Probing the mechanism of CO2 capture in diamine-appended metal-organic frameworks using measured and simulated X-ray spectroscopy
Probing the mechanism of CO2 capture in diamine-appended metal-organic frameworks using measured and simulated X-ray spectroscopy
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DOI：
10.1039/c5cp02951a
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发表时间：
2015-01-01
影响因子：
3.3
通讯作者：
Kortright, Jeffrey B.
中科院分区：
文献类型：
--
作者：
Drisdell, Walter S.;Poloni, Roberta;Kortright, Jeffrey B.
二胺附加的金属-有机骨架在碳捕获应用中显示出巨大的前景，这是由于最近被归因于一种不寻常的阶梯状吸附行为，即吸附的二氧化碳分子插入金属-氮键形成有序的氨基甲酸铵链[McDonald等人，自然，2015,519,303]。我们通过原位X射线吸收光谱和密度泛函理论计算对这种机制进行了详细的研究。在mMAN-Mg(2)(Dobpdc)和mman-mN-2(Dobpdc)中，CO_2在N和O-K边都有明显的光谱变化，这些变化是基于三种潜在吸附结构的计算光谱来估算的。计算表明，观察到的光谱变化来自于特定的电子态，这些电子态是准三角平面氨基甲酸酯物种的特征，它是氢键连接到铵离子上的。这消除了所研究的三种结构中的两种，并证实了插入机制。我们注意到X射线吸收光谱对这一机制插入步骤的特殊敏感性，这为研究气体吸附的微妙化学变化的技术奠定了基础。
Diamine-appended metal-organic frameworks display great promise for carbon capture applications, due to unusual step-shaped adsorption behavior that was recently attributed to a cooperative mechanism in which the adsorbed CO2 molecules insert into the metal-nitrogen bonds to form ordered ammonium carbamate chains [McDonald et al., Nature, 2015, 519, 303]. We present a detailed study of this mechanism by in situ X-ray absorption spectroscopy and density functional theory calculations. Distinct spectral changes at the N and O K-edges are apparent upon CO2 adsorption in both mmen-mg(2)(dobpdc) and mmen-Mn-2(dobpdc), and these are evaluated based upon computed spectra from three potential adsorption structures. The computations reveal that the observed spectral changes arise from specific electronic states that are signatures of a quasi-trigonal planar carbamate species that is hydrogen bonded to an ammonium cation. This eliminates two of the three structures studied, and confirms the insertion mechanism. We note the particular sensitivity of X-ray absorption spectra to the insertion step of this mechanism, underpinning the strength of the technique for examining subtle chemical changes upon gas adsorption.