21908
J. Phys. Chem. B 2005, 109, 21908-21914
Water Mediated Proton Transfer in a Mesostructured Aluminosilicate Framework: An ab Initio Molecular Dynamics Study Hong Li,*,† S. D. Mahanti,† and Thomas J. Pinnavaia‡ Department of Physics and Astronomy and Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824 ReceiVed: July 25, 2005; In Final Form: September 27, 2005
The proton transfer process mediated by water molecules adsorbed in an aluminosilicate framework has been studied using ab initio molecular dynamics simulations. This investigation has been carried out using a quasione-dimensional model simulating the mesoporous aluminosilicate channel structures. The effects of both the water loading and temperature of the system have been considered. At low coverage (one water molecule per acid site), the hydroxonium ion (H3O)+ is found to be a transition state, in agreement with earlier studies on zeolites. At a higher water coverage (two water molecules per acid site), the (H5O2)+ species and the hydrogen bonded “neutral complex” structure are both found to be stable complexes at finite temperatures. The vibrational frequency spectrum is simulated by performing a Fourier transform of the velocity autocorrelation function (VAF), and the peak positions in the VAF are compared with IR measurements and zero-temperature calculations.
I. Introduction Crystalline zeolites have been widely used as adsorbents, catalysts, and molecular sieves due to their uniform pore size and large internal surface area. In the past decade, mesoporous aluminosilicate (AS) materials with quasi-one-dimensional channel structures (MCM-41 and Al-MSU-S for instance) have attracted considerable attention due to their larger channel size (>1.5 nm) and their potential applications in the petroleum industry for cracking large chemical species.1-3 By incorporating protozeolitic nanocluster precursors, also known as “zeolite seeds”, into the AS framework, the hydrothermal stability of the framework is found to be greatly enhanced in comparison to its analogous derivatives assembled from conventional aluminate and silicate precursors. The hydrolytic stability of these materials in terms of energetics has been recently investigated using cluster models to simulate the mesoporous AS material incorporating five- and six-membered Si-O rings.4 The cluster model was found to be quite reasonable. Another area of great practical interest is the mechanism of proton transfer inside mesoporous channels in the AS framework and microporous channels in zeolites.5-24 When water molecules are adsorbed inside the zeolite channels, proton transfer from an acidic hydroxyl group attached to the framework to water and from water back to the framework are important steps in acid catalysis by zeolites. Early IR experiments5-8 supported the transfer of the acidic framework proton to water at low water coverage (
