Heterogeneous catalysis (C9981) 1.Basic principles, thermodynamics, and kinetics 2.Active sites and catalyst evaluation Definition •Catalyst is… –A substance that speeds up a chemical reaction –Without being consumed or changed Výsledek obrázku pro beaker Výsledek obrázku pro beaker Výsledek obrázku pro beaker Výsledek obrázku pro beaker + CATA + CATA Výsledek obrázku pro snail Výsledek obrázku pro fastest animal in the world •We get our products –In shorter time; at lower rxn temperature; at lower pressure –cheaper; economical; ecological Types of catalyst (based on phase) •Homogeneous •Heterogeneous •Enzymatic Výsledek obrázku pro beaker + CATA Enzymatic catalysis •Cytochrome P450 Související obrázek Catalytic cycle Enzymatic catalysis •Cytochrome P450 Výsledek obrázku pro cytochrome p450 Výsledek obrázku pro cytochrome p450 active site Active site Homogeneous catalysis •Gas phase • 1/ Cl + O3 → ClO3 ClO3 → ClO + O2 ClO + O → Cl + O2 Total: O3 + O → 2 O2 2/ Isomerization of hydrocarbons with HF (both in gaseous phase) •Liquid phase • 1/ Esterification of acids with alcohols catalyzed by H+ (e.g. H2SO4, H3PW12O40) Výsledek obrázku pro grubbs catalyst 2/ Olefin metathesis over Grubbs catalyst (organometallics dissolved in liquid solvent along with precursors •A catalyst and rxn mixture in one unique phase Heterogeneous catalysis •A catalyst is present in a different, distinct phase in contrary to the rxn mixture •Gas phase rxn mixture over solid catalyst –Olefin metathesis Ethene + Butene Propene material??? Heterogeneous catalysis •A catalyst is present in a different, distinct phase in contrary to the rxn mixture •Gas phase rxn mixture over solid catalyst –Alcohol dehydration Výsledek obrázku pro phosphotungstic acid Methanol Dimethylether H3PW12O40; a molecular compound, not a material! Heterogeneous catalysis •A catalyst is present in a different, distinct phase in contrary to the rxn mixture •Liquid phase rxn mixture over solid catalyst –Alcohol dehydration Výsledek obrázku pro phosphotungstic acid Butanol Dibutylether + Butenes H3PW12O40; insoluble molecular compound Heterogeneous catalysis •A catalyst is present in a different, distinct phase in contrary to the rxn mixture •Liquid phase rxn mixture over solid catalyst –Olefin metathesis Longer olefins Longer olefins Heterogeneous vs. Homogeneous catalysis Výsledek obrázku pro grubbs catalyst •Well defined = easier to characterize the active site, describe reactivity, and evaluate catalytic performance • •Ill defined = active site??? = more than one type of active sites??? = number of active sites??? • Heterogeneous vs. Homogeneous catalysis Výsledek obrázku pro grubbs catalyst •One phase with reactants = higher probability that all necessary species meet and reactJ •Distinct phase = diffusion??? = adsorption??? = desorption??? • Heterogeneous vs. Homogeneous catalysis Výsledek obrázku pro grubbs catalyst •Separation??? •Reusability/Price??? •Toxicity??? •Batch rxns only??? •Distinct phase = easy to separate = reusable/recyclable = clean products = continuous processes • Heterogeneous vs. Homogeneous catalysis Výsledek obrázku pro grubbs catalyst Homogeneous and Heterogeneous catalysis are complementary to each other We choose either hetero or homo based on the rxn we want to catalyze (pros and cons) Catalytic cycle •A + B → P Ea >> Ea(cata); k = A∙e–Ea/RT ΔG = ΔG(cata); ΔG = –RT∙lnK Thermodynamics vs. kinetics •Addition of catalyst into the rxn mixture –does change „k“ –does not change „K“ A → P, speeded up by addition of a catalyst K = 1; ΔG = 0 kJ mol–1; final rxn mixture: 1A + 1P The catalyst is then equally efficient in catalysing the opposite process P → A; final rxn mixture: 1A + 1P Thermodynamics vs. kinetics •If a catalyst is efficient in dehydration –then it can be possibly used in hydration •If a catalyst is efficient in hydrogenation –then it can be possibly used in dehydrogenation I.e.: It depends on ΔG of studied reaction, whether the catalyst hydrogenates or dehydrogenates. We know how to shift equilibrium of chemical reactionJ (#GenChem). This is not a job of the catalyst. Thermodynamics vs. kinetics •Reaction coordinate: A multidimensional problem Thermodynamics vs. kinetics •Reaction coordinate: A multidimensional problem –e.g. selective oxidation reactions (Table 1) •We are not shooting for the most stable product (thermodynamically); Difficult! Mountain cottage Spa resort Thermodynamics vs. kinetics •Reaction coordinate: A multidimensional problem –How is it possible that we sometimes end up in a mountain cottage and not in the spa resort??? –We need a catalyst! –The catalyst will lower the activation energy of our way to the mountain cottage (beautiful views, crazy friends,…) –The catalyst will increase the activation energy of our way to the spa resort –E.g. The catalyst has to increase the rate of hiking (selective oxidation) and decrease the rate of hiking (total oxidation). What a difficult job! Thermodynamics vs. kinetics •Reaction coordinate: A multidimensional problem Thermodynamics vs. kinetics •Reaction coordinate: From multidimensional to two-dimensional problem Thermodynamics vs. kinetics •Transitions state vs. Intermediate Thermodynamics – heterogeneous catalysis •A + B → P 1 1: Adsorption of reactants on catalyst surface (hetero) / Formation of reaction intermediate ABC (enzymes and homo) In heterogeneous catalysis always thermodynamically favorable (coordinatively unsaturated surface + adsorbate) Let‘s imagine weak interaction of A, B, and catalyst surface…??? Let‘s imagine very strong interaction of catalyst surface, A, and/or B…?? 2 3 1 Thermodynamics •A + B → P 1 2 3 3: Desorption of products from catalyst surface (hetero) / Rupture of product intermediate PC (enzymes and homo) Let‘s imagine very strong interaction of P with catalyst surface…??? Thermodynamics •Interactions of A,B, and P with catalyst not too weak, not too strong (= physi/chemisorption on catalyst surface) Sabatier‘s principle; Volcano plot sabatier_principle Thermodynamics vs. kinetics •A + B → P 1 2 3 Before going any further… Steps 1 and 3 are equally important to step 2 in catalysis . Adsorption and desorption of A, B, and P might have a significant effect on Ea. In kinetic studies in heterogeneous catalysis we estimate only apparent activation energy (Eapp), where all these contributions are included. Thermodynamics vs. kinetics •Importance of reactant adsorption and product desorption –Example: Ethylbenzene dehydrogenation to styrene The best catalyst Thermodynamics vs. kinetics •A + B → P 1 2 3 What is preceding step 1 and following step 3 ? Diffusion! Diffusion of A, B, and P might have a significant effect on Ea as well. In kinetic studies in heterogeneous catalysis we estimate only apparent activation energy (Eapp), where all these contributions are included. Thermodynamics vs. kinetics •Another representation of the complexity of heterogeneous catalytic reaction Adsorption, desorption, and diffusion of A, B, and P might have a significant effect on Ea. Thermodynamics vs. kinetics •Another representation of the complexity of heterogeneous catalytic reaction Adsorption, desorption, and diffusion of A, B, and P might have a significant effect on Ea. Kinetics •We know kineticsJ (#ChemKin) •Rxns of zero order, first order, and second order • • • • • •But in catalysis also 0.39 order, –0.5 order… (many elementary rxns, fight for active site between reactants and products,…!) Výsledek obrázku pro zero order kinetics Výsledek obrázku pro first order kinetics Thermodynamics vs. kinetics •A + B → P 1 2 3 2: In order to decrease Ea for step 2 we need to design an active site of the catalyst as good as possible What is the active site??? How do we measure the „goodness“ of the active site??? Active sites Výsledek obrázku pro grubbs catalyst Výsledek obrázku pro cytochrome p450 Výsledek obrázku pro cytochrome p450 active site Active sites •Active sites are often dynamic: They has to be created first! Active site/catalyst evaluation •Turn-over frequency (TOF; [s–1]) = number of catalytic cycles performed by 1 active site per time unit •Precise numbers for homogeneous and enzymatic catalysis • Numbers for heterogeneous catalysis??? TOF [s–1] Hetero: ~1–100 s–1 Homo: ~10–1000 s–1 Enzymes: ~10000–1000000 s–1 Active site/catalyst evaluation •Turn-over number (TON; [-]) = number of catalytic cycles performed by 1 active site before deactivation (~lifetime) •Precise numbers for homogeneous and enzymatic catalysis • Numbers for heterogeneous catalysis??? Active site/catalyst evaluation •Selectivity is ability of catalyst to form one product from a pool of products (possibly many) • • • • • •Selectivity (S; [%]) = number of D molecules produced / R molecules converted reactant R Active site/catalyst evaluation •Enantiomeric enrichment is ability of catalyst to form selectively one enantiomer •Enantiomeric enrichment (ee; [%]) = S(PR) – S(PS) (e.g. Final mixture consists of 90 % PR and 10 % PS ee = 80 %) S [%] ee [%] Hetero: 1–100 1–100 Homo: > 90 > 70 Enzymes: > 99.99 > 99.99 Active site/catalyst evaluation •Selectivity is ability of catalyst to form one product from a pool of products (possibly many) •Specificity is ability of catalyst to react with one reactant among many (similar) reactants Active sites Výsledek obrázku pro grubbs catalyst Výsledek obrázku pro cytochrome p450 Výsledek obrázku pro cytochrome p450 active site Electronic and steric properties do play an important role! (TOF, TON, Selectivity, ee, specificity). Catalyst evaluation •Conversion (C; [%]) = number of R molecules converted / number of R molecules introduced •Yield (Y; [%]) = number of P molecules produced / number of R molecules introduced •Selectivity (S; [%]) = number of P molecules converted / number of R molecules converted • R → P + P‘ Catalyst evaluation •Productivity (STandard Yield, STY; [g g–1 s–1]) = amount of product P produced per unit of time and unit of catalyst mass = high conversion, high selectivity, low catalyst mass, high flow = industrial viewpoint • R → P + P‘ Catalyst evaluation •Homework Catalyst evaluation •Ecological point of view •Atom efficiency [%] = mass of desired product / total mass of all products •E factor [-] = mass of all wastes (solvents, gases, unreacted precursors, byproducts) / mass of desired products •EQ factor [-] = E factor, mass of each waste multiplied by Q (environmental unfriendliness), e.g. Q = ~0 for water, ~ 1 for NaCl, and ~1000 for toluene… • Catalyst evaluation •Ecological point of view •Atom efficiency [%], E factor [-], EQ factor [-] Gas phase, flow mode Liquid phase, toluene as solvent, batch mode