The value-added utilization of glycerol for the synthesis of glycerol carbonate catalyzed with a novel porous ZnO catalyst

Abstract

In the carbonylation reaction, a novel porous ZnO was prepared by a calcination method, and the raw material Zn glycerolate platelets were prepared via the glycerol approach, which could make use of a by-product of glycerol. To elucidate their composition, morphology, and properties, the resulting materials were characterized by FT-IR, XRD, SEM, BET, XPS, TPD and TG. The results showed that the catalyst was porous and irregularly shaped with appropriate acid and base properties; moreover, it displayed better catalytic performance for the synthesis of glycerol carbonate. The highest glycerol carbonate yield reached 85.97% of ZnO from zinc glycerolate under the optimal reaction conditions of 5.0 wt% of catalyst, 1 : 1.5 molar ratio of glycerol to urea and reacting at 140 °C for 6 h under 1 kPa. Comparing the three catalysts ZnO, zinc glycerolate and ZnO from zinc glycerolate, the maximum glycerol carbonate yield was 85.97% with the ZnO from zinc glycerolate as the catalyst under optimized operating conditions. Compared with the conventional ZnO, the as-prepared catalyst embodied in its porosity, acidity and basicity. The catalyst maintained excellent catalytic performance after 5 cycles with almost no loss of catalytic activity. This study revealed that ZnO from a zinc glycerolate catalyst is highly active, highly recyclable, remarkably stable, and environmental friendly for industrial applications. Overall, this new material overcomes the limitation of glycerol application and will have a good potential for industrial application.

---

1. Introduction

The sharp increase in demand for fuels and the increase in environmental problems, coupled with the reduction of crude oil reserves, have attracted peoples' attention about renewable energy.¹ Biodiesel as a renewable energy has developed very well in recent years.² In fact, the production of biodiesel is still relatively far away from the intended target, which is due to the high cost of production. In order to maintain the balance of the existing glycerol market, improve the added value of glycerol, and promote the development of the biodiesel industry, many researchers focus on the study of valued-added glycerol derivatives. As a result, glycerol as a biodiesel by-product is a viable method to reduce the cost of biodiesel production.^3–5

Conversion processes for glycerol as a renewable and cheap raw chemical for high value-added products have attracted more and more peoples' attention.⁶ Glycerol carbonate is a small molecule with a bifunctional compound containing a dioxolane ring and hydroxyl group.^7,8 The bifunctional compound as reactive sites make glycerol carbonate a raw material for the synthesis of chemical intermediates achievable. Glycerol carbonate can react both as a nucleophile through its hydroxyl group, and as an electrophile through its ring carbon atoms. Glycerol carbonate can be used in solvents, beauty and personal care products, chemical intermediates, and polymers such as hyper-branched polyethers, polycarbonates, polyurethanes and non-isocyanate polyurethanes. Glycerol carbonate has many potential applications in manufacturing useful materials where it is used as a carrier in pharmaceutical preparations, lithium and lithium-ion batteries, and solid laundry detergent compositions.⁹

The synthesis of glycerol carbonate is carried out directly from glycerol and subcritical CO₂ using Cu/La₂O₃,¹⁰ nBu₂SnO,¹¹ and La₂O₂CO₃–ZnO¹² catalysts. Glycerol carbonate can be synthesized through the reaction of glycerol with CO and O₂ in the presence of PdCl₂(phen)/KI catalysts.¹³ Glycerol carbonate could be synthesized with several different synthetic routes such as the transesterification reaction of glycerol and dimethyl carbonate or diethyl carbonate using K₂CO₃,¹⁴ CaO¹⁵ and hydrotalcite¹⁶ and the carbonylation reaction of glycerol and urea in the presence of catalysts such as metal oxides,^17,18 WO₃/SnO₂,¹⁹ ionic liquids,²⁰ lanthanum compounds,²¹ gypsum based catalyst,²² Zn–Al mixed-oxide catalyst,²³ WO₃/TiO₂,²⁴ tantalum in heteropoly tungstate catalysts,²⁵ γ-zirconium phosphate,^26–28 calcined Zn hydrotalcite,²⁹ Co₃O₄/ZnO,³⁰ gypsum and gold-based catalysts.^31,32 CO and phosgene do not agree with the requirements of sustainable development. The use of carbon dioxide as a carbonating agent required high temperature and high pressure and the yield for glycerol carbonate was very low for practical purposes.

In our study, a novel solid catalyst was used for catalyzing glycerol and urea to synthesize the high value-added glycerol carbonate. Attempts are being focused on using urea due to its cheaper cost and safe handling. Furthermore, ammonia that is formed can be easily converted to urea since urea synthesis is performed from ammonia and carbon dioxide. The performances of the three catalysts ZnO, zinc glycerolate, and ZnO from zinc glycerolate were compared under optimized operating conditions. The catalyst was also characterized by FT-IR, XRD, SEM, BET, TPD, XPS and TG. In addition, the combination of a weak Lewis acid and Lewis base was found in the carbonate formation. In the reaction, glycerol can be used not only as the reactant but also as the catalyst precursor.

2. Experimental section

2.1 Materials

Anhydrous glycerol of high purity (>99%), urea (>99%), absolute ethanol (>99%), acetic acid (99%), zinc nitrate hexahydrate Zn(NO₃)₂·6H₂O (99%), zinc acetate dihydrate Zn(CH₃COO)₂·2H₂O (99%), oxalic acid (99%), potassium hydroxide KOH (99%) and zinc chloride (99%) were obtained from Sinopharm Chemical Reagent Co, Ltd, China.

2.2 Catalyst preparation

Zn(CH₃COO)₂·2H₂O (2 g) was mixed with glycerol (20 g) at 160 °C under reflux for 5 h. The resulting mixture was transferred into a Teflon-lined stainless steel autoclave and kept at 150 °C for 18 h. The product mixture was filtered, washed with distilled water, and dried at 80 °C in air overnight. Then, a white fine powder of Zn glycerolate was obtained. The as-prepared samples were labeled as ZMG and were heated with different thermal procedures (i.e. heating rates). Finally, the synthesized precursor ZMG was decomposed to ZnO by calcination at 450 °C in an N₂ flow for 2 h (heating temperature rate was set to 0.5 °C min⁻¹). The optimal thermal method was conducted at 450 °C at 0.5 °C min⁻¹.

In the 0.5 mol L⁻¹ zinc nitrate solution, potassium hydroxide was added to prepare zinc hydroxide. The synthesized precursor Zn(OH)₂ was decomposed to ZnO by calcination at 450 °C in air for 2 h.

2.3 Catalyst characterization

For the solid catalyst, 1 mg of sample and 200 mg of KBr were ground completely and pressed into thin disks. Subsequently, the FT-IR of samples were tested by a FTLA2000 Fourier transform infrared spectrometer (Canadian ABB) with a scanning range 4000–500 cm⁻¹.

X-ray diffraction (XRD) patterns were examined with a Bruker D8 Advance powder diffractometer using a Cu Kα radiation source (λ = 1.5406 Å) at 40 kV and 40 mA from 10° to 90° with a scan rate of 4° min⁻¹.

The morphology of the catalyst was evaluated by field emission scanning electron microscopy (FE-SEM) on a Hitachi S-4800.

The specific surface area and pore diameter of the catalysts were measured according to the Brunauer–Emmet–Teller (BET) method with nitrogen adsorption–desorption with an ASAP 2020 instrument (Micromeritics, USA).

Temperature programmed desorption of CO₂ (CO₂-TPD) was measured to determine the basicity and base intensity distribution of the catalysts. The sample was pretreated under a helium flow at 300 °C for 1 h. Subsequently, it was cooled to 50 °C to adsorb CO₂. After adsorption of CO₂ for 30 min, the catalyst was pretreated under a helium flow at 100 °C for 1 h to remove the adsorbed CO₂ from the sample surface. A desorption curve was recorded from 100 °C to 800 °C at 10 °C min⁻¹.

Temperature programmed desorption of NH₃ (NH₃-TPD) was performed to determine the acidity and acid intensity distribution of the catalysts. The sample was pretreated under a helium flow at 300 °C for 1 h. Subsequently, it was cooled to 50 °C to adsorb NH₃. After adsorption of NH₃ for 30 min, the catalyst was pretreated under a helium flow at 100 °C for 1 h to remove the adsorbed NH₃ from the sample surface. A desorption curve was recorded from 100 °C to 800 °C at 10 °C min⁻¹.

X-ray photoelectron spectroscopy (XPS) analysis was conducted using an ESCALAB 250 Xi (Thermo, USA) X-ray photoelectron spectrometer with an Al Kα line as the excitation source (hν = 1484.6 eV) and adventitious carbon (284.6 eV for binding energy) was used as a reference to correct the binding energy of the sample.

TG analysis was performed with a STA409 instrument in dry air at a heating rate of 20 °C min⁻¹.

2.4 Catalytic reaction

The carbonylation reaction was carried out in a 100 mL three-necked flask with a magnetic stirrer. An aliquot of 25 mmol glycerol, 37.5 mmol urea and a certain amount of prepared catalysts (5 wt% to glycerol) was added into the flask. The carbonylation reaction was performed at 140 °C for 6 h under stirring, under reduced pressure of under 1 kPa. After completion of the reaction, the mixtures were cooled to ambient temperature. Then, 15 mL of methanol was used to wash the catalyst. Subsequently, the reaction mixtures were separated from the catalyst by centrifugation and dried for 12 hours. The supernatant was used for gas chromatography analysis. The washed catalyst was used in the next circulation.

2.5 Product analysis

The supernatant was analyzed by gas chromatography 9790II. A quantitative analysis method of glycerol and glycerol carbonate was performed using a SE-54 capillary column with an FID detector using tetraglycol as an internal standard.^33–36 The injection volume of the sample was 0.2 μL. The temperature program increased from 60 °C to 260 °C, using a silyl reagent in the analysis. The silylation agent^37,38 composed of 4 mL DMF, 4 mL dioxane amine hexamethyldisilazane and 0.1 mL trimethylchlorosilane. A 100 μL aliquot of the sample was derivatized by adding the silylation reagent in a tube. Subsequently, the tube was vibrated for 1 min and allowed to stand. The supernatant was analyzed by gas chromatography.

3. Results and discussion

3.1 Characterization of catalysts

3.1.1. FT-IR analysis. Before the detailed results with the three solid catalysts are presented, the activity of several heterogeneous catalysts was compared. The yield to glycerol carbonate was <100%, mainly due to the formation of a by-product. From the test results, ZnO from ZMG had a better catalytic activity due to its porous structure. The results are displayed in Fig. 1. The FTIR spectra of ZnO, ZMG and ZnO from ZMG catalysts are shown in Fig. 1. Zn glycerolate platelets were obtained via the glycerol-mediated synthesis utilizing a solution of Zn(CH₃COO)₂ in glycerol that was heated to 150 °C. At about 120 °C, the solution became turbid, which indicates the precipitation of solid Zn glycerolate. Infrared spectra (FT-IR) of the as-prepared ZMG catalyst with a plate-like structure were also in complete agreement with the reference data of Zn glycerolate. The narrowing of the glycerol bands in Zn glycerolate was due to the crystalline nature of the material. The characteristic signals of the alcoholic C–O stretching mode were present at 1054 and 1124 cm⁻¹. The peaks at 1468 and 1675 cm⁻¹ were attributed to the O–H bending mode. The broad absorption peak at 3443 cm⁻¹ was attributed to the –OH group, such as water molecules in the catalyst interlayer region, glycerol O–H stretching mode and hydrogen bonding. The band at 1943 cm⁻¹ was assigned to the C–O stretching mode, where the oxygen atom is involved in a hydrogen bond, proving the existence of the zinc glycerolate phase. The characteristic signals of the Zn–O in zinc glycerolate were present at 650.89 cm⁻¹ in Fig. 2(a), indicating the presence of a Zn–O bond in the sample. The peak of ZnO from ZMG was consistent with ZnO. In fact, ZnO from ZMG and ZnO had characteristic signals at 650.89 cm⁻¹. The surface of the ZnO sample had bending and stretching vibrational peaks for a hydroxyl group at 1632.78 cm⁻¹ and 1115.46 cm⁻¹ in Fig. 2(b) and (c). In air, the sample surface adsorbed water, resulting in the formation of the hydroxyl peak.

Fig. 1 Comparison of activity of three catalysts (reaction conditions: glycerol/urea = 1 : 1.5, catalyst dosage 5.0 wt%, 140 °C, 6 h, 1 kPa).

Fig. 2 FTIR spectra of three catalysts: (a) ZMG; (b) ZnO from ZMG; (c) ZnO.

3.1.2. XRD analysis. The three different solid base catalysts with XRD patterns is shown in Fig. 3. The XRD profile of the synthesized ZMG with typical layered compounds showed an intense basal diffraction attributed to the first basal peak in the region from 11° at 2θ values. From Fig. 3(b), compared with the standard card, the prominent crystalline phase of ZMG catalysts are attributed to Zn glycerolate. The XRD pattern for the solid catalyst presented characteristic peaks located at 2θ = 11.08°, 17.03°, 20.9°, 27.79°, 36.67°, 38.83°, according to the JCPDS file no. 23-1975, which are assigned to the crystal phase of Zn glycerolate (Zn(C₃H₆O₃)).^39–42

Fig. 3 XRD patterns of three solid catalysts (a) ZnO obtained from ZMG; (b) ZMG; (c) ZnO.

Upon subsequent heating of the sample to 450 °C, the evolution of the ZnO phase (JCPDS file no. 65-3411)⁴³ occurred, as seen in Fig. 3(b). Fig. 3(b) depicts the diffraction profile of the calcined sample, and all the peaks were well indexed to the ZnO phase (JCPDS file no. 65-3411; Fig. 3(b)). ZnO that was calcined from ZMG with a platelet-like structure had the larger surface area. ZnO obtained from Zn(OH)₂ was identified as the ZnO phase (JCPDS file no. 65-3411) in Fig. 3(c). However, the morphology of ZnO obtained from Zn(OH)₂ was inferior to ZnO obtained from ZMG.

3.1.3. SEM analysis. The morphology of the precursor ZMG before and after calcination analysed by scanning electron microscopy is shown in Fig. 4. The surface morphologies of the ZMG samples were recorded using field emission scanning electron microscopy (FE-SEM) analysis (Fig. 4(a) and (b)). It can be clearly seen that a large number of plate-like microcrystals of zinc glycerolate were formed. In addition, according to the SEM images, the morphology of ZMG was plate-like, irregular and polygonal. From the figure, we can see that after calcination, the sample had a porous layer structure Fig. 4(c) and (d).

Fig. 4 SEM of three solid catalysts (a and b) ZMG; (c and d) ZnO from ZMG; (e and f) ZnO.

We speculate that the major change in morphology of the sample can be explained due to the loss of organic ligands, resulting from the phase transition from ZMG to ZnO at high temperature, which was consistent with the XRD analysis as described earlier.³⁹ At the same time, the micrographs reveal that most of them are porous and irregularly shaped. The precursor ZMG after calcination transformed into ZnO. ZMG with plate-like morphology and its layered structure resulted in poor catalytic performance due to the absence of porosity in the plate. The precursor ZMG after calcination had a porous layer structure that contributed to its excellent catalytic performance. The morphology of ZnO displayed that the samples were stacked together in Fig. 4(c) and (d). That resulted in the formation of nonporous materials, which was the reason of poor catalytic performance.

3.1.4. BET analysis. Three different catalysts were analyzed by BET analysis (Table 1). The specific surface area and pore size were described by the N₂ adsorption isotherm. Structural features improved significantly due to calcination, which may be attributed to the improved crystalline phase of the catalyst. Based on pore size, the catalyst is divided into microporous, mesoporous and macroporous regions. Moreover, the high absorption at low relative pressure represents the microporous nature of materials. Among the three catalysts, the ZnO from ZMG catalysts possess the largest pore diameters and specific surface areas. As seen from the BET analysis, the pore size of ZnO from the ZMG catalyst was 48.156 nm, which matched with the characteristics of mesoporous materials, suggesting that the catalyst was composed of mesoporous structures. It was observed that the surface area of ZMG was less than ZnO from ZMG, which could be attributed to the porous structure. The specific surface area and pore volume of ZnO obtained from Zn(OH)₂ was poor (Table 1). Mesoporous materials with relatively large specific surface area, pore size and regular pore structure can handle larger molecules or groups and improve the diffusion rate of the reactants. The BET results are consistent with the results of SEM. The porous structure of catalyst makes the catalyst a promising candidate for the industrial production of glycerol carbonate.

Table 1 BET surface area, pore volume and pore diameter of three catalysts

Catalyst	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
ZnO from ZMG	13.71	0.04531	48.15
ZMG	3.698	0.03752	16.15
ZnO	1.178	0.004923	7.568

---

3.1.5. Acid and base properties of catalysts. CO₂-TPD and NH₃-TPD were adopted to measure the basicity and the acidity of the catalysts. Largely, the catalytic performance of catalyst was affected by the basicity and acidity of catalyst (Fig. 5(a) and (b)). The CO₂ desorption capacity is proportional to the amount of base and the CO₂ desorption temperature represents the strength of basic sites. The CO₂ desorption curve is shown in Fig. 5(a). The ZnO catalyst exhibited a broad desorption peak at 400 °C. These desorption peaks are related to the weak basic sites in the catalyst. ZnO from the ZMG catalyst also presented a broad desorption peak in at 400 °C. The integrated area of ZnO from the ZMG catalyst was 4041.614. The integrated area of the ZnO catalyst was 2916.536. Thus, from the integrated area, ZnO from the ZMG catalyst had a high peak area, which showed a stronger basicity.

Fig. 5 (a) TPD of CO₂ profiles of two catalysts: ZnO and ZnO from ZMG solid catalysts. (b) TPD of NH₃ profiles of two catalysts: ZnO and ZnO from ZMG solid catalysts.

The NH₃ desorption capacity is proportional to the amount of acid and the NH₃ desorption temperature represents the strength of acid sites. For acidity, the NH₃ desorption curve is shown in Fig. 5(b). The ZnO catalyst exhibited a broad desorption peak in the range of 250–350 °C. These desorption peaks are related to the weak acid sites in the catalyst. ZnO from ZMG catalyst also presented two broad desorption peak in 200–500 °C and 550–800 °C. These desorption peaks at 200–500 °C are related to the weak acid sites in the catalyst and the desorption peaks at 550–800 °C are related to the strong acid sites in the catalyst. The NH₃ desorption integrated area of the ZnO from the ZMG catalyst was 5413.714. The NH₃ desorption integrated area of the ZnO catalyst was 3132.736. Thus, from the integrated area, ZnO from ZMG catalyst had a high peak area, which showed a stronger acidity. It can been seen from the integrated area that the acidic property was nearly close to the basic property of ZnO from ZMG. In the carbonylation reaction, the Lewis acid catalyst activated the urea carbonyl group and the hydroxyl group of glycerol was activated by the conjugated basic sites. The results showed that the adequate combination of a weak Lewis acid and a Lewis base plays an important role in the high yield of glycerol carbonate.

The amount of base and acid of ZnO was inferior to that of ZnO from ZMG.

3.1.6. XPS analysis. To gain more information about the ZnO nanoparticle, XPS was performed.⁴⁴ Fig. 6(a) exhibited the XPS survey spectrum of the ZnO nanoparticle. Fig. 6(a) shows that XPS consists of Zn, O and trace amounts of carbon, which were mainly attributed to adventitious hydrocarbon from the XPS itself. Fig. 6(b) showed that the peaks of the Zn 2p were found to be at 1021.68 eV and 1044.3 eV for Zn 2p_1/2 and Zn 2p_3/2, respectively.⁴⁵ We obtained the binding energy of ZnO in the experiment, which was consistent with previously reported values in the literature for Zn 2p for ZnO. No obvious shift of the Zn 2p peaks is observed and the peak positions closely match with the standard values for ZnO, indicating that Zn atoms are in the 2+ oxidation state.⁴⁶ In Fig. 6(c), the peak of O 1s was located at about 530.45 eV. The peak at 530.45 eV could be attributed to the bond formation between O and Zn. Namely, the peak at 530.45 eV was attributed to O²⁻ ions in ZnO.⁴⁷

Fig. 6 XPS of ZnO from ZMG solid catalysts.

3.1.7. TG analysis. Thermal decomposition of Zn glycerolate had only a significant weight loss, which corresponded to the decomposition of organic compounds with a weight loss rate of 37.28% (Fig. 7). From the DTG curves of Zn glycerolate, there was a maximum weight loss for Zn glycerolate at 412 °C. Namely, at this temperature, the decomposition of Zn glycerolate was fastest. This value matched exactly when assuming a thermal decomposition of Zn glycerolate in air under the formation of ZnO. As can be seen from the TG analysis, a temperature of 450 °C was required for complete decomposition. Moreover, to prepare a catalyst with excellent hole-shaped morphology, we needed to control the calcination rate. A high temperature and high calcination rate will lead to the catalyst sintering. Therefore, Zn glycerolate was calcined at 450 °C with a calcination rate of 0.5 °C min⁻¹.

Fig. 7 Thermogravimetry of Zn glycerolate (total weight: 8 mg, heating rate: 20 °C min⁻¹).

3.2 Catalytic activity

3.2.1. Effect of reaction temperature. The reaction temperature was one of the most important factors investigated, which could largely affect the carbonylation reaction. The catalytic activities of the ZnO catalysts were studied with the reaction temperatures varying from 120 to 160 °C. The yield of glycerol carbonate is shown in Fig. S1.† Increasing the reaction temperature would improve the catalytic reactivity. The results showed that the yields of glycerol carbonate firstly increased with an increase in reaction temperature and then decreased with further heating with a turning point of 140 °C. In summary, the optimum reaction temperature is 140 °C. Chemical equilibrium constant calculations show that the high temperature and low pressure are appropriate to synthesize glycerol carbonate by glycerol and urea.

3.2.2. Effect of urea-to-glycerol molar ratio. It is well known that the carbonylation reaction is an equilibrium reaction and that the excess amount of glycerol can drive the equilibrium toward product formation. In this reaction, glycerol reacted with urea in a molar ratio 1 : 1.5 at 140 °C in the presence of a catalyst and usually under vacuum to shift the thermodynamic equilibrium by continuously removing the ammonia formed. The carbonylation reaction was performed by varying the urea to glycerol molar ratios from 1 : 1 to 3 : 1 and at 140 °C until the completion of the reaction (see Fig. S2 in ESI†). In the first stage, the yields of glycerol carbonate gradually increased until the maximum values were reached. Further increases in the molar ratio of urea-to-glycerol beyond its optimum value would decrease the yield of glycerol carbonate. Hence, a 1.5 : 1 molar ratio of urea to glycerol was selected for optimizing urea-to-glycerol molar ratio parameters.

3.2.3. Effect of reaction time and dosage of catalyst. The reaction time was known to be one of the important parameters, which could affect the fame yield. The glycerol carbonate yields were investigated by varying the reaction time from 3 to 7 h. As shown in Fig. S3,† the maximum yield of glycerol was obtained at 6 h. As depicted, the formation of GC proceeds in two consecutive steps. Carbamate was formed in the first step of the reaction and glycerol carbonate was the end-product.⁴⁸ With further increase in the reaction time, the generated glycerol carbonate would decompose into glycidol. Hence, the optimal reaction time for glycerol carbonate production was 6 h. The catalyst amount varied between 0.023 and 0.253 g (1–11 wt% with respect to glycerol amount), whereas the other reaction conditions were fixed, with a molar ratio of glycerol/urea of 1 : 1.5, reaction temperature of 140 °C, 1 kPa and reaction time of 6 h in Fig. S4.† According to Fig. S4,† it is clear that addition of catalyst ameliorated the yield of GC.

3.3 Catalyst stability tests

The stability of the catalyst ZnO from ZMG with porous, irregular shaped was examined by testing the reusability of the catalyst. The recyclability of the catalyst is very important for industrial and technological applications. Catalyst recycling is an important step as it minimizes the cost of the process. The ZnO catalyst in each reaction was separated by the separator, washed with methanol and dried fully before reutilization. The stability of the catalyst was investigated (Fig. 8). The glycerol carbonate yield could remain 70.85% after the catalyst was repeatedly used for five times. Recycling experiments were executed by employing it in the consecutive reaction for 6 times under the same conditions. After each reaction, the catalyst was fully washed with methanol over again and dried in an oven overnight. In order to further explore the superiority of the ZnO catalyst on the catalytic performance, a comparison of ZnO, pure ZnO and ZMG had been repeated. The retained performance illustrated that the activity of catalyst still exists after the circulation. The catalyst still showed excellent catalytic performance without significantly weakening. The particle size of the porous ZnO catalyst was maintained between 220 nm and 235 nm. The polydispersibility of the porous ZnO catalyst was less than ZnO, which demonstrated the excellent dispersion performance of the catalyst (Fig. S5†). As seen by the XRD characterization, the structure of the catalyst was essentially maintained during the recycling tests, and it was helpful for the good recycling performance. Therefore, the porous ZnO catalyst that is highly active, highly recyclable, remarkably stable, and environmental friendly can be appropriately applied in industrial settings.

Fig. 8 Comparison of reusability of three catalysts (reaction conditions: glycerol/urea = 1 : 1.5, catalyst dosage 5.0 wt%, 140 °C, 6 h, 1 kPa).

4. Conclusions

In short, the heterogeneous catalysts prepared in this study can be applied for the synthesis glycerol carbonate in industrial production. Compared with different catalysts, the maximum glycerol carbonate yield is 85.97% with ZnO from ZMG as the catalyst under optimized operating conditions. Reaction tests indicated that the catalysts with appropriate acid and base properties were favorable for the synthesis of glycerol carbonate. The catalyst still has excellent catalytic performance after 5 cycles with almost no loss of catalytic activity. The study revealed that the ZnO catalysts have the advantages of high activity, high recyclability, high stability, and environmental friendliness that can be appropriately applied in industrial settings.

A green co-generation of high purity of zinc glycerolate (ZMG) and glycerol carbonate (GC) was established using glycerol as the main raw material. In this study, glycerol can be relevant for applications such as reactants and as a raw material to prepare the catalyst. Furthermore, the reaction product of ammonia can also be used in the next reaction to synthesize urea. We hope that our work can make a contribution to the industrialization of glycerol carbonate.

Acknowledgements

The financial support from the National Natural Science Foundation of China (NSFC) (No. 21306063), the Fundamental Research Funds for the Central Universities (JUSRP51623A), the Key Research and Development Program of Jiangsu Province (Industry Outlook and Common Key Technologies) (BE2015204), the Fundamental Research Funds for the Central Universities (JUSRP51507), the and MOE & SAFEA for the 111 Project (B13025) are gratefully acknowledged.

References

1. A.-R. Go, Y.-R. Lee and Y.-H. Kim, et al., Enzyme Microb. Technol., 2013, 53(3), 154–158.
2. H.-S. Jung, Y.-R. Lee and D. Kim, et al., Enzyme Microb. Technol., 2012, 51(3), 143–147.
3. N.-T. Nguyen and Y. Demirel, Int. J. Chem. React. Eng., 2011, 9.
4. M.-M. Fan, J. Yang, P.-P. Jiang, P.-B. Zhang and S.-S. Li, RSC Adv., 2013, 3, 752.
5. F.-X. Yang, M.-A. Hanna and R.-C. Sun, Biotechnol. Biofuels, 2012, 5, 13.
6. W.-K. Teng, G.-C. Ngoh, R. Yusoff and M.-K. Aroua, Energy Convers. Manage., 2014, 88, 484–497.
7. M.-O. Sonnati, S. Amigoni and E.-P. Givenchy, et al., Green Chem., 2013, 15, 283.
8. A. Dibenedetto, A. Angelini and M. Aresta, et al., Tetrahedron, 2011, 67, 1308–1313.
9. J. Ochoa-Gómez, O. Gómez-Jiménez-Aberasturi, C. Ramírez-López and M. Belsué, Org. Process Res. Dev., 2012, 16, 389–399.
10. J. Zhang and D.-H. He, J. Colloid Interface Sci., 2014, 419, 31–38.
11. J. George, Y. Patel, S.-M. Pillai and P. Munshi, J. Mol. Catal. A: Chem., 2009, 304, 1–7.
12. H.-G. Li, D.-Z. Gao, P. Gao, F. Wang and N. Zhao, et al., Catal. Sci. Technol., 2013, 3, 2801.
13. J.-L. Hua, J.-J. Li, Y.-L. Gua, Z.-H. Guan and W.-L. Mo, et al., Appl. Catal., A, 2010, 386, 188–193.
14. G. Rokicki, P. Rakoczy, P. Parzuchowski and M. Sobiecki, Green Chem., 2005, 7, 529–539.
15. J.-R. Ochoa-Gómez, O.-A. Gómez-Jiménez-Aberasturib and B. Maestro-Madurgab, et al., Appl. Catal., A, 2009, 366(2), 315–324.
16. A. Takagaki, K. Iwatani, S. Nishimura and K. Ebitani, Green Chem., 2010, 12, 578–581.
17. Y. Patel, J. George, S.-M. Pillaic and P. Munshi, Green Chem., 2009, 11, 1056–1060.
18. M.-G. Alvarez, M.-S. Anna, S. Contreras and J.-E. Sueiras, Chem. Eng. J., 2010, 161(3), 340–345.
19. M. Srinivas, G. Raveendra, G. Paramesw, P.-S. Sai Prasad, S. Loridant and N. Lingaiah, J. Chem. Sci., 2015, 127(5), 897–908.
20. J.-J. Chen, C. Wang, B. Dong, W.-G. Leng, J. Huang, R. Ge and Y.-N. Gao, Chin. J. Catal., 2015, 36, 5336–5343.
21. D.-F. Wang, X.-L. Zhang, C.-L. Liu and T.-T. Cheng, React. Kinet., Mech. Catal., 2015, 115, 597–609.
22. N.-A. Zuhaimi, V.-P. Indran, M.-A. Deraman, N.-F. Mudrikah, G.-P. Maniam, Y.-H. Taufiq-Yap and M.-H. Rahim, Appl. Catal., A, 2015, 502, 312–319.
23. Y.-B. Ryu, J.-S. Kim, K.-H. Kim, Y. Kim and M.-S. Lee, Res. Chem. Intermed., 2016, 42, 83–93.
24. K. Jagadeeswaraiah, C. Ramesh Kumar, A. Rajashekar, A. Srivani and N. Lingaiah, Catal. Lett., 2016, 146, 692–700.
25. M. Sharath Babu, A. Srivani, G. Parameswaram, G. Veerabhadram and N. Lingaiah, Catal. Lett., 2015, 145, 1784–1791.
26. Y.-Y. Wang and X.-J. Cao, Bioresour. Technol., 2011, 102(22), 10173–10179.
27. L.-B. Li, W.-Y. Zhang, N. Zhao, W. Wei and Y.-H. Sun, Catal. Today, 2006, 115(1–4), 111–116.
28. M. Aresta, A. Dibenedetto, F. Nocito and C. Ferragina, J. Catal., 2009, 268(1), 106–114.
29. M.-J. Climent, A. Corma, P.-D. Frutos, S. Iborra, M. Noy, A. Velty and P. Concepción, J. Catal., 2010, 269(1), 140–149.
30. F. Rubio-Marcos, V. Calvino-Casilda, M.-A. Bañares and J.-F. Fernandez, J. Catal., 2010, 275(2), 288–293.
31. C. Hammond, J.-A. Lopez-Sanchez, M. H. A. A. Rahim and N. Dimitratos, et al., Dalton Trans., 2011, 40, 3927.
32. N. A. S. Zuhaimi, V. P. Indran, M. A. Deraman and N. F. Mudrikah, et al., Appl. Catal., A, 2015, 502, 312–319.
33. H.-J. Wang and P.-F. Lu, J. Chem. Eng. Data, 2012, 57, 582–589.
34. L.-Y. Wang, Y. Liu, C.-L. Liu, R.-Z. Yang and W.-S. Dong, Sci. China: Chem., 2013, 56, 10.
35. Y. Wang, C.-L. Liu, J.-H. Sun, R.-Z. Yang and W.-S. Dong, Sci. China: Chem., 2015, 58, 4.
36. G. Rokicki, P. Rakoczy, P. Parzuchowski and M. Sobiecki, Green Chem., 2005, 7, 529–539.
37. J.-L. Hua, J.-J. Li, Y.-L. Gua and Z.-H. Guan, et al., Appl. Catal., A, 2010, 386, 188–193.
38. P.-F. Lu, H.-J. Wang and K.-K. Hu, Chem. Eng. J., 2013, 228, 147–154.
39. J. Das and D. Khushalani, J. Phys. Chem. C, 2010, 114(17), 8114.
40. D.-M. Reinoso, D.-E. Damiani and G.-M. Tonetto, Appl. Catal., B, 2014, 144, 308–316.
41. S. Fujita, Y. Yamanishi and M. Arai, J. Catal., 2013, 297, 137–141.
42. F.-S. Lisboa, F.-R. Silva, L.-P. Ramos and F. Wypych, Catal. Lett., 2013, 143, 1235–1239.
43. X. Collard, A. Comès and C. Aprile, Catal. Today, 2015, 241, 33–39.
44. N.-K. Divya and P.-P. Pradyumnan, Mater. Sci. Semicond. Process., 2016, 41, 428–435.
45. K. Ravichandran, K. Subha and N. Dineshbabu, J. Alloys Compd., 2016, 656, 332–338.
46. M. Shatnawi, A.-M. Alsmadi, I. Bsoul and B. Salameh, et al., J. Alloys Compd., 2016, 655, 244–252.
47. H.-R. Madan, S.-C. Sharma, U.-D. Suresh and Y.-S. Vidya, et al., Spectrochim. Acta, Part A, 2016, 152, 404–416.
48. L.-G. Wang, Y.-B. Ma, Y. Wang, S.-M. Liu and Y.-Q. Deng, Catal. Commun., 2011, 12(15), 1458–1462.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14288e

---