Liao er qing hua wu cui hua er yang hua tan jia qing de li lun yan jiu

Ph.D.Carbon dioxide is potentially one of the cheapest carbon feedstocks for the synthesis in industrial chemical production. One typical reaction is hydrogenation of carbon dioxide into formic acid catalyzed by Ru dihydride phosphine complexes. Despite many theoretical studies, there are still many important problems unresolved in the catalytic cycles. By using DFT calculations, a systematic investigation is done in this thesis to understand the underlying mechanisms of carbon dioxide hydrogenation catalyzed by Ru dihydride complexes.First, the catalytic cycle for carbon dioxide hydrogenation catalyzed by (PMe₃)₄RuH₂ in supercritical fluid is investigated. By comparing with the mechanisms previously reported, we demonstrate that as the first reaction step, a carbon dioxide goes through a direct attack on the Ru-H bond to form a formate ion, rather than the previous suggestion that CO₂ takes up an empty coordination site produced by the dissociation of a PMe₃ ligand. Then, the formate ion binds to Ru as a bidentate ligand, resulting in dissociation of a PMe₃ ligand, which is accelerated in supercritical fluid due to the entropy effect. Finally, there is a hydrogen insertion step and a PMe₃ ligand re-coordinates to Ru to release the formic acid product. A kinetic model is also worked out which is consistent with the experiment.Second, the reaction cycle for another similar catalyst (dmpe)₂RuH₂ with dmpe = Me₂PCH₂CH₂PMe₂ is also calculated. As dmpe is a bidentate ligand and binds more strongly to Ru, there is little entropy enhancement when the formate ion becomes a bidentate ligand. The catalytic cycle will go through direct carbon dioxide insertion followed by direct hydrogen insertion without a bidentate formate ion as an intermediate. It produces a larger overall activation energy for (dmpe)₂RuH₂ than that for (PMe₃)₄RuH₂ and a relatively low experimental efficiency. However, the bidentate formate ion intermediate still plays a crucial role in cis-trans transformation for the (dmpe)₂RuH₂ catalyst.Third, the acceleration effect of trace water as addictive in the carbon dioxide hydrogenation catalyzed by (PMe₃)₄RuH₂ is theoretically investigated. AIMD results show that water molecules can solvate around the Ru catalyst, but there is no significant change in the activation barriers. Only after the first catalytic cycle, it becomes dynamically possible for a water molecule to be coordinated to the Ru center. Such a water complex would dramatically accelerate carbon dioxide insertion with the hydrogen bond, and lower the overall activation energy.Finally, the acceleration effect of methanol addictive in the carbon dioxide hydrogenation catalyzed by (PMe₃)₄RuH₂ is also theoretically investigated. When methanol is used as an addictive, the overall mechanism is similar to the case of water, although it performs better due to its higher solubility in supercritical carbon dioxide. The esterification process between methanol and formic acid is also examined. The esterification barrier is a little lower in the presence of Ru catalyst.對於化工生產合成來說二氧化碳是一種潛在的低成本碳原料。其中一個典型反應就是在釕二氫化物的催化下二氧化碳加氫生成甲酸。虽已有很多相關的理論研究，但是在催化循環中依然有很多疑問有待解決。通過密度泛函理論計算，在本論文中對釕二氫化物催化下二氧化碳加氫的深層機制進行了系統研究。首先，我們研究了在超臨界相中四（三甲基膦）二氫化釕((PMe₃)₄RuH₂)催化下二氧化碳加氫的反應循環。跟之前報到的反應機理相比，我們論證了作為反應的第一步二氧化碳應該直接進攻釕-氫鍵形成甲酸根，而非之前的二氧化碳佔用由三甲基膦配體解離形成的空配位點的提議。之後甲酸根作為一個雙齒配體和釕結合並導致一個三甲基膦配體解離，在超臨界流中這個過程由於熵效應的原因而被加速。最後一個氫分子加入並伴隨著一個三甲基膦配體重新配合到釕上進而釋放甲酸產物。相應的動力學模型也被建立了，且和實驗結果相符合。其次，我們也計算了作為相似催化劑，(dmpe)₂RuH₂ (dmpe=Me₂PCH₂CH₂PMe₂ )的反應循環。因為dmpe是一個雙齒配體而且和釕結合更緊密，在甲酸根 變成雙齒配體的時候只有很少的熵加成作用。催化循環會遵循著直接的二氧化碳的加入過程並接著在沒有雙齒甲酸根作為中間體的情況下直接加入氫氣。相比 於(PMe₃)₄RuH₂，對於(dmpe)₂RuH₂這就產生了一個更高的總體反應能壘和一個 相對較低的催化效率。然而對於(dmpe)₂RuH₂催化劑來說，雙齒甲酸根中間體依 然在順式(cis)-反式(trans)轉換中扮演著至關重要的作用。再其次，我們在理論上研究了在(PMe₃)₄RuH₂催化下二氧化碳加氫的反應中 痕量水添加劑的加速作用。第一性分子動力學模擬的結果顯示水分子會溶解在釕催化劑周圍，但是對其活化勢壘而言並沒有明顯的改變。只有在第一個催化反應循環之後，動力學上一個水分子才有可能配位到釕中心上。由於氫鍵的作用這個水配位化合物可以顯著地加速二氧化碳的加入過程並且降低整體的活化能。最後，我們在理論上研究了在(PMe₃)₄RuH₂催化下二氧化碳加氫的反應中甲 醇添加劑的加速作用。當甲醇作為添加劑使用時，由於甲醇在超臨界二氧化碳中的溶解性高導致其效果相比水更好，但整體的反應機理和水的情況是相似的。甲醇和生成的甲酸的酯化過程也做了考察。在有釕催化劑存在的條件下，這個酯化反應的勢壘會略微變低。Xia, Guangjie."October 2017."Thesis Ph.D. Chinese University of Hong Kong 2018.Includes bibliographical references.Abstracts also in Chinese.Title from PDF title page (viewed on 19, April, 2021).Xia, Guangjie