Xianlong Du

Direct Methylation of Amines with Carbon Dioxide and Molecular Hydrogen using Supported Gold Catalysts

The N-methylation of amines with CO2 and H2 is an important step in the synthesis of bioactive compounds and chemical intermediates. The first heterogeneous Au catalyst is reported for this methylation reaction with good to excellent yields. The average turnover frequency (TOF) based on surface Au atoms is 45 h(-1) , which is the highest TOF value ever reported for the methylation of aniline with CO2 and H2 . Furthermore, the catalyst is tolerant toward a variety of amines, which includes aromatic, aliphatic, secondary, and primary amines. Preliminary mechanistic studies suggest that the N-alkyl formamide might be an intermediate in the N-methylation of amine process. Moreover, through a one-pot process, it is possible to convert primary amines, aldehydes, and CO2 into unsymmetrical tertiary amines with H2 as a reductant in the presence of the Au catalyst.

Research Progress on the Indirect Hydrogenation of Carbon Dioxide to Methanol

Methanol is a sustainable source of liquid fuels and one of the most useful organic chemicals. To date, most of the work in this area has focused on the direct hydrogenation of CO2 to methanol. However, this process requires high operating temperatures (200-250 °C), which limits the theoretical yield of methanol. Thus, it is desirable to find a new strategy for the efficient conversion of CO2 to methanol at relatively low reaction temperatures. This Minireview seeks to outline the recent advances on the indirect hydrogenation of CO2 to methanol. Much emphasis is placed on discussing specific systems, including hydrogenation of CO2 derivatives (organic carbonates, carbamates, formates, cyclic carbonates, etc.) and cascade reactions, with the aim of critically highlighting both the achievements and remaining challenges associated with this field.

Direct methylation of N-methylaniline with CO2/H2 catalyzed by gold nanoparticles supported on alumina

Small gold nanoparticles (∼3 nm) loaded onto various supports have been prepared by a deposition–precipitation method and studied for direct methylation of N-methylaniline with CO2/H2. Among the catalysts examined, an acid–base bifunctional support γ-alumina supported gold catalyst (Au/γ-Al2O3) exhibits the best catalytic performance. Au/γ-Al2O3 catalysts with controlled mean Au particle sizes (1.8–8.3 nm) have also been successfully prepared by regulating the concentration of HAuCl4 in solution, aging time, aging temperature and mole ratio of urea to gold in the process of deposition–precipitation with urea. The turnover frequency (TOF) values for direct methylation of N-methylaniline with CO2/H2 increase on decreasing the mean size of Au nanoparticles (from 8.3 to 1.8 nm), showing that methylation of N-methylaniline with CO2/H2 is a structure-sensitive reaction. A fast increase in TOF occurs when the mean Au particle size becomes smaller than 3 nm. Through TEM (transmission electron microscope), gold L3-edge XAFS (X-ray absorption fine structure) and CO2- and NH3-TPD (temperature programmed desorption) analysis, we can conclude the Au particle sizes, oxidation state of the gold species and acid–base properties of the supports are responsible for the high catalytic activity of direct methylation of N-methylaniline with CO2/H2.

Efficient Hydrogenation of Alkyl Formate to Methanol over Nanocomposite Copper/Alumina Catalysts

The production of methanol, an important fuel and chemical feedstock, from carbon dioxide is an important process for CO2 utilization. We describe herein a mild and efficient method for the indirect hydrogenation of carbon dioxide to methanol via a CO2‐derived formate ester intermediate by using a simple heterogeneous catalyst system comprising Cu highly dispersed in an alumina matrix under solvent‐free conditions. This catalyst is also effective for the hydrogenation of other formate esters, such as ethyl formate, propyl formate, and butyl formate.

Efficient Reduction of CO2 to CO Using Cobalt–Cobalt Oxide Core–Shell Catalysts

The route of converting CO2 to CO by reverse water-gas shift (RWGS) reaction is of particular interest due to the direct use of CO as feedstock in many significant industrial processes. Here, an engineered cobalt-cobalt oxide core-shell catalyst (Co@CoO) with nanochains structure has been made for the efficient reduction of CO2 to useful CO. Owing to the excellent performance for H2 activation of metal nanoparticles and the enhanced absorption and activation for CO2 molecule of defective metal oxides, the unique synergistic effect of metallic Co and encapsulating coordinatively unsaturated CoO species shows high performance for clean generation of CO under moderate and practical conditions. Furthermore, with N-dopant into the defective CoO shell, the Co@CoO-N achieves the highest conversion of 19.2 % and an exceptional CO evolution rate of 96 mL min-1  gcat-1 at 523 K with a gas hourly space velocity (GHSV) of 42,000 mL gcat-1  h-1 , which is comparable with the previously reported materials under identical conditions.

Prominent Electron Penetration through Ultrathin Graphene Layer from FeNi Alloy for Efficient Reduction of CO2 to CO

The chemical transformation of CO2 is an efficient approach in low-carbon energy system. The development of nonprecious metal catalysts with sufficient activity, selectivity, and stability for the generation of CO by CO2 reduction under mild conditions remains a major challenge. A hierarchical architecture catalyst composed of ultrathin graphene shells (2-4 layers) encapsulating homogeneous FeNi alloy nanoparticles shows enhance catalytic performance. Electron transfer from the encapsulated alloy can extend from the inner to the outer shell, resulting in an increased charge density on graphene. Nitrogen atom dopants can synergistically increase the electron density on the catalyst surface and modulate the adsorption capability for acidic CO2 molecules. The optimized FeNi3 @NG (NG=N-doped graphene) catalyst, with significant electron penetration through the graphene layer, effects exceptional CO2 conversion of 20.2 % with a CO selectivity of nearly 100 %, as well as excellent thermal stability at 523 K.

Efficient Photocatalytic Reduction of CO2 Using Carbon-Doped Amorphous Titanium Oxide

CO2‐related solar to chemical conversions have gained extensive interest due to the great concerns on renewable energy utilization. Here, we have demonstrated a new synthetic route to C‐doped amorphous titanium oxide using a facile citric acid assisted sol‐gel method for efficient photocatalytic reduction of CO2. The synthesized amorphous material exhibits a mesoporous structure with high specific surface area and a significantly narrowed band gap of 2.1 eV, which are crucial for solar light harvesting and adsorption/chemical activation of CO2 for energy transformation. The amorphization, mesoporous structure, and the band structure of the C‐doped samples were also successfully tuned by controlling the annealing temperatures. The optimized catalyst annealed at 300 °C shows the highest specific surface area, favorable visible‐light response as well as the considerable performance for CO2 photoreduction. Moreover, the further treatment of Al reduction can induce numerous surface oxygen vacancies on the amorphous sample and thus efficiently restrain the recombination of photogenerated carriers. Of significant importance is that the Al‐reduced catalyst achieves excellent performance with the space‐time yield of CH4 and CO of 4.1 and 2.5 μmol g−1 h−1 for solar light, and 0.53 and 0.63 μmol g−1 h−1 for visible light, respectively. This sample is also stable for photocatalytic CO2 transformation.