DIASTEREOMER CONTROL IN THE HYDROGENATION OF O- AND P-TOLUIDINE OVER RUTHENIUM CATALYSTS Текст научной статьи по специальности «Химические науки»

КАТАЛИЗ

CATALYSIS

The article has entered in publishing office 10.01.10. Ed. reg. No. 694 Статья поступила в редакцию 10.01.10. Ред. рег. № 694

DIASTEREOMER CONTROL IN THE HYDROGENATION OF o- AND ^-TOLUIDINE OVER RUTHENIUM CATALYSTS

E. Gebauer-Henke, L. Blumenthal, A. Prokofieva, H. Vogt, G. Voss, T.E. Müller*

CAT Catalytic Center, ITMC, RWTH Aachen University, Worringerweg 1, D-52074 Aachen, Germany Tel.: +492418028593 / Fax: +492418022593 *E-mail: [email protected]

Referred: 25.01.10 Expertise: 30.01.10 Accepted: 05.02.10

The liquid phase hydrogenation of toluidine was studied as a model reaction for the reduction of more complex aromatic amines used in the pharmaceutical industry. In this study, the hydrogenation of o- and/>-toluidine was investigated in detail using a Ru/C catalyst, with special emphasis on the reaction kinetics. With respect to product selectivity, two parameters were investigated: the chemo selectivity towards 2- and 4-methylcyclohexylamine (relative to secondary amines), and the diastereoselectivity towards the cis- and trans-products.

Keywords: ruthenium, nanoparticles, hydrogenation, heterogeneous catalyst, o-, p-toluidine, diastereomer control.

КОНТРОЛИРУЕМОЕ ОБРАЗОВАНИЕ ДИАСТЕРЕОМЕРОВ В РЕАКЦИИ ГИДРИРОВАНИЯ о- И р-ИЗОМЕРОВ ТОЛУИДИНА В ПРИСУТСТВИИ РУТЕНИЕВОГО КАТАЛИЗАТОРА

Е. Гебауэр-Хенке, Л. Блюменталь, А. Прокофьева, Х. Фогт, Г. Фосс, Т.Э. Мюллер

Заключение совета рецензентов: 25.01.10 Заключение совета экспертов: 30.01.10 Принято к публикации: 05.02.10

Жидкофазное гидрирование толуидинов было изучено в качестве модельной реакции для восстановления более сложных ароматических аминов, используемых в фармакологической индустрии. В настоящей работе детально изучалось гидрирование о- и р-изомеров толуидина, особое внимание было уделено изучению кинетики реакции. Были исследованы параметры, влияющие на образование продуктов, такие как селективность по отношению к 2- и 4-метилцикло-гексиламину (в сравнении с образованием вторичных аминов) и диастереоселективность относительно цис- и транс-продуктов.

Ключевые слова: рутений, наночастицы, гидрогенизация, гетерогенный катализатор, о-, р-толуидин, диастереометрический контроль.

Ewa Gebauer-Henke was born in Lodz, Poland in 1978. She received her Diploma at the Technical University of Lodz in 2002 for studies on the selective reduction of a,|3-unsaturated aldehydes. She joined the research group of J. Rynkowski and received her Ph.D. degree in 2008 for her work on supported catalysts in the selective hydrogenation of crotonaldehyde in gas and liquid phase. In 2008, she moved to RWTH Aachen University for postdoctoral research on amine synthesis and selective hydrogenation over heterogeneous catalysts. She has co-authored 15 publications. Her current research interest is focused on heterogeneous catalysis, synthesis and characterization of catalysts, multiphase reactions, and selective hydrogenation reactions in gas and liquid phase.

Ewa Gebauer-Henke

Lena Blumenthal was born in München, Germany in 1987. She is a student of RWTH Aachen University and finished her Bachelor thesis in 2009 at CAT Catalytic Center, RWTH Aachen University. The topic was catalyst development and reaction engineering aspects of the hydrogenation of aromatic amines with heterogeneous catalysts.

Lena Blumenthal

Angelina Prokofieva

Henning Vogt

Georg Voss

Thomas E. Müller

Angelina Prokofieva was born in Kiev, Ukraine in 1982. She received her Master degree at the National Taras Schevchenko University in Kiev in 2003 for studies on the coordination polymers of W,W-dimethyl-4,4'-bipyrazolyl with 3d-metals. Afterward she joined the research group of Prof. Meyer at the Georg-August Universität Göttingen, Germany and received her Ph.D. degree in 2008 for her work on the C-C coupling of phenols catalyzed by dinuclear copper complexes. In 2008, she moved to RWTH Aachen University for postdoctoral research at CAT Catalytic Center to develop new bimetallic redox catalysts for C-C coupling reactions. She was awarded with three scholarships during her PhD studies. Her current research interest is focused on homogeneous catalysis, synthesis and characterization of catalysts, oxidation reactions.

Henning Vogt was born in Koblenz, Germany in 1975. He received his university degree in chemistry from RWTH Aachen University in 2002. During his postgraduate studies with Stefan Brase at the University of Bonn, he worked on the development of new organocatalytic amination reactions and obtained his Ph.D. in 2006. He then moved to the Technical University of Denmark for a postdoctoral stay with Robert Madsen, where he developed a new catalytic system for the direct synthesis of amides from alcohols and amines. Since early 2008, he is holding a postdoctoral position at CAT Catalytic Center, RWTH Aachen University. His current work focuses on catalytic polymerization and amination reactions.

Georg Voss was born in Bonn, Germany in 1984. He is a student of RWTH Aachen University and worked in 2009 as research student at CAT Catalytic Center, RWTH Aachen University on kinetic investigations of the hydrogenation ofp- and o-toluidine over heterogeneous ruthenium catalysts.

Thomas E. Müller was born in Landshut, Germany in 1967. He received his undergraduate education at LMU München and ETH Zürich. For his Diploma project, he worked with D. M. L. Goodgame, IC London, on coordination polymers. After returning to Switzerland, he received his Diploma in 1991. He joined the research group of D. M. P. Mingos at IC London and received his Ph.D. degree in 1995 for studies on polyaromatic phosphines and their coordination to noble metals. In 1995, he moved to the University of Sussex to pursue postdoctoral research on fullerenes and nanotubes. For his habilitation, he joined the group of J. A. Lercher at TU München in 1998. In 2003, he continued at TU München as Privatdozent. After visiting professor positions at NUS Singapore (2005) and the University of Tokyo (2005), he accepted the position as head of CAT Catalytic Center, RWTH Aachen University, in 2007. He has published more than 50 papers, mainly in the field of catalyst immobilization, amine synthesis, and mechanistic studies on hydroamination, reductive amination, and hydrogenation of nitriles. His current research interest is focused on homogeneous catalysis with transition metal complexes, immobilization of homogeneous catalysts, supported ionic liquids and metal nanoparticles, multiphase reactions, and building block systems for heterogeneous catalysis.

Introduction

Today, heterogeneous catalysts find increasing attention in fine chemical industry, due to the many advantages, such as the ease of handling, separation, recovery, and recyclability, their high stability, as well as environmental aspects [1]. In the production of fine chemicals, the control of the stereochemistry in various heterogeneous catalysed reactions has become a subject of considerable interest [2]. The chemoselective and stereoselective catalytic hydrogenation of aromatic compounds is an important example. The products, alicyclic amines, are used in the synthesis of pesticides,

plasticizers, explosives, metal corrosion inhibitors and sweetening agents, but also as intermediates in the pharmaceutical industry [3].

In contrast to the hydrogenation of aniline, there are only few reports on the hydrogenation of substituted aromatic amines, such as toluidines. Metal oxide catalysts have been reported [4] but metal catalysts based on Co, Ni, Ru and Pd are used most frequently for the hydrogenation of aromatic amines [5-10]. A particular challenge arises from controlling the chemo- and stereoselectivity of the reaction. Friefelder [11] found that an alkyl substituent on the aromatic ring has very little effect on aniline hydrogenation when ruthenium

International Scientific Journal for Alternative Energy and Ecology № 4 (84) 2010

© Scientific Technical Centre «TATA», 2010

dioxide is used as the catalyst. Ranade investigated the diasteroselective hydrogenation of o-toluidine using (S)-proline as a chiral auxiliary [12].

Products reported to arise from aniline hydrogenation include cyclohexylamine, dicyclohexylamine, A-phenyl-cyclohexylamine, diphenylamine, ammonia, benzene, cyclohexane, cyclohexanol, and cyclohexanone. The catalyst and the reaction conditions determine the rate of product formation and the selectivity. In comparison to supported Ni, supported Ru catalysts provide higher selectivity in liquid-phase hydrogenation of aniline to cyclohexylamine [13-15]. Thus, the selectivity increased by approximately 60% in water and 15-20% in organic

solvents. With a Ru/Al2O3 catalyst, cyclohexylamine was obtained with a selectivity of around 90%. Other products observed were dicyclohexylamine, cyclohexanol, and cyclohexanone. Hydrogenation with a Rh/Al2O3 catalyst in water gave cyclohexylamine as the main product with selectivities of 64-79%; the side-products were mainly dicyclohexylamine and cyclohexanol. These reports indicate that the product selectivity is strongly dependent on the choice of the catalyst and the reaction conditions. Narayanan proposed a reaction scheme for the vapour-phase hydrogenation of aniline over Ni/Al2O3 and Co/Al2O3 catalysts (Scheme 1) [7], whereas a different pathway was assumed for hydrogenation over Rh/Al2O3 [5, 16].

Scheme 1. Reaction sequence proposed for the hydrogenation of aniline over Ni/Al2O3 and Co/Al2O3 catalysts [7]

For the hydrogenation of aromatic amines, it was demonstrated that the addition of salts to the reaction mixture can strongly influence both conversion and selectivity towards the primary amine. Nishimura showed that addition of LiOH-H2O leads to full selectivity towards the primary amine, as well as significantly shortened reaction times [17]. The same behaviour was observed by Kim et al. upon addition of NaNO2 or NaNO3 to the reaction mixture [18]. Addition of these salts provided higher conversion, while suppressing the formation of side-products. After addition of those salts to the reaction mixture, the primary amine was obtained with 100% selectivity in the hydrogenation of 6is-(4-aminophenyl)methane (MDA) and 1,4-diaminobenzene (PDA).

In this study, we decided to explore the hydrogenation of toluidine, as it provides particular challenges with respect to chemo- and diastereoselectivity:

- the aromatic ring can be fully or partially hydrogenated;

- the amino group can be cleaved off or may be susceptible to parallel or consecutive reactions;

- the methyl group adds cis/trans diastereomerism to the hydrogenated product.

Here, we report on the liquid-phase hydrogenation of o- and /-toluidine on a Ru/C catalyst placing particular emphasis on the reaction kinetics and the diastereoselectivity (Eq. 1 and 2).

H,C

nh2

H2, cat.

H

trans

(1)

(2)

Experimental

Results and discussion

Materials and instrumentation All chemicals (o- or p-toluidine, 99.7%), hexadecane and THF were obtained from Sigma-Aldrich and used as received. The Ru/C catalyst (nominal ruthenium content 5 wt %) used throughout the study was obtained as a powder. The catalyst was characterised by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a Spectro Ciros Vision ICP-OES. For high resolution transmission electron microscopy (HR-TEM) a JEOL JEM 2000 FX II electron microscope was used, which was equipped with an EDX normal SiLi -detector (Tracor TN 5502), PEELS (Gatan 666), Slow-Scan CCD-Camera (Gatan 690), heating stage (JEM EM-SHH4; < 800 °C), tilting angles: ±25° (X and Y), and a point-to-point resolution of 0.28 nm. The H2 chemisorption isotherms and the BET surface area (measured at 77 K) were obtained on a Sorptomatic 1900 (Thermo Electron S. p. A.). The data concerning the Ru/C catalyst are summarized in Table 1. Gas chromatographic analyses were carried out using a Hewlett Packard 6890 device equipped with HP 7683 autosampler, 50m-CP-Sil-Pona-CB GC column and flame ionisation detector.

Table 1

Characterisation of the Ru/C catalyst used in this study

Specific BET surface area [m2/g] 851.4

Metal content [wt.%] 3.95

Metal surface area [m2/g] 28.1

Mean particle diameter (active phase) a [nm] 2-5

Mean particle diameter (active phase) b [nm] 0.7

As catalyst for the core hydrogenation of o- and p-toluidine, Ru/C was chosen. High-resolution transmission electron microscopy (TEM) revealed the catalyst to contain ruthenium clusters in the range of 2.7 to 4.4 nm in diameter, which are located on the surface of the carbon support (Fig. 1, a). An energy-dispersive X-ray spectroscopy (EDX) analysis of a chosen area of the investigated catalyst confirmed that only ruthenium was present as the metal (Fig. 1, b).

a based on statistical evaluation of the particle size in the electron microscopy images; b based on hydrogen chemi-sorption data assuming spherical metal particles.

Hydrogenation experiments Kinetic studies were performed in 200 ml stainless steel autoclaves equipped with mechanical stirrer for gas entrainment, heating mantel and sampling outlet. Typically, the reactor vessel was charged with the substrate o- or p-toluidine (Sigma-Aldrich, 99.7%), the Ru/C catalyst (0.5 mol %, 0.5 wt % Ru), and hexadecane as GC-standard (Sigma-Aldrich) together with 140 mL THF (Sigma-Aldrich). The vessel was stirred at 500 rpm and heated to the reaction temperature (90, 100, 110, or 120 °C). The reaction was started by pressurising the reactor with hydrogen to 100 bar. During the reaction, the hydrogen pressure was maintained constant. Samples were obtained at regular time intervals for gas chromatography (GC). The product selectivities were calculated as the ratio of the amount of the particular product to the total amount of products formed.

Fig. 1. Typical TEM image (top) and EDX characterisation (bottom) of the Ru/C catalyst

Hydrogenation of p-toluidine The hydrogenation of p-toluidine was carried out at four different temperatures, 90, 100, 110, and 120 °C (Table 2). The best results were obtained for experiments performed at 120 °C, where the highest conversion (49%) and the best selectivity to 4-methylcyclohexylamine (68%) was obtained. At lower temperatures, the selectivity to the primary amine was in the range 37 to 56%.

Analysis of the time-concentration diagram (Fig. 2, a) revealed a nonlinear decrease of the substrate concentration and a nonlinear increase in primary amine concentration. The shape of the curve of conversion as a function of time (Fig. 2, b) resembles a logarithmic increase up to 20% conversion, while it shows a more linear dependence at higher conversions.

International Scientific Journal for Alternative Energy and Ecology № 4 (84) 2010

© Scientific Technical Centre «TATA», 2010

Table 2

Results of p-toluidine hydrogenation on Ru/C (pH? = 100 bar, 180 min)

Temperature, °C Conversion, % Selectivity, %

primary amine Secondary amine Side-products

cis trans total

90 24.0 17.6 37.7 55.3 36.1 8.6

100 26.8 11.5 25.8 37.3 52.3 10.4

110 33.3 18.1 37.5 55.6 35.1 28.7

120 48.8 29.0 39.2 68.2 12.3 19.5

100-

^ 80-

o 60-

40-

20-

50

40

Г 30

о '«

5 20

>

с

о 10

primary amine (cis) primary amine (trans) p-toluidine intermediate 1 intermediate 2 intermediate 9 sec. amine (cis) sec. amine (trans)

0 20 40 60 80 100 120 140 160 180 200

Time [min]

a

20 40 60 80 100 120 140 160 180 200

Time [min]

b

100

Ф </)

80

60

a: о

П

oz

20

♦ primary amine (cis+trans)

♦ secondary amine (cis+trans) ■ intermediate 1

♦ intermediate 2 4 intermediate 9

20 40 60 80 100 120 140 160 180 200 Time [min]

c

5n

4-

* 34

2-

1-

• •

20 40 60 80 100 120 140 160 180 200

Time [min]

d

Fig. 2. Hydrogenation of p-toluidine at 120° C (pH. = 100 bar)

8 40

0

0

0

0

0

0

We assume that the kinetic regime changes from first order to zero order in /-toluidine. The formation of the primary amine was clearly favoured with a final selectivity around 68% (Fig. 2, c). The final selectivity towards the secondary amine was about 13%.

Various unknown reaction intermediates and side products were formed. The formation of the primary amine seems to be correlated with the formation of one particular intermediate (no. 9), which decreased in concentration, while the concentration of the primary amine increased. As indicated by the plot given in Fig. 2, d, primary and secondary amines were formed in parallel. The relatively

low conversion compared to o-toluidine hydrogenation (vide infra) may be indicative of inhibition by either the main product or any of the side products adsorbing strongly on the catalyst surface, thus displacing ¿»-toluidine. This explanation would be consistent with the higher basicity of alkylamines relative to arylamines (for comparison [19]: pKa(cyclohexylamine) = 10.7, pKa(aniline) = 4.63).

The temperature dependence of selectivity and the reaction rate was also evaluated. The conversion increased exponentially with the reaction temperature (Fig. 3). While 24% conversion was obtained after 180 min at 90 °C, the conversion was 49% at 120 °C. From the

temperature dependence, the apparent activation energy was calculated to 34.7 kJmol-1. Concerning diastereo-selectivity, the trans/cis ratio was constant over the temperature range (2). The trans-primary amine was favoured with 26-39% selectivity, while the cis-primary amine was formed with a selectivity of 12-21%.

100n 90 80 70 60 50 40 30 20 10

0

Primary amine (trans) Primary amine (cis)

50

40

30

20

О

0

з

<

r

1 о

90

100

110

120

Temperature [ C]

Fig. 3. Influence of the reaction temperature on the conversion (right axis) and the selectivity to cis- and frans-4-methyl-cyclohexylamine (left axis) in the hydrogenation of p-toluidine (pH2 = 100 bar, 180 min)

Hydrogenation of o-toluidine To obtain more general information about the reaction kinetics and the selectivity in the hydrogenation reaction, the experiments were repeated using o-toluidine as substrate (Table 3 and Fig. 4). At 120 °C, full conversion was achieved after 180 min and the primary amine obtained with high selectivity (95%). At lower temperatures, the conversion decreased and the selectivity to the primary amine was slightly lower (89-93%).

Table 3

Results of o-toluidine hydrogenation on Ru/C (pH2 = 100 bar, 180 min)

Temperature, °C Conversion, % Selectivity, %

Primary amine Secondary amine

cis trans total

90 61.5 21.2 69.3 90.5 9.5

100 70.1 22.0 70.8 92.8 7.2

110 98.1 23.1 66.2 89.3 10.8

120 100 25.9 68.9 94.8 5.2

The reaction profile is typical for a first order reaction operated in the batch mode (Fig. 4, a). The concentration of o-toluidine decreased exponentially, while the concentration of the primary amine increased in parallel. The conversion showed an exponential increase to 100% (Fig. 4, b).

100

„ 80-

Primary amine (cis) Primary amine (trans) p-Toluidine intermediate 1 intermediate 2 intermediate 10 Sec. amine (cis) Sec. amine (trans)

60

5 40

о О

20-

20 40 60 80 100 120 140 160 180 200 Time [min]

a

20 40 60 80 100 120 140 160 180 200 Time [min]

b

100-,

I

80

— 60

40

20

-*—*—*—*-

+ Primary amine (cis+trans)

♦ Secondary amine (cis+trans) ■ intermediate 1

♦ intermediate 2 ► intermediate 10

lié* «ttt-t-'--

- г~г. у.,.,:, . ; . ;

♦ ♦

-r-

20 40 60 80 100 120 140 160 180 200 Time[min]

c

40-,

35-

T 30-

z

к 25-

T

T' 20-

к

о 15-

га

о: 10-

5-

0-

0 20 40 60 80 100 120 140 160 180 200

Time [min]

d

Fig. 4. Hydrogenation of o-toluidine at 120 °C (pH2 = 100 bar)

0

0

0

International Scientific Journal for Alternative Energy and Ecology № 4 (84) 2010

© Scientific Technical Centre «TATA», 2010

The primary amine was formed with a selectivity of 95%, while the secondary amine was obtained with a selectivity of 5%. The ratio of the selectivity between the primary and the secondary amine (Fig. 4, d) increased rapidly at shorter reaction times to around 16 at longer reaction times (>40 min). This is indicative of a consecutive reaction, whereby the secondary amine is formed as consecutive product of the primary amine.

Compared to /-toluidine hydrogenation, fewer side-products were formed in the hydrogenation of o-toluidine. The initial rate of reaction was higher and 100% conversion were obtained after 180 min (120 °C) indicating the absence of product inhibition. In contrast to /-toluidine, the primary and secondary amines were not formed in parallel. In fact, the secondary amine appears to be a consecutive product, when o-toluidine was used as substrate.

The reaction temperature was varied to evaluate the temperature dependence of the selectivity and the reaction rate (Fig. 5). The conversion towards the primary amine increased exponentially with the temperature, and was close to 100% for the experiment carried out at 110° C. The apparent activation energy derived from the initial rates was calculated to 37.2 kJmol-1, which is very close to the apparent activation energy of /-toluidine hydrogenation (34.7 kJ mol-1). The selectivity towards the cis- and trans- primary amine was stable. 7rans-2-methylcyclo-hexylamine was favoured with 66-70% selectivity, while cis-2-methylcyclohexylamine was obtained with a selectivity of 21-26%, corresponding to a trans/cis ratio of 3.5. Thus, the preference for the trans--primary amine was significantly higher than in case of /-toluidine (trans/cis ratio 2.0).

10090 SO ^ 70 £ SO | 60

I 40 « 30

20 10

0

Primary amine (trans) Primary amine (cis)

100

90

S0

70 C

о

S0 3 <

r

60 i

40 o 3

30 ]

20

10

0

90

100

110

120

Temperature [ C]

Fig. 5. Influence of the reaction temperature on the conversion (right axis) and the selectivity to cis- and trans-2-methylcyclohexylamine (left axis) in the hydrogenation of p-toluidine (Ph2 = 100 bar, 180 min)

Comparison of Substrates The results of o- and p-toluidine hydrogenation are summarized in Table 4. The conversion and the selectivity towards the primary amine were very high for o-toluidine (pH2 = 100 bar, 120 °C, 180 min), while p-toluidine gave a maximum conversion and selectivity to

primary amine of 50%. Besides the formation of larger amounts of side products, p-toluidine revealed a parallel formation of primary and secondary amine. Using o-toluidine as the substrate, the secondary amine was formed as a consecutive product with a selectivity of 5%. The trans-product was favoured for both substrates, albeit with a higher trans/cis ratio for products arising from o-toluidine.

Table 4

Summary of the results for the hydrogenation of p- and o-toluidine (pH2 = 100 bar, 180 min)

Substrate p-toluidine o-toluidine

Conversion ~ 50% ~ i00%

Selectivity towards rnh2 ~ 50% ~ 95%

Formation of RNH2/R2NH parallel reactions R2NH consecutive product

Ratio trans/cis for RNH2 2.0 3.5

Apparent activation energy 35 kJ/mol 37 kJ/mol

The activation energies for both substrates were essentially the same. This suggests that the rate-determining step - which is reflected by the apparent activation energy - may be equal for both isomers. Thus, the rate-determining step appears to be independent of the position of the methyl group. In agreement with the literature [20], this is most likely the first hydrogenation step whereby the first hydrogen atom binds to the aromatic ring.

In terms of diastereoselectivity during the core hydrogenation of aromatic rings, it is to be expected that the unsaturated CC bonds in the ring remain adsorbed on the ruthenium surface. In this case, all hydrogen atoms should approach the metal surface from the same side, corresponding to an all-cis configuration in the product. In order for two substituents to end up in trans-position to each other, the molecule has to "flip ovef' on the catalyst surface. A methyl-group in ortho-position is expected to hinder the molecule to flip over the side, while it may enhance a turn over the amino-function. The differences between the two different substrates lead to the assumption that there may be a difference in the adsorption mode of the intermediate on the catalyst surface in dependence of the position of the methyl group.

We speculate that two factors control rate and diastereoselectivity:

- the methyl-group in para-position leads to the formation of more side products, which adsorb strongly to the catalyst surface and lead to inhibition;

- the methyl group in ortho-position weakens the adsorption of the amino function to the catalyst surface. In consequence, the competing adsorption of the aromatic ring becomes more likely, leading to faster hydrogenation of o-toluidine.

Conclusion

In this study, the core hydrogenation of p- and o-toluidine on a Ru/C catalyst was investigated with particular emphasis on reaction kinetics and product selectivity. We have successfully undertaken two sets of experiments, each set involving hydrogenation experiments at four different temperatures, while the one set dealt with the hydrogenation of p-toluidine and the other with the hydrogenation of o-toluidine. Both substrates revealed the same apparent activation energy, which indicates that the rate-determining step is not affected by the position of the methyl group. We assume that the rate-determining step is the first hydrogenation step, whereby the aromatic ring reacts with the first hydrogen atom. Furthermore, conversion, selectivity towards primary amine, and trans/cis ratio are higher for o-toluidine. Thus, the position of the methyl group affects the formation the formation of the trans-product and side products. At the present stage, we can only assume that product inhibition and a difference in the adsorption mode are influencing factors in the core hydrogenation of toluidines. Further work is directed at clarifying the difference in the adsorption mode of the substrates on the catalyst surface to evaluate how this affects the formation of side products or the cis/trans-sterochemistry.

References

1. Hindle K.T., Jackson S.D., Stirling D., Webb G. The hydrogenation of para toluidine over rhodium/silica: The effect of metal particle size and support structure // J. Catal. 2006. Vol. 241. P. 417-425.

2. Konuspaev S.R., Zhanbekov K.N., Kul'kova N.V., Murzin D.Yu. Kinetics of 4-tert-Butylphenol Hydrogenation over Rhodium // Chem. Eng. Technol. 1997. Vol. 20. P. 144-148.

3. Casey J.P. Amines, cycloaliphatic, in Kirk-Othmer. Encyclopedia of Chemical Technology, 5th Edition, 2004. Vol. 2. P. 498-518.

4. Devereux J.M., Payne K.R., Peeling E.R.A. Catalytic Hydrogenation. Part I. The hydrogenation of unsaturated amines over platinic oxide // J. Chem. Soc. (Resumed). 1957. P. 2845-2851.

5. Mink G., Horvath L. Hydrogenation of aniline to cyclohexylamine on NaOH promoted or lanthana supported nickel // React. Kinet Catal Lett. 1998. Vol. 65. P. 59-65.

6. Vishwanathan V., Narayanan S. A direct correlation between dispersion, metal area, and vapour phase hydrogenation of aniline; a first report // J. Chem. Soc., Chem. Commun. 1990. Vol. 1. P. 78-80.

7. Narayanan S., Unnikrishnan R. Comparison of hydrogen adsorption and aniline hydrogenation over co-precipitated Co/Al2O3 and Ni/Al2O3 catalysts // Chem. Soc., Faraday Trans. 1997. Vol. 93. P. 2009-2013.

8. Narayanan S., Unnikrishnan R., Vishwanathan V. Nickel-alumina prepared by constant and varying pH method: Evaluation by hydrogen-oxygen chemisorption and aniline hydrogenation // Appl. Catal. A. Gen. 1995. Vol. 129. P. 9-19.

9. Barbier J., Pitara E.L., N'Zemba B., Barbot F. Maginiac, catalytic hydrogenation of aniline in aqueous media // J. Mol. Catal. A Chem. 1996. Vol. 106. P. 235-240.

10. Greenfield H. Hydrogenation of aniline to cyclohexylamine with platinum metal catalysts // J. Org. Chem. 1964, Vol. 29, P. 3082-3084.

11. Freifelder M., Stone G.R. Reductions with ruthenium catalyst. III. Hydrogenation of nuclear substituted anilines // J. Org. Chem. 1962. Vol. 27. P. 3568-3572.

12. Ranade V.S., Prins R. Diastereoselective hydrogenation of (S)-proline-2-methylanilide // J. Catal. 1999. Vol. 185. P. 479-486.

13. Hagihara H., Echigoya E. // Bull. Chem. Soc. Jap. 1965. Vol. 38. P. 2094.

14. Nishimura S., Shu T., Hara T., Takagi Y. // Bull. Chem. Soc. Jap. 1966. Vol. 39. P. 329.

15. Narayanan S., Unnikrishnan R.P. // J. Chem. Soc., Faraday Trans. 1997. Vol. 93(10). P. 2009-2013.

16. Mailyubaev B.T., Ualikhanova A. // Russ. J. Appl. Chem. 1994. Vol. 66. P. 1739.

17. Nishimura S., Kono Y., Otsuki Y., Fukaya Y. The ruthenium-catalyzed hydrogenation of aromatic amines promoted by lithium hydroxide // Bull. Chem. Soc. Jpn. 1971. Vol. 44. P. 240-243.

18. Kim H.S., Seo S.H., Lee H., Lee S.D., Kwon Y.S., Lee I.-M. Ru-catalyzed hydrogenation of aromatic diamines: The effect of alkali metal salts // J. Mol. Catal. A Chem. 1998. Vol. 132. P. 267-276.

19. http://ifs.massey.ac.nz/outreach/resources/ chem/ orgbases.php#M.

20. Bianchini C., Meli A., Vizza F. Hydrogenation of arenes and heteroaromatics. Handbook of Homogeneous Hydrogenation, 2007. Vol. 1. P. 455-488.

International Scientific Journal for Alternative Energy and Ecology № 4 (84) 2010

© Scientific Technical Centre «TATA», 2010