Synthesis and Characterization of Zirconia Supported Silicotungstic

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Kinetics, Catalysis, and Reaction Engineering

Synthesis and Characterization of Zirconia Supported Silicotungstic Acid for Ethyl Levulinate Production Shireen Quereshi, Ejaz Ahmad, Kamal K. Pant, and Suman Dutta Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01659 • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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Synthesis and Characterization of Zirconia Supported Silicotungstic Acid for Ethyl Levulinate Production Shireen Quereshiab†, Ejaz Ahmadb†, Kamal Kishore Pantb, Suman Duttaa* a

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

Department of Chemical Engineering, Indian Institute of Technology, Delhi, India *Corresponding

Authors: [email protected], [email protected] † Equal Contribution Authors

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Abstract: Keggin silicotungstic acid (HPS) catalyst was heterogenized by loading (10-40 wt.%) over zirconia support, and resulting catalysts were named as ESZN-1 (10 wt.%), ESZN-2 (20 wt.%), ESZN-3 (30 wt.%) and ESZN-4 (40 wt.%) respectively. After that, synthesized catalysts were characterized using several tools and techniques which revealed that the Keggin structure of parent HPS catalyst remained intact after heterogenization. Eventually, synthesized catalysts 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 110 oC temperature in 30 minutes in the presence of 100 mg ESZN-4 in a solution containing levulinic acid and ethanol in 1:43 ratio. A kinetic study on LA conversion in the presence of ESZN-4 catalyst revealed a pseudo first order mechanism for ethyl levulinate synthesis. Keywords: levulinic acid, ethyl levulinate, zirconia, silicotungstic acid, alcoholysis 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 suitable acid catalyst have been reported1. However, most favored route for EL production is LA esterification possibly due to availability of technologies for large scale production of LA2–4. Indeed, the United States Department of Energy (US DOE) and National Renewable Energy Laboratory (NREL) have mentioned levulinic acid among top 12 building chemicals derived from the biomass5. 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 have been reported6,7. For example, ZrO2 supported sulfated catalysts have been reported among highly active catalysts for the LA esterification reaction8,9. Like sulfated catalysts, sulfonated catalysts are other 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 loofah spongederived carbon sulfonic acid at 80 oC in 12 hours in a round bottom flask reactor10. Similarly, Oliveira et al. have synthesized sulfonated carbon nanotubes and tested in a round bottom flask type reactor at 70 oC for 5 hours to achieve more than 50% LA conversion11. Nevertheless, Keggin heteropolyacid (HPA) catalysts remained 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 phase12. 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 99 mol% ML yield in 3 hours at 65 oC in 1:7 LA to methanol molar ratio13. Similarly, Wu et al. have reported synthesis and application of 3D graphene anchored aerogel phosphotungstic acid (HPW) to cause 89.1% LA conversion at 80 oC in 9 hours in the presence of 45 wt.% HPW loaded catalyst14. Dharne et al. have reported 97 mol% butyl levulinate yield in the presence of clay supported dodecatungstophosphoric acid at 120 oC reaction temperature in 4 hours reaction time15. Furthermore, Pasquale et al. have reported more than 90 mol% LA conversion at 78 oC reaction temperature for 10 hours in the presence of Wells Dawson phosphotungstic acid supported over silica16. Like 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 esters17–20. For


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example, Hilmioglu group have loaded silicotungstic acid over hydroxyethyl cellulose to synthesize a novel pervaporation catalytic membrane reactor for the application in levulinic acid esterification reaction17. Both the reaction and separation of products takes place simultaneously in these systems which make it unique and cost effective19. Interestingly, the addition of zirconium oxide to the membranes further improves their activity and cause up to 90% LA conversion in 7 hours at 75 oC temperature18. 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 high activity of the heterogenized catalysts followed by a kinetic study on LA conversion. 2. Materials & 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% & 40% by wt.%) on ZrO2 support via 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 mL and 100 mL water respectively in separate beakers. After that, the mixture containing ZrO2 was placed on a magnetic stirrer at 40 oC under continuous stirring (600 rpm), and silicotungstic acid was added dropwise. This process took two hours for completion. Post this; the resulting mixture was aged for 24 hours 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 300 oC temperature for 5 hours. A typical procedure for the synthesis of the catalysts is shown in supplementary 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 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 operating at room temperature and 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 Micro-Raman spectrometer, inVia reflex Raman spectroscopy system integrated with FTIR IlluminatIR II™ module combined with research grade Leica microscope. BET Surface area, pore size, and pore volumes of degassed catalyst samples were measured using Micromeritics ASAP 2010 surface area analyzer. Similarly, acidities of the catalysts were measured using a Micromeritics 2720 TPX system. Firstly, catalyst samples were degassed at 200 oC for 6 hours and then subjected to saturation with ammonia (5% NH3, balance helium) at 115 oC for one hour. Post this, physiosorbed ammonia was removed by flushing Helium gas for one hour, and the catalyst sample temperature was cooled to 50 oC. After that, thermal programmed desorption of ammonia was carried out by the increasing temperature of the


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catalysts samples from 50 oC to 850 oC at a rate of 10 oC/min, and desorbed ammonia was recorded using a thermal conductivity detector (TCD). Morphology of synthesized catalysts was studied using a Scanning Electron Microscope (SEM, Zeiss EVO 50) and high-resolution 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 300 oC temperature and withhold 30 bar 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 300 rpm stirring speed. Post reaction, the product sample was cooled down to 50 oC inside the reactor cavity using a compressor delivering cooling air at 5 bar 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, Agilent make HPLC 1200 infinity series equipped with an autosampler, Refractive Index (RI) detector and Aminex HPX-87H column (300 X 7.8 mm) was used. General program for all reactants conversion and products quantification was fixed to 50 oC

for 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 minutes whereas 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 have reported earlier21. The LA conversion, EL selectivity, and EL yield were calculated as follows: % LA Conversion =

% EL Selectivity =

(𝐼𝑛𝑖𝑡𝑖𝑎𝑙 ― 𝐹𝑖𝑛𝑎𝑙) 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐿𝐴 𝑖𝑛 𝑚𝑚𝑜𝑙𝑒𝑠 𝑋100 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐿𝐴 𝑖𝑛 𝑚𝑚𝑜𝑙𝑒𝑠

𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐸𝐿 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑖𝑛 𝑚𝑚𝑜𝑙𝑒𝑠 𝑋100 (𝐼𝑛𝑖𝑡𝑖𝑎𝑙 ― 𝐹𝑖𝑛𝑎𝑙) 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐿𝐴 𝑖𝑛 𝑚𝑚𝑜𝑙𝑒𝑠

% EL Yield =

𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐸𝐿 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑖𝑛 𝑚𝑚𝑜𝑙𝑒𝑠 𝑋100 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐿𝐴 𝑖𝑛 𝑚𝑚𝑜𝑙𝑒𝑠

𝑇𝑢𝑟𝑛𝑜𝑣𝑒𝑟 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 (𝑇𝑂𝐹) =

(𝐼𝑛𝑖𝑡𝑖𝑎𝑙 ― 𝐹𝑖𝑛𝑎𝑙) 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐿𝐴 𝑖𝑛 𝑚𝑚𝑜𝑙𝑒𝑠 𝐻𝑃𝑆 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑚𝑚𝑜𝑙𝑒𝑠 𝑋 𝑡𝑖𝑚𝑒 (𝑚𝑖𝑛)

3. Results & 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 wt.%, 20 wt.%, 30 wt.% and 40 wt.% parent Keggin type silicotungstic acid (HPS) loaded over ZrO2 (ESZN-1, ESZN-2, ESZN-3, and ESZN-4) are shown in Figure- 1a, 1b, 1c, and 1d respectively. It was observed that the parent Keggin HPS was finely dispersed over ZrO2 after synthesis in all the catalysts. In addition, FESEM EDX mapping was carried out for surface of the one sample as shown in supplementary information Figure-S2 which showed the uniform elemental distribution of all the metals, thereby confirming excellent dispersion of parent Keggin HPS catalyst in the ZrO2 support. 7 ACS Paragon Plus Environment

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Similar observations have been made by Amin and co-workers for the synthesis of zirconia supported phosphotungstic acid which indicates a uniform and homogenous distribution of phosphotungstic acid ZrO2 surface22.

Figure-1 SEM images of (a) ESZN-1, (b) ESZN-2, (c) ESZN-3 and, (d) ESZN-4 taken at 50K magnification To further confirm these observations, XRD diffraction patterns of ZrO2 and silicotungstic acid loaded over ZrO2, catalysts ESZN-1, ESZN-2, ESZN-3, and ESZN-4 are illustrated in Figure-2. Same 2θ values 24.1o, 28.3o, 31.5o, 34.3o, and 49.4o, were measured for all the catalysts which indicate monoclinic phases of zirconia as dominant23. Nevertheless, the presence of peaks at 30.0o, 35.3o, and other 2θ values indicates the presence of tetragonal and cubic phases in the minor amount24,25. 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 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.

Figure-2 XRD patterns of HPS, ZrO2, and ZrO2 supported Keggin type 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 cm1,

971 cm-1, 917 cm-1, 783 cm-1 are characteristics 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 original properties Keggin properties of the parent catalyst.

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

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

The further confirmative analysis was performed using Raman scattering spectroscopy which results are plotted in Figure-4. The characteristic peaks of the primary Keggin structure of parent HPS are visible at 996 cm-1, 927 cm-1, 887 cm-1 representing 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 cm-1 and 175 cm-1 Raman shifts. Overall, the results obtained from Raman scattering spectroscopy further confirms that the primary Keggin structure of parent HPS remained intact on heterogenization over ZrO2 support. Nevertheless, a slight shift in W-O-W bridging bonds peaks was measured as shown in Figure-4 due to the possible weak interaction between parent HPS and ZrO2 support. Similarly, few other negligible peaks were observed in heterogenized catalysts, possibly arising from weak interactions between the parent Keggin type HPS catalyst and support ZrO2.


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Figure-5 (a) Structure of parent Keggin type silicotungstic acid and, (b) Plausible catalysts support interaction between parent HPS and ZrO2 support Notably, previous studies on heterogenization of various heteropolyacid over metal oxides suggest the formation of 2D structure albeit primary Keggin structure was intact on the loading of Si-ZrO2 over phosphotungstic acid26. Based on 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 parent HPS is shown in Figure-5a in which central Si has connected four oxygen atoms along with four hydrogen atoms.

Figure-6 HR-TEM images of ESZN-4 (a) 100 nm, (b) 50 nm, (c) 2 nm and, (d) SAED 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 interacts 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 ZrO2 matrix during synthesis via weak hydrogen bonding. Similarly, some of the parent HPS and ZrO2 matrix may connect via weak acid-base interaction. Indeed, similar interactions for metal oxide support and Keggin type heteropolyacid have been reported in the literature27–29. Also, it is evident from the 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, 10 ACS Paragon Plus Environment


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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, ZrO2 SAED diffraction pattern forms a circle which is also present in present image30,31. 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 parent catalyst over ZrO2 support. Brunauer Emmett Teller (BET) method was used to calculate specific surface area from N2 physisorption data of 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 Angstrom diameters. 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 catalysts22. On the contrary, no clear trend was observed in the BJH desorption diameter of the newly synthesized catalysts. Table -1 Textural properties and acidity of the synthesized catalysts Catalysts

BET

BJH

Pore

Weak

Medium

Strong

Total

Surface

desorption

Volume

Acidity

Acidity

Acidity

Acidity

Area

Diameter

(cm3/g)

(mmol/g)

(mmol/g)

(mmol/g)

(mmol/g)

(m2/g)

(nm) 11 ACS Paragon Plus Environment

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ESZN-1

29.4

7.8

0.06

0.31

0.12

0.09

0.52

ESZN-2

18.3

6.2

0.02

0.38

0.15

0.18

0.71

ESZN-3

13.8

8.0

0.01

0.62

0.08

0.22

0.92

ESZN-4

1.2

30.6

0.01

0.68

0.07

0.51

1.26

HPS

1.64

44.3

0.009

0.60

0.00

0.82

1.42

ZrO2

33.4

9.2

0.08

0.10

0.00

.00

0.10

Figure-7 Ammonia temperature programmed desorption profiles (NH3-TPD) of synthesized catalysts Ammonia temperature programmed desorption (NH3 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 oC were assigned to weak acidity whereas the peaks obtained at 550 oC was assigned to medium acidity. Similarly, peaks obtained beyond 620 oC desorption temperature were assigned to strong acidity. The quantitative acidity data of all catalysts are given in Table-1. It is evident from the Table-1 that both strong and total acidity increases with increase in HPS loading which shows a uniform distribution of the parent catalyst over ZrO2 support. 3.2. Catalysts Performance Evaluation On performing LA esterification reaction in the presence of support alone without parent HPA, a 14 mol% LA conversion with 100 mol% EL selectivity at 110 oC temperature in 30 minutes 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 oC for 30 minutes caused 96% LA conversion (TOF



= 6.4 min-1) and 94% yield. Thus, 110 oC operating temperature was selected for the performance evaluation of synthesizes 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, increase in HPS loading to 20 wt.% (ESZN-2), 30 wt.% (ESZN-3), and 40 wt.% (ESZN-4) over ZrO2 support 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 110 oC temperature in 30 minutes, 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 most preferred catalyst for further study.

Figure-8 (a) Catalysts Screening, LA 2 mmol, LA: EtOH 1:43, Temp. 110 oC, Time 30 minutes, Catalysts 100 mg at 300 rpm (b) Effect of Stirring Speed, LA 2 mmol, LA: EtOH 1:43, Temp. 110 oC, Time 30 minutes, ESZN-4 100 mg (c) Effect of LA to EtOH Ratio LA 2 mmol, Temp. 110 oC, Time 30 minutes, ESZN-4 100 mg and, (d) Effect of Reactant Concentration, LA: EtOH 1:43, Temp. 110 oC, Time 30 minutes, ESZN-4 100 mg 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. Maximum 83 mol% LA conversion was measured with similar EL yield when experiments were performed at 110 oC temperature for 30 minutes 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 condition. A considerable increase in LA conversion (90 mol%) was measured as compared to LA conversion 13 ACS Paragon Plus Environment

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(83 mol%) measured in the absence of stirring in the reaction mixture. Interestingly, no significant change in LA conversion and EL yield were measured when stirring speed was changed from 300 rpm to 600 rpm 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 non-stirred experiments as compared to stirred experiments could be due to non-uniform heat distribution for non-stirred 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 300 rpm stirring speed. 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 to 1:51 as shown in Figure-8c. Nevertheless, varying LA: EtOH molar ratio did not cause a considerable change in both the LA conversion and the EL yield. Interestingly, Yadav and coworkers have reported similar observations on the application of excess alcohol in esterification reaction32. Authors have reported that excess alcohol occupies all the catalytic sites, thereby making levulinic acid as a limiting reactant which in turn makes it a pseudo 1st order reaction. Therefore, 1:43 LA to EtOH molar ratio was chosen for further experiments considering microwave reactor limitations which yields most consistent results in the 5 mL volume which is equivalent to 1:43 molar ratio of LA to EtOH. Table -2 Comparison of conversion values with literature Reactant

Catalyst

Experimental

Conversion Reactor

Ref.

Conditions LA

40

wt.% T= 65 oC, Catalyst= >80% 14 ACS Paragon Plus Environment

Reflux (Conventional

33


HPS/SiO2 LA

15

104 mg, Time= 6 h

wt.% T= 78 oC, Cat. /LA 93%

HPW/DH-

ratio= 0.2, Time= 4 h

Page 16 of 37

Heating) Reflux (Conventional

34

Heating)

ZSM-5 LA

oC,

7.2ArSO3H-

T= 78

Cat= 2 >94%

Si(Et)Si-Ph-

wt.%, Time= 4 h


35

Heating)

NTs LA

H2SO4

T= 78

oC,

Cat= 2 >98%

wt.%, Time= 1 h LA Furfuryl

20


HPS/SiO2

ratio= 0.2, Time= 2 h

Heating)

Ti-HPW

T= 120 oC, Cat. = 100%

Sealed

2.25 wt.%, Time= 30

(Conventional

min

Heating)

40

wt.% T= 110 oC, Cat= 100 >90%

HPS/ZrO2

35

Heating)

wt.% T= 84 oC, Cat. /LA >91%

Alcohol LA


mg, Time= 30 min

Reactor

Microwave Reactor

36

37

This Study

Similarly, the effect of reactant concentration was studied by varying LA feed between 0.5 mmol to 2.5 mmol which results 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 in the microwave reactor in 5 mL ethanol at 110 oC temperature for 30 minutes in the presence of 100 mg heterogenized HPS catalyst ESZN-4. However, the LA conversion decreased from 95 mol% and 90 mol% as LA feed concentration was increased to 1.5 mmol to 2 mmol respectively. Further, the increase in LA feed concentration to 2.5 mmol reduced conversions to 85 mol%. LA feed concentration 2 mmol was considered optimum considering 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, to be noted that the


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experiments were performed at a higher temperature in the present study as compared to 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 pressure rise due to heating effects, thus the possibility of pressure effect may exist which can be an interesting area of study in the future. Earlier reports suggest that the change in reactor type from the conventional batch reactor to the 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 for at least 5 times with minor loss in the activity as shown in Figure-9. Before application of the used catalysts, the catalysts were washed twice with ethanol and dried at 200 oC 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 nearly 10.2% loss in the activity. One possible reason for the loss in the activity can be attributed to leaching on parent catalyst in the solvent.

Figure-9 Recyclability of the catalyst ESZN-4, (LA 2 mmol, LA: EtOH 1:43, Temp. 110 oC, Time 30 minutes, Catalysts 100 mg at 300 rpm) 6.4. Kinetic Study Kinetic study experiments for LA conversion reaction were performed at different temperatures and time as shown in Figure-10a. In general, 2 mmol of LA was charged into the microwave reactor vial along with 100 mg ESZN-4 catalyst and 5 mL ethanol (LA: EtOH, 1:43 molar ratio) at 300 rpm. Post this, series of experiments were performed at 70 oC, 80 oC, 90 oC, 100 oC, and 16 ACS Paragon Plus Environment


110 oC reaction temperature each for 5, 10, 15, 20, 25 and 30 minutes of reaction time. An increasing trend in LA conversion was observed at lower temperatures throughout the reaction whereas experiments at higher temperature showed a linear increase in LA conversion only for first 15 minutes and started slowing down after possibly due to reason that it started approaching equilibrium condition after 15 minutes at higher temperatures (100 oC-110 oC).

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

From Figure-10a, it is also evident that the difference in LA conversion at lower temperature (70 oC-90 oC)

was less as compared to difference in LA conversion at 100 oC-110 oC 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 the catalyst. EL yield was 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 irradiations. Also, it is well known that the esterification reactions follows first-order kinetics38–43. Considering the absence of mass transfer and diffusion limitations, Yadav and co-workers have reported a dual site monofunctional Langmuir-Hinshelwood-Hougen-Watson model which follows chemisorption, and adsorption mechanism32 for which herein, LA is denoted by L, Alcohol (ethanol) is denoted by A, EL by E, Water by W and vacant sites by *. Firstly, levulinic acid L and Ethanol A get adsorbed on the active sites of the catalyst as follows:


Page 18 of 37

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L+ * A+ *

𝐾1

L*

(1)

A*

(2)

𝐾2

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

L* + A*

𝐾3

E* + W*

(3)

Eventually, adsorbed species undergoes desorption step to yield ethyl levulinate E and molecule W as follows:

E*

1𝐾 4

W*

E+*

1𝐾 5

(4)

W+*

(5)

From equation 1, 2, 3, 4, and 5, the total concentration of sites is:

CT = C* + CL* + CA* + CE* + CW*

(6)

or CT = C* + K1 CL C* + K2 CA C* + K4 CE C* + K5 CW C*

(7)

Assuming that adsorption and desorption steps in equilibrium, equation (7) can be written as follows:

C* =( 1 + 𝐾 𝐶

1 𝐿

𝐶𝑇

(8)

+ 𝐾2𝐶𝐴 + 𝐾4𝐶𝐸 + 𝐾5𝐶𝑊)

Since reaction takes place via adsorption and desorption routes, thereby leading to the possibility of mostly surface controlled mechanism, the rate of reaction of levulinic acid L can be written as:

-rL = -

𝑑𝐶𝐿 𝑑𝑡

= K3 CL* CA* - K3’ CE* CW*

(9)



-


(

𝐾3 𝐾1𝐾2𝐶𝐿𝐶𝐴 ―

=

)𝐶

𝐾4𝐾5𝐶𝐸𝐶𝑊 𝐾3

2 𝑇

(10)

( 1 + 𝐾1𝐶𝐿 + 𝐾2𝐶𝐴 + 𝐾4𝐶𝐸 + 𝐾5𝐶𝑊)2

For a reaction which is far away from equilibrium, equation (10) can be written as:

-


=

𝐾3𝐶2𝑇𝐾1𝐾2𝐶𝐿𝐶𝐴 ( 1 + ∑𝐾𝑖𝐶𝑖)

(11)

2

Replacing K3CT2K1K2 with KR 𝑑𝐶𝐿

𝐾𝑅𝐾2𝐶𝐿𝐶𝐴

(12)

- 𝑑𝑡 = 2 ( 1 + ∑𝐾𝑖𝐶𝑖) or 𝑑𝐶𝐿

𝑑𝑋𝐿

- 𝑑𝑡 = 𝑐𝐿𝑂 𝑑𝑡 = 𝐾𝑅 ( 𝑐𝐿𝑂 ― 𝑐𝐿𝑂𝑋𝐿)(𝑐𝐴𝑂 ― 𝑐𝐿𝑂𝑋𝐿)

= 𝐾𝑅 𝑐𝐿𝑂 (1 ― 𝑋𝐿)(𝑀 ― 𝑋𝐿) 𝑑𝐶𝐿

𝑑𝑋𝐿

(13) (14)

- 𝑑𝑡 = 𝑐𝐿𝑂 𝑑𝑡 = 𝐾𝑅 𝑐𝐿𝑂𝑀 (1 ― 𝑋𝐿)

Since 𝐾𝑅 𝑐𝐿𝑂 is constant, therefore it can be represented by a single term k and XL can be 𝑐𝐴 𝑂

simply written as X whereas M= 𝑐𝐿 , and, (M-𝑋)≅𝑀 owing to M ≫ 20 values 𝑂

𝑑𝑋𝐿 𝑑𝑡

(15)

= 𝑘(1 ― 𝑋) 𝑐𝐴𝑂

Where M= 𝑐 , when M ≫ 20, (M-𝑋)≅𝑀 and thus 𝐾𝐶 = 𝐾𝑅2𝑀 𝐿𝑂 Therefore, it becomes a pseudo-first order reaction which can be written as follows:

-ln (1-𝑋) = 𝑘 𝑡

(16)

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


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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 38.43 kJ/mol which indicates that the reaction is kinetically controlled. The results obtained are in line with earlier reports which suggests that esterification reaction having an activation barrier above 20 kJ/mol are kinetically controlled44. Conclusions 10 wt.%, 20 wt.%, 30 wt.% and 40 wt.% silicotungstic acid was successfully loaded over zirconium oxide support without losing its Keggin structure. The ESZN-4 catalyst (having 40 wt.% HPS) yielded more than 90 mol% EL at 110 oC reaction temperature in 30 minutes in the presence of 100 mg 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 LangmuirHinshelwood-Hougen-Watson model based kinetic study revealed that LA esterification reaction followed first order rate in the presence of ESZN-4 catalysts and activation barrier was measured 38.43 kJ/mol for LA to EL conversion reaction.

Supporting information: Catalysts Synthesis Procedure Diagram (Figure S1); FESEM Mapping Profile of ESZN Catalyst (Figure S2)

Declaration Authors delcares no conflict of interest.



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Figure-1 SEM images of (a) ESZN-1, (b) ESZN-2, (c) ESZN-3 and, (d) ESZN-4 taken at 50K magnification 125x102mm (96 x 96 DPI)


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Figure-2 XRD patterns of HPS, ZrO2, and ZrO2 supported Keggin type silicotungstic acid 118x90mm (96 x 96 DPI)



Figure-3 FTIR spectra of the parent silicotungstic acid and synthesized catalysts 127x90mm (96 x 96 DPI)


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Figure-4 Raman spectra of parent Keggin type silicotungstic acid and synthesized catalysts 130x98mm (96 x 96 DPI)



Figure-5 (a) Structure of parent Keggin type silicotungstic acid and, (b) Plausible catalysts support interaction between parent HPS and ZrO2 support 165x90mm (96 x 96 DPI)


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Figure-6 HR-TEM images of ESZN-4 (a) 100 nm, (b) 50 nm, (c) 2 nm and, (d) SAED 157x109mm (96 x 96 DPI)



Figure-7 Ammonia temperature programmed desorption profiles (NH3-TPD) of synthesized catalysts 120x84mm (96 x 96 DPI)


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Figure-8 (a) Catalysts Screening, LA 2 mmol, LA: EtOH 1:43, Temp. 110 oC, Time 30 minutes, Catalysts 100 mg at 300 rpm (b) Effect of Stirring Speed, LA 2 mmol, LA: EtOH 1:43, Temp. 110 oC, Time 30 minutes, ESZN-4 100 mg (c) Effect of LA to EtOH Ratio LA 2 mmol, Temp. 110 oC, Time 30 minutes, ESZN4 100 mg and, (d) Effect of Reactant Concentration, LA: EtOH 1:43, Temp. 110 oC, Time 30 minutes, ESZN4 100 mg 163x141mm (96 x 96 DPI)



Figure-9 Recyclability of the catalyst ESZN-4, (LA 2 mmol, LA: EtOH 1:43, Temp. 110 oC, Time 30 minutes, Catalysts 100 mg at 300 rpm) 125x84mm (96 x 96 DPI)


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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 152x139mm (96 x 96 DPI)


Synthesis and Characterization of Zirconia Supported Silicotungstic

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