Ind. Eng. Chem. Res. 2006, 45, 4589-4596
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Limonene Epoxidation: Diffusion and Reaction over PW-Amberlite in a Triphasic System R. Barrera Zapata, A. L. Villa,* and C. Montes de Correa Departamento de Ingenierı´a Quı´mica, UniVersidad de Antioquia, Calle 67 N° 53-108, Medellı´n, Colombia
The effect of several reaction parameters on limonene epoxidation and hydrogen peroxide decomposition over PW-Amberlite was studied. Parameters evaluated were as follows: amount of catalyst (8-28 g/L), average particle diameter (110-720 µm), and concentrations of acetonitrile (7.4-15.7 mol/L), limonene (0.27-1.04 mol/L), hydrogen peroxide (0.73-1.92 mol/L), water (5.84-7.39 mol/L), and limonene epoxide (0-0.17 mol/L). The initial limonene reaction rate exhibits a maximum at 71 wt % acetonitrile (18 wt % hydrogen peroxide), 6.5 times higher than that obtained under biphasic conditions. According to WeiszPrater criterion, mass transfer limitations were insignificant for catalyst particle sizes smaller than 425 µm. External mass transfer limitations were avoided by using a stirring speed of 1000 rpm. The apparent activation energy of limonene epoxidation was 25 kJ/mol, three times lower than that previously found in a biphasic system. Empirical reaction rates of limonene epoxidation and hydrogen peroxide decomposition are proposed. 1. Introduction
2. Experimental Section
Limonene is an abundant monoterpene extracted from citrus oil, which is usually epoxidized for obtaining fragrances, perfumes, and food additives.1 High limonene conversion (>80%) and limonene epoxide selectivity (>90%) have been obtained over PW-Amberlite using aqueous H2O2 as the oxidizing agent and acetonitrile as the solvent.2 A kinetic study of limonene epoxidation over PW-Amberlite using a homogeneous liquid phase, i.e., biphasic system, was previously reported.3 Upon addition of 81 wt % acetonitrile, reaction rate is first order with respect to limonene, oxidant, and catalyst concentration. Notwithstanding, because of environmental and economic reasons, it is desirable to minimize the amount of solvent so a triphasic reaction system consisting of two liquid phases and a solid catalyst may be obtained. Overall reaction over PW-Amberlite should be different in a triphasic system compared to that observed in a biphasic one. Catalytic activity differences in biphasic and triphasic systems have been reported. Bhaumik and Kumar4 found higher benzyl alcohol oxidation with H2O2 over TS-1 under triphasic (89.6%) compared to biphasic conditions (12.5%). Product selectivities were also dependent on the number of phases. In another report, it was found that reaction rate for benzene hydroxylation with H2O2 over TS-15 increases between 15 and 25 times when a biphasic system is used in comparison to a triphasic system. However, in certain organic substrate oxidations over TS-1,6 reaction rate increased 3-10 times when a triphasic system was used instead of a biphasic one. Here, we report the influence of several reaction parameters for limonene epoxidation with aqueous H2O2 over PW-Amberlite in a triphasic system. Parameters studied were as follows: temperature, limonene and hydrogen peroxide concentrations, solvent, catalyst amount, catalyst recycling, and the presence of water and limonene epoxide. Internal mass transfer limitations were tested following the Weisz-Prater criterion.
2.1. Materials. All commercial products were used without further purification. Products used include aqueous hydrogen peroxide (30 wt %, J. T. Baker), (R)-(+) limonene (97 wt %, Sigma-Aldrich), acetonitrile (99.5 wt %, Merck), Amberlite IRA-900 ionic form chloride (Sigma), tetrabutylammonium hydrogen sulfate (97 wt %, Aldrich), tungstic acid (99 wt %, Aldrich), phosphoric acid (85 wt %, Merck), sodium nitrate (95 wt %, Merck), and acetone (99.8 wt %, Merck). 2.2. Catalyst Preparation. PW-Amberlite catalyst was synthesized as reported elsewhere.7 First, the complex PW4O24[(C4H9)4N]3 was prepared and heterogenized on previously crushed and sieved NO3-Amberlite IRA-900, taking care of preserving complex peroxo groups. Following this procedure, 0.55 mmol of PW4 (active complex, S) per gram of dry material were heterogenized.3,7 2.3. PW-Amberlite Swelling Experiments. Ion-exchange resins are functionalized cross-linked polymers. They are glassy in their dry form and become rubberlike in contact with polar solvents; the swelling degree depends on the interaction between solvent and resin.8 The relationship between swelling and reaction mixture composition was determined as follows: 0.1 g of dry catalyst was placed into a glass cylinder (70 × 7.0 mm) and, after compacting by centrifugation, solid height was measured (ho); then, the homogeneous liquid phase (see Table 1) was added and the final solid height (hf) was measured after ∼8 days when no changes were observed. The percentage of swelling, % H, was estimated according to eq 1.
* Corresponding author. Tel.: +574 2106606. Fax: +574 2106609. E-mail:
[email protected].
%H )
hf × 100% ho
(1)
2.4. Catalyst Density. The density of catalyst samples having different particle sizes was determined by placing a known amount of sample in a 5 mL graduated cylinder filled with water. Once catalyst was fully swollen (∼8 days), the displaced volume of water was measured. This value was taken as a rough estimate of the catalyst volume.
10.1021/ie060098b CCC: $33.50 © 2006 American Chemical Society Published on Web 05/28/2006
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Table 1. Swelling of PW-Amberlite water mass fraction (wt/wt), W
limonene mass fraction (wt/wt), L
acetonitrile mass fraction (wt/wt), A
limonene epoxide mass fraction (wt/wt), E
swelling (% H)
0.042 0.223 1 0 0 0 0 0.076 0.105 0.146
0.089 0.375 0 1 0 0 0.126 0.321 0.036 0
0.868 0.057 0 0 1 0 0.874 0.176 0.860 0.853
0 0.345 0 0 0 1 0 0.426 0 0
16.7 30.1 45.5 9.1 6.8 1.5 18.3 33.9 22.0 14.8
2.5. Catalytic Experiments. Typically, required amounts of limonene, acetonitrile, and hydrogen peroxide were mixed at room temperature and added to 8 mL magnetically stirred glass flasks immersed in a thermally controlled oil bath at the desired temperature; then, catalyst was added to start the reaction. Temperature was maintained within (0.5 K using an IKA ETSD4 fuzzy controller. Flasks were sampled at different times, between 0 and 120 min. Catalytic tests were carried out using catalyst samples of average particle diameter between 110 and 720 µm and concentrations varying between 8 and 28 g/L; acetonitrile (7.4-15.7 mol/L), limonene (0.27-1.04 mol/L), hydrogen peroxide (0.73-1.92 mol/L), water (5.84-7.39 mol/ L), and limonene epoxide (0-0.17 mol/L) were used. An electronic balance accurate to (0.1 mg was employed for weighing. After reaction, samples from each liquid phase (aqueous and organic) were carefully taken by means of syringes and analyzed by gas chromatography. 2.6. Catalyst Recycling. Catalyst was recovered by filtration, washed with acetone, and dried at room temperature before being recycled. 2.7. Analyses. Liquid samples were analyzed using a gas chromatograph (Varian Star, 3400) equipped with a flame ionization detector (FID). A 50 m DB-1 capillary column was used to separate limonene and its epoxide; column temperature was increased from 120 to 160 °C at a heating rate of 10 °C/ min; injector temperature was 200 °C; and nitrogen (7.7 cm3/ min) was used as carrier gas. Each sample (1.0 µL) was analyzed three times. A confidence interval of 95% and a normal distribution of data were assumed. Uncertainties were estimated by calculating standard deviation, using a factor of 1.960, recommended by the NIST (National Institute of Standard and Technology)9,10 for a confidence interval of 95%. Estimated uncertainties in initial reaction rate of limonene disappearance (mol/(h g)) were (1.5 × 10-4 and (4.32 × 10-4 for organic and aqueous phase, respectively. H2O2 concentration at different times was determined by cerimetric titration using a 0.1 N CeSO4 aqueous solution. Limonene epoxide was the only product detected; therefore, we assumed that H2O2 was consumed in two reactions (see eqs 2 and 3). The uncertainty in the determination of H2O2 decomposition (mol/(h g)) rate was estimated as (2.65 × 10-4.
H2O2 + C10H16 f C10H16O + H2O
(2)
1 H2O2 f H2O + O2(g) 2
(3)
If -rLa and -rLo represent initial limonene reaction rates for limonene epoxidation in the aqueous and the organic phase, respectively, and -rP2 and -rP3 represent H2O2 disappearance reaction rates for reactions 2 and 3, respectively, the total
Figure 1. Effect of stirring speed on initial limonene epoxidation rate over PW-Amberlite: [C10H16]0 ) 0.66 mol/L, [H2O2]0 ) 1.33 mol/L, [H2O] ) 5.84 mol/L, [cat] ) 18 g/L, catalyst particle size ) 250-425 µm, [CH3CN] ) 14.05 mol/L, 306 K. Table 2. Effect of Catalyst Particle Size on the Initial Reaction Rates of Limonene Epoxidation over PW-Amberlite initial reaction rate of limonene epoxidation, mmol/(h g) dry cata fraction
particle size range (µm)
average diameter, 2rdry catalyst (µm)
aqueous phase
organic phase
bulk
I II III IV V VI
600-830 425-600 250-425 180-250 125-180 90-125
715 512 337 215 152 107
3.77 3.78 4.13 4.15 4.11 3.98
3.8 3.54 3.77 3.78 3.80 3.85
7.57 7.32 7.9 7.93 7.91 7.83
a Reaction conditions: [C H ] ) 0.66 mol/L, [H O ] ) 1.33 mol/L, 10 16 0 2 2 0 [H2O] ) 5.84 mol/L, [cat] ) 18 g/L, [CH3CN] ) 14.05 mol/L, 306 K.
disappearance reaction rate of H2O2, -rPb, can be expressed by eq 4.
-rPb ) -rP2 + (-rP3)
(4)
On the other hand, assuming that only limonene epoxide is obtained, H2O2 consumed by limonene oxidation (-rP2) is estimated from eq 5 and H2O2 decomposition rate (-rP3) is estimated from eq 4.
-rP2 ) -rLa + (-rLo)
(5)
3. Results and Discussion 3.1. Mass Transfer Resistances. First, internal and external diffusion limitations on initial reaction rate were investigated. Stirring speed was varied (see Figure 1) for determining external mass transfer resistances. Above 800 rpm, no effects on limonene epoxidation with aqueous H2O2 over PW-Amberlite under triphasic conditions were observed. Thus, stirring speed was set at 1000 rpm for all further experiments in order to avoid external mass transfer resistances. The effect of internal diffusion on initial reaction rates was determined by measuring reaction rates for different average catalyst particle sizes (see Table 2). It was observed that initial reaction rates did not vary when catalyst particle sizes were smaller than 425 µm. 3.1.1. Weisz-Prater Criterion. The Weisz-Prater criterion was used to estimate intraparticle mass-transfer limitations. The “true” particle radius (R) was determined from swelling experiments (see Table 1). An empirical correlation, between swelling (percent) and mixture composition was obtained by multiple linear regression using Polymath 5.1 (see eq 6). The correlation coefficient (0.92) was greater than that reported by Pa¨a¨kko¨nen and Krause11 (0.42) for isoamylene etherification over a cationexchange resin. As is observed in Table 1, limonene epoxide
Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006 4591 Table 3. Estimated Densities of Swollen PW-Amberlite (g/mL) ( 0.08 for Different Particle Sizes particle size range (µm)
density
600-830 425-600 250-425 180-250 125-180 90-125
2.23 2.30 2.25 2.35 2.20 2.20
D°1-2(cm2/s) )
component
molar volume (cm3/mol)
ref
viscosity (cP)
ref
limonene water acetonitrile
192.2 18.11 57.4
12 12 12
1.08 1.0 0.35
13 14 15
does not appreciably contribute to resin swelling; so, it was not considered in this correlation.
(6)
where W, L, and A denote mass fraction of water, limonene, and acetonitrile, respectively. In the isoamylene etherification study,11 the true particle radius R was calculated by eq 7.
R)
3V2 V1
2/3
(10)
µ-0.2661 ) µref-0.2661 +
T - Tref 233
(11)
where µref is the known viscosity at Tref. To avoid nonideality implicit in estimations of diffusion coefficients at infinite dilution,17-19 the Vignes equation20 (eq 12) was used to correct liquid-phase activities.
D1-2 )
[( ) ]( ) D°2-1 D°1-2
x1
d ln a D° d ln X 1-2
(12)
where a and X denote liquid-phase activity and molar fraction of solute in reaction mixture, respectively. Liquid-phase activities were calculated using the UNIQUAC model.21 The equation of Kooijman and Taylor19 (eq 13) was further used for estimating diffusion coefficients of a multicomponent mixture:
Dij ) (D°ij)Xj(D°ji)Xi
(D°ikD°jk)X /2 ∏ k)1 k
(13)
k*i,j
x 3
µ2V11/3
1+
where µ is the viscosity (cP) and T is the temperature (K). Viscosity at the reaction temperature, 306 K, was estimated by the Reid equation12 (eq 11).
Table 4. Molar Volumes and Viscosities of Limonene, Water, and Acetonitrile at 293 K
%H ) 44.7W + 9.2L + 14.9A
[ ( )]
(8.2 × 10-8)T
swelling (%) × swelling in pure methanol ()113%) mean radius of the fractions (7)
where swelling (%) was calculated with an empirical correlation similar to eq 6 and swelling in pure methanol (113%) corresponds to the maximum swelling that was obtained with a pure compound in the swelling experiments.11 Notwithstanding, we found that, depending on the composition of the reaction mixture, eq 7 may give a value of the swollen resin radius smaller than that of the dry resin, which has no physical meaning. Thus, R was calculated from eq 8, which gives a good representation of resin swelling phenomena.
(
R ) rdry catalyst 1 +
%H 100
)
(8)
where rdry catalyst was obtained from Table 2. The highest PW-Amberlite swelling was 45.5% (see Table 1). From Table 3, a catalyst density, Fpart, of 2.23 g/cm3 was used as a representative value. The effective diffusion coefficient of limonene-water and acetonitrile-water, required to estimate Weisz-Prater criterion, was determined using the Othmer-Thakar equation16 (eq 9) recommended for diluted aqueous solutions.12
D°1-2 (cm2/s) ) (14 × 10-5)µw -1.1V1-0.6
(9)
where D°1-2 (cm2/s) is the diffusivity coefficient at infinite dilution of solute (1) in water (2), µw is the water viscosity (cP), and V is the molar volume (cm3/mol). Table 4 lists molar volumes and viscosities of reaction mixture components at 293 K. The Scheibel equation17 (eq 10) recommended for solutions of nonaqueous solvents11,12 was used to estimate the binary diffusivities of water-limonene and water-acetonitrile, as well as those of acetonitrile-limonene and limonene-acetonitrile.
where i, j, and k are mixture components limonene (L), water (W), and acetonitrile (A), respectively. For example, the calculated limonene bulk diffusion coefficient in water and acetonitrile (DL-W,A) was 5.06 × 10-6 cm2/s. Pore volume (Vp ) 0.42 cm3/g) obtained from N2 adsorption was used for the porosity calculation, and a value of 0.48 was obtained by eq 14.22
mVp
)
mVp +
( )
(14)
m Fpart
where Fpart is particle density in g/cm3 and m is amount of catalyst used during N2 adsorption (0.0411 g). A tortuosity factor, τ, of 2.17 was estimated using a model proposed by Dogu and Dogu23,24 (eq 15) recommended for solid porosities, , higher than 0.476.
τ)
3 2/3 1 - π (1 - ) 4π
[
]
(15)
The effective diffusion coefficient was determined by eq 16, obtaining a value of 1.24 × 10-6 cm2/s.
DeL ) DL-W,A τ
(16)
The Weisz-Prater criterion (Φ), eq 17, establishes that, if Φ , 1, there are not diffusional problems.11,18,25
Φ)
-robsFpartR2 DeiCi
(17)
where -robs is the bulk observed reaction rate in mol/(g s), R is in cm, Fpart is in g/cm3, Dei is the effective diffusivity of component i in the reaction mixture (cm2/s), and Ci is the concentration of component i in mol/cm3.
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Figure 2. Estimated Weisz-Prater criterion: [C10H16]0 ) 0.66 mol/L, [H2O2]0 ) 1.33 mol/L, [H2O] ) 5.84 mol/L, [cat] ) 18 g/L, [CH3CN] ) 14.05 mol/L, 306 K. Table 5. Estimated Weisz-Prater Criterion and Effectiveness Factors at 306 Ka particle size range (µm)
[CH3CN] (mol/L)
swelling (%)
R (cm)
rate (mmol/(g h))
Weisz
η
600-830 425-600 250-425 180-250 125-180 90-125
14.7 14.4 14.3 14.4 14.4 14.4
19.0 19.7 19.5 19.7 19.7 19.7
0.043 0.031 0.020 0.013 0.009 0.006
3.10 7.34 7.20 7.93 7.91 7.83
1.73 1.63 0.72 0.31 0.16 0.08
0.53 0.93 1.00 1.00 1.00 0.99
a Reaction conditions: [C H ] ) 0.66 mol/L, [H O ] ) 1.33 mol/L, 10 16 0 2 2 0 [H2O] ) 5.84 mol/L, [cat] ) 18 g/L.
Figure 2 shows the Weisz-Prater criterion profile vs catalyst particle sizes. It is observed that, for particles with average diameters smaller than 425 µm, the Weisz-Prater criterion is
