Li Chenchen, Lu Dongmei, Wu Chao
Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an 710054, China.
Department of Chemistry, School of Science, Xi'an Jiaotong University, Xi'an 710049, China.
Phys Chem Chem Phys. 2020 May 27;22(20):11354-11361. doi: 10.1039/d0cp01493a.
Some singly charged ionic liquids (ILs) have been reported to absorb multi-molar CO2. However, the conventional acid(CO2)-base(anion) interaction picture leads to too weak CO2 binding to support the high uptake. Later, a so-called "cation-channel" mechanism assuming the cation-to-anion proton transfer successfully explains the over equimolar CO2 uptake of some phosphonium-based ILs. Here, by employing the density functional theory (DFT) calculations, we extend the proton transfer mechanism to incorporate imidazole- and ammonium-based ILs as well. For imidazole-based ILs, carbene molecules formed after the proton transfer can react strongly with CO2. More importantly, for ammonium-based ILs, the proton transfer process is feasible only with the help of CO2 molecules. Furthermore, compared to the one IL ion pair model, the model consisting of two IL ion pairs can result in stronger CO2 absorption because it can describe the intermolecular hydrogen bonds more appropriately, especially after incorporating CO2 molecules. The relative acidity and basicity of cations and anions in ILs may be crucial for understanding their functionalization as ILs.
据报道，一些单电荷离子液体（ILs）能够吸收多摩尔的二氧化碳。然而，传统的酸（二氧化碳）-碱（阴离子）相互作用模型表明，二氧化碳的结合力过弱，无法支持如此高的吸收量。后来，一种假设阳离子到阴离子质子转移的所谓“阳离子通道”机制成功解释了一些基于鏻的离子液体对二氧化碳的超等摩尔吸收现象。在此，通过运用密度泛函理论（DFT）计算，我们将质子转移机制扩展到咪唑基和铵基离子液体。对于咪唑基离子液体，质子转移后形成的卡宾分子能与二氧化碳发生强烈反应。更重要的是，对于铵基离子液体，质子转移过程只有在二氧化碳分子的帮助下才可行。此外，与单离子对模型相比，由两个离子对组成的模型能够更恰当地描述分子间氢键，尤其是在引入二氧化碳分子后，因此对二氧化碳的吸收更强。离子液体中阳离子和阴离子的相对酸度和碱度对于理解它们作为离子液体的功能化可能至关重要。
