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Aluminosilicates

Discussion Questions

  • What are aluminosilicates?
  • What are zeolites?
  • What are some of the applications of zeolites?

Aluminum Silicates

While alumina, Al2O3, and silica, SiO2, are two most abundant minerals of the earth crust, a the class of minerals containing  both silicon and aluminum oxides, aluminum silicates, exist. Many minerals contain aluminum and silicon oxides. For example, three minerals andalusite, sillimanite, and kayanite all have the same chemical composition of Al2O(SIO4). Topaz is closely related in chemical composition, Al2O(SIO4)(OH,F)2.

Beryl, Be3Al2(Si6O18, contains rings of (SiO3)6 type. It is usually found in cavities of granite. This mineral is also known as aquamarine, a precious stone. This is the main source of Be metal. A closely related mineral is cordierite, Al3(Mg, Fe)2(SI5AlO18).

Layer aluminum silicates such as kaolinite goup of minerals of Al4(Si4O10)(OH)8 is resulted from weathering of feldspar group of minerals, (K, Na)(AlSi3O8, or Ca(Al2Si2O8. The koalinite group is an important clay mineral. The picture shown here is the structure of kaolinite. Two layers are shown here, the bottom octahedral layer represent the oxygen atoms and the 6-coordinated Al atoms or ions. Some of these oxygen atoms are shared with 4-coordionated silicon on the top layer, each tetrahedron represent a SiO4 group. Zeolites are the most important group of aluminum silicates. About 40 natural zeolites are known and 150 zeolites have been synthesized. Jadeite is often confused with nephrite. These are aluminum silicates.

Aluminosilicates?

Aluminosilicate minerals are silicates when some of the \(Si^{4+}\) ions in silicates are replaced by \(Al^{3+}\) ions. For each \(Si^{4+}\) ion replaced by an \(Al^{3+}\), the charge must be balanced by having other positive ions such as Na+, K+, and \(Ca^{2+}) ions. Feldspar group and zeolites are typical aluminosilicates by this definition. Examples of minerals belong to the feldspar group include:

  • Sanidine, [(K,Na)AlSi3O8]4
  • Orthoclase, [(K, Na)AlSi3O8]4
  • Albite, [NaAlSi3O8]4
  • Anorthite, Ca[Al2Si2O8]

The alkali ions are held in place to balance the charges due to the presence of \(Al^{3+}\) ions instead of \(Si^{4+}\) ions. The \(Al^{3+}\) ions seem replace \(Si^{4+}\) ions in the chains of corner shared tetrahedra of \(SiO_4\) groups. However, the bonding between Al and Si can be different. Silicon atoms or ion tend to be bonded to 4 oxygen atoms in a tetrahedral fashion, but aluminum ions tend to be bonded to 6 oxygen atoms in an octahedral fashion.

Zeolites

Zeolite is a class of hydrated aluminosilicates found in certain volcanic rocks. Geologists thought these minerals were interesting because they consist of large cage-like structures with open channel ways. Today, these materials are highly valued for their applications.

Jaujasite is a representative zeolite. This link represented its structure using large polyhedra. All lines in the structure shown are oxygen bridges -O-, the oxygen atom is located not on the line but somewhere next to it. The angle -O- of the two bonds is about 110ø. Points where lines meet are the locations of Si4+ that are surrounded tetrahedrally by four oxygen atoms. They can be substituted by Al3+. As a result, the frame work is negatively charged. Thus, zeolites can trap positive ions: H+, Na+, K+, Ca2+, Cu2+ or Mg2+.

Figure 1: Crystal structure of  Faujasite. Image used with permission of Hermann Luyken via wikipedia.

The name 'zeolite' is said to have its origin in the two Greek words zeo and lithos which mean 'to boil' and 'a stone'. The phenomena of melting and boiling at the same time is a novel property. The name 'zeolite' was first used by the Swedish mineralogist Cronstedt to describe stilbite, the first recognized mineral zeolite, which was discovered in 1756. Over 100 years later, the reversible desorption/adsorption of water in this mineral was recognized.

The faujasite framework consists of "sodalite" cages which are connected through hexagonal prisms (Figure 1). The pores are arranged perpendicular to each other. The pore, which is formed by a 12-membered ring, has a relatively large diameter of 7.4 Å. The inner cavity has a diameter of 12 Å and is surrounded by 10 sodalite cages (Figure 2).

Figure 2: The structure of an individual cage of  Faujasite. Image used with permission of wikipedia.

While you are admiring this beautiful picture of faujasite, remember that the oxygen atoms have two unshared electron pairs in addition to the (Al,Si)-O-Si(or Al) bonds. Thus the oxygen atoms are sites to interact with positive site of molecules that passes by these structures. At present over 150 synthetic zeolites & zeotypes and 40 natural zeolites are known. Synthesis of zeolite is a very active field of study.

Aluminosilicates have three major minerals: Andalusite, sillimanite, and kyanite. Zeochem has been developing and manufacturing molecular sieve adsorbents since 1977. Simply put, their adsorbents are used to "screen" out impurities from a variety of applications by attracting and trapping the targeted contaminants. For example, in natural gas processing, molecular sieves are used to remove specific molecules from the gas stream to allow for more efficient downstream processing. Faujasite is a typical zeolite.

Applications of Zeolites?

As you have read above that there are many different kinds of zeolites, each with a definite structure and associate with it are unique properties. In terms of applications, we are assuming zeolites as porous aluminosilicates with large tunnels and cages for a fluid (gas and liquid) to pass through. The applications are based on the interactions between the fluid phase and the atoms or ions of the zeolites. In general terms, zeolites have many applications:

  1. As selective and strong adsorbers: remove toxic material, selective concentrate a particular chemical, as Molecular Sieve. This link will be a very good to discuss zeolites. Currently, the site is under construction, but it has a very good framework. Even many deorderants are zeolite type.
  2. As selective ion exchangers: for example used in water softener.
  3. Superb solid acid catalysts, when the cations are protons H+. As catalysts, their environmental advantages include decreased corrosion, improved handling, decreased environmentally toxic waste and minimal undesirable byproducts.
  4. As builder: a material that enhance or protecting the cleaning power of a detergent. Sodium aluminosilicate is an ion exchange builder often used in laundry detergent as a builder. A builder inactive the hardness of water by either keeping calcium ions in solution, by precipitation, or by ion exchange.

Example 1

How much zeolite A in its proton, \(H^+\), form would be needed to soften completely a cubic meter of water, if the hardness of the water was equivalent to 123 ppm of dissolved \(CaCO_3\). Assume 80% of the protons will be used in this exchange.

SOLUTION
We have discussed the solution of this problem type in class. The formula for the proton form zeorlite A is \(H_{12}[(AlO_2)_{12}(SiO_2)_{12}] \cdot 27H_2O\), and its formula (molecular) weight is

\[(1+27+32+28+32)*12+27*18 = 1,926\; g/mol\]

Note the following:

The formula mass of \(CaCO_3\)

\[(40+12+48) = 100\; g/mol\]

1 m3 H2O = 103 L H2O = 106 mL H2O = 106 g H2O
123 ppm CaCO3 = 123 g per 106 g of water.

            1 mol CaCO3   2 mol H+  1 mol z-A  1926 g z-A 100
123 g CaCO3 ----------- ----------- ---------- ---------- ---
            100 g CaCO3 1 mol CaCO3 12 mol H+  1 mol z-A   80

= 494 g zeolite A

That 80 % of protons of the zeolite A is used means that we require a little more zeolite A than stoichiometric quantities.

DISCUSSION
Zeolites are aluminosilicates with open frames strcutures discussed above. Replacement of each Si atom by an Al atom in silicates results in having an extra negative charge on the frame. These charges must be balanced by trapping positive ions: H+, Na K+, Ca2+, Cu2+ or Mg2+. Water molecules are also trapped in the frame work of zeolites.

In this example, we assume that when we soak the zeolite in water containing Ca2+, and Mg2+ ions, these ions are more attrative to the zeolite than the small, singly charged protons. We further assumed that 80 percent of the protons in zeolite are replaced by other ions.

Example 2

A water conditioner contains 10 kg of zeolite A in its sodium, Na+, form. Assume that 80% of the sodium ions have been replaced by divalent ions, Ca2+, Cu2+ or Mg2+. In the regeneration process, only 20% of the sodium ions are used to replace the divalent ions. How much sodium salts is required in the regeneration process?

SOLUTION
The molar mass of Na-zeolite A is

(23+27+32+28+32)*12+27*18 = 2190 g/mol = 2.19 kg/mol,

and the molar mass of NaCl is

58.5 g/mol.

            1 mol  0.8*12 mol NaCl 100 58.5 g NaCl
10 kg z-A -------- --------------- --- -----------
          2.190 kg   1 mol z-A      20  1 mol NaCl

   = 12822 g NaCl = 12.8 kg NaCl

DISCUSSION
How much salt is required if 60% of the sodium ions are effectively used to replace all the divalent ions?

Questions

  1. What is the preferred coordination around the Si4+ ions in aluminosilicates?

    Skill - Describe the bonding of silicon from its electronic configurations (atomic orbitals).

  2. The Si4+ ions in silica are replaced by what ions to form aluminosilicates?

    Skill - Describe the formation of aluminosilicates.

  3. For every 10 Si4+ ion replaced by 10 Al3+ ions, how many Ca2+ ions are required to balance the charge in silicates?

    Skill - Apply the principle of charge conservation to the chemical problems.

  4. Is the cage-like framework of zeolite charged? If so, what type, positive or negative and why?

    Skill - Explain zeolite properties in based on electronic structures of atoms.

  5. The truncated octahedral structure known as the sodalite cage is one of the basic units for zeolite. Every vertex of the truncated octahedron represent a Si (or Al) atom. How many such atoms are there in a sodalite cage?

    Skill - Describe the structure of the sodalite cage.

  6. The truncated octahedral structure known as the sodalite cage is one of the basic units for zeolite. Every line of the cage represent a -O- linkage. How many oxygen atoms are there in a sodalite cage?

    Skill - Workout the stoichiometry of some zeolite.

  7. The mineral sodalite has a formula, Nax[(AlO2)6(SiO2)6]Cl2 what is the value of x?

    Skill - Determine the stoichiometry by balancing the charge.

Solutions

  1. Tetrahedrally coordinated to 4 oxygen atoms.
  2. Al(III) atoms.
  3. Five calcium ion will be trapped.
  4. The zeolite frameworks are negatively charged.
  5. 24 Si atoms
  6. 36
  7. 8

Contributors