Solid catalysts, adsorption

Heterogeneous catalysis most often involves gaseous reactants being adsorbed on the surface of a solid catalyst. Adsorption refers to the collection of one substance on the surface of another substance absorption refers to the penetration of one substance into another. Water is absorbed by a sponge. 

A general mechanism of surface catalysis involves (a) diffusion of reactants to the surface of the catalyst, (b) a fast reaction between the molecules of the reactants and the atoms in the surface of the solid catalyst (adsorption), followed by (c) the formation of the transition state (the rate-determining step), which then decomposes rapidly to the catalyst and the products. For example, at about BOOOK the homogeneous decomposition of hydrogen iodide,... 

Measurements of the true reaction times are sometimes difficult to determine due to the two-phase nature of the fluid reactants in contact with the solid phase. Adsorption of reactants on the catalyst surface can result in catalyst-reactant contact times that are different from the fluid dynamic residence times. Additionally, different velocities between the vapor, liquid, and solid phases must be considered when measuring reaction times. Various laboratory reactors and their limitations for industrial use are reviewed below. 

Dynamic Methods for Characterization of Adsorptive Properties of Solid Catalysts L. POLINSKI AND L. NaPHTALI Enhanced Reactivity at Dislocations in Solids... 

Adsorption of reactants on the surface of the catalyst is the first step in every reaction of heterogeneous catalysis. Flere we focus on gases reacting on solid catalysts. Although we will deal with the adsorption of gases in a separate chapter, we need to discuss the relationship between the coverage of a particular gas and its partial pressure above the surface. Such relations are called isotherms, and they form the basis of the kinetics of catalytic reactions. 

In comparison with the case of a gas phase molecule that reacts in a monomole-cular reaction on a solid catalyst, the reciprocal of the Michaelis constant takes the place of the equilibrium constant of adsorption in the Langmuir-Hinshelwood equations. 

Fig. 3.1 (Kapteijn et al., 1999) shows the model commonly u.sed to pre.sent a reversible reaction (A B) taking place on the surface of a solid catalyst. Three elementary steps are distinguished, i.e. adsorption of A on an active site, reaction of this adsorbed complex to adsorbed complex B, and desorption of B from the active site.

From these results, one can understand that the liquid-liquid interface can assist effectively in the interfacial reaction through the adsorption of extractants like a solid catalyst. The whole extraction scheme of the chelate extraction system is represented in Scheme 1. 

Figure 1.6 Top Low-temperature nitrogen adsorption ( ) and desorption (x) isotherms measured on a calcined SBA-15 mesoporous silica solid prepared using an EO20PO70EO20 block copolymer [54]. Bottom Pore size distribution derived from the adsorption isotherm reported at the top [54]. A high surface area (850 m2/g), a uniform distribution of cylindrical nanopores (diameter —90 A), and a large pore volume (1.17 cm3/g) were all estimated from these data. These properties make this material suitable for use as support in the preparation of high-surface-area solid catalysts. (Reproduced with permission from The American Chemical Society.)...

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

In a heterogeneous process, a sequence of steps involving diffusion and chemical reaction is involved. For example, a reactant must diffuse to the surface of a solid catalyst before adsorption and chemical reaction can occur. Either diffusion or the chemical reaction may be rate-controlling and, over a sufficiently large temperature range, the Arrhenius plot of In k vs. 1/T is no longer straight, but curved. 

There can be, however, no doubt that in catalytic processes, purely physical factors play an important role, in addition to the chemical valence forces. This is particularly true for the solid catalysts of heterogeneous reactions for which the properties of surfaces, as the seats of catalytic action are of prime importance. The total surface areas, the fine structure of the surfaces, the transport of reactants to and from surfaces, and the adsorption of the reactants on the surfaces, can all be considered as processes of a predominantly physical nature which contribute to the catalytic overall effect. Any attempt, however, to draw too sharp a line between chemical and physical processes would be futile. This is illustrated clearly by the fact that the adsorption of gases on surfaces can be described either as a mere physical condensation of the gas molecules on top of the solid surface, as well as the result of chemical affinities between adsorbate and adsorbent. Every single case of adsorption may lie closer to either one of the hypothetical extremes of a purely physcial or of a purely chemical adsorption, and it would be misleading to maintain an artificial differentiation between physical and chemical factors. 

Previously mentioned properties can be conveniently investigated by studying the adsorption of a suitably chosen probe molecule on the solid. Adsorption influences all phenomena depending on the properties of the surface (e.g., it constitutes the primary step in corrosion as well as the prerequisite for every catalytic reaction involving solid catalysts). 

Nuclear magnetic resonance (NMR) is another powerful technique to study solid acid catalysts. Advanced NMR methods such as magic-angle spinning (MAS) of solids have increased the capability of this technique to study acid sites in solid acid catalysts [80]. For example, H MAS NMR technique performed on the solid catalysts after activation and upon adsorption allows the detection of the signals due to the magnetic resonance of the protons... 

As already mentioned, the first step in any heterogeneous catalytic reaction is the adsorption of a gas molecule onto a solid surface. Adsorption heat measurements can provide information about the adsorption process not available using other surface analytical tools. For example, differential heat measurements can provide valuable insights into sites distribution on the catalyst surface as well as quantitative information on the changes in catalyst particle surface chemistry that result from changes in particle size or catalyst support material [148-150],... 

The most spectacular solid catalyst reported in the literature is probably the sulfonated carbon. Indeed, in contrast to all other solid catalysts, sulfonated carbon was able to hydrolyze, in water, cellulose to soluble 1,4-p-glucan with high yield. The presence of hydroxyl groups on the carbon surface was found to be crucial and allows a better adsorption of cellulose on the catalyst surface. However, the amount of catalyst used is unacceptable for an industrial application and much effort is stiU needed. 

Dynamic Methods for Characterization of Adsorptive Properties of Solid Catalysts... 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

By in situ MAS NMR spectroscopy, the Koch reaction was also observed upon co-adsorption of butyl alcohols (tert-butyl, isobutyl, and -butyl) and carbon monoxide or of olefins (Ao-butylene and 1-octene), carbon monoxide, and water on HZSM-5 (Ksi/ Ai — 49) under mild conditions (87,88). Under the same conditions, but in the absence of water (89), it was shown that ethylene, isobutylene, and 1-octene undergo the Friedel-Crafts acylation (90) to form unsaturated ketones and stable cyclic five-membered ring carboxonium ions instead of carboxylic acids. Carbonylation of benzene by the direct reaction of benzene and carbon monoxide on solid catalysts was reported by Clingenpeel et al. (91,92). By C MAS NMR spectroscopy, the formation of benzoic acid (178 ppm) and benzaldehyde (206 ppm) was observed on zeolite HY (91), AlC -doped HY (91), and sulfated zirconia (SZA) (92). 

In some cases the estimated temperature of preparation of solid catalysts seems to be close to 0 and therefore in accordance with the foregoing working hypothesis moreover, adsorption measurements on some catalysts show a dependence on the temperature of catalyst pretreatraent in accordance with a Boltzmann distribution of their surface centers. However for catalysts such as chlorides (15) and oxides of the type Me2O3 (7), which were first considered to be suitable objects for rationalizing the compensation effect, Equations (10) and (17), which interrelate the overall rate constant k with the temperature 9 of the pretreatment of the catalyst, did not fit the experimental data. 

Allied with the diffraction methods, such as low-energy electron diffraction (LEED) and photoelectron diffraction (PED), which can also be applied in single-crystal research, these advances have led to much better interpretations of the vibrational spectra of chemisorbed hydrocarbons in terms of the structures of the surface species. The new results have in turn led to the possibility of reassessing more reliably earlier interpretations of the infrared or Raman spectra of adsorbed hydrocarbons on the finely divided metal samples (usually oxide supported) that are more closely related to working solid catalysts. Such spectra are more complicated because of the occurrence of a variety of different adsorption sites on the metal particles, with the consequence that the observed pattern of absorption bands frequently arises from overlapping spectra from several different surface species. 

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