inorganic carbonates [M(M′ 2 )CO 3 ], carbamates (the ammonium salt HRN–CO 2
−
+
H 3 RN, or the HRN–CO 2 R′ species, both derivatives of the labile carbamic acid
HRN–CO 2 H), organic carbonates [(RO) 2 C=O, putative esters of carbonic acid,
(HO) 2 C=O], hydrogencarbonates (MHCO 3 ). In this class of products fall also
polycarbonates obtained by co-polymerization of CO 2 with epoxides.
B. Carboxylation of organic substrates
In the carboxylation of organic substrates, a C–C bond is formed and the oxidation
state of C goes down to +3 from +4. Examples are (i) reactions in which CO 2 reacts
with energy-rich molecules such as olefins, alkynes, dienes (conjugated and
cumulated) to afford specialty chemicals such as lactones, pyrones, esters, among
others; (ii) insertion of CO 2 into C–H bonds with formation of acids (Eq. 9.1).
RÀH þ CO 2 ! RCOOH
ð9:1Þ
The former reactions occur under quite mild conditions and can be exploited in
an energy system based on fossil-C, the latter (Eq. 9.1) are quite common in Nature
and are of great interest for Industry as their implementation would reduce the
C-footprint and environmental impact of chemicals such as organic acids, as will be
discussed in next paragraphs.
C. Reduction to energy products
This class of reactions includes the conversion into chemicals in which the C-atom
has a lower oxidation state than +3: from +2 (e.g., CO) to −4 (e.g., CH 4 ). The
energy content of the resulting chemical (and, thus, the energy to be delivered to
CO 2 for its conversion) depends on a series of factors, among which the oxidation
state of the C-atom in end species, the number of C–H bonds formed, the number of
C–C bond formed.
Table 9.3 shows the thermodynamic properties of some molecules derived from
CO 2 upon hydrogenation. Data show that there is not a linear relationship between
the change of the oxidation state with respect to CO 2 (Column 3) and the change of
Free Gibbs energy (Column 7) moving from CO 2 to CH 4 . A peculiar situation is
represented by methanol and ethene in which the C-atom presents an oxidation state
equal to-2, but the two compounds present different energy content per C-atom due
to the different environments. Let us now consider some chemical uses of CO 2 .
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9 Circular Economy and Carbon Dioxide Conversion
−
+
H 3 RN, or the HRN–CO 2 R′ species, both derivatives of the labile carbamic acid
HRN–CO 2 H), organic carbonates [(RO) 2 C=O, putative esters of carbonic acid,
(HO) 2 C=O], hydrogencarbonates (MHCO 3 ). In this class of products fall also
polycarbonates obtained by co-polymerization of CO 2 with epoxides.
B. Carboxylation of organic substrates
In the carboxylation of organic substrates, a C–C bond is formed and the oxidation
state of C goes down to +3 from +4. Examples are (i) reactions in which CO 2 reacts
with energy-rich molecules such as olefins, alkynes, dienes (conjugated and
cumulated) to afford specialty chemicals such as lactones, pyrones, esters, among
others; (ii) insertion of CO 2 into C–H bonds with formation of acids (Eq. 9.1).
RÀH þ CO 2 ! RCOOH
ð9:1Þ
The former reactions occur under quite mild conditions and can be exploited in
an energy system based on fossil-C, the latter (Eq. 9.1) are quite common in Nature
and are of great interest for Industry as their implementation would reduce the
C-footprint and environmental impact of chemicals such as organic acids, as will be
discussed in next paragraphs.
C. Reduction to energy products
This class of reactions includes the conversion into chemicals in which the C-atom
has a lower oxidation state than +3: from +2 (e.g., CO) to −4 (e.g., CH 4 ). The
energy content of the resulting chemical (and, thus, the energy to be delivered to
CO 2 for its conversion) depends on a series of factors, among which the oxidation
state of the C-atom in end species, the number of C–H bonds formed, the number of
C–C bond formed.
Table 9.3 shows the thermodynamic properties of some molecules derived from
CO 2 upon hydrogenation. Data show that there is not a linear relationship between
the change of the oxidation state with respect to CO 2 (Column 3) and the change of
Free Gibbs energy (Column 7) moving from CO 2 to CH 4 . A peculiar situation is
represented by methanol and ethene in which the C-atom presents an oxidation state
equal to-2, but the two compounds present different energy content per C-atom due
to the different environments. Let us now consider some chemical uses of CO 2 .
146
9 Circular Economy and Carbon Dioxide Conversion