We first describe materials made from a precursor, known to produce high surface areas when used in templated syntheses, that we have shown also produces very high surface areas (&gt; 1200 m2/g) in syntheses which use no template and no organic solvents or complex drying steps. This is followed by an explanation of this behavior that generalizes to other materials. In sol-gel routes to porous materials, the precursors are hydrolyzed and then condensed to form a 3D polymeric network. During drying, the network substantially collapses due to strong capillary forces, unless something is done to prop it up. Our hypothesis is that, in the absence of pore-templating guest molecules or complex solvent-removal processes, the network must reach a critical level of rigidity in order to maintain a porous structure. We used rigidity theory to calculate a level of condensation present at the critical level. For such a network to become critically rigid before collapse, it must form quickly (kinetics) and be stable (thermodynamics) under the conditions of drying. We developed a descriptor, based on Rigidity Theory or Constraint Counting (J.C. Maxwell, 1864), for the level of solvent remaining in the gel at the rigidity transition. We further hypothesize that the amount of solvent remaining when rigidity is reached determines the total porosity as the originally solvent-filled volume becomes empty. We show that the theory successfully distinguishes between precursors that exhibit very high levels of porosity and surface area (&gt; 1000 m2/g), those that exhibit intermediate levels, and those that substantially collapse (&lt; 50 m2/g). The theory is simple to implement for many precursors, and is being used to predict new precursors that might make useful materials.

Predicting surface area in green synthesis of sol-gel materials

255th National Meeting (2018)

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