Thermodynamics of silica depolymerization with alcohols

摘要

Experimental and computational results describe the outlook for silica (polymeric SiO2) depolymerization with alcohols. The resonance energy of the Si-O-Si segment, calculated as 8.0 kcal/mol, or 16.0 kcal/mol of SiO2, explains the thermodynamic stability of silica. Acid-catalyzed alkylorthosilicate metathesis reactions between Si(OMe)(4) and diols indicate silicon's thermodynamic preference for diols because of favorable entropic chelation. Conversion to Si(OCH2CH2O)(2) with ethylene glycol is 74.6% and conversion to Si(OCH2CH2CH2O)(2) with 1,3-propanediol is 66.2%. The enthalpic stability of Si(OR)(4) species correlates to two main factors: (1) the O-Si-O bond angle, with a compression force constant of k(Theta-comp) = 0.0315 (kcal/mol)(degrees)(-2) and expansion force constant of k(Theta-exp) = 0.0167 (kcal/mol)(degrees)(-2); and (2) the C-O-Si-O dihedral angle, dictated by stabilizing O n -> Si-O sigma* anomeric interactions of 3.5 kcal/mol in the model com- pound Si(OMe)(4). Computationally, silica depolymerization with methanol is never exergonic, but depolymerization with 1,3-propanediol, forming Si(OCH2CH2CH2O)(2) and two equivalents of water, is exergonic above 99 degrees C in water. (C) 2020 Elsevier Ltd. All rights reserved.