(2el) Advanced Porous Materials for Molecular Discrimination of Light Hydrocarbons at Sub-Angstrom Precision

Research Interests: Molecular discrimination of light hydrocarbons via metal-organic framework (MOFs), polyMOFs and zeolite-templated carbon (ZTC)

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. Among various industry sectors, crystallization and distillation process nearly consumed 40% of the total energy consumption. Especially, fractionization and reforming process of fossil fuel accompanied tremendous energy consumption due to thermally-induced phase change. As an alternative, adsorption-based separation technologies using porous adsorbents were adopted in practical industries. Development of target-selective and robust adsorbents was essential for practical use of adsorptive separation. In recent years, tremendous researches have been reported for separation of olefin/paraffin, xylene isomers and additional mixture product stream derived from light hydrocarbons refinement. Versatile types of adsorbents were utilized including zeolite, metal-organic framework (MOF) (1–3), covalent-organic framework (COF) (4) and carbonaceous materials (5). Within their characteristic micropores, it was proven that discrimination of component species was feasible through molecular sieving, shape-selective adsorption and favorable adsorbate-adsorbent interaction. However, adsorption-based separation of light hydrocarbon mixtures still remains as challenging task for practical application. Design of functional porous materials with pertinent chemistry was one of the most significant factor for selective adsorption. Metal-organic frameworks (MOF) were highly functional porous materials with large surface area, fabricated by regular arrangement of various metal and ligands. Pore size and surface chemistry of MOFs have significant effect on selective adsorption, and it was the one of our main research objectives that development of novel strategy for modulating pore dimension of MOFs.

Among versatile MOFs, M-MOF-74 [M₂(dobdc), M=Mg,Mn,Fe,Co,Ni,Cu, and Zn; dobdc^4- = 2,5-dioxido-1,4-benzenedicarboxylate] has been used in olefin/paraffin separations and complex light hydrocarbon gases (C₁-C­₃) due to facile synthesis, high density open metal sites and large surface areas (6). The compound known as 2,5-dioxido-1,4-benzenedicarboxylate (dobdc4−) is utilized as a linker in the construction of MOF-74. This linker possesses both rigidity and sufficient length to create a pore aperture of considerable size (11.9 Ã…). Consequently, it facilitates rapid movement of light hydrocarbon molecules (typically 3−6 Ã… in kinetic diameters) into the pores. However, due to the large size of the pore aperture, it lacks the ability to distinguish between molecules of different sizes and shapes, resulting in limited selectivity in terms of kinetics. This trade-off between sorption selectivity and diffusion selectivity poses a constraint on the use of MOF-74 for hydrocarbon separation applications. One potential solution to this issue is to select a different MOF with a smaller aperture size. However, this approach usually leads to a decrease in the capacity of adsorbed light hydrocarbon gas molecules. To address this challenge, we proposed novel design that modifying the original structure of MOF-74 to reduce its aperture size to the ultramicropore range (<6 Å), while minimizing the loss of micropore volume. Based on density functional theory (DFT) calculation, 2,4,6-tri(4-pyridyl)-1,3,5-triazine (tpt) was selected to reduce the pore aperture size of the pristine MOF-74 structure via the size-matched ligand partitioning. According to simulation results, the partitioned MOF, called tpt-Mg-MOF-74 possessed the capability to sort light hydrocarbons with different kinetic diameters (4.0 Å for ethane and 4.2 Å for propane). Interestingly, tpt-Mg-MOF-74 can empirically separate ethane/propane with and diffusion selectivity of ~49.(7) Notably, in spite of the substantial increase in kinetic selectivity, the reduction in pore volume was minimized. Designed ligand insertion strategy proved that conventional trade-off can be overcome, which is typically associated between kinetic selectivity and adsorption capacity.

As an alternative approach for pore size modulation of MOFs, preliminary polymerized ligand species was utilized for adjustment of pore dimension. Hybrid material of polymer and MOF, called PolyMOFs was new type of porous material that combines metal-organic frameworks (MOFs) with polymers.(8) 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). 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. 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). Even for multicomponent liquid mixture adsorption at room temperature, 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). Use of preliminary polymerized organic ligand was proven to effective strategy for both pore size adjustment and stability enhancement. (9)

MOFs were powerful tool for selective gas adsorption, however its poor stability was one of major obstacles for practical usage, originated from the use of organic ligand as building blocks. Thus, we attempted to expand the types of porous materials, having robustness such as carbon-based materials which showed thermal and chemical resistance. Zeolite-templated carbon (ZTC) was highly porous carbonaceous material derived from zeolite scaffolds. (10) By introduction of fluidized bed reactor, few hundred gram scale ZTC can be obtained in our lab scale, and its Brunauer-Emmett-Teller (BET) surface area was up to 2500 m²/g with narrow micropores. With adequate functionalization, modified ZTC has been tested for the corresponding applications: separation of alkyne/olefin, separation of product stream in C₁ chemistry. Owing to its expansive surface area, well-defined channels, and remarkable stability, modified zeolite-templated carbons (ZTCs) possess the potential to undergo rigorous testing under practical, demanding conditions. Consequently, they are envisioned as promising candidates for the development of advanced functional porous materials in the forthcoming generation.

In conclusion, our study explores the realm of cutting-edge functional porous materials, with a particular emphasis on their aptitude for the selective adsorption of light hydrocarbon species, thereby enabling energy-efficient separation of these compounds. Through an adequate design strategies, ranging from metal-organic frameworks (MOFs) to meticulously engineered microporous carbons, we tailored the target materials to optimize the desired performance characteristics. Furthermore, we explored the integration of these advanced porous materials into compact pellet or fiber formats, thereby conferring heightened practicality and applicability for industrial-scale implementation. Our ongoing research endeavors are dedicated to the advancement of highly discerning materials. Ultimate goal is to supplant the conventional distillation-driven separation process with a superior adsorption-based counterpart. By continually refining the selectivity of materials through our investigations, we strive to pave the way for a transformative shift in the field of separation technologies.

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