Glycothermal reaction

As discussed in Section 1 of this chapter, the prefix solvo- means any kinds of solvent in this section, however, solvothermal is used to refer to reactions in organic solvents. When alcohols and glycols are used as the reaction media, the reactions are called alcohothermal and glycothermal reactions, respectively. 

Hydrothermal synthesis of a-alumina has been well studied. Since the hydro-thermal reaction of aluminum compound yields boehmite at relatively low temperatures (approximately 200°C), transformation of boehmite was examined and it was reported that more than 10 hours is required for complete conversion into a-alumina, even with a reaction at 445°C in a 0.1 N NaOH solution and in the presence of seed crystals. On the other hand, under glycothermal conditions, a-alumina is formed at 285°C for 4 h. The equilibrium point between diaspore (another polymorph of AlOOH) and a-alumina under the saturated vapor pressure of water was determined to be 360°C. 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 a-alumina requires a much higher temperature. Since boehmite is slightly less stable than diaspore, the hypothetical equilibrium point between boehmite and a-alumina would be lower than that for diaspore-alumina. However, a-alumina would not be formed by a hydrothermal reaction at such a low temperature as has been achieved in the glycothermal reaction. 

Cho et al. reported that a-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 a-alumina. They also reported that hydroxyl ions, acetic acid, and pyridine added to the glycothermal reaction system affect the morphology of the a-alumina particles because of their preferential adsorption to a specific surface. ... 

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. ° 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-0) facilitate cleavage of the bond between acetate and RE ions. 

Glycothermal reaction of two starting materials gives various crystalline mixed oxides, and some of the mixed oxides synthesized by the glycothermal method are summarized in Table According to our working hypoth-... 

For auother possible reactiou mechauism for the formatiou of mixed oxides, binary alkoxide (glycoxide) may be formed prior to the formation of the mixed oxide. In fact, various binary glycoxides have been prepared and their crystal structures have been elucidated. Formation of BaTiOj by the glycothermal reaction may be explained by this mechanism, because Ba-Ti binary alkoxide is well established. However, formation of garnet phases (Sections III.C.l through III.C.3) cannot be explained by this mechanism because addition of seed crystals, in some cases, gives a product with a chemical composition completely different from that of the product obtained without the addition of the seed crystals. 

Kominami et al. ° reported that the amorphous product obtained by solvothermal decomposition of La(OiPr)3 and Fe(OBu)3 in toluene crystallized into perovskite-type LaFe03 using glycothermal treatment, while direct glycothermal reaction of the two starting materials yielded a mixture of La(OH)3 and magnetite. 

Table 1 lists the support used in the present study. TIO is a Reference Catalyst of the Catalysis Society of Japan (JRC-TIO-4, equivalent to P-25) [9]. ST (supplied from Catalysts and Chemicals Ind. Co., Ltd.) was prepared by the coprecipitation method. GT and XG were prepared by the thermal reaction of titanium tetraisopropoxide and tetraethylorfriosilicate in glycols (glycothermal reaction) [7]. HT was prepared by the hydrolysis of titanium alkoxide in toluene with water that was dissolved from gas phase at high temperature [8]. 

Figure 2. Kinetic evaluation of the glycothermal reaction at 300°C by phase identification via X-ray diffraction. (A) Precursor material. (B) 2 hours. (C) 6 hours. (D) 12 hours. (Reproduced with permission from ref. 13. Copyright 1996 The American Ceramic Society.)...

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 a-alumina formation rather than the retardation expected from the equilibrium point of view. 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 a-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 a-alumina. Thus conversion of this compound into a-alumina has a much larger driving force. The smaller crystallite size of the glycol derivative of boehmite also contributes to the instability of the intermediate. 

Monodispersed particles are formed for garnet with smaller RE ions. Mon-odispersed 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.Once nucleation occurs in the glycothermal synthesis of gallium... 

Hydrothermal synthesis is also known mainly by chemists as solvothermal synthesis, a boarder term meaning any chemical reaction in the presence of a solvent in supercritical or near supercritical conditions. Likewise, there are other terms such as glycothermal, alcothermal, ammonothermal, depending npon the type of solvent nsed in such reactions. However, the purpose behind using these different solvents in the chemical reactions is essentially to bring down the pressure and tanperature conditions. 

Previous work (13,14) has shown that the glycothermal synthesis process, a liquid phase precipitation at elevated temperatures under autogeneous pressure using a glycol as solvent, for a-Al203 is mediated by the formation of a precursor pseudo-boehmite phase (15). The pseudo-boehmite is thermodynamically unstable under the reaction conditions, and transforms to the stable a-Al203 (similar to the formation of alumina in the hydrothermal system (16)). Thus, the overall reaction is as follows. 

LeChatelier s principle favors the generation of the product in a non-aqueous environment. In hydrothermal processing, the solvent is a product and the level of a-AI2O3 permitted in the reaction is inhibited. Hydrothermal processing produces alumina at 405°C and 34.5 MPa in a reaction time of days to weeks. In contrast, glycothermal processing of alumina proceeds at a minimum temperature of 270°C, 4.1 MPa and 12 hours. 

Glycothermal synthesis seems to be very attractive method for catalytic materials preparation, especially when microwave heating was applied. Research cany on this field indicate on many advantages of this method as short synthesis time, phase purity with better yield, homogeneity and high reproducibility [1-3], Various glycols may be used as reaction medium, and different reaction parameters (temperature, pressure, time, and others) can be applied what has influence on the properties of obtained materials [4], As a result, it provides direct and effective one step route to prepare nanomaterials of well-controlled properties, which can be used as catalyst or catalyst support... 

The aim of the current woik was to prepare and characterise Z11AI2O4 using microwave-assisted glycothermal method, and test Pt and Pd catalysts supported on it in total oxidation of diluted isobutane, selected as a VOC molecule. Reaction parameters were optimized to obtain low crystalline zinc aluminate spinel with high specific surface area... 

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