(520i) Molecular Dynamics Simulations of Aldol Condensation Catalyzed By Alkylamine-Functionalized Crystalline Silica Surfaces

Molecular
Dynamics Simulations of Aldol Condensation Catalyzed by
Alkylamine-Functionalized Crystalline Silica Surfaces

Ki
Chul Kim,¹ Eric G. Moschetta,² Christopher W. Jones,²
and Seung Soon Jang¹

¹School of Material
Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive NW,
Atlanta, GA 30332-0245, USA

²School of Chemical
and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive
NW, Atlanta, GA 30332-0100, USA

Abstract

Molecular
dynamics simulations are performed to investigate the cooperatively catalyzed
aldol condensation between acetone and 4-nitrobenzaldehyde on alkylamine (or
alkylenamine)-grafted silica surfaces, focusing on the mechanism of the
catalytic activation of the acetone and 4-nitrobenzaldehyde by the acidic
surface silanols followed by the nucleophilic attack of the basic amine
functional group toward the activated reactant. From the analysis of the
correlations between the catalytically active acid-base sites and reactants, it
is concluded that the catalytic cooperativity of the acid-base pair can be
affected by two factors:  1) the
competition between the silanol and the amine (or enamine) to form a hydrogen
bond with a reactant and 2) the flexibility of the alkylamine (or alkylenamine)
backbone. Increasing the flexibility of the alkylamine facilitates the
nucleophilic attack of the amine on the reactants. From the molecular dynamics
simulations, it is found that C3 propylamine and C4 butylamine linkers exhibit
the highest probability of reaction, which is consistent with the experimental
observation that the activity of the aldol reaction on mesoporous silica
depends on the length of alkylamine grafted on the silica surface. This
simulation work serves as a pioneering study demonstrating how the molecular
simulation approach can be successfully employed to investigate the cooperative
catalytic activity of such bifunctional acid-base catalysts.