Synthesis and Characterization of Zirconia Supported Silicotungstic

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

Synthesis and Characterization of Zirconia Supported Silicotungstic Acid for Ethyl Levulinate Production Shireen Quereshi,†,‡,§ Ejaz Ahmad,‡,§ Kamal Kishore Pant,*,‡ and Suman Dutta*,† †

Department of Chemical Engineering, Indian Institute of Technology (ISM), Dhanbad, India Department of Chemical Engineering, Indian Institute of Technology, Delhi, India

‡

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S Supporting Information *

ABSTRACT: A Keggin silicotungstic acid (HPS) catalyst was heterogenized by loading (10−40 wt %) over zirconia support, and the resulting catalysts were named ESZN-1 (10 wt %), ESZN-2 (20 wt %), ESZN-3 (30 wt %), and ESZN-4 (40 wt %). After that, synthesized catalysts were characterized using several tools and techniques which revealed that the Keggin structure of the parent HPS catalyst remained intact after heterogenization. Eventually, synthesized catalyst performance evaluation tests were performed for the production of ethyl levulinate from biomass-derived levulinic acid under microwave heating irradiations. Under optimum operating conditions, more than 90% LA conversion with 100% EL selectivity was obtained at a 110 °C temperature in 30 min in the presence of 100 mg of ESZN-4 in a solution containing levulinic acid and ethanol in a 1:43 ratio. A kinetic study on LA conversion in the presence of the ESZN-4 catalyst revealed a pseudo-first-order mechanism for ethyl levulinate synthesis. loofah sponge-derived carbon sulfonic acid at 80 °C in 12 h in a round-bottom flask reactor.10 Similarly, Oliveira et al. have synthesized sulfonated carbon nanotubes and tested them in a round-bottom flask type reactor at 70 °C for 5 h to achieve more than 50% LA conversion.11 Nevertheless, Keggin heteropolyacid (HPA) catalysts remained the most interesting out of all the reported catalysts for the production of ethyl levulinate. In contrast, recycling of HPA is a tedious and expensive task due to its solubility in the solvent phase.12 Thus, attempts have been made to heterogenize the HPA catalysts by loading over different metal supports. For example, Su et al. have heterogenized phosphotungstic acid over zirconia support for the production of methyl levulinate (ML) from levulinic acid and measured more than a 99 mol % ML yield in 3 h at 65 °C at a 1:7 LA to methanol molar ratio.13 Similarly, Wu et al. have reported synthesis and application of 3D graphene

1. INTRODUCTION Ethyl levulinate is an essential chemical that can be produced from a wide range of biorenewable resources. In this regard, production of ethyl levulinate (EL) from biomass, carbohydrates, 5-hydroxymethylfurfural, 5-ethoxymethylfurfural, and levulinic acid (LA) in the presence of a suitable acid catalyst have been reported.1 However, the most favored route for EL production is LA esterification, possibly due to availability of technologies for large scale production of LA.2−4 Indeed, the United States Department of Energy (U.S. DOE) and National Renewable Energy Laboratory (NREL) have mentioned levulinic acid among the top 12 building chemicals derived from the biomass.5 Therefore, the production of EL from LA under mild reaction conditions is more promising as compared to any other feedstock. Accordingly, a wide range of catalytic processes have been reported for the synthesis of EL from LA.6,7 For example, ZrO2 supported sulfated catalysts have been reported among highly active catalysts for the LA esterification reaction.8,9 Like sulfated catalysts, sulfonated catalysts are another important class which has been widely used in the LA esterification reaction. For example, Li et al. have reported up to 91% LA conversion in the presence of © XXXX American Chemical Society

Special Issue: Biorenewable Energy and Chemicals Received: March 26, 2019 Revised: May 16, 2019 Accepted: May 20, 2019

A

DOI: 10.1021/acs.iecr.9b01659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

operating at room temperature for 64 scans (resolution 4) over the range of 400−4000 cm−1. The vibrational frequencies of all the catalysts were scanned at room temperature in the range of 200−1200 cm−1 using a Micro-Raman spectrometer and inVia reflex Raman spectroscopy system integrated with an FTIR IlluminatIR II module combined with a research grade Leica microscope. The BET surface area, pore size, and pore volumes of degassed catalyst samples were measured using a Micromeritics ASAP 2010 surface area analyzer. Similarly, acidities of the catalysts were measured using a Micromeritics 2720 TPX system. First, catalyst samples were degassed at 200 °C for 6 h and then subjected to saturation with ammonia (5% NH3, balance helium) at 115 °C for 1 h. After this, physiosorbed ammonia was removed by flushing with helium gas for 1 h, and the catalyst sample temperature was cooled to 50 °C. After that, thermal programmed desorption of ammonia was carried out by increasing the temperature of the catalyst samples from 50 to 850 °C at a rate of 10 °C/min, and desorbed ammonia was recorded using a thermal conductivity detector (TCD). The morphology of synthesized catalysts was studied using a scanning electron microscope (SEM, Zeiss EVO 50) and highresolution transmission electron microscope (HR-TEM, Tecnai G2 20). 2.4. Experimental Methods. All experiments were performed in a microwave reactor (Make: Anton Paar, Model: Monowave 300) equipped with a 10 mL reaction vial, automatic temperature controller, and pressure sensor, which can operate at a 300 °C temperature and withhold 30 bar of pressure. In a typical experimental procedure, unless stated otherwise, 2 mmol of reactant levulinic acid was charged into the vial along with 5 mL of ethanol, 100 mg of ESZN catalysts, and a magnetic bead for stirring. The desired temperature was set into the reactor with the fastest heating rate mode at a 300 rpm stirring speed. Post reaction, the product sample was cooled down to 50 °C inside the reactor cavity using a compressor delivering cooling air at 5 bar of pressure. After that, the product mixture was filtered to separate the catalyst. Subsequently, the collected liquid sample was analyzed using high-performance liquid chromatography (HPLC) and gas chromatography equipped with a flame ionization detector (GC-FID) according to the standard protocols discussed in subsequent sections. 2.5. Product Analysis Methods. For reactant and products analysis, an Agilent HPLC 1200 infinity series equipped with an autosampler, refractive index (RI) detector, and Aminex HPX-87H column (300 × 7.8 mm) was used. The general program for all reactants conversion and products quantification was fixed to 50 °C for the column and RI detector temperature. The eluant (5 mM sulfuric acid) flow rate was fixed to 0.60 mL/min, whereas the sample injection volume was fixed to 2 μL. Before analysis, the RI detector was purged for at least 10 min, whereas the column was washed according to the standard program after the analysis of each sample. To further confirm the results, liquid samples obtained from optimized experiments were also analyzed with the help of GC-FID according to the method we reported earlier.21 The LA conversion, EL selectivity, and EL yield were calculated as follows:

anchored aerogel phosphotungstic acid (HPW) to cause 89.1% LA conversion at 80 °C in 9 h in the presence of a 45 wt % HPW loaded catalyst.14 Dharne and Bokade have reported a 97 mol % butyl levulinate yield in the presence of clay supported dodecatungstophosphoric acid at a 120 °C reaction temperature for a 4 h reaction time.15 Furthermore, Pasquale et al. have reported more than 90 mol % LA conversion at 78 °C reaction temperature for 10 h in the presence of Wells Dawson phosphotungstic acid supported over silica.16 Like the HPW catalyst, silicotungstic acid is another vital member of the heteropolyacid family which has been explored widely in the esterification of LA to produce a variety of esters.17−20 For example, the Hilmioglu group has loaded silicotungstic acid over hydroxyethyl cellulose to synthesize a novel pervaporation catalytic membrane reactor for application in the levulinic acid esterification reaction.17 Both the reaction and separation of products take place simultaneously in these systems, which makes it unique and cost-effective.19 Interestingly, the addition of zirconium oxide to the membranes further improves their activity and causes up to 90% LA conversion in 7 h at a 75 °C temperature.18 Herein, the present study reports a relatively simple process for synthesis, characterization, and application of zirconia supported Keggin silicotungstic acid for the production of ethyl levulinate from levulinic acid. Several characterization techniques have been employed for insight into the catalytic properties responsible for the high activity of the heterogenized catalysts followed by a kinetic study on LA conversion.

2. MATERIALS AND METHODS 2.1. Materials. All chemicals used in the present study were procured from reputed vendors except heterogenized catalysts ESZN-1, ESZN-2, ESZN-3, and ESZN-4. The Keggin type silicotungstic acid (H4SiW12O40·nH2O, 99.9% trace metal basis), ZrO2 (99.9% trace metal basis), levulinic acid (LA, 99% purity), and ethyl levulinate (EL, 99% purity) were procured from Sigma-Aldrich, whereas alcohols including methanol, ethanol, propanol, butanol, amyl alcohol, octanol, and HPLC grade water (each >99% purity) were procured from Merck. 2.2. Catalysts Synthesis Procedure. All catalysts were synthesized using Keggin type silicotungstic acid (10%, 20%, 30%, and 40% by wt %) on ZrO2 support via the wet impregnation method and named ESZN-1, ESZN-2, ESZN-3, and ESZN-4, respectively. Initially, both Keggin silicotungstic acid and ZrO2 were measured and charged separately in 10 and 100 mL of water, respectively, in separate beakers. After that, the mixture containing ZrO2 was placed on a magnetic stirrer at 40 °C under continuous stirring (600 rpm), and silicotungstic acid was added dropwise. This process took 2 h for completion. After this, the resulting mixture was aged for 24 h in the beaker. The precipitates of the resultant mixture were obtained by evaporation of the aged aqueous solution in a rotary evaporator followed by calcination at a 300 °C temperature for 5 h. A typical procedure for the synthesis of the catalysts is shown in Supporting Information Figure S1. 2.3. Catalyst Characterization Methods. All the synthesized catalysts were characterized using several tools and techniques. X-ray diffraction patterns of the synthesized catalysts were recorded using a Rigaku benchtop instrument, model Miniflex-600, having Kα radiation (40 kV, 15 mA, NaI scintillation detector with graphite monochromator). FTIR spectra were obtained using a Nicolet iS50 instrument B


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and homogeneous distribution of phosphotungstic acid ZrO2 surface.22 To further confirm these observations, XRD diffraction patterns of ZrO2 and silicotungstic acid loaded over ZrO2 and catalysts ESZN-1, ESZN-2, ESZN-3, and ESZN-4 are illustrated in Figure 2. The same 2θ values 24.1°, 28.3°,

%LA conversion (initial − final) concentration of LA in millimoles = initial concentration of LA in millimoles × 100 %EL selectivity concentration of EL produced in millimoles = (initial − final) concentration of LA in millimoles × 100 %EL yield =

concentration of EL produced in millimoles initial concentration of LA in millimoles × 100

turnover frequency (TOF) =

(initial − final) concentration of LA in millimoles HPS concentration in millimoles × time (min)

3. RESULTS AND DISCUSSION 3.1. Catalysts Characterization Results. The surface morphologies of the synthesized catalysts were analyzed using scanning electron microscopy (SEM) to study the subsequent effects of the loading of the parent catalyst in different concentrations over ZrO2. The catalysts with 10, 20, 30, and 40 wt % parent Keggin type silicotungstic acid (HPS) loaded over ZrO2 (ESZN-1, ESZN-2, ESZN-3, and ESZN-4) are shown in Figures 1a, b, c, and d, respectively. It was observed that the

Figure 2. XRD patterns of HPS, ZrO2, and ZrO2 supported Keggin type silicotungstic acid.

31.5°, 34.3°, and 49.4° were measured for all the catalysts which indicate monoclinic phases of zirconia as dominant.23 Nevertheless, the presence of peaks at 30.0°, 35.3°, and other 2θ values indicates the presence of tetragonal and cubic phases in a minor amount.24,25 On the contrary, Keggin type silicotungstic acid diffraction peaks were not significantly visible in synthesized catalysts, thereby confirming uniform and homogeneous distribution over ZrO2 support. However, one or two peaks at different 2θ values than that of zirconia were visible in the ESZN-4 catalyst, thus indicating a possible weak interaction between the parent Keggin HPS and the ZrO2 support. The other reason could be due to the XRD peaks of silicotungstic acid. A comparative FTIR study of parent silicotungstic acid (HPS) and heterogenized catalysts, i.e., ESZN-1, ESZN-2, ESZN-3, and ESZN-4, is shown in Figure 3. The bands measured at 1017 cm−1, 971 cm−1, 917 cm−1, and 783 cm−1

Figure 1. SEM images of (a) ESZN-1, (b) ESZN-2, (c) ESZN-3, and (d) ESZN-4 taken at 50K magnification.

parent Keggin HPS was finely dispersed over ZrO2 after synthesis in all the catalysts. In addition, FESEM EDX mapping was carried out for the surface of the one sample as shown in Supporting Information Figure S2, which showed the uniform elemental distribution of all the metals, thereby confirming excellent dispersion of the parent Keggin HPS catalyst in the ZrO2 support. Similar observations have been made by Amin and co-workers for the synthesis of zirconia supported phosphotungstic acid, which indicates a uniform

Figure 3. FTIR spectra of the parent silicotungstic acid and synthesized catalysts. C


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interactions between the parent Keggin type HPS catalyst and support ZrO2. Notably, previous studies on heterogenization of various heteropolyacid over metal oxides suggest the formation of a 2D structure, albeit the primary Keggin structure was intact on the loading of Si-ZrO2 over phosphotungstic acid.26 On the basis of similarities in results obtained for parent catalyst HPS and ZrO2 using XRD, ATR-FTIR, and Raman scattering spectroscopy, which in turn suggested weak interaction possibilities between the support and parent HPS terminal WO bond, the plausible structure of the synthesized catalysts is illustrated in Figure 5b. The structure of the parent HPS is shown in Figure 5a in which central Si has connected four oxygen atoms along with four hydrogen atoms. Interestingly, when parent HPS is loaded over the support ZrO2, it interacts with ZrO2 via three different routes. The very first possibility is that bridging oxygen atoms of ZrO2 interact with bridging W−O−W bonds of parent HPS to form a weak covalent bond W−O−Zr. The second possibility is that the proton of the parent HPS interacts with the hydroxyl group trapped in the ZrO2 matrix during synthesis via weak hydrogen bonding. Similarly, some of the parent HPS and ZrO2 matrices may connect via weak acid−base interaction. Indeed, similar interactions for metal oxide support and Keggin type heteropolyacid have been reported in the literature.27−29 Also, it is evident from Figure 6a that parent catalyst HPS was uniformly distributed over ZrO2 support without the formation of an agglomerate. A more detailed image taken at 50 nm using HR-TEM (Figure 6b) further showed the homogeneous distribution of the parent catalyst HPS. Moreover, results obtained from HR-TEM (Figure 6c) showed different lattice fringes with different orientations, thereby indicating the formation of particles with different sizes. Moreover, a strong diffraction pattern was observed which indicates the crystalline nature of the synthesized catalyst. In general, a ZrO2 SAED diffraction pattern forms a circle which is also present in the present image.30,31 However, some scattered bright spots were also observed which have possibly arisen due to the presence of tungsten. Overall, the SAED image shown in Figure 6d further confirms the uniform distribution of metals owing to the homogeneous distribution of the parent catalyst over the ZrO2 support. The Brunauer−Emmett−Teller (BET) method was used to calculate specific surface area from N2 physisorption data of

are characteristic peaks of the Keggin unit, i.e., vibrational stretching of terminal WO bonds, tetrahedral central atom Si−O bonds, and bridging W−O−W bonds of the parent HPS catalyst. Identical peaks were obtained for heterogenized catalysts, which further confirms that the primary Keggin structure of the HPS remained unchanged during the synthesis process, thereby possessing the original Keggin properties of the parent catalyst. A further confirmative analysis was performed using Raman scattering spectroscopy, the results of which are plotted in Figure 4. The characteristic peaks of the primary Keggin

Figure 4. Raman spectra of parent Keggin type silicotungstic acid and synthesized catalysts.

structure of the parent HPS are visible at 996 cm−1, 927 cm−1, and 887 cm−1 representing a terminal WO bond, central atom Si−O bonds, and bridging W−O−W bonds, respectively. Similarly, out of plane W−O−W bridging bonds’ vibrational frequencies were measured at 216 and 175 cm−1 Raman shifts. Overall, the results obtained from Raman scattering spectroscopy further confirm that the primary Keggin structure of parent HPS remained intact on heterogenization over a ZrO2 support. Nevertheless, a slight shift in W−O−W bridging bonds peaks was measured as shown in Figure 4 due to a possible weak interaction between parent HPS and ZrO2 support. Similarly, few other negligible peaks were observed in heterogenized catalysts, possibly arising from weak

Figure 5. (a) Structure of parent Keggin type silicotungstic acid and (b) plausible catalyst support interaction between parent HPS and ZrO2 support. D


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Figure 6. HR-TEM images of ESZN-4 (a) 100 nm, (b) 50 nm, (c) 2 nm, and (d) SAED.

Table 1. Textural Properties and Acidity of the Synthesized Catalysts catalysts

BET surface area (m2/g)

BJH desorption diameter (nm)

pore volume (cm3/g)

weak acidity (mmol/g)

medium acidity (mmol/g)

strong acidity (mmol/g)

total acidity (mmol/g)

ESZN-1 ESZN-2 ESZN-3 ESZN-4 HPS ZrO2

29.4 18.3 13.8 1.2 1.64 33.4

7.8 6.2 8.0 30.6 44.3 9.2

0.06 0.02 0.01 0.01 0.009 0.08

0.31 0.38 0.62 0.68 0.60 0.10

0.12 0.15 0.08 0.07 0.00 0.00

0.09 0.18 0.22 0.51 0.82 0.00

0.52 0.71 0.92 1.26 1.42 0.10

the synthesized catalyst as shown in Table 1, whereas pore diameters were measured using the BJH method. Similarly, pore volumes of the synthesized catalysts were calculated using BJH desorption cumulative pore volume of pores between 17 and 3000 Å in diameter. It was found that the BET surface area of the synthesized catalysts decreases with an increase in parent catalyst HPS loading. The decrease in surface area with higher HPS loading can be attributed to the blockage of some pores of the support ZrO2, which in turn also reduces the pore volume of the synthesized catalysts. It is well-known that the higher loading of parent catalyst causes blockage of the pore and reduces the surface area of the metal oxide supported heteropolyacid catalysts.22 On the contrary, no clear trend was observed in the BJH desorption diameter of the newly synthesized catalysts. Ammonia temperature-programmed desorption (NH 3 TPD) was carried out to study the acidic properties of the catalysts. The TPD profile of synthesized catalysts is depicted in Figure 7, which shows at least three major peaks which are described as weak, medium, and strong acid sites. The peaks obtained at 270 °C were assigned to weak acidity, whereas the peaks obtained at 550 °C were assigned to medium acidity. Similarly, peaks obtained beyond 620 °C desorption temperature were assigned to strong acidity. The quantitative acidity data of all catalysts are given in Table 1. It is evident from Table 1 that both strong and total acidity increase with an increase in HPS loading, which shows a uniform distribution of the parent catalyst over a ZrO2 support. 3.2. Catalysts Performance Evaluation. On performing the LA esterification reaction in the presence of support alone without parent HPA, a 14 mol % LA conversion with 100 mol

Figure 7. Ammonia temperature-programmed desorption profiles (NH3-TPD) of synthesized catalysts.

% EL selectivity at a 110 °C temperature in 30 min as shown in Figure 8a was measured, which suggests that zirconia in the absence of Keggin type silicotungstic acid also catalyzed the LA esterification. On the contrary, experiments performed in the presence of pure HPS catalyst at 110 °C for 30 min caused 96% LA conversion (TOF = 6.4 min−1) and 94% yield. Thus, a 110 °C operating temperature was selected for the performance evaluation of syntheses of ESZN-1, ESZN-2, ESZN-3, and ESZN-4 catalysts. It was observed that a 10 wt % HPS loading on the ZrO2 support (ESZN-1) caused 55 mol % LA (TOF = 10.5 min−1) conversion with 100% EL selectivity. Subsequently, an increase in HPS loading to 20 wt % (ESZN-2), 30 wt % (ESZN-3), and 40 wt % (ESZN-4) over ZrO2 support E


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Figure 8. (a) Catalyst screening, LA 2 mmol, LA/EtOH 1:43, temp. 110 °C, time 30 min, catalysts 100 mg at 300 rpm. (b) Effect of stirring speed, LA 2 mmol, LA/EtOH 1:43, temp. 110 °C, time 30 min, ESZN-4 100 mg. (c) Effect of LA to EtOH ratio, LA 2 mmol, temp. 110 °C, time 30 min, ESZN-4 100 mg. (d) Effect of reactant concentration, LA/EtOH 1:43, temp. 110 °C, time 30 min, ESZN-4 100 mg.

caused 63 mol % (TOF = 6.0 min−1), 75 mol % (TOF = 4.7 min−1), and 90 mol % (TOF = 4.3 min−1) LA conversion, respectively, as shown in Figure 8a under identical reaction conditions. Although parent catalyst HPS caused maximum LA conversion (96 mol %) at a 110 °C temperature in 30 min, it is readily soluble in reaction media, which makes catalyst and product separation a challenging task. Therefore, HPS loaded heterogeneous catalyst ESZN-4 was chosen as the most preferred catalyst for further study. In general, the application of heterogeneous catalysts faces mass transfer and diffusion limitation challenges. Thus, experiments were performed at varying stirring speeds (0− 900 rpm) to eliminate the external mass transfer limitations. A maximum 83 mol % LA conversion was measured with similar EL yield when experiments were performed at a 110 °C temperature for 30 min without stirring in the reaction mixture, as shown in Figure 8b. Afterward, stirring speed was set to 300 rpm, and experiments were performed under identical reaction conditions. A considerable increase in LA conversion (90 mol %) was measured as compared to LA conversion (83 mol %) measured in the absence of stirring in the reaction mixture. Interestingly, no significant change in LA conversion and EL yield was measured when stirring speed was changed from 300 to 600 and 900 rpm as shown in Figure 8b. The results obtained indicate the absence of mass transfer and diffusion limitations in the synthesized catalyst. Thus, a considerable difference in LA conversion and EL yield observed for nonstirred experiments as compared to stirred experiments could be due to nonuniform heat distribution for nonstirred experiments. Overall, no significant change in LA conversion at different stirring speeds confirms the absence of

any mass transfer or diffusion limitations. Therefore, further experiments were performed at a 300 rpm stirring speed. The LA to ethanol molar ratio is another critical parameter which may affect the reaction kinetics. Notably, the use of excess alcohol is necessarily required to maintain the reaction in the forward direction. Therefore, LA esterification experiments were performed with different LA/EtOH molar ratios varying between 1:25 and 1:51, as shown in Figure 8c. Nevertheless, varying the LA/EtOH molar ratio did not cause a considerable change in both the LA conversion and the EL yield. Interestingly, Yadav and Yadav have reported similar observations on the application of excess alcohol in the esterification reaction.32 Authors have reported that excess alcohol occupies all the catalytic sites, thereby making levulinic acid a limiting reactant, which in turn makes it a pseudo-firstorder reaction. Therefore, a 1:43 LA to EtOH molar ratio was chosen for further experiments considering microwave reactor limitations, which yields the most consistent results in the 5 mL volume, which is equivalent to a 1:43 molar ratio of LA to EtOH. Similarly, the effect of reactant concentration was studied by varying the LA feed between 0.5 and 2.5 mmol, the results of which are plotted in Figure 8d. Nearly 100 mol % LA conversion and EL yields were measured when 0.5 and 1 mmol of LA was fed into the microwave reactor in 5 mL of ethanol at a 110 °C temperature for 30 min in the presence of 100 mg of heterogenized HPS catalyst ESZN-4. However, the LA conversion decreased from 95 mol % to 90 mol % as LA feed concentration was increased to 1.5 to 2 mmol, respectively. Further, the increase in LA feed concentration to 2.5 mmol reduced conversions to 85 mol %. An LA feed F


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Industrial & Engineering Chemistry Research Table 2. Comparison of Conversion Values with Literature reactant LA LA LA LA LA furfuryl alcohol LA

catalyst 40 wt % HPS/SiO2 15 wt % HPW/DH-ZSM-5 7.2ArSO3H−Si(Et)Si-PhNTs H2SO4 20 wt % HPS/SiO2 Ti-HPW 40 wt % HPS/ZrO2

experimental conditions

conversion

reactor

T = 65 °C, catalyst = 104 mg, time = 6 h T = 78 °C, cat./LA ratio = 0.2, time = 4 h T = 78 °C, cat = 2 wt %, time = 4 h

>80% 93% >94%

reflux (conventional heating) reflux (conventional heating) reflux (conventional heating)

T = 78 °C, cat = 2 wt %, time = 1 h T = 84 °C, cat./LA ratio = 0.2, time = 2 h T = 120 °C, cat. = 2.25 wt %, time = 30 min T = 110 °C, cat = 100 mg, time = 30 min

>98% >91% 100% >90%

reflux (conventional heating) reflux (conventional heating) sealed reactor (conventional heating) microwave reactor

ref 33 34 35 35 36 37 this study

was charged into the microwave reactor vial along with 100 mg of the ESZN-4 catalyst and 5 mL of ethanol (LA/EtOH, 1:43 molar ratio) at 300 rpm. After this, series of experiments were performed at 70 °C, 80 °C, 90 °C, 100 °C, and 110 °C reaction temperatures each for 5, 10, 15, 20, 25, and 30 min of reaction time. An increasing trend in LA conversion was observed at lower temperatures throughout the reaction, whereas experiments at higher temperatures showed a linear increase in LA conversion only for the first 15 min and started slowing down afterward, possibly due to it having started approaching equilibrium conditions after 15 min at higher temperatures (100−110 °C). From Figure 10a, it is also evident that the difference in LA conversion at lower temperatures (70−90 °C) was less as compared to the difference in LA conversion at a 100−110 °C reaction temperature, which is due the fact that a higher conversion in less time at elevated temperatures causes less availability of reactants per active site of the catalysts. Therefore, the reaction proceeds at a relatively faster rate when a low reactant concentration is available for the same amount of catalyst. EL yield was the same as that of LA conversion, as shown in Figure 10b, which suggests that no secondary reactions take place during the LA esterification reaction in the presence of ESZN-4 catalyst in ethanol media under microwave heating irradiation. Also, it is well-known that the esterification reactions follows first-order kinetics.38−43 Considering the absence of mass transfer and diffusion limitations, Yadav and Yadav have reported a dual site monofunctional Langmuir−Hinshelwood− Hougen−Watson model, which follows a chemisorption and adsorption mechanism32 in which, herein, LA is denoted by L, alcohol (ethanol) is denoted by A, EL by E, water by W, and vacant sites by *. First, levulinic acid L and ethanol A get adsorbed on the active sites of the catalyst as follows:

concentration of 2 mmol was considered optimum considering the 90% LA conversion. The LA conversion measured in the presence of heterogenized silicotungstic acid is comparable with the reported values in the literature in the presence of different catalysts, as shown in Table 2. However, it is to be noted that the experiments were performed at a higher temperature in the present study as compared to in the reported literature; thus a shorter time was needed to complete the reaction. Moreover, all experiments were performed in a closed vessel system capable of withstanding a pressure rise due to heating effects, thus the possibility of a pressure effect may exist, which can be an interesting area of study in the future. Earlier reports suggested that the change in reactor type from a conventional batch reactor to a membrane reactor can improve the LA conversion by >50−60%.17 Also, it is to be noted that LA feed was measured on a weight basis (2 mmol equivalent to 232.11 mg) due to its highly viscous nature, which causes an error in volumetric measurements in practical operations. Thus, care should be taken while measuring levulinic acid on a molar basis for experiments. Also, recyclability studies suggest that the catalysts can be recycled at least five times with a minor loss in activity as shown in Figure 9. Before application of the used

Figure 9. Recyclability of the catalyst ESZN-4 (LA 2 mmol, LA/ EtOH 1:43, temp. 110 °C, time 30 min, catalysts 100 mg at 300 rpm).

K1 L + * ⇔ L*

(1)

K2 A+*⇐ ⇒ A*

(2)

After this, adsorbed levulinic acid L* and ethanol A* react to yield adsorbed ethyl levulinate E* and water W* on the surface of the catalyst as follows:

catalysts, the catalysts were washed twice with ethanol and dried at 200 °C for removal of alcohols and moisture. In the first catalyst application cycle, 89.8% LA conversion was measured, which decreased to 80.6% in the fifth cycle, which is a nearly 10.2% loss in activity. One possible reason for the loss in activity can be attributed to leaching on the parent catalyst in the solvent. 6.4. Kinetic Study. Kinetic study experiments for the LA conversion reaction were performed at different temperatures and times, as shown in Figure 10a. In general, 2 mmol of LA

L* + A*

K3 ⇔

E* + W*

(3)

Eventually, the adsorbed species undergoes a desorption step to yield ethyl levulinate E and molecule W as follows: 1/K4 E* ⇐=⇒ E + * G

(4) DOI: 10.1021/acs.iecr.9b01659 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 10. (a) Effect of reaction temperature and time on LA conversion. (b) Effect of reaction temperature and time on EL yield. (c) Pseudo first order fitting curve of LA conversion and (d) Arrhenius plot for activation barrier calculation.

1/K5 W* ⇐⇒ = W+*

(5)

−

From eqs 1, 2, 3, 4, and 5, the total concentration of sites is C T = C + C L * + CA * + C E * + C W * *

−

or CT = C + K1CLC + K 2CA C + K4CEC + K 5C W C * * * * *

(7)

−

(

K4K5C EC W K3

2

(1 + ∑ K iCi)

(12)

dC L dX =c LO L = KR (c LO − c LOXL)(c A O − c LOXL) dt dt = KR c LO(1 − XL)(M − XL) (13)

−

dC L dX = c LO L = KR c LOM(1 − XL) dt dt

(14)

Since KRcLO is constant, it can therefore be represented by a single term k, and XL can be simply written as X whereas M = cAO/cLO and (M − X) ≅ M owing to M ≫ 20 values

(9)

dXL = k(1 − X ) dt

)

K3 K1K 2C LCA − C T2 dC L − = dt (1 + K1C L + K 2CA + K4C E + K5C W )2

KR K 2C LCA

(8)

Since the reaction takes place via adsorption and desorption routes, thereby leading to the possibility of a mostly surface controlled mechanism, the rate of reaction of levulinic acid L can be written as dC L = K3C L *CA * − K3′C E *C W * dt

dC L = dt

or

Assuming that adsorption and desorption steps are in equilibrium, eq 7 can be written as follows:

−rL = −

(11)

Replacing K3CT2K1K2 with KR

(6)

CT C = * (1 + K C + K C + K C + K C ) 1 L 2 A 4 E 5 W

K C 2K K C C dC L = 3 T 12 2 L A dt (1 + ∑ K iCi)

(15)

where M = cAO/cLO, when M ≫ 20, (M − X) ≅ M and thus KC = KR2M. Therefore, it becomes a pseudo-first-order reaction, which can be written as follows:

(10)

For a reaction which is far away from equilibrium, eq 10 can be written as H


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Industrial & Engineering Chemistry Research −ln(1 − X ) = kt

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The −ln(1 − X) vs time data for less than 50 mol % LA conversion obtained from experiments at different reaction temperatures and times are plotted in Figure 10c. Fitting of experimental data in the model confirms that the overall LA esterification reaction follows first order kinetics. Furthermore, Arrhenius plot ln k vs 1/T (in K) is illustrated in Figure 10d, which was also used to calculate the activation barrier. Eventually, the activation barrier for the reaction was calculated as 38.43 kJ/mol, which indicates that the reaction is kinetically controlled. The results obtained are in line with earlier reports, which suggests that the esterification reaction having an activation barrier above 20 kJ/mol is kinetically controlled.44

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CONCLUSIONS Here, 10 wt %, 20 wt %, 30 wt %, and 40 wt % silicotungstic acid was successfully loaded over a zirconium oxide support without losing its Keggin structure. The ESZN-4 catalyst (having 40 wt.% HPS) yielded more than 90 mol % EL at a 110 °C reaction temperature in 30 min in the presence of 100 mg of catalyst in a microwave reactor. The LA conversion and EL yield reached nearly 100 mol % when LA feed concentration was reduced to 0.5 mmol. A Langmuir− Hinshelwood−Hougen−Watson model-based kinetic study revealed that the LA esterification reaction followed a first order rate in the presence of ESZN-4 catalysts, and the activation barrier was measured as 38.43 kJ/mol for the LA to EL conversion reaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01659.

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Catalysts synthesis procedure diagram (Figure S1); FESEM mapping profile of ESZN catalyst (Figure S2) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ejaz Ahmad: 0000-0002-6142-5331 Kamal Kishore Pant: 0000-0002-0722-8871 Suman Dutta: 0000-0002-2695-8108 Author Contributions §

Equal contribution authors

Notes

The authors declare no competing financial interest.

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REFERENCES

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Synthesis and Characterization of Zirconia Supported Silicotungstic

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