Synthesis and evaluation of novel materials for gas separation membranes through thermal rearrangement processes of ortho-functionalized polyimides

Resumen

The main goal of this thesis work is in the framework of I+D research effort made by many laboratories to reduce the effects of the CO2 emission into the atmosphere, separating CO2 from gas mixtures where it is an undesirable component. There is an evidence, already universally admitted, that CO2 in the air is one of the most relevant responsible of the greenhouse effect and the global warming. Industrial activity, mainly the use of fossil fuels such as gasoline, gas or coal, is the chief responsible for the increase in CO2 content in the air from 280 ppm to 400 ppm in the last 150 years, giving rise to an increment in the average temperature of the Earth of approximately 0.85 ° C with respect to the levels of the preindustrial era, and the forecasts announce an increase of 40 % in the energy demand between 2007 and 2030. Several technology solutions including the development of renewable energies and the capture and storage of CO2 are scheduled in order to mitigate the remarkable increase in the greenhouse effect, which is a serious threat to live in the Earth. Polymeric membranes for gas separation have experienced a continuous growth thanks to new materials and engineering development. Thus, it can be said that gas separation membranes offer an alternative energetically more favorable, less polluting, and simpler and more modular than classical gas separation procedures, such as cryogenic distillation or large adsorption/absorption columns. Among the great number of tested polymers, polyimides have been particularly interesting due to its permeation properties (productivity and selectivity), and also because they have physical properties especially suitable for making membranes. As a consequence of the continued search of polymers with enhanced permeation properties, two families of new rigid porous materials have drawn interest from the scientific and practical point of view. They are called PIMs, polymers of intrinsic microporosity, and TR polymers which are polymers obtained by thermal transposition from polymer precursors, in particular functionalized polyimides. The intrinsic microporosity was defined as a continuous network of interconnected intermolecular microcavities, which is formed as a consequence of the shape and rigidity of the polymer structure conferred by their kinked backbone (spiro-type structure) and highly restricted backbone rotational movements. Thus, these rigid ladder polymers prevent efficient chain packing and provide extraordinarily high free volume and high surface areas, resulting in permeability values above 1000 Barrers. In the case of TR polymers, the thermal rearrangement process in solid state, originates conformational and chemical changes at high temperature that also produces an increment and redistribution of the fractional free volume, resulting in permeability values close to or greater than 1000 Barrers, together with lower physical aging and low plasticization tendency. TR membranes show excellent separation performance for CO2/CH4 mixtures due to their unusual microstructure, where cavity size and bimodal cavity distribution can be changed by the choice of the precursor structure and thermal treatment protocol. This thesis work is aimed to study deeply these TR materials and contribute to their development as precursors of gas separation membranes. The work is mainly focused on aspects of preparation and evaluation of new TR polymers, and on the detailed study of the mechanisms that govern the rearrangement process giving rise to the final materials by thermal treatments. The most innovative features of this thesis can be summarized in the following paragraphs: • Synthesis and evaluation of TR polymers from commercial monomers not used previously, such as 2,4-diaminophenol and 4,6-diaminoresorcinol. The goal of this study consisted of exploring the capability of these new TR polymers in gas separation materials, as well as studying the influence of the amount of OH groups present in the precursor polyimides in the final properties of the TR materials. Two new polyimides having ortho-hydroxyl groups to the nitrogen of the imide moiety were attained via classical two-step polycondensation, using an in situ base-assisted silylation method. Both polyimides were processed in the form of films with good mechanical properties and, subsequently, underwent thermal rearrangement in the solid state at temperatures above 350 ºC, yielding polybenzoxazoles (TR-PBO) or polyimide-polybenzoxazoles (TR-PI-PBO). The gas separation properties of this set of TR polymer films were evaluated after different treatment temperatures showing a very good balance between permeability and selectivity and placing these materials above the 2008 Robeson limit for some gas pairs. Consequently, these TR membranes derived from inexpensive monomers can be considered as excellent materials for gas separation. • Synthesis and evaluation of TR polymers from a newly prepared monomer, 3,3´-diamino-4,4´-dihydroxybiphenyl, isomer of 3,3'-dihydroxy-4,4'-diaminobiphenyl, which has been widely used as a precursor monomer for TR-PBOs. The 3,3´-diamino-4,4´-dihydroxybiphenyl monomer has been obtained by a short, low-cost and highly efficient synthetic methodology. The synthetized monomer and the commercial isomeric one were reacted with 6FDA using the in situ silylation of the diamine in a first stage and azeotropic cycloimidization in a second stage to form two o-hydroxypolyimides having high molecular weights. Extensive characterization and physical properties measurements were carried out for hydroxypolyimides and for the corresponding thermally obtained polybenzoxazoles, showing a differential behavior attributed to the different orientation of imide and phenylene groups in the precursor monomers. • Synthesis and evaluation of TR polymers from diamines with different substituents in ortho position, in particular, -OH, -OAc and –OMe substituents. In this study, the preparation of soluble ortho-methoxy polyimides made from 6FDA and 3,3ʹ-dimethoxybenzidine, which is a comparatively low-priced monomer, has been reported. The conversion of this precursor into TR-PBO at high temperatures was the main goal of this study and the focus was to relate the time-temperature schedule and the final properties of the films fabricated from the precursor. Special attention was given to the possible reactions that occurred alongside the thermal treatment. Results were compared with these of ortho-hydroxy and ortho-acetylpolyimides of similar structure, comparing the permeation properties of the final films. A dramatic change in permeation properties was observed. Unlike ortho-hydroxypolyimides, ortho-methoxypolyimides did not seem to be converted into PBOs, but the starting polyimide remained in a great proportion after the thermal treatment, even after the formation of benzoxazole and lactam units. In this work, based on the accepted mechanistic paths and also on recent literature, a plausible molecular simulation justification is proposed in which the presence of protons or radical hydrogen atoms plays a critical role in benzoxazole formation. Gas permeation of the TR polymers reported here are attractive and compare fairly well with other TR-PBOs, exhibiting P(CO2) up to 540 Barrers and selectivities of 30 for the CO2/CH4 gas pair. As a resume, it can be concluded that TR-PBOs and TR-PI-PBOs, with excellent permeation properties for the separation of CO2 from gas mixtures, can be attained by a convenient thermal treatment of novel poly-o-hydroxypolyimides based in original hydroxydiamines. TR-polymers with a varied proportion of imide, benzoxazole and lactone/lactam moieties can also be prepared from poly-o-acetylimides and poly-o-methoxyimides by thermal treatment up to 450 ºC. Their permeation properties, and particularly their separation capabilities for CO2, compare fairly well with those of previously reported TR-PBOs and greatly exceed the permeation properties of most polyimides and other technical polymers tested with this purposes so far. Therefore, it should be stated that the targets proposed in the research project have been accomplished. Thus, new generations of gas separation membranes with excellent gas separation properties have been obtained by using low-cost monomers. The optimization of the polycondensation reaction has permitted to obtain high molecular weight ortho-substituted polyimides that undergo thermal rearrangement to polybenzoxazoles in solid state to yield membranes with adequate mechanical properties, which can be employed in industrial gas purification. The combination in the same monomer of TR-able and non TR-able functionalities has permitted to obtain relationships between the structures of the final TR-PBO materials and their physical and gas separation properties. Also, a new approach to TR-materials has been developed by using ortho-methoxy polyimides. This latter approach can be expanded to a new generation of membranes by affordable modification of the ortho-hydroxy groups to other functionalities, which should lead to materials with improved properties: lower thermal rearrangement temperature, better mechanical properties and enhanced gas separation properties.