In
this paper, we present a comparative study of the adsorption
behavior of short chain alcohols (pure and in aqueous solution) into
silicalite-1 (MFI-type zeolite) and silicalite-2 (MEL-type zeolite).
For quite some time, silicalite-1 has been the reference material
to address the problem of adsorptive-based separation, mostly for
hydrocarbon mixtures. Interestingly, being structurally close to silicalite-1,
adsorption studies using silicalite-2 are scarce and to the best of
our knowledge, a comparative study of their behavior for alcohol–water
mixtures has not been published to date. We have here resorted to
molecular simulation techniques to analyze the adsorption and diffusion
phenomena in both zeolites at 25 and 50 °C for pure methanol,
ethanol, 1-butanol, and water, and for some relevant compositions
of alcohol/water mixtures. In addition to the dilute regime in the
mixture, our study ranges from intermediate alcohol concentrations
to alcohol-rich phases, relevant to alcohol purification processes.
Besides, we have performed volumetric and calorimetric measurements
of single-component adsorption of alcohols in pure silica MEL zeolite,
which were used to validate the model potentials used in the simulations.
We observe that the zigzag channels of MFI zeolite are most likely
responsible for its somewhat higher affinity for alcohols. This leads
to higher adsorption selectivities when compared to those of MEL zeolite.
We have also found that the choice of water model strongly conditions
water coadsorption into the zeolites and subsequently the predictions
of the adsorbent’s selectivity in alcohol/water systems. Despite
considerable differences for adsorbed pure components, diffusivities
of alcohol and water adsorbed from mixtures are relatively similar,
as a consequence of the strong hydrogen bonds between hydroxyl groups
and water.