Professor of Chemistry
Humboldt University Berlin
Ab initio free energy calculations for adsorption and reactions in nanoporous systems
Wednesday, April 26, 2017
to 5:00 PM
180 Dell Butcher Hall
6100 Main St
Houston, Texas, USA
The ab initio prediction of reaction rate or equilibrium constants for systems with hundreds of atoms with an accuracy that is comparable to experiment is a challenge for computational quantum chemistry. We present a divide-and-conquer strategy that departs from a potential energy surface obtained by standard density functional theory with inclusion of dispersion. The energies of the reactant and transition structures are refined by wave-function-type electron correlation calculations for the reaction site. Thermal effects and entropies are calculated from vibrational partition functions. Anharmonic frequencies are calculated for each vibrational mode separately. For a key reaction of an industrially relevant catalytic process, the methylation of small alkenes over zeolites, we obtain results that agree within chemical accuracy limits with experiment. Deviations are within one order of magnitude for rate constants (free energies), and pre-exponential factors (entropies), and within 4 kJ/mol for enthalpy barriers.
This methodology also yields chemically accurate (± 4 kJ/mol or less) free energies of adsorption (Henry constants) for small molecules on Brønsted sites in zeolites, or on metal ion and linker sites on the internal surfaces of metal organic frameworks (MOF).
The accurate ab initio prediction of adsorption isotherms and selectivities with no other input than the atomic positions is prerequisite to a rational design of materials for gas storage, e.g. for energy carriers such as H2, and separation, e.g. removal of CO2 from CH4. This we achieve with Grand Canonical Monte Carlo (GCMC) simulations on a lattice of adsorption sites. The Hamiltonian is defined by Gibbs free energies of adsorption on individual sites and lateral interaction energies (adsorbate-adsorbate) calculated ab initio.
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