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Description
Flashcards by Sulivan González, updated more than 1 year ago
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Created by Sulivan González
over 8 years ago
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| Question | Answer |
| Benefits of Heterogeneous Catalysts (3 P) | - Easier recovery (filtration) -- Easier to use in continuous (flow) reactors - Easier to handle (eg. strong acids) - Nearly all are porous: v. important property |
| Surface area formula | r = radius n = no. of pores l = length of cube |
| IUPAC definition... size for: 1) Micropores 2) Mesopores 3) Macropores | 1) <2.0 nm 2) 2 - 50 nm 3) >50 nm |
| Variable surface chemistry | |
| 2D Materials: - Clays | |
| Clays - neutral, no catalyst activity --> Isomorphous substitution: inserts charge into system: more active catalyst | |
| Clays: - Effect of isomorphous substituion | - Bronsted acid: Charge polarises OH - No direct \(H^{+}\) exchange as it will dissolve the material |
| Clays: - Effect of isomorphous substituion 2 | - Hydrated interlayer gaps reduce catalyst activity --> DRY - BUT, drying makes interlayers come closer together, so slower catalysis --> COMPROMISE |
| Catalyst activation | |
| Pillaring cations Layer collpase - problem especially w/ high charge density cations | Replace a (eg) \(Na^{+}\) w/ this: Gap maintained even when heated (dried) |
| Reactions of clays | |
| Other layered materials | |
| 3D Catalysts - Microporous systems | |
| Catalytic sites within zeolites | |
| Acidic Zeolites | |
| Pore size and diffusion | Aperture diameter used to define narrowest entry point - Ring size defined by no. of O atoms present (= no. of Si + heteroatoms) - Apreture dimensions between 0.4-0.75 nm |
| Pore size and diffusion 2: (3 P) | - Molecular: pores >> molecule - Knudsen: pores > molecule - Configurational: pores \(\simeq\) molecule |
| Diffusion in porous materials depend on: (3 P) | 1) Pore diameter 2) Connectivity (move & connect between pores: more degrees of freedom --> better catalytic activity) 3) Tortuosity |
| 5 stages involved in a liquid phase reaction using a porous solid catalyst - NOTE: for gas phase reactions, thickness of hydrodynamic layer = 0, so 1) and 5) can be omitted | 1) Diffusion of reactants through hydrodynamic layer 2) Diffusion of reactants through pore system 3) Reaction 4) Diffusion of products through pore system 5) Diffusion of products through hydrodynamic layer - For 1) & 2), parameters influencing the rate are: Thickness of hydrodynamic layer & pore size/geometry respectively |
| 5 stages involved in a liquid phase reaction using a porous solid catalyst 2 - Optimizing performance: | - 1) & 5) --> Heating: viscosity drops, rate increases. Stirring: hydrodynamic layer decreases, rate increases - 3) If high \(E_a\) --> heating increases rate - Low \(E_a\) suggests process is diffusion controlled - To distinguish between each: If RDS is 1), 5), 2) & 4), stirring will increase rate for 1) and 5), NOT for 2) and 4) NOTE: Increasing temperature does NOT increase rate of 2) and 4) |
| Test for diffusion control | - Diffusion dependent on pore size & pore size is not temperature dependent. |
| How is pore size measured (2 P): | - X-ray diffractions (work for crystalline systems) - Porosimetry: Gives pore size of porous solids. Adsrobtion of gas (eg N2) onto a solid over a range of partial pressures at the b.p of the gas Stage 1: low partial pressure - monolayer of gas on surface (surface area calc.) Stage 2: higher partial pressure - pores fill with liquid N2 to stabilise the meniscus (pore size calc.) |
| How is pore size measured - Porosimetry | |
| Measuring acidity of zeolites | - Desorption of ammonia: Higher temp of desorp. higher the acidity - Acidic strength \(\propto\) 1/Al content - Concentration of sites \(\propto\) Al content |
| Shape selectivity - Key property of zeolites (eg H-ZSM5) | Selectivity based on shape/size rather than electronic properties |
| Reactant selectivity | |
| Other uses of zeolites | AS a host for catalytic species (reactants can enter the supercage) |
| Titanium silicates (TS-1): Oxidation catalyst | |
| Titanium silicates (TS-1): Oxidation catalyst 2 | |
| Titanium silicates (TS-1) w/ 2 Ti: Oxidation catalyst | |
| Mesoporous systems - Larger templates give unstable materials. Solution... | Micelle Templated Silicas (MTS) |
| Addition of Heteroatoms to MTS | |
| ALMA industrial process Catalyst = \((VO)_2P_2O_7\) | |
| What is meant by isomorphous substitution in context of porous solids | |
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