p-아미노페놀 합성 연구 · 는 액상 수소화/산 복합 촉매 반응에 대해서...

工學碩士學位請求論文

복합 촉매 반응을 이용한 유기/수용액상

p-아미노페놀 합성 연구

p-Aminophenol synthesis by dual catalytic reactions

in an organic/aqueous system

2002년 2월

仁荷大學校大學院

化學工學科 (化學工學專攻)

閔慶一

工學碩士學位請求論文

복합 촉매 반응을 이용한 유기/수용액상

p-아미노페놀 합성 연구

p-Aminophenol synthesis by dual catalytic reactions

in an organic/aqueous system

2002년 2월

指導敎授安和承

이 論文을 工學碩士學位論文으로 提出함

仁荷大學校大學院

化學工學科 (化學工學專攻)

閔慶一

이 論文을 閔慶一 의 碩士學位論文으로 認定함

2002년 2월

主審

副審

委員

I

요 약

현재 기존의 정밀 화학 중간체에 대한 생산 공정들은

보다 환경 친화적이고 에너지 효율이 높은 화학공정으로 대체

되어 나가는 추세에 있으며, 새로운 대체 촉매/담체의 개발과

반응 scheme의 다양화가 활발히 진행되고 있다.

PAP(p-aminophenol)는 해열진통제로 수요가 큰 Tylenol

의 원료인 acetaminophen합성의 중간체로서 중요하며, 선택성

향상과 공정상의 문제 해결에 대한 연구가 진행되어왔으나, 관

련된 정보는 주로 특허로서 발표되었을 뿐, 반응의 인과 관계

를 밝히는 체계적인 연구 결과는 한정되어 있다. PAP(p-

aminophenol)의 상업적인 합성에서는, 유기상에서 담지 귀금속

촉매에 의한 반응물인 nitrobenzene의 수소화 반응과 수상에서

산 촉매에 의한 재배열반응의 두 단계가 한 반응기 내에서 동

시에 진행된다. 본 연구에서는 의약적으로 중요한 전구체인

PAP의 합성을 위해 유기상/수상의 biphasic 반응계에서 진행되

는 액상 수소화/산 복합 촉매 반응에 대해서 효과적인 촉매의

함량, 반응온도, 산의 농도, 첨가제(phase transfer agent, selectivity

enhancing agent)등의 다양한 반응 인자들을 조절하여 반응속도

및 PAP의 선택도에 대한 최적 반응 조건을 찾았다. 또한, 반응

II

물/생성물의 확산 저항을 줄이기 위하여 메조 크기의 세공분포

도가 높은 탄소 분자체를 합성한 후 백금 촉매의 담체로 사용

하여 실험한 결과 전환율/선택성에서 현저히 향상된 결과를 얻

었다.

III

ABSTRACT

P-aminophenol(PAP) is an important synthesis intermediate

for acetaminophen which is the raw material for Tylenol. Whilst much

work has been done to improve PAP selectivity, little systematic work

enabling us to gauge the effects of various reaction/synthesis

parameters in overall perspective has been reported, except for limited

number of patents. For commercial synthesis of p-aminophenol(PAP), a

biphasic reaction scheme which composed of an organic nitrobenzene

phase in which hydrogenation reaction is taking place using Pt/C

catalyst and an aqueous phase in which Bamberger rearrangement

reaction is taking place by inorganic acid catalyst in a single reactor is

employed. In this work, the effects of various synthesis parameters

involved in this complex system, such as Pt/C catalyst loading, reaction

temperature, acid catalyst concentration, degree of mechanical mixing,

and additives (phase transfer agent, selectivity enhancing agent) on

PAP synthesis was investigated to come up with the optimum set of

reaction conditions. A novel carbon supports, having a regular

mesopores of 3~5nm diameter was prepared using mesoporous silica

IV

matrix as a template. Pt impregnated on novel carbon support was

prepared and tested for liquid phase hydrogenation of nitrobenzene.

Significantly enhanced activities were observed for these mesopore-

supported Pt catalysts compared with active carbon supported catalysts

commercially available. It is believed that these enhancements in

activity are due to increased metal dispersions, and concurrent

reduction in pore diffusion resistance due to mesopore structure of

novel carbon supports.

V

TABLE OF CONTENTS

요 약

ABSTRACT

TABLE OF CONTENTS

LIST OF FIGURES AND TABLES

1. INTRODUCTION

2.EXPERIMENTAL

2.1. Apparatus and Procedure of Nitrobenzene hydrogenation

2.2. Preparation of Mesoporous carbon and Supported pt catalysts

3. RESULTS AND DISCUSSION

3.1. Effect of biphasic reaction parameters on PAP synthesis

3.1.1. Effect of catalyst concentration

3.1.2. Effect of reaction temperature

3.1.3. Effect of H2SO4 concentration in aqueous phase

3.1.4. Effect of DMSO (dimethyl sulfoxide)

Ⅰ

Ⅲ

Ⅴ

Ⅶ

1

7

11

7

13

17

13

21

24

13

VI

3.1.5. Effect of phase transfer agent (N,N-dimethyl-n-

dodecylamine)

3.2. Effect of stirring speed and mesoporous carbons (CMK-1,CMK-3)

3.2.1. Characterization of CMK-1 and CMK-3

3.2.2. Performance of Pt/mesoporous carbons(CMK-1, CMK-3) vs.

commercial Pt/C

4. CONCLUSION

REFERENCES

28

32

32

42

47

49

VII

LIST OF FIGURES AND TABLES FIGURES

Fig. 1.1. A scheme of the nitrobenzene hydrogenation

Fig. 1.2. A scheme of the Bamberger Rearrangement reaction

Fig. 2.1. The experimental apparatus for the biphasic synthesis of

PAP

Fig. 3.1. Cumulative H2 uptake profile of nitrobenzene

hydrogenation with different catalyst loading

Fig. 3.2. Effect of catalyst loading on nitrobenzene conversion and

selectivity to PAP


hydrogenation at different reaction temperature

Fig. 3.4. Effect of reaction temperature on nitrobenzene conversion

and selectivity to PAP


hydrogenation with different H2SO4 concentration

Fig. 3.6. Effect of H2SO4 concentration on nitrobenzene conversion


VIII


hydrogenation with different amounts of DMSO

Fig. 3.8. Effect of DMSO on nitrobenzene conversion and selectivity

to PAP


hydrogenation with different amounts of phase transfer

agent

Fig. 3.10. Effect of phase transfer agent on nitrobenzene conversion


Fig. 3.11. XRD patterns of CMK-1 and its silica template MCM -48

Fig. 3.12. Argon adsorption-desorption isotherm of CMK-1

Fig. 3.13. SEM(a) and TEM(b) micrographs of CMK-1

Fig. 3.14. TEM images of 5% Pt/CMK-1(a) and 5% Pt/C(b)

Fig. 3.15. XRD patterns of CMK-3

Fig. 3.16. Nitrogen adsorption-desorption isotherm of CMK-3

Fig. 3.17. TEM micrographs of CMK-3


hydrogenation with different Pt loadings of CMK-1

IX

TABLES

Table 2.1. Standard reaction conditions of nitrobenzene

hydrogenation

Table 2.2. Standard conditions and setup for HPLC analysis

Table 3.1. Performance comparison of Pt(2%)/CMK-1 with

commercial Pt(5%)/C

Table 3.2. Catalytic performance of Pt(2%)/CMK-1, 3 for

nitrobenzene hydrogenation at different stirring speed

1

1. Introduction

Those chemical processes involved in the preparation of fine

chemical intermediates are undergoing a transition period in which

increasing emphasis is placed upon energy efficiency and

environmentally benign features. Emergence of new catalyst or support

materials as well as the noble reaction schemes proposed such as

biphasic/multi-phasic contacting patterns has been proved to be very

useful in this regard.

Fine chemical products are mainly produced under

homogeneous conditions. Nowadays, simplification of separation

process and catalyst recycle with anchoring active components on

inorganic oxides or polymer supports via immobilizing of catalysts has

been steadily progressed. But it has little practical use because of the

frequent leaching of the catalyst active components from the support

materials. Consequently, biphasic reaction scheme including an

organic/aqueous phase or an organic/inorganic phase is receiving

increased attention [1]. Generally, the biphasic reaction scheme offers

environmental and other significant advantages including avoidance of

the use of a toxic or environmentally troublesome solvent, easy of

2

catalyst recovery and substrate recycle, and the attainment of high

reactivity, selectivity and reproducibility under mild reaction conditions.

Recently, in response to those biphasic reaction scheme advantages, a

new reactor concept named as the Biphasic Countercurrent Extractive

Column Reactor is being developed. This reaction concept is cyclic in

nature and involves simultaneous reaction, product removal and

catalyst recycling [2]. In this study, nitrobenzene hydrogenation was

conducted in biphasic mode of organic/aqueous phase and the optimum

set of biphasic reaction operating conditions was studied to maximize

the selectivity to desired product, p-aminophenol(PAP).

P-aminophenol(PAP) is an important synthesis intermediate

for acetaminophen which is the raw material for Tylenol. Whilst much

work has been done to improve PAP selectivity, little systematic work

enabling us to gauge the effects of various reaction/synthesis

parameters in overall perspective has been reported, except for limited

number of patents. For commercial synthesis of p-aminophenol(PAP), a

biphasic reaction scheme which composed of an organic nitrobenzene

phase in which hydrogenation reaction is taking place using Pt/C

catalyst [3,4] and an aqueous phase in which Bamberger rearrangement

reaction is taking place by inorganic acid catalyst in a single reactor is

3

employed [5,6].

According to this reaction scheme summarized in Fig. 1. 1, p-

aminophenol(PAP) is prepared by successive hydrogen reduction of

nitrobenzene(NB) to N-phenylhydroxylamine(PHA) followed by the

extraction of the PHA intermediate into an aqueous sulfuric acid [7,8]

phase, where the PHA undergoes acid-catalyzed rearrangement

-H 2O

NO 2 NO

H+

-H 2O

NHOH NH 2

OH

NH 2

-H2O

Pt/C, H2 Pt/C, H2

Pt/C, H2

Nitrobenzene Nitrosobenze ne N-Phenylhydroxylamine(PHA) P-Aminophenol

Aniline

PAP

Aqueous Organic

Fig. 1.1. A scheme of nitrobenzene hydrogenation

4

(Bamberger rearrangement) reaction to yield PAP as a soluble sulfate

salt. Aniline is the major side product formed by over-reduction

whereas o-aminophenol is a rearrangement isomer of PAP. Coupled

products are also formed by cross coupling of amino compounds.

Various metal catalysts have been screened in terms of their

effectiveness in reducing nitrobenzene to PHA as opposed to aniline,

and 5 % Pt/C was reported to be the best catalyst among those tested

[3]. A commercial producer, Mallinckrodt, also employs the 5 % Pt/C

catalyst in its current PAP synthesis process.

Acid catalyzed rearrangement of n-phenylhydroxylamine

(PHA) to p-aminophenol(PAP) is the second component of the overall

reaction of interest. This particular rearrangement is generally referred

to as the Bamberger rearrangement. Bamberger was the first to do

extensive research on this reaction. His work lead to the understanding

of the mechanism involved in the acid catalyzed rearrangement of PHA

to PAP. The following is the postulated reaction scheme for the

Bamberger rearrangement [9].

5

According to Fig. 1.2, the rearrangement occurs in an

intermolecular fashion and the oxygen of PAP came from the aqueous

solvent. Detailed kinetic studies show that the reaction is first order in

acid and that the formation of nitrene is the rate-determining step of the

overall rearrangement reaction [10,11]. This nitrene is subsequently

subjected to nucleophilic attack by water to form p-aminophenol.

Bamberger rearrangement is usually carried out using aqueous sulfuric

NHOH N+H

NH

H

N

H

H

+

+

NH 2

NH 2

OH

OH

H +

H2 O

H 2O

H+

H2 O

H+

O-A minophenol

P -A minophenol

PHA

Nitren e in term ediate

Fig. 1.2. A scheme of the Bamberger Rearrangement reaction

6

acid above 10% concentration.

The general principle of the process to improve selectivity to

p-aminophenol is relatively well established. But the optimization of

the biphasic process needs further investigation. In this work, we

investigated the effects of various synthesis parameters involved in this

complex system, such as Pt/C catalyst loading, reaction temperature,

acid catalyst concentration, stirring speed, and additives (phase transfer

agent, selectivity enhancing agent) on PAP synthesis to come up with

the optimum set of reaction conditions. An ordered mesoporous carbon

molecular sieves designated as CMK-1 and CMK-3 were prepared

using silica matrixes (MCM-48, SBA-15) as a template. These

materials synthesized are of high purity and possess regular ordered

mesopores, which may prove to be advantageous in liquid phase

reactions as a catalyst support. Pt supported on mesoporous carbons

(CMK-1, CMK-3) were prepared by incipient wetness method and

tested for commercially important liquid phase hydrogenation reaction

of nitrobenzene. Comparison was made with the performance of

commercially available activated carbon-supported precious metal

catalysts.

7

2. Experimental

1.1. Apparatus and Procedure of Nitrobenzene

Hydrogenation

The experimental apparatus for p-aminophenol(PAP) synthesis

is represented in Fig. 2.1. It consists of a 250-ml, three-necked Morton

flask reactor, a waterbath in which the reactor sits, and a hydrogen

delivery system and a constant-pressure manometric unit to which the

reactor is hooked up and by which the synthesis reaction can be

monitored via hydrogen uptake. Stirring is provided by a mechanical

stirrer whose shaft passes through a vacuum adapter attached to the

middle neck of the reactor. The hydrogen delivery system consists of an

hydrogen tank, a pressure regulator, a delivery line, and three-way

valve with an outlet hooked up to the hydrogen burette of the

manometric unit and the other outlet hooked up to the reactor. The

hydrogen burette is connected via flexible tubing to a second identical

burette that serves as the atmospheric reservoir for the manometer

liquid. The two burettes are attached to a pair of reciprocating arms that

moves up and down in a countervailing manner over a pair of flywheels.

8

Fig

. 2.1

. The

exp

erim

enta

l app

arat

us fo

r th

e bi

phas

ic s

ynth

esis

of P

AP

9

By continuously adjusting the relative height of the two burettes during

the reaction, a constant manometric head can be maintained and the gas

uptake can be conveniently read off the liquid level in the hydrogen

burette.

A typical reaction procedure is as follows: The reactor was

charged with 40mmol(4.924g) of nitrobenzene and 60g of acid solution

and desired amounts of ingredient, i.e., additives (phase transfer agent,

DMSO) and catalyst. Table 1 shows the standard reaction conditions.

The reactor was placed in the constant-temperature waterbath and

purged with hydrogen for about 10 minutes and then allowed to

equilibrate to the set temperature. Following temperature equilibration,

the stirrer was started. The hydrogen uptake of the reaction was

monitored continuously by means of the manometer unit until the

reaction reached completion. Upon completion of the reaction, reaction

products were poured into a separate funnel and standing for overnight

results in organic/aqueous phase separation. The aqueous phase was

sampled. The sample were diluted with water (1: 8000) and then

analyzed by HPLC. Table 2 shows HPLC standard conditions and

setup.This system was convenient because all of the major products of

the reaction system were soluble in the aqueous phase. Therefore, after

10

Table 2.1. Standard reaction conditions of nitrobenzene

hydrogenation

Parameter Spec.

Nitrobenzene 40 mmol

H2SO4(10wt.%) solution 60 g

Pt(5wt.%)/C 30 mg

Pressure 1 atm

Temperature 80 ℃

Stirring Rate 900 rpm

Reaction time 4 h

Table 2.2. Standard conditions and setup for HPLC analysis

Parameter Spec.

Column SYMMETRY C18

UV detector 240 nm

Carrier fluid 50:50 = Acetonitril : Water

with KH2PO4 buffer (0.01mol)

Flow rate 1 ml/min

11

dilution, one was directly able to analyze the reaction products without

further processing. Most experiments on hydrogenation showed that the

main reaction products observed were p-aminophenol and aniline.

Other by-products were too small to analyze and the presence of

phenylhydroxylamine(PHA) could not be detected, indicate that the

rearrangement of PHA to PAP is an instantaneous reaction in the

aqueous phase.

2.2. Preparation of mesoporous carbon supported Pt

catalysts

Ordered mesoporous molecular sieves with carbon framework,

designated as CMK-1 was prepared using the procedure of Ryoo et al

[12]; 5.0g of the dehydrated cubic (Ia3d) phase mesoporous silica

molecular sieve MCM-48 was well mixed with an aqueous solution of

6 g sucrose, 0.7g sulfuric acid, and 30 g distilled water. The resulting

viscous mixture was dried at 373 K in a drying oven and the oven

temperature was increased subsequently to 433 K. The silica host

containing partially decomposed sucrose after the heating was

contacted again with aqueous solution made up of 3.75 g sucrose, 0.4 g

12

H2SO4, and 30 g water. After heating to 433 K again, the powder was

further heated to 1073-1373 K under vacuum using a fused quartz tube

equipped with fritted disks. Finally, the carbon-silica composite was

washed twice using HF solution to remove the silica at 373 K. The

carbon product was then filtered and dried for subsequent use. Another

mesoporous carbon designated as CMK-3 synthesis procedure was

similar to CMK-1 except that the silica template is SBA-15 instead of

MCM-48 [13]. The mesopore structure of the samples prepared was

confirmed by X-ray diffraction using Ni-filtered CuKα radiation

(Philips, PW-1700), and the morphology of the samples was examined

by SEM (Hitachi, X-650) and TEM (Philips, CM 200). The specific

surface area and average pore diameters were determined by N2

physisorption with the BET method at liquid nitrogen temperature

using a Micromeretics ASAP 2000 automatic analyzer. Pt catalyst

supported on CMK-1 and CMK-3 was prepared by incipient wetness

technique to 1-5 wt % loading using H2PtCl6 dissolved in acetone as

metal precursors and reduced in H2 at 473 K for 2h. The metal

dispersions were measured using H2 chemisorption at room

temperature.

13

3. Results and discussion

3.1. Effects of biphasic reaction parameters on PAP

synthesis

3.1.1. Effect of catalyst concentration

Fig. 3.1 shows H2 uptake profiles of the nitrobenzene

hydrogenation with different catalyst loading in the range of 10mg to

30mg of commercial Pt(5%)/C purchased from Aldrich 4h reaction.

The effects of catalyst loading on the nitrobenzene conversion and p-

aminophenol(PAP) selectivity is shown in Fig. 3.2. Nitrobenzene

conversion was measured both by H2 uptake amounts and by products

analysis by HPLC, which matched very well. Nitrobenzene conversion

was found to increase almost linearly as 5% Pt/C loading increases, but

the selectivity to PAP was decreased from 82 to 72% after 4h reaction.

30mg of Pt(5%)/C produced the highest PAP yield among the catalysts

tested and chosen as a base catalyst concentration. This trend in

biphasic reaction with respect to catalyst loading is a consequence of

14

the yield depending in part on the availability of hydrogen at the

catalyst surface. In other words, the increase in catalyst loading results

in increased conversion rate along with a concurrent increase in aniline

yield, and PAP selectivity drops as conversion increases.

15

0 50 100 150 200 2500

400

800

1200

1600

Cum

ulat

ive

H2

upta

ke (m

l)

Time (min)

30mg 15mg 10mg


hydrogenation with different catalyst loading

Reaction conditions: Temperature, 353K;

Pressure, 1 atm;

Nitrobenzene, 40mmol

10% H2SO4 solution, 60g

Catalyst loading, 30mg;

Stirring speed, 900rpm

16

Fig. 3.2. Effect of catalyst loading on nitrobenzene

conversion and selectivity to PAP

Reaction conditions: Temperature, 353K;

Pressure, 1 atm;





0 10 20 300

20

40

60

Pt(5%)/C catalyst loading, g

Con

vers

ion

of N

B, %

0

70

75

80

85

Sel

ectiv

ity to

PA

P, %

17

3.1.2. Effect of reaction temperature

The reaction was also studied at different reaction

temperatures in the range 313 ~ 353K. Fig. 3.3, and Fig. 3.4 showed

that NB conversion and the PAP selectivity increased as the reaction

temperature increase from 313 to 353K. However, the extent of

increase of NB conversion and the total H2 uptake after 4h reaction

with reaction temperature decreased steadily as the temperature

increases. Intrinsic hydrogenation rate constant will increase with

increasing temperature. At the same time, when temperature is

increased the vapor pressure of the solvent will increase and

consequently results in the reduction of the hydrogen pressure in the

reactor which is maintained at a constant total pressure of 1atm. Since

the hydrogenation reaction rate to aniline is more negatively influenced

by low hydrogen pressure at higher temperatures, the extent of increase

of NB conversion was slightly increased at higher temperatures

accompanied by increase of selectivity to PAP by inhibiting the over

reduction of the intermediate, PHA to aniline. On the other hand, the

selectivity to PAP decreased sharply at temperatures below 343K.

Apparently, it seems PHA does not undergo appreciable rearrangement

18

in 10% aqueous sulfuric acid concentration at temperatures below

343K.

19

0 50 100 150 200 2500

400

800

1200

1600

Cum

ulat

ive

H2 u

ptak

e (m

l)

Time (min)

313K 333K 343K 353K


hydrogenation at different reaction temperature

Reaction conditions: Pressure, 1 atm;





20

320 340 3600

20

40

60

80

Reaction temperature, K

Con

vers

ion

of N

B, %

0

60

65

70

75

80

Sel

ectiv

ity to

PA

P, %

Fig. 3.4. Effect of reaction temperature on nitrobenzene


Reaction conditions: Pressure, 1 atm;





21

3.1.3. Effect of H2SO4 concentration in aqueous phase

Bamberger rearrangement is usually carried out using sulfuric

acid catalyst. In this study, experiments were carried out at different

sulfuric acid concentrations in the range 7 ~ 16(wt)% concentration in

aqueous phase. Fig. 3.5 and Fig. 3.6 show H2 uptake profiles during the

reaction and NB conversion and PAP selectivity after 4h reaction,

respectively. When the acid concentration was too low (7wt%), both of

the conversion and the selectivity decreased owing to the slow

rearrangement of synthesis intermediate, n-phenylhydroxylamine(PHA).

When the acid concentration was too high (>13wt%), the NB

conversion after 4h reaction decreased from 66 to 57%. It seems that a

reduced sulfuric acid partly functions as a catalyst poison for Pt/C and

leads to a reduction of its hydrogenation activity [5]. The yield of PAP

was favored by sulfuric acid concentration about 10wt%, and

subsequently chosen as the standard condition.

22

0 50 100 150 200 2500

400

800

1200

1600

Cum

ulat

ive

H2 u

ptak

e (m

l)

Time (min)

7% 10% 13% 16%


hydrogenation with different H2SO4 concentration

Reaction conditions: Temperature, 353K

Pressure, 1 atm;


H2SO4 solution, 60g


23

8 12 160

20

40

60

80

H2SO

4 Concentration, %(w/w)

Con

vers

ion

of N

B, %

0

60

65

70

75

80

Sel

ectiv

ity to

PA

P, %

Fig. 3.6. Effect of H2SO4 concentration on nitrobenzene



Pressure, 1 atm;


H2SO4 solution, 60g


24

3.1.4. Effect of DMSO(dimethyl sulfoxide)

Many chemicals have been found which improve selectivity of

partial hydrogenation of nitrocompounds [3]. Many amines and sulfur

compounds was screened and found that they did increase selectivity of

PHA relative to aniline, but the hydrogenation rate were impractically

low, for most of them poisoned the catalyst easily. One exception was

dimethylsulfoxide (DMSO). It was effective in increasing the yield of

PHA without significantly reducing the rate. In this work, the effect of

DMSO concentration on PAP selectivity was also investigated. As

show in Fig. 3.7, the introduction of DMSO, indeed, resulted in

substantial reduction in NB conversion. However, PAP selectivity

enhancement was noticeable as described in Fig. 3.8. By adding 15mg

of DMSO, the selectivity to PAP was increased dramatically from 72%

without DMSO to 88%. This PAP enhancement was accompanied by

reduction in conversion from 66 to 47%. Further increase in amount of

DMSO produced little improvement in PAP selectivity, whilst sharp

decrease in the rate was observed. Thus it was established that even

small additions of DMSO to the reaction mixture have a potent effect

on the selectivity of the hydrogenation. There was also a limiting value

25

of an amount of DMSO. Added amounts of DMSO had relatively

little effect above 15mg for PAP selectivity. It was reported that these

results were due to an unfavorable competition of PAP with DMSO for

catalyst sites [3]. Therefore, DMSO promotes increase of the selectivity

to PAP by inhibiting the reduction of PHA to aniline.

26

0 50 100 150 200 2500

400

800

1200

1600

Cum

ulat

ive

H2 u

ptak

e (m

l)

Time(min)

15mg 30mg 60mg - mg


hydrogenation with different amounts of DMSO


Pressure, 1 atm;




27

0 20 40 600

20

40

60

80

100

DMSO, g

Con

vers

ion

of N

B, %

0

70

80

90

100

Sel

ectiv

ity to

PA

P, %

Fig. 3.8. Effect of DMSO on nitrobenzene conversion



Pressure, 1 atm;




28

3.1.5. Effect of Phase transfer agent (N,N-dimetyl-n-

dodecylamine)

The rate of reaction in this process substantially depends on

the solubility of the nitrobenzene or its wettability by the aqueous

system. However, since this solubility or wettability of the nitrobenzene

is exceptionally low, this matter represents a critical point that makes

the usability of the Bamberger reaction more difficult and limits and in

some cases completely prevents it. For the reasons mentioned, in the

past there has been sustained effort to improve the wetting of the

organic nitrocompound and in this way to exert a favorable influence

on the course of Bamberger rearrangement [14,15,16]. In this work, the

experiments was carried out with N,N-dimethyl-n-dodecylamine as

phase transfer agent to improve the reaction rate and selectivity. The

effect of N,N-dimethyl-n-dodecylamine on the biphasic hydrogenation

reaction is presented in Fig. 3.9, and Fig. 3.10. By adding initial 0.2g of

phase transfer agent, the NB conversion was increased dramatically

from 66 to 75% compared without a phase transfer agent after 4h

reaction. Thereafter, the conversion was increased modestly from about

87 to about 92%. The selectivity to PAP was also increased from 72 to

29

76%. Whilst this improvement in PAP selectivity is only moderate,

concurrent increase of PAP selectivity at high conversion is a

significant improvement in the operation in biphasic reaction. As

shown in Fig. 3.2 earlier, normal operation of biphasic reaction without

a phase transfer agent usually results in substantial decrease in PAP

selectivity at the expense of improved conversion. It was observed that

the rate and selectivity were significantly increased with only a small

addition of phase transfer agent and the 0.06g phase transfer agent was

roughly limiting value. It was reported that phase transfer catalyst

which can transport protons from the aqueous phase to the nitrobenzene

reaction phase allows the Bamberger rearrangement reaction to better

compete with the PHA reduction aniline [14]. It can also exert an

influence on the reaction rates by affecting the solubility of the

nitrobenzene or its wettability by the aqueous system on the interface.

30

0 50 100 150 200 2500

400

800

1200

1600

2000

2400

Cum

ulat

ive

H2 u

ptak

e (m

l)

Time(min)

20mg 30mg 40mg 60mg - %


hydrogenation with different amounts of phase transfer agent


Pressure, 1 atm;





31

0.00 0.02 0.04 0.060

20

40

60

80

100

Phase transfer agent, g

Con

vers

ion

of N

B, %

0

70

75

80

85

90

Sel

ectiv

ity t

o P

AP

, %Fig. 3.10. Effect of phase transfer agent on nitrobenzene



Pressure, 1 atm;





32

3.2. Effect of stirring speed and mesoporous carbons

(CMK-1, CMK-3)

3.2.1 Characterization of CMK-1 and CMK-3.

1) CMK-1: Fig. 3.11 shows the XRD patterns of the

mesoporous carbon CMK-1 and its silicate template MCM-48. The

patterns for MCM-48 host consist of six to eight distinguishable peaks,

which can be indexed to different (hkl) reflections of cubic structure.

Upon removing silica template by HF, two distinct (1,1,0) and (2,1,0)

peaks appeared, which indicates systematic transformation of the pore-

filled carbon to a new ordered cubic structure [12]. Argon adsorption-

desorption isotherm at 87 K and the corresponding pore size

distribution analysis for CMK-1 gave a sharp inflection characteristic

of the capillary condensation with average pore diameter of 3.0 nm as

shown in Fig. 3.12. CMK-1 also contained micropores of 0.5-0.8 nm in

diameter in addition to the 3.0nm mesopores, which are believed to

exist in the amorphous carbon framework prior to the removal of silica

wall. The corresponding micro and mesopore volumes were 0.3 and 1.1

cm3/g, respectively and BET surface area of the sample was ca.1350 m2.

33

The ordered mesopore structure of CMK-1 was confirmed by TEM

micrograph image and the SEM showed porous carbon particles

retaining the truncated rhombic dodecahedral shape of MCM-48

template as shown in Fig.3.13. The increase of Pt dispersion on the

Pt/CMK-1 carbon was confirmed by TEM images shown in Fig. 3.14.

The extremely small Pt clusters supported on the CMK-1 carbon were

almost invisible by TEM under the present experimental conditions,

whereas Pt clusters greater than 2nm in diameters are clearly seen in

the case of the Pt/C sample.

34

2 4 6 8 1 0

M C M -4 8

C M K -1

2 θ ( d e g r e e s )

Fig. 3.11. XRD patterns of CMK-1 and its silica

template MCM -48.

35

Fig. 3.12. Argon adsorption-desorption isotherm of

CMK-1

Relative Pressure (P/P0)

0.0 0.2 0.4 0.6 0.8 1.0

Am

ou

nt

Ad

sorb

ed (

cm3 S

TP

g-1

)

0

300

600

900Adsorption Desorption

Pore Size (nm)2 4 6 8

Po

re S

ize

Dis

trib

utio

n(c

m3 g

-1 n

m-1

)

36

Fig. 3.13. SEM(a) and TEM(b) micrographs of CMK-1

(a)

1 µm

(b)

30 nm

37

a)

b)

Fig. 3.14. TEM images of 5% Pt/CMK-1(a) and 5% Pt/C(b)

38

2) CMK-3: The XRD patterns of the mesoporous carbon

CMK-3 shown in Fig. 3.15, which can be assigned to (1,0,0), (1,1,0),

and (2,0,0) diffractions of hexagonal structure similar to the case of

SBA-15 appeared, which indicates that the structural symmetry for

silica template is retained in CMK-3 [13] due to cross linking of the

carbon framework. This is because the main mesoporous channels in its

silica template SBA-15 are interconnected through micropores inside

the walls of the main channels. Nitrogen adsorption-desorption

isotherm at 77 K and the corresponding pore size distribution analysis

for CMK-3 indicate that the carbon is mostly nanoporous with quit

narrow pore-size distribution centered at 4.5 nm as shown in Fig.3.16.

The total pore volume was 1.3 cm3/g, and BET surface area of the

sample was ca.1520 m2. The ordered mesopore structure of CMK-3

was confirmed by TEM micrograph image as shown in Fig. 3.17. As

the TEM images show, the structure of the CMK-3 was the hexagonal

pore arrangement and exactly an inverse replica of SBA-15.

39

Fig. 3.15. XRD patterns of CMK-3

2θ (degrees)

1 2 3 4 5 6 7 8

Inte

ns

ity

(a

. u

.)

40

Fig. 3.16. Nitrogen adsorption-desorption

isotherm of CMK-3

Relative Pressure (P/P0)

0.0 0.2 0.4 0.6 0.8 1.0

Am

ount

Ads

orbe

d (c

m3 S

TP g

-1)

0

300

600

900

AdsorptionDesorption

Pore Size (nm)2 4 6 8 10 12 14

Por

e S

ize

Dis

trib

utio

n(c

m3 g

-1 n

m-1

)

41

Fig. 3.17. TEM micrographs of CMK-3

42

3.2.2. Performance of Pt/mesoporous carbons(CMK-1, CMK-3) vs.

commercial Pt/C

Potential advantages of mesoporous less tortuous carbon as a

catalyst support for handling liquid phase reaction of macromolecules

were emphasized earlier by stiles [17]. In this regard, new mesoporous

molecular sieve carbons designated as CMK-1 or CMK-3 by Ryoo[12,

13 ] are worth considering for the biphasic synthesis of PAP to promote

the mass transfer of reactants in liquid phase by mesoporous with

narrow pore size distribution. Mesoscopically ordered nanoporous (or

mesoporous) carbon molecular sieve designated as CMK-1 was

synthesized by carbonizing sucrose inside the pores of the MCM-48

mesoporous silica molecular sieve. The CMK-1 carbon, obtained after

subsequent removal of the silica template with HF or NaOH solution,

exhibited three narrow X-ray powder diffraction (XRD) peaks

indicating the highly ordered pore arrangement. In a similar fashion,

ordered nanoporous carbon material designated as CMK-3 was

synthesized by using the ordered mesoporous silica molecular sieve

SBA-15 instead of MCM-48. CMK-1 has a mean mesopore diameter of

3nm and cubic in structure, whilst CMK-3 has a larger mean mesopore

diameter of 4.5nm and 2D hexagonal in structure. These differences in

43

pore diameter and structure may lead to different performances as a

catalyst support in liquid phase hydrogenation reaction. Fig. 3.18 shows

H2 uptake profiles of the reaction runs with 1-5% Pt/CMK-1 and 5%

Pt/C (Aldrich). Steady state H2 uptake rate estimated by taking an

initial slope of H2 uptake to 100 min. and conversion/selectivity data

are summarized in Table 3.1. Nitrobenzene conversion increased from

53 to 81% as Pt loading increases from 1 to 5% on CMK-1

accompanied by decreases in selectivity to p-aminophenol from 88 to

72 %. P-aminophenol yield was best with 2 % Pt/CMK-1, which

displayed catalytic activity equivalent to the commercial 5% Pt/C but

with significantly improved selectivity from 72 to 84 %. At the same

5 % metal loading, steady state H2 uptake rate for Pt/CMK-1 was 56 %

higher than commercial 5% Pt/C with the total H2 uptake being ca.

20 % higher. Selected H2 chemisorption studies on these catalyst

samples indicated the sorption stoichiometry of ca. 1.5 H/Pt for 2%

Pt/CMK-1, which remarkably indicates dispersion close to 100 %. High

level of metal dispersion was maintained to 5wt% metal loading with

1.3 H/Pt, whereas 5% Pt/C(Aldrich) gave H/Pt of 0.8. Apparently, one

reason for the improved performance of Pt/CMK-1 is due to its

substantially higher level of Pt dispersion. This difference in Pt

44

dispersion was shown clearly in Fig. 3. 14. Having found that 2%

Pt/CMK-1 gave conversion close to that of 5 % Pt/C, the effect of

mechanical stirring speed on the catalytic performance of these two

catalysts were compared. In addition, 2% Pt/CMK-3 prepared by using

SBA-15 as the template was also compared. The results are

summarized in Table 3.2. Whilst the conversion increases with stirring

speed was almost identical in three cases, significant improvement in p-

aminophenol selectivity was obvious for the 2% Pt/CMK-1, CMK-3 as

the stirring speed increases from 500 to 900 rpm. Apparently, diffusion

resistance becomes a dominant factor in this biphasic mode of reaction,

and mesopore structure of the CMK-1 and CMK-3 proved to be very

useful in transporting N-phenylhydroxylamine to aqueous phase in

which acid-catalyzed rearrangement reaction to p-aminophenol takes

place. 2%Pt/CMK-3, at 500rmp in particular, produced highest

conversion among the three catalysts tested without any loss in

selectivity. Larger pores of CMK-3 are also superior to CMK-1 in

promoting diffusion rates of reactants.

45

0 50 100 150 200 2500

400

800

1200

1600

2000

Cum

ulat

ive

H2 u

ptak

e (m

l)

Time (min)

5% Pt/C 1% Pt/CMK-1 2% Pt/CMK-1 5% Pt/CMK-1


hydrogenation with different Pt loading of CMK-1


Pressure, 1 atm;




46

Table 3.1. Performance comparison of Pt(2%)/CMK-1 with

commercial Pt(5%)/C.

Catalysts Conversion of N.B(%)

H2 uptake rate(ml/min)

Selectivity to PAP(%)

Pt(5%) /C 66 10.0 72 Pt(1%) /CMK-1 53 7.4 88 Pt(2%) /CMK-1 67 11.0 84 Pt(5%) /CMK-1 81 15.6 72

Table 3.2. Catalytic performance of Pt(2%)/CMK-1, 3 for

nitrobenzene hydrogenation at different stirring speed.

Catalyst Pt(5%)/C

Commercial Pt(2%)/CMK-1 Pt(2%)/CMK-1

rpm Conversion ( %)

Selectivity ( %)

Conversion ( %)

Selectivity ( %)

Conversion ( %)

Selectivity ( %)

900 66 72 67 84 73 81 700 62 69 62 76 66 73 500 28 60 27 61 36 64

47

4. Conclusion

1. The selective conversion of nitrobenzene (N.B) to p-

aminophenol(PAP) via platinum-on-carbon catalyzed

hydrogenation and acid-catalyzed rearrangement was studied in

biphasic process. The effects of various synthesis parameters

involved in this complex system such as Pt/C catalyst loading,

reaction temperature, acid catalyst concentration, stirring speed,

and additives (phase transfer agents or selectivity enhancing

agents) on PAP synthesis to come up with the optimum set of

reaction conditions were investigated.

2. Novel carbon supports, CMK-1and CMK-3, with regular mesopore

structure having an average pore diameter of 3 and 4.5nm was

prepared using MCM-48 and SBA-15 respectively, as a template.

1~5% Pt impregnated on CMK-1 and CMK-3 was prepared by

incipient wetness technique and tested for hydrogenation of

nitrobenzene. Compared with Pt supported on active carbon

commercially available, Pt supported on CMK-1 and CMK-3

performed significantly enhanced catalytic activities. The CMK-3

48

carbon support having the largest average pore diameter among

those three supports was best in terms of PAP yield. These

enhancements in activity are believed to be due to an increased

metal dispersion, and concurrent reduction in pore diffusion

resistance of the mesoporous structure of CMK-1 and CMK-3.

49

References

[1] B.Cornils and W.A.Herrmnn, “Aqueous-phase organometallic

catalysis : concept and application, Wiley-VCH (1998)

[2] P.K.Lim, North Carolina State University, private communication

(2000)

[3] Rylander, P., Karpenko, I., Pond, G. "Selectivity in Hydrogenation

over Platinum Metal Catalysts: Nitroaromatics", Annals of New

York Academy of Sciences, 172, 266-275 (1970)

[4] S.L.Karwa and R.A.Rajadhyaksha, "Selective Catalytic

Hydrogenation of Nitrobenzene to Phenylhydroxylamine," Ind.

Eng. Chem. Res., 26, 1746-1750 (1987)

[5] Rylander, P. N., Karpenko, I. M., and Pond, G. R., U.S. Patent

3,715,397, Feb. 6, 1973

[6] Rylander, P. "Catalytic Hydrogenation in Organic Syntheses" (1979)

50

[7] Benwell, N. R. W., Br. Patent 1,181,969, Feb. 18, 1970

[8] Michel Gubelmann, Paris, Christian Moliverney, Lyons, U.S. Patant

5,288,906 (1994)

[9] March, J., Advanced Organic Chemistry. New York: Wiley-

Interscience (1992)

[10] Shine,H., Aromatic Rearrangements. New York: Elsevier

Publishing Company (1967)

[11] Fishbein, J., McClelland, R., "Azide Ion Trapping of the

Intermediate in the Bamberger Rearrangement. Lifetime of a Free

Nitrenium ion in Aqueous Solution", Journal of the American

Chemical Society, 109, 2824-2825 (1987)

[12] R.Ryoo, S.H.Joo and S. Jun, J. Phy.Chem.B, 103(37) 7743 (2000)

[13] R.Ryoo, S.Jun, and S.H.Joo, J. Am.Chem.Soc. 122, 10712-10714

(2000)

51

[14] Hwang, J., Chang, K., Chen, C., Juang, T., “Selectivity in the

Phase Transfer Catalytic Hydrogenation of Nitrobenzene by

Pt/Carbon”, The Chinese Pharmaceutical Journal, 44, 475-485

(1992)

[15] Douglas C. Miller, St. Louis, Mo. Patent 5,312,991 (1994)

[16] Edward T, Derrenbacker, St. Louis, Mo. Patent 4,307,249 (1981)

[17] A. B. Stiles, Catalyst Supports and Supported Catalysts.

Butterworth Publishers, 136 (1987)

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