Gold Functionalized Supported Ionic Liquids Catalyst for CO Oxidation.

Catalysts 2011, 1, 52-68; doi:10.3390/catal1010052 catalysts ISSN 2073-4344 www.mdpi.com/journal/catalysts Article Gold Functionalized Supported Ionic Liquids Catalyst for CO Oxidation Svetlana Ivanova, Luis Francisco Bobadilla, Anna Penkova, Francisca Romero Sarria, Miguel Angel Centeno * and Jose Antonio Odriozola Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Avda Americo Vespucio 49, Sevilla 41092, Spain; E-Mails: [email protected] (S.I.); [email protected] (L.F.B.); [email protected]c.es (A.P.); [email protected] (F.R.S.); [email protected] (J.A.O.) * Author to whom correspondence should be addressed; E-Mail: [email protected]. Received: 21 September 2011; in revised form: 17 October 2011 / Accepted: 17 November 2011 / Published: 25 November 2011 Abstract: The present study tries to give an insight to the combination of the homogeneous and heterogeneous catalytic properties in a new class of materials. Well dispersed gold nanoparticles on an ionic liquid layer supported on a mineral carrier have been prepared. This work is concentrated on the characterizations and understanding of the interactions between all the components of the catalytic system. The application of the materials in the reaction of oxidation of carbon monoxide shows rather unexpected results—a good catalytic activity completely independent of the temperature. Keywords: inorganic compounds; organic compounds; nanostructures; chemical synthesis; catalytic properties 1. Introduction Ionic liquids (IL) have attracted a significant scientific interest in the last decade due to their unique properties: relatively wide electrochemically stable window; good electric conductivity; high ionic mobility; broad range of room temperature liquid compositions; negligible vapor pressure; and excellent chemical and thermal stability [1,2]. Many of these materials have been applied in different fields ranging from material chemistry to electrochemistry [3,4] and organic synthesis [5] to separation approaches [6–8], as well as in catalytic applications [2,5,9] and nanostructured material synthesis [10]. OPEN ACCESS Catalysts 2011, 1 53 One of the key challenges in the application research of the ionic liquids has been the availability and the cost issues. Therefore, the concept of immobilized ionic liquids supported on the surface of various materials offers an attractive opportunity to study the performance of ionic liquids cost effectively [1], and brings many advantages, such as easier separation of the catalyst from the reaction media and a possible re-utilization of the catalyst [9]. In addition, the high relative viscosity of many room temperature ionic liquids, promises the formation of a thin layer on the surface, facilitating the diffusion and mass transfer in gas phase reactions [1]. Moreover, ionic liquids can also be used for the formation and stabilization of metal nanoparticles [11,12]. Ionic liquids are known to possess an intrinsic high charge and polarizability allowing them to create electrostatic and steric stabilization for metal nanoparticles [8]. For instance, platinum, rhodium and iridium nanoparticles are prepared in 1-butyl-3-methyl-imidazolium ILs with control of size, near-monodispersity, shape and stabilization, and applied in catalysis [13]. The astonishing discovery of Haruta et al. [14] that very small gold nanoparticles exhibit catalytic activity for the oxidation of CO at sub-ambient temperature provokes an enormous scientific interest and makes gold the most popular metal for this catalytic application. However, the use of gold colloids for producing the active supported catalyst is not as popular as the deposition precipitation method. The advantage of using the colloidal route for preparing supported gold catalysts lies in the great number of possibilities to change the conditions of preparation in order to obtain metal particles with a narrow size distribution, and/or different particle shape [15,16]. There is a general agreement on the fact that the gold nanoparticles are kinetically unstable and have higher tendencies for agglomeration even in the presence of different stabilizing agents [17–19]. When the standard 1-butyl-3-methylimidazolium ionic liquids are used, it seems to be insufficient to prevent the agglomeration of the gold nanoparticles [20]. However, by the changes in the ionic liquid structure, addition of the ether functionalities, the presence of the ionic liquid protecting layer has been observed around the gold nanoparticles [21]. The aim of this work is to study the effect of the presence of ionic liquid on the gold based systems in the reaction of CO oxidation. The expected formation of a thin supported layer on a mineral carrier will facilitate the diffusion and mass transfer in the heterogeneous catalytic reaction and will also guarantee better dispersion and stabilization of freshly prepared gold colloids. 2. Results and Discussion 2.1. Preparation 2.1.1. Ionic Liquid Preparation The halide free ionic liquid 1-butyl-3-methylimidazolium methanesulfonate has been prepared by the method proposed by Cassol et al. [22]. The used reactants were methanesulfonyl chloride (Alfa Aesar), triethylamine (Alfa Aesar), 1-methylimidazole (Aldrich), 1-butyl-3-methylimidazolium methanesulfonate (Fluka), n-butanol, dichloromethane, acetone and distilled water. The method goes first through the synthesis of butyl methanesulfonate, as described below. Catalysts 2011, 1 54 2.1.2. Preparation of Butyl Methanesulfonate Methanesulfonyl chloride (61.1 g, 0.53 mol) was added slowly, under vigorous stirring, to a solution of n-butanol (39.4 g, 0.53 mol) and triethylamine (53.9 g, 0.53 mol) in dichloromethane (0.5 L). An external water-ice bath was used to control the reaction mixture temperature between 10–20 °C. After addition, stirring was continued for further 2 h at room temperature. Water (200 mL) was added; the aqueous layer containing the triethylammonium chloride by-product was separated; the organic layer was washed with water (200 mL) and dried with sodium carbonate. Solvent evaporation followed by reduced pressure distillation of the residue afforded the desired butyl methanesulfonate, as a colorless liquid; yield: 55.14 g (73%). 2.1.3. Preparation of 1-Butyl-3-methylimidazolium Methanesulfonate Butyl methanesulfonate (55.14 g, 0.36 mol) was mixed with 1-methylimidazole (29.74 g, 0.36 mol) and the reaction mixture was kept at room temperature by means of an external water bath. After 24 h, one crystal of 1-butyl-3-methyl imidazolium methanesulfonate was added and the resulting crystalline reaction mass was kept at room temperature for a further 120 h. Recrystallization was performed twice using acetone as a solvent (120 mL; from reflux temperature to freezer temperature overnight). After vacuum drying, colorless and very hygroscopic crystals of 1-butyl-3-methylimidazolium methanesulfonate were obtained; yield: 77.4 g (95%). 1H-NMR was performed of the synthesized ionic liquid by dissolving it in CDCl 3 and compared to the 1H-NMR spectra of the commercially available 1-butyl-3-methylimidazolium methanesulfonate (Fluka, Louis, MO, USA). The obtained spectra were identical (not shown). 2.1.4. Supports A commercial gamma aluminium oxide (Spheralite 505) was used after milling and sieving with the fraction 125–250 μm retained. In order to facilitate the ionic liquid impregnation on the support surface, the number of the anchorage sites should be increased. For this purpose the phosphoric acid treatment was chosen for modification. Five grams of γ-Al 2 O 3 were dispersed in 100 mL of 1 mol L −1 orthophosphoric acid solution at 75 °C during 4 h [23]. Then, the solution was filtered and the resulting solid was dried at 100 °C for 3 h and subsequently calcined at 400 °C for 4 h. The resulting solid will be named γ-Al 2 O 3 -MOD. 2.1.5. Ionic Liquid Impregnation Impregnation of 20 wt% ionic liquid onto support surface was performed in ethanol solution in an ultrasonic bath for 10 min. followed by ethanol evaporation under reduced pressure at room temperature. 2.1.6. Gold Deposition In all preparations: HAuCl 4 (Alfa Aesar, 45.92% Au) was used as gold precursor. Catalysts 2011, 1 55 The gold colloids were produced by two different methods: (i) the standard Turkevich method [24]; and (ii) reduction of the gold precursor by sodium borohydride [25]. (i) In the Turkevich method, an aqueous 2 × 10 −4 M gold solution was heated to boiling and then, the support was added followed by 1 mL 34 mM solution of sodium citrate. The mixture was vigorously stirred for 5 min and 1 mL of 34 mM oxalic acid solution was added to assure the complete formation of gold colloids. Then the suspension was filtered and dried at 80 °C overnight. The sample will be named as Au/support (citrate). As the application of this method is possible only in hot aqueous solutions in order to prevent the risk of the ionic liquid removal during the preparation, a room temperature method of synthesis has been applied. (ii) In this method, a gold aqueous solution with 2 × 10 −4 M concentration was mixed with the support and, 2 min later, 0.01 g of sodium borohydride was added. In these conditions, the gold reduction occurs immediately and the solution was kept at room temperature one night. The solution was then filtered and dried at 80 °C overnight. The sample will be named as Au/support (NaBH 4 ). As the room temperature reduction by sodium borohydride permits the utilization of other solvents, an additional preparation has been carried out by modifying the solvent, water being replaced by acetone. This allows us to diminish the immediate reduction by the addition of NaBH 4 and to keep the ionic liquid on the support, as it is not soluble in this solvent. The sample will be named as Au/support (NaBH 4 -acetone). The gold loading was fixed to 0.8% for all obtained samples. 2.2. Results and Discussion The specific surface area of the supports before and after modification and its point of zero charge are presented in Table 1. Table 1. Support and catalysts characteristics. Support Specific Surface Area, m 2 g −1 Pore Volume, cm 3 g −1 PZC, pH γ-Al 2 O 3 251 0.47 8.0 γ-Al 2 O 3 MOD 195 0.59 5.4 20ΙL /γ-Al 2 O 3 MOD 19 0.09 - Au/20IL/γ-Al 2 O 3 MOD (citrate) 252 0.51 - Au/20IL/γ-Al 2 O 3 MOD (NaBH 4 ) 131 0.45 - Au/20IL/γ-Al 2 O 3 MOD (NaBH 4 -acetone) 84 0.29 Figure 1 presents the pore volume distribution as a function of pore diameter for the bare and modified support. For the bare support, a bimodal distribution was observed with an average pore size of 67 Å. The modification of the surface by phosphoric acid provokes a general increasing of the average size of the pores to 110 Å which results in higher pore volume but in lower surface caused by the blocking of some mesopores. Catalysts 2011, 1 56 Figure 1. Pore distribution on gamma alumina before and after modification. 0,00 0,02 0,04 0,06 0,08 0,10 0,12 10 1 10 2 10 3 0,00 0,02 0,04 0,06 0,08 γ -Al 2 O 3 Pore volume, cm 3 /g Pore Diameter, A γ -Al 2 O 3 MOD The strength of the interaction of a Lewis base (PO 43− ) with the hard Lewis acid sites characteristic for alumina depends on the electron density and the polarizability of the base [26]. Lower polarizability and higher electron density lead to stronger interaction. The strength of interactions of the bases is as follows: Phosphate > fluoride > citrate > sulfate > acetate > formate > nitrate > chloride The biggest effect on the surface and, which is more important, the biggest possibility to enhance interaction modifier/alumina, should be observed when the phosphoric acid is applied and this was confirmed by the observation of significant decrease in the point of zero charge (Table 1). The characterization by IR spectroscopy of the modified support compared to bare unmodified alumina (Figure 2(A,B)) reveals a new band at 1100 cm −1 attributed to the P–O groups [27,28]. Figure 2. FTIR spectra of (A) bare alumina; (B) phosphate-modified alumina; (C) supported ionic liquid and (D) 1-butyl-3-methylimidazolium methanesulfonate. 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 Absorbance 1000 1500 2000 2500 3000 3500 Wavenumbers (cm -1 ) 1100 3400 1640 (D) (C) (B) (A) 2800 2900 3000 3100 3200 x10 3017 3090 3146 2962 2935 2874 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 Absorbance 1000 1500 2000 2500 3000 3500 Wavenumbers (cm -1 ) 1100 3400 1640 (D) (C) (B) (A) 2800 2900 3000 3100 3200 2800 2900 3000 3100 3200 x10 3017 3090 3146 2962 2935 2874 Catalysts 2011, 1 57 As claimed in the literature [29], the anions can be adsorbed in an oxide–water interface in an inner- or outer-sphere mode. Inner-sphere surface complexes are created as a result of direct chemical bond formation between adsorbing anion (Lewis base) and the metal ion at the surface (Lewis acid). In the case of anions with more than one donor atom (e.g., phosphate), several coordination geometries are possible: monodentate, chelating bidentate, or bridging bidentate. Other -sphere surface complexes are formed as a result of electrostatic attraction of opposite charges. Busca et al. [30] reported a selective interaction of the phosphoric acid on the basic sites which excludes the formation of H-bonded phosphate species, preventing the formation of the two types of bidentates. The formation of only ionically bonded species can be envisaged (Scheme 1, row 1). Scheme 1. Support modification and interaction with 1-butyl-3-methylimidazolium methanesulfonate. One can suppose that in this case, the biggest part of the phosphates is chemically bonded but the formation of the hydrogen bonded phosphates cannot be excluded. However, the phosphoric acid treatment does not provoke any chemical changes and the only detectable phase by XRD was γ-Al 2 O 3 and no formation of aluminium phosphate was observed (Figure 3(A,B)). Catalysts 2011, 1 58 Figure 3. XRD patterns of (A) γ-Al 2 O 3 ; (B) γ-Al 2 O 3 MOD; (C) 20IL/γ-Al 2 O 3 MOD; (D) Au/20IL/γ-Al 2 O 3 MOD (citrate); (E) Au/20IL/γ-Al 2 O 3 MOD (NaBH 4 ); (F) Au/20IL/γ-Al 2 O 3 MOD (NaBH 4 —acetone). 0 102030405060708090100110 0 100 200 300 400 500 600 700 800 γ -Al 2 O 3 Au D F E C B A Intensity, a.u. 2 Θ , degree However, the acid treatment of the support provokes changes in its morphology as could be seen from the SEM images (Figure 4(A,B)). Figure 4. Scanning electron microscopy images of (A) γ-Al 2 O 3 ; (B) γ-Al 2 O 3 MOD; (C,D) 20IL/γ-Al 2 O 3 MOD. C Catalysts 2011, 1 59 The bare alumina presents a heterogeneous morphology, composed of a mixture of grains and plates, which became rather laminar and ordered after acidic treatment but seemed to lose a part of its crystallinity. 2.3. Ionic Liquid Deposition The deposition of the ionic liquids depends on the specific surface of the support [9,31]. A high specific surface area provides a large number of adsorption sites and will increase the quantity of the adsorbed ionic liquid. De Castro et al. [9] have found a 20 weight percent maximal value for the quantity of deposited ionic liquids adsorbed on the gamma alumina surface. However, in this study the modification of alumina provides additional adsorption sites resulted in 50 wt% maximal ionic liquid loading. Nevertheless, this sample, was not be used since the possibility of the formation of multiple layers of ionic liquid on the support are higher with the increase of the deposited amount. The lower the quantity of ionic liquid adsorbed, the higher the possibility of a good anchoring and subsequent fine dispersion of the gold nanoparticles. That is why a sample with 20 wt% deposited ionic liquid was chosen for the study. The specific surface area changes dramatically when the ionic liquid is supported, a value of one tenth being obtained (Table 1). However, the outgas treatment provokes ionic liquid melting and the pores filling, which results in a lower surface area and a quasi inexistent pore volume. As the ionic liquid did not possess a crystalline structure, no structural changes have been observed (Figure 3). However, heterogeneity of the surface was observed. In some places, the morphology seems to change and to recover the initial morphology of the non-treated alumina (Figure 4(C)) while, in other parts, it keeps the morphology of the modified alumina (Figure 4(D)). This interesting observation led us to suggest that the part of the support surface is liberated from the weakly bonded phosphates through the H-bond and which are now bonded to the ionic liquids as a counter ion. 2.4. Gold Addition The gold amount was fixed to 0.8 wt% and verified by X-ray fluorescence. The specific surface area and its pore volume of the gold prepared samples are listed in Table 1. When citrate was used for the gold reduction, the sample recovers the specific surface area of the initial alumina indicating the complete liberation of the surface by ionic liquid loss during the deposition procedure. The use of NaBH 4 as a reducing agent at room temperature in gold solutions, no matter the solvent, increases the surface and pore volume compared to those of the supported ionic liquid system demonstrating the presence of the ionic liquid in the samples. As expected, the lower surface area of the sample prepared in acetone solution indicates the higher amount of the ionic liquid present in the sample. In aqueous media, no matter the reducing agent, the diffractions of metallic gold have not been observed (Figure 3(D,E)). This could be provoked either by the lower particle size or by the low quantity of the gold present in the sample (less than 1%). Nevertheless, the sample prepared in acetone solution clearly presents the diffraction lines attributed to metallic gold suggesting far bigger particle size. In addition, the diffraction lines of the gamma alumina become more stretched and intensive and a new diffraction peak appears at 2Θ = 32° Catalysts 2011, 1 60 (Figure 3(F)). This line has been reported by De Yu et al. [32] for calculated gamma alumina XRD pattern, although, to our knowledge, no experimental XRD pattern has yet been reported. The utilization of acetone in combination with a strong reducing agent seems to influence the crystallization of gamma alumina. The gold particles size has been studied by Transmission Electron Microscopy (Figure 5). Figure 5. TEM images of (A) Au/20IL/γ-Al 2 O 3 MOD (citrate); (B) Au/20IL/γ-Al 2 O 3 MOD (NaBH 4 ); (C) Au/20IL/γ-Al 2 O 3 MOD (NaBH 4 —acetone); (D) Diffraction of the gold crystals of Au/20IL/γ-Al 2 O 3 MOD (NaBH 4 —acetone). In aqueous solutions the change of the reducing agent from citrate to NaBH 4 did not seem to influence the gold particle size. However, it should be noted that the restrictions of the microscope did not permit observation of particles smaller than 3 nm. The use of water as a solvent resulted in lower gold particle sizes at around 6.5 nm for both samples. The change to acetone provokes an increase of the particle size to 9.5 nm in average but the agglomerates with average size higher than 40 nm have been observed and excluded from the particle size distribution. For the samples prepared in the gold solutions at room temperature (Figure 5(B,C)), the presence of a fine organic layer can also be observed, which confirms the presence of IL. The IR study of the supported ionic liquid compared to the IR spectra of the synthesized unsupported ionic liquid is presented in Figure 2(C,D). For the IL only, the bands at 3146, 3090 and 3017 cm −1 are attributed to the ν(CH) vibrations of C 4 , C 5 and C 2 (as shown in Scheme 1, on the right) of the imidazolium ring and those at 2962, 2935 and 2874 cm −1 corresponds to ν(CH) of butyl and methyl groups bonded to the ring [33,34]. On the base of theoretical calculations of IR spectra of an Catalysts 2011, 1 67 22. Cassol, C.C.; Ebeling, G.; Ferrera, B.; Dupont, J. A simple and practical method for the preparation and purity determination of halide-free imidazolium ionic liquids. Adv. Synth. Catal. 2006, 348, 243–248. 23. Francisco, M.S.P.; Cardoso, W.S.; Gushikem, Y.; Landers, R.; Kholin, Y.V. Surface modification with phosphoric acid of SiO 2 /Nb 2 O 5 prepared by the sol-gel method: Structural-textural and acid sites studies and an ion exchange model. Langmuir 2004, 20, 8707–8714. 24. Turkevich, J. Colloidal Gold part I. Historical and preparative aspects, morphology and structure. Gold Bull. 1985, 18, 86–91. 25. Douglas, F.; Yañez, R.; Ros, J.; Marín, S.; de la Escosura-Muñiz, A.; Alegret, S.; Merkoci, A. Silver, gold and the corresponding core shell nanoparticles: Synthesis and characterization. J. Nanopart. Res. 2008, 10, 97–106. 26. Nawrocki, J.; Dunlap, C.; McCormick, A.; Carr, P.W. Part II. Chromatography using ultra-stable metal oxide-based stationary phases for HPLC. J. Chromatogr. A 2004, 1028, 1–30. 27. Mekhemer, G.A.H.; Nohman, A.K.H.; Fouad, N.E.; Khalaf, N.E. Surface to bulk characterization of phosphate modified aluminas. Colloids Surf. A Physicochem. Eng. Aspects 2000, 161, 439–446. 28. Gu, W.; Shen, M.; Chang, X.; Wang, Y.; Wang, J. Gelification process to prepare phosphate modified alumina: Study on structure and surface properties. J. Alloys Compd. 2007, 441, 311–316. 29. Kasprzyk-Hordern, B. Chemistry of alumina, reactions in aqueous solution and its application in water treatment. Adv. Colloid Interface Sci. 2004, 110, 19–48. 30. Busca, G.; Ramis, G.; Lorenzelli, V.; Rossi, P.F.; La Ginestra, A.; Patrono, P. Phosphoric acid on oxide carriers. 1. Characterization of silica, alumina, and titania impregnated by phosphoric acid. Langmuir 1989, 5, 911–916. 31. Mehnert, C.P. Supported ionic liquid catalysts. Chem. Eur. J. 2005, 11, 50–56. 32. De Yu, L.; O’Connor, B.H.; Roach, G.I.D.; Cornell, J.B. Crystal structure communication. Acta Crystallogr. A Found. Crystallogr. 1990, 46, C61. 33. Heimer, N.E.; Del Sesto, R.E.; Meng, Z.; Wilkes, J.S.; Carper, W.R. Vibrational spectra of imidazolium tetrafluoroborate ionic liquids. J. Mol. Liq. 2006, 124, 84–95. 34. Dieter, K.M.; Dymek, C.J.; Heimer, N.E.; Rovang, J.W.; Wilkes, J.S. Ionic structure and interactions in 1-methyl-3-ethylimidazolium chloride-aluminum chloride molten salts. J. Am. Chem. Soc. 1988, 110, 2722–2726. 35. Talaty, E.R.; Raja, S.; Storhaug, V.J.; Dolle, A.; Carper, W.R. Raman and infrared spectra and ab initio calculations of C 2–4 MIM imidazolium hexafluorophosphate ionic liquids. J. Phys. Chem. B 2004, 108, 13177–13184. 36. Katsyuba, S.A.; Zvereva, E.E.; Vidis, A.; Dyson, P.J. Application of density functional theory and vibrational spectroscopy toward the rational design of ionic liquids. J. Phys. Chem. A 2007, 111, 352–370. 37. Romero-Sarria, F.; Penkova, A.; Martinez, L.M.; Centeno, M.A.; Hadjiivanov, K.; Odriozola, J.A. Role of water in the CO oxidation reaction on Au/CeO 2 : Modification of the surface properties. Appl. Catal. B Environ. 2008, 84, 119–124. 38. Carriazo, G.; Martínez, L.M.; Odriozola, J.A.; Moreno, S.; Molina, R.; Centeno, M.A. Gold supported on Fe, Ce, Al, pillared bentonites for CO oxidation reaction. Appl. Catal. B Environ. 2007, 72, 157–165. Catalysts 2011, 1 68 39. Date, M.; Ichihashi, Y.; Yamashita, T.; Chiorino, A.; Boccuzzi, F.; Haruta, M. Performance of Au/TiO 2 catalyst under ambient conditions. Catal. Today 2002, 72, 89–94. 40. Date, M.; Haruta, M. Moisture effect on CO oxidation over Au/TiO 2 catalyst. J. Catal. 2001, 201, 221–224. 41. Ivanova, S.; Petit, C.; Pitchon, V. Application of alumina supported gold-based catalysts in total oxidation of CO and light hydrocarbons mixture. Catal. Today 2006, 113, 182–186. 42. Ivanova, S.; Petit, C.; Pitchon, V. A new preparation method for the formation of gold nanoparticles on an oxide support. Appl. Catal. A Gen. 2004, 267, 191–201. 43. Tsuchiya, H.; Imanari, M.; Ishihara, S.; Nakakoshi, M.; Nishikawa, K.; Seki, H.; Tashiro, M. NMR study for self-aggregation of 1-butyl-3-methylimidazolium bromide in aqueous solution. Anal. Sci. 2008, 24, 1369–1371. 44. Schubert, M.M.; Hackenberg, S.; Van Veen, A.C.; Muhler, M.; Plzak, V.; Behm, R.J. CO oxidation over supported gold catalysts—“Inert” and “active” support materials and their role for the oxygen supply during reaction. J. Catal. 2001, 197, 113–122. 45. Santos, C.S.; Baldelli, S. Surface orientation of 1-methyl-, 1-ethyl-, and 1-butyl-3-methylimidazolium methyl sulfate as probed by sum-frequency generation vibrational spectroscopy. J. Phys. Chem. B 2007, 111, 4715–4723. © 2011 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).