Solid micellar catalysts: exploring the stability of Ru(III) single-sites in amorphous silica

Abstract

Amorphous silica plays an important role as a catalyst support due to its structural and chemical properties, though its complexity poses challenges identifying and characterizing active sites. Computational models capturing the heterogeneity of amorphous silica-supported materials are crucial in understanding these systems and their catalytic behavior. Ru^(III)@MCM-41, the first example of a solid micellar catalyst, consists of Ru^(III) sites and silanoxo basic sites incorporated into the walls of amorphous MCM-41 and stabilized by quaternary ammonium surfactant molecules (CTA⁺) in the pores. To link structure and activity, computational models were developed to elucidate the nature and activity of these Ru^(III) single-sites. The amorphous silica framework offers a wide heterogeneity of sites where ruthenium can be located. To understand how site distribution affects the activity and stability of Ru^(III) sites, a periodic model of an amorphous hydroxylated silica surface was used. The model includes 27 potential Si-to-Ru^(III) exchange and multiple Ru^(III) grafting sites, from bulk-like Si(O)₄ to single and geminal silanol groups (Si(O)₃(OH) and Si(O)₂(OH)₂, respectively). The coordination environment, ring strain, and the hydrogen bonding interactions influence the stability and activity of the Ru^(III) single-sites. Incorporating Ru^(III) in amorphous silica is highly favorable, with Si-to-Ru^(III) exchange energies up to −248 kJ mol⁻¹ in the presence of the CTA⁺ surfactant. Replacing the surfactant counterion with H⁺ results in a less favorable incorporation of Ru^(III), with exchange energies up to −118 kJ mol⁻¹, highlighting the surfactant's stabilizing role. Grafting of Ru^(III) onto silanoxo groups is also favorable in the presence of surfactant. However, space restrictions and the limited availability of neighboring silanol groups at an optimal distance made the formation of Ru^(III)(O)₄ challenging, instead favoring less stable Ru^(III)(O)₂(OH)₂ species. Our calculations suggest that Ru^(III) is predominantly present as Ru^(III)(O)₄(H₂O)₂ or Ru^(III)(O)₃(OH)(H₂O)₂ at sites within large Si-membered rings near hydroxylated regions. Various Ru^(III)-hydride species were calculated, with formation energies ranging from +21 to −113 kJ mol⁻¹. Interestingly, some highly stable Ru^(III) sites also show favorable hydride formation, making them both synthesizable and active.