Catalytic Homogeneous Hydrogenation of CO to Methanol via Formamide

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.

Biomimetic Frustrated Lewis Pair Catalysts for Hydrogenation of CO to Methanol at Low Temperatures

The industrial production of methanol through CO hydrogenation using the Cu/ZnO/Al2O3 catalyst requires harsh conditions, and the development of new catalysts with low operating temperatures is highly desirable. In this study, organic biomimetic FLP catalysts with good tolerance to CO poison are theoretically designed. The base-free catalytic reaction contains the 1,1-addition of CO into a formic acid intermediate and the hydrogenation of the formic acid intermediate into methanol. Low-energy spans (25.6, 22.1, and 20.6 kcal/mol) are achieved, indicating that CO can be hydrogenated into methanol at low temperatures. The new extended aromatization-dearomatization effect involving multiple rings is proposed to effectively facilitate the rate-determining CO 1,1-addition step, and a new CO activation model is proposed for organic catalysts.

Hydrogen Activation with Ru-PN3P Pincer Complexes for the Conversion of C1 Feedstocks

The hydrogenation of C1 feedstocks (CO and CO2) has been investigated using ruthenium complexes [RuHCl(CO)(PN3P)] as the catalyst. PN3P pincer ligands containing amines in the linker between the central pyridine donor and the phosphorus donors with bulky substituents (tert-butyl (1) or TMPhos (2)) are required to obtain mononuclear single-site catalysts that can be activated by the addition of KOtBu to generate stable five-coordinate complexes [RuH(CO)(PN3P-H)], whereby the pincer ligand has been deprotonated. Activation of hydrogen takes place via heterolytic cleavage to generate [RuH2(CO)(PN3P)], but in the presence of CO, coordination of CO occurs preferentially to give [RuH(CO)2(PN3P-H)]. This complex can be protonated to give the cationic complex [RuH(CO)2(PN3P)]+, but it is unable to activate H2 heterolytically. In the case of the less coordinating CO2, both ruthenium complexes 1 and 2 are highly efficient as CO2 hydrogenation catalysts in the presence of a base (DBU), which in the case of the TMPhos ligand results in a TON of 30,000 for the formation of formate.

In Situ Modification of CuO-Fe2O3 by Nonthermal Plasma: Insights into the CO2-to-CH3OH Hydrogenation Reaction

The hydrogenation of CO₂ to CH₃OH on the binary mixed metal oxides of CuO-Fe₂O₃ under nonthermal plasma discharge has been reported in this study. The catalysts are synthesized using the sol-gel route and characterized by XRD, FTIR, SEM, and XPS techniques. The impact of CuO mixing with Fe₂O₃ on CO₂ conversion and CH₃OH yield has been investigated. Herein, we have compared two distinct techniques, namely thermal and plasma catalytic processes. The overall outcome shows that the CO₂ conversion and CH₃OH production increase with an increase in CuO mixing with Fe₂O₃. The synthesized catalyst does not show significant CO₂ conversion and CH₃OH formation in the thermal catalytic process (100-250 °C). Interestingly, when plasma discharge is combined with thermal heating, CO₂ conversion and CH₃OH production significantly improve. The plasma discharges in the CO₂/H₂ gas stream, at low temperatures (<200 °C), reduce Cu⁺² to Cu⁺¹ and Fe⁺³ to Fe⁺², which could probably enhance the CO₂ conversion and CH₃OH production. Among the catalysts prepared, 15% CuO-Fe₂O₃ exhibited the best catalytic activity with 13.2% CO₂ conversion, 7.3% CH₃OH yield, and a space-time yield of 13 mmol_CH3OH/h g_cat, with 4.67 kJ/L of specific input energy (SIE). The CH₃OH space-time yield is 2.9-fold higher than that of the commercial catalyst Cu/ZnO/Al₂O₃, which is operated at 30 °C with 45.45 kJ/L SIE.

Hydrogenation of Carboxylic Acids, Esters, and Related Compounds over Heterogeneous Catalysts: A Step toward Sustainable and Carbon-Neutral Processes

The catalytic hydrogenation of esters and carboxylic acids represents a fundamental and important class of organic transformations, which is widely applied in energy, environmental, agricultural, and pharmaceutical industries. Due to the low reactivity of the carbonyl group in carboxylic acids and esters, this type of reaction is, however, rather challenging. Hence, specifically active catalysts are required to achieve a satisfactory yield. Nevertheless, in recent years, remarkable progress has been made on the development of catalysts for this type of reaction, especially heterogeneous catalysts, which are generally dominating in industry. Here in this review, we discuss the recent breakthroughs as well as milestone achievements for the hydrogenation of industrially important carboxylic acids and esters utilizing heterogeneous catalysts. In addition, related catalytic hydrogenations that are considered of importance for the development of cleaner energy technologies and a circular chemical industry will be discussed in detail. Special attention is paid to the insights into the structure-activity relationship, which will help the readers to develop rational design strategies for the synthesis of more efficient heterogeneous catalysts.

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.

Hydrogenation of CO2 or CO2 Derivatives to Methanol under Molecular Catalysis: A Review

The atmospheric CO2 concentration has been continuously increasing due to fossil fuel combustion. The transformations of CO2 and CO2 derivatives into high value-added chemicals such as alcohols are ideal routes to mitigate greenhouse gas emissions. Among alcohol products, methanol is very promising as it fulfills the carbon neutral cycle and can be used for direct methanol fuel cells. Herein, we summarize the recent progress in the hydrogenation of CO2 or CO2 derivatives to methanol, and focus on those systems with homogeneous catalysts and molecular hydrogen as the reductant. Discussions on the catalytic systems, efficiencies, and future outlooks will be given.

Homogeneous Catalysis for Sustainable Energy: Hydrogen and Methanol Economies, Fuels from Biomass, and Related Topics

As the world pledges to significantly cut carbon emissions, the demand for sustainable and clean energy has now become more important than ever. This includes both production and storage of energy carriers, a majority of which involve catalytic reactions. This article reviews recent developments of homogeneous catalysts in emerging applications of sustainable energy. The most important focus has been on hydrogen storage as several efficient homogeneous catalysts have been reported recently for (de)hydrogenative transformations promising to the hydrogen economy. Another direction that has been extensively covered in this review is that of the methanol economy. Homogeneous catalysts investigated for the production of methanol from CO2, CO, and HCOOH have been discussed in detail. Moreover, catalytic processes for the production of conventional fuels (higher alkanes such as diesel, wax) from biomass or lower alkanes have also been discussed. A section has also been dedicated to the production of ethylene glycol from CO and H2 using homogeneous catalysts. Well-defined transition metal complexes, in particular, pincer complexes, have been discussed in more detail due to their high activity and well-studied mechanisms.

Ascendancy of Nitrogen Heterocycles in the Computationally Designed Mn(I)PNN Pincer Catalysts on the Hydrogenation of Carbon Dioxide to Methanol

The development of sustainable catalysts to get methanol from CO₂ under milder conditions and without any additives is still considered an arduous task. In many instances, transition-metal-catalyzed carbon dioxide to formic acid formation is more facile than methanol formation. This article provides comprehensive density functional theoretic investigations of six new Mn(I)PNN complexes, which are designed to perform CO₂ to methanol conversion under milder reaction conditions. All these six catalysts have similar structural features except at terminal nitrogen, -N (1), where adenine-inspired nitrogen heterocycles containing pyridine and pyrimidine moieties are attached to instill an electron withdrawing effect on the central metal and thus to facilitate dihydrogen polarization during the catalyst regeneration. All these computationally modeled Mn(I)PNN complexes demonstrate the promising catalytic activity to get methanol through cascade catalytic cycles at 298.15 K. The metal-ligand cooperative (MLC) as well as noncooperative (NC) pathways are investigated for each catalytic cycle. The NC pathway is the preferred pathway for formic acid and formaldehyde formation, whereas methanol formation proceeds through only the MLC pathway. Different nitrogen heterocycles attached to the -N (1) terminal manifested a considerable amount of impact on the Gibbs free energies, overall activation energies, and computed turnover frequencies (TOFs). Among all the catalysts, SPCAT02 provides excellent TOFs for HCO₂H (500 151 h^-1), HCHO (11 912 h^-1), and CH₃OH (2 372 400 h^-1) formation at 50 °C. SPCAT04 is found to be a better catalyst for the selective formation of formic acid formation at room temperature than the rest of the catalysts. The computed TOF results are found reliable upon comparison with experimentally established catalysts. To establish the structure-activity relationship, the activation strain model and Fukui function calculations are performed on all the catalysts. Both these studies provide complementary results. The present study revealed a very important finding that a more electrophilic metal center could facilitate the CO₂ hydrogenation reaction robustly. All computationally designed catalysts could be cheaper and better alternatives to convert CO₂ to methanol under mild reaction conditions in an aqueous medium.

Efficient Base-Free Aqueous Reforming of Methanol Homogeneously Catalyzed by Ruthenium Exhibiting a Remarkable Acceleration by Added Catalytic Thiol

Production of H2 by methanol reforming is of particular interest due the low cost, ready availability, and high hydrogen content of methanol. However, most current methods either require very high temperatures and pressures or strongly rely on the utilization of large amounts of base. Here we report an efficient, base-free aqueous-phase reforming of methanol homogeneously catalyzed by an acridine-based ruthenium pincer complex, the activity of which was unexpectedly improved by a catalytic amount of a thiol additive. The reactivity of this system is enhanced by nearly 2 orders of magnitude upon addition of the thiol, and it can maintain activity for over 3 weeks, achieving a total H2 turnover number of over 130 000. On the basis of both experimental and computational studies, a mechanism is proposed which involves outer-sphere dehydrogenations promoted by a unique ruthenium complex with thiolate as an assisting ligand. The current system overcomes the need for added base in homogeneous methanol reforming and also highlights the unprecedented acceleration of catalytic activity of metal complexes achieved by the addition of a catalytic amount of thiol.

Alcohol-Assisted Hydrogenation of Carbon Monoxide to Methanol Using Molecular Manganese Catalysts

Alcohol-assisted hydrogenation of carbon monoxide (CO) to methanol was achieved using homogeneous molecular complexes. The molecular manganese complex [Mn(CO)2Br[HN(C2H4P i Pr2)2]] ([HN(C2H4P i Pr2)2] = MACHO- i Pr) revealed the best performance, reaching up to turnover number = 4023 and turnover frequency 857 h-1 in EtOH/toluene as solvent under optimized conditions (T = 150 °C, p(CO/H2) = 5/50 bar, t = 8-12 h). Control experiments affirmed that the reaction proceeds via formate ester as the intermediate, whereby a catalytic amount of base was found to be sufficient to mediate its formation from CO and the alcohol in situ. Selectivity for methanol formation reached >99% with no accumulation of the formate ester. The reaction was demonstrated to work with methanol as the alcohol component, resulting in a reactive system that allows catalytic "breeding" of methanol without any coreagents.

Carbon monoxide and hydrogen (syngas) as a C1-building block for selective catalytic methylation

A catalytic reaction using syngas (CO/H2) as feedstock for the selective β-methylation of alcohols was developed whereby carbon monoxide acts as a C1 source and hydrogen gas as a reducing agent. The overall transformation occurs through an intricate network of metal-catalyzed and base-mediated reactions. The molecular complex [Mn(CO)2Br[HN(C2H4P i Pr2)2]] 1 comprising earth-abundant manganese acts as the metal component in the catalytic system enabling the generation of formaldehyde from syngas in a synthetically useful reaction. This new syngas conversion opens pathways to install methyl branches at sp3 carbon centers utilizing renewable feedstocks and energy for the synthesis of biologically active compounds, fine chemicals, and advanced biofuels.

Homogeneous and heterogeneous catalytic reduction of amides and related compounds using molecular hydrogen

Catalytic hydrogenation of amides is of great interest for chemists working in organic synthesis, as the resulting amines are widely featured in natural products, drugs, agrochemicals, dyes, etc. Compared to traditional reduction of amides using (over)stoichiometric reductants, the direct hydrogenation of amides using molecular hydrogen represents a greener approach. Furthermore, amide hydrogenation is a highly versatile transformation, since not only higher amines (obtained by C–O cleavage), but also lower amines and alcohols, or amino alcohols (obtained by C–N cleavage) can be selectively accessed by fine tuning of reaction conditions. This review describes the most recent advances in the area of amide hydrogenation using H₂ exclusively and molecularly defined homogeneous as well as nano-structured heterogeneous catalysts, with a special focus on catalyst development and synthetic applications.

Catalytic hydrogenation of amides is a pivotal chemical transformation for both research labs and chemical production in industry. Here, the authors comprehensively review this topic by including state-of-art homogeneous and heterogeneous catalysts that can hydrogenate amides and related compounds.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Selective Room-Temperature Hydrogenation of Amides to Amines and Alcohols Catalyzed by a Ruthenium Pincer Complex and Mechanistic Insight

We report a room-temperature protocol for the hydrogenation of various amides to produce amines and alcohols. Compared with most previous reports for this transformation, which use high temperatures (typically, 100–200 °C) and H₂ pressures (10–100 bar), this system proceeds under extremely mild conditions (RT, 5–10 bar of H₂). The hydrogenation is catalyzed by well-defined ruthenium-PNNH pincer complexes (0.5 mol %) with potential dual modes of metal–ligand cooperation. An unusual Ru-amidate complex was formed and crystallographically characterized. Mechanistic investigations indicate that the room-temperature hydrogenation proceeds predominantly via the Ru–N amido/amine metal–ligand cooperation.

Molecularly Defined Manganese Catalyst for Low-Temperature Hydrogenation of Carbon Monoxide to Methanol

Methanol synthesis from syngas (CO/H₂ mixtures) is one of the largest manmade chemical processes with annual production reaching 100 million tons. The current industrial method proceeds at high temperatures (200-300 °C) and pressures (50-100 atm) using a copper-zinc-based heterogeneous catalyst. In contrast, here, we report a molecularly defined manganese catalyst that allows for low-temperature/low-pressure (120-150 °C, 50 bar) carbon monoxide hydrogenation to methanol. This new approach was evaluated and optimized by quantum mechanical simulations virtual high-throughput screenings. Crucial for this achievement is the use of amine-based promoters, which capture carbon monoxide to give formamide intermediates, which then undergo manganese-catalyzed hydrogenolysis, regenerating the promoter. Following this conceptually new approach, high selectivity toward methanol and catalyst turnover numbers (up to 3170) was achieved. The proposed general catalytic cycle for methanol synthesis is supported by model studies and detailed spectroscopic investigations.