6 Catalyst Shapes and Production of Heterogeneous Catalysts

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223 6 Catalyst Shapes and Production of Heterogeneous Catalysts 6.1 Catalyst Production [1, T41] Industrial catalysts are generally shaped bodies of various forms, e. g., rings, spheres, tablets, pellets (Fig.

1 223 6 Catalyst Shapes and Production of Heterogeneous Catalysts 6.1 Catalyst Production [1, T41] Industrial catalysts are generally shaped bodies of various forms, e. g., rings, spheres, tablets, pellets (Fig. 6-1). Honeycomb catalysts, similar to those in automobile catalytic converters, are also used. The production of heterogeneous catalysts consists of numerous physical and chemical steps. The conditions in each step have a decisive influence on the catalyst properties. Catalysts must therefore be manufactured under precisely defined and carefully controlled conditions [14]. Since even trace impurities can affect catalyst performance, strict quality specifications apply for the starting materials. Successful catalyst production is still more Fig. 6-1 Various shaped catalyst bodies (BASF, Ludwigshafen, Germany) Industrial Catalysis: A Practical Approach, Second Edition. Jens Hagen Copyright # 2006 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim ISBN:

2 224 6 Catalyst Shapes and Production of Heterogeneous Catalysts of an art than a precise science, and much company know-how is required to obtain catalysts with the desired activity, selectivity, and lifetime. Depending on their structure and method of production, catalysts can be divided into three main groups [8]: Bulk catalysts Impregnated catalysts Shell catalysts Bulk catalysts are mainly produced when the active components are cheap. Since the preferred method of production is precipitation, they are also known as precipitated catalysts. Precipitation is mainly used for the production of oxidic catalysts and also for the manufacture of pure support materials. ne or more components in the form of aqueous solutions are mixed and then coprecipitated as hydroxides or carbonates. An amorphous or crystalline precipitate or a gel is obtained, which is washed thoroughly until salt free. This is then followed by further steps: drying, shaping, calcination, and activation (Scheme 6-1) Solutions of two or more salts Mixing + NaH (Na C ) 2 3 Precipitation Amorphous or cryst. solid or gel Washing Drying Grinding Shaping Calcination Activation Scheme 6-1 Production of a precipitated catalyst [7] Precipitated catalyst The production conditions can influence catalyst properties such as crystallinity, particle size, porosity, and composition. In the shaping step, the catalyst powder is plastified by kneading and pelletized by extrusion or pressed into tablets after addition of auxiliary materials (Fig. 6-2). The influence of the shaping process on the mechanical strength and durability of the catalyst should not be underestimated. When reactors are filled with catalyst, a dropping height of 6 8 m is usual, and bed heights of up to 10 m are possible. Furthermore, industrial catalysts are subject to high temperatures and also often to changing temperatures.

3 6.1 Catalyst Production 225 Fig. 6-2 Production of noble metal catalysts at the company Degussa, Hanau Wolfgang, Germany Typical examples of precipitated catalysts are: Iron oxide catalysts for high-temperature C conversion (Fe 2 3 with addition of Cr 2 3 ) Catalysts for the dehydrogenation of ethylbenzene to styrene (Fe 3 4 ) Highly homogeneous catalysts can be obtained by using mixed salts or mixed crystals as starting materials, since in this case the ions are already present in atomically distributed form. Readily decomposible anions such as formate, oxalate, or carbonate are advantageous here. Examples: Cu(H)NH 4 Cr 4 as a precursor for copper chromite (Adkins catalyst) Ni 6 Al 2 (H) 16 C 3 7 4H 2 decomposes to give a supported Ni/Al 2 3 catalyst ne of the best known methods for producing catalysts is the impregnation of porous support materials with solutions of active components [9,10]. Especially catalysts with expensive active components such as noble metals are employed as supported catalysts. A widely used support is Al 2 3. After impregnation the catalyst particles are dried, and the metal salts are decomposed to the corresponding oxides by heating. The process is shown schematically in Scheme 6-2. In the impregnation process, active components with thermally unstable anions (e. g., nitrates, acetates, carbonates, hydroxides) are used. The support is immersed

4 226 6 Catalyst Shapes and Production of Heterogeneous Catalysts Precipitation of support, e.g., Al 2 3 Washing and drying Shaping of support Impregnation with solutions of the active components Drying Decomposition (calcination) Activation (reduction) Supported metal catalyst Scheme 6-2 Production of supported metal catalysts by impregnation in a solution of the active component under precisely defined conditions (concentration, mixing, temperature, time). Depending on the production conditions, selective adsorption of the active component occurs on the surface or in the interior of the support. The result is nonuniform distribution. To achieve the best possible impregnation, the air in the pores of the support is removed by evacuation, or the support is treated with gases such as C 2 or NH 3 prior to impregnation. After impregnation, the catalyst is dried and calcined. For large-scale manufacture the so-called incipient wetness impregnation (also called pore volume, or dry or capillary impregnation) is the most advantageous method. In this approach the support is brought into contact with a solution the volume of which corresponds to the total pore volume of the solid and which contains the appropriate amount of precursor compound. The principle of this method is shown in Figure 6-3. If catalysts with high loadings of the active compounds are to be made, limited solubility of the precursor compound may cause problems, and multiple impregnations may have to be applied. With incipient wetness impregnation, even precursor compounds which do not interact with the support can be deposited when the solvent is removed during a subsequent drying procedure. This can be illustrated with Figure 6-4. The rate of drying depends on the temperature and the gas throughput. From Figure 6-4 it can be seen that the rate of drying strongly affects the metal distribution of the catalyst particles. There can be obtained catalysts with egg-yolk, egg-shell, and homogeneous metal distributions. Calcination is heat treatment in an oxidizing atmosphere at a temperature slightly higher than the intended operating temperature of the catalyst. In calcina-

5 6.1 Catalyst Production 227 Fig. 6-3 Principle of catalyst preparation by incipient wetness impregnation Fig. 6-4 Influence of the rate of drying on the profile of pores and particles tion numerous processes can occur that alter the catalyst, such as formation of new components by solid-state reactions, transformation of amorphous regions into crystalline regions, and modification of the pore structure and the mechanical properties. In the case of supported metal catalysts, calcination leads to metal oxides as catalyst precursors, and these must subsequently be reduced to the metals. This reduction can be performed with hydrogen (diluted with nitrogen), C, or milder reducing agents such as alcohol vapor. In some cases reduction can be carried out in the production reactor prior to process start-up. Here temperature control is a problem.

6 228 6 Catalyst Shapes and Production of Heterogeneous Catalysts Impregnated catalysts have many advantages compared to precipitated catalysts. Their pore structure and specific surface area are largely determined by the support. Since support materials are available in all desired ranges of surface area, porosity, shape, size, and mechanical stability, impregnated catalysts can be tailor-made with respect to mass transport properties [9]. In individual cases it is possible to achieve almost molecular distribution of the active components in the pores. As a rule, however, the active substance is distributed in the form of crystallites with a diameter of nm. This fine distribution on the support not only ensures a particularly favorable surface to volume ratio and hence makes good use of the active components, some of which are expensive, it also reduces the risk of sintering. In general, with increasing loading, catalyst activity eventually reaches a limiting value. Therefore, for economic reasons the catalyst loading is % for noble metals, and 5 15 % for other metals. Examples of industrial impregnated catalysts are: Ethylene oxide catalysts in which a solution of a silver salt is applied to Al 2 3 Catalysts in the primary reformer of ammonia synthesis, with % Ni on a-al 2 3 Catalysts for the synthesis of vinyl chloride from acetylene and HCl: HgCl 2 / activated carbon; HgCl 2 is applied from aqueous solution Catalysts in which the active component is a finely divided metal are often pyrophoric. The catalyst can be better handled after surface oxidation of the active component (passivation). Reactivation is then carried out in the start-up phase under process conditions. Shell catalysts consist of an compact inert support, usually in sphere or ring form, and a thin active shell that encloses it [4]. Since the active shell has a thickness of only mm, the diffusion paths for the reactants are short. There are many heterogeneously catalyzed reactions in which it would be advantageous to eliminate the role of pore diffusion. This is particularly important in selective oxidation reactions, in which further reactions of intermediate products can drastically lower the selectivity. An example is acrolein synthesis: two catalysts with the same active mass but different shell thicknesses differed greatly in selectivity at the high conversions desired in industry (Fig. 6-5). Therefore, if acrolein synthesis is to be operated economically, the shell thickness must be optimized. The best known method for producing shell catalysts is the controlled short-term immersion of strongly adsorbing support materials. A well-known example is the platinum shell catalyst, which can easily be prepared with low loading and a high degree of dispersion. The support is immersed in solution of hexachloroplatinic acid (H 2 PtCl 6 ), and an outer layer of adsorbed PtCl 27 4 ions is formed. The adsorption of the hexachloroplatinic acid is so fast that diffusion of the solution into the pores is rate-determining. The treated catalyst particles are then dried without washing and calcined to generate the metal [T35]. Figure 6-6 shows how different impregnation techniques can be used to obtain supported catalysts with special distributions of the metal.

selectivity of acrolein synthesis (BASF,

400 Selectivity [%] at 99% conversion 89 82 a b c

6-6 Different metal distributions in pellets of

(Degussa, Hanau-Wolfgang, Germany) a) Shell

7 6.1 Catalyst Production 229 Fig. 6-5 Cross section of a shell catalyst (magnification 186). Influence of the shell thickness on the selectivity of acrolein synthesis (BASF, Ludwigshafen, Germany): Shell thickness [µm] Selectivity [%] at 99% conversion a b c d e Fig. 6-6 Different metal distributions in pellets of diameter 6 mm consisting of a metal on a support (Degussa, Hanau-Wolfgang, Germany) a) Shell catalyst with normal shell thickness b) Shell catalyst with an extremely thin shell c) Shell catalyst with a thick shell d) Impregnated catalyst e) Catalyst with ring distribution

230 6 Catalyst Shapes and Production of Heterogeneous Catalysts The advantages of shell catalysts are short transport or diffusion paths, a pore structure independent of the support, and better heat

8 230 6 Catalyst Shapes and Production of Heterogeneous Catalysts The advantages of shell catalysts are short transport or diffusion paths, a pore structure independent of the support, and better heat transport in the catalyst layer. Examples of industrial applications of shell catalysts are: Selective oxidation reactions, e. g., production of acrolein from propene and of phthalic anhydride from o-xylene Purification of automobile exhaust gases Selective oxidation of benzene to maleic anhydride: vanadium molybdenum oxide on fused corundum (catalytically inactive support without pores) Autothermal decomposition of liquid hydrocarbons on Ni/a-Al 2 3 shell catalysts (high selectivity for lower alkenes [4] In this chapter we have seen how the different steps of catalyst production can affect the functional properties of catalysts, such as activity and selectivity, and their morphology (Fig. 6-7). Fig. 6-7 Modern catalyst production plant (BASF, Ludwigshafen, Germany) Because of the numerous influencing parameters, prediction of the catalytic properties is not possible. They can only be determnined by measurement of the reaction kinetics. This makes it clear why catalyst production is based on special company know-how and that not all details are publicized.

9 6.2 Immobilization of Homogeneous Catalysts Immobilization of Homogeneous Catalysts As we have seen in Chapter 3, the industrial use of homogeneous catalysts often leads to problems with catalyst separation and recycling, recovery of the often valuable metal, and short catalyst lifetimes. Therefore, in the last twenty years or so, extensive studies have been carried out on the development of heterogenized homogeneous catalysts, which are intended to combine the advantages of homogeneous catalysts, in particular high selectivity and activity, with those of heterogeneous catalysts (ease of separation and metal recovery). Hence attempts are made to convert organometallic complex catalysts to a form that is insoluble in the reaction medium. This is generally achieved by anchoring a suitable molecule on an organic or inorganic polymer support. In the following, we will discuss such methods for obtaining immobilized homogeneous catalysts, which are also known as fixed catalysts or hybrid catalysts, and the potential applications of this intersting class of catalysts [3]. To come to the most important point first: the ideal immobilized metal complex for industrial appplications has not yet been found, as is shown by weighing up the advantages and disadvantages of this type of catalyst. Advantages: 1) Separation and recovery of the catalyst from the product stream is straightforward. This is the main advantage of heterogenization. 2) Mutifunctional catalysts can be obtained in which more than one active component is bound to a carrier. 3) Highly reactive, coordinatively unsaturated species that can not exist in solution can be stabilized by heterogenization. Disadvantages: 1) The immobilized homogeneous catalysts are not sufficiently stable. The valuable metal is continuously leached and carried away with the product stream. 2) The problems of homogeneous catalysts, such as corrosion, catalyst recovery, and catalyst recycling, have so far not been satisfactorily solved. 3) Lower catalytic activity than homogeneous catalysts because of: poor accessibility of the active sites for the substrate, steric effects of the matrix, incompatibility of solvent and polymer, deactivation of active centers. 4) Inhomogeneity due to different linkages between support matrix and complex. Particularly intensive investigations have been carried out on catalysts for reactions with C or alkenes. These reactions, which are typical transition metal catalyzed conversions, provide the best possibility for assessing the properties of heterogenized catalysts. Examples are given in the following overview (Table 6-1). All the examples show that the reaction mechanisms with homogeneous and heterogeneous catalysis are in many respects similar. However, care must be taken in

10 232 6 Catalyst Shapes and Production of Heterogeneous Catalysts comparing soluble and matrix-bound catalysts, since the matrix can be regarded as a ligand. Thus at least one coordination site of the complex catalyst is no longer available for the catalytic cycle. It is difficult to find the corresponding ligands required for a comparison. For example, a monodentate phosphine ligand like PPh 3 is not directly comparable to a polystyrene matrix with phosphine groups. For meaningful comparisons, the less common multidentate ligands must be used in solution. Table 6-1. Comparison of homogeneous and heterogenized catalysts in industrial reactions Reaction Homogeneous catalyst Heterogenized catalyst Hydroformylation of olefins (oxo synthesis) xidation of olefins (Wacker process) Carbonylation of methanol to acetic acid Co or Rh complex [PdCl 4 ] 2 [Rh(C) 2 I 2 ] + HI Co or Rh complex on polymer or Si 2 support matrix PdCl 2 on support matrix RhCl 3 on activated carbon or [RhCl(C)PR n ] on modified polystyrene Hydrogenation of olefins [Rh(PPh 3 ) 3 Cl] [Rh(PPh 3 ) n Cl] on polymer support There are four basic ways of fixing transition metal complexes on a matrix: 1) Chemical bonding on inorganic or organic supports 2) Production of highly dispersed supported metal catalysts 3) Physisorption on the surface of oxidic supports (supported solid phase catalysts, SSPC) 4) Dissolution in a high-boiling liquid that is adsorbed on a porous support (supported liquid phase catalysts, SLPC) The immobilization of organometallic complexes on inorganic or organic supports is the most widely used method. Basically the supports act as high molecular mass ligands and are obtained by controlled synthesis. The bonding can be ionic or coordinative. The main aim of the process is to bind the complexes on the solid surface in such a manner that its chemical structure is retained as far as posssible. A common method is the replacement of a ligand by a bond to the surface of the solid matrix. This means that a reactive group must be incorporated in the surface during production of the support. Numerous polymer syntheses and orgamometallic syntheses are available for the construction of functionalized supports; Equation 6-1 gives just one example.

11 6.2 Immobilization of Homogeneous Catalysts 233 PCl 3 /AlCl 3 PCl 2 (6-1) Polymer chain (polystyrene) 2 RLi PR 2 Here triphenylphosphine, the most important ligand in organometallic catalysis, is coupled to the benzene rings of cross-linked polystyrene. An anchored catalyst is then formed by coordination of the phosphine group to the metal center of a rhodium complex (Eq. 6-2). PPh 2 RhL x Ph P Ph (6-2) RhL x The degree of swelling of this copolymer in organic solvents is controlled by means of the amount of divinylbenzene. Hard copolymers of this type take up metal complexes only on the surface. The physical properties of the support can be varied by means of the polymerization method; the metal loading can also be controlled well. There are many reactions available for applying the organometallic complexes to the surface. Two examples are shown in Equations 6-3 and 6-4. CH +RuH 2 (PPh 3 ) 4 C RuH(PPh 3 ) 3 (6-3) CH 2 Cl + NaMn(C) 5 CH ΝaCl 2 Mn(C) 5 (6-4) Disadvantages of the organic polymer supports are low mechanical durability (e. g., in stirred tank reactors), poor heat-transfer properties, and limited thermal stability (up to max. 1508C).

12 234 6 Catalyst Shapes and Production of Heterogeneous Catalysts There are also several methods available for producing inorganic supports. Here we will discuss a few basic methods. The most important method is the reaction of inorganic supports having surface hydroxyl groups with metal alkyls (Eq. 6-5). H CH 2 C 6 H 5 Ti(CH 2 C 6 H 5 ) Mg 4 Mg Ti (6-5) H CH 2 C 6 H 5 Alkoxides and halides can also be attached to surfaces. Subsequent hydrolysis and dehydration lead to terminal metal oxo structures (Eq. 6-6). Si H H H MoCl 5-3HCl Si 180 C -H 2 Cl Mo Cl Si +2 H 2-2HCl M Si Mo H H (6-6) Such immobilized molybdenum oxide catalysts are active in selective oxidation reactions. For example, methanol can be oxidized with air to methyl formate at ca. 500 K with % selectivity [T22]. The catalyst obtained from g-al 2 3 and tetrakis(z 3 -allyl)dimolybdenum (Eq. 6-7) is considerably more active in ethylene hydrogenation and olefin metathesis than the catalysts prepared by conventional fixation of [Mo(C) 6 ] followed by calcination. Al H H H H 3 ( -C 3 H 5 ) 4 Mo 2 0 C Al C 3 H 5 Mo C 3 H 5 C 3 H 5 Mo C 3 H 5 2 Mo Al 0 C Mo C H C Al Mo Al Mo Mo Mo (6-7) rganofunctional polysiloxanes are a versatile group of catalysts developed by the company Degussa [13]. These are solids with a silicate framework obtained by hydrolysis and polycondensation of organosilicon compounds (Eq. 6-8).

13 6.2 Immobilization of Homogeneous Catalysts 235 X X (CH 2 ) 3 (CH 2 ) 3 +H 2 (CH 2 ) 3 (CH 2 ) 3 Si(R) 3 Si(R) 3 +RH Si(H) Si(H) 3 3 X (6-8) - H 2 + H 2 (CH 2 ) 3 (CH 2 ) 3 Si Si X = functional group: sulfane, phosphine, amine This class of substances is characterized by broad chemical modifiability, a high capacity for functional groups, high temperature and ageing resistance, and insolubility in water and organic solvents. The heterogenized organopolysiloxane catalysts are marketed as abrasion-resistant spheres of various particle sizes. In particular the phosphine complexes of Ru, Pd, Ir, and Pt are interesting catalysts for hydrogenation, hydroformylation, carbonylation, and hydrosilylation Highly Dispersed Supported Metal Catalysts [T22] This method is used to obtain a very fine distribution of metal on a support by decomposition of organometallic compounds (so-called grafted catalysts). For example, by treating Ti 2 with Z 3 -allyl complexes of rhodium followed by decomposition, highly active hydrogenation and hydrogenolysis catalysts are obtained (Eq. 6-9). Similar catalysts based on polysiloxanes are produced by Degussa; Pd, Rh, and Pt systems are available. Rh(C 3 H 5 ) 3 + H Ti H C 3 H 5 Rh 273 K 293 K Ti H 2 H Rh Ti (6-9) K H 2 (Rh) n Ti (Rh) n = small aggregates of 25 or more Rh atoms with particle diameters of ca. 1.4 nm SSP Catalysts [6, 11] In this group of catalysts, organometallic complexes are anchored on the inner surface of porous supports, mainly by physisorption. These catalysts can be used as catalyst beds through which the reaction medium flows. For example, the complex

14 236 6 Catalyst Shapes and Production of Heterogeneous Catalysts [Rh(Z 3 -C 3 H 5 )(C)(PPh 3 ) 2 ] is adsorbed on g-al 2 3 and used as a hydrogenation catalyst. The fixed complexes often exhibit considerably lower activity and selectivity than in the homogeneous phase, and this limits their range of applications. The SLP catalysts are a better alternative SLP Catalysts [11, 15] In this process a solution of the complex in a high-boiling solvent spreads out on the inner surface of a porous support, which generally consists of an inorganic material such as silica gel or chromosorb. The reaction takes place in the liquid film, which the starting materials reach by diffusion. The products are also transported away by diffusion out of the film, which is retained on the support. The use of SLP catalysts is generally restricted to the synthesis of low-boiling compounds. xo synthesis with SLP catalysts has been the subject of much interest. An example is the hydroformylation of propene with [RhH(C)(PPh 3 ) 3 ] in liquid triphenylphosphine on g-al 2 3. The starting material and the C 4 aldehyde are present in the gas phase. In a pilot plant at DSM, low selectivity was found and diffusion problems were encountered. Further examples are the oxidation of ethylene to acetaldehyde with aqueous solutions of PdCl 2 and CuCl 2 on kieselguhr, and the oxychlorination of alkenes with a CuCl 2 /CuCl/KCl/rare earth halide melt on silica gel [T22]. From these examples, most of which are based on laboratory investigations, it becomes clear that heterogenization is not a general method for solving problems in catalysis. It is, however, an interesting addition to the spectrum of catalytic methods. Finally we shall discuss some examples in which heterogenized catalysts have been successfully used in industrial processes. Chromium complexes on the basis of chromocene or chromium salts on Si 2 are used for the polymerization of a-olefins and for the production of linear polyethylene in the Phillips process. The structure of the active surface species is unknown. Heterogenized titanium complexes are used for the polymerization of propylene and give high yields of isotactic polypropylene [T31]. Another example for the use of a multifunctional solid catalyst is the Aldox process for the production of 2-ethylhexanol (Eq. 6-10). CH 3 CH CH 2 +C+H 2 CH 3 CH 2 CH 2 CH 1 2 2x H 2 CH 3CH 2 CH 2 CH C CH CH 2 CH H 2 CH 3 CH 2 CH 2 CH 2 CH CH 2 H CH 2 CH 3 (6-10) In industry the hydroformylation (reaction 1) is catalyzed by Rh or Co complexes in solution. The aldol condensation (reaction 2) is acid or base catalyzed, and the hydrogenation of the unsaturated aldehyde (reaction 3) is catalyzed by metals such

15 Exercises for Chapter C CH 2 P Rh P CH 2 Cl CH 2 N H C 2 H 5 Fig. 6-8 Multifunctional, polymer-fixed solid catalyst for the Aldox process [16] as nickel. n this basis a catalyst with a metal function (Rh) and a base function (amine) was developed (Fig. 6-8), and is active for the formation of 2-ethylhexanol. The rhodium center catalyzes the hydroformylation and the partial hydrogenation of the aldol product, in which the aldehyde group is retained, while the amino group catalzes the aldol condensation [16]. These examples show that the area of heterogenization of catalysts represents an enormous potential for research. Some of these catalysts show high activities under mild conditions with interesting and sometimes unexpected selectivities. The processes for the production of these catalysts, the investigation of their precise structures, and the elucidation of their reaction mechanisms are still at an early stage. It would seem that the use of heterogenized catalysts is best suited to small molecules (oxidation, hydrogenation), and that inorganic supports are more promising than organic supports. The field of heterogenization has led to a closer approach between heterogeneous and homogeneous catalysis. " Exercises for Chapter 6 Exercise 6.1 Which are the main physical properties of a catalyst that are influenced by the production conditions? Exercise 6.2 What are the advantages of impregnated catalysts compared with precipitated catalysts?

16 238 6 Catalyst Shapes and Production of Heterogeneous Catalysts Exercise 6.3 Name porous supports with which impregnated catalysts can be manufactured. Exercise 6.4 Which two types of support are preferentially used for oxidation catalysts? Exercise 6.5 For which reactions are supported catalysts impregnated near the surface particularly suitable? Exercise 6.6 a) Why do monolith and honeycomb catalysts have to be coated before they are loaded with catalyst? b) What is this initial coating called? Exercise 6.7 a) What are the advantages of shell catalysts compared to bulk catalysts? b) What is the preferred support material for shell catalysts? Exercise 6.8 Why have numerous dinuclear and multinuclear metal complexes (clusters) been tested in the synthesis of gycol from C/H 2? Exercise 6.9 What are the advantages of heterogenized metal catalysts compared to conventional heterogeneous catalysts? Exercise 6.10 A phosphine-modified plastic matrix is treated with iron pentacarbonyl. What reaction can be expected? PR 2 +Fe(C) 5? Exercise 6.11 What are the disadvantages of organic polymer supports for the production of immobilized homogeneous catalysts? Exercise 6.12 How are SLP catalysts produced?

Catalytic Chemistry. Bruce C. Gates. John Wiley & Sons, Inc. New York Chichester Brisbane Toronto Singapore. University of Delaware ^.'-'.

Catalytic Chemistry. Bruce C. Gates. John Wiley & Sons, Inc. New York Chichester Brisbane Toronto Singapore. University of Delaware ^.'-'. : s / ; '.... ;. : : ^.'-'. Catalytic Chemistry Bruce C. Gates University of Delaware John Wiley & Sons, Inc. New York Chichester Brisbane Toronto Singapore Contents List of Notation xix 1 INTRODUCTION