Immobilizing Polyether Imidazole Ionic Liquids on ZSM-5 Zeolite for the Catalytic Synthesis of Propylene Carbonate from Carbon Dioxide

Traditional ionic liquids (ILs) catalysts suffer from the difficulty of product purification and can only be used in homogeneous catalytic systems. In this work, by reacting ILs with co-catalyst (ZnBr2), we successfully converted three polyether imidazole ionic liquids (PIILs), i.e., HO-[Poly-epichlorohydrin-methimidazole]Cl (HO-[PECH-MIM]Cl), HOOC-[Poly-epichlorohydrin-methimidazole]Cl (HOOC-[PECH-MIM]Cl), and H2N-[Poly-epichlorohydrin-methimidazole]Cl (H2N-[PECH-MIM]Cl), to three composite PIIL materials, which were further immobilized on ZSM-5 zeolite by chemical bonding to result in three immobilized catalysts, namely ZSM-5-HO-[PECH-MIM]Cl/[ZnBr2], ZSM-5-HOOC-[PECH-MIM]Cl/[ZnBr2], and ZSM-5-H2N-[PECH-MIM]Cl/[ZnBr2]. Their structures, thermal stabilities, and morphologies were fully characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The amount of composite PIIL immobilized on ZSM-5 was determined by elemental analysis. Catalytic performance of the immobilized catalysts was evaluated through the catalytic synthesis of propylene carbonate (PC) from CO2 and propylene oxide (PO). Influences of reaction temperature, time, and pressure on catalytic performance were investigated through the orthogonal test, and the effect of catalyst circulation was also studied. Under an optimal reaction condition (130 °C, 2.5 MPa, 0.75 h), the composite catalyst, ZSM-5-HOOC- [PECH-MIM]Cl/[ZnBr2], exhibited the best catalytic activity with a conversion rate of 98.3% and selectivity of 97.4%. Significantly, the immobilized catalyst could still maintain high heterogeneous catalytic activity even after being reused for eight cycles.


Preparation of HO-[PECH-MIM]Cl
1, 2-Dichloroethane (19.6 mL), ethylene glycol (2.8 mL) and boron trifluoride etherate (2 mL) were added into a flask equipped with a thermometer, stirrer, and condensing tube. N2 was purged into the system to replace the air inside. The mixture was stirred at room temperature. After stirring for 30 min, the temperature of the reaction system was reduced to 0 °C in an ice bath, and then epichlorohydrin (ECH) (39.2 mL) and 1, 2dichloroethane (15 mL) were mixed and slowly added to the reaction system. After 3 h the solution became a colorless and viscous liquid. Then, the reaction mixture was transferred to a separatory funnel and washed with distilled water for three times, and then transferred to a distillation flask and distilled under the pressure of 0.09 MPa at 70 °C for 3 h to remove low-boiling solvents and organics, resulting in a refined polyepichlorohydrin (PECH). Toluene (50 ml) and PECH (40 g) were added into a round-bottom flask. After stirring for 8 min, Nmethylimidazole (65 ml) was added to the mixture. The mixture was stirred at 50 °C for 10 h and then cooled to room temperature, followed by the addition of ether (50 ml). The reaction mixture was transferred to a distillation flask and distilled under the pressure of 0.1 MPa at 90 °C to remove the solvent and low-boiling organic matters to obtain the polyether imidazole ionic liquid (PIIL), namely HO-[PECH-MIM]Cl.

Preparation of HOOC-[PECH-MIM]Cl
PECH (8.2 g) and chloroacetic acid (18 g) were added into a flask equipped with a condenser tube and added acetonitrile (50 ml). The mixture was stirred at 80 °C for 1 h and then Na2CO3 (4 g) was added into the system. After 6~8 h, the pH of the reaction system was adjusted to 3~4 by adding hydrochloric acid (7 mL), and acetonitrile was removed by rotary evaporation under the pressure of 0.09 MPa at 70 °C, and washed with distilled water for 3 to 5 times to remove a small amount of low molecular weight substances and excess chloroacetic acid. After that, a small amount of water was removed under the pressure of 0.09 MPa at 80 °C,

Preparation of H2N-[PECH-MIM]Cl
A mixture of toluene and PECH in a mass ratio of 2:1 was added into a flask equipped with reflux condensation, and the mixture was stirred. The bromine was slowly added to the mixture in a separatory funnel.
The reaction was continued at room temperature for 2 h after the addition was completed, and then the reaction was continued for 3 h by gradually increasing the temperature and maintaining the temperature (not exceeding 50 °C). Excessive ammonia was slowly added to the solution, and the reaction was continued until the color of the solution changed from reddish brown to colorless, the temperature was maintained at 30 °C, and after 3 h, the oily liquid was separated using a separatory funnel. Then the oily liquid was washed with deionized water for 5 times and then toluene and small molecular substances were removed on a rotary vap under the pressure of 0.09

Typical Procedure for the Synthesis of PC from PO and CO2
The catalyst (2.5% mass fraction of the PO) was added into autoclave (300 ml). Replaced internal air with nitrogen after sealed the reactor. CO2 was filled in the autoclave through the intake bypass, when the pressure was 1.5 MPa, PO (150 ml) were added by a metering pump. The flow of CO2 was regulated. The reaction temperature (130 °C), pressure (2.5MPa), and stirring rate (200 r/min) were set. Cooling water was turned on to cool the system to room temperature and the pressure was relieved when the flow of CO2 was zero. To purify prepared PC, the product was added into distilled bottle for vacuum distillation under the condition of temperature (135°C) and pressure (0.09 MPa), the residual catalyst was reused. The obtained colorless liquid was PC. The PC was weighed and the purity was determined by gas chromatography.

Molecular weights of PIILs
Using tetrahydrofuran as solvent and molecular weight of polystyrene (PS) as the contrast, the molecular weights of three PIILs were determined by gel permeation chromatography (GPC). Figure 6 showed the mass distribution of three PIILs. The