Catalyst Design and Preparation

7/29/2019 Catalyst Design and Preparation

1/16

Catalyst Design and Preparation

Dr. King Lun Yeung

Department of Chemical Engineering

Hong Kong University of Science and Technology

CENG 511Lecture 3

Design of Catalyst

(1) Stoichiometric analysis of target reaction

(2) Thermodynamic analysis

(3) Molecular mechanism

(4) Surface mechanism

(5) Reaction pathway(6) Catalyst properties

(7) Catalytic materials

(8) Propose a catalyst


2/16

Case Study

Methane Partial Oxidation to Formaldehyde

CH4

+ O2

CH2

O + H2

O H = -76.8 kcal/mol

G = -70.9 kcal/mol

Current Technology

CH4 + H2O CO + 3H2

CO + 2H2 CH3OH

CH3OH + 0.5 O2

CH2O + H2O

Poor efficiency

high energy and material cost

Stoichiometric Analysis-1

(1) List all possible stoichiometric chemical equations

(2) Calculate the H and G of reaction

(3) Chemical bond changes

Primary ReactantsCH4

O2

Reactant Self-interactions

2CH4 C2H6 + H2 DH G = 8.5 kcal/mol

2CH4 C2H4 + 2H2 DH G = 12.8 kcal/mol

2CH4 C2H2 + 3H2 DH G = 22.2 kcal/mol

O2 = none


3/16


Reactant Cross-interactions

CH4 + 0.5 O2 CH3OH OI G = -20.6 kcal/mol

CH2

O + H2

OI, DH G = -20.0 kcal/mol

CO + 2H2 OI, DH G = -43.1 kcal/mol

CH4 + O2 CH2O + H2O OI, DH, O G = -70.9 kcal/mol

HCOOH + H2 OI, DH, O G = -67.0 kcal/mol

CO + H2 + H2O OI, DH, O G = -87.3 kcal/mol

CO2 + 2H2 OI, DH, O G = -90.5 kcal/mol

CH4 + 1.5O2 CH2O + H2O2 OI, DH, O G = -31.0 kcal/mol

HCOOH + H2O OI, DH, O G = -119.8 kcal/mol

CO + 2H2

O OI, DH, O G = -136.5 kcal/mol

CO2 + H2 + H2O OI, DH, O G = -139.8 kcal/mol


Reactant Cross-interactions

CH4 + 2O2 HCOOH + H2O2 OI, DH, O G = -98.6 kcal/mol

CO + H2O2 + H2O OI, DH, O G = -118.7 kcal/mol

CO2 + 2H2O OI, DH, O G = -189.5 kcal/mol

Reactant-Product interactions

CH4 + C2H6 C3H8 + H2 DH, A G = 16.6 kcal/mol

CH4 + C2H4 C3H8 A G = 4.5 kcal/mol

CH4 + CH3OH C2H5OH + H2 DH, A G = 10.5 kcal/mol

etc.


4/16

Thermodynamic Analysis

(1) Assess thermodynamic feasibility (rank by G)

(2) Rank and group reactions with common trend

CH4 + 2O2 CO2 + 2H2O OI, DH, O G = -189.5 kcal/mol

CH4 + O2 CH2O + H2O OI, DH, O G = -70.9 kcal/mol

CH4 + O2 HCOOH + H2 OI, DH G = -67.0 kcal/mol

CH4 + 0.5 O2 CH2O + H2 OI, DH G = -20.0 kcal/mol

CH4 + 0.5 O2 CH3OH OI G = -20.6 kcal/mol

CH2O CO + H2 DH G = -17.0 kcal/mol

CH3OH CH2O + H2 DH G = 2.0 kcal/mol

Reaction Mechanism

(1) Visualize molecular events leading to formation of desired product(s)

(2) Eliminate non-plausible pathways

CH4 + 0.5 O2 CH2O + H2

Surface Mechanism

(1) Guess the most plausible surface mechanism that lead to the desired product(s)

(2) Research know adsorption, molecular configurations of reactants and products

CH4 CH3-S CH2-S O2 2O-S

CH3OH

CH2O, H2


5/16

Reaction Pathways

(1) Based on the analysis of surface mechanism establish the desired reaction

pathways for the reaction

CH4 + 0.5 O2 CH2O + H2 OI, DH G = -20.0 kcal/mol

(1) Must promote oxygen insertion (OI)

(2) Must be a mild dehydrogenation (DH)

(3) Must prevent strong dehydrogenation

(4) Must prevent oxidation

CH4

H CH3

O2

O O

O

CH3

H

CH2O

Catalyst Properties

(1) Identify the desired catalyst properties based on surface mechanism/reaction

pathway

(1) Oxygen adsorption site leading to dissociated and immobile oxygen species

(2) Mild dehydrogenation to produce CH3(3) Adjacent sites to facilitate final dehydrogenation

CH4

H CH3

O2

O O

O

CH3

H

CH2O


6/16

Catalyst Selection

(1) Based on knowledge of catalyst materials

(1) Mild dehydrogenating catalysts

Usually oxide catalysts, metals are strong DH catalyst

Cu2+, Ni2+, Fe3+, Mn2+, V3+, V5+, Ti4+

(2) Mild oxidation catalysts

Sc3+, Ti4+, V3+, Cr3+, Fe2+, Zn2+, Zr3+, Nb3+, Mo6+

(3) Low mobility

Co3O4 > MnO2 > NiO > CuO > Fe2O3 > Cr2O3 > V2O5 > MoO3(4) Hard to reduce

CoAl2O4, NiAl2O4, ZnTiO4

Bond G.C. Catalysis by Metals, Academic Press (1962)

Krylov O.V. Catalysis by Non-metals, Academic Press (1970)

Propose a Catalyst

Mild DH

Fe3+

V3+

V5+

Ti4+

Mild OI

Sc3+ V3+

Ti4+

Fe2+

Zn2+

Zr3+

Nb3+

Mo6+

Possible Catalysts

Single

TiO2, V2O3

MixedTiO2 + MoO3V2O3 + ZnO

Complex

Fe2O3Zn TiO3


7/16

Catalyst Preparation

(1) Unsupported Catalyst

are typically usually very active catalyst that do not require high

surface areae.g., Iron catalyst for ammonia production

are usually used for high temperature applications

e.g., refractory aluminates for catalytic combustion

intrinsically have a large surface area

e.g., gamma alumina catalyst for isomerization

clay catalyst for hydrogenation

(2) Supported Catalystrequires a high surface area support to disperse the primary

catalyst, the support may also act as a co-catalyst or secondary

catalyst for the reaction

Unsupported Catalyst

Typical preparation methods


8/16


Required preparation steps



(1) Fusion Method


9/16



(2) Precipitation and Co-precipitation Methods




10/16



Preparation of aluminum oxide



(3) Sol-gel synthesis


11/16



(3) Sol-gel synthesis

Silica-alumina acid catalyst



(4) Frame Pyrolysis

Fumed silica

(a) vaporizer(b) mixing chamber

(c) burner

(d) cooling section

(e) separation

(f) deacidification

(g) hopper

(h) compactor


12/16

Frame Pyrolysis (Fumed Silica)

(a) 380 m2g-1

(b) 300

(c) 200

(d) 90

Supported Catalyst

Maintains large catalyst surface area and prevents sintering during high

temperature operation


13/16

Supported Catalyst

Typical support materials

Support Materials


14/16

Metal Ion Distribution in Support Pellet

Supported Catalyst

Weak Interaction Interaction


15/16

Catalyst-Support Interactions

Supported phase-support interaction

(transition layer attachment)

Monolayer formation

Bilayer formation

Catalyst-Support Interactions

Formation of solid solution Formation of new compounds

Grafted catalyst


16/16

Supported CatalystTypical preparation methods

(1) Precipitation method

Precipitation Method

Catalyst Design and Preparation

Documents

Transcript of Catalyst Design and Preparation