Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Discrete Polyoxopalladates as Molecular Precursors for Supported Palladium Metal Nanoparticles as Hydrogenation Catalysts Wassim W. Ayass,† Juan F. Miñambres,†,∥ Peng Yang,†,⊥ Tian Ma,† Zhengguo Lin,†,△ Randall Meyer,§ Helge Jaensch,‡ Anton-Jan Bons,‡ and Ulrich Kortz*,† †
Department of Life Sciences and Chemistry, Jacobs University, Campus Ring 1, 28759 Bremen, Germany Corporate Strategic Research, ExxonMobil Research and Engineering, Annandale, New Jersey 08801, United States ‡ Global Chemical Research, ExxonMobil Chemical Europe Inc., 1831 Machelen, Belgium Downloaded via CLARKSON UNIV on April 6, 2019 at 03:10:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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S Supporting Information *
ABSTRACT: We have used discrete polyoxopalladates(II) (POPs) of the MPd12X8 nanocube- and Pd15X10 nanostar-types (M = central metal ion, X = capping group) as molecular precursors (diameter ca. 1 nm) for the formation of supported (SBA-15) metallic nanoparticles. These materials proved to be highly active in the hydrogenation of o-xylene. The characterization of such hydrogenation catalysts revealed that the average size of the resulting alloy particles is quite uniform with diameters ranging from 1 to 3 nm (indicating little to no agglomeration). The central transition-metal ion Mn+ (MnII, FeIII, CoII, NiII, CuII, ZnII, PdII) in the POP structure and also the nature of the capping group (AsO43−, SeO32−, PO43−, phenyl-AsO32−) influence the resulting catalytic performance.
10 years, >70 derivatives have been reported. This young field of research has been reviewed twice in recent years.4 In the very first reported POP, [PdII13O8(AsO4)8H6]8− (Pd13As8), the 13 PdII ions exhibit square-planar coordination geometry and form a distorted cuboid structure with eight external AsO43− capping groups.3 In 2009, it was demonstrated that the arsenate capping groups in Pd13As8 can be replaced by selenite or phenylarsonate, resulting in [Pd II 13 O 8 (O 3 Se IV ) 8 ] 6− (Pd 13 Se 8 , with 6-coordinated central Pd II ion) and [PdII13O8(O3AsPh)8]6− (Pd13(AsPh)8, with 8-coordinated central PdII ion), respectively.5 In the following years, a procedure was developed allowing to replace the central PdII ion in the Pd13L8 (L = capping group) nanocube by lanthanide ions,6a and also by transition-metal ions,6b−e and eventually even by alkaline earths metal ions.6f In 2010, the first polyoxoaurate(III) was reported,6g and in 2012 the first mixed palladium-gold polyoxo-noble-metalate.6h In 2016, the AgI-capped palladate cube was reported.6i This shows that to date the class of cuboid POPs is large, with different capping groups, central metal ion guests, and d8 metal addenda. The first POP with a structure different from the nanocube was the phosphate-capped Pd15 nanostar, [Pd0.4Na0.6⊂Pd15O40(PO)10H6.6]12− (Pd15.4P10), which is a co-crystallized, nonseparable mixture of [NaPd15P10O50]19− (Pd15P10) and [PdPd15P10O50]18− (Pd16P10) in a ratio of 3:2. 7a The selenite-capped analogue [Pd 15 Se 10 O 40 ] 10−
1. INTRODUCTION Key challenges in designing the “perfect” heterogeneous metal catalyst for a specific reaction include metal utilization (dispersion) and control of selectivity through tuning of both the physical and electronic structures of metal clusters and support. Therefore, there is much scientific interest in investigating the effect of composition and cluster size on the activity and selectivity of metal catalysts.1a,b The reactivity of a metal catalyst can sometimes be changed by varying only one single atom. As an example, this was seen for icosahedral Au and Pt clusters with dopants where the conversion of styrene oxidation increased from 59 to 91% by just replacing one gold atom with platinum.1c Another important factor is the exact metal surface structure, which has motivated researchers to investigate varying/developing catalyst synthetic methods, thus influencing the structure of the catalyst to form core−shell structures, homogeneously alloyed structures (solid solutions), porous structures, or other unique structures which may give rise to specific reactivity.2 Finally, the post-synthetic treatment (activation) will depend on the nature and composition of the materials used and will further significantly influence the catalytic performance. From this knowledge of design principles, it can be expected that the ability to broadly vary polyoxometalate (POM) composition and to prepare supported metal clusters with well-defined stoichiometry and size may allow to prepare closely related systems and to study their activity and/or selectivity. POMs comprising exclusively palladium(II) addenda (polyoxopalladates, POPs) were discovered in 2008,3 and in the last © XXXX American Chemical Society
Received: December 17, 2018
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DOI: 10.1021/acs.inorgchem.8b03513 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
k2, and k3-weighted Fourier transform for the fits. See Supporting Information for more information. 2.4. Catalytic Hydrogenation. A model substrate should provide useful information about the catalytic system in terms of reactivity, stereochemistry, isomerization, possible transalkylation, cracking propensity, etc. Therefore, o-xylene hydrogenation to cis/trans-1,2dimethylcyclohexane (1,2-DMCH) was selected as the reaction model to investigate and rationalize the key features (shape, size, composition) of POPs as noble-metal-based heterogeneous hydrogenation precatalysts. The reaction was carried out in a 100 mL stainless steel, high-pressure Parr Compact reactor equipped with a magnetically coupled stirrer drive, ensuring a well-mixed environment of reactants (Figure S2). The reaction mixture containing 3.5 mL of oxylene (0.029 mmol, 0.569 M) in 47.5 mL hexane and 50 mg of activated supported POP (5 wt %) was stirred at 950 rpm at 180 °C. The absence of diffusion limitations was confirmed by the fact that a similar reaction rate was observed when stirring at 1500 rpm at 180 °C. The same reaction at 300 °C was found to be diffusion-controlled (Figure S3). The autoclave was purged with H2 and then heated and pressurized to the desired temperature set point (180 or 300 °C) and pressure (80 or 90 bar), respectively. After calcination, the precatalyst was reduced in situ in the reactor. For the reaction at 180 °C or (300 °C), 50 bar of H2 was introduced in the reactor at 140 °C or (250 °C), and then the mixture was stirred for 1 min to reduce the catalyst. This allows for a temperature increase, since this process is exothermic. Once the temperature set point is reached, the needed pressure is introduced, and the reaction starts once the stirring is turned on. Milder reaction conditions (i.e., stoichiometric amounts of H2 around 40 bar) resulting in long reaction times could have been used,9 but a higher pressure was preferred in order to push the reaction forward according to Le Châtelier’s principle. In order to ensure catalyst recyclability, a new portion of substrate (3.5 mL) was added into the reactor following all catalytic runs (i.e., running more than one cycle). The blank reaction on the SBA-15-apts support alone at both temperatures did not show any hydrogenation activity. The reaction performed in the compact reactor was followed by H2 consumption and gas chromatography (GC) analysis (Figures S4− S5). A Shimadzu GC-2010 equipped with a flame ionization detector (FID) was used to measure substrate conversion and selectivity of the obtained products via a HP-5 column (15 m × 0.25 mm, I.D. 0.25 μm), providing a good separation of the reaction products. The carrier gas was He. This overall procedure ensured good reproducibility of the catalytic experiments. Hot filtration experiments indicated that the reaction is heterogeneous (Figure S6).10
(Pd15Se10) was later synthesized by Predieri’s and Hu’s groups, respectively.6d,7b A barium(II)-centered POP nanostar6f as well as a silver(I)-centered and capped POP nanostar are also known.6i All of the above-mentioned discrete POPs could potentially be considered as molecular precursors for the formation of monodisperse noble-metal nanoparticle catalysts. Metal catalysts displaying activity in the metallic state (e.g., transition metals in groups VIIIB and IB) are useful for chemical transformations such as hydrogenations, dehydrogenations, and naphtha reforming.8 Heterogeneously catalyzed hydrogenations are common in industry. There are a few reports on using POPs as homogeneous catalysts.3,6b However, until very recently, no heterogeneous catalysts based on POPs have been reported.6j Hence, we decided to investigate the immobilization of POPs on a mesoporous support, followed by chemical reduction leading to discrete metal nanoparticles. These prepared heterogeneous catalysts were then used for the hydrogenation of organic substrates.
2. EXPERIMENTAL SECTION 2.1. Materials. All reagents and chemicals were of high-purity grade and were used as purchased from Sigma-Aldrich without further purification. The commercial palladium 5 wt % (dry basis) on wet activated carbon support Pd/C was used as is (product number: 330116). 2.2. Synthesis of Supported POP Precatalysts on SBA-15apts. All POPs were prepared following published procedures (see Supporting Information). The respective POP was dissolved in 100 mL of water, resulting in a colored solution. While stirring, the mesoporous support (SBA-15) modified by 3-aminopropyltriethoxysilane (apts) was slowly added to the POP solution so that the respective amounts were 95 and 5 wt %. The pH of the POP solution was ca. 6, which is sufficient to protonate the amino group in SBA-15apts (see Figure S1). The exact quantities depend on the desired amount of supported catalyst to be prepared. For example, to a solution of 25 mg of POP dissolved in 100 mL water, 475 mg of SBA15-apts was added to yield ca. 500 mg of POP@SBA-15-apts. The mixture was stirred for 24 h at 40 °C and then filtered and washed three times with water. The filtrate was colorless, indicating that the POP was quantitatively loaded onto the SBA-15-apts support. The product was air-dried and then collected (yield: 95−100%). Elemental analysis confirmed the expected percentage of POP loading. See Supporting Information for details regarding the synthesis procedures used for SBA-15-apts support and POPs. 2.3. Characterization. The surface area of the supported catalysts was determined with a gas adsorption analyzer for surface area measurements. Nitrogen physisorption isotherms were measured at 77 K using a Quantachrome AUTOSORB 1 apparatus, and this was used to determine the Brunauer−Emmett−Teller (BET) surface area. The samples were degassed at 70 °C for 24 h. The specific surface areas were evaluated with the BET method in the P/P0 range of 0.05− 0.35. Elemental analyses of the prepared supported POPs were performed at ExxonMobil Chemical Europe Inc., Belgium. A thermal analysis instrument SDT Q600 was used to determine the thermal stability of the materials under N2 flow (100 mL/min) up to 800 °C. Transmission electron microscopy (TEM) images were collected on an FEI Tecnai G2 F20ST TEM/STEM operated at 200 kV. Images were obtained in scanning (STEM) mode using a high-angle annular dark-field (HAADF) detector. Energy dispersive X-ray spectroscopy (TEM-EDS) spectra and hyperspectral element maps were acquired using an EDAX Inc. detector. X-ray absorption spectroscopy (XAS) measurements were performed at Argonne National Laboratory at the 20 BM and 9 BM beamlines. In all experiments, a metal foil was used for calibration. The Demeter program was used to fit the data to obtain coordination numbers and bond distances using a combined k,
3. RESULTS AND DISCUSSION The structures of the POP nanocubes Pd13As8 and MPd12Se8 as well as the nanostar Pd15P10 are shown in Figure 1. In the course of this work, we first synthesized several POP nanocubes with various capping groups and central metal ion guests, MPd12X8 (M = MnII, FeIII, CoII, NiII, CuII, ZnII, PdII; X = Se, P, As, AsPh) as well as the two POP nanostars Pd15X10 (X = Se, P), and also SBA-15-apts, in all cases following published procedures (for details see Experimental Section and
Figure 1. Ball-and-stick representation of the POP nanocubes Pd13As8 (left) and MPd12Se8 (middle) and the nanostar Pd15P10 (right). Color code: Pd (blue), As (light green), O (red), central metal guest M (dark green), Se (orange), and P (pink). B
DOI: 10.1021/acs.inorgchem.8b03513 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
in situ at 250 °C (similar to the reaction scenario at 300 °C), followed by a reaction temperature of 180 °C. Reduction of the central metal atom (Cu, Ni) was somewhat retarded compared to reduction of Pd (particularly in the case of Ni, see Figure S8). Reducing the catalyst at a higher temperature improved the reaction time at 180 °C by 26%, hinting at the formation of a more active catalyst structure at a higher reduction temperature, thus also explaining the higher reactivity at 300 °C. The same experiment was also performed for CuPd12Se8 and an improvement of 31% was observed (Figure S9). X-ray absorption spectroscopy (XAS) was used to characterize the structure of such more active catalysts (vide infra). In order to find out if the chemical nature of the capping group on the POP has an influence on the resulting catalytic activity, we decided to study this phenomenon in some detail. 3.3. Role of Capping Groups for Supported MPd12X8 Nanocubes. 3.3.1. Pd13X8 and MnPd12X8. The ease of removal of capping groups and the effect of the remaining capping group elements on the catalytic activity were investigated for the Pd13X8 and MnPd12X8 POP nanocubes. Table 1 shows the optimal catalytic activities achieved using
Supporting Information). Then we immobilized the POPs on the support (at ca. 5 wt %) and tested their activity as catalysts for the hydrogenation of o-xylene. We discovered that appropriate pretreatments were extremely important (see Supporting Information) and also that the nature and composition of the capping groups as well as the central metal ion guest are essential. We used extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopic techniques to investigate the structure−activity relationships for this class of catalysts and to characterize the resulting Pd metallic clusters. 3.1. Immobilization of POPs on SBA-15-apts. Elemental analysis of the supported POPs on SBA-15-apts confirmed a loading of 4.9 ± 1 wt %. Elemental analysis calculated (found): Pd, 2.27 (2.23 wt %). The as-synthesized materials showed no catalytic activity, and we revealed that a thermal pretreatment or a calcination step was required before observing any catalytic activity. 3.2. Catalytic Activity of Supported MPd12Se8 Nanocubes (M = MnII, FeIII, CoII, NiII, CuII, ZnII, PdII). Initially, we decided to investigate the catalytic activity (o-xylene hydrogenation) of the selenite-capped nanocubes MPd12Se8 (M = MnII, FeIII, CoII, NiII, CuII, ZnII, and PdII) on the support (5 wt %), abbreviated as MPd12Se8-SBA-15-apts. All precatalysts were air-calcined at 550 °C for 4.5 h prior to their in situ reduction (XANES data verifying the reduction is shown in Figure S7). Figure 2 shows the catalytic results at two different
Table 1. o-Xylene Hydrogenation Results at 300 °C, 90 bar, 1500 rpm Using Activated POP Nanocube Catalysts (Loading 5 wt %) Pd13X8 (X = Se, PhAs, As) and MnPd12X8 (X = Se, PhAs, P) supported catalyst Pd13(PhAs)8 Pd13As8 Pd13Se8 MnPd12(PhAs)8 MnPd12Se8 MnPd12P8
treatment methoda
conv. (%)
time (min)
Sc/t
1 2 3+2 1 3+1 1 4 1
36 97 ∼100 ∼100 63 98 ∼100 ∼100
2700 1072b 960 50 1000 32 32 38
40/60 38/62 40/60 43/57 40/60 42/58 43/57 43/57
Method 1: Air calcination at 550 °C, 4.5 h. Method 2: Air calcination at 650 °C, 4.5 h. Method 3: Hydrazine chemical reduction (see Supporting Information). Method 4: Air calcination at 300 °C, 3 h. Error bar ±2 min. bThis reaction was performed at 120 bar. a
Figure 2. Reaction times for o-xylene hydrogenation at 180 °C, 80 bar (error bar ±20 min) and 300 °C, 90 bar (error bar ±2 min) using MPd12Se8-SBA-15-apts (5 wt %) calcined at 550 °C for 4.5 h. The average cis/trans selectivity ratio (Sc/t) of 1,2-DMCH is 45/55 at 180 °C and 43/57 at 300 °C (see also Table S1, which summarizes measured conversions and cis/trans selectivity ratios).
these POPs with arsenate, phenyl-arsonate, and phosphate capping groups activated under different conditions. The selenite- and phosphate-capped POPs showed the fastest reaction rates and exhibited even the same rate for MnPd12X8. Replacing the central PdII ion by MnII in the case of Pd13Se8 resulted in an almost 50% improvement of activity (reduction of reaction time) at 300 °C, which highlights the important role of the 3d central metal ion guest. The phenyl-arsonatecontaining Pd13(AsPh)8 and MnPd12(AsPh)8 showed a slow reaction rate compared to the selenite/phosphate-capped POP analogues, with MnPd12(PhAs)8 (63% conversion after 1000 min) performing better than Pd13(PhAs)8 (36% conversion after 2700 min). The arsenate-capped Pd13As8 achieved complete conversion, but only after ∼1000 min. It turns out that the selenite and phosphate capping groups can essentially be eliminated completely (>99%), unlike arsenate. Elemental analysis of MnPd12P8 calcined at 300 °C showed a P content decrease by 50% (elemental analysis found: P, 0.22 wt %). However, washing the sample further with water showed no remaining P (
