1. BASIC PRINCIPLES OF PILLARED CLAYS
1.2.5. Polyoxycations
Polyoxycations of various metals are by far the most popular pillaring species that are described in PILC literature. Their success should be attributed to the following properties: (1) most of them are easy to prepare in a reproducible way; (2) they carry a positive charge; (3) they exchange well with the Na+ ions between the clay sheets; (4) they are readily converted by heating into very stable metal oxide pillars; (5) they tightly connect adjacent clay layers to each other; (6) they are large and give good interlayer distances.
First of all, alumina- and iron-pillaring species will be presented in detail, because this research work was focused these kind of polyoxycations. Subsequently, a review about the other kind of polyoxycations will be given.
1.2.5.1. The pillaring precursor [Al13]7+
The intercalation of clays with polyoxycations of Al3+ was not only one of the first inorganic PILCs, it is also the most investigated PILC till now. The main pillaring agent that is responsible for the 10 Å interlayer distance in alumina pillared clays, is the [AlO4Al12(OH)24+x(H2O)12-x](7-x)+ (briefly Al13).
Figure 1.6. Line Drawing of the [Al13O4(OH)24(H2O)12]7+ Keggin ion
This polyoxycations is the result of a controlled hydrolysis of Al3+ salts. The pH measurements give direct information on the hydrolysis process. The structure of aluminum-polyoxycations was characterized by several methods [66], namely such as NMR, SAXS, Raman Spectroscopy, Light Scattering, Ultracentrifugation and Size Exclusion or Gel Permeation Chromatography. From all these data, the Al13-oligomer was described as a Keggin ion [67, 68], which has the shape of a prolate spheroid, that
consists of one central AlO4 tetrahedron, surrounded by twelve octahedra of aluminum OH groups are assumed). The tetrahedral structure is revealed in the macrostructure of this Keggin ion crystal [69]. Figure 1.7. shows the SEM picture of Al13-crystals.
Although the ideal formula represents a charge of 7+, it may be changed upon hydrolysis of the ion. By modeling the deprotonation reaction with alkalimetric titration data, it was found that the hydrolysis reaction could best be described as a sequence of six two-deprotonation steps. This phenomenon occurs also if the Keggin ions are intercalated between the clay layers [70, 71].
Depending on the preparation procedure, one finds various aluminum monomers or polymers in the solution. In general, five methods are used to prepare hydrolyzed species of [Al(OH)3]n, sometimes described as ring polymers from the basic ring unit [Al6(H2O)12(OH)12]6+ or from the polymerization of Al13 entities [73, 74].
Figure 1.7. SEM photograph of Al13-crystals after Molinard [72]
Figure 1.8. Speciation of 10-2M aluminum for [OH] / [Al] up to 2.5 after Bottero et al.
[73,74]
The controlled hydrolysis of AlCl3 solution with NaOH in the range 1.5 < [OH] / [Al] <
2.3 should result in a solution with Al13-oligomers as the main species (Figure 1.8.).
The exact composition and distribution of the species in solution however, depends on the Al3+ concentration, the temperature and the hydrothermal treatment, the age of the solution and the aging temperature, the source of Al3+ and the preparation method.
Since the charge of the Keggin ion depends on the degree of hydrolysis and therefore the pH, it is possible to charge this value in a controlled way. Evidence for this charge variation was found by Vaughen [75]. Aluminum(III) hydrolysis and precipitation in the presence of acetic acid and oxalic acid have been studied by combining potentiometric titration and liquid-state 27Al NMR [76]. Different aluminum species have thus been identified and followed according to the pH and the L / M (ligand to metal) ratio:
unreacted and hydrolyzed monomers, complexed forms, and the Al13 tridecamers were directry recorded by NMR, and a “solid” species, not detected by liquid-state NMR, has been determined by difference. The level of perturbation of aluminum hydrolysis by organic acids can be related to the strength of complexation. Thus, the speciation depends on differential acid-base affinity between aluminum and carboxylate, and aluminum and hydroxyl. The nature of the “solid” species is hypotetical. 27Al NMR studies of aluminum clusters have one major drawback [77]. At lower field strengths, some aluminum nuclei may not be detected. Al nuclei having large quadrupole coupling constants (QCC), as a result of low coordination symmetries, can give signals broadened beyond detection [78]. Solid state 27Al NMR detects nuclei with high QCCs provided that sufficiently large magnetic field strengths are used. In research work of Parker et al. [77] the species of Al13 tridecamer were examined by performing 27Al NMR on the solutions during aging and by studying the precipitated sulfate salts via solid state 27Al NMR and powder XRD. There was no evidence of M incorporation; only Al13
was found. The XRD patterns are indexable in the cubic F¯ 43m space group. In fact, different polymorphic structures of sulfate salt can be obtained, depending on solution pH.
The pillared clays obtained with the Keggin-ion exhibit Brönsted or Lewis acidity, or both, depending on the nature of the clay. This acidity is believed to be a consequence of the dehydration / dehydroxylation reaction that occurs upon calcination of the intercalated polycation at elevated temperatures [79]:
[Al13O4(OH)24(H2O)12]7+ 6.5 Al2O3 + 7 H+ + 20.5 H2O (1.2.-7)
The physical properties of pillared clays that most significantly affect their catalytic behavior are porosity and acidity.
1.2.5.2. The iron pillaring precursors
The aqueous chemistry of Fe(III) is known to yield polymeric cations [80-84] of substantial size and there have been reported attempts to intercalate such ions in the interlayers of smectite clays [85-88]. The characterized crystalline iron(III) oxides and hydrous oxides are Fe2O3 and FeO(OH), each of which is polymorphic. The phases
LPSRUWDQW LQ WKH K\GURO\VLV DQG SUHFLSLWDWLRQ RI )H,,,VDOWVDUH - FeO(OH), goethite;
-)H22+DNDJDQHLWHDQG -)H22+OHSLGRFURFLWHDVZHOODV -Fe2O3, hematite.
The iron hydrous oxides are built of double chains of edge-shared Fe(O,OH)6
octahedra (Figure 1.9.). In contrast, the structure of Fe2O3 does not contain this double chain element. In aqueous Fe(III) salt solutions the following species exist: Fe3+, Fe(OH)2+, Fe(OH)2
+, Fe(OH)2
4+, and Fe2O4+. A survey of Fe(III) coordination in oxides, hydroxides, and aqua species shows that Fe(III) in acidic oxide environments, including aqueous solutions, is octahedrally coordinated [89]. Only in basic environments does Fe(III) occur in tetrahedral coordination to a significant extent;
example include Na5FeO4. The hydrolysis of inorganic Fe(III) solutions consists of several steps: (1) formation of low-molecular-weight species; (2) formation of a red cationic polymer; (3) aging of the polymer, with eventual conversion to oxide phases;
and (4) precipitation of oxide phases directly from low-molecular-weight precursors.
Figure 1.9. The schematic crystal structure of proposed akaganeite type of iron-pillaring complex.
Most of the hydrolysis studies have been conducted of solutions of Fe(III) nitrate, perchlorate, or chloride in the concentration range 10-3-10-1 M, at room temperature.
The behavior of hydrolyzed Fe(III) solutions depends on the nature and mode of addition of basic reagents. Ordinary mixing of solutions of alkali hydroxide or ammonia with Fe(III) solutions results Fe(III) in immediate formation of a precipitate; if the amount of base added corresponds to PROEDVHSHUPROHLURQWKHSUHFLSLWDWHUH -dissolves. For reliable characterization of the hydrolysis process, it is necessary to add base so that precipitation does not occur. At higher temperatures the aging processes are accelerated. The pH of hydrolyzed Fe(III) solutions is observed to decrease with time. Polymer produced in chloride solutions consists of spherical particles like those obtained in nitrate or perchlorate solutions. The course of pH changes on aging is accelerated by the presence of Cl¯ . Substitution of Cl¯ by OH¯ in the polymer may contribute to the accelerated pH decrease. The lifetime of the polymer decreases with increasing temperature, so that titrations conducted at 90ºC result in precipitation without formation of soluble polymer. The solids precipitated have studied during high-temperature hydrolysis without addition of base by XRD. In the presence of chloride,
DNDJDQHLWH - FeO(OH) is produced. The precipitates obtained by addition of bases to Fe(III) solutions under a wide variety of conditions are amorphous to X-rays and are not stoichiometric hydroxides. Aging of the amorphous gels requires years at room temperature, and hours to days at 100ºC; the product formed are goethite or hematite.
Polymerization of iron typically begins at low pH (<1.5) and propagates by deprotonation of coordinated water molecules (olation) and hydroxylgroups (oxolation) as illustrated in Eqs.(1.2.-8) and (1.2.-9):
The hydrolysis reactions of Fe(III) can lead to discrete spherical polycations as large as 30 Å in diameter. Aggregation of the spheres produces rods and eventually rafts of rods. The polymerization process is dependent on base to metal ratio, temperature, and nature of the counterion, pH, and other factors. Studies carried out on fully
hydrated precipitates using X-ray diffraction showed that the structural continuity exists [90-91]. Rightor et al. [92] found that the iron oxide pillared clays formed by the reaction of sodium montmorillonite with hydrolyzed Fe3+-solutions depended critically on the hydrolysis conditions. The basal spacings of iron oxide pillared clays (Fe-PILC) between 18 and 29 Å may be obtained by using iron(III)nitrate, chloride or perchlorate, hydrolyzed with sodium carbonate. Bradley and co-workers found [93] that Fe13-ion formed by hydrolysis of Fe(III) solution. It appeared that this Fe13-ion(7+) is extremely unstable and decomposes very rapidly in solution. The additional of small amounts of Fe(II) ions seems to stabilize the structure somewhat, and allowed it to be used as a clay mineral pillaring agent. These results suggest very strongly that analogous Fe13
and Ga13 species form upon the base hydrolyses of aqueous iron(III) and gallium(III) solutions.
1.2.5.3. Review of other kind of precursors
Some structures of the pillaring precursor cations are elucidated already, while others are still subject of discussion.
Ga
Some work has been done on Ga-PILCs [94,95]. The gallery heights are somewhat more than 10 Å at room temperature around 8 Å at 600ºC. At higher temperatures, a collapse of the PILC occurs. The surface area of this Ga-PILC drops from 230 m2/g at room temperature to 170 m2/g at 600ºC. It is worth noting that the pillaring precursor cations of the Ga-PILC are identical in structure to those of the alumina pillared clay, namely [GaO4Ga12(OH)24(H2O)12]7+. This PILC seems to have a very good catalytic performance.Zr
Small but stable interlayer species are formed by zirconium tetramers [Zr4(OH)8+x(H2O)16-x](8-x)+ [96]. Depending on the method of preparation, which is most often the hydrolysis of ZrOCl2 (zirconylchloride), these Zr-PILCs exhibit spacings between 3 and 14 Å and surface areas between 100 and 400 m2/g [97-100]. They might remain stable up to 700ºC [101].Ti
Very large basal spacings (almost 30 Å) on the other hand are obtained by the intercalation of TiO2 pillars. The final Ti-PILC is prepared by calcination of clay, intercalated with hydrolyzed TiCl4 or Ti-alkoxides [102-106]. The polymers are described as [TiO(OH)2]n and surface areas up to 350 m2/g are reported. The pore volume and surface area seems to remain constant to about 700ºC.Bi
Hydrolysis of bismuth perchlorate results in the formation of [Bi6(OH)16]2+polycations (Figure 1.10.). Bi-PILCs have interlayer spacings of about 6 Å and a surface are of approximately 80 m2/g. The porosity drops steadily upon heating the PILC at temperatures above 200ºC [107].
Figure 1.10. The [Bi6(OH)16]2+ polycations