Control of Competing Hydrogenation of Phenylhydroxylamine to

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Control of Competing Hydrogenation of Phenylhydroxylamine to Aniline in a Single-Step Hydrogenation of Nitrobenzene to p-Aminophenol Jayprakash M. Nadgeri, Narayan S. Biradar, Priyanka B. Patil, Sachin T. Jadkar, Ajit C. Garade, and Chandrashekhar V. Rode* Chemical Engineering and Process Development Division National Chemical Laboratory, Dr. Homi Bhabha Road, Pune - 411008, India ABSTRACT: Two steps involving catalytic hydrogenation of nitrobenzene to phenylhydroxylamine (PHA) in acid medium and its rearrangement to p-aminophenol (PAP) were studied separately in a batch reactor, using a well-characterized 3% Pt/C catalyst. The first step of hydrogenation of nitrobenzene to PHA could be carried out at 303 K and a H2 pressure of 0.69 MPa with complete conversion of nitrobenzene, while the achieved selectivity to PHA was higher than 90% with some formation of aniline, even at lower temperature. The second step of PHA rearrangement to PAP could be achieved under a hydrogen atmosphere at elevated temperature of 353 K to give a maximum selectivity to PAP of 74%.

1. INTRODUCTION p-Aminophenol (PAP) is an important starting material for the manufacture of analgesic and antipyretic drugs such as paracetamol, acetanilide and phenacetin. It is also used as an anticorrosion agent in paints and fuels, in the dye industry, as wood stain (imparting a roselike color to timber), and as dyeing agents for fur and feathers.1
3 Conventionally, PAP is manufactured from p-nitrochlorobenzene or p-nitrophenol. Both these are multistep processes involving Fe/HCl reduction (Bechamp process). The conventional processes generate large amount of Fe/FeO sludge (1.2 kg/kg of product), which cannot be recycled apart from other drawbacks.4 The environmentally benign process is a single-step catalytic hydrogenation of nitrobenzene (NB) to PAP in the presence of an aqueous acid medium, as shown in Scheme 1. Mallinckrodt is the largest producer of PAP via the catalytic hydrogenation route, having increased its capacity from 30 million pounds in 1991 to 70 million pounds in 2005.5 The catalytic process minimized the effluent disposal problems to a great extent, and it also gives improved process economics, because of easy recovery of pure PAP. The Pt/C catalyst, along with a mineral acid, was first reported by Henke and Vaughen for nitrobenzene hydrogenation to PAP, followed by reports on other catalysts, such as PtO2, Pd, Mo, etc.6
8 Several other patents and publications mainly describe the use of cationic surfactants and solid acids and also the influence of process variables on the rate of hydrogenation and selectivity to PAP.9
15 This process for PAP from nitrobenzene involves a two-step reaction carried out in a single reactor. Initial reduction of nitrobenzene gives phenylhydroxylamine (PHA) as an intermediate, which undergoes a Bamberger rearrangement in the presence of an aqueous acid to give PAP (see Scheme 1). The formation of aniline via further hydrogenation of PHA is the major competing reaction that affects the selectivity to PAP. In order to achieve the highest selectivity to PAP, the suppression of the competing hydrogenation of PHA to aniline is highly desirable. In this paper, we report, for the first time, the study of two steps separately, viz, r 2011 American Chemical Society

(i) partial hydrogenation of NB to PHA, and (ii) rearrangement of PHA to PAP. For this purpose, the hydrogenation of NB to PHA was carried out under very mild temperature (T = 303 K) and pressure (pH2 = 0.69
4.1 MPa) conditions, and its acid rearrangement to PAP was observed at an elevated temperature of 353 K, both in the presence of hydrogen and in an inert atmosphere (Argon) under atmospheric pressure conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. Nitrobenzene, aniline, and sulfuric acid (98%) were procured from SD Fine Chemicals, Ltd. (India). Standard p-aminophenol was purchased from Aldrich (Bangalore, India). Standard phenylhydroxylamine (PHA) was prepared separately via the reduction of NB with zinc dust in the presence of an aqueous solution of ammonium chloride.16 2.2. Catalyst Preparation. For the preparation of 3% Pt/C catalyst, 0.159 g of H2PtCl6 3 6H2O was dissolved in a minimum amount of water, ensuring the complete dissolution of the precursor. Agitation was done with a magnetic stirring bar. Under stirring, 2 g of a slurry of carbon support prepared in water was added to the above solution, and the temperature was maintained at 353 K. After 1 h, 3 mL of formaldehyde was added under stirring, as a reducing agent. The reaction mixture was then cooled and filtered to obtain the catalyst, which was dried at room temperature under vacuum. In a similar way samples of 1%, 2%, and 5% Pt supported on carbon catalysts were prepared. 2.3. Experimental Setup. The reactions were carried out in a 3  10
4m3-capacity high-pressure hastelloy reactor supplied by Parr Instrument Co. (USA). It was equipped with a cooling coil, a gas inlet/outlet, liquid sampling system, automatic temperature Received: December 21, 2010 Accepted: February 28, 2011 Revised: February 19, 2011 Published: March 22, 2011 5478

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Scheme 1. Reaction Scheme for Catalytic Hydrogenation of Nitrobenzene to p-Aminophenol

control, variable agitation speed, a safety rupture disc, a hightemperature cutoff, and pressure recording by a transducer. The reactor was connected to a hydrogen reservoir that was held at a pressure higher than that of the reactor, through a constantpressure regulator. Hydrogen gas was supplied from this reservoir to the reactor through a nonreturn valve. The gas consumed during the course of the reaction was monitored by pressure drop in the reservoir vessel, using a transducer. 2.4. Catalytic Activity. In a typical hydrogenation experiment, 0.0813 mol of nitrobenzene, 0.035 g of Pt/C catalyst, and 10 g of sulfuric acid were charged, and the total reaction volume was made to be 1  10
4 m3 with water. The contents were first flushed with nitrogen and then with hydrogen. After the desired temperature was attained, the system was pressurized with hydrogen to a level required for the experiment, and the reaction was started by switching the stirrer on. The progress of the reaction was monitored by the observed pressure drop in the reservoir vessel, as a function of time. The contents were cooled to room temperature. After completion of the reaction, hydrogen was vented out safely and again, the temperature was increased to 353 K for the conversion of PHA to PAP. The progress of the reaction was monitored by intermittent sampling of the liquidphase analysis. The contents were cooled to room temperature and discharged after completion of the reaction. Hydrogenation catalyst was separated from the reaction crude by filtration, and its recyclability was demonstrated in our previous publication.4 After completion of the reaction, the crude was neutralized with NH3 to give ammonium sulfate, which is a sellable product; hence, water will be the only effluent. 2.5. HPLC Analysis. A Hewlett
Packard Model 1050 liquid chromatograph that was equipped with an ultraviolet (UV) detector was employed for the analysis. HPLC analysis was performed on a 25-cm RP-18 column that was supplied by Hewlett
Packard. The product and reactant were detected using a UV detector at λmax = 254 nm, using 35% acetonitrile þ 5% pH 7 buffer þ 60% deionized water as the mobile phase at a column temperature of 298 K and flow rate of 1 mL/min in isocratic mode. Twenty microliters (20 μL) of the reaction mixture were pipetted out into a 10-mL volumetric flask and diluted with the mobile phase. Diluted samples of 10 μL were injected into the column, using an auto sampler (Model HP 1100). 2.6. Catalyst Characterization. BET surface areas of the 1%
5% Pt/C catalysts have been measured by means of N2 adsorption

Figure 1. XRD patterns of Pt loadings of 1%
5%.

at 77 K performed on a Quantachrome Model CHEMBET 3000 instrument. TPR experiments were also performed on a Quantachrome Model CHEMBET 3000 instrument. In the TPR experiment, a U-tube (Quartz tube) was filled with solid catalyst. This sample holder was positioned in a furnace equipped with a temperature control. A thermocouple is placed in the solid for temperature measurement. An equal quantity of fresh vacuumdried catalyst was taken in the U-tube. Initially, a flow of inert gas (nitrogen) was passed through the U-tube to remove the air present in the lines and heated in a N2 atmosphere with a flow rate of 60 mL/min to 523 K for 30 min, to remove the moisture and surface impurities present on the sample, and further cooled to room temperature. The nitrogen then was replaced by a mixture of 5% H2 in N2 for the TPR experiment, and the hydrogen uptake was measured using a thermal conductivity detector. X-ray diffraction (XRD) patterns were recorded on a PAnalytical PXRD system (Model X-Pert PRO-1712), using Nifiltered Cu KR radiation (λ = 0.154 nm) as a source (current intensity, 30 mA; voltage, 40 kV) and X-celerator detector. The samples were scanned in the 2θ range of 30°
80°. The species present on the surface were identified by their characteristic 2θ values of the relevant crystalline phases. The software program X-Pert High Score Plus was employed to subtract the contribution of the Cu KR2 line prior to data analysis. X-ray photoelectron spectra were recorded using an ESCA-3000 (VG Scientific, Ltd., England) with a 9-channeltron CLAM4 analyzer under a vacuum of better than 1  10
8 Torr, using Mg KR radiation (1253.6 eV) and a constant pass energy of 50 eV. The binding energy values were charge-corrected to the C1s signal (284.6 eV).

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Powder XRD patterns of Pt/C with 1%
5% metal loading are shown in Figure 1. The reflection peaks at 2θ of 39°, 46°, and 68° are based on the 111, 200, 220 reflection peaks for metallic platinum, representing the typical character of a crystalline Pt face-centered-cubic (FCC) lattice structure.17 The broader peaks were observed in the case of 1% and 2% Pt loadings, indicating lower crystallinity than that observed for higher metal loadings of 3% and 5%.18 This was in accordance 5479

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Table 1. BET Surface Area, Degree of Dispersion, and Particle Size of Pt in Pt/C Catalyst for Various Metal Loadings Pt metal

surface area

degree of

particle

loading (%)

(m2/g)

dispersion (%)

size (nm)a

0 1

1100 703

74

8.6

2

656

73

8.9

3

620

69

10.5

5

482

51

13.0

a

Particle size of Pt metal in Pt/C catalyst is calculated using the Scherrer equation.

Figure 3. XPS spectra for the 3% Pt/C catalyst.

Figure 2. TPR profiles for metal loadings of 1%
5%.

with the increase in particle size from 8 nm to 13 nm, with an increase in metal loading from 1% to 5%. The specific BET surface areas of carbon and Pt/C with Pt loadings varying from 1% to 5% are shown in Table 1. The BET surface area of carbon decreased consistently from 1100 to 482 m2/g as the Pt loading increased from 1% to 5%. The decrease in BET surface area indicates that Pt was well-dispersed on the carbon support.19 The extent of Pt metal dispersion on carbon support was determined by the pulse titration method. The term “dispersion” refers to the ratio of the number of metal atoms on the surface to the total number of metal atoms present. It was found that, as the metal loading was increased from 1% to 3%, the dispersion of Pt marginally decreased from 74% to 69% and, with further increase in Pt loading, from 3% to 5%, resulting in a decrease in dispersion, from 69% to 51% (see Figure 13, presented later in this work). This decrease in dispersion was due to the increase in particle size with increasing Pt metal loading, as evidenced by XRD analysis. The TPR profile of Pt/C catalyst showed two major peaks at 363 K and 513
523 K in Figure 2. The peak at 363 K could be due to the hydrogen consumption by the oxygen attached to Pt as a superficial Pt oxide or unreduced Pt species.18,20 It was observed that, as the metal loading increased from 1% to 5%, the formation of unreduced Pt species increased and it also showed the increase in peak intensity of PtO at 363 K. The peak at 523 K was attributed to the reduction of Pt4þ species.21

Since hydrogenation was carried out at low temperature (303 K), the PtO and PtO2 may not be the active species and only metallic Pt0 could be playing a major role in the hydrogenation of nitrobenzene to PHA and aniline. Figure 3 presents XPS spectra of the 3% Pt/C catalyst in which the Pt 4f spectrum of the catalyst shows Pt in different oxidation states. The most intense doublet for Pt 4f7/2 and Pt 4f5/2 (71.4 and 74.7 eV) is due to metallic Pt. The second set of doublets for Pt 4f7/2 and Pt 4f5/2 (73.4 and 76.7 eV) could be assigned to the Pt2þ chemical state as PtO or Pt(OH)2.22,23 The third doublet of Pt is weakest in intensity, and occurred due to the binding energies for Pt 4f7/2 and Pt 4f5/2 (75.5 and 78.8 eV), which could be assigned to the Pt4þ chemical state. The Pt 4f spectrum of the Pt/C catalyst was fitted with two pairs of overlapping Gaussian curves, and the percentage of the Pt metal species was calculated from the relative areas of these peaks. These data revealed that 49.7% was the Pt metal, 31.5% was divalent Pt-oxide (PtO), and the remaining 18.8% of the Pt was present in a higher oxidation state (which was expected to be platinum dioxide, PtO2).24 The XPS spectra did not show the presence of a Cl
peak, which ensured the complete consumption of chloroplatinic acid during the catalyst preparation. 3.2. Catalyst Activity Measurement. Catalytic hydrogenation of nitrobenzene to PAP involves four phases, viz, an organic phase of nitrobenzene, an aqueous phase of dilute sulfuric acid, a solid catalyst, and hydrogen as a gas phase, as shown schematically in Figure 4.4,13 It was observed that the Pt/C catalyst remained within the nitrobenzene (organic layer) interface. As the reaction proceeded, the intermediate phenylhydroxylamine migrated outward from the organic phase into the aqueous sulfuric acid phase, and the Bamberger rearrangement proceeds to give p-aminophenol. An increase in nitrobenzene conversion obviously leads to a decrease in nitrobenzene concentration during the reaction, which results in an increase in the catalyst: nitrobenzene ratio. PHA formed in this catalyst-rich environment immediately undergoes further hydrogenation to give aniline, which is a competiting reaction with the Bamberger rearrangement to give PAP. The effects of various parameters on the conversion of NB and PHA, and the selectivities to PHA, PAP, and aniline, are described in the following sections. 3.2.1. Effect of Reaction Time. Figure 5 shows a typical conversion, selectivity vs time profile for the partial hydrogenation 5480

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Figure 4. Gas, solid, organic, and aqueous phases involved in the hydrogenation of nitrobenzene to PAP via PHA.

Figure 5. Bamberger rearrangement facilitated at high temperature. (Reaction conditions: nitrobenzene, 0.0813 mol; acid, 10 g; water, 84 g; 3% Pt/C, 0.035 g; temperature, 303 K; H2 pressure, 0.69 MPa; and agitation speed, 1000 rpm.)

of nitrobenzene to PHA at 303 K and a pressure of 0.69 MPa in a biphasic mixture consisting of nitrobenzene and aqueous sulfuric acid using 3% Pt/C. As can be seen from this figure, at the end of the first hour of the reaction, a PHA selectivity as high as 95% was obtained, with a nitrobenzene conversion of >35%. As the reaction proceeded, almost-complete conversion of nitrobenzene was obtained after ∼3 h; however, an appreciable amount of aniline started to form while PAP formation was 97%. Before increasing the temperature, hydrogen gas was released in order to enhance selectivity to PAP. As soon as the reaction temperature was increased to 353 K, PHA rearrangement was observed to give PAP (74%) after 8 h,

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Figure 6. PHA:aniline ratio. (Reaction conditions: nitrobenzene, 0.0813 mol; acid, 10 g; water, 84 g; 3% Pt/C, 0.035 g; temperature, 303 K; H2 pressure, 0.69 MPa; and agitation speed, 1000 rpm.)

indicating that the Bamberger rearrangement was facilitated at higher temperature. The effect of reaction time on PHA/aniline ratio at 303 K is shown in Figure 6. During the first hour of the reaction and up to 40% conversion of nitrobenzene, the PHA:aniline ratio was as high as 22; however, as the nitrobenzene conversion increased to >95% after 2 h, the PHA:aniline ratio decreased to ∼3 and remained almost constant at this value. Thus, over a period of time, further hydrogenation of PHA to aniline was the much more predominant reaction than the rearrangement of PHA to PAP. 3.2.2. Effect of Hydrogen Pressure on PHA Selectivity. Since the first step involves the hydrogenation of nitrobenzene to PHA, the effect of hydrogenation pressure on catalyst activity and selectivity to both PHA and aniline was also studied at 303 K, and the results are shown in Figure 7. As the H2 pressure increased from 0.69 to 4.1 MPa (6-fold), the turnover frequency (TOF, which is expressed as the ratio of the number of moles of substrate converted to those of active metal used, per unit time) also increased linearly, from 2.4  103 h
1 to 12.25  103 h
1. The selectivity to PHA decreased from 70% to 63% as the H2 pressure increased from 0.69 MPa to 4.1 MPa. Because of the increase in H2 pressure, a greater concentration of surface hydrogen becomes available on the active catalyst sites, facilitating the hydrogenation of PHA to aniline; hence, the selectivity to aniline marginally increased from 27% to 29%. 3.2.3. Effect of Acid Concentration. Although the first step involves hydrogenation of nitrobenzene to PHA, this hydrogenation must be carried out in the presence of an acid; otherwise, only aniline formation was observed in the absence of acid, even at 303 K. Hence, it was necessary to study the effect of acid concentration on the selectivity to PHA. This study was carried out by varying the acid concentration in a range of 5% to15% (w/w), keeping other reaction conditions constant, and the results are shown in Figure 8. The conversion of nitrobenzene remained almost constant (>99%) while the selectivity to PHA increased from 30% to 70% as the acid concentration increased from 5% to 10%. At higher acid concentrations of 15%, rearrangement of PHA to PAP was initiated even at a lower temperature of 303 K, hence lowering the PHA selectivity from 72% to 66%. Thus, an acid concentration of 10% w/w was found to be the optimum for the highest selectivity of PHA. 5481

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Figure 9. Effect of nitrobenzene concentration. (Reaction conditions: acid, 10 g; water, 84 g; 3% Pt/C, 0.035 g; temperature, 303 K; H2 pressure, 0.69 MPa; and agitation speed, 1000 rpm.) Figure 7. Effect of pressure on PHA selectivity. (Reaction conditions: nitrobenzene, 0.0813 mol; acid, 10 g; water, 84 g; 3% Pt/C, 0.035 g; temperature, 303 K; agitation speed, 1000 rpm; turnover frequency (TOF), 103 h
1.)

Figure 10. Bamberger rearrangement facilitated under an inert atmosphere. (Reaction conditions: nitrobenzene, 0.0813 mol; acid, 10 g; water, 84 g; 3% Pt/C, 0.035 g; temperature, 303 K; H2 pressure, 0.69 MPa; and agitation speed, 1000 rpm.)

Figure 8. Effect of acid concentration on PHA selectivity. (Reaction conditions: nitrobenzene, 0.0813 mol; water, 84 g; 3% Pt/C, 0.035 g; temperature, 303 K; H2 pressure, 0.69 MPa; and agitation speed, 1000 rpm.)

3.2.4. Effect of Nitrobenzene Concentration. Typical results of the effect of initial concentration of nitrobenzene in the range of 0.04
0.12 mol on conversion and selectivity pattern are shown in Figure 9. Marginal lowering of the nitrobenzene conversion (from 99% to 97%) was observed as the nitrobenzene concentration increased from 0.04 mol to 0.12 mol. Selectivity to PHA remained almost constant at ∼72%, up to a nitrobenzene concentration of 0.08 mol, while it decreased to 50% as the nitrobenzene concentration increased further, to 0.12 mol, due to the formation of both aniline and PAP.

3.2.5. Bamberger Rearrangement with Nitrobenzene as a Substrate. In situ rearrangement of PHA to PAP was studied during nitrobenzene hydrogenation under inert (argon) atmosphere, as well as under hydrogen atmosphere. Under inert atmosphere, a PAP selectivity of 77% was achieved (see Figure 10) while under H2 atmosphere, the selectivity to PAP was reduced to 74% (see Figure 5), because of the higher amount of aniline formed. Since the first step essentially involves the hydrogenation of nitrobenzene to PHA, further hydrogenation of PHA to aniline also is initiated, which could only be controlled by keeping the lower temperature of 303 K. 3.2.6. Effect of Hydrogen Pressure on PAP Selectivity. Figure 11 shows the effect of H2 pressure on the PAP selectivity during the rearrangement of PHA at 353 K. PAP selectivity decreased from 74% to 57% as the H2 pressure increased from 0.69 MPa to 4.1 MPa, because of a competitive hydrogenation of 5482

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Figure 11. Effect of pressure on PAP selectivity. (Reaction conditions: nitrobenzene, 0.0813 mol; acid, 10 g; water, 84 g; 3% Pt/C, 0.035 g; temperature, 353 K; and agitation speed, 1000 rpm.) Figure 13. Effect of metal loading on PHA selectivity, dispersion, and TOF. (Reaction conditions: nitrobenzene, 0.0813 mol; acid, 10 g; water, 84 g; Pt/C, 0.035 g; temperature, 303 K; H2 pressure, 0.69 MPa; agitation speed, 1000 rpm.)

Table 2. Bamberger Rearrangement with PHA as a Substrate Selectivity sr. no.

variables

conversion PAP aniline byproduct

1 2

pH2 = 0.69 MPa inert atm.

100 100

3

pH2 = 0.69 MPa, without acid

100

70 100

10

20

100

a

Reaction conditions: phenylhydroxylamine, 0.0917 mol; acid,10 g; water, 84 g; 3% Pt/C, 0.035 g; temperature, 353 K; H2 pressure, 0.69 MPa; and agitation speed, 1000 rpm.

Figure 12. Effect of catalyst loading on PHA selectivity. (Reaction conditions: nitrobenzene, 0.0813 mol; acid, 10 g; water, 84 g; 3% Pt/C; temperature, 303 K; H2 pressure, 0.69 MPa; and agitation speed, 1000 rpm.)

PHA to aniline reaction at 353 K. This clearly shows that a selectivity to PAP as high as 74% could be achieved only if the reactor is depressurized after the first hydrogenation step carried out at 303 K, and the next step of Bamberger rearrangement is continued under H2 atmosphere by enhancing the temperature to 353 K. 3.2.7. Effect of Catalyst Loading. Effect of catalyst loading on selectivity to PHA was studied in the range of 0.02
0.105 g by keeping the other reaction parameters constant for a 3% Pt/C catalyst, and the results are shown in Figure 12. It was found that, at lower catalyst loading (0.02 g), the selectivity toward PHA was 72%, whereas, as the catalyst loading increased from 0.035 g to 0.105 g, the selectivity toward PHA decreased from 70% to 60%. This was due to more catalyst active sites being available for further hydrogenation of PHA to aniline. 3.2.8. Effect of Metal Loading. The fffect of platinum metal loading on carbon in a range of 1%
5% on selectivity to PHA,

TOF, and metal dispersion was also studied, and the results are shown in Figure 13. For all the metal loadings, conversion of nitrobenzene remained constant, while the selectivity to PHA increased from 55% to 70% as the metal loading increased from 1% to 3%. At a higher metal loading of 5%, the selectivity to PHA decreased to 65% . An increase in the Pt content from 1% to 5% produced a dramatic improvement in the catalytic activity (TOF) from 0.27  103 h
1 to 5.8  103 h
1. Here, note that, with a higher Pt loading of 5%, the selectivity to PHA decreased as the catalytic activity increased. Therefore, it is very critical to control the hydrogenation rate of phenylhydroxylamine to aniline by maintaining an appropriate Pt metal loading. 3% Pt/C catalyst showed the better activity (2.45  103 h
1) with the highest selectivity (70%) to PHA. 3.2.9. Bamberger Rearrangement with PHA as a Substrate. A few experiments were carried out with PHA as a substrate under three different conditions in order to understand whether the product distribution is different from that when nitrobenzene was used as a substrate. With PHA as a substrate, under hydrogenation conditions and in the presence of H2SO4, the major product formed was PAP, while aniline selectivity was only 10% with the formation of a coupling product up to 20% (entry 1, Table 2). Contrary to the reaction with nitrobenzene as a substrate, in the present case, PHA was completely soluble in an aqueous medium; hence, although PHA rearrangement to PAP was 5483

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Industrial & Engineering Chemistry Research facilitated, the coupled product formation was also more dominant than the hydrogenation to aniline. As expected, complete selectivity to PAP was obtained in the presence of an inert atmosphere (entry 2, Table 2), whereas only aniline was obtained under hydrogenation conditions without acid (entry 3, Table 2).

4. CONCLUSION p-Aminophenol via the catalytic hydrogenation of nitrobenzene was studied, involving first the hydrogenation of nitrobenzene to an intermediate PHA at 303 K, followed by its rearrangement at elevated temperature of 353 K. An aqueous acid medium also was found to be essential for the first hydrogenation step; however, an increase in temperature from 303 K to 353 K initiates the rearrangement of PHA to PAP. Initially, the selectivity to PHA was as high as 95% at 36% conversion of nitrobenzene, which then decreased to 75% for complete conversion of nitrobenzene, because of the competing hydrogenation of PHA to give aniline. An increase in H2 pressure also had a dramatic effect in lowering the PHA selectivity, because of the availability of a higher concentration of surface hydrogen, causing the further hydrogenation of PHA to aniline. Rearrangement of PHA in an inert atmosphere (argon) gave the highest selectivity of 77% to PAP, compared to the rearrangement carried out under a hydrogen atmosphere (74%). The TOF value increased from 0.27  103 h
1 to 5.8  103 h
1 as the metal loading increased from 1% to 5%. The selectivity to PHA decreased from 72% to 60% as the catalyst loading increased from 0.02 g to 0.105 g, because of the hydrogenation of PHA to aniline becoming a more dominant reaction. ’ AUTHOR INFORMATION Corresponding Author

*Fax: þ91 20 2590 2621. E-mail: [email protected].

’ ACKNOWLEDGMENT One of the authors (J.M.N.) is grateful to the Council of Scientific and Industrial Research for the award of a Senior Research Fellowship. ’ ABBREVIATIONS PAP = p-aminophenol PHA = phenylhydroxylamine NB = nitrobenzene TOF = turnover frequency, h
1; TOF = (converted moles of substrate)/[(actual moles of catalyst used for the reaction)  time (h)]

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