MOF基Pd单位点催化CO酯化制碳酸二甲酯

MOF-Stabilized Pd Single Sites for CO Esterification to Dimethyl Carbonate

碳酸二甲酯(DMC)是一种重要的工业产品, 被广泛应用于制备锂离子电池溶剂和聚碳酸酯等化工产品. 一氧化碳(CO)酯化反应制DMC是一种新型的碳一技术路线, 具有成本低、反应效率高和安全等优势, 近年来引起了广泛的研究兴趣. 许多研究表明, Pd单位点催化剂(Pd SSCs)可以催化该反应高选择性合成DMC, 却易被反应物CO还原成Pd纳米颗粒, 从而导致DMC选择性快速下降. 因此, 设计合成稳定的Pd SSCs显得尤为重要. 通常, 研究人员通过增加Pd原子与载体之间的相互作用来稳定催化剂. 在各种载体中, 金属有机框架材料(MOFs)由于具有比表面积大、稳定性好、可修饰性强等特点, 是稳定单金属位点的理想载体. 选择自身较为稳定且易修饰的UiO-66-NH_(2)作为载体, 采用后合成修饰的方法将吡啶-2-甲醛引入UiO-66-NH_(2)的孔中. 吡啶-2-甲醛通过与氨基进行反应连接在UiO-66-NH_(2)的框架上, 进而创造了相近的双氮原子位点共同鳌合Pd原子, 记为PdⅡ-UiO-66-X%. 该催化剂在反应中, 展现出极高的DMC选择性, 同时也具备良好的催化稳定性, 在70 h的连续评价中, 依旧能够保持85%以上的DMC选择性. 该工作这一发现对构筑稳定Pd SSCs高选择性催化合成DMC提供了新思路.

Pd single site catalysts (Pd SSCs) are recognized to be efficient for CO esterification to a valuable product, dimethyl carbonate (DMC), but the tendency of reduction to Pd nanoparticles in CO atmosphere under reaction process leads to a reduced selectivity and limits the industrial application. Hence, the development of appropriate supports to stabilize Pd single sites is of great importance. Metal-organic frameworks (MOFs) are expected to become one of the most ideal choices for supporting SSCs due to their large specific surface area, good stability, and ease of modification. Herein, a stable and highly modifiable MOF, UiO-66-NH_(2), has been selected as the support. The pyridine-2-formaldehyde is grafted to UiO-66-NH_(2) by post-synthetic modification, connecting to the skeleton of UiO-66-NH_(2) via amine aldehyde condensation. In this way a pair of adjacent N atom sites are created to jointly chelate Pd(Ⅱ) species, yielding PdⅡ-UiO-66-X%. The Pd SSCs exhibit high DMC selectivity (>98%) in the CO esterification reaction. Moreover, PdII-UiO-66-X% shows excellent stability and more than 85% DMC selectivity can be maintained in 70 h continuous test, thanks to the good dispersion and strong interaction of Pd(Ⅱ) species and the MOF support. As a control, the Pd(Ⅱ) species is supported on ZrO_(2) to give PdⅡ/ZrO_(2). Obviously, it is difficult for ZrO_(2) to disperse Pd(Ⅱ) species and to strongly interact with them due to the absence of binding groups. As a result, PdII/ZrO_(2) is rapidly reduced by CO during the reaction, resulting in decreased DMC selectivity. In addition, the signals of key intermediates are detected and the reaction mechanism on PdⅡ-UiO-66-X% is proposed based on in-situ diffuse reflectance infrared Fourier transform (in-situ DRIFT) spectra. The successful fabrication of PdII-UiO-66-X% with high selectivity and stability provides great opportunity for CO esterification to DMC.