Toward Intrinsic Catalytic Rates and Selectivities of Zeolites in the Presence of Limiting Diffusion and Deactivation
Toward Intrinsic Catalytic Rates and Selectivities of Zeolites in the Presence of Limiting Diffusion and Deactivation
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DOI：
10.1021/acscatal.3c03559
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发表时间：
2023-09
期刊：
影响因子：
12.9
通讯作者：
Cole W. Hullfish;Jun Zhi Tan;Hayat I. Adawi;Michele L. Sarazen
中科院分区：
文献类型：
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作者：
Cole W. Hullfish;Jun Zhi Tan;Hayat I. Adawi;Michele L. Sarazen
利用可再生（即生物质）和封闭碳（即二氧化碳和废塑料）原料以原子效率和能源效率生产燃料和化学品的目标需要有效的催化剂。为了设计出能够最大限度地利用所有活性位点、提高所需反应途径的选择性并在多个生命周期中保持稳定的有效催化剂，需要对其性能进行分子了解。然而，我们的许多目标转化涉及复杂的反应网络，其中给定催化剂中活性位点之间的通信影响测量的速率和选择性，从而混淆了内在动力学的解释。当这些位点处于微孔环境中时，例如沸石和某些金属有机框架（MOF），最初进入晶体或位点之间的扩散变得更加相关，因为随着分子尺寸接近限制空隙的尺寸，有效扩散率会降低。此外，扩散增强的次级反应可以使测量的产物选择性远离活性位点固有控制的选择性。因此，表观选择性偏向于形成具有相对较低出口壁垒的产物的反应（例如，通过异构化）。这种选择有时会减轻失活，但在扩散限制的烃转化过程中，由于碳质污垢的活性位点堵塞或孔隙闭塞，失活也可能加剧。因此，反应-扩散-失活现象的严格反卷积对于提取支持下一代催化剂设计的固有速率和选择性至关重要。克服所述限制的一种催化剂设计策略是引入辅助孔隙率以增加反应物和产物物质的有效扩散率。例如，可以通过微孔载体的直接合成或合成后修饰来引入中孔。 1− 4 由于扩散限制反应系统内的选择性取决于产物逸出的难易程度和大体积过渡态的形成，因此引入二次介孔可以因扩散增强而增加周转率，并因晶内停留时间减少（从而减轻二次反应）而影响产物分布。中孔也影响结焦失活，因为微孔或中孔中的相对焦炭积累影响活性位点密度和扩散。 5− 10 这些后果可能会使（分级）沸石中反应扩散失活现象的处理变得具有挑战性，但当焦炭沉积重新分布到中孔时，也可以延长催化剂寿命并延长微孔催化作用。在这个观点中，我们讨论了如何从沸石上碳氢化合物反应的四个不同例子中的反应-扩散-失活反卷积中获得基本见解。我们首先探讨了烯烃低聚过程中烃链的增长如何与二次 β-断裂竞争，分别降低和增加产物扩散率。随着沸石孔径和连通性的变化，这些对扩散率的竞争效应作为反应进程的函数，对反应性和选择性具有重大影响。其次，我们重点介绍分级（微孔-介孔）沸石，它通过提高质子的催化效率和延迟失活的动力学表现（即抑制表观速率常数）来减轻反应-扩散-失活的协同性。为了解决分级沸石的广泛合成问题，我们描绘了一种简单的反应扩散分析方案，以确定是否选择了（后）合成介孔……
The goal of atom-and energy-efficient production of fuels and chemicals from renewable (ie, biomass) and closed-carbon (ie, CO2 and waste plastic) feedstocks requires effective catalysts. In order to design effective catalysts that utilize all active sites to their fullest extents, increase selectivity to desired reaction pathways, and remain stable for multiple life cycles, a molecular understanding of their performance is needed. However, many of our target conversions involve complex reaction networks where communication between active sites in a given catalyst affects measured rates and selectivities, thus obfuscating interpretation of intrinsic kinetics. When these sites are in microporous environments, such as zeolites and certain metal− organic frameworks (MOFs), the initial ingress into a crystal or diffusion between sites becomes even more relevant because effective diffusivities decrease as molecule sizes approach that of confining voids. Further, diffusionenhanced secondary reactions can shift measured product selectivities away from those intrinsically governed by the active site. Apparent selectivities consequently skew toward reactions that form products with relatively low egress barriers (eg, through isomerization). This selection sometimes mitigates deactivation, but deactivation can also be exacerbated during diffusion-limited hydrocarbon conversions due to active site blockage or pore occlusion by carbonaceous foulants. Thus, rigorous deconvolution of reaction− diffusion− deactivation phenomena is vital for the extraction of intrinsic rates and selectivities that will support next-generation catalyst design. One catalyst design strategy to overcome said limitations is the introduction of auxiliary porosity to increase effective diffusivities of reactant and product species. For example, mesopores may be introduced through direct synthesis or postsynthetic modification of microporous supports. 1− 4 Since selectivities within diffusion-limited reaction systems depend on the ease of product egress and formation of bulky transition states, introduction of secondary mesoporosity can increase turnovers due to enhanced diffusion and impact product distributions due to reduced intracrystalline residence times (which mitigate secondary reactions). Mesopores also influence deactivation by coking because relative coke accumulations in micropores or mesopores affect both active site density and diffusion. 5− 10 These consequences can make treatments of reaction− diffusion− deactivation phenomena in (hierarchical) zeolites challenging but can also extend the catalyst lifetimes and prolong micropore catalysis when coke deposition is redistributed to mesopores. In this Viewpoint, we discuss how fundamental insights are gained from deconvolution of reaction− diffusion− deactivation in four different examples of hydrocarbon reactions on zeolites. We first explore how hydrocarbon chain growth during alkene oligomerization competes with secondary β-scission to decrease and increase product diffusivities, respectively. These competing effects on diffusivity as a function of reaction progress have significant implications on reactivity and selectivity as the zeolite pore size and connectivity change. Second, we highlight hierarchical (microporous− mesoporous) zeolites, which mitigate the cooperativity of reaction− diffusion− deactivation by increasing catalytic efficiency of protons and delaying kinetic manifestations of deactivation (ie, dampened apparent rate constants). To grapple with the wide synthetic scope of hierarchical zeolites, we delineate a straightforward reaction− diffusion analysis scheme to determine whether selected (post) synthetic mesopore …