Caratterizzazione di membrane metalliche e ionomeriche per applicazioni energetiche

The work of this thesis has been focused on the characterization of metallic membranes for the hydrogen purification from steam reforming process and also of perfluorosulphonic acid ionomeric (PFSI) membranes suitable as electrolytes in fuel cell applications. The experimental study of metallic membranes was divided in three sections: synthesis of palladium and silver palladium coatings on porous ceramic support via electroless deposition (ELD), solubility and diffusivity analysis of hydrogen in palladium based alloys (temperature range between 200 and 400 °C up to 12 bar of pressure) and permeation experiments of pure hydrogen and mixtures containing, besides hydrogen, also nitrogen and methane at high temperatures (up to 600 °C) and pressures (up to 10 bar). Sequential deposition of palladium and silver on to porous alumina tubes by ELD technique was carried out using two different procedures: a stirred batch and a continuous flux method. Pure palladium as well as Pd-Ag membranes were produced: the Pd-Ag membranes’ composition is calculated to be close to 77% Pd and 23% Ag by weight which was the target value that correspond to the best performance of the palladium-based alloys. One of the membranes produced showed an infinite selectivity through hydrogen and relatively high permeability value and is suitable for the potential use as a hydrogen separator. The hydrogen sorption in silver palladium alloys was carried out in a gravimetric system on films produced by ELD technique. In the temperature range inspected, up to 400°C, there is still a lack in literature. The experimental data were analyzed with rigorous equations allowing to calculate the enthalpy and entropy values of the Sieverts’ constant; the results were in very good agreement with the extrapolation made with literature data obtained a lower temperature (up to 150 °C). The information obtained in this study would be directly usable in the modeling of hydrogen permeation in Pd-based systems. Pure and mixed gas permeation tests were performed on Pd-based hydrogen selective membranes at operative conditions close to steam-reforming ones. Two membranes (one produced in this work and another produced by NGK Insulators Japan) showed a virtually infinite selectivity and good permeability. Mixture data revealed the existence of non negligible resistances to hydrogen transport in the gas phase. Even if the decrease of the driving force due to polarization concentration phenomena occurs, in principle, in all membrane-based separation systems endowed with high perm-selectivity, an extensive experimental analysis lack, at the moment, in the palladium-based membrane process in literature. Moreover a new procedure has been introduced for the proper comparison of the mass transport resistance in the gas phase and in the membrane. Another object of study was the water vapor sorption and permeation in PFSI membranes with short and long side chains was also studied; moreover the permeation of gases (i.e. He, N2 and O2) in dry and humid conditions was considered. The water vapor sorption showed strong interactions between the hydrophilic groups and the water as revealed from the hysteresis in the sorption-desorption isotherms and thermo gravimetric analysis. The data obtained were used in the modeling of water vapor permeation, that was described as diffusion-reaction of water molecules, and in the humid gases permeation experiments. In the dry gas experiments the permeability and diffusivity was found to increase with temperature and with the equivalent weight (EW) of the membrane. A linear correlation was drawn between the dry gas permeability and the opposite of the equivalent weight of PFSI membranes, based on which the permeability of pure PTFE is retrieved in the limit of high EW. In the other hand O2 ,N2 and He permeability values was found to increase significantly, and in a similar fashion, with water activity. A model that considers the PFSI membrane as a composite matrix with a hydrophilic and a hydrophobic phase was considered allowing to estimate the variation of gas permeability with relative humidity on the basis of the permeability in the dry PFSI membrane and in pure liquid water.

Polymeric membranes for CO2 separation: effect of aging, humidity and facilitated transport

Polymeric membranes represent a promising technology for gas separation processes, thanks to low costs, reduced energy consumption and limited waste production. The present thesis aims at studying the transport properties of two membrane materials, suitable for CO2 purification applications. In the first part, a polyimide, Matrimid 5218, has been throughout investigated, with particular reference to the effect of thermal treatment, aging and the presence of water vapor in the gas transport process. Permeability measurements showed that thermal history affects relevantly the diffusion of gas molecules across the membrane, influencing also the stability of the separation performances. Subsequently, the effect of water on Matrimid transport properties has been characterized for a wide set of incondensable penetrants. A monotonous reduction of permeability took place at increasing the water concentration within the polymer matrix, affecting the investigated gaseous species to the same extent, despite the different thermodynamic and kinetic features. In this view, a novel empirical model, based on the Free Volume Theory, has been proposed to qualitatively describe the phenomenon. Moreover, according to the accurate representation of the experimental data, the suggested approach has been combined with a more rigorous thermodynamic tool (NELF Model), allowing an exhaustive description of water influence on the single parameters contributing to the gas permeation across the membrane. In the second part, the study has focused on the synthesis and characterization of facilitated transport membranes, able to achieving outstanding separation performances thanks to the chemical enhancement of CO2 permeability. In particular, the transport properties have been investigated for high pressure CO2 separation applications and specific solutions have been proposed to solve stability issues, frequently arising under such severe conditions. Finally, the effect of different process parameters have been investigated, aiming at the identification of the optimal conditions capable to maximize the separation performance.

Synthesis, Modification and Characterization of Polymeric Membranes for CO2 Separation

This doctorate focused on the development of dense polymeric membranes for carbon capture, mostly in post combustion applications, and for natural gas sweetening. The work was supported by the European Project NANOMEMC2 funded under H2020 program. Different materials have been investigated, that rely on two main transport mechanisms: the solution-diffusion and the facilitated transport. In both cases, proper nano-fillers have been added to the matrix, in order to boost the mechanical and permselective properties of the membranes. Facilitated transport membranes were based on the use of was polyvinylamine (PVAm), as main matrix with fixed-site carriers, and L-Arginine as mobile carrier; the filler, used mostly as reinforcer, was carboxymethylated nanocellulose (cNFC). Humid test showed interesting results, and especially the blend made of PVAm/cNFC/Arg in weight ratio 27,5/27,5/45 crossed the Robeson CO2/N2 upper bound, representing current state of the art membranes, with a CO2 permeability of 271 Barrer and CO2/N2 selectivity of 70. Solution diffusion membranes were based on Pebax®2533 matrix which was added with three different graphene oxide (GO)-based materials, namely pristine GO, Porous Graphene Oxide (PGO) and a GO functionalized with polyetheramine (PEAGO). All of them provided a modest but clear increment of permeability of the Pebax matrix, from plus 2% (GO) to plus 8% (PGO), with no change in selectivity. The gas tested with this type of composites were CO2 and N2, for Post combustion capture applications. Pebax®2533 was also chemically modified, obtaining the product called “Benzoyl-P2533”, that was fully characterized, and tested in term of permeation using five gas: CO2, N2, CH4, O2, and He. Modified material showed an increment of the overall permeability of the material of a fair 10% for all gases tested, apart from helium, that increased of almost 50%.

Novel membranes for hemodialysis application

The present work is focused on the synthesis and characterization of novel materials for hemodialysis applications. Cellulose acetate was chosen as base polymer for the preparation of porous Mixed Matrix Membrane adsorbers (MMMAs) and for the synthesis of hybrid ultrafiltration membranes. Hemodialysis is a renal replacement therapy used to eliminate,the waste products and excess fluids accumulating in the blood of people affected by an end stage renal disease. The main environmental drawback associated to it is the large water consumption. The MMMAs were prepared with the porpoise of eliminating waste metabolites (uremic toxins) from the spent dialysate solution, with the prospective limiting the consumption of water related to the process. Batch tests of MMMAs showed that the removal of uric acid is almost complete while the one of urea and creatinine is limited to a 20/30 %. The thinking behind the concept of MMMAs was aimed to develop a small a lab scale chromatographic cartridge to continuously remove uremic toxins from an aqueous feed solution. The cartridge was packed with MMMAs and tested with a mixture of toxins. Experiments results shown a promising removal capability of the system even if the necessity of a higher surface area to achieve better efficiency is denoted. The other important issue related to hemodialysis is the assessment of an overall mass transfer rates in hemodialyzers. The mass transfer correlations proposed in literature do not take into account the effect of permeation and are developed for turbulent flow regime. Therefore, hybrid cellulose acetate/Silica ultrafiltration membranes were prepared to characterize a surrogate system of an artificial kidney (AK) in terms of fluid mechanics and mass transfer. The effect of surface roughness and suction on the velocity profiles was determined and a new dimensionless mass transfer correlation accounting for permeation was developed.

Photoactive (supra)molecular devices in double layer membranes

Biological systems are complex and highly organized architectures governed by non-covalent interactions responsible for the regulation of essential tasks in all living organisms. These systems are a constant source of inspiration for supramolecular chemists aiming to design multicomponent molecular assemblies able to perform elaborated tasks, thanks to the role and action of the components that constitute them. Artificial supramolecular systems exploit non-covalent interactions to mimic naturally occurring events. In this context, stimuli-responsive supramolecular systems have attracted attention due to the possibility to control macroscopic effects through modifications at the nanoscale. This thesis is divided in three experimental chapters, characterized by a progressive increase in molecular complexity. Initially, the preparation and studies of liposomes functionalized with a photoactive guest such as azobenzene in the bilayer were tackled, in order to evaluate the effect of such photochrome on the vesicle properties. Subsequently, the synthesis and studies of thread-like molecules comprising an azobenzene functionality was reported. Such molecules were conceived to be intercalated in the bilayer membrane of liposomes with the aim to be used as components for photoresponsive transmembrane molecular pumps. Finally, a [3]rotaxane was developed and studied in solution. This system is composed of two crown ether rings interlocked with an axle containing three recognition sites for the macrocycles, i.e. two pH-switchable ammonium stations and a permanent triazolium station. Such molecule was designed to achieve a change in the ratio between the recognition sites and the crown ethers as a consequence of acid-base inputs. This leads to the formation of rotaxanes containing a number of recognition sites respectively larger, equal or lower than the number of interlocked rings and connected by a network of acid-base reactions.

Novel asymmetric geopolymer membranes for oil/water emulsions separation

In this study, it was investigated the possibility of using a geopolymeric membrane as an alternative to the expensive ceramic ones. The goal was to synthesise a low-cost membrane made entirely of geopolymer that can perform equally to commercial membranes. This study initially investigated the feasibility of preparing a microporous support suitable for microfiltration through casting and pressing techniques. Subsequently, a selective geopolymeric layer was developed and deposited on the support, with the capability to operate within the microfiltration range and to effectively separate oil from oil-water emulsions. In order to evaluate the performance, the properties of the geopolymeric supports obtained through pressing were carefully evaluated during the experimentation phase investigating the effect of varying parameters such as sodium silicate content, water content, and applied pressure. The results obtained from these evaluations showed that it is possible to produce supports with excellent porosity and highly controlled narrow pore size distributions. The most promising geopolymeric pressed support was then used for the deposition of a selective layer on its surface. Following physical characterization, it was confirmed that the resulting geopolymer membrane was suitable for use in the microfiltration range. Subsequently, the membrane was tested for its ability to separate oil from water using various emulsions prepared with different surfactants at different concentrations and pH. The results revealed that the fluxes were highly dependent on the electrostatic interaction between the membrane and the emulsion, with best results being obtained with emulsions prepared using anionic surfactants. The rejection rate of the membrane was also found to be extremely high, with values over 95%, comparable to a commercial ceramic membrane. This suggests that geopolymer membranes are suitable alternatives to ceramic membranes, offering the added benefits of lower cost and reduced environmental impact during production.

Study and development of BaCe0.65Zr0.20Y0.1503-Δ (BCZY) – Gd0.2Ce0.8O2-Δ (GDC) dense ceramic membranes sysyem for high temperature H2 purification

The first main conclusion drawn from this dissertation concerns the amount of Pt deposited on the asymmetric layer of membrane produced by tape casting porosity shaping method. Three different amounts were investigated (0.15, 1.5 and 4.5 mg cm-2 ). The most optimal performance, based on H2 permeation performances, was attained when 1.5 mg cm-2 of Pt was deposited on the porous layer, resulting in a 0.642 mL min-1 cm-2 permeated H2 when 80% H2 in He was employed as the feed. Pt deposition method is influenced by the concentration of the Pt precursor, which results in different morphology of the catalyst. The second development focused on further optimization on tape casting membranes concerning the solvent employed for the Pt catalyst deposition. The same concentration of Pt was employed, depositing 1.5 mg cm-2 on the porous side of the membrane, but a mixture of acetone and water was employed as solvent. This mixture allowed the suppression of effects leading to poorly dispersed particles. As a result, it was possible to achieve 0.74 mL min-1 cm-2 at 750°C with 50% H2 in He. Lastly, first-ever permeation performance measurements into an innovative ceramic membrane type for hydrogen separation was investigated. In-depth research was done on a group of hierarchically-structured BaCe0.65Zr0.20Y0.15O3-δ(BCZY) - Gd0.2Ce0.8O2-δ(GDC) membranes produced by freeze casting porosity shaping method. Membranes were investigated observing the effect of deposition solvent and the effect of porous layer thickness. Employing a mixture of Acetone and water resulted in better hydrogen permeation at temperatures (T > 650°C), reaching 0.26 mL min-1 cm-2 at 750°C with 50% H2 in He. The reduction of porous layer thickness led to a hydrogen flow of 0.33 mL min-1 cm-2 , at 750°C with 50% H2 in He.