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Effects of potassium on propylene epoxidation by molecular oxygen on Cu2O (111): a DFT study

机译：钾对分子氧对Cu2O丙烯环氧化的影响（111）：DFT研究

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摘要

Propylene epoxidation catalyzed by cuprous oxide (Cu2O) with molecular oxygen is significant in the industrial field, and strategies to improve the selectivity of the target product propylene oxide (PO) are highly desired. In the present work, spin-polarized density functional theory (DFT) calculations with Hubbard U correction were employed to investigate the effects of potassium on propylene epoxidation by molecular oxygen on a Cu2O (111) surface. The mechanism for propylene epoxidation can adopt two parallel pathways, an allylic hydrogen stripping (AHS) process and an epoxidation process, and acrolein, PO, propanal, and acetone can be generated. The results calculated here indicated that the valence of the absorbed O2 on the Cu2O (111) surface could be altered by the presence of potassium (K), and the absorbed O2 can be identified as a peroxygen anion (O22−) on a K-modified Cu2O (111) surface. For the primary chemistry of the propylene epoxidation mechanism, it was shown that the AHS pathway was hindered upon the addition of potassium due to the strong K–O bond as well as the steric effects of potassium, thus favoring the epoxidation process. From analysis of the energetic factor for controlling PO and acetone formation (i.e., secondary chemistry in the propylene epoxidation mechanism), it can be found that the binding strength difference between the C2H5OO˙ and species (EOOC2H5 − ECH3) had an increased effect because of the adjacency of K, which is available to improve the selectivity of PO. The effective free energy barriers for acrolein, PO, propanal, and acetone formation are 1.07, 1.05, 1.32, and 2.08 eV, respectively, on the K-modified Cu2O (111) surface, whereas they are 0.43, 0.66, 0.69, and 0.62 eV, respectively, on a pure Cu2O (111) surface, indicating that the PO formation selectivity increased in the presence of K, although its activity is decreased to some extent. Moreover, microkinetic analysis was carried out to simulate propylene epoxidation on a Cu2O (111) surface with and without K modification, and the results demonstrated that the crucial, competitive step is the first H-stripping step in the AHS process on the Cu2O (111) surface, and the rate-determining step is the generation of OOMMP2 on the K-modified Cu2O (111) surface for the formation of PO. From this study, it was found that the selectivity of PO can be improved upon the use of an alkali metal promoter, either via decreasing the catalytic activity of the first H-stripping step (reducing the oxygen basic strength) or via increasing the oxametallacycle formation activity.
机译：用分子氧催化的丙烯环氧化在工业领域很重要，并且高度期望提高靶标丙烷氧化物（PO）的选择性的策略。在目前的工作中，采用Hubbard U校正的自旋密度功能理论（DFT）计算来研究钾对分子氧对Cu2O（111）表面丙烯环氧化的影响。丙烯环氧化的机制可以采用两种平行途径，一个烯丙基氢剥离（AHS）过程和一个环氧化过程，并且可以产生丙烯醛，PO，丙酮和丙酮。此处计算的结果表明，Cu2O（111）表面吸收的O2的价可以通过钾（K）的存在来改变，并且可以将吸收的O2鉴定为在K-上的过氧阴离子（O22-）。修饰的Cu2O（111）表面。对于丙烯环氧化机制的主要化学性质，证明AHS途径由于强大的K – O键以及钾的空间作用而阻碍了AHS途径，因此有利于环氧化过程。从控制PO和丙酮形成的能量因子的分析（即丙烯环氧机制中的二级化学），可以发现，C2H5OOU和物种之间的结合强度差异（EOOC2H5 -ECH3）具有增加的作用，因为效果增加K的邻接性，可用于提高PO的选择性。丙烯醛，PO，丙酮和丙酮形成的有效自由能屏障分别为1.07、1.05、1.32和2.08 eV，在K-Modified Cu2O（111）表面上，而它们为0.43、0.66、0.66、0.69和0.62和0.62 EV分别在纯Cu2O（111）表面上，表明在存在K存在下，PO形成选择性提高，尽管其活性在一定程度上降低。此外，进行了微动力分析以模拟有或没有K修饰的Cu2O（111）表面上的丙烯环氧化，结果表明，至关重要的，竞争性的步骤是CU2O上AHS过程中的第一个H-Stripping步骤（111 ）表面，确定的速率步骤是在k修饰的Cu2O（111）表面上产生的Oommp2，以形成PO。从这项研究中，发现使用碱金属启动子可以提高PO的选择性，要么是通过减少第一个H stripping步骤的催化活性（降低氧基本强度），也可以通过增加Oxametallacycle形成活动。

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