Platinum Group Metal Phosphides as Heterogeneous Catalysts for the

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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 Gläser, Maria Dimitrakopoulou, Xing Huang, Annette Trunschke, Marc Georg Willinger, Robert Schloegl, Frank Rosowski, and Stephan A Schunk ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00499 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017

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Platinum group metal phosphides as heterogeneous catalysts for the gas-phase hydroformylation of small olefins Luis Alvarado Rupflin a, Jaroslaw Mormul a*, Michael Lejkowski a, Sven Titlbach a, Rainer Papp b, Roger Gläser c, Maria Dimitrakopoulou d, Xing Huang d, Annette Trunschke d, Marc Georg Willinger d, Robert Schlögl d,e, Frank Rosowski b,e, Stephan A. Schunk a* a

hte GmbH, 69123 Heidelberg, Germany; b BASF SE, 67056 Ludwigshafen, Germany; c

University Leipzig, 04103 Leipzig, Germany; d Department of Inorganic Chemistry, Fritz-HaberInstitute of the Max-Planck-Gesellschaft, 14195 Berlin, Germany; e BasCat, UniCat BASF Jointlab, Technical University Berlin, 10623 Berlin, Germany *corresponding authors E-Mail for Stephan A. Schunk: [email protected] E-Mail for Jaroslaw Mormul: [email protected]

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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 (TEM) and by infrared (IR) 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 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 give evidence that high temperature reduction leading to highly crystalline Rh2P nano-particles is key to achieving active, selective and long-term stable catalysts.

KEYWORDS Hydroformylation, heterogeneous, rhodium, phosphide, nanoparticles, ethylene

Introduction Since its discovery in 1938 by Otto Roelen the 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 midand 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


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modified with phosphine or phosphite ligands.[3-5] A challenge accompanied with 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, a great care has to be taken in order 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 which would also lead to new solutions with regard to process design. In the last decades a number of research activities in academia and industry were focused on the conceptual development of heterogeneous or heterogenized hydroformylation catalysts. Among others 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 probably catalyzed by molecular rhodium species being present in pseudo-liquid films on the catalyst surface.[10-12] A topic that attracted a lot of attention is the supported ionic liquid phase (SILP) process where the catalyst is dissolved in an ionic liquid which 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 which accumulate in the ionic liquid and poison the catalyst. Such condensation products can be removed from the SILP, followed by a regain in activity, but the regeneration is technically demanding.


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Platinum group metal phosphides are known to be useful heterogeneous catalysts and have been

applied

since

years

in

academia

especially for hydrodesulfurization

(HDS),

hydrodenitrogenation (HDN) and hydrodeoxygenation (HDO) reactions.[21-28] Rh2P crystallizes in an 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 phosphorous 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 1. Comparison of the structures of metallic rhodium (A, fcc) and Rh2P (B, anti-fluorite). The phosphorous 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 can also be aspired that through the interaction between phosphorous and rhodium at the surface of Rh2P non-zero-valent Rhodium-


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species are present. Additionally it can be assumed that the environment for rhodium in surface sites on Rh2P may be created that also 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 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.

Results and discussion After impregnation of the support material SiO2 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 4 different temperatures in the range 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 XRD analysis (Figure 2).


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Figure 2: X-ray diffraction patterns of Rh/P samples reduced at 4 different temperatures (250 °C, 500 °C, 750 °C 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 to 900°C). As expected, the phosphorous-free sample (Rh140) clearly shows the formation of metallic rhodium nanoparticles. The diffraction patterns of the phosphorous-containing samples (Rh/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 the 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 phosphorous on the SiO2 support with an average diameter between 2-5 nm and no evidence for the presence of metallic rhodium nanoparticles was found, we therefore interpret these nanoparticles as precursor of Rh2P with adequate stoichiometry which at higher temperatures topo-


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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 a cubic structure.


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Figure 3: Top: TEM analysis of Rh/P samples which were reduced at different temperatures (a: 250 °C; b: 500 °C; c: 750 °C; 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 a cubic structure.


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The samples reduced at 500 °C and 900 °C were also analyzed by IR absorption after CO exposure (Figure 4).

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 4 distinct CO signals in the region around 2000 cm-1. Rh/P900: Spectrum of a sample which was reduced at 900 °C showing just one signal at 2070 cm-1. Carbon monoxide adsorption on rhodium phosphide has not been investigated so far 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 cm-1 and 1912 cm-1 due


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to terminal (Rh-CO) and bridged bonded (Rh2CO) CO, respectively, are typical for carbon monoxide adsorbed on metallic rhodium nanoparticles, while a doublet at 2107 and 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 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, since 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). To test the catalytic performance of the Rh2P samples gas-phase hydroformylation experiments were performed with ethylene and propylene as the feed gas. Ethylene was chosen as 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 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.


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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 the H2-partial pressure (180 °C, 1 % C2H4, 49 % CO, 0-35 % N2, 10 % Ar). B: effect of the 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 (gas hourly space velocity: GHSV=2500 h-1, 180 °C, 7.5-9.4 % H2, 65.6-77.8 % CO, 10 % Ar).


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Starting with a H2 content of 10 % (= 5 bar) and a CO content of 49 % (= 24.5 bar) an ethylene conversion of about 50 % was observed together with a propionaldehyde selectivity of about 84 %. Increasing the hydrogen content to 40 % resulted in a conversion of about 90 % and a slight decrease in the propionaldehyde selectivity to about 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. Increase of 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 variation of the ethylene partial pressure an optimum for the propionaldehyde selectivity was identified in the range around 5-10 % with regards to ethylene content, while 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 around 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 5-15h-1 at 2 bar and were extrapolated by Bell et al. to be in the range of 300 h-1 at 10 bar.[10] But 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


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of approximately 190 h-1. For comparison: Typical homogeneous catalysts of industrial relevance show initial TOF values in the hydroformylation of 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 30.000 h-1 but drop within minutes to around 10.000 h-1.[33] Water can be a side product of the reaction formed by aldol condensation of propionaldehyde. In order to suppress 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 the selectivity towards the desired aldehyde (Figures 6A and 6B). In the temperature range to about 200 °C the formation of ethane and condensation products even was completely suppressed by the addition of water. Since the activity of the catalyst was also lowered it might be possible that water blocks certain surface sites which are mainly active in the hydrogenation of the educt olefin.


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Figure 6: A: hydroformylation of ethylene without addition of water, catalyst: Rh2P 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, gas hourly space velocity: GHSV=2400 h−1, 1 % C2H4, 69 % CO, 10 % H2, 10 % H2O, 10 % Ar). C: comparison of six catalysts which were prepared under different conditions in the hydroformylation of ethylene (210 °C, gas hourly space velocity: GHSV=3000 h−1, 20 bar, 10 % C2H4, 10 % H2, 50 % CO, 20 % H2O, 10 % Ar) – note that for the pure rhodium catalyst due to the low productivity selectivity may be overestimated. D: X-ray diffraction patterns of different Rh/P catalysts after testing.


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The catalytic experiment with water was performed for more than 1000 hours with no significant loss in activity or selectivity. Further experiments were carried out without addition of hydrogen to the educt gas mixture, keeping up the partial 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 which were prepared under different conditions. We found that both the catalyst activity and selectivity were increased by raising the reduction temperature from 250 °C 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 like the catalyst which was reduced at 250 °C (Rh/P250). Impregnation of SiO2 with rhodium nitrate without addition of phosphoric acid (Rh) led to a material which 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 which 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 the 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 nano-particles 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


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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 (about 10 % selectivity at 3 % hydrogen content and 30 % conversion of the educt olefin, see Supporting Information). Like in the experiments without addition of methanol propionaldehyde and ethane were obtained as main products. Therefore, we suggest a reaction mechanism in which the ester is not formed via alkoxycarbonylation but via dehydrogenation of an intermediate hemiacetal (Figure 7). H2/CO

O

MeOH

H

OH

-H2

O

O O

Figure 7. Proposed mechanism for the formation of methyl propionate by dehydrogenation of the intermediate semi-acetal. 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 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 towards propionaldehyde, in the best cases greater than 80 %. By addition of water to suppress the formation of condensation products we surprisingly found that the hydrogenation of ethylene can


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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 semi-acetals. 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. These catalysts described here do to our knowledge outperform up to date any competing inorganic materials like metals, carbides, oxides and sulfides or the like in the from the point of view of catalytic functionality, activity and selectivity in the described target reactions. Experimental 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


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was dried at 80 °C for 16h, followed by heat treatment at 240 °C in air to decompose the 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 fines 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 equals 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. 16 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 on-line GC techniques (Agilent 6890N with a DB 1 Column (60m x 0,32mm x 3µm) and an FID for hydrocarbons and a second column (Molsieve5A-Porabond) with a TCD for CO, H2 and Ar). In-situ DRIFTS (diffuse reflectance infrared Fourier-transform spectroscopy) measurements were conducted using an Agilent Cary 680 FTIR spectrometer equipped with a 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 high-temperature reaction chamber HVC-000-4 with ZnSe windows) was used. The amount of the catalyst was approximately 60 mg. Before the CO adsorption, the catalysts were pretreated in the reaction


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chamber with Ar (50 ml/min) at 150 °C for 1 h in order to remove the physically adsorbed water and residual impurities. As a next step the samples were cooled down to room temperature followed by evacuation (residual pressure 2.7x10-5 mbar). DRIFT spectra of sequential dosing of CO (0.05-5 mbar equilibrium pressure) were collected at 313 K.

ASSOCIATED CONTENT Supporting Information with results of the hydroformylation of propylene and the methoxycarbonylation of ethylene available as (PDF).

AUTHOR INFORMATION Corresponding Author * E-Mail for Stephan A. Schunk: [email protected] * E-Mail for Jaroslaw Mormul: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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Platinum Group Metal Phosphides as Heterogeneous Catalysts for the

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