Lee Byeongno, Stowe Haley M, Lee Kyu Hyung, Hur Nam Hwi, Hwang Son-Jong, Paek Eunsu, Hwang Gyeong S
Department of Chemistry, Sogang University, Seoul 04107, Korea.
Phys Chem Chem Phys. 2017 Sep 13;19(35):24067-24075. doi: 10.1039/c7cp03803h.
Aqueous amines are currently the most promising solution for large-scale CO capture from industrial sources. However, molecular design and optimization of amine-based solvents have proceeded slowly due to a lack of understanding of the underlying reaction mechanisms. Unique and unexpected reaction mechanisms involved in CO absorption into aqueous hydrazine are identified using H, C, and N NMR spectroscopy combined with first-principles quantum-mechanical simulations. We find production of both hydrazine mono-carbamate (NH-NH-COO) and hydrazine di-carbamate (OOC-NH-NH-COO), with the latter becoming more populated with increasing CO loading. Exchange NMR spectroscopy also demonstrates that the reaction products are in dynamic equilibrium under ambient conditions due to CO exchange between mono-carbamate and di-carbamate as well as fast proton transfer between un-protonated free hydrazine and mono-carbamate. The exchange rate rises steeply at high CO loadings, enhancing CO release, which appears to be a unique property of hydrazine in aqueous solution. The underlying mechanisms of these processes are further evaluated using quantum mechanical calculations. We also analyze and discuss reversible precipitation of carbamate and conversion of bicarbonate to carbamates. The comprehensive mechanistic study provides useful guidance for optimal design of amine-based solvents and processes to reduce the cost of carbon capture. Moreover, this work demonstrates the value of a combined experimental and computational approach for exploring the complex reaction dynamics of CO in aqueous amines.
水性胺类目前是从工业源大规模捕获二氧化碳最有前景的解决方案。然而，由于对潜在反应机理缺乏了解，基于胺的溶剂的分子设计和优化进展缓慢。利用氢、碳和氮核磁共振光谱结合第一性原理量子力学模拟，确定了二氧化碳吸收到水合肼中所涉及的独特且意想不到的反应机理。我们发现生成了肼单氨基甲酸盐（NH-NH-COO）和肼二氨基甲酸盐（OOC-NH-NH-COO），随着二氧化碳负载量增加，后者的含量增多。交换核磁共振光谱还表明，由于单氨基甲酸盐和二氨基甲酸盐之间的二氧化碳交换以及未质子化的游离肼和单氨基甲酸盐之间的快速质子转移，反应产物在环境条件下处于动态平衡。在高二氧化碳负载量下，交换速率急剧上升，促进了二氧化碳的释放，这似乎是肼在水溶液中的独特性质。使用量子力学计算进一步评估了这些过程的潜在机理。我们还分析和讨论了氨基甲酸盐的可逆沉淀以及碳酸氢盐向氨基甲酸盐的转化。这项全面的机理研究为基于胺的溶剂和工艺的优化设计提供了有用的指导，以降低碳捕获成本。此外，这项工作证明了结合实验和计算方法探索二氧化碳在水性胺类中复杂反应动力学的价值。
