Platinum Group Metal Phosphides as Heterogeneous Catalysts for the

Research Article pubs.acs.org/acscatalysis

Platinum Group Metal Phosphides as Heterogeneous Catalysts for the Gas-Phase Hydroformylation of Small Olefins Luis Alvarado Rupflin,† Jaroslaw Mormul,*,† Michael Lejkowski,† Sven Titlbach,† Rainer Papp,‡ Roger Glas̈ er,§ Maria Dimitrakopoulou,∥ Xing Huang,∥ Annette Trunschke,∥ Marc Georg Willinger,∥ Robert Schlögl,∥,⊥ Frank Rosowski,‡,⊥ and Stephan A. Schunk*,† †

hte GmbH, 69123 Heidelberg, Germany BASF SE, 67056 Ludwigshafen, Germany § University Leipzig, 04103 Leipzig, Germany ∥ Department of Inorganic Chemistry, Fritz-Haber-Institute of the Max-Planck-Gesellschaft, 14195 Berlin, Germany ⊥ BasCat, UniCat BASF Jointlab, Technical University Berlin, 10623 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: A method for the synthesis of highly crystalline Rh2P nanoparticles on SiO2 support materials and their use as truly heterogeneous single-site catalysts for the hydroformylation of ethylene and propylene is presented. The supported Rh2P nanoparticles were investigated by transmission electron microscopy and by infrared analysis of adsorbed CO. The influence of feed gas composition and reaction temperature on the activity and selectivity in the hydroformylation reaction was evaluated by using high throughput experimentation as an enabling element; core findings were that beneficial effects on the selectivity were observed at high CO partial pressures and after addition of water to the feed gas. The analytical and performance data of the materials gave evidence that high temperature reduction leading to highly crystalline Rh2P nanoparticles is key to achieving active, selective, and longterm stable catalysts. KEYWORDS: hydroformylation, heterogeneous, rhodium, phosphide, nanoparticles, ethylene

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INTRODUCTION Since its discovery in 1938 by Otto Roelen, homogeneously catalyzed hydroformylation of olefins has become one of the most important large-scale industrial processes.1,2 Like in the early years of development, today cobalt carbonyl complexes are still used as catalysts for mid- and long-chain olefins, whereas for the short-chain olefins like ethylene and propylene, cobalt catalysts were quickly replaced by inherently more active and selective rhodium complexes modified with phosphine or phosphite ligands.3−5 A challenge accompanied by using the distinctly more expensive noble metal in a homogeneously catalyzed process is an effective separation of the catalyst from the reaction products. This explains why rhodium is only used for the hydroformylation of low-boiling compounds where an economic distillative separation is possible at all, or alternatively, the process is run biphasic keeping the catalyst in one phase. However, great care has to be taken to minimize rhodium loss even in the ppm range. A technical solution involving a heterogeneously catalyzed hydroformylation reaction with the catalyst in the solid phase and the products in the gas phase could offer interesting alternatives that could also lead to new solutions with regard to process design. In the last few decades, a number of research activities in academia and © 2017 American Chemical Society

industry have focused on the conceptual development of heterogeneous or heterogenized hydroformylation catalysts. Among other rhodium complexes, pure or ligand-modified rhodium nanoparticles immobilized on inorganic support materials were tested as catalysts.6−9 Aside from compromises with regard to lower turnover frequencies (TOFs) observed compared to homogeneous catalyst analogues, it could also be shown that the reaction is likely catalyzed by molecular rhodium species being present in pseudoliquid films on the catalyst surface.10−12 A topic that has attracted a lot of attention is the supported ionic liquid phase (SILP) process where the catalyst is dissolved in an ionic liquid that is supported on a specific support material.13−20 Compared to other heterogenized catalysts, the reported TOF values are initially high but drop over time. The reason for this decline in activity is due to the formation of condensation products that accumulate in the ionic liquid and poison the catalyst. Such condensation products can be removed from the SILP, followed by improved activity, but the regeneration is technically demanding. Received: February 14, 2017 Revised: April 7, 2017 Published: April 10, 2017 3584

DOI: 10.1021/acscatal.7b00499 ACS Catal. 2017, 7, 3584−3590

Research Article

ACS Catalysis Platinum group metal phosphides are known to be useful heterogeneous catalysts and have been applied for years in academia, especially for hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodeoxygenation (HDO) reactions.21−28 Rh2P crystallizes in a fluorite-like structure with bulk Rh atoms surrounded by four phosphorus atoms and surface Rh atoms surrounded by two coordinating phosphorus atoms. Looking at the surface of crystallographic low index planes of platinum metal phosphides, especially Rh2P, the environment of the metal atoms at the surface of the solid can be viewed as sterically similar to the metal phosphorus interaction in bisphosphine- and bisphosphite-modified rhodium complexes, which are used as efficient homogeneous catalysts for hydrogenation and especially for hydroformylation reactions (Figure 1) and is in contrast to the situation in rhodium metal.

Figure 2. X-ray diffraction patterns of Rh/P samples reduced at 250, 500, 750, and 900 °C. The sample without phosphoric acid (Rh140) shows the reflexes of metallic Rh nanoparticles. The formation of Rh2P is clearly favored at higher reduction temperature (500−900 °C).

P250−Rh/P900) show that an increase in reduction temperature favors the formation of nanoparticles of rhodium phosphide (Rh2P); in the diffraction patterns, no clear evidence for metallic rhodium nanoparticles can be found. Samples of the supported rhodium phosphide materials were analyzed by transmission electron microscopy (TEM, Figure 3). For sample Rh/P250, a clear assignment of the diffraction pattern is difficult: TEM/EDX analysis of the material, which was reduced at 250 °C, shows finely dispersed nanoparticles consisting of both rhodium and phosphorus on the SiO2 support with an average diameter between 2 and 5 nm, and no evidence for the presence of metallic rhodium nanoparticles was found; we therefore interpret these nanoparticles as precursors of Rh2P with adequate stoichiometry, which at higher temperatures topo-tactically converts into more crystalline Rh2P. By increasing the reduction temperature, an increase of the particle diameter to up to 12 nm was observed. HRTEM analysis of the sample reduced at 900 °C revealed Rh2P nanoparticles of high crystallinity. The labeled lattice fringes with a d-spacing of 2.75 Å fit well to the (002) planes of Rh2P with cubic structure. The samples reduced at 500 and 900 °C were also analyzed by IR absorption after CO exposure (Figure 4). Carbon monoxide adsorption on rhodium phosphide has not been investigated to date by infrared spectroscopy. FTIR studies of nickel and molybdenum phosphides revealed that CO adsorption on P-terminated surfaces is very weak.29−31 A band at ∼2000 cm−1 was assigned to PCO complexes.30,31 Such a surface species was not observed in the present study. The spectra of CO adsorbed at 300 K on alumina-supported molybdenum phosphide show a band at 2037 cm−1 that is very similar to CO adsorption bands on noble metals. The peak is attributed to CO linearly adsorbed on Mo atoms of the MoP surface.29 On silica-supported nickel phosphide Ni2P, formation of Ni(CO)4 is observed (νCO = 2050−60 cm−1). Furthermore, bands due to C−O stretching vibrations at 2083−2093 cm−1 and around 1900 cm−1 are assigned to terminally and bridged bonded CO on Niδ+ sites, respectively.30,31 The spectrum of the low-temperature sample (Rh/P500) revealed different CO coordination modes. The bands at 2073 and 1912 cm−1 due to terminal (Rh-CO) and bridged bonded (Rh2CO) CO, respectively, are typical for carbon monoxide adsorbed on metallic rhodium nanoparticles, whereas doublet at 2107 and

Figure 1. Comparison of the structures of metallic rhodium (A, fcc) and Rh2P (B, antifluorite). The phosphorus atoms increase the distance between two rhodium atoms from 0.268 nm (metallic rhodium) to 0.275 nm (Rh2P, 001 plane).

Rhodium is not zero-valent in bulk Rh2P; therefore, it is possible that, through the interaction between phosphorus and rhodium at the surface of Rh2P, non-zero-valent rhodium species are present. Additionally, it can be assumed that the environment for rhodium in surface sites on Rh2P may be created that, from an electronic point of view, have similarity to the environment in molecular complexes of rhodium with organic phosphine ligands. The assumption made here is that, on a molecular level, a surface structure can be present, which may unite the above-mentioned features and has similarity with a situation related to the structural picture sketched in Figure 1. Although phosphides show high catalytic activity in different reactions involving hydrogen activation, they have never been considered as candidates for the hydroformylation of lower olefins. Herein, we want to show the usefulness of supported rhodium phosphide catalysts for the hydroformylation of ethylene and propylene as highly active, selective, and durable catalysts.

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RESULTS AND DISCUSSION After impregnation of the support material SiO 2 with Rh(NO3)3 and phosphoric acid using the incipient wetness method and drying, the samples were reduced in a forming gas stream (5% H2 in Ar) at four different temperatures between 250 and 900 °C. For purposes of comparison, a sample with only Rh(NO3)3 was reduced at 140 °C. After reduction, the materials were characterized by X-ray powder diffraction (XRD) analysis (Figure 2). As expected, the phosphorus-free sample (Rh140) clearly shows the formation of metallic rhodium nanoparticles. The diffraction patterns of the phosphorus-containing samples (Rh/ 3585


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distinct CO signal at 2070 cm−1, which indicates the presence of only one site for CO adsorption. The peak is attributed to CO linearly bonded to single Rh atoms (Rh-CO) on the surface of Rh2P. Other species, such as bridged-bonded CO and Rh1+(CO)2, are present only in negligible amounts. The CO molecules are adsorbed only weakly because the peak disappears upon evacuation. This structural feature is unique to samples reduced at 900 °C and can be linked to the properties that this sample shows in the hydroformylation reaction (see Figure 6 and below). For the catalytic performance of the Rh2P samples to be tested, gas-phase hydroformylation experiments were performed with ethylene and propylene as the feed gas. Ethylene was chosen as the substrate because even at a pressure of 50 bar it can be assured that all reactants and products are handled above their respective dew points above 150 °C; this excludes that liquid films of educt or product molecules can be formed on the catalyst surface, which might act as a liquid reservoir for homogeneous Rh complexes. Figure 5 shows the results of ethylene hydroformylation at 50 bar total pressure for varying gas compositions and temperatures. During the studies of the partial pressure variation, nitrogen was used as a substituting gas for either carbon monoxide or hydrogen to keep the other gases at constant partial pressure. The effect of the hydrogen partial pressure is shown in Figure 5A. Starting with a H2 content of 10% (= 5 bar) and a CO content of 49% (= 24.5 bar), an ethylene conversion of ∼50% was observed together with a propionaldehyde selectivity of ∼84%. Increasing the hydrogen content to 40% resulted in a conversion of ∼90% and a slight decrease in the propionaldehyde selectivity to ∼75%. In all experiments, ethane was observed as the main side product according to our results by undesired hydrogenation of the educt ethylene. Figure 5B shows the effect of the CO partial pressure on the activity and selectivity. Increasing the CO content from 40 to 70% had a negative impact on the activity but led to an increase in propionaldehyde selectivity from 65 to 73%. Temperature increase from 170 to 240 °C had a strong positive influence on the ethylene conversion (25 → 91%) and at the same time decreased the selectivity for the hydroformylation product (Figure 5C). By varying the ethylene partial pressure, the optimum for the propionaldehyde selectivity was identified in the range around

Figure 3. (top) TEM analysis of Rh/P samples reduced at (a) 250, (b) 500, (c) 750, and (d) 900 °C. Red arrows depict Rh2P nanoparticles. By increasing the reduction temperature, a slight increase in particle size can be observed. (bottom) HRTEM analysis of a single particle from the sample reduced at 900 °C revealing high crystallinity of Rh2P with cubic structure.

2037 cm−1 is assigned to the symmetric and antisymmetric stretching mode, respectively, of germinal dicarbonyl species Rh1+(CO)2.32 The weak band at 2152 cm−1 may be attributed to a monocarbonyl Rhn+CO (1 ≤ n ≤ 3). The adsorption complexes are stable upon evacuation. In contrast, the high temperature sample (Rh/P900) surprisingly showed just one

Figure 4. (Rh/P500) IR spectrum after CO adsorption (equilibrium pressure provided in the legend) of a sample reduced at 500 °C showing at least four distinct CO signals in the region around 2000 cm−1. (Rh/P900) Spectrum of a sample that was reduced at 900 °C showing just one signal at 2070 cm−1. 3586


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Figure 5. Results of the hydroformylation of ethylene with Rh2P on SiO2 reduced at 900 °C (50 bar total pressure, gas hourly space velocity (GHSV) = 2773 h−1). (A) Effect of H2 partial pressure (180 °C, 1% C2H4, 49% CO, 0−35% N2, 10% Ar). (B) Effect of CO partial pressure (220 °C, 10% C2H4, 10% H2, 0−30% N2, 10% Ar). (C) Effect of the reaction temperature (10% C2H4, 10% H2, 70% CO, 10% Ar). (D) Effect of the ethylene partial pressure (GHSV = 2500 h−1, 180 °C, 7.5−9.4% H2, 65.6−77.8% CO, 10% Ar).

ethylene that are one magnitude higher.2,3 To the best of our knowledge, the highest reported TOF values for homogeneous rhodium catalysts are reported around 30000 h−1 but drop within minutes to around 10000 h−1.33 Water can be a side product of the reaction formed by aldol condensation of propionaldehyde. For suppressing the formation of condensation products, 10% of water was added to the feed stream. To our surprise, the addition of water did not only reduce the amount of condensation products, it also strongly enhanced selectivity toward the desired aldehyde (Figure 6A and B). In the temperature range to ∼200 °C, the formation of ethane and condensation products was even completely suppressed by the addition of water. Because the activity of the catalyst was also lowered, it might be possible that water blocks certain surface sites that are mainly active in the hydrogenation of the educt olefin. The catalytic experiment with water was performed for more than 1000 h with no significant loss of activity or selectivity. Further experiments were carried out without the addition of hydrogen to the educt gas mixture, keeping up the partial

5−10% with regards to ethylene content, whereas no significant changes in the ethylene conversion were observed (Figure 5D). Compared to typical homogeneous catalysts currently used in industry, supported Rh2P shows a higher tendency for hydrogenation of the educt olefin, which can be suppressed by increasing the CO partial pressure. Under optimal conditions (CO/H2 ratio of 5−10), a propionaldehyde selectivity of higher than 80% can be reached. The Rh/P catalysts showed turnover frequencies (TOFs) in the range of ∼20 h−1 (based on total rhodium content), which is lower than the reported values for SILP (∼800 h−1).13−20 Values for supported rhodium complexes are on the order of 5−15 h−1 at 2 bar and were extrapolated by Bell et al. to be in the range of 300 h−1 at 10 bar.10 However, in contrast to the SILP system and supported complexes, rhodium complexes only the surface Rh atoms of the Rh/P catalysts are accessible by the substrate. Therefore, we measured the amount of accessible surface Rh atoms by CO adsorption and calculated a theoretical TOFsurf of ∼190 h−1. For comparison, typical homogeneous catalysts of industrial relevance show initial TOF values in the hydroformylation of 3587


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Figure 6. (A) Hydroformylation of ethylene without addition of water with Rh2P as catalyst on SiO2 reduced at 900 °C (50 bar, gas hourly space velocity (GHSV) = 2400 h−1, 1% C2H4, 79% CO, 10% H2, 10% Ar). (B) Hydroformylation with the addition of water (50 bar, GHSV = 2400 h−1, 1% C2H4, 69% CO, 10% H2, 10% H2O, 10% Ar). (C) Comparison of six catalysts prepared under different conditions in the hydroformylation of ethylene (210 °C, GHSV = 3000 h−1, 20 bar, 10% C2H4, 10% H2, 50% CO, 20% H2O, 10% Ar). Note that for the pure rhodium catalyst, because of the low productivity, selectivity may be overestimated. (D) X-ray diffraction patterns of different Rh/P catalysts after testing.

pressure of water in the feed; neither propionaldehyde nor hydrogen were detected after the reaction, which excludes activity of the catalyst in the water-gas shift reaction. Figure 6C shows the activity and selectivity of six catalysts prepared under different conditions. We found that both the catalyst activity and selectivity were increased by raising the reduction temperature from 250 to 900 °C (Rh/P250 → Rh/P900). Another sample, for which the SiO2 carrier material was impregnated with rhodium nitrate and phosphoric acid but without a subsequent heat treatment in H2/N2 (Rh/H3PO4), gave a similar result to that of the catalyst reduced at 250 °C (Rh/P250). Impregnation of SiO2 with rhodium nitrate without the addition of phosphoric acid (Rh) led to a material that was not active in the catalytic experiment. XRD analysis of the spent catalysts revealed the reason for these results (Figure 6D). Compared with the fresh catalysts, the XRD analysis of the spent catalysts clearly shows a partial decomposition of Rh2P to elemental rhodium in the samples that were reduced at lower temperatures. Only the Rh/P900 sample showed full stability over several hundred hours during the catalytic test. The stability of the phosphide nanoparticles and the absence of metallic rhodium seem to be key for the

superior activity and selectivity in the hydroformylation reaction. Apparently, this stability can be reached by thermal annealing under reducing conditions leading also to supported rhodium phosphide nanoparticles of apparently high crystallinity, which can be seen from the electron diffraction pattern in Figure 3. Additionally, the catalysts were successfully tested in the hydroformylation of propylene to butyraldehyde, and again a positive effect was observed by the addition of water. At temperatures below 200 °C, hydroformylation selectivities of more than 90% to butyraldehydes with an n/iso ratio of 2.4 were reached (see Supporting Information for further details). Finally, the Rh2P catalysts were tested in the methoxycarbonylation of ethylene by exchanging hydrogen in the gas mixture with methanol to obtain the respective methyl ester. No ester formation was observed under these conditions, but to our surprise, a small amount of the desired product could be detected when hydrogen gas was added to the mixture (∼10% selectivity at 3% hydrogen content and 30% conversion of the educt olefin, see Supporting Information). Like in the experiments without the addition of methanol, propionaldehyde and ethane were obtained as the main products. 3588


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Rh salt and reduction with 5% H2 in N2 at 180 °C for 6 h. As a last step, the samples were sieved to remove the fine particles that are formed during the impregnation and reduction processes. Samples were characterized by XRD, TEM, and CO absorption. The activity tests in the hydroformylation reaction were performed in a fixed bed reactor (ID = 3.6 mm, ED = 10.0 mm, L = 290 mm) with 0.5 mL of the respective catalyst (length of the catalyst bed in the reactor = 50 mm). Below and above the catalyst, the reactor was filled with quartz to keep the catalyst in place.34,35 The reactors were accommodated in a heating furnace of a parallelized reactor system with an isothermal zone of 50 mm. Sixteen reactors with different catalysts were placed in the oven and tested under the same process conditions (pressure, temperature, and feed composition). The flow of the gases CO (Praxair, purity 3.0), H2 (Praxair, purity 3.0), Ar (Praxair, purity 5.0), C2H4 (Praxair, purity 2.7), and C3H6 (Praxair, purity 2.5) was controlled by a mass flow controller, and the pressure was controlled by a pressure valve and pressure hold-up gas. The concentration of reaction educts and products was measured using online GC techniques (Agilent 6890N with a DB 1 column (60 m × 0.32 mm × 3 μm) and an FID for hydrocarbons and a second column (Molsieve5A-Porabond) with a TCD for CO, H2, and Ar). In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) measurements were conducted using an Agilent Cary 680 FTIR spectrometer equipped with an MCT detector at a spectral resolution of 2 cm−1 and accumulation of 512 scans. An in situ cell (Harrick Praying MantisTM diffuse reflectance attachment DRP-DI8 in combination with a hightemperature reaction chamber HVC-000-4 with ZnSe windows) was used. The amount of the catalyst was ∼60 mg. Before the CO adsorption, the catalysts were pretreated in the reaction chamber with Ar (50 mL/min) at 150 °C for 1 h to remove the physically adsorbed water and residual impurities. As a next step, the samples were cooled to room temperature followed by evacuation (residual pressure = 2.7 × 10−5 mbar). DRIFT spectra of sequential dosing of CO (0.05−5 mbar equilibrium pressure) were collected at 313 K.

Therefore, we suggest a reaction mechanism in which the ester is not formed via alkoxycarbonylation but via dehydrogenation of an intermediate hemiacetal (Figure 7).

Figure 7. Proposed mechanism for the formation of methyl propionate by dehydrogenation of the intermediate semiacetal.

In summary, we could show a simple method for the synthesis of highly crystalline Rh2P nanoparticles on a SiO2 support. A high dispersion of the formed nanoparticles was found by TEM analysis. IR measurements after CO adsorption were performed and have shown that, after reduction at high temperature, the material features only a single site for CO coordination. We suggest that this finding can be taken as evidence that Rh2P in the form of highly crystalline supported nanoparticles features surface sites that allow catalytic functionality similar to that of a bisphosphine- or bisphosphite-modified rhodium complex. The materials were tested in the hydroformylation of ethylene and showed good activity together with a good selectivity toward propionaldehyde, in the best cases greater than 80%. Through the addition of water to suppress the formation of condensation products, we surprisingly found that the hydrogenation of ethylene can be completely suppressed at reaction temperatures below 200 °C. A possible explanation is that water selectively blocks the reactions sites responsible for hydrogenation and/or positively influences product desorption. By testing differently prepared catalysts, we found that a high reduction temperature leads to improved catalytic performance and enhanced stability in the hydroformylation. XRD analysis of spent catalyst proved that this can be attributed to an increased stability coupled to higher crystallinity of the catalyst, whereas the other Rh/P samples partly decomposed to metallic rhodium. Supported Rh2P also displayed attractive performance in the hydroformylation of propylene, and again a positive influence of high CO contents and water addition was observed. Experiments for the methoxycarbonylation of ethylene indicated that supported Rh2P is also active in the dehydrogenation of semiacetals. We believe the findings illustrated in this paper are of importance to the community as, to our knowledge, the best performing rhodium phosphide catalysts show outstanding catalytic behavior that can be linked to the findings of materials properties. The catalysts described here, to our knowledge, to date outperform any competing inorganic materials including metals, carbides, oxides, sulfides and the like from the point of view of catalytic functionality, activity, and selectivity in the described target reactions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b00499.

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EXPERIMENTAL SECTION The support material SiO2 (Cariact Q20, 120 m2) was supplied by Fuji Silysia Chemical Ltd. Prior to use, it was crushed and sieved to a fraction of 125−160 μm. A solution of H3PO4 and Rh(NO3)3 with a Rh:P ratio of 2 was prepared and impregnated upon incipient wetness on SiO2. The Rh loading was 5 mmol (Rh)/g (SiO2). After impregnation, the sample was dried at 80 °C for 16 h, followed by reductive heat treatment with 5% H2 in N2 for 6 h at different temperatures. For comparison, Rh/SiO2 was prepared using a similar method. After impregnation, the Rh/SiO2 was dried at 80 °C for 16 h, followed by heat treatment at 240 °C in air to decompose the

Results of the hydroformylation of propylene and methoxycarbonylation of ethylene (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jaroslaw Mormul: 0000-0002-4104-6860 Xing Huang: 0000-0002-8700-0606 Annette Trunschke: 0000-0003-2869-0181 Notes

The authors declare no competing financial interest. 3589


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Platinum Group Metal Phosphides as Heterogeneous Catalysts for the

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