Surface properties and catalytic activity of sulfated rutile - Springer Link

Jointly published by Akadémiai Kiadó, Budapest and Kluwer Academic Publishers, Dordrecht

React.Kinet.Catal.Lett. Vol. 79, No. 1, 27-33 (2003)

RKCL4135 SURFACE PROPERTIES AND CATALYTIC ACTIVITY OF SULFATED RUTILE S. Sugunan*, T. Radhika and H. Suja Department of Applied Chemistry Cochin University of Science and Technology Kochi-682 022 Received May 27, 2002 In revised form August 6, 2002 Accepted September 19, 2002

Abstract The present work reports an investigation of physico-chemical properties and catalytic activity of the pure, sulfated and iron containing sulfated rutile. The catalysts were characterized by X-ray Diffraction, FT-IR, BET surface area, TG and NH3-TPD measurements. The catalytic activity of the prepared catalysts towards liquid phase Friedel-Crafts alkylation of toluene with benzyl chloride was tested. Keywords: Sulfated oxides, sulfated titania, sulfated rutile, solid acids

INTRODUCTION The synthesis of environmentally friendly solid super acids as catalysts is desirable for replacing the conventional liquid acid catalysts such as H2SO4, HF, AlCl3 and BF3 because of its hazardous properties like corrosivity, high sensitivity to water, etc. [1,2]. Recently, sulfated metal oxides (SMO) mainly of ZrO2, TiO2, Fe2O3, SnO2, Al2O3, HfO2 and SiO2 have been the focus of intensive research due to its high efficiency in catalyzing the hydroisomerization of paraffins [3-7]. The importance of solid acid catalysis in petroleum refining has led to extensive studies on the acidic properties, the nature of the acid site and its catalytic action [8]. Parsulescu et al. reported that the catalytic activity of sulfated zirconia significantly depends on the preparation method and the 0133-1736/2003/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved.

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SUGUNAN et al.: SULFATED OXIDES

method of activation [9]. Properties of sulfated systems can be improved by the addition of some promoters such as iron, manganese, tungsten, platinum, nickel and molybdenum [10]. Titania, classified as a solid acidic oxide in both the anatase and rutile crystallographic forms, has long been known to possess catalytic activity, although anatase was found to be more active than rutile [11]. A molybdenum incorporated sulfated titania catalyst is much more active than molybdated titania for the reduction of NO with NH3 and the higher activity is due to the acidity enhanced by the interaction of TiO2 with SO42- species [12]. For the isomerization of 2-pinene to camphene and tricyclene SO42-/TiO2 showed higher activity than SO42-/ZrO2 [13]. In the present work, we have prepared sulfated and iron containing sulfated rutile catalysts with different iron loadings and performed various physicochemical characterizations. The materials were tested as catalysts for Friedel-Crafts alkylation of toluene with benzyl chloride. EXPERIMENTAL The sulfated rutile was prepared from titanium dioxide (preheated at 300oC for 2 h) by wet impregnation method using 1N H2SO4 [14]. Iron modified rutile catalysts were prepared similarly by a single step wet impregnation using 1N H2SO4 and ferric nitrate solution. The samples after overnight drying at 373 K were calcined at 823 K for 3 h. Percentage of iron loading was varied from 2 to 10 and the samples were denoted generally as xFeST where ‘x’ stands for percentage of iron. XRD patterns were recorded on a Rigaku (D.Max.C) X-ray diffractometer using Cu Kα radiation (λ=1.54nm) with Ni filter and a count time of 0.5 s at each step. The BET surface area and pore size distribution of all the samples were acquired on a Micromeritics Gemini apparatus. FT-IR spectra were recorded on a Jasco FT-IR-5300 instrument. TG analysis was performed using Metler Toledo Starsystem instrument in N2 atmosphere. Temperature programmed desorption data using ammonia as the probe molecule (NH3-TPD) was obtained by desorbing ammonia at various temperatures after adsorption on the catalyst at room temperature.

Catalytic activity tests Liquid phase benzylation of toluene with benzyl chloride was carried out in a 50 mL R.B flask fitted with a spiral condenser. The temperature was maintained using an oil bath. In a typical run, toluene and benzyl chloride in a specific molar ratio (5:1) was added to 0.1 g of the catalyst and the reaction mixture was


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magnetically stirred. At the end of the reaction, the mixture was separated from the solid catalyst by filtration. The filtrate was analyzed by Chemito 8610 GC equipped with a FID and SE-30 column. RESULTS AND DISCUSSION Powder X-ray diffraction was used for the crystal phase identification of the catalysts studied (Fig. 1). The peak corresponding to rutile phase (2θ=27.5o) was identified in the XRD pattern of pure TiO2. Intensity of the peaks was lowered for sulfate treated samples indicating low crystallinity. In the case of iron containing systems the peak intensity was further reduced. Absence of specific peaks corresponding to Fe2O3 indicates the presence of iron in highly dispersed form on the surface. Even at 10% iron loading peaks corresponding to Fe2O3 could not be detected and the only effect observed in the XRD pattern was the lowering of the peak intensities.

d

intensity(arb.units)

c

b

a

20

30

40

50

60

2θ degrees

Fig. 1. XRD profiles of a) TiO2, b) ST, c)2FeST and d) 10FeST

70

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Table 1 depicts the surface area and pore volume of the samples calcined at 823 K. As evident from the data, sulfated oxides showed a higher surface area compared to pure titania. The retention of surface area by the sulfated samples after high temperature calcination can be explained on the basis of the higher resistance to sintering acquired by doping with sulfate ions. Incorporation of iron resulted in lowering of surface area. As the iron content increased the surface area values showed a gradual increase up to 6% iron loading and thereafter it declined. The lowering of surface area may be accounted on the basis of agglomerization of iron particles on the surface. Maximum dispersion may be obtained at an optimum iron loading where maximum surface area is obtained. The total pore volume of samples measured followed the same trend as in the case of surface area.

Table 1 Surface area and pore volume values

Catalyst

Surface area (m2/g)

Pore volume (10-2 cm3/g)

TiO2 ST 2FeST 4FeST 6FeST 8FeST 10FeST

2.63 11.78 2.79 8.01 12.65 8.03 4.27

0.09 0.71 0.11 0.48 0.80 0.52 0.22

FT-IR spectra of the catalysts synthesized are shown in Fig. 2. In the case of sulfated systems, the peak at 1107 cm-1 can be assigned to S=O group [15]. The absence of peaks at around 1400 cm-1 suggests that the added sulfate exists as SO32- groups rather than as SO42- species [16]. The bands around 1626 cm-1 and 3441 cm-1 correspond to the bending and stretching modes of the -OH groups of water molecules present in the catalysts. No significant difference could be observed in the spectra of iron incorporated systems. Thermal analysis of the samples showed an initial weight loss at around 100oC, which may be assigned to the loss of surface, adsorbed water of hydration. The weight loss around 500oC may be assigned to the evolution of structural water and also due to the dehydroxylation of the catalyst. For sulfated samples, the dip was observed after 700oC indicating the decomposition of the sulfate species. The presence of iron moieties lends special stabilization to the sulfate species and shifts the decomposition to higher temperatures.


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Fig. 2. FT-IR spectra of a) TiO2, b) ST, c) 2FeST, d) 6FeST and e) 10FeST

Acidity measurement by ammonia TPD Temperature Programmed Desorption (TPD) experiments using ammonia as adsorbate was performed on the catalysts to compare their acidity and the results are summarized in Table 2. The amount of ammonia desorbed in the temperature ranges 100-200, 200-300 and 300-500oC were taken as a measure of weak, medium, and strong acid sites, respectively. From the data, it is clear that total acidity of the sulfated sample was considerably higher than that of the pure. An enhancement in the amount of both strong and weak acid sites was observed after incorporation of sulfate and iron species. The total acidity values obtained from ammonia TPD measurements were comparable for the sulfated and iron incorporated systems. For low percentage of iron, total acidity is found to be lower than that of sulfated rutile. As the iron content increases the total acidity was also found to increase.

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Table 2 Ammonia -TPD measurements Catalyst

Weak

Medium

Strong

Total


0.1444 0.3310 0.1118 0.2022 0.1621 0.2346 0.2582

0.1238 0.2896 0.2606 0.1349 0.1737 0.2639 0.231

0.00 0.0965 0.0279 0.0337 0.0589 0.1026 0.1359

0.2682 0.7171 0.2654 0.3708 0.3947 0.6011 0.6251

The percentage conversion and product selectivity for alkylation of toluene with benzyl chloride using prepared catalysts are shown in Table 3.

Table 3 Friedel-Crafts alkylation over sulfated and iron modified sulfated catalysts Catalyst


Time (min)

60 60 60 60 60 45 45

Conversion (%) 99.17 15.05 26.25 78.21 99.08 100

Selectivity (%) MAP

DAP

89.57 100 100 100 100 92.39

10.43 7.61

The percentage conversion and relative product selectivity can be correlated with the surface acidic properties of the catalysts. It is evident from the table that pure titania was totally inactive towards the alkylation reaction while the modified systems exhibited high percentage conversion under the reaction conditions studied. Sulfated titania showed an enhanced activity which is in agreement with the surface area and acidity measurements. Contrary to our expectations, incorporation of iron in small amounts had a negative influence on the percentage conversion. This can be visualized in the light of the low acidity values of low loaded iron systems when compared with simple sulfated system.


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The activity of the catalysts increased as the incorporated metal percentage increased. For higher loadings of iron such as 8FeST and 10FeST the reaction was completed within 45 minutes. The increase in activity may be considered to be a combined effect of the acidic properties as well as the structural influence of the transition metal species. The high activity of the iron promoted systems may be attributed to the redox properties of iron as well as its strong acidity as evident from TPD measurements. In comparison with other systems, in the case of ST and 10FeST a small amount of dialkylated products were also formed. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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