Ab Initio Molecular Dynamics Study of the Dehydroxylation Reaction in a Smectite Model

Abstract

We have studied the dehydroxylation reaction in a smectite model, with the purpose of determining the influence of the octahedral layer composition and the possible role of the interlayer cations. Employing ab initio molecular dynamics and with the help of metadynamics for accelerating the reaction, we study the two possible mechanisms for the formation of the first water molecule. In the first mechanism, the migration of the proton across the octahedral vacancy takes place by means of a stable intermediate in which the proton is coordinated with an apical oxygen. The formation of this intermediate is not possible without a local deformation of the tetrahedral sheet, in which a Si−O basal bond is broken so that the Si coordinates with the O that releases the H, stabilizing its residual charge. In turn, the basal oxygen that loses the Si is stabilized by the nearest interlayer Na atoms. This tetrahedral deformation is permanent at this step of the reaction. In the second mechanism the proton migrates to the neighboring OH group in the same octahedral units. This takes place without reaction intermediates and it does not imply any permanent distortion in the structure, though it is observed that the environment of a tetrahedral Si spontaneously fluctuates between a tetrahedral and a ditrigonal bipyramidal arrangement. ■ INTRODUCTIONThe high temperature transformations of phyllosilicates play an important role in ceramic technology, in the design of new materials, in chemical and nuclear waste management, as well as in many geological processes such as earthquakes and vulcanism. Of special relevance is dehydroxylation, which has been linked to the phenomenology of diverse sediments overpressurization, to petroleum diffusion, to the smectite-illite transition, and to the rheology of sediments in the subduction zones. 1,2 Phyllosilicates are mainly composed of two structural units: SiO 4 tetrahedra and MO 6 octahedra, where M = Al 3+ , Fe 3+ , Mg 2+ , and Fe 2+ . The SiO 4 tetrahedra are linked and arranged in hexagons, forming the so-called tetrahedral or T-sheet. The MO 6 octahedra are edge linked, forming the octahedral or Osheet, and one distinguishes between the trioctahedral and the dioctahedral series. In the former all octahedral sites are occupied by divalent cations (mainly Mg 2+ or/and Fe 2+ ), whereas in the latter one out of three octahedra is left empty while the other two are occupied by trivalent cations (mainly Al 3+ and Fe 3+ ). T-and O-sheets are linked by sharing a plane of oxygens, called apical oxygens (O ap ). Nonapical oxygens of the O-sheet form hydroxyl groups, which can be located at the opposite sides of the octahedral vacancy or in the same side (trans-vacant and cis-vacant configurations, respectively). The T-and O-sheets can be associated in TO or TOT units, which are stacked along the c crystallographic axis leaving an interlayer space. TO and TOT phyllosilicates are also referred to as 1:1 and 2:1 phyllosilicates, respectively. Isomorphic cation substitutions are possible: for instance...

NMR Studies of the Dehydroxylation and Rehydroxylation (RHX) of Clays with Respect to the RHX Dating of Ceramic Materials

“…In addition, we conclude that in the tetrahedral layer a bridged hydroxyl group in the arrangement Al···HO–Si does not exist, because an 27 Al coupled signal at about 4 ppm cannot be observed. The structure of smectites during RHX was studied using ab initio molecular models by Muñoz-Santiburcio et al Their calculations show no presence of bridging OH groups.…”

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“…Continuing with the same idea that phyllosilicates are clear evidence for aqueous alteration during the accretion of CCs, the literature contains a set of computational chemistry studies based on ab initio molecular dynamic simulations essentially focused on the dehydroxylation of two phyllosilicates: pyrophillite (Molina-Montes et al, 2008a;Molina-Montes et al, 2008b;Molina-Montes et al, 2010) and smectite (Muñoz-Santiburcio et al, 2012;Muñoz-Santiburcio et al, 2016). These authors identified different competitive paths for the dehydroxilation of these hydrous minerals, but the major conclusion is that in order to carry out the reaction high temperatures (1500 K) are required because of the high activation energies (about 60 kcal mol -1 for pyrophillite and 50 kcal mol -1 for smectite).…”

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