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.

Methanol synthesis via CO2 hydrogenation is a key step in methanol-based economy. This reaction is catalyzed by supported copper nanoparticles and displays strong support or promoter effects. Zirconia is known to enhance both the methanol production rate and the selectivity. Nevertheless, the origin of this observation and the reaction mechanisms associated with the conversion of CO2 to methanol still remain unknown. Here, we present a mechanistic study of the hydrogenation of CO2 on Cu/ZrO2. Using kinetics, in situ IR and NMR spectroscopies and isotopic labeling strategies, we examined the surface intermediates during CO2 hydrogenation at different pressures. Combined with DFT calculations, we show that formate species is the reaction intermediate and that the zirconia/copper interface is a key for its conversion to methanol.The catalytic hydrogenation of carbon dioxide to methanol is a key process in the sustainable methanol-based economy. [1] While copper-based catalysts are highly active for this transformation, [2] their activity and selectivity strongly depend on the support and/or the promoters. Understanding the copper-support interaction -its effect on the activity and product selectivity -has been a very intensive field of research over the last decade. While the reaction mechanisms and the nature of the active sites on Cu/ZnO systems have been extensively investigated, [3] copper supported on zirconia and related materials also exhibits high activity and selectivity in CO2 hydrogenation to methanol (Eq. 1) by minimizing the formation of CO, a byproduct often resulting from the competitive reverse water-gas shift reaction (Eq. 2). [4] CO2 + 3H2 = CH3OH + H2O ∆rH° (500 K) = -62 kJ.mol -1 (1) CO2 + H2 = CO + H2O ∆rH° (500 K) = +40 kJ.mol -1 (2)Although the copper-zirconia interface was proposed to play a key role in the selective formation of methanol, [4c, 4e-g] the active site and the reaction mechanism, including the role of the interface on methanol selectivity, are still not understood. In fact, mechanistic investigations using Diffuse Reflectance IR Fourier Transform spectroscopy (DRIFTS) led to opposite conclusions: formate is an intermediate in methanol formation [4c, 4d] vs. CO2 is first reduced to CO that is in turn hydrogenated to methanol through a carboxyl intermediate. [4f] Herein, by using a combined experimental and computational approach on realistic models, we investigated the reaction mechanism of CO2 hydrogenation to methanol on a Cu/ZrO2 catalyst. Kinetic investigation, in situ and ex situ spectroscopies -FTIR and NMRtogether with isotopic labeling and computational modelling showed that methanol is a primary product formed by the hydrogenation of formate as a reaction intermediate. First, narrowly dispersed copper nanoparticles supported on monoclinic zirconia were prepared by a molecular approach. [5] Grafting of [Cu(O t Bu)]4 on the surface hydroxyl groups of the support ( Figure S1-S2, Scheme S1) followed by a treatment under H2 at 500 °C for 5 h [6] yields smal...

Natural methane hydrates are believed to be the largest source of hydrocarbons on Earth. These structures are formed in specific locations such as deep-sea sediments and the permafrost based on demanding conditions of high pressure and low temperature. Here we report that, by taking advantage of the confinement effects on nanopore space, synthetic methane hydrates grow under mild conditions (3.5 MPa and 2°C), with faster kinetics (within minutes) than nature, fully reversibly and with a nominal stoichiometry that mimics nature. The formation of the hydrate structures in nanospace and their similarity to natural hydrates is confirmed using inelastic neutron scattering experiments and synchrotron X-ray powder diffraction. These findings may be a step towards the application of a smart synthesis of methane hydrates in energy-demanding applications (for example, transportation).

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Interplay between three important reaction parameters (pressure, temperature, and space velocity) in stoichiometric hydrogenation of carbon dioxide (CO 2 :H 2 =1:3) was systematically investigated using a commercial Cu/ZnO/Al 2 O 3 catalyst. Their impacts on reaction performance and important ranges of process conditions towards full one-pass conversion of CO 2 to methanol at high yield were rationalized based on the kinetics and thermodynamics of the reaction. Under high-pressure condition above a threshold temperature, the reaction overcomes kinetic control, entering thermodynamically controlled regime. Ca. 90% CO 2 conversion and >95% methanol selectivity was achieved with a very good yield (0.9-2.4 g MeOH g cat-1 h-1) at 442 bar. Such high-pressure condition induces the formation of highly dense phase and consequent mass transfer limitation. When this limitation is overcome, the advantage of high-pressure conditions can be fully exploited and weight time yield as high as 15.3 g MeOH g cat-1 h-1 could be achieved at 442 bar. Remarkable advantages of high-pressure conditions in the terms reaction kinetics, thermodynamics, and phase behavior in the aim to achieve better methanol yield are discussed.