Metal-organic framework (MIL-101) stabilized ruthenium nanoparticles: Highly efficient catalytic material in the phenol hydrogenation

Highlights

• Ru/MIL-101 synthesized via gas phase infiltration followed by H₂ reduction method.

• Ru/MIL-101 acted as highly active and selective catalyst in phenol hydrogenation.

• Ru/MIL-101 showed great stability throughout the catalytic reusability.

Abstract

Ruthenium(0) nanoparticles stabilized by MIL-101 metal-organic framework (Ru/MIL-101) were prepared via gas phase infiltration of Ru(cod) (cot) (cod = 1,5-cyclooctadiene, cot = 1,3,5-cyclooctatriene) followed by hydrogenolysis of Ru(cod) (cot)@MIL-101 at 3 bar H₂ and 323 K. The resulting material was characterized by using various analytical tools including ICP-OES, EA, P-XRD, XPS, DR-UV-VIS, SEM, BFTEM, HRTEM, STEM-EDX, CO-chemisorption and N₂-adsorption–desorption technique, which revealed that the formation of ruthenium(0) nanoparticles (4.2 ± 1.2 nm) mainly exist on the surface of MIL-101 by keeping the host framework intact. The application of Ru/MIL-101 in catalysis by considering their activity, selectivity and reusability was demonstrated in the phenol hydrogenation under mild conditions. Ru/MIL-101 acted as active (lower-limit TOF = 29 mol cyclohexanone/mol Ru × h; corrected TOF = 88 mol cyclohexanone/mol Ru × h at ≥ 90% conversion) and selective (≥90%) catalyst in the hydrogenation of phenol to cyclohexanone in water at 323 K and 5 bar initial H₂ pressure. More importantly, the resulting ruthenium(0) nanoparticles in Ru/MIL-101 were found to be highly durable throughout the catalytic reuse in the phenol hydrogenation (retain ≥85% of their inherent activity and selectivity at 5th reuse), which makes Ru/MIL-101 a reusable catalytic material for the liquid phase mediated catalytic transformations.

Introduction

In the last decade, metal nanoparticles have been broadly investigated in the development of nanocatalysts, which have enhanced catalytic performances as compared to their bulk-counterparts because they have much higher surface-to-volume ratio, thus, larger fraction of catalytically active atoms exist on their surface [1], [2]. For all that metal nanoparticles are considered as thermodynamically unstable against to agglomeration into bulk form due to their large surface areas and high surface energies. At this concern, the suitable protecting ligands or polymers are widely used in their synthesis to avoid their aggregation [3]. However, the agglomeration of metal nanoparticles ultimately to the bulk form even in the presence of best stabilizing agents [4], [5] is still the most important concern that should be surmounted in their catalytic applications. In addition, it is another critical matter to obtain pure active metal surfaces by staying away from surface contamination resulting from surface protecting groups, which often lead to a decrease in the catalytic performance resulting from the blocking of active sites. In this context, the utilization of porous solid matrices as host material for the immobilization of guest metal nanoparticles allows the generation of specific surfactant-free active sites with the advantages of preventing particle aggregation [1], [2], [3], [4], [5].

In this context, porous solid materials like zeolites [6], [7], carbon-based materials [8], [9], and minerals [10], [11] have been widely used to synthesis of stable metal nanoparticles within their porous matrices [12]. Besides these porous materials, more recent studies [13], [14], [15] have also indicated that metal-organic frameworks (MOFs), which are highly crystalline hybrid materials that combine metal ions with rigid organic ligands [16], can also be considered as suitable host materials to stabilize guest metal nanoparticles. Indeed, MOFs can be used as more suitable support material for metal nanoparticles with respect to other porous solids as they allow more flexible and systematic modification of the pore structure by the proper selection of the structural subunits and their connected way ligands [13], [14], [15], [16]. Furthermore, the stabilization of metal nanoparticles within the structure of MOFs produces solid catalytic materials, which can help us in the kinetic control of the catalytic reactions. In this context, the selection of MOFs type properly depending on the reaction conditions is the most critical step for the employment of MOFs as supports for metal nanoparticles immobilization as only a few MOFs with suitable pore structures are presently known for their thermal/chemical stability. In this line, the results of recent studies have shown that chromium(III) terephthalate ([Cr₃F(H₂O)₂O{O₂CC₆H₄(CO₂)}₃.nH₂O] framework; MIL-101; MIL: Materials Institut Lavosier), which was first reported by Ferey and co-workers in 2005 [17], can be used as a suitable host material for the stabilization of guest metal nanoparticles as it is stable in water even under very acidic conditions and can show thermal stability up to 300 °C under air [18]. MIL-101 has a large surface area (∼4100 m² g⁻¹) and contains two different types of cages with diameters of 29 and 34 Å, which have pore apertures of 12 and 16 Å, respectively. These unique properties of MIL-101 encouraged us to focus on the use of the MIL-101 matrix in the stabilization of transition metal nanoparticles (Fig. 1).

Up to date MIL-101 has been employed as a suitable host material for the stabilization and catalytic applications of ligand-free Pt [19], Pd [20], [21], [22], AuNi [23], AuPd [24], AgPd [25], PdNi [26] and Ru [27] nanoparticles. Xu and co-workers have used “double-solvents” method to yield MIL-101 encapsulated Pt [19], AuNi [23], and AgPd [25] nanoparticles and these materials have been used as active catalysts in the hydrolysis of ammonia-borane [19], [23] and one-pot cascade reactions [25]. Kempe et al. have achieved the fabrication of bimetallic PdNi nanoparticles within the cavities of MIL-101 by the simultaneous deposition of [(C₅H₅)Pd(C₃H₅)] and [(C₅H₅)₂Ni] precursors followed by their dihydrogen reduction. The resulting PdNi@MIL-101 has been employed as a catalyst in the catalytic reduction of 3-heptanone [26]. El-Shall and co-workers have developed a microwave-assisted chemical reduction approach to encapsulate Pd nanoparticles within the cavities of MIL-101, and have tested their catalytic performance in CO oxidation [22]. Chang, Férey, and co-workers have realized the synthesis of well-dispersed Pd nanoparticles stabilized by ethylenediamine (ED) grafted MIL-101 framework [20] and these Pd nanoparticles can act as highly active and durable catalyst in the Heck type coupling reactions. Lastly, Luo et al. have successfully immobilized ultra fine ruthenium(0) nanoparticles inside the pores of MIL-101 by using a simple liquid impregnation method and these MIL-101 encapsulated ruthenium(0) nanoparticles achieve to catalyze the hydrolytic dehydrogenation of ammonia-borane [27].

Herein, we report the synthesis, characterization and catalytic application of ruthenium(0) nanoparticles stabilized by MIL-101 framework, hereafter referred to as Ru/MIL-101. Ru/MIL-101 were prepared by following the procedure comprising of (i) gas phase infiltration of Ru(cod) (cot) (cod = 1,5-cyclooctadiene, cot = 1,3,5-cyclooctatriene), which is a well-known ruthenium precursor for metal–organic chemical vapor deposition (MOCVD) [28], into MIL-101 framework to yield Ru(cod)cot)@MIL-101, (ii) followed by hydrogenolysis of Ru(cod)cot)@MIL-101 at 3 bar H₂ and 323 K to form Ru/MIL-101. The characterization of the resulting material by inductively coupled plasma optical emission spectroscopy (ICP-OES), powder X-ray diffraction (P-XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV–visible spectroscopy (DR-UV-VIS), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), bright-field transmission electron microscopy (BFTEM), high resolution-TEM (HRTEM), scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDX), CO-chemisorption and N₂-adsorption–desorption analyses is revealing the formation of 4.2 ± 1.2 nm ruthenium(0) nanoparticles mainly exist on the surface of MIL-101 without distorting the host material framework. The catalytic performance of these ruthenium(0) nanoparticles in terms of activity, selectivity and selectivity has been demonstrated in the catalytic hydrogenation of phenol under mild conditions (at 323 K and 5 bar initial H₂ pressure). They achieved the hydrogenation of phenol to cyclohexanone at high activity and selectivity. More importantly, Ru/MIL-101 material was found to be highly stable throughout the catalytic reuse, which makes it a reusable catalyst in the phenol hydrogenation.