Mechanisms of Catalytic Reactions and Characterization of Catalysts

Catalyst affects only the rate of the reaction,i.e.Kinetics. What is a Catalyst ? Catalyst is a substance that increases the rate of the reaction at which a chemical system approaches equilibrium , without being substantially consumed in the process. Catalyst affects only the rate of the reaction,i.e.Kinetics. It changes neither the thermodynamics of the reaction nor the equilibrium composition.

Chemical Reaction Thermodynamic Thermodynamics says NOTHING about the rate of a reaction. Thermodynamics : Will a reaction occur ? Kinetics : If so, how fast ?

C (graphite)  C (diamond) Kinetic Vs. Thermodynamic A reaction may have a large, negative DGrxn, but the rate may be so slow that there is no evidence of it occurring. Conversion of graphite to diamonds is a thermodynamic favor process (G -ve ). C (graphite)  C (diamond) Kinetics makes this reaction nearly impossible (Requires a very high pressure and temperature over long time)

Kinetic Vs. Thermodynamic Reaction Profile Reaction path for conversion of A + B into AB

Activation Energy Definition Activation Energy : The energy required to overcome the reaction barrier. Usually given a symbol Ea or ∆G≠ The Activation Energy (Ea) determines how fast a reaction occurs, the higher Activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction

Activation Energy Catalyst Affect Catalyst lowers the activation energy for both forward and reverse reactions.

Activation Energy Catalyst Affect This means , the catalyst changes the reaction path by lowering its activation energy and consequently the catalyst increases the rate of reaction.

How a Heterogeneous Catalyst works ? Substrate has to be adsorbed on the active sites of the catalyst

Surface process bulk process Absorption and Adsorption H2 absorption  palladium hydride H H2 adsorption on palladium Surface process bulk process

Adsorption In physisorption The bond is a van der Waals interaction adsorption energy is typically 5-10 kJ/mol. ( much weaker than a typical chemical bond ) many layers of adsorbed molecules may be formed.

Adsorption For Chemisorption The adsorption energy is comparable to the energy of a chemical bond. The molecule may chemisorp intact (left) or it may dissociate (right). The chemisorption energy is 30-70 kJ/mol for molecules and 100-400 kJ/mol for atoms.

Characteristics of Chemi- and Physisorptions E(d) physisorption atomic chemisorption DE(ads) < DE(ads) Physisorption Chemisorption small minima large minima weak Van der Waal formation of surface attraction forces chemical bonds physisorption/ desorption chemisorption CO

Adsorption and Catalysis Adsorbent: surface onto which adsorption can occur. example: catalyst surface, activated carbon, alumina Adsorbate: molecules or atoms that adsorb onto the substrate. example: nitrogen, hydrogen, carbon monoxide, water Adsorption: the process by which a molecule or atom adsorb onto a surface of substrate. Coverage: a measure of the extent of adsorption of a specie onto a surface H adsorbate adsorbent H coverage q = fraction of surface sites occupied

Adsorption Mechanisms Langmuir-Hinshelwood mechanisms: 1. Adsorption from the gas-phase 2. Desorption to the gas-phase 3. Dissociation of molecules at the surface 4. Reactions between adsorbed molecules Two Questions: Is the reaction has a Langmuir-Hinshelwood mechanism? What is the precise nature of the reaction steps? Cannot be solved without experimental or computational studies

Example The Reaction A2 + 2B = 2AB Langmuir-Hinshelwood mechanisms Example The Reaction A2 + 2B = 2AB may have the following mechanism A2 + * = A2* A2* + * = 2A* B + * = B* A* + B* = AB* + * AB* = AB + *

The last step cannot occur in a Langmuir-Hinshelwood mechanism Adsorption Mechanisms Eley-Rideal mechanism: Adsorption from the gas-phase Desorption to the gas-phase Dissociation of molecules at the surface Reactions between adsorbed molecules Reactions between gas and adsorbed molecules The last step cannot occur in a Langmuir-Hinshelwood mechanism

Eley-Rideal mechanism Example The reaction A2 + 2B = 2AB may have the following Eley-Rideal mechanism A2 + * = A2* A2* + * = 2A* A* + B = AB + * where the last step is the direct reaction between the adsorbed molecule A* and the gas-molecule B.

Eley-Rideal or Langmuir-Hinshelwood? For the Eley-Rideal mechanism: the rate will increase with increasing coverage until the surface is completely covered by A*. For the Langmuir-Hinshelwood mechanism: the rate will go through a maximum and end up at zero, when the surface is completely covered by A*. This happens because the step B + * = B* cannot proceed when A* blocks all sites. The trick is that the step B + * = B* requires a free site.

Catalyst Preparation (1) Unsupported Catalyst Usually very active catalyst that do not require high surface area e.g., Iron catalyst for ammonia production (Haber process) (2) Supported Catalyst requires a high surface area support to disperse the primary catalyst the support may also act as a co-catalyst (bi-functional) or secondary catalyst for the reaction (promoter)

Supported Catalyst Highly dispersed metal on metal oxide Nickel clusters SiO2

Molecules in Zeolite Cages and Frameworks + p-xylene ZSM-5 Y-zeolite Paraffins

What is ZSM-5 Catalyst ? It is an abbreviation for (Zeolite Scony Mobile Number 5 ) First synthesized by Mobil Company in 1972 It replaces many Homogeneous Catalysts were used in many petrochemical processes ZSM-5 has two diameters for its pores : d1= 5.6 Å , d2= 5.4 Å Where as, Zeolite Y has a diameter = 7.4 Å

Properties ZSM-5 The ZSM-5 zeolite catalyst is used in the petroleum industry for hydrocarbon interconversion. ZSM-5 zeolite is a highly porous aluminosilicate with a high silica/alumina ratio. It has an intersecting two-dimensional pore structure. The aluminum sites are very acidic. The acidity of the zeolite is very high. The reaction and catalysis chemistry of the ZSM-5 is due to this acidity.

Structure of ZSM-5

Aromatic Isomerization Observed by Haag and Co-workers P-Selectivity is achieved due to the high diffusion coefficient of P-Substituted Molecules Relative to that of Ortho or Meta. A side reaction in homogenous solution phase isomerization is the Bimolecular disproportionation of Xylene. This Can not occur when using zeolite such as ZSM-5 since the pore size will not allow for the bulky bimolecular transition state necessary for disproportionation.

Relative Diffusion Coefficient + p-xylene ZSM-5 1 m-Xylene o-Xylene p-Xylene Relative Diffusion Coefficient Compound > 10000

Effective Diameter of intracrystalline Cavity (nm) 15000 0.70 Mordenite > 1.2 0.80 Effective Diameter of intracrystalline Cavity (nm) > 45000 HY 10000 ZSM-4 ZSM-5 Kdis/Kiso Zeolite Catalyst Type 1000 0.60 Selectivities of acidic zeolite for disproportionation and isomerization of Xylene. The HZSM-5 Catalyst is preferred, minimizing the bimolecular disproportionation reaction by virtue of restricted transition state selectivity.

Schematic diagram of product shape selectivity: Para-xylene diffuses preferentially out of the zeolite channels p-xylene m-xylene

Acidity of the Catalyst Vs. Acid Sites Theoretically, As the Acid Sites increase the Acidity increases Experimentally, As the Acid Sites increase the Acidity decreases How we can understand this behavior?

Acidity of the Catalyst Vs. Acid Sites Al is the Acidic Site But, Si is more electronegative than Al B C N O F Al Si P S Cl Electronegativity Effect O O Al Si Al H+ Si Si Makes the release of the H+ faster since it attracts Electrons toward it which make the proton away of it

Acidity of the Catalyst Vs. Acid Sites It is possible to conclude that: the acid strength of a site will increase when there is a decrease in the number of Al atoms in the Next Nearest Neighbor position of the Al atom. So, the strongest type of framework Bronsted site is A completely isolated Al tetrahedron which have zero NNN or (0NNN)

Acidity Characterization of a Catalyst Acidity of the catalysts can be assess by: I. Temperature Programmed Desorption (TPD) II. Fourier Transformation Infrared spectroscopy ( FTIR) III. Induced Laser spectroscopy (IL)

I. Temperature Programmed Desorption (TPD) Pure carrier gas (typically helium) flows over the sample as the temperature is raised to desorb the previously adsorbed gas e.g. NH3 2. This characteristic "fingerprint" for each catalyst,used to determine: the distribution of acid-site strength if ammonia is the sorbed gas, or the distribution of basic sites if carbon dioxide is the sorbed gas.

I. Temperature Programmed Desorption (TPD) Time(min)

II. Fourier Transformation Infrared spectroscopy ( FTIR) By this method we assess acidic site: Bronsted acidic site or Lewis acidic site For example: if pyridine is the probe molecule, It will give peaks at: 1. 1540 cm-1 for Bronsted site 2. 1450 cm-1 for Lewis site

II. Fourier Transformation Infrared spectroscopy ( FTIR)

II. Fourier Transformation Infrared spectroscopy ( FTIR)

III. Induced Laser spectroscopy (IL) To assess the acidity by IL: 1. Probed molecule has to be prepared in different acidic concentration solutions 2. Life time measurements has to run onto each concentration 3. Calibration curve between life time Vs. concentration of the acid

III. Induced Laser spectroscopy (IL)

Conclusion There is no single method that can be used to determine all aspects of acidity for a solid i.e. nature, strength and the number of acid sites Each method measures only certain aspects and,therefore, the application of many methods is desirable. THE END

THANK YOU End of the Catalysis and Catalysts References A.Corma,Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem. Rev.1995, 95, 559-614. Bruce C. gates, Catalytic Chemistry.1992. Zaki Seddigi, Characterization of the acidic properties of zeolite and their catalytic behavior in the synthesis of MTBE. 1994. Ali El-Rayyes, Study of the photochemical properties of some aromatic compound on molecular sieves using a picosecond pulse laser system. 2001. Keith Laidler, Chemical Kinetics, 1987.