Introduction to Catalysis

What is a “Catalyst” A catalyst (Greek: καταλύτης, catalytēs) is a substance that accelerates the rate of a chemical reaction without itself being transformed or consumed by the reaction. (thank you Wikipedia) k(T) = k0e-Ea/RT Ea′ < Ea k0′ > k0 k′ > k ΔG = ΔG Ea Ea′ A + B A + B + catalyst ΔG ΔG C C + catalyst uncatalyzed catalyzed

Catalysts Open Up New Reaction Pathways ‡ H O H2C C O CH3 OH C C CH3 CH3 CH2 CH3 ‡ propanone propenol propenol propanone

Catalysts Open Up New Reaction Pathways + H2O C CH2 CH3 OH− −OH− Base catalyzed O OH rate = k[OH−][acetone] C C CH3 CH3 CH2 CH3 propanone propenol ‡ ‡ propenol intermediate propanone

Catalysts Open Up New Reaction Pathways ‡ ‡ propenol different intermediate propanone propenol O OH propanone rate = k[H3O+][acetone] C C Acid catalyzed CH3 CH3 CH2 CH3 H3O+ −H3O+ OH C + CH3 CH3 + H2O

Types of Catalysts - Enzymes The “Gold Standard” of catalysts Highly specific Highly selective Highly efficient Catalyze very difficult reactions N2  NH3 CO2 + H2O  C6H12O6 Works better in a cell than in a 100000 l reactor Triosephosphateisomerase “TIM” Cytochrome C Oxidase Highly tailored “active sites” Often contain metal atoms

Types of Catalysts – Organometallic Complexes Perhaps closest man has come to mimicking nature’s success 2005 Noble Prize in Chemistry Well-defined, metal-based active sites Selective, efficient manipulation of organic functional groups Various forms, especially for polymerization catalysis Difficult to generalize beyond organic transformations Polymerization: Termination:

Types of Catalysts – Homogeneous vs. Heterogeneous Zeolite catalyst Catalyst powders Homogeneous catalysis Single phase (Typically liquid) Low temperature Separations are tricky Heterogeneous catalysis Multiphase (Mostly solid-liquid and solid-gas) High temperature Design and optimization tricky

Types of Catalysts: Crystalline Microporous Catalysts Regular crystalline structure Porous on the scale of molecular dimensions 10 – 100 Å Up to 1000’s m2/g surface area Catalysis through shape selection acidity/basicity incorporation of metal particles 10 Å 100 Å Zeolite (silica-aluminate) MCM-41 (mesoporous silica) Silico-titanate

Types of Catalysts: Amorphous Heterogeneous Catalysts Amorphous, high surface area supports Alumina, silica, activated carbon, … Up to 100’s of m2/g of surface area Impregnated with catalytic transition metals Pt, Pd, Ni, Fe, Ru, Cu, Ru, … Typically pelletized or on monoliths Cheap, high stability, catalyze many types of reactions Most used, least well understood of all classes SEM micrographs of alumina and Pt/alumina

Important Heterogeneous Catalytic Processes Haber-Bosch process N2 + 3 H2 → 2 NH3 Fe/Ru catalysts, high pressure and temperature Critical for fertilizer and nitric acid production Fischer-Tropsch chemistry n CO + 2n H2 → (CH2)n + n H2O , syn gas to liquid fuels Fe/Co catalysts Source of fuel for Axis in WWII Fluidized catalytic cracking High MW petroleum → low MW fuels, like gasoline Zeolite catalysts, high temperature combustor In your fuel tank! Automotive three-way catalysis NOx/CO/HC → H2O/CO2/H2O Pt/Rh/Pd supported on ceria/alumina Makes exhaust 99% cleaner

Heterogeneous Catalytic Reactors Design goals rapid and intimate contact between catalyst and reactants ease of separation of products from catalyst Packed Bed (single or multi-tube) Fluidized Bed Slurry Reactor Catalyst Recycle Reactor

Automotive Emissions Control System “Three-way” Catalyst CO  CO2 HC  CO2 + H2O NOx  N2 Monolith reactor Most widely deployed heterogeneous catalyst in the world – you probably own one! Pt, Rh, Pd Alumina, ceria, lanthana, …

Length Scales in Heterogeneous Catalysis Chemical adsorption and reaction Mass transport/diffusion

Characteristics of Heterogeneous Supported Catalysts Surface area: Amount of internal support surface accessible to a fluid Measured by gas adsorption isotherms Loading: Mass of transition metal per mass of support Dispersion: Percent of metal atoms accessible to a fluid M M M support

Rates of Catalytic Reactions Pseudo-homogeneous reaction rate r = moles / volume · time Mass-based rate r′ = moles / masscat · time r′ = r / ρcat Heterogeneous reactions happen at surfaces Area-based rate r′′ = moles / areacat · time r′′ = r′ / SA, SA = area / mass Heterogeneous reactions happen at active sites Active site-based rate Turn-over frequency TOF = moles / site · time TOF = r′′ / ρsite TOF (s−1) Hetero. cats. ~101 Enzymes ~106

Hetrogeneous Catalysis- Milestones in Evolution-1 1814- Kirchhoff-starch to sugar by acid. 1817-Davy-coal gas(Pt,Pd selective but not Cu,Ag,Au,Fe) 1820s –Faraday H2 + O2 H2O(Pt);C2H4 and S 1836- Berzelius coins”Catalysis”; 1860-Deacon’s Process ;2HCl+0.5O2  H2O + Cl2; 1875-Messel.SO2  SO3 (Pt); 1880-Mond.CH4+H2O  CO+3H2(Ni); 1902-Ostwald-2NH3+2.5O2 2NO+3H2O(Pt); 1902-Sabatier.C2H4+H2  C2H6(Ni). 1905-Ipatieff.Clays for acid catalysed reactions; isomerisation, alkylation, polymerisation.

Milestones in Evolution-2 1910-20: NH3 synthesis (Haber,Mittasch) ; Langmuir 1920-30-Methanol syn(ZnO-Cr2O3); Taylor;BET 1930-Lang-Hinsh &Eley -Rideal models ;FTsyn;EO; 1930-50:Process Engg; FCC / alkylates;acid-base catalysis;Reforming and Platforming. 1950-70: Role of diffusion; Zeolites, Shape Selectivity; Bifunctional cata;oxdn cat-HDS; Syngas and H2 generation. 1970- Surface Science approach to catalysis(Ertl) 1990 - Assisted catalyst design using : -surface chem of metals/oxides, coordination chemistry - kinetics,catalytic reaction engg - novel materials(micro/mesoporous materials)

Catalysis in the Chemical Industry Hydrogen Industry(coal,NH3,methanol, FT, hydrogenations/HDT,fuel cell). Natural gas processing (SR,ATR,WGS,POX) Petroleum refining (FCC, HDW,HDT,HCr,REF Petrochemicals(monomers,bulk chemicals). Fine Chem.(pharma, agrochem, fragrance, textile,coating,surfactants,laundry etc) Environmental Catalysis(autoexhaust, deNOx, DOC)

Some Developments in Industrial catalysis-1 1900- 1920s Industrial Process Catalyst 1900s:CO + 3H2  CH4 + H2O Ni Vegetable Oil + H2  butter/margarine Ni 1910s:Coal Liquefaction Ni N2 +3 H2  2NH3 Fe/K NH3 NO NO2 HNO3 Pt 1920s: CO +2 H2  CH3OH (HP) (ZnCr)oxide Fischer-Tropsch synthesis Co,Fe SO2  SO3 H2SO4 V2O5

Heterogeneous Catalysis. Some Challenges Ahead Selective oxdn of long chain paraffins to terminal alcohols/ald/acids; CH4 CH3OH. Activation of CO2 & its use as raw material; CO2 + H2O/ CH3OH/C2H5OH  C2 + Chiral catalysis with high ee. H2 generation from H2O without using HC . Photocatalysis with Sunlight.

Industrial catalysis-2 1930s and 1940s 1930s:Cat Cracking(fixed,Houdry) Mont.Clay C2H4 C2H4O Ag C6H6  Maleic anhydride V2O5 1940s:Cat Cracking(fluid) amorph. SiAl alkylation (gasoline) HF/acid- clay Platforming(gasoline) Pt/Al2O3 C6H6 C6H12 Ni

Industrial catalysis-3 1950s C2H4 Polyethylene(Z-N) Ti C2H4 Polyethylene(Phillips) Cr-SiO2 Polyprop &Polybutadiene(Z-N) Ti Steam reforming Ni-K- Al2O3 HDS, HDT of naphtha (Co-Mo)/Al2O3 C10H8  Phthalic anhydride (V,Mo)oxide C6H6  C6H12 (Ni) C6H11OH C6H10O (Cu) C7H8+ H2 C6H6 +CH4 (Ni-SiAl)

Industrial catalysis-4 1960s Butene Maleic anhydride (V,P) oxides C3H6 acrolein (BiMo)oxides C3H6  acrylonitrile(ammox) -do- Bimetallic reforming PtRe/Al2O3 Metathesis(2C3 C2+C4) (W,Mo,Re)oxides Catalytic cracking Zeolites C2H4 vinyl acetate Pd/Cu C2H4  vinyl chloride CuCl2 O-Xylene Phthalic anhydride V2O5/TiO2 Hydrocracking Ni-W/Al2O3 CO+H2O H2+CO2 (HTS) Fe2O3/Cr2O3/MgO --do-- (LTS) CuO-ZnO- Al2O3

Industrial catalysis-5 1970s Xylene Isom( for p-xylene) H-ZSM-5 Methanol (low press) Cu-Zn/Al2O3 Toluene to benzene and xylenes H-ZSM-5 Catalytic dewaxing H-ZSM-5 Autoexhaust catalyst Pt-Pd-Rh on oxide Hydroisomerisation Pt-zeolite SCR of NO(NH3) V/ Ti MTBE acidic ion exchange resin C7H8+C9H12 C6H6 +C8H10 Pt-Mordenite

Industrial catalysis-6 1980s Ethyl benzene H-ZSM-5 Methanol to gasoline H-ZSM-5 Vinyl acetate Pd Oxdn of t-butanol to MMA Mo oxides Improved Coal liq NiCo sulfides Syngas to diesel Co HDW of kerosene/diesel.GO/VGO Pt/Zeolite MTBE cat dist ion exchange resin Cyclar Ga-ZSM-5 Oxdn of methacrolein Mo-V-P heteropolyacid N-C6 to benzene Pt-L zeolite

Industrial catalysis-7 1990+ DMC from acetone Cu chloride NH3 synthesis Ru/C Phenol to HQ and catechol TS-1 Isom of butene-1(MTBE) H-Ferrierite Ammoximation of cyclohexanone TS-1 Isom of oxime to caprolactam TS-1 Ultra deep HDS Co-Mo-Al Olefin polym Supp. metallocene cats Ethane to acetic acid Multi component oxide Fuel cell catalysts Rh, Pt, ceria-zirconia Cr-free HT WGS catalysts Fe,Cu- based

Industrial catalysis-8 2000 + Solid catalysts for biodiesel - solid acids, Hydroisom catalysts Catalysts for carbon nanotubes - Fe (Ni)-Mo-SiO2

Catalytic Reaction Kinetics Why catalytic reaction kinetics Derivation rate expressions Simplifications Rate determining step Initial reaction rate Limiting cases Temperature dependency Pressure dependency Examples

Reactor design equation conversion i stoichiometric coefficient i rate expression ‘space time’ catalyst effectiveness

Simple example: reversible reaction A B ‘Elementary processes’ A + * k 1 - B 2 3 1. 2. 3. A B 1 3 2 A* B* ‘Langmuir adsorption’

Elementary processes Rate expression follows from rate equation: At steady state: Eliminate unknown surface occupancies

Elementary processes contd. Site balance: (7.5) Steady-state assumption: (7.6-7) Rate expression: (7.9)

Quasi-equilibrium / rate-determining step r = r+2 - r-2

Rate expression r.d.s. Rate determining step: Eliminate unknown occupancies Quasi-equilibrium: So:

Rate expression, contd. Substitution: where: Unknown still q*

Rate expression, contd. Site balance: Finally:

Other rate-determining steps Adsorption r.d.s Surface reaction r.d.s. Desorption r.d.s.

Langmuir adsorption Uniform surface (no heterogeneity) Constant number of identical sites Only one molecule per site No interaction between adsorbed species 0.2 0.4 0.6 0.8 1 0.1 100 10 pA (bar) KA (bar-1) A + * A*

Thermodynamics Equilibrium constant Reaction entropy Reaction enthalpy Adsorption constant Adsorption entropy, <0 (J/mol K) Adsorption enthalpy,<0 (J/mol) atm-1

Multicomponent adsorption / inhibition Langmuir adsorption Inhibitors

Dissociative adsorption H2 + 2* 2H* Two adjacent sites needed

Langmuir-Hinshelwood/Hougen-Watson models (LHHW) includes NT, k(rds) For: A+B C+D pApB-pCpD/Keq molecular: KApA dissociative: (KApA)0.5 = 0, 1, 2 number of species in r.d.s.

Initial rate expressions Forward rates Product terms negligible Adsorption Surface reaction Desorption (K2 and KApA0 >>1) T1 T1 T1 T2 r0 T2 T2 T3 T3 T3 pA0

Ethanol dehydrogenation Franckaerts &Froment Cu-Co cat. C2H5OH « CH3CHO + H2 Model: 1. A + * « A* 2. A* + * « R* + S* (r.d.s.) 3. R* « R + * 4. S* « S + * = Derive rate expression =

Initial rates - linear transformation Ethanol dehydrogenation Full expression Initial rate After rearrangement linear form: linear least squares fit trends, positive parameters

Partial benzene hydrogenation Ru-catalyst - clusters of crystallites Slurry reaction, elevated pressures Water-salt addition increases selectivity + 2 H + H 2 2 Ru Salt-water Adsorption / Desorption properties affected

Dual site models: A + B C (r.d.s.)

Surface occupancies Empty sites: Occupied by A: Occupied by B:

Dual site models, contd. Number of neighbouring sites (here: 6)

Thermodynamics Equilibrium constant Adsorption constant Reaction entropy Reaction enthalpy Adsorption constant Adsorption entropy, <0 (J/mol K) Adsorption enthalpy,<0 (J/mol) atm-1

Multicomponent adsorption / inhibition Langmuir adsorption Inhibitors

Dissociative adsorption H2 + 2* 2H* Two adjacent sites needed

Langmuir-Hinshelwood/Hougen-Watson models (LHHW) includes NT, k(rds) For: A+B C+D pApB-pCpD/Keq molecular: KApA dissociative: (KApA)0.5 = 0, 1, 2 number of species in r.d.s.

Verwerking p. 11 t/m 13

Initial rate expressions Forward rates Product terms negligible Adsorption Surface reaction Desorption (K2 and KApA0 >>1) T1 T1 T1 T2 r0 T2 T2 T3 T3 T3 pA0

Ethanol dehydrogenation Franckaerts &Froment Cu-Co cat. C2H5OH « CH3CHO + H2 Model: 1. A + * « A* 2. A* + * « R* + S* (r.d.s.) 3. R* « R + * 4. S* « S + * = Derive rate expression =

Initial rates - linear transformation Ethanol dehydrogenation Full expression Initial rate After rearrangement linear form: linear least squares fit trends, positive parameters

Initial rates - CO hydrogenation over Rh Van Santen et al. Kinetic model 1. CO + * « CO* 2. CO* + * ® C* + O* (r.d.s.) 0.2 0.4 0.6 0.8 1.0 Occupancy (-) 400 450 500 550 600 Temperature (K) 200 800 Rate Initial rate

Temperature and Pressure Dependence Verwerking p. 18 t/m21 *A *A# *B k- # k+ kbarrier

Limiting cases - forward rates Surface reaction r.d.s. 1. Strong adsorption A A* # Ea2 A* B*

Limiting cases - forward rates Surface reaction r.d.s. 2. Weak adsorption A* # A(g) + * Ea2 HA A*

Limiting cases - forward rates Surface reaction r.d.s. 3. Strong adsorption B A* # Ea2 B + *+ A HA - HB A* B* + A

Cracking of n-alkanes over ZSM-5 J. Wei I&EC Res.33(1994)2467 Ea2 Eaobs DHA Carbon number kJ/mol 200 100 -200 -100

Observed temperature behaviour T higher coverage lower Highest Ea most favoured Change in r.d.s. 1/T ln robs desorption r.d.s. adsorption r.d.s.

‘Kinetic Coupling’ two kinetically significant steps Pt-catalysed dehydrogenation of methylcyclohexane: M  T + H2 Two kinetic significant steps: * + M  .... T*  T + * mari no inhibition by e.g. benzene T* much higher than equilibrium with gas phase T

Sabatier principle - Volcano plot Rate Heat of adsorption

Summary Langmuir adsorption Rate expression uniform sites no interaction adsorbed species constant number of sites Rate expression series of elementary steps steady state assumption site balance quasi-equilibrium / rate determining step(s) initial rates simpler mechanism kinetics

Catalysed N2O decomposition over oxides Winter, Cimino Rate expressions: 1st order strong O2 inhibition moderate inhibition Also: orders 0.5-1 water inhibition = Explain / derive =

N2O decomposition over Mn2O3 Vannice et al. 1995 2 N2O 2N2 + O2 Kinetic model 1. N2O + * « N2O* 2. N2O* ® N2 + O* 3. 2 O* « 2* + O2 Rate expression

N2O decomposition over Mn2O3 Vannice et al. 1995 order N2O ~0.78 Eaobs= 96 kJ/mol Oxygen inhibition 0.0 2.0 4.0 6.0 8.0 10.0 p O2 / kPa 0.1 0.2 0.3 0.4 r / 10 -6 mol.s -1 .g 648 K 638 K 623 K 608 K 598 K pN2O = 10 kPa = Explain =

N2O decomposition over Mn2O3 Vannice et al. 1995 Kinetic model 1. N2O + * « N2O* 2. N2O* ® N2 + O* 3. 2 O* « 2* + O2 Values Rate expression = Thermodynamically consistent =

N2O decomposition over ZSM-5 (Co,Cu,Fe) Kapteijn et al. 11th ICC,1996 2 N2O 2N2 + O2 Kinetic model 1. N2O + * ® N2 + O* 2. N2O + O*® N2 + O2 + * Rate expression no oxygen inhibition

N2O decomposition over ZSM-5 (Co,Cu,Fe) Kapteijn et al. 11th ICC,1996 2 4 6 8 10 p(O ) / kPa 0.0 0.2 0.4 0.6 0.8 1.0 X(N O) Fe-ZSM-5 Co-ZSM-5 Cu-ZSM-5 743 K 833 K 793 K 733 K 773 K 688K Oxygen inhibition model 1. N2O + * ® N2 + O* 2. N2O + O*® N2 + O2 + * 3. O2 + * « *O2 Rate expression

Effect of CO on N2O decomposition CO + O*® CO2 + * CO + * « CO* (Cu+) CO removes oxygen from surface so ‘enhances’ step 2, oxygen removal now observed: rate of step 1 r1 = k1 NT pN2O increase: ~2, >3, >100

Effect of CO on N2O decomposition rate without CO rate with CO ratio = 1 + k1/k2 and: So k1/k2 = : 1 Co >2 Cu >100 Fe 0.7 >0.9 >0.99

Apparent activation energies N2O decomposition CO/ N2O = 2 Fe Cu

Apparent activation energies N2O decomposition CO/ N2O = 0 Cu Fe

Complex kinetics HDN of Quinone over NiMo/Al2O3 (Prins & Jian, Zurich) Kinetic scheme Purpose: Kinetics of reaction Effects functions Ni and Mo Addition role of P

Complex kinetics Subscheme research: HDN of OPA Not observed intermediate, not significant

Complex kinetics HDN of OPA Derived global scheme: How can this ‘direct’ step be rationalised?

Complex kinetics HDN of OPA (Jiang & Prins) Reaction modelling space time (cs) 10 20 30 40 50 60 0.0 0.2 0.4 0.6 0.8 1.0 1 2 3 4 5 OPA NiMo one site model 370C Partial pressure (kPa) OPA PB PCHE PCH strong adsorption N-containg species plug flow reactor excellent fit

Complex kinetics HDN of OPA Competitive parallel steps Direct global routes OPA + * OPA* PB + * PCHA* PCHA + * PCHE* PCHE + * PCH + * HCs not adsorbed (weakly compared to N-s) kb ka slow Fast reaction steps Only traces found kc kd ke The direct route to PCH Other hydrogenation functional sites ?

Rate expressions Steady state assumption Site balance (one site) Strong adsorption N-species parallel reactions Q: explain zero order OPA direct route from PCHE

‘Kinetic coupling’ two steps kinetically significant Decomposition of ammonia over Mo (low p, high T) 2NH3 -> N2 + 3H2 Steps: 2NH3 + * -> 2N* + 3H2 2N* -> N2 surface concentration N much higher than equilibrium with N2 pressure ‘fugacity of N* corresponds with virtual fugacity N2

Virtual fugacity, kinetic coupling Aromatization light alkanes over zeolite Alkanes -> Aromatics + Hydrogen Cracking yields high H*, so high fugacity H* H* not in equilibrium with H2 -> low aromatics selectivity Addition of Ga provides escape route for H* zeolite: alkane -> 2H* + ..... Ga: 2H* -> H2 Kinetic coupling used to increase reaction selectivity for aromatics

Kinetic coupling between catalytic cycles effect on selectivity Hydrogenation: butyne -> butene -> butane A1 A2 A3 butyne and butene compete for the same sites but: K1 >> K2 resulting high selectivity for butene (desired) possible even when k2 > k1 since: Meyer and Burwell (JACS 85(1963)2877) mol%: 2-butyne 22.0 cis-2-butene 77.2 trans-2-butene 0.7 1-butene 0.0 butane 0.1

Kinetic coupling between catalytic cycles effect on selectivity Bifunctional catalysis: Reforming Isomerization n-pentane: n-C5 -> i-C5 low concentration close proximity Pt-function: n-C5 -> n-C5= surface diffusion Acid function: n-C5= -> i-C5= Pt-function: i-C5= -> i-C5 Catalytic cycles on different catalysts Affect selectivity: modify surface (change adsorption properties) modify fluid phase (change adsorption properties) benzene hydrogenation M. Soede

Competitive adsorption Selective hydrogenation aromatics S.Toppinen,Thesis 1996 Ni-alumina trilobe catalyst 3 mm particles 40 bar H2 125oC semi-batch reactor Consecutive conversion behaviour rate constants ~ similar adsorption constants decrease Propose a rate expression to account for this effect

Partial benzene hydrogenation Ru-catalyst - clusters of crystallites Slurry reaction, elevated pressures Water-salt addition increases selectivity + 2 H + H 2 2 Ru Salt-water Adsorption / Desorption properties affected

Dual site models: A + B C (r.d.s.)

Surface occupancies Empty sites: Occupied by A: Occupied by B:

Dual site models, contd. Number of neighbouring sites (here: 6)