MODELING OF NANOSCALE TRANSPORT IN MESOPOROUS MEMBRANES

Mesoporous membranes with pore sizes in the range 2-50 nm provide an energy efficient alternative for separation of mixtures such as CO2 from stack effluents and volatile organic compounds (VOC) from air. Transport mechanisms such as capillary condensation, Knudsen diffusion and surface adsorption help in enrichment of a more condensable component based on the bulk mixture thermodynamics, surface chemistry and geometry of the mesopores. Despite the progress in synthesis techniques, design of better mesoporous materials for targeted separations is still a challenge due to the absence of clear design rules. Modeling techniques can be used to quantify the relevant transport processes and determine the correlations between mesopore properties and separation performance. Continuum modeling requires predetermined transport models while molecular simulations are computationally too intensive for realistic membrane processes. Dynamic mean field theory (DMFT), a coarse-grained lattice based theory, was employed to model nonequilibrium transport in mesoporous membranes. DMFT was used to model permporometry, an experimental technique for pore size distribution measurement. It involves light gas permeation in presence of condensable vapor under near equilibrium conditions. Detailed study of transport revealed a maximum of light gas flux in the layer adjacent to the strongly adsorbed surface layer of heavy component. A highly nonequilibrium process of VOC recovery, which involves passing a mixture of light gas and condensable vapor through the mesopores under significant pressure gradient, was also modeled. Nonequilibrium steady states with capillary condensation confined to high pressure feed/inlet side (partial capillary condensation) of the mesopores were found. The conditions required for existence of these states were then investigated in single component systems. Equilibrium adsorption/desorption behavior of silica monoliths with disordered porous structure was studied using mean field theory. Dual control volume grand canonical molecular dynamics (DCV-GCMD), a combination of grand canonical Monte Carlo and molecular dynamics, was used to evaluate DMFT and study single component systems under confinement. Investigation of experimentally predicted phenomenon of enhanced flux of pure component in the presence of condensed fluid in the pore with DCV-GCMD revealed an increased surface density in the vapor region of pores with partial capillary condensation

Modeling Fixed Bed Membrane Reactors for Hydrogen Production through Steam Reforming Reactions: A Critical Analysis

Membrane reactors for hydrogen production have been extensively studied in the past years due to the interest in developing systems that are adequate for the decentralized production of high-purity hydrogen. Research in this field has been both experimental and theoretical. The aim of this work is two-fold. On the one hand, modeling work on membrane reactors that has been carried out in the past is presented and discussed, along with the constitutive equations used to describe the different phenomena characterizing the behavior of the system. On the other hand, an attempt is made to shed some light on the meaning and usefulness of models developed with different degrees of complexity. The motivation has been that, given the different ways and degrees in which transport models can be simplified, the process is not always straightforward and, in some cases, leads to conceptual inconsistencies that are not easily identifiable or identified

The physics of water leaks and water nanoflows

The encapsulation of devices sensitive to moisture is necessary to prolong lifetimes under adverse environmental conditions. Therefore, quantifying moisture flow is important in design and verification of the encapsulations. Gaseous flows have been studied after Knudsen’s paper appeared in 1909, with one important exception: water vapour. A recent unexpected finding from Holt et al. concerned ultra-fast water and air flows in carbon nanotubes. While Gruener and Huber did not obtain ultra-fast nitrogen flows in silicon nanotubes. This leaves us to concern main effective factors for flows in tubes. We use a theory of extended Navier-Stokes equations, having one equation for all flow regimes with an empirical parameter (Cha and McCoy theory), for predicting flow rates of nitrogen and water vapour through a 25 μm diameter silica glass cylindrical tube under isothermal condition. We measure nitrogen flow rates through microtubes across a wide range of Knudsen number (0.0048 ~ 12.4583) using a two-chamber method. We find that the nitrogen flow obeys the Cha and McCoy theory with values of the tangential momentum accommodation coefficient (TMAC) α= 0.91 at small Kn and α close to one at large Kn, consistent with the redefinition of α by Arya et al. We obtain fast transport of water vapour compared to the predictions from the Cha and McCoy theory over a range of pressures using the two-chamber method and a mass loss method. We attribute the excess flows to: (1) a thin adsorbed layer of chain-like water on the walls reducing the TMAC at low pressures; (2) liquid or two-phase flow appearing for inlet pressure close to saturation pressure. A theory for TMAC is developed based on the Langmuir adsorption. We measure interdiffusive flow rates of water vapour in atmospheric air for the first time using the mass loss method and compare experimental results with ideal gas interdiffusive flow theory. We find interdiffusive flows of water vapour in air agree with the theory except for the case where water vapour partial pressures are close to the saturation pressure. Liquid or two-phase flow causes an enhancement of the interdiffusive flow by up to three orders of magnitude. Using the available theories we predict the dominant flow types as a function of channel diameter and make recommendations on the moisture hermeticity testing in devices

Characterization of Mesoporous Materials Via Fluorescent Spectroscopic Methods

There are three components that need to be understood to create new porous membranes for industrial applications. 1.) To understand appropriate synthesis conditions to create a successful membrane system. 2.) To understand how the microstructures generated in synthesis affect the transport properties of that system. 3) To be able to characterize the heterogeneity of the fabricated membrane’s transport and physical structure. Presented within this manuscript are new characterization methods to increase the understanding in membrane technology. It will be demonstrated that the novel application of standard fluorescent methods and the development of new fluorescent methods techniques allows for the measurement of molecular interactions and transport properties on length scales capable of providing valuable information in the field of membrane science, as well as expanding new applications in fluorescent techniques

NIRT: gated transport through carbon nanotube membranes

Issued as final reportUniversity of California, Berkele

Molecular Simulation of Diffusive Mass Transport in Porous Materials

Ever increasing control over the shape and form of a material's nanoscale features provokes the pursuit of a detailed understanding for the main factors influencing fluid transport. It is sought to facilitate the intelligent design of novel materials used in membrane separation processes. In addition to a strong dependence on molecular mobility, mass transport is heavily influenced by thermodynamic effects. Isolating thermodynamic and mobility effects is useful to understand the significant driving forces for mass transport through porous materials and their selective characteristics. However, experimental techniques are limited in probing this behaviour at the nanometre scale. In response to experimental challenges, the present study makes extensive use of the ability of molecular simulations to reflect the molecular character of nanoscale diffusion and identify equilibrium and transport properties individually. First, this work investigates diffusive mass transport inside a planar slit pore focusing on the influence of solid-fluid interactions, pore width, and fluid density. The influence of solid-fluid interactions, in particular, have often been neglected in studies of mass transport in porous solids. The vast variety of functionalised nano-materials is virtually endless and has spurred interest in this area. Equilibrium simulations were employed to determine self- and collective diffusivities and Grand Canonical insertions were used for the determination of thermodynamic factors. In addition, this work showcases the implementation of a highly efficient Non-Equilibrium Molecular Dynamics (NEMD) method through which effective transport was studied. The method was used to determine effective diffusivities which incorporate thermodynamic effects, the dominating contribution to transport for dense fluids. It is well suited to observe effective fluid transport in confined spaces as opposed to measuring self-diffusion, a measure for single-particle mobility only. The method is effective in studying mass transport in model systems as well as more realistic, complex geometries. As a second exemplary case, gas permeation through an atomistically detailed model of a high free-volume polymer was simulated explicitly with the NEMD approach. In addition to determining permeability and solubility directly from NEMD simulations, the results also shed light on the permeation mechanism of the penetrant gases, suggesting a departure from the expected pore-hopping mechanism due to the considerable accessibility of permeation paths.Open Acces

Multi-Scale Computational Studies of Calcium (Ca\u3csup\u3e2+\u3c/sup\u3e) Signaling

Ca2+ is an important messenger that affects almost all cellular processes. Ca2+ signaling involves events that happen at various time-scales such as Ca2+ diffusion, trans-membrane Ca2+ transport and Ca2+-mediated protein-protein interactions. In this work, we utilized multi-scale computational methods to quantitatively characterize Ca2+ diffusion efficiency, Ca2+ binding thermodynamics and molecular bases of Ca2+-dependent protein-protein interaction. Specifically, we studied 1) the electrokinetic transport of Ca2+ in confined sub-µm geometry with complicated surfacial properties. We characterized the effective diffusion constant of Ca2+ in a cell-like environment, which helps to understand the spacial distribution of cytoplasmic Ca2+. 2) the association kinetics and activation mechanism of the protein phosphatase calcineurin (CaN) by its activator calmodulin (CaM) in the presence of Ca2+. We found that the association between CaM and CaN peptide is diffusion-limited and the rate could be tuned by charge density/distribution of CaN peptite. Moreover, we proposed an updated CaM/CaN interaction model in which a secondary interaction between CaN’s distal helix motif and CaM was highlighted. 3) the roles of Mg2+ and K+ in the active transport of Ca2+ by sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump. We found that Mg2+ most likely act as inhibitor while K+ as agonist in SERCA’s transport process of Ca2+. Results reported in this work shed insights into various aspects of Ca2+ signaling from molecular to cellular level

Synthesis and Molecular Transport Studies in Zeolites and Nanoporous Membranes

The advent of nanoporous materials such as zeolites and nanoporous membranes has provided cost-effective solutions to some of the most pressing problems of the 20th century such as the conversion of crude oil into fuels and valuable chemicals. Hierarchical zeolites and mesoporous inorganic membranes are showing great promise in addressing new problems such as the conversion of biomass into value-added chemicals and development of energy-efficient separation processes. The synthesis and fundamental aspects of molecular transport in these new materials with hierarchical porosities need to be better understood in order to rationally develop them for these desired applications. Pore narrowing and pore blockage have been proposed to cause the significantly slower than expected diffusion in hierarchical zeolites and zeolite nanoparticles. In the first part of this work, the diffusion of cyclohexane and 1-methylnaphthalene is studied in MCM-41, SBA-15 and conventional as well as hierarchical silicalite-1 zeolite. The role of sorbate-sorbent interactions is investigated and surface diffusion-mediated pore re-entry into micropores is proposed to cause the slower overall diffusion in these materials. Previous molecular transport studies in zeolites have been limited to the MFI zeolite framework, mainly due to ease of synthesis of siliceous MFI in comparison to other siliceous zeolites. Additionally, the requirement of fluoride for the synthesis of siliceous zeolites makes practical applications of these materials difficult. The second part of this work addresses these problems by developing a general, fluoride-free method for the synthesis of siliceous zeolites. The dry gel conversion (DGC) method is used to synthesize 2 new siliceous zeolites for the first time without using fluoride. Mechanistic aspects of siliceous zeolite synthesis, the DGC method in particular, are studied and employed to further improve the synthesis method. Mesoporous inorganic membranes have ideally suited properties for separations such as low pressure drop and thermal as well mechanical stability. However, two challenges impede their applications – the large-scale synthesis of defect-free mesoporous membranes and the development of a fundamental understanding of molecular transport in them. In the third part of this thesis, a new, scalable synthesis method with superior coverage is demonstrated for the synthesis of hybrid mesoporous silica-anodized aluminium oxide (AAO) membranes. Steady state non-equilibrium capillary condensation is studied in detail using the permeation of butane through AAO membranes. New aspects of this phenomenon are reported and experimental evidence is found in support of a partial capillary condensed state of a mesopore stabilized by molecular transport