605
2.1 Chemical Absorption
The CO 2 is separated from the flue gas by passing through a continuous scrubbing
system that contains an absorber and a desorber. The absorption processes utilize
the reversible chemical reaction of CO 2 with an aqueous alkaline solvent, usually an
amine [26]. In the desorber, the absorbed CO 2 is stripped, and a pure stream of CO 2
is sent for compression. The technique is suitable for post-combustion CO 2 capture,
and this process was initially established in 1930 to capture the CO 2 from ammonia
plant. They mostly used ammonia, aqueous amines, and alkali for CO 2 capture from
flue gases [30].
The CO 2 absorption mostly occurs in an aqueous medium [26, 31]. Among the
alkali, monoethanolamine (MEA) was mainly adopted for the removal of CO 2 from
natural gas and hydrogen. In view of amine, the piperazine in aqueous solution
exhibits better performance in the CO 2 capture. Moreover, the amount of CO 2
absorbed on piperazine is more than twice compared to MEA [32, 33]. There are
three amine-based technologies (such as Fluor Econamine FG Plus
SM
(EFG
+
) technology, KM-CDR, and the Kerr-McGee/AGG Lummus Crest (KMALC) technology) for the absorption of CO 2 , under moderate working conditions. All these
amines are efficient at ambient pressure for CO 2 capture [31, 34, 35]. In addition to
organic amines, the aqueous ammonia can also be employed for chemisorption of
CO 2 from flue gas and natural gas streams. This process relies upon a temperature
swing to cycle between ammonium carbonate and ammonium bicarbonate. The
major drawbacks include their low CO 2 adsorption capacity and the associated high
cost as major challenges to commercialize. The drawbacks may be possible to overcome using the amine-grafted and amine-impregnated materials by enriching the
amine or nitrogen groups [34, 35].
2.2 Chemical Adsorbents: Amine-Impregnated
and Amine-Grafted Material
To enhance the adsorption capacity, the amines are generally loaded on solid material. Among the various amines tested, polyethyleneimine (PEI)-impregnated mesoporous silica, MCM-41, etc. are showing well adsorption capacity. A higher PEI
loading significantly enhanced CO 2 adsorption capacity together with the decreases
of surface area, pore size, and pore volume of the PEI-impregnated supports. The
highest CO 2 adsorption capacity of 3.02 mmol/g was achieved for the PEIimpregnated MCM-41 (PEI/MCM-41) with 75 wt% of PEI loading [36].
Although high CO 2 adsorption capacity was observed in amine-impregnated
materials, the lack of thermal stability in desorption needs to be overcome. The
amine-impregnated materials are further improved as amine-grafted solids in which
the amine is covalently impregnated to the intra-channel surface of mesoporous
silica. There are various amine-functionalized silica materials used for the
Efficient Homogeneous Catalysts for Conversion of CO 2 to Fine Chemicals
2.1 Chemical Absorption
The CO 2 is separated from the flue gas by passing through a continuous scrubbing
system that contains an absorber and a desorber. The absorption processes utilize
the reversible chemical reaction of CO 2 with an aqueous alkaline solvent, usually an
amine [26]. In the desorber, the absorbed CO 2 is stripped, and a pure stream of CO 2
is sent for compression. The technique is suitable for post-combustion CO 2 capture,
and this process was initially established in 1930 to capture the CO 2 from ammonia
plant. They mostly used ammonia, aqueous amines, and alkali for CO 2 capture from
flue gases [30].
The CO 2 absorption mostly occurs in an aqueous medium [26, 31]. Among the
alkali, monoethanolamine (MEA) was mainly adopted for the removal of CO 2 from
natural gas and hydrogen. In view of amine, the piperazine in aqueous solution
exhibits better performance in the CO 2 capture. Moreover, the amount of CO 2
absorbed on piperazine is more than twice compared to MEA [32, 33]. There are
three amine-based technologies (such as Fluor Econamine FG Plus
SM
(EFG
+
) technology, KM-CDR, and the Kerr-McGee/AGG Lummus Crest (KMALC) technology) for the absorption of CO 2 , under moderate working conditions. All these
amines are efficient at ambient pressure for CO 2 capture [31, 34, 35]. In addition to
organic amines, the aqueous ammonia can also be employed for chemisorption of
CO 2 from flue gas and natural gas streams. This process relies upon a temperature
swing to cycle between ammonium carbonate and ammonium bicarbonate. The
major drawbacks include their low CO 2 adsorption capacity and the associated high
cost as major challenges to commercialize. The drawbacks may be possible to overcome using the amine-grafted and amine-impregnated materials by enriching the
amine or nitrogen groups [34, 35].
2.2 Chemical Adsorbents: Amine-Impregnated
and Amine-Grafted Material
To enhance the adsorption capacity, the amines are generally loaded on solid material. Among the various amines tested, polyethyleneimine (PEI)-impregnated mesoporous silica, MCM-41, etc. are showing well adsorption capacity. A higher PEI
loading significantly enhanced CO 2 adsorption capacity together with the decreases
of surface area, pore size, and pore volume of the PEI-impregnated supports. The
highest CO 2 adsorption capacity of 3.02 mmol/g was achieved for the PEIimpregnated MCM-41 (PEI/MCM-41) with 75 wt% of PEI loading [36].
Although high CO 2 adsorption capacity was observed in amine-impregnated
materials, the lack of thermal stability in desorption needs to be overcome. The
amine-impregnated materials are further improved as amine-grafted solids in which
the amine is covalently impregnated to the intra-channel surface of mesoporous
silica. There are various amine-functionalized silica materials used for the
Efficient Homogeneous Catalysts for Conversion of CO 2 to Fine Chemicals