General Information About Pyrolysis

C H A P T E R

1 General Information About Pyrolysis

S U B C H A P T E R

1.1

Introductory Information About Pyrolysis

WHAT IS PYROLYSIS? The chemical decomposition of a compound caused by temperatures above 250–300°C is commonly known as pyrolysis [1]. The decomposition takes place because chemical bonds have limited thermal stability and can break due to heat. This type of decomposition usually leads to the formation of smaller molecules, although the resulting fragments may sometimes interact and further generate larger compounds compared to the starting molecule. Some molecules decompose due to heating at even lower temperatures than 250–300°C, but these processes are typically indicated as thermal decompositions and not as pyrolysis. Pyrolysis can be applied to any molecule, nonpolymeric or polymeric, in a solid, liquid, or gaseous phase. Pyrolysis can be performed on pure compounds as well as on mixtures, and the resulting fragments may further react and generate many other compounds. For nonvolatile molecules such as polymers, the effect of heat is complete or incomplete pyrolysis of the sample. However, for smaller molecules that have some volatility, it is very

Pyrolysis of Organic Molecules https://doi.org/10.1016/B978-0-444-64000-0.00001-9

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1. GENERAL INFORMATION ABOUT PYROLYSIS

common that the compound to be pyrolyzed can also distill during heating; it is present in the final product of heating, known as pyrolyzate. Pyrolysis does not involve, in principle, any reagents. However, certain thermal decompositions at temperatures above 300°C can be performed intentionally in the presence of a reagent, such as hydrogen, oxygen, water, and tetramethylammonium hydroxide, leading to special types of pyrolysis. The presence of catalysts during pyrolysis can modify the course of the process, and catalytic pyrolysis has very important industrial applications. Pyrolysis is frequently associated with burning. However, burning is a more complex process that consists of combustion (reaction with oxygen or another comburant), pyrolysis due to the heat generated by combustion, volatilization, steam distillation, aerosol formation (including even solid aerosol particles), etc. Organic materials generate by burning compounds such as H2O, CO2, CO, and N2 (as a result of combustion) as well as many other compounds produced by pyrolysis, volatilization, partial combustion, etc. A clear separation of the processes of pyrolysis and combustion during burning is not really possible because the formation of free radicals during the reaction with oxygen can be involved in the pyrolytic decomposition of molecules. Also, the free radicals formed from molecules by heat (pyrolysis) are typically the initiators of the combustion process.

TYPES OF PYROLYTIC PROCESSES Pyrolytic processes may be performed intentionally at the laboratory or industrial scale, or may take place unintentionally. A broad classification of pyrolysis types can be based on the conditions in which the process takes place, as follows: (1) industrial scale preparative pyrolysis, (2) analytical pyrolysis, (3) preparative pyrolysis at laboratory scale, (4) pyrolysis associated with burning, and (5) pyrolysis performed to simulate a specific process. (1) Industrial scale preparative pyrolysis is part of several of the most important industrial activities including the processing of oil, natural gas, coal, and different types of waste (e.g., used automobile tires). These procedures have enormous economic implications. For example, ethylene is obtained by pyrolytic processes mainly from naphtha, ethane, or propane. Ethylene is an important starting material for polyethylene, acetaldehyde, ethanol, ethyl chloride, ethylene oxide, ethylbenzene, vinyl acetate, etc., and its worldwide production in 2016 reached more than 150
106 tons, with continued prospective to grow. Besides ethylene, other olefins of industrial importance are obtained by pyrolytic procedures. Pyrolytic processes are also used for the production of aromatic hydrocarbons from heavier naphtha feed or even from crude oil. Typical for pyrolysis performed for industrial purposes is that it is frequently performed in the presence of reagents and/or catalysts. Crude oil processing utilizes a variety of reagents such as hydrogen or water; various catalysts for obtaining specific chemicals such as ethylene, propylene, butylene, 1,3-butadiene, acetylene, and cycloolefins; and aromatic hydrocarbons such as benzene, toluene, and naphthalene. Pyrolysis associated with catalytic hydrogenation or dehydrogenation is a common operation for the production of hydrocarbons from naptha or other sources. The addition of hydrogen in the presence of catalysts enhances the yield of C7–C9 hydrocarbons from the heavier oils while the

TYPES OF PYROLYTIC PROCESSES

3

pyrolytic dehydrogenation of cycloalkanes in the presence of catalysts leads to the formation of benzene, toluene, and xylenes. Pyrolytic techniques in the presence of catalysts also are used for the purpose of hydrocarbon isomerization, which can generate gasoline with a higher octane number. Pyrolysis in the presence of steam and catalysts is used in the oil industry for modifying the chemical composition of different oil fractions. Such important activities at the industrial scale are associated with a large body of information published in dedicated journals such as Fuel, Fuel Processing Technology, Energy, Energy Sources, and Petroleum Science and Technology, with dedicated books (e.g., [2,3]), peer-reviewed articles (e.g., [4–18]), and information on the Internet. One additional industrial field where pyrolysis techniques are used successfully is the production of fuels from renewable sources such as vegetable oils [19,20]. Some aspects regarding the chemistry and mechanisms of pyrolysis of compounds involved in these important practical applications will be discussed in this book, but an in-depth presentation of the subject of industrial scale preparative pyrolysis is beyond the purpose of the present material. (2) Analytical pyrolysis is performed at laboratory scale on very small amounts of material with the purpose of finding the pyrolysis products for specific compounds or for the identification of a material when its nature is unknown. The main subject of this present book is analytical pyrolysis of nonpolymeric molecules, which will be further discussed in detail. The identification of the compounds generated by pyrolysis from a specific compound has significant implication in the understanding of potential health and environmental hazards posed by pyrolysis products. When practiced on volatile molecules, analytical pyrolysis can be associated with simple distillation of the compound to be pyrolyzed. The identification of unknown materials following pyrolysis is practiced in particular on polymers or other materials that are not amenable for other analytical techniques such as gas chromatography mass spectrometry (GC/MS), liquid chromatography mass spectrometry (LC/MS), or spectroscopic techniques. In analytical pyrolysis, the instrument in which the pyrolysis is performed (pyrolyzer) is typically coupled with an analytical instrument. The most common instrument coupled with a pyrolyzer is a GC/MS because the fragments generated from pyrolysis are typically volatile and capable of being analyzed by this technique. Other instruments, such as infrared spectrometers (IR), are also used for the analysis of pyrolyzates. Analytical pyrolysis of polymers is not discussed in this book. However, the subject can be found in numerous other materials such as peer-reviewed articles in dedicated journals such as Journal of Analytical and Applied Pyrolysis, books (see e.g., [21–23]), and on the Internet. (3) Preparative pyrolysis at laboratory scale is a technique used in certain special syntheses. Different types of instrumental setups were designed for performing pyrolysis and capturing the pyrolysis products containing the desired product (see e.g., [24]). Laboratory synthesis using pyrolytic procedures is beyond the purpose of this book, and the subject is discussed in detail in other publications (see e.g., [25,26]). (4) Burning is a complex process that includes pyrolysis in addition to combustion and other processes. Burning can take place intentionally (such as fuel burning) or unintentionally (such as accidental forest fires). Intentional burning is associated with pyrolytic processes taking place during fuel burning, burning of various types of waste, and cigarette burning. Numerous sources of information are dedicated to each of these subjects such as

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1. GENERAL INFORMATION ABOUT PYROLYSIS

journals (e.g., Fuel, Energy Sources, Energy & Fuels, Waste Management & Research, Beitr€ age zur Tabakforschung International, etc.), books (e.g., [27]), numerous peer-reviewed papers (e.g., [28–32]), and Internet information. A common process that may involve pyrolysis related to burning is the cooking of food. During this process, the temperatures applied to food may be rather high (e.g., during meat broiling), and some of the resulting pyrolysis products may have adverse health effects. The formation during cooking of polycyclic aromatic hydrocarbons [33], certain heterocyclic amines [34], acrylamide [35,36], etc. has been frequently studied and is the subject of numerous publications (see e.g., [24]). Unintentional burning also involves pyrolytic processes. A common example is that of wood burning in unintentional fires (e.g., [37,38]) taking place for example during forest fires (e.g., [39,40]). Unintentional burning may generate toxic pyrolysis products even more than intentional burning. In intentional burning, some parameters of the process can be controlled such as the burning temperature or the amount of air/oxygen that is provided for the increase of the combustion component and reduction of pyrolysis (e.g., [41]). The subject of pyrolysis products from unintentional burning is beyond the purpose of this book. However, the study of pyrolysis products of individual compounds that are discussed in this book has implications in the understanding of chemicals generated during burning. (5) Pyrolysis can be performed with the goal of simulating particular processes that occur in practice. Such pyrolysis may use specific experimental setups or sometimes may use analytical pyrolysis instrumentation. Examples include the study of flames [42], the study of the burning process in a cigarette [43,44], the evaluation of the degradation products of illicit drugs that are smoked [45], etc.

References 1.1 [1] Anon, Nomenclature and terminology for analytical pyrolysis (IUPAC recommendations 1993), J. Anal. Appl. Pyrolysis 31 (1995) 251–256. [2] L.F. Albright, B.L. Crynes (Eds.), Industrial and Laboratory Pyrolysis, ACS Symposium Ser., vol. 32, ACS, Washington, 1976. [3] L.F. Albright, B.L. Crynes, W. Corcoran (Eds.), Pyrolysis: Theory and Industrial Practice, Academic Press, New York, 1983. [4] I. Ziegler, R. Fournet, P.M. Marquaire, J. Anal. Appl. Pyrolysis 73 (2005) 212. [5] I. Ziegler, R. Fournet, P.M. Marquaire, J. Anal. Appl. Pyrolysis 73 (2005) 231. [6] A.F. Makhov, N.P. Smirnov, B.Y. Risov, A.I. Stekhun, L.E. Pokhitun, G.F. Unger, K.N. Mamaeva, L.N. Andreeva, Chem. Technol. Fuels Oil 9 (1973) 422. [7] N. Mostoufi, R. Sotudeh-Gharebagh, M. Ahmadpour, J. Eyvani, Chem. Eng. Technol. 28 (2005) 174. [8] M. Ohshima, H. Nakagawa, I. Hashimoto, U. Ohkamo, G. Suzuki, Y. Kawabata, J. Process Control 6 (1996) 309. [9] H. Manafzadeh, S.M. Sadrameli, J. Towfighi, Appl. Therm. Eng. 23 (2003) 1347. [10] G. Gasco, G.G. Blanco, F. Guerrero, A.M. Mendez, J. Anal. Appl. Pyrolysis 74 (2005) 413. [11] S.M. Sadrameli, A.E.S. Green, J. Anal. Appl. Pyrolysis 73 (2005) 305. [12] D.M. Matheu, G.M. Grenda, J. Phys. Chem. A 109 (2005) 5332. [13] D.M. Matheu, G.M. Grenda, J. Phys. Chem. A 109 (2005) 5343. [14] B.-f. Li, X.-h. Li, W.-y. Li, J. Feng, Fuel Process. Technol. 166 (2017) 69. [15] J.E. Gwyn, Fuel Process. Technol. 70 (2001) 1. [16] E. Joo, S. Park, M. Lee, Ind. Eng. Chem. Res. 40 (2001) 2409. [17] R. Khan, J. Chu, J. Margrave, R. Hauge, R. Smalley, Energy Sources 27 (2005) 279. [18] X.-H. Meng, J.-S. Gao, L. Li, C.-M. Xu, Pet. Sci. Technol. 22 (2004) 1327.

MAIN TYPE OF CHEMICAL REACTIONS IN PYROLYSIS

[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45]

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A. Demirbas, Biodiesel. A Realistic Fuel Alternative for Diesel Engines, Springer, London, 2008. D.S. Scott, P. Majerski, J. Piskorz, D. Radlein, J. Anal. Appl. Pyrolysis 51 (1999) 23. S.C. Moldoveanu, Analytical Pyrolysis of Natural Organic Polymers, Elsevier, Amsterdam, 1998. S.C. Moldoveanu, Analytical Pyrolysis of Synthetic Organic Polymers, Elsevier, Amsterdam, 2005. S. Tsuge, H. Ohtani, C. Watanabe, Pyrolysis-GC/MS Data Book of Synthetic Polymers, Pyrograms, Thermograms and MS of Pyrolyzates, Elsevier, Amsterdam, 2011. S.C. Moldoveanu, Pyrolysis of Organic Molecules With Applications to Health and Environmental Issues, first ed., Elsevier, Amsterdam, 2010. R.F.C. Brown, Pyrolytic Methods in Organic Chemistry, Academic Press, New York, 1980. B. Zou, Pyrolysis, in: Z.L. Wang, Y. Liu, Z. Zhang (Eds.), Handbook of Nanophase and Nanostructured Materials Synthesis, Kluwer Academic/Plenum Publishers, Dordrecht, 2003. J.J. Santoleri, J. Reynolds, L. Theodore, Introduction to Hazardous Waste Incineration, Wiley, New York, 2000. W.J. Catallo, C.H. Kennedy, W. Henk, S.A. Barker, S.G. Grace, A. Penn, Environ. Health Perspect. 109 (2001) 965. G.I. Ksandopulo, L.I. Kopylova, Combust. Explos. Shock Waves 40 (2004) 535. M.H. Topal, J. Wang, Y.A. Levendis, J.B. Carson, J. Jordan, Fuel 83 (2004) 2357. B. Leckner, L.E. Amand, K. Lucke, J. Werther, Fuel 83 (2004) 477. L. Bartonova, Z. Klika, D.A. Spears, Fuel 86 (2007) 455. W.M. Baird, S.L. Ralston, Carcinogenic polycyclic aromatic hydrocarbons, in: G.T. Bowden, S.M. Fischer (Eds.), Comprehensive Toxicology Chemical Carcinogens and Anticarcinogens, vol. 12, Elsevier, New York, 1997, pp. 171–200. F. Toribio, R. Busquets, L. Puignou, M.T. Galceran, Food Chem. Toxicol. 45 (2007) 667. E. Tareke, P. Rydberh, P. Karlsson, S. Eriksson, M. T€ ornqvist, Chem. Res. Toxicol. 13 (2000) 517. S.C. Moldoveanu, A.R. Gerardi, J. Chromatogr. Sci. 49 (2011) 234. G. Barrefors, G. Petersson, Chemosphere 30 (1995) 1551. M.J. Kleeman, J.J. Schauer, G.R. Cass, Environ. Sci. Technol. 33 (1999) 3516. J.A. Prange, C. Gaus, R. Weber, O. P€apke, J.F. M€ uller, Environ. Sci. Technol. 37 (2003) 4325. X. Wang, P. Thai, M. Mallet, M. Desservettaz, D.W. Hawker, M. Keywood, B. Miljevic, C. Paton-Walsh, M.J. Gallen, J.F. Mueller, Environ. Sci. Technol. 51 (2017) 1293, https://doi.org/10.1021/acs.est.6b03503. Z. Yufeng, D. Na, L. Jihong, X. Changzhong, Renew. Energy 28 (2003) 2383. K.A. Magrini, R. Follett, J. Kimble, M.F. Davis, E. Pruessner, Soil Sci. 172 (2007) 659. S.J. Stotesbury, H. Digard, L.J. Willoughby, A. Couch, Beitr. Tabakforsch. Int. 18 (1999) 147. R.R. Baker, L.J. Bishop, J. Anal. Appl. Pyrolysis 71 (2004) 223. P.S. Cardona, A.K. Chaturvedy, J.W. Soper, D.V. Canfield, Forensic Sci. Intern. 157 (2006) 46.

S U B C H A P T E R

1.2

Chemical Reactions Occurring During a Pyrolytic Process

MAIN TYPE OF CHEMICAL REACTIONS IN PYROLYSIS Simple pyrolysis, not performed in the presence of special reagents or of catalysts, may involve a variety of simultaneous reactions with the formation of different fragment

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1. GENERAL INFORMATION ABOUT PYROLYSIS

compounds. The structure of these fragments depends on the initial compound (parent compound) and also on pyrolysis conditions. When pyrolysis is performed intentionally, these conditions are usually selected to diminish the variability of the results and to simplify the pyrolysis outcome. This can be achieved, for example, by selecting a precise temperature of pyrolysis, a precise heating rate, and a precise length of pyrolysis time. In analytical pyrolysis, the process is commonly performed in an inert atmosphere such as helium. In attempts to simulate a process such as burning, the pyrolysis is conducted in gases such as air or in mixtures of oxygen and nitrogen (or other inert gas) with a specific composition. The pyrolysis process can be more than the decomposition of one single molecular species because the products of the initial decomposition may undergo further reactions under the influence of heat. Also, the initial process may consist of a unique reaction or may have multiple reaction paths occurring simultaneously. Particularly for pyrolysis in solid or liquid phase, a variety of chemical interactions may occur. Even the cooling rate of the pyrolyzate may influence the pyrolysis outcome. In spite of the complexity of pyrolytic reactions, the main pyrolyzates are typically obtained by the cleavage of one or only a few bonds from the initial compound and of some of the pyrolysis fragments. Therefore, some resemblance between the initial molecule and the pyrolysis products is frequently obvious. This resemblance typically decreases when the pyrolysis is performed at higher temperatures and smaller fragments are generated. Besides the formation of the main pyrolyzate compounds, it is common that pyrolysis also generates a number of minor components. They are formed either by reaction paths different from the main ones, by further fragmentation of the initial pyrolysis products, or by reactions between fragments. The major and minor components of a pyrolyzate may form together a rather complex mixture. The whole molecular structure as well as particular functional groups may contribute to the outcome of the pyrolysis. Due to many factors influencing the pyrolysis result, there are considerable differences in the pyrolysis outcome, even within the same class of molecules. Sometimes, even the compounds from a homolog series may lead to different results. However, some specific molecular moieties are more susceptible to thermal decomposition (e.g., diazo, alcohol, etc.) and in such cases, the members of a homolog series may have a very similar behavior during pyrolysis. The decomposition mechanism plays a major role regarding the pyrolysis outcome. For example, when the formation of free radicals occurs, the result is typically more complex than in cases when a concerted mechanism is responsible for the changes during pyrolysis. The individual reaction types taking place during pyrolysis can be studied independently. The main types of chemical reactions taking place during pyrolysis can be classified as follows: (1) elimination reactions, (2) fragmentations, (3) rearrangements, and (4) other reaction types. However, during pyrolysis, the decomposition of the same molecular species may take place by several mechanisms, which adds to the complexity of the pyrolysis products. (1) Elimination reactions include α-eliminations, β-eliminations, 1,3-eliminations, and 1,neliminations. α-Eliminations involve two leaving groups connected to the same carbon (an α-carbon is the carbon that attaches a functional group). They are encountered in some pyrolytic reactions where the more common β-eliminations are not possible. βEliminations, with two groups lost from adjacent atoms, are the most common type. Such eliminations usually involve a heteroatom, but certain alkyl-aromatic hydrocarbons may

MAIN TYPE OF CHEMICAL REACTIONS IN PYROLYSIS

7

also undergo β-eliminations. 1,3-Eliminations and 1,n-eliminations are not very common, and they involve leaving groups from more distant atom positions. The common mechanisms in eliminations are either Ei or radicalic. In the Ei mechanism of a βelimination, two vicinal substituents on an alkane framework leave simultaneously via a cyclic transition state to form an alkene in a syn elimination. The cyclic transition state can be four, five, or six membered. The schematic of a β-elimination with Ei mechanism and the formation of a six-member transition state is indicated below:

ð1:2:1Þ

During pyrolytic reactions of the Ei type, if a double bond is present, the formation of a conjugate system is preferred if sterically possible. Otherwise, the orientation in the pyrolytic elimination is random and is determined by the number of β-hydrogens. The newly formed double bond goes more toward the least substituted carbon (Hofmann’s rule). In the bridged systems, the double bond is formed away from the bridgehead. Also, for the Ei mechanism in a planar molecule, a cis β-hydrogen is required. In cyclic systems, with a cis hydrogen on only one side of the molecule, the double bond will be formed from this hydrogen [1]. However, when there is a six-membered transition state, this does not necessarily mean that the leaving groups must be cis to each other because six-membered states are not completely coplanar. Because the axial (a) groups alternate in direction, two axial groups in β-position are always trans. Therefore, if the leaving group is axial (a), then the hydrogen must be equatorial (e) to the leaving group because the transition state cannot be realized when the groups are both axial (and trans to each other). When the leaving group is equatorial, it can form a transition state with a β-hydrogen that is either axial (cis) or equatorial (trans). A common type of mechanism found to operate in pyrolytic eliminations involves free radicals. First, an initiation occurs by pyrolytic cleavage, followed by propagation and termination with the result of molecular fragment formation. A schematic example of free radical elimination is presented as follows: Initiation R2 CH-CH2 X ! R2 CH-CH2  + X Propagation R2 CH-CH2 X + X ! R2 C -CH2 X + HX R2 C -CH2 X ! R2 C¼CH2 + X Termination 2 R2 C -CH2 X ! R2 C¼CH2 + R2 CX-CH2 X 2 X  ! X2 R2 CH-CH2  + X ! R2 CH-CH2 X

(1.2.2)

The main result of the reaction (excluding small amounts of other compounds resulting from termination reactions, and the potential formation of larger molecules) can be written as follows: R2 CH-CH2 X ! R2 C ¼ CH2 + HX

(1.2.3)

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1. GENERAL INFORMATION ABOUT PYROLYSIS

Free radical eliminations are frequent at temperatures between 600°C and 900°C. The initiation reaction typically takes place with a higher probability at the bonds with a lower energy. However, at higher temperatures, even bonds with higher energy are cleaved. The stability of the free radicals that are formed in these reactions also plays an important role in the initiation mechanism. Propagation reactions can have various paths. This explains in part the complexity of pyrolyzates, even from simple molecules such as aliphatic hydrocarbons that generate other aliphatic compounds as well as unsaturated and aromatic hydrocarbons (and also char). Frequently, pyrolysis takes place in several stages, with further decomposition of the compounds generated in the first stage. In general, propagation reactions with free radical formation, besides intermolecular reactions, may include intramolecular reactions. Molecules with longer chains may display β-scissions and radical back biting, which may generate various isomers. A radical back-biting process is indicated below: ð1:2:4Þ

Some reactions during pyrolysis have a higher probability than others, depending on the molecular structure of the initial compound and of the products. Also, the elimination reactions taking place with a free radical mechanism can be followed by rearrangements also involving free radicals. Less frequently, the mechanisms in eliminations can be of the E1 or E2 type. For these eliminations, a proton is pulled by a (Lewis) base and an X group departs simultaneously from the molecule. The differentiation E1 or E2 is based on the kinetics reaction order. (2) Fragmentations are reactions in which a molecule AB is broken into several smaller parts. Because the cleavage of a single two-electron bond A-B generates either ions or radicals (in some special cases, the cleavage generates carbenes or nitrenes, with these compounds typically suffering rearrangements), the formation of neutral new molecules A and B (with complete number of electrons) is not possible without the transfer of one part of molecule A to molecule B, or vice versa. Among fragmentations are the retro-ene reactions exemplified by reaction (1.2.5), retro Diels-Alder condensation exemplified by reaction (1.2.6), and retro aldol condensations exemplified by reaction (1.2.7).

ð1:2:5Þ

ð1:2:6Þ

9

MAIN TYPE OF CHEMICAL REACTIONS IN PYROLYSIS

ð1:2:7Þ

Extrusion reactions can also be considered a type of fragmentation. This type of reaction can be written schematically as follows: X-A-Y ! X-Y + A0

(1.2.8)

Deacarboxylation of β-unsaturated acids can be viewed as an extrusion reaction: ð1:2:9Þ

(3) Rearrangement reactions are also common in some pyrolytic reactions. Various types of rearrangements may take place at elevated temperatures, with the end result of these types of reactions usually being compounds more stable than the initial one (such that the reaction is thermodynamically favored). Among the rearrangement-type reactions that can be indicated: 1,2-migrations (exemplified by reaction 1.2.10), rearrangements in compounds with strained bond angles (e.g., in cyclopropane, cyclobutane), electrocyclic rearrangements, sigmatropic rearrangements, double bond migrations, etc. [2].

ð1:2:10Þ

One special type of rearrangement consists of steric changes. The steric changes may affect geometric isomerism (exemplified by reaction 1.2.11 for the change of trans-coniferyl alcohol into cis-coniferyl alcohol) or chiral isomerism (exemplified by reaction 1.2.12 for the equilibrium between (S)-nicotine and (R)-nicotine).

ð1:2:11Þ

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1. GENERAL INFORMATION ABOUT PYROLYSIS

ð1:2:12Þ

Chiral isomers do not differ energetically, and the reaction does not favor a specific enantiomer. (4) Other reaction types that may take place during pyrolysis include oxidation/reduction, substitutions, additions, Diels-Alder condensations, etc. [2].

ANALYTICAL PYROLYSIS IN THE PRESENCE OF ADDITIONAL REACTANTS OR WITH CATALYSTS Pyrolysis in the presence of additional reactants or with catalysts is common for industrial pyrolysis as well as for some specific types of pyrolysis that attempt to simulate processes such as fuel burning, smoking, etc. However, the use of reactants and/or catalysts during pyrolysis can also be useful for analytical purposes. For example, some compounds generated by pyrolysis have polar hydrogen atoms and are not volatile enough to be analyzed by GC/ MS. For enhancing the volatility of the pyrolyzate, a reagent can be added together with the sample such that during pyrolysis, the reagent will methylate the active hydrogens in the pyrolysis products. The most common reagent used for in situ methylation is tetramethylammonium hydroxide (TMAH) [3–6] although other methylating reagents can be used (e.g., tetramethylammonium acetate) [5,6]. Attempts to use in situ silylation (formation of trimethylsilyl derivatives of compounds with active hydrogens) has also been attempted although offline silylation of pyrolyzates is a more successful procedure for forming silyl derivatives [7]. Pyrolysis for analytical purposes in the presence of TMAH was frequently reported [8]. This book will discuss some aspects of pyrolysis in the presence of reagents such as H2O or O2 for individual classes of compounds. Also, some aspects regarding the pyrolysis in the presence of a catalyst will be presented.

PYROLYSIS OF MIXTURES OF COMPOUNDS The pyrolysis of mixtures of compounds is a complex problem, with numerous parameters being involved in determining the outcome of the process. Both physical parameters, such as experimental conditions in which pyrolysis takes place, and chemical parameters affect the pyrolytic outcome. The nature of the participating compounds as well as the pyrolysis mechanism are very important factors, and the result of pyrolysis is a combined effect of physical and chemical factors. Due to the complexity of the problem of pyrolysis of compound mixtures, a systematic approach of the subject is difficult and only occasional examples are discussed in this book.

11

THERMODYNAMIC ASPECTS OF THE PYROLYTIC PROCESS

References 1.2 [1] J. March, Advanced Organic Chemistry, Wiley, New York, 1992. [2] S.C. Moldoveanu, Pyrolysis of Organic Molecules With Applications to Health and Environmental Issues, first ed., Elsevier, Amsterdam, 2010. [3] J.M. Challinor, J. Anal. Appl. Pyrolysis 20 (1991) 15. [4] J.M. Challinor, J. Anal. Appl. Pyrolysis 61 (2001) 3. [5] T. Ohra-aho, J. Ropponen, T. Tamminen, J. Anal. Appl. Pyrolysis 103 (2013) 31. [6] F. Shadkami, R. Helleur, J. Anal. Appl. Pyrolysis 89 (2010) 2. [7] S.C. Moldoveanu, V. David, Sample Preparation in Chromatography, Elsevier, Amsterdam, 2002. [8] D.C. Waggoner, P.G. Hatcher, J. Anal. Appl. Pyrolysis 122 (2016) 289–293.

S U B C H A P T E R

1.3

Thermodynamic and Kinetic Aspects of the Pyrolytic Process

THERMODYNAMIC ASPECTS OF THE PYROLYTIC PROCESS A simple pyrolysis can be indicated by the following chemical reaction: Δ

X > bB + cC + dD + …

(1.3.1)

In reaction (1.3.1), X is a starting compound that forms the products B, C, D, …, under the influence of heat (indicated in equilibrium 1.3.1 by Δ). In thermodynamics, a chemical reaction can be characterized by the Gibbs free energy (or free enthalpy) G, which has the expression: G ¼ H  TS

(1.3.2)

where H is the enthalpy (H ¼ E + pV) measured in Joules J (SI units) and represents the sum of the system internal energy E and pV (p is pressure and V is volume), T is temperature measured in degrees Kelvin K, and S is the entropy measured in J/deg. (SI units). Because thermodynamic functions involving the internal energy are difficult to measure in an absolute amount, only the changes in these functions are typically of interest. For any change at constant temperature, expression (1.3.2) can be written in the form (where standard values are noted with 0 and are considered at 1 atm): ΔG0 ¼ ΔH0  TΔS0

(1.3.3)

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1. GENERAL INFORMATION ABOUT PYROLYSIS

Expression (1.3.3) indicates that a change in free enthalpy is equal to the change in enthalpy minus an energy part that is unavailable for work (captured as heat in a reversible process where ΔS ¼ ΔQ/T, and where ΔQ is the change in the heat of the system). Also, expression (1.3.3) shows that ΔG0 depends explicitly on temperature (in fact it also depends implicitly on temperature through ΔH0 and ΔS0 that show some changes with temperature, e.g., [1]). The variation in the free enthalpy accompanying chemical reactions is obtained from the difference in the sum of standard free enthalpies of the products and the sum of standard free enthalpies of the reactants, and for reaction (1.3.1), the change in free enthalpy is given by the expression: X ΔG0products  ΔG0X (1.3.4) ΔG0 ¼ all products

Similar formulas are valid for enthalpy and entropy, expressed for reaction (1.3.1) by the formulas: X 0 ΔH0 ¼ ΔHproducts  ΔHX0 (1.3.5) all products

ΔS0 ¼

X

ΔS0products  ΔS0X

(1.3.6)

all products

For all compounds X, B, C, … in gas phase, the free enthalpy expressed by formula (1.3.4) is related to the equilibrium constant K for the reaction (1.3.1). The formula for the equilibrium constant is the following: Kp ¼

pbB pcC pdD … pX

(1.3.7)

where pX is the partial pressure of component X, pB is the partial pressure of component B, etc. (expression 1.3.7 has an equivalent for solutions where pressures are replaced by activities [2]). The relation between Kp and ΔG0 is given by the formula (see e.g., [1]): log Kp ¼ 

ΔG0 RT

(1.3.8)

(R is the gas constant, R ¼ 8.31446 J/deg./mol ¼ 1.987 cal/deg./mol). When a chemical reaction reaches equilibrium, thermodynamic factors control the reaction outcome. The calculation of a free enthalpy for a chemical reaction allows the prediction of the reaction course, and the reactions are displaced toward the formation of products when ΔG0 < 0. For equilibrium, ΔG0 ¼ 0, the temperature satisfying this condition is defined as the ceiling temperature Tc. From expression (1.3.3), the formula for the ceiling temperature is the following: Tc ¼

ΔH0 ΔS0

(1.3.9)

The ceiling temperature Tc can be considered the temperature at which a pyrolytic process will reach equilibrium. It may be seen, therefore, as a recommended temperature for pyrolysis. The values for Tc can be obtained using experimental values for ΔH0 and ΔS0 when these are known, or only estimated values based on the expressions (1.3.5), (1.3.6) for ΔH0 and ΔS0.

BOND DISSOCIATION ENERGY FROM THERMOCHEMICAL DATA

13

For the molecules involved in these expressions, approximation formulas and experimental data are available for the calculation of ΔH0 and ΔS0 (see e.g., [3]). Such approximation formulas are based, for example, on the additive fragments contributions and corrections due to structural factors (see e.g., [3–5]). Although in principle the above theory should hold true for any system, its application to condensed phases may be accompanied by effects difficult to account for, such as phase change, melting, cage effects [1], etc. Also, while pyrolysis at Tc assures only Kp ¼ 1, usually a close to complete decomposition of compound X is desired. For this reason, temperatures at least 100°C higher than Tc frequently must be used as practical values of the temperature in the pyrolysis of a specific compound. At higher temperatures, the yield of pyrolysis products is higher, which can be an advantage, but also additional chemical reactions may occur such as thermal decomposition of the fragments initially formed from the compound X. For this reason, in practice the selection of the temperature for pyrolysis is frequently made empirically, for example based on previous experiments or based on similarity with a known pyrolysis process.

BOND DISSOCIATION ENERGY FROM THERMOCHEMICAL DATA Evaluation of bond dissociation energies is a typical application of thermochemical information related to the prediction of pyrolysis outcome. Although the fragmentation in pyrolysis does not take place only at the weaker bond, the weaker ones typically have a higher probability of breaking. The information on bond dissociation energy may indicate which bond is more likely to break in a molecule and can be used to estimate the temperature range where the process occurs. Bond energy is defined as the energy required to separate an isolated molecule (i.e., in gas phase) after the cleavage of a bond into two fragments (atoms or radicals) at infinite distance. A reaction with bond dissociation and the formation of free radicals can be written as follows: XY!X + Y

(1.3.10)

where indicates a free radical. In practice, instead of bond energy ΔE, the value of bond enthalpy ΔH is measured. The standard enthalpy of a bond is the enthalpy required to break the bond (usually at 25°C and 1 atm). Assuming the initial molecule and its fragments to be perfect gases, the following formula can be written for the difference between enthalpy and energy: X pV  pVXY ¼ Δn RT (1.3.11) ΔH  ΔE ¼ •

fragments

where Δn is the variation in the number of moles during fragmentation. The variation in the number of moles with the formation of two fragments is Δn ¼ 1, and for 298.1°C, ΔH0 ΔE0 ¼ 2.4789 kJ/mol ¼ 0.592 kcal/mol. True bond energy should be taken at 0 K and not at 298.3 K, which also introduces a small difference between bond energy and bond enthalpy. However, these differences are very small compared to the value of the bond dissociation energy, and, in general, the bond dissociation energy is taken as equal to the negative value for the enthalpy ΔH0XY (heat) of formation of the bond X Y. This value is considered a direct measure

14

1. GENERAL INFORMATION ABOUT PYROLYSIS

of the bond strength (It should be noted that because no work is performed in this process, the variation in enthalpy ΔH is equal to the variation in heat ΔQ). The enthalpy of bond dissociation is given by the expression: 0 0 ¼ ΔHX0  + ΔHY0   ΔHXY ΔHXY

(1.3.12)

Several average bond energies (enthalpies evaluated at 25°C) for different bond types are given in Table 1.3.1. In addition to the nature of the atoms forming the bond, the bond strength depends on the rest of the molecule. The values from Table 1.3.1 give only an estimate for a specific bond strength. More precise bond dissociation energies can be given when the nearest neighbors of the dissociating bond are specified, and such values are reported in the literature [1]. TABLE 1.3.1

Average Bond Energies (Enthalpies at 298.15 K) in kcal/mol

Single Bonds

ΔH0 (kcal/mol)

Single Bonds

ΔH0 (kcal/mol)

Multiple Bonds

ΔH0 (kcal/mol)

HdH

104.2

BdF

150

C]C

146

CdC

83

BdO

125

N]N

109

NdN

38.4

CdN

73

O]O

119

OdO

35

NdCO

86

C]N

147

FdF

36.6

CdO

85.5

C]O (CO2)

192

SidSi

52

OdCO

110

C]O (aldehyde)

177

PdP

50

CdS

65

C]O (ketone)

178

SdS

54

CdF

116

C]O (ester)

179

CldCl

58

CdCl

81

C]O (amide)

179

BrdBr

46

CdBr

68

C]O (halide)

177

IdI

36

CdI

51

C]S (CS2)

138

HdC

99

CdB

90

N]O (HONO)

143

HdN

93

CdSi

76

P]O (POCl3)

110

HdO

111

CdP

70

P]S (PSCl3)

70

HdF

135

NdO

55

S]O (SO2)

128

HdCl

103

SdO

87

S]O (DMSO)

93

HdBr

87.5

SidF

135

P]P

84

HdI

71

SidCl

90

P^P

117

HdB

90

SidO

110

CO

258

HdS

81

PdCl

79

C^C

210

HdSi

75

PdBr

65

N^N

226

HdP

77

PdO

90

C^N

213

KINETIC FACTORS IN PYROLYTIC REACTIONS

15

For example, when the dissociating bond is in conjugation with a π electron system, the dissociation energy must be decreased with about 16 kcal/mol for a phenyl ring and with about 10 kcal/mol for a double bond. Several other procedures are reported in the literature for the estimation of bond energies besides the use of expression (1.3.12) with known values for the heats of formation of the participating molecule and free radicals. Among these procedures are those based on fitting a Morse potential [6,7] based on infrared (IR) stretching frequencies data [8], using kinetic reaction rate values [9] based on ionization potentials of radicals, or using empirical correlations between a specific molecular property and the distance between the atoms in the molecule [10]. Direct evaluation of bond energies is also possible based on the electron population analysis (electron density between atoms) [11–14].

KINETIC FACTORS IN PYROLYTIC REACTIONS The reaction rate of a chemical process where X is a reactant and Y is one of the products is defined as the variation of the concentration of X or of Y versus time. The reaction rate can be expressed using the following formula: 

d½X d½Y ¼ dt dt

(1.3.13)

(where square brackets indicate molar concentration). For the reaction rate depending only on the concentration of species X, the reaction rate is described by the formula: 

d½X ¼ k½X  dt

(1.3.14)

and the kinetic of the reaction is indicated as of the first order (the rate constant k is measured in s1). It is possible that the rate of a reaction depends on the concentrations of the X and Y reactant species simultaneously, with this type of reaction following second-order kinetics. Note that the rate constant k, in this case, has different units from those of the rate constant for the first-order kinetics. Some chemical reactions have a reaction rate of the form: 

d½ X  ¼ k½Xn dt

(1.3.15)

where the value of n is indicated as reaction order, and can be an integer or a fraction. Information on the kinetics of many reactions is available in the literature (see e.g., [15]). Many kinetic data are published in dedicated journals, (e.g., Journal of Physical and Chemical Reference Data). For chemical reactions, including pyrolysis, that involve only one starting molecular species, it can be assumed that the process can take place only when the molecules collide. Following this collision, an intermediate state called an activated complex is formed. The difference in the energy of the isolated reactants and the energy of the activated complex, which is the maximum energy that the system should pass through to form the products, is indicated as the activation energy ΔE# (the symbol Δ for the difference is sometimes

16

1. GENERAL INFORMATION ABOUT PYROLYSIS

neglected). This activation energy is related to the constant k in expression (1.3.14) or (1.3.15) by the following formula:   E# (1.3.16) k ¼ A exp  RT In formula (1.3.16), A is a parameter (expressed in s1 for first-order kinetics) related to the frequency of collisions during the reaction and is indicated as frequency factor. Expression (1.3.16) is known as the Arrhenius reaction rate equation, and it shows how the reaction rate depends explicitly on temperature and the activation energy of the reaction (although showing less variation with T, both A and E# are not strictly temperature independent [16]). For many pyrolytic reactions, the values for the activation energies E# and of the frequency factor A are not known. Such values can be obtained, for example, from thermogravimetric studies (see e.g., [17]). For a pyrolytic reaction with first-order kinetics in which a compound X is decomposed in gaseous fragment molecules, instead of the molar concentration [X] in Eq. (1.3.14), the weight of the sample W can be used (e.g., by multiplying [X] with molecular weight and the volume of the sample). In this case, formula (1.3.14) can be written in the form:   dW E# W (1.3.17) ¼ k W ¼ A exp  RT dt The integration of relation (1.3.17) for the interval of time 0 to tf (final or total time of pyrolysis) will give the following expression:     Wf E# (1.3.18) tf ¼ exp A exp  W0 RT Expression (1.3.18) indicates the dependence of the final weight Wf of the sample as a function of pyrolysis temperature T, final pyrolysis time tf, activation energy E#, frequency factor A, and initial sample weight W0. An exemplification of the results for Eq. (1.3.18) is given in Fig. 1.3.1A for the dependence of residual weight fraction Wf/W0 (in %) on temperature when tf ¼ 5 s the frequency factor A ¼ 1000 s1 and E# has three different values. Fig. 1.3.1B shows the dependence of Wf/W0 (in %) on total pyrolysis time tf for E# ¼ 12.5 kcal/mol, A ¼ 1000 s1, and T ¼ 500°C. In many pyrolytic reactions, not all pyrolysis products are volatile and a residue with the weight Wr remains after pyrolysis (e.g., in the form of char). In such cases, the variation of sample weight during pyrolysis can be approximated with the formula:     Wf  Wr E# (1.3.19) tf ¼ exp A exp  W0  Wr RT For a 99% of the initial loss Wf/W0 ¼ 0.01 (no residue assumed), expression (1.3.18) can be used to determine a kinetically required temperature Tk for a specific pyrolysis. This temperature (in K) is given by the expression: E#  Tk ¼  R ln tf A  1:527

(1.3.20)

KINETIC FACTORS IN PYROLYTIC REACTIONS

(A)

17

(B)

FIG. 1.3.1 Exemplification of the dependence of Wf/W0% on temperature T for three different values of E# (graph A) and on pyrolysis total time tf (graph B).

Both the ceiling temperature Tc and the kinetically required temperature Tk are seldom calculated, and the temperature for pyrolysis is usually selected empirically. However, the estimation of Tk may be necessary for the case of pyrolysis processes known to have large E# values or when the pyrolysis time tf is intended to be very short. The study of both thermodynamic and kinetic factors in pyrolysis can be performed using much more elaborate models. Various such studies can be found in the dedicated literature (see e.g., [3]). The kinetics equation of the type described by expression (1.3.17) is commonly applied for describing the overall reaction kinetics during pyrolysis. However, this equation provides only an approximation for a case in which the process is not composed of a single reaction [18]. The pyrolysis of solid samples is usually a complicated process, and rel. (1.3.17) may lead to erroneous results. The simpler relations valid for the kinetics in homogeneous systems do not fit well with the experimental data for solid samples. Factors related to heterogeneous reactions must be taken into account in this case. A series of models has been developed for a better description of the process and can be found in the dedicated literature [19–21]. An empirical approach [22] for describing the kinetics of the pyrolytic reactions in a solid state is to use a parametric equation that includes formulas for all possible categories of kinetics mechanisms known to occur for the chemical reactions of solid samples. Considering the conversion α ¼ 1  W/W0, which is the mass fraction of the reacted substance at the time t, the empirical kinetics equation for heterogeneous systems can be expressed in the general form: dα ¼ k f ðαÞ dt

(1.3.21)

where k ¼ rate constant given by Arrhenius equation and f (α) ¼ function that can be chosen of the form: f ðαÞ ¼ ð1  αÞn αm ½ ln ð1  αÞr

(1.3.22)

18

1. GENERAL INFORMATION ABOUT PYROLYSIS

The terms in Eq. (1.3.22) attempt to describe the reaction rate controlled by the movement of the phase boundary, diffusion, nucleation in solid state, etc., and different values (including zero values) for m, n, and r were proposed in the literature [23].

KINETICS OF GAS PHASE PYROLYTIC REACTIONS In the condensed phase, the pyrolysis process is typically complicated, sometimes involving the reaction between two molecules of the same species. However, in the gas phase, the initial step of pyrolysis can be truly a unimolecular reaction (see e.g., [24]). In order for molecule A to undergo a fragmentation of the type: A!B+C

(1.3.23)

it is necessary that the molecule achieves a sufficient level of energy. In the gas phase, this energy is generated by inelastic collisions, either with other A molecules or with an inert molecule M (Lindemann theory). Labeling as M either the same or a different (inert) molecule, the collision process can be described as follows: k1

! A + M    A∗ + M

(1.3.24)

k2

where A* designates the molecule with sufficient energy to dissociate, k1 is the reaction rate constant for the forward reaction, and k2 is the reaction rate of the deexcitation process. Some of the A* molecules will react, generating the products shown below: k3

A∗ ! B + C

(1.3.25)

For gas components, the partial pressures pA, pB, … can be related to molar concentrations by the expression: pA ¼ RT [A], pB ¼ RT [B], … and the variation in the concentration of B can be described by the equation: d½B d½A ¼ ¼ k3 ½A ∗  dt dt

(1.3.26)

On the other hand, the change in the concentration of A* obtained based on reactions (1.3.24) and (1.3.25) is described by the equation: d½A∗  ¼ k1 ½A½M  k2 ½A∗ ½M  k3 ½A∗  dt

(1.3.27)

A  In stationary state conditions, the concentration of [A*] does not change, d½dt ¼ 0 and from expression (1.3.27), the following expression can be written: ∗

½A∗  ¼

k1 ½A½M k2 ½ M  + k3

(1.3.28)

Expression (1.3.28) for [A*] can be introduced in Eq. (1.3.26) to give: d½A k1 k3 ½M  ½A ¼ dt k2 ½M + k3

(1.3.29)

KINETICS OF GAS PHASE PYROLYTIC REACTIONS

19

From expression (1.3.29), an “effective unimolecular rate coefficient” kuni can be defined by the formula: kuni ¼ 

k1 k3 ½M  k2 ½ M  + k3

(1.3.30)

This effective rate coefficient kuni is dependent on the concentration (pressure) of the participating gases (except for some limiting cases). When molecules M are those of the reacting gas, [M] ¼ [A] and kuni are dependent on the pressure of A (reacting gas). In a limiting case in formula (1.3.30), k2 [M] is much higher than k3 (the deactivation process is much more important then the decomposition) and k3 is neglected in the denominator. Eq. (1.3.30) is reduced in this case to the following expression: d½A k1 k3 ½A ¼ k2 dt

(1.3.31)

Expression (1.3.31) indicates that at a given high pressure of the gas undergoing a unimolecular reaction, the kinetic of the reaction is of the first order and the change in concentration depends linearly on the pressure of the gas. The rate constant in this case is known as the “limiting high pressure rate constant” k∞, where: kuni ¼ k∞ ¼

k1 k3 k2

(1.3.32)

The other limiting case is obtained when the value of k2 [M] in expression (1.3.30) is much smaller than k3 and the reaction becomes of the second order: d½A ¼ k1 ½M½A dt

(1.3.33)

The reaction rate k1 is indicated in this case as a “low-pressure” rate constant k0, and the reaction is of the second order. The values of gas pressures where the reaction rate kuni becomes k∞ are dependent on temperature. An example of expected variation of kuni with the gas pressure (at a given temperature) is depicted in Fig. 1.3.2. The pressure of gas necessary to reduce the rate constant to one half its limiting high-pressure value k∞ is indicated as “falloff pressure” p1/2, and the region between the high-pressure limit and the low-pressure limit is indicated as the “falloff region.” At pressures intermediate to the high- and low-pressure limits (in the falloff region), the rate constant can be estimated by the formula (where M and A can be the same type of molecules): kuni ¼ 

k0 k∞ ½M k0 ½M + k∞

(1.3.34)

This formula can be modified to generate a more accurate description of the values of kuni in the falloff region by using the “Troe” parameters [25]. Expression (1.3.34) has significant practical applications in the industrial pyrolysis of gases.

20

FIG. 1.3.2

1. GENERAL INFORMATION ABOUT PYROLYSIS

Expected variation of k with the gas pressure at a fixed temperature in a unimolecular reaction.

References 1.3 [1] S.C. Moldoveanu, Analytical Pyrolysis of Synthetic Organic Polymers, Elsevier, Amsterdam, 2005. [2] S.C. Moldoveanu, V. David, Sample Preparation in Chromatography, Elsevier, Amsterdam, 2002. [3] S.C. Moldoveanu, Pyrolysis of Organic Molecules With Applications to Health and Environmental Issues, first ed., Elsevier, Amsterdam, 2010. [4] S.W. Benson, Method for the Estimation of Thermochemical Data and Rate Parameters, Wiley, New York, 1976. [5] J.B. Pedley, R.D. Naylor, S.P. Kirby, Thermodynamic Data of Organic Compounds, Chapman and Hall, London, 1986. [6] H.B. Schlegal, S. Wolfe, F. Bernardi, Can. J. Chem. 53 (1975) 3599. [7] E.S. Apostolova, A.V. Tulub, J. Struct. Chem. 38 (1997) 212. [8] D.C. Nonhebel, J.C. Walton, J. Chem. Soc. Chem. Commun. (1984) 731. [9] E.T. Denisov, V.E. Tumanov, Usp. Khim. 74 (2005) 905. [10] A. Cherkasov, M. Jonsson, J. Chem. Inf. Model. 40 (2000) 1222. [11] G.V. Gibbs, F.C. Hill, M.B. Boisen, R.T. Downs, Phys. Chem. Miner. 25 (1998) 585. [12] D. Stalke (Ed.), Electron Density and Chemical Bonding II: Theoretical Charge Density Studies, Springer, Hedelberg, 2012. [13] S. Moldoveanu, A. Savin (Eds.), Aplicatii in Chimie ale Metodelor Semiempirice de Orbitali Moleculari, Academiei RSR, Bucuresti, 1980. [14] E.G. Lewars, Computational Chemistry, third ed., Springer, Heidelberg, 2016. [15] NIST, n.d., http://kinetics.nist.gov (Accessed 26 February 2018). [16] M.L. Poutsma, Energy Fuels 4 (1990) 113. [17] S.R. Dharwadkar, M.D. Kharkhanavala, Thermal Analysis, vol. 2, Academic Press, New York, 1969. [18] J.A. Moens, G.F. Froment, J. Anal. Appl. Pyrolysis 3 (1981/1982) 187. [19] S.C. Moldoveanu, Analytical Pyrolysis of Natural Organic Polymers, Elsevier, Amsterdam, 1998. [20] R. Font, I. Martin-Gullon, M. Esperanza, A. Fullana, J. Anal. Appl. Pyrolysis 58–59 (2001) 703. [21] P. Mousques, J.L. Dirion, D. Grouset, J. Anal. Appl. Pyrolysis 58–59 (2001) 733. [22] V. Swaminathan, N.S. Madhavan, J. Anal. Appl. Pyrolysis 3 (1981) 131. [23] J. Sestak, G. Berggren, Thermochim. Acta 3 (1971) 1. [24] R.N. Rogers, L.C. Smith, Anal. Chem. 39 (1967) 1024. [25] W.C. Gardiner Jr., J. Troe, In: W.C. Gardiner Jr. (Ed.), Combustion Chemistry, Springer-Verlag, New York, 1984 (Chapter 4).

PARAMETERS THAT CHARACTERIZE ANALYTICAL PYROLYSIS

21

S U B C H A P T E R

1.4

Instrumentation Used in Analytical Pyrolysis

PARAMETERS THAT CHARACTERIZE ANALYTICAL PYROLYSIS Analytical pyrolysis is typically performed in “flash mode” and takes place in an inert atmosphere, such as helium. In most analytical pyrolyzers, the inert gas flows through the pyrolysis chamber and carries the pyrolysis products to the analytical instrument (e.g., a GC/ MS system). Flash pyrolysis is intended to rapidly reach a specific isothermal temperature and is performed with a fast rate of temperature increase (on the order of 10–20°C/ms). After the final pyrolysis temperature is attained, the temperature is essentially constant (isothermal pyrolysis) for a specific period of time. The targeted isothermal condition for flash pyrolysis is referred to as equilibrium temperature (Teq) or final pyrolysis temperature. Most of the pyrolytic process is assumed to take place at Teq. Besides Teq, another important parameter to be selected in analytical pyrolysis is the temperature rise time (TRt). This parameter measures the time necessary for the heating element of the pyrolyzer to reach Teq. TRt depends on Teq and the rate of temperature increase β or heating rate (in °C/ms). Temperature rise time TRt  (Teq  T0)/β where T0 is the initial temperature (°C) of the pyrolyzer. During the temperature rise period (TRt), the sample is exposed to lower temperatures. When the increase in temperature is linear, its variation during the TRt can be expressed by the formula:   (1.4.1) T ° C ¼ T0 + βt where t is time (in ms). The heating rate β is a parameter that can be set in some types of pyrolyzers. However, the actual heating rate of the actual sample is typically slower than the heating rate set for the instrument. The actual sample heating depends on criteria such as the mass of the sample, the construction of heating components, the sample container, etc., that consume heat for increasing the temperature. Because different temperatures may favor the formation of different compounds, TRt is important because the sample is exposed to different temperatures during TRt. Exposure to different temperatures may favor a different path of decomposition, and the elimination of such differences is highly desirable. An exemplification of the variation of residual fraction of sample weight W/W0 as a function of temperature for 2.0 s heating time of two hypothetical reactions (one with A ¼ 100 s1 and E# ¼ 10 kcal/mol and the other with A ¼ 12,000 s1 and E# ¼ 18.5 kcal/mol) is shown in Fig. 1.4.1.

22

1. GENERAL INFORMATION ABOUT PYROLYSIS

FIG. 1.4.1

Variation of residual mole fraction W/W0 as a function of temperature for 2 s heating time of two hypothetical reactions with different A and E# values.

For a pyrolysis that targets Teq ¼ 800°C when the heating takes place slowly and the sample is kept at temperatures below 600°C for a longer time, Fig. 1.4.1 shows that the reaction with A ¼ 100 s1 and E# ¼ 10.0 kcal/mol is favored. For a rapid heating time that reaches Teq ¼ 800°C very fast, the other reaction is favored. Therefore, differences in the composition of pyrolyzates may occur for different heating rates (usually expressed in °C/ms). The goal in flash pyrolysis is to have a very short TRt such that the decomposition of the sample takes place virtually in isothermal conditions at Teq. The isothermal conditions are usually easier to reproduce than a specified variable heating. However, even isothermal requirements may not always be simple to achieve for large samples or in pyrolyzers of specific construction because it takes time for the sample (and sample holder) to reach Teq. The sample is pyrolyzed for a specific period of time indicated as total pyrolysis time (or total heating time THt). THt includes TRt and the time the sample is kept at Teq. The total heating time THt used for pyrolyzing the sample is also a parameter that must be set for most pyrolyzers. The THt value may determine if the whole sample is pyrolyzed or not, and for certain samples if additional reactions take place after the initial decomposition because the sample is heated too long. Times in the range of 10–20 s are common for THt. However, the true time duration for heating the sample depends on the flow conditions in the pyrolytic chamber. Most pyrolysis systems are connected on line with a GC/MS system, and a gas (e.g. He) flows through the pyrolytic chamber sweeping the pyrolysis products toward the GC/MS system. The true residence time of the sample at Teq temperature may be much shorter than TRt, affecting significantly the pyrolysis outcome. Besides Teq, TRt, and THt, several other parameters may influence the pyrolysis outcome. One of these parameters is the amount of sample taken for pyrolysis. It is common in analytical pyrolysis to have a small amount of sample (between 0.1 and 2–3 mg). Also, depending on the pyrolyzer construction, it may be required that certain zone temperatures of the pyrolyzer are set to avoid, for example, condensation of the pyrolysis products (for example, the housing temperature Thou). Also, certain times are selected for equilibrating the sample. For example, for a filament pyrolyzer (a pyrolyzer where the heating of the sample is achieved using a heated filament) among the parameters to be selected are the time used to purge the sample with an inert gas before pyrolysis and the postpyrolysis time necessary for all the pyrolysis products to be transferred to the analytical instrument.

EVAPORATION OF SAMPLE BEFORE PYROLYSIS

23

Besides flash pyrolysis, other types of analytical pyrolysis are known. One example is fractionated pyrolysis (or step-wise pyrolysis), in which the same sample is pyrolyzed at different temperatures (in increasing order) for different times in order to study special fractions of the pyrolyzate. Temperature-programmed pyrolysis is another special type in which the sample is heated at a controlled rate β within different temperature ranges.

EVAPORATION OF SAMPLE BEFORE PYROLYSIS Heating of a sample that has a boiling point lower than the temperature where decomposition starts will produce rapid sample vaporization. The flow of inert gas through the pyrolysis system may carry the undecomposed compound into the analytical instrument, indicating that the compound does not decompose completely at the selected temperature. This effect is exemplified in Fig. 1.4.2 for a hypothetic case of two pyrolytic decompositions, and assuming a linear rate of sample evaporation. FIG. 1.4.2 Variation of residual weight fraction W/W0 for three processes: pyrolysis with A ¼ 500 s1 and E# ¼ 10.5 kcal/mol, evaporation with a linear rate, and pyrolysis with A ¼ 12,000 s1 and E# ¼ 20.0 kcal/mol.

FIG. 1.4.3

Contribution of evaporation versus pyrolysis of volatile compounds with low activation energy for pyrolysis at the true temperature of the sample.

24

1. GENERAL INFORMATION ABOUT PYROLYSIS

FIG. 1.4.4 Contribution of evaporation versus pyrolysis of volatile compounds with high activation energy for pyrolysis at the true temperature of the sample.

The graphs from Figs. 1.4.3 and 1.4.4 show the contribution of each process at a specific temperature. For the process having E# ¼ 10.5 kcal/mol, pyrolysis plays a more important role than evaporation while for the compound with E# ¼ 20.0 kcal/mol, evaporation is more important, and the result of the heating of the sample is a large amount of undecomposed compound. For all volatile samples, the evaluation of decomposition by pyrolysis must take into consideration this effect. The pyrolyzate in such cases will be formed from both pyrolysis products and the initial compound that was transferred in the pyrolyzate by distillation. The proportion of evaporation versus pyrolysis for volatile compounds is strongly influenced by the heating rate (β). In order to diminish evaporation for a pyrolysis process with high activation energy, a fast heating is necessary. For a volatile compound that is easily decomposed, the diminishing of evaporation can be achieved by selecting a lower pyrolysis temperature. For nonvolatile samples, the evaporation or boiling does not affect the result of heating, but part of the material may remain undecomposed if the pyrolysis time is not sufficiently long. A considerable number of small molecules suffer both evaporation and decomposition during heating. In order to study only the pyrolysis for volatile compounds, this should be performed in a sealed container in static conditions. Even in such cases, part of the initial material to be pyrolyzed may remain undecomposed.

THE CHOICE OF PARAMETERS FOR ANALYTICAL PYROLYSIS The specific parameters for pyrolysis are set mainly depending on the pyrolysis purpose. Equilibrium temperature Teq depends on the objectives of performing analytical pyrolysis, which are the following: (1) the evaluation of all pyrolysis products of a specific compound, (2) the chemical identification of a material that it is not amenable for other analytical techniques, (3) the identification of the compounds generated at a specific temperature from a material, (4) simulation of a specific pyrolytic process (e.g., cigarette smoking), and (5) desorption of compounds retained on a given support when the pyrolysis instrument is used as a desorber and not for pyrolysis purposes. (1) For the evaluation of all pyrolysis products of a specific compound, the equilibrium pyrolysis temperature Teq must be selected higher than the ceiling temperature Tc (see

THE CHOICE OF PARAMETERS FOR ANALYTICAL PYROLYSIS

(2)

(3)

(4)

(5)

25

expression 1.3.9) and higher than the kinetically required temperature Tk (see expression 1.3.20). For verifying a complete decomposition, several trials can be made, usually in the range of 600–900°C. Higher equilibrium temperatures Teq typically generate more decomposition products, but the composition of the pyrolyzate may also change with the temperature. This takes place because either depending on the temperature the pyrolysis takes different decomposition paths, or because the fragments initially formed continue the decomposition process, or both. In some instances, pyrolysis of the same material can be performed at a number of set temperatures, and the resulting pyrolyzates can be compared. Also, the thermograms of the analyzed material may be used as guidance for selecting the appropriate temperature for pyrolysis (see e.g., [1,2]). The knowledge of the pyrolysis products of a given compound (or material) may be necessary for the assessment of toxicity or of the impact on the environment of the pyrolyzate. Analytical pyrolysis with the goal of identifying unknown compounds or materials (sometimes composite materials) is usually applied when the compound subject to pyrolysis cannot be analyzed by other analytical techniques, either chromatographic or spectroscopic. This is the case of certain insoluble and nonvolatile compounds, polymers, and even composite materials. The analysis of pyrolyzate can be done using an analytical instrument hyphenated with the pyrolyzer such as a GC/MS. Some analytical pyrolyzers also have the capability to collect the pyrolyzate on a cold trap or on a trap containing an adsorbing material (such as Tenax-TA, Porapak, etc.). The trap is “online” with the analytical instrument. From the cold trap or adsorbing material, the pyrolyzate can be desorbed almost instantaneously using rapid heating. This procedure allows the use of a longer time for the sample decomposition (e.g., using long TRt) with accumulation of the pyrolyzate in the trap while the transfer to the analytical instrument is made as a narrow injection. The temperature used for pyrolysis when the goal is compound identification is typically selected such that maximum structural information is obtained about the sample, and when this is not known, Teq is selected high enough to assure enough sample decomposition. Temperatures in the range of 600–900°C are commonly selected. For comparison with the results reported in the literature, the same temperature as utilized in the literature must be applied to the sample. In some cases, TIC (total ion chromatogram) can be used as a fingerprint for a specific material (see e.g., [1]). The analysis of compounds generated by pyrolytic decomposition at a specific temperature is a common use of analytical pyrolysis. Knowing the temperature at which a material or compound is going to be heated is frequently important for practical purposes to know if any decomposition takes place, and if it takes place, which compounds are generated. In such cases, the Teq is set equal or slightly higher than the temperature of interest. For the simulation of a specific pyrolytic process, the Teq is selected based on experimental data for the process to be simulated. It is common that simulation of a pyrolytic process requires a different protocol than flash pyrolysis, for example with several heating ramps and specific heating times. The desorption of compounds retained on a given support (e.g., poly(2,6-diphenyl-pphenylene oxide) or Tenax-TA, alumina, silica), when the pyrolyzer is used as a desorber should be performed at a temperature Teq high enough to allow the desorption, but where the support is perfectly stable (e.g., for Tenax-TA not higher than 350°C).

26

1. GENERAL INFORMATION ABOUT PYROLYSIS

COMMON PYROLYSIS INSTRUMENTATION USED FOR ANALYTICAL PYROLYSIS A variety of instruments are used to perform analytical pyrolysis. In these instruments, the pyrolysis is performed, setting the desired parameters (Teq, TRt, THt, etc.). Typically, the pyrolysis products are further transferred into an analytical instrument (e.g., a GC/MS). The main types of pyrolyzer instruments are (1) filament pyrolyzers, (2) oven pyrolyzers, (3) Curie point pyrolyzers, and (4) other types. (1) Filament pyrolyzers are based on heating the sample with a coil or ribbon filament that can reach the desired pyrolysis temperature when electrical current from a controller is passed through it. The sample holder is usually a quartz tube or boat where the small sample is deposited. The filament can be wrapped directly around the quartz tube containing the sample, and the whole assembly is placed in a housing and heated to avoid condensation. It is very common that pyrolysis is performed in an inert atmosphere such as a flow of helium (other gases can be used). The helium can also be used as a carrier gas flowing to the measuring instrument (e.g., GC/MS). When the pyrolysis atmosphere is selected to be different from the carrier gas, after pyrolysis is performed, the desired carrier gas can be used for flushing the pyrolysis products to the analytical instrument (e.g., a GC). The schematic diagram of a simple filament pyrolyzer is shown in Fig. 1.4.5. The temperature of the sample is controlled depending on the electric current passing through the filament. The heat is generated in accordance with Joule’s law for a resistive conductor:   (1.4.2) Q ¼ I 2 Rt ¼ V 2 t =R The electric current source has the intensity of the current initially boosted at high levels to achieve rapid heating, and then decreased to maintain the desired temperature. The filament pyrolyzers may reach temperatures as high as 1100°C with a heating rate of 20°C/ ms. In order to obtain a correct Teq, modern equipment uses a feedback controlled temperature system (see e.g., [3]). Several other procedures for precise temperature control of the filament are available, such as the use of optical pyrometry or thermocouples [4,5]. With the special construction of a pyrolyzer of the type indicated in Fig. 1.4.3, the

FIG. 1.4.5

The simplified scheme of a pyrolyzer (based on the design of a heated filament system made by CDS Inc.). Heated housing is set at a Thou temperature.

COMMON PYROLYSIS INSTRUMENTATION USED FOR ANALYTICAL PYROLYSIS

27

heated housing of the pyrolyzer must be set at a Thou such that condensation on the wall of the chamber is avoided. Filament-heated pyrolyzers with a different construction are also commercially available. For example, the heating can be achieved with a filament at the exterior of an additional quartz tube in which the tube containing the sample is placed. The schematic drawing of such a pyrolyzer is shown in Fig. 1.4.6. The pyrolysis chamber in the pyrolyzer shown in Fig. 1.4.6 is bypassed by the flow of gas going to the analytical instrument (e.g., a GC/MS) when the sample is loaded. During pyrolysis, the chamber is included in the gas circuit using the switch valves. After pyrolysis, the gas is switched back to bypass the pyrolysis chamber and the pyrolyzed sample is discarded. The loading of the sample in the pyrolyzer shown in Fig. 1.4.6 can be done automatically using an autosampler. Instruments having a trapping capability between the pyrolyzer and the analytical instrument are also commercially available. (2) Oven pyrolyzers (or furnace pyrolyzers) are devices used in both flash pyrolysis and slow gradient pyrolysis. For flash pyrolysis, the common principle of use is to keep the furnace at the desired temperature and to suddenly introduce the sample into the furnace. The heating of the furnace can be done using electrical heating, which is controlled using thermocouples and feedback systems to maintain the correct temperature. An inert gas flow is commonly passed through the furnace to sweep the pyrolysis products into the analytical instrument. For analytical purposes, small furnaces with low dead volumes are used. Several designs were used for furnace pyrolyzers, with a successful one being a vertical furnace that allows the sample to be dropped from a cool zone into a heated zone (see e.g., [6]). A simplified diagram of a microfurnace pyrolyzer is shown in Fig. 1.4.7. In the pyrolyzer shown schematically in Fig. 1.4.7, the sample (placed in a small quartz tube) is dropped into the furnace (set at the desired temperature) and pyrolyzed, with the pyrolysis

FIG. 1.4.6

The simplified schematics of a pyrolyzer with a heating coil outside a second quartz tube (based on the design of a heated filament system made by CDS Inc.). The two switch valves from this figure are in fact joined in a single six-port valve.

28

1. GENERAL INFORMATION ABOUT PYROLYSIS

FIG.

1.4.7 Simplified microfurnace pyrolyzer.

schematics

of

a

products flushed into the analytical instrument (e.g., a GC/MS). After the sample is pyrolyzed, it is ejected by switching the flow of the gas upward in the pyrolysis chamber. (3) Curie point pyrolyzers use heating based on the property of ferromagnetic conductors to heat rapidly at a specific temperature by interaction with a high frequency (radio frequency, RF) electromagnetic field. The temperature (Curie point) depends on the composition of the ferromagnetic conductor. When the Curie point temperature is attained, the ferromagnetic property disappears such that the material does not heat beyond that value. For pyrolysis, the sample is placed in close contact with the ferromagnetic conductor, which can be shaped into different forms such as a wire, ribbon, folded ribbon, or cylinder to properly hold the sample. The sample and its holder are maintained in a stream of inert gas in a similar way as for other pyrolyzers. The housing where the sample and its ferromagnetic holder are introduced is also heated to avoid condensation of the pyrolyzate but without decomposing the sample before pyrolysis. The heating of the conductor and subsequently of the sample can be realized by the high frequency electromagnetic field with a very short TRt, commonly between 10 and 100 ms. Lists of Curie point temperatures for various alloys are available in the literature [3]. (4) Other types of pyrolyzers utilized for analytical pyrolysis purposes are also known [7]. Some instruments are commercially available, and others are custom-made for specific purposes. Among these are laser pyrolyzers [8], “in column” type pyrolyzers [9], and shock-tube pyrolyzers used in particular for studying the pyrolysis of gases at high temperatures [10]. Shock-tube pyrolysis producing short exposure times at high temperatures (above 1000 K) were employed [10,11]. A typical shock tube is a metal tube in which a gas at low pressure and a gas at a very high pressure are separated by a diaphragm, as shown in Fig. 1.4.8. The tube dimensions and the length of the low-pressure and high-pressure sections can vary.

ANALYSIS OF PYROLYZATES

FIG. 1.4.8

29

Simplified diagram of a shock tube.

It is common to indicate the low-pressure gas as driven gas and the high-pressure gas as driver gas. The diaphragm separating the gases is eliminated suddenly (e.g., by rupture). The driver gas is allowed to burst into the driven gas, producing a shock wave that travels into the low-pressure section of the shock tube. An intermediate section sometimes is used to separate the high-pressure chamber from the low-pressure chamber. The shock wave increases the temperature and pressure of the driven gas and induces a flow in the direction of the shock wave. The behavior of the shock wave (density, speed, pressure) usually is calculated using the Rankine-Hugoniot equation. Once the incident shock wave reaches the end of the shock tube, it is reflected back into the already heated gas, resulting in a further increase in the temperature and pressure of the gas. This can create a hightemperature and high-pressure zone in the driven gas. The type of pyrolyzer influences (unfortunately) the pyrolysis outcome. In spite of selecting similar parameters for the pyrolysis process, specific construction characteristics influence in particular the heating rate. Special procedures are utilized in some pyrolyzers for measuring the heating rate of the actual sample. Also, the purity of the inert gas used for the pyrolysis atmosphere influences the pyrolysis outcome. When this gas contains a low level of air, for example, it is possible that some combustion may take place. The free radicals formed during combustion may influence the pyrolysis outcome, and the amount of sample undergoing true pyrolysis is smaller. This effect is significantly increased when oxygen is purposely added to the gas in which the pyrolysis takes place.

ANALYSIS OF PYROLYZATES Pyrolyzates generated with one of the devices previously described are typically transferred with a flow of gas “online” into an analytical instrument. Because the pyrolyzates may consist of a complex mixture of molecules, the most common type of analysis involves separation (usually chromatographic) in instruments such as a gas chromatograph/mass spectrometer. The typical chemical complexity of pyrolyzates requires an efficient separation for the analysis, and that explains the common use of chromatography coupled with the analytical pyrolysis. Because low molecular mass fragments are frequently generated in the pyrolytic process, GC is the technique usually applied for the separation. The use of MS as the detection tool provides very good sensitivity in addition to the identification capability. This sensitivity is necessary, mainly because the amount of sample used in analytical pyrolysis is very small (up to a few mg). Also, the analysis of trace components in pyrolyzates is possible

30

1. GENERAL INFORMATION ABOUT PYROLYSIS

only by use of very sensitive detection such as that offered by MS. The qualities of pyrolysis GC/MS (Py-GC/MS) make this technique the most convenient and widely utilized in practice. Various ionization techniques applied in association with Py-GC/MS are reported in the literature (e.g., [7]). The most common ionization method is electron impact with the detection of positive ions (EI +). Chemical ionization (CI) is used sometimes, but CI spectra interpretation is difficult because of the lack of fragmentation and because the reproducibility in CI can be affected by the experimental conditions in which the spectra are generated. Other ionization techniques are sometimes used for ion generation in the MS, for example with the goal of producing simpler spectra and achieving some specificity without the help of the GC separation [7]. Besides the use of GC/MS as a pyrolysis analyzer, other analytical instruments were used for the same purpose as previously indicated. These include chromatographic instruments further connected to a measuring device (such as GC, GC/MS, LC, LC/MS) or spectroscopic instruments such as MS without including a chromatographic separation, or IR. Direct interface of the pyrolyzer with spectroscopic instruments must take into consideration that the pyrolyzate can be a complex mixture of molecules. In the case of using only a mass spectrometer for detection (with no GC), milder ionization techniques were sometimes employed for reducing the fragmentation that occurs in an MS with typical electron impact ionization at 70 eV, such as resonance enhanced multiphoton ionization (REMPI) [12]. In addition to “online” analysis of pyrolyzates, “offline” analysis is sometimes performed. A number of possibilities for collecting the pyrolyzate offline have been reported. The pyrolyzate can be collected in an offline trap. Once trapped, the sample can be processed, for example by extraction with a solvent, and analyzed offline. The analysis can be performed on the pyrolyzate as is, or after sample processing using for example derivatization. Offline silylation of pyrolyzates is a common derivatization technique for further GC/MS analysis. Besides GC or GC/MS, the offline-collected pyrolyzate can be analyzed using LC, LC/MS, or other techniques. Collection of the pyrolyzate on an SPME fiber followed by offline desorption and GC/MS analysis also have been reported [13].

CONDITIONS FOR PYROLYSIS EXPERIMENTS IN THIS BOOK A variety of experimental conditions for pyrolysis and for the procedure to analyze the pyrolyzate are reported in the literature (see e.g., [7]). The inclusion of analytical procedure together with the condition for pyrolysis is very important because very frequently not all pyrolysis products are analyzed. For example, when using GC/MS coupled online with a pyrolyzer, nonvolatile components of the pyrolyzate are not analyzed and frequently not even reported as present in the pyrolyzate. The selection of the chromatographic column in the GC/MS system also provides a specific window for the range of analytes detected in the pyrolyzate. For these reasons, the result regarding pyrolysis of the same compound may be reported as producing results with a number of differences. In the present book, in addition to the data collected from the literature, some original results are reported. These results were obtained using several experimental setups. The pyrolyzers used for obtaining the original results were all filament type Pyroprobe 1000, Pyroprobe 2000 with an AS 2500 autosampler, and Pyroprobe 5000 Series, Model 5200, (all

31

CONDITIONS FOR PYROLYSIS EXPERIMENTS IN THIS BOOK

CDS Analytical, CDS). Pyrolysis was performed typically in flash mode and the Teq was selected specifically for a certain compound (e.g., at 700°C or at 900°C). The pyrolyzates were analyzed online using a 6890/5973 GC/MS instrument (Agilent) or were collected in a cooled (uncoated) capillary and analyzed offline after dissolution in a solvent or after derivatization. The GC/MS analysis of the pyrolyzates can be done using a variety of conditions that can be adjusted to obtain the most information about the pyrolyzate. However, for better standardization of the pyrolyzate analysis, when possible the online analyzes were performed using one unique set of GC/MS parameters, as described in Table 1.4.1. The DB-1701 type column (Agilent/J&W Scientific) has medium polarity and separates well low molecular weight components of the pyrolyzates. However, for compounds with high boiling points, for very polar compounds, or for compounds that decompose when heated, the separation on this column is not adequate, and other experimental setups for the GC/MS side of the pyrolysis must be selected. The offline experiments usually were done when a derivatization of the pyrolyzate was necessary in order to analyze very polar compounds. Derivatization also was used when the pyrolysis products were expected to have high boiling points or when traces of a specific compound or group of compounds were analyzed using a dedicated procedure. The pyrolyzate was collected in a short uncoated 0.53 mm capillary column that was cooled in iced water. The typical derivatization was trimethylsilylation. For this derivatization, the collected material was treated in the capillary with 0.15 mL of a mixture of one part dimethylformamide TABLE 1.4.1

Typical Parameters for the GC/MS Online Analysis of Pyrolyzates

Parameter

Description

Parameter

Description

GC column

DB-1701

Flow mode

Constant flow

Column dimensions

60 m long, 0.25 mm id.

Flow rate

1.1 mL/min

Film thickness

1.0 μm

Nominal initial pressure

17.5 psi

Initial oven temperature

37°C

Split ratio

70:1

Initial time

4.0 min

Split flow

76.0 mL/min

Oven ramp rate

2°C/min

GC outlet

MSD

Oven final first ramp

60°C

Outlet pressure

Vacuum

Final time first ramp

0 min

MSD transfer line temperature

280°C

Oven ramp rate

5°C/min

Ion source temperature

230°C

Oven final temperature

280°C

Quadrupole temperature

150°C

Final time

20 min

MSD EM offset

250 V

Total run time

75.5 min

MSD solvent delay

2.0 min

Inlet temperature

280°C

MSD acquisition mode

TIC

Inlet mode

Split

Mass range

29–550 a.u.

Carrier gas

Helium

32 TABLE 1.4.2

1. GENERAL INFORMATION ABOUT PYROLYSIS

Typical Parameters for the Offline GC/MS Analysis After TMS Derivatization of Pyrolyzates

Parameter

Description

Parameter

Description

GC column

DB-5MS

Inlet mode

Split

Column dimensions

30 m long, 0.25 mm id.

Injection volume

1.0 μL

Film thickness

0.25 μm

Flow mode

Constant flow

Initial oven temperature

50°C

Flow rate

1.0 mL/min

Initial time

0.5 min

Nominal initial pressure

7.57 psi

Oven ramp rate 1

3°C/min

Split ratio

30:1

Oven final first ramp

200°C

Split flow

28.8 mL/min

Final time first ramp

0 min

GC outlet

MSD

Oven ramp rate 2

4°C/min

Outlet pressure

Vacuum

Oven final temperature

300°C

MSD transfer line temperature

280°C

Final time second ramp

10 min

Ion source temperature

230°C

Oven ramp rate 3

4°C/min

Quadrupole temperature

150°C

Oven final temperature

300°C

MSD EM offset

100 V

Final time third ramp

10 min

MSD solvent delay

7.0 min

Total run time

101.75 min

MSD acquisition mode

TIC

Inlet temperature

300°C

Mass range

29–800 a.u.

(DMF) and two parts bis(trimethylsilyl)-trifluoroacetamide (BSTFA). The samples were transferred into GC vials, kept at 76°C (in a heating block) for 30 min, and allowed to cool at room temperature for another 30 min. The analyzes were done using a GC/MS with parameters given in Table 1.4.2. Other GC/MS procedures for pyrolyzate analysis that was used in special cases were locally described. Qualitative identifications of the peaks generated in the pyrograms for the original experiments described in this book have been done almost exclusively using mass spectral identifications. The mass spectral libraries used for these identifications included NIST14 [14], Wiley275, and Wiley7n [15]. Some identifications in the tables were only tentative and were indicated with “?.” Associated with the chemical identifications for many compounds, the CAS Registry Numbers (CAS#) were indicated [16]. The CAS# was not available for some compounds, and this was indicated with “N/A” (not available). The quantitation using pyrolysis can be done following the typical procedures in GC/MS analyzes, after generating a calibration curve between the MS response (peak area count) and a specific amount of analyte in the pyrolyzer. However, with complex pyrolyzates it is very difficult to generate such calibrations for a large number of compounds. An estimation of the levels of various compounds in the pyrogram can be done by simply comparing the peak area counts of the compound of interest with that of a peak generated by a compound present in a

REFERENCES 1.4

33

known amount (e.g., a standard that does not decompose by pyrolysis). Another procedure is to make estimations based on areas normalized by the sum of all peak areas in the chromatogram. Because the response (signal/quantity) in GC/MS is not equal for different compounds, these procedures are not a true quantitation. The estimation based on normalized peaks by the total peak areas in the chromatogram and by the molecular weight of the individual compound is used in this book for providing a rough estimation of the moles level of different compounds in the pyrogram. However, it must be noted that these estimations sometimes can be misleading because differences in the response factors in the analytical procedures can be quite large for different compounds. Because the purity of the parent compounds used in pyrolysis experiments is not always perfect, some compounds detected in the pyrolyzates may come from the impurities in the initial sample and are not a result of the pyrolysis process. Therefore, it is not always possible to know the origin of a pyrolyzate component without a preliminary analysis of the initial sample. The samples that were used in pyrolysis experiments and are described in this book were frequently subjected to a GC/MS analysis. For this purpose, solutions of the compound taken for pyrolysis were injected in a GC/MS system working in conditions virtually identical to those described in Table 1.4.1.

References 1.4 [1] S. Tsuge, H. Ohtani, C. Watanabe, Pyrolysis-GC/MS Data Book of Synthetic Polymers, Pyrograms, Thermograms and MS of Pyrolyzates, Elsevier, Amsterdam, 2011. [2] S.C. Moldoveanu, Analytical Pyrolysis of Synthetic Organic Polymers, Elsevier, Amsterdam, 2005. [3] S.C. Moldoveanu, Analytical Pyrolysis of Natural Organic Polymers, Elsevier, Amsterdam, 1998. [4] I. Tyden-Ericsson, Chromatographia 6 (1973) 353. [5] I. Ericsson, J. Anal. Appl. Pyrolysis 2 (1980) 187. [6] http://www.lqa.com/frontier-pyrolyzer-epapy-3030d/ (Accessed 26 February 2018). [7] S.C. Moldoveanu, Pyrolysis of Organic Molecules With Applications to Health and Environmental Issues, first ed., Elsevier, Amsterdam, 2010. [8] N.E. Vanderborgh, C.E. Roland Jones, Anal. Chem. 55 (1983) 527. [9] T. Go´recki, J. Poerschmann, Anal. Chem. 73 (2001) 2012. [10] Y. Hidaka, K. Sato, Y. Henmi, H. Tanaka, K. Inami, Combust. Flame 118 (1999) 340. [11] J.D. Anderson, Hypersonic and High Temperature Gas Dynamics, AIAA, Reston, VA, 2000. [12] R. Zimmermann, R. Dorfner, A. Kettrup, J. Anal. Appl. Pyrolysis 49 (1999) 257. [13] S.C. Moldoveanu, V. David, Sample Preparation in Chromatography, Elsevier, Amsterdam, 2002. [14] https://www.nist.gov/srd/nist-standard-reference-database-1a-v17 (Accessed 26 February 2018). [15] http://olabout.wiley.com/WileyCDA/Section/id-406117.html (Accessed 26 February 2018). [16] https://www.cas.org/content/chemical-substances/faqs (Accessed 26 February 2018).