(219e) Assessment of Options for Determining the Total Adsorption Uptake from Liquid Solution: Alkane-?,?-Diols/(Water or Ethanol) Onto Silicalite-1
Experiments and molecular simulations are employed to probe the total uptake of binary mixtures containing an alkane-Î±,Ï-diol with four or five carbon atoms solvated in water or ethanol onto all-silica MFI-type zeolite. One of several assumptions (ranging from no solvent adsorption to complete pore filling) must be made to determine the total uptake of solution-phase adsorption in batch experiments, as only the bulk solution-phase is open to direct measurement. The diol loadings from aqueous solution obtained using different assumptions differ significantly (more than 10% relative deviation) at both low and high concentrations (exceeding one molecule per unit cell at high concentrations), with consensus among approximations only occurring at intermediate concentrations. Different assumptions yield water loadings at low diol concentrations that vary from 0 up to 60 molecules per unit cell and can even result in negative values at intermediate and high concentrations, making a determination of the adsorption selectivity impossible. The diol loadings obtained from Gibbs ensemble simulations with transferable force fields agree very well with those determined experimentally. The water loadings from these molecular simulations follow chemical intuition for adsorption onto a hydrophobic, all-silica zeolite with zero defects. This includes almost no water loading at low diol loading (or concentration), significant water co-adsorption at intermediate diol loading, and displacement of water as saturation loading of the diol is approached. It is demonstrated that a co-adsorption model based on the simulated equilibria can be used to map the experimental adsorption of water without any significant change in diol loading over the entire concentration range. Ideal adsorbed solution theory is found to vastly overestimate the adsorption selectivity for the aqueous diol solutions. The experimental determination of the total uptake for ethanolic solutions is less fraught with errors (with exception of the no-solvent-adsorption and volume-change-by-solute-adsorption assumptions) where the solvent fills the pores until being displaced by the solute at higher concentrations. This work is expected to inform future experimental approaches and thermodynamic models for determining the total uptake from liquid solution onto nanoporous materials.