Cooperative Role of Water Molecules during the Initial Stage of Water-Induced Zeolite Dealumination
Cooperative Role of Water Molecules during the Initial Stage of Water-Induced Zeolite Dealumination
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DOI：
10.1021/acscatal.9b00307
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发表时间：
2019-05
期刊：
影响因子：
12.9
通讯作者：
K. Stanciakova;B. Ensing;Florian Göltl;R. E. Bulo;B. Weckhuysen
中科院分区：
文献类型：
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作者：
K. Stanciakova;B. Ensing;Florian Göltl;R. E. Bulo;B. Weckhuysen
了解水诱导沸石脱铝对于控制沸石催化剂材料的水热稳定性至关重要。在这里，我们探讨了脱铝过程，重点研究了ZSM-5晶体的密度泛函理论模型中的第一个Al-O(H)键断裂步骤，每个活性中心存在一个和两个水分子。我们确定了一组由两种不同类型的反应组成的四种可能的反应机理。在前三种机理中，Al-O(H)键的断裂是由水分子的吸附和解离引起的。第四种机理不同，导致了不同的反应产物，暗示了一种替代的后续机理。在这种能量非常有利的情况下，Al-O(H)键的断裂是由两个水分子的非解离吸附引起的。因此，我们假设所提出的机制是可行的第一步脱铝。这意味着所有Al-O(H)键的断裂机制都是从亚稳态的水吸附模式开始的，而最稳定的模式中的水重组需要在Al-O(H)键的水解之前发生。我们认为，这种重排的可行性(铝的可及性)是不同地点相对发生脱铝的决定因素之一。我们进一步建立了Al位脱铝敏感性与反应条件之间的关系，该关系可进一步用于沸石的后合成处理，以控制Al的分布，从而控制催化剂的水热稳定性。
Understanding water-induced zeolite dealumination is crucial for control of the hydrothermal stability of zeolite-based catalyst materials. Here we explore the dealumination process, focusing on the first Al–O(H) bond-breaking step in a density functional theory model of a ZSM-5 crystal in the presence of a single and two water molecules per active site. We identify a set of four possible reaction mechanisms consisting of two different types of reactions. In the first three proposed mechanisms, Al–O(H) bond breaking is induced by adsorption and dissociation of an incoming water molecule. The fourth mechanism is different and leads to a different reaction product, suggesting an alternative follow-up mechanism. In this energetically very favorable case, the breaking of the Al–O(H) bond is induced by nondissociative adsorption of two water molecules. We therefore assume that the proposed mechanism is a viable first dealumination step. This implies that all Al–O(H) bond-breaking mechanisms are initiated from metastable water adsorption modes, and water reorganization from the most stable mode needs to occur prior to hydrolysis of the Al–O(H) bond. We suggest that the feasibility of this rearrangement (Al accessibility) is one of the determining factors for the relative occurrence of dealumination at different sites. We further establish a correlation between the Al site susceptibility toward dealumination and reaction conditions, which can be further used during postsynthetic treatment of the zeolite to control Al distribution and thus hydrothermal stability of the catalyst.