Membrane design and characterisation by incorporation of ionic liquids

Dissertação apresentada para obtenção do Grau de Doutor em Engenharia Química, especialidade de Operações Unitárias e Fenómenos de Transferência, pela Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia

This thesis concerns the design and characterisation of new membranes prepared by incorporation/immobilisation of ionic liquids (ILs). These membranes were developed aiming for two distinct applications: fuel cells and CO2 separation from gas streams. In the first part of this PhD thesis, Nafion membranes were modified by incorporating different types of ionic liquids cations, with different degrees of concentration, in order to evaluate their potential use for fuel cells. The characterisation of the modified Nafion/IL cation membranes was performed using the following techniques: XPS (X-ray photoelectron spectroscopy), EIS (Electrochemical Impedance Spectroscopy), proton NMR relaxometry and thermogravimetry analysis. Methanol and gas crossover studies were also performed for these modified Nafion/IL cation membranes. It has been shown that it is possible to obtain membranes with tailored properties, depending on the IL cation incorporated and its degree of incorporation. Moreover, the introduction of the bulky IL cations leads to changes in the membrane water content and in the water degree of structuring. Modifications in the water structuring improved the stability of the modified membranes at high temperatures (up to 200 °C) and reduced both their methanol and gas crossover. However, this modification appears to decrease the proton mobility (at room temperature) when compared to that of an unmodified Nafion membrane. In the second part of this Thesis, different ionic liquids were immobilised in the porous structure of polymeric porous membranes, in order to evaluate their potential use for gas separation processes. Studies were performed to assess the supported ionic liquid membranes (SILMs) stability, using membrane supports of different chemical nature. Moreover, permeabilities were measured for pure gases, humidified gases, and gas mixtures. It was concluded that the SILMs are stable (for difference pressures up to 2 bar) and highly selective for CO2, when compared with N2 and CH4. The presence of water vapour in the gas stream is an important factor, since it decreases their selectivity. In this work, the results obtained for the CO2/CH4 separation are rather promising suggesting SILMs may be potentially applied for this specific gas separation. This work shows that by using ionic liquids in both membrane types – Nafion membranes and polymeric porous membranes – the properties of the membranes can be tuned and improved according with the application envisaged, through a careful selection of the IL structure and its concentration.