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17The article has entered in publishing office 13.01.10. Ed. reg. No. 698
Статья поступила в редакцию 13.01.10. Ред. рег. № 698
UNEXPECTEDLY HIGH CATALYTIC ACTIVITY OF RUTHENIUM CATALYSTS IN THE HYDROGENATION OF NITROBENZENE
A. Kraynov12, E. Gebauer-Henke1, W. Leitner1,23'*, T.E. Müller1'*
1CAT Catalytic Center, ITMC, RWTH Aachen University, Worringerweg 1, D-52074 Aachen, Germany.
* E-mail: [email protected] 2Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany 3Max Planck Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
Referred: 26.01.10 Expertise: 31.01.10 Accepted: 06.02.10
The performance of ruthenium nanoparticles as quasi-homogeneous catalyst for the concurrent hydrogenation of the nitro group and the aromatic ring in nitrobenzene was compared to a classic carbon supported Ru/C catalyst. When the tartaric acid and glycine based ionic liquids [TA2-][N^8888]2 and [Gly-][N+8888], respectively, were used for stabilizing the ruthenium nanoparticles, catalytic activity and selectivity to cyclohexylamine were similar to the Ru/C catalyst. In contrast, Ru nanoparticles stabilised with the dimethylglycine based ionic liquid [Me2Gly-][N+8888] provided high selectivity to aniline. With Ru/C, the hydrogenation of nitrobenzene was faster than of aniline. These observations were rationalised in terms of binding strength of the stabiliser and intermediates to the ruthenium surface and consequent changes in the elementary steps.
Keywords: ruthenium, nanoparticles, heterogeneous catalysis, hydrogenation, nitrobenzene, aniline, cyclohexylamine, ionic liquid, stabilization, surface modification.
ПРОЯВЛЕНИЕ НЕОБЫЧНО ВЫСОКОЙ АКТИВНОСТИ РУТЕНИЕВЫХ КАТАЛИЗАТОРОВ ПРИ ГИДРИРОВАНИИ НИТРОБЕНЗОЛА
А. Крайнов, Е. Гебауэр-Хенке, В. Ляйтнер, Т.Э. Мюллер
Заключение совета рецензентов: 26.01.10 Заключение совета экспертов: 31.01.10 Принято к публикации: 06.02.10
Было произведено сравнение поведения классического нанесенного на углерод рутениевого катализатора и наночас-тиц рутения, используемых как квазигомогенный катализатор в совместном гидрировании нитрогруппы и ароматического кольца нитробензола. Когда ионные жидкости [TA2-][N+8888]2 и [Gly-][N+8888], основанные на винной кислоте и глицине, соответственно, были использованы для стабилизации наночастиц рутения, каталитическая активность и селективность по отношению к циклогексиламину были близки данным, полученным для Ru/C. В противоположность этому наночасти-цы рутения, стабилизированные ионной жидкостью [Me2Gly-][N+8888], основанной на диметилглицине, демонстрируют высокую селективность по отношению к анилину. Эти наблюдения были объяснены в терминах силы адсорбционного взаимодействия стабилизатора и промежуточных продуктов с поверхностью рутения и последовательных изменений в этапах реакции.
Ключевые слова: рутений, наночастицы, гетерогенный катализ, гидрогенизация, анилин, циклогексиламин, ионная жидкость, стабилизация, модификация поверхности.
Alexander Kraynov was born in 1980 in Orenburg, Russia. He studied physics at Novosibirsk State University and received his bachelor and master degree working at Boreskov Institute of Catalysis. In 2003, he started his Ph.D. in chemistry at Jacobs University Bremen and in 2006 joined the group of Prof. Leitner at RWTH Aachen University. Since 2007, he works at CAT Catalytic Center as a postdoctoral researcher. His current research interest lies in the field of ionic liquids, nanosized materials and catalysis.
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,P-unsaturated aldehydes. She joined the research group of Prof. 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
Walter Leitner
Professor Walter Leitner was born in 1963. He studied chemistry before completing his doctorate at the University of Regensburg, to which he returned after a one-year postdoctoral stay at the University of Oxford as a Liebig-Fellow. In 1991, he joined the newly formed Working Group "CO2 chemistry" of the Max Planck Society at the University of Jena, where he obtained his "Habilitation" in 1995. After nearly seven years in senior positions at the Max Planck Institute for Coal Research in Mülheim an der Ruhr, he accepted an appointment to the Chair of Technical Chemistry and Petrochemistry of the RWTH Aachen University in 2002. In addition to activities on the board of CAT Catalytic Center as scientific director, the German Society for Catalysis (GeCATS) and the Sustainable Chemistry Section of the German Chemical Society (GDCh), he is also the scientific editor of Green Chemistry. His scientific interests are centred on homogeneous catalysis with transition metal complexes and the use of supercritical carbon dioxide for environmentally benign chemical processes. He has been awarded the Wöhler Prize by the German Chemical Society for his innovative contribution to the development of sustainable chemical processes.
Thomas E. Müller
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
Alicyclic amines are important intermediates in the chemical and pharmaceutical industry [1]. They are obtained readily by hydrogenation of the aromatic ring in substituted anilines [2-9]. Frequently, the anilines are obtained by reduction of the corresponding nitro aromatics (Scheme 1) [10-12].
1 3H2, -2H2O
jy Catalyst
no2
3H,
Catalyst
Scheme 1. Synthesis of alicyclic methylcyclohexylamine by reduction of nitrotoluenes
We, thus, became interested in heterogeneous-catalytic protocols allowing the simultaneous reduction of the nitro group and aromatic ring in nitrotoluidines. Controlling the chemoselectivity with respect to formation of primary and secondary amines and alcohols, the cis/trans stereochemistry in the primary amines, and the enantioselectivity in this reaction is of particular interest in the production of pharmaceuticals [13].
To our knowledge, the open literature provides no examples of the direct transformation of nitrotoluenes to
alicyclic amines using heterogeneous catalysts. Also for the hydrogenation of nitrobenzene to cyclohexylamine, only few reports have been published [14, 15]. We report herein on a comparison of the performance of ionic liquid stabilised ruthenium nanoparticles as quasi-homogeneous catalyst and a conventional supported Ru/C nanocomposite catalyst in the hydrogenation of nitrobenzene to cyclohexylamine evaluating the impact of the dimensional factor and of additives on catalytic activity and selectivity.
Experimental
Materials and instrumentation
The chemicals 6/'s-(2-methylallyl)(1,5-cyclo-octadiene)ruthenium(II), ruthenium on activated charcoal (5 wt%), (R^)-tartaric acid, glycine, dimethylglycine, Ambersep 900 OH anion exchange resin, tetraoctylammonium bromide, acetophenone were obtained from Aldrich and used as received. Silica gel 60 (0.04-0.063 mm) was obtained from Merck. Solvents were reagent grade, dried and distilled before use following standard procedures.
For gas chromatographic analyses, a TRACE GC ULTRA from Thermo Scientific instrument equipped with a 50 m CP-Sil-PONA-CB capillary column and FID detector was used.
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Preparation of ionic liquids For the preparation of a methanolic solution of tetraoctylammonium hydroxide, a chromatographic column (100 ml volume) was filled half with the hydroxide loaded anion exchange resin Ambersep 900 OH. The resin was washed slowly with distilled water (300 ml) and subsequently with aqueous methanol (100 ml, 60%). Tetraoctylammonium bromide (4.035 g, 7.38 mmol) dissolved in aqueous methanol (170 ml, 60%) was passed over the column during 1 hour. The resin was washed with aqueous methanol (ca. 100 ml, 60%) until no more hydroxide was determined in the eluent by analysis with a solution of AgNO3 in HNO3. Similarly, the absence of bromide ions in the combined fractions was confirmed by analysing a sample with a solution of AgNO3 in water.
The ionic liquid й/5-tetraoctylammonium (RR)-tartrate [TA2-][N+8888]2 was obtained by addition of (RR)-tartaric acid (0.554 g, 3.69 mmol) to the solution of tetraoctylammonium hydroxide in aqueous methanol under constant stirring. Finally, the solvent was removed in a rotary evaporator and the ionic liquid dried under high vacuum (10-2 mmHg, 8 h, 60° C). The ionic liquids tetraoctylammonium glycinate [Gly-][N+8888] and tetraoctylammonium dimethylglycinate [Me2Gly-][N+8888] were prepared accordingly. All ionic liquids were obtained in quantitative yield.
Preparation of Ru nanoparticles 5/'s-(2-methylallyl)(1,5 -cyclooctadiene)ruthenium(II) (52.3 mg, 0.163 mmol, corresponding to 16.54 mg Ru) was mechanically mixed with [TA2-][N+8888]2 at room temperature in a small glass bottle. The mixture was transferred into a custom-build high pressure metal autoclave. The autoclave was purged with argon and pressurised with 100 bar hydrogen. The autoclave was placed in an oil bath and maintained at 100° C for 16 hours. A dark brown liquid was obtained and exposed at 60° C to high vacuum for 6 hours to remove volatile side products. The mixture containing Ru nanoparticles was used directly as quasi-homogeneous catalyst. The Ru nanoparticles in the ionic liquids [Gly-][N+8888] and [Me2Gly-][N+8888] were prepared accordingly.
Catalyst characterization Transmission electron microscopy (TEM) images were taken on a JEOL 2000 FX II instrument at the Central Facility for Electron Microscopy (GfE), RWTH Aachen University. Specimens were prepared by placing a drop of the colloidal solution or suspension onto a copper grid with a perforated carbon film subsequently allowing the solvent to evaporate.
The metal content of the Ru/C catalyst was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a Spectro Ciros Vision, ICP-OES. The H2 chemisorption isotherms and the BET surface area (measured at 77 K) were obtained on a Sorptomatic 1900 (Thermo Electron S. p. A.) instrument.
Hydrogenation reactions In a typical experiment, substrate, catalyst, external standard (n-dodecane) and magnetic stirring bar were placed in a 20 ml stainless steel autoclave. The latter was purged with argon and pressurized to 100 bar of hydrogen pressure and transferred into an oil bath preheated to the desired reaction temperature. After 16 hours, the gas pressure was released, the autoclave opened and THF added to the reaction mixture. The stability of colloidal Ru nanoparticles was evaluated after the reaction as absence of a precipitate. The mixture was then filtered over a bed of silica gel in case of the colloidal Ru catalyst or using a 0.45 ^m syringe filter in case of the Ru/C catalyst.
Results and discussion
The performance of ionic liquid stabilised ruthenium nanoparticles in the hydrogenation of nitrobenzene was evaluated in comparison to a conventional supported Ru/C catalyst. The ruthenium nanoparticles were prepared by dissolution of 6/'s-(2-methylallyl)(1,5-cyclooctadiene)ruthenium(II) in an ionic liquid and reduction to elemental ruthenium with molecular hydrogen. To evaluate the effect of the stabiliser [16], we decided to employ three different neutral and basic ionic liquids (Table 1), i.e., ¿/^-tetraoctylammonium tartrate [TA2-][N+8888]2, tetraoctylammonium glycinate [Gly-][N+8888], and tetraoctylammonium dimethylglycinate [Me2Gly-][N+8888] (Scheme 2) as modifier for the Ru nanoparticles.
Table 1
Functional groups of the ionic liquids used in stabilisation of Ru nanoparticles and pKa value of the corresponding acid
Ionic liquid Functional group I pKa Functional group II pKa Ref.
[TA2-][N+8888]2 2 x -COO- 2.98, 4.34 -OH — [17]
[Gly-][N+8888] -COO- 2.35 -nh2 9.78 [18]
[Me2Gly-][N+8888] -COO- 2.35 -NMe2 9.89 [18]
[Gly-][N+8888]
C8H17
2 C8H17 N+ C8H17
C8H17 C8H-|7 N+ C8H-|7
C8H17 C8Hi7 N+ C8H17
CftH
8n17
c«H
8n17
cftH
8n17
Scheme 2. Ionic liquids used for stabilisation of the Ru nanoparticles
Ionic liquid stabilized ruthenium nanoparticles The size of the ruthenium nanoparticles was evaluated with transmission electron microscopy (TEM). The [TA2"][N+8888]2 stabilized Ru nanoparticles (Fig. 1) had average size of 2.5 nm and narrow size distribution with standard deviation of about 0.5 nm. The particles were well-dispersed and no obvious coagulation of the nanoparticles was found.
Fig. 1. TEM images of the Ru nanoparticles in [TA2"][N+8888h before (left) and after (right) the use as hydrogenation catalyst
40-
ra 30
20-
10
Fig. 2. Size distribution histogram of the [TA2"][N+8888]2 stabilised Ru nanoparticles before and after use as catalyst in acetophenone hydrogenation
It is important to note that under the typical reaction conditions of hydrogenation (pH2 = 100 bar, T = 100° C) [19] the mean size of the particles remained practically unchanged and no agglomeration was observed (Fig. 2).
The Ru nanoparticles in [Gly"][N+8888] or [Me2Gly"][N+8888] (Fig. 3) had a size in the range of 3.06.0 and 2.0-4.5 nm. The particles were also well-dispersed. Only a small number of larger particles with up to 8 nm size were found.
4.0 4.5 Size, nm
20 -
s
le icl
tar15 p
10
7
Size, nm
e
CL
2 3 4 5 6 7 8
Size, nm
Fig. 3. Size distribution histograms and TEM images (insert) of the Ru nanoparticles in [Gly"][N+8888] (upper) and [Me2Gly"][N+8888] (down)
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2
3
4
5
6
5
0
0
Characterization of the conventional Ru/C catalyst The results of characterization of the Ru/C catalyst are summarized in Table 2.
Table 2
Characterization of the Ru/C catalyst used in this study
Fig. 4. TEM image of the Ru/C catalyst. The scale bar is 50 nm for the overview image and 5 nm for the inset
The material had a slightly lower ruthenium content (3.95 wt.%) than stated by the supplier. Electron microscopy revealed ruthenium particles of 2-5 nm mean particle diameter (Fig. 4). The dispersion of the metal particles was found to exceed 100% reflecting an uncertainty in the binding mode of hydrogen frequently observed for ruthenium [20]. The dispersion was calculated assuming that one hydrogen atom chemisorbs per ruthenium surface atom, but additional hydrogen might bind in bridging positions and as sub-surface hydrogen. In consequence, also the mean particle diameter calculated from the hydrogen chemisorption data was smaller than determined by TEM analysis.
Hydrogenation with Ru nanoparticles
Ruthenium nanoparticles, which were stabilized with neutral and basic ionic liquids, were used as a quasi-homogeneous catalyst in the hydrogenation of nitrobenzene (1) in comparison to the conventional Ru/C catalyst (vide infra).
With [TA2-][N+8888]2 and [Gly-][N+8888] stabilized Ru nanoparticles, cyclohexylamine was obtained as the main product (79 and 75% selectivity, respectively). Surprisingly, aniline was obtained with high selectivity (80%), when the [Me2Gly-][N+8888] stabilised Ru nanoparticles were used. Dicyclohexylamine and cyclohexanol were obtained as side products (Table 3).
In the case of the [TA2-][N+8888]2 stabilized Ru nanoparticles (entry 1), the selectivities towards cyclohexylamine, dicyclohexylamine and cyclohexanol (79, 4, and 13%, respectively) were similar to the conventional Ru/C catalyst (78, 6, and 17%, respectively). For the glycine based ionic liquid [Gly-][N+8888] (entry 2), the selectivities were analogous (75, 13, and 6%, respectively) and some aniline (6%) was obtained. In contrast, aniline was the main product (80% selectivity) in case of the dimethylglycine based ionic liquid [Me2Gly-][N+8888] (entry 3), while no cyclohexylamine, dicyclohexylamine or cyclohexanol were obtained.
Metal content [wt.%] 3.95
Specific BET surface area [m2/g] 851.4
Metal surface area a [m2/g] 28.1
Dispersion b [%] 132.6
Mean particle diameter (active phase) c [nm] 0.7
Mean particle diameter (active phase) d [nm] 2-5
ameasured by hydrogen chemisorption; formal value based on hydrogen chemisorption; cbased on hydrogen chemisorption data assuming spherical metal particles; dbased on statistical evaluation of particles size in the electron microscopy images.
(1)
Table 3
Conversion and selectivity in the hydrogenation of nitrobenzene using ionic liquid stabilized Ru nanoparticles
Entry Ionic liquid Catalyst [mol%] Conversion [%] Selectivity* [%]
CHA DCHA CHO ANI
1 [TA2-][N+8888]2 0.5 100 79 4 13 0
2 [Gly-][N+8888] 0.5 100 75 13 6 6
3 [Me2Gly-][N+8888] 0.14 67 0 0 0 80
*Cyclohexylamine (CHA), dicyclohexylamine (DCHA), cyclohexanol (CHO), aniline (ANI)
A1
We speculate that the ionic liquid, in which the nanoparticles were prepared, remained coordinated to the surface of the Ru nanoparticles. In this respect, it is known that modification of the catalyst surface can strongly change activity and selectivity [21]. Strongly coordinating stabilizers, in particular, compete with the substrate for coordination, and may lead to reduced activity. In the hydrogenation of nitrobenzene, aniline was observed in the product mixture with Ru nanoparticles stabilized with [Gly-][N+8888] or [Me2Gly-][N+8888], which had an amino function in the anion. In particular, when the Ru nanoparticles were stabilized with [Me2Gly-][N+8888] containing a tertiary amino group, aniline was formed in high selectivity. Apparently, the selectivity towards aniline increased with increasing basicity and, as we suppose, coordination strength of the anion of the ionic liquid. The ruthenium surface may have been partially poisoned and the reaction stopped at the intermediate product aniline. With respect to the formation of cyclohexanol, a base catalysed hydrolysis of cyclohexylamine or an intermediate product by the by-product water may have occurred.
Hydrogenation with Ru/C To evaluate the effect of the ionic liquid stabilizer in the catalysis with ruthenium nanoparticles relative to a classic carbon support, nitrobenzene was
hydrogenated also over a conventional Ru/C catalyst (Table 4). At 80 °C, cyclohexylamine was obtained with 78% selectivity, while dicyclohexylamine and cyclohexanol were obtained as side-products. At higher temperatures (135 °C), the selectivity to the target product cyclohexylamine (entry 2) was significantly reduced, while dicyclohexylamine and three unidentified compounds were obtained in significant amounts. A reference experiment (entry 3) showed that cyclohexylamine reacts to dicyclohexylamine at 135 °C, while cyclohexylamine was stable at 80 °C. Therefore, further hydrogenation reactions were performed at 80 °C.
It is interesting to note that the hydrogenation of aniline (entry 4) under the same reaction conditions resulted in significantly lower conversion (48%), reduced selectivity towards cyclohexylamine (63%) and higher amounts of dicyclohexylamine (33%) compared to nitrobenzene. Doubling the catalyst loading resulted in increased conversion of aniline (from 48 to 76%), slight decrease in the selectivity to dicyclohexylamine (from 33 to 24%), while the selectivity to cyclohexylamine remained nearly unchanged. The moderate activity of the Ru/C catalyst in the hydrogenation of aniline is not surprising, and hence this reaction is carried out typically at temperatures higher than 100° C [22].
Table 4
Conversion and selectivity for the hydrogenation of nitrobenzene and reference compounds over Ru/C
Entry Substrate Temp. [°C] Catalyst [mol%] Conversion [%] Selectivity* [%]
CHA DCHA CHO ANI
1 Nitrobenzene 80 0.5 100 78 6 17 0
2 Nitrobenzene 135 0.8 100 6 47 12 0
3 Cyclohexylamine 135 0.8 58 - 52 0 0
4 Aniline 80 0.5 48 63 33 0 -
5 Aniline 80 1.0 76 66 24 0 -
* Cyclohexylamine (CHA), dicyclohexylamine (DCHA), cyclohexanol (CHO), aniline (ANI)
In spite of the fact that the hydrogenation of nitrobenzene requires more reaction steps than the hydrogenation of aniline (Scheme 1), the hydrogenation of the nitrobenzene was faster and full conversion was obtained more quickly, while aniline was not fully converted under the same reaction conditions. To the best of our knowledge, this is a new and unexpected effect, which deserves further investigation. At the moment, we speculate that the elementary steps on the catalyst surface are distinctly different in nitrobenzene and aniline hydrogenation.
A possible reaction pathway (Scheme 3, path A) consists of the formation of aniline and its subsequent reduction to cyclohexylamine. However, this sequence is not compatible with the enhanced reaction rate.
Scheme 3. Analysis of the most likely reaction sequences of nitrobenzene hydrogenation. Note that each reaction comprises adsorption on ruthenium, several elementary steps on the catalyst surface and desorption of the reaction product(s)
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Aniline may be prone to slow hydrogenation as it coordinates to ruthenium preferentially via the amine group, while binding via the aromatic system is necessary for core hydrogenation of the aromatic ring. The direct hydrogenation of nitrobenzene to cyclohexylamine avoids the formation of free aniline (Scheme 3, path B). In this direct pathway all elementary reaction steps occur on the catalyst surface and no intermediates desorb from the catalyst surface. However, we cannot rule out at the present stage that the reaction proceeds via nitrocyclohexane [23], i.e., the aromatic ring is hydrogenated first, and the nitro group thereafter (Scheme 3, path C). It is also not excluded, that one of the intermediates or by-products formed in this reaction, e.g., nitrosobenzene or water, respectively, may function as a cocatalyst and lead to an enhanced reaction rate and improved selectivity towards cyclohexylamine. Note that the presence of water in combination with NaNO2 as additive led to enhanced activity of Ru/C for the hydrogenation of 4,4'-methylenedianiline [24].
Conclusions
Ruthenium nanoparticles, which were stabilized with neutral and basic ionic liquids, were used successfully as catalysts in the hydrogenation of nitrobenzene. The performance was compared to a conventional Ru/C catalyst. The [TA2-][N+8888]2 and [Gly-][N+8888] stabilized Ru nanoparticles provided similar conversion of nitrobenzene and the same high selectivity to cyclohexylamine as the Ru/C catalyst. In contrast, the [Me2Gly-][N+8888] stabilised Ru nanoparticles provided aniline as the main product at lower conversion of nitrobenzene. We speculate that this difference is caused by the presence of the Me2Gly- anion, which remained coordinated to the catalyst surface. Note that the size of the ruthenium nanoparticles was comparable in all four catalysts. Thus, the particular performance of the [Me2Gly-][N+8888] stabilized Ru nanoparticles must be associated with the presence of the modifier. Surprisingly, the Ru/C catalyst showed higher activity and selectivity towards cyclohexylamine, when the reaction was started from nitrobenzene, rather than from aniline. We speculate that for nitrobenzene hydrogenation an alternative reaction pathway on the ruthenium surface circumvents the formation of free aniline. Coordination of aniline via the amine group poisons the catalyst surface and hinders binding of the aromatic system, which reduces the rate of hydrogenation of the aromatic core. The origin of these changes in the reaction pathways will be investigated further in a more detailed study.
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