Sieving di-branched from mono-branched and linear alkanes using ZIF-8: experimental proof and theoretical explanation

Abstract

We study the adsorption equilibrium isotherms and differential heats of adsorption of hexane isomers on the zeolitic imidazolate framework ZIF-8. The studies are carried out at 373 K using a manometric set-up combined with a micro-calorimeter. We see that the Langmuir model describes well the isotherms for all four isomers (n-hexane, 2-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane). The linear and mono-branched isomers adsorb well, but 2,2-dimethylbutane is totally excluded. Plotting the differential heat of adsorption against the loading shows an initial plateau for n-hexane and 2-methylpentane. This is followed by a slow rise, indicating adsorbate-adsorbate interactions. For the di-branched isomers the differential heat of adsorption decreases with loading. To gain further insight, we ran molecular simulations using the grand-canonical Monte Carlo approach. Comparing the simulation and the experimental results shows that the ZIF framework model requires blocking of the cages, since 2,2-dimethylbutane cannot fit through the sodalite-type windows. Practically speaking, this means that ZIF-8 is a highly promising candidate for enhancing gasoline octane numbers at 373 K, as it can separate 2,2-dimethylbutane and 2,3-dimethylbutane from 2-methylpentane. Our results prove the potential of ZIF-8 as a new adsorbent that can be employed in the upgrade of the Total Isomerization Process for the production of high octane number gasoline, by blending di-branched alkanes in the gasoline.

Characteristic features of molecular transport in MOF ZIF-8 as revealed by IR microimaging

“…This is about 3 Å larger than the selective size and more than twice the XRD-window size. Moreover, the work of Ferreira et al [19] suggests, that the selective size depends on temperature. At 373 K 2,2-dimethylbutane and 2,3-dimethylbutane could be separated effectively suggesting an selective aperture size of up to 6 Å.…”

“…In the following, just a few of them shall be mentioned. Thus, molecules with critical diameter substantially exceeding the XRD window size of ZIF-8 (3.4 Å) were found to be adsorbed [15][16][17][18][19][20]. This is attributed to the flexibility of the ZIF-8 structure.…”

Separation of Hexane Isomers in ZIF-8 by Fixed Bed Adsorption

“…Dubbeldam et al 21 reported a mixed sorption hierarchy while Herm et al, 22 by introducing intracrystalline diffusional resistances, stated a normal sorption hierarchy. Thereafter, Ferreira et al 23 studied the hexane isomers sorption under static conditions where only the dibranched isomer 22DMB was not adsorbed. Zhang et al 24 also studied the sorption of hexane isomers under static conditions and completely different results were obtained, being the dibranched isomer 23DMB most preferable adsorbed than its respective monobranched and linear isomers.…”

Theoretical Study of Effective Parameters in Catalytic Growth of Carbon Nanotubes

“…The kinetic diameter is a reflection of the smallest effective dimension of a given molecule (Figure 2 of Ref. ). This parameter is used for determining the number of molecules that cover the surface of the catalyst.…”