Pyrolytic formation of C19 isoprenoid hydrocarbons from dihydrophytol: In relation to the genesis of pristane in petroleum

Article abstract of DOI:10.1246/cl.2000.206

This study was concluded to elucidate a pathway for formation of C₁₉ isoprenoid hydrocarbons (isops) in petroleum from chlorophylls. C₁₉ isops are predominantly produced when dihydrophytol is heated at 320°C for a period ranging from 1 to 5 h under vacuum while C₂₀ isops are predominantly produced when chlorophyll a or phytol is heated. A radical chain reaction of decomposition of dihydrophytol is proposed as plausible pathway for producing C₁₉ isops.

Full text of DOI:10.1246/cl.2000.206

206
Chemistry Letters 2000
Pyrolytic Formation of C19 Isoprenoid Hydrocarbons from Dihydrophytol:
in Relation to the Genesis of Pristane in Petroleum
Mariko Ishiwatari,* Keita Yamada,† and Ryoshi Ishiwatari†
Engineering Research Institute, School of Engineering, Graduate School of Tokyo University, Yayoi, Bunkyo-ku, Tokyo 113-8656
†Department of Chemistry, School of Science, Graduate School of Tokyo Metropolitan University, Hachioji, Tokyo 192-0397
(Received November 11, 1999, CL-990968)
chla5 and phytol8 showed that relationship in carbon isotopic
ratios between C19 isops and C20 isops produced on heating is
similar to that observed in common petroleum.15,16
This study was concluded to elucidate a pathway for for-
mation of C19 isoprenoid hydrocarbons (isops) in petroleum
from chlorophylls. C19 isops are predominantly produced when
dihydrophytol is heated at 320˚C for a period ranging from 1 to
5 h under vacuum while C20 isops are predominantly produced
when chlorophyll a or phytol is heated. A radical chain reac-
tion of decomposition of dihydrophytol is proposed as plausible
pathway for producing C19 isops.
As to the mechanism of C19 isops and C20 isops from
chlorophyll pigments, it is reasonable to consider that: the pres-
ence of C-C double bond (b in Figure 1) in phytyl side chain is
favorable for the formation of C20 isops on decomposition
because C20 isops are formed via β scission of the double
bond.3 Actually, the ratios of C19 isops over C20 isops are less
than 1 in the heating experiments of chla or phytol.2-4,7
Formation of C19 isops is hindered by the double bond; that is
scission of C-C bonds at α position of double bond, which
results in the formation of C19 isops, is unfavorable. Therefore
we predict that dihydrophytol (DHP) would yield larger amount
of C19 isops on heating. To verify our prediction, we heated
DHP at 320 ˚C for 1, 2.5 or 5 h in a sealed Pyrex glass ample
under vacuumed pressure in this study.
DHP was prepared by the following procedure. (1) Phytol
was hydrogenated by bubbling H2 gas with PtO2 in hexane for
10 min. (2) Since the hexane solution obtained was too emulsi-
fied for the catalyst to be removed by filtration, hexane was
evaporated and the residue was dissolved in benzene/ methanol
(3:1). (3) After the catalyst was removed, benzene/methanol
was evaporated from the filtrate and the residue was dissolved
in hexane, which was passed through silica-gel column with
hexane and methanol successively. (4) DHP was obtained as a
methanol fraction, which was confirmed by GC-MS analysis.
About 10 mg DHP thus prepared was heated in a Pyrex
glass ample (ca. 4 mm i.d. x ca. 100 mm) under conditions
shown above. The pyrolyzates were analyzed by GC-MS.
Major products were pristane, 1-pristene, phytane and phytenes
and some unidentified compounds as exemplified in Figure 2.
Smaller molecular weight compounds were not analyzed since
we focused only on the yield ratio of C19/C20 isops. As Table 1
shows, the total yield of C19 isops is larger than that of C20
isops, and the ratio of former to the latter increases with heating
time.
The ratio of pristane (2,6,10,14-tetramethylpentadecane:
saturated C19 isoprenoid hydrocarbon) to phytane (3,7,11,15-
tetramethylhexadecane: saturated C20 isoprenoid hydrocarbon)
in petroleum and geolipids (organic solvent extracts of sedi-
mentary rocks) is often used as an indicator to assess oxic/anox-
ic conditions of sedimentary environment.1 Phytyl side chain (a
in Figure 1) of chlorophyll a (chla, Figure 1) has been hypothe-
sized1 as a possible source of both pristane and phytane. A
number of heating experiments of chla2-7 and phytol (3,7,11,15-
tetramethyl-2-hexadecenol)8-10 have been carried out so far for
proving this hypothesis.
Since C20 isoprenoid hydrocarbons (phytenes and phytadi-
enes) are major products in these experiments, chla is common-
ly recognized as a major precursor of phytane. However, a
yield of C19 isoprenoid hydrocarbons (pristenes) is considerably
smaller than that of C20 isoprenoid hydrocarbons (isops) in all
experiments, which cannot explain wide ranges of ratios of
pristane to phytane observed for petroleum/geolipids (0.1-
12).1,9,11,12 Therefore, precursor of pristane is a controversial
issue. Goossens et al. proposed tocopherol as an alternative
precursor of C19 isops because a large amount of 1-pristene was
obtained on its flash pyrolysis.13
Recent development of isotopic ratio mass spectrometry
(IRMS) of organic compounds revealed that carbon isotopic
ratios of phytane and pristane from the same geological sam-
ples are similar in many cases, implying that these compounds
are of the same origin, chlorophylls.14 Our study of heating of
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
207
account for the formation of most hydrocarbons in petroleum.17
In conclusion, the results of the present study implies,
therefore, that DHP can be an important precursor of pristane in
petroleum/geolipids. DHP is a reduction product of phytol,
which is produced by cleavage of the phytyl side chain of
chlorophylls. For the production of DHP, the following
processes are known to play an important role under oxic con-
dition: a grazing of planktonic detritus by benthos,18 reduction
through the guts of copepods,19 or microbiological reduction of
phytol in early diagenesis.20
We hypothesize consequently that large ratios of pristane
over phytane frequently observed in petroleum/geolipids is
brought about by the following steps: (1) Formation of DHP
from phytol (hydrolysis product of chla) by microbial reaction
under oxic conditions, (2) formation of both C19 and C20 isops
by geothermal heating of DHP or DHP-incorporated kerogen,21
and (3) subsequent reduction of C19 and C20 isops.
The yield of C19 isops predominates that of C20 isops in all
cases, and the ratio of the former to the latter increases with
increase of heating time as we predicted. This result can
explain the ratios of pristane to phytane in petroleum/geolipids.
Moreover, carbon isotope compositions of pristane and phytane
which were obtained by reduction of C19 and C20 isops, give
similar values (difference within 0.5‰). The scheme shown
below is a plausible reaction for thermal decomposition of
DHP. The radical chain reaction sufficiently explains the for-
mation of C19 isops as well as C20 isops. Similar reactions are
assumable for the formation of C19 isops in sediment because
thermal processes including radical and catalytic reactions may
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