Lanthanum Mixed-Oxide Catalyst for the Synthesis of Glycerol

Article pubs.acs.org/IECR

Zinc/Lanthanum Mixed-Oxide Catalyst for the Synthesis of Glycerol Carbonate by Transesterification of Glycerol Dheerendra Singh,† Bhoja Reddy,‡ Anuradda Ganesh,† and Sanjay Mahajani*,‡ †

Department of Energy Science & Engineering and ‡Department of Chemical Engineering, Indian Institute of Technology, Bombay, Mumbai, India S Supporting Information *

ABSTRACT: ZnO/La2O3 mixed oxides are prepared by a coprecipitation method at different molar ratios and used as catalysts for the synthesis of glycerol carbonate by transesterification of glycerol with dimethyl carbonate (DMC). X-ray diffraction, N2 adsorption, transmission electron microscopy, scanning electron microscopy, and thermal-programmed desorption methods were used for characterization of the catalysts. The surface area and porosity of the catalysts prepared by the precipitation method proved to be better than those prepared by combustion and modified-citrate methods. Zn4La1 and Zn2La1 (mole ratio Zn:La of 4:1 and 2:1, respectively) were found to be the best proportions in view of both higher activity and higher selectivity. The rates offered by these catalysts were substantially high compared to reported Mg/La and other catalysts. The effects of parameters such as the temperature, catalyst loading, and mole ratio of DMC to glycerol were examined, and a suitable kinetic model was proposed for the Zn4La1 catalyst.

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INTRODUCTION

An increased production of biodiesel has rapidly increased the worldwide production of glycerol (GLY), which is a valueadded coproduct, produced at a rate of 10% of biodiesel production. GLY finds application in the synthesis of various fine chemicals such as acrolein, 1,3-propanediol, docosahexaenoic acid, polyglycerols, and glycerol carbonate (GC). GC is a versatile fine chemical with many potential applications. It is widely used as an adhesive, surfactant, and elastomer. It can also serve as a useful solvent in the paint industry and in lithium-ion batteries and finds use in the synthesis of polymers, such as polyesters, polycarbonates, polyurethanes, polyamides, surfactants, and lubricating oils. GC can be synthesized from GLY via different paths. The conventional method is direct carbonation of GLY with phosgene or carbon monoxide and oxygen.1 Both phosgene and carbon monoxide are toxic and unsafe; hence, there is a need to find a suitable replacement. GC can also be produced by carbonation of GLY with carbon dioxide (CO2) using tin (Sn) catalysts [nBu2Sn(OMe)2]2 but with a smaller GC yield. A much safer and greener route for GC production is the reaction of GLY and dimethyl carbonate (DMC) or of GLY and urea. The former, being a neat and relatively fast reaction, holds promise if DMC is available at a reasonable price. The transesterification reaction (see eqs 1 and 2) is catalyzed mainly by a base material. In a side reaction, GC further decomposes over the strong basic catalyst to form glycidol, which is another value-added product if recovered in pure form. © XXXX American Chemical Society

Table S1 in the Supporting Information (SI) summarizes the various catalysts reported for transesterification of GLY with DMC and their respective performances. The calcium (Ca)based catalysts, such as CaO,3 CaO/Al2O3, and Mg1+xCa1−xO2, showed a high catalytic activity. However, Ca species dissolved in the GLY solution by forming a calcium glycerol complex.18 Potassium (K)-based catalysts, such as KF/γ-Al2O3,10−12 Potassium fluoride (KF)-modified hydroxyapatite,7 and K2CO3/MgO,8 and sodium (Na)-based catalysts, such as NaOH/γ-Al2O3,8 NaAlO2,9 and Na-based zeolite,6 have also been used. KF-modified hydroxyapatite can be recycled four or five times without much change in GLY conversion; 111 ppm/ g of K leached into the product mixture after the third run. The instability of K- and Na-based catalysts over a long duration is the main concern in the liquid-phase reaction. Mixed-oxide catalysts such as MgO/La2O314,15 and Mg/Zr/Sr13 exhibit relatively poor performance in terms of the selectivity of GC compared with hydrotalcite4,5 or any of the other basic catalysts reported in Table S1 in the SI. Lipase,16,17 like any other enzyme catalyst, requires a long reaction time and an additional Special Issue: Ganapati D. Yadav Festschrift Received: March 18, 2014 Revised: May 26, 2014 Accepted: May 28, 2014

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dx.doi.org/10.1021/ie5011564 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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emission microscope (HR-TEM; JEM-2100F; JEOL, USA) provided TEM analysis using an accelerating voltage of 200 kV. SEM analysis (JSM-7600F; JEOL, USA) was used to determine the surface morphology. The catalyst powder was directly sprinkled over the carbon tape, and images were taken under optimum operating conditions. A well-dispersed solution was prepared by adding a small amount of catalyst powder to ethanol and sonicating it for 10−15 min. One drop of the dispersed solution was placed on a TEM grid and dried under an IR lamp for 30 min. A TPDRO instrument (Thermo 1100, Thermo Scientific, USA) using 10% CO2 in helium (He) as a probe molecule for basicity provided TPD analysis. The catalysts used for TPD were pretreated with 5% H2/He at a flow rate of 20 mL/min and a temperature ramp rate of 20 °C/ min up to 350 °C. The temperature was then raised to 400 °C and maintained for 60 min under an argon flow rate of 20 mL/ min. This pretreatment ensured that the metal oxide surface was free from impurities such as chlorides or nitrates. For TPD analysis of La2O3, Zn6La1, Zn4La1, Zn2La1, MgO, and Mg6La1, the samples were first heated to 600 °C and the CO2 pulses were introduced at room temperature until saturation. The temperature was varied (50−600 °C) at a rate of 5 °C/min. Energy-dispersive X-ray (EDX) spectrometry and inductively coupled plasma atomic emission spectroscopy (ICP-AES) confirmed metal loading. The leaching of metal oxide during the course of the reaction was studied by using ICP-AES (ARCOS, M/s. Spectro, Germany). Reaction Procedure and Analysis. Reactions under different conditions were carried out in a 300 mL batch autoclave (model 4843; Parr Instrument Co., USA; maximum temperature, 350 °C; maximum pressure, 199 bar). After transesterification, the reaction mixture was centrifuged to separate the solid catalyst. The samples were analyzed on a gas chromatograph (Mak Analytica, India) using a Restek capillary column (Stabilwax, 30 m × 0.53 mm × 1 μm film thickness). The oven temperature was varied from 60 to 230 °C while maintaining both the injector and detector (FID) at 240 °C. nButanol was used as an external standard.

solvent. Basic oxide with relatively strong basic sites and high stability in the liquid-phase reaction could be the best choice for this reaction. The present work is undertaken to examine this possibility. ZnO/La2O3 mixed oxides with different molar ratios were reported by Yan et al.19 for simultaneous esterification and transesterification of waste vegetable oil. This catalyst was tested continuously for 70 days in a fixed-bed continuous reactor; leaching was negligible, giving a consistent performance. The stability of metal oxide in the liquid phase is a very important criterion in selection of the catalyst. In the present work, we investigated a few d-block metal oxides, viz., ZnO, PbO, and MgO, and also some f-block metal oxides, viz., CeO2, Ce2O3, and La2O3, as catalysts for this reaction. La2O3 was combined with ZnO to increase the surface area and improve the catalytic activity. Mixed oxides of zinc (Zn) and lanthanum (La) were prepared at three different molar ratios (2:1, 4:1, and 6:1). They offered reasonably high catalytic activity in terms of GLY conversion and also high selectivity for GC. Parametric studies were performed to examine the effects of temperature, catalyst loading, and GLY-to-DMC mole ratio, and a suitable kinetic model is proposed to explain the data.

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EXPERIMENTAL WORK Materials. Glycerol (GLY; >99.5% pure), zinc nitrate, magnesium nitrate, and sodium bicarbonate, all analytical reagent grade, were procured from Merck Chemicals, India Ltd. Dimethyl carbonate (DMC; >99% pure) was procured from SD Fine-Chem Ltd., and lanthanum nitrate (>99% pure) was obtained from KEM Light Laboratories Pvt. Ltd. Catalyst Preparation. Metal oxide and mixed metal oxide catalysts were prepared by combustion,20 a modified-citrate technique21 and coprecipitation methods. The coprecipitation method used NaHCO3 as a precipitating agent. In a 1 M solution of metal nitrate, a saturated solution of NaHCO3 was added at 90 °C, and the solution was stirred at high rpm (1500) to break the nucleation. The mixture was filtered, and the solid obtained was dried at 110 °C for 14 h. The catalyst was calcined from 110 to 500 °C at a rate of 10 °C/min and maintained at 500 °C for 4 h. Zinc oxide and lanthanum oxide catalysts with molar ratios of 2:1, 4:1, and 6:1 were prepared and labeled Zn2La1, Zn4La1, and Zn6La1, respectively. Similarly, Mg6La1 was prepared with a 6:1 molar ratio of magnesium oxide to lanthanum oxide. It is a reported catalyst for the reaction of interest, and hence its performance is compared with the new Zn/La catalysts developed in the present work. The other metal oxides CeO2, Ce2O3, and PbO, used for comparison, were prepared by a precipitation method in a similar way. Methods of Catalyst Characterization. Techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermal-programmed desorption/thermal-programmed reduction (TPD/TPR), and N2 adsorption, based on Brunauer− Emmett−Teller (BET) theory, were used for characterization of the catalysts. N2 adsorption−desorption isotherms, measured at 77 K on an ASAP-2020 instrument (V3.01H; Micromeritics, USA), provided the surface area. Approximately 0.2 g of powder was degasified at 400 °C for 400 min and then used for analysis. For XRD, the catalyst powder was analyzed with an X’Pert diffractometer (PANalytical, UK). The data were collected over a 2θ range of 5−90 °C using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. A high-resolution field-

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CATALYST CHARACTERIZATION XRD of La2O3, Zn2La1, Zn4La, Zn6La1, and ZnO. La2O3, Zn2La1, Zn4La1, Zn6La1, and ZnO were prepared by the precipitation method and calcined at 500 °C. Figure 1 shows the XRD patterns of La2O3, Zn2La1, Zn4La1, Zn6La1, and ZnO. The diffraction peaks of ZnO obtained in pure ZnO and in Zn/La mixed oxides are at 31.9, 34.7, 36.4, 47.7, 56.8, and 63.10, which correspond to the lattice planes (100), (002), (101), (102), (110), and (103), respectively. The XRD patterns of ZnO are consistent with the values in the JCPDS 36-1451 database. Zn2La1 and Zn4La1 exhibit both ZnO and La2O3 phases with a low intensity of the La2O3 XRD peaks for Zn6La1. The XRD pattern of lanthanum oxide shows the mixed phases of La2O2CO3 (JCPDS 83-1355) and La2O3 (JCPDS 83-1344), in which the La2O2CO3 phase is in larger abundance. The XRD patterns for Zn2La1 and Zn4La1 show a dominant La2O3 phase; the patterns for Zn6La1 show only the La2O3 phase. The average crystallite sizes of ZnO in the Zn/La mixedoxide catalysts were calculated by the Scherrer equation. The sizes of ZnO in the Zn2La1, Zn4La1, and Zn6La1 crystallites were 11.64, 14.86, and 11.32 nm, respectively. XRD of MgO, Mg6La1, and La2O3. MgO, Mg6La1, and La2O3 were prepared by the precipitation method and calcined B



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the P method. The surface area of MgO (183.5 m2/g) was higher with the P method than with MCT (144.4 m2/g). Table S2 in the SI shows that the catalysts prepared by the P method are better in terms of surface area and porosity compared to those prepared by other methods. SEM and TEM. SEM images of La2O3, MgO, Mg6La1, Zn6La1, Zn4La1, and Zn2La1 are shown in parts a−f of Figure 3, respectively. Figure 3a shows that the La2O3 particles are in the form of flakes, with their size varying from 0.2 to 2 μm. MgO and Mg6La1 particles (Figure 3b,c) are spherical, with the particle size varying from 15 to 30 nm. The SEM and TEM images of Zn6La1, Zn4La1, and Zn2La1, shown in Figures 3d− f and 4c−f, reveal spherical particles forming chainlike agglomerates. The particle size increases with an increase in the proportion of La2O3 in Zn/La mixed oxides from 6:1 to 2:1. Figure 4 shows TEM images of Mg6La1, MgO, Zn6La1, Zn4La1, and Zn2La1. The particle size of the Mg6La1 catalyst varies from 15 to 40 nm, and that of MgO varies from 15 to 25 nm. The particle size varies with the lanthanum oxide composition. The particle size of Zn6La1 is 15−22 nm, while that of Zn2La1 ranges from 20 to 40 nm. Although the catalyst particle size can be obtained by both TEM and SEM methods, we find an isolated particle in TEM, with very good contrast, and hence we believe that the TEM results are more reliable. Figure 4d shows the elemental mapping of Zn and La in the Zn6La1 catalyst, which indicates a uniform distribution. The red color represents zinc oxide, and green represents lanthanum oxide. TPD. CO2-TPD was used to analyze the basic-site density and distribution. Figure 5 shows the results of pulse chemisorption of CO2 at room temperature on a Zn4La1 catalyst pretreated at different temperatures. At 25 °C, the adsorption of CO2 on the Zn4La1 surface is 121.9 μmol/g; CO2 adsorption increases with the pretreatment temperature, reaching a maximum in the range 500−600 °C and then decreasing. The other catalysts also exhibit similar behavior; i.e., the optimum pretreatment temperature range corresponds to 500−600 °C. CO2 pulses were introduced at room temperature until saturation. The temperature was varied (50−600 °C) at a rate of 10 °C/min in the presence of He gas (20 mL/min flow). Table 1 shows the results of CO2 pulse chemisorption for the different catalysts. The distribution of basic sites was calculated by Gaussian deconvolution of the TPD signal. An example of the TPD trace and deconvolution of the Zn4La1 sample is shown in Figure 6. The baseline correction was followed by the Gaussian fitting using Originpro 8.5 software for the catalyst sample. The percentages of weak and strong basic sites were calculated from the area obtained under the curves for the two peaks shown in Figure 6. The first peak corresponds to weak basic sites and the second to strong basic sites. The ZnO catalyst showed low CO2 adsorption (4.43 μmol/g), whereas ZnO/La2O3 mixed oxides showed higher CO2 adsorption. The adsorption of CO2 on the catalyst surface depends on the proportion of La2O3 in the mixed oxide and dispersion of the catalyst. Zn6La1, which has the highest surface area (50.8 m2/ g), showed high CO2 adsorption even with a lower proportion of La2O3 (25 wt %). Zn2La1, despite a higher La2O3 content (50 wt %), showed relatively lower CO2 adsorption, probably because of its relatively smaller surface area (25.5 m2/g). CO2TPD of ZnO, which has low CO2 adsorption (4.43 μmol/g), did not show any peak in the desorption profile. The extents of CO2 adsorption for MgO and Mg6La1 are 319.01 and 448.49

Figure 1. XRD of ZnO, La2O3, and Zn/La mixed oxides (*, La2O2CO3 phase; #, La2O3 phase; α, ZnO phase).

at 500 °C. Figure 2 shows the XRD patterns of MgO, Mg6La1, and La2O3. Mg6La1 exhibits both La2O3 and MgO peaks,

Figure 2. XRD patterns of the MgO, Mg6La1, and La2O3 catalysts (α, MgO phase; *, La2O2CO3 phase; #, La2O3 phase).

indicating that the catalyst is a mixture of lanthanum and magnesium oxides. Lanthanum oxide exists as the mixed phases of La2O2CO3 (JCPDS 83-1355) and La2O3 (JCPDS 83-1344). The XRD patterns reveal that La2O3 dominates over La2O2CO3 in the Mg6La1 catalyst. The average crystallite sizes of MgO in the pure and Mg6La1 mixed oxides were calculated by the Scherrer equation as 7.03 and 6.43 nm, respectively. Surface Area (BET). As mentioned before, the oxides and mixed oxides MgO, La2O3, ZnO/La2O3, and MgO/La2O3 were prepared by three different methods, viz., a modified-citrate technique (MCT), combustion (Comb.), and precipitation (P) method. Table S2 in the SI shows the BET surface areas, external surface areas, pore volumes, and average pore sizes of the catalysts prepared by different methods. Catalysts prepared by MCT and Comb. showed less porosity (the BET and external surface areas are comparable) than those prepared by C



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Figure 3. SEM images of (a) La2O3, (b) MgO, (c) Mg6La1, (d) Zn6La1, (e) Zn4La1, and (f) Zn2La1.

μmol/g, respectively, which were comparatively higher than those of other catalysts.

rates in terms of both conversion and selectivity to GC, mixed oxides of Zn and La offered even more promising results, giving 99% conversion and over 97% selectivity in 2 h. Knowing that the mixed oxides of Zn and La offer much improved performance, we varied the proportion of individual metals and compared the activities of Zn6La1, Zn4La1, Zn2La1, and La2O3 for the reaction of interest. Zn4La1 and Zn2La1 were found to perform much better than the other two catalysts (Figure 7a). Figure 7b shows that the GC selectivities in the presence of Zn2La1 and Zn4La1 were comparable and much higher than those of all other catalysts including Zn6La1. Zn6La1 offered 87.5% conversion with 96.1% selectivity in 4 h, compared with 97.7% conversion and 98.5% selectivity for Zn4La1 in only 1 h. The performance of Mg6La1 was slightly better than that of MgO, in terms of both conversion and selectivity. The initial reaction rate with the Zn4La1 catalyst was found to be 1.6 times higher than that for the Mg6La1 catalyst under similar conditions, even if CO2 adsorption on Mg6La1 (Table 1) was more than that on Zn4La1. This means that the sites on Zn/La oxides are more active for this reaction than those on Mg/La oxides. It may be noted that Simanjuntak et al.15 studied the Mg3La1 catalyst (the molar ratio of Mg/La is 3:1), which offered a GLY conversion of 70% with 92% selectivity of GC at 85 °C in 1 h. At this point, we relate the CO2-TPD results in Table 1 to the activities observed for the different catalysts of interest. It may be seen that both Zn4La1 and Zn2La1, which show comparable performances, have approximately similar extents of CO2 adsorption and distribution of strong and weak sites. Furthermore, it can be seen that the side reaction of the formation of glycidol from GC is related to the number of strong basic sites. The La2O3 catalyst has the highest number of

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LEACHING TEST The extent of leaching of La in the product mixture is crucial: less leaching allows a catalyst to last longer. A total of 2 mL of the reaction mixture, which consisted of DMC, GLY, methanol (METH), and GC, along with leached metal ions, was dissolved in 10 mL of distilled water. The extents of leaching from the catalysts La2O3, Zn2La1, Zn4La1, and Zn6La1 were measured by ICP-AES, which was first calibrated for La metal in an aqueous solution. Using this calibration, the quantitative analysis for La metal for different samples was performed. Table 2 shows the results of La metal leaching in the product for the different catalysts. The catalyst used in the autoclave was in powder form. The leaching of La2O3 in pure form exceeded that of the mixed-oxide catalysts. Increasing the zinc oxide composition reduced La leaching further. The reduced leaching for mixed oxides is attributed to metal−metal interaction.22 It is reported in the literature, based on X-ray photoelectron spectroscopy analysis, that there exists metal−metal interaction between La and Zn.19

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ACTIVITIES OF DIFFERENT CATALYSTS Transesterification of GLY with DMC in the absence of a catalyst is very slow; the reaction thus requires a catalyst to achieve substantial rates. To determine the activities of the catalysts, all of the reaction runs were performed under otherwise similar conditions. Table 3 shows that ZnO, CeO2, and Ce2O3 offered only 3−4% conversion of GLY, whereas MgO, PbO, and La2O3 showed a significant increase in the catalytic activity in terms of conversion at the end of 4 h. While the mixed oxides of magnesium (Mg) and La showed improved D



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Figure 4. TEM images of (a) Mg6La1, (b) MgO, (c) Zn6La1, (e) Zn4La1, and (f) Zn2La1 and (d) elemental mapping of Zn6La1.

Table 1. Adsorption of CO2 for Different Catalysts and the Distribution of Basic Sites

catalyst

CO2 adsorbed (μmol/g)

BET surface area (m2/g)

area % of weak basic sites

area % of strong basic sites

CO2 adsorbed on strong basic sites (μmol/g)

ZnO Zn6La1 Zn4La1 Zn2La1 La2O3 MgO Mg6La1

4.43 251.86 255.77 229.10 286.43 319.01 448.49

3.35 50.84 33.54 25.46 4.19 183.53 141.83

92.00 80.44 75.60 63.23 89.92 91.80

8.00 19.56 24.40 36.76 10.08 8.20

20.15 48.60 55.90 105.30 32.16 36.78

interpretation is based on limited data, and more investigations are necessary to support this inference. We now compare the activities of basic sites in each catalyst for the conversion of GLY. If we assume the number of active centers (the basic sites) to be proportional to the extent of CO2 adsorbed, then one can calculate the site activity in the

Figure 5. Pulse adsorption of CO2 for the Zn4La1 catalyst at different pretreatment temperatures.

strong basic sites (105.30 μmol/g) and shows maximum glycidol formation compared to other catalysts. This E



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Figure 6. TPD trace and deconvolution of the desorption curve for the Zn4La1 catalyst.

Table 2. Comparison of La Leaching in Different Catalysts with a Reaction Temperature of 130 °C, Mole Ratio (DMCto-GLY) of 4:1, and Catalyst Loading of 0.5 wt % catalyst

La (ppm)

La2O3 Zn2La1 Zn4La1 Zn6La1

1.4 0.45 0.36 0.31

Table 3. Catalytic Activities of Different Catalysts for GLY Transesterification with DMCa series no.

catalyst

GLY convn (%)

selectivity of GC (%)

yield of GC (%)

1 2 3 4 5 6 7 8 9

uncatalyzed CeO2 Ce2O3 MgO PbO ZnO La2O3 Mg6La1 *Zn4La1

3.04 3.5 2.95 96.62 52.46 4.1 97.3 98.12 98.5

100 100 100 96 95.99 100 84.8 96.31 97.2

3.04 3.5 2.95 92.75 50.35 4.1 82.5 94.5 95.7

Figure 7. (a) GLY conversion and (b) GC selectivity versus conversion of GLY using La2O3, MgO, Mg/La, and Zn/La mixedoxide catalysts with a reaction temperature of 150 °C, a DMC-to-GLY mole ratio of 5:1, and a catalyst loading of 0.5 wt % at 1000 rpm.

Reaction conditions: temperature, 150 °C; DMC-to-GLY mole ratio, 6:1; catalyst loading, 0.5 wt %; reaction time, 4 h. For *Zn4La1, the reaction time was 2 h. a

respective catalyst in terms of the reaction rate per mole of CO2 adsorbed (eq 3), a number that is equivalent to the turnover frequency (TOF). Figure 8 shows the activities of different catalysts, which follow the trend Zn2La1 > Zn4La1 > Zn6La1 > Mg6La1 > MgO. We have not included La2O3 in this analysis because its BET surface area (Table 1) is much lower than the one calculated by CO2-TPD (shown in the SI section S3), indicating the possibility of multilayer adsorption or subsurface diffusion of CO2 in La2O3.

Figure 8. Site activities (rate per unit CO2 adsorbed) of different catalysts at a reaction temperature of 150 °C, a DMC-to-GLY mole ratio of 5:1, and a catalyst loading of 0.5 wt % at 1000 rpm.

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CATALYST REUSABILITY The catalyst reusability was examined in an autoclave at 150 °C. Catalyst recovered from the first run was washed twice with acetone and dried for 6 h in an oven at 90 °C. The same procedure was followed for consecutive cycles. The conversion, under otherwise similar conditions, drops from 97.3 to 63.96% in the fourth reuse (Figure 9). It is anticipated that the reduced activity of the reused catalyst results from deposition of the

site activity =

initial rate of reaction (CO2 adsorbed per g of catalyst)(g of catalyst)

(3) F



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Effect of the Temperature. The reactions were performed over a temperature range of 120−140 °C. Figure 11a shows the

Figure 9. Reusability of the Zn4La1 catalyst at 150 °C with a catalyst loading of 1.0 wt % in 80 min and a mole ratio of DMC-to-GLY of 4:1.

reaction product(s) on the catalytic sites. To confirm this, after the fourth cycle, the catalyst was calcined at 500 °C for 3 h to oxidize the deposited hydrocarbons on the catalyst surface. It can be seen that the catalyst can be completely regenerated by calcination.

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REACTION KINETICS WITH ZN4LA1 In this section, we present the detailed kinetic studies in the presence of the Zn4La1 catalyst, which along with Zn2La1 exhibited the best performance in terms of both activity and selectivity. General Course of the Reaction. Figure 10 shows the general course of the reaction. This reaction mixture consisted

Figure 11. (a) Conversion of GLY at different temperatures and (b) selectivity of GC versus conversion of GLY at different temperatures with a DMC-to-GLY mole ratio of 4:1 and a catalyst loading of 0.5 wt % at 1000 rpm.

conversion of GLY at different temperatures: the rate of the reaction increases with an increase in the temperature. Figure 11b shows the selectivity of GC versus conversion of GLY at different temperatures. GC selectivity was not very sensitive to a change in temperature under the conditions of interest. Effect of the Catalyst Loading. The experiments were performed at three catalyst loadings: 0.25, 0.5, and 1.0 wt %. Figure 12a shows an increase in the conversion of GLY with an increase in the catalyst loading from 0.25 to 1.0 wt % (w/w). Figure 12b shows the trend in selectivity to GC versus conversion of GLY. The selectivity to GC is not much affected by the catalyst loadings; however, it can be seen in the case of a catalyst loading of 1.0 wt % that the selectivity of GC decreases drastically once the main reaction reaches near-equilibrium. Effect of the Mole Ratio. The reactions were performed at mole ratios (DMC-to-GLY) of 2:1, 4:1, and 6:1. Figure 13 shows the results. The rate increases when the mole ratio increases from 2:1 to 6:1. Interestingly, the GC selectivity was found to be lower at lower mole ratios. This is because the rate of decomposition of GC is proportional to its instantaneous concentration of carbonate and high mole ratios result in a dilution effect, thereby suppressing the side reaction.

Figure 10. General course of the reaction on the Zn4La1 catalyst at 140 °C with a mole ratio of DMC-to-GLY of 4:1 and a catalyst loading of 0.5 wt %.

of four main components (GLY, DMC, METH, and GC) and only one side product, i.e., glycidol. The mole fractions of DMC and METH were very high compared to those of GLY, GC, and glycidol. Hence, for clarity, the reaction progress is shown only in terms of the mole fractions of GLY, GC, and glycidol with respect to the reaction time. Figure 10 shows that formation of glycidol increases with an increase in conversion. Initially, glycidol formation is small (before 60 min), but after GLY conversion reached near-equilibrium (at 60 min), glycidol formation picks up because of decomposition of GC. G



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Figure 13. (a) Conversion of GLY at different mole ratios of DMC-toGLY and (b) selectivity of GC versus conversion of GLY at different mole ratios with a reaction temperature of 130 °C and a catalyst loading of 0.5 wt % at 1000 rpm.

Figure 12. (a) Conversion of GLY at different catalyst loadings and (b) selectivity of GC versus conversion of GLY at different catalyst loadings with a reaction temperature of 130 °C and a mole ratio of 4:1 at 1000 rpm.

Kinetic Modeling. Transesterification of GLY with DMC is a reversible reaction. The reaction scheme (eqs 4 and 5) consists of two steps. In the first step, GLY and DMC form GC and METH; in the second step, GC decomposes over the catalyst surface to form glycidol and CO2. Decomposition of GC over the catalyst surface is irreversible.

1 dnDMC = rGLY Wcat. dt

(4)

rGLC =

(5)

(6)

(7)

1 dnGC Wcat. dt

rMETH =

catalyst, f3

glycerol carbonate ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ glycidol + CO2

rDMC =

= k f1xGLYx DMC − k f2xGCxMETH 2 − k f3xGC

catalyst, f1 f2

1 dnGLY = k f2xGCxMETH 2 − k f1xGLYx DMC Wcat. dt

rGC =

glycerol + dimethyl carbonate HoooooooooI glycerol carbonate + 2methanol

rGLY =

1 dnMETH = −2rGLY Wcat. dt

1 dnGLC = k f3xGC Wcat. dt

(8)

(9)

(10)

where

The reactions were performed at different speeds of agitation of 600, 800, and 1000, and it was found that, beyond 800 rpm, there is no further effect of agitation on the rate of reaction showing no external mass-transfer limitations. In this reaction, the catalyst was taken in powder form with a particle size of less than 30 nm, for which the value of the Weisz−Prater parameter is much less than 1, indicating that internal diffusion limitations can also be neglected. The rates of consumption of GLY and DMC are given by eqs 6 and 7; the rates of formation of METH, GC, and glycidol are given by eqs 8−10.

k f1 = k f10 exp( −Ef1/RT ) k f2 = k f2 0 exp( −Ef2 /RT ) k f3 = k f30 exp( −Ef3/RT )

The kinetic parameters are estimated by minimizing the sum of squares of error between the calculated values of the mole fractions of different components and that observed through the experiments. H



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Table 4. Estimated Values of the Kinetic Parameters for the Reaction of Interest series no.

reaction

kf0 (kmol/kg·s)

E (kJ/mol)

MSE

1

DMC + GLY → GC + METH

1.18 × 1010

98.3

3.79 × 10−3

2

GC + METH → DMC + GLY

1.04 × 107

77.5

3.79 × 10−3

3

GC → GLC + CO2

2.78 × 109

127.7

1.08 × 10−10

min ϕ =

∑ samples

(xi ,cal − xi ,exp)2

activity of the catalyst. The rate of GLY conversion increased with temperature, catalyst loading, and DMC-to-GLY molar ratio. The selectivity is higher at high mole ratios of DMC-toGLY. A kinetic model, which considers the reaction to be reversible and of the second-order with respect to GLY and DMC, was proposed, and its predictions agreed well with the experimental results. Decomposition of carbonate followed first-order kinetics. The activation energy values of the forward and backward reactions of GC were estimated to be 98.3 and 77.5 kJ/mol, respectively; the same for glycidol formation was 127.7 kJ/mol.

(11)

MATLAB function “nlinfit”, which is based on the Levenberg− Marquardt algorithm, is used in regression analysis. The estimated values of the relevant parameters are given in Table 4. The activation energy values for all of the reactions are very high (98.3 kJ/mol), which proves that our assumption of neglecting mass-transfer or internal diffusion limitations was correct. There is not much literature reported on the kinetic modeling of this reaction. Herseczki et al.23 conducted the reaction at much lower temperatures (60−80 °C) and considered this to be a two-step reaction. In the first step, an intermediate is formed and converts into GC in the second step. In our case, we did not detect any such intermediate product during the reaction. A parity plot for mole fractions of DMC, GLY, METH, GC, and GLC for the runs performed at different temperatures, catalyst loadings, and mole ratios of DMC-to-GLY is shown in Figure 14. The parity plot shows good agreement between the experimental data and model prediction.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1 and S2 and calculation of the La2O3 catalyst surface area using an adsorbed CO2 molecule from CO2-TPD analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +9122 2576 7246. Fax: +9122 2572 6895. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge SAIF, IIT, Bombay, India, for their help in the characterization of catalysts and Ashutosh Namdeo for his assistance in TPD analysis.

■ Figure 14. Parity plot comparing the predicted and experimental values of the mole fractions of different components in all of the experiments.

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CONCLUSION ZnO/La2O3 mixed oxide offers a higher reaction rate than the reported MgO/La2O3 with comparable selectivity of GC. Leaching of La2O3 in the reaction medium was less for ZnO/ La2O3 mixed oxide. Zn4La1 and Zn2La1 were found to be the most promising catalysts, giving reasonably high selectivity and conversion. It appears that the activity for glycidol formation is proportional to the number of strong basic sites. The performance of the Zn4La1 catalyst was evaluated in detail. It was reused four times, and the activity decreased during each cycle because of adsorption of the carbonaceous side products on the catalytic basic sites. However, calcination restored the

NOMENCLATURE Ef activation energy of the forward reaction, kJ/mol kf0 Arrhenius preexponential factor for the forward reaction rate constant, kmol/min·L kf1 forward reaction rate constants, kmol/kg·min kf2 backward reaction rate constants, kmol/kg·min n initial molar holdup, kmol ri rate of the reaction of species i, kmol/min R ideal gas constant, kJ/kmol·K t time, min T temperature, K xi mole fraction of species i in the liquid phase Wcat. weight of the catalyst, kg

Abbreviations

DMC dimethyl carbonate GLY glycerol METH methanol GC glycerol carbonate GLC glycidol TPD temperature-programmed desorption XRD X-ray diffraction TEM transmittance electron microscopy SEM scanning electron microscopy I



Article

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ICP-AES inductive couple plasma atomic emission spectroscopy BET Brunauer−Emmett−Teller FID flame ionization detector MSE mean of squares of errors MCT modified-citrate technique Comb combustion P precipitation TOF turnover frequency

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Lanthanum Mixed-Oxide Catalyst for the Synthesis of Glycerol

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