Visible-Light-Driven Production of Solar Fuels Catalyzed by Nanosized Porphyrin-Based Metal–Organic Frameworks and Covalent–Organic Frameworks: A Review

Abstract

Porphyrin-based materials are excellent chromophores because they strongly absorb visible light and their relatively low-energy lowest unoccupied molecular orbitals thermodynamically favor photoinduced electron transfer. They can generate charge-transfer excited states with and without cocatalyst(s) and ease energy transfer and ultrafast excitation energy migration. Combined with synthetic accessibility, these qualities make them ideal building blocks for porous metal− organic framework (MOF)-and covalent−organic framework (COF)-based photocatalysts to produce solar fuels. This review first describes the structures of the most common porphyrinic MOFs and COFs and their excited-state properties and semiconducting behavior as well as that of derived composites. The generally accepted mechanisms of formation of H 2 , CH 4 and derivatives, and N 2 are then reviewed, followed by the detailed examples of nano-MOFs and nano-COFs used for the said purpose: characteristic parameters such as rates of production, turnover numbers (TONs), turnover frequencies (TOFs), and apparent quantum efficiencies are described and compared. This shows that porphyrin-based MOFs and COFs are efficient solar-fuelproducing photocatalysts, with characteristics comparable to those of nonporphyrinic MOF and COF photocatalysts, although, on some occasions, the rates of production fall short of record values. Conversely, porphyrinic MOFs and COFs exhibit the greatest TONs and TOFs of any solar-fuel-producing MOFs or COFs but still face shortcomings concerning selectivity in CH 4 production because of the many possible side products. Importantly, while the best rate of photoproduction of solar fuels has been observed from nanoscale photocatalysts, there seem to be no drastic differences in the rate (within μmol −1 h −1 and mmol g −1 h −1 ) between nanoscale and microscale heterogeneous photocatalysts. This observation suggests that the more active sites are mostly located at or near the surface of the particles. Overall, nanosized porphyrin-based MOFs and COFs show rich and promising photocatalytic properties for generating solar fuels but still have room for improvement.

Topology- and wavelength-governed CO2 reduction photocatalysis in molecular catalyst-metal–organic framework assemblies

“…The latter is in line with previous reports, and suggest two competing CO 2 reduction mechanisms concomitantly occurring in Re-222. 17,18,39,46 The corresponding Re-224-based experiments showed overall lower TONs and rates than for Re-222 but similar reactivity trends favouring formate over CO production, i.e., $2 mmol CO g −1 h −1 and $9 mmol HCOO − g −1 h −1 (Fig. 3a, b and Table S3 †).…”

“…The latter is in line with previous reports, and suggest two competing CO 2 reduction mechanisms concomitantly occurring in Re-222. 17,18,39,46 The corresponding Re-224-based experiments showed overall lower TONs/rates than for Re-222 but similar reactivity trends favouring formate over CO production, i.e., ~2 µmol CO •g −1 •h −1 and ~9 µmol HCOO −•g −1 •h −1 (Figures 3a-b, Table S3). While interfacing the molecular catalyst amid the MOF significantly increases its lifetime compared to when employed in homogenous conditions, its catalytic involvement is limited as formate evolution prevails.…”

“…Luminescence and catalytic experiments together provide consistent evidence that irradiation during photocatalysis with λ > 430 nm in PCN assemblies mainly results in excitation of the Soret-band (S 0 -S 2 transition, Figures S6-S8) upon which the S 2 state is rapidly quenched by electron transfer to the Zr 6 -oxonode (Figure 6c). 17,18,39 Here, bimolecular reaction with the diffusing SED is likely to proceed subsequently to regenerate the ground state (Figure S13) or detrimental charge recombination occurs. 19 Selective CO 2 -to-formate reduction proceeds via reduced Zr(III) centres upon collecting another electron.…”

“…6c). 17,18,39 Here, bimolecular reaction with the diffusing SED is likely to proceed subsequently to regenerate the ground state (Fig. S13 †) or detrimental charge recombination occurs.…”

“…The assemblies consist of a fac-ReBr(CO) 3 (qtpy) (1, qtpy = 4,4':2',2":4,4'"-quaterpyridine) CO 2 reduction catalyst and Zn-metalated 5,10,15,20-tetrakis(4carboxylphenyl)porphyrin (2-Zn) linker-and Zr 6 -oxo node-based MOFs, i.e., PCN-222 and PCN-224 (Figures 1b-c). 15 These MOFs were chosen as model systems for their broad visible-light absorption, [16][17][18] and their reported directional exciton migration and energy funnelling to acceptors. 19,20 While 1 has not been applied in photocatalytic CO 2 reduction to date, its derivatives are known for selective CO formation under homogeneous and heterogeneous conditions, with turnover numbers (TONs) ranging from ~10 to 4500 in the presence of sacrificial electron donors (SEDs).…”