Quantifying the Influence of Water on the Mobility of Aluminum Species and Their Effects on Alkane Cracking in Zeolites
Quantifying the Influence of Water on the Mobility of Aluminum Species and Their Effects on Alkane Cracking in Zeolites
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DOI：
10.1021/acscatal.1c01138
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发表时间：
2021-06
期刊：
影响因子：
12.9
通讯作者：
T. Pham;Vy T. Nguyen;Bin Wang;Jeffery L. White;S. Crossley
中科院分区：
文献类型：
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作者：
T. Pham;Vy T. Nguyen;Bin Wang;Jeffery L. White;S. Crossley
框架外铝（EFAL）在工业上的重要反应如烷烃裂解中的作用已被广泛讨论和争论。人们早就知道，水处理会影响框架铝位点，在某些情况下，可以改变活性。然而，不太清楚的是结构修饰与重要反应（如烷烃裂解和异构化）的反应活性之间的直接关系。对水在沸石改性过程中所起的多种作用及其对反应速率的影响的集体认识正在不断发展。研究人员提出，靠近br / nsted酸位（BAS）的晶格外Al可以改变与表面中间体和动力学相关过渡态相关的能量，从而提高烷烃裂解反应的速率。然而，水对这些框架外氧化铝物种迁移产生高活性位点的动力学作用知之甚少，这是本研究的重点。将水以可控脉冲的方式引入具有不同Si/Al比和EFAL密度的ZSM-5沸石中，利用正己烷裂解活性的响应来研究新活性位点的产生。脉冲技术允许水剂量解耦、晶格重排和干燥，从而能够量化与新活性位点产生相关的活化能，而不会损失结晶度或总BAS密度。此外，通过减去与分离的BAS相关的反应速率的贡献，估计与新创建的位点相关的反应速率。结果表明，高活性位点裂解所需的能垒远低于传统Brønsted位点（75 vs 110 kJ/mol）。这些新位点产生的温度依赖性表明，在水蒸气存在的情况下，与它们产生相关的动力学步骤的活化能为44 kJ/mol，据我们所知，这在以前还没有被量化过。据报道，虽然水蒸气对于这些新的活性位点的产生是必不可少的，但它也与这些位点结合并强烈抑制裂解速率。这些发现澄清了一些关于水在活动增强中的作用的相互矛盾的报告。
The role of extra-framework Al (EFAL) species on industrially important reactions such as alkane cracking has been extensively discussed and debated. It has long been known that water treatments influence the framework aluminum sites and, in some cases, can modify activity. What is less understood, however, is the direct relationship between the structural modifications and reactivity of important reactions such as alkane cracking and isomerization. The collective understanding of the multiple roles that water plays in the modification of zeolites and influence on reaction rates is continuously evolving. Extra-lattice Al species in close proximity to a framework Brønsted acid sites (BAS) have been proposed to modify the energies associated with surface intermediates and kinetically relevant transition states, which results in an enhancement in the rates of alkane cracking reactions. However, the kinetic role of water on the migration of these extra-framework alumina species to generate highly active sites is less understood and is the focus of this study. Water is introduced in controlled pulses to ZSM-5 zeolites with various Si/Al ratios and EFAL densities, with responses inn-hexane cracking activity used to investigate the generation of new active sites. A pulse technique allows decoupling of water dosing, lattice rearrangement, and drying, thereby enabling the quantification of activation energies associated with the generation of new active sites without losses in crystallinity or total BAS density. Further, by subtraction of the contributions to the reaction rate associated with isolated BAS, the reaction rate associated with the newly created sites is estimated. The results show that the energy barrier required for cracking on highly active sites is much lower than that observed on traditional Brønsted sites (75 vs 110 kJ/mol). The temperature dependence for the generation of these new sites reveals a 44 kJ/mol activation energy for the kinetically relevant step associated with their generation in the presence of water vapor, which to the best of our knowledge has not been previously quantified. It is reported that, while water vapor is essential for the generation of these new active sites, it also binds to these sites and strongly inhibits the cracking rate. These findings clarify some of the conflicting reports regarding the role of water in activity enhancement.