The accuracy challenge of the DFT-based molecular assignment of 13C MAS NMR characterization of surface intermediates in zeolite catalysis,Physical Chemistry Chemical Physics

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The accuracy challenge of the DFT-based molecular assignment of 13C MAS NMR characterization of surface intermediates in zeolite catalysis
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2020-09-30 , DOI: 10.1039/d0cp04439c
Alexander A. Kolganov _{1,

2,

3,

4} , Anton A. Gabrienko _{1,

2,

3,

4,

5} , Ivan Yu. Chernyshov _{4,

6,

7,

8,

9} , Alexander G. Stepanov _{1,

2,

3,

4,

5} , Evgeny A. Pidko _{4,

6,

7,

8,

9}

Affiliation

The influence of the model and method choice on the DFT predicted ¹³C NMR chemical shifts of zeolite surface methoxide species has been systematically analyzed. Twelve ¹³C NMR chemical shift calculation protocols on full periodic and hybrid periodic–cluster DFT calculations with varied structural relaxation procedures are examined. The primary assessment of the accuracy of the computational protocols has been carried out for the Si–O(CH₃)–Al surface methoxide species in ZSM-5 zeolite with well-defined experimental NMR parameters (chemical shift, δ(¹³C) value) as a reference. Different configurations of these surface intermediates and their location inside the ZSM-5 pores are considered explicitly. The predicted δ value deviates by up to ±0.8 ppm from the experimental value of 59 ppm due to the varied confinement of the methoxide species at different zeolite sites (model accuracy). The choice of the exchange–correlation functional (method accuracy) introduces ±1.5 ppm uncertainty in the computed chemical shifts. The accuracy of the predicted ¹³C NMR chemical shifts for the computational assignment of spectral characteristics of zeolite intermediates has been further analyzed by considering the potential intermediate species formed upon methane activation by Cu/ZSM-5 zeolite. The presence of Cu species in the vicinity of surface methoxide increases the prediction uncertainty to ±2.5 ppm. The full geometry relaxation of the local environment of an active site at an appropriate level of theory is critical to ensure a good agreement between the experimental and computed NMR data. Chemical shifts (δ) calculated via full geometry relaxation of a cluster model of a relevant portion of the zeolite lattice site are in the best agreement with the experimental values. Our analysis indicates that the full geometry optimization of a cluster model at the PBE0-D3/6-311G(d,p) level of theory followed by GIAO/PBE0-D3/aug-cc-pVDZ calculations is the most suitable approach for the calculation of ¹³C chemical shifts of zeolite surface intermediates.

中文翻译：

沸石催化中表面中间体的13C MAS NMR表征基于DFT的分子分配的精度挑战

系统地分析了模型和方法选择对DFT预测的沸石表面甲醇盐物种的¹³ C NMR化学位移的影响。考察了十二周期¹³ C NMR化学位移计算方案，该方案针对具有各种结构弛豫程序的完整周期和混合周期群集DFT计算。已对ZSM-5沸石中的Si–O（CH ₃）–Al表面甲醇盐物种进行了计算协议准确性的初步评估，并具有明确定义的实验NMR参数（化学位移，δ（¹³C）值）作为参考。明确考虑了这些表面中间体的不同构型及其在ZSM-5孔中的位置。由于在不同沸石位点处的甲醇氧化物种类不同，预测的δ值与59 ppm的实验值相差高达±0.8 ppm（模型精度）。交换相关函数（方法的准确性）的选择在计算出的化学位移中引入了±1.5 ppm的不确定性。预测的准确性¹³通过考虑Cu / ZSM-5沸石甲烷活化后形成的潜在中间体，进一步分析了用于计算沸石中间体光谱特征的C NMR化学位移。在表面甲醇盐附近存在Cu物种，将预测不确定性提高到±2.5 ppm。在适当的理论水平下，活性部位局部环境的完全几何弛豫对于确保实验和计算NMR数据之间的良好一致性至关重要。通过计算的化学位移（δ）沸石晶格位点相关部分的团簇模型的完全几何弛豫与实验值最一致。我们的分析表明，在PBE0-D3 / 6-311G（d，p）的理论水平上对聚类模型进行完整的几何优化，然后进行GIAO / PBE0-D3 / aug-cc-pVDZ计算是最合适的方法。计算沸石表面中间体的¹³ C化学位移。

更新日期：2020-10-20

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