**1. Introduction**

Industrial selective hydrogenation processes mostly make use of solid catalysts and threephase reactors (slurry or packed bed). Most catalysts are of the supported metal kind,and the metal carriers are either completely inorganic (e.g., alumina, titania, silica) or organic (e.g., resins, activated carbons).

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Nowadays, a great variety of materials are being used as support, such as alumina, mordenite, silica, titania, magnesia, zeolites, refractory metal oxides of groups III, IV, V, VI, VIII, activated carbons, and polymers.

The preparation of the support is an important step in the manufacture of commercial catalysts. The size and shape of the supports are determined by the kind of reactor and the set of reaction conditions [1].

Supported metal catalysts for industrial reactors are preferably available in the physical form of cylinders, tablets, spheres, rings, fibers, cloths, etc. The form of the support in the catalyst preparation step is important since it is closely related to the structure of the final catalyst.

At the time of choosing a catalyst, its mechanical properties cannot be disregarded. Catalysts are generally placed inside the packed beds, commonly 10 m or more in height. This arrangement makes the bottom pellets support the weight of the bed above and thus need a high crushing resistance [1]. If the resistance is not high enough, the pelletsget broken and smaller particles and dust are generated. As a result the pressure drop in the packed bed gradually increases until the bed is plugged; the unit must then be shut down for maintenance. Thus, the smallest particles of catalyst could be eluted from reactor with solvents and reaction products. This leads to an activity loss and a decrease of catalyst life.

In the preparation of the inorganic carriers employed in the fabrication of supported metal catalysts, the support is first obtained in the form of a powder or tiny particles, which are mixed with water and/or organic compounds. The mixture is then extruded, granulated, or pelletized to give it the final shape [1, 2]. Once the material has the desired shape, it is dried at temperatures between 353 and 673 K for one or more hours. Then, depending on its nature and final use, the support is calcined at higher temperatures to give mechanical resistance [1–9]. This procedure is long and energy consuming. However, the preparation of organic materials is energetically less costly.

Carbons and polymers are commonly used among the organic supports. Carbons are obtained by controlled combustion in oxygen-poor atmosphere and the final product can be surface treated to modify its catalytic properties. When carbon particles of a certain shape and size are needed, the carbon material must be agglomerated. Polymeric supports are mainly obtained by reactions in emulsions and suspensions. Carbons and polymeric supports have the common disadvantage of low mechanical resistance.

Among the supported metal catalysts we prefer the catalysts with metal "profile" that comprise an inert support and a metal phase of specially tailored distribution. Depending on this distribution catalysts are classified as "egg-yolk," "egg-white," or "egg-shell". They are usually available in the form of rings, spheres, tablets, or pellets [10].

Metal-supported catalysts of the egg-shell type are mainly used in heterogeneous catalytic reactions in which the mass or heat transfer limitations have an important effect on the activity and selectivity to the desired products. They also tend to prevent or minimize the deactivation phenomena. Their main advantages are low mass transfer resistance, support-independent pore structure, and high heat transfer rate at the catalyst surface [2].

It is known that a uniform access of reactants to active sites is important for getting high activity and selectivity in chemical processes [11]. For example, consecutive reactions of the form A→B→C in which product C should be suppressed or minimized are of great industrial interest. Due to the interaction of intraparticle diffusion and reaction, the occurrence of C can be minimized in catalysts where the active sites are preferentially located on a surface layer [11–13].

Nowadays, a great variety of materials are being used as support, such as alumina, mordenite, silica, titania, magnesia, zeolites, refractory metal oxides of groups III, IV, V, VI, VIII, activated

The preparation of the support is an important step in the manufacture of commercial catalysts. The size and shape of the supports are determined by the kind of reactor and the set of reaction

Supported metal catalysts for industrial reactors are preferably available in the physical form of cylinders, tablets, spheres, rings, fibers, cloths, etc. The form of the support in the catalyst preparation step is important since it is closely related to the structure of the final catalyst.

At the time of choosing a catalyst, its mechanical properties cannot be disregarded. Catalysts are generally placed inside the packed beds, commonly 10 m or more in height. This arrangement makes the bottom pellets support the weight of the bed above and thus need a high crushing resistance [1]. If the resistance is not high enough, the pelletsget broken and smaller particles and dust are generated. As a result the pressure drop in the packed bed gradually increases until the bed is plugged; the unit must then be shut down for maintenance. Thus, the smallest particles of catalyst could be eluted from reactor with solvents and reaction

In the preparation of the inorganic carriers employed in the fabrication of supported metal catalysts, the support is first obtained in the form of a powder or tiny particles, which are mixed with water and/or organic compounds. The mixture is then extruded, granulated, or pelletized to give it the final shape [1, 2]. Once the material has the desired shape, it is dried at temperatures between 353 and 673 K for one or more hours. Then, depending on its nature and final use, the support is calcined at higher temperatures to give mechanical resistance [1–9]. This procedure is long and energy consuming. However, the preparation of organic materials is

Carbons and polymers are commonly used among the organic supports. Carbons are obtained by controlled combustion in oxygen-poor atmosphere and the final product can be surface treated to modify its catalytic properties. When carbon particles of a certain shape and size are needed, the carbon material must be agglomerated. Polymeric supports are mainly obtained by reactions in emulsions and suspensions. Carbons and polymeric supports have the common

Among the supported metal catalysts we prefer the catalysts with metal "profile" that comprise an inert support and a metal phase of specially tailored distribution. Depending on this distribution catalysts are classified as "egg-yolk," "egg-white," or "egg-shell". They are usually

Metal-supported catalysts of the egg-shell type are mainly used in heterogeneous catalytic reactions in which the mass or heat transfer limitations have an important effect on the activity and selectivity to the desired products. They also tend to prevent or minimize the deactivation phenomena. Their main advantages are low mass transfer resistance, support-independent

products. This leads to an activity loss and a decrease of catalyst life.

carbons, and polymers.

182 New Advances in Hydrogenation Processes - Fundamentals and Applications

energetically less costly.

disadvantage of low mechanical resistance.

available in the form of rings, spheres, tablets, or pellets [10].

pore structure, and high heat transfer rate at the catalyst surface [2].

conditions [1].

Egg-shell catalysts are used with great benefits in reactions of selective hydrogenation [14– 16], Fischer-Tropsch synthesis [17], methane reforming [18, 19], partial oxidation of methane [20], etc.

It is also known that diffusional restrictions decrease the chemical reaction rate and negatively affect the selectivity to the desired products. These phenomena are mainly enhanced in processes using pelletized catalysts in packed beds, such as Fischer-Tropsch synthesis [17] or selective hydrogenation reactions [14–16]. In these reactions the maximum yields and selectivities are obtained with egg-shell catalysts in which the active phase is located on the external surface of the pellet, where the thickness of the active phase plays an important role [17].

Making egg-shell catalysts with common supports is not a simple process because there are many preparation variables that have to be carefully controlled: pH and viscosity of the impregnating solution, concentration of the metal salts in solution, time of contact of the impregnating solution with the support, temperature during impregnation, drying temperature, and calcination temperature. Other complexities arise from the phenomena involved in the grafting of the active phase, the variable nature of the impregnating solution (aqueous, organic, mixed phases), etc. [12, 13, 17, 20–30].

The field of materials science has grown significantly in the past few decades. This growth has been partly due to the development of complex composite materials that combine different properties of the individual components, sometimes with convenient synergysm of physical and chemical features, thus leading to materials with unique properties. Composite-based catalysts can have enhanced mechanical resistance, selective adsorption properties, and unique selectivity for regio, stereo, or enantio hydrogenation reactions, etc.

Composite supports using organic (polymer) and inorganic components are of special interest for modern chemistry and material science because of their potential use in photochemistry, nanoelectronics, optics, and catalysis. This chapter focuses on composite supports for heterogeneous catalysts. In these supports the inorganic phase can be supplied by metal oxide or metal particles or their mixture. The organic phase is supplied by polymers of different chemical nature, with chosen surface functional groups available for acting as active sites for adsorption and catalysis.

Synthesis and use of metal catalysts supported over functional polymers or their mixture with other inorganic materials are the areas of focus. Organometallic or metal complex catalysts are not included in this work, though they can be mentioned for the sake of discussion when reviewing the chemistry of the active sites.

Two main advantages of functional polymers when used as supports of metal particles with catalytic properties are described below:


Pd/composite catalysts with egg-shell structure can be easily obtained because the hydrophillic-hydrophobic action of the support regulates the penetration of the metal (active phase). The good mechanical properties of these materials give them an advantage over traditional supports where their use is intended for packed bed or basket reactors.

To evaluate the properties of the metal/composite egg-shell catalysts, the test reactions of selective hydrogenation of styrene to ethylbenzene, 1-heptyne to 1-heptene, 3-hexyne to 3 hexene, 2,3-butanodione to 3-hydroxy-2-butanone and ethyl piruvate to (R)-ethyl lactate are used.
