(455a) Development of Cellulose-Based Composite Adsorbents for CO2 Capture

Currently, a major environmental concern across the world is the increase in carbon dioxide (CO₂) emissions due to anthropogenic activities arising from agriculture, transportation, and industries such as cement, petrochemical, iron, and steel. On the other hand, the accumulation of waste paper in landfills is a growing concern that needs to be addressed. Several CO₂ reduction technologies are currently being investigated to offset the negative impact caused due to CO₂. Among them, the most prominent technology has been Carbon Capture, Utilization, and Storage (CCUS). At present, absorption using liquid amines are being utilized widely in industries to capture CO₂ from flue gas but it has various limitations such as high regeneration energy requirements, high corrosion, and limited CO₂ loading. As a result, alternate methods are being studied extensively.

This research aims at exploring the use of cellulose-based solid adsorbents in capturing CO₂ by reusing and recycling waste paper. Office paper comprises 85% cellulose which is extracted using an alkali and bleaching treatment in this study. Alkali treatment is done to remove hemicellulose while bleaching eliminates lignin. Cellulose is the most abundant biopolymer in the world which is inexpensive, non-toxic, and can be easily functionalized with CO₂-philic functionalities due to the presence of abundant hydroxyl groups. To further enhance the CO₂ uptake of cellulose-based materials, by creating more functional groups, higher specific surface area, and porosity; aerogels are synthesized, and the pristine cellulose is combined with graphene oxide (GO) which is a 2D material comprising hydroxyl, alkoxy, carbonyl, and carboxyl functional groups. In this study, the aerogels are prepared by dissolving cellulose in a NaOH/urea/ DI water solvent which is subsequently solvent exchanged with organic solvents and vacuum freeze-dried. The synthesized cellulose-GO aerogels were evaluated for CO₂ adsorption as a function of temperature and absolute pressure. Furthermore, the cellulose-GO aerogels were functionalized with an amino silane called 3-aminopropyltriethoxysilane (APTES) to study the effect of introducing different concentrations of APTES in adsorbing CO₂ at different temperatures and absolute pressure. To functionalize cellulose and GO, the wet gel is grafted with APTES dissolved in a hydroalcoholic mixture. By functionalizing the aerogel with APTES, it is observed that the aerogel can uptake a good amount of CO₂ at low pressures which makes the material promising for low-concentration CO₂ capture such as Direct Air Capture (DAC) applications. The study has analyzed the effect of different GO and APTES concentrations on CO₂ uptake. The optimized non-functionalized cellulose-GO aerogel displaying physisorption behavior has an uptake of 0.48 mmol/g at 1 bar and 25^oC. On the other hand, the improvised APTES grafted cellulose-GO aerogel which presented chemisorption characteristics has a CO₂ uptake of 2.52 mmol/g at 1 bar and 25^oC. Figure 1 represents the adsorption isotherm of cellulose, cellulose-GO, and cellulose-GO-APTES aerogels.

The cellulose-GO aerogel and the APTES functionalized cellulose-GO aerogels had also been studied for their kinetics, regeneration, and cyclability for up to 10 cycles by using both Pressure Swing Adsorption (PSA) and Temperature Pressure Swing Adsorption (TPSA), selectivity to nitrogen (N₂), water adsorption and CO2 adsorption at humid conditions. In addition, the CO₂ adsorption of the aerogels had been tested at 40^oC and 60^oC. As expected, the APTES functionalized cellulose-GO aerogels had higher CO₂ uptake at higher temperatures while the non-functionalized cellulose-GO aerogels had lower uptake as the temperature increased. Furthermore, the functionalized samples had much faster kinetics, higher selectivity against N_2, and showed good regeneration ability while performing TPSA. Due to the hydrophilic nature of both cellulose and GO it was observed that the non-functionalized sample had a higher water adsorption capacity and the CO₂ uptake in humid conditions at 25^oC dropped to 0.4 mmol/g at 1 bar. However, due to the APTES grafting the aerogel had some hydrophobic nature due to which it adsorbed much lesser water than the non-functionalized sample. At humid conditions, the APTES functionalized sample had a lesser CO₂ adsorption capacity than the sample tested at dry conditions however, the uptake was still much higher than the cellulose-GO aerogel.

Other crucial parameters such as surface area, pore volume, pore size, and heat of adsorption have also been estimated. It is observed that the synthesized aerogels are mesoporous in nature. An important observation that was made is that the cellulose-GO aerogel had a much lower surface area than the APTES functionalized sample. This is because the cellulose-GO aerogel had a much higher average pore size and with the introduction of APTES the average pore size decreased thus, as a result of having a greater number of pores with a smaller size the surface area increased. The heat of adsorption for the aerogels was estimated using the Clausius-Clapeyron equation and the value obtained for the APTES functionalized samples was 35.95 kJ/mol while the non-functionalized sample gave a value of 14.28 kJ/mol. Both values represent how chemisorption requires higher heat of adsorption than physisorption and are within the range reported in the literature. Finally, the modeling of the CO₂ isotherms at 25^oC was done using Langmuir and Freundlich models. From the curve fitting it has been observed that for the cellulose-GO aerogel both the Langmuir and Freundlich models were promising. On the contrary, for the APTES functionalized samples the Langmuir model overestimated the CO₂ capacity at low pressures and underestimated the values at higher pressures. But by using the Freundlich model the error associated with the adsorption capacity was much lesser and “n” which represents the affinity between the adsorbent and the adsorbate had a value of 5.32. Since a value of n between 2 to 8 represents high affinity and heterogenicity of the sites in the adsorbent it is concluded that the functionalized adsorbent is better modeled using the Freundlich model.

In conclusion, this works focuses on the development of a cellulose-based composite adsorbent for CO₂ capture by recycling paper and functionalizing it with APTES which is an area that has not been explored. The synthesized aerogel which is functionalized displays promising CO₂ adsorption at different conditions and when compared with similar such cellulose-based adsorbents the values are consistently making it applicable for both low CO₂ concentration sources such as DAC and industrial emissions capture.