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## Über dieses Buch

This book provides an analysis of the reaction mechanisms relevant to a number of processes in which CO2 is converted into valuable products. Several different processes are considered that convert CO2 either in specialty chemicals or in bulk products or fuels. For each reaction, the mechanism is discussed and the assessed steps besides the dark sites of the reaction pathway are highlighted. From the insertion of CO2 into E-X bonds to the reduction of CO2 to CO or other C1 molecules or else to C2 or Cn molecules, the reactions are analysed in order to highlight the known and obscure reaction steps. Besides well known reaction mechanisms and energy profiles, several lesser known situations are discussed. Advancing knowledge of the latter would help to develop efficient routes for the conversion of CO2 into valuable products useful either in the chemical or in the energy industry. The content of this book is quite different from other books reporting the use of CO2. On account of its clear presentation, “Reaction Mechanisms in Carbon Dioxide Conversion” targets in particular researchers, teachers and PhD students.

## Inhaltsverzeichnis

### Chapter 1. The Carbon Dioxide Molecule

Abstract
The basic aspects of the reactivity of carbon dioxide (CO2) are featured in this chapter and related to the electronic structure of the molecule. The electronic properties of neutral CO2 are compared with those of the radical ions $${\mathrm{CO}}_2^{-}$$ and $${\mathrm{CO}}_2^{+}$$. The potential of a few spectroscopic techniques (infrared, ultraviolet–visible, nuclear magnetic resonance) in the characterization of CO2 states is also highlighted.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 2. CO2 Coordination to Metal Centres: Modes of Bonding and Reactivity

Abstract
The modes of coordination of carbon dioxide (CO2) to metal centres are presented in this chapter. The coordination at both room temperature and low temperature in gas matrices is discussed with the reactivity of the coordinated cumulene. X-ray diffraction structural data are presented and discussed together with spectroscopic properties of the complexes.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 3. Interaction of CO2 with Electron-Rich Moieties

Abstract
In this chapter the direct, non-metal-mediated interaction of carbon dioxide with electron-rich elemental or molecular species is discussed. Anionic species such as H, OH, and R3C and covalent species such as amines have been taken into consideration, in view of their relevance to systems of potential or real industrial interest.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 4. Insertion of CO2 into E–X Bonds

Abstract
This chapter deals with the “insertion” reactions of carbon dioxide (CO2) into E–X bonds, where E and X represent several different (sets of) atoms, such as M–H, M–OH, M–C, M–OR, M–O2, M–N, M–P, C–C, C–N, Si–H, and M–M (M = metal). Such reactions are relevant to catalysis for the formation of new bonds in which CO2 may be implied (C–C bonds or C–E bonds) and thus to the conversion of CO2 into added-value chemicals. The insertion product can be thermodynamically and kinetically stable or labile, offering in the latter case the opportunity of a catalytic path.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 5. Interaction of CO2 with C–C Multiple Bonds

Reactions with Olefins, Cumulated and Conjugated Dienes, and Alkynes
Abstract
In the presence of low-valent transition metal centers, CO2 can react with unsaturated organic substrates, such as alkenes, alkynes, and conjugated and cumulated dienes, to give carboxylated products. This chapter focuses on the main mechanistic features of these carboxylation processes. A key step of these transformations is the metal-promoted oxidative coupling reaction between CO2 and the unsaturated substrate. The mechanistic details of this step are highlighted and the relevance and role of CO2 coordination to metal center in these reactions is argued.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 6. Reaction Mechanisms in the Direct Carboxylation of Alcohols, Polyols, Cyclic Ethers, and Cyclic Amines to Afford Monomeric Compounds and Polymeric Materials

Abstract
This chapter deals with the utilization of CO2 in the carboxylation of alcohols, diols, polyols, and epoxides to create a variety of compounds such as linear carbonates, cyclic monomeric carbonates, and polycarbonates. Homogeneous, heterogenized, and heterogeneous catalysts are described. The problem of “water elimination” is considered and routes for water-trapping discussed. DFT calculations used to support the reaction mechanism are presented with the identified transition states relevant to various mechanistic scenarios.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 7. Carbon Dioxide Conversion in High Temperature Reactions

Abstract
This chapter deals with high temperature reactions in which carbon dioxide (CO2) is used either as dehydrogenation agent (“soft oxidant”) or hydrogen user (“reduction to other C1 molecules”). The first part of the chapter covers reactions such as the oxidative coupling of methane, the dehydrogenation of alkanes to olefins, the dehydrogenation of ethylbenzene to styrene, and, finally, CO2 reforming of methane.
The second part covers the reaction of CO2 with hydrogen in processes such as the reverse water gas shift reaction and the synthesis of both methanol and dimethyl ether.
All processes discussed here already have an industrial exploitation or are related to processes on stream, so that a large set of data is available in the literature. This chapter illustrates the building-up of knowledge for demonstrating the reaction mechanism in processes in which very often several conversion routes of CO2 coexist and the role of support and catalysts is essential for addressing the reaction in one or another direction.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 8. One- and Multi-electron Pathways for the Reduction of CO2 into C1 and C1+ Energy-Richer Molecules: Some Thermodynamic and Kinetic Facts

Abstract
This chapter deals with the mechanism of reduction of “free” and coordinated CO2 by electron transfer. One-e and multi-e transfer pathways are compared energetically and their role in the conversion of CO2 into higher energy C1 or C1+-species is highlighted. The state of the knowledge is presented through the analysis of reference cases.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 9. Enzymatic Conversion of CO2 (Carboxylation Reactions and Reduction to Energy-Rich C1 Molecules)

Abstract
This chapter deals with the enzymatic conversion of CO2. It covers the two aspects of the fixation of the entire CO2 molecule into substrates (carboxylation) and the reduction of CO2 to other C1 (or C2) energy-richer molecules. The known mechanisms are discussed and barriers to exploitation at the industrial level highlighted.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Chapter 10. Thermodynamics and Applications of CO2 Hydrates

Abstract
Gas hydrates are clathrate solid crystalline compounds consisting of a lattice formed by water molecules and entrapped gas molecules inside. They are stable under high pressure and low temperature. CO2 hydrates, specifically, are composed of CO2 as the guest molecule and water as the host molecule. CO2 hydrates have a number of applications including CO2 capture, cold storage, CO2 sequestration, and, lately, the direct displacement of methane hydrates with CO2 to simultaneously produce methane and sequester CO2. This chapter provides a comprehensive overview of the fundamentals of CO2 hydrates. The first section gives a general introduction and some basic concepts of gas hydrates. Section 10.2 shifts the focus to the microscopic perspective, looking into how gas hydrates form, the three structures of gas hydrates, and the characteristics of CO2 hydrates. From there onward, the text focuses specifically on CO2 hydrates. The physical properties of CO2 hydrates are considered in Sect. 10.3. Section 10.4 deals with the phase equilibrium of CO2 hydrate. Experimental methods and the phase diagram are shown in this section. The last section covers the applications of CO2 hydrates, including the formation and dissociation of CO2 hydrates, ocean sequestration, and the CH4 replacement in hydrates by CO2, which is an attractive potential method to produce natural gas.
Michele Aresta, Angela Dibenedetto, Eugenio Quaranta

### Backmatter

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