REACTIONS WITH POROUS SOLID CATALYSTS

Catalysis by solids depends on the amount of surface exposed to the fluid. Large specific surface is obtained with small particles, but primarily with highly porous structures. For instance, to achieve 1 m2/cc the diameter of a sphere must be reduced to 6(10-4) cm, but porous catalysts may have several hundred m2/cc. Practical limitations exist to the smallness of particles that can be used, such as pressure drop and entrainment. In fixed or moving beds, particle diameters are several millimeters, in fluidized beds they may be less than 0.1 mm. 

Internal surface of porous particles also has limitations. Diffusional resistances of participants may be such that only a fraction of the pore surfaces is accessed, resulting in a waste of expensive catalyst, or undersizing of equipment that is designed for full utilization of the catalytic surface. Appraising the effectiveness of internal surface is the main thesis of this chapter. 

Catalyst manufacturing processes usually make particles in a distribution of sizes, although special shapes such as Raschig rings or cylinders made by extrusion and other shapes made by stamping may be quite uniform. Size distribution is measured by sieving or elutriation. A mean diameter is a convenient quantity. The kind of mean value that is applicable when surface is the main property of interest is the volume-surface mean that is applied in P7.01.09. 

Measurement of other properties also is treated in Chapter 6. Pore volume is measured with helium and mercury pososimeters that together measure the empty space between particles and within particles. P6.01.05 is an example 

Shapes of pores have a great effect on diffusion through them. They are greatly varied and usually cannot be observed directly for commercial materials. For theoretical comparisons they may be assumed parallel cylinders of some mean diameter. Diffusion experments also have been performed with parallel small capillaries. 

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Power-law kinetic rate expressions can frequently be used to quantify homogeneous reactions. However, many reactions occur among species in different phases (gas, liquid, and solid). Reaction rate equations in such heterogeneous systems often become more complicated to account for the movement of material from one phase to another. An additional complication arises from the different ways in which the phases can be contacted with each other. Many important industrial reactors involve heterogeneous systems. One of the more common heterogeneous systems involves gas-phase reactions promoted with porous solid catalyst particles. 

Multiphase reactors include, for instance, gas-liquid-solid and gas-liq-uid-liquid reactions. In many important cases, reactions between gases and liquids occur in the presence of a porous solid catalyst. The reaction typically occurs at a catalytic site on the solid surface. The kinetics and transport steps include dissolution of gas into the liquid, transport of dissolved gas to the catalyst particle surface, and diffusion and reaction in the catalyst particle. Say the concentration of dissolved gas A in equilibrium with the gas-phase concentration of A is CaLt. Neglecting the gas-phase resistance, the series of rates involved are from the liquid side of the gas-liquid interface to the bulk liquid where the concentration is CaL, and from the bulk liquid to the surface of catalyst where the concentration is C0 and where the reaction rate is r wkC",. At steady state,... 

The immobilized-catalysts are confined to a region in space defined by the dimensions of the polymer particle. Reactant(s) must diffuse ftom the external surface to the catalytic sites within the particle before any chemical reaction can occur. This sequential process, mass transfer with reaction, has been treated extensively for catalytic reactions in porous solids (13,14,15). A limited number of studies have shown that the mathematical formalism which is applied to heterogeneously-catalyzed reactions can be used to interpret mass transfer with reaction in immobilized catalysts which employ polymers as supports (11,16,17). 

In reactions using amorphous solids as catalysts, where reaction occurs at the surface of the solid, the mobility of the reactants is reduced from three dimensions to two. In effect this concentrates the reactants in small, localised areas and thus raises the rate of reaction. In lamella or porous solids the limitations may be even greater. One type of catalysis common with porous solids when two species are in competition in a reaction involves biasing the process in favour of one species by use of a solid that allows only one of the competitors to enter the pores. This essentially separates the two species, allowing a selective reaction to occur. It is also possible to control the outcome of a reaction by selecting one of a set of possible products on the basis of its shape. Quite commonly reactions produce more than one product, or isomers of the same material, that have different physical shapes. By restricting the space in which the reaction occurs it is sometimes possible to control which product is formed or which can escape from the pores of the solid. 

With triethylbenzylammonium bromide (TEBA) as the catalyst on a macro-porous styrene support cross-linked with 12% of divinylbenzene, the following kinetic rate constants k have been obtained (Table 6). It is noteworthy to observe that the apparatus FB-UM gives a rate constant very close to that of the SR-UM with the advantage of avoiding pulverization of the catalyst. Therefore, the FB-UM apparatus can be proposed for any reaction using a solid catalyst and an ultrasonic source to increase the reactivity with respect to silent conditions. ... 

Raney nickel A porous solid catalyst made from an activated alloy of nickel and aluminium. The nickel is the catalytic metal with the aluminium as the structural support It was developed by American mechanical engineer Murray Raney (1885-1966) in 1926 for the hydrogenation of vegetable oil and is now used in hydrogenation reactions in various forms of organic synthesis. It is widely used as an industrial catalyst for the conversion of olefins and acetylenes to paraffins, nitriles, and nifro compounds to amines, and benzene to cyclohexane amongst others. 

A rather new concept for biphasic reactions with ionic liquids is the supported ionic liquid phase (SILP) concept [115]. The SILP catalyst consists of a dissolved homogeneous catalyst in ionic liquid, which covers a highly porous support material (Fig. 41.13). Based on the surface area of the solid support and the amount of the ionic liquid medium, an average ionic liquid layer thickness of between 2 and 10 A can be estimated. This means that the mass transfer limitations in the fluid/ionic liquid system are greatly reduced. Furthermore, the amount of ionic liquid required in these systems is very small, and the reaction can be carried in classical fixed-bed reactors. 

There are, however, two broad classes of exceptions to this conclusion. The first comes with the slow reaction of a gas with a very porous solid. Here reaction can occur throughout the solid, in which situation the continuous reaction model may be expected to better fit reality. An example of this is the slow poisoning of a catalyst pellet, a situation treated in Chapter 21. 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

Fig. 9. Electric field E and concentration C of a reactant for a fast reaction catalyzed by a porous solid for transmission and ATR geometries. The dotted line represents the IR beam path. The electric field E is represented as a solid line, and the concentration C of a hypothetical reactant is represented as a dashed line. In the ATR experiment, the electric field is evanescent and decays exponentially with distance z from the surface of the IRE. In the transmission experiment, the electric field decreases as a consequence of absorption. The two techniques sample the catalyst differently.

Functionalized polymers are of interest in a variety of applications including but not limited to fire retardants, selective sorption resins, chromatography media, controlled release devices and phase transfer catalysts. This research has been conducted in an effort to functionalize a polymer with a variety of different reactive sites for use in membrane applications. These membranes are to be used for the specific separation and removal of metal ions of interest. A porous support was used to obtain membranes of a specified thickness with the desired mechanical stability. The monomer employed in this study was vinylbenzyl chloride, and it was lightly crosslinked with divinylbenzene in a photopolymerization. Specific ligands incorporated into the membrane film include dimethyl phosphonate esters, isopropyl phosphonate esters, phosphonic acid, and triethyl ammonium chloride groups. Most of the functionalization reactions were conducted with the solid membrane and liquid reactants, however, the vinylbenzyl chloride monomer was transformed to vinylbenzyl triethyl ammonium chloride prior to polymerization in some cases. The reaction conditions and analysis tools for uniformly derivatizing the crosslinked vinylbenzyl chloride / divinyl benzene films are presented in detail. 

The problem is also more complex when heterogeneous catalysed reactions are considered. With porous catalyst pellets, reaction occurs at gas- or liquid-solid interfaces at the outer or inner sphere. When the reactants diffuse only slowly from the bulk phase to the exterior surface of the catalyst, gas or liquid film resistance must be taken into account. Pore diffusion resistance may be involved when the reactants move through the pores into the pellet. 

When the catalyst is a porous solid, most of the surface area of the catalyst is the surface area of the inner surface of the pores. Therefore, most of the reaction proceeds in the pore. Gas molecules are transferred to the outer surface of the catalyst by diffusion. Generally speaking, the diffusion is faster than the diffusion inside the pores. Gas molecules collide with the inner wall of the pore before they collide with another molecule for the porous catalyst having an average pore radius rp of a few nm. Such diffusion is called Knudsen diffusion and its diffusion constant D is given by ... 

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