II. CHOICE OF THE REACTION MEDIUM

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Chemical Processing of Ceramics, Second Edition



examine the equilibrium conversion of minerals. Today researchers seek more

rapid conversion to synthesize materials, and therefore adequate synthesis of the

precursors by, for example, the coprecipitation method and the sol-gel method

become more important. Addition of salt, acid, or base may facilitate the reaction

or alter the morphology of the products. These materials are called mineralizers.

Fluoride ions sometimes have a drastic effect on the hydrothermal synthesis of

materials. Besides the excellent article by Somiya, Roy, and Komarneni in this

book, many review articles have appeared on hydrothermal synthesis;28–32 therefore this technique will not be discussed further in this chapter.

2. Ammonia

Besides water for hydrothermal reactions, liquid ammonia (bp, 78°C; Tc, 132°C;

Pc, 113 atm) is also used for the solvothermal synthesis of nitrides. Metastable

or otherwise unobtainable nitride materials were reported to be formed by this

method.33–35 Ammonium and amide (NH2) ions are the strongest acid and base,

respectively, for the liquid ammonia system, and therefore ammonium salt acts

as the acid mineralizer,36 while amide ion can be prepared by addition of alkali

metals to the solvent. Since ammonia has a low boiling point, the reaction pressure

is usually quite high.

3. Other Inorganics

Sulfur dioxide (bp, −10°C; Tc, 157.5°C; Pc, 78 atm) is another interesting inorganic solvent. This compound has a high dielectric constant and low basicity

(actually, it acts as an acid). To the best of my knowledge, there have been no

articles that apply this solvent for the solvothermal synthesis of inorganic materials. However, the highly corrosive nature of this solvent may limit its use in

autoclaves.

Hydrofluoric acid (bp, 19.5°C; Tc, 188°C; Pc, 64 atm), nitrogen dioxide (bp,

21°C; Tc, 158.2°C; Pc, 100 atm; in equilibrium with N2O4), sulfuric acid (decomposition at 280°C), and polyphosphoric acid are candidates for solvents in solvothermal reactions, and the reactions of these solvents will produce a variety of

products that cannot be prepared by any other methods. For example, Bialowons

et al.37 reported that solvothermal treatment of (O2)2Ti7F30 in anhydrous HF at

300°C yielded single crystals of TiF4. Solvothermal reactions in these solvents

may produce fruitful results and a new field seems to be awaiting many researchers.



B. ORGANIC MEDIUM

1. General Considerations

Various organic solvents have been applied for the synthesis of inorganic materials. Because most of inorganic synthesis researchers are not familiar with

organic solvents, some important features are summarized here.



© 2005 by Taylor & Francis Group, LLC



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Since organic compounds easily react with oxygen in highly exothermic

reactions (i.e., combustion of organics), the gas phase in the reaction vessel must

be completely purged with an inert gas such as nitrogen. From a thermodynamic

point of view, all organic compounds have an inherent tendency to decompose

into carbon, hydrogen (or water, if the organic compound has oxygen atoms),

nitrogen, and so on, at high temperatures. Methane is the most stable aliphatic

hydrocarbon, however, it can decompose into carbon and hydrogen at temperatures above 360°C in the presence of a suitable catalyst such as nickel or iron.38

Therefore most of the organic compounds, even though they may be stable at

room temperature, act as reducing agents at high temperatures. To avoid reduction

during the course of the reaction, the reaction can be carried out in the presence

of air in the gas phase in the autoclave. For example, microcrystalline α-LiFe5O8

powders were directly synthesized by a reaction of FeCl2 · 4H2O and lithium metal

in ethylenediamine at 120°C.39 In this reaction, one-fifth of the iron ions must be

oxidized by air. In this reaction system, however, the gas phase in the autoclave

can be an explosive mixture. Since an electric spark is required to ignite the

explosive mixture, the reaction can be carried out without any problems. However,

even if one run of the experiment is conducted without any troubles, one should

not think that another run of the same experiment can be carried out safely,

because static electricity may supply sufficient energy to explode the gas mixture.

Nor should one scale up the reaction, as an increase in the gas phase volume

drastically increases the risk of explosion.

Carbon nanotubes can be prepared using solvothermal reactions,19,20 however,

inherent decomposition of organic compounds into carbon and hydrogen is not

yet utilized in the solvothermal synthesis of nanotubes, but hexachlorobenzene

was reduced by alkali metal according to the following reaction:

C6Cl6 + K → KCl + 6/nCn (carbon nanotubes).

Absolutely dried organic solvents are highly hygroscopic, even though the

solubility of water in organic solvent is quite low. The solubility of water increases

with an increase in the reaction temperature.

2. Paraffins

Paraffins have low dielectric constants and therefore are essentially inert in inorganic synthesis. Because of the relatively low solubility of water, the activity of

water in these solvents easily approaches unity, and when inorganic particles are

present in the medium, water is easily adsorbed on the particles, where water

may facilitate hydrothermal conversion of the inorganic particles (see Section

III.A.1 in this chapter).

n-Hexane (this is the common name; the International Union of Pure and

Applied Chemistry [IUPAC] name is hexane) is one of the most common aliphatic

solvents; however, this compound has a specific toxicity that is not shown by

branched C6 hydrocarbons nor by other straight-chain paraffins such as pentane



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Chemical Processing of Ceramics, Second Edition



and heptane. The origin of this toxicity is well established. Through the metabolic

system of human beings, this compound is oxidized by oxygenase to 2,5-hexanedione, which attacks the peripheral nervous system.40 Therefore the author

recommends that readers use heptane in place of hexane.

n-Paraffins with various carbon numbers are commercially available. Since

the boiling point of the organic solvent usually increases with an increase in

molecular weight, one can carry out the solvothermal reaction at relatively low

pressure by using higher paraffins such as tetradecane (bp = 253°C) or hexadecane

(bp = 287°C). However, higher straight-chain paraffins have quite low autoignition points (201°C for hexadecane), thus contact of heated paraffins with air or

leaking of these solvents from the autoclave will cause spontaneous ignition.

When higher hydrocarbons are desired for use as reaction solvents, mineral

oil (bp = 260 to 330°C) is recommended because the autoignition temperature

of mineral oil (260 to 370°C) is much higher than those of the n-paraffins. Since

it is produced in large quantity, it is quite cheap, and is a mixture of various

branched aliphatic hydrocarbons.

3. Aromatic Hydrocarbons

Aromatic hydrocarbons are relatively inert and have slightly higher base and

dielectric constants as compared with the paraffins. Benzene is the simplest

aromatic hydrocarbon and is thermally stable at high temperatures. Some

researchers favor the use of benzene for the medium of solvothermal reactions.

For example, the reaction of GaCl3 with LiN3 in benzene at 300°C yields gallium

nitride (GaN).41 Similarly the reaction of TiCl4 with NaN3 yields TiN.42 The

authors of these reports called this reaction procedure a “benzene-thermal” reaction. However, benzene is highly toxic and it causes fatal damage to hemopoietic

organs. The author strongly recommends that readers use toluene in place of

benzene. Although toluene also attacks the nervous system, the methyl group in

toluene is easily oxidized by oxygenase and the thus-formed benzoic acid reacts

with glutathione and is digested.

Solvothermal reaction (thermal decomposition) of metal alkoxides in toluene

usually yields the corresponding metal oxides (see Section III.B.1). Aromatic

hydrocarbons are favored for this reaction over aliphatic hydrocarbons because

of the higher solubility of the precursors in the former than those in the latter.

Xylenes (dimethylbenzenes) are also suitable solvents for the solvothermal synthesis.

Naphthalene is solid at room temperature, but is known to be a good solvent

for various organic reactions. When this solvent is used for the synthesis of

inorganic materials, it may cause problems in washing out the solvent by other

low boiling point solvents such as acetone and methanol. However, naphthalene

easily sublimes, and therefore this solvent can be eliminated by sublimation. In

this regard, this method may avoid the coagulation of particles during the drying

stage, which is caused by the surface tension of the liquid remaining between

the product particles.



© 2005 by Taylor & Francis Group, LLC



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4. Alcohols

Solvothermal reactions in alcohols are sometimes called “alcohothermal” reactions; this word is derived from alcoholysis based on the similarity between

“hydrothermal” and “hydrolysis.” Alcohols are the most common solvents for

sol-gel synthesis. Primary alcohols are fairly stable at higher temperatures (up to

360°C) and therefore are widely used for the solvothermal reactions.43 For example, amorphous gel derived by hydrolysis of metal alkoxides can be crystallized

by solvothermal treatment in alcohols. Since lower alcohols (methanol, ethanol,

and 1-propanol) are completely miscible with water, water molecules present in

the precursor gel may be replaced with the solvent alcohols. Therefore the precursor gel is easily dispersed in the solvent, where crystallization takes place.

Detailed mechanisms for the formation of crystals are not yet fully elucidated.

Crystallization of metal oxides is usually reported to take place by dissolutionrecrystallization mechanisms, but the mechanism seems to depend on the gel

structure. Moreover, water molecules dissolved from the gel in the reaction

medium may facilitate crystallization of the product. More discussion is given in

Section III.D of this chapter.

Note that the primary alcohols corrode aluminum,44 and therefore aluminum

cannot be used as the sealing material for the autoclave when solvothermal

reactions in primary alcohols are performed.

Since noble metal ions have a high tendency to be reduced to the metallic

state, noble metal particles are easily formed by heating noble metal compound

in alcohols at relatively low temperatures.45 In these reactions, alcohols are oxidized to the corresponding aldehydes and then to the carboxylic acids.

Secondary alcohols such as 2-propanol (the common name is isopropyl

alcohol; some researchers use “isopropanol,” but this name is created by confusion of the common and IUPAC names) and 2-butanol are easily dehydrated

at temperatures of 200 to 300°C, liberating olefins and water. Metal alkoxide

can be hydrolyzed by the thus-formed water. This method was first reported by

Fanelli and Burlew46 (see Section III.B.8). Since water is formed homogeneously in the reaction system, this method can be regarded as the sol-gel

version of the homogeneous precipitation method.47 The usual precipitation

method uses the precipitation reagent (acid or alkali), and addition of these

reagents to the reaction system causes heterogeneity in pH in the system because

of a large difference in pH between the system and the reagent. On the other

hand, in the homogeneous precipitation method, pH of the system is homogeneously changed by, for example, the hydrolysis of urea intentionally added to

the reaction system.

Dehydration of alcohols proceeds by heterolytic cleavage of the C–O bond,

yielding carbocation and hydroxide anion, and the dehydration rate is determined

by the stability of the thus-formed carbocation. Therefore tertiary alcohols such

as tert-butyl alcohol (2-methyl-2-propanol) are more easily dehydrated. When

these solvents are used for the solvothermal reaction, the essential nature of the

reaction may be identical to that of the hydrothermal reaction.



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Chemical Processing of Ceramics, Second Edition



5. Glycols

The solvothermal reaction in glycols is called a “glycothermal” reaction. Ethylene

glycol (1,2-ethanediol) is the simplest glycol. This compound is stable at high

temperatures. The hydroxyl group is an electron-withdrawing group (electrons

are withdrawn through the C–O covalent bond) and the carbocation formed by

the cleavage of a C–O bond is destabilized by the intramolecular hydroxyl group.

Therefore dehydration of ethylene glycol barely proceeds.

This molecule has two hydroxyl groups that can donate their lone-pair electrons to those that are electron deficient, such as a metal cation. Thus it forms

stable chelates with many metal ions and easily dissolves various metal salts. A

number of glycolato complexes have been prepared and their crystal structures

determined.48–50

One of the most significant results in solvothermal chemistry reported thus

far is the synthesis of silica-sodalite, reported by Bibby and Dale,13 which has

never been prepared in aqueous systems. Ethylene glycol was used as the solvent

in this synthesis, and it was shown that ethylene glycol is trapped in the cage of

the sodalite.51

Ethylene glycol can be used to reduce metal ions to the metallic state. All

the noble metals can be easily reduced by heating the metal salt in glycol at

temperatures of 120 to 200°C,5,6,52,53 and therefore reactions can be carried out

in an open system. Nickel is also reduced, but iron is not reduced to the metallic

state. This method was first reported by Figlarz et al.,5 and they called this method

the polyol process.

The author synthesized various mixed oxides using 1,4-butanediol. A more

detailed discussion of this solvent is given in Sections III.B.9 and III.C.

6. Cyclic Ethers

Tetrahydrofuran (THF) and 1,4-dioxane are the most common cyclic ethers. Since

the basicity of cyclic ethers is approximately 1 pKa unit higher than acyclic ethers,

these compounds are widely used as solvents in many organic reactions. However,

it should be noted that peroxide is formed by the reaction with molecular oxygen.

The peroxides formed from these solvents are relatively stable and therefore can

accumulate during storage. Once the reagent bottle is opened, the remaining solvent

should be discarded. Old reagent should not be used because accumulated peroxide

easily explodes on heating. It should be noted that tetrahydrofuran can polymerize

with acid compounds in an exothermic reaction, and therefore acid compounds

should be avoided in the reaction system. Some mixed metal oxides such as silicaalumina and titania-silica have a highly acidic surface and the reaction systems

which may produce these oxides in tetrahydrofuran should be avoided.

7. Carboxylic Acids and Esters

Carboxylic acids can be used as the media for the solvothermal synthesis of

inorganic materials. Synthesis of magnetite by thermal decomposition of an iron

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extract from the aqueous solution with a carboxylic acid has been reported (see

Section III.B.4 of this chapter).54 In organic chemistry, it is well known that carboxylic salts of divalent metal cations decompose into ketone yielding carbon dioxide.

Essential oils and vegetable oils are carboxylic acid esters of glycerin. In an

inert atmosphere, these compounds are fairly stable at high temperatures. When

they are heated in an open system, however, they are gradually oxidized by air

and begin to decompose before they evaporate. When aluminum hydrogels are

dropped into heated vegetable oils, alumina can be formed with evaporation of

water. In this method, hydrothermal conditions are partially formed; thus boehmite particles are formed which combine with primary particles of amorphous

alumina, and after calcinations, γ-alumina granules with sufficient mechanical

strength are formed.

8. Amines

Ammonia as the solvothermal medium was described in Section II.A.2. Alkyl

amines are similar to alcohols: lone-pair electrons on the nitrogen atom can

stabilize metal cations, while protons on the nitrogen atom can stabilize anions

by the formation of hydrogen bonding. The C–N bonds are more stable than the

C–O bonds, and alkyl amines have higher thermal stabilities than the corresponding alcohols. Various amines are commercially available and a variety of amines

have been used as templates (structure-directing agents) in the hydrothermal

crystallization of zeolites and open-framework metal oxides and phosphates.55,56

a. Hydrazine hydrate (H2N–NH2 · H2O)

It was reported that tetragonal SnO powders with a disc-like morphology were

solvothermally prepared from stannous oxalate (SnC2O4) in hydrazine hydrate at

50°C–200°C.57 However, hydrazine hydrate decomposes on heating or exposure

to ultraviolet light to form ammonia, hydrogen, and nitrogen, therefore great care

must be taken to prevent overheating of the reaction system. Moreover, contact

with cadmium, gold, brass, molybdenum, and stainless steel containing greater

than 0.5% molybdenum may cause rapid decomposition of hydrazine.58 A Teflonlined autoclave should be used and this solvent should not be used at high

temperatures. The autoignition temperature is reported to be 280°C. Similarly,

all the compounds that contain N–N, N–O, N–Cl, O–O, and Cl–O bonds can

explosively decompose, and therefore great care should be taken when using

these compounds.

b. Ethanolamine (2-aminoethanol), diethanolamine

(2,2’-iminobisethanol), and triethanolamine

(2,2’,2’’-nitrilotrisethanol)

All these compounds are thermally stable and therefore are good solvents for

solvothermal reactions. Since the amino group also donates its lone-pair electrons

to Lewis acid (metal cation), metal cations are stabilized and triethanolamine is

a tetradentate ligand. Laine et al.59 reported that some metal oxides and hydroxides

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Chemical Processing of Ceramics, Second Edition



such as silica and aluminum hydroxide are completely dissolved in ethylene

glycol in the presence of triethanolamine by refluxing the solvent with continuous

distilling of the water produced by formation of triethanolamine complex of the

metal ions. They used dissolved complex as the precursor for mixed oxides via

spray pyrolysis.59–62

c.



Ethylenediamine, diethylenetriamine,

triethylenetetramine

These solvents dissolve the chalcogen elements sulfur, selenium, and tellurium,

and therefore can be used for the solvothermal synthesis of metal chalcogenides

by the reaction of metal oxalate with chalcogens at 120°C–200°C.63–65 It has been

reported that the reaction also proceeds in tetrahydrofuran or pyridine, but the

reaction in polyamines having more than two chelating atoms proceeds more

completely.65

9. Other Nitrogen-Containing Compounds

a. Acetonitrile

Acetonitrile has medium basicity and medium polarity, and therefore this compound is used for the synthesis of inorganic compounds such as polyoxometalates.15,66–68

b. Nitromethane

This solvent is unique because it is poorly basic but has a high polarity. However,

the nitro group is thermally unstable and therefore this solvent should not be

heated. Nitrobenzene has a similar specificity; moreover, this compound is more

toxic than nitromethane. Another compound having similar properties is sulforane. This compound is synthesized by the reaction of 1,3-butadiene and sulfur

dioxide followed by hydrogenation of the C=C double bond and is now widely

used for extraction of aromatic compounds from the residue of steam cracking

for the synthesis of lower olefins. The thermal stability of sulforane (decomposes

slightly at 285°C) is much higher than that of nitromethane.69

10. Dipolar Aprotic Solvents

Dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and hexamethylphosphoramide (C6H18N3OP; HMPA) are highly basic and have high dielectric

constants. Because of the high basicity of these solvents, cations are highly

solvated but anions are left unsolvated.70 Therefore anions in these solvents have

high reactivities. Only a few papers have dealt with the effect of these solvents

in inorganic synthesis.

Since these solvents have high affinities to protein, they have a high toxicity

to liver. Because of the relative low vapor pressures of these solvents at room

temperature, there is relatively low risk when these solvents are used at lower

temperatures. However, solvothermal reactions use these solvents at high temperatures, and therefore small leaks from the autoclave can cause severe damage

© 2005 by Taylor & Francis Group, LLC