(402e) Adsorptive Separation of Xylene Isomers Via Polymer-MOF Hybrid Material

According to the Department of Energy in United States, total energy consumption in U.S Chemical industry was nearly 3200 trillion British thermal unit (BTU) per year. Considering that crystallization and distillation process nearly consumed 40% of the total energy consumption, there needs critical solution to effectively reduce energy requirement in these processes. Distillation process was intensively utilized for crude oil fractionization. Among the products, xylenes were one of the most significant precursor materials, for producing a plasticizer and several polymers. Among the isomers, p-xylene (p-X) occupied nearly 85% of annual xylene production amount as major feedstock for the production of polyesters and polyamides.(1) Since the raw xylenes were obtained as mixture of p-xylene, m-xylene (m-X), o-xylene (o-X) and ethylbenzene (EB), separation of xylene isomers with successive isomerization was definitely required. To separate xylene isomers with their boiling point differences, more than 150 theoretical plates were needed to isolate o-X and up to 360 plates for further separation of m-X and p-X due to their nearly identical boiling points. Given that melting point difference is larger than boiling point, fractional crystallization method was adopted as an alternative. However, only 60-70% of p-X was recovered due to the existence of eutectic point and operation temperature of 220K also requires intensive energy consumption. For these reason, fractional crystallization was only applied for separation of p-X rich feed stock and accounts for 25% of the p-X production.(2) Rather, so-called simulated moving bed (SMB) process was developed utilizing p-xylene selective porous materials as a more energy-efficient separation technology. In SMB process, type of adsorbent was one of the most significant factor determining the separation efficiency. Typically, zeolites such as cation-exchanged faujasite (FAU) and ZSM-5 have been used for the SMB process due to their moderate selectivity with robustness.(3) However, limited selectivity and functional tunability of zeolites necessitate the emergence of advanced porous materials. Over the past decade, versatile porous materials including metal-organic framework (MOF) (4–7), covalent-organic framework (COF) (8) and coordination polymers (9) were reported for the separation of xylene isomers. However, adsorptive separation of xylene isomers still remains as highly challenging task, due to their nearly identical physical properties.

Herein, we used the hybrid material of polymer and MOF, called PolyMOFs as a precise molecular sieves for size-selective separation of xylene isomers. PolyMOF was new type of porous material that combines metal-organic frameworks (MOFs) with polymers.(10) Instead of using ligand monomer (1,4-benzenedicarboxylic acid, bdc), preliminary polymerized ligands (polymeric-bdc, pbdc-xa) were used as building units for polyMOFs. Term “x” denotes the number of -CH₂- group between H₂bdc units and can be controlled by using the dibromoalkane with different length (x=5 to 7). Synthesized polymeric ligands were characterized by ¹H solution nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Despite of kinetically and entropically challenging polymer-to-MOF synthesis, polymeric ligands can be crystallized into polyIRMOF-1-xa which have identical crystallinity with IRMOF-1. Among the polyMOFs, polyIRMOF-1-7a exhibited the highest crystallinity with a large surface area (~1000 m²/g). It showed reduced pore size (5.8Å) compared to prototype IRMOF-1 (10Å) due to the alkyl chain inside of the pore. According to empirical pore volume data, newly designed molecular structure of polyIRMOF-1-7a was also suggested based on density functional theory (DFT) calculation.

Given that the molecular size range of xylene isomers was from 5.8 to 6.8 Å, polyIRMOF-1-7a was tested as potential adsorbent for the selective adsorption of p-xylene over other isomers. In vapor phase adsorption, polyIRMOF-1-7a showed exceptional high p-xylene uptake (1.65 mmol g^-1) at relative pressure of 0.85 compared to other isomers (0.03, 0.08 and 0.14 mmol g^-1 for o-xylene, m-xylene and ethyl benzene, respectively). Additionally, polyIRMOF-1-7a showed substantial adsorption amount of benzene (3.5 mmol g^-1) and toluene (1.99 mmol g^-1) which have similar or slightly smaller molecular size than p-xylene (~5.8Å). For precise comparison, IRMOF-1derivative (denoted as IRMOF-1-C₇) was additionally synthesized, where only two bdc units were preliminarily cross-linked by hepamethylene linker, as polyIRMOF-1-7a. Interestingly, IRMOF-1-C₇ showed non-selective adsorption of xylene isomers, indicating that polymerized ligand had significant effect on pore size reduction, which was the key point for discrimination of xylene isomers. To prove the practical separation ability of polyIRMOF-1-7a, we performed multicomponent liquid mixture adsorption at room temperature. Remarkably, overall p-xylene selectivity toward other isomers was up to 12 for equimolar ternary liquid mixtures (p-X/m-X/o-X) and 9.1 for equimolar quaternary mixtures (p-X/m-X/o-X/EB). Separation factors for p-xylene was comparable to recently reported highly selective porous materials.(11, 12) Not only for selectivity, but also chemical and thermal stability of adsorbents were significant factors for practical application. In polyMOFs, existence of alkyl chain not only reduced the pore size but also enhanced the stability by increasing the hydrophobicity.(10) After the xylene adsorption, polyIRMOF-1-7a maintain their crystallinity and its thermal stability in ambient air condition up to 450K was also confirmed by supramolecular crystallography (SMC) beamline.

To sum up, polymer-MOF hybrid material was successfully synthesized and characterized and its crystal structure was newly designed. Sub-angstrom size-based separation of xylene isomers was feasible in polyIRMOF-1-7a for both single-component vapor phase adsorption and ternary/quaternary liquid mixture batch adsorption. In addition to its exceptional separation efficiency, the remarkably stable polyIRMOF-1-7a exhibits promise as a prospective candidate for the separation of C₈ aromatics. The hybridization of polymers with MOFs presents an appealing strategy for pore size modulation and can augment its durability compared to conventional MOFs. Although the synthesis of polyMOFs is currently limited, the discovery of additional varieties of polyMOFs is anticipated in the future, thereby enabling various separations or other potential applications.

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