Homogeneous (De)hydrogenative Catalysis for Circular Chemistry – Using Waste as a Resource

Abstract

Increasing production and usage of several consumer products and energy sources have resulted in the accumulation of a substantial amount of waste products that are toxic and/or difficult to biodegrade, thus creating a severe threat to our planet. With the recently advocated concepts of circular chemistry, an attractive approach to tackle the challenge of chemical waste reduction is to utilize these waste products as feedstocks for the production of useful chemicals. Catalytic (de)hydrogenation is an atom‐economic, green and sustainable approach in organic synthesis, and several new environmentally benign transformations have been reported using this strategy in the past decade, especially using well‐defined transition metal complexes as catalysts. These discoveries have demonstrated the impact and untapped potential of homogeneous (de)hydrogenative catalysis for the purpose of converting chemical wastes into useful resources. Four types of chemical waste that have been (extensively) studied in recent years for their chemical transformations using homogeneous catalytic (de)hydrogenation are CO2, N2O, plastics, and glycerol. This review article highlights how these chemical wastes can be converted to useful feedstocks using (de)hydrogenative catalysis mediated by well‐defined transition metal complexes and summarizes various types of homogeneous catalysts discovered for this purpose in recent years. Moreover, with examples of hydrogenative depolymerization of plastic waste and the production of virgin plastic via dehydrogenative pathways, we emphasize the potential applications of (de)hydrogenation reactions to facilitate closed‐loop production cycles enabling a circular economy.

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

“… 83 Application of (de)hydrogenation reactions to convert waste to useful chemical resources to enable a circular economy has been recently reviewed. 84 Cost-effective and sustainable demonstration of H 2 as a clean energy carrier to manifest the hydrogen economy faces two major challenges: (a) sustainable production of renewable H 2 and (b) efficient storage of H 2 .…”

Mechanistic Investigations of Ruthenium Catalyzed Dehydrogenative Thioester Synthesis and Thioester Hydrogenation

“… 16 Not only are these methodologies to construct and deconstruct thioesters unprecedented, but also the formation and consumption of H 2 gas is of substantial interest with respect to atom economical synthesis, hydrogen storage, and a circular economy. 17 …”

Direct synthesis of polyureas from the dehydrogenative coupling of diamines and methanol

“…13 Catalytic dehydrogenation is a green and atom-economic approach in organic synthesis, and several new green protocols based on acceptorless dehydrogenative coupling have been developed in the past. 18 For example, in relevance to the current report, the synthesis of urea derivatives has been reported by the catalytic dehydrogenative coupling of methanol and amines, independently by the groups of Hong, 19 Gunanathan, 20 Bernskoetter, 21 and Milstein. 22 The concept of acceptorless dehydrogenative coupling has also been employed for the synthesis of useful polymers such as polyesters, 23 and nylons.…”