Natural photosynthetic systems utilize the sun’s energy to transform carbon dioxide and water into carbohydrates, nature’s stored solar fuel. Artificial photosynthetic assemblies need to be able to absorb sunlight, transfer absorbed energy, transport charge, oxidize water and reduce carbon dioxide to a fuel source. Metal-Organic Frameworks (MOFs) can be functionalized to provide an efficient, heterogeneous system for artificial photosynthesis.
Ruthenium based water oxidation catalysts are common in the field of homogenous catalysis. These single site catalysts can achieve high turnover frequencies but suffer from the typical drawbacks of homogeneous catalysis, primarily rapid catalyst deactivation. Zirconium-based MOFs such as the UiO series can be used to incorporate ruthenium catalysts into the MOF backbone while retaining catalytic activity. The Morris group has shown that these loaded MOFs result in higher catalytic production on a per area basis and greater stability in comparison to the homogeneous analog.
One of the most important components of an artificial photosynthetic system is the light absorbing antenna which has ability to absorb solar energy and direct it towards catalytic centers. Porphyrin-based MOFs have been investigated as a possible antenna system due to their abilities to absorb light and act as an energy cascade system when incorporated into an ordered system. MOFs allow for the ordered spacing and arrangement of porphyrins to study energy transfer properties for use as an antenna system.
The Morris group uncovered the fundamental mechanism of charge transport through MOFs, termed redox hopping, in early work. We now are interested in understanding how molecular and structural MOF properties change the efficiency of the process. Ultimately, through systematic variations of the molecular species in the MOFs, redox electrolytes, and MOF pores/channels, the Morris group is developing a set of design rules for sufficient charge transport through MOFs to facilitate artificial photosynthetic catalysis.
Although many types of CO2 reduction catalysts exist, metal cyclams have the potential to be highly selective, efficient catalysts. The Morris group recently reported a new zirconium-based MOF, VPI-100 with metallo-cyclams incorporated. These MOFs are active for CO2 reduction similar to that observed for cyclams in homogeneous solution. Additionally, CO2 can be added to strained epoxides to produce useful industrial chemicals, rather than saturating the market with carbon-based fuels.
Long-lived, charge-separated states are critical to the realization of both photovoltaic and photocatalytic assemblies. The Morris group recently demonstrated the ability to control the lifetime of photo-induced charge separated states by employing a charge-transfer-induced, spin-transition (high-spin to low-spin transition) in a class of manganese poly(pyrazolyl)borates. Our current research effort is motivated by the need to understand the role of molecular reorganization and metallic spin state in controlling charge-separated-state lifetimes and catalytic selectivity/efficiency.
Zirconium (and Hafnium) based metal-organic frameworks (MOFs) have shown immense promise in the sorption and solution-phase degradation of chemical warfare agents (CWAs). Their activity has been attributed to open coordination sites on the MOF nodes and the porous structure of MOFs which enables rapid diffusion to these sites. That said, recent studies show that only 30% of the MOF’s surface area is utilized during sorption and reactivity. Additionally, the products of CWA hydrolysis poison the catalysts during gas-phase reactivity. The Morris group aims to address these challenges through synthetic manipulation of MOF structure and active-site engineering. Thw work is highly collaborative with joint efforts in the John Morris group (VT, Chemistry), Ayman Karim group (VT, Chemical Engineering), Diego Troya group (VT, Chemistry), Yue Wu group (UNC, Physics), Karl Johnson group (Pitt, Chemical Engineering), and the Chemical and Biological Center in Edgewood, MD.