Role of Third Phase in Intensification of Reaction ... - ACS Publications

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Ind. Eng. Chem. Res. 2007, 46, 8448-8458

Role of Third Phase in Intensification of Reaction Rates and Selectivity: Phase-Transfer Catalyzed Synthesis of Benzyl Phenyl Ether Ganapati D. Yadav* and Omprakash V. Badure Department of Chemical Engineering, UniVersity Institute of Chemical Technology, UniVersity of Mumbai, Matunga, Mumbai-400 019, India

In the current work, the merits of the creation of a third phase in a typical biphasic reaction have been illustrated. The advantages of liquid-liquid-liquid phase-transfer catalysis (L-L-L PTC) have been brought out over liquid-liquid phase-transfer catalysis (L-L PTC) by considering the etherification of phenol by benzyl chloride to benzyl phenyl ether. L-L-L PTC is a novel strategy for waste reduction and improving profitability, in which a catalyst-rich middle phase is formed between the other two phases, wherein the main reaction takes place and intensifies the rates of reaction as well as offers better selectivity including catalyst reusability, unlike in the L-L PTC. The etherification of phenol with benzyl chloride under L-L PTC is accompanied by side reactions that lower the selectivity, and the catalyst cannot be recovered but wasted as an effluent, causing a load on the environment. However, the transformation of L-L PTC into L-L-L PTC leads to 100% conversion of the limiting reactant benzyl chloride with 100% selectivity to benzyl phenyl ether. The catalyst-rich phase is recovered and reused to some extent. This also helps in waste minimization, which is a major theme of green chemistry. The current work deals with the effects of different kinetic and processes parameters on enhancement in rates and selectivities in L-L-L PTC over L-L PTC. A mathematical model is also developed. 1. Introduction Multiphase reactions involving catalysis are omnipresent in a variety of industries. These reactions involve multiphase mass transfer, and thus, different reactor configurations influencing the selectivity of the desired product are possible,1 which was not well-appreciated in the fine chemical industries until recently.2,3 It has been a common practice in the fine chemical industry to homogenize reaction mixtures by using exotic solvents, to overcome mass transfer effects, notwithstanding the environmental and safety aspects. During the past few years, several new strategies have been evolved to engineer reactions from different perpectives, be they safety and hazard, green chemistry, combinatorial chemistry, asymmetric synthesis, or materials chemistry.4 The existence of the multiple phases need not be integral to the process, and instead, one may intentionally impose an additional liquid phase to accrue several benefits like higher selectivity and yield, use of cheaper chemicals, adoption of milder conditions, and ease of separation from toxic mixture, as epitomized by phase-transfer catalysis (PTC).5 It is now utilized in hundreds of processes, from research chemistry to full-scale production, where the benefits of faster, cleaner, and more selective reactions are required.6,7 It is, thus, prudent to modify the ubiquitous PTC from green chemistry and wasteminimization angles to suit the industry. A majority of PTC reactions are conducted under liquidliquid (L-L) conditions, and the catalyst is normally not recovered and reused, since the catalyst cost/(kg of product) is very measly. In industrial practice, the catalyst is extracted by several water-washes of the organic phase, and the voluminous aqueous effluent is sent for treatment. The separation of catalyst from the reaction mixture can be achieved by extraction, distillation, and adsorption, and all these are energy-intensive.6 * To whom correspondence should be addressed. Tel.: 91-22-410 2121. Fax: 91-22-410 2121. E-mail: [email protected], gdyadav@ udct.org.

Liquid-liquid-solid (L-L-S) PTC is one approach where the expensive catalyst is bound to the solid matrix like polymeric resin6,7 or clays8 and can be recovered, which is however fraught with intraparticle diffusion limitations.9,10 Still a better approach is the capsule membrane phase-transfer catalysis (CMPTC) where thin capsules are bound with the catalyst and the diffusion barriers are overcome.11-13 The best way to intensify reaction rates and to reuse the catalyst partly will be to create a separate catalyst-rich third liquid phase between the aqueous and organic phases during the reaction of L-L PTC. This phase would become the main reaction phase, and it not only would intensify the rates of reaction but also improve the selectivity of the desired product, apart from the economical and environmental benefit. The liquid-liquid-liquid (L-L-L) PTC is very attractive in a number of reactions of industrial relevance.13-26 The purpose of this paper is to show the prowess of L-L-L PTC in comparison with the L-L PTC with a practical example. The etherification or O-alkylation of phenol with benzyl chloride was chosen as a model system that also has practical utility. This reaction has been studied by McKillop et al.,27 who developed a simple and efficient process for the synthesis of ethers of both simple and hindered phenols, using L-L PTC, which involves alkylation of the phenoxide ion with an alkyl halide or sulfate ester using quaternary ammonium salts in a methylene chloride-water system at room temperature. Although the ether yields are high, they are still byproducts formed in the reaction including C-alkylation and hydrolysis of the alkylating agent. Merker and Scott28 reported the use of triethylamine as a catalyst at 150-190 °C and a 46% yield of benzyl phenyl ether, whereas Hwu et al.29 studied the kinetics of synthesis of some ethers by using trialkyl amines as catalyst under L-L PTC. This paper deals with mechanistic models of L-L-L PTC vis-a`-vis L-L PTC in the etherification process, along with the kinetics of the reactions.

10.1021/ie070180m CCC: $37.00 © 2007 American Chemical Society Published on Web 10/25/2007

Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8449

Figure 1. L-L-L PTC: possibilities of contributions to overall reaction rate by three parallel reaction mechanisms.

Scheme 1 . Effect of Third Phase on Selectivity in O-Alkylation of Phenol with Benzyl Chloride in Comparison with BI-liquid PTC (Complete Selectivity to Benzyl Phenyl Ether Is Realized under L-L-L PTC)

2. Experimental Section 2.1. Chemicals and Catalyst. Benzyl chloride, toluene, phenol, sodium hydroxide, benzyl alcohol, benzyl phenyl ether, and dibenzyl ether of analytical-reagent (AR) grade were obtained from M/S s. d. Fine. Chem. Pvt. Ltd., Mumbai, India. Tetrabutylammonium bromide (TBAB) of pure grade was obtained as a gift sample from M/s Dishman Pharmaceuticals and Chemicals Ltd., Ahmedabad, India. 2.2. Experimental Setup and Procedure. The reaction was studied in a 5 cm i.d. fully baffled mechanically agitated glass

reactor of 150 cm3 total capacity that was equipped with a 6-blade-pitched turbine impeller and a reflux condenser. The reactor was kept in an isothermal oil bath whose temperature was maintained at a desired value, and the reaction mixture was agitated mechanically with the help of an electric motor. Typical runs were conducted with 0.02 mol of benzyl chloride dissolved in toluene to make up the volume of the organic phase to 50 cm3 and 0.02 mol of phenol and 0.025 mol of NaOH dissolved in water to make up the volume of the aqueous phase to 50 cm3 with TBAB as the catalyst at 80 °C. The amounts of the

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Figure 3. Effect of speed of agitation on conversion of benzyl chloride: benzyl chloride ) 0.02 mol; organic phase made up to 50 cm3 with toluene; phenol ) 0.02 mol; TBAB ) 9.007 × 10-3 mol; NaOH ) 0.025 mol; aqueous phase made up to 50 cm3 with water; NaCl ) 0.26 mol; and temperature ) 80 °C.

Figure 2. Photograph of the L-L-L PTC reactor (stationary stirrer) at 80 °C.

2.5. Reaction Scheme. Scheme 1 depicts the reaction products under L-L and L-L-L PTC processes. 3. Results and Discussion

catalyst and NaCl determined the formation of the third liquid phase, and separate experiments were conducted to arrive at the window of operation. A typical three-phase reaction contained benzyl chloride (0.02 mol) with organic phase made up to 50 cm3 with toluene, and the aqueous phase was made of 0.02 mol of phenol, 9.007 × 10-3 mol of TBAB, 0.02 mol of NaOH, and 0.2629 mol of NaCl in 50 cm3 of solution at 80 °C. Upon mixing these two mixtures, the third phase would appear, and its volume was 7 cm3 with a density of 0.8723 g/cm3. 2.3. Method of Analysis. Samples of the organic phase were withdrawn periodically and analyzed by gas chromatography on a Chemito 8510 model. A 4 m × 3.18 mm internal diameter stainless steel column packed with 5% OV-101 on chromosorb WHP was used for analysis in conjunction with a flame ionization detector. The conversion was based on the disappearance of benzyl chloride, and synthetic mixtures were used to quantify the data. In the case of L-L PTC, benzyl alcohol, dibenzyl ether, and (p-benzyl) benzyl phenyl ether were formed as byproducts. Under three liquid phases, only benzyl phenyl ether was produced. The products were confirmed by GC-MS. The conversion is defined with reference to the limiting reactant, which was benzyl chloride in most cases, that is, the amount of benzyl chloride consumed at a given time to that taken at zero time. The selectivity of benzyl phenyl ether, the desired product, is defined as the ratio of the moles formed at a given time to the moles of benzyl chloride reacted. The rate of reaction is defined as the moles reacted per unit time per unit volume of the particular phase. The initial rates were calculated from conversion profiles from the average slope within 5 min of reaction time. 2.4. Determination of Third Phase Composition. The formation and stability of the third phase was verified independently at the reaction temperature by a trial-and-error procedure. In a typical experiment, the composition of the third phase was analyzed on gas chromatography and Karl Fischer apparatus. The third-phase volume was 7.0 cm3 with a density of 0.8723 g/cm3 containing 41.98% toluene, 35.66% TBAB, 6.56% benzyl chloride, 2.28% phenol, 3.61% benzyl phenyl ether, and 9.91% water by weight. The amount of water present in the third phase was analyzed by Karl Fischer apparatus.

Several experiments were done to understand the critical parameters of maintaining three immiscible phases at 80 °C under agitation. The composition was such that the fidelity of three separate phases was maintained at all temperatures between 50 and 90 °C, during intense agitation and without agitation. When the agitation was stopped, the reaction mass had droplets of organic phase covered by a thin dark film in a pool of aqueous phase, which was a continuous phase (Figure 1), which would slowly disintegrate, and the three phases would emerge as shown in the photograph (Figure 2). Even after cooling the mixture to room temperature, the phases would retain their identity. For the three-phase system, the organic reactant benzyl chloride is distributed between an organic phase and a third phase, but it is not in an aqueous phase. Scheme 2 shows the mechanism of the tri-liquid-phase reaction, for which different reactions take place in different phases and the products are transferred across the interfaces. The locale of the rate-controlling reaction (reaction b) is the third phase, and no side reactions occur here since benzyl chloride is not allowed to transfer to the aqueous-third-phase interface. Scheme 3 demonstrates why there are side products in the biphasic system, wherein the rate-controlling reaction takes place in the organic phase (reaction b). However, the formation of Q+OH- ion pair due to ion exchange (reaction c in Scheme 3) near the interface influences the selectivity. This ion pair has a very limited partitioning into the organic phase; hence, benzyl chloride is hydrolyzed to benzyl alcohol at the interfacial region (reaction d), and subsequently, a series of reactions take place such as e, f, and g, in that order. As a result, byproducts are formed, which were benzyl alcohol (2), (pbenzyl) benzyl phenyl ether (3), and dibenzyl ether (4). The comparison of Schemes 2 and 3 also reveals why there is 100% selectivity. In a typical case, selectivity to benzyl phenyl ether was 70% 3.1. Effect of Speed of Agitation. To ascertain the influence of mass transfer resistance of the reactants to the reaction phase, the speed of agitation was varied in the range of 800-1200 rpm under otherwise similar conditions in the presence of TBAB as the catalyst while maintaining the three phases at 50 °C

Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8451 Scheme 2. L-L-L PTC Mechanism (The Main Reaction (b) Occurs in the Third Phase near the Third Phase-Organic Phase Interface; There Is No Transfer of R′Cl from the Bulk Third Phase to the Aqueous-Third Phase Interface, and Hence, No Hydrolysis and Subsequent Byproducts Occur)

Scheme 3. L-L PTC Mechanism (The Formation of Byproducts Is Clearly Demonstrated Here, the Formation of Which is Due to the in situ Generation of Q+OH- (Reaction c) and Subsequent Reactions d, e, f, and g in That Order)

(Figure 3). The conversion was found to be practically the same at 1000 and 1200 rpm. A further increase in the speed of agitation to 1500 rpm had practically no effect on the conversion. So there was no mass transfer resistance, and all further experiments were conducted at 1000 rpm. A typical analysis was done for mass transfer rates and overall reaction rates by using theoretical correlations on overall mass transfer coefficients also to demonstrate that there were no mass transfer effects beyond 800 rpm. 3.2. Effect of Catalyst Concentration. The comparison of rates of reactions using three liquid phases over two liquid phases in PTC was done by conducting several experiments at

different catalyst concentrations. Using the same concentrations of phenol, benzyl chloride, and sodium hydroxide, the effect of catalyst concentration was studied in the range of 6.2 × 10-5 to 3.79 × 10-3 mol of TBAB at 80 °C, under L-L PTC (Figure 4a). In the L-L PTC, the catalyst is distributed in aqueous and organic phases. This is governed by complex anion-exchange reactions across the interface as shown in Scheme 3, which results in series and parallel reactions. Three distinct phases appeared when the concentration of TBAB was raised to 9.01 × 10-3 mol with the addition of 0.26 mol of NaCl. Thus, the concentration of TBAB was varied from 9.01 × 10-3 to 17.99 × 10-3 mol, keeping all other parameters constant under

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Figure 4. Comparison of effect of catalyst concentration on conversion of benzyl chloride in L-L versus L-L-L PTC. (a) L-L PTC: benzyl chloride ) 0.02 mol; organic phase made up to 25 cm3 with toluene; phenol ) 0.02 mol; NaOH ) 0.025 mol; aqueous phase made up to 25 cm3 with water; temperature ) 80 °C; agitation speed ) 1000 rpm. (b) L-L-LPTC: benzyl chloride ) 0.02 mol; organic phase made up to 50 cm3 with toluene; phenol ) 0.02 mol; NaOH ) 0.025 mol; aqueous phase made up to 50 cm3 with water; NaCl ) 0.26 mol; temperature ) 80 °C; agitation speed ) 1000 rpm.

Figure 5. Effect of sodium hydroxide on conversion of benzyl chloride: benzyl chloride ) 0.02 mol; organic phase made up to 50 cm3 with toluene; phenol ) 0.02 mol; TBAB ) 9.007 × 10-3 mol; aqueous phase made up to 50 cm3 with water; NaCl ) 0.26 mol; speed of agitation ) 1000 rpm; temperature ) 80 °C.

Figure 6. Effect of sodium chloride on conversion of benzyl chloride: benzyl chloride ) 0.02 mol; organic phase made up to 50 cm3 with toluene; phenol ) 0.02 mol; TBAB ) 9.007 × 10-3 mol; NaOH ) 0.025 mol; aqueous phase made up to 50 cm3 with water; speed of agitation ) 1000 rpm; temperature ) 80 °C.

L-L-L PTC (Figure 4b). The rates of reaction are dramatically increased by orders of magnitude, and almost complete conversions could be realized for 9.01 × 10-3 mol of TBAB with 100% selectivity of desired product. This shows that the locale of the reaction is the middle catalyst-rich phase or the third phase. The conversion of L-L PTC into the L-L-L PTC happens at a higher concentration of the catalyst along with added NaCl. It is not just the higher conversion due to the higher amount of catalyst present but also the intensification of the rate and the total suppression of side reactions leading to 100% selectivity of the product. Just below the critical concentration when the two-phase environment is maintained, there is no increase in the rate, but when it crosses the critical value, there is a dramatic increase in the rate due to creation of a different microenvironment of the third phase, which enhances the rate. The same ion pair (RO-Q+) with the nucleophile when present in the organic phase, as in L-L PTC, becomes more active, in the third phase whose composition suggests that it is more polar in nature (Scheme 2). Partial recovery of the catalyst in the form of the third phase and also by repeated but limited use of catalyst in the aqueous phase, which is discussed separately, is beneficial

from a waste-minimization viewpoint, as against the total wastage of the catalyst in the L-L PTC process. 3.3. Effect of Concentration of NaOH. The concentration of sodium hydroxide has a pronounced effect on the formation of the third liquid (catalyst-rich) phase as well as on the rate of reaction. The effect of NaOH concentration was studied in the range of 0.025-0.06 mol (Figure 5). It was found that, as the concentration of NaOH was increased from 0.025 to 0.04 mol, the rate of the reaction also increased. As the NaOH concentration increases, the aqueous phase becomes saturated and a higher amount of Q+ salt goes into the third phase, leading to an increase in the reaction rates. Beyond 0.04 mol, the rate of reaction remains almost constant. Since the formation of sodium phenoxide is complete with the mole ratio of phenol to NaOH over 1:2, and the equilibrium is shifted totally to the right in reaction a of Scheme 2, there is no effect of additional NaOH on the rate. Besides, the aqueous phase gets completely saturated with Na+ ions, and thus, the anion-exchange reaction is at equilibrium and all RO- in the form of RO-Q+ is totally transferred into the third phase. 3.4. Effect of Sodium Chloride. Sodium chloride concentration was varied between 0.194 to 0.394 mol under otherwise similar conditions. The conversion is plotted as a function of


RO-Na+(aq) + Q+Cl-(aq) a Q+O-R(aq) + Na+Cl-(aq)

(1)

(2) Transport of Q+Cl- , RO-Na+, and Q+O-R from the aqueous phase into the third liquid phase with equilibrium constants K1, K2, and K3, respectively:

Q+Cl-(aq) a Q+Cl-(th) K1 )

Cth QCl Caq QCl

Q+O-R(aq) a Q+O-R(th) K2 )

Cth QOR Caq QOR

RO-Na+(aq) a RO-Na+(th) K3 ) Figure 7. Effect of mole ratio on conversion of benzyl chloride: organic phase made up to 50 cm3 with toluene; TBAB ) 9.007 × 10-3 mol; aqueous phase made up to 50 cm3 with water; NaCl ) 0.26 mol; temperature ) 80 °C; speed of agitation ) 1000 rpm. Symbols indicate mole ratio of phenol/benzyl chloride.

time (Figure 6). The role of NaCl is to salt out the catalyst along with the nucleophile in the form of the RO-Q+ ion pair from the aqueous phase into the third phase as well as to increase the rate of aqueous phase ion-exchange reaction, which leads to equilibrium. There are already Na+ ions from NaOH that also increase the ionic strength of the aqueous phase, which enhances the rate in that phase. The rate increases as the amount of NaCl is increased. Addition of NaCl beyond saturation concentration has no significant effect on the rate of reaction because it forms an additional solid phase. 3.5. Effect of Mole Ratio. Effect of mole ratio of phenol/ benzyl chloride was studied in the range of 1:1 to 2:1 (with a fixed phenol-to-NaOH mole ratio of 1:1.25). Figure 7 shows the conversion of benzyl chloride as a function of time. With an increase in the mole ratio of phenol/benzyl chloride, the initial rate was observed to increase. It was also observed that, beyond a certain limit, there was no effect of increased concentration of sodium phenoxide on reaction rate since the reaction becomes zero order, as will be discussed in the following section. 3.6. Kinetic Model. The reaction mechanism for L-L-L PTC system is shown in Scheme 2. Since the selectivity to benzyl phenyl ether (R′OR) (1) was 100%, the mechanism depicted there is straightforward. The sodium phenoxide ion pair (RO-Na+) generated in situ in the aqueous phase is equivalent to moles of phenol due to an excess of sodium hydroxide, and the anion exchange takes place between the quaternary salt (Q+Cl-) and RO-Na+ in the aqueous phase to form an ion pair (RO-Q+), which is then transferred into the third liquid phase. This is an equilibrium reaction, and all the species involved have equilibrium concentrations. The substrate benzyl chloride R′Cl is transferred from the organic phase to the third phase, where the subsequent reaction occurs between RO-Q+ and R′Cl to produce the desired product R′OR (1), which is then transferred to the organic phase. Thus, the overall reaction is L-L-L PTC

RONa(aq) + R′Cl(org) 98 R′OR(org) + NaCl(aq) The steps involved in the overall reaction are as follows: (1) Ion-exchange reaction of RO-Na+ and Q+Cl- in the aqueous phase to form the ion pair with the nucleophile RO-Q+′:

Cth RONa Caq RONa

(2)

(3)

(4)

(3) Ion-exchange reaction of RO-Na+ and Q+Cl- can also take place in the third liquid phase to form RO-Q+ and Na+Cl-.

RO-Na+(th) + Q+Cl-(th) f RO-Q+(th) + Na+Cl-(th) (5) NaCl(th) a NaCl(aq) K4 )

Caq NaCl Cth NaCl Corg QOR

QOR(th) a QOR(org) K5 )

Cth QOR

(6)

(7)

(4) Reaction of Q+RO- with R′Cl in the third liquid phase: kth

Q+OR-(th) + R′Cl(org) 98 R′OR(th) + Q+Cl-(th)

(8)

There is a very insignificant contribution by the reaction in the organic phase, although it is shown in Scheme 2 for the sake of clarity and to compare it with a L-L PTC process. The critical analysis of rate data suggested that the conversions of benzyl chloride were linear in time, suggesting zero-order reaction up to a certain time, and then it followed an exponential pattern, indicating apparent first order. It was neither first order nor zero order over the entire range of concentrations in L-L-L PTC, whereas a first-order equation was found to fit the rate data of L-L PTC. The rate of formation of R′OR can be written from eq 8, as follows. At the same time, the stoichiometry suggests that, for every 1 mol of R′Cl, 1 mol of product R′OR is formed. Thus, the rate of formation of the product per unit volume of the third phase is given by th dCR′OR th ) kthCth QORCR′Cl dt

(9)

Further,

K7 )

th CR′Cl org CR′Cl

and

K8 )

th CR′OR org CR′OR

(10)

The fractional conversion of R′Cl (designated as A) is given by

XA )

NA0 - NA NA0

where “0” denotes the zero time or initial condition.

(11)

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Figure 8. Comparison of effect of temperature on conversion of benzyl chloride in L-L and L-L-L PTC. (a) L-L PTC: benzyl chloride ) 0.02 mol; organic phase made up to 25 cm3 with toluene; phenol ) 0.02 mol; NaOH ) 0.025 mol; TBAB ) 3.1 × 10-4 mol; aqueous phase made up to 25 cm3 with water; speed of agitation ) 1000 rpm. (b) L-L-L PTC: benzyl chloride ) 0.02 mol; organic phase made up to 50 cm3 with toluene; phenol ) 0.02 mol; NaOH ) 0.025 mol; TBAB ) 9.007 × 10-3 mol; aqueous phase made up to 50 cm3 with water; NaCl ) 0.26 mol; speed of agitation ) 1000 rpm.

The rate of formation of R′OR in mol/time is equal to that of reaction of R′Cl as shown below. th -dNA dNR′OR dCR′OR th th )V ) VthkthCth (12) QORCR′Cl ) dt dt dt

-dNA org ) VthkthCth QORK7CA dt

[(

Cth QOR )

Nth QOR th

)( )]

) (

Vth Vth 1 - η th 1 1 + aq + + aq CQORVaq K2 V K1 V η

(19)

NQtot V

)

[(

1 Vth + aq K2 V

Vaq 1 Vth 1 - η + + aq K1 V η

)( )]

) (

(20)

-dNA kthK7Nth QORNA ) org dt (V + K Vth) 7

NQtotVth

NA

(14)

(Vorg + K7Vth)

kthK7Nth -dNA VthkthK7Cth QORNA QORNA ) ) org th org dt (V + K V ) (V + K Vth) 7

NQtot )

(13)

The volumes of the aqueous, third, and organic phases are given by Vaq, Vth, and Vorg, respectively. The total number of moles of A at any time are distributed between the organic and third org phases and none are in the aqueous phase, NA ) Nth A + NA and th org th org th org org th K7 ) CR′Cl/CR′Cl ) CA /CA ) NA V /NA V ; therefore, the following can be derived:

Corg A )

Substituting eqs 2, 3, and 18 into eq 17, the following is obtained.

(15)

7

The mass balance for the catalyst (NQtot, the total moles added initially), which is distributed in six different species in the three phases, is as follows: aq th th th NQtot ) Vaq(Caq QOR + CQCl) + V (CQOR + CQCl) + org Vorg(Corg QOR + CQCl) (16)

As stated earlier, the contribution of the organic-phase reaction is negligible since the amount of Q in the organic phase is negligible, and thus, eq 16 becomes: th th aq th NQtot ) Vaq(Caq QOR + CQCl) + V (CQOR + CQCl)

(17)

Let the amount of catalyst in the third phase be denoted by th th th th Cth Q ) CQOR + CQCl, CQOR ) ηCQ

(18)

In eq 18, η is the molar ratio of Q+ in the form of QOR at any time in the third phase.

)

kthK7NA org (V + K7Vth)

-dNA ) dt

[(

th

1 V + K2 Vaq

Vaq 1 Vth 1 - η + + aq K1 V η

)( )]

) (

RkthK7NANQtot 1 1 1-η Vorg(1 + K7β) +R + +R K2 K1 η

[(

) (

Vth Vaq

and β )

)( )]

(21) (22)

where

R)

Vth Vorg

(23)

In eq 22 above, R, β, K7, and kth are constants and, hence, can be suitably combined as an apparent rate constant kapp. The catalysts quantity added, NQtot, is also constant. In terms of fractional conversion, now eq 22 can be written as follows,

dXA ) kapp(1 - XA)NQtot dt

(24)

When the third phase was saturated with A, which happened during the first 20-30 min of reaction time leading to almost 30-50% conversion (all figures show it), depending on


conditions, it was observed that the conversions were linear in time. Thus, from eq 9 or eq 12, the following is obtained:

dXA ) k0NQtot ) k′0 ) const. dt

(25)

Integrating eq 25 leads to

XA ) k′0NQtott

(26)

Thus, conversion of A is linear in time with apparent zeroorder reaction. Integrating eq 24 gives

-ln(1 - XA) ) kappNQtott

(27)

The above model can be fitted to the collected data. The L-L PTC model is not developed here since it has been reported in our earlier work.30

-dCorg A org ) korgCorg A CQOR dt Ke )

Figure 9. Kinetic plot for L-L PTC: benzyl chloride ) 0.02 mol; organic phase made up to 25 cm3 with toluene; phenol ) 0.02 mol; NaOH ) 0.025 mol; TBAB ) 3.1 × 10-4 mol; aqueous phase made up to 25 cm3 with water; speed of agitation ) 1000 rpm.

(28)

aq Corg QOR CCl‚ aq Corg QCl CRO-

(29)

If NQtot is the total amount of catalyst (mol) added to the reaction mixture initially and φ is the fraction of the catalyst cation Q+ distributed in the organic (reaction) phase, then org org ) φNQtot (Corg QCl + CQOR)V

(30)

The above rate equation can be written for equimolar quantities of substrate and nucleophile as

dXA ) korg dt

(

φCQtot(1 - XA) XA 1 1+ Ke (1 - XA)

)

(31) Figure 10. Arrhenius plot for L-L PTC.

Integration of the above equation gives

ln(1 - XA)[1 - Ke] +

XA (1 - XA)

) KekorgφCQtott (32)

aq aq When Ke is large and the ratio of CCl -/CRO- is fairly constant, then it becomes a pseudo-first-order kinetics,

dXA ) k′appCQtot(1 - XA) dt

(33)

which on integration gives

-ln(1 - XA) ) k′appCQtott

(34)

In the above derivation, it is assumed that no byproducts are formed during the initial period and the kinetics of the consumption of benzyl chloride could be found for L-L PTC. 3.7. Effect of Temperature. The effect of temperature on the reaction between phenol and benzyl chloride was studied under both L-L PTC and L-L-L PTC. The temperature was varied from 50 to 90 °C. It was observed that the rates of reaction increased with increasing temperature under L-L PTC and L-L-L PTC (Figure 8, parts a and b).

3.7.1. L-L PTC Kinetics. Invoking the above theory, typical first-order plots were made for L-L PTC (Figure 9). At high temperature, the data fit is not so good since the byproduct formation increases. The Arrhenius plot is shown in Figure 10. The apparent energy of activation has been found to be 13.4 kcal/mol for L-L PTC. 3.7.2. L-L-L PTC Kinetics. For a control experiment, both zero-order and first-order kinetic plots are given in parts a and b of Figure 11, respectively. The data fit very well. For sake of brevity, the zero-order behavior for which conversion is linear with time is shown by straight lines for initial periods in Figure 3 (for 1000 and 1200 rpm), Figure 4b , Figure 5 (0.04 mol), Figure 6 (0.26 mol), Figure 7 (for three mole ratios of phenol to BzCl), and Figure 8b (all temperatures). The zero-order rate constants were used to make the Arrhenius plot (Figure 12) from which the apparent energy of activation of 7.84 kcal/mol is obtained. This value can now be compared with an apparent activation energy of 13.4 kcal/mol obtained by using L-L PTC. The reduction in activation energy is due to the change of the locale of reaction from the organic phase to the third phase. This also suggests a change in the activity of the ion pair in the third phase in relation to that in the organic phase of L-L PTC. 3.8. Reusability Study. After completion of the kinetic run, the stirring was stopped and the reaction mixture was cooled.

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Figure 11. L-L-L PTC (a) zero-order kinetics up to 20 min and (b) first-order kinetics thereafter: benzyl chloride ) 0.02 mol; organic phase made up to 50 cm3 with toluene; phenol ) 0.02 mol; NaOH ) 0.025 mol; TBAB ) 9.007 × 10-3 mol; aqueous phase made up to 50 cm3 with water; NaCl ) 0.26 mol; speed of agitation ) 1000 rpm.

Figure 12. Arrhenius plot for L-L-L PTC.

When phases were clearly separated, the organic phase containing the product was removed. The reusability study was done in two ways, one in which only the catalyst-rich phase was reused and another in which the aqueous phase was also recycled along with the third phase. 3.8.1. Method A: Reuse of Third Phase Only. The third phase was separated from the previous experiment and reused by adding fresh aqueous- and organic-phase reactants. In this method, the third phase was used three times starting with the fresh use. There is distribution of catalyst in the catalyst-rich third phase and organic phase and the aqueous phase. Since both the aqueous and organic phases were freshly used, there was some loss of the catalyst with discarded organic and aqueous phases. Therefore, there is once again distribution of catalyst available from the third phase to the organic phase, which reduces the volume of the third phase and, hence, the rates of reaction. The subsequent experiments with replenished aqueous and organic phases had, in fact, a lower quantity of available catalyst than the previous run, and hence, rates of reaction decreased correspondingly (Figure 13). 3.8.2. Method B: Reuse of Third and Aqueous Phases Only. Fresh toluene was added along with a known quantity of benzyl chloride, for which the third phase as well as the aqueous phase from the previous experiment were used. In this method, the catalyst and the aqueous phase were reused five times and

Figure 13. Reuse of catalyst phase only: benzyl chloride ) 0.02 mol; organic phase made up to 50 cm3 with toluene; phenol ) 0.02 mol; NaOH ) 0.025 mol; TBAB ) 9.007 × 10-3; aqueous phase made up to 50 cm3 with water; NaCl ) 0.26 mol; speed of agitation ) 1000 rpm; temperature ) 80 °C.

the organic phase was replenished every time. The organic phase containing the product with an insignificant quantity of catalyst was removed each time. Therefore, when the second run was conducted with 0.02 mol of benzyl chloride, almost all catalyst was available for reaction, and therefore, the loss in conversion is marginal (Figure 14). The catalyst is distributed in three phases, and the purpose of this study was to get evidence as to whether the catalyst can be reused and which way. Thus, replenishing the organic phase without removal of the aqueous phase helps. Only addition of sodium phenoxide is required to maintain the rate of reaction. The catalyst is required in the aqueous phase also (reaction a in Scheme 2). 4. Comparison of L-L vs L-L-L PTC In the case of two liquid phases (the normal L-L PTC), at a catalyst concentration just below the critical concentration required to generate three distinct liquid phases, the reaction reaches a maximum of 85% conversion in 3 h at 80 °C and the selectivity of benzyl phenyl ether was 70%. On the contrary, with the creation of the third phase, by further addition of either sodium chloride or catalyst beyond the


Nomenclature and Symbols

Figure 14. Reuse of catalyst and aqueous phase: benzyl chloride ) 0.02 mol; organic phase made up to 50 cm3 with toluene; speed of agitation ) 1000 rpm; temperature ) 80 °C.

critical value, the conversion of benzyl chloride and the selectivity of benzyl phenyl ether was 100%. Thus, there is a manyfold increase in the productivity of the same reactor with an improvement in total selectivity. This third phase was reusable many times as discussed earlier but preferably along with the aqueous phase. The activation energy value is also reduced substantially in L-L-L PTC. 5. Conclusion L-L PTC can be easily converted to L-L-L PTC, to recover and reuse catalyst as a waste-minimization strategy and also to enhance selectivity, thereby improving profitability, reduction in reactor volume, and operating costs. The novelties of phase-transfer catalyzed reaction etherification of phenol with benzyl chloride were studied in detail in a triphasic L-L-L PTC system with TBAB as the catalyst. Mechanistic models have been proposed to account for the calculation of the rate constant. Different parameters such as speed of agitation, temperature, catalyst loading, reusability of catalyst-rich phase, salt concentration, and mole ratio have been studied. The type and concentration of catalyst and the amount of sodium chloride are important factors that influence the formation of the third liquid (catalyst-rich) phase and the distribution of catalyst. It was observed that the reaction rates were intensified and there was an improvement in selectivity in the L-L-L PTC vis-a`-vis the L-L PTC. A detailed kinetic study has been done for the above system. There is a reduction in apparent activation energy for the L-L-L PTC vis-a`-vis L-L PTC, thereby suggesting ehanced activity of the ion pairs in the third phase due to the change in polarity of the microenvironment. Acknowledgment G.D.Y. acknowledges support for personal Chair from Darbari Seth endowment, and O.V.B. thanks UGC for the award of a SRF. Thanks are also due to Purdue University for inviting G.D.Y. as a distinguished visiting scholar under the President’s Asian Visitor program, which enabled him to spend considerable time on creativity.

A ) benzyl chloride Corg A ) concentration of benzyl chloride in the organic phase, mol/cm3 of organic phase th CA ) concentration of benzyl chloride in the third phase, mol/ cm3 of third phase - + (also QOR) in aqueous Caq QOR ) concentration of RO Q phase, mol/cm3 of aqueous phase + Caq QCl ) concentration of Q Cl (also QCl) in aqueous phase, 3 mol/cm of aqueous phase - + Cth QOR ) concentration of RO Q (also QOR) in third phase, 3 mol/cm of third phase + Cth QCl ) concentration of Q Cl (also QCl) in third phase, mol/ cm3 of third phase - + Corg QOR ) concentration of RO Q (also QOR) in organic phase, mol/cm3 of organic phase + Corg QCl ) concentration of Q Cl (also QCl) in organic phase, 3 mol/cm of organic phase Ka-aq ) ion-exchange reaction equilibrium constant for aqueous phase (reaction a) Ka-th ) ion-exchange reaction equilibrium constant for third phase (reaction a) Ki ) distribution constants, as given below aq K1 ) Cth QCl/CQCl th K2 ) CQOR/Caq QOR aq K3 ) Cth RONa/CRONa aq th K4 ) CNaCl/CNaCl th K5 ) Corg QOR/CQOR org th K6 ) CQCl/CQCl th org K7 ) CR′Cl /CR′Cl th org K8 ) CR′OR/CR′OR korg ) second-order rate constant of forward reaction in organic phase, cm3/mol‚s (L-L PTC) org aq aq Ke ) Corg QOR/CQCl‚CCl-/CRO- ) overall ion-exchange reaction equilibrium constant (L-L PTC) kapp ) apparent first-order rate constant, cm3/((mol of catalyst)‚ s) k′app ) apparent first-order rate constant k0 ) zero-order constant, mol-1 s-1 k′0 ) apparent zero-order constant , s-1 korg ) organic-phase reaction rate constant, cm3/((mol of catalyst)‚s) kth ) rate of reaction in the third phase, cm3/(mol of catalyst s) NQtot ) total moles of catalyst added to the system, mol Nth A ) moles of A in third phase, mol Norg A ) moles of A in organic phase, mol Nth QOR ) moles of QOR in third phase, mol R′Cl ) benzyl chloride ROH ) phenol R′OR ) benzyl phenyl ether t ) time of reaction, s Vaq ) volume of aqeous phase, cm3 Vorg ) volume of organic phase, cm3 Vth ) volume of third phase, cm3 XA ) (NA0 - NA)/NA0, fractional conversion Greek Symbols R ) Vth/Vaq ) ratio of third to aqueous phase volumes β ) Vth/Vorg ) ratio of third to organic phase volumes th th + η ) Cth QOR/(CQOR + CQCl) ) molar ratio of Q in the form of QOR at any time in the third phase

8458


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ReceiVed for reView January 31, 2007 ReVised manuscript receiVed September 20, 2007 Accepted September 21, 2007 IE070180M

Role of Third Phase in Intensification of Reaction ... - ACS Publications

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