III. SOLVOTHERMAL SYNTHESIS OF METAL OXIDES

32



Chemical Processing of Ceramics, Second Edition

MOn/2



M(OH)n



Solvothermal

dehydration



Solvothermal

oxidation

Laine

method



MXn



M (metal)

Solvothermal

reduction

M(OR)n



H2O (hydrolysis)

Alkoxide method

(Sol-gel method)



Precipitation

method

(Hydrolysis)



CVD method



Amorphous

M(OH)n

H2O

Forced hydrolysis



Solvothermal

crystallization



Nonhydrolytic

Sol-gel method



Amorphous

MOn/2

Solvothermal

decomposition

of alkoxide



Crystalline

MOn/2

Solvothermal method



Calcination



Methods closely connected with

solvothermal method



Other method



FIGURE 2.1 Solvothermal routes to synthesize crystalline particles of metal oxide.



1. Solvothermal Dehydration of Aluminum Hydroxide

in Alcohols

Solvothermal reaction of gibbsite in ethanol gives boehmite (AlOOH; a polymorph of aluminum oxyhydroxide), which is also obtained by hydrothermal

reaction of the same starting material. The product is comprised of randomly

oriented, thin, small crystals of boehmite. This result suggests that a dissolutionrecrystallization mechanism takes place during the conversion.

The reaction of gibbsite in methanol occurs by a different route. The product

is AlO(OCH3) and has a boehmite-like structure. Since the product has an essentially identical morphology to that of the starting material, Kubo and Uchida73

concluded that the reaction takes place by means of a solid state reaction in which

methanol diffuses into the gibbsite structure with the aid of the Hedvall effect.74

Since the molecular sizes of ethanol and higher alcohols are larger than that of

methanol, the former solvents cannot diffuse into the gibbsite structure.

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Solvothermal Synthesis



33



With an increase in the carbon number of the alcohol, the yield of boehmite

decreases and the yield of χ-alumina increases.72 The latter compound is the

thermal dehydration product of gibbsite,75 and the highest yield of χ-alumina is

observed in pentanol. The morphology of the product obtained in pentanol is

identical to that of the starting material. This feature (pseudomorphism) is typical

for thermal dehydration of metal hydroxides and suggests that a solid state

transformation mechanism takes place. 76 Since higher alcohols have progressively

lower dielectric constants, the dissolution rate of the starting material in higher

alcohols decreases and thermal dehydration of gibbsite becomes significant in

these solvents. Therefore higher alcohols have essentially no effect on product

formation. However, the solvent decreases the fugacity of water and prevents the

hydrothermal conversion of gibbsite, as discussed later.

The reaction in higher alcohols is greatly affected by the particle size of the

starting materials (Table 2.1).72 When coarse gibbsite is used, the yield of boehmite increases. If boehmite were to be formed by the dissolution-recrystallization

mechanism, the yield of boehmite would decrease with an increase in the particle

size of the starting material; this is not the case. Formation of boehmite can be

explained by the intraparticle hydrothermal reaction mechanism, originally proposed by de Boer et al.77,78 for the formation of boehmite by thermal dehydration

of gibbsite. It is well known that when coarse gibbsite is thermally dehydrated,

boehmite is formed, and that formation of boehmite stops when formation of χalumina starts. According to the mechanism, thermal dehydration starts at the

“hot spot” of the particle and water molecules formed by dehydration cannot

diffuse out from the particles. Therefore a hydrothermal condition is achieved

inside the particle and boehmite is formed. When thermal dehydration yielding

χ-alumina starts, pore systems are formed inside the originating particle and water

that facilitates the hydrothermal conversion into boehmite diffuses out from the

particles; therefore formation of boehmite ceases. Formation of boehmite from

coarse gibbsite under the solvothermal conditions in higher alcohols can be

explained similarly.

Further increases in the carbon number of alcohol result in an increase in the

yield of boehmite.72 Since the reaction was carried out in a closed system, the

water formed by the dehydration of gibbsite is adsorbed on the surface of the

starting material, where water facilitates the hydrothermal conversion of gibbsite

into boehmite. Actually, when the same starting material is heated in a closed

vessel in the absence of any solvent, complete conversion into boehmite is

observed. This result shows that the alcoholic solvent somehow retards the hydrothermal conversion of gibbsite. Solvent molecules (higher alcohols) lower the

fugacity (activity) of water by forming strong hydrogen bonds between alcohol

and water molecules. This effect becomes less significant with an increase in the

carbon number of alcohol, and therefore the yield of boehmite increases with an

increase in the carbon number of alcohol. The reaction mechanisms for the

alcohothermal dehydration of gibbsite are summarized in Table 2.1.79

The reaction in an open system gives a completely different result. Since the

boiling point of water is much lower than that of higher alcohols, water formed

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Chemical Processing of Ceramics, Second Edition



TABLE 2.1

Solvothermal Reaction of Gibbsite in Alcohols and Mineral Oil at 250°C

for 2 h

Yield (%) of the Product from

Gibbsite with a Particle Size of

Solvent



Product



80 µm



8 µm



Mechanism for the

Formation of the

Product



Water

Ethanol

Propanol



Boehmite

Boehmite

Boehmite

χ-alumina

Boehmite

χ-Alumina

Boehmite

χ-Alumina

Boehmite

χ-Alumina

Boehmite

χ-Alumina

Boehmite



100

100

99

0

98

0

51

16

51

0

86

3

100



100

100

70

24

56

38

21

41

27

30

41

53

100



DP(sol)a

DP(sol)

DP(sol), (IPH)b

Solid state reaction

DP(sol), IPH

Solid state reaction

IPH

Solid state reaction

IPH, HYDc

Solid state reaction

HYD, IPH

Solid state reaction

HYD



Butanol

Pentanol

Hexanol

Octanol

Mineral oil

a



Dissolution-precipitation mechanisms: dissolution of aluminum species in the solvent used for the

solvothermal reaction.

b Intraparticle hydrothermal reaction originally proposed by de Boer et al. 77,78 for the formation of

boehmite during thermal dehydration of coarse gibbsite.

c Hydrothermal reaction takes place because of the water adsorbed on the particles surface: Therefore

crystallization of the product occurs by means of the dissolution-precipitation mechanism.



by partial thermal dehydration of gibbsite will escape from the reaction system,

and therefore simple thermal dehydration will occur.72

2. Alcohothermal Dehydration of Hydroxides of Metals

Other Than Aluminum

Although the solvothermal dehydration of metal hydroxides depends on the

solubility of the precursor in the reaction medium and the ease of thermal

dehydration of the precursors, the essential features are similar to those for

aluminum hydroxide discussed in the previous section. Yin et al.80 examined the

phase transformation of H2TiO9 · nH2O (prepared from fibrous K2Ti4O9) by solvothermal reaction in methanol and ethanol at 200 to 325°C and compared them

with those by calcination in air or hydrothermal reaction. Under the solvothermal

conditions, it transformed to monoclinic TiO2 and then anatase, while under

hydrothermal conditions or calcination in air it transforms into H2Ti8O17 before

transformation into monoclinic TiO2. All the products retained a fibrous morphology similar to that of K2Ti4O9, which was used as the starting material.

© 2005 by Taylor & Francis Group, LLC



Solvothermal Synthesis



35



However, the microstructure of the fibers changed significantly; therefore they

concluded that the phase transformation proceeded by a dissolution-reprecipitation mechanism. They also reported that anatase obtained by alcohothermal

reactions had much higher photocatalytic activity than Degussa P25 titania.81,82



3. Solvothermal Dehydration of Aluminum Hydroxide

in Glycols and Related Solvents

When a solvent having an electron-donating group, such as glycol, amino alcohol,

or alkoxyethanol, is used as the solvent for the solvothermal dehydration of

gibbsite, a completely different reaction occurs. The product is the alkyl (glycol)

derivative of boehmite (AlO(OH)x(O(CH2)nX)y; x + y = 1; X = OH, NR2, or OR),

in which solvent molecules are incorporated between the boehmite layers through

the covalent bonds.17,83 The fact that the product has Al–O–C bonds17,83 indicates

that equilibrium occurs between aluminum hydroxide and alkoxide84 and that

alkoxide can be formed from hydroxide at high temperatures:



→

M(OH)n + nROH ← M(OR)n + nH2O





∆H > 0, ∆S > 0. (2.2)



Formation of alkoxide from hydroxide is a reverse reaction of hydrolysis of

alkoxide, which proceeds easily at room temperature and is a highly exothermic

reaction (therefore Equation 2.2 has a positive reaction enthalpy). However, metal

hydroxide is usually solid and has a polymeric M–(OH)–M network, while metal

alkoxide usually has oligomeric structure. Therefore the former compound has

lesser freedom (lower entropy). Consequently the unfavorable enthalpy term is

overcome by the entropy term at high temperatures and equilibrium is attained.

The difference between the reactions in simple alcohols and in glycols can

be attributed to the stabilization effects of the intramolecular electron-donating

group on the intermediate and to the decrease in the activity (fugacity) of water

due to solvation by the highly hydrophilic solvent molecules.

The particle size of the starting material has a significant effect on this

reaction.85 When gibbsite with a particle size less than 0.2 µm is used, complete

conversion into boehmite derivatives is attained, while the use of coarser gibbsite

results in an increase in the yield of well-crystallized boehmite formed by the

intraparticle hydrothermal reaction. An increase in the particle size of gibbsite

also increases recovery of the starting material. Randomly oriented thin plates of

boehmite derivatives are formed on the surface of the pseudomorphs of the

originating gibbsite particles. An important point here is that the product is not

formed in the bulk of the solvent. Two explanations are possible: the gibbsite

surface acts as the nucleation site of the product, or the gradient of the concentration of water, which declines from the surface of the particle to the bulk solvent,

causes precipitation of product particles near the surface of the originating particles. The product particles formed on the originating gibbsite crystals prohibit

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Chemical Processing of Ceramics, Second Edition



dissolution of the starting material and a certain amount of the starting material

remains unconverted even with prolonged reaction times.

4. Glycothermal Synthesis of α-Alumina

When microcrystalline gibbsite (less than 0.2 µm) is allowed to react in higher

glycols such as 1,4-butanediol, 1,5-pentanediol, or 1,6-hexanediol at higher temperatures (approximately 300°C), α-alumina is formed.7 This was the first example of thermodynamically stable metal oxide crystallized in organic solvent at a

temperature lower than that required by the hydrothermal reaction. The α-alumina

is formed from the glycol derivative of boehmite as an intermediate and product

particles are hexagonal plates with a relatively large surface area. This indicates

that the dissolution-recrystallization mechanism occurs.

Hydrothermal synthesis of α-alumina has been well studied. Since the hydrothermal reaction of aluminum compound yields boehmite at relatively low temperatures (approximately 200°C),86 transformation of boehmite was examined

and it was reported that more than 10 hours is required for complete conversion

into α-alumina, even with a reaction at 445°C in a 0.1 N NaOH solution and in

the presence of seed crystals.87 On the other hand, under glycothermal conditions,

α-alumina is formed at 285°C for 4 h. The equilibrium point between diaspore

(another polymorph of AlOOH) and α-alumina under the saturated vapor pressure

of water was determined to be 360°C.88 However, near the equilibrium point, the

transformation rate is very sluggish, and only a small conversion of diaspore is

observed. Therefore complete conversion of diaspore into α-alumina requires a

much higher temperature. Since boehmite is slightly less stable than diaspore,

the hypothetical equilibrium point between boehmite and α-alumina would be

lower than that for diaspore-alumina. However, α-alumina would not be formed

by a hydrothermal reaction at such a low temperature as has been achieved in

the glycothermal reaction.

The difference between two reactions may be attributed to the activity of

water present in the reaction system, since the overall reaction is the dehydration

reaction (Equation 2.1). However, intentional addition of a small amount of water

caused enhancement of α-alumina formation rather than the retardation expected

from the equilibrium point of view.89 Another important factor is the difference

in the thermodynamic stabilities of the intermediates between glycothermal and

hydrothermal reactions; that is, the glycol derivative of boehmite vs. well-crystallized boehmite. The latter compound is fairly stable and therefore conversion

of this compound into α-alumina has only a small driving force. On the other

hand, the glycol derivative of boehmite has Al–O–C bonds and therefore is more

unstable with respect to α-alumina. Thus conversion of this compound into αalumina has a much larger driving force. The smaller crystallite size of the glycol

derivative of boehmite also contributes to the instability of the intermediate.

The reaction is strongly affected by the particle size of the starting material.89

When gibbsite with a particle size less than 0.2 µm is used, complete conversion

into α-alumina is attained. When coarse gibbsite is used for the reaction, the



© 2005 by Taylor & Francis Group, LLC



Solvothermal Synthesis



37



product is χ-alumina together with well-crystallized boehmite. α-Alumina was

not formed at all.89 As mentioned in the previous section, formation of the

intermediate, the glycol derivative of boehmite, ceases at a certain point in the

reaction because the originating gibbsite particles are covered with the crystals

of the glycol derivative of boehmite. Higher reaction temperatures cause thermal

dehydration of gibbsite into χ-alumina, which seems to facilitate epitaxial decomposition of the glycol derivative of boehmite into a transition alumina. Therefore,

when coarser gibbsite is used as the starting material, α-alumina cannot be

obtained, even at higher temperatures.

There are two other important points in this reaction.89 Water formed as a

byproduct in the formation of the glycol derivative of boehmite facilitates crystallization of α-alumina by increasing the dissolution rate. Intentional addition

of a small amount of water actually lowers the α-alumina crystallization temperature; however, excess water causes the formation of boehmite, which is fairly

stable under the reaction conditions and contaminates the product. Another point

is the presence of α-alumina nuclei in the starting gibbsite sample. The starting

gibbsite is commercially produced by milling coarser gibbsite. It is known that

prolonged grinding or milling of gibbsite particles causes mechanochemical transformation into α-alumina.90 Therefore α-alumina-like domains are formed at this

stage and act as the nuclei of α-alumina in the glycothermal reaction. Bell et al.91

also addressed the importance of seed particles on the size of hexagonal αalumina platelets.

Although the glycol derivative of boehmite is also prepared from aluminum

alkoxides (see Section III.B.9), this product cannot transform into α-alumina

because of the absence of α-alumina nuclei and water. However, the reaction of

a mixture of gibbsite (microcrystalline) and aluminum alkoxide in the presence

of a small amount of water gives α-alumina with larger crystallite sizes than

those obtained from gibbsite alone.

Cho et al.92 reported that α-alumina is formed from aluminum hydroxide

prepared by precipitation with potassium hydroxide. However, when alkaline

hydroxide is used as the precipitation agent, alkali cations are incorporated into

the product, and commercial gibbsite samples are always contaminated with a

small amount of sodium ions. Therefore their starting material seems to be

contaminated with potassium, and the presence of potassium ions in their precursor seems to play an important role in the nucleation of α-alumina. They also

reported that hydroxyl ions, acetic acid, and pyridine added to the glycothermal

reaction system affect the morphology of the α-alumina particles because of their

preferential adsorption to a specific surface.93



B. SOLVOTHERMAL DECOMPOSITION



OF



METAL ALKOXIDES



1. Metal Alkoxide in Inert Organic Solvents

Thermal decomposition of metal alkoxides gives the corresponding metal

oxides. This reaction is usually applied for the synthesis of oxide films or

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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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