Metal Alkoxides in Inert Organic Solvent: Synthesis of Mixed Oxides

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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Solvothermal decomposition of stannous oxalate (SnC2O4) yielding tetragonal

SnO powders was also reported.57

5. Cupferron Complexes

Rockenberger et al.119 described a general route for the synthesis of dispersible

nanocrystals of transition metal oxides. Their route involves the decomposition

of cupferron (N-nitroso-N-phenylhydroxylamine) complexes of metal ions such

as Fe3+, Cu2+, and Mn3+ in the high-temperature solvent trioctylamine at 250 to

300°C, obtaining the oxides γ-Fe2O3, Cu2O, and Mn3O4, respectively, of 4 to 10

nm in diameter. The products are crystalline and are dispersible in organic solvent.

By addition of a threefold volume excess of methanol, the nanocrystals can be

reprecipitated. Particles with the smallest average size (4 nm) were synthesized

by lowering the reaction temperature and/or lowering the injected precursor

concentration, and average particle diameter was controlled by subsequent injection of the precursor. The decompositions takes place at sufficiently low temperatures, and a capping agent such as a long-chain amine can be employed to

prevent sintering and to prepare well-dispersed nanoparticles.

Thimmaiah et al.120 and Guatam et al.121 extended the method of Rockenberger

et al. by replacement of relatively expensive (and toxic) amines with toluene

using a closed system, and showed that mixed transition metal oxides such as

the spinel CoFe2O4 can also be prepared.



6. Solvothermal Decomposition of Alkoxide Followed by

Removal of Organic Media in a Supercritical or

Subcritical State

Removal of the organic solvent at the solvothermal temperature is an interesting

modification of this type of reaction. Since inert organic solvent usually has a

relatively low critical point, the reaction temperature may be in the supercritical

or subcritical region. The removal of the organic phase directly from the reaction

vessel at the reaction temperature gives well-divided powders of the product.122,123

When a product is washed with water and then dried, coagulation of the

product particles occurs in the drying stage. The surface tension of the water

remaining between the product particles pulls the particles closer as the drying

proceeds,124 producing tightly coagulated products. When water is replaced with

an organic solvent, this coagulation may be loose because the surface tension of

organic solvent is less than that of water. However, when product is dried with

supercritical fluid, coagulation of the particles can be avoided.125 This process is

called supercritical drying and the product is called an aerogel.126 Thus the

procedure provides a convenient method for the synthesis of oxide powders using

only one reaction vessel, combining solvothermal synthesis and the supercritical

drying process.

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Solvothermal Synthesis



43



7. Metal Alkoxide in Alcohols

When metal alkoxides are allowed to react in alcohols, an alkoxyl exchange

reaction takes place at lower temperatures:84



→

M(OR)n + nR′OH ← M(OR′)n + nROH.





(2.9)



Therefore the composition of alkyl groups in the coordination sites of the metal

is determined by the relative number of the two alkyl groups in the reaction

system and by the relative volatility of the two alcohols. When the alcohol

derived from the alkoxide has a low boiling point, the alcohol will evaporate

and escape from the reaction system to the gas phase in the reaction vessel,

and therefore the corresponding alkyl groups are completely expelled from the

alkoxide.

The reaction of aluminum isopropoxide in primary alcohols yields the alkyl

derivative of boehmite, in which the alkyl moieties derived from the solvent

remain while expelling all the alkyl groups derived from the alkoxide.127

Straight-chain primary alcohols with a carbon number up to 12 were examined,

and a linear relationship between the basal spacing and carbon number of the

alcohols was observed. Note that when the reaction is carried out in 2-ethyl1-hexanol, the product is χ-alumina, indicating that this solvent behaves as an

inert solvent.127

As described in Section III.B.1, decomposition of primary alkoxides in inert

organic solvents requires temperatures much higher than 300°C, but in alcohols

they may decompose at relatively low temperatures. The carbocation formed by

the heterolytic cleavage of the C–O bond is only poorly solvated in the inert

organic solvent; therefore the reaction barely proceeds. On the other hand, in

alcohols, carbocation is solvated, which lowers the activation energy for the

decomposition of alkoxide. For example, aluminum ethoxide does not decompose

in toluene at 300°C, while it does decompose in ethanol, yielding the alkyl

derivative of boehmite.

Barj et al.43 and Pommier et al.128 examined thermal decomposition of

Mg[Al(O-sec-Bu)4]2 in ethanol: decomposition starts at 283°C, yielding an

essentially amorphous product, while the reaction at 360°C yields partially

crystallized MgAl2O4 spinel. The product consisted of secondary particles with

a size of 3 µm, which is easily disaggregated by ultrasonic treatment into 0.02

µm primary particles. They also reported that thermal dehydration of ethanol

becomes significant at temperatures higher than 360°C. Therefore crystallization of this product seems to occur with the aid of water formed by dehydration

of the solvent.

Wang et al.129 reported the reaction of TiCl4 in various alcohols at 100°C

and 160°C. Methanol, ethanol, 1-propanol, and 2-propanol gave anatase, and

butanol and octanol yielded rutile, while ethylene glycol yielded a mixture of

rutile and anatase. It is known that TiCl2(OR)2 can be readily formed when

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Chemical Processing of Ceramics, Second Edition



TiCl4 is introduced into alcohols with liberation of HCl.130 The products consist

of spherical particles, but agglomerated tenuous fibers were obtained in octanol

(rutile) and 2-propanol (anatase). The quite low crystallization temperature of

titania as well as the low decomposition temperature of the alkoxides derived

from TiCl4 are rather surprising. However, water, formed by dehydration and

etherification of alcohols (the authors detect ethers by GC-MS) with the aid of

dissolved HCl as a catalyst, possibly hydrolyzes the alkoxide and facilitates

hydrothermal crystallization of the product. Moreover, HCl may catalyze the

cleavage of Ti–O–R bonds (Equation 2.10) and also mediates the crystallization

of titania through the bond breaking and forming equilibrium shown in

Equation 2.11. They also noted that the phase formed by the reaction is affected

by the concentration of HCl, with a lower concentration of HCl favoring the

formation of anatase.

≡TiOR + H+ → ≡Ti-O(H+)-R → ≡TiOH + R+



(2.10)





→

≡Ti-O-Ti + HX ← ≡TiOH + XTi





(2.11)



8. Reaction of Alkoxide in Secondary Alcohols

Reaction of alkoxide in secondary alcohols gives a completely different route

than that occurring in inert organic solvents. Secondary alcohols are easily dehydrated at higher temperatures, yielding water and olefins, and water can hydrolyze

the alkoxide.46 In the CVD reaction, Takahashi et al.131 observed that decomposition of titanium isopropoxide in the presence of isopropyl alcohol occurs at

lower temperatures because of the formation of water from alcohol. This hydrolysis route may compete with the thermal decomposition route; however, even

when tert-alkoxide is used as the starting material, the alkoxyl exchange reaction

proceeds at lower temperatures and therefore the hydrolysis route seems to be

the predominant one. Water formed by dehydration of solvent alcohols also

facilitates hydrothermal crystallization of the hydrolysis product of alkoxide (see

Section III.E). When the reaction in primary alcohol is carried out at high temperatures (greater than 360°C) and/or in the presence of acid catalyst, a similar

route can be expected.

Fanelli and Burlew46 first applied this method for the formation of alumina

by the reaction of aluminum sec-butoxide in 2-butanol at 250°C and reported the

formation of alumina with a quite large surface area. They pointed out that this

method could be regarded as the sol-gel version of the homogeneous precipitation

method (see Section II.B.4).

Courtecuisse et al.132 reported a flow reactor for the formation of titania by

decomposition of titanium isopropoxide in 2-propanol at 260 to 300°C. They

reported that the rate-determining step is the thermal dehydration of titanium



© 2005 by Taylor & Francis Group, LLC



Solvothermal Synthesis



45



hydroxide formed by the hydrolysis of the precursor alkoxide.133 They also found

that an increase in the supercritical fluid density decreased the overall reaction

rate, but adequate explanation was not given by the authors.

9. Reaction of Alkoxide in Glycols

The reaction of aluminum alkoxides in glycol yields the glycol derivative of

boehmite.17,134,135 The crystallite size of the product increased in the following

order: HO(CH2)2OH < HO(CH2)3OH < HO(CH2)6OH < HO(CH2)4OH. The physical properties of the products and the aluminas derived by calcination thereof

varied according to this order.134,136 This result suggests that development of the

product structure is controlled by the heterolytic cleavage of the O–C bond of

the glycoxide intermediate, >Al-O-(CH2)nOH, formed by alkoxyl exchange

between aluminum alkoxide and the glycol used as the medium. The presence

of an electron-withdrawing group, that is, a hydroxyl group, near the O–C bond

retards the formation of carbocation, thus only poorly crystallized product is

obtained in ethylene glycol. On the other hand, the largest crystallite size, obtained

by the use of 1,4-butanediol (1,4-BG), is interpreted by the ease of the cleavage

of the O–C bond due to participation of the intramolecular hydroxyl group,137

which yields an aluminate ion (>Al-O) and protonated tetrahydrofuran

(Equation 2.12). A similar medium effect was also observed for the glycothermal

treatment of other alkoxides:

CH2–CH2

>M–O–CH2–CH2–CH2–CH2–OH → M–O + CH2

–



CH2



(2.12)



+



OH



It must be noted that both the reactions of aluminum hydroxide and alkoxide

in glycol yielded the glycol derivative of boehmite with identical morphology.17,83,134 As discussed in Section III.A.3, equilibrium between hydroxide-alcohol and alkoxide-water (Equation 2.2) is attained at high temperatures.84 Because

of this equilibrium, glycoxide (a kind of alkoxide) is generated from the hydroxide

under glycothermal conditions, and therefore partially hydrolyzed alkoxide can

be used for the reaction. (Complete hydrolysis of aluminum alkoxides yields

microcrystalline boehmite [pseudoboehmite], the structure of which is fairly

stable, and therefore fully hydrolyzed alkoxide does not give the desired product

by the glycothermal reaction.) Actually the glycothermal reaction does not require

any precautions for handling alkoxides, and reproducible results are obtained

even without purification of the starting alkoxides.138 (As for zirconium alkoxide,

the polycondensation reaction of the Zr-OH group proceeds rapidly as compared

with the alkoxyl exchange reaction; as a result, partial hydrolysis of zirconium

alkoxide severely affects the physical properties of the glycothermal products.)96,138–141



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Various oxides such as ZrO2,96,141 TiO2,85 ZnO,142 and Nb2O599 have been

prepared by the glycothermal reaction of the corresponding alkoxides. Kang et

al.143 reported that anatase (122 m2/g) obtained by glycothermal (1,4-butanediol)

treatment of titanium isopropoxide at 300°C has considerably higher photocatalytic activity than the catalyst prepared by the sol-gel method.



C. GLYCOTHERMAL SYNTHESIS



OF



MIXED METAL OXIDES



1. Rare Earth Aluminum Garnets

Since metaloxo anion (>M-O) is expected to be formed by decomposition of the

glycoxide intermediate derived from alkoxide and 1,4-butanediol, the presence

of metal cation that gives basic oxides would give M–O–M bonds. According to

this working hypothesis, we examined the reaction of aluminum isopropoxide

(AIP) with yttrium acetates in 1,4-BG at 300°C and found the formation of

crystalline yttrium aluminum garnet (YAG).8 The hydrothermal reaction of

pseudoboehmite (hydrolyzed product of AIP) with yttrium acetate at 300°C

yielded boehmite together with a small amount of YAG. Single-phase YAG was

not obtained, even with prolonged reaction time.8,144 The difference between

glycothermal and hydrothermal reactions can be attributed to the different stabilities of the intermediate phases, that is, the glycol derivative of boehmite vs. wellcrystallized boehmite, which is easily formed by hydrothermal reaction of aluminum compounds (see Section III.A.4).

Similarly the reaction of the stoichiometric mixture of AIP and rare earth

(RE) acetate (Gd–Lu) gives the corresponding rare earth aluminum garnet

(REAG) in single phase.144 Synthesis of single-phase REAG by the reaction of

mixed alumina and RE oxide powders normally requires a temperature higher

than 1600°C with a prolonged heating period.145 Homogeneous mixing of aluminum and RE atoms in the starting materials can lower the crystallization temperature of REAG,146–149 but these materials still require calcination temperatures

higher than 800°C to crystallize the REAG phase.

The glycothermal reaction of rare earth acetate alone yields RE(OH)2(OAc),

REO(OAc) (two polymorphs), and RE(OH)(OAc)2, depending on the ionic size

of the RE ion.150,151 The acetate ions are not completely expelled from the coordination sites of the RE ion. However, in the presence of aluminum alkoxide as

the starting material, acetate ions are fully eliminated from the product. Therefore

anionic species (that is, >Al-O) facilitate cleavage of the bond between acetate

and RE ions.

The reaction of samarium or europium acetate with AIP produced SmAG or

EuAG, although the product was contaminated with RE acetate oxide

(RE(CH3COO)O).144 The reaction of AIP with neodymium acetate gave only

Nd(CH3COO)O as the sole crystalline product. The thermodynamic stabilities of

the garnet phases depend on the ionic size of the RE element, and REAGs were

reported to be thermodynamically stable for all the RE elements from terbium

to lutetium.152 Therefore all the thermodynamically stable REAGs were prepared

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Solvothermal Synthesis



47



by the glycothermal method. GdAG (metastable phase) has been prepared by

many researchers, but synthesis of SmAG and RuAG has never been reported by

any other methods.

Hydrothermal synthesis of single-phase REAG requires higher temperatures (350 to 600°C) and pressures (70–175 MPa),153,154 although Mill' reported

that the lower temperature limit for the formation of YAG was 280°C.153 He

also reported that with an increase in the ionic size of the RE element, the

lower temperature limit increased. The REAG with the largest RE ion size

that has been thus far prepared hydrothermally is TbAG,153 and it was reported

that 420 to 450°C was required for the formation of this garnet. Therefore

there seems to be no possibility that SmAG and EuAG can be prepared by the

hydrothermal method, because the ionic size of these elements is much larger

than that of Tb.

2. Rare-Earth (Nd-Lu) Gallium Garnets

The reaction of RE (Nd-Lu) acetates with Ga(acac)3 in 1,4-BG at 300°C yielded

the corresponding RE gallium garnets (REGGs).21,156 The garnet phases were

reported to be thermodynamically stable for all the RE elements from samarium

to lutetium,157 and all of the stable REGGs were prepared by the glycothermal

method. The reaction at 270°C also gave phase pure REGGs for Sm-Lu, but

the reactions at 250°C gave amorphous products except for the reaction of

yttrium, which gave yttrium gallium garnet (YGG). Hydrothermal reaction of

Ga(acac)3 with RE acetate under conditions identical to the glycothermal reaction (except reaction medium) gave -Ga2O3 together with a small amount of

the garnet phase.21

The reaction of praseodymium and cerium acetates with Ga(acac)3 gave

RECO3(OH) as the sole crystalline product. When the reaction was carried out

in the presence of gadolinium gallium garnet (GGG) seed crystals, garnets were

crystallized in spite of the fact that unit cell parameters of these garnets are much

larger than those of GGG.21 These results suggest that under glycothermal conditions, nucleation is the most difficult process, and that once nucleation takes

place, crystal growth proceeds easily.

The particles of gallium garnets with large RE ions are spherical (0.5–2 µm)

and the surface of the particles is smooth. On the other hand, the surface of the

particles (0.1–0.3 µm) of the gallium garnets of terbium and RE elements having

ionic sizes smaller than terbium is rough, with apparent polycrystalline outlines.

However, high-resolution images of the latter type of particles show that a whole

particle is covered with a single lattice fringe, indicating that each particle is a

single crystal grown from only one nucleus. The authors concluded that the latter

type of morphology is formed by quite rapid crystal growth.21

Monodispersed particles are formed for garnet with smaller RE ions.21 Monodispersed particles can be prepared if a burst of nuclei is formed at the early

stage of the reaction and if nucleation does not take place during the crystal

growth stage.158 Once nucleation occurs in the glycothermal synthesis of gallium

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