25th International Meeting on Organic Geochemistry IMOG 2011

O-10
Sulfur species as facilitators for water splitting to react with
organic matter under medium temperatures
Ward Said-Ahmad 1 , Alon Amrani 2 , Zeev Aizenshtat 1
1 The Hebrew University of Jerusalem, Institute of Chemistry and Casali Institute, 2 Jerusalem, Israel, Institute
of Earth Sciences, The Hebrew University of Jerusalem, 91904 israel (corresponding author:
zeev@vms.huji.ac.il)
It has been suggested that water can act as hydrogen
and oxygen donor during thermal maturation of
sedimentary organic matter (OM).
The major hurdle to explain the water chemical
activity during the catagenetic stages or under
laboratory hydrous pyrolysis conditions is finding the
path lowering the very high activation energy required
for its splitting.
Homolytic cleavage of neat water without catalysis
require steam at >1200 0 C or ��
form H . and OH . radicals. Such high levels of energy
are not possible at natural depositional environments.
It is known that metals and sulfur (S) species can
catalyze the splitting of water even under mild
temperatures. Experiments performed under very
basic conditions (1 Molar NaOH) with elemental S
show that disproportionation occur at temperatures of
100-200ºC to the following species: S -2 ; Sn -2 ; SO3 -2 ;
S2O3 -2 and SO4 -2 (Lin et al., 2004).
In the present study we attempt to understand the role
of S species in the activation of water toward organic
matter in natural environments. We performed a set of
experiments to examined the effect of pH (from 4
to13), temperature (20-250°C), and the water-organic
interface by using emulsifiers.
The experiments between several S species (H2S ,
HS - ,Sx 2- , SO4 -2 ) and water performed with and
without the presence of organic model compounds.
In all the experiments we examined the organic and
inorganic products distribution and their δ 34 S values.
Our results show that elemental S in water
disproportionate above 120°C at pH 8-8.5 without the
presence of organic matter. As the pH and
temperature elevated this reaction is faster. In the
presence of organic compounds the disproportionate
of elemental S is significantly faster even at relatively
low pH and low temperature (80°C). This reaction
yields a variety of organosulfur compounds as well as
oxidized compounds (non-S) such as ketones (Fig.1).
The sulfurized and oxidized organic compounds seem
to form in parallel if no ammonia is present.
There is 5-13‰ fractionation between elemental S
and H2S which decrease as a function of temperature.
Organosulfur compounds and polysufides exhibit
smaller fractionation relative to elemental S of 0-3‰.
The sign of this fractionation is changing with the
temperature and organic substrate and the chemical
environment (such as the presence of emulsifiers).
This fractionation sign differences helps us to
determine formation pathway of certain products
(Amrani et al., 2008).
In summary, this study indicates that elemental S
catalyzes the splitting of water even at relatively low
temperatures. The hydroxyl radical which is form
(HO . ) react with both organic compounds and sulphur
species to produce oxidized compounds such as
ketones and oxosulfur (SnOx, x,n=1-4) as well as
radical sulphur species. S species such as thyil
radical (HS . ) reacts with organic compounds to form
organosulfur compounds.
Figure 1.
Conversion(%)
100
90
80
70
60
50
40
30
20
10
0
H2O +SReagent
130 o C
0 50 100 150 200 250 300 350 400
Time (h)
Hongfei Lin, Zhixia Li, Kazuyuki Tohji, Noriyoshi Tsuchiya and Nakamichi, 2004.
Proceeding of the Yamasaki 14th <strong>International</strong> Conference on the Properties of
Water and Steam in Kyoto , 356-368.
Amrani, A., Ma,Q., Said Ahmad, W. Aizenshtat, Z. and Tang, Y. 2008. Sulfur
isotope fractionation during incorporation of sulfur nucleophiles into organic
compounds. Chemical Communication. 11, 1356 – 1358.
HS
H2O H2S(aq) pH=8.5
Na2Sx(aq) pH=13
S8 pH=8.5
Na2Sx(aq) pH=8.5
(NH4)2Sx(aq) pH=8.5
S
S
S
S
O
S
S
S
S
S
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