Utilization of carbon dioxide (CO2) has become an important global issue due to the significant and continuous rise in atmospheric CO2 concentrations, accelerated growth in the consumption of carbon-based energy worldwide, depletion of carbon-based energy resources, and low efficiency in current energy systems. The barriers for CO2 utilization include: (1) costs of CO2 capture, separation, purification, and transportation to user site; (2) energy requirements of CO2 chemical conversion (plus source and cost of co-reactants); (3) market size limitations, little investment-incentives and lack of industrial commitments for enhancing CO2-based chemicals; and (4) the lack of socio-economical driving forces. The strategic objectives may include: (1) use CO2 for environmentally-benign physical and chemical processing that adds value to the process; (2) use CO2 to produce industrially useful chemicals and materials that adds value to the products; (3) use CO2 as a beneficial fluid for processing or as a medium for energy recovery and emission reduction; and (4) use CO2 recycling involving renewable sources of energy to conserve carbon resources for sustainable development. The approaches for enhancing CO2 utilization may include one or more of the following: (1) for applications that do not require pure CO2, develop effective processes for using the CO2-concentrated flue gas from industrial plants or CO2-rich resources without CO2 separation; (2) for applications that need pure CO2, develop more efficient and less-energy intensive processes for separation of CO2 selectively without the negative impacts of co-existing gases such as H2O, O2, and N2; (3) replace a hazardous or less-effective substance in existing processes with CO2 as an alternate medium or solvent or co-reactant or a combination of them; (4) make use of CO2 based on the unique physical properties as supercritical fluid or as either solvent or anti-solvent; (5) use CO2 based on the unique chemical properties for CO2 to be incorporated with high 'atom efficiency' such as carboxylation and carbonate synthesis; (6) produce useful chemicals and materials using CO2 as a reactant or feedstock; (7) use CO2 for energy recovery while reducing its emissions to the atmosphere by sequestration; (8) recycle CO2 as C-source for chemicals and fuels using renewable sources of energy; and (9) convert CO2 under either bio-chemical or geologic-formation conditions into "new fossil" energies. Several cases are discussed in more detail. The first example is tri-reforming of methane versus the well-known CO2 reforming over transition metal catalysts such as supported Ni catalysts. Using CO2 along with H2O and O2 in flue gases of power plants without separation, tri-reforming is a synergetic combination of CO2 reforming, steam reforming and partial oxidation and it can eliminate carbon deposition problem and produces syngas with desired H2/CO ratios for industrial applications. The second example is a CO2 "molecular basket" as CO2-selective high-capacity adsorbent which was developed using mesoporous molecular sieve MCM-41 and polyethylenimine (PEI). The MCM41-PEI adsorbent has higher adsorption capacity than either PEI or MCM-41 alone and can be used as highly CO2-selective adsorbent for gas mixtures without the pre-removal of moisture because it even enhances CO2 adsorption capacity. The third example is synthesis of dimethyl carbonate using CO2 and methanol, which demonstrates the environmental benefit of avoiding toxic phosgene and a processing advantage. The fourth example is the application of supercritical CO2 for extraction and for chemical processing where CO2 is either a solvent or a co-reactant, or both. The CO2 utilization contributes to enhancing sustainability, since various chemicals, materials, and fuels can be synthesized using CO2, which should be a sustainable way in the long term when renewable sources of energy are used as energy input.
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