New Directions in Sustainable Catalysis by Metal Complexes

Periodic Reporting for period 4 - SUSCAT (New Directions in Sustainable Catalysis by Metal Complexes)

Periodo di rendicontazione: 2021-01-01 al 2022-06-30

Currently, many of the processes employed by the chemical, pharmaceutical and agrochemicals industries produce waste. This waste has to be treated in order not to harm the environment, posing a substantial economic burden and depletion of our resources. Moreover, most processes are based on non-sustainable chemicals, such as fossil-derived compounds.
In addition, industrial and laboratory catalytic processes are often based on compounds of noble metals. Noble metals are expensive, rare and their compounds are often toxic. Hence there is a strong need to develop catalytic processes based on earth-abundant metals

Treatment of plastic waste, such as nylon waste, is also an important issue.

Regarding energy-related issues, hydrogen gas, which has the highest gravimetric energy capacity, is regarded as an ideal alternative for overcoming the drawbacks of fossil fuel technology. However, its direct utilization is challenging, due to the low volumetric energy density of hydrogen gas, requiring its use under high pressure or cryogenically, which consume part of the energy potential of hydrogen, and may pose safety issues. An attractive approach to solve this challenge is the developments of liquid organic hydrogen carriers, capable of generation of hydrogen gas when needed by use of a catalyst, and can be readily regenerated using hydrogen under mild conditions. The ability to store and transport hydrogen within a Liquid Organic Hydrogen Carrier (LOHC) at standard temperatures and pressures provides many advantages over current hydrogen storage methods. Most importantly, the need for large storage tanks is eliminated, major changes in the fuelling infrastructure are avoided and the risk for explosion is minimized.
In another direction, Formic acid (FA) is a promising hydrogen carrier that can play a role in the overall implementation of a hydrogen economy. However it is important to generate H2 gas from neat FA without any solvent and/or additive, for which existing systems are scarce.

In view of the issues mentioned above there is an urgent need for the design and development of new, sustainable and green catalytic reactions, which do not generate waste, use sustainable starting materials, proceed under mild conditions, and do not harm the environment. Moreover, replacing the direct use of hydrogen gas by organic liquids capable of chemically generating hydrogen gas when needed, and being safely regenerated using hydrogen, is highly desirable.

We have designed and developed new, sustainable and green catalytic reactions, which do not generate waste, use sustainable starting materials such as alcohols, proceed under mild conditions, and do not harm the environment. These processes are based on novel pincer-type metal complexes that we have developed. Several of these processes generate hydrogen gas, valuable by itself, or consume it, leading to unprecedented, industrially relevant methodologies in chemical synthesis. Our research has had a strong impact on the rapid development of (a) sustainable catalysis by pincer complexes of earth-abundant metals (b) use of water as a sustainable oxidant of organic compounds, liberating H2, and other novel sustainable reactions. In sustainable energy directions, we have developed novel Liquid Organic Hydrogen Carriers (LOHC), an efficient process for methanol reforming , and a process for dehydrogenation of neat formic acid.

Specific examples, catalyzed by novel pincer complexes, include:

1. New environmentally benign, sustainable synthetic reactions catalyzed by pincer complexes of earth-abundant metals, capable of metal-ligand cooperation:
(a) Manganese is the second most abundant metal on earth’s crust. Examples of novel reactions catalyzed by Mn complexes: (a) waste-free synthesis of amides by coupling of amines with alcohols or esters, or by reaction of benzyl alcohols with ammonia, evolving hydrogen gas. Noteworthy, amide synthesis is one of the most frequently used synthetic operations, but it generatess waste. (b) Mild hydrogenation of the CO2-derived organic carbonates to methanol and alcohols, providing a green two-step process to the generation of methanol fuel from CO2, using a base-metal (rather than ruthenium) (c) C–C bond-formation via α-alkylation of ketones, amides, and esters, using primary alcohols, are synthetically important reactions.
(b) iron complexes: (a) Selective hydrogenation of nitriles to imines, avoiding over hydrogenation (b) Preparation of valuable formamides by N-formylation of amines with H2 and CO2.
(c) Cobalt complexes: Synthesis of benzimidazoles by coupling of aromatic diamines and alcohols generating H2. Benzimidazole and its derivatives are important building blocks for the pharmaceutical industry, because of their prominent biological activity such as antihistaminic, anticancer activity

2. Oxidation of organic compounds using water as oxidant evolving hydrogen gas rather than using polluting, toxic oxidation reagents or potentially unsafe oxygen pressure: (a) oxidation of alkenes by water forming ketones; while the scope is currently limited, further development is warranted; Noteworthy, such a process using oxygen pressure is practiced industrially (“Wacker Process”). (b) Ester synthesis from enol ethers and water c) Selective oxidative deamination of amines by water to form ketones or carboxylic acids. (d) oxidation of the biomass derived Furfural and 5-hydroxymethyl furfural to the corresponding carboxylic acids, with various potential applications, such as synthesis of biomass-derived polyesters.

3. Other synthetically important sustainable transformations catalyzed by ruthenium pincer complexes, liberating hydrogen gas: (a) Synthesis of thioesters by coupling of thiols and alcohols. Thioesters are valuable building blocks in synthetic processes towards heterocyclic compounds and new materials. (b) synthesis of N-heteroaromatics by coupling of ammonia with diols (c) synthesis of oxalamides by coupling of ethylene glycol and amines (d) Mild synthesis of amides under ambient conditions by coupling of alcohols and amines, enabling waste-free preparation of amide-functionalized pharmaceuticals (e) synthesis of primary amides from alcohols and ammonia, and via coupling of epoxides with amines

4. Treatment of environmental hazardous agents: (a) Abatement of the potent greenhouse gas nitrous oxide (N2O) by hydrogenation or hydrosilylation, as well as by oxidation of CO (b) Methodology for Use of formamides as surrogates for highly toxic isocyanates (c) Hydrogenative depolymerization of nylons, forming aminoalcohols, providing a method for cyclical treatment of nylon waste.

5. Novel Liquid organic hydrogen carriers (LOHCs): Employing our pincer ruthenium catalysts, we have developed several new LOHCs, including a novel, efficient and reversible liquid-to-liquid organic hydrogen carrier system based on cheap, readily available and renewable ethylene glycol, enabling efficient discharge and loading of hydrogen under relatively mild conditions using a single ruthenium pincer catalyst, with a high theoretical hydrogen storage capacity of 6.5 wt% exceeding the DOE target for light vehicles of 5.5 wt%.

6. Highly efficient, additive-free, dehydrogenation of neat formic acid, capable of generation high hydrogen pressure, catalyzed by a ruthenium pincer complex.

All of the accomplishments listed above are signjficantly beyond the state of the art.

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Periodic Reporting for period 4 - SUSCAT (New Directions in Sustainable Catalysis by Metal Complexes)

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