Distribution and Transport of CO2 in Hydrated Hyperbranched Poly(ethylenimine) Membranes: A Molecular Dynamics Simulation Approach

CO2 Uptake and Stability Enhancement in Vinyltrimethoxysilane-Treated SBA-15 Solid Amine-Based Sorbents

Silica-supported amine absorbents, including materials produced by tethering aminosilanes or infusion of poly(ethyleneimine), represent a promising class of materials for CO2 capture applications, including direct air and point source capture. Various silica surface treatments and functionalization strategies are explored to enhance stability and CO2 uptake in amine-based solid sorbent systems. Here, the synthesis and characterization of novel vinyltrimethoxysilane-treated Santa Barbara Amorphous-15 (SBA-15) supports and the corresponding enhancement in CO2 uptake compared to various SBA-15-based control supports are presented. The relationship between CO2 diffusion and amine efficiency in these systems is explored using a previously reported kinetic model. The synthesized materials are characterized with CO2 and H2O isotherms, diffuse reflectance infrared Fourier transform spectroscopy, 1H T1-T2 relaxation correlation NMR, and rapid thermal cycling experiments. The novel support materials are shown to enable high amine efficiencies, approaching a fourfold improvement over standard SBA-15-supported amines, while simultaneously exhibiting excellent stability when cycled rapidly under humid conditions. As the poly(ethyleneimine) loadings are held constant across the various samples, enhancements in CO2 uptake are attributed to differences in the way the poly(ethyleneimine) interacts with the support surface.

Probing the Distribution and Mobility of Aminopolymers after Multiple Sorption-Regeneration Cycles: Neutron Scattering Studies

Solid-supported amines are effective CO2 adsorbents capable of capturing CO2 from flue gas streams (10-15 vol % CO2) and from ultradilute streams, such as ambient air (∼400 ppm CO2). Amine sorbents have demonstrated promising performance (e.g., high CO2 uptake and uptake rates) with stable characteristics under repeated, idealized thermal swing conditions, enabling multicycle application. Literature studies suggest that solid-supported amines such as PEI/SBA-15 generally exhibit slowly reducing CO2 uptake rates or capacities over repeated thermal swing capture-regeneration cycles under simulated DAC conditions. While there are experimental reports describing changes in supported amine mass, degradation of amine sites, and changes in support structures over cycling, there is limited knowledge about the structure and mobility of the amine domains in the support pores over extended use. Furthermore, little is known about the effects of H2O on cyclic applications of PEI/SBA-15 despite the inevitable presence of H2O in ambient air. Here, we present a series of neutron scattering studies exploring the distribution and mobility of PEI in mesoporous silica SBA-15 as a function of thermal cycling and cyclic conditions. Small-angle neutron scattering (SANS) and quasielastic neutron scattering (QENS) are used to study the amine and H2O distributions and amine mobility, respectively. Applying repeated thermal swings under dry conditions leads to the thorough removal of water from the sorbent, causing thinner and more rigid wall-coating PEI layers that eventually lead to slower CO2 uptake rates. On the other hand, wet cyclic conditions led to the sorption of atmospheric water at the wall-PEI interfaces. When PEI remains hydrated, the amine distribution (i.e., wall-coating PEI layer thickness) is retained over cycling, while lubrication effects of water yield improved PEI mobility, in turn leading to faster CO2 uptake rates.

Distribution and Transport of CO2 in Hyperbranched Poly(ethylenimine)-Loaded MCM-41: A Molecular Dynamics Simulation Approach

Fossil fuel use is accelerating climate change, driving the need for efficient CO2 capture technologies. Solid adsorption-based direct air capture (DAC) of CO2 has emerged as a promising mode for CO2 removal from the atmosphere due to its potential for scalability. Sorbents based on porous supports incorporating oligomeric amines in their pore spaces are widely studied. In this study, we investigate the intermolecular interactions and adsorption of CO2 and H2O molecules in hyperbranched poly(ethylenimine) (HB-PEI) functionalized MCM-41 systems to understand the distribution and transport of CO2 and H2O molecules. Density Functional Theory (DFT) is employed to compute the binding energies of CO2 and H2O molecules with HB-PEI and MCM-41 and to develop force field parameters for molecular dynamics (MD) simulations. The MD simulations are performed to examine the distribution and transport of CO2 and H2O molecules as a function of the HB-PEI content. The study finds that an HB-PEI content of approximately 34 wt % is thermodynamically favorable, with an upper limit of HB-PEI loading between 45 and 50 wt %. The distribution of CO2 and H2O molecules is primarily determined by their adsorptive binding energies, for which H2O molecules dominate the occupation of binding sites due to their strong affinity with silanol groups on MCM-41 and amine groups of HB-PEI. The HB-PEI content has a considerable impact on the diffusion of CO2 and H2O molecules. Furthermore, a larger number of water molecules (higher relative humidity) reduces the correlation of CO2 with the MCM-41 pore surface while enhancing the correlation of CO2 with the amine groups of the HB-PEI. Overall, the presence of H2O molecules increases the CO2 correlation with the amine groups and also the CO2 transport within HB-PEI-loaded MCM-41, meaning that the presence of H2O enhances the CO2 capture in the HB-PEI-loaded MCM-41. These findings are consistent with experimental observations of the impact of increasing humidity on CO2 capture while providing new, molecular-level explanations for the macroscopic experimental findings.

The selective heating effect of microwave irradiation on a binary mixture of water and polyethylene oxide: a molecular dynamics simulation approach

In this study, we investigate the molecular mechanisms of a microwave-driven selective heating process by performing molecular dynamics simulations for three different systems including pure water, pure polyethylene oxide (PEO), and water-PEO mixed systems in the presence of a microwave with two different intensities of electric field such as 0.001 V Å^-1 and 0.01 V Å^-1 at a frequency of 100 GHz. First, from performing molecular dynamics simulations of CO and CO₂ in the presence of the microwave, it is confirmed that the molecular dipole moment is responsible for the rotational motion induced by the oscillating electric field. Second, by analyzing the MD simulations of the pure water system, we discover that the dipole moment of water exhibits a time lag with respect to the microwave. During the heating process, however, the temperature, kinetic, and potential energies increase synchronously with the oscillating electric field of the microwave, showing that the heating of the water system is caused by the molecular reaction of water to the microwave. Comparing the water-PEO mixed system to the pure water and pure PEO systems, the water-PEO mixed system has a higher heating rate than the pure PEO system but a lower heating rate than the pure water system. Therefore, we conclude that heating the water-PEO mixed system is driven by water molecules selectively activated by microwave irradiation. We also calculate the diffusion coefficients of water molecules and PEO chains by describing their mean square displacements, demonstrating that the diffusion coefficients are increased in the presence of microwaves for both water and PEO in pure and mixed systems. Lastly, during the microwave heating process, the structures of the water-PEO mixed system are altered as a function of the intensity of electric field, which is mainly driven by the response of water molecules.

Thermoresponsive Conductivity of Graphene-Based Fibers

Smart materials are versatile material systems which exhibit a measurable response to external stimuli. Recently, smart material systems have been developed which incorporate graphene in order to share on its various advantageous properties, such as mechanical strength, electrical conductivity, and thermal conductivity as well as to achieve unique stimuli-dependent responses. Here, a graphene fiber-based smart material that exhibits reversible electrical conductivity switching at a relatively low temperature (60 °C), is reported. Using molecular dynamics (MD) simulation and density functional theory-based non-equilibrium Green's function (DFT-NEGF) approach, it is revealed that this thermo-response behavior is due to the change in configuration of amphiphilic triblock dispersant molecules occurring in the graphene fiber during heating or cooling. These conformational changes alter the total number of graphene-graphene contacts within the composite material system, and thus the electrical conductivity as well. Additionally, this graphene fiber fabrication approach uses a scalable, facile, water-based method, that makes it easy to modify material composition ratios. In all, this work represents an important step forward to enable complete functional tuning of graphene-based smart materials at the nanoscale while increasing commercialization viability.