Additionally, both isotherms show a non-closing hysteresis, which is known for non-rigid polymers and can be attributed to pore blocking or swelling effects as well as capillary condensation. The pore volume of P-Ru(PPh3)3 in nitrogen physisorption is almost double that of P-PPh3. This difference may be due to the increased incorporation of flexible phosphine units in P-PPh3, diminishing the rigidity required for the formation of a porous structure. This relation is consistent with the findings of Dawson et al. that the porosity of an HCP decreased as the proportion of heteroatom building units in the benzene-based framework increased. The catalysts were examined in the base-mediated hydrogenation of CO2 to formate in the aqueous phase (Scheme 2) and referenced to their homogeneous and heterogeneous analogues (Fig. ). The catalytic results highlight comparable productivities for P-Ru(PPh3)3 and P-PPh3-Ru, accompanied by a continuous decrease in catalytic activity over the course of the reaction. Since the reaction rate is not limited by thermodynamic constraints, the flattening of the kinetic curve is presumably due to deactivation of the catalytically active species. Regarding reference catalysts, the commercial Ru@C catalyst is inactive, while no significant amounts of formic acid are detected, when the Ru-free polyphosphine P-PPh3 is applied. BCMBP-Ru, consisting of a Ru-impregnated framework of internally crosslinked biphenyl units with no phosphine centers present, shows only low activity. Hence, it appears that both, the Ru species and an available phosphine ligand, contribute crucially to the formation of the catalytically active species. Accordingly, the molecular precursor Ru(NO)(NO3)3 slightly catalyzes the hydrogenation of CO2, while the addition of the water-soluble derivative of PPh3 (TPPTS) forms an active homogeneous catalyst. This in situ formed, dissolved Ru catalyst exhibits a comparable activity to the solid P-Ru(PPh3)3 and P-PPh3-Ru, although its deactivation is retarded. Overall, the reference catalysts demonstrate the excellent activity of our heterogenized catalysts and strongly suggest the involvement of ruthenium and phosphine in their catalytically active species.