Carbon Monoxide (C1) Chemistry Flashcards
What is the trans effect?
- Affects square planar and some octahedral complexes, describing how certain ligands influence the substitution rate of the ligand positioned trans to them
- It is where a strong trans-directing ligand accelerates the substitution of a ligand opposite it
- These ligand include: CN⁻>CO>C₂H₄>PR₃>H⁻>CH₃⁻
A group 13 metal with 3 M-L bonds is…
Lewis acidic (Electron acceptor)
What is the Fischer-Tropsch reaction with CO
- The FT reaction is a catalytic process that converts CO and H₂ into hydrocarbons
- mCO + nH₂ (m and n are variable)
- Produces liquid alkanes (petrol), alkenes, methanol and higher alcohols
“Syngas” is the feedstock for the Fischer-Tropsch reaction
Where does most of it come from?
- Mostly from fossil fuels
This is a simplified mechanism for FT synthesis to make alcohol product
Identify the intermediates
Identify the different steps of the Fischer-Tropsch from Coal Feedstock
(As well as a way to make CO, you also need a way to make hydrogen)
The Water-Gas Shift Reaction is a method for….
…industrial scale production of H₂
(not really ideal as the main way to produce hydrogen as one mole of CO₂ is producted too)
Fill in the gaps of the intermediates of the Water-Gas Shift reaction
- Energetic driver of removal of CO₂
- Loss of hydrogen is a reductive elimination
Fill in the gaps for the lastest approach using Iridium for the Water-Gas Shift reaction
- Catalyst starts off as Ir(I), d⁸ - square planar
- Iodide trans to carbonyl due to trans effect as CO is a good π-acceptor
The Monsanto Acetic Acid process came about in the 1970s which involved the Homogeneously catalysed carbonylation of MeOH to form acetic acid
It uses Rhodium catalyst with iodide [RhI₂(CO)₂], why?
- Much more active thhan other halogens
- Good soft liands for soft Rh(I)
- Produces the fastest oxidative addition step, likely to do with the trans effect
What is the drawback of the Monsanto Acetic Acid process?
- Reactivity falls MeOH > EtOH > PrOH
- CO and Me have strong trans affect, thats why they are a ‘fac’ complex
- Me-CO-I is a very weak bond, hence is hydroloysed easily
Which step is therefore the rate determining step?
- The RDS needs to contain the catalyst and MeI
What problem with the Monsanto Acetic Acid Process is the corrosive nature of HI/Acetic acid solution requires an expensive plant
Why?
- High water content (15wt%) required to maintain the soluability of the Rh catalyst
- The energy needed to remove the water from the acetic acid out is great
How was the water content problem in the Monsanto Acetic Acid Process attempted to be overcome?
- Hoechst Celanese add an inorganic promoter to drive water content down to 5wt% and improve process
- Tennessee-Eastman Acetic Anhydride Process: replaces methanol with methylacetate to produce acetic anhydride
The Mnsanto Acetic acid process has been superseded by the BP Cativa Process
What changes were made?
- Based on a related Iridium Catalyst:
- [IrI₂(CO)₂]⁻ outperfoms the Rhodium system
- Allows water content of only 0.5wt%
What are the benefits of lower water content (than Monsanto Process) in the reaction mixture?
- Lower number of drying columns needed (lower costs)
- Decreased by-product formation (e.g. propionic acid)
- Suppresses the Water-Gas-Shift reaction
What was enabling the lower water content in the BP Cativa Process
- There was a significant contribution from a neutral species so that it doesn’t need water to support the cation
- The neutral species was created from the original Monsanto catalyst by removing an iodide
- In addition, as Ir is lower down in the periodic table, it is more stabilised in the higher oxdiation state, so we dont need a negative charge to promote it
- The addition of CO to the IR catalyst is very slow
- This is because if you take a single bond between two identical atoms, it gets weaker as you go down the periodic table
- How is this overcome?
- Ir-Me bond is slightly strong, requiring greater energy to hop it across to the π-antibonding orbital
- This reaction is made more favourable by making the π-star orbital of CO better electron acceptors
- Which a lewis acid is used to remove I⁻, removing the -tve charge
Here is a high pressure IR reaction Monitoring
Describe what is occuring
- Can see if IR bands in the carbonyl region going down as a new product forms
- Form both fac and mer isomers, BUT a lot more of the fac one occurs
Here is another IR reaction monitoring using isotopic labelling
Describe what is happening?
- This is the water-gas-shift reaction
- The water is labelled with ¹⁸O
- And catalyst is labelled with ¹⁴C
- It shows the CO₂ comes from the CO which have been liberated
- Trace water causes by-product from the system - via nucleophilic attack on CO ligand
