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

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 isothe...

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Bibliographic Details
Published in:Physical chemistry chemical physics : PCCP 2013-06, Vol.15 (22), p.8795-8804
Main Authors: FERREIRA, Alexandre F. P, MITTELMEIJER-HAZELEGER, Marjo C, GRANATO, Miguel Angelo, DUARTE MARTINS, Vanessa F, RODRIGUES, Alírio E, ROTHENBERG, Gadi
Format: Article
Language:English
Subjects:
Alkanes
Alkanes - chemistry
Chemistry
Computer simulation
Exact sciences and technology
Gasoline
General and physical chemistry
Heat of adsorption
Isomers
Isotherms
Mathematical models
Models, Molecular
Molecular Dynamics Simulation
Monte Carlo Method
Octane number
Surface physical chemistry
Zeolites - chemistry
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Summary: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.
ISSN:1463-9076
1463-9084
DOI:10.1039/c3cp44381g