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Application of carbon nanotube coated aluminosilicate beads as “support on support” catalyst for hydrogenation of nitrobenzene
Journal of Industrial and Engineering Chemistry 79 (2019) 307–313
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Application of carbon nanotube coated aluminosilicate beads as “support on support” catalyst for hydrogenation of nitrobenzene László Vanyoreka , Ádám Prekoba , EmÅke Sikoraa , Edina Reizera , Gábor Muránszkya , Ferenc Kristályb , Béla Viskolcza , Béla Fisera,c,* a b c
Institute of Chemistry, University of Miskolc, Miskolc-Egyetemváros 3515, Hungary Institute of Mineralogy and Geology, University of Miskolc, Miskolc-Egyetemváros 3515, Hungary Ferenc Rákóczi II, Transcarpathian Hungarian Institute, Beregszász, Transcarpathia 90200, Ukraine
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 April 2019 Received in revised form 1 July 2019 Accepted 4 July 2019 Available online 12 July 2019
Nitrogen-doped bamboo-like carbon nanotube (N-BCNT) coating was synthesized onto the surface of zeolite beads by using Catalytic Chemical Vapour Deposition (CCVD) method to develop a “support on support” (SoS) system. These complex structured materials were used as supports during the preparation of hydrogenation catalysts. Rhodium, palladium and platinum nanoparticles were deposited homogeneously onto the surface of the N-BCNTs of the SoS (final metal content 2 wt%). The catalytic activity of these samples was compared in the hydrogenation of nitrobenzene. The Pt/N-BCNT-zeolite sample was the most active (182 mol nitrobenzene after 30 min). The activity of the other two catalysts at 20 bar was well below this value, 99.5 mol after 60 min and 96 mol after 120 min for Pd and Rh, respectively. The aniline selectivity was different for the three catalysts and they facilitate the formation of various by-products (e.g. N-methylaniline, cyclohexylamine). The usage of the Pd and Pt/NBCNT-zeolite catalysts are more convenient, as only one main by-product was formed. It was confirmed that the zeolite supported N-BCNTs are efficient catalyst supports in hydrogenation processes. Furthermore, by using this special SoS structure to support the catalytic metals the applicability is widened and the catalyst removal is easier. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: N-BCNT Rhodium Palladium Platinum Aniline SoS catalyst
Introduction Nitrobenzene has an important role in the chemical industry including the production of dyes, drugs, and explosives as it is the starting material of aniline synthesis. The first technology to create aniline involves a reduction of nitrobenzene by iron in the presence of hydrochloric acid (Béchamp process) [1]. In this case, iron oxide was formed as a by-product, which was later used as a pigment. There are other methods for producing aniline, for example, the amination of phenol with ammonia in the presence of hydrogen [2]. However, the most commonly and widely used technology is the reduction of nitrobenzene in gas or liquid phase [3–5]. A wide variety of catalysts can be used to promote this process, which
* Corresponding author at: Institute of Chemistry, University of Miskolc, MiskolcEgyetemváros 3515, Hungary. E-mail addresses: [email protected] (L. Vanyorek), [email protected] (Á. Prekob), [email protected] (E. Sikora), [email protected] (E. Reizer), [email protected] (G. Muránszky), [email protected] (F. Kristály), [email protected] (B. Viskolcz), fi[email protected] (B. Fiser).
usually consists of a catalytically active metal (e.g. Ni, Pd, Pt, Cu) and a carrier (e.g. carbon, alumina, silica, zeolites). Carbon nanotubes (CNTs) have fascinating properties, including extraordinary mechanical strength, good chemical stability and large surface area, which makes them perfect catalyst carriers [6]. CNTs do not contain micropores, therefore the catalysis can be more effective and more resistant to oxidation processes. The characteristics of carbon nanotubes can be tuned by incorporating heteroatoms into their structure [7]. Such incorporation can be done by using nitrogen-containing carbon compounds as starting materials for the synthesis of the CNTs, which will lead to the formation of N-doped carbon nanotubes (N-CNT) [8]. Incorporation of nitrogen into the CNT structure will lead to the formation of a bamboo-like structure (nitrogen-doped bamboo-like carbon nanotube, N-BCNT). N-BCNTs have more defect sites than their single- or multi-walled counterparts, and due to the incorporated nitrogen atoms, they also have high energy adsorption sites which are excellent spots for catalytically active metal particles [9,10]. Several studies have shown that reactions can be catalyzed successfully by using CNT-supported metal catalysts [11]. Functionalized carbon nanotube supported Ru catalysts were used
https://doi.org/10.1016/j.jiec.2019.07.006 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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in sorbitol hydrogenolysis to glycols [12], while Pd, Pt, and Nicontaining CNT catalysts were efficiently used in nitrocyclohexene hydrogenation [13]. N-doped CNT based catalysts were applied in oxygen and CO2 reduction reactions [14,15]. N-BCNT supported catalysts have also achieved remarkable results in the field of alternative diesel production (e.g. iron-based Fischer–Tropsch catalysts) [16]. Pd, Pt, Ru, and Rh catalysts supported on carbon (C), silica (SiO2) or alumina (Al2O3) were tested in nitrobenzene hydrogenation [17]. The catalytic activity changed by the employed metal (Pt > Pd > Ru, Rh), and carriers (C > Al2O3, SiO2) [17]. Nitrobenzene hydrogenation was also carried out by using CNT supported Pt [18], Pd and Fex-Pd catalyst [19]. The bimetallic Pt–Fe/CNT catalysts showed much higher activity than the Pt/CNTs due to the promoter effect of iron [20]. The reduction of nitrobenzene (NB) to aniline, which is one of the most important products in the chemical industry, could lead to a wide variety of by-products and intermediates depending on the reaction conditions. In the case of applying CNT supported catalyst, azoxybenzene (AOB) [18] and phenylhydroxylamine (PHA) [19] were formed as intermediates. PHA could occur by using activated carbon as catalyst support [18]. The application of Fex-Pd has promoted the formation of nitrosobenzene during the hydrogenation reaction [19]. A comparable amount of aniline and azobenzene were produced with a Pt/C catalytic system in ethanol [21]. Hydroxylamine and azoxybenzene have been produced as intermediates with Pd/C catalyst [22]. Carbon nanotube and activated carbon supported Pt catalysts were compared in nitrobenzene hydrogenation, the Pt/CNT exhibit higher activity than the Pt/AC catalysts [23]. Oxygen-containing functional groups on the surface of carbon nanotubes could affect the particle size of the deposited catalytically active metals, and in some case, the catalytic activity can be decreased [24]. Application of cobaltcontaining, oxygen and nitrogen functionalized carbon nanotubes as a catalyst will also initiate NB hydrogenation [25]. By using According to Li et al. when Pt/CNT is applied as a catalyst in NB hydrogenation only azoxybenzene can be detected as intermediate, which contradicts with the results of Sun et al. who detected nitrosobenzene (NOB) next to AOB [26,27]. Sun et al. found, that the route from NOB to aniline is less important to the production of aniline when Pt/CNT used [27]. During their experiments, NOB was used as a reactant for aniline synthesis, and mostly (95%) converted to AOB, while phenylhydroxylamine did not form. Furthermore, increasing rection time initiated the hydrogenation of AOB towards aniline [27]. Carbon nanotube supported catalysts are catalytically active and can be efficiently applied in aniline synthesis. However, CNTs can form stable dispersion in the liquid reaction media (SI Fig. 1), thus their separation is difficult [19,26]. This difficulty can be avoided, by using a special “support on support”, or SoS system based on zeolite bead supported nanotube supports (SI Fig. 1). Rhodium, palladium and platinum nanoparticles were deposited onto nitrogen-doped bamboo-like carbon nanotube (N-BCNT) coated zeolite beads. Thus, metal-containing SoS catalyst has been developed. The catalytic activity of these catalysts was studied and compared in nitrobenzene hydrogenation at various reaction conditions.
production. The next step was the synthesis of the catalytically active metal particles starting from anhydrous palladium (II)acetate (Pd(OAc)2, Fluorochem Ltd.), rhodium (II)-acetate dimer ([Rh(OAc)2]2, Sigma Aldrich), and hexachloroplatinic acid (H2PtCl6, Reanal). In the catalytic tests, methanol (WVR) was applied as a solvent, nitrobenzene (WVR) and 4.5 hydrogen (Messer) was used as reactants to produce aniline. Synthesis of the catalysts As catalyst support, N-BCNT coated zeolite beads were used, which were prepared according to a previously optimized procedure [8]. The zeolite beads (19.6 g) were added to the aqueous solution of nickel-nitrate hexahydrate (1.98 g Ni(NO3)2 6 H2O) the nickel content was kept to 2 wt%. The water from the beads was evaporated by rotary vacuum evaporator, thereafter the nickel salt impregnated zeolites were dried at 120 C overnight. The impregnated zeolite beads were used as catalysts in the Catalytic Chemical Vapour Deposition (CCVD) method to synthesize N-BCNTs. In each case, 20 g catalyst was placed in a quartz reactor and the synthesis was carried out for 60 min at 750 C. The carbon source was butylamine, which was added by a syringe pump (6 ml/h). The butylamine was evaporated in the frontsection of the quartz reactor, and carrier gas (nitrogen, 100 ml/min) took it to the catalyst bed. After the CCVD method, the N-BCNT coated zeolite beads were decorated with noble metal particles to achieve the final SoS catalysts. During the catalyst’s preparation, the metal precursors (palladium-acetate, rhodium-acetate, and hexachloroplatinic acid) were solved in distilled water. The theoretical noble metal content was 2 wt% in each case (Table 1), which was monitored by elemental analysis (ICP-OES) measurement. The N-BCNT coated 4A zeolite beads were added to the solution of metal precursors. The solvents evaporated after 30 min by using a vacuum rotary evaporator, and thus, the Rh, Pd, and Pt were impregnated into the N-BCNT/Zeolite beads. The impregnated beads were dried at 393 K overnight. Then, the samples were heat-treated in a nitrogen flow at 673 K temperature for 20 min. In the final step, the catalytically active form of the samples was achieved by hydrogenating them for one hour at 673 K. Characterization of the catalysts After the CCVD (Catalytic Chemical Vapour Deposition) synthesis of the carbon nanotubes, the presence of the N-BCNTs on the surface of the zeolite beads was confirmed by Hitachi S 4800 scanning electron microscope (SEM), which was equipped with energy dispersive EDAX X-ray spectrometer. During the sample preparation for the SEM analysis, Carbon tape rubber was used. The catalysts were examined by SEM-EDX as well. Netzsch Tarsus TG 209 thermo-microbalance was used to analyze the carbon nanotube content of the CNT/zeolite through thermogravimetric analysis (TGA). Nitrogen (4.5) and oxygen (5.0) mixture was applied as the oxidative atmosphere in the TGA measurements. The flow rate was set to 6 ml min1 and 14 ml min1, for the oxygen and nitrogen, respectively. The heating rate was 10 K min1, in the 308–1073 K temperature range. The
Materials and methods Materials Nickel(II)-nitrate hexahydrate (Ni(NO3)2x6H2O) as a catalyst, 4A zeolite beads (2 mm diameter) as support materials, patrol as solvent (a mixture of aliphatic alcohols, Molar Chem.) and nbutylamine (Sigma Aldrich) as carbon source was used for N-BCNT
Table 1 Composition of the catalysts. Catalytically active metal precursor weight (g) Weight of catalyst support (g) Palladium (II)-acetate Rhodium (II)-acetate dimer Hexachloroplatinic acid
0.84 0.85 1.06
19.60 19.60 19.60
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presence of catalytically active noble metals was confirmed by X-ray diffractometry (Bruker Advance D8). The Pd/N-BCNT-zeolite SoS catalyst was measured by Micromeritics ASAP sorptometer, and the BET surface was found 15.6 m2/g. Catalytic tests The catalytic hydrogenation was carried out in a Büchi Uster Picoclave reactor, in a 200 ml stainless steel vessel with heating jacket. An excess of hydrogen was present, and the hydrogen pressure was 5, 10 and 20 bar and the reactions were carried at 323 K. Sampling took place after the beginning of hydrogenation at 5, 10, 15, 20, 30, 60, 120, 180, 240 min. The concentration of nitrobenzene was 0.25 mol l1 in methanol. The total amount of the solution was 150 ml during each test. The 2 g catalyst was used in the reactions. Aniline formation was followed by applying Agilent 7890A gas chromatograph coupled with Agilent 5975C Mass Selective Detector. Analytical standards (aniline, nitrobenzene, nitrosobenzene, azoxybenzene, dicyclohexylamine, o-toluidine, cyclohexylamine and n-methylaniline) were provided by Dr. Ehrenstorfer and Sigma Aldrich. The efficiency of the catalytic hydrogenation was compared by calculating the conversion, X% of nitrobenzene based on the following equation (Eq. (1)): X % ¼
consumed nnitrobenzene 100 initial nnitrobenzene
ð1Þ
The selectivity, S% of the catalysts was calculated as follows (Eq. (2)): S % ¼
naniline 100 nnitrobenzene
ð2Þ
where naniline and nnitrobenzene are the corresponding chemical amounts of the compounds. The following equation (Eq. (3)) was used to calculate the yield (Y, mol %) of the species.: Y % ¼
synthetized nproduct 100 theoritical nproduct
ð3Þ
Results Characterization of the N-BCNT coated zeolite beads After the CCVD synthesis, the surface of the zeolite beads was studied by scanning electron microscopy (SEM) in order to confirm
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the presence of nanotubes. The SEM images verified that the zeolite beads are abundantly coated with carbon nanotubes (SI Fig. 2a). This observation was further studied by TGA analysis and the carbon content was determined (SI Fig. 2b). The mass change of the N-BCNT coated zeolite beads is related to the oxidation of carbon in the system which is identical to the carbon nanotube content and it was 2.6 wt%. The initial ignition temperature of the bamboo-shaped carbon nanotubes was 669 K. There was another mass loss on the initial section of the TG curve, which can be attributed to the elimination of adsorbed water from the zeolite structure and from the nanotube surface. Characterization of the Rh, Pd, and Pt-decorated N-BCNT coated zeolite nanocomposite The noble metal decorated SoS catalysts were examined by SEM and EDX (Fig. 1a–f). The Rh nanoparticles are located on the NBCNT surface in the rhodium-containing sample and they have an average diameter (dmean) of 6.1 nm (Fig. 1a). The size distribution of the particles covered a relatively narrow range. The surface of nanotubes is richly coated by Rh nanoparticles. Rhodium was also identified on the EDX spectrum alongside with other elements which belong to the zeolite core (Fig. 1d). In the case of the Pd containing catalyst, the average size of the particles was 8.3 nm (Fig. 1b). The palladium identified on the X-ray spectrum and the typical peaks of the zeolite are also visible (Fig. 1e). The Pt catalyst has small metal particles on its surface with a size of about 5–10 nm and the catalytically active metal is well dispersed (Fig. 1c). The energy dispersive spectrometry demonstrates the presence of platinum and other elements of the zeolite (Fig. 1f). The samples were examined by X-ray spectrometry (XRD) as well. The XRD shows, that in each case the elemental rhodium, palladium, and platinum were observable. On the diffractogram of the 2% Rh/N-BCNT-Zeolite catalyst, the reflexion of the elemental Rh with (111), (200) and (220) Miller indexed crystal plains can be found at 40.96, 47.8 and 69.7 2Q degree, respectively (SI Fig. 3a). In case of the 2% Pd/N-BCNT-Zeolite sample three crystal plains were identified Pd (111), Pd (200) and Pd (220) and the corresponding reflexion peaks are located at 40.1, 46.6 and 68.1 2Q degrees (SI Fig. 3b). The peaks which are typical for the Pt nanoparticles (111), (200) and (220) have been found on the diffractogram of the platinum-containing sample at 39.7, 47.4 and 67.2 2Q degrees, respectively (SI Fig. 3c). C (002) and C (101) crystal planes were also found due to the presence of carbon nanotubes in the
Fig. 1. SEM image and EDX spectrum of the 2% Rh/N-BCNT-Zeolite (a, d), 2% Pd/N-BCNT-Zeolite (b, e) and the 2% Pt/N-BCNT-zeolite (c, f) catalysts.
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nanocomposites and the corresponding reflexions can be seen at 26.3 and 44.4 2Q degrees in each case. In summary, the XRD results confirmed, that the three noble metals are in catalytically active forms, exactly in their elemental state on the surface of the catalysts. Results of the catalytic measurements The developed catalyst was found to be more active during the nitrobenzene (NB) hydrogenation. The 2% Pt/N-BCNT-zeolite catalyst reached 99.8% conversion after 1 h at 5 bar hydrogen pressure (Fig. 2a). The maximum conversion in the case of the palladium-containing sample after 1 h (5 bar) was 99.7%. By applying of rhodium nanocomposite 97.4% was reached at 5 bar pressure, after 4 h. The Pt nanocomposite catalyst was the most catalytically active, 180 mol (nNB) of the reactant was converted with 1 mol of platinum, within one hour (Fig. 2d). The difference in electron configuration and size of the metal atoms are two factors which could be responsible for the different activity of the catalysts. The strength of the interaction between the nitrogen of aniline and the noble metals are highly influenced by the abovementioned factors and could be the reason behind the outstanding activity of Pt. By increasing hydrogen pressure up to 10 bar, the conversion rate increased in case of all catalysts (Fig. 2b). The Pt-containing catalyst resulted in 99.8% NB conversion after 30 min, so 182 mol nitrobenzene converted by 1 mol Pt, within 30 min hydrogenation (Fig. 2e). The nitrobenzene conversion was lower with the same amount of rhodium in a similar system. By further increasing the pressure up to 20 bar, the conversion also increased and within 15 min it has reached 95.6% in case of the platinum catalyst (Fig. 2c). The conversion on the Rh/N-BCNTzeolite was increased from 60% to 95%, after 80 min. In the case of the Pd/N-BCNT-zeolite catalyst, 20 min shorter reaction time was enough to compare to lower pressure to achieve maximum nitrobenzene transformation. In the case of the Pd and Rh catalysts, 99.5 mol (after one hour) and 96 mol (after two hours) nitrobenzene were converted by 1 mol catalytically active metal. For the Pt
catalyst, this parameter was almost the same as in the case of 10 bar hydrogen pressure (Fig. 2f). In terms of selectivity 84% aniline was reached with the 2% Pd/ N-BCNT-zeolite catalyst at 5 bar after 80 min hydrogenation, and it did not change when 10 or 20 bar pressure was applied (Fig. 3a–c). The aniline selectivity was higher (98.1%) with the Rh containing samples at 5 bar, but the reaction time was longer (4 h). However, the pressure increase was counterproductive and at 10 and 20 bar, the achieved selectivity is only 88% and 53%, even after running the reaction for four hours. The aniline selectivity at 5 and 10 bar pressure was reached the maximum (99.2% and 98.9%) after 80 and 20 min in case of the 2% Pt/N-BCNT-zeolite, at 20 bar this was achieved after 60 min. The concentration of the synthesized aniline was decreased by in pressure, and reaction time. This can be explained by aniline polymerization, which was also confirmed by the mass-spectra of the species. On the mass spectra obtained from the UPLC-MS chromatogram the following m/z ratios belong to the polyaniline structure [C6H4NH]n: 183.3 (n = 2), 274.2 (n = 3), 365.3 (n = 4), 456.3 (n = 5), 547.5 (n = 6) (SI Fig. 4). Thus, 5 bar is optimal for the aniline synthesis, because increasing pressure and reaction time facilitates polyaniline formation. Several intermediates/by-products such as N-methylaniline (NMA), azoxybenzene (AOB), cyclohexylamine (CHA), dicyclohexylamine (DCHA), o-toluidine (OTD) and nitrosobenzene (NOB) were formed during the hydrogenations in different concentrations (Figs. 4a–i and 5 ). In the case of the 2% Pd/N-BCNT-zeolite catalyst, during aniline production, one intermediate and one by-product, nitrosobenzene and N-methylaniline (11.7 mmol/dm3) were formed at 5 bar (Fig. 4a). The same Pd catalyst at 10 bar converted nitrobenzene into aniline and NMA (11.4 mmol/dm3) (Fig. 4d). By increasing the pressure to 20 bar NMA was still formed (10.2 mmol/dm3) and its concentration did not change significantly, while the NOB was eliminated after 120 min hydrogenation. The Pt-containing catalyst was less selective towards aniline, and several other components were detected next to the mainproduct (Fig. 4b, e, h). At 5 bar CHA, AOB, OTD, and NOB were formed (Fig. 4b). CHA, AOB and NOB were intermediates, which
Fig. 2. Nitrobenzene conversion and converted moles of nitrobenzene on the surface of 1 mol catalytic metal of the nanocomposite at 5 bar (a and d), 10 bar (b and e) and 20 bar (c and f). Blue square — 2% Pd/N-BCNT-zeolite; white circle — 2% Pt/N-BCNT-zeolite; red triangle — 2% Rh/N-BCNT-zeolite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Aniline selectivity of the catalysts by applying 5 bar (a), 10 bar (b) and 20 bar (c) hydrogen pressure. Blue square — 2% Pd/N-BCNT-zeolite; white circle — 2% Pt/N-BCNTzeolite; red triangle — 2% Rh/N-BCNT-zeolite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Concentration of the intermediates and by-products at different hydrogen pressure produced by using the prepared catalysts: 2% Pd/N-BCNT-zeolite at 5 bar (a), 10 bar (d) and 20 bar (g); 2% Pt/N-BCNT-zeolite at 5 bar (b), 10 bar (e) and 20 bar (h); 2% Rh/N-BCNT-zeolite at 5 bar (c), 10 bar (f) and 20 bar (i).
reacted forward to produce aniline at the end of the hydrogenation. By increasing the pressure to 10 bar AOB and NOB were formed again as intermediates, but they have converted to aniline within less than an hour, while CHA did not form (Fig. 4e). At 10 and 20 bar pressure, NMA was not detected unlike in the case of the Pd/NBCNT-zeolite catalyst but, new by-product such as o-toluidine appeared (6.4 and 5.9 mmol/dm3) (Fig. 4e, h). The transformation rate of the two intermediates increased with the pressure (Fig. 4h), but the OTD content did not change significantly.
In the case of the 2% Rh/N-BCNT-zeolite catalyst 5 intermediates and by-products, AOB, NOB, NMA, CHA, and DCHA has been detected (Fig. 4c, f, i). The NOB (10.3 mmol/dm3) and the AOB (71 mmol/dm3) have not converted into aniline at 5 bar, and instead of that, NMA (9.6 mmol/dm3) other aromatic compounds (CHA and DCHA, 32 and 4 mmol/dm3, respectively) which were not detected in cases of the Pd and Pt catalyst appeared in the reaction mixture after 120 min (Fig. 4c). This suggests, that the Rh catalyst opens new reaction pathways. By increasing the pressure to 10 bar,
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Fig. 5. Proposed mechanism of nitrobenzene hydrogenation including the detected structures for all three studied SoS catalysts (Pd — green frame, Pt — black-dashed frame, Rh — grey). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
NOB and AOB converted to aniline, but the amount of the other byproducts only slightly has changed. The concentration of NMA and CHA decreased to 7.9 and 22.5 mmol/dm3, while the DCHA increased up to 5 mmol/dm3. (Fig. 4f and i). Further, an increase in the pressure to 20 bar caused a significant increase in the yield of CHA and DCHA up to 66 and 9.6 mmol/dm3, respectively. However, the other components were eliminated after 180 min. All in all, we can conclude that the Pd/N-BCNT-zeolite catalyst was the most selective as it facilitates the formation of only aniline and one byproduct (NMA). The process of nitrobenzene hydrogenation to aniline was first described by Haber [28] and later it was modified and new proposals were developed [29]. These proposals are not completely applicable in this case. Thus, based on the results of the catalytic measurements, a reaction mechanism was envisaged which includes all the structures detected during the nitrobenzene hydrogenation (Fig. 5). This complex reaction mechanism starts with nitrobenzene (NB). After two hydrogen additions and one water elimination in the third step, the formation of nitrosobenzene (NOB) can be achieved, which was detected in the presence of each catalyst. The reaction continues with another H addition, which is followed by a water elimination resulting in this way the indirect formation of azoxy–azobenzene (AOB). With the help of an additional water molecule, the reaction route goes on with the split of azoxy–azobenzene, reaching in this way the main product of this study, aniline (AN). However, aniline can be formed also indirectly from azoxy–azobenzene, resulting in orto-toluidine (OTD) as well, which was a by-product in case of the Pt catalyst (black-dotted line). Since, at the ends of nanotubes, numerous highly reactive carbenes could be found, orto-toluidine (OTD), as well as N-methylaniline (NMA), can be formed directly from aniline with the addition of a carbene. The reaction which gives Nmethylaniline can occur by applying both Pd and Rh catalysts. In previous studies, NMA was not detected when carbon supported Pd was applied [29,30,31]. Cyclohexilamine (CHA) was detected by
applying Rh catalyst, and can be formed by the further hydrogenation of aniline. By applying Rh another molecule appeared, dyciclohexylamine (DCHA) which can be reached from aniline through a four-step reaction. CHA and DCHA were reached previously by using nickel and palladium-containing catalysts (Pd/Al2O3 and Ni/Al2O3) [32–35]. Conclusion In our work, special “support on support” (SoS) catalysts have been developed which include nitrogen-doped bamboo-like carbon nanotube (N-BCNT) coated zeolite beads as catalyst supports for rhodium, palladium and platinum nanoparticles. The surface of the zeolite beads was abundantly covered by NBCNTs. On the surface of the zeolite supported N-BCNTs noble metal nanoparticles were deposited by using a hydrogenation procedure. The SEM images confirmed that the distribution of nanoparticles on the surface of the three catalysts was homogenous. The catalytic activity of the three samples was compared at different pressures (5, 10 and 20 bar) at 323 K by using the nitrobenzene (NB)/niline (AN) hydrogenation reaction. The Pt/NBCNT-zeolite sample was the most active catalytically (182 mol nitrobenzene after 30 min). The activity of the other two catalysts at 20 bar was well below this, 99.5 mol after 60 min and 96 mol after 120 min for the 2% Pd/N-BCNT-zeolite and 2% Rh/N-BCNTzeolite, respectively. Although the catalytic activity is lower, the Pd containing catalyst can be a convenient choice for the NB/AN hydrogenation, as only one main by-product was formed (N-methylaniline). The selectivity of the Rh catalyst is unsuitable to produce aniline, because several side-reaction pathways appeared, which led to various by-products, such as cyclohexylamine, dicyclohexylamine, N-methylaniline, and polyaniline derivates as well. Reaction pathways are proposed which describe the formation of all detected structures for all three studied SoS catalysts. All in all, the SoS system is more convenient to use
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compared to other catalysts, because it is easy to treat and remove from the reaction medium and the formation of a stable dispersion of the carbon nanotubes can be avoided. Conflict of interest On behalf of all authors, the corresponding author states that there is no conflict of interest. Acknowledgements This research was supported by the European Union and the Hungarian State, co-financed by the European Regional Development Fund in the framework of the GINOP-2.3.4-15-2016-00004 project, aimed to promote the cooperation between the higher education and the industry. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jiec.2019.07.006. References [1] G. Wegener, M. Brandt, L. Duda, J. Hofmann, B. Klesczewski, D. Koch, R.J. Kumpf, H. Orzesek, H.G. Pirkl, C. Six, C. Steinlein, M. Weisbeck, Appl. Catal. A Gen. 221 (1–2) (2001) 303, doi:http://dx.doi.org/10.1016/S0926-860X(01)00910-3. [2] Y. Ono, H. Ishida, J. Catal. 72 (1981) 121. [3] C.S. Srikanth, V.P. Kumar, B. Viswanadham, A. Srikanth, K.V.R. Chary, J. Nanosci. Nanotechnol. 15 (7) (2015) 5403, doi:http://dx.doi.org/10.1166/jnn.2015.9872. [4] D.J. Collins, A.D. Smith, B.H. Davis, Ind. Eng. Chem. Prod. Res. Dev. 21 (2) (1982) 279, doi:http://dx.doi.org/10.1021/i300006a016. [5] V. Hatziantoniou, B. Andersson, N.H. Schoon, Ind. Eng. Chem. Process Des. Dev. 25 (4) (1986) 964, doi:http://dx.doi.org/10.1021/i200035a021. [6] Y. Yan, J. Miao, Z. Yang, F.-X. Xiao, H. Bin Yang, B. Liu, Y. Yang, Chem. Soc. Rev. 44 (10) (2015) 3295, doi:http://dx.doi.org/10.1039/C4CS00492B. [7] K. Fujisawa, T. Tojo, H. Muramatsu, A.L. Elías, S.M. Vega-Díaz, F. Tristán-López, J. H. Kim, T. Hayashi, Y.A. Kim, M. Endo, M. Terrones, Nanoscale 3 (10) (2011) 4359, doi:http://dx.doi.org/10.1039/c1nr10717h. [8] L. Vanyorek, G. Muranszky, E. Sikora, X. Pénzeli, Á. Prekob, A. Kiss, B. Fiser, B. Viskolcz, J. Nanosci. Nanotechnol. 19 (1) (2019) 429, doi:http://dx.doi.org/ 10.1166/jnn.2019.15776. [9] Y. Yang, G. Lan, X. Wang, Y. Li, Chin. J. Catal. 37 (8) (2016) 1242, doi:http://dx. doi.org/10.1016/S1872-2067(16)62459-2.
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Application of carbon nanotube coated aluminosilicate beads as “support on support” catalyst for hydrogenation of nitrobenzene László Vanyoreka , Ádám Prekoba , EmÅke Sikoraa , Edina Reizera , Gábor Muránszkya , Ferenc Kristályb , Béla Viskolcza , Béla Fisera,c,* a b c
Institute of Chemistry, University of Miskolc, Miskolc-Egyetemváros 3515, Hungary Institute of Mineralogy and Geology, University of Miskolc, Miskolc-Egyetemváros 3515, Hungary Ferenc Rákóczi II, Transcarpathian Hungarian Institute, Beregszász, Transcarpathia 90200, Ukraine
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 April 2019 Received in revised form 1 July 2019 Accepted 4 July 2019 Available online 12 July 2019
Nitrogen-doped bamboo-like carbon nanotube (N-BCNT) coating was synthesized onto the surface of zeolite beads by using Catalytic Chemical Vapour Deposition (CCVD) method to develop a “support on support” (SoS) system. These complex structured materials were used as supports during the preparation of hydrogenation catalysts. Rhodium, palladium and platinum nanoparticles were deposited homogeneously onto the surface of the N-BCNTs of the SoS (final metal content 2 wt%). The catalytic activity of these samples was compared in the hydrogenation of nitrobenzene. The Pt/N-BCNT-zeolite sample was the most active (182 mol nitrobenzene after 30 min). The activity of the other two catalysts at 20 bar was well below this value, 99.5 mol after 60 min and 96 mol after 120 min for Pd and Rh, respectively. The aniline selectivity was different for the three catalysts and they facilitate the formation of various by-products (e.g. N-methylaniline, cyclohexylamine). The usage of the Pd and Pt/NBCNT-zeolite catalysts are more convenient, as only one main by-product was formed. It was confirmed that the zeolite supported N-BCNTs are efficient catalyst supports in hydrogenation processes. Furthermore, by using this special SoS structure to support the catalytic metals the applicability is widened and the catalyst removal is easier. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: N-BCNT Rhodium Palladium Platinum Aniline SoS catalyst
Introduction Nitrobenzene has an important role in the chemical industry including the production of dyes, drugs, and explosives as it is the starting material of aniline synthesis. The first technology to create aniline involves a reduction of nitrobenzene by iron in the presence of hydrochloric acid (Béchamp process) [1]. In this case, iron oxide was formed as a by-product, which was later used as a pigment. There are other methods for producing aniline, for example, the amination of phenol with ammonia in the presence of hydrogen [2]. However, the most commonly and widely used technology is the reduction of nitrobenzene in gas or liquid phase [3–5]. A wide variety of catalysts can be used to promote this process, which
* Corresponding author at: Institute of Chemistry, University of Miskolc, MiskolcEgyetemváros 3515, Hungary. E-mail addresses: [email protected] (L. Vanyorek), [email protected] (Á. Prekob), [email protected] (E. Sikora), [email protected] (E. Reizer), [email protected] (G. Muránszky), [email protected] (F. Kristály), [email protected] (B. Viskolcz), fi[email protected] (B. Fiser).
usually consists of a catalytically active metal (e.g. Ni, Pd, Pt, Cu) and a carrier (e.g. carbon, alumina, silica, zeolites). Carbon nanotubes (CNTs) have fascinating properties, including extraordinary mechanical strength, good chemical stability and large surface area, which makes them perfect catalyst carriers [6]. CNTs do not contain micropores, therefore the catalysis can be more effective and more resistant to oxidation processes. The characteristics of carbon nanotubes can be tuned by incorporating heteroatoms into their structure [7]. Such incorporation can be done by using nitrogen-containing carbon compounds as starting materials for the synthesis of the CNTs, which will lead to the formation of N-doped carbon nanotubes (N-CNT) [8]. Incorporation of nitrogen into the CNT structure will lead to the formation of a bamboo-like structure (nitrogen-doped bamboo-like carbon nanotube, N-BCNT). N-BCNTs have more defect sites than their single- or multi-walled counterparts, and due to the incorporated nitrogen atoms, they also have high energy adsorption sites which are excellent spots for catalytically active metal particles [9,10]. Several studies have shown that reactions can be catalyzed successfully by using CNT-supported metal catalysts [11]. Functionalized carbon nanotube supported Ru catalysts were used
https://doi.org/10.1016/j.jiec.2019.07.006 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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in sorbitol hydrogenolysis to glycols [12], while Pd, Pt, and Nicontaining CNT catalysts were efficiently used in nitrocyclohexene hydrogenation [13]. N-doped CNT based catalysts were applied in oxygen and CO2 reduction reactions [14,15]. N-BCNT supported catalysts have also achieved remarkable results in the field of alternative diesel production (e.g. iron-based Fischer–Tropsch catalysts) [16]. Pd, Pt, Ru, and Rh catalysts supported on carbon (C), silica (SiO2) or alumina (Al2O3) were tested in nitrobenzene hydrogenation [17]. The catalytic activity changed by the employed metal (Pt > Pd > Ru, Rh), and carriers (C > Al2O3, SiO2) [17]. Nitrobenzene hydrogenation was also carried out by using CNT supported Pt [18], Pd and Fex-Pd catalyst [19]. The bimetallic Pt–Fe/CNT catalysts showed much higher activity than the Pt/CNTs due to the promoter effect of iron [20]. The reduction of nitrobenzene (NB) to aniline, which is one of the most important products in the chemical industry, could lead to a wide variety of by-products and intermediates depending on the reaction conditions. In the case of applying CNT supported catalyst, azoxybenzene (AOB) [18] and phenylhydroxylamine (PHA) [19] were formed as intermediates. PHA could occur by using activated carbon as catalyst support [18]. The application of Fex-Pd has promoted the formation of nitrosobenzene during the hydrogenation reaction [19]. A comparable amount of aniline and azobenzene were produced with a Pt/C catalytic system in ethanol [21]. Hydroxylamine and azoxybenzene have been produced as intermediates with Pd/C catalyst [22]. Carbon nanotube and activated carbon supported Pt catalysts were compared in nitrobenzene hydrogenation, the Pt/CNT exhibit higher activity than the Pt/AC catalysts [23]. Oxygen-containing functional groups on the surface of carbon nanotubes could affect the particle size of the deposited catalytically active metals, and in some case, the catalytic activity can be decreased [24]. Application of cobaltcontaining, oxygen and nitrogen functionalized carbon nanotubes as a catalyst will also initiate NB hydrogenation [25]. By using According to Li et al. when Pt/CNT is applied as a catalyst in NB hydrogenation only azoxybenzene can be detected as intermediate, which contradicts with the results of Sun et al. who detected nitrosobenzene (NOB) next to AOB [26,27]. Sun et al. found, that the route from NOB to aniline is less important to the production of aniline when Pt/CNT used [27]. During their experiments, NOB was used as a reactant for aniline synthesis, and mostly (95%) converted to AOB, while phenylhydroxylamine did not form. Furthermore, increasing rection time initiated the hydrogenation of AOB towards aniline [27]. Carbon nanotube supported catalysts are catalytically active and can be efficiently applied in aniline synthesis. However, CNTs can form stable dispersion in the liquid reaction media (SI Fig. 1), thus their separation is difficult [19,26]. This difficulty can be avoided, by using a special “support on support”, or SoS system based on zeolite bead supported nanotube supports (SI Fig. 1). Rhodium, palladium and platinum nanoparticles were deposited onto nitrogen-doped bamboo-like carbon nanotube (N-BCNT) coated zeolite beads. Thus, metal-containing SoS catalyst has been developed. The catalytic activity of these catalysts was studied and compared in nitrobenzene hydrogenation at various reaction conditions.
production. The next step was the synthesis of the catalytically active metal particles starting from anhydrous palladium (II)acetate (Pd(OAc)2, Fluorochem Ltd.), rhodium (II)-acetate dimer ([Rh(OAc)2]2, Sigma Aldrich), and hexachloroplatinic acid (H2PtCl6, Reanal). In the catalytic tests, methanol (WVR) was applied as a solvent, nitrobenzene (WVR) and 4.5 hydrogen (Messer) was used as reactants to produce aniline. Synthesis of the catalysts As catalyst support, N-BCNT coated zeolite beads were used, which were prepared according to a previously optimized procedure [8]. The zeolite beads (19.6 g) were added to the aqueous solution of nickel-nitrate hexahydrate (1.98 g Ni(NO3)2 6 H2O) the nickel content was kept to 2 wt%. The water from the beads was evaporated by rotary vacuum evaporator, thereafter the nickel salt impregnated zeolites were dried at 120 C overnight. The impregnated zeolite beads were used as catalysts in the Catalytic Chemical Vapour Deposition (CCVD) method to synthesize N-BCNTs. In each case, 20 g catalyst was placed in a quartz reactor and the synthesis was carried out for 60 min at 750 C. The carbon source was butylamine, which was added by a syringe pump (6 ml/h). The butylamine was evaporated in the frontsection of the quartz reactor, and carrier gas (nitrogen, 100 ml/min) took it to the catalyst bed. After the CCVD method, the N-BCNT coated zeolite beads were decorated with noble metal particles to achieve the final SoS catalysts. During the catalyst’s preparation, the metal precursors (palladium-acetate, rhodium-acetate, and hexachloroplatinic acid) were solved in distilled water. The theoretical noble metal content was 2 wt% in each case (Table 1), which was monitored by elemental analysis (ICP-OES) measurement. The N-BCNT coated 4A zeolite beads were added to the solution of metal precursors. The solvents evaporated after 30 min by using a vacuum rotary evaporator, and thus, the Rh, Pd, and Pt were impregnated into the N-BCNT/Zeolite beads. The impregnated beads were dried at 393 K overnight. Then, the samples were heat-treated in a nitrogen flow at 673 K temperature for 20 min. In the final step, the catalytically active form of the samples was achieved by hydrogenating them for one hour at 673 K. Characterization of the catalysts After the CCVD (Catalytic Chemical Vapour Deposition) synthesis of the carbon nanotubes, the presence of the N-BCNTs on the surface of the zeolite beads was confirmed by Hitachi S 4800 scanning electron microscope (SEM), which was equipped with energy dispersive EDAX X-ray spectrometer. During the sample preparation for the SEM analysis, Carbon tape rubber was used. The catalysts were examined by SEM-EDX as well. Netzsch Tarsus TG 209 thermo-microbalance was used to analyze the carbon nanotube content of the CNT/zeolite through thermogravimetric analysis (TGA). Nitrogen (4.5) and oxygen (5.0) mixture was applied as the oxidative atmosphere in the TGA measurements. The flow rate was set to 6 ml min1 and 14 ml min1, for the oxygen and nitrogen, respectively. The heating rate was 10 K min1, in the 308–1073 K temperature range. The
Materials and methods Materials Nickel(II)-nitrate hexahydrate (Ni(NO3)2x6H2O) as a catalyst, 4A zeolite beads (2 mm diameter) as support materials, patrol as solvent (a mixture of aliphatic alcohols, Molar Chem.) and nbutylamine (Sigma Aldrich) as carbon source was used for N-BCNT
Table 1 Composition of the catalysts. Catalytically active metal precursor weight (g) Weight of catalyst support (g) Palladium (II)-acetate Rhodium (II)-acetate dimer Hexachloroplatinic acid
0.84 0.85 1.06
19.60 19.60 19.60
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presence of catalytically active noble metals was confirmed by X-ray diffractometry (Bruker Advance D8). The Pd/N-BCNT-zeolite SoS catalyst was measured by Micromeritics ASAP sorptometer, and the BET surface was found 15.6 m2/g. Catalytic tests The catalytic hydrogenation was carried out in a Büchi Uster Picoclave reactor, in a 200 ml stainless steel vessel with heating jacket. An excess of hydrogen was present, and the hydrogen pressure was 5, 10 and 20 bar and the reactions were carried at 323 K. Sampling took place after the beginning of hydrogenation at 5, 10, 15, 20, 30, 60, 120, 180, 240 min. The concentration of nitrobenzene was 0.25 mol l1 in methanol. The total amount of the solution was 150 ml during each test. The 2 g catalyst was used in the reactions. Aniline formation was followed by applying Agilent 7890A gas chromatograph coupled with Agilent 5975C Mass Selective Detector. Analytical standards (aniline, nitrobenzene, nitrosobenzene, azoxybenzene, dicyclohexylamine, o-toluidine, cyclohexylamine and n-methylaniline) were provided by Dr. Ehrenstorfer and Sigma Aldrich. The efficiency of the catalytic hydrogenation was compared by calculating the conversion, X% of nitrobenzene based on the following equation (Eq. (1)): X % ¼
consumed nnitrobenzene 100 initial nnitrobenzene
ð1Þ
The selectivity, S% of the catalysts was calculated as follows (Eq. (2)): S % ¼
naniline 100 nnitrobenzene
ð2Þ
where naniline and nnitrobenzene are the corresponding chemical amounts of the compounds. The following equation (Eq. (3)) was used to calculate the yield (Y, mol %) of the species.: Y % ¼
synthetized nproduct 100 theoritical nproduct
ð3Þ
Results Characterization of the N-BCNT coated zeolite beads After the CCVD synthesis, the surface of the zeolite beads was studied by scanning electron microscopy (SEM) in order to confirm
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the presence of nanotubes. The SEM images verified that the zeolite beads are abundantly coated with carbon nanotubes (SI Fig. 2a). This observation was further studied by TGA analysis and the carbon content was determined (SI Fig. 2b). The mass change of the N-BCNT coated zeolite beads is related to the oxidation of carbon in the system which is identical to the carbon nanotube content and it was 2.6 wt%. The initial ignition temperature of the bamboo-shaped carbon nanotubes was 669 K. There was another mass loss on the initial section of the TG curve, which can be attributed to the elimination of adsorbed water from the zeolite structure and from the nanotube surface. Characterization of the Rh, Pd, and Pt-decorated N-BCNT coated zeolite nanocomposite The noble metal decorated SoS catalysts were examined by SEM and EDX (Fig. 1a–f). The Rh nanoparticles are located on the NBCNT surface in the rhodium-containing sample and they have an average diameter (dmean) of 6.1 nm (Fig. 1a). The size distribution of the particles covered a relatively narrow range. The surface of nanotubes is richly coated by Rh nanoparticles. Rhodium was also identified on the EDX spectrum alongside with other elements which belong to the zeolite core (Fig. 1d). In the case of the Pd containing catalyst, the average size of the particles was 8.3 nm (Fig. 1b). The palladium identified on the X-ray spectrum and the typical peaks of the zeolite are also visible (Fig. 1e). The Pt catalyst has small metal particles on its surface with a size of about 5–10 nm and the catalytically active metal is well dispersed (Fig. 1c). The energy dispersive spectrometry demonstrates the presence of platinum and other elements of the zeolite (Fig. 1f). The samples were examined by X-ray spectrometry (XRD) as well. The XRD shows, that in each case the elemental rhodium, palladium, and platinum were observable. On the diffractogram of the 2% Rh/N-BCNT-Zeolite catalyst, the reflexion of the elemental Rh with (111), (200) and (220) Miller indexed crystal plains can be found at 40.96, 47.8 and 69.7 2Q degree, respectively (SI Fig. 3a). In case of the 2% Pd/N-BCNT-Zeolite sample three crystal plains were identified Pd (111), Pd (200) and Pd (220) and the corresponding reflexion peaks are located at 40.1, 46.6 and 68.1 2Q degrees (SI Fig. 3b). The peaks which are typical for the Pt nanoparticles (111), (200) and (220) have been found on the diffractogram of the platinum-containing sample at 39.7, 47.4 and 67.2 2Q degrees, respectively (SI Fig. 3c). C (002) and C (101) crystal planes were also found due to the presence of carbon nanotubes in the
Fig. 1. SEM image and EDX spectrum of the 2% Rh/N-BCNT-Zeolite (a, d), 2% Pd/N-BCNT-Zeolite (b, e) and the 2% Pt/N-BCNT-zeolite (c, f) catalysts.
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nanocomposites and the corresponding reflexions can be seen at 26.3 and 44.4 2Q degrees in each case. In summary, the XRD results confirmed, that the three noble metals are in catalytically active forms, exactly in their elemental state on the surface of the catalysts. Results of the catalytic measurements The developed catalyst was found to be more active during the nitrobenzene (NB) hydrogenation. The 2% Pt/N-BCNT-zeolite catalyst reached 99.8% conversion after 1 h at 5 bar hydrogen pressure (Fig. 2a). The maximum conversion in the case of the palladium-containing sample after 1 h (5 bar) was 99.7%. By applying of rhodium nanocomposite 97.4% was reached at 5 bar pressure, after 4 h. The Pt nanocomposite catalyst was the most catalytically active, 180 mol (nNB) of the reactant was converted with 1 mol of platinum, within one hour (Fig. 2d). The difference in electron configuration and size of the metal atoms are two factors which could be responsible for the different activity of the catalysts. The strength of the interaction between the nitrogen of aniline and the noble metals are highly influenced by the abovementioned factors and could be the reason behind the outstanding activity of Pt. By increasing hydrogen pressure up to 10 bar, the conversion rate increased in case of all catalysts (Fig. 2b). The Pt-containing catalyst resulted in 99.8% NB conversion after 30 min, so 182 mol nitrobenzene converted by 1 mol Pt, within 30 min hydrogenation (Fig. 2e). The nitrobenzene conversion was lower with the same amount of rhodium in a similar system. By further increasing the pressure up to 20 bar, the conversion also increased and within 15 min it has reached 95.6% in case of the platinum catalyst (Fig. 2c). The conversion on the Rh/N-BCNTzeolite was increased from 60% to 95%, after 80 min. In the case of the Pd/N-BCNT-zeolite catalyst, 20 min shorter reaction time was enough to compare to lower pressure to achieve maximum nitrobenzene transformation. In the case of the Pd and Rh catalysts, 99.5 mol (after one hour) and 96 mol (after two hours) nitrobenzene were converted by 1 mol catalytically active metal. For the Pt
catalyst, this parameter was almost the same as in the case of 10 bar hydrogen pressure (Fig. 2f). In terms of selectivity 84% aniline was reached with the 2% Pd/ N-BCNT-zeolite catalyst at 5 bar after 80 min hydrogenation, and it did not change when 10 or 20 bar pressure was applied (Fig. 3a–c). The aniline selectivity was higher (98.1%) with the Rh containing samples at 5 bar, but the reaction time was longer (4 h). However, the pressure increase was counterproductive and at 10 and 20 bar, the achieved selectivity is only 88% and 53%, even after running the reaction for four hours. The aniline selectivity at 5 and 10 bar pressure was reached the maximum (99.2% and 98.9%) after 80 and 20 min in case of the 2% Pt/N-BCNT-zeolite, at 20 bar this was achieved after 60 min. The concentration of the synthesized aniline was decreased by in pressure, and reaction time. This can be explained by aniline polymerization, which was also confirmed by the mass-spectra of the species. On the mass spectra obtained from the UPLC-MS chromatogram the following m/z ratios belong to the polyaniline structure [C6H4NH]n: 183.3 (n = 2), 274.2 (n = 3), 365.3 (n = 4), 456.3 (n = 5), 547.5 (n = 6) (SI Fig. 4). Thus, 5 bar is optimal for the aniline synthesis, because increasing pressure and reaction time facilitates polyaniline formation. Several intermediates/by-products such as N-methylaniline (NMA), azoxybenzene (AOB), cyclohexylamine (CHA), dicyclohexylamine (DCHA), o-toluidine (OTD) and nitrosobenzene (NOB) were formed during the hydrogenations in different concentrations (Figs. 4a–i and 5 ). In the case of the 2% Pd/N-BCNT-zeolite catalyst, during aniline production, one intermediate and one by-product, nitrosobenzene and N-methylaniline (11.7 mmol/dm3) were formed at 5 bar (Fig. 4a). The same Pd catalyst at 10 bar converted nitrobenzene into aniline and NMA (11.4 mmol/dm3) (Fig. 4d). By increasing the pressure to 20 bar NMA was still formed (10.2 mmol/dm3) and its concentration did not change significantly, while the NOB was eliminated after 120 min hydrogenation. The Pt-containing catalyst was less selective towards aniline, and several other components were detected next to the mainproduct (Fig. 4b, e, h). At 5 bar CHA, AOB, OTD, and NOB were formed (Fig. 4b). CHA, AOB and NOB were intermediates, which
Fig. 2. Nitrobenzene conversion and converted moles of nitrobenzene on the surface of 1 mol catalytic metal of the nanocomposite at 5 bar (a and d), 10 bar (b and e) and 20 bar (c and f). Blue square — 2% Pd/N-BCNT-zeolite; white circle — 2% Pt/N-BCNT-zeolite; red triangle — 2% Rh/N-BCNT-zeolite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Aniline selectivity of the catalysts by applying 5 bar (a), 10 bar (b) and 20 bar (c) hydrogen pressure. Blue square — 2% Pd/N-BCNT-zeolite; white circle — 2% Pt/N-BCNTzeolite; red triangle — 2% Rh/N-BCNT-zeolite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Concentration of the intermediates and by-products at different hydrogen pressure produced by using the prepared catalysts: 2% Pd/N-BCNT-zeolite at 5 bar (a), 10 bar (d) and 20 bar (g); 2% Pt/N-BCNT-zeolite at 5 bar (b), 10 bar (e) and 20 bar (h); 2% Rh/N-BCNT-zeolite at 5 bar (c), 10 bar (f) and 20 bar (i).
reacted forward to produce aniline at the end of the hydrogenation. By increasing the pressure to 10 bar AOB and NOB were formed again as intermediates, but they have converted to aniline within less than an hour, while CHA did not form (Fig. 4e). At 10 and 20 bar pressure, NMA was not detected unlike in the case of the Pd/NBCNT-zeolite catalyst but, new by-product such as o-toluidine appeared (6.4 and 5.9 mmol/dm3) (Fig. 4e, h). The transformation rate of the two intermediates increased with the pressure (Fig. 4h), but the OTD content did not change significantly.
In the case of the 2% Rh/N-BCNT-zeolite catalyst 5 intermediates and by-products, AOB, NOB, NMA, CHA, and DCHA has been detected (Fig. 4c, f, i). The NOB (10.3 mmol/dm3) and the AOB (71 mmol/dm3) have not converted into aniline at 5 bar, and instead of that, NMA (9.6 mmol/dm3) other aromatic compounds (CHA and DCHA, 32 and 4 mmol/dm3, respectively) which were not detected in cases of the Pd and Pt catalyst appeared in the reaction mixture after 120 min (Fig. 4c). This suggests, that the Rh catalyst opens new reaction pathways. By increasing the pressure to 10 bar,
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Fig. 5. Proposed mechanism of nitrobenzene hydrogenation including the detected structures for all three studied SoS catalysts (Pd — green frame, Pt — black-dashed frame, Rh — grey). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
NOB and AOB converted to aniline, but the amount of the other byproducts only slightly has changed. The concentration of NMA and CHA decreased to 7.9 and 22.5 mmol/dm3, while the DCHA increased up to 5 mmol/dm3. (Fig. 4f and i). Further, an increase in the pressure to 20 bar caused a significant increase in the yield of CHA and DCHA up to 66 and 9.6 mmol/dm3, respectively. However, the other components were eliminated after 180 min. All in all, we can conclude that the Pd/N-BCNT-zeolite catalyst was the most selective as it facilitates the formation of only aniline and one byproduct (NMA). The process of nitrobenzene hydrogenation to aniline was first described by Haber [28] and later it was modified and new proposals were developed [29]. These proposals are not completely applicable in this case. Thus, based on the results of the catalytic measurements, a reaction mechanism was envisaged which includes all the structures detected during the nitrobenzene hydrogenation (Fig. 5). This complex reaction mechanism starts with nitrobenzene (NB). After two hydrogen additions and one water elimination in the third step, the formation of nitrosobenzene (NOB) can be achieved, which was detected in the presence of each catalyst. The reaction continues with another H addition, which is followed by a water elimination resulting in this way the indirect formation of azoxy–azobenzene (AOB). With the help of an additional water molecule, the reaction route goes on with the split of azoxy–azobenzene, reaching in this way the main product of this study, aniline (AN). However, aniline can be formed also indirectly from azoxy–azobenzene, resulting in orto-toluidine (OTD) as well, which was a by-product in case of the Pt catalyst (black-dotted line). Since, at the ends of nanotubes, numerous highly reactive carbenes could be found, orto-toluidine (OTD), as well as N-methylaniline (NMA), can be formed directly from aniline with the addition of a carbene. The reaction which gives Nmethylaniline can occur by applying both Pd and Rh catalysts. In previous studies, NMA was not detected when carbon supported Pd was applied [29,30,31]. Cyclohexilamine (CHA) was detected by
applying Rh catalyst, and can be formed by the further hydrogenation of aniline. By applying Rh another molecule appeared, dyciclohexylamine (DCHA) which can be reached from aniline through a four-step reaction. CHA and DCHA were reached previously by using nickel and palladium-containing catalysts (Pd/Al2O3 and Ni/Al2O3) [32–35]. Conclusion In our work, special “support on support” (SoS) catalysts have been developed which include nitrogen-doped bamboo-like carbon nanotube (N-BCNT) coated zeolite beads as catalyst supports for rhodium, palladium and platinum nanoparticles. The surface of the zeolite beads was abundantly covered by NBCNTs. On the surface of the zeolite supported N-BCNTs noble metal nanoparticles were deposited by using a hydrogenation procedure. The SEM images confirmed that the distribution of nanoparticles on the surface of the three catalysts was homogenous. The catalytic activity of the three samples was compared at different pressures (5, 10 and 20 bar) at 323 K by using the nitrobenzene (NB)/niline (AN) hydrogenation reaction. The Pt/NBCNT-zeolite sample was the most active catalytically (182 mol nitrobenzene after 30 min). The activity of the other two catalysts at 20 bar was well below this, 99.5 mol after 60 min and 96 mol after 120 min for the 2% Pd/N-BCNT-zeolite and 2% Rh/N-BCNTzeolite, respectively. Although the catalytic activity is lower, the Pd containing catalyst can be a convenient choice for the NB/AN hydrogenation, as only one main by-product was formed (N-methylaniline). The selectivity of the Rh catalyst is unsuitable to produce aniline, because several side-reaction pathways appeared, which led to various by-products, such as cyclohexylamine, dicyclohexylamine, N-methylaniline, and polyaniline derivates as well. Reaction pathways are proposed which describe the formation of all detected structures for all three studied SoS catalysts. All in all, the SoS system is more convenient to use
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