a reduction of nitrotoluenes by aqueous ammonium sulfide kinetic study.pdf

8/20/2019 A Reduction of Nitrotoluenes by Aqueous Ammonium Sulfide Kinetic Study.pdf

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Phase Transfer Catalyzed Reduction of Nitrotoluenes by Aqueous

Ammonium Sulfide: Kinetic Study

Sunil K. Maitya

, Narayan C. Pradhan*, b

, Anand V. Patwardhanc

a Department of Chemical Engineering, IIT, Kharagpur, India, Email: [email protected]

b Department of Chemical Engineering, IIT, Kharagpur, India, Email: [email protected]

c Department of Chemical Engineering, IIT, Kharagpur, India, Email: [email protected]

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

Keywords: Liquid-liquid phase transfer catalysis; Ammonium sulfide; Nitrotoluene.

AbstractThe reduction of nitrotoluenes (o-, m- and p-) using aqueous ammonium sulfide as the reducing agent was

carried out in an organic solvent, toluene, under liquid-liquid mode with phase transfer catalyst (PTC),tetrabutylammonium bromide (TBAB). The selectivity of toluidines was found to be 100%. The reaction rate ofm-nitrotoluene was found to be highest among the three nitrotoluenes followed by p- and o-nitrotoluene. The

effects of different parameters such as speed of agitation, catalyst concentration, sulfide concentration,concentration of nitrotoluene, and temperature on the reaction rate and conversion were studied. The rate ofreaction of nitrotoluene was found to be proportional to the concentration of catalyst, to the square of theconcentration sulfide, and to the cube of the concentration of nitrotoluene. The apparent activation energy forthis kinetically controlled reaction was estimated as 19.43, 21.45 and 25.54 kcal/mol for ONT, PNT and MNT,respectively.

1. Introduction

Phase transfer catalysts (PTC) are widely usedto intensify otherwise slow heterogeneous reactions

involving an organic substrate and an ionicreactant, either dissolved in water (liquid-liquid) or present in solid state (liquid-solid). Phase transfercatalysis is now an attractive technique for organicsynthesis because of its advantages of simplicity,reduced consumption of organic solvent and raw

materials, mild operating conditions, and enhancedreaction rates and selectivity. Among severalvarieties of phase transfer catalysts, quaternaryammonium salts are most preferred for their better

activity and ease of availability.

Two mechanisms, interfacial and bulk, aregenerally used for liquid-liquid phase- transfercatalysis based on the lipophilicity of thequaternary cation. The bulk mechanism, as

suggested by Starks [1] and Starks and Liotta [2], isapplicable to catalysts that are not highly lipophilicor that can distribute themselves between theorganic and the aqueous phase, such as benzyltriethyl ammonium, dodecyl trimethyl ammonium,and tetrabutyl ammonium salts. In the interfacial

model, catalysts such as tetrahexyl ammonium andtrioctyl methyl ammonium salts remain entirely in

the organic phase because of their high lipophilicityand exchange anions across the liquid-liquidinterface [3]. Tetrabutyl ammonium bromide

(TBAB) has been reported to be the most activePTC among six different catalysts used to intensify

the reaction of benzyl chloride with solid sodiumsulfide [4]. Several researchers have also carried

out reduction of nitroarenes by sodium sulfideusing TBAB as PTC [5-8]. However, there is noreported work in the literature on the preparation ofaryl amines using aqueous ammonium sulfide in presence of phase transfer catalyst, TBAB.

The reduction of nitrotoluenes to thecorresponding amines is commercially veryimportant as the products toluidines have widecommercial applications as intermediates for dyes,agrochemicals and pharmaceutical products. Theuse of ammonium sulfide as a reducing agent is of

considerable practical value due to some inherentadvantages of the method over other conventional processes. For example, catalytic hydrogenationrequires more expensive equipment and hydrogenhandling facility; additional problems arise due tocatalyst preparation, catalyst poisoning hazards and

the risk of reducing other groups. Although thereduction by iron is reserved for small-scalecommercial applications, it cannot be used forreduction of a single nitro group in a polynitrocompound, 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 ofazoxy and azo compounds, besides being

Presented at

SCHEMCON, Indian Institute of Technology, Guwahati, 2005.Sunil K. Maity is presently with NIT-Rourkela, India. ; [email protected]

This paper is Archived in dspace@nitr ; http://dspace.nitrkl.ac.in/dspace



expensive. The present work is concerned with thereduction of nitrotoluenes (o-, m- and p-) by usingaqueous ammonium sulfide as the reducing agent

under two phase condition with TBAB as the PTC.The reduction reaction of nitroarenes by

negative divalent sulfur (sulfide, hydrosulfide and

polysulfides) is called Zinin reduction [9]. Sodiumsulfide, sodium disulfide and ammonium sulfideare most commonly used for this purpose. The

reduction of nitroarenes by aqueous ammoniumsulfide follows the stoichiometry of Eq. 1 [10, 11].

ArNO2 + 3HS− + H2O→ ArNH2 + 3S+ 3HO− (1)

Development of ammonium sulfide or

hydrogen sulfide based processes is also very muchwelcome in the refining industry. With gradualdecline of light and easy-to-process crude oils,refiners throughout the world are forced to process

heavy crudes containing high amount of sulfur andnitrogen. In addition, refiners are forced to

hydrotreat such crude to bring down the sulfur andnitrogen levels to those prescribed byenvironmental protection agencies. Duringhydrotreatment of heavy and sour crude, largequantities of hydrogen sulfide and ammonia are

produced. The stream containing these gases arefirst scrubbed with water to remove ammonia andthen sent through amine treating unit to removehydrogen sulfide, which is further processed in the

Claus unit to produce elemental sulfur [12]. Thiselemental sulfur is mainly used for sulfuric acid

production and to some extent in the rubberindustry. Due to very high production ratecompared to the consumption rate, refineries processing sour crude are facing severe problem indisposing elemental sulfur produced in their sulfur

recovery units (SRUs). Therefore, any process,which could consume hydrogen sulfide, will bevery much helpful to the refining industry inalleviating the sulfur disposal problem.

Several researchers used sodium sulfide anddisulfide for the reduction of nitroarenes tocorresponding amines both in presence and absenceof PTC and also under different modes (liquid-

liquid or solid–liquid). Hojo et al. [13] studied thekinetics of the reduction of nitrobenzene byaqueous methanolic solutions of sodium disulfide.Bhave and Sharma [14] studied the kinetics of two- phase reduction of aromatic nitro compounds (e.g.,

m-nitrochlorobenzene, m-dinitrobenzene and p-nitroaniline) by aqueous solutions of sodium

monosulfide and sodium disulfide. Pradhan andSharma [5] studied the reaction ofnitrochlorobenzene with sodium sulfide both in the presence and absence of a PTC. In the solid-liquidmode, the reactions of o- and p-nitrochlorobenzene

gave 100% chloroanilines in the absence of a

catalyst and 100% dinitrodiphenyl sulfides in presence of a catalyst. The reaction of m-

nitrochlorobenzene with solid sodium sulfide,

however, was reported to give m-chloroaniline asthe only product even in the presence of a catalyst.In the liquid-liquid mode, all three substrates gaveonly amine as the product both in the presence andabsence of a catalyst. Pradhan [6] has studied the

reduction of nitrotoluenes (o-, m- and p-) bysodium sulfide to corresponding aminotoluenes, both in the liquid-liquid and solid-liquid modeswith phase transfer catalyst, TBAB. In the liquid-

liquid mode, the reactions of all three nitrotolueneswere reported to be kinetically controlled. In solid-

liquid mode, the reactions of o- and p-nitrotolueneswere kinetically controlled whereas that of m-nitrotoluene was reported as mass transfercontrolled. Yadav et al. [7] have reported thekinetics and mechanisms of L–L PTC reduction of

p-nitroanisole by sodium sulfide to p-anisidine.It is evident from the above discussion that

there is hardly any information in the literature onthe reduction of nitrotoluenes by aqueous

ammonium sulfide. Considering the importance ofthe system, a detailed study was, therefore, performed and reported in the present work for thereduction of nitrotoluenes to produce commerciallyimportant toluidines. The effect of various

parameters on the reaction rate and conversionwere studied.

2. Experimental

2.1 Chemicals Toluene (≥99%) of LR grade and LIQR

ammonia (~26%) of analytical grade were procuredfrom S. D. Fine Chemicals Ltd., Mumbai, India.

Nitrotoluenes (>99%) of synthesis grade were purchased from Loba Chemie Pvt. Ltd., Mumbai,India. Tetrabutylammonium bromide (TBAB) wasobtained from SISCO Research Laboratories Pvt.

Ltd., Mumbai, India.

2.2 EquipmentThe reactions of nitrotoluenes with aqueous

ammonium sulfide were carried out batch wise in afully baffled mechanically agitated glass reactor ofcapacity 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 of1.5 cm from the bottom, was used for stirring thereaction mixture. The reactor was kept in a constanttemperature water bath whose temperature could be

controlled within ±1oC.

2.3 Preparation of ammonium sulfide solutionFor the preparation of ammonium sulfide

solution, around 15% ammonia solution was

prepared first by adding suitable quantity of LIQRammonia in distilled water. Then H2S gas was

SCHEMCON, IIT Guwahati, 2005.



reaction was, therefore, considered as 2nd orderwith respect to sulfide concentration. However, forthe reduction of nitroarenes with aqueous sodium

sulfide, the reaction rate was reported to be firstorder with the sulfide concentration [7, 14]. Therate was reported to be proportional to the square of

the concentration of sodium disulfide [13].

0 50 100 150 200 250 3000

5

10

15

20

25

30

35

40

45

C o n v e r s i o n ( % )

Reaction time (min)

m-nitrotoluene

p-nitrotoluene

o-nitrotoluene

Figure 1: Comparisons among the nitrotoluenes.

volume of organic phase = 5×10-5 m3; nitrotoluene

= 1.17 kmol/m3; TBAB = 9.3×10-2

kmol/m3 of

organic phase; volume of aqueous phase = 5×10-5 m3; concentration of sulfide = 2.2 kmol/m3;concentration of ammonia = 5.62 kmol/m3;temperature = 323 K; speed of agitation =1500

rev/min.

800 1000 1200 1400 1600 1800 2000 2200 2400 2600

0.0

0.5

1.0

1.5

4.0

4.5

5.0

5.5

6.0

ONT1

PNT2

MNT3

R e a c t i o n r a t e x 1 0 4

( k m o l / m 3

s )

Speed of agitation (rev/min)

Figure 2. Effect of speed of agitation. matchingconversion = 20%. volume of organic phase =

volume of aqueous phase = 5×10-5 m3; speed ofagitation = 1500 rev/min; 1,3temperature = 323 K.1,3TBAB = 9.3×10-2 kmol/m3 of organic phase;ONT = 1.35 kmol/m3; 1concentration of ammonia =

8.03 kmol/m3;

1concentration of sulfide = 2.6

kmol/m3; PNT = 1.17 kmol/m3; 2TBAB = 12.4×10-

2 kmol/m

3 of organic phase;

2concentration of

sulfide = 2.9 kmol/m3;

2,3concentration of ammonia

= 8.42 kmol/m3; 2temperature = 333 K; MNT = 1.7kmol/m3; 3concentration of sulfide= 3.5 kmol/m3.

Table 2. Effect of sulfide concentrationa

Sulfide concentration,kmol/m

3

Rate of reaction×104,

kmol/m3s

2.85 1.57

3.42 2.303.94 3.467.77 8.64

a Matching conversion = 5%; volume of organic

phase = 5×10-5 m3; MNT = 1.7 kmol/m3; TBAB =

9.3×10-2 kmol/m3 of organic phase; volume of

aqueous phase = 5×10-5 m3; concentration ofammonia = 8.42 kmol/m

3; temperature = 323 K;

speed of agitation =1500 rev/min.

3.5 Effect concentration of nitrotolueneThe effect of concentration of ONT on the

conversion is shown in Figure 3. The conversion as

well as the reaction rate increases with increase in

concentration of ONT. The order of the reactionwith respect to ONT concentration was obtained as3.16 which is close to third order. However, for the

reduction of nitroarenes by aqueous sodium sulfide,the reported order is unity with respect to the

concentration of p-nitroanisole [7] andnitroaromatics [14]. The rate was also reported as proportional to the concentration of nitrobenzenefor its reduction with sodium disulfide [13].

0 50 100 150 200 250

0

5

10

15

20

25

30

35

40

ONT, kmol/m3

1.021.35

1.69

2.03

C o n v e r s i o n o f O N T ( % )

Reaction time (min)

Figure 3. Effect of concentration of ONT.

volume of organic phase = 5×10-5 m3; TBAB =

9.3×10-2 kmol/m3 of organic phase; volume of

aqueous phase = 5×10-5

m3; concentration of

ammonia = 8.03 kmol/m3; concentration of sulfide

= 2.6 kmol/m3; temperature = 323 K; speed ofagitation =1500 rev/min.

3.6 Effect of temperatureThe effect of temperature on the rate of

reaction of nitrotoluenes with aqueous ammoniumsulfide was studied in the range of 303-333K in presence of catalyst, TBAB. 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 the Figure 4. The apparent activation




energy for this kinetically controlled reaction wascalculated from the slope of the straight lines as19.43, 21.45 and 25.54 kcal/mol for ONT, PNT and

MNT, respectively.

3.00x10-3

3.08x10-3

3.15x10-3

3.23x10-3

3.30x10-3

-13.5

-13.0

-12.5

-12.0

-11.5

-11.0

-10.5

-10.0

-9.5

-9.0

-8.5 Linear fit

Exptl. r 2 AE(kcal/mol)

MNT1

0.96 25.54

PNT2 0.99 21.45

ONT3 0.96 19.43

L n ( I n i t i a l r a t e ,

k m o l / m 3 s )

1/T(K)

Figure 4. Arrhenius plot. volume of organic phase

= 5×10-5 m3; volume of aqueous phase = 5×10-5 m3;concentration of ammonia = 8.03 kmol/m3; TBAB

= 9.3×10-2 kmol/m3 of organic phase; speed ofagitation =1500 rev/min. 1MNT = 1.35 kmol/m3;concentration of sulfide = 2.8 kmol/m3. 2PNT=1.46 kmol/m3; concentration of sulfide = 2.6

kmol/m3.

3ONT = 1.35 kmol/m

3; concentration of

sulfide = 2.6 kmol/m3.

4. Conclusions

The reduction of nitrotoluenes by aqueousammonium sulfide to the corresponding toluidineswas studied under liquid–liquid mode in presence

of PTC, TBAB. The selectivity of toluidines was100%. The MNT was found to be the most reactive

among the nitrotoluenes followed by PNT andONT. The reaction was found to be kineticallycontrolled with apparent activation energies of25.54, 21.45 and 19.43 kcal/mol for MNT, PNT

and ONT, respectively. The rate of reduction ofnitrotoluene was found to be proportional to theconcentration of catalyst, square of theconcentration of sulfide, and to the cube of the

concentration of nitrotoluenes.

AcknowledgmentSunil K. Maity is thankful to the All India Councilof Technical Education (AICTE), New Delhi,

India, for the award of the National Doctoral

Fellowship.

NomenclatureMNT m-nitrotolueneONT o-nitrotoluene

PNT p-nitrotolueneTBAB tetrabutylammonium bromide

PTC phase transfer catalyst

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