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This review reports recent achievements in dimethyl ether (DME) synthesis via CO2 hydrogenation. This gas-phase process could be considered as a promising alternative for carbon dioxide recycling toward a (bio)fuel as DME. In this view, the production of DME from catalytic hydrogenation of CO2 appears as a technology able to face also the ever-increasing demand for alternative, environmentally-friendly fuels and energy carriers. Basic considerations on thermodynamic aspects controlling DME production from CO2 are presented along with a survey of the most innovative catalytic systems developed in this field. During the last years, special attention has been paid to the role of zeolite-based catalysts, either in the methanol-to-DME dehydration step or in the one-pot CO2-to-DME hydrogenation. Overall, the productivity of DME was shown to be dependent on several catalyst features, related not only to the metal-oxide phase—responsible for CO2 activation/hydrogenation—but also to specific properties of the zeolites (i.e., topology, porosity, specific surface area, acidity, interaction with active metals, distributions of metal particles, …) influencing activity and stability of hybridized bifunctional heterogeneous catalysts. All these aspects are discussed in details, summarizing recent achievements in this research field.

The catalytic hydrotreatment of Kraft lignin using sulfided NiMo and CoMo catalysts on different acidic and basic supports (Al2O3, ZSM-5, activated carbon (AC) and MgO-La2O3) was studied in the absence of a solvent. Experiments were carried out in a batch set-up at a reaction temperature of 350 °C, 4 h and 100 bar initial H2 pressure. The catalysts before and after reaction were characterized by X-ray diffraction, temperature programmed desorption of ammonia/CO2, BET surface area and scanning electron microscopy. The liquid products were fractionated and analyzed extensively by different techniques such as GPC, GC-MS-FID, GC-TCD, FT-IR, 13 C-NMR and elemental analyses. Two dimensional gas chromatography (GCxGC-FID) was applied to identify distinct groups of compounds (aromatics, alkylphenolics, alkanes) after reaction, and product quantification was performed based on this method. Catalyst activity is a function of the support and increased in the order Al2O3 < ZSM-5 < AC=MgO-La2O3. In addition, the support also largely influenced the extent of depolymerization and monomer yield. The highest lignin oil yields were obtained using the sulfided NiMo supported on activated carbon and MgO-La2O3. The highest total monomer yield 26.4 wt.% on lignin intake, which included 15.7 wt.% alkyl-phenolics was obtained using the sulfided NiMo/MgO-La2O3 catalyst.

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