Intercalation Properties

Over time, research on layered phosphates has moved from the study of their ion exchange properties to that of their intercalation properties in great part determined by the presence of Br+nsted acid groups in the interlayer region. Both a- and y-zirconium phosphates are excellent intercalating agents of Lewis bases. The intercalation chemistry of the former has been more widely investigated and we will be mainly concerned with a-zirconium phosphate. Many molecules belonging to various classes of organic compounds (alkanols, glycols, alkyl and aryl amines, heterocyclic bases, aminoacids and dyes) have been intercalated. The corresponding intercalation compounds have been characterized for composition and arrangement of the guest molecules in the interlayer region. Table 3 gives the interlayer distance and composition of some typical examples of intercalation compounds.

Let us examine in more detail the intercalation of n-alkylamines that leads to the formation of compounds containing two moles of guest per mole of host, according to the reaction a-Zr(HPO4)2 • H2O # 2RNH p a-Zr(HPO4)2 • 2RNH2 • H2O

where R is the n-alkyl-chain. The reaction proceeds stepwise with the formation of different phases. At low amine loading we observe the formation of a phase with interlayer distance 10.4 A and the alkyl-chain axis is almost parallel to the layer plane (see Figure 10A). At half intercalation, the alkyl chains are arranged as a monolayer of extended molecules with the chain axes inclined by 55° with respect to the layer plane (see Figure 10B). At full intercalation n-al-kylamines give rise to compounds in which the inorganic layer regularly alternates with a bilayer of al-kylamines with the n-alkyl chain in trans-trans conformation (see Figure 10C). The terminal -NH2 groups are protonated by the hydrogenphosphate groups.

Intercalation compounds with a,œ-alkyldiamines contain one mole of guest per formula weight. The guest molecules are arranged as a monolayer of extended chains and the terminal -NH2 groups interact with the P-OH groups belonging to two-faced layers. Alkanols and glycols produce intercalation compounds whose composition and arrangement of guest species are similar to those found in alkyl mono-amines and diamines, respectively. However, direct intercalation is prevented by the lower basicity of the alkanol OH group, compared to that of the NH2 group. It is necessary to use as precursors pre-swelled zirconium phosphates.

Table 3 Interlayer distances and guest contents of intercalation compounds of a-zirconium phosphate

Guest molecule

mol Guest/

Interlayer

mol a-ZrP

distance (A)

Methylamine

2.0

12.1

Ethylamine

2.0

14.8

Propylamine

2.0

17.6

Penthylamine

2.0

21.5

Diethylamine

1.0

12.7

Dipropylamine

1.0

15.7

Dioctylamine

0.8

26.8

Aniline

2.0

18.4

p-Methoxyaniline

2.0

21.7

Benzylamine

2.0

19.1

Benzylethylamine

2.0

22.4?

Ephedrine

2.0

22.0

Histamine

1.9

20.5

Pyridine

0.95

10.9

Pyrazole

0.75

10.8

Imidazole

0.95

10.7

3-Methylpyrazole

0.98

12.1

1-Methylimidazole

0.58

10.4

Benzimidazole

1.90

20.4

Pyridazine

0.64

10.8

Pyrimidine

0.71

11.1

Pyrazine

0.78

10.8

2,2'-Bipyridyl

0.25

10.9

1,10-Phenantroline

0.5

13.6

2,9-Dimethylphenantroline

0.5

14.6

Ethanol

14.2

1-Propanol

16.6

1-Butanol

18.7

1-Octanol

26.7

Isopropanol

15.6

2-Methyl-1-propanol

17.5

3-Methyl-1-butanol

19.2

Benzyl alcohol

21.0

Diethylene glycol

10.5

Acetone

9.9

Acetylacetone

13.5

Acetonitrile

11.3

Urea

0.9

9.9

a-Alanine (DL)

0.5

12

Phenylalanine (DL)

1.7

23.2

Histidine (DL)

0.9

16.2

Crystal violet

0.5

22

Rhodamine

0.66

24.7

Heterocyclic bases give rise to non-stoichiometric intercalation compounds and the heterocyclic ring is positioned parallel to the layer plane. For the arrangement of other intercalated guests the reader is referred to recent reviews given in the Further Reading section. Materials with special properties have also been obtained by intercalation. Porphyrins and metalloporphyrins, thionine, methylene blue and rhodamine have been intercalated in a-zirconium phosphate and the materials obtained have been investigated for their optical properties. The possibility of intercalating dyes and of controlling, at least to

Figure 10 Arrangement of n-alkyl monoamines intercalated into a-Zr(HPO4)2• H2O: (A) alkyl-chain axis parallel to the layer plane. (B) Monolayer of extended molecules in trans-trans conformation. (C) Bilayer of extended molecules in trans-trans conformation.

some extent, molecular orientation is of interest in the preparation of new composite materials for nonlinear optic applications. Intercalation of weak Br+nsted bases was found to enhance the proton conduction of the host and some of the compounds obtained have been used as active components in solid-state electrochemical gas sensors. Molecular and chiral recognition properties have been induced in layered zirconium phosphate by the intercalation of suitable receptors such as aminated ^-cyclodex-trins, crown ethers or a Pirkle receptor.

Much attention is presently being paid to the possibility of performing reactions in the interlayer region. Polymerization, induced by chemical, thermal or photochemical treatment, of pyrrole, aniline, propargylamine or s-aminocaproic acid intercalated in layered phosphates, produces interesting composite materials in which the inorganic layers regularly alternate with the polymers formed in the inter-layer region.

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