Butylation of Phenylacetonitrile in an Oscillatory Baffled Reactor

In this paper, we report our investigation of a liquid-liquid phase-transfer catalysis (PTC) of the butylation of phenylacetonitrile in a novel oscill...
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Ind. Eng. Chem. Res. 2005, 44, 8663-8670

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Butylation of Phenylacetonitrile in an Oscillatory Baffled Reactor B. Wilson,† D. C. Sherrington,‡ and X. Ni*,† Centre for Oscillatory Baffled Reactor Applications (COBRA), School of Engineering and Physical Science, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, U.K., and Department of Pure and Applied Chemistry, Strathclyde University, Glasgow G1 1XJ, Scotland, U.K.

In this paper, we report our investigation of a liquid-liquid phase-transfer catalysis (PTC) of the butylation of phenylacetonitrile in a novel oscillatory baffled reactor. We evaluated the reaction kinetics by looking at the order of the reaction and examined the effect of operating conditions as well as the PTC catalysts on the conversion. The results demonstrated that the conversion is highly dependent upon the operating conditions for all the different PTC catalysts used. Such a dependence provides an effective means of increasing the productivity of the butylation of phenylacetonitrile without involving the design and testing of new catalysts. Introduction Many desirable reactions cannot be brought about because two reactants are inaccessible to each other; frequently, the first reagent resides in an organic liquid phase while the other belongs to an aqueous solution. One of a handful of techniques available to overcome this physicochemical barrier is that of phase-transfer catalysis (PTC). A catalytic amount of a phase-transfer agent can be used to cross the barrier, transporting reactive anions from, typically though not exclusively, the aqueous phase into the organic phase where, with the transferred species in an active state, the reaction proceeds in the organic phase, producing the desired products.1 There are many factors that affect a PTC reaction scheme, for example, (a) the choice of catalysts, (b) the choice of solvent, (c) the presence of water, (d) mixing, (e) the choice of anion and counteranion, and (f) the choice of base. Among the listed parameters, only mixing is related to chemical engineering while the rest are associated with chemistry factors. Not surprisingly, research into PTC has predominately been carried out by chemists, and the typical reactor has been mechanically or magnetically stirred vessels.2-6 Regarding agitation in PTC reactions, the issue of the greatest relevance is what advantage can mixing offer beyond that provided by the optimization of other experimental parameters? We argue that mixing is important in a PTC process as it promotes contact between liquid/liquid, solid/liquid, or gas/liquid/solid phases, creating an interface through which part of the PTC reaction takes place. In this paper, we report our evaluation of the importance of mixing upon the performance of the alkylation of phenylacetonitrile, a liquid-liquid PTC scheme, in a novel oscillatory baffled reactor (OBR). The OBR is based on the concept of superimposing fluid oscillation onto a cylindrical column containing periodically placed orifice baffles. The key feature of the OBR is that mixing can be controlled to a very high degree of precision, either in a dynamic sense, by * Corresponding author. Tel.: 00441314513781. Fax: 0044131451312. E-mail [email protected]. † Heriot-Watt University. ‡ Strathclyde University.

Figure 1. Mechanism of mixing in an oscillatory baffled column.

altering the oscillation frequency and amplitude, or by customizing the baffle geometry. This allows a wide range of mixing conditions to be achieved, from “soft” mixing, exhibiting plug flow characteristics, to the most intense mixing, corresponding to mixed flow.7-9 The mechanism of mixing can be understood with the help of Figure 1. The essential feature is that sharp edges (provided by the baffles) are presented transverse to an oscillating, fully reversing flow. Because the motion is periodic and fully reversing, there are two half cycles, each containing flow acceleration and deceleration corresponding to a sinusoidal velocity-time function. On each flow acceleration, vortex rings form downstream of the baffles. A peak velocity is reached, and then, as the flow decelerates, the vortices are swept into the bulk and subsequently unravel with the bulk flow acceleration in the opposite (axial) direction. It is the strong radial velocities, arising from the repeating cycles of vortex formation and of similar magnitude to the axial velocities, which give uniform mixing in each interbaffle zone and cumulatively along the length of the column.10 The power dissipation rate, , in an OBR can be evaluated via a quasi-steady-state approach, taking into consideration the effects of static, inertial, and frictional forces, assuming that there is a uniform velocity through-

10.1021/ie048855y CCC: $30.25 © 2005 American Chemical Society Published on Web 03/09/2005

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Figure 2. Diagram of a PTC scheme operating with an interfacial mechanism.

out the column, as follows:11,12

)

(

)

2FN 1 - R2 3 3 P ) xo ω (W kg-1) FV 3πC 2 R2 D

(1)

where P/V is the power density per unit volume (W m-3), F the density of the liquid (kg m-3), xo the amplitude of oscillation (m), f the frequency of oscillation (Hz), N the number of baffles per unit length of OBC (m-1), CD the coefficient of discharge of the baffles ()0.7 normally), and R the baffle-free area ratio [R ) (Do/D)2], where Do is the orifice hole diameter (m). The product of xof is the oscillatory velocity (m s-1). Reaction Mechanism The PTC reaction in question is the butylation of phenylacetonitrile, predominately producing a monosubstituted product; the working principle for this type of PTC reaction is illustrated in Figure 2, where the interface between the two liquid phases is a key element for more than one of the crucial steps in the overall reaction.13-18 There are essentially three steps to this reaction, identified by the numbers in Figure 2 and summarized as follows: (1) The organic phase substrate (RHorg) is deprotonated at the interface by a concentrated aqueous phase base (OHaq), forming an anion (R1int). (2) The anion (R1int) is paired at the interface and moved into the organic phase by a catalyst (QXorg), typically a quaternary onium species residing in the organic phase, to form R1Qorgsa catalyst-anion pair. (3) The paired anion (R1Qorg) then reacts with an alkylating agent in the organic phase (R2Xorg) to form the organic phase product R1R2org and return the catalyst to its original form of QXorg. The organic phase reactants in this reaction consist of bromobutane, BuBr, the alkylating agent; phenylacetonitrile, PhCH2CN, the organic phase substrate; and the catalyst, tetrabutylammonium bromide (TBAB), a typical quaternary onium salt often used in phasetransfer catalysis, labeled QBr in Figure 2. The main product of the organic phase butylation reaction is 2-phenylhexanenitrile, PhBuCHCN, and the other product is the regenerated catalyst QBr. The sole aqueous phase reagent is sodium hydroxide in solution (50% w/w), represented by NaOH in Figure 2. The product

Figure 3. Schematic of the OBR (not to scale).

in the aqueous phase is sodium bromide, represented as NaBr in Figure 2, which pairs the Na+ ion produced from the interfacial deprotonation of PhCH2CN by NaOH with the halide anion (Br-) leaving the catalyst. Experimental Facilities The batch OBR used in this study was made of a vertical stainless steel jacketed column of 22.3 mm in diameter and 300 mm in height with a working fluid capacity of 44 mL, as shown in Figure 3. The shell side consisted of two sections; the lower section was used for heating the reactor and was 95 mm in diameter and 315 mm in height. Hot water was used as the heating source and circulated via a pump from a Hakke W 45 bath with a DL30 temperature control unit incorporated. The upper cooling section acted as a condenser to minimize loss of vapor during the reaction. Tap water at room temperature was used for the purpose of cooling. The upper section for cooling was 55 mm in diameter and 120 mm in height. There were no baffles in the upper section and no air entrainment to the system. Essentially, the PTC reaction takes place in the lower section of the reactor while the upper section is free of any reaction. In the following text, “the reactor” refers to the lower section of the assembly. Two sample ports of thin bore (inside diameter of 1.6 mm) piping were attached to the reactor by twinned compression fittings. A PTFE coated septum was then fitted to the end of the piping of each sample port by a

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third compression fitting, creating a reliable seal. These two tube-side sampling ports can be used for sampling, using 19G syringe needlework, as well as for temperature measuring, using a K-type thermocouple. A further two ports were available on the shell side, as indicated in Figure 3, for the purpose of supplying heating and cooling water. A set of two orifice baffles was used in the batch OBR. The baffles were made of a stainless steel plate, 22.15 mm in diameter and 2.1 mm in thickness. The baffle spacing was 33.45 mm, that is, 1.5 times the column diameter, and the baffle-free area was 34%. Fluid oscillation was achieved by moving the set of baffles up and down the column at the top of the unit via an oscillatory unit consisting of an electric motor with a flywheel, an inverter, and a bell house linkage (Figure 3). The speed of the motor can be controlled to generate an oscillation frequency from 1 to 8 Hz. Oscillation amplitudes of xo ) 1-20 mm center-to-peak can be obtained by adjusting the eccentric position of the bell house linkage on the flywheel.

justification of the interfacial mechanism has been centered on the extremely low mutual solubility of the reagents in both phases and the strong dependence of the rate of butylation on the intensity of agitation and, thus, the liquid-liquid interfacial area. The combination of these observations led to the identification of the ratelimiting step as the interfacial deprotonation step (step 1 of Figure 2), and this remained so for a decade (approximately 1975-1984) until Halpern et al. conducted experiments to investigate the presence of the kinetic isotope effect in the butylation of PhCH2CN.15 Due to an absence of this effect, the conclusion was that the first step is a prerequisite for the interfacial mechanism to operate but it is a fast equilibrium step in comparison to step 2, making the second step the ratelimiting one. A further observation made within the investigation was that the presence of a lipophilic anion (i.e. X-) might be critical to the progress of the reaction. This is because the lipophilicity of the anion maintains a certain concentration of the catalyst-anion pair (QPhCHCN) in the organic phase. The overall reaction of this PTC scheme can be represented by

Experimental Procedure

PhCH2CN + BuBr 9 8 NaOH

TBAB

(aq)

The procedure of the experiment is as follows. The aqueous phase reagent, sodium hydroxide solution (50% w/w), and the organic phase reactant, bromobutane (BuBr), were preheated to a reaction temperature in the OBR. Three temperatures of 55, 70, and 80 °C were used in this work. At the same time, the catalyst TBAB was dissolved in the organic substrate (PhCH2CN) in a separate preheated vessel. The PhCH2CN plus the dissolved TBAB catalyst were then added to the OBR by injection through the top port of the reactor. Fluid oscillation started with a preset oscillation amplitude and frequency. The length of a typical experiment was 2 h 10 min with samples extracted at 1, 5, 15, 60, and 130 min. The number of samples for extraction was limited to ensure that less than 10% of the organic phase volume was removed over the course of the reaction. The extracted samples then went through two stages of processing prior to GC analysis. The first stage was the separation of the aqueous and organic phases, which was achieved by releasing the sample into approximately 1 mL of diluted (0.01 M) hydrochloric acid at room temperature. This step also quenches the temperature of the mixture, minimizing further reaction. The second stage involved drawing the organic phase liquid from the upper layer of the separation using a micropipet with a narrow bore tip. This was then diluted into ether, from which 0.25 µL was manually measured by a microsyringe and charged into the GC column (a Perkin-Elmer GC-2500 machine with a 2 m long, 2 mm internal diameter column packed with SP 2100, 80100 mesh Supelcoport packing). Thus, with the use of appropriate standard chromatograms for calibration, the concentration of reagents in the organic phase can be measured and the reaction rate determined. Concentrations obtained from the GC are with a 95% level of confidence. Butylation of Phenylacetonitrile. The reaction of butylation of phenylacetonitrile by bromobutane in the presence of an aqueous sodium hydroxide solution is a widely studied example of PTC by the interfacial mechanism;13 although the mechanics and kinetics, in particular, have been subjected to rigorous debate,14-18 the

PhCH(Bu)CN + NaBr + H2O (2) where PhCH2CN ) organic phase substrate, BuBr ) organic phase alkylating agent, Ph(Bu)CHCN ) product in organic phase, NaBr ) product in aqueous phase, and TBAB ) phase-transfer catalyst. The two organic reactants on the left-hand side (LHS) of the reaction together with the catalyst (TBAB) react with the sodium hydroxide solution presented at the interface, producing products in the organic and aqueous phases. Additional TBAB catalyst could also be generated as follows: NaOH

8 PhCH2COONa + NH3 PhCH2CN 9 H O 2

(3)

NH3 + 4BuBr + 3OH- f Bu4N+Br- + 3H2O + 3Br(4) The Bu4N+Br- might then act as a phase-transfer catalyst in the overall reaction (Bu)4N+

8 PhCH2CN + BuBr 9 NaOH PhCH(Bu)CN + NaBr + H2O (5) where PhCH2CN ) organic phase substrate, PhCH2COONa ) phenylacetic acid sodium salt, NH3 ) ammonia, BuBr ) organic phase alkylating agent, OH- ) hydroxyl group from aqueous phase base, H2O ) water, Bu4N+Br- ) in situ generated TBAB, NaBr ) sodium bromide, and PhCH(Bu)CN ) 2-phenylhexane nitrile. The catalyst generated this way may contribute to the overall reaction of eq 2. In reality however, the amount of the catalyst so generated is much smaller than that of the TBAB intentionally added.16,17 Figure 4 shows the concentration of PhCH2CN against time for a control reaction where no TBAB is added as a catalyst and where catalytic TBAB is produced only via the reactions of eqs 3 and 4. It can be seen that the consumption of PhCH2CN is much smaller for the catalyst TBAB generated via the alternative route than it is when TBAB is added intentionally. The conversion shown in

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Figure 4. Control reaction of BuBr with PhCH2CN involving no added TBAB (xo ) 10 mm, f ) 6.5 Hz, T ) 80 °C).

Figure 6. Conversion of PhCH2CN versus oscillatory velocity in the PTC reaction with BuBr and PhCH2CN in the presence of NaOH.

Figure 5. Dimensionless conversion versus time for the reaction of BuBr with PhCH2CN in the OBR (xo ) 10 mm, T ) 80 °C).

Figure 4 is about 20% on the basis of the initial concentration of 0.0041 mol cm-3 (based upon organic phase volume only) of PhCH2CN. The truth of the matter is that no one knows whether the two sets of reactions take place simultaneously or sequentially within this PTC scheme. Furthermore, there is no possible means to distinguish the contribution by each type. Consequently, the small contribution from any in situ generated catalyst will be neglected in the analysis to follow. The rate equation of the reaction of eq 2 can then be written as

-rPhCH2CN ) -

dCPhCH2CN dt

) kCPhCH2CNCBuBr

(6)

Let xPhCH2CN ) 1 - [CPhCH2CN/(CPhCH2CN)0] be the conversion of the organic reagent PhCH2CN, and M ) (CBuBr)0/ (CPhCH2CN)0, where the subscript “0” denotes the initial concentration. By integrating eq 6, it arrives at

ln

[

M - xPhCH2CN

M(1 - xPhCH2CN)

]

) [(CBuBr)0 - (CPhCH2CN)0]kt (7)

Thus, a plot of the LHS of eq 7 against time should, if the reaction is second-order, yield a straight line with a slope of ((CBuBr)0 - (CPhCH2CN)0)k, where k is the observed second-order rate constant. Figure 5 presents a typical example of such a plot, based upon a corresponding set of concentration profiles.

Figure 7. Effect of oscillation frequency on conversion of PhCH2CN in the PTC reaction with BuBr and PhCH2CN in the presence of NaOH (xo ) 5 mm).

Clearly, the reaction follows a second-order behavior. Interestingly, the slope varies with the oscillation intensity, that is, frequency. Given the constant initial concentration, the results suggest that the observed reaction rate constant depends on the oscillation intensity and, in turn, the fluid mechanical conditions. The effect of the oscillatory velocity (xof) on the conversion of the organic phase reactant, PhCH2CN, is shown in Figure 6. It is clear that the final fractional conversion of PhCH2CN increases with increasing oscillatory velocity and that the effect of temperature on conversion is of little significance. This indicates a domain where mass transfer is controlling this PTC reaction. Keeping one parameter fixed at a time can also provide the individual influence of the oscillation amplitude or frequency on the conversion. Figures 7 and 8 are the corresponding plots. It can be seen that the individual effects of the oscillation amplitude and frequency on the conversion of PhCH2CN are more or less similar, with a similar slope. In the plot of (xPhCH2CN)final versus the power dissipation rate to a power of -0.4, that is, -0.4, the data fit well to a straight line, as shown in Figure 9. This may not be a coincidence, however; a number of studies into liquid-liquid dispersions and droplet size distribution in batch OBRs19-21 as well as in reciprocating plate columns (RPCs)22-25 and stirred tank reactors (STRs)26-34

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Figure 8. Effect of oscillation amplitude on conversion of PhCH2CN in the PTC reaction with BuBr and PhCH2CN in the presence of NaOH (f ) 4 Hz).

Figure 9. Final fractional conversion of PhCH2CN versus energy dissipation rate to the power of -0.4 in the PTC reaction with BuBr and PhCH2CN in the presence of NaOH.

have confirmed a correlation between the Sauter mean droplet size, d32, and the energy dissipation rate as

d32 ) K′-0.4

(8)

where K′ is a constant parameter describing the characteristics of liquid-liquid dispersion in a given system and d32 ) ∑(n′idi3)/∑(n′idi2), where n′i and d are the droplet number and diameter (m) respectively. Note that eq 8 was derived for a system of a dispersed volume phase fraction of 0.38.19 In this work, the volume fraction is around 0.36, which is very close. The same power index to the dissipation rates can be seen here, although droplet sizes were not studied in this project and the volume fraction of the organic to aqueous phases in this PTC reaction is different from that of the droplet works. The main connection is that, for the same power dissipation rate in a system, the conversion of the organic phase reaction in the PTC reaction scheme behaves similarly to that of the mean droplet size in liquid-liquid dispersion (that is, the decrease in the conversion with the increase of -0.4 is due to the increase of d32 via the decrease of interfacial area), and the availability of the interfacial area is a key step in the PTC reaction with the interfacial mechanism. This will be discussed later.

Figure 10. Second-order apparent rate constant versus oscillatory velocity over the temperature range 55-80 °C for the PTC reaction with BuBr and PhCH2CN in the presence of NaOH. Table 1. Summary of Rate Constants for the PTC Reaction with BuBr and PhCH2CN in the Presence of NaOH (from Ref 17 and This Work)

system

temp range (°C)

k (L2 mol-2 min-1)

ultrasonic mixer (20 kHz) magnetically stirred flask (1000 rpm) OBR

40-70 40-70 55-70

0.211-1.14 0.066-0.468 0-1.32

Effect of Operating Parameters upon Observed Rate Constant. The dependence of the experimentally observed overall rate constant on the oscillation conditions was identified previously in Figure 5 and is compiled into a single graph in Figure 10. Such a dependence is indeed surprising, as the rate constant should, in theory, stay constant throughout the reaction. Similar studies16,17 on the identical PTC reaction also showed such a dependence in two different reactors, that is, an ultrasonic mixer and a magnetic bar stirred flask. Table 1 lists the range of the second-order rate constants by previous researchers. The question is, what causes this and why? From the well-known Arrhenius equation,

kc ) Ae-(Ea/RT)

(9)

the rate constant, kc, is influenced by temperature, T, and the activation energy, Ea, while the universal gas constant, R, and the frequency factor, A, remain constant. From the point of view of reaction kinetics, k should be a sole function of temperature. With regard to a PTC reaction by the interfacial mechanism, on the other hand, in addition to the rate constant, that is, the chemical kinetics, there is also the aspect of mass transfer. For such PTC schemes, the effects of both mass transfer and chemical kinetics should be considered jointly.35 Taking a generic case, as illustrated by Figure 11, for the dispersion and reaction of a component “a” diffusing from the organic phase through to the aqueous phase, the rate of reaction at the interface can be expressed by

(-r)reaction ) kcCai

(10)

where (-r)reaction is the reaction rate (mol m-3 s-1), kc the first-order rate constant (s-1), and Cai the concentration of “a” at the interface (mol m-3).

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Figure 11. Generic representation of the diffusion of a component “a” from the organic to the aqueous phase through the diffusion film for an aqueous phase reaction.

The mass transfer rate from the organic phase to the interface is

(-r)mass transfer ) kLaM(CSAT - Cai)

(11)

where kL is the mass transfer coefficient (m s-1), aM the interfacial area per unit volume (m-1), and CSAT the equilibrium concentration of “a” at the interface (mol m-3). At steady state, the two rates are equal and the interfacial concentration of component “a” becomes

CSAT Cai ) kc 1+ kLaM

Figure 12. Rate-limiting step 2 in the reaction of BuBr with PhCH2CN in the presence of NaOH and TBAB.

12, which can be written as

PhCHCNNaINT + QBrINT f NaBraq + QPhCHCNorg (13) The associated reaction rate expression is

-rstage B ) k′INTCNaPhCHCNINTCQBrINT (12)

For convenience, let 1/ko ) 1/[1 + (kc/kLaM)], where ko is the overall rate constant. The relative magnitudes of the reaction constant (kc) and the mass transfer coefficient (kLaM) then determine which (chemical kinetics or mass transfer) controls the overall PTC reaction. For a mass transfer controlled PTC reaction, for example, kc is large and kLaM is small, that is, kc/kLaM . 1, Cai f 0. This means that it is very difficult for the reactants either to get to the interface or to have a large enough interface available. The mode of mixing will affect both aspects, hence the kLaM, and the balance of kc/kLaM. From the viewpoint of the interfacial liquidliquid PTC mechanism, other researchers have identified the rate-limiting step in this PTC scheme to be the second step, that is, the pairing of PhCH-CN at the interface with a catalyst (QBr) to create an organophilic catalyst-anion pair of QPhCHCN. In fact, the second step can further be broken up into three stages: A, B, and C, as shown in Figure 12, where A represents the mass transfer of QBr from the bulk of the organic phase to the interface; B the reaction between QBr and PhCHCNNa, producing QPhCH-CN and NaBr at the interface; and C the mass transfer of QPhCHCN from the interface to the bulk. When mass transfer is the rate-limiting step, this refers to either stage A or C in Figure 12; that is to say, the step of either getting the reactant (QBr) to the interface or getting the catalyst-anion pair of QPhCHCN away from the interface is the controlling step. The mass transfer limiting in this PTC reaction can also result from the reaction at the interface; this would refer to stage B in Figure

(14)

where CNaPhCHCNINT is the interfacial concentration of NaPhCHCN (mol cm-3 interfacial layer), CQBrINT the interfacial concentration of QBr (mol cm-3 interfacial layer), and k′INT the reaction rate constant for the interfacial reaction (cm3 mol-1 s-1). The availability of the interface for the reaction will be the key element for such a reaction as the concentration of NaPhCHCN is directly proportional to the interfacial concentration of PhCH2CN; the rate expression can then be rewritten as

-rstage B ) k′INTCPhCH2CNINTCQBrINT

(15)

The concentrations at the interface are given as

CPhCH2CN ) nPhCH2CNθPhCH2CNaM

(16)

CQBr ) nQBrθQBraM

(17)

where ni is the number of moles present per unit area of the interface and θi the fraction of the interfacial area occupied by a species. Combining these expressions, the rate equation for stage B takes the form of

-rstage B ) k′INTnPhCH2CNθPhCH2CNnQBrθQBraM2

(18)

This results in the rate constant of reaction, k′INT, being expressed in units of m2 mol-1 s-1, that is, in terms of the interfacial area; consequently, the reaction rate depends on the area available. For mass transfer controlled PTC reaction schemes, the overall rate constant will be affected by the intensity of mixing at the interface. Thus, the variation of rate constant seen in this work (Figure 10) may be explained. In turn, this

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ing conditions can be a valuable method for improving PTC reactions, in addition to studies of alternative catalysts. Acknowledgment The authors would like to thank the EPSRC for financial support throughout this project. Nomenclature

Figure 13. Fractional conversion of PhCH2CN versus reaction time in the reaction of BuBr with PhCH2CN in the presence of NaOH and TBAB in the OBR (xo ) 10 mm, f ) 6.5 Hz, T ) 55 °C).

highlights the importance of mixing in liquid-liquid PTC reactions that follow the interfacial mechanism. Effect of PTC Catalyst. The PTC catalyst is usually a type of salt, for example, either an ammonium or a phosphonium catalyst, and the selection of it is generally a balance between cost, activity, and stability; for example, phosphonium salts are considered to be a choice that is more active and of comparable stability but more expensive. TBAB has been the PTC catalyst used in this study so far, and it is an ammonium salt. In addition to this, tetrabutylphosphonium bromide (TBPB) and benzyltributylammonium chloride (BTAC) were the two further options. Both TBAB and TBPB belong to a similar family of soluble catalyst, varying in the N+ and P+ centers, while BTAC is a different soluble catalyst with a different lipophilic halide (Clin place of Br-) and a benzyl ring in place of one alkyl chain. The inclusion of BTAC also allows for comparison of the relative importance of catalyst species to this liquid-liquid PTC by the interfacial mechanism. The kinetic behavior of the new PTC catalysts was confirmed by following the same procedure as that described earlier (i.e. eq 9). Figure 13 compiles such profiles for all three soluble catalysts employed. Trial conditions were consistent at 55 °C, xo ) 10 mm, and f ) 6.5 Hz for all experiments. It can be seen that the overall trend in the conversion produced for each type of catalyst is similar with two distinct sections. The initial rapid increase in conversion, continuing up to around 20 min of the reaction time, is replaced by an almost linear trend as time increases. These linear sections appear parallel for all three catalysts. Overall, the effect of PTC catalysts on conversion is very small for the duration of the experiments. This suggests that the mass transfer limiting behavior is applicable to all three types of PTC catalysts. Conclusions The investigation of the liquid-liquid PTC reaction of the butylation of phenylacetonitrile in the OBR has demonstrated that the conversion is highly dependent upon the operating conditions, that is, mixing, for all three types of soluble PTC catalysts. Such increases in conversion are readily achieved simply by adjusting the operating parameters. Using OBR and varying operat-

A ) frequency factor in Arrhenius equation aM ) interfacial area per unit volume, m-1 Ci ) concentration of species i, mol cm-3 CD ) orifice discharge coefficient ()0.7) D ) column diameter, m Do ) orifice diameter, m di ) droplet diameter, m d32 ) Sauter mean droplet diameter, m Ea ) energy of activation, kJ mol-1 f ) oscillation frequency, s-1 K′ ) constant in Sauter mean diameter correlation k ) observed second-order rate constant, cm3 mol-1s-1 kc ) chemical reaction rate constant, units are order dependent kINT ) reaction rate constant for interfacial reaction, m2 mol-1s-1 k′INT ) reaction rate constant for interfacial reaction in terms of interfacial volume, cm3 mol-1 s-1 ko ) overall reaction rate constant, units are order dependent kL ) mass transfer coefficient, m s-1 M ) concentration ratio between reagents ni ) interfacial concentration of species i, mol m-2 n′i ) droplet number N ) number of baffles per meter, m-1 P ) power, W R ) universal gas constant -ri ) reaction rate expressed with respect to i, mol m-3 s-1 Reo ) oscillatory Reynold’s number ()2πfxoDF/µ) St ) Strouhal number ()D/4πxo) t ) time, s T ) temperature, K V ) volume, m3 xi ) fractional conversion of species i xo ) oscillation amplitude, m Greek Letters R ) fractional baffle-free area  ) energy dissipation rate, W kg-1 φ ) phase fraction of the dispersed phase µ ) dynamic viscosity, kg m-1 s-1 F ) density, kg m-3 ω ) angular frequency of oscillation ()2πf), rad s-1 θi ) fraction of the interfacial area occupied by species i

Literature Cited (1) Starks, C. M.; Liotta, C. L.; Halpern, M. Phase Transfer Catalysis, Fundamentals, Applications and Industrial Perspectives; Academic Press: New York, 1994. (2) Solaro, R.; D’Antone, S.; Chiellini, E. Heterogeneous ethylation of phenylacetonitrile. J. Org. Chem. 1980, 45, 4179-4183. (3) Makosza, M. Reactions of carbanions and halogenocarbenes in two-phase systems. Russ. Chem. Rev. (Engl. Transl.) 1977, 46, 2174-2202. (4) Mason, D.; Magdassi, S.; Sasson, Y. Interfacial activity of quaternary salts as a guide to catalytic performance in phasetransfer catalysis. J. Org. Chem. 1990, 55, 2714-2717.

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Received for review November 26, 2004 Revised manuscript received January 27, 2005 Accepted January 31, 2005 IE048855Y