From WolfWikis

Zeolites

Figure 1: A β-cage, common in many zeolite structures and describable as a truncated octahedron. O has been omitted, and the vertices represent Al/Si.

Zeolites are porous aluminosilicate materials that can be described by the general formula:

               An[AlnSim-nO2m].xH2O

where A is an extraframework cation of valence +1 or +2. The extraframework cations usually serves as templates for synthesis and may also be readily exchanged. The framework in a zeolite structure refers to the connectivity of the Si or Al atoms. The ratio of Si/Al in a zeolite sample is important. The minimum Si/Al ratio is 1 due to the Loewenstein rule which states that there can be no Al-O-Al bonding in a zeolite sample[1]. The open porous nature of the zeolite structure is one of its most important properties. Open pores and voids within the structure provide places for mobile cations and water to reside. A few examples of the open nature of the zeolite structure are shown in Figure 2.

Figure 2: Crystal structures of ITA, CHA, and MFI from left to right.

The zeolite framework consists of cross-linked TO₄ tetrahedra where T is Al or Si. Each T atoms occupy four connected vertices of a three dimensional network and the oxygen occupy two connected positions between the four connected vertices[1]. Having been derived from silicate type network structures, the O/T ratio in a zeolite structure is always equal to 2[1]. The O-T-O bond angle (α in Figure 1) is close to the ideal tetrahedra bond angle of 109.5°. The T-O-T bond angle (β in Figure 1) is much more flexible than the O-T-O bond angle and is usually around 140° to 165°[1].

Figure 3: TO₄ tetrahedra where α is the O-T-O bond angle and β is the T-O-T bond angle

The TO₄ tetrahedra are often referred to as the primary building units of zeolite structures. Primary building units are linked together to form secondary building units. The secondary building units consist of n-ring structures which can contain as many as 20 tetrahedra and as little as 4[2]. This is shown schematically in Figure 4. Each corner in the secondary building units represents the center of a tetrahedra.

Figure 4: The secondary building units of zeolite structures[4].

Secondary building units can be linked to form cages or channels within the structure. Connecting rings of different sizes leads to many different structures. The notation nⁱmⁱ is often used to describe the number and type of rings that make up a cage within a structure. An example of the notation is shown in Figure 5 with the cancrinite cage structure. The cancrinite cage structure is described with the notation 4⁶6⁵ which means that it is composed of six 4-rings and five 6-rings.

Figure 5: Cancrinite cage structure built from six 4-ring and five 6-ring secondary building units.

The aluminosilicate cages and n-rings connect to form a three dimensional net type structure like those shown in Figure 2. Void spaces in these nets can be classified as pores and channels, were channels describe pores that extend infinitely in one direction [1]. Extraframework cations reside within the pores and channels of the zeolite structure to balance the negative charge of the aluminosilicate framework[8]. These cations can be readily exchanged for cations of the same charge. The exchange properties of zeolites have many commercial uses such as builder agents in detergents that soften the water by removing calcium ions, which lets the detergents work more effectively[4].

Zeolites can also be used in acid catalyzed reactions where the extrframework cation is exchanged for a proton. The acid stability of a particular zeolite structure can be determined from its Si/Al ratio[1]. Zeolites with an Si/Al ratio equal to about 1 are not stable in acidic solutions with a pH lower than 4 while zeolites with a Si/Al ratio greater than 5 are have a higher acid resistance and are stable[1]. An example of a zeolite structure with a high Si/Al ratio used in acid catalyzed reactions is ZSM-5 (shown in Figure 2 as MFI structure type)[13].

For more information on zeolite structures visit the IZA Structure Database.

SOD "Sodalite"

Figure 12: The sodalite structure, where the vertices of teal polyhedrons (β-cages) are composed of Al/Si. Oxygen and guest atoms have been omitted.

Sodalite, a natural product, is the simplest zeolite. As seen in Figure 12, its structure [13] consists of a body centered cubic arrangement of β-cages (see Figure 1). Like all zeolites, the aluminosilicate framework is capable of ion exchange via intercalation. This intercalation takes place through 6-ring windows between any particular β-cage and one of its eight nearest-neighbor cages.

There has been recent interest in the strudy sodalite with guest clusters of semiconducting material, such as InAs or GaN, within the cages. It was expected that quantum confinement should dictate the size of the bandgap, i.e. the smaller the cage, the larger the gap.

Calculations [14] demonstrated that confinement is not the primary factor in bandgap size; Ga or In tends to bond to oxygen in the aluminosilicate framework, yielding bandgaps largely dictated by chemistry between the host and guest. More extensive calculations [15] which varied the cluster size, while holding type of guest constant, suggest the most significant factor was strain on the semiconducting guest. As researchers attempted to fill the cages with larger clusters, the bandgap increased in analogy to similar behavior observed upon applying pressure to the bulk semiconductor in question.

While sodalite is ideal for computational modelling due to the small size of the unit cell, it is most likely not the ideal zeolite for this type of application. It will be of interest to follow the progress of semiconducting nanostructures built around zeolite templates.

LTA "Zeolite Type A"

Zeolite Type A is quite common, and derives its name from widespread use as a molecular sieve. The structure [13] can be viewed, as in Figure 13, as a primitive cubic arrangement of β-cages which pseudo-cornershare (from the perspective of β-cages as truncated octahedra) through 4-4 structural units. Alternatively, one can view Zeolite Type A as a primitive arrangement of larger polyhedra, termed α-cages. β-cages connect to α-cages via shared 6-rings, while the α-cages interconnect with each other through shared 8-rings.

Recent and interesting research on Zeolite Type A involves in-situ X-Ray diffraction during crystallization [16]. It was suggested that the surface of interest for studying crystallization dynamics involved the interface between precursor zeolite gel and the nucleated crystal.

Figure 13: The structure of Zeolite Type A as a network of β-cages, (teal) which interconnect via 4-4 units (maroon). The center of each β-cage resides on the corner of the crystallographic cell.

Figure 14: A 2x2x1 supercell of Zeolite Type A, depicted as a network of α-cages. One can see the inverse surface of a β-cage in the middle of the figure.

FAU "Faujasite"

Fig 15: The structure of Faujasite as a diamond-like network of β-cages which connect via 6-6 units. The inverse structure around the beta cages, again a diamond network, is the three-dimensional network of channels.

Faujasite, a naturally occurring zeolite, is a common structure for molecular sieves. In particular, Linde Type-X and Type-Y sieves both share this structure, where the latter is Si rich and thus can incorporate fewer guest cations. The structure [13] can be viewed in Figure 15 as a diamond-like lattice of β-cages which connect via 6-6 structural units.

Faujasite could alternatively be described by a network of much larger cages (when compared to either α- or β-cages) which do not lend themselves to polyhedral representation. Due to the size of the pores (12-rings) connecting these larger cages, the vacancies between β-cages are typically described as three-dimensional channels rather than a network of cages. Among the simple zeolites, Faujasite-type zeolites are most frequently seen in industrial settings, such as petroleum cracking, due to the presence of these large channels.

Erionite

Figure 6: Erionite crystal structure where red atoms are Si/Al, Dark blue atoms are Ca, light blue atoms are O.

Erionite (ERI) is a natural zeolite type mineral with the formula ( Ca, Mg, Na₂, K₂)_4.5[( AlO₂)₉(SiO₂)₂₇] • 27H₂O. The structure is described as being composed of columns of cancrinite cages linked by double 6-rings of tetrahedra[7]. The adjacent columns of alternating cancrinite cages and double 6-ring tetrahedra are linked by single 6-ring tetrahedra. This linking forms larger cages parallel to the c-axis known as erionite cages (Figure 6)[1]. The 6-ring stacking sequence along the c-axis of erionite is AABAAC[1]. Often times, erionite is seen inter grown with offretite due to similarities in their stacking sequences[7]. Offretite is a member of the erionite group of minerals. Intergrowths between two zeolite structures are common within zeolite families where particular members of the family differ only in their stacking sequences in one direction[1]. The stacking sequence of erionite, AABAAC and offerite, AABAAB are shown in Figure 7.

Figure 7: Stacking sequence of erionte (top) and offerite (bottom). Cancrinite cages are also shown as alternating in the columns with double 6-rings. Blue represents the AA stacking portion, green represents the B stacking portion and pink represents the pink stacking portion of the structure. Picture adapted from "Zeolites", Williams, C.

Three different types of erionite structures have been identified, erionite-K, erionite-Ca, and erionite-Na[7]. The difference between the structures has to do with its most abundant extra framework cation. Erionite's large 8-ring channel openings parallel to the c-axis are shared by two erionite cages. The possible cation extra framework sites in the erionite structure are shown in Figures 4 and 6 as the blue Ca cations. The Ca and Mg cations in each erionite cage that share the large 8-ring opening must be equal in order to fulfill the charge balance in the structure[7]. Each cancrinite cage is occupied by one K atom in all of the erionite structure types.

Figure 8: Crystal structure of erionite looking down the C axis.

The name erionite comes from the Greek word for wool because of its wool fiber like appearance[5]. Erionite naturally occurs in parts of Europe and the western United States[6]. Erionite has recently been classified as the most carcinogenic fiber known to man and has been described as being more carcinogenic than asbestos[5]. Similar to asbestos, erionite causes mesothelioma in certain individuals.

The chemistry involved with the development of mesothelioma after inhaling erionite fibers involves the incorporation of iron into the crystal structure via ion exchange once in the lungs. The iron initially acquired by proteins forms an erionite-Fe³⁺ complex[5]. This complex is then reduced to an erionite-Fe²⁺ complex by antioxidants in the lungs[5]. This leads to the production of hydroxyl radicals which are toxic in the body.

Gismondine - CaAl₂Si₂O₈·4H₂O

Gismondine is a zeolite structure with monoclinic P2₁/c symmetry. It was named after an Italian mineralogist Carlo Giuseppe Gismondi (1762-1824). It is one of 17 structure types formed by “double crankshafts” of 4-membered rings. The double crankshaft structure is shown in Figure 9. 4-membered rings of AlO₄ and SiO₄ tetrahedra in the a-and c-directions are linked to form 8-membered rings. Figure 10 and 11 show these rings. These 8-ring openings have a diameter of ~3.0x4.5Å. Ca²⁺ ions that occupy this cavity coordinate to framework oxygen atoms, and are surrounded by three water molecules, and two partially filled water sites, giving a total of 4 water molecules per unit cell [11]. The unit cell contains 2 AlO_4/2 tetrahedra and 2 SiO_4/2 tetrahedra. Gismondine not only describes this specific structure, but also describes a family of zeolite structures with this basic framework. Within this family are zeolite structures with cations other than Ca²⁺ in the 8-ring cavity. One of the most commonly studied variations of the gismondine structure involves dehydration.

After dehydrating for one hour in vacuum, symmetry and structure are very similar, but slightly decrease in volume (0.6%). Water sites and full/partial occupation were very similar. Dehydration for 24 hours causes much larger deformation in the framework and a decrease in volume of 17%. The unit cell after 24 hours of dehydration must double in size due to deformation to contain twice the AlO₄ tetrahedra and SiO₄ tetrahedra and twice as many Ca²⁺. The deformation causes the "double crankshaft" chains to zigzag. One chain is rotated from the normal gismondine and alternately parallel to the a- and c- axes. The channel system becomes severely squashed and decreases in size with dehydration [12].

Figure 9: Gismondine double crankshaft.

Figure 10: Gismondine ring structure viewed down a axis.

Figure 11: Gismondine ring structure viewed down c axis.

Other "double crankshaft" zeolites include amicite, garronite, gobbinsite, and gismondine derivates with cations other than Ca²⁺ in the 8-ring cavity.

References

[1] Lobo, R. F., "Handbook of Zeolite Science and Technology" Marcel Dekker Inc. 2003.

[2] Williams, C.D. Zeolites, University of Wolverhampton.

[3] <http://www.ch.ic.ac.uk/vchemlib/course/zeolite/structure.html> Accessed 5/2/07

[4] Proctor & Gamble, <http://www.scienceinthebox.com/en_UK/glossary/builders_en.html> Accessed 5/2/07.

[5] Ruda, T. A., Dutta, P. K.,Environ. Sci. Technol. 39, 6147-6152, 2005.

[6] Carbone, M., et.al, Nature Reviews: Cancer, 7, 147-154, 2007.

[7] Alberti, A., et.al., Zeolites, 19, 349-352, 1997.

[8] Rao, C. N. R.; Gopalakrishnan, J. New Directions in Solid State Chemistry, 1997, Cambridge University Press, 41-45.

[9] Cheetham, A. K.; Ferey, G.; Loiseau, T. Angew. Chem. Int. Ed., 1999, 38, 3268-3292.

[10] IZA structure database <http://www.iza-structure.org/databases/>

[11] Lewis, Dewi W. et al. Molecular Simulations, 2002, 28, 649.

[12] Vezzalini, Giovanna and Simona Quartieri. Zeolites, 1993, 13, 34.

[13] W.M. Meier, D.H. Olson and Ch. Baerlocher, "Atlas of Zeolite Structure Types", Elsevier Science (2001)

[14] A. Trave, F. Buda, A. Fasolino, Phys Rev Lett 77 27 (1996) 5405

[15] F. Buda, A. Fasolino, Phys Rev B 60 8 (1999) 6131

[16] R. Grizetti, G. Artioli, "Micropor. and Mesopor. Mater." 54 (2002) 105