L11: Carbonylation of methanol, alkene polymerisation

Question 1

How is acetic acid produced industrially?

Answer 1

• Direct carbonylation of methanol
• Monsanto; original process using Rhodium catalyst
• Cativa; modern version with similar conditions but a cheaper Iridium catalyst, w/ an additional promoter

Question 2

Monsanto process: processes in each cycle

Answer 2

• Cycles are inter-dependent (iodide and carbonylation cycles)
• Iodide cycle…
  A. Nucleophilic attack by I-, substitutes for OH2+ in acidic conditions
  B. Hydrolysis of acetyl iodide
• Carbonylation…
  1. Oxidative addition of CH₃I (RDS)
  2. Methyl migration
  3. Carbonyl addition
  4. Reductive elimination

Question 3

Monsanto process: explain the use of Me-I and omission of R3P ligand

Answer 3

• Me-I is preferred over other alkyl halides since rate of oxidative addition to 16 e- M centres depends on R-X bond strength
• R-I bond is weaker than R-Br
• Although R3P ligands would make the M centre more e- rich and speed up oxidative addition, but under the reaction conditions the modified catalyst soon reverts so is too short-lived to be useful

Question 4

Why is Rh preferred over other group 9 metals in the Monsanto process

Answer 4

• Cobalt is much less active so higher temperature and pressure would be required for MeOH carbonylation
• Iridium would have a much higher oxidative addition step (~150x) but the subsequent methyl migration becomes the rds as rates decrease from 5d to 3d due to decreasing M-R bond strength up the group (3000x slower for Ir vs Rh)

Question 5

Promoter in cativa process

Answer 5

• Ru(CO)₄I₂ or Hg₂
• Dramatically accelerates methyl migration
• Acts as a reversible iodide acceptor and aids substitution of an iodide ligand prior to Me migration

Question 6

Alkene polymerisation catalysts (prior and post Ziegler Natta)

Answer 6

• Originally, LDPE was produced form ethene using radical polymerisation via R₂O₂ initiator a 2000 atm and 200 degrees
• LDPE is of poor quality and branching is inevitable
• Ziegler Natta produced HDPE from ethene (LA catalyst, 25 degrees, 1 atm) and isotactic polypropylene from propene (same conditions)

Question 7

Z-N catalyst; type of catalyst, formation, role of AlEt₃, key features

Answer 7

• Heterogenous, surface sites of solid TiCl₃ particles formed by in situ reduction of TiCl₄ w/ AlEt₃, which reduces it and activates the surface by exchange of Cl for Et
• Early transition M centre
• alkyl group
• vacant coordination site

Question 8

Mechanism employed by Z-N catalyst

Answer 8

• Cossee-Arlman mechanism
• Termination of the alkyl chain is either beta-elimination or cleavage of M-R using H₂
• Regulating the amount of H₂ in the feed gives some control over polymers RMM

Question 9

Why does Z-N catalysis produce highly stereoregular propylene?

Answer 9

• ‘Head-to-tail’ coupling of the propene units occurs because steric interactions at rhe M centre strongly favour primary M alkyl formation
• Only isotactic PPE is formed at the active sites on the TiCl₃ surface

Question 10

Metallocene catalysts for Z-N

Answer 10

• Homogenous analogues for Z-N system
• Metallocene dihalides of Ti, Zr, Hf activated by MAO are highly active for alkene polymerisation (Zr best)
• MAO: methylaluminoxane

Question 11

Exception to rule: atactic polymers from Z-N

Answer 11

• Catalysed by (Cp)₂MCl₂ pre-catalysts; no control over tacticity

Question 12

ansa-metallocenes in Z-N

Answer 12

• Used to control polymer tacticity
• Normally, whether the Me group of the eta-2 propene points up or down it has identical steric interactions with the Cp rings; orientations equally favoured, only atactic PPE produced
• With ansa metallocenes, it produces either isotactic or syndiotactic (depends on type)