Continuous Flow, Circulating Microwave System and Its Application in

May 22, 2008 - Continuous Flow, Circulating Microwave System and Its Application in Nanoparticle Fabrication and Biodiesel Synthesis. Yoni Groisman an...
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J. Phys. Chem. C 2008, 112, 8802–8808

Continuous Flow, Circulating Microwave System and Its Application in Nanoparticle Fabrication and Biodiesel Synthesis Yoni Groisman and Aharon Gedanken* Department of Chemistry, Kanbar Laboratory for Nanomaterials, Institute of Nanotechnology and AdVanced Materials, Bar-Ilan UniVersity, Ramat-Gan, Israel 52900 ReceiVed: February 17, 2008; ReVised Manuscript ReceiVed: March 9, 2008

The current research is an attempt to combine reactions in a domestic microwave oven (DMO) with a circulating pump in order to create a continuous circulating flow microwave system. This system allows the reagents and products to circulate in and out of the microwave reactor until the end of the synthesis. In fact, this is the first attempt of its kind to build a continuous flow microwave system for production of nanosized particles. The continuous DMO system has several advantages over batch microwave ovens, such as its ability to work with large quantities of reagents, the possibility to withdraw samples of the product without stopping the process, and the constant stirring of the reagents during the reaction. Various nanoparticles with different particle sizes were prepared. Another reaction that was tested in this study was the synthesis of biodiesel. Complete conversion of commercially available vegetable oils to fatty-acid esters and glycerol was accomplished in the newly designed system. Introduction In the past decade, there has been increasing interest by many research groups in the use of microwave irradiation instead of conventional heating in various chemical reactions. While the exact nature of microwave effects is still being debated,1 in many cases microwave-assisted chemical reactions are superior to other synthetic techniques in the fields of organic, inorganic, and analytical chemistry.2–5 The number of chemists using MW is growing, and we believe that this trend will continue in the near future. As compared to conventional synthetic methods,6,7 some of the controlled microwave heating processes showed a dramatic reduction of reaction times, increased product yields, and a higher purity of products as a result of reducing unwanted side reactions. Microwave-assisted processes rely mostly on the ability of the reaction mixture (especially the solvent) to absorb microwave energy, taking advantage of the microwave dielectric heating, which is the result of dipolar–polarization or ionic conduction mechanisms.6–9 The conventional heating of a sample has a few significant drawbacks compared to microwave irradiation. These disadvantages are heterogenic heating of the surface, limitations dependent on the thermal conductivity of materials, specific heat, and density.10,11 In a regular heating process the external surface of the heated sample has a higher temperature, and the heat flows toward the internal parts, but when using microwave irradiation, the opposite is true. In addition, the superheating effect and local hot spot effect also contribute to the microwave’s performance.12 Aside from the great advantages of microwave-assisted reactions, there are also a few drawbacks. The most significant of them is the inability to work with large quantities of materials. Microwave synthesis is not easily scalable from laboratory small-scale synthesis to industrial multikilogram production.13 The most significant limitation of the scale up of this technology * To whom correspondence should be addressed. E-mail: gedanken@ mail.biu.ac.il.

is the penetration depth of MW radiation into the absorbing materials, which is only a few centimeters, depending on their dielectric properties.14 The microwave power density inside a large batch reactor of more than 1 L volume will be much lower than on the surface. Therefore, materials in the center of the reaction vessel are heated only by convection and not by microwave dielectric heating. When trying to heat large quantities of materials, additional problems arise. As the volume of the mixture increases, the energy required for heating it also increases and higher radiation intensity is needed. The regular batch microwave reactors provide an average output of 1000 W from standard air-cooled magnetrons, which are capable of effectively heating mixtures of up to 500 mL. If the intensity of the microwave reactor increases to 5000 W, more sophisticated oilor water-based cooling is required, thus increasing the complexity, size, and cost of the instrument.15 The safety aspect is another reason for rejecting microwave reactors in industry. Processing comparatively large volumes under pressure in batch microwave reactors is not safe because any malfunction or rupture of a large pressurized reaction vessel may have significant consequences. These problems can be solved using a continuous flow microwave reactor where the reaction mixture flows through the microwave, thus avoiding the problem of penetration depth as well as other problems.16,17 In recent years there have been several developments in this field. Some research groups successfully made flow microwave reactor prototypes and used them to conduct organic synthesis.18–20 None of the above-mentioned instruments18–20 can be used for production of nanoparticles. The existing Con-Flow instruments, though effectively conducting short organic synthesis, are incapable of producing nanosized inorganic materials because their synthesis requires a high and intensive irradiation for long periods of time. The irradiation time in the existing Con-Flow instruments18–20 is not long enough to initiate particle crystallization. In addition, the prolonged and narrow tubing, with which the existing reactors are equipped, does not fit this kind of reaction because the solid aggregates will plug it. Most

10.1021/jp801409t CCC: $40.75  2008 American Chemical Society Published on Web 05/22/2008

Continuous Flow, Circulating Microwave System

Figure 1. Scheme of a continuous flow, circulating microwave: (1) reagent vessel, (2) tubing, (3) circulating pump, (4) reactor vessel, (5) microwave oven, (6) thermometer cell, (7) faucet cell, (8) reflux column, (9) brass choke, (10) heating/stirring plate.

of the mentioned systems are equipped with HPLC pumps that do not suit solid products. Reactions yielding solid products require different kinds of pumps with a much higher capacity and wide tubing in order to avoid plugging of the system. The existing systems18–20 are adapted to small-scale reactions and do not allow scale up of the process from laboratory synthesis to industrial large scale. In this paper we report on the development of a new continuous flow microwave reactor that is capable of producing nanoparticles in addition to organic reactions such as biodiesel synthesis. The reactor was also successful in polymer coating with nanoparticles. It is a cost-effective apparatus made from a DMO system. After the primary construction of the system, it was further improved and upgraded with additional accessories required for nanoparticle synthesis. Its characteristics were then checked and measured, and it was further optimized to improve its efficiency. When the Circulating Con-Flow System (Figure 1) operates, the reagent mixture flows from the reagent’s tank to the reactor flask inside the MW, where it is irradiated, and the continuous flow returns it to the tank. Some particles are already crystallized at the first cycle, return to the reagent’s tank, and continue to flow in the system. We noticed that the early formed particles continue to grow in the next cycles, which means that whenever they reach the oven, they serve as seeds for further nucleation. We demonstrated the operation of the Con-Flow system by carrying out a few well-known reactions. For example, fabrication of Ag nanoparticle was carried out by employing the polyol method21,23 using different stabilizers for formation of stable colloidal solutions. We also used the system for coating polymer beads by Ag nanoparticles.24 Fe3O4 nanoparticles were synthesized without a stabilizer25 using a permanent magnet to control the particle size. A continuous flow biodiesel synthesis reaction was successfully performed using commercially available vegetable oils as reagents. This was done by the trans-esterification of triglycerides under basic conditions.22 Experimental Section Instrument. The continuous flow MW system (Con-Flow) is presented in Figure 1. The system was built from commercially available parts. The microwave oven is a domestic oven (SHARP model R-390f) equipped with a 12 cm brass choke to avoid radiation leakage. The pipes were made of regular laboratory-available quartz and Teflon tubing with 5 mm internal and 8 mm external diameters, respectively. This tubing

J. Phys. Chem. C, Vol. 112, No. 24, 2008 8803 allows the flow of large amounts of liquid and wet solids. The pump used is a laboratory-available pump, Divac 1.2, which allows a solvent flow of approximately 1 L/min. The circulating pump is responsible for the flow of the reactants and products. Working at this high speed prevents precipitation of the solid products on the tubing. Broad Teflon tubing enables a smooth flow and allows the transfer of large quantities of solid aggregates immersed in the liquid fluids. The Teflon pipes reduce adhesion of the products to the pipes and prevent plugging of the system. The volume of the reagents container is 500 mL. A boiling flask with a volume of 100 mL was kept in the center of the oven. This large volume was adjusted for the prolonged irradiation of the fluid. When the liquid mixture flows out of the reactor toward the circulating pump, its temperature is measured by a thermometer installed in the pipes. A valve allows samples to be removed from the flow without stopping it. Gases can be bubbled through the system whenever inert conditions are required. The reactants and products keep circulating through the system. Circulation is necessary because one cycle cannot provide the energy required for crystallization of the nanoparticles. When the reagent/product mixture returns to the reagent vessel, the larger, heavier nanoparticles will precipitate and not return to the reaction vessel. On the other hand, smaller particles will continue their flow and revisit the MW oven. In this way control over the particle size can be achieved. In an earlier version of the system several problems occurred, the most serious of which was the pump’s inability to avoid precipitation of particles on the tubing, which led to blocking of the system. In addition, at temperatures higher than 200 °C some sharp curves along the Teflon pipes bent, which led to cessation of the reagent’s flow. These problems were overcome by placing the microwave oven on its side in order to equalize the heights of the reaction vessel, feeding tube coming from the reagent tank, and suction line. The length of all the tubing was minimized, and the right-angled Teflon bends were replaced by quartz. These modifications led to a drastic decrease in the amount of product precipitating on the Teflon tubing. The main advantage of the new Con-Flow system over batch MW, when a comparison between the systems is made, is the ability to work with larger amounts of reactants and obtain more products. The new system also makes it possible to remove samples from the flow without stopping the reactions and measure reaction temperature immediately after the flow exits the reactor. In addition, it provides a constant mixing of the materials during the reaction as a result of the flow. Finally, the system is cost effective. The drawbacks noticed during operation of the system were as follows. (1) The system has to be washed by the solvent after each reaction in order to remove the leftovers, which is a costly operation. (2) It is impossible to conduct small-scale reactions, namely, with volumes under 200 mL. (3) This ConFlow system is not designed to run reactions resulting in heavy precipitating products such as CuInSe2. The nanoparticles that have been prepared in this study have already been synthesized before using MW radiation. The only novel synthesis is that of the biodiesel. The reason we choose to concentrate on well-known MW reactions is that the focus of this paper is the Con-Flow system and its advantages. We are also aware that today single-mode MW ovens are available. However, using a simple and cheap multimode domestic oven will be of interest to scientists from poor countries. Moreover, the current design should be considered as a prototype for further development of the instrument.

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Figure 2. (A) Change in the Ag1+ concentration. (B) Yield of the polyol reaction as a function of time. (C) Product yield at various flow speeds. (D) Comparison of the efficacy at various microwave intensities of the Con-Flow system (3) and laboratory batch ovens: domestic (2) and Mars (1).

Cautionary note: Since conventional microwave ovens are not made for laboratory use we recommend the following precautions. (1) The experiments should be conducted in a hood. (2) A distillation column should be connected to the system to avoid splashes of the liquids. Boiling stones would also help to avoid splashes of the liquids. Reactions. The products of the new Con-Flow system were compared with the products of the same reactions preformed in batch microwaves. These ovens were either a domestic microwave equipped with a choke and a distillation column or a single-mode laboratory oven (Mars 5, CEM Corp.). All reagents were purchased from Aldrich Co. They were of the highest purity, commercially available, and used without further purification: AgNO3 (silver nitrate, >99%), Fe(CH3COO)2 (iron(II) acetate), KSCN (potassium thiocyanate), EtOH (ethanol), EG (ethylene glycol), PEG (poly(ethylene glycol)). The stabilizers were PVP (polyvinyl-pyrrolidone), PVA (poly(vinyl alcohol)), linoleic acid, and sodium linoleate. The system’s efficacy was measured by conducting an Ag reduction following the polyol reaction (Figure S1, see Supporting Information). The polyol reaction used 5 g of AgNO3 which was dissolved in 0.5 L of EtOH by heating and stirring. The system was turned on, and the silver was reduced by the ethanol under microwave irradiation. Kinetic measurements were also conducted when ethylene glycol was the reducing agent in the polyol reaction. To follow the reaction’s kinetics, samples were removed every 5–10 min without stopping the process. The concentration of

the unreacted Ag+ ions was measured by volumetric titration with potassium thiocyanate (KSCN) according to the Folgard method (Figure 2A). Several sets of experiments were conducted in order to find the optimal operating conditions and efficacy of the ovens. The silver powder product was dissolved by HNO3 (5 N) and titrated with KSCN. Ag starts to crystallize after 5 min of irradiation, and the effective time for attaining maximum efficacy is 80 min. After that the increase in the Ag weight is negligible (Figure 2B). The influence of the flow speed of the mixtures on the efficacy was measured (Figure 2C). At higher flow rates the pump power is raised, thus reducing sedimentation and improving the production yield. In addition, several experiments were done to compare the efficacy of the Con-Flow system with laboratory batch ovens at various microwave intensities (Figure 2D). Comparing the efficiency of the Con-Flow system with that of the batch ovens revealed that the reaction yield is higher in the latter. We explain this low yield of the Con-Flow system as resulting from partial sedimentation of the reaction product in the Con-Flow system. On the other hand, the reaction volumes of the batch ovens were 10 times smaller (only 50 mL for batch ovens and 500 mL for the Con-Flow system), but the durations of the reactions were only 5 times shorter (15 min for a domestic oven, 10 min for a Mars oven, and 60 min for the Con-Flow system). The primary purpose of this Circulating Con-Flow Microwave system was fabrication of nanoparticles.23 We also synthesized

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Figure 3. TEM image of the first reaction set results. Con-Flow produced Ag nanoparticles using PVA. Reaction results are particles with 3 nm average size.

Figure 4. TEM image of the second reaction set results. Con-Flow produced Ag nanoparticles using PVP. Reaction results are various size particles with 12 nm average size.

Figure 5. TEM image of the third reaction set results. Con-Flow produced Ag nanoparticles using linoleic acid as a surfactant. Reaction results are particles with 5.5 nm average size.

Ag nanoparticles using various stabilizers. The origin of the Ag+ was AgNO3. In the first experiment, 5 g of AgNO3 was dissolved in 500 mL of EG by heating and stirring. Then, 3 g of PVA was added, and the system was operated for 60 min at 80% power (products are presented in Figure 3). In the second experiment, EtOH was used as a reducing agent instead of EG, and the stabilizer was PVP. Here, the oven was operated at 70% power because of the volatility of the ethanol (products are presented in Figure 4). The third synthesis of Ag nanoparticles was conducted under the same conditions, using 12 mL of linoleic acid as a surfactant with 10 g of sodium linoleat21 (products are presented in Figure 5). We also examined use of the Con-Flow system for coating silver nanoparticles on the surface of 3 mm polymer beads by employing the polyol method.24 PMMA (poly(methyl methacrylate)), PC (polycarbonate bisphenol A), and Nylon 6 with

grains 3 mm in diameter were supplied by the Palram Co., Ltd. (Israel), and used as received. In the different experimental set ups, 30 g of each polymer was placed in the reaction boiling flask, after which 300 mL of H2O, 100 mL of EtOH, 100 mL of PEG, 7 mL of NH3, and 5.8 g of AgNO3 were mixed in the reagent’s tank. This mixture was designed for an effective polymer coating. PEG easily reduces the silver ions, but it cannot be used as a single solvent because of its high boiling temperature (300 °C) that might decompose the polymers. For this reason, a large amount of water is added to form the solvent mixture. The polymers used are hydrophobic, and thus, addition of EtOH is needed to connect the polymer surface and the solvent. An inert environment was achieved by bubbling argon through the tubing for 30 min. PEG reduces the silver ions, but this process is quite sluggish. This is the reason for addition of

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Figure 6. SEM images of polymer coated by Ag nanoparticles. The samples were taken after 35 min: (1) PMMA, (2) PC, (3) Nylon 6.

TABLE 1: Size of the Ag Particles Coated on PMMA, PC, and Nylon 6 after 20, 35, and 45 Minutes reaction time (min) 20

35

45

coating particle size (nm) PMMA polycarbonate Nylon 6

43 50 44

73 80 60

80 105 114

NH3. The ammonia helps in forming [Ag(NH3)2]2, a stable complex. Under these conditions reduction of the silver ions by PEG can be performed much faster. In each experiment, coated polymer samples were removed after 20, 35, and 45 min of the process (Figure 6) and examined for the amount of coated silver. Fabrication of magnetite Fe3O4 nanoparticles was performed using Fe(CH3COO)2 as the precursor.25 A 300 mL amount of H2O was bubbled with argon for 30 min. To avoid oxidation, 10.5 g of Fe(CH3COO)2 was dissolved in water in a glovebox. A strong permanent magnet was attached to the reagent’s tank wall. We assigned the magnet to attract the large particles, allowing the small nanoparticles to further flow through the system. The same reaction was carried out without a magnet but with 1.8 g of PVP as a stabilizer. The biodiesel reaction22 was carried out using commercially available Canola and Sunflower oil. The reaction is a transesterification of triglycerides with methanol using KOH as the catalyst. In different experiments, 500 mL of oil (460 g) was mixed with 135 mL of methanol containing 5 g of KOH, which was previously dissolved. The MeOH/oil (triglyceride) molar ratio, 6:1, allows methanol to be in double excess for each fatty acid, thus pushing the reaction equilibrium toward the product.

After the reaction, the products were separated to the glycerol phase and to the methyl ester of fatty acids phase (biodiesel). To compare the resulting yields with the batch microwave system, several batch reactions were conducted. In the first set, the same reaction but with one-tenth of all the materials was done by batch microwave. In the second set, the quantity of the material was the same as in the Con-Flow system, but it was conducted by a batch oven. The X-ray diffraction (XRD) pattern of the products was measured with a Bruker AXS D* Advance powder X-ray diffractometer (using Cu KR radiation 1.5418). TEM images were obtained with the use of a JEOL-JEM 100SC electron microscope operating at an accelerating voltage of 100 kV. SEM pictures were performed on a JEOL-JSM-840 Scanning Electron Microscope. Results and Discussion Fabrication of Silver Nanoparticles.21,23 The results of the reactions described in the experimental section are probed by XRD (Figure S1, Supporting Information) and TEM measurements. Circulating Con-Flow microwave results showed the successful preparation of small silver nanoparticles with effective stabilizer-controlled particle size and good dispersion. These results are very similar to the results obtained by batch microwaves. The main difference is in the quantity of the product and the reaction times. As mentioned in the Introduction, batch ovens allow effective irradiation of only ∼50 mL, while in the new system the volume is about 500 mL and higher. Thus, the amount of product is about 10 times larger, and so the scale up of the process was carried out effectively. Although the product yield in the Con-Flow system was 10-fold higher,

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Figure 7. TEM image of Fe3O4 nanoparticles produced using a strong magnet. Average particle size is 6.5 nm.

Figure 8. TEM image of Fe3O4 nanoparticles produced without a magnet: (A) using a PVP stabilizer, average size of 10.5 nm; (B) without stabilizers, average size of 20 nm.

the reaction times are only 5 times longer than in the batch MW ovens. This difference derived from a higher irradiation surface area and constant stirring during the process in the ConFlow system. Although the tubing has to be rinsed after each reaction in order to wash out the precipitating metallic residue, the new system allows effective scale up of the nanoparticle production process. In Figures 3, 4, and 5 we present the TEM images of the silver products obtained with PVA, PVP, and linoleic acid, respectively. We also present a histogram describing the distribution of the particle sizes. The smallest particles (3 nm) are obtained when PVA is used as the stabilizer. In all three cases, spherical particles are obtained, and aggregates are also detected in the three syntheses. The size of the largest aggregates is less than 30 nm. They may be formed when the liquid flows again through the MW oven. Another possible reason is that the current system enables one to work using higher concentrations than the previous batch studies. Polymer Coating. The results of polymer coating by silver nanoparticles using the Con-Flow system are depicted in Figure 6. The PMMA, PC, and Nylon grains were coated by silver nanoparticles using the Polyol method, as described in the Experimental Section. The smallest nanoparticles (in short radiation times) and most homogeneous coatings are obtained for the Nylon 6 beads. This may be related to the existence of a very active amide group as part of the polymer, which interacts with the silver particles. Table 1 summarizes the dependence of the particle size of the silver as a function of the nature of the coated polymer and duration of the reaction, as calculated from the SEM images. The resulting coating seems to be full and effective, with small particle size dispersion, similar to results obtained by batch laboratory microwaves.

Magnetite Nanoparticle Production by Con-Flow Synthesis. In the synthesis of the ferromagnetic Fe3O4, we followed a microwave-assisted process described elsewhere using Fe(CH3COO)2 as a precursor.25 The novel idea in the current experiment is an attempt to attract the newly created particles out of the flow using a strong magnet. Our assumption was that the larger particles would be removed from the flow, while the smaller ones would continue to stream and grow until they reached the desired size, when they would be attracted by the magnet. The magnetic force exerted on the nanoparticle became larger than the force of the stream. This will enable us to control the size of the nanoparticle without stabilizers. The resulting TEM images show that the smallest particles were obtained using a magnet (6.5 nm), but when using PVP, the particles were about 10 and 20 nm without any sizecontrolling technique. Using a magnet with the Circulating ConFlow system allows us to control the magnetite particle size without a stabilizer. It appears that magnetic removal of larger particles out of the stream prevents their further growth. Biodiesel Synthesis. To demonstrate the multifunctionality nature of the apparatus, organic reactions were also conducted. Synthesis of biodiesel from vegetable oil is a well-known reaction and has been conducted many times by conventional heating methods. Recently, batch microwave biodiesel synthesis was also conducted,22 but a continuous flow method has not yet been introduced. Here we report on the Con-Flow synthesis of biodiesel from a vegetable oil origin. Con-Flow microwave synthesis will allow large-scale biodiesel production at reduced cost. The reaction process was described in the Experimental Section. The product yield in the Con-Flow system was 92% for canola and 89% for sunflower oil. To compare the product yield of the Con-Flow system with a batch oven, several batch

8808 J. Phys. Chem. C, Vol. 112, No. 24, 2008 reactions were conducted. Although the batch reaction with onetenth of the materials (50 mL of oil) gave a good 97% yield, the reaction with the same amount of reagents (500 mL of oil) resulted in only a 64% yield. This proves the inability of batch microwave ovens to conduct large-scale reactions and emphasizes the advantage of the Con-Flow method. When dealing with a larger scale, the low efficacy of the batch oven resulted from the limited penetration of the microwave radiation. The inner volume of the bulk (500 mL of oil) is heated by a conventional method. An additional aspect that improves the Con-Flow performance is the constant mixing of the reagents, as a result of the flow. While operating the Con-Flow system for the biodiesel reaction the electricity consumption was measured, and it was found that 930 W were consumed for completion of the reaction (7 min). Conclusions In the current article we demonstrated the use and application of a continuous circulating-flow system built around a DMO. While working with large-scale amounts of precursors, the various reactions experimented on in the system showed good efficiency. The advantages and disadvantages of the apparatus were presented and discussed. The reaction yielding biodiesel has never been experimented on in a continuous flow MW reaction. Supporting Information Available: XRD patters of the silver and magnetite nanoparticles synthesized by the Con-Flow system are available. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) De La Hoz, A.; Diaz-Ortiz, A.; Moreno, A. Chem. Soc. ReV. 2005, 34, 164. (2) Galema, S. A. Chem. Soc. ReV. 1997, 26, 233. (3) Lidström, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225. (4) Whittaker, A. G.; Mingos, D. M. P. J. Chem. Soc., Dalton Trans. 2000, 1521. (5) Mingos, D. M. P. Res. Chem. Intermed. 1994, 20, 85.

Groisman and Gedanken (6) (a) Loupy, A. MicrowaVes in Organic Synthesis, 1st ed.; WileyVCH: Weinheim, Germany, 2002. (b) Hayes, B. L. MicrowaVe Synthesis: Chemistry at the Speed of Light; CEM Publishing: Matthews, NC, 2002. (c) Komarneni, S.; Roy, R.; Li, Q. H. Mater. Res. Bull. 1992, 27, 1393. (d) Komarneni, S.; Li, Q. H.; Roy, R. J. Mater. Chem. 1994, 4, 1903. (e) Newalkar, B. L.; Olanrewaju, J.; Komarneni, S. J. Phys. Chem. B 2001, 105, 8356. (7) Kappe, C. O. Angew. Chem. Int. Ed. 2004, 43, 6250. (8) Gabriel, C.; Gabriel, S.; Grant, E. H.; Halstead, B. S.; Mingos, D. M. P. Chem. Soc. ReV. 1998, 27, 213. (9) Mingos, D. M. P.; Baghurst, D. R. Chem. Soc. ReV. 1991, 20, 1. (10) Carslaw, H. S.; Jaeger, J. C. Conduction of Heat in Solids; Oxford University Press: Oxford, England, 1959. (11) Metaxas, A. C. Foundations of Electroheat; Wiley: New York, 1996. (12) Kingston, H. M.; Jassie, L. B. Introduction to MicrowaVe Sample Preparation: Theory and Practice; American Chemical Society: Washington DC, 1988. (13) Kremsner, J. M.; Stadler, A.; Kappe, C. O. Top. Curr. Chem. 2006, 266, 233. (14) Kingston, H. M.; Haswell, S. J. MicrowaVe-Enhanced Chemistry Fundamentals, Sample Preparation and Applications; American Chemical Society: Washington, DC, 1997. (15) Ngoc, T. L.; Roberts, B. A.; Strauss, C. R. MicrowaVes in Organic Synthesis, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2006. (16) Jas, G.; Kirschning, A. Chem. Eur. J. 2003, 9, 5708. (17) Kirschning, A.; Solodenko, W.; Mennecke, K. Chem. Eur. J. 2006, 12, 5972. (18) He, P.; Haswell, S. J.; Fletcher, P. D. Appl. Catal., A 2004, 274, 111. (19) Jachuck, R. J. J.; Selvaraj, D. K.; Varma, R. S. Green Chem. 2006, 8, 29. (20) Wilson, N. S.; Sarko, C. R.; Roth, G. Org. Process. Res. DeV. 2004, 8, 535. (21) Wang, X.; Zhuang, J.; Peng, Q. Nature 2005, 437, 121–124. (22) (a) Vicente, G.; Martinez, M.; Aracil, J. J. Am. Oil Chem. Soc. 2005, 86, 1057. (b) Liu, E.; Lopez, D. Y. J.; Bruce, E.; Goodwin, D. A. Ind. Eng. Chem. Res. 2005, 44, 5353. (c) Nicholas, E.; Leadbeater, L.; Stencel, M. Energy Fuels 2006, 20, 2281–2283. (d) Kremsner, M.; Stadler, A.; Kappe, C. O. Top. Curr. Chem. 2006, 266, 233. (23) (a) Komarneni, S.; Li, D.; Newalkar, B.; Katsuki, H.; Bhalla, A. Langmuir 2002, 18, 5959–5962. (b) Palchik, O.; Kerner, R.; Gedanken, A.; Weiss, A. M.; Slifkin, M. A.; Palchik, V. J. Mater. Chem. 2001, 11, 874–878. (c) Kotlyar, A.; Perkas, N.; Amiryan, G.; Meyer, M.; Zimmermann, W.; Gedanken, A. J. Appl. Polym. Sci. 2007, 104, 2868–2876. (24) Irzh, A.; Perkas, N.; Gedanken, A. Langmuir 2007, 23, 9891–9897. (25) (a) Pol, S. V.; Pol, V. G.; Gedanken, A.; Felner, I.; Sung, M. Inorg. Chem. 2007, 46, 4951–4959. (b) Ben-Ishay, M. L.; Gedanken, A. Langmuir 2007, 23, 5238–5242. (c) Vijayakumara, R.; Koltypin, Y.; Felner, I.; Gedanken, A. Mater. Sci. Eng. 2000, 286, 101–105.

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