Synergistic CO2-Sieving from Polymer with Intrinsic Microporosity Masking Nanoporous Single-Layer Graphene

High-flux nanoporous single-layer graphene membranes are highly promising for energy-efficient gas separation. Herein, in the context of carbon capture, a remarkable enhancement in the CO(2)selectivity is demonstrated by uniquely masking nanoporous single-layer graphene with polymer with intrinsic microporosity (PIM-1). In the process, a major bottleneck of the state-of-the-art pore-incorporation techniques in graphene has been overcome, where in addition to the molecular sieving nanopores, larger nonselective nanopores are also incorporated, which so far, has restricted the realization of CO2-sieving from graphene membranes. Overall, much higher CO2/N(2)selectivity (33) is achieved from the composite film than that from the standalone nanoporous graphene (NG) (10) and the PIM-1 membranes (15), crossing the selectivity target (20) for postcombustion carbon capture. The selectivity enhancement is explained by an analytical gas transport model for NG, which shows that the transport of the stronger-adsorbing CO(2)is dominated by the adsorbed phase transport pathway whereas the transport of N(2)benefits significantly from the direct gas-phase transport pathway. Further, slow positron annihilation Doppler broadening spectroscopy reveals that the interactions with graphene reduce the free volume of interfacial PIM-1 chains which is expected to contribute to the selectivity. Overall, this approach brings graphene membrane a step closer to industrial deployment.

Etching nanopores in single-layer graphene with a sub-angstrom resolution in molecular sieving

Shiqi Huang

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

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

Gas Separation Membranes with Atom-Thick Nanopores: The Potential of Nanoporous Single-Layer Graphene

Kumar Varoon Agrawal, Deepu Joseph Babu, Kuang-Jung Hsu, Cédric Karel J Van Goethem, Luis Francisco Villalobos Vazquez de la Parra

Gas separation is one of the most important industrial processes and is poised to take a larger role in the transition to renewable energy, e.g., carbon capture and hydrogen purification. Conventional gas separation processes involving cryogenic distillation, solvents, and sorbents are energy intensive, and as a result, the energy footprint of gas separations in the chemical industry is extraordinarily high. This has motivated fundamental research toward the development of novel materials for highperformance membranes to improve the energy efficiency of gas separation. These novel materials are expected to overcome the intrinsic limitations of the conventional membrane material, i.e., polymers, where a longstanding trade-off between the separation selectivity and the permeance has motivated research into nanoporous materials as the selective layer for the membranes. In this context, atom-thick materials such as nanoporous single-layer graphene constitute the ultimate limit for the selective layer. Gas transport from atom-thick nanopores is extremely fast, dependent primarily on the energy barrier that the gas molecule experiences in translocating the nanopore. Consequently, the difference in the energy barriers for two gas molecules determines the gas pair selectivity. In this Account, we summarize the development in the field of nanoporous single-layer graphene membranes for gas separation. We start by discussing the mechanism for gas transport across atom-thick nanopores, which then yields the crucial design elements needed to achieve high-performance membranes: (i) nanopores with an adequate electron-density gap to sieve the desired gas component (e.g., smaller than 0.289, 0.33, 0.346, 0.362, and 0.38 nm for H2, CO2, O2, N2, and CH4, respectively), (ii) narrow pore size distribution to limit the nonselective effusive transport from the tail end of the distribution, and (iii) high density of selective pores. We discuss and compare the state-of-the-art bottom-up and top-down routes for the synthesis of nanoporous graphene films. Mechanistic insights and parameters controlling the size, distribution, and density of nanopores are discussed. Fundamental insights are provided into the reaction of ozone with graphene, which has been successfully used by our group to develop membranes with record-high carbon capture performance. Postsynthetic modifications, which allow the tuning of the transport by (i) tailoring the relative contributions of adsorbed-phase and gas-phase transport, (ii) competitive adsorption, and (iii) molecular cutoff adjustment, are discussed. Finally, we discuss practical aspects that are crucial in successfully preparing practical membranes using atom-thick materials as the selective layer, allowing the eventual scale-up of these membranes. Crack-and tear-free preparation of membranes is discussed using the approach of mechanical reinforcement of graphene with nanoporous carbon and polymers, which led to the first reports of millimeter-and centimeter-scale gas-sieving membranes in the year 2018 and 2021, respectively. We conclude with insights and perspectives highlighting the key scientific and technological gaps that must be addressed in the future research. Superscript/Subscript Available

AMER CHEMICAL SOC2022