Topic 5 - Framework Solids Flashcards
What different types of frameworks are there?
COFs - Covalent Organic Frameworks
MOFs - Metal-Organic Frameworks
Zeolites
PCPs - Porous Coordination Polymers
HOFs - H-bonded Organic Frameworks
What are zeolites?
Zeolites are crystalline microporous aluminosilicates
What is the general formula for zeolites?
n is the valence of M cations
- x:y (Al:Si) ration is variable
What’re the primary building blocks of zeolites?
Corner-sharing [SiO4]4-
[AlO4]5- tetrahedral with oxygen bridges
Whats the use of zeolites?
Desiccants
Water Softening
Environmental remediation
Hydrocarbon separation
Heterogenous catalysis
How are zeolites used as desiccants?
Small pore zeolites selectively adsorb polar molecules due to their hydrophilicity
- size selective due to pore sizes
How are zeolites used to soften water?
Na+ ions in zeolite exchange for cations such as; Mg2+ / Ca2+
- Mg2+ / Ca2+ cause ‘hard water’
Avoids use of polyphosphates
- environmental problems (algal blooms)
How are zeolites used in environmental remediation?
Heavy metal, radioactive contamination, removal
Zeolites can show high affinities for various metal cations
- used after Chernobyl disaster as cation-exchange agents to clean up 134Cs, 137Cs & 90Sr.
How are zeolites used in hydrocarbon separations?
Small pore zeolites can accommodate linear molecules, but exclude branched molecules
- shape selectivity
Window diameter can be tuned, to an extent, to allow access by specific molecules
How are zeolites used in heterogenous catalysis?
Zeolites can act as Bronsted acid catalysts (H+ counter ion forms hydroxylated bridge)
Or Lewis acids (open Si/Al site from heating Bronsted acid framework)
Shape and size selectivity can also promote selectivity of reaction
Catalytic metals, Pd2+ or Ni2+ ions, or nanoparticles of Pt/Pd can be exchanged or loaded into the structure
How can zeolite’s properties be changed?
Al/Si ratio
Changing counter cation
Templating
What effect does changing the Al/Si ratio have on zeolite properties?
Structure type
Charge on framework
- more Al = greater charge/more polar
- hydrophilicity
- available pore volume (number of cations)
- chemical stability of framework
What effect does changing the counter cation have on zeolite properties?
Affects channel & cage size
- affecting chemical properties; acidity, polarity, catalytic
How is templating used to tune zeolite properties?
Specific large cations present during synthesis
Used to promote desired structure type
Also used to stabilise high Al/Si ratio
What are Metal-Organic frameworks (MOFs)?
They are crystalline framework solids composed of metal centres and organic linker units
How are MOFs and zeolites similar?
The framework for MOFs can be represented as a network:
- Nodes are metal ions or clusters of ions
- Edges are organic linkers
Whats the use of MOFs?
They can exhibit large open pores within the crystal structure
Enormous scope for modulating properties of material
- metal centres, linker and defect formation can all be varied
Whats the structure of MOFs?
Polydentate organic linkers coordinate to metal centres and form the framework
- carboxylate linkers most common, various can be used tho
Describe the structure of silicates (SiO2) in zeolites.
Composed of corner-sharing [SiO4]4- tetrahedra
- variable number of corners shared
Si-O-Si link is flexible
Many possible configurations
What does replacement of Si(IV) by Al(III) result in?
Makes framework anionic
Cavities in 3D structure host charge-balancing cations, such as Na+
What are secondary building units (SBUs) in zeolites?
Tetrahedra link together to form SBUs
- finite units with ring structures
Tetrahedral centres are shown as vertices or nodes
- oxygen bridges are shown as straight lines that form edges of the shapes
Whats the range of angles accessible by the oxygen bridges (Si/Al-O-Si/Al)?
Oxygen bridge is relatively flexible
Angles between 120˚ and 180˚
What is Lowenstein’s rule?
That Al-O-Al linkages do not occur
- can have Si-O-Si
(Al-O-Al do sometimes occur tho)
What is a sodalite cage also known as? Whats its shape?
A ß-cage
Shape is a truncated octahedron
Whats the internal diameter of a sodalite cage?
6Å - large enough to host small molecules
- must enter through windows
What does the sodalite cage structure consist of?
Cage framework consists of 24 tetrahedra in linked 4- and 6-rings
Whats the structure of Sodalite (mineral)?
Composed of sodalite units fused together via 4-ring faces of the ß-cages
Whats the size of the 4-ring faces of sodalite cages?
Approximately 2.6Å
- restricts access to internal 6Å central cavity
Whats the structure of Zeolite A?
Composed of sodalite units linked via oxygen bridges connecting the 4-rings
Central cavity (11.4Å) is linked to six other 8-ring faces
- large truncated cuboctahedron
8-ring faces form orthogonal channels (4.1Å diameter)
Whats the structure of Zeolite X/Y?
Sodalite units linked via oxygen bridges connecting 6-rings
- tetrahedral geometry around the ß-cage
Tetrahedral sodalite units enclose large central cavity (11.8Å)
- accessed by 12-ring windows (7.4Å)
Large pore zeolites
Whats unique about the structure of ZSM-5?
ZSM-5 is a pentasil zeolite
- contains chains of pentasil units
Chains join together to form layers
Whats the narrowest channel in ZSM-5?
The 10-rings
- 5.5Å diameter
Medium pore zeolite
What does reticular chemistry describe?
Chemical components form nodes and linkers in a network
Geometrical diversity
- varieties of different metal ions & organic linkers
Allows for ‘controlled’ assembly of extended structures
How can the channel sizes be changed in MOFs while retaining the same framework type?
By swapping the organic linker unit
- modulates the pore size and chemical environment of guests
Why are Zeolitic Imidazolate Frameworks (ZIFs) a good alternative to Zeolites?
Because the bridge angles are approximately the same
- highly stable configuration
What’re the applications of MOFs?
Molecular separation
Sequestration - e.g. removal of of radioactive material
Ionic (proton) conductors
Catalysis
Ferroelectrics
Magnetic materials
Drug delivery
Detection & sensing
How can the properties of MOFs be tuned?
Changing metal centres
Changing organic linker
Defect engineering
How can metal centres be changed in MOFs to tune its properties?
Change the structure type
- changing coordination
Catalytic properties
- e.g. open metal sites
How can the organic linkers be changed in MOFs to tune its properties?
Endless possibilities:
Change structure type
- changes denticity (the number of atoms with which a ligand binds to a metal ion)
Maintain structure type
- change pore dimensions
Change chemical environment in pores
- hydrophobicity, catalytic, polarity, specific chemical functionalities
With what type of MOFs/ZIFs is defect engineering employed?
With amorphous MOFs/ZIFs
