Chemical Engineering July 2019 - 32

Facts At Your Fingertips
Heterogeneous catalyst
reaction mechanisms
Surface-Mediated Heterogeneous Catalysis
Department Editor: Scott Jenkins
H
eterogeneous catalysis in industrial
processes involves a
complicated
set
of
physical
and chemical phenomena that help
lead to products. This one-page reference
provides information about the
formation of products in an industrial
process using solid catalyst materials.
Catalyst materials
Industrially, there are several important
classes of heterogeneous catalysts,
including metals, aluminosilicates
and organometallic materials. Industrial
catalysts are typically porous solid
materials, or are chemicals containing
such materials. Solid catalysts exhibit
specificity for particular reactions and
selectivity for certain desired products,
that in most cases, cannot practically
be achieved without catalysts.
And because they are present in a different
phase than the reactants (solid
versus fluid), heterogeneous catalysts
are easily separated from the reaction
mixture. Ideal catalysts allow reactions
to proceed at suitable rates under
conditions that are economically
profitable, and at as low a temperature
and pressure as possible.
Surface-mediated reactions
Because of the presence of different
phases, heterogeneous-catalyzed reactions
involve transport of reactants
to the catalyst surface, as well as adsorption
onto, and desorption from,
the catalyst surface. All reactants and
products must make their way to and
from the catalytically active surface,
and this mass transport can strongly
affect apparent reaction rates and selectivity.
Several general mechanisms
for heterogeneously catalyzed reactions
are shown in the box. Individual
steps for adsorption to the catalyst
surface, reaction and desorption, are
shown for a monomolecular reaction
and two types of bimolecular reactions.
The terms ka, kr and kd refer to
the rate constants for each process.
Surface area
To increase the chances of reactants
encountering a catalyst active
site, catalysts are highly porous and
32
are designed with large internal surface
areas. For example, zeolites are
a particular class of aluminosilicates
with well-defined microporous crystalline
structures. Many different zeolites
have been developed because
of the different ways in which the atoms
can be arranged. Zeolite materials
can allow large vacant spaces in
the three-dimensional structure that
leaves room for cations, such as Na+
and Ca2+, and molecules such as
water. The void spaces in zeolites are
interconnected and form long channels
and pores which vary in size
among different types of zeolites.
These materials are widely used to
catalyze a range of important reactions,
such as fluid catalytic cracking
(FCC), toluene disproportionation, aromatic
alkylation, methanol-to-hydrocarbon
(MTH) conversions and more.
Catalyst action
Catalysts increase reaction rates by
providing a pathway, at the molecular
level, for the reaction to proceed at a
lower activation energy than the uncatalyzed
reaction. A transition state
is a conceptual construct that can be
thought of as a transient, activated
complex through which reactant molecules
are transformed into products.
The catalyst stabilizes the transition
state in some way, lowering the energy
required to transform reactants to
products, without itself being chemically
changed by the reaction.
Catalyst deactivation
Two primary mechanisms contribute
to catalyst deactivation. Solid catalysts
can be deactivated by fouling,
which involves the formation of carbonaceous
deposits on the catalyst
surface (coking). These deposits,
formed by undesirable decomposition
of organic compounds, can block
the pores of a catalyst, and prevent
access to active sites. Catalysts can
also be deactivated by poisoning,
which occurs when impurities, such
as sulfur, trace metals and others
contained in the feed material, attach
to the surface of the catalyst and prevent
adsorption of the reactants.
Monomolecular
Agas
Aads
Pads
Agas
kr
kd
k+a
k-a
Aads + Bgas
kd
Pads
k+a Aads Adsorption
k-a
Pads Reaction
Pgas Desorption
Bimolecular (one reactant adsorbed on catalyst
surface, one gas phase)
Aads
kr
k-r
Pgas
Bimolecular (both reactants adsorbed on catalyst
surface)
Agas
k+a Aads
k-a
Bgas
k+b
k-b
Aads + Bads
kd
Pads
Bads
kr
k-r
Pgas
Industrial catalytic reactors
Although industrial catalytic reactors
exist in a wide range of types, shapes
and sizes, one method of categorizing
them is by the size of catalyst
particles. Large-particle catalysts are
generally kept stationary, and the reaction
mixture passes through the
bed of particles. The particles are
usually greater than 2 mm in size.
For small
catalyst
particles, the
flowing reaction mixture suspends
the solid catalysts, such that the solids
behave like a fluid. The catalyst
must be separated from the reaction
mixture at the reactor exit. Particle
sizes in this case are generally in the
range from 10 µm to 1 mm.
Because most chemical reactions
involve either heat being evolved or
absorbed, one challenge in heterogeneous
reactors is to maintain the
optimal temperature within the catalyst
bed and catalyst beads so that
the catalyst is fully effective.
n
References
1. Wijngaaden, R.J., Kronberg, A. and Westerterp, K.R. " Industrial
Catalysis: Optimizing Catalysts and Processes, "
Wiley-VCH, Weinheim, Germany, 1998.
2. Center for Industry Education and Collaboration, University
of York, Catalysis in Industry, The Essential Chemical
Industry, www.essentialchemicalindustry.org, 2013.
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