1,721,066 research outputs found
Dispersible exfoliated zeolite nanosheets and their application in high performance zeolite membrane
No full textUniversity of Minnesota Ph.D. dissertation. October 2013. Major: Chemical Engineering. Advisors: Michael Tsapatsis, Lorraine F. Francis. 1 computer file (PDF); xv, 105 pagesIn the wake of the energy crisis, an efficient separation technology such as membrane is required to replace the energy intensive processes like distillation. High performance zeolite membrane can be fabricated by coating of a thin film of high-aspect-ratio zeolite nanosheets on a porous support. However, the synthesis of highly crystalline and morphologically intact zeolite nanosheets by the direct hydrothermal synthesis has been challenging. Successful reports on the synthesis of zeolite nanosheets by the exfoliation of their layered structure exist, but the synthesis routes provided in these reports often lead to significant damages to the structure and the morphology of nanosheets.
This dissertation focuses on the development of a scalable method for the synthesis of zeolite nanosheets, while preserving their structure and the morphology. MWW and MFI nanosheets were prepared by polymer melt compounding of their layered precursors with polystyrene. Zeolite nanosheets were extracted out of the polymer matrix by solution processing of the zeolite-polymer nanocomposite. Exfoliated nanosheets and their coatings were then characterized by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). A compact, oriented, 300-nm thick zeolite film was fabricated on a symmetric alumina support by a one-step filter coating method. This nanosheet film demonstrated molecular sieving capabilities after a mild hydrothermal treatment.
Density gradient centrifugation was used to purify the zeolite nanosheets from the polymer matrix, and the large unexfoliated particles, resulting in a two-fold increase in the yield of nanosheets in the final coating suspension. Sub-100 nm thick films of these nanosheets were made on a symmetric alumina supports. Nanosheet films with thickness ranging from 10 nm to 100 nm were prepared on an asymmetric silica supports. In-plane secondary growth of these films by the impregnation growth method led to b-oriented, 100-150 nm thick zeolite film that separated xylene isomers with separation factors of 100-800, while providing a high permeance of p-xylene (4 x 10-7 moles/m2-s-Pa).</Agrawal, Kumar Varoon. (2013). Dispersible exfoliated zeolite nanosheets and their application in high performance zeolite membrane. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/160750
Etching nanopores in single-layer graphene with a sub-angstrom resolution in molecular sieving
No full textSingle-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.LA
A holistic approach for incorporating sub-angstrom gas-sieving pores in single-layer graphene: from atomic simulations to large area membranes
No full textThe ultimate thinness of graphene pores makes them ideal as a high-flux selective layer for manipulating molecular transport. However, due to the limited understanding of the formation of ångstrom (Å)-scale pores, controlling pore size and understanding gas transport across realistic graphene pores remains challenging. This thesis focuses on understanding pore formation in graphene to develop membranes with a high density of pores. Building on this understanding, it then seeks to reveal gas transport mechanisms across realistic pores to guide experimental design.
A novel photonic gasification method to form gas-sieving pores is introduced. It is first demonstrated that during oxidation, oxygen clusters form and then evolve into a core/shell structure, with an ether core surrounded by an epoxy group. This organization is driven by an attempt to minimize lattice strain. The core of the oxygen cluster was then selectively gasified at room temperature using 3.2 eV light, leading to pore at the cluster's center. By leveraging oxidation temperature, the feasibility of maintaining a narrow pore size distribution, independent of pore density, is demonstrated. A simultaneous increase in gas permeance and gas pair selectivity was observed, overcoming a common trade-off and long-standing challenge in the field. Ultrahigh H2 permeance of 12000 gas permeation units (GPU), along with a highly attractive H2/N2 selectivity of 50, was achieved.
Building on the understanding of the 2D pore formation mechanism mentioned above, molecular dynamics (MD) simulations were conducted to explore CO2/O2 mixture separation across realistic graphene pores prepared by oxidation where semiquinone groups are present at the pore edge. Herein, using molecular dynamics (MD) simulations, we show that C=O displays a remarkable molecular-interaction-dependent dynamic motion, leading to a distribution in PLD comparable to the size differences between CO2, O2, and N2. Dynamic open and closed pore states are observed in small pores, making impermeable pores CO2-permeable. The strong molecular interaction eliminates effusive transport, resulting in selective gating of CO2 from O2 and N2, even from large PLD pores expected to be nonselective. Finally, transition-state-theory calculations validated against MD simulations reveal the immense potential of porous graphene for carbon capture beyond the state-of-the-art membranes. These insights will inspire improved graphene membrane design, pushing the carbon capture frontier.
Finally, the selective transport of NH3 from graphene pores was explored. Attractive separation performance from centimeter-scale porous graphene was achieved with NH3 permeance of 16000 GPU accompanying NH3/N2 selectivity up to 105. MD simulations revealed competitive adsorption between NH3 at graphene pores, where NH3 transports solely by an adsorbed-phase pathway blocking the access for N2. Both simulations and experiments are showing that 2D graphene pores are promising to find application in NH3 purification.LA
Functionalized Å-scale Pores in Graphene for Carbon Capture
Full text linkSingle-layer graphene, hosting a high density of functionalized molecular-sieving atom-thick pores, is considered to be an excellent material for gas separation membranes. These functionalized atom-thick pores enable the shortest transport pathway across the membrane and competitive sorption of the target molecules over the unwanted molecules. However, incorporating high-density gas-sieving pores with sub-angstrom resolution and functionalization of heteroatoms on the 2D pores have been major challenges.
This dissertation focuses on the development of a gas separation single-layer graphene membrane hosting gas-sieving and functional groups rich 2D pores. The gas-sieving pores were incorporated by O3-etching chemistry assisted by mathematical modeling to predict the gas-sieving defect density and the pore size distribution. This approach enabled a well-control of the nucleation and pore expansion rate, yielding the attractive CO2 permeance of 4400 gas permeation units (GPU; 1 GPU = 3.35×1010 mol m-2 s-1 Pa-1) and CO2/N2 separation factor of 33.4. A nucleation-decoupled expansion achieved by using dilute O3 was utilized for a slow pore expansion. The high throughput graphene membranes functionalized with CO2-philic polymer achieved an attractive carbon capture performance.
Graphene membranes hosting N-substituted 2D pores have been developed to promote selective and rapid transport of CO2 across membranes using competitive sorption and gas-sieving. N-substituted 2D pores, derived by a facile reaction of oxidized graphene with NH3, show a rapid and quantitatively reversible complexation of CO2 with pyridinic N by cycles of adsorption and desorption in the spectroscopy. The phenomenon is also visualized by microscopy where pores are observed occupied and empty upon adsorption and desorption, respectively. The N-substituted 2D pores exhibit strong competitive sorption for CO2, resulting in an excellent carbon capture performance, even in the dilute CO2 source. It led to a large CO2/N2 separation factor (close to 2000) while the atomic selective layer resulted in high CO2 permeance (up to 50000 GPU). Additionally, CO2-complexation of amine groups, such as -NHCOO-, -NH3+, on the pore edges resulted in a narrower electron-density gap in the 2D pores, thereby leading to effective size-sieving separation between H2/N2 and O2/N2, with a molecular sieving resolution of 0.2 Å. The resulting graphene membrane reached O2/N2 selectivity of 6.0 with a corresponding O2 permeance of 1630 GPU, and H2/N2 selectivity of 53.5 with corresponding H2 permeance of 14460 GPU.
Finally, high-performance centimeter-scale membranes were achieved thanks to the uniform and scalable chemistry of gas phase O3 etching and NH3 treatment. The graphene membranes mechanically supported by Poly(1-trimethylsilyl-1-propyne) (PTMSP) on the porous support were demonstrated for carbon capture. Both CO2-philic polymer-functionalized and pyridinic-N-substituted graphene membranes show excellent CO2 capture permeance for post-combustion and dilute CO2 sources.
Overall, incorporating gas-sieving 2D pores on the graphene lattice and functional heteroatoms/functional groups at 2D pores provides new directions to enhance the performance of membranes, sensors, and catalysts.LA
Mechanistic study on the evolution of vacancies in graphene by oxidation by scanning tunneling microscopy
No full textNanostructured graphitic materials, including graphene hosting Å to nanometer-sized pores, have attracted attention for various applications such as separations, sensors, and energy storage. Graphene with Å-scale pores is a promising next-generation material for molecular sequencing and membrane-based separation.
Our group has developed a method to generate tunable Å-scale pores using O3. This dissertation characterizes gas-sieving nanopores in the graphitic lattice by O3 etching using low-temperature scanning tunneling microscopy (LTSTM) and X-ray photoelectron spectroscopy (XPS). A systematic study was conducted to optimize parameters for graphene porosity during the O3-led gasification reaction using a millisecond gasification reactor (MGR). LTSTM showed that high temperature increases the nanopore density.
To understand the mechanism of O3-led gasification, LTSTM was used to investigate the formation of linear oxygen clusters on the graphitic lattice. While advances in the application of oxidized graphene have been made, the mechanism is not well understood. Combining LTSTM imaging and van-der-Waals density-functional theory (vdW-DFT) calculations, the atomic structure of the linear oxygen cluster was analyzed. Results show that the linear oxygen cluster is formed by the assembly of cyclic epoxy trimers that propagate along the graphitic armchair direction.
Defect formation and unzipping of the graphitic lattice have various applications that depend on the evolution of epoxy clusters. To understand how cluster evolution affects morphology and pore size, LTSTM was used to study cluster evolution upon heat treatment. Circular oxygen clusters were formed after heat treatment, and three distinct nanostructures corresponding to three stages of evolution were observed. The observation of cyclic epoxy trimers validated theoretical predictions, as they minimize energy in the cyclic structure. The cyclic epoxy trimers grew into honeycomb superstructures and formed larger clusters of approximately 2-3 nm. LTSTM observations revealed that vacancy defects evolved only in the larger clusters, highlighting the role of lattice strain in defect generation.
A novel, scalable method to generate nanometer-sized pores in graphene lattice was developed based on decoupling nanopore nucleation and expansion. Mild oxidation temperature was used to graft nanosized epoxy clusters as nucleation sites without forming pores. In situ gasification of these clusters inside a transmission electron microscope showed precise nanopore generation. The method was manipulated to form Å-scale pores that effectively sieve gas molecules based on size, paving the way for independent control of pore size and density.
The effect of STM bias voltage and polarity on epoxy clusters was studied. A large negative bias voltage (e.g., -2 V) caused epoxy desorption, while a large positive bias voltage (e.g., +2 V) enabled imaging of the epoxidized surface. Surface morphologies at low (e.g., +0.05 V) and high bias voltages (e.g., +2 V) were analyzed by studying graphene nanoribbon electronic structures. These findings could be applied as a novel STM tip-induced nanolithography for graphitic lattice patterning in the future.
This dissertation provides an integral study of oxygen clusters to elucidate the mechanism of graphitic lattice oxidation, which could enrich the understanding of this field and provide new insights for future applications of graphitic-materials-based devices.LA
Crystallization of Size-Selective Nanopores in Graphene for Gas Separation
Full text linkNanoporous single-layer graphene (N-SLG) membranes, owing to their single-atom thinness, have the potential to exceed the permeance and selectivity limits of gas separation membranes. However, two key issues in the top-down N-SLG synthesis need to be addressed to achieve scalable, high-performance membranes: a) reproducible synthesis of high-quality SLG film by chemical vapor deposition (CVD) on a low-cost Cu substrate, and b) introducing high-density of pores with a narrow pore-size-distribution (PSD). This dissertation addresses these issues by developing a method to prepare smooth and oriented Cu foil by a facile approach to obtain high-quality SLG membranes. On the fundamental science front, it explores two novel methods of tuning the PSD in graphene for gas separation.
Low-cost Cu foils are rough, and result in membranes with large nonselective intrinsic vacancy defects which hampers their application in gas separation. Herein, we conduct a systematic high-temperature annealing study on two separate, commercial, low-cost Cu foils leading to their transformation to smooth Cu(111), decreasing their surface roughness by ~ 2-fold. The smooth, oriented Cu foils yielded SLG with a significantly lower defect density with ID/IG ratio decreasing from 0.18 ± 0.02 to 0.04 ± 0.01. The intrinsic defects in these SLG films were H2 selective with H2 permeance reaching 1000 gas permeation units (GPU; flux normalized with transmembrane pressure difference) and attractive H2/CH4 and H2/C3H8 selectivities of 13 and 26, respectively.
To decouple the pore nucleation and expansion in SLG, we utilize CO2 as a mild etchant to expand the existing pores at temperatures ranging 750 â 1000 °C. CO2 could uniformly expand the intrinsic pores in SLG in a controlled manner, down to a few à /min, without nucleating new defects in the SLG basal plane. Furthermore, we revealed two distinct kinetic zones for the reaction of CO2 with graphene edges, with the transition happening around the pore diameter of ~ 2 nm. Etching rate of the larger expanded-pores was constant and independent of the pore size. The expansion was thermally activated with an activation energy of 2.71 eV, consistent with the literature based on ab-initio calculation for CO2 dissociative chemisorption on zigzag edges. In comparison, the etching rate was an order of magnitude slower in smaller pores indicating that geometrical confinement in smaller pores play an important role. An exponential relation between the density of expanded pores and etching temperature, with an activation energy of 3.58 eV, was observed.
Next, we develop a novel method to fabricate N-SLG with a high pore-density while maintaining a narrow PSD. We exposed the highly porous SLG (treated by O2 plasma) with a broad PSD to graphene CVD condition in presence of both CH4 and CO2. The pore expansion (as a result of etching) and shrinkage (as a result of growth) reached a comparable rate. Moreover, CO2 suppressed the graphene grain nucleation, leading to high-quality graphene synthesis. Membranes with H2 permeance reaching 10000 GPU and H2/C3H8 selectivity of 26 were fabricated by optimizing the CH4/CO2 ratio at 800°C.
In summary, we address the current obstacles in SLG membrane development by utilizing CO2 in graphene CVD environment to tune the PSD of SLG membranes. Moreover, a simple annealing method to optimize the morphological and crystallographic properties of Cu foils for high-quality SLG synthesis is proposed.LA
Mass transport of water vapor and ions from Å-scale graphene nanopores
No full textHydrodynamics at the nanoscale involves both fundamental study and application of fluid and mass transport phenomena in nanometer-sized confinements. Nanopores in single-layer graphene can be highly attractive for exploring the molecular transport of gas and water molecules and hydrated ions at the ultimate scales of pore size and pore length. However, the experimental data is limited, and the state-of-the-art artificial nanopores still underperform compared to biological channels in cellular membranes.
This dissertation focuses on developing ultimate graphene nanopore devices to study mass transport phenomena under controlled spatial confinement. We first investigated the kinetics of liquidâ vapor transport from nanoscale confinements which is attractive for novel evaporation and separation applications; however, it is not explored at the ultimate confinement limit, i.e., at the atomic-thick and à -scale nanopore placed at the liquidâ vapor interface. We show that the evaporation flux from such nanopores increases with decreasing pore size by up to one order of magnitude relative to the bare liquidâ vapor interface. Molecular dynamics simulations reveal that oxygen-functionalized nanopores render rapid rotational and translational dynamics to water molecules by reducing and shortening the lifetime of waterâ water hydrogen bonds.
Graphene nanopores also enable the study of ion transport across sub-nanometer-scale 2D confinements. We produce tailor-made nanopores approaching the size of hydrated ions by decoupling the pore nucleation and expansion. Monovalent metal ions are efficiently sieved from divalent ions, with K+/Mg2+ selectivity up to 70 and Li+/Mg2+ selectivity up to 50, corresponding to a sieving resolution of 1 Ã . Mitigating the non-selective pore formation further enhance the ion-sieving performance, reaching K+/Mg2+ selectivity up to 350 and Li+/Mg2+ selectivity up to 260. The pore size and structure allow adjusting the diffusion of ions across the nanopores, suggesting that the sterically induced partial dehydration process may play an important role in the observed cation selectivities. These selectivities were realized from centimeter-scale suspended graphene membranes, prepared in crack-free fashion by using dual layer reinforcement strategy where the first layer is 200-nm-thick nanoporous carbon (NPC) film hosting 20 nm pores which ensures a conformal contact and reinforcement of the graphene film and the second (top) layer is Nafion.
Finally, a dual layer reinforcement is also demonstrated for preparing crack-free centimeter-scale gas separation membranes to utilize the full potential of graphene nanopores for energy-efficient applications. The bottom layer of the composite film is NPC film while the top layer is made of a 500-nm thick multi-walled carbon nanotube (MWNT) film with a pore size ranging from 200 to 300 nm. The obtained selectivities from crack-free centimeter-scale graphene membranes for H2/CH4 and H2/CO2 are 11â 23 and 5â 8, respectively, which is significantly higher than the corresponding Knudsen selectivities.
Overall, this dissertation presents a graphene nanopore toolkit for studying fluid mechanics at the ultimate scales. The findings of enhanced water evaporation rate and ion selectivity using the nanopore platform could enrich our understanding of mass transport under extreme confinement and open new opportunities for a range of separation applications.LA
Crystal, defect, and morphology engineering of porous two-dimensional materials for ionic separation
No full textOrdered two-dimensional (2D) materials hosting Å-scale pores are highly promising for enabling challenging separation, thanks to well-defined pore geometry resulting in tight confinement of ions when hosted inside the pore. In addition, the 2D nature of these materials allows one to engineer thin films which are ideal for membrane-based separation while achieving a high ion flux. This dissertation investigates the structural characterization and ion-sieving applications of two classes of crystalline porous 2D materials, based on graphitic carbon nitride (g-C3N4) and metal-organic frameworks (MOFs) with well-defined pore architectures.
The dissertation begins with g-C3N4 crystallized as poly(triazine imide) or PTI via a molten salt synthesis route. It is noted for its high pore density consisting of an array of 3.4-Å-sized pores which are ideal for size sieving of small molecules and ions. However, a significant challenge arises from the ions (primarily Li+ and Cl-) intercalated in as-synthesized PTI. In particular, Li+ positions itself inside the plane of the pore occluding the pore for transport. Addressing this, this research introduces acid treatment to modulate the ion depletion level in PTI, effectively substituting Li+ in the pore space with H+ and reducing Cl- concentration in the gallery space. A series of characterization tools is used to reveal, for the first time, the coexistence of AA' stacking of layers with the open pore channels aligned along c-axis, facilitating transport and AB stacking of layers, wherein adjacent PTI layers are shifted by a/3 and 2b/3 along the two crystallographic directions, resulting in closed channels. A notable increase in proton conductance through PTI layers is observed at higher depletion levels.
Next, the dissertation investigates the synthesis of unit-cell-thick non-van der Waals (n-vdW) quasi 2D MOF films, particularly focusing on UiO-66-NH2, featuring well-defined pore structure of 6 Å, optimally positioned within the range for efficient ion-ion separation. Our study introduces an aqueous synthesis route, enabling the formation of ultrathin, crystalline n-vdW quasi 2D UiO-66-NH2 films under ambient conditions. This approach pinpointed an optimal condition with ultra-dilute precursor concentrations in a low pH regime to foster controlled nucleation and decelerate the reaction kinetics. In addition, the use of crystalline single-layer graphene as a substrate serves the crystallographic registry, promoting an exclusive in-plane growth. The thickness of these films can be controlled through synthesis time, ranging from half to two unit cell. The preferred orientation of UiO-66-NH2 along 200 lattice plane is revealed, shedding light on the structure correlation between graphene and UiO-66-NH2, where the lattice mismatch is minimal. The resulting UiO-66-NH2 films demonstrate a remarkable ion-ion separation performance and stability over 5 weeks.
Overall, this dissertation highlights the correlation between structure and performance in nanoporous materials, paving the way for future breakthroughs in membrane technology. The research emphasizes the extraordinary potential of ultrathin 2D nanoporous materials for molecular separation, opening up new possibilities for the development of highly efficient, selective, and durable membranes for diverse applications.LA
Two-dimensional atomically bridged nanoporous silicate-based membranes for gas separation
No full textThe 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.
VI
The 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]LA
Molecular simulation on two-dimensional nanoporous material for energy-efficient separation
No full textThe energy-consumption by the chemical and petrochemical industry in the European Union (EU) accounts for about quarter of its energy footprint. Approximately half of this energy goes toward chemical separation, currently dominated by thermally-driven processes such as distillation and absorption and extraction. Based on the recent strategic energy technology plan (SET-plan) focusing on the energy-efficiency and the environment, EU needs to cut down the carbon emissions by 60-80% by 2050. Keeping this in mind, it is crucial to develop energy-effective separation processes. Partial or complete replacement of the thermally-driven separation processes by the membrane-based separation has been estimated to save up to 90% of energy consumption in separation.
Two-dimensional (2D) nanoporous materials are emerging membrane platform for molecular separation offering ultrahigh permeance with an attractive molecular selectivity. In general, the 2D membranes can be classified in two categories. The first category is impermeable to molecules (for example, graphene), except when their pores are incorporated by a chemical or a physical etching. Here, the biggest bottleneck has been identifying a suitable pore-structure, and then incorporating similar pore-structure with precision (narrow pore-size-distribution) at a moderate to high density. We are highly interested in understanding a possible pore-structures in graphene that are stable and are attractive for molecular separation. For this, a fundamental understanding of the molecule-pore interaction is crucial. For example, hundreds of different pore-structures (number of missing carbon, zig-zag vs. armchair edge, pore-edge-functionalization) can be envisioned. Here, the most important question is that which lattice structure is the most promising one for a given separation. Since there are a number of promising nanoporous 2D materials and a number of possible pore-structures in graphene, it is highly important to efficiently screen the lattice structure. This thesis will investigate this employing ab-initio density functional theory (DFT) and classical molecular dynamic simulation. One can make a rough estimate by analyzing the size, topology, density, and chemical composition of the nanopore, however, an accurate prediction of the separation performance (permeance and selectivity) can only be made by calculating the entropic and enthalpic changes incurred during adsorption and at the transition state.
The second category of 2D material are intrinsically nanoporous two-dimensional structure from the family of inorganic materials (zeolites, metal-organic frameworks, carbon-nitride, etc.), obtained from the exfoliated of their layered precursors. Recently, by screening a database of 2D materials, the group of Prof. Marzari (THEOS), has identified a set of nearly 200 inorganic nanoporous layered materials that seem to be attractive for molecular separations based on a purely geometric survey of materials in the Crystallographic Open Database (COD) and the Inorganic Crystal Structure Database (ICSD). In this thesis, we will study their separation efficiency using DFT and classical molecular dynamic simulation to discover new potentially interesting nanoporous 2D materials. Overall, the thesis will establish to a number of nanoporous 2D structures for energy-efficient molecular separation.LA
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