Insights into the Synthesis of Ethyl Levulinate ... - ACS Publications

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Insights into the Synthesis of Ethyl Levulinate under Microwave and Nonmicrowave Heating Conditions Ejaz Ahmad, Md. Imteyaz Alam, K.K. Pant,* and M. Ali Haider* Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016, India

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S Supporting Information *

ABSTRACT: The effects of microwave and nonmicrowave heating methods on the synthesis of ethyl levulinate (EL) from levulinic acid (LA) have been investigated in the present study. The levulinic acid esterification experiments were performed in the presence of silicotungstic acid catalyst and ethanol in a microwave and nonmicrowave instant heating reactor. An experimental fit of experimental data in a kinetic model suggested that LA esterification follows a pseudo-first-order reaction mechanism. Consequently, activation barriers were calculated (44−45 kJ/mol) and a negligible difference was found for LA esterification reaction performed in both the reactors. Nevertheless, slightly higher LA conversions were measured under microwave irradiations as compared to experiments performed in a nonmicrowave instant heating reactor. Thus, series of experiments were performed to study the (i) nonthermal and (ii) thermal effects of microwave heating irradiations. Eventually, it was found that the enhanced LA conversion in microwave reactor could be due improved heat transfer into the reaction mixture owing to direct heating by microwaves, as compared to conventional conductive and convective heat transfer in the instant heating reactor. In addition, experiments performed under different operating conditions such as at varying stirring speed, varying sample volume, and different catalyst concentration and reactant concentration indicated the absence of nonthermal microwave effects.

1. INTRODUCTION Ethyl levulinate (EL) is an emerging biomass-derived platform chemical widely used as a fuel additive,1 fragrance and flavoring agent,2 solvent,3 and a precursor for gamma valerolactone4 and to produce other important classes of chemicals.5 Moreover, blending of ethyl levulinate in fossil-based existing diesel and gasoline improve overall fuel properties6 besides reducing particulate matter emissions and smoke number.7 Furthermore, a computational study on EL blending has shown promising results as a replacement for carcinogenic aromatics and MTBE used in gasoline, thereby creating a new window of opportunity toward the development of reformulated “green gasoline”.8 The EL and other alkyl levulinates can be synthesized through alcoholysis of lignocellulosic biomass or biomass-derived platform chemicals in the presence of a suitable acid catalyst.5 However, direct conversion of biomass to ethyl levulinate is a tedious task because of the presence of lignin9 and formation of undesired products such as humin. On the contrary, several efficient and sustainable technologies have been developed to produce a vital platform chemical levulinic © XXXX American Chemical Society

acid (LA) from biomass, and biomass-derived other molecules via a series of reactions involving depolymerization, hydrolysis, isomerization, dehydration, and rehydration.10 Indeed, LA is included among most promising platform chemicals identified by the U.S. Department of Energy11 and further appreciated by other prominent research groups working in this area.12,13 Notably, LA on esterification yields EL in one pot process under appropriate reaction conditions in the presence of a suitable catalyst in ethanol. It is well-known that the EL yield from LA and other biorenewable resources depends on several factors such as the type and acid strength of catalysts, reaction time and temperature, as well as the molecular structure of reactants.14 Special Issue: Biorenewable Energy and Chemicals Received: Revised: Accepted: Published: A

February 28, 2019 May 1, 2019 May 6, 2019 May 6, 2019 DOI: 10.1021/acs.iecr.9b01137 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

absorption of all the microwaves in the silicon carbide vessel is not possible, and thus the possibility of microwaves passing partially into the reaction mixture cannot be ruled out.30 Furthermore, different vessel materials and dimensions can contribute significantly to the overall reaction efficiency because of the resonant nature of the microwave field.31 The above cited reasons necessitate further studies on the effect of microwave irradiation and conventional heating in esterification reactions. Thus, the present study investigates the effects of microwave and nonmicrowave heating conditions on the LA esterification reaction in the presence of an acid catalyst. The Anton Parr monowave-50 (NMW) reactor has a conventional but instant electrical heating system and resembles the Anton Parr monowave-300 microwave reactor (MW) regarding constructions and other features that have been used in the present study. Both the reactors have similar design, heating, and control features although the mode of heating is different in the conventional monowave-50 reactor, which is heated using electrical rods that can produce a maximum 315 W of power, whereas the monowave-300 uses heavy power magnetron for microwave irradiations up to 850 W. A special single cavity design of the monowave-300 reactor ensures high field density availability, thereby causing quick heating of the sample. An in-built IR system was used for the temperature measurement in the monowave-300, whereas a PT-100 temperature sensor was used in the monowave-50 reactor. All experiments were performed under identical operating parameters in both the MW and NMW reactors using silicotungstic acid catalyss,t which showed that enhanced LA conversion under microwave irradiations can be explained in terms of thermal effects only.

For example, Popova et al. reported the application of sulfated tin oxide (SO42−/SnO2) for LA esterification and measured up to 77% LA conversion at 343 K in 420 min.15 The authors have stated a combined synergistic effect of Lewis and Brønsted acid sites as a key factor for the LA conversion. However, a major challenge associated with SO42−/SnO2 catalyst was leaching of active acid sites, for which authors measured up to 12% loss in three reactions. Nevertheless, the catalyst leaching challenges can be overcome by replacing a sulfate group with a sulfonic group loaded over a suitable support such as SBA-15; however, this comes at the cost of LA conversion for experiments performed under similar conditions.16 Recently, Luan et al. reported up to 98% LA conversion with 100% EL selectivity under reflux reaction conditions in the presence of quinaldic acid heterogenized silicotungstic acid.17 The authors measured a similar value for LA conversion in one of the experiments performed in the presence of parent silicotungstic acid. However, a detailed study on LA esterification and parameters optimization is yet to be done for the effects of unsupported silicotungstic acid catalyst. Similarly, it is reported that the heating method is another critical factor that affects esterification reaction rate and efficiency with respect to reactant conversion as well as desired product selectivity.18 The fundamental of microwave heating lies in the fact that the microwave radiation heats only solvents and reactants without heating the wall of the vessel while passing through it.19 On the contrary, a conventional heating system transfers heat via conduction and convection which also heats the wall of the vessel, thus a slow heating rate.20 Thus, application of microwave heating irradiations has been attempted by prominent research groups worldwide in a variety of reactions to improve the overall efficiency of the process and shorten the reaction time.21 Eventually, several reason such as minimized temperature gradient,22 the interaction between reactants and microwaves,23 reduced local heating effects,24 microwave dielectric heating, solvents superheating effects, selective heating, molecular radiation, and elimination of wall effects25 have been hypothesized to explain the enhanced efficiency of the process carried out under microwave heating irradiations. Nevertheless, the researcher community is yet to reach a conclusive decision on the effect of microwave irradiations.25−28 In this regard, Yadav and co-workers have studied the effect of microwave irradiation (CEM make microwave reactor) and conventional heating (water bath heated over a Remi magnetic stirrer) system for synthesis of butyl levulinate and measured approximately equal activation barrier under both, although the reaction under microwave irradiation proceeded at a faster rate.29 The authors suggested that the increase in preexponential factor of the Arrhenius equation under microwave irradiation may have caused an enhanced conversion and product formation rate. However, uniform heat distribution through a heating mantle to the water bath is complex and challenging, and thus a comparative study was needed for further validation using a setup with more similarity and resemblance to a microwave reactor. Koshima and co-workers studied the effects of microwave irradiations and conventional heating system using a quartz vessel and a silicon carbide vessel in the microwave reactor.18 The authors noticed improvement in the ester yield when experiments were performed in a quartz vessel, which allows passage of microwaves, as compared to experiments performed in the silicon carbide, which reportedly does not allow microwaves to pass. However, complete

2. MATERIALS AND METHODS 2.1. Materials. All reagents and chemicals (with 99.9% purity) were procured commercially from reputed vendors and used without further purification and modification. The silicotungstic acid, levulinic acid, and ethyl levulinate were procured from the Sigma-Aldrich Chemicals Pvt. Ltd. (India), and alcohols used in the present study were procured from the Merck, India. HPLC grade water was purchased from the Rankem India whereas syringe filters (0.2-μm nylon filter) were procured from Axiva India. EL synthesis experiments were performed in monowave-50 (NMW) and monowave-300 (MW) reactors (Make: Anton Parr). Both, the monowave-50 and monowave-300 reactors are equipped with an inbuilt magnetic stirrer to maintain uniform temperature distribution in the reaction mixture. The 10 mL borosilicate glass vial with 2 mL minimum and 6 mL maximum operational volume was used in both the reactors. In general, 5 mL of alcohol volume was used for the experimental purpose, as suggested in previous reports by different research groups for microwave effects study.32,33 The monowave-300 reactor is reported to have better output because of its high power density.34 2.2. Experimental Procedure. The reactions were performed in synthesis reactors, comprising a stainless steel heating jacket or microwave irradiation source, and integrated pressure and temperature control system. In a typical experiment, LA (2 mmol, 232.22 mg), analytical grade pure ethanol (5 mL), and silicotungstic acid (0.01 mmol, 28.78 mg) were charged in a glass tube (10 mL capacity). A magnetic bead was kept inside the tube for properly stirring the reaction mixture during the experiment. The vials containing the reactant and catalyst mixture were closed and kept in the cavity B

DOI: 10.1021/acs.iecr.9b01137 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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present study. In particular, the peaks obtained at wavenumbers 798 and 878 cm−1 shows the presence of edgesharing and corner-sharing W−O−W bonds, respectively, whereas peaks obtained at wavenumbers 926 and 1018 cm−1 shows the presence of Si−O stretching mode bonds of the Keggin structure of the silicotungstic acid. Similarly, peaks obtained at wavenumber 980 cm−1 shows the presence of W O bond, thereby further confirming the Keggin structure of the silicotungstic acid. In addition, an out-of-plane O−H bond shows a peak obtained at wavenumber 1630 cm−1, which indicates that employed silicotungstic acid could have gained some moisture from the atmosphere during the analysis. The results obtained are in line with earlier reports, which suggests that the Keggin structure of the parent anion [SiW12O40]4− shows characteristic bands between wavenumbers 700 and 1100 cm−1 because of vibrations of edge-sharing W−O−W and corner-sharing octahedra WO6 linked to tetrahedra SiO4.39 To further confirm the Keggin structure of the employed silicotungstic acid, the Raman shift was measured. Accordingly, the distinct peaks of the primary Keggin structure were found for the Si−O central atom at 927 cm−1, for terminal WO bonds at 996 cm−1, and for bridging W−O−W bonds at 887 cm−1, along with out-of-plane stretching vibrations of W−O− W between 170 and 220 cm−1 Raman shift values as shown in Figure S3. Thus, the results obtained from the Raman analysis further confirms the Keggin structure of the silicotungstic acid used in the present study. Moreover, nitrogen physisorption studies revealed 1.64 m2/g BET surface area, which is comparable with the earlier reported results.40 Furthermore, ammonia temperature-programmed desorption study was performed in a micromeritics TPx analyzer (chemisorb 2720) equipped with a thermal conductivity detector. In result, four distinct peaks at 198, 288, 412, and 598 °C were observed, representing weak (peaks at 198 and 288 °C), medium (peak at 412 °C), and strong (peak at 598 °C) acidities of the catalyst as shown in Figure S4. Accordingly, total 0.60 mmol/g weak plus medium acidity, and 0.84 mmol/g strong acidity was measured respectively, thereby resulting in 1.44 mmol/g total acidity of the silicotungstic acid. Overall, a greater number of strong acid sites were measured for the silicotungstic acid as compared to weak acid sites. Thus, it can act as a suitable catalyst for the esterification reaction. 3.1.2. Effect of Microwave (MW) vs Nonmicrowave (NMW) Instant Heating System. Preliminary experiments were performed to screen the effect of microwave irradiation and nonmicrowave instant heating reactor at 353 K temperature for 5−90 min in the presence of 0.01 mmol silicotungstic acid for esterification of 2 mmol LA into EL in 5 mL ethanol. Initially, 16 and 10% LA conversion with 100% EL selectivity was measured under microwave irradiation (MW) and nonmicrowave instant heating system (NMW), respectively, as shown in Figure 5 when experiments were performed for 5 min. On extending the reaction time from 5 min until 90 min, a further increase in LA conversion was observed under both MW and NMW reactors. Eventually, maximum 85% LA conversion was achieved under microwave irradiations at 353 K temperature in 90 min whereas NMW reactor caused 79% LA conversion under identical reaction conditions. The results obtained show enhanced LA conversion under microwave irradiation as compared to a nonmicrowave heating system, as shown in Figure 1. It is argued that there could be three possible effects under microwave irradiations, namely (i) thermal effects, (ii) specific

of the corresponding monowave-50 or monowave-300 reactor. Prior to pressing the start button, the desired temperature, time, and rpm were set to begin the reaction. The heating parameter was set “as fast as possible” in both the reactors, which allows for reaching the desired reaction temperature in the shortest possible time (as shown in Figure S5). Eventually, the product was allowed to cool to room temperature and the product mixture was characterized. It should be noted that all experiments were performed in batch reactors separately at all temperatures and each reaction interval. 2.3. Products Analysis. The obtained product mixtures were filtered through a 0.2-μm nylon filter and analyzed using high-performance liquid chromatography (HPLC, Agilent make 1200 infinity series) equipped with Aminex HPX-87H column, UV and RI detectors followed by subsequent confirmation using gas chromatography equipped with a flame ionization detector (GC-FID) and DB-5MS column. Also, the effect of reaction time, temperature, feed concentration, catalyst concentration, and feed to ethanol ratio were studied. Eventually, reaction time was varied at each temperature to develop a suitable kinetic model. The reactant conversion, desired product selectivity, and yields were calculated as follows: %conversion =

CAo − CA 100 CAo

%product selectivity =

%product yield =

CB 100 CAo − CA

CB 100 CAo

Where, CAo and CA, are initial and final concentration of the reactant in mmoles and CB is a concentration of product formed in mmoles. However, nearly 100% EL selectivity was measured for all experiments; thus, EL yield was equal to LA conversion in both the reactors.

3. RESULTS AND DISCUSSIONS 3.1.1. Catalyst Characterization Results. The silicotungstic acid, in general, is represented by the formula H4SiW12O40·nH2O, where n can be 0, 6, 14, or 24 depending upon the methods of preparation and drying. 35 The silicotungstic acid employed in the present study was anhydrous and used on an as-received basis without further modification. However, it was noticed that the silicotungstic acid absorbs moisture rapidly when left open in the air. Thus, XRD analysis was performed to study the structure of the silicotungstic acid, and it was noticed that diffraction pattern obtained (as shown in Figure S1) matches the diffraction pattern reported for the anhydrous silicotungstic acid.35 Moreover, peaks obtained between 7 and 10°, 15 and 21°, 26 and 30°, and 35 and 38° exhibit the secondary structure of the silicotungstic acid crystals formed by Keggin anions.36,37 The results obtained were further confirmed to detect the presence of the Keggin units with the help of FTIR and Raman spectroscopy, as shown in Figures S2 S3, respectively. In general, the characteristic peaks of silicotungstic acid in the FTIR are found between 780 and 1018 cm−1 wavenumbers.38 The peaks obtained at 795, 787, 926, 980, and 1018 cm−1 wavenumbers as shown in Figure S2 confirm the Keggin structure of the silicotungstic acid employed in the C


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during the reaction. Overall, enhanced LA conversion under microwave irradiations can be attributed to thermal effects alone, as no significant difference in LA conversion was measured for longer reaction times in both MW and NMW reactors. The results obtained are in line with the majority of published articles, which suggests that enhanced conversion in microwave reactors can be explained in terms of kinetics/ thermal effects alone.42 A further detailed kinetic analysis on the effect of microwave heating irradiations and conventional instant heating system is discussed in subsequent sections. 3.1.3. Effect of Stirring Speed and Sample Volume. The silicotungstic acid readily solubilizes in the solvent media to form a homogeneous reaction mixture. On the contrary, it is found in a solid state at room temperature when not mixed in the liquid solvent media, which necessitates studying the effect of mass transfer limitations for a detailed understanding of the role of catalysts physical properties during the LA esterification reaction. Furthermore, it is reported that single mode microwave reactors cause the formation of hot zones and cold zones into the reaction mixture because of the localized heating mechanism.31 In contrast, heat is transferred to the reaction media via conduction and convection in the NMW reactor, which essentially requires efficient stirring for homogeneous distribution of heat.43 Therefore, experiments were performed at different stirring speeds in both the MW and NMW reactor to study the effect of mass transfer limitations as well as localized heating. Interestingly, no significant change in LA conversion was measured with respect to change in the stirring speeds from 300 to 900 rpm of the reaction mixture, as shown in Figure 2.

Figure 1. LA conversion measured for experiments performed in an MW and NMW reactor at 353 K temperature for 5−90 min in the presence of 2 mmol of LA, 0.01 mmol of silicotungstic acid catalyst, and 5 mL of ethanol at 300 rpm stirring speed.

thermal effects, and (iii) nonthermal effects, that cause enhanced activity of the reaction.41 Thermal effects are primarily caused by improved heat transfer under microwave irradiations, thereby causing enhanced bulk temperature, whereas specific thermal effects are caused by hot spots and liquid overheating. On the contrary, nonthermal microwave effects are hypothesized to be caused by either an increase in pre-exponential factor or decrease in activation energy under microwave irradiations. Interestingly, it was noticed that the difference in LA conversion (MW-NMW) was significantly higher and in increasing order, i.e., 6, 13, and 14% for 5, 15, and, 30 min reaction time, respectively; thereafter, no definite trend for the difference in LA conversion (MW-NMW = 7, 8, 6%,and 5% for 45, 60, 75, and 90 min reaction time, respectively) was observed. Thus, it is hypothesized that higher LA conversion for the first 30 min for experiments performed in the MW reactor could be due to an improvement in heat distribution with time that reaches maximum heat uniformity in 30 min. Moreover, no significant difference in LA conversion in MW and NMW reactors was measured post 30 min of reaction time, which further confirms our hypothesis of thermal effects. The results obtained are in line with earlier reports, which suggests that the homogeneity in heat distribution under microwave irradiation improves with time.31 It is worth noting that the heat transfer in the NMW reactor proceeds via electrical energy conversion into heat energy followed by heat conduction through a glass wall and thus subsequent transfer into the liquid reactant mixture via convection. Therefore, achieving complete homogeneous and uniform heat distribution in the NMW reactor might have taken more time as compared to the MW reactor because of heat loss and wall effects.42 Therefore, a higher LA conversion was measured in the MW reactor, providing core heating, as compared to NMW reactor having possible wall effects during the initial 30 min of reaction time. Therefore, it is hypothesized that uniform heat distribution in an NMW reactor took a longer time as compared to the MW reactor under identical experimental conditions. Moreover, the EL yield remained equal to LA conversion, indicating that the product did not decomposed

Figure 2. Effect of stirring speed on LA conversion for experiments performed in an MW and NMW reactor at 353 K temperature for 5− 90 min in the presence of 2 mmol of LA, 0.01 mmol of silicotungstic acid catalyst, and 5 mL of ethanol at 300−900 rpm stirring speed.

The results obtained indicate the absence of external mass transfer limitations. It was found that the stirring speed between 300 and 900 rpm does not cause any kind of temperature gradient, uneven power density distribution, or formation of hot zones and cold zones in the reaction mixture. Thus, it is concluded that the stirring speed does not cause any significant variation LA conversion under both the MW and NMW reactions, and thus the possibility of nonthermal effects was absent. Under optimum operating conditions at 353 K for D


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Figure 3. Effect of ethanol volume on LA conversion for experiments performed in (a) MW and (b) NMW reactor at 353 K temperature for 5−90 min in the presence of 2 mmol of LA, 0.01 mmol of silicotungstic acid catalyst, and 3−6 mL of ethanol at 300 rpm stirring speed.

Figure 4. Effect of feed concentration on LA conversion for experiments performed in (a) MW and (b) NMW reactor at 353 K temperature for 5− 90 min in the presence of 1−5 mmol of LA, 0.01 mmol of silicotungstic acid catalyst, and 5 mL of ethanol at 300 rpm stirring speed.

5−90 min, no significant change in LA conversion (less than 1%) was measured with varying stirring speeds for experiments performed in both the MW and NMW reactors. One possible reason could be attributed to a nonsignificant change in LA conversion with varying stirring speeds is well mixing of the reaction mixture at 300 rpm which in turn causes uniform bulk temperature.33 Overall, it was found that stirring at a rate of 300 rpm is sufficient for LA esterification conversion in both the MW and NMW reactors. It is worth noticing that the microwave setup comes with a cylindrical vessel, which is in contrast to round-bottom flask typically used in conventional oil baths. Moreover, it is reported that stirring in MW reactors is less optimal as compared to conventional oil bath reactors and is a function of sample volume.44 The cylindrical shape of the vessel was the basis for studying the effect of microwave heating and nonmicrowave heating on varying sample volume, although we used the cylindrical vessel in both the MW and NMW reactors, so the shape effect if any could be relative in both. The results obtained for experiments performed in different

ethanol volumes are plotted in Figure 3a, b. It was found that the change in ethanol volume did not have any significant impact on the LA conversion in the MW and NMW reactors, which confirms efficient mixing of the reactant, catalysts, and solvent in the reaction mixture at given stirring speed. Stoichiometrically, one mole of ethanol is required for the conversion of one mole of LA to produce one mole of EL. However, providing excess alcohol (LA:ethanol molar ratio) helps to keep the esterification reaction in the forward direction. On the contrary, an LA to ethanol molar ratio beyond 1:20 does not make any significant changes in the esterification reaction, and thus overall reaction behaves as a pseudo-first order reaction.45 Therefore, all experiments were performed in excess alcohol (LA: thanol ratio 1:42) to avoid reverse reaction and to be in the first-order regime. 3.1.4. Effect of Reactant and Catalyst Concentration. Effect of reactant concentration was studied in the MW and NMW reactors at 353 K for 5−90 min reaction time, and these results are presented in Figure 4. It is evident from Figure 4a, b that LA conversion under microwave irradiation was relatively E


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Figure 5. Effect of catalyst concentration on LA conversion for experiments performed in (a) MW and (b) NMW reactor at 353 K temperature for 5−90 min in the presence of 2 mmol of LA, 0.005−0.02 mmol of silicotungstic acid catalyst, and 5 mL of ethanol at 300 rpm stirring speed.

Figure 6. Effect of temp. and time on LA conversion for experiments performed in (a) MW and (b) NMW reactor at 333−383 K temperature for 5−90 min in the presence of 2 mmol of LA, 0.005−0.02 mmol of silicotungstic acid catalyst, and 5 mL of ethanol at 300 rpm stirring speed.

in both the MW and NMW reactors as shown in Figure 5a, b. For a reaction time of 5 min, LA conversion increased by 8% under microwave irradiations (from 8 to 16% LA conversion) when the silicotungstic acid concentration was increased from 0.005 to 0.01 mmol. Subsequently, LA conversion increased by 13% (from 16 to 29%) and 17% (from 29 to 46%) when silicotungstic acid concentration was increased from 0.01 to 0.0125 mmol and 0.0125 to 0.02 mmol, respectively. However, no such trend in LA conversion for varying catalyst concentration was observed at longer reaction times for higher catalyst concentration beyond 0.01 mmol silicotungstic acid. In contrast, the enhancement trend in LA conversion remained higher for a longer duration for 0.01 mmol silicotungstic acid, thus 0.01 mmol catalyst concentration was chosen as the optimum quantity for the kinetic study purpose. Under optimum operating conditions, 85 and 79% LA conversions were measured in the presence of 0.01 silicotungstic acid in MW and NMW reactors, respectively, at 353 K temperature in 90 min. A double fold increase in silicotungstic acid concentration (0.02 mmol) further enhanced LA conversion to 98 and 91% in the MW and NMW reactors, respectively.

higher than the experiments performed in the nonmicrowave heating conditions. Interestingly, it was found that the LA conversion decreases in LA concentration and vice versa for experiments performed in both the MW and NMW reactors. A maximum of 94 and 89% LA conversion was measured at 353 K temperature in 90 min reaction time in the MW and NMW reactors, respectively, when 1 mmol of LA was used in the feed. Increasing LA concentration to 2 mmol in the feed mixture caused a significant reduction in LA conversion, lowering the LA conversion to a maximum 85 and 79% in the MW and NMW reactors, respectively. Eventually, increasing LA feed to 5 mmol resulted in a decrease in LA conversions to 56%and 53% in the MW and NMW reactors, respectivel,y at this concentration under identical experimental conditions. The decrease in LA conversion with an increase in concentration can be correlated with the availability of catalytic sites available for the reaction and thus catalyst concentration. Further, experiments were performed by varying catalysts concentration from 0.005 to 0.02 mmol, and it was found that LA conversion increases with increase in catalyst concentration F


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Figure 7. First-order kinetic plot of −ln (1 − XA) vs time for LA conversion experiments performed in (a) MW and (b) NMW reactor at 333−363 K temperature for 5−90 min in the presence of 2 mmol of LA, 0.01 mmol of silicotungstic acid catalyst, and 5 mL of ethanol at 300 rpm stirring speed.

Overall, a higher LA conversion was measured for all experiments performed in the MW reactor as compared to experiments performed in the NMW reactor. 3.1.5. Effect of Reaction Temperature. Reaction temperature plays an imperative role in determining the overall efficacy of the chemical processes. Thus, experiments were performed over a wide temperature range from 333 to 383 K for a period of 5 to 90 min in the presence of 0.01 mmol silicotungstic acid catalysts for the conversion of 2 mmol LA in 5 mL ethanol. It was found that the LA conversion improved with an increase in reaction temperature and showed a maximum 97% LA conversion at 383 K temperature in 90 min under microwave irradiations as shown in Figure 6a. These trends indicate that a higher temperature is favorable for higher LA conversion in shorter reaction time. However, the LA conversion rate started decreasing at elevated temperatures (363−383 K) when experiments were performed for longer run times. One possible reason could be the LA conversion reaches equilibrium, thus lowering conversion. Nevertheless, the overall increase in LA conversion with an increase in reaction temperature indicated a kinetically controlled reaction.45 A similar improvement in LA conversion was noticed with an increase in reaction temperature when experiments were performed in the NMW instant heating reactor. At 383 K reaction temperature, maximum 90% LA conversion (as shown in Figure 6b) was measured when experiments were performed for 90 min, which is slightly less (7%) than the LA conversion measured (97%) for experiments performed in the MW reactor under identical experimental conditions. Overall, LA conversion remained slightly higher for experiments performed in the MW reactor as compared to experiments performed in the NMW instant heating reactor. One possible reason could be less uniformity in heat distribution in the NMW reactor as compared to the MW reactor in a short duration of time, and thus the significant difference in LA conversion in both the reactors for shorter reaction time. On the contrary, prolonged reaction time improves uniformity in heat distribution in the NMW instant heating reactor, and thus LA conversion difference as compared to that of the MW reactor decreases. 3.2. Kinetic Study. It was observed that the LA esterification reaction was independent of mass transfer limitations as discussed in the stirring speed effect. Thus, a

pseudohomogeneous model was selected for the kinetic study of the LA esterification reaction for both the MW and NMW instant heating reactors. Also, two trends in LA conversion were observed: (i) initial reaction rate and (ii) equilibriumcontrolled region. Thus, initial reaction rates were considered for the kinetic study, which is also suggested in in earlier reports for different reactions by research groups worldwide.46−49 In general, the esterification reaction proceeds as follows: k1 A + BHooIC + D k2

(1)

Where A, B, C, and D represent LA, ethanol, EL, and water molecules respectively, whereas k1 and k2 are forward and reverse reaction constants, respectively. Thus, the rate of reaction can be calculated as follows: −rA = k1CAa C Bb − k 2CCc C Dd

(2)

However, in the present case, LA conversion is the ratedetermining step owing to excess of alcohol used in the reaction which in turn keeps the reaction in the forward direction and makesthe reaction zero order with respect to alcohol.50 Therefore, eq 2 can be written as i dC y −jjj A zzz = k1CAn k dt {

(3)

Because esterification reactions are elementary and follow firstorder with respect to each reactant when alcohol is not used in excess,51 the order of the reaction becomes 1, in this case, because only LA conversion is the rate-determining step. Thus, eq 3 can be written as follows: i dx y −CAojjj A zzz = k1CAo(1 − XA ) k dt {

(4)

Which can be reduced to

i dx y −jjj A zzz = k1(1 − XA ) k dt {

(5)

Equation 5 can be solved to yield: G


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Industrial & Engineering Chemistry Research −ln(1 − XA ) = k1t

reactors was measured, which further confirms the absence of nonthermal microwave effects. The results obtained are in line with earlier reports, which suggests that nonthermal microwave effects possibly do not exist.25 Thus, improved LA conversion under microwave irradiations can be attributed to the better heat transfer that occurs directly as compared to the NMW reactor, in which heat is transferred through conduction and convection and hence has a slow rate of heat transfer in bulk. The rapid heating and improved heat transfer could be the reason that may lead to higher conversions and yields in a shorter time.52 A similar trend was observed in the present study results, where the difference in LA conversion for the MW and NMW reactors reduces significantly for longer reaction times at elevated temperatures, thereby indicating that heat transfer in the NMW reactor improves with time, thus showing only thermal effects under microwave irradiation for the LA esterification reaction. Furthermore, the measured activation barrier for LA esterification reaction was low (44−45 kJ/mol), which might be one of the reasons for the absence of nonthermal microwave effects, which are usually dominant for reactions having activation barriers higher than 83 kJ/mol.53 In addition, experiments for the present study have been carried out at a fixed microwave frequency (2.4 GHz), which is one of the major limitations of the monowave-300 reactor. Therefore, a further study on the influence of varying microwave frequency would be interesting to further explore the effects of microwave and nonmicrowave heating conditions.

(6)

Because only k1 is the rate constant in eq 6, it can be simply written as −ln(1 − XA ) = kt

(7)

A plot of−ln (1 − XA) vs t is shown in Figure 7a, b for experiments performed in both the MW and NMW instant heating reactor. It is evident that the LA conversion data fits well in the first order, thus validates the considered model. The rate respective rate constants values were calculated from the slope of the −ln (1 − XA) vs t graph at different operating temperatures. The measured rate constants were found to be the same order of magnitude, i.e., 1 × 10−4/s, although rate constants for experiments performed in the MW reactor were slightly higher (1.4−1.7 times) as compared to rate constants calculated for experiments performed in the NMW reactor. The measured difference in rate constants were not significant and too low to assign any nonthermal microwave-effects. Indeed, this small improvement in rate constant could have been caused by better heat transfer in the MW reactor. Thus, the activation barriers were calculated using the Arrhenius equation as follows to further study the effect of microwave and nonmicrowave heating on the LA esterification reaction: i −E y k = A expjjj a zzz k RT {

(8)

Which can be written as i E y lnk = jjj− a zzz + lnA k RT {

4. CONCLUSION Synthesis of ethyl levulinate from levulinic acid in the presence of silicotungstic acid catalyst was studied under microwave irradiation and a conventional but instant heating closed reactor system. The present study attempted to explore the effect of microwave and conventional heating on the levulinic acid esterification reaction. In general, only minor differences in LA conversion under microwave irradiation (97% at 383 K in 90 min) and the nonmicrowave instant heating reactor (90% at 383 K in 90 min) were measured, thereby eliminating the possibility of nonthermal microwave effects in the LA esterification reaction. Accordingly, the improved LA conversion measured in the microwave reactor is attributed to the purely thermal effects such as better heat transfer in the microwave reactor, which transfer heats directly into the bulk as compared to a nonmicrowave reactor, in which heat transfer takes place via conduction and convection. The results obtained from the kinetic study support the claim, as similar activation barriers (44 ± 2 kJ/mol for MW and 45 ± 3 kJ/mol for NMW) were measured for experiments performed in both the reactors. Thus, a conventional heating reactor is preferable for LA esterification reactions as compared to a microwave reactor, which is expensive and yet to be tested at commercial scale.

(9)

Figure 8. Arrhenius plot for LA conversion experiments performed in the MW and NMW reactor at 333−363 K temperature for 5−90 min in the presence of 2 mmol of LA, 0.01 mmol of silicotungstic acid catalyst, and 5 mL of ethanol at 300 rpm stirring speed.

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ASSOCIATED CONTENT

S Supporting Information *

()

A subsequent plot of lnk vs T is shown in Figure 8 for experiments performed in both the MW and NMW reactor to calculate corresponding activation energies. Interestingly, no significant difference in activation energies for LA esterification reaction in MW (44 ± 2 kJ/mol) and NMW (45 ± 3 kJ/mol)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01137. Figures S1−S5 (PDF) H


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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. *Email: [email protected]. ORCID

Ejaz Ahmad: 0000-0002-6142-5331 Md. Imteyaz Alam: 0000-0001-9749-4040 K.K. Pant: 0000-0002-0722-8871 M. Ali Haider: 0000-0002-8885-5454 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.A. thanks Hindustan Petroleum Corporation Limited (HPCL) and the Prime Minister Fellowship (D.O. No. DST/SSK/SERB−CII-FeII/2014) for funding his doctoral studies. K.K.P. thanks DST/SERB (EMR/2015/001959) for providing project funding for this study.

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