Carbon molecular sieve membranes for gas separation

Abstract

Membrane separations are simple, energy efficient processes, which can be economically competitive with traditional separation technologies. In the case of gas separation both dense and porous materials have been developed for different application where hydrogen production is one of the most important niches of development. Hydrogen is being one of the most important vectors to develop alternative clean power generation sources. Nowadays, a lot of processes require the fabrication of pure hydrogen for efficiency and better performance. Different materials have been reported as gas separation membranes but still numerous problems related to stability, cost and fabrication must be overcome. The actual goal is to achieve materials that report good separation properties in new type of configuration facing industrial applications. Carbon molecular sieve membranes (CMSM) achieve high separation factors and permeance values than polymeric membranes. During the last 30 years they have gained importance due to their excellent performance as gas separation membranes. However, most research work has been focused on flat or hollow fiber configurations and minor attention has been done to supported CMSM. The main reason is due the difficulties associated to fabricate a defect free membrane using a highly reproducible fabrication method that allow to obtain a carbon layer after one polymer precursor coating step. In tubular configuration, these hybrid membranes are suitable for scaling up towards industrial applications, being more competitive than commercial unsupported hollow fiber membranes and films, especially under high pressure and temperature. The main objective of this work was to explore alternative fabrication methods for the fabrication of supported CMSM. In order to achieve this objective polyimide was coated over inorganic supports using two different approaches. The two methods reported in this thesis were spinning-coating and dip-coating. The idea of spinning¬coating was adapted from fabrication of supported carbon planar film. In this work it was developed the same idea coating TiO2 tubular supports under rotation with polyimide (Matrimid®). The thickness of the carbon membranes was controlled adjusting the viscosity of the polymeric solution, and after an exhaustive solvent i elimination it was possible to obtain a defect free carbon membrane. The influence of methanol washing, pyrolysis temperature (550-700ºC), and presence of the support allowed to extracting conclusions about the characteristics of the carbon material. Single gas permeance of H2, CO, CO2, N2, CH4 were obtained and ideal selectivity computed from this measurements indicated the presence of pinholes on the carbon membrane. However, the characterization of this carbon obtained after 550º and 700º C by adsorption-desorption analysis allowed to confirm the microporosity of the carbon layer. As an important contribution of this work the influence of the support as pore modifier of the carbon structure is presented after analysis of supported and unsupported samples. Different characterization techniques are presented and integrated in this work to analyze the microporous character of the carbon layer (immersion calorimetry, AFM) and to evaluate the mesoporous characteristics of the asymmetric membrane (liquid-liquid displacement porosimetry). An additional coating procedure with polydimethylsiloxane (PDMS) was performed to decrease the influence of pinholes which caused a permeance decrease but increase on ideal selectivity values over Knudsen theoretical index. As a second fabrication technique, the modification of Al2O3 inorganic support allowed to achieve microporosity in the support that allowed the fabrication of CMSM by dip¬coating procedure. Similarly to the dip-coating method, viscosity and polymer concentration were optimized in order to achieve high ideal separation factors for hydrogen pairs. For the type of membranes obtained by this method single gas permeance of H2, He, CO2, O2, N2, CH4, Propane, n-butane, 1-butene, SF6 was performed. Influence of pyrolysis temperature, aging, non-solvent immersion, and support were also studied as pore modifier of the carbon membrane. However, for these membranes the characterization was focused on the effect on permeance and selectivity more than in the characterization of the material. The findings described in this PhD thesis open new perspectives for alternative fabrication techniques of CMSM. This work reports not only the permeance and selective properties of CMSM as the traditional approaches rule. Moreover, brings how each fabrication variable could affect the final properties of the membrane. Integration of structure and properties are presented as an alternative strategy to design new pore architecture on CMSM.