Benzotriazolium Ionic Liquid Immobilized on Periodic Mesoporous Organosilica as an Effective Reusable Catalyst for Chemical Fixation of CO2 into Cyclic Carbonates

A type of dichloro(dimethoxyethane)nickel anionic benzotriazolium ionic liquid-functionalized periodic mesoporous organosilicas were synthesized and tested as effective and practical heterogeneous catalysts in the cycloaddition of CO2 with epoxides. The catalyst PMO@ILC4H10O2NiCl3(1.0) showed brilliant catalytic activity for the synthesis of cyclic carbonates with high yields and selectivities under solventand cocatalyst-free conditions. We also found that the catalytic activity could be significantly influenced by the hydroxyl groups sites of periodic mesoporous organosilica and the active sites (hydroxyl groups/ dichloro(dimethoxyethane)nickel anion) of the benzotriazoliumcation ionic liquid, probably due to an intensification of intramolecular synergistic effect. The catalytic process displayed ease of recovery, excellent stability and recyclability for at least five runs without significant loss of its catalytic activity.

Ionic liquids (ILs) have attracted much attention because of the excellent properties and have meaningful applications in many fields, such as catalysis, chemical conversion, etc. [32][33][34] Studies involving the utilization of ionic liquids as catalysts for the transformation of CO2 into cyclic carbonates have also been reported. [35][36][37] Although, these functionalized ILs catalysts are quite effective but their practical applications are restricted by some defects in the separation and recyclability. Hence, the development of easy recovery and recyclable ionic liquid-based heterogeneous catalysts are always in demand. Immobilization of Ionic liquids onto porous solid supports to explore heterogeneous supported ILs has gained comprehensive attention. [38][39][40][41] Among these soild supports, periodic mesoporous organosilicas (PMOs) have attracted increasing attention because of their combined advantages of large specific surface areas, tunable pore sizes, uniform distribution of functional groups, chemical and thermal stabilities, as well as highly ordered mesostructure properties. [42][43][44][45][46][47][48][49][50] As part of our ongoing interest in the development of efficient and environmentally friendly catalytic systems, herein, we intend to perform immobilization of benzotriazolium ionic liquid onto periodic mesoporous organosilica to design multifunctional immobilized ionic liquids. The obtained PMO@ILC4H10O2NiCl3 with different ionic liquid concentration have been employed as heterogeneous and recyclable catalysts in the cycloaddition of CO2 with epoxides under cocatalyst-and solvent-free conditions (Scheme 1). Additionally, the recyclability and reusability of the catalyst was also investigated.

RESULTS AND DISCUSSION
The catalytic activities of PMO@ILC4H10O2NiCl3(x) were tested in the model reaction of CO2 cycloaddition with propylene oxide to produce propylene carbonate. As shown in Table 1 [6][7][8]. Therefore, PMO@ILC4H10O2NiCl3(1.0) is thought to the suitable catalyst for the reaction. Then, the effect of catalyst dosage on the cycloaddition was examined. The yield of propylene carbonate increased with the catalyst amount of the catalyst was increased from 5 mg to 20 mg (Table 1, entries 4, 10-12), while a further increase in the catalyst amount did not gave more product (Table 1, entries 13 and 14). Therefore, the best result was obtained with 20 mg catalyst.
The effect of the reaction temperature on the cycloaddition was studied over the catalyst PMO@ILC4H10O2NiCl3(1.0), and the results are revealed in Figure 1. As shown in the figure, the yield and selectivity of propylene carbonate were significantly increased with the increase of reaction temperature. While, the temperature was further increased from 110°C to 130°C, the yield and selectivity of propylene carbonate showed a slight decrease. The reason may be that the overly high temperatures can increase the occurrence of side reactions of isomerization and ring opening of propylene oxide, which was determined by GC analysis. These results indicated that the suitable temperature was 110 °C. The Scheme 1. Catalytic synthesis cyclic carbonates from CO2 and epoxides with PMO@ILC4H10O2NiCl3.  effects of initial pressure on the cycloaddition was also studied ( Figure 2). As shown in the figure, the yield and selectivity of propylene carbonate were strengthened gradually with the CO2 pressure was increased from 0.2 MPa to 1.0 MPa. However, the yield and selectivity of propylene carbonate showed a visible decrease when CO2 pressure was above 1.0 MPa. The reason may be that too high CO2 pressure retard the interaction of propylene oxide, CO2 and PMO@ILC4H10O2NiCl3(1.0), which reduced the concentration of propylene oxide and lead to low yields and selectivities. [13][14][15][16] It can be observed that the proper pressure was 1.0 MPa.
The stability and reusability are important properties for the designed catalyst PMO@ILC4H10O2NiCl3(1.0), which was evaluated in the cycloaddition of propylene oxide with       (Figure 3). The catalyst could be easily recovered by simple filtration, and then reused directly in the following runs. The results demonstrated that the catalyst can be recycled for five consecutive runs without significant loss in catalytic activity. In addition, FT-IR spectra for the recovered catalyst after five runs was similar to that of fresh catalyst, indicating that its characteristic framework did not change significantly during the reaction (Figure 4). Furthermore, a hot filtration test confirmed that the reaction follows a heterogeneous pathway and no obvious active species leaching was present in the catalytic process ( Figure 5). These results clearly illustrating the excellent stability and recyclability of the designed catalyst.
The versatility of the catalyst PMO@ILC 4H10O2NiCl3(1.0) for the cycloaddition of CO2 with different substituted epoxide substrates were studied under the optimal reaction conditions, and corresponding results were detailed in Table 2. The CO2 cycloaddition with different terminal epoxides containing electron withdrawing and electron donating groups exhibited good to excellent yields. All the terminal cyclic carbonates were obtained with good to high yields (84.0 %~97.6 %) and excellent selectivity (> 99 %) within the reaction time of less than 5 h. However, the reaction of cyclohexene oxide requires a longer time of 5 h to obtain a good yield ( Table 2, entry 5), which may be ascribed to the steric hindrance obstructed the nucleophilic attack of anion.
On the basis of the above results and those previously reported works, [12][13][14][15][18][19][20][21]    anion on the less sterically hindered carbon atom of epoxide generates the intermediate ii. Then, there is nucleophilic attack of the oxygen anion (ii) on the C atom of the activated CO2, forming the acyclic carbonate anion iii. Finally, cyclic carbonate is formed by subsequent intramolecular cyclic step, and the catalyst is regenerated to promote the next catalytic cycle.

CONCLUSION
In conclusion, this work demonstrates the synthesis of a type of multifunctional periodic mesoporous organosilicagrafted benzotriazolium ionic liquids PMO@ILC4H10O2NiCl3 possessing dichloro(dimethoxyethane)nickel anion and hydroxyl functionalized groups. The presence of an intramolecular synergistic effect of hydroxyl groups and dichloro(dimethoxyethane)nickel anion of ionic liquid makes them suitable as effective heterogeneous catalysts in the cycloaddition of CO2 to epoxides. The catalytic results demonstrated that PMO@ILC4H10O2NiCl3(1.0) showed higher activity than other ionic liquid concentration immobilized ILs catalysts. Moreover, PMO@ILC4H10O2NiCl3(1.0) with excellent stability is a reusable catalyst that can be recycled for five consecutive times without significant loss of activity. These discoveries suggests that PMO@ILC4H10O2NiCl3(1.0) can act as a potentially active novel heterogeneous catalyst for the cycloaddition.