Single-phase catalysts

During the last two decades it has been found that there is a special group of chemical reactions, essentially redox reactions, for which the catalytic influence of solids can be interpreted in terms of the catalyst s electronic structure and the controlled variations of that structure. The study of single-phase catalysts and the relationship between function and electronic structure of solid state catalysts show that redox reactions may be divided into two classes. Donor reactions are reactions in which the rate-determining step involves an electron transition from the reactant molecule to the catalyst acceptor reactions are those where the reactant must accept electrons from the catalyst in order to form the activated state. Broadly speaking, donor reactions mobilize reducing agents like... 

Figure 1. Schematic representation of the vital role of solid-state reactions in preparing single-phase catalysts.

The question arises, why do bi- or multi-phasic catalysts generally show better activity and selectivity than the active phase alone The aim of this paper is to answer this question by exploring the role of interfacial effects. We shall examine first how the thermodynamic and structural properties of one phase influence its interactions, not only with the gaseous reactants, but also with coexisting solid phases as a result of its bulk, surface, and defect structure. We will also examine the conditions necessary for these interactions and set up a structural classification of the main components of mild oxidation catalysts. This will lead finally to a discussion of the role of interfacial effects in catalyst performance using some illustrative examples. Thermodynamic and Structural Properties of Single Phase Catalysts... 

Synthesis of single-phase Au-Pd catalysts exhibited higher activity and reusability than random Au-Pd catalysts in the selective oxidation of glycerol. By comparison, the catalytic performance of Au-Pt and Au-Pd single-phase catalysts in the selective oxidation of various primary alcohols (benzyl alcohol, cinnamyl alcohol and 1-octanol), showed that Au-Pd catalysts were far more active than Au-Pt catalysts (Eq. 22.13). A significant improvement in catalytic activity was found when water, instead of toluene, was used as a solvent, with TOF values increasing by a factor of 1.5-6. Moreover, using the same preparation method, they demonstrated... 

Figure 13.4 XRD patterns of single phase catalysts (a)Ml and (b) M2 enlarged patterns between 5 and 24° with indexation based on [69] (Data taken from Baca, M., Pigamo, A., Dubois, J.L., and Millet, J.M.M. Top. Catal. 2003, 23, 39-46. With permission.)...

The Tokuyama Soda single-step catalyst consists of a zirconium phosphate catalyst loaded with 0.1—0.5 wt % paHadium (93—97). Pilot-plant data report (93) that at 140°C, 3 MPa, and a H2 acetone mole ratio of 0.2, the MIBK selectivity is 95% at an acetone conversion of 30%. The reactor product does not contain light methyl substituted methyl pentanes, and allows MIBK recovery in a three-column train with a phase separator between the first and second columns. 

Since the catalyst is concentrated and operates in the ionic phase, and also probably at the phase boundary, reaction volumes in the biphasic technology are much lower than in the conventional single-phase Dimersol process, in which the catalyst concentration in the reactor is low. As an example, the Difasol reactor volume can be up to 40 times lower than that classically used in the homogeneous process. 

Allyl alcohol isomerization is typically conducted as a single-phase reaction, needing efforts for separation of the catalyst [110, 113], One driver was to exploit a catalyzed liquid/liquid route with aqueous (catalytic) and organic phases as commonly employed in the chemical industry. 

Higher selectivity, easier processing, use of inexpensive solvents, use of cheaper chemicals, and ease of heat removal have been realized through phase-transfer catalysis (PTC). It appears that no catalytic method has made such an impact as PTC on the manufacture of fine chemicals (Sharma, 1996). Many times we benefit by deliberately converting a single-phase reaction to a two-phase reaction. Consider catalysis by. sodium methoxide in a dry organic. solvent. This can invariably be made cheaper and safer by using a two-pha.se. system with a PT catalyst. 

Reaction conditions Catalyst H(CO)Rh(TPPTS)3, Rh 0.089 mmol, 1-octene/Rh = 426, CO/H2 (molar ratio) = 1, P(C0/H2) 400 psi, T = 100°C. Single-phase heptane only solvent, catalyst H(CO)Rh(PPh3)3. Linear/branched. 

It may be recalled that in homogeneous reactions all reacting materials are found within a single phase, be it gas, liquid or solid if the reaction is catalytic, then the catalyst must also be present within the phase. Thus, there are a number of means of defining the rate of a reaction the intensive measure based on unit volume of the reacting volume (V) is used practically exclusively for homogeneous systems. The rate of reaction of any component i is defined as... 

Our very first experiments with the reactor depicted in Figure 5.4.1 were carried out with a 15% Pt-Y-Al203 single cylindrical catalyst pellet [10-12], The acquisition time of 2D images of an axial slice at that time was about 260 s. Despite this, the first direct MRI visualization of the operation of a model gas-liquid-solid reactor has revealed the existence of large gradients of the liquid phase content within the catalyst pellet upon imbibition of liquid a-methylstyrene (AMS) under conditions... 

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