Heterogeneous Catalysis SLC.pdf

CH3F0 –Heterogeneous Catalysis SLC Benji Moore [email protected] If you have any questions, please email me! Volcano Plot Question – 2022 Paper Volcano Plot Question – 2022 Paper di) SiO2 acts as support dii) Rh is very expensive relative to Co (if you get a similar question but instead o...

CH3F0 –Heterogeneous Catalysis SLC Benji Moore [email protected] If you have any questions, please email me! Volcano Plot Question – 2022 Paper Volcano Plot Question – 2022 Paper di) SiO2 acts as support dii) Rh is very expensive relative to Co (if you get a similar question but instead of Rh the catalyst is Ir, Ru, Pt, Au, Ag etc. stating cost as reason is extremely valid) + Co results in an increase in activity diii) Rh adsorbs the reactants weakly on its own (hence on 60% conversion), Co adsorbs the reactants strongly therefore adding Co increases the activity of the catalyst (100% conversion at 20% Co). If too much Co is added, the reactants are adsorbed too strongly and therefore even though a chemical transformation has taken place, the products cannot be released div) Vary the partial pressures of H2 LH kinetics ER kinetics rate rate po LH Kinetics: Rate depends on both substrates, and they compete Bimolecular RDS ER Kinetics: Rate depends on one substrate Unimolecular RDS po dv) One way would be add amorphous silica onto the surface thereby preventing any surface catalytic activity resulting in reactants going through the pores of the material. Another option would be increasing crystallite size, larger crystal ⇒ reactants go through pores. Zeolite Features – 2021 Paper Zeolite Features – 2021 Paper bi) Zeolites are aluminosilicate frameworks. The essential components are Si, Al and M where M = monovalent cation (Na+, K+) or divalent cation (Ca2+, Mg2+) Think of the structure as SiO2 doped with Al. As Al is 3+ while Si is 4+ you form an anionic framework. This anionic charge requires you to have something to balance it. The monovalent or divalent cations balance the anionic framework charge and sit in the pores of the framework. This charge balance concept is important when considering Zeotypes (e.g., Si(IV)O2 = Si2(IV)O4 = Al(III)P(V)O4) Zeolite Features – 2021 Paper bii) High Al content implies greater negative charge on the [Al xSi2-xO4] framework which implies we require more cations (Mx) to charge balance. For ion exchange you want a lot of ions available for exchange and therefore you have high Al content Zeolites. For molecular sieves, you want the pores to be open for products to be separated. Therefore, having low Al content Zeolites means that you require less cations for charge balance and therefore you have more pores available as sieves. biii) Brønstead Acidity ⇒ give up a proton 1) Substitute the monovalent cation for ammonium 2) Heat the material, this drives off ammonia and leaves a proton Zeolite Selectivity – 2019 Paper Zeolite Features – 2019 Paper bi) Same as question above bii) This question makes reference to building units. In Zeolites, the primary building unit (PBU) is the corner sharing AlO4 and SiO4 tetrahedra. These units can be linked together to form a secondary building unit (SBU). These can be linked together to form even larger units (e.g. sodalite β cage) PBU SBU More complex building blocks (e.g. sodalite β cage) Zeolite Features – 2019 Paper bii) In MFI, multiple 5-1 SBU are attached together to form a larger pentasil building block. Multiple pentasil building blocks can then link together and form the 3D structure. More straight forward for BEA, you can simply link sodalite cages with double-six-ring units Zeolite Features – 2021 Paper biii) In all cases, the diffusion barrier for MFI is higher than for BEA, why? Well BEA is a 12-ring system which ⇒ larger pores (more open structure). As such it can accommodate for the branched isomers much better than MFI. 2-methylhexane & 3-methylhexane have the exact same structure with the methyl having moved along 1 carbon which ⇒ similar kinetic diameter ⇒ very little difference in the diffusion energy barrier. 2,2-dimethylpentane & 3,3-dimethylpentane have the exact same structure with the two methyl groups having moved along 1 carbon ⇒ similar kinetic diameter ⇒ very little difference in the diffusion energy barrier. 2,3-dimethylpentane & 2,4-dimethylpentane have different structures. In the 2,3-dimethylpentane case we have a sort of trans linkage with the methyl groups while in the 2,4-dimethylpentage case we have a cis linkage. As such the kinetic diameter for 2,3-dimethylpentane is larger and therefore has a larger diffusion energy barrier. biv) If dimethylpentanes ratios similar ⇒ we do not see reactant shape selectivity. No reaction occurs therefore we observe product shape selectivity bv) MFI has zig-zag channels and a smaller pore diameter which implies reactants are trapped in pores and can undergo successive cracking resulting in smaller hydrocarbons Thank you for coming! Good luck for your exams! If you have any questions about the material covered or anything else regarding the module, feel free to email me!