Solvent-free glycerol transesterification with propylene carbonate to

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Solvent-Free Glycerol Transesterification with Propylene Carbonate to Glycerol Carbonate over a Solid Base Catalyst Sharda Kondawar and Chandrashekhar Rode* Chemical Engineering and Process Development Division, Council of Scientific and Industrial Research (CSIR)-National Chemical Laboratory (NCL), Dr. Homi Bhabha Road, Pashan, Pune 411008, India S Supporting Information *

ABSTRACT: Glycerol transesterification using propylene carbonate (PC) to glycerol carbonate (GC) could be efficiently performed under solvent-free conditions using solid base as catalysts involving non-noble metal oxide in combination with hydrotalcites (HTs). Among all of the catalysts studied for transesterification, the best result was obtained over a calcium-doped hydrotalcite (Ca-HT) catalyst, giving 84% conversion of glycerol and almost complete GC selectivity. The crystal structure of HT was modified by incorporation of Ca and La into HT, as revealed by X-ray diffraction studies. The temperature-programmed desorption of carbon dioxide study confirmed the presence of the highest basic site density in terms of 1.94 mmol of CO2 desorbed/g of catalyst, responsible for its higher transesterification efficiency of the Ca-HT catalyst. The Fourier transform infrared spectroscopy study showed peaks at 3036 and 3042 cm−1 for Ca-HT and lanthanum-doped hydrotalcite (La-HT), respectively, confirming the presence of hydrogen bonding between water and interlayer carbonate anions responsible for abstracting proton from the primary hydroxyl group of glycerol to attack over carbonyl carbon of PC. The presence of intercalated carbonate ions is also confirmed by the Raman study, in both HT and Ca-HT catalysts and even after use of the CaHT catalyst. The thermogravimetry−differential thermal analysis study evidenced the higher thermal stability of the Ca-HT (T4 = 765 °C) catalyst than that of parent HT with a Mg/Al ratio of 3:1 (T4 = 630 °C). Various process conditions, such as the temperature, molar ratio of glycerol/PC, and catalyst loading, significantly influenced conversion and selectivity of glycerol and GC, respectively. under harsh conditions of temperature (180 °C) and pressure (5 MPa), giving only 40% conversion, which is too low for practical purposes.7,8 Mixed oxides of La−Zn were also proposed in the direct glycerol carboxylation but resulted in a very poor yield of only 14%.9 Dibenedetto et al. reported CeO2−Al2O3 and CeO2− Nb2O5 catalysts for direct carboxylation of glycerol with CO2 but resulted in a lower conversion because of thermodynamic limitation (ΔG = 23.92 kJ mol −1).10,11 The glycerol transesterification route to GC is generally catalyzed by basic catalysts, as exemplified by several researchers. Some of these include Mg/ Al/Zr catalysts, giving 86% yield of GC,12 while K2CO3/MgO, KF-modified hydroxyapetite, and an enzymatic catalyst, such as lipase, have also been studied for the transesterification reaction.13−15 Glycerol transesterification with urea over the 2.5% Au/ZnO catalyst studied by Hammond et al. was found to give 88% conversion of glycerol and much lower selectivity (56%) to GC as a result of the formation of substituted carbamates.16 Another Zr-based catalyst reported by Ferragina and co-workers exhibited 80% glycerol conversion with GC selectivity of >99% for carbonylation of glycerol using urea,17 whereas Co3O4−ZnO mixtures were also employed for glycerol transesterification using urea, giving 69% conversion with complete selectivity to GC.18 Another route of glycerol carbonation with phosgene involves use of toxic and hazardous carbon monoxide, while direct carboxylation with CO2 requires a

1. INTRODUCTION Manufacturing of biodiesel via transesterification of triglycerides, although a widely practiced alternative sustainable energy source, is also accompanied by the co-generation of glycerol; this needs to be used in a most profitable way. In recent years, increasing biodiesel manufacture has led to a huge amount of glycerol availability, which prompted the researchers to focus on its downstream processing to valuable products.1 Among several options, cyclic carbonate of glycerol is one such valuable product and is having widespread industrial applications as surfactants, adhesive, ink, paint, lubricant, and also a moisturizing agent for making health care products. It can also be used as an electrolyte in lithium batteries by replacing flammable ethylene carbonate. It is also useful being a green solvent for the manufacture of polymers, such as urethane,2 while it can serve as a monomer for making polycarbonates and polyurethanes.3 Glycerol carbonate (GC) is also one of the novel components in the gas separation membranes.4 In the literature, several approaches are reported for converting glycerol to GC. Some such attempts include (i) reacting glycerol with phosgene, (ii) direct glycerol carbonylation in the presence of carbon monoxide and oxygen, (iii) carboxylation of glycerol with CO2, and (iv) glycerol transesterification using urea, dimethyl carbonate, and propylene carbonate (PC). Among these, carbonation and carbonylation of glycerol with phosgene and CO have been reported using homogeneous Pd (1,10-phenanthroline) and metallic catalysts (e.g., Cu), respectively,5,6 whereas carboxylation of glycerol with CO2 has been reported by Aresta et al. using a Sn-based catalyst © XXXX American Chemical Society

Received: January 5, 2017 Revised: February 22, 2017 Published: February 22, 2017 A

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Energy & Fuels Scheme 1. Two-Step Route for the Synthesis of GC from Epoxide

studied. Catalyst stability was confirmed by three successive activity testing runs over the same catalyst under the same conditions of reaction.

key challenge of activation of a chemically inert CO2 molecule, thus restricting the practicality of these approaches. Although, carbonylation of glycerol with urea has better potential of commercialization, the removal of ammonia under reduced pressure becomes inevitable to drive the reaction equilibrium to the product side.19 In the case of transesterification with dimethyl carbonate, a high glycerol/dimethyl carbonate (DMC) ratio, high temperature, longer reaction time, and use of additional solvents still are the major issues to be resolved. Its quite surprising that there are hardly any reports on glycerol transesterification with PC, except that Zephirin et al. employed Amberlyst-15 and ZSM-5 catalysts with glycerol conversion of 61%.20 The unique feature of glycerol transesterification with PC is that along with GC, a formation of 1,2-propanediol (1,2-PDO) with a global demand of ∼1.5 million tons per year which is mainly the glycerol hydrogenolysis product, having wide-range industrial applications as a functional fluid, in paint, liquid detergents, health care products, and foodstuff and for the synthesis of polyesters.21 More importantly, it can be recycled to produce PC by its alcoholysis with urea in the presence of Pb−Zn mixed metal carbonate as a catalyst.22 The current market price of PC is U.S. $1450/MT, which becomes significantly appreciated to U.S. $4000/MT for GC as the marketable product.23 In this transesterification of glycerol with PC, the complete atom economy can be achieved because both GC and 1,2-PDO are the useful products, while for only GC as the product, 60% atom economy is possible as per eq 1. % atom economy =

2. EXPERIMENTAL SECTION 2.1. Materials. Glycerol (98%) and hexahydrate of magnesium nitrate were purchased from Merck, India. Al(NO3)3·9H2O was purchased from Loba Chemie, India. While nitrate precursors of Ca and La and NaOH were purchased from Thomas Baker, India. PC was obtained from Himedia Laboratories, India, and used as received. 2.2. Catalyst Preparation. La2O3, CaO, MgO, HT (Mg/Al = 3), Ca-HT (Ca/Mg/Al of 3:3:1), and La-HT (Mg/Al/La = 0.75:0.023:0.02) were prepared by the method of co-precipitation.26,27 Typically, weighed amounts of Mg(NO3)2·6H2O and Al(NO3)3·9H2O were dissolved in deionized water to make the resepective solutions of 0.75 and 0.25 M and then allowed to precipitate out using 2 M K2CO3 and 2 M KOH solutions by maintaining the pH at 10. The suspension was stirred for 18 h at ambient temperature. Thus, the precipitate formed was filtered followed by washing with deionized water several times and dried in an oven (110 °C) extending to 12 h. It was then calcined in air up to 450 °C for 3 h. The catalysts thus prepared were designated as CaO, MgO, La2O3, HTWC (Mg/Al = 3, as synthesized), and HTC (Mg/Al = 3, after calcination). For the synthesis of Ca-HT (Ca/Mg/Al = 3:3:1) and La-HT (Mg/Al/La = 0.75:0.023:0.02), along with 0.75 M Mg(NO3)2·6H2O and 0.25 M Al(NO3)3·9H2O, 0.75 M Ca(NO3)2·4H2O and 0.02 M La(NO3)3·6H2O were also added, respectively. 2.3. Characterization. X-ray diffraction (XRD) characterization was performed using the PAnalytical PXRD system (X’Pert PRO-1712), using Ni-filtered Cu Kα (λ = 0.154 nm) as an X-ray source (current intensity, 30 mA; voltage, 40 kV) with an X-accelerator detector. Scanning of samples was performed in the 2θ range of 10−90°. Thermogravimetric analysis (TGA) was performed on PerkinElmer TGA-7 at a scan rate of 10 °C min−1 in N2 from room temperature to 800 °C. The temperature-programmed desorption of carbon dioxide (CO2-TPD) was performed using a Micromeritics 2720 (Chemisoft TPx). To assess the basicity of the catalysts, the TPD procedure was as follows: (i) pretreatment of the samples under He (25 mL min−1) from room temperature to 200 °C, (ii) CO2 adsorption at 40 °C, and (iii) CO2 desorption at 10 °C min−1, starting from the adsorption temperature to 700 °C. Infrared (IR) spectra were recorded on a PerkinElmer (Spectrum One) Fourier transform infrared spectroscopy (FTIR) instrument in the range of 400−4000 cm−1, with accumulation of 20 scans and 4 cm−1 resolutions. The Raman spectra were recorded on a Horiba JY Lab RAM HR800 micro-Raman spectrometer with 17 mW and 632.8 nm laser excitation. Morphology of HT and Ca- and Ladoped HT catalysts was studied by scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) analysis was performed on LEO−LEICA STEREOSCAN 440 for the presence of Ca and La loading. Transmission electron microscopy (TEM) characterization was performed on a JEOL model, JEM 1200 electron microscope operated at an accelerating voltage of 120 kV. For this purpose, ethanol suspension of the sample was prepared by ultrasonication and the carbon-coated copper grid was used for mounting the samples. 2.4. Catalyst Performances. The transesterification reaction was carried out in a round-bottom flask at ambient pressure. A typical reaction protocol, involved charging 12 mmol of glycerol, 12 mmol of PC, and 0.1 g of catalyst to a two-neck round-bottom flask equipped

molecular mass of desired product molecular mass of all reactants × 100

(1)

The catalyst development for transesterification of glycerol with PC also has another great advantage that the cascade route for GC synthesis from CO2 can be worked out starting from propylene oxide and CO2, as shown in Scheme 1. There are several reports in the literature on CO2 carboxylation of propylene oxide to PC.24,25 Hence, our approach for GC synthesis covers two green chemistry aspects: first, it is carried out under solvent-free conditions, and second, it is atomeconomic. This is the first report on a solid base catalyst for glycerol transesterification with PC as a carbonylating agent. Several metal oxides, such as CaO, MgO, and La2O3, along with Ca- and La-incorporated hydrotalcite (HT) catalysts were also prepared and evaluated for their activity. Calcium leaching was overcome by doping Ca into HT, making it truly heterogeneous and stable. A detailed characterization of the catalysts and its correlation with the observed activity were the main objectives of this work. In addition, the influence of various process conditions, such as the temperature, varying ratios of glycerol/PC, and catalyst loading, on glycerol conversion and GC selectivity was also B

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Figure 1. (A) XRD patterns of HT samples: (a) HT-WC (as synthesized), (b) HT-C (calcined), (c) Ca-HT, and (d) La-HT [■, La(OH)3; ●, La2O2CO3] and (B) XRD patterns of metal oxide samples: (a) MgO, (b) La2O3, and (c) CaO.

Figure 2. TG−DTA profiles for prepared catalysts. with a coiled condenser. The desired temperature was attained by means of a silicon oil bath, and the agitation speed was maintained at 1000 rpm using a magnetically driven agitator. For the reactions carried out in the presence of dimethylformamide (DMF) solvent, samples of liquid were directly analyzed by gas chromatography, whereas the reactions carried out under solvent-free conditions, initial as well as the sample after the reaction, were diluted with methanol (keeping the same dilution factor for all of the samples) and then analyzed by Shimatzu 2025 gas chromatography equipped with a HP-FFAP column (30 m length × 0.53 mm inner diameter × 1 μm film thickness) and flame ionization

detector (FID). All of the products were identified by gas chromatography−mass spectrometry (GC−MS) (Table S1 and Figure S1 of the Supporting Information). The mass balance calculated in each experiment matched >95%. The glycerol conversion, GC selectivity, and turnover frequency (TOF) were calculated as follows:

percent conversion of glycerol =

C

moles of glycerol reacted × 100 initial moles of glycerol (2) DOI: 10.1021/acs.energyfuels.7b00034 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Profiles of CO2-TPD for (a) HT samples and (b) metal oxide. percent selectivity of GC =

moles of desired product × 100 ∑ moles of all products

reflections at 2θ = 23.2°, 29.3°, 36.2°, 39.6°, 43.4°, and 57.7° corresponding to (105), (108), (116), (118), (0015), and (303) planes (JCPDS 86-0383). Figure 1B displays the XRD patterns of MgO, La2O3, and CaO samples. MgO showed characteristic reflections at 2θ = 36.9°, 42.8°, 62.3°, 74.6°, and 78.6° arising from (111), (200), (220), (311), and (222) planes, respectively (JCPDS 78-0430), whereas La2O3 exhibited hexagonal symmetry showing reflections at 2θ = 27.2°, 28.2°, 31.5°, 31.9°, 50.0°, 69.7°, and 77.3° as a result of (002), (101), (102), (200), (203), and (212) planes, respectively. In the case of CaO, reflections at 2θ = 32.3°, 37.4°, 54.0°, 67.4°, and 88.8° correspond to (111), (200), (220), (222), and (331) planes, respectively (JCPDS 82-1691). The weight loss profiles and heat flow versus temperature determined by TGA for HT without calcination and after calcination and for Ca-HT catalyst samples are displayed in Figure 2. The first step weight loss of 10% observed at 400 °C, as a consequence of the removal of residual carbonate in the sample (Figure 2a).30 The total weight loss of 40% was observed for HT (Mg−Al = 3:1), as shown in Figure 2b. However, the total percent weight loss of Ca-HT was 35% (Figure 2c). Hence, from the thermogravimetry−differential thermal analysis (TG−DTA) study, it can be concluded that the thermal stability of Ca-HT (T4 = 765 °C) was higher compared to that of parent HT with a Mg−Al ratio of 3:1 (T4 = 630 °C). The CO2-TPD profiles are shown in Figure 3. As already known, the HTs after calcination generate varying active sites, such as low-coordinated O2− and surface OH− groups and M+− O2−, which are responsible for strong Lewis basicity, weak Brønsted basicity, and medium-strength Lewis basicity, respectively.31 Figure 3a clearly shows that all three CO2-TPD profiles for HT, La-HT, and Ca-HT have a characteristic broad peak covering a wide range of temperatures of 100−700 °C, suggesting that there is a wide distribution of basic strength on the surface.32 In the case of La-HT and Ca-HT, CO2-TPD profiles showing a peak possessing the highest basic site density were found to be shifted to a higher temperature range compared to HT (at 275 °C) with peak maxima at 450 and 630 °C for Mg and Al mixed oxides, respectively. It might be because of

(3)

TOF =

mgly Xgly Mgly 100Acat t

(4)

where mgly is the initial moles of glycerol (mol), Xgly is the conversion of glycerol (%), Mgly is the molecular weight of glycerol (g/mol), Acat is the catalyst amount (g), and t is the reaction duration (h).

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Among all of the catalysts screened for transesterification of glycerol with PC, Ca-HT and La-HT catalysts exhibited the best performance in terms of glycerol conversion and GC selectivity. Therefore, these catalysts were characterized in detail, and the results are discussed below. XRD patterns of hydrotalcite without calcination (HT-WC), hydrotalcite calcined at 450 °C (HT-C), calcium-doped hydrotalcite (Ca-HT), and lanthanum-doped hydrotalcite (LaHT) samples are depicted in Figure 1A. The as-prepared HT showed characteristic reflections at 2θ = 11.6°, 23.5°, 35.0°, 39.6°, 47.1°, 60.7°, and 62.1° arising from (003), (006), (012), (015), (018), (010), and (113) planes, respectively [Joint Committee on Powder Diffraction Standards (JCPDS) 702151]. The calcined HT sample also exhibited all of the reflections corresponding to all of the above planes but with decreased intensity.27 Decomposition of HT occurs during calcination with the evolution of water and CO2 molecules, leading to the formation of micropores within the structure with the development of Mg−Al mixed oxide phases. Interestingly, Ca and La doping affected the crystal structure of HT, and hence, the intensity of XRD peaks dramatically decreased with the incorporation of Ca and La.28 In addition, the La-HT sample exhibited peaks at 2θ = 34.1° (JCPDS 23-0435) and 28° and 47.1° (JCPDS 75-1900) corresponding to La2O2CO3 and La(OH)3 phases, respectively. Although Al and La showed the same oxidation state (+3), the ionic radius for La3+ is nearly onefourth (0.013 nm) that of Al3+ (0.054 nm), because of which La most likely resides in the interlayer space of HT in the form of an oxide/hydroxide phase instead of being replaced by Al3+. On the other hand, in case of the Ca-HT catalyst, the ionic radius of Ca2+ is nearly about double (0.114 nm) that of Mg2+ (0.086 nm). Hence, it is more likely that Mg2+ is replaced by Ca2+ in the layered double hydroxide (LDH) framework with characteristic D

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Energy & Fuels Table 1. CO2-TPD Results of Prepared Catalysts temperature-range-wise desorption of CO2 (mmol g−1) number

catalyst

weak, 400 °C

total CO2 desorbed (mmol g−1)

1 2 3 4 5 6

HT La-HT Ca-HT MgO CaO La2O3

0.1409 0.2978 0.0758

0.1503 0.2931 0.1466 0.2101 1.714 0.004

0.0181

0.3093 0.5909 1.9424 0.4366 2.0139 1.559

0.0772 0.885

replacement of Mg2+ by Ca2+ and Al3+ by La3+ for Ca-HT and LaHT catalysts, respectively.33 Therefore, it could be inferred that Mg or Al was partially substituted by ions of S block (Ca) and inner transition metal (La) that had significantly influenced the basic strength of the catalysts (Figure 3a). Total CO2 desorbed in the case of La-HT and Ca-HT was 0.590 and 1.942 mmol g−1, respectively (Table 1). More interestingly, in the case of the CaHT catalyst, the highest intense peak with 1.942 mmol g−1 CO2 desorption was found to be in the temperature range of 400−600 °C (Table 1). CO2-TPD profiles of different metal oxides in Figure 3b indicate that the basic site density of metal oxide was in the order of CaO > La2O3 > MgO, with a total CO2 desorbed 2.013, 1.559, and 0.4366 mmol g−1, respectively, which is in good agreement with the activity profiles (Table 2).

(panels a−c of Figure 4), and also a shoulder peak at 3034 cm−1 was arising as a result of hydrogen bonding between water and interlayer carbonate anions (Figure 4a), which was found to be slightly shifted to higher values of 3036 and 3042 cm−1 for CaHT and La-HT, respectively. The peak arising at 1365−1370 cm−1 was the major absorption band of the carbonate anion, which was found in the case of HT and La-HT samples. The split in double bands indicates that there was lowering of carbonate anion symmetry. The sharp peak observed at 760 cm−1 in Ca-HT and La-HT profiles was due to the translational vibration mode of the hydroxyl group influenced by alumina.30 Structural analysis of the catalyst was studied further by Raman spectroscopy. Raman spectra of HTC and Ca-HT fresh and spent catalysts in the range of 400−4000 cm−1 are shown in Figure 5. In the case of the calcined HT sample, the presence of CO32− and NO3− was confirmed by sharp bands around 1084 and 719 cm−1, respectively (Figure 5a).34 At a lower wavenumber region of the HTC sample spectra, the two bands at around 500 and 427 cm−1 were due to Al−O−Al and Mg−O−Mg linkages, respectively. The Raman spectra of the Ca-HT catalyst (Figure 5b) showed a slightly shifted band position for CO32− and NO3− at around 1086 and 711 cm−1, respectively. The Ca-HT catalyst showed two intense bands at around 530 cm−1 attributed to Al− O−Al linkage and 430 cm−1 as a result of Mg−O−Mg linkage in HT. The bands at a lower wavenumber of 395 and 358 cm−1 were arising as a result of the Ca−O stretching mode of vibration in the HT structure (Figure 5b),35 while in the case of the spent catalyst, intensity of all of the bands appeared to be decreased, particularly for Al−O−Al and Mg−O−Mg linkages (Figure 5c). This confirmed that the structural deformation occurred in the HT lattice under reaction conditions. Thus, from the Raman study, the presence of intercalated carbonate ions was confirmed in both HT and Ca-HT catalysts and even after use of the Ca-HT catalyst (Scheme 2). The morphologies of the prepared HT catalysts were studied by SEM. Figure 6a is a high-vacuum SEM image of HT, which showed layered HT sheets. It was observed from panels a and b of Figure 6 that HT showed lamellar morphology, which was the characteristic of a layer structure. The Ca-HT catalyst was found to consist of agglomerates of hexagonal, plate-like crystals of 0.2− 0.5 μm in size. From SEM images of Ca-HT and La-HT (panels a and b of Figure 6), it was found that some pores were formed after the calcination of the HT catalyst (shown by the arrows). This was due to removal of interlayer OH− and CO32− anions during calcination of (Ca-HT and La-HT) catalysts, which was also confirmed by the TG−DTA study.36 Calcium and lanthanum doping in the HT framework was confirmed by EDX of Ca-HT and La-HT catalysts (panels c and d of Figure 6). Figure 7 shows TEM images of HT before and after calcination along with Ca-HT. The morphology of LDH powders changed drastically with aging temperature, as shown in panels a and b of Figure 7. LDH particles after calcination at 450 °C resulted in

Table 2. Catalyst Screening for Transesterification of Glycerol in the Presence of DMFa entry

catalyst

percent conversion (%)

percent selectivity (%)

1 2 3 4 5

CaO HT (3) MgO La2O3

30 58 52 40 29

96 99 99 98 96

1.720 0.2265 0.2227 0.670

a

Reaction conditions: glycerol/PC molar ratio, 2; catalyst dose, 0.1 g; DMF, 5 mL; temperature, 70 °C; and reaction time, 2 h.

Figure 4 displays IR spectra of the HTC, Ca-HT, and La-HT in the range of 400−4000 cm−1. The broad band around 3400− 3450 cm−1 in all of the samples was due to the stretching of hydrogen-bonded hydroxyl groups and the H2O molecule

Figure 4. FTIR spectra of HT samples. E

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Figure 5. Raman plots of the prepared catalyst.

and glycerol conversion because the trend was found to be in the following order: CaO > La2O3 > MgO > HT (3), which is consistent with CO2-TPD trends (Figure 3b). Exceptionally, HT showed higher activity that might be because HT has a lamellar sheet-like structure and provides easy accessibility of glycerol and PC molecules to interact with active sites of the catalyst to produce GC. 3.2.1. No Solvent Studies. Because the organic solvents as a result of their volatile nature create environmental problems, their elimination is highly desirable.37 Therefore, transesterification of PC with glycerol was carried out under solvent-free reaction conditions, using a solid base catalyst. The ratio of PC/ glycerol was maintained at 1:1 in all of the transesterification reactions. Among all of the catalysts shown in Table 2, the highest activity was observed for CaO with 80% conversion of glycerol and 98% GC selectivity within 5 h at 80 °C temperature (entry 1 in Table 3). In contrast to this, ZrO2 being an acidic oxide showed inferior activity of only 43% conversion of glycerol with 97% selectivity toward GC (entry 4 in Table 3). Ca-HT also showed good conversion of about 67% and GC selectivity of 99% (entry 2 in Table 3). Although CaO showed the highest activity, it was not recyclable because of the formation of soluble calcium glycerate in the reaction crude after the first use.38 Simanjuntak et al. investigated that the transesterification reaction in the presence of CaO as a catalyst is catalyzed by the formation of homogeneous species giving turbidity to reaction crude, which is also observed in our case with 4.2 mg L−1 of Ca leaching, as confirmed by inductively coupled plasma optical emission spectrometry (ICP−OES) analysis (entry 1 in Table 3).

Scheme 2. Schematic Representation of the Prepared Catalyst

platelet morphology (Figure 7b). The lattice spacing for the (012) plane was found to be 2.47 nm, obtained from the fringe pattern of the Ca-HT catalyst (Figure 7c). 3.2. Activity Studies. Glycerol transesterification with PC gave GC and 1,2-PDO over several metal oxides (CaO, MgO, and La2O3) and HTs (HT, Ca-HT, and La-HT) screened, and these results are displayed in Table 2. Interestingly, this reaction showed about 30% conversion without a catalyst but with low selectivity to GC with the formation of other side products, involving 2,6-dihydroxy-3,5-dimethyl-4-oxyheptane and 2,6dihydroxy-3- oxyheptane ( entry 1 in Table 2 and Scheme 3). Among the screened catalysts (basic metal oxides and HTs), CaO and HT (Mg/Al = 3) gave higher conversion of about 58 and 52%, respectively, with almost complete selectivity to GC (entries 2 and 3 in Table 2, respectively), whereas MgO and La2O3 showed somewhat lower conversion of 40 and 29%, respectively (entries 4 and 5 in Table 2, respectively). The basicity of the catalyst was found to facilitate the selectivity to GC F

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Figure 6. SEM images of (a) Ca-HT and (b) La-HT and EDX for (c) Ca-HT and (d) La-HT.

Figure 7. HRTEM images of (a) HTWC (without calcination), (b) HTC (calcined), and (c) Ca-HT and (d) selected area electron diffraction (SAED) pattern of Ca-HT.

G

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Energy & Fuels Scheme 3. Schematic Representation of Other Possible Byproduct Formation during Transesterification of Glycerol to GC

Figure 9. Effect of the reaction temperature. Reaction conditions: glycerol/PC molar ratio, 1; catalyst dose, 0.1 g; and reaction time, 5 h.

decreased as a result of the formation of side products, such as 2hydroxyethylene oxide, 2,6-dihydroxy-3,5-dimethyl-l-4-oxyheptane, and 2,6-dihydroxy-3-oxyheptane (Table S1 and Figure S1 of the Supporting Information). Hence, a further study of process optimization was performed at 160 °C. 3.2.3. Mole Ratio of Glycerol/PC. The mole ratio of glycerol/ PC was varied from 0.5 to 2, and these results are displayed in Figure 10. Because transesterification of glycerol required 1 mol

Table 3. Catalyst Screening for Transesterification of Glycerol without Solventa entry

catalyst

percent conversion (%)

1 2 3 4

CaO Ca-HT La-HT ZrO2

80 67 60 43

percent selectivity (%)

percent yield (%)

metal leaching (Ca) (ppm)

98 99 91 97

78 66 54 41

4.227 ND

a

Reaction conditions: glycerol/PC molar ratio, 1; catalyst dose, 0.1 g; temperature, 80 °C; and reaction time, 5 h.

Hence, the catalyst was heterogenized by incorporating Ca in the HT framework, giving glycerol conversion of 67% and GC selectivity of 99% (entry 2 in Table 3). The catalyst stability was evident by a clear reaction crude of the Ca-HT-catalyzed reaction (Figure 8).

Figure 10. Effect of the glycerol/PC ratio. Reaction conditions: catalyst dose, 0.1 g; temperature, 160 °C; and reaction time, 5 h.

of PC for each mole of glycerol to form 1 mol of GC, it was observed that glycerol conversion and GC yield increased initially from 78 to 84% for 0.5 to 1 molar ratio of glycerol/PC. However, glycerol conversion abruptly decreased from 84 to 52% for a further increase in the glycerol/PC ratio from 1 to 2 while maintaining the complete selectivity to GC. This could be because of competitive adsorption of excess glycerol, thus decreasing the possibility of PC adsorption on the catalyst surface, resulting in lowering of glycerol conversion.39 3.2.4. Catalyst Loading Study. Influence of the Ca-HT catalyst amount was observed by varying its loading in the range of 0.05−0.2 g. Figure 11 showed that, when the catalyst amount was increased up to 0.1 g, both the GC yield and glycerol conversion (up to 84%) enhanced with complete selectivity. A further increase in the catalyst loading did not greatly affect the glycerol conversion. An increase in the Ca-HT catalyst loading

Figure 8. Product mixtures of glycerol transesterification with CaO and Ca-HT as catalysts: (A) product mixture with CaO as the catalyst, (B) STD mixture of the reactant (glycerol and PC), and (C) product mixture with Ca-HT as the catalyst.

3.2.2. Temperature Effect. The study of the influence of the reaction temperature on glycerol transesterification with PC was conducted in the range of 80−180 °C (Figure 9). The conversion of glycerol increased linearly from 67 to 84% by increasing the temperature up to 160 °C with almost complete GC selectivity (99%), and glycerol conversion remained constant beyond 160 °C, up to 180 °C. However, selectivity to GC marginally H

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Figure 11. Effect of the catalyst loading. Reaction conditions: glycerol/ PC molar ratio, 1; temperature, 160 °C; and reaction time, 5 h.

increased the number of basic sites (increase of the active sites), which, in turn, enhanced the catalytic performance. For a catalyst loading higher than 0.1 g, agglomeration of the catalyst might result in the blockage of the active sites, thereby restricting the interaction of catalyst and substrate molecules. 3.2.5. Reaction Time Study. The reaction time for glycerol transesterification with PC over the Ca-HT catalyst was optimized by monitoring the reaction at regular time intervals, keeping all of the other parameters constant. Variation of glycerol conversion with respect to the reaction time is shown in Figure 12. Glycerol conversion was found to increase with time with Figure 13. Recycle study of the catalyst: Reaction conditions: glycerol/ PC molar ratio, 1; catalyst dose, 0.1 g; temperature, 160 °C; and reaction time, 5 h.

be constant (99%). The Raman study of the spent catalyst (Figure 5c) showed the decrease in intensity of peaks corresponding to CO32− and Mg−O−Mg and Al−O−Al linkage bond stretching vibration. The decline in activity of spent Ca-HT might be from the structural deformation and physical handling losses also. Nevertheless, selectivity to GC was not affected during the recycle experiments. From this, it can be inferred that the structural changes influenced the extent of reaction without affecting the reaction pathway. The reaction mechanism of the transesterification catalyzed by basic sites of the catalyst is proposed in Scheme 4. In the first step, basic sites of the catalyst, particularly the −OH− group present in the cationic interlayer and/or “CaO”, which is highly basic, abstract acidic proton of the primary hydroxyl group of glycerol, producing alkoxy species. This is followed by the attack of nucleophilic alkoxy species on the carbonyl carbon of PC, which becomes activated on the acidic sites (M+2 = Mg/Ca) of the catalyst, resulting into a tetrahedral intermediate. The intermediate, thus formed, further rearranges to give 1,2-PDO. Finally, GC formation takes place, which detaches from the catalyst surface, making it available for the next cycle of reaction.

Figure 12. Effect of the reaction time: Reaction conditions: glycerol/PC molar ratio, 1; temperature, 160 °C; and catalyst dose, 0.1 g.

almost complete selectivity to GC. Increasing the reaction time after 5 h did not affect the glycerol conversion. Hence, 5 h of reaction time was found to be the optimum reaction time. The stability of the Ca-HT catalyst was determined by three successive recycle experiments conducted in the following manner: the catalyst after completion of the first experiment of transesterification was recovered by filtration, washed several times with methanol, dried in an oven at 110 °C for 2 h, followed by calcination at 450 °C for 3 h, and then reused again. A similar procedure was repeated for the next transesterification experiments, the results of which are shown in Figure 13. Glycerol conversion was observed to decrease from 84 to 80% after the third reuse experiment; nevertheless, GC selectivity was found to

4. CONCLUSION A series of metal oxides (CaO, MgO, and ZrO2) alone and HTs without and with Ca and La cations (HT, Ca-HT, and La-HT) were developed and evaluated for their performance in transesterification of glycerol with PC in solvent-free conditions. Among all of the prepared catalysts, the highest TOF of 1.8 h−1 I

DOI: 10.1021/acs.energyfuels.7b00034 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Scheme 4. Mechanism of Solid-Base-Catalyzed Transesterification of Glycerol

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corresponding structures of products and reactants (Figure S1) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chandrashekhar Rode: 0000-0002-2093-2708 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Sharda Kondawar acknowledges the Council of Scientific and Industrial Research, New Delhi, India, for the award of senior research fellowship.

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was achieved with 84% glycerol conversion and complete selectivity to GC over the Ca-HT catalyst within 5 h of reaction time. Although CaO showed the highest activity, the formation of soluble calcium glycerate prevented its recovery and recyclability. This was overcome by its heterogenization with HT (Ca-HT). The use of PC for transesterification also affords the formation of 1,2-PDO, having a wide range of industrial applications. The glycerol conversion was found to be in the following order: CaO > La2O3 > MgO > HT (3), which was in good agreement with the CO2-TPD results. Exceptionally, HT showed higher activity that might be because HT is having a lamellar sheet-like structure and provides easy accessibility of glycerol and PC molecules to interact with active sites of the catalyst to produce GC. Both the XRD and Raman spectroscopy studies evidenced Ca doping into HT. The highest activity of the Ca-HT catalyst was due to the lamellar structure of the HT framework, which provided easy access of the reactant molecule to form the desired product, and the formation of the LDH structure was confirmed by SEM and XRD studies. The presence of OH−, CO32−, and NO3− anions in the interlayer of the HT structure was confirmed by FTIR and Raman spectroscopy, which were responsible for abstraction of acidic proton from glycerol for initiation of the reaction. CO2-TPD results reveal that the Ca-HT catalyst showed the highest basic site density in terms of CO2 desorbed 1.94 mmol g−1 required for glycerol transesterification with PC. The catalytic activity of prepared CaHT is comparable to the homogeneously acting CaO catalyst in the reaction medium; however, the heterogeneous Ca-HT catalyst was successfully recyclable 3 times in the glycerol transesterification reaction.

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NOMENCLATURE GC = glycerol carbonate PC = propylene carbonate HT = hydrotalcite DMF = dimethylformamide LDH= = layered double hydroxide REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00034. Structure and International Union of Pure and Applied Chemistry (IUPAC) names of compounds confirmed by GC−MS (Table S1) and MS spectra for the ions and J

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DOI: 10.1021/acs.energyfuels.7b00034 Energy Fuels XXXX, XXX, XXX−XXX