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Conversion of levulinic acid to ethyl levulinate using tin modified silicotungstic acid supported on Ta2O5
Catalysis Communications 134 (2020) 105864
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Short communication
Conversion of levulinic acid to ethyl levulinate using tin modified silicotungstic acid supported on Ta2O5 Parameswaram Ganji, Sounak Roy
T
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Department of Chemistry, Birla Institute of Technology and Science, Pilani, Hyderabad Campus, Jawahar Nagar, Hyderabad 500078, Telangana, India
ARTICLE INFO
ABSTRACT
Keywords: Silicotungstic acid Tin Ta2O5 Esterification Levulinic acid Ethyl levulinate
Tin modified silicotungstic acid (STA) deposited on Ta2O5 was explored as suitable catalyst for the conversion of levulinic acid to ethyl levulinate. The microwave synthesized catalysts of varying amounts of STA were characterised by N2 adsorption, powder XRD, FT-IR, FE-SEM, XPS, pyridine-FT-IR, TGA-DTA and NH3-TPD. Among the synthesized catalysts, 50% Sn2STA/Ta2O5 catalyst exhibited excellent catalytic activity for the esterification of levulinic acid providing 78% of ethyl levulinate yield and stable recyclability up to three cycles. The rate of esterification was 6.6 × 10−3 mol/g/min, which is significantly higher compared to reported values in the open literature. Mechanistic investigations revealed that the high catalytic activity was influenced by the presence of appropriate Brønsted acid sites, surface area and porosity.
1. Introduction Depletion of non-renewable fossil fuels, global warming and other environmental hazards have resulted in a paradigm shift towards the generation of fuels and chemicals from renewable materials. Against this background, many researchers are relentlessly working in finding varieties of biomass platform chemicals and synthesis routes to produce useful key industrial chemicals and transportation fuels. According to National Renewable Energy Laboratory reports (Denver, USA), the lignocellulose biomass derived levulinic acid (LA) is one of the top-12 priority platform chemicals, since the presence of two functional moieties in this γ-keto acid imparts it the potential to render a variety of useful chemicals on subjection to different reactions [1,2]. The esterification of LA with alcohol attracted much attention as these levulinates or esters consist of versatile chemical feedstocks with numerous potential applications in the flavouring and fragrance industry, or as a blending component in biodiesel fuel [3–8]. Among them, ethyl levulinate (EL) is one of the most important ester as blending of 5 wt% of EL in diesel leads to important modification in the fuel properties, such as high lubricity, stability in flashpoint, reduced sulfur content, and improved viscosity [6,9,10,11]. EL can be efficiently produced by esterification of LA with ethanol with traditional catalysts, like strong Brønsted liquid acids (such as H2SO4, HCl, and polyphosphoric acid) [12,13]. Nevertheless, liquid acid-catalysed homogeneous esterification processes suffer from severe corrosion, waste and safety problems, handling and recycling issues
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[13,14,15]. Development of separable and eco-friendly recyclable solid acid catalysts is therefore one of the key technologies to establish environmentally sustainable approaches for the LA esterification processes. In the category of solid acid catalysts, heteropoly acids have gained significant attention because of their super strong Brønsted acidity and special Keggin-type structural properties [7,9,16,17]. However, the poor chemical stability, high solubility in polar media and small specific surface area have limited their industrial applications. Dispersing the heteropoly acid on a porous support recently has attracted much attention [9,11,18–20]. In spite of some minor improvements, largely the catalysts are expensive, cumbersome to synthesize, performances are poor, and, therefore, it would be a novel challenge to exploit a fine catalyst to improve the synthetic efficiency of EL. In this work we report for the first time a novel microwave-assisted hydrothermal synthesis method of a tin (Sn) incorporated heteropolyacid supported on another water tolerant solid Brønsted acid catalyst, like Ta2O5, Nb2O5 and Al2O3, and studied their performances towards esterification of renewable LA with ethanol as an alkylating agent. Among the synthesized catalysts, tin incorporated silicotungstic acid over Ta2O5 was found to have excellent yield and very high initial rate for the catalytic formation of EL from LA in ethanolic solvent, as compared to the open literature. The performances of these synthesized catalysts were correlated with the influence of surface acidity, porosity and surface hydrophobicity.
Corresponding author. E-mail address: [email protected] (S. Roy).
https://doi.org/10.1016/j.catcom.2019.105864 Received 4 September 2019; Received in revised form 22 October 2019; Accepted 24 October 2019 Available online 31 October 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 134 (2020) 105864
P. Ganji and S. Roy
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Fig. 1. %-Yield of EL and Al over unsupported and supported oxide catalysts after 3 h of esterification of LA at 70 °C; Inset: The reaction scheme with products distribution.
2. Experimental
The weak and strong acidic sites on the surface of the catalysts were evaluated by temperature-programed desorption of ammonia (NH3TPD), and FT-IR spectra of adsorbed pyridine over the as-prepared materials, details of which were reported earlier [21,22]. The NH3-TPD is one of the most conventional methods for characterizing the overall acidity of the catalysts. The strong basicity and small molecular size of ammonia facilitate identification of acidic sites present even on the surface of the small pores in the solids. The NH3-TPD of catalytic samples was carried out in a BELCAT II instrument (Japan). To screen the as prepared catalysts for esterification, LA and ethanol were taken in a round bottom flask connected with a reflux condenser. Typically, the substrate LA (7 g) and ethanol were charged together into the flask followed by the addition of the as prepared catalyst (0.05 g). Esterification of LA was performed at 70 °C for 3 h using excess of ethanol with a molar ratio of ethanol:LA = 5:1. Typically, the amount of ethanol was 17.5 mL in a reaction. During reaction the aliquots were periodically withdrawn and filtered prior to their analysis by gas chromatography (GC). Reaction samples were analysed by GC (Agilent 3900 Gas chromatograph) using a capillary EB-WAX column (30 m × 0.32 mm × 0.25 μm), nitrogen as a carrier gas within the programmed temperature range of 50–210 °C and the use of a FID detector. The %-yield of the reaction product was calculated by using the following Eq. (1):
Tin (II) chloride, silicotungstic acid (H4[W12SiO40], STA, 99.9%), tantalum oxide, niobium oxide, alumina, levulinic acid (99.9%), Ethyl Levulinate (99.9%) and ethanol were obtained from M/s E. Merck, Mumbai (India). Tin incorporated STA on various solid acidic supports, like Ta2O5, Nb2O5 and Al2O3 were prepared by a microwave assisted hydrothermal method. The detailed synthetic procedure is reported in previous publications [21,22]. In a typical process, calculated amount of aqueous solution of SnCl2.2H2O was added drop wise to the required amount of STA in de-ionised water, and the resulting mixture was subjected to heating in a G30 vial at 70 °C for 2 h in the microwave system (Anton Parr, Monowave 300, GmbH, Europe) in order to replace the two protons of STA and produce the Sn2STA solid acid. After the microwave treatment, the obtained partially soluble solid acid Sn2STA was dried at 60 °C overnight in a hot air oven. For the Sn2STA/support (support = Ta2O5, Nb2O5 and Al2O3) solids, the synthesized Sn2STA was taken in deionised water with the oxidic support and subjected to microwave heating in a similar method as stated above. The amount of Sn2STA was varied from 15 to 60 wt% with respect to the oxide support. For specific surface area (SSA, m2/g) and pore size distribution estimations of the synthesized catalysts, N2 physisorption isotherms were measured at 77 K with a Microtrac BELSORP mini-II surface area analyser after using the BET method. Phase identification and degree of crystallinity were recorded on a Rigaku ULTIMA-IV powder XRD operated at 40 kV and 30 mA using CuKα radiation as an X-ray source. The diffractograms were recorded with a step size of 0.02° and scan rate of 2° min−1. Thermo Scientific K-ALPHA surface analysis spectrometer with X-ray source of Al Kα radiation (1486.6 eV) was used to record XPS spectra of the synthesized catalysts. Binding energies are reported with respect to C (1 s) at 284.8 eV. The FT-IR spectra (transmission mode) of the synthesized materials were recorded by JASCO FT-IR-4200 instrument at room temperature. The synthesized catalysts were mixed with KBr, grounded with mortar and pestle, and a pellet was made afterwards to record the FTIR spectrum in transmittance mode with a resolution of 4 cm−1. TGA was performed using a Shimadzu DTG-60 instrument in the temperature range of 30–700 °C and a heating rate of 10 °C min−1 under N2 gas atmosphere. The surface morphology of the powders was evaluated by using FE-SEM (FEI, Apreo Model).
%Yield =
Actual Yield × 100% Theoretical Yield
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The actual yield was calculated experimentally with the help of a calibration curve of the EL concentration (ppm) vs area response under the GC peak. 3. Results and discussion As Nb2O5.nH2O, Ta2O5.nH2O and Al2O3 exhibit excellent Brønsted acidity and are effective for esterification, olefin hydration, and alcohol dehydration [23], these solid materials were chosen as support materials in the present investigation. Successful synthesis of 50% Sn2STA over Ta2O5, Nb2O5 and Al2O3 was confirmed by powder diffraction (Fig. S1, ESI). The esterification of LA to produce EL using these synthesized catalysts was evaluated under the above-mentioned 2
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Fig. 2. (a) FT-IR spectra of catalysts after adsorption of pyridine at room T. (b) NH3-TPD profiles of 50 wt% Sn2STA/Nb2O5, 50 wt% Sn2STA/Al2O3, 50 wt% Sn2STA/ Ta2O5. Blank data is provided in Fig. S3(a-b) of ESI.
experimental conditions, and the obtained results are presented in Fig. 1. The GC profiles of formation of LA and AL are shown in Fig. S2 (ESI). Apparently, there is formation of two products, the primary product EL and the by-product angelica lactone (AL). AL is formed due to an acid catalysed dehydration followed by cyclization (see inset of Fig. 1), which is also reported in the literature [24–26]. It is evident from these results that Ta2O5 support outperforms the other two supports tested as well as the unsupported Sn2STA solid. The yield of EL was found to be 78% over the 50% Sn2STA/Ta2O5 catalyst, whereas it was only 33 and 23% over the 50% Sn2STA/Nb2O5 and 50% Sn2STA/ Al2O3, respectively. On the other hand, the unsupported Sn2STA showed that the %-yield of EL is only 19%. The selectivity of EL over AL was also found to be higher (96%) over the 50% Sn2STA/Ta2O5 catalyst compared to the other two catalysts (ca. 47% EL selectivity over 50% Sn2STA/Al2O3 and 76% EL selectivity over 50% Sn2STA/Nb2O5). As the surface acidity plays a key role in determining the catalytic performance for the present catalytic system, to probe the higher efficiency of the Ta2O5 support, the surface acidity of the synthesized catalysts was evaluated. Fig. 2a presents pyridine adsorbed FT-IR spectra, and Fig. 2b NH3-TPD profiles of the three investigated catalysts. The major IR bands observed at 1441, 1482 and 1536 cm−1 correspond to surface Lewis acid sites, both Brønsted and Lewis type acid sites, and Brønsted acid sites, respectively [21,22]. It is evident from Fig. 2a that Ta2O5-supported catalyst possesses very strong bands due to the presence of Brønsted acid sites, concluding the highest Brønsted acidity over the 50% Sn2STA/Ta2O5 among the three catalysts. Control experiments of adsorbed pyridine FT-IR spectra of the three bare supports, only showed the 1441 cm−1 peak (Fig. S3a, ESI). The NH3-TPD profiles shown in Fig. 2b also support the view that the acidity is in the following order: Ta2O5 > Nb2O5 > Al2O3. The total amount of NH3 desorbed equivalent with the total surface acidity, was found to be 1.39, 0.78 and 0.55 mmol/g over the 50% Sn2STA/Ta2O5, 50% Sn2STA/Nb2O5 and 50% Sn2STA/Al2O3 catalyst, respectively. Therefore, the highest yield of EL with high selectivity can be directly correlated with the surface acidity exhibited by the 50% Sn2STA/Ta2O5 solid. It should be noted that the bare supports did not show any significant NH3 desorption (Fig. S3b, ESI). As the Ta2O5 support was found to have the best performance compared to the other support, the next exploration was to correlate the surface concentration of Sn2STA over the Ta2O5 support with catalytic performance. The loading of Sn2STA was varied from 15 to 60 wt%, and the corresponding powder diffraction profiles of the synthesized catalysts are presented in Fig. 3. All the synthesized supported catalysts showed good crystallinity as evident from the sharp diffraction peaks recorded. In the top panel of Fig. 3, the three main diffraction peaks
Fig. 3. XRD patterns of 15–60 wt% Sn2STA/Ta2O5, Ta2O5, and STA solid catalysts. The shown (001), (100) and (101) planes correspond to Ta2O5 (JCPDS 19–1299).
appeared at 2θ = 10.3°, 25.6° and 34.7° and which can be assigned to the diffraction lines of body centred cubic secondary structure of the Keggin ion of STA. Ta2O5 showed major characteristic diffraction peaks at 2θ = 22.75°, 28.38° and 36.79° corresponding to the (001), (100) and (101) planes, respectively, which are due to the hexagonal phase of Ta2O5 (JCPDS 19–1299). After using a lower loading of Sn2STA over Ta2O5 (15 and 40 wt%), the STA peaks were obscured. However, with increasing loading of Sn2STA the characteristic peaks were clearly observed, indicating the retention of the Keggin structure in the Sn2STA/ Ta2O5 solid. The structural information was also corroborated from FTIR analyses (see Fig. S3c, ESI). All the synthesized catalysts exhibited the characteristic FT-IR bands of SieO, terminal W]O, inter-octahedral W–O–W, and intra-octahedral W–O–W at 1020, 980, 877 and 3
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778 cm−1, respectively, suggesting the intact Keggin structure after exchange of protons with Sn in the secondary structure of STA [27,28]. Elemental concentration measured by XRF was found in accordance with the experimental conditions applied (Table S1, ESI). The thermal stability of prepared catalysts was evaluated by TGA and DTA, and results are shown in Fig. S4 (ESI). The graph shows the typical TG/DTA patterns for the Keggin-type heteropolyacid. The initial major weight loss below 100 °C is related to the elimination of adsorbed water, which is responsible for the crystallization of heteropolyanions into a hydrate. The second weight loss just above 100 °C could be due to the evolution of CO2 and decomposition of oxycarbonates to get a stable catalyst. The final weight loss at elevated temperatures could be due to the decomposition of heteropolyanion, with crystallization of oxides. Apparently, the catalyst is thermally stable at the operating reaction temperature. The oxidation state(s) of these surface active sites was explored by XPS, and results are shown in Fig. S5 (ESI). The survey spectra did show existence of Sn 3d and O 1 s XPS peaks. The core level spectra of Sn in Sn2STA showed two peaks at 487.3 and 495.7 eV corresponding to 3d5/2 and 3d3/2 of Sn4+. It must be noted that the binding energy of Sn4+ in STA is slightly higher than the reported binding energies of Sn4+ in SnO2, indicating more ionic environment of Sn in STA than in SnO2. There is also a minor shoulder at slightly lower binding energy signifying existence of little amount of Sn2+ in the sample. The surface textural features of the catalysts were probed by nitrogen adsorption−desorption. Interestingly, the specific surface area increased with increasing Sn2STA loading on the Ta2O5 support, and a highest surface area of 59.8 m2/g was achieved with 50 wt% Sn2STA loading. Specific pore volume and mean pore diameter were also found to be highest with 50 wt% Sn2STA loading. To understand the catalyst's surface morphology, FE-SEM images of the as prepared Sn2STA/Ta2O5 catalysts were obtained and these are shown in Fig. S6 (ESI). The average secondary particle size of Ta2O5 was found to be ~ 300 nm, and it is evident from the micrographs that with the increase in Sn2STA loading the surface of Ta2O5 is covered with more Sn2STA particles. The Sn2STA particles are discrete up to 50 wt% of loading, and further increase in loading makes them further agglomerated. The mean size of Sn2STA secondary particles were ~ 50 nm. The microwave synthesized catalysts with varied loading of Sn2STA
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deposited on Ta2O5 were explored for the catalytic esterification up to 3 h of reaction, and results are shown in Fig. 4. With the lower loading of 15 wt% Sn2STA the catalyst showed only 25% of EL yield and a corresponding 93% selectivity. Thus, there is 7% selectivity for the byproduct AL. By further increasing Sn2STA loading to 40 wt%, the catalyst did not show any significant change in the EL yield; however, the selectivity towards EL was poor, and 15% of AL was obtained. Further increase in loading of heteropoly acid sites on the Ta2O5 support surface (50 wt% Sn2STA/Ta2O5) resulted in a 78% yield of EL with as high as 97% selectivity towards EL. With a further increase of Sn2STA loading to 60%, the EL yield decreased to 37%, whereas the AL yield was increased to 14%. It is apparent from these results that an optimum loading is about 50% Sn2STA over the Ta2O5 support that shows excellent LA conversion to EL with higher selectivity under the experimental conditions applied. Further time-dependent experiments on the progress of reaction were conducted over the 50 wt% Sn2STA/Ta2O5, and results are shown in the inset of Fig. 5. It is shown that the concentration of LA decreases with time as the % formation of EL is increased, while AL shows a volcanic hump against time. The initial rate of conversion of LA was calculated and found to be 6.6 × 10−3 mol/g/ min. In order to rationalize the catalytic performance dependence on the surface acidity, pyridine adsorbed FT-IR and NH3-TPD experiments were conducted and data are shown in Fig. 5. It is apparent from the intensity of IR bands at 1545, 1445 and 1490 cm−1 in Fig. 5a, that the Brønsted/Lewis site ratio increased with increasing Sn2STA loading up to 50 wt% and with further increase of it the intensity of both IR bands decreased. The uniform distribution of Sn2STA up to 50 wt% loading on the surface of Ta2O5 (vide FE-SEM) might be responsible for the increase of Brønsted and Lewis acid sites concentration, and with a higher loading it might form multilayers that could block access to some acid sites of Sn2STA from the catalyst support, thus deteriorating the catalytic efficiency [9]. The desorbed NH3 against temperature traces of the synthesized catalysts show very good surface acidic strengths for the catalytic materials. For all the catalysts, there is a minor low-temperature desorption peak, and a larger peak above 500 °C (Fig. 5b). The amount of ammonia desorbed at a given temperature range is taken as a measure of the acid site concentration, whereas the temperature ranges at which most of the ammonia is desorbed indicates the acid strength
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Variation in Sn2STA loading over Ta2O5 Fig. 4. %-Yield of LA conversion to EL against varied amount of Sn2STA deposited on Ta2O5. Time dependent reaction profile with % conversion of LA and % formation of El and AL over the 50 wt% Sn2STA/Ta2O5 catalyst is shown in inset graph. 4
Catalysis Communications 134 (2020) 105864
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Fig. 5. (a) Room temperature FT-IR spectra of pyridine–adsorbed on 15–60 wt% Sn2STA/Ta2O5. About 1 μL of pyridine was dropped on KBr pellets of Sn2STA/Ta2O5 before the recording of spectra. (b) NH3-TPD profiles of 15–60 wt% Sn2STA/Ta2O5 catalysts. NH3 was passed over the catalysts at a flow rate of 10 cc/min for 1 h before desorption. Blank data is provided in ESI.
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distribution. The total amount of NH3 desorbed from the catalysts was found to be 0.45, 0.95, 1.39, and 1.11 mmol/g for the 15, 40, 50 and 60 wt% Sn2STA/Ta2O5, respectively. It can be easily concluded from pyridine adsorbed FT-IR and NH3-TPD studies that 50 wt% Sn2STA/ Ta2O5 has the optimum ratio of Brønsted to Lewis acid sites due to the uniform distribution of Sn2STA over the Ta2O5 support, parameter that makes this particular catalyst composition efficient for LA esterification with high selectivity to EL. To benchmark the performance of the synthesized catalyst, the results were compared with reported values in existing literature and which are comprehensively presented in Table S2 (ESI). Literature shows that a wide range of heteropoly acids like HPA-H4SiW12O40 and H3PMo12O40 were used to modify the SiO2 surface and tested for the catalytic esterification of LA [29]. However, after prolonged 6–10 h reaction and high catalyst loading, the conversion was only 75%, and the product yield was as low as 67%. The H/BEA zeolite modified with NaOH was applied as catalyst for LA esterification with ethanol, where the LA conversion was only 40% [11,30]. In another study, ZrO2/SBA15 catalysts were synthesized by the impregnation method with moderate acidic sites [24]. The ZrO2 modified zeolite catalyst could only convert 50% of LA at a very high temperature, ca. 250 °C. A significant conversion of LA with high selectivity of EL was only achieved in a flow reactor under vapour phase conditions over a WO3-incorporated SBA16 catalyst [31]. Based on the above discussion, it is apparent that the present tin modified silicotungstic acid/Ta2O5 catalyst is by far the best catalytic material, which could efficiently convert LA to EL with 97% selectivity at the moderate temperature of 70 °C within 3 h of duration. In addition to the catalytic activity, we have also scrutinized the stability and reusability of the 50 wt% Sn2STA/Ta2O5 catalyst. To explore the recyclability performance, the catalyst was subjected to multiple cycles under the same reaction conditions, where after each run, the catalyst was filtered, washed with acetone, and without any further treatment was used for the next cycle. The conversion of LA was found to be 81, 76 and 74%, and the percentage of EL yield was 78, 71 and 68% after the 1st, 2nd and 3rd cycle, respectively.
yield and nearly complete selectivity, and with an initial conversion rate of 6.6 × 10−3 mol/g/min. Additionally, the catalyst showed a stable recyclability up to three cycles. The catalytic performance of the 50 wt% Sn2STA/Ta2O5 was strongly favourable when compared with the literature. This work paves a path for alternative heterogeneous catalytic materials to inorganic acids used for the synthesis of EL. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgements P. G. thanks SERB and DST, India, for the financial support in the form of N-PDF grant [SERB/F/2788/20l8-2019, SERB/F/877/20192020]. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.105864. References [1] J.J. Bozell, G.R. Petersen, Technology development for the production of biobased products from biorefinery carbohydrates-the US Department of Energy’s “top 10” revisited, Green Chem. 12 (2010) 539–554. [2] G.W. Huber, S. Iborra, A. Corma, Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering, Chem. Rev. 106 (2006) 4044–4098. [3] D.J. Braden, C.A. Henao, J. Heltzel, C.C. Maravelias, J.A. Dumesic, Production of liquid hydrocarbon fuels by catalytic conversion of biomass-derived levulinic acid, Green Chem. 13 (2011) 1755–1765. [4] C. Antonetti, D. Licursi, S. Fulignati, G. Valentini, A. Raspolli Galletti, New frontiers in the catalytic synthesis of levulinic acid: from sugars to raw and waste biomass as starting feedstock, Catalysts 6 (12) (2016) 196–224. [5] A. Corma, S. Iborra, A. Velty, Chemical routes for the transformation of biomass into chemicals, Chem. Rev. 107 (2007) 2411–2502. [6] J.J. Bozell, Connecting biomass and petroleum processing with a chemical bridge, Science 329 (5991) (2010) 522–523. [7] D.R. Fernandes, A.S. Rocha, E.F. Mai, C.J.A. Mota, V. Teixeira da Silva, Levulinic acid esterification with ethanol to ethyl levulinate production over solid acid catalysts, Appl. Catal. A 425−426 (2012) 199–204. [8] H. Joshi, B.R. Moser, J. Toler, W.F. Smith, T. Walker, Ethyl levulinate: a potential bio-based diluent for biodiesel which improves cold flow properties, Biomass Bioenergy 35 (7) (2011) 3262–3266. [9] K.Y. Nandiwale, S.K. Sonar, P.S. Niphadkar, P.N. Joshi, S.S. Deshpande, V.S. Patil, V.V. Bokade, Catalytic upgrading of renewable levulinic acid to ethyl levulinate biodiesel using dodecatungstophosphoric acid supported on desilicated H-ZSM-5 as catalyst, Appl. Catal. A 460−461 (2013) 90–98. [10] C. Chang, G. Xu, X. Jiang, Bioresour. Technol. 121 (2012) 93–99. [11] G. Pasquale, P. Vazquez, G. Romanelli, G. Baronetti, Catal. Commun. 18 (2012) 115–120. [12] H.J. Bart, J. Reidetschlager, K. Schatka, A. Lehmann, Kinetics of esterification of
4. Conclusions A novel microwave assisted hydrothermal method was employed to prepare Sn- exchanged STA supported on Ta2O5. 50 wt% of Sn2STA loading on the Ta2O5 support not only resulted in high surface area and porosity, but also showed optimum ratio of Brønsted to Lewis acid sites due to the uniform distribution of the active Sn-exchanged heteropoly acid over the support oxide. The resulting 50 wt% Sn2STA supported catalyst achieved high conversion of LA to EL with as high as 78% of 5
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hydroxymethylfurfural, RSC Adv. 8 (50) (2018) 28461–28471. [22] G. Parameswaram, S.A. Roy, Trade-off between acidic sites and crystallinity of WO3- TiO2 catalyst towards dehydration of glucose to 5-hydroxymethylfurfural, Energy Fuel 33 (6) (2019) 5293–5303. [23] T. Okuhara, Water-tolerant solid acid catalysts, Chem. Rev. 102 (2002) 3641. [24] E.S. Sankar, V. Mohan, M. Suresh, G. Saidulu, B.D. Raju, K.R. Rao, Vapor phase esterification of levulinic acid over ZrO2/SBA-15 catalyst, Catal. Commun. 75 (2016) 1–5. [25] F. Su, Q. Wu, D. Song, X. Zhang, M. Wang, Y. Guo, Pore morphology-controlled preparation of ZrO2-based hybrid catalysts functionalized by both organosilica moieties and Keggin-type heteropoly acid for the synthesis of levulinate esters, J. Mater. Chem. A 1 (2013) 13209–13221. [26] C.G. Lima, J.L. Monteiro, T. de Melo Lima, M. Weber Paixão, A.G. Corrêa, Angelica lactones: from biomass-derived platform chemicals to value-added products, ChemSusChem 11 (1) (2018 Jan 10) 25–47, https://doi.org/10.1002/cssc. 201701469. [27] S. Borghese, A. Blanc, P. Pale, B. Louis, Dalton Trans. 40 (2011) 1220. [28] K.V. Rao, P.S. Prasad, N. Lingaiah, Solvent-free hydration of alkynes over a heterogeneous silver exchanged silicotungstic acid catalyst, Green Chem. 14 (5) (2012) 1507–1514. [29] K. Yan, G. Wu, J. Wen, A. Chen, One-step synthesis of mesoporous H4SiW12O40SiO2 catalysts for the production of methyl and ethyl levulinate biodiesel, Catal. Commun. 34 (2013) 58–63, https://doi.org/10.1016/j.catcom.2013.01.010. [30] C.R. Patil, P.S. Niphadkar, V.V. Bokade, P.N. Joshi, Esterification of levulinic acid to ethyl levulinate over bimodal micro–mesoporous H/BEA zeolite derivatives, Catal. Commun. 43 (2014) 188–191. [31] S.S. Enumula, V.R. Gurram, R.R. Chada, D.R. Burri, S.R. Kamaraju, Clean synthesis of alkyl levulinates from levulinic acid over one pot synthesized WO3-SBA-16 catalyst, J. Mol. Catal. A Chem. 426 (2017) 30–48.
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Short communication
Conversion of levulinic acid to ethyl levulinate using tin modified silicotungstic acid supported on Ta2O5 Parameswaram Ganji, Sounak Roy
T
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Department of Chemistry, Birla Institute of Technology and Science, Pilani, Hyderabad Campus, Jawahar Nagar, Hyderabad 500078, Telangana, India
ARTICLE INFO
ABSTRACT
Keywords: Silicotungstic acid Tin Ta2O5 Esterification Levulinic acid Ethyl levulinate
Tin modified silicotungstic acid (STA) deposited on Ta2O5 was explored as suitable catalyst for the conversion of levulinic acid to ethyl levulinate. The microwave synthesized catalysts of varying amounts of STA were characterised by N2 adsorption, powder XRD, FT-IR, FE-SEM, XPS, pyridine-FT-IR, TGA-DTA and NH3-TPD. Among the synthesized catalysts, 50% Sn2STA/Ta2O5 catalyst exhibited excellent catalytic activity for the esterification of levulinic acid providing 78% of ethyl levulinate yield and stable recyclability up to three cycles. The rate of esterification was 6.6 × 10−3 mol/g/min, which is significantly higher compared to reported values in the open literature. Mechanistic investigations revealed that the high catalytic activity was influenced by the presence of appropriate Brønsted acid sites, surface area and porosity.
1. Introduction Depletion of non-renewable fossil fuels, global warming and other environmental hazards have resulted in a paradigm shift towards the generation of fuels and chemicals from renewable materials. Against this background, many researchers are relentlessly working in finding varieties of biomass platform chemicals and synthesis routes to produce useful key industrial chemicals and transportation fuels. According to National Renewable Energy Laboratory reports (Denver, USA), the lignocellulose biomass derived levulinic acid (LA) is one of the top-12 priority platform chemicals, since the presence of two functional moieties in this γ-keto acid imparts it the potential to render a variety of useful chemicals on subjection to different reactions [1,2]. The esterification of LA with alcohol attracted much attention as these levulinates or esters consist of versatile chemical feedstocks with numerous potential applications in the flavouring and fragrance industry, or as a blending component in biodiesel fuel [3–8]. Among them, ethyl levulinate (EL) is one of the most important ester as blending of 5 wt% of EL in diesel leads to important modification in the fuel properties, such as high lubricity, stability in flashpoint, reduced sulfur content, and improved viscosity [6,9,10,11]. EL can be efficiently produced by esterification of LA with ethanol with traditional catalysts, like strong Brønsted liquid acids (such as H2SO4, HCl, and polyphosphoric acid) [12,13]. Nevertheless, liquid acid-catalysed homogeneous esterification processes suffer from severe corrosion, waste and safety problems, handling and recycling issues
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[13,14,15]. Development of separable and eco-friendly recyclable solid acid catalysts is therefore one of the key technologies to establish environmentally sustainable approaches for the LA esterification processes. In the category of solid acid catalysts, heteropoly acids have gained significant attention because of their super strong Brønsted acidity and special Keggin-type structural properties [7,9,16,17]. However, the poor chemical stability, high solubility in polar media and small specific surface area have limited their industrial applications. Dispersing the heteropoly acid on a porous support recently has attracted much attention [9,11,18–20]. In spite of some minor improvements, largely the catalysts are expensive, cumbersome to synthesize, performances are poor, and, therefore, it would be a novel challenge to exploit a fine catalyst to improve the synthetic efficiency of EL. In this work we report for the first time a novel microwave-assisted hydrothermal synthesis method of a tin (Sn) incorporated heteropolyacid supported on another water tolerant solid Brønsted acid catalyst, like Ta2O5, Nb2O5 and Al2O3, and studied their performances towards esterification of renewable LA with ethanol as an alkylating agent. Among the synthesized catalysts, tin incorporated silicotungstic acid over Ta2O5 was found to have excellent yield and very high initial rate for the catalytic formation of EL from LA in ethanolic solvent, as compared to the open literature. The performances of these synthesized catalysts were correlated with the influence of surface acidity, porosity and surface hydrophobicity.
Corresponding author. E-mail address: [email protected] (S. Roy).
https://doi.org/10.1016/j.catcom.2019.105864 Received 4 September 2019; Received in revised form 22 October 2019; Accepted 24 October 2019 Available online 31 October 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 134 (2020) 105864
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Fig. 1. %-Yield of EL and Al over unsupported and supported oxide catalysts after 3 h of esterification of LA at 70 °C; Inset: The reaction scheme with products distribution.
2. Experimental
The weak and strong acidic sites on the surface of the catalysts were evaluated by temperature-programed desorption of ammonia (NH3TPD), and FT-IR spectra of adsorbed pyridine over the as-prepared materials, details of which were reported earlier [21,22]. The NH3-TPD is one of the most conventional methods for characterizing the overall acidity of the catalysts. The strong basicity and small molecular size of ammonia facilitate identification of acidic sites present even on the surface of the small pores in the solids. The NH3-TPD of catalytic samples was carried out in a BELCAT II instrument (Japan). To screen the as prepared catalysts for esterification, LA and ethanol were taken in a round bottom flask connected with a reflux condenser. Typically, the substrate LA (7 g) and ethanol were charged together into the flask followed by the addition of the as prepared catalyst (0.05 g). Esterification of LA was performed at 70 °C for 3 h using excess of ethanol with a molar ratio of ethanol:LA = 5:1. Typically, the amount of ethanol was 17.5 mL in a reaction. During reaction the aliquots were periodically withdrawn and filtered prior to their analysis by gas chromatography (GC). Reaction samples were analysed by GC (Agilent 3900 Gas chromatograph) using a capillary EB-WAX column (30 m × 0.32 mm × 0.25 μm), nitrogen as a carrier gas within the programmed temperature range of 50–210 °C and the use of a FID detector. The %-yield of the reaction product was calculated by using the following Eq. (1):
Tin (II) chloride, silicotungstic acid (H4[W12SiO40], STA, 99.9%), tantalum oxide, niobium oxide, alumina, levulinic acid (99.9%), Ethyl Levulinate (99.9%) and ethanol were obtained from M/s E. Merck, Mumbai (India). Tin incorporated STA on various solid acidic supports, like Ta2O5, Nb2O5 and Al2O3 were prepared by a microwave assisted hydrothermal method. The detailed synthetic procedure is reported in previous publications [21,22]. In a typical process, calculated amount of aqueous solution of SnCl2.2H2O was added drop wise to the required amount of STA in de-ionised water, and the resulting mixture was subjected to heating in a G30 vial at 70 °C for 2 h in the microwave system (Anton Parr, Monowave 300, GmbH, Europe) in order to replace the two protons of STA and produce the Sn2STA solid acid. After the microwave treatment, the obtained partially soluble solid acid Sn2STA was dried at 60 °C overnight in a hot air oven. For the Sn2STA/support (support = Ta2O5, Nb2O5 and Al2O3) solids, the synthesized Sn2STA was taken in deionised water with the oxidic support and subjected to microwave heating in a similar method as stated above. The amount of Sn2STA was varied from 15 to 60 wt% with respect to the oxide support. For specific surface area (SSA, m2/g) and pore size distribution estimations of the synthesized catalysts, N2 physisorption isotherms were measured at 77 K with a Microtrac BELSORP mini-II surface area analyser after using the BET method. Phase identification and degree of crystallinity were recorded on a Rigaku ULTIMA-IV powder XRD operated at 40 kV and 30 mA using CuKα radiation as an X-ray source. The diffractograms were recorded with a step size of 0.02° and scan rate of 2° min−1. Thermo Scientific K-ALPHA surface analysis spectrometer with X-ray source of Al Kα radiation (1486.6 eV) was used to record XPS spectra of the synthesized catalysts. Binding energies are reported with respect to C (1 s) at 284.8 eV. The FT-IR spectra (transmission mode) of the synthesized materials were recorded by JASCO FT-IR-4200 instrument at room temperature. The synthesized catalysts were mixed with KBr, grounded with mortar and pestle, and a pellet was made afterwards to record the FTIR spectrum in transmittance mode with a resolution of 4 cm−1. TGA was performed using a Shimadzu DTG-60 instrument in the temperature range of 30–700 °C and a heating rate of 10 °C min−1 under N2 gas atmosphere. The surface morphology of the powders was evaluated by using FE-SEM (FEI, Apreo Model).
%Yield =
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The actual yield was calculated experimentally with the help of a calibration curve of the EL concentration (ppm) vs area response under the GC peak. 3. Results and discussion As Nb2O5.nH2O, Ta2O5.nH2O and Al2O3 exhibit excellent Brønsted acidity and are effective for esterification, olefin hydration, and alcohol dehydration [23], these solid materials were chosen as support materials in the present investigation. Successful synthesis of 50% Sn2STA over Ta2O5, Nb2O5 and Al2O3 was confirmed by powder diffraction (Fig. S1, ESI). The esterification of LA to produce EL using these synthesized catalysts was evaluated under the above-mentioned 2
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Fig. 2. (a) FT-IR spectra of catalysts after adsorption of pyridine at room T. (b) NH3-TPD profiles of 50 wt% Sn2STA/Nb2O5, 50 wt% Sn2STA/Al2O3, 50 wt% Sn2STA/ Ta2O5. Blank data is provided in Fig. S3(a-b) of ESI.
experimental conditions, and the obtained results are presented in Fig. 1. The GC profiles of formation of LA and AL are shown in Fig. S2 (ESI). Apparently, there is formation of two products, the primary product EL and the by-product angelica lactone (AL). AL is formed due to an acid catalysed dehydration followed by cyclization (see inset of Fig. 1), which is also reported in the literature [24–26]. It is evident from these results that Ta2O5 support outperforms the other two supports tested as well as the unsupported Sn2STA solid. The yield of EL was found to be 78% over the 50% Sn2STA/Ta2O5 catalyst, whereas it was only 33 and 23% over the 50% Sn2STA/Nb2O5 and 50% Sn2STA/ Al2O3, respectively. On the other hand, the unsupported Sn2STA showed that the %-yield of EL is only 19%. The selectivity of EL over AL was also found to be higher (96%) over the 50% Sn2STA/Ta2O5 catalyst compared to the other two catalysts (ca. 47% EL selectivity over 50% Sn2STA/Al2O3 and 76% EL selectivity over 50% Sn2STA/Nb2O5). As the surface acidity plays a key role in determining the catalytic performance for the present catalytic system, to probe the higher efficiency of the Ta2O5 support, the surface acidity of the synthesized catalysts was evaluated. Fig. 2a presents pyridine adsorbed FT-IR spectra, and Fig. 2b NH3-TPD profiles of the three investigated catalysts. The major IR bands observed at 1441, 1482 and 1536 cm−1 correspond to surface Lewis acid sites, both Brønsted and Lewis type acid sites, and Brønsted acid sites, respectively [21,22]. It is evident from Fig. 2a that Ta2O5-supported catalyst possesses very strong bands due to the presence of Brønsted acid sites, concluding the highest Brønsted acidity over the 50% Sn2STA/Ta2O5 among the three catalysts. Control experiments of adsorbed pyridine FT-IR spectra of the three bare supports, only showed the 1441 cm−1 peak (Fig. S3a, ESI). The NH3-TPD profiles shown in Fig. 2b also support the view that the acidity is in the following order: Ta2O5 > Nb2O5 > Al2O3. The total amount of NH3 desorbed equivalent with the total surface acidity, was found to be 1.39, 0.78 and 0.55 mmol/g over the 50% Sn2STA/Ta2O5, 50% Sn2STA/Nb2O5 and 50% Sn2STA/Al2O3 catalyst, respectively. Therefore, the highest yield of EL with high selectivity can be directly correlated with the surface acidity exhibited by the 50% Sn2STA/Ta2O5 solid. It should be noted that the bare supports did not show any significant NH3 desorption (Fig. S3b, ESI). As the Ta2O5 support was found to have the best performance compared to the other support, the next exploration was to correlate the surface concentration of Sn2STA over the Ta2O5 support with catalytic performance. The loading of Sn2STA was varied from 15 to 60 wt%, and the corresponding powder diffraction profiles of the synthesized catalysts are presented in Fig. 3. All the synthesized supported catalysts showed good crystallinity as evident from the sharp diffraction peaks recorded. In the top panel of Fig. 3, the three main diffraction peaks
Fig. 3. XRD patterns of 15–60 wt% Sn2STA/Ta2O5, Ta2O5, and STA solid catalysts. The shown (001), (100) and (101) planes correspond to Ta2O5 (JCPDS 19–1299).
appeared at 2θ = 10.3°, 25.6° and 34.7° and which can be assigned to the diffraction lines of body centred cubic secondary structure of the Keggin ion of STA. Ta2O5 showed major characteristic diffraction peaks at 2θ = 22.75°, 28.38° and 36.79° corresponding to the (001), (100) and (101) planes, respectively, which are due to the hexagonal phase of Ta2O5 (JCPDS 19–1299). After using a lower loading of Sn2STA over Ta2O5 (15 and 40 wt%), the STA peaks were obscured. However, with increasing loading of Sn2STA the characteristic peaks were clearly observed, indicating the retention of the Keggin structure in the Sn2STA/ Ta2O5 solid. The structural information was also corroborated from FTIR analyses (see Fig. S3c, ESI). All the synthesized catalysts exhibited the characteristic FT-IR bands of SieO, terminal W]O, inter-octahedral W–O–W, and intra-octahedral W–O–W at 1020, 980, 877 and 3
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778 cm−1, respectively, suggesting the intact Keggin structure after exchange of protons with Sn in the secondary structure of STA [27,28]. Elemental concentration measured by XRF was found in accordance with the experimental conditions applied (Table S1, ESI). The thermal stability of prepared catalysts was evaluated by TGA and DTA, and results are shown in Fig. S4 (ESI). The graph shows the typical TG/DTA patterns for the Keggin-type heteropolyacid. The initial major weight loss below 100 °C is related to the elimination of adsorbed water, which is responsible for the crystallization of heteropolyanions into a hydrate. The second weight loss just above 100 °C could be due to the evolution of CO2 and decomposition of oxycarbonates to get a stable catalyst. The final weight loss at elevated temperatures could be due to the decomposition of heteropolyanion, with crystallization of oxides. Apparently, the catalyst is thermally stable at the operating reaction temperature. The oxidation state(s) of these surface active sites was explored by XPS, and results are shown in Fig. S5 (ESI). The survey spectra did show existence of Sn 3d and O 1 s XPS peaks. The core level spectra of Sn in Sn2STA showed two peaks at 487.3 and 495.7 eV corresponding to 3d5/2 and 3d3/2 of Sn4+. It must be noted that the binding energy of Sn4+ in STA is slightly higher than the reported binding energies of Sn4+ in SnO2, indicating more ionic environment of Sn in STA than in SnO2. There is also a minor shoulder at slightly lower binding energy signifying existence of little amount of Sn2+ in the sample. The surface textural features of the catalysts were probed by nitrogen adsorption−desorption. Interestingly, the specific surface area increased with increasing Sn2STA loading on the Ta2O5 support, and a highest surface area of 59.8 m2/g was achieved with 50 wt% Sn2STA loading. Specific pore volume and mean pore diameter were also found to be highest with 50 wt% Sn2STA loading. To understand the catalyst's surface morphology, FE-SEM images of the as prepared Sn2STA/Ta2O5 catalysts were obtained and these are shown in Fig. S6 (ESI). The average secondary particle size of Ta2O5 was found to be ~ 300 nm, and it is evident from the micrographs that with the increase in Sn2STA loading the surface of Ta2O5 is covered with more Sn2STA particles. The Sn2STA particles are discrete up to 50 wt% of loading, and further increase in loading makes them further agglomerated. The mean size of Sn2STA secondary particles were ~ 50 nm. The microwave synthesized catalysts with varied loading of Sn2STA
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deposited on Ta2O5 were explored for the catalytic esterification up to 3 h of reaction, and results are shown in Fig. 4. With the lower loading of 15 wt% Sn2STA the catalyst showed only 25% of EL yield and a corresponding 93% selectivity. Thus, there is 7% selectivity for the byproduct AL. By further increasing Sn2STA loading to 40 wt%, the catalyst did not show any significant change in the EL yield; however, the selectivity towards EL was poor, and 15% of AL was obtained. Further increase in loading of heteropoly acid sites on the Ta2O5 support surface (50 wt% Sn2STA/Ta2O5) resulted in a 78% yield of EL with as high as 97% selectivity towards EL. With a further increase of Sn2STA loading to 60%, the EL yield decreased to 37%, whereas the AL yield was increased to 14%. It is apparent from these results that an optimum loading is about 50% Sn2STA over the Ta2O5 support that shows excellent LA conversion to EL with higher selectivity under the experimental conditions applied. Further time-dependent experiments on the progress of reaction were conducted over the 50 wt% Sn2STA/Ta2O5, and results are shown in the inset of Fig. 5. It is shown that the concentration of LA decreases with time as the % formation of EL is increased, while AL shows a volcanic hump against time. The initial rate of conversion of LA was calculated and found to be 6.6 × 10−3 mol/g/ min. In order to rationalize the catalytic performance dependence on the surface acidity, pyridine adsorbed FT-IR and NH3-TPD experiments were conducted and data are shown in Fig. 5. It is apparent from the intensity of IR bands at 1545, 1445 and 1490 cm−1 in Fig. 5a, that the Brønsted/Lewis site ratio increased with increasing Sn2STA loading up to 50 wt% and with further increase of it the intensity of both IR bands decreased. The uniform distribution of Sn2STA up to 50 wt% loading on the surface of Ta2O5 (vide FE-SEM) might be responsible for the increase of Brønsted and Lewis acid sites concentration, and with a higher loading it might form multilayers that could block access to some acid sites of Sn2STA from the catalyst support, thus deteriorating the catalytic efficiency [9]. The desorbed NH3 against temperature traces of the synthesized catalysts show very good surface acidic strengths for the catalytic materials. For all the catalysts, there is a minor low-temperature desorption peak, and a larger peak above 500 °C (Fig. 5b). The amount of ammonia desorbed at a given temperature range is taken as a measure of the acid site concentration, whereas the temperature ranges at which most of the ammonia is desorbed indicates the acid strength
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Variation in Sn2STA loading over Ta2O5 Fig. 4. %-Yield of LA conversion to EL against varied amount of Sn2STA deposited on Ta2O5. Time dependent reaction profile with % conversion of LA and % formation of El and AL over the 50 wt% Sn2STA/Ta2O5 catalyst is shown in inset graph. 4
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Fig. 5. (a) Room temperature FT-IR spectra of pyridine–adsorbed on 15–60 wt% Sn2STA/Ta2O5. About 1 μL of pyridine was dropped on KBr pellets of Sn2STA/Ta2O5 before the recording of spectra. (b) NH3-TPD profiles of 15–60 wt% Sn2STA/Ta2O5 catalysts. NH3 was passed over the catalysts at a flow rate of 10 cc/min for 1 h before desorption. Blank data is provided in ESI.
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distribution. The total amount of NH3 desorbed from the catalysts was found to be 0.45, 0.95, 1.39, and 1.11 mmol/g for the 15, 40, 50 and 60 wt% Sn2STA/Ta2O5, respectively. It can be easily concluded from pyridine adsorbed FT-IR and NH3-TPD studies that 50 wt% Sn2STA/ Ta2O5 has the optimum ratio of Brønsted to Lewis acid sites due to the uniform distribution of Sn2STA over the Ta2O5 support, parameter that makes this particular catalyst composition efficient for LA esterification with high selectivity to EL. To benchmark the performance of the synthesized catalyst, the results were compared with reported values in existing literature and which are comprehensively presented in Table S2 (ESI). Literature shows that a wide range of heteropoly acids like HPA-H4SiW12O40 and H3PMo12O40 were used to modify the SiO2 surface and tested for the catalytic esterification of LA [29]. However, after prolonged 6–10 h reaction and high catalyst loading, the conversion was only 75%, and the product yield was as low as 67%. The H/BEA zeolite modified with NaOH was applied as catalyst for LA esterification with ethanol, where the LA conversion was only 40% [11,30]. In another study, ZrO2/SBA15 catalysts were synthesized by the impregnation method with moderate acidic sites [24]. The ZrO2 modified zeolite catalyst could only convert 50% of LA at a very high temperature, ca. 250 °C. A significant conversion of LA with high selectivity of EL was only achieved in a flow reactor under vapour phase conditions over a WO3-incorporated SBA16 catalyst [31]. Based on the above discussion, it is apparent that the present tin modified silicotungstic acid/Ta2O5 catalyst is by far the best catalytic material, which could efficiently convert LA to EL with 97% selectivity at the moderate temperature of 70 °C within 3 h of duration. In addition to the catalytic activity, we have also scrutinized the stability and reusability of the 50 wt% Sn2STA/Ta2O5 catalyst. To explore the recyclability performance, the catalyst was subjected to multiple cycles under the same reaction conditions, where after each run, the catalyst was filtered, washed with acetone, and without any further treatment was used for the next cycle. The conversion of LA was found to be 81, 76 and 74%, and the percentage of EL yield was 78, 71 and 68% after the 1st, 2nd and 3rd cycle, respectively.
yield and nearly complete selectivity, and with an initial conversion rate of 6.6 × 10−3 mol/g/min. Additionally, the catalyst showed a stable recyclability up to three cycles. The catalytic performance of the 50 wt% Sn2STA/Ta2O5 was strongly favourable when compared with the literature. This work paves a path for alternative heterogeneous catalytic materials to inorganic acids used for the synthesis of EL. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgements P. G. thanks SERB and DST, India, for the financial support in the form of N-PDF grant [SERB/F/2788/20l8-2019, SERB/F/877/20192020]. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.105864. References [1] J.J. Bozell, G.R. Petersen, Technology development for the production of biobased products from biorefinery carbohydrates-the US Department of Energy’s “top 10” revisited, Green Chem. 12 (2010) 539–554. [2] G.W. Huber, S. Iborra, A. Corma, Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering, Chem. Rev. 106 (2006) 4044–4098. [3] D.J. Braden, C.A. Henao, J. Heltzel, C.C. Maravelias, J.A. Dumesic, Production of liquid hydrocarbon fuels by catalytic conversion of biomass-derived levulinic acid, Green Chem. 13 (2011) 1755–1765. [4] C. Antonetti, D. Licursi, S. Fulignati, G. Valentini, A. Raspolli Galletti, New frontiers in the catalytic synthesis of levulinic acid: from sugars to raw and waste biomass as starting feedstock, Catalysts 6 (12) (2016) 196–224. [5] A. Corma, S. Iborra, A. Velty, Chemical routes for the transformation of biomass into chemicals, Chem. Rev. 107 (2007) 2411–2502. [6] J.J. Bozell, Connecting biomass and petroleum processing with a chemical bridge, Science 329 (5991) (2010) 522–523. [7] D.R. Fernandes, A.S. Rocha, E.F. Mai, C.J.A. Mota, V. Teixeira da Silva, Levulinic acid esterification with ethanol to ethyl levulinate production over solid acid catalysts, Appl. Catal. A 425−426 (2012) 199–204. [8] H. Joshi, B.R. Moser, J. Toler, W.F. Smith, T. Walker, Ethyl levulinate: a potential bio-based diluent for biodiesel which improves cold flow properties, Biomass Bioenergy 35 (7) (2011) 3262–3266. [9] K.Y. Nandiwale, S.K. Sonar, P.S. Niphadkar, P.N. Joshi, S.S. Deshpande, V.S. Patil, V.V. Bokade, Catalytic upgrading of renewable levulinic acid to ethyl levulinate biodiesel using dodecatungstophosphoric acid supported on desilicated H-ZSM-5 as catalyst, Appl. Catal. A 460−461 (2013) 90–98. [10] C. Chang, G. Xu, X. Jiang, Bioresour. Technol. 121 (2012) 93–99. [11] G. Pasquale, P. Vazquez, G. Romanelli, G. Baronetti, Catal. Commun. 18 (2012) 115–120. [12] H.J. Bart, J. Reidetschlager, K. Schatka, A. Lehmann, Kinetics of esterification of
4. Conclusions A novel microwave assisted hydrothermal method was employed to prepare Sn- exchanged STA supported on Ta2O5. 50 wt% of Sn2STA loading on the Ta2O5 support not only resulted in high surface area and porosity, but also showed optimum ratio of Brønsted to Lewis acid sites due to the uniform distribution of the active Sn-exchanged heteropoly acid over the support oxide. The resulting 50 wt% Sn2STA supported catalyst achieved high conversion of LA to EL with as high as 78% of 5
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