Suib New and Future Developments in Catalysis

Activation of Carbon Dioxide
1. Auflage 2013
ISBN: 978-0-444-53883-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 658 Seiten

New and Future Developments in Catalysis is a package of books that compile the latest ideas concerning alternate and renewable energy sources and the role that catalysis plays in converting new renewable feedstock into biofuels and biochemicals. Both homogeneous and heterogeneous catalysts and catalytic processes will be discussed in a unified and comprehensive approach. There will be extensive cross-referencing within all volumes.This volume presents a complete picture of all carbon dioxide (CO2) sources, outlines the environmental concerns regarding CO2, and critically reviews all current CO2 activation processes. Furthermore, the volume discusses all future developments and gives a critical economic analysis of the various processes. - Offers in-depth coverage of all catalytic topics of current interest and outlines future challenges and research areas - A clear and visual description of all parameters and conditions, enabling the reader to draw conclusions for a particular case - Outlines the catalytic processes applicable to energy generation and design of green processes

Autoren/Hrsg.

Suib, Steven L

Leseproben

Chapter 1

Catalytic Processes for Activation of CO2

Narcís Homsa,b, Jamil Toyirc and Pilar Ramírez de la Piscinaa,    aDepartament de Química Inorgànica and Institut de Nanociència i Nanotecnologia, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain, bCatalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Barcelona, Spain, cUniversité Sidi Mohamed Ben Abdellah, Faculté Polydisciplinaire, B.P 1223, Taza, Morocco

1.1 Introduction

CO2 can be considered a carbon source rather than just a greenhouse gas (GHG) that needs to be mitigated. As a result of tough policies on GHG emissions, CO2 has turned out to be a strategic carbon source, available as free or even with a financial return if adequate technology is applied for CO2 capture and recycling. From this perspective, an attractive opportunity is emerging for a new CO2-based economy. The chemical recycling of CO2 and its reuse in the production of fuels or other chemicals is attractive from both an environmental perspective and in terms of fossil source independency. CO2 is released as a by-product in the chemical and petrochemical industry and in fermentation processes and currently, new technological possibilities based on membranes and absorbents are opening up for its separation and capture. High purity CO2 is available from such sources; indeed, some such emissions are already used in the production of chemicals such as CO2 from the ammonia industry which is fed to urea plants or CO2 from fermentation which is recycled into the beverage industry after appropriate cleaning. Moreover, large-scale emissions of CO2 come from power plants fueled by natural gas and from the cement industry; in these cases undesired impurities must be taken into account before the CO2 becomes useful. However, these sources can be considered an opportunity for large-scale recycling of CO2. In what follows we will consider different options for the reuse of CO2 by means of chemical recycling.

We must bear in mind that CO2 is classified as a stable, almost inert molecule. To activateCO2, it is necessary to overcome its considerable Gibbs energy of formation: . Nevertheless, CO2 use is thermodynamically possible via numerous reactions involving its reduction or its incorporation into other compounds.

The reduction of CO2 by the cleavage of the CO bond(s) and the formation of new C—H or C—C bond(s) can be accomplished using a reductant. In this case, the action of an appropriate catalyst can convert CO2 into a potential energy vector. This recycling pathway requires a large input of energy and usually H2 as the reductant, and leads to fuels such as methane and methanol. In this context, two aspects must be taken into account: the production of H2 and the energy source. If the energy comes from renewable sources and the H2 does not come from fossil fuels, does the overall process merit further interest. Nowadays, we can engage optimized electrolysis of water to supply the H2, thus avoiding the fossil fuel starting material. The use of non-constant wind energy or electrical energy produced at night can be used to obtain H2, thereby matching the necessity to store such excess energy. In fact, storage of H2 itself is necessary; one nice way to accomplish this is to incorporate the hydrogen into a liquid fuel. Then, since the products obtained from the catalytic hydrogenation of CO2 can be directly used as fuels, overall this process could be seen as a means of electrical energy storage and transport. However, the recycling of CO2 to fuels implies its immediate release into the environment.

The incorporation of CO2 into chemicals as a C1 building block is the other approach to its reuse. In this case, lower energy input is required since the whole CO2 molecule is used without the need to cleave the CO bond(s). However, to accomplish a desired reaction, the action of a catalyst is also usually necessary. In this CO2 recycling pathway, different parameters need to be taken into account to determine the viability of a proposed process. The amount of CO2 fixed and the overall energy balance of the complete chemical route, together with the value of the product produced, could indicate how feasible this process is. Large-scale production of highly valued polymers could be performed using recycled CO2, thus also diminishing the use of fossil resources. In this way, if highly stable long-life materials are produced, the CO2 storage could be very long term and therefore the release of CO2 into the environment could be very slow.

In recent years, extensive research has been carried out into these aspects of CO2 reuse and numerous publications have appeared [1–12]. This chapter therefore presents recent advances in the conversion of CO2 into fuels and desirable chemical products by using heterogeneously catalyzed processes, which are summarized in Figure 1.1.

Figure 1.1 Catalytic routes for CO2 activation in heterogeneous phase leading to fuels and chemicals.

The chapter is divided in different sections. After the Introduction, the second section is dedicated to the reactions of CO2 with hydrogen; particular attention is paid to large-scale processes that produce synfuels: methanol, dimethyl ether (DME), methane and higher hydrocarbons and alcohols. Then, CO2-reforming processes of methane and alcohols, and dehydrogenation processes using CO2 are presented in Section 1.3. Catalytic processes based on CO2 insertion into organic compounds to form carbonates or carboxylic acids are discussed in Section 1.4. In the last part, future trends in the catalytic conversion of CO2 are presented with a particular focus on processes that aim to produce strategic chemicals or fuels using renewable energy sources.

1.2 Reactions of CO2 with hydrogen

1.2.1 Hydrogenation of CO2 to Methanol

Methanol is extensively used as a starting material for the production of several bulk chemicals and liquid fuels [2]. Furthermore, the use of methanol as a medium for the storage and transportation of hydrogen and as fuel in direct methanol fuel cells (DMFC) could increase demand for it in a future.

Nowadays, methanol production uses syngas (a CO/CO2/H2 mixture) produced from fossil sources and Cu/ZnO-based catalysts. However, methanol production from CO2 and hydrogen (Eq. (1.1)) is now considered a serious alternative which could contribute to mitigate CO2. In a recent book, Nobel Laureate Olah [6] set out the concept of the “methanol economy” based on the production of methanol by chemical recycling of CO2.

(1.1)

In fact, initial attempts to produce methanol in industry were performed in the 1920s using CO2 from fermentation processes. The catalysts used at that time were similar to those currently commercialized for the production of methanol from syngas [6]. However, to match the performances of the process using syngas, the process of methanol synthesis from CO2 and H2 requires a highly effective catalyst in terms of activity, stability, and selectivity. Achieving high selectivity is difficult because the formation of products other than methanol, such as hydrocarbons and higher alcohols, is thermodynamically favored. Moreover, the reverse water-gas shift reaction (RWGS, Eq. (1.2)) may be kinetically favored under methanol synthesis conditions.

(1.2)

The most important research into methanol production from CO2 and H2 has been reviewed in a number of recent publications [2,11–13]. Although a wide range of metal-based catalysts have been studied for methanol synthesis, particular attention has been paid to copper-based catalysts [11]. Table 1.1 compiles several representative reported catalysts.

Table 1.1

Catalytic Behavior of Various Catalysts for Methanol Synthesis from CO2 and H2

aPrepared by coprecipitation and tested at bench scale for a long-term operation.

bPrepared by co-impregnation, short-term operation.

cPrepared by incipient wetness, short-term operation.

dPrepared by leaching, initial activity.

In particular, Cu/ZnO-based catalysts are among the most useful systems for the catalytic hydrogenation of CO2 to methanol [13–17]. The addition of other components such as ZrO2, Al2O3, Ga2O3, and SiO2 has been extensively studied [16,18]. Saito et al. developed high performance multicomponent catalysts based on Cu/ZnO/ZrO2/Al2O3 and prepared via co-precipitation using metal nitrates and sodium carbonate [16]. Several functions have been proposed for ZnO in these co-precipitated Cu/ZnO-based catalysts: (i) ZnO may favor the formation of appropriate precursors during preparation of the catalyst, which leads to higher dispersion; (ii) in the presence of Al2O3, ZnO may show refractory behavior and attenuate the unavoidable agglomeration of Cu particles which takes place during a long-term operation; (iii) ZnO may avoid the poisoning of Cu particles by feed gas impurities such as sulfides and chlorides; and (iv) ZnO, as a basic oxide, may neutralize the acidity of Al2O3, thus preventing the transformation of methanol to DME. However, in such catalysts, the effect of the texture of ZnO on the interaction with Cu and on the dispersion of Cu particles is difficult to determine. The activity of the multicomponent catalyst is proportional to the surface area of copper. The...