Molecular sieve

Summary

A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecules migrates through the stationary bed of porous, semi-solid substance referred to as a sieve (or matrix), the components of the highest molecular weight (which are unable to pass into the molecular pores) leave the bed first, followed by successively smaller molecules. Some molecular sieves are used in size-exclusion chromatography, a separation technique that sorts molecules based on their size. Other molecular sieves are used as desiccants (some examples include activated charcoal and silica gel). The pore diameter of a molecular sieve is measured in ångströms (Å) or nanometres (nm). According to IUPAC notation, microporous materials have pore diameters of less than 2 nm (20 Å) and macroporous materials have pore diameters of greater

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https://en.wikipedia.org/wiki/Molecular_sieve

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Two-dimensional atomically bridged nanoporous silicate-based membranes for gas separation

Mostapha Dakhchoune

The synthesis of molecular-sieving two-dimensional zeolitic membranes by the assembly of crystalline building blocks without resorting to the secondary growth process is highly desirable. The precise pore size for molecular sieving, ultrathin thickness, and high thermal and chemical stability make zeolite nanosheets attractive for a number of gas separation applications. However, preparing ultrathin membranes in a scalable way can only be achieved with a secondary growth-free approach, and this remains a grand challenge. Overall, there are four major drawbacks for the synthesis and scale-up of these type of membranes: i) the preservation of the crystallinity of the nanosheets after exfoliation; ii) the non-reproducibility of the secondary growth method; iii) the development of low-cost and scalable support for the ultrathin films, and iv) the implementation of a facile and scalable membrane fabrication methods.This dissertation focuses on the development of ultrathin zeolitic membranes employing 0.8-nm-thick crystalline nanosheets from the sodalite zeolite precursor RUB-15 that hosts hydrogen-sieving six-membered rings (6-MRs) of SiO4 tetrahedra. The hydrothermal synthesis of the layered RUB-15 followed by the cation exchange chemistry to increase the d spacing between the layers facilitated the polymer blend-based exfoliation of the layered precursor leading to the first report of highly crystalline RUB-15 nanosheets where the 6-MRs were clearly visible with high-resolution transmission electron microscopy. Highly dispersed RUB-15 nanosheets in polar solvents allowed their facile assembly via vacuum filtration into 100-300 nm-thick continuous films on top of porous supports. Detailed transport studies of such as-filtered membranes revealed the presence of two different transport pathways for gas molecules: 1) the H2-selective 6-MRs and 2) the interlayer galleries, which allow He, H2, and CO2 molecules to permeate freely. The latter dominated the transport in as-filtered films, which displayed a molecular cutoff of 3.6 Å yielding a H2/N2 and H2/CH4 selectivities above 20. Non-H2-selective pathways [interlayer galleries] were eliminated by topotactic condensation of the terminal silanol groups. Upon calcination, defective [SiO3][O-] units were converted into fully coordinated silicon tetrahedra [SiO4], diminishing the interlayer gaps and yielding H2/CO2 selectivities above 100, demonstrating the effective suppression of the interlayer transport and highlighting the selective role of the 6-MRs in the temperature range 25-300 °C. This is the first report of high-performance two-dimensional zeolitic membranes without the need for the secondary growth process able to efficiently sieve light gases.VIThe scale-up of thin supported membranes relies on the quality of the underlying support. A scalable polymeric support was developed to support uniform RUB-15 films. The support was synthesized via non-solvent induced phase separation (NIPS) of polybenzimidazole AM Fumion® polymer on a low-cost stainless steel mesh. The support possesses a smooth surface, high porosity, and thermal and mechanical stability. However, the high calcination temperature of RUB-15 membranes prohibits its employment as support. For this, two new routes for removing the residual template and surfactant were developed to enable the use of the polymeric PBI-AM supports for the future scale up of RUB-15 membranes. [CONTINUED]

EPFL2022

Rapid Gas Transport from Block-Copolymer Templated Nanoporous Carbon Films

Kumar Varoon Agrawal, Mostapha Dakhchoune, Xuekui Duan, Kuang-Jung Hsu, Marina Micari, Luis Francisco Villalobos Vazquez de la Parra, Jing Zhao

Porous carbon films, attributed to their superior thermal and chemical robustness, are attractive for a number of applications. In the context of molecular separation, a major focus has been on films where the effective pore diameter is lower than 1 nm, e.g., carbon molecular sieves. Only a handful of reports are available on carbon films hosting 2-3 nm size pore channels where gas transport mainly takes place by Knudsen diffusion in contrast to activated transport. Recently, we reported nanoporous carbon (NPC) films, by the pyrolysis of phase-separated blockcopolymer/turanose films, as a gas-permeable mechanical reinforcement for crack-free synthesis of single-layer graphene membranes. However, a dedicated study on the nanostructure and transport properties of the standalone NPC film has been missing. Herein, we show that the NPC film has a perforated lamellar (PL) nanostructure where molecular transport is limited by an interlamellar spacing of similar to 2 nm. The unique PL nanostructure of the NPC film originates from its precursor, i.e., a block-copolymer stabilized by hydrogen bonding with a carbohydrate additive, where the latter also acts as the main carbon-forming agent. This nanostructure is highly sensitive to the carbohydrate/block-copolymer ratio and gives way to a lacey structure below a ratio of 2:1. The transport of gases through the interlamellar spacing takes place predominantly in the Knudsen regime, determined by their molecular mass. Attributed to a thickness of 100 nm, the film yields extremely rapid gas transport with a H-2 permeance over two million gas permeation units (GPU) and H-2/CO2 selectivity over 4.5 in a temperature range of 25-300 degrees C. These properties make the NPC film a promising membrane support and a good choice for the mechanical reinforcement for high-permeance twodimensional membranes for gas separation.

AMER CHEMICAL SOC2021

Single-layer graphene (SLG) membranes, hosting molecular-sieving nanopores have been regarded as the ultimate gas separation membranes, attributing to the fact that they are the thinnest possible molecular barrier. However, the expected attractive performance for gas separation from one-atom-thick graphene membranes has rarely been demonstrated experimentally. There are two major bottlenecks to realize this high-performance graphene membrane: 1) crack-free fabrication of large membrane area; 2) incorporation of high-density nanopores with a narrow pore-size distribution in the otherwise impermeable graphene lattice. This dissertation focuses on the development of a crack-free transfer method and highly-precise pore-etching chemistry to realize high-performance single-layer graphene membranes for gas separation. A novel nanoporous carbon-assisted transfer method was developed to mechanically reinforce the atom-thick graphene layer during its transfer from catalytic Cu foil to a porous substrate, yielding a relatively large area (millimeter-scale) crack-free graphene membrane. This enabled, for the first time, the observation of the gas-sieving behavior through the intrinsic defects in the chemical vapor deposition (CVD) derived polycrystalline SLG. Attractive H2 permeance and molecular-sieving selectivity between H2 (kinetic diameter of 2.89 Å) and CH4 (kinetic diameter of 3.80 Å) were achieved by the graphene film. A scalable gas-phase millisecond ozone-based carbon gasification chemistry was developed to realize a controllable etching of graphene lattice. High-density (1012 cm-2) gas-sieving nanopores with narrow pore-size distribution were observed by scanning tunneling microscopy and aberration-corrected high-resolution transmission electron microscopy. A model based on the kinetics of ozone etching was built to optimize the incorporation of CO2-sieving pores on graphene. The nanoporous single-layer graphene (N-SLG) membranes accomplished an effective separation between CO2 (3.30 Å) and O2 (3.46 Å), corresponding to 0.2 Å molecular sieving resolution. Furthermore, ozone-based pore-edge functionalization chemistry and oxygen-based slow etching method were developed to adjust the molecular cut-off within the sub-angstrom for tuning the gas separation performance. The resulting N-SLG membrane reached O2/N2 selectivity of 3.4 with corresponding O2 permeance of 1300 gas permeation units (GPU; 1 GPU = 3.35"×" 10-10 mol m-2 s-1 Pa-1), and CO2/N2 selectivity of 21.7 with corresponding CO2 permeance of 11850 GPU. These are, so far, the best membrane performance for the post-combustion carbon capture, and will likely tilt the capture technology toward the membrane-based process. The developed transfer method and ozone-based pore-edge functionalization chemistry are universal tools for high-performance carbon-based membrane fabrication. Accordingly, a sub-200 nm defect-free carbon molecular sieve membrane was successfully fabricated with a tunable pore-size distribution, resulting in attractive gas separation performance as well.

EPFL2021