Carbon dioxide is abundant, non-toxic, inexpensive, and a renewable source of carbon. This makes CO2 the most coveted compound by Green Chemistry enthusiasts. Industries are always on the lookout for ways that will enable effective use of CO2 to act as synthetic building blocks for producing fuels like Methane, Di-methyl-ether and Mthanol and fine-chemicals. Furthermore, CO2 conversion could also help reduce atmospheric CO2 levels, popularly known as “Green House Gas”, and thus, protect climate.
Nature has been highly successful in using CO2 as synthetic building blocks in photosynthesis. For decades, scientists have been trying to understand this phenomenon at a molecular level. Such studies have proved useful in developing biomimetic catalysts for CO2 conversion. Chlorophyll (Porphyrin molecules) in green plants helps convert incident sunlight and atmospheric CO2 into sugars (energy). So, this makes them promising target for testing activation catalysts for CO2 adsorption. Effective CO2 adsorption using man-made catalysts is indeed our end-goal. Lot of research is being conducted in this area to further the economic viability of the processes that utilize CO2. Several companies are pursuing the idea / concept of thermochemical and electrochemical conversion of CO2 into chemical feedstock or polymers. Research and development are currently focused on increasing the catalyst life and bringing down the temperature of conversion.
Future research must emphasize on the rational design of highly active catalyst, in order to satisfy the economic development of CO2 conversion. But, the development of such efficient catalysts requires a complete understanding of CO2 and CO2-catalyst interactions.
In order to develop such a catalyst, following points should be considered,
- CO2 has strong affinity towards nucleophiles and electron-donating reagents, due to the electron deficiency of its carbonyl-carbon, If the designed catalyst has nitrogen or base-functionalty (basic), it will have an increased affinity towards CO2 (eg, Porphyrin, Grignard reagents). Such catalysts lead to the synthesis of acid-derivatives.
- With low-valent metals and alkene, CO2 undergoes “oxidative cycloaddition”
- New CO2 soluble catalysts may increase efficiency.
- Homogeneous catalysis in compressed CO2, may increase selectivity.
- Catalyst in supercritical CO2 may also increase stability. It is essential to make use of CO2, based on the unique physical properties as that of the supercritical fluid, either as a solvent, or as an anti-solvent, or reactant, or a combination of all.
- Photoelectrochemistry, the study of using solar energy to split CO2, is an emerging method for clean production of chemicals. It is also important to develop catalysts (Semiconducting materials) for the electrochemical conversion of CO2.
- Use of high-energy starting materials may ease the catalyst role.
- The catalyst will be more efficient if it has both CO2 adsorption and activation functionality. Eg, designer MOF that contains Lewis-base sitewill donate electron to CO2, in contrast to the Lewis-acids sites in traditional MOFs for adsorbing CO2.
- Computational tool such as Density Function Theory (DFT) may help improve the catalytic activity, or, find a new catalyst.
- Chemical reactions can also benefit from using CO2 as a mild oxidant, or as a selective source of O2 atoms because dissociation of CO2 on a catalyst-surface could produce active O2 species.
The trend towards conversion of CO2 to useful chemicals and fuels will probably intensify in the near future. This could in turn lead to effective management to tackle climate change and energy crisis.
As George Whitesides emphasizes, managing CO2 and conversion into useful chemicals and energy will be the reinvention of chemistry, and it is also chemistry/ molecular solution to the important problem facing the society. He says
“Some of the most interesting problems in science, and many of the most important facing society, need chemistry for their solution. Examples include: understanding life as a network of chemical reactions; interpreting the molecular basis of disease; global stewardship; the production, storage, and conservation of energy and water; and the management of carbon dioxide."
Issues pertaining to CO2 is truly global, and a major opportunity for the development of sustainable energy options and environmental preservation. So, use of CO2 for the synthesis of useful chemicals & fuels will mark a new field in chemistry. It is important to establish university-industry-collaboration to search for new-reactions & new-catalysts in this field.
To Readers: If you have experience with CO2 conversion reaction or specific catalyst, please feel free to comment.