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Catalyst materials & properties

Textbook 2014 114 Pages

Engineering - Chemical Engineering

Excerpt

Table of contents

Preface

List of Figures

List of Tables

1. Introduction
1.1 Catalyst Performance

2. Catalyst materials
2.1 Makeup of a typical heterogeneous catalyst
2.2 Active catalytic phases
2.2.1 Selecting the active catalysts
2.3 Carriers (Supports)
2.3.1 Alumina
2.3.2 Silicas
2.3.3 Carbons
2.3.4 Titania
2.3.5 Synthesis of zeolites and mesoporous materials
2.3.6 Other supports
2.3.6.1Monolith Support
2.3.7 Selecting the right support
2.4 Promoters
2.5 Inhibitors

3. Catalyst Properties
3.1 Introduction
3.2 Physical, mechanical and chemical properties
3.2.1 Physical and mechanical properties, their definition and their
Importance
3.2.2 Chemical Properties
3.2.3 Dynamic properties
3.3 Catalyst characterization

4. References

List of Figures

1 Important properties of an industrial catalyst

2 Elements of the period table finding application as catalytic phases, carriers or promoters

3 A thermostable carriers prevents sintering of the active material

4 Model for the hydrogenation of acetone (SMSI effect)

5 Alumina comes in many different forms; γ -Al2O3 being the most often used catalyst support

6 Lattice structure of spinel MgAl2O

7 Production of Alumina

8 Schematic representation of the formation of the different forms of alumina

9 The influence of sodium oxide impurity and types of alumina hydrate on thermal stability of alumina produced from the hydrate

10 The effect of lanthanum oxide on thermal stability of γ –Al2O

11 Production of silica (Porous SiO­2) by sol gel route

12 Gelation

13 Scheme for production of silica

14 The surface of crystoballite is sometimes used as a structural model for silica

15 Typical 29Si-NMR spectrum of a potassium silicate solution SiO2/K2O = 1.

16 Schematic representation of the formation of silica gel

17 Schematic representation of the structure of pyrogenic silica

18 Production of Activated carbon

19 Reaction composition diagram for the synthesis of zeolite A and zeolite Y

20 Photograph of monolithic catalyst supports with different cell densities

21 Pressure drop vs specific geometric surface area of monolithic structures, spheres and rings. Data are for air at 20°C and 1 bar and a superficial gas velocity of 1 m/s (at STP). Monolithic data are for commercial and developmental products (the notation is cell density in cpsi and wall thickness in 1/1000 in.)

22 (a) Monolithic structures of various shapes. Square channel cordierite structures (1, 3, 5, 6), internally finned channels (2) washcoated steel monolith (4) (b) Square channel cordierite monoliths with cell densities of 200, 400 and 600 cells per square inch (cpsi)

23 Schematic representation of a honeycomb monolith catalyst

24 (a) Monolith shapes (b) Monolith cross section with triangular cell shape

25 Photograph of gas and liquid flow in a 50-mm-diameter monolith with 400 cpsi (the channel diameter is 1 mm)

26 Gas/liquid distributions for different monolithic channel geometries under film flow conditions

27 Flooding curves for different monolithic packings operated under countercurrent gas/liquid flow. Symbols: (a) 200 cpsi, 1849 m2/m3, 31% solid fraction, hydraulic diameter 1.49 mm (b) 100 cpsi, 1354 m2/m3, 26% solid fraction, hydraulic diameter 2.18 mm (c) 50 cpsi, 964 m2/m3, 25% solid fraction, hydraulic diameter 3.11 mm. The system is water/air at ambient conditions

28 Comparison of flooding curves for monoliths and other conventional packings operated under countercurrent gas/liquid flow. The system n -decane/air is at ambient conditions

29 Catalyst monolith

30 Catalyst monolith channel showing diffusion paths and flow profiles

31 Diagram of preparation paths for extruded monolithic catalysts

32 Schematic representation of catalyst preparation by the modified sol–gel method

33 A porous alumina support particle with well dispersed active sites (left), and zoom-in view of a single active site (right)

34 Pores may vary in size, shape, and connectivity: (a) channel/cage structures (b) polygonal capillaries (c) ink bottle pores (d) laminae (e) slit pores

35 The action of potassium promoters in the dissociative chemisorption of N2 on iron catalysts

36 Triangular concept for catalyst design

37 (a) Conversion, density and porosity relationships for typical pellets (b) Typical pellet crush strength versus particle porosity

38 Typical reactor pressure drop versus particle diameter for particulate catalysts of different geometries

39 Distribution of solid, pore and void volumes in a microscopic section of a typical catalyst particle

40 (a) Schematic of pore size breakdown in a catalyst particle (b) Expanded idealized view of pores in a catalyst particle

41 The relationship between fraction of atoms at the surface and the particle radius

42 Schematic representation of a solid catalyst crystal surface

43 Important classes of catalytic reactions of hydrocarbons and examples of each; hydrogenation and dehydrogenation are typically catalyzed by metals; hydrogenolysis (cracking), isomerization and dehydrocyclization are typically catalyzed by acid sites in high surface area oxides

44 Schematic showing Lewis and Bronsted acid sites on an aluminosilicates surface

45 High temperature water gas shift catalyst, iron oxide promoted with chromia; 5 x 5 mm pellets

46 Two forms of steam reforming catalysts. Outside diameter = 16 mm, thickness 10 mm

47 Various forms of steam reforming catalysts. For the 7-hole catalyst, outside diameter = 16 mm, hole diameter = 3.5 mm, thickness = 8 to 11 mm

48 Extrudates of different sizes with a trilobal cross section, termed trilobes

49 Twisted trilobes

List of Tables

1 Components of a typical heterogeneous catalyst: Material types and examples

2 Active catalytic phases and reactions they typically catalyze

3 A general classifications of hydrogenation reactions with the applicable catalysts

4 Relative activities of metal catalyst for hydrogenating unsaturated hydrocarbons

5 Oxide catalysts classified in order of decreeing activities for the oxidation of hydrogen containing compounds

6 A poster of catalyst by order of increasing acid activity, as obtained for isomerization, polymerization and cracking

7 Important catalyst supports and their applications

8 Example uses of supports in selected catalytic reactions

9 Selection of catalyst supports

10 Typical physical properties of common carriers (supports)

11 Hydrogenolysis of ethane on supported nickel catalysts (10 % Ni)

12 Dehydrogenation of cyclohexane to benzene on supported platinum catalysts at 773 K

13 Hydrogenation of CO on supported Pd catalysts

14 Relative activities of supported Rh catalysts in the hydrogenation of CO

15 Influence of support materials on the hydrogenation of CO with rhodium catalysts

16 Supported copper catalysts for the hydrogenation of CO

17 SMSI effect in the hydrogenation of acetone to isopropanol on supported platinum catalyst

18 Physical and structural characteristics of common aluminium oxides

19 Properties of shaped fumed silica supports

20 Characteristics of carbon support materials with indicative figures on surface areas readily obtainable

21 Examples of Monolithic Structures, Materials and Their Use

22 Comparison of ceramic and metallic monolith catalyst properties

23 Physical data of ceramic and metal substrates for 400 cpsi monolith catalyst

24 Comparison of multiphase reactors

25 Examples of promoters in the chemical industry

26 Physical and mechanical Properties: Definitions and Specifications

27 Constants and Planar densities useful in estimating dispersion, surface area and average crystallite diameter in supported metals

28 Chemical Properties: Definitions and Specification

29 Qualitative ranking of aqueous and solid acids (listed in decreasing order of acid strength)

30 Dynamic (Catalytic) Properties of Catalysts: Definitions and Specifications

31 Catalyst characteristics and methods for their investigation

1. Introduction

Heterogeneous and homogeneous catalysts used commercially are chemically and physically complex, sophisticated materials based on over a century of catalytic art, technology and science. Because of the chemical and physical complexity of catalysts, catalyst research and development are highly multidisciplinary endeavors which require knowledge of chemistry, chemical engineering, material science and physics. This is particularly true in design, development and preparation of new catalysts, a process, which requires detailed knowledge of catalyst material, catalyst properties and art/science of catalyst preparation [1].

1.1 Catalyst Performance

Factors which Affect the Catalyst Performance

Catalysis is a multidisciplinary subject that has a lot of aspects. It is obvious that a good catalyst should possess high activity. A high activity allows relatively small reactor volumes, short reaction times and operation under mild conditions. High selectivity is often more important than high activity. Furthermore, a catalyst should maintain its activity and selectivity over a period of time i.e. it should have sufficient stability. In summary, important properties of an industrial catalyst are shown in Figure 1[2].

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Figure 1 Important properties of an industrial catalyst [2]

Catalysts are developed for specific processes e. g. for a specific reaction in a specific reactor under specific reaction conditions. Therefore, there are many requirements for an industrial catalyst:

– High activity/unit of reaction volume
– High selectivity with reference to the desired product at the required conversion in the reactor
– Sufficient stability with regard to deactivation
– Possibilities for regeneration especially for fast deactivation processes
– Reproducible production method
– Sufficient thermal stability against sintering, structural change or loss via gas phase (e.g. if H2O vapor is produced as side product)
– High compressive strength (with reference to the catalyst bed and shape of the catalyst)
– High resistance against mechanical stress [2, 3]
The catalytic performance can be affected by many influences such as
– Active phase (metal, metal oxide; type, morphology …)
– Support (type, texture, chirality …)
– Environment of the reaction (solvent etc)
– Promoters (inorganic, organic, chiral)
– Inhibitors

For a good understanding of catalysis it is crucial to have a good idea of the structure (both chemical and physical) of a catalyst. The properties of a catalyst can be manipulated by any process that alters the properties of its surface because the nature of the individual sites at the surface is responsible for the activity, selectivity and stability of the catalyst [2].

2. Catalyst materials

Catalysts are complex, high tech, high surface area materials.

2.1 Makeup of a typical heterogeneous catalyst

A typical heterogeneous catalyst is comprised of three components: (1) an active catalytic phase (2) a promoter, which increases activity and/or stability (3) a high surface area carrier (support) which serves to facilitate the dispersion and stability of the active catalytic phase. Material types and examples of these three catalyst component are listed in table 1

Table 1 Components of a typical heterogeneous catalyst: Material types and examples [1]

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aThe term base metal derives from the jewelry industry where Fe, Ni etc. serve as the base metal for coating with noble metals such as Au, Pt and Rh.

bRefers to a high surface area carrier or matrix which is an integral part of the catalyst; this carrier is distinct from low surface area metal or ceramic monolithic supports or substrates upon which catalysts are sometimes coated

Active phase (metal, metal oxides and metal sulfides) are typically dispersed in the pores of support in the form of nano-particles which are 1-50 nm in diameter; the surface of these crystallites contains sites (atoms or collection of atoms) active for catalyzing various reactions.

Promoters are added in relatively small quantities (i.e. 1- 5%) to enhance and/or maintain texture or catalytic surface area and/or to chemically increase catalytic activity. For example, Al2O3 is added in small quantities (0.5 – 1 wt %) as a texture promoter (not support) to the unsupported ammonia synthesis catalyst to facilitate its preparation and to maintain active iron surface over long periods of reactions. K2O, a chemical promoter, is added to the ammonia synthesis catalyst to increase the activity of iron. The support refers to high surface area carriers (typically oxides of carbon) that facilitate preparation of well dispersed catalytic phase and improved the thermal stability of these phase over long periods of time.

Note: Low surface area support or substrates in monolithic form are not catalyst carriers in the same sense, but rather substrates upon which catalysts are coated.

Elements of the periodic table that are important as catalysts, promoters, supports and poisons are highlighted in figure 2.

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Figure 2 Elements of the period table finding application as catalytic phases, carriers or promoters [1]

2.2 Active catalytic phases

Transition metals as well as their oxides, sulfides, carbides and nitrides are unique in their abilities to catalyze chemical reactions primarily due to their multiplicity of low energy surface electronic states, which can readily donate or accept electrons in the process of making or breaking bonds at a surface table 2 gives the example of a very general correlation establishing a correspondence between the three large classes of solid catalysts, metals, semi conductor oxides and isolating oxides and the families to which they apply.

Table 2 Active catalytic phases and reactions they typically catalyze [1, 4]

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2.2.1 Selecting the active catalysts

Proceeding to table 3 and more restricted grouping of hydrogenation reactions, the correspondence becomes more precious; according to the type of chemical bond to be activated the table shows the group of promoters that are recommended for use with the catalytic metal. One see that the catalysts are the most part metals of group VIII of the periodic table and the promoters are mostly sulfides or oxides of either those metals or metals from group VIA with the selection varying from one family of hydrogenation to another.

Table 3 A general classifications of hydrogenation reactions with the applicable catalysts [4]

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If within the area of hydrogenation one further limits to the better known reactions such as hydrogenation of unsaturated hydrocarbons on reduced metals, the data available from the literature allows one to rate the various metals according to their activity for each one of a group of hydrogenation (Table 4). In spite of the similarities between the various bonds to be activated, table 4 shows that the different metals exhibit a certain specificity by being relatively more or less active than each other. Moreover, this specificity becomes striking when one compares the hydrogenation of olefins to the isomerization of the position of the olefins double bond which take place under the same operating conditions. The relative activities of the metals are almost the inverse.

Table 4 Relative activities of metal catalyst for hydrogenating unsaturated hydrocarbons [4]

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This type of comparison between the catalysis of one family by order of activity for a given reaction is available in all areas of catalysis. Table 5 presents oxide catalysts in order of decreasing activity for the oxidation of various molecules containing hydrogen.

Table 5 Oxide catalysts classified in order of decreeing activities for the oxidation of hydrogen containing compounds [4]

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Again table 6 compares some acid catalysts according to their activity in three reactions that each takes place according to a mechanism involving the carbonium ion as intermediate form. For these reactions, the activities of the catalytic agents occur in the same order and this order would remain the same for analogous reactions such as the isomerization of xylenes, reactions of hydrogen transfer hydrations, alkylation and so forth [4].

Table 6 A poster of catalyst by order of increasing acid activity as obtained for isomerization, polymerization and cracking [4]

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2.3 Carriers (Supports)

The surface interactions hold the key to the catalyst’s activity, selectivity and stability. Unlike the situation for molecular catalysts, the bulk parameters such as particle size, shape and mechanical strength are crucial here. Particle size is directly related to the accessible active surface area. Platinum, for example, is an excellent catalyst for alkane dehydrogenation but it is also very expensive (ca. €32/g or $44/g in February, 2007). Suppose you use a cubic centimeter of Pt as your catalyst. This cube would weigh 21.3 g and cost about €700 ($1000). The available surface area of such a cube would be only 5 cm2 because one side would have to be attached to the reactor. Now, if you break this cube into 1012 smaller cubes, each of which has 1 mm sides, the total available surface area would be 5 · 1012 mm2 or 50 000 cm2. State of the art catalyst synthesis methods can produce particles as small as 2 nm with correspondingly huge surface areas. Interestingly, reducing the particle size can also cause undesired effects, due to surface penetration by foreign atoms. One reason why Pt is an excellent oxidation catalyst is that O2 is readily adsorbed on Pt particles, dissociating to two O atoms which react with a variety of substrates. When the particles are very small (<10 nm), they react with oxygen and form platinum oxide. The resulting strong Pt-O bond lowers the catalytic activity, despite the large metal surface area.

Placing all these small particles in one reactor at high temperatures would cause very high back-pressures and agglomeration. This can be avoided by coating the active metal particles on a (porous) support with a high surface area e.g., silica, alumina, zeolites, or carbon. The shape and mechanical strength of the catalyst particles are also important especially in large-scale applications (refinery reactors can be 20 m high, holding as much as 50 tons of catalyst). They determine the packing in the reactor and ultimately the flow of the gaseous reactants and products to and from the catalyst [5].

Many catalysts are prepared by adding small amount of the catalytically active component to a fairly large amount of a more or less inert solid called the support [6].

Support materials are vehicles for the active phase. They exercise several functions among which are the maximization of the surface area of the active phase by providing a large area over which it may be spread and allowing the catalyst to be cast into the form of coarse bodies suitable for use in technical reactors. The active phase usually constitutes between 0.1 and 20% of the total catalyst mass and is normally in the form of very small crystallites, e.g. 1 nm Pt crystals or 50 nm Ag particles. The support is often supposed to be catalytically inactive by itself, however in partnership with the active phase it can participate in the total reaction in an important way (e.g. so-called bi-functional catalysis) [7].

Small metal particles are often unstable and prone to sintering, particularly at the temperatures typical of catalytic reactions. Therefore, heterogeneous catalysts used in industry consist of relatively small particles stabilized in some way against sintering. This can be achieved by adding so-called structural promoters or by applying particles inside the pores of an inert support. All kinds of materials that are thermally stable and chemically relatively inert can be used as supports [3].

Supported catalysts represent the largest group of heterogeneous catalysts and are of major economic importance, especially in refinery technology and the chemical industry. Supported catalysts are heterogeneous catalysts in which small amounts of catalytically active materials, especially metals are applied to the surface of porous, mostly inert solids the so-called supports [2].

The early concept of a support or a carrier was of an inert substance that provided a means of spreading out an expensive catalyst ingredient such as platinum for its most effective use or a means of improving the mechanical strength of an inherently weak catalyst. However, the carrier may actually contribute catalytic activity depending on the reaction and reaction conditions and it may react to some extent with other catalyst ingredients during the manufacturing process. It can also help stabilize the catalytically active structure [8].

The support serves the following important functions:

1. The support with its large surface area allows the active component to spread thus exposing a large proportion of the latter to the reactants. This minimizes the amount of the active component needed and is particularly important when the active component is expensive (e.g. platinum metals)
2. The support holds on its surface the micro-crystalline particles of the active component and prevents its sintering.
3. The support surface may interact with the active component to form surface complexes that have better catalytic activity and selectivity than either that of the support or the active component.
4. The porous nature of the support may control the transport of the reactant and the product molecules affecting the overall conversion [6].

Catalyst supports are generally granular, but they may also be fibrous (like asbestos) or monolithic. They include naturally occurring materials like kieselguhr (diatomaceous earth), clay, bentonite (all hydrosilicates), asbestos, zeolites etc. or synthetic materials like silica, alumina, other oxides, activated carbon, synthetic zeolites etc. [13]. The supports can have special forms such as pellets, rings, extrudates and granules. Typical catalyst supports are porous solids such as aluminum oxides, silca gel, MgO, TiO2, ZrO2, aluminosilicates, zeolites, activated carbon and ceramics. Table 7 lists widely used catalyst supports [4]. In special cases where an unrestricted flow of reacting gases is important, monoliths are applied. Table 8 gives an overview of supports used in catalysts for various reactions [3].

Catalyst carriers (supports) are porous, high surface area metal oxides or carbons having significant pore volume and capacity for preparing and preserving stable, well-dispersed catalytic phases during reaction. Surface areas of these materials range from about 1.5 to 1500 m2/g, pore volume are generally 0.4-1 cm3/g and pore diameters range from 0.4 to 2000 nm. Other less common but still important commercial carriers includes MgO, titania, aluminosilicates and calcium aluminates [1].

Table 7 Important catalyst supports and their applications [2]

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Table 8 Example uses of supports in selected catalytic reactions [3]

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Typical substances that find wide use as high-area supports include silica gel and γ-alumina, which can be obtained with surface areas in the range 100-800 m2/g. Materials used as low surface area supports ( 1 m2/g) include α-alumina and mullite (alumina-silica). These oxidic support materials are in fact ceramics with good thermal stability, which is relevant to obtain thermostable catalysts (Figure 3). Carbon is an attractive carrier because it is chemically inert in (aqueous) liquids, which allows one to use it in liquid-phase catalysis for fine chemicals synthesis [7].

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Figure 3 A thermostable carriers prevents sintering of the active material [9]

The carrier may be used as pellets or powders to be impregnated. A powdered carrier may be incorporated into a mixture to be precipitated or the carrier may itself be precipitated from solution in the manufacturing process. Some substances such as colloidal alumina or colloidal silica may play a double role acting as a binding agent in catalyst manufacture and as a carrier in the ultimate product. Aluminium in the γ form is intrinsically weakly acidic but such a substance may be a truly inert carrier for many reactions. In other cases it can be used by itself as a catalyst, as in dehydration of an alcohol. High area carriers are sometimes loosely referred to as “active” carriers in contrast to low area “inert” carriers but this usage may be misleading [8].

What are the reasons for the predominant use of supported catalysts in industry?

Costs:

The catalytically active components of supported catalysts are often expensive metals. Since this active component is applied in a highly dispersed form, the metal represents only a small fraction of the total catalyst mass. For example, the metals Rh, Re and Ru are highly effective hydrogenation catalysts for aromatic hydrocarbons. They are sometimes used in mass fractions as low as 0.5 % on Al2O3 or activated carbon.

Activity:

The high activity leads to fast reaction rates, short reaction times and maximum throughput.

Selectivity:

Selectivity facilitates the following: maximum yield, elimination of side products and lowering of purification costs; it is the most important target parameter in catalyst development.

Regenerability:

Regenerability helps keep process costs low.

Which factors influence these properties? The main factors are the choice of the most suitable support material and the arrangement of the metal atoms in the pore structure of the support. In choosing catalyst supports, numerous physical and chemical aspects and their effects must be taken into account (Table 9).

The tasks of catalyst supports are as follows:

– Fixation of the active components
– Formation of high dispersed particles of the active component
– Stabilization of the active component
– Enlargement of the specific surface area

Table 9 Selection of catalyst supports [2]

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The selection of a carrier is based on its having certain desirable characteristics. In addition to possible chemical effects, certain physical properties are important:

1. Inertness to undesired reactions
2. Desirable mechanical properties including attrition resistance hardness and compressive strength
3. Stability under reaction and regeneration conditions
4. Surface area (high surface area is usually but not always desirable)
5. Porosity including average pore size and pore size distribution (high area imples fine pores but relatively small pores such as < 2nm, may become plugged in catalyst preparation especially if high loading are sought)
6. Low cost

Of a wide variety of possible materials, only three combine the foregoing characteristics in an optimum way and therefore they account for most uses. These are alumina, silica and activated carbon; titanium also has some limited uses. Of the first three, alumina is the most widely used industrially. Magnesia generally has poor strength and zinc oxide tends to be reduced. Chromium tends to cause dehydration and its acidity can cause undesirable reactions to occur. Zirconia, although more expensive, is stable at high temperatures and is stable in alkaline media. For a low area carrier, α-alumina or a magnesia alumina silicate have been used.

A necessary requirement for any carrier is resistance to sintering under reaction conditions. The temperature at which lattices begin to be appreciably mobile is sometimes termed the Tammann temperature and that at which surface atoms become significantly mobile, the hitting temperature. For example compound without phase changes on heating and of low vapor pressure, the Tammann temperature is very approximately 0.5 Tm and hitting temperature about 0.3 Tm where Tm is the melting point in absolute units. Consequently suitable carriers must usually have fairly high meting points as a minimum. Appreciable mobility appears at about Tm/3 for metals, so group IB metals (Cu, Ag, AU) which have melting points in neighborhood of 1300 K, must almost always be supported or have textural promoters incorporated with them in order for high area to be maintained. The transition metal, iron, cobalt and nickel with melting points of about 1800 K will become mobile at temperatures above roughly 250 to 300 0C, the platinum group metals melt at high temperatures but are usually supported for economy [8].

Table 10 Typical physical properties of common carriers (supports) [1]

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The main function of the catalyst support is to increase the surface area of the active component. Catalytic activity generally increases with increasing catalyst surface area, but a linear relationship cannot be expected since the reaction rate is often strongly dependent on the structure of the catalyst surface. However, in many reactions, the selectivity decreases when the catalytic surface is enlarged. As a general rule, catalysts for the activation of hydrogen (hydrogenation, hydrodesulfurization, hydrodenitrogenation) require high support surface areas while selective oxidations (e. g., olefin epoxidation) need small support surface areas to suppress problematic side reactions.

The choice of the appropriate catalyst support for a particular active component is important because in many reactions the support can significantly influence the reaction rate and the course of the reaction. The nature of the reaction system largely determines the type of catalyst support.

If a support material with a large surface area such as activated carbon is used as support, then the metal is present as discrete crystallites, only a few atomic layers thick, with a very high surface area.

In batch liquid-phase reactions, powder supports are used exclusively, whereas in gas-phase and continuous liquid-phase reactions (trickle columns), supports in pellet or granule form can be employed.

The pore structure of the support can also have an influence on the role of the active component, since the course of the reaction is often strongly dependent on the rate of diffusion of the reactants. Furthermore, the size of the support surface can limit the exploitable metal concentration.

Many commercially available catalyst supports, for example, activated carbon and alumina, are offered in various particle sizes, each having a series of different specific surface areas and pore size distributions.

The choice of catalyst support may be restricted by the reaction conditions. Thus the support must be stable under the process conditions and must not interact with the solvent and the starting materials. Depending on the process, supported catalysts can have a low (e. g. 0.3 % Pt/Al2O3, 15 % Ni/Al2O3) or a high loading (e. g. 70 % Ni/Al2O3, Fe/Al2O3).

In supported metal catalysts, the support does not only ensure high dispersion of the metal; there are also interactions between metal and support due to various physical and chemical effects:

– Electronic effects: electron transfer up to formation of chemical bonds
– Adhesive forces (Van der Waals forces)
– Formation of reduced support species on the metal surface
– Formation of new phases at the boundary surface

Electronic effects result from the n - or p -type semiconductor properties of the support material. The interactions can impair the chemisorption capability and effectiveness of a catalyst as well as restricting the mobility of the disperse phase and delaying its sintering.

In the last few years, the concept of strong metal–support interaction (SMSI) has gained considerable importance. It was introduced in 1978 to explain certain peculiarities in the chemisorption of H2 and CO on TiO2-supported platinum group metals. The catalysts were subjected to high-temperature reduction with H2 (400 0C), after which a strong decrease in the adsorption capacity for H2, CO and NO was found. The effect is also exploited in chemical syntheses: platinum group metals on TiO2 can considerably influence the catalytic activity and product selectivity in the hydrogenation of CO.

The following examples discuss the industrial use of supported catalysts and the above-mentioned metal–support interactions.

Hydrogenation is one the oldest and most widely used applications for supported catalysts. The usual metals are Co, Cu, Ni, Pd, Pt, Re, Rh, Ru, and Ag. There are numerous catalysts for special applications. Most hydrogenation catalysts consist of an extremely fine dispersion of the active metal on activated carbon, Al2O3, aluminosilicates, zeolites, kieselguhr, or inert salts such as BaSO4. An example is the selective hydrogenation of chloronitrobenzene (Equation 1).

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.….. (1)

Usually, palladium catalysts are used for the industrial hydrogenation of nitro compounds but Pd is also an excellent catalyst for the dehydrochlorination reaction, so that aniline is predominantly formed. Therefore, a new, high-selectivity Pt/C catalyst was developed which gives the desired product o -chloroaniline without affecting the rate of hydrogenation.

In the dehydrogenation of cyclohexanone derivatives (Equation 2), an activated carbon support in which the platinum is uniformly distributed in the support structure is recommended. With increasing ordering of the metal, the catalyst exhibits an increasing metal dispersion and therefore a higher resistance to thermal sintering. Sintering would lead to crystal growth and deactivation of the catalyst.

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.….. (2)

The hydrogenolysis of ethane on supported nickel catalysts is a good example for the influence of the degree of dispersion of the metal (Table 11). It is known that nickel is more highly dispersed on SiO2 than on Al2O3 and at the same time there is an influence on the crystallite form. A further influence is due to the acid centers of aluminum oxide which lead to more extensive coke formation, deactivating the nickel catalyst.

Table 11 Hydrogenolysis of ethane on supported nickel catalysts (10 % Ni) [2]

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The dehydrogenation of cyclohexane to benzene can be explained well in terms of electronic effects (Table 12). The benzene selectivity decreases on going from TiO2 to SiO2 and this corresponds to the decreasing n character of the support material. Apparently, weak n -type semiconductor oxides are the most effective supports for this reaction. In contrast, the strong n -type semiconductor ZnO which has a higher electron concentration than TiO2 gives no reaction.

Table 12 Dehydrogenation of cyclohexane to benzene on supported platinum catalysts at 773 K [2]

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Extensive investigations have been carried out on the industrially important hydrogenation of CO. High activities and selectivities for the formation of methanol were found for the catalysts Pd on La2O3, MgO, or ZnO, but high activities and selectivities for the formation of methane with Pd on TiO2 or ZrO2 (Table 13). It is no surprise that a high proportion of dimethyl ether is formed with the acidic support Al2O3. However, these investigations did not take degree of dispersion of the metal into consideration.

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The hydrogenation of CO can be influenced by means of the support composition and by varying the degree of dispersion of the metal. Thus it is assumed that for metals of Groups 8–10, a low degree of dispersion favors formation of hydrocarbons, and a high degree of dispersion, the formation of oxygen-containing compounds.

Relative activities in CO hydrogenation measured for supported rhodium catalysts are listed in Table 14. These experimental findings are supported by H2 chemisorption measurements and active rhodium centers.

Table 14 Relative activities of supported Rh catalysts in the hydrogenation of CO [2]

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In another investigation with supported rhodium catalysts, it was found that the oxidation state of the rhodium influences the type of chemisorption of CO and hence the product distribution according to Equation 3.

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.. (3)

Thus dissociative chemisorption of CO leads to hydrocarbons, and associative chemisorption to alcohols as final product (Table 15).

Table 15 Influence of support materials on the hydrogenation of CO with rhodium catalysts [2]

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In CO hydrogenation with supported copper catalysts (Table 16), the results were explained in terms of electronic effects of the support material. The differing CO hydrogenation activity of the catalysts reflects the electronic interaction between the Cu particles on the surface and the support. With p -type semiconductors such as Cr2O3 and ZrO2, which have higher work functions than copper metal, higher activity than with pure copper is observed. This is explained by the fact that in this case, electron density can flow from copper to the support. With the insulators SiO2 and Al2O3, the activity corresponds roughly to that of copper; no electrons can be taken up by the support. In the case of n -type semiconductors such as TiO2 and MgO, charge transfer from copper to support cannot take place, and the catalytic activities are lower than with pure copper.

Table 16 Supported copper catalysts for the hydrogenation of CO [2]

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a TON= mol CO/atom surface metal × s; H2/CO = 3; flow arte 60 mL/min, normal pressure, 275 0C; all catalysts have approximately the same particle size

The next example shows how catalyst bifunctionality can arise from the support material. Platinum metal dehydrogenates napthenes to give aromatic compounds, but it is not able to isomerize or cyclize n -alkanes. This function is adopted by the Al2O3 support with its acidic properties. The cooperation of the two catalyst components is shown schematically for the reforming of n -hexane in Scheme 2.

Scheme 1 Reforming of n -hexane on a Pt/Al2O3 supported catalyst [2]

It was shown that neither Pt nor the support material Al2O3 can isomerize the alkane starting material. However, acidic Al2O3 centers can isomerize n -alkenes which are then hydrogenated to isoalkanes on Pt. During the activation phase of the catalyst, chlorine is added to achieve the necessary acidity.

The final examples deal with SMSI effects. In the hydrogenation of CO on Pt/TiO2 catalysts, a 100-fold increase in catalyst turnover number was observed after high-temperature reduction. In the high-temperature reduction, the chemisorption capacity for both starting materials, CO and H2, was drastically lowered, but no sintering of the metal occurred. It has been shown that partially reduced TiO x species are distributed over the Pt surface. Interestingly, in spite of the higher catalyst activity, higher activation energy was measured rather than a lower one.

A further example is the model reaction of hydrogenation of acetone to isopropanol (Equation 4).

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Kinetic measurements on a Pt catalyst showed no dependence on the size of the crystallites. On an inert SiO2 support the catalyst turnover number remained virtually constant over the particle size range 2–1000 nm; that is, the reaction is structure- insensitive. With a TiO2 support, the TON was increased by a factor of 500 following high-temperature reductions (Table17).

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Details

Pages
114
Year
2014
ISBN (eBook)
9783656685852
ISBN (Book)
9783656685845
File size
6.1 MB
Language
English
Catalog Number
v275585
Institution / College
Nirma University – Institute of Technology, Nirma University, Ahmedabad, Gujarat
Grade
Tags
catalyst

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Title: Catalyst materials & properties