The Role of Water in Carbon Dioxide Adsorption in Porphyrinic Metal-Organic Frameworks

Capturing and converting CO2 in photoactive, porous materials to mimic photosynthesis is not only an appealing but also an important approach to combat increasing CO2 concentrations. Porphyrinic Zr-based metal-organic frameworks (MOFs) incorporate a photoactive dye in a porous material and are therefore promising candidates for artificial photosynthesis. The photochemical conversion of CO2 relies on the interactions between CO2, the proton source H2O, and the photoactive sites in the MOF pores. However, our understanding of these interactions and the photoreaction mechanism is still rather limited. Here, we studied the initial step of the artificial photosynthesis: CO2 sorption and activation in the presence of water. We have developed a combined vibrational and visible spectroscopic setup to simultaneously monitor the adsorption of CO2 into porphyrinic Zr-MOFs, namely PCN-222 and PCN-223, and the photophysical changes in the porphyrin linker as function of water concentration. Computational simulations further corroborate the experimentally obtained adsorption enthalpies. We found a shift of CO2 sorption site from the Zr-cluster to the center of the porphyrin macrocycle when PCN-MOFs were brought in contact with humidity greater than the water pore condensation concentration. The shift in sorption site was accompanied by a bending of the porphyrin macrocycle, which induced a change in color and, thus, a change in spectral overlap with light. Furthermore, CO2/H2O competition experiments revealed that the exchange of CO2 by H2O is pore-size-dependent. Therefore, both humidity and pore-size allow to tune the CO2 sorption site, CO2 capacity, and light harvesting in porphyrinic MOFs, which are key factors for CO2 photoreduction.