Novel Concepts in Catalysis and Chemical Reactors

Improving the Efficiency for the Future

Andrzej Cybulski, Jacob A. Moulijn, Andrzej Stankiewicz, Andrzej Cybulski, Jacob A. Moulijn, Andrzej Stankiewicz

• Publisher: Wiley-VCH
• Year: 2011
• ISBN:
  9783527641536
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

The chemical process industry faces a tremendous challenge of supplying a growing and ever more demanding global population with the products we need. The average efficiency at which resources are converted into the final products is however still dramatically low. The most obvious solution is to carry out chemical conversions at much higher yields and selectivity and this is where active and selective catalysts and efficient chemical reactors play a crucial role. Written by an international team of highly experienced editors and authors from academia and industry, this ready reference focuses on how to enhance the efficiency of catalysts and reactors. It treats key topics such as molecular modeling, zeolites, MOFs, catalysis at room temperature, biocatalysis, catalysis for sustainability, structured reactors including membrane and microchannel reactors, switching from batch to continuous reactors, application of alternative energies and process intensification. By including recent achievements and trends, the book provides an up-to-date insight into the most important developments in the field of industrial catalysis and chemical reactor engineering. In addition, several ways of improving efficiency, selectivity, activity and improved methods for scale-up, modeling and design are presented in a compact manner.

1

Molecular Catalytic Kinetics Concepts

Rutger A. van Santen

1.1 Key Principles of Heterogeneous Catalysis

We discuss the following topics in the subsequent sections:

• Sabatier principle and volcano curve;
• Brønsted-Evans-Polanyi (BEP) linear activation energy-reaction energy relationships;
• compensation effect in catalytic kinetics;
• micropore size dependence in zeolite catalysis;
• structure sensitivity and insensitivity in transition-metal catalysis;
• transition-state stabilization rules.

The molecular interpretation of major topics in catalytic kinetics will be highlighted based on insights on the properties of transition-state intermediates as deduced from computational chemical density functional theory (DFT) calculations.

1.2 Elementary Rate Constants and Catalytic Cycle

A catalytic reaction is composed of several reaction steps. Molecules have to adsorb to the catalyst and become activated, and product molecules have to desorb. The catalytic reaction is a reaction cycle of elementary reaction steps. The catalytic center is regenerated after reaction. This is the basis of the key molecular principle of catalysis: the Sabatier principle. According to this principle, the rate of a catalytic reaction has a maximum when the rate of activation and the rate of product desorption balance.

The time constant of a heterogeneous catalytic reaction is typically a second. This implies that the catalytic event is much slower than diffusion (10⁻⁶ s) or elementary reaction steps (10⁻⁴ − 10⁻² s). Activation energies of elementary reaction steps are typically in the order of 100 kJ mol⁻¹. The overall catalytic reaction cycle is slower than elementary reaction steps because usually several reaction steps compete and surfaces tend to be covered with an overlayer of reaction intermediates.

Clearly, catalytic rate constants are much slower than vibrational and rotational processes that take care of energy transfer between the reacting molecules (10⁻¹² s). For this reason, transition reaction rate expressions can be used to compute the reaction rate constants of the elementary reaction steps.

Eyring’s transition-state reaction rate expression is

(1.1a)

(1.1b)

Q^# is the partition function of transition state and Q₀ isthepartitionfunctionof ground state, k is Boltzmann’s constant, and h is Planck’s constant.

The transition-state energy is defined as the saddle point of the energy of the system when plotted as a function of the reaction coordinates illustrated in Figure 1.1.

Figure 1.1 Transition-state saddle point diagram. Schematic representation of potential energy as a function of reaction coordinate.

Γ is the probability that reaction coordinate passes the transition-state barrier when the system is in activated state. It is the product of a dynamical correction and the tunneling probability. Whereas statistical mechanics can be used to evaluate the pre-exponent and activation energy, Γ has to be evaluated by molecular dynamics techniques because of the very short timescale of the system in the activated state. For surface reactions not involving hydrogen, Γ is usually close to 1.

Most of the currently used computational chemistry programs provide energies and vibrational frequencies for ground as well as transition states.

A very useful analysis of catalytic reactions is provided for by the construction of so-called volcano plots (Figure 1.2). In a volcano plot, the catalytic rate of a reaction normalized per unit reactive surface area is plotted as a function of the adsorption energy of the reactant, product molecule, or reaction intermediates.

Figure 1.2 Volcano plot illustrating the Sabatier principle. Catalytic rate is maximum at optimum adsorption strength. On the left of the Sabatier maximum, rate has a positive order in reactant concentration, and on the right of Sabatier maximum the rate has a negative order.

A volcano plot correlates a kinetic parameter, such as the activation energy, with a thermodynamic parameter, such as the adsorption energy. The maximum in the volcano plot corresponds to the Sabatier principle maximum, where the rate of activation of reactant molecules and the desorption of product molecules balance.

1.3 Linear Activation Energy-Reaction Energy Relationships

The Sabatier principle deals with the relation between catalytic reaction rate and adsorption energies of surface reaction intermediates. A very useful relation often exists between the activation energy of elementary surface reaction steps, such as adsorbate bond dissociation or adsorbed fragment recombination and corresponding reaction energies. These give the Brønsted-Evans-Polanyi relations.

For the forward dissociation reaction, the BEP relation is

(1.2a)

Then for the backward recombination reaction Eq....

Table of contents

• Cover
• Title
• Copyright
• Preface
• List of Contributors
• 1: Molecular Catalytic Kinetics Concepts
• 2: Hierarchical Porous Zeolites by Demetallation
• 3: Preparation of Nanosized Gold Catalysts and Oxidation at Room Temperature
• 4: The Fascinating Structure and the Potential of Metal-Organic Frameworks
• 5: Enzymatic Catalysis Today and Tomorrow
• 6: Oxidation Tools in the Synthesis of Catalysts and Related Functional Materials
• 7: Challenges in Catalysis for Sustainability
• 8: Catalytic Engineering in the Processing of Biomass into Chemicals
• 9: Structured Reactors, a Wealth of Opportunities
• 10: Zeolite Membranes in Catalysis: What Is New and How Bright Is the Future?
• 11: Microstructures on Macroscale: Microchannel Reactors for Medium- and Large-Size Processes
• 12: Intensification of Heat Transfer in Chemical Reactors: Heat Exchanger Reactors
• 13: Reactors Using Alternative Energy Forms for Green Synthetic Routes and New Functional Products
• 14: Switching from Batch to Continuous Processing for Fine and Intermediate-Scale Chemicals Manufacture
• 15: Progress in Methods for Identification of Micro- and Macroscale Physical Phenomena in Chemical Reactors: Improvements in Scale-up of Chemical Reactors
• Index