Gas-Phase Pyrolytic Reactions

Synthesis, Mechanisms, and Kinetics

Nouria A. Al-Awadi

• Publisher: Wiley
• Year: 2019
• Print ISBN: 9781118057476
• eBook ISBN: 9781119010746
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Offers a physical organic chemistry and mechanistic perspective of the chemistry of thermal processes in the gas phase

The book looks at all aspects of the chemical processing technique called gas-phase pyrolysis, including its methodology and reactors, synthesis, reaction mechanisms, structure, kinetics, and applications. It discusses combinations of pyrolytic reactors with physiochemical techniques, routes for and reactions for the synthesis of organic compounds, and the control of reaction rates.

Gas-Phase Pyrolytic Reactions: Synthesis, Mechanisms, and Kinetics starts with in-depth chapter coverage of static pyrolysis, dynamic flow pyrolysis, and analytical pyrolysis. It then examines synthesis and applications, including flash vacuum pyrolysis in organic synthesis, elimination of HX, elimination of CO and CO 2, pyrolysis of Meldrum's acid derivatives, and elimination of N 2. A chapter on reaction mechanism comes next and includes coverage of retero-ene reaction and reactive intermediates. Following that are sections covering: structure/reactivity correlation, functional group & structural frame interconversions; gas-phase pyrolysis of hydrazones and phosphorus Ylides; and more.

• Deals with a growing area of chemistry and engineering interest that fits under the practices of green and sustainable chemistry
• Addresses several important aspects: methodology and reactors, synthesis, reaction mechanisms, structure, kinetics, and applications
• Reviews general methods of pyrolysis techniques
• Sets out the fundamentals and advantages of gas-phase pyrolysis in a way that illustrates its wide potential applications

Gas-Phase Pyrolytic Reactions: Synthesis, Mechanisms, and Kinetics will appeal to organic chemists, physical organic chemists, chemical engineers and anyone interested in green/sustainable chemistry, chemical synthesis, or process chemistry.

1
Methodologies and Reactors

Chapter Menu

1. Static Pyrolysis
   1. Sealed-Tube Reactor
   2. Static Apparatus
2. Dynamic Flow Pyrolysis
   1. Flash Vacuum Pyrolysis
   2. Synthetic Applications of FVP
   3. Gas-Flow Pyrolysis vs. STP
   4. Limitations of FVP
   5. Spray Pyrolysis
3. Falling-Solid Pyrolysis
   1. Analytical Pyrolysis
   2. Pyrolysis Gas Chromatography (Py-GC)
   3. Pyrolysis Mass Spectrometry
   4. FVP with Spectroscopy
   5. Catalytic Gas-Phase Pyrolysis
4. References

This chapter presents an overview of pyrolytic reactions, which may be carried out either in sealed tubes (static reactors) or in flow systems, including flash vacuum pyrolysis (FVP) reactors, the use of static pyrolysis (STP) in kinetic investigation, and why flow systems were used in organic synthesis. It also covers the combination of the various pyrolytic reactors and online systems with advanced physiochemical techniques. A comparison of each type of pyrolytical methodology has also been given.

1.1 Static Pyrolysis

In a static reaction, the substrate is heated continuously in a solid phase, in solution, or in a gas phase in a sealed vessel. This type of pyrolysis is performed in furnace pyrolyzers. The sample is heated for a relatively long period of time, generally at a relatively low temperature (below 450 °C). The pyrolysis products are further analyzed, commonly by an offline analytical technique such as HPLC, GC, GC/MS, IR, or LCMS. The residence time of the substrate in the hot zone plays a crucial role in determining the nature of the products formed. The longer the contact time, the higher the probability that the primary products will undergo secondary reactions. This technique has long been used for a large number of classical thermal reactions: for example, eliminations, fragmentations, and rearrangements, including pericyclic processes. Thermally labile products, however, cannot be isolated using STP, as these may undergo further inter‐ and intramolecular reactions. STP is the right choice for pyrolyzing substrates with low volatility or which involve intermolecular reactions or reactive intermediates. Nevertheless, static reactors are widely used in the study of gas kinetics [1–4].

We, in our laboratory, have successfully used this technique to study gas‐phase kinetics, where the ultimate product formation was not disturbed by secondary reactions due to a relatively long residence time of the substrate in the hot zone. The two types of static reactors that were used for kinetic studies are discussed next.

1.1.1 Sealed‐Tube Reactor

This system consists of two parts: the oven pyrolyzer and the reaction tube.

1.1.1.1 Pyrolyzer

The pyrolyzer is a custom‐made unit made from a cylinder of an insulated aluminum block, which can be heated to any preselected temperature up to 530 °C. Aluminum is chosen for this purpose because of its high thermal conductivity, which ensures an exceptionally low temperature gradient throughout the block. The temperature is controlled by a precision temperature regulator set to provide a 0.1 °C incremental change achieved by a digital switch, which gives an overall temperature output with an accuracy of ±0.5 °C. The actual pyrolysis temperature [5a, b is measured by a platinum resistance thermocouple within the pyrolyzer unit, very close to the reaction vessel, which is connected to a microprocessor thermometer.

The block was hollowed where necessary to fit the pyrex reaction vessel and the tip of the platinum resistance thermocouple for actual reaction temperature read‐out; the latter was fitted in a hole drilled diagonally along the cylindrical axis (Figure 1.1).

Figure 1.1 Schematic diagram of the pyrolyzer.

1.1.1.2 Reaction Tube

The pyrex reaction tube shown in Figure 1.2 is used for both kinetic studies and product analysis. Samples of the starting material in very dilute solution together with an internal standard are introduced into the reaction tube, which is placed in liquid nitrogen in order to freeze the contents of the tube. The tube is then sealed under vacuum to eliminate the possibility of combustion reactions and ensure unimolecularity and conversion of the substrate into vapor prior to reaction. The sealed tube with the sample is then placed in the niche in the pyrolysis unit set to the preselected temperature.

Figure 1.2 Schematic diagram of the pyrolysis tube.

1.1.1.3 Kinetic Studies

A stock solution (7 ml) is prepared by dissolving 6–10 mg of the substrate in acetonitrile to give a concentration of 1000–2000 ppm. An internal standard is then added, the amount of which is adjusted to give the desired peak area ratio of substrate to standard (2.5,1) in a HPLC/GC analysis. The solvent and the internal standard are selected so that both are stable under the conditions of pyrolysis, and so that they do not react with either the substrate or the products. Generally, the compounds used as ...

Table of contents

• Cover
• Table of Contents
• Preface
• List of Abbreviations
• About the Author
• 1 Methodologies and Reactors
• 2 Synthesis and Applications
• 3 Reaction Mechanism
• 4 Structure/Reactivity Correlation
• 5 Functional Group and Structural Frame Interconversions
• 6 Gas‐Phase Pyrolysis of Hydrazones
• 7 Gas‐Phase Pyrolysis of Phosphorus Ylides
• Index
• End User License Agreement