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CRE II Heterogeneous Catalysis
Prof. K.K.Pant
Department of Chemical EngineeringIIT Delhi.
[email protected]
mailto:[email protected]:[email protected]
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-Catalyst
Allow reactions occur under a milderconditions, e.g. at lower
temperatures for
those heat sensitive materials
It is important to remember that the useof catalyst DOES NOT
vary DG& Keqvalues of the reaction concerned, it
merely change the PACE of the process.
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Whether a reaction can proceed or not and
to what extent a reaction can proceed is
solely determined by the reaction
thermodynamics, which is governed by the
values of DG& Keq, NOT by the presence of
catalysts. The reaction thermodynamics provide the
driving force for a rxn; the presence of
catalysts changes the way how driving force
acts on that process.
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e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g)
DG373=151 kJ/mol (100 C) & DG973
=-16 kJ/mol (700 C)
=>At 100C, DG373=151 kJ/mol > 0.There is no thermodynamic
driving force, the
reaction wont proceed with or without a
catalyst
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At 700C, DG373= -16 kJ/mol < 0. The
thermodynamic driving force is there. However,
simply putting CH4and CO2together in a
reactor does not mean they will react. Without
a proper catalyst heating the mixture in reactor
results no conversion of CH4and CO2at all.
When Pt/ZrO2or Ni/Al2O3is present in thereactor at the same
temperature, equilibrium
conversion can be achieved (
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Why R& D in catalysis is important?
90 % of chemical industry involve products
made using catalysts (food, fuels,
polymers, textiles, pharma/
agrochemicals,etc)
For discovery/use of alternate sources ofenergy/fuels/ raw
material for chemical
industry.
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For preparation of new materials (organic &
inorganic-eg: Carbon Nanotubes).
For Pollution control-Global warming.
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Catalysis is multidisciplinary
( physics, chemistry, Material science
(nanomaterial) & chem. Engg. )
The catalyst is an inorganic solid;
Catalysis is a surface phenomenon; solid state
and surface structures play important roles.
Adsorption, desorption and reaction are subject
to thermodynamic, transport and kinetic controls(chem engg);
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Adsorbate-substrate and adsorbate -
adsorbate interactions are both electrostatic
and chemical (physical chemistry).
The chemical reaction is organic chemistry.
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Concept of Green catalysis
Technology is called Greenif it uses raw
materials efficiently, such that the use of
toxic and hazardous reagents and
solvents can be avoided while formation of
waste or undesirable by-products is
minimized.
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Concept of Green catalysis
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Basic concept of green catalysis
1. Indicators to measure the efficiency and
environmental impact of a reaction.
Atom Efficiency: is the molecular weight of
the desired product divided by the total
molecular weight of All Products.
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Concept of Atom efficiency & E -Factor
Atom efficiency =
The molecular weight of thedesired product
The total weight of all
products.
Another useful indicator of environmental
acceptability is the E factor- the weight
of waste or undesirable by product by the
Weight of the desired product.
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Mass balances of alternative routes inchemical processing can be
compared using
measures E factor and mass index. The E
factor :Ratio of Waste [kg] to Product[kg]), is
an output orientated indicator, whereas the
Mass index (Ratio of all Raw materials [kg] to
the Product [kg]) is an input oriented indicator.
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E factor:
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For example the conventional oxidation of
a secondary alcohol
3C6H5CHOHCH3+ 2Cr2O3+ 3H2SO4
3C6H5COCH3+ Cr2(SO4)3+ 6H2O
360 392Atom efficiency of 360/860 = 42%.
C6H5CHOHCH3+ 1/2O2
C6H5COCH3+ H2O
Atom efficiency of 120/138 = 87%, with
water as the only by product.
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There is a substantial increase in E factors
on going downstream from bulk chemicals(50) and
specialties (25>100).
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Action of Catalysts
Catalysis action - Reaction kinetics andmechanism
Catalyst action leads to the rate of a
reaction to change. This is realised by
changing the course of reaction (comparedto non-catalytic
reaction)
Forming complex with reactants/
products, controlling the rate ofelementary steps in the
process.
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For comparative measurements, such as
catalyst screening, determination of process
para-meters, optimization of catalyst
production conditions, and deactivation
studies, the following activity measures can
be used:Conversion under constant reaction
conditions
Space velocity for a given, constant
conversionSpacetime yield
Temperature required for a givenconversion
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Catalysts are often investigated in
continuously operated test reactors, inwhich the conversions
attained at constant
space velocity are compared .
The space velocity: is the volume flow rate
V0, relative to the catalyst mass mcat:
(V0/mcat)
(made under constant conditions of startingmaterial ratio,
temperature, and pressure).
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Often the performance of a reactor is given
relative to the catalyst mass or volume,
so that reactors of different size or
construction can be compared with one
another.
This quantity is known as the spacetimeyield STY
STY= Desired product quantity/ Catalyst
vol. time
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Comparison of catalyst activities
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The suitability of a catalyst for an industrialprocess depends
mainly on the following
three properties:
ActivitySelectivity
Stability (deactivation behavior)
The question which of these functions is the
most important is generally difficult toanswer because the
demands made on the
catalyst are different for each process.
Mode of Action of Catalysts
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Activity
Activity is a measure of how fast one or more reactions
proceed in the presence of the catalyst. Activity can be
defined in terms of kinetics or from a more practically
oriented viewpoint.
In a formal kinetic treatment, it is appropriate to
measure reaction rates in the temperature and
concentration ranges that will be present in the reactor.
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Every catalytic reaction is a sequence of
elementary steps, in which reactant moleculesbind to the
catalyst, where they react, after which
the product detaches from the catalyst, liberating
the latter for the next cycle.
Steps of Catalytic Reactions
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Potential energy diagram of a heterogeneous catalytic
reaction
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Surface Area
Factors affecting react io n rate:
Concentrations of reactants
CatalystTemperature
Surface area of solid reactants or
catalyst
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Turnover frequencies, Rates and
numbers
CATALYSIS IS A KINETICPHENOMENON
Sequence of elementary steps at steady state:
diffusion (bulk, film, surface) - adsorption-
reaction-desorption- diffusion
TOF= number of product molecules formed per unit
area per sec(molecules.cm-2.sec-1)
TOF= number of product molecules formed peractive siteper
sec(molecules.sec-1) onlyif active
site is known.
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TOT= 1/TOF = turnover time, time necessary
to form a product molecule(sec);
TOR= Turnover rate = TOF X Surface area
TON= TOF X total reaction time;
TON must be >100 to be industrially useful.
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Conversions, Rates and Rate
constants
Conversion = % Reactant converted;
Reaction rate = kpX f(Pi) or kcX f(Ci)
k = A exp(-
E
#
/RT); A is temp independent. TOFs between 0.0001 and 100 in
industry; Temp
adjusted to get the desired rates.
E#~ 35-45 Kcal/mol for isom, cyclisation,
cracking, dehydo / hydrogenolysis; HighT needed.
E#~ 6-12 Kcal/mol for hydrogenation;
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Chemical Kinetics
Collision Theory
Collisions between reacting molecules are
necessary before a reaction can occur.
Only those collisions having sufficient energy
are effective in bringing about a reaction
activation energy.
Colliding molecules must be properly oriented
with respect to one another for the reaction to
take place.
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SelectivityThe selectivity (Sp) of a reaction is the fraction of
the
starting material that is converted to the desired product
P.
It is expressed by the ratio of the amount of desired
product to the reacted quantity of a reaction A. In
addition to the desired reaction, parallel and sequential
reactions can also occur.
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npnp vAvp
Sp= = mol/mol or %nA,o - nA nA,o - nA vp
vA
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Presently the efficient use of raw materials and
energy is of major importance, and it is prefer-
able to optimize existing processes than to
develop new ones.
For various reasons, the target quantities
should be given the following order of priority:
Selectivity >Stability> Activity
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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, Hydrotreating,
Hydrocracking, Reforming, Alkylation etc.
etc.)
Petrochemicals(monomers, bulk chemicals).
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Fine Chem.(pharma, agrochem, fragrance,
textile, coating, surfactants,laundry etc)
Environmental Catalysis (auto exhaust,
deNOx, )
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Types of Catalysts & Catalytic
Reactions
The types of catalysts
Classification based on the its physical
state, a catalyst can be
gas
liquid
solid
Classification based on the substances
from which a catalyst is made
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Inorganic (gases, metals, metal
oxides, inorganic acids, bases etc.)
Organic (organic acids, enzymes etc.)
Types of catalysts
Classification based on the ways
catalysts work Homogeneous - both catalyst and all
reactants/products are in the same
phase (gas or liq)
Heterogeneous - reaction system
involves multi-phase (catalysts +
reactants/products)
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Classification of Catalysts
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Comparison between Homogeneous
and heterogeneous catalysts
