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Insights into the metal salt catalyzed ethyl levulinate synthesis from biorenewable feedstocks
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Insights into the metal salt catalyzed ethyl levulinate synthesis from biorenewable feedstocks Shireen Quereshi a , Ejaz Ahmad b , K.K. Pant b,∗ , Suman Dutta a,∗ a b
Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, 826004, India Department of Chemical Engineering, Indian Institute of Technology, Delhi, 110016, India
a r t i c l e
i n f o
Article history: Received 30 July 2016 Received in revised form 5 December 2016 Accepted 9 December 2016 Available online xxx Keywords: Ethyl levulinate Metal salts Esterification 5-Hydroxymethylfurfural Levulinic acid Biofuel
a b s t r a c t Ethyl levulinate (EL) synthesis from biorenewable feedstocks in the presence of metal salts and under nonmicrowave instant heating reaction environment has been studied. Initial screening suggested CuCl2 and FeCl3 as an efficient catalyst for EL synthesis from 5-hydroxymethylfurfural (HMF), thereby resulting into 47.5% and 43.3% EL yield respectively at 160 ◦ C in 5 min. Interestingly, environmentally benign and widely available metal salt, AlCl3 , was found to yield comparable EL (39.0%) when experiments were performed under identical reaction conditions. Therefore, further study was performed using AlCl3 to estimate the effect of operating parameters and catalyst concentration over EL yield and HMF conversion. Thereafter, a comparative study on the effect of microwave vs non-microwave instant heating reaction environment has been done to develop a sustainable process for EL synthesis. Consequently, mechanistic insights on reactant conversion and product yield have been attempted to understand various pathways followed. Thereafter, effect of reaction temperature, catalyst loading, ethanol concentration and heating rates were explored. EL synthesis from HMF under instant heating reaction conditions in the presence of metal salts is reported for the first time. We anticipate that present work would be helpful in providing insights on effects of instant heating method and operating conditions for EL synthesis from HMF. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Rapid growth of modern society has intensified the usage of conventional fossil fuel, thereby causing swift decline in the fuel reserves and substantial increase in environmental damages. Therefore, current research focus has been shifted towards development of sustainable process to obtain fuel and chemicals from biorenewable resources [1–11]. In this regard, lignocellulosic biomass consisting of lignin, cellulose and hemicellulose is widely used to produce an array of chemicals and fuel [12]. However, lignin acts as binding material that prevents cellulose and hemicellulose from taking part in further reactions to produce desired chemicals. Therefore, lignin is removed at first step using various technologies. Thereafter, cellulose and hemicellulose are depolymerized to produce HMF and furfural (FA) respectively via a series of reactions [13]. Both HMF and FA are considered among to be important class of building block chemicals for high value applications [14]. On fur-
∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (K.K. Pant), [email protected] (S. Dutta).
ther dehydration under acidic conditions, HMF yields levulinic acid (LA) which itself is a major building chemical [15]. However, separation of levulinic acid and HMF is challenging task which may limit its feasibility for scale up [16]. Albeit HMF is a major platform chemical obtained from biomass resources, its applicability as a biofuel is yet to be proven. However, HMF may be used to produce a wide range of fuel additives such as 2,5-dimethylfuran, 5-ethoxymethylfurfural (EOMF) and alkyl levulinates [17]. Specifically, biomass derived alkyl levulinates are widely used as fuel and fuel additive [18,19]. In this regard, ethyl levulinate has shown considerable promises, both as fuel and building block chemical. On blending 10% EL with regular diesel, 41% reduction in engine out smoke number is measured [20]. In addition, significant reduction in cloud point (4–5 ◦ C), pour point (3–4 ◦ C) and cold filter plugging point (3 ◦ C) is measured with 20% EL blend [21]. Moreover, it is possible to reduce sulfur emission in diesel engines by addition of EL with other co-additives [22]. Alternatively, EL can be upgraded to produce a diverse platform chemical gamma valerolactone via hydrogenation reaction [23–25]. In general, EL is conventionally used in plasticizers, flavor and fragrance industry in very less quantity [26]. A catalytic method for bulk pro-
http://dx.doi.org/10.1016/j.cattod.2016.12.019 0920-5861/© 2016 Elsevier B.V. All rights reserved.
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duction of EL may open up a window of opportunity in the area of biorenewable energy and chemicals research. Interestingly, an array of catalytic materials have been used for the synthesis of EL from various bio renewable feedstocks [27–34]. In this regard, a recent review by our group can be referred for further study on catalytic and mechanistic insights on EL synthesis [35]. On the contrary, HMF as feedstock is least explored, necessitating the development of a sustainable process to produce EL from it. For example, Neves et al. have reported a maximum 21% EL yield from HMF using zeolite based catalyst for 48 h at 140 ◦ C [36]. Authors have measured 100% HMF conversion, albeit EL yield remained low in all reaction conditions. The possible reason could be attributed to formation of by-products such as lactides. The authors have also summarized, major catalysts applied for EL synthesis from HMF, such zeolites, MCM, zirconia, sulfated zirconia, SBA-15, heteropolyacids, and ionic liquids. A maximum 75% EL yield was measured using sulfonated SBA-15 catalysts in 24 h at 140 ◦ C by Riisager and coworkers [37]. On the contrary, up to 80% and 97% EL yield obtained when metal salt catalyzed experiments were performed on furfural alcohol (FAL) and LA respectively, under microwave irradiation [38,39]. It is reported that microwave heating have several advantages that leads to higher product yield. For example, rapid heating suppresses formation of byproducts whereas conventional heating promotes by-product formation due to slow heating and increased time to reach the desired temperature[40,41]. In contrast, Dutta et al. have observed formation of EL in significant quantity (25% EL yield) in an oil bath reactor for 8 h in the presence of metal salt based catalyst [42]. This indicates that metal salts can be effective catalyst for EL synthesis from HMF under non-microwave reaction conditions for a short reaction time. Moreover, Yang et al. have obtained 12% EL yield under microwave irradiation in 15 min at 160 ◦ C which is comparatively lower than the EL yield obtained by conventional heating method [43]. Nevertheless, microwave irradiation based EL synthesis technologies are yet to be tested for a large scale production. Thus, it is essential to develop non-microwave based sustainable technologies which has greater acceptance at industrial scale. To best of our knowledge, no efforts has been made to attempt EL synthesis in the presence of metal salts under non-microwave instant heating conditions. Thus, objective of present study was to synthesize EL from HMF under instant heating under non-microwave reaction environment in the presence of a suitable metal salt catalyst. Thereafter, effect of operating conditions such as reaction time and temperature, catalyst and feedstock concentration, effect of alcohol chain length on EL yield were studied. In addition, effect on EL yield with other biorenewable feedstocks such as LA, glucose, fructose and FAL was studied. Moreover, effects of heating ramp rate in non-microwave reaction environment have been explored. Consequently, attempts have been made to develop a plausible reaction mechanism for EL production from HMF under different heating conditions. Finally, a comparative study on the effect of non-microwave reaction environment vs. microwave reaction environment for HMF conversion and EL yield have been attempted. We believe that the present study would be helpful in providing insights on advantage of instant heating process and basis for further study under non-microwave instant heating condition for ethanolysis of HMF to produce EL.
2. Experimental 2.1. Materials The metal salts (AlCl3 , CrCl3 , FeCl3 , SnCl4 , ZnCl2 , CuCl2 , ZrOCl2 , LiCl) tested herein were procured from Thermo-Fischer scientific, India and used without further modifications. On
the contrary, HZSM-5 with varying silica alumina ratio and Lanthanum loaded HZSM-5 were synthesized in our lab. All feedstocks, Ethyl levulinate, Levulinic acid, 5-hydroxymtehylfurfural, 5-ethoxymethylfurfural and furfural alcohol were purchased from Sigma-Aldrich, USA and used without further purification. Methanol, ethanol, butanol and other alcohols were procured from Merck, India and used directly for the esterification reaction.
2.2. Experimental setup and esterification reaction All experiments were performed in a non-microwave synthesis reactor supplied by Anton Paar, USA (Model: Monowave-50). The reactor consists of a 315W stainless steel heating jacket along with integrated pressure and temperature control system (Refer to Fig. S-1 in SI). Experiments using non-microwave instant heating reactor (Monowave 50) is employed for the first time. In a typical experimental procedure, desired amount of HMF (0.5 mmol, 63 mg) and catalysts (0.2 mmol) were charged into the high pressure quartz vials. Thereafter, 4.5 ml ethanol was added into feed-catalyst mixture and a magnetic bead was kept inside the vials for stirring purpose. The reactor temperature was set to 160 ◦ C and as fast as possible mode of heating was selected to reach the desired temperature. In general, it took 2 min to reach the desired temperature. Thereafter, 5–10 min hold time was set for the reaction to occur. Consequently, reaction mixture was cooled down by inbuilt cooling system provided with the reactor. Post this, ethanol concentration was varied to study the effect of feed to ethanol ratio. Similarly, catalyst concentration, reaction temperature, reaction time and stirring speed were varied to study their effect on EL yield. Moreover, feedstock and alcohols composition was varied to study the effect on EL yield. For a comparative study, experiments were also performed using Anton Paar, USA, (model: Monowave 300) microwave reactor. All experiments were repeated twice for accuracy of the results and average values are reported herein. In some cases, like with zeolite catalysts, reactions were performed thrice to recheck the activity of the catalysts.
2.3. Product analysis Product mixture obtained after ethanolysis reaction were charged into a rotary evaporator (Make: Buchi, Switzerland, Model: R210) for separation of the catalysts. Earlier studies by prominent research groups in this area suggest that DB-5 and DB-5MS capillary columns are suitable for EL and related compounds determination using gas chromatography technique [44–49]. Thus, product mixture was analyzed in a GC-FID system (Nucon, India, Model: 5765) provided with DB-5MS capillary column (30 m length × 0.25 mm id × 0.25 m film, Agilent India) and flame ionization detector. The injector and detector temperature was kept at 250 ◦ C and 240 ◦ C respectively approximately close to previously reported methods [47,48]. Minor changes in reported program were made based on different preliminary studies at our lab for temperature optimization and an optimized program was developed. Initial temperature was set to 60 ◦ C and thereafter, gradually increased to 180 ◦ C with a ramp rate of 10 ◦ C/minute. The oven temperature was held for 1 min at 180 ◦ C. Post this, final oven temperature i.e. 240 ◦ C was achieved in 4 min with a ramp rate of 15 ◦ C/minute and held for 3 min at the final temperature. Nitrogen (30 ml/min) was used as a carrier gas whereas hydrogen and zero air were used as flame generation gases with a flow rate of 30 ml/min and 300 ml/min respectively. The split ratio of the sample was 1:30 in GC-FID system. The GC-FID was calibrated using external standard method. All samples were analyzed thrice for the better accuracy in results and averaged val-
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ues were considered for the calculation. Thereby, isolated product yield was calculated as follows: EL Yield Moles of EL ProducedMoles of total < feed < Conversion Moles of total Feed − Moles of unreacted Feed Moles of total Feed
2.4. Products conformational analysis The qualitative analysis of product mixtures were performed using GC–MS system (Perkin Elmer, USA, Model: Clarus 600), provided with an ESI detector. For a better product separation during the analysis, GC-FID program was kept same as that of GC-FID. However, 4 min solvent delay was kept during data acquisition to remove ethanol peaks from the GC–MS. Similar to GC-FID analysis for product quantification, DB-5 MS capillary column of identical specification was used in GC–MS instrument. In addition, nuclear magnetic resonance spectrometry (1 H, 9.4 T, 400 MHz and 13 C, 100 MHz) studies were performed using Bruker NMR spectrometer (model: AscendTM 400) to confirm structural conformation of the product. All the samples were prepared in CDCl3 solvent whereas TMS was used as internal standard. Based on m/z ratio obtained from GC–MS and ␦ ppm values obtained from NMR spectrometer, structural conformation were determined. In addition, GC–MS results were further verified using GC–MS (Shimadzu QP-2010) with thermal desorption system TD 20 at Advanced Instrumentation Research Facility in JNU, Delhi. 3. Results and discussion 3.1. Catalyst selection Firstly, experiments were performed on a variety of metal salts and zeolites to select suitable catalyst for synthesis of EL from HMF (Table 1). The reactions were performed, as per method described in experimental section, in an Aton Paar instant heating reactor (Monowave-50) at 160 ◦ C for 5 min. First experiment was performed in the absence of any catalyst to determine self-catalyzed reactivity of the feedstock. However, neither HMF conversion nor EL yield was detected, suggesting HMF do not reacts in the absence of a suitable acid catalyst (Table 1, entry 1). On the contrary, maximum 93.4% HMF conversion and 47.5% EL yield was measured, when experiments were performed in the presence of CuCl2 in 5 min at 160 ◦ C reaction temperature (Table 1, entry 2). Similar, result (43.2% Table 1 List of Catalysts Screened for HMF Conversion and EL Synthesis. S.N.
Catalyst
Catalysts Loading (mmol)
%HMF Conversion
%EL Yield
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Blank CuCl2 FeCl3 AlCl3 SnCl4 ZnCl2 CrCl3 LiCl ZrOCl2 H-Beta HZSM-5 (1) HZSM-5 (2) HZSM-5 (3) HZSM-5 (4) La-HZSM-5
– 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 30 mg 30 mg 30 mg 30 mg 30 mg 30 mg
0.0 93.4 73.9 59.5 40.8 31.85 14.7 8.0 33.7 19.9 38.8 29.9 32.3 11.6 12.94
0.0 47.5 43.2 39.0 13.04 0.0 10.0 0.0 9.9 0.0 0.0 0.0 0.0 0.0 0.0
Experimental conditions, feed: 0.5 mmol HMF, ethanol: 4.5 ml, temp. 160 ◦ C, reaction time: 5 min, heating rate: as fast as possible, stirring rate: 300 RPM.
3
EL yield, Table 1, entry 3) was obtained on ethanolysis of HMF in the presence of FeCl3 catalyst. The result obtained was comparable with data reported in literature for EL synthesis from a similar molecule furfural alcohol [38]. The EL yield obtained in the presence of both CuCl2 and FeCl3 , indicates that Lewis acidity (of Fe3+ and Cu2+ ) could be a major reason for such high EL yield [39]. A next very active catalyst was found to be AlCl3 (Al3+ ), yielding 39.0% EL under identical reaction conditions (Table 1, entry 4). In general, synthesis of EL from HMF essentially requires both Brønsted as well as Lewis acidity caused due to the application of metal salts. The metal part of the salts creates a Brønsted acid environment via ethanolysis/hydrolysis of M+ ions that helps in de-ethanolysis/dehydration of intermediates formed [50]. Whereas the Lewis acid center of metal salts coordinates with ring oxygen attached to HMF, thus leading to the formation of stable intermediate species which in result lowers the overall activation barrier for dehydration reaction [51]. However, results obtained with CrCl3 were in contrast to the anticipated values, thereby leading to conclusion that presence of Brønsted and Lewis acidity alone cannot be accounted for EL yield. Instead, there could be other factors that govern the desired product yield. One such possibility could be optimal ratio of Bronsted and Lewis acidity ratio [44]. The other factor could be different reaction mechanism, resulting into different activation barriers for each mechanism followed. For example, EL synthesis from HMF may follow two reaction pathways i.e. (i) conversion of HMF into LA and then to EL and/or (ii) conversion of HMF to EL via EOMF route (Fig. 1) [35]. In former route, HMF rehydrates to yield levulinic acid, which on further ethanolysis converts to EL. However, LA was not detected in the product mixture, thus possibility of this reaction route is minimum. Route R1 essentially requires presence of water for conversion of HMF into LA which in turn converts to EL on ethanolysis. Thus, limited availability of water molecules in the reaction mixture may be a possible reasons for not following this reaction pathway. On the contrary, EOMF as major intermediate and EL as product were detected in GC–MS data (Figs. S-2 & S-3 in SI), thus confirming the later reaction route followed. We already have discussed this route in detail in earlier study by our group [35]. Besides, EOMF and EL, several other products in minor quantity were also detected in GC–MS and NMR analysis (Figs. S-4, S-5, S-8, S-8 and S-9). Based on appreciable results obtained in the presence of CuCl2 , FeCl3 and AlCl3 , other metal salts were explored which are summarized in Table 1. However, no significant EL yield was obtained in the presence of ZnCl2 (Zn2+ ), possibly due to its weak acidity. To further confirm the effect of Brønsted and Lewis acid role in EL synthesis, we have employed zeolite based H-beta, HZSM-5 and lanthanum, loaded HZSM-5 catalyst with varying silica to alumina ratio (SAR) for a comparative study. In this case, no significant EL yield was detected, albeit 38.8% HMF conversion was measured (Table 1, entry 11). One possible reason could be the tendency of zeolites to promote polymerization reaction [36]. Huang et al. in a similar study for methyl levulinate production from furfural alcohol have measured very low (1% and 4.9%) yield using H-beta and HZS-5 zeolites respectively under microwave irradiation [38]. In our case, since AlCl3 yielded comparable EL and less by product formation, thus, next set of experiments were performed using AlCl3 catalysts for further study. Moreover, EOMF was not detected in product when experiments were performed using other metal salts except AlCl3 and ZrOCl2 . Absence of EOMF suggests that there could be other reaction pathway which does not essentially involve the formation of EOMF. However, since catalyst AlCl3 was mainly considered for studying effect of other parameters, we may present a more conclusive discussion later, on the basis of results obtained from DFT analysis of reactant and intermediates interaction with different metal salts and change in activation barrier.
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Fig. 1. Plausible Reaction Mechanisms for EL Synthesis from HMF.
Fig. 2. Effect of Reaction Temperature on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Ethanol: 4.5 ml, AlCl3 : 0.2 mmol, Reaction time: 5 min, Heating rate: as fast as possible, Stirring rate: 300 RPM
3.2. Effect of reaction temperature The role of temperature is crucial for any process to determine its techno-economic feasibility as it directly accounts for energy consumption during the reaction. Thus, experiments were performed by varying reaction temperature between 100 ◦ C–160 ◦ C for 5 min in the presence of AlCl3 . In general, it was observed that increase in temperature resulted into linear increase in HMF conversion and EL yield (Fig. 2). A maximum of 39.0% EL yield was measured in 5 min at 160 ◦ C reaction temperature whereas; a minimum of 14% EL was measured under identical reaction conditions. Thus, it can be concluded that higher reaction temperature accelerates the reaction rates. Similar trends in levulinate yield is reported by prominent researchers for methyl levulinate and ethyl levulinate respectively [38,43]. On the contrary, higher reaction temperature beyond a certain limit may promote the formation of byproducts. However, to check this hypothesis, more experiments needs to be performed at higher temperatures. We have not explored temperature beyond 160 ◦ C due to specified limits of the instrument.
Fig. 3. Effect of Reaction Time on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Ethanol: 4.5 ml, AlCl3 : 0.2 mmol, Reaction temp: 160 ◦ C, Heating rate: as fast as possible, Stirring rate: 300 RPM
mediates and byproducts. Therefore, experiments were carried out to study the effect of reaction time on EL yield and HMF conversion. It was observed that EL yield decreases with increase in reaction time (Fig. 3). The EL yield decreased from 39.0% in 5 min to 25.7% in 20 min at 160 ◦ C reaction temperature in the presence of AlCl3 . The results obtained were in line with the trend reported by Huang et al. for methyl levulinate synthesis from FAL [38]. The authors have suggested that levulinate yield increases to a maximum limit within five minutes of reaction time and starts decreasing after. Thus, it can be concluded that a longer reaction time leads to different reaction pathways and promotes byproducts formation. Possibly, a longer reaction time could have been the reason behind low EL from HMF by Yang et al. (12% EL in 15 min at 160 ◦ C) under microwave irradiation reaction environment [43]. Therefore, we attempted to perform experiments for less than 5 min time, however Monowave 50 takes 2–3 min minimum to reach the desired temperature and for pressure stabilization, thus results obtained were not encouraging and reproducible. It would be interesting to study the effect of reaction time below 5 min on EL yield, if desired reaction temperature can be reached within seconds like microwave reactor.
3.3. Effect of reaction time 3.4. Effect of catalyst concentration Reaction time is another important parameter that can be held responsible for overall energy consumed in the process. In addition, variations in reaction time may lead to formation of different inter-
Effect of catalyst concentration over HMF conversion and EL yield has been studied by varying catalyst amount from 0.1 mmol
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Fig. 4. Effect of Catalyst Concentration on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Ethanol: 4.5 ml, Reaction time: 10 min, Reaction temp: 160 ◦ C, Catalyst: AlCl3 , Heating rate: as fast as possible, Stirring rate: 300 RPM
to 0.4 mmol (Fig. 4). In present study, we performed experiments at 160 ◦ C in the presence of 0.1 mmol to 0.4 mmol AlCl3 catalyst. Initially, EL yield increased from 33.1% to 42.6% when AlCl3 concentration was increased from 0.1 mmol to 0.3 mmol. Enhancement in EL yield could be attributed to increase in acidity (H+ ion concentration created by hydrolysis of cations) due to higher catalyst loading. Interestingly, no significant improvement in EL yield was observed by increasing the catalyst concentration beyond 0.3 mmol, thus suggesting requirement of an optimum acid loading for better EL yield. A maximum of 43.5% EL yield was measured in the presence of 0.4 mmol AlCl3 , at 160 ◦ C reaction temperature. On the contrary, HMF conversion was found to be enhanced from 50.0% to 70% with the increase in catalyst concentration from 0.1 mmol to 0.2 mmol. Beyond this, HMF conversion was found to be unaffected with increase in catalyst concentration up to 0.3 mmol, thereby suggesting a catalyst loading around 0.2 mmol is sufficient for optimum EL yield and HMF conversion. 3.5. Effect of ethanol concentration It is important to determine the optimum feed ratio of the reactant to enhance the sustainability of the overall process. In general, a higher HMF concentration promotes self-polymerization reaction in the presence of an acid catalyst. Thus, experiments were performed by varying ethanol concentration and keeping HMF concentration constant under identical reaction conditions. Fig. 5 depicts effect of ethanol concentration on overall EL yield and HMF conversion. Interestingly, EL yield was found to be increasing (from 27.5% to 37.5%) with increase in ethanol concentration. In contrast, HMF conversion was found to be decreasing (from 81.8% to 70.18%) with increase ethanol concentration. The result obtained suggests that higher EL formation is favored in the presence of higher alcohol concentration albeit; a minor decrease in HMF conversion was measured. The improved EL yield could be attributed to inhibition of undesired reaction due to excess availability of ethanol for reaction. In addition, low HMF concentration due to excess ethanol could have suppressed self-polymerization reaction, thus resulting into overall low HMF conversion.
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Fig. 5. Effect of Ethanol Concentration on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Reaction time: 10 min, Reaction temp: 160 ◦ C, Catalyst: 0.2 mmol AlCl3 , Heating rate: as fast as possible, Stirring rate: 300 RPM Table 2 Effect of Heating Rate on EL Yield and HMF Conversion. S.N.
Effect of rate
% HMF Conversion
% EL Yield
1. 2. 3.
As fast as possible 5 ◦ C/min 15 ◦ C/min
70.1 82.9 41.2
37.1 30.8 37.9
Experimental conditions, feed: 0.5 mmol HMF, ethanol: 4.5 ml, temp. 160 ◦ C, reaction time: 10 min, AlCl3 : 0.2 mmol, stirring rate: 300 RPM.
3.6. Effect of stirring speed & heating rate In heterogeneous systems, stirring speed plays a crucial role in overcoming mass transfer limitations. In general, most of the metal salts dissolved in reaction mixture, thus we believed that external and intrinsic mass transfer limitations should not affect the HMF conversion and EL yield. To confirm this hypothesis, experiments were performed by varying stirring speed from 300 RPM to 1200 RPM at 160 ◦ C. Maximum 37.5% EL yield was measured in the presence of 0.2 mmol AlCl3 at 160 ◦ C reaction temperature and 300 RPM. However, significant reduction in EL yield was measured when experiments were performed at a speed of 600 RPM. Post this, we did not observed any major deviation in EL yield and HMF conversion due to variation of stirring speed from 600 RPM to 1200 RPM (Fig. 6). Albeit minor changes in both EL yield as well as HMF conversion was observed. Low EL yield could be because of lesser contact time between the catalyst and the reactant due to very high turbulence, thus suppressing the formation of intermediates for further reaction. Similarly, experiments were performed to study the effect of heating method on EL yield and HMF conversion. It was observed that instant heating (as fast as possible) and fast heating (5 ◦ C/min) yielded a higher HMF conversion and amount EL (Table 2) whereas slow heating method resulted into less HMF conversion, thus yielding less EL. Moreover, decrease in ramp rate also led to decrease in EL yield from 38.0% to 31.2% which indicates instant heating method have advantage over slow heating processes. Interestingly, EL yield was higher at low HMF conversion which indicates that more free acid sites could have been available for conversion of pro-
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Fig. 6. Effect of Stirring Speed on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Ethanol: 4.5 ml, Reaction time: 10 min, Reaction temp: 160 ◦ C, Catalyst: 0.2 mmol AlCl3 , Heating rate: as fast as possible
duced intermediates to EL. On the contrary, when HMF conversion was high, more number of acid sites could have been occupied in HMF to intermediates conversion, thereby resulting into less number of acid sites available for conversion of produced intermediates to final product EL.
Fig. 7. Effect of Substrates on EL Yield. Experimental conditions, Feed: Various, Ethanol: 4.5 ml, Reaction time: 10 min, Reaction temp: 160 ◦ C, Catalyst: 0.2 mmol AlCl3 , Heating rate: as fast as possible, Stirring rate: 300 RPM
and proven as compared to microwave system, thus present study may open a new window of opportunity in this area.
3.8. Catalyst activity on different substrate 3.7. Comparison between non-microwave vs. microwave irradiation reaction environment The effects of microwave and non-microwave instant heating reaction environment were studied by performing optimized experiments in a microwave reactor (Monowave 300, Anton Paar, USA). We did not found any major deviation in the EL yield obtained from both the reactors when experiments were performed in the presence of AlCl3 catalyst. The major advantage associated with microwave reactor is the reduced reaction time and lower byproduct formation due to powerful agitation and localized heating that ultimately helps to achieve improved EL yield. Therefore, creating identical reaction conditions (powerful stirring, instant and localized heating) may help in getting similar yield. The results obtained under non-microwave instant heating environment (in Monowave 50) were relatively comparable with microwave reactor in terms of EL yield. However, HMF conversion was measured to follow similar trend comparable to results obtained under nonmicrowave reaction environment (Table 3). These results suggest that non-micorwave instant heating in the presence of AlCl3 catalyst is helpful in improving EL yield. In addition, it can be considered as an alternative to microwave reactors which are difficult to scale up. Nevertheless, scale up of non-microwave system is quite easier
In a critical review on catalytic synthesis of EL, we suggested that molecular structure of the reactant plays a crucial role in determining overall EL yield [35]. To confirm this hypothesis, experiments were performed using various substrates in the presence of efficient and environmentally benign catalyst AlCl3 [35]. Cellulose ethanolysis in the presence of AlCl3 catalyst yielded minimum EL 11.3% followed by glucose and fructose that yielded 17.9% and 23.3% EL respectively at 160 ◦ C at 300 RPM. On the contrary, EL yield above 90% obtained when LA and FAL were used as feedstock (Fig. 7). The results obtained were in line with our hypothesis that downstream feedstock such as FAL and LA yields more EL. On the contrary, when we start moving upstream feedstock from LA such fructose, glucose, cellulose etc., the EL yields decreases in the same order. The obtained EL yields were 11.3%, 18.8%, 23.3%, 25%, 37.17%, 91% and 96% for cellulose, starch, glucose, fructose, HMF, FAL and LA respectively. The measured EL yield clearly indicates the effect of reactant structure, thereby leading to a conclusion that moving down from upstream feedstock to downstream feedstock increases EL yield due to structural effect of the feedstocks. The other possible reason could be lack of high strong acidity for conversion of glucose, starch and cellulose, thus resulting into less dehydration of the reactants for further conversion. Similarly, conversion of large molecules may
Table 3 Comparison between Microwave vs. Non-microwave Instant Heating Reaction Environment. S.N
1. 2. 3. 4. 5.
Reactant
HMF HMF HMF HMF HMF
Catalyst
AlCl3 SnCl4 ZnCl2 CrCl3 ZrOCl2
Yield%
Conversion%
Monowave 50
Monowave 300
Monowave 50
Monowave 300
39.0 13.0 0.0 10.0 9.9
41.9 37.3 43.0 53.8 41.8
59.5 40.8 31.9 14.7 33.7
43.3 57.6 45.9 46.7 38.3
Experimental conditions, feed: 0.5 mmol HMF, ethanol: 4.5 ml, temp. 160 ◦ C, reaction time: 5 min, Catalysts: 0.2 mmol, stirring rate: 300 RPM.
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be limited due to intrinsic mass transfer diffusional limitations at low stirring speed. 4. Conclusion and outlook The catalytic conversion of 5-hyroxymethylfurfural using commercially available metal salt was studied under microwave and non-microwave instant heating reaction environment. The maximum EL yield of 47.5%, 43.5% and 39.0% was obtained using CuCl2 , FeCl3 and AlCl3 respectively at 160 ◦ C in 5 min. EL yield further improved to 43.5% in the presence of 0.4 mmol of AlCl3 at 160 ◦ C in 10 min. In general, EL yield was found to be increasing with increase in reaction temperature and the catalyst concentration whereas longer reaction time led to a slight reduction in EL yield. The comparative study of non-microwave vis-à-vis microwave instant heating reaction environment suggests HMF conversion is non-micorwave instant heating reaction environment may yield comparable results to microwave reactors for EL synthesis from HMF in the presence of AlCl3 . Future studies will be directed towards understanding multiple reaction pathways, and effect of dielectric constant of the reactants structure thereby affecting EL yield in the presence of different metal catalysts.
[9]
[10]
[11]
[12]
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[14]
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[17]
Acknowledgements [18]
One of the Authors, Ejaz Ahmad would like to thank Prime Minister Fellowship (D.O. No. DST/SSK/SERB-CII-FeII/2014) and Hindustan Petroleum Corporation Limited (HPCL Green R&D Center) for providing fellowship to pursue his doctoral studies. Department of science and technology (DST), government of India for funding via extra mural research grant (EMR/2015/001959) to carry out this research work is gratefully acknowledged. The authors would like to thank Anton Paar, Gurgaon India for providing prototype of the reactor Monowave-50 to perform experiments at our lab. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.12. 019.
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Insights into the metal salt catalyzed ethyl levulinate synthesis from biorenewable feedstocks Shireen Quereshi a , Ejaz Ahmad b , K.K. Pant b,∗ , Suman Dutta a,∗ a b
Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, 826004, India Department of Chemical Engineering, Indian Institute of Technology, Delhi, 110016, India
a r t i c l e
i n f o
Article history: Received 30 July 2016 Received in revised form 5 December 2016 Accepted 9 December 2016 Available online xxx Keywords: Ethyl levulinate Metal salts Esterification 5-Hydroxymethylfurfural Levulinic acid Biofuel
a b s t r a c t Ethyl levulinate (EL) synthesis from biorenewable feedstocks in the presence of metal salts and under nonmicrowave instant heating reaction environment has been studied. Initial screening suggested CuCl2 and FeCl3 as an efficient catalyst for EL synthesis from 5-hydroxymethylfurfural (HMF), thereby resulting into 47.5% and 43.3% EL yield respectively at 160 ◦ C in 5 min. Interestingly, environmentally benign and widely available metal salt, AlCl3 , was found to yield comparable EL (39.0%) when experiments were performed under identical reaction conditions. Therefore, further study was performed using AlCl3 to estimate the effect of operating parameters and catalyst concentration over EL yield and HMF conversion. Thereafter, a comparative study on the effect of microwave vs non-microwave instant heating reaction environment has been done to develop a sustainable process for EL synthesis. Consequently, mechanistic insights on reactant conversion and product yield have been attempted to understand various pathways followed. Thereafter, effect of reaction temperature, catalyst loading, ethanol concentration and heating rates were explored. EL synthesis from HMF under instant heating reaction conditions in the presence of metal salts is reported for the first time. We anticipate that present work would be helpful in providing insights on effects of instant heating method and operating conditions for EL synthesis from HMF. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Rapid growth of modern society has intensified the usage of conventional fossil fuel, thereby causing swift decline in the fuel reserves and substantial increase in environmental damages. Therefore, current research focus has been shifted towards development of sustainable process to obtain fuel and chemicals from biorenewable resources [1–11]. In this regard, lignocellulosic biomass consisting of lignin, cellulose and hemicellulose is widely used to produce an array of chemicals and fuel [12]. However, lignin acts as binding material that prevents cellulose and hemicellulose from taking part in further reactions to produce desired chemicals. Therefore, lignin is removed at first step using various technologies. Thereafter, cellulose and hemicellulose are depolymerized to produce HMF and furfural (FA) respectively via a series of reactions [13]. Both HMF and FA are considered among to be important class of building block chemicals for high value applications [14]. On fur-
∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (K.K. Pant), [email protected] (S. Dutta).
ther dehydration under acidic conditions, HMF yields levulinic acid (LA) which itself is a major building chemical [15]. However, separation of levulinic acid and HMF is challenging task which may limit its feasibility for scale up [16]. Albeit HMF is a major platform chemical obtained from biomass resources, its applicability as a biofuel is yet to be proven. However, HMF may be used to produce a wide range of fuel additives such as 2,5-dimethylfuran, 5-ethoxymethylfurfural (EOMF) and alkyl levulinates [17]. Specifically, biomass derived alkyl levulinates are widely used as fuel and fuel additive [18,19]. In this regard, ethyl levulinate has shown considerable promises, both as fuel and building block chemical. On blending 10% EL with regular diesel, 41% reduction in engine out smoke number is measured [20]. In addition, significant reduction in cloud point (4–5 ◦ C), pour point (3–4 ◦ C) and cold filter plugging point (3 ◦ C) is measured with 20% EL blend [21]. Moreover, it is possible to reduce sulfur emission in diesel engines by addition of EL with other co-additives [22]. Alternatively, EL can be upgraded to produce a diverse platform chemical gamma valerolactone via hydrogenation reaction [23–25]. In general, EL is conventionally used in plasticizers, flavor and fragrance industry in very less quantity [26]. A catalytic method for bulk pro-
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2
duction of EL may open up a window of opportunity in the area of biorenewable energy and chemicals research. Interestingly, an array of catalytic materials have been used for the synthesis of EL from various bio renewable feedstocks [27–34]. In this regard, a recent review by our group can be referred for further study on catalytic and mechanistic insights on EL synthesis [35]. On the contrary, HMF as feedstock is least explored, necessitating the development of a sustainable process to produce EL from it. For example, Neves et al. have reported a maximum 21% EL yield from HMF using zeolite based catalyst for 48 h at 140 ◦ C [36]. Authors have measured 100% HMF conversion, albeit EL yield remained low in all reaction conditions. The possible reason could be attributed to formation of by-products such as lactides. The authors have also summarized, major catalysts applied for EL synthesis from HMF, such zeolites, MCM, zirconia, sulfated zirconia, SBA-15, heteropolyacids, and ionic liquids. A maximum 75% EL yield was measured using sulfonated SBA-15 catalysts in 24 h at 140 ◦ C by Riisager and coworkers [37]. On the contrary, up to 80% and 97% EL yield obtained when metal salt catalyzed experiments were performed on furfural alcohol (FAL) and LA respectively, under microwave irradiation [38,39]. It is reported that microwave heating have several advantages that leads to higher product yield. For example, rapid heating suppresses formation of byproducts whereas conventional heating promotes by-product formation due to slow heating and increased time to reach the desired temperature[40,41]. In contrast, Dutta et al. have observed formation of EL in significant quantity (25% EL yield) in an oil bath reactor for 8 h in the presence of metal salt based catalyst [42]. This indicates that metal salts can be effective catalyst for EL synthesis from HMF under non-microwave reaction conditions for a short reaction time. Moreover, Yang et al. have obtained 12% EL yield under microwave irradiation in 15 min at 160 ◦ C which is comparatively lower than the EL yield obtained by conventional heating method [43]. Nevertheless, microwave irradiation based EL synthesis technologies are yet to be tested for a large scale production. Thus, it is essential to develop non-microwave based sustainable technologies which has greater acceptance at industrial scale. To best of our knowledge, no efforts has been made to attempt EL synthesis in the presence of metal salts under non-microwave instant heating conditions. Thus, objective of present study was to synthesize EL from HMF under instant heating under non-microwave reaction environment in the presence of a suitable metal salt catalyst. Thereafter, effect of operating conditions such as reaction time and temperature, catalyst and feedstock concentration, effect of alcohol chain length on EL yield were studied. In addition, effect on EL yield with other biorenewable feedstocks such as LA, glucose, fructose and FAL was studied. Moreover, effects of heating ramp rate in non-microwave reaction environment have been explored. Consequently, attempts have been made to develop a plausible reaction mechanism for EL production from HMF under different heating conditions. Finally, a comparative study on the effect of non-microwave reaction environment vs. microwave reaction environment for HMF conversion and EL yield have been attempted. We believe that the present study would be helpful in providing insights on advantage of instant heating process and basis for further study under non-microwave instant heating condition for ethanolysis of HMF to produce EL.
2. Experimental 2.1. Materials The metal salts (AlCl3 , CrCl3 , FeCl3 , SnCl4 , ZnCl2 , CuCl2 , ZrOCl2 , LiCl) tested herein were procured from Thermo-Fischer scientific, India and used without further modifications. On
the contrary, HZSM-5 with varying silica alumina ratio and Lanthanum loaded HZSM-5 were synthesized in our lab. All feedstocks, Ethyl levulinate, Levulinic acid, 5-hydroxymtehylfurfural, 5-ethoxymethylfurfural and furfural alcohol were purchased from Sigma-Aldrich, USA and used without further purification. Methanol, ethanol, butanol and other alcohols were procured from Merck, India and used directly for the esterification reaction.
2.2. Experimental setup and esterification reaction All experiments were performed in a non-microwave synthesis reactor supplied by Anton Paar, USA (Model: Monowave-50). The reactor consists of a 315W stainless steel heating jacket along with integrated pressure and temperature control system (Refer to Fig. S-1 in SI). Experiments using non-microwave instant heating reactor (Monowave 50) is employed for the first time. In a typical experimental procedure, desired amount of HMF (0.5 mmol, 63 mg) and catalysts (0.2 mmol) were charged into the high pressure quartz vials. Thereafter, 4.5 ml ethanol was added into feed-catalyst mixture and a magnetic bead was kept inside the vials for stirring purpose. The reactor temperature was set to 160 ◦ C and as fast as possible mode of heating was selected to reach the desired temperature. In general, it took 2 min to reach the desired temperature. Thereafter, 5–10 min hold time was set for the reaction to occur. Consequently, reaction mixture was cooled down by inbuilt cooling system provided with the reactor. Post this, ethanol concentration was varied to study the effect of feed to ethanol ratio. Similarly, catalyst concentration, reaction temperature, reaction time and stirring speed were varied to study their effect on EL yield. Moreover, feedstock and alcohols composition was varied to study the effect on EL yield. For a comparative study, experiments were also performed using Anton Paar, USA, (model: Monowave 300) microwave reactor. All experiments were repeated twice for accuracy of the results and average values are reported herein. In some cases, like with zeolite catalysts, reactions were performed thrice to recheck the activity of the catalysts.
2.3. Product analysis Product mixture obtained after ethanolysis reaction were charged into a rotary evaporator (Make: Buchi, Switzerland, Model: R210) for separation of the catalysts. Earlier studies by prominent research groups in this area suggest that DB-5 and DB-5MS capillary columns are suitable for EL and related compounds determination using gas chromatography technique [44–49]. Thus, product mixture was analyzed in a GC-FID system (Nucon, India, Model: 5765) provided with DB-5MS capillary column (30 m length × 0.25 mm id × 0.25 m film, Agilent India) and flame ionization detector. The injector and detector temperature was kept at 250 ◦ C and 240 ◦ C respectively approximately close to previously reported methods [47,48]. Minor changes in reported program were made based on different preliminary studies at our lab for temperature optimization and an optimized program was developed. Initial temperature was set to 60 ◦ C and thereafter, gradually increased to 180 ◦ C with a ramp rate of 10 ◦ C/minute. The oven temperature was held for 1 min at 180 ◦ C. Post this, final oven temperature i.e. 240 ◦ C was achieved in 4 min with a ramp rate of 15 ◦ C/minute and held for 3 min at the final temperature. Nitrogen (30 ml/min) was used as a carrier gas whereas hydrogen and zero air were used as flame generation gases with a flow rate of 30 ml/min and 300 ml/min respectively. The split ratio of the sample was 1:30 in GC-FID system. The GC-FID was calibrated using external standard method. All samples were analyzed thrice for the better accuracy in results and averaged val-
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ues were considered for the calculation. Thereby, isolated product yield was calculated as follows: EL Yield Moles of EL ProducedMoles of total < feed < Conversion Moles of total Feed − Moles of unreacted Feed Moles of total Feed
2.4. Products conformational analysis The qualitative analysis of product mixtures were performed using GC–MS system (Perkin Elmer, USA, Model: Clarus 600), provided with an ESI detector. For a better product separation during the analysis, GC-FID program was kept same as that of GC-FID. However, 4 min solvent delay was kept during data acquisition to remove ethanol peaks from the GC–MS. Similar to GC-FID analysis for product quantification, DB-5 MS capillary column of identical specification was used in GC–MS instrument. In addition, nuclear magnetic resonance spectrometry (1 H, 9.4 T, 400 MHz and 13 C, 100 MHz) studies were performed using Bruker NMR spectrometer (model: AscendTM 400) to confirm structural conformation of the product. All the samples were prepared in CDCl3 solvent whereas TMS was used as internal standard. Based on m/z ratio obtained from GC–MS and ␦ ppm values obtained from NMR spectrometer, structural conformation were determined. In addition, GC–MS results were further verified using GC–MS (Shimadzu QP-2010) with thermal desorption system TD 20 at Advanced Instrumentation Research Facility in JNU, Delhi. 3. Results and discussion 3.1. Catalyst selection Firstly, experiments were performed on a variety of metal salts and zeolites to select suitable catalyst for synthesis of EL from HMF (Table 1). The reactions were performed, as per method described in experimental section, in an Aton Paar instant heating reactor (Monowave-50) at 160 ◦ C for 5 min. First experiment was performed in the absence of any catalyst to determine self-catalyzed reactivity of the feedstock. However, neither HMF conversion nor EL yield was detected, suggesting HMF do not reacts in the absence of a suitable acid catalyst (Table 1, entry 1). On the contrary, maximum 93.4% HMF conversion and 47.5% EL yield was measured, when experiments were performed in the presence of CuCl2 in 5 min at 160 ◦ C reaction temperature (Table 1, entry 2). Similar, result (43.2% Table 1 List of Catalysts Screened for HMF Conversion and EL Synthesis. S.N.
Catalyst
Catalysts Loading (mmol)
%HMF Conversion
%EL Yield
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Blank CuCl2 FeCl3 AlCl3 SnCl4 ZnCl2 CrCl3 LiCl ZrOCl2 H-Beta HZSM-5 (1) HZSM-5 (2) HZSM-5 (3) HZSM-5 (4) La-HZSM-5
– 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 30 mg 30 mg 30 mg 30 mg 30 mg 30 mg
0.0 93.4 73.9 59.5 40.8 31.85 14.7 8.0 33.7 19.9 38.8 29.9 32.3 11.6 12.94
0.0 47.5 43.2 39.0 13.04 0.0 10.0 0.0 9.9 0.0 0.0 0.0 0.0 0.0 0.0
Experimental conditions, feed: 0.5 mmol HMF, ethanol: 4.5 ml, temp. 160 ◦ C, reaction time: 5 min, heating rate: as fast as possible, stirring rate: 300 RPM.
3
EL yield, Table 1, entry 3) was obtained on ethanolysis of HMF in the presence of FeCl3 catalyst. The result obtained was comparable with data reported in literature for EL synthesis from a similar molecule furfural alcohol [38]. The EL yield obtained in the presence of both CuCl2 and FeCl3 , indicates that Lewis acidity (of Fe3+ and Cu2+ ) could be a major reason for such high EL yield [39]. A next very active catalyst was found to be AlCl3 (Al3+ ), yielding 39.0% EL under identical reaction conditions (Table 1, entry 4). In general, synthesis of EL from HMF essentially requires both Brønsted as well as Lewis acidity caused due to the application of metal salts. The metal part of the salts creates a Brønsted acid environment via ethanolysis/hydrolysis of M+ ions that helps in de-ethanolysis/dehydration of intermediates formed [50]. Whereas the Lewis acid center of metal salts coordinates with ring oxygen attached to HMF, thus leading to the formation of stable intermediate species which in result lowers the overall activation barrier for dehydration reaction [51]. However, results obtained with CrCl3 were in contrast to the anticipated values, thereby leading to conclusion that presence of Brønsted and Lewis acidity alone cannot be accounted for EL yield. Instead, there could be other factors that govern the desired product yield. One such possibility could be optimal ratio of Bronsted and Lewis acidity ratio [44]. The other factor could be different reaction mechanism, resulting into different activation barriers for each mechanism followed. For example, EL synthesis from HMF may follow two reaction pathways i.e. (i) conversion of HMF into LA and then to EL and/or (ii) conversion of HMF to EL via EOMF route (Fig. 1) [35]. In former route, HMF rehydrates to yield levulinic acid, which on further ethanolysis converts to EL. However, LA was not detected in the product mixture, thus possibility of this reaction route is minimum. Route R1 essentially requires presence of water for conversion of HMF into LA which in turn converts to EL on ethanolysis. Thus, limited availability of water molecules in the reaction mixture may be a possible reasons for not following this reaction pathway. On the contrary, EOMF as major intermediate and EL as product were detected in GC–MS data (Figs. S-2 & S-3 in SI), thus confirming the later reaction route followed. We already have discussed this route in detail in earlier study by our group [35]. Besides, EOMF and EL, several other products in minor quantity were also detected in GC–MS and NMR analysis (Figs. S-4, S-5, S-8, S-8 and S-9). Based on appreciable results obtained in the presence of CuCl2 , FeCl3 and AlCl3 , other metal salts were explored which are summarized in Table 1. However, no significant EL yield was obtained in the presence of ZnCl2 (Zn2+ ), possibly due to its weak acidity. To further confirm the effect of Brønsted and Lewis acid role in EL synthesis, we have employed zeolite based H-beta, HZSM-5 and lanthanum, loaded HZSM-5 catalyst with varying silica to alumina ratio (SAR) for a comparative study. In this case, no significant EL yield was detected, albeit 38.8% HMF conversion was measured (Table 1, entry 11). One possible reason could be the tendency of zeolites to promote polymerization reaction [36]. Huang et al. in a similar study for methyl levulinate production from furfural alcohol have measured very low (1% and 4.9%) yield using H-beta and HZS-5 zeolites respectively under microwave irradiation [38]. In our case, since AlCl3 yielded comparable EL and less by product formation, thus, next set of experiments were performed using AlCl3 catalysts for further study. Moreover, EOMF was not detected in product when experiments were performed using other metal salts except AlCl3 and ZrOCl2 . Absence of EOMF suggests that there could be other reaction pathway which does not essentially involve the formation of EOMF. However, since catalyst AlCl3 was mainly considered for studying effect of other parameters, we may present a more conclusive discussion later, on the basis of results obtained from DFT analysis of reactant and intermediates interaction with different metal salts and change in activation barrier.
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Fig. 1. Plausible Reaction Mechanisms for EL Synthesis from HMF.
Fig. 2. Effect of Reaction Temperature on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Ethanol: 4.5 ml, AlCl3 : 0.2 mmol, Reaction time: 5 min, Heating rate: as fast as possible, Stirring rate: 300 RPM
3.2. Effect of reaction temperature The role of temperature is crucial for any process to determine its techno-economic feasibility as it directly accounts for energy consumption during the reaction. Thus, experiments were performed by varying reaction temperature between 100 ◦ C–160 ◦ C for 5 min in the presence of AlCl3 . In general, it was observed that increase in temperature resulted into linear increase in HMF conversion and EL yield (Fig. 2). A maximum of 39.0% EL yield was measured in 5 min at 160 ◦ C reaction temperature whereas; a minimum of 14% EL was measured under identical reaction conditions. Thus, it can be concluded that higher reaction temperature accelerates the reaction rates. Similar trends in levulinate yield is reported by prominent researchers for methyl levulinate and ethyl levulinate respectively [38,43]. On the contrary, higher reaction temperature beyond a certain limit may promote the formation of byproducts. However, to check this hypothesis, more experiments needs to be performed at higher temperatures. We have not explored temperature beyond 160 ◦ C due to specified limits of the instrument.
Fig. 3. Effect of Reaction Time on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Ethanol: 4.5 ml, AlCl3 : 0.2 mmol, Reaction temp: 160 ◦ C, Heating rate: as fast as possible, Stirring rate: 300 RPM
mediates and byproducts. Therefore, experiments were carried out to study the effect of reaction time on EL yield and HMF conversion. It was observed that EL yield decreases with increase in reaction time (Fig. 3). The EL yield decreased from 39.0% in 5 min to 25.7% in 20 min at 160 ◦ C reaction temperature in the presence of AlCl3 . The results obtained were in line with the trend reported by Huang et al. for methyl levulinate synthesis from FAL [38]. The authors have suggested that levulinate yield increases to a maximum limit within five minutes of reaction time and starts decreasing after. Thus, it can be concluded that a longer reaction time leads to different reaction pathways and promotes byproducts formation. Possibly, a longer reaction time could have been the reason behind low EL from HMF by Yang et al. (12% EL in 15 min at 160 ◦ C) under microwave irradiation reaction environment [43]. Therefore, we attempted to perform experiments for less than 5 min time, however Monowave 50 takes 2–3 min minimum to reach the desired temperature and for pressure stabilization, thus results obtained were not encouraging and reproducible. It would be interesting to study the effect of reaction time below 5 min on EL yield, if desired reaction temperature can be reached within seconds like microwave reactor.
3.3. Effect of reaction time 3.4. Effect of catalyst concentration Reaction time is another important parameter that can be held responsible for overall energy consumed in the process. In addition, variations in reaction time may lead to formation of different inter-
Effect of catalyst concentration over HMF conversion and EL yield has been studied by varying catalyst amount from 0.1 mmol
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Fig. 4. Effect of Catalyst Concentration on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Ethanol: 4.5 ml, Reaction time: 10 min, Reaction temp: 160 ◦ C, Catalyst: AlCl3 , Heating rate: as fast as possible, Stirring rate: 300 RPM
to 0.4 mmol (Fig. 4). In present study, we performed experiments at 160 ◦ C in the presence of 0.1 mmol to 0.4 mmol AlCl3 catalyst. Initially, EL yield increased from 33.1% to 42.6% when AlCl3 concentration was increased from 0.1 mmol to 0.3 mmol. Enhancement in EL yield could be attributed to increase in acidity (H+ ion concentration created by hydrolysis of cations) due to higher catalyst loading. Interestingly, no significant improvement in EL yield was observed by increasing the catalyst concentration beyond 0.3 mmol, thus suggesting requirement of an optimum acid loading for better EL yield. A maximum of 43.5% EL yield was measured in the presence of 0.4 mmol AlCl3 , at 160 ◦ C reaction temperature. On the contrary, HMF conversion was found to be enhanced from 50.0% to 70% with the increase in catalyst concentration from 0.1 mmol to 0.2 mmol. Beyond this, HMF conversion was found to be unaffected with increase in catalyst concentration up to 0.3 mmol, thereby suggesting a catalyst loading around 0.2 mmol is sufficient for optimum EL yield and HMF conversion. 3.5. Effect of ethanol concentration It is important to determine the optimum feed ratio of the reactant to enhance the sustainability of the overall process. In general, a higher HMF concentration promotes self-polymerization reaction in the presence of an acid catalyst. Thus, experiments were performed by varying ethanol concentration and keeping HMF concentration constant under identical reaction conditions. Fig. 5 depicts effect of ethanol concentration on overall EL yield and HMF conversion. Interestingly, EL yield was found to be increasing (from 27.5% to 37.5%) with increase in ethanol concentration. In contrast, HMF conversion was found to be decreasing (from 81.8% to 70.18%) with increase ethanol concentration. The result obtained suggests that higher EL formation is favored in the presence of higher alcohol concentration albeit; a minor decrease in HMF conversion was measured. The improved EL yield could be attributed to inhibition of undesired reaction due to excess availability of ethanol for reaction. In addition, low HMF concentration due to excess ethanol could have suppressed self-polymerization reaction, thus resulting into overall low HMF conversion.
5
Fig. 5. Effect of Ethanol Concentration on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Reaction time: 10 min, Reaction temp: 160 ◦ C, Catalyst: 0.2 mmol AlCl3 , Heating rate: as fast as possible, Stirring rate: 300 RPM Table 2 Effect of Heating Rate on EL Yield and HMF Conversion. S.N.
Effect of rate
% HMF Conversion
% EL Yield
1. 2. 3.
As fast as possible 5 ◦ C/min 15 ◦ C/min
70.1 82.9 41.2
37.1 30.8 37.9
Experimental conditions, feed: 0.5 mmol HMF, ethanol: 4.5 ml, temp. 160 ◦ C, reaction time: 10 min, AlCl3 : 0.2 mmol, stirring rate: 300 RPM.
3.6. Effect of stirring speed & heating rate In heterogeneous systems, stirring speed plays a crucial role in overcoming mass transfer limitations. In general, most of the metal salts dissolved in reaction mixture, thus we believed that external and intrinsic mass transfer limitations should not affect the HMF conversion and EL yield. To confirm this hypothesis, experiments were performed by varying stirring speed from 300 RPM to 1200 RPM at 160 ◦ C. Maximum 37.5% EL yield was measured in the presence of 0.2 mmol AlCl3 at 160 ◦ C reaction temperature and 300 RPM. However, significant reduction in EL yield was measured when experiments were performed at a speed of 600 RPM. Post this, we did not observed any major deviation in EL yield and HMF conversion due to variation of stirring speed from 600 RPM to 1200 RPM (Fig. 6). Albeit minor changes in both EL yield as well as HMF conversion was observed. Low EL yield could be because of lesser contact time between the catalyst and the reactant due to very high turbulence, thus suppressing the formation of intermediates for further reaction. Similarly, experiments were performed to study the effect of heating method on EL yield and HMF conversion. It was observed that instant heating (as fast as possible) and fast heating (5 ◦ C/min) yielded a higher HMF conversion and amount EL (Table 2) whereas slow heating method resulted into less HMF conversion, thus yielding less EL. Moreover, decrease in ramp rate also led to decrease in EL yield from 38.0% to 31.2% which indicates instant heating method have advantage over slow heating processes. Interestingly, EL yield was higher at low HMF conversion which indicates that more free acid sites could have been available for conversion of pro-
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Fig. 6. Effect of Stirring Speed on EL Yield and HMF Conversion. Experimental conditions, Feed: 0.5 mmol HMF, Ethanol: 4.5 ml, Reaction time: 10 min, Reaction temp: 160 ◦ C, Catalyst: 0.2 mmol AlCl3 , Heating rate: as fast as possible
duced intermediates to EL. On the contrary, when HMF conversion was high, more number of acid sites could have been occupied in HMF to intermediates conversion, thereby resulting into less number of acid sites available for conversion of produced intermediates to final product EL.
Fig. 7. Effect of Substrates on EL Yield. Experimental conditions, Feed: Various, Ethanol: 4.5 ml, Reaction time: 10 min, Reaction temp: 160 ◦ C, Catalyst: 0.2 mmol AlCl3 , Heating rate: as fast as possible, Stirring rate: 300 RPM
and proven as compared to microwave system, thus present study may open a new window of opportunity in this area.
3.8. Catalyst activity on different substrate 3.7. Comparison between non-microwave vs. microwave irradiation reaction environment The effects of microwave and non-microwave instant heating reaction environment were studied by performing optimized experiments in a microwave reactor (Monowave 300, Anton Paar, USA). We did not found any major deviation in the EL yield obtained from both the reactors when experiments were performed in the presence of AlCl3 catalyst. The major advantage associated with microwave reactor is the reduced reaction time and lower byproduct formation due to powerful agitation and localized heating that ultimately helps to achieve improved EL yield. Therefore, creating identical reaction conditions (powerful stirring, instant and localized heating) may help in getting similar yield. The results obtained under non-microwave instant heating environment (in Monowave 50) were relatively comparable with microwave reactor in terms of EL yield. However, HMF conversion was measured to follow similar trend comparable to results obtained under nonmicrowave reaction environment (Table 3). These results suggest that non-micorwave instant heating in the presence of AlCl3 catalyst is helpful in improving EL yield. In addition, it can be considered as an alternative to microwave reactors which are difficult to scale up. Nevertheless, scale up of non-microwave system is quite easier
In a critical review on catalytic synthesis of EL, we suggested that molecular structure of the reactant plays a crucial role in determining overall EL yield [35]. To confirm this hypothesis, experiments were performed using various substrates in the presence of efficient and environmentally benign catalyst AlCl3 [35]. Cellulose ethanolysis in the presence of AlCl3 catalyst yielded minimum EL 11.3% followed by glucose and fructose that yielded 17.9% and 23.3% EL respectively at 160 ◦ C at 300 RPM. On the contrary, EL yield above 90% obtained when LA and FAL were used as feedstock (Fig. 7). The results obtained were in line with our hypothesis that downstream feedstock such as FAL and LA yields more EL. On the contrary, when we start moving upstream feedstock from LA such fructose, glucose, cellulose etc., the EL yields decreases in the same order. The obtained EL yields were 11.3%, 18.8%, 23.3%, 25%, 37.17%, 91% and 96% for cellulose, starch, glucose, fructose, HMF, FAL and LA respectively. The measured EL yield clearly indicates the effect of reactant structure, thereby leading to a conclusion that moving down from upstream feedstock to downstream feedstock increases EL yield due to structural effect of the feedstocks. The other possible reason could be lack of high strong acidity for conversion of glucose, starch and cellulose, thus resulting into less dehydration of the reactants for further conversion. Similarly, conversion of large molecules may
Table 3 Comparison between Microwave vs. Non-microwave Instant Heating Reaction Environment. S.N
1. 2. 3. 4. 5.
Reactant
HMF HMF HMF HMF HMF
Catalyst
AlCl3 SnCl4 ZnCl2 CrCl3 ZrOCl2
Yield%
Conversion%
Monowave 50
Monowave 300
Monowave 50
Monowave 300
39.0 13.0 0.0 10.0 9.9
41.9 37.3 43.0 53.8 41.8
59.5 40.8 31.9 14.7 33.7
43.3 57.6 45.9 46.7 38.3
Experimental conditions, feed: 0.5 mmol HMF, ethanol: 4.5 ml, temp. 160 ◦ C, reaction time: 5 min, Catalysts: 0.2 mmol, stirring rate: 300 RPM.
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be limited due to intrinsic mass transfer diffusional limitations at low stirring speed. 4. Conclusion and outlook The catalytic conversion of 5-hyroxymethylfurfural using commercially available metal salt was studied under microwave and non-microwave instant heating reaction environment. The maximum EL yield of 47.5%, 43.5% and 39.0% was obtained using CuCl2 , FeCl3 and AlCl3 respectively at 160 ◦ C in 5 min. EL yield further improved to 43.5% in the presence of 0.4 mmol of AlCl3 at 160 ◦ C in 10 min. In general, EL yield was found to be increasing with increase in reaction temperature and the catalyst concentration whereas longer reaction time led to a slight reduction in EL yield. The comparative study of non-microwave vis-à-vis microwave instant heating reaction environment suggests HMF conversion is non-micorwave instant heating reaction environment may yield comparable results to microwave reactors for EL synthesis from HMF in the presence of AlCl3 . Future studies will be directed towards understanding multiple reaction pathways, and effect of dielectric constant of the reactants structure thereby affecting EL yield in the presence of different metal catalysts.
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Acknowledgements [18]
One of the Authors, Ejaz Ahmad would like to thank Prime Minister Fellowship (D.O. No. DST/SSK/SERB-CII-FeII/2014) and Hindustan Petroleum Corporation Limited (HPCL Green R&D Center) for providing fellowship to pursue his doctoral studies. Department of science and technology (DST), government of India for funding via extra mural research grant (EMR/2015/001959) to carry out this research work is gratefully acknowledged. The authors would like to thank Anton Paar, Gurgaon India for providing prototype of the reactor Monowave-50 to perform experiments at our lab. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.12. 019.
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