Symmetry breaking in nanostructure development of carbogenic molecular sieves: Effects of morphological pattern formation on oxygen and nitrogen transport

A comprehensive study has been undertaken to establish the primary factors that control transport of oxygen and nitrogen in polymer-derived carbogenic molecular sieves (CMS). Characterization of the nanostructure of CMS derived from poly(furfuryl alcohol) (PFA) indicates that significant physical and chemical reorganization occurs as a function of synthesis temperature. Spectroscopic measurements show a drastic decrease in oxygen and hydrogen functionality with increasing pyrolysis temperature. Structural reorganization and elimination of these heteroatoms lead to a measurable increase in the unpaired electron density in these materials. High-resolution transmission electron microscopy and powder neutron diffraction are used to probe the corresponding changes in the physical structural features in the CMS. These indicate that as the pyrolysis temperature is increased, the structure of the CMS transforms from one that is disordered and therefore highly symmetric to one that is more ordered on a length scale of 15 Å and hence less symmetric. This structural transformation process, one of symmetry breaking and pattern formation, is often observed in other nonlinear dissipative systems, but not in solids. Symmetry breaking provides the driving force for these high-temperature reorganizations, but unlike most dissipative systems, these less-symmetric structures remain frozen in place when energy is no longer applied. The impact of these nanostructural reorganizations on the molecular sieving character of the CMS is studied in terms of the physical separation of oxygen and nitrogen. These results show that the effective diffusivities of oxygen and nitrogen in the CMS vary by more than an order of magnitude across the range of synthesis temperatures studied. Although the electronic nature of the CMS leads to higher equilibrium capacity for oxygen, it is the physical nanostructure which governs the separation of these two molecules. It is concluded that the primary separation mechanism is steric and configurational in nature, a conclusion in good agreement with the general features of the kinetic hypothesis conjectured by earlier workers.