Pebax/Modified Cellulose Nanofiber Composite Membranes for Highly Enhanced CO₂/CH₄ Separation

This work explored the use of biomass-derived cellulose nanofibers as an additive to enhance the separation performance of Pebax membranes for the removal of CO₂ from biogas. Succinate functional groups were modified on the cellulose nanofiber (SCNF) to incorporate more CO₂-attracting functional groups before they were added to the polymer matrix. A small addition of SCNF up to 0.5 wt % had no significant impact on the polymer chain packing of Pebax but significantly enhanced the tensile strength and separation performance in both CO₂ permeability and CO₂/CH₄ selectivity. On the other hand, increasing the SCNF addition amount above 1 wt % resulted in a slight alternation of membrane microstructure, i.e., lowering crystallinity, stiffer structure, and reduced tensile strength. At high loading, the CO₂ permeability and CO₂/CH₄ selectivity of the composite membrane were, however, found to decline. This behavior is explained by a greater propensity for interaction among the CO₂-attracting functional groups of SCNF and Pebax at elevated SCNF loadings, leading to fewer functional groups available for CO₂ sorption. The optimal 0.5% SCNF loading (Pebax/SCNF-0.5) demonstrated a CO₂ permeability of 263.8 Barrer and selectivity of 19.9 under 4 bar pressure and an operating temperature of 30 °C. These separation performances increased by 29.69% permeability and 39.04% selectivity compared with those of pure Pebax. These highly impressive results corresponded to the increases in the levels of CO₂ dissolution and diffusion via hydrophilic SCNF nanofillers in Pebax. This work could strongly advance the research and development of gas separation technology based on polymeric membranes with the utilization of biobased nanofillers for energy and environmental sectors.