(520a) Understanding the Unique Sorption of Alkane-?,?-Diols in All-Silica Zeolites
Adsorption equilibria of alkane-Î±,Ï-diols (diols) from aqueous solution onto all-silica MFI-type zeolite (silicalite-1) with three to six carbons are obtained by simulations and experiments. After an initial slow rise, the diol isotherms (which show agreement between both approaches) exhibit steep changes in loading and eventually reach 7 to 10 molecules per unit cell (molec/uc). Thermodynamic and structural information, obtained from the simulations, allow for a mechanistic understanding of these adsorption equilibria. The step in the isotherms is caused by cooperative hydrogen-bonding that leads to a minimum in the free energy of transfer for the diol from vapor to zeolite. At T = 323 and 348 K, the steep change for the pentane-1,5-diol (C5) adsorption isotherm is found to be a phase transition from a low-dense phase to an ordered solid phase with a loading of 8 molec/uc. At saturation loading, nearly all C5 and C6 diols are involved in a percolating hydrogen-bonding network. With increasing temperature, the C5 isotherm becomes less steep, as the minimum in the free energy of adsorption decreases in depth while the relative population of hydrogen bonds decreases. However, the isotherm and adsorption onset do not shift significantly to higher concentration because the magnitudes of the free energies of solvation and adsorption decrease by similar and, hence, compensating amounts. The sensitivity of the calculated selectivities for C5 diol over water to the rigid MFI framework structure adopted in the simulations is investigated by randomly perturbing the framework atoms or cell parameters of experimentally determined structures. The steep change in the isotherm of C5 diol from aqueous solution is also found to occur on many other all-silica zeolites in the database available from the Structure Commission of the International Zeolite Association (http://www.iza-structure.org/databases/).