The Brønsted acidity of synthetic mica–montmorillonite (SMM) clay was studied by periodic DFT calculations. Different structural models were compared to determine the Brønsted acidity of protons of the SMM clay based on (i) isomorphous substitution of Si⁴⁺ by Al³⁺ in the tetrahedral silicate layer and additional NiF-doping (ii) in the platelets and (iii) at the edge terminations of the clay platelets. The acid strength was judged from the computed adsorption energies of ammonia and pyridine. The SMM acidity is mainly determined by the composition of the clay platelets. The strongest acidity is found in structures in which octahedral [AlO]⁺ is replaced by [NiF]⁺ adjacent to tetrahedral [Si–(OH)–Al] moieties in the tetrahedral layer. For the Brønsted acid sites in the interlayer of SMM, modification with either Ni²⁺ or F⁻ in the octahedral layers has only a minor influence on the acidity. Our data indicate that Brønsted acid sites, properly modified in the second coordination shell by electron-withdrawing F, in the interlayer and at defect sites at the edges of clay platelets (intralayer sites) can contribute to the enhanced acidity in NiF-modified SMM. Although the predicted acidity of SMM by ammonia adsorption is higher than that of faujasite zeolite, the reactivity judged from propene protonation demonstrates that zeolites are more reactive than clays. This difference seems to be the result of the curved nature of the micropores of zeolites, which stabilizes the transition states for an acid-catalyzed reaction more than flat surfaces of clays do.

中文翻译：

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NiF改性的合成云母-蒙脱土粘土的布朗斯台德酸度提高的起源†

通过定期的DFT计算研究了合成云母-蒙脱石（SMM）粘土的布朗斯台德酸度。比较不同的结构模型以确定SMM粘土的质子的Brønsted酸度，这是基于（i）四面体硅酸盐层中的Al ³⁺对Si ⁴⁺的同构取代以及对血小板的附加NiF掺杂和（iii） ）在粘土片的边缘终止处。由计算出的氨和吡啶的吸附能来判断酸强度。SMM的酸度主要由粘土片的组成决定。在八面体[AlO] ⁺被[NiF] ⁺取代的结构中发现最强的酸度在四面体层中与四面体[Si-（OH）-Al]部分相邻。在SMM，变形例的层间或者与镍的布朗斯台德酸位点²⁺或F ^-在八面体层中对酸度的影响很小。我们的数据表明，在中间配位层和粘土薄片边缘的缺陷部位（层内部位），通过吸电子F在第二配位壳中适当修饰的布朗斯台德酸性部位可有助于NiF改性SMM中增强的酸度。尽管通过氨吸附预测的SMM酸度比八面沸石高，但从丙烯质子化判断的反应活性表明，沸石比粘土更具反应性。这种差异似乎是沸石微孔弯曲性质的结果，它使酸催化反应的过渡态比粘土的平坦表面更稳定。