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CHEM313 > Topic 5 - Framework Solids > Flashcards

Topic 5 - Framework Solids Flashcards

1
Q

What different types of frameworks are there?

A

COFs - Covalent Organic Frameworks

MOFs - Metal-Organic Frameworks

Zeolites

PCPs - Porous Coordination Polymers

HOFs - H-bonded Organic Frameworks

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2
Q

What are zeolites?

A

Zeolites are crystalline microporous aluminosilicates

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3
Q

What is the general formula for zeolites?

A

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

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4
Q

What’re the primary building blocks of zeolites?

A

Corner-sharing [SiO4]4-

[AlO4]5- tetrahedral with oxygen bridges

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5
Q

Whats the use of zeolites?

A

Desiccants

Water Softening

Environmental remediation

Hydrocarbon separation

Heterogenous catalysis

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6
Q

How are zeolites used as desiccants?

A

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

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7
Q

How are zeolites used to soften water?

A

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

Avoids use of polyphosphates
- environmental problems (algal blooms)

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8
Q

How are zeolites used in environmental remediation?

A

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.

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9
Q

How are zeolites used in hydrocarbon separations?

A

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

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10
Q

How are zeolites used in heterogenous catalysis?

A

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

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11
Q

How can zeolite’s properties be changed?

A

Al/Si ratio

Changing counter cation

Templating

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12
Q

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

A

Structure type

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

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13
Q

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

A

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

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14
Q

How is templating used to tune zeolite properties?

A

Specific large cations present during synthesis

Used to promote desired structure type

Also used to stabilise high Al/Si ratio

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15
Q

What are Metal-Organic frameworks (MOFs)?

A

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

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16
Q

How are MOFs and zeolites similar?

A

The framework for MOFs can be represented as a network:

  • Nodes are metal ions or clusters of ions
  • Edges are organic linkers
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17
Q

Whats the use of MOFs?

A

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

18
Q

Whats the structure of MOFs?

A

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

19
Q

Describe the structure of silicates (SiO2) in zeolites.

A

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

Si-O-Si link is flexible

Many possible configurations

20
Q

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

A

Makes framework anionic

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

21
Q

What are secondary building units (SBUs) in zeolites?

A

Tetrahedral 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

22
Q

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

A

Oxygen bridge is relatively flexible

Angles between 120˚ and 180˚

23
Q

What is Lowenstein’s rule?

A

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

(Al-O-Al do sometimes occur tho)

24
Q

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

A

A ß-cage

Shape is a truncated octahedron

25
Q

Whats the internal diameter of a sodalite cage?

A

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

26
Q

What does the sodalite cage structure consist of?

A

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

27
Q

Whats the structure of Sodalite (mineral)?

A

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

28
Q

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

A

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

29
Q

Whats the structure of Zeolite A?

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)

30
Q

Whats the structure of Zeolite X/Y?

A

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

31
Q

Whats unique about the structure of ZSM-5?

A

ZSM-5 is a pentasil zeolite
- contains chains of pentasil units

Chains join together to form layers

32
Q

Whats the narrowest channel in ZSM-5?

A

The 10-rings
- 5.5Å diameter

Medium pore zeolite

33
Q

What does reticular chemistry describe?

A

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

34
Q

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

A

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

35
Q

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

A

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

36
Q

What’re the applications of MOFs?

A

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

37
Q

How can the properties of MOFs be tuned?

A

Changing metal centres

Changing organic linker

Defect engineering

38
Q

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

A

Change the structure type
- changing coordination

Catalytic properties
- e.g. open metal sites

39
Q

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

A

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

40
Q

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

A

With amorphous MOFs/ZIFs

CHEM313 (9 decks)

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