Tandem Synthesis of Glycidol via Transesterification of Glycerol with ...

Research Article pubs.acs.org/journal/ascecg

Tandem Synthesis of Glycidol via Transesterification of Glycerol with DMC over Ba-Mixed Metal Oxide Catalysts Sharda E. Kondawar, Chetana R. Patil, and Chandrashekhar V. Rode* Chemical Engineering and Process Development Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune 411008, India S Supporting Information *

ABSTRACT: Glycerol carbonate (GC) and glycidol (GD) are commercial products possible from glycerol transformation, which has become a subject of great importance. Among several basic catalysts screened in this work, BaO showed the highest glycerol conversion of 71% with almost complete selectivity to GC. A tandem synthesis of GD with a selectivity as high as 80% with 98% glycerol conversion could be achieved with mixed oxides of Ba and lanthanides (La and Ce) prepared by the coprecipitation method. Although BaO alone showed the highest basicity as measured by CO2 TPD, tuning of basicity by incorporation of CeO2 resulted in the formation of GD. Incorporation of Ba into the ceria matrix induced oxygen vacancies in the cerium oxide material. The presence of u″/v″ doublets at 888.7 and 903.2 eV, respectively, in XPS of the Ba−Ce sample also confirmed the oxygen vacancies in the lattice. In this tandem approach to GD, the subsequent decarboxylation of initially formed GC was due to the presence of a CeO2 lattice with defects, which is known to be the best for CO2 adsorption. Increase in both catalyst loading and temperature showed a dramatic enhancement in GD selectivity. A plausible reaction pathway for the transesterification of glycerol with DMC to give GC followed by its decarboxylation to GD is also proposed based on the structural characterization and activity studies. KEYWORDS: Glycerol, Transesterification, DMC, Glycerol carbonate, Glycidol, Basicity

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INTRODUCTION One of the renewable fuels used in large quantities worldwide is biodiesel which also augments the efforts in minimizing the green house gas emission arising from burning of fossil fuels. The production of biodiesel has increased exponentially in recent years in the United States, Europe, Asia, and Brazil. The process of biodiesel production involves the transesterification of triglycerides of both plant and animal origin with methanol. During transesterification, the formation of glycerol as a byproduct in large quantities (10% of biodiesel) offers a great opportunity to derive several valuable products from this highly reactive triol available at a cheap price. Prior to the growth in biodiesel production, the glycerol price tag was $ 1.5/kg which dramatically decreased to $ 0.66/kg and then to $ 0.11/kg, after 2007.1 Since a very small fraction of glycerol goes directly in applications related to health care products, it becomes highly essential to explore its transformation to other value-added chemicals having commercial applications. Several such products are possible which include 1,2 and 1,3-diols, ethylene glycol, acetol, dihydroxy acetone, hydrogen, ethanol, monodi, triglycerol ethers, etc.2,3 On the other hand, glycerol conversion to glycerol carbonate (GC) and glycidol (GD) has emerged in very recent years, despite wide range applications owing to their unique properties of low volatility, biodegradability, and low toxicity. GC has been already incorporated in the portfolio of companies such as Huntsman and Ube Industries Ltd. to be © 2016 American Chemical Society

used as a polar solvent for polymer synthesis and also in personal care products.4 GC is one of the components in membranes serving for gas separation applications. More importantly, by possessing both carbonyl as well as hydroxyl functionalities within one molecule, it can serve as a monomer for the synthesis of variety of polyesters, polycarbonates, and polyurethanes.5−7 Similarly GD obtained from GC is a raw material for the manufacture of polyglycerols, glycidyl ethers, and pharmaceuticals, as well as in perfumes and cosmetics, detergents, paints, de-emulsifiers, and dye-leveling agents.8 It can also serve as a plastic modifier, surfactant, and fire retardant.9−11 Glycidol can be a substituent for epichlorohydrin as the homopolymerization of glycidol has been used to introduce functional groups for “click” coupling reactions, biodegradable cleavage sites, and redox-responsive ferrocene moieties into hyperbranched polyether structures.12 In addition, use of glycidol as a monomer can eliminate HCl generation associated with epichlorohydrin. The market share of GD as a monomer has a great potential growth once a direct and cheaper route for its synthesis is developed. Conventional synthesis of GC via glycerol carbonylation either with phosgene or carbon monoxide13 suffers from a Received: October 18, 2016 Revised: November 22, 2016 Published: December 13, 2016 1763

DOI: 10.1021/acssuschemeng.6b02520 ACS Sustainable Chem. Eng. 2017, 5, 1763−1774

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration of Synthesis of Ba-Mixed Oxide Catalysts by Coprecipitation Method

serious drawback of high risk of handling and environmental pollution as both phosgene and CO are extremely toxic in nature.14,15 Dibenedetto et al. reported CeO2/ Al2O3 and CeO2/ Nb2O5 as supported catalysts for direct carboxylation of glycerol to yield >99% glycerol carbonate.16 At present, commercial production of GD is through epoxidation of allyl alcohol and/or reaction of glycerol with HCl to give 3-chloro1,2-propane diol followed by ring closure in the presence of a strong base.17−19 Both the routes have major disadvantages of multistep synthesis eventually leading to higher product costs and generation of stoichiometric inorganic wastes, thus, far from sustainability. A two-step approach for GD involves first GC formation via catalytic transesterification or by urea carbonylation followed by GC decarboxylation over another catalyst.20 However, decarboxylation is a separate step in this approach and usually carried out using a homogeneous catalyst under relatively harsh conditions that can lead to polymerization of the reactive glycidol.21 Direct GD formation was observed in GC synthesis via a glycerol transesterification route over basic catalysts but as a byproduct in very small amounts.22 The GD selectivity could be enhanced to 51% with 95% glycerol conversion by employing a basic ionic liquid catalyst. However, the cost of the ionic liquid, its tedious preparation procedure, inability to recover IL, and hence poor recyclability make it almost impractical to use from a large scale synthesis perspective.23

As far as GC synthesis via transesterification is considered, several types of heterogeneous basic catalysts have been reported. For example, Simanjuntak et al. reported CaO as a catalyst for transesterification of glycerol with DMC, which showed a 94% conversion and almost complete selectivity toward GC, but the catalyst was rapidly deactivated due to the formation of calcium glycerate complexes.24 Some other catalysts include Mg−Al hydrotalcite, KF/hydroxyapetite, NaOH/γAl2O3, K2CO3/MgO, KF/Al2O3, and ionic liquids for the transesterification route for the selective synthesis of GC.23,25−28 Mixed metal oxides like Mg/Al/Zr, Mg/Zr/Sr, and ZnO/La2O3 were also employed for the transesterification of glycerol,29−31 but all these catalytic systems have some other drawbacks like poor reusability, use of high DMC:glycerol ratio, and/or longer reaction times. Hence, the present study was undertaken to develop a highly efficient, cheap, and CO2-tolerant heterogeneous catalyst for the direct synthesis of GD under mild reaction conditions. To the best of our knowledge, for the first time, we are reporting mixed oxides of Ba and lanthanides as efficient catalysts for the one-pot synthesis of glycidol starting from glycerol and DMC. Initially, we screened a series of mixed metal oxides involving a combination of alkaline earth metals, like Ba and transition metal oxides, to get selective formation of glycerol carbonate, and later, by replacing transition metals with more basic lanthanides and altering the reaction conditions, we could maneuver the selectivity toward glycidol. 1764


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analyzed by gas chromatography to monitor the progress of the reaction. The gas chromatograph (Shimatzu 2025) was equipped with an HP-FFAP column (30 m length × 0.53 mm ID ∼ 1 μm film thickness) and a flame ionization detector. Injection and detector temperatures were kept at 300 °C. The column temperature started at 80 °C, and then it was increased at a rate of 20 °C min−1 to the temperature of 200 °C, holding at this temperature for 30 min. Activity of the catalyst was based on conversion of the limiting reagent under standard reaction conditions. (B) One-pot synthesis of glycidol was carried out by transesterification of glycerol with DMC, followed by decarboxylation of GC in a 50 mL Parr autoclave. In a typical experiment, 2 g of glycerol (21.73 mmol) and 5.87 g of DMC (65.21 mmol) charged to the reactor with 5% weight loading of catalyst with respect to glycerol and 10 mL of DMF as a solvent were mixed together under 1000 rpm stirring at a 120 °C temperature for 90 min. After the reaction, the mixture was cooled, and then the catalyst was separated by filtration, washed with DMF, dried at 110 °C, and calcined at 650 °C for 3 h. The reaction mixture was analyzed by gas chromatography to monitor the progress of the reaction as mentioned above. Identification of products was done by HPLC and 13C NMR spectroscopy (SI, Figures S1c and S3c). HPLC analysis was carried out by using HPLC (Agilent 1260) equipped with an RI detector and Hi-Plex H column at 60 °C. A solution of 0.005 M aqueous sulfuric acid was used as the mobile phase with the flow rate of 0.7 mL min−1. The glycerol conversion and product selectivity were calculated by using the following equations:

EXPERIMENTAL SECTION

Materials. Glycerol (98% pure) was purchased from Merck, India. Dimethyl carbonate, cerium oxide, and lanthanum oxide were purchased from Aldrich Chemicals, Bangalore, India. Lanthanum nitrate, barium nitrate, zinc nitrate, cerium nitrate, neodymium nitrate, zirconium nitrate, aluminum nitrate, sodium hydroxide, and potassium hydroxide were purchased from Thomas Baker, India. Dimethylformamide was purchased from TCI Chemicals, India. All chemicals were used as received without further purification. Catalysts Preparation. All of the mixed metal oxide catalysts were prepared by the coprecipitation method (Scheme 1). Initially, 100 mmol of Ba (NO3)2 (5.22 g) in 200 mL of DI water (deionized water) and 100 mmol of La (NO3)3·6H2O (8.66 g) in 200 mL or 100 mmol of Ce (NO3)3·6H2O (8.66 g) in 200 mL or 100 mmol of Nd (NO3)3· 6H2O (8.76 g) in 200 mL of DI water were dissolved separately (for 1:1 Ba−La, Ba−Ce, and Ba−Nd catalysts). Also, 50 mL of 1 M NaOH and 50 mL of 0.26 M K2CO3 solutions were prepared separately. Initially, 2 mL of 10% aqueous solution of polyvinyl pyrolidone (M.W. 40,000) was added slowly to the sodium hydroxide solution. To this mixture, Ba (NO3)2 and La (NO3)3 or Ce (NO3)3 or Nd (NO3)3 solutions were added dropwise by means of an addition funnel. The solution was mixed perfectly by a magnetic stirring apparatus (1000 rpm) at room temperature for 3 h, and pH was maintained in the range of 9−10 by adding alkali. Then obtained precipitate was filtered and washed with DI water. It was dried at 110 °C for 24 h and then calcined at 600 °C for 3 h. Prepared catalysts were named as Ba−La (1−1), Ba−La (1−3), Ba−Ce (1−1), Ba−Ce (1−3), and Ba−Nd (1−1), respectively. Catalyst Characterization. X-ray diffraction (XRD) patterns were recorded on a P Analytical PXRD system (Model X-Pert PRO-1712), using Ni filtered Cu Kα radiation (λ= 0.154 nm) as an X-ray source (current intensity, 30 mA; voltage, 40 kV) and an X-accelerator detector. The samples were scanned in a 2θ range of 10°−90° with a step size (2θ) of 0.008° and scanning rate of 5°/min. The species present in the catalysts were identified by their characteristic 2θ values of the relevant crystalline phases. The software program X-Pert High Score Plus was employed to subtract the contribution of the Cu Kα 2 line prior to data analysis. The BET surface area and CO2 TPD experiments of the catalysts were determined by the N2 adsorptiondesorption technique using a Micromeritics-2720 (Chemisoft TPx) volumetric instrument. In order to evaluate basicity of the catalysts, carbon dioxide TPD measurements were carried out by (i) pretreating the samples from room temperature to 200 °C under helium flow rate of 25 mL min−1, (ii) adsorption of carbon dioxide at 40 °C, and (iii) desorption of carbon dioxide with a heating rate of 10 °C min−1 starting from the adsorption temperature to 700 °C. X-ray photoelectron spectroscopy (XPS) data were collected on a VG Scientific ESCA-3000 spectrometer using nonmonochromatized Mg Kα radiation (1253.6 eV) at a pressure of about 1× 10−9 Torr (pass energy of 50 eV, electron take off angle 55), and overall resolution, ∼0.7 eV, was determined from the full width at half-maximum of the 4f 7/2 core level of the gold surface. The error in the binding energy values were within 0.1 eV. The binding energy values were chargecorrected to the C 1s signal (285.0 eV).The Raman spectra of the sample were recorded on a Horiba JY Lab RAM HR800 micro-Raman spectrometer with 17 mW, 632.8 nm laser excitation. Transmission electron microscopy (TEM) analysis was performed on a Jeol Model JEM 1200 electron microscope operated at an accelerating voltage of 120 kV. A small amount of specimen was prepared by ultrasonically suspending the powder sample in ethanol, and drops of the suspension were deposited on a carbon-coated copper grid dried at room temperature before analysis. Catalytic Reaction. (A) The transesterification of glycerol was carried out in a 50 mL round-bottomed flask equipped with a condenser under vigorous stirring. In a typical run, 2 g of glycerol (21.73 mmol) and 5.87 g of DMC (65.21 mmol) were charged to round-bottomed flask along with 100 mg of catalyst with 5 mL of DMF as a solvent. The reaction was carried out at 70 °C for a 3 h reaction time. The reaction mixture was cooled, and the sample was

moles of glycerol reacted × 100 initial moles of glycerol

(1)

moles of one product × 100 ∑ moles of all the products

(2)

Conversion (%) =

Selectivity (%) =

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RESULTS AND DISCUSSION Catalyst Characterization. After screening of several catalysts involving the alkaline earth metal Ba with other transition metal oxides for transesterification of glycerol with DMC, it was found that replacing transition metals with more basic lanthanides gave excellent selectivity toward glycidol. Hence, only these catalysts have been characterized in detail, and the results are discussed below. Table 1 shows the BET surface areas of different Ba-based catalysts. The Ba−Nd (1−1) catalyst showed the highest

Table 1. Textural Properties of Prepared Catalysts catalyst

BET surface area (m2 g−1)

average pore diameter (Å)

pore volume (cm3 g−1)

Ba−La (1−1) Ba−Ce (1−1) Ba−Nd (1−1)

47 61 128

6.37 6.02 5.625

0.0123 0.0166 0.0329

surface area of 128 m2 g−1, while barium mixed with cerium and lanthanum catalysts showed much lower surface area values of 61 and 47 m2 g−1, respectively. The pore volume was found to increase progressively as lanthanides were changed from La to Nd in the barium catalysts (0.012−0.032 cm3 g−1), while the reverse trend was observed in the case of pore diameter. The wide-angle X-ray diffraction patterns of the prepared catalysts are shown in Figure 1. In the cases of Ba−La (1−1) and Ba−La (1−3) catalysts, all the reflections observed were corresponding to the La2O3 phase. Thus, the peaks at 2θ = 29.9° (0 1 1), 39.5° (0 1 2), 55.3° (1 1 2), 52.0° (1 0 3), and 66.8° (0 1 4) could be attributed to the hexagonal lanthanum oxide (PCPDF no. #831344). In addition, the peaks observed 1765


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Figure 1. Wide angle XRD patterns of (a) Ba−La (1−1), (b) Ba−La (1−3), (c) Ba−Ce (1−1), and (d) Ba−Ce (1−3).

at 2θ = 29.1° (1 2 1), 42.4° (4 1 1), 44.4° (4 2 1), 50.4° (1 6 1), and 75.5° (8 0 1) evidenced the formation of a new orthorhombic BaLa2O4 phase (PCPDF no. #421500). The sharp peaks observed at 2θ = 28.6° (1 1 0) and 50.4° (2 0 1) confirmed the presence of BaO phase (PCPDF no. #850418).32 In the cases of Ba−Ce (1−1) and Ba−Ce (1−3) catalysts, sharp peaks present at 2θ = 28.6° (1 1 1), 33.1° (2 0 0), 47.5° (2 2 0), 59.2° (2 2 2), and 79.2° (4 2 0) were attributed to the cubic phase of CeO2 (PCPDF no. #750076). Interestingly, the presence of the orthorhombic BaCeO3 phase was also confirmed by strong XRD reflections at 2θ = 28.6° (1 1 2), 42.6° (2 2 1), 56.1° (1 3 3), and 68.3° (4 2 1) (PCPDF no. #701429).33 In both the ceria-containing Ba catalysts, the tetragonal BaO phase was present, which gave reflections at 2θ = 28.6° (1 1 0), 50.4° (2 0 1), 59.2° (2 2 0), 76.7° (2 1 2), and 88.5° (4 0 0) (PCPDF no. #850418). The N2 adsorption studies of the Ba−La catalyst showed a type I adsorption isotherm which was confirmed by the continuous uptake of N2 with a characteristic hysteresis (Figure 2A) loop extending from P/P0 = 0.2−1, which is a typical feature of microporous material (pore size Ba−Ce (1−1) > Ba−Ce (1−3) > Ba−La (1−3) > Ba−La (1−1). Raman spectra of ceria and Ba−Ce catalysts are displayed in Figures 4A and B, respectively. Figure 4A showing the stretching mode (f2g mode) of vibration, located at 466.5 cm−1, confirmed the presence of the cubic fluorite structure of ceria.34 While for the Ba−Ce (1−1) catalyst, a blue shift was observed with an f2g symmetric stretching mode at 462.4 cm−1 because of the modification of the ceria lattice with Ba2+, which also promotes the formation of oxygen vacancies (Figure 4B).35 In addition, one broad peak observed at 600 cm−1 could be ascribed to O2− vacancies generated in the CeO2 lattice arising due to the replacement of Ce (IV) by Ce(III). From the Raman studies, it can be concluded that incorporation of Ba into the ceria matrix did not distort the crystal structure of the cerium oxide but induced oxygen vacancies in the cerium oxide material. On the basis of the above characterization results, a schematic of the structural representation of the prepared barium cerium-mixed metal oxide catalyst is shown in Scheme 2. 1766


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2p orbital to the empty 4f orbital of the Ce (IV) species, whereas the u′/v′ doublet is observed due to photoemission from Ce (III) cations. The highest binding energy peaks are located at 916.2 and 898 eV with 18.2 eV spin orbital splitting, which is in good agreement with the literature.40 The presence of the u″/v″ doublet confirms that the sample is having some oxygen vacancies in the lattice.41 Hence, it confirmed the presence of tetravalent and trivalent Ce species, with a Ce4+/ Ce3+ ratio of 0.83 in the Ba−Ce (1−1) catalyst. An XPS of O 1s of the Ba−Ce (1−1) catalyst (Figure 6C) displays a dominant peak at 529.8 eV, which could be attributed to the oxygen present in the CeO2, whereas a small peak at a higher binding energy of 531.4 eV could be due to Ce2O3, and another peak at 532.6 eV might be due to the adsorbed water molecule.42 The O 1s core level peak is reported at 529 eV, but for the Ba−Ce (1−1) catalyst, the O 1s peak was shifted to the higher binding energy of 529.8 eV due to the presence of the covalent Ce−O−Ba species.40 From the XPS spectra (Figures 5 and 6) of all the lanthanides containing the Ba catalyst, it could be concluded that except for the ceria-containing Ba sample, all other samples of Ba having lanthanum and neodymium oxides possessed a +3 oxidation state, while Ce was present in both +4 as well as +3 oxidation states in the Ba−Ce sample. The HR-TEM image (Figure 7A) showed the mixed cubic, oval, and spherical morphologies of the microporous Ba−Ce (1−1) catalyst. The fringe pattern (Figure 7B) of the Ba−Ce (1−1) catalyst exhibited the most exposed dominant plane as (2 0 0) with a lattice space of 0.27 nm, which was in good agreement with the values obtained from the XRD study. The SAED pattern of the Ba−Ce (1−1) catalyst in Figure 7C clearly shows the crystalline nature of particles having a diffraction plane of (1 3 3) corresponding to the BaCeO3 phase as confirmed by XRD (PCPDF no. #701429). Catalytic Performance. Transesterification of Glycerol with DMC to GC. Transesterification of glycerol with DMC primarily gave glycerol carbonate (GC) for which several mono- and mixed metal oxides were screened, and the results are presented in Table 3. This reaction did not proceed without any catalyst (Table 3, entry 1), indicating that the basic catalyst is necessary for the activation of the glycerol molecule through deprotonation of its acidic −OH group followed by its attack on carbonyl functionality of DMC (Scheme 3a). Among several basic catalysts screened in this work, BaO showed the highest glycerol conversion of 71% with almost complete selectivity to GC, while the lowest conversion of 40% was observed in the case of ZrO2 (Table 3, entries 2 and 6, respectively). All the catalysts screened here showed almost complete selectivity (99%) toward GC except the Ba-mixed oxide catalysts. ZnO also showed more or less similar activity as shown by BaO, while the activity decreased to 50% for the MgO catalyst. ZrO2 and Al2O3 exhibited much lower activities in terms of glycerol conversion (∼42%), as both of these having acidic character-

Figure 3. CO2 TPD profiles of the prepared catalysts.

Figure 5 shows X-ray photoelectron spectra of the prepared catalysts in which a doublet for La2O3 at binding energies of 842.7 and 846 eV was observed clearly for the Ba−La (1−1) catalyst (Figure 5A).36 The 3d spectra of neodymium oxide in the Ba−Nd (1−1) catalyst showed a peak at 988 eV with a shoulder peak at a higher binding energy of 992 eV (Figure 5B),37 indicating that both La and Nd were in a +3 oxidation state in Ba−La (1−1) and Ba−Nd (1−1) catalysts, respectively. Figure 6A represents the XPS spectrum for Ba 3d of the Ba−Ce (1−1) catalyst which showed two dominant peaks for the 3d5/2 and 3d3/2 spin orbital state at 779.2 and 795 eV, respectively, assigned to the Ba2+ species.38 XPS peak fitting studies were performed to quantify the cerium ion formation and to determine the peak positions of the various components. Ce 3d XPS (Figure 6B) showed two multiplets (u and v) in the range of 930−885 eV for the Ba−Ce (1−1) catalyst, which corresponded to the spin orbital splitting 3d3/2 and 3d5/2, respectively.39 The peaks at 882.2, 885.4, 888.7, and 898 eV are denoted as v, v′, v″, and v‴, whereas peaks at 900.8, 903.2, 907, and 916.2 eV are denoted as u, u′, u″, and u‴ respectively. The highest intense peaks, viz. v‴ and u‴, are due to the primary photoemission from the Ce (IV) O2 final state. The u/v and u″/v″ doublets are observed due to the shakedown feature arising from transfer of one or two electrons from the filled O Table 2. CO2 TPD Results of Prepared Catalysts

distribution of basic sites (mmol g−1) catalyst

40−200 °C

200−400 °C

400−600 °C

total CO2 desorbed mmol g−1

BaO Ba−Ce (1−1) Ba−Ce (1−3) Ba−La (1−3) Ba−La (1−1)

0.00819 0.0573 0.0566 0.0154 0.0105

0.8512 0.0585 0.1350 0.1709 0.1681

0.1318 0.2565 0.0724 0.00029 0.00022

0.9911 0.3723 0.2640 0.1865 0.1788

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Figure 4. Raman spectra for (A) CeO2 and (B) Ba−Ce (1−1) catalysts.

Scheme 2. Schematic of Structural Representation of Barium Cerium-Mixed Metal Oxide Catalyst

istics43 showed lower capability of deprotonation for hydroxyl of glycerol. Considering the similar activities of BaO and ZnO, their combination was expected to achieve higher activity; however, the glycerol conversion obtained (55% conversion) was much lower than that of their individual activities with 93% selectivity toward GC and 7% toward GD (Table 3, entry 8). However, the combination of BaO and MgO resulted in achieving 67% conversion, which was much higher than individual MgO but somewhat lower than BaO, suggesting

the individual BaO would be a better choice (Table 3, entry 7). Interestingly, the MgO−ZrO2, glycerol conversion obtained was 50%, somewhat higher than that obtained for only ZrO2 (Table 3, entry 10). BaO with lanthanides (La and Ce oxides) prepared by the coprecipitation method exhibited higher selectivity to GD 25% and 36%, respectively, with glycerol conversion in the range of 45%−58% under the same reaction conditions (Table 3, entries 11 and 12). 1768


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Figure 5. 3d XPS spectrum of (A) La 3d and (B) Nd 3d.

One-Pot Tandem Synthesis of Glycidol from Glycerol and DMC. Although both glycerol transesterification to GC and its further decarboxylation to glycidol are base-catalyzed reactions, surprisingly even the highly basic catalyst like BaO, which showed excellent conversion for the transesterification reaction, did not give GD formation at 70 °C (Table 3). Similarly, rare earth metals also known to possess basic characteristics arising due to the low charge distributed over large cations alone do not produce glycidol.31 Hence, we thought it appropriate to harness the synergism of some of the rare earth metal oxides with BaO for direct conversion of glycerol to glycidol. For this purpose, several mixed metal oxides, such as Ba−La, Ba−Ce, and Ba−Nd with 1:1 and 1:3 varying ratios, were prepared and screened for the one-pot synthesis of glycidol from glycerol and DMC. As discussed above, the preliminary results over Ba−La (1−1) and Ba−Ce (1−1) at 70 °C were very encouraging, giving significant selectivity of 25% and 36% to GD, respectively. In order to enhance both GC conversion and GD selectivity, first the reaction temperature was increased from 70 to 120 °C, which resulted in achieving almost complete glycerol conversion for both Ba−Ce (1−1) and Ba−La (1−1) catalysts (Table 4, entries 1 and 2). As expected, a significant increase in GD selectivity up to 80%, particularly for Ba−Ce (1−1), was observed (Table 4, entry 1). Ba−Nd (1−1) showed a similar conversion of 98% with GD selectivity higher than that for Ba− La (1−1) but still lower that of the Ba−Ce (1−1) catalyst (Table 4, entry 3). The highest activity of the Ba−Ce (1−1) catalyst at higher temperature could be explained based on its

Figure 6. XPS spectrum of A) Ba 3d, B) Ce 3d and C) O 1s.

high strength of medium basic sites in terms of 0.2565 mmol g−1 CO2 desorbed in the temperature range of 400−600 °C (Table 2) as compared to other catalysts studied in this work. It was then thought appropriate to study the effect of higher concentration of corresponding lanthanides with Ba. However, higher concentrations of Ce in Ba−Ce resulted in considerable lowering of the GD selectivity to 54% with 43% GC (Table 4, entry 4). Although, the Ba−Ce (1−3) catalyst showed the presence of the BaCeO3 phase, higher GD selectivity was not observed. This might be because of the lower basicity of Ba−Ce (1−3) as compared to that of Ba−Ce (1−1) as revealed from 1769


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Figure 7. HRTEM images of (A) Ba−Ce (1−1) catalyst (with 10 nm resolution), (B) Ba−Ce (1−1) catalyst (with 5 nm resolution), and (C) SAED pattern of Ba−Ce (1−1) catalyst.

parameters were also studied for this catalyst. The effect of catalyst loading on conversion and selectivity of glycerol is shown in Figure 8. As the catalyst loading increased from 0.025 to 0.1 g, glycerol conversion also increased from 63% to 98%, resulting in a decrease in GC selectivity from 53% to 18% with a simultaneous increase in GD selectivity from 39 to 80%. The increase in glycerol conversion was mainly because of the higher availability of active catalytic sites with increasing the catalyst concentration. It also indicates that the decarboxylation step was significantly accelerated with more availability of basic sites. Figure 9 shows the influence of temperature on glycerol conversion and product selectivities in glycerol transesterification reaction. Glycerol conversion increased by 7% with an increase of 20 °C temperature from 80 to 100 °C, with higher selectivity of 56% and 42% toward GC and GD, respectively, at 80 °C. The selectivity pattern was found to be reversed at 100 °C (42% and 56% toward GC and GD, respectively). With a further increase in temperature to 120 °C, a dramatic increase in both glycerol conversion (98%) and GD selectivity (80%) was observed. Hence, 120 °C was considered as the optimum reaction temperature for the one-pot tandem synthesis of GD by the glycerol−DMC transesterification reaction. In order to study the stability of this heterogeneous catalyst, its recycling experiments were carried out in the following way: after the first run with a fresh Ba−Ce (1−1) catalyst, it was filtered out and washed several times with DMF, dried at 100 °C for 2 h, and then reused after calcination at 600 °C for 3h (Figure 10). The procedure was followed for three subsequent experiments, and the results are shown in Figure 10. The conversion of glycerol was found to decrease marginally from 98% to 96% after the third recycle, which was due to handling losses of the catalyst. However, selectivity to GC and GD remained almost constant (18% and 80%, respectively). In order to confirm further the stability of the catalysts, a hot filtration test was also carried out, and the results are shown in (SI, Figure S4). Both these observations proved that the catalyst was completely heterogeneous under the reaction conditions. On the basis of the characterization and activity results discussed above, a plausible mechanistic pathway forming GC is depicted in Scheme 3a. It is proposed that the reaction is initiated by abstracting an acidic proton of the primary hydroxyl group of glycerol by the Lewis basic site BaCeO3 phase (O δ−) of the Ba−Ce catalyst with a simultaneous activation of the carbonyl carbon of DMC by the Lewis acidic site (Ce4+) of the catalyst (I). It is then followed by the attack of oxygen of the primary hydroxyl group on the carbonyl carbon of DMC with

Table 3. Catalyst Screening for Transesterification of Glycerola selectivity (%) catalyst

conv. (%)

GC

GD

− BaO MgO ZnO Al2O3 ZrO2 Ba−Mg (1−1) Ba−Zn (1−1) Mg−Zn (1−1) Mg−Zr (1−1) Ba−Ce (1−1) Ba−La (1−1)

− 71 50 67 42 40 67 55 50 50 58 45

− 99 99 99 99 99 94 93 99 99 63 75

− 1 1 1 1 1 6 7 1 1 36 25

a

Reaction conditions: catalyst, 100 mg; glycerol, 2 g; DMC, 5.8 g, (1:3); solvent, DMF, 5 mL; temperature, 70 °C; 3 h.

CO2 TPD (Table 2). Among lanthanides, ceria possesses slight acidity as well; hence, the basicity of Ba−Ce decreased with higher Ce concentration.44 On the other hand, by increasing the La concentration, glycerol conversion decreased to 90% with higher selectivity toward GD (Table 4, entry 5), which can be explained by the marginal increase in the basicity with an increase in La concentration (Table 2, entry 4) unlike Ba−Ce catalysts. The higher activity of Ba−Ce (1−1) for catalyzing the transesterification and decarboxylation reaction could be because of the formation of BaCeO3 as revealed by XRD. This was also in accordance with the fact that glycerol to GD synthesis requires basic sites and among all the prepared catalysts, Ba−Ce (1−1) showed the highest basicity in terms of 0.3723 mmol g−1 CO2 desorbed.30 As observed from HR-TEM, the most exposed plane of CeO2 was (2 0 0), which confirmed the presence of Ce4+ species. The Raman study also showed the presence of oxygen vacancies in the ceria lattice, which play an important role in the decarboxylation step to adsorb CO2 liberated in the decarboxylation step.35 Some control experiments with bare CeO2 and BaO revealed that ceria being less basic than barium oxide gave 70% glycerol conversion with only 10% selectivity to GD, while 98% glycerol conversion with much higher (29%) selectivity was achieved with BaO (Table 4, entries 6 and 7, respectively). However, bare La2O3 gave only 22% selectively GD and 77% selectively GC with 70% conversion of glycerol (Table 4, entry 8). Since Ba−Ce (1−1) catalyst showed the highest GD selectivity (80%), the effect of two important reaction 1770


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Scheme 3. Plausible Mechanism for (a) Transesterification of Glycerol with DMC Followed by (b) Decarboxylation of GC to Glycidol

decarboxylation of GC is shown in Scheme 3(b), according to which abstraction of the proton of the primary hydroxyl by the Lewis basic site of the catalyst takes place (IV). In the next step, the C−O bond cleavage at the 3−4 and 1−2 positions of the ring takes place because of the O − attack on the C4 carbon atom owing to the formation of three-member cyclic GD (V). Strong basic sites present in the catalyst accelerate the

an elimination of the methoxy group in the form of methanol (II). Subsequently, the oxygen of the secondary hydroxyl of glycerol attacks the carbonyl carbon of DMC, already activated by the Lewis acidic sites (Ce4+) of the catalyst to form GC with a loss of another molecule of methanol. In the final step, cyclization occurs leading to the formation of GC and regeneration of the catalyst (III). GD formation via 1771


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ACS Sustainable Chemistry & Engineering Table 4. Catalyst Screening for Tandem Approach To Produce Glycidola selectivity (%) catalyst

conv. (%)

GC

GD

Other

Ba:Ce (1:1) Ba:La (1:1) Ba:Nd (1:1) Ba:La (1:3) Ba:Ce (1:3) BaO CeO2 La2O3

98 97 98 90 96 98 70 70

18 45 34 39 43 70 89 77

80 52 63 59 54 29 10 22

2 3 3 2 3 1 1 1

a

Reaction conditions: catalyst, 100 mg; glycerol, 2 g; DMC, 5.8 g (1:3); solvent, DMF, 10 mL; temperature, 120 °C; 90 min.

Figure 10. Recycle study of transesterification of glycerol. Reaction conditions: catalyst, 100 mg; glycerol, 2 g; DMC, 5.8 g (1:3); solvent, DMF, 10 mL; temperature, 120 °C; 90 min.

oxygen active species along with carbon monoxide gas (Scheme 3b) as per the Kroger Vink notation.47 Almost complete conversion of glycerol with excellent selectivity to GD (80%) was obtained with Ba−Ce (1−1) catalyst.

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CONCLUSIONS In this work, several single metal as well as mixed metal oxide catalysts of Ba and lanthanides (La, Ce, and Nd) were prepared by the coprecipitation method and evaluated for their activity for one-pot synthesis of glycidol by transesterification of glycerol with DMC. Among the single metal oxides, BaO with the highest basicity as measured by CO2 TPD showed maximum conversion of glycerol (71%) with selective formation of glycerol carbonate. Interestingly, BaO in combination with lanthanides, particularly Ba−Ce (1−1), was found to be the best catalyst for glycerol transesterification to GC followed by its decarboxylation to give glycidol (80%). XRD analysis revealed the formation of the BaCeO3 phase along with the BaO species in this mixed oxide catalyst with a maximum basicity of 0.3723 mmol g−1. The Raman study showed the presence of a cubic fluorite structure of CeO2 with oxygen vacancies induced due to incorporation of Ba2+ inside the lattice. In the Ba−Ce (1−1) sample, Ce 3d XPS showed two multiplets (u and v) in the range of 930−885 eV, corresponding to the spin orbital splitting 3d3/2 and 3d5/2, respectively. The highest intense peaks, viz., v‴ and u‴, at 916.2 and 898 eV, respectively, were due to the primary photoemission from the Ce (IV) O2 final state. The u/v and u″/v″ doublets were observed due to a shakedown feature arising from transfer of one or two electrons from the filled O 2p orbital to the empty 4f orbital of the Ce (IV) species, whereas the u′/v′ doublets at 885.4 and 903.2 eV were due to photoemission from Ce (III) cations. The presence of u″/v″ doublets corroborated the finding of the Raman study that the sample was having oxygen vacancies in the lattice. The strong basicity alone with the presence of oxygen vacancies in the ceria lattice played an important role in the decarboxylation step to adsorb CO2 liberated in the decarboxylation step. The suggested reaction pathway involved an abstraction of the acidic proton of primary −OH of glycerol by the Lewis basic site BaCeO3 phase (O δ−) of the Ba−Ce followed by the attack

Figure 8. Effect of catalyst loading for transesterification of glycerol. Reaction conditions: glycerol, 2 g; DMC, 5.8 g (1:3); solvent, DMF, 10 mL; temperature, 120 °C; 90 min.

Figure 9. Effect of temperature on catalytic activity. Reaction conditions: catalyst, 100 mg; glycerol, 2 g; DMC, 5.8 g (1:3); solvent, DMF, 10 mL; 3h.

decarboxylation step. The driving force for the decarboxylation is the presence of the CeO2 lattice with defects which is known to be the best for CO2 adsorption45 which leads to forming glycidol despite the higher strain of the three-member ring as compared to the five-member ring of GC.46 Istadi et al. reported that a highly basic catalyst surface adsorbs carbon dioxide which is then activated on oxygen vacancies to form 1772


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of oxygen of the primary hydroxyl group on the carbonyl carbon of DMC to form GC with a loss of another molecule of methanol. GD formation via decarboxylation of GC can take place by abstraction of the proton of the primary hydroxyl by the Lewis basic site of the catalyst with a subsequent cleavage of the C−O bond at the 3−4 and 1−2 positions of the ring resulting in the formation of three-member cyclic GD.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02520. GC, HPLC, 13C NMR spectra, hot filtration test, and effect of time on catalytic activity. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 (020) 25902349. ORCID

Chandrashekhar V. Rode: 0000-0002-2093-2708 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.E.K. thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, for the award of a senior research fellowship, and C.R.P. thanks DST New Delhi for the award of a women’s fellowship.

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ABBREVIATIONS DMC, dimethyl carbonate; DMF, dimethylformamide; GC, glycerol carbonate; GD, glycidol

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