Materials Transactions, Vol. 50, No. 8 (2009) pp. 1964 to 1968 #2009 The Japan Institute of Light Metals

Preparation of Aluminumtriethoxide by Application of Aluminum Corrosion

Osami Seri and Daichi Sasaki*

Muroran Institute of Technology, Faculty of Engineering, Muroran 050-8585, Japan

Aluminumtriethoxide were obtained when aluminum powder was refluxed in dehydrated containing aluminum chloride for 10 ks. It is shown that aluminumtriethoxide powder made in our laboratory coincided with that supplied in commercial market by similar pattern of XRD-analysis, particle size distribution analysis and particle morphology observation. It is considered that aluminumtriethoxide in our laboratory was electrochemically interpreted as corrosion product which has been oxidized in ethanol containing aluminum chloride as catalyst. [doi:10.2320/matertrans.L-M2009815]

(Received January 5, 2009; Accepted April 20, 2009; Published July 25, 2009) Keywords: aluminum triethoxide, corrosion product, electrochemical, chloride ions, oxidation-reduction reaction

1. Introduction From the electrochemical point of view, both of the preparation of aluminum alkoxide and corrosion phenomena It is well known that aluminum oxide (alumina) is one of aluminum must be interpreted as same oxidation-reduction of the most important materials for industrial fields, such reaction of electrochemical process. The only difference as furnace-construction materials, abrasives and catalyst between them is their environments, the former is and carriers. Also the alumina is widely used in our dairy life, the latter is water. The above interpretation leads to an idea due to its superiorities not only in heat-resistance, corrosion that electrochemical analysis and its consideration used in resistance, and electric insulation, but also medical harm- corrosion phenomena in water environment can be applied to lessness. There are many preparation processes for alumina the preparation for aluminum alkoxide. such as Bayer process, heat decomposition of alum and In order to obtain the electrochemical possibility for low- sol-gel method.1) cost preparation of aluminum alkoxide, dissolution behavior Among them, it is well known that sol-gel method is a of aluminum in an alcohol environment has been electro- relatively new method for alumina preparation. It is pointed chemically investigated. Trial and error experiments guides out that the preparation of alkoxide for sol-gel method has to new findings that aluminum alkoxide (aluminum trieth- some merits and demerits.2) The merits are followings: it is oxide) has been synthesized in an ethanol with an addition possible ‹ to synthesize at low temperature, › to obtain of aluminum chloride. We report a fundamental technology homogeneity in microscopic order, fi to synthesize various for aluminum alkoxide production in low-cost and simple alkoxides with new compositions, fl to prepare a ceramics handlings.4) with fine particle size, to gain good efficiency in production process. The demerits are followings: it may 2. Experiments occur that ‹ organic matters are sometimes left in product, › composition of alkoxide may be changed to another 2.1 Ingredients compounds during sintering process, fi shrinkages in size Aluminum powder (Kanto Chemical Co. Ltd, 99.99 and change of shape may happen during sintering, and fl mass%, Al), aluminumtriethoxide (Kanto Chemical Co. generally too expensive. Ltd, 95.0 mass%, Al(OC2H5)3), anhydrous aluminum chlo- 3) As an example of aluminum alkoxides aluminum ride (Kanto Chemical Co. Ltd, 98.0 mass%, AlCl3), dehy- propoxide has been obtained as following processes. drated ethanol (Kanto Chemical Co. Ltd, 99.5 volume%, Roughly speaking, its process has three steps: step1 is that C2H5OH) and de-ionized water were used. metallic aluminum powder and isopropanol are mixed together and then refluxed in a reaction tower. Step2 is that 2.2 Preparation of aluminumalkoxide (aluminumtrieth- the mixed slurry obtained in step1 are heated and kept oxide) until the slurry will change into aluminum isopropoxide. Metallic aluminum powder (3 g) and AlCl3 (30 g) were Step3 is that the alkoxide in step2 is distilled into pure mixed, and then completely blended by using a magnetic alkoxide and dried to solid powder in a vacuum atmos- mixing mortar for 1.8 ks. The powder above was immediately phere. In the above process the aluminumisopropoxide is mixed and then poured into a flask with 300 ml volume which prepared by heating an isopropanol environment with has contained 200 ml ethanol. The flask was refluxed by a addition of mercury chloride. It is often said that any of mantle heater for about 10 ks. It has been confirmed that gas aluminum alkoxide now in use will be expensive for evolved in the refluxing process was gas by a gas utilization in industrial fields.2) Because aluminum alkoxides chromatograph analysis. After the refluxing process, grey- have some faults such as complicated process in removing colored solution was obtained. The solution was heated again environmental poison such as mercury and low efficiency to be solid state in an electric-heating apparatus at the in manufacturing. temperature of 351 K. For the purpose of following inspec- tions and analyses, the solid state substance was crushed *Undergraduate Student, Muroran Institute of Technology and re-grinded by the magnetic mixing mortar. Preparation of Aluminumtriethoxide by Application of Aluminum Corrosion 1965

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(b) Fig. 2 (a) Distribution of alumina particle size (made by this paper). (b) Distribution of alumina particle size (in the Japanese market).

Fig. 1 (a) XRD analysis of Al(OC2H5)3 which are calcinated in various sintering temperatures (made in our laboratory). (b) XRD analysis of Al(OC2H5)3 which are calcinated in various sintering temperatures (in the 1473 K completely coincided with that of -alumina. Japanese market). Comparing curves in Fig. 1(a) to that in Fig. 1(b), it is clear that X-ray curves of our alkoxide powder calcinated at 573, 873, 1173 and 1473 K show complete agreement with that 2.3 Measurements of the aluminum triethoxide in market, except for low range The aluminum alkoxide obtained by our method was at 298 K. identified by using XRD analysis, particle size distribution analysis and particle morphology observation. For this 3.2 Particle size distribution purposes, a desktop X-ray diffraction apparatus (Rigaku We compared the particle size distributions between our Co. Ltd, MiniflexII), automatic particle measurement aluminum alkoxide powder and the aluminum triethoxide apparatus (Nikkisou Co. Ltd, MT3000) and transmission powder in market. After the powder (3 g) was poured into electron microscope (JOEL Co. Ltd, JEM-2000FX) were beaker with 100 ml ethanol, the beaker was strongly shaken used. for about 600 s by ultrasonic apparatus and then measured its particle size distribution by the automatic particle 3. Results measurement apparatus. Figure 2(a) and Fig. 2(b) show typical examples of our powder and commercial powder, 3.1 XRD analysis5,6) respectively. Figure 2(a) shows that frequency curve may Ten kinds of powders, which are aluminum alkoxide be roughly divided into three areas: area ‹ submicron powders made in our laboratory and aluminumtriethoxide (0:21 mm) distribution, area › several micron (110 mm) powders supplied as a commercial product, were calcinated distribution and area fi several dozens micron (3080 mm) at various temperature (298 K of room temperature, 573 K, distribution. The accumulation curve of area ‹ and area › 873 K, 1173 K and 1473 K) by electric furnace for about in Fig. 1(a) tells us that almost all of powders were in size 3.6 ks and then XRD-analyzed. of submicron order. The characteristics of the area › Figure 1(a) shows XRD-analysis results of our laboratory exhibit shape with gentle and gradual slope distribution, but powders and Fig. 1(b) shows commercial powders. The X- not the well-known normal distribution. The area fi in ray profile patterns at temperatures of 298 K, 573 K and Fig. 2(a) shows almost same shape of that of Fig. 2(b); both 873 K in Fig. 1(a) show typical amorphous structures of indicate the sharp normal distribution patterns. It was found aluminum , because their profiles indicate gentle that the differences between Fig. 2(a) and Fig. 2(b) lie on and gradual flat pattern but not crystallographic peak two experimental facts; there are disappearance of the patterns. The curve at 1173 K has two peaks at the scattering area ‹ of Fig. 1(a), and whole shape of the distributions in angle of around 2 ¼ 46 and 2 ¼ 67 which may be crystal Fig. 2(b) is shifted to right direction side from that of the structure -alumina.5) It was found that X-ray profile curve at Fig. 2(a). 1966 O. Seri and D. Sasaki

Overview Detail

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Overview Detail

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Fig. 3 (a) TEM observation of alumina particles (made by this paper). (b) TEM observation of alumina particles (in the Japanese market).

3.3 Particle morphology important methods to prepare materials for ceramics, glasses, The particle shape of aluminum alkoxide powder in our , etc. and the metal alkoxide is important precursor laboratory and the aluminumtriethoxide in market were material for the method. Roughly summarizing, there are two observed by TEM. Figure 3(a) and Fig. 3(b) show each ways to obtain alkoxide materials: one is by oxidation- typical examples. reduction reaction and another is by substitution reaction. Left-hand side photo in Fig. 3(a), which is our aluminum Two ways for the alkoxide reactions above are generally alkoxide powder, shows various size particles; submicron expressed as followings. and several micron size particles are scattered. Right-hand M þ nROH ! M(OR)n þ n/2H2 "ð1Þ side photo in Fig. 3(a), which is detailed photo, is composed MCl þ nROH ! M(OR) þ nHCl ð2Þ with gathering of some 0:20:5 mm size particles by n n observing the shadows of the particles. In Fig. 3(b) which where, M is metal elements, R is alkyl group. is aluminumtriethoxide powder in market, it is often The oxidation-reduction reaction expressed in eq. (1) confirmed that larger size particles are also constructed by will proceed in quantitative manner, but unfortunately gatherings of some smaller size (submicron size) particles. oxidation-reduction reaction of eq. (1) is restricted to a few metals such as lithium and .2) The substitution 4. Discussion reaction of eq. (2) will proceed in poor reaction efficiency, since very few chloride ions of metal chloride MCln can be 4.1 Synthesis of metal alkoxide2) substituted with alkyl function (-OR) and also long reaction It is well-known that the sol-gel method is one of the most time is needed. Preparation of Aluminumtriethoxide by Application of Aluminum Corrosion 1967

4.2 Aluminum hydroxide and aluminum alkoxide as Table 1 Estimated values of chemical thermodynamic properties in Fig. 4. corrosion product 0 1 f G /kJmol The well-known reaction of aluminum corrosion in water Formula sate Remarks 298 K 351 K is expressed as a following reaction. Al cr 0 0 Al þ 3H2O ¼ Al(OH)3 þ 3/2H2 "ð3Þ Al3þ aq 485 478 Et-OH l 175 156 ethanol The aluminum hydroxide Al(OH)3, and hydrogen gas H2, as corrosion product will be obtained. Aluminum in an Et-O aq 102 95 ethylate, estimated value alcohol environment will be corroded as the following H2 g0 0 and then an aluminum alkoxide and Cl aq 131 125 AlCl3 cr 692 617 hydrogen gas H2 will be obtained as corrosion product. When an alcohol is ethanol, corrosion product will be aluminumtriethoxide Al(O-Et)3.

Al þ 3Et-OH ¼ Al(O-Et)3 þ 3/2H2 "ð4Þ 0.0 At 351K Equation above will be divided into two reactions; anodic - and cathodic reactions as followings, EtOH+e EtO +1/2H2 -0.5 At 298K

3þ / V vs. SHE Al ! Al þ 3e ð5Þ E A 3Et-OH þ 3e ! 3Et-O þ 3/2H "ð6Þ 2 - -1.0 Al+3Cl AlCl3+3e

It is well known for corrosion scientists and engineers that the Potential, corrosion rate of eq. (3) will be accelerated when the water Al Al3+ +3e 7) environment was contaminated with chloride ions. It is -1.5 electrochemically recognized that anodic new reaction 10-5 10-4 10-3 10-2 10-1 between aluminum and chloride ions will be formed when Current density, i / A · cm-2 an environment contains chloride ions. For example, a Fig. 4 Schematic explanation of corrosion of aluminum in an ethanol following anodic reaction will occur as an new anodic environment. reaction. Al þ 3Cl ! AlCl þ 3e ð7Þ 3 the specific resistance of aluminum oxide: kcm, l is Same anodic reaction of the eq. (7) will occur even if thickness of Al2O3 oxide film: cm, iCl;L is diffusion limit an environment is an alcohol which contains chloride ions. current density of chloride ion.: mAcm2.9–12) The anodic reaction above must be rapid reaction, so-called Hence, the polarization curves in eqs. (5), (6) and (7) will ‘‘fast system’’,8,9) because it has experimentally been found be plotted by substituting numerical values, which are that the eq. (7) has been severely attack reaction for tabulated in Table 1, for items in eqs. (8), (9) and (10). aluminum. The eqs. (5), (6) and (7) will simultaneously Figure 4 shows result.8,12) It is glanced that the anodic and vigorously occur when an environment will be heated. reaction curves of the eq. (10) governs the almost of anodic Since the relationship between eq. (5) and eq. (7) is char- reaction. acterized as mutual and competitive reactions. A reaction The spontaneous corrosion reaction occurs at a cross point which shows more rapid reaction rate will determine at which the curves of eq. (9) and eq. (10) intersects dominant rate of whole anodic reaction. The relation between together. In order to make the rate of the eq. (4) fast, it is electrode potential and current density (eqs. (5), (6) and (7)), necessary to increase the current density at the point . which is expressed as respective partial polarization curve, Above leads to more fundamental ways which are to increase are followings,8–11) the potential difference between eq. (9) and eq. (10) and to 8 decrease the gradient of both of eq. (9) and eq. (10) curves. > RT 1 RT > E ¼ E; þ ln þ ln i þ i Al2O3 l > Al/Al3þ a a Namely, it must be most effective way when larger values > 3F kAl3þ 3F ; RT 2 ; RT > of ðE þ ln k ÞðE þ ln kCl Þ are > (8) Et-O /Et-OH 2F EtO Al/AlCl3 F > necessary condition, and smaller values of both dE=d of <> ia ; RT 2 3RT E ¼ E þ ln k k lnði Þ eq. (10) and dE=di of eq. (9) is sufficient condition. Et-O /Et-OH 2F EtO H2 2F c c > > (9) > 4.3 Characteristics of aluminumtriethoxide made in our > RT RT 1 > ; > E ¼ EAl/AlCl þ ln kCl þ ln laboratory > 3 :> F F ðiCl ;L iaÞ Aluminumtriethoxide is used in various industrial fields (10) such as catalyst of polymerization reactions, transmaterials ; ; ; where, E 3þ , E and E is standard formal for esterification reactions, cross-linker substances for paints Al/Al Et-O /Et-OH Al/AlCl3 electrode potential: V vs.SHE, F is Faraday constant: and material of ceramics or medicines.13) In Japan only one 96:5 103 Cmol1, R is gas constant: 8.31 Jmol1K1, T company is recorded as the aluminumtriethoxide manufac- 13) 3þ is absolute temperature: K, kAl , kEt-O , kH2 and kCl are the turers, but detailed information about the aluminumtrieth- coefficients regarding diffusion of Al3þ ion, Et-O ion, oxide production is not opened. The main reasons why the hydroxide gas and Cl ion in ethanol: Acmmol1, Al2O3 is only one company supplies the aluminumtriethoxide may 1968 O. Seri and D. Sasaki be have some disadvantages such as complicated production commercial market showed almost same results of XRD process, long times for , safety and easy handling analysis, particle size distribution and particle morphology problems and so on. observation. In the corrosion science and technology point The merits and demerits of our aluminumtriethoxide of view, it was considered that the aluminumtriethoxide production may be as follows: obtained above is interpreted as aluminum corrosion product (Merits) corroded in ethanol. It is possible to analyze corrosion (1) It is possible to prepare by simple equipments. reaction of aluminum in ethanol: the electrochemical (2) It is possible to make the aluminumtriethoxide easy; procedure for corrosion reaction of aluminum in water can at low temperature (usually under about 373 K) and in apply to the preparation of aluminumtriethoxide in ethanol. a short time (less than a few hours). (3) It is possible to quantitatively evaluate the production Acknowledgements process and estimate quantity of the aluminumtrieth- oxide by assistance of corrosion science and engineer- We wish to express our sincere thanks to Dr. N. Masuko ing knowledge. (Professor Emeritus of Tokyo University) for useful advice (Demerits) and encouraging comments. (1) It is necessary that a corrosion-resistant material will be needed for plant and equipment materials due to the chloride ion. REFERENCES (2) It will be necessary to have a closed system process due to the evolution and spread of organic gases such as 1) K. Katayama, T. Ohkura, K. Hashimoto and Y. Yamashita: Kohgaku no tameno Mukizairyokagaku, (Science-sha, Japan, 2006) pp. 166–167. ethyl chloride and diethyl . 2) S. Sakka: Sol-Gel hou no Kagaku, (Agune-shoufusha, Japan, 2006) It is difficult to evaluate the advantage/disadvantage pp. 216–219. between our aluminum triethoxide method and commercial 3) D. C. Brandley, R. C. Mehrotra and D. P. Gaur: Metal Alkoxides, method in details, because the present preparation methods in (Academic Press, 1978) 5–58. the market are absolutely unclear. But it will be expected that 4) Japanese patent: JP2008121135(A). 5) The Japan Institute of Light Metals Report: Aluminum no cost will be lower by our method, because our merit (1) and Kinouhimaku to sono Ouyou, (The Japan Institute of Light Metals, (2) are more superior ways to have cost-cutting process. And 1985), pp. 1–9. our method will have more advantageous in environmentally 6) S. Ono: Kagaku Kohgyo 18 (1968) 384–360. harmless process such as mercury-free process and various 7) Japan Society of Corrosion Engineering: Kinzoku no Fushoku Q&A, production routes when quantitative analysis of merit (3) will (Maruzen, Japan, 1988) pp. 47–162. 8) O. Seri and Y. Kido: Mater. Trans. 50 (2009) 1433–1439. be carried out in careful manner. 9) G. Charlot, J. Badoz-Lambling and T. Tremillon: Electrochemical Reactions, (Elsevier Publishing Company, 1962) pp. 46–89. 5. Conclusion 10) O. Seri: Kinzoku Zairyo no Fushoku to Boushoku no Kiso, (Seizando- shoten, Japan, 2006) pp. 92–117. When metallic aluminum powder were poured into 11) O. Seri and Y. Kido: 113th Keikinzoku Kohen Taikai Yokoh-shu, (the Japan Institute of Light Metals, 2006) pp. 80–81. dehydrated ethanol with AlCl3 and then refluxed for about 12) ASM: J. Phys. Chem., Ref. Data, vol. 11 Suppl. 2 (1982). 3 h, aluminumtriethoxide were obtained. Aluminumtrieth- 13) The Chemical Daily: Chemicals, (Kagaku-kohgyo-nipposha, 2007) oxide powder made in our laboratory and that supplied in pp. 997–998.