Probing the Relationship between Silicalite‑1
Defects and Polyol Adsorption Properties
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Abstract
The relationship between polyol adsorption
affinity and silanol defect density was investigated through the development
of vapor and aqueous adsorption isotherms on silicalite-1 materials
which vary in structural and surface properties. Silicalite-1 crystals
prepared through alkaline synthesis, alkaline synthesis with steaming
post-treatment, and fluoride synthesis routes were confirmed as crystalline
mordenite framework inverted (MFI) by SEM and XRD and were shown to
contain ∼8.5–0 silanol defects per unit cell by <sup>29</sup>Si MAS, <sup>1</sup>H MAS, and <sup>1</sup>H–<sup>29</sup>Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm
at 10<sup>–3</sup> <i>P</i>/<i>P</i><sub>0</sub> evolved with a decrease in silanol defects, and, through
features in the XRD and <sup>29</sup>Si MAS NMR spectra, it is postulated
that the hysteresis is the result of an orthorhombic–monoclinic
symmetry shift with decreasing silanol defect density. Gravimetric
and aqueous solution measurements reveal that propylene glycol adsorption
at 333 K is promoted by silanol defects, with a maximum 20-fold increase
observed for aqueous adsorption at ∼10<sup>–3</sup> g/mL
with an increase from ∼0 to 8.5 silanols per unit cell. A comparison
of vapor and aqueous propylene glycol adsorption isotherms on defect-free
silicalite-1 at 333 K, both of which exhibit the Type-V character,
indicates that water enhances adsorption by a factor of ∼2
in the Henry’s Law regime. Henry’s constants for aqueous
C<sub>2</sub>–C<sub>4</sub> polyol adsorption (concentrations
below 0.004 g/mL) at 298 K are shown to have a linear dependence on
the silanol defect density, demonstrating that these molecules preferentially
adsorb at silanol defects at dilute concentrations. This systematic
study of polyol adsorption on silicalite-1 materials highlights the
critical role of defects on adsorption of hydrophilic molecules and
clearly details the effects of coadsorption of water, which can guide
the selection of zeolites for separation of biomass-derived oxygenates