Effects of Void Environment and Acid Strength on Alkene Oligomerization Selectivity
Effects of Void Environment and Acid Strength on Alkene Oligomerization Selectivity
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DOI：
10.1021/acscatal.6b02128
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发表时间：
2016-10-01
期刊：
影响因子：
12.9
通讯作者：
Iglesia, Enrique
中科院分区：
文献类型：
--
作者：
Sarazen, Michele L.;Doskocil, Eric;Iglesia, Enrique
通道连通性、空隙环境和酸强度对轻质烯烃转化过程中低聚、β-断裂和异构化反应相对速率的影响（乙烯、丙烯、异丁烯; 2-400 kPa烯烃; 473-533 K）的微孔板上进行了检测（TON、MFI、莫尔、BEA、FAU）和中孔（无定形二氧化硅氧化铝（SiAI）、MCM-41、Keggin POM）布朗斯台德酸，具有广泛的限制空隙和酸强度。在所有的压力和转化条件下，以及在所有的催化剂上，无论它们的酸强度、空隙大小或骨架连接性如何，Skirt和区域异构体都平衡，这与醇盐中间体的快速氢化物和甲基位移以及它们的快速吸附/解吸步骤一致。这种平衡是显而易见的，从详细的化学形态的产品，也从分子内同位素混乱的所有低聚物形成的2-C-13-丙烯的TON，MFI，SiAI，和POM集群。以前的主张，通过形状选择性的影响赋予空隙环境的低聚催化骨架异构体的动力学控制可能使用不准确的列表热力学，因为我们在这项研究中显示。然而，空隙环境影响在这些酸催化烯烃反应中形成的链的尺寸分布。一维微孔铝硅酸盐主要形成真正的低聚物，预期来自给定反应物烯烃的二聚和随后的低聚事件的那些;这样的链被保留，因为它们不能生长到将抑制它们通过这些框架中的基本上圆柱形的通道扩散的尺寸。无定形SiAl和胶体二氧化硅支持的POM簇含有非常不同的强度的酸位点;两者都表现出整个空隙空间的大小变化，但在长度尺度远大于分子直径，从而通过允许它们在β-断裂事件之前排出空隙来保留真正的低聚物。强度非常不同的中孔酸（POM、SiAl）给出类似的真实异构体选择性，如在具有不同杂原子的MFI结构（X-MFI，其中X = Al、Ga、Fe、B）上也观察到的，其在酸强度上也不同;这种不敏感性反映了涉及类似离子对过渡态的低聚和β-断裂反应，因此类似地取决于共轭阴离子的稳定性。三维微孔框架包含大于其互连路径的空隙，这是交叉通道和笼形窗口结构的固有结果。因此，低聚物可以达到限制其通过互连扩散的尺寸，直到β-断裂事件形成更小和更快的扩散链。这些起伏是分子尺度的，它们的大小（这里定义为沿沿着晶内扩散路径的最大尺度与最小尺度的比率）决定了低聚断裂循环对产物的尺寸分布的贡献程度。这些贡献在由2-C-13-丙烯形成的每个分子中的C-13原子的链尺寸和数目接近其二项式分布的程度上是明显的，正如它们在具有显著波动的微孔酸上所做的那样。这些结论的一般性质是显而易见的，从类似的影响，空隙形状和连通性和酸强度对乙烯，丙烯和异丁烯反应物的选择性。
The effects of channel connectivity, void environment, and acid strength on the relative rates of oligomerization, beta-scission, and isomerization reactions during light alkene conversion (ethene, propene, isobutene; 2-400 kPa alkene; 473-533 K) were examined on microporous (TON, MFI, MOR, BEA, FAU) and mesoporous (amorphous silica alumina (SiAI), MCM-41, Keggin POM) Bronsted acids with a broad range of confining voids and acid strength. Skeletal and regioisomers equilibrate under all conditions of pressure and conversion and on all catalysts, irrespective of their acid strength, void size, or framework connectivity, consistent with rapid hydride and methyl shifts of alkoxides intermediates and with their fast adsorption desorption steps. Such equilibration is evident from detailed chemical speciation of the products and also from intramolecular isotopic scrambling in all oligomers formed from 2-C-13-propene on TON, MFI, SiAI, and POM clusters. Previous claims of kinetic control of skeletal isomers in oligomerization catalysis through shape-selective effects conferred by void environments may have used inaccurate tabulated thermodynamics, as we show in this study. The void environment, however, influences the size distribution of the chains formed in these acid-catalyzed alkene reactions. One-dimensional microporous aluminosilicates predominantly form true oligomers, those expected from dimerization and subsequent oligomerization events for a given reactant alkene; such chains are preserved because they cannot grow to sizes that would inhibit their diffusion through essentially cylindrical channels in these frameworks. Amorphous SiAl and colloidal silica-supported POM clusters contain acid sites of very different strength; both exhibit size variations across the void space, but at length scales much larger than molecular diameters, thus preserving true oligomers by allowing them to egress the void before beta-scission events. Mesoporous acids of very different strength (POM, SiAl) give similar true isomer selectivities, as also observed on MFI structures with different heteroatoms (X-MFI, where X = Al, Ga, Fe, B), which also differ in acid strength; this insensitivity reflects oligomerization and beta-scission reactions that involve similar ion-pair transition states and therefore depend similarly on the stability of the conjugate anion. Three-dimensional microporous frameworks contain voids larger than their interconnecting paths, an inherent consequence of intersecting channels and cage window structures. As a result, oligomers can reach sizes that restrict their diffusion through the interconnections, until /3-scission events form smaller and faster diffusing chains. These undulations are of molecular dimensions and their magnitude, which is defined here as the ratio of the largest scale to the smallest scale along intracrystal diffusion paths, determines the extent to which oligomerization scission cycles contribute to the size distribution of products. These contributions are evident in the extent to which chain size and the number of C-13 atoms in each molecule formed from 2-C-13-propene approach their binomial distributions, as they do on microporous acids with significant undulations. The general nature of these conclusions is evident from the similar effects of void shape and connectivity and of acid strength on selectivity for ethene, propene, and isobutene reactants.