Article pubs.acs.org/Organometallics
Synthesis, Characterization, and Catalytic Properties of Novel SingleSite and Nanosized Platinum Catalysts Matteo L. M. Bonati,† Thomas M. Douglas,† Sander Gaemers,*,† and Neng Guo‡ †
Hull Research and Technology Centre, BP Chemicals Ltd., Saltend, Hull HU12 8DS, United Kingdom BP Products North America Inc., 150 West Warrenville Road, Naperville, Illinois 60563, United States
‡
S Supporting Information *
ABSTRACT: Novel single-site platinum catalysts have been synthesized by reacting platinum(II) organometallics with partially dehydroxylated silica. The resulting materials have been characterized by various methods such as IR, MAS NMR, and EXAFS. Further, the single-site platinum catalysts were calcined in air to remove the ligand and produce nanosized platinum particles, that were characterized by TEM and H2 chemisorption. All catalysts were tested for the hydrogenation of toluene. The single-site platinum catalysts were less active than a commercial Pt/SiO2 catalyst with comparable platinum loading, and this has been ascribed to ligand effects. Conversely, the nanosized platinum catalysts were more active than the commercial Pt/SiO2 catalyst due to their high dispersion and small particle sizes.
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INTRODUCTION
Site-isolated late-transition-metal catalysts can be prepared via protonolysis of the transition-metal center by surface OH groups and elimination of a suitable leaving group. This synthetic approach has previously been demonstrated for palladium complexes on dehydroxylated silica (Scheme 1)19 and platinum complexes on SBA-15 (Scheme 1).23 The resulting materials have been shown to be catalytically active for both alkene hydrogenation (Pt23) and the cyclization of aminoalkynes (Pd19) under mild conditions. These supported
Single-site supported organometallic catalysts represent a bridge between homogeneous and heterogeneous catalysts. Such site-isolated materials may provide useful insight into the reactivity at metal−surface boundary sites, act as model systems for supported nanoparticles, and may also present a route to the selective and controlled formation of nanosized supported metal particles. The grafting of early-transition-metal complexes onto oxide surfaces, including structural characterization and observations of unique reactivity, is an area that was first explored about four decades ago. The pioneers in this area were researchers at ICI1,2 and in Novosibirsk;3,4 both groups investigated supported “early”-transition-metal complexes for olefin polymerization. Despite the lack of advanced surface characterization techniques, their interpretations have been shown to be essentially correct. Since the early 1980s, research has expanded to include “late”-transition-metal-supported complexes such as rhodium supported on silica5,6 and even heavier elements such as U and Th organometallic complexes supported on alumina.7 Recent reviews focus on how surface organometallic species can bridge the gap between homogeneous and heterogeneous catalysis,8 and on the theory and simulation of catalysis onto surface organometallics.9 Although catalysis utilizing late transition metals on oxide supports has been extensively studied,10−13 much less attention has been given to catalysis with and characterization of isolated, mononuclear late-transition-metal centers.14−20 Some published examples include single-site ruthenium and palladium catalysts for the oxidation of water21 and cyclotrimerization of acetylene,22 respectively. © 2012 American Chemical Society
Scheme 1. Typical Grafting Reaction to a Silica Surface of (a) a Palladium Complex19 and (b) a Platinum Complex23
Received: August 19, 2011 Published: July 19, 2012 5243
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The collected gas sample was then analyzed by GC using an Agilent 6890 RGA (Refinery Gas Analyzer) equipped with a TCD and a packed column containing molecular sieve 13X from Analytical Controls. The analysis was carried out isothermally at 60 °C and using He as carrier gas. NMR Spectroscopy. Solution NMR spectra were recorded on a Bruker 300 MHz spectrometer. Chemical shifts are quoted in ppm against tetramethylsilane (TMS, external). 1H spectra were referenced against the residual solvent protio signal, and 31P{1H} spectra were referenced against 85% H3PO4 (external). All coupling constants are quoted in Hz. Solid-state NMR spectra were obtained using a Varian VNMRS spectrometer operating at 100.56 MHz for 13C and 161.87 MHz for 31 P. Samples were loaded in a 6 mm (o.d.) rotor sealed with a tightfitting Teflon cap. All samples were packed under N2 and immediately transferred to the spectrometer, where N2 was used as the rotor spin gas. 13C MAS NMR experimental parameters: spin rate 6800 Hz, contact time 3.00 ms, acquisition time 20.0 ms, recycle 1.0−5.0 s. 31P MAS NMR experimental parameters: spin rate 6800 Hz, contact time 5.00 ms, acquisition time 20.0 ms, recycle 3.0 s. Spectral referencing is with respect to external TMS (neat) and 85% H3PO4 for 13C and 31P, respectively. FTIR Spectroscopy. All spectra were recorded on a Bruker Tensor 27 spectrophotometer. The samples were loaded between two KBr disks in a N2-filled glovebox and transferred to the spectrophotometer, and spectra were obtained by averaging 16 scans collected at 4 cm−1 resolution. The spectra were baseline corrected and normalized against the silica overtone signals. Inductively Coupled Plasma-Optical Emission Spectrometry. The Pt loading of each sample was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). Samples were prepared by accurately weighing 100 mg of supported catalysts into a PTFE beaker and removing the silica support by digestion with hydrofluoric acid. The resulting residue was subsequently dissolved using aqua regia over a period of 2 days at room temperature. The resulting solution was diluted to a known volume and analyzed by ICP-OES using a Thermo Scientific IRIS Advantage ICP. The instrument was calibrated against externally certified reference standards. EXAFS Experiments. EXAFS Pt L3 edge data for supported and unsupported Pt catalysts was collected at the Materials Research Collaborative Access Team (MRCAT) beamline at the Advanced Photon Source, Argonne National Laboratory. The setup used was similar to that outlined by Castagnola.29 In a nitrogen-filled Innovative Technology glovebox with a large-capacity recirculator (0.1 ppm of O2 and 1.6 ppm of H2O), a predetermined amount of each sample was loaded as a self-supporting wafer without binder in the channels (i.d. = 4 mm) of a stainless steel multisample holder. The sample holder was then placed in the center of a quartz tube, equipped with gas and thermocouple ports and Kapton windows. While an inert atmosphere was maintained, the sample holder was transferred into the experiment hutch and mounted on a positioning platform. The beam was set to scan the sample cell holder by manipulating the position of the platform. Once the sample positions were fine-tuned, the EXAFS spectra were recorded in transmission mode at room temperature. A platinum-foil spectrum was measured simultaneously with each sample spectrum for energy calibration purposes. Spectra for each sample were collected from 11 364 to 12 418 eV. Phase shifts and backscattering amplitudes of Pt−Cl and Pt−N were obtained from Na 2PtCl4 and Pt(NH3) 4 Cl 2, respectively. These values were subsequently utilized for Pt−P and Pt−C(O) contributions, respectively. Standard procedures based on WINXAS 3.2 software were used to fit the acquired EXAFS data. The EXAFS coordination parameters were obtained by a least-squares fit in k and R space of the isolated nearest neighbor, k2-weighted Fourier transform data.30 The quality of the fits is equally good with both k1 and k3 weightings. Catalyst Testing. In a typical catalytic run, the catalyst was pressed into pellets, ground, and sieved (212−500 μm average particle size) prior to loading into a tubular fixed bed reactor. A precatalyst bed of corundum, separated with glass wool from the catalyst, was charged on
late transition metals are relatively thermally stable (supported Pt up to 150−200 °C) with moderate air sensitivity (Pd systems can be recycled by filtration in air, showing only slight loss in activity).19,23 In addition to the synthesis of supported single-site catalysts, transition-metal organometallic complexes have also been used for the synthesis of nanoparticles and supported nanoparticles. For example, Chaudret24 has shown the synthesis of watersoluble platinum nanoparticles starting from [Pt(COD)(CH3)2], and the same organometallic precursor has been employed for the synthesis of platinum nanoparticles supported on organic aerogels.25 This paper describes the synthesis, characterization, and initial catalyst testing of two novel, site-isolated platinum complexes supported on partially dehydroxylated silica. Our study further includes the careful thermal decomposition of these single-site materials in order to obtain supported nanoparticles of controlled dimensions and with enhanced catalytic activity.
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EXPERIMENTAL SECTION
Materials. All manipulations, unless otherwise stated, were performed under an atmosphere of nitrogen using standard Schlenkline and glovebox techniques. Glassware was oven-dried at 130 °C overnight prior to use. CH2Cl2, THF, MeCN, Et2O, pentane, hexane, and toluene were purchased from Aldrich as anhydrous solvents in Sure/Seal containers and were degassed using a stream of nitrogen and stored over activated 3 Å molecular sieves. CD2Cl2 was stirred overnight with CaH2 and then vacuum-transferred to a clean Schlenk flask and stored over activated 3 Å molecular sieves. Published literature methods were used for the preparation of the starting materials [Pt(dppe)(CH3)2]26 and Mg(CH2C6H5)2(THF)2.27 Aerosil200 was obtained from Degussa; [Pt(COD)(CH3)2] was purchased from Strem Chemicals. Dehydroxylation of Aerosil-200 Silica. The method employed for the partial dehydroxylation of silica is based on a literature procedure shown to effectively reduce the number of surface silanol groups without changing its surface area.28 Aerosil-200 (25 g) was loaded into a quartz vessel equipped with a greaseless (Viton “O”-ring) ball and socket joint and a J. Young tap. Vacuum was applied (5 × 10 −1 mbar) for 1 h at 25 °C, after which the temperature was increased to 500 °C (ramp rate 10 °C/min) and held for 15 h. The resulting dehydroxylated silica is referred to as SiO2500. The remaining number of silanol groups was determined to be 0.41 mmol of −OH/g by reaction of Mg(CH2C6H5)2(THF)2 with SiO2500 27 and quantification of the toluene evolved by 1H NMR spectroscopy.23 This value is very similar to that reported in the literature (0.40 mmol/g) and confirms a coverage of 1.2 OH groups/nm2.19 Preparation of SiO2500-Supported Pt Complexes. CH2Cl2 (200 mL) was added to SiO2500 (5 g; 2 mmol of OH), and the resulting suspension was stirred for 30 min. A solution of either [Pt(COD)(CH3)2] (167 mg, 0.5 mmol) or [Pt(dppe)(CH3)2] (311 mg, 0.5 mmol) in CH2Cl2 (50 mL) was slowly added to the suspension. The resulting slurry was stirred for 16 h and filtered, and the solid was washed with CH2Cl2 (3 × 50 mL) and dried in vacuo to give a white solid. The material prepared using [Pt(COD)(CH3)2] was termed Pt(COD)/SiO2 (0.46 wt % Pt); the material prepared using [Pt(dppe)(CH3)2] was termed Pt(dppe)/SiO2 (1.66 wt % Pt). Further preparations were carried out using a different impregnation time and a different organometallic-precursor loading in such a way as to obtain materials with similar Pt loadings. Pt(COD)/SiO2 with a higher Pt loading (0.95 wt %) than was obtained using the recipe above was prepared by stirring a slurry containing 167 mg of [Pt(COD)(CH3)2] for 2 days, and Pt(dppe)/SiO2 with a lower Pt loading (0.83 wt %) was prepared by stirring a slurry containing 207 mg of [Pt(dppe)(CH3)2] for 16 h. After the synthesis of Pt(COD)/SiO2, the headspace was tested for the presence of methane using a 20 mL gastight syringe. 5244
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top of the catalyst bed. The catalyst bed was supported in a quartz tube by a porosity 1 quartz frit. In the case of the supported organometallics (Pt(COD)/SiO2 and Pt(dppe)/SiO2) the above procedure was conducted in a N2-filled glovebox; the reactor was then covered with Parafilm, removed from the glovebox, and immediately placed under flowing N2 in a Carbolite clamshell furnace. The base of the reactor was connected to a liquid collection vessel (immersed in an ice bath). After a thorough N2 purge, the gas flow was switched to H2 at P = 1 bar and GHSV = 1440 h−1 and the furnace was set to heat to the desired reaction temperature with a 5 °C/min ramp rate. Once the system was stabilized at the desired reaction temperature, the toluene liquid feed was delivered at LHSV = 1 h−1 using a Harvard syringe drive. After 3 h the reactor was cooled under a stream of H2. The liquid products were collected and analyzed by gas chromatography using an Agilent 7890 with ALS fitted with a J&W DB5MS 30 m × 0.25 mm × 0.25 μm column. The activities of the new catalysts were compared to that of a commercial catalyst (Sigma-Aldrich 1 wt % Pt on SiO2) tested under identical conditions. Formation of Nano-Sized Supported Platinum Particles. Samples of Pt(COD)/SiO2 (1 g) and Pt(dppe)/SiO2 (1 g) were both heated in air to 500 °C following this temperature program: (a) from room temperature to 90 °C with a 20 min ramp rate and 120 min dwell time, (b) then heated to 110 °C in 20 and 120 min dwell time, (c) and then heated to 500 °C in 80 and 120 min dwell time and subsequently cooled to room temperature. The resulting materials were characterized by TEM analysis, EDX spectroscopy, and H2 chemisorption. TEM images were recorded using a JEOL 2000FX instrument. Samples were prepared by either setting in Araldite resin overnight at 80 °C followed by slicing with a diamond knife (microtomed) or by grinding between glass slides and loading directly onto a TEM grid. H2 chemisorption experiments were performed using an Altamira Instruments AMI-200 apparatus equipped with a TCD. The samples (100 mg) were placed in a U-shaped quartz tube in an electrical furnace and heated to 400 °C (1 °C/min) under 4% (v/v) H2 in N2 and maintained at 400 °C for 1 h. They were then cooled to 50 °C (still under a flow of 4% (v/v) H2 in N2) and then purged with N2 for 1.5 h. The sample was then ramped at 10 °C/min up to 400 °C under a 30 mL/min N2 flow. Platinum dispersion was calculated by taking the integral of the TCD signal over the desorption range and assuming a stoichiometric ratio of H2/Pt = 0.5.
Figure 1. Plot showing the change in concentration of [Pt(dppe)(CH3)2] and [Pt(COD)(CH3)2] upon grafting to dehydroxylated silica in CD2Cl2 solution. The data were derived from NMR analysis; for experimental conditions see text.
signal at δ 0.20 ppm was observed, consistent with dissolved CH4.31 After 30 h of reaction at room temperature the signals attributed to the dissolved [Pt(dppe)(CH3)2] complex were reduced significantly (∼60%) relative to the CDHCl2 signal. The loss in signal intensity is attributed to the grafting reaction of the complex to the silica surface. A concomitant increase in the signal at δ 0.2 ppm is observed as methane is formed. This is a clear indication of reaction of the platinum organometallic complex with the dehydroxylated silica surface. The in situ grafting experiment was repeated using [Pt(COD)(CH3)2], also shown in Figure 1. The reduction rate in [Pt(COD)(CH3)2] signals in the 1H NMR spectrum was significantly slower (70 h to reach 60% reduction) than was observed for [Pt(dppe)(CH3)2]. This is consistent with reports for mononuclear Pd complexes,19 where complexes with more basic ligands undergo a more facile reaction with the silica surface. To confirm the reaction between the silica surface and [Pt(COD)(CH3)2], the headspace was sampled and analyzed by off-line GC (see the Supporting Information). The results confirmed the formation of methane and, hence, reaction of the complex with the silica surface; the molar ratio between the amount of evolved methane and the amount of platinum in the sample was found to be 0.83. This suggests that Pt(COD)/ SiO2 is composed of Pt(COD)(CH3) fragments that are η1 coordinated to the silica surface (vide infra). Both SiO2500 and the two surface complexes Pt(COD)/SiO2 and Pt(dppe)/SiO2 were analyzed by FTIR (Figure 2).
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RESULTS AND DISCUSSION The materials presented herein have been prepared by reacting dehydroxylated silica with various Pt precursors ([Pt(COD)(CH3)2] and [Pt(dppe)(CH3)2]) in CH2Cl2. The grafting reaction was typically carried out by stirring overnight, filtering, washing with CH2Cl2, and drying in vacuo. The resulting materials have been characterized using a range of techniques, the results of which are described in the following sections. Catalyst Characterization. ICP-OES analysis determined the platinum content of Pt(COD)/SiO2 to be 0.46 wt % and that of Pt(dppe)/SiO2 to be 1.66 wt %. These results are consistent with the difference in the residual silanol signals seen in the IR spectra (see Figure 2). To explain the different platinum loadings, the relative rates of the grafting reaction of the two precursor complexes were determined by solution NMR spectroscopy. A CD2Cl2 solution of [Pt(dppe)(CH3)2] or [Pt(COD)(CH3)2] (2 equiv/surface −OH) was added to a solid sample of SiO2500, and the resulting mixture was sealed in a J. Young NMR tube. Note that in this case the molar ratio between the organometallic complex and the surface −OH is higher than in the actual preparation SiO2500-supported Pt complexes (see the Experimental Section). The grafting reaction was followed by in situ 1H NMR spectroscopy over a period of 72 h at room temperature (Figure 1). In addition to signals for the [Pt(dppe)(CH3)2] complex and the residual proton signal of CDHCl2 (δ 5.31 ppm), a weak
Figure 2. Partial IR spectra of SiO2500, Pt(COD)/SiO2, and Pt(dppe)/ SiO2, showing variation in −OH stretching with loading, and the fingerprint region. 5245
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The spectrum of SiO2500 features a sharp signal at 3747 cm−1, consistent with the −OH stretching mode for isolated and geminal (up to 30%) silanol groups.32 The broad bands (not shown) at 1977, 1868, and 1647 cm−1 are consistent with overtones and combinations of Si−O−Si fundamental modes.33 The IR spectrum of Pt(COD)/SiO2 shows a significant reduction in the signal from the isolated silanol (65% of the signal in SiO2500), consistent with organometallic immobilization at these sites. The silanol signal remains due to the low Pt to −OH molar ratio (1:4) used in the synthesis of these materials. In addition, weak signals at 1485 and 1434 cm−1 consistent with a Pt(η4-COD) complex were observed. The olefinic stretches are slightly shifted from the alkene signals observed for both free COD (1487 and 1426 cm−1) and the precursor complex [Pt(COD)(CH3)2] (1477 and 1429 cm−1),34 consistent with chemical reaction of the organometallic complex with the silica support. The IR spectrum of Pt(COD)[OSi(OtBu)3]/SBA-15 reported by Tilley et al. exhibits similar alkene signals at 1475 and 1435 cm−1.23 In the IR spectrum of Pt(dppe)/SiO2 the isolated silanol signal (35% of the signal in SiO2500) is lower than that observed for Pt(COD)/SiO2, which is consistent with a higher Pt loading of Pt(dppe)/SiO2 vs Pt(COD)/SiO2 (vide infra). The three signals at 1486, 1438, and 1414 cm−1 were also slightly shifted in comparison to the precursor material (1483, 1435, and 1422 cm−1, respectively). Typically, a P−Ph group gives rise to a sharp IR band in the region of 1440 cm−1, and C−H deformations give rise to a medium signal in the region between 1470 and 1430 cm−1.35 As already observed in the case of Pt(COD)/SiO2, this shift is consistent with the hypothesis of a reaction between the complex and the silica surface. The 13C MAS NMR spectrum of Pt(COD)/SiO2 is shown in Figure 3. The resonance signals of Pt(COD)/SiO2 were
The 13C MAS NMR spectrum of Pt(dppe)/SiO2 is shown in Figure 4. Two aromatic signals at δ 128 and 132 ppm are
Figure 4. Solid-state CP/MAS 13C{1H} NMR spectrum of Pt(dppe)/ SiO2.
clearly visible (spinning sidebands at δ 62 and 200 ppm). These signals account for 87% of the signal in the spectrum, and this is close to what was expected from dppe (86%, 24 out of 28 carbons in the molecule). There is a further signal between δ 20 and 40 ppm attributable to the dppe CH2 (accounting for 8% of the total signal; 7% expected). In principle, there should be a signal accounting for ∼3% of total intensity for the methyl group (if one is present) or 0% if none are present. There is a small signal near δ 0 ppm which could indicate a small proportion of the supported complex is η1-bound to the silica surface or alternatively that there is unreacted [Pt(dppe)(CH3)2] physisorbed on the silica surface. The 31P{1H} spectrum of Pt(dppe)/SiO2 is shown in Figure 5 (gray spectrum). This is comprised of three lines (δ = 48, 35,
Figure 3. Solid-state CP/MAS 13C{1H} NMR spectrum of Pt(COD)/ SiO2.
observed by 13C{1H} CP-MAS NMR at δ 115 (CC, spinning sideband at δ 48 ppm), δ 76 (CC), δ 30 (CH2), and δ 0 ppm (CH3) (Figure 3). The two signals at δ 115 and 76 ppm are assigned to cyclooctadiene alkene resonances.36 The observation of two alkene signals indicates the presence of a nonsymmetric environment around platinum. Despite the nonsymmetric environment, only one broad cyclooctadiene alkyl signal is observed at δ 30 ppm, presumably because of coincident alkyl signals. Similar signals have been observed in PtMeCl(COD):36 δ 83.8 (CC), δ 32.0 (CH2), and δ 5.5 ppm (CH3). Together with the presence of a methyl signal at δ 0 ppm, this indicates that Pt(COD)/SiO2 is η1-bound to the silica surface and retains one methyl group out of the two in the parent material [Pt(COD)(CH3)2].
Figure 5. Solid-state 31P{1H} (gray) and 31P{1H,195Pt} (black) MAS NMR spectra of Pt(dppe)/SiO2. Peaks marked with an asterisk are spinning sidebands.
and ∼23 ppm) together with spinning sidebands to high and low frequency of these. Figure 5 also shows a spectrum with both proton and platinum decoupling (31P{1H,195Pt}, black spectrum). This spectrum clearly indicates that the signal at δ 23 ppm can be assigned to the satellite arising from 1JP−Pt coupling. Under 195Pt decoupling conditions this line is absent and the intensity of the signal at δ 35 ppm increases, consistent with 195Pt decoupling. The 1JP−Pt coupling constant is approximately 4200 Hz, as measured between the peak at δ 23 and the peak at δ 35 5246
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(multiplied by 2 for the P−Pt coupling). The large 1JP−Pt coupling constant is consistent with cleavage of the M−CH3 bond and coordination to a low-trans-influence ligand. For example, the 31P{1H} NMR spectrum of the Pt silsesquiloxane complex in Figure 6 exhibits the coupling 1JPt−P = 3773 Hz and a chemical shift of δ 27.1 ppm.37
Scheme 2. Proposed Reaction with Dehydroxylated Silica (SiO2500) of (a) [Pt(COD)(CH3)2] and (b) [Pt(dppe)(CH3)2]a
Figure 6. Example of an η2-coordinated Pt(dppe) fragment where P is trans to a M−O bond.37
Spectral deconvolution of the 31P{195Pt} spectrum enabled the relative intensities of the δ 35 and 48 ppm signals to be determined. The δ 35 ppm signal accounts for ∼69% of the intensity in the spectrum (including its associated spinning sidebands), with the δ 48 ppm signal accounting for ∼31%. As the ratio does not satisfy the molecular stoichiometry, this suggests that the two lines may not belong to different phosphorus chemical environments in the same single-site organometallic complex. It should be noted that crosspolarization experiments suffer from varying signal intensities due to magnetization transfer efficiencies and varying relaxation times. However, for phosphorus environments within the same molecule and with similar relationships to protons (as is the case in the ligand used), the expectation is equal-intensity lines for equal numbers of phosphorus atoms. The different relative intensities observed, therefore, suggest phosphorus environments in different molecular arrangements. The signal at δ 48 ppm in the 31P{1H} NMR spectrum of Pt(dppe)/SiO2 is coincident with that observed for the organometallic precursor complex [Pt(dppe)(CH3)2]. This observation suggests that unreacted [Pt(dppe)(CH3)2] may remain in the final sample. Interestingly, repeated washings with CH2Cl2 or the use of lower complex concentrations did not significantly alter the peak ratio. The signals in the 31P{1H} NMR spectrum at δ 35 and 23 ppm are consistent with a new species with the general formula [Pt(dppe)(X)2], where X is a weak-trans-influence ligand that results in a large 1JPt−P = 4200 Hz coupling constant. The possibility of this signal resulting from the species [Pt(dppe)(X)(Y)] (with coincident 31P environments) appears less likely, due to the relatively high intensity observed for the δ 23 ppm signal (16% of the entire Pt signal). Given that the natural abundance of 195Pt is ∼34%, a complex with the general formula [Pt(dppe)(X)2] would be expected to give rise to two platinum satellites with relative intensities of 17%, while [Pt(dppe)(X)(Y)] would result in four smaller satellite signals, accounting for ∼8% each. In conclusion, the data presented here support the identification of Pt(COD)/SiO2 as composed of Pt(COD)(CH3) fragments that are η1-coordinated to the silica surface (Scheme 2a). This assertion apparently contradicts the recent findings by Basset,36 who did not observe any reaction between dehydroxylated silica and [Pt(COD)(CH3)2]. However, this discrepancy could be the result of the different ratios of reagents used. Also, differences in the thermal treatments
a
In the case of [Pt(dppe)(CH3)2], available data cannot definitively exclude the possibility that a small proportion of η1-coordinated material is present on the silica surface.
employed to dehydroxylate silica affect its reactivity toward organometallics, as observed by the same author in a previous paper.38 In the case of Pt(dppe)/SiO2, an interpretation of the data indicates that it is predominantly composed of a mixture of Pt(dppe) fragments that are η2-coordinated to the silica surface (Scheme 2b) and unreacted [Pt(dppe)(CH3)2]. However, the possibility that a small proportion of η1-coordinated material is present on the silica surface cannot be dismissed. Note that η2 coordination requires a pair of hydroxyl groups. Even though the IR spectrum (see Figure 2) shows no or little evidence for H bonding, there is a growing body of evidence that the hydroxyls on partially dehydroxylated silicas are actually not isolated.39 EXAFS Analysis. EXAFS analysis of the single-site complexes was carried out to confirm changes in the coordination environment of the platinum atom upon reaction with the dehydroxylated silica support. To validate the accuracy of reference-compound-generated phase shift and backscattering amplitude for Pt−P, Pt−C, and Pt−O scattering, unsupported catalyst precursors [Pt(dppe)(CH3)2] and [Pt(COD)(CH3)2] were examined also. Table 1 summarizes the fitting results of the EXAFS spectra. For [Pt(dppe)(CH3)2] (entry 1 and Figure S3 (Supporting Information)), the coordination numbers of C and P were fixed to be 2.0 and 2.0, respectively. The fit average bond distances of 2.21 and 2.14 Å for Pt−P and Pt−C, respectively, match closely with published solid-state crystal structure data (2.25 Å for Pt−P and 2.11 Å for Pt−CH3).24 The model compound [Pt(COD)(CH3)2] with three distinct Pt−C scattering shells was also investigated. The EXAFS spectrum of [Pt(COD)(CH3)2] can be fit using model parameters of 2.0, 4.0, and 4.0 C atoms at approximately 2.05, 2.22, and 3.08 Å distances from the Pt atom, respectively (Table 1, entry 3 and Figure S4 (Supporting 5247
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Figure S6 (Supporting Information) shows the Fourierfiltered (1.2−2.0 Å) EXAFS spectrum of Pt(COD)/SiO2 and its curve fit. The Pt−CH2(COD) scattering shell was omitted, since it is at a bond distance similar to that of Pt−Si scattering, which is approximately 3.34 Å, as observed in the model compound Pt(COD)[OSi(OtBu)3]2. EXAFS fitting results suggested that Pt(COD)/SiO2 can be described with a model of 2.4 C(O) and 3.7 C atoms at distances of 2.05 and 2.16 Å away from the Pt center, respectively. In comparison to [Pt(COD)(CH3)2], the much shorter Pt−CH2(COD) bond distance is in excellent agreement with that determined in Pt(COD)[OSi(OtBu)3]2 (2.12 and 2.16 Å),23 suggesting that one or possibly two Pt−CH3 bonds were protonated by surface silanol groups. Formation of Pt Nanoparticles. Both Pt(COD)/SiO2 and Pt(dppe)/SiO2 were then calcined to 500 °C in air following the method described in the Experimental Section. Even though treatment under H2 would typically be used, the much simpler procedure of calcining in air gave unexpectedly good results. Transmission electron microscopy (TEM) images of Pt(COD)/SiO2 showed no platinum particles, suggesting that decomposition and agglomeration of the precursor complex to platinum nanoparticles (>2 nm) had not occurred during the grafting reaction (Figure 7). TEM analysis of the same material after calcination at 500 °C for 2 h in air confirmed the formation of platinum nanoparticles (8−12 nm). The composition of the nanoparticles was confirmed by energy dispersive X-ray (EDX) analysis as platinum. Both Pt(COD)/SiO2 and Pt(dppe)/SiO2 were subjected to the same calcination procedure and the resulting samples analyzed by TEM (Figure 8). It is evident that, although Pt(COD)/SiO2 has a lower platinum loading, calcination results in larger platinum particles (9 ± 3 nm). In contrast, despite having a much higher platinum loading, the calcination of Pt(dppe)/SiO2 results in much smaller particles (4 ± 2 nm). This effect is attributed to the strength of the interaction of the metal complex with the surface. The more basic dppe ligand results in a stronger Pt−O bond and hence a less mobile platinum center. For comparison, a commercially available 1 wt % Pt/SiO2 catalyst (Figure 9) contains much larger particles and irregular agglomerates of particles. Hydrogenation of Toluene. The catalytic activity of the new single-site Pt catalysts and nanoparticles was evaluated for the hydrogenation of toluene. Hydrogenation reactions were carried out using samples of Pt(COD)/SiO2 and Pt(dppe)/
Table 1. EXAFS Fitting Results for Unsupported and Supported Pt Complexes entry
complex
1
Pt(dppe) (CH3)2
2
Pt(dppe)/ SiO2
3
Pt(COD) (CH3)2
4
Pt(COD)/ SiO2
CN (±20%)
R (±0.02 Å)
σ2 (Å2 × 103)
Pt−C
2.0
2.14
8.9
6.1
Pt−P Pt−C(O)
2.0 1.8
2.21 2.04
1.0 10.0
−4.5 9.3
Pt−P Pt−C
2.0 2.0
2.18 2.05
4.2 3.7
−11.2 4.7
Pt−C Pt−C Pt−C(O)
4.0 4.0 2.4
2.22 3.08 2.05
4.2 13.3 1.2
0.3 0.9 3.1
Pt−C
3.7
2.16
8.0
−2.6
scatter
ΔE0 (eV)
Information)). These values are in close agreement with the single-crystal diffraction data for [Pt(COD)(CH3)2] (2.06 Å for Pt−CH3 and 2.22−2.25 Å for Pt−CH(COD))26 and [Pt(COD)(PhCH3)2] (2.26−2.28 Å for Pt−CH(COD) and 3.03− 3.15 Å for Pt−CH2(COD)).40 Due to the large variation in the Pt−CH2(COD) bond distances, the σ2 parameter of 0.013 Å2 is substantially larger than those of the other two scattering shells. The close agreement between our data and the published data lends confidence to our method of EXAFS data analysis. Figure S5 (Supporting Information) shows the EXAFS spectrum of Pt(dppe)/SiO2 after Fourier transformation and its curve fit. To separate out the contribution from Pt−Si scattering, the spectrum was filtered from 1.4 to 2.2 Å. In comparison to the precursor Pt(dppe)(CH3)2, there are still 1.8 C or O atoms and 2.0 P atoms around Pt. However, the Pt− C(O) and Pt−P bond distances have decreased from 2.14 to 2.04 Å and from 2.21 to 2.18 Å, respectively (entry 1 vs entry 2). The significantly shortened Pt−C(O) distance is believed to be due to the formation of a covalent Pt−O−Si bond, which has a distance of ∼2.01 Å in the model compound Pt(COD)[OSi(OtBu)3]2.23 It was also observed that there is an increase in the σ2 parameter for both Pt−C and Pt−P scattering, indicating a greater disorder arising from the inhomogeneity of the SiO2 environment around Pt centers. The smaller than usual E0 value of −11.2 eV for Pt−P scattering might originate from the fact that Pt−Cl scattering phase and amplitude were used to fit Pt−P scattering.
Figure 7. TEM images of Pt(COD)/SiO2 both before (left) and after (right) calcination for 2 h at 500 °C in air. 5248
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Figure 8. TEM images of Pt(COD)/SiO2 (left) and Pt(dppe)/SiO2 (right) after calcination at 500 °C in air for 2 h.
Figure 9. TEM images of a commercially available 1 wt % Pt/SiO2 catalyst.
ligand making the supported Pt complex harder to reduce under hydrogen.41 In contrast, the hydrogenation activity of Pt(COD)/SiO2 could be due to the rapid hydrogenation of the COD ligand under reaction conditions with the concurrent formation of Pt(0), similar to that observed using supported bis(allyl)iridium complexes.30 The hydrogenation of toluene with Pt(dppe)/SiO2 after calcination resulted in more than 99% conversion of toluene with 100% selectivity to methylcyclohexane. The hydrogenation of toluene with Pt(COD)/SiO2 resulted in 60% conversion of toluene with 100% selectivity to methylcyclohexane. Both Pt(COD)/SiO2 and Pt(dppe)/SiO2 after calcination thus show
SiO2, both synthesized with a Pt loading close to 1 wt %, and a commercial 1 wt % Pt/SiO2 catalyst was used for comparison; the results are shown in Table 2. The hydrogenation of toluene over the 1 wt % Pt/SiO2 catalyst resulted in 26% conversion of toluene with 100% selectivity to methylcyclohexane (liquid sample). In comparison, under the same conditions the hydrogenation of toluene over Pt(COD)/SiO2 resulted in 7% conversion of toluene with 100% selectivity to methylcyclohexane (in the liquid sample). In the case of Pt(dppe)/SiO2, no conversion of toluene was observed under the reaction conditions tested. We suggest that the lack of hydrogenation activity of Pt(dppe)/SiO2 is likely due to the electron-rich dppe 5249
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Table 2. Hydrogenation of Toluene over Different PlatinumSupported Silica Catalystsa cat. Pt/SiO2 Pt(COD)/SiO2 Pt(dppe)/SiO2 Pt(COD)/SiO2, after calcination Pt(dppe)/SiO2, after calcination
Pt loading (wt %)
toluene conversn (%)
methylcyclohexane selectivity (%)
dispersion (%)
1 0.95 0.83 0.95
26 7 0 60
100 100 0 100
7.5
10.2
0.83
>99
100
37.3
ASSOCIATED CONTENT
S Supporting Information *
Text, a table, and figures giving details on the head space methane quantification and EXAFS curve fits. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Mike Dixon for the TEM images and Dr. David Apperley at the National Solid State NMR service at Durham University for the MAS NMR spectra. We also thank Dr. Darren Cook, Prof. Vernon Gibson, Dr. John Shabaker, and Dr. Glenn Sunley for their help in reviewing the paper. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.
Experimental conditions: T = 150 °C, t = 3 h, GHSV(H2) = 1440 h−1, LHSV(toluene) = 1 h−1.
a
higher activity than the commercial catalyst. This is consistent with the higher dispersion and lower particle size of Pt(dppe)/ SiO2 versus Pt(COD)/SiO2 and of both materials versus the commercial catalyst.
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CONCLUSIONS
REFERENCES
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Addition of the platinum(II) organometallic precursors [Pt(COD)(CH3)2] and [Pt(dppe)(CH3)2] to dehydroxylated silica results in a reaction at the silica surface to give novel materials with Pt loadings ranging from 0.46 to 1.66 wt %, depending on the ligand type, ligand concentration, and reaction time. The [Pt(dppe)(CH3)2] precursor affords a significantly higher loading than [Pt(COD)(CH3)2] under identical reaction conditions. This observation is ascribed to a kinetic effect, since increasing the reaction time of [Pt(COD)(CH3)2] with the dehydroxylated silica was shown to increase the platinum loading. The available data support the characterization of Pt(COD)/ SiO2 and Pt(dppe)/SiO2 as novel surface-bound platinum complexes. Solid-state 31P NMR suggests that Pt(dppe)/SiO2 may be a mixture of the precursor complex [Pt(dppe)(CH3)2] and a new surface-bound platinum complex. On the basis of EXAFS fitting it is shown that the single-site complexes retain coordination characteristics similar to those of the starting materials. Careful thermal decomposition of Pt(COD)/SiO2 and Pt(dppe)/SiO2 shows that a high degree of control can be obtained over particle size and size distribution of the resulting platinum nanoparticles. Pt(dppe)/SiO2 contains platinum nanoparticles with higher dispersion and lower particle size compared to Pt(COD)/SiO2. Pt(COD)/SiO2 is capable of catalytically hydrogenating toluene at 150 °C, while under the same conditions Pt(dppe)/ SiO2 shows no reactivity. When the supported platinum nanoparticles are tested, a significant increase in the reactivity is observed. Pt(COD)/SiO2 and Pt(dppe)/SiO2 after calcination have higher hydrogenation activity than a commercial Pt/ SiO2 catalyst of comparable platinum loading. Pt(dppe)/SiO2 is more active than Pt(COD)/SiO2 because of the higher dispersion and lower particle size of the supported platinum nanoparticles. 5250
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