Mitigation of CO2by Chemical Conversion: Plausible Chemical Reactions and Promising Products

A critical literature analysis was conducted on the viable usage of CO2 in the framework of the attempts to reduce the emissions of CO2 into the atmosphere from various processes or the rate of increase of the concentration of CO2 in the atmosphere. Applications based on the physical properties of CO2 are summarized first. Major examples are applications in supercritical extraction, enhanced oil recovery, and use as inert gas in fire extinguishing and safety application in industry. The various possibilities of use in chemical applications are systematically discussed. CO2 can react with several hydrocarbons and nitrogen-containing compounds (NH3, amines, imines). It can be used as a weak acid or as an oxidizing agent. It can be reduced electrochemically, photochemically, or chemically or by a syngas route. A variety of products can be manufactured from CO2, e.g., acids, alcohols, esters, lactones, carbamates, urethanes, urea derivatives, various copolymers, and polymers. In particular, polycarbonates are attractive products. Nevertheless, currently, less than 1% of the CO2 emitted is used in chemical reactions. To reduce the emission of CO2 substantially, only those reactions by which CO2 is used to produce bulk chemicals are relevant, whereas those useful in the manufacture of fine chemicals are not important in this respect. CO2 conversion to MeOH represents one of the most important options in CO2 mitigation. MeOH can be used as additive to fuels or as a substitute for motor fuels. An increasingly important application is in the production of MTBE (methyl tert-butyl ether), DMC (dimethyl carbonate), or DME (dimethyl ether) which are major components in modern gasoline to boost octane number. An interesting application of MeOH is the usage as fuel for cars via in situ decomposition into syngas; this results in enhancement of the energy efficiency. From MeOH several useful products can be made. Another reaction of importance that has been commercialized is the so-called dry reforming in which methane reacts with CO2, giving synthesis gas. Some processes are commercialized and the technology is mature. An example is a process in which a mixture of biomass and fossil fuels is converted into methanol and carbon which is stored. It is obvious that thermodynamically most processes using CO2 are not favorable for CO2 mitigation. This does not imply that processes are never feasible. Special situations do exist where the usage of CO2 in chemical applications is attractive. Examples are processes where CO2 is available at a high temperature, where an external heat source is present without a good outlet for the heat, processes where CO2 leads to the right stoichiometry or reaction environment. In these cases a high potential for an efficient process exists. An elegant example is "chemical cooling" of hot gases in the chemical process industry. A promising application is the direct substitution of certain chemicals by a reaction product of CO2; an example is the replacement of the hazardous phosgene by urea, which is produced from CO2, in the production of organic carbonates. A trivial case constitutes processes based on renewable energy sources; these processes, although perhaps inefficient, can lead to a reduction of CO2 emissions.