Membrane-based separations are considered an energy-efficient alternative to absorption and distillation for carbon capture from industrial flue gases and natural gas sweetening. The performance of a membrane in a gas separation process depends critically on two properties: permeability, which influences the productivity of the process, and perm-selectivity, which determines process efficiency and product purity. An ideal membrane would combine high permeability with high perm-selectivity. However, these two properties typically compete: polymers with higher permeability often exhibit lower perm-selectivity, and vice versa. This trade-off is represented graphically for a specific gas pair, with the perm-selectivity of available polymer materials plotted against their permeability, forming a line known as the Robeson upper bound. This line was originally proposed as a way to generalize the trends in the observed experimental data. However, its existence was later justified based on theoretical considerations. The Robeson bond defines the practical performance limit of membranes for various gas pairs in separation processes.