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Aluminosilicates

  • Page ID
    39419
  • 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

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