WO2007091085A1 - Removal of transition metal contamination - Google Patents

Removal of transition metal contamination Download PDF

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Publication number
WO2007091085A1
WO2007091085A1 PCT/GB2007/000458 GB2007000458W WO2007091085A1 WO 2007091085 A1 WO2007091085 A1 WO 2007091085A1 GB 2007000458 W GB2007000458 W GB 2007000458W WO 2007091085 A1 WO2007091085 A1 WO 2007091085A1
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Prior art keywords
transition metal
process according
solution
agent
water
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PCT/GB2007/000458
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French (fr)
Inventor
David William Knight
Ian Rhys Morgan
Anthony John Proctor
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University College Cardiff Consultants Ltd
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Publication of WO2007091085A1 publication Critical patent/WO2007091085A1/en

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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J31/00Catalysts comprising hydrides, coordination complexes or organic compounds
    • B01J31/40Regeneration or reactivation
    • B01J31/4015Regeneration or reactivation of catalysts containing metals
    • B01J31/4023Regeneration or reactivation of catalysts containing metals containing iron group metals, noble metals or copper
    • B01J31/4038Regeneration or reactivation of catalysts containing metals containing iron group metals, noble metals or copper containing noble metals
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J31/00Catalysts comprising hydrides, coordination complexes or organic compounds
    • B01J31/40Regeneration or reactivation
    • B01J31/4015Regeneration or reactivation of catalysts containing metals
    • B01J31/4053Regeneration or reactivation of catalysts containing metals with recovery of phosphorous catalyst system constituents
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22BPRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B11/00Obtaining noble metals
    • C22B11/04Obtaining noble metals by wet processes
    • C22B11/042Recovery of noble metals from waste materials
    • C22B11/048Recovery of noble metals from waste materials from spent catalysts
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22BPRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B3/00Extraction of metal compounds from ores or concentrates by wet processes
    • C22B3/20Treatment or purification of solutions, e.g. obtained by leaching
    • C22B3/22Treatment or purification of solutions, e.g. obtained by leaching by physical processes, e.g. by filtration, by magnetic means, or by thermal decomposition
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22BPRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B7/00Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
    • C22B7/009General processes for recovering metals or metallic compounds from spent catalysts
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2231/00Catalytic reactions performed with catalysts classified in B01J31/00
    • B01J2231/50Redistribution or isomerisation reactions of C-C, C=C or C-C triple bonds
    • B01J2231/54Metathesis reactions, e.g. olefin metathesis
    • B01J2231/543Metathesis reactions, e.g. olefin metathesis alkene metathesis
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2531/00Additional information regarding catalytic systems classified in B01J31/00
    • B01J2531/80Complexes comprising metals of Group VIII as the central metal
    • B01J2531/82Metals of the platinum group
    • B01J2531/821Ruthenium
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J31/00Catalysts comprising hydrides, coordination complexes or organic compounds
    • B01J31/16Catalysts comprising hydrides, coordination complexes or organic compounds containing coordination complexes
    • B01J31/24Phosphines, i.e. phosphorus bonded to only carbon atoms, or to both carbon and hydrogen atoms, including e.g. sp2-hybridised phosphorus compounds such as phosphabenzene, phosphole or anionic phospholide ligands
    • B01J31/2404Cyclic ligands, including e.g. non-condensed polycyclic ligands, the phosphine-P atom being a ring member or a substituent on the ring
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J38/00Regeneration or reactivation of catalysts, in general
    • B01J38/48Liquid treating or treating in liquid phase, e.g. dissolved or suspended
    • B01J38/50Liquid treating or treating in liquid phase, e.g. dissolved or suspended using organic liquids
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J38/00Regeneration or reactivation of catalysts, in general
    • B01J38/48Liquid treating or treating in liquid phase, e.g. dissolved or suspended
    • B01J38/64Liquid treating or treating in liquid phase, e.g. dissolved or suspended using alkaline material; using salts
    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02PCLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00Technologies related to metal processing
    • Y02P10/20Recycling

Definitions

  • This invention relates to the removal of metal contamination in the form of complexes and decomposition products thereof from the products of synthetic organic processes and, in particular, to the removal of transition metal catalyst species or residues from homogeneous product mixtures.
  • transition metals In synthetic organic chemistry, especially for the synthesis of biologically-active compounds for pharmaceutical use, transition metals have widespread potential as catalysts for hydrogenation, oxidation and other reactions, including reactions such as olefin metathesis reactions.
  • cross metathesis methodologies have been shown to be highly effective routes in the synthesis of insect pheromones, flavour and fragrance compounds and valuable synthetic intermediates such as novel ⁇ , ⁇ -unsaturated carbonyl systems. Such reactions are recognised as offering a novel and highly efficient way in which olefmic bonds are created and functionalised.
  • reaction products are ultimately intended for ingestion as pharmaceutical compounds, it is important to reduce to a minimum the quantity of any transition metal residues, including spent catalyst and catalyst by-products, present in the reaction product; by way of example, palladium has been used extensively on a research scale in various catalytic reactions, especially as a hydrogenation catalyst, and ruthenium is used in olefin metathesis reactions but such reactions are potentially compromised for use on an industrial scale due to the difficulty of removal of toxic catalytic residues from catalytic reaction mixtures. FDA recommended levels are now routinely less than 10 ppm at least.
  • Methods which have been reported for removal of catalytic residues include the use of scavenger resins; the immobilisation of organometallic catalysts on solid supports including organic polymers and inorganic materials such as alumina, zeolites or silica; micro-encapsulation on, for example, a cross-linked polyurea matrix; the use of chelating reagents and exposure to lead(IV) acetate and chromatography procedures, for example using mixtures of dimethyl sulphoxide and triphenylphosphine.
  • Catalytically-active transition metals typically include metals of the platinum group, which are expensive. There is therefore an economic incentive for providing for their residues to be removed from reaction solutions for recovery and re-use purposes.
  • transition metal residues can be removed from catalytic reaction mixtures, remaining therein in very low levels, by a work-up process which comprises reacting the product solution with an oxidising agent and filtration of the reaction solution, with, in some examples, autocatalytic removal of the oxidising agent.
  • the present invention provides a process for removal of transition metal catalyst species from solutions of products obtained from a synthetic organic process, the method comprising:
  • Oxide products of transition metals in the context of the present invention include water- soluble ionic species or hydroxides that precipitate from solution and are thus capable of being removed for example by filtration or other process, for example centrifugation.
  • the water-immiscible solvent comprises such solvents as petrols, ethers such as diethyl ether or t-butyl methyl ether, benzene, toluene, ethyl acetate and/or chlorinated hydrocarbons such as dichloromethane or chloroform and the water- soluble reducing agent may comprise sodium sulfite or a similar reducing agent such as sodium thiosulphate.
  • the formed transition metal oxide can catalyse the decomposition of the oxidising agent spontaneously.
  • the transition metal comprises one or more metals selected from Groups 8, 9, 10 and 11 of the Periodic Table, especially including Ru, Pd, Pt, Rh, Ir, Ni and Cu. It has been found that oxidic products of these metals act as redox catalysts for reduction of the oxidising agent and are susceptible of removal from the reaction solution by filtration through, for example, a plug or pad of silica gel.
  • Oxidising agents for use in the process of the invention preferably comprise peroxy compounds such as hydrogen peroxide and percarbonate, perborate and peroxysuphate compounds, for example as their sodium salts.
  • the oxidising agent is preferably added as an aqueous solution to the organic reaction medium and stirred or otherwise agitated to effect reaction contact with the catalyst residue.
  • Hydrogen peroxide is generally preferred for economic reasons and because its by-products are water and oxygen, posing no environmental or toxicity problems in disposal.
  • Nitrogen or other inert gas may be added to the reaction vessel or other elements of the apparatus to minimise any risk of explosion. However, where solid oxidising agents are used, these may be added direct to the organic reaction solution.
  • phase transfer catalysts may be beneficial in either a liquid: liquid or liquid: solid system, in increasing the rate of oxidation of catalyst residue and reducing down time.
  • the process preferably includes the addition to the reaction solution, after the oxidising agent has oxidised the transition metal species, of a solution of a quenching agent to prevent the oxidising agent from oxidising the intended reaction product.
  • the quenching agent is also added as an aqueous solution.
  • the quenching agent preferably comprises a neutral inorganic salt which is inert to oxidising and reducing agents; sodium sulphite is an example of a quenching agent suitable for use with a hydrogen peroxide oxidising agent.
  • Catalyst residues removed from reaction solutions by a process according to the invention may be recoverable for re-use.
  • a final feature of the peroxide method is that various other transition metal catalyst species such as phosphine ligands, so often a constituent part of this type of catalyst, are also rapidly oxidised to the corresponding phosphine oxides. These are much more polar than the initial trivalent phosphines and are also removed, usually during the final filtration through silica gel.
  • transition metal catalyst species such as phosphine ligands, so often a constituent part of this type of catalyst, are also rapidly oxidised to the corresponding phosphine oxides. These are much more polar than the initial trivalent phosphines and are also removed, usually during the final filtration through silica gel.

Abstract

A process for removal of transition metal catalyst species from solutions of products obtained from a synthetic organic process, comprises treating a solution of the contaminated product, dissolved in a water-immiscible solvent, with an oxidising agent in which the transition metal catalyst species is rapidly oxidised, resulting in transition metal oxide products; washing the treated solution with an aqueous water-soluble reducing agent, in order to remove excess oxidant, and treating the resulting suspension to remove the transition metal oxides and allow for their recovery. Preferably, the transition metal comprises one or more metals selected from Groups 8, 9, 10 and 11 of the Periodic Table, especially including Ru, Pd, Pt, Rh, Ir, Ni and Cu. Oxidic products of these metals act as redox catalysts for reduction of the oxidising agent and are susceptible of removal from the reaction solution by filtration through, for example, a plug or pad of silica gel.

Description

Removal of Transition Metal Contamination
This invention relates to the removal of metal contamination in the form of complexes and decomposition products thereof from the products of synthetic organic processes and, in particular, to the removal of transition metal catalyst species or residues from homogeneous product mixtures.
In synthetic organic chemistry, especially for the synthesis of biologically-active compounds for pharmaceutical use, transition metals have widespread potential as catalysts for hydrogenation, oxidation and other reactions, including reactions such as olefin metathesis reactions. For example, cross metathesis methodologies have been shown to be highly effective routes in the synthesis of insect pheromones, flavour and fragrance compounds and valuable synthetic intermediates such as novel α, β-unsaturated carbonyl systems. Such reactions are recognised as offering a novel and highly efficient way in which olefmic bonds are created and functionalised. However, especially where the reaction products are ultimately intended for ingestion as pharmaceutical compounds, it is important to reduce to a minimum the quantity of any transition metal residues, including spent catalyst and catalyst by-products, present in the reaction product; by way of example, palladium has been used extensively on a research scale in various catalytic reactions, especially as a hydrogenation catalyst, and ruthenium is used in olefin metathesis reactions but such reactions are potentially compromised for use on an industrial scale due to the difficulty of removal of toxic catalytic residues from catalytic reaction mixtures. FDA recommended levels are now routinely less than 10 ppm at least. Methods which have been reported for removal of catalytic residues include the use of scavenger resins; the immobilisation of organometallic catalysts on solid supports including organic polymers and inorganic materials such as alumina, zeolites or silica; micro-encapsulation on, for example, a cross-linked polyurea matrix; the use of chelating reagents and exposure to lead(IV) acetate and chromatography procedures, for example using mixtures of dimethyl sulphoxide and triphenylphosphine. Again, however, although such procedures can be successful on a laboratory scale, they are not amenable for use on an industrial scale for various reasons such as the use of large quantities of toxic chelating reagents or merely high expense. Catalytically-active transition metals typically include metals of the platinum group, which are expensive. There is therefore an economic incentive for providing for their residues to be removed from reaction solutions for recovery and re-use purposes.
We have now found that transition metal residues can be removed from catalytic reaction mixtures, remaining therein in very low levels, by a work-up process which comprises reacting the product solution with an oxidising agent and filtration of the reaction solution, with, in some examples, autocatalytic removal of the oxidising agent.
In one aspect, the present invention provides a process for removal of transition metal catalyst species from solutions of products obtained from a synthetic organic process, the method comprising:
treating a solution of the contaminated product, dissolved in a water-immiscible solvent, with an oxidising agent in which the transition metal catalyst species is rapidly oxidised, resulting in transition metal oxide products;
washing the treated solution with an aqueous water-soluble reducing agent, in order to remove excess oxidant, and
treating the resulting suspension to remove the transition metal oxides and allow for their recovery.
Oxide products of transition metals in the context of the present invention include water- soluble ionic species or hydroxides that precipitate from solution and are thus capable of being removed for example by filtration or other process, for example centrifugation.
In the method of the invention, the water-immiscible solvent comprises such solvents as petrols, ethers such as diethyl ether or t-butyl methyl ether, benzene, toluene, ethyl acetate and/or chlorinated hydrocarbons such as dichloromethane or chloroform and the water- soluble reducing agent may comprise sodium sulfite or a similar reducing agent such as sodium thiosulphate. In some cases, the formed transition metal oxide can catalyse the decomposition of the oxidising agent spontaneously. Preferably, the transition metal comprises one or more metals selected from Groups 8, 9, 10 and 11 of the Periodic Table, especially including Ru, Pd, Pt, Rh, Ir, Ni and Cu. It has been found that oxidic products of these metals act as redox catalysts for reduction of the oxidising agent and are susceptible of removal from the reaction solution by filtration through, for example, a plug or pad of silica gel.
Oxidising agents for use in the process of the invention preferably comprise peroxy compounds such as hydrogen peroxide and percarbonate, perborate and peroxysuphate compounds, for example as their sodium salts. The oxidising agent is preferably added as an aqueous solution to the organic reaction medium and stirred or otherwise agitated to effect reaction contact with the catalyst residue. Hydrogen peroxide is generally preferred for economic reasons and because its by-products are water and oxygen, posing no environmental or toxicity problems in disposal. Nitrogen or other inert gas may be added to the reaction vessel or other elements of the apparatus to minimise any risk of explosion. However, where solid oxidising agents are used, these may be added direct to the organic reaction solution. The use of phase transfer catalysts may be beneficial in either a liquid: liquid or liquid: solid system, in increasing the rate of oxidation of catalyst residue and reducing down time.
The process preferably includes the addition to the reaction solution, after the oxidising agent has oxidised the transition metal species, of a solution of a quenching agent to prevent the oxidising agent from oxidising the intended reaction product. Preferably, the quenching agent is also added as an aqueous solution. The quenching agent preferably comprises a neutral inorganic salt which is inert to oxidising and reducing agents; sodium sulphite is an example of a quenching agent suitable for use with a hydrogen peroxide oxidising agent.
Catalyst residues removed from reaction solutions by a process according to the invention may be recoverable for re-use.
Embodiments of the invention will now be described by way of example, in which reaction solutions from an olefin metathesis reaction using a ruthenium-based complex (Grubbs catalyst) as catalyst were subjected to oxidation in air (as a control experiment) and using a hydrogen peroxide solution (see Samples 1 and 2 in Table 1 below). Samples 3, 4 and 5 relate to further experiments on ruthenium olefin metathesis reaction solutions at different catalyst loadings, Sample 3 being a low-loading and Samples 4 and 5 being a high-loading solution. Sample 4 was following one treatment with peroxide solution; Sample 5 concerned a second treatment.
In all experiments, a dichloromethane solution of the metathesis product was washed with a 15% solution of hydrogen peroxide, quenched of any excess peroxides from the organic phase by addition of an aqueous solution of sodium sulphite and the product filtered through silica gel. Visual inspection of the products indicated removal of all coloured by-products and no indication of oxidation of the reaction product was observed by 1H NMR analysis, the product being an olefin which, potentially, is reactive towards peroxides. For analysis purposes, the product was digested with aqua regia and analysed by ICPMS (Inductively Coupled Mass Spectrometry).
As can be seen, a straight comparison of air oxidation with peroxide on the same reaction sample produced an improvement, in terms of residual ruthenium present in the reaction solution, of a factor of approximately 100. In Samples 3 to 5, the product of Sample 3 had ruthenium below those recommended for oral doses for pharmaceutical products according to the EMA regulations; a similar result was obtained for the high-loading Sample 5 after the second treatment with peroxide.
Table 1
Figure imgf000005_0001
In carrying out the experiments, it was observed that the aqueous solution of peroxide effervesced after the work-up procedure, indicating peroxide decomposition by autocatalytic quenching by ruthenium oxide products. The invention is preferably only for the treatment of contaminated organic solutions of products and may not be suitable for the purification of polymeric materials.
Further preliminary experiments using a representative range of transition metal complexes have shown the method to be equally effective for the removal of contamination by palladium, platinum, iridium, nickel, copper, silver, rhodium and rhenium derived from catalytic species from a range of organic products. Levels as low as those achieved in the ruthenium removal experiments have been reached.
A final feature of the peroxide method is that various other transition metal catalyst species such as phosphine ligands, so often a constituent part of this type of catalyst, are also rapidly oxidised to the corresponding phosphine oxides. These are much more polar than the initial trivalent phosphines and are also removed, usually during the final filtration through silica gel.

Claims

Claims
1. A process for removal of transition metal catalyst species from solutions of products obtained from a synthetic organic process, the method comprising:
treating a solution of the contaminated product, dissolved in a water- immiscible solvent, with an oxidising agent in which the transition metal catalyst species is rapidly oxidised, resulting in transition metal oxide products;
washing the treated solution with an aqueous water-soluble reducing agent, in order to remove excess oxidant, and
treating the resulting suspension to remove the transition metal oxides and allow for their recovery.
2. A process according to claim 1, in which the water-immiscible solvent comprises petrols, ethers, benzene, toluene, ethyl acetate and/or a chlorinated hydrocarbon.
3. A process according to claim 1 or claim 2, in which the water-soluble reducing agent comprises sodium sulfite or sodium thiosulphate.
4. A process according to any preceding claim, in which the formed transition metal oxide catalyses the decomposition of the oxidising agent spontaneously.
5. A process according to any preceding claim, in which the transition metal comprises one or more metals selected from Groups 8, 9, 10 and 11 of the Periodic Table.
6. A process according to any preceding claim, in which the oxidising agent comprises peroxy, perborate or peroxysuphate compounds.
7. A process according to any preceding claim including the use of a phase transfer catalyst.
8. A process according to any preceding claim, in which a solution of a quenching agent is added to the reaction solution after the oxidation step.
9. A process according to claim 8, in which the quenching agent is added as an aqueous solution.
10. A process according to claim 8 or claim 9, in which sodium sulphite is added as the quenching agent and hydrogen peroxide is used as the oxidising agent.
11. A process according to any preceding claim, in which the transition metal catalyst species includes phosphine ligands.
12. Transition metal catalyst species when removed from a synthetic reaction solution and recovered by a process according to any of claims 1 to 11.
PCT/GB2007/000458 2006-02-09 2007-02-09 Removal of transition metal contamination WO2007091085A1 (en)

Applications Claiming Priority (2)

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GB0602663A GB0602663D0 (en) 2006-02-09 2006-02-09 Removal of catalyst species
GB0602663.7 2006-02-09

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4945075A (en) * 1989-12-04 1990-07-31 Eastman Kodak Company Oxidative secondary rhodium recovery process
WO1991007372A1 (en) * 1989-11-20 1991-05-30 Eastman Kodak Company Process for the recovery of catalyst values
US6608027B1 (en) * 1999-04-06 2003-08-19 Boehringer Ingelheim (Canada) Ltd Macrocyclic peptides active against the hepatitis C virus
WO2005056182A1 (en) * 2003-12-08 2005-06-23 Boehringer Ingelheim International, Gmbh Removal of ruthenium by-product by supercritical fluid processing

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1991007372A1 (en) * 1989-11-20 1991-05-30 Eastman Kodak Company Process for the recovery of catalyst values
US4945075A (en) * 1989-12-04 1990-07-31 Eastman Kodak Company Oxidative secondary rhodium recovery process
US6608027B1 (en) * 1999-04-06 2003-08-19 Boehringer Ingelheim (Canada) Ltd Macrocyclic peptides active against the hepatitis C virus
WO2005056182A1 (en) * 2003-12-08 2005-06-23 Boehringer Ingelheim International, Gmbh Removal of ruthenium by-product by supercritical fluid processing

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
AHN Y M ET AL: "A convenient method for the efficient removal of ruthenium byproducts generated during olefin metathesis reactions", ORGANIC LETTERS, ACS, WASHINGTON, DC, US, vol. 3, no. 9, 3 May 2001 (2001-05-03), pages 1411 - 1413, XP002278989, ISSN: 1523-7060 *
ASTRUC DIDIER: "The metathesis reactions: from a historical perspective to recent developments", NOUVEAU JOURNAL DE CHIMIE - NEW JOURNAL OF CHEMISTRY, CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIC, PARIS, FR, vol. 29, no. 1, 21 December 2004 (2004-12-21), pages 42 - 56, XP002402438, ISSN: 0398-9836 *
PAQUETTE L A ET AL: "A convenient method for removing all highly-coloured byproducts generated during olefin metathesis reactions", ORGANIC LETTERS, ACS, WASHINGTON, DC, US, vol. 2, no. 9, 4 May 2000 (2000-05-04), pages 1259 - 1261, XP002278990, ISSN: 1523-7060 *

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