Flashcards
FLASHCARDS - Fundamentals of Catalytic Processes
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- 1What is a catalyst? (3 MP)It increases the rate at which a chemical rxn approaches equilibrium, without itself being consumed or changed in the process
- 2For a catalysed process... (3 P)1) The rxn rate constant obeys the Arrhenius Eq. & provides lower energy rxn pathways
2) It does not circumvent normal thermodynamic predictions
3) It does not alter equilibrium constants - 3Importance of catalysts... (4 P)1) 90% chemical processes use catalysts
2) Changes in catalyst influence greatly rates and selectivities of rxns
3) Reactor design associated w/ optimizing performance of catalyst
4) Catalytic Cycle:
reactants + catalyst --> complex
complex --> products + catalyst - 4Classes of catalyst: (4 P) 1) Heterogeneous (4 P) 2) Homogeneous (4 P) 3) Bio-catalysts (2 P) 4) Phase transfer (2 P)1) Range of different active sites / Active site immobilised on solid support / Tuneable selectivity / Easily separated
2) Organometallic complexes widely used/ More active than heterogeneous / High selectivity / Difficult to separate
3) Enzymes, bugs / Highly selective
4) Reagent soluble in separate phase to substrate / use PTC to transfer reagent into organic - 5Advantages & disadvantages of a homogeneous acid catalyst over a heterogeneous acid catalyst
- 6Catalyst Efficiency: (2 P) 1) Turn over number (TON) 2) Turn over frequency (TOF) --> (used to compare catalysts)1) No. of rxns a single site can achieve before becoming inactive
- max turnovers (assumes 100% yield) = equivalents rectant/equivalents catalyst
- actual turnovers = max turnovers x yield
2) No. of rxns per site per unit time
- TOF = actual turnovers/time for rxn (hour) - 7Reactions in a catalytic converter: 1) CO 2) \(CH_4\) 3) \(NO_x\)
- 8Limitations with catalytic converters
- 9How to change the rate with catalysts: (2 P)1) Increasing the surface area (eg. smashing the catalyst), allowing more active sites - especially true for heterogeneous catalysts
2) Changing shape of catalyst (eg making surface rougher) - can create defects on surface at which catalysis can occur, increasing activity - 10Catalytic converter problems: (3 P) 1) Operating temperature (2 MP) 2) Catalyst poisons 3) Use of precious metals (4 MP)1) Inactive when cold, activity destroyed if too hot
2) Catalyst poisons (eg Zn in cars etc.)
3) Problems of stability, dispersion, lifetime and elemental sustainability - 11Metal recovery: Phytoremediation - use of plants for environmental clean-up
- 12Type of rxn: 1) General acid catalysis 2) Specific acid catalysis1) Rxn catalysed by any acid species (Bronsted and Lewis), generally HA
2) Rxn catalysed by one acidic species only, generally \(H_3O^{+}\) - 13Mechanism of rxn: 1) Specific acid catalysis - \(H^{+}\) transfers before rds, forming an intermediate - 2 slow rds 2) General acid catalysis - \(H^{+}\) transfers in rds (slow) - 1 slow rds
- 14Mechanism of rxn 2: Specific acid catalysis (less species, generally in RB flask)
- 15Mechanism of rxn 3: General acid catalysis (more species, generally in enzymatic catalysis)
- 16Mechanism of acid-catalyzed hydrolysis of ester (Specific acid catalysis) (NOTE: 2 slow steps, both rates increased when catalyzed) (NOTE difference if no acid catalyst present)
- 17Strenghts of acids in... Specific acid catalysisSpecific acid MUST be strong acid
- 18Strenghts of acids in... General acid catalysisGeneral acid CAN be weaker acid, but stronger means more efficient rxn
- 19Henderson–Hasselbalch equation (for dilute aqueous solutions) --> large Ka = strong acid --> small pKa = strong acid (inverse)pH is the standard method for measuring acidity in AQUEOUS systems
- 20Mechanism of proton transfer: (3 equilibriums)
- 21Rate of proton transfer 1Proton transfer in org. systems can be slower than in aq.
The C-H bond breaking process is slow due to bond length & bond angle change
- 22Rate of proton transfer 2- Nevertheless, diffusion of \(H^{+}\) in water is much faster than diffusion of other ions
- There is rapid proton transfer in hydrogen bonded systems in water
- 23Rate of proton transfer 3
- 24Rate of proton transfer 4 --> Intramolecular H-bonding: reduces rate
- 25Rate of proton transfer 5rate of proton transfer much slower When electronic reorganisation is required leading to change in bond lengths/angles (eg keto-enol tautomerism)
- 26Does NOT affect rate of proton transferElectrostatic interactions, eg having \(Na^{+}\) \(Cl^{-}\) in solution
(NOTE: but may affect the rxn itself) - 27Proton transfer in polarisable systems
- 28Effect of water on dissociation of Bronsted acidsOrder of acidities based on amount of water required for complete dissociation to \(H_5O_2^{+}\)
--> more water required = more acidic - 29Ideal conditions mean... (2 P)- Low acid concentrations (< 1M)
- Full dissociation of acid - 30Bronsted acid catalysis means...- Catalyst source of \(H^{+}\), typically \(H_5O_2^{+}\) in water under ideal conditions, which somehow lowers \(E_a)\)
- Substrates are protonated
- Rates of rxn increase w/ increase in acidity (slope = 1), up to diffusion control. - 31Bronsted Catalysis LawUnder ideal conditions (dilute aq. solutions), stronger acids increase the rate, but to the limit of diffusion control
- 32Bronsted Catalysis Law for non-ideal conditions, ie. non-aqueous & non-protonic aicdsPUT MORE... NON-AQ COND, NON-PROT ACIDS ETC.
- 33Bronsted Catalysis Law for non-ideal conditions, ie. non-aqueous & non-protonic aicds 2
- 34Hammett Acidity Function Can be used for measuing acidities of: - Lewis acids - V. concentrated solutions , including superacids (ie, low H2O in system, not being able to transfer H+ throughout rxn)
- 35Hammett Acidity Function in dilute solutions (works v. well too)
- 36Hammet acidity function (\(H_O\)) 2:- Can replace the pH in concentrated solutions
- Measures tendency of a solution to protonate a NEUTRAL base
- Superacidity at \(H_O\) \(\geq\) 12 - 37Hammet acidity function plot: (effect of H2O/AH on \(H_O\))
- 38Solvent ("medium") effects on proton transferDo rxn in solvent that helps catalysis (eg hydrolysis of esters in aqueous, to move H+ throughout)
- 39Proton transfer in non-aqueous solvents --> Acid-base complexes can have enough stability to allow detailed spectroscopic studies & isolation
- 40Reactivity of hydrogen bonded complexes (in NON-aqueous conditions) --> Hydrogen bonded complexes can act as v. reactive in-situ sources of nucleophiles
- 41Catalytic function of the proton in organic chemistry - Electronic distortion
- 42Catalytic function of the proton in organic chemistry - Leaving group stabilisation
- 43Catalytic function of the proton in organic chemistry - Addition of H+ to electron system (eg \(\pi\) - system)
- 44Definitions: 1) Lewis 2) Lewis acids 3) Lewis bases1) Capability of coordinating with unshared electron pairs
2) Have a vacant orbital which permits coordination of molecules w/ unshared electron pairs
3) Have unshared electron pairs available for donation - 45Lewis acids- \(M^(n+)\) or similar coordinating to site of high e- density
- Non-protonic (eg \(AlCl_3\)), & bulkier (than \(H^+\)), so rxns more subject to steric limitations
- Active intermediate is complex with Lewis base, ie. \(BH^+\)
- Water can change nature of Lewis acid catalysts
- Rate/kinetic plot can be > 1, ie. rates can more than double on doubling the quantity of Lewis acid - 46Mechanism of Friedel-Crafts reactions: Effect of water: - Weakens overall acidicity, reducing rate - Changes nature of catalyst --> no longer a catalyst
- 47Mechanism of Friedel-Crafts reactions 2: Effect of water: - Weakens overall acidicity, reducing rate - Changes nature of catalyst --> no longer a catalyst
- 48Heterogeneous Catalysts - Solid Acids: Inorganic solids possessing Lewis or Bronsted acid sites.- Balance of acid site type (eg through heating) & strength critical for catalyst selectivity
- Porosity & high surface area) v. important as het. catalysts are diffusion limited and need to move in & out of micropores
- 49Heterogeneous Catalysts 2: (NOTE: TS--1 Catalyst is same structure but with Titanium instead of Silicon)
- 50Heterogeneous Catalysts 2: another version
- 51Immobilised aluminium chloride (\(AlCl_3\))
- 52Product isolation with: 1) \(AlCl_3\) 2) Heterogenous catalyst (solid acids) Note: Solid acids catalyst can be reused, as opposed to AlCl3 which gets quenched1) - Quench with water, neutralise HCl, \(Al(OH)_3\) formed, organic product extracted
2) Simply filter
--> In both cases, distil to produce the pure product - 53Heterogeneous Catalysts - Deactivation (3 P) (eg in Friedel-Crafts reactions)
- 54Heterogeneous Catalysts - Reactivation (3 P)
- 55Heterogeneous Catalysts - Problems (2 P)
- 56Heterogeneous Catalysts - Alkylation mechanism (can get double addition, see shape (size) selectivity)
- 57Heterogeneous Catalysts 2 - Alkylation isomers
- 58Heterogeneous Catalysts - Shape (size) Selectivity (better w/ solid acids)
- 59Mechanism for catalysed polymerisation of styrene with: - \(AlCl_3\) - \H^+\)
- 60Heterogeneous Catalysts - Solid Bases: Inorganic solids possessing basic sites. - Michael Reaction mechanism
- 61Heterogeneous Oxidations (using eg TS-1 - Enichem)Want to replace Cr(VI) with a heterogeneous process employing \(O_2\) or \(H_2O_2\)
- 62Green oxidations
- 63Things to take care w/ Heterogeneous catalysts (3 P):- Stability of catalyst
- Leaching & product contamination
- Selectivity - 64Green processing - The Substrate- Better use a renewable resource
- Care w/ oxygen content of bio-renewables
- Care w/ physical state of bio-resource
- Care w/ type of reactor
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- FLASHCARDS - Fundamentals of Catalytic Processes, Catalysis (5L + 1W)
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- Sulivan González
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