Lecture 17: Clays and Zeolites

Before coming to class:
  1. Read the information on this page.

  2. Read Layered Structures: Clays and Lubricants by M. Hewat, 1998

  3. Read Zeolite Catalysts and Sieves: Microscopic Architecture by M. Hewat, 1998

  4. Watch the 15 minute video lecture Clays and Zeolites on the MediaSite page.



Clays are sheet silicates of a special sort.
  1. Imagine a layer of a sheet silicate with all of the unshared oxygens pointed up. Because of the empirical formula ([Si4O11]6-) the layer has a negative charge of -1.5 for every silicon.

  2. Now imagine a second layer above the first but pointed in the opposite direction so that all of its unshared oxygens are pointed down.

  3. Next think about Al+3 cations each bonding to 3 of the unshared oxygens in the bottom layer and 3 unshared oxygens in the top layer. (This removes -0.5 charge from each Si.)

  4. All oxygen atoms are now shared. Each Si(IV) is surrounded by 4 oxygens and each Al(III) is surrounded by 6 oxygens. This "sandwich" is held together by strong covalent bonds.

  5. There remains a net -1 charge per Si atom. To balance this charge, there are cations such as Mg+2, Ca+2, Fe+3, Ni+2, Cu+2, Na+, K+ in layers outside the sandwich. The cations can be surrounded by water molecules and these cations are held only electrostatically.


Clays consist of stongly-bonded and weakly bonded layers. Muscovite mica is an example of a simple clay with the formula KAl2Si3AlO10.(OH)2. It is composed of sandwiches of octahedrally coordinated Al3+ between two layers of aluminosilicate sheets.

The sandwiches are weakly connected together by potassium cations. The potassium cations are coordinated to 3 oxygens in one layer and 3 in the next. These bonds are long, relatively weak, and easily broken.


When the charge on the layers is reduced, the interaction between layers is also reduced and the interlayer cations can be hydrated.

Look at the smectite clay montmorillonite [(Mg0.33Al1.67)Si4O10(OH)2]Na0.33. The spacing between layers can expand and contract depending on the amount of water, even though the layers themselves remain intact.

This expansion causes structural damage to buildings on soils with a high smectite clay content. Montmorillonite is also called "Fuller's earth" and was originally used remove grease from cloth. It is also used as an absorbant.

Clays have the capacity to absorb not only inorganic but also organic materials, providing novel functional organic/inorganic hybrid materials.

Investigations into their fundamental and molecular structure as well as their various characteristic properties can reveal their great potential for the development of new devices and systems composed of functionalized organic and inorganic compounds.

Aluminosilicates

The cations Al+3 ions (67 pm) and Si+4 (54 pm) have nearly the same size. When some Al(III) ions are substituted for Si(IV) in silicate minerals the resulting mineral is an aluminosilicate. The increase in the ration of Al/Si happens naturally over time as the mineral is exposed to water because silicate anions are more soluble in water than aluminate anions.

There is a difference in change. For every substitution of Al+3 for Si+4, the negative charge increases by 1.

The feldspars are aluminosilicates with composition that varies from M1[AlSi3O8] (M1= K+, Na+) to M2[Al2Si2O8] (M2= Ba+2, Ca+2).



Soil Weathering

Silicates and aluminosilicates change over time, especially in the presence of acidic rain. The more basic orthosilicates progressively condense until, ulimately, silicon dioxide is formed.

Why does this matter? Metal ions, such as K+, Fe2+, Mg2+, are important to growing plants. Fertile soil is a mix of organic matter, silicates, and clays with these metal ions complexed. As plants grow, the cations are exchanged in solution so that they can be absorbed. Decaying plants return the nutrient cations to the soil.

Clays are excellent ion exchangers. They readily pick up metal cations and release them. Clay, along with organic matter stabilizes the soil. The soil in Illinois is rich with metals, clay, and organic matter and so it is very good for growing corn and soybeans year after year. Farmers add fertilizers to replenish nitrates and phosphates because clay binds only cations not these anions.

What about the rainforest in South America? Farmers in Brazil who clear forest and remove the organic matter can grow crops on the soil only for a few years. The soil quickly looses it fertility and its ability to hold water. This is because the "young soil" reacts with the acidic rain to release cations. The rainy conditions wash the cations away. Organic matter rapidly decomposes in the hot, wet conditions and important metal cations in it are also washed away. The quantity of 3-dimensional, condensed, aluminosilicates increases as the quantity of simpler silicates and clays decreases.

Zeolites

The Zeolite Group of Minerals

The zeolites are minerals with important chemical applications. They are framework silicates consisting of interlocking tetrahedrons of SiO4 and AlO4. In other words, the basic structure is like beach sand (SiO2) with Al(III) substituting for some Si(IV). SiO2 is neutral but every aluminum atom adds a -1 charge to the structure and there must be cations present to balance the charge.

Zeolites have large vacant spaces or cages in their structures that allow space for large cations such as sodium, potassium, barium and calcium and even relatively large molecules and cation groups such as water, ammonia, carbonate ions and nitrate ions. In the more useful zeolites, the spaces are interconnected and form long wide channels of varying sizes depending on the mineral. These channels allow the easy movement of the resident ions and molecules into and out of the structure. Zeolites are characterized by their ability to lose and absorb water without damage to their crystal structures. The large channels explain the consistent low specific gravity of these minerals.

Natural zeolites typically form in the cavities, or vesicles, of volcanic rocks. Structural variations in natural zeolites include:.
  1. There are chain-like structures whose minerals form acicular or needle-like prismatic crystals, ie natrolite.
  2. Sheet-like structures where the crystals are flattened platy or tabular with usually good basal cleavages, ie heulandite.
  3. And framework structures where the crystals are more equant in dimensions, ie Chabazite.


Example: ANALCIME, NaAlSi2O6H2O


Another commone mineral is sodalite, Na6[Al6Si6O24](H2O)2.

This zeolite is made up of sodalite cage units. The image show here represents one sodalite cage with each line being a [Al/Si-O-Al/Si] unit.

In sodalite, these cages are linked directly to one another to produce 4-sided open channels with a 220 pm diameter.
Other zeolites are formed by connecting sodalite cages through spacer units. This picture shows the basic structure of Linde A zeolite, Na8[Al8Si40O96](H2O)24. Formed by linking the square faces of sodalite cages.Note that in picture of zeolites and related materials, each line connects a Si/Al to another Si/Al with an oxygen in the center of the line. We can't tell where the aluminum atoms are in the structure.

The 8-sided channel has a diameter of 410 pm.
Faujacite has a 12-sided channel with a diameter of 850 pm. This results from linking the hexagonal faces of sodalite cages.


Some application of zeolites include:
  1. Absorption - They can absorb a variety of materials due to a very large surface area, in such processes as drying, purification and separation, without chemical or physical change in the Zeolite structure. Zeolites trap gases in the void porous cavities. This enables them to be used in odour absorbing air filter media.

  2. Catalysis - Zeolites can be shape selective catalysts on the basis of molecular diameter and are used in petroleum refining and syn fuels production(e.g. changing alcohol into petrol in Brazil). We'll dscuss this next week.

  3. Ion Exchange
      i. They have replaced phosphates as water softening agents.

      ii. Treatment of radioactive wastes.
      Zeolites have been used for many years to remove Caesium and Strontium radioisotopes from nuclear waste. They are very resistant to the high doses of radiation present and are very selective ion exchangers, able to remove dangerous isotopes which may be present in only minute concentrations.

      iii. Removal of ammonia and ammonium ions from waste water.
      Zeolites can reduce the concentration by over 90%.

      iv. Zeolites are used in modern detergents as 'builders'.
      They remove calcium and magnesium ions from the washing water to prevent their precipitation by surfactants.


  4. Heat Storage - Unlike other methods of storing solar heat (i.e. Water tanks), containers for zeolites do not require insulation. When heated, zeolites release water vapour. As long as they staydry, they can store large amounts of heat for months. When zeolites are allowed to absorb moisture, they release their stored heat. However, because of the relatively high cost, zeolite heat storage is not yet available for residential use.