25th International Meeting on Organic Geochemistry IMOG 2011

P-362
The reaction of elemental sulfur with organic compounds:
formation of benzothiophenes and dibenzothiophenes
Zhibin Wei 1 , Scott Northrop 2 , Heather Rehmer 3 , Steve MacFarland 3 , Glenn Otten 4 ,
Clifford Walters 5 , Paul Mankiewicz 1 , Marlene Madincea 4
1 ExxonMobil Exploration Company, Houston, United States of America, 2 ExxonMobil Development
Company, Houston, United States of America, 3 ExxonMobil Production Company, Houston, United States of
America, 4 ExxonMobil Upstream Research Company, Houston, United States of America, 5 ExxonMobil
Research & Engineering Company, Houston, United States of America (corresponding
author:zhibin.wei@exxonmobil.com)
Dibenzothiophene (DBT) can be formed at low
temperatures from aromatic hydrocarbons upon
treatment with sulfur with the aid of catalyst (e.g.,
AlCl3). Many organic compounds can cyclize to form
thiophenes when heated with sulfur or H2S. Sulfur is
known to abstract hydrogen from organic compounds
at high temperatures to produce fragments that
ultimately give stable products by aromatization, ring
formation, or dimerization. However, it is unclear that
benzothiophenes (BTs) and DBTs can be formed
from smaller ring cyclics (e.g., cyclohexane),
aromatics (e.g., toluene) when heating with sulfur at
higher temperatures. To address this issue, heating
experiments were conducted on a variety of individual
reagents including cyclohexane, xylenes,
trimethylbenzenes, toluene, and cyclohexamylamine
in the presence of sulfur using sealed gold tubes to
investigate if BTs and DBTs can be generated. In
addition, these reagents were reacted as mixtures
with cyclohexylamine to determine if the yields of
these thiophenic compounds increase. The
experiments were conducted at different
temperatures and time scales to determine the
influece of these parameters on BT and DBT yields
and distributions.
Our results show that DBT is not observed when
cyclohexane is reacted with sulfur at 190 °C (Fig. 1).
However, DBT and 2,5-dimethylthiophene and are
generated in significant yields as the reaction
temperature is elevated to 250 °C and the yield of
DBT slightly increases with increasing reaction time.
Although cyclohexanethiol is readily formed at 190 °C,
it is very reactive and tends to disappear to form
benzenethiol at 250 °C as reaction proceeds.
Diphenyl sulfide and diphenyl disulfide are both
formed at higher temperature, but are nearly absent
at lower temperature. Sulfur can cause
dehydrogenation of organic compounds, cyclization,
and in some instances, reduction. Upon treatment
with elemental sulfur at 190 °C and 250 °C, xylenes
did not form DBTs. However, BTs are formed at both
heating temperatures. A fair quantity of 4,5-
dihydrobenzothiophene is produced from the reaction
of xylenes and sulfur at 250 °C for 1 day. As the
reaction proceeds, sulfur abstracts more hydrogen
from this compound, leading to the formation of BT.
Trimethylbenzene generated abundant alkylated BTs
but no DBTs upon treatment with sulfur.
Cyclohexylamine forms much more BTs and DBTs
than cyclohexane in the presence of sulfur. The
yields of the BTs and DBTs are improved by the
presence of cyclohexylamine and the reactions are
considerably accelerated due to the heterolytic
cleavage of the covalent S-S bond in the cyclic S8
molecule with the formation of polysulfide ions. These
ions undergo hemolytic cleavage with the formation of
free radicals which then serve as initiators of
dehydrogenation. The occurrence of dimeric products
(e.g., diphenyl sulfide) is consistent with radical
reactions. In addition, the yields of these thiophenic
compounds slightly increase with increasing reaction
time.
Relative abundance
A
B
C
SH
SH
S
S
S
SH
SH
S
SH
SH
HS
SH
HS
HS
20 40
Retention time (minute)
SH
Fig. 1. Total ion chromatograms of the reaction
products from cyclohexane and elemental sulfur at (A)
190°C for 7 days, (B) 250°C for 1 day and (C) 250°C
for 7 days
S
SH
SH
s s
s s
s s
s s
S SH
S S
S
SH S
S
S
S
S
S
S
492