Characterization of Carbon Molecular Sieves Using Methane and Carbon Dioxide as Adsorptive Probes

Abstract

Nitrogen adsorption at 77 K is the current standard means for pore size determination of adsorbent materials. However, nitrogen adsorption reaches limitations when dealing with materials such as molecular sieving carbon with a high degree of ultramicroporosity. In this investigation, methane and carbon dioxide adsorption is explored as a possible alternative to the standard nitrogen probe. Methane and carbon dioxide adsorption equilibria and kinetics are measured in a commercially derived carbon molecular sieve over a range of temperatures. The pore size distribution is determined from the adsorption equilibrium, and the kinetics of adsorption is shown to be Fickian for carbon dioxide and non-Fickian for methane. The non-Fickian response is attributed to transport resistance at the pore mouth experienced by the methane molecules but not by the carbon dioxide molecules. Additionally, the change in the rate of adsorption with loading is characterized by the Darken relation in the case of carbon dioxide diffusion but is greater than that predicted by the Darken relation for methane transport. Furthermore, the proposition of inkbottle-shaped micropores in molecular sieving carbon is supported by the determination of the activation energy for the transport of methane and subsequent sizing of the pore-mouth barrier by molecular potential calculations.

Slow diffusion of methane in ultra-micropores of silicon carbide-derived carbon

“…Figure 15 Interestingly, both the ultra-micropores and the larger particle scale micropores have nearly similar heats, which at first appeared surprising. However, this is readily reconciled with the variation of interaction energy of CH 4 , and the resulting heat of adsorption, with carbon slit pore size, estimated by Rutherford et al [77] and replotted in the inset in Figure 15 As further cross-validation with simulation, Figure 15(c) compares the overall Henry constants from grand canonical Monte Carlo simulation [66] and the present measurements.…”

“…These values are significantly larger than the activation energy for micropore diffusion of methane molecules in microporous carbon of 11.7 kJ/mole found by Prasetyo et al [76]. On the other hand they are close to the activation energy of 40 kJ/mole for CH 4 diffusion in Takeda 3 Å carbon molecular sieve found by Rutherford et al [77], and slightly smaller than the activation energy of 50 kJ/mole for methane transport in a different carbon molecular sieve determined by Reid and Thomas [78]. The large activation energies obtained from the CH 4 diffusivity in the grains suggests the presence of constrictions and internal pore-mouth barriers that affect the entry of CH 4 molecules into the ultra-micropores.…”

“…The large activation energies obtained from the CH 4 diffusivity in the grains suggests the presence of constrictions and internal pore-mouth barriers that affect the entry of CH 4 molecules into the ultra-micropores. In support it is noted that based on the variation of fluid solid interaction potential energy with pore size Rutherford et al [77] estimate an effective aperture size of 0.32 nm, for the activation barrier of 40 kJ/mole found by them for methane diffusion in Takeda result in support is that the lower activation energies of 15.6 and 14.2 kJ/mol found here for the larger particle-scale micropores, are close to the value of 11.7 kJ/mole found by Prasetyo et al [76], suggesting that these pores are of a similar size range as the micropores in traditional activated carbon. This is indeed evident from the PSDs in Figure 6, showing the pore size range to be similar to that of many commercial activated carbons [36,60].…”

Adsorption equilibrium and transport kinetics for a range of probe gases in Takeda 3A carbon molecular sieve

“…Several studies have shown that the pore mouth creates a barrier to penetration which generates a resistance in series with the micropore diffusion process through the grains [8][9][10][11][12]. Small molecules experience only a small activation energy for transport through this barrier as is exemplified by carbon dioxide transport in CMS [13]. Experimental studies have shown that the uptake of large molecules has kinetic dependence described by the linear driving force (LDF) model [14][15][16][17][18].…”

Preparation and characterization of carbon molecular sieve membranes based on resorcinol–formaldehyde resin

“…However, when ultramicroporosity is involved some diffusional limitations occur and adsorption of carbon dioxide at 0 ºC is a good alternative to overcome this problem [43,44]. The adsorption equilibrium isotherms of CO2 at 0 ºC for CMSM 500 and CMSM 550 are plotted in Fig.…”