2
1 Overview of Direct Methane Conversion to Chemicals …
Although fossil fuel reserves are dwindling, the use of fossil resources such as
natural gas to obtain primary energy and chemical products cannot be replaced until
new technologies such as solar light and artificial photosynthesis have been developed
and implemented. Thus, to maintain a comfortable lifestyle, human beings will have
to continue to consume finite oil, coal, and natural gas resources for the foreseeable
future. The use of the abundant and inexpensive gas methane as a raw material for
energy and chemical products has been proposed as a means of transitioning away
from the current dependence on crude oil, which is localized in a limited number
of geographic areas. Thus, methane is important in terms of establishing energy
security, continuing economic development, and maintaining a comfortable lifestyle
for humans.
However, industrial processes for the direct conversion of methane to chemicals
have not yet been developed. For scientists and engineers, the establishment of nextgeneration processes for direct methane conversion represents a grand challenge in
chemistry, independent of trends in the larger world such as the discovery of shale gas
and the drive to reduce carbon dioxide emissions. The authors are of the view that the
development of direct methane conversion is crucial, and is highly preferable to the
use of CO 2 , the most thermodynamically stable chemical, as a chemical feedstock, as
CO 2 conversion inherently requires high energy consumption. Therefore, we herein
focus on the conversion of methane to value-added chemicals, especially on the direct
conversion of methane to methanol and higher hydrocarbons (such as propylene)
bypassing the stage of synthesis gas (CO and H 2 ) production.
1.2 Industrial Methane Conversion for the Production
of Chemicals
Although natural gas can be converted into various chemicals such as methanol
(Fig. 1.1), the industrial processes of methane conversion to chemicals are limited
(Fig. 1.2) and classified into two categories. The first category is the indirect conversion via synthesis gas (mixture of CO and H 2 ), the production of which accounts for
~96% of total methane consumption for the production of chemicals (right side of
Fig. 1.2), and the second one is direct conversion to produce chemicals (left side of
Fig. 1.2). Herein, the term “direct methane conversion” refers to processes that do
not involve the production of synthesis gas as an intermediate. In other words, “direct
methane conversion” implies that reforming agents, i.e., H 2 O and CO 2 , are not added.
In indirect methane conversion for chemical production, methane is first converted
to synthesis gas. Technologies for the production of synthesis gas are well established
and in current industrial use [7]. Synthesis gas, produced by the steam reforming and
partial oxidation of methane, is used for the production of methanol/hydrocarbons
via Fischer–Tropsch synthesis [8] and NH 3 . In direct methane conversion, ~70%
of methane is used for ammonia synthesis as a hydrogen source, ~20% is used for
methanol synthesis, and ~10% is used for Fischer–Tropsch synthesis.
1 Overview of Direct Methane Conversion to Chemicals …
Although fossil fuel reserves are dwindling, the use of fossil resources such as
natural gas to obtain primary energy and chemical products cannot be replaced until
new technologies such as solar light and artificial photosynthesis have been developed
and implemented. Thus, to maintain a comfortable lifestyle, human beings will have
to continue to consume finite oil, coal, and natural gas resources for the foreseeable
future. The use of the abundant and inexpensive gas methane as a raw material for
energy and chemical products has been proposed as a means of transitioning away
from the current dependence on crude oil, which is localized in a limited number
of geographic areas. Thus, methane is important in terms of establishing energy
security, continuing economic development, and maintaining a comfortable lifestyle
for humans.
However, industrial processes for the direct conversion of methane to chemicals
have not yet been developed. For scientists and engineers, the establishment of nextgeneration processes for direct methane conversion represents a grand challenge in
chemistry, independent of trends in the larger world such as the discovery of shale gas
and the drive to reduce carbon dioxide emissions. The authors are of the view that the
development of direct methane conversion is crucial, and is highly preferable to the
use of CO 2 , the most thermodynamically stable chemical, as a chemical feedstock, as
CO 2 conversion inherently requires high energy consumption. Therefore, we herein
focus on the conversion of methane to value-added chemicals, especially on the direct
conversion of methane to methanol and higher hydrocarbons (such as propylene)
bypassing the stage of synthesis gas (CO and H 2 ) production.
1.2 Industrial Methane Conversion for the Production
of Chemicals
Although natural gas can be converted into various chemicals such as methanol
(Fig. 1.1), the industrial processes of methane conversion to chemicals are limited
(Fig. 1.2) and classified into two categories. The first category is the indirect conversion via synthesis gas (mixture of CO and H 2 ), the production of which accounts for
~96% of total methane consumption for the production of chemicals (right side of
Fig. 1.2), and the second one is direct conversion to produce chemicals (left side of
Fig. 1.2). Herein, the term “direct methane conversion” refers to processes that do
not involve the production of synthesis gas as an intermediate. In other words, “direct
methane conversion” implies that reforming agents, i.e., H 2 O and CO 2 , are not added.
In indirect methane conversion for chemical production, methane is first converted
to synthesis gas. Technologies for the production of synthesis gas are well established
and in current industrial use [7]. Synthesis gas, produced by the steam reforming and
partial oxidation of methane, is used for the production of methanol/hydrocarbons
via Fischer–Tropsch synthesis [8] and NH 3 . In direct methane conversion, ~70%
of methane is used for ammonia synthesis as a hydrogen source, ~20% is used for
methanol synthesis, and ~10% is used for Fischer–Tropsch synthesis.