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Methods of separation and underground storage of CO2 released through the burning of fossil fuels have gained notoriety as strategies for combating climate change Membrane-based separations are one way in which large reductions in the cost of CO2 separation can be realized. The carbon separation and sequestration-oriented Integrated Gasification Combined Cycle (IGCC) process requires a membrane capable of separating CO2 and H2 with high efficiency.
In recent years, it has been reported that zeroeth generation polyamidoamine (PAMAM) dendrimers can have excellent CO2/N2 selectivity. We later reported further improvement in CO2/N2 and CO2/H2 selectivity resulting from substitution of the PAMAM dendrimer with hydroxyl surface groups (2). In this report, a new dendrimer has been synthesized and evaluated for its separation performance and H2O absorption characteristics.
The chemical structure of the dendrimer under evaluation is shown in Figure 1. In this work, a new dendrimer, 6OH-TA, was synthesized with a larger than usual number of hydroxyl groups per unit mass.
The dendrimer was cast into porous, hydrophilic PVDF (pore diameter 0.1um, membrane thickness 125um) and CO2/H2 separation performance was measured at 25C with a helium sweep gas at isobaric pressure. The supply gas was a mixture of CO2 (5%) and H2 (95%) with a fixed relative humidity and was used at atmospheric pressure. To measure water absorption, the dendrimer was held in a container under constant temperature and humidity and tested at 25C over a relative humidity range from 40-99%. The amount of water absorbed was calculated from the change in weight.
3. Results and Discussion
The relationship between relative humidity and separation performance for each dendrimer is shown in Figure 2. For each dendrimer, the CO2 permeability increased with the increase in the relative humidity of the supply gas. However, the largest permeability values were measured at relative humidities of 75-80%. The newly developed dendrimer, 6OH-TA, shows better CO2/H2 separation performance than the 0OH-, 4OH-PAMAM dendrimers with a separation factor of 1300 at a relative humidity of 75%.
The relationship between water absorption and relative humidity for each of the dendrimers is shown in Figure 3. For each of the dendrimers, the amount of water absorbed increased as the relative humidity increased, with a particularly large increase above 90% relative humidity. In addition, water absorption was higher in 6OH-TA than in the other dendrimers.
In this presentation, information on the chemical structures of the dendrimers as well as on the relationship between CO2 separation performance and the condition of the humidified dendrimers will be reported.
This research was supported by a grant from the Ministry of Economy, Trade and Industry.