Mechanism of CO2 hydrogenation to formates by homogeneous Ru-PNP pincer catalyst: from a theoretical description to performance optimization
Mechanism of CO2 hydrogenation to formates by homogeneous Ru-PNP pincer catalyst: from a theoretical description to performance optimization
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DOI：
10.1039/c4cy00568f
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发表时间：
2014-01-01
影响因子：
5
通讯作者：
Pidko, Evgeny A.
中科院分区：
文献类型：
--
作者：
Filonenko, Georgy A.;Hensen, Emiel J. M.;Pidko, Evgeny A.
用密度泛函理论方法研究了在DBU碱助催化剂存在下，吡啶基Ru-PNP催化剂上CO2加氢反应的反应机理。分析了在反应条件下可能存在的三种可能促进的反应通道，即脱芳烃化合物2以及协同CO2(3)和H-2(4)加成的产物。结果表明，双氢Ru-PNP络合物4提供了唯一的最低能量的反应途径，包括极化络合物5*的直接有效无障碍氢解。在这种情况下，反应速度由Ru-H的CO2活化控制，该反应以大约20kJ的摩尔(-1)的非常低的势垒进行。稳定的甲酸络合物5的形成阻碍了催化反应的进行。在这种情况下，反应速率受Ru-Ocho配位键上的H-2插入所控制，预测其势垒为65kJ·mol(-1)。密度泛函计算表明，可以通过改变反应混合物中H-2的分压来控制对特定路线的偏好。在富氢条件下，前者更容易催化。专门的动力学实验验证了这些理论预测。在不同H_2/CO_2摩尔比下测得的表观活化能与计算值完全一致。RU-PNP是一种高活性的CO2加氢催化剂，在高温下可达到10(6)h(-1)的转化频率。此外，在超过H-2时可获得的反应速率与温度的关系很小，这表明在接近环境温度的情况下可以有效地进行二氧化碳加氢。
The reaction mechanism of CO2 hydrogenation by pyridine-based Ru-PNP catalyst in the presence of DBU base promoter was studied by means of density functional theory calculations. Three alternative reaction channels promoted by the complexes potentially present under the reaction conditions, namely the dearomatized complex 2 and the products of cooperative CO2 (3) and H-2 (4) addition, were analysed. It is shown that the bis-hydrido Ru-PNP complex 4 provides the unique lowest-energy reaction path involving a direct effectively barrierless hydrogenolysis of the polarized complex 5*. The reaction rate in this case is controlled by the CO2 activation by Ru-H that proceeds with a very low barrier of ca. 20 kJ mol(-1). The catalytic reaction can be hampered by the formation of a stable formato-complex 5. In this case, the rate is controlled by the H-2 insertion into the Ru-OCHO coordination bond, for which a barrier of 65 kJ mol(-1) is predicted. The DFT calculations suggest that the preference for the particular route can be controlled by varying the partial pressure of H-2 in the reaction mixture. Under H-2-rich conditions, the former more facile catalytic path should be preferred. Dedicated kinetic experiments verify these theoretical predictions. The apparent activation energies measured at different H-2/CO2 molar ratios are in a perfect agreement with the calculated values. Ru-PNP is a highly active CO2 hydrogenation catalyst allowing reaching turnover frequencies in the order of 10(6) h(-1) at elevated temperatures. Moreover, a minor temperature dependency of the reaction rate attainable in excess H-2 points to the possibility of efficient CO2 hydrogenation at near-ambient temperatures.