Introduction

Palladium catalysts supported are used in various processes including amination, Heck and Suzuki coupling, hydrogenation, hydrogen production and dehydrogenation reactions [1-15]. The Pd/SiO2 is a classical model of Pd catalysts with SiO2 as the “inert” oxide support [16]. SiO2 resists to reduction and has low surface acidity, making it relatively inert compared to other oxide supports, such as TiO2 and Al2O3 [17]. These characteristics make Pd/SiO2 an ideal starting point for study of the catalytic role of Pd [18,19]. It is well known that several steps in the catalyst preparation process strongly influence particle size, including the support composition, metal salt, precursor deposition method, metal loading, pH, drying conditions, calcinations temperature, and reduction temperature, among others [20,21]. The combination of an ionic liquid with a solid support material is emerging as a new alternative for the immobilization of transition metal catalyst precursors [22,23]. Imidazolium Ionic Liquids (ILs) possess pre-organized structures mainly through hydrogen bonds which induce structural directionality [24]. These IL structures can adapt or be adaptable to many species, as they provide hydrophobic or hydrophilic regions, and a high directional polarizability [25,26]. This structural organization of ILs can be used as “entropic drivers” for spontaneous, well-defined and extended ordering of nanoscale structures. Indeed, the unique combination of adaptability towards Other molecules and phases associated to the strong hydrogen-bond driven structure makes ionic liquids potential key tools in the Preparation of a new generation of chemical nanostructures such as template porous silica prepared in a sol-gel process [27-30]. The Metal Nanoparticles (MNPs) with small diameter and narrow size distribution can be prepared by simple H2 reduction of metal compounds or decomposition of organometallic species dissolved in ILs [31-32]. In several cases the MNPs are not stable and tend to aggregate [33]. Alternatively, these nanoparticles can be used in conjunction with other stabilizers or be easily transferred to other organic and inorganic supports to generate more stable and active catalysts [34-37] The metal nanoparticles/ionic liquid/stabilizer combination usually exhibits an excellent synergistic effect that enhances activity of the catalyst. So could be prepared more efficient and stable catalytic systems using the generation of metal nanoparticles associated with silica using ILs as templates for both catalytic partners i.e. the metal nanoparticles and the silica support [38-42]. We present herein our results, which show that palladium nanoparticles synthesized in BMI.BF4, BMI.PF6 and PMI.Si.(OMe)3.Cl ionic liquids can be applied for the generation of the heterogeneous catalyst (Pd/ILs/SiO2) via sol-gel processes. The heterogeneous catalyst formed (Pd/ILs/SiO2) was applied in hydrogenation reactions studies. 

Experimental 

All experiments were performed in air, except for the synthesis of the Pd NPs. The Pd NPs and the halide-free BMI.PF6, BMI.BF4 and PMI.Si.(OMe)3.Cl ionic liquids were prepared according to literature procedure [33,43]. Solvents, alkenes, and arenes were dried with the appropriate drying agents and distilled under argon prior to use. All other chemicals were purchased from commercial sources and used without further purification. Gas chromatography analysis was performed with a Hewlett-Packard-5890 gas chromatograph with an FID detector and a 30 m capillary column with a dimethylpolysiloxane stationary phase. The nanoparticles formation and hydrogenation reactions were carried out in a modified Fischer–Porter bottle immersed in a silicone oil bath and connected to a hydrogen tank. The temperature was maintained at 75° C by a hot-stirring plate.

Synthesis of Palladium Nanoparticles (Pd NPs) supported in silica and ILs: Silica supporting Pd NPs/ILs/SiO2 were prepared by the solgel method under acidic conditions. Typical procedure for acid catalysis: 10 mL of tetraethoxy orthosilicate (9.34 g, 45 mmol) was introduced in a Becker under vigorous stirring at 60° C. The Pd NPs/ILs (10 mg, 0,05 mmol) dispersed in BMI.PF6, BMI.BF4 and PMI.Si.(OMe)3.Cl ionic liquids (1 mL, 5.1 mmol) and ethanol (5 mL). This solution was submitted to stirring and sonication for 2 min and then added to the solution containing TEOS. Consecutively, an acid solution (HF) was added as acid catalyst. The temperature was kept at 60° C for 18 h. The resulting material was washed several times with acetone and dried under vacuum. Typical procedure for base catalysis: 10 mL of TEOS (9.34 g, 45 mmol) was added to ethanol (5 mL), containing the ionic liquids (1 mL, 5.1 mmol) and previously isolated Pd NPs (10 mg, 0.05 mmol). Then ethanol (95 mL) and ammonium hydroxide (20 mL) were added. The mixture was kept under stirring for 3 h at room temperature and left to stand for a further 18 h. The resulting xerogel was filtered and washed with acetone and dried under vacuum for 1 h. 

X-Ray Diffraction (XRD)

The phase structures were characterized by of XRD Pd NPs. For XRD analysis, the nanoparticles were isolated as a fine powder and placed on the specimen holder. The XRD experiments were performed in a SIEMENS D500 diffractometer equipped with a curved graphite crystal using radiation Cu K ∞ (λ = 1.5406 Å). The diffraction data were collected at room temperature in Bragg-Brentano geometry ϴ-2 ϴ. The equipment was operated at 40 kV and 20 mA with a scan range between 20° and 90°. The diffractograms were obtained with a constant step Δ2ϴ = 0.05. The indexation of Bragg reflections was obtained by fitting a pseudo-Voigt profile using the code FULPROFF code.37 Nanoparticles Pd/ILs/SiO2 were analyzed on a glass substrate. 

Elemental analysis (CHN)

The organic phases present in the xerogels were analyzed using CHN elemental Perkin Elmer elemental CHNS/O analyzer, model 400. Triplicate analysis of the samples, previously heated at 100° C under vacuum for 1 h, was carried out. 

Rutherford Backscattering Spectrometry (RBS)

Palladium loadings in catalysts were determined by RBS using He+ beams of 2.0 MeV incidents on homogeneous tablets of the compressed (12MPa) catalyst powder. The method is based on the determination of the number and energy of the detected particles, which are elastically scattered in the Coulombic field of the atomic nuclei in the target. In this study, the Pd/Si atomic ratio was determined by the heights of the signals corresponding to each of the elements in the spectra and converted to wt% Pd/ILs/SiO2. For an introduction to the method and applications of this technique, the reader is referred elsewhere. 

Nitrogen adsorption-desorption isotherms

The adsorption–desorption isotherms of previous degassed solids (150° C) were determined at liquid nitrogen boiling point in a volumetric apparatus, using nitrogen as probe. The specific surface areas of xerogels were determined from the t-plot analysis and pore size distribution was obtained using the BJH method. A homemade equipment with a vacuum line system employing a turbo-molecular Edwards vacuum pump was used. The pressure measurements were made using a capillary Hg barometer and a Pirani gauge. 

Scanning Electron Microscopy (SEM) and Electron Dispersive Spectroscopy (EDS) elemental analysis

The materials were analyzed by SEM using a JEOL model JSM 5800 with 20 kV and 1000 magnification. The same instrument with was used for the EDS with a Noran detector (20 kV and acquisition time of 100 s and 1000 magnification). 

Transmission Electron Microscopy (TEM) analysis

The morphologies and the Electron Diffraction (ED) patterns of the obtained particles were determined on a JEOL JEM-2010 equipped with an EDS system and a JEOL JEM-120 EXII electron microscope, operating at accelerating voltages of 200 and 120 kV, respectively. The TEM samples were prepared by deposition of the Pd NPs or Pd/ILs/SiO2 isopropanol dispersions on a carbon-coated copper grid at room temperature. The histograms of the nanoparticle size distributions were obtained from the measurement of around 300 diameters and reproduced in different regions of the Cu grid assuming spherical shapes. 

Catalytic Hydrogenations

The catalysts (150 mg) were placed in a Fischer–Porter bottle and the alkene or arene (12.5 mmol) was added. The reactor was placed in an oil bath at 75° C and hydrogen was admitted to the system at constant pressure (4 atm) under stirring until the consumption of hydrogen stopped. The organic products were recovered by decantation and analyzed by GC. 

Results and Discussion 

The sol-gel process involves a chemical approach for the synthesis of stable oxide materials, this process allows us to obtain solid products by creating an oxide network via progressive polycondensation reactions in a liquid medium [41]. The steps involved consist of hydrolysis and condensation. The reactions are affected by the nature of the catalyst. Therefore, in the present study, two main routes were evaluated: (i) an acid-catalyzed one using either HF, or (ii) a basecatalyzed approach, using NH4OH as catalyst. In both routes, the hydrolysis and condensation of Tetraethoxy Orthosilicate (TEOS) were performed in the presence of Pd NPs, which were prepared by hydrogen reduction (4 atm) of Pd2(dba)3 dissolved in the ionic liquids at 75° C [27]. These nanoparticles obtained presented 4.8 nm of diameter. Figure 1 shows the XRD pattern of Pd NPs and encapsulated in silica matrix showing the diffraction planes of silica and Platinum (Pd/ILs/SiO2). This material was obtained by sol-gel synthesis under acidic conditions using the liquids amount of Pd (0) < 0.2% compared to silica. These Pd nanoparticles were first isolated from the IL to be characterized by XRD (Figure 1A). Through the analysis of X-Ray Diffraction (XRD), is possible identified crystalline palladium in the isolated powder. The characteristics diffraction lines (111, 220, 200, 311) of metallic Pd can be observed in the diffraction pattern (Figure 1A). The Pd NPs obtained presented 4.8±0.4 nm with a narrow range of diameter distribution. The Figure 1B show the Pd/ILs/SiO2 XRD, diffraction lines (111 and 220) were detected in the sample. TEM analysis of the synthesized Pd NPs show that the particles display a spherical shape. The mean diameter observed was 4.8±0.4 nm Pd NPs estimated from ensembles of 300 particles found in an arbitrary chosen area of the enlarged micrographs. The evaluation of their characteristic diameter results in a monomodal particle size distribution (Figure 2A). Figure 2B show the obtained particle size distributions that can be reasonably well fitted by a Gaussian curve.

Figure1: XRD analysis of: A) Pd NPs (4.8 nm) and B) Pd NPs/IL/SiO2.

Figure2: SEM micrographs obtained of: A) Pd NPs/ILs/SiO2 and B) EDX corresponding.Table 1 present the elemental analysis of the resulting silica. Carbon andnitrogen content was taken as a sign for the presence of ionic liquids onceencapsulated within the silica network. Based on the results shown in Table 1,higher ionic liquids contents were incorporated with the acid catalysts. It isworth noting that in the case of acidic conditions, hydrolysis is faster thancondensation.

Table 1:Elemental analysis of samples Pd/ILs/SiO2

The rate of condensation slows down with increasing number of siloxane linkages around a central silicon atom. This leads to formation weakly branched polymeric networks. The condensation, in case of basic conditions, is accelerated relative to hydrolysis. The rate of condensation increases with increasing number of siloxane bridges, result in highly branched networks are formed [42,43]. In the present case, based on the carbon and nitrogen contents, it seems that the resulting weakly branched structure generated in the presence of acid catalyst (either HF) guarantees the constraint of the ionic liquids. Rutherford Backscattering Spectrometry (RBS) was used in the determination of the metal contents. The Table 1 show that the immobilized Pd content is roughly the same for silica prepared by both routes, corresponding to ca. 65-75% of the initial Pd content employed in the synthesis. The metal distribution in the support was determined by SEM-EDX analyses. Mapping showed a homogeneous Palladium distribution in the silica grains, independently of the preparative route. The Figure 2 shows SEM micrograph of Pd/ILs/SiO2 synthesized using acid conditions by sol-gel method. The micrograph show lighter regions, indicating the presence of platinum metal nanoparticles on the silica matrix (gray regions). The elemental composition of the region focused on the micrograph confirms this structure. Samples Pd/ILs/SiO2 were analyzed by the scanning point and area exposed to the electron beam. All selected areas showed the presence of palladium in the silica matrix. In the micrograph, the metal is identified by the bright regions in contrast to the array of silicon that has the dark background. Figure 3 illustrates the micrography of Pd/ILs/SiO2 prepared by both routes, acid and basic. According to Figure 3, particle morphologies are in accordance to that usually observed for pure silica synthesized by these routes. In the case of acid-catalyzed conditions, a less organized, platelike structure was observed, while in the case of basic conditions, spherical particles were obtained. It is worth noting that smaller particles were produced in the latter case.

Figure 3: Micrographsobtained by SEM of the resulting xerogels: (A) Pd/ILs/ SiO2/HF/(acid) and B) Pd/ILs/ SiO2/NH4OH (basic

Transmission Electron Microscopy (TEM) was also employed for the characterization of the supported catalyst. Figure 4 shows the micrograph of the Pd/ILs/SiO2, the mean size of which was shown to be ca. 4.8 nm. It is very likely that the presence of ionic liquids affords stability, avoiding sintering of the metallic particles.

Figure 4: Micrographs obtained by TEM of Pd/ILs/SiO2/HF and histogram of diameter distribution.

The textural properties were further characterized by nitrogen adsorption. Specific area was calculated by the BET method, while pore diameter, by the BJH one (Table 2). According to Table 2, silica prepared in the absence of palladium present higher specific area (ca. 100 m2 g −1 ). The introduction of nanoparticles during the synthesis, independently of the synthetic route, led to a reduction in the specific area. The pore diameter was demonstrated to be smaller for the materials was used NH4OH as catalyst. The pore volume was shown to be independent of the presence of Pd in acidic or basic conditions. The supported catalysts were evaluated in hydrogenation reactions. Table 3 presents data regarding 1-decene, cyclohexene and benzene hydrogenation reactions. For comparative purposes we also included the data concerning the catalytic activity of isolated Pd NPs [27]

Table 2:Surface area, pore volume and average pore diameter of SiO2/ILs supporting PdNPsa.

A)    Reactions conditions: sol-gel method, constant hydrogen pressure (4 atm), 75 °C ratio [alkene/Arene]/[Pd/SiO2] = 1250/1, added Pd/SiO2 (150 mg, 0.010 mol Pd NPs followed by 12.5 mmol of alkenes or arenes. B) grafting method constant hydrogen pressure (4 atm), 75 °C ratio [alkene/Arene]/Pd/SiO2] = 625/1, SiO2 added (150 mg, 0.025 mmol Pd followed by 12.5 mmol the arenes used. C) Pd nanoparticles (5 mg. Relation [Arene]/[Pd]] = 250/1, added Pd (5 mg. D) Pd NPs [alkene/arene]/[metal(0)]= 250/1) followed by 12.5 mmol of alkenes or arenes.

Table3: Hydrogenation of alkenes by encapsulated Pd/ILs/SiO2a and Pd NPsb. 

Table 3 shows the results obtained in the hydrogenation reactions using the system Pd/SiO2. Is possible to observe that all the supported systems were more active than those constituted of isolated Pd NPs were. Among the silica-based systems, those prepared under acidic conditions are the most active, exhibiting higher TOF in comparison to those of isolated Pd NPs. The denser and bulkier structure generated under basic conditions might have afforded less active systems as shown by some clues. First, the ionic liquids content, which seems to be important in order to guarantee stability for the nanoparticles, was lower for these systems. Besides, according to porosimetric measurements, the pore diameter was much smaller for the SiO2/ILs/PdNPs/NH4OH system. Palladium encapsulated particles, in spite of a slightly higher content in comparison to that afforded with an acid catalyst (Table 3), might be not accessible in the supported systems prepared under basic conditions. The hydrogenation of simple arenes and alkenes by SiO2/ILs/Pd NPs/HF depends on steric hindrance at the C=C double bond and follows the same trend as observed with classical palladium complexes in homogeneous conditions, that is, the reactivity follows the order: terminal-internal. 

Conclusions

Palladium nanoparticles dispersed in ionic liquids and functionalized ionic liquids (SiO2/ILs/Pd NPs) can be easily immobilized within a silica network when prepared by the sol-gel method (acid or base catalysis). The palladium content in the resulting xerogels was shown to be independent of the preparative route, but acidic conditions afforded higher encapsulated ionic liquid content and xerogels with larger pore diameter, which in turn might have guaranteed higher catalyst activity in the hydrogenation of arenes and alkenes. The use of ionic liquids for the preparation of both nanoparticles and silica affords encapsulated SiO2/ILs/Pd NPs materials with different morphology, texture, and catalytic activity. This combination exhibits an excellent synergistic effect that enhances the stability and activity of the Pd NPs in hydrogenation catalysts. All the supported systems were more active than that constituted of isolated Pd NPs for the hydrogenation of arenes and alkenes. In particular, the silica-based systems prepared under acidic conditions were shown to be the most active, exhibiting higher TOF. The denser and bulkier silica structure generated under basic conditions (less active catalytic system) incorporated less ionic liquids. A high level of ionic liquids incorporation seems to be important in order to guarantee stability for the palladium nanoparticles. 

Acknowledgements

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Corresponding author 

Carla Scheerena Laboratory of Catalysis, School of Chemistry and Food, Federal University of Rio Grande - FURG, Rua Barão do Caí, 125, CEP 95500-000, Santo Antônio da Patrulha, RS, Brazil. Email: carlascheeren@gmail.com 

Citation

Hartza T, Fischera D, de Fragaa K and Scheerena C. Ionic liquids/SiO2 supporting Pd nanoparticles: efficient catalysts in hydrogenation reaction (2021) Nanomaterial Chem Technol 2: 8-12.

Keywords

Nanoparticles, Material, Chromatography