Carbons and Carbon Supported Catalysts in Hydroprocessing
174
Carbons and Carbon Supported Catalysts in Hydroprocessing
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Carbons and Carbon-Supported Catalysts in Hydroprocessing
By Edward Furimsky
The Royal Society of Chemistry
Copyright © 2008 Edward FurimskyAll rights reserved.
ISBN: 978-0-85404-143-5
CHAPTER 1
Introduction
Carbon materials have been attracting attention as potential supports in heterogeneous catalysis. Thus, only in 2006, the number of articles dealing with various types of catalysts supported on carbon approached 1000. Among these, only a fraction was devoted to hydroprocessing catalysts. It is, however, emphasized that interest in carbonss as supports for hydroprocessing catalysts began more than two decades ago. The available information indicates some beneficial effects, although overall, there might be some limitations on the use of carbon materials as the supports for hydroprocessing catalysts.
Carbons that are used industrially exist in a highly ordered crystalline form (diamond and graphite) and a less ordered amorphous form. Figure 1 depicts models of these carbons. Amorphous forms of carbons such as carbon black (CB) and activated carbon (AC) have been used in various industrial applications most extensively. Novel carbon materials, e.g., carbon nanotubes (CNT), fullerenes, etc. have been developed. The information on the individual types of carbon is so extensive that a separate book can be written on each of them.
In catalysis, AC, CB, CB composites (CBC), graphite and graphitized materials have been attracting attention as potential supports for precious metals containing catalysts used for hydrogenation (HYD) of various organic compounds. Because of a weak interaction, a true alloy phase can be created from different metals on some carbon surfaces. This enhances the dispersion of metals and their utilization during alloy catalysis. To some extent, surface defects on carbon supports may be responsible for the interaction with metals. Such alloys cannot be formed on oxidic supports because of their much stronger interaction. An increasing number of studies indicating potential application of carbon supports, particularly those of AC and CB, in hydroprocessing catalysis have been noted. Carbon fibers and CNT have been attracting attention as well, whereas so far little information supports the use of fullerenes in hydro-processing applications.
Although the primary focus of this review was carbon and carbon-supported catalysts, attempts have been made to identify the difference in the effect of carbon supports compared with the oxidic supports, particularly that of γ-Al2O3. It has been noted that many studies had the same objective. For this purpose, the difference in catalyst activity and stability was estimated using both model compounds and real feeds under variable conditions. The conditions applied during the preparation of carbon-supported catalysts have received attention as well. This included various methods of pretreatment of carbon supports to enhance catalyst performance. In spite of all these efforts, commercial utilization of the carbon-supported catalysts in hydroprocessing is rather limited. In this regard, additional research may be needed to identify suitable applications.
Because of the neutral nature and little interaction with active metals, carbon supports are suitable to study the structure of active phase without interference as is usually the case of oxidic supports. Consequently, the understanding of the active phase in hydroprocessing catalysts was significantly advanced. Carbons alone exhibit activity in some hydroprocessing reactions. The ability of carbons to adsorb and activate hydrogen may be the origin of their catalytic activity.
CHAPTER 2Industrial Carbons
A cursory account of the carbon types (AC, CB, CBC, CNT, fullerenes and graphite) that have been attracting attention for potential applications in hydroprocessing catalysis is given, with focus on the properties and methods of preparation, as well as some industrial applications.
2.1 Carbon Black
Figure 1 shows that CB is an amorphous solid characterized by degenerate or imperfect graphitic structures. In these structures, the angular displacement of one layer with respect to another is random and the layers overlap irregularly thus, forming a turbostratic structure. Within the particles of CB, the crystallites are arranged randomly. The microstructure of CB aggregates consists of a concentric arrangement of layer planes, with the interior of the aggregate being less ordered than the exterior. Also, the interior is more chemically reactive and has a lower density. Thus, during exposure to O2, the oxidation begins at the interior of the aggregate. Structure, determined by the size and shape, as well as the number of particles per aggregate, is another important parameter of CB. The structure influences packing and volume of voids in the aggregate. Chemically, carbon blacks contain about 99% of carbon with hydrogen, oxygen, sulfur, nitrogen and ash accounting for the rest. The content of the noncarbon components determines the surface reactivity of CB. This depends on the method of preparation and the origin of the feed from which CB was made. The particle diameter of most of the CBs is less than 0.5 mm, i.e. a large portion of the CB particles is in the nanosize range.
Carbon black is produced by partial combustion or pyrolysis of hydrocarbon liquids or gases, although attempts have been made to produce carbon black from coal. Particle size, structure (aggregate size) and surface area are among the important properties of CB. Structure refers to the size of the primary aggregates. Thus, CB consisting of many prime particles with extensive branching and chaining is referred to as a high structure, while CB with fewer particles forming more compact units as low-structure blacks. The amorphous nature of CBs results from a short residence time (<1 s) in the reaction zone, i.e. not enough time was left for crystallization, in spite of rather high temperatures employed (~1200 K).
Several dozen grades of CB have been available commercially. Among them, a high-abrasion grade accounts for almost half of the CBs production. Other grades include super-abrasion, intermediate super-abrasion, general purpose, high modulus, semi-reinforcing and fast extrusion CBs. Large volumes of CB have been consumed in the production of rubber (tire and nontire) and other plastics. This is followed by the printing industry for production of various inks. The commercial production of CB has been dominated by an oil-furnace process. In this case, a heavy feed is pyrolyzed with the aid of heat produced by combustion of natural gas. High yield (45 to 65%) and a wide range of grades can be prepared by this process. The gas-furnace process has been gradually displaced by the oil-furnace process. The former is based on the partial combustion of natural gas in the refractory lined reactor. In this case, yields of blacks are less than 30%.
In hydroprocessing catalysis, carbon blacks can be used either directly by slurrying with a feed or used for the preparation of CBC that are suitable supports for the catalyst preparation. The properties of some commercial CBs are shown in Table 1 and that of CBC prepared from the former in Table 2. The latter were prepared by the method developed by Schmitt et al. based on the mixing CB with a binder (e.g., partially polymerized furfuryl alcohol) followed by a heat treatment at 383 K and additional heat treatment at 923 K in a flow of nitrogen. The properties of the CBCs could be further modified by oxidative treatment (e.g., with HNO3).
The use of CB as pore-forming material during preparation of the γ-Al2O3 support represents another application in hydroprocessing catalysis. In this case, CB of various particle size is mixed with γ-Al2O3. After forming the particle shape of interest (e.g., extrudates) the γ-Al2O3 is calcined (at ~820K) to remove all CB. The size of the resulting pores left behind depends on the particle size of CB. γ-Al2O3 supports varying widely in pore-size distribution can be prepared using this method.
2.2 Activated Carbon
Activated carbon is another amorphous, noncrystalline form of carbon possessing a large number of micropores and a high surface. The latter may exceed 1000 m2/g. Properties of AC depend on pore volume and pore-size distribution, as well as on the functional groups on the surface. Typically, pore size varies between 10 to 100 Å. If present, pores greater than 100 Å serve as channels for molecules entering micropores. Besides porosity, other important physical parameters include particle-size distribution, attrition resistance, hardness and density. Chemical properties of AC include ultimate analysis, ignition temperature, ash and moisture content. Depending on the applications, industrial AC are produced in the form of powder, granules, pellets and extrudates. Extrudates are produced by pulverizing AC, mixing with a binder and extruding. To enhance performance, AC is impregnated with various chemicals, i.e. zinc salts, iodine and phosphorus compounds, elemental sulfur, iron salts, silver, etc.
Low-cost feedstocks such as wood, nut shells, coal, petroleum coke, waste materials, etc. can be used for the preparation of AC. Depending on the feedstock and preparation conditions, a great degree of variance in porosity of AC can be established. Typically, the wood-derived AC is known for its extensive macroporous structure, whereas the coal-based AC can adsorb high molecular substances because of the suitable mesoporosity. Microporous AC can be prepared from the nut-shells. Two principal methods for AC preparation include thermal activation and chemical activation. The former is carried out in two stages, i.e. carbonization followed by activation. In the first stage, the feedstock is pyrolyzed to drive off volatiles and to produce a high carbon content char. The char is subsequently activated (from about 800 to 1400K) using an oxidizing medium such as steam, CO2 and diluted air. During activation, oxidizing gas reacts with the char to form gaseous products (CO, CO2 and H2). At the same time, channels and pores are created in the interior of the char particle. For some applications, the AC prepared by activation is subjected to an additional treatment, i.e. washing with water, nitric acid, hydrochloric acid, phosphoric acid, etc. to remove impurities. For feedstocks such as sawdust and peat, an AC can be prepared by chemical activation. In this case, the feedstocks are mixed with dehydration agents (zinc chloride, phosphoric acid, sulfuric acid, etc.) to chemically decompose the feedstock. Typically, the plastic mass prepared by mixing the feeedstock with a chemical agent is kneaded before being extruded, dried and calcined. The extrudates are then activated at about 900 K. During activation the chemical agent, e.g., zinc chloride, is recovered and recycled. Rotary kilns are the most common types of reactors used for the preparation of AC, although fluidized-bed reactors have been used as well.
With respect to industrial applications, ACs are grouped into gas-phase and liquid-phase types. The former produced in a larger particle size (granular), are used for removal of contaminants and condensible species from various gaseous streams and effluents. Mostly in a powdered form, AC is used in liquid-phase applications to remove contaminants, e.g., water purification. Recently, attempts have been made to use AC for removal of the most re- fractory multi-ring thiophenic compounds from middle distillates. In these applications, the following selectivity order of the S-heterorings has been established: BT < DBT < 4-MDBT < 4,6-DMDBT. This order was maintained regardless of the origin of AC. Apparently, the adsorption was dominated by the molecular volume of the compounds. This suggests that the interaction with the surface was more physical rather than chemical. Therefore, surface properties such as surface area, pore volume and size distribution may determine the efficiency of AC utilization. The same was confirmed in the study of Zhou et al. using a model diesel fuel mixture and real diesel fuel. Using a similar approach, Kim and Song used the mixture of DBT, 4,40DMDBT, indole, quinoline, naphthalene and 1-methyl naphthalene. In this study, the AC alone, as well as the AC loaded with metals such as Cu, Ce, Ni, Fe and Ag were tested. AC can be readily impregnated with the salts of catalytically active metals providing that a suitable impregnation solution was used. The properties of AC that were tested as supports for hydroprocessing catalysts are shown in Table 3. These results indicate a significant variability in the pore volume and size distribution between the two samples of AC. Table 4 compares elemental analysis and physical properties of several carbons, i.e. nanoparticles of carbon black (Ketjen black), granular AC particles of a moderate and large surface area (Diahope, BP2000 and Max sorb 3060) and the pitch-based AC fibers (ACF-OG). Compared with Table 3, a significantly lower ash content of these carbons should be noted. Moreover, relatively large content of O + S in some carbons in Table 4 suggests that these elements may play some role during the impregnation of these carbons with active metals. The presence of the O-containing groups (e.g., hydroxyl, carboxyl, carbonyl, arylether, etc.) on the surface of AC was reported by Solar et al., although the stability of such groups under typical hydroprocessing conditions has not yet been investigated. Similarly as CB, AC can be used as a pore-forming material during the preparation of the γ-Al2O3 supports varying widely in pore-size distribution.
2.3 Carbon Nanomaterials
This group of carbon materials includes nanotubes and nanofibers. These materials have been attracting attention because of their rather unique properties, i.e. unusual strength as well as a high electrical and thermal conductivity.
Carbon nanotubes are made up of a rolled-up graphite sheet and are available as single-walled (SWCNT) or multiwalled nanotubes (MWCNT). The methods of preparation include arc discharge, laser ablation and catalytic chemical vapor deposition. The CNT with regular turbostratic structures not covered with amorphous carbon could be prepared by selecting a suitable catalyst and experimental conditions. It is believed that graphite can be partially converted to CNT by applying suitable radiation with the aim of removing the aromatic sheet from the basal plane. Apparently, there is a driving force for rolling of such sheets into CNT.
Carbon nanotubes can be readily dispersed in a solvent using ultrasound. However, because of a strong van der Waals forces, they can quickly aggregate and precipitate. This problem can be alleviated by various pretreatments. For example, Table 5 shows the effect of HNO3 on properties of nanotubes. An increase in the content of carboxylic, lactone and hydroxyl groups was noted. At the same time, the total amount of base was decreased to zero. However, the CNT prepared by the template technique could be dispersed in water without requiring any pretreatment.
The evaluation of CNT and the CNT-supported catalysts for potential application in hydroprocessing catalysis deserves attention. So far, this topic may still be in the early stages of research, although some initial attempts to use nanotubes as the support for the preparation of hydroprocessing catalysts have been noted. For example, the recent information indicates on potential applications of the CNT and carbon nanofibers (CNF) in catalysis mainly as supports. In this regard, the CNT and CNF with macroscopic shaping appear to be promising supports for catalysts being used either in a gas-phase or trickle-bed mode. This shaping ensures stabile physical and chemical properties. Also, when used in fixed-bed reactors, the problems associated with diffusion and pressure drops are much less evident.
2.4 Fullerenes
Fullerenes represent a new form of carbon made up of 60 carbons (C60) connected together by hexagons and pentagons as in a soccer ball. They are commercially available from several suppliers with prices steadily decreasing because of improvements in the methods of preparation. Several books on various aspect of fullerenes have been published including an extensive review on fullerenes and fullerene-based materials in catalysis. The review published by Olah et al. focused on reactivity of fullerenes. Reactions included reduction, oxidation, alkylation and related reactions, reactions with neutral bases, cycloaddition reactions, epoxidation and oxygenation, halogenation, Friedel–Crafts fullerylation of aromatics, fulleration of aromatics, reactions with free radicals and formation of metallic complexes. The reactivity for so many reactions suggests that various modifications of the surface of fullerenes are possible. An anionic form of fullerenes is a strong reducing agent and can catalyze the reduction of nitrogen to ammonia. There is little evidence indicating the use of fullerenes and/or fullerenes-supported catalysts in the studies on hydroprocessing reactions. This may be attributed to a low surface area and the lack of stability of the metal/C60 materials. However, the oxide-C60 materials may enhance the complexation with metals, although this may be affected at temperatures exceeding 600 K.
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Excerpted from Carbons and Carbon-Supported Catalysts in Hydroprocessing by Edward Furimsky. Copyright © 2008 Edward Furimsky. Excerpted by permission of The Royal Society of Chemistry.
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