Abstract
Spherical carbon molecular sieves (CMS) have selective adsorptive properties which are suitable for separation and purification of gas mixtures. Precise methods of characterization are needed to understand the performance of CMS in separation processes. To this end, the pore size distribution (PSD) of four carbon molecular sieves were evaluated experimentally using immersion calorimetry and complemented with gas adsorption measurements at cryogenic temperatures for N2, O2 and Ar, and at 273 K for CO2. Theoretical pore size distributions were estimated using two-dimensional non-local Density Functional Theory (2D-NLDFT) models. Calorimetry results showed that B and C samples had a narrow pore size distribution with pores below 0.7 nm. Meanwhile, the pore size distributions calculated from O2 and Ar adsorption isotherms, gave an apex in the 0.5–0.6 nm region for all the carbons together with a growing development of porosity at around 0.8 nm and above for carbons A and D. The agreement observed between experiments and theory confirmed the validity of the theoretical 2D-NLDFT models to anticipate the PSD. Carbon C with pores exclusively below 0.7 nm separated CO2 and CH4 while carbon D with pores in the supermicroporous region separated propane and propylene chromatographically.
Highlights
Gas phase separations using porous carbon particles are possible thanks to their tailored textural properties [1]
The surface chemistry analysis of the carbon materials showed that the adsorbents contained oxygen in the surface which was in agreement with the oxygen content of mildly oxidized carbons [27]
The combination of gas adsorption with immersion calorimetry into liquids of different kinetic diameter allowed the evaluation of textural properties in carbon materials
Summary
Gas phase separations using porous carbon particles are possible thanks to their tailored textural properties (well-defined pore size window and pore shape) [1]. Small scale packed bed systems in analytical applications have made use of porous carbons [2]. Since carbon molecular sieves have been efficient means of separation of molecules with similar molecular weights and chemistries [3], these carbons were effective adsorbents of gas molecules such as CO2, CH4, and light hydrocarbons [4]. The separation of small molecules with similar kinetic diameters has been a challenge for the current separation systems [8]. The design of adsorbents with tuned textural characteristics and pre-defined surface chemistry aims for more efficient separations. The availability of precise methods of characterization would help to speed up the development of effective adsorbents for difficult separations
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