Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low-Temperature Catalysis in the Context of the Methanol Economy.

The traditional economy based on carbon-intensive fuels and materials has led to an exponential rise in anthropogenic CO2 emissions. Outpacing the natural carbon cycle, atmospheric CO2 levels increased by 50 % since the pre-industrial age and can be directly linked to global warming. Being at the core of the proposed methanol economy pioneered by the late George A. Olah, the chemical recycling of CO2 to produce methanol, a green fuel and feedstock, is a prime channel to achieve carbon neutrality. In this direction, homogeneous catalytic systems have lately been a major focus for methanol synthesis from CO2 , CO and their derivatives as potential low-temperature alternatives to the commercial processes. This Review provides an account of this rapidly growing field over the past decade, since its resurgence in 2011. Based on the critical assessment of the progress thus far, the present key challenges in this field have been highlighted and potential directions have been suggested for practically viable applications.

Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol.

The first example of an alkali hydroxide-based system for CO2 capture and conversion to methanol has been established. Bicarbonate and formate salts were hydrogenated to methanol with high yields in a solution of ethylene glycol. In an integrated one-pot system, CO2 was efficiently captured by an ethylene glycol solution of the base and subsequently hydrogenated to CH3OH at relatively mild temperatures (100-140 °C) using Ru-PNP catalysts. The produced methanol can be easily separated by distillation. Hydroxide base regeneration at low temperatures was observed for the first time. Finally, CO2 capture from ambient air and hydrogenation to CH3OH was demonstrated. We postulate that the high capture efficiency and stability of hydroxide bases make them superior to existing amine-based routes for direct air capture and conversion to methanol in a scalable process.

Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol.

Amine-assisted homogeneous hydrogenation of CO2 to methanol is one of the most effective approaches to integrate CO2 capture with its subsequent conversion to CH3OH. The hydrogenation typically proceeds in two steps. In the first step the amine is formylated via an in situ formed alkylammonium formate salt (with consumption of 1 equiv of H2). In the second step the generated formamide is further hydrogenated with 2 more equiv of H2 to CH3OH while regenerating the amine. In the present study, we investigated the effect of molecular structure of the ruthenium pincer catalysts and the amines that are critical for a high methanol yield. Surprisingly, despite the high reactivity of several Ru pincer complexes [RuHClPNP R(CO)] (R = Ph/ i-Pr/Cy/ t-Bu) for both amine formylation and formamide hydrogenation, only catalyst Ru-Macho (R = Ph) provided a high methanol yield after both steps were performed simultaneously in one pot. Among various amines, only (di/poly)amines were effective in assisting Ru-Macho for methanol formation. A catalyst deactivation pathway was identified, involving the formation of ruthenium biscarbonyl monohydride cationic complexes [RuHPNP R(CO)2]+, whose structures were unambiguously characterized and whose reactivities were studied. These reactivities were found to be ligand-dependent, and a trend could be established. With Ru-Macho, the biscarbonyl species could be converted back to the active species through CO dissociation under the reaction conditions. The Ru-Macho biscarbonyl complex was therefore able to catalyze the hydrogenation of in situ formed formamides to methanol. Complex Ru-Macho-BH was also highly effective for this conversion and remained active even after 10 days of continuous reaction, achieving a maximum turnover number (TON) of 9900.

Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy.

Herein we report an efficient and recyclable system for tandem CO2 capture and hydrogenation to methanol. After capture in an aqueous amine solution, CO2 is hydrogenated in high yield to CH3OH (>90%) in a biphasic 2-MTHF/water system, which also allows for easy separation and recycling of the amine and catalyst for multiple reaction cycles. Between cycles, the produced methanol can be conveniently removed in vacuo. Employing this strategy, catalyst Ru-MACHO-BH and polyamine PEHA were recycled three times with 87% of the methanol producibility of the first cycle retained, along with 95% of catalyst activity after four cycles. CO2 from dilute sources such as air can also be converted to CH3OH using this route. We postulate that the CO2 capture and hydrogenation to methanol system presented here could be an important step toward the implementation of the carbon neutral methanol economy concept.

Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol.

A novel hydrogen storage system based on the hydrogen release from catalytic dehydrogenative coupling of methanol and 1,2-diamine is demonstrated. The products of this reaction, N-formamide and N,N'-diformamide, are hydrogenated back to the free amine and methanol by a simple hydrogen pressure swing. Thus, an efficient one-pot hydrogen carrier system has been developed. The H2 generating step can be termed as "amine reforming of methanol" in analogy to the traditional steam reforming. It acts as a clean source of hydrogen without concurrent production of CO2 (unlike steam reforming) or CO (by complete methanol dehydrogenation). Therefore, a carbon neutral cycle is essentially achieved where no carbon capture is necessary as the carbon is trapped in the form of formamide (or urea in the case of primary amine). In theory, a hydrogen storage capacity as high as 6.6 wt % is achievable. Dehydrogenative coupling and the subsequent amide hydrogenation proceed with good yields (90% and >95% respectively, with methanol and N,N'-dimethylethylenediamine as dehydrogenative coupling partners).

Tertiary Amine-Ethylene Glycol Based Tandem CO2 Capture and Hydrogenation to Methanol: Direct Utilization of Post-Combustion CO2.

Carbon dioxide capture using tertiary amines in ethylene glycol solvent was performed under ambient conditions. Subsequently, the CO2 captured as alkyl carbonate salts was successfully hydrogenated to methanol, in the presence of H2 gas and Ru-Macho-BH catalyst. A comprehensive series of tertiary amines were selected for the integrated capture and conversion process. While most of these amines were effective for CO2 capture, tetramethylethylenediamine (TMEDA) and tetramethylbutanediamine (TMBDA) provided the best CH3 OH yields. Deactivation of the base due to side reactions was significantly minimized and substantial base regeneration was observed. The proposed system was also highly efficient for CO2 capture from a gas mixture containing 10 % CO2 , as found in flue gases, followed by tandem conversion to CH3 OH. We postulate that such high boiling tertiary amine-glycol systems as dual capture and hydrogenation solvents are promising for the realization of a sustainable and carbon-neutral methanol economy in a scalable process.

Oxidation-Resistant, Cost-Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air.

CO2 adsorbents based on the reaction of pentaethylenehexamine (PEHA) or tetraethylenepentamine (TEPA) with propylene oxide (PO) were easily prepared in "one pot" by impregnation on a silica support in water. The starting materials were readily available and inexpensive, facilitating the production of the adsorbents on a large scale. The prepared polyamine/epoxide adsorbents were efficient in capturing CO2 and could be regenerated under mild conditions (50-85 °C). They displayed a much-improved stability compared with their unmodified amine counterparts, especially under oxidative conditions. Leaching of the active organic amine became minimal or nonexistent after treatment with the epoxide. The adsorption as well as desorption kinetics were also greatly improved. The polyamine/epoxide adsorbents were able to capture CO2 from various sources including ambient air and indoor air with CO2 concentrations of only 400-1000 ppm. The presence of water, far from being detrimental, increased the adsorption capacity. Their use for indoor air quality purposes was explored.

Paradoxical Effect of Trehalose on the Aggregation of α-Synuclein: Expedites Onset of Aggregation yet Reduces Fibril Load.

Aggregation of α-synuclein is closely connected to the pathology of Parkinson's disease. The phenomenon involves multiple steps, commenced by partial misfolding and eventually leading to mature amyloid fibril formation. Trehalose, a widely accepted osmolyte, has been shown previously to inhibit aggregation of various globular proteins owing to its ability to prevent the initial unfolding of protein. In this study, we have examined if it behaves in a similar fashion with intrinsically disordered protein α-synuclein and possesses the potential to act as therapeutic agent against Parkinson's disease. It was observed experimentally that samples coincubated with trehalose fibrillate faster compared to the case in its absence. Molecular dynamics simulations suggested that this initial acceleration is manifestation of trehalose's tendency to perturb the conformational transitions between different conformers of monomeric protein. It stabilizes the aggregation prone "extended" conformer of α-synuclein, by binding to its exposed acidic residues of the C terminus. It also favors the ß-rich oligomers once formed. Interestingly, the total fibrils formed are still promisingly less since it accelerates the competing pathway toward formation of amorphous aggregates.