(342b) Novel Strategy to Control the Pore Structure and Surface Chemistry of Cellulose-Based Carbon Molecular Sieve Membranes

The hastening of climate change and global warming caused by the greenhouse gas emissions to the atmosphere is a pressing issue for our society. The anthropogenic nature of this problem requires the development of new technological solutions to achieve the carbon neutrality goals implemented by the European Union for 2050 [1]. Hydrogen or biomethane are considered the vectors for this technological transition in the energy, transport and industrial systems [2]. When compared with the conventional separation technologies for CO₂ capture, air enrichment or H₂ recovery, such as cryogenic distillation, chemical absorption or pressure-swing adsorption (PSA), membrane separation technologies are the most energy efficient, versatile, low capital cost and environmentally friendly [3-5]. Carbon molecular sieve (CMS) membranes appear the most promising alternative not only for their better trade-off between permeability and selectivity but also for their chemical robustness: they work very well even in extremely harsh chemical, environmental, high-pressure and temperature applications [5-7]. CMS membranes possess a rigid pore structure with a bimodal pore size distribution: the micropores (< 2 nm) provide the sorption sites and the ultramicropores (< 0.7 nm) enable the molecular sieving mechanism [8]. This pore structure, when tuned properly for the desired separation, is responsible for the membrane's high permeability and selectivity [5]. CMS membranes are fabricated by the controlled carbonization of a polymeric precursor. Rodrigues et al. [9] reported for the first time a tailor-made precursor from cellulose dissolved in a mixture of ionic liquid/dimethyl sulfoxide (DMSO). These CMS membranes withstand permeate steams with a relative humidity close to 100 % without suffering pore blockage, even at room temperature. In addition to the proper selection of polymeric precursor, selection of adequate carbonization conditions and post-carbonization treatments, the optimization of the precursor preparation conditions and pre-carbonization treatments can also be used to improve the performance of the final CMS membranes. For addressing the challenge of preparing CMS membranes with the desired pore size distribution and with very high porosity, this work proposes the development of different strategies to control the structural and surface chemistry of the cellulose-based CMS membranes to the desired different separations.

Experimental

CMS membranes were fabricated by controlled carbonization conditions at 550 ºC from an optimized ionic liquid cellulose-based precursor [10]. Small molecules with chemical affinity to cellulose were initially added to the cellulose/IL/DMSO polymeric precursor solution. Furthermore, the cellulose polymers were immersed in baths containing plasticizers. By immersing CMS membrane precursors (which are saturated in water) in a solution with a plasticizer, an exchange of solvents takes place. These solvents should be released during drying and carbonization, altering the micropore structure. The separation performance of such membranes was evaluated with pure gases at 25 ºC, following a procedure described elsewhere [10].

Results and discussion

The results of the single-component gas permeation of the prepared CMS membranes with the additives added to the precursor solution are shown in Figure 1A. The addition of any additive increased the permeability of the CMS membranes to hydrogen, but the same behavior was not observed for the other gases. The addition of 1 wt.% urea produced CMS membranes with higher permeabilities than the CMS membrane without additive. Compared with the addition of propylene glycol and glycerol, it appears that the molecule with the highest molecular mass and boiling point (glycerol) produced CMS membranes with higher permeabilities than propylene glycol. This result leads us to conclude that these additives can function as functionalizing agents for CMS membranes as well as porogenic agents, as their incorporation can change the membrane gas permeabilities. Regarding the permselectivities of the CMS membranes produced (Figure 1B), it is verified that the addition of propylene glycol increased 19-fold the H₂/CH₄ selectivity relative to the CMS membrane without additive and increased 4-fold the CO₂/CH₄ permselectivity. Therefore, the concentration of propylene glycol in the polymeric precursor solution was increased and the CMS membranes produced presented permeabilities higher than the membrane with 1 wt.% of propylene glycol. However, the selectivities of the CMS membrane produced with 2 wt.% propylene glycol decreased. These results indicate that increasing the concentration of the additive acts as a porogenic agent and allows for the adjustment of the size of the ultramicropores for the desired separations.

Additionally, the prepared cellulose films (without any additive on the precursor solution) were immersed in different soften baths containing ethylene glycol, triethylene glycol, urea and xylitol. In Figure 1C are presented the results of single gas permeation of CMS membranes prepared from cellulose films with a dip-coating of 5 wt.% additives for 30 minutes. Membranes prepared with ethylene glycol presents higher permeabilities. CMS membranes prepared with the other additives have lower permeabilities to carbon dioxide and oxygen than membranes without the additive. Therefore, it turns out that the addition of different additives produces CMS membranes with different performances. The permselectivities and the respective Robeson Index of the CMS membranes prepared are listed in Table 1. This table highlights several promising results: the membranes prepared with urea presents an O₂/N₂ permselectivity of 198; while the membrane prepared with ethylene glycol has very exciting CO₂/CH₄ and CO₂/N₂ permselectivities. CMS membranes prepared with xylitol shows interesting results for carbon dioxide removal from flue gas. These plasticizers seam to act as protector of the membrane pores from collapsing during drying and carbonization. As described by other authors, glycerol acts as a membrane pore radius-maintaining agent by filling the micropores [11].

Conclusions

The two strategies proposed demonstrate to be extremely effective in controlling the structure and surface chemistry of membranes for various separations. Both techniques are simple and inexpensive to perform. Applying the different protocols described above, it is possible to tune the pores in order to make the prepared materials more permeable and/or selective for the desired separation. In the presentation of this work, an in-depth study of the characterization of the materials prepared will be carried out with the aim of understanding and optimizing the action of different additives in the formulation of the polymeric precursor of carbon membranes. Furthermore, the permeation results at high temperatures and different pressures of the prepared CMS membranes will be present.

Acknowledgments

Araújo is grateful to the Portuguese Foundation for Science and Technology (FCT) for the doctoral grant (reference SFRH/BD/143598/2019) supported by POPH/FSE. G.Bernardo thanks the Portuguese Foundation for Science and Technology (FCT) for the financial support of his work contract through the Scientific Employment Stimulus Individual Call – (CEEC_IND/02039/2018). This work was financially supported by LA/P/0045/2020 (ALiCE), UIDB/00511/2020 and UIDP/00511/2020 (LEPABE), funded by national funds through FCT/MCTES (PIDDAC).

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