aromatization of organic matter induced by the presence of clays during flash pyrolysis-gas...

Aromatization of organic matter induced by the presence

of clays during flash pyrolysis-gas chromatography–mass

spectrometry (PyGC–MS)

A major analytical artifact

Pierre Faure a,*, Laurence Schlepp a, Laurence Mansuy-Huault a, Marcel Elie a,Emilie Jarde a, Manuel Pelletier b

a UMR 7566 CNRS G2R and CREGU, Universite Henry Poincare-Nancy I, BP 239, 54506 Vandoeuvre les Nancy Cedex, Franceb UMR 7569 CNRS LEM, Institut National Polytechnique de Lorraine, 15 avenue du Charmois, BP 40, 54501 Vandoeuvre les-Nancy Cedex, France

Received 17 September 2004; accepted 2 February 2005

Available online 23 May 2005

This paper is dedicated to Laurence Schlepp deceased in August 2001.

Abstract

The macromolecular characterization of fossil or recent organic matter in soils and sediments is frequently carried out using flash

pyrolysis-gas chromatography—mass spectrometry (PyGC–MS). Such analyses providing information on the organic matter structure and

reactivity, can be applied on isolated organic materials or on raw samples. Nevertheless, working on pure organic material implies heavy and

time-consuming pre-treatments (humic substance isolation, mineral removal by acids), which can induce molecular alteration of the initial

organic matter. On the contrary, PyGC–MS analysis on raw sample avoids heavy preparation. However, previous studies have shown that

mineral matrixes and especially clay can induce analytical artifacts.

The aim of this study is to test clay influences during the flash pyrolysis of pure organic compounds, which can be generated during

pyrolysis of natural macromolecules and humic acid. Moreover, flash pyrolysis was carried out with and without a methylation derivative so as

to evaluate its ability to limit clay effects.

Pyrolysis of Na-smectite/model compounds mixtures indicates that clays intensify recombination reactions. For instance, in the presence

of Na-smectite, aromatization increases as a function of the organic compound polarity. Especially, alkyl-benzenes and polycyclic aromatic

hydrocarbons are generated during pyrolysis of alkanol, alkanal and especially alkanoic acid.

The pyrolysis of humic acid in the presence of clays was also carried out and reveals that clays significantly affect initial molecular

distribution especially by intense aromatization. Moreover, the aromatization degree is dependant on the nature of the clay, Na-smectite

exhibiting the most intense influence on the molecular signature.

Whatever the nature of the organic matter (model compounds or humic acid), the thermally assisted hydrolysis and methylation allows to

limit clay interactions and seems a promising way of raw sediments and soils investigation.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Flash pyrolysis; Clay minerals; Humic acid; Model compounds; Analytical artifact; Aromatization

www.elsevier.com/locate/jaap

J. Anal. Appl. Pyrolysis 75 (2006) 1–10

1. Introduction

The analysis of macromolecular units from fossil

(kerogen, asphaltene, coal . . .) or recent sedimentary organic

* Corresponding author. Tel.: +33 3 83 68 47 40; fax: +33 3 83 68 47 01.

E-mail address: [email protected] (P. Faure).

0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jaap.2005.02.004

matter (humic and fulvic acids, humin, lignin, cellulose,

proteins, sugars, chlorophyll . . .) is frequently carried out in

order to better understand their structure, their origin and their

reactivity.

Analyses of variable complexity can be used in order to

better know macromolecules: (i) chemical characterization

P. Faure et al. / J. Anal. Appl. Pyrolysis 75 (2006) 1–102

(elemental analyses, carbon organic content, Rock Eval

pyrolysis . . .) [1,2], (ii) spectroscopic investigation (infrared,nuclear magnetic resonance, ultraviolet, . . .) [3–7] and (iii)

molecular analyses. More generally, a combination of these

complementary analytical approaches is applied because of

the very high complexity of macromolecular organic matter.

Among these different analysis types, flash pyrolysis-gas

chromatography–mass spectrometry (PyGC–MS) is fre-

quently used for the characterization of fossil [8–14] or

recent [15–26] macromolecular organic matter encountered

in soils and sediments.

However, different methods of PyGC–MS are used on

organic matter depending of its origin: (i) analyses of isolated

organic fraction (demineralization) frequently used for

kerogens, (ii) analyses of specific organic matter fractions

(humic and fulvic acids) applied to soils or (iii) direct

pyrolysis of raw samples (organic matter and mineral phases

together) [27–30] for rapid investigation of polluted soils and

sediments. Characterization of isolated organic matter or

specific organic fractions imply preliminary demineralization

steps (generally hydrochloric and fluoridric acid treatments)

or humic material isolation (base and acid extraction) which

are time consuming and can induce alteration of the initial

macromolecules [31,32]. Nevertheless, such treatments

prevent from any mineral interference during analysis. On

the contrary, pyrolysis of raw samples (organic–mineral

mixtures) is rapid and avoids modification of the organic

structure during acid treatments. However, the presence of

minerals and especially clays, frequent in soils and sediments

can induce analytical artifacts [33–38].More recently, several

authors [39,40] have demonstrated that clay minerals

(kaolinite and montmorillonite) can modify the molecular

fingerprint of model compounds (alkanols and alkanoic

acids), chlorophyll-a and soil organic matter (grass material)

during flash pyrolysis.

The aimof thiswork is to evaluate the influenceof different

clay minerals and sediments (talc, Na-smectite, Ca-smectite

and illite) during PyGC–MS of different organic compounds.

Theminerals were chosen in order to evaluate the influence of

the specific area, the nature of the interlayer ion and the charge

degree of sheets. Several model compounds (alkanes,

alkanols, alkanones, alkanoic acids and alkanals), typical

compounds potentially generated during flash pyrolysis of

macromolecules, were investigated. Moreover, humic acid

was used in order to investigate the consequences of clay

minerals on the flash pyrolysis of macromolecular organic

matter encountered in soils, as well as on free molecules,

which occur in association with macromolecules. Such

experimental approach deals with the identification of (i)

minerals, which induce the most intense interferences on the

organic geochemical signature and (ii) organic compounds,

which are the most sensitive to clay during flash pyrolysis.

Moreover, thermally assisted hydrolysis methylation–

GC–MS (THM–GC–MS) is tested as clay artifact inhibitor.

Indeed, THM–GC–MS was already recommend for char-

acterization of recent organic matter in order to improve

chromatographic resolution (increase of the volatility) [41]

and for limiting recombination processes (protection of

carboxyl groups). For instance, in the presence of sulphur,

pyrolysis of triglycerides and fatty acids produces cycliza-

tion and aromatization of the aliphatic hydrocarbons chains

with concomitant decarboxylation, yielding homologous

series of alkybenzenes [21]. On the contrary, THM–GC–MS

prevents decarboxylation by protecting carboxyl groups,

inhibiting sulfur catalytic effect [42]. As for the presence of

sulfur, THM–GC–MS can be efficient to inhibit clay

interactions and needs to be evaluated.

2. Methods and materials

2.1. Samples

2.1.1. Mineral phases

Four different mineral phases were used in this study:

(i) 2
:1 phyllosilicate, illite du Puy (France) [43]. The
structural formula of the illite deduced from microp-

robe analyses is the following:

ðSi3:45Al0:55ÞIVðAl1:21Fe0:39Mg0:40Ti0:05ÞVI

O10ðOHÞ2K0:62Na0:09Ca0:01

The smectite, a Wyoming montmorillonite, was
(ii)
supplied by CECA (Paris, France) in the form of a

<2 mm suspension in Na phosphate at pH 10. The

sample was prepared by dispersing the clay three times

in a NaCl 1 M solution, which was stirred for 24 h, and

centrifuged at 30,000 � g for 20 min. In order to

remove quartz and other impurities, the centrifuged

fraction was washed by dispersion in ultra-pure water,

and centrifuged again. Five washing cycles were then

carried out until a chloride-free supernatant was

obtained (silver nitrate test). The structural formula

was obtained from chemical and electron microprobe

analyses [44]:

ðSi3:88Al0:12ÞIVðAl1:53Mg0:257Fe2þ

0:006Fe3þ

0:207ÞVI

O10ðOHÞ2Na0:38�nH2O

The Ca-smectite was prepared by dispersing the Na-
(iii)
homoionic clay three times in a CaCl2 1 M solution,

which was stirred for 24 h, and centrifuged at

30,000 � g for 5 min. Five washing cycles were then

carried out until a chloride-free supernatant was

obtained (silver nitrate test). The structural formula

was obtained from chemical and electron microprobe

analyses:

ðSi3:91Al0:09ÞIVðAl1:55Mg0;22Fe3þ

0:19Ti0:01ÞVI

O10ðOHÞ2Ca0:19�nH2O

A talc. The purity of the talc was controlled by X-ray
(iv)
diffraction and revealed the occurrence in low

P. Faure et al. / J. Anal. Appl. Pyrolysis 75 (2006) 1–10 3

Table

Speci

step v

Miner

Illite

Na-sm

Ca-sm

Talc

Table 2

Formula and molecular weight of the five model compounds used in this

study

Compounds Formula Molecular weight

(g/mol)

Undecane C11H24 156

1-Undecanol C11H24O 172

Undecanal C11H22O 170

2-Undecanone C11H22O 170

Undecanoic acid C11H22O2 186

proportion of chlorite (less than 5%). The structural

formula of the talc is:

ðSi4ÞIVðMg3ÞVIO10ðOHÞ2

These four mineral phases were ground (<500 mm)

and extracted in soxhlet with chloroform for 48 h in

order to clean from any organic contamination. Their

texture was characterized in the dry state using a

conventional step-by-step volumetric adsorption equip-

ment (B.E.T.) in order to determine complete nitrogen

gas adsorption–desorption cycles and determine the

specific area (Table 1).

The homoionic Na-smectite due to its intermediate

specific area has been selected as reference mineral for

the organic model compounds experimentation.

2.1.2. Organic phases

(i) In order to test major types of organic compounds

encountered in natural organic matter, several model

compounds have been selected: n-alkanes, alkanoic

acid, 2-alkanone, alkanal and 1-alkanol. The selected

compounds correspond to linear chain with 11 carbons

(Table 2).

(ii) T
he macromolecule used in this work, is a humic acid,
sodium salt (Aldrich, reference: H 1.675-2). The

elemental analysis of the humic acid is: %C = 38.78;

%O = 30.30; %H = 3.14; %N = 0.56; %S = 0.21. This

humic acid was not pre-extracted in order to evaluate

influence of clay mineral on free as well as bound

organic materials.

2.2. Sample preparation

2.2.1. Model organic compounds

Each model compound was dissolved in dichloromethane

except the carboxylic acid, which was dissolved in a mixture

methanol/dichloromethane (50/50, v/v). The concentration of

each solution was equal to 100 mg/ml. The preparation

depended on the pyrolysis type: (i) for puremodel compounds

pyrolysis, silica filter, previously extracted in soxhlet with

chloroform for 48 h in order to clean from any organic

contamination, were impregnated with the solution and dried

under argon. A mass balance of the filter before and after

impregnation allowed to determine the pure compound mass;

(ii) for model compound/clay mineral mixture, 1 ml of the

solution was added to 1 g of each mineral phase in order to

perform a good impregnation of the mineral particles.

1

fic areas of clay minerals deduced by using a conventional step-by-

olumetric adsorption equipment (B.E.T.)

al phases Specific area (m2/g)

122

ectite 40

ectite 20

4.6

2.2.2. Humic acid

In the case of humic acid (powder), 100 mg, previously

ground (<100 mm), were added to 1 g of each mineral

phase. Mixtures were then homogenized. Solvents were let

to evaporated before analysis.

All the organic matter/mineral mixture corresponds to

mineral/organic ratio equal to 10.

2.3. Flash pyrolysis-gas chromatography–mass

spectrometry

Pyrolysis of samples was performed with a CDS

Analytical 2000 pyroprobe. Impregnated filters were

inserted in quartz tube so as to load the same amount of

pure compounds. In the case of solid samples, the same mass

(500 mg) was directly loaded in quartz tubes.

Quartz tubes were then heated at 620 8C during 20 s. The

generated products were analyzed by on-line gas chromato-

graphy–mass spectrometry (HP 5890 Serie II GC coupled to

a HP 5971mass spectrometer), using a split-splitless injector

(splitless injection), a 60 m DB-5 J&W, 0.25 mm i.d.,

0.1 mm film fused silica column. After cryofocusing

(�30 8C), the GC oven was temperature programmed from

�30 to 40 8C at 10 8C/min and 40 to 300 8C at 5 8C/min

following by a stage at 300 8C during 10 min (constant

helium flow of 1.4 ml/min).

2.4. Thermally assisted hydrolysis methylation–GC-MS

The organic/mineral samples as well as impregnated

filters were mixed with tetramethylammonium hydroxide (1/

10, w/w) according to the procedure of Hatcher and Clifford

[18]. The mixture of sample and TMAH was then placed

into a quartz tube and the THM–GC–MS analyses were

carried out using the same PyGC–MS apparatus described in

the above section. On-line pyrolysis at 620 8C was

performed during 20 s. After cryofocusing (0 8C), the oventemperature program was 0 8C for 1 min, 70 8C/min to

40 8C followed by an isothermal stage at 40 8C for 6 min,

then 6 8C/min to 300 8C, followed by an isothermal stage at

300 8C for 20 min (constant helium flow of 1.4 ml/min).

2.5. Pyrogram peaks identification

Compounds were identified based on their mass spectra

and GC retention time with reference to the Wiley and U.S.


National Bureau of Standards computerized mass spectral

libraries. The identifications were also based on comparisons

with publishedmass spectra of pyrolysis compounds of humic

acids and fossil organic matter [12,16–19,28,45,46].

2.6. Pyrogram peaks quantification

Integration of mass chromatograms with relevant ‘‘m/z’’

values was performed and the peak areas obtained were

multiplied by a correction factor, which takes into account

differences in mass spectrometric responses for various

compounds. The correction factor was calculated from the

mass spectrum of each authentic compound by taking the

inverse of the percentage of the total ion current of the

relevant ‘‘m/z’’ value and multiplying it by 100 [13,26]. As a

result of this mathematical procedure, distribution patterns

were obtained showing the relative concentrations of the

different compounds.

Fig. 1. Pyrograms of (a) pure 2-undecanone, (b) pure undecanal, (c) pure 1-

undecanol and (d) pure undecanoic acid. See identification in Table 3.

3. Results

3.1. Model organic compounds

Three different pyrolysis conditions were used for each

organic model compound: (i) pure compounds pyrolysis, (ii)

pyrolysis of model compounds/Na smectite mixture (10/90,

m/m) and (iii) pyrolysis of model compounds/Na-smectite/

TMAH mixture (5/45/50, m/m/m).

Whatever the pyrolysis condition, the n-undecane is not

affected and produces exclusively the initial compound by

thermal desorption. On the contrary, the other model

compounds yield byproduct depending of the pyrolysis

conditions.

3.1.1. Pure compounds pyrolysis

Whatever the nature of the model compound, pyrograms

are dominated by the initial compound (Fig. 1a–d).

However, in each case, pyrolysis byproducts are observed

in low proportion. Especially, n-alk-1-enes (^) containing

6–9 carbon number are detected in each pyrogram. 2-

Undecanone pyrogram exhibits nonanal (* C9) whereas

undecanal pyrogram exhibits decanal (* C10), 1-undecanol

(& C11) and n-decane (^ C10) in low abundance.

1-undecanol pyrogram contains undecanal (*C11) and

1-decene (^ C10) as for undecanoic acid program but this

latter exhibit a more complex distribution characterized by

the occurrence of decanal (* C10) and undecanal (* C11),

alkanoic acids (*) containing 8–10 carbon atoms, 3-

substitued g-lactone (gL C11) and undecanoic acid-methyl

ester (* C11ME) (Fig. 1d; Table 3).

3.1.2. Pyrolysis of pure compounds/Na smectite mixture

Whereas 2-undecanone (Fig. 2a) seems not to be affected

by the presence of clay, the other model compounds exhibit

contrasting pyrograms (Fig. 2b–d). In the case of undecanal

and 1-undecanol (Fig. 2b and c, respectively), the initial

product predominates. However, pyrolysis byproducts

corresponding to alkenes (5; different positional isomer

of n-alkenes and iso-alkenes), n-alkanes (^) and especially

alkyl-benzene (Ci F) are observed. Moreover, 1-undecanol

pyrograms exhibit a high proportion of different positional

isomers of n-undecene and iso-undecenes (5 C11) and n-

undecane (^ C11) (Fig. 2c). Naphthalene (N) is detected in

the undecanal pyrogram (Fig. 2b). The undecanoic acid

pyrogram is extremely different (Fig. 2d). On one hand, the

proportion of the initial compounds is low in the pyrogram.

Minor pyrolysis byproducts such as alkenes (5), n-alkanes

(^), undecanoic acid-methyl ester (* Ci–ME) and

undecanitrile (C11–Ni) occur. On the contrary, aromatic

compounds, especially benzene (F), alkyl-benzene (Ci F),

naphthalene (N) and alkyl-naphthalene (Ci–N) are observed

as predominant compounds and phenanthrene (P) is also

detected.


Table 3

Identification of components in pyrograms of Figs. 1–3

Symbole Compounds

Ci 2-Alkanone (i = carbon number)

* Ci Alkanal (i = carbon number)

* Ci Alkanoic acid (i = carbon number)

* Ci–ME Alkanoic acid, methyl ester (i = carbon number)

* Ci–ME Alkenoic acid, methyl ester (i = carbon number)

& Ci Alkanol (i = carbon number)

& Ci–ME Alkanol, methyl ether (i = carbon number)

^ Ci n-alkane (i = carbon number)

^ Ci n-alk-1-ene (i = carbon number)

5 Ci Different positional isomer of n-alkenes and iso-alkenes

(i = carbon number)

F Benzene

Ci F Alkyl-benzene (i = substitution degree)

N Naphthalene

Ci–N Alkyl-naphthalene (i = substitution degree)

P Phenanthrene

Ph Phenol

Ci–Ph Alkyl-phenol (i = substitution degree)

Ci–Ni Alkyl-nitrile (i = carbon number)

1 PR Prist-1-ene

2 PR Prist-2-ene

Ph 2-Pentadecanone, 6,10,14-trimethyl-

1 Styrene

2 Benzene methoxy

3 C1–benzene methoxy

4 Butanedioic acid, dimethyl ester

5 Benzoic acid, methyl ester

6 C2–benzene methoxy

7 Benzene dimethoxy

8 C1–benzene dimethoxy

9 Benzoic acid, methoxy methyl ester

10 Benzene trimethoxy

11 C1–benzene trimethoxy

12 Benzoic acid, dimethoxy methyl ester

13 Propenoic acid, dimethoxyphenyl, methyl ester

Fig. 2. Pyrograms of (a) a mixture Na-smectite/2-undecanone (90/10, w/w),

(b) a mixture of Na-smectite/undecanal (90/10, w/w), (c) a mixture of Na-

smectite/1-undecanol (90/10, w/w) and (d) a mixture of Na-smectite/

undecanoic acid (90/10, w/w). See identification in Table 3.

3.1.3. Thermally assisted hydrolysis methylation of pure

compounds/Na smectite mixture

2-Undecanone is weakly influenced by the occurrence of

TMAH. The initial product predominates and minor

pyrolysis byproducts corresponding to methylated alkanoic

acids (* Ci–ME) with 8–10 carbon number are detected

(Fig. 3a).

The undecanoic acid occurs in the pyrogram only in the

methylated form (* Ci–ME) without any pyrolysis

byproducts (Fig. 3d). On the contrary, 1-undecanol is both

observed in methylated (& C11–ME) and initial compound

configuration (& C11) (Fig. 3c). Moreover, as for

undecanoic acid, the pyrolysis of 1-undecanol does not

produce byproducts during the pyrolysis. On the contrary,

undecanal pyrolysis produces various byproducts including

alkenes (5), n-alkanes (^), alkanals (*) and methylated

alkanoic acids (* Ci–ME) whereas the initial product is not

observed (Fig. 3b).

3.2. Humic acid

In order to evaluate the influence of different clay mineral

on natural organic matter pyrolysis, several samples

corresponding to pure humic acid (HA), talc/HA, Ca-

smectite/HA, illite/HA and Na-smectite/HA mixtures were

pyrolysed without and with TMAH.

3.2.1. Pyrolysis of pure humic acid

The pyrolysis of pure humic acid (Fig. 4a) yields various

compounds which can be grouped in three different families:

(i) n-alk-1-ene/n-alkane doublets (^, ^) with 8–29 carbon

showing a limited odd over even predominance in the

C24–C29 range, (ii) alkyl-benzenes (Ci F), naphthalene (N)

and alkyl-naphthalene (CiN) and (iii) phenol (Ph) and

alkyl-phenols (Ci–Ph). Moreover, prist-1-ene (1PR) and

2-pentadecanone, 6,10,14-trimethyl ( Ph) occur and prist-

2-ene is detected in very low intensity.


Fig. 3. Pyrograms of (a) a mixture of Na-smectite/2-undecanone/TMAH

(45/5/50, w/w/w), (b) a mixture of Na-smectite/undecanal/TMAH (45/5/50,

w/w/w), (c) a mixture of Na-smectite/1-undecanol/TMAH (45/5/50, w/w/

w) and (d) a mixture of Na-smectite/undecanoic acid/TMAH (45/5/50, w/w/

w). See identification in Table 3.

Fig. 4. Pyrograms of (a) pure humic acid, (b) a mixture talc/humic acid (90/

10, w/w), (c) a mixture Ca-smectite/humic acid (90/10, w/w), (d) a mixture

illite/humic acid (90/10, w/w) and (e) a mixture Na-smectite/humic acid

(90/10, w/w). See identification in Table 3.

3.2.2. Thermally assisted hydrolysis methylation of pure

humic acid

The chromatogram of the thermally assisted hydrolysis

methylation of humic acid (Fig. 5a) is characterized by a

complex distribution dominated by long chain alkanol

methyl ether (& Ci–ME; fatty alcohol) and long chain

alkanoic acids, methyl ester (* Ci–ME; fatty acids) in the

range C22–C30 with an even over odd carbon number

predominance. Such compounds are typical of plant waxes

such as leaf cuticular waxes [47]. Methoxy-benzenes (2,3,6),

dimethoxy-benzenes (7,8) and trimethoxy-benzene (10,11)

occur and are typical of lignin-derived pyrolysis byproducts

[16,19,22].

3.3. The influence of clay minerals

3.3.1. Pyrolysis humic acid/mineral phases mixture

The humic acid/talc pyrogram resembles to the pure acid

humic (Fig. 4b). However, the proportion of the n-alk-1-ene/


Fig. 5. Pyrograms of (a) a mixture humic acid/TMAH (50/50, w/w) and (b) a mixture Na-smectite/humic acid/TMAH (45/5/50, w/w/w). See identification in

Table 3.

n-alkane doublets is lower compare to aromatic compounds.

Moreover, prist-2-ene (2PR), observed in low proportion in

the pure humic acid pyrogram, occurs in the presence of talc.

In the case of humic acid/Ca-smectite, n-alk-1-enes

disappear whereas the aromatic compounds proportion

increases compared to n-alkanes (Fig. 4c). Prist-1-ene and

prist-2-ene are still observed whereas 2-pentadecanone,

6,10,14-trimethyl is absent.

The pyrogram of humic acid/illite mixture (Fig. 4d) is

characterized by the absence of n-alk-1-ene, the disappear-

ance of n-alkanes with long chain length (longer than 20

carbons), the absence of pristene and a high predominance

of the aromatic hydrocarbons.

The humic acid/Na-smectite mixture pyrogram (Fig. 4e)

exhibits a distribution quite surprising. Indeed, n-alk-1-enes

and n-alkanes are absent from the pyrogram whereas alkyl-

benzenes, alkyl-naphthalene and phenanthrene (P) predo-

minate.

3.3.2. Thermally assisted hydrolysis methylation of

humic acid/Na smectite mixture

The humic acid/Na smectite mixture pyrogram in the

presence of TMAH is close to the pure humic acid (Fig. 5b)

with the same predominance of alkanols and alkanoic acids.

Prist-1-ene and 2-pentadecanone, 6,10,14-trimethyl occur

whereas prist-2-ene is absent. However, some differences

are noticeable. Alkyl-benzene containing 3–6 carbons in the

alkyl chain are detected whereas n-alk-1-ene/n-alkane

doublets in the range C9–C11 are absent.

4. Discussion

4.1. Model organic compounds

Whatever the nature of the model compounds, pure

compounds pyrolysis induce essentially their thermo-

desorption, whereas production of byproducts are low in

amount and correspond essentially to n-alk-1-enes (Fig. 1a–

d) excepted for the carboxylic acid which exhibits a more

complex distribution (Fig. 1d). Especially, n-alk-1-enes

occurrence in relatively high proportion in the pyrograms

(Fig. 1d) suggests that alkanoic acids decarboxylate during

pyrolysis, yielding n-alk-1-enes with aliphatic chain lengths

shorter than the initial alkanoic acid. On the contrary, the

presence of Na-smectite induces drastic modification of the

product generated during flash pyrolysis except for alkanone

(Fig. 2a–d). Na-smectite induces formation of aromatic units

(alkyl-benzene and naphthalene) as well as different

positional isomers of n-alkenes and iso-alkenes. Therefore,

pyrolysis of alkanol, alkanal and alkanoic acid in the

presence of Na-smectite leads to intense aromatization


Fig. 6. Evolution of aliphatic hydrocarbons, aromatic hydrocarbons and polar compounds proportion in pyrograms for the pure humic acid and the different

clay mineral/acid humic mixtures.

processes. Clay minerals and especially smectite have

already been described as dehydration catalysts of alkanol

[40]. This dehydration conducts to the formation of n- and

iso-alkenes and probably aromatic units result from

‘‘scrambling’’ of alkenes. This cyclization and aromatiza-

tion processes already observed in the presence of sulfur

[42], occurs in the presence of Na-smectite.

On the contrary, TMAH seems to limit these aromatiza-

tion processes and limits the catalytic affect of clays

(Fig. 3a–d). Especially, alkanoic acids and alkanols are

preserved and no aromatic units are generated. Only

undecanal, even in the presence of TMAH, remains affected

by the occurrence of Na-smectite.

Fig. 7. Evolution of the n-alk-1-enes and n-alkanes proportion for the pu

4.2. Humic acid

The different compounds detected in each pyrogramwere

identified and quantified (see Section 2.6). These com-

pounds were grouped in three families: aliphatic hydro-

carbons, aromatic hydrocarbons and polar compounds and

plotted in a triangular diagram (Fig. 6). A progressive

aromatization is observed from the pure humic acid to the

humic acid/Na-smectite mixture. As a matter of fact, these

experiments reveal the increasing influence of clay mineral

depending of their nature. Whereas talc exhibits a limited

influence on humic acid pyrolysis, Ca-smectite, and

essentially illite and Na-smectite modify the molecular

re humic acid and the different clay mineral/acid humic mixtures.


distribution increasing the aromatic proportion (until 80% of

aromatic hydrocarbons whereas raw acid humic acid

contains less than 10% of these latters).

The evolution of the relative abundance of n-alk-1-enes

and n-alkanes (Fig. 7) reveals a progressive disappearance of

n-alk-1-ene with an increased aromatization accompanied

by a disappearance of n-alkanes. Therefore, aromatic units

(alkyl-benzene, alkyl-naphthalene and phenanthrene) seem

to be generated on the expenses of n-alk-1-enes and then n-

alkanes.

The relative proportion of the prist-1-ene (1PR) and prist-

2-ene (2PR) depends on the presence and the nature of clay

minerals. The (1PR/(1PR + 2PR)) ratio is equal to 0.94 for

the pure humic acid (HA) pyrogram. This ratio decrease

progressively with the HA–talc (0.31) and HA–Ca-smectite

(0.28) mixtures. With illite and Na-smectite, 1PR and 2PR

are absent. Several authors [38,48] suggest that the presence

of clay minerals favors the transformation of prit-1-ene to

prist-2-ene during pyrolysis, clays acting as isomerization

catalysts [38]. As a matter of fact, our results confirm that

clay minerals influence 1PR and 2PR proportions during

flash pyrolysis. Moreover, the intensity of the clay catalytic

properties is dependant on the clay type.

4.3. Clay mineral influence

Two different mechanisms can be involved in the

recombination reactions:

(i) M
any properties of clay minerals can be derived from
their crystal structures and crystal chemistry [49]. They

reflect the state and the distribution of the electrostatic

charge of the structure layers. The negative charge is a

result of the ionic substitution in the octahedral and

tetrahedral sheets of clay minerals. As a matter of fact,

charged sheets can favor recombination reactions. Illite

exhibits the higher negative charge (0.73; Table 1) but

does not correspond to the clays, which show the more

important interaction with the humic acid.

(ii) T
he high specific area of some clays used in this study
favors the contact with different molecules generated

during flash pyrolysis. As a matter of fact, recombina-

tions can be accelerated. However, the Na-smectite

which exhibits the higher aromatization degree, has a

less important specific area than illite.

As a matter of fact, the mechanism involved during flash

pyrolysis and especially aromatization is difficult to identify.

Layer charge and specific area probably play amajor role but

their role need to be programs.

These results suggest that the organic matter sensitivity

during flash pyrolysis in the presence of clay minerals is

dependant on (i) the clay mineral type and (ii) the organic

molecules generated during the pyrolysis. Alkanols and

alkanoic acids are particularly sensitive to the presence of

clay and can induce generation of aromatic units such as

alkyl-benzene and polycyclic aromatic hydrocarbons (alkyl-

naphthalene, phenanthrene in this study). Such aromatiza-

tion is observed for pure compounds as well as humic acid,

this latter producing alkanols and alkanoic acids in high

proportion during THM–GC–MS.

5. Conclusion

All these results suggest that flash pyrolysis of recent

organic matter rich in alkanoic acids and/or alkanols, free or

bound to the macromolecular structure in the presence of

clay minerals can induce major interpretation errors,

especially concerning the identification of contaminants.

For instance, in environmental investigations, polycyclic

aromatic hydrocarbons (HAP) and the BTX (benzene,

toluene and xylene), included in the European and American

priority contaminant lists are important targets. This type of

analytical artifact can conduct to describe soils or sediments

as polluted whereas it is not the case. However, the nature of

the OM–clay interaction nature needs be precised.

On the contrary, thermally assisted hydrolysis methyla-

tion (THM–GC–MS), already recommended for preventing

cyclization and aromatization of fatty acids in the presence

of sulfur [42], needs to be used in order to limit

aromatization artifacts if demineralization of samples is

not carried out. As a matter of fact, molecular investigation

of macromolecules in raw matrixes by PyGC–MS contain-

ing clays should not be carried out unless thermally assisted

hydrolysis and methylation is used, this later inhibiting

generation of analytical artifacts due to clays.

Acknowledgment

The authors acknowledge R. Mosser-Ruck for chemical

and electron microprobe analyses on the different clay

minerals.

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aromatization of organic matter induced by the presence of clays during flash pyrolysis-gas...

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