(228d) Advanced Textural Characterization of Carbon Molecular Sieves Complementing the Development of Material Structure and Process Performance Correlations

As a consequence of the tailored pore structure, carbon molecular sieves (CMS) enable separations of industrially relevant gas mixtures, including O₂/N₂, CO₂/CH₄, N₂/CH₄, or CO₂/N₂ by exploiting a significant difference in mass transfer rates among adsorptives. Thus, PSA/VSA techniques utilize primarily that effect for the efficient production of purified gas streams. The proper design of kinetic-based separation processes requires an extended, application-performance-oriented information about the selected adsorbent, as adsorption capacity, mass transfer kinetics, selectivity, or regenerability. However, as these material characteristics result from the adsorbent structural properties, specifically surface-topology and surface-chemistry, acquiring that structure-oriented information shall not be omitted, as it is fundamentally in control of the final performance of PSA/VSA units.

The reliable textural characterization of CMS materials remains challenging due to their complex pore network, consisting of macropores and ultramicropores connected by very narrow necks. Physisorption of argon and nitrogen at 87 K and 77 K, respectively, remains one of the most widely applied techniques within this context, however, it does not allow for an accurate and comprehensive characterization of CMS, given substantial diffusion limitations within the narrow neck at cryogenic temperatures. Thus, a selection of suitable probe molecules – considering their size, shape, and polarity – as well as the choice of appropriate testing conditions given available validated mathematical solutions, is not trivial. Moreover, an additional complexity dimension to the stated problem lies within the lack of standardized characterization protocols to determine the specific CMS structure-oriented properties beyond the simple assessment of their pore size distribution, as the distribution of neck sizes including their distinct geometry, specific location/orientation of deposited pyrocarbon layers forming the neck, or type/quantity/location/orientation of surface functional groups as well as associated charge distribution, especially around the entrance of ultramicropores.

To address these challenges, we have developed a novel methodology targeted towards a comprehensive and detailed textural and surface-chemistry characterization of CMS materials, based on the combination of gas and vapor physisorption of proper adsorptives at unique testing conditions, coupled with advanced data reduction methods. Moreover, to conclude about the material structural changes during its production, characterization tests are performed on both the microporous carbon precursor as well as the CMS adsorbent, specifically at individual stages of the manufacturing procedure chain. On top of that, we demonstrate the unique correlations among different material assessment strategies, namely textural-based and process-based methods, particularly to highlight the importance of diverse complementary techniques towards a holistic understanding of CMS synthesis-structure-performance association.

Combining the expertise on production strategies of CMS with advanced textural characterization and process-based assessment enables the fabrication of improved adsorbents in favor of PSA/VSA systems exhibiting reduced CAPEX and/or OPEX as well as the design of novel materials for unique applications.