Topic 5 - Framework Solids

Question 1

What different types of frameworks are there?

Answer 1

COFs - Covalent Organic Frameworks

MOFs - Metal-Organic Frameworks

Zeolites

PCPs - Porous Coordination Polymers

HOFs - H-bonded Organic Frameworks

Question 2

What are zeolites?

Answer 2

Zeolites are crystalline microporous aluminosilicates

Question 3

What is the general formula for zeolites?

Answer 3

n is the valence of M cations
- x:y (Al:Si) ration is variable

Question 4

What’re the primary building blocks of zeolites?

Answer 4

Corner-sharing [SiO4]4-

[AlO4]5- tetrahedral with oxygen bridges

Question 5

Whats the use of zeolites?

Answer 5

Desiccants

Water Softening

Environmental remediation

Hydrocarbon separation

Heterogenous catalysis

Question 6

How are zeolites used as desiccants?

Answer 6

Small pore zeolites selectively adsorb polar molecules due to their hydrophilicity
- size selective due to pore sizes

Question 7

How are zeolites used to soften water?

Answer 7

Na+ ions in zeolite exchange for cations such as; Mg2+ / Ca2+
- Mg2+ / Ca2+ cause ‘hard water’

Avoids use of polyphosphates
- environmental problems (algal blooms)

Question 8

How are zeolites used in environmental remediation?

Answer 8

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.

Question 9

How are zeolites used in hydrocarbon separations?

Answer 9

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

Question 10

How are zeolites used in heterogenous catalysis?

Answer 10

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

Question 11

How can zeolite’s properties be changed?

Answer 11

Al/Si ratio

Changing counter cation

Templating

Question 12

What effect does changing the Al/Si ratio have on zeolite properties?

Answer 12

Structure type

Charge on framework
- more Al = greater charge/more polar
- hydrophilicity
- available pore volume (number of cations)
- chemical stability of framework

Question 13

What effect does changing the counter cation have on zeolite properties?

Answer 13

Affects channel & cage size
- affecting chemical properties; acidity, polarity, catalytic

Question 14

How is templating used to tune zeolite properties?

Answer 14

Specific large cations present during synthesis

Used to promote desired structure type

Also used to stabilise high Al/Si ratio

Question 15

What are Metal-Organic frameworks (MOFs)?

Answer 15

They are crystalline framework solids composed of metal centres and organic linker units

Question 16

How are MOFs and zeolites similar?

Answer 16

The framework for MOFs can be represented as a network:

• Nodes are metal ions or clusters of ions
• Edges are organic linkers

Question 17

Whats the use of MOFs?

Answer 17

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

Question 18

Whats the structure of MOFs?

Answer 18

Polydentate organic linkers coordinate to metal centres and form the framework
- carboxylate linkers most common, various can be used tho

Question 19

Describe the structure of silicates (SiO2) in zeolites.

Answer 19

Composed of corner-sharing [SiO4]4- tetrahedra
- variable number of corners shared

Si-O-Si link is flexible

Many possible configurations

Question 20

What does replacement of Si(IV) by Al(III) result in?

Answer 20

Makes framework anionic

Cavities in 3D structure host charge-balancing cations, such as Na+

Question 21

What are secondary building units (SBUs) in zeolites?

Answer 21

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

Question 22

Whats the range of angles accessible by the oxygen bridges (Si/Al-O-Si/Al)?

Answer 22

Oxygen bridge is relatively flexible

Angles between 120˚ and 180˚

Question 23

What is Lowenstein’s rule?

Answer 23

That Al-O-Al linkages do not occur
- can have Si-O-Si

(Al-O-Al do sometimes occur tho)

Question 24

What is a sodalite cage also known as? Whats its shape?

Answer 24

A ß-cage

Shape is a truncated octahedron

Question 25

Whats the internal diameter of a sodalite cage?

Answer 25

6Å - large enough to host small molecules - must enter through windows

Question 26

What does the sodalite cage structure consist of?

Answer 26

Cage framework consists of 24 tetrahedra in linked 4- and 6-rings

Question 27

Whats the structure of Sodalite (mineral)?

Answer 27

Composed of sodalite units fused together via 4-ring faces of the ß-cages

Question 28

Whats the size of the 4-ring faces of sodalite cages?

Answer 28

Approximately 2.6Å - restricts access to internal 6Å central cavity

Question 29

Whats the structure of Zeolite A?

Answer 29

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)

Question 30

Whats the structure of Zeolite X/Y?

Answer 30

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

Question 31

Whats unique about the structure of ZSM-5?

Answer 31

ZSM-5 is a pentasil zeolite - contains chains of pentasil units Chains join together to form layers

Question 32

Whats the narrowest channel in ZSM-5?

Answer 32

The 10-rings - 5.5Å diameter Medium pore zeolite

Question 33

What does reticular chemistry describe?

Answer 33

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

Question 34

How can the channel sizes be changed in MOFs while retaining the same framework type?

Answer 34

By swapping the organic linker unit - modulates the pore size and chemical environment of guests

Question 35

Why are Zeolitic Imidazolate Frameworks (ZIFs) a good alternative to Zeolites?

Answer 35

Because the bridge angles are approximately the same - highly stable configuration

Question 36

What’re the applications of MOFs?

Answer 36

Molecular separation Sequestration - e.g. removal of of radioactive material Ionic (proton) conductors Catalysis Ferroelectrics Magnetic materials Drug delivery Detection & sensing

Question 37

How can the properties of MOFs be tuned?

Answer 37

Changing metal centres Changing organic linker Defect engineering

Question 38

How can metal centres be changed in MOFs to tune its properties?

Answer 38

Change the structure type - changing coordination Catalytic properties - e.g. open metal sites

Question 39

How can the organic linkers be changed in MOFs to tune its properties?

Answer 39

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

Question 40

With what type of MOFs/ZIFs is defect engineering employed?

Answer 40

With amorphous MOFs/ZIFs