Zeolite A

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Structural formula
General
Surname	Zeolite A
other names	Molecular sieve A LTA (linden type A) MS 4A Sasil ® ZEOLITE ( INCI )
Molecular formula	Na 12 ((AlO 2 ) 12 (SiO 2 ) 12 ) 27H 2 O
Brief description	colorless solid
External identifiers / databases
CAS number 1318-02-1 EC number 215-283-8 ECHA InfoCard 100,013,895 Wikidata Q191323
properties
Molar mass	2191.05 g mol −1
Physical state	firmly
solubility	almost insoluble in water
safety instructions
GHS labeling of hazardous substances no GHS pictograms H and P phrases H: no H-phrases P: no P-phrases
Toxicological data	> 5110 mg kg −1 ( LD 50 ,  rat ,  oral )
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Zeolite A is a synthetic, colorless, crystalline aluminosilicate and in its hydrated sodium form has the empirical formula Na ₁₂ ((AlO ₂ ) ₁₂ (SiO ₂ ) ₁₂ ) · 27 H ₂ O. Various variants of this are hidden under the collective name zeolite A Connection. They all have the same aluminosilicate lattice, but instead of sodium ions contain other ions such as potassium or calcium. Since the structure of the zeolite lattice is independent of the water content, the anhydrous forms are also counted as zeolite A. Zeolite A belongs to the group of zeolites , but is not a mineral .

Zeolite A is used in its sodium form as a softening agent , in its dehydrated (= dehydrated, activated) forms as a drying agent. It is used in detergents as a softener, in laboratories as a dehydrating agent for solvents, for drying gases (e.g. air or natural gases) and is used for the technical separation of straight-chain and branched alkanes . Zeolite A's wide utility has made this chemical a mass chemical. Several hundred thousand tons are produced every year.

structure

introduction

Fig. 1: Structure of zeolite A.

A zeolite has a framework structure of AlO ₄ - and SiO ₄ - tetrahedra . They form a covalent lattice with cavities that usually contain water. This lattice is a huge anion. The cavities therefore contain a corresponding number of cations, which are dissolved in the inner water or coordinated on the inner walls. Figure 1 shows a representative section of the lattice structure ( unit cell ). In the next section, the unit cell is built up in a few thought steps in order to be able to understand the usual form of presentation of the unit cells as well as the physical and chemical properties.

Structural consideration

The anionic lattice

Fig. 2: Sodalite cage

Fig. 3: Sodalite cage with Al and Si

Fig. 4: The alpha cage (highlighted in brown, in the middle of the picture)

Fig. 5: Zeolite A: a huge multi-anion

A zeolite has a framework structure of AlO ₄ - and SiO ₄ - tetrahedra in the ratio 1: 1. Here, in the alternating aluminum - and silicon -atoms each other by oxygen atoms connected. Since the tetrahedra share an oxygen atom at each point of connection, there are 2 oxygen atoms on each Al and Si atom. A balance of the oxidation states shows that every Al atom of the covalent structure generates a negative charge. This charge is generated by ionic bonds with cations such as B. Na ⁺ balanced. (Oxidation states: Si +4, Al +3 and O −2; SiO ₂ = 0; AlO ₂ = −1).

The connection of the Al and Si tetrahedra leads to a three-dimensional structure. A multitude of structures can be formed from tetrahedra, which explains the large number of different zeolites. In the case of zeolite A, the so-called sodalite cage (Fig. 2) is formed. Figure 3 shows the positions of Al and Si. Oxygen atoms are not shown here. One oxygen is located near the red lines drawn between the atoms. This illustration shows only three bonding directions of the Al and Si tetrahedra. The fourth direction allows a link with other sodalite cages. Fig. 1 shows the connection of eight cages, as they are in Zeolite A. When the sodalite cages are linked, a new cage is formed in the center, which is called the alpha cage. In Figure 4 this cage is shown more clearly raised. The unit cell represents only a small section of the zeolite. Figure 5 shows eight linked unit cells. If many unit cells are placed next to one another, a microscopic, cube-shaped crystal is formed. Each crystal is a single huge multi-anion.

The cations

The charge of the multi-anion is balanced by an ionic bond to form cations. In an anhydrous zeolite, the cations occupy very fixed coordination positions in the vicinity of the Si _n Al _n O _2n rings. Figure 6 shows the position of a cation on the 12-ring of the sodalite cage. In the case of zeolite A with singly charged cations, the number of 12 rings is insufficient. The large rings of the alpha cage must also be used. Cations at this position block part of this large opening. With doubly charged cations, the number of 12 rings is sufficient. The ring of the alpha cage remains unused. This occupancy or non-occupancy of the alpha cage and the size of the cations is decisive for the use of zeolite A as a molecular sieve (see below).

If the zeolite contains water, the cations are very mobile. They are (almost) dissolved in the water and can therefore be exchanged for other cations. This property is crucial for use as a water softener (see below). Here, 2 Na ^{+ is} exchanged for one Ca ²⁺ , since Ca ^{2+ has} a higher affinity for the zeolite than the Na ⁺ cation.

If sodium zeolite A is suspended in pure water, the pH value of the water rises to pH = 10 to 10.5 (with 3 g in 100 ml). The zeolite added Na ⁺ to the water and absorbed H ⁺ ions in return. The H ⁺ ions were previously part of H ₂ O. OH ^- remains, the pH has risen. This reaction is also an ion exchange of the zeolite.

Fig. 6: Sodalite cage with Al, Si, O and Me ⁺

The water

The water in zeolite A is in the sodalite and alpha cages. The water molecules are (almost) freely mobile, but interact strongly with the cations and the anionic zeolite lattice. The vapor pressure of the water in the zeolite is therefore significantly lower than that of free water (see capillary effect ). The water can be removed from the zeolite by heating without changing or destroying the lattice structure. When the zeolite is heated, it appears that the solid is boiling. In the process, gaseous water flows out of the solid and whirls up the crystals ( boiling stones ).

synthesis

Fig. 7: Electron microscope picture of zeolite A crystals

The synthesis can be carried out from a mixture of aluminum and silicon hydroxides in sodium hydroxide solution at temperatures between 50 and 90 degrees Celsius. The hydroxides can be freshly prepared separately from aluminum powder and tetraethylorthosilicate using sodium hydroxide solution and then mixed. An excess of aluminum is advantageous.

${\displaystyle \mathrm {12\ Al(OH)_{3}+12\ Si(OH)_{4}+12\ NaOH\longrightarrow Na_{12}((AlO_{2})_{12}(SiO_{2})_{12})+48\ H_{2}O} }$

The crystals can be seen in Figure 7 and are usually between 0.5 and 10 µm in size.

use

Desiccant

Zeolite A as a desiccant : The alpha cage and the sodalite cage have enough free internal volume and sufficiently large ring openings (also called pores or channels) to absorb or release water molecules. If the zeolite does not contain any water, there is a high tendency to absorb water. The uptake is an exothermic reaction and leads to the hydration of the cations and the anionic framework structure. The water can easily be removed reversibly under vacuum and at elevated temperature . Zeolite A is available in the form of small spheres, which are formed from the small crystals and binders. They can be dried with little dust or separated by decanting from a solvent that has been dried with it.

Water softening: ion exchange

Zeolite sodium-A as a water softener: In the hydrated form of the zeolite, the cations in the cavities are similar to those in a solution. You are mobile and not tied to one place. They can be exchanged for other cations when the zeolite is suspended in water. Ca ²⁺ is preferred. In doing so, 2 Na ⁺ ions are released to maintain the charge balance ( ion exchange ). In the aqueous phase (= in the water) there is no longer any free Ca ²⁺ available. B. in detergents lead to the formation of insoluble lime soaps . Since the exchange is an equilibrium reaction , the Ca ^{2+ can be} removed from the zeolite by Na ⁺ if Na ^{+ is offered} in high concentration in the aqueous phase (regeneration).

Zeolite A is the common substitute for phosphates in laundry detergents, is considered environmentally neutral and is non-toxic. It was first introduced at Henkel by Milan Schwuger and Heinz-Gerd Smolka in 1972, and the first phosphate-free detergents came onto the market in 1977. Its disadvantage is that it occurs in sewage sludge. Since a mineral is fundamentally non-biodegradable, it increases the amount of sludge left behind.

Selectivity in ion exchange:

Ag ⁺ > Na ⁺ > K ⁺ > NH ₄ ⁺ > Li ⁺

Ba ²⁺ > Ca ²⁺ > Cd ²⁺ > Mg ²⁺

Molecular sieve

Zeolite A as a molecular sieve : Technical separation of straight-chain and branched alkanes with anhydrous zeolite A. Straight-chain ( n -) alkanes can penetrate the zeolite through the opening of the alpha cage. Branched ( iso ) alkanes are bulkier and are not absorbed. n -alkanes are, so to speak, screened out. The size of the opening in the alpha cage is not only determined by the fixed ring size, but is also influenced by the cations of the zeolite. In the case of Na-A in anhydrous zeolite, Na ⁺ ions use this space as a very firm place of attachment, since there is no water to dissolve. The opening seems to have gotten smaller. In the case of Ca-A, the Ca ²⁺ ions do not use this space. The full ring opening is available. The available ring opening in Ångström gives the molecular sieves (MS) their name: MS 5A (calcium form), MS 4A (sodium form), MS 3A (potassium form).

Type	Adsorbed (= absorbed) substances	Applications
3A	H 2 O, NH 3	Drying of methanol, ethanol and gases (e.g. H 2 )
4A	CO 2 , H 2 S, ethene, ethane (plus compounds mentioned above in the table)	Standard for drying polar and non-polar solvents, drying saturated, gaseous hydrocarbons
5A	n -alkanes, n -propanol, n -butanol (plus compounds mentioned above in the table)	Sorption of alkanethiols and H 2 S from natural gases, separation of n -alkanes in aromatics and iso- alkanes .

Biocide depot

Biocide deposit silver zeolite A: By exchanging ionswith Ag ⁺ , silver ions can easily be stored in the zeolite. Silver is very preferred to be inserted. When used as an additive in surface coatings, silver is released in low doses. Silver ions have biocidal properties (see oligodynamics ).

Web links

• The International Zeolite Association (IZA): Structural data for zeolites
• School experiment: ion exchange through zeolite A ( Memento from June 17, 2008 in the Internet Archive )

Individual evidence

1. ↑ entry to ZEOLITE in the CosIng database of the European Commission, accessed on February 26 2020th
2. ↑ ^{a b c} Datasheet Zeolite A (PDF) from Merck , accessed on January 19, 2011.
3. ^ Entry on zeolite in the GESTIS material database of the IFA , accessed on December 14, 2012(JavaScript required) .