This behaviour (P_CO2 > P_H2) is referred to as 'reverse-selectivity' and is reported for different polymeric materils (see Lau CH, Li P, Li F, Chung T-S, Paul DR. Reverse-selective polymeric membranes for gas separations. Prog. Polym. Sci. 2013;38:740–766.)

The main reason, as also indicated in the case of the the anionic MOF membranes, is the stronger interaction between the quadrupolar CO2 molecules and the membrane material. Accordingly, the CO2 permeability is increased with respect to other gases.

Some polar groups (e.g., the electronegative nitrogen atom in the polar nitrile groups) within a polymer matrix preferentially interact with the electrophilic carbon atom in CO2, leading to a predominantly larger solubility. In a similar way, in the anionic MOF membranes there is a preferential interaction of CO2 with the extra-framework imidazolium cations.

Both normal and reverse size selectivities of membranes for permeation of gases have a simple thermodynamic explanation:

1) Normal size selectivity. If the diffusion occurs only in liquid phase of gas solution in membrane polymer, the permeability is proportional to an effective thermodynamic activity of the gas at its given pressure. This activity is the higher for the gases with the higher codensability, i.e. with larger and more polar molecules. See the reference in Paola's answer.

2) Reverse selectivity. This takes place if the membrane has an intrinsic porosity. In this case, permeation resembles a retention process in gas chromatography defined by gas-liquid or gas-solid partition coefficient. In this process, gases with the lower condensability have a lower retention and a higher permeability.

3) Mixed cases. Glassy polymers as membrane materials may behave both ways with different ratios depending on the gas and polymer molecular structures, hence the complicated non-monotonous permeation/size relationships discussed in the cited review of C.H. Lau et al.