Industrial solid metal catalysts: an overview.
Guido Busca Diparti Dipartimen mento to di Ingegn Ingegneri eria a Chimic Chimica a e di Proces Processo, so, Univer Universit sità à di Genov Genova, a, P.le P.le J.F. J.F. Kennedy 1, I-16129, Genova, Italy Phone +39-010-353-6024; fax +39-010-353-6028 +39-010-353-6028 ; e-mail :
[email protected] [email protected] Contents.
1. Introduction. 2. Metal catalysts for hydrogenation reactions. 2.1 Hydrogenation of hydrocarbons. 2.2 Hydrogenation of oxygenated compounds. 2.3 Hydrogenation of nitrogen containing compounds. 3. Metal catalysts for dehydrogenation reactions. 3.1 Dehydrogenation of alkanes to olefins and dienes. 3.2 Dehydrogenation of olefins to dienes. 3.3 Aromatization. 3.4 Dehydrogenation of alcohols to carbonyl compounds. 4. Steam reforming and related catalysts 4.1 Steam reforming of natural gas and other hydrocarbons. 4.2 Water gas shift. 4.3 Steam reforming of methanol. 4.4 Steam reforming of ethanol. 5 Metals in acid and base catalysts. 5.1 Noble metal doping of acid catalysts. 5.2 Alkali metal basic catalysts. 6. Metal catalysts for oxidation reactions. 6.1 Selective and preferential oxidations. 6.2 Catalytic combustion for energy generation. 6.3 Total oxidation of VOC’s. 6.4 Oxidation of ammonia. 6.5 Catalytic converters. 6.6 Diesel aftertreatment.
Abstract The available data on solid metal catalysts used in industry are summarized and briefly discussed. The typical behaviour/application of the different metals and the real use of different supports/carriers are specified. The important role in the production of real catalysts of secondary components such as activators, is also underlined. Data concerning the physical properties of the catalysts, such as support morphology, metal loading, presence of additives, when available, are briefly discussed. The still open controversies concerniong the real state of the catalyst in action, the nature of the active sites and the resulting mechanisms are cited. The still large need of further investigation is evidenced. evidenced.
1. Intr Introd oduc uctitioon As it is well known, a predominant part of industrial chemical processes, in particular those of large scale refinery and petrochemistry production, are performed in the presence of catalysts. Among catalytic processes, a predominant part is performed in heterogeneous catalytic conditions, i.e. with solid catalysts, reactant reactant mixture being in the liquid, gaseous, or mixed liquid and gaseous phase. Several kinds of solid catalysts find industrial application in chemical industry [ 1]. From the point of view of their virtual chemical chemical composition, composition, solid catalysts catalysts are based based on oxides [ 2] (such as most of industrial acid [ 3] and basic catalysts [ 4] as well as many oxidation catalysts [5]), sulphide catalysts (mostly hydrotreating catalysts [ 6]), metal halides (used as acidic catalysts, for polymerization, ,halogenation transhalogenation and oxychlorination ) and those based on metals. Catalysis by metals has been the object of enormous efforts, as shown in many publications and books [ 7-9]. Metal catalysts may be bulk or supported, Enormous number of studies have been published concerning the effects of different catalyst characteristics (support type, composition and morphology, dispersion of the metals, size and shape of metal particles, role of additives). Indeed we refer here to virtual catalysts composition, because the actual composition of catalysts may vary with the reaction medium and frequently is not defined easily. In particular, metal catalysts may indeed change significantly their surface and even bulk composition depending on the reactant environment: as for example, in the presence of oxidants such as oxygen and also water, they may be oxidized to some extent, in the presence of carbon oxides or hydr hydroc ocar arbo bons ns they they may may be carb carbur uriz ized ed to some some exte extent nt,, in the the pres presen ence ce of sulp sulphu hur r compounds they may be sulphided to some extent, etc. Surfac Surfacee scienc sciencee studie studiess helped helped very very much much to the unders understan tandin dingg of metal metal catal catalysi ysis. s. Recognition of this work is represented by the Nobel Prize 2007 for Chemistry, awarded to Gerhard Ertl, the most eminent surface scientist. Experimental methods for metal –based
Abstract The available data on solid metal catalysts used in industry are summarized and briefly discussed. The typical behaviour/application of the different metals and the real use of different supports/carriers are specified. The important role in the production of real catalysts of secondary components such as activators, is also underlined. Data concerning the physical properties of the catalysts, such as support morphology, metal loading, presence of additives, when available, are briefly discussed. The still open controversies concerniong the real state of the catalyst in action, the nature of the active sites and the resulting mechanisms are cited. The still large need of further investigation is evidenced. evidenced.
1. Intr Introd oduc uctitioon As it is well known, a predominant part of industrial chemical processes, in particular those of large scale refinery and petrochemistry production, are performed in the presence of catalysts. Among catalytic processes, a predominant part is performed in heterogeneous catalytic conditions, i.e. with solid catalysts, reactant reactant mixture being in the liquid, gaseous, or mixed liquid and gaseous phase. Several kinds of solid catalysts find industrial application in chemical industry [ 1]. From the point of view of their virtual chemical chemical composition, composition, solid catalysts catalysts are based based on oxides [ 2] (such as most of industrial acid [ 3] and basic catalysts [ 4] as well as many oxidation catalysts [5]), sulphide catalysts (mostly hydrotreating catalysts [ 6]), metal halides (used as acidic catalysts, for polymerization, ,halogenation transhalogenation and oxychlorination ) and those based on metals. Catalysis by metals has been the object of enormous efforts, as shown in many publications and books [ 7-9]. Metal catalysts may be bulk or supported, Enormous number of studies have been published concerning the effects of different catalyst characteristics (support type, composition and morphology, dispersion of the metals, size and shape of metal particles, role of additives). Indeed we refer here to virtual catalysts composition, because the actual composition of catalysts may vary with the reaction medium and frequently is not defined easily. In particular, metal catalysts may indeed change significantly their surface and even bulk composition depending on the reactant environment: as for example, in the presence of oxidants such as oxygen and also water, they may be oxidized to some extent, in the presence of carbon oxides or hydr hydroc ocar arbo bons ns they they may may be carb carbur uriz ized ed to some some exte extent nt,, in the the pres presen ence ce of sulp sulphu hur r compounds they may be sulphided to some extent, etc. Surfac Surfacee scienc sciencee studie studiess helped helped very very much much to the unders understan tandin dingg of metal metal catal catalysi ysis. s. Recognition of this work is represented by the Nobel Prize 2007 for Chemistry, awarded to Gerhard Ertl, the most eminent surface scientist. Experimental methods for metal –based
materi materials als charac character teriza izatio tionn underw underwent ent furthe furtherr develo developme pments nts in recent recent years, years, with with the develo developme pment, nt, among among other other techn techniqu iques, es, of high high resolu resolutio tionn electr electron on micros microscop copies ies [10], monocrystal and surface layers spectroscopies [ 11], theoretical calculations [12]. These techniques allowed to develop refined models for active sites and reaction mechanisms of metal catalysis, supposed to occur essentially on the surface of metal crystals. However some of these techniques sometimes fail in elucidating the role of the supports in supported metal catalysis and in determining the existence and the possible role of metal species different from bulk crystals, i.e. dispersed clusters or isolated atom and ion species and support-metal interfaces, which are instead observed by other techniques such as, e.g. infrared spectroscopy [ 13]. Indeed Indeed,, even for well establ establish ished ed industr industrial ial processes authors still disagree on the real state of the working catalyst, the nature of the active sites sites and reaction mechanisms. On the frame of a systematic investigation on the surface chemistry of metal catalysts, aimed at checking the possible role of metal-support interfaces, we wanted to distinguish from the very many catalyst formulations which have been the object of academic studies, thos thosee wh whic ichh find find actu actual al indu indust stririal al appl applic icat atio ion. n. This This wo work rk is not not easy easy beca becaus usee data data concerning industrial catalyst formulations are frequently not available. However, we believe a synthetic analysis of the available data concerning catalysts which find actual industrial application would allow us to obtain some interesting conclusion. 2. Metal catalysts catalysts for hydrogena hydrogenation tion reactions. reactions. Hydrogenation reactions are exothermic equilibrium reactions, thus performed at the lowest temperature allowed by the catalyst activity, and at medium to high pressure. Most of them are performed in the presence of solid metal-based catalysts, most frequently in multiple fixed beds with inter-bed cooling by heat exchange or quenching, in gas/solid or gas/liquid/solid conditions, or in slurry conditions, depending from the volatility of the reactants as well as from fr om reaction temperature and pressure. 2.1.Hydrogenation of hydrocarbons. Many metals, including platinum group metals, nickel, cobalt, iron, copper, etc. are active in hydrocarbon hydrogenation. However, some noble metals are far more active, but quite easily deactivated in particular by sulphur compounds. For these reasons the catalyst composition for hydrocarbon hydrogenation is strongly dependent on the amount of sulphur impurities in the feed. Palladium, and, to a lesser extent, platinum are the choice
catalysts for hydrogenation of clean feeds while nickel is mostly used for hydrogenation of medium sulphur containing feeds. Bimetallic noble metal catalysts or other modified noble metal catalysts may also have interesting resistance to medium-low concentrations of sulphur. Feeds which are heavily contaminated by sulphur may be hydrogenated over typica typicall hydro hydrotre treati ating ng sulphi sulphide de cataly catalysts sts,, such such as alumin alumina, a, silica silica-al -alumi umina na or zeolite zeolite supported Ni-W, Ni-Mo or Co-Mo catalysts [14], or bulk transition metal sulphides [ 15,16], among which RuS2 is reported to be the most active. 2.1.1. Hydrogena Hydrogenation tion of acetylen acetylenic ic compounds compounds.. The hydrogenation of acetylenic hydrocarbons is performed mostly to destroy these compounds present in small amounts in different flows. This subject has been thoroughly reviewed some years ago by Molnár et al. [ 17]. Hydrogenation of acetylene in the C2 cut from steam cracking is performed to reduce this noxious impurity in polymer-grade ethylene (from near 1 % to < 5 ppm, frequently < 0.5 ppm) [18]. The feed is usually previously cleaned by caustic washing, thus allowing the use of noble metal catalysts [19]. The reaction should convert most of acetylene, with low conversion of it or of ethylene to high molecular weight hydrocarbons (“green oil”), with high stability towards poisoning by traces of CO and sulphur in the feed. Thus the metal should be very active and the support very inactive. For these reasons, Pd is the preferred metal today, the loading is quite small, e.g. 0.04 % wt, the support being a low surface area alumina, e.g.α-Al2O3 with ca. 20 m2/g [20] but silica-supported Pd catalysts are also proposed [21], that may reduce the oligomerization activity thus leading to a low green oil formation [22]. The reaction is performed in the gas phase at 25-100 °C and 20-35 bar using multiple fixed catalyst beds (from one to four beds, depending on the initial acetylene concentration in the feed) catalysts with intermediate cooling [18 [ 18].]. Acetylene is hydrogenated much faster than ethylene when both are present, because the acetylene occupies the active adsorption sites preferentially. However, over the same catalyst ethylene or other olefins may be also hydrogenated to alkanes. It seem seemss that that the the most most usua usuall cata cataly lyst st comp compos osititio ionn for for this this appl applic icat atio ionn (tai (tail-l-en endd configuration, sulphur free feed) is today Ag-promoted Pd/Al 2O3, although the actual concentration of Ag may be higher than that of Pd (es. Pd 0.03 %, Ag 0.18 % for the Süd Chemie OleMax 201 catalyst [23]). Silver concentration must be optimal to increase selectivity selectivity without without relevant relevant decreas in activity. Gold is also reported reported ter for
Pd, while potassium, and chromium have been reported as additional or alternative promoters [21 [21].]. Acetylene hydrogenation reaction is performed to purify HCl flow coming from ethylene dichloride (EDC) cracking, in the process of production of vinyl chloride monomer (VCM), e.g. in the Vinnolit process [ 24]. In fact HCl is recycled to ethylene oxychlorination reactor to reproduce EDC. The presence of acetylene here would result in the production of polychlorinated byproducts. The catalysts Noblyst® E 39 H and Noblyst® E 39KHL [ 25], produced by Evonik, are specific for this application: they are both reported to be constituted by Pd on silica. May be the different reactivity of the support with respect to HCl is a key feature here. Hydrogenation of methylacetylene and propadiene in the steam cracking C3 cut or after propane dehydrogenation must be performed to limit their content in polymer-grade propylene. This reaction may either be performed in the gas phase with multiple beds or in the liquid phase in a single bed. In both cases promoted Pd/Al 2O3 is used but the Pd content for liquid phase use must be much higher (0.3 %) than for gas phase use (0.03 %) [23]. 23]. In gas-phase hydrogenation the reaction is controlled by means of the operating temperature, which may be between 50 and 120 °C, 15-20 bar, depending on the preparation and aging state of the catalyst. With liquid-phase hydrogenation the reaction is controlled by the hydrogen partial pressure. The operating temperature of 15 –25 °C is considerably lower in this case [17 [17,,18]. 18]. Hydrogenation of C2 and C3 acetylenics may also be performed simultaneously in the so called front-end hydrogenation configuration [ 26,27], in the gas phase with three fixed beds and intermediate cooling. The catalysts appear to be, in all cases, based on promoted Pd/Al2O3 with small Pd content. If the feed, however, contains significant amounts of sulphur compounds, a multipromoted NiO catalyst supported on Silica-alumina (0.6-2.6 % NiO), as the OleMax 101, 102 and 103 catalysts, are proposed by Süd Chemie [23 [ 23].]. Hydrog Hydrogena enatio tionn of α-acet α-acetyle ylenic nic compoun compounds ds (1-butyn (1-butynee and vinylace vinylacetyl tylene ene)) may be performed with 100 % yield to purify the steam cracking C4 cut in order to simplify the further extraction of 1,3-butadiene (e.g. by N-methyl pyrrolidone). This occurs in the UOP KLP process [28,29], using a single fixed bed of the KLP-60 TM catalyst, catalyst, Cu-Ni/Al Cu-Ni/Al2O3 ,in the liquid phase. Temperatures are not allowed to exceed 85 °C. The reactor pressure is set to maintain the reaction mixture in the liquid phase. Two or three reactors are used one of which is in regenerati regeneration on by solvent solvent wash. wash. The catalyst selectively selectively converts converts acetylenes, acetylenes,
with a significant amount being converted to butadiene. The process produces a high quality butadiene product, with typically less than 5 ppm acetylenes. Other major catalyst producers offer Pd based catalysts for the hydrogenation of butynes. Süd Chemie offers a 0.2 % Pd/Al 2O3 catalyst (Ole Max 400 and 353) [23] while Axens offers a bimetallic Pd-containing catalyst (LD 277) [30]. Alves et al. [31] recently described and tested a commercial catalyst for this reaction, 0.21 % Pd/Al2O3, 70 m2/g total surface area, 3.6 nm Pd crystal size, 27 % Pd dispersion. Another important reaction is the hydrogenation of phenylacetylene impurities in styrene, just after the ethylbenzene dehydrogenation step. This reaction [32] is carried out in liquid phase at 25-80 °C, 1-4 bar, over eggshell 0.3 % Pd/Al 2O3 catalyst (like for the Süd Chemie DMaxPA catalyst [23]) although other noble metal catalysts, such as 0.5 wt % Pt/ γ -Al2O3 (ESCAT-26, Engelhard) have also been reported to have excellent activity [ 33]. 2.1.2. Hydrogenation of conjugated dienes and olefins. Several industrial hydrocarbon flows must be purified from dienes and other unsaturated hydrocarbon impurities, to avoid formation of oligomers or unwanted products in further steps. C4 cuts from Fluid Catalytic Cracking can be used to produce 1-butene, e.g. as a comonomer of ethylene in the production LLDPE. Hydrogenation is needed to reduce butadiene content to the ppm range, achieving high 1-butene yields, and minimizing 2butene and butane formation. Conventional Pd-only catalysts have poor butene-1 selectivity because they significantly promote 1-butene isomerization to 2-butene. Axens developed a special catalyst based on Pd and a promoter on an alumina carrier, endowing the catalyst with very high intrinsic selectivity to 1-butene (LD 271 [30]). A similar behaviour is claimed for Süd Chemie for some catalyst of the G-58 family (now Olemax 200, likely Pd-Ag/Al2O3 [23]). Pd/Al2O3 catalysts from BASF [22] are also used in the SELOP C4 ® process to hydrorefine C4 cuts. The Hüls Selective Hydrogenation Process (SHP) was developed for hydrogenation of dienes and acetylenes in C3-C5 olefin fractions to their respective mono-olefins. The conversion of dienes and acetylenes is nearly 100%. There is minimal loss of olefins via nonselective saturation reactions. H-15 SHP catalyst [ 34] is nickel based, sulphur resistant catalyst optimized for use in Hüls Selective Hydrogenation process units. The H-15 catalyst can provide an economical operation at feed sulphur levels up to 300 ppm.
Total saturation of a C4 olefins cut to a stream containing mainly butanes is one of the options. This stream can then be recycled to the cracking furnaces as its ethylene yield is higher than that of naphtha. Another option is to sell it as LPG. Pd based catalysts such as Axens LD 265 [30] are characterized by efficient hydrogenation activity and high long-term stability. Pyrolysis gasoline (pygas), a byproduct in steam cracking of naphtha or light gas oil, are characterized by high aromatics content (up to 88 %) and by chemical instability, due to the presence of highly unsaturated compounds such as diolefins and alkenylaromatics. Options for pygas upgrading include preparing it for use as a gasoline pool component or recovering the aromatics in the pygas. In both cases it must be stabilized by hydrogenation. Hydrogenation of pyrolysis gasoline [17,35] is performed usually in two steps. The first stage consists in a Selective Hydrogenation Unit (SHU), operated in the liquid phase and at a low temperature of 40-100 °C in which about 90% of diolefins and styrene but only less than 10% of monoolefins are removed. In the first stage, both 0.2-0.4 % Pd/Al2O3 and Ni catalysts are able to carry out the required reactions, i.e., the selective hydrogenation of diolefins and alkenylaromatics such as styrene, while minimizing olefin hydrogenation and avoiding aromatics hydrogenation. Süd Chemie offers 0.3-0.4 % Pd/Al 2O3 catalysts [23]. BASF SELOP C5+ hydrogenation process treats gasoline over Pd H O-22 containing 0.25 wt.% Pd on high surface area alumina [17]. Similar hydrogenation processes may also be performed with reformate gasolines to stabilize them [35]. Johnson Matthey offers for first stage pyrolysis gas hydrogenation the HTC NI 200 (12 % Ni on alumina [36]), which is available in three forms – oxidic (OX), pre-reduced and air passivated (RP) and pre-reduced, air passivated and pre-sulphided (RPS) [ 37]. Due to the wide pore structure and small nickel crystals providing high strength, activity, selectivity and poison resistance, together with lower gum/polymer formation and side reactions, Nickel catalysts can offer comparable performance to palladium catalysts. Nickel catalysts contain 20 to 50 times more active metal atoms than Pd catalysts. For this reason, Ni catalysts may be more tolerant to poisons than Pd catalysts, and therefore they are preferred in case of highly contaminated feedstocks [30]. The conversion of olefins from pyrolysis gasoline is completed in the second step which is essentially a hydrodesulphurization unit working at higher temperature and pressure (280 to 330 °C, 25-30 bar).
Small amounts of straight chain C10 to C13 di-olefins are selectively hydrogenated to the mono-olefins in the feed to alkylation reactor in the process of production of Linear Alkyl Benzene (LAB). The reaction is carried out in a liquid phase at 170-230 °C and 10-20 bar on nickel – alumina (Ni/Al 2O3) catalyst [38,39].
2.1.3. Hydrogenation of olefins. 0.3 % Pt/Al2O3 catalysts are used in the hydrogenation of isooctene to isooctane [23], obtained by isobutene dimerization, in the “indirect alkylation process” . The reaction can be carried out in the liquid phase at 70-130 °C, 10-30 bar [ 40]. Hydrogenation of the byproduct α-methyl-styrene (AMS) is performed in the cumene synthesis processes using Ni- or Pd-based catalysts, on alumina or on carbon supports [ 41] where the catalyst can be used in a fixed bed system in the liquid phase, or in a trickle bed system with both vapor and liquid phases. The Noblyst® H 14108 catalyst from Evonik [25] suggested for this application, is based on Pd/Al 2O3. Palladium catalyst having a metal content of between 1 and 5% is noted as being preferred, for use at temperatures of 24-50 °C and pressures of 1.5-5 bar. A double bed reactor with a Ni catalyst on the first bed and a noble metal catalyst in the second is a possible option [ 42]. 2.1.4. Hydroisomerization of olefins. C4-cut treatment from steam creacking, pre-treatment of HF alkylation unit feedstocks from an FCC unit may have the objective to produce 2-butene. Particularly developed palladium catalysts, such as Axens LD 267 R, achieve efficient residual butadiene removal with a high isomerization of 1-butene to 2-butene that approaches the thermodynamic equilibrium. Over-hydrogenation to butanes is held to a minimum [ 43]. The catalyst’s selectivity for isomerization and olefin recovery is obtained by a specific pre-treatment step that moderates the catalyst hydrogenation activity. A further advantage of this pretreatment is that the catalyst is delivered in its reduced form. The result is that the activation step during start-up can be performed quickly and easily. 2.1.5. Aromatics saturation Benzene full hydrogenation can either be performed to produce cyclohexane, thus using a pure benzene feed, or to reduce benzene content in a gasoline fraction. Similarly,
hydrogenation of naphthalenes (aromatics saturation) is performed to reduce the content of polyaromatics in Diesel fuels. The activity of heterogeneous metal catalysts for the hydrogenation of aromatics was reported to be in order Rh > Ru >> Pt >> Pd >> Ni > Co [44,45] However, the sulphur resistance of pure noble metals is quite limited, but significantly improves using acidic supports like zeolites (e.g. Ultra Stable Y, USY), and appropriate alloying of noble metals such as Pt-Pd alloys. Typical hydrotreating catalysts based on alumina supported NiMo or NiW sulphides are used with sulphur rich feeds. Thus, also in this case the choice of the catalyst can depend largely from the sulphur content in the feed. The conventional benzene reduction process in gasoline employs a hydrogenation unit downstream from the reformate splitter. Pt/Al2O3 catalysts are used in the BenSat™ benzene saturation technology from UOP [ 46]. Recently the previous UOP H-8™ catalyst has been updated in response to a demand for improved economics. The new BenSat process uses a new catalyst, the UOP H-18 TM catalyst, resulting in lower catalyst volume, reduced recycle, and lower precious metal requirements. According to UOP patents, its preferred form, the alumina support will comprise spheres having a surface area of from about 160 to 200 m2/g with an apparent bulk density of from about 0.45 to 0.6. The platinum metal may be present on the catalyst in a concentration of from 0.375 to 0.75 wt. %.[47]. According to people from UOP, unlike nickel-based catalysts, platinum-based saturation catalysts are not permanently poisoned by sulfur or heavies upsets and do not cause cracking to light ends. Also GT-BenZap(SM) process from GTC Technology uses Pd based catalyst [48]. Süd Chemie offers Ni-based catalysts (the NiSAT® family, with NiO content from 43 % to 77 %) for naphtha dearomatisation [23]. For the same reaction Axens proposes the BenfreeTM reactive distillation process, integrating reactive hydrogenation with the reformate splitter column, thus reducing equipment and associated costs. The catalyst is AX 744 is a nickel catalyst, recommended for its high resistance to poisons when the feed contains a few ppm of sulphur or other contaminants [49]. Also CDHydro ® process from CDTECH is of the reactive distillation type [50]. Industrial production of cyclohexane is achieved either with liquid phase or with gas phase hydrogenation of pure benzene. Gas phase hydrogenation of benzene can be carried out with noble metal catalysts, mainly platinum supported over alumina, etc., at 200° C. temperatures and about 30 atm pressures. In these cases, however, secondary reactions occur producing undesired contaminants especially methylcyclopentane (MCP). Some
improvement has been achieved using nickel instead of platinum catalysts, thus meeting quality requirements although not being able to take full advantage of caloric energy generated by the process due to the need to work at relatively low temperatures. Here, there is also relatively lower reaction speed compared to hydrogenation at higher temperatures, thus, involving larger equipment. An alternative approach to limit MCP formation consists in performing the reaction in the presence of ammonia or nitrogenated organic bases as inhibitors, being incorporated in a 0.2 to 100 ppm concentration [ 51]. As an alternative, cyclohexane is produced in liquid phase like through the process developed years ago by the Institut Francais du Petrole wherein benzene and hydrogenrich gas is fed to a liquid-phase reactor containing Raney nickel catalyst at 200-225 °C and 50 bar [52]. The nickel suspension is circulated to improve heat removal, the benzene being completely hydrogenated in a second fixed-bed reactor. Although a good quality cyclohexane is achieved in this way, the inconvenience of a continuous consumption of catalyst is added to the generation of residues (exhausted catalyst) that forces to a later disposition of said catalyst as dangerous residue. However, the most serious disadvantage of this technology lies in the fact that low reaction temperature prevents the full exploitation of the enormous amount of caloric energy generated by hydrogenation, which in gas phase process is totally recoverable. Axens-IFP developed a new process for the production of high purity cyclohexane by benzene hydrogenation with a first homogeneous liquid phase step in the presence of a soluble nickel catalyst and a second fixed bed vapour phase “polishing” step [ 53]. Benzene hydrogenation to cyclohaxane may also be done in conditions of catalytic distillation [54]. The partial saturation of benzene to cyclohexene is also of industrial interest. For this reaction ruthenium catalysts offer the best selectivity. In the Asahi Chem. Process Ru/La2O3-ZnO catalysts are used [55]. Hydrodearomatization of napthalenes in gasoils may be needed in the near future to fulfil the more stringent limits of these polyaromatics in commercial Diesel fuels [56]. One step technologies (with dearomatization and desulphurization performed together over sulphide catalysts) or two-stage technologies (that perform deep aromatics reduction with noble metal catalysts after HDS of the feed with sulphide catalysts) may be applied. This is due to the deactivation of noble metal by sulphur compounds. The rate of aromatic saturation is up to about 70-90% for two-stage processes in contrast with the 40-55% HDA level of one-stage processes using sulphide catalysts. High intrinsic activity and sulfur tolerance may be enhanced by deposition of noble metals on acidic, high surface area supports,
e.g., a large-pore zeolites beta and HY. It has been established that noble metals deposited on acidic supports show higher turnover frequencies (TOF) than when they are supported on non-acidic carriers [57,58]. Catalytic systems based on zeolite-supported noble metals, such as Pt-Pd/H-USY, are now under development [ 59], as the ASATTM catalysts from Süd Chemie [60]. Saturation of higher polyaromatics can be performed to upgrade lube oils to medicinal grade white oil: the MAXSAT process from ExxonMobil uses a noble metal containing molecular sieve catalyst [61]. 2.1.6. Naphthene ring opening The selective ring opening (SRO) of cyclic paraffins, in particular tetralins [ 62] and decalins, is an useful reaction in upgrading cetane qualities of Diesel fuels [56]. This reactivity is usually performed together with aromatic saturation in new refinery processes still under development. It has been shown that the most active catalysts for these reactions contain Iridium metal [63]. Typical reaction conditions are 250-350 °C with 30 bar hydrogen. A promising catalyst formulation has been reported to be 1 % Ir supported on mesoporous silica-alumina [64]. 2.1.7. Hydrodealkylation Hydrodealkylation of toluene to benzene and methane is performed industrially to increase benzene production in the “aromatics loop”. The reaction is highly exothermic and the typical operating conditions are 550 °C to 660 °C, and 20 to 70 bar. The typical catalysts are be based on chromia-alumina [65]. Ethylbenzene hydrodealkylation is performed as a first step of the xylenes isomerisation process to increase p-xylene production in the aromatics loop. This is the case of the XyMax process from ExxonMobil [ 66], where the two reactions occur in series in two beds located in the same reactor. The ethylbenzene hydrodealkylation catalyst is actually a dealkylation catalyst based on zeolites, like mordenite [ 67], with a noble metal content to hydrogenate ethylene to ethane, working at 370-430 °C and 5-15 bar. In a recent patent a catalyst is described based on H-ZSM5 zeolite with SiO 2 /Al2O3 70-100, crystal size 0.2 to 0.8 microns, containing 0.075 to 0.5 wt. % Pt, bound with an inorganic matrix, such as alumina, extruded to a 1/16-in. diameter extrudate. Hydrogen to ethylbenzene mole ratios are most preferably between 1.0 and 3.0 [68]
2.2.Hydrogenation of oxygenated compounds. 2.2.1. Hydrogenation of carbon oxides. 2.2.1.1. Methanation. Methanation, i.e. the synthesis of methane from hydrogenation of COx is performed industrially according to two main configurations, both using, usually, Ni/Al2O3 –based catalysts [69,70]. Low temperature methanation allows to destroy carbon oxides impurities in hydrogen to avoid the presence of such a oxygenated poisons from hydrogenation catalysts such as e.g. ammonia synthesis catalysts. To do this, Topsøe produces the prereduced PK-7R catalyst, constituted by 23 % Ni wt on an alumina carrier, which ensures that CO and CO 2 are fully converted to methane at an operating temperature of 190°C [ 71]. The composition is similar to that of Süd Chemie METH 134 catalyst. These catalyst cannot be used below 190-200 °C to avoid the formation of Ni(CO)4. For very low temperature methanation Süd Chemie sells a 0.3 % Ru/Al2O3 catalyst (METH 150), active even below 170 °C [23]. Methanation can also be performed using syngases rich in carbon oxides, like those arise from coal or biomass gasification, to convert them into Substitute or Synthetic Natural Gas (SNG) [72]. In this case, the exothermicity of the reaction causes the temperature to increase, interbed cooling being needed to control it into limits to make the reaction thermodynamically favoured. Nickel based catalyst on stabilized support. The high temperature methanation catalyst Topsøe MCR is reported to have stable activity up to 700 °C [73]. Publications report about the use of a 22% Ni catalyst on a stabilized support, with a surface are decreasing from 50 m 2/g (fresh) to 30 m2/g (used) [74]. Until the 1970s the mechanism of methanation was supposed to occur through oxygenated intermediates [ 75]. More recently, it has been found that dissociation of CO over metals such as Ni and Co, may produce surface carbide species whose hydrogenation gives rise to methane and higher hydrocarbons. The via-carbide mechanism has been mostly considered recently for methanation over pure Ni catalysts [76-77] but also over Ni–silica [78] as well as Ni/MgAl2O4 catalysts [77]. Ir spectroscopy methods provide evidence of thye easy formation of oxygenate intermediates and, in particular, of the evolution of methoxy groups into methane by hydrogenation over NiAl2O3 [79]. 2.2.1.2.
Fischer Tropsch synthesis.
The Fischer-Tropsch Synthesis (FTS) has been demonstrated to be a suitable process to produce sulphur, nitrogen and aromatics-free liquid fuels from syngases, not only produced from coal gasification (CTL: Coal-To-Liquids), as originally done, but also from cleaner sources i.e. natural gas (GTL: Gas-To-Liquids), and, potentially, from biomasses (BTL: Biomass-To-Liquids). Discovered by Franz Fischer and Hans Tropsch in 1923 [ 80,81], FTS process has been developed at the industrial level to produce synthetic fuels and chemicals, starting from coal in Germany up to the end of World War II and in South Africa mainly during the period of the embargo [82]. Now FTS is the object of much interest and new plants at the industrial level or at the pilot plant level have been built [ 83]. Ruthenium is reported to be the most active metal in the CO hydrogenation, allowing CO/H 2 conversions higher than that obtained with Fe or Co, leading to lower FTS fuels production costs and producing high molecular weight paraffinic waxes, with typical values of chain growth probability (α) of 0.85-0.95 [84,85]. Furthermore, Ru catalysts can operate in the presence of high partial pressure of water (FTS co-product), and other oxygenate-containing atmospheres, which is an important property to convert syngas obtained from biomass [86,87]. However, iron and cobalt catalysts seem to be those used in industrial application. The low-temperature Fischer Tropsch process, performed at 200-250 °C, 20-30 bar over Cobased catalysts (Co-LTFT) represents today a promising option to convert syngas produced from natural gas into of middle distillate and hydrocarbon waxes both applicable to produce high cetane number and sulphur-free Diesel fuels [ 88]. Among the different metals active as FT catalysts, Cobalt represents the best choice for this application [89] for its moderate price, high activity with stoichiometric syngases arising from methane steam reforming, good selectivity to high molecular weight paraffins and low selectivity to methane, olefins and oxygenates. Among possible supports, alumina is a good choice because of its availability with texture suitable to Slurry Bed Catalytic Reactors (SBCRLTFT). The cobalt FT catalysts are usually supported on high surface area alumina (150200 m2/g) and typically contain 15 ÷ 30% weight of cobalt. To stabilize them and decrease selectivity to methane, these catalysts also contain small amounts of noble metal promoters [90] (typically 0.05-0.1% weight of Ru, Rh, Pt or Pd) and / or an oxide promoter as well (zirconia, lantania, cerium oxide, 1-10%). The role of noble metal doping has been supposed to be to improve the reducibility of cobalt, while the support and the oxide promoters may have a role in controlling the size of the cobalt metal to an optimal value. Authors agree that cobalt metal should represent the active phase in FT, the formation of
cobalt carbide [91] and oxide (which may arise from reaction with water [ 92]) representing deactivation processes. Via-carbide mechanisms have been found for Fischer Tropsch reaction over Co monocrystals [ 93] and seem to be today preferred for FT reaction over Cobased catalysts [94,95]. However, mechanisms through oxygenates are also being proposed for the main reaction to hydrocarbons [96,97] or for ways to oxygenates [ 98]. The High Temperature Fischer Tropsch process (HTFT), performed on iron-based catalysts at 300 °C, and the low temperature process performed with iron catalysts (FeLTFT), are also useful options [99]. Due to their high activity for the water gas shift reaction, iron catalysts are more indicated for coal-based FT syntheses. They produce more oxygenates and a slightly different molecular weight distribution for hydrocarbons [99]. A typical iron FT catalyst contains also few percents of silica, copper and potassium. Copper is added to aid in the reduction of iron, while silica is a structural promoter added to stabilize the surface area but may also have a chemical effect on the catalyst properties. Potassium is considered to increase the catalytic activity for FTS and water–gas shift reactions, to promote CO dissociation and enhance chain growth, increasing olefin yield and lowering the CH4 fraction [100,101]. Under reaction conditions, the catalyst converts into mixtures of carbides, like χ -Fe5C2 and ε -Fe2.2C, and magnetite Fe3O4, with only small amounts of α -Fe. Carbidic rather than metallic catalysis should really occur [100]. 2.2.1.3. Methanol synthesis. Syngases with opportune stoichiometries may be converted into methanol [ 102]. Today reaction is performed at 200-250 °C, 50-150 bar, in the so-called low temperature synthesis process [ 103,69] . Most of commercial catalysts today are based on the Cu-Zn-Al system, prepared by coprecipitation, with Cu:Zn atomic ration in the 2-3 range, and minor alumina amounts [104]. Cu-Zn-Cr and Cu-Zn-Cr-Al systems have also been considered and used at the industrial level [ 105,106]. The MK-121 catalyst from Topsøe contains, in the unreduced form, > 55 % wt CuO; 21-25 % ZnO, 8-10 % Al 2O3 in the fresh catalyst, graphite, carbonates, moisture balance [107]. Also the BASF Catalysts (such as S3-85 (83 m2/g [108]) and S3-86 [22] and Süd Chemie MegaMax® 700 catalysts are reported to be constituted by CuO/ZnO/Al2O3 [23]. The formulation of KATALCOJM methanol synthesis catalysts [ 109] is based on a coppercontaining mineral in which a controlled proportion of the cations have been replaced by zinc ions. The active phase is supported on a specifically designed zinc aluminate
throughout operating life, while allowing reactant gases access to the active copper metal surface. Micro-crystalline zinc oxide is also present to protect the copper metal surface sites from poisons such as sulphur and chlorine compounds. A further unique aspect of this catalyst, patent protected, is that magnesia is added to modify the catalyst structure during manufacture to maximize activity throughout catalyst life. Several authors [110-111] agree that the actual reactant for methanol synthesis over Cu– ZnO–Al2O3 is CO2, usually present with a concentration near 10 % in the reactant syngas, although the reaction can also be performed with CO only. Recent data suggested that, for methanol synthesis, the more reducing the gas the more active the catalyst, and the more dispersed the zerovalent copper particles [ 112,113] , where the reaction should occur. However, CO2 should oxidize copper [ 114] so providing the active sites for methanol synthesis [69,111]. Most authors agree that the reaction should occur through formation of formates and their hydrogenolysis to methoxy groups [69,103,111,115,116] i.e. through oxygenated intermediates. 2.2.1.4. Methanol and higher alcohol syntheses. Modified methanol and Fischer Tropsch synthesis catalysts, usually containing alkali, allow the production of methanol-higher alcohol mixtures. These mixtures can be used as octane boosters of gasoline, in contrast to the pure methanol/gasoline mixture that tends to split in two phases in the presence of moisture. In the Octamix process from Lurgi [69] an alkalized Cu-ZnO-Al2O3 or Cu-ZnO-Cr 2O3 low temperature methanol synthesis catalyst is used at 285-300 °C and 60-90 bar, giving rise to methanol / ethanol / 2-propanol /isobutanol mixtures, while in the Snamprogetti MAS process [ 117] K-doped ZnO-ZnCr 2O4 catalysts work at 350-400 °C and 100-150 bar, producing essentially methanol-isobutanol mixtures. This mixture can be used to produce ethers with the Ethermix process [ 118]. In an alternative process, Substifuel from IFP, a alkalized modified Fischer Tropsch catalyst (alkali doped Co-Cu-ZnO-Al2O3) is used at 270-320 °C and 60-100 bar, to produce linear higher alcohols /methanol mixtures [119]. Alkali doping should promote C-C bond formation in all cases, but with different mechanisms. In the case of the IFP process the mechanism is similar to that of Fischer-Tropsch linear hydrocarbon synthesis. In the case of the Lurgi and Snamprogetti process aldol condensation of a acetaldehyde-like intermediate is likely. 2.2.2. Specific hydrogenation of C=C double bonds in unsaturated oxygenates.
Several processes imply the hydrogenation of aromatic or olefinic multiple CC bonds without reducing co-present oxygenated functions. Among quite old processes we can mention the hydrogenation of C=C double bonds of unsaturated fatty acids chains in converting vegetable oils into margarine. Small crystal size nickel on high surface area and pore volume alumina, silica-alumina, or silicas including diatomite, working at 180230 °C, 2-6 bar, represent the traditional catalysts [ 120], like the catalysts of the PRICAT series from Johnson Matthey [ 121], and NYSOSEL from BASF [122]. As for example, Pricat 9910 is reported to be constituted by 22% Ni, 4% SiO2 dispersed in hydrogenated edible fats [123]. Batch processes are most commonly used in the oil industry. The carbon double bonds are partially or fully saturated during the hydrogenation. Concomitantly, the catalytic isomerization of naturally occurring cis fatty acids (CFAs) to trans fatty acids (TFAs), which have negative health effects, takes place. The selectivity of the process represents a considerable challenge, aiming to enhance the hydrogenation rate of the double bonds saturation and simultaneously to suppress the isomerization. Doping nickel catalysts with silver seems to reduce cis-trans FA isomerization [124]. Recently, new catalysts containing 0.2 % Pd on boron treated alumina have been proposed to avoid possible contamination of margarine by nickel [125]. Supported noble metals, particularly Pd, allow the selective hydrogenation of α ,βunsaturated carbonyl compounds on the C=C double bond [ 126]. A typical example is the synthesis of Methyl-Isobutyl Ketone (MIBK) by hydrogenation of Mesityl Oxide (MO) previously produced by acetone condensation. Pd catalysts allow the highly selective hydrogenation of the C=C bond without any reduction of the C=O bond (150-200 °C, 3-10 bar). Nowadays, MIBK is industrially obtained from acetone in one step in trickle-type reactors at low temperatures (120-140 °C) and high pressures (10–100 bar) on multifunctional catalysts such as Pd or Pt supported on sulfonated resins, which contain condensation, dehydration, and hydrogenation functions [ 127]. Both the aldol condensation and the dehydration reactions are reversible under these reaction conditions but the catalyst shifts the equilibrium in favor of MO by irreversibly hydrogenating it to MIBK. The full reaction may also be performed in a single liquid phase reactor using Mg-Al hydrotalcite with 0.1% Pd [128]. Similarly, 0.1 wt % Pd added to 4% Na/SiO 2 catalysts allow the one-step synthesis of the saturated aldehyde 2-ethylhexanale with 85-95% selectivity, by self-condensation of n-butanal at 350-400 °C in the presence of hydrogen [ 129,130]. Many other industrially significant examples may be cited. Among others, the selective hyd tio of phenol to loh mixtur (an inte diate in the Nylon6
synthess) is performed industrially over Pd- based catalysts [ 131,132] such as Pd/Ca-Al2O3 [133,134] or Pd/C [55] at 140-170 °C, 1-20 bar. Pd/C catalyst was used to produce cyclohexane carboxylic acid from the hydrogenation of benzoic acid at 170 °C and 10-17 bar [136]. 2.2.3. Partial hydrogenations of oxygenates. Several processes imply the partial hydrogenations of oxygenated compounds (such as carbonyl compounds or esters) to alcohols, thus retaining the C-O single bond. Most of these processes are currently performed over copper-containing catalysts. Three predominant catalytic systems are used for these processes: those based on copper chromite, those based on copper/zinc oxide and those based on Zn-free copper on alumina [135]. Copper/silica, nickel/silica and nickel/alumina catalysts are also offered e.g. by Johnson Matthey [37]. Copper chromites, mostly with large copper excess with respect to the spinel stoichiometry, are the traditional catalysts for the production of fatty alcohols by hydrogenation either of triglyceride fats or of fatty acid methyl esters (FAMEs) at 200 °C, 200-250 bar [55]. Copper chromites are generally promoted by barium and or manganese. As for example, the fixed bed hydrogenation of furfural to furfuryl alcohol can be performed on the Süd Chemie G22F catalyst whose composition is CuO 38 %, Cr 2O3 37 %, BaO 11%, silica-balance. For the slurry phase furfural hydrogenations Süd Chemie G99D catalyst is proposed whose composition is CuO 46 %, Cr 2O3 44 %, MnO2 4% with a high surface area of 70 – 80 m 2/g [23]. Copper zinc oxide catalyst 33% CuO, 66 % ZnO, entirely Cr-free, is used in the hydrogenation of butyraldehyde and 2-ethyl-hexenal to the corresponding alcohols and for the hydrogenation of maleic acid dimethyl esters to 1,4butandiol, in substitution of copper chromite [55,
136
]. A catalyst with different
composition, 64% CuO, 24 % ZnO, Al2O3 balance, is used for the gas-phase hydrogenation of maleic anhydride to 1,4-butandiol. 2-Ethylhexanale is selectively converted into 2-ethylhexanol by hydrogenation over Cu-ZnO-Al2O3 catalyst at 130 °C [129,130].
For the hydrogenation of maleic anhydride, a catalyst constituted by CuO 56 %, MnO2 10 %, alumina balance can be used. By variation of the standard catalyst, selectivity can be shifted either in the direction of the diols, or the intermediate γ-butyrolactone, or
In the case of the oxo-synthesis products, in general two steps are performed, like in the case of the Johnson Mattey Oxo Alcohols Process [137] a Cupper-Zinc catalyst (PRICAT CZ 29/2) is used to hydrogenate oxo-aldehydes to oxo-alcohols, while a Nickel catalyst (HTC-400) is used later to hydrorefine the product with removal of traces of carbonyls. Mn-promoted CuO/Al2O3 catalyst can be used for gas phase hydrogenations of oxo aldehydes. NiO 68%, CuO 3%, silica balance is usually applied in specific trickle phase oxo aldehyde hydrogenation processes [23]. 2.2.4. Full hydrogenation and hydrodeoxygenation of oxygenates to hydrocarbons. In recent years efforts have been devoted to the conversion of renewable feedstocks into fuels. Among these techniques, the catalytic conversion of fats and oils in the presence of hydrogen is found to represent a promising option. ENI and UOP jointly developed the Ecofining process [138,139] and Neste Oil developed the NExBTL diesel producing process, both consisting in the production of hydrocarbons in the Diesel fuel range by total hydrogenation of vegetable oils. In patents [ 140,141], typical hydrodesulphurization catalysts like sulphided Ni-Mo/Al2O3 or alumina supported noble metals (Pt, Pd) are cited as the catalysts. Hydrogenation conditions include a temperature of 200°C to 300°C and a hydrogen partial pressure of 35-70 bar. 2.3.Hydrogenation of nitrogen containing compounds. Ammonia synthesis is one of the few industrial processes were unsupported metals are used [142]. The traditional catalyst, still used in most plants, is based on bulk iron, containing several elements for activation and stabilization. It was originally obtained from a swedish magnetite mineral. The catalyst may be charged in the oxidized form (either magnetite Fe3O4 or wüstite FeO) or in the prereduced form, both available commercially from most producers. Topsøe sells the KM1 unreduced catalyst, 91-95 % wt of iron oxides with 5-9 % wt K 2O, Al2O3, CaO and SiO2, as well as the prereduced KM1R catalyst (89-93 % Fe,FeO). The working temperature range is 340-550 °C, with pressure range 80-600 bar [ 143]. The catalysts of the KATALCOJM 35 series and KATALCO JM S6-10 series proposed by Johnson Matthey [144] are based on magnetite multi-promoted and/or stabilized by K2O, Al2O3 and CaO. To achieve higher activity at low pressures in the region of 80 - 150 bar, KATALCOJM 74-1 catalysts containing also CoO as a promoter are proposed alternatively.
Süd Chemie proposes the AmoMax 10 catalyst which is a wüstite-based ammonia synthesis catalyst (98% FeO plus promoters [23]) that, according to the firm, features significantly higher activity than magnetite-based catalysts. This high activity level is also evident at low operating temperatures, allowing improved conversion at thermodynamically more favorable conditions. AmoMax 10 is available in oxide and pre-reduced, stabilised form. BASF Ammonia Synthesis Catalyst is a multi-promoted iron based catalyst: in its commercial form it contains ≥ 67% of iron in metal form with ≥ 2.3 % Al2O3, ≥ 2.1 % of promoter I and small amounts of oxidized iron, and three other promoters [ 145]. In the nineties, Kellogg Brown and Root (KBR) developed a new process based on Ru based catalysts denoted as KAAP™ (Kellogg Advanced Ammonia Process). The proprietary KAAP™ catalyst, which is manufactured and guaranteed by BASF Catalysts LLC under exclusive license to KBR, consists of ruthenium on a stable, high-surface-area graphite carbon base [ 146]. According to the literature it should contain alkali and alkali earth promoters such as K, Cs, Ba [ 147,148]. The KAAP™ catalyst is reported to have an intrinsic activity ten to twenty times higher than conventional magnetite catalyst. This allows operation at 90 bar synthesis loop pressure, which is one-half to two-thirds the operating pressure of a conventional magnetite ammonia synthesis loop. At this low pressure, only a single-casing synthesis gas compressor is needed and pipe wall thicknesses are reduced. This results in savings in plant capital equipment and operating costs. 2.3.1. Sinthesis of amines by hydrogenation of nitriles and amination of alcohols and carbonyl compouds. Amines may be produced by different methods. Among the most common amination of alcohols [149], reductive amination of carbonyl compounds and hydrogenation of nitriles [55,150]. Heterogeneous catalysed amination of alcohols is established as the most important industrial process for manufacture of different aliphatic and aromatic amines. The reaction takes place in the presence of hydrogen at about 5 – 250 bar and about 100 – 250 °C, depending on the catalyst and on whether the liquid-phase or the gas-phase process is used. Ni, Co and Cu catalysts are mostly used [ 151]. For ethylamines and propylamines, supported nickel or cobalt catalysts [55], are preferred. For long chain amines, copper
catalysts [152] such as copper chromites are used. Noble metal catalysts, in fact, have a tendency to cause C – C or C – N bond cleavage and can hence give lower selectivities. Reductive alkylation is an effective method to synthesize primary and higher amines from aldehydes and ketones and hence is commercially used for the synthesis of a variety of fine and specialty chemicals. The amine (or ammonia) is condensed with a carbonyl group to yield the intermediate imine (a Schiff’s base) which can then be catalytically hydrogenated to the desired amine. The main challenge in these types of reaction is the selectivity to the desired amine while minimizing the reduction of the carbonyl group. Raney type nickel or cobalt catalysts are used for this reaction [ 55]. Platinum group metals are capable of facilitating reductive alkylation of alkyl and aryl amines with ketones or aldehydes. Evonik has developed several carbon supported Pd and Pt-based catalysts that can effect high conversion and selectivity. Platinum is more effective than palladium in alkylating amines and carbonyls of higher molecular weight. Typical operating conditions for reductive alkylation include pressure of 5-35 bars and temperature of 50-150°C. Hydrogenation of adiponitrile to hexamethylene diamine (a monomer for Nylon 6,6) is performed in slurry phase over Ni Raney at 120-150 °C – 5000 psig [136]. Promoted Raney Nickel are usually promoted either by Cr and Fe [55] or by molybdenum and alumina, such as the BASF Actimet 8040P catalyst (92v % Ni, 1 % Mo, 7 % Alumina, Average Particle Size 35 microns). This catalyst is supplied immersed in water to protect it from air oxidation. In a dry state, it is pyrophoric [153]. Ni-SiO2 with variable Ni content 70-35% can be used for hydrogenation of long chain nitriles [23]. 2.3.2. Hydrogenation of nitroaromatics to aromatic amines. Hydrogenation of nitrobenzene to aniline is performed with different processes and reactor systems. Slurry phase processes apply mostly Ni based catalysts such as Ni-Silica containing small amounts of iron. Hydrogenation in fixed bed liquid phase in a plug flow reactor (Dupont-KBR process [ 154]) uses a of a noble- metal-on carbon catalyst. A similar process is reported to be carried out in liquid phase at 75°C, 500 psig over carbon supported palladium to produce toluene diamine from dinitrotoluene [136]. Nitro groups are readily hydrogenated. However selectivity may become an issue if functional groups like double bonds, triple bonds or halogen substituents, aromatic rings, carbonyls and even oximes are present [ 55]. Vanadium promoted Pt catalysts (1-3 % Pt, 0.5-2 % V) are available from Evonik for this application [155]
Gas phase process mostly use Mn and Ba- promoted copper chromite. The process is conducted at temperature values within 250–350 °C, low pressure values (below 10 atm) but with a high excess of hydrogen (up to 1:100 ÷ 1:200 molar NB/H2) to make the NB conversion practically complete 3. Metal catalysts for dehydrogenation reactions. Dehydrogenations are endothermic equilibrium reactions, thus carried out at moderately high temperature and usually not far from ambient pressure. Most of dehydrogenation catalysts are based on metals, mostly the same used also for hydrogenations, which are in fact the reverse reactions. However, several oxides (like chromia-aluminas and zinc oxide) also show catalytic behavior forthese reactions. 3.1.Dehydrogenation of alkanes to olefins and dienes. Although alkalized chromia-aluminas represent the main alkane dehydrogenation catalysts, Pt/Al2O3 catalysts are also quite largely used in this field, beingmore stable at higher temperature and in the presence of steam. In fact alkalised 0.3-0.5 % Pt/Al 2O3 (such as the UOP OleflexTM DeH-14 catalyst) with Sn, Zn and/or Cu as promoters acts as the catalyst in the Oleflex light paraffin dehydrogenation technology and in the Pacol long linear paraffin dehydrogenation technology, both from UOP [ 156]. Such catalysts may contain less than 1 % of both Pt metal and alkali wt/wt, and additionally an activator metal, on a medium surface area alumina (100-200 m2/g) [157]. Pt-Sn/Al2O3 catalyst is also used in the IFP process for heavy paraffin dehydrogenation [ 158] at 450-475 °C, 2.5 atm. It has been established that relative to mono metallic Pt/Al2O3 catalyst, bi metallic Pt–Sn/Al2O3 promotes desorption of olefins and hence may follow a different mechanism. 3.2.Aromatization. Catalytic naphtha reforming is a main industrial process converting aliphatic hydrocarbons into mononuclear aromatics both in refinery (to produce high-octane gasoline) and in petrochemistry, to produce benzene, toluene and xylenes (BTX). Bifunctional metal-acid catalysts are typically used for naphtha catalytic reforming [ 159]: the acidic function allows isomerisation and cyclization of paraffins while the noble metal function catalyzes dehydrogenation to aromatics. In typical reforming catalyst formulations the metal function is provided by Pt, which is supported over the acid function, chlorinated gamma alumina 150 – 300 m2/g). In the more t multimetallic catalysts, the catalytic
properties of Pt is improved by the addition of another metal, such as usually Re, with the further addition of some other element among Sn, Si, Ge, Pb, Ga, In, Ir, Th La, Ce, Co, Ni [160]. Also at least 10 wppm of one or more alkaline earth metals selected from Ca, Mg, Ba, and Sr, wherein the total amount of modifier does not exceed about 5000 wppm can be present [161].These additives modify the activity, selectivity and stability of the catalyst. The effects of the additives are multiple. (i) They decrease the deep dehydrogenation capacity of Pt and thus decrease the formation of unsaturated coke precursors. (ii) They decrease the hydrogenolysis capacity and therefore also decrease the formation of light gases. (iii) They modify the concentration of surface hydrogen. This has an effect on the relative production of different reaction intermediates and therefore on the final reaction selectivity. (iv) A portion of the additives remains oxidized on the surface and modifies the amount and strength of the acid sites of the support. The typical composition of an industrial catalyst, such as R-98 catalyst from UOP to be used in fixed-bed, semi-regenerative reforming units, the claimed composition is 0.25 % wt Pt, 0.25 % wt Rh, proprietary promoters [162]. For application in continuous regenerative reforming R-232 (0.375 % Pt) and R-234 (0.290 % Pt) catalysts are produced by UOP [ 163]. 3.3.Dehydrogenation of alcohols to carbonyl compounds. The industrial synthesis of some carbonyl compounds is performed by dehydrogenation of the corresponding alcohols. ZnO based catalysts or the more active Cu-based catalysts, such as Cu-ZnO, Cu/SiO 2, Cu-Al2O3 or copper chromite catalysts at 200-450 °C are employed to this purpose [ 164]. The dehydrogenation of 2-butanol to methyl-etyhylketone (MEK) represents one of the most important processes of this kind. The Deutsche Texaco process employs a copper based catalyst at 240 – 260 °C under normal pressure [ 165]. Copper on silica [166,167] or Cu-Zn-Al catalyst [168] are reported for this reaction. The dehydrogenation of cyclohexanol to cyclohexanone may be performed at 230-
300°
C
with
a
CuO(33%)-ZnO
catalyst
while
the
cyclododecanol to cyclododecanone is performed with a
dehydrogenation
of
CuO(31%)-ZnO catalyst
[23].
4. Steam reforming and related catalysts Several important reactions imply the use of water to produce hydrogen. In these reactions water formally acts as an oxidant. These processes are very relevant just in relation to hydrogen production technologies.
4.1.Steam reforming of natural gas and other hydrocarbons. Hydrogen is mostly produced today through steam reforming of hydrocarbons, usually natural gas [169,170], performed at 1000-1200 K. The industrial catalysts are invariably based on Ni supported on an alumina-based carrier, usually stabilized by the presence of alkali and/or alkali earth cations [172]. Typical methane steam reforming catalysts contain 10-25 % wt Ni, 70-85 % Al 2O3 or aluminate supports and up to 5 % K, Ba, Ca [103,169,170]. These catalysts that typically work in the presence of much steam at 700900 °C, 30-50 bar, need an important refractory character. The support phase is a refractory oxide such as alpha-alumina, Mg aluminate spinel MgAl 2O4, calcium aluminate CaAl12O19 calcium-potassium aluminate CaK2Al22O34 [23].The catalyst composition varies slightly if the feed shifts from methane, to other light alkanes or naphtha. In fact coking of these catalysts increases with molecular weight of the feed. For this reason catalysts for naphtha steam reforming contain more alkali metal and also more Nickel (25 %). These catalysts, however, are reported to be rapidly irreversibly deactivated in case of daily start-up and shut-down operation, typical in case of hydrogen and Fuel Cells domestic use. In this case, Ni catalysts doped with small amounts of noble metals are reported to display an “intelligent” behaviour, with suppression of such deactivation phenomena [173]. Modern plants contain a previous adiabatic fixed bed prereforming reactor, where higher molecular weight are reformed. These catalysts contain much more Ni (NiO 45-60 %), with other additional components (some silica and chromia), Oxygen- blown autothermal reformers and secondary reformers require a mixed loading of catalysts, comprising an active heat shield (mostly Ni on alfa alumina) and a reforming catalyst of excellent physical stability and thermoshock resistance. The production of low H2/CO ratio syngases through the dry reforming of methane, i.e. the reaction of methane with carbon dioxide, is also a potentially useful process. Coking in this case is a more serious problem, and, for this reason, noble metal catalysts, which suffer coking less than Ni based ones, have longer life [ 174-176]. Also in this case bimetallic catalysts, such as Pt-doped Ni catalysts may have optimal behavior [ 177]. 4.2.Water gas shift. While high temperature water gas shift are essentially oxidic system based on Fe-Cr oxides, low temperature water gas shift (LTWGS) catalysts are thought to be constituted
by copper metal with ZnO and Al 2O3 components. Topsøe sells three different catalysts for the SCR process: besides the “classical” Cu-ZnO-Al 2O3 (40 % Cu) catalyst, Cs-promoted Cu-ZnO-Al2O3 (40 % Cu) is produced to limit the by product formation thus allowing lower methanol levels in the process condensate and CO 2 stream leaving the plant, and CuZnO-Cr 2O3 (14 % Cu) as a guard catalyst located on the top of the bed to protect copper/zinc/aluminum against chlorine poisoning. These catalysts work at 185 - 275°C [178]. Cu-ZnO-Al2O3 and alkali-promoted Cu-ZnO-Al2O3 are also sold by Johnson Matthey. According to these producers [ 179] alkali doping suppresses methanol formationand also boosts poisons pick-up, resulting in the highest poison capacity of any commercially available low temperature shift catalyst. Süd Chemie sells for LTWGS 58 % CuO/ 31 % ZnO / 11 % Al2O3, as well as a promoted catalyst with 1 % promoter [23]. A commercial low-temperature shift catalyst, BASF K3110 contains nominally 40 % CuO, 40 % ZnO, 20 % Al2O3, BET area 102 m2/g, pore volume 0.35 ml/g, copper area 9.83 m2/g, dispersion 4.8%, copper crystallite size 219 Ǻ [180]. BASF patented recently a new preparation method to produce water gas shift catalysts that, in the unreduced form, have 50.5-52.3 wt % CuO, 27.1-29.6 wt % ZnO, 18.8-19.7 wt % Al2O3, 0.08-0.1 wt % Na [181]. According to the literature, two different mechanisms may justify the WGS reaction over Cu-Zn-Al catalysts: i) the redox mechanism, where water oxidizes the catalyst producing hydrogen and CO re-reduced it producing CO 2. ii) the “associative” mechanism through formate species, where water hydrates the surface producing active OH species. In both cases the active catalyst surface is expected to be somehow oxidized [ 182-184]. While widely used in large hydrogen and ammonia production plants, such catalysts cannot be applied for small scale devices like those are concerned for fuel cells technologies. This is due to the need of a activation step carried out in the reformer by slow and careful addition of H2 carefully controlling the exotherm to avoid sintering, the problems related to their pyroforic behavior, and their low catalytic activity implying too big reactor beds. Thus, more active and safe low temperature WGS catalysts are needed for these applications [185]. Zn-free new Cu-Al2O3-CuAl2O4 water-gas shift seem to have advantages among which they can be activated by calcination [ 186]. The most promising types of WGS catalysts, and those most extensively studied for these applications, are based on supported noble metals, like Pt/CeO2-Al2O3 based catalysts [ 187], in monolytic structures. However, instability of this catalyst under fuel processing conditions has been a recurring problem. There is
much debate over what deactivation mechanisms are actually involved, and research on the Pt-CeO2 based catalyst continues with a particular emphasis on increasing catalytic activity and stability. Catalysts based on gold, such as Au-Fe 2O3, Au-CeO2 and Au-TiO2 find high activity and commercial interest for LTWGS [188]. 4.3.Steam reforming of methanol. Methanol is a liquid fuel which shows potentiality as an “hydrogen vector” for small scale stationary production of hydrogen [ 189]. More than ten medium-size plants (100-1000 Nm3/h) are in operation worldwide for the stationary production of hydrogen from methanol for chemical and electronic industry with the Tøpsoe “package hydrogen plants” [190,191]. Methanol and water vapour are fed into a multitubular reactor heated externally by a hot oil circulation. Although the Tøpsoe process is reported to be “based on methanol decomposition”, the published data suggest that the reaction mostly occurs through endothermic steam reforming of methanol (MSR). BASF patented for this reaction catalysts with composition CuO 67 %wt, ZnO 26.4 %wt, Al 2O3 6.6 %wt (atomic ratios 65:25:10) with surface areas 80-100 m 2/g [192]. Catalysts with similar compositions (30-50 % wt/wt as CuO) to those of water gas shift catalysts are tested in the literature for the reaction [193-196], such as BASF F3-01 and K3-110 (CuO 40 wt%, ZnO 40 wt%, Al 2O3 20 wt %) catalysts, have been used in several methanol steam reforming studies [ 197]. Also in the case of methanol steam reforming, the real oxidation state of the Cu-Zn-Al catalysts is object of discussion. Methanol is also assumed to be a suitable liquid source for on-board production of hydrogen by steam reforming in the case of fuel cell electric engine cars and boats [ 189]. As already discussed, however, catalysts based on the the Cu-ZnO-Al 2O3 system, due to their pyrophoric behaviour and the need of careful and long start-up and shut-down procedures, cannot find application in fuel cell technologies. Catalysts based on oxidesupported Pd-Zn alloys have been developed for this application [198]. 4.4. Steam reforming of ethanol. The production of hydrogen from renewables would result in the release of the energy production from fossil fuels and, simultaneously, in the reduction of greenhouse gases emissions [199]. With this in mind, efforts have been devoted to the development of efficient catalysts for the steam reforming of bioethanol, i.e. ethanol produced by fe tatio of st ch ich tabl of obtained fr lig ellulosi
materials. Also in this case, Ni-based catalysts have been tested and found very active [200-203]. The formation of methane is a critical aspect, because it limits the formation of hydrogen at low and intermediate temperature, or implies the use of even higher temperatures to steam reform it to produce high hydrogen yields. In these conditions, the water gas shift equilibrium limits hydrogen yield unavoidably. A second low temperature WGS reaction step should be necessary to complete hydrogen production. Recent studies [demonstrated that cobalt catalysts may give better performances than Ni catalysts in particular at lower temperature and that Ni-Co alloy catalysts [204-206] allow the best catalytic behavior among non-noble metal catalysts. Noble metals are actually even more active for ESR and may be preferable for small ESR monolytic devices to produce hydrogen for fuel cells. Noble metal catalysts (such as those based on Rh-Pt) are less susceptible to coking, more easily regenerated, and also more sulphur tolerant in case of use of gasoline-bioethanol blends [ 207]. 5. Metals in acid and base catalysts. 5.1.Noble metal doping of acid catalysts Several acidic catalysts may contain very small amounts of noble metals (usually Pt) and work in the presence of hydrogen. This is mostly due to the activity of Pt in hydrogenating carbonaceous materials and unsaturated hydrocarbons, which may prevent catalyst coking. A typical case is that of catalysts for light paraffin skeletal isomerisation, performed in refinery. The oldest and still most active catalysts for this reaction are based on Ptcontaining chlorided alumina, which work at 130-180 °C in C4 and C5/C6 paraffin isomerization, but are very sensitive to poisons. Among others, Akzo Nobel and Total developed new catalysts in two separate grades, AT-2 for C4 and AT-2G for C5/C6 isomerization. Both grades are amorphous, bifunctional catalysts, consisting of platinum on chlorided alumina. UOP I-82, I-84, I-122 and I-124 are also amorphous, chlorided alumina catalysts containing platinum, used for the different versions of paraffins isomerisation processes [208]. The amount of Pt is different being tailored in relation to the amount of benzene and sulphur I the feed. Less active but more robust catalysts based on zeolites have been developed for light paraffin isomerization. Dealuminated mordenite (MOR) is the basic structure of some commercial catalysts [209,210,211], based on alumina-bound Pt-H-MOR with SiO 2/Al2O3 ~ 1517. These catalysts are active in the 250-280 °C range, thus at a definitely higher
temperature than those based on chlorided alumina, when the thermodynamics is less favourable, but are more stable and more environmentally friendly. Dealumination until a framework/extraframework Al ratio ~ 3 improves the catalytic activity [ 212]. This catalyst may be applied e.g. with the IPSORB process of IFP [ 213]. This agrees with the quite easy diffusion of branched molecules in the main channels of mordenite. Zeolite omega (isostructural with mazzite, MAZ) is the basic structure of other zeolitic C4C6 paraffin scheletal isomerization catalysts cited under development by Süd Chemie as HYSOPAR® catalysts, reported to be characterized by their outstanding tolerance of feedstock poisons such as sulphur (even more tha 100 ppm) and water with very high catalyst lifes [210,211]. The catalyst is alumina-bound Pt-H-MAZ with Si/Al ∼ 16, working at 250 °C with WHSV 1.5 h -1 and a H2/hydrocarbon ratio of 4. Pt and hydrogen have the effect of reducing coking e hydrodesulphurizing S-containing compounds. It may be applied in the so-called CKS ISOM process licenced by Kellogg, Brown and Root. This catalyst is reported to be more effective than Pt-H-MOR commercial catalysts, and more stable than the catalysts based on chlorided aluminas. Sulphated zirconia and tungstated zirconia-based catalysts have also been developed for light paraffin isomerization, presenting intermediate activity and stability. Pt-promoted sulphated zirconia catalysts are commercialized, e.g. by Süd Chemie (HYSOPAR-SA catalysts) [210,211]. They work at 180-210 °C with moderate limits in the allowed feed purity and possible regeneration. The so-called EMICT (ExxonMobil Isomerization Catalyst Technology) catalyst, based on promoted WO 3-ZrO2, is reported to be very effective in C5C6 paraffin skeletal isomerization at 175-200 °C even in the presence of 20 ppm water and to be fully regenerable [214]. Pt and Mn promoted WO3-ZrO2 catalysts are very active, e.g. in the isomerization of n-hexane at 220-250 °C [ 215]. Also catalysts for xylenes isomerization, toluene disproportionation to benzene and xylenes, as well as for transakylation reactions usually contain small amounts of noble metals (usually Pt) and work in the presence of hydrogen. They are based on zeolites like ZSM5 and/or mordenite and work in contact with gas phase hydrocarbons at 380-500 °C, 5-40 bar [67]. 5.2.Alkali metal containing basic catalysts. Alkali metal work as catalysts, or better as initiators, in some basic catalyzed reaction of industrial interest: working in non-protic solvents, they abstract very weakly acidic hydrogen atoms forming carbanions which initiate chain reactions. They may be supported
on solid supports. For selective position isomerization of complex olefins, strong bases are needed working at lower temperature. Sumitomo developed Na/NaOH/Al 2O3 catalysts for the isomerization of 5-vinylbicyclo-[2.2.1]-hept-2-ene (VBH) into 5-ethylidenebicyclo[2.2.1]-hept-2-ene (EBH), and of 2,3-dimethyl-1-butene into 2,3-dimethyl-2-butene [216,217]. The side alkylation of toluene and xylenes with olefins is performed with alkali metals as initiators. The real catalysts are the benzyl-sodium organometallics that attack the olefin and are regenerated by transmetallation. The synthesis of isobutylbenzene (an important intermediate for the production of the non-steroidal antiinfiammatory agent ibuprofen) from toluene and propilene is performed at 200-250°C in the presence of an alkali metal such as the Na-K alloy [218] or of an organometallic compound [219,220] under ∼ 30 atm of olefin. Heterogeneous catalysts constituted by potassium on alumina and on magnesia [ 221] or cesium on nanoporous carbon are also active for this reaction [ 222]. Similar reactions occur with ethylene [223]. similar industrial processes, ortho-xylene reacts with 1,3-butadiene forming 5-tolyl-2-pentene with fixed bed K/CaO (AMOCO process) or Na/Na 2CO3 catalysts at 140 °C (Teijin process) [216,217]. 6. Metal catalysts for oxidation reactions. Oxidation of hydrocarbons and of other organic compounds is normally exothermic noninvertible reaction, and is consequently carried out at moderate or high temperature. Catalysts allow to reduce reaction temperature and to orientate selectivity. 6.1.Selective and preferential oxidations. Selective oxidations are mostlt catalyzed by complex oxides including vanadium, molybdenum, tungsten, iron, chromium, as the main redox elements. However, several metal catalysts find application for selective oxidation of hydrocarbons. Among the largest productions, ethylene selective oxidation to ethylene oxide (EO) is performed over silver based catalysts. Typical catalysts may contain 8-15% by weight of silver deposed over low surface area alpha-alumina, 0.5-1.3 m 2 /g with a porosity of about 0.2-0.7 cc/g. The catalyst may contain several promoters such as 500-1200 ppm alkali metal (mostly cesium), 5-300 ppm by weight of sulphur as cesium or ammonium sulphate, 10-300 ppm of fluorine as ammonium fluoride or, alkali metal fluoride [224]. Similar catalysts are used when oxidation is performed with air or with oxygen at 250-280 °C, ~ 20 bar. Silver, either in the form of bulk crystals (granules with 30-35 cm 2/g surface area [225]) or of gauzes (high purity silver, < 100 ppm trace metals, with 0.1-2 mm sieve, 1.3-1.8 g/cm3 bulk
density [226,227]), is also used for methanol oxidative dehydrogenation to formaldehyde. The reaction takes place in the presence of oxygen and steam at 620-700 °C, the active species being considered to be adsorbed oxygen on silver metal [ 228]. Vinyl acetate monomer (VAM) is usually produced by the catalysed, vapour phase reaction of acetic acid with ethylene and oxygen in a fixed bed tubular reactor using a supported noble metal catalyst. The reaction is carried out at 175-200 °C and 5-9 bar pressure usually over silica supported Pd-Au bimetallic catalysts promoted with potassium acetate [229], like Noblyst® EJK 3017 catalyst from Evonik [25]. More recently, a new catalyst with a similar composition has been developed by Johnson Matthey to be used in the new fluidised-bed ‘Leap Process’ developed by BP [ 230]. The catalytic partial oxidation (CPO) of methane, has received considerable attention for synthesis gas production because it provides close to 100% methane conversion and >90% synthesis gas yields in millisecond contact times [ 231]. Compared to contact times of seconds in a steam reformer, CPO reactors can be three orders of magnitude smaller processing the same amount of synthesis gas. In addition to reduced investment costs, methane CPO supplies a H2/CO ratio of 2/1, which is favorable for methanol or FischerTropsch synthesis. Alternative applications are to produce hydrogen in refineries and filling stations [232]. Ni and Rh-based catalysts have been identified to be the most promising CPO catalysts [233,234], the support being a refratory ceramic material such as α -Al2O3, magnesia or zirconia. In contrast to supported Ni catalysts, however, Rh-based catalysts display both high activity and stability during the catalytic partial oxidation of methane to synthesis gas. Rh catalysts also show higher resistance to carbon deposition and to sulphur poisoning. However, due to the high cost of Rh, its use for commercial application is one of the key issue. Ni catalysts deactivate due to metal evaporation and formation of NiO and NiAl 2O4. Pt and Ir also showed high stability, but significantly lower conversions and selectivities compared to Rh catalysts. Rapid deactivation was also observed in case of Pd coated monoliths as a result of coke deposition. Results clearly pointed out that the final performance of the supported Rh catalysts are strongly influenced by the conditioning procedure. Important processes of reconstruction of Rh particles arising from their interaction with the reacting mixture and surface Ccontaining species certainly occur [ 235,236]. Data also suggest that several elements, such as cobalt, may act as promotors of Rh/MgO catalysts for CPO [ 237]. Several processes
have been developed at the nearly industrial stage, such as CONOCO PHILLIPS COPOXTM and ENI SCT-CPO [232]. The preferential oxidation of CO in the presence of hydrogen (PROX) represents an important step in the synthesis of pure hydrogen for application in low temperature fuel cells [238]. Noble metals are active for this reaction at very low temperature (100-200 °C). Commercial catalysts are typically based on Pt/ γ -Al2O3. The commercial SELECTOXO™ process, developed in the early 1960s by Engelhard, uses a catalyst [ 239] containing from 0.3 to 0.5% platinum and 0.03% iron dispersed on alumina support tablets or pellets by wet impregnation of the alumina with a solution of platinum and iron salts. The SELECTOXO™ catalyst material would be used at temperatures not higher than 125° C and that a higher drying temperature would detrimentally affect the platinum. Mitsubishi Gas [240] disclosed the use of Pt with copper, as well as Pt with Mn, Ni or Co, for a PROX catalyst that is applied to fuel cells. In a more recent patent from Engelhard (now BASF), a catalyst suitable for a preferential oxidation is described where platinum is present in an amount of about 2%, copper is present in an amount of about 8%, and iron is present in an amount of 0.10-1.5% by weight [241]. K is also reported to act as a promoter of Pt/Al 2O3 [242]. Supported gold was quite recently discovered to act as excellent catalyst for PROX as well as for the low temperature (200 K) CO oxidation in waste gases [243-245]. The very high catalytic activity appears when gold particles smaller 50-30 Å are prepared, corresponding to a contraction of the Au-Au inter-atomic distance, and this seems to be independent from the support used [246]. In spite of this, it seems that the catalytic activity do strongly depend on the support. Au/TiO2 seems to be the most active in CO oxidation while Au/Al 2O3 seems to be usually considered to be poorly active [ 247]. However, in the presence of hydrogen the activity of both Au/Al2O3 and Au/TiO2 are reported to be significantly promoted and may become similar [248,249]. Several other semiconducting supports besides titania such as zirconia, ceria and iron oxide and thir mixtures have been tested and found acting as activators of gold with respect to isolating supports such as silica and alumina. Gold catalysts present a very high activity at low temperatures. These systems are strongly dependent on the preparation method and in general they present deactivation problems and penalization of the selectivity by hydrogen oxidation at temperatures higher than 80ºC. Supported gold is also reported to represent a promising catalyst for several other oxidation reactions [188], such as e.g. gold supported on carbon for glycerol oxidation [250 251].
The direct synthesis of hydrogen peroxide from hydrogen and oxygen [ 252], which could substitute the older antraquinone based process, may be performed over noble metal catalysts. The catalysts described in the literature are based on noble metals or combinations thereof supported on a great variety of substrates such as alumina, silica, and carbon. Palladium is the most suitable active metal in most catalyst formulations. Acids are often incorporated into the reaction medium in addition to the heterogeneous. Degussa-Headwaters developed a production plant for the direct synthesis of hydrogen peroxide coupled with propylene oxide production. The patents of these companies reveal that palladium nanoparticles deposited on carbon and an appropriate solvent (in general an alcohol), as well as the use of concentrations of hydrogen outside the flammability limits, are appropriate for industrial H2O2 production. 6.2.Catalytic combustion for energy generation. Methane, natural gas, syngas or hydrogen combustion for energy generation may be performed in the presence of a catalyst to reduce temperature as well as NOx formation. Palladium based catalysts appear to be the most active catalysts in methane combustion [253-255]. Alumina is the most largely used support, although addition of ceria is reported to be beneficial [256]. Zirconia- [257] and zirconia-ceria [258] supported catalysts have also been investigated. Despite there are still some divergences in literature concerning mainly which is the most active state of the Pd-based catalysts for CH 4 oxidation, e.g. metallic Pd, PdO or a mixed phase Pd 0/PdOx, the active phase of Pd oxidation catalysts is mostly identified as PdO, which is known to decompose into Pd metal in the range 650-850 °C, depending on oxygen pressure and reactive gas mixture. The transformation of PdO into Pd is reported to negatively affect catalytic reaction by lowering conversion, CH 4 combustion activity being reversibly restored upon re-oxidation of Pd to PdO. Howevere, it is evident that Pd and PdO are not the only species present on the catalysts, and that Pd catalysts work also in conditions where Pd metal is the predominant species. Recent surface science studies showed that the chemistry of these systems is indeed very complex [258-260]. The combination of Pd/Al2O3 with other active metallic component, such as e.g. Rh, Cu, Ni, may significantly improve low-temperature performances. 6.3.Total oxidation of VOC’s. Catalytic combustion [261] represents the alternative to thermal incineration to destroy highly concentrated VOC-containing streams, allowing heat recovery. The advantages are
represented by the lower NOx emissions and the lower amount of fuel needed, both associated with the lower combustion temperature obtained in the presence of catalysts, i.e. 300-500 °C. However, also in this case the heat can be recovered and utilized, but is obviously at lower temperature. The drawbacks are represented by a more complex reactor design, by the cost of the catalyst and sometimes by the relatively low catalyst lifetime. In fact combustion catalysts can be deactivated in particular when the feed contains sulphur, nitrogen and chlorine. In this case either regeneration procedures are needed, or the lifetime of the catalyst is low. Moreover, partial oxidation of the organic contaminants can occur and this can give rise to low concentration of partially oxidized species in the out-stream that can be even more toxic than the original contaminant [ 262]. Catalyst producers propose different catalyst formulations for combustion of different contaminants [263]. Most of commercial combustion catalysts are based on supported noble metals, which are needed to burn refractory compounds such as hydrocarbons. Alumina is the most frequent support, due to its stability at the required temperature. This is the case of the CK-304 and CK-307 catalysts (Pd/Al 2O3 and Pt/Al2O3, respectively, reported to be useful for most applications) from Topsøe [73]. Johnson Matthey PURAVOC catalysts contain 0.3-0.5 % wt noble metal, Pt, Pd, Pt-Pd, or Rh, on alumina. The BASF RO-25 catalyst, specific for VOC combustion, is reported top contain 0.5 % Pd on alumina, with 109 m2/g [264]. While noble metals are normally used for hydrocarbons combustion, base metal oxides can be used when the VOC to be destroyed are organic oxygenates or N- compounds. Among oxides which are active in VOC catalytic combustion [ 265,266], manganese, copper and chromium- containing materials, and/or their combination, such as copper chromites, may be doped with small amounts of Pt, seem to be the most active [ 73]. Quite refractory oxide supports or matrices, such as alumina or silica, zirconia, aluminate spinels, βaluminas, perovskites, are applied. Particular care must be devoted to chlorided VOC’s. In fact chlorine is reported to poison noble metal catalysts. The most common support for noble metal VOC removal catalysts is alumina, although this support is often the cause for deactivation of the catalyst by halogen species, the formed aluminum halides blocking the active species. Therefore, much research has been made into the development of alternative supports for combustion catalysts. According to several authors, also base metals, including manganese, are poisoned by chlorine. In contrast to this, manganese based catalysts (such as the CK395 catalyst from Topsøe [73], and the HPM (Z-2) catalyst from Matrostech, which is >
10% Mn2O3 on alumina with 25 - 35 m 2/g [267]), are suggested as special catalysts for chlorinated VOC destruction. Among the most important issues in this field, the catalytic combustion of chlorinated dioxins in waste incinerator waste gases. As reviewed some year ago [268] , catalytic dioxin abatement is usually performed in the DeNOx SCR reactor. Typical SCR catalysts based on V 2O5-WO3-TiO2 are reported to be active for catalyst destruction. Vanadium rich V2O5-WO3-TiO2 catalysts (8% vanadium) are reported [ 269] to represent the catalyst component of Remedia ® catalytic filters (W.L. Gore) working at 200 °C. Alternatively, noble-metal based catalysts (such as CK-306 (Cr-Pd/Al2O3)and CK-307 (Pt/Al2O3) from Topsøe [73] and EnviCat ® HHC from SüdChemie [23]) are used to catalytically destroy dioxins. These catalyst can be put as monolytic layers at the bottom of the downflow SCR reactor [270]. 6.4.Oxidation of ammonia. Ammonia oxidation by air to nitrogen monoxide is performed industrially as the first step in the production of nitric acid (Ostwald process) [ 271]. The reaction is carried out at 800-1000 °C, 1-12 bar with 10-14 % NH 3 in air using Pt-based gauze pads [ 272], in order to reduce the contact time to limit the further reaction of NO with ammonia and oxygen, to N2. To reduce Pt loss as volatile PtO, other metals such as Rh and Pd are usually alloyed to Pt. The typical composition may be 5-10 % Rh, 0-15 % Pd, sometimes 0.5 % Ru. Microaddition of other elements such as B and Y are reported to limit crystal growth, with a positive effect on catalytic activity [ 273]. The gauzes, of the woven or the knitted type, are produced using 0.06-0.07 mm diameter wires. In recent years, the selective catalytic oxidation (SCO) of ammonia to nitrogen became an important reaction to limit ammonia slip from processes such as the Selective Catalytic Reduction of NOx by ammonia (in the treatment of waste gases fom power stations) as well as from the combustion of biomass-derived gases. Several catalysts have been studied in the academic research, containing base and noble metal catalysts [ 274]. Haldor Topsøe [275] patented a process for catalytic low temperature oxidation of ammonia in an off-gas at a temperature of 200--500 °C. The catalysts used were silica supported Cu, Co and Ni oxides doped with small amount of noble metals(100-2000ppm). The selectivity to nitrogen can be improved by sulphating the catalysts during or prior to contact with ammonia containing gas from 26-53% to 78-99%. A more recent patent from BASF reports on a layered catalyst constituted by gammaalumina (60-300 m2/g) with 0.5-4 % wt of Pt and 0.5 to 4 % of vanadia all deposed over the
surface of a monolytic structure, ceramic or metallic. The reactivity is studied with a 15 ppm NH3 containing feed , 70000 VHSV at 275 °C [ 276]. A further recent patent from BASF concerns an ammonia selective oxidation catalyst containing a zeolite (including mordenite, ferrierite, zeolite Y or beta zeolite), from about 0.02 wt. % to about 0.20 wt. % of a precious metal, e.g., platinum, and a base metal, preferentially copper oxide. Typically, the amount of base metal compound added to the ammonia oxidation catalyst ranges from about 5 wt. % to about 16 wt. % when zeolite Y is used and from about 3 wt. % to about 8 wt. % when mordenite, beta, ferrierite or ZSM-5 zeolites are used [277]. 6.5.Catalytic converters. The aftertreatment of Otto-cycle gasoline engines is satisfactorily achieved by the so called Three Way Catalysts (TWC), a technology developed after the seventies allowing the efficient abatement of unburnt hydrocarbons (HC), CO, and NOx. The original TWC composition was Pt-Rh on alumina, deposed on ceramic monolyths. In recent years [ 278], typical TWC formulations have included Pd as the active metal, ceria–zirconia as promoters according to the Oxygen Storage Capacity (OSC) of ceria and the thermal stability of zirconia, and alumina as support as well as other minor components mainly present in order to enhance thermal stability. Perovskite materials can also be present to help limiting of noble metal sintering. The use of the different noble metal formulation (PtRh, Pd-Rh, Pd only) is due in part to purely economic reasons, resulting from the high cost and scarcity of Rh and from the variable relative prices of Pd and Pt. The addition of gold could improve activity at low temperature [279]. Modern advanced TWCs are very thermally stable so they can be mounted directly on the exhaust manifold. Here, with a rapid engine start-up strategy catalyst operating temperatures are quickly achieved after the engine is started. Improvements in catalyst technologies and optimized engine calibrations allowed a substantial reduction of noble metal content in the converter, now even less than 1 g. Zoning also allows to optimize the use of noble metals, with higher loading on the front of catalyst, to aid light-off, and lower amounts in the rear portion where mostly Rh is deposed. 6.6.Diesel engines aftertreatment. Several different systems are under development to reduce the emissions of NOx, hydrocarbons and particulate matter from Diesel and other lean burn engines. Among
these, the SCR systems (Selective Catalytic Reduction of NOx by ammonia or urea), uses oxide catalysts (V-W-TiO2 or Fe zeolite catalysts) [280]. Pt-based or Pt-Pd-based porous ceramic wall-flow Diesel particulate filters (DPF) are largely used today to abate particulate matter from Diesel engine waste gases [ 281,282]. At quite high car speeds these systems allow “passive regeneration” behaviour, where NO 2 is formed from NO and oxygen and oxidizes soot, by reducing to nitrogen. In general, periodical “active regeneration” steps are also needed, where oxygen is fed allowing particulate matter burning at 550-600°C. Pt-Ba/Al2O3 or Pt-K/Al2O3 sorbents-catalysts, typically containing ~ 1 % Pt and ~ 8 % Ba wt/wt, or ~ 8 % K wt/wt, are used for “NOx storage-reduction” (NSR), a technology, first developed by Toyota in the early eighties [ 283,284], to reduce the NOx content in waste gases of lean-burn gasoline and Diesel engines. A following evolution of this technology is the newer DPNR (Diesel Particulate-NOx Reduction) process, also developed by Toyota [285,286]. It allows the simultaneous abatement of both particulate and NO x due to the development of a new catalytic filter and a new diesel combustion technology. The catalytic materials for both NSR and DPNR consist in porous ceramic filters coated with a catalytic layer constituted by a high surface area support (typically γ -alumina), a noble metal (usually Pt) and an alkaline or alkali-earth metal oxide which presents a high NOx-storage capacity, most frequently Ba or K oxide. These catalytic systems work under cyclic conditions, alternating long lean phases (during which NOx are adsorbed on the catalyst in the form of nitrate /nitrites) with short phases in rich condition (during which the nitrate-nitrite species are reduced to molecular nitrogen). However, adsorption implies the conversion of NO into NO2 catalyzed by Pt, which also catalyse the reaction to N 2 in the rich step. In recent years it has been shown that La 1-xSr xCoO3 and La 0.9Sr 0.1MnO3 perovskites may substitute Pt both from DPF and Lean NOx Traps, allowing NO-to-NO 2 conversions and NOx reduction performance comparable to that of commercial platinum-based systems [287]. 7. Conclusions. The data summarized above show how predominant is the application of two noble metals, platinum and palladium, over all other metals usually applied in heterogeneous metallic catalysis (Table 1). These two elements have a very wide application in all fields such as hydr atio (p tial d full), dehydr atio d total idatio in fi
petrochemistry, fine chemistry and environmental catalysis. Among other platinum group metals (PGMs), whose application in homogeneous catalysis is very relevant [ 288], all other have more limited and specific applications in the field of heterogeneous metal catalysis. Rhodium finds very relevant application in the three way catalyst technology as well as in methane partial oxidation to syngas (CPO) and is an additive for silver in ammonia oxidation catalysts, iridium plays a peculiar role in selective ring opening of naphthenes, ruthenium is the base of new catalysts for ammonia synthesis but is also very active for low temperature methanation and Fischer Tropsch synthesis. Osmium, instead, does not seem to find important application in the field of solid metal catalysts. Rhenium, which belongs to group VII B (7), is almost always present as an additive in naptha catalytic reforming catalysts, The discovery of new preparation technique revealed quite recently the possible new applications of supported gold in oxidation catalysis, such as for the low temperature and the preferential oxidation of CO and the water gas shift , in particular in the field of fuel cells technologies. Very important is also the role of other group IB (11) metals, i.e. silver and copper. Silver catalysts play a very relevant role in ethylene and methanol partial oxidation, as well as an additive e.g. of supported Pd hydrogenation catalysts. Copper finds very large application in methanol synthesis, low temperature water gas shift, methanol steam reforming, selective hydrogenation and dehydrogenation of oxygenated compounds. The other group VIIIB (8-10) elements (Fe, Co, Ni) have also wide application as metal catalysts. The use of bulk iron catalysts is very relevant for ammonia synthesis and HT Fischer Tropsch synhesis, Cobalt mostly for LT Fischer Tropsch synthesis and amination, nickel in almost all fields of hydrogenation catalysis and sometimes also in oxidation catalysis. As for other metal elements, their electropositive character hampers their use as metal catalysts in most conditions. A particular case is that of alkali metals which are used in non-protic highly reducing environments as basic catalysts/initiators. The above data also show how predominant is the use of supported metals with respect to bulk metals, not only for precious metals but also for base metals. In fact bulk iron with dopants finds large application in ammonia synthesis as well as Fischer Tropsch synthesis, bulk nickel (Raney or normal silica-containing type) finds application in hydrogenation, bulk silver (as crystals or gauzes) find application in oxidative dehydrogenation of methanol to formaldehyde, platinum gauzes find application in
ammonia oxidation for nitric acid production. In spite of this, most metal catalysts are supported, allowing the formation of small size, large surface area crystals. Among supports, the very large predominance of aluminas is evident. These supports range from high surface area spinel type gamma-aluminas (> 200 m 2/g) to medium surface area mainly theta aluminas (100 m 2/g) to low or very low surface area corundum- alfaalumina (20-0.5 m2/g). This depends from the temperature at which catalysis must be performed: obviously, the higher the temperature, the more the stability needed of the support. Thus highly refractory low surface area supports are used at higher temperatures (e.g. 500-1000 °C). Thermal stability of aluminas is usually favoured by adding silica (like in the case of the Siralox materials from Sasol [ 289]) or by adding potassium or calcium ions. On the other hand, the catalytic activity of the support must usually be low, and this also induces the use of basic components such as alkali- or alkali earth ions. The need for both properties (high refractory behaviour and low reactivity) is the reason for the use of alkali and alkali earth aluminates such as spinels and beta aluminas as supports, such as for Ni-based steam reforming catalysts (800-900 °C) or for both noble and base metal catalysts for catalytic combustion. The use of silica as a support for metal catalysts is also quite common, being at least in part due to the lower reactivity of high surface area silica with respect to high surface area alumina. Other supports which are the object of many academic studies, such as titania and zirconia, seem to still have scarce application in the field of metal catalysis at least for large refinery and petrochemistry processes. Other metal oxides, such as in particular ceria and the more stable ceria-zirconia mixed oxides, find increasing application as components of catalysts, due to their oxygen storage capacity. Indeed, it seems that metal catalysts for smaller fine chemistry applications [55] may have larger variety of supports and metals used than bigger processes. As said, platinum and palladium find widespread application as the active phase in industrially relevant supported metal catalysts. However, as it is well known, their application is hampered by their high costs. In recent years strong efforts have been devoted to develop methods allowing the decrease of precious metal content in industrial catalysts. This is mostly obtained by increasing the catalytic activity of these metals by using appropriate activator dopants. For these reasons, in many cases multimetallic catalysts have been developed, allowing high activity with quite small precious metal content. This is the case, in particular, of three way catalysts used in catalytic convertes, where the amount of precious metal has been decreased considerably in recent years.
Although very impotant improvements have been made on the synthesis, characterization and mechanistic studies concerning metal catalysts, actually many points still reserve further investigation. One of the main points concerns the real state of the catalytic surface upon reaction as well as the nature of the active sites. It is in fact clear that the reaction environment has a very important effect on the state of the metal surface. As mentioned in the text, still divergences occur among scientists concerning the oxidation state of several metal catalysts during oxidation reactions. This problems also concerns reaction performed with water vapour and carbon dioxide as a reactant or as a product, such as steam reforming, water gas shift, methanation and Fischer Tropsch syntheses, methanol syntheses. A similar problem concerns the role of the different carbidic species in reactions involving CO and hydrocarbons as reactants. Also the nature of adsorbed hydrogen on metals does not seems to be still fully established. Indeed, some investigation techniques, such as e.g. IR spectroscopy, tend to reveal a very complex situation of the surface of metal catalysts. As for example, the presence of several different oxidized species (isolated cations with different oxidation states, oxide layers and bulk oxides) is usually detected on even deeply reduced supported metal catalysts, and even more in the presence of steam or oxygen. Other techniques, such as theoretical studies and surface monocrystal spectroscopies, instead, tend to give a simpler idea of the catalytic phenomenon. These techniques are largely used today although not infrequently refer data obtained in conditions which are very far from those of the industrial catalytic reaction. Although in situ and operando techniques tend to reduce the gap between the conditions of most investigation techniques and real use of catalysts, the large use of other techniques may do the reverse. This point justifies the need further investigation in the field.
Table 1. Metals mostly involved in heterogeneous metallic catalysis. Group I raw IIraw III raw
VIIB 7 Mn Re
VIIIB 8 Fe
9 Co
10 Ni
Ru
Rh
Pd
Os
Ir
Pt
In Italic: Platinum Group Metals (PGMs)
IB 11 Cu Ag Au
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