908
Ind. Eng. Chem. Res. 1998, 37, 908-913
Reaction Kinetics of Benzylation of Benzene with Benzyl Chloride on Sulfate-Treated Metal Oxide Catalysts S. N. Koyande, R. G. Jaiswal, and R. V. Jayaram* Department of Chemical Technology, University of Bombay, Mumbai 400 019, India
Sulfate-promoted metal oxides of ZrO2 (SZ) and Fe2O3 (SF) and the mixed oxide ZrO2-Fe2O3 (SZF) have been prepared by coprecipitation and wet impregnation techniques. The synergistic effect of the dopant metal oxide in the matrix of the parent sulfated oxide, SO42-/ZrO2 (SZ), has been studied for the alkylation reaction of benzene with benzyl chloride. An attempt has been made to decrease the polyalkylation observed on the highly reactive catalyst, SO42-/Fe2O3(SF). Detailed kinetic study of the reaction has been carried out to evaluate the kinetic parameters. Introduction Hino and Arata reported that zirconium oxide modified with sulfate ions (SZ) on the surface developed strong acidic properties and unique acid catalytic activity.1 Since the original report several sulfate treated metal oxides have been identified as solid superacids. They effectively catalyze many commercially important reactions like isomerization of lower paraffins at mild ambient conditions.2 However, these catalysts are not much employed in the chemical manufacturing industry as it is a common observation that they lose their activity in due course due to thermal as well as chemical decompositions.3,4 Hence, to increase the stability and to a certain extent the activity of these catalysts, doping of different metal cations into the matrix of the parent oxide has been attempted by several researchers.5-9 Recently Hsu et al. have synthesized SZ incorporated with Fe and Mn. These catalysts can isomerize nbutane to isobutane at room temperature and more so can also be easily regenerated. It has also been claimed that the activity of SZ doped with Fe and Mn is due to redox-active metal sites.10 SZ containing a small amount of WO3 has also been found to have enhanced acid strength and reusability.11 Pt doping of SZ has also been attempted for the hydrocarbon conversions.12,13 In the present investigation we have tried to modify SZ with Fe2O3 to improve its activity and reusability. Reaction kinetics of alkylation of benzene with benzyl chloride have been investigated on ZrO2(SZ), Fe2O3(SF), and also the mixed oxide SO42-/Fe2O3-ZrO2(SZF).
Figure 1. XRD of SO42-/Fe2O3.
Figure 2. XRD of SO42-/ZrO2.
Experimental Methods (A) Catalyst Preparation. (i) Preparation of SZ/ SF. An appropriate amount of ZrOCl2‚8H2O/Fe(NO3)3‚ 9H2O was dissolved in a minimum quantity of deionized water. To this solution, excess ammonia was added with constant stirring (pH 10.0). The hydroxide thus precipitated was filtered and washed repeatedly with deionized water until the filtrate was free of Cl-/NO3ions. The sample was then dried in an air oven at 120 °C for 24 h. Sulfate treatment to this was given by treating 2.0 g of the powdered sample with 30 mL of 1 N H2SO4 on a filter paper. The hydroxide thus treated * Corresponding author. Fax: 91-22-4145614. E-mail:
[email protected].
Figure 3. XRD of SO42-/ZrO2/Fe2O3 (prepared by coprecipitation).
was kept in an air oven at 120 °C for 4 h and calcined in air in a Pyrex tube for 3 h at 500 °C. (ii) Preparation of SZF. (a) By Coprecipitation. Appropriate amounts of the respective salts, i.e., ZrOCl2 and Fe(NO3)3‚9H2O were dissolved in a minimum amount of deionized water. The solution was filtered to remove undissolved impurities, if any. Aqueous NH3 was added to precipitate out the hydroxides simulta-
S0888-5885(97)00306-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/03/1998
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 909
Figure 6. FTIR spectrum of SO42-/ZrO2. Figure 4. XRD of SO42-/ZrO2-Fe2O3 (prepared by impregnation).
Figure 7. FTIR spectrum of SO42-/ZrO2-Fe2O3. Figure 5. FTIR spectrum of SO42-/Fe2O3.
neously. The solution was again filtered and washed repeatedly with deionized water to remove Cl- and NO3- ions. Precipitation of the hydroxide was carried out at pH 10.0. (b) By Wet Impregnation. A total of 2.4 g of ZrOCl2 was dissolved in deionized water. The hydroxide was precipitated using aqueous ammonia. The solution was then filtered, and the residue was dried in an air oven. A total of 3.09 g of Fe(NO3)3‚9H2O dissolved in a minimum quantity of deionized water was poured over the hydroxide and the mixture stirred to form a slurry which was dried in an oven until the water was completely removed to give a free-flowing powder. The mixed oxide prepared by both methods was then dried in an air oven for 24 h at 120 °C. To this was given sulfate treatment as described earlier. The sample was further dried at 120 °C for 4 h and then calcined at 500 °C for 3 h. (B) Characterization of the Catalysts. The catalysts prepared were characterized by XRD and FTIR spectroscopic techniques. XRD patterns were recorded on a JOEL JDX 8030 X-ray diffractometer using CuKR radiation (λ ) 1.54 Å) with Ni filter. All diffractograms were recorded at room temperature. XRD patterns are given in Figures 1-4. The FTIR spectra of the calcined and degassed catalyst materials were recorded using a Jasco FTIR spectrophotometer. The spectra were recorded at room temperature by a KBr pellet method and are shown in Figures 5-7. (C) Reaction Setup. The reactions were carried out in a fully baffled three-necked glass reactor of 5 cm internal diameter and 150 mL capacity. A pitched four-
bladed stirrer was placed at a distance of 2.5 mm from the bottom of the reactor for agitating the reaction mixture and was driven mechanically. The temperature was controlled ((1 °C) by means of a thermostatic oil bath in which the reactor was immersed. All chemicals used were of A.R. grade and were dried with suitable drying agents. In all the reactions, (except for the reactions to study the effect of mole ratio), 0.03 M benzyl Chloride and excess benzene (0.18 M) were taken together in the reactor. An appropriate amount of the catalyst predried in an air oven for 30 min at 120 °C was then transferred to the reactor. The mixture was then stirred at the desired temperature at a constant rpm and drawn out periodically for analysis. The products were analyzed with the aid of GLC (Chemito 2865) fitted with a SE 30 column or by chemical analysis. The activity of the catalyst was calculated in terms of percentage conversion to diphenylmethane (DPM). (D) Kinetic Studies. A detailed kinetic analysis of the alkylation reaction (benzylation of benzene with benzyl chloride) was carried out, and the kinetic parameters were evaluated. Percentage yields were calculated in terms of the amount of the alkylated product (DPM). It was confirmed that there were no parallel reactions in the conditions chosen for kinetic studies, other than the alkylation reaction. This conclusion was based on the material balance obtained between unreacted benzyl chloride and that converted to DPM, which was also confirmed by quantitative TLC analysis. The reactions were carried out at 60, 70, and 80 °C, and the reaction mixture was drawn out from time to time and analyzed.
910 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 Table 1. IR Frequencies and Assignments for Possible Sulfate Coordination to a Metal Oxide catalyst SO2-/ZrO2 (SZ) SO2-/Fe2O3 (SF)
SO2-/ZrO2-Fe2O3 (SZF) (coprecipitation) SO2-/ZrO2-Fe2O3 (SZF) (impregnation)
wavenumber (cm-1) 1140 1085 1010 1260 1140 1100-1125 1020 1210 1060-1135 1000 1160
possible structure bridged bidentate chelating bidentate
chelating bidentate
bridged as well as chelating bidentate
1090-1125 1060,1020
Results and Discussion (A) Characterization of the Catalysts. (i) XRD. SZ and SF were found to be partially crystalline. SZF prepared by coprecipitation revealed a single phase and was amorphous in nature. It was observed that the sulfation produced zirconia of tetragonal form. SZF was amorphous, suggesting that the presence of Fe2O3 decreased the crystallinity further. Catalyst prepared by the impregnation method (SZF) showed distinct phases of SF and SZ (Figures 1-4). (ii) FTIR. FTIR spectra (Figures 5-7) show the characteristic sulfate coordination to metal oxides as reported by Yamaguchi.14 The observations are given in Table 1. It can be seen that SZ shows a bridged bidentate structure. SF and SZF prepared by coprecipitation show chelating bidentate structures. SZF prepared by the impregnation method shows a mixture of bridged bidentate and chelating structures. (B) Effect of Various Parameters on the Alkylation Activity. (i) Effect of the Method of Preparation. It was observed that the catalyst prepared by the impregnation method was very similar to SF in terms of reactivity, while that prepared by coprecipitation showed a synergistic effect on the activity of SZ. The decrease in activity with time observed in the case of SF was also taken care of when Fe2O3 is doped in the SZ matrix by coprecipitation (Figure 8) (ii) Effect of External Mass-Transfer Resistance. The effect of external mass-transfer resistance was studied at different agitation speeds to find whether the same levels of conversion were obtained. The speed of agitation was varied over a range of 500-1500 rpm under otherwise similar conditions. A catalyst loading of 5% w/w (based on benzyl chloride) and reflux temperature was chosen for this purpose. It was observed that the speed of agitation had no effect on conversion beyond 1200 rpm on all these catalysts. Thus, there was no limitation of external mass transfer beyond this speed. Further experiments were conducted above this speed (1500 rpm). (iii) Effect of Intraparticle Resistance (Pore Diffusional Resistance). For the catalysts SZ, SZF, and SF with an average particle size of 164 µm, 100 nm, and 150 µm, the effectiveness factor was calculated. Since benzene was taken in excess and the alkyl chloride was a limiting reactant in all these cases, the concentration of the latter may not be uniform within the catalyst particles due to intraparticle diffusional resistance.
Figure 8. Effect of the method of preparation on the activity of SZF. Table 2. Kinetic Parameters Evaluated of the Catalysts SZ, SF, and SZF catalyst SZ SF SZF
Ea (kcal/mol)
A (min-1)
70 °C
K (min-1) 80 °C
90 °C
28.0 5.6 20.0
3.16 × 2.0 × 102 3.9 × 1012
0.0525 0.0468 5.44
0.9471 0.4983 4.72
1.096 0.115 6.97
1016
To account for this type of intraparticle resistance, differential equations have been developed earlier.15 In the case of SZ, SZF, and SF, the values of the Theile modulus (φ) and effectiveness factor (η) were calculated according to the following equations:
φ) η)
( )( ) R Fkr 3 De
1/2
(51[cot 3φ + 31φ])
(1) (2)
The effective diffusivity (De) of benzyl chloride (BzCl) into benzene (Bz) was calculated using the WilkinsonChan equation
(XM)0.5 T De ) 7.4 × 10-8 Vb0.6 µ
(3)
where X ) association constant of Bz ) 1, M ) molecular weight of Bz ) 78.11, T ) temperature, Vb ) molar volume of BzCl, and µ ) viscosity of Bz. The average particle size was calculated by Scherrer’s formula from the X-ray line width as
D)
0.9λ β cos θ
(4)
where λ ) 1.5 Å, β ) line width in radians, and 2θ ) angle of diffraction. The Theile modulus and the effectiveness factor as given by eqs 1 and 2 are found to be 0.023 and 1, respectively, at reflux temperature for SZ and SZF. Thus, in both systems there was no intraparticle diffusional resistance. This fact was further confirmed by
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 911
the high Ea values obtained (Table 2), suggesting that the surface reaction could be rate controlling. However, in case of SF, η was much less than 1. Also, the Ea value was much lower (
