Developing quantitative synthesis-structure-function relations for framework aluminum arrangement effects in zeolite acid catalysis
Developing quantitative synthesis-structure-function relations for framework aluminum arrangement effects in zeolite acid catalysis
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DOI：
10.1016/j.jcat.2021.04.027
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发表时间：
2021-05-25
影响因子：
7.3
通讯作者：
Gounder, Rajamani
中科院分区：
文献类型：
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作者：
Bickel, Elizabeth E.;Nimlos, Claire T.;Gounder, Rajamani
铝硅酸盐沸石长期以来被认为表现为固体布朗斯台德酸的催化多样性套件[1]，[2]，[3]，部分原因是它们的骨架拓扑结构具有宽范围的尺寸（直径< 2 nm）和形状（例如，通道，口袋，笼）[4]，[5]，其通过形状选择性和扩散相关现象影响反应性[3]，[6]，[7]。Haag及其同事在沸石酸催化的早期历史中进行的一项里程碑式的研究报告了一种结构-功能关系，该关系表明沸石中的质子位点具有相同的反应性，基于在组成广泛变化的H-MFI沸石（Si/Al= 15- 10，000）中随Al含量（每克）线性增加的正己烷裂化速率（每克）[8]。作者得出结论，这些数据反映了MFI中26个独特骨架O原子中的催化等效质子[8]，但也承认另一种解释，其中所研究的沸石样品含有类似的催化不同质子分布，尽管它们的组成不同[9]。沸石合成的最新进展[10]已经提供了获得在不同的骨架位置[11]，[12]和排列[13]，[14]中具有Al取代的沸石材料的途径，导致了催化多样性可以由沸石晶格中晶体学上不同的四面体位点（T-位点）和位点集合中Al的特定位置和排列引起的报道。Al位置决定晶格O原子能够容纳电荷补偿质子活性位点，进而决定活性中间体和过渡态所在的微孔空隙，从而决定它们通过限制效应稳定的程度[15]，[16]。此外，不同相对接近度的Al-Al位点对集合体（即Al-（O-Si）x-O-Al; x≥ 1）导致不同的更替率（例如，烷烃裂解[17]，[18]，烯烃低聚[19]，[20]）和产物选择性（例如甲醇制烯烃[21]，[22]）用于与工业过程相关的酸催化反应，强调了具有不同Al位置和排列的沸石的设计是用于商业应用的催化剂开发中的新兴策略。此外，骨架Al排列影响交换的金属离子和络合物的结构和形态，所述金属离子和络合物是甲烷部分氧化[23]，[24]，[25]，[26]和脱氢芳构化[27]，[28]，[29]，烷烃脱氢[30]，[31]，[32]以及氮氧化物的各种氧化还原和分解反应[33]的活性位点的前体。[34]、[35]、[36]、[37]、[38]、[39]。我们关于Al在不同T位点和空隙环境中的位置对沸石酸催化的影响的观点可以在其他地方找到[16]，并且该观点仅关注不同Al-Al位点对集合对酸催化的影响。
Aluminosilicate zeolites have long been recognized to behave as a catalytically diverse suite of solid Brønsted acids [1],[2],[3], in part because their framework topologies possess a wide range of micropore sizes (< 2 nm in diameter) and shapes (eg, channels, pockets, cages)[4],[5], which influence reactivity via shape-selective and diffusion-related phenomena [3],[6],[7]. A landmark study by Haag and co-workers in the early history of zeolite acid catalysis reported a structure–function relation that suggested proton sites in zeolites were equally reactive, based on n-hexane cracking rates (per gram) that increased linearly with Al content (per gram) among H-MFI zeolites of widely varying composition (Si/Al= 15–10,000)[8]. The authors concluded that these data reflected catalytically-equivalent protons among the 26 unique framework O atoms in MFI [8], but also acknowledged an alternate explanation wherein the zeolite samples studied contained similar distributions of catalytically distinct protons, despite their varying composition [9].Recent advances in zeolite synthesis [10] have provided access to zeolite materials with Al substituted in distinct framework locations [11],[12] and arrangements [13],[14], resulting in reports that catalytic diversity can arise from the specific locations and arrangements of Al among crystallographically distinct tetrahedral sites (T-sites) and site ensembles in zeolite lattices. Al location determines the lattice O atoms capable of hosting charge-compensating proton active sites, and in turn the microporous voids within which reactive intermediates and transition states reside, and thus the extent to which they are stabilized by confinement effects [15],[16]. Furthermore, Al-Al site pair ensembles of differing relative proximity (ie, Al-(O-Si) x-O-Al; x≥ 1) have been reported to result in different turnover rates (eg, alkane cracking [17],[18], alkene oligomerization [19],[20]) and product selectivities (eg, methanol-to-olefins [21],[22]) for acid-catalyzed reactions relevant to industrial processes, highlighting that the design of zeolites with different Al siting and arrangement is an emerging strategy in catalyst development for commercial applications. Additionally, the framework Al arrangement influences the structure and speciation of exchanged metal ions and complexes that are precursors to active sites for methane partial oxidation [23],[24],[25],[26] and dehydroaromatization [27],[28],[29], alkane dehydrogenation [30],[31],[32], and various redox and decomposition reactions of nitrogen oxides [33],[34],[35],[36],[37],[38],[39]. Our perspective on the effects of Al location among distinct T-sites and void environments on zeolite acid catalysis can be found elsewhere [16], and this Perspective focuses solely on the effects of different Al-Al site pair ensembles on acid catalysis.