Preparation of SO₄²⁻/TiO₂-La₂O₃ solid superacid and its catalytic activities in acetalation and ketalation

Preparation of SO 4 2− /TiO 2 -La 2 O 3 solid superacid and its catalytic activities in acetalation and ketalation 1 first-author †E-mail:yangshuijin@163.com †E-mail:yangshuijin@163.com yangshuijin@163.com 1 2 Department of Chemistry and Environmental Engineering, Hubei Normal University, Huangshi 435002, China Department of Chemistry and Environmental Engineering, Hubei Normal University, Huangshi 435002, China College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China SO 4 2− /TiO 2 -La 2 O 3 , a novel solid superacid, was prepared and its catalytic activities at different synthetic conditions are discussed with esterification of n -butanoic acid and n -butyl alcohol as probing reaction. The optimum conditions have also been found, mole ratio of n (La 3+ ): n (Ti 4+ ) is 1:34, the soaked consistency of H 2 SO 4 is 0.8 mol/L, the soaked time of H 2 SO 4 is 24 h, the calcining temperature is 480 °C, the calcining time is 3 h. Then it was applied in the catalytic synthesis of ten important ketals and acetals as catalyst and revealed high catalytic activity. Under these conditions on which the molar ratio of aldehyde/ketone to glycol is 1:1.5, the mass ratio of the catalyst used in the reactants is 0.5%, and the reaction time is 1.0 h, the yields of ketals and acetals can reach 41.4%~95.8%. SO 4 2− /TiO 2 -La 2 O 3 Rare earth Solid superacid Catalysis INTRODUCTION 1 There is considerable interest in synthesizing acetals and ketals as important perfume substances and industrial raw materials of organic synthesis (Loader and Anderson, 1978 1978 ; Dhrubojyoti et al., 1999 1999 ; Wang et al., 1992b 1992b ). Concentrated sulfuric acid and other analogous Brönsted acids used as catalysts in industrial processes for many years, exposed their defects, such as the corrosion of the reactors, difficulties in product separation and catalyst recovery, pollution by acidic waste water, poor quality of the products, etc. The alternative catalysts, for example, transition metal salt (Dhrubojyoti et al., 1999 1999 ; Li and Cheng, 2001 2001 ), TiSiW 12 O 40 /TiO 2 (Yang et al., 2003 2003 ), tungstosilicic acid supported on active carbon (Yang et al., 2005a 2005a ), montmorillonite K 10 (Li et al., 1996 1996 ), solid superacid (Wang et al., 1992b 1992b ), have been developed for environmental protection and techno-economical profit. Among substitutes, much attention has been paid to solid superacids in various industrial processes due to their successful application in several important acid-catalyzed reactions (esterification, acelation) (Wang et al., 1992b 1992b ; Kirbaslar et al., 2001 2001 ; Hess and Kemnitz, 1997 1997 ). Lu and Zu ( 2002 2002 ) reported that the SO 4 2− -M x O y -type solid superacid has excellent catalytic activity for esterification due to its super acidity in heterogeneous systems. However, the structural stability and the catalytic activity should be further improved. It was suggested that intermingling SO 4 2− -M x O y with transition metal oxides was a possible method for solving the above problems (Ma and Li, 2002 2002 ; Yang et al., 2005b 2005b ). We found it had better catalytic activity when La 2 O 3 was selected. In this paper, we report the compositing method for a new kind of solid superacid-SO 4 2− /TiO 2 -La 2 O 3 and discuss its catalytic activity under different synthetic conditions, esterification of n -butanoic acid and n -butyl alcohol as probing reaction. Solid superacid-SO 4 2− /TiO 2 -La 2 O 3 was used as catalyst to synthesize acetals and ketals for the first time and the corresponding catalytic activity was investigated. EXPERIMENTAL DETAILS 1 Materials and measurements 2 All chemicals were commercial products of reagent grade. The X-ray powder diffraction pattern of the powder samples was measured by a Rigaku model XRD-6000 X-ray diffractometer employing Cu K α radiation with the accelerating voltage 40 kV and applied current of 30 mA, respectively in scanning range of 2 θ of 10°~70°. The infrared spectra were recorded on a Nicolet 5DX FT-IR spectrophotometer with liquid membrane in the 400~4000 cm −1 region. 1 H NMR spectra were determined on a MERCURY-VX 300 MHz spectrometer with tetramethylsilane (TMS) as internal standard in CDCl 3 . The SO 4 2− and La 2 O 3 content in the SO 4 2− /TiO 2 -La 2 O 3 were determined by chemical analysis. Preparation of SO 4 2− /TiO 2 -La 2 O 3 2 SO 4 2− /TiO 2 -La 2 O 3 was prepared as follows. Hydroxides or oxides of Ti and La were obtained by dissolving chloride of Ti and La in water, hydrolyzing with aqueous ammonia to pH=8~9 in solution, washing the precipitate, drying at 110 °C, and powdering it. The precipitates were treated with sulfate ion by immersing the dried Ti-La-O substrate in 0.8 mol/L H 2 SO 4 solution for 24 h, then filtered, dried, and calcinated in furnace at 480 °C for 3 h. Finally the catalysts were stored in a desiccator until use. The microstructure of SO 4 2− /TiO 2 -La 2 O 3 was identified by means of FT-IR and XRD. Determination of sulphate content 2 Sulphate-modified TiO 2 -La 2 O 3 samples calcined at different temperatures were fused by m (ZnO): m (Na 2 CO 3 )=4:1, and extracted with water. The filtrate was titrated for sulphate estimation by the standard procedure (Gao et al., 1994 1994 ). Acid strength measurement of catalysts 2 The acid strengths of the catalysts were examined by the colour change method using the Hammett indicator (Gao et al., 1992 1992 ), when a powdered sample was added to an indicator dissolved in dry benzene. Synthesis of acetals and ketals 2 The reaction was carried out in a three-neck flask equipped with a stirrer, a reflux condenser and a thermometer. Certain amounts of ethyl acetoacetate, cyclohexanone, butanone, butyraldehyde, benzaldehyde were added separately with alcohol (ethylene glycol or 1,2-propylene glycol) and the catalyst. The solution was heated to boiling and refluxed until no water flowed off, then distilled after ridding the water anhydrous magnesium sulfate to obtain the desired product. RESULTS AND DISCUSSION 1 Solid-state structure of the complexes 2 1. Infrared spectra of SO 4 2− /TiO 2 -La 2 O 3 3 Fig. 1 1 shows the IR spectra of SO 4 2− /TiO 2 -La 2 O 3 . Infrared spectrum of solid samples (Fig. 1 1 ) showed that the characteristic peaks of SO 4 2− /TiO 2 -La 2 O 3 are at 1379 cm −1 , 1132 cm −1 and 1043 cm −1 , indicating sulphurous species on the catalyst surface (Gao et al., 1996 1996 ) and provide the corner of strong acid by induction, with the result according with reports of SO 4 2− -M x O y -type solid superacid in (Yin et al., 1994 1994 ). Generally, there are two absorption bands in the infrared spectrum of SO 4 2− -M x O y -type solid super acid at 900~1300 cm −1 and 1300~1400 cm −1 , which can be attributable to the vibration of S=O being similar to that of sulfate and sulfuric ester, respectively. The absorption peaks at low frequency partly show double bond in the former and fully show double bond at high frequency in the latter. And we can surmise that adsorbed water exists in the catalyst compound as shown by the strong and broad peaks at 1636 cm −1 and 3415 cm −1 attributable to the bending vibrations of H-O-H bond and the stretching vibrations of O-H. The peak at 1383 cm −1 is distinguished from the 1630~1620 cm −1 sulphate pears for the double coordination between metal ions and SO 4 2− 2. X-ray structure of SO 4 2− /TiO 2 -La 2 O 3 3 The X-ray powder diffraction spectrum of solid samples (Fig. 2 2 ) suggests anatase crystalline phase in both SO 4 2− /TiO 2 and SO 4 2− /TiO 2 -La 2 O 3 having the same value of diffraction peaks. We did not find the diffraction peaks that La 2 O 3 should display, indicating that no sole crystalline phase caused by La 2 O 3 and La is dispersed on the catalyst surface in case the amount of La is below a threshold value (Pan and Xie, 2001). The mass fraction of SO 4 2− and La 2 O 3 in the SO 4 2− /TiO 2 -La 2 O 3 are about 9.48% and 0.70%, respectively. 3. Acid strengths of the catalysts 3 The acid strengths of the catalysts were examined by the colour change method using the Hammett indicator (2,4-dinitroluene ( H o =−14.52), 1,3,5-trinitrotoluene ( H o =−16.04)). It was seen that the Hammett acidity function of SO 4 2− /TiO 2 -La 2 O 3 is −16.04 < H o < −14.52. Effect of preparation conditions on the activity of SO 4 2− /TiO 2 -La 2 O 3 catalysts 2 The reaction was carried out in a three-neck flask equipped with a stirrer, a reflux condenser and a thermometer. Certain amounts of n -butanoic acid and n -butyl alcohol were added with molar ratio of acid to alcohol being 1.3:1 separately, mass ratio of the catalyst used in the reactants being 1.5%. The solution was heated to boiling in the range of 94 °C to 112 °C and refluxed for 1 h. To find the optimum reaction factors affecting activities of SO 4 2− /TiO 2 -La 2 O 3 , we studied the effects of mole ratio of n (La 3+ ): n (Ti 4+ ) included in compound, soaked consistency of H 2 SO 4 , impregnation time of H 2 SO 4 , calcination temperature and burnt time on the results shown in Fig. 3 3 . The preparation conditions are important factors affecting the SO 4 2− /TiO 2 -La 2 O 3 properties. The results in Fig. 3a 3a show influences of the mole ratio of n (La 3+ ): n (Ti 4+ ) on the catalytic activity of SO 4 2− /TiO 2 -La 2 O 3 prepared under the following conditions: impregnation time in 0.8 mol/L H 2 SO 4 , 24 h; calcinations temperature of SO 4 2− /TiO 2 -La 2 O 3 , 480 °C; burnt time, 3 h. The results showed that the SO 4 2− /TiO 2 -La 2 O 3 catalytic activity under mole ratio of n (La 3+ ): n (Ti 4+ )=1:34 is relatively higher. The results in Fig. 3b 3b and Fig. 3c 3c show the influences of the H 2 SO 4 solution concentration used and the impregnation time on the SO 4 2− /TiO 2 -La 2 O 3 activities, showing that immersion of TiO 2 -La 2 O 3 in 0.8 mol/L H 2 SO 4 for 24 h is appropriate for the preparation of SO 4 2− /TiO 2 -La 2 O 3 catalyst, which show higher esterification activity, indicating that a proper amount of SO 4 2− on TiO 2 -La 2 O 3 is necessary to achieve high activity. If the SO 4 2− amount added to the catalyst surface is more or less than the proper amount, high SO 4 2− /TiO 2 -La 2 O 3 activity cannot be achieved. The results in Fig. 3d 3d and Fig. 3e 3e show influences of calcination temperature and the burnt time on the SO 4 2− /TiO 2 -La 2 O 3 activities. It is known that the calcination temperature in the preparation of a catalyst and the burnt time are key factors, which not only can affect the SO 4 2− amount added to the catalyst surface but the activity of the catalysts as well. The results showed that the catalytic activity of SO 4 2− /TiO 2 -La 2 O 3 calcined at about 480 °C for 3 h is higher than that calcined at other temperatures, and that above 480 °C this sulphureous complex gradually decomposes. We changed only one of the five reaction factors and kept the others constant in the experimental runs, then the optimum conditions were selected. That is, the mole ratio of n (La 3+ ): n (Ti 4+ ) is 1:34, the soaked consistency of H 2 SO 4 is 0.8 mol/L, impregnation time of H 2 SO 4 is 24 h, the calcination temperature is 480 °C, the burnt time is 3 h. Comparsion of catalytic activity of different catalysts 2 Table 1 1 summarizes the activities of SO 4 2− /TiO 2 -La 2 O 3 and H 2 SO 4 , H 3 PW 12 O 40 (Zhang et al., 1993 1993 ) for esterification of n -butanoic acid and n -butyl alcohol. The activity of catalysts and other data were obtained at the temperature of the reactant solution reflux. Catalyst Molar ratio of alcohol to acid Reaction time (min) The highest reaction temperature (°C) Yield (%) SO 4 2− /TiO 2 -La 2 O 3 1.3:1 60 114 94.7 H 2 SO 4 1.0:1 120 158 72.1 H 3 PW 12 O 40 1.0:1 113 140 62.5 SO 4 2− /TiO 2 -La 2 O 3 has the most intense catalytic activity among these H 2 SO 4 and H 3 PW 12 O 40 acid catalysts, even if compared on the basis of the catalyst weight and reaction time. Acetals and ketals 2 Some equations and experiment results on the synthetic acetals and ketals are listed: listed: The yield of ten classes of acetals and ketals under typical reaction conditions (molar ratio of aldehyde/ketone to glycol is 1:1.5, the quantity of catalyst is equal to 0.5% of feed stocks, the reaction time is 1.0 h) can reach 41.4%~95.8%. The amount of reactant of aldehyde/ketone was fixed in 0.2 mol with cyclohexane as water-removed reagent. Results of these synthesis reactions are summarized in Table 2 2 . http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23537290&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23537290&type=small http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23537290&type=middle http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23537290&type=small http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23537292&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23537295&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23537292&type=middle From the data listed in the table, we can find that SO 4 2− /TiO 2 -La 2 O 3 was highly active, leading to yields of ten ketals and acetals up to 95.8% and no less than 41.4% after 1.0 h. Moreover, the reaction temperature was controlled at the reflux temperature and the amount of SO 4 2− /TiO 2 -La 2 O 3 was much lower than that of other catalyst used in these reactions (Wang et al., 1992a 1992a ; Zhang et al., 1995 1995 ). All these helped to solve the technical problems of industrial processes. When the temperature rose, we observed the solution of reagents became dark in color because temperature above the optimum condition would decrease the activity of catalyst. More reaction time was needed to keep the yield but the activity began to decrease after two hours. Comparing the 2 alcoholic reactants, the special steric hindrance and the density of electron atmosphere on oxygen atoms exerted opposite effects on yield. The former kept oxygen atoms from approaching carbonium ions and the C-O binding was difficult to form. But the latter decreased the density of electron atmosphere on oxygen atoms bonded with carbonium ions so as to alleviate the rejection of the electron atmosphere of two oxygen atoms. To reactions 3a and 3j 3a and 3j , the special steric hindrance disadvantageous effect and took the leading role. For the rest, the density of electron atmosphere on oxygen atom predominated. CONCLUSION 1 The following conclusions can be drawn from this study: (1) The optimum synthetic conditions of SO 4 2− /TiO 2 -La 2 O 3 are: mole ratio of n (La 3+ ): n (Ti 4+ ) is 1:34, the soaked consistency of H 2 SO 4 is 0.8 mol/L, impregnation time of H 2 SO 4 is 24 h, the calcination temperature is 480 °C, the calcining time is 3 h. (2) The molar ratio of aldehyde/ketone to glycol is 1:1.5, the mass ratio of the catalyst used in the reactants is 0.5%, and the reaction time is 1.0 h, the yields of ketals and acetals can reach 41.4%~95.8%. (3) The esterification of n -butanoic acid and n -butyl alcohol as probing reaction showed efficiently highly catalytic activity of SO 4 2− /TiO 2 -La 2 O 3 at its optimum conditions as catalyst. At the same time, it also shows much higher catalytic activity than other solid acid catalysts, such as H 2 SO 4 , H 3 PW 12 O 40 , and so on. Valuable products, ten important ketals and acetals have been obtained at high yield. References D Dhrubojyoti , DP Laskar , SS Jagir . Cadmium iodide catalyzed and efficient synthesis of acetals under microwave irradiations . Chemistry Letters , 1999 . 332 ( 8 ): 1283 - 1284 . . Cadmium iodide catalyzed and efficient synthesis of acetals under microwave irradiations Z Gao , JM Chen , Y Tang . Studies on the formation of ZrO 2 /SO 4 2− superacid system . Chem J Chinese Universities , 1992 . 13 ( 12 ): 1498 - 1502 . (in Chinese) . Studies on the formation of ZrO2/SO42− superacid system Z Gao , JM Chen , Y Tang . 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Catalytic synthesis of acetals and ketals with heteropoly-acid 10.1631/jzus.2006.B0553 Project supported by the National Natural Science Foundation of China (No. 20471044), and the Natural Science Foundation of Hubei Province (No. 2005ABA053), China 2005-11-16 2006-04-17 2006-06-14 2022-01-05 Copyright © 2006, Journal of Zhejiang University Science 2006 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 07 07