Dr
Lukáš Grajciar
(Department of Physical and Macromolecular Chemistry, Charles University in Prague)
The silicates, based on a tetrahedron-shaped anionic (SiO_4)^(4-) group, is one of the
most abundant classes of compounds on Earth being heavily used in industry.
However, the silicates, and in particular porous silicates doped with aluminium (zeolites), are
often being degraded and eventually destroyed upon prolonged exposure to humidity
[1]. Besides the negative effects of prolonged exposure to water, it is possible to
direct hydrolysis, which can lead to synthesis of new materials such as hierarchical
layered silicates with micro- and meso-porosity [2].
Therefore, with a goal to hinder or direct the silicate hydrolysis, numerous first
principle studies have focused on understanding the hydrolysis mechanism. However,
they were limited to static calculations not properly accounting for water dynamics in
a fully solvated material at realistic conditions [3]. In this study, we employed a
(biased) ab initio molecular dynamics to accurately incorporate entropic effects and
most importantly to allow for explicit treatment of water molecules in interaction with a
zeolites, since water is not just the solvent but also the reactant.
New hydrolysis paths, specific for solvated material, have been found, which include
Grotthuss proton-hopping mechanism or water splitting on the T-O-T (T=Si, Ge, Al)
bonds with protons solvated in the surrounding water (see Figure). These new
mechanisms have lower reaction barriers than the most stable mechanism proposed
previously using limited solvation model [2]. In addition, the new mechanisms can
help understand the feasibility of directed partial hydrolysis on the basis of different
accessibility of (TO_4)^(4-) tetrahedra and germanium clustering. Although, the current
study considered a specific porous silicate, the UTL zeolite, we expect the new
mechanisms to be of importance for other silicates as well.
Summary
Zeolites are some of the most important industrial catalysts. However, these microporous materials are often being degraded and eventually destroyed upon prolonged exposure to humidity. Surprisingly, the hydrolysis mechanism is not well understood at the atomistic level.
It is the goal of the present investigation to shed light on the mechanism of zeolite hydrolysis based on state-of-the-art computational techniques. A combination advanced biased molecular dynamics simulations has been be used to study these reactive events at realistic conditions. New favourable hydrolysis paths, specific for highly solvated material, have been discovered.In addition, the effects of solvation level, temperature, active site accessibility or heteroatom concentration (Ge, Al) have been considered.
References
[1] A. Vjunov et al. Chem. Mater. (2015), 27, 3533
[2] W. J. Roth, P. Nachtigall, R. E. Morris, J. Čejka Chem. Rev. (2014), 114, 4807
[3] S. Malola, S. Svelle, F. Bleken, O. Swang Angew. Chem. Int. Ed. (2012), 51, 652
Dr
Lukáš Grajciar
(Department of Physical and Macromolecular Chemistry, Charles University in Prague)
Dr
Christopher J. Heard
(Department of Physical and Macromolecular Chemistry, Charles University)
Ms
Mengting Jin
(Department of Physical and Macromolecular Chemistry, Charles University)
Prof.
Petr Nachtigall
(Department of Physical and Macromolecular Chemistry, Charles University)
