SCIENCE CHINA Chemistry • ARTICLES •

doi: 10.1007/s11426-013-4918-5 doi: 10.1007/s11426-013-4918-5

An efficient catalytic system for the synthesis of glycerol carbonate by oxidative carbonylation of glycerol WANG LiYan, LIU Yan, LIU ChunLing, YANG RongZhen & DONG WenSheng* Key Laboratory of Applied Surface and Colloid Chemistry (SNNU), MOE, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, China Received January 31, 2013; accepted March 21, 2013

Glycerol carbonate was synthesized by the oxidative carbonylation of glycerol catalyzed by the commercial Pd/C with the aid of NaI. High conversion of glycerol (82.2%), selectivity to glycerol carbonate (>99%), and TOF (900 h–1) were obtained under the conditions of 5 MPa (pCO:pO2= 2:1), 140 ºC, 2 h. The highly active palladium species were generated in situ by dissolution from the carbon support and stabilized by re-deposition onto the support surface after the reaction was finished. Palladium dissolution and re-deposition were crucial and inherent parts of the catalytic cycle, which involved heterogeneous reactions. This Pd/C catalyst could be recycled and efficiently reused for four times with a gradual decrease in activity. Moreover, the influences of various parameters, e.g., types of catalysts, solvents, additives, reaction temperature, pressure, and time on the conversion of glycerol were investigated. A reaction mechanism was proposed for oxidative carbonylation of glycerol to glycerol carbonate. glycerol, glycerol carbonate, oxidative carbonylation, Pd/C catalyst

1 Introduction In recent years, the increasing use and production of biodiesel has resulted in an increase of glycerol production and price decline, which makes glycerol a particularly attractive building block for the synthesis of other valuable chemical products [1–4]. Glycerol can be converted into several important chemicals including acrolein by acid-catalysis [5], propylene glycol and 1,3-propanediol by hydrogenolysis and glyceric acid by oxidation [6–10]. Glycerol carbonate is one of the most attractive derivatives of glycerol. Due to its low toxicity, low evaporation rate, low flammability, and moisturizing ability, glycerol carbonate is used as a wetting agent in cosmetics and carrier solvent for medical preparations. It can be used as protic solvent (in resins and plastics), additive, surfactant, adhesive, paint, lubricant and electro-

*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2013

lyte. It is also an important intermediate of polymers such as polycarbonate, polyester, polyurethane, and polyamide [11, 12]. Traditionally, cyclic carbonates have been prepared by reaction of glycols with phosgene, but due to the high toxicity and corrosive nature of phosgene, alternative routes such as transesterification reaction of dialkyl or alkykene carbonates to obtain cyclic carbonates have been explored. The main methods reported in the literature for the preparation of GC are based on the reaction of glycerol with (a) a carbonate source (phosgene, a dialkyl carbonate or an alkylene carbonate) [13, 14], (b) urea [15, 16], (c) carbon dioxide [17, 18], (d) carbon monoxide and sulfur [19], (e) carbon monoxide and oxygen [20–25]. Among these methods, the oxidative carbonylation of glycerol has been regarded as a highly efficient approach to obtain glycerol carbonate because of the wide availability of CO and the attractiveness of the process in light of its high atom economy (87%) [21]. Recently, a few catalyst systems have been chem.scichina.com

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found effective in catalyzing the oxidative carbonylation of glycerol to glycerol carbonate. For example, Mizuno et al. found that by combining the selenium-catalyzed carbonylation of glycerol in N,N-dimethyl formamide (DMF) with the oxidation of the resulting selenocarbonate salt with molecular oxygen, glycerol carbonate was obtained in good yield (83~84%) [22]. However, an equimolar amount of selenium with glycerol was needed during the reaction, which makes the process uneconomic for large-scale applications. Li et al. explored a homogeneous PdCl2(phen)/KI (phen=1,10phenanthroline) catalyst system to synthesize glycerol carbonate by the oxidative carbonylation of glycerol [21]. Müller et al. reported that a Wacker-type catalyst system, i.e., Pd(OAc)2/Mn(acac)3/KBr catalyzed the oxidative carbonylation of glycerol to glycerol carbonate [23]. Gabriele et al. reported that PdI2/KI catalyst was also active in the oxidative carbonylation of glycerol [24]. However, the catalytic activities of these existing homogeneous catalyst systems are unsatisfactory. Besides, the homogeneous system suffers from difficulty in the separation and recovery of catalysts from the reaction mixtures. Hence, a heterogeneous catalyst is highly desired. Very recently, Li et al. developed a new zeolite-Y-confined Pd catalyst, PdCl2(phen) @Y [25]. With the aid of CuI, this catalyst exhibited a comparable activity to its homogeneous counterpart and could be reused five times without significant decrease in activity. A high turnover frequency of 317 h–1 (TOF, mol converted glycerol/mol Pd∙h) was obtained. In the present study, we found that the commercially available Pd/C catalyst could be an efficient catalyst for the synthesis of glycerol carbonate by the oxidative carbonylation of glycerol with the aid of NaI. A turnover frequency as high as 900 h–1 (TOF, mol converted glycerol/mol Pd∙h) has been obtained under the optimum conditions.

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purchased from Northwest Institute For Non-ferrous Metal Research (Xi’an, China). 2.2

Characterization

Transmission Electron Microscopy (TEM) micrographs were obtained by using JEM2010 instrument. Powder X-ray diffraction (XRD) was performed on a Rigaku D/ MAX2550VB X-ray diffractometer using a Cu Kα source. X-ray Photoelectron Spectroscopy (XPS) was performed on an Axis Ultra, Kratos (UK) system using Al Ka source (15 kV, 1486.6 eV). The vacuum in the spectrometer was 10−9 Torr. The binding energy was calibrated relative to the C1s peak (284.6 eV). The content of Pd was determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, model Atom Scan 16, TJA Corp.). 2.3

Reaction test and product analysis

The catalytic reactions were carried out in a 35 mL stainless steel autoclave equipped with a mechanical stirrer. In a typical experiment, 40 mg Pd/C, 17.1 mg (0.114 mmol,) NaI, 15 ml N,N-dimethyl-acetamide (DMA) solvent, and 2.30 g (25 mmol) glycerol were charged into the reactor. The autoclave was purged three times with CO and then pressurized to 5.0 MPa with CO and O2 (pCO : pO2= 2:1) at room temperature. The reaction was carried out at 140 °C for 2 h. After the reaction, the reactor was cooled down to below 10 °C with an ice-water mixture and depressurized in a fume hood. An internal standard of n-butyl alcohol was added in the liquid product. Quantitative analysis was performed using an Agilent 6820 GC with a HP-50 capillary column (30 m×0.32 mm ×0.5 mm) and the FID.

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Results and discussion

2 Experimental 2.1

Chemicals and catalysts

All chemicals are analytical grade unless otherwise stated and used without further purification. DMA, glycerol, NaI and NaBr were purchased from Fuchen Chemical Reagent (Tianjin, China). PPh3 was purchased from Shanghai Chemical Reagent Co., Ltd.. PdCl2, n-butanol, KI, NaCl, NaOH, Et3N, DMF, DMSO, and PEG200 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 1,4-Dioxane was purchased from Bodi Chemical Reagent (Tianjin, China). O2 (>99.99%) and CO (>99.99%) were supplied by the Xi’an MESSER Gas Company. Pd/C (3 wt% Pd loading, 237515-10G, BET surface area 662.7 m2 g−1, pore volume: 0.52 m2 g−1, average pore diameter: 3.5 nm) was purchased from Aldrich. PdO, Pd(OAc)2, Pd(PPh3)4, Pd(NH3)4Cl2, Pd(PPh3)2Cl2, and K2PdCl4 were

3.1

The catalytic performance of different catalysts

Glycerol carbonate was prepared by the oxidative carbonylation of glycerol with CO and O2 using different catalysts, i.e., Pd(OAc)2, K2PdCl4, PdCl2, Pd(PPh3)2Cl2, Pd(NH3)4Cl2, Pd(PPh3)4, and Pd/C with NaI as a promoter in DMA at 140 ºC. The results are summarized in Table 1. As shown in Table 1, simple palladium salts, i.e., Pd(OAc)2, K2PdCl4, PdCl2, can catalyze the oxidative carbonylation of glycerol; the complex of PdCl2, i.e., Pd(NH3)4Cl2 and Pd(PPh3)2Cl2, are also active in the reaction. The complex Pd(PPh3)4 showed higher activity in the reaction, and after the reaction the color of the mixture was colorless, suggesting that Pd(PPh3)4 may function as molecular species (homogenous catalysts) rather than as heterogeneous catalysts via decomposing to palladium nanoparticles under the reaction conditions. When the commercial Pd/C was used as the catalyst, the optimum catalytic performance was obtained with

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Table 1 Comparison of different Pd-catalysts in the oxidative carbonylation of glycerola) Entry 1 2 3 4 5 6 7

Catalyst Pd/C Pd(PPh3)4 Pd(OAc)2 K2PdCl4 PdCl2 Pd(PPh3)2Cl2 Pd(NH3)4Cl2

Conversion (%) 82.2 80.4 79.1 78.7 76.6 76.7 75.6

Selectivity (%) 99.6 95.3 95.6 95.8 95.2 97.6 90.8

TOFb) (h–1) 900 881 867 862 839 840 829

a) Reaction conditions: solvent DMA 15 ml, glycerol 2.30 g (25 mmol), glycerol:Pd=2190 (molar ratio), NaI/Pd = 10:1 (molar ratio), 5 MPa, pCO:pO2 = 2:1, 140 °C, 2 h, stirring speed 500 rpm. b: TOF (mol converted glycerol/mol Pd∙h).

a conversion of glycerol of 82.2%, selectivity to glycerol carbonate of 99.6%, and TOF of 900 h–1, followed by those of Pd(PPh3)4, Pd(OAc)2, K2PdCl4, Pd(PPh3)2Cl2~PdCl2, and Pd(NH3)4Cl2. The results suggest that Pd0 species are better catalyst precursors for the oxidative carbonylation of glycerol in the present cases. Previously, PdCl2/KI was used as a catalytic system for the oxidative carbonylation of glycerol in N,N-dimethyl formamide (DMF) at 140 ºC by Li et al. A turnover frequency of 58 h–1 (TOF, mol converted glycerol/mol Pd∙h) was obtained under the conditions of 140 °C, 3.0 MPa (pCO:pO2=2:1) for 2 h [21]. However, a turnover frequency of 839 h–1 for PdCl2/NaI catalyst was obtained under the conditions used in the present study. The results suggest that reaction pressure, solvent and additive have huge influence on the catalytic performance of the catalyst. Hence, in the following section, the influences of various process parameters, e.g., types of solvents, additives, reaction temperature, pressure, and time on the conversion of glycerol were investigated. 3.2

The influences of additives

Table 2 shows the influences of additives in the oxidative carbonylation of glycerol on the Pd/C catalyst. In the presence of NaOH additive, only 13.1% glycerol conversion and 19.6% selectivity of glycerol carbonate were obtained. For organic base additives, i.e., Et3N and PPh3, the conversion of glycerol was 22.1% and 31.2%, and the selectivity to glycerol carbonate was 57.2% and 53.5%, respectively. For NaCl and NaBr additives, the glycerol conversion was 35.8% and 47.9%, and the selectivity to glycerol carbonate was 72.2% and 79.8%, respectively. Whereas in the presence of NaI, 82.2% conversion of glycerol and 99.6% selectivity to glycerol carbonate were obtained. The results indicate that the presence of I－ is essential for achieving a good catalyst performance in the reaction. The quite unique role of iodide salts in catalysis has also been found in many other reactions [26, 27]. We also investigated the influence of KI as an additive, and 79.5% conversion of glycerol and 92.6% selectivity to glycerol carbonate were obtained, indicating that cations also have influence on the reaction.

When NaI/Pd molar ratio increased from 10 to 20 (entry 8 in Table 2), both the conversion of glycerol and the selectivity to glycerol carbonate decreased slightly. On the other hand, when NaI/Pd molar ratio decreased from 10 to 5 and 1 (entry 9, 10 in Table 2), both the conversion of glycerol and the selectivity to glycerol carbonate decreased. 3.3

The influences of solvents

The oxidative carbonylation of glycerol by using Pd/C-NaI catalyst system in the different solvents, i.e., DMF, ethanol, PEG-200, DMSO and 1,4-dioxane was carried out to investigate the influence of solvents. The reaction was carried out at 140 °C for 2 h; the results are shown in Table 3. In aprotic high boiling point solvent DMF (b.p. 152.8 °C), the conversion of glycerol was 82.8% and the selectivity of glycerol carbonate was 84.1%, which were lower than those in DMA. The reaction was also performed in ethanol and PEG-200 separately. The corresponding conversion of glycerol was 53.3% and 43.2%, and the selectivity to glycerol carbonate was 72.6% and 71.3%, respectively. In DMSO, the conversion of glycerol was 39.3%; however, no glycerol carbonate was generated. Under the same reaction condition, a glycerol conversion of 58.2% and glycerol carbonate selectivity of 7.3% were obtained in 1,4-dioxane. 3.4

The influence of reaction temperature

Figure 1 depicts the effect of reaction temperature on the conversion of glycerol and the selectivity to glycerol carbonate. It shows that both the conversion of glycerol and the selectivity to glycerol carbonate increase initially with increasing reaction temperature, and then gradually decrease. When the reaction was conducted at 100 °C for 2 h, the conversion of glycerol was 49.5% with 92.3% glycerol carbonate selectivity. When the reaction temperature reached 140 °C, the conversion of glycerol increased to 82.2%, and the selectivity of glycerol carbonate increased to 99.6%. When the reaction temperature was further increased from 140 to 160 °C, the conversion of glycerol decreased from 82.2% to 76.5%, and the selectivity of glycerol carbonate decreased from 99.6% to 85.9%, suggesting

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Table 2

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Influences of additives on the catalytic performance of Pd/C in the oxidative carbonylation of glycerola)

Entry 1 2 3 4 5 6 7 8c) 9d) 10e)

Additive NaOH Et3N PPh3 NaCl NaBr KI NaI NaI NaI NaI

Conversion (%) 13.1 22.1 31.2 35.8 47.9 79.5 82.2 80.6 71.3 67.7

Selectivity (%) 19.6 57.2 53.5 72.2 79.8 92.6 99.6 98.0 88.8 77.8

TOFb) (h–1) 143 241 342 392 525 872 900 883 771 741

a) Reaction conditions: solvent DMA 15 ml, glycerol 2.30 g (25 mmol), glycerol:Pd=2190 (molar ratio), additive/Pd=10:1 (molar ratio), 5 MPa, pCO:pO2=2:1, 140 °C, 2 h, stirring speed 500 rpm. b): TOF (mol converted glycerol/mol Pd∙h). c): NaI /Pd = 20:1 (molar ratio). d): NaI/Pd = 5:1 (molar ratio). e): NaI/Pd = 1:1 (molar ratio).

Table 3

Solvent effect in the oxidative carbonylation of glycerola)

Entry 1 2 3 4 5

Solvent DMF Ethanol PEG200 DMSO 1,4-dioxane

Conversion (%) 82.8 53.3 43.2 39.3 58.2

Selectivity (%) 84.1 72.6 71.3 0 7.3

a) Reaction conditions: solvent 15 ml, glycerol 2.30 g (25 mmol), glycerol:Pd=2190 (molar ratio), NaI/Pd = 10:1 (molar ratio), 5 MPa, pCO:pO2=2:1, 140 °C, 2 h, stirring speed 500 rpm.

Figure 1 Effect of temperature on oxidative carbonylation of glycerol catalyzed by Pd/C/NaI. Reaction conditions: solvent DMA 15 ml, glycerol 2.30 g (25 mmol), glycerol:Pd=2190 (molar ratio), NaI/Pd=10:1 (molar ratio), 5 MPa, pCO:pO2=2:1, 2 h, stirring speed 500 rpm.

that higher temperature is unfavorable for the formation of glycerol carbonate. This is probably due to the fact that the solubility of CO and O2 decreases with increasing temperature. The fresh and used Pd/C catalysts at 140 and 160 °C, respectively, were characterized by XRD, TEM, XPS and ICP analysis. Figure 2 presents XRD patterns of the catalysts. The fresh catalyst does not show any diffraction peaks from palladium, suggesting that palladium is highly dispersed on the surface of carbon support. For the used catalysts at 140 and 160 °C, sharp diffraction peaks assigned to metallic Pd are observed, indicating that the aggregation

Figure 2 XRD pattern of the fresh and used Pd/C catalysts: (a) the fresh; (b) the used at 140 ºC; (c) the used at 160 °C.

and growth of Pd particles after the reaction. Figure 3 shows TEM images of these catalysts. It is seen that Pd particles with an average size of ~2.5 nm are distributed well on carbon support over the fresh catalyst. However, for the used catalyst at 140 °C, the growing and aggregation of Pd particles occurred after the reaction. For the used catalyst at 160 °C, few Pd particles were observed by TEM. Figure 4 and Table 4 show XPS results of the fresh and used catalyst at 140 °C. XPS analysis confirmed that only metallic Pd was detected on the fresh Pd/C catalyst. However, for the used

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Figure 3

Table 4

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HRTEM images of the fresh and used Pd/C catalysts: (a) the fresh; (b) the used at 140 °C; (c) the used at 160 °C.

XPS analysis results of the fresh and used Pd/C catalysts Catalyst Fresh

Used at 140 °C

Figure 4

Binding energy (eV) Pd3d5/2 I3d 335.3 (Pd0) 335.6 (Pd0: 36.0%) 618.4 (I: 17.4%) 336.8 (Pd2+: 64.0%) 619.6 (I0: 82.6%)

XPS analysis of Pd/C catalysts: (a) the fresh; (b) and (c) used at 140 °C.

Pd 1.10

Surface atomic composition (%) O C Cl 11.32 86.24 1.34

0.90

7.38

91.15

0

I 0 0.58

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catalyst at 140 ºC, both Pd0 and PdII were found. Besides, I0 and I−1 were also detected on the surface of carbon support, suggesting that during the reaction some Pd0 was oxidized into PdI2 and after the reaction PdI2 was re-deposited on the surface of the catalyst. In addition, some KI was oxidized into I0 during the reaction. For the used catalyst at 160 ºC, no XPS signal from palladium was detected. ICP analysis confirmed that no Pd leaching was observed after the reaction at both 140 and 160 ºC for 2h. The results suggest that Pd particles may enter the micropores of carbon support via dissolution and re-deposition process, which will be explained in detail later. 3.5

The influence of pressure

Figure 5 depicts the effect of pressure on the conversion of glycerol and the selectivity to glycerol carbonate in different reaction times. From Figure 5, it can be observed that when the reaction was conducted under 3 MPa for 2 h, the conversion of glycerol was 65.4% with 92.5% glycerol carbonate selectivity; with increasing the pressure to 4 MPa, the conversion of glycerol increased to 74.6%, and the selectivity to glycerol carbonate increased to 99.2%. With further increasing pressure to 5 MPa, the conversion of gly-

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cerol increased to 82.2% with 99.6% glycerol carbonate selectivity. Under the conditions of 140 ºC and 5 MPa, with increasing reaction time both the conversion of glycerol and the selectivity to glycerol carbonate increased slightly, and then decreased slightly. The highest glycerol conversion and the selectivity to glycerol carbonate were obtained with 2 h reaction time. 3.6

We also examined the influences of CO/O2 partial pressure ratio and stirring speed on the oxidative carbonylation of glycerol, and the results are summarized in Table 5. As shown in Table 5, with increasing the stirring speed from 500 to 800 rpm, both the conversion of glycerol and the selectivity to glycerol carbonate change slightly, indicating that mass transfer effect is unimportant beyond 500 rpm. From Table 5, it can also be observed that with increasing the CO/O2 partial pressure ratio, the conversion of glycerol decreases apparently, whereas the selectivity to glycerol carbonate decreases slightly, indicating that high CO/O2 partial pressure ratio suppresses the conversion of glycerol to glycerol carbonate. This is probably due to the fact that at high CO partial pressure, the oxidation of Pd0 in the catalytic cycle could be suppressed, leading to inhibition of catalyst activity. We also conducted the reaction under 5 MPa of a 4:1 CO/air mixture which is beyond the explosion limits using the Pd/C-NaI catalytic system, and only 7.0% conversion of glycerol and 85.3% selectivity to glycerol carbonate were obtained (see Entry 9 in Table 5). 3.7

Figure 5 Effect of pressure on oxidative carbonylation of glycerol catalyzed by Pd/C/NaI. Reaction conditions: solvent DMA 15 ml, glycerol 2.30 g (25 mmol), glycerol:Pd=2190 (molar ratio), NaI/Pd=10:1 (molar ratio), pCO/pO2=2:1, 140 °C, 2 h, stirring speed 500 rpm.

Table 5

The influence of CO/O2 partial pressure ratio

Reusability of catalysts

The recyclability of Pd/C catalyst was examined for the oxidative carbonylation of glycerol. Figure 6 shows the results obtained after four catalytic cycles. It should be noted that addition of NaI was necessary in each run. A gradual decrease in activity was observed during four successful runs and the fourth run corresponded to 65.2% conversion with ~100% selectivity to glycerol carbonate. The recycled catalyst after the fourth run was characterized by TEM, XPS,

Influence of CO/O2 mole ratio and stirring speed on the oxidative carbonylation of glycerola Entry 1 2b 3 4 6 7 8 9c

pCO/pO2 2:1 2:1 4:1 8:1 10:1 12:1 15:1 –

Conversion (%) 82.2 82.8 70.4 51.6 41.0 19.0 10.3 7.0

Selectivity (%) 99.6 97.2 92.7 95.3 89.9 87.2 86.3 85.3

a) Reaction conditions: solvent DMA 15 ml, glycerol 2.30 g (25 mmol), glycerol:Pd=2190 (molar ratio), NaI/Pd = 10:1 (molar ratio), 5 MPa, 140 ºC, 2 h, stirring speed 500 rpm. b: stirring speed 800 rpm. c: pCO/pAir=4:1.

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Figure 6 The recycling of Pd/C and NaI catalyst system. Reaction conditions: solvent DMA 15 ml, glycerol 2.30 g (25 mmol), glycerol:Pd=2190 (molar ratio), NaI/Pd=10:1 (molar ratio), 5 MPa, pCO:pO2=2:1, 140 °C, 2 h, stirring speed 500 rpm.

Figure 7 Time-dependent correlation of palladium leaching with the progress of the reaction. Reaction conditions: solvent DMA 15 ml, glycerol 2.30 g (25 mmol), glycerol:Pd=2190 (molar ratio), NaI/Pd=10:1 (molar ratio), 5 MPa, pCO:pO2=2:1, 140 °C, 2 h, stirring speed 500 rpm.

XRD and ICP analysis. Both TEM and XPS did not find Pd species present on the surface of carbon support. However, the metallic Pd diffraction peaks assigned to (111), (200), (220) and (311) were clearly observed in the XRD pattern. ICP analysis confirmed that this used catalyst contained 2.24% palladium. The results further suggest that Pd particles may enter the micropores of carbon support via dissolution and re-deposition process. The loss of catalytic activity after the fourth run was probably due to the reduction of active Pd species on the surface of carbon support because of leaching of palladium. 3.8 Proposed mechanism for the oxidative carbonylation of glycerol In order to verify whether the oxidative carbonylation of glycerol catalyzed by Pd/C is heterogeneous or homogene-

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ous, we investigated Pd leaching during the reaction. The heterogeneous Pd/C catalyst was filtered while hot after 30 min reaction (ca. 42% glycerol conversion) and the filtrate was allowed to react for a further 90 min under the same conditions. The conversion of glycerol was 78.3% after that time. ICP analysis confirmed that the amount of Pd leaching in the solution was near 33%. The results suggest that dissolution of palladium species from the surface of the solid support led to the formation of the catalytically active species in solution. We also carried out the reaction in different reaction times to investigate the influence of time on the conversion of glycerol and Pd leaching. The results are summarized in Figure 7. It shows that after reaching the reaction temperature of 140 °C, about 31% palladium was leached from the surface of the carbon support. Simultaneously, the conversion of glycerol was 29.6%. With prolonging the reaction time the Pd leaching increased slightly, while the conversion of glycerol increased quickly. After 60 min the amount of Pd species in the solution decreased, and after 2 h no Pd species in the solution was detected, indicating that the palladium was completely re-deposited onto the support after the reaction was finished. The solid catalyst functions as a reservoir for catalytically active molecular palladium species in solution. The similar phenomenon was also observed by Köhler et al. in Heck reaction and Suzuki cross-coupling reaction [28, 29]. The preceding results have confirmed that the dissolved molecular palladium is the catalytically active species. It is known that under oxidant conditions, Pd0, in the presence of iodide anions, can be oxidized to palladium iodide [30]. Our XPS analysis also confirmed that during the reaction, I– was oxidized to I0, and Pd0 was oxidized to PdI2. Hence, the reaction mechanism for the oxidative carbonylation of glycerol to glycerol carbonate catalyzed by Pd/C-NaI could be expressed as follows (anionic iodide ligands are omitted for clarity), which is quite similar to that proposed by other authors for PdI2-KI catalyst system [24]. 2KI + 0.5O2 → I2 + K2O Pd0 + I2 → PdI2 C3H8O3 + CO + PdI2 → C4H6O4 + Pd0 + 2HI K2O + 2HI → 2KI + H2O

4 Conclusions This study showed that glycerol carbonate can be readily synthesized from the catalytic oxidative carbonylation of glycerol using the commercially available Pd/C catalyst with the aid of NaI. High conversion of glycerol (82.2%), selectivity to glycerol carbonate (>99%), and TOF (900 h–1) were obtained under the conditions of 5 MPa (pCO:pO2= 2:1), 140 °C, 2 h. The dissolved molecular palladium species from the solid carbon support represents the catalytically

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active species. At the end of the reaction, the dissolved palladium was re-deposited onto the support. Palladium dissolution and re-deposition are crucial and inherent parts of the catalytic cycle, which involves heterogeneous reactions. This Pd/C catalyst could be recycled and efficiently reused for four times with a gradual decrease in activity. This work was financially supported by the National Natural Science Foundation of China (20976101), the Program for Key Science & Technology Innovation Team of Shaanxi Province (2012KCT-21), and the Program for Changjiang Scholars and Innovative Research Team in University of China (IRT1070). 1

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