Using degrees of rate control to improve selective n-butane oxidation over model MOF-encapsulated catalysts: sterically-constrained Ag3Pd(111).
Using degrees of rate control to improve selective n-butane oxidation over model MOF-encapsulated catalysts: sterically-constrained Ag3Pd(111).
复制标题
使用速率控制程度来改善 MOF 封装催化剂模型的选择性正丁烷氧化：空间约束的 Ag3Pd(111)。
作者：
Sean T. Dix;Joseph K. Scott;Rachel B. Getman;C. Campbell
封装在金属有机框架 (MOF) 内的金属纳米粒子在催化金属附近提供空间限制，可以提高选择性，就像在酶中一样。开发了一种微动力学模型，用于在 MOF 封装的双金属纳米粒子模型上用 O2 将正丁烷区域选择性氧化为 1-丁醇。该模型由 Ag3Pd(111) 表面组成，表面装饰有 2 个原子厚的（固定）氦原子环，形成了与普通 MOF 大小相似的人造孔，在空间上限制了吸附的反应中间体。动力学参数基于使用密度泛函理论 (DFT) 计算的能量。使用速率控制程度 (DRC) 分析，在 423 K 下分析微动力学模型，以确定主要途径以及哪些物质（反应机制中吸附的中间体和过渡态）具有对不同产物的反应速率最敏感的能量。该分析表明，吸附的氧原子 O* 有助于 C-H 键的活化。不幸的是，O* 也从吸附的 1-丁醇和丁氧基中夺取 H，导致丁醛成为唯一重要的产物。这建议（1）添加水以产生更多的 OH*，从而抑制这些产生 OH* 的不需要的步骤，以及（2）消除大部分 O2 压力以减少 O* 覆盖率，从而也抑制这些步骤。结合不断增加的丁烷压力，这极大地提高了 1-丁醇的选择性（从 0 到 95%）和速率（每秒每个位点 2 个分子）。此外，产品中每个氧原子的 O2 消耗量减少了 40%。在这些条件下，丁烷中的末端H直接消除到Pd位点，所得吸附的丁基与OH*结合得到所需的1-丁醇。这些结果表明，DRC 分析为优化催化工艺条件提供了一种强有力的方法，并且有时可以通过使用 O2 和 H2O 的混合物作为氧化剂来实现高选择性氧化。基于相关但假设的催化剂的第二个微动力学模型的 DRC 分析进一步证明了这一点，其中两个步骤的活化能被修改。
Metal nanoparticles encapsulated within metal organic frameworks (MOFs) offer steric restrictions near the catalytic metal that can improve selectivity, much like in enzymes. A microkinetic model is developed for the regio-selective oxidation of n-butane to 1-butanol with O2 over a model for MOF-encapsulated bimetallic nanoparticles. The model consists of a Ag3Pd(111) surface decorated with a 2-atom-thick ring of (immobile) helium atoms which creates an artificial pore of similar size to that in common MOFs, which sterically constrains the adsorbed reaction intermediates. The kinetic parameters are based on energies calculated using density functional theory (DFT). The microkinetic model was analysed at 423 K to determine the dominant pathways and which species (adsorbed intermediates and transition states in the reaction mechanism) have energies that most sensitively affect the reaction rates to the different products, using degree-of-rate-control (DRC) analysis. This analysis revealed that activation of the C-H bond is assisted by adsorbed oxygen atoms, O*. Unfortunately, O* also abstracts H from adsorbed 1-butanol and butoxy as well, leading to butanal as the only significant product. This suggested to (1) add water to produce more OH*, thus inhibiting these undesired steps which produce OH*, and (2) eliminate most of the O2 pressure to reduce the O* coverage, thus also inhibiting these steps. Combined with increasing butane pressure, this dramatically improved the 1-butanol selectivity (from 0 to 95%) and the rate (to 2 molecules per site per s). Moreover, 40% less O2 was consumed per oxygen atom in the products. Under these conditions, a terminal H in butane is directly eliminated to the Pd site, and the resulting adsorbed butyl combines with OH* to give the desired 1-butanol. These results demonstrate that DRC analysis provides a powerful approach for optimizing catalytic process conditions, and that highly selectivity oxidation can sometimes be achieved by using a mixture of O2 and H2O as the oxidant. This was further demonstrated by DRC analysis of a second microkinetic model based on a related but hypothetical catalyst, where the activation energies for two of the steps were modified.