Separation and Molecular-Level Segregation of Complex Alkane Mixtures in Metal−Organic Frameworks

Abstract

Metal-organic frameworks (MOFs) are a new class of nanoporous materials that have good stability, high void volumes, and welldefined tailorable cavities of uniform size.1 MOFs consist of metal oxide vertices interconnected by rigid or semirigid organic molecules having terminal functional groups that bind to the metal corners. The pores of MOFs can be systematically varied by a judicious choice of linker molecules and/or metal corners. Moreover, the higher void volumes in comparison to, for example, zeolites could potentially lead to very efficient separation processes. In this paper, we show that MOFs are highly selective for separating alkanes based on the degree of branching in complex, multicomponent mixtures. This is potentially useful for removing low research octane number (RON) alkanes from a mixed alkane stream. In addition, we report an unusual molecular-level segregation of molecules based on their degree of branching.The separation of alkane mixtures using nanoporous materials is up to date usually achieved by selective adsorption within zeolitic materials. Calero et al. 2 and Krishna et al. 3 showed using simulations that linear, monomethyl, and dimethyl alkanes in the 5-7 carbon atom range can be separated using silicalite-1 (MFItopology) by exploiting configurational entropy effects. There are only a few papers on separation of alkanes in MOFs. Düren and Snurr simulated methane/n-butane mixtures in a family of related MOFs and found that selectivity for n-butane varied strongly with the MOF linker molecule. 4 Several groups simulated mixtures of branched and linear alkanes in IRMOFs but did not observe significant selectivities for these structures. 5 Experimentally, Chen et al. demonstrated that linear alkanes can enter the pores of MOF-508, but branched alkanes cannot.6 Interestingly, recently Barcia et al. found experimentally that hexane could be kinetically separated from 3-methylpentane and 2,2-dimethylbutane by fixed bed adsorption using a Zn 2 (1,4-bdc) 2 (dabco)] MOF, 7 where "bdc" denotes 1,4-benzenedicarboxylate and "dabco" denotes 1,4-diazabicyclo[2.2.2]-octane. In this work we refer to this structure, synthesized by Dybtsev et al., 8 as MOF-1. Figure 1 shows a cartoon of the structure. For the adsorbates in this study, the structure is essentially a one-dimensional channel system. MOF-1 is thermally stable and easily reactivated and reused. To investigate the use of MOF-1 as an adsorbent for alkane separations relevant to industrial processes, we simulated a multicomponent mixture in the C 5 -C 7 range using grand canonical Monte Carlo (GCMC). Details are given in the Supporting Information. We note that the simulations are performed using a rigid framework, although flexibility for MOFs might be more important than in zeolites. However, for alkanes we expect the differences to be small. Figure 2 shows the absolute adsorption isotherms predicted for a 13-component alkane mixture in MOF-1. The numbers at the right side of the figure are the research octane numbers. The figure shows ...

RASPA: molecular simulation software for adsorption and diffusion in flexible nanoporous materials

“…This allows one to check and study 'segregation' of molecules. [92] Figures 3 and 11 have been made using RASPA and VTK. To visualise molecules inside the structure, the pore walls can be rendered transparent.…”

Modeling adsorption equilibria of xylene isomers in a microporous metal–organic framework

“…Dubbeldam et al [38] have reported using grand canonical Monte Carlo simulations (GCMC), that MOF 1 presents two energetically different sites of adsorption. At low loading, the bulkier molecules adsorb along the 7.5 Â 7.5 Å 1D channels, preferentially near the ''boxes'' formed by the ''BDC'' ligands.…”

“…The fitted parameters obtained are reported in Table 2. We take site A to be the one with higher Langmuir constant, which according to Dubbeldam et al [38] represents sorption along the 7.5 Â 7.5 Å 1D channels near the ''boxes''. The lines in Fig.…”

“…For the adsorbates in this study, the structure of MOF 1 is in effect a one-dimensional channel system, despite the three dimensional structure, and is thermally stable. Dubbeldam et al have reported that MOF 1 presents two energetically different sites of adsorption [38].…”

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“…[24] Finally, PSi is readily functionalized by a large variety of molecular structures [25][26][27][28][29] and is highly chemically reactive. [30] It has also been suggested that nanoporous materials can preferentially adsorb different species of organic molecules, [31] indicating that the right preparation of PSi could serve as a selective detector of explosive molecules.…”