Selectivity Engineering of 2,6-Diisopropylphenol in Isopropylation of

Cs2.5H0.5PW12O40/K-10 catalyst, which is a synergistic combination of cesium-modified hetero- .... activity.27 Supporting DTP on K-10 clay14,15 rectif...
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Selectivity Engineering of 2,6-Diisopropylphenol in Isopropylation of Phenol over Cs2.5H0.5PW12O40/K-10 Clay Ganapati D. Yadav* and Sanket S. Salgaonkar Department of Chemical Engineering, University Institute of Chemical Technology (UICT), University of Mumbai, Matunga, Mumbai 400 019, India

Propofol (2,6-diisopropylphenol), a very important drug, is typically synthesized by the isopropylation of phenol over an acid catalyst. This process consists of two series reactions, namely, the formation of 2-isopropylphenol and its further isopropylation to 2,6- and 2,4diisopropylphenols. The selectivity to 2,6-diisopropylphenol depends on a number of parameters, and the alkylating agent, type of catalyst, and temperature play a major role. With 2-propanol (IPA) as the alkylating agent, its dehydration to propylene and the subsequent formation of isopropyl ether (DIPE) need to be considered. The fact that isopropyl ether, formed in situ, is itself a good alkylating agent further complicates the reaction network. Modeling this intricate system requires extensive experimental data free from mass-transfer limitations. Thus, the alkylation of both phenol and monoisopropylphenol was studied in the liquid phase in an autoclave at 200 °C using IPA and DIPE independently as alkylating agents, over 20% (w/w) Cs2.5H0.5PW12O40/K-10 catalyst, which is a synergistic combination of cesium-modified heteropolyacid and clay. This catalyst is better than zeolites reported in the literature. The effect of various operating parameters and catalyst reusability was also investigated. Mathematical models were proposed to probe into the intricate reaction kinetics and mechanism consistent with the experimental results. 1. Introduction Alkylphenols are commercially very important chemicals that are produced by Friedel-Crafts alkylation of phenol, cresols, or xylenols with olefins, alcohols, or halogenated hydrocarbons. The Friedel-Crafts catalysts mainly used in industry are sulfuric acid, phosphoric acid, p-toluenesulfonic acid, BF3, AlCl3, FeCl3, ZnCl2, etc., and these are highly polluting. In today’s context of strict environmental regulations, the use of solid acids with enhanced selectivity of the desired product in alkylation of phenols will be advantageous. Several products are likely to be produced depending on the nature of the substituted phenol, alkylating agent, solvent, catalyst, temperature, and pressure. Typically, the catalyst combines with the alkylating agent to form a stable (branched) alkyl cation, which reacts with the nucleus preferentially in the positions ortho and para to the OH group. Further, particularly under mild conditions, etherification of the OH group also occurs. The initial isomer distribution is kinetically controlled and decided by (i) the position and inductive effect of the alkyl group(s) already in the ring, (ii) the reactivity of the alkyl cation, (iii) the steric situation on the ring, and (iv) the alkylating agent. Because monoalkylation products are generally more reactive than the starting phenol, they are further alkylated, provided steric hindrance is absent. The ratio of monoalkylphenols to di- and trialkylphenols is proportional to the ratio of phenol to the alkylating agent. Rearrangements (isomerizations, transalkylations, and disproportionations) also occur during further reaction times, and the product composition shifts in the direction of a thermodynamically more stable equilibrium state, in which phenols * To whom correspondence should be addressed. Tel.: 9122-2410 2121 or 2414 5616. Fax: 91-22-2410 2121 or 2414 5614. E-mail: [email protected] or [email protected].

with m-alkyl groups are finally formed. During this process, phenol ethers first rearrange to alkylphenols, the amount of di- and trialkylphenols is lowered, and the proportion of o-alkylphenols is shifted in favor of the p-alkylphenols, provided that the para position is free and is not sterically hindered. Some of the side reactions are (i) dealkylations, (ii) di- and oligomerizations of the olefin used as the alkylating agent or formed by dealkylation, (iii) isomerization and cleavage in the alkyl groups (particularly in the case of larger and branched alkyl groups or olefins), (iv) readdition of the olefins formed by cleavage or di- or oligomerization, (v) dehydrogenation of the alkyl to alkenyl groups and their reaction with olefins to form indanols, and (vi) reactions of the catalyst with reaction components.1 Isopropylphenols are formed by the reaction of phenol with propylene or 2-propanol (IPA) in the presence of Lewis or Brønsted acids such as alumina.2 Industrially, 2-isopropylphenol is produced from a liquid-phase reaction of phenol and propylene using γ-Al2O3 at 250-300 °C and 10 MPa. If a phenol to propylene molar ratio of 1.3:1 is used, 2-isopropylphenol is obtained with a selectivity of about 85% at complete propylene conversion.3 2,6-Diisopropylphenol is formed as a byproduct (10-12%) in this process along with smaller amounts of 4-isopropylphenol (1-2%) and isopropyl phenyl ether. The proportion of ether formed increases considerably at temperatures below 250 °C. Monoisopropylphenol mixtures enriched in the 4 isomer can be synthesized from phenol and propylene or IPA over zeolite catalysts.4,5 When a 1:1 molar mixture of phenol and IPA is passed over a ZSM-5-catalyst at 250 °C and a liquid hourly space velocity of 1 h-1, a phenol conversion of 20% and a selectivity for 4-isopropylphenol of 63% are achieved.5 Exclusive C alkylation of phenol with propylene over H-β and H-USY6 resulted in 80% and 71% conversion

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with 56% and 37% 2-isopropylphenol selectivity and 30% and 31% diisopropylphenol selectivity at 423 K. H-ZSM-5 gives 40% conversion with 40% monoselectivity and only 6% diselectivity at 573 K. Cs+ ion exchange7 of H-ZSM-5 resulted in a 50% decrease in the original selectivity, while a Na ion exchange8 increased the conversion only by 4%. Mg-Al hydrotalcites are reported9,10 to convert 46% phenol with 58% orthoselectivity at a temperature of 673 K. The use of ZnO-Fe2O3 as the catalyst is also reported with an orthoselectivity of 11% and a conversion of 12%, also at a temperature of 623 K.11 Both mono- and diisopropylphenols are very valuable intermediates. The isopropylphenols find extensive applications in flavoring agents, adhesives, agricultural chemicals, pharmaceuticals, etc., while 2,6-diisopropylphenol, the active ingredient in Diprivan, is the world’s most widely used intravenous anaesthetic, with annual worldwide sales in excess of $350MM.12 Moreover, the isopropyl group on the ring can be subjected to make phenolcarboxylic acids and also esters, opening up avenues for a great many synthetic routes. Thus, it is clear that the catalysts explored so far been have been restricted only to monoselectivity, and there exists tremendous scope to develop catalysts for isopropylation reaction of phenol with excellent conversion and selectivity toward diisopropylphenol at comparatively low temperatures with ease of preparation and reusability. Heteropolyacids (HPAs) with the Keggin structure, especially dodecatungstophosphoric acid (DTP), which possesses the highest Brønsted acidity, are good catalysts.13 However, low surface area, rapid deactivation, and relatively poor stability are some of the major problems associated with HPAs in bulk form. The novelties of synergism between HPAs and acid-treated clays were brought out by us for the first time,14,15 including other systems of industrial relevance.16-22 The clay-supported catalysts have lower surface areas in the range of 130-170 m2 g-1, depending on the amount of loading, in comparison with 230 m2 g-1 of K-10 clay. The surface area can be increased by using supports such as mesoporous silica,23,24 mesoporous aluminosilicates,25 alumina and carbon,26 and zirconia,27 and partial substitution of protons of HPAs with Cs+ converts them to materials with higher surface area and improved thermal stability.13 However, alumina and zirconia tend to decompose HPAs because of their basic nature, resulting in the deformation of the parent Keggin structure and causing a reduction in the overall activity.27 Supporting DTP on K-10 clay14,15 rectifies this problem, and we have also reported a novel method of supporting Cs-modified DTP on K-10 clay, which keeps the Keggin anion intact and results in a very active and selective catalyst useful in a variety of industrially important reactions29,30 and regioselective separations.31 So, it was thought desirable to investigate the efficacy of Cs2.5H0.5PW12O40/K-10 under milder operating conditions, in the liquid-phase isopropylation of phenol with IPA and diisopropyl ether (DIPE) as alkylating agents. Extensive experiments were conducted to establish the effect of alkylating agents and operating parameters on conversion and selectivity. Various reaction mechanisms were proposed, and mathematical models so developed were used to validate experimental data and to provide insight into rate-controlling steps.

2. Experimental Section 2.1. Chemicals. Phenol, DIPE, and IPA were obtained from E. Merck Ltd., Mumbai, India, and K-10 clay was obtained from Fluka, Seelze, Germany. Cesium chloride and DTP (all AR grade) were procured from s.d. Fine Chem. Pvt. Ltd., Mumbai, India, and 2-isopropylphenol was supplied by Sigma Aldrich, Dorset, U.K. 2.2. Experimental Setup. Isopropylation of phenol with Cs2.5H0.5PW12O40/K-10 catalyst was conducted in a 300 mL Parr autoclave fully equipped with a temperature and speed indicator and controller. 2.3. Reaction Procedure. A total of 150 mL of the reaction mixture containing the desired proportion of phenol, alkylating agent, and catalyst was fed into the reactor and stirred at a desired speed after the reaction mixture reached the desired temperature. The temperature was maintained within (0.5 °C of the set value. The reaction time was 4 h. 2.4. Method of Analysis. Clear liquid samples were withdrawn at regular intervals by reducing the speed of agitation momentarily to zero and allowing the catalyst to settle at the bottom of the reactor. The samples were analyzed on a Chemito model 8510 gas chromatograph equipped with a 10% SE-30 column (3.175 mm diameter × 4 m length). A standard calibration method with synthetic mixtures was used for quantification of data. The reaction products were confirmed by gas chromatography-mass spectrometry (GC-MS). 3. Results and Discussion 3.1. Catalyst Characterization. The catalyst was fully characterized, and the details are reported recently by us.28-31 Only a few salient features are reported here. Crystallinity and textural patterns of the catalysts predicted from X-ray diffraction data of Cs2.5H0.5PW12O40/ K-1028 show that DTP is crystalline while K-10 is amorphous. The diffractogram obtained suggested that, although the Cs2.5H0.5PW12O40 salt loses some of its crystallinity in the process of supporting it on K-10, the Keggin structure of DTP remains intact. The Fourier transform infrared analysis29 fortified the preservation of the keggin structure of DTP in the catalyst. A characteristic split in the WdO band of Cs2.5H0.5PW12O40 suggested the existence of direct interaction between the Keggin polyanion and Cs+. The scanning electron micrographs28 reveal that both K-10 and 20% (w/w) DTP/K-10 samples possess rough and rugged surfaces, whereas 20% (w/w) Cs2.5H0.5PW12O40/K-10 shows a smoother surface because of a layer of “Cs salt of DTP” over the external surface of K-10. The Brunauer-Emmett-Teller surface area of 20% (w/w) Cs2.5H0.5PW12O40/K-1031 was measured to be 207 m2 g-1, and the pore volume and pore diameter were 0.29 cm3 g-1 and 58 Å, respectively. The adsorptiondesorption isotherm for Cs2.5H0.5PW12O40/K-10 showed that they have the form of a type IV isotherm with the hysteresis loop of type H3, which is a characteristic of a mesoporous solid. 3.2. Catalytic Activity. With IPA as the alkylating agent, a conversion of 50% of phenol was achieved at 473 K with a 1:4 mole ratio of phenol/IPA and a catalyst loading of 0.02 g cm-3 at 1000 rpm. Selectivities of 60%, 30%, and 10% toward isopropyl phenyl ether, isopropyl-

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Figure 1. Effect of the alkylating agent on the conversion of phenol: [, IPA; 9, DIPE. Reaction conditions: mole ratios, phenol: IPA ) 1:4 and phenol:DIPE ) 1:2; speed of agitation, 1000 rpm; temperature, 473 K; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

Figure 2. Effect of the alkylating agent on the selectivity of phenol isopropylation: 9, isopropyl phenyl ether; 2, monoisopropylphenol; [, diisopropylphenol; - - -, IPA; s, DIPE. Reaction conditions: mole ratios, phenol:IPA ) 1:4 and phenol:DIPE ) 1:2; speed of agitation, 1000 rpm; temperature, 473 K; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

phenol, and diisopropylphenol, respectively, were realized. The proportion of ortho/meta/para monoisopropylphenol isomers was 10:1:2.5. The comparatively higher amount of para substitution compared to that at the meta position is attributed to the mesoporosity of the catalyst. Diisopropylphenol consisted mainly of 2,6 and 2,4 isomers. Nearly 40% of the IPA was converted to DIPE. Reaction products were also confirmed by GCMS. An independent IPA dehydration study conducted at 473 K over the same catalyst showed the formation of DIPE with 40% selectivity at 90% conversion. Because this DIPE generated in situ is itself a good alkylating agent, capable alkylating phenol, it was thought worthwhile to examine the product distribution by conducting separate experiments with it. A considerable enhancement in the conversion of phenol was observed with DIPE as an alkylating agent. With a molar ratio of phenol:DIPE ) 1:2, under otherwise similar operating conditions a conversion of 90% was obtained. The C alkylation was also improved with a selectivity of isopropylphenol and diisopropylphenol of 40% each being achieved. Comparative plots showing the effect of IPA and DIPE as the alkylating agents on conversion and selectivity are given in Figures 1 and 2, respectively. The conversion and diselectivity obtained

Figure 3. Effect of the speed of agitation: [, 800 rpm; 9, 1000 rpm; 2, 1200 rpm. Reaction conditions: mole ratio, phenol:IPA ) 1:4; temperature, 473 K; catalyst loading, 0.02 g cm-3; total reaction volume,150 mL.

is much more than the maximum conversion reported in the literature with H-β and H-USY catalysts. Now, because DIPE is a better alkylating agent than IPA and is formed in situ when IPA is used, the influence of all operating conditions was studied only for the alkylation of phenol with IPA. 3.3. Mass-Transfer Considerations. This reaction is a typical solid-liquid slurry reaction involving the transfer of phenol, the limiting reactant IPA (A), and phenol (B) from the bulk liquid phase to the catalyst wherein external mass transfer of reactants to the surface of the catalyst particle, followed by intraparticle diffusion, adsorption, surface reaction, and desorption, takes place. The influence of external solid-liquid masstransfer resistance must be ascertained before a true kinetic model could be developed. Depending on the relative magnitudes of the external resistance to mass transfer and reaction rates, different controlling mechanisms have been put forward.23 The liquid-phase diffusivity values of the reactants A (IPA) and B (phenol), denoted by DAB and DBA, were calculated by using the Wilke-Chang equation24 as 4.979 × 10-5 and 4.24 × 10-5 cm2 s-1, respectively. The solid-liquid masstransfer coefficients for both A and B were calculated from the limiting value of the Sherwood number (e.g., ShA ) kSL-Adp/DAB) of 2. The actual Sherwood numbers are typically higher by 1 order of magnitude in wellagitated systems, but for conservative estimations, a value of 2 is taken.25 The solid-liquid mass-transfer coefficient kSL-A and kSL-B values were obtained as 9.96 × 10-2 and 8.48 × 10-2 cm s-1, respectively. The initial rate of the reaction was calculated from the conversion profiles. 3.4. Effect of the Speed of Agitation. To assess the role of external mass transfer on the reaction rate, the effect of the speed of agitation (Figure 3) was studied. The speed of agitation was varied from 800 to 1200 rpm. It was observed that the conversion of phenol was practically the same in all cases. Thus, it was ensured that external mass-transfer effects did not influence the reaction. Hence, all further reactions were carried out at 1000 rpm. 3.5. Effect of the Catalyst Loading. In the absence of external mass-transfer resistance, the rate of reaction is directly proportional to the catalyst loading based on the entire liquid-phase volume. The catalyst loading was varied over a range of 0.01333-0.026 g cm-3 on the basis of the total volume of the reaction mixture. Figure 4 shows the effect of the catalyst loading on the conversion of phenol. The conversion increased with an increase in the catalyst loading, which is due to the

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Figure 4. Effect of the catalyst loading: [, 0.01333 g cm-3; 9, 0.02 g cm-3; 2, 0.026 g cm-3. Reaction conditions: mole ratio, phenol:IPA ) 1:4; speed of agitation, 1000 rpm; temperature, 473 K; total reaction volume, 150 mL.

proportional increase in the number of active sites. However, beyond a catalyst loading of 0.02 g cm-3, there was no significant increase in the conversion, and hence all further experiments were carried out at this catalyst loading. At steady state, the rate of external mass transfer (i.e., from the bulk liquid phase in which A and B are located with concentrations [A0] and [B0], respectively) to the exterior surface of the catalyst is proportional to aP, the exterior surface area of the catalyst where the concentrations of A and B are [AS] and [BS], respectively. For a spherical particle, aP is also proportional to w, the catalyst loading per unit of liquid volume. It is possible to calculate the values of [AS] and [BS]. For instance, kSL-AaP{[A0] - [AS]} ) robs at steady state ) 8.455 × 10-8 mol cm-3 s -1. From the calculations, it was observed that [AS] ≈ [A0]; similarly, [BS] ≈ [B0]. Thus, any further addition of catalyst is not going to be of any consequence for external mass transfer. 3.6. Proof of Absence of Intraparticle Resistance. Because the average particle size of 20% (w/w) Cs2.5H0.5PW12O40/K-10 was found to be in the range of 2-10 µm and the catalyst is amorphous in nature, it was not possible to study the effect of the catalyst particle size on the rate of reaction. The average particle diameter of Cs2.5H0.5PW12O40/K-10 used in the reactions was 0.001 cm, and thus a theoretical calculation was done based on the Wietz-Prater criterion to assess the influence of intraparticle diffusion resistance.18 According to the Wietz-Prater criterion, the dimensionless parameter CWP, which represents the ratio of the intrinsic reaction rate to the intraparticle diffusion rate, can be evaluated from the observed rate of reaction, the particle radius (RP), the effective diffusivity of the limiting reactant (De), and the concentration of the reactant at the external surface of the particle. If CWP ) -robsFRP2/De[AS] . 1, then the reaction is limited by severe internal diffusional resistance. If CWP , 1, then the reaction is intrinsically kinetically controlled. The effective diffusivity of IPA (DeA) inside the pores of the catalyst was obtained from the bulk diffusivity (DAB), porosity (), and tortuosity (τ) as 1.493 × 10-5 cm2 s-1 where DeA ) DAB/τ. In the present case, the value of CWP was calculated as 4.012 67 × 10-7 for the initial observed rate, which is much less than 1, and this signifies the absence of resistance due to intraparticle diffusion; therefore, the reaction could be considered as an intrinsically kinetically controlled reaction. Further proof of the absence of intraparticle

Figure 5. Effect of the mole ratio on phenol conversion: 9, 1:1; b, 1:2; 2, 1:4; [, 1:6. Reaction conditions: temperature, 473 K; catalyst loading, 0.02 g cm-3; speed of agitation, 1000 rpm; total reaction volume, 150 mL.

Figure 6. Effect of the temperature on the cracking of IPA. Reaction conditions: speed of agitation, 1000 rpm; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

diffusion resistance was obtained through the study of the effect of temperature, and it will be evaluated later. 3.7. Effect of the Mole Ratio. The mole ratio of phenol to IPA was varied from 1:1 to 1:6 (Figure 5) under otherwise similar operating conditions. As the concentration of IPA is increased with respect to the concentration of phenol, an increase in the conversion of phenol and the rate of reaction is observed. This is because at the lower concentration of phenol more active sites of catalyst are available for the surface adsorption of IPA, which results in quantitatively large formation of propylene and DIPE to react with phenol. However, there was no significant increase in the conversion of phenol by increasing the mole ratio from 1:4 to 1:6. Thus, all of the subsequent reactions were carried out with a mole ratio of 1:4. 3.8. Effect of the Temperature. Intrinsically kinetically controlled reactions show a significant increase in the conversion profile with temperature. Because almost all mass-transfer limitations were eliminated, the effect of the temperature was studied on all three reaction steps. 3.8.1. Dehydration of IPA. The IPA dehydration reaction (Figure 6) was essentially completed within 1 h, with the conversion values rising by 5% with every 10 °C rise in the temperature. Because propylene is difficult to sample and quantify, the concentrations of IPA and DIPE were first quantified by GC and then a mass balance was established to calculate the concentration of propylene. Figure 7 shows the selectivity profile to favor propylene formation initially but it soon drops steeply and plateaus off. The formation of DIPE increased significantly with temperature.

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Figure 7. Effect of the temperature on the selectivity in the cracking of IPA: [, 443 K; 9, 453 K; 2, 463 K; b, 473 K; - - -, diisopropyl ether; s, propylene. Reaction conditions: speed of agitation, 1000 rpm; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

Figure 8. Effect of the temperature on the isopropylation of phenol with IPA: [, 463 K; 9, 473 K; 2, 453 K; b, 443 K. Reaction conditions: mole ratio, phenol:IPA ) 1:4; temperature, 473 K; catalyst loading, 0.02 g cm-3; speed of agitation, 1000 rpm; total reaction volume, 150 mL.

Figure 9. Effect of the temperature on the selectivity in the isopropylation of phenol with IPA: ], 443 K; 0, 453 K; 4, 463 K; O, 473 K; - - -, isopropyl phenyl ether; s, monoisopropylphenol; -‚-, diisopropylphenol. Reaction conditions: mole ratio, phenol: IPA ) 1:4; temperature, 473 K; catalyst loading, 0.02 g cm-3; speed of agitation, 1000 rpm; total reaction volume, 150 mL.

3.8.2. Isopropylation of Phenol with IPA. Figures 8 and 9 depict the effect of the temperature on the conversion and selectivity of the isopropylation reaction with IPA as the alkylating agent. It was observed that there was a steady rise in the conversion profile until 463 K. A further 10 °C rise to 473 K nearly doubled the rate of reaction. The selectivity toward 2-isopropylphenol increased marginally from 60% at 443 K to 64%

Figure 10. Effect of the temperature on the isopropylation of 2-isopropylphenol with IPA: +, 443 K; ×, 453 K; 4, 458 K; ], 463 K; O, 473 K; 0, 468 K. Reaction conditions: mole ratio, oisopropylphenol:IPA ) 1:4; speed of agitation, 1000 rpm; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

Figure 11. Effect of the temperature on the conversion of phenol with DIPE: 9, 473 K; 2, 463 K; [, 453 K; b, 433 K. Reaction conditions: mole ratio, phenol:DIPE ) 1:2; speed of agitation, 1000 rpm; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

at 463 K and dropped to 58% at 473 K. There was a 7% increase in the selectivity of monoisopropylphenol from 34% at 443 K to 41% at 473 K, while the diisopropylphenol selectivity dropped from 6% to 1% over the same temperature range. The decrease in isopropyl phenyl ether selectivity is due the commonly observed acid-catalyzed rearrangement of alkyl phenyl ethers to alkyl phenols.37 The drop in the diisopropylphenol selectivity suggests that the rate of formation of isopropylphenol from direct alkylation of phenol and from the rearrangement of isopropyl phenyl ether is much higher than its own rate of alkylation. Independent experiments of isopropylation of 2-isopropylphenol (2-IPP) with IPA were undertaken to obtain further insight on the selectivity patterns. 2-Isopropylphenol was taken as a model mono isomer to study the temperature dependency of this reaction. As shown in Figure 10, the conversion of IPA increased from 2% at 443 K to 15% at 473 K. This conversion level shows that isopropylphenol is not very reactive with IPA and that the rate of its isopropylation is indeed less than its rate of formation. 3.8.3. Diisopropylation of Phenol with DIPE. The effect of the temperature on the conversion and selectivity of isopropylation of phenol with DIPE is plotted in Figures 11 and 12. The conversion obtained was as high as 90%, which was nearly double the value obtained with IPA. Moreover, there was a drastic reduction in the selectivity of isopropyl phenyl ether from 57% at 443 K to 16% at 473 K. With regards to the C alkylation, the mono- and diisopropylphenol selectivities were 44%

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Figure 12. Effect of the temperature on the selectivity of phenol isopropylation with DIPE: ], 443 K; 4, 463 K; O, 473 K; - - -, isopropyl phenyl ether; s, monoisopropylphenol; -‚-, diisopropylphenol. Reaction conditions: mole ratio, phenol:DIPE ) 1:2; speed of agitation, 1000 rpm; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

Figure 13. Isopropylation of 2-isopropylphenol with DIPE: 9, 473 K; [, 463 K; b, 453 K; 2, 433 K. Reaction conditions: mole ratio, 2-isopropylphenol:DIPE ) 1:2; speed of agitation, 1000 rpm; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

and 40%, respectively. To investigate the cause of a drastic increase in the dialkylation selectivity, independent studies of isopropylation of 2-isopropylphenol were carried out. Figure 13 shows the effect of the temperature on the conversion of isopropylation of 2-isopropylphenol with DIPE. The conversion increased from 30% at 443 K to 70% at 473 K. The temperature dependency of both of these reaction proves that DIPE is a much better alkylating agent than IPA and can be used to achieve a high dialkyl selectivity. 3.9. Reaction Mechanism and Kinetics. We have recently reported alkylation of several substituted aromatics using linear and branched alcohols and ethers using a variety of solid acid catalysts, wherein a free olefin and water are generated along with the C- and/ or O-alkylated products. In the condensation reaction of two alcohols, it was found that symmetrical or asymmetrical ethers are formed (e.g., methyl tert-butyl ether (MTBE) from methanol and tert-butyl alcohol) in which sulfated zirconia, ion-exchange resin, and 20% (w/w) Cs2.5H0.5PW12O40/K-10 clay were employed as catalysts. With IPA as an alkylating agent, the formation of DIPE could be witnessed depending on the reaction conditions and types of catalysts.

As seen in Figure 1, the rate of isopropylation of phenol with DIPE is much higher than that with IPA, which is in consonance with the results obtained when alkylation of phenol was studied by using both MTBE and tert-butyl alcohol over 20% (w/w) DTP/K-10 clay.18,43 Moreover, when mesitylene was reacted with IPA over UDCaT-4, the reaction was found to follow second order. Thus, it is seen that the alkylation with alcohols does not follow a simple reaction pathway and our current rate data needed testing of different hypotheses. Several models were tried to fit the experimental data, which were collected in the absence of any external masstransfer and intraparticle diffusion limitations. The dehydration of IPA in the vapor phase was found to obey an overall second-order kinetics with a second order in IPA for two sets of temperature ranges studied, contrary to the expected first-order kinetics for weakly adsorbed species. Further, it was found that the alkylation of mesitylene also followed a second-order reaction. Three cases are considered here: (i) dehydration of IPA to propylene and water, (ii) alkylation of phenol, and (iii) dialkylation of phenol. 3.9.1. IPA Dehydration.

The analysis of earlier published reports on the dehydration of ethanol and IPA39,40 suggests that there is a production of ether and also two types of sites should be involved in the reaction. Thus, a model based on two catalytic sites was proposed, according to which IPA (A) gets adsorbed onto two different sites, S1 and S2. These two adsorbed species participate in the reaction. Assuming that the rate-determining step is the reaction of AS1 and AS2 to form DIPE and water as the surface complexes (ES1 and WS2, respectively) and ES1 subsequently decomposes instantly into propylene (P), we get K1-A

A + S1 798 AS1 K2-A

A + S2 798 AS2 KSR1

AS1 + AS2 98 ES1 + WS2 KSR2

ES1 98 2P + WS1

(1) (2) (3) (4)

The site balance for this case is

Ct-S1 ) CV-S1 + CA-S1 + CES1 + CWS1

(5a)

Ct-S2 ) CV-S2 + CA-S2 + CWS2

(5b)

The following adsorption equilibria for different species hold: K1-W

W + S1 798 WS1 K2-W

W + S2 798 WS2 K1-E

E + S1 798 ES1

(6a) (6b) (6c)

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Figure 15. Validation of the mathematical model for isopropylation of phenol with IPA for various catalyst loadings: ], 0.01333 g cm-3; 0, 0.02 g cm-3; 4, 0.026 g cm-3.

Figure 14. Validation of the mathematical model for IPA dehydration: ], 473 K; O, 463 K; 0, 453 K; 4, 443 K.

Thus, the rate of formation of propylene is -rP′ ) KSR1K1-ACAK2-ACACTS1CTS2 (1 + K1-ACA + K1-WCW)(1 + K2-ACA + K2-WCW + K2-ECE)

(7) When the adsorption constants of all species are very weak, eq 7 is reduced to

rP′ ) kPCA2

(8)

where

Figure 16. Validation of the mathematical model for isopropylation of phenol with IPA for various mole ratios of phenol:IPA: [, 1:1; 9, 1:2; b, 1:4; 2, 1:6.

Analogously, the site balance can be written to obtain -rB′ )

kP ) KSR1K1-AK2-ACT-S1CT-S2

(9)

Thus, a plot of rP against CA2 (Figure 14) was made to get an excellent fit, thereby supporting the model. This is an overall second-order reaction for weak adsorption of IPA. 3.9.2. Isopropylation of Phenol.

kSR2CT-S1CT-S2K1-ACAK2-BCB (1 + K1-ACA + K1-WCW + K1-ECE)(1 + KBCB + KD-SCD)

(11) With weak adsorption of all species, it gives the following:

-rB′ ) kBCACB

(12)

kB ) kSR2CT-S1CT-S2K1-AK2-B

(13)

where

As is validated above, IPA dehydration follows second-order kinetics by adsorption of IPA on two adjacent sites S1 and S2 and the product ether (E) is formed, which is decomposed instantaneously to propylene (P). We have also reported earlier32 the dehydration of IPA over a broad range of temperatures (110-150 and 180-220 °C), which showed that DIPE, although formed at lower temperature, cracks faster than IPA. Thus, in the temperature range studied, the rate of alkylation is not controlled by the dehydration rate but alkylation of phenol adsorbed on site S2 with IPA adsorbed on adjacent site S1 as shown below. kSR2

BS2 + AS1 98 DS2 + WS1

(10)

Writing in terms of conversion and further integration results in

[

ln

M - XB

M(1 - XB)

]

) (M - 1)kBCB0t

(14)

ln[(M - XB)/M(1 - XB)] is plotted against t at various catalyst loadings (Figure 15), mole ratios (Figure 16), and temperatures (Figure 17). It is seen that the experimental data validate the model very well. Similarly, the model of isopropylation of 2-isopropylphenol with IPA was developed (eq 15) and validated with the experimental data (Figure 18).

ln

[

M - XC

M(1 - XC)

]

) (M - 1)kCCC0t

(15)

Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1713

Figure 17. Validation of the mathematical model for isopropylation of phenol with IPA for various temperatures: 9, 443 K; 2, 453 K; [, 463 K; O, 473 K.

Figure 18. Validation of the mathematical model for isopropylation of 2-isopropylphenol with IPA: +, 443 K; ×, 453 K; 4, 458 K; 0, 463 K; *, 468 K; O, 473 K.

3.9.3. Diisopropylation of Phenol with DIPE.

Because ethers are direct alkylating agents, a thirdorder model, second order in phenol and first order in DIPE, was tested, analogous to our previous investigations38 on the reaction of benzyl ether with toluene. According to the proposed mechanism, isopropyl ether [E] gets adsorbed onto an acidic site [S] and forms a complex [ES], which reacts with two adsorbed molecules of phenol to give 2 mol of alkylated product and 1 mol of water [W]. KE

E + S 798 ES KB

B + S 798 BS kSR3

2BS + ES 98 2CS + WS

(16) (17)

KC

KW

WS 798 W + S

-rB )

(1 + KBCB + KECE + KCCC + KWCW)3

(23)

kB ) kSR4KBKECT3

(24)

where

Writing in terms of conversion, we get

(25)

Upon separation of variables and further integration, we get

[

]

XB M(1 - XB) 1 ln + ) kBCB02t 2 M X (1 - XB)(M - 1) (M - 1) B (26)

(19)

Thus, a plot of 1/(M - 1)2 ln[M(1 - XB)/(M - XB)] + XB/(1 - XB)(M - 1) against t (Figure 19) gave a good fit, validating the model developed. A similar model was developed for the isopropylation of 2-isopropylphenol with DIPE. As shown in Figure 20, the experimental data give a very good fit; the model final equation is given below.

(20) (21)

For the alkylation of phenol with DIPE

kSR4KBKECT3CECB2

-rB′ ) kBCECB2

(18)

The following adsorption equilibria are defined:

CS 798 C + S

For weak adsorption of all species

dXB/dt ) kBCB02(1 - XB)2(M - XB)

A total site balance gives

Ct ) CV + CBS + CES + CCS + CWS

Figure 19. Validation of the mathematical model for isopropylation of phenol with DIPE: b, 433 K; [, 453 K; 2, 463 K; 9, 473 K.

(22)

[

]

XC M(1 - XC) 1 ln + ) kCCC02t 2 M X (1 X )(M 1) (M - 1) C C (27) Finally, Arrhenius plots (Figure 21) were made for all of the reactions, and the Arrhenius parameters are given in Table 1.

1714

Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005

Figure 20. Validation of the mathematical model for isopropylation of 2-isopropylphenol with DIPE: 9, 443 K; 2, 453 K; [, 463 K; O, 473 K.

Figure 22. Catalyst reusability: [, fresh catalyst; 9, first reuse; 2, second reuse. Reaction conditions: mole ratio, phenol:IPA ) 1:4; speed of agitation, 1000 rpm; temperature, 473 K; catalyst loading, 0.02 g cm-3; total reaction volume, 150 mL.

4. Conclusion

Figure 21. Arrhenius plot for isopropylation of phenol and 2-isopropylphenol: ], IPA dehydration; *, Ph + IPA; 4, 2-IPP + IPA; 0, Ph + DIPE; ×, 2-IPP + DIPE. Table 1. Arrhenius Parameters for Isopropylation Reactions

reaction

K

E (kcal mol-1)

IPA dehydration isopropylation of phenol with IPA isopropylation of 2-IPP with IPA isopropylation of phenol with DIPE isopropylation of 2-IPP with DIPE

3.37 × 1017 mol-1 cm3 min-1 2.08 × 1014 mol-1 cm3 min-1

36.37 32.48

1.57 × 1013 mol-1 cm3 min-1

30.83

9.51 × 1018 mol-2 cm6 min-1

22.23

1.61 × 1022 mol-2 cm6 min-1

29.80

3.10. Reusability and Stability of 20% (w/w) Cs2.5H0.5PW12O40/K-10 Catalyst. The stability of the active species in solution has been of concern for solid acids, especially for supported materials. The potential use of 20% (w/w) Cs2.5H0.5PW12O40/K-10 catalyst has been well explored in a variety of reactions including alkylation31 and acylation21 with no significant loss in catalyst activity after repeated use even in the presence of corrosive and strong reagents such as benzoyl chloride.30 The change in the texture of the catalyst from white to gray suggested deactivation due to coking. Reactivation of the catalyst was done by maintaining the catalyst in a solution of IPA at reflux temperature for 4 h. The catalyst regained nearly 75% of its initial activity. The conversion dropped approximately 10% with every use. Figure 22 depicts the observed conversion profiles.

Liquid-phase isopropylation of phenol using 20% (w/ w) Cs2.5H0.5PW12O40/K-10 is accomplished with better conversions and 2,6-diisopropyl selectivities than those reported so far with any other catalyst. At 473 K, a 1:2 molar ratio of phenol/DIPE with a catalyst loading of 0.02 g cm-3 resulted in a phenol conversion of 90%. The selectivities toward mono- and dialkylated products was 44% and 40%, respectively. The conversion and diselectivity with DIPE are nearly double those achieved with IPA. This shows that DIPE formed in situ enhances the isopropylation activity significantly. Moreover, the deactivated catalyst is easily rejuvenated and can be used thrice. The reactions were found to be intrinsically kinetically controlled with second-order kinetics for both DIPE dehydration and alkylation reactions with IPA. Alkylation with DIPE, however, follows a third-order kinetics. Comprehensive mathematical models were developed for all of the reaction steps and validated with experimental results. Acknowledgment G.D.Y. gratefully acknowledges the funding from Darbari Seth Professorship Endowment. Nomenclature A ) IPA aP ) surface area of the catalyst (cm) B ) phenol C ) isopropylphenol Ci ) concentration of species i (mol cm-3) CV-Si ) concentration of vacant sites of type i D ) diisopropylphenol De ) effective diffusivity (cm2 s-1) Dij ) diffusivity of i in j (cm2 s-1) dP ) diameter of the catalyst particle (cm) E ) diisopropyl ether K ) adsorption/mass transfer/reaction rate constant M ) mole ratio O ) isopropyl phenyl ether ri ) rate of reaction of species i (mol cm-3 s-1) RP ) radius of the catalyst particle (cm) S ) vacant catalyst sites Shi ) Sherwood number of species i t ) time (min) W ) water Xi ) conversion of species i Greek Symbols  ) porosity of the catalyst

Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1715 τ ) tortuosity of the catalyst F ) density (gm cm-3) Subscripts SR ) surface reaction T ) total catalyst sites i-j ) adsorption of species j on sites of type i SL-i ) solid-liquid mass-transfer coefficient of species i 0 ) initial concentration

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Received for review September 7, 2004 Revised manuscript received December 8, 2004 Accepted December 21, 2004 IE049141Q