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Core 6 > Catalysis > Flashcards

Catalysis Flashcards

1
Q

What is a catalyst?

A

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

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2
Q

What are some examples of catalysts in history?

A

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

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3
Q

Why are catalysts important?

A

Achieve faster rates

Process occurs under milder conditions

Greater control of selectivity

Reduce environmental impact

Reduces cost

Asymmetric catalysts can form selective chiral molecules

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4
Q

Generally how do catalysts speed up reactions?

A

Provide new pathway wth lower energy:

Enhance polarisation of a bond - catalysis of chlorination of benzene

Stabalise transition state

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5
Q

How does a catalyst affect thermodynamics?

A

Will not alter the thermodynamics of a reaction

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6
Q

How does a catalyst affect the Gibbs free energy pathway?

A

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

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7
Q

What is the change in enthalpy of activation?

A

thedifference in enthalpy betweene the ground state and the transition state in a chemical reaction

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8
Q

What is Gibbs free energy of activation?

A

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

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9
Q

What makes a good catalyst?

A

Active, Selective, Safe

Reusable, Robust, Inexpensive

Predictable, environmentally benign and sustainable

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10
Q

What is catalytic activity?

A

RAte of consumption of reactant

Sometimes stated as formation of product

Units mol s-1 (katal also used 1 katal - 1 mol s-1)

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11
Q

What is selectivity?

A

Fraction of the total products which a particular substance represents

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12
Q

What is the catalytic lifetime?

A

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

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13
Q

What are the methods of catalyst deactivation?

A

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

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14
Q

How can we compare catalyst activity?

A

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

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15
Q

How do we calculate theoretical TON and TOF?

A

Theoretical TON =- equivalents of limiting reagent / equivalents of catalyst

Theoretical TOF = TON / duration of reaction

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16
Q

What are the principles of green chemistry?

A

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

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17
Q

Is catalysis green?

A

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

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18
Q

How dow e consider the sustainability of catalysts?

A

how the catalyst is used

The energy consumption required fro preperation / recovery

the impact on the environment

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19
Q

What can be used to quantify / qualify environmental impact of cartalysts?

A

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

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20
Q

What are the classifications of catalysts?

A

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

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21
Q

What are examples of homogenous catalysts?

A

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

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22
Q

What is the Monsanto Process?

A

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

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23
Q

What happens in the rds of the Monsanto process?

A

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%)

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24
Q

What is the Cativia Process?

A

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

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25
Q

Mechanism of Cativia process?

A
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26
Q

What is an asymmetric catalyst?

A

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

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27
Q

Whatis the Hoffman-La Roche L-Dope process?

A

Method led to a racemic mixture resolved using column chromatography

heterogenous Pd/C catakyst used in hydrogenation step

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28
Q

What is the asymmetric synthesis for L-Dopa developped by Knowles and Monsanto?

A

IMproved process - forms only one enantiomer

Uses Rh organomettalic catalyst for hydrogenation step

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29
Q

How do enzymes act as catalysts?

A

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)

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30
Q

Mechanism for enzyme action?

A

Reacting groups brought togetehr in correct orientation

TS and intermediates stabilised by van der Waals, electrostatic and hydrogen bodning interactions

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31
Q

What are the limitations of enzymatic catalysts?

A

pH dependent

solvent choice limitted

Denaturation

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32
Q

What are heterogenous catalysts?

A

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

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33
Q

Why is the surface of the catalyst so reactive?

A

The surface has high energy because it is exposed and has low coordination number

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34
Q

What are some examples of heterogenous catalysts?

A

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

35
Q

What are the advantages of heterogenous catalysts?

A

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

36
Q

What are the main types of hetergenous catalyst?

A

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…

37
Q

How do we classify the size of pores?

A

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

38
Q

What is the role of the catalytic support?

A

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

39
Q

What are catalytic converters?

A

Reduce emissions from internal combustion engine - convert SOx, NOx , CO and hydrocarbons to less harmful gases

40
Q

What kind of catalytst do catalytic converters use?

A

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

41
Q

What are nano particles?

A

particles between 1 and 100 nanoparticles

42
Q

Why are metal particles dispersed on ceramic structure in catalytic converters?

A

To reduce loss in activity caused by sintering of the metal particles

Allows for the transort of large volumes of gas

43
Q

What are the reactions occurring in a catalytic converter?

A

2CO + O2 —-(Pt/Pd)–> 2CO2

CxH2x+2 + 2xO2 —(Pt/Pd)—> xCO2 + 2xH2O

2NOx —(Rh/Pt)—> N2 + xO2

44
Q

What are the advantages and disadvantages of catalytic converters?

A

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

45
Q

What are zeolites?

A

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

46
Q

What is the structure of zeolites?

A

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

47
Q

What is the role of Al3+ in zeolites?

A

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

48
Q

How are zeolites selective?

A

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

49
Q

How are zeolites synthesised?

A

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

50
Q

How are zeolites used?

A

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

51
Q

What is a mechanism of reactions at solid surface?

A

1) substrate diffusion
2) substrate adsorption
3) Surface diffusion
4) Surface reaction to form adsorbed
5) Product desorbed and product diffusion away from surface

52
Q

How does a substance diffusion?

A

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

53
Q

How are substrates adsorbed onto a surface oir pore of a support?

A

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

54
Q

What are the types adsorption?

A

Physisorption

Chemisorption

55
Q

What is the physisorption?

A

Involves weak van der Waals interactions and electrostatic polarisation effects

Eads = 10-40 kJ mol-1

No chemical bonds formed

56
Q

WHat is chemisorption?

A

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

57
Q

What is the migration of adsorbed substrates?

A

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

58
Q

How does the selectivity of heterogenous catalysts compare with homogenous?

A

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

59
Q

How does strength of adsorption affect selectivity?

A

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

60
Q

What are the types of active sites?

A

Acidic/basic sites

Metal particles:

  • finely divided on solid support or as nanoparticles
  • active catalysts for a range of processes
61
Q

What factors affect catalytic activity of solid catalysts?

A

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

62
Q

What are promoters?

A

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

63
Q

What are poisons?

A

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

64
Q

What are nanoparticles?

A

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

65
Q

What do we have consider when using supported nanoparticles?

A

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

66
Q

Why does the form of the metal matter?

A

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

67
Q

How can nanoparticles affect the activity?

A

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

68
Q

How is light scattering and electron microscopy used to test for nanoparticles?

A

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

69
Q

How are kinetic studies used to test for nanoparticles?

A

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

70
Q

What is a tethered catalyst?

A

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

71
Q

What are the advantages of supported catalysts?

A

Easy in situ synthesis of silica supported silver nanoparticles

Easy separation of catalyst from reaction products

72
Q

Why are acid catalysts important?

A

Most widely used

Often expensive

73
Q

How do we classify acids?

A

Bronstead-Lowry (H2SO4) or Lewis (AlCl3)

All Bronstead-Lowry are Lewis

Better classification: protic or non-protic

H2SO4 - protic

AlCl3 - non protic

74
Q

Are acid catalysts homo or hetero genous?

A

Traditional acid catalysts are homogenous

Modern acid catalysts are often heterogenous

75
Q

What is the difference between specific or general acid catalyst?

A

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

76
Q

What is the mechanism of proton transfer?

A

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

77
Q

What are the factors that affect rate of proton transfer?

A

Intramolecular H-bonding

Requirements for electronic reorganisation - causes changes to bond lengths/angles

solvent effects - fast proton transfer in water

78
Q

What is the correlation between acid strength and rate of catalysed reaction?

A

Stronger the acid catalyst the faster the rate of reacgtion until the limit of diffusion control is reached

79
Q

What effects can solvents have?

A

Rate increased when protonated transition state is solvated more than acidic proton

Greater proton stabilisation = lower reactivity of catalyst

80
Q

How are Lewis acid catalysed reactions usuallyu conducted?

A

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

81
Q

Which famous reaction are Lewis acids used as catalysts?

A

Freidel Crafts - AlCl3

82
Q

What are examples of solid acid catalysts?

A

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)

83
Q

What are ther advantages and disadvantages of solid acid catalysts?

A

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

Core 6 (7 decks)

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