Review Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Recent Advances in Bifunctional Catalysts for the Fischer−Tropsch Process: One-Stage Production of Liquid Hydrocarbons from Syngas Daniela Xulú Martínez-Vargas,*,† Ladislao Sandoval-Rangel,† Omar Campuzano-Calderon,† Michel Romero-Flores,† Francisco J. Lozano,† K. D. P. Nigam,†,‡ Alberto Mendoza,† and Alejandro Montesinos-Castellanos*,† †
Tecnologico de Monterrey, Escuela de Ingeniería y Ciencias, Ave. Eugenio Garza Sada 2501, Monterrey 64849, N.L., Mexico Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016, India
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‡
S Supporting Information *
ABSTRACT: Fischer−Tropsch (FT) synthesis is an important reaction for the alternative production of high-quality liquid fuels, which promote sustainable development by enabling economical decarbonization. The one-stage FT process involves both hydrocarbon growth and hydrocracking/isomerization in a single step and is more energy and cost efficient than the twostage process. Bifunctional catalysts, composed of acid and metal sites, are needed in order to achieve one-step synthesis. In this review, we discuss the state of the art concerning the use of bifunctional catalysts for the one-stage FT process, focusing on the effect of metal and acid sites on the catalytic performance. Several supports with potential utility for the one-stage FT process were analyzed, including zeolites, clays, alumina, silica, aluminosilicates, and carbons, and the advantages and disadvantages of each are evaluated.
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INTRODUCTION The use of fossil fuels in energy and chemical production is a globally significant concern.1 The widespread utilization of fossil fuels is responsible for different forms of atmospheric pollution, with greenhouse gas (GHG) emissions being among the most important, since approximately 65% of the anthropogenic emissions are attributed to fossil fuels.2 This causes global warming, leading to climate change.3,4 The transportation sector is the second most significant consumer of global energy (after only the industrial sector),4,5 using fossil fuels such as gasoline (associated with C5−C11 range), diesel (associated with C12−C20 range), liquefied petroleum gas (associated with C3−C4 range), compressed natural gas (associated with C1−C2 range), and jet fuel (associated with C9−C16 range).6,7 Emerging economies have the highest fuel demands due to the necessity of personal mobility and freight transportation. In order to overcome the problem of pollution © XXXX American Chemical Society
without affecting economic development, more efficient alternative energy sources have been proposed to replace classic fossil fuels for energy generation and transportation (the dominant source of GHG emissions).4 Although research into the application of renewable energy sources is currently being conducted, there is still much to be done, since nonrenewable energy resources still account for more than 80%8 of the global primary energy consumption, despite the fact that the raw materials are expected to run out in the near future.9,10 For example, although the cost of electric Special Issue: Biorenewable Energy and Chemicals Received: Revised: Accepted: Published: A
March July 1, July 3, July 3,
1, 2019 2019 2019 2019 DOI: 10.1021/acs.iecr.9b01141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
energy density compared with the raw material.16,29 BTL fuels can be produced by a range of technologies, which are shown and compared in Table 1. From the aforementioned BTL technologies, FT was found to be a feasible and environmentally benign way for the production of liquid fuels, which have higher quality owing to the absence of aromatics, sulfur, and nitrogen compounds.34,35 Recent advances focused on the direct production of highquality liquid hydrocarbons from syngas by performing both hydrocarbon growth and hydrocracking/isomerization processes in a single catalyst and reactor (“one-stage”), which has the potential to be more energy and cost efficient than the twostage process. This one-stage process is achieved with a catalyst that contains within its structure both FT active sites and acid sites (usually provided by the support); therefore, although the support is considered inert for many applications, it is active in the one-stage FT process. It is important to highlight that at present only cobalt-based catalysts employed industrially are prepared as supported catalysts, while the iron-based catalysts are fused bulky iron with an alkali promoter. Although cobaltbased catalysts have been mainly studied as bifunctional FT catalysts, supported iron catalysts have also been explored and considered for this application. The one-stage FT process requires using a more selective catalyst, producing chains of hydrocarbons with desired qualities, such as liquid range hydrocarbons (gasoline, C5−C11; diesel, C12−C20), with high Cisoparaffins/Cn‑paraffins (Ciso/Cn) ratios. The second step (upgrading) is also less demanding than in the two-stage process. This one-stage process simplifies the post-FT processing operations and decreases the operating and capital costs while facilitating small-scale applications in remote locations.36,37 A catalyst with bifunctional properties could enable the above process. Typically, bifunctional catalysts are defined as having two different active metals (bimetallic), but the application of this term to the FT process is different. The so-called bifunctional catalysts for FT are composed of metal active sites for the hydrogenation of CO to form hydrocarbon chains (FT functionality) and acid sites where hydrocracking and isomerization reactions occur immediately after chain formation (Figure 1).38,39 Some authors have employed Fe-, Co-, Ni-, and Ru-based catalysts, since these metals are known to be active in FT synthesis.14,37,40,41 One of the first uses of a bifunctional catalyst in the FT process was reported in 1962,42 with further work taking place in the following years, focusing mainly on the use of zeolites to provide the acid sites.43 In 1980s and 1990s, Rao et al.44 and Udaya et al.45 pioneered bifunctional catalysts for FT, using Co with a ZSM-5 zeolite for the first time, resulting in a change in the gasoline range selectivity, breaking the classical Anderson−Schulz−Flory (ASF) product distribution. It has since been shown that using this type of catalyst with acidic properties promotes greater selectivity toward hydrocarbons within the gasoline and/or diesel range, which can be compared with conventional catalysts, using supports such as SiO2, Al2O3, and activated carbon (AC).46 Few review papers have addressed the topic of bifunctional catalysts for FT processes, for which current works focus mainly on the use of zeolites as supports.37,39,47−49 However, there has been a growing interest in bifunctional catalysts (zeolites and others) for the one-stage FT process during the past decade, especially in the last five years (2014−2018), indicated by an increase of 62% in the number of published research papers on the topic compared with the previous fiveyear period (2009−2013) (Figure 2). The aim of this review is
vehicles will likely soon be similar to that of conventional vehicles,11 important barriers12 such as operating costs, the limited range of vehicles, restricted charging infrastructure, and the high cost of battery improvements reduce their potential for wide diffusion, therefore undermining the considerable changes in liquid fuel consumption that could result from their widespread use.13 Liquid hydrocarbons can also be obtained via the Fischer− Tropsch (FT) process; this is a well-established technology, where the main raw material for the process is syngas (a mixture of H2 and CO, mostly used in a 2:1 molar ratio and usually obtained from either carbon or natural gas).14 This process consists of two stages. In the first stage, the FT reaction, a catalyst is used to produce predominantly straight-chain hydrocarbons, which can be paraffins, olefins, and to a lesser extent oxygenated products and (at sufficiently high operating temperatures) even aromatics. Water and/or carbon dioxide are also produced as main products (typically around 99% of all oxygen from CO dissociation is discarded as water in the case of cobalt catalysts and a significant portion as carbon dioxide in the case of iron catalysts) as a result of the water− gas shift reaction.15 Then, in the second stage, the products are upgraded by hydrotreating (hydrocracking) or separated to obtain liquid fuels and/or other valuable chemicals.14,16 The FT process allows the conversion of syngas to light hydrocarbons (C1−C4), premium middle distillate cuts (for example, naphtha, jet fuel, and diesel), and heavier cuts like solid waxes C22+ or heavy aromatics depending of the process parameters (i.e., temperature, pressure, and type of catalysts).16 High temperatures (using alkali-promoted iron catalysts) promote the formation of light hydrocarbons, where high selectivity to olefins and aromatics leads to the production of high valuable chemicals instead of hydrocarbon fuels. On the other hand, low temperatures increase selectivity to long chain waxes, from which high quality middle distillates can be produced via hydrocracking.15 Recently, there has been an increasing interest in diverse technologies in the industrial field for obtaining liquid hydrocarbons. In addition to conventional technologies, such as carbon-to-liquid and gas-to-liquid conversion, processes such as biomass-to-liquid (BTL) or waste-to-liquid have been developed and explored. Residual biomass from agricultural production has shown promise as a replacement for fossil fuel resources because of its abundance and low cost.1,17−19 Efforts to increase the economic feasibility of biomass application have focused on the optimal selection of biomass,20 its geographical distribution, and its seasonality.21 Research is being directed toward the use of residual biomass derived from agroindustrial waste, uncultivable or inedible raw materials because of ethical issues, and economic considerations regarding increases in the price of feedstocks when they are used for energy production. The synthesis of fuels from residual biomass not only replaces crude oil as the raw material but also promotes the production and use of green fuels,22−24 which in turn favors sustainable development by fostering economical decarbonization.25,26 BTL processes are considered to be an attractive industrial alternative, since they decrease the use of fossil fuels and reduce CO2 emissions by fossil fuel replacement (if 20% of all fuels were produced from biomass-based carbon-neutral sources, CO2 emissions would be reduced by 15%).4,27,28 BTL processes are used to synthesize highly valuable liquid hydrocarbons, which are advantageous because of their superior B
DOI: 10.1021/acs.iecr.9b01141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Brief description
Oil transesterification (from seeds) with alcohol using acid or alkali catalysis
C
Synthesis of methanol and dimethyl ether (in a single oxygenate synthesis reactor) from biomass-produced syngas, followed by transformation into gasoline
Topsøe integrated gasoline synthesis
Conversion of syngas (produced by gasification) to higher hydrocarbons, with Co or Fe usually required as a catalyst
Synthesis of methanol from syngas (obtained from biomass), followed by conversion into hydrocarbons and water over a ZSM-5 catalyst
Methanol to hydrocarbons (MTH)
FT
Treatment of biomass in the absence of O2 at 450−500 °C, at heating rates of 103−104 K/s, with a vapor residence time of 1 s
Conversion of solid biomass into liquid fuel under increased pressure (150−200 bar) in the presence of hydrogen and a catalyst at 330−370 °C
Direct liquefaction
Fast pyrolysis
Extraction of the lipid fraction of algae for the production of biodiesel via chemical processing
Bio-oil (algae)
Bioethanol Fermentation of organic matter rich in glucose or sucrose (corn, sugar cane) by (agricultural crops) microorganisms
Transesterification (vegetable oils)
Technology
Table 1. Comparison of BTL Technologies4,30−33 Advantages
•Intermediate methanol production and storage needs are eliminated. •It requires lower investment and operating costs compared with MTH. •It is very well established, based on CH4 reforming or coal gasification systems. •FT fuels are sulfur-free and contain low concentrations of aromatics and nitrogen.
•The gasoline produced is composed mainly of isoparaffins and aromatics with a low benzene content and zero sulfur.
•Pyrolytic oil is the main product.
•The products have a high heating value (e.g., biocrude).
•It is less toxic than petroleum fuel. •It is readily biodegradable. •It produces less airborne pollutants. •The oil yield/hectare is 10−20 times higher for algal biodiesel compared with oil crops.
•Gasoline with a higher octane number is produced.
•Bioethanol is used worldwide.
•The properties of biodiesel are very similar to those of conventional diesel fuel.
Disadvantages
•FT for BTL technology is currently under development. •There are very few commercial installations.
•The growth media can be contaminated by microorganisms. •It is highly dependent on environmental variations. •The CO2 necessary for the growth of algae is provided by fossil fuels. •The production of microalgae is more expensive than growing oil crops. •It requires extremely high pressures. •It is not commercially viable. •It is experimentally challenging. •Separation of solids from liquids requires an extra production step. •There is a cost associated with the use of H2. •There are problems of energy efficiency and scaling to commercial sizes. •There is a high content of oxygen and H2O in pyrolytic oils. •High temperature (400−500 °C) and pressure are required. •There are high capital investment and operating costs. •Very limited data are currently available on this process. •Its efficiency is lower than that of FT.
•A large amount of wastewater is separated. •The catalyst and products need to be cleaned. •A large amount of energy input is required. •Protein and glycerin need to be efficiently recovered. •Current production relies on starch and sugars, but there has been a debate concerning its sustainability. •The separation cost to produce fuel-quality ethanol is high. •There is competition with food production.
Industrial & Engineering Chemistry Research Review
DOI: 10.1021/acs.iecr.9b01141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 1. Schematic illustration of the one-stage FT process: hydrocarbon chain growth followed by hydrocracking and isomerization.
1.1. FT Active Sites. Producing hydrocarbons through FT reactions requires the activation of syngas (H2 and CO). The activation of H2 and CO on metal surfaces is related to the dissociative chemisorption of molecular species to yield atomic H, C, and O species (Figure 3). Due to the fact that the dissociation of H2 is easier than that of CO, the activity of the metal sites for the FT reaction is mostly dependent on their ability to activate CO.48 Transition metals such as Fe, Co, and Ru interact strongly with CO at their surface and, therefore, activate the dissociation of CO, promoting the hydrogenation reaction.50 The ideal FT active site has to favor both C−H and C−C coupling; if only C−H coupling occurs, the main product would be CH4. The application of a variety of active metals (e.g., Co, Fe, Ru, Ni, and Rh) has been studied as a potential catalyst for the FT reaction at industrial and bench scales. Table 2 shows a comparison between the characteristics of these transition metals; Ru and Rh are the most expensive materials, which limits their application as FT active sites at the industrial level. While Ni is much less expensive than Co, a critical disadvantage is its high selectivity to methane and light hydrocarbons. On the other hand, Co is very active for FT synthesis, having high selectivity toward long-chain paraffins. Fe is less active than Co, but its low price makes it attractive for industrial-scale applications. Fe also shows good selectivity to olefins and valuable chemicals such as alcohols (which may be a disadvantage if the main objective is to obtain fuel precursors), with activity toward the water−gas shift (WGS) reaction. Bifunctional Fe-based catalysts also have enhanced selectivity to C5+ hydrocarbons. Therefore, the most usual FT active sites are Co and Fe; this section focuses on these two transition metals. For a Co-based catalyst, the Co0 particles are considered as the active sites for CO hydrogenation. Deactivation has been observed due to the oxidation and carburization of metallic cobalt particles under FT reaction conditions.48,59−63 Cobalt oxides (CoOx) and carbides (Co2C) are related to activity loss.64 However, Zhong et al.65 and Li et al.66 reported in recent research the activity of the Co2C nanoprism, which is selective toward smaller olefins (C2−C4). Other inactive species identified in Co-based catalysts include CoAl2O4 and Co2O4Si, which are formed due to strong metal−support interactions (MSI) and may be present when using conventional supports, such as Al2O3 and SiO2, respectively.67,68 Although catalyst deactivation may vary according to the process,
Figure 2. Number of publications since 2007 on bifunctional catalysts for the one-stage FT process.
to provide a perspective on recent advances in the use of bifunctional catalysts for the one-stage FT process, focusing on works published since 2007 and addressing both zeolites and other support materials. This review presents a wide variety of different supports that have been used as bifunctional catalysts for the one-stage FT process. A description of the active sites and their role in the reaction is given in the Section 1. A detailed discussion on the diverse characteristics of supports, such as their physical (texture, particle size, and morphology) and chemical (total acid content, acid site strength, and reducibility grade) properties, and a comparison between the performances of bifunctional catalysts for the one-stage FT process are provided in Section 2, together with new trends in supports, including zeolites, clays, alumina, silica, aluminosilicates, and carbon. Finally, Section 3 provides an outlook of the advantages and disadvantages of each support together with possible future work and potential applications of these materials.
1. ACTIVE SITES OF BIFUNCTIONAL CATALYSTS As described above, bifunctional catalysts for the one-stage FT process are composed of a metallic active site for CO hydrogenation (in this work, metallic sites are termed “FT active sites”, since they are responsible for hydrocarbon growth) and an acid site for hydrocracking and isomerization. This section provides a description of the role of each active site for both functions. D
DOI: 10.1021/acs.iecr.9b01141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 3. Scheme of FT synthesis mechanism using a Co active site.
conditions; however, it actively transforms into FeC. This FeC is demonstrated to be the active site since when a mostly Fe3O4 catalyst was used, CO conversion did not exceed 10%, but when FeC was present, conversion increased up to 90%. More recently, Schulz58 also showed that an Fe-based catalyst consisting mainly of Fe0 shows essentially no activity to FT synthesis, until FeC starts forming, again confirming that this is the main active phase. Finally, an explanation of the more prominent formation of FeC compared with Fe0 under FT conditions was provided by Cheng et al.,48 showing that the thermodynamics of FeC and metallic Fe formations favor the presence of iron carbide (Ea FeC = 43.9−69.0 kJ/mol; Ea Fe = 89.1 ± 3.8 kJ/mol). Although it has been extensively documented that FeC is the main active species for Fe-based catalysts applied to the FT process, there may be several iron carbide species present, each of which presents a different activity behavior. Transformations
mechanism, and catalyst design, Co-based catalysts have generally been reported to experience a slower deactivation process compared to Fe-based materials, promoting higher lifetimes for Co-based catalysts51,69,70 due to the lower reoxidation and coke deposition rates of Co-based catalysts compared with Fe-based catalysts.51,69 For Fe-based catalysts, the main active species for FT are not metallic Fe0 or Fe3O4 but rather iron carbide.58 For example, Davis71 provided at least three arguments that proved that FeC is the main active species for the FT process: (a) At low temperatures, metallic Fe0 is stable only in an essentially pure H2 atmosphere, which is not the case for FT conditions. (b) Under FT conditions, there is Fe0 present at the start of the reaction; however, with increasing time of stream, almost all Fe0 oxidizes to produce Fe3O4, which, on activation with CO, generates a mostly FeC surface in the catalyst particles. (c) Fe3O4 is basically inert (compared to FeC) in FT under low-temperature E
DOI: 10.1021/acs.iecr.9b01141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 2. Characteristics of Metal-Based Catalysts for FT Synthesis48,51−58 Metal-based catalyst
Active species
Co
Co0, Co2C nanoprism
CoOx, CoAl2O4, Co2O4Si, Co3C
Fe
χ-Fe5C2, έ-Fe2.2C
θ-Fe3C, FexC, Fe3O4 (amorphous)
Ru
Ru0
RuOx
Ni Rh
Ni0 Rh0
NiOx RhOx
Inactive species
Advantages
Market pricea (USD/metric ton)
Disadvantages
•Higher activity and selectivity •Much more expensive than Fe and Ni to long-chain paraffins •Low FT temperature •Greater resistance to deactivation than Fe •Used at the industrial scale •Low price and high availability •Problems such as attrition, catalyst loss, and difficulty of catalyst/wax separation in slurry reactors •Low ratio of H2/CO •Higher yield of alcohols compared with Co •Selectivity to olefins •Used at the industrial scale •The most FT active metal •Expensive and limited availability •Easily reduced •Selective mainly to paraffins •FT active •Selectivity to methane or
