Silica dissolution catalyzed by NaOH: Reaction kinetics and energy barriers simulated by quantum mechanical strategies

Abstract

Quartz dissolves in Na⁺ solutions much faster than in pure water though the experimental activation energies are essentially the same. To elucidate the mechanism, silica dissolution was simulated using Gaussian 03 software at B3LYP/3-21G* initially and the B3LYP/6-311G(2d,p) levels, applied to a modeled silica Si₄O₆(OH)₄ in the presence of an NaOH molecule and up to five water molecules. Each water molecule was added one by one to the system and approached the surface silicon in successive geometry optimizations. The atomic positions of the sodium hydroxide, the water molecules and the surface SiO₃(OH) moiety were varied to mimic the surface reaction whereas the positions of the remaining Si₃O₃(OH)₃ atoms were frozen to represent the bulk structure throughout the geometry optimization.
  In the first step a single H₂O molecule was added to the system [Si₄O₆(OH)₄ + NaOH]. None of the Si-O bonded interactions were ruptured by the intrusion of the water molecule but the surface Si was stabilized in the Q³Si site (connected to three Si-O-Si bridges), being coordinated by five oxygen atoms. The energy barrier was 63 kJ/mol.
  In each of the second and third steps one more water molecule was introduced to the system. One Si-O bonded interaction of the Si-O-Si bridges was ruptured to make Q²Si and Q¹Si sites in the second and third steps, respectively, and the energy barriers were low (22-29 kJ/mol).
  In the fourth and fifth steps, the added water molecules were prevented by the sodium ion from reaching the last Si-O-Si bridge, leaving the Si in the Q¹Si site.
  The basis set was raised to the 6-311G(2d,p) level and applied to the 63 kJ/mol barrier found in the first step to determine the maximum energy barrier in the series of reactions. The barrier increased to 88 kJ/mol (82 kJ/mol in enthalpy), which is still in the range of experimental activation energies of 46-96 kJ/mol.
  In summary sodium works to stabilize the surface silicon in penta-coordination with the energy barrier, 82 kJ/mol, resulting in longer Si-O distances and the weakening of the bonded interactions. This makes the Si-O rupture easier and faster, which gives insight as to how the presence of alkali enhances silica dissolution.