Methanol Synthesis
(from Xin-Mei Liu, G. Q. Lu, Zi-Feng Yan, and Jorge Beltramini, Ind. Eng. Chem. Res.
2003, 42, 6518-6530)
With the rapid changes and development in modern industry, the energy and
environmental issues have become two major concerns. Methanol has been a common
chemical feedstock for several important chemicals such as acetic acid, methyl tert-butyl
ether (MTBE), formaldehyde, and chloromethane for over 30 years. Methanol, being a clean
liquid fuel, could provide convenient storage of energy for fuel cell applications, particularly
in transportation and mobile devices. When used as a fuel, it is a cleaner energy compared
with most other sources. For these reasons, methanol synthesis is still attracting interest
despite the fact that the current technology process was developed a long time ago.
Commercially, methanol has been produced from syngas using natural gas or coal, mainly
containing CO and H2 along with a small amount of CO2.2 Methanol synthesis through
hydrogenation of carbon dioxide has attracted continuous worldwide research interest in the
past 15 years because of its environmental impact. The potential use of CO2, the most
important greenhouse gas, as an alternative feedstock replacing CO in the methanol
production has received attention as an effective way of CO2 utilization. A recent study
showed that a mixture with a proper proportion of CO2 and CO not only can increase the
yield of methanol but also decreases the apparent activation energy of the reaction. Of special
interest is that the presence of CO2 could maintain the active copper sites in the oxidation
state or prevent an over-reduction of the ZnO component when Cu/Zn catalysts are used
during methanol synthesis.
The active site is yet to be fully understood. It is generally accepted that the
coordination, chemisorption, and activation of carbon monoxide and the homogeneous
splitting of hydrogen take place on Cu0 or Cu+ and that the heterogeneous splitting of
hydrogen, which provides Hä+ and Hä- in the catalytic process, takes place on
ZnO. However, different points of view on the nature and valence of copper sites still
exist. Some researchers pointed out that metallic Cu atoms were uniformly active for
methanol synthesis. Pan et al found that the activity of the catalyst is directly proportional
to the surface area of metallic Cu and methanol is formed on a metallic Cu surface of a
Cu-based catalyst. Similarly, Deng et al. reported that the catalytic activity of a
Cu/ZnO/Al2O3 ternary catalyst for carbon dioxide hydrogenation increases with an
increase in the metallic copper surface area, reached a maximum, and then decreased at a
Cu/ZnO molar ratio of 8. Rasmussen investigated methanol synthesis by hydrogenation
of CO2 on Cu(100) at total pressures of 1-4 bar and reported that metallic copper is actual
an active site. They formulated a kinetic model for the elementary steps of methanol
synthesis. It indicated that the calculated reaction rate and the activation energy for
methanol synthesis were in accord with those measured on Cu(100). They excluded the
possibility of Cu+ active sites because only metallic copper was observed on the Cu(100)
surface used. Similarly, Askgaard et al supported the metallic Cu model by applying a
kinetic model based on experimental data to the methanol synthesis reaction using a Cu
single crystal. However, in the presence of CO2 and with a large fraction of the Cu0
surface covered by oxygen-containing species, the catalytic activity toward methanol
synthesis is found to be independent of the Cu0 surface area. This finding can be
explained by the fact that the Cu+ sites might be acting as active sites in methanol
synthesis. Subsequently, Herman et al reported that active Cu+ ion sites are dissolved on
the surface of the ZnO matrix. They based their findings on the fact that there was not
any new phase other than the normal crystal structure of Cu metal and zinc oxide that
coexisted in the mixed Cu/ZnO catalysts. It can be deduced that the promotion effect
could only stem from the catalytic activity of a solid solution such as Cu+/ZnO and active
centers of the Cu-based catalyst should be Cu+ ions dissolved in ZnO.
Traditionally, zinc oxide is a good hydrogenation catalyst that activates hydrogen by
heterogeneous splitting, giving rise to ZnH and OH. Furthermore, hydrogen spillover has
been observed with ZnO acting as a reservoir of hydrogen for the hydrogenation of CO
over Cu surfaces. Dennison et al. observed that hydrogen is adsorbed on ZnO to a much
greater extent when Cu is present in the catalyst. On the other hand, Burch found that
hydrogen spillover from Cu to ZnO occurs very rapidly from a partially oxidized Cu
surface but only to a very small extent from a fully reduced copper surface. Moreover,
the hydrogen atoms were trapped at surface defects or at interstitial sites of ZnO, but they
were not held too strongly. This means that a possible role of ZnO might serve as a
reservoir to provide H atoms for subsequent hydrogenation of adsorbed reaction
intermediates.