Interfacial Catalysis PDF 2024-2025

Molecular catalysis Interfacial catalysis N. de Viguerie 2024-2025 1 reactions at the liquid-liquid interface: The interface between hydrophilic and hydrophobic liquids can be used to combine immiscible reaction partners or to protect sensitive partners,...

Molecular catalysis Interfacial catalysis N. de Viguerie 2024-2025 1 reactions at the liquid-liquid interface: The interface between hydrophilic and hydrophobic liquids can be used to combine immiscible reaction partners or to protect sensitive partners, f hydrolysis for example. Nano/micro-compartments: Phase-transfer catalysis emulsions micelles microemulsions > Interfacial catalysis 2 Interfacial Catalysis A. Introduction 1) Definition of the interface Small, poorly defined thickness surface through which all the exchanges of matter and energy between the two phases occur. The interface is in a particular state that can be called a phase boundary. 2) Interface types: Solid-solid Solid-liquid Solid-gaz Liquid-liquid Liquid-gas 3 3) Reactivity at interface The reaction conditions of a molecule adsorbed at an interface are very different from those which it undergoes in each phase. In the interfacial environment: - Higher concentrations  Oil interface  effect on rates water - Specific permittivity [c]  solvent effect - Well-defined molecular orientations  selectivity effect The molecular organization thus obtained will lead to reactions of the same type as those observed in biological systems (for example, in cell membranes). 4  Schematic representation of reactions at the liquid–liquid interface. Published in: Keti Piradashvili; Evandro M. Alexandrino; Frederik R. Wurm; Katharina Landfester; Chem. Rev. 2016, 116, 2141-2169. DOI: 10.1021/acs.chemrev.5b00567 5 Copyright © 2015 American Chemical Society 4) Advantages Easy to conduct reactions under milder/safer reaction conditions Increased rates of very slow reactions High yields and purity of products High reactivity and selectivity Simplicity of the procedure Frequently simplified separation of products (The extraction has already begun! Highly scalable Low energy consumption and low investment cost Minimization of industrial waste  Concept similar to heterogeneous catalysis 6 Examples of application Ruhrchemie/Rhône–Poulenc hydroformylation of propene (the oxo process) Producing over 450 000 tons/year of aldehydes A general schematic process for aqueous biphasic catalysis: Water-soluble phosphines overall yield >99% Catalytic cycle n/iso aldehyde ratio =96 : 4.  high activity and selectivity with easy separation by decantation 7 5) Concepts for interfacial catalysis Two limits exploited in catalysis at liquid-liquid interfaces: - Macroscopically heterogeneous media  phase-transfer catalysis  Pickering emulsions - Microscopically heterogeneous media: lyotropic phases  micellar catalysis (micelles, vesicles, microemulsions) 8 5) Concepts for interfacial catalysis Two limits exploited in catalysis at liquid-liquid interfaces:  Macroscopically heterogeneous media  Microscopically heterogeneous media: lyotropic phases  phase-transfer catalysis  micellar catalysis (micelles, vesicles, microemulsions)  Pickering emulsions PHASE-TRANSFER CATALYSIS crown ethers MICROEMULSION CATALYSIS onion salts EMULSION CATALYSIS PICKERING-ASSISTED CATALYSIS MICELLAR CATALYSIS - Kinetic effects Nano/micro-compartments (one of the phases is water) - Influence on reactional mechanism - Orientation effect 5) Concepts for interfacial catalysis Two limits exploited in catalysis at liquid-liquid interfaces:  Macroscopically heterogeneous media  Microscopically heterogeneous media: lyotropic phases  phase-transfer catalysis  micellar catalysis (micelles, vesicles, microemulsions)  Pickering emulsions PHASE-TRANSFER CATALYSIS crown ethers MICROEMULSION CATALYSIS onion salts EMULSION CATALYSIS PICKERING-ASSISTED CATALYSIS MICELLAR CATALYSIS - Kinetic effects Nano/micro-compartments (one of the phases is water) - Influence on reactional mechanism - Orientation effect - Macroscopically heterogeneous media - Microscopically heterogeneous media System Concept Surface-active or phase- transfer agent Biphasic (demixed Phase-transfer agent catalysis Onion salts, crown ethers system) PART 1 Microdispersed Emulsion Surfactant- Surfactants or polymers (0,5-200 µm) catalysis assisted catalysis PART 2 Pickering- Catalytic amphihilic assisted nanoparticles catalysis Nanodispersed Micellar catalysis Surfactants (5-100 nm) PART 3 Microemulsion catalysis Surfactants/co- surfactants 11  PART 1 Phase-transfer catalysis 12 B. Phase-Transfer Catalysis (PTC) 1) Definition Consider the following reaction: reflux 1-2 days No reaction Insoluble in water Soluble in water Soluble in organic phase Insoluble in organic phase Near 100% yield in 2-3h Bimolecular reactions in which both reactant molecules cannot solubilize in the same phase. The phase-transfer catalyst (R4N+) facilitates the migration of a reactant (CN-) in a heterogeneous system from one phase (water) into another phase (organic) where reaction can take place. “A molecular shuttle bus". Phase-transfer catalysis or PTC refers to the acceleration of the reaction by the phase-transfer catalyst. 13 2) The Mechanisms of PTC R-Cl (R4N+,CN-) R-CN (R4N+,Cl-) Organic phase Complexant Q+ (org) Anion X- Interface M+ metallic cation Aqueous phase (aq) Na+Cl- R4N+CN- Na+CN- R4N+Cl- The reaction occurs in at least two steps: - Step 1: The intrinsic reaction - Step 2: The transfer step Two mechanisms: - Extraction mechanism (rate determining step: step 1) - Interfacial mechanism (rate determining step: step 2) 14 2) The Mechanisms of PTC a) Extraction mechanism Initial state: MY and QX in water + AX in organic phase : AX organic reactant Y - nucleophilic reactant M+ metallic cation Organic phase Q+ phase transfer catalyst (org) (ammonium, phosphonium, ether Interface crown…) AY organic product Aqueous phase X- Anion (aq) - Q+ extracts Y- from aqueous phase to transfer into organic phase  ion pair (Q+, Y-) - The reaction occurs in organic phase -Ion pair (Q+, X-) extracts X- from organic phase to transfer into aqueous phase  Stark Mechanism 15 2) The Mechanisms of PTC a) Extraction mechanism  Selective extraction of ion pairs Organic phase (org) Interface Aqueous phase (aq) There are 2 transfers: : Extraction constants of the two ion pairs: K: efficiency 16 2) The Mechanisms of PTC a) Extraction mechanism  extraction of ion pairs Factors favoring the transfer step: - Salting-out effect (saline aqueous phase) - Nature of Q+ o Lipophilicity o Relative size of Q+ and Y-: similar sized favors extraction - Nature of X- : hydrophilicity - Nature of Y-: large weakly-hydrated or large organic anions such as perchlorate, iodide and phenolate are easily transferred - Difference of solvatation energy of X- and Y- in water and organic solvent -Interfacial area  tiny droplets (highly apolar organic solvents, stirring, presence of surfactants) 17 2) The Mechanisms of PTC a) Extraction mechanism  Factors governing the rate of intrinsic step - Reactivity of Y- Ionic radii (Q+,Y-) The difference in ionic radii can be translated into ionic interaction energies by simple Coulombic calculations. tight ion pair loose ion pair 18 2) The Mechanisms of PTC a) Extraction mechanism  Factors governing the rate of intrinsic step - Reactivity of Y- Cation size and coulombic interaction energies of bromide salts As the cationic radius of the quaternary salt increases, the activating effect becomes larger. 19 2) The Mechanisms of PTC a) Extraction mechanism  Factors governing the rate of intrinsic step - Solvent Anion in a protic solvent  hydrogen-bonding anion solvation  dissociation of salts in free ions Anion in a aprotic solvent  anion not solvated  binds to cation  neutralization of charge : - aprotic and polar solvent  more solvated cation (complexation of cation)  free anion  Y- more reactive - aprotic and apolar solvent  less solvated cation ions pairs  more soluble in organic phase - Temperature  can greatly influence the intrinsic reaction step (most quaternary ammonium salts decompose at higher temperature) 20 2) The Mechanisms of PTC b) Interfacial Mechanism Example: alkylation of weakly acid organic compounds H+ is extracted at the interface: M+aq + -OHaq + HAorg (M+ A-)int + H2O (step 1) The role of catalyst Q+X- is double: -To stabilize of A- by Q+ - To detach A- from interface: (M+ A-)int + (Q+ X-)org (Q+ A-)org + M+aq + X-aq (Step 2) The alkylation takes place in organic phase : (Q+ A-)org + RXorg RAorg + (Q+ X-)org Q+X- +RX Organic phase Q+X- (org) H-A I A- II Q+A- III R-A + Q+X- Interface Aqueous phase HO- M+ M+ M+X- (aq) Q+X- : 2wt% 3) Typical phase-transfer catalysts Onium salts: Ammonium salts and phosphonium salts R R R N R R P R X R R Br X = Br, F, OH, HSO 4.. -Alkyl chain length can be easily modulated -Ability to introduce functional group (chiral for ex) Crown ether + KY - ability to complex cation (PTC solid/liquid) Cryptate 22 4) Applications of PTC  PTC is particularly useful for reactions of organic anions with nonpolar organic reactants PTC is also applicable for reactions in which anions are intermediates for generating other active species such as carbenes, nitrenes and organometallic reagents 1. Alkylations C-alkylations: C- from C-H pKa>22-23  concentrated NaOH  interfacial mechanism 2. Substitutions 3. Aldol and related condensations 4. Carbene reactions 5. Oxidations and reductions 6. Organometallic transformations 23 a) Sustainable Oxidations Oxidants: H2O2 and NaOCl: oxidation under mild reaction conditions without metallic waste O2: ideal oxidant but poor selectivity due to competition combustion KMnO4: stable, water-soluble and good oxidant KHSO5 : stable, water-soluble and environmentally safe oxidant. D. C. M. Albanese; F. Foschi; M. Penso; Org. Process Res. Dev. 2016, 20, 129-139. 24  Hydrogen Peroxide: Cyclohexene Route to Adipic Acid Adipic acid 3.5 million tons manufactured mainly for nylon-6.6 production (Manufactured into: Carpet fiber, Airbags, Apparel, Tires, Ropes,…) Classical commercial process relies on a hazardous cyclohexane air oxidation providing a mixture of cyclohexanol and cyclohexanone, followed by nitric acid oxidation that generates large amounts of the greenhouse gas N2O as byproduct. 25 Ryoji Noyori , Masao Aoki and Kazuhiko Sato, Chem. Commun., 2003, 1977-1986 Hydrogen Peroxide: Cyclohexene Route to Adipic Acid Yield 90% R4N+HSO4– : Phase-Transfer Catalyst H2O2 : H2O as the only byproduct, high tunability of the reaction parameters, relatively cheap Na2WO4: Co-catalyst  accelerated the oxidation process - Without organic solvent - Catalytic oxidation - Higher yields K Sato, M. Aoki, R. Noyori, Science 1998, 281, 1646-1647 26 Role of Na2WO4 Catalytic cycle of alcohol oxidation. Q+ = CH3(n-C8H17)3N+ Na2WO4 + 2H2O2 Na2[WO(O2)2(OH)2] +H2O. Co catalyst precursor bisperoxotungstate A pH>4: A dominant (feebly active species) 0.4