Etherification of β-Naphthol with Alkanols Using Modified Clays and

The efficacy of solid acid catalysts such as acid-treated clays (Filtrol-24 and K-10), heteropolyacids (dodecatungstophosphoric acid, dodecatungstosil...
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Ind. Eng. Chem. Res. 1998, 37, 3358-3365

Etherification of β-Naphthol with Alkanols Using Modified Clays and Sulfated Zirconia G. D. Yadav* and M. S. Krishnan Chemical Engineering Division, University Department of Chemical Technology (UDCT), University of Mumbai, Matunga, Mumbai 400 019, India

Alkyl 2-naphthyl ethers are an important class of precursors for the manufacture of fine chemicals useful as perfumery and pharmaceutical chemicals. The efficacy of solid acid catalysts such as acid-treated clays (Filtrol-24 and K-10), heteropolyacids (dodecatungstophosphoric acid, dodecatungstosilicic acid, dodecaphosphomolybdic acid), zeolites (X, Y, mordenite, ZSM-5), and sulfated zirconia was tested in the etherification of β-naphthol with methanol. Among these catalysts, dodecatungstophosphoric acid supported on K-10 clay was found to be the best. The rates of etherification with other aliphatic alcohols over the same catalyst at 150 °C were in the following order n-BuOH > 2-PrOH > MeOH > EtOH > n-PrOH. A kinetic model was built for these reactions. The catalyst can be easily recovered and reused without any significant loss in activity. Introduction The ethers of β-naphthol are commercially very attractive due to their extensive applications in the fine chemical industry. For instance, naphroxen (6-methoxy-R-methyl-2-naphthaleneacetic acid), which is an antiinflammatory, analgesic, and antipyretic drug, is produced from β-naphthyl methyl ether as the starting material. This ether is synthesized from the reaction of β-naphthol with methanol. The alkyl ethers of β-naphthol particularly methyl, ethyl, and isobutyl ethers known respectively as Yara Yara (or nerolin-I), Bromelia (or Nerolin-II, Nerolin Bromelia), and Fragrol (or Nerolin-Fragrol) are important perfumery and flavoring compounds (Arctander, 1969; Bedoukian, 1986). To a lesser extent, the propyl and n-butyl ethers of β-naphthol are used in perfumes. The current practice for the manufacture of these ethers is based on homogeneous acid-catalyzed reactions which are not in consonance with the worldwide thrust on ecofriendly processes. Solid acid catalysts of different kinds have gained considerable importance in the synthesis of fine chemicals, due to such factors as the use of milder reaction conditions, shorter workup times, favorable selectivity of the desired product, cheap materials of construction, reuse of catalysts, their ecofriendliness, and elimination of hazardous conditions or processes. Besides, the corrosion problem associated with homogeneous catalysis is also avoided due to the binding of acidity within the porous matrix of the solid catalysts (Yadav and coworkers, 1993, 1994, 1995, 1996, 1997). These include zeolites and zeotypes, clays and pillared clays, ionexchange resins, supported heteropolyacids (HPA), and other modified metal oxides such as sulfated zirconia. Thus, studies in the etherification of β-naphthol with different alkanols, in the presence of solid acid catalysts, appear to be relevant, having both academic and industrial context. * To whom correspondence should be addressed. Fax: 9122-4145614. Telephone: 91-22-4145616. E-mail: [email protected].

The preparation and characterization of heteropolyacids supported on K-10, a commercially available acidtreated montmorillonite clay, were explored in this laboratory for the methyl tert-butyl ether (MTBE) synthesis from tert-butyl alcohol with methanol, and these were found to be very efficient and selective (Yadav and Kirthivasan, 1995). Further, dodecatungstophosphoric acid (DTP/K-10) was found to be the best catalyst among HPA/C, ion-exchange resins, and K-10 clay alone for the etherification of phenethyl alcohol with different alkanols (Yadav and Bokade, 1996). Thus, the use of these catalysts in the etherification of β-naphthol merited attention. The general industrial procedure for making the ethers of phenols and naphthols is by heating them with the alkali solution of alkyl halides (Arctander, 1969; Bedoukian, 1986). It is also a very useful laboratory technique. However, in practice it is considered more economical to use the Gattermann method of heating β-naphthol with an alcohol in the presence of concentrated hydrochloric acid or, preferably, sulfuric acid. Another route involves the reaction of β-naphthylsodium with dimethyl sulfate in dilute aqueous alkali. Ethers of phenols can also be obtained in good yields by treating them with trialkyl phosphates, and the same procedures can be applied in the preparation of other ethers (Bedoukian, 1986). This paper presents extensive studies in the etherification of β-naphthol with alkanols such as methanol, ethanol, n-propanol, 2-propanol, and n-butanol by using a variety of solid acid catalysts, such as Filtrol-24, K-10 clay, K-10-supported heteropolyacids such as dodecatungstophosphoric acid (DTP/K-10), dodecatungstosilicic acid (DTS/K-10), and dodecaphosphomolybdic acid (DPM/ K-10), zeolites X, Y, mordenite, and ZSM-5, and sulfated zirconia. The effect of various parameters was studied with the etherification of β-naphthol with methanol as a model system in a mechanically agitated slurry reactor. Experimental Methods Chemicals and Catalysts. All chemicals and catalysts were procured from firms of repute: β-naphthol

S0888-5885(97)00814-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/11/1998

Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3359 Table 1. Physical Properties of Modified Clays and Sufated Zirconia catalyst

surface area, m2/g

CEC, mequiv/100 g

pore volume, cm3/g

Filtrol-24 K-10 20% DTP/K-10 S-ZrO2

350 230 230 92

16 35 39

0.32 0.36 0.95

(AR, 99% purity), dodecatungstophosphoric acid (DTP), dodecatungstosilicic acid (DTS), dodecaphosphomolybdic acid (DPM), and the alcohols (AR, 99% purity) were procured from M/s s.d. Fine Chemicals Ltd., Mumbai, India, and were used as such. β-Naphthyl alkyl ethers were procured from M/s Industrial Perfumes Ltd., Mumbai, India. Filtrol-24 (Engelhart Inc.) and K-10-montmorillonite (Fluka) were taken as such and dried in an oven for 2 h prior to use. Zeolites Y, X, ZSM-5, and mordenite (all from M/s Associated Chemical Co., Mumbai, India) were converted to the H-form by treatment with ammonium nitrate followed by calcination at 550 °C for 3 h. Swy-2 (a Wyoming Na+-montmorillonite) was obtained from Clay Minerals Society, Source Clay Minerals Repository, Missouri University, Columbia, MO. K-10 is a commercially available acid-activated clay, derived from Bavarian montmorillonite Tonsil-13. It has a relatively high alumina content at 14% with a surface area of 220-270 m2 g-1, compared to the natural montmorillonite, and has a reduced exchange capacity of 50-60 mequiv/100 g. The so-called incipient wetness technique was employed to prepare the K-10 clay supported heteropolyacids such as dodecatungstophosphoric acid (DTP), dodecatungstosilicic acid (DTS), and dodecaphosphomolybdic acid (DPM) as given by Yadav and Kirthivasan (1995). An appropriate quantity of montmorillonite clay (K-10) was taken and evacuated in an oven for 20 min, to which the corresponding heteropolyacid dissolved in methanol was added dropwise under continuous stirring to make 20% HPA/K10. The catalysts were dried at 120 °C for 2 h, calcined at 260 °C for an additional 3 h, stored in sealed bottles, and dried prior to use. The sulfated zirconia catalyst was prepared by dissolving the required quantity of zirconium oxychloride in deionized water, followed by precipitation using an aqueous solution of ammonia at room temperature at a constant pH of 9.0. The precipitate, zirconium hydroxide, was washed with deionized water to remove the chloride ions until the filtrate gave a negative chloride test to AgNO3 treatment. The precipitate was dried in an oven at 120 °C for 24 h, and it was crushed to make a powder. The sulfate impregnation was performed by passing 1 N H2SO4 (15 cm3/g) through the dried powder uniformly spread on a filter paper. It was dried in an oven at 120 °C for 1 h and calcined at 600 °C for 3 h. Table 1 shows the physical properties of these catalysts. The catalysts have been characterized fully as reported earlier (Khumbar et al., 1989; Yadav and Kirthivasan, 1997). Experimental Setup and Reaction Procedure. A typical procedure is given for the reaction of β-naphthol with methanol. The reaction was carried in a Parr autoclave of 100-cm3 capacity. Five grams of β-naphthol, 30 g of methanol, and 0.7 g of the particular catalyst were charged to the autoclave. The total volume of the reaction mixture, which was allowed to reach the desired temperature, was 42 cm3, and the

Figure 1. 1H NMR of β-naphthyl methyl ether [2-methoxynaphthalene]. Solvent: CCl4. (a) 3.8 (3 H); (b) 6.8-7.3 (4 H); (c) 7.57.8 (3 H).

initial sample was collected. Agitation was then commenced at a known speed. Samples were withdrawn periodically for analysis. In most cases, methanol was taken in far molar excess over β-naphthol to drive the equilibrium toward ether formation. It was interesting to note that the reaction mixture developed an intense odor of 2-methyl naphthyl ether as the reaction proceeded. Analysis The analysis was carried out by gas chromatography (Perkin-Elmer, model 8350) by using a flame ionization detector. A stainless steel column (3.2 mm i.d. × 2 m long) packed with 10% OV-17 on chromosorb WHP was used for analysis. Pure samples of the reactants and products and their synthetic mixtures were used to calibrate the chromatograms and to deduce quantitative information. Isolation of Products. The unreacted β-naphthol and the product ether dissolved in the excess quantity of alcohol were removed by evaporating the alcohol in a rotavac. The solids were then treated with 5% w/w aqueous alkali to extract unreacted β-naphthol, and benzene was added to partition the β-naphthyl alkyl ether in the benzene phase. The organic layer was repeatedly washed with water and separated out. A brownish, shiny product was obtained after removing the benzene by using a rotavac. To remove the brownish color in the product, a charcoal treatment was given to the product. The product was dissolved in CCl4 and heated with predried activated charcoal. The charcoal was filtered off in the hot condition, and the filtrate was evaporated to dryness. A pure white crystalline solid product was obtained. The final product was characterized by melting point, FT-IR, and 1H NMR analyses along with a commercial sample. Figure 1 shows the 1H NMR spectrum of the isolated β-naphthyl methyl ether product. Both the spectrum and the melting point matched those of the commercial pure sample. Theoretical Aspects of a Solid Acid Catalyzed Liquid-Phase Reaction. The solid acid catalyzed etherification is a typical solid-liquid slurry reaction for which some theoretical work has emanated from this laboratory (Yadav and Bokade, 1996; Yadav and Krishnan, 1998). The theory takes into account the evaluation of the intrinsic kinetics of the reaction.

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(iii) Desorption of E and W from the catalyst surface to the exterior of the catalyst.

Figure 2. Concentration profiles for reactants and products in etherification of β-naphthol. A: β-naphthol. B: alcohol. E: β-naphthyl alkyl ether. W: water.

The etherification reaction involves two organic-phase reactantssA (β-naphthol) and B (methanol)sand the desired product E (β-naphthyl methyl ether) and W (coproduct water) as given in reaction a.

Although A and B are liquid-phase reactants, they need to diffuse to the interior surface of the catalyst. There is a likelihood of a concentration gradient for β-naphthol in the catalyst pores, in comparison with the alkanol taken in stoichiometric excess. Figure 2 depicts the various concentration profiles. At steady state the rate of mass transfer per unit volume of the liquid phase can be given by RA (gmol/ cm3‚s).

RA ) kSL-Aap{[A0] - [AS]}

(1)

(iv) Transport of desorbed E and W from the exterior surface through the liquid film surrounding the catalyst particle to the bulk liquid phase. The reaction can take place according to the Langmuir-Hinshelwood-Hougen-Watson (LHHW) model or the Eley-Rideal model, if the adsorption is significant. If the adsorption terms are insignificant, it could be a simple power-law model. The analysis of collected data suggested that the adsorption terms were insignificant. Thus, the following equation was found to hold.

RA ) ηkR2w[AS][BS]

(3)

rate of surface reaction per unit volume in the presence of intraparticle diffusion where kR2 is the second-order rate constant [(cm3/gmol)(cm3/g)(1/s)]. Eliminating the unknown surface concentrations of the reactants, [AS] and [BS], a quadratic equation in RA is obtained. Equation 3 is a power-law model. When the surface reaction is very rapid with respect to the diffusion either of A and B or of A alone, several interesting cases emerge depending on the concentrations of A and B in the liquid phase. The current investigation of reactions presented in this paper deals with a situation where the surface reaction was much lower than the diffusional steps, and thus the following equations are relevant, i.e., if

1 1 1 . and ηkR2w[AS][BS] kSL-AaP[A0] kSL-BaP[B0]

(4)

rate of diffusion of A from the bulk liquid phase to the catalyst surface

then the surface reaction is the controlling mechanism and the overall rate of the reaction will be the same as that given by the surface reaction, i.e.

RA ) zkSL-Bap{[B0] - [BS]}

RA ) ηkR2w[AS][BS]

(2)

rate of diffusion of B from the bulk liquid phase to the catalyst surface As far as the surface reaction is concerned, the following mechanism is envisaged. (i) Adsorption of A on the catalyst surface (M+), where M+ represents a general acid site.

(ii) Reaction between chemisorbed β-naphthylcarbenium ion with the liquid-phase alcohol ROH by the Eley-Rideal mechanism.

(5)

The influence of intraparticle diffusion, reflected in the effectiveness factor (η) can be studied by varying the particle size and also the reaction temperature. To discern the controlling mechanism, it was desirable to study the effect of various parameters on the overall rate of reaction. Since one of the components is always taken as the limiting reactant, its concentration may not be uniform within the catalyst particle due to intraparticle diffusional resistance in contrast to that of the component used in excess. To account for this resistance, differential equations for intraparticle diffusion with reaction on the surface of the catalyst need to be solved and these types of problems are well-known (Doraiswamy and Sharma, 1984).

Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3361

When B is taken in far excess, eq 5 becomes

RA ) ηkR1w[AS]

(6)

Once again the concentration [AS] can be eliminated to get RA as

RA ) [A0]w{(FPdP)/(6kSL-A) + 1/(ηkR1)}-1

(7)

because

aP ) 6w/(FPdP)

(8)

kR1 ) kR2[B0]

(9)

Depending on the relative magnitudes of kSL-A and kR1, the controlling resistance can be found. In the absence of both external and internal resistances to mass transfer, it is possible to determine the intrinsic kinetics. Accordingly, for a pseudo-first-order reaction

-d[A0]/dt ) kR1w[A0] ) k1[A0]

(10)

Figure 3. Effect of different catalysts on conversion of β-naphthol. Catalyst loading: 2.0% w/w. Temperature: 150 °C. Speed of agitation: 600 rpm. Pressure: 980 kPa. β-Naphthol: 5 g. Methanol: 30 g. Total volume: 42 cm3. Table 2. Rate Constants for Various Catalysts for the Etherification of β-Naphthol at 150 °C

where

k1 ) kR1w

(11)

Integrating eq 10, the following is obtained:

-ln(1 - XA) ) k1t

(12)

where XA is the fractional conversion of A. Thus, a plot of -ln(1 - XA) against time t will give a slope which represents kl and from which kR1 can be determined. The above theory is developed for a single liquid phase containing the reactants, the products, and the solid catalyst. The water of reaction was miscible in the organic phase in the presence of methanol. When there is a multiphase system such as L-L-S, the diffusion of one of the reactants into the other liquid phase which acts as a continuous phase and contains the catalyst needs to be considered. Results and Discussion Methanol was always taken in far excess, as mentioned earlier, over β-naphthol, and the preliminary experiments were conducted under otherwise similar conditions of reactant concentration (or mole ratio), catalyst loading, particle size, speed of agitation, and temperature except the type of catalyst. Efficacy of Various Catalysts. Different solid catalysts were used to assess their efficacy in this reaction as mentioned earlier. A 2.0% w/w loading of catalyst based on the organic mass of the reaction mixture was employed at a mole ratio of methanol to β-naphthol of 27:1 at 150 °C and an agitational speed of 600 rpm. It was expected to have no influence of external and/or internal resistance to mass transfer so that a comparative study on the basis of intrinsic kinetics could be made. Further, as will be explained later, indeed there was no influence of mass transfer on the rate of reaction of β-naphthol with methanol. Figure 3 shows the plots of conversion of β-naphthol, the limiting reagent, against time for the various catalysts.

catalyst

kR2, (cm3/g)(cm3/gmol) s-1

20% DTP/K-10 Filtrol-24 20% DTS/K-10 S-ZrO2 20% DPM/K-10 K-10

13.72 3.23 2.69 2.15 1.61 0.27

It was found that the zeolites X, Y, mordenite, and ZSM-5 and the raw clay Na+-montmorillonite (Swy2) did not show any activity to this reaction. The 20% DTP/K-10 showed very good activity followed by Filtrol24 compared to other catalysts. The activity of the various catalysts is evaluated on the basis of the second-order rate constants [kR2] that are obtained under the kinetically controlled mechanism. Table 2 shows the values of kR2 for various catalysts. The activity of the catalysts is as follows: DTP/K-10 > Filtrol-24 > DTS/K-10 > S-ZrO2 > DPM/ K-10 > K-10. The evaluation of kinetics was done by studying the effects of different parameters on the rate of reaction. Filtrol-24 was chosen as the catalyst, and the kinetic model so developed was extended to other catalysts. Efficacy of Type HPA/K-10. Figure 4 shows the efficacy of different HPAs used for support on the clay. The 20% DTP/K-10 showed better activity than the other two catalysts, DTS/K-10 and DPM/K-10, with the same amount of HPA loading. The acidic properties of HPAs in solutions have been tabulated by Kozhevnikov and Matveev (1983). In ethanol, the respective pK1, pK2, and pK3 values of DTP are 1.6, 3.0, and 4.1; for DTS, 2, 3.96, and 6.3; and for DPM, 1.8, 3.41, and 5.1. The deposition of these HPAs on K-10 could perhaps be assumed to retain the same strength and hence DTP/ K-10 is a better catalyst. Effect of Speed of Agitation. The effect of speed of agitation was varied in the range of 400-800 rpm with Filtrol-24 as a catalyst. As shown in Figure 5, the conversions of β-naphthol were unaffected by the speed in the range of 400-800 rpm. Therefore, further

3362 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998

Figure 4. Effect of different HPAs supported on K-10 on conversion of β-naphthol. Catalyst loading: 2.0% w/w. Temperature: 150 °C. Speed of agitation: 600 rpm. Pressure: 980 kPa. β-Naphthol: 5 g. Methanol: 30 g. Total volume: 42 cm3.

Figure 6. Effect of particle size on conversion of β-naphthol. Catalyst: Filtrol-24 (2.0% w/w). Temperature: 150 °C. Pressure: 980 kPa. Speed of agitation: 600 rpm. β-Naphthol: 5 g. Methanol: 30 g. Total volume: 42 cm3.

per unit liquid volume, was calculated from

aP ) 6w/(FPdP) as 0.6374 cm-1 Thus

kSL-AaP[A0] ) 1.593 × 10-6 gmol cm-1 s-1 and

kSL-BaP[B0] ) 9.468 × 10-7 g/mol cm-1 s-1 Thus, a typical initial rate of reaction was calculated as 2.064 × 10-8 gmol/cm3‚s. Therefore,

1 1 1 . and ηkR2w[AS][BS] kSL-AaP[A0] kSL-BaP[B0] Figure 5. Effect of speed of agitation on conversion of β-naphthol. Catalyst: Filtrol-24 (2.0% w/w). Temperature: 150 °C. Pressure: 980 kPa. Particle size: 600 µm. β-Naphthol: 5 g. Methanol: 30 g. Total volume: 42 cm3.

experiments were conducted at 600 rpm to be on the safer side. Since alcohol [B] was chosen in far excess, there is a possibility of diffusional resistance for transfer of β-naphthol [A] through the liquid film around the catalyst and then inside the pores. According to the model used for this analysis, the liquid-phase diffusivity values DAB and DBA were required and these values were calculated by using the Wilke-Chang equation (Reid et al., 1977). The value of DAB, the diffusivity of A in B at 150 °C, was calculated as 9.077 × 10-5 cm2/s. The values of solid-liquid mass-transfer coefficients for A, kSL-A, were calculated by assuming the Sherwood number, Sh ) kSLdP/D ) 2, where D is the diffusivity of the reactant in the liquid phase.. It should be noted that the actual Sherwood number could be much higher, but for orders of magnitude calculations, it is safe to take the lowest Sherwood number. Thus, kSL-A was found as 3.026 × 10-3 cm/s for a particle size of Filtrol24 of 0.06 cm. The value of aP, the particle surface area

(13)

i.e., 4.845 × 107 . 6.28 × 105 and 1.06 × 106. The above inequality demonstrates that there is an absence of resistance due the solid-liquid external mass transfer and the rate may be either surface reaction controlled or intraparticle diffusion controlled. Effect of Intraparticle Diffusion. The effect of intraparticle diffusional resistance was studied by taking three different particle size ranges as shown in Figure 6. For an average particle size of less than 600 µm, there was no effect of particle size on the conversion of β-naphthol. This would suggest that the effectiveness factor for this reaction is almost unity. As will be shown later the effectiveness factor was indeed unity. Thus, there was no intraparticle diffusion and the entire process becomes surface reaction controlled, where the chemisorbed A reacts with B on the interior surface of the particle. The percent of active sites on the exterior surface of particles is negligible. This was further confirmed by studying the effect of temperature on the rate of reaction which gives the energy of activation. Effect of Catalyst Loading. As per eq 5, in the absence of mass-transfer resistance, the rate of reaction is directly proportional to catalyst loading based on the entire liquid-phase volume. The catalyst loading was

Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3363

Figure 8. Effect of concentration of β-naphthol on initial rate of reaction with methanol. Temperature: 150 °C. Catalyst: Filtrol24 (2.0% w/w). Speed of agitation: 600 rpm. Particle size: 600 µm. Total volume: 42 cm3. Figure 7. Plot of pseudo-rate constant k1 vs catalyst loading (g/ cm3). Catalyst: Filtrol-24. Temperature: 150 °C. Speed of agitation: 600 rpm. Particle size: 600 µm. Pressure: 980 kPa. β-Naphthol: 5 g. Methanol: 30 g. Total volume: 42 cm3.

varied from 0.1% to 3.0% w/w of total organic mass, which corresponds to w of 8.33 × 10-3-2.50 × 10-2 g/cm3. Either the initial rate of reaction or the pseudo-rate constant k1 could be plotted against w. It is seen that the rate is linearly proportional to w (Figure 7). More appropriately,

kSL-AaP{[A0] - [AS]} ) ηkR1w[AS]

(14)

kSL-AdP )2 DAB

(15)

but

aP )

6w FPdP

and

For

[A0] [AS]

kR1FPdP2 12DAB

)1+

(16)

When intraparticle diffusion is present,

φ)

[ ]

dP kR1FP 6 De-AB

1/2

and

η ) 1/φ

(17)

where

De-AB ) DAB/τ [A0] [AS]

)1+

[x

(18)

]

kR1DABFP dP τ

(19)

where  ) porosity and τ ) tortuosity. Thus, using the appropriate values, it is seen that eq 16 is valid, giving [A0] ≈ [AS]; that is, the rate is linearly proportional to catalyst loading in the absence of any intraparticle diffusion (Figure 7). Effect of Mole Ratio. The concentration of methanol had an influence on the reaction rate and on the conversion. The β-naphthol concentration was varied

Figure 9. Typical pseudo-first-order (integrated form) plots for the effect of different catalysts on the reaction of β-naphthol with methanol. Catalyst loading: 2.0% w/w. Speed of agitation: 600 rpm. Pressure: 980 kPa. β-Naphthol: 5 g. Methanol: 30 g. Total volume: 42 cm3.

from 1.652 × 10-3 to 2.0714 × 10-4 g mol/cm3. The plot of β-naphthol concentration and the corresponding initial rates (Figure 8) showed that the rate of the reaction is linear to the concentration of β-naphthol. Kinetics of the Reaction. When both external and internal mass-transfer resistances are absent, the kinetics of the surface reaction by using the power law model can be established. When the molar ratio of β-naphthol to methanol M is 1:1, eq 5 is integrated to get

XA ) [A0]ikR2wt 1 - XA

(20)

By making a plot of XA/(1 - XA) vs t, we get a straight line whose slope represents kR2. When M * 1, the

3364 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998

Figure 10. Effect of temperature on conversion of β-naphthol. Catalyst: Filtrol-24 (2.0% w/w). Speed of agitation: 600 rpm. Particle size: 600 µm. β-Naphthol: 5 g. Methanol: 30 g. Total volume: 42 cm3.

Figure 11. Typical first-order plot for the reaction of β-naphthol with methanol. Catalyst: Filtrol-24 (2.0% w/w). Speed of agitation: 600 rpm. Particle size: 600 µm. β-Naphthol: 5 g. Methanol: 30 g. Total volume: 42 cm3.

integration of the above equation leads to the following equation:

[

ln

M - XA

]

M(1 - XA)

) [A0]i(M - 1)kR2wt

(21)

Thus, plots can be made of the left-hand side term of eq 21 against time to determine kR2, which has units (cm3/gmol)(cm3/g of catalyst)(1/s). When M is very high, the rate constant kR1 is a pseudoconstant.

kR1 ) kR2[B0]

(22)

The integration of a relevant equation leads to

-ln(1 - XA) ) k1t

(23)

k1 ) kR1w ) kR2[B0]w

(24)

where

Figure 12. Arrhenius plot of ln kR2 vs 1/T for the reaction of β-naphthol with methanol.

Figure 13. Effect of different alcohols on conversion of β-naphthol. Catalyst: Filtrol-24 (2.0% w/w). Temperature: 150 °C. Speed of agitation: 600 rpm. Particle size: 600 µm. β-Naphthol: 5 g. Total volume: 42 cm3.

In the present case plots were made of -ln(1 - XA) vs t to determine k1 from which kR1, and hence kR2, were calculated for each catalyst. Figure 9 shows this plot. Effect of Temperature. The effect of temperature on conversion under otherwise similar conditions was studied in the range of 120-170 °C as shown in Figure 10. It is seen that the conversion increases with temperature. The plots of -ln(1 - XA) vs time indicate a straight-line behavior typical of a pseudo-first-order reaction (Figure 11). The rate constants k1 at 120, 150, and 170 °C were 0.2 × 10-3, 1.2 × 10-3, and 6.1 × 10-3 s-1, respectively, from which kR2 values were calculated. The Arrhenius plot is shown in Figure 12 wherein ln kR2 is plotted against 1/T to get the activation energy, Ea, as 23.23 kcal/gmol, which confirms that the reaction is intrinsically kinetically controlled. Effect of Different Alcohols. The etherification of β-naphthol with ethanol and n-propanol at 150 °C proceeded smoothly, and as shown in Figure 13, the

Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3365 Table 3. Rate Constants for the Etherification of β-Naphthol of Different Alcohols at 150 °C for Filtrol-24 catalyst

kR2, (cm3/g)(cm3/gmol)(1/s)

methanol ethanol n-propanol i-propanol n-butanol

5.076 2.403 0.215 5.610 8.547

rates were in the following order:

methanol > ethanol > n-propanol The kR2 values calculated were given in Table 3. However, in the case of n-BuOH and 2-PrOH, there was substantial dehydration of these alcohols, leading to the formation of isobutylene and propylene. The pressures were found to shoot to 3500 kPa. The rate of reaction was also enhanced, and formation of c-alkylated product in conjunction with the ether was noticed. The rate of reaction was therefore

n-BuOH > 2-PrOH > MeOH > EtOH > n-PrOH Odor Evaluation. The odor evaluation for methyl and ethyl ethers was performed by experts, and the perfume grades were found to match with those of the commercial samples. Conclusions The etherification of β-naphthol with methanol and other alcohols was studied systematically using different solid acid catalysts such as Filtrol-24, K-10, DTP/K-10, DTS/K-10, DPM/K-10, zeolites X, Y, mordenite, and ZSM-5, and sulfated zirconia. Among these catalysts DTP/K-10 was found to be the best catalyst for this reaction. Zeolites and the raw montmorillonite clay Swy2 did not show any activity to this reaction. The products were isolated and conformed by melting point, FT-IR, and 1H NMR analyses. The products obtained through isolation were very well matching with perfumery-grade standard samples brought from the industry. A kinetic model was developed from the experimental data and proved that this reaction was intrinsically kinetically controlled. Acknowledgment M.S.K. acknowledges the University Grants Commission (UGC) for awarding the Senior Research Fellowship. G.D.Y. acknowledges a research grant from the All India Council for Technical Education (AICTE), New Delhi, India. Notation A ) β-naphthol [A] ) concentration of A in catalyst pores, gmol/cm3 [A0] ) concentration of A at the solid (catalyst) surface, gmol/cm3 aP ) solid-liquid interfacial area, cm2/cm3 B ) alkanol [B0] ) concentration of B in the bulk liquid phase, gmol/ cm3 [BS] ) concentration of B at the solid-liquid interface, gmol/cm3 CA ) concentration of β-naphthol, gmol/cm3 CB ) concentration of alkanol, gmol/cm3 dP ) particle diameter, cm

De ) effective diffusivity, cm2/s Ea ) energy of activation, kcal/gmol KA ) adsorption equilibrium constant for A k1 ) pseudo-first-order rate constant, cm3/gmol‚s kR1 ) first-order rate constant, (cm3/gmol)(cm3/g)(1/s) kSL-A, kSL-B ) solid-liquid mass-transfer coefficients, respectively, cm/s RA ) rate of reaction for A, i.e., β-naphthol, gmol/cm3‚s w ) catalyst loading, g/cm3 XA ) fractional conversion of β-naphthol Greek Letters FP ) density of catalyst particle, g/cm3 η ) effectiveness factor  ) porosity φ ) Thiele modulus τ ) tortuosity

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Received for review November 24, 1997 Revised manuscript received April 16, 1998 Accepted April 29, 1998 IE970814X