Bridging hydrodyl groups in zeolitic catalysts: a computer simulation of their structure, vibrational properties and acidity in protonated faujasites (HY zeolites)

Proton-energy differences, ammonia adsorption, and D/H-exchange barriers for methane at selected isolated Bronsted sites in zeolites FAU, MFI, BEA, ERI, and CHA are studied by combined quantum-chemical−classical (QM/MM) calculations in an attempt to understand the factors that determine the reactivity at these Bronsted sites. The barrier of the D/H-exchange reaction for methane was found to correlate well with the calculated ammonia chemisorption energy, but even better with the O−Al−O angle of the free zeolite Bronsted site the reaction is taking place on, provided the Si−O−Al−O−Si moiety over which the reaction takes place is more or less collinear. The barrier is considerably higher if this collinearity is weaker, which may be explained by the necessity of costly backbone distortions to accommodate the geometrical requirements of the transition state. This is confirmed by similarly strong correlations with the O−Al−O angle change going from the free acid site to zeolite−ammonium ion bidentate structure...

Zeolite structure and reactivity by combined quantum-chemical- classical calculations

ZEOLITES are microporous aluminosilicates which, in their protonated form, act as solid catalysts1, and are widely used in the oil and petrochemical industries for processes such as cracking, isomerization and alkylation if hydrocarbons2. The proposed mechanisms3–5 of these processes mostly involve proton transfer and formation of carbenium or carbonium ions as reactive intermediates, but the detailed function of the zeolite and in particular the relation between acidity and catalytic activity is not well understood. Here we report experimental and theoretical studies of denterium–hydrogen exchange between deuterated methane and protonated zeolites — a prototypical hetero-geneous catalytic reaction between a hydrocarbon and an acid zeolite. We monitored this slow exchange reaction in two different zeolites using infrared spectroscopy, and used ab initio quantum chemistry calculations to determine both the reaction mechanism and the acidity–activity relationship. Combining our theoretical results with recent estimates8–11 of the acidity differences within zeolites enables us to reproduce the experimentally observed reaction rates and thus to obtain a detailed microscopic picture of this heterogeneous catalytic process.

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Understanding the acid behavior of zeolites from theory and experiment

An attempt to rationalize stretching frequencies of lattice hydroxyl groups in hydrogen-zeolites

A neutron diffraction study of H, Na-Y zeolites

Quantum chemical calculations of the physical characteristics of the hydroxyl groups of zeolites

The CNDO/2 method is employed in the study of the acid strength of O−H groups of the HY and HX zeolites using their cluster models of the Si4Al2O6(OH)
 12
 2−
 and Si3Al3O6(OH)
 12
 3−
 type. It is shown that the O−H groups of the HY zeolites are of more acidic character than those of the HX zeolites. The reasons of the heterogeneity of O−H groups are shortly discussed on the basis of the theoretical results.

V. Bosáček

Papers

Quantum chemical calculations of the physical characteristics of the hydroxyl groups of zeolites

Quantum chemical study of the physical characteristics of the hydroxy groups of zeolites, III