Differential adsorption of phenol derivatives onto a polymeric sorbent: A combined molecular modeling and experimental study

Oxygenated aromatic compounds (OACs) are used for the synthesis of a variet y of commercial products. Lignin from wood and other plant products are pot ential sources of OACs, but it is difficult to separate the mixtures of OAC s found in digests of these raw materials. One promising separation approac h involves the use of selective adsorption resins, such as the acrylic este r sorbent XAD-7. It has been shown previously that this sorbent binds the p ara isomer of one OAC, methoxyphenol, more favorably that the ortho isomer when hexane is used as the solvent. The present study uses a combination of molecular modeling and experiment to elucidate the mechanism of this selec tivity. The calculations yield good agreement with experimental binding aff inities and indicate that hydrogen bonding is the dominant mode of adsorpti on of para-methoxyphenol onto XAD-7 from hexane. In contrast, ortho-methoxy phenol appears to form an intramolecular hydrogen bond that weakens the int ermolecular hydrogen bond to the sorbent. As a consequence, ortho-methoxyph enol binds less strongly, and its association is dominated by van der Waals interactions and three-centered hydrogen bonds. This result is supported b y quantum mechanical calculations and infrared spectroscopic experiments. I t is also found that when water is the solvent, hydrogen bonding becomes an insignificant adsorption mechanism, and both molecules bind to the resin v ia nonpolar interactions. This explains the loss of selectivity that is obs erved in both experiment and calculation.