Catalysis Flashcards
What is a catalyst?
Substance that increases the rate of reaction without modifying the overall standard Gibbs energy change
Catalyst speeds up the approach of equilibrium but does not affect the position
Lowers activation barriers
Not used or changed by reaction
What are some examples of catalysts in history?
Sulphuric acid as catalyst to form ether
Nickel used in hydrogenation
Wacker process - Pd used to make ethanal
Ziegler-Natter catalysts ued to polymerise terminal alkynes
Zeolites used to isomerise or alkylate
Why are catalysts important?
Achieve faster rates
Process occurs under milder conditions
Greater control of selectivity
Reduce environmental impact
Reduces cost
Asymmetric catalysts can form selective chiral molecules
Generally how do catalysts speed up reactions?
Provide new pathway wth lower energy:
Enhance polarisation of a bond - catalysis of chlorination of benzene
Stabalise transition state
How does a catalyst affect thermodynamics?
Will not alter the thermodynamics of a reaction
How does a catalyst affect the Gibbs free energy pathway?
Change in Gibbs free activation is lowered
No high peaks or deep trophs
Stable/non reactive intermediates are not formed
Release of product of thermodynamically feasible
What is the change in enthalpy of activation?
thedifference in enthalpy betweene the ground state and the transition state in a chemical reaction
What is Gibbs free energy of activation?
The difference in free energy between the TS and ground state of reactants
Catalyst provides path with lower free energy of activation - can alter enthalpy or entropy
Calculated from experimental rate cnstants via absolute rate equation
Can use computational cham to estimate energies of reactants intermediates, TS and products and differences between them
What makes a good catalyst?
Active, Selective, Safe
Reusable, Robust, Inexpensive
Predictable, environmentally benign and sustainable
What is catalytic activity?
RAte of consumption of reactant
Sometimes stated as formation of product
Units mol s-1 (katal also used 1 katal - 1 mol s-1)
What is selectivity?
Fraction of the total products which a particular substance represents
What is the catalytic lifetime?
Catalysts can lose acitivity or selectivity over time
For recycled catalysts we want long lifetimes
For unrecycled catalysts we want it to last long enough for the reaction
What are the methods of catalyst deactivation?
Chemical, mechanical or thermal
Denaturation - enzymes
Poisons - block active site
Mechanical wear - chrushing or pores/channels, removal of catalytic material from surface
Chemical etching - removal of catalytic material from surface
Sooting - coking, active sites become blocked with carbonaceous material
How can we compare catalyst activity?
turnover frequency or turnover number
(TON) - the number of molecules reacing per actuve site (before it becoems inactive) to afford desired product
If this is low the catalyst may be inactive or redily deactivated
TON can be reduced by boccurrence of side reactions
TOF - Ther number of molecules reacting per active site per unit of time to afford the desired product
Highly reactive catalyst has high TON and TOF
How do we calculate theoretical TON and TOF?
Theoretical TON =- equivalents of limiting reagent / equivalents of catalyst
Theoretical TOF = TON / duration of reaction
What are the principles of green chemistry?
Prevent waste, atom economy,
Less harzardous chemical syntheses
Designing safer, benign chemical, use safer auxilaries (new catalysts)
Design for energy efficiency, Use renewable feedstocks, reduce derivatives, Use catalysis vs stochiometric
Design for degredation, realtime analysis for pollution prevention, inherently safer chemistry for accident prevention
Is catalysis green?
Catalysts allow milder conditions, reduce waste (vs stochiometric, greater selectivity, simply purification)
Rely on rare transition metals (Pd / Pt)
Catalysts can be hazardous to health / environment
Must consider the whole process
How dow e consider the sustainability of catalysts?
how the catalyst is used
The energy consumption required fro preperation / recovery
the impact on the environment
What can be used to quantify / qualify environmental impact of cartalysts?
Green chemistry metrics:
yeild, conversion, selectivity
E factor (E factor = total waste / product) - considered solvent use in terms of mass but not type of solvent
Atom economy - measures efficientcy of reaction - assumes 100% yield
Reaction mass efficiency - considers yield
Solvent classification - impact on human and environmental health
Renewable intensity = Mass of all renewably driven materials used / mass of product
What are the classifications of catalysts?
Homogenous - reagents and catalyst arein the same phase
Heterogenous - reagents and catalyst arein different phases
Most catalysts are classified as as homo or heterogenous but the nature of a catalyst is not always known
Some arereferred to as hybrid catalysts (polymer supported ctalaysts) - combine propoerties of both classes
These catalysts are heterogenous as they are in a different phase to reagents but have defined active sites
What are examples of homogenous catalysts?
Simple acids / bases - acid catalysed hydrolysis
Organocatalysts - proline derived compounds
MOst often solubl in liquids
Ligands influence steric and electronic environemnt of active site
Often single or defined active site
What is the Monsanto Process?
Process to make acetic acid
Uses rhodium catalyst and operates at high pressure and temperature
Catalyst is coordinately unsaturated
Rhodium can adopt different stable oxidation states and coordination numbers
What happens in the rds of the Monsanto process?
Oxidation states: RhI —> RhIII
CN: 4—> 6
TVEC: 16 —-> 18
Both reactants can coodrinate to rhodium
Synergic M-L bonding leads to weakening and lengthening of the CO bond
High activity and selectivity (over 99%)
What is the Cativia Process?
Process for synthesis of ethanoic acid
Iridium catalyst used - more active than rhodium catalyst
Oxidative addition of methyl iodide (rds) is 150 times faster to iridium catalyst than rhodium catalyst
Mechanism of Cativia process?
What is an asymmetric catalyst?
a type of catalyst in which a chiral catalyst directs the formation of a chiral compound so that formtion of one stereoisomer is favoured
e.g synthesis of L-DOPA
Whatis the Hoffman-La Roche L-Dope process?
Method led to a racemic mixture resolved using column chromatography
heterogenous Pd/C catakyst used in hydrogenation step
What is the asymmetric synthesis for L-Dopa developped by Knowles and Monsanto?
IMproved process - forms only one enantiomer
Uses Rh organomettalic catalyst for hydrogenation step
How do enzymes act as catalysts?
Large proteins that catalyse to high degree of activity and specificity
Active sites are highly specific in binding subrastrates and hist-guest interactions determined by covalent and non covalent interactions
Product has lower affinitiy for active site than substrate
All organisms depend on metallo enzymes - B12 (Co)
Mechanism for enzyme action?
Reacting groups brought togetehr in correct orientation
TS and intermediates stabilised by van der Waals, electrostatic and hydrogen bodning interactions
What are the limitations of enzymatic catalysts?
pH dependent
solvent choice limitted
Denaturation
What are heterogenous catalysts?
Catalyst in different phase than reangents - often solid catalyst with liquid or gaseous reagents.
May inherently contain catalytic sites or be a support for a catalytic species (nanoparticles of Pd on carbon)
Catalysis occurs on the curface, in pores or at defective sites
Why is the surface of the catalyst so reactive?
The surface has high energy because it is exposed and has low coordination number
What are some examples of heterogenous catalysts?
clays, metal surfaces, zeolites and metal organic frameworks
May have many different active sites but there arealso examples which have highly ordered structures with well defined active site such as zeolites and MOFs
What are the advantages of heterogenous catalysts?
facile separation of the catalyst is highly beneficial for ease of purification, catalyst recovery and green credentials of the process
Reaction occurs at interface between phases
Heterogenously catalysed processes may be structurallu sensitve or insensitive - structure insensitive reactions exxhit nearly identical trunover frequencies independent of the structure of the catalyst
What are the main types of hetergenous catalyst?
Finely divided solid where the active sites are located on the surface of the particles - pallaidium on carbon, palladium on silica
Catalysts where the active sites are ;locatd at internal surfaces such as within pores or cavities - zeolites, mesoporous silicates…
How do we classify the size of pores?
Microporous material: material containing pores which are less than 2nm in diameter
Mesoporous: material containig pores with diameters between 2 and 50 nm
Macroporous meterial: a material containing pores which are larger than 50 nm in diameter
Solid catalysts can contain a variety of pore sizes
What is the role of the catalytic support?
The surface area of a cataltic metal particle can determine the activity
As the surface is a region of high energy there will be a thermodynamic driving force to minimise this energy
This can occur in many way - sintering of particles
The suppoort prevents sintering (fusing) which would cause a reduction of loss of activity
What are catalytic converters?
Reduce emissions from internal combustion engine - convert SOx, NOx , CO and hydrocarbons to less harmful gases
What kind of catalytst do catalytic converters use?
Silica or alumina deposited onto a honeycomb stainless steel or ceramic structure
There are many parallel channels separated by thin walls that are coated with the catalytic active substance - mixture of platinum, rhodium and palladium nanoparticles
What are nano particles?
particles between 1 and 100 nanoparticles
Why are metal particles dispersed on ceramic structure in catalytic converters?
To reduce loss in activity caused by sintering of the metal particles
Allows for the transort of large volumes of gas
What are the reactions occurring in a catalytic converter?
2CO + O2 —-(Pt/Pd)–> 2CO2
CxH2x+2 + 2xO2 —(Pt/Pd)—> xCO2 + 2xH2O
2NOx —(Rh/Pt)—> N2 + xO2
What are the advantages and disadvantages of catalytic converters?
Adv: successfully reduces emissions of harzardous exhaust gasses
Higb activity
Dis: Performance of catalyst is strongly dependant on temperature - The majority of pollutants are emitted just after a ‘cold’ start when both the engine and the catalyst are cold
Deactivation of catalyst at high temp - 1200 degrees celcius - poisons
Use of precious metals - limitted resource/expensive
What are zeolites?
Aluminosilicates with highly organised structures which includes cavities which are occupied by large ions and water molecules
Naturally occurring zeolites are found in cracks or cavities in basaltic or volcanic rocks
What is the structure of zeolites?
Highly orders 3D crystalline structure
Most zeolites are aluminosilicates based on tetrahedral buildimhg blocks of [SiO4] and [AlO4]
Prepared as fine crystals containng large regular channels and or cages called micropores (high surface area)
Channels are large enough to allow passge of gues species
What is the role of Al3+ in zeolites?
Distorts lattice allowing for new crystals structures and pore shapes and sizes
Lower charge so must be balanced by another cation (often alkali metal (Na)) which are loosely bound and exchangeable
The ion and water in the pores have considerable freedom of movement which allows for ion-exchange and reversible dehydration
How are zeolites selective?
Only absorb molecules or elute products which are small enough to fit within channels
Different products have different shapes and sizes - if product doesn’t fit well it will diffuse slowly and may undergo futher reaction - transform into less hindered species and be eluted or large molecules or larger moleculkes get trapped in porse - contribute to catalyst deactivation
Shape selectivity can be exploited as different crystalline forms have different channels sizes
How are zeolites synthesised?
high cost - long synthestic processes + materials
Synthesised by crystallisation organic template used (alkyl ammonium compound) to facilitate pore formation
synthesise different zeolites by varying the crystallisation conditions (time, pH, temp), order of mixing and template molecule
Organictemplate and any other organic residues are removed through heating in calcination step
How are zeolites used?
As hetergenous catalysts
favourable thermal, stability, regenerability, long lifetime, ease of separation
USed fo catalytic cracking - heavy oils converted to more valuable lighter products
Used in alkylation of aromatic
Acid catalysts - protonated zeolites are strong Bronsted acids
What is a mechanism of reactions at solid surface?
1) substrate diffusion
2) substrate adsorption
3) Surface diffusion
4) Surface reaction to form adsorbed
5) Product desorbed and product diffusion away from surface
How does a substance diffusion?
Mass transfer of reactasnts from the bulk fluid to the external surface of the catalyst
The reactnats can then diffuse further from the external surface into and through any pore within the catalyst
Diffusion-controlled reactions occur so quickly that the reaction rate is the rate of transported of the reacatnats through the reaction medium
How are substrates adsorbed onto a surface oir pore of a support?
Adsorption is the attachments of atoms, ions or molecules to the surface of a solid
This is thermodynmaically favourable
Adsorption is essential for hertogenous catalysis to occur but if it is too strong - prevent further reaction because actuive sites arr blocked
Strength of the interaction depends on both substrate and surface
What are the types adsorption?
Physisorption
Chemisorption
What is the physisorption?
Involves weak van der Waals interactions and electrostatic polarisation effects
Eads = 10-40 kJ mol-1
No chemical bonds formed
WHat is chemisorption?
Chenical bonds formed between the surface and the substrate
Thuis may also lead to bond breaking within substrate
e.g dissociative adsorption of O2 onto Pt
Eads = 100-500 kJ mol-1
What is the migration of adsorbed substrates?
Adsorbed substrates migrate over catalyst surface
Allows substrates to find each other
Also referred to as diffucion
If the energy barrier for this process is low –> high rate
How does the selectivity of heterogenous catalysts compare with homogenous?
hetergoenous selectivity often lower than homogenous
This is because of the presence of multiple active sites - different reactivity
Zeolitesexert greater control over selectivity by acting as molcular sieves
How does strength of adsorption affect selectivity?
If two reactions can happen such as the hydrogenation of CO to either methanol or methane the strength of binding can affect which product if formed
Weak binding = methanol
Strong = methane
Different supports can bind with different strengths to reactants
Cu = weak to CO whereas Ni = string to CO
What are the types of active sites?
Acidic/basic sites
Metal particles:
- finely divided on solid support or as nanoparticles
- active catalysts for a range of processes
What factors affect catalytic activity of solid catalysts?
Lifetime
Surface area/Porosity - greater surface area/no. pores faster rate
Particle size - larger p[article = smaller surface area to volume ratio and lower rate
Strength of surface adsorption
What are promoters?
Substances that enhance the activity of catalysts when added as a minor component (-1%)
Commonly electropositive elements such as K, Cs, LA
May affect selectivity by enhancing rate of a particular reaction pathway or catalyst lifetime either by reducing rate of sintering or the build up of poisons
What are poisons?
Tend to be electronegative elements such as Cl, S, P, C
They inhibit catalysis by blocking acive sites on the surface
Poisons may ahve a beneficial effect on selectivity
What are nanoparticles?
Nanoscale species made up of pure metals or their compiunds (oxides, hydroxides, sulfides, phosphates, fluorides)
Metal nanoparticles dispersed on the surface form the basis for many catalytic systems but can also form in situ under certain conditions
Homogenous organometallic catalyst may decompose under the reaction conditions to form free metal atoms which can sgreegate to small nanoparticles
What do we have consider when using supported nanoparticles?
Whether the true active catalyst is heterogenous or the catalyst is soluble in the reaction media or functioning via a release and capture mechanism.
Large metal particles may leach single metal atoms which are highly reactive and could be involved in catalysis
Why does the form of the metal matter?
The nature of the catalyst determines the mechanism of catalysis - whether catalyst is mononuclear or surface
This in turn will affect the activity and selectivity of the catalysed process and what factors would be effective in improving it
How can nanoparticles affect the activity?
The larger the particle the smaller the surface area to volume ratio and the lower the catalytic activity#
Metal atoms on different crystallographic faces of a metal nanoparticle pack differently - tuning the shape of metal nanoparticles to expose different crystallographic faces to give different types of catalytic sites
How is light scattering and electron microscopy used to test for nanoparticles?
Used to detect whether nanoparticles and present and gather information about their size and shape
Cannot provide information as to whether the particles detected are involved in catalysing the process
How are kinetic studies used to test for nanoparticles?
Reactions catalysed by nanoparticles often exhibit irreproducible kinetics
Induction periods may be observed which indicate the formation of catalytic nanoparticles in situ
Sigmoidal curves: initiation, catalysis and completion
What is a tethered catalyst?
Catalysts can be heterogenised by thd trapping or attaching the active molecular catalyst the active molecular cataklyst onto surfaces or pores of solid supports (silica/alumina/ceria)
Combine high selectivity typical of homogenous catalysis with the facile separation of heterogenous catalysts
Applicability can be limitted by leaching of the catalyst to a lower activity when the supported catalyst is recovered and reused
What are the advantages of supported catalysts?
Easy in situ synthesis of silica supported silver nanoparticles
Easy separation of catalyst from reaction products
Why are acid catalysts important?
Most widely used
Often expensive
How do we classify acids?
Bronstead-Lowry (H2SO4) or Lewis (AlCl3)
All Bronstead-Lowry are Lewis
Better classification: protic or non-protic
H2SO4 - protic
AlCl3 - non protic
Are acid catalysts homo or hetero genous?
Traditional acid catalysts are homogenous
Modern acid catalysts are often heterogenous
What is the difference between specific or general acid catalyst?
Specific: proton is transgferred before the rds usually this occurs under strongly acidic conditions
Genral: proton is transferred during rds usually under weak acidic conditions
What is the mechanism of proton transfer?
Step 1: step is diffusion controlled
Step 2: general trends:
- proton tranfer to C is slow
- proton transfer to N, o or S is fast
- proton transfer to N or O frokm H3O+ is diffusion controlled
Step 3: diffusion controlled
What are the factors that affect rate of proton transfer?
Intramolecular H-bonding
Requirements for electronic reorganisation - causes changes to bond lengths/angles
solvent effects - fast proton transfer in water
What is the correlation between acid strength and rate of catalysed reaction?
Stronger the acid catalyst the faster the rate of reacgtion until the limit of diffusion control is reached
What effects can solvents have?
Rate increased when protonated transition state is solvated more than acidic proton
Greater proton stabilisation = lower reactivity of catalyst
How are Lewis acid catalysed reactions usuallyu conducted?
in non-aqueous / aprotic solvent systems
Presence of water can lead to complex Lewis/Bronstead behaviour
Sterics must be considered as Lewis acids are bulkier
Which famous reaction are Lewis acids used as catalysts?
Freidel Crafts - AlCl3
What are examples of solid acid catalysts?
Acidic zeolithes - exchangable cation can be swapped for a proton leading to high acidity
AlCl3 - supported on Silica (reacts with silanol groups forming a covalent link to the solid support nd liberating HCl)
What are ther advantages and disadvantages of solid acid catalysts?
Safer to handle
Less water sensitive
Can be recoveredby filtration and reused
Less reactive
Higher temperatures and longer reaction times may be required
Pores may becomes blocked
