shell ruthenium-halloysite nanocatalysts for hydrogenation of

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Core/shell ruthenium - halloysite nanocatalysts for hydrogenation of phenol Vladimir Vinokurov, Aleksandr Glotov, Yaroslav Chudakov, Anna Stavitskaya, Evgenii Ivanov, Pavel Gushchin, Anna Zolotukhina, Anton Maximov, Eduard Karakhanov, and Yuri M. Lvov Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03282 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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Industrial & Engineering Chemistry Research

Core/shell ruthenium-halloysite nanocatalysts for hydrogenation of phenol Vladimir Vinokurov*,†, Aleksandr Glotov†, Yaroslav Chudakov†, Anna Stavitskaya†, Evgenii Ivanov†, Pavel Gushchin†, Anna Zolotukhina‡, Anton Maximov‡, Eduard Karakhanov‡, Yuri Lvov†,§ †

Department of Physical and Colloid Chemistry, Gubkin University, 119991 Moscow, (Russian Federation)

‡

Department of Petroleum Chemistry and Organic Catalysis, Moscow State University, 119991, Moscow (Russian Federation) §

Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272, USA

*

Vladimir Vinokurov, tel.: +7 499 507 85 64, e-mail: [email protected]

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ABSTRACT: Halloysite tubular nanoclay was applied as a template for synthesis of ruthenium core-shell composite catalysts for the first time. 50-nm diameter ceramic tubular systems with metal seeded interior were produced. Procedure for the metal deposition and prior halloysite modification had a significant influence on properties of the catalyst and, as a consequence, on its activity in hydrogenation of phenol. Cyclohexanol was the main reaction product, but its yield depended on the substrate conversion and nanoarchitectural composition of the catalysts used. Maximum catalytic activity (turnover frequency, TOF) achieved was 17282 h-1 in terms of hydrogen uptake per surface Ru atoms. Substrate selectivity of halloysite-based catalysts was also demonstrated at the comparative hydrogenation of phenol and various cresols. KEYWORDS: halloysite clay nanotubes, Ru nanoparticles, hydrogenation, catalyst core-shell

INTRODUCTION Catalytic hydrogenation of phenol is an important chemical process for synthesis of cyclohexanone and cyclohexanol, which are the main intermediates for Nylon 6 and Nylon 66 production1. Besides, phenol and its derivatives represent an important group of refractory organic pollutants and are considered as an environment hazardous toxin2. Therefore, catalytic hydrogenation of phenol has been extensively studied3. Various metals (Pd, Pt, Rh, Ni) were tried as catalysts, but ruthenium was proven to be the most efficient in the hydrogenation of phenol under different reaction conditions4. Ruthenium particles of different size and shape, such as chain-like nanoparticles or Ru-nanocolloids with narrow size distribution were synthesized for phenol hydrogenation to cyclohexanone and cyclohexanol5. Supported heterogeneous catalysts with silica, alumina, titania and carbon material carriers (graphene, carbon nanotubes and nanospheres) are considered the most prospective5-6. Ru/Al2O3 showed a conversion of 82 % after 1 hour under mild conditions (80° C and 2 MPa of H2), but this catalyst has shown poor selectivity (67 % on cyclohexanone)7. A comparison of Ru/H-ZSM-5 (with different Si/Al ratio), Ru/SiO2 and Ru/Al2O3 catalysts in hydrogenation of phenol at 150 °C and 5 MPa of H2 was presented8. For all samples, 100% conversion of phenol was observed. Herein, cyclohexanol was the predominant product in the case of Ru/Al2O3 or Ru/SiO2 catalysts (selectivity 97-98 %), while Ru/HZSM-5 favored deoxygenation and preferential cyclohexane formation. These results were explained as follows: if zeolite was used as a carrier, cyclohexanol, obtained after hydrogenation of phenol with ruthenium nanoparticles, was subsequently subjected to dehydration on acidic centers of H-ZSM-5, giving cyclohexene as a product. It contains a C=C double bond and underwent hydrogenation again, resulting in cyclohexane. Metal organic frameworks9, diamond-like porous aromatic frameworks10, and cross-linked dendrimer matrices were also tested11. The main feature of these supports, containing organic moieties, is an ability to swell or contract under the influence of solvent, pressure and temperature and, therefore, allows some control on the reaction rate and product distribution12. A hydrogenation of phenol in the presence of ruthenium, supported on MIL-101 metalorganic framework, was studied by Ertas et al.9. Among the other solvents (ethanol, tetrahydrofuran, dichloromethane), water was the best for reaction carrying out under mild conditions (conversion 90 % and selectivity also of 90 % at 50 ºC and 5 bars of H2). Porous aromatic frameworks were proposed as carriers for ruthenium catalysts10. With this it was possible to achieve phenol conversion of 86 % (80° C, 10 bar H2) within 6 hours with 2 ACS Paragon Plus Environment

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cyclohexanol as the main product. Catalysts, based on carbon nanotubes, have already proved themselves in hydrogenation of arenes and phenols13, as well as cross-coupling reactions14. Another type of mesoporous carrier is a tubular allotropic form of kaolin clay called halloysite. It is a 50-nm diameter natural aluminosilicate multiwall tube with the chemical formula Al2(OH)4Si2O5 × nH2O15. The unique feature of halloysite clay is a 15-nm diameter lumen formed by rolled alumosilicate sheets: this lumen may be loaded with metal complexes and nanoparticles for nanoconfined chemical reactions, allowing the adjustment of the reaction rate and product selectivity. This halloysite clay is a cheap, stable and biocompatible material15. We suggest abundantly available halloysite clay nanotubes (HNT) as a promising mesoporous support for heterogeneous catalysts with improved mechanical properties and dispersity. Thus, halloysitesupported Pd, Au, Ag, Pt nanoparticles have already shown high catalytic activity in redox reactions16. The synthesis of Ru nanoparticles, deposited on halloysite, was performed using the wetness impregnation method. The catalyst oxidates CO in hydrogen-rich atmosphere; and its activity was comparable with commercial Ru/Al2O3 or Ru/SiO217. In the present work we describe new nanocatalysts, based on ruthenium nanoparticles, impregnated in halloysite nanotubes, for hydrogenation of phenol in aqueous media. For the first time the ruthenium nanoparticles with a narrow size distribution were synthesized predominately inside the halloysite lumen. Synthesized nanomaterials demonstrated good catalytic activity in hydrogenation of phenol and possibility for recycling. EXPERIMENTAL SECTION Chemicals. For synthesis of halloysite-based materials the following reagents were used: ruthenium (III) chloride RuCl3 (Aldrich, 45–55 % Ru content), Halloysite (Aldrich), furfural 2-C4H3OC(=O)H (99% (Sigma-Aldrich), hydrazine hydrate N2H4×H2O (Aldrich). The following solvents were used: ethanol (96 %) The following reagents were used as substrates and reference substances: phenol C6H5OH (Reachim, purum); cyclohexanol C6H11OH (Ferak, Rein); cyclohexanone C6H10O (JenapharmApolda, pro analysi); 2-cyclohexenone (Aldrich, ≥ 95%). Analyses and Instrumentations. Transmission electron microscopy (TEM) analysis was performed using a JEM-2100 JEOL microscope with electron tube voltage of 100 kV. The count of particle sizes was performed by processing the obtained microimages using Image-Pro Plus 6.0 program X-ray photoelectron (XPS) studies were performed using a VersaProbe II ULVAC-PHI instrument, equipped with a photo-electronic analyzer with retarding potential OPX-150. To excite photoelectrons, aluminum anode X-ray radiation was used (AlKα = 1486.6 eV) with a tube voltage of 12 kV and emission current of 20 mA. The calibration of photoelectron peaks was performed along with the N 1s line with binding energy of 399.9 eV. Quantitative determination of ruthenium in the samples was conducted by using X-ray fluorescence analysis on instrument ARL Perform’X (Thermo Fisher Scientific, New Wave). Chromatographic analysis of the reaction products was performed using a ChromPack CP9001 gas chromatograph equipped with a flame ionization detector and a 25 m × 0.15 mm column containing a grafted CP-SIL 5 CB phase. The chromatograms were recorded and analyzed in a computer using the program Maestro 1.4. Conversion was determined by the change in the relative area (%) of substrate and products peaks. 3 ACS Paragon Plus Environment


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Synthesis of halloysite-based Ru catalysts. To prepare Ru-1 catalyst, 5 g of halloysite were dispergated in 30 ml of furfural and held out in ultrasound bath at 30 0C for 30 minutes. The mixture obtained was centrifuged (7500 rpm, for 4 minutes) and washed out with ethanol (25 mL) twice. Then the halloysite, impregnated with furfural, was dispersed in 20 mL of hydrazine hydrate and held out in ultrasound bath at 30 0C for 30 minutes and then heated up to 80 0C. Then the material obtained was centrifuged and washed out with 30 mL of ethanol; this procedure was repeated three times. After that the resultant composite was dried at 50 0C for 12 hours. Thus, halloysite, impregnated with Schiff base, was dispersed in solution of 25 mg (0.12 mmol) of RuCl3 in 30 mL of ethanol. The mixture prepared was boiled for 5 minutes, then cooled down to room temperature, washed with ethanol three times and centrifuged. To reduce Ru (III) to Ru0 in the composite obtained, a solution of NaBH4 in water (300 mg in 20 mL) was used. Thus synthesized catalyst was washed with distilled water, centrifuged and dried at 50 0C overnight. The product was obtained as a slightly grey powder. The yield was 81%. Ru content, wt. %: 0.82. XPS, at. % (eV): 25.5 (Ca2+, Ca 3p, 0.3%); 74.9 (Al2O3, Al 2p, 12.5%); 103.1 (SiO2, Si 2p, 11.8%); 119.9 (Al2O3, Al 2s); 154.3 (SiO2, Si 2s); 280.2 (Ru0 or Ru/RuOx, Ru 3d5/2, 0.4%), 281.7 (RuCl3, Ru 3d5/2, 0.7%), 284.3 (Ru0 or Ru/RuOx, Ru 3d3/2), 285.6 (RuCl3, Ru 3d3/2); 284.7 (-CH2–CH2-, C 1s, 6.4%), 285.5 (-CH2–N-, C 1s, 1.3%), 286.5 (-C=C–O-, C 1s, 1.2%), 287.9 (-C(=O)-, C 1s, 0.6%), 289.2 (–C(=O)-, -C=N-, C 1s sat, 0.3%), 291.0 (-C=C-/Ar, C 1s sat, 0.5%); 347.9 (Ca2+, Ca 2p); 399.1 (N 1s, 1%); 532.7 (Al2O3/SiO2, O 1s, 61.6%); 1072.5 (Na+, Na 1s, 1.3%). Ru-2 catalyst was synthesized analogously to the previous procedure, with the following exceptions. 2 g of halloysite were used as initial material, and 20 mL of 20 ml of hydrazine hydrate were taken as initial substances. Then 20 mL of furfural were added to obtain the Schiff base. 25 mg (0.12 mmol) of RuCl3 in 30 mL of ethanol and 300 mg of NaBH4 in 20 mL of water were used to obtain Ru nanoparticles in modified halloysite. The final product was obtained as a grey powder with a yield of 80%. Ru content, wt. %: 1.25%. XPS, at. % (eV): 74.2 (Al2O3, Al 2p, 13.3%); 103.2 (SiO2, Si 2p, 13.2%); 119.2 (Al2O3, Al 2s); 154.2 (SiO2, Si 2s); 280.0 (Ru0, Ru 3d5/2, 0.033%), 281.6 (RuCl3 or Ru (III)/Ru (IV) O,N-bound, Ru 3d5/2, 0.0046%), 282.6 (RuOx/Ru or RuCl3×xH2O, RuCl3 N-bound, Ru 3d5/2, 0.0054%), 283.3 (RuO3, Ru 3d5/2, 0.007%), 284.2 (Ru0, Ru 3d3/2), 285.7 (RuCl3 or Ru (III)/Ru (IV) O,N-bound, Ru 3d3/2), 286.7 (RuOx/Ru or RuCl3×xH2O, RuCl3 N-bound, Ru 3d3/2), 287.5 (RuO3, Ru 3d3/2); 285.0 (CH2–CH2-, C 1s, 5.29%), 286.5 (-C=C–O/N-, -CH2–C(=O)-, C 1s, 1.05%), 288.0 (-CH2–C(=O)O-, -C=C–C(=O)-, -C=C–NH2+-, C 1s, 0.74%), 292.2 (-C=C–C(=O)-, C 1s sat, 0.12%); 293.2 (K+, K 2p, 0.1%); 532.2 (Al2O3/SiO2, O 1s, 64.7%); 1072.2 (Na+, Na 1s, 1.4%). Synthesis of Ru-4 catalysts was performed similar to Ru-2. A main difference was the use of two cycles for Ru nanoparticles impregnation and their subsequent reduction. At first, 2 g of halloysite, 30 mL of hydrazine hydrate, 20 mL of furfural in 30 mL of ethanol, 25 mg (0.12 mmol) of RuCl3 in 30 mL of ethanol and 300 mg of NaBH4 in 20 mL of water were taken as initial substances. After that, the halloysite obtained was again dispersed in a solution of RuCl3 (25 mg, 0.12 mmol) in 30 mL of ethanol, stirred for 30 min and washed 2 times with ethanol. A solution of 300 mg of NaBH4 in 20 mL of water was used to reduce Ru (III) in Ru0. A final product was obtained as a dark grey powder with a yield of 72%. Ru content, wt. %: 2.4% 4 ACS Paragon Plus Environment

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XPS, at. % (eV): 24.8 (Ca2+, Ca 3p, 0.1%); 74.8 (Al2O3, Al 2p, 13.0%); 102.8 (SiO2, Si 2p, 14.1%); 118.8 (Al2O3, Al 2s); 153.8 (SiO2, Si 2s); 280.0 (Ru0, Ru 3d5/2, 0.064%), 281.0 (RuO2 or Ru (II) Nbound, Ru 3d5/2, 0.009%), 281.9 (RuOx/Ru or RuCl3×xH2O, Ru 3d5/2, 0.017%), 284.1 (Ru0, Ru 3d3/2), 285.1 (RuO2 or Ru (II) N-bound, Ru 3d3/2), 286.1 (RuOx/Ru or RuCl3×xH2O, Ru 3d3/2); 283.1 (Ru≡C, C 1s, 0.28%), 284.9 (-CH2–CH2-, C 1s, 2.46%), 286.0 (-C=C–O/N-, -CH2–C(=O)-, C 1s, 0.95%), 287.1 (-CH2–C(=O)-, -C=C–O→Ru, C 1s, 1.24%), 288.9 (-C(=O)OH-, -CH2C(=O)H→MOx, C 1s, 0.55%), 290.9 (-C=C–C(=O)-, -C=C–NH, C 1s sat, 0.13%); 399.8 (N 1s, 0.6%); 531.8 (Al2O3/SiO2, O 1s, 66.1%); 1071.8 (Na+, Na 1s, 0.8%). Ru-3 was synthesized similar to Ru-2, with the same quantities of initial substances and solvents. Herein furfural was replaced by salicylic aldehyde, and hydrazine hydrate was replaced by urea. A final product was obtained as a light grey powder with a yield of 77%. Ru content, wt. %: 0.64 Catalytic tests. To study catalytic activity of the materials synthesized, the required amount of catalyst, 300 mg of phenol and 300 µL of water were placed into a thermostated steel autoclave, equipped with a glass vial insert and a magnetic stirrer. The autoclave was sealed, filled with hydrogen up to a pressure of 3 MPa and connected to the thermostat. Reactions were conducted at 80 0C for 1, 3 or 6 hours. After that the reactor was cooled below room temperature and depressurized. The reaction products were analyzed by gas-liquid chromatography. Catalytic activity (TOF = turnover frequency) was calculated as in terms of hydrogen uptake by substrate per mole of ruthenium per hour according to the formula:

TOF(H 2 ) =

ν substr * ω * ν H ν Ru * t

2

where ω is substrate conversion, expressed in unit fractions. The reuse of Ru-2 catalysts was conducted according to the following procedure. Catalyst (5 mg), phenol (300 mg), and water (300 µL) were placed into the thermostated steel autoclave equipped with a glass vial insert and a magnetic stirrer. The autoclave was sealed, filled with hydrogen to a pressure of 3 MPa and connected to the thermostat. The reaction was carried out for 6 hours at 80 0C. After that the reactor was cooled below room temperature and depressurized. The reaction mixture was diluted with acetone (2 mL) and left overnight. Then the mixture was separated from the catalyst by decantation and analyzed by gas-liquid chromatography. The catalyst located in the vial was used for the next reaction cycle without additional loading, drying or regeneration. RESULTS AND DISCUSSION Synthesis of halloysite-based core-shell nanocatalysts. To synthesize halloysite based-catalysts, halloysite carrier was first impregnated with aldehyde and amine compounds to form Schiff bases inside. These are needed for the effective capture of Ru3+ ions from solution and for loading within the lumen of halloysite tubes. Ruthenium ions were deposited in halloysite by incipient wetness impregnation, RuCl3 solution in ethanol and aqueous NaBH4 as a reducing agent were used. The synthesized catalysts were characterized by nitrogen low-temperature adsorption/desorption (NLTAD), transmission electron microscopy (TEM) and X-ray photoelectron 5 ACS Paragon Plus Environment


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spectroscopy (XPS). Some of the physical and chemical properties of the catalysts obtained are presented in Table 1. Metal content in the catalysts correlates with the selected impregnation technique: the higher concentration of Schiff bases on the inner halloysite surfaces favored better Ru-complexation. As a consequence, larger ruthenium content in the catalysts was obtained. It was approximately 1.5 times higher for Ru-2 (1.25 %), characterized by a two times larger ratio of immobilized Schiff base to initial halloysite carrier, as compared to Ru-1 (0.82 %). In turn, Ru-4, subjected to double impregnation procedure, contained 2.4 wt. % of ruthenium, as compared with Ru-2. Ru-3 was characterized by the lowest metal content (0.64 %). An impregnation method as well as the carrier structure also influenced the synthesized particle size distribution (Figures 1-4). Thus, bimodal particle distribution with well-defined maxima at 0.8 and 1.5 nm is the characteristic of Ru-1 sample. Particle sizes here have a wide uniform distribution (Figure 1). A bimodal particle size distribution is typical for Ru-3, which was also impregnated only once (Figure 3). Ru-3 contains a higher portion of small particles (0.8 nm in radius, 64 vs. 43 %, Table 1). Ruthenium nanoparticles here are not uniformly distributed and aggregated in island-like structures (Figure 3). One may assume that impregnation with salicylic aldehyde and urea occurred not regularly, and nanoparticles were immobilized only in the Schiff bases locations. Ru-2 and Ru-4 catalysts, with the higher Schiff base loading were characterized by uniform particle distribution through the carrier volume (Figures 2 and 4). The particles formation and growth in case of Ru-2 has occurred not only simultaneously, but also in a different ligand microenvironment. Ru-4 is characterized by a maximum 2.4 wt. % ruthenium content and has uniform, almost monomodal particle distribution with the well-defined maximum at 1.25 nm (Figure 4). Incorporation of Ru3+ ions and their subsequent reduction by sodium borohydride did not lead to destruction of the nanotube structure, that was confirmed by nitrogen low temperature adsorption / desorption and TEM imaging. Both initial halloysite and Ru-halloysite catalysts were characterized by adsorption isotherms of IV type (Figure S1), typical for mesoporous adsorbents.


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Table 1. Physical and chemical properties of the catalysts synthesized Surface concentrations by XPS, atomic % Entry

Catalyst

Ru, wt. %

r, nm

Ru 3d5/2 valency states by XPS, at. % (eV)

Ru

Si

Al

C

O

N

Ru0, Ru/RuOx

RuO2 or Ru (II) N-bound

RuCl3 or Ru (III)/Ru (IV) O,N-bound

RuOx/Ru or RuCl3×xH2O

RuO3

1

Ru-1

0.82

0.81±0.01 (43%) 1.55±0.09 (56%) 3.33±0.85 (1%)

1.1

11.8

12.5

10.4

61.6

1.0

36.2 (280.2)

-

63.8 (281.7)

-

-

2

Ru-2

1.25

1.69±0.27 (100%)

0.06

13.2

13.3

7.2

64.7

-

66.3 (280.0)

-

9.3 (281.6)

10.9 (282.6)

13.7 (283.3)

3

Ru-3

0.64

0.82±0.02 (64%) 1.54±0.05 (33%) 2.59±0.13 (3%)

4

Ru-4

2.40

1.27±0.10 (95.5%) 3.65±0.21 (4.5%)

0.09

14.1

13.0

5.6

66.1

0.2

70.7 (280.0)

10.1 (281.0)

-

19.2 (281.9)

-

7

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At the same time, modification of halloysite with Schiff bases, followed by the innermost Ru nanoparticles formation, resulted in significant reduction of the surface area and pore volume (Table S1). The maximum decrease in porosity was observed for Ru-1 formulation, while the minimum was obtained for Ru-2 and Ru-4 (Table S1). According to the XPS data, ruthenium in core-shell samples is presented both in zero-valent18 and oxidized states (RuO2, RuOx), as well as chlorides, bound with water and amine ligands19 (Table 1, Figure S2). This pattern is typical for RuCl3, deposited onto Al2O3 or aluminosilicate clays and reduced thereafter20. XPS has revealed the presence of -CH2–N-, -C=C–O- and –C(=O)- fragments21. The first of them may be attributed to alkyl-substituted amines resulted from catalysts reduction by sodium borohydride. The latter apparently belongs to unreacted furfural moieties. Admixtures, like Na, Ca, and Fe typical for halloysite clays22, were also detected, not exceeding 2 wt. %. A significant difference in surface ruthenium content between Ru-1, Ru-2 and Ru-4 should be noted (Table 1). In the highest Ru-1 it reaches 1.1 atomic %; Ru in this sample is mostly presented in oxidized form: the ratio of [Ru0]/[RuOx + RuClx] was ~0.6. For Ru-2 and Ru-4, surface ruthenium content was less than 0.1 %, however [Ru0]/[RuOx + RuClx] ratio reached 1.8 and 2.4; and [Ru0]/[RuOx + RuClx] strongly correlates with surface metal content. This data can be explained in terms of carrier nanoarchitecture: ruthenium located in the deeper interior of halloysite is less subjected to oxidation in air.

16

Relarive abundance, %

14 12 10 8 6 4 2 0 0

1

2

3

4

5

r, nm

Figure 1. TEM image of Ru-1 halloysite clay nanotubes and particle size distribution.

18 16

Relative abundance, %

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14 12 10 8 6 4 2 0 0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

r, nm

Figure 2. TEM image of Ru-2 halloysite clay nanotubes e and particle size distribution.


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25

20

15

10

5

0 0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

r, nm


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

15

10

5

0 0

1

2

3

4

5

r, nm


Hydrogenation of phenol in the presence of Ru-halloysite based catalysts. The Ru-halloysite nanocomposite catalysts were tested in aqueous phenol hydrogenation. The substrate concentration was 1 mg / µL of water, as in our previous works, devoted to two-phase Ru-catalyzed hydrogenation11, 23 . The use of water, in a weight ratio to substrate of 1:1, promotes Ru catalysis, reaching 10–1000-fold improvement in the reaction rate, as was already proven24. The results of phenol hydrogenation are presented in Table 2. Catalytic activity (TOF (H2)), expressed as turnover frequencies, was calculated in terms of hydrogen uptake by substrate per mole of ruthenium per hour according to the formula:

TOF(H 2 ) =

ν substr * ω * ν H ν Ru * t

2

where: ω is a substrate conversion, expressed in unit fractions; νsubstr, νRu and νH2 are the quantities of substrate, ruthenium and hydrogen consumed, expressed in moles; t is reaction time, expressed in hours. Hydrogen uptake was calculated as a sum of hydrogen moles, required e.g. for cyclohexanol and cyclohexanone formation, multiplied by corresponding selectivities, expressed in unit fractions, according to common reaction (Scheme 1). 9 ACS Paragon Plus Environment


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OH

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OH

+ (3*Sel. CyOH + 2*Sel. Cy=O)*H2

[catalyst]

O

+ Sel. CyOH (%) 3 moles of H2 needed

Sel. Cy=O (%) 2 moles of H2 needed

Scheme 1. Typical reaction for calculation of hydrogen consumed.

TOFS (H2) as surface turnover frequency was calculated, taking into account particle dispersity, DM, showing the ratio of surface metal atoms to those in particle volume. Herein:

=

and =

,

,

where d is mean particle diameter, expressed in nanometers, and 0.75 is the reduced factor for ruthenium, corresponding to the ratio of atomic metal phase volume vM to an average useful atomic area aM on the nanoparticle surface25, that can be calculated according to the formula: vM Ar = aM N A * ρ * aM ,

where Ar is the relative atomic mass, NA is Avogadro’s number and ρ is the metal density. As seen from Table 2, conversions of phenol within 6 hours are similar for catalysts Ru-1 and Ru-2, reaching 100 % at catalyst loading of 15 mg per 300 mg of phenol and 60-70 % at catalyst loading decreased three times. Ru-1 had the maximum activity, reaching 8210 h-1, due to the higher phenol to ruthenium ratio (Table 2). Ru-1 was also tested in hydrogenation of cresols. An introduction of a methyl group to the aromatic ring resulted in sharp decrease of conversion from 100 to approximately 30% within 6 hours at substrate/Ru ratio of 2600-2800, that may be attributed to the helical multilamellar structure of halloysite tubes. The reaction yield was also strongly dependent on the relative positions of the methyl and hydroxyl groups in a cresol molecule. Thus, it was only 4% for o-cresol, but reached 28 and 33% for p-cresol and m-cresol. The main reaction product was methylcyclohexanol, its portion increased in the following order: o-cresol (61%) < m-cresol (90%) < p-cresol (99%). Simultaneously, the tendency to undergo hydrogenolysis was revealed, that is typical for heterogeneous Ru-catalysts based on SiO2 and/or Al2O3 carriers26. Propensity of cresols to hydrogenolysis in the presence of Ru-1, on the contrary, increased in the order: p-cresol (0%) < m-cresol (~ 10%) < o-cresol (~ 39%); and the main hydrogenolysis product was phenol.


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Table 2. Hydrogenation of phenol with halloysite-based Ru catalysts Entry

1

2

3

4

Catalyst

Ru-1

Ru-2

Ru-3

Ru-4

Ru, wt. %

0.82

1.25

0.64

2.4

r, nm

0.81±0.01 (43%) 1.55±0.09 (56%) 3.33±0.85 (1%)

1.69±0.27 (100%)

0.82±0.02 (64%) 1.54±0.05 (33%) 2.59±0.13 (3%)

1.27±0.10 (95.5%) 3.65±0.21 (4.5%)

DM

Catalyst loading, mg

PhOH/Ru, mol/mol

The yield of phenol hydrogenation

TOF (H2), h-1

TOFS (H2), h-1

15

2620

Conversion 100% Cyclohexanol 79% Cyclohexanone 21%

1126

3755

5

7858

Conversion 63% Cyclohexanol 98% Cyclohexanone 1% 2-cyclohexenone 0.5% Cyclohexane 0.5%

2463

8210

15

1718

Conversion 100% Cyclohexanol 100%

859

3905

5

5155

Conversion 60% Cyclohexanol 93% Cyclohexanone 7%

1510

6865

15

3356

Conversion 80% Cyclohexanol 99.5% 2-cyclohexenone 0.5%

1340

3942

5

10068

Conversion 5% Cyclohexanol 56.5% Cyclohexanone 43.5%

215

635

15

895


447

1625

5

2685


1342

4880

0.30

0.22

0.34

0.28

Reaction conditions are: 300 mg of PhOH, 300 µL of H2O, 80 0С, 6 hours, 3 MPa of H2.

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100

100

80

80

Conversion, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Conversion, %


60

40

20

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60

40

20

0 0

1

2

3

4

5

0 2000

4000

Catalyst loading, mg per 300 mg of PhOH

a)

6000

8000

10000

12000

14000

PhOH/Ru, mol/mol

b)

Figure 5. Plot of phenol conversion vs. catalyst loading (a) and PhOH/Ru molar ratio (b) in the presence of Ru-4. Reaction conditions are: 80 0C, 3 MPa of H2, 3 h., m(PhOH) = m(H2O).

Ru-3 was characterized by the least metal content and the worst particle distribution through the carrier lumen (Figure 3). It revealed the lowest activity, giving 80 % within 6 hours at phenol/Ru = 3356 and only 5 % at phenol/Ru = 10068. Thus, we have an induction period on the activity vs. ratio of substrate to catalyst. The presence of an induction period is typical for ruthenium catalysts and connected with adsorbed water and oxidized species on the surface of ruthenium particles27. Ru-4, with the highest metal content, gave 100% conversion even with a catalyst loading ratio of 5 mg per 300 mg of phenol. Therefore this catalyst was additionally tested at higher phenol/Ru ratios and less reaction times. As seen from Figure 5, Ru-4 gave a conversion of 85 % within 3 hours at loading of 5 mg per 300 mg of phenol (PhOH/Ru ~ 2685), 53 % at loading of 3 mg (PhOH/Ru ~ 4475) and only 6.5 % at loading of 1 mg (PhOH/Ru ~ 13425). This sharp downfall in conversion at high (> 10000), but not highest substrate to catalyst ratios may be attributed to the tubular multilayer halloysite structure, making inner catalytic centers less accessible for the substrate, when the last appears in large excess. In this situation, halloysite-based catalysts may be compared with those, based on dendrimers, for which similar decrease in conversion at high substrate to catalyst ratios, especially for high dendrimer generations (when metallocenters become less accessible), was also observed11, 23. With the conversion decreased, changes in product distribution were observed.Thus, at conversion of 85 % (PhOH/Ru ~ 2685) cyclohexanol was the only reaction product, and at 53 % (PhOH/Ru ~ 4475) cyclohexanol portion was 95 %, with cyclohexanone being a minor reaction product. At 6.5 % (PhOH/Ru ~ 13425) cyclohexanol portion dropped to 20 %, other products being cyclohexanone (43 %) and 2cyclohexenone (37 %). For a PhOH/Ru ratio of ~ 4475, reaction kinetics in the presence of Ru-4 was additionally investigated. It was found, that a decrease in reaction time from 3 hours to 1 hour resulted in conversion of 38 %, with a cyclohexanol portion of 85 % and TOFs of 17282 h-1. Further increase of reaction time did not result in any significant upturn of conversion: it was only 59 % within 6 hours, with a cyclohexanol portion of 97 %. So high activity for Ru-4 in comparison with Ru-1 and Ru-2 may be attributed both to the regular particle distribution inside the halloysite channels and to significant prevalence of Ru0 in the nanoparticles surface (Table 1, Figures 4, S2, c). 12 ACS Paragon Plus Environment

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Table 3 summarizes data on activity Ru-containing heterogeneous catalysts on hydrogenation of phenol, both synthesized in this work and described in the literature. As one can see, halloysite-based catalysts are significantly superior for the most part of traditional heterogeneous catalysts, like Ru/C1, 28, Ru/Al2O36b or Ru/SiO28a, 29. Table 3. Comparison in activities for Ru heterogeneous catalyst in hydrogenation of phenol. Conversion of phenol, %

Reaction conditions

TOFs (H2), h-1

Ref.

63 (6 h.)

Cyclohexanol 98% Cyclohexanone 1% 2-cyclohexenone 0.5% Cyclohexane 0.5%

8210

This work

80 0C, 3 MPa of H2, water

59 (6 h.)

Cyclohexanol 97% Cyclohexanone 3%

17282

This work

Ru@γ-Al2O3, microvawe irradiated

160 0C, 5 MPa of H2, cyclohexane

100 (3 h)


16365

5

4

Ru/Al2O3

40 0C, 2 MPa of H2, ethanol

95 (0.5 h.)

2-methylcyclohexanol 100%*

220**

6b

5

Ru/MCM-41


99.5 (4 h.)

Dodecahydro-bisphenol A 92%*** Hexahydro-bisphenol A 92%***

1430

29

6

Ru/Al2O3


90 (2 h.)


31919

7

7

Ru/H-ZSM-5

150 °C, 5 MPa of H2, water

100 (2 h.)

Cyclohexanol 0.5% Cyclohexane 96%

6860

8a

8

Ru/C

275 °C, 10 MPa of H2

100 (5 h.)

Cyclohexanol 33% Cyclohexane 66%

774**

8b

9

Ru/C

60 °C, 0.5 MPa of H2, i-PrOH

92 (4 h.)

Cyclohexanol 100%

74**

28

10

Ru/N-doped carbon

40 °C, 0.5 MPa of H2, water

96 (2 h.)


291

13b

11

Ru/PAF-30


100 (1 h.)

Cyclohexanol 100%

8100

10

12

Ru/PS

80 °C, 5 MPa of H2, THF

100 (7 h.)

Cyclohexanol 100%

5847**

31

13

Ru/MIL-101

50 °C, 0.5 MPa of H2, water

90 (4 h.)


132

9

14

G1-HMDI-Ru


92 (2 h.)

Cyclohexanol 100%

14060

11

15

G2-dendrmeso-SiO2-Ru


100 (6 h.)

Cyclohexanol 100%

10394

30

Entry

Catalyst

1

Ru-1 (on halloysite)


2

Ru-4 (on halloysite)

3

Products

* o-cresol was used as a substrate; ** data on particle diameters are unavailable, TOF value is presented without dispersity account; *** bisphenol A was used as a substrate.



They are also more efficient than various new hybrid materials, such as polyaromatic10 and metal organic frameworks9 as well as dendrimer-based matrices11, 30. In addition, being inexpensive with natural availability and utilizing a simple procedure for a metal deposition, halloysite is attractive not only for academic research but also for large scale industrial applications. Recycling test. Ru-2 was tested for the possibility of recycling in the hydrogenation of phenol at substrate/Ru ratio of 5155. The recycling test was performed according to the standard procedure for hydrogenation of phenols in the presence of ruthenium catalysts11. Because of the slow sedimentation of catalyst particles from the aqueous phenol solution and its hydrogenation products formed during the reaction, the latter was additionally diluted with acetone; held out for a night, after which time the reaction solution was decanted and new portions of phenol and water were added to the remaining catalyst. Results are presented in Figure 6.

70 60

Conversion, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 40 30 20 10 0 1

2

3

4

Cycle

Figure 6. Recycling of Ru-2 catalyst in hydrogenation of phenol. Reaction conditions are: 5 mg of catalyst, 300 mg of phenol, 300 µL of H2O, 80 0C, 30 atm. of H2, 6 hours.

As one can see, there is a sharp decrease in conversion after the first reaction cycle that may be attributed to some metal leaching. We assume, that washed out nanoparticles were located mainly on the halloysite surface. It is favorable for nanoparticles encased inside of halloysite tubes to remain inside the lumen, thus providing sustained activity in the following reaction cycles. Besides, substrate access to the inner catalytic centers is retarded, which along with decrease in total metal loading, results in a drop of conversion. ICP, XPS and TEM of the recycled Ru-2 catalyst have supported our assumption. Indeed, particles in the catalyst appeared as much more rarely and less uniformly distributed (Figure 7), that is result of metal leaching and the main reason of the sharp downfall in the conversion. Herein mean particle size slightly decreased (1.2 nm vs. 1.6 nm, Figures 2, S3) with the tendency for retained particles agglomeration also observed.


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Figure 7. TEM images of the recycled halloysite Ru-2 catalyst.

Weight Ru content in the catalyst after recycling was only 0.16% vs. 1.25% before (Tables 1, S2). Analogously dropped was the surface Ru content according XPS data (0.03 at. % after vs. 0.06 at. % before, Tables 1, S2). Herein a slight correlation between catalyst activity, expressed in the phenol conversion, and Ru surface content may be noticed for first and subsequent cycles (Figure 6). In spite of the fact, that the ratio of [Ru0]/[RuOx+RuClx] species appeared as similar both for fresh and for used catalysts, a pattern for the oxidized species has been changed, resulting in less amount of Ru oxides or Ru–O bonds, but higher one for Ru complexes with N-containing ligands, i.e. Schiff bases (Figure S4, Table S2), that indirectly correlated with the TEM data: the more local ligand to metal ratio, the less particles formed11. CONCLUSIONS For the first time ruthenium core–clay shell nanocatalysts based on tubular halloysite have been synthesized, characterized, and tested in phenol hydrogenation, producing cyclohexanol as a major reaction product. The halloysite nanotubes’ modification by Schiff bases and subsequent metal deposition significantly improved the physical and chemical properties of the resulted nanocomposite catalysts (Ru content, particle size and distribution through the carrier nanotubes, Ru valency state) and their activity in the phenol hydrogenation. The most active were the catalysts with uniform 2-3 nm in diameter particle distribution encased inside the clay tubes; these formulations used furfural and hydrazine hydrate to modify the initial carrier. The rate and yield of reaction were strongly dependent on the clay to catalyst ratio and the substrate structure (substrate selectivity): decrease in ruthenium loading and replacement of phenol with cresol led to a significant downfall in the conversion. For halloysite-based ruthenium catalysts some metal leaching was observed, which may be attributed to poorly retained nanoparticles located outside of the nanotubes, but further these Ru-coreshell nanocatalysts can be recycled without loss of activity. All this, combined with halloysite natural origin and cheapness, as well as with improved mechanical properties and dispersity, makes halloysite-based materials prospective for the future application in the industrial catalysis. 15 ACS Paragon Plus Environment


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ASSOCIATED CONTENT Supporting Information. SI contains information about adsorption properties of halloysite-based catalysts before and after Ru impregnation, and XPS spectra of the catalysts synthesized. ACKNOWLEDGEMENTS This work was supported by the Ministry of Education and Science of the Russian Federation (Grant No 14.Z50.31.0035). A.S., V.V. and Y.L. thank Y. Darrat and M. Kotelev for their input in the work. REFERENCES (1) a) Chwang, W. K.; Nowlan, V. J.; Tidwell, T. T. J. Am. Chem.Soc. 1977, 99, 7233-7238; b) Parshall, G. W.; Ittel, S. D. The applications and chemistry of catalysis by soluble transition metal complexes in Homogeneous Catalysis, 2nd ed., Wiley, New York, 1992, 342 p. (2) a) Xiang, Y.; Ma, L.; Lu, C.; Zhang, Q.; Li, X. Aqueous system for the improved hydrogenation of phenol and its derivatives. Green Chem. 2008, 10, 939-943; b) Lu, F.; Liu, J.; Xu, J. Synthesis of chainlike Ru nanoparticle arrays and its catalytic activity for hydrogenation of phenol in aqueous media. Mater. Chem. Phys. 2008, 108, 369-374. (3) Zhong, J.; Chen, J.; Chen, L. Selective hydrogenation of phenol and related derivatives. Catal. Sci. Technol. 2014, 4, 3555-3569. (4) a) Shafaghat, H.; Sirous Rezaei, P.; Daud, W. M. A. W. Catalytic hydrogenation of phenol, cresol and guaiacol over physically mixed catalysts of Pd/C and zeolite solid acids. RSC Adv. 2015, 5, 33990-33998; b) Park, C.; Keane, M. A. Catalyst support effects: gas-phase hydrogenation of phenol over palladium. J. Coll. Interface Sci. 2003, 266, 183-194; c) Li, Y.; Xu, X.; Zhang, P.; Gong, Y.; Li, H.; Wang, Y. Highly selective Pd@mpg-C3N4 catalyst for phenol hydrogenation in aqueous phase. RSC Adv. 2013, 3, 1097310982; d) Rode, C. V.; Joshi, U. D.; Sato, O.; Shirai, M. Catalytic ring hydrogenation of phenol under supercritical carbon dioxide. Chem. Commun. 2003, 1960-1961; e) Shin, E.-J.; Keane, M. A., Gas-Phase Hydrogenation/Hydrogenolysis of Phenol over Supported Nickel Catalysts. Ind. Eng. Chem. Res. 2000, 39, 883-892; f) Li, G. F.; Han, J. Y.; Wang, H.; Zhu, X. L.; Ge, Q. F. Role of Dissociation of Phenol in Its Selective Hydrogenation on Pt(111) and Pd(111). ACS Catal. 2015, 5, 2009-2016; g) Singh, N.; Song, Y.; Gutierrez, O. Y.; Camaioni, D. M.; Campbell, C. T.; Lercher, J. A. Electrocatalytic Hydrogenation of Phenol over Platinum and Rhodium: Unexpected Temperature Effects Resolved. ACS Catal. 2016, 6, 7466-7470; h) Bayram, E.; Finke, R. G. Quantitative 1,10-Phenanthroline Catalyst-Poisoning Kinetic Studies of Rh(0) Nanoparticle and Rh-4 Cluster Benzene Hydrogenation Catalysts: Estimates of the Poison K-association Binding Constants, of the Equivalents of Poison Bound and of the Number of Catalytically Active Sites for Each Catalyst. ACS Catal. 2012, 2, 1967-1975. (5) Raspolli Galletti, A. M.; Antonetti, C.; Longo, I.; Capannelli, G.; Venezia, A. M. A novel microwave assisted process for the synthesis of nanostructured ruthenium catalysts active in the hydrogenation of phenol to cyclohexanone. Appl. Catal. A: Gen. 2008, 350, 46-52. (6) a) Phaahlamohlaka, T. N.; Kumi, D. O.; Dlamini, M. W.; Jewell, L. L.; Coville, N. J. Ruthenium nanoparticles encapsulated inside porous hollow carbon spheres: A novel catalyst for Fischer–Tropsch synthesis. Catal. Today 2016, 275, 76-83; b) Solladié-Cavallo, A.; Baram, A.; Choucair, E.; NorouziArasi, H.; Schmitt, M.; Garin, F. Heterogeneous hydrogenation of substituted phenols over Al2O3 supported ruthenium. J. Mol. Catal. A: Chem. 2007, 273, 92-98; c) Karakhanov, E. A.; Glotov, A. P.; Nikiforova, A. G.; Vutolkina, A. V.; Ivanov, A. O.; Kardashev, S. V.; Maksimov, A. L.; Lysenko, S. V., 16 ACS Paragon Plus Environment

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