Ind. Eng. Chem. Res. 2008, 47, 9081–9089
9081
Ionic Liquid as Catalyst for Solid-Liquid Phase Transfer Catalyzed Synthesis of p-Nitrodiphenyl Ether Ganapati D. Yadav* and Bhavana G. Motirale Department of Chemical Engineering, UniVersity Institute of Chemical Technology (UICT), UniVersity of Mumbai, Matunga, Mumbai-400 019, India
The novelties of solid-liquid phase transfer catalyzed synthesis of p-nitrodiphenyl ether from pnitrochlorobenzene and potassium phenoxide were studied in detail with ionic liquids as phase transfer catalysts among others. Tetradecyl(trihexyl)phosphonium bromide was found to be the best catalyst leading to 100% selectivity toward the desired product p-nitrodiphenyl ether. Ionic liquids offer excellent conversions and selectivity, stability at high temperatures, and reusability in this reaction. A mathematical model was developed to study the kinetics of the reaction and used to extract both the rate constant and ion-exchange equilibrium constant. The contribution of the uncatalyzed reaction was also considered in this model. The activation energy and Gibb’s free energy for a combined ion exchange were also calculated. Microwave irradiation was also employed as an effective alternative to conventional heating. 1. Introduction Ionic liquids have several potential benefits as solvents, catalysts, and electrolytes,1-3 in petrochemicals, heavy chemicals, fine chemicals, agrochemicals, pharmaceuticals,4 and nuclear industries.5 Several reports and reviews have appeared on ionic liquids delineating a gamut of properties and uses.6-9 Ionic liquids have successfully been studied in several reactions such as Friedel-Crafts,10 Diels-Alder,11 Suzuki coupling,12 Heck coupling,13-15 chlorination,16 polymerization,17 cracking,18 oxidation,19 and hydrogenation.20 Ionic liquids coupled with phase transfer catalysis (PTC) are currently used in several fields like electrochemistry21 and liquid crystals.22 Enzymatic catalysis is also studied in ionic liquids.23,24 Ionic liquids enable good control of product distribution,25 enhanced rates,26,27 ease of product recovery,28 catalyst recycling, and immobilization.29,30 In the current work, the novelties of ionic liquids as phase transfer catalysts are examined in the synthesis of p-nitrodiphenyl ether. p-Nitrodiphenyl ether (p-NDPE) is a key intermediate for the synthesis of herbicides, antihelmintic drugs,31 local anesthetics,32 fungicides,33 bactericides,34 and polyimide end-capping agents.35 The commonly used process for the synthesis of p-NDPE involves refluxing of potassium phenoxide with p-chloronitrobenzene (p-NCB) in chlorobenzene with copper powder as a catalyst.36 A solventless synthesis of p-nitrodiphenyl ethers by aromatic nucleophilic substitution reaction of p-NCB with different phenoxides has been reported.37 Mil’to et al.38,39 have studied the kinetics of p-NCB reactions with substituted phenolates. PTC has been employed for the preparation of phenolic ethers.40 The preparative synthesis of nitroaryl ethers is achieved by the in situ generation of phenoxides from corresponding phenols and alkaline agents using PTC.41,42 p-NCB in chlorobenzene as solvent was reacted with aqueous potassium phenoxide with tetrabutyl ammonium bromide (TBAB) as catalyst using liquid-liquid (L-L) PTC. However, the rates of L-L PTC processes were lower, and p-nitrophenol was also produced as a byproduct since hydrolysis occurs prominently * Author to whom correspondence should be addressed. E-mail:
[email protected];
[email protected]. Tel.: +91-22-2414-5616. Fax: +91-22-2414-5614. Telefax: +91-22-410 2121.
under the basic conditions. CYPHOS are a new class of ionic liquids (IL) based on quaternary phosphonium salts introduced by Cytec Industries, Canada.43 The compositions of CYPHOS ILs are comprised of tetraalkyl phosphonium cations pairing with anions such as halides and hexafluorophosphate. Tundo et al. have carried out nucleophilic substitution reactions in ionic liquids, as many of the ionic liquids used fulfill all the criteria required as phase transfer catalysts.44,45 p-NDPE is synthesized by cumbersome methods, which can be replaced by solid-liquid (S-L) PTC most effectively, and it is the subject matter of this paper. The advantages of using S-L PTC include intensification of rates, enhanced selectivity, and suppression of hydration of reactant which normally happens in L-L PTC. Many halogen-containing aromatics lead to corresponding phenols and ethers in L-L PTC due to the presence of water, which is suppressed in S-L PTC. In this work, p-NDPE was synthesized by the reaction of solid potassium phenolate with p-NCB in toluene with quaternary phosphonium ionic liquids as catalyst. A kinetic model is also developed, by which both the reaction rate constant and the equilibrium constant are determined simultaneously. The model also accounts for an uncatalyzed reaction which cannot totally be discounted at very high temperatures. 2. Experimental Section 2.1. Materials. All chemicals and solvents used in this study were commercially available as analytical reagents and used without further purification. p-Nitrochlorobenzene (p-NCB), o-nitrochlorobenzene (o-NCB), m-nitrochlorobenzene (m-NCB), and p-nitrodiphenyl ether (p-NDPE) were obtained from Fluka Chemicals, Buch, Switzerland. Phenol and PEG 400 were procured from M/s Merck India Ltd., Mumbai. India. Potassium hydroxide (AR grade) was purchased from s. d. Fine Chemicals, Mumbai. Ethyl triphenyl phosphonium bromide (ETPPB) and tetrabutyl ammonium bromide (TBAB) were obtained as a gift sample from M/s Dishman Pharmaceuticals and Chemicals, Ahmadabad, India. Phosphonium-based ionic liquids called CYPHOS IL were supplied by Cytec Industries, Canada. 2.2. Experimental Procedure. The reaction was carried out in a Parr autoclave of 100 cm3 capacity with an internal diameter of 0.05 m. A four-bladed pitched turbine impeller was employed
10.1021/ie800340j CCC: $40.75 2008 American Chemical Society Published on Web 10/09/2008
9082 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Scheme 1. Reaction Scheme for the Synthesis of p-Nitrodiphenyl Ether
for agitation. The temperature was maintained at (1 °C of the desired value with the help of an in-built PID controller. In a typical experiment, 0.01 mol of potassium phenolate, 0.01 mol of p-NCB, and 3.3 × 10-4 mol of catalyst were charged to the autoclave at 150 °C. Toluene was used as a solvent to make the volume of organic phase 50 cm3. An initial sample was withdrawn. Agitation was then commenced at 1200 rpm and the reaction conducted for 3 h. Samples were withdrawn periodically for analysis. 2.3. Analysis. Analysis was performed by GC (Chemito model 8510) by using a 4 m × 3.8 mm stainless steel column packed with 10% SE 30 on Chromosorb WHP, coupled with a flame ionization detector. The injector and detector temperatures were kept at 300 °C. The oven temperature was programmed from 130 °C up to 295 °C with a ramp rate of 20 °C/min. Nitrogen gas was used as a carrier gas at 30 cm3/min flow rate. Synthetic mixtures of the reactant and internal standard were used to calibrate the chromatograms and quantify the data. Upon completion of the reaction, the reaction mixture was washed with water to remove unreacted potassium phenoxide and then distilled under reduced pressure to separate the product, which was characterized by GC-MS. Both analyses showed that only p-NDPE was the sole product. 2.4. Isolation of Product. After completion of the reaction, the mixture was filtered to remove unreacted potassium salt of phenol and solid KCl. The filtrate contained the catalyst, p-CNB, and the product. The mass was washed with water to remove any disssolved salt. Toluene was recovered by distillation of the filtrate. Unreacted p-CNB and p-NDPE were separated by fractional distillation under vacuum. The remaining residue consisted of the catalyst. 3. Results and Discussion 3.1. Kinetics and Mechanism. The overall reaction is as shown below kt
RX(org) + MY(s) 98 RY(org) + MX(s)
(1)
where RX, MY, RY, and MX represent p-CNB, potassium phenolate, p-NDPE, and KCl, respectively. 3.2.1. Uncatalyzed Reaction. Preliminary experiments suggested that at high temperature the solubility of the solid reactant potassium phenolate (MY) increases substantially and can contribute to the overall reaction up to 10% at low catalyst concentration over the entire period. The contribution of the uncatalyzed reaction cannot be neglected and hence is accounted for. MY is sparingly soluble in the organic phase, and the organic phase gets saturated with it. The dissolved MY thus reacts with RX in the organic phase in the absence of any catalyst. K2
{\} M+Y(org) M+Y(s)
(2)
kuc
RX(org) + MY(org) 98 RY(org) + MX(org)
(3)
The coproduct salt MX precipitates out. It has a finite solubility in the organic phase. Thus 1 ⁄ K4
{\} M+X(s) M+X(org)
(4)
The corresponding equilibrium constants are given by K2 ) K4 )
[M+Y-]org [M+Y-]s [M+X-]org [M+X-]s
(5)
(6)
3.2.2. S-L PTC Mechanism. In S-L PTC, the reaction takes place in anhydrous conditions, and both solid and liquid phases are dry. The first step in the reaction involves the transport of a reactant anion from the solid phase to the organic phase by a phase transfer cation. This is an organophilic cation which exchanges the anion with the phenate to form the ionpair. This is organophilic and freely transported within the organic phase. There could be a resistance associated with the transfer of this ion-pair across the film surrounding the solid particle. The second step involves the reaction of the transferred anion with the reactant located in the organic phase. The reactant anion must be in the active form. Finally, the third step involves the transport of the product anion by the phase transfer cation to the solid phase and the transport of another reactant into the organic phase. The overall reaction is presented in Scheme 1. Potassium phenoxide [M+Y-] is the solid suspended in the organic phase. First, the cation of the ionic liquid [Q+] diffuses to the solid surface [M+Y-] through the film surrounding the particle, which is analogous to the typical S-L PTC mechanism using quaternary onium salts. The ion-exchange reaction can occur either in the liquid film surrounding the particle or outside the film in the bulk liquid phase (both are examples of homogeneous solubilization). Another possibility is that the exchange takes place on the surface of the particle (heterogeneous solubilization for sparingly soluble particles). The second step in the reaction involves the transport of a reactant anion (nucleophile, Y-) by the cationic portion of the ionic liquid [Q+] to the bulk phase as an ion pair [Q+Y-]. There could be a resistance associated with the transfer of this ion pair across the liquid film next to solid-liquid interface. The solid particles of the reactant provide a very large surface area per unit volume of the organic phase. The reaction can occur on the surface of the particle if it is poorly soluble in the liquid, or they could dissolve in the liquid due to ion-exchange reaction. The third step involves the reaction of the [Q+Y-] with the reactant (RX) located in the organic phase. There are several possibilities by which this reaction can occur. Finally, the fourth step involves the transport of the coproduct anion [X-], the leaving group,
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9083
by the phase transfer cation to the solid and the transport of another nucleophile [Y-] into the organic phase. The solid reactant is in equilibrium with its solution in the organic phase as given by eq 2. The nucleophile is in the form of a metal salt which is transferred to the organic phase where it also forms an ion pair with the quaternary salt. The catalyst [Q+X-] reacts with the dissolved part of the solid reactant in the organic phase and is represented in the scheme below. K7
[Q+X-]org + [M+Y-]org {\} [Q+Y-]org + [M+X-]org
K7 )
K7 )
(7)
(9)
[Q+X-]org · K2[M+Y-]s
( )
-
[Q Y ]org ) Ke[Q X ]org
(10)
[M Y ]s [M+X-]s
( ) K2K7 K4
R ) Ke (11)
(12)
The initial quantity of catalyst Q0 ) [Q+Y-]org + [Q-X-]org
[M+Y-]s +
(14)
The net rate of reaction of RX in the organic phase is given by two parallel reactionssuncatalyzed (uc) and phase transfer catalyzed (PTC) as shown below
[
-d[RX]org rnet ) runcat + rPTC ) dt * ) kuc[RX]org[M+Y-]org + kPTC[RX]org[Q+Y-]org(15)
+
[M Y ]org ) [M
* Y-]org + [Q+Y-]org
(16)
+
[M
* Y-]org ) [M+Y-]org - [Q+Y-]org
) K2[M+Y-]s - [Q+Y-]org
+
+
-
[M X ]s
-
[Q0 - [Q+Y-]org] -
[Q Y ]org ) [Q0 - [Q Y ]org] * R [Q+Y-]org ) R ) Ke
[Q0]R 1+R
[M+Y-]s +
-
[M X ]s
[
kucK2(1 - XA)(M - XA) + (kPTC - kuc)[Q0]Ke
] ]
(M - XA) (1 - XA) XA (28) [RX]0 (M - XA) 1 + Ke XA
k1 ) (kPTC - kuc)[Q0]Ke[RX]0
(17)
(18) (19) (20) (21)
[
kucK2 (kPTC - kuc)[Q0]Ke
(30)
]
(31)
On separation of variables, the following equation is obtained
∫
XA
+
(26)
(29)
k2 ) [M+Y-]s
(M - XA) XA
where
From eqs 11 and 12 [Q+Y-]org ) Ke
) Ke
(M - XA) (1 - XA) XA )k1 k2(1 - XA)(M - XA) + (M - XA) [RX]0 1 + Ke XA
The total quantity of MY in the organic phase is in two forms -
[Q0]R (25) 1+R
dXA ) kucK2[RX]0(1 - XA)(M - XA) + (kPTC - kuc) × dt (M - XA) [Q0]Ke XA (27) (1 - XA) (M - XA) 1 + Ke XA
)(kPTC - kuc)[Q0]Ke[RX]0 [Q+X-]org ) [Q0 - [Q+Y-]org]
-
[M X ]s
(13)
Thus
(24)
From eq 21
-
where
+
(23)
dXA ) kucK2[RX]20(1 - XA)(M - XA) + dt
s
+
(22)
) fraction conversion of RX
NRX0
(kPTC - kuc)[RX]0(1 - XA)
K2K7 [M+Y-]s [Q+X-]org + K4 [M X ]
Ke )
NRX0 - NRX
) initial mole ratio
(kPTC - kuc)[RX]org[Q+Y-]org [RX]0
[Q+Y-]org · K4[M+X-]s
-
NRX0
-d[RX]org ) kuc[RX]org{K2[M+Y-]s - [Q+Y-]org} + dt kPTC[RX]org[Q+Y-]org
(8)
[Q+X-]org[M+Y-]org
+
+
XA )
NMY0
) kucK2[RX]org[M+Y-]s +
[Q+Y-]org[M+X-]org
[Q+Y-]org )
M)
0
[
dXA (M - XA) (1 - XA) XA k2(1 - XA)(M - XA) + [RX]0 (M - XA) 1 + Ke XA
]
) k1t
(32)
The solution of the above equation is given in the Appendix
9084 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
and is as follows
(
)
M - XA + M [cXA(1 - Ke) + cKeM + 1] C ln c(1 - Ke) [cKe(M) + 1] ) t (42) k1
-A ln(1 - XA) - B ln
(
)
The constants are defined in the equation. Since the constants a, b, c, A, B, and C contain kuc, kPTC, and Ke, either they should be known independently to fit the above equation or their values could be extracted by using polymath software. The uncatalyzed reaction follows a simple pseudofirst-order kinetics since the organic phase is always saturated with the solid potassium phenoxide. -d[RX]org * ) kuc[RX]org[M+Y-]org dt
(43)
which on integration leads to the following -ln(1 - XA) ) kuc[M+Y-]*t ) k′uct
(44)
kuc[M+Y-]* ) k′uc ) a pseudofirst-order constant
(45)
where
When the uncatalyzed reaction is insignificant, the overall rate of reaction is due to the PTC reaction as follows -d[RX]org ) kPTC[RX]org[Q+Y-]org dt
(46)
The above equation can be solved analytically as given in our earlier work46,47 to get the following
[(
(
M - XA M ln M-1 M
)
)] ( -
)
1 - Ke + KeM × M-1 ln(1 - XA) ) kPTCQ0t(47)
Eq 47 is manipulated to the following to extract both Ke and kPTC as follows
[
](
(M - XA) 1 - Ke + KeM M ) + ln(1 - XA) M (M - 1)kPTCQ0 t (48) M ln(1 - XA)
ln
[
)
]
[
]
A plot of ln( M - XA / M ) / ln(1 - XA) versus t / ln(1 XA) should give Slope )
)
[ (
] )
(M - 1)kPTCQ0 M 1 - Ke + KeM M
and
Intercept
(49)
Thus, both Ke and kr can be obtained from the slope and intercept from knowledge of M and Q0. Further, for equimolar quantities of substrate and nucleophile, the following form of the integrated equation results.
(
)
XA + (1 - Ke) ln(1 - XA) ) krPTCQ0t 1 - XA
For M ) 1 (50)
Eq 50 is also manipulated to the following
[ )] (
XA 1 - XA t + (1 - Ke) ) kPTCQ0 ln(1 - XA) ln(1 - XA)
[
]
(51)
A plot of ( XA / 1 - XA ) / ln(1 - XA) versus t / ln(1 XA) will give a straight line with Slope ) krQ0 and Intercept ) (1 - Ke). The above model was validated by studying the effects of various parameters on the conversion and rates of reaction of p-NCB. 3.3. Effect of Various Catalysts. First, it was thought desirable to study whether the reaction occurs without adding any catalyst. It is observed that even in the absence of PTC a conversion of about 5-10% occurs in a total reaction time of 3 h. This is probably because of a small but finite solubility of potassium phenoxide in the organic phase. The efficacy of various catalysts such as ethyl (trihexyl) phosphonium bromide (ETPPB), polyethylene glycol (PEG 400), tetradecyl (trihexyl) phosphonium chloride (CYPHOS IL 101), tetradecyl (trihexyl) phosphonium bromide (CYPHOS IL 102), and tetradecyl (trihexyl) phosphonium decanoate (CYPHOS IL103) is shown in Scheme 2. All reactions were carried out at 150 °C and at a speed of 1200 rpm under otherwise similar conditions without the influence of mass transfer resistance, which will be discussed later. The activities of the various catalysts based on initial rates and final conversions are in the following order (Figure 1): tetradecyl (trihexyl) phosphonium bromide (IL 102) (max.) > tetradecyl (trihexyl) phosphonium chloride (IL 101) > tetradecyl (trihexyl) phosphonium decanoate (IL 103) > ETPPB > PEG400 (least). The phosphonium salts are better than PEG400. The reactivity of the phosphonium-based ionic liquids differs because of the anion attached to the phosphonium cation. IL 102 is the tetradecyl (trihexyl) phosphonium cation paired with a bromide anion that has higher affinity toward the phenoxide anion and Scheme 2. CYPHOS Ionic Liquids Used as Phase Transfer Catalysts
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9085
Figure 1. Effect of different PTC and ionic liquid on percentage conversion of p-NCB. p-NCB, 0.01 mol; potassium phenate, 0.01 mol; toluene made up to 50 cm3; catalyst concentration, 6.66 × 10-6 mol/cm3; reaction temperature, 150 °C; speed of agitation, 1200 rpm. (, tetradecyl (trihexyl) phosphonium bromide (IL 102); b, tetradecyl (trihexyl) phosphonium chloride (IL 101); 2, tetradecyl (trihexyl) phosphonium decanoate (IL 103); ×, ETPPB; *, PEG 400; 9, no catalyst.
Figure 2. Effect of speed of agitation on conversion of p-NCB. p-NCB, 0.01 mol; potassium phenate, 0.01 mol; catalyst IL 103, 6.66 × 10-6 mol/ cm3; toluene made up to 50 cm3; reaction temperature, 150 °C; -, 800 rpm; 2, 1000 rpm; (, 1200 rpm.
thus increases its concentration in the organic phase which leads to the highest conversion and selectivity. Hence, further experiments were conducted with CYPHOS IL 102 as catalyst. The uncatalyzed reaction leads to a conversion of 9.6% after 3 h and hence was independently studied, which will be discussed later. 3.4. Effect of Speed of Agitation. The reaction was carried out at three different speeds of agitation from 800 to 1200 rpm with tetradecyl (trihexyl) phosphonium bromide as the catalyst. As shown in Figure 2, there was an increase in the conversion from 800 to 1000 rpm. The initial rate of reaction was the same at 1000 and 1200 rpm, but the final conversion at 1200 rpm was higher. A theoretical calculation was done on the results at 1200 rpm to find the rate of mass transfer, and it was compared with the observed rate. The rate of mass transfer was much higher than the observed rate of reaction. This indicated the absence of external mass transfer resistance beyond 1200 rpm. All subsequent reactions were carried out with 1200 rpm while assessing the effect of other variables on the rate of reaction. Further, this was confirmed by studying the effect of temperature which will be discussed later. 3.5. Effect of Catalyst Concentration. The concentration of the tetradecyl (trihexyl) phosphonium bromide was varied from 6.66 × 10-6 to 2.66 × 10-5 mol/cm3 under otherwise
Figure 3. Effect of catalyst concentration on conversion of p-NCB. p-NCB, 0.01 mol; potassium phenoxide, 0.01 mol; toluene made up to 50 cm3; catalyst, IL102; reaction temperature, 150 °C; speed of agitation, 1200 rpm. -, uncatalyzed; (, 6.66 × 10-6 mol/cm3; ×, 1.33 × 10-5 mol/cm3; +, 2 × 10-5 mol/cm3; 2, 2.66 × 10-5 mol/cm3.
Figure 4. Effect of mole ratio of p-NCB to potassium phenoxide on conversion of p-NCB. Mole ratio, p-NCB:potassium phenoxide; toluene made up to 50 cm3; CYPHOS IL102, 2 × 10-5 mol/cm3; reaction temperature, 150 °C; speed of agitation, 1200 rpm. Mole ratio: (, 1:1; +, 1:1.5; -, 1:2; 2, 1:2.
similar conditions (Figure 3). It was observed that the conversion increased with an increase in the amount of catalyst. This is typical of a PTC reaction in the absence of mass transfer resistance. 3.6. Effect of Mole Ratio. The effect of mole ratio of p-NCB to potassium phenoxide was studied from 1:1 to 1:2.5, keeping the total volume of the reaction mass constant made at 50 cm3 with toluene and tetradecyl (trihexyl) phosphonium bromide as the catalyst. The concentration of potassium phenoxide was varied from 4.4 × 10-4 to 1.1 × 10-3 mol/cm3. The catalyst concentration was kept at 2 × 10-5 mol/cm3 (Figure 4). There was a sharp increase in conversion from mole ratio of 1:1 to 1:2. This is due to the corresponding increase in the concentration of the ion pair in the organic phase. Not much difference was seen in the final conversion when mole ratio varied from 1:2 to 1:2.5 because the organic phase was almost saturated with [Q+Y-]. The mole ratio was kept at 1:2 for the remaining experiments. At a mole ratio of 1:2.5, the conversion obtained was 80% with 100% selectivity toward the ether.
9086 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Figure 5. PTC Reaction: effect of temperature on conversion of p-NCB. p-NCB, 0.01 mol; potassium phenoxide, 0.02 mol; toluene made up to 50 cm3; CYPHOS IL102, 2 × 10-5 mol/cm3; speed of agitation, 1200 rpm. (, 125 °C; 2,150 °C; -, 175 °C; +, 200 °C.
Figure 6. Uncatalyzed Reaction: effect of temperature on conversion of p-NCB. p-NCB, 0.01 mol; potassium phenoxide, 0.02 mol; toluene made up to 50 cm3; speed of agitation, 1200 rpm. -, 150 °C; 2, 175 °C; +, 200 °C.
Figure 8. Arrhenius plot for uncatalyzed reaction.
Figure 9. Model fitting for PTC reaction: plot of LHS of eq 42 versus time. p-NCB, 0.01 mol; potassium phenate, 0.02 mol; toluene made up to 50 cm3; CYPHOS IL102, 2 × 10-5 mol/cm3; speed of agitation, 1200 rpm. -, 150 °C; -175 °C, +, 200 °C. Table 1. Values of Various Constants Obtained by Nonlinear Regression to the Analytical Solution, Equation 46 temperature A B C c Ke k1
Figure 7. Model validation at different temperatures for uncatalyzed reaction. -, 150 °C; 2, 175 °C; +, 200 °C.
3.7. Effect of Temperature. The effect of temperature was studied from 125 to 200 °C with tetradecyl (trihexyl) phosphonium bromide as the catalyst (Figure 5). It was found that the conversion increased substantially with an increase in temperature, suggesting that the reaction is intrinsically kinetically controlled. Since the contribution of uncatalyzed reaction was significant, it was studied at different temperatures in the same range (Figure 6).
200 °C 10.91 -24. 56 1.95 10.62 0.263 3.86 × 10-3
175 °C 1.44 -5.53 10.41 4.81 0.261 6.95 × 10-4
150 °C 0.57 -3.48 8.99 0.089 0.060 8.0 × 10-5
3.8. Model Validation. The kinetic model uncatalyzed reaction (eq 44) was tested against the experimental data which shows an excellent fit (Figure 7). The Arrhenius plots were also made to obtain the activation energy as 10.35 kcal/mol (Figure 8). When the contribution of uncatalyzed reaction was neglected at 150 °C, there was a reasonable fit; however, the model was less accurate at higher temperatures. Hence, the experimental data for all catalyzed reactions were fitted by invoking eq 45 using nonlinear regression (Polymath 5.1 software). The rate constant for the uncatalyzed reaction was used in these calculations, and all other constants were established (Table 1 lists these constants). There was an excellent fit (Figure 9). The Arrhenius plot gives an activation energy of 9.9 kcal/ mol (Figure 10). This value suggests that there was no mass transfer resistance in the reaction and that the reaction was kinetically controlled. A further proof of the validity of the model was done by making a parity plot for the simulated fractional conversion versus experimental fractional conversion
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9087
Figure 10. Arrhenius plot for PTC reaction. p-NCB, 0.01 mol; potassium phenate, 0.02 mol; toluene made up to 50 cm3; CYPHOS IL102, 2 × 10-5 mol/cm3; reaction temperature, 150 °C; speed of agitation, 1200 rpm.
Figure 13. Catalyst reusability study. p-NCB, 0.01 mol; potassium phenoxide, 0.02 mol; reaction temperature, 200 °C; CYPHOS IL102, 2.67 × 10-05 mol/cm3; toluene made up to 50 cm3. 2, fresh catalyst; -, first reuse; (, second reuse.
Figure 11. Parity plot of simulated fractional conversion versus experimental fractional conversion of p-NCB. Figure 14. Comparison of reactivity of other isomers of chloronitrobenzenes. Potassium phenoxide, 0.02 mol; reaction temperature, 200 °C; CYPHOS IL102, 2.66 × 10-5 mol/cm3; toluene made up to 50 cm3. +, m-NCB; ×, p-NCB; 2, o-NCB.
Figure 12. Gibb’s free energy plot.
(Figure 11). There is a very good agreement. The Gibb’s free energy plot was made as depicted in Figure 12. The Gibb’s free energy was calculated as -1.6 kcal/mol. This value is a combination of different equilibrium constants K2, K3, and K4 and is reasonable. 3.9. Catalyst Reusability Study. In this study, the catalyst tetradecyl (trihexyl) phosphonium bromide was separated from the organic layer by differential vacuum distillation after the first run and directly used for the next run, without any further purification or addition of fresh catalyst (Figure 13). During differential vacuum distillation, toluene, p-NCB, and p-NDPE
Figure 15. Effect of microwave irradiation on the synthesis of p-nitro diphenyl ether. p-NCB, 0.01 mol; potassium phenoxide, 0.02 mol; reaction temperature, 120 °C; CYPHOS IL102, 2.66 × 10-5 mol/cm3; toluene made up to 50 cm3. 2, conventional heating; (, microwave heating.
were separated, and the catalyst along with KCl remained in the bottom. The remaining reaction mass was used for the consecutive runs. This contained KCl in the reaction mass. After the second run, the conversion was found to be less, suggesting there was some catalyst loss during the separation process. The catalyst could be used twice maintaining its original activity during recycling.
9088 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
3.10. Effect of Isomer. The reactivity of o-NCB and m-NCB was also studied. Since the reactivity of m-NCB was very low at 150 °C, the experiments were done at 200 °C. o-NCB proves to be the most reactive isomer among all three isomers. m-NCB shows the least reactivity as compared to p-NCB and o-NCB. It is postulated that the higher actiVity of the ortho deriVatiVe can be explained with reference to the role of the nitro group, which makes the chloro group more Vulnerable to attack by a bulky quaternary phosphonium phenoxide (Figure 14). 3.11. Effect of Microwave Irradiation. We have studied the effect of microwave irradiation on phase transfer catalyzed reactions. To find out the efficacy of microwave irradiation on S-L PTC reactions, which was brought out by us in some of our earlier work,48-50 the O-alkylation of p-NCB was also conducted with microwave irradiation using a commercial microwave reactor of CEM USA (Discover model). p-NDPE was the sole product (Figure 15). A comparison with conventional heating is also done. Microwave irradiation has many advantages such as low power input per unit volume of the reaction mixture and high reactivity of ion pairs in the bulk phase due to increased collision rates.
B.G.M. gratefully thanks University Grants Commission for the award of a senior research fellowship which enabled him to carry out this work. G.D.Y. acknowledges support of the Darbari Seth professor endowment. This paper is dedicated to Professor Arvind Varma on his sixtieth birthday. He has been a rare combination of academic excellence and administrative capabilities and a great friend. Appendix XA
0
[
b ) [RX]0Ke
(35)
c ) k2[RX]0
(36)
∫
XA
0
[aXA + b(M - XA)]dXA ) (1 - XA)(M - XA)[cXA + cKe(M - XA)] + 1] BdXA XA AdXA XA + 0 + 0 (1 - XA) (M - XA) CdXA XA ) k1t(37) 0 [cXA + cKe(M - XA)] + 1]
∫
∫
∫
A)
a + b(M - 1) (M - 1)[c + cKe(M - 1) + 1]
(38)
aM (1 - M)[cM + 1]
(39)
B) C)
[
(a - b)(1 + cKe) + bM(c(1 - Ke)) (40) [Mc(1 - Ke) - (1 + cKe)][c(1 - Ke) - (1 + cKe)]
The solution is
(
)
M - XA C × + M c(1 - Ke) [cXA(1 - Ke) + cKeM + 1] ln [cKe(M) + 1]
-A ln(1 - XA) - B ln
(
Dividing by k1, the final equation is
(
-A ln(1 - XA) - B ln
(
)]
) k1t(41)
)
M - XA + M
[cXA(1 - Ke) + cKeM + 1] C ln c(1 - Ke) [cKe(M) + 1]
)
k1 ) t
(42)
The left-hand side of eq 42 can be plotted against t to get a straight line passing through the origin provided the various constants are independently known.
Acknowledgment
∫
(34)
where a, b, and c are constants. Equation 33 needs to be solved by parts.
4. Conclusion The solid-liquid phase transfer catalyzed reaction of pnitrochlorobenzene and potassium phenate was studied by using various phase transfer catalysts among which the phosphonium ionic liquids are better. Tetradecyl (trihexyl) phosphonium bromide (CYPHOS IL 102) was found to be the best catalyst among the available CYPHOS IL series: it gives 100% selectivity toward the desired product. A mathematical model was developed to study the kinetics of the reaction, and it can be used to extract both the rate constant and ion-exchange equilibrium constant. The contribution of uncatalyzed reaction was also accounted for. A catalyst reusability study was successfully done. Microwave irradiation coupled with ionic liquid as PTC proves to be an efficient and rapid alternative process for the synthesis of p-nitro diphenyl ether.
a ) [RX]0
dXA (M - XA) (1 - XA) XA k2(1 - XA)(M - XA) + [RX]0 (M - XA) 1 + Ke XA
]
) k1t
(32)
Nomenclature [Q+Y-] ) Ion-pair of organophilic cation and phenate anion Q+ ) Cation of ionic liquid Q0 ) Total concentration of ionic liquid in organic phase, mol/ cm3 [Q+X-] ) Ion-pair of organophilic cation and leaving anion Ke ) Ion exchange equilibrium constant, dimensionless Kt ) Rate of reaction in the organic phase, cm3/(mol · s) kuc ) Rate constant for uncatalyzed reaction, cm3/(mol · s) kPTC ) Rate of PTC reaction in the organic phase, cm3/(mol · s) K1, K2, K4 ) Various equilibrium constants NRX0 ) Initial mole of RX at time t ) 0, mol RX ) p-Nitrochlorobenzene XA ) Fractional conversion of RX M ) Initial mole ratio of potassium phenolate to p-NCB, dimensionless
Eq 32 is simplified to the following
∫
XA
0
[RX]0XA + [RX]0Ke(M - XA)dXA ) ((1 - XA)(M - XA)[k2[RX]0XA + [RX]0k2Ke(M - XA)] + 1])
∫
XA
0
aXA + b(M - XA)dXA ) k1t ((1 - XA)(M - XA)[cXA + cKe(M - XA)] + 1]) (33)
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ReceiVed for reView March 12, 2008 ReVised manuscript receiVed July 24, 2008 Accepted July 28, 2008 IE800340J
