Catalysis

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Catalysis: Midterm Review

LECTURE 1:
Sept 6th 2012
What is catalysis, Importance of catalysis, Industrial relevance, presentation of the course

What is Catalysis:
Catalyst:
* catalyst is substance that increases the rate at which a chemical reaction approaches equilibrium, but is not consumed in the process * not reagent or product; noted above equation arrow
* participates in kinetic equations, but not in eq constant

Catalytic Cycle:
* succession of chemical changes undergone by the catalyst until it recovers its initial state

Catalysis=Kinetic phenomenon
* catalyst creates alternative path for reaction, more complex but energetically much more favorable * smaller activation energy for catalytic reaction larger reaction rate * Catalyst changes the kinetics but not the thermodynamics * accelerates both forward and reverse reaction to the same extent

Some catalysts:
* enzyme (biocatalyst)
* copper zine crystallites on silica
* homogenous acid catalysts
* L-proline (organocatalyst)
* Zeolite (crystalline aluminosilicate)
* (R,R,)-DiPAMP-Rh (organometallic complex)

Homo/hetero-genous/bio-catalysis:

HOMOGENOUS:
* catalyst and reagents are in the same phase
* in solution – soluble and/or liquid reagents react in the liquid phase * Can be: organic molecules, transition metal complexes, inorganic salts, lanthanide complexes * Powers:
* Efficiency of an homogeneous process
* Fine tuning of activity with the choice of ligands
* Greater selectivity (region, chemo selectivity)
* Better potential for asymmetric catalysis
* Better understanding of the catalyst structure
* Limitation:
* Problematic separation
* Catalyst Cost – metal and ligands expensive
* Homogenous catalysis used for complex reations in fine and pharmaceutical industry

HETEROGENOUS:
* catalyst and reagents are in a different phase
* maximize the surface
* maximize the number of active sites
* allows for continuous processes or plug flow reactors
* Kinetics: two major models
* Langmuir-Hinshelwood: adsorption of both reagents, diffusion, reaction, desorption * Eley-Rideal: adsorption, reaction between adsorbed species and gas/liq phase reagents * POWERS:
* Separation is made easy
* Heterogeneous catalysts are usually very temperature and pressure stable * Solventless reactions possible
* Flow reaction possible
* LIMITATIONS:
* Poorer understanding of the catalyst structure
* Poorer control over selectivity
* Catalysis used broadly in industry

BIOCATALYSIS:
* Catalysis performed by enzymes
* Can be whole cells or purified enzymes
* Technical enzymes: enzymes extracted from cells
* WHOLE CELLS:
* Advantageous if co factors are needed
* PURIFIED ENZYMES:
* Closer to classical chemistry
* Higher volumetric production
* Less waste
* Less rick of having other enzymes consume the product
* An emerging field:
* several major companies (DSM, RHODIA, BASF) places biocatalysis as strategic project * novozymes is the world’s largest enzyme manufacturer
* example: blue jeans – BASF indigo (1867), improved 1990’s KOH/NAOH – NANH2, 2002 genecor E.Coli, faded using hypochlorites * POWERS:
* Mild reaction
* Water as solvent
* Biocompatibility, biodegradability of catalysts
* High selectivity
* High TOF
* LIMITATIONS:
* Large Quantity of waste generated
* Catalyst deactivation
* Substrate selective
* Used in food and commodity companies, their importance is growing

Evaluation of catalysts:
* TURN OVER NUMBER (TON) = total number of cycles
* TURN OVER FREQUENCY (TOF) = number of cyles/time TOF=TON/time * HOMOGENOUS:
* TON=nformedproduct/ncatalyst=number of cycles
* TOF=TON/time
* HETEROGENOUS:
* TON= nformed product/weightcatalyst (mol/g)
* TOF=TON/time...
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