CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201301001
Micrometer-Scale Mixing with Pickering Emulsions: Biphasic Reactions without Stirring Wenjuan Zhang, Luman Fu, and Hengquan Yang*[a] A general strategy that avoids stirring for organic/aqueous reactions involving solid catalysts is reported. The strategy involves converting a conventional biphasic system into a Pickering emulsion phase with micrometer-scale droplets ensuring good mixing. In test reactions, nitrotoluene reduction and epoxidation of allylic alcohols, the reaction efficiency is comparable to conventional stirrer-driven biphasic catalysis reaction systems. Short diffusion distances, arising from the compartmentalization of densely packed droplets, play an important role in boosting the reaction efficiency.
Organic–aqueous biphasic reactions are ubiquitous in laboratory- and industry-scale syntheses.[1] Owing to the incompatibility in such reaction systems, especially for biphasic reactions involving solid catalysts, vigorous stirring is required to ensure suspension of solid catalyst particles and sufficient mixing of reactants, mitigating mass-transport resistance.[2] However, such vigorous stirring not only requires a large energy input, but causes fragmentation of catalyst particles because of repeated collisions and persistent abrasion, also.[3] Recently developed microfluidic and micromixing technologies open a promising avenue towards stirring-free reactions by enforcing continuous contact between immiscible reactants and catalysts in micrometer-sized spaces.[4] Chen and Nardello-Rataj found that the introduction of surface-active molecules to biphasic systems remarkably accelerates reactions under static conditions.[5] However, these techniques or methods require either specialized devices or the introduction of special additives. Ohtani et al. synthesized an amphiphilic solid catalysts able to suspend itself at the organic/aqueous phase boundary, and obtained an encouraging reaction efficiency without any stirring.[6] The reaction interface area created by the phase boundary, however, is very limited, and impedes the practical application of this technology. Accordingly, general and effective methods to realize stirring-free (static) processes are still a challenging task, although it seems beyond vision of researchers. Nano- to micrometer-sized particles with moderately wettable surfaces (i.e., interfacial activity) prefer to attach at an oil/ water interface, leading to Pickering emulsions [oil-in-water (o/ w) or water-in-oil (w/o)].[7] Pickering emulsions are emerging as [a] W. Zhang, L. Fu, Prof. Dr. H. Yang School of Chemistry and Chemical Engineering Shanxi University Wucheng Road 92,Taiyuan 030006 (PR China) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301001.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
an interesting platform for the design of efficient biphasic catalysis system because they can offer a large oil/water interface area.[8] This benefit has been demonstrated in hydrogenation,[8a] epoxidation,[8d] and enzyme catalysis.[8i] Moreover, Pickering emulsions offer long-term stability against particle sedimentation and macroscopic phase separation. These merits of Pickering emulsions may address the obstacles encountered in static biphasic reactions, such as suspension of solid catalysts and reactant mixing. Herein, we demonstrate a general strategy for static biphasic reactions that does not required specialized devices or special additives, and is based on transferring a biphasic system to one Pickering emulsion phase. This strategy allows to achieve reaction efficiencies comparable to conventional stirring-driven biphasic reactions. As Figure 1 shows, for a given organic/ aqueous biphasic reaction (a), one can formulate a Pickering emulsion phase by using an interfacially active solid catalyst. In this Pickering emulsion phase (e.g., a w/o emulsion in Figure 1), solid catalyst particles are homogeneously distributed on the surface of densely packed small droplets, not only creating a large reaction interface area but also yielding a stable three-dimensional network. The entire reaction system
Figure 1. The schematic description of a static Pickering emulsion reaction system A + B!C, where A and B represent reactant molecules and C is the product. a) A given organic/aqueous biphasic reaction. b) The formulated Pickering emulsion phase in the presence of an interfacially active solid catalyst. c) Reactant molecules A and B compartmentalized in the interspace of the adjacent droplets and in the droplet meet at the catalyst-covered interface through molecular self-diffusion. d) A catalyst particle. The gray dots on the particle represent catalytically active centers. The catalyst particle is partly immersed in organic medium and water.
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CHEMSUSCHEM COMMUNICATIONS is thereby uniformly compartmentalized into a network of numerous micrometer-scaled regions. As a result, the incompatible reactant molecules A (confined in the spaces between adjacent droplets) and B (confined within the droplets) are sufficiently mixed on a micrometer scale. Furthermore, the diffusion distances of these two molecules for reaction are significantly shortened because of the compartmentalization effect. The combination of large reaction interface area, high level of mixing, and short diffusion distances enables the reaction to proceed efficiently by autodiffusion of reactant molecules. An energy-consuming stirring process is thus avoided. We began our study by comparing our proposed static Pickering emulsion reaction with a reported phase-boundary reaction and a biphasic reaction under static conditions. We chose the palladium-catalyzed nitroarene reduction with NaBH4 as model reaction to evaluate the reaction efficiency of these three systems. This reaction is a typical biphasic reaction in which nitroarene is dissolved in an organic phase and NaBH4 in water. By depositing palladium nanoparticles onto triamineoctyl-bifunctionalized, 250–350 nm-sized SiO2 microspheres, which has proven interfacially active for the formation of a Pickering emulsion,[9] we obtained the solid catalyst Pd/SM-1 for this reaction (Pd loading: 0.44 wt %). We also synthesized its counterpart catalyst Pd/SM-2 by depositing palladium nanoparticles onto these same SiO2 microspheres but functionalized only with triamine groups (Pd loading: 0.77 wt %). The proposed structures and elemental compositions of Pd/SM-1 and Pd/SM-2 are shown in the Supporting Information (Figure S1), along with transmission electron microscopy (TEM) images, energy-dispersive X-ray (EDX) spectra, and N2 sorption isotherms (Figures S2–S4). Because Pd/SM-2 does not have hydrophobic octyl groups, unlike Pd/SM-1, it is highly hydrophilic and therefore interfacially inactive towards stabilizing Pickering emulsions. The palladium particles of these two catalysts were of similar size, thereby ensuring comparable intrinsic activity. By varying the solid catalyst (Pd/SM-1 or Pd/SM-2) and pretreatment conditions, we obtained the desired three reaction systems (Figure 2; the actual catalyst distributions in the reaction systems are shown in Figure S5 in the Supporting Information). After vigorously stirring the organic/aqueous mixture formulated according to the footnote of Figure 2 in the presence of Pd/SM-1 for 2 min (2000 rpm, emulsification by stirring), the biphasic system was successfully transferred to one Pickering emulsion phase. Numerous closely packed emulsion droplets were observed in this emulsion phase and the average size of the emulsion droplets was ca. 380 mm (Supporting Information, Figure S6). The emulsion type was confirmed to be w/o by a drop test. Also, using Pd/SM-1 but with gentle stirring (300 rpm for 2 min), the reaction system was just the reported phase boundary catalysis system,[6] in which Pd/SM-1 is located at the organic/aqueous phase boundary. Using Pd/SM-2, the reaction system rapidly split into two phases after stirring was stopped (2000 rpm), and Pd/SM-2 gradually deposited at the bottom of reaction vessel owing to its strong hydrophilicity. After standing for 60 min (without further stirring), these three systems gave strikingly different conversions (Figure 2). The static Pickering emulsion catalysis system afforded as high as 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. Reactivity comparison of m-nitrotoluene reduction in three different systems under static conditions. Reaction conditions: 2 mL of H2O, 0.7 mL of toluene, 1 mmol of m-nitrotoluene, 2 mmol of NaBH4, solid catalyst containing 1 mmol Pd. Pickering emulsion catalysis: Pd/SM-1 is dispersed at the surface of emulsion droplets. Phase-boundary catalysis: Pd/SM-1 is located at the two phase boundary. Biphasic catalysis: Pd/SM-2 is deposited at the bottom of the reaction vessel.
97 % conversion of m-nitrotoluene (a blank experiment revealed that the reaction does not occur in the absence of Pd catalyst). However, only 12 % and less than 1 % conversion were obtained in the phase boundary catalysis system and the biphasic catalysis system under the static conditions, respectively. Obviously, the reaction efficiency of the explored Pickering emulsion catalysis system is nearly one order of magnitude higher than that of the phase-boundary catalysis system, and two orders of magnitude higher than that of the biphasic catalysis system. These comparisons demonstrate the outstanding efficiency of the static Pickering emulsion reaction. To clarify the role of the presence of emulsion droplets, we examined the reaction efficiency of the static Pickering emulsion systems with different droplet sizes. To obtain this goal, we varied the amount of solid catalyst because it has already been established that the emulsion droplet diameter decreases as the amount of solid particles increases.[10] As displayed in Figure S7 (Supporting Information), when the Pd/SM-1 amount was varied from 0.3 wt % to 0.6 wt %, 0.9 wt %, 1.2 wt %, and 1.8 wt % (with respect to water), the average droplet diameter decreased from 1050 mm to 800, 510, 380, and 260 mm. Further increasing the catalyst amount caused polydispersity of emulsion droplets. The kinetic profiles for these static Pickering emulsion systems are shown in Figure 3 A. Obviously, the reaction rate increases with increasing the amount of Pd/SM-1. According to the kinetic profiles (before the conversion reachChemSusChem 2014, 7, 391 – 396
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www.chemsuschem.org gests that the reaction efficiency is governed by other factors besides the reaction interface area. This is directly supported by results that the reaction area-normalized efficiency increases with decreasing the droplet size (Supporting Information, Figure S8). The difference in the diffusion distance of reactant molecules may be responsible for the variation of catalyst mass-normalized efficiency because the mass transfer in the static system depends on the molecular self-diffusion that is driven by the concentration difference at the reaction interface and in the dispersed or continuous phase. This speculation can be rationalized as follows: The time for the reactant molecules to reach the reaction interface can be estimated according to:[11] t D ¼ L2 =D
Figure 3. Kinetic data for the static Pickering emulsion systems with different amounts of Pd/SM-1. A) The kinetic profiles of m-nitrotoluene reduction (the reaction conditions are the same as Figure 2, except the varied catalyst amount). a) 0.3 wt % Pd/SM-1; b) 0.6 wt % Pd/SM-1; c) 0.9 wt % Pd/SM-1; d) 1.2 wt % Pd/SM-1; e) 1.8 wt % Pd/SM-1. B) Plots of S and T versus droplet diameter. T = moles of the converted m-nitrotoluene per catalyst mass per reaction time (mol g1 h1), where T is estimated before the conversion reaches 20 %; S = the droplet surface area per catalyst mass (m2 g1).
es 20 %), we can estimate the catalyst mass-normalized efficiency T (the reaction efficiency per gram of solid catalyst, see the footnotes of Figure 3). It is interesting to find that T gradually increases with increasing the catalyst amount, which differs from the conventional biphasic systems. That is, the catalyst mass-normalized efficiency T increases as the droplet diameter decreases, as shown in Figure 3 B. In parallel, it is found that catalyst mass-normalized reaction area S (the reaction interface area created by per gram of solid catalyst, i.e., the total reaction interface area/the weight of solid catalyst. Please see the footnotes of Figure 3 and Equation (1) in the Supporting Information) does not vary much despite the change of the emulsion droplet size, as shown in Figure 3 B. This is not surprising because all catalyst particles are thermodynamically preferable to be distributed on the droplet surface in the densely arranged fashion. The surface coverage of above 0.9 that is estimated according to Equation (2) in the Supporting Information supports that catalyst particles are densely arranged on the droplet surface. The combining consideration of these two plots (T versus droplet diameter and S versus droplet diameter) allows us to conclude that for these systems the solid catalyst has the same ability to create reaction interface area, but exhibits different catalysis efficiency. The conclusion further sug 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
In this equation, tD is diffusion time, L is diffusion distance, and D is the molecular diffusion coefficient. As shown in Figure 1, due to the compartmentalization of densely packed droplets, the diffusion distances of the reactant molecules A and B to the droplet surface (reaction interface) are both related to the droplet diameter. The smaller the droplet is, the shorter the diffusion distance is. The shorter diffusion distance leads to the shorter time that the reactant molecules take to reach the reaction interface. This can well explain the dependence of catalyst mass-normalized efficiency (T) on the droplet diameter. Typically, the diffusion coefficient of molecules in a solution is in the range of 1 109–1 1010 m2 s1.[12] The time that the reactant molecules take to reach the reaction interface in the above systems is estimated to be in the range of ca. 1–23 min (Table S1 in Supporting Information). In principle, this relatively short diffusion time suggests the feasibility of our static strategy. To further validate the diffusion distance effects, we formulated another set of Pickering emulsion systems using 0.6 wt % solid catalyst plus different amounts of the support (the support triamine-octyl-bifunctionalized silica helps Pd/SM-1 emulsify the system, but it per se is inactive for this reaction). By this way, we could tune the emulsion droplet size and therefore regulated the diffusion distance of reactant molecules, while keeping the catalyst amount unchanged. When the support amount was changed from 0.3 wt % to 0.6 wt % and 1.2 wt % (with respect to water), the corresponding droplet size decreased from 500 mm to 380 and 250 mm (Figure S9 in Supporting Information). These Pickering emulsion systems show significantly different reaction efficiency although the catalyst amount is the same (Figure S10 in Supporting Information). The smaller droplet led to the higher catalyst mass-normalized efficiency (Figure S11 in supporting information). These results also confirm the impact of the molecular diffusion distance on reaction efficiency. The reaction efficiency of the static Pickering emulsion strategy is further highlighted by its comparison with the stirring Pickering emulsion system and the conventional stirrer-driven biphasic system. The kinetic profiles of m-nitrotoluene reduction in these three systems are shown in Figure 4 A. The reaction rate of the static Pickering emulsion system is comparable ChemSusChem 2014, 7, 391 – 396
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accomplished by stirring within ca. 2 min. After standing for 60 min, the conversions reached 99 % and 91 %, respectively. The reaction efficiency does not significantly decrease as the reaction scale increases. This may be explained as a result of the fact that the Pickering emulsion phase and emulsion droplet size were well maintained after scaling-up (Supporting Information, Figure S13). In this regard, it is superior to the conventional stirring-driven biphasic reactions, of which the reaction efficiency often decreases after scaling-up because it is difficult to mix all parts of the reaction system for a large reaction vessel. These results suggest that our strategy is potentially useful for practical Figure 4. The results of the reduction of nitroarene to arylamine in three different systems. A) The kinetic profiles synthesis although heat transfer of m-nitrotoluene reduction. a) the static emulsion system using Pd/SM-1; b) the emulsion system under stirring for this strategy needs further in(800 rpm) using Pd/SM-1; c, the conventional stirring-driven biphasic system using Pd/SM-2 (800 rpm). B) Comparison of the reaction efficiency of the static Pickering emulsion system with the biphasic system stirred at different vestigation. The heat transfer for set speeds (1 h). C) Results for other nitroarenes (2 h). The reaction conditions are the same as those in Figure 3. those exothermic reactions may D) The recycling results of the static emulsion system (60–80 min). be addressed by optimization of the shape of the reaction vessel. The static Pickering emulsion strategy is extendable to other biphasic reactions, for example, to the Pickering emulsion system under stirring (800 rpm, the epoxidation of allylic alcohols with H2O2 (an important rea magnetic stir bar with length of 10 mm). Notably, the static Pickering emulsion system proceeds faster than the convenaction for producing epoxides). Polyoxometalate [POM, tional stirrer-driven biphasic system (800 rpm, using Pd/SM-2 Na12WZn3(ZnW9O34)2] is an efficient catalyst for this reaction.[13] as catalyst). Impressively, its reaction efficiency is even compaIn order to create an interfacially active solid POM catalyst, we rable to the vigorous-stirring-driven biphasic system (Figused a bifunctional reagent, [3-(trimethoxysily)propyl]-octadeure 4 B). For the other investigated substrates, the static Pickercyldimethylammonium chloride, to modify the aforementioned ing emulsion system also exhibits reaction efficiency comparasilica microsphere through silylation. The presence of quaterble to (or higher than) the Pickering emulsion system under nary ammonium sites on the silica microsphere surface allows stirring (although stirring can promote the molecular diffusion, us to immobilize POM by ionic interactions, thereby obtaining it also causes the droplet breakage) and the Pd/SM-2-catalyzed a solid catalyst POM/SM-3. The interplay between the hydroconventional biphasic system under stirring (Figure 4 C). These phobic alkyl chains, a hydrophilic ammonium salt, and POMs comparisons convincingly demonstrate the high reaction effion the surface result in a moderate wettability (interfacial activciency of the static Pickering emulsion reaction. These results ity) for the formation of Pickering emulsions. For comparison also reveal that for the Pickering emulsion systems, the high with the conventional stirring-driven biphasic reaction, we also level of mixing, large reaction interface area and short diffusion prepared its counterpart catalyst POM/SM-4 (POM is loaded on distance make stirring unnecessary to some extent. This is pro3-(trimethoxysily)propyl-dimethylammonium chloride-functionfoundly advantageous over the conventional biphasic system alized silica microsphere). Owing to the strong hydrophilicity, that requires vigorous stirring during the whole reaction POM/SM-4 is unable to stabilize Pickering emulsion (even with course to prevent the macroscopic phase separation and catavigorous stirring). The proposed structures, elemental composilyst sedimentation. Furthermore, at the end of reaction, Pd/ tion, TEM images and N2 sorption isotherms of POM/SM-3 and SM-1 could be recovered from the Pickering emulsion phase POM/SM-4 are supplied in Figure S14–16 in Supporting Inforthrough centrifugation. The reaction efficiency had no signifimation. cant decrease during the consecutive reaction cycles (FigA Pickering emulsion phase was obtained after stirring the ure 4 D. The morphology of the emulsion droplets was mainreaction system in the presence of POM/SM-3 (2000 rpm for tained, as Figure S12 in Supporting Information showed). 2 min. Droplets with diameters of 200 ~ 400 mm were clearly We next scaled up the reduction reaction to 40 and 400 mL. observed, as shown in Figure S17 in Supporting Information). For the scaled-up reactions, the emulsification was still readily The comparative results of the static Pickering emulsion epoxi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1. Epoxidation of allylic alcohols in (A) a static Pickering emulsion system, and (B) a conventional stirrer-driven biphasic system (800 rpm).
Entry 1[a]
[a]
2
3[a]
[b]
4
5[c]
Substrates
Conditions[a]
T [8C]
Conv. [%]
A
50
99
B
50
93
A
50
97
B
50
98
A
50
98
B
50
93
A
50
71
B
50
63
A
70
74
B
70
63
[a] 1 mmol of substrate, 1 mL of ethyl acetate, solid catalyst containing 0.5 mmol POM, 2 mL of water, and 0.34 g of aqueous 30 wt % H2O2. [b] 0.5 mmol of substrate, 1 mL of ethyl acetate, 2 mL of water, solid catalyst containing 0.5 mmol POM, and 0.17 g of aqueous 30 wt % H2O2. [c] 0.5 mmol of substrate, 1 mL of toluene, 2 mL of water, solid catalyst containing 0.5 mmol POM, and 0.17 g of aqueous 30 wt % H2O2.
dation (A) and the conventional stirring-driven biphasic epoxidation (B, using POM/SM-4 as catalyst) are summarized in Table 1. For all the investigated allylic alcohols, the static Pickering emulsion system afforded good to excellent conversions within 5 h (the selectivity for all epoxides was more than 97 %). Its conversions are comparable to (or higher than) those of the conventional stirring-driven biphasic reactions (800 rpm). Although POM/SM-3 is found to be less active towards terpenes and terminal alkenes (similarly to unsupported Na12WZn3(ZnW9O34)213b), these comparisons in terms of allylic alcohols sufficiently demonstrate the effectiveness of the static Pickering emulsion strategy. In summary, we demonstrate a general strategy to conduct organic/aqueous reactions involving solid catalysts without stirring. The strategy is based on transferring a conventional biphasic reaction into a system comprising one Pickering emulsion phase. The static Pickering emulsion system exhibits a much higher reaction efficiency than reported phase-boundary catalysis systems. Impressively, our investigations show that the reaction efficiency of the static Pickering emulsion is comparable to that of the conventional stirrer-driven biphasic systems. The short diffusion distance arising from the compartmentalization of densely packed droplets plays an important role in boosting the reaction efficiency. In view of the low energy input, high reaction efficiency, and simple work-up, we envision that this strategy will bring biphasic reactions with fundamental innovations toward more green and sustainable chemistry. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Experimental Section Typical procedure for nitroarene reaction in the static Pickering emulsion system: 0.024 g of solid catalyst was added into 2 mL of water. NaBH4 (2 mmol), toluene (0.7 mL) and nitroarene (1 mmol) were then added into the above suspension. After stirring for 2 min (2000 rpm, emulsification), the reaction proceeded under the static conditions. The conversion of nitroarene was monitored by gas chromatography (GC). Preparation procedures for Pd/SM-1 and Pd/SM-2 can be found in the Supporting Information. Typical procedure for epoxidation of allylic alcohols in the static Pickering emulsion system: the given amounts of water, ethyl acetate, POM/SM-3 (or POM/SM-4), olefins, and H2O2 were added into a test tube (their detailed amounts can be found in the footnotes of Table 1), and were stirred vigorously for 2 min to form one Pickering emulsion phase (2000 rpm). After the reaction system was left to stand for a given time, the organic layer was isolated through centrifugation. The conversion was determined with GC analysis. The preparation procedures of POM/SM-3 and POM/ SM-4 are included in the Supporting Information.
Acknowledgements This work is supported by the Natural Science Foundation of China (20903064, 21173137), Program for New Century Excellent Talents in University (NECT-12-1030), Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi (2011002), and Middle-aged Innovative Talents of Higher Learning Institutions of Shanxi (20120202). Keywords: biphasic catalysis · diffusion · kinetics · phasetransfer catalysis · synthetic methods [1] a) S. Minakata, M. Komatsu, Chem. Rev. 2009, 109, 711 – 724; b) F. Jo, Acc. Chem. Res. 2002, 35, 738 – 745; c) C. J. Li, L. Chen, Chem. Soc. Rev. 2006, 35, 68 – 82; d) X. F. Wu, J. L. Xiao, Chem. Commun. 2007, 2449 – 2466; e) H. X. Wang, H. Q. Yang, H. R. Liu, Y. H. Yu, H. C. Xin, Langmuir 2013, 29, 6687 – 6696. [2] a) X. H. Li, W. L. Zheng, H. Y. Pan, Y. Yu, L. Chen, P. Wu, J. Catal. 2013, 300, 9 – 19; b) A. H. G. Cents, D. W. F. Brilman, G. F. Versteeg, Ind. Eng. Chem. Res. 2004, 43, 7465 – 7475; c) W. H. Chong, L. K. Chin, R. L. S. Tan, H. Wang, A. Q. Liu, H. Chen, Angew. Chem. 2013, 125, 8732 – 8735; Angew. Chem. Int. Ed. 2013, 52, 8570 – 8573. [3] T. J. Lin, X. Meng, L. Shi, Ind. Eng. Chem. Res. 2012, 51, 13123 – 13143. [4] a) K. Jhnisch, V. Hessel, H. Lçwe, M. Baerns, Angew. Chem. 2004, 116, 410 – 451; Angew. Chem. Int. Ed. 2004, 43, 406 – 446; b) R. L. Hartman, J. P. McMullen, K. F. Jensen, Angew. Chem. 2011, 123, 7642 – 7661; Angew. Chem. Int. Ed. 2011, 50, 7502 – 7519; c) B. Zhao, J. S. Moore, D. J. Beebe, Science 2001, 291, 1023 – 1026. [5] a) H. Y. Fu, M. Li, H. Chen, X. J. Li, J. Mol. Catal. A 2006, 259, 156 – 160; b) L. Leclercq, R. Company, A. Mhlbauer, A. Mouret, J. M. Aubry, V. Nardello-Rataj, ChemSusChem 2013, 6, 1533 – 1540. [6] a) H. Nur, S. Ikeda, B. Ohtani, Chem. Commun. 2000, 2235 – 2236; b) S. Ikeda, H. Nur, T. Sawadaishi, K. Ijiro, M. Shimomura, B. Ohtani, Langmuir 2001, 17, 7976 – 7979; c) Y. K. Takahara, S. Ikeda, S. Ishino, K. Tachi, K. Ikeue, T. Sakata, T. Hasegawa, H. Mori, M. Matsumura, B. Ohtani, J. Am. Chem. Soc. 2005, 127, 6271 – 6275. [7] a) R. Aveyard, B. P. Binks, J. H. Clint, Adv. Colloid Interface Sci. 2003, 100, 503 – 546; b) S. Tsuji, H. Kawaguchi, Langmuir 2008, 24, 3300 – 3305; c) N. X. Yan, M. R. Gray, J. H. Masliyah, Colloids Surf. A 2001, 193, 97 – 107; d) L. A. Fielding, S. P. Armes, J. Mater. Chem. 2012, 22, 11235 – 11244; e) E. S. Read, S. Fujii, J. I. Amalvy, D. P. Randall, S. P. Armes, Langmuir 2004, 20, 7422 – 7429.
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Received: September 19, 2013 Revised: October 29, 2013 Published online on December 16, 2013
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Micrometer-Scale Mixing with Pickering Emulsions: Biphasic Reactions without Stirring Wenjuan Zhang, Luman Fu, and Hengquan Yang*[a] A general strategy that avoids stirring for organic/aqueous reactions involving solid catalysts is reported. The strategy involves converting a conventional biphasic system into a Pickering emulsion phase with micrometer-scale droplets ensuring good mixing. In test reactions, nitrotoluene reduction and epoxidation of allylic alcohols, the reaction efficiency is comparable to conventional stirrer-driven biphasic catalysis reaction systems. Short diffusion distances, arising from the compartmentalization of densely packed droplets, play an important role in boosting the reaction efficiency.
Organic–aqueous biphasic reactions are ubiquitous in laboratory- and industry-scale syntheses.[1] Owing to the incompatibility in such reaction systems, especially for biphasic reactions involving solid catalysts, vigorous stirring is required to ensure suspension of solid catalyst particles and sufficient mixing of reactants, mitigating mass-transport resistance.[2] However, such vigorous stirring not only requires a large energy input, but causes fragmentation of catalyst particles because of repeated collisions and persistent abrasion, also.[3] Recently developed microfluidic and micromixing technologies open a promising avenue towards stirring-free reactions by enforcing continuous contact between immiscible reactants and catalysts in micrometer-sized spaces.[4] Chen and Nardello-Rataj found that the introduction of surface-active molecules to biphasic systems remarkably accelerates reactions under static conditions.[5] However, these techniques or methods require either specialized devices or the introduction of special additives. Ohtani et al. synthesized an amphiphilic solid catalysts able to suspend itself at the organic/aqueous phase boundary, and obtained an encouraging reaction efficiency without any stirring.[6] The reaction interface area created by the phase boundary, however, is very limited, and impedes the practical application of this technology. Accordingly, general and effective methods to realize stirring-free (static) processes are still a challenging task, although it seems beyond vision of researchers. Nano- to micrometer-sized particles with moderately wettable surfaces (i.e., interfacial activity) prefer to attach at an oil/ water interface, leading to Pickering emulsions [oil-in-water (o/ w) or water-in-oil (w/o)].[7] Pickering emulsions are emerging as [a] W. Zhang, L. Fu, Prof. Dr. H. Yang School of Chemistry and Chemical Engineering Shanxi University Wucheng Road 92,Taiyuan 030006 (PR China) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301001.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
an interesting platform for the design of efficient biphasic catalysis system because they can offer a large oil/water interface area.[8] This benefit has been demonstrated in hydrogenation,[8a] epoxidation,[8d] and enzyme catalysis.[8i] Moreover, Pickering emulsions offer long-term stability against particle sedimentation and macroscopic phase separation. These merits of Pickering emulsions may address the obstacles encountered in static biphasic reactions, such as suspension of solid catalysts and reactant mixing. Herein, we demonstrate a general strategy for static biphasic reactions that does not required specialized devices or special additives, and is based on transferring a biphasic system to one Pickering emulsion phase. This strategy allows to achieve reaction efficiencies comparable to conventional stirring-driven biphasic reactions. As Figure 1 shows, for a given organic/ aqueous biphasic reaction (a), one can formulate a Pickering emulsion phase by using an interfacially active solid catalyst. In this Pickering emulsion phase (e.g., a w/o emulsion in Figure 1), solid catalyst particles are homogeneously distributed on the surface of densely packed small droplets, not only creating a large reaction interface area but also yielding a stable three-dimensional network. The entire reaction system
Figure 1. The schematic description of a static Pickering emulsion reaction system A + B!C, where A and B represent reactant molecules and C is the product. a) A given organic/aqueous biphasic reaction. b) The formulated Pickering emulsion phase in the presence of an interfacially active solid catalyst. c) Reactant molecules A and B compartmentalized in the interspace of the adjacent droplets and in the droplet meet at the catalyst-covered interface through molecular self-diffusion. d) A catalyst particle. The gray dots on the particle represent catalytically active centers. The catalyst particle is partly immersed in organic medium and water.
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CHEMSUSCHEM COMMUNICATIONS is thereby uniformly compartmentalized into a network of numerous micrometer-scaled regions. As a result, the incompatible reactant molecules A (confined in the spaces between adjacent droplets) and B (confined within the droplets) are sufficiently mixed on a micrometer scale. Furthermore, the diffusion distances of these two molecules for reaction are significantly shortened because of the compartmentalization effect. The combination of large reaction interface area, high level of mixing, and short diffusion distances enables the reaction to proceed efficiently by autodiffusion of reactant molecules. An energy-consuming stirring process is thus avoided. We began our study by comparing our proposed static Pickering emulsion reaction with a reported phase-boundary reaction and a biphasic reaction under static conditions. We chose the palladium-catalyzed nitroarene reduction with NaBH4 as model reaction to evaluate the reaction efficiency of these three systems. This reaction is a typical biphasic reaction in which nitroarene is dissolved in an organic phase and NaBH4 in water. By depositing palladium nanoparticles onto triamineoctyl-bifunctionalized, 250–350 nm-sized SiO2 microspheres, which has proven interfacially active for the formation of a Pickering emulsion,[9] we obtained the solid catalyst Pd/SM-1 for this reaction (Pd loading: 0.44 wt %). We also synthesized its counterpart catalyst Pd/SM-2 by depositing palladium nanoparticles onto these same SiO2 microspheres but functionalized only with triamine groups (Pd loading: 0.77 wt %). The proposed structures and elemental compositions of Pd/SM-1 and Pd/SM-2 are shown in the Supporting Information (Figure S1), along with transmission electron microscopy (TEM) images, energy-dispersive X-ray (EDX) spectra, and N2 sorption isotherms (Figures S2–S4). Because Pd/SM-2 does not have hydrophobic octyl groups, unlike Pd/SM-1, it is highly hydrophilic and therefore interfacially inactive towards stabilizing Pickering emulsions. The palladium particles of these two catalysts were of similar size, thereby ensuring comparable intrinsic activity. By varying the solid catalyst (Pd/SM-1 or Pd/SM-2) and pretreatment conditions, we obtained the desired three reaction systems (Figure 2; the actual catalyst distributions in the reaction systems are shown in Figure S5 in the Supporting Information). After vigorously stirring the organic/aqueous mixture formulated according to the footnote of Figure 2 in the presence of Pd/SM-1 for 2 min (2000 rpm, emulsification by stirring), the biphasic system was successfully transferred to one Pickering emulsion phase. Numerous closely packed emulsion droplets were observed in this emulsion phase and the average size of the emulsion droplets was ca. 380 mm (Supporting Information, Figure S6). The emulsion type was confirmed to be w/o by a drop test. Also, using Pd/SM-1 but with gentle stirring (300 rpm for 2 min), the reaction system was just the reported phase boundary catalysis system,[6] in which Pd/SM-1 is located at the organic/aqueous phase boundary. Using Pd/SM-2, the reaction system rapidly split into two phases after stirring was stopped (2000 rpm), and Pd/SM-2 gradually deposited at the bottom of reaction vessel owing to its strong hydrophilicity. After standing for 60 min (without further stirring), these three systems gave strikingly different conversions (Figure 2). The static Pickering emulsion catalysis system afforded as high as 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. Reactivity comparison of m-nitrotoluene reduction in three different systems under static conditions. Reaction conditions: 2 mL of H2O, 0.7 mL of toluene, 1 mmol of m-nitrotoluene, 2 mmol of NaBH4, solid catalyst containing 1 mmol Pd. Pickering emulsion catalysis: Pd/SM-1 is dispersed at the surface of emulsion droplets. Phase-boundary catalysis: Pd/SM-1 is located at the two phase boundary. Biphasic catalysis: Pd/SM-2 is deposited at the bottom of the reaction vessel.
97 % conversion of m-nitrotoluene (a blank experiment revealed that the reaction does not occur in the absence of Pd catalyst). However, only 12 % and less than 1 % conversion were obtained in the phase boundary catalysis system and the biphasic catalysis system under the static conditions, respectively. Obviously, the reaction efficiency of the explored Pickering emulsion catalysis system is nearly one order of magnitude higher than that of the phase-boundary catalysis system, and two orders of magnitude higher than that of the biphasic catalysis system. These comparisons demonstrate the outstanding efficiency of the static Pickering emulsion reaction. To clarify the role of the presence of emulsion droplets, we examined the reaction efficiency of the static Pickering emulsion systems with different droplet sizes. To obtain this goal, we varied the amount of solid catalyst because it has already been established that the emulsion droplet diameter decreases as the amount of solid particles increases.[10] As displayed in Figure S7 (Supporting Information), when the Pd/SM-1 amount was varied from 0.3 wt % to 0.6 wt %, 0.9 wt %, 1.2 wt %, and 1.8 wt % (with respect to water), the average droplet diameter decreased from 1050 mm to 800, 510, 380, and 260 mm. Further increasing the catalyst amount caused polydispersity of emulsion droplets. The kinetic profiles for these static Pickering emulsion systems are shown in Figure 3 A. Obviously, the reaction rate increases with increasing the amount of Pd/SM-1. According to the kinetic profiles (before the conversion reachChemSusChem 2014, 7, 391 – 396
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www.chemsuschem.org gests that the reaction efficiency is governed by other factors besides the reaction interface area. This is directly supported by results that the reaction area-normalized efficiency increases with decreasing the droplet size (Supporting Information, Figure S8). The difference in the diffusion distance of reactant molecules may be responsible for the variation of catalyst mass-normalized efficiency because the mass transfer in the static system depends on the molecular self-diffusion that is driven by the concentration difference at the reaction interface and in the dispersed or continuous phase. This speculation can be rationalized as follows: The time for the reactant molecules to reach the reaction interface can be estimated according to:[11] t D ¼ L2 =D
Figure 3. Kinetic data for the static Pickering emulsion systems with different amounts of Pd/SM-1. A) The kinetic profiles of m-nitrotoluene reduction (the reaction conditions are the same as Figure 2, except the varied catalyst amount). a) 0.3 wt % Pd/SM-1; b) 0.6 wt % Pd/SM-1; c) 0.9 wt % Pd/SM-1; d) 1.2 wt % Pd/SM-1; e) 1.8 wt % Pd/SM-1. B) Plots of S and T versus droplet diameter. T = moles of the converted m-nitrotoluene per catalyst mass per reaction time (mol g1 h1), where T is estimated before the conversion reaches 20 %; S = the droplet surface area per catalyst mass (m2 g1).
es 20 %), we can estimate the catalyst mass-normalized efficiency T (the reaction efficiency per gram of solid catalyst, see the footnotes of Figure 3). It is interesting to find that T gradually increases with increasing the catalyst amount, which differs from the conventional biphasic systems. That is, the catalyst mass-normalized efficiency T increases as the droplet diameter decreases, as shown in Figure 3 B. In parallel, it is found that catalyst mass-normalized reaction area S (the reaction interface area created by per gram of solid catalyst, i.e., the total reaction interface area/the weight of solid catalyst. Please see the footnotes of Figure 3 and Equation (1) in the Supporting Information) does not vary much despite the change of the emulsion droplet size, as shown in Figure 3 B. This is not surprising because all catalyst particles are thermodynamically preferable to be distributed on the droplet surface in the densely arranged fashion. The surface coverage of above 0.9 that is estimated according to Equation (2) in the Supporting Information supports that catalyst particles are densely arranged on the droplet surface. The combining consideration of these two plots (T versus droplet diameter and S versus droplet diameter) allows us to conclude that for these systems the solid catalyst has the same ability to create reaction interface area, but exhibits different catalysis efficiency. The conclusion further sug 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
In this equation, tD is diffusion time, L is diffusion distance, and D is the molecular diffusion coefficient. As shown in Figure 1, due to the compartmentalization of densely packed droplets, the diffusion distances of the reactant molecules A and B to the droplet surface (reaction interface) are both related to the droplet diameter. The smaller the droplet is, the shorter the diffusion distance is. The shorter diffusion distance leads to the shorter time that the reactant molecules take to reach the reaction interface. This can well explain the dependence of catalyst mass-normalized efficiency (T) on the droplet diameter. Typically, the diffusion coefficient of molecules in a solution is in the range of 1 109–1 1010 m2 s1.[12] The time that the reactant molecules take to reach the reaction interface in the above systems is estimated to be in the range of ca. 1–23 min (Table S1 in Supporting Information). In principle, this relatively short diffusion time suggests the feasibility of our static strategy. To further validate the diffusion distance effects, we formulated another set of Pickering emulsion systems using 0.6 wt % solid catalyst plus different amounts of the support (the support triamine-octyl-bifunctionalized silica helps Pd/SM-1 emulsify the system, but it per se is inactive for this reaction). By this way, we could tune the emulsion droplet size and therefore regulated the diffusion distance of reactant molecules, while keeping the catalyst amount unchanged. When the support amount was changed from 0.3 wt % to 0.6 wt % and 1.2 wt % (with respect to water), the corresponding droplet size decreased from 500 mm to 380 and 250 mm (Figure S9 in Supporting Information). These Pickering emulsion systems show significantly different reaction efficiency although the catalyst amount is the same (Figure S10 in Supporting Information). The smaller droplet led to the higher catalyst mass-normalized efficiency (Figure S11 in supporting information). These results also confirm the impact of the molecular diffusion distance on reaction efficiency. The reaction efficiency of the static Pickering emulsion strategy is further highlighted by its comparison with the stirring Pickering emulsion system and the conventional stirrer-driven biphasic system. The kinetic profiles of m-nitrotoluene reduction in these three systems are shown in Figure 4 A. The reaction rate of the static Pickering emulsion system is comparable ChemSusChem 2014, 7, 391 – 396
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accomplished by stirring within ca. 2 min. After standing for 60 min, the conversions reached 99 % and 91 %, respectively. The reaction efficiency does not significantly decrease as the reaction scale increases. This may be explained as a result of the fact that the Pickering emulsion phase and emulsion droplet size were well maintained after scaling-up (Supporting Information, Figure S13). In this regard, it is superior to the conventional stirring-driven biphasic reactions, of which the reaction efficiency often decreases after scaling-up because it is difficult to mix all parts of the reaction system for a large reaction vessel. These results suggest that our strategy is potentially useful for practical Figure 4. The results of the reduction of nitroarene to arylamine in three different systems. A) The kinetic profiles synthesis although heat transfer of m-nitrotoluene reduction. a) the static emulsion system using Pd/SM-1; b) the emulsion system under stirring for this strategy needs further in(800 rpm) using Pd/SM-1; c, the conventional stirring-driven biphasic system using Pd/SM-2 (800 rpm). B) Comparison of the reaction efficiency of the static Pickering emulsion system with the biphasic system stirred at different vestigation. The heat transfer for set speeds (1 h). C) Results for other nitroarenes (2 h). The reaction conditions are the same as those in Figure 3. those exothermic reactions may D) The recycling results of the static emulsion system (60–80 min). be addressed by optimization of the shape of the reaction vessel. The static Pickering emulsion strategy is extendable to other biphasic reactions, for example, to the Pickering emulsion system under stirring (800 rpm, the epoxidation of allylic alcohols with H2O2 (an important rea magnetic stir bar with length of 10 mm). Notably, the static Pickering emulsion system proceeds faster than the convenaction for producing epoxides). Polyoxometalate [POM, tional stirrer-driven biphasic system (800 rpm, using Pd/SM-2 Na12WZn3(ZnW9O34)2] is an efficient catalyst for this reaction.[13] as catalyst). Impressively, its reaction efficiency is even compaIn order to create an interfacially active solid POM catalyst, we rable to the vigorous-stirring-driven biphasic system (Figused a bifunctional reagent, [3-(trimethoxysily)propyl]-octadeure 4 B). For the other investigated substrates, the static Pickercyldimethylammonium chloride, to modify the aforementioned ing emulsion system also exhibits reaction efficiency comparasilica microsphere through silylation. The presence of quaterble to (or higher than) the Pickering emulsion system under nary ammonium sites on the silica microsphere surface allows stirring (although stirring can promote the molecular diffusion, us to immobilize POM by ionic interactions, thereby obtaining it also causes the droplet breakage) and the Pd/SM-2-catalyzed a solid catalyst POM/SM-3. The interplay between the hydroconventional biphasic system under stirring (Figure 4 C). These phobic alkyl chains, a hydrophilic ammonium salt, and POMs comparisons convincingly demonstrate the high reaction effion the surface result in a moderate wettability (interfacial activciency of the static Pickering emulsion reaction. These results ity) for the formation of Pickering emulsions. For comparison also reveal that for the Pickering emulsion systems, the high with the conventional stirring-driven biphasic reaction, we also level of mixing, large reaction interface area and short diffusion prepared its counterpart catalyst POM/SM-4 (POM is loaded on distance make stirring unnecessary to some extent. This is pro3-(trimethoxysily)propyl-dimethylammonium chloride-functionfoundly advantageous over the conventional biphasic system alized silica microsphere). Owing to the strong hydrophilicity, that requires vigorous stirring during the whole reaction POM/SM-4 is unable to stabilize Pickering emulsion (even with course to prevent the macroscopic phase separation and catavigorous stirring). The proposed structures, elemental composilyst sedimentation. Furthermore, at the end of reaction, Pd/ tion, TEM images and N2 sorption isotherms of POM/SM-3 and SM-1 could be recovered from the Pickering emulsion phase POM/SM-4 are supplied in Figure S14–16 in Supporting Inforthrough centrifugation. The reaction efficiency had no signifimation. cant decrease during the consecutive reaction cycles (FigA Pickering emulsion phase was obtained after stirring the ure 4 D. The morphology of the emulsion droplets was mainreaction system in the presence of POM/SM-3 (2000 rpm for tained, as Figure S12 in Supporting Information showed). 2 min. Droplets with diameters of 200 ~ 400 mm were clearly We next scaled up the reduction reaction to 40 and 400 mL. observed, as shown in Figure S17 in Supporting Information). For the scaled-up reactions, the emulsification was still readily The comparative results of the static Pickering emulsion epoxi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1. Epoxidation of allylic alcohols in (A) a static Pickering emulsion system, and (B) a conventional stirrer-driven biphasic system (800 rpm).
Entry 1[a]
[a]
2
3[a]
[b]
4
5[c]
Substrates
Conditions[a]
T [8C]
Conv. [%]
A
50
99
B
50
93
A
50
97
B
50
98
A
50
98
B
50
93
A
50
71
B
50
63
A
70
74
B
70
63
[a] 1 mmol of substrate, 1 mL of ethyl acetate, solid catalyst containing 0.5 mmol POM, 2 mL of water, and 0.34 g of aqueous 30 wt % H2O2. [b] 0.5 mmol of substrate, 1 mL of ethyl acetate, 2 mL of water, solid catalyst containing 0.5 mmol POM, and 0.17 g of aqueous 30 wt % H2O2. [c] 0.5 mmol of substrate, 1 mL of toluene, 2 mL of water, solid catalyst containing 0.5 mmol POM, and 0.17 g of aqueous 30 wt % H2O2.
dation (A) and the conventional stirring-driven biphasic epoxidation (B, using POM/SM-4 as catalyst) are summarized in Table 1. For all the investigated allylic alcohols, the static Pickering emulsion system afforded good to excellent conversions within 5 h (the selectivity for all epoxides was more than 97 %). Its conversions are comparable to (or higher than) those of the conventional stirring-driven biphasic reactions (800 rpm). Although POM/SM-3 is found to be less active towards terpenes and terminal alkenes (similarly to unsupported Na12WZn3(ZnW9O34)213b), these comparisons in terms of allylic alcohols sufficiently demonstrate the effectiveness of the static Pickering emulsion strategy. In summary, we demonstrate a general strategy to conduct organic/aqueous reactions involving solid catalysts without stirring. The strategy is based on transferring a conventional biphasic reaction into a system comprising one Pickering emulsion phase. The static Pickering emulsion system exhibits a much higher reaction efficiency than reported phase-boundary catalysis systems. Impressively, our investigations show that the reaction efficiency of the static Pickering emulsion is comparable to that of the conventional stirrer-driven biphasic systems. The short diffusion distance arising from the compartmentalization of densely packed droplets plays an important role in boosting the reaction efficiency. In view of the low energy input, high reaction efficiency, and simple work-up, we envision that this strategy will bring biphasic reactions with fundamental innovations toward more green and sustainable chemistry. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Experimental Section Typical procedure for nitroarene reaction in the static Pickering emulsion system: 0.024 g of solid catalyst was added into 2 mL of water. NaBH4 (2 mmol), toluene (0.7 mL) and nitroarene (1 mmol) were then added into the above suspension. After stirring for 2 min (2000 rpm, emulsification), the reaction proceeded under the static conditions. The conversion of nitroarene was monitored by gas chromatography (GC). Preparation procedures for Pd/SM-1 and Pd/SM-2 can be found in the Supporting Information. Typical procedure for epoxidation of allylic alcohols in the static Pickering emulsion system: the given amounts of water, ethyl acetate, POM/SM-3 (or POM/SM-4), olefins, and H2O2 were added into a test tube (their detailed amounts can be found in the footnotes of Table 1), and were stirred vigorously for 2 min to form one Pickering emulsion phase (2000 rpm). After the reaction system was left to stand for a given time, the organic layer was isolated through centrifugation. The conversion was determined with GC analysis. The preparation procedures of POM/SM-3 and POM/ SM-4 are included in the Supporting Information.
Acknowledgements This work is supported by the Natural Science Foundation of China (20903064, 21173137), Program for New Century Excellent Talents in University (NECT-12-1030), Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi (2011002), and Middle-aged Innovative Talents of Higher Learning Institutions of Shanxi (20120202). Keywords: biphasic catalysis · diffusion · kinetics · phasetransfer catalysis · synthetic methods [1] a) S. Minakata, M. Komatsu, Chem. Rev. 2009, 109, 711 – 724; b) F. Jo, Acc. Chem. Res. 2002, 35, 738 – 745; c) C. J. Li, L. Chen, Chem. Soc. Rev. 2006, 35, 68 – 82; d) X. F. Wu, J. L. Xiao, Chem. Commun. 2007, 2449 – 2466; e) H. X. Wang, H. Q. Yang, H. R. Liu, Y. H. Yu, H. C. Xin, Langmuir 2013, 29, 6687 – 6696. [2] a) X. H. Li, W. L. Zheng, H. Y. Pan, Y. Yu, L. Chen, P. Wu, J. Catal. 2013, 300, 9 – 19; b) A. H. G. Cents, D. W. F. Brilman, G. F. Versteeg, Ind. Eng. Chem. Res. 2004, 43, 7465 – 7475; c) W. H. Chong, L. K. Chin, R. L. S. Tan, H. Wang, A. Q. Liu, H. Chen, Angew. Chem. 2013, 125, 8732 – 8735; Angew. Chem. Int. Ed. 2013, 52, 8570 – 8573. [3] T. J. Lin, X. Meng, L. Shi, Ind. Eng. Chem. Res. 2012, 51, 13123 – 13143. [4] a) K. Jhnisch, V. Hessel, H. Lçwe, M. Baerns, Angew. Chem. 2004, 116, 410 – 451; Angew. Chem. Int. Ed. 2004, 43, 406 – 446; b) R. L. Hartman, J. P. McMullen, K. F. Jensen, Angew. Chem. 2011, 123, 7642 – 7661; Angew. Chem. Int. Ed. 2011, 50, 7502 – 7519; c) B. Zhao, J. S. Moore, D. J. Beebe, Science 2001, 291, 1023 – 1026. [5] a) H. Y. Fu, M. Li, H. Chen, X. J. Li, J. Mol. Catal. A 2006, 259, 156 – 160; b) L. Leclercq, R. Company, A. Mhlbauer, A. Mouret, J. M. Aubry, V. Nardello-Rataj, ChemSusChem 2013, 6, 1533 – 1540. [6] a) H. Nur, S. Ikeda, B. Ohtani, Chem. Commun. 2000, 2235 – 2236; b) S. Ikeda, H. Nur, T. Sawadaishi, K. Ijiro, M. Shimomura, B. Ohtani, Langmuir 2001, 17, 7976 – 7979; c) Y. K. Takahara, S. Ikeda, S. Ishino, K. Tachi, K. Ikeue, T. Sakata, T. Hasegawa, H. Mori, M. Matsumura, B. Ohtani, J. Am. Chem. Soc. 2005, 127, 6271 – 6275. [7] a) R. Aveyard, B. P. Binks, J. H. Clint, Adv. Colloid Interface Sci. 2003, 100, 503 – 546; b) S. Tsuji, H. Kawaguchi, Langmuir 2008, 24, 3300 – 3305; c) N. X. Yan, M. R. Gray, J. H. Masliyah, Colloids Surf. A 2001, 193, 97 – 107; d) L. A. Fielding, S. P. Armes, J. Mater. Chem. 2012, 22, 11235 – 11244; e) E. S. Read, S. Fujii, J. I. Amalvy, D. P. Randall, S. P. Armes, Langmuir 2004, 20, 7422 – 7429.
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Received: September 19, 2013 Revised: October 29, 2013 Published online on December 16, 2013
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