Lecture 6 Flashcards
Why is high pressure equipment used?
- for gas liquid reactions
Why is pressurised equipment dangerous?
• Dangerous due to stored enegy and risk of explosion/ gas release if the equipment ruptures
- Many gasses used in synthesis are flammable, toxic and corrosive
- Each 1 mL gas stores 12 kJ energy
How does flow chemistry minimise the dangers of pressurised reactions?
• Flow chemistry minimises the risk associated with this reaction by decreasing the volume of the system and therefore the amount of stored energy
How do TBR/ PBR work?
- uses the downward movement of a liquid and gas over a packed bed of catalyst particles.
- It is used extensively used in processing plants.
How does a bubble column work?
- A bubble column reactor consists of a cylindrical columns
- gas in-let is at the bottom of the column and causes a turbulent stream to enable an optimum gas exchange.
- The mixing is achieved by the gas sparging and it requires less energy than mechanical stirring.
- The liquid can be in parallel flow or counter-current.
- Better surface areas and mass transfer in solution
Describe the different types of gas flow regime
- Can have flow where gas doesn’t touch the sides of a vessel
- Segmented Flow: As the size of the bubbles increase there is distinct separation of sections of liquid
- Annular flow: fast gas flow that breaks up segments with a thin coating of liquids on the inside of a channel. The contact surface area is extremely high
How is flow regime determined?
by the physical properties of the liquid and the gas, with flow rate being very influential
Describe a minature falling flow reactor
-liquid enters at one end and gas at the other. - Liquid flows down channels that have a gas layer above them leading to much more efficient mixing
How do annular flow reactors work?
- up to 5m of reactor channel on an inch size reactor.
- A constant flow of gas forces liquid through the reactor with a very low residence time.
- Generating segmented flow means that the reaction coil can be shorter
Why is fluorination hard in bulk conditions?
- Elemental fluorine is extremely reactive and oxidising and requires specialist equipment and training
- Selectivity can be difficult to predict and control
- Reactions requite care because hydrocarbons can uncontrollably burn in F2 analogous to combustion with oxygen
Where do fluorination reactions occur?
- at the gas liquid phase interface
- due to the poor solubility of fluorine
How do flow reactors improve fluorination?
• very high specific phase interfaces that serve to form the very thin liquid layer on the surface of the micro- structures, both heat transport and mass transport are intensified such that the formation of hot spots is suppressed and the selectivity of the reaction is improved.
- high conversion rates
- tailored selectivity (below 40% due to limiting chemistry)
- reaction time can be vaired
- improved space time yields
How are flow reactors adapted for fluorination?
- Ni or polymer reaction channels instead of glass to prevent unwanted reactions
- Flourine can be diluted with nitrogen to lower reactivity
What specific reactors are god for fluorination?
- Both falling film and microbubble reactors offer advantages for fluorination
- Selectivity better in a falling film reactor much higher than normal bubble reactors for toluene
Why are hydrogenation reactions useful?
- catalytic hydrogenation reactions are commonly used for saturating double bonds and effecting functional group conversions
- Hydrogenation processes are of enormous industrial importance and are used on large scales in the petrochemical, fine chemical and food industries.
What are traditional catalytic methods for hydrogenation?
- Heterogeneous catalytic hydrogenation methods were established in the early 1900’s followed by advances in homogenous catalysis using transition metal catalysts in 1960’s.
- The discovery of asymmetric transition metal hydrogenation reactions soon followed and paved the way for highly efficient syntheses of single enantiomers.
- Today, catalytic hydrogenation research continues at a pace with recent notable advances using cheaper transition metal catalysts, such as iron and cobalt, and also in the area metal free hydrogenations.
What do modern catalytic hydrogenation reactions involve?
• Common with all hydrogenation methods involving molecular hydrogen is the need to ensure efficient mass transport of hydrogen gas into the liquid phase.
Why are hydrogenation reactions dangerous?
- Owing to the poor solubility of molecular hydrogen in common organic solvents, hydrogenation processes are typically performed at elevated pressures and temperatures
- Such elevated temperatures and pressures have associated safety and cost implications and require speciality high pressure equipment.
- One of the key drivers for the development new catalysts is to achieve efficient conversions at lower pressures and temperatures
Why is the use of molecular hydrogen green?
- good atom economy
- use of renewable feedstock
How is wilkinsons catalyst used in micro reactors?
• Wilkinson was developed by adding a black coating to the inside of microchannels with a polymer of a Rhodium catalyst which was solid, turning the catalysis from homogeneous to heterogenous
How can alkene/alkyne reactions and deptrotection reactions be done in flow?
- glass microchannel reactor with Pd0 immobilized on the inner surface of the channel.
- good yield
- production rate 140000 times higher than batch
- no Pd leaching therefore reusable reactors
What are tube in tube reactions useful for?
- toxic gas reactions
- help hydrogenation reactions
How can flow help the reduction of imines or nitriles to amines/imines?
- H-cube reactor
- supported precious metal catalyst (Pd/C, Pt/ C, or Rh/C). 30–80 bar of hydrogen pressure at 60– 80 oC
- stereoselectivity variations were observed depending on the metal catalyst and the temperature/pressure of the hydrogenation reaction.
How is selectivity tuned in ethyl nicotine?
selectivity between partial and full hydrogenation could be tuned depending on the hydrogen pressure, solvent, and the choice of supported metal catalyst
How can deuteration occur?
In the H-Cube hydrogen gas is generated by electrolysis of water changing the hydrogen source to deuterated water allows the preparation of deuterated derivatives.
Why is palladium based carbonylation important?
- highly versatile
- effective synthetic route to an range of molecule
- one pot reactions
- readily available precursors
Why can’t palladium based carbonylation be widely used?
- safety concerns with CO
- not as well categorised as other palladium based reactions
- yield depends substrate and CO pressure, solvent and reaction temperature
- emphasis place on development of catalyst that allows lower temps and pressures
Describe carbonylation in flow
- low yield and selectivity but higher than batch reactions
- segmented flow set up
How is carbonylation affected by flow regime?
see notes
