Glycerol Carbonate Synthesis by Hierarchically ... - ACS Publications

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Glycerol Carbonate Synthesis by Hierarchically Structured Catalysts: Catalytic Activity and Characterization Praveen Kumar,*,†,‡ Patrick With,‡ Vimal Chandra Srivastava,† Roger Glas̈ er,‡ and Indra Mani Mishra†,§ †

Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India Institute of Chemical Technology, Universität Leipzig, Linnéstraße 3, 04103 Leipzig, Germany § Department of Chemical Engineering, Indian School of Mines, Dhanbad 826004, Jharkand, India ‡

S Supporting Information *

ABSTRACT: The surge in biodiesel production in recent years has resulted in enhanced research interest in the conversion of glycerol to other valuable chemicals such as glycerol carbonate (GLC). In the present study, the catalytic transesterification of glycerol with dimethyl carbonate (DMC) for the production of GLC was studied with calcium−lanthanum mixed-oxide catalysts at different Ca/La molar ratios. These catalysts were synthesized using an exo- and endotemplating method. The physicochemical characteristics of the catalysts were determined using powder X-ray diffraction (XRD), N2 sorption, scanning electron microscopy (SEM), and temperature-programmed desorption (TPD) of CO2 and NH3. The transesterification of glycerol was found to be highly dependent on the basicity of the catalysts. The catalyst with a Ca/La molar ratio of 3 (3CaLa) showed the highest glycerol conversion and GLC yield. Optimization of the reaction parameters and reusability of the catalyst were studied with the 3CaLa catalyst in terms of glycerol conversion, GLC yield, and turnover frequency (TOF). At the optimum operating conditions, namely, a reaction time of 90 min, a DMC/glycerol molar ratio of 5, a catalyst mass of 0.217 g, and a temperature of 90 °C, the glycerol conversion and GLC yield were found to be 94% and 74%, respectively, with a reaction rate of ∼0.14 mol L−1 h−1 (with respect to glycerol).

1. INTRODUCTION The extensive production of biodiesel to satisfy worldwide demand has resulted in the production of significant amounts of glycerol as a byproduct. This accumulated glycerol is available in the market at low prices.1,2 In recent years, research emphasis has focused on the utilization and conversion of glycerol to other valuable chemicals such as glycerol carbonate (GLC). GLC, a nontoxic and water-soluble liquid, is an important component in beauty and personal-care products and is also indirectly used for the synthesis of surfactants and chemical intermediates.3−8 GLC can be prepared from glycerol using various pathways depending on the type of feed materials that act as the source of the carbonyl functionality. Feed materials such as phosgene, carbon dioxide, carbon monoxide, urea, and dialkyl or alkylene carbonates react with glycerol to form GLC. Traditional GLC synthesis routes using glycerol together with phosgene or carbon monoxide have been abandoned because of their high toxicity and unsafe reactants.9−11 Another method of GLC synthesis, namely, the carbonation of glycerol with carbon dioxide, suffers from high pressure requirements, thermodynamic limitations, and low GLC yields, which dramatically increase the production costs.12,13 Glycerolysis of urea is another method for the synthesis of GLC. However, this reaction requires the continuous removal of the ammonia gas produced during the reaction so as to increase the GLC yield. Moreover, the formation of undesirable side products such as biuret and isocyanic acid decrease the rate of GLC formation.14 A much safer and “greener” alternative for GLC synthesis is the transesterification of glycerol with dimethyl carbonate (DMC). In this process, the catalysts/solvents can both be easily © 2015 American Chemical Society

separated, and no side products are formed during the reaction.15 Among the known synthesis routes, the transesterification reaction is most promising for the synthesis of GLC and has gained recent research attention. Numerous catalysts have been investigated for the transesterification of glycerol with DMC in homogeneous and heterogeneous reaction phases to produce GLC. A few studies reported >90% glycerol conversion using CaO-based catalysts for the transesterification of glycerol; however, the reusability of CaO was poor.16−18 A decrease in the CaO specific surface area due to agglomeration and blockage of the active sites caused an appreciable decrease in the catalytic activity of CaO during its reuse in the transesterification reaction.18 Other catalysts such as hydrotalcites,19 LiNO3/Mg4AlO5.5,20 KNO3/CaO,21 Mg− La,22 Mg−Ca,23 and MgO24 have also been used for the synthesis of GLC by the transesterification of glycerol with DMC. Hydrotalcite catalysts have also been used for other transesterification reactions.25−29 The disadvantages of many of these catalysts include low reusability, long reaction times, the need for more than one solvent, and high energy consumption due to high reaction temperatures duing GLC synthesis. Ca and La mixed-metal oxides not only have higher chemical stability than the single-metal oxides, but also exhibit higher contents of acidic and basic sites, which is essential for the transesterification reaction.30−34 A few authors have studied Ca and La mixed-oxide (CaLa) catalysts and investigated them for Received: Revised: Accepted: Published: 12543

September 29, 2015 November 19, 2015 November 23, 2015 November 23, 2015 DOI: 10.1021/acs.iecr.5b03644 Ind. Eng. Chem. Res. 2015, 54, 12543−12552

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Industrial & Engineering Chemistry Research

Pluronic F-127 (10.08 g) as a triblockcopolymer (TBC) was added [such that the molar ratio of TBC to calcium and lanthanum, TBC/(Ca + La), was 0.02] to the sol as an endotemplate. It is known that addition of a TBC as an endotemplate during synthesis increases porosity. In the present study, the molar ratio of TBC to calcium + lanthanum [nTBC/n(Ca + La)] was kept at 0.2, which has been reported to give better porosity during the synthesis of catalysts by the procedure employed.46 This mixture was again kept in the ultrasonic bath for 2 h to dissolve the Pluronic. Polymer-based spherical activated carbon (PBSAC) activated at 110 °C for 24 h was used as an exotemplate. Specifically, 4.42 g of PBSAC was added to the nanoparticle sol, which was then dried at 60 °C for 12 h. The nanoparticle-loaded PBSAC was further calcined at 600 °C for 5 h under an air flow (40 cm3 min−1). The heating rate was 3 °C min−1 from room temperature to 600 °C with a holding time of 1 h at 100 °C and 5 h at 600 °C. After calcination, white CaLa mixed-oxide spheres were obtained. The synthesized catalysts are denoted as 1CaLa, 2CaLa, and 3CaLa for catalysts having Ca/La molar ratios of 1, 2, and 3, respectively. For characterization and catalytic experiments, the CaLa mixed oxides were sieved in the range of 0.2−0.4 mm. 2.2. Catalyst Characterization. Catalysts were characterized by XRD, N2 sorption, CO2 TPD, FTIR spectroscopy, SEM, and TEM. XRD patterns were recorded on a Siemens D5000 diffractometer with Cu Kα radiation (λ = 0.15406 nm) at 40 kV, using a step size of 0.02° over a 2θ range of 5−100°. The crystalline phases were analyzed using PANalytical X’pert High Score software with reference from the International Centre for Diffraction Data/Joint Committee on Powder Diffraction Standards (ICDD/JCPDS) database. The average crystallite sizes were calculated using the Scherrer equation

different reactions (Table S1, Supporting Information) such as the transesterification of natural oils for biodiesel synthesis and the etherification of glycerol to diglycerol.35−45 Table S1 (Supporting Information) provides a comparative assessment of CaLa catalysts and their applications. To the best of our knowledge, porous CaLa mixed-oxide catalysts have not yet been studied in GLC synthesis from the transesterification of glycerol. Thus, in the present work, the effects of the Ca/La molar ratio, reaction conditions, and reuse on the catalytic activity for the transesterification of glycerol to GLC were investigated. The synthesis of porous CaLa mixed oxides was conducted using a combined exo- and endotemplate approach as reported by With et al.46 The textural and structural properties of the CaLa catalysts were characterized by N2 sorption, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and the actual metal composition was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The contents of basic and acidic sites per unit surface area of the synthesized catalysts were determined by the temperatureprogrammed desorption (TPD) of CO2 and NH3, respectively. The dependence of the catalyst activity on the reaction conditions was studied batchwise under various reaction conditions including reaction temperature (T), DMC/glycerol molar ratio (nDMC/nglycerol), catalyst mass (mcatalyst), and reaction time (tr). The reusability of the catalysts was also checked.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Calcium nitrate tetrahydrate [Ca(NO3)2·4H2O, 99%], lanthanum nitrate hexahydrate [La(NO3)3·6H2O, 99%], glycerol, and dimethyl carbonate (DMC, 99%) were purchased from Sigma-Aldrich GmbH (Munich, Germany). Ammonia solution (25 wt % in H2O) and nitric acid (65 wt % in H2O) were purchased from Merck GmbH (Darmstadt, Germany). Carbon spheres were kindly supplied by Blücher GmbH (Erkrath, Germany) [Brunauer−Emmett− Teller (BET) surface area = 1748 m2 g−1, Barrett−Joyner− Halenda (BJH) volume = 2 cm3 g−1, diameter = 0.45−0.5 mm), whereas Pluronic F-127 was purchased from BASF (Ludwigshafen am Rhein, Germany). A standard sample of glycerol carbonate (GLC) was purchased from Sigma-Aldrich GmbH (Munich, Germany). All chemicals used were of analytical (AR) grade. CaLa catalysts with different target molar ratios (Ca/La = 1, 2, and 3) were synthesized using 2.36, 4.72, and 7.08 g, respectively, of Ca(NO3)2 and 3.25 g of La(NO3)3 as precursors. For this synthesis, the appropriate amounts of Ca(NO3)2 and La(NO3)3 were dissolved separately in 100 cm3 of demineralized water, and the solutions were mixed together under continuous stirring at room temperature. Liquid ammonia solution was added dropwise to the precursor solution over a period of 0.5 h until the pH reached ∼8.5 and a white precipitate formed. The mixture was aged for 2 h under further stirring, and the precipitate was filtered and washed with demineralized water until the pH of the filtrate became neutral. Finally, the filter cake was transferred to a 200 cm3 polypropylene (PP) bottle, and demineralized H2O was added until a total weight of 30 g was obtained. Then, 2.5 cm3 of HNO3 (65 wt % in H2O) was added. The filled PP bottle was transferred to an ultrasonic bath (Sonorex RK1, BANDELIN Electronic GmbH, Berlin, Germany), where it was kept for 4 h until a clear nanoparticle sol was formed.

L=

Kλ β cos θ

(1)

where L is the average particle size, K is the Scherrer constant (0.94), λ is the wavelength of X-ray radiation (λ = 0.154051 nm), θ is the scattering angle of the main reflection, and β is the full width of the reflection at half-maximum (fwhm). ICP analysis was performed to determine the actual metal composition in the CaLa-based catalysts using a Perkin-Elmer OPTIMA 8000 model inductively coupled plasma optical emission spectrometer. Textural characteristics of the samples were determined by N2 sorption at −195 °C using a Micromeritics ASAP 2020 apparatus. Each sample was pretreated with nitrogen gas for 6 h at 200 °C to remove any absorbed impurities before the experiments. The specific surface areas of the samples were determined using the Brunauer−Emmett−Teller (BET) method47 in the relative pressure range of p/p0 = 0.05−0.35. The specific pore volumes and average pore widths were calculated using the Barrett−Joyner−Halenda (BJH) method48 applying the desorption branch of the isotherms. Estimation of the irregularity or roughness of the catalyst surface was compared using the fractal dimension factor D. The D value was estimated using the desorption branch of the isotherm applying the Frenkel−Halsey−Hill (FHH) equation49 ⎛ P ⎞D−3 q = K ln⎜ 0 ⎟ ⎝P⎠ qe

(2)

where q is the amount of N2 adsorbed at equilibrium pressure P, P0 is the saturated pressure, qe is the amount adsorbed to fill 12544

DOI: 10.1021/acs.iecr.5b03644 Ind. Eng. Chem. Res. 2015, 54, 12543−12552

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Figure 1. SEM images: (a) Initial carbon sphere and (b) 1CaLa, (c) 2CaLa, and (d) 3CaLa catalysts.

the micropore volume, K is a constant, and D is the fractal dimension. The logarithmic plot of q/qe versus P0/P exhibited linear behavior, and D was calculated from the slope of the line. The surface roughness, irregular nature, and surface smoothness depend on the D value. If D = 2, then the surface is considered to be perfectly smooth, whereas if D = 3, then the surface is considered to be very irregular or rough. The basic sites per unit surface area and the basic site densities of the synthesized materials were determined by CO2 TPD using a Micromeritics Chemisorb 2720 instrument fitted with a thermal conductivity detector (TCD). For each TPD run, 50 mg of catalyst was placed in a quartz U-tube and pretreated at 200 °C under helium flow (20 cm3 min−1) for 6 h. Adsorption of CO2 was performed at room temperature with a flow rate of 20 cm3 min−1 for 30 min, followed by a helium purge for 1 h at room temperature to remove the physisorbed CO2. The desorption process was recorded in the temperature range of 50−900 °C at a heating rate of 10 °C min−1 under a helium flow (20 cm3 min−1), and the evolved CO2 was monitored by TCD. The morphologies of the catalysts and their elemental compositions were determined by SEM and energy-dispersive X-ray (EDX) spectroscopy (QUANTA 200 FEG, FEI, Eindhoven, The Netherlands). TEM images and corresponding selected-area electron diffraction (SAED) patterns were obtained using a TECNAI G2 20S-TWIN microscope (FEI, Eindhoven, The Netherlands) with LaB6 as the cathode (point resolution, 0.24 nm; line resolution, 0.14 nm) at 200 kV. For TEM analysis, the catalysts were dispersed in ethanol solution and sonicated for 30 min. Afterward, a drop of the dispersion was placed on a TEM copper grid. The surface functional groups of the synthesized catalysts were studied by FTIR

spectroscopy (Magna 760, Thermo Nicolet, Madison, WI). KBr pellets containing the catalysts were used for the FTIR measurements with a KBr pellet as the reference. FTIR spectra were recorded in the wavenumber range of 400−4000 cm−1. 2.3. Catalytic Activity. The transesterification reaction was carried out batchwise in a 50 cm3 Teflon-lined stainless-steel autoclave fitted with a magnetic stirrer, a heating plate, and a thermocouple. DMC and glycerol together with the catalyst were placed in the autoclave, which was then sealed. The catalyst mass was set to 10.8% of the weight of glycerol, which was equal to 0.217 g. The desired reaction temperature was attained at a heating rate of 10 °C min−1 under stirring rate of 250 rpm. At this temperature, no detectable pressure was generated from the reaction mixture. After a reaction time of 90 min, the reaction was stopped, and the product mixture was cooled to room temperature within 30 min using ice cubes. The catalyst was then separated from the reaction mixture by centrifugation, and a liquid sample was taken for product analysis. For the reuseability experiments, the 3CaLa catalyst was separated from the reaction mixture by filtration. The recovered catalyst was washed with 200 cm3 of methanol several times and dried at 120 °C for 12 h.21 DMC and methanol were separated using a Rotavapor rotary evaporator (Büchi, Flawil, Switzerland) at 0.3 bar and 90 °C. The product was analyzed by gas chromatography (GC) using an HP 5890 gas chromatograph (Hewlett-Packard) equipped with an HP-5 capillary column (30 m × 0.25 mm, 0.25 μm) and a flame ionization detector (FID). The column temperature was initially kept at 75 °C with a holding time of 2 min. Afterward, the temperature was increased at a rate of 25 °C min−1 to 225 °C, and after that, the heating rate was 10 °C min−1 to a final temperature of 275 °C. The injector and detector temperatures 12545


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Industrial & Engineering Chemistry Research were 260 and 270 °C, respectively. The experimental runs were repeated three times (reproduceable results with a maximum deviation of 5%). The turnover frequency (TOF) was calculated as TOF =

reflections for the CaO cubic phase (JPDS No. 00-043-1001) and the La2O3 hexagonal phase [space group P3m1 (164), JPDS No. 00-073-2141] were found. This suggests that the synthesized materials consisted of separate solid solutions of CaO and La2O3 crystal phases, consistent with the results of another publication.45 The average crystallite sizes were determined from the most intense reflections (2θ = 29°) in the XRD patterns. Using the Scherrer equation, the average crystallite sizes of 1CaLa, 2CaLa, and 3CaLa were found to be 16, 15, and 27 nm, respectively. Additionally, a TEM image (Figure 2b) of catalyst 3CaLa confirms the presence of spherical crystallites with sizes in the range of 15−40 nm. The corresponding selected-area electron diffraction (SAED) indexing pattern of 3CaLa confirms the presence of the crystalline phases identified by XRD. The surface functional groups on the surfaces of the 1CaLa, 2CaLa, and 3CaLa catalysts were analyzed by FTIR spectroscopy. All three catalysts exhibited comparable FTIR patterns (Figure 3) and, thus, contain the same surface funtional groups. Peaks at 994 and 865 cm−1 are due to La−O stretching vibrations, and the peak at 657 cm−1 is due to the bending vibration of La−O.50,51 The peaks at 617 and 595 cm−1 correspond to the bending and stretching vibrations of Ca2+− O2−.51 These results confirm the presence of CaO and La2O3 in all of the catalysts and are consistent with XRD analysis. The dependence of the specific surface area and pore volume variation on the pore size is shown in Figure S1 (Supporting Information), and detailed textural data on the catalysts are provided in Table 1. All catalysts showed type II isotherms according to the IUPAC classification, with an H1 hysteresis loop at p/p0 > 0.7, a characteristic of mesoporous materials.45 The BET surface areas of 1CaLa, 2CaLa, and 3CaLa were found to be 30, 45, and 33 m2 g−1, respectively, whereas the BJH pore volumes of all three catalysts were comparable, in the range of 0.07−0.08 m2 g−1. Luo et al.41 reported surface areas in the range of 10−17.5 m2 g−1 for CaLa catalysts with different Ca/La ratios, whereas Käßner and Baerns45 reported surface areas in the range of 1.1−2.5 m2 g−1. For the synthesized catalysts, the fractal dimension was found to be 2.42 for 1CaLa, 2.54 for 2CaLa, and 2.41 for 3CaLa. This shows that all of the catalysts had similar surface heterogeneities, with maximum variations of 5%. In the present study, the Ca/La molar ratios in the mixedoxide catalysts were analyzed by ICP-OES and EDX spectroscopy (Figure S2, Supporting Information). The structural chemical compositions of the catalysts are reported in Table S2 (Supporting Information). The chemical compositions of the synthesized catalysts were relatively close to the desired initial metal compositions. The basic surface properties of the CaLa catalysts were characterized by CO2 TPD (Figure 4). Catalysts are assumed to contain weak, medium, and strong basic sites if they exhibit desorption peaks in the ranges of 450 °C, respectively. All three mixed oxides exhibited similar CO2 TPD profiles with a main peak at 740−760 °C (Figure 4a and Table 1). The weight loss due to CO2 evoluted because of the decomposition of surface CaCO3 during the heating process was studied for all the three synthesized catalysts according to the method reported earlier, and the results are shown in Figure 4b.27,52−54 It is clear that the peaks at 740−760 °C should not be assigned as basic sites because they are due to the decomposition of CaCO3. Therefore, the actual basicity of the as synthesized catalysts was determined after subtracting

mglycerol XglycerolMglycerol 100mcatalyst t

(2)

where mglycerol is the initial amount of glycerol (moles), Xglycerol is the conversion of glycerol, Mglycerol is the molecular weight of glycerol, mcatalyst is the mass of catalyst used in the reaction, and t is the reaction time.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Porous CaLa mixed oxides were synthesized using PBSAC as an exotemplate and Pluronic F-127 as an endotemplate. Most importantly, the spherical particle morphology of the activated carbon was transferred to the CaLa mixed oxides (Figure 1). From the SEM images of the as-synthesized mixed oxides (Figure 1), it can be seen that the size distribution of the resulting particles was in the range dp = 0.2−0.4 mm. Therefore, the size was smaller than that of the initial PBSAC (Figure 1a). Possible reasons for this decrease in size were discussed in a previous publication on the synthesis of highly porous zirconia spheres using PBSAC as the exotemplate.46 Furthermore, a higher magnification of the CaLa mixed oxides shows significant macroporosity in the particles. Interestingly, the macropore morphology changed from slits to spongelike pores when the molar Ca/La ratio was increased from 1 to 3. The XRD patterns of 1CaLa, 2CaLa, and 3CaLa mixed oxides are shown in Figure 2a. In all three materials, the

Figure 2. (a) XRD patterns of calcium−lanthanum mixed oxides and (b) TEM image and SAED pattern of 3CaLa. 12546


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Figure 3. FTIR spectra of the synthesized 1CaLa, 2CaLa, and 3CaLa catalysts.

decreases with an increase in the amount of La in the catalyst. This is in agreement with the results shown here, where the basic sites per unit surface area of the CaLa catalysts continuously decreased with an increase in the amount of La and a decrease in the amount of Ca. The acidic properties of the synthesized CaLa-based catalysts were estimated using NH3 TPD, and the results are shown in Figure 4c. The strengths of the acidic sites are reported in Table 1. All of the catalysts were found to contain strong Brønsted acid sites. The 3CaLa catalyst had highest number of acidic sites per unit surface area in the strong region. The total amount of NH3 desorbed and the total amount of acidic sites per unit surface area were found to be in the following order: 3CaLa > 1CaLa > 2CaLa. Therefore, 3CaLa had the highest amount of acidic sites, and 2CaLa had the lowest amount of acidic sites. Becker and Baerns55 reported the maxium acidity for a Ca−La mixed-oxide catalyst having 20% La. The prepared 3CaLa catalyst exhibited the highest amounts of acidic and basic sites per unit area, and thus, this catalyst can act as the best acid− base bifunctional catalyst among the synthesized catalysts. 3.2. Catalytic Activity for the Transesterification of Glycerol with DMC. In the present sudy, the synthesized CaLa mixed oxides were used in the transesterification of DMC with glycerol to obtain GLC at 90 °C. Because an excess of DMC drives the reaction toward the transesterified product, a DMC/glycerol molar ratio of 5 was applied. It should be mentioned that, in the absence of a catalyst, negligible GLC formation of ∼1% after 4 h was observed. The glycerol conversions, GLC yields, and TOFs for the CaLa catalysts are shown in Figure 5. It can be seen that the glycerol conversion and the GLC yield increased when the Ca/La molar ratio increased from 1 to 3. The TOF values for 1CaLa, 2CaLa, and 3CaLa were found to be 5.15, 5.49, and 5.78 h−1, respectively. The TOF values obtained in the present study are similar to those reported in the literature. Parameswaram et al.56 reported TOF values of 0.5−16.11 h−1 using catalysts such as MgO, MgO-ZrO2, and MgO−ZrO2−SrO2. Similarly, Xu et al.57 reported a TOF of 0.96 h−1 using an ionic liquid for the transesterification of ethylene carbonate (EC)/propylene

Table 1. Properties of 1CaLa, 2CaLa, and 3CaLa Catalysts property

1CaLa

2CaLa

3CaLa

crystallite sizea (nm) lattice constant da (nm) BET surface area (m2 g−1) specific pore volumeb (cm3 g−1) average pore diameterc (nm) CO2 evolution decompositiond (mmol g−1) CO2 TPDe (mmol g−1)

16 0.3100 30 0.07 28 0.270 (771 °C) 0.498 (776 °C) 0.228

15 0.3099 45 0.08 15 0.241 (771 °C) 0.642 (764 °C) 0.401

27 0.2995 33 0.07 24 0.268 (771 °C) 0.771 (768 °C) 0.503

3.125

2.316

5.493

7.6

8.91

15.24

0.1042

0.0515

0.1665

total basic site density (mmol g−1) total acidic site densityf (mmol g−1) basic sites per unit surface area (μmol m−2) acidic sites per unit surface area (mmol m−2) a

Unit cell parameter; crystallite size was calculated using the Scherrer equation. bBJH desorption cumulative pore volume for pores in the range of 1.7−300 nm. cBJH desorption average pore diameter. dCO2 evolution during decomposition; maximum temperature given in parentheses. eCO2 TPD values calculated using the peak area at the maximum temperature, which is given in parentheses. fNH3 TPD values calculated using the peak area at the maximum temperature, which is given in parentheses.

the CO2 intensity values from the decomposition of surface CaCO3 for all the three catalysts. The amounts of basic sites per unit surface area of the catalysts were in the order 1CaLa (0.228 μmol g−1) < 2CaLa (0.401 μmol g−1) < 3CaLa (0.503 μmol g−1). Thus, the amount of basic sites per unit mass of catalyst increased, as expected, with increasing molar Ca/La ratio in the catalysts, which is desirable for the transesterification reaction. Weak basicity in the lower temperature range is due to the interaction between CO2 and the weak basic surface hydroxyl groups, and strong basic sites at higher temperatures are due to the interaction between CO2 and O−. Käßner and Baerns45 reported that, for CaLa mixed-oxide catalysts, the basicity 12547


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Figure 4. (a) CO2 TPD, (b) CO2 evolution during decomposition, and (c) NH3 TPD of 1CaLa, 2CaLa, and 3CaLa mixed-oxide catalysts.

Scheme 1. Proposed Reaction Mechanism for the Transesterification of Glycerol with DMC to Glycerol Carbonate

Figure 5. DMC yield, glycerol conversion, and TOF values over 1CaLa, 2CaLa, and 3CaLa catalysts. Reaction conditions: nDMC/nglycerol = 5, mcatalyst = 0.217 g, T = 90 °C, tr = 90 min.

carbonate (PC) with methanol. Kumar et al.27 reported TOF values of 1.50−5.99 h−1 using Cu−Zn−Al (hydrotalcite-like compound, HTlc) catalysts for DMC synthesis by the transesterification reaction. The mechanism of GLC formation through the transesterification of glycerol with DMC is shown in Scheme 1.16 According to this mechanism, the basic sites of the CaLa catalyst react with glycerol to extract H+ ions from the primary

hydroxyl groups of glycerol. The glyceroxide anions (C3H7O3−) react with DMC to form hydroxyl alkyl carbonate, which is an unstable intermediate that is converted to GLC and methanol. In this transesterification reaction, the main function of the solid catalyst is to support the abstraction of H+ from glycerol 12548


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Figure 6. Effects of various parameters on the transesterification of glycerol with DMC using 3CaLa catalyst: (a) effect of time at nDMC/nglycerol = 5, and T = 90 °C, mcatalyst = 0.217 g; (b) effect of catalyst mass at nDMC/nglycerol = 5, tr = 90 min, and T = 90 °C; (c) effect of nDMC/nglycerol molar ratio at T = 90 °C, tr = 90 min, and mcatalyst = 0.217 g; and (d) effect of temperature at nDMC/nglycerol = 5, tr = 90 min, and mcatalyst = 0.217 g.

Simanjuntak et al.22 and Parameswaram et al.56 reported similar trends for glycerol transesterification with DMC using Mg/La and Mg/Zr/Sr mixed-oxide catalysts, respectively. The infuence of catalyst mass was investigated in the range of 5.5−21.75 wt % relative to the glycerol amount, keeping the other reaction parameters constant (i.e., T = 90 °C, DMC/ glycerol = 5, tr = 90 min) (Figure 6b). It can be seen that, as the 3CaLa catalyst mass increased from 0.11 to 0.43 g, the glycerol conversion increased from 65% to 94% and the GLC yield increased from 32% to 79%. This can be attributed to the increase in the overall number of basic sites. A catalyst amount greater than 0.217 g did not influence the glycerol conversion or GLC yield after the 90-min reaction time. Liu et al.58 used various types of hydrotalcites for the transesterification of glycerol and reported that blocking of the pores and, thus, lack of access to the active sites of the catalysts due to agglomeration at higher catalyst masses and increased external mass-transfer resistance in the glycerol, DMC, and catalyst triphase system did not allow for an increase in catalytic efficiency beyond a certain value. The effect of the DMC/glycerol molar ratio was studied in the range of 1−6 using the same reaction conditions (i.e., T = 90 °C, tr = 90 min) and a constant catalyst mass of 0.217 g (Figure 6c). An increase in the DMC/glycerol molar ratio from 1 to 5 increased the GLC yield from 32% to 76%, the glycerol conversion from 80% to 94%, and the TOF from 4.9 to 5.78 h−1. The results were similar at a DMC/glycerol molar ratio of

by the basic sites so as to form glycerol anion. The higher the basicity of the catalyst, the more negative the charge of the glyceroxide anion (C3H7O3−), and consequently, the lower the free energy of the reaction. The catalytic activity (in terms of GLC yield) was found to be in the following order: 1CaLa (61%) < 2CaLa (70%) < 3CaLa (74%). The 3CaLa mixed oxide had highest number of basic sites per unit surface area among all of the studied catalysts and, consequently, showed the highest GLC yield. These results are in agreement with literature findings.20,40,56 The analogous compounds 4CaLa and 5CaLa were also tested for their catalytic activity and showed similar activity or a marginal decrease as compared to 3CaLa (Figure S3, Supporting Information). Overall, 3CaLa gave the highest activity, and it was further used to study the effects of the reaction conditions such as reaction temperature, catalyst amount, reaction time, and DMC/glycerol molar ratio during GLC formation from glycerol and DMC. The effect of reaction time (30−240 min) during the transesterification of glycerol with DMC with the 3CaLa catalyst is shown in Figure 6a. The GLC yield and glycerol conversion increased from 30% and 65%, respectively, at 30 min to 78% and 94%, respectively, at 90 min. The reaction rate (with respect to glycerol) under these conditions was approximately 0.14 mol L−1 h−1. A further increase in the reaction time beyond 90 min showed no effect on the GLC yield or glycerol conversion. This indicates that the reaction over the tested catalysts reached equilibrium within 90 min. 12549


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mechanical strength of the catalyst particles is likely to be higher in packed-bed reactors because of the absence of strring in packed beds. The reused catalysts were also characterized by XRD and CO2 TPD. No differentiable change in the XRD profile of the reused catalyst was observed (Figure S5, Supporting Information). Also, only a marginal difference in basicity was observed after five cycles of usage of 3CaLa catalyst (Figure S6 and Table S3, Supporting Information). Pure CaO and La2O3 oxide were also catalytically studied at 90 °C in the transesterification of DMC with glycerol to obtain GLC. A DMC/glycerol molar ratio of 5 was used in the reaction. The glycerol conversion, GLC yield, and TOF values for the CaO and La2O3 catalysts and the reusability of CaO and La2O3 were examined by utilizing them in five consecutive batch reactions (see Figure S4, Supporting Information). Upon reuse, the activities of pure CaO and La2O3 decreased significantly. It can be seen that the glycerol conversion and GLC yield decreased after each cycle for pure CaO and La2O3 catalysts. Thus, the activities of the pure oxides are as poor compared to that of the CaLa mixed-oxide catalysts. A number of investigators have previously used various catalysts for GLC production using transesterification reactions. Uncalcined Mg−Al and CaO catalysts exhibited about 75% and 38% GLC yields, respectively, with N,N-dimethylformamide as the organic solvent.60 Promoted nickel hydrotalcite and MgO catalysts showed 56% and 12.1% GLC yields, respectively, with 100% and 12.1% glycerol conversions, respectively, without use of any organic solvent.16 MZS-311 catalysts provided a 56% GLC yield and 96% glycerol conversion.61 The catalytic activity of the CaLa mixed-oxide catalyst (without the use of any solvent during the reaction) is better than or comparable to those reported in the literature. However, it should be noted that the maximum GLC yield and glycerol conversion in these literature studies were obtained under different optimum reaction conditions, and thus, it is not possible to directly compare the catalytic activities of these different oxide catalysts.

6. The glycerol conversion does not increase beyond a certain value at higher DMC/glycerol molar ratios because of two counterbalancing effects: On one hand, an increase in molar ratio favors a higher conversion by shifting the equilibrium of the section step to the right (see the mechanism in Scheme 1), but on the other hand, an increase in the molar ratio decreases the glycerol concentration, leading to a lower reaction rate.16 Malyaadri et al.59 and Takagaki et al.60 also reported a DMC/ glycerol molar ratio of 5 to be optimal for the synthesis of GLC using Mg/Al/Zr oxides and Mg/Al hydrotalcite catalysts, respectively. The influence of the reaction temperature on the activity of the 3CaLa catalyst was studied in the temperature range of 60− 140 °C (Figure 6d). It can be clearly seen that an optimum glycerol conversion of 95% was obtained at 90 °C with a maximum GLC yield of 74% (TOF = 5.78 h−1). A further increase in the reaction temperature did not affect the conversion but caused a decrease in the GLC yield from 78% to 48% (from 90 to 140 °C). This could be due to dehydrogenation and condensation reactions of the byproduct methanol.58 An other reason could be the conversion at higher temperature of GLC by a decarboxylation reaction in the basic medium into glycidol, which is then converted into polyglycerol by base-catalyzed ring-opening polymerization.61 In other studies, comparable reaction temperatures of 85,22 90,24 and 100 °C60 using MgO, hydrotalcite, and transition-metal doped hydrotalcite catalysts were reported to be optimum in terms of GLC yield. Reusability experiments were carried out using 3CaLa. For these experiments, the catalyst was used in four consecutive batch reactions (Figure 7). The observed GLC yield decreased

4. CONCLUSIONS In the present study, calcium and lanthanum mixed-oxide catalysts were synthesized using a combined exo- and endotemplating approach with varying Ca/La molar ratio (denoted as 1CaLa, 2CaLa, and 3CaLa). Thus, porous and spherical-shaped CaLa oxides were obtained using polymerbased activated carbon spheres as the exotemplate and Pluronic as the endotemplate. The crystallite size and specific surface area were found to increase with increasing Ca/La ratio. The glycerol conversion and GLC yield were found to increase with increasing amount of basic sites per unit surface area of the catalysts. The 3CaLa catalyst showed the highest density of strong basic sites among all of the catalysts synthesized and, consequently, showed the highest GLC yield. The glycerol conversion, GLC yield, and GLC selectivity using 3CaLa catalyst were found to increase with increasing reaction temperature, catalyst mass, reaction time, and DMC/glycerol molar ratio up to the optimum reaction conditions of a reaction temperature of 90 °C, a catalyst mass of 0.217 g, a reaction time of 90 min, and a DMC/glycerol molar ratio of 5. Under the optimum reaction conditions, the glycerol conversion, GLC yield, and GLC selectivity were found to be 94%, 74%, and 90%, respectively. The 3CaLa catalyst showed good stability and reusability. The present study shows that GLC, a valuable chemical, can be produced by the transesterification of glycerol

Figure 7. Reusability of 3CaLa catalyst in the transesterification of glycerol with DMC (nDMC/nglycerol = 5, mcatalyst= 0.217 g, T = 90 °C, tr = 90 min).

slightly with increasing reuse cycle. The GLC yield and TOF in the fourth run were found to be 67% and 5.46 h−1, respectively. This loss in catalytic activity of the 3CaLa catalyst might be due to the deposition of reaction products and blockage of the pores and active sites during glycerol conversion to GLC.62,63 Pure CaO and La2CO3 were also tested, and they showed poor recyability (Figure S4, Supporting Information). It must also be mentioned that a minor portion of the spherical catalyst particles disintegrated during the four batches; however, the 12550


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with dimethyl carbonate (DMC) using CaLa catalysts with good yield and selectivity.

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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/acs.iecr.5b03644. Comparison of previous studies on synthesis and characterization of Ca−La catalysts for various applications; nitrogen sorption isotherms and pore size distributions for Ca−La catalysts; elemental analysis of CaLa catalysts; crystalline size, CO2 TPD, and XRD patterns of reused catalysts; reuseability of CaO and La2O3 catalysts in the transesterification of glycerol with DMC (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-1332-285889. Fax: +91-1332-276535. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.K. is thankful to Deutscher Akademischer Austausch Dienst (DAAD), Germany, for providing financial support to carry out this work under a Sandwich Model Scholarship.

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REFERENCES

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