Influence of substituents on the reduction of aromatic nitrocompounds over supported palladium catalysts

Applied Catalysis, 10 (1984) 347-359 Elsevier Science Publishers B.V., Amsterdam

INFLUENCE

OF SUBSTITUENTS

PALLADIUM

CATALYSTS

M.A. ARAMENDIA, Department

347 - Printed

ON THE REDUCTION

V. BORAU, 3. GOMEZ,

of Organic

Chemistry,

in The Netherlands

OF AROMATIC

C. JIMENEZ

NITROCOMPOUNDS

OVER SUPPORTED

J.M. MARINAS

and

Faculty of Sciences,

Cordoba

University,

Cordoba,

Spain.

(Received

12 December

1983, accepted

1 March

1984)

ABSTRACT This work reports the activity of several catalysts of Pd supported on both SiOz-AlP04 and a sepiolite in the reduction of aromatic nitrocompounds, both with gaseous hydrogen and by hydrogen transfer. The reaction is insensitive to structure in the absence of diffusion phenomena. The kinetic expression found for the hydrogenation of nitrobenzene by hydrogen transfer shows zero order for both the donor and the acceptor. The order with respect to hydrogen increases with temperature in the reduction of nitrobenzene with gaseous hydrogen. The reduction of variously substituted nitrobenzenes yields, selectively, their corresponding anilines. This reaction is favoured by electronwithdrawing groups attached to the ring. The results obtained conform the Hammett equation.

INTRODUCTION Most of the papers

in the literature

volve the use of silica, We have recently

silica-alumina,

reported

Pd on A1P04, Si02-A1P04

dealing

[I] the activity

and A1P04-AlZOS,

with supported

activated

charcoal

of new catalysts

in the reduction

metal catalysts

or zeolites

in-

as supports.

obtained

by supporting

of aromatic

derivatives

with formic acid and triethylamine. Hydrogenation-dehydrogenation ent acceptor

DHx + n A

__$

is a process

studied

[4-81,

of nitrobenzene iron and cobalt

used as donors,

This work compares

although

the activity

of nitrobenzene,

deals with the different 0166-9834/84/$03.00

over heterogeneous phtalocyanines

Cyclohexene

[I,81 have also been successfully

genation

between

some time ago by Braude and Linstead

have been used for this purpose. entially

transfer

a donor and a differ-

n AHx + D

formed the reduction Noble metals

by hydrogen

molecule:

formic

supported

per-

Pd catalysts.

[9] and other catalysts

and cyclohexadiene

[lo]

have been prefer-

acid [6] and triethylammonium

formiate

employed. of various

Pd supported

both with pressurized variables

[Z]. These authors

affecting

0 1984 Elsevier Science

hydrogen

the process

Publishers

B.V.

catalysts

in the hydro-

and by transfer.

It also

as well as with the influ-

348

ence of substituents

on the rate of reaction

Finally, a mechanism

is proposed

of the nitrocompounds.

to account

for the results obtained.

EXPERIMENTAL Supports The following denominated

supports

perties were reported

from Vallecas

for which various

groups and molecules

and accounting

(Madrid).

structural

have been proposed with varying hydroxyl

(8O:ZO by weight)

oxide, whose synthesis

in a propylene

in earlier work [II,121 and a sepiolite

TOLSA S.A. and obtained silicates

have been used: a Si02-Alp04

P2, gellified

Sepiolites

formulas

stoichiometry

for their peculiar

water,

[13,14].

depending

pro-

supplied

by

magnesium

(OH)y(H20)z

in the number of protons,

properties

at 300°C prior to use, thus obtaining

support

are hydrated

such as Si ,2030Mg8Hx

of crystallization

system

and texture

superficial

on their origin

The support was calcined

a solid with a stable

structure.

Catalysts We have used a series of catalysts Catalyst

Pd3P2, consisting

Pd3Sep, consisting catalyst

synthesized

on the aforementioned

of 3 wt% of Pd deposited

of 3 wt% of Pd deposited

(Fluka, Ref. 75992) including

on the P2 support;

on sepiolite;

catalyst

a 5 wt% of Pd, supported

supports: catalyst

Pd5C, commercial

on activated

charcoal. Catalysts

Pd3P2 and Pd3Sep were obtained

by impregnation

with aqueous

solutions

calcined in the air from room of Pd(NH3)4C12. Prior to use, the solids were -1 temperature to 300°C at a heating rate of 1°C min , and a hydrogen stream of -1 60 ml min was subsequently passed at 300°C for 1.5 hours. L-termed

catalysts

(I'C min-')

were

calcined

A hydrogen

The commercial

catalyst

The theoretical

metal

Determination

Transmission

Electron

synthesized

by Atomic Absorption

of the metal particle

The metal particle

reduced

linearly

for 30 min.

with

than 1%.

size

size has been determined Microscopy

is in agreement

with an error smaller

both by X-ray diffraction

(TEM). The results

and by

found by both procedures

are

coincident.

The Transmission Philips EM-300 were prepared determined

and subsequently

was used untreated.

loading of the systems

the actual value, determined

practically

analogously

from room temperature to 3OO"C, keeping this temperature -1 was used in the process. stream of 60 ml min

Electron

instrument,

Microscopy

working

by an extractive

by counting

using a spherical

600-800

particle

experiments

at 100 kV, with a resolution

replica method, particles.

model.

were carried

out with a of 3 8. Samples

and metal size distribution

The metallic

was

surface was determined

349 TABLE

1

Activity

of the catalysts

synthesized

in the hydrogenation V

Catalyst

I

of nitrobenzene.

/mol s-1m-2 x 205 Pd

d/I (X-ray)

SMet./m2gpA

Pd3Sep L

94

53.1

7.93

1.63

Pd3Sep

85

58.7

a.40

7.80

Pd3P2 L

76

65.7

7.95

0.65

Pd5C

79

63.0

7.15

0.08

aReaction 1)

conditions:

With hydrogen: solvent

2)

Hydrogen

methanol,

By hydrogen solvent

b

pressure

5 bar, nitrobenzene

total volume 20 ml, reaction

transfer:

Donor I-phenylethanol,

dimethylformamide,

total volume

VI (H2) and VI (T): Initial reaction transfer

concentration

temperature

nitrobenzene

10 ml, reaction

rate with gaseous

0.25 M,

27°C. concentration

temperature

hydrogen

IM,

150°C.

and by hydrogen

respectively.

X-ray diffractograms the Cu Ka radiation, was made

VI (T)b

VI (H2)b

were taken with a Philips

with

graphically,

1130/00/60

x = 1.5418 i. The measurement

with an angle of 2e = 40.10"

of Pd and a value of 0.89 was taken

apparatus

of the half-peak

corresponding

for the constant

by using width

to plane

(l,l,l)

K of the Debye-Scherrer

equation.

Apparatus The reduction a Parr-type The reaction pressure

gauge coupled

matrass

at regular

fitted

to the hydrogen

The initial

hydrogen

cylinder

measured

by the

(f 0.1 bar).

of time. The reaction mixture (150°C) was reached

(+l"C) controls.

in pressure

was performed at atmospheric

pressure,

allowed

using a two-

one to draw samples

was stirred magnetically

by submerging

the matrass

and

in a thermo-

bath.

rate method was used in both cases

data, employing

has been carried out in

rate and temperature

to a cooler and a coil which

temperature

silicone

with gaseous

with shaking

by reading the variations

by transfer

intervals

the reaction statted

provided

was tracked

The reduction mouthed

of nitrocompounds

reactor

conversion

data smaller

for the calculation

of kinetic

than 8%.

Product analysis The reaction standards

products were analyzed

to characterize

them. Their

by gas chromatography nature was confirmed

The reaction yield was the corresponding

aniline

using high purity by mass spectrometry.

in every case and neither

by-

350 TABLE 2 Specific

catalytic

activity

of various

catalysts V /mol s-'m-2Pd x IO5 I

Catalyst

a/i

% Pd

SMet/m2g-'Pd

VI (H2)

VI (T)

Pd,Sep

1

54b

92.4

8.33

7.87

Pd2Sep

2

82b

61.6

8.12

7.67

Pd3Sep

3

a5c

58.7

8.40

7.81

Pd4Sep

4

94c

53.1

8.18

7.40

Pd5Sep

5

IO!?

47.5

a.27

7.92

aReaction

conditions

identical

to those in Table

bDetermined

by TEM

'determined

by X-ray diffraction.

products

nor reaction

nitroaniline carbonyl

intermediates

selectively.

Alcohols

I

were detected. used as donors

m-Dinitrobenzene

yielded

m-

gave rise to their corresponding

compounds.

RESULTS AND DISCUSSION Catalysts Table 1 shows the results the catalysts

synthesized

nitro-benzene

with gaseous

The catalysts

of particle

and compares hydrogen

synthesized

thermal treatment

or reduction

the activity

of the catalysts with gaseous

without

in the transfer

Specific

catalytic

reduction,

gen and by transfer.

surface,

in the synthetic

marked

differences

for which catalyst

shows the greatest

of various

regardless

procedure.

of the

Although of

appear when they are Pd3Sep

activity

(reduced at 300°C

of all. Hence we chose

experiments.

catalysts

in the reduction

The results,

ing of the catalyst

used. However,

pendent of the metal hydrogen

synthesized

of nitrobenzene

obtained

surface of the catalysts

over sepiolite

hydro-

in Table 2.

decreases

catalytic

the hydrogenation

as supp-

both with gaseous

are displayed

synthesized

the specific

loading, whether

with the metal

activity

process

load-

found is inde-

is carried out

or by transfer,

In view of the results under the reaction

for

of

activity

activity

ort has been determined

The metallic

determined

is very similar when used in the hydrogenation

for most of the subsequent

The specific

with gaseous

undergone

hydrogen,

a heating gradient)

this catalyst

surface

in the hydrogenation

and by transfer.

show similar metallic

nitrobenzene employed

size and metallic

their activity

it may be stated that the hydrogenation

conditions

studied

is, in principle,

a process

of nitrobenzene rather

insensi-

351 TABLE 3 Influence Catalyst

of the solvent on the reduction

of nitrobenzene

with gaseous

Pd3Sepa.

V /mol s-'m-'Pd x lo5 I

E (25°C)

Methanol

8.40

32.63

1.70

Ethanol

5.90

24.30

1.69

I-Propanol

4.72

20.10

1.68

2-Propanol

4.45

18.30

1.66

1-Butanol

4.20

17.10

1.66

I-Pentanol

3.43

13.90

5 bar, nitrobenzene

aHydrogen

pressure:

reaction

temperature:

tive to structure

according

to Boudart's

size is not very wide.

substantial

widening

3 and 4 show the results

Solvents

By modifying

[151, although

the reduction particle

the range

temperature,

no

size has been observed.

derived

We may therefore

intermediates An increase is observed increase,

generated

think that the solvent on the surface

dielectric

anisole

of the solvent

be evaluated

a complete

iple, solubility

effects, factors

to evaluate

We conclude

in the process

dipole-dipole

interaction

related

dielectric

constant.

Another

solvents pairs

transfer.

(1,4-

The action

being inert

cannot be assumed.

In princ-

effects with the rest of the

to its structure,

in short, comprises

etc., should be re-

all these effects,

separately.

solvents,

is in agreement

with free electron

in a simple way since, although

pasiveness

directly

the

of the medium

as solvents.

for monoalkylaromatic

used in the hydrogen

that, under our reaction

tained with various

constant

with alcohols

and solvents

The action of a given solvent,

difficult

and dipole moments

in the course of the re-

plays the role of stabilizing

obtained

is observed

constants)

in the reaction,

entropic

constants

with the dielectric

the results

and dimethylformamide)

cannot

of nitrobenzene

of the catalyst.

in the rate of reaction

this time with polarity,

products,

in the hydrogenation

from the solvent were detected

when comparing

(with similar dioxane,

obtained

used.

were used in terms of their dielectric

[16]. No products

which

classification

of the range of the metal

in terms of the solvent

garded.

1 M, total volume: 20 ml,

influence

Tables

action.

concentration:

27°C.

of particle

Solvent

hydrogen.

charged

conditions

with the correlations This conclusion

and in view of the results

intermediates observed

are involved

with the polarity

can only be stabilished

ob-

in the process, and the

by comparing

solvents

352

TABLE 4 Influence Catalyst

of the solvent

on the reduction

of nitrobenzene

by hydrogen

transfer.

Pd3Sep.a

V /mo? s-'m-' Pd x IO5 I

Solvent

E

u/Debye

Benzene

0.83

Toluene,

2.08

2.4 (25°C)

0.37

Ethylbenzene

2.48

2.4 (20°C)

0.58

Cumene

2.92

2.4 (20°C)

0.65

Tertbutylbenzene

3.05

1,4-Dioxane

3.23

Anisole

4.18

Dimethylformamide

7.80

aHydrogen

donor:

10 ml, reaction

1

I-phenylethanol temperature:

x1o-3 Cm01 -1

2.3 (20°C)

0.70 2.2 (25°C)

1

hydrogen.

Correlation

0.00 1.38

38.0 (25°C)

3M, nitrobenzene

concentration:

3.82

TM, total volume:

150°C.

S)

W2)

FIGURE

0.00

between

=

2.84

l/VI and l/W for nitrobenzene

x

10

-4

mol

reduction

s

-1

-1 Scat.

with gaseous

353 TABLE 5 Initial rates of nitrobenzene methanol.

Total volume:

reduction

with hydrogen

/mol s

20 ml. Reaction

CN02-@l

-1 -2 mPd x 105. Solvent

pH*

temperature

/"C

/bar

18

27

37

0.25

5

7.01

8.40

9.71

12.05

0.50

5

7.08

8.40

9.56

11.93

1.00

5

6.98

8.11

9.50

11.88

1.50

5

6.91

8.23

9.47

'1.74

2.00

5

6.89

8.08

9.58

11.83

1.00

4

5.86

7.03

8.11

9.34

1.00

6

7.77

9.28

11.12

13.55

1.00

7

8.63

10.48

'2.47

15.73

/In01 1-l

47

TABLE 6 Partial

reaction

n (reaction

similar

orders found

in nitrobenzene

reduction

T/"C

order)

0.68

18

0.71

27

0.78

37

0.92

47

in nature

(for which many of the aforementioned

such as the different in that of aromatic rather different

alcohols

solvents

The results obtained

by transfer.

such as anisole, relationship

a multistep

generated

of the catalyst constant

or

to

or N,N-dimethylformamide,

mechanism

of nitrobenzene

by both proced-

in which the reaction

are stabilized

intermediates

by an increase

in the pol-

of the medium.

tests

Blank tests were performed obtaining

hydrogen

be extended

to the preceding.

ures takes place through on the surface

it cannot

1,4-dioxane

are quite alike)

with gaseous

However,

show that the hydrogenation

arity or in the dielectric

effects

used in the hydrogenation

hydrocarbons

which bear no structural

Kinetics

with hydrogen

kinetic

throughout

the temperature

data, in order to check the absence

those inherent

to the catalytic

of the reactor

in any of the systems

process.

Neither

used proved

range studied,

of phenomena

the support to be active

prior to

other than

nor any other parts in the reduction

of

nitrobenzenes. No diffusion

phenomena

were observed

when shaking

at a rate of over 200 shakes

354

TABLE 7 Apparent

energies

of activation

in nitrobenzene

Ea/kJ mol-'

reduction

with hydrogen

P,,?/bar

12.4

4

14.1

5

14.5

6

15.9

7

TABLE 8 Initial rates of nitrobenzene phenylethanol

min-'

reduction

as donor. Reaction

by transfer

/mol s

-1 -2 mPd x 105, using l-

temp. = 150°C.

[N02-@l

[ Dador]

/mol 1-l

/mol 1-l

VI (T) x IO5

0.97

2.35

7.81

0.97

3.00

7.81

0.97

3.65

7.90

0.97

4.29

7.90

0.49

3.00

7.67

1.46

3.00

7.71

1.95

3.00

7.81

in . the hydrogenation

no interparticle

reactor or over 300 rpm in the transfer

diffusion

occurred

while working

with catalysts

reactor and

sifted through

70-400 mesh. It was checked the reaction

that the rate of the hydrogen

in the hydrogenation rates of reaction

reactor.

obtained

The rate of the transfer

hydrogen

Figure

from the gas phase to

step of the process when working

1 shows the relationship

for different

of hydrogen

rived from the intercept, Therefore,

transfer

medium was not the rate-determining

catalyst

weights

from the gaseous

is much greater

between

(t/VI vs l/W) at 27°C.

phase to the catalyst,

than the rate of the overall

it may be stated that the process

the initial

is not governed

de-

reaction.

by the diffusion

of

to the surface of the catalyst.

Under working

conditions

gen and in that by transfer, the amount of catalyst Several experiments

we have proved that, both in the reduction the initial

rate of reaction

increases

with hydro-

linearly with

used. have been carried

determine

kinetic data corresponding

hydrogen.

Nitrobenzene

concentrations

out with catalyst

to nitrobenzene ranging

Pd3Sep in order to

reduction

with pressurized

from 0.25 - 2.00 mol l-', hydrogen

355

TABLE 9 Initial rates of hydrogenation

of various

aromatic

nitrocompounds.

Catalyst:

Pd3Sep.a

Nitrocompound

0

VI (H2)

VI (T)

x 105 /mol s-'me2 Pd

/mol s

-1 -2 mpd

-0.66

4.76

0.33

p-Hydroxynitrobenzene

-0.37

6.12

1.29

p-Methylnitrobenzene

p-Aminonitrobenzene

-0.17

7.49

3.81

Nitrobenzene

0.00

8.40

7.81

p-Acetylnitrobenzene

0.50

8.60

7.95

m-Dinitrobenzene

0.71

8.69

8.14

aReaction 1)

2)

Solvent methanol,

concentration

By hydrogen

phenylethanol,

pressures

5

conditions:

With hydrogen:

benzene

x 10

total volume

0.25 M, hydrogen

transfer:

Solvent

nitroderivative

pressure

dimethylformamide,

concentration

from 4 - 7 bar and temperatures

The initial

rates obtained

The reaction

20 ml, temperature

temperature

15O"C, donor

i-

1 M.

between

in the experiments

rate found conforms

27"C, nitro-

5 bar.

18 - 47°C have been employed.

are listed in Table 5.

an equation

such as:

V = K (PH )" 2 where

K is an apparent

respect

to hydrogen

The partial equation,

reaction

rate constant

rate, n, analytically

of temperature,

Table 7 shows the values of apparent

the Ea. The variation the apparent

several

benzene

E, corresponding

expressions

(PH)

KH2 l+KH 2

(P*) H2

range

pressure,

the greater

to hydrogen

and that of

have been also observed

by Coenen et al. [17]

with a Langmuir-Hinselwood

may be formulated

from the above (see Table 6).

to the temperature

in liquid phase over a Ni supported

found are consistent

the expression:

V=K

calculated

it increases

of the partial order with respect

energy of activation

The results which

with which

It may be seen that the higher the hydrogen

on hydrogenating

order with

pressure.

order of reaction

is a function

of 27 - 47°C.

and n is the reaction

catalyst. type mechanism

[18]. Our experimental

results

for

conform

356

log

v

I

‘V

12

(H2)

0,05

FIGURE 2

Correlation

nitrobenzene

between

Log (VI/VIo) and 0 for various

v

I

‘V

13

'. 0p2

3

Correlation

nitrobenzene

0,02

substrates,

taking

as reference.

log

FIGURE

P-

between

as reference.

CT)

f

=

0,02

Log (VI/VIA and o for various

substrates,

taking

357

HO-C-CH :

LOW

STEP

Steps

Quick

---2

R

H2N

C6H5-CHOH-CH

FIGURE 4

3

CeH5-CO-CH

3

Scheme of reaction.

coefficient

all the temperatures Table 8 gathers reduction

the concentration

better than 0.99 and with a positive

intercept

for

studied. the results of the kinetics

by transfer

experiments

using the same catalyst.

range studied,

the reaction

ration of both the donor and the acceptor, catalyst

+

3

with a correlation

benzene

3

performed

It may be seen that, within

rate is independent

depending

for nitro-

exclusively

of the concenton the amount of

used.

Casiraghi by transfer

et al. [19], who have studied with alcohols,

the acceptor

from's given concentration

Hydrogenations Variously

of various substituted

in the hydrogenation substituents

kinetics

of carbonyl

compounds

for both the donor and

of reactants.

nitrobenzenes nitrobenzenes

reactor

attached

the hydrogenation

suggest a zero order

have been subjected

and by transfer,

to evaluate

to hydrogenation, the influence

to the ring. Table 9 shows the results

both

of the

of the initial

rate

for both procedures. The results

obtained

of the substituent.

Thus, nitrocompounds

to the ring show greater nitrocompounds benzene

of the rate of reaction

show a dependence

with electron-withdrawing

rates of reaction

with electron-releasing

than unsubstituted

substituents

on the nature groups attached

nitrobenzene,

have smaller

whereas

rates than nitro-

itself.

Figures

2 and 3 show how the results obtained

in the form:

Log VI/VI0 =

PU

conform

the Hamnett

equation

(20)

358 It can be seen that in both cases the reaction of electron-donors

than to the presence

is more sensitive

of electron

acceptors,

to the presence

as it may be inferr-

ed from the values of p found. The overall

results

of nitrocompounds

found in this work allow to conclude

by transfer

is a multi-step

reaction,

ion from the donor being the slow step of the process

that the hydrogenation

the transfer

according

of a hydride

to the scheme shown

in Figure 4. The charged stabilized

intermediates

by an increase

present on the surface

in the polarity

nature of R should contribute rise to reaction withdrawing

groups have the opposite

conclusion

ion from the alcohol

for the hydrogenation

Essentially,

the mechanism

and Manassen

a classical

the reduction mechanism

electron-

of alcohols

by quinones

and suggested

over magnesium

with that suggested

of nitrobenzene

of nitrobenzenes

et al. [22] have come to the same

of acetone

coincides

to those observed

by transfer,

namely

oxide.

by Bar-Iland

with 1,4-cyclohexadiene.

with pressurized

of the Horiuti-Polanyi

is also the transfer

hydrogen

and assuming

type [23], the results obtained

to state that both the nature of the solvent and that of the reagent effects

giving

Conversely,

step of the process

[213. Takezawa

[9] for the reduction

Regarding

The electron-releasing

effect.

by transfer

proposed

should be

this kind of intermediates,

the hydrogenation

in which the rate-determining

of a hydride

of the solvent.

to instabilize

rates smaller than that of nitrobenzene.

Ohki et al. have studied a mechanism

of the catalyst

stabilizing

allow

have anologous

the reaction

intermed-

iates.

ACKNOWLEDGEMENT This study has been carried Investigation

Cientifica

out with a grant from the "Comision

y Tecnica",

thank the firm "Tolsa S.A."

project

number:

0291/82.

(Madrid) for the sepiolite

Asesora

de

We should like to

supports

supplied.

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