The Promoter Role of Amines in the Condensation of Silicic Acid: A First-Principles Investigation

Abstract

Though well-recognized, the molecular-level understanding of the multifunctional roles of amines in the condensation of polysilicic acids, which is one of the key processes in hydrothermal synthesis of zeolites, is still limited. Taking ethylamine as a prototype, we investigated the mechanism of polysilicic acid condensation in the existence of organic amines in aqueous solution with extensive first-principles-based calculations. Because of the high proton affinity, ethylamine exists as amine silicates and alters the subsequent condensation mechanisms from a 1-step lateral attack mechanism accompanied with simultaneous intermolecular proton transfer in neutral aqueous solution to a 2-step S_N2-like mechanism. Specifically, the 5-coordinated Si species that were not observed on pathways of condensation in neutral solution are effectively stabilized by the ethylamine cations as intermediates, and the barriers for condensation of ortho-silicic acid are significantly reduced from 133 kJ/mol in neutral solution to 58 and 63 kJ/mol for formation of the 5-coordinated Si intermediate and proton transfer for water release, respectively. Similar variations of mechanisms and barriers for condensation were also observed in the formation of cyclic trimers as well as linear and cyclic tetramers of ortho-silicic acids. Based on these, it was proposed that apart from acting as structure-directing agents, pore fillers, and pH adjusters, organic amines can also function as promoters in the condensation of polysilicic acids.

Figure 1

Optimized structures for the hydrogen-bond (HB) complexes formed by water (a–c), water and ethylamine (d–f), ortho-silicic acid and water (g–j), disilicic acid and water (k), ethylamine (l,m), between silicic acids (n,o), and between ethylamine and silicic acids (p–s). The Si, O, H, N, and C atoms are in yellow, red, white, blue, and gray, respectively.

Figure 2

Potential energy surface condensation of two ortho-silicic acid molecules in the existence of ethylamine. The Si, O, H, N, and C atoms are in yellow, red, white, blue, and gray, respectively.

Figure 3

Potential energy surface of condensation between a trimer and a monomer of ortho-silicic acid molecules in the existence of ethylamine. The Si, O, H, N, and C atoms are in yellow, red, white, blue, and gray, respectively.

Figure 4

Potential energy surface for formation of a cyclic trimer from a linear trimer of ortho-silicic acid in the existence of ethylamine. The Si, O, H, N, and C atoms are in yellow, red, white, blue, and gray, respectively.

Figure 5

Potential energy surface for formation of a cyclic tetramer from a linear tetramer of ortho-silicic acid in the existence of ethylamine. The Si, O, H, N, and C atoms are in yellow, red, white, blue, and gray, respectively.