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Facts At Your Fingertips: Surface-Mediated Heterogeneous Catalysis

By Scott Jenkins |

Heterogeneous 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, krand 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 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 Ca 2+, 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.

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 10µm 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. 

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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