利用^(13)C NMR技术探究叔胺溶液中HCO_3^-的生成

Investigation of formation of bicarbonate in tertiary amines using ^(13)C NMR technique

利用13C NMR技术对CO2捕获叔胺溶剂进行了碳元素的定量研究,主要考察了对胺溶剂解吸热影响较大的HCO3-的生成规律。重点对叔胺分子结构中羟基官能团(OH)、羟烷基数目、烷基支链及氮原子(N)所连接烷基链大小对胺溶剂生成HCO3-的影响。在20℃条件下分别对1 mol·L-1具有不同CO2负载的1-二甲基氨基-2-丙醇(1DMA2P)、N-甲基二乙醇胺(MDEA)、3-二甲氨基-1-丙醇(3DMA1P)、二乙氨基-2-丙醇(1DEA2P)、N,N-二乙基乙醇胺(DEEA)、N,N-二甲基乙醇胺(DMMEA)及三乙醇胺(TEA)溶液进行了13C NMR测试研究。实验结果显示:在相同浓度的叔胺水溶液中,同一CO2负载下的叔胺-CO2-水体系中HCO3-的含量顺序为:DMMEA>MDEA>3DMA1P>1DMA2P>TEA>DEEA>1DEA2P。通过对各叔胺分子结构中N原子的电子云密度大小及空间位阻效应分析,得出如下结论:3DMA1P分子中OH官能团离N原子的距离大于其在DMMEA分子中的距离,导致其生成了较少的HCO3-;DMMEA分子中N原子上连接的烷基链小于DEEA分子中N原子上的烷基链,导致DMMEA溶液中生成了更多HCO3-;MDEA分子中羟烷基数目少于TEA分子中的羟烷基数目,且MDEA比TEA多了一个甲基,导致MDEA溶液中含有更多的HCO3-;3DMA1P相比1DMA2P、DEEA相比1DEA2P分子中都少了一个甲基支链,导致3DMA1P溶液相比1DMA2P溶液、DEEA溶液相比1DEA2P溶液生成了更多的HCO3-。

The amount of bicarbonate is a very important factor for energy consumption in solvent regeneration in CO2 capture process. More bicarbonate in rich amines will lead to lower energy cost in CO2 desorption process. To get a preciously knowledge of the formation of bicarbonate in tertiary amines for design a better absorbent for CO2 capture, a series of tertiary amines such as N-diethylethanolamine （DEEA）,1-dimethylamino-2-propanol （1DMA2P）, 1-diethylamino-2-propanol （1DEA2P）, 3-dimethyl-amino-1-propanol （3DMA1P）, N-methyldiethanolamine （MDEA）, dimethylmonoethanolamine （DMMEA） and triethanolamine （TEA） with various CO2 loading at 1 mol·L^-1 were investigated using ^13C NMR technology at 293.15 K. The amount of bicarbonate was calculated by the chemical shift of bicarboante/carboante in ^13C NMR spectra. The results showed that the order of the amount of bicarbonate in those tertiary amines is DMMEA 〉 MDEA 〉 3DMA1P 〉 1DMA2P 〉 TEA 〉 DEEA 〉 1DEA2P. Considering the effects of electron density of nitrogen atom （N） and the steric hinderance in those tertiary amines to the formation of bicarbonate in those aqueous tertiary amines solution, it can be concluded that 1） the aqueous 3DMA1P solution produced more bicarbonate compared to DMMEA for its nearer distance of OH to N; 2） the less hydroxyalkyl and one more methyl in amine molecular structure connected to N in MDEA compared to TEA led to more bicarbonate generation; 3） a smaller alkyl connected to N in DMMEA molecular structure compare to DEEA resulted in more bicarbonate was generated in aqueous DMMEA solution; and 4） one more methyl branch existed in 1DMA2P and 1DEA2P molecular structures compared to 3DMA1P and DEEA, respectively, leading to less bicarbonate generation.