MATERIALS DESIGN AND HYBRIDIZATION FOR GREEN CARBON DIOXIDE CAPTURE

Abstract

To provide an impetus for a green technology like membranes for carbon dioxide (CO2) capture, the trade-off relationship between gas permeability and selectivity in membranes must be overcome. An elegant and simple approach to overcome this trade-off relationship is to transform the chemical structures of current polymeric materials in a fashion that enhances both gas permeability and gas selectivity.

In this work, the trade-off relationship between gas permeability and selectivity in glassy polymeric materials for e.g. polyimides is abated via an innovative technique ¿ vapor phase modification that is traditionally used in lithography and nanotube alignment. Traditionally, solution crosslinking is used to overcome the aforementioned trade-off between gas permeability and selectivity. The main problem with solution cross-linking is its inability to maintain structural integrity in hollow fiber membranes, the preferred physical configurations of membranes in the industry. By converting the polyimide structures into dense polyamide structures via the vapor of crosslinking agents, gas selectivity was enhanced whilst retaining structural integrity in hollow fiber membranes.

In another part of this work, material design and hybridization was used to improve the CO2 permeation properties of polyethylene oxide (PEO), a well-known material with high CO2 affinity. Silicate nanoparticles were fabricated in-situ a PEO-based matrix via a sol-gel approach. The CO2 transport and CO2/H2 separation properties of this organic-inorganic material resemble current state-of-the-art cross-linked PEO rubbers. However, the CO2 transport properties of this organic-inorganic material remain inferior to polydimethylsiloxane ¿ the preferred gas separation material in current industries. To enhance its CO2 and CO2/H2 transport and separation properties, alkyl chains were grafted onto the main chains of the organic-inorganic material. The resultant nanohybrid material possesses CO2 permeability that is similar to polydimethylsiloxane. However, the CO2/H2 selectivity of this material is much higher than those observed in polydimethylsiloxane, and is comparable with state-of-the-art cross-linked PEO rubbers.

Synthesis parameters like water/silicon ratio, organic/inorganic ratio, graft content and sol-gel synthesis conditions like hydrolysis durations and condensation durations were discovered to be fundamental to attune the CO2 transport and separation properties of the nanohybrid materials. To obtain optimal nanohybrid membranes, an intricate combination of synthesis parameters and conditions is required. This work reveals that by combining 30 minutes of hydrolysis duration with 1 hour of condensation duration for the synthesis of nanohybrid membranes consisting 80 wt.% polyether diamines, 20 wt.% alkoxysilanes and 20 wt.% alkyl chain grafts (with respect to the weight of the base material) could yield a CO2 permeability of about 2000 Barrer and CO2/H2 and CO2/N2 selectivities of 10.5, and 55.6 respectively. The high CO2 transport and separation properties of these nanohybrid membranes have pushed the boundaries of using polymeric membranes for CO2 capture to newer heights.