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

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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.