Microkinetic theory

The diffusion microkinetic theory, where high membrane partitioning of lipophilic bases into phospholipids is used to explain the long duration of action seen after administration of compound into the airway smooth muscle [9]. 

The compound displayed promising ex vivo duration of action in guinea pig trachea strips as well as a good systemic pharmacokinetic profile in terms of in vivo clearance and bioavailability from fraction swallowed (CL 130ml/min/kg F <5%). From overlays with salmeterol, and also the feet that 17 is isohpophiUc with salmeterol, it was concluded that both the exosite and the microkinetic theories could potentially explain its apparent duration of action and 17 was selected as a clinical candidate. 

Chemical reactions obey the rules of chemical kinetics (see Chapter 2) and chemical thermodynamics, if they occur slowly and do not exhibit a significant heat of reaction in the homogeneous system (microkinetics). Thermodynamics, as reviewed in Chapter 3, has an essential role in the scale-up of reactors. It shows the form that rate equations must take in the limiting case where a reaction has attained equilibrium. Consistency is required thermodynamically before a rate equation achieves success over tlie entire range of conversion. Generally, chemical reactions do not depend on the theory of similarity rules. However, most industrial reactions occur under heterogeneous systems (e.g., liquid/solid, gas/solid, liquid/gas, and liquid/liquid), thereby generating enormous heat of reaction. Therefore, mass and heat transfer processes (macrokinetics) that are scale-dependent often accompany the chemical reaction. The path of such chemical reactions will be... 

Key Words Ethylene epoxidation, Silver, Bimetallic catalyst design, Copper, Density functional theory, Microkinetic modeling. 2008 Elsevier B.v. 

Thus, it is adequate to determine the energetic characteristics of the elementary reactions based on the Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) method developed by Shustorovich [2], while the pre-exponential factors may be estimated simply from the transition-state theory [4,26]. Here we employ, for illustrative purposes, a simplified version of a microkinetic WGSR model developed by us earlier [14],... 

Shustorovich (5) has reviewed the detailed method and the equations for calculating heat of chemisorption and activation barriers by BOC. The multidimensional activation energies were calculated in the present work and the activation energies are listed in Table 1. The initial preexponential factors were estimated by transition-state theory, employing reasonable chemical assumptions about surface mobility. Dumesic et al. (3) summarized typical ranges of these values used in microkinetic analysis studies. For the reaction A +B —>C +D the preexponential factor is typically 10 s", assuming immobile surface intermediates without rotation. 

Recently, semiempirical methods based on DFT calculations have been developed for catalyst screening. These methods include linear scaling relationships [41, 42] to transfer thermochemistry from one metal to another and Brpnsted-Evans-Polanyi (BEP) relationships [43 7]. Here, these methods and also methods for estimation of the surface entropy and heat capacity are briefly discussed. Because of their screening capabilities, semiempirical methods can be used to produce a first-pass microkinetic model. This first-pass model can then be refined using more detailed theory aided by analytical tools that identify key features of the model. The empirical bond-order conservation (BOC) method, which has shown good success in developing microkinetic models of small molecules, has recently been reviewed [11] and will not be covered here. 

The models developed here account for unmeasurable intermediates such as adsorbed ions or free radicals. Microkinetic analysis, pioneered by Dumesic and cowokers"", is an example of this approach. It quantifies catalytic reactions in terms of the kinetics of elementary surface reactions. This is done by estimating the gas-phase rate constants from transition state theory and adjusting these constants for surface reactions. For instance, isobutane cracking over zeolite Y-based FCC catalysts has 21 reversible steps defined by 60 kinetic parameters." The rate constants are estimated from transition state theory... 

This book aims to present the fundamentals of catalysis and applications illustrated with experiments performed in our laboratory, trying to understand why select the catalysts and processes. We seek to split the text into two parts. The first part presents the fundamentals addressing the activity patterns, adsorption-desorption phenomena, and advanced theories (Chaps. 1-5). The second part presents the most important conventional methods of characterizing properties (Chap. 6) the important methods of preparation with pre/posttreatment (Chap. 7) the most important traits (Chap. 8), with examples and practices spectroscopic characterizations, even in situ (Chaps. 8-12) Nanostructured catalysts (Chap. 13) the microkinetic chemistry and surface mechanisms (Chap. 14), and finally the evaluation of an industrial catalyst process (Chap. 15). 

Quantum chemistry approaches, such as density functional theory (DFT), can be used within the framework of microkinetic modeling to estimate thermochemical parameters, like enthalpies and entropies, as well as kinetic parameters like activation energies and frequency factors of chemical reactions. Some examples of the values of pre-exponential factors for different types of surface reactions are presented in Table 3.4. The appealing feature of DFT is in the description of the system of interacting fermions by their density rather than through its many-body wave function, lowering substantially the computational cost. 

In the first chapter, Lars Grabow (University of Houston) discusses the screening of catalysts through the use of first-principles methods. Using density functional theory (DFT), key descriptors and scaling relationships can be identified and incorporated with an appropriate microkinetic model. Such an approach allows for the rapid screening of materials based on DFT calculations. 

H. Falsig, Understanding Catalytic Activity Trends for NO Decomposition and CO Oxidation using Density Functional Theory and Microkinetic Modeling , PhD Thesis, Technical University of Denmark, Kongens Lyngby, Denmark, 2010. 

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