537
the mixture of CO, H 2 and unreacted CO 2 is sent to the methanol reactor where
methanol is produced over a Cu/ZnO/ZrO 2 /Ga 2 O 3 catalyst.
Generally, hydrogenation of CO 2 requires the use of renewable energy for hydrogen production to ensure that the overall process results in a net reduction of carbon
dioxide when compared to conventional fossil energy–driven processes. For example, the George Olah renewable methanol plant in Iceland is the first commercialscale CO 2 hydrogenation plant to methanol with a capacity of about 4000 tons/year
of methanol [16]. The process uses industrial CO 2 streams and hydrogen which is
generated by electrolysis of water. This process is feasible only in Iceland due to the
availability of cheap renewable Icelandic geothermal power [16, 32]. Opportunities
also exist for hydrogenation of CO 2 in scale when waste hydrogen from the chloralkali industry is used [33].
Industrial-scale CO 2 incorporation into polymers has been achieved. The Asahi
kasei process [34, 35] incorporates CO 2 into bisphenol A polycarbonate product in
three steps. In the first step, CO 2 reacts with ethylene oxide to produce ethylene
carbonate. Ethylene carbonate is subsequently reacted with methanol to produce
dimethyl carbonate and ethylene glycol. In the third step, dimethyl carbonate is
reacted with phenol to produce diphenyl carbonate. In a final polymerisation step,
bisphenol A polycarbonate is produced by the reaction of bisphenol A and diphenyl
carbonate. A 50,000 ton/year polycarbonate plant based on the Asahi Kasei process
has been successfully constructed and commissioned in Taiwan since June 2002.
Another major route for the catalytic conversion of CO 2 is by the reforming of
methane with carbon dioxide to produce synthesis gas (a mixture of H 2 and CO).
Reforming processes involving CO 2 include dry reforming and tri-reforming. The
reaction of methane and carbon dioxide is called dry reforming of methane. When
steam and oxygen are added to dry reforming, it is termed tri-reforming. Methane
as a raw material is readily sourced from natural gas, bio-gas and land fill gas, and
is relatively inexpensive. Synthesis gas is an intermediate which is utilized in several downstream processes depending on its CO to H 2 ratio [36]. Catalyst stability
due to coking remains a persistent challenge for dry reforming but tri-reforming
minimizes the incidence of coking, making tri-reforming a more practical conversion process at scale [37].
Direct reduction of CO 2 with CH 4 into a variety of gaseous and liquid products
via plasma-catalysis has been reported recently [38]. However, this process is still
under development in the lab scale and energy requirement for maintaining a plasma
may be prohibitive for large-scale implementation of this process.
Among the different approaches for CO 2 conversion into products of value, catalytic conversion has the potential for implementation at scale. Compared to hydrogenation processes that rely on relatively expensive hydrogen, reforming processes
with CO 2 have better prospects for industrial-scale implementation. Generally,
reforming processes require three major steps for CO 2 conversion into high value
products: carbon dioxide capture, reforming, and synthesis gas conversion as shown
in Fig. 2.
Carbon dioxide capture has been discussed in Sect. 1.4. Captured CO 2 is mixed
with compressed methane, preheated, and sent to the CO 2 reformer where reforming
Synthesis of Novel Catalysts for Carbon Dioxide Conversion to Products of Value
the mixture of CO, H 2 and unreacted CO 2 is sent to the methanol reactor where
methanol is produced over a Cu/ZnO/ZrO 2 /Ga 2 O 3 catalyst.
Generally, hydrogenation of CO 2 requires the use of renewable energy for hydrogen production to ensure that the overall process results in a net reduction of carbon
dioxide when compared to conventional fossil energy–driven processes. For example, the George Olah renewable methanol plant in Iceland is the first commercialscale CO 2 hydrogenation plant to methanol with a capacity of about 4000 tons/year
of methanol [16]. The process uses industrial CO 2 streams and hydrogen which is
generated by electrolysis of water. This process is feasible only in Iceland due to the
availability of cheap renewable Icelandic geothermal power [16, 32]. Opportunities
also exist for hydrogenation of CO 2 in scale when waste hydrogen from the chloralkali industry is used [33].
Industrial-scale CO 2 incorporation into polymers has been achieved. The Asahi
kasei process [34, 35] incorporates CO 2 into bisphenol A polycarbonate product in
three steps. In the first step, CO 2 reacts with ethylene oxide to produce ethylene
carbonate. Ethylene carbonate is subsequently reacted with methanol to produce
dimethyl carbonate and ethylene glycol. In the third step, dimethyl carbonate is
reacted with phenol to produce diphenyl carbonate. In a final polymerisation step,
bisphenol A polycarbonate is produced by the reaction of bisphenol A and diphenyl
carbonate. A 50,000 ton/year polycarbonate plant based on the Asahi Kasei process
has been successfully constructed and commissioned in Taiwan since June 2002.
Another major route for the catalytic conversion of CO 2 is by the reforming of
methane with carbon dioxide to produce synthesis gas (a mixture of H 2 and CO).
Reforming processes involving CO 2 include dry reforming and tri-reforming. The
reaction of methane and carbon dioxide is called dry reforming of methane. When
steam and oxygen are added to dry reforming, it is termed tri-reforming. Methane
as a raw material is readily sourced from natural gas, bio-gas and land fill gas, and
is relatively inexpensive. Synthesis gas is an intermediate which is utilized in several downstream processes depending on its CO to H 2 ratio [36]. Catalyst stability
due to coking remains a persistent challenge for dry reforming but tri-reforming
minimizes the incidence of coking, making tri-reforming a more practical conversion process at scale [37].
Direct reduction of CO 2 with CH 4 into a variety of gaseous and liquid products
via plasma-catalysis has been reported recently [38]. However, this process is still
under development in the lab scale and energy requirement for maintaining a plasma
may be prohibitive for large-scale implementation of this process.
Among the different approaches for CO 2 conversion into products of value, catalytic conversion has the potential for implementation at scale. Compared to hydrogenation processes that rely on relatively expensive hydrogen, reforming processes
with CO 2 have better prospects for industrial-scale implementation. Generally,
reforming processes require three major steps for CO 2 conversion into high value
products: carbon dioxide capture, reforming, and synthesis gas conversion as shown
in Fig. 2.
Carbon dioxide capture has been discussed in Sect. 1.4. Captured CO 2 is mixed
with compressed methane, preheated, and sent to the CO 2 reformer where reforming
Synthesis of Novel Catalysts for Carbon Dioxide Conversion to Products of Value