Synthesis of titanium tetraalkoxides from hydrous titanium dioxide and dialkyl carbonates Eiichi Suzuki, Satoru Kusano, Hiroshi Hatayama, Masaki Okamoto and Yoshio Ono Department of Chemical Engineering, T okyo Institute of T echnology, Ookayama,Meguro-ku, T okyo 152, Japan The reaction of hydrous titanium dioxide (TiO2·nH2O, n=0.15–1.23) and dialkyl carbonates at 453–573 K oVers a convenient synthetic method for titanium tetraalkoxides which are free from chlorine-containing impurities.Thus, hydrous titanium dioxide was almost completely converted into Ti(OEt)4 by its reaction with diethyl carbonate at 493 K for 16 h. The reaction proceeded faster in the presence of a sodium hydroxide catalyst. From the hydrous titanium dioxide and dipropyl carbonate, Ti(OPrn)4 was obtained in a high yield.Metal alkoxides are important chemicals as starting materials heating under reduced pressure at 303 K to dryness and then at 353 K. The value of n did not change by this catalyst- for preparing ceramics by the sol–gel method. One of the methods to prepare metal alkoxides is the reaction of metal loading procedure. A 20 mmol portion of hydrous titanium dioxide (1.96 g for chloride with an alcohol or with sodium alkoxide.1 Metal oxides could be starting materials for chlorine-free n=1) and diethyl carbonate (24.4 cm3, 200 mmol) were introduced into a 120 cm3 autoclave.After the atmosphere was synthesis of metal alkoxides, if they were reactive enough towards appropriate organic compounds. The reactivity of a replaced with nitrogen, the autoclave was heated to the desired temperature at a heating rate of 90 K h-1 and reactions were metal oxide, silicon dioxide, has been demonstrated.Rosenheim et al.2 reported the transformation of silica into hexacoordi- conducted under autogenous pressure with stirring. After cooling, the reaction mixture was transferred into a round-bot- nated dianion complexes using pyrocatechol as a complexing agent under basic conditions.Laine et al.3,4 reported the tomed flask (capacity 50 cm3) in a nitrogen atmosphere, and titanium tetraethoxide was separated by distillation under formation of pentacoordinated silicates from silica, ethylene glycol and a base. Silicon alkoxides can be obtained by the reduced pressure. The product thus isolated was weighed to determine its yield.The product, titanium tetraethoxide, was reaction of silica with alcohols such as ethanol using potassium hydroxide as a catalyst under the conditions where water is identified by 1H NMR and IR absorption spectroscopy.† For the reactions of hydrous titanium dioxide with dipropyl continuously removed by azeotropic distillation.5 Ono and coworkers6–8 reported that silicon alkoxides can be easily carbonate, the same reaction and analytical procedures as those with diethyl carbonate were adopted, except that the synthesized by the reactions of silica gel with gaseous dialkyl carbonates in the presence of an alkali hydroxide [eqn. (1)].amounts of hydrous titanium dioxide and dipropyl carbonate charged into the autoclave were 15 mmol (1.47 g for n=1.0) SiO2+2ROC(NO)OR�Si(OR)4+2CO2 (R=Me, Et) and 23.2 cm3 (150 mmol), respectively.(1) In this work, we report the synthesis of titanium tetraalkox- Results and Discussion ides by the reaction of hydrous titanium dioxide (TiO2·nH2O, n=0.15–1.23) with dialkyl carbonates. The reaction is given Reaction of hydrous titanium oxide with diethyl carbonate by eqn. (2). (a) EVect of reaction temperature.The reaction of hydrous TiO2·nH2O+(2+n)R¾OC(NO)OR¾� titanium dioxide (TiO2·H2O) with diethyl carbonate was carried out at various temperatures for 16 h. Fig. 1 shows the Ti(OR¾)4+(2+n)CO2+2nR¾OH (R¾=Et, Prn) (2) yield of titanium tetraethoxide, Ti(OEt)4, as a function of This reaction oVers a simple method to synthesize chlorine- reaction temperature.At 433 K, Ti(OEt)4 was not obtained. free titanium tetraalkoxides. At 453 K, the yield was 77% and reached ca. 95% at 493 and 533 K. The yield slightly decreased at 573 K. Thus, in the temperature range 493–533 K, practically complete conversion Experimental of hydrous titanium dioxide to Ti(OEt)4 can be attained. Hydrous titanium dioxide was prepared as follows. To a Fig. 2 shows the change in the yield of Ti(OEt)4 with reaction 160 cm3 portion of an aqueous solution of titanium(IV) sulfate, time at 453 and 493 K.At 453 K, the presence of the induction with stirring, was added dropwise aqueous ammonia until the time was clear. The yield was almost negligible over the first pH reached 7.0. After the suspension of the precipitate was 4 h, and then sharply increased with reaction time to reach ca.kept stirring overnight at a constant pH of 7.0, the precipitate 60% at 10 h. The yield only slightly increased to 65% by was separated by filtration, washed with water four or five times, and then dried in an ambient atmosphere at 383 K for † The IR spectrum for the product obtained in this work is in 1–18 h. The hydrous titanium dioxide (TiO2·nH2O) thus agreement with that of the published data (T he Aldrich L ibrary of FT - IR Spectra Edition 1, vol. 1, ed. C. J. Pouchert, 1985), except that obtained had n in the range 0.15–1.23 depending on the drying several minor bands in the published spectrum are missing. Titanium time, as determined by thermal gravimetric analysis. tetraethoxide containing chlorine-containing impurities from a com- When a catalyst, for instance, sodium hydroxide, was used, mercial source, which had been produced from titanium tetrachloride, a 5 mass% portion (based on TiO2) of the catalyst was showed these minor bands together with the bands observed for the supported by an impregnation method: hydrous titanium product obtained in this work.This indicates that the bands which dioxide was immersed for 1 h in an aqueous solution of the are missing for the sample prepared in this work are probably attributed to chlorine-containing titanium species.catalyst (as small an amount of water as possible), followed by J. Mater. Chem., 1997, 7(10), 2049–2051 2049Fig. 3 EVect of n-value in TiO2·nH2O on the titanium tetraethoxide Fig. 1 Reaction-temperature dependence of the titanium tetraethoxide yield.Reaction conditions: TiO2·nH2O 1.65–2.04 g (20 mmol based on yield. Reaction conditions: TiO2·nH2O (n=1.0) 1.96 g (20 mmol based TiO2), diethyl carbonate 24.4 cm3 (200 mmol), reaction time 16 h, and on TiO2), diethyl carbonate 24.4 cm3 (200 mmol), and reaction time reaction temperature 453 K (#), 473 K (6) and 493 K (%). 16 h. Alkali-metal chlorides, NaCl and KCl, exhibited a retarding eVect, the yield being 72 and 71%, respectively. Sodium hydroxide is the most eVective catalyst.Fig. 2 also shows the time courses of the Ti(OEt)4 yield obtained for the reactions using the NaOH catalyst. Here the promoting eVect of NaOH is clearly seen; at 453 K, the induction time was shortened, and higher yields of Ti(OEt)4 were obtained.At 493 K, a very high yield of 96% was attained after only 6 h. (d) EVect of diethyl carbonate/hydrous titanium dioxide ratio. The eVect of the molar ratio of diethyl carbonate to hydrous titanium dioxide (TiO2·nH2O, n=0.94) on the Ti(OEt)4 yield was examined. The reactions were carried out at 503 K for 4 h Fig. 2 Time course of the titanium tetraethoxide yield in the presence and the results are shown in Fig. 4. Without a catalyst, a high and absence of sodium hydroxide catalyst. Reaction conditions: molar ratio of 10 was required to obtain a high yield of TiO2·nH2O (n=0.70) 1.85 g (20 mmol based on TiO2), diethyl carbon- Ti(OEt)4. Upon loading NaOH as a catalyst on hydrous ate 24.4 cm3 (200 mmol), and reaction temperature 453 K (#, $) and titanium dioxide, a molar ratio of 6 was suYcient to obtain a 493 K (%, &).Open and closed symbols represent the yield obtained high yield. for the reactions in the absence and presence of 5 mass% of a sodium Hydrous titanium dioxide is considered to consist of hydroxide catalyst, respectively. flocculates of small anatase crystals9 and the strre of hydrous titanium dioxide is schematically shown in Fig. 5.10 It contains a number of hydroxy groups as well as water further increasing the reaction time to 16 h. The presence of molecules coordinated to titanium ions. an induction time indicates that the reaction proceeds through Under the reaction conditions, the hydroxy groups on the an intermediate rate (or species) before the monomeric surface may react with diethyl carbonate to form ethoxy Ti(OEt)4 is finally formed.At 493 K, no induction period was observed. The yield monotonously increased with reaction time, and reached 95% at 16 h. (b) EVect of dehydration of hydrous titanium dioxide. The eVect of the extent of dehydration of hydrous titanium dioxide (TiO2·nH2O) on the yield of Ti(OEt)4 was examined. The extent of dehydration was varied by changing the time of drying the samples at 383 K, with n varying from 0.15 to 1.23.The reactions were carried out at 453, 473 and 493 K for 16 h. As shown in Fig. 3, at each reaction temperature, higher yields of Ti(OEt)4 were obtained with samples with higher n, i.e. with lower degree of dehydration. (c) EVect of the catalysts. In the reaction of silica gel with dialkyl carbonates, alkali-metal hydroxides and halides are eVective catalysts.6,7 Therefore, these compounds were tested as catalysts for the reaction of hydrous titanium dioxide with Fig. 4 EVect of molar ratio of diethyl carbonate (DEC) to hydrous diethyl carbonate. After the catalysts were supported by 5 titanium dioxide on the titanium tetraethoxide yield. Reaction con- mass%, the reactions were carried out at 493 K for 4 h.When ditions: TiO2·nH2O (n=0.94) 0.484 g (5 mmol based on TiO2), diethyl the catalyst was LiOH, NaOH, KOH and CsOH, the yield carbonate 2.4–6.1 cm3 (20–50 mmol), reaction time 4 h, and reaction of Ti(OEt)4 was 91, 93, 90 and 82%, respectively, while temperature 503 K. Open and closed symbols represent the yield the reaction using no catalyst gave a 79% yield.All the obtained for the reactions in the absence and presence of 5 mass% of a sodium hydroxide catalyst, respectively. alkali-metal hydroxides examined showed a promoting eVect. 2050 J. Mater. Chem., 1997, 7(10), 2049–2051Conclusion Titanium tetraethoxide (or tetrapropoxide) is prepared by the reaction of hydrous titanium dioxide and diethyl (or dipropyl) carbonate.The reaction proceeds almost to completion at 493–533 K after 16 h. Titanium tetraalkoxide is formed by the successive cleavage of TiMOMTi bonds in hydrous titanium dioxide by reaction with dialkyl carbonate molecules with formation of TiMOEt bonds and release of carbon dioxide. Use of a catalyst such as sodium hydroxide remarkably reduces the time required to accomplish the reaction.Reaction of the catalyst with hydrous titanium dioxide presumably occurs, giving rise to the cleavage of the TiMOMTi bond and simultaneously, the formation of a TiMOEt bond. The catalyst may facilitate the formation of an EtO group from a diethyl Fig. 5 Structure of hydrous titanium dioxide10 carbonate molecule, which attacks the TiMOMTi bond to form the TiMOEt bond.groups, ethanol and carbon dioxide [eqn. (3)]. TiMOH+EtOC(NO)OEt�TiMOEt+EtOH+CO2 (3) References The water molecules coordinated to titanium ions readily react 1 D. C. Bradley, R. C. Mehrotra and D. D. Gaur, Metal Alkoxides, with diethyl carbonate to give ethanol and carbon dioxide. Academic Press, London, 1978, p. 23. 2 A. Rosenheim, B. Raibmann and G. Schendel, Z. Anorg. Allg.H2O+EtOC(NO)OEt�2EtOH+CO2 (4) Chem., 1931, 196, 160. By these reactions, the hydroxy groups and the water molecules 3 R. M. Laine, K. Y. Blohowiak, T. R. Robinson, M. L. Hoppe, P. Nardi, J. Kampf and J. Uhm, Nature (L ondon), 1991, 353, 642. in hydrous titanium dioxide are scavenged as ethanol, and the 4 K. Y. Blohowiak, D. R. Treadwell, B. L. Mueller, M. L. Hoppe, reaction system is always kept free from water, which may S.Jouppi, P. Kansal, K. W. Chew, C. L. S. Scotto, F. Babonneau, promote the recondensation of the alkoxide product, Ti(OEt)4. J. Kampf and R. M. Laine, Chem. Mater., 1994, 6, 2177. 5 D. L. Bailey and F. M. O’Connor, US Pat., 2 881 198, 1959. Synthesis of titanium tetrapropoxide 6 E. Suzuki, M. Akiyama and Y. Ono, J. Chem. Soc., Chem. Commun., 1992, 136. Hydrous titanium dioxide (TiO2·nH2O, n=0.70) reacted with 7 Y. Ono, M. Akiyama and E. Suzuki, Chem. Mater., 1993, 5, 442. dipropyl carbonate to aVord titanium tetrapropoxide, 8 M. Akiyama, E. Suzuki and Y. Ono, Inorg. Chim. Acta, 1993, Ti(OPrn)4. The reaction at 493 K for 16 h without catalyst 207, 259. 9 W. F. Sullivan and S. S. Cole, J. Am. Ceram. Soc., 1959, 42, 127. gave an 84% yield of Ti(OPrn)4. When sodium hydroxide was 10 F. Inoue and M. Kaneko, Kogyo Kagaku Zasshi, 1971, 74, 591. used as a catalyst (5 mass%), reaction at 493 K for 6 h gave a yield of 92%. The product was identified by IR and 1H NMR spectra. Paper 6/08327G; Received 10th December, 1996 J. Mater. Chem., 1997, 7(10), 2049–2051