Chemically accurate predictions for the effect of water on the structure and reactivity of zeolite catalysts

我们的目标是量子化学从头计算预测酸性沸石催化剂的水合和水解，以及水对其在工业相关的烃合成和转化反应中的反应性的影响。如果能够以化学精确度（能量±4 kJ/mol）对具有桥连Si-O（H）-Al基团作为活性位点的理想沸石结构进行预测，则与实验不一致的地方在于所用样品的缺陷或其他缺陷。我们专注于在合成过程中，合成后脱铝或催化转化过程中在水热条件下形成的骨架外铝氧-羟（EFA 1）物种的存在。目前占主导地位的计算方法，密度泛函理论与一个额外的参数化分散项（DFT-D），不产生可靠的结果，沸石的反应性和它们与水的相互作用。质子化水团簇的吸附能和稳定性被高估，能垒过低。O-H键太长，太弱，当与H键接合时拉伸太多，导致O-H伸缩频率的太大的红移。因此，该项目成功的关键是实施混合QM：QM方法（QM -量子力学），该方法将反应/吸附位点的二阶Møller-Plesset微扰理论（MP2）和耦合簇（CC）理论与完整周期结构的DFT-D相结合，适用于现实模型（1，000个原子）。沸石中烃反应的先前实施方案（混合MP2：DFT-D+ΔCC方法）已经显示出对于基元反应和吸附步骤产生化学准确的结果。通过实施和测试的方法，我们将生成一组广泛的化学准确数据，作为选择更广泛应用所需的计算效率更高（DFT-D）方法的参考。对于H-MFI沸石中不同类型的表面OH基团，通过可靠的QM：QM结构，计算的O-H伸缩频率和1H-NMR化学位移将允许将“自由”和H-键合的OH基团分配到不同的晶体位置或EFA 1物种。我们也将能够拒绝或接受建议EFA 1结构模型的基础上预测的光谱特征和实验之间的分歧或协议。我们将研究水与理想沸石结构的相互作用，以及与水解的Si-O-Al桥和其他EFA 1模型。比较将与观察到的等温线和量热数据使用一个新的多网站吸附模型。

We aim at quantum chemical ab initio predictions for hydration and hydrolysis of acidic zeolite catalysts and on the effect of water on their reactivity in industrially relevant hydrocarbon synthesis and transformations reactions. If predictions can be made for ideal zeolite structures with bridging Si-O(H)-Al groups as active sites with chemical accuracy (±4 kJ/mol for energies) disagreement with experiment points to defects or other imperfections of the samples used. We focus on the presence of extra-framework aluminium oxo-hydroxo (EFAl) species which form under hydrothermal conditions during synthesis, post-synthesis dealumination or catalytic conversion. The currently dominating computational approach, density functional theory with an additional parameterized dispersion term (DFT-D), does not yield reliable results for the reactivity of zeolites and their interaction with water. Adsorption energies and the stability of protonated water clusters are overestimated, and energy barriers are too low. O–H bonds are too long, too weak and too much stretched when engaged in H bonds which results in much too large red-shifts of O–H stretching frequencies. Crucial for the success of this project is therefore the implementation of a hybrid QM:QM method (QM – quantum mechanics) which combines second order Møller-Plesset perturbation theory (MP2) and Coupled Cluster (CC) theory for the reaction/adsorption site with DFT-D for the full periodic structure and is applicable to realistic models (1,000 atoms). Previous implementations for hydrocarbon reactions in zeolites (hybrid MP2:DFT-D+ΔCC method) have been shown to yield chemically accurate results for elementary reaction and adsorption steps. With the implemented and tested methods, we will generate an extensive set of chemically accurate data as reference for the selection of computationally more efficient (DFT-D) methods that are needed for broader application. With reliable QM:QM structures for different types of surface OH groups in zeolite H-MFI, calculated O–H stretching frequencies and 1H-NMR chemical shifts will permit to assign “free” and H-bonded OH groups to different crystallographic positions or to EFAl species. We will be also able to reject or accept suggested EFAl structure models based on disagreement or agreement between predicted spectroscopic signatures and experiment. We will study the interaction of water with ideal zeolite structures as well as with hydrolysed Si-O-Al bridges and other EFAl models. Comparison will be made with observed isotherms and calorimetric data using a new multi-site adsorption model.