First principles, nonlocal density functional theory (DFT) calculations within the cluster approximation have been carried out in order to model the formation of Ti active sites in the pores of mesoporous silicas by the reaction of (η 5 -C 5 H 5 ) 2 TiCl 2 with terminal surface silanols. In line with recent experimental observations (Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature, 1995, 378, 159), the calculations predict that, after calcination, the dominant surface Ti species is the (≡SiO) 3 TiOH complex. However, our results imply that there could be an appreciable concentration of the (≡SiO) 2 Ti(OH) 2 complex present. The (≡SiO) 2 TidO species is energetically unfavorable. Also in line with experiment, our calculations predict that the dominant surface Ti complex is formed Via a (≡SiO) 3 Ti(η 5 -C 5 H 5 ) intermediate. Calculated structures of this Ti intermediate are not, however, in agreement with the experimental EXAFS structure, and it is suggested that the cause of this difference is the use of an oversimplified model during the EXAFS fit. Computational studies of the hydrolysis and hydration of Ti centers provide a possible rationale for the uncertainties in previous studies concerning the nature and coordination of such sites in materials synthesized by different methods and studied under a variety of conditions.

Quantum-chemical studies of ethene, propene and isobutene chemisorption at an aluminosilicate BrÔnstedsite in the zeolite chabazite are reported. Comparison of the results using di †erent cluster models and a acid qm/mm (quantum mechanical/molecular mechanical) embedded cluster approach are compared and contrasted. As in previous studies, the activation barriers for the chemisorption process leading to a surface alkoxide are found to follow a carbenium ion trend, i.e. ethene [ propene [ isobutene. In contrast to previous studies, however, results indicate that the stability of the alkoxide is also very sensitive to a number of factors, the dominant one being steric interactions with the acid site, i.e. the stability order is ethene [ propene [ isobutene. This steric e †ect and other, less dominant, contributions are only observed when host environment e †ects are included in the model, in the present case via constraints on the cluster boundaries and via the qm/mm embedded-cluster approach. The possible formation of stable carbenium ions in the pores of acidic zeolites is discussed.

Quantum chemical calculations of the reaction of two methanol molecules at model aluminosilicate Bransted acid sites are reported. The reaction, involving dehydration of one methanol molecule, resulting in formation of a surface bound CH, group was found to have an activation barrier of 130-160 kJ mol-' at the MP2/6-31G**//HF/3-21G level of theory when calculated using model acid sites containing one and two aluminosilicate tetrahedral centres. The calculated activation barrier was found to be substantially lower than those reported previously for a similar reaction involving only one methanol molecule. Different '2T' models are used to analyse the importance of the difference in the proton affinity of neighbouring lattice oxygen sites involved in the dehydration process. The calculated difference in the reaction energetics strongly suggests that, as reported previously by Kramer et al., (G. J. Kramer, R. A. van Santen, C. A. Emeis and A. K. Nowak, Nature (London) 1993, 363, 529) the difference in proton affinities of lattice oxygen sites involved in a concerted reaction is an important, structure-sensitive parameter in zeolite reactivity.

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