International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 10 No: 06 5 100506-3939 IJBAS-IJENS © December 2010 IJENS I J E N S Preparation and characterization of sulfated titania catalysts for the isomerisation of citronellal S. N. Aisyiyah Jenie*, Dona S. Kusuma, Anis Kristiani, Joddy A. Laksmono, Silvester Tursiloadi Research Centre for Chemistry, Indonesian Institute of Sciences-LIPI, Kawasan PUSPIPTEK, Serpong, Tangerang 15314 Indonesia, Ph. : +62 21 7560929, Fax: +62 21 7560549 Mesoporous sulfated titania xerogels with structural and catalytic properties were prepared by the modified sol-gel method using surfactant as templates. The sulfate loading of these catalyst materials were varied in the range of 0 to 50 wt% SO 4 2- . Characterization of the sulfated titania were evaluated by transmission electron microscopy, N 2 adsorption and X-ray diffraction. The addition of sulfate has decreased the crystalline size and induces the structural transformation from rutile to anatase. The best sample catalyst was attained at TS5 containing 5 wt% of sulfate and having equal strength of both Brønsted and Lewis acid site. Sulfated titania show good activity towards citronellal isomerization at 95°C using toluene as solvent. Conversion of citronellal correlates with the the surface acidity of the catalyst and the use of toluene as solvent in the reaction, while the selectivity towards (-)-isopulegol depends on the corresponding acid site strength. Keywords—catalyst, citronellal, isopulegol, sulfated titania I. INTRODUCTION Recently, non-polluting and efficient catalytic technologies are much required, considering that environmental restrictions on emissions are covered in several legislations throughout the world. Solid acids have begun to replace highly corrosive, hazardous and polluting liquid acids. The substitution of homogeneous liquid acids by heterogeneous solid superacids as catalysts is expected to ease their separation from reacting mixture, allowing continuous operation as well as regeneration and neutralization of the catalyst [1] and lowering the cost of process installation and maintenance [2]. Solid acids leading to better regio- and stereoselectivity have been extensively studied, which is effected not only by the acid strength but also the type of acidity (Brønsted or Lewis)[2-4,6]. Sulfated metal oxides have been tested as catalysts. The most studied sulfated oxides are ZrO 2 and TiO 2 . Sulfated titania or its mixture with other oxide such as ZrO 2 , has been found to be efficient catalysts for isomerization, alkylation, Friedel-Crafts acylation, esterifaction, photocatalytic oxidation and reduction of NOx [1,2,7-9]. The acid strength for the sulfated metal oxides is high, stronger than 100% sulfuric acid [2,4]. It has been reported that for SO 4 2- /TiO 2 and SO 4 2- /ZrO 2 , the Hammet acidity is, H 0 ≤ -14.52 and H 0 ≥ -14.52 respectively, as compared with that of H 2 SO 4 , H 0 ≤ -12 [7]. Sulfated metal oxides has been found to catalyze the isomerization or cyclisation of citronellal to isopulegol. Isopulegol is an important intermediate in the manufacture of menthols (C 10 H 20 O), which have a characteristic peppermint odor. Of the eight optically active menthols, only (-)-menthol, derived from the hydrogenation of (-)-isopulegol, possesses the characteristic peppermint odour and exerts a cooling effect [6, 10,11]. Previous reports have shown the isomerization of d-citronellal using sulfated zirconia/ SO 4 2- ZrO 2 as catalyst [3,4]. Yadav and Nair (1996) has reported that besides the Brønsted acid sites are more needed compared to the Lewis acid site, the pore size of the catalyst should also be taken into account during the isomerization process [3]. Chuah and coworkers (2001), on the other hand have reported that active catalyst are the ones possessing strong Lewis and weak Brønsted acidity [4]. The sulfated zirconia system has been studied most comprehensively because of its acidity being the highest of the sulfate oxides [3-5]. However, previous works also show that isomerization of citronellal do not require such high acidity values, which frequently induce side reactions [4]. Therefore in this work, we propose the use of sulfated titania for the isomerization process. Besides the acidity properties, the physical properties of the catalyst are also important. The properties of catalysts strongly depend on the preparation procedures. The high porosity and the large specific surface area of materials prepared by the sol-gel method make them very attractive from a catalytic point of view [12]. The sol-gel method not only allows good control of the characteristics of the support, but also offers the possibility of preparing oxide-supported metal catalysts from a homogenous solution containing both the catalytic metal precursor and the support precursor [13]. A metal alkoxide undergoes hydrolysis and subsequent condensation in an alcohol solvent, forming a polymer oxide network entraining the alcohol. For catalytic uses, the liquid solvent must be removed from the alcogel. In conventional drying, a liquid-vapor interface forms in the pores and the corresponding surface tension collapses the oxide network, thereby reducing the surface area [14]. To prevent the collapsing of the oxide network, most work usually apply the removal of solvents by supercritical drying [13,14]. Another different approach during the removal of solvents is by using surfactants as a template to fabricate porous materials and to improve the alcogel properties [13,15]. By immersing the wet alcogels in surfactant solution before drying, the oxide network will be prevented from collapsing. Catalysts prepared using TiO 2 as supports are at least two or three times more active than the equivalent Al 2 O 3 supported catalyst, measured on either a surface area, weight, or atomic basis. However, because of the typical low surface area of these supports, usually less than 50 m 2 /g, the volume activity is much lower than that of Al 2 O 3 supports, which usually have *Corresponding author: S. N. Aisyiyah Jenie (e-mail: siti045@lipi.go.id).