Synthesis and characterization of catalysts obtained by trifluoromethanesulfonic acid immobilization on zirconia

MARINA GORSD; MIRTA BLANCO; LUIS PIZZIO

Louvain-la-Neuve, Bélgica

Simposio; 10th International Symposium on the "Scientific Bases for the Preparation of Heterogeneous Catalysts; 2010

The trifluoromethanesulfonic acid (CF3SO3H) has a highly acidic nature and excellent thermal stability; it also has good resistance to reductive and oxidative dissociation, with no generation of fluoride ions. The trifluoromethanesulfonic acid was used as an efficient homogeneous catalyst, but has environmental disadvantages because it generates high amount of wastes. An interesting alternative is the trifluoromethanesulfonic acid heterogeneization by immobilization on an adequate support. There are few works on this subject, though it can be mentioned its immobilization on titania [1]. On the other hand, zirconia (zirconium oxide) is an interesting material to be used as catalyst support due to its thermal stability in different atmospheres. The most common methods that can be used to obtain zirconia are the sol-gel method, the micellar technique or the mechanochemical synthesis [2]. Zirconia is frequently prepared by micellar method, while the sol-gel method from an alkoxide is less used [3]. Its acid properties can be modified by addition of cationic or anionic substances, such as sulfate or tungstate [4]. The addition of Keggin heteropolyacids, though less studied, leads to materials with high acidity [5]. We present here the physicochemical and textural characteristics of catalysts obtained from the impregnation of trifluoromethanesulfonic acid on mesoporous zirconia. Zirconia was synthesized from zirconium propoxide via sol-gel reactions catalyzed by HCl, using urea as a pore-forming agent. Urea was removed by extraction with water, the solid was dried at room temperature for 24 h, and calcined at 100, 200, 300, and 400 ºC for 24 h, thus obtaining the ZrTX samples, where X is the calcination temperature. These samples were impregnated with trifluoromethanesulfonic acid in toluene at reflux temperature. Afterwards, the acid loosely adsorbed on the solid was removed by extraction with dichloromethane and diethyl ether. These samples will be named TriZrTX. Mesoporous materials were obtained, with a mean pore diameter (DP) higher than 3.7 nm, which increased with the thermal treatment temperature (Table 1), while the specific surface area (SBET) and the microporosity (SMicro) decreased. The textural properties of the catalysts were mainly the same as those of the supports. The amount of acid attached on the support (NTri), determined by elemental analysis, decreased with the thermal treatment temperature. This effect may be explained if an electrostatic type interaction is assumed, due to proton transfer to the -OH groups on the support surface. As a result of the support dehydroxylation during the thermal treatment, the amount of OH groups to be protonated decreases, and therefore NTri diminishes. Table 1. Support textural properties and CF3SO3H amount in the catalysts. Sample SBET (m2/g) SMicro (m2/g) DP (nm) Sample NTri (mmol CF3SO3H/g) ZrT100 192 88 3.7 TriZrT100 0.91 ZrT200 132 50 4.7 TriZrT200 0.53 ZrT300 78 17 5.5 TriZrT300 0.41 ZrT400 20 0 14.2 TriZrT400 0.10 By XRD it was observed that the samples have amorphous characteristics, with any line indicating the presence of crystalline phases. The characteristic bands of the S═O bond and C-F bond stretchings are observed in the FT-IR spectra of the catalysts. From DSC-TGA, it can be established that they are thermally stable up to 250 ºC. The potentiometric titration with n-butylamine indicated that the catalysts present very strong acid sites, nearly independent of NTri, and a number of acid sites that decreases with the thermal treatment temperature in an almost linear relation with the NTri decrease. The obtained results showed that this trifluoromethanesulfonic acid heterogeneization on zirconia lead to potentially appropriate catalysts to be used in acid reactions thus contributing with the field of clean processes. References [1] L. Pizzio, Mater. Lett. 60 (2006) 3931. [2] M. Fernández-García, A. Martínez-Arias, J. C. Hanson, J. A. Rodríguez, Chem. Rev. 104 (2004) 4063. [3] X. Qu, Y. Guo, Ch. Hu, J. Molec. Catal. A 262 (2007) 128. [4] G. D. Yadav, J. J. Nair, Micropor. Mesopor. Mater. 33 (1999) 1. [5] L. Pizzio, P. Vázquez, C. Cáceres, M. Blanco, Catal. Lett. 77 (2001) 233.