Ionic liquid mediated CO2 activation for DMC synthesis

Journal of Natural Gas Chemistry 21(2012)476–479

Ionic liquid mediated CO2 activation for DMC synthesis Jun Du∗ , Jing Shi,

Zhengfei Li, Zuohua Liu, Xing Fan, Changyuan Tao

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, China [ Manuscript received November 1, 2011; revised December 19, 2011 ]

Abstract Promoted catalytic reaction between methanol and CO2 for dimethyl carbonate (DMC) synthesis is conducted over K2 CO3 /CH3 I catalyst in the presence of ionic liquid under microwave irradiation. The effect of ionic liquids incorporated with microwave irradiation on the yield of DMC is investigated. DMC was found to form at lower temperature in a relative short time, which indicated an enhanced catalytic process by ionic liquid. Among the ionic liquids used, 1-butyl-3-methylimidazolium chloride is the most effective promoter. Density functional theory calculations indicate that CO2 bond lengths and angles changed due to the molecular interaction of ionic liquid and CO2 , resulting in the activation of CO2 molecules and consequently the acceleration of reaction rate. Key words dimethyl carbonate; direct synthesis; microwave; ionic liquids; K2 CO3 /CH3 I; CO2

1. Introduction Carbon dioxide can be used as an attractive C1 building block since it is highly functional, abundant, inexpensive, nontoxic and environmentally friendly. Industrial chemical processes with carbon dioxide as starting materials have drawn considerable attention [1]. One of the most promising utilization of carbon dioxide is the synthesis of dimethyl carbonate (DMC), which has shown extensive applications [2,3]. The synthesis of DMC has been well developed, such as methanol phosgenation [4], oxidative carbonylation of methanol [5], oxidative carbon monoxidemethyl nitrite processes [6], transesterification of alkene carbonate with methanol [7,8], as well as the directly catalytic synthesis of DMC from methanol and CO2 . Although methodologies such as phosgenation are credited for their high productivity of DMC, the toxicity of phosgene both to health and environment limits its applications for cleaner synthesis. For DMC synthesis technologies, direct reaction of CO2 and methanol is the most attractive route from the viewpoint of green chemistry and sustainable development. A number of catalysts have been reported for this reaction, including carbonate salts [9], metal oxides [10−13], organometallic compounds [14] etc. However, the methanol conversion rates and the selectivity to DMC remain low due to the thermodynamic limitation of the reaction, even in the presence of dehydrating agents such as CaCl2 [15], 2,2-dimethoxy propane

(DMP) [10], acetals [16,17] and molecular sieves [18]. The hydrolysis of catalysts caused by the coproduced water was postulated to be one of the reasons for low catalytic activities [16]. Efforts have been made to enhance the methanol conversion and DMC selectivity, such as H2 O removal [16], base additives selection [19−21], catalyst modification [22]. Among those, improving catalysts is proved to be the most effective route. Recent years, ionic liquid has been reported to be of great potential for developing clean and highly efficient catalytic technologies [23−25]. Under microwave irraidaion, ionic liquids are usually effective microwave adsorbents [26]. There have been much research work reported the facilitation effects of microwave irradiation on organic reactions [27−29]. In this paper, with K2 CO3 as catalyst, ionic liquid coupling with microwave irradiation was applied for the activation of CO2 and the promotion of catalytic reaction of DMC synthesis. A detailed DFT study on the modeling of the interaction between IL and CO2 was also reported here. 2. Experimental 2.1. DMC synthesis CH3 I was synthesized according to the procedures reported in the literature [30]. Ionic liquids 1-butyl-3methylimidazolium chloride ([Bmim]Cl), 1-butyl-3-methyl-

∗

Corresponding author. Tel: +86-23-65102531; Fax: +86-23-65105106; E-mail: [email protected] This work was supported by the Natinal Science Foundation (NSFC 21006130) and the Key Research Programs of Chongqing Science and Technology Commission Foundation (CSTC, 2009AB4012). Copyright©2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(11)60393-9

Journal of Natural Gas Chemistry Vol. 21 No. 4 2012

imidazolium tetrafluoroborate ([Bmim]BF4 ) and 1-butyl-3methylimidazolium hexafluorophosphates ([Bmim]PF6 ) purchased from Henan Lihua pharmaceutical company, China, were used in experiment. The other reagents were used as received without further purification. Direct synthesis of DMC from methanol and carbon dioxide was performed in a 100 mL PTEF digestion vessel. After CH3 I, methanol, K2 CO3 and one kind of ILs were charged into the vessel, purged the vessel with CO2 , and then sealed. The vessel was brought into the microwave oven and the reaction was started by turning on the microwave irradiation (2.45 GHz). The microwave power was set as 160 W. The dosage of CH3 I, as well as the molar ratio between methanol and ionic liquid was investigated. The reaction was stopped by turning off the microwave irradiation. After the reactants cooled to near room temperature, the content was distilled and the distillate was analyzed by a gas chromatograph (GC-1100) equipped with a Poropak Q column and TCD detector. The yield of DMC was calculated as the amount of DMC (in molar) produced per molar catalyst per hour. The blank reaction was performed under the same reaction conditions, but without adding of ionic liquid. 2.2. Computation simulation of the reaction The interaction between ionic liquid molecules and carbon dioxide molecules, and the effect of ionic liquid on the solubility of CO2 and stability were calculated based on B3LYP density functional theory (DFT), with Gaussian 03 program package [31].

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increased firstly with increasing amount of iodide under the reaction conditions, but it turned down to decline as CH3 I reached 0.6 mL. So the amount of CH3 I was optimized as 0.6 mL (9.6 mmol) in this reaction condition. 3.2. Ef fects of ionic liquids A blank test was carried out under the same reaction conditions, but without the addition of any ionic liquid. As result, dimethyl carbonate was not detected in the product. It indicated that with CO2 as reactant, methanol carbonylation reaction couldn’t proceed without ILs, even under microwave irradiation. The influences of [Bmim]Cl, [Bmim]BF4 and [Bmim]PF6 on the direct synthesis DMC from carbon dioxide and methanol are shown in Figure 2. The presence of [Bmim]Cl and [Bmim]BF4 in reaction system, increased the turnover frequency of methanol to DMC separately. It was deduced that “hot spot effect” may be aroused under this microwave irradiation coupling ILs [32], consequently promoted the turnover of methanol to DMC. Booske and co-workers have proposed that the microwave can localize resonant coupling of microwave energy to weak-bond surface and point defect modes [33,34]. In this work, microwave dielectric heating may result in selective heating of catalytic sites with respect to their surroundings, since both K2 CO3 and ILs are strong microwave absorbers, thus leading to “molecular hot spots” in this reaction system.

3. Results and discussion 3.1. Ef fects of methyl iodide amount on DMC yield The effects of methyl iodide on direct synthesis of DMC were investigated in the presence of [Bmim]Cl (3.27 mmol), K2 CO3 (4.7 mmol) and methanol (0.247 mol) under 4 min microwave irradiation, as displayed in Figure 1. The DMC yield

Figure 2. Effects of ILs amount on DMC yield. Reaction conditions: K2 CO3 4.7 mmol, CH3 OH 247 mmol, CH3 I 9.6 mmol, microwave power 160 W, microwave irradiation intermittently for 4 min

Figure 1. Methyl iodide amount vs DMC yield. Reaction conditions: K2 CO3 4.7 mmol, CH3 OH 247 mmol, [Bmim]Cl = 0.6 g (3.27 mmol), microwave power input 160 W, microwave irradiation intermittently for 4 min

Thereby, one of the IL effects on the reaction process was simulated as following. ILs may act as reaction medium, regulating intermediate polarity and the dipole moment of the transition state, which enhanced the selective absorption of microwave energy and increased the solubility of CO2 in reaction system. Consequently, it reduced the interfacial reaction activation energy and increased the transportation of CO2 , and the numbers of reactive molecules collision between CO2 and CH3 OH, which happened around K2 CO3 molecules (Figure 3), thus improving the catalytic activity.

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Figure 3. Scheme of IL promoted catalytic reaction process

3.3. Modeling of CO2 activation by IL Blanchard and co-worker had reported that CO2 can be dissolved in different ionic liquids [35], and ionic liquids absorbed CO2 mainly through the combination of IL’s cations and CO2 molecules. Theoretical calculation and simulation have provided an important opportunity to study SO2 absorption [36,37], and modeling of the IL’s for HCl solubility [38] had been reported. Herein, CO2 was carried out using B3LYP density functional theory and compared with those of [Bmim]Cl-CO2 , [Bmim]BF4 -CO2 and [Bmim]PF6 -CO2 ; 6−31 G (d, p) basis sets were employed for all atoms. The modeling of the IL by DFT method revealed possible optimized structures as shown in Figure 4 that developed upon absorption of CO2 at room temperature; it is obvious that [Bmim]PF6 and CO2 associated to form a pair with a long distance. Bond distances and bond

angles are exhibited in Table 1. Compared with isolated CO2 molecule, the average distance of O(1)–C(2) bond in˚ to 1.17476 A ˚ and the bond angle of creased from 1.16916 A O(1)–C(2)–O(3) decreased from 176.72976o to 174.20707o in IL-CO2 systems, which means that CO2 molecules are adsorbed on IL. Therefore, the sequence of interaction intensity between IL and CO2 can be deduced as: [Bmim]ClCO2 >[Bmim]BF4 -CO2 >[Bmim]PF6-CO2 . So the amount of CO2 adsorption onto IL molecule has a direct influence on DMC yield. The reaction mechanism of DMC synthesis catalyzed by K2 CO3 was proposed by Fang and Fujimoto [9], which included the activation of methanol to methoxy on the base sites of the catalyst, followed by the insertion of carbon dioxide and the production of DMC. It has been announced that the basicity of the catalyst is very important to the activation of methanol. In this work, we found that adding ionic liquids can affect the production of DMC. According to the calculation results, CO2 was activated in a considerable degree by interaction with ILs (Table 1), and this would consequently facilitate the insertion step of the catalytic reaction. So, IL coupling microwave irradiation in the presence of base catalyst may not change the catalytic reaction mechanism, in which the most probable way involved is CO2 activation. And with the energy provided by microwave radiation, ILs promote the mass transfer between the materials, and consequently, increase the probability of molecular collisions and reactivity of CO2 . Table 1. Effects of ionic liquids on bond distances and bond angles of CO2 Ionic liquids CO2 (gas phase) [Bmim]PF6 [Bmim]BF4 [Bmim]Cl

˚ Bond distance (A) O(1)−C(2) C(2)−O(3) 1.16916 1.16916 1.17231 1.16583 1.17341 1.16487 1.17476 1.16558

Bond angle (o ) O(1)−C(2)−O(3) 180 176.72976 175.46180 174.20707

Figure 4. The representative optimized structures [Bmim]Cl-CO2 , [Bmim]BF4 -CO2 and [Bmim]PF6 -CO2

4. Conclusions DMC is synthesized via the IL promoted carbonylation reaction of methanol and CO2 , utilizing microwave irradiation as the thermal energy source, which is applied to provide the

greatest extent of complete reaction and shorten the reaction time by the “microwave dielectric heating” effect. Adding ionic liquids can improve the yields of DMC, but may not change the catalytic reaction mechanism. Ionic liquid acts as a reaction medium of strong microwave adsorption ability and

Journal of Natural Gas Chemistry Vol. 21 No. 4 2012

increasing CO2 solubility. Meanwhile, IL interacts in a considerable degree with CO2 molecule resulting in the activation of CO2 molecule by altering the angle and length of C = O bond, which is proposed as important kinetic as well as thermodynamic factors to facilitate the catalytic synthesis of DMC with methanol and CO2 as starting materials. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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