energyfromrenewableresourcestomitigatethepossibledevastatingeffectof ... Thebookisintendedtobeacomprehensiveandcutting-edgecollectionoffundamentaland ... andnovelcatalyticprocessesastransitiontocleanenergyage.
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It is an honor to present this IGI Global Series book, Advanced Solid Catalysts for Renewable Energy Production. This book is mainly inspired on the increasing demand of our society to consume green energy from renewable resources to mitigate the possible devastating effect of greenhouse gas emissions could have over humankind existence and also on the principal role that catalysis would play to fulfill this demand. The book is intended to be a comprehensive and cutting-edge collection of fundamental and applied studies enlightening specific aspects of the utilization of renewable resources, catalyst synthesis and novel catalytic processes as transition to clean energy age.
SETTING THE SCENE Heterogeneous catalysis plays a major role in environmental technology, energy conversion–production and chemical manufacture. It is estimated that about 80% of the industrial catalysis involves solid catalysts and the remaining corresponds to homogeneous catalysts (17%) and biocatalysts (3%) (de Jong, 2009). This reflects the importance of research, development and manufacture of (solid) catalysts in this growing industry. The global heterogeneous catalyst demand in 2015 was approximately US$20 billion and the main application of solid catalysts was for mobile emission control (Bravo-Suárez, Chaudhari, & Subramaniam, 2013). It is expected that the average annual growth rate (AAGR) for environmental solid catalyst substantially increase relative to the refining and petrochemical sectors in the coming decades. In fact, the recent progresses for producing renewable energy (bio-gases/hydrogen, bio-fuels, fuel cells, solar energy, etc.) are mainly based on catalytic processes (e.g., heterogeneous/homogeneous catalysis, electrocatalysis, photocatalysis and bio-catalysis) using not only innovative synthesis method to produce (nano)materials/catalysts but also computational chemistry and operando spectroscopy to develop advanced catalysts by design rather than trial and error. The solid catalysts for commercial applications need to be an active, selective and stable material for a determined catalytic process. The best synthesis method must be able to produce a catalytic material with appropriate textural properties (i.e., sufficiently high surface area and uniform pore distribution), suitable mechanical strength and high attrition resistance. In Figure 1 is given a trilobe-shaped catalyst and its cross section together a mesoscopic-sized cartoon to illustrate the multiple scales present in solid catalysts. It is also shown the chemical and physical steps involved in the heterogeneous catalysis: Interphase and intraparticle diffusion steps for the reactants (steps 1 and 2) and the product (steps 7 and 6), respectively. Furthermore, the chemical adsorption of the reactants onto the active sites (step 3), the chemical reaction on the surface of the catalyst (step 4) and desorption of the product from the surface
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of the catalyst surface (step 5) are also illustrated (Kapteijn, Martin, & Moulijn, 1993).The catalytically active sites (1-10 nm) inside the pores of support particles (μm) and the molecular transport occur at the mesoscopic length scale, whilst the chemical adsorption and reactions take place in the (sub)nanometer level. The pressure drops, mechanical strength and attrition resistance are associated to size and shape of the catalyst body, the scale is between centimetre (laboratory reactor) and meter (industrial reactor) (i.e., macroscopic length scale) (Chorkendorff & Niemantsverdriet, 2003).
ENERGY PRODUCTION ON SOLID CATALYSTS Fossil Fuels Nowadays, the availability of energy comes mainly from crude oils, and heterogeneous catalysis plays a major role not only for the refining process to obtain the fossil fuels but also for mitigating the environmental impact of the greenhouse gases emissions produced upon the energy conversion to usable forms. The most important processes and the catalysts normally employed in the crude oil refining to make liquid transportation fuels are summarized in Table 1 (Chorkendorff & Niemantsverdriet, 2003). Figure 1. Different length scales of metal catalyst structure and sequence of physical and chemical reaction steps involved in the heterogeneous catalysis. (1) Diffusion of reactants from the gas or liquid phase to the external surface of the porous catalyst particle; (2) intraparticle diffusion of the reactants through the catalyst pores to the internal active sites; (3) adsorption of reactants onto the metal sites; (4) chemical reaction on the catalyst surface; (5) desorption of products from the surface of the metal catalyst; (6) intraparticle diffusion of the products from the catalyst internal pores to the external surface of the catalyst particle; (7) diffusion of the products from the external surface of the particle to the bulk of the fluid.
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Alumina-supported Co- and/or Ni-promoted Mo and W catalysts and zeolite-type catalysts play an important role in various refinery processes employed for the production of fuels. Metal catalysts are important for enhancing the octane number of gasoline and generate hydrogen by reforming of hydrocarbons, while acid supports and zeolite catalysts are employed for hydrocracking (or hydroprocessing) and catalytic cracking, respectively. The valorization of unconventional resources such as extra heavy crudes, shale gas and even coal has required the development of advanced catalysts. The production of medium distillates from extra heavy oils required the development of nanosized MoS2 catalyst (Bellussi, et al., 2013), whilst the upgrading of shale gas and coal to liquid fuels is carried out on advanced Fisher-Tröpsch Co catalyst (Zhang, et al., 2014).
Renewable Fuels The main driving force of energy-related catalysis is currently the transition from fossil fuel-based energy to a renewable-centered energy system. A perspective of this process is a transient biomass-based refinery (i.e., bio-refinery) evolving toward a permanent solar energy-fueled society or fossil-fuel-free energy economy (Abate, Centi, & Perathoner, 2015). The utilization of biomass to produce bio-fuels (and/or chemicals) through new, or even conventional, catalytic processes would need the development of multifunctional advanced catalysts, whose physical and chemical properties such as particle size, shape, porosity and the crystalline phases need to be carefully controlled. This is particularly relevant for lignin depolymerization to bio-fuels, since it would require tandem catalytic reactions for deoxygenation of C-O-C and C-O-H bonds and ring-opening reactions, followed by C-C bond formation to produce branched alkanes into the range of biofuels (Xu, et. al., 2014; Rinaldi, et al., 2016). Another example is the synthesis of biodiesel that is usually carried out in batch reactor with liquid catalysts. An alternative process is the production of biodiesel through heterogeneous catalysis in continuous flow reactors to avoid the separation issues of the catalyst and enhance productivity by process intensification. However, Table 1. Major refinery processes for fuel production using solid catalysts
Adapted from Chorkendorff & Niemantsverdriet, 2003.
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the difficulties to design acid/base solid catalysts with appropriate pore network and surface properties represent major challenges in this area (Lee, et al., 2014; Lima, et al., 2016). The natural gas, whose main component is methane, as a source of clean fossil energy alongside the large reserves of shale gas, coalbed methane and methane hydrate, including biomethane production, have driven the development of new catalytic routes that valorize methane through non-oxidative processing that decrease anthropogenic CO2 emissions (Schwach, Pan, & Bao, 2017). This represents an important step toward a renewable-based energy system supported by the catalytic conversion of biomethane to liquid fuels (or chemicals). A schematic representation of the direct and indirect routes to convert methane to fuels and chemicals of interest for the fuel industry (e.g., aromatics and olefins) is given in Figure 2. It is worth remarking that methane is also commercially converted to additional value-added chemicals such as halocarbons, acetylene and carbon disulfide, whilst the direct conversion of methane to oxygen-containing compounds (i.e., methanol, formaldehyde and acetic acid), light hydrocarbon (C2 and C3), hydrogen, olefins and aromatics are currently under the development stage (Abate, Centi, & Perathoner, 2015; Holmen, 2009; Alvarez-Galvan, et al., 2011). The indirect routes are commercially well-established technologies that involve the steam methane reforming (SMR) to syngas (CO+H2) and subsequently its conversion to liquid fuels via Fischer-Tropsch synthesis (FTS) or oxygenated fuels (methanol/dimethyl ether) or even hydrocarbon fuel (i.e., gasoline), see Figure 2. Hydrogen production is enhanced through the water-gas shift reaction (WGSR) and then it is used for various processes (i.e., ammonia synthesis, methanol synthesis, hydroprocessing). The indirect routes also produce large amount of anthropogenic CO2 emissions because of their oxidative character. The capture and storage of CO2 (Leung, Caramanna, & Maroto-Valer, 2014) and/or its catalytic conversion to valuable products (France, et al., 2015) would play a major role to mitigate the greenhouse gas emission from these catalytic pathways. On the other hand, the direct routes generate power by the methane combustion (MC) reaction and unsaturated chemicals through the conversion of methane to olefins, aromatics and hydrogen (MTOAH). Hydrogen can be also produced by the direct conversion of methane to hydrogen (MTH). These routes, with the exception of methane combustion (MC), are apFigure 2. Valorisation of methane to fuels and unsaturated chemicals through direct and indirect routes: MTOAH (methane to olefins, aromatics and hydrogen), MC (methane combustion), MTH (methane to hydrogen), SMR (steam methane reforming), FTS (Fischer-Tropsch synthesis), STO (syngas to oxygencontaining compounds), WGSR (water gas-shift reaction), MTG (methanol to gasoline)
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parently eco-friendly processes due to non-oxidative character of the reaction atmosphere. Nevertheless, Life-Cycle Analysis of greenhouse gas emissions is required, since the MTH and MTOAH are carried out at severe condition of temperatures (above 1000 °C). It should be mentioned that the utilization of carbon-free fuel (i.e., hydrogen) from the catalytic decomposition of hydrocarbons to high-purity hydrogen and residual carbon as by-product (GonzalezCortes, et al., 2016; Jie, et al., 2017) could be possible to mitigate the CO2 emission not only from the main sources of hydrogen production reactions (i.e., steam methane reforming and coal gasification) but also from the energy-generation process (i.e., fuel combustion) of mobile sources. Another convenient alternative for hydrogen production is the utilization of renewable biomass resources through the catalytic reforming of oxygenated compounds in the context of fuel cell vehicles and electricity generation (Li, Li, & Gong, 2016).
SYNTHESIS OF ADVANCED SOLID CATALYSTS The traditional synthesis of solid catalysts can be divided into two categories: Methods in which the active phase is generated as a solid phase by either precipitation or a decomposition reaction and methods in which the active phase is introduced and fixed onto a porous solid by a process intrinsically dependent on the surface of the support (Scharz, et al., 1995). The former method is mainly used to synthesize bulk catalysts and the latter produces supported catalysts that contain an active phase typically dispersed on catalyst carrier (or support) and a possible promoter. In the context of supported catalysts, two main approaches can be distinguished. The first consists of depositing metal precursor(s) onto the support (e.g., impregnation methods) to produce, after a calcination step, nano-crystalline metal oxides, which can then be transformed to metal, metal carbide (nitride, phosphide, sulphide) after a catalyst activation step. The second approach is based on transforming the metal precursor into the required active component such as metal, metal oxide (or metal hydroxide) or metal sulfide during the deposition process (e.g., deposition-precipitation methods). Other methods to synthesize advanced solid catalysts such as co-precipitation, sol-gel synthesis, flame hydrolysis, electrostatic adsorption, ion exchange (Heinrichs, et al., 2007; de Jong, 2009; Hagen, 2015) and combustion synthesis (Gonzalez-Cortes, & Imbert, 2013) could be also classified into these categories. New approaches have been developed to synthesize advanced solid catalysts. Particularly impressive is the synthesis of single-atom catalysts that involves the atomic dispersion of metal on an appropriate support to prevent atom aggregation and leads to a narrow distribution of single-atom sizes (Yang, et al., 2013). This approach has been successfully applied to synthesise advanced catalyst for CO oxidation (Qiao, et al., 2011), hydrogen production from water-gas shift reaction (Lin, et al., 2013) and even nonoxidative methane conversion to olefins, aromatics and hydrogen (Guo, et al., 2014). This last reaction was carried out on a single-site iron nano-stabilised on a matrix of silica that prevents coke deposition due to the absence of adjacent active sites as illustrated in Figure 3a. Another approach is the synthesis of multifunctional catalyst operating in cascade reactions, in which each subsequent reaction catalyzed by a particular active phase occur as a consequence of the chemical functionality generated in the previous chemical reaction (Climent, et. al., 2014). An interesting application of this approach is the direct production of liquid fuels from CO2 hydrogenation on multifunctional Na–Fe3O4/HZSM-5 catalyst though three consecutive processes: (1) The water gas-shift reaction (WGSR), (2) Fischer-Tropsch synthesis (FTS) and (3) the oligomerization, isomerization and aromatization (OIA) reactions; see Figure 3b. The xviii
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Figure 3. (a) Nonoxidative conversion of methane to olefins, aromatics and hydrogen on single iron site of 0.5% Fe @ SiO2, adapted from Guo et al. (2014). (b) CO2 Hydrogenation to gasoline fuel through a Na–Fe3O4/Zeolite multifunctional catalyst. The cascade process occurs in three consecutive processes: Initially, a partial reduction of CO2 to CO via water gas-shift reaction (WGSR), then, the hydrogenation of CO to α-olefins trough Fischer-Tropsch synthesis (FTS) and finally the oligomerization, isomerization and aromatization (OIA) reactions on acid sites to produce gasoline-range hydrocarbons. Adapted from Wei et al. (2017).
CO2 was directly converted to gasoline fuel (C5–C11) with hydrocarbon selectivity up to 78% at a CO2 conversion of 22% under industrial relevant conditions (Wei, et al., 2017).
ABOUT THIS BOOK The replacement of non-renewable crude oils by renewable resources has been addressed in recent years, particularly in developed countries. Its main driving force has been the increasing demand and limited reserves of fossil fuels, the concerns related to the environmental impact, greenhouse gas emissions and also the need of securing energy supplies. The book aims at highlighting the innovative approaches to produce green energy through heterogeneous catalysis, electrocatalysis and photocatalysis mainly from renewable resources. The book was divided in four sections to highlight not only the utilization of renewable resources and solid catalysts to produce biofuels, but also the key role of catalyst synthesis, photocatalysis and electrocatalysis to disrupt the traditional way to produce energy.
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The first section called “Production of Renewable and Non-Renewable Fuels” starts describing the advances in theoretical studies of solid catalysts for renewable energy production (Chapter 1), particularly for biodiesel production, reforming of methane and hydrogen storage. The pretreatment of lignocellulosic biomass is an important step for the production of biofuels. This issue is addressed in Chapter 2, in which chemical, physical and biological pretreatments are described. The authors conclude that a combination of two or more pretreatment methods may be necessary to enhance the overall performance of lignocellulosic biomass to biofuels. The production of ultraclean transportation fuels from the deep hydrotreating of renewable and non-renewable feedstocks is widely described in Chapter 3. The authors review the current status of the hydrotreating process and the challenges for the valorization of non-renewable and renewable feedstocks with high content of heteroatoms, metals and unsaturated poly-aggregate compounds (asphaltenes and lignin). It is also outlined the importance to develop advanced catalysts based on abundant metals, rather than precious metals, and multifunctional properties with sufficient activity and selectivity in hydrodeoxygenation of bio-oils. The second section called “Catalysis and Biodiesel Production” initially describes the production of biodiesel through non-conventional feedstock and technologies in Chapter 4. The authors address the utilization of non-traditional feedstocks (i.e., waste cooking oil, non-edible oils, animal fats and algae), highlight the advantages of developing novel processes based on microwaves and ultrasound and the importance of byproduct recovery to enhance the energy footprint and economics of current biodiesel production. Chapter 5 mainly describes the utilization of mesoporous silicas as basic heterogeneous catalysts for biodiesel production and also compares them with conventional homogenous catalysts. The synthesis, characterization and performance in the transesterification reaction for as-synthesized M41S silicas with their pores occluded with organic cations, and functionalized silicas, with accessible pores were also described. An overview of the advantages and disadvantages of homogeneous and heterogeneous catalysts for biodiesel production is given in Chapter 6. This chapter highlights the leaching and deactivation of the solid catalysts during the transesterification reaction as major factors that challenge the application of heterogeneous catalysts in biodiesel production. A third section called “Synthesis of Solid Catalysts” starts describing in Chapter 7 the fundaments of ultrasound and its application in the synthesis of nanostructured materials and catalysts through sonochemical and ultrasonic spray pyrolysis routes. The authors remark that the particular physico-chemical properties that ultrasound-assisted synthesis gives to the synthesized materials, allow them to be highly efficient in various renewable energy applications. The precipitation method is an important approach to produce solid catalysts as described Chapter 8. This method is used to synthesize Zr-Fe catalysts able to maximize the hydrogen production in the reforming of natural gas process through the water-gas shift reaction. This chapter highlights the effect of zirconia on the textural and catalytic properties of magnetite and its potential to make the iron catalyst more resistant to sintering and over-reduction during industrial operation conditions. The solution combustion synthesis (SCS) of LaSrNiAl perovskite-type structures and their performances in the dry reforming of methane are discussed in Chapter 9. The authors analyze the effect of the combustion fuel (i.e., glycine and sucrose), ignition or heating source (i.e., furnace and microwave radiation) and nickel content on the dry reforming of methane to hydrogen. They remark the fact that SCS approach can tune the dry reforming of methane and the reverse water-gas shift reactions by varying the combustion fuels.
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The last section called “Photocatalysis and Electrocatalysis” describes in Chapter 10 the photocatalytic reduction of carbon dioxide on titania and the influence of various efficient co-catalysts and dopants on TiO2-based photocatalysts, which has been described as the most widely used photocatalysts. The authors state that low yield of product, the utilization of solar light, instead of UV light; alongside the mechanism of the CO2 photoreduction, among others areas; deserve further investigation to efficiently photoreduce carbon dioxide to high value chemicals. The synthesis and utilization of electrocatalysts for green energy production is comprehensively discussed in Chapter 11. The authors integrate the source of energy, capture, conversion, storage, supply and applications of energy under the umbrella of “Electrochemistry in Energy” and remark the key role of the electrocatalysis (Catalysis-Electrochemistry symbiosis) on developing clean and green energy accessible to the increasing population worldwide. In Chapter 12, the Voltammetry of Immobilized Micro Particles (VIMP) and the Electrochemical Scanning Microscopy (SECM) are described as important tools to characterise solid catalysts with potential for renewable energy production. The authors state that these two techniques can be used to study compact solid surfaces or powder with catalytic and electrocatalytic activities and opens a window of opportunities to gain further insight into the development of advanced solid catalysts. Sergio Gonzalez-Cortes Oxford University, UK Oxford, September 15, 2017
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