Start Date: September 2012
T. Daniel Stack and Christopher Chidsey, Department of Chemistry, Stanford University
The goal of this research is to develop energy-efficient catalysts capable of electrocatalytic reductive coupling of carbon dioxide (CO2) to oxalate. Oxalate can be further reduced to other products that can be used for liquid fuel production or in alternative, sustainable syntheses of industrially important chemicals produced on the million-ton scale.
The production of carbon-based fuels and chemicals from renewable energy and materials is important for a sustainable energy-based society. Transforming surplus solar energy into high energy density, storable fuels can help address the intermittent and diffuse availability of sunlight. One approach is to develop energy-efficient processes that convert CO2, an abundant and renewable carbon source, into fuels or industrial chemicals.
The synthesis of long-chain carbon products and fuels from CO2 is significantly limited by the extreme challenges associated with carbon-carbon (C-C) bond formation under energy-efficient conditions. The simplest C-C bond-forming reaction with CO2 produces oxalate, an industrially important chemical that can be further transformed to other value-added chemicals, such as ethylene glycol, ethylene and more complex hydrocarbons (Figure1).1,2
The reductive coupling of CO2 is the critical step in an electrochemical pathway to oxalate. Most electrochemical reductions of CO2 non-selectively form a mixture of products through multiple proton-coupled electron transfer processes. Copper electrodes can reduce CO2 in methanol to ethylene. Developing catalysts capable of efficiently coupling CO2 to yield first oxalate and then other compounds would provide a game-changing strategy to make value-added chemicals independent of fossil fuel reserves.
The proposed research will involve a variety of laboratory and analytical approaches including: ligand synthesis; homogeneous catalyst screening; catalyst imprinting and immobilization on electrode surfaces; and mechanistic analyses, along with complementary density functional theory (DFT) calculations. The overall goal is to develop robust and efficient copper complexes capable of the electrocatalytic reduction of CO2 with minimal energy (Figure 2).
The design principles will include ligation with charge-neutral amines to assure modest CuII/I redox potentials for energy-efficient processes and a macrocyclic arrangement of the amines to assure durable copper-based electrocatalysts. The initial research will include a modular synthetic approach to a family of ligands along with screening of the resulting CuI complexes for CO2 coupling under homogeneous conditions. To inform the design of more advanced catalysts, kinetic studies will be done in conjunction with DFT-reaction profiles. Promising candidates will be synthesized with copper complexes immobilized as discrete electrocatalysts onto electroactive heterogeneous surfaces, which may ultimately be more amenable for applications at scale.
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