cm−1 for all types of membranes. Spectra b and d of Figure 3 show the cross-linked pristine and catalytic HEC membranes, respectively. The hydroxyl groups of H3PO4 react with the hydroxyl group of HEC, and a covalent bond formed. The peak at 1260 cm−1 is confirmed as the specific absorbance of the −P−O−C− bond. The PO group is observed at 960 cm−1. Spectra c and d of Figure 3 show the uncross-linked and cross-linked catalytic HEC membranes, respectively. FTIR of silicotungstic acid has four characteristic bands at 798, 882, 919, and 970 cm−1. They are assigned to W−Oc−W, W−Ob−W, Si−O, and WO, respectively.17,20,24,25 3.2. SEM. Panels a and b of Figure 4 show a silicotungsticacid-coated HEC catalytic membrane.

where ms and md are the masses of the membrane in the swollen and dry statuses. The membranes were cut into small pieces, and dry weights were determined. Small pieces of the membranes were immersed in the aqueous ethanol solutions of different concentrations. The membranes were taken away from the mixture and dried after weighed regularly. The membrane pieces were then quickly put back in the mixture. The experiment was duplicated until the membranes stayed at a constant weight.26 Figure 5 shows how the degree of swelling changes with time and cross-linking method. When the degree of cross-linking increases, the cross-linker hinders the mobility of polymer C

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Figure 5. Water sorption characteristics of HEC membranes of different cross-linking methods.

Figure 6. Effect of the time on conversion of LA (T = 70 °C, and Ccat = 8 g/L).

chains. In this experiment, glutaraldehyde causes a higher crosslinking degree. When glutaraldehyde is used as a cross-linking agent, rigidity of the membrane increases. This situation results in decreasing the diffusivity. Accessibility of active sites of the membranes decreases. Therefore, conversion decreases. H3PO4 causes a lower cross-linking degree. Lower crosslinking provides free volume for diffusivity of reactants. Therefore, reactants easily diffuse through the catalytic membrane, and conversion increases. High conversions are also obtained by removing the water from a larger free volume of the membrane more easily. Because of these reasons, H3PO4 is chosen as the cross-linking agent. 3.4. Synthesis of Fuel Bioadditive EL. Synthesis of EL was taken place in a stirred batch reactor and in a PVCMR. The esterification reaction was performed using a catalytic membrane. In this study, while the reaction time and reusability of the catalytic membrane were determined in a batch reactor, the effects of the reaction parameters, such as the reaction temperature, catalyst concentration of the membrane, and initial molar ratio on LA conversion, were examined for both the batch reactor and PVCMR. 3.4.1. Effect of the Reaction Time on the Synthesis. The effect of the reaction time that is between 0 and 5 h was determined for the synthesis of EL at the different initial molar ratios of LA/ethanol (M = 2:1 and 3:1) in a stirred batch reactor, keeping the other reaction parameters constant (Ccat = 0.8 g/L, and T = 70 °C). Esterification reactions are equilibrium-limited, reversible reactions. Therefore, it is very difficult to obtain a really high conversion value, and it may also take a long time. Figure 6 presents the results of LA conversion versus time in the batch reactor with catalytically active membranes at different molar ratios. It shows that the conversion of LA rapidly increased in the reaction time range of 0−5 h with the increase in the initial molar ratio. There was a great increment from 0 to 5 h. The composition change of solution with time was determined by gas chromatography (GC) in a batch reactor. The composition in the reactor was not prominently changed. The reaction time was determined according to the batch reactor experiments. LA reached the equilibrium conversion after 5 h in the batch reactor. After 5 h, the conversion of LA increased slowly and remained constant. The conversion value is approximately 40% at equilibrium. This value can indicate an

important change from the point of composition. Therefore, the reaction time was selected as 5 h. 3.4.2. Effect of the Reaction Temperature on the Synthesis. The temperature of the reaction varied between 50 and 70 °C to assess its effect on the synthesis of EL in a stirred batch reactor and in a PVCMR. The experiments were carried out at M = 1:1 molar ratios of LA/ethanol and catalyst concentration of 8 g/L. The temperature has a great effect on the rate of reaction at which maximum conversion is achieved.27 The conversion values shown in Figure 7 remark that the conversion is sensitive

Figure 7. Effect of the reaction temperature on conversion of LA (Ccat = 8 g/L, and M = 1:1).

to changes in the temperature. Esterification of LA with ethanol is an endothermic reaction.3 Increasing the temperature is favorable for the acceleration of the forward reaction. Therefore, as the temperature increases, the reaction balance shifts in favor of products and conversion increases. An increment of production of EL and water is observed as the temperature increases. In contrast to the batch reactors, the conversion in the PVCMR with catalytically active membranes was higher. The LA conversion reaches 30.78 and 96.46% in the batch reactor D

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Figure 8. Effect of the reaction temperature on (a) total flux and (b) water selectivity (Ccat = 8 g/L, and M = 1:1).

and PVCMR at 70 °C, respectively. Using the PVCMR leads to an increase in LA conversion values by approximately 60%. This result is due to the effects of the temperature and the removal of water. In this study, the HEC composite catalytic membrane was used. HEC has hydroxyl groups that interact with water. Water was removed from the reaction mixture by the HEC membrane. In this case, reaction equilibrium changed; the forward reaction occurred more rapidly than the backward reaction.28 An increase in the temperature induced not only an acceleration of the esterification reaction rate but also acceleration in the rate of pervaporation. The water concentration has a higher value in higher reaction temperatures. In this situation, water permeation could be easy and total flux increased. The temperature also affects the chain mobility, flexibility, and free volume of the HEC membrane.29,30 The membrane was swollen and water-diffused easily through the membrane. Increasing the degree of the swelling of the membrane caused the diffusion of the other components from the membrane. As seen in Figure 8a, while total flux increased, in Figure 8b, selectivity of water decreased with the temperature. 3.4.3. Effect of the Catalyst Concentration of the Membrane on the Synthesis. LA was reacted with ethanol in the absence of any catalyst. The conversion value was obtained as 8% at the end of 24 h. It was understood that the reaction could not be catalyzed by itself. The catalyst loading in the membrane was investigated in the range of 2−8 g/L while keeping the other parameters constant. To determine the influence of the catalyst concentration on LA conversion, the initial molar ratio of ethanol/LA was fixed as 1:1 and the temperature was fixed as 70 °C. The experimental results for conversion of LA in the batch reactor and PVCMR over various catalyst concentrations were presented in Figure 9. The catalyst concentration is defined as the proportion of the amount of catalyst to polymeric membrane solution volume. Catalytic membranes were used as the catalyst in the stirred batch reactor and PVCMR. It can be seen that, as the concentration of catalyst increased, conversion increased, because the amount of catalyst enhances accessibility of active sites of the catalytic membrane. Increasing active sites exposes more catalyst particles to contact with reactants.31,32 Reactants were converted to products, and conversion of LA increased. As seen in Figure 9, when the catalyst concentration increased from 2 to 8 g/L, LA conversion increased from 75.84 to 96.48% in the PVCMR. In the stirred batch reactor, the highest

Figure 9. Effect of the catalyst concentration of the membrane on conversion of LA (M = 1:1, and T = 70 °C).

conversion was obtained as 30.78% at 8 g/L catalyst concentration. Increasing the catalyst concentration tends to increase the production rate of water. The water amount was continually increased at the beginning of the reaction. As the water concentration increased, the concentration gradient increased. The concentration gradient is a driving force in a PVCMR process. An increment concentration gradient results in increasing of flux.33 The synthesis of EL is a reversible equilibrium-limited esterification process. When the water is separated from the reaction medium by PVCMR, the possibility of ester hydrolysis disappeared. A high water concentration causes swelling of the hydrophilic HEC membrane and free volume expansion. Then, the permeation of other components, such as ethanol, becomes easier, and flux increases. Water selectivity decreased with time. The total flux and water selectivity values were seen from panels a and b of Figure 10. 3.4.4. Effect of the Initial Molar Ratio on the Synthesis. Esterification reactions are carried out in the initial molar ratio from M = 1:1 to 3:1 while keeping the reaction temperature at 70 °C and the catalyst concentration at 8 g/L. Figure 11 shows the effect of the different initial molar ratios of the reactants on the conversion of LA. From this figure, it can be seen that an increase of the initial molar ratio leads to an increase of the LA conversion. Esterification reactions are reversible and equilibrium-limited E

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Figure 10. Effect of the catalyst concentration of the membrane on (a) total flux and (b) water selectivity (M = 1:1, and T = 70 °C).

Catalytic membranes can be easily separated from the reaction medium, and reutilization experiments can be performed. Catalyst recycling is an important factor for principles of green chemistry. Therefore, catalytic stability and reusability of the catalytic membrane are important properties for the catalysts.37−39 The LA conversion versus runs are presented in Figure 12. The reutilization of the catalytic membrane was researched in

Figure 11. Effect of the initial molar ratio on conversion of LA (T = 70 °C, and Ccat = 8 g/L).

reactions. The reaction equilibrium may be influenced by removing one product from the reaction mixture or using an excess of one reactant.34−36 Both procedures result in obtaining good yields of ester with respect to Le Chatelier’s principle.3 In this experiment, to shift the reaction equilibrium toward the formation of the EL, excess ethanol was used in the stirred batch reactor. In the PVCMR, removal of water was used to obtain good conversion values of LA to EL. As seen in Figure 11, conversion values of LA were not much different in a batch reactor. Conversion was increased slightly with an increase of the initial molar ratio. However, for the PVCMR, there was a significant improvement in conversion values. The initial molar ratio has no more impact on the pervaporation rate. Therefore, the influence of the initial molar ratio on conversion was not studied in the PVCMR. The maximum conversion is up to 96.48% when the reaction was carried out at M = 1:1 in the PVCMR. When the initial molar ratio changed from M = 1:1 to 3:1, conversion of LA increased from 30.06 to 35.27% in the batch reactor. 3.4.5. Catalytic Membrane Reusability and Stability. Catalytic membranes have been synthesized and investigated for the synthesis of EL. Catalytic membranes are environmental friendly, cheap, recyclable, and reusable catalysts. Esterification of LA is usually carried out in the liquid phase using homogeneous acid catalysts in the literature. These catalysts have some disadvantages. The catalytic membranes were used as green alternatives to homogeneous catalysts in this study.

Figure 12. Catalytic membrane reusability and stability in the batch reactor (T = 70 °C, M = 1:1, and Ccat = 8 g/L).

the reaction of LA with ethanol in the presence of 8 g/L catalyst concentration of the membrane in the batch reactor. The catalytic membrane can be easily handled and separated from the reaction medium. The recovered catalyst was tested consecutively 5 times. A similar LA conversion was obtained from the first to the fifth use. The recovered catalytic membrane showed the same catalytic activity. This result demonstrates the high stability of the catalytic membrane. From these results, it can be concluded that the catalytic membrane can be reused with no loss in catalytic activity during the 5 cycles.

4. CONCLUSION EL is an important renewable fuel bioadditive. In this study, silicotungstic-acid-loaded catalytic membranes were prepared and used for EL production. The catalytic membrane was found to be very active and stable. The effects of different variables, F

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such as the temperature, catalyst concentration, and initial molar ratio of Ethanol\LA, were investigated to calculate LA conversion. In comparison of our results to other data in the literature, PVCMRs are excellent candidates for synthesis of EL. Furthermore, it can be seen that the PVCMR has higher efficiency with respect to the conventional batch reactor. Optimum reaction conditions were determined as T = 70 °C, M = 1:1, and Ccat = 8 g/L. Under these conditions, permeation composition was 4 wt % EL, 6 wt % LA, 20 wt % ethanol, and 70 wt % water. These results indicate that the PVCMR is a potential green technology to the traditional production technology. The usage of the PVCMR for the synthesis of EL is economical and friendly to the Earth technology for an industrial scale in the future.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +902623033545. E-mail: [email protected] and/or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported financially by the Scientific and ̇ AK, Project Technological Research Council of Turkey (TÜ BIT 114M147) and Kocaeli University Scientific Research Projects Unit (Project 2014/076).

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NOMENCLATURE α = selectivity ai = initial acidity (mol) at = acidity at time t (mol) Ccat = catalyst concentration (g/L) EL = ethyl levulinate Fa and Fb = weight fractions of i and j components in the feed J = flux (kg m−2 h−1) LA = levulinic acid ms = weights of swollen membranes md = weights of dried membranes M = initial molar ratio Pa and Pb = weight fractions of i and j components in the permeate PVCMR = pervaporation catalytic membrane reactor S = effective membrane area (cm2) t = time (h) x = conversion of levulinic acid wp = permeate weight (g)

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b02911 Energy Fuels XXXX, XXX, XXX−XXX