Catalysis Flashcards

Question

Mechanism of Cativia process?

Answer 1

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

Answer 2

Method led to a racemic mixture resolved using column chromatography heterogenous Pd/C catakyst used in hydrogenation step

Answer 3

IMproved process - forms only one enantiomer Uses Rh organomettalic catalyst for hydrogenation step

Answer 4

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)

Answer 5

Reacting groups brought togetehr in correct orientation TS and intermediates stabilised by van der Waals, electrostatic and hydrogen bodning interactions

Answer 6

pH dependent solvent choice limitted Denaturation

Answer 7

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

Answer 8

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

Answer 9

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

Answer 10

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

Answer 11

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...

Answer 12

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

Answer 13

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

Answer 14

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

Answer 15

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

Answer 16

particles between 1 and 100 nanoparticles

Answer 17

To reduce loss in activity caused by sintering of the metal particles Allows for the transort of large volumes of gas

Answer 18

2CO + O₂ ----(Pt/Pd)--\> 2CO₂ C_xH_2x+2 + 2xO₂ ---(Pt/Pd)---\> xCO₂ + 2xH₂O 2NO_x ---(Rh/Pt)---\> N₂ + xO₂

Answer 19

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

Answer 20

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

Answer 21

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

Answer 22

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

Answer 23

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

Answer 24

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

Answer 25

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

Answer 26

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

Answer 27

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

Answer 28

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

Answer 29

Physisorption Chemisorption

Answer 30

Involves weak van der Waals interactions and electrostatic polarisation effects E_ads = 10-40 kJ mol^-1 No chemical bonds formed

Answer 31

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 E_ads = 100-500 kJ mol^-1

Answer 32

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

Answer 33

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

Answer 34

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

Answer 35

Acidic/basic sites Metal particles: - finely divided on solid support or as nanoparticles - active catalysts for a range of processes

Answer 36

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

Answer 37

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

Answer 38

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

Answer 39

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

Answer 40

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

Answer 41

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

Answer 42

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

Answer 43

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

Answer 44

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

Answer 45

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

Answer 46

Easy in situ synthesis of silica supported silver nanoparticles Easy separation of catalyst from reaction products

Answer 47

Most widely used Often expensive

Answer 48

Bronstead-Lowry (H₂SO₄) or Lewis (AlCl₃) All Bronstead-Lowry are Lewis Better classification: protic or non-protic H₂SO₄ - protic AlCl₃ - non protic

Answer 49

Traditional acid catalysts are homogenous Modern acid catalysts are often heterogenous

Answer 50

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

Answer 51

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

Answer 52

Intramolecular H-bonding Requirements for electronic reorganisation - causes changes to bond lengths/angles solvent effects - fast proton transfer in water

Answer 53

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

Answer 54

Rate increased when protonated transition state is solvated more than acidic proton Greater proton stabilisation = lower reactivity of catalyst

Answer 55

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

Answer 56

Freidel Crafts - AlCl₃

Answer 57

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)

Answer 58

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

Catalysis Flashcards

(83 cards)