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Preparation of Tio2 Using Supercritical CO2 Antisolvent Precipitation (SAS): A Support for High Activity Gold Catalysts
Scientific Bases for the Preparation of Heterogeneous Catalysts E.M. Gaigneaux et al. (Editors) © 2006 Elsevier B.V. All rights reserved.
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Preparation of TiO2 Using Supercritical CO2 Antisolvent Precipitation (SAS): A Support for High Activity Gold Catalysts Zi-Rong Tang, Jonathan K. Bartley, Stuart H. Taylor, Graham J. Hutchings Cardiff University, School of Chemistry, Main Building, Park Place, Cardiff, CFIO 3AT, United Kingdom.
Abstract A supercritical anti-solvent precipitation technique has been used to prepare a novel Titania catalyst support. The Titania precursor was prepared by precipitating TiO (acac) 2 from a solution of methanol using supercritical carbon dioxide at 110 bar and 40°C. The surface area of the supercritical precursor was 160 m2g"! and this decreased to 35 m2g"! after calcination, although there was no significant reduction of particle size. The new titania support was used to prepare a supported gold catalyst and this was tested for ambient temperature carbon monoxide oxidation. The supercritical catalyst demonstrated notably high activity when compared with catalysts prepared by other nonsupercritical methods. 1. Introduction In recent decades, highly active catalysts of gold on metal oxides have attracted much attention since Haruta and co-workers found that the gold nanoparticles deposited on semiconductor transition-metal oxides, such as TiO2, exhibited surprisingly high catalytic activity for CO oxidation, even at a temperature as low as -77 "C.1 This has led to extensive research on various catalytic reactions by highly dispersed gold catalysts, such as epoxidation propene,2 selective oxidations of alkenes3 such as cyclohexene and ciscyclooctene, and the purification of hydrogen in fuel cell.4'3 Even though the catalytic mechanism of gold catalysts is still the subject of debate, a number of
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researchers have clearly shown that the nature of oxide support is one of the key factors affecting the catalytic activity and stability of supported gold catalysts.6"" Therefore, it is of great interest to develop oxide supports with unique surface properties for gold catalysts. The use of supercritical CO2 (scCO2) as an antisolvent for the controlled precipitation of materials from conventional solvents is a novel technique. It has been successfully used to produce a range of materials including polymers, pharmaceutical chemicals, explosives, superconductors and some catalysts.12"19 In an SAS process, scCO2 is used to reduce the solvation power of conventional solvents so that the solutes precipitate. Particularly, the diffusivity of sc CO2 is about two orders of magnitude larger than those of liquids and mass transfer from sc CO2 to liquid phase is so fast that it facilitates the production of very small particles of the solute contained in the liquid phase. Previously, using this precipitation method, we have successfully produced vanadium phosphate catalysts. In this paper, we present a novel synthesis of TiO2 support by precipitation using supercritical CO2 as an antisolvent. We found that supercritical treated supports can remarkablely enhance the catalytic activity of gold nanoparticles for low temperature CO oxidation. 2. Experimental 2.1. SAS apparatus The scheme of the SAS apparatus is shown in Figure 1. The apparatus comprises of two HPLC pumps (Jasco, PU-980 for solution and PU-1580-CO2 for CO2), a back pressure regulator (Jasco, BP-1580-81), a precipitation vessel and a GC oven.
GO Ovpn
Cu/Mn Salt Solution
P: HPLC pump PV; precipitation vessel BPR: Back pressure regulator
Figure 1. Schematic of the apparatus for the precipitation using SAS process
TiO2 using supercritical CO precipitation (SAS) Preparation of ofTiO2 CO22 antisolvent precipitation
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CO2 was pumped as a liquid using one HPLC pump over the critical pressure (72 bar), which was maintained by a BPR. It was passed through a length of coiled tubing in the GC oven and it was heated through its critical point. The starting solution was pumped through a fine capillary (220 nm id) into the precipitation vessel by the other HPLC pump. As the solution exited the capillary, the solvent and CO2 diffuse into each other, reducing the solubility and the solutes precipitate. A stainless steel filter (500 nm) was placed at the bottom of the vessel to collect the precipitate. 2.2. Catalyst preparation Synthesis of the TiO2 precursor (labeled as scTiO(acac)3) was carried out in the SAS apparatus. Supercritical CO2 was pumped at pressures of up to 110 bar with the flow rate of 7 ml min"1. The whole system was held at 40 °C. Initially pure methanol was pumped through a fine capillary into the precipitation vessel at a flow rate around 0.1 ml min"' for 25 min in co-current mode with supercritical CO2 in order to obtain steady-state conditions in the vessel. After the initial period, the flow of liquid solvent was stopped and the solution of TiO(acac)2 in methanol(13.33 mg ml"1) was delivered at 0.1 ml min"' flow rate. The system pressure and temperature were maintained constant during the course of feeding the solution and CO2. As the solution exited the capillary, precipitation occurred. When all the solution had been processed, scCO2 was pumped for a further hour to wash the vessel in case the residual methanol condensed during the depressurization and partly solubilized the precipitated powder modifying its morphology. When the washing process was completed, the CO2 flow rate was stopped and the vessel was depressurized to atmospheric pressure and the light green precipitate was collected. Experiments were conducted for 20 h, which resulted in the synthesis of approximately 0.7 g of solid. The precursor was calcined to give scTiO2 at 400 °C for 2 h with the ramp of 10 °C min"1. As a comparison, untreated TiO2 (labeled as unTiO2) was produced by the direct calcination of as-received TiO(acac)2 under the same conditions. Gold was deposited on the surfaces of scTiO2 and unTiO2 supports via the following deposition-precipitation procedure: A slurry containing scTiO2 (200 mg) in distilled water was adjusted to pH 2 by the addition of dilute HC1. When the pH was stable, a solution of HAuCl4 in distilled water was added into the slurry. After adjusting the slurry to pH = 1 0 with 2M Na2CO3, the slurry was stirred for 20 h at room temperature. The solid was filtered and washed until free of chloride and then dried at 100 °C overnight. 2.3. Catalyst Characterisation Samples were characterized by powder X-ray diffraction using an Enraf Nonius PSD 120 diffractometer with a monochromatic CuK source operated at 40 keV and 30 mA. Surface areas of the catalysts were determined by multipoint nitrogen adsorption at -196 °C and data were treated in accordance
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with the BET method. The gold loadings were determined using a Varian 55B atomic absorption spectrometer. FT-IR spectra were recorded on a Perkin Elmer series 2000 FT-IR spectrometer. Raman spectra were obtained using a Renishaw Ramanscope Spectrograph fitted with an Ar+ laser(A, = 514.532 nm). Scanning Electronic Microscope (SEM) was performed using a Hitachi S246ON instrument operating at 20 kV on gold coated powder samples. 2.4. Catalyst Testing The catalysts were tested for CO oxidation using a fixed-bed laboratory microreactor (3 mm id), operated at atmospheric pressure. Typically CO (0.5% CO in synthetic air) were fed to the reactor at controlled rates of 22.5 ml min"1 using mass flow controllers and passed over the catalyst of 50 mg (GHSV = 17,000 h"1). The catalyst temperature was maintained at 25 °C by immersing the quartz bed in a thermostatically controlled water bath. The products were analyzed using on-line gas chromatography. 3. Results and Discussion The powder XRD result for the supercritical product is shown in Figure 2. It can be seen that the supercritical precursor was completely amorphous by X-ray diffraction, in contrast with the crystalline TiO(acac)3 before supercritical processing. BET surface area measurements indicated that the supercritical precursor had very high surface area up to 160 m2g"', whereas the titanium salt exhibited a very low surface area of 4 m2 g"'.
sc-TiO(acac)9
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2theta Figure 2 XRD patterns of scTiO(acac)2 and the untreated titanium salt
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TiO2 using supercritical CO precipitation (SAS) Preparation of ofTiO2 CO22 antisolvent precipitation
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The FT-IR spectra of the precursor and the untreated salt are shown in Figure 3. The titanium salt has the typical bands of J3 -diketones, with main peaks at 1585 and 1533 cm"1, corresponding to C=O and C=H stretching vibrations, while, bands associated with the vibration of Ti-O bonds can be clearly observed between 1000 cm"1 and 500 cm"'. However, following the supercritical process, scTiO(acac)2 has features in the spectra associated with basic carbonate salts. The peak at 1584 cm"1 can be assigned to C=O asymmetric stretching and the peak at 1445 cm"' as a shoulder is assigned to C=O symmetric stretching. In addition, no bands associated with Ti-0 bond vibrations can be detected in the range of 1000 cm"1 and 500 cm"1.
untreated TiO(acac)
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Figure 3 IR spectra of scTiO(acac)2 and the untreated titanium salt
The SEM image of the precursor reveals that the individual particles have a spherical morphology with a slight degree of aggregation and an average particle size of around 100 nm (Figure 4A).
Figure 4 SEM images of scTiO(acac)2 and scTiO2
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Following calcination, the XRD patterns of scTiO2 showed that it is crystallised with broad peaks characteristic of the pure anatase phase (Figure 5). The SEM image of scTiO2 (Figure 4B) indicates that the particles have no regular shape and aggregate to form larger particles. The particle sizes of the scTiO2 materials appear similar to the precursor and remains around 100 nm, whereas, the surface area dramatically decreased to 35 m2g"'.
Figure 5 XRD patterns of scTiO2 and unTiO2
Gold was deposited on the surfaces of the scTiO2 and unTiO2 materials by a deposition-precipitation process. The gold loadings of Au/scTiO2, and Au/unTiO2 determined by atomic absorption spectroscopy (AAS) were 0.8 wt% and 1 wt%.
Au/scTiOz
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Figure 6 Catalytic performance of Au/scTiC>2 and Au/unTiC>2
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The catalytic activity of the gold on titania catalysts were tested for CO oxidation using a fixed-bed laboratory microreactor. As shown in the Figure 6, the gold catalyst supported on scTiO2 exhibits extremely high catalytic activity with 100% conversion. In contrast, when gold is deposited on the untreated TiO2, the conversion of CO was only about 10%. The results show that the scTiO2 support can remarkably improve the catalytic activity of gold nanoparticles. Since scTiO2 and untreated TiO2 have similar crystalline structures, with an anatase phase, we think the great improvement should be due to the unique surface properties of the nano-sized scTiO2 support produced by the supercritical process. The stronger interaction between gold nanoparticles and scTiO2 might exist and play an important role in the enhancement of catalytic activity. Further experiments, such as TEM and XPS, are under way to clarify the surface properties of scTiO2. 4. Conclusion A novel process for the synthesis of TiO2 has been developed using a supercritical antisolvent precipitation method. The TiO2 prepared was used to support gold nanoparticles which are used as catalysts for low temperature CO oxidation. The catalytic data show the activity and stability for CO oxidation of gold catalyst supported on this support is much better than the catalyst by depositing Au on the regular TiO2 derived from the direct calcination of titanium oxide acetylacetonate. Our studies show that the green process using supercritical CO2 as an antisolvent has significant potential to produce novel supports that can be employed to prepare highly active catalyst for some given reactions. Thus, it provides another effective route to prepare catalyst supports for not only gold catalysts but also other supported metal catalyst. References 1. M. Haruta, T. Kobayashi, H. Sano andN. Yamada, Chem. Lett., 1987, 16, 405. 2. A. K. Sinha, S. Seelan, S. Tsubota and M. Haruta, Angew. Chem., Int. Ed,2004, 43, 1546. 3. M. D. Hughes, Y. -J. Xu, P. Jenkins, P. McMorn, P. Landon, D. I. Enache, A. F. Carley, G. A. Attard, G. J. Hutchings, F. King, E. H. Stitt, P. Johnston, K. Griffin and C. J. Kiely, Nature, 2005, 437,1132. 4. P. Landon, J. Ferguson, B. E. Solsona, T. Garcia, A. F. Carley, A. A. Herzing, C. J. Kiely, S. E. Golunski and G. J. Hutchings, Chem. Commun., 2005, 27, 3385. 5. B. T. Qiao and Y. Q. Deng, Chem. Commun., 2003,17, 2192. 6. M. M. Schubert, S. Hackenberg, A. C. van Vee, M. Muhler, V. Plzak and R. J. Behm, J. Cato/., 2001,197, 113 7. S. K. Sharkhutdinov, R. Meyer, M. Naschitzki, M. Baumer and H. -J. Freund, Catal. Lett., 2003,86,211. 8. S. Ami, F. Morfin, A. J. Renouprez and J. L. Rousset, J. Am. Chem. Soc. 2004,126, 1199.
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9. S. Carrettin, P. Concepcion, A. Corma, J. M. Lopez Mieto and V. F. Puntes, Angew. Chem., Int. Ed, 2004, 43, 2538. 10. J. Guzman and A. Corma, Chem. Commun. 2005, 743 11. W. Yan, S. M. Mahurin, Z. Pan, S. H. Overbury, and S. Dai, J. Am. Chem. Soc. 2005, 127, 10480. 12. P. M. Gallagher, M. P. Coffey, V. J. Krukonis, N. Klasutis, ACS Symposium Series {Supercrit. FluidSci. Techno!.), 1989, 406, 334. 13. D. J. Dixon, G. Luna-Bercenas, K. P. Johnston, Polymer, 1994, 35, 3998. 14. A. O'Neil, C. Wilson, J. M. Webster, F. J. Allison, Howard JAK, M. Poliakoff, Angew. Chem. Int. Ed., 2002, 20, 3796. 15. C. N. Field, P. A. Hamley, J. M.Webster, D. H. Gregory, J. J. Titman, M. Poliakoff, J. Am. Chem. Soc. 2000, 11, 2480. 16. E Reverchon, Delia Porta C, Di Trolio A, Pace S, bid. Eng. Chem. Res. 1998, 3, 952. 17. E. Reverchon, G. D. Porta, D. Sannino, P. Ciambelli, Powder Technology, 1999, 102, 127.
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Preparation of TiO2 Using Supercritical CO2 Antisolvent Precipitation (SAS): A Support for High Activity Gold Catalysts Zi-Rong Tang, Jonathan K. Bartley, Stuart H. Taylor, Graham J. Hutchings Cardiff University, School of Chemistry, Main Building, Park Place, Cardiff, CFIO 3AT, United Kingdom.
Abstract A supercritical anti-solvent precipitation technique has been used to prepare a novel Titania catalyst support. The Titania precursor was prepared by precipitating TiO (acac) 2 from a solution of methanol using supercritical carbon dioxide at 110 bar and 40°C. The surface area of the supercritical precursor was 160 m2g"! and this decreased to 35 m2g"! after calcination, although there was no significant reduction of particle size. The new titania support was used to prepare a supported gold catalyst and this was tested for ambient temperature carbon monoxide oxidation. The supercritical catalyst demonstrated notably high activity when compared with catalysts prepared by other nonsupercritical methods. 1. Introduction In recent decades, highly active catalysts of gold on metal oxides have attracted much attention since Haruta and co-workers found that the gold nanoparticles deposited on semiconductor transition-metal oxides, such as TiO2, exhibited surprisingly high catalytic activity for CO oxidation, even at a temperature as low as -77 "C.1 This has led to extensive research on various catalytic reactions by highly dispersed gold catalysts, such as epoxidation propene,2 selective oxidations of alkenes3 such as cyclohexene and ciscyclooctene, and the purification of hydrogen in fuel cell.4'3 Even though the catalytic mechanism of gold catalysts is still the subject of debate, a number of
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researchers have clearly shown that the nature of oxide support is one of the key factors affecting the catalytic activity and stability of supported gold catalysts.6"" Therefore, it is of great interest to develop oxide supports with unique surface properties for gold catalysts. The use of supercritical CO2 (scCO2) as an antisolvent for the controlled precipitation of materials from conventional solvents is a novel technique. It has been successfully used to produce a range of materials including polymers, pharmaceutical chemicals, explosives, superconductors and some catalysts.12"19 In an SAS process, scCO2 is used to reduce the solvation power of conventional solvents so that the solutes precipitate. Particularly, the diffusivity of sc CO2 is about two orders of magnitude larger than those of liquids and mass transfer from sc CO2 to liquid phase is so fast that it facilitates the production of very small particles of the solute contained in the liquid phase. Previously, using this precipitation method, we have successfully produced vanadium phosphate catalysts. In this paper, we present a novel synthesis of TiO2 support by precipitation using supercritical CO2 as an antisolvent. We found that supercritical treated supports can remarkablely enhance the catalytic activity of gold nanoparticles for low temperature CO oxidation. 2. Experimental 2.1. SAS apparatus The scheme of the SAS apparatus is shown in Figure 1. The apparatus comprises of two HPLC pumps (Jasco, PU-980 for solution and PU-1580-CO2 for CO2), a back pressure regulator (Jasco, BP-1580-81), a precipitation vessel and a GC oven.
GO Ovpn
Cu/Mn Salt Solution
P: HPLC pump PV; precipitation vessel BPR: Back pressure regulator
Figure 1. Schematic of the apparatus for the precipitation using SAS process
TiO2 using supercritical CO precipitation (SAS) Preparation of ofTiO2 CO22 antisolvent precipitation
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CO2 was pumped as a liquid using one HPLC pump over the critical pressure (72 bar), which was maintained by a BPR. It was passed through a length of coiled tubing in the GC oven and it was heated through its critical point. The starting solution was pumped through a fine capillary (220 nm id) into the precipitation vessel by the other HPLC pump. As the solution exited the capillary, the solvent and CO2 diffuse into each other, reducing the solubility and the solutes precipitate. A stainless steel filter (500 nm) was placed at the bottom of the vessel to collect the precipitate. 2.2. Catalyst preparation Synthesis of the TiO2 precursor (labeled as scTiO(acac)3) was carried out in the SAS apparatus. Supercritical CO2 was pumped at pressures of up to 110 bar with the flow rate of 7 ml min"1. The whole system was held at 40 °C. Initially pure methanol was pumped through a fine capillary into the precipitation vessel at a flow rate around 0.1 ml min"' for 25 min in co-current mode with supercritical CO2 in order to obtain steady-state conditions in the vessel. After the initial period, the flow of liquid solvent was stopped and the solution of TiO(acac)2 in methanol(13.33 mg ml"1) was delivered at 0.1 ml min"' flow rate. The system pressure and temperature were maintained constant during the course of feeding the solution and CO2. As the solution exited the capillary, precipitation occurred. When all the solution had been processed, scCO2 was pumped for a further hour to wash the vessel in case the residual methanol condensed during the depressurization and partly solubilized the precipitated powder modifying its morphology. When the washing process was completed, the CO2 flow rate was stopped and the vessel was depressurized to atmospheric pressure and the light green precipitate was collected. Experiments were conducted for 20 h, which resulted in the synthesis of approximately 0.7 g of solid. The precursor was calcined to give scTiO2 at 400 °C for 2 h with the ramp of 10 °C min"1. As a comparison, untreated TiO2 (labeled as unTiO2) was produced by the direct calcination of as-received TiO(acac)2 under the same conditions. Gold was deposited on the surfaces of scTiO2 and unTiO2 supports via the following deposition-precipitation procedure: A slurry containing scTiO2 (200 mg) in distilled water was adjusted to pH 2 by the addition of dilute HC1. When the pH was stable, a solution of HAuCl4 in distilled water was added into the slurry. After adjusting the slurry to pH = 1 0 with 2M Na2CO3, the slurry was stirred for 20 h at room temperature. The solid was filtered and washed until free of chloride and then dried at 100 °C overnight. 2.3. Catalyst Characterisation Samples were characterized by powder X-ray diffraction using an Enraf Nonius PSD 120 diffractometer with a monochromatic CuK source operated at 40 keV and 30 mA. Surface areas of the catalysts were determined by multipoint nitrogen adsorption at -196 °C and data were treated in accordance
Z.-R. Tang et al. Z.-R. Tangetal.
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with the BET method. The gold loadings were determined using a Varian 55B atomic absorption spectrometer. FT-IR spectra were recorded on a Perkin Elmer series 2000 FT-IR spectrometer. Raman spectra were obtained using a Renishaw Ramanscope Spectrograph fitted with an Ar+ laser(A, = 514.532 nm). Scanning Electronic Microscope (SEM) was performed using a Hitachi S246ON instrument operating at 20 kV on gold coated powder samples. 2.4. Catalyst Testing The catalysts were tested for CO oxidation using a fixed-bed laboratory microreactor (3 mm id), operated at atmospheric pressure. Typically CO (0.5% CO in synthetic air) were fed to the reactor at controlled rates of 22.5 ml min"1 using mass flow controllers and passed over the catalyst of 50 mg (GHSV = 17,000 h"1). The catalyst temperature was maintained at 25 °C by immersing the quartz bed in a thermostatically controlled water bath. The products were analyzed using on-line gas chromatography. 3. Results and Discussion The powder XRD result for the supercritical product is shown in Figure 2. It can be seen that the supercritical precursor was completely amorphous by X-ray diffraction, in contrast with the crystalline TiO(acac)3 before supercritical processing. BET surface area measurements indicated that the supercritical precursor had very high surface area up to 160 m2g"', whereas the titanium salt exhibited a very low surface area of 4 m2 g"'.
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2theta Figure 2 XRD patterns of scTiO(acac)2 and the untreated titanium salt
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The FT-IR spectra of the precursor and the untreated salt are shown in Figure 3. The titanium salt has the typical bands of J3 -diketones, with main peaks at 1585 and 1533 cm"1, corresponding to C=O and C=H stretching vibrations, while, bands associated with the vibration of Ti-O bonds can be clearly observed between 1000 cm"1 and 500 cm"'. However, following the supercritical process, scTiO(acac)2 has features in the spectra associated with basic carbonate salts. The peak at 1584 cm"1 can be assigned to C=O asymmetric stretching and the peak at 1445 cm"' as a shoulder is assigned to C=O symmetric stretching. In addition, no bands associated with Ti-0 bond vibrations can be detected in the range of 1000 cm"1 and 500 cm"1.
untreated TiO(acac)
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Figure 3 IR spectra of scTiO(acac)2 and the untreated titanium salt
The SEM image of the precursor reveals that the individual particles have a spherical morphology with a slight degree of aggregation and an average particle size of around 100 nm (Figure 4A).
Figure 4 SEM images of scTiO(acac)2 and scTiO2
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Following calcination, the XRD patterns of scTiO2 showed that it is crystallised with broad peaks characteristic of the pure anatase phase (Figure 5). The SEM image of scTiO2 (Figure 4B) indicates that the particles have no regular shape and aggregate to form larger particles. The particle sizes of the scTiO2 materials appear similar to the precursor and remains around 100 nm, whereas, the surface area dramatically decreased to 35 m2g"'.
Figure 5 XRD patterns of scTiO2 and unTiO2
Gold was deposited on the surfaces of the scTiO2 and unTiO2 materials by a deposition-precipitation process. The gold loadings of Au/scTiO2, and Au/unTiO2 determined by atomic absorption spectroscopy (AAS) were 0.8 wt% and 1 wt%.
Au/scTiOz
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Figure 6 Catalytic performance of Au/scTiC>2 and Au/unTiC>2
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The catalytic activity of the gold on titania catalysts were tested for CO oxidation using a fixed-bed laboratory microreactor. As shown in the Figure 6, the gold catalyst supported on scTiO2 exhibits extremely high catalytic activity with 100% conversion. In contrast, when gold is deposited on the untreated TiO2, the conversion of CO was only about 10%. The results show that the scTiO2 support can remarkably improve the catalytic activity of gold nanoparticles. Since scTiO2 and untreated TiO2 have similar crystalline structures, with an anatase phase, we think the great improvement should be due to the unique surface properties of the nano-sized scTiO2 support produced by the supercritical process. The stronger interaction between gold nanoparticles and scTiO2 might exist and play an important role in the enhancement of catalytic activity. Further experiments, such as TEM and XPS, are under way to clarify the surface properties of scTiO2. 4. Conclusion A novel process for the synthesis of TiO2 has been developed using a supercritical antisolvent precipitation method. The TiO2 prepared was used to support gold nanoparticles which are used as catalysts for low temperature CO oxidation. The catalytic data show the activity and stability for CO oxidation of gold catalyst supported on this support is much better than the catalyst by depositing Au on the regular TiO2 derived from the direct calcination of titanium oxide acetylacetonate. Our studies show that the green process using supercritical CO2 as an antisolvent has significant potential to produce novel supports that can be employed to prepare highly active catalyst for some given reactions. Thus, it provides another effective route to prepare catalyst supports for not only gold catalysts but also other supported metal catalyst. References 1. M. Haruta, T. Kobayashi, H. Sano andN. Yamada, Chem. Lett., 1987, 16, 405. 2. A. K. Sinha, S. Seelan, S. Tsubota and M. Haruta, Angew. Chem., Int. Ed,2004, 43, 1546. 3. M. D. Hughes, Y. -J. Xu, P. Jenkins, P. McMorn, P. Landon, D. I. Enache, A. F. Carley, G. A. Attard, G. J. Hutchings, F. King, E. H. Stitt, P. Johnston, K. Griffin and C. J. Kiely, Nature, 2005, 437,1132. 4. P. Landon, J. Ferguson, B. E. Solsona, T. Garcia, A. F. Carley, A. A. Herzing, C. J. Kiely, S. E. Golunski and G. J. Hutchings, Chem. Commun., 2005, 27, 3385. 5. B. T. Qiao and Y. Q. Deng, Chem. Commun., 2003,17, 2192. 6. M. M. Schubert, S. Hackenberg, A. C. van Vee, M. Muhler, V. Plzak and R. J. Behm, J. Cato/., 2001,197, 113 7. S. K. Sharkhutdinov, R. Meyer, M. Naschitzki, M. Baumer and H. -J. Freund, Catal. Lett., 2003,86,211. 8. S. Ami, F. Morfin, A. J. Renouprez and J. L. Rousset, J. Am. Chem. Soc. 2004,126, 1199.
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9. S. Carrettin, P. Concepcion, A. Corma, J. M. Lopez Mieto and V. F. Puntes, Angew. Chem., Int. Ed, 2004, 43, 2538. 10. J. Guzman and A. Corma, Chem. Commun. 2005, 743 11. W. Yan, S. M. Mahurin, Z. Pan, S. H. Overbury, and S. Dai, J. Am. Chem. Soc. 2005, 127, 10480. 12. P. M. Gallagher, M. P. Coffey, V. J. Krukonis, N. Klasutis, ACS Symposium Series {Supercrit. FluidSci. Techno!.), 1989, 406, 334. 13. D. J. Dixon, G. Luna-Bercenas, K. P. Johnston, Polymer, 1994, 35, 3998. 14. A. O'Neil, C. Wilson, J. M. Webster, F. J. Allison, Howard JAK, M. Poliakoff, Angew. Chem. Int. Ed., 2002, 20, 3796. 15. C. N. Field, P. A. Hamley, J. M.Webster, D. H. Gregory, J. J. Titman, M. Poliakoff, J. Am. Chem. Soc. 2000, 11, 2480. 16. E Reverchon, Delia Porta C, Di Trolio A, Pace S, bid. Eng. Chem. Res. 1998, 3, 952. 17. E. Reverchon, G. D. Porta, D. Sannino, P. Ciambelli, Powder Technology, 1999, 102, 127.