Textbook: Fogler, Elements of Chemical Reaction Engineering Chapter 10 Catalysis and Catalytic Reactors Catalysts Catalytic reactions Rate expression Mechanism Reactor design Chemical Vapor Deposition (CVD) Deactivation Reactors-packed bed, fluidized bed, moving bed Chapter 11 External Diffusion Effects on Heterogeneous Reactions Diffusion Fundamentals Binary Diffusion External Resistance to Mass Transfer Chapter 12 Diffusion and Reaction Diffusion and reaction in spherical catalyst pellets Internal effectiveness factor Overall effectiveness factor Determination of limiting situations from reaction data Grading Quiz (every Tuesday) 20% Final Exam 75% Participation 5% Homework 0% 101 Chapter 10 Catalysis and Catalytic Reactors Objective: To develop an understanding of catalysts: define a catalyst and describe its properties reaction mechanism: describe the steps in a catalytic reaction and apply the concept of a rate-limiting step to derive a rate law catalytic reactor design Contents: Catalyst definitions and properties Steps in catalytic reactions Rate expression and parameters Catalyst deactivation Catalytic reactors Conversion or catalyst weight for reactors Chemical vapor deposition ( CVD ) References: 1. 2. G. C. Bond: Heterogeneous Catalysis, Oxford, 1987. (: , ) 3. 370 (92 10 ) ! 1021. Catalysts A catalyst is a substance that affects the rate of a reaction but emerges from the process unchanged. A catalyst changes a reaction rate by promoting a different molecular path (mechanism) for the reaction. A catalyst makes it possible to obtain an end product by a different pathway with a lower energy barrier; it can affect both the yield and the selectivity. A catalyst changes only the rate of a reaction; it does not affect the equilibrium. * Two types of catalysis (1.) Homogeneous catalysis: the catalyst is in solution with at least one of the reactants Example: oxo process (2.) Heterogeneous catalysis: involves more than one phase usually the catalyst is a solid Example: dehydrogenation of cyclohexane 103 The simple and complete separation of the fluid product mixture from the solid catalyst makes heterogeneous catalysis economically attractive, especially because many catalysts are quite valuable and their reuse is demanded. Only heterogeneous catalysts will be considered in this chapter. Catalyst Properties -(interface)a large interfacial area()-0.6 cm3/g4 nm 300 m2/g(10 g catalyst posses more surface area than a football field) Porous: interfacial area is provided by an inner porous structure. Molecular sieves: pores are so small that they will admit small molecules but prevent large ones from entering. e.g. clays and zeolites . (see Fig. 10-2) Nonporous (monolithic ): some catalysts are sufficiently active so that the effort required to create a porous catalyst would be wasted. Supported: a catalyst consists of minute particles of an active materials dispersed over a less active substance called a support. The active material is a metal or alloy. Pt on Al2O3 (p.683) or Ni on SiO2 Unsupported Support ; ; ; (dual-function catalyst Pt on Al2O3 ) 104 Figure 10.2 One example of the high selectivity of zeolite catalysis is the formation of xylene from toluene and methane. Catalyst Deactivation 1. Poisoning : the irreversible deposit of a foreign material on the active site. e.g. thiophine on Ni catalyst ; quinoline on Al2O3 2. Coking or fouling : the deposit of carbonaceous material ( as in the catalytic cracking of naphthas ). 3. Aging : sinter or structure change; a gradual change in surface crystal structure Chemical Properties: (1.) Activity (Turnover frequency): one parameter used to quantify the activity of a catalyst is TOF . (2.) Selectivity (vs. yield) (3.) Stability (deactivation) Active site: an active site is a point on the catalyst surface that can form strong chemical bonds with an adsorbed atom or molecule. Dispersion (D): the dispersion of active metal on a catalyst is the fraction of the metal atoms deposited that are on the 105surface. Turnover frequency: no. of molecules reacting per active site per second at the condition of the experiment. Deactivation mechanisms: A) Coke formation, B) Poisoning, C) Sintering of the active metal particles, and D) Sintering and solid-solid phase transitions of the washcoat () and encapsulation of active metal particles Example 10-1 (p.651) TOF The Fischer-Tropsch synthesis was studied using a commercial 0.5 wt% Ru on -Al2O3. The catalyst dispersion percentage of atoms exposed, determined from hydrogen chemisorption, was found to be 49%. At a pressure of 988 kPa and a temp. of 475K, a turnover frequency, fCH4, of 1060.044 s-1 was reported for methane. What is the rate of formation of methane, rM, in mole/sg of catalyst (metal plus support)? Solution Chemisorption is the type of adsorption that affects that rate of chemical reaction. Fig. 10-4 Range of turnover frequency as a function of temp. and type of rxns. 107Classification of Catalysts - Metals Hydrogenation, hydrogenolysis, oxidation Metal oxides Partial oxidation, dehydrogenation Acids Hydration, polymerization, cracking Metal + Acid Isomerization, hydrogenlysis Table 10-1. Types of reactions and representation catalysts (p. 655) Reforming Catalyst One common reforming catalyst is Pt on Al2O3. This is a bifunctional catalyst that can be prepared by exposing alumina pellets to a chloroplatinic acid solution, drying, and then heating in air 775 to 875 K for several hrs. Next, the material is exposed to hydrogen at 725 to 775 K to produce very small clusters of Pt on Al2O3. These clusters have sizes on the order of 1 nm. 1082. Steps in a Catalytic Reaction Isomerization : A B Table 10-2. Steps in a catalytic reaction 1. Mass transfer (diffusion) of the reactant (species A) from the bulk fluid to the external surface of the catalyst pellet (Chap. 11) 2. Diffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surface (Chap. 12) 3. Adsorption of reactant A onto the catalyst surface 4. Reaction on the surface of the catalyst (A B) 5. Desorption of the products (B) from the surface 6. Diffusion of the products from the interior of the pellet to the pore mouth at the external surface 7. Mass transfer of the products from the external pellet surface to the bulk fluid In this chapter we focus on: 3. Adsorption 4. Surface reaction 5. Desorption 109(1) Adsorption Isotherms: The amount of a gas adsorbed on a solid at different pressures but at one temp. A + S AS active site Ct = molar conc. of active sites per unit mass of catalyst, mole/g cat (= no. of active sites per unit mass / Avagadros no.) Cv = molar conc. of vacant sites, mole/g cat Pi = partial pressure of species i in the gas phase, atm Cis = surface conc. of sites occupied by species i, mole/g cat. A. Molecular Adsorption (Nondissociated Adsorption) At equilibrium, the net rate of adsorption 0: rAD = 0 (If kA is very large, rAD/kA 0) CCOS = KACvPCO Ct = CV + CCOS = CV ( 1+ KA PA ) CV = Ct1 + KA PA CO ACO A tS COP KP K CC+=1. Evaluation of constants 1010B. Dissociative Adsorption CCS = COS At equilibrium, rAD = 0: For CCS = COS , COS = (KAPCO)1/2CV Site Balance: Ct = CV + CCS + COS = CV + 2 CCS = CV (1+2 (KAPCO)1/2) 2 / 12 / 1) ( 2 1) (CO At CO AOSP KC P KC+= Evaluation of constants 2 / 12 / 12 / 12 1COt A t OSCOPC K C CP+ = Adsorption of H2 and CO Nondissociated H2 + S H2S CO+S COS Dissociated H2 + 2S 2HS CO+2S CS+OS Homework (): If both A and B are adsorbed as molecules, the adsorption of both A and B are first-order processes. : CAS = KAPACt1 + KAPA + KBPB 1011(2) Surface Reaction (Langmuir-Hinshelwood Mechanisms) (A) Single-Site Mechanism A S B S (B) Dual-Site Mechanism for one reactant AS + S B S + S (C) Dual -Site mechanism for two reactants AS + B S C S + D S (D) Dual-Site Mechanism for two reactants adsorbed on different type of sites A S + B S ' C S ' + D S ' (E) Reaction between an adsorbed molecule and a molecule in the gas phaseEley-Rideal mechanism A S + B (g) C S D (g)C S + (3) Desorption A S A + S The products of the surface reaction adsorbed on the surface are subsequently desorbed into the gas phase. (4) Rate-Limiting Step (RLS) Rate-Controlling Step (RCS) Rate-Determining Step (RDS) When heterogeneous reactions are carried out at steady state, the rates of each the three reaction steps in series (adsorption, surface reaction, and desorption) are equal to one another: rA= rAD = rs = rD 1012However, one particular step in the series is usually found to be rate-limiting or rate-controlling. 10133. Synthesizing a Rate Law (Rate Expression) Example: Decomposition of cumene () to form benzene & propylene C6H5CH (CH3)2C6H6 + C3H6C B P Adsorption: Surface reaction: 1014 At Steady state, We wish to determine which step is rate-limiting. We first assume one of the steps to be rate-limiting and then formulate the reaction rate law in terms of the partial pressures of the species. If the predicted rate varies with pressure in the same manner as the rate observed experimentally, the implication is that the assumed mechanism is correct. (1) If Adsorption of cumene is the RLS ++Because we can not measure CV or CCS, we must replace these variables with PC, PB and PP. ++For adsorption-limited reaction, kA is small and kS and kD are large. The ratios rS/kS and rD/kD are very small (~0), whereas the ratio Benzene Desorption: 1015rAD/kA is relatively large. Hint: Using rS/kS ~ 0 ~ rD/kD to find CBS and CCS in terms of partial pressures. After combining (10-33) and (10-35): Replacing CCS in the (10-25) by (10-36): Site balance: Substituting (10-35) and (10-36) into (10-40): Combining (10-41) and (10-37), assuming the adsorption of cumene is RLS: Sketch a plot of the initial rate as a function of the partial pressure of 1016cumene, PC0. Initially, PP = PB = 0. (2) If surface reaction is the RLS Site balance: Substituting for CBS and CCS The initial rate (PP = PB = 0) is 1017 linearly independent of PC0 (3) If the desorption is the RLS Combining (10-28) and (10-47): Site balance: 1018Replacing CV in (10-48) by (10-49) and multiplying the numerator and denominator by PP: The initial rate (PP = PB = 0): (4) Summary of the cumene decomposition The experimental observation of rC0 as a function of PC0: The rate law derived by assuming the surface reaction is rate-limiting agrees with the data. 1019Rate limiting step Rate expression Adsorption Surface rxn Desorption As a function of PC0, the initial rate will be Adsorption - rC0 CtkAPC0 Surface rxn -rC0 KCkSCtPC0 /(1 + KCPC0) Desorption -rC0 kDCt (PC0 = yC Pt0) Eqn 10-42 (adsorption of C is the RLS) i. PC Eqn 10-44 (Surface rxn is the RLS) i. PP Eqn 10-50 (desorption of B is the RLS) i. PB : (1.) adsorption of A RLS PA (2.) desorption of B RLS PB (3.) surface rxn RLS, surface rxn component partial pressure 1020 (4.) component partial pressure (page 689) The Plot of rC0 vs. PC0Decomposition of Cumene (Without inhibitors; initially no B and P) 1.Adsorption of cumene is the RLS -rC0 = CtkAPC0 2.Surface rxn is the RLS -rC0= KCkSCtPC0/(1 + KCPC0) 3. Desorption of benzene is the RLS -rC0 = kDCt Experimental observation * Adsorption or desorption is not the RLS * The surface rxn is the RLS 1021 1022(5) Rate laws derived from the Pseudo-Steady-State hypothesis (PSSH) The PSSH, the net rate of formation of reactive intermediates was assumed to be zero. Each species adsorbed on the surface is a reactive intermediate. ++Use the PSSH when: 1. Some steps are irreversible. 2. Two or more steps are rate-limiting. The rate law for the irreversible surface rxn: The net rate of the intermediate is zero: Substituting for CNS in (10-55): Site balance: After substituting for CNS and CIS, solving for CV, which we then substitute in the rate law: 1023 Take the surface rxn step to be rate-limiting; kS is much smaller than kN. (10-63) is identical to the one derived assuming that and . ps. this method also works well for a single RLS. This technique is preferred if two or more steps are RLS or if some of the steps are irreversible or if none of the steps are RLS. 10244. Heterogeneous Data Analysis for Reactor Design Deduce rate law; Find mechanism; Evaluate rate law; Design reactor Hydrodemethylation of toluene C6H5CH3H2 C6H6CH4 T H2 B M To design a packed-bed reactor and CSTR to process a feed consisting of 30 T45 H2and 25 inert. T is fed at a rate of 50minmol at 640C and 40 atm. To design the reactors, we must first determine the rate expression from the differential reactor (p. 281) data presented in Table 10-5 clinoptilolite (crystalline silica-alumina) 1025Deducing a rage law from the experimental data 1 Dependence on M: if M were absorbed on the surface, PM would appear in the denominator. From run 1 and 2: 4 rT Methane is not (or weakly) adsorbed (R1) or goes directly into the gas phase similar to propylene in the cumene decomposition. (R2) 2 Dependence on B: From runs 3 and 4 (10-68) suggests that B is adsorbed on the catalyst surface. 3 Dependence on T: at low conc. of T (runs 10 and 11), the rate increases with PT; while at high conc. (runs 14 and 15), the rate is independent of PT. Combine (10-68) and (10-69): 4 Dependence on H: From runs 7, 8 and 9, We conclude that the reaction is first order in H2. H2 is either not adsorbed on the surface or its coverage of the surface is extremely low. (1