B. SOLVOTHERMAL DECOMPOSITION OF METAL ALKOXIDES

38



Chemical Processing of Ceramics, Second Edition



TABLE 2.2

Phases Formed by Solvothermal Decomposition of Alkoxides

and Acetylacetonates

Starting Material



Reaction

Temperature



Product



Reference



Aluminum isopropoxide

Aluminum n-butoxide

Aluminum tert-butoxide

Zirconium isopropoxide

Zirconium n-propoxide

Zirconium acetylacetonate

Zirconium tert-butoxide

Titanium isopropoxide

Titanium oxyacetylacetonate

Titanium tert-butoxide

Niobium n-butoxide

Tantalum n-butoxide

Iron acetylacetonate

Iron (n-butoxide)

Lanthanum isopropoxide



300

300

300

300

300

300

200

300

300

300

300

300

300

300

300



χ-Alumina

No reaction

Amorphous

Tetragonal zirconia

No reaction

Tetragonal zirconia

Amorphous

No reaction

Anatase

Anatase

Amorphous

Amorphous

Magnetite

Hematite + magnetite

Lanthanum hydroxide



95

95

95

96

96

96

96

97

97

98

99, 100

101

102

103

103



aerosols by the chemical vapor deposition (CVD) method.94 Whereas CVD

reactions of metal alkoxides under reduced pressure usually produce amorphous

products, solvothermal reactions of secondary alkoxides in inert organic solvents such as toluene, in some cases, give crystalline products (Table 2.2). For

example, thermal decomposition of aluminum isopropoxide and zirconium

isopropoxide in toluene at 300°C yields χ-alumina95 and tetragonal zirconia,96

respectively. This means that solvent molecules present in the reaction system

facilitate crystallization of the product.

The primary alkoxides of these metals do not decompose at 300°C, as decomposition of these compounds requires much higher temperatures, whereas tertiary

alkoxides decompose at much lower temperatures, yielding amorphous products.

These results indicate that heterolytic cleavage of the C–O bond yielding carbocation and metaloxo anion (>M–O; Equation 2.3) is the key step, and stability

of the carbocation determines the reactivity of the metal alkoxides:95

M(OR)n → >M–O– + R+



(2.3)



However, this reaction is strongly affected by the metal cation of the alkoxide; thus Nb(OBu)5 decomposes in toluene at 573 K yielding amorphous Nb2O5

powders,100 whereas Ti(O-iPr)4 is not decomposed under the same reaction



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Solvothermal Synthesis



39



conditions.97 The factors of metal cations controlling decomposition of alkoxides are not yet fully elucidated, but electronegativity as well as the oligomeric

structure of the alkoxide seem to determine the thermal decomposition reactivity

of the alkoxides.

Solvothermal decomposition of titanium tert-butoxide98 and titanium oxyacetylacetonate (TiO(acac)2)97 in toluene at 300°C yields nanocrystalline anatase.

It should be noted that the lowest temperature required for the formation of

crystalline titania by CVD synthesis was reported to be 400 to 450°C.104–106

One of the limitations of this method (thermal decomposition of alkoxide in

inert organic solvent) is the prerequisite of purification of the starting alkoxides

(see for comparison, the glycothermal reaction described in Section III.B.9), since

the alkoxide is easily hydrolyzed in moist air. Note, however, that fairly good

reproducibility may be obtained by using a fresh reagent from a brand-new

reagent bottle and discarding the remaining reagent.

This method is closely related to the nonhydrolytic sol-gel method.107 For

example, titania is prepared by etherolysis/condensation of TiCl4 by diisopropyl

ether (Equation 2.4) or by direct condensation between TiCl4 and Ti(O-iPr)4

(Equation 2.5). Detailed chemistry of the reaction was examined by means of

nuclear magnetic resonance (NMR), and it has been reported that the true precursors are titanium chloroisopropoxides in equilibrium through fast ligand

exchange reactions.108 A variety of metal oxides,109,110 nonmetal oxides,111 multicomponent oxides112,113 were studied, and the nonhydrolytic sol-gel method was

surveyed by Vioux and Leclercq.107

TiX4 + 2ROR → TiO2 + 4RX



(2.4)



TiX4 + Ti(OR)4 → 2TiO2 + 4RX



(2.5)



Trentler et al.114 reported synthesis of “hydroxyl-free” anatase nanocrystals by

the nonhydrolytic sol-gel method (Equation 2.4). Titanium halide was mixed with

distilled trioctylphosphine oxide (TOPO) in heptadodecane and heated to 300°C

under dry nitrogen and a titanium alkoxide was then rapidly injected into the hot

solution. Anatase with a crystallite size less than 10 nm in diameter was obtained,

although there was considerable size distribution. They reported that the reaction

rate dramatically increased with greater branching of R, and suggested that an SN1

mechanism takes place, consistent with similar low-temperature reactions.115 This

result is also consistent with our conclusion that heterolytic cleavage of the C–O

bond (Equation 2.3) is the key step for the solvothermal decomposition of metal

alkoxide. However, from a physical organic chemistry viewpoint, it is widely

accepted that high temperatures favor an elimination reaction (Equation 2.1) over

a substitution reaction (SN1), suggesting formation of olefins rather than alkyl

halide. Although titanium isopropoxide does not decompose in inert organic solvents at the reaction temperature (300°C),95 the presence of a small amount of

hydrogen halide formed by decomposition of TiX4 or RX possibly catalyzes the



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40



Chemical Processing of Ceramics, Second Edition



decomposition of metal alkoxide. Thus this reaction procedure does not ensure the

formation of “hydroxyl-free” product because of the following reactions:

Ti(OR)4 → Ti(OH)4 + olefins



(2.6)



Ti(OH)4 → TiO2 + 2H2O.



(2.7)



Rather, the observed lack of surface hydroxyl groups may be due to the open

reaction system, where surface hydroxyl groups are dehydrated as water is eliminated from the reaction system.

Trentler et al.114 also proposed that nanocrystalline products are obtained at

elevated temperatures because TiX4 serves as a crystallization agent as well as a

reactant. They pointed out the importance of a chemical reversibility, that is, Ti–O

bond breaking and forming, that would erase defects incorporated into growing

titania crystals. This statement seems to be made because they do not know that

the solvothermal reaction of titanium tert-butoxide and titanium oxyacetylacetonate in toluene yields nanocrystalline anatase.97,98 However, this point is closely

connected with one of the important features of solvothermal products, and

therefore will be discussed here.

In hydrothermal reactions, dissolution-deposition equilibrium takes place.

Dissolution of adatoms on the surface occurs preferentially, while preferential

adsorption of ions at the vacancies of the surface proceeds. Therefore a nearly

perfect surface is formed. Since a nearly perfect growing surface is created,

crystals formed by the hydrothermal reaction usually contain fewer defects

than the crystals formed by other methods. On the other hand, under solvothermal conditions, dissolution of oxide materials into the organic solvent

barely takes place, and therefore the product usually contains various types of

crystal defects.



2. Metal Alkoxides in Inert Organic Solvent: Synthesis of

Mixed Oxides

Thermal decomposition of two starting materials in inert organic solvent may

provide a convenient route for the synthesis of mixed oxides or precursor of

mixed oxides. For example, when a mixture of aluminum isopropoxide and

tetraethoxysilane (tetraethylorthosilicate) in a 3:1 ratio is decomposed in toluene,

an amorphous product is obtained.116 Note that thermal decomposition of the

former compound alone yields χ-alumina,95 while the latter compound alone does

not decompose at the reaction temperature.116 Mullite is crystallized by calcination

of the product at 900°C.116 It is known that the crystallization behavior of mullite

from the precursor gel depends on the homogeneity of mixing of aluminum and

silicon atoms in the precursor: when the precursor gel has atomic scale homogeneity, mullite is crystallized at 900°C, and the gel with homogeneity in a nano

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Solvothermal Synthesis



41



scale causes crystallization of silicon-aluminum spinel at around 900°C, while

heterogeneous gel requires 1300°C for crystallization of mullite.117,118 Therefore

atomic scale homogeneity is attained in the solvothermal product, even though

the reaction procedure is quite simple.

The solvothermal decomposition of a mixture of La(O-iPr)3 and Fe(OBu)3 in

toluene yields an amorphous product, whereas the reaction of individual starting

materials yielded crystalline La(OH)3 and a mixture of α-Fe2O3 and Fe3O4,

respectively.103 Calcination of the amorphous product at 550°C yields crystalline

LaFeO3 (perovskite). Low crystallization temperature also suggests high homogeneity of the solvothermal product.

3. Metal Acetylacetonate in Inert Organic Solvent

Besides alkoxides, acetylacetonates are also used as the starting materials for the

synthesis of oxides. Titania (anatase) is obtained by decomposition of titanium

oxyacetylacetonate (TiO(acac)2) in toluene at 300°C.97 Similarly solvothermal

treatment of Fe(III) acetylacetonate in toluene yields microcrystalline magnetite.102 One of the drawbacks of the use of acetylacetonate may be formation of

various high boiling point organic by-products via aldol-type condensation of the

acetylacetone. Actually more than 50 compounds are detected by gas chromatography-mass spectrometry (GC-MS) analysis of the supernatant of the reaction,

some of which are phenolic compounds and are hardly removed from the oxide

particles by washing with acetone.97

4. Metal Carboxylates

Konishi et al.54 reported thermal decomposition of iron carboxylate: Fe(III) was

extracted from an aqueous solution using Versatic 10 (tertiary monocarboxylic

acids) and the organic layer was diluted with Exxsol D80 (aliphatic hydrocarbons:

bp 208 to 243°C). The organic solution was then filtered through glass filter paper

and passed through phase separating paper to remove physically entrained water.

Then the organic solution was solvothermally treated. Magnetite particles about

100 nm in size were formed when the solution was heated at 245°C, but in the

presence of intentionally added water, hematite (α-Fe2O3) contaminated in the

product, while pure hematite was formed at a lower temperature in the presence

of a larger amount of water. The carboxylic acid serves as a reducing agent and

is partially decomposed into carbon dioxide:

RCO2– → R · + CO2 + e–.



(2.8)



The strategy of their research is solvent extraction (hydrometallurgy) from

mineral resources followed by thermal decomposition of the extracts directly.

Therefore they used a rather special carboxylic acid, Versatic 10.

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