Ab initio screening of cation-exchanged zeolites for biofuel purification

Abstract

Using periodic density functional theory calculations combined with four dispersion-correction schemes, we have investigated the adsorption of phenol, toluene and water for various cation-exchanged faujasite zeolites. In the context of purifying the biofuels derived from the catalytic cracking of lignocellulosic bio-oils in a fluid catalytic cracking (FCC) unit, our aim was to find a suitable material with a higher affinity for phenol versus toluene and water, which are also present in the biofuel charge. In order to identify the most promising adsorbent materials, we have undertaken a full screening of monovalent cations which can be incorporated into the faujasite-type zeolite (FAU). We have evidenced that phenol can be adsorbed in two modes: O-interaction (where phenol is adsorbed via its oxygen) and π-interaction (where phenol is adsorbed via its aromatic ring). The screening of the interaction energies of the different exchanged zeolites showed that over HFAU the phenol is selectively adsorbed via O-interaction, contrary to what is observed over alkali and transition metal exchanged zeolites where the π-interaction dominates for the adsorption of phenol, leading to weaker interaction with the zeolite compared to toluene. On the Lewis modeled acid sites, namely the defect Lewis acid site (D-LAS) and the extraframework Lewis acid site (EX-LAS), the two interaction modes of phenol show almost equivalent interaction energies. In terms of selectivity, HFAU is the most selective to phenol adsorption towards O-interaction even in the presence of toluene and H₂O molecules. Both adsorption modes of phenol present a higher interaction energy than those of toluene and H₂O once adsorbed over an EX-LAS. The alkali and metallic cations have a high affinity for toluene molecules, which prohibit their use for the biofuel purification process.