Ab initio quantum mechanical studies of the kinetics and mechanisms of quartz dissolution: OH- catalysis

The generally accepted mechanisms of quartz dissolution in basic pH solutions can be summarized as (a) direct attack by H2O on a negatively charged surface site and (b) catalysis by hydroxide ion (OH-) on a neutral surface site. In order to test these proposals and to understand the full dynamics of the dissolution processes from first principles, we have carried out high level ab initio molecular orbital calculations to study the following reactions: (HO)(3)Si-O-Si(OHO- + H2O --> (HO)(3)Si-OH + O---Si(OH)(3) (HO)(3)Si-O-Si(OH)(3) + OH- --> (HO)(3)Si-OH + O---Si(OH)(3). Disilicic acid, (HO)(3)Si-O-Si(OH)(3), and its -1 deprodonated form, (HO)(3)Si-O-Si(OHO-, have been chosen to simulate the neutral and negatively charged quartz surface site, respectively. Their reactions with OH- and H2O, which lead to the hydrolysis of the Si-O bond, have then been thoroughly investigated. Based on our ab initio calculations, we have discovered some interesting consequences: 1) OH- attack on the hydroxyl surface will lead to the deprotonation of the surface by transferring one H to the OH-, resulting in a H-bonded H2O adsorption onto a negatively charged Si-O- site. This process is equivalent to H2O attack on the negatively charged Si-O- surface. 2) The next step involves the formation of a negatively charged fivefold coordinated Si species; this step has to overcome a large energy barrier (18.91 kcal/mol). The fivefold coordinated Si species has almost the same potential energy as the reaction precursor-the hydrogen-bonded H2O adsorption minimum-but it significantly weakens the Si-O bond. 3) The final step is the rupture of the Si-O-Si bond to form =Si-OH ...-O-Si=, with a much smaller energy barrier (4.39 kcal/mol). Therefore, our ab initio results uncover two adsorption minima in series before the cleavage of the Si-O bond and suggest that it is the formation of the adsorption fivefold coordinated Si species, not the actual hydrolysis step, that controls the activation energy and the rate of quartz dissolution in basic pH solutions. The calculated activation energy, 18.91 kcal/mol, is similar to the one (24 kcal/mol) in H3O+ catalyzed hydrolysis of Si-O-Si bond, while much smaller than that (29 kcal/mol) in pure H2O hydrolysis (see Xiao and Lasaga, 1994a), indicating the role that OH- as well as H3O+ play in catalyzing quartz dissolution. The normal mode analysis of the relevant transition states indicate that, even though H transfer has been involved in the overall reaction, the dominant mode in the rate determining step is the forming and breaking of the Si-O bond. This will lead to a small hydrogen kinetic isotope effect, which agrees well with recent experimental results at basic pH.