WO2009122149A1 - Method for the preparation of supported catalyst using noble metal nanoparticles and catalyst so obtained - Google Patents

Method for the preparation of supported catalyst using noble metal nanoparticles and catalyst so obtained Download PDF

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WO2009122149A1
WO2009122149A1 PCT/GB2009/000821 GB2009000821W WO2009122149A1 WO 2009122149 A1 WO2009122149 A1 WO 2009122149A1 GB 2009000821 W GB2009000821 W GB 2009000821W WO 2009122149 A1 WO2009122149 A1 WO 2009122149A1
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noble metal
metal nanoparticles
catalyst
ionic liquid
sol
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PCT/GB2009/000821
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French (fr)
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Jairton Dupont
Sandra Shirley Ximeno Chiaro
Marcos Alexandre Geleysky
Flavio André PAVAN
João Henrique ZIMNOCH DOS SANTOS
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Petroleo Brasileiro S.A. - Petrobras
Benson, John, Everett
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Publication of WO2009122149A1 publication Critical patent/WO2009122149A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/08—Silica
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/46—Ruthenium, rhodium, osmium or iridium
    • B01J23/464—Rhodium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J31/00—Catalysts comprising hydrides, coordination complexes or organic compounds
    • B01J31/02—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides
    • B01J31/0272—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides containing elements other than those covered by B01J31/0201 - B01J31/0255
    • B01J31/0274—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides containing elements other than those covered by B01J31/0201 - B01J31/0255 containing silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J31/00—Catalysts comprising hydrides, coordination complexes or organic compounds
    • B01J31/02—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides
    • B01J31/0277—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides comprising ionic liquids, as components in catalyst systems or catalysts per se, the ionic liquid compounds being used in the molten state at the respective reaction temperature
    • B01J31/0278—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides comprising ionic liquids, as components in catalyst systems or catalysts per se, the ionic liquid compounds being used in the molten state at the respective reaction temperature containing nitrogen as cationic centre
    • B01J31/0281—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides comprising ionic liquids, as components in catalyst systems or catalysts per se, the ionic liquid compounds being used in the molten state at the respective reaction temperature containing nitrogen as cationic centre the nitrogen being a ring member
    • B01J31/0284—Catalysts comprising hydrides, coordination complexes or organic compounds containing organic compounds or metal hydrides comprising ionic liquids, as components in catalyst systems or catalysts per se, the ionic liquid compounds being used in the molten state at the respective reaction temperature containing nitrogen as cationic centre the nitrogen being a ring member of an aromatic ring, e.g. pyridinium
    • B01J35/23—
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/0072—Preparation of particles, e.g. dispersion of droplets in an oil bath
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/03—Precipitation; Co-precipitation
    • B01J37/036—Precipitation; Co-precipitation to form a gel or a cogel
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/06—Washing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/16—Reducing
    • B01J37/18—Reducing with gases containing free hydrogen
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H01M4/923—Compounds thereof with non-metallic elements
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • This invention relates to the field of the preparation of a supported catalyst comprising nanoparticles of a noble metal.
  • the method includes dispersion of the particles into an ionic liquid and incorporation into a matrix by a sol-gel process, in a single stage.
  • the usual methods for stabilising metal nanoparticles involve the use of protective agents, such as binders with functional groups, surfactants and polymers, which have the disadvantage of causing the loss of catalytic activity as they can occupy the active catalytic sites, apart from the difficulty of removing them from the reaction medium, suggesting improvements such as those obtained through the use of ionic liquids, as will be described below.
  • protective agents such as binders with functional groups, surfactants and polymers
  • Ionic liquids as defined in Patent US 7,252,791, are salts or mixtures of salts having a melting point below 100 0 C. Salts of this type known in the literature are derived from ammonium, phosphonium, pyridine or imidazolium cations associated with anions, such as halogen-stannates, halogen-aluminates, hexafluoro- phosphates or tetrafluoroborates .
  • the most investigated and used ionic liquids are those based on 1, 3-dialkyl-imidazolium cations, because they have unique physical and chemical properties such as low vapour pressure, a liquid state over a wide temperature range (close to ambient temperature) , sufficiently low viscosity ( ⁇ 800 mm 2 /s at 20 0 C); they are non-flammable, thermal and electrochemical stability are more favourable than for common solvents, there is the possibility of a wide range of organic and inorganic compounds whose solubilities can be adjusted by selecting the alkyl groups bound to the imidazolium ring or the nature of the ion, typically non- coordinating liquids which are easily prepared from commercial reagents using conventional processes of synthesis . These unique properties confer operational advantages in a large number of industrial applications such as for example their use as a "green" solvent to replace conventional organic solvents, and in catalytic processes which are selective at ambient temperature under more economical energy conditions.
  • metal nanoparticles dispersed in ionic liquids show significant improvements in stability when applied in catalytic systems, in some cases they still tend to aggregate. On the other hand the nanoparticles can be easily incorporated into matrices to produce more stable catalysts having greater catalytic activity.
  • the sol-gel process is based on the preparation of a "network" of oxides created by reactions involving the hydrolysis and condensation of molecular precursors, generally alkoxides or inorganic salts, both reactions being affected by the presence of acid or basic catalysts.
  • Homogeneous catalysts whose catalytic activity takes place at molecular level, and heterogeneous catalysts in which catalytic activity is governed by surface phenomena, have been incorporated into matrices by sol-gel processes using ionic liquids and metal precursors such as [RhCl (PPh 3 ) 3 ] and Rh (AcAc) 3 .
  • ionic liquids and metal precursors such as [RhCl (PPh 3 ) 3 ] and Rh (AcAc) 3 .
  • Rh (AcAc) 3 Rh
  • These catalysts are prepared by covalent bonding of the ionic liquids to the surface of the matrix, or simply depositing catalytically active species on the surface of the matrix, usually silica or polymer materials.
  • Patent US 2007/0101824 describes a method for producing nanoparticles incorporated in a matrix, preferably carbon, using a metal precursor dispersed in an ionic liquid, in which the particle size is reduced through heating by microwaves.
  • the compositions so produced can be used as catalysts, fuel cells, super- capacitors, and components for batteries and the like.
  • Patent US 6,686,308 describes a method for the preparation of a supported catalyst comprising metal nanoparticles having a mean size of 3 nm or less, supported in a matrix, preferably carbon.
  • the metal nanoparticles content of the supported catalyst is 30% m/m.
  • the metals may be selected from: palladium, platinum, ruthenium, rhodium, iridium, osmium, molybdenum, tungsten, iron, nickel and tin.
  • this invention relates to a method for the preparation of a supported catalyst comprising noble metal nanoparticles, the particles being dispersed in an ionic liquid and incorporated in a matrix by a sol-gel process, in a single stage.
  • the method comprises taking noble metal nanoparticles, dispersing the nanoparticles in an ionic liquid and a co-solvent, and encapsulating the nanoparticles in a solid matrix through the sol-gel process .
  • the method provides a supported catalyst having a proportion of noble metal nanoparticles within the range between 0.10% and 0.15% m/m in relation to the supported catalyst.
  • the supported catalysts so obtained are quite homogeneous and have a regular distribution of noble metal nanoparticles in the solid matrix and have greater catalytic activity than isolated noble metal nanoparticles .
  • Figure 1 shows a typical diffractogram of Rh(O) nanoparticles obtained by reducing the RhCl 3 .nH 2 ⁇ precursor in BMI. BF 4 ionic liquid as described in Example 1.
  • Figure 2 shows a diffractogram of Rh(O) nanoparticles encapsulated in a silica matrix, prepared by the acid route, using BMI. BF 4 as the ionic liquid and a quantity of Rh(O) less than 2% in relation to the silica, as described in Example 2.
  • Figure 3 shows a micrograph of Rh(O) /SiO 2 synthesised using BMI. BF 4 by the sol-gel method in acid medium, a synthesis according to the method described in Example 2.
  • Figure 4 shows a micrograph of Rh(O) /Si ⁇ 2 synthesised using BMI. BF 4 by the sol-gel method in basic medium, a synthesis according to the method described in Example 3.
  • Figure 5 illustrates the chemical mapping of Rh(O) /SiO 2 synthesised using ' BMLBF 4 by the sol-gel method in acid medium, a synthesis according to the method described in Example 2.
  • Figure 6 illustrates the chemical mapping of Rh(O) /SiO 2 synthesised using BMLBF 4 by the sol-gel method in basic medium, a synthesis according to the method described in Example 3.
  • Figure 7 illustrates transmission electron microscopy and a size distribution histogram for the Rh(O) /SiO 2 catalyst synthesised using BMLBF 4 by the sol-gel method in acid medium, a synthesis according to the method described in Example 2.
  • Figure 8 illustrates the hydrogenation reaction of 1-hexene (-•-) and cyclohexene (-!-) catalysed by Rh(O) /SiO 2 /HF under a pressure of 4°C to 75°C, and an aromatic hydrocarbon/Rh (0) ratio of 1179.
  • Figure 9 shows electron transmission microscopy and a size distribution histogram for isolated nanoparticles of Rh(O) prepared in BMLBF 4 ionic liquid and redispersed in isopropanol.
  • the method described below comprises the preparation of a supported catalyst using noble metal nanoparticles dispersed in an ionic liquid and incorporated in a matrix through a sol-gel process, in a single stage.
  • the supported catalyst obtained has noble metal nanoparticles having a mean size within the range from 3 nm to 6 nm and a noble metal nanoparticle concentration in the supported catalyst within the range from 0.10% to 0.15% (m/m) .
  • the particle size is preferably a particle diameter of substantially spherical particles, and where the particles are not substantially spherical it is preferably the diameter of notional particles of the same volume as that of the actual particles .
  • the method according to this invention comprises the stages of: a) providing noble metal nanoparticles having a mean particle size within the range from 3 nm to 6 nm, b) providing a mixture comprising an ionic liquid and a co-solvent, c) obtaining a dispersion by adding noble metal nanoparticles to the mixture obtained in b) , d) adding a solid matrix molecular precursor to the dispersion obtained in c) , e) adding a catalyst for the sol-gel process, f) recovering a supported catalyst comprising noble metal nanoparticles dispersed in ionic liquid and incorporated in a solid matrix.
  • Noble metal nanoparticles which are useful for the method according to this invention may be obtained by any of the methods available in the state of the art for noble metals selected from: iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, platinum and palladium. Preferably, rhodium.
  • the use of nanoparticles of noble metals in elemental form represents a great advantage ever preparing supported catalysts obtained from metal precursors. When metal precursors are incorporated into a solid matrix there is an additional stage of reducing the metal to its elemental form, and the possibility of interaction between the metal precursor and the solid phase affecting the final size of the particles obtained. This is avoided through directly using nanoparticles of noble metal in elemental form without the additional stage of reducing the metal.
  • the noble metal nanoparticles must be dispersed in ionic liquids with the addition of an alcohol, preferably ethanol, which acts as a co-solvent having the function of reducing the viscosity of the ionic liquid and thus assisting handling.
  • an alcohol preferably ethanol
  • the useful proportion for achieving this objective is 10% to 20% of ionic liquid in relation to the quantity of co- solvent.
  • the ionic liquids act in steric and electrostatic stabilisation of the noble metal nanoparticles mainly because of their high charge and polarisation.
  • the ionic liquids useful for the method are derivatives of the 1-3-dialkylimidazolium cation associated with poorly coordinating anions, preferably PF 6 " , BF 4 " , CF 3 SO 3 " , (CF 3 SO 2 ) 2 N " .
  • the choice of anion used is based on the characteristics desired for the ionic liquid, as a correct choice of anion will influence the physical and chemical properties of the liquid such as viscosity, solvating power, catalytic activity, hydrophobicity and melting point.
  • the useful salts derived from the 1-3-dialkylimidazolium cation have pre-organised structures which can be adapted to different species, because they have both hydrophobic and hydrophilic regions, and high polarisability .
  • This structural organisation acts as an "entropy signpost" for solid matrices on the nanoscale, such as those prepared by a sol-gel process .
  • the chemistry of a sol-gel process is based on the hydrolysis and condensation of molecular precursors.
  • the stage of hydrolysing a metal alkoxide takes place by reacting this with water, generating a hydroxy group (M-OH) .
  • This reaction is based on the nucleophilic addition of the water molecule to the metal atom.
  • the second stage in the sol-gel process comprises condensing the M-OH species, giving rise to the formation of -M-O-H- bonds, which after various condensation stages result in a MOn "network".
  • a catalyst which may be an acid or a base.
  • Useful acids for this invention may be any inorganic acids, preferably HF or HCl.
  • bases NH 4 OH is preferably used.
  • the excess ionic liquid can be separated off and recovered by any known methods such as solvent extraction or calcination.
  • solvents used for extraction acetone is preferably used. • Calcination is carried out at a temperature of 450 0 C.
  • the supported catalysts prepared in this way have noble metal nanoparticles having a particle size in the range from 3 nm to 6 run, as shown in Figure 7, with no change in relation to the mean particle size of the noble metal used for the said method, illustrated in Figure 9, which is an indication that there is no sintering, and therefore of the stabilisation provided by the ionic liquids .
  • the noble metal nanoparticles are homogeneously distributed throughout the solid matrix, mainly because of the stabilisation provided by the ionic liquid.
  • Catalysts produced according to the method of this invention may be used in hydrogenation reactions, as the ionic liquid present in the pores of the solid matrix has the property of satisfactorily dissolving unsaturated hydrocarbons, but not alkanes, making the selective reduction of dienes to simple olefins (hydrogenation) possible.
  • the catalytic activity of these catalysts is greater than that of isolated noble metal nanoparticles, as shown in Table 2, as well as making it possible to use small quantities of noble metal in the catalyst without losing its catalytic activity, as illustrated in Table 3.
  • This example illustrates the preparation and isolation of rhodium nanoparticles.
  • Rh nanoparticles were carried out in a modified Fischer-Porter ® reactor with an inlet for introducing the substrate and/or removing aliquots of the substrates for analysis by gas chromatography.
  • a typical preparation approximately 0.026 g (0.1 mmol) of RhCl 3 .3H 2 ⁇ was dissolved in 3 mL of methanol, followed by the addition of 1 mL of the ionic liquid
  • Rh(O) particles obtained according to the method described in Example 1 and shown in Figure 1, together with the Debye-Scherrer equation, enables us to calculate the size of the Rh(O) nanoparticles.
  • the calculated value was a mean diameter of 5.8 run.
  • This example illustrates the preparation of Rh(O) nanoparticles supported on a silica matrix by the sol- gel process via the acid route.
  • TEOS tetraethoxysiloxane
  • the mixture was initially homogenised by gentle magnetic stirring (10 minutes) , followed by the application of ultrasound (2 minutes) and was finally allowed to stand in a semi- sealed beaker at a temperature of 60 0 C (18 hours) .
  • the resulting material was washed with acetone several times, dried under vacuum and then used in a catalytic process. Extraction with acetone may be used to remove the liquid. Calcination 450 0 C for 6 hours can also be used to extract the ionic liquid.
  • Figure 2 shows the amorphous nature of the silica produced in this way (halo centred on 22°) and the presence of rhodium is confirmed by the diffraction planes for Rh(O) (plane (1 1 1) at 41°C and plane (2 0
  • Rh(O) particles on the silica matrix was found to be relatively homogeneous, as shown in Figure 5, by the chemical mapping of Rh(O) /SiO 2 synthesised with BMLBF 4 using the sol-gel method in acid medium.
  • This example illustrates the preparation of Rh(O) nanoparticles supported on a silica matrix by the sol- gel process using the basic route.
  • TEOS tetraethoxysiloxane
  • BMLBF 4 ionic liquid
  • Rh(O) nanoparticles 10 mg
  • 95 mL of ethanol was added to the solution and finally 20 mL of NH 4 OH.
  • the mixture was kept gently stirred for 3 hours at ambient temperature and subsequently allowed to stand for 18 hours .
  • the solid material obtained was filtered, then washed with acetone and dried in a vacuum atmosphere for 1 hour. Extraction with acetone may be used to remove the liquid. Calcination at 450 0 C for 6 hours can also be used to extract the ionic liquid.
  • This example relates to the use of supported catalysts prepared according to the method described in this invention in aromatic hydrocarbon hydrogenation reactions .
  • Aromatic hydrocarbon hydrogenation reactions in heterogeneous medium were carried out in a Fischer- Porter ® reactor modified with an inlet to add the substrate or ionic liquid and/or to remove samples for chromatographic analysis.
  • Rh/SiO 2 Rh(O) ⁇ 0.2%) were placed in the reactor followed by the addition of 12.5 mmol of substrate (decene, cyclohexene and benzene) in an aromatic hydrocarbon/Rh ratio of 695 (12.5 mmol/0.018 mmol).
  • substrate decene, cyclohexene and benzene
  • the hydrogen pressure was held constant at 4 atm and the temperature at 75°C.
  • Progress of the reaction was monitored through consumption of the hydrogen present in a container coupled to a pressure transducer connected to the computer.
  • the conversion of substrate to product was analysed by gas chromatography.
  • Table 2 shows data relating to the hydrogenation of decene and cyclohexene. Data relating to the catalytic activity of isolated Rh(O) nanoparticles have been included for comparison.
  • the reaction conditions used were as follows: temperature 75°C; P H2 (4 atm, constant pressure) ; conversion (100%) ; TOF - turnover frequency - defined here as mol of hydrogenated product per mol of Rh(O) catalyst per minute; c (1) Rh (0) /HF/BMI .BF 4 (100 mg, 0.11% of Rh(O)) 12.5 mmol of alkene; d (2) Rh(O) /NH 4 OH/BMI. BF 4 (100 mg, 0.14% of Rh(O)) 12.5 mmol of arene; Rh(O) nanoparticles (3 mg, 0.03 mmol),
  • Table 3 shows data relating to the hydrogenation of benzene using the supported catalyst obtained via the acid route. Data relating to the catalytic activity of isolated Rh(O) nanoparticles have been included for comparison.

Abstract

A method for the preparation of a supported catalyst comprising noble metal nanoparticles incorporated in a solid matrix through a sol-gel process in ionic liquid medium is described.

Description

METHOD FOR THE PREPARATION OF SUPPORTED CATALYST USING NOBLE METAL NANOPARTICLES AND CATALYST SO OBTAINED
FIELD OF INVENTION This invention relates to the field of the preparation of a supported catalyst comprising nanoparticles of a noble metal. The method includes dispersion of the particles into an ionic liquid and incorporation into a matrix by a sol-gel process, in a single stage.
BACKGROUND OF THE INVENTION
The simplification of reaction processes or systems in which selectivity and/or yield are low, or in which energy expenditure is very high, has led to a search for new catalysts.
Of the various lines of catalyst development, the use of noble metal nanoparticles having particles of size varying between 1 nm and 100 run has proved to be promising, especially as regards activity and selectivity. However, use of these particles as catalysts has been rendered difficult because of their poor stability, given that they are only kinetically stable and therefore have to be stabilised so that they do not undergo sintering, that is agglomeration of the metal particles, which would result in the loss of catalytic activity.
The usual methods for stabilising metal nanoparticles involve the use of protective agents, such as binders with functional groups, surfactants and polymers, which have the disadvantage of causing the loss of catalytic activity as they can occupy the active catalytic sites, apart from the difficulty of removing them from the reaction medium, suggesting improvements such as those obtained through the use of ionic liquids, as will be described below.
Ionic liquids, as defined in Patent US 7,252,791, are salts or mixtures of salts having a melting point below 1000C. Salts of this type known in the literature are derived from ammonium, phosphonium, pyridine or imidazolium cations associated with anions, such as halogen-stannates, halogen-aluminates, hexafluoro- phosphates or tetrafluoroborates . The most investigated and used ionic liquids are those based on 1, 3-dialkyl-imidazolium cations, because they have unique physical and chemical properties such as low vapour pressure, a liquid state over a wide temperature range (close to ambient temperature) , sufficiently low viscosity (< 800 mm2/s at 200C); they are non-flammable, thermal and electrochemical stability are more favourable than for common solvents, there is the possibility of a wide range of organic and inorganic compounds whose solubilities can be adjusted by selecting the alkyl groups bound to the imidazolium ring or the nature of the ion, typically non- coordinating liquids which are easily prepared from commercial reagents using conventional processes of synthesis . These unique properties confer operational advantages in a large number of industrial applications such as for example their use as a "green" solvent to replace conventional organic solvents, and in catalytic processes which are selective at ambient temperature under more economical energy conditions.
Jairton Dupont et al . , in their article "Catalytic Applications of Metal Nanoparticles in Imidazolium Ionic Liquids", Chemistry (Weinheim) , v.13, pp32-39 (2007) describe the preparation of metal nanoparticles in ionic liquids derived from the chloride cation of 1,3-dialkyl imidazolium and their use as catalysts for reactions under multiphase conditions. The combination of high charge and polarisation in the ionic liquids acts to improve both the steric and the electrostatic stability of the metal nanoparticles, so that in comparison with conventional catalysis these exhibit a potential advantage in catalytic reactions such as hydrogenation . Although metal nanoparticles dispersed in ionic liquids show significant improvements in stability when applied in catalytic systems, in some cases they still tend to aggregate. On the other hand the nanoparticles can be easily incorporated into matrices to produce more stable catalysts having greater catalytic activity. •
Among the processes available for obtaining matrices, or supports, for catalysts, the sol-gel process stands out. The sol-gel process is based on the preparation of a "network" of oxides created by reactions involving the hydrolysis and condensation of molecular precursors, generally alkoxides or inorganic salts, both reactions being affected by the presence of acid or basic catalysts.
Homogeneous catalysts, whose catalytic activity takes place at molecular level, and heterogeneous catalysts in which catalytic activity is governed by surface phenomena, have been incorporated into matrices by sol-gel processes using ionic liquids and metal precursors such as [RhCl (PPh3) 3] and Rh (AcAc) 3. These catalysts are prepared by covalent bonding of the ionic liquids to the surface of the matrix, or simply depositing catalytically active species on the surface of the matrix, usually silica or polymer materials.
Patent US 2007/0101824 describes a method for producing nanoparticles incorporated in a matrix, preferably carbon, using a metal precursor dispersed in an ionic liquid, in which the particle size is reduced through heating by microwaves. The compositions so produced can be used as catalysts, fuel cells, super- capacitors, and components for batteries and the like.
Patent US 6,686,308 describes a method for the preparation of a supported catalyst comprising metal nanoparticles having a mean size of 3 nm or less, supported in a matrix, preferably carbon. The metal nanoparticles content of the supported catalyst is 30% m/m. The metals may be selected from: palladium, platinum, ruthenium, rhodium, iridium, osmium, molybdenum, tungsten, iron, nickel and tin.
Nevertheless, there is in the literature no description or suggestion of a method of preparing a supported catalyst comprising noble metal nanoparticles dispersed in an ionic liquid and incorporated in a matrix through a sol-gel process in a single stage, so that the ionic liquid has the simultaneous functions of stabilising the noble metal nanoparticles and of acting as a mould for both the noble metal nanoparticles and the matrix, this method being described and claimed below.
SUMMARY OF THE INVENTION In a general way, this invention relates to a method for the preparation of a supported catalyst comprising noble metal nanoparticles, the particles being dispersed in an ionic liquid and incorporated in a matrix by a sol-gel process, in a single stage. The method comprises taking noble metal nanoparticles, dispersing the nanoparticles in an ionic liquid and a co-solvent, and encapsulating the nanoparticles in a solid matrix through the sol-gel process . The method provides a supported catalyst having a proportion of noble metal nanoparticles within the range between 0.10% and 0.15% m/m in relation to the supported catalyst.
The supported catalysts so obtained are quite homogeneous and have a regular distribution of noble metal nanoparticles in the solid matrix and have greater catalytic activity than isolated noble metal nanoparticles .
BRIEF DESCRIPTION OF THE DRAWINGS
The characteristics of the method for preparing supported catalysts to which this invention relates will be better understood from the following detailed description of the figures referred to below, which form an integral part of this description.
Figure 1 shows a typical diffractogram of Rh(O) nanoparticles obtained by reducing the RhCl3.nH2θ precursor in BMI. BF4 ionic liquid as described in Example 1.
Figure 2 shows a diffractogram of Rh(O) nanoparticles encapsulated in a silica matrix, prepared by the acid route, using BMI. BF4 as the ionic liquid and a quantity of Rh(O) less than 2% in relation to the silica, as described in Example 2.
Figure 3 shows a micrograph of Rh(O) /SiO2 synthesised using BMI. BF4 by the sol-gel method in acid medium, a synthesis according to the method described in Example 2.
Figure 4 shows a micrograph of Rh(O) /Siθ2 synthesised using BMI. BF4 by the sol-gel method in basic medium, a synthesis according to the method described in Example 3. Figure 5 illustrates the chemical mapping of Rh(O) /SiO2 synthesised using ' BMLBF4 by the sol-gel method in acid medium, a synthesis according to the method described in Example 2.
Figure 6 illustrates the chemical mapping of Rh(O) /SiO2 synthesised using BMLBF4 by the sol-gel method in basic medium, a synthesis according to the method described in Example 3.
Figure 7 illustrates transmission electron microscopy and a size distribution histogram for the Rh(O) /SiO2 catalyst synthesised using BMLBF4 by the sol-gel method in acid medium, a synthesis according to the method described in Example 2.
Figure 8 illustrates the hydrogenation reaction of 1-hexene (-•-) and cyclohexene (-!-) catalysed by Rh(O) /SiO2/HF under a pressure of 4°C to 75°C, and an aromatic hydrocarbon/Rh (0) ratio of 1179.
Figure 9 shows electron transmission microscopy and a size distribution histogram for isolated nanoparticles of Rh(O) prepared in BMLBF4 ionic liquid and redispersed in isopropanol.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The method described below comprises the preparation of a supported catalyst using noble metal nanoparticles dispersed in an ionic liquid and incorporated in a matrix through a sol-gel process, in a single stage. The supported catalyst obtained has noble metal nanoparticles having a mean size within the range from 3 nm to 6 nm and a noble metal nanoparticle concentration in the supported catalyst within the range from 0.10% to 0.15% (m/m) . The particle size is preferably a particle diameter of substantially spherical particles, and where the particles are not substantially spherical it is preferably the diameter of notional particles of the same volume as that of the actual particles . The method according to this invention comprises the stages of: a) providing noble metal nanoparticles having a mean particle size within the range from 3 nm to 6 nm, b) providing a mixture comprising an ionic liquid and a co-solvent, c) obtaining a dispersion by adding noble metal nanoparticles to the mixture obtained in b) , d) adding a solid matrix molecular precursor to the dispersion obtained in c) , e) adding a catalyst for the sol-gel process, f) recovering a supported catalyst comprising noble metal nanoparticles dispersed in ionic liquid and incorporated in a solid matrix.
Noble metal nanoparticles which are useful for the method according to this invention may be obtained by any of the methods available in the state of the art for noble metals selected from: iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, platinum and palladium. Preferably, rhodium. The use of nanoparticles of noble metals in elemental form represents a great advantage ever preparing supported catalysts obtained from metal precursors. When metal precursors are incorporated into a solid matrix there is an additional stage of reducing the metal to its elemental form, and the possibility of interaction between the metal precursor and the solid phase affecting the final size of the particles obtained. This is avoided through directly using nanoparticles of noble metal in elemental form without the additional stage of reducing the metal.
The noble metal nanoparticles must be dispersed in ionic liquids with the addition of an alcohol, preferably ethanol, which acts as a co-solvent having the function of reducing the viscosity of the ionic liquid and thus assisting handling. The useful proportion for achieving this objective is 10% to 20% of ionic liquid in relation to the quantity of co- solvent. In this case the ionic liquids act in steric and electrostatic stabilisation of the noble metal nanoparticles mainly because of their high charge and polarisation.
Preferably the ionic liquids useful for the method are derivatives of the 1-3-dialkylimidazolium cation associated with poorly coordinating anions, preferably PF6 ", BF4 ", CF3SO3 ", (CF3SO2) 2N". The choice of anion used is based on the characteristics desired for the ionic liquid, as a correct choice of anion will influence the physical and chemical properties of the liquid such as viscosity, solvating power, catalytic activity, hydrophobicity and melting point.
The useful salts derived from the 1-3-dialkylimidazolium cation have pre-organised structures which can be adapted to different species, because they have both hydrophobic and hydrophilic regions, and high polarisability . This structural organisation acts as an "entropy signpost" for solid matrices on the nanoscale, such as those prepared by a sol-gel process . The chemistry of a sol-gel process is based on the hydrolysis and condensation of molecular precursors.
Molecular precursors useful for the method according to this invention are metal alkoxides of formula M(OR)n (R = methyl, ethyl, propyl, isopropyl, butyl, t-butyl and others), preferably of the alkoxy siloxanes group, more specifically tetramethoxysiloxane and tetraoxysiloxane .
The stage of hydrolysing a metal alkoxide takes place by reacting this with water, generating a hydroxy group (M-OH) . This reaction is based on the nucleophilic addition of the water molecule to the metal atom. The second stage in the sol-gel process comprises condensing the M-OH species, giving rise to the formation of -M-O-H- bonds, which after various condensation stages result in a MOn "network".
The hydrolysis and condensation reactions involved in the polymerisation process are carried out in the presence of a catalyst, which may be an acid or a base. Useful acids for this invention may be any inorganic acids, preferably HF or HCl. Among the bases, NH4OH is preferably used.
The choice of acid or basic reaction conditions has an influence on the morphology of the supported catalyst obtained by this method, as we can see in the figures. In Figure 3, supported catalysts on silica prepared in BMI-BF4 under acid conditions have an irregular morphology, comprising submicrometre particles, while in Figure 4 the catalysts prepared in BMI. BF4 under basic conditions approximate more to a spherical shape, and are also of slightly greater size than those obtained when acid catalyst is used.
Once preparation of the supported catalyst has been completed, the excess ionic liquid can be separated off and recovered by any known methods such as solvent extraction or calcination. Among the solvents used for extraction, acetone is preferably used. • Calcination is carried out at a temperature of 4500C. The supported catalysts prepared in this way have noble metal nanoparticles having a particle size in the range from 3 nm to 6 run, as shown in Figure 7, with no change in relation to the mean particle size of the noble metal used for the said method, illustrated in Figure 9, which is an indication that there is no sintering, and therefore of the stabilisation provided by the ionic liquids .
The noble metal nanoparticles are homogeneously distributed throughout the solid matrix, mainly because of the stabilisation provided by the ionic liquid.
Catalysts produced according to the method of this invention may be used in hydrogenation reactions, as the ionic liquid present in the pores of the solid matrix has the property of satisfactorily dissolving unsaturated hydrocarbons, but not alkanes, making the selective reduction of dienes to simple olefins (hydrogenation) possible.
The catalytic activity of these catalysts is greater than that of isolated noble metal nanoparticles, as shown in Table 2, as well as making it possible to use small quantities of noble metal in the catalyst without losing its catalytic activity, as illustrated in Table 3.
EXAMPLES
The examples below illustrate application of the method to the preparation of supported catalyst according to this invention without thereby representing a limitation on its scope.
EXAMPLE 1
This example illustrates the preparation and isolation of rhodium nanoparticles.
The preparation of Rh nanoparticles was carried out in a modified Fischer-Porter® reactor with an inlet for introducing the substrate and/or removing aliquots of the substrates for analysis by gas chromatography. In a typical preparation approximately 0.026 g (0.1 mmol) of RhCl3.3H2θ was dissolved in 3 mL of methanol, followed by the addition of 1 mL of the ionic liquid
(BMLBF4) . The methanol was removed under reduced pressure (0.1 mbar) for 30 minutes at ambient temperature. The system was immersed in silicone oil, kept at 75°C with constant stirring, and subjected to 4 atm pressure of molecular hydrogen. The temperature was held constant through a digital controller immersed in the silicone bath. After 1 hour the mixture darkened. The resulting dark solution, comprising nanoparticles, was centrifuged at 3500 rpm for 3 minutes and washed at least 5 times with methanol to remove the ionic liquid. The supernatant solution was removed and the dry solid was obtained under reduced pressure.
The X-ray diffractogram for Rh(O) particles obtained according to the method described in Example 1 and shown in Figure 1, together with the Debye-Scherrer equation, enables us to calculate the size of the Rh(O) nanoparticles. The calculated value was a mean diameter of 5.8 run.
EXAMPLE 2
This example illustrates the preparation of Rh(O) nanoparticles supported on a silica matrix by the sol- gel process via the acid route.
In a typical preparation approximately 10 mL of tetraethoxysiloxane (TEOS) was placed in a beaker fitted with a magnetic stirrer and heating (600C) . Nanoparticles of Rh(O) (10 mg) were prepared previously and redispersed in 1 mL of BMI. BF4, 5 mL of ethanol were added to the solution comprising the ionic liquid and nanoparticles. After being subjected to ultrasound for 2 minutes, this was added to the beaker containing the TEOS. Finally an acid solution containing 2 mL of water and 05 mL of HF was added. The mixture was initially homogenised by gentle magnetic stirring (10 minutes) , followed by the application of ultrasound (2 minutes) and was finally allowed to stand in a semi- sealed beaker at a temperature of 600C (18 hours) . The resulting material was washed with acetone several times, dried under vacuum and then used in a catalytic process. Extraction with acetone may be used to remove the liquid. Calcination 4500C for 6 hours can also be used to extract the ionic liquid.
Figure 2 shows the amorphous nature of the silica produced in this way (halo centred on 22°) and the presence of rhodium is confirmed by the diffraction planes for Rh(O) (plane (1 1 1) at 41°C and plane (2 0
0) at 47°) .
The distribution of Rh(O) particles on the silica matrix was found to be relatively homogeneous, as shown in Figure 5, by the chemical mapping of Rh(O) /SiO2 synthesised with BMLBF4 using the sol-gel method in acid medium.
In Figure 7 the average size of the Rh particles immobilised on the silica is 4.1 ± 0.6 nm.
EXAMPLE 3
This example illustrates the preparation of Rh(O) nanoparticles supported on a silica matrix by the sol- gel process using the basic route.
In a typical preparation approximately 10 mL of tetraethoxysiloxane (TEOS) were introduced into a beaker equipped with magnetic stirring, to which an alcoholic solution (5 mL) containing 1 mL of ionic liquid (BMLBF4) and previously isolated Rh(O) nanoparticles (10 mg) were added. 95 mL of ethanol was added to the solution and finally 20 mL of NH4OH. The mixture was kept gently stirred for 3 hours at ambient temperature and subsequently allowed to stand for 18 hours . The solid material obtained was filtered, then washed with acetone and dried in a vacuum atmosphere for 1 hour. Extraction with acetone may be used to remove the liquid. Calcination at 4500C for 6 hours can also be used to extract the ionic liquid.
Relative homogeneity in the distribution of the Rh(O) particles on the silica matrix was observed as shown in Figure 6, through the chemical mapping of Rh(O) /SiO2 synthesised using BMLBF4 by the sol-gel method in basic medium.
EXAMPLE 4
This example relates to the use of supported catalysts prepared according to the method described in this invention in aromatic hydrocarbon hydrogenation reactions .
Aromatic hydrocarbon hydrogenation reactions in heterogeneous medium were carried out in a Fischer- Porter® reactor modified with an inlet to add the substrate or ionic liquid and/or to remove samples for chromatographic analysis. In a typical reaction, approximately 100 mg of Rh/SiO2 (Rh(O) < 0.2%) were placed in the reactor followed by the addition of 12.5 mmol of substrate (decene, cyclohexene and benzene) in an aromatic hydrocarbon/Rh ratio of 695 (12.5 mmol/0.018 mmol). During the reaction the hydrogen pressure was held constant at 4 atm and the temperature at 75°C. Progress of the reaction was monitored through consumption of the hydrogen present in a container coupled to a pressure transducer connected to the computer. The conversion of substrate to product was analysed by gas chromatography.
In Figure 8, for the conversion of cyclohexene (- !-) to approximately 98% cyclohexane a reaction time of 50 minutes was required for a catalyst obtained according to the method described in Example 2.
Figure imgf000013_0001
Table 1 shows the results of hydrocarbon hydrogenation using Rh/Siθ2 as a catalyst under the reaction conditions: hydrogen pressure 4 atm, 75 ± 3°C, hydrocarbon / Rh/SiO2 ratio = 695 mol/mol.
These results demonstrate the technical advantages of the method of preparing a supported catalyst according to this invention, as it yields a product which if applied in hydrogenation reactions provides high conversion percentages, always above 90%. The supported catalyst prepared under acid conditions (HF as catalyst) proved more active than supported catalysts prepared under basic conditions (NH4OH as catalyst) , due to the smaller concentration of encapsulated ionic liquid obtained under basic conditions, which is responsible for the stability of the noble metal nanoparticles .
Figure imgf000014_0001
Table 2 shows data relating to the hydrogenation of decene and cyclohexene. Data relating to the catalytic activity of isolated Rh(O) nanoparticles have been included for comparison.
The reaction conditions used were as follows: temperature 75°C; PH2 (4 atm, constant pressure) ; conversion (100%) ; TOF - turnover frequency - defined here as mol of hydrogenated product per mol of Rh(O) catalyst per minute; c (1) Rh (0) /HF/BMI .BF4 (100 mg, 0.11% of Rh(O)) 12.5 mmol of alkene; d (2) Rh(O) /NH4OH/BMI. BF4 (100 mg, 0.14% of Rh(O)) 12.5 mmol of arene; Rh(O) nanoparticles (3 mg, 0.03 mmol),
6.25 mmol of alkene/ 9 nanoparticles and Rh(O) (3 mg, 0.03 mmol), 12.5 mmol of alkene.
TABLE 3
Feed Catalytic Substrate Rh (O) Time Conv. % TOF system mmol (mirT1)
1 "Rh (O) /SiO2/HF Benzene 0 . 001 18 98 11 hours
2 Rh ( O ) Benzene 0. 01 14 100 90 hours
Table 3 shows data relating to the hydrogenation of benzene using the supported catalyst obtained via the acid route. Data relating to the catalytic activity of isolated Rh(O) nanoparticles have been included for comparison.
These results show that the supported catalyst prepared by the acid route is active for the benzene hydrogenation reaction, even if the Rh(O) is present in very small quantities.

Claims

1. Method for the preparation of a supported catalyst comprising noble metal nanoparticles through a sol-gel process, characterised in that it comprises the stages of: a) providing noble metal nanoparticles having a mean particle size within the range from 3 nm to 6 nm, b) providing a mixture comprising an ionic liquid and a co-solvent, c) obtaining a dispersion by adding noble metal nanoparticles to the mixture obtained in b) , d) adding a solid matrix molecular precursor to the dispersion obtained in c) , e) adding a catalyst for the sol-gel process, f) recovering a supported catalyst comprising noble metal nanoparticles dispersed in ionic liquid and incorporated in a solid matrix.
2. Method according to Claim 1, characterised in that the noble metal is selected from iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, platinum and palladium.
3. Method according to Claim 1, characterised in that the noble metal is rhodium.
4. Method according to Claim 1, characterised in that the ionic liquid is a derivative of the 1-3- dialkylimidazolium cation associated with a weakly coordinating anion.
5. Method according to Claim 4, characterised in that the anion is selected from PF6 ", BF4 ", CF3SO3 " or (CF3SO2) 2N".
6. Method according to Claim 1, characterised in that the ionic liquid is in a proportion varying between 1% and 20% (v/v) in relation to the co-solvent.
7. Method according to Claim 1, characterised in that the co-solvent is an alcohol.
8. Method according to Claim 8, characterised in that the alcohol is ethanol.
9. Method according to Claim 1, characterised in that the molecular precursor is an alkoxide.
10. Method according to Claim 10, characterised in that the alkoxide is preferably a metal alkoxide.
11. Method according to Claim 11, characterised in that the metal alkoxide is preferably an alkoxysiloxane .
12. Method according to Claim 11, characterised in that the alkoxysiloxane is preferably tetraethoxysiloxane (TEOS) .
13. Method according to Claim 1, characterised in that the catalyst for the sol-gel process is an inorganic acid.
14. Method according to Claim 13, characterised in that the acid is HF.
15. Method according to Claim 13, characterised in that the acid is HCl.
16. Method according to Claim 1, characterised in that the catalyst for the sol-gel process is a base, preferably NH4OH.
17. Method according to Claim 1, characterised in that the noble metal nanoparticles in the supported catalyst have a mean size in the range between 3 nm and 6 ran.
18. Method according to Claim 1, characterised in that the concentration of noble metal nanoparticles in the supported catalyst lies within a concentration range between 0.10% and 0.15% (m/m) .
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