Znd. Eng. Chem. Res. 1988,27, 541-548
541
KINETICS AND CATALYSIS Alkylation of p -Cresol with Isobutene Catalyzed by Cation-Exchange Resins: A Kinetic Study E. Santacesaria,*t R. Silvani, P. Wilkinson, and S. Carrii Dipartimento di Chimica Fisica Applicata del Politecnico di Milano, Piazza Leonard0 da Vinci, 32-20133 Milano, Italy
In this paper the kinetics of the reactions occurring in the alkylation of p-cresol with isobutene, catalyzed by cation-exchange resins, have been studied. These reactions are the monoalkylation, the successive dialkylation, and the dimerization and trimerization of isobutene. The kinetics have been studied in a slurry reactor, completely mixed and operating under batch conditions for the liquid phase and continuous conditions for the gas phase. The influence of mass transfer on reaction rates has been carefully considered with experiments independent of kinetic runs. Vapor-liquid equilibrium of isobutene, in the reaction mixture, has also been studied for determining the gas-liquid interface concentration of this reagent. A second-order kinetic law has been found suitable to describe the behavior of all mentioned reactions. A kinetic model, containing both reaction and mass-transfer rates, has been formulated for interpreting kinetic data. Kinetic parameters have been evaluated by fitting experimental data. The alkylation of p-cresol (P) with isobutene (I) is industrially important in the production of butylated hydroxytoluene (BHT), a substance largely employed as an antioxidant. The reaction is normally carried out in the presence of sulfuric acid or cation-exchange resin as catalyst (Kirk and Othmer, 1978 Weinrich; 1943). Despite the importance of the process in industry, very few papers have been devoted, in the literature, to the kinetics of this reaction (Gehlawat and Sharma, 1970; Unni and Bahtia, 1982). In the present paper, therefore, the kinetic aspects of all the reactions occurring in the alkylation of p-cresol with isobutene catalyzed by the cation-exchange resin Amberlyst 15 will be examined. The influence of mass transfer on reaction rates has been tested by determining, independently, the kinetic runs, mass-transfer coefficients, and isobutene solubilities. In particular, vapor-liquid equilibrium, for isobutene dissolved in the reaction mixtures, has been studied by determining, experimentally, the activity coefficients of isobutene in the binary mixtures with respectively the reagent p-cresol and the main products of the reaction 2-tert-butyl-p-cresol (M) and BHT. Intraparticle diffusion has been taken into account by calculating the catalyst efficiencies for the involved reactions. Kinetic parameters have been determined by fitting experimental data of kinetic runs.
Experimental Section (A) Apparatus, Methods, and Reagents. Kinetic runs have been performed in the semibatch reactor shown in Figure 1; this is continuous for the gaseous reagent and batch for the liquid phase. The overall volume of the reactor was about 400 cm3, and it was normally filled with 140 cm3 of p-cresol; other t Present address: Cattedra di Chimica Industriale dell'Universia di Napoli, Via Mezzocannone 4,80134 Napoli, Italy.
Table I. Characteristics of t h e Employed Catalyst, Amberlyst 15, and of the Reactor of Figure 1 mean diameter of particles, cm 0.07 porosity 0.30-0.35 specific surface area, m2/g 40-50 acidity related to dry wt, mequiv/g 4.9 internal diameter of the reactor, cm 8 3.8 diameter of the impeller, cm
characteristics are collected in Table I. Small samples of the reaction mixture were withdrawn, at different times, and analyzed by a FID gas chromatograph. A column of Apiezon (15% by weight) on Chromosorb W, 3-m length, kept at 180 "C, was employed. Helium, with a feed rate of 30 cm3/m, was the carrier gas. Kinetic runs have been performed at different temperatures, stirrer speeds, and sizes of the catalyst particles. The reactor of Figure 1 was equipped with a stirrer of a particular shape. This stirrer operates in such a way that both fluid phases can be considered completely mixed and very high interfacial areas can be developed relating to the rotating speed. The effect of the rotating speed of the stirrer on gasliquid mass-transfer rates can be determined by the sulfite methods (Charpentier, 1981; Linek and Vacek 1981). This can be done by comparing the absorption rates of oxygen in aqueous solutions of sodium sulfite that are stirred at different rotating speeds with the rate obtained in a solution of the same type that is well stirred but doesn't produce cavitation, that is, in a fluid dynamic reference condition. In the last case, the interfacial area is known. If the rotating speed is increased with respect to the reference condition, both the gas-liquid interface area and the mass-transfer coefficient increase. But, if we consider the value of the mass-transfer coefficient constant and equal to the one valid in the reference condition, a change in the absorption rate of oxygen with stirring speed can be at-
0888-5885/88/2627-0541$01.50/0@ 1988 American Chemical Society
542 Ind. Eng. Chem. Res., Vol. 27, No. 4,1988
Le
,o
out
=A out
t I;\
50
150
100
200
t (mtnutes)
Figure 2. Kinetic run performed, at 35 "C,with 1100 rpm stirrer speed in the presence of 40 g of catalyst. Points are experimental; curves are calculated. x 10'
moles
I\ Figure 1. Scheme of the reactor employed 1and 2 are respectively inlet and outlet of the thermostating fluid; 3 and 4 are inlet and outlet of the isobutene gas stream; 5 is a magnedrive stirrer with holes in the turbine for cutting gas bubbles and producing high interface area; 6 is the thermometer; 7 is Teflon baffles.
tributed to a single parameter that can be defined as an apparent interfacial area. The mass-transfer coefficient in the reference conditions can be determined by recording the transient absorption of oxygen, in a solvent, by employing an amperometric electrode like the one suggested by Clark (1959) and by interpreting the obtained results. This mass-transfer coefficient must be recalculated taking into account differences in molecular properties of isobutene with respect to oxygen and of the reaction mixture with respect to the employed solvent by applying the relationship suggested by Wilke and Chang (1955). The product of the mass-transfer coefficient, determined in the described way, and the apparent interfacial area gives the correct mass-transfer parameter for any rotating speed of the stirrer. Vapor-liquid equilibrium of isobutene was studied by measuring, at different temperatures, the activity coefficients, at infiiite dilution, of isobutene in binary mixtures with respectively the reagent p-cresol and the main products BHT and M. These measurements were made by employing the stripping method suggested by Leroi et al. (1977). This method consists of dissolving a small amount of isobutene in the chosen solvent and then stripping it with helium. An on-line gas chromatographic sampling valve allows us to analyze the time decay of the isobutene concentration in the helium stream. According to Leroi et al. (1977), if In (S/S,) versus time is plotted, straight lines should be obtained whose slopes would be related to the activity coefficients at infinite dilution, in agreement with the relationship In @/So) = -(FPIo/RTN)y"t
(1)
Cation-exchange resin of the type Amberlyst 15 was employed as the catalyst. The catalyst was supplied by the Rohm and Haas Company. The main properties of this catalyst are summarized in Table I. Negligible deactivation
--I
BHTO
I \
o/
. D
t (minutes)
Figure 3. Kinetic run performed, at 50 "C, with 1100 rpm stirrer speed in the presence of 40 g of catalyst. Point are experimental; curves are calculated. moles x 10'
130
100
50
50
100
150
t (minutes)
Figure 4. Kinetic run performed, at 65 "C, with 1100 rpm stirrer speed in the presence of 40 g of catalyst. Pointa are experimental; curves are calculated.
occurs during the runs, as has been verified by titrating active acid sites before and after each run. Only fresh or renewed catalyst was employed in any kinetic run. All the reagents employed in this work were from the Carlo Erba Company and were of the highest degree of purity.
Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 543 analysis. On the other hand, the good agreement obtained in the mass balance on aromatics, as appears in the fitting of experimental runs, clearly indicates a negligible contribution of these side reactions. Another important reaction, the transalkylation of BHT, BHT P s 2M is implicit in the scheme, because it can be obtained by subtracting reaction 3 from reaction 2. A t last there is no evidence of 0-alkylation. Preliminary runs performed with isobutene, diluted with nitrogen, showed a first-order dependence of the reaction rates with respect to the gaseous reagent. Since no inhibitory effect appeared, due to the reagents or products, it is reasonable to assume a second-order kinetic law for all the mentioned reactions in agreement with Gehlawat and Sharma (1970) and Unni and Bahtia (1982). This kinetic law can be justified by assuming, for the aromatic alkylations, the validity of a Rideal mechanism of the type
moles x 10'
I
I
i.
+
I
t h n u tes)
+
H+(Cat)-
I+(Cat)-
(6)
Figure 5. Kinetic run performed, at 65 OC,with 1600 rpm stirrer speed in the presence of 40 g of catalyst. Points are experimental; curves are calculated. motes x lo2
CH, (M)
If in reaction 7 the first step is slow, reaction rates can be expressed as r = k,[R]t9 19 being the surface coverage degree, that is, the number of active sites occupied by the adsorbed isobutene carbocations related to the overall sites. Therefore,
8 = - b[II 1 b[I]
+
Introducing t (minutes)
Figure 6. Kinetic run performed, at 65 O C , with 1600 rpm stirrer speed in the presence of 40 g of catalyst with mean diameter of 0.042 cm instead of 0.07 cm.
(B) Results and Discussion. (1) Reaction Pathway and Kinetic Model. By observing the kinetic runs performed, reported in Figure 2-6, it is possible t~ suggest the following reaction scheme: monoalkylation dialkylation dimerization trimerization
P
M
+I
M
+ I A BI-IT 21r3-D
D+AT
(2)
(3) (4)
(5)
Very small amounts of oligomers, greater than the trimer, were sometimes found. In this case, we considered only a trimerization reaction, considering as trimer the other oligomers. Molecular rearrangement of C8 and C12has been observed but not considered in the kinetic model. The presence of alkylated molecules containing C8 or C12 groups has not been observed in the chromatographic
t9
in the reaction rate expression, we obtain [RI [I1 r = k,b- 1 b[I] = keff[RI[I1
+
where b[I]
