P-Site Structural Diversity and Evolution in a Zeosil Catalyst
P-Site Structural Diversity and Evolution in a Zeosil Catalyst
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DOI：
10.1021/jacs.0c11768
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发表时间：
2021-01-25
影响因子：
15
通讯作者：
Scott, Susannah L.
中科院分区：
文献类型：
--
作者：
Jain, Sheetal K.;Tabassum, Tarnuma;Scott, Susannah L.
磷改性的硅质沸石或P-沸石催化生物质衍生物选择性脱水为平台化学品，如对二甲苯和1，3-丁二烯。在这些反应过程中产生的水是催化活性的关键因素，但水解对结构，酸度和活性位点分布的影响在很大程度上是未知的。在这项研究中，P-网站的全硅自柱撑pentasil（P-SPP）与低P-负载（Si/P = 27），确定了固态P-31 NMR使用频率选择性检测。该技术解决了与固相共价结合的P-位点的重叠信号，以及限制在沸石中但不连接到沸石的低聚物。动态核极化提供了进行Si-29过滤的P-31检测和P-31-P-31相关实验所需的灵敏度。上述技术使我们能够区分P-O-Si键的位点和P-O-P键的位点。光谱揭示了一个以前不欣赏的多样性的P-网站，包括证据表面结合的低聚物。在干燥的P-沸石中，基本上所有的P-位点都锚定在固相上，包括含有[Si-O-P-O-P-O-Si]基序的单核位点和双核位点。当暴露在潮湿环境中时，即使在室温下，完全凝聚的网站也会迅速演变。部分水解的物种具有宽范围的酸度，从其计算的LUMO能量推断。一些P-O-Si键的初始裂解导致表面结合的单-和寡聚P-位点的混合物随着酸度的增加而逐渐形成。随后的P-O-P裂解导致酸性降低，因为P-位点最终转化为H3 PO 4。鉴定P-沸石中的酸性中心并描述其结构和稳定性的能力将在通过调节其水含量来控制微孔催化剂的活性方面发挥重要作用。
Phosphorus-modified siliceous zeolites, or P-zeosils, catalyze the selective dehydration of biomass derivatives to platform chemicals such as p-xylene and 1,3-butadiene. Water generated during these reactions is a critical factor in catalytic activity, but the effects of hydrolysis on the structure, acidity, and distribution of the active sites are largely unknown. In this study, the P-sites in an all-silica self-pillared pentasil (P-SPP) with a low P-loading (Si/P = 27) were identified by solid-state P-31 NMR using frequency-selective detection. This technique resolves overlapping signals for P-sites that are covalently bound to the solid phase, as well as oligomers confined in the zeolite but not attached to the zeolite. Dynamic Nuclear Polarization provides the sensitivity necessary to conduct Si-29-filtered P-31 detection and P-31-P-31 correlation experiments. The aforementioned techniques allow us to distinguish sites with P-O-Si linkages from those with P-O-P linkages. The spectra reveal a previously unappreciated diversity of P-sites, including evidence for surface-bound oligomers. In the dry P-zeosil, essentially all P-sites are anchored to the solid phase, including mononuclear sites and dinuclear sites containing the [Si-O-P-O-P-O-Si] motif. The fully-condensed sites evolve rapidly when exposed to humidity, even at room temperature. Partially hydrolyzed species have a wide range of acidities, inferred from their calculated LUMO energies. Initial cleavage of some P-O-Si linkages results in an evolving mixture of surface-bound mono- and oligonuclear P-sites with increased acidity. Subsequent P-O-P cleavage leads to a decrease in acidity as the P-sites are eventually converted to H3PO4. The ability to identify acidic sites in P-zeosils and to describe their structure and stability will play an important role in controlling the activity of microporous catalysts by regulating their water content.