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Publicly Available Published by De Gruyter May 5, 2020

Formation of ruthenium nanoparticles inside aluminosilicate nanotubes and their catalytic activity in aromatics hydrogenation: the impact of complexing agents and reduction procedure

Anna Stavitskaya , Aleksandr Glotov EMAIL logo , Kristina Mazurova , Vladimir Nedolivko , Pavel Gushchin , Wei Huang , Eduard Karakhanov and Vladimir Vinokurov

Abstract

Ruthenium particles with size from 1 to 7 nm were formed by reduction of ruthenium complexes with urea, ethylenediaminetetraacetic acid, acetone azine, 1,2-Bis(2-furylmethylene)hydrazine) inside halloysite nanotubes. Catalysts of different morphology with Ru content from 0.75 to 0.93 %wt. were obtained using NaBH4 or H2 as reducing agents and tested in benzene hydrogenation as a model reaction. NaBH4 reduced catalysts showed similar catalytic activity with 100 % benzene conversion after 1.5 h. Reduction with H2 resulted in a decrease of catalytic activity for all samples. High benzene conversion was achieved only in the case of 1,2-Bis(2-furylmethylene)hydrazine and ethylenediaminetetraacetic acid. It was concluded that the thermal stability of complexing agents plays a key role in activity of catalysts reduced with H2.

Introduction

Ruthenium based catalysts are widely investigated in many industrial focused processes, including selective catalytic reactions such as hydrogenation, oxidation, epoxidation and metathesis [1], [2], [3], [4]. For hydrogenation catalysts key factors are: metal valence state, dispersity and stability to avoid nanoparticles leaching and sintering [5], [6], [7]. The nature, textural and structural properties of the support strongly effect these parameters; consequently, selection of the carrier becomes the key option in designing the best catalyst [8], [9], [10].

Zeolites, modified carbon, silica or alumina, metal-containing and organic polymers are commonly used as carriers for ruthenium catalysts [11], [12], [13]. The formation of stable nanoparticles (NPs) on support is impossible without strong metal-surface interaction or particle stabilization agents. Metal nanoparticles may be stabilized by addition of polymers, surfactants or ligands, which allow to control their size, shape, dispersion, and also their surface state [14]. The use of proper support and additional stabilization with complexing agents may result in very efficient and stable catalysts. An interesting and green approach was presented in [15] where well-dispersed Ru nanoparticles supported on a nitrogen-doped carbon material were obtained from ruthenium chloride and dicyanamide in a facile and scalable method.

Multiwalled aluminosilicate nanotubes (halloysite) with different inside-outside chemistry are a focus of interest as templates for Ru nanoparticles formation [16], [17], [18], [19]. Halloysite is a clay mineral one of the forms of which is mesoporous nanotubes resulted from rolling of kaolin plates under favorable geological conditions [20]. The inner space of nanotubes consists of alumina and has a positive charge, but the outer surface formed by silica has a negative charge. The important factor is that the positive inner surface together with a mesoporous lumen size about 20 nm make loading of metal ions inside the tubes difficult. Special procedures and manipulations being needed for this to be achieved [21], [22], [23], [24]. Nevertheless, selective intercalation of nanoparticles in the mesopores of halloysite results in the efficient catalysts with higher stability than materials with particles on the outer surface.

There are several methods for selective embedding nanoparticles inside the lumen of clay nanotubes. The first one refers to stabilization of preformed nanoparticles with negatively charged agents and pumping them inside. This method was applied to load nanotubes with NiCu NPs to produce very stable oxidation catalyst, operating at temperatures up to 500°C [25]. Gold nanopeapods were formed inside halloysite lumen by reduction of HAuCl4 with citric acid in organic media comprising oleylamine and oleic acid as stabilizers [26]. Another way refers to loading of negatively charged surfactants inside the clay tubes lumen. For example iron oxide nanorods were formed through bonding of iron ions with ethylenediaminetetraacetic acid (EDTA) inside halloysite [27].

It is known that azines form stable metal complexes and can be used for nanoparticles synthesis [28], [29]. Ketazines obtained on the basis of various alcohols and hydrazine hydrate act as stabilizing agents [30], [31]. In our recent works we developed a new method of nanoparticles formation inside halloysite tubes lumens using azines assisted loading of metal ions [32], [33]. Compositions obtained by this technique are more efficient in catalytic reactions than industrial catalysts [18], [23].

This work is focused on more detailed investigation of these new strategies for selective loading of Ru complexes into the internal cavity of halloysite nanotubes with further formation of NPs by reduction. Here we show the effect of the reduction procedure and composition of complexing agents on catalytic performance of produced materials in reaction of benzene hydrogenation.

Experimental

Materials

Aluminosilicate nanotubes (Merck, ≥98), ethylenediaminetetraacetic acid (EDTA) (Sigma-Aldrich, ≥99.5%), hydrazine hydrate solution (RusChem, 78–82%), furfural (RusChem, pure), urea (RusChem, pure), ruthenium chloride (III), (Aurat, 46.5%), sodium borohydride powder (RusChem, pure).

Intercalation of complexing agents inside aluminosilicate nanotubes

Loading of urea or EDTA

Halloysite (1 g) was dispersed in urea or EDTA water solution (30 mg/ml) and kept in ultrasonic bath for 1 h. The dispersion was centrifuged (7000 rpm for 2 min), the precipitate was separated and washed with distilled water to remove excess of urea or EDTA. The washing cycle was repeated 3 times.

Azines formation inside aluminosilicate nanotubes

Halloysite (1 g) was dispersed in hydrazine hydrate (20 ml) and kept in an ultrasonic bath for 30 min. After the mixture was centrifuged at 7000 rpm for 2 min, the precipitate was washed with ethanol to remove an excess of hydrazine hydrate from the outer surface of the aluminosilicate tubes. Then, 20 ml of acetone of furfural solution in ethanol was gradually added to the precipitate and kept in an ultrasonic bath for 1 h to react. After reaction, the mixture was centrifuged at 7000 rpm for 2 min washed with ethanol, the washing cycle was repeated 3 times. As a result of the reaction of acetone with hydrazine acetone azine (azine 1) was formed. Furfural reaction with hydrazine gave 1,2-Bis(2-furylmethylene)hydrazine (azine 2) inside aluminosilicate nanotubes.

Formation of Ru complexes inside alumonisilicate nanotubes

The halloysite modified with complexing agents was dispersed in 30 ml of the ruthenium chloride water/ethanol solution (0.7 mg/ml). The mixture was kept in an ultrasonic bath for 30 min to form ruthenium complexes inside the halloysite nanotubes. Then the mixture was centrifuged (7000 rpm for 2 min) and washed with water. The washing/centrifugation cycle was repeated two times.

Ruthenium complexes reduction with NaBH4

NaBH4 (0.5 g) was dissolved in distilled water and used to reduce ruthenium complexes obtained inside halloysite nanotubes. Halloysite powder with ruthenium complexes of different composition was added to the resulting solution. The reduction was carried out with vigorous stirring at room temperature. The reduced samples were centrifuged and washed with distilled water, and the procedure being performed 3 times to remove the reaction by-products. Materials were dried at 70°C for 24 h. Dry powders were named as Ru-1(1)-Ru-4(1) where first number is a reduction procedure index and the second number is a number of complexing agent (Fig. 1).

Fig. 1: 
Structural formula of complexing agents used for Ru NPs formation and stabilization inside aluminosilicate nanotubes.
Fig. 1:

Structural formula of complexing agents used for Ru NPs formation and stabilization inside aluminosilicate nanotubes.

Ruthenium complexes reduction in a flow of hydrogen

The reduction of Ru complexes formed inside nanotubes in a hydrogen flow was carried out on the AutoChem HP2950 (Micromeritics). Ruthenium loaded clay (0.1 g) was placed in a quartz reactor, purged in argon flow (30 ml/min) at 50°C for 1 h. The reduction was carried out in a flow of the H2 (8%Vol)+Ar gas mixture (30 ml/min) up to 400°C with a heating rate of 10°C/min. Resulted powders were named as Ru-1(2)-Ru-4(2).

Methods

To investigate the mechanism of ruthenium complexes reduction inside the tubes, the programmed hydrogen reduction (TPR) method was used. The reduction of all samples was carried out on the AutoChem HP2950 (Micromeritics). The test sample (0.05 g) was placed in a quartz reactor, purged in argon flow (30 ml/min) at 50°C for 1 h. The reduction was carried out in a flow of the H2 (8%Vol)+Ar gas mixture (30 ml/min). To obtain the TPR curve, the sample was heated up to 400°C with a heating rate of 10°C/min.

A transmission electron microscope (TEM) (JEM-2100, JEOL, Japan) was used to study the morphology of catalysts. The particle size distribution was studied using the Image J program and statistical data processing.

Thermogravimetric analysis (TGA) was performed on a TA Instruments SDT Q600 analyzer. The sample analyzed was put in a platinum crucible and gradually heated up to 800°C with a temperature ramp of 10°C·min−1.

Quantitative determination of ruthenium in the samples was conducted by using an energy dispersive X-ray fluorescence (XRF) spectrometer Thermo Scientific ARL Quant’X in vacuum. The results were processed using the standard-less UniQuant method.

Catalytic experiment

Hydrogenation of benzene in the presence of halloysite based catalysts was carried out in a Parr Series 5000 Multiple Reactor System in stainless steel batch reactors equipped with a Teflon inlet according to the literature procedures [5], [34]. During catalytic experiment, the sequence of operations was as follows. The reactor was loaded with 1.5 ml (0.0169 mol) of benzene and 60 mg of the catalyst. The autoclave was sealed, filled with hydrogen up to pressure of 3.0 MPa and installed into the oven with a thermocouple. Then the hydrogenation reaction was conducted at 80°C for 3 h with sampling for analysis every 20 min. After reaction, the autoclave was cooled down to the room temperature and depressurized. The catalyst was removed from reaction products via centrifugation. The reaction products were analyzed to control the completeness of the reaction.

Products analysis

Hydrogenation products were analyzed on a Chromos GC-1000 chromatograph equipped with a flame-ionization detector and a MEGA-WAX Spirit capillary column at 70°C (isotherm). The conversion was calculated as a ratio between cyclohexane and sum of unreacted benzene and cyclohexane. Experimental error does not exceed 1%.

Results and discussions

The in situ metal nanoparticles formation in aluminosilicate nanotubes by metal complexes reduction could be applied in various fields including catalysts preparation. Herein, we discuss synthesis procedure and show how complexing and the reduction procedure may influence morphology and the catalytic activity of Ru NPs selectively formed inside positively charged mesoporous lumen of the tubular clay.

The synthesis procedure is shown on Scheme 1 and is a variation of methods described earlier [23], [33], [34]. At first, complexing agents were loaded or synthesized inside nanotube. Addition of ruthenium precursor solution results in metal complex formation inside the lumen, and characterized by rapid absorption of precursor. The ligands may be of different composition, wherein four chemicals being used. The choice of ligands (Fig. 1) is based on molecular size, molecular charge, chelating ability. Urea molecules have a small size allowing easy penetration of ligands into the cavity of aluminosilicate nanotubes. It easily forms ruthenium complex [35], [36]. The structure of the EDTA allows selective sorption on the inner surface of halloysite due to electrostatic attraction to the positively charged inner surface of the tubes [27]. EDTA ruthenium complexes show high catalytic activity in many reactions [37], [38]. The azines (acetone azine, 1,2-Bis(2-furylmethylene)hydrazine), used previously, show their suitability for monodispersed ruthenium particles formation inside the halloysite nanotube [18], [23], [33], [39], [40], [41], [42], [43].

Scheme 1: 
Synthesis of Ru NPs inside aluminosilicate nanotubes using complexing agent for Ru loading.
Scheme 1:

Synthesis of Ru NPs inside aluminosilicate nanotubes using complexing agent for Ru loading.

To determine the loading efficiency of organic molecules inside the tubes, as well as their thermal stability, TGA/DSC analysis was performed (Fig. 2). Halloysite is characterized by three thermal effects: dehydration at ~105°C, dehydroxylation at ~475°C, and deformation of nanotubes at ~1000°C [44]. The selected analysis mode shows two steps of water evaporation from surface at ~105°C and water desorption from interlayers at ~475°C. At the first thermal transition corresponding to physically adsorbed water removal from inner surface up to 120°C, weight loss is ~3%. In the dehydroxylation step, starting around 450°C, the weight loss is ~10%.

Fig. 2: 
TGA analysis of pristine halloysite (hall) and halloysite modified with urea (hall_urea), EDTA (hall_EDTA), acetone azine (hall_azine 1), 1,2-Bis(2-furylmethylene)hydrazine (hall_azine 2).
Fig. 2:

TGA analysis of pristine halloysite (hall) and halloysite modified with urea (hall_urea), EDTA (hall_EDTA), acetone azine (hall_azine 1), 1,2-Bis(2-furylmethylene)hydrazine (hall_azine 2).

TGA curves of halloysite intercalated with EDTA or urea were alike to halloysite one. Two main stages of weight loss were observed at about 100°C and 470°C; they are generally attributed to water loss from halloysite, but also include organic substrate decomposition [45]. For EDTA and urea it is hard to calculate ligand loading due to overlapping of their decomposition processed with halloysite dehydration, and low loading efficiency. After dehydroxylation stage, the lowest mass loss is observed for hall-urea material. That could be an evidence of replacement of water by urea of the surface and between interlayers. In case of azine 2, approximate loading efficiency was ~1%. Halloysite modified with azine 1 was characterized by thermal decomposition at 70°C with the weight loss of ~4%, which may be caused by decomposition of acetone azine to acetone and hydrazine [46]. In this case, we assume maximum ligand loading efficiency of about ~4%. According to DSC data, thermal decomposition of the ligands occur at 150–175°C for urea (1), 230–250°C for EDTA (2), 120–190°C for acetone azine (3), 180–270°C for 1,2-Bis(2-furylmethylene)hydrazine (4).

Figure 3 reveals the process of Ru-complexes reduction in a flow of hydrogen over the temperature range 60–340°C. As shown on Fig. 3, several reduction stages took place for all samples corresponding the stepwise Ru3+ reduction to Ru2+ followed by reduction thereof to Ru0 [47], [48], [49], [50]. TPR curve of Ru-1 shows temperature peaks at 113°C and 241°C, Ru-2 at 99°C, 242°C and 289°C, for Ru-3 and Ru-4 characteristic peaks are observed at 126°C with a shoulder, 235°C and 121°C, 234°C, respectively. In contrast to other samples, reduction of Ru-2(2) produced with EDTA shows small peak at 99°C that may be a result of reduction of well-dispersed RuxOy oxide species, probably, being formed on the outer surface of nanotubes during oxidation of ruthenium chloride by air even at room temperature. High temperature peak at 289°C may be attributed to the reduction of ruthenium species after thermal decomposition of EDTA.

Fig. 3: 
TPR curves of ruthenium complexes loaded inside halloysite nanotubes using as complexing agents: Urea (Ru-1), EDTA (Ru-2), acetone azine (Ru-3), 1,2-Bis(2-furylmethylene)hydrazine (Ru-4).
Fig. 3:

TPR curves of ruthenium complexes loaded inside halloysite nanotubes using as complexing agents: Urea (Ru-1), EDTA (Ru-2), acetone azine (Ru-3), 1,2-Bis(2-furylmethylene)hydrazine (Ru-4).

Elemental analysis showed that loading efficiency of the ruthenium chloride inside the tubes was quite similar for all complexing agents (Table 1). After reduction, the ruthenium concentration in catalysts reached 0.74–0.92% wt., meaning high efficiency of the ruthenium uptake from stock solution. It is possible that amount of loaded ligand plays a role in the ruthenium uptake. For the azines the percentage of the ruthenium salt loaded inside the tubes was higher, according to element analysis of the samples after reduction, and equals ~90% for azine 1 and ~85% for azine 2. Urea and EDTA modified samples demonstrated about 75% of the loading efficiency.

Table 1:

The ruthenium loaded halloysite catalysts characterization.

Sample name Complexing agent Ru content (% wt.) Particles size distribution after complex reduction (nm) Average size of particles after complex reduction (nm)
Ruthenium complex reduction with NaBH4
 1 Ru-1(1) Urea 0.75 1.5–5.1 2.8
 2 Ru-2(1) EDTA 0.74 1.1–7.2 3.9
 3 Ru-3(1) Acetone azine 0.92 1.2–4.6 2.5
 4 Ru-4(1) 1,2-Bis(2-furylmethylene)hydrazine 0.85 1.1–4.1 2.7
Ruthenium complex reduction with H2
 5 Ru-1(2) Urea 0.76 1.5–8.4 3.5
 6 Ru-2(2) EDTA 0.76 1.6–7.0 4.3
 7 Ru-3(2) Acetone azine 0.93 1.2–4.4 2.4
 8 Ru-4(2) 1,2-Bis(2-furylmethylene)hydrazine 0.86 1.9–5.1 3.3

The difference in structure and chemical composition of the complexing agents was revealed in the morphology of Ru NPs, formed inside the tubes after reduction. This difference may be observed in Fig. 4 and in Table 1.

Fig. 4: 
Morphology of Ru@hallloysite catalysts and Ru NPs size distribution.
Fig. 4:

Morphology of Ru@hallloysite catalysts and Ru NPs size distribution.

Figure 4 presents TEM images of the samples. Ru NPs formed inside alunimosilicate nanotubes were observed after proposed synthesis procedure. From TEM, one may conclude that the loading of urea or EDTA, further complex formation and reduction, results in less homogeneous distribution of particles within the samples. Nanotubes without any loading as well as tubes with very high ruthenium content are observed (Fig. 4).

Different ruthenium particles size distribution was obtained after reduction of complexes in hydrogen flow (Tmax=400°C) (Fig. 4). The use of acetone azine as the ligand made it possible to obtain small particles characterized by the narrow size distribution from 1.2 to 4.4 nm. Ru-4(2), obtained by reduction of the ruthenium complex in hydrogen flow, had an average particle size of 3.3 nm. When urea and EDTA were applied, wider particle size distributions were observed an average particle size of 3.5 and 4.3 nm, respectively.

Figure 5 shows the benzene hydrogenation activity for ruthenium catalysts based on alunimosilicate nanotubes with Ru NPs inside the lumen. The blue curves indicate reaction kinetics for materials Ru-1(1)-Ru-4(1) reduced with NaBH4 aqueous solution. The red curves show benzene hydrogenation kinetics in Ru-1(2)-Ru-4(2) reduced in a flow of hydrogen in the temperature up to 400°C. Under reaction conditions, all the catalysts produce cyclohexane as the only product.

Fig. 5: 
Kinetics of benzene hydrogenation over Ru@halloysite catalysts (a) urea (Ru-1), (b) EDTA (Ru-2), (c) acetone azine (Ru-3), (d) 1,2-Bis(2-furylmethylene)hydrazine (Ru-4) prepared by various reduction procedures (blue – NaBH4, red – H2).
Fig. 5:

Kinetics of benzene hydrogenation over Ru@halloysite catalysts (a) urea (Ru-1), (b) EDTA (Ru-2), (c) acetone azine (Ru-3), (d) 1,2-Bis(2-furylmethylene)hydrazine (Ru-4) prepared by various reduction procedures (blue – NaBH4, red – H2).

Reduction of ruthenium complexes with NaBH4 aqueous solution with the formation of NPs inside clay tubes gave active catalysts with 100% substrate conversion within first 1.5 h of reaction. Conversion reached 100% for Ru-1(1) after 40 min, Ru-2(1) after 40 min, Ru-3(1) after 80 min and Ru-4(1) after 60 min. In the case of catalysts with modified with urea and EDTA, the benzene hydrogenation rate was slightly higher than in case of azines modified ruthenium samples (Table 1). The ligand loading according to TGA (Fig. 2) was close to 0.5% in case of urea and EDTA modified catalysts and this fact may be a reason to high reaction rates thanks to better reduction with NaBH4. When azines were used as ligands higher ruthenium content attributed to higher ligands loading may result in only partial reduction of ruthenium in sodium borohydride water solution taking into account hydrophobicity of azines. It could be concluded that in the higher row of catalyst their activity in benzene hydrogenation decrease in the range: Ru-2(1)=Ru-1(1)>Ru-4(1)>Ru-3(1).

Samples in which ruthenium complexes were reduced using H2 were characterized by lower reaction rate in comparison to similar catalytic systems obtained using NaBH4 as a reducing agent. The highest degree of substrate conversion equal to 97% was reached using Ru-4(2) (Fig. 5d). For Ru-1(2), Ru-2(2) and Ru-3(2) benzene conversion is 60%, 85% and 40%, respectively after 3 h of reaction. Decomposition of the ligands during the reduction in the hydrogen flow at temperatures up to 400°C generally leads to decrease in catalytic activity. This is especially visible in the case of acetone azine. Decomposition of ligands destabilizes nanoparticles and causes its agglomeration during reaction. EDTA and azine 2 are decomposed at higher temperatures and thus activity of particles remains higher (Fig. 5b, d). It could be concluded that in the lower row of catalyst their activity in benzene hydrogenation decreases in the range: Ru-4(2)>Ru-2(3)>Ru-1(1)>Ru-3(2).

Figure 6 shows the comparison of Ru-3(2) and Ru-(4)2 samples after hydrogenation reaction. In Ru-3(2) reaction led to aggregation of particles into clusters resulting in decrease in a surface of ruthenium available for benzene sorption. Such particles aggregation happened due to thermal decomposition of organic molecules under reduction conditions, and seems to be the main reason to the decrease in catalytic activity of Ru-3(2) and Ru-1(2). In Ru-(4)2 with thermally stable azine 2 particles are still well dispersed and stabilized inside nanotubes after reaction (Fig. 6). This is in agreement with the conclusion about effect of thermal stability of ligands on catalytic activity of Ru@halloysite catalysts.

Fig. 6: 
Morphology of Ru-3(2) and Ru-4(2) samples after benzene hydrogenation reaction.
Fig. 6:

Morphology of Ru-3(2) and Ru-4(2) samples after benzene hydrogenation reaction.

In order to prove the efficiency of the catalysts preparation method we carried out the recycling of Ru-2(1) one of the most efficient of prepared catalysts. In the second reaction cycle the conversion of benzene reached 99% after 2 h of reaction. The induction period of 90 min was observed cased by sample reduction after recycling procedure. The speed of reaction was comparable to the first reaction cycle meaning the stability of synthesized material.

Conclusion

New synthesis strategy of ruthenium nanoparticles inside aluminosilicate nanotubes was proposed. The influence of complexing agents and reduction procedure on catalytic activity of ruthenium catalysts in reaction of benzene hydrogenation was revealed. It could be concluded that the ruthenium content and particles size distribution did not play a key role in catalytic activity in benzene hydrogenation under studied conditions. The main factors were the reduction procedure and the complexing agents used for particles stabilization. It was found that when NaBH4 was used as a reducing agent, the ligands composition has no significant effect on catalysts activity. When the reduction took place in a flow of hydrogen under high temperatures catalysts activity correlated with the thermal stability of the complexing agents.


Article note

A collection of papers from the 18th IUPAC International Symposium Macromolecular-Metal Complexes (MMC-18), held at the Lomonosov Moscow State University, 10–13 June 2019.


Acknowledgements

This work was supported by the Russian Science Foundation (Funder Id: http://dx.doi.org/10.13039/501100006769, project № 19-19-00711). The authors thank Valentine Stytsenko for his input in this work.

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Published Online: 2020-05-05
Published in Print: 2020-06-25

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