Adsorption and separation of linear and branched alkanes on carbon nanotube bundles from configurational-bias Monte Carlo simulation

The adsorption and separation of linear $\left({\mathrm{C}}_1\text{−}n{\mathrm{C}}_5\right)$ and branched (${\mathrm{C}}_5$ isomers) alkanes on single-walled carbon nanotube bundles at 300 K have been studied using configurational-bias Monte Carlo simulation. For pure linear alkanes, the limiting adsorption properties at zero coverage exhibit a linear relation with the alkane carbon number; the long alkane is more adsorbed at low pressures, but the reverse is found for the short alkane at high pressures. For pure branched alkanes, the linear isomer adsorbs to a greater extent than its branched counterpart. For a five-component mixture of ${\mathrm{C}}_1\text{−}n{\mathrm{C}}_5$ linear alkanes, the long alkane adsorption first increases and then decreases with increasing pressure, but the short alkane adsorption continues increasing and progressively replaces the long alkane at high pressures due to the size entropy effect. All the linear alkanes adsorb into the internal annular sites with preferred alignment parallel to the nanotube axis on a bundle with a gap of 3.2 Å, and also intercalate the interstitial channels in a bundle with a gap of 4.2 Å. For a three-component mixture of ${\mathrm{C}}_5$ isomers, the adsorption of each isomer increases with increasing pressure until saturation, though $n{\mathrm{C}}_5$ increases more rapidly with pressure and is preferentially adsorbed due to the configurational entropy effect. All the ${\mathrm{C}}_5$ isomers adsorb into the internal annular sites on a bundle with a gap of 3.2 Å, but only $n{\mathrm{C}}_5$ also intercalates the interstitial channels on a bundle with a gap of 4.2 Å. This work suggests the possibility of separating alkane mixtures based on differences in either size or configuration, as a consequence of competitive adsorption on the carbon nanotube bundles.

Figure 1

A periodic hexagonal nanotube bundle. The rectangle illustrates the simulation box in the $x$ and $y$ dimensions.

Figure 2

Limiting adsorption properties of the pure alkanes (${\mathrm{C}}_1,{\mathrm{C}}_2,{\mathrm{C}}_3,n{\mathrm{C}}_4,n{\mathrm{C}}_5,i{\mathrm{C}}_5$, and $neo{\mathrm{C}}_5$) on a bundle with $g=3.2\mathrm{\AA }$. (a) Isosteric heats, (b) Henry constants, (c) adsorption entropies. Dotted lines are drawn to guide the eye.

Figure 3

(Color online) Adsorption isotherms of the pure linear alkanes (${\mathrm{C}}_1,{\mathrm{C}}_2,{\mathrm{C}}_3,n{\mathrm{C}}_4$, and $n{\mathrm{C}}_5$) on a bundle with $g=3.2\mathrm{\AA }$.

Figure 4

(Color online) Adsorption isotherms of the pure ${\mathrm{C}}_5$ isomers ($n{\mathrm{C}}_5,i{\mathrm{C}}_5$, and $neo{\mathrm{C}}_5$) on a bundle with $g=3.2\mathrm{\AA }$.

Figure 5

(Color online) Adsorption of a mixture of linear alkanes $({\mathrm{C}}_1:{\mathrm{C}}_2:{\mathrm{C}}_3:n{\mathrm{C}}_4:n{\mathrm{C}}_5=5:4:3:2:1)$ on a bundle with $g=3.2\mathrm{\AA }$. (a) Isotherms, (b) selectivities with respect to ${\mathrm{C}}_1$.

Figure 6

(Color online) Adsorption of a mixture of linear alkanes $({\mathrm{C}}_1:{\mathrm{C}}_2:{\mathrm{C}}_3:n{\mathrm{C}}_4:n{\mathrm{C}}_5=5:4:3:2:1)$ on a bundle with $g=3.2\mathrm{\AA }$. (a) Radial density distributions, (b) angular distributions with respect to the $z$ axis.

Figure 7

(Color online) Radial density distributions for the adsorption of a mixture of linear alkanes $({\mathrm{C}}_1:{\mathrm{C}}_2:{\mathrm{C}}_3:n{\mathrm{C}}_4:n{\mathrm{C}}_5=5:4:3:2:1)$ on a bundle with $g=4.2\mathrm{\AA }$.

Figure 8

(Color online) Adsorption of a mixture of ${\mathrm{C}}_5$ isomers $(n{\mathrm{C}}_5:i{\mathrm{C}}_5:neo{\mathrm{C}}_5=1:1:1)$ on a bundle with $g=3.2\mathrm{\AA }$. (a) Isotherms, (b) selectivities with respect to $neo{\mathrm{C}}_5$.

Figure 9

(Color online) Radial density distributions for the adsorption of a mixture of ${\mathrm{C}}_5$ isomers $(n{\mathrm{C}}_5:i{\mathrm{C}}_5:neo{\mathrm{C}}_5=1:1:1)$ on a bundle with $g=3.2\mathrm{\AA }$.

Figure 10

(Color online) Radial density distributions for the adsorption of a mixture of ${\mathrm{C}}_5$ isomers $(n{\mathrm{C}}_5:i{\mathrm{C}}_5:neo{\mathrm{C}}_5=1:1:1)$ on a bundle with $g=4.2\mathrm{\AA }$.