589
4.6 Challenges Associated with Carbon Dioxide Reduction
The generation of charge carriers limits CO 2 photoreduction process. It is a reversible process and is accompanied by bulk or surface recombination of electrons and
holes to generate heat energy, thereby decreasing the efficiency of the conversion
process. Even if the reduction process is feasible, the conversion of CO 2 to CO 2
•─
requires higher energy due to its more reduction potential and therefore becomes the
rate-controlling step in most of the proposed pathways for CO 2 reduction. Controlling
the selectivity of the final product is an essential factor due to several possible products, and re-oxidation of the formed products is also a crucial parameter. The quantum efficiency depends on irradiations and reactor design. Designing a reactor to
uniformly distribute the catalyst and illuminate the larger area for high activity also
plays a crucial role. Separation of formed products is also a critical factor that would
govern the scale-up of the process.
5 DME: A Valuable Methanol Derivative
Synthetic fuels from non-conventional and renewable resources are going to replace
the lion-share petroleum-derived fuels in the transportation sector. Steady energy
supply from various energy sources and shifts to renewable energies are required to
sustain the activities of mankind. Along with bioethanol and biodiesel, dimethyl
ether (an important derivative of methanol) is also gaining importance as an alternative synthetic fuel. DME can be used as a cleaner highly efficient fuel with reduced
SOx, NOx, and particulate matter. It does not possess any large toxicity issues and
can be proficiently reformed to H 2 at lower temperatures. DME can be produced
from natural gas, coal, biomass, and also from heavy oil. Currently, almost all DME
is produced through dehydration of methanol with well-established infrastructure
for production.
There are two methods under development: the two-step process (indirect
method) of adding a dehydration step to the latter part on industrial methanol processing and the one-step (direct method) of synthesizing DME directly from syngas
(mixture of H 2 and CO). Methanol dehydration reaction occurs over solid acid catalysts such as γ-alumina, zeolites, and aluminum silicates. For direct synthesis of
DME from syngas, catalysts with two active centers, one for methanol synthesis and
one for methanol dehydration, are required. The direct method of DME synthesis
from syngas is considered to be more feasible as compared to the indirect process.
The main reasons are as follows:
• Thermodynamic thresholds associated with CO conversion to methanol could be
surmounted by the synergistic effect between methanol synthesis and its dehydration catalyst components.
Perspectives in Carbon Oxides Conversion to Methanol/Dimethyl Ether: Distinctive…
4.6 Challenges Associated with Carbon Dioxide Reduction
The generation of charge carriers limits CO 2 photoreduction process. It is a reversible process and is accompanied by bulk or surface recombination of electrons and
holes to generate heat energy, thereby decreasing the efficiency of the conversion
process. Even if the reduction process is feasible, the conversion of CO 2 to CO 2
•─
requires higher energy due to its more reduction potential and therefore becomes the
rate-controlling step in most of the proposed pathways for CO 2 reduction. Controlling
the selectivity of the final product is an essential factor due to several possible products, and re-oxidation of the formed products is also a crucial parameter. The quantum efficiency depends on irradiations and reactor design. Designing a reactor to
uniformly distribute the catalyst and illuminate the larger area for high activity also
plays a crucial role. Separation of formed products is also a critical factor that would
govern the scale-up of the process.
5 DME: A Valuable Methanol Derivative
Synthetic fuels from non-conventional and renewable resources are going to replace
the lion-share petroleum-derived fuels in the transportation sector. Steady energy
supply from various energy sources and shifts to renewable energies are required to
sustain the activities of mankind. Along with bioethanol and biodiesel, dimethyl
ether (an important derivative of methanol) is also gaining importance as an alternative synthetic fuel. DME can be used as a cleaner highly efficient fuel with reduced
SOx, NOx, and particulate matter. It does not possess any large toxicity issues and
can be proficiently reformed to H 2 at lower temperatures. DME can be produced
from natural gas, coal, biomass, and also from heavy oil. Currently, almost all DME
is produced through dehydration of methanol with well-established infrastructure
for production.
There are two methods under development: the two-step process (indirect
method) of adding a dehydration step to the latter part on industrial methanol processing and the one-step (direct method) of synthesizing DME directly from syngas
(mixture of H 2 and CO). Methanol dehydration reaction occurs over solid acid catalysts such as γ-alumina, zeolites, and aluminum silicates. For direct synthesis of
DME from syngas, catalysts with two active centers, one for methanol synthesis and
one for methanol dehydration, are required. The direct method of DME synthesis
from syngas is considered to be more feasible as compared to the indirect process.
The main reasons are as follows:
• Thermodynamic thresholds associated with CO conversion to methanol could be
surmounted by the synergistic effect between methanol synthesis and its dehydration catalyst components.
Perspectives in Carbon Oxides Conversion to Methanol/Dimethyl Ether: Distinctive…