Photochemical Carbon Dioxide Capture and Conversion by Metal-Organic Frameworks

Although photocatalytic CO2 reduction using several zirconium metal-organic frameworks (MOFs) has been reported, mechanistic understanding of photocatalytic CO2 reduction in zirconium MOFs with photoresponsive ligands is lacking. An often proposed generalized pathway involving ligand-to-metal charge transfer to form high energy Zr(III) intermediates is inconsistent with ligand-centered frontier orbitals. The mechanism of photochemical CO2 reduction to formate by PCN-136, a Zr-based metal−organic framework (MOF) that incorporates light-harvesting nanographene ligands, has been investigated using steady-state and time-resolved spectroscopy and density functional theory (DFT) calculations. The catalysis was found to proceed via a “photoreactive capture” mechanism, where Zr-based nodes serve to capture CO2 in the form of Zr-bicarbonates, while the nanographene ligands have a dual role of absorbing light and storing one-electron equivalents for catalysis. We also find that the process occurs via a “two-for-one” route, where a single photon initiates a cascade of electron/hydrogen atom transfers from the sacrificial donor to the CO2-bound MOF. Direct photoreduction of MOF-bound bicarbonate to formate provides further support of the proposed mechanism. The new mechanism proposed in this study elucidates a low-energy photoreactive CO2 capture pathway with an energy barrier surmountable by visible light and may henceforth provide guidance for the design of CO2 reduction MOF photocatalysts. In a follow-up study, we demonstrate a facile approach to optical gap tuning via postsynthetic modifications of pbz-MOF-1, a Zr-based MOF with polyphenylene ligands. A simple reaction of pbz-MOF-1 with FeCl3 was shown to induce three different chemical reactions: oxidative dehydrogenation, chlorination, and one-electron oxidation of the ligands. The result of these reactions was a gradual decrease in the optical gap from 2.95 eV to as little as 0.69 eV. Steady-state and time-resolved optical spectroscopy, mass spectrometry, and electron paramagnetic resonance spectroscopy, coupled with density functional theory calculations, provide insights into the chemical transformations that affect the optical properties of the MOF. In a latest study, we synthesized a new MOF using a ligand with a perchlorinated, contorted hexabenzocoronene core. Preliminary transient absorption studies on the Zr-MOF show long-lived excited states. Additionally, the light-responsive Zr-MOF has been shown to photochemically reduce CO2 to CO and formate.