Reduction of Nitrotoluenes by H2S-rich Aqueous Ethanolamine Solution: A Viable Alternative to Claus Process

Sunil K. Maity, Narayan C. Pradhan*, Anand V. Patwardhan Department of Chemical Engineering, Indian Institute of Technology, Kharagpur-721302, India.

*Corresponding author: Phone: +91-3222-283940; Fax: +91-3222-255303; E-mail: [email protected]

Abstract The reduction of nitrotoluenes (o-, m- and p-) by H2S-rich aqueous monoethanolamine (MEA) was carried out in an organic solvent, toluene, under liquidliquid mode with phase transfer catalyst, tetrabutylammonium bromide (TBAB). The selectivity of toluidines was found to be 100%. The reaction rate of m-nitrotoluene (MNT) was found to be highest among the three nitrotoluenes followed by p- and onitrotoluene (PNT and ONT). The reaction was found to be kinetically controlled with the apparent activation energies of 18.2, 21.1, and 21.7 kcal/mol for MNT, PNT, and ONT respectively. The effects of different parameters such as TBAB concentration, sulfide concentration, concentration of nitrotoluene, MEA concentration, and elemental sulfur loading on the reaction rate and conversion were studied to establish the mechanism of the reaction. The rate of reaction of nitrotoluene was found to be proportional to the concentration of catalyst, to the square of the concentration sulfide, and to the cube of the concentration of nitrotoluenes. A generalized empirical kinetic model was developed to correlate the experimentally obtained conversion versus time data for the three nitrotoluenes. Keywords: Hydrogen sulfide; Amine treatment unit; Nitrotoluenes; Liquid-liquid phase transfer catalysis; Kinetics.

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1. Introduction During the course of many processes in the petroleum and coal processing industry, one or more gaseous streams containing hydrogen sulfide (H2S) are quite commonly produced. Also in the natural gas industry where the H2S content of certain gas streams recovered from natural gas deposits in many parts of the world is often too high for commercial acceptance. The removal of H2S from these gaseous streams can be desirable for a variety of reasons. (1) H2S is odiferous in nature, corrosive in presence of water, poisonous in very small concentrations. Therefore, it must be almost completely removed from the gas streams before its use and preferably before transport. (2) If these gaseous streams are to be burned as a fuel, the removal of H2S from the fluid stream may be necessary to prevent environmental pollution owing to the resultant sulfur dioxide. (3) The presence of H2S in the refinery gas streams can cause a number of detrimental problems in subsequent processing steps such as: corrosion of process equipment, deterioration and deactivation of catalysts, undesired side reactions, etc. It is, therefore, desirable to remove H2S from the gas stream while affording the opportunity to generate useful products capable of being used in further reactions, processes or other activities. The H2S from these gaseous streams is conventionally removed through amine treating unit and then processed in the Claus unit [1] to produce elemental sulfur. However, there are several disadvantages of air oxidation of H2S which include loss of a valuable hydrogen source, requirement of precise air rate control, removal of trace sulfur compounds from spent air, and an upper limit on the ratio of carbon dioxide (CO2) to H2S to name a few. Therefore, the development of viable alternative processes for the conversion of H2S to produce commercially important chemicals with co-production of

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elemental sulfur is highly desired in the process industry, particularly in the refineries handling sour crudes. Although both ammonia- and alkanolamine-based processes are in use for removal of acid constituents (H2S and CO2) from gas streams, alkanolamine-based process got wide commercial acceptance as the gas treating art because of its advantages of low vapour pressure (high boiling point) and ease of reclamation [1]. The low vapour pressure of alkanolamines can make the operation more flexible in terms of operating pressure, temperature and concentration of alkanolamine, in addition to negligible vaporization loses.

The present work was, therefore, undertaken to promote the

alkanolamine-based process with the objective of better utilization of H2S component of the gas stream. Obviously, in this case the costly regeneration of the H2S-rich amine solution and subsequent complicated Claus process could be avoided and at the same time some commercially important products, toluidines, could be obtained with coproduction of elemental sulfur (as obtained in the Claus process). Although triethanolamine (TEA) [2-4] is one of the first amines used for the removal of H2S and CO2 from gas streams, it is superseded by aqueous monoethanolamine (MEA) and diethanolamine (DEA) because of their higher rate of reaction with the acid gases. Aqueous MEA have been widely used due to their high reactivity, low solvent cost, ease of reclamation, low absorption of hydrocarbons, and low molecular weight (that result high solution capacity at moderate concentrations). On the other hand, DEA, which is less corrosive than MEA, is a better choice for treating gas streams containing appreciable amounts of COS and CS2 as it is much less reactive with these impurities compared to primary amines like MEA. Lot of study was done on the

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equilibrium solubility of pure H2S [5-8], mixture of acid gases (H2S and CO2) [7-9], and the mathematical representation of the experimental solubility data for H2S, CO2 and their mixture [10-15] using aqueous MEA. Much work was also reported on the use of aqueous DEA [7, 13-21]. However, no attempt was made in the past to utilize the H2Srich aqueous alkanolamine to produce value-added chemicals like toluidines. Considering the importance of the system, the present work was undertaken using most commonly used amine, MEA. Aqueous methyldiethanolamine (MDEA) has been reported to remove H2S selectively from a gas stream containing both CO2 and H2S [22-24]. In addition to its selectivity, other advantages of MDEA include low vapor pressure, highly resistant to thermal and chemical degradation, and low heat capacity and enthalpy of reaction with H2S and CO2. The selectivity of H2S removal could be greatly improved by using MDEA in non-aqueous solvents like N-methylpyrolidone, ethylene glycol etc. [24-25]. The applicability of the present process for gas streams containing both H2S and CO2 largely depends on this selectivity. Therefore, extensive research is needed in this field using various H2S-selective alkanolamines as the solvent. The reduction reaction of nitroarenes by negative divalent sulfur (sulfide, hydrosulfide and polysulfides) is called Zinin reduction [26]. Sodium sulfide, sodium disulfide, and ammonium sulfide are most commonly used for this purpose. Several researchers studied the kinetics of the reduction of nitroarenes using sodium disulfide [27-28] and sodium sulfide [28-32] both in absence and presence of phase transfer catalyst (PTC) [29-32] and also under different modes (solid–liquid [29-30] or liquidliquid [29-32]). The preparation of aryl amines using aqueous ammonium sulfide [33],

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alcoholic ammonium sulfide [34-35], and ammonium sulfide prepared from the equivalent amounts of ammonium chloride and crystalline sodium sulfide dissolved in ammonium hydroxide or alcohol [36-37] are reported in the literature. However, since the invention of Zinin reduction, there is no reported work in the literature on the use of alkaline medium other than ammonium hydroxide for absorption of H2S and reduction of nitroarenes using this solution. In the present work, industrially relevant alkaline medium such as aqueous MEA has been used in the Zinin reduction. The Zinin reduction is of considerable practical value due to some inherent advantages of the method over other conventional processes. For example, catalytic hydrogenation requires more expensive equipment and hydrogen handling facility; additional problems arise due to catalyst preparation, catalyst poisoning hazards and the risk of reducing other groups. Although the reduction by iron is reserved for small-scale commercial applications, it cannot be used for reduction of a single nitro group in a polynitro compound, nor it can be used on substrates harmed by acid media (e.g., some ethers and thioethers). Metal hydrides, e.g., lithium aluminum hydride, generally converts nitro compounds to mixtures of azoxy and azo compounds, besides being expensive. The reduction of nitrotoluenes to the corresponding amines is commercially very important as the products toluidines have wide applications as intermediates for dyes, agrochemicals and pharmaceutical products. The present work is concerned with the reduction of nitrotoluenes (o-, m- and p-) by H2S-rich aqueous MEA under two phase condition in presence of PTC. The advantages of the use of PTCs in carrying out multiphase reactions efficiently are well recognized. The use of these catalysts to enhance otherwise slow two-phase

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reduction of aromatic nitro compounds may be very interesting from a commercial point of view. Among several varieties of PTCs, quaternary ammonium salts are most preferred for their better activity and ease of availability. Tetrabutylammonium bromide (TBAB) has been reported to be the most active PTC among six different catalysts used to intensify the reaction of benzyl chloride with solid sodium sulfide [38]. The TBAB was therefore used in this study as PTC. In H2S-rich aqueous MEA, the sulfide ions (S2-) and hydrosulfide ions (HS-) remain in equilibrium as represented by Scheme 1 [14]. The similar ionic equilibrium exists in the aqueous ammonium sulfide as well. Therefore the behavior of H2S-rich aqueous MEA is expected to be similar to that of ammonium sulfide. However, due to the existence of two different ions (sulfide and hydrosulfide) in the H2S-rich aqueous MEA and aqueous ammonium sulfide; the properties of these reducing agents are expected to be different from the other reducing agents like sodium sulfide and disulfide. RNH2 + H2O ⇌ RNH3+ + HOH2O⇌H+ + HOH2S ⇌ H+ + HSHS-⇌ H+ + S2Scheme 1 The overall stoichiometry of the Zinin’s original reduction of nitrobenzene by aqueous ammonium sulfide is given by Eq. 1 [26]. This stoichiometry is also applicable for reduction of nitroarenes by sodium sulfide [28-32].

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4ArNO2 + 6S2− + 7H2O → 4ArNH2 + 3S2O32− + 6HO−

(1)

For the preparation of p-aminophenylacetic acid from p-nitrophenylacetic acid using aqueous ammonium sulfide, it was reported that the sulfide ions were oxidized to elemental sulfur instead of thiosulfate following stoichiometry of Eq. 2 [33]. The similar stoichiometry was reported for the reduction of 2-bromo-4-nitrotoluene by alcoholic ammonium sulfide [35]. The formation of elemental sulfur was reported for the preparation of 3-amino-5-nitrobenzyl-alcohol using ammonium sulfide prepared from ammonium chloride and crystalline sodium sulfide dissolved in methanol [37]. ArNO2 + 3HS− + H2O → ArNH2 + 3S+ 3HO−

(2)

The overall stoichiometry of the reduction reaction using disulfide as the reducing agent is as follows [27-28]: ArNO2 + S22− + H2O → ArNH2 + S2O32− + HO−

(3)

Therefore, the two different reactions leading to the formation of either elemental sulfur or thiosulfate may be operative for the reduction of nitroarenes with H2S-rich aqueous MEA. A detailed study of such reactions is, therefore, not only commercially important, it is academically interesting too.

2. Experimental 2.1 Chemicals Toluene (≥99%) and ethanolamine (≥98%) of synthesis grade were procured from Merck (India) Ltd., Mumbai, India. Nitrotoluenes (>99%) of synthesis grade were purchased from Loba Chemie Pvt. Ltd., Mumbai, India. Tetrabutylammonium bromide (TBAB) was obtained from SISCO Research Laboratories Pvt. Ltd., Mumbai, India.

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2.2 Equipment The reactions of nitrotoluenes with H2S-rich aqueous MEA were carried out batch wise in a fully baffled mechanically agitated glass reactor of capacity 250 cm3 (6.5 cm i.d.). A 2.0 cm-diameter six-bladed glass disk turbine impeller with the provision of speed regulation, located at a height of 1.5 cm from the bottom, was used for stirring the reaction mixture. The reactor was kept in a constant temperature water bath whose temperature could be controlled within ±1oC.

2.3 Preparation of H2S-rich aqueous MEA For the preparation of H2S-rich aqueous MEA, around 30 wt% MEA was prepared first by adding suitable quantity of MEA in distilled water. Then H2S gas was bubbled through this aqueous MEA in a 250 cm3 standard gas-bubbler. The gas bubbling was continued until the desired sulfide concentration was obtained in the aqueous MEA.

2.4 Experimental procedure In a typical run, 50 cm3 of the aqueous phase containing a known concentration of sulfide was charged into the reactor and kept well agitated until the steady-state temperature was reached. Then the organic phase containing measured amount of nitrotoluene, catalyst (TBAB) and solvent (toluene), kept separately at the reaction temperature, and was charged into the reactor. The reaction mixture was then agitated at a constant speed. About 0.5 cm3 of the organic layer was withdrawn at a regular interval after stopping the agitation and allowing the phases to separate. 2.5 Analysis

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All the samples from the organic phase were analyzed by gas-liquid chromatography (GLC) using a 2 m × 3 mm stainless steel column packed with 10% OV17 on Chromosorb W (80/100). A Chemito Model 8610 GC interfaced with Shimadzu CR6A Chromatopac data processor was used for the analysis. The column temperature was programmed with an initial temperature at 1500C and increased at 150C/min to 2400C. Nitrogen was used as the carrier gas with a flow rate of 15 cm3/min. Injector temperature of 2700C was used during the analysis. An FID detector was used at the temperature of 2700C. Initial sulfide concentrations were determined by the standard iodometric titration method [40].

3. Results and discussion 3.1 Effect of speed of agitation The effect of speed of agitation on the rate of reaction of nitrotoluenes (o-, m- and p-) was studied in the range 1000-2500 rev/min under otherwise identical experimental conditions in presence of PTC, TBAB as shown in Fig. 1. As it is evident from the figure, the variation of reaction rate with speed of agitation is so small that the reactions may be considered as kinetically controlled for all the nitrotoluenes. All other experiments were performed at 1500 rev/min in order to avoid the effects of mass transfer resistance on the reaction kinetics.

3.3 Effect of temperature The effect of temperature on the rate of reaction of nitrotoluenes with H2S-rich aqueous MEA was studied in the range of 313-343 K under identical experimental

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conditions in presence of catalyst, TBAB, as shown in Table 1. The reaction rate increases with temperature for all the nitrotoluenes as it is observed from the table. The initial rates were calculated at different temperatures and Arrhenius plot of Ln (initial rate) against 1/T (K-1) was made as shown in Fig. 2. The apparent activation energy for this kinetically controlled reaction was calculated from the slope of the straight lines as 18.2, 21.1 and 21.7 kcal/mol for MNT, PNT and ONT, respectively. The high values of apparent activation energy confirm that the reaction systems are kinetically controlled.

3.3 Comparison of reactivities of nitrotoluenes As it is observed from the Table 1, the reaction rates of nitrotoluenes follow the order of m-nitrotoluene (MNT) > p-nitrotoluene (PNT) > o-nitrotoluene (ONT) in presence of PTC, TBAB, in the temperature range studied. From this observation, it can be concluded that the presence of electron donating group like methyl group in the aromatic ring reduces the reaction rate more when it is present at the ortho and para positions, i.e., positions of high electron density, compared to its presence at the meta position (site of low electron density). Pradhan [30] also reported a similar trend of reactivity for the reduction of nitrotoluenes by sodium sulfide using TBAB in the liquidliquid mode.

3.4 Effect of TBAB Loading The effect of catalyst (TBAB) loading on conversion of PNT was studied in the concentration range of 3.1×10-2 to 12.4×10-2 kmol/m3 of organic phase as shown in Fig. 3. The study was also conducted in the absence of catalyst as shown in the figure. As it is

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observed from the figure, the conversion of PNT is only about 1 % in absence of catalyst whereas it is around 47% with maximum concentration of catalyst tried after 150 minutes of reaction under otherwise identical experimental conditions. It is observed from Table 2 that the rate of reaction of PNT in the absence of TBAB is very low compared to that in the presence of TBAB resulting in very high enhancement factors. This shows the importance of PTC in enhancing the rate of the reaction under investigation. In order to determine the order of the reaction with respect to TBAB concentration, the initial reaction rate was calculated at different TBAB concentration and the plot of Ln(initial rate) against Ln(TBAB concentration) (Fig. 4) was made. From the slope of the linear fit line, the order of the reaction with respect to TBAB concentration was obtained as 1.12, which is close to unity. Yadav et al. [31] also reported similar observation for the reduction of p-nitroanisole by sodium sulfide in presence of PTC, TBAB.

3.5 Effect concentration of p-nitrotoluene The effect of concentration of PNT on the conversion was studied at four different concentrations in the range of 0.87-1.75 kmol/m3 in presence of TBAB under otherwise identical experimental conditions as shown in Fig. 5. The conversion (so also reaction rate) of PNT increases with increase in concentration of the PNT. From the plot of Ln(initial rate) against Ln(PNT concentration) (Fig. 6), the order of the reaction with respect to PNT concentration was obtained as 3.12 which is close to third order. Same order was observed for other two nitrotoluenes (MNT and ONT) as well. However, for the reduction of nitroarenes by aqueous sodium sulfide, the reported order is unity with

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respect to the concentration of p-nitroanisole [31] and nitroaromatics [28]. The rate was also reported as proportional to the concentration of nitrobenzene for its reduction with sodium disulfide under two-phase conditions [27].

3.6 Effect of sulfide concentration Fig. 7 shows the effect of sulfides concentration in the aqueous phase on the conversion of MNT. With increase in the concentration of sulfides, the conversion of MNT as well as the reaction rate increases, as it is evident from the figure. From the plot of Ln(initial rate) against Ln(initial sulfide concentration) (Fig. 8), the order of the reaction with respect to sulfide concentration was obtained as 1.64. Since this value is closer to integer, 2, the reaction was, therefore, considered as 2nd order with respect to sulfide concentration. However, for the reduction of nitroarenes with aqueous sodium sulfide, the reaction rate was reported to be first order with the sulfide concentration [28, 31]. The rate was also reported to be proportional to the square of the concentration of sodium disulfide [27].

It is worthy to mention here that the nature of the curve obtained in this reaction is ‘S’ type which is typical of autocatalytic reaction where the rate of reaction increases with increase in the concentration of catalyst formed by the reaction and then the rate of reaction decreases with the depletion of the reactants as observed in the Fig. 3, 5, and 7. This phenomenon was only observed for low concentration of one of the components: nitrotoluenes, sulfide and catalyst or the conditions that favor low initial reaction rate. However, for high concentration, this phenomenon could not be observed as it occurred

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within a very short period of time. For o- and p-nitrotoluenes, this phenomenon was found to occur even at relatively high concentrations because of their slow reaction rates. The exact reason for this nature of curve will be explained when the effect of elemental sulfur loading on the reaction will be discussed.

3.7 Effect of MEA concentration Although MEA as such does not take part in the reaction with nitrotoluenes, it affects the equilibrium among MEA, H2S, and water, which results two active anions, sulfide (S2-) and hydrosulfide (HS-), in the aqueous phase as shown in the Scheme 1. These two active anions participate in two different reactions (Eqs.1 and 2). In presence of base, MEA, dissociation equilibrium shifts toward more ionization and the concentration of sulfide ions relative to hydrosulfide ions in the aqueous phase increases with increase in MEA concentration. Therefore, only by changing the MEA concentration with constant sulfide concentration in the aqueous phase, it would be easy to prove the existence of two different reactions. To study the effect of MEA concentration, H2S-rich aqueous MEA of different MEA concentrations (but constant sulfide concentration) was prepared by taking 30 cm3 of H2S-rich aqueous MEA (with known sulfide and MEA concentration) and then adding various proportions of pure MEA and distilled water to it in such a way that the total volume became 50 cm3 in all the cases. The effect of MEA concentration on the conversion of MNT is shown in Fig. 9. With increase in concentration of MEA, conversion of MNT was found to decrease up to

14

a certain reaction time; beyond that opposite trend was observed, i.e., with higher MEA concentration higher MNT conversion was achieved (Fig. 9). It is expected to get around 54% conversion of MNT if the reaction follows the stoichiometry of Eq. 1 and it would be around 27% if the stoichiometry of the Eq. 2 is considered, for complete conversion of sulfide in both cases. However, after long reaction run, maximum 34% conversion of MNT was achieved with the maximum MEA concentration used in this study as shown in the figure. From this result it is clear that the first reaction (Eq. 1) is also operative in the Zinin reduction as proposed by Zinin in 1842 [26]. These results are in complete disagreement with some of the recent works with ammonium sulfide [33, 35, 37] that proposes reaction of Eq. 2 as the solely operative one. From the same figure, it is also seen that reaction reaches equilibrium well before the expected conversion from stoichiometry of Eq. 1. The lowering of conversion may be due to the existence of reaction of Eq. 2 in addition to the reaction of Eq. 1. The formation of elemental sulfur will be confirmed when the effect of elemental sulfur loading will be discussed. Since, the formation of elemental sulfur was not reported anywhere in the literature for the reduction of nitroarene with sodium sulfide, it could be thought that the reaction via the transfer of sulfide ions follows the stoichiometry of Eq. 1. The concentration of sulfide ions (S2-) increases with increase in the concentration of MEA for fixed sulfide concentration. Thus, with increase in MEA concentration, there is an increase in the reaction via the transfer of sulfide ions following the stoichiometry of Eq. 1, which results higher conversion of MNT at higher MEA concentration.

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3.8 Effect of elemental sulfur loading Elemental sulfur in the solution of ammonia and hydrogen sulfide is known to form ammonium polysulfides [39], (NH4)2Sn where 2 ≤ n ≤ 6, which is also one of the reducing agents of the Zinin’s reduction. A similar behavior is also expected with aqueous MEA. Since, the formation of elemental sulfur was reported for this reaction [33, 35, 37], here we examined the effect of externally added elemental sulfur on the reaction rate and conversion of MNT. In this experiment, elemental sulfur was first dissolved in the H2S-rich aqueous MEA and then used for the reaction following the same procedure as described earlier. The color of the H2S-rich aqueous MEA is greenish yellow. However, after dissolution of elemental sulfurs to this solution, the color of the solution became reddish brown. During the reaction run also, initially the color of the solution was unaffected but later on it changed rapidly from greenish yellow to reddish brown. This color change indicates the formation of elemental sulfur (and polysulfides) during the reaction. The characteristic reddish brown color of the polysulfide, which develops as the reaction proceeds, is useful in indicating the extent of the reaction. The effect of elemental sulfur loading on conversion of MNT is shown in Fig. 10. It is clearly observed from the figure that initially the reaction rate (or conversion of MNT) increases with increasing elemental sulfur loading. However, the overall conversion of MNT decreases with increase in the elemental sulfur loading. As it is observed from the nature of the curves in the figure, the reaction rate gradually decreases with conversion of MNT in the presence of elemental sulfur whereas the nature of the curve is ‘S’ type in the absence of elemental sulfur.

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These observations are in support of the formation of elemental sulfur and can also be used to explain the ‘S’ type of curve as discussed in the previous section. Therefore, it can be said that the reaction rate increases with the build up of elemental sulfur concentration (Eq. 2) as the reaction proceeds and then falls with the depletion of the reactants resulting in ‘S’ type of curve. The rise in rate of reaction with elemental sulfur loading may be due to the fact that the reaction via the transfer of hydrosulfide and sulfide ions is slow compared to polysulfide ions formed by the reaction of elemental sulfur with H2S-rich aqueous MEA. As reported by Hojo et al. [27], disulfide reduces nitrobenzene much more rapidly than that by sulfide The overall conversion of MNT decreases with increase in elemental sulfur loading as observed from the figure. This may be due to the formation of polysulfide in addition to disulfide (which is only transferred and react with nitrotoluenes to form thiosulfate and toluidines according to the stoichiometry of Eq. 3) [35]. From the results of Fig. 9, it is observed that conversion of MNT remains closer to the value as governed by stoichiometry of Eq. 2. Therefore, it can be concluded that the reaction follows the stoichiometry of Eq. 2 predominantly. It can also concluded from this results that it is preferred to carry out the reaction with high sulfide loading (low MEA concentration) in the aqueous phase in order to get the elemental sulfur predominantly instead of thiosulfate at the cost of low overall conversion of nitrotoluenes.

4. Kinetic modeling

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The kinetics and mechanism of a variety of phase transfer catalyzed SN2 type of reactions, in liquid-liquid [41], solid-liquid [42], and liquid-liquid-liquid [43] mode, and some oxidation reactions [41,44] are well documented. However, such information on Zinin reduction is very limited. Although, a lot of published works are there on Zinin reduction but the exact mechanism of this important reaction is still not clear. Probably the first product is a nitroso compound, which is rapidly reduced to hydroxylamine and then to amine [26]. The rate-determining step is considered to be the attack of negative divalent sulfur on the nitro group as no intermediate compounds are observed to form during the reaction. The mechanism and kinetic scheme developed by Yadav et al. [31] for the reduction of p-nitroanisole by aqueous sodium sulfide under liquid-liquid mode in presence of PTC, TBAB was found to be not applicable in the case of reduction of nitrotoluenes by H2S-rich aqueous MEA. The hydrosulfide (HS-) and sulfide (S2-) ions present in the aqueous phase readily form ion pairs [Q+HS− and Q+S2-Q+], with quaternary cations, [Q+] and are transferred to the organic phase and reduce the nitrotoluenes following the stoichiometry of Eq.2 and Eq.1, respectively. The polysulfide is formed by the reaction of elemental sulfur (Eq.1) with H2S-rich aqueous MEA. It is mentioned in the earlier discussion that the overall conversion of MNT decreases with increase in elemental sulfur loading because of the formation of higher amount of polysulfides (other than disulfide) which are not easily transferred to the organic phase. Only disulfide ions form ion pair, [Q+S22-Q+] and are transferred to the organic phase and reduce the nitrotoluenes following Eq. 3. Development of fundamental kinetic model for this system is a difficult task because of these complexities involved and poor knowledge of the system. In this work,

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an empirical kinetic model applicable for all the nitrotoluenes was, therefore, developed to correlate the experimentally obtained time versus conversion data. Since, the concentration of MEA in the aqueous phase was kept around 30 vol%, its effect was not incorporated in this kinetic model. Based on the experimental facts, the rate of reduction of nitrotoluenes (−rA ) is expressed by the following equation. 2

2

− rA = k1C A3 C S C C + k 2C A3 C S C C C B

(4)

where CA and CC are the concentrations of nitrotoluenes and catalyst, TBAB in the organic phase, respectively. The 2nd term in the above rate expression take care of ‘S’ nature of curves due to the formation of elemental sulfur during the reaction as discussed previously. Since, the course of reduction follows the stoichiometry of Eq. 2 predominantly; the concentration of sulfide (CS) and elemental sulfur (CB) in the aqueous phase are obtained from the overall mass balance based on the same stoichiometry as given by the following expressions. C S = C SO − 3 f (C AO − C A )

(5)

C B = 3 f (C AO − C A )

(6)

where CSO and CAO represent the initial concentrations of sulfide and nitrotoluenes, respectively and f is the ratio of volume of organic phase to that of aqueous phase. A non-linear regression algorithm was used for parameter estimation. The optimum values of the rate constants (k1 and k2) of the nitrotoluenes reactions were estimated by minimizing the objective function (E) as given by the equation n

[{

} {

E = ∑ (− rA ) pred i − (− rA )exp t i =1

19

}]

2

i

(7)

The optimum values of the rate constants, k1 and k2, are listed in Table 3. Fig. 11 represents the comparison of the calculated conversions of nitrotoluenes based on these rate constants and experimentally obtained conversions. Good agreement was observed between the predicted and experimental conversions.

5. Conclusions The reduction of nitrotoluenes by H2S-rich aqueous MEA to the corresponding toluidines was studied under liquid–liquid mode in presence of PTC, TBAB. The selectivity of toluidines was 100%. The MNT was found to be the most reactive among the nitrotoluenes followed by PNT and ONT. The reaction was found to be kinetically controlled with apparent activation energies of 18.2, 21.1, and 21.7 kcal/mol for MNT, PNT and ONT, respectively. The rate of reduction of nitrotoluene was found to be proportional to the concentration of catalyst, to the square of the concentration of sulfide, and to the cube of the concentration of nitrotoluenes. The process was found to follow a complex mechanism involving three different reactions. Based on the detailed kinetic study and proposed mechanism, a general empirical kinetic model was developed. The developed model, applicable to all nitrotoluenes, predicts the conversions of nitrotoluenes reasonably well.

Acknowledgment Sunil K. Maity is thankful to the All India Council for Technical Education (AICTE), New Delhi, India, for the award of the National Doctoral Fellowship during the tenure of this work.

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Nomenclature DEA

diethanolamine

MDEA

methyldiethanolamine

MEA

monoethanolamine

MNT

m-nitrotoluene

ONT

o-nitrotoluene

PNT

p-nitrotoluene

PTC

phase transfer catalyst

TBAB

tetrabutylammonium bromide

TEA

triethanolamine

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21

[11] G. Astarita and D. W. Savage, Chem. Eng. Sci. 37(5) (1982) 677. [12] L. Kaewsichan, O. Al-Bofersen, V. F. Yesavage and M. S. Selim, Fluid Phase Equilibria 183–184 (2001) 159. [13] D. M. Austgen, G. T. Rochelle, X. Peng and C. Chen, Ind. Eng. Chem. Res. 28 (1989) 1060. [14] R. H. Weiland, T. Chakravarty and A. E. Mather, Ind. Eng. Chem. Res. 32 (1993) 1419. [15] N. A. Al-Baghli, S. A. Pruess, V. F. Yesavage and M. S. Selim, Fluid Phase Equilibria 185 (2001) 31. [16] J. II Lee, F. D. Otto and A. E. Mather, J. Chem. Engg. Data 18(1) (1973) 71. [17] J. II Lee, F. D. Otto and A. E. Mather, J. Chem. Engg. Data 18(4) (1973) 420. [18] J. I. Lee, F. D. Otto and A. E. Mather, Can. J. Chem. Engg. 52 (1974) 125. [19] D. Lal, F. D. Otto and A. E. Mather, Can. J. Chem. Engg. 63 (1985) 681. [20] R. Sidi-Boumedine, S. Horstmann, K. Fischer, E. Provost, W. Fürst and J. Gmehling, Fluid Phase Equilibria 218 (2004) 149. [21] G. Vallée, P. Mougin, S. Jullian and W. Fürst, Ind. Eng. Chem. Res. 38 (1999) 3473. [22] B. P. Mandal, A. K. Biswas and S. S. Bandyopadhyay, Separation and Purification Technology 35 (2004) 191. [23] M. Bolhàr-Nordenkampf, A. Friedl, U. Koss and T. Tork, Chemical Engineering and Processing 43 (2004) 701. [24] H. Xu, C. Zhang and Z. Zheng, Ind. Eng. Chem. Res. 41 (2002) 2953. [25] H. Xu, C. Zhang and Z. Zheng, Ind. Eng. Chem. Res. 41 (2002) 6175.

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[26] W. G. Dauben, Organic Reactions, John Wiley & Sons, Inc.: New York, Vol. 20, p455-481, 1973. [27] M. Hojo, Y. Takagi and Y. Ogata, J. Am. Chem. Soc. 82 (1960) 2459. [28] R. R. Bhave and M. M. Sharma, J. Chem. Tech. Biotechnol. 31 (1981) 93. [29] N. C. Pradhan and M. M Sharma, Ind. Eng. Chem. Res. 31 (1992) 1606. [30] N. C. Pradhan, Indian J. Chem. Technol. 7 (2000) 276. [31] G. D. Yadav, Y. B. Jadhav and S. Sengupta, Chem. Eng. Sci. 58 (2003) 2681. [32]

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[33] H. Gilman, Organic Syntheses. Collective volume 1, John Wiley & Sons, Inc., NewYork, p-52, 1941. [34] E. L. Cline and E. E. Reid, J. Am. Chem. Soc. 49 (1927) 3150. [35] H. J. Lucas and N. F. Scudder, J. Am. Chem. Soc. 50 (1928) 244. [36] M. J. Murray and D. E. Waters, J. Am. Chem. Soc. 60 (1938) 2818. [37] W. R. Meindl, E. V. Angerer, H. Schonenberger and G. Ruckdeschel, J. Med. Chem. 27 (1984) 1111. [38] N. C. Pradhan and M. M. Sharma, Ind. Eng. Chem. Res. 29 (1990) 1103. [39] P. Dubois, J. P. Lelieur and G. Lepoutre, Inorg. Chem. 27 (1988) 1883. [40] W. W. Scott, Standard Methods of Chemical Analysis, Van Nostrand: New York, 1966, 6th ed., Vol. IIA, p 2181. [41] J. A. B. Satrio and L. K. Doraiswamy, Chem. Eng. Sci. 57 (2002) 1355. [42] S. D. Naik and L. K. Doraiswamy, Chem. Eng. Sci. 52(24) (1997) 4533. [43] G. D. Yadav and S. S. Naik, Catalysis Today 66 (2001) 345.

23

[44] G. D. Yadav and B. V. Haldavanekar, J. Phys. Chem. A 101 (1997) 36.

24

Table 1. Effect of temperature on the reaction rate of nitrotoluenesa Temperature,

a

Rate of reaction ×106, kmol/m3 s

K

MNT

PNT

ONT

313

7.2

2.8

1.1

323

22.0

11.0

3.5

333

39.3

27.3

15.6

343

82.5

54.1

31.3

Matching conversion=5%; speed of agitation=1500

rev/min; all other conditions are same as in Fig. 1.

25

Table 2. Effect of TBAB loading on reaction rate of PNT a

a

TBAB concentration×102,

Reaction rate×104,

kmol/m3 of org. phase

kmol/m3s

0.0

0.01

-

3.1

0.26

26

6.2

0.50

50

9.3

0.79

79

12.4

1.00

100

Enhancement factor

Matching PNT conversion=5%; all other conditions are same as in Fig. 3.

26

Table 3. Activation energy and pre-exponential factors for rate constants of nitrotoluenes k1 = A10 Exp(− AE1 ) T

k 2 = A20 Exp(− AE 2 ) T

A10 [(kmol/m3)-5s-1]

AE1 (kcal/mol)

A20 [(kmol/m3)-6s-1]

AE2 (kcal/mol)

MNT

1.26×107

18.0

1.94×109

19.2

PNT

1.17×109

21.2

1.95×105

13.6

ONT

7.50×108

21.4

8.76×104

13.4

27

9.00

MNT PNT ONT

8.50

8.25

5

3

Reaction rate×10 (kmol/m s)

8.75

4.50 4.25 4.00 3.75 3.50 1000

1500

2000

2500

Speed of agitation (rev/min)

Fig. 1. Effect of speed of agitation. Volume of organic phase = 5×10-5 m3; nitrotoluenes concentration = 1.46 kmol/m3; TBAB concentration= 6.2×10-2 kmol/m3 of org. phase; volume of aqueous phase = 5×10-5 m3; concentration of MEA = 5.01 kmol/m3; sulfide concentration= 1.85 kmol/m3; temperature = 343 K.

28

-9.0

Linear fit 2 Exptl. r MNT 0.99 PNT 0.99 ONT 0.95

-9.5 -10.0

3

Ln(initial rate, kmol/m s)

-10.5 -11.0 -11.5 -12.0 -12.5 -13.0 -13.5 -14.0 -14.5 -15.0 2.9x10

-3

-3

3.0x10

-3

-3

3.0x10

3.1x10

-3

3.1x10

-3

3.2x10

-1

1/T (K ) Fig. 2. Arrhenius plot. All conditions are same as in Table 1.

29

-3

3.2x10

60 2

50 45

Conversion of PNT (%)

3

TBAB×10 , kmol/m of org. phase 0.0 3.1 6.2 9.3 12.4

55

40 35 30 25 20 15 10 5 0 0

20

40

60

80

100

120

140

Reaction time (min) Fig. 3. Effect of TBAB loading. Volume of organic phase = 5×10-5 m3; PNT concentration = 1.46 kmol/m3; volume of aqueous phase = 5×10-5 m3; concentration of MEA = 5.01 kmol/m3; sulfide concentration= 2.25 kmol/m3; temperature = 333 K; speed of agitation =1500 rev/min.

30

-10.2

Experimental 2 Linear fit (r =0.99)

-10.4

3

Ln(initial rate, kmol/m s)

-10.6 -10.8 -11.0 -11.2 -11.4 -11.6 -11.8 -12.0 -3.6

-3.4

-3.2

-3.0

-2.8

-2.6

-2.4

-2.2

-2.0

3

Ln(TBAB concentration, kmol/m of org. phase) Fig. 4. Plot of Ln(initial rate) vs. Ln(TBAB concentration). All conditions are same as in Fig. 3.

31

40 35

3

PNT (kmol/m ) 0.87 1.17 1.46 1.75

Conversion of PNT(%)

30 25 20 15 10 5 0 0

20

40

60

80

100

120

140

Reaction time (min)

Fig. 5. Effect of PNT concentration. Volume of organic phase = 5×10-5 m3; TBAB = 6.2×10-2 kmol/m3 of organic phase; volume of aqueous phase = 5×10-5 m3; concentration of MEA = 5.01 kmol/m3; sulfide concentration= 2.25 kmol/m3; temperature = 333 K; speed of agitation =1500 rev/min.

32

-10.0

Experimental 2 Linear fit (r =0.99 )

3

Ln(initial rate, kmol/m s)

-10.5

-11.0

-11.5

-12.0

-12.5 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

3

Ln(initial PNT concentration, kmol/m )

Fig. 6. Plot of Ln(initial rate) vs. Ln(PNT concentration). All conditions are same as in Fig. 5.

33

55

3

Sulfide (kmol/m ) 0.86 1.29 1.72 2.15

50 45

Conversion of MNT(%)

40 35 30 25 20 15 10 5 0 0

50

100

150

200

250

300

350

Reaction time (min) Fig. 7. Effect of sulfide concentration. Volume of organic phase = 5×10-5 m3; MNT = 1.60 kmol/m3; TBAB = 9.3×10-2 kmol/m3 of organic phase; volume of aqueous phase = 5×10-5 m3; concentration of MEA = 5.01 kmol/m3; temperature = 333 K; speed of agitation =1500 rev/min.

34

-10.0

Experimental 2 Linear fit (r =0.99)

-10.2

3

Ln(initial rate, kmol/m s)

-10.4 -10.6 -10.8 -11.0 -11.2 -11.4 -11.6 -11.8 -0.2

0.0

0.2

0.4

0.6

0.8 3

Ln(initial sulfide concentration, kmol/m ) Fig. 8. Plot of Ln(initial rate) vs. Ln(sulfide concentration). All conditions are same as in Fig. 7.

35

35

Conversion of MNT (%)

30

25

20

15

3

10

MEA (kmol/m ) 3.00 6.34 9.68

5

0 0

100

200

300

400

500

600

700

800

Reaction time (min)

Fig. 9. Effect of MEA concentration. Volume of organic phase = 5×10-5 m3; MNT = 1.60 kmol/m3; TBAB = 9.3×10-2 kmol/m3 of organic phase; volume of aqueous phase = 5×10-5 m3; concentration of sulfide = 1.29 kmol/m3; temperature = 333 K; speed of agitation =1500 rev/min.

36

40

Conversion of MNT (%)

35 30 25 20

Elemental Sulfur (gm) 0.0 0.5 1.0 1.5

15 10 5 0 0

25

50

75

100 125 150 175 200 225 250 275 300

Reaction time (min) Fig. 10. Effect of elemental sulfur loading. Volume of organic phase = 5×10-5 m3; MNT concentration = 1.46 kmol/m3; TBAB concentration= 6.2×10-2 kmol/m3 of org. phase; volume of aqueous phase = 5×10-5 m3; concentration of MEA = 5.01 kmol/m3; sulfide concentration= 1.85 kmol/m3; temperature = 333 K; speed of agitation =1500 rev/min.

37

45 40

ONT PNT MNT

Predicted conversion (%)

35 30 25 20 15 10 5 0 0

5

10

15

20

25

30

35

40

45

Experimental conversion (%) Fig. 11. Comparison of calculated and experimental conversions of nitrotoluenes. All conditions are the same as in Table 1.

38

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