Global challenges and strategies for control, conversion and Utilization of CO_2 for sustainable development involving energy, catalysis, adsorption and chemical processing

摘要

Utilization of carbon dioxide (CO_2) has become an important global issue due to the significant and continuous rise in atmospheric CO_2 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 CO_2 utilization include: (1) costs of CO_2 capture, separation, purification, and transportation to user site; (2) energy requirements of CO_2 chemical conversion (plus source and cost of co-reactants); (3) market size limitations, little investment-incentives and lack of industrial commitments for enhancing CO_2-based chemicals; and (4) the lack of socio-economical driving forces. The strategic objectives may include: (1) use CO_2 for environmentally-benign physical and chemical processing that adds value to the process; (2) use CO_2 to produce industrially useful chemicals and materials that adds value to the products; (3) use CO_2 as a beneficial fluid for processing or as a medium for energy recovery and emission reduction; and (4) use CO_2 recycling involving renewable sources of energy to conserve carbon resources for sustainable development. The approaches for enhancing CO_2 utilization may include one or more of the following: (1) for applications that do not require pure CO_2, develop effective processes for using the CO_2-concentrated flue gas from industrial plants or CO_2-rich resources without CO_2 separation; (2) for applications that need pure CO_2, develop more efficient and less-energy intensive processes for separation of CO_2 selectively without the negative impacts of co-existing gases such as H_2O, O_2, and N_2; (3) replace a hazardous or less-effective substance in existing processes with CO_2 as an alternate medium or solvent or co-reactant or a combination of them; (4) make use of CO_2 based on the unique physical properties as supercritical fluid or as either solvent or anti-solvent; (5) use CO_2 based on the unique chemical properties for CO_2 to be incorporated with high 'atom efficiency' such as carboxylation and carbonate synthesis; (6) produce useful chemicals and materials using CO_2 as a reactant or feedstock; (7) use CO_2 for energy recovery while reducing its emissions to the atmosphere by sequestration; (8) recycle CO_2 as C-source for chemicals and fuels using renewable sources of energy; and (9) convert CO_2 under either bio-chemical or geologic-formation conditions into "new fossil" energies. Several cases are discussed in more detail. The first example is til-reforming of methane versus the well-known CO_2 reforming over transition metal catalysts such as supported Ni catalysts. Using CO_2 along with H_2O and O_2 in flue gases of power plants without separation, tri-reforming is a synergetic combination of CO_2 reforming, steam reforming and partial oxidation and it can eliminate carbon deposition problem and produces syngas with desired H_2/CO ratios for industrial applications. The second example is a CO_2 "molecular basket" as CO_2-selective high-capacity adsorbent which was developed using mesoporous molecular sieve MCM-41 and polyethylenimine (PEI).