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 internal standards are chlorobenzene, 1,3-dichlorobenzene, and 1,2,4-trichlorobenzene. Each solution is filtered to ensure that a homogeneous solution is obtained.

The reaction rate is obtained by tracing the rate of disappearance of the substrate with respect to the internal standard as follows:

An aliquot (0.2?ml) of each solution containing the substrate and the internal standard is pipetted into the reaction tube, which is then placed in the pyrolyzer for six minutes under non-thermal conditions. The sample is then analyzed using the HPLC/GC probe, and the standardization value (Ao) is calculated.

Several HPLC/GC measurements are obtained with a consistency =98%. The temperature of the pyrolysis block is then raised until approximately 10% pyrolysis is deemed to have occurred over a given period of residence time. This process is repeated after each 10-15?°C rise in temperature of the pyrolyzer until =84% pyrolysis has taken place. The relative ratios of the integration values of the sample and the internal standard (A) at the pyrolysis temperature are then calculated. A minimum of two kinetic runs are carried out at each 10-15?°C rise in the temperature of the pyrolyzer to ensure reproducible values of (A). Kinetic runs are also repeated in the presence of a radical inhibitor, such as cyclohexene or toluene, depending on the temperature range of pyrolysis, to check the possibility of radical mechanisms, and by using a pyrolysis tube packed with helices to increase the surface area (which helps to ascertain that there are no surface reactions).

1.1.1.4 Treatment of Kinetic Results

For the first-order reaction at a given temperature, the integrated rate equation is given by Eq. (1.1):

The variation of the rate constant (k) with temperature can be expressed satisfactorily by the logarithmic form of the Arrhenius equation Eq. (1.2) to give Eq. (1.3):

where:

• k = rate constant for first-order reaction (s-1)
• A = frequency or pre-exponential factor (s-1)
• E a = energy of activation (kcal?mol-1)
• R = gas constant (1.98722?cal?K-1 mol-1)
• T = absolute temperature (K)

The plot of log k versus 1/T gives a straight line with a slope equal to (-E a/2.303?R) and intercept equal to (log A), from which the energy of activation (E a) and the frequency factor (A) can be calculated.

The entropy of activation is temperature-dependent [7] and is determined from the log A factor at the appropriate temperature using Eq. (1.4):

where:

• K?=?Boltzman constant
• h?=?Plank constant

1.1.2 Static Apparatus

The apparatus shown in Figure 1.3 used for determining the reaction kinetics in the static system consists of a cylindrical reaction vessel of approximately 185?ml volume with two chambers divided by a very thin stainless steel diaphragm equipped with a platinum contact on the outside, and sealed with a copper gasket crushed between knife edges, to give more reliable seal; the diaphragm acts as a null-point gauge [8].

Figure 1.3 Schematic diagram of the static pyrolysis (STP) apparatus.

The reaction vessel is made of stainless steel, for its good resistance to hot acids and hardness to metal seals, which is of prime importance. To avoid leaks, parts are machined from a single piece of steel or by electron-beam welding of the joints. On one end of the cylindrical reactor are two valves, the injection and evacuation valves, each being perpendicular to the cylinder end, and screwed in to finger tightness to provide a metal-to-metal seal after...