Intensification of Rates and Selectivity Using Tri-liquid versus Bi-liquid

Aug 29, 2006 - The L−L PTC reactions are conducted under milder conditions, using less-expensive solvents at much faster reaction rates and improved...
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Ind. Eng. Chem. Res. 2007, 46, 2951-2961

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Intensification of Rates and Selectivity Using Tri-liquid versus Bi-liquid Phase Transfer Catalysis: Insight into Reduction of 4-Nitro-o-xylene with Sodium Sulfide Ganapati D. Yadav* and Sharad V. Lande Department of Chemical Engineering, UniVersity Institute of Chemical Technology (UICT), UniVersity of Mumbai, Matunga, Mumbai 400 019, India

Reactions in three immiscible liquid phases are attractive, and one of the phases can be the locale of the reaction, which will have a dramatic effect on the product distribution in complex reactions. Thus, converting a bi-liquid (L-L) system into tri-liquid (L-L-L) phases is of considerable scientific and commercial interest. Such systems encounter mass-transfer resistances for transfer across two interfaces and the kinetic analysis becomes difficult. In the case of phase transfer catalysis (PTC), a majority of preparatory and industrial reactions are conducted in two phases. The L-L PTC reactions are conducted under milder conditions, using less-expensive solvents at much faster reaction rates and improved selectivities to desired products. However, the phase transfer catalyst is not recovered but treated as waste, because the quantities are very small and do not contribute much to the expensive product. L-L PTC can be elegantly modified to convert it to L-L-L PTC, to recover and reuse the catalyst and also to enhance selectivity, thereby improving profitability and environmental benefits. 3,4-Dimethyl aniline is a useful starting material for the production of riboflavin (vitamin B2) and also is used as a precursor for many drugs. It can be produced via the reduction of 4-nitroo-xylene in toluene with aqueous sodium sulfide, using tetrabutylammonium bromide (TBAB) as a catalyst under L-L-L PTC at 80 °C. Comparison between tri- and bi-liquid phases was also conducted, and it was observed that, under L-L-L conditions, the rates of reaction of the formation of 3,4-dimethyl aniline had been dramatically enhanced. The kinetics and mechanism of complex L-L-L PTC processes have been explored in detail. The yield, based on the initial amount of reactant, was >95%, and the selectivity was 100%. 1. Introduction Phase transfer catalysis (PTC) is now a commercially mature discipline, with more than 600 applications, covering a wide spectrum of industries, such as pharmaceuticals, agrochemicals, perfumes, flavors, dyes, polymers, pollution control, etc. However, most of the reactions are performed under biphasic conditions, and reuse of the catalyst is not attempted.1-6 Yadav and co-workers have studied extensively PTC that has involved multistep complex reactions in liquid-liquid, liquid-liquidliquid, solid-liquid (ω phase)-liquid, and liquid-liquid-solid systems.7-16 The inherent problems associated with the recovery and reuse of the catalyst under liquid-liquid-phase transfer catalysis (L-L PTC) can be overcome by either immobilizing the catalyst on a porous solid (organic/inorganic) support, which brings into picture intraparticle diffusion resistance, or generating a separate middle catalyst phase between the aqueous phase and the organic phase. The middle liquid phase must be the reaction phase. The novelty of liquid-liquid-liquid phase transfer catalysis (L-L-L PTC) is that the catalyst forms the third phase, and this catalyst-rich phase can be reused repeatedly. The third liquid phase appears when the amount of catalyst exceeds a critical amount beyond which the rate of reaction increases sharply. The concentration of aqueous and organic phase reactants and the structure and concentration of the catalyst are very critical to obtain three immiscible phases. The third liquid phase is the main reaction phase, which must have a density value between the aqueous and organic phases and the correct lipophilicity * To whom correspondence should be addressed. Tel.: +91-222410-2121; Fax: +91-22-2414 5614. E-mail: [email protected], [email protected].

and hydrophilicity balance (HLB).17,18 Because the catalyst forms the third liquid phase, recovery of the catalyst is easier and it can be reused. The major advantages of L-L-L PTC over normal PTC are (i) increased rate of reaction; (ii) easier catalyst recovery and reuse; (iii) high yields, purity, and selectivity toward desired products; (iv) low investment cost; (v) low energy consumption; (vi) minimization of industrial wastes; and (vii) the catalyst does not need to be bound to a solid support. Hence, the attendant difficulties of reduced activity due to interparticle diffusional resistance and mechanical strength can be avoided. This is major advantage of L-L-L over liquid-liquid-solid (L-L-S) PTC. However, the L-L-L PTC suffers from following disadvantages: (i) more catalyst (PTC) is required, which is expensive, and (ii) the method is not applicable for systems where a very high temperature is required to perform the reaction. As the temperature increases, the stability of the third liquid phase decreases. However, if the catalyst is stable, then at the end of the reaction, it could be easily separated into a third phase at lower temperature for recovery and reuse. Selective reduction of nitro compounds that contain multifunctional groups to the corresponding amino derivatives, which are commercially important, has been a major challenge in organic synthesis.19-23 Numerous techniques and reagents have been developed on the laboratory scale for the nitro reduction, and some of these have been scaled up; however, most of them still have limitations, in regard to safety and handling. The reduction of nitroxylenes and other substituted nitroaromatics to the corresponding amines can be affected by aqueous inorganic sulfides and polysulfides, and the rates of reductions are amenable to intensification under PTC. There are several reports on phase-transfer-catalyzed sulfide reductions in the literature.24-26 The reductions of o-chloronitrobenzene27 and

10.1021/ie060646l CCC: $37.00 © 2007 American Chemical Society Published on Web 08/29/2006

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p-chloronitrobenzene with sodium sulfide/disulfide have been performed using polar solvents such as dimethyl formamide (DMF) and N-methylpyrrolidone (NMP), with good yields of the corresponding products. Nonionic and ionic surfactants have also been used to catalyze the reaction of chloronitrobenzenes with alkali disufide to give dinitrodiphenyl disulfide.28 Uncatalyzed reductions of nitro-aromatics29 such as m-nitrochlorobenzene, m-dinitrobenzene, and p-nitroaniline by aqueous solutions of sodium monosulfide and disulfide have been reported, which have also been studied under biphasic PTC with sodium sulfide, with tetrabutylammonium bromide (TBAB) as the catalyst.30,31 Many aromatic amines exhibit biological activities and have a multitude of industrial applications, such as intermediates for the synthesis of dyes, pharmaceuticals, and agrochemicals. 3,4Dimethylaniline is one such important molecule, which is a precursor for the production of riboflavin (vitamin B2) and other drugs. It has been synthesized via several routes, which include those involving hydrogenation of 4-nitro-o-xylene (a) at 90100 °C with hydrogen at pressures >100 atm in a stirred autoclave, using a Raney nickel catalyst;32 (b) over homogeneous Ru or Fe catalyst complexes (e.g., RuCl2 (PPh3)3, Fe(Co)3 ((PPh3)2, etc., with at least 50% conversion and 80% amine selectivity);33 (c) with triethylsilane in the presence of Wilkinson’s catalyst;34 and (d) with ∼2.0 equiv sodium trimethylsilanethiolate in anhydrous 1,3-dimethyl-2-imidazolidione at 185 °C in a sealed tube for 24 h with 86% yields.35 It is also prepared from 3,4-xylenol with NH3 or an amine at 50-300 atm and 300-400 °C in the presence of a small amount of chloride or other salts of NH3 or amine36 and also by hydrogenating 2-chloromethyl-4-nitro-toluene over a Raney nickel catalyst in an alcohol medium in the presence of alkaline agents at 100-120 °C and pressures of >35 atm.37 3,4-Dimethyl aniline was synthesized via the reaction of sodium sulfonates of o-xylene with soda amide in liquid ammonia at >80 °C.38 3,4-Dimethyl benzoic acid is also converted to 3,4-dimethyl aniline via reaction with NH3 and CuO at 210-215 °C for 30 min with 50% yield and 95% selectivity.39 Except for hydrogenation using supported metals, all other routes are not industrially relevant. However, hydrogenation is energyintensive. The literature review suggests that there is still hope to develop simpler and better methods of preparation of 3,4dimethyl aniline, and the current work addresses this problem through the use of multiphase transfer catalysis. This paper delineates the novelties of the kinetics and mechanism of the L-L-L PTC reduction of 4-nitro-o-xylene to 3,4-dimethyl aniline and covers the determination of process parameters and the waste reduction strategy, which will be useful for industrial applications. 2. Experimental Section 2.1. Chemicals and Catalysts. 4-Nitro-o-xylene, sodium sulfide (flakes), and toluene (all of analytical reagent (AR) grade) were obtained from Merck, India, Ltd., Mumbai, India. TBAB was obtained as a gift sample from M/s Dishman Pharmaceuticals and Chemicals Ltd., Ahmedabad, India. 2.2. Experimental Procedure. The reactions were studied in a 5.0-cm-inner-diameter (id) fully baffled mechanically agitated contactor with a total capacity of 100 cm3, which was equipped with a six-blade turbine impeller. The reactor was kept in a constant temperature water bath. Typical runs were conducted by taking 0.1125 mol of sodium sulfide in water and adjusting the volume to 25 cm3. To this solution was added 0.005 mol of TBAB catalyst. The organic phase, which was comprised of 0.0075 mol of 4-nitro-o-xylene, was adjusted to

Figure 1. Photograph of the tri-liquid phase transfer catalysis (L-L-L PTC) at 80 °C. (The stirrer and other parts are removed to show clarity in a quiescent pool.)

a volume of 15 cm3 with toluene. All the typical reactions were performed at 80 °C and 1500 rpm. The said composition of the reaction mixture created three distinct and stable phases under the operating conditions (see Figure 1). The overall reaction is given below, and preliminary experiments have suggested that the locale of the reaction was the middle phase and both the top and bottom phases could be replaced without any difficulty, making repeated reuse of the catalyst possible.

In the L-L-L system, 3,4-dimethyl aniline, selectively, was the only product obtained. 2.3. Analysis and Isolation of Product. Samples were withdrawn periodically, and gas chromatography (GC) analyses were performed (Chemito Model 8610) using a stainless steel column (3.25 mm × 2 m) that was packed with a liquid stationary phase of 10% SE-30, which was used for flame ionization detection (FID) analysis. The quantification was done by calibration, using synthetic mixtures. Initial rates of the reactions were also evaluated. At the end of the reaction, the organic phase, which contained 3,4-dimethyl aniline and toluene, was washed with copious quantities of water, to remove any traces of catalyst and the solvent was distilled under vacuum to get a pure semisolid product. It was also confirmed by gas chromatography-mass spectroscopy (GC-MS). 3. Results and Discussions Several experiments were conducted 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 in the

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Figure 2. L-L-L PTC, showing the possible contributions to the overall reaction rate by three parallel reaction mechanisms.

range of 50-80 °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 2), which would slowly disintegrate, and the three phases would emerge, as shown in Figure 1. Even after the mixture was cooled to room temperature, the phases would retain their identity. 3.1. Overall Stoichiometry. The reduction reaction of nitroarenes by negative divalent sulfur (sulfide, sulfahydrate, and polysulfide) is called Zinin reduction. The overall stoichiometry of this reaction, using sulfide as the reducing agent, is as follows:

4C6H5NO2 + 6S2- + 7H2O f 4C6H5NH2 + 3S2O32- + 6OHObviously, this reaction involves a complex mechanism and the rate-determining step must be evaluated. 3.2. Mechanism of L-L-L PTC. Because the stoichiometry of the overall reaction suggests a complex mechanism, the following steps were envisaged.6 S is a multivalent species, and, thus, different monovalent anions, such as HS-, HSO-, HSO2-, and HSO3-, can be formed sequentially, which are capable of producing ion pairs with single quaternary cations more readily than those multivalent anions, which require ion pairing with multiple quaternary cations. In the aqueous phase, S2- reacts with OH- to form HS-. Thus, Q+HS-, which is readily formed in the aqueous phase, is transferred to the middle (third) phase, where several consecutive and parallel reactions occur, as shown in Figure 3. The contribution to the overall rate of the reaction can be shown by three mechanisms. Because the creation of a third catalyst-rich phase is known to enhance the rate of the reaction, it should be the main reaction phase into which the substrate from the organic phase and the nucleophile from the aqueous phase are transferred. However, the aqueous phase reagent, in the form of the ion pair, can also transfer across the third-phase/organic-phase interface and there is a possibility of some reaction occurring in the organic phase. Similarly, the reaction can occur near the interface between the third phase and the aqueous phase on the aqueous phase side, because of very limited solubility of the substrate in the aqueous phase.

However, one should note that the substrate would have already reacted in the third phase. Thus, the contribution of the reaction in the aqueous phase is negligible. Because the third phase is generated beyond the critical concentration of the catalyst, the locale of the PTC reaction that occurs in the organic phase in the biphasic condition is shifted to the third phase. Depending on the amount of catalyst beyond the critical value, the contribution to the overall rate of reaction would be either from totally by the reaction in the third phase and the organic phase. The thickness of the third phase also matters, because the reactive species must diffuse from the organic phase to the third phase as an ion pair. The mass-transfer coefficients (kL) for these two species are also different, as shown in Figure 4. Thus, the kL1 and kL2 values must be very high, such that there is no resistance to the transfer of the organic phase reagent from the third phase and that of the aqueous phase reagent from the aqueous phase into the middle phase. When the reagents are in the middle phases, the reaction should proceed smoothly. Depending on the relative rates of mass transfer of reaction, different mechanisms would prevail. Just below the critical concentration of third-phase formation, the biphasic reaction has its maximum conversion. In the absence of any mass-transfer limitations, the reaction is kinetically controlled and occurs in the organic phase. However, after the formation of the third phase, there is a likelihood of masstransfer resistance setting in, because of higher rates of reaction in the third phase, which depletes the concentration of the reactants in the third phase. The model of the reaction shows that the quaternary cation of the catalyst (Q+) pairs up with different anions for its reaction with the substrate; however, most of it remains in the form of Q+HS- in the reaction medium and makes a cycle of the catalyst in that form among the phases. 3.3. Kinetics of the Reaction. In the absence of any masstransfer resistance, where the kL1, kL2, kL3, and kL4 values are very high and the reaction is controlled by intrinsic kinetics, a model can be developed from first principles methodology. In the current case, the net rate of reaction of 4-nitro-o-xylene (A) is distributed between an organic phase and a third (middle) phase but not in the aqueous phase, because of its insolubility in that phase.

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Figure 3. Mechanism of reduction of nitro-aromatics to amino-aromatics under L-L-L PTC.

Figure 4. Various mass-transfer steps in L-L-L PTC: film model. The term δ represents the thickness of the liquid film.

Let NAi be the number of moles of A taken initially, NAOi the number of moles of A in the organic phase, and NAti the number of moles of A in the third phase. The distribution coefficient for A between the third phase and the organic phase (because A is practically absent in the aqueous phase) is

KA )

CAt C AO

)

N At V O N At ) NAO Vt NAOβ

(1)

where

β)

Vt VO

(2)

NAt ) NAOt + KANAOt

(3)

NAt ) NAOt(1 + NAOt)

(4)

and NQi is the number of moles of catalyst added to the system

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at t ) 0. The catalyst is distributed in all of the phases as ion pairs. The ion pairs are Q+SH- (B) and Q+HSO3- (C).

NQi ) CBa + CBo + CCa + CCo + CCt

The total rate of reaction of A is the summation of that in the third phase and in the organic phase:

(5)

-

dNAt 1 ) rate of reaction in the third phase dt Vt

The various distribution coefficients for the catalyst can be defined as follows. KtBa is defined as the distribution coefficient of B between the third phase and the aqueous phase:

KtBa )

CBt CBa

)

NBt Va NBa Vt

)

) KtCAtCBt ) Kt

NBt

(6)

NBaR

-

Let

-

Vt )R Va

(19)

NAt NBt

(20)

Vt Vt

dNAo 1 NAo NBo ) Kt dt Vo Vo Vo

(21)

NAtNBt NAoNBt dNA ) Kt + Ko dt Vt Vt

Vt )β Vo

(22)

)

1 [K N N + KoNAtNBtβ] Vt t At Bt

)

NQi - NCtφC KoβNAo NBt 1 B KtKAβNAo + Vt φB Vt Ktoβ

and

then,

(18)

[

(23)

(

)

B

]

(24)

where

Va β ) Vo R KtBo )

CBt CBo

)

NBt Vo NBt ) NBo Vt NBoβ

to R NBa KB ao ) ) ) KB ) CBo NBo Va β NBo Kta

CBa

NBa Vo

(7)

φC ) 1 +

1 1 + t ta KCR KCoβ

(25)

(8)

φB ) 1 +

1 1 + t ta KBR KBoβ

(26)

-

(9)

NAtNBt NAoNBo dNA ) Kt + Ko dt Vt Vo

(27)

B

KtCa

)

KtCa ) KtCo )

But,

NCt

(10)

NCaR

NBo )

NCt

(11) -

NCt

)

KtBa R

+

[

NBt

(12)

NCoβ

)

KtCo

KtBoβ

) NBt 1 +

+ NBt +

KtBoR

+

KtNAtNBt Vt

+

KtNAoNBt KtBoVt

(30)

Now,

(14) NCt

KtBoβ

+ NC t

] [

1 1 1 1 + t + NC t 1 + t + t ta o o KBR KBβ KBR KBoβ

(15)

]

(16)

Therefore,

N Bt )

(29)

(13)

KtCa

NCt

N At NAo NBt Vo dNA ) Kt NBt + Ko dt Vt Vo K t o Vt B

NQi ) NBa + NBo + NBt + NCo + NCt NB t

(28)

B

NCoR

KaCo )

NB t V t K t o Vo

Vt NAt ) KAβNAo ) KA NAo Vo Therefore,

-

koNAo NBt dNA ktKA ) NAoNBt + t dt Vo K o Vt )

NQt - NCt{1 + [1/(KtCa R)] + [1/(KtC0β)]} 1 + [1/(KtBa R)] + [1/(KtBoR)]

(17)

(31)

(

)

(32)

B

ko ktKA + t NAoNBt Vo K oV

(33)

B t

where NA is the total moles of A and NAo is the number of moles of A in the organic phase.

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NA ) NAo (1 + KAβ) Therefore,

-

( ) (

)

dNA ko dNA ktKA ) + t NAoNBt ) (1 + KAβ) dt dt Vo KBoVt (34)

Thus,

-

dNAo dt

)

(

)

ko ktKA 1 + t NAoNBt V (1 + KAβ) o KBoVt

(35)

Now,

N Bt )

NQi - NCt{1 + [1/(KtCa R)] + [1/(KtCoβ)]} 1 + [1/(KtCa R)] + [1/(KtCoβ)]

(36)

The previously described equation can be integrated under a certain set of conditions. It is necessary to have a priori knowledge of the various equilibrium constants and the phase ratios R and β. In the present case, R and β were measured experimental values. An order-of-magnitude analysis shows the following:

KtBa . 1

(37a)

KtBo . 1

(37b)

Figure 5. Comparison of L-L PTC versus L-L-L PTC reduction of 4-nitro-o-xylene to 3,4-dimethylaniline. Conditions: 4-nitro-o-xylene concentration, 0.0075 mol; sodium sulfide concentration, 0.1125 mol; TBAB concentration, 0.002 mol for L-L) and 0.005 mol for L-L-L; toluene volume, 15 cm3; water volume, 25 cm3; and temperature, 80 °C.

For a fixed value of NQi (the catalyst added to the system), the equation can be integrated to the following form:

-ln(1 - XA) )

kt N t V t Qi

) k1t

Therefore,

φB≈ ≈ 1

(38)

KtCa , 1

(39a)

KtCo , 1

(39b)

Thus,

φC ≈

1 1 + KtCa R KtCoβ

(40)

Therefore,

NBt≈ ≈ NQi -

NC t

(41)

φC

Now, because φC is very high, we can neglect second term of the equation. Therefore,

N B t ≈ N Qi dNAo dt

)

(

(42)

)

ko ktKA 1 + t NAoNBt (1 + KAβ) Vo KBo Vt

(43)

Now, KA ≈ 100 or, thus, the contribution of the organic phase reaction is negligible. The aforementioned equation can be reduced to

-

dNA kt ) NAoNQi dt Vt

(44)

(45) (46)

where k1 is a pseudo first-order rate constant:

k1 )

and

-

()

()

kt N Vt Qi

4. Validation of the Model The previously discussed theory was tested against experimental data. Thus, the effect of various parameters on the rate of reaction was studied in detail. 4.1. Comparison of L-L versus L-L-L. 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 conversion of 78% in 6 h at 80 °C. In contrast, with the creation of the third phase, either by further addition of sodium sulfide and catalyst beyond the critical value, the conversion increased to 95.8% conversion within 1 h and the selectivity to 3,4-dimethyl aniline was 100%. Thus, there is an 6-fold increase in productivity of the same reactor with total selectivity. This third phase was reusable many times, as will be discussed later. The comparative plots show the observed trend (see Figure 5). Thus, further experiments were performed in L-L-L PTC. 4.2. 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 1000-1800 rpm, under otherwise similar conditions, in the presence of TBAB catalyst. The conversion is plotted as a function of time in Figure 6. This observation implies that there was no resistance to external mass transfer beyond 1500 rpm. Therefore, further experiments were conducted at 1500 rpm.

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Figure 6. Effect of the speed of agitation on conversion: (0) 1000 rpm, (9) 1200 rpm, ([) 1500 rpm, and (O) 1800 rpm. Conditions: 4-nitro-oxylene concentration, 0.0075 mol; sodium sulfide concentration, 0.1125 mol; TBAB concentration, 0.005 mol; toluene phase volume, 15 cm3; water volume, 25 cm3; and temperature, 80 °C.

Figure 8. Plot of initial rate versus catalyst loading.

Figure 9. Kinetic plot of -ln(1 - XA) versus time at different catalyst quantities taken initially: ([) 0.0015 mol, (2) 0.0025 mol, (0) 0.005 mol, and (O) 0.0075 mol.

Figure 7. Effect of catalyst loading on conversion: ([) 0.0015 mol, (2) 0.0025 mol, (0) 0.005 mol, and (O) 0.0075 mol. Conditions: 4-nitro-oxylene concentration, 0.0075 mol; sodium sulfide concentration, 0.1125 mol; toluene phase volume, 15 cm3; water volume, 25 cm3; speed of agitation, 1500 rpm; and temperature, 80 °C .

4.3. Effect of Catalyst Loading. Because the catalyst concentration is the most important parameter in L-L-L PTC, its effect was studied systematically. The catalyst concentration was varied over a range of 0.0015-0.0075 mol, under otherwise similar conditions. It was observed that the formation of the third phase occurs only after a certain critical amount of catalyst was added to the reaction mixture. In the present case, the formation of the third phase occurs at the catalyst concentration of 0.005 mol TBAB. At any catalyst concentration below this value, the third phase disappears and the system becomes an L-L PTC system. Figure 7 shows the percentage conversion, as a function of time, at different catalyst concentrations. Formation of the third phase was observed only at higher catalyst concentrations, i.e., at a concentration of 0.005 mol; at any other quantity of catalyst below this value, the third phase disappeared

and the system would convert itself into an L-L PTC system and the conversions would decrease. When the catalyst concentration increased from 0.0015 mol to 0.0075 mol, the rate of reaction increased substantially. The initial rate of reaction is plotted against time in Figure 8; this figure shows that the rate increases very sharply under L-L-L conditions, and it shows that there is no dependence of the rate of reaction on catalyst concentration, which is characteristic of L-L PTC. According to the theory, plots of -ln(1 - XA) versus time were made with catalyst loading as a parameter (Figure 9). The lines pass through the origin, confirming the first-order dependence of the rate on catalyst loading. Furthermore, it also shows that the catalyst is mainly in the third phase. The slopes of the lines were plotted against catalyst loading NQi (see Figure 10) to obtain the rate constant: kt ) 13.42 s-1. 4.4. Effect of Phase Volume Ratio (Aqueous:Organic). The effect of the volume ratio of aqueous phase to organic phase on the conversion of 4-nitro-o-xylene was studied, keeping the concentration of both the reactants constant in both phases. The chosen volume ratios of aqueous phase to organic phase were 1:1, 3:5, and 5:3. The concentration of 4-nitro-o-xylene in the organic phase was kept at 0.0075 mol and the concentration of

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Figure 10. Plot of the pseudo-first-order rate constant versus catalyst quantity.

Figure 11. Effect of phase volume ratio (aqueous:organic) on conversion, for aqueous:organic phase ratios of ([) 1:1, (9) 5:3, and (O) 3:5. Conditions: 4-nitro-o-xylene concentration, 0.0075 mol; sodium sulfide concentration, 0.1125 mol; speed of agitation, 1500 rpm; and temperature, 80 °C.

sodium sulfide in the aqueous phase was 0.1125 mol. In the aforementioned experiments, there was a marginal but critical change in the volume of the third phase. The best results were obtained with a standard aqueous:organic phase ratio of 5:3, which was chosen for all reactions, as shown in Figure 11. 4.5. Effect of Sodium Sulfide Concentration. The sodium sulfide loading concentration was varied over a range of 0.0375-0.125 mol under similar reaction conditions. It was found (Figure 12) that, as the sodium sulfide concentration increased, the rate of the reaction increased. There is a greater concentration of [Q+HS-], which gets immediately extracted. When the sodium sulfide concentration is much less, the volume of the third phase is also less, because the aqueous phase is not saturated with sodium sulfide and there is no salting out of TBAB. However, as the sodium sulfide concentration is increased, it becomes saturated and more and more catalyst is pushed into the catalyst phase. After a certain concentration, the formation of HS- remains constant and, therefore, no further increases in the rate of reaction is observed. This is consistent with the theory.

Figure 12. Effect of sodium sulfide concentration on conversion: ([) 0.0375 mol, (9) 0.0725 mol, (2) 0.1125 mol, and (O) 0.125 mol. Conditions: 4-nitro-o-xylene concentration, 0.0075 mol; sodium sulfide concentration, 0.1125 mol; toluene volume, 15 cm3; water volume, 25 cm3; speed of agitation, 1500 rpm; and temperature, 80 °C.

Figure 13. Effect of 4-nitro-o-xylene concentration on conversion: ([) 0.0025 mol, (0) 0.0037 mol, (2) 0.0075 mol, and (+) 0.015 mol. Conditions: sodium sulfide concentration, 0.1125 mol; toluene volume, 15 cm3; water volume, 25 cm3; speed of agitation, 1500 rpm; and temperature, 80 °C.

4.6. Effect of 4-Nitro-o-xylene Concentration. The effect of varying the 4-nitro-o-xylene concentration was studied using concentrations in the range of 0.0025-0.015 mol, under the same conditions, as shown in Figure 13. It is observed that, as the concentration increases, the rate of reaction increases, except for the 0.015 mol concentration, for which the rate decreases, which is due to less availability/concentration of the HS- ion. A plot of -ln(1 - XA) against time is shown at various 4-nitroo-xylene concentrations in Figure 14, to validate the first-order rate. 4.7. Effect of Temperature. The effect of temperature was studied at 50, 60, 70, 80, and 90 °C (Figure 15). It was observed that, at 50 °C, the reaction rate was slow. As the temperature increased, the reaction rate intensified, as expected. The three phases were maintained at all temperatures. 4.8. Reusability of Catalyst. After completion of the kinetic run, the phases reappeared. When the phases were clearly

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Figure 14. Kinetic plot of -ln(1 - XA) versus time at different concentrations of 4-nitro-o-xylene: ([) 0.0015 mol, (2) 0.0025 mol, (0) 0.005 mol, and (O) 0.0075 mol.

Figure 15. Effect of temperature on conversion: (2) 50 °C, ([) 60 °C, (+) 70 °C, (O) 80 °C, and (0) 90 °C. Conditions: 4-nitro-o-xylene concentration, 0.0075 mol; sodium sulfide concentration, 0.1125 mol; toluene volume, 15 cm3; water volume, 25 cm3; and speed of agitation, 1500 rpm.

separated, the organic phase that contained the product was removed. The reusability studies can be conducted in two ways, either using the catalyst-rich phase alone or using the catalystrich phase and the aqueous phase together. 4.8.1. Method A: Reuse of the Third Phase Only. The third phase separated from the previous experiment and was reused by adding fresh aqueous and organic phase reactants. In this method, the third phase was used three times, starting with the fresh use. As explained previously, there is a distribution of catalyst in the catalyst-rich third phase, as well as in the organic phase and the aqueous phase. Because both the aqueous and organic phases were freshly used, there was a loss of catalyst with the discarded organic and aqueous phases. Thus, there is, once again, a 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 the reaction. The subsequent experiment with replenished aqueous and organic phases had,

Figure 16. Effect of reusability of the catalyst phase on conversion: (+) fresh, (9) first use, (2) second use, and ([) third use. Conditions: 4-nitroo-xylene concentration, 0.0075 mol; sodium sulfide concentration, 0.1125 mol; toluene volume, 15 cm3; water volume, 25 cm3; speed of agitation, 1500 rpm; and temperature, 80 °C.

Figure 17. Effect of reusability of the catalyst phase and aqueous phase on conversion: (O) fresh, (0) first use, (2) second use, (+) third use, (9) fourth use, and ([) fifth use. Conditions: 4-nitro-o-xylene concentration, 0.0075 mol; toluene volume, 15 cm3; speed of agitation, 1500 rpm; and temperature, 80 °C.

in fact, less available catalyst than the previous run and, hence, the rates of reaction decreased correspondingly (see Figure 16). 4.8.2. Method B: Reuse of the Catalyst and the Aqueous Phase. Fresh organic solvent was added, along with a known quantity of 4-nitro-o-xylene, for which the third phase as well as the aqueous phase from the previous experiment was used. In this method, the catalyst and the aqueous phase were reused up to five times, and the organic phase was replenished every time. Figure 17 shows the conversion after 1 h in each of these cases. The top organic phase that contained the product with an insignificant quantity of catalyst was removed, because the catalyst in the third phase (the middle phase), as well as in the bottom aqueous phase, was primarily in the form of an ion pair, [Q+SH-]. Therefore, when the second run is conducted with

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can be elegantly modified to convert it to tri-liquid (L-L-L) PTC, to recover and reuse the catalyst and also to enhance selectivity, thereby improving profitability and environmental benefits. This work has addressed the novelties of the kinetics and mechanism of the synthesis of 3,4-dimethyl aniline from the reduction of 4-nitro-o-xylene under L-L-L PTC. A detailed kinetic study has been conducted for the aforementioned system. The effects of different parameters such as speed of agitation, temperature, catalyst loading, reusability of the catalyst-rich phase, phase volume ratio, and mole ratio were studied. The catalyst can be reused, and the various rate constants and equilibrium constants could be determined. It was observed that the reaction rates were intensified in the L-L-L PTC, vis-a`vis, the L-L PTC. It leads to 100% selectivity of 3,4dimethylaniline. Acknowledgment Figure 18. Validity of the kinetic model at various temperatures: (2) 50 °C, ([) 60 °C, (+) 70 °C, (O) 80 °C, and (0) 90 °C. Conditions: 4-nitroo-xylene concentration, 0.0075 mol; sodium sulfide concentration, 0.1125 mol; toluene volume, 15 cm3; water volume, 25 cm3, and speed of agitation, 1500 rpm.

It has been a privilege and joy to be associated with Professor M. M. Sharma, first as a doctoral student and later as a colleague. He was instrumental in introducing me to the wonderful world of PTC in 1975. We were the first pair of chemical engineers contributing to PTC. The mechanism called the Yadav-Sharma mechanism of S-L PTC is well-known for homogeneous solubilization, and we had the first commercial success with my doctoral work. I dedicate this paper to him on the occasion of his 70th birthday (G.D.Y.). Nomenclature

Figure 19. Arrhenius plot for L-L-L PTC.

0.0075 g mol/cm3 of 4-nitro-o-xylene, almost all of the catalyst is available for reaction and, therefore, the loss in conversion is marginal. 4.9. Activation Energy. As discussed in section 4.7, the effect of temperature was studied and plots of -ln(1 - XA) against time were made to validate the first-order rate (Figure 18). The calculated rate constants were used to make an Arrhenius plot (Figure 19) from which an activation energy of 6.9 kcal/mol was obtained, which also suggests that there was no masstransfer resistance for the transfer of reactants to the middle phase and products into the respective organic and aqueous phases. 5. Conclusion The bi-liquid (L-L) phase transfer catalysis (PTC) reactions are conducted under milder conditions, using less-expensive solvents at much faster reaction rates and improved selectivities to desired products and, thus, by definition, are waste reduction processes. However, the phase transfer catalyst is not recovered but treated as waste, because the quantities are very small and do not contribute much to the expensive product. L-L PTC

CAO ) feed concentration of 4-nitro-o-xylene in the organic phase, mol/cm3 CA-O ) concentration of 4-nitro-o-xylene in the organic phase, mol/cm3 CA-t ) concentration of 4-nitro-o-xylene in the third phase, mol/ cm3 KA ) distribution coefficient of 4-nitro-o-xylene between the organic phase and the third phase NAOi ) number of moles of 4-nitro-o-xylene in the organic phase initially NQi ) feed mole of catalyst added to the system initially, mol NAt ) number of moles of 4-nitro-o-xylene in the third phase NCt ) number of moles of C(QHSO3-) in the third phase NBt ) number of moles of B(QHS-) in the third phase KtBa ) distribution coefficient of B between the third phase and the aqueous phase KtBo ) distribution coefficient of B between the third phase and the organic phase KaBo ) distribution coefficient of B between the aqueous phase and the organic phase KtCa ) distribution coefficient of C between the third phase and the aqueous phase KtCo ) distribution coefficient of C between the third phase and the organic phase KaCo ) distribution coefficient of C between the aqueous phase and the organic phase Kt ) rate constant of the third phase KO ) rate constant of the reaction of the organic phase t ) time of reaction (s) VO ) volume of organic phase (cm3) Vt ) volume of third phase (cm3) Literature Cited (1) Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis, 3rd Edition; VCH: New York, 1993.

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ReceiVed for reView May 23, 2006 ReVised manuscript receiVed July 22, 2006 Accepted July 25, 2006 IE060646L