Catalytic dehydrogenative coupling and reversal of methanol-amines: advances and prospects

The development of efficient hydrogen release and storage processes to provide environmentally friendly hydrogen solutions for mobile energy storage systems (MESS) stands as one of the most challenging tasks in addressing the energy crisis and environmental degradation. The catalytic dehydrogenative coupling of methanol and amines (DCMA) and its reverse are featured by high capacity for hydrogen release and storage, enhanced capability to purify the produced hydrogen, avoidance of carbon emissions and singular product composition, offering the environmentally and operationally benign strategy of overcoming the challenges associated with MESS. Particularly, the cycle between these two processes within the same catalytic system eliminates the need for collecting and transporting spent fuel back to a central facility, significantly facilitating easy recharging. Despite the promising attributes of the above strategy for environmentally friendly hydrogen solutions, challenges persist, primarily due to the high thermodynamic barriers encountered in methanol dehydrogenation and amide hydrogenation. By systematically summarizing various reaction mechanisms and pathways involving Ru-, Mn-, Fe-, and Mo-based catalytic systems in the development of catalytic DCMA and its reverse and the cycling between the two, this review highlights the current research landscape, identifies gaps, and suggests directions for future investigations to overcome these challenges. Additionally, the critical importance of developing efficient catalytic systems that operate under milder conditions, thereby facilitating the practical application of DCMA in MESS, is also underscored.

A Self-Separating Multiphasic System for Catalytic Hydrogenation of CO2 and CO2 -Derivatives to Methanol

Catalytic conversion of CO₂ and hydrogen to methanol was achieved in a self-separating multiphasic system comprising the tailor-made complex [Ru(CO)ClH(MACHO-C₁₂ )] (MACHO-C₁₂ =bis{2-[bis(4-dodecylphenyl)phosphino]ethyl}amine) in n-decane as the catalyst phase. Effective catalyst recycling was demonstrated for the carbonate and the amine-assisted pathway from CO₂ to methanol. The polar products MeOH or MeOH/H₂ O generated from the catalytic reactions spontaneously formed a separate phase, allowing product isolation and catalyst separation without the need for any additional solvent. In the amine-assisted hydrogenation of CO₂ , the catalyst phase was recycled over ten subsequent runs, reaching a total turnover number to MeOH of 19200 with an average selectivity of 96 %.

Advancements and Challenges in Reductive Conversion of Carbon Dioxide via Thermo-/Photocatalysis

Carbon dioxide (CO₂) is the major greenhouse gas and also an abundant and renewable carbon resource. Therefore, its chemical conversion and utilization are of great attraction for sustainable development. Especially, reductive conversion of CO₂ with energy input has become a current hotspot due to its ability to access fuels and various important chemicals. Nowadays, the controllable CO₂ hydrogenation to formic acid and alcohols using sustainable H₂ resources has been regarded as an appealing solution to hydrogen storage and CO₂ accumulation. In addition, photocatalytic CO₂ reduction to CO also provides a potential way to utilize this greenhouse gas efficiently. Besides direct CO₂ hydrogenation, CO₂ reductive functionalization integrates CO₂ reduction with subsequent C-X (X = N, S, C, O) bond formation and indirect transformation strategies, enlarging the diverse products derived from CO₂ and promoting CO₂ reductive conversion into a new stage. In this Perspective, the progress and challenges of CO₂ reductive conversion, including hydrogenation, reductive functionalization, photocatalytic reduction, and photocatalytic reductive functionalization are summarized and discussed along with the key issues and future trends/directions in this field. We hope this Perspective can evoke intense interest and inspire much innovation in the promise of CO₂ valorization.

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.

Acid-assisted hydrogenation of CO2 to methanol using Ru(II) and Rh(III) RAPTA-type catalysts under mild conditions

A highly efficient homogeneous catalyst system for production of CH3OH from CO2 using single molecular defined ruthenium and rhodium RAPTA-type catalysts [Ru(η6-p-cymene)X2(PTA)] (X = I(1), Cl(2); PTA = 1,3,5-triaza-7-phosphaadamantane) and rhodium catalysts [Rh(η5-C5Me5)X2(PTA/PTA-BH3)] (X = Cl(3), H(4) and PTA-BH3, H(5)) developed in acidic media under mild conditions. A TON of 4752 is achieved using a [Ru(η6-p-cymene)I2(PTA)] catalyst which represents the first example of CO2 hydrogenation to CH3OH using single molecular defined Ru and Rh RAPTA-type catalysts.

Homogeneous and heterogeneous catalysts for hydrogenation of CO2 to methanol under mild conditions

This review summarizes the concepts, mechanisms, drawbacks and challenges of the state-of-the-art catalysis for CO₂ to MeOH under mild conditions. Thoughtful guidelines and principles for future research are presented and discussed.

CO2 Reduction to Methanol in the Liquid Phase: A Review

Excessive carbon dioxide (CO₂ ) emissions have been subject to extensive attention globally, since an enhanced greenhouse effect (global warming) owing to a high CO₂ concentration in the atmosphere could lead to severe climate change. The use of solar energy and other renewable energy to produce low-cost hydrogen, which is used to reduce CO₂ to produce bulk chemicals such as methanol, is a sustainable strategy for reducing carbon dioxide emissions and carbon resources. CO₂ conversion into methanol is exothermic, so that low temperature and high pressure are favorable for methanol formation. CO₂ is usually captured and recovered in the liquid phase. Herein, the emerging technologies for the hydrogenation of CO₂ to methanol in the condensed phase are reviewed. The development of homogeneous and heterogeneous catalysts for this important hydrogenation reaction is summarized. Finally, mechanistic insight on CO₂ 's conversion into methanol over different catalysts is discussed by taking the available reaction pathways into account.