(27f) Facilitated Transport Nanocellulose Membranes Enhanced By Amine Moieties for CO2 Separation

As a response to the growing emissions of carbon dioxide from anthropogenic sources, more efficient and straightforward technologies are needed, in order to separate it from flue gases. Gas separation membranes, in this concern, do represent a viable and explored alternative and several solutions have reached promising results; regular solution-diffusion based materials, however, tend to suffer from an inherent trade-off between permeability and selectivity [1]. Facilitated transport membranes (FTMs), on the other hand, represent a class of films, where CO₂ can selectively react with aminated functional groups, embedded within the polymeric matrix, and be efficiently transported on the permeate side [2]. Most promising materials for facilitated transport membranes production, however, show low mechanical stability, if not crosslinked or mixed with a supporting polymer.

Recently, another option to stabilize such materials was found in the blending of nanocellulose fibers (NFC), a 1-D nanomaterial with ever growing applications, due to its large availability, renewability and stable mechanical properties. Several works have been indeed carried on in the last years, focusing on exploring its gas separation capabilities, both in pure form and when combined with functionalized molecules [3].

To better understand the potential of nanocellulose in the preparation of facilitated transport membranes, a novel nanocomposite material was developed by coupling carboxymethylated nanocellulose (CMC-NFC) with polyvinylamine (PVAm) and L-Arginine, which represent very well-known fixed and mobile carrier system for CO₂ transport across FTMs.

Membranes were initially fabricated without the aminated polymer via a solvent casting protocol and nanocellulose was combined directly with different amounts of Arginine, an amino acid carrying 3 amine moieties in its molecular structure.

Permeation tests were conducted at low temperature (35 °C, 1 bar a) and variable humidity conditions. These showed a strong improvement of both selectivity and permeability once high loadings of the carrier were reached. Pure CMC-NFC presented a top CO₂ permeability of 29 Barrer and an ideal CO₂/N₂ selectivity of 56, but, when 45 wt% of the amino acid was added, carbon dioxide permeability increased 7-fold, up to 225 Barrer and selectivity up to 186. Both results were obtained in humidity saturated conditions.

To further increase the membrane stability, PVAm was then employed in the blend, which also served as an additional amine source. Once the two polyelectrolytes (carboxymethylated nanocellulose and polyvinylamine) were mixed together, indeed they were able to form a strong ionic interaction and a stable suspension, which could be successfully casted in self-standing films.

Despite being formed by two water soluble materials, the final composite exhibited very good resistance to water, even in liquid form, without losing integrity. Since FTMs must work in large presence of humidity, this significantly improved stability in a highly hydrated environment, respect to pure CMC-NFC and to its mixture with L-Arginine.

Thanks to its improved stability, this new nanocomposite was tested at both low (35 °C) and high (>90 °C) temperature, employing pure gases and mixtures (10 % CO₂, 90 % N₂) at medium-low pressure (1-3 bar a). At 35 °C, self-standing films, containing both PVAm and the amino acid, exhibited good, but not exceptional, performances, topping at 130 Barrer and a CO₂/N₂ selectivity of 58 in saturated conditions.

In an attempt to increase the membrane performances, new permeation tests were carried out in a different system, raising the temperature up to 96°C. Only CMC-NFC/PVAm and CMC-NFC/PVAm/L-Arginine membranes were tested in this condition as CMC-NFC/L-Arginine composite were not able to withstand that operative condition due to intrinsic fragility. Once the temperature was raised, interestingly, a significant increment of membrane properties was observed, which resulted particularly elevated for the ternary system. Membranes loaded with L-Arginine indeed increased their CO₂ permeability up to 600 Barrer at 96 °C, while in similar conditions films made exclusively by fixed carriers could not exceed 200 Barrer. Moreover, selectivity with respect to N₂ was substantially increased with values for the different materials, always higher than 80. This behavior can be considered a strong indication of transport facilitation, since the kinetic of the reactions occurring is heavily dependent on temperature and speeds up significantly with it as the theory suggests [2].

As of now, research is focused in developing supported thin films of the material, to better understand its potential in industrial scale carbon capture applications.

In conclusion, a new polyelectrolyte-based nanocomposite membrane, relying on facilitated transport mechanisms, was successfully fabricated by the combination of carboxymethylated nanocellulose and polyvinylamine. Moreover, the addition of a mobile carrier such as L-Arginine was able to greatly improve the permeation and separation performances of the material, reaching very interesting results, especially at high temperature. Due to its improved stability, tunable performances, high workability and lack of harmful solvents, this material could find its way in many gas separation applications, starting from carbon dioxide capture from flue gas.

[1] L.M. Robeson, The upper bound revisited, J. Memb. Sci. 320 (2008) 390–400. doi:10.1016/j.memsci.2008.04.030.

[2] R. Rea, M.G. De Angelis, M.G. Baschetti, Models for facilitated transport membranes: A review, Membranes (Basel). 9 (2019). doi:10.3390/membranes9020026.

[3] D. Venturi, D. Grupkovic, L. Sisti, M.G. Baschetti, Effect of humidity and nanocellulose content on Polyvinylamine-nanocellulose hybrid membranes for CO2capture, J. Memb. Sci. 548 (2018) 263–274. doi:10.1016/j.memsci.2017.11.021.