Download The behaviour of esters in the presence of

Journal of Analytical and Applied Pyrolysis, 26 ( 1993) 175- 184
175
Elsevier Science Publishers B.V., Amsterdam
The behaviour of esters in the presence of
tetramethylammonium
salts at elevated temperatures;
flash pyrolysis or flash chemolysis?lv2
J.W. de Leeuw * and M. Baas
Netherlandr Institute for Sea Research (NIOZ), Division of Marine Biogeochemistry,
P.O. Box 59, 1790 AB Den Burg, Texel (Netherlands)
(Received November 2, 1992; accepted in final form January 26, 1993)
ABSTRACT
Flash-heating experiments with model compounds such as tomato cutin, octadecyl octadecanoate, hexadecanol, triacontanol and L-cc-lecithin with and without tetraalkylammonium
salts, in particular tetramethylammonium
hydroxide (TMAH), show that we have to
discriminate between pyrolysis products sensu strict0 and products resulting from bond
breaking induced by chemical reagents at elevated temperatures. This discrimination is
required to improve the structural elucidation of the macromolecules to be analyzed.
Esters; flash chemolysis; flash pyrolysis; pyrolysis; tetramethylammonium
salts.
INTRODUCTION
To improve the gas chromatographic separation of relatively polar pyrolysis products, attempts have been made to perform so-called on-line derivatization of such pyrolysis products generated during flash pyrolysis-gas
chromatography
(Py-GC) or flash pyrolysis-gas chromatography/mass
spectrometry (Py-GC/MS)
analyses of macromolecular organic matter.
Challinor [l] noticed that synthetic polymers subjected to pyrolysis conditions in the presence of tetraalkylammonium
salts such as tetramethylammonium hydroxide (TMAH) yielded methyl esters and methyl ethers. He
suggested that fatty acids and alcohols are produced as primary pyrolysis
products and that these compounds were derivatized, i.e. methylated in this
case. Other workers [2-61 have reported on the methylation of free acids
* Corresponding author.
’ NIOZ Division of Marine Biogeochemistry Contribution 280.
* Presented at: Pyrolysis ‘92-Proceedings
of the 10th International Conference on Fundamental Aspects, Processes and Applications of Pyrolysis, Hamburg, Germany, September
28-October 2, 1992.
01652370/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
176
J. W. de Leeuw and hf. Baas / J. Anal. Appl. Pyrolysis 26 (1993) IV-
184
with tetraalkylammonium
hydroxides at elevated temperatures. It was
shown that free carboxylic acids are quantitatively converted to the corresponding tetramethyl~monium
salts which yield methyl esters and
trimethylamine between about 250 and 350°C. This procedure was sometimes referred to as “pyrolytic methylation” and was applied more recently
to improve the gas chromatographi~ resolution of mixtures of fatty acids. In
a number of more recent papers [7-lo] natural esters were converted to
their methyl esters by tetramethyl- or other tetraalkylammonium hydroxides
under pyrolysis conditions. It was indicated that the methyl esters were the
result of transeste~fi~ation reactions via intermediate tetramethyla~onium hydroxide salts and were not generated by methylation of pyrolytically
formed fatty acids, although the genesis of free carboxylic acids from esters
by pyrolysis is well known [ 1l].
Based on the above-mentioned observations it is not completely clear if
tetraalkylammonium
hydroxide salts under pyrolytic conditions act as
chemical reagents inducing bond breaking within the macromolecular substances to be analysed (chemolysis) or if they derivatize, i.e. methylate,
pyrolysis compounds which are generated from the macromolecular substances by bond breaking induced by heat only (pyrolysis).
In the study presented here a number of selected model compounds
were subjected to pyrolysis conditions with and without TMAH to determine whether the products obtained have to be considered as pyrolysis
or chemolysis products. Such discrimination is important to enable a
correct molecular characterization of the macromolecular substances based
on the products formed. ~urthe~ore,
if we are dealing with thermally
assisted chemolysis, many new possibilities complementary to analytical
pyrolysis will become available for characterizing complex mixtures of
synthetic polymers and biomacromolecules
using all kinds of specific
chemical reagents.
EXPERIMENTAL
Curie point pyrolysis-gas
chromatography (Curie point Py
-GC)
The samples were pressed onto flattened ferromagnetic wires at a Curie
temperature of 358 and 610°C. Approximately 2 ~1 of a 25% w/w solution
of tetraammonium hydroxide (TMAH) in water was used as the reagent for
in situ methylation. A high frequency generator (Fischer 9425) was used to
induce the magnetic field. The gas chromatograph (Hewlett-Packard HP5890) was equipped with a cryogenic unit and programmed from 0°C
(5 min) to 300°C (10 min) at a rate of 3”C/min. Separation of the products
was achieved by a 25 m fused silica capillary column coated with CP-Sil 5
(0.32 mm i.d.; film thickness 0.45 pm). Helium was used as carrier gas. The
temperature of the flame ionization detector was 320°C.
J. W. de Leeuw and M. Baas 1 J. Anal. Appl. Pyrolysis 26 (1993) 175- 184
Curie point pyrolysis-gas chromatography/mass
(Curie point Py - GCIMS)
111
spectrometry
Pyrolysis was performed as described above. The column was directly
inserted into the EI ion source of a VG-70SE double focusing mass spectrometer (mass range m/z 40-800; cycle time 1.8 s; ionization energy 70 eV).
RESULTS
AND
DISCUSSION
Figure 1 shows the gas chromatogram of the pyrolysate of the natural
polyester cutin isolated from the fruit of Lycopersicon esculentum, the
tomato plant. This pyrolysate has already been analysed by Tegelaar et al.
[ 111 and the pyrolysis products were explained by assuming six-membered
rearrangement reactions occurring at the ester linkages, resulting in the
formation of di-unsaturated fatty acids and mono-unsaturated
mid-chain
hydroxy fatty acids corresponding to the building blocks of cutin. For
convenience the mechanisms are schematically indicated in Fig. 2 [I]. It is
emphasized that the terminal ester-linked alcohol moieties yield terminal
alkene moieties and that free alcohol groups at C9 or Cl0 are not dehydrated during pyrolysis. It has been speculated that the ratio of pyrolysis
products with a mid-chain hydroxy group and those with a mid-chain
double bond is an indicator for the degree of cross-linking in these natural
TOMATO CUTIN
Py-GCFID (610-c)
4
c,
C,B C-C-yo-Co-C*-
c=c
I
OH
OH
I
I
C,BC-c
OH
-c40
‘OH
10
\
Fig. 1. Gas chromatogram
of the pyrolysis
cutin isolated from L. esculentum.
products
obtained
from the natural
polyester
178
J. W. de Leeuw and M. Baas / J. Anal. Appl. Pyrolysis
\;/o-;
-
CtiO
IO
I'O-_c/
16
O\
o#=m
/o--
10
1
16
OH
Fig. 2. Proposed pyrolysis mechanism
hexadecenoic
acids from the polyester
TOMATO
26 (1993) 175-184
CUTIN
for the genesis of hexadecadienoic
cutin.
MeOC
Py(Me)-GCFID (610°C)
MeOC -
C=$,y
acids and hydroxy-
$0
-c
16
OMe
I
’ OMe
C
MeOC
40
‘OMe
\
/
-C,,
C
90
‘OMe
Y
OH
I
c,,c
C
tieOC,,-C,:
fro
‘OMe
OMe
40
*
HC=,6-C,oC,
Fig. 3. Gas chromatogram
of the products obtained from the natural polyester
from L. esculentum in the presence of TMAH under pyrolysis conditions.
OMe
cutin isolated
polyesters. p-Vinylphenol may originate from esterified p-coumaric acid
moieties, which generate p-coumaric acid as a primary pyrolysis product
that subsequently undergoes decarboxylation [ 111. Figure 3 shows the gas
J. W. de Leeuw and M. Baas / J. Anal. Appl. Pyrolysis 26 (1993) 17% 184
119
chromatogram of the compounds obtained by flash heating the tomato cutin
sample in the presence of TMAH under the same conditions as mentioned
above for the pyrolysis experiment. The compounds indicated were identified
by comparison of their mass spectra and GC retention times with those of
standards. The major compounds are dimethoxymethyl esters and monohydroxymonomethoxymethyl
esters. Products with mid-chain and/or terminal
double bonds are minor compounds. The virtual absence of these latter
compounds, which were the dominant products generated by pyrolysis (cf.
Fig. l), and the abundant presence of methoxy and hydroxy compounds
clearly shows that pyrolysis has occurred not at all or only to a minor extent.
Most of the products can be explained by assuming a hydrolysis reaction
followed by quantitative methylation of the fatty acid moieties as already
suggested by Dworzanski et al. [9] and partial methylation of the hydroxyl
groups. It is also noteworthy to indicate the presence of the dimethyl ester
and the 16-methoxymethyl ester, products not encountered in the pyrolyzate
obtained without TMAH. The assumption that p-vinylphenol in the pyrolyzate originated from p-coumaric acid moieties was substantiated by the
relatively abundant presence of ~-methoxycinnamic
acid methyl ester. To
further investigate the mechanism of demacromolecularization
in the presence of TMAH through a hydrolysis-methylation
pathway, a C36 saturated
wax ester, octadecyl octadecanoate, was subjected to pyrolysis/evaporation
conditions with and without TMAH. Figure 4 shows the GC traces of
reaction mixtures obtained from experiments performed with ferromagnetic
wires at Curie temperatures of 358 and 610°C. The results of the experiments
performed without TMAH are very similar and clearly indicate that the wax
ester does not pyrolyze but simply evaporates. The peaks in the middle of
the chromatogram correspond to impurities present in the wax ester sample.
Their relative abundances are greatly overestimated because the high boiling
point of the intact wax ester severely hampers a quantitative transfer from
the pyrolysis chamber to the capillary column. Direct on-column injection
of the wax ester sample substantiated the presence of small amounts of
these impurities. The product mixture obtained in the presence of TMAH
using a ferromagnetic wire at a Curie temperature of 358°C shows the
abundant presence of octadecanoic acid methyl ester, octadecanol and
1-methoxyoctadecane
next to the intact wax ester. The same results
were obtained v+th a ferromagnetic wire at a Curie temperature of 61O”C,
although the wax ester was barely recognizable. These results clearly show
that no pyrolysis occurred in all ex~riments with the wax ester; in the
absence of TMAH only evaporation takes place whereas in the presence of
TMAH hydrolysis and partial methylation of the hydrolysis products occurs
as indicated in Fig. 5. It should be noted that in the presence of TMAH using
a 358°C wire competition between hydrolysis-methylation
and evaporation
takes place whereas hydrolysis-methylation
is virtually the only process if
more energy is available in the case of a 610°C wire.
180
J. W. de Leeuw and M. Baas / J. Anal. Appl. Pyrolysis 26 (1993) 175- 184
C36:O WAXESTER
PY-GC (S5S/610%)
C36:O WAXESTER
Py(Me)-GC (35e%)
‘t
i
I
t
C3S:O WAXESTER
tin
Py(Me)-GC (610%)
I
t
;
C , ,,-OMe
.
-
t
retention time Fig. 4. Gas chromatograms of the products from octadecyl octadecanoate
pyrolysis conditions with and without TMAH.
generated under
J. W. de Leeuw and M. Baas 1 J. Anal. Appl. Pyrolysis 26 (1993) 175- 184
‘I,~,-
181
$0
"-0-C
0-
I
H
18 37
(CH,),N+
OH-
+
(CH3)4N+
C,,H,-b-0-C,,H,
I
OH
1
‘17~35-
//O
‘\OH
+
//O
C,7H35-C\o-
-O -
Cl 857
-4
Ho
-
5
e.%7
_
W,)4N+
I
C,7H,-C90
‘O-N+(CH
I
//O
C17H~-c’O-CH
C,,H,,OH
+
(CHJ4N+OH-
)
34
n
+
3
=
(CH&N
C,8H370-N+(CH3)4
+
H,O
n
5eH370Me
+
(CH,),N
Fig. 5. Mechanisms of methyl ester and methyl ether formation.
To investigate the methylation behaviour of alcohols, 1-hexadecanol and
I-tricosanol were subjected to pyrolysis conditions with and without
TMAH. Figure 6 shows the GC traces of the I-hexadecanol experiments.
The alcohol evaporates with and without TMAH. Obviously, the energy
required for evaporation is insutkient to methylate the alcohol. In the case
of the I-triacontanol the GC data (Fig. 7) showed that the energy required
for evaporation of this higher boiling alcohol is similar to the amount of
energy needed to methylate the alcohol. As a result, the alcohol is partly
methylated. In a separate experiment it was shown that the C30 alcohol also
became partly methylated in the presence of methanol. Experiments with
J. W. de Leeuw and M. Baas 1 J. Anal. Appl. Pyrolysis 26 (1993) 175-184
182
i3,CH2OH
Cl 6 ALCOHOL
Py-GC-FID (610%)
I
I
Cl 6 ALCOHOL
Py(Me)-GC-FID (610%)
-
retentiontime
Fig. 6. Gas chromatograms
obtained after heating I-hexadecanol with and without TMAH.
TMAH and hexadecanoic acid showed, as expected, a quantitative conversion to the methyl ester.
Because natural samples may contain compounds with tetraai~ylammonium moieties it was interesting to study the behaviour of a lecithin
J. W. de Leeuw and M. Baas 1 J. Anal. Appl. Pyrolysis26 (1993) 115-184
183
C30 ACOHOL
Py-GC-FIR (610%)
Cz9H 59CH20Me
‘20 ALCOHOL
Py(Me)-GC-FID (610°C)
retentiontime -
Fig. 7. Gas chromatograms
obtained after heating I-triacontanol
with and without TMAH.
(L-a-phosphatidylcholine),
a very common cell membrane component of
many organisms. CC/MS analysis of the product mixture revealed the
abundant presence of both the free carboxylic acids and their methyl esters
184
J. W. de L.eeuw and M. Baas / J. Anal. Appl. Pyrolysis
26 (1993) 175-184
indicating that the carboxylic acid moieties were partly transesterified by the
trimethylammonium
group.
The data presented above clearly show that we have to discriminate
between pyrolysis products sense strict0 and products generated through a
reaction between the macromolecules and a specific chemical reagent at an
elevated temperature. The latter process may be indicated by the term
the~ally
assisted chemolysis (TAC). As in the case of lecithin it can
become difficult to refer to the products as pyrolysis products or TAC
products. Although this situation may be encountered in natural samples, it
is important to discriminate as much as possible between pyrolysis and
chemolysis products in order to optimally reconstruct the molecular structures present in the samples analysed. In fact, pyrolysis and TAC data
complement each other so that the combined data enhance the reliability
of the macromolecular structures proposed. The results with TMAH illustrate that it will be worthwhile exploring thermally assisted chemolytic
approaches using a variety of specific reagents in the future to better
characterize macromolecules.
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