I. A molecular dimer of aluminum chloride in gas phase has the structure: [VISUAL] II. Aluminum chlo — Analytical Chemistry Chemistry Question
Structure and synthesis of Al-Keggin ion
I. A molecular dimer of aluminum chloride in gas phase has the structure:
[VISUAL]
II. Aluminum chloride dissociates in basic solution yielding several Al-polycations. A typical Al-Keggin ion with the molecular formula of [Al13O28H24.12H2O]Cln was formed at a hydrolysis ratio [OH-] / [Al3+] of 1.5 to 2.5. It is only composed of tetrahedral and octahedral Al cations. The 27Al NMR spectrum of an Al13 ion is shown below.
[VISUAL]
The sharp signal at 64 ppm is due to the very symmetrical environment of the corresponding Al atom(s) in the Keggin cation.
III. Al13-Keggin ions have recently been prepared by solid - solid interaction referred to as mechanochemical synthesis [J. Catal. 245 (2007) 346; Inorg. Chem. Commun. 11 (2008) 1125]. A mixture of (NH4)2CO3/[Al(H2O)6]Cl3 was blended together in a silicon carbide crucible (with the inner volume of 17 cm3) in the presence of three hardened silicon carbide balls (with the radius of 0.542 cm) in atmospheric condition (25 oC, 1 atm). The milling was kept for a period of time at 25 oC until the pressure gauge remains at constant value of 2.50 atm (Hint: The volume of solids is negligible).
What is the hybridization of the Al atoms in a dimer?
Model Answer
Al2Cl6 has sp3 hybridization
Determine the distance between two Al atoms.
Model Answer
Al – Al = 2(Al – Cl) cos(39.5) = 2 × 221 pm × 0.772 = 325.63 pm
Determine the absolute value of (n) in the Al13 Keggin ion.
Model Answer
n = 7+
Assign 27Al NMR signals in the spectrum to the appropriate Al cation(s) in the right figure. [VISUAL]
Model Answer
The Aloctahedral / Altetrahedral is estimated ~ 12/1. The center Al atom at number 7 is tetrahedral; the other atoms are octahedral.
In an Al13-Keggin ion, Al tetrahedron(s) is(are) only linked with other Al atoms by the oxygen bridge. Propose the structural formula for the Al13-Keggin ion. [VISUAL]
Model Answer
The Al-Keggin cation structure is composed of one Altetrahedral cation surrounded by four oxygens. This Al atom is located centrally and caged by 12 octahedral AlO6 units linked to one another by the neighboring oxygen atoms. The formula can be expressed as (AlO4Al12(OH)24(H2O)12)7+.
Determine the number of oxygen atoms bridging adjacent octahedra.
Model Answer
There are a total of 24 bridging oxygen atoms that link the 12 adjacent atoms.
Write down the overall equation to prepare Al-Keggin ions from the reaction between NaOH and AlCl3 solution.
Model Answer
13 AlCl3 + 32 NaOH + 8 H2O = [(AlO4Al12(OH)24(H2O)12)7+]Cl7 + 32 NaCl
Write down the overall reaction between aluminum hexahydrate chloride and ammonium carbonate to yield the Al13 -Keggin ions.
Model Answer
13 Al [(H2O)6]Cl3 + 16 (NH4)2CO3 + 24 H2O = [(AlO4Al12(OH)24(H2O)12)]Cl7 + 32 NH4Cl + 16 CO2 ↑ + 54 H2O
Determine the number of molecular Al13-Keggin cations in the crucible.
Model Answer
Volume of a ball = (4/3) π r3 = (4/3)× 3.14 × (0.542)3 = 0.667 cm3.
Volume of 3 balls = 3 × 0.667 = 2.00 cm3.
Inner volume of crucible = 15 cm3
Volume of gas = 13 – 2 = 13 cm3.
P V = n R T → n = 1 × 13 × 10-3 / 0.082 × 298 = 5.32 × 10-4 mol.
After reaction:
gases before CO2 = +n n n∑
P V = ngases R T → n = (2.50 atm) ×(13.10-3 L) / (0.082 L.atm.mol-1.K-1) × (298 K) = 1.33 × 10-3 mol
CO2 = n_after - n_before = 1.33 × 10-3 - 5.23 × 10-4 = 8 × 10-4 mol.
Amount of substance of Al13-Keggin cation = 8 ⋅ 10-4 / 16 = 5 ⋅ 10-5 mol.
Number of Al13-Keggin cations = 5 ⋅ 10-5 mol × 6.023 ⋅ 1023 = 3 ⋅ 1019 ionic molecules