Starting with coal, followed by petroleum oil and natural gas, the utilization of fossil fuels has allowed the fast and unprecedented development of human society. However, the burning of these resources in ever increasing pace is accompanied by large amounts of anthropogenic CO2 emissions, which are outpacing the natural carbon cycle, causing adverse global environmental changes, the full extent of which is still unclear. Even through fossil fuels are still abundant, they are nevertheless limited and will, in time, be depleted. Chemical recycling of CO2 to renewable fuels and materials, primarily methanol, offers a powerful alternative to tackle both issues, that is, global climate change and fossil fuel depletion. The energy needed for the reduction of CO2 can come from any renewable energy source such as solar and wind. Methanol, the simplest C1 liquid product that can be easily obtained from any carbon source, including biomass and CO2, has been proposed as a key component of such an anthropogenic carbon cycle in the framework of a “Methanol Economy”. Methanol itself is an excellent fuel for internal combustion engines, fuel cells, stoves, etc. It's dehydration product, dimethyl ether, is a diesel fuel and liquefied petroleum gas (LPG) substitute. Furthermore, methanol can be transformed to ethylene, propylene and most of the petrochemical products currently obtained from fossil fuels. The conversion of CO2 to methanol is discussed in detail in this review.

Recycling of carbon dioxide to methanol and derived products – closing the loop

The recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO2 to formate/formic acid, methanol, and dimethyl ether are thoroughly reviewed, with special emphasis on thermodynamics and catalyst design considerations. After introducing the main motivation for the development of such processes, we first summarize the most important aspects of CO2 capture and green routes to produce H2. Once the scene in terms of feedstocks is introduced, we carefully summarize the state of the art in the development of heterogeneous catalysts for these important hydrogenation reactions. Finally, in an attempt to give an order of magnitude regarding CO2 valorization, we critically assess economical aspects of the production of methanol and DME and outline future research and development directions.

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Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes.

The article discusses oxidation catalysis by substitutional cation doping of binary oxides. Substitutional cation doping is not the only possibility. One can imagine that replacing some anions with other anions may also be beneficial. There is evidence that the presence of small amounts of halogen in the feed or on the oxide surface improves its catalytic activity. It is very likely that doped oxide catalysts have been used before the concept was formulated explicitly. Most oxide catalysts have low levels of impurities that may be substitutional dopants. If they segregate at the surface, they can affect the catalytic activity without our knowledge even though their net concentration is very low. It is also possible that the 'as-prepared' catalyst is a doped oxide that, under reducing reaction conditions, is converted to very small metallic dopant clusters supported on the host oxide. The physical and chemical properties of such clusters are different from those of a bulk metal, and it is difficult to distinguish them from a doped oxide.

Catalysis by doped oxides.

Nanocrystals of MgO and CaO have been prepared by a modified aerogel/hypercritical drying/dehydration method. For nanocrystalline MgO (AP-MgO) surface areas ranged from 250 to 500 m2/g, whereas for AP-CaO 100−160 m2/g. These materials have been compared with more conventional (CP) microcrystalline samples of lower surface area with regard to (1) morphology (AP-samples (autoclave preparation) are tiny polyhedral crystallites, while CP-samples (conventional preparation) are larger, hexagonal platelets and cubes); (2) residual surface OH (AP-samples have less acidic OH, which are more isolated from each other; (3) acid gas adsorption (AP-samples adsorb more SO2 and CO2 at low pressures and room temperature and prefer monodentate rather than bidentate adsorption modes, but at higher pressures CP-samples adsorb more SO2 and HCl apparently due to the formation of more well ordered multilayers); (4) destructive adsorption of organophosphorus compounds and chlorocarbons (AP-samples are superior due to higher surf...

Nanocrystals as Stoichiometric Reagents with Unique Surface Chemistry

Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H2) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis–structure–function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer–Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer–Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO2 and recycle side-products in the process are described in the third section.

Status and prospects in higher alcohols synthesis from syngas

The catalytic conversion of methanol to dimethyl ether (DME) has been studied over a series of solid-acid catalysts, such as γ-Al2O3, H-ZSM-5, amorphous silica-alumina, as well as titania modified zirconia. All the catalysts are active and selective for DME formation. The apparent activation energy for DME formation over γ-Al2O3 is ca. 25 kcal/mol, a value that increases to ca. 37 kcal/mol upon the addition of 23 Torr of H2O to the reagent. The rate of methanol dehydration decreases with increasing acidity (silica content) over the amorphous silica-alumina catalysts. Although H-ZSM-5 with Si/Al = 25 is the most active among the catalysts tested, the DME selectivity is only 20% at 280°C, a typical temperature used in the syngas-to-methanol process. An amorphous silica-alumina catalyst with 20 wt.-% silica content (SIRAL20) exhibits the best catalytic performance of those tested at 280°C.

Synthesis of dimethyl ether (DME) from methanol over solid-acid catalysts

Ten wt.-% Pd/Cab-O-Sil reduced at 300°C has been found to be an effective catalyst for the catalytic dehydration of methanol to dimethyl ether (DME). The presence of hydrogen in the reagent stream inhibited the catalytic activity, but increased the stability of the catalyst. High reaction temperature and low methanol partial pressure did not favor DME formation.

Catalytic dehydration of methanol to dimethyl ether (DME) over Pd/Cab-O-Sil catalysts

The Effect of Chloride Ions on a Li+-MgO Catalyst for the Oxidative Coupling of Methane

AtT ⩽ 650 °C carbon dioxide either formed during reaction or added to the system increases the selectivity for the desired hydrocarbon products during the oxidative coupling of methane and the oxidative dehydrogenation of ethane reaction over Li+/MgO catalysts. Similarly, CO2 inhibits secondary reactions of CH3-radicals with the surface of the Li+/MgO. The improved selectivities are attributed to the poisoning effect that CO2 has on the secondary reactions of alkyl radicals with the surface.

Effect of carbon dioxide on the selectivities obtained during the partial oxidation of methane and ethane over Li+/MgO catalysts

The apparent activation energies for both the generation of CH{sub 3}{sup {sm_bullet}} radicals and the overall consumption of CH{sub 4} during the oxidative coupling reaction are strongly influenced by the presence of CO{sub 2}. The intrinsic activation energy for the information of CH{sub 3}{sup {sm_bullet}} radicals over Li/MgO catalysts is 26 {plus_minus} 2 kcal/mol, a value that increases to 37 {plus_minus} 3 kcal/mol upon the addition of only 0.04 Torr of CO{sub 2} to the reagents. The activation energy approaches a limiting value of ca. 50 kcal/mol at a CO{sub 2} partial pressure of {ge}1 Torr. Thus, under typical catalytic reaction conditions for the oxidative coupling of methane (e.g., 190 Torr of CH{sub 4}, 95 Torr of O{sub 2}), sufficient CO{sub 2} would be produced to attain this limiting value of E{sub a}. However, by operating at sufficiently low partial pressures and conversion levels, an E{sub a} of 32 kcal/mol was obtained during the oxidative coupling reaction. Temperature-programmed desorption of CO{sub 2} from Li/MgO produces a peak at 630{degrees}C, which is consistent with the role of CO{sub 2} as a poison for the catalytic reactions. 15 refs., 4 figs.

Mingting Xu

Papers

Synthesis of dimethyl ether (DME) from methanol over solid-acid catalysts

Catalytic dehydration of methanol to dimethyl ether (DME) over Pd/Cab-O-Sil catalysts

The Effect of Chloride Ions on a Li+-MgO Catalyst for the Oxidative Coupling of Methane

Effect of carbon dioxide on the selectivities obtained during the partial oxidation of methane and ethane over Li+/MgO catalysts

Effect of carbon dioxide on the activation energy for methyl radical generation over Li/MgO catalysts