Synergistic Effects of Rh-catalyst on Particle Stabilized Pickering

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Synergistic Effects of a Rhodium Catalyst on Particle-Stabilized Pickering Emulsions for the Hydroformylation of a Long-Chain Olefin Dmitrij Stehl,† Nataša Milojević,‡ Sebastian Stock,† Reinhard Schomäcker,‡ and Regine von Klitzing*,† †

Department of Physics, Technische Universität Darmstadt, Darmstadt, Germany Department of Chemistry, Technische Universität Berlin, Berlin, Germany

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/05/18. For personal use only.

‡

ABSTRACT: Hydrophilic silica nanoparticles (100 nm in length and of 20 nm diameter) and larger hollow Halloysite nanotubes (HNTs; 800 nm in length with an outer diameter of 50 nm and an inner diameter of 15 nm) are used to stabilize an oil-in-water emulsion. These particle-stabilized Pickering emulsions (PEs) are used for the hydroformylation of a long-chain olefin (1-dodecene). Rhodium (Rh) and the water-soluble ligand sulfonated 4,5-bis(diphenylphosphino)9,9-dimethylxanthene are used as catalyst. The emulsions are prepared by sonication and Ultra-Turrax in a specially designed vessel to protect the catalyst from oxygenation and to control the temperature of each sample during the preparation process. The Rh catalyst shows interfacial active behavior and strongly influences the mean droplet size of the emulsions, stability, wettability, and energy of detachment. Further, the Rh catalyst stabilizes an emulsion even in the absence of particles. In a mixture of Rh catalyst and particles, both attach at the interface if the droplet size is in a magnitude of micrometers. These PEs show a monotonous droplet decrease with increasing particle concentration. It is shown that hydroformylation is possible in all emulsions stabilized by the Rh catalyst, silica nanoparticles, or HNTs. However, the conversions in the emulsions are different. The highest conversion is observed in silica-stabilized emulsions with above 40 wt % after a reaction time of only 5 h. Further, high selectivity for aldehyde was observed for all emulsions. A model for the behavior of the emulsions in the reactor is postulated. Interestingly, the emulsions stabilized by the Rh catalyst and silica nanoparticles are destroyed after the reaction, but the HNTs-stabilized PEs remained stable.

1. INTRODUCTION Nearly 90% of chemical reactions are catalyzed. Homogeneously catalyzed reactions are selective, with a highly active catalyst and a well-known reaction, but only 15% of catalytic reactions occur via homogeneous catalysis. This is because of the complicated recycling process and leaching of expensive metals that are components of a homogeneous catalyst. This economic reduction is the reason why heterogeneous catalysts are used instead of homogeneous ones. Nearly 80% of chemical reactions are heterogeneously catalyzed because of the simple recycling process. Hydroformylation is one of the important homogeneously catalyzed reactions. In this reaction, olefins (alkenes) are converted to aldehydes in the presence of a mixture of carbon monoxide (CO) and H2 (syngas). This was discovered by Otto Roelen in 1938 (Figure 1).1,2 Usually, a product mixture is obtained from the reaction that contains the linear aldehyde, which is the most desired product for further industrial applications.3 This homogeneously catalyzed reaction is also subject to the recycling problem. For this reason, Ruhrchemie invented a process concept for the © XXXX American Chemical Society

hydroformylation of olefins in a two-phase system with a water-soluble rhodium (Rh) catalyst.4 The Rh catalyst becomes water-soluble, for example, by the ligand sulfonated 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (SulfoXantPhos, SX). However, one drawback of this process is its limitation to short-chain olefins (C5) because of the poor water solubility of long-chain olefins. Therefore, long-chain olefins must still be hydroformylated in a homogeneous phase, resulting in a difficult recycling of the precious catalyst from the product.5,6 Therefore, emulsion systems stabilized by surfactants or particles, so-called Pickering emulsions (PEs),7 have the potential to perform hydroformylation8−11 and subsequent separation of the catalyst. Particle-stabilized emulsions were Special Issue: Reinhard Schomacker Festschrift Received: Revised: Accepted: Published: A

September 20, 2018 November 15, 2018 November 21, 2018 November 21, 2018 DOI: 10.1021/acs.iecr.8b04619 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Schematic illustration of the hydroformylation of an olefin with the Rh catalyst.

first reported by Ramsden in 190312 and studied systematically by Pickering.13 Although PEs have existed for over 100 years, industrial applications in the chemical industry are rare. PEs are much more stable against coalescence and against several environmental stressors like the pH, salt, or temperature14 in comparison to surfactant-stabilized emulsions. The energy of detachment (ΔGparticles) of the particles attached at the interface is several thousand kT.15 This forms the basis of why PEs can be used for chemical reactions.16−19 Several studies have shown that PEs can also be used for the hydroformylation of long-chain olefins.20,21 Furthermore, it is possible to combine the high selectivity and high activity of the homogeneous catalysis and the efficient separation of the heterogeneous catalysis in PEs. Because of the high stability of PEs, the product and catalyst phases can be easily separated, by membrane filtration, for example,22−24 and fulfill the requirements of green chemistry.25,26 In this study, two hydrophilic particles, silica nanoparticles, and Halloysite nanotubes (HNTs) were used to stabilize dodecene (oil)-in-water emulsions. Silica nanoparticles are fumed silica with an amorphous silicon oxide (SiO2) structure.27 The particles are produced by flame hydrolysis.28 The shape of the particles is a combination of primary smaller particles to a larger one, which is why the arrangement of the primary particles is arbitrary29 and silica nanoparticles have an irregular shape. The silica nanoparticles are used in many industrial products and applications. HNTs are a natural clay30 and are rolled 1:1 aluminosilicates31 with a molecular formula of Al2Si2O5(OH)4·nH2O,32 similar to kaolinite sheets.33 These HNTs have a tubular shape with SiO2 tetrahedra outside and aluminum oxide octahedra inside the tubes.34 HNTs show a negative ζ potential at pH 4−10.35 The shape is much more regular and is significantly larger in comparison to that of the silica nanoparticles, and they are hollow. The lengths of the HNTs are 500−1000 nm36 and their outer diameters are 40− 80 nm with inner diameters of 10−15 nm.37 HNTs have many advantages. They are cheap, biocompatible,38 and available in thousands of tons and can be accessed for potential industrial applications. Further, they may be loaded with chemical agents (anticorrosions, antioxidants, antimicrobes, flame retardants, proteins, or other drugs) for functional nanocomposites39 and can also be used as membrane composites for filtration applications.40 In an emulsion system, the catalyst should be surface-active to facilitate the hydroformylation. This essential property is known but rarely studied. Especially, the influence of a surfaceactive catalyst on the properties of PEs like the mean droplet size, energy of detachment, and mechanical stability in the presence of the catalyst is rarely investigated. Further, the combination of a surface-active catalyst and surface-active particles in PEs, which might induce synergistic effects from the point of view of a hydroformylation, is not studied in detail. In particular, these synergistic effects are essential for optimization of the industrial applied hydroformylation of long-chain olefins.

In the present study, a Rh-SX catalyst was used, and it was studied how Rh-SX affects the energy of detachment, mean droplet size, and stability of the PEs and how these parameters influence the hydroformylation of a long-chain olefin (1dodecene). Further, a mechanism of the PEs behavior during the hydroformylation was proposed in a stirred batch reactor.

2. EXPERIMENTAL SECTION 2.1. Materials. Fumed silica provided from Wacker (HDK N20) and HNTs from Henan Province in China were applied to stabilize PEs against coalescence. Water (ρ = 18.2 MΩ·cm at 25 °C; purified with a three-stage Milli-Q system) and 1dodecene (>94%) purchased from Merck were used. Rh(acac)(CO)2 (Rh precursor) was contributed by Umicore. As a ligand, SX was purchased from Molisa GmbH. SX was synthesized by the procedure of Goedheijt et al.41 For all surface tension measurements, dodecane (>99%) was purchased from Sigma-Aldrich. Argon 5.0 obtained from Linde was used to purge the samples. The syngas mixture for the hydroformylation was CO (purity 1.6):H2 (purity 3.0) = 1:1. All chemicals were used without any further purification. 2.2. Preparation Methods. 2.2.1. Preparation of the PEs. Catalyst Preparation (Rh-SX). A total of 0.1 mmol (26.2 mg) of Rh(acac)(CO)2 and 0.4 mmol (313.9 mg) of SX were weighed in a Schlenk tube. The ligand-to-metal ratio (SX:Rh) was held constant at 4:1. The tube was evacuated three times and purged with argon (Ar). Afterward, 6 g of degassed water was added through a septum to the powder. To form Rh-SX, the mixture was stirred at 700 rpm overnight at room temperature. Emulsion Preparation by Ultra-Turrax (UT; Low Energy Input). The UT from IKA T25 digital with a S25N-10G dispersing element was used to disperse the oil phase. First, 13 g of water, without or with Rh-SX, was added to 80 mg (0.50 wt %, referred to the whole mass of the sample) of the particles and sonicated for 5 min. Afterward, 3 g of 1-dodecene (water:oil (w:o) = 4.3:1) was added to the suspension and homogenized for 5 min at 10000 rpm. Additional steps during the preparation for hydroformylation were carried out to exclude the contact between air and Rh-SX. First, 0.50 wt % (referring to the whole mass of the sample) of the particles was placed in a Schlenk flask and purged with N2. Afterward, the degassed Rh-SX solution (cSX = 1.5 × 10−2 mol/L; w:o = 4.3:1) was added to the particles and dispersed in a sonication bath for 5 min. The suspension was carefully transferred into the vessel and covered with a cap. The vessel was purged with Ar, 1-dodecene (w:o = 4.3:1) was added to the suspension, and the mixture was homogenized. Emulsion Preparation by Sonication (High Energy Input). A Sonifier W-250 (20 kHz, Pmax = 200 W) sonicator from Branson was used to homogenize the emulsions. First, 80 mg (0.50 wt %, referring to the whole mass of the sample) of the particles was inserted into the vessel. A total of 13 g of water, with or without Rh-SX, was added to the particles. Afterward, 3 g of 1-dodecene (w:o = 4.3:1) was added to the suspension, B

DOI: 10.1021/acs.iecr.8b04619 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. Custom-designed vessel to prepare PEs by temperature control and gas purging.

tip. The holding plate for the tip is at the top of the spacer so the depth of the tip into the sample can be adjusted. 2.2.3. Hydroformylation. Hydroformylation was performed at 100 °C, 15 bar, and 1200 rpm in a batch reactor with a paddle stirrer.14 The olefin (substrate; S) to catalyst (Rh-SX; C) mole ratio was for all samples S:C = 340:1. The hydroformylation ran for a total time of 24 h and was stopped after 24 h. The samples were taken hourly in the first 5 h, and the last sample of the hydroformylation was taken after 24 h. 2.3. Methods for Characterization. 2.3.1. Electron Microscopy. Scanning Electron Microscopy (SEM). A Hitachi SU8030 scanning electron microscope with a cold-field emitter at the Zentrum für Elektrononenmikroskopie at Technische Universität (TU) Berlin was used. The samples were measured in a top view with an acceleration voltage of 3 kV. Transmission electron microscopy [TEM; JEM 2100, JEOL GmbH (Eching, Germany)] at the Joint Laboratory for Structural Research [JLSR; Humboldt Universität (HU) Berlin, TU Berlin, Helmholtz-Zentrum Berlin (HZB)] was used to analyze the shape and size of the nanoparticles. For the sample preparation, a copper grid with a carbon support film (200 mesh) was used. A total of 5 μL of the particle suspension was dropped onto the grid. The excess liquid was removed with filter paper after 30 s. The prepared grid was inserted into the EM21010 sample holder, and the sample holder was inserted into the transmission electron microscope. Measurements were performed at an acceleration voltage of 200 kV and recorded with a bottom-mounted camera system. Cryo-TEM. The specimens for cryo-TEM were vitrified by plunging the samples into liquid ethane using an automated

and the PEs were homogenized by sonication for 5 min (with respect to pulse on) with a 2 s pulse on, a 3 s pulse off, and an amplitude of 50%. The whole homogenization procedure lasted 12.5 min. During the preparation, the sample was stirred at 350 rpm and cooled at ∼8 °C using a Huber KISS K6 cryostat. Additional steps during the preparation for hydroformylation were carried out to exclude the contact between air and Rh-SX. A total of 0.50 wt % (referring to the whole mass of the sample) of the particles was added to the vessel. The temperature of the vessel was adjusted to 8 °C and purged with Ar (quality = 5.0). A degassed Rh-SX solution (cSX = 1.5 × 10−2 mol/L) and 1-dodecene (w:o = 4.3:1) were added to the particles. The sonicator was coupled to the vessel in order to sonicate the mixture. 2.2.2. Designed Vessel for Inert and TemperatureControlled PEs Preparation by Sonication. A special vessel (Figure 2) was designed for the inert PEs preparation. The inner smaller stainless steel flask is fitted into the larger outer container, and they are connected with screws. There is space between the flasks for the cooling water for the closed-cycle cooling system. A sealing ring is inserted at the flange. The next element is a stainless steel cap with two holes, the smaller of which is for the temperature sensor and the bigger one for the sonicator tip. The cap is also screwed on to the flask, and a sealing ring lies between them. The cap has a gas connection to purge the sample during the preparation. The inert gas goes through a distribution line to two visible vents inside the inner flask. At the top of the cap is placed a spacer for the sonicator C


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per minute were increased stepwise until the first significant differences between the released oil and residual emulsion fractions were observed. The samples containing Rh-SX did not show any significant changes at 14g. 2.3.6. Gas Chromatography (GC). GC was used to analyze the mass fraction of the oil phase during hydroformylation. A RTX-5MS capillary column with a length of 30 m, an internal diameter of 0.25 mm, and df = 0.25 μm was used. The temperature program is shown in Table 1.

plunge freezer (Virtobot Mark IV, FEI Deutschland GmbH, Frankfurt, Germany). A total of 4 μL of the sample solution was pipetted onto lacey carbon grids (200 mesh, Science Services, Munich, Germany), which were pretreated for 15 s with glow discharge. The liquid was blotted with filter paper. After vitrification, the specimen was inserted into a precooled high-tilt cryo transfer sample holder (Gatan 914, Gatan, Eching, Germany) and transferred into the JEOL JEM 2100 microscope (JEOL GmbH, Eching, Germany), which was operated at 200 kV. All images were recorded digitally using a bottom-mounted CMOS camera system (Tem-Cam-F416, TVIPS, Gauting, Germany) and processed with a digital imaging processing system (EM-Menu4.0, TVIPS, Gauting, Germany). The measurements were carried out at JLSR (HU Berlin, TU Berlin, HZB). Cryo-SEM. Cryo-SEM measurements were carried out at the Zentrum für Werkstoffanalytik Lauf GmbH. A LEO 1530 VP with an ALTO 2500 cryo unit from Gatan was used as the electron microscope. PEs were frozen in N2 slush at −212 °C. The sample was quickly transferred into the cryo unit and fractured. Afterward, the sample was sublimated at −88 °C for 6 min at 0.2 mbar and sputtered with AuPd (≈10 nm). The sample was analyzed at −150 °C at 10−5 mbar. 2.3.2. Bright-Field Microscopy. A Axio Imager A1 brightfield microscope from Zeiss was used to measure the droplet size of the PEs. The sample was shaken by hand to disperse the emulsion phase, and 3 μL of the sample was pipetted onto a glass slide and immediately carefully covered with a 0.17-mmthick cover glass. Measurements were carried out with a 50/ 0.70 objective in transmission mode. The automated analysis software from SOPAT GmbH42 was used to count the droplets and determine the droplet diameter. 2.3.3. Du Noüy Ring Method. A D-CAT 11 device from DataPhysics was used to determine the surface tension (γair/water). Measurements were carried out with a Du Noüy ring made of platinum/iridium with a diameter of 18.7 mm, a height of 25 mm, and a wire thickness of 0.37 mm. After every measurement was obtained in different concentrations, the ring was first washed with Milli-Q water and then dipped in a MilliQ/ethanol mixture (3:1), which was burned off until the ring turned orange. Afterward, the ring was used for the next measurement. 2.3.4. Drop-Shape Analysis. Sessile Drop. The contact angle (θ) was measured with a DSA100 device from Krüss. A clean glass plate was used as a substituent for the silica particles. For HNTs, a pallet was pressed with 1.3 GPa of pressure in a press. The sample was placed on a quartz cuvette and covered with dodecane. Afterward, a water droplet of 5 μL was deposited onto the surface of the sample. The shape of the droplet was fitted with an ellipse, and the contact angle was determined with a tangential method. Pendant Drop. To measure the interfacial tension γoil/water, a PAT-1 tensiometer from Sinterface was used. A quartz cuvette (32 × 35 × 35 mm3) was completely filled with water, and a bent needle (ϕ = 0.49 mm) was dipped into it. A 3 μL dodecane droplet was generated on the tip of the needle and analyzed. 2.3.5. Stability Measurements. After sample preparation by UT, the emulsions were immediately transferred to a centrifuge tube and centrifuged for 1 min at 300 rpm (14g) for emulsions without Rh-SX and 1100 rpm (196g) for emulsions with a Rh-SX solution (cSX = 1.5 × 10−2 mol/L). The height of the released phase was analyzed. The revolutions

Table 1. Temperature Program of the RTX-5MS Capillary Column level

rate [°C/min]

Tstart [°C]

Tfinal [°C]

holding time [min]

1 2 3

10 30

120 120 160

160 330

5.5 2.0 15

3. RESULTS 3.1. Nanoparticles. Hydrophilic HNTs and hydrophilic silica nanoparticles were used to stabilize the emulsion against coalescence during hydroformylation. In Figure 3, the structure

Figure 3. SEM and TEM images of silica (SEM = top, left; TEM = bottom, left) and HNTs (SEM = top, right; TEM = bottom, right).

of the nanoparticles is shown. Silica nanoparticles show an irregular shape in Figure 3, left (top, SEM; bottom, TEM). The structure results from a combination of primary spheres to a major secondary structure. The mean length is 100 ± 10 nm, and the mean diameter is 20 ± 5 nm. In contrast, HNTs (Figure 3, right) have a mean length of 800 ± 200 nm, which is 8 times longer than that of the silica particles. The outer mean diameter of the HNTs is 50 ± 5 nm, and the inner mean diameter is 15 ± 2 nm. The shape is much more regular and shows a rolled structure. The structure of the HNTs is shown in Figure 3, right (top, SEM; bottom, TEM). All particles are dispersible in water. 3.2. Effect of Rh-SX. 3.2.1. Interfacial Active Catalyst. Drop-shape analysis was carried out to determine the behavior of Rh-SX as an interfacial active detergent. The concentrations D


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Figure 4. (A) Surface tension against air measured with the Du Noüy ring method for the Rh-SX solution containing SX and a pure ligand SX solution. (B) Interfacial tension γoil/water between dodecane and water containing Rh-SX (cSX = 1.5 × 10−2 mol/L) measured with the pendant drop method. All concentrations (A + B) are given with respect to the ligand SX concentration and a fixed SX:Rh ratio of 4:1.

Figure 5. Cryo-TEM images of oil droplets containing Rh-SX in the water phase (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1; w:o = 4.3:1). The samples were immediately analyzed after sample preparation: (A) without particles; (B) in the presence of silica nanoparticles (csilica = 0.50 wt %); (C) in the presence of HNTs (cHNTs = 0.50 wt %). All samples were prepared by sonication (high energy input).

sonication lead to a droplet diameter of 300 nm (Figure 5), which is independent of the particle type. Measurements were taken directly after preparation of the emulsions. The images indicate that the particles are not attached at the oil droplets but dispersed in the continuous water phase. The droplets are stabilized against coalescence for months even without particles at the interface. However, reducing the energy input during the preparation (samples were prepared by UT) leads to an increase of the droplet size even if the water phase contains Rh-SX (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1). The droplet size regarding both particle-stabilized emulsions is on the order of micrometers and is shown in Figure 6. Further, it was observed that silica and HNTs are attached at the interface for the lower energy input (UT). 3.3. Effect of the Particle Concentration. The measured dependence of the droplet mean diameter from the particle concentration is shown in Figure 7A, B. All samples were prepared by UT (low energy input) to ensure the attachment of the particles at the micrometer-sized droplets. In the case of PEs prepared with silica but without Rh-SX, the droplet diameter decreases from 97 μm (csilica = 0.20 wt %) to 16 μm (csilica = 0.80 wt %), and in the case of HNTs prepared PEs, the droplet diameter decreases from 45 to 19 μm for the same mass content regime as that with silica. If the water phase contains Rh-SX (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1), the droplet size is 18 μm for csilica = 0.20 wt % and decreases to 10 μm with an increase in the silica concentration to 0.80 wt %.

(mol/L) of the mixed Rh-SX solution referred to the concentration of SX, and the SX:Rh ratio was fixed at 4:1, as was later used for hydroformylation. Figure 4A shows the surface tension γair/water of the Rh-SX and ligand solutions. The comparison of γair/water as a function of the concentration shows no difference between the samples. All concentrations of the mixed Rh-SX solutions referred to the ligand SX and are expressed as cSX. In the concentration range (0−3.2) × 10−4 mol/L, the surface tension corresponds to that of water (72.5 mN/m). Increasing the concentration to 3.1 × 10−3 mol/L causes a decrease of γair/water to nearly 55 mN/m. Further increasing the Rh-SX concentration above 3.1 × 10−3 mol/L does not affect γair/water. The kind of fluctuation between the concentrations 3.1 × 10−3 and 8.3 × 10−3 mol/L is a hint for sample impurities. Figure 4B shows the interfacial tension γoil/water between the dodecane droplet and water phase. Dodecane was chosen instead of dodecene because it was obtained with high purity. Without Rh-SX, γoil/water is constant at nearly 50 mN/m and does not change with time. If the water phase contains Rh-SX (cSX = 1.5 × 10−2 mol/L), then γoil/water decreases to 22 mN/m and decreases further with time. Within the period of the experimental time, a constant value is not reached. γoil/water is 14 mN/m after 20 min. 3.2.2. Effect of the Energy Input during the PEs Preparation Depending on the Catalyst. Substituting the water phase with the Rh-SX/water mixture (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1) and preparing the emulsions by E


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interfacial tension measurement between pure dodecane and water (Figure 4B) was used for all ΔGparticles calculations. The measured contact angle for the different samples is shown in Figure 8, and the resulting ΔGparticles is summarized in Table 2. ÄÅ ÅÅ ry i ΔGparticles = 2rlγoil/waterÅÅÅÅsin α − α cos αjjj1 + zzz ÅÅÇ l{ k ÉÑ 2 r cos α sin α ÑÑÑ ÑÑ + ÑÑ l (1) ÑÖ

Figure 6. Freeze-fraction cryo-SEM measurements of silica-stabilized PEs (left) and HNTs-stabilized PEs (right). The particle concentration for both samples is 0.50 wt % (w:o = 4.3:1). The water phase contains Rh-SX in a concentration of 1.5 × 10−2 mol/L (SX:Rh = 4:1) and refers to SX. In every picture, the oil droplets are covered by the particles. The samples were prepared by UT (low energy input).

Also, at the HNTs-stabilized PEs, Rh-SX leads to a reduction of the droplet diameter to 20 μm for cHNTs = 0.20 wt %. Increasing the HNTs concentration to 0.80 wt % causes a monotonous reduction of the droplet diameter to 9 μm. A turbid water phase below the emulsion phase was observed for all samples, which is a hint for an excess of particles in the water phase. Because the partition coefficient between the interface and water phase is not known, a theoretical calculation of the coverage was not possible. Further, the fraction of the emulsion phase with respect to the whole sample was constant for all particle concentrations. 3.4. Energy of Detachment. The nanoparticle energy of detachment (ΔGparticles) at the interface can be calculated with eq 1.33,43 It should be mentioned that a rod shape was assumed for the silica nanoparticles. In eq 1, r is the particle radius (silica = 10 nm; HNTs = 25 nm), l is the particle length (silica = 100 nm; HNTs = 800 nm), γoil/water is the interfacial tension between water and dodecene, and α is the three-phase contact angle. As a γoil/water value, approximately 49 mN/m from the

Figure 8. Sessile-drop measurement to determine the three-phase contact angle α. For all measurements, the sample was completely covered with dodecane and a water droplet was placed onto the sample: (A) glass plate (≙ silica particles) without Rh-SX in the water droplet; (B) glass plate (≙ silica particles) with Rh-SX (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1) in the water droplet; (C) HNTs pellet without Rh-SX in the water droplet; (D) HNTs pellet containing RhSX (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1) in the water droplet.

The contact angle for silica was 50° and was higher in comparison to that of the HNTs (α = 30°). A further comparison of the energies of detachment for different particles showed that the larger structural size of the HNTs resulted in a higher energy of detachment. Silica has a ΔGparticles value of 7188 kT and HNTs of 34192 kT and is 4.76 times higher. The addition of Rh-SX (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1) to the water phase results in a reduction of the

Figure 7. (A) Mean droplet diameter (dmean) as a function of the particle concentration (wt % with respect to the whole sample mass; w:o = 4.3:1) prepared by UT. (B) Inset of A of the samples stabilized by silica or HNTs. The water phase additionally contains Rh-SX (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1). F


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Industrial & Engineering Chemistry Research Table 2. Three-Phase Contact Angles α and the Corresponding Energy of Detachment ΔGparticles Calculated with Equation 1 at a Temperature of 298.15 K and with γoil/water = 49 mN/m for All ΔGparticles Calculationsa sample

α [deg]

ΔGparticles [kT]

silica silica + Rh-SX HNTs HNTs + Rh-SX

50 29 30 27

7188 1422 34192 25009

the mechanical stress increases from 14g to 196g even if ΔGparticles is reduced. The samples containing Rh-SX did not show any significant changes of the fractions at 14g. Additionally, the differences between the released/residual phases are much more pronounced. The released oil phase for the emulsion without particles is 11%, that for silica + Rh-SX is 8%, and that for HNTs + Rh-SX only is 2%. The residual emulsion phase fraction is 8% for the emulsion without particles and 14% for silica + Rh-SX. The highest mechanical stress stability was observed for a (HNTs + Rh-SX)-stabilized emulsion with a residual emulsion fraction of 22%. 3.6. Hydroformylation of a Long-Chain Olefin. First, hydroformylation was studied with HNTs-stabilized PEs because of the higher stability of this emulsion. A comparison of the resulting conversions after 24 h of reaction time for variations in the energy input is shown in Figure 10A. The sample prepared by the UT (low energy input) shows the lowest conversion of 1-dodecene of 9 wt % [turnover frequency (TOF) = 1 h−1; n:iso = 99]. Increasing the energy input by preparing the sample with sonication leads to a drastic increase in the conversion to 42 wt % (TOF = 18 h−1; n:iso = 70). All emulsions were stable for months after the reaction. To understand the process in the reactor in detail, emulsions were prepared by sonication and the time dependence of the conversion was measured. The resulting conversions over 5 h of reaction time are shown in Figure 10B. It has to be mentioned that these reactions were also running for 24 h in total. The emulsion stabilized by silica nanoparticles shows the highest conversion of 42 wt % (TOF = 55 h−1; n:iso = 61) after 5 h, which is almost 2 times higher than that of the reference system without particles. The conversion for the emulsion without particles is 24 wt % (TOF = 26 h−1; n:iso = 79). In contrast, stabilization by HNTs decreases the conversion to 13 wt % (TOF = 18 h−1: n:iso = 58), which is 2 times lower than that for the reference system and 3 times lower than that for silica-stabilized PEs. A reduction of the conversion after nearly 2.5 h was observed for all samples. It has to be mentioned that the n:iso ratio increases from 58 (5 h of reaction time; Figure 10B) to 70 (24 h of reaction time; Figure 10A) for the HNTs-stabilized PEs prepared by sonication because of the increase of tridecanal with the reaction time; however, the value for isoaldehyde (Figure 11)

The Rh-SX concentration was cSX = 1.5 × 10−2 mol/L (SX:Rh = 4:1).

a

contact angle for silica particles to 39° and also in a reduction of ΔGparticles to 1422 kT, which is 5 times lower than that without Rh-SX. However, the addition of Rh-SX slightly decreases the contact angle of the HNTs sample to 27°. Because of this, ΔGparticles slightly decreases to 25009 kT, which is 1.4 times lower than that without Rh-SX. Consequently, ΔGparticles of HNTs + Rh-SX in comparison to that of silica + Rh-SX is about 18 times higher. 3.5. Mechanical Stability. To ensure attachment of the particles at the interface, the samples were prepared by UT. After a creaming time of nearly 5 min, 25% of the emulsion fraction without any oil fraction at the top was observed for all samples. To analyze the stability of the PEs under mechanical stress, the freshly prepared PEs by UT were centrifuged. The mechanical stress was increased stepwise until the first significant differences of the released oil and residual emulsion fractions were observed. Figure 9 shows the results after centrifugation for the released oil and residual emulsion fractions, respectively; there are no significant differences for silica- and HNTs-stabilized PEs. For silica-stabilized PEs, the released oil phase corresponds to 10%, while it is around 12% for HNTs, which is not significantly higher. The residual emulsion phase for silica amounts to 9% and that for HNTs to 10%. At all samples, the fraction at the bottom is the turbid water phase. It was shown earlier that it is possible to use RhSX (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1) as a detergent to stabilize an emulsion even without particles. A strong increase in the stability of the particle-stabilized emulsions can be observed if the water phase additionally contains Rh-SX, and

Figure 9. Stability measurements of silica/HNTs-stabilized PEs against coalescence under mechanical stress (the particle concentration was 0.50 wt %): (A) without Rh-SX, centrifugation at 300 rpm (14g) for 1 min; (B) with Rh-SX (cSX = 1.5 × 10−2 mol/L; SX:Rh = 4:1), centrifugation at 1100 rpm (196g) for 1 min. The dashed line (A + B) represents the emulsion fraction of the noncentrifuged emulsions (25%). All PEs were prepared with UT. G


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Figure 10. (A) Hydroformylation of 1-dodecene after 24 h of reaction time for HNTs-stabilized PEs (0.50 wt %) prepared by UT (low energy input) and for PEs prepared by sonication (high energy input). (B) Comparison of the conversion of 1-dodecene during 5 h of reaction time (the total reaction time was 24 h) by stabilizing the emulsion with silica (circles, 0.50 wt %) and HNTs (triangles, 0.50 wt %) and without particles (square, reference system). All samples in part B were prepared by sonication. 0.50 wt % referred to the whole mass of the sample. All samples contained a Rh-SX concentration of cSX = 1.5 × 10−2 mol/L (SX:Rh = 4:1; S:C = 340:1).

observed. Silica-stabilized PEs show the highest slope of the linear fit, followed by the reference system (emulsion without particles). The lowest slope was observed at HNTs-stabilized PEs. The third regime is between 2.5 and 5 h (linear fit between t = 3 and 5 h). In this regime, the conversions of all emulsions are reduced in comparison to the second regime. The particle-stabilized PEs show a stronger reduction of the conversion in the third regime than the reference system without particles. In all emulsions, the reaction reacts further with time up to 24 h of reaction time. The reactions were stopped after 24 h. Figure 11 shows the component composition of the oil phase in the reactor as a function of the reaction time. The curve progressions of the product (tridecanal) correlate with the behavior of the conversion in Figure 10B and show the same behavior for all samples. The amounts of side products (isododecene, isoaldehyde, and dodecane) are nearly the same for the reference system (emulsion without particles) and for the PEs stabilized by HNTs. In silica-stabilized PEs, nearly the same amounts of isoaldehyde and dodecane were observed; however, a higher amount of isododecene was measured. This is nearly 3 times higher in comparison to the other two emulsions. In all samples, the amounts of the side products do not increase with time and the selectivities for the aldehydes are nearly the same (Table 3). The state of the emulsions after the reaction is summarized in Table 3. The droplets of the reference system without

is nearly constant. Further, all n:iso ratios are comparable with each other because of the low amount of isoaldehyde (Figure 11) and the error of the method of analysis (GC).

Figure 11. Comparison of the component composition of the oil phase during 5 h of reaction time (the total reaction time was 24 h) by stabilizing the emulsion with silica (0.50 wt %) and HNTs (0.50 wt %) and without particles (reference system). All samples were prepared by sonication. 0.50 wt % referred to the whole mass of the sample. All samples contain a Rh-SX concentration of cSX = 1.5 × 10−2 mol/L (SX:Rh = 4:1; S:C = 340:1).

Table 3. State of the Emulsion Phase after the Hydroformylation of 1-Dodecene, Calculated TOF, and Selectivity for Aldehydesa emulsion/PEs stabilized by without particles (sonicated, reference system) silica (sonicated) HNTs (sonicated) HNTs (UT)

Further, the conversions in Figure 10 B can be separated into three parts. The first regime is up to 1 h. In this regime, the conversions slowly increase with time and are nearly the same for all emulsions. The second regime is between t = 1 and 2.5 h (linear fit between t = 1 and 2 h). In general, in this part, the conversions show a strong increase for all emulsions; however, differences between the three emulsions were

stable

TOFb [h−1]

selectivityc [%]

no

26

84

no yes yes

55 18 1

77 86 75

a The samples were prepared by sonication or UT. bTOF = nsubst.reacted/nRh‑SX·t; determined after a reaction time of 2 h. c Selectivity for aldehydes.

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Deshiikan et al.45 In this work, also the pendant-drop method was used to determine the interfacial tension. In a certain SDS concentration and in the presence of KCl, the interfacial tension shows a similar time-dependent decrease of the interfacial tension and a comparable final value. Opposite to the present study (even after 20 min, a constant value was not reached), constant values were reached in the regime of seconds. This different behavior might be explained by the different molecule structures and sizes of SDS and/or Rh-SX. As a consequence of its surface activity, Rh-SX can stabilize an emulsion without any particles. Rh-SX might molecularly adsorb at the interface or assemble to aggregates, as shown in the work of Sihler and Ziener.46 In the study, it was shown that an oil-in-water emulsion can be stabilized by a fluorescence dye fluorescein. It was suggested that fluorescein aggregates at the interface and stabilizes the emulsion. The mean droplet diameter of the emulsions prepared with sonication (high energy input) containing Rh-SX is about 300 nm immediately after the preparation, irrespective of the fact that particles were added or not. The high energy input disperses the oil phase into the water phase, but coalescence might occur. In the case of PEs, the coverage of the droplets by the particles might reduce the coalescence of the droplets. It was observed that, at a droplet size of 300 nm, neither silica nor HNTs is attached at the droplet surface (Figure 5). The droplet size seems to be too small and the droplet curvature too high, so the particles can adsorb at the droplet surface. In contrast, particles can adsorb at larger droplets pronounced at lower energy input (Figure 6). The mean droplet size decreases monotonously with increasing particle concentrations and shows a typical behavior in PEs33,47 for both particle types. With increasing concentration of the particles, a larger total surface can be occupied and the surface increases; i.e., the droplets become smaller. It is assumed that the two types of particles lead to different coverage of the droplets because of their different sizes and aspect ratios. This might lead to a difference in the droplet size between silica- and HNTs-stabilized PEs. Further, it should be mentioned that a turbid water phase in the PEs was observed for both types of particles. It is a hint that not all particles are adsorbed at the droplet surface and still dispersed in the water phase. Because of this, it was not possible to calculate the absolute extent of the coverage. The substitution of the water phase by a Rh-SX/water mixture led to a reduction of the droplet diameter for both types of particles due to a reduction of the surface tension as explained above. The general behavior in which the droplet size decreases with increasing particle concentration remained the same. However, the droplet size in the presence of Rh-SX became similar for both particles. The surface-active Rh-SX decreased the interfacial tension, which led to reduced droplet sizes. The fact that both Rh-SX and the particles reduce the droplet size is a strong hint for a synergistic effect of both compounds. Sabouni and Gomaa48 used metal− organic frameworks (MOFs) to stabilize PEs and demonstrated that the addition of the surfactant Tween-20 to an MIL-101 (MIL = Matérial Institut Lavoisier)-stabilized PEs leads to a narrow droplet distribution. In the study, it was assumed that the surfactant and particles show a synergistic effect on the stability. Hence, reduction of the coalescence was assumed. Adding Rh-SX to the water phase causes a reduction of the three-phase contact angle for silica from 50° to 29°. In contradiction to expectations, the contact angle for HNTs was

particles completely coalesce, and two separated phases of water and oil were observed. Also, the silica-stabilized PEs coalesce completely and show phase separation. Only the HNTs-stabilized emulsion is stable, independent of the preparation method, with creaming at the top of the sample; however, it shows lower conversion.

4. DISCUSSION To stabilize PEs against coalescence, fumed silica and HNTs were used. Pristine silica and HNTs are dispersible in the water phase. However, these hydrophilic particles have different sizes and shapes. Silica particles were assumed as nanorods with a secondary, irregular shape composed of primary spheres that are connected to each other. In contrast, HNTs are rolled, hollow nanotubes with a regular shape. HNTs are much longer than silica (800 vs 100 nm). The larger size leads to a larger area occupied by one HNT at the water/oil interface, explaining the higher detachment energy of HNTs in comparison to silica. Both types of hydrophilic particles stabilize an oil-in-water (o/w) emulsion and fulfill the Bancroft rule. Binks and Yin44 showed that hydrophilic silica cannot stabilize PEs of a nonpolar oil (dodecane, 99%). However, the addition of different dialkyl adipates at a low concentration leads to stable o/w PEs. The stability of these PEs increases with a decrease in the molecule chain because of the increased solubility of the adipate in the water phase. The explanation was provided by an in situ modification of the hydrophilic particles, which leads to hydrophobization of the particles forwarded by hydrogen bonds between the hydroxyl groups on the particles and the carbonyl group on the adipate. In the present study, no further additives in the nonpolar 1-dodecane (oil phase) were used, but stable PEs were obtained. It is assumed that the impurities of dodecene used in the present study hydrophobized the particles sufficiently and led to stable PEs. It was observed that the used Rh-SX exhibits an underestimated surface activity. First, the surface activity was compared between Rh-SX [Rh(acac)(CO)2 + SX] and the ligand SX. Their aqueous solutions show a decrease in the surface tension with respect to that of water. The surface activity is dominated by SX because of its amphiphilic character. The surface activity of Rh-SX is explained by the chemical structure of the SX. The two sulfonic acid groups are the hydrophilic head groups, while the remainder of the molecule acts as the hydrophobic segment. Acetylacetone (acac), which is released by the formation of Rh-SX [Rh(acac)(CO)2 + SX] and is part of the Rh-SX solution, has no effect on the surface tension. Further, the complexation with Rh does not affect the hydrophilic/hydrophobic balance of the ligand SX. The interfacial tension between dodecane and water is also affected by Rh-SX. Within the first minutes after droplet formation, the interfacial tension strongly decreases with time because of the adsorption of Rh-SX to the droplet surface. After 15−20 min, an interfacial tension of about 15 mN/m was reached, which is well below the value of the pure dodecane/water sample (49 mN/m). Pogrzeba et al.1 also observed a reduction of the interfacial tension to nearly the same value (15 mN/m) as that in this study by adding the same Rh-SX to a 1-dodecne/water system (cSX = 1 × 10−2 mol/L; SX:Rh = 4:1). However, a time-dependent interfacial tension decrease was not observed. The interfacial behavior of Rh-SX is comparable with that of the sodium dodecyl sulfate (SDS)/n-dodecane/water system, which was studied by I


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the interface, the van der Waals interaction acts as an attractive force. In addition, HNTs show significant attractive capillary forces, which lead to self-assembled bundles. In the case of silica, the capillary forces are significantly lower and even negligible. Adding the anionic Rh-SX leads to a change in the equilibrium, resulting from these forces. The anionic Rh-SX, which is also adsorbed at the interface, leads to an additional electrostatic repulsion between the particles. Reynaert et al.51 studied the effect of the surfactant SDS on the closely packed sulfonated polystyrene particle monolayer at a decane/water interface. Results showed that the addition of SDS to the particle monolayer also containing 0.5 M NaCl leads to a more open structure. This effect was explained by the increase in electrostatic repulsion and the decrease in capillary attraction. In particular, the increased particle−particle distance, which is an effect of a change in wettability, and the reduction of the capillary forces by adding SDS were cited as the reasons for the expansion of the monolayer. Besides this, ΔGparticles is higher for HNTs-stabilized emulsions compared to silica-stabilized PEs, which also improve the stability against coalescence. Further, it was assumed that ΔGparticles influences the movement of the particles at the interface. Dai et al.52 used laser scanning confocal microscopy to show that the diffusion of single particles at an emulsion interface depends on the oil-phase viscosity, particle size, and particle wettability and may be described by the Stokes−Einstein relationship. Because of the different wettabilities of the particles and their different values for ΔGparticles, the movement barrier for silica particles is lower in compensation for HNTs. Thus, the layer at the interface favors expansion of the particle monolayer and, with it, the interfacial catalytic reaction. Pristine HNTs-stabilized PEs prepared by low energy input (UT) leads to an unsatisfied conversion of 1-dodecene in hydroformylation. The droplets have a bigger size in comparison to PEs prepared by high energy input (sonication), which leads to a smaller internal interface, and are from the beginning covered by the HNTs. This leads to interruption of the contact between Rh-SX and 1-dodecene. An increased energy input during the PEs preparation leads to a drastic increase in conversion because of the decreased droplet size by an increase in the energy input during the PEs preparation. In the case of sonication, the mean droplet size is on the order of 300 nm for PEs containing Rh-SX. Further, it was shown that the emulsion stabilized only by Rh-SX can also be used for hydroformylation. Tao et al.20 also observed a conversion of 1dodecene in a biphasic system using a Rh-SX catalyst. Additionally, Zhao et al.21 showed conversion of 1-octene in a biphasic system by using a Rh-TPPTS [TPPTS = tris(3sulfophenyl)phosphine trisodium salt] catalyst. In the present study, the emulsion without the particles is disrupted and a phase separation was observed after the reaction. The emulsions in the reactor were stirred at 1200 rpm. Because of this mechanical stress on the droplets, an equilibrium was provided between coalescence and stabilized droplets, as shown in previous work.14 The emulsion only stabilized by RhSX showed the lowest stability against coalescence as the sample was centrifuged (Figure 9B). Because of this, the droplets in the reactor began to coalesce by stirring in the reactor, and a reduction of the conversion is observed after nearly 2.5 h of reaction time. Adding particles to the emulsion changes the ratio between coalescence and the stability of the droplets in the reactor. Directly after the preparation, the oil

almost the same as that obtained by adding Rh-SX to the water phase, with only a slight decrease from 30° to 27°. The contact angles of silica or HNTs correlate with the droplet sizes of the particle-stabilized PEs. The difference in the contact angle leads to a difference in the droplet size. However, a similar contact angle leads to a similar droplet size of the particlestabilized PEs. This might be a hint that the wettability of the particles correlates with the droplet size. It was observed that the stability was similar for silica- or HNTs-stabilized PEs in the absence of Rh-SX. The amounts of the released oil and residual emulsion phases after centrifugation at 14g for both particles were nearly the same, although the energy of detachment was higher for HNTsstabilized emulsions (34192 kT) in comparison to that of silica (7188 kT) because of their different sizes. It can be assumed that the energy of detachment using silica was still high enough to stabilize the emulsion phase against the mechanical stress caused by the centrifuge. Obviously, a further increase of ΔGparticles provided by the larger HNTs has no further effect on the stability of the emulsions. Further, the stability against coalescence can be a function of the coverage. Gautier et al.49 demonstrated that stronger covered PEs are more stable against compression than lower covered PEs. Adding Rh-SX to the PEs caused a drastic increase in the stability of the emulsions and a synergistic effect between RhSX and particles. This effect was observed for PEs of both particle types; however, ΔGparticles is reduced because of the lower three-phase contact angle and lower interfacial tension. Binks et al.50 observed a similar behavior in a tricaprylin oil-inwater emulsion containing a mixture of hydrophilic silica particles (the same as that in this study) and nonionic surfactant. It was shown that the stability of the droplets against coalescence increases if the surfactant and particles are simultaneously emulsified; however, ΔGparticles decreases. It decreases at a certain surfactant concentration because domination of the interfacial tension decrease is affected by the surfactant. This contradiction was explained by the fact that the theoretical calculation of ΔGparticles takes only the radius of one single particle into account; however, the real assembly of the particles at the interface is not known. In the study, it was mentioned that an increase in the particle radius by flocculation would lead to a higher calculation of ΔGparticles. Further, in the present study, the droplet size decreases if the emulsions contain Rh-SX, which also might increase the stability in comparison to the PEs without Rh-SX. The detailed effects of the catalyst on the particle structure and adsorption phenomena are not completely understood and have to be studied further. Further, significant differences in the stability were identified between the emulsions. The emulsion without particles shows the smallest residual emulsion phase, followed by silica + RhSX. The (HNTs + Rh-SX)-stabilized emulsion shows the highest residual emulsion phase and the highest stability against coalescence. The increase of the stability by using particles in comparison to the emulsion without particles indicates the adsorption of Rh-SX at the droplets and the simultaneous attachment of the particles at the oil/water interface. The higher stability for (HNTs + Rh-SX)-stabilized emulsions compared to (silica + Rh-SX)-stabilized emulsions might be explained by the higher coverage of the oil droplets by HNTs. The coverage of the droplets is an equilibrium between several attractive and repulsive forces. While electrostatic forces result in a repulsive force between the particles at J


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at silica is higher in comparison to that of the HNTs at the same used mass of particles, which may lead to the strong isomerization. Nevertheless, all emulsions showed a high selectivity for aldehydes of nearly 80%. All emulsions show good n:iso ratios, which are nearly comparable for all samples. Comparing the hydroformylation in PEs with heterogeneous catalysis, which makes separation of the catalyst after reaction easier but shows lower selectivity, activity, and leaching of the catalyst during dissociation of the complex,54−56 demonstrates that PEs combine the advantages of homogeneous and heterogeneous catalysis.

droplets are not covered with the particles because the droplets are too small, and a maximum contact is possible between the oil droplets and Rh-SX, which leads to a fast increase of the conversion. The mechanical stress causes a coalescence of the droplets, and the droplet size increases. Above a certain droplet size (in the micrometer range), the particles start to attach at the oil droplets and stabilize the droplets against coalescence (Figure 12). However, the particles interrupt the contact

5. CONCLUSION The present study shows strong synergy between a surfaceactive Rh catalyst and nanoparticles (silica and HNTs) for stabilization of o/w PEs. The PEs are used for the hydroformylation of a long-chain olefin (1-dodecene). Applying high energy input for PEs leads to droplet sizes on the order of nanometers. However, at this droplet size, the particles are not attached at the interface. Particle attachment occurs if the droplet size is on the order of micrometers. This might mimic the situation in the reactor if the droplets are small at the beginning of the reaction and become larger over time because of coalescence. For PEs stabilized by hydrophilic nanoparticles such as silica or HNTs, the mean droplet size decreases by adding the catalyst to the system. This is retraced to the surface activity of the catalyst and its ability to adsorb at the particles surface, which reduces the energy of detachment. The general behavior that the droplet size decreases with increasing particle concentration is preserved. Also, the mechanical stability of the PEs shows a strong synergistic effect and increases in the presence of catalyst and nanoparticles. An intermediate stability of the PEs seems to be favorable for high conversion. If the stability is too low, the droplets break fast and the contact between the catalyst and 1dodecene is low at the reduced interface. On the other hand, if the stability is too high, which is assumed to be caused by a high coverage of the droplet surface, the particles seem to block transfer between the catalyst and 1-dodecene. This also results in a low conversion. A high PEs stability is important for catalyst separation for methods like filtration. Nevertheless, a high selectivity for aldehyde was given for all emulsions. This demonstrates the complex counterplay between the emulsion (de)stability, conversion, and separation of the catalyst. A specific adjustment of the particle properties toward intermediate stability might lead to an intensification of the hydroformylation process, including separation in PEs, and to a possible industrial application of PEs even for other reaction systems.

Figure 12. Schematic illustration of the proposed mechanism of a Pickering emulsion in a reactor during hydroformylation of a longchain olefin.

between Rh-SX and 1-dodecene, which might lead to a slower increase of the conversion. In silica-stabilized PEs, the counterplay between the coalescence, stability, mobility of the particles at the interface, and density of the particle monolayer provides the highest conversion of 42 wt % (TOF = 55 h−1) after 5 h. However, the stability against coalescence at a stirring speed of 1200 rpm in the reactor is too small, and the emulsion breaks after the reaction. This leads to a phase separation between the oil and water. Because of their capillary force and higher energy of detachment, HNTs are less mobile than silica particles and are closer packed at the interface. This leads to the lowest conversion of 13 wt % (TOF = 18 h−1) after 5 h because of HNTs interruption of the contact between Rh-SX and the oil phase. However, the stability is higher than that of PEs prepared with silica nanoparticles, and the PEs are still stable after the reaction. Both the emulsion without particles and the PEs stabilized by HNTs showed comparable low amounts of side products (isododecene, isoaldehyde, and dodecane) in the oil phase. In the case of silica-stabilized PEs, the amounts of isoaldehyde and dodecane were low; however, the amount of isododecene was from the beginning of the hydroformylation nearly 3 times higher in comparison to the other two emulsions. The amounts of the side products do not increase with the reaction time and are nearly constant for all emulsions. This is a hint that the isomerization of 1-dodecene to isododecene in silica-stabilized PEs is affected by the silica particles during the preparation process. Gallaway and Murray53 showed that olefins can isomerize by the passage of olefins through a silica gel column. This isomerization might be affected by the silanol groups of the fumed silica particles (HDK N20). Because of the smaller size of the silica particles, the effective number of silanol groups

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49-(0)61511625647. ORCID

Dmitrij Stehl: 0000-0003-0168-040X Reinhard Schomäcker: 0000-0003-3106-3904 Regine von Klitzing: 0000-0003-0555-5104 Notes

The authors declare no competing financial interest. K


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ACKNOWLEDGMENTS

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REFERENCES

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This study was carried out in the frame of the Collaborative Research Centre/Transregio (CRG/TR) 63 “Integrated Chemical Processes in Liquid Multiphase Systems” (Subprojects B6 and A2), financed by the Deutsche Forschungsgemeinschaft (German Research Council). We thank Wacker Chemie AG for the silica particles, Maren Lehman for the cryo-TEM measurements, and Lena Hohl for analyzing the microscope images.

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