(71c) Adsorption of Low Molecular Weight Perfluoroalkanes on Nanoporous Carbon

Perfluorohydrocarbons (PFCs), particularly lower molecular weight ones such as perfluoromethane (CF4) and perfluoroethane (C2F6), are powerful greenhouse gases (?super? greenhouse gases), i.e., gases which when present in the troposphere have a particular ability to absorb the outgoing infrared radiation, thus causing a temperature increase in the planet. The total worldwide emissions of PFCs are small in comparison to carbon dioxide, but their extreme chemical stability makes their persistence in the atmosphere several orders of magnitude longer in time. The global warming potential (GWP) index of these compounds is more than 10000 times higher than that of CO2. Therefore, these compounds, even with relatively small emissions, have the potential to influence climate far into the future, deeming them to be, in essence, a serious environmental problem to consider.

Although some industries have voluntarily phased out the use of PFCs, some new emerging technologies have been finding unique uses for them, particularly in the area of therapeutic medicine. They have also been suggested as markers for measuring leakage in CO2 sequestration reservoirs. Several recovery methods are available, such as low-temperature distillation (cryogenic recovery), membrane separation, and pressure swing adsorption. Activated carbon adsorption traps may be considered due to their low cost, portability and effectiveness. However, here the choice of adsorbent is critical, since relative adsorption is strongly dependent on the pore size distribution, the nature of the precursor carbon, and the activation process, which determines the type of active sites left inside the porous matrix. Typically, a thorough, expensive, and not always successful experimental search for the appropriate adsorbent and separation conditions is needed. Thus, a previous screening process performed by way of a more economical route is desired. Molecular simulation poses as an excellent alternative to determine the rough pore size and operating conditions that will favor this particular separation processes. In that context, this work is focused on the molecular-level physical details of the adsorption of CF4, C2F6, and their mixtures with nitrogen (which would be the main component of an atmospheric stream) on model graphite slit micropores.

Grand Canonical Monte Carlo simulations are reported for low molecular weight perfluorocarbons (C2F6 and CF4) and their dilute mixtures in N2 adsorbing unto slit-like graphite pores. Intermolecular potentials are taken from recently parameterizations that take into account the molecules as united atom groups and recognize explicitly the presence (in the case of the C2F6) of a point quadrupole moment. Graphite nanopores are modeled using Steele's potential.

We studied a model ambient condition stream with a 10% molar composition of C2F6 or CF4 in N2 at a total pressure of 0.1 MPa and at temperatures of 250 and 300 K and report the adsorption unto graphite slit pores of different widths spanning from 0.7 nm to 4 nm.

The adsorption selectivity towards the PFC is shown to be large, reaching values on the order of 1000. However, pore size and temperature are seen to be determining factors in obtaining these values. Both pressure and temperature can have noticeable effects on the preferential adsorption. Particularly, maxima in the excess adsorption and the selectivity curves are observed in a range between 0.7 and 1.5 nm, and a close analysis of the equilibrium configurations suggests the formation of a liquid-like layering which enhances the adsorption. This layering is seen to come from surface confinement effect and breaks down at larger pore sizes. It is a distinct phenomena unrelated to capillary condensation.

Results are shown for the adsorption isotherms, selectivity and excess adsorption curves as a function of pore width, pressure and temperature. Snapshots of the evolution of the adsorption process are shown and discussed in detail.

The results are consistent with experimental CF4/N2 selectivities in commercial activated carbons with reported pore sizes > 2 nm. However, these results indicate that optimal separation is attained at smaller pore widths than those commonly employed for commercial purposes. In fact, increases in selectivities on the order of 100 may be obtained with a suitable choice of pore size for the adsorbent, making nanoporous carbons a much more selective solvent than zeolites or membranes for which selectivities on the order of 10-20 are reported.

The results shown indicate the enormous potential of adsorption on nanoporous carbons for air remediation applications and suggest the range of parameter values of pore sizes, temperatures, etc., in which such carbons should be synthesized and/or used. We also show how molecular simulation may give physically meaningful guidelines on the design of appropriate adsorbents.