Indian Joumal of Chemical Technol ogy Vol. 7, November 2000, pp. 280-285

Hydrogenation of nitrobenzene using polymer bound Ru(III) complexes as catalysts

D R Patel &. R N Ram *

Department of Chemistry, Facult y of Science, M S University of Baroda. Bamda 390 002, India

Nccci, ·ct! II March 199Y; acccp1ed 20 Jun e 2000

Polymer bound ruthenium 2-aminobutanol complexes were synthesized using 2% and 8% copolymer of styrene divinylbenzene by sequenti al at tachment or bifunctional group, followed by treatment or metal salt with the functionali sed polymer beads. The various physico-chemi ca l properti es or the above beads such as surface area, moisture content , bulk density and swelling were studied. The thermal stability or these catalys ts was investi gated by DTA-TG anal yses and the morphology of the catalysts was seen using scanning electron microscope. Probable structure based on spectroscopic studies has been proposed and catalytic act ivity was evaluated for the model reaction of hydrogenation or nitrobenzene. The effect on the rate or va rious factors such as the concentration of the catalyst and substrate, nature or the so lvent and temperature of the system was seen. The stability of the syn thesi zed catalysts was checked by studying the life cycle or the catalyst. The homogeneous counterpart of thi s complex has been synthesised and the act ivity of· the catalys t was compared with the heterogenized homogeneous system.

The hydrogenation of organic compound using polymer supported chira l catalysts is a subject of both academic and commercial importance. Severa l studies have been reported usi ng group (VIII) metal complexes chelating as well as chiral catal ysts for hydrogenation reactions in homogeneous medium 1

-3

.

The ma in problem of thi s type of the catalysts was found to be their separation from the reaction system and having low catalytic acti vity. However, when these catalysts were loaded on some sol id support , the activity was observed to be very high. The first supported chiral catal ys ts was reported in 197745

. A number of pnpers have been published using diamines as metal chclating complex supported on polymer as catalysts fo r hydrogenation as well as oxidation reactions and a considerable catalytic activ ity was observed6

-9

. In the present investigation it was therefore, thought over to synthesize the metal complexes over polymer matrix using nitrogen and oxygen bidentate ligand chimi compound and to investigate the catalytic acti vi ty for the hydrogenation of nitrobenzene to aniline which are used widely as dye-intermediate in the synthesis of dyes and drugs. The influence on the rate of hydrogenation of various factors such as the concentration of catal ys t and substrate, nature of the solvent as well as the tempera ture of the system was seen . The recycling efficiency or the catalysts has also been studied 111

order to examine the stabil it y of the cata lysts.

Experimental Procedure

Materials and equipment Styrene divinylbenzene co-polymer (XAD-2)

supplied by Fluka-AG was purified by soxhlet ex traction method using benzene-ethanol (I: I )

mixture and dried at 70°C. THF, dioxane, methanol, ncetone, nitrobenzene, I ,2-dichloroethane and 2-aminobutanol were distill ed before use. The detail s 0f the procedures are given elsewhere 10

. Aluminium chl oride was purifi ed by sublimati on. RuCid H20 (obtained from Johnson Matthey Inc., New Jersey) was used as such. 8% cross-linked chloromethylated PS-DYB was obtained as gift sample from Jon Exchange (India) Ltd . Bombay.

Ultra violet reflectance spectra Ultra violet reflecta!lce spectm of the solid samples

were recorded Oil a Shimadzu-UY-240 instrument using BaS04 as standard . Infmred spectra were recorded on a Beckmann IR-4220 instrument and surface area of the supports as well a-; the catalysts was determined by BET method at liquid nitrogen temperature by use of Carlo-Erba 1800. ES R was scanned on a Bruker ES P-300 K band spectrometer us ing a l 00 K Hz field modulation . The experiment was carried out on powdered sample at 298 K in nitrogen atmos here. Elcmemal analyses of the catalysts at different stages of ancl ori ng were canied

PATEL & RAM: HYDROGENATION OF NITROBENZENE 281

out in the laboratory using Coleman ana lyser. The thermal stability of the catalysts was evaluated using Shimadzu thermal analyser DT-30. The change in morphology of the catalysts and the supports has been studied using scanning electron microscope (JSM­T300) . Swelling studies of the supports as well as catalysts were carried out using polar and non-polar sol vents at a constant temperature. The detailed procedure has been described earli er11

.

Synthesis of the catalysts Chloromethylation of polymer support- Styrene

divinyl benzene co-polymer (XAD-2) was washed with distilled water, water-ethanol mi xture ( I: I) and finally with ethanol. The adsorbed materi al (if any) was removed by extracting the polymer using sox hlet extractor with ethanol-benzene ( I: I) mixture for a total period of 24 h. The polymer beads were chloromethylated by p-Formaldehyde, l ,2 dichloroethane using AICI 3 as catal yst. The reacti on was carried out at reflux ed temperature 12 and dry HCI gas was bubbled through the mixture for a total peri od of 48 h. The above polymer was washed in sequence with water-dioxane mixture ( I: I), water-dioxane +10% HCI, methanol and finally with di stilled water till the complete removal of free chloride ion. The chloromethylation was confirmed by estimating the chlorine content present on the surface of the polymer. For thi s, a fixed amount of polymer beads were heated with sodium peroxide for three hours. The resulting so lution was treated with 4% AgN03

solution to prec ipitate out the chloride ion as sil ver chloride and fina ll y the chl orine content was calculated gravimetrica l! / ' . It was found to be 6.0% for XAD-2. 8% cross-l inked chl oromethylated PS­DVB was supplied by Ion Exchange (India) Ltd. Mumbai . The chlorine content was estimated using above procedu re and found to be 17 .5%.

Ligand introduction onto the funct ionalised polymer- In order to enhance the ac tivity of the polymer beads, it was treated with the Nal solution prepared in acetone-dioxane mixture (3: I), pu ri fied by soxhlet extraction usi ng acetone as a solvent and dried at 80°C fo r I 0 h. The polymers, thus processed were refluxed with 2-amino butanol for the li gand introduction using THF as solvent for a maximum period of 48 h and finally they were washed with THF 1 ~. The introduction of li gand onto polymer matrix was confirmed by nitrogen est imation and was found to be 1.23% and 3.68% for catalysts D and R, respectively .

Formation of 111etal complex on the .fitnctionalized polymer-A 25 g sample of the functionalized polymer was kept in contact with 80 mL of ethanol for 30 min in a round bottomed flask. An ethanolic so lution of RuCI ,.3H20 (0.25 H/v) was placed in a pressure equali sing funne l, fitted to the reacti on vessel by means of a standard joint, the end being closed with a stopcock. The solution was added to the reaction vessel over a period of 30 min . The reacti on mixture was initially reflu xed for 6 h and then allowed to stirr for the react ion at room temperature fo r seven days. The beads were filtered , washed with ethanol, dried in vacuum at 80°C fo r 24 h and finally stored in vacuum desiccator. Ruthenium content was estimated by refluxing a fixed amount of metal supported polymer beads with concentrated HCI of AR grade for 5 h. It was fi ltered, washed thoroughly in a 50 mL measuring flask and finally made up to the mark. The metal content present in so lution was estimated by spectrophotometric method using 2,4,6-trin itro pyrimidine as a complexing agent 15

•

Synthesis of unbound complex-In order to synthes ize [Ru (lll )L2AB Cl3] complex in the so lution phase, RuCI 3.3H20 (0.0 I M in I 00 mL ethanol) was placed in a rou nd bottomed flask and L2AB (0.0 I M in I 00 mL ethanol) was added by addition funnel with constant stirring for 4 h at room temperature. The band observed at 340 nm using visible spectrophotometer, indicates the formation of the metal complex in the so lution and was used as such to in vesti gate the catalytic activity of the reacti on under study.

Nomenclature-The catalysts are designated by the general formula NPML where N: Percent cross link, P: Co-po lymer of styrene divinyl benzene (PS-DVB), M: Metal atom, L: Type of li gand .

lt may be defined as:

Catalyst D : 2P[Ru(III)L2ABCI 3];

Catalys t R : 8P[Ru(III)L2ABCI.,];

Catalyst H : [Ru(TII)L2ABCI,]

Hydrogenation procedure-The hydrogenati on reaction was carried out at atmospheric pressure in a magnetically stirred glass reactor in a chemically contro lled regime. The rate of hydrogen consumption was measured using a glass manometric apparatus. The detailed experimental set up and hydrogenation procedure were described elsewhere 16

•

282 INDIAN J. CHEM . TECHNOL , NOVEMBER 2000

Results and Discussion Phys ica l properties of the support and catal ysts are

given in Tab le I (a). A decrease in su rface area was observed after loading metal ions on polymer support. Th is might be clue to the block ing of the pores of the polymer support as well as the steric hi ndrance after introduction of the li gand as well as the metal ions present on the surface. The swell ing studi es of the support as well as the catalysts were carried out using ten different solvents. The results are summari sed in Table I (b). On increasing the degree of cross- linking of polymer support , a decrease in swe lling was observed which might be clue to the ri gidity of the support. The opt imum solvent is one that combines good swell ing ability and hi gh polarity . Methanol was found to be a good solvent by causi ng greater swelling and larger capacity for the dissolut ion of the substrate . Morphology of the support and catalyst was observed using SEM. Fig. I (a & c) indicate the SEM of 2% support and the catalyst respecti vely . The white spot on the photograph of c is indicative of the

Tab le I (a)-Physical properties or the supports and cat al ysts

Physical Propcrtit:s 2'1!' 8% Cat~dyst D Cat:~ l yst R

Pore volume 0.420 0.04 0.59:1 0.159 (cm3 g- 1)

Su rface arc:~ 330.0 37.37 203.3 34.70 (m2 g-1

) NTP

Apparent bulk densi ty 0.425 0.475 (g cm3

)

Mo isture content 1.75 3.96 (Wt%)

Table I (h)-Swelling studies o r the supports as we ll as cat alysts (mol%)

Solven t 2% X'ln Cata lyst D Cawlyst R

Water 4 .03 3.H3 3.6 1 3.73 Methanol 3. 12 2.52 n5 2.36 Ethanol 2.30 I. R I J.R:l 1.50 Di oxane 1.84 1.42 1.36 1.15 DM F 1.52 1.27 1.24 1.00 Acetone 1.37 1.10 1.0 1 0 .9 1 THF 1.06 1.00 0.92 0. 90 Benzene 1.00 0.83 0.91 0 .74 Cyclohexanc 0 .8H 0.68 0.52 0.44 11- Heptane 0. 73 0.52 0.44 0.41

dispersion of the li gand and the metal ions. Similar results were obtained in case of 8% crosslinked support where a hi gher di spersi on was observed. Elemental analyses of chl oromethylatecl polymer, li ganclecl polymer and anchored catalys ts show the lower leve l of anchoring of the meta l ions on the funct ionali secl polymer Table 2. However anchoring of the metal complex on to the polymer was confirmed by comparati ve spectral studies of the catalyst. The var ious IR frequencies obtained for the liganclecl polymer and the catalysts are given in Table 3. A shi ft observed from 3319 to 3365 cm- 1 in the case of -NH group and from 1488 to 1492 cm- 1 in case of -OH group is indicative of the formation of the coordination bond. The UY re flectance spectroscopy gave a broad peak at 360 nm for catalyst D and 376 nm fo r cata lyst R wh ich are indicati ve of the d-d transition of Ru (III). The g value obtained from ESR study was found to be 1.99 which shows that Ru is present in +3 oxidation state.

,

c d r

;.

d Fig. !- Scanning Elec tron Mi crograph s or (a) 2% suppo rt. (b)

8% support (c) Cat alyst D and (d) Cntal yst R

Tab le 2- Eiemcntal an alyses at the di llcrt:nt stages or syn th t:s is (wt')(·)

Cat. A B c c H Cl c H N c 1-1 N Ru (g/g)x I o•

D H0.9 7.4 6.0 X4.6 X.2 I .2 87.4 ?\. i 1.1 :u R 72.9 5.9 17.5 X5.X 7.1 3.6 X7.9 6.3 3.3 3.2

A : After chloromcthylation. 13 : After ligand introduction, C: Arter complex formation.

PATEL & RAM: I-IYDROGE ATION OF NITROB ENZENE 283

Table 3-IR rrequencies or liganded polymer, and catalysts D and R

C-. (cm- 1) N-1-1 (cm- 1

) - 01-1 (cm- 1) Ru-0 (cm- 1

) Ru-N (crn- 1)

Liganded Polymer

Catalyst D

Catalyst R

1047 !062 1056

3319

3365 3383

Thermogravimetric analysis of the catalysts and the support was carried out from room temperature to

500°C with a heat ing rate of I 0°C per minute (Fig. 2). It was fou nd that in both cases polymer degradation starts above 140°C. The weight loss be low thi s t.::mperature may be due to moisture content. Hence it was ensured that the polymer bound catalysts can be

used for catalytic studies below 140°C. Based on spectral evidence, a probabl e structure of the catalysts may be as shown in Scheme I.

Hydrogenation of nitrobenzene-The kinetics of the hydrogenation of nitrobenzene to ani line was investigated fo r supported as wel l as unbound catalysts. The data for the hydrogenation reaction were obtained in a kinetic regime using a stirring

speed of 650 rpm in the temperature range 30-45°C. In each case, the uptake of hydrogen with time was found to increase fast initially and after that a plateau was observed. The initial rate of hydrogenation reaction was calculated from the slope of the tangent of the plot of vo lume of H2 uptake as a function of time. A summary of the resul ts are g iven in Table 4 .

Influence of the catalyst and the substrate concentration-The study was made while varying the concentration of the catalyst (i.e. Ru content) from 1.58xl0-5 to 2.77±10-5 mol L- 1 in case of catalyst D and 1.60 to 2.81xl0-5 mol L- 1 in case of catalyst R. In case of unbound catalyst H , the Ru content was found suitable from 5.0 to 12.5x l0-4 mol L- 1

• The experiment was carried out by maintaining a constant concentration of the substrate i.e. 9.72xl0-1 mol L- 1 at

a constant tempe rature 35°C. The order of reaction calculated, form the log- log plot of concentration of the cata lyst versus rate was observed to be fractional (Fig. 3). This indicates that the metal ions, which are present on the surface are exposed for the catalytic action. Similar results were found when the concentration of the nitrobenzene was varied from 4 .83 to 19.40x I o-1 mol L- 1 keeping concentration of the catalysts constant at 1.58x I o-5 mol L -I for cata lyst

D and 1.60xl0-5 mol L- 1 for catalyst R. For all sets of the experiments, the resu lts are summarized in Table 4. In this case also, the order of reaction calculated from the slope of the log-log plot of

1-+88

1492 1496

28l)

304 315 3 15

Tim e) mt'n

10 20 30 40

278 279

50

100 200 300 400 500 Temperature., °C

Fi g.2- DTA-TGA curves or Catalyst D

(!)-cH2-CI

CH2-CH3

~G)-CH2-NH-~H-CH2-0H THF,80 C

1 Ru03

CH2-CH3

I ~CH2-Ni+----CH-- CH2

~l--L rl '-....()

Scheme (I)

[Nitrobenzene] against the initial rate of the reaction and a fraction al order was obtained with respect to the concentration of the substrate. The resu lts are given in Table 4 . This indicates that the interact ion of the substrate is possible with the metal ions present on the surface.

284 I DI AN J. CHEM . TEC HNOL. , NOVEMBER 2000

Table 4-Summary or kinetics or hyd rogenation of nit robenzene ro r supponed catalysts in meth anol at at mospheric pressure Volume or methanol used: 20 mL

Concentration or Ru (lll)on the surrace

(mol L- 1)x iO'

1.5!:: 1.9X 2.38 2.77 1.58

1. 58

Cata lystR 1.60 2.01 2.41 2.XI

1.60

1.60

-4.8 -4.7

[ itrobenzencJ (mol L- 1)x l0 1

9.72

4 .83 9.72 14.50 19.40

9.72

972

4.83 9.72 14.50 19.40

9.72

().,8- , I

.85-41·5

, ~.s l -0.95 j

-1 I ! -1.05 i I I L·----·--·-~---------1.....1-!

log[catalyst]

Fig. 3-Log- log plot or Init ial rate vcrsu~ I Catalyst I

Effect of temperature -The rate of hydrogenation was carried out for supported as well as unbound

complex in the range of 30-45°C and the results are summarized in Table 4. In order to compare the results with unbound complex, the study was made using sarne amount of the catalyst and it was found to be difficult to measure hydrogen uptake. However when a higher amount was used for the same purpose, a lower rate was observed (Tabl e 5 ). The e nergy of

Temperatu re ("C)

.15

35

30 35

40 45

35

35

30 35

Rate or react ton (mL min- 1)x iO"

8.8 10.0 11 .0 12.3 S.O 8.X 11 .5 12.6

7.5 8.S

10.1-> 13.0

11 .2 12.2 13.3 1-Ul

9.70 11 .2 12.6 13.H

8.9 I 1.2

Order of reactiOn

0.60

0.35

0. -1 1

0.30

act ivation calcu lated from the linear plot of log k against I IT was found to be 6.6, 7.0 and 7.2 kcal/mol for cata lysts D. R and H respective ly ; the entropy of activation calculated from the rate data was found to

be -52.0, -50.2 and -51.2 eu . The negative values of ent ropy of activation corresponds to loss of freedom due to fixation of catalyst mol ecu le on the polymeric support .

Effect of solvent-The catalytic activity of the cata lys ts has been seen in the liquid phase, it is important to investi ga te the role of the solvents . The study was carried out using different solvents. The resu lts are summarised in Table 6. The enhancement in the reaction rate using polar solvents may be due to the swel ling of the po lymer support and hence the availability of more catalytic sites on the surface of po lymer.

L(fe cycle of tile catalyst-One of the ways in which the polymer supported catalyst can loose its activity is by loosing of Ru (III), which is brought abom by leaching of the metal complex or reduction to free metal. In most cases the polymer bound complex detaches and the metal is leached out in solution 17 ·'~.

PATEL & RAM : HYDROGENATIO 1 OF NITROBENZENE 285

Table 5- Summary of kineti cs of hydrogen of nitrobenzene using unbound catalyst in meth anol at atmospheric pressure Volume of methanol as solven t: 20 mL

Cat a I yst J-1 I Ruthenium! molL - 1 x l0~

!Nit robenzene] mol L- 1x l03

Temperature (o C)

Rate of reaction (mL min- 1)x tO"

Order of reacti on

5.00

7.50

10.00

12.50

7.50

7.50

9.72

4.83 9.72

14.50

19.40

9.72

Table 6- Etlect of solvent on the kinet; cs of hydroge nati on of nitrobenzene at atmospheri c pressure

[Nitrobenzene]: <J.72x l0-3 mol L- 1, Volume of so lvent: 20 mL,

Temperature : 35°C

Catalys t

D

R

I Catalyst] mol L- 1x iW0

1.58

1.60

Sol velll

Methanol

Ethanol

THF

Benzene

!\I ethanol

Et hanol

THF

Benzene

Rate of react ion mL min- 1xto"

8.8

7.0 6.(,

5.0

11.2

10.2

S.R

6.5

The effect is enhanced when the compl ex is anchored through monodentate li gand. It has been repo rted by

JR Drago et al. that polymer complexes anchored through chelating ligand s a re more stable under reaction condi tions. In order to study the activ ity of the catalyst, the recycl ing effic iency of the catalyst was tested (Table 7). lt was found that the catalyst was stab le up to five cyc les .

Conclusion On the basis o f above results, it is concluded that

the polymer bound ruthenium catal ysts have a higher catalyt ic act ivity than their homogeneous counterparts even with higher concentration of metal content in later one. This may be cl ue to the fo rmation of the coorclmati vel y unsaturated metai spec ies wh ich an be more easily obtained in the polymer bound moiety than in the monomeric analogs. Catal) st R wa~ found

35

35

30

35 40

45

5.4

6.6 0.64

9.0

10.0

6.0

6.6 0.33

8.8

9.7

5.6

6.6

8.4

10.0

Table 7-Recycling efli ciency of catalysts D and R for hydrogenation of nitrobenzene at 35°C at atmospheric pressure

Concentrat ion of catalys t D: 1.58 x to-5 mol L- 1• concentration

of catalyst R : 1.60 x I o-5mol L - I : concentration of nitrobenzene : 9.72 x 10-3 mol L- 1

: Total time on stream: 10 h, volume of methanol : 20 mL.

Cata lys t Time Rate of reaction (min) (mL min- 1)x I 02

D 60 8.8 (8.6)

120 8.8 (8.5)

180 8.6 (o.2l

240 8.3 (8.0)

300 7.6 (7 .3)

R 60 11.0 ( 10.8)

120 11.0 (10.7)

180 10.8 (10.2)

240 10.3 (9 7)

300 10. 1 (9.3)

Values ment ioned in parentheses indicate data for used catalysts.

to be superi or and ex hibits higher catalytic activity for hydrogenation of nitrobenzene than the catalyst D. T he DTA-TG studies of the supports and the catal ysts show that the syn thesised cataly ·ts could be used

safely below 140°C. From the recycli ng efficiency it was concluded that the catalyst can be reused up to five cycles .

Acknowledgement The authors a re thankful to Prof. A G K. Rakshit,

Head Chemistry Department , Catalysis Group (R&D) I.P.C.L., Baroda, Ion Exc hange India Ltd . Bombay as well as CSIR (New Delhi) for providing fellowshi to one of us (DRP).

286 INDIA J. CHEM . TECH OL., NOVEMBER 2000

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