Metallic Nanoparticles Hosted in Mesoporous Oxide Thin Films for Catalytic Applications (2006)

Hybrid materials in catalysis: Mesoporous oxide thin layers hosting metallic nanoparticles were used as catalytic materials for both glycol oxidation and allylic amination (see picture). Despite the low amount of metal used in each run, high conversion rates could be observed within a few days of operation, revealing the high activity of the hosted nanoparticles. In a recent review on catalytic applications of molecular imprinting, Becker and Gagné defined the Holy Grail of catalysis in the following way: “We seek catalysts that have high reactivities, longetivities, and selectivities, are easy to separate from the products of the reaction, and are tolerant to water, the atmosphere, and contaminants in the feedstocks. Oh yes, we’d like them to be inexpensive and never pollute. We still have not found this catalyst, though we keep trying.”1 Fighting on all of those fronts at the same time is probably too ambitious, and it would be probably more efficient to address these requirements in succession. Therefore, not surprisingly, much attention has been paid to the recovery and recycling of catalysts. Some years ago, Chemical Reviews devoted a whole issue (under the supervision of J. A. Gladysz) to recoverable reactants and catalysts.2 Many technical solutions were developed to immobilize catalysts3, 4 including biphasic catalysis5 or the use of supercritical fluids6, 7 and ionic liquids.8 Solids supports, from polymers9, 10 to clays,11, 12 and from dendrimers13 to zeolites,14 have been extensively studied. Mesoporous materials, however, have proven to be the most successful supports. Indeed, they can feature a large variety of geometries (for a fine tuning of the access to the catalytic center), chemical composition (for a fine control of the physicochemical properties of the walls), and overall forms (e.g., powders, membranes, and monolites for fitting manipulation requirements).15 Their functionalization is a relatively easy task and many catalysts were thus tethered on such materials.16 Even the usual drawback of activity loss upon immobilization has recently been circumvented.17, 18 Unfortunately, the mesoporous oxides used are generally powders that are difficult to handle. Furthermore, they are subject to attrition in batch reactors and induce strong pressure drops when used in continuous-flow reactors. Therefore, our interest has focused on the use of mesoporous thin layers as catalyst supports. Indeed, evaporation-induced self-assembly (EISA[19, 20]) allows the synthesis of thin layers with controlled chemical composition and tailored pore size and geometry. Moreover, this technique can be applied to nearly any substrate in order to suit industrial requirements for catalyst supports.21, 22 The range of available chemical compositions for such layers broadens every day, and recently some very promising structures (from a catalytic point of view) were reported.23, 24 However, the quantity of deposited material, and thus the amount of catalyst per layer, remains low by construction. Therefore, the use of thin layers as catalyst supports has been restricted to applications with very long timescales, such as photocatalysis,25–27 or to films deposited on substrates featuring intrinsically broad surface areas, such as sand28 or fibers.29 Such a strategy must be improved to enable the design of catalytically active thin layers, otherwise known as “catalytic beakers”. The use of very active catalysts has been envisaged to overcome this low-loading issue. In this respect, metallic nanoparticles (NPs) immobilized in mesoporous thin films appear to be ideal candidates for thin-layer-supported catalysis. Herein, we report on the significant enhancement in catalytic activity (compared with nonsupported NPs) we have observed while using gold NPs loaded in titania thin layers and palladium NPs loaded in silica thin layers. These two systems were used in the gycol oxidation and allylation of amines, respectively. Employing supported gold NPs to catalyze oxidation reactions was an obvious choice as gold-catalyzed reactions are very popular30 and the dependence of the support on their activity is well documented.31 Palladium NPs, on the other hand, are well-known catalysts for arene hydrogenation and recently proved to be effective catalysts for Suzuki (interestingly, the immobilization of PdNPs on polymers proved to be very successful in this specific case[32]) and Heck couplings,33–36 and for allylic substitution processes.37, 38 The transition-metal-catalyzed allylic substitution reaction has become a very powerful method in recent years for the preparation of secondary and tertiary allylic amines.39 This reaction seemed to be a suitable test for the systems being developed. Synthetic approaches that would enable a fine control of the metal loading, the particle size and shape, the material mesostructure, and the overall particle repartition within the material were sought. The use of presynthesized nanoparticles with well-defined properties followed by materials synthesis through sol–gel methods appeared to be a convenient approach. Indeed, this methodology enables the formation of a mesoporous material around particles of any size and shape. Furthermore, dip-coating techniques are available for providing well-defined thin layers on glass wafers. Recently we described the synthesis of mesoporous silica thin films loaded with gold NPs via EISA for chemical sensing purposes.29 The same methodology was used to yield gold NPs supported on titania thin films (AuNP@TiO2) and palladium NPs supported on silica thin films (PdNP@SiO2). The AuNPs used here were stabilized with phosphinine ligands, the phosphorus equivalents of pyridines. Their synthesis afforded monodisperse NPs with a diameter of 6 nm.40 After synthesis and washing, the AuNPs were redispersed in a titania-containing sol with an amphiphillic block copolymer (P123) as template. (The sol had the following molar composition: TiCl4/EtOH/H2O/P123=1:40:10:0.01; Au/TiO2=1:20 w/w.) Films were then synthesized by a procedure adapted from the work of Crepaldi et al.41 The sol was dip-coated at a relative humidity of 30 %. After drying at this rate for 5 min, the humidity was raised to 80 % for 1 min. The films were consolidated under N2 at 120 °C for 24 h, in order to increase the degree of condensation of the inorganic framework. X-ray diffraction (XRD) measurements showed that the films exhibited a porous Im3n structure. Film thickness was determined through ellipsometry and reached 300 nm. Figure 11 shows transmission electron microscopY (TEM) and high-resolution (HRTEM) images of the layers confirming the proposed structure and displaying a satisfactory repartition of the particles in the inorganic matrix. TEM (top) and HRTEM (bottom) images of typical samples of AuNP@TiO2. The palladium nanoparticles were synthesized by decomposition of bis(dibenzylideneacetone)palladium(0) [Pd(dba)2] under H2 (3 bar) at room temperature in tetrahydrofuran (THF) in the presence of 0.4 equivalents of PPh3 as a ligand.42 The resulting NPs exhibited a mean diameter of 5.6±1.5 nm. The obtained black powder was then redispersed in a sol containing the silica source (tetraethyl orthosilicate, TEOS) and cethyltrimethylammonium bromide (CTAB) as a surfactant for dip coating, so as to obtain a nanoparticle/silica ratio of 1:25. The composition of the sol was Si(OEt)4/H2O/HCl/EtOH/CTAB=1:5:0.05:20.4:0.14. This mixture proved to be very efficient in yielding optical-quality mesoporous thin layers with a cubic structure, as shown by Cagnol et al.20, 43 Following this procedure, the films were dip-coated at a humidity of 30 %. After drying of the films the humidity was raised to 80 % for 5 min. The resulting films were aged at 30 % humidity for 15 min and consolidated at 100 °C under nitrogen overnight. The film thickness was found to be 240 nm. XRD measurements showed that the films featured a typical cubic Pm3n phase and the presence of the NPs in the layer could be observed by TEM (Figure 22). In a previous study the specific surface area of a silica thin film, which was synthesized using the same methodology but without NPs, was evaluated by ellipsometric porosimetry and reached ≈700 m2 cm−3.44 TEM image of PdNP@SiO2. In both cases, the presence of ligands on the NP surfaces after film formation was not checked because of the low metal loading of the film, which prevented a satisfactory FTIR analysis. However, in previously reported experiments, we demonstrated that the EISA techniques were mild enough to enable the ligands to remain untouched on the NPs surface. In these cases, both gold or palladium NPs were immobilized in mesoporous oxide powders.40, 45 Prior to the catalytic tests, the surfactants were removed by thoroughly washing the samples with ethanol. The catalytic activity of the AuNP@TiO2 films was tested for the oxidation of glycol, which is a classical reaction for gold nanoparticles stabilized on metal-oxide supports.46–48 The protocol used is similar to that previously reported. An aqueous solution of glycol was placed in an autoclave with one equivalent of NaOH as a base (required to promote the initial step of the reaction).46 The films were placed in small vials to protect them from the stirring bar and immersed into the solution. The solution was stirred for 6 days at 70 °C under an oxygen pressure of 3 bar. The conversion of the glycol to glycolic acid was total. Considering the wafer size (2.5 cm×2.5 cm×300 nm×2 sides) and the initial Au/TiO2 ratio (1:20), one can calculate a total metal amount of 2×10−7 mol per wafer. The turnover frequency (TOF) is thus above 290 h−1. This value reveals that our NPs are highly active and is consistent with previous work.47 For the allylic nucleophilic substitution tests, benzylamine was used as nucleophile (Scheme 1). Indeed, other nucleophiles, such as malonates, have to be activated with bases,49 which are incompatible with silica matrices. Amines were better suited for that purpose because they are known not to require any basic co-reagent.39 A single 1 cm2 coated wafer was used in the experiments. Since the thickness of the film was ≈240 nm, it is estimated that 0.07 μg of palladium was employed per run. As a reference, nonimmobilized NPs were used for the same reaction (this catalytic system is referenced to as PdNP). Both experiments were carried out in THF at room temperature (the results are summarized in Table 1). Allylic amination of cinnamyl acetate by benzylamine. Catalyst TOF[a] [day−1] Amount of product A [%] Amount of product B [%] PdNP 4 34 66 SiO2 0 0 0 PdNP@SiO2 2300 100 0 PdNP@SiO2[b] 2000 100 0 As can be seen from these data, the comparison with homogeneous conditions (based on metal loading) is very surprising. The immobilized NPs are three orders of magnitude more active than their homogeneous counterparts. The process is also much more selective as the regioselectivity is totally in favor of the linear compound. Both features may rely on so called “confinement effects”. Indeed such effects have been reported previously in zeolites to account for the observed activities and selectivities,50 and in mesoporous systems to account for strong activity increases.51 In conclusion, we have shown that the immobilization of very active catalytic entities enables a shift for single thin oxide layers from the field of ‘ideal catalyst supports’ to that of ‘real catalytic materials’. Moreover, the significant impact of the support on the activity of the catalysts is interesting from both an academic and industrial point of view. Further studies will now focus on the extension of this approach towards other catalytic transformations with synthetic relevance. All solvents and reagents were purchased from Aldrich and used as received. Small-angle XRD spectra were recorded on a Brucker D8 spectrometer. TEM samples were visualized with a JEOL 100 cxII microscope. Ellipsometry measurements were performed on a UV/Vis (from 240 to 1000 nm) variable-angle spectroscopic ellipsometer (VASE) from Woolam, and the data analysis was calculated using the WVase32 software. PdNP: [Pd(dba)2] (28.7 mg, 0.05 mmol) and triphenylphosphine (5.2 mg, 0.02 mmol) were dissolved in THF (20 mL). The resulting mixture was stirred in a 50 mL stainless steel autoclave for 12 h under hydrogen (4 bar). The resulting black powder was filtered and triply washed with pentane (10 mL). Yield: 7 mg (68 %). AuNP@TiO2: AuNPs were synthesized according to an established procedure.40 After synthesis and washing, the dry AuNPs (12 mg of Au, 0.0625 mmol) were redispersed in a titania-containing sol (6.8 mL; Ti/EtOH/H2O/P123=1:40:10:0.01). Films were then produced by dip-coating at a humidity of 30 % (at a dipping rate of 4 mm s−1). After drying for 5 min, the humidity was raised to 80 % for 1 min. The thin layers were then consolidated for 24 h at 120 °C under nitrogen. The surfactant was removed by generous washing with EtOH prior to the catalytic tests. PdNP@SiO2: PdNPs (14 mg) were dispersed into the dip-coating sol (6.8 mL). The composition of the sol was TEOS/H2O/HCl/EtOH/CTAB=1:5:0.05:20.4:0.14. Films were the synthesized following the procedure of Cagnol et al.20, 43 The resulting thin layers were then consolidated at 100 °C overnight under nitrogen. The surfactant was removed by generous washing with EtOH prior to the catalytic tests. Oxidation reaction: A stainless steel autoclave was charged with a magnetic stirrer, a solution of glycol (520 mg, 8.4 mmol) and NaOH (346 mg, 8.65 mmol) in water (20 mL). A 2.5×2.5 cm2 glass wafer coated on both sides with a AuNP@TiO2 film was cut into three pieces and placed into small open vials dipped into the solution. The autoclave was pressurized under 3 bar of oxygen. The solution was then stirred for 6 days at 70 °C. After reaction, cooling, and depressurization, the solution was neutralized before being analyzed by NMR spectroscopy. Allylic amination: In typical runs, a 1 cm² wafer was placed in a Schlenk tube. A solution of cinnamyl acetate (176 mg, 1 mmol) and benzylamine (214 mg, 2 mmol) in THF (5 mL) was then added. The reaction mixture was then stirred at room temperature for 6 days. The reaction products were analyzed by high-pressure liquid chromatography (HPLC) and 1H NMR spectroscopy. 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