An Introduction of CO2 Conversion by Dry Reforming with Methane and New Route of Low-Temperature Methanol Synthesis

Abstract

Carbon dioxide is one of the highest contributors to the greenhouse effect, as well as a cheap and nontoxic building block for single carbon source chemistry. As such, CO₂ conversion is one of most important research areas in energy and environment sciences, as well as in catalysis technology. For chemical conversion of CO₂, natural gas (mainly CH₄) is a promising counterpart molecule to the CO₂-related reaction, due to its high availability and low price. More importantly, being able to convert CH₄ to useful fuels and molecules is advantageous, because it is also a kind of “greenhouse effect” gas, and can be an energy alternative to petroleum oil. In this Account, we discuss our development of efficient catalysts with precisely designed nanostructure for CO₂ reforming of CH₄ to produce syngas (mixture of CO and H₂), which can then be converted to many chemicals and energy products. This new production flow can establish a GTL (gas-to-liquid) industry, being currently pushed by the shale gas revolution. From the viewpoint of GTL industry, developing a catalyst for CO₂ reforming of CH₄ is a challenge, because they need a very high production rate to make the huge GTL methane reformer as small as possible. In addition, since both CO₂ and CH₄ give off carbon deposits that deactivate non-precious metallic catalysts very quickly, the total design of catalyst support and supported metallic nanoparticles is necessary. We present a simple but useful method to prepare bimodal catalyst support, where small pores are formed inside large ones during the self-organization of nanoparticles from solution. Large pores enhance the mass transfer rate, while small pores provide large surface areas to disperse active metallic nanoparticles. More importantly, building materials for small pores can also be used as promoters or cocatalysts to further enhance the total activity and stability. Produced syngas from methane reforming is generally catalytically converted in situ via one of two main routes. The first is to use Fischer–Tropsch synthesis (FTS), a process that catalytically converts syngas to hydrocarbons of varying molecular weights. The second is methanol synthesis. The latter has better atomic economy, since the oxygen atom in CO is included in the product and CO₂ can be blended into syngas as a reactant. However, production of methanol is very inefficient in this reaction: only 10–15% one-pass conversion typically at 5.0–10.0 MPa and 523–573 K, due to the severe thermodynamic limitations of this exothermal reaction (CO + 2H₂ = CH₃OH). In this Account, we propose and develop a new route of low-temperature methanol synthesis from CO₂-containing syngas only by adding alcohols, including methanol itself. These alcohols act as homogeneous cocatalysts and the solvent, realizing 70–100% one-pass conversion at only 5.0 MPa and 443 K. The key step is the reaction of the adsorbed formate species with alcohols to yield ester species at low temperatures, followed by the hydrogenation of ester by hydrogen atoms on metallic Cu. This changes the normal reaction path of conventional, high-temperature methanol synthesis from formate via methoxy to methanol.