of 4,6-dimethyl-dibenzothiophene - ETH E

DISS. ETH NO. 16434
STUDY OF THE DEEP HYDRODESULFURIZATION
OF 4,6-DIMETHYL-DIBENZOTHIOPHENE
OVER PT AND PD CATALYSTS
A dissertation submitted to the
SWISS FEDERALINSTITUTE OF TECHNOLOGYZÜRICH
for the degree of
Doctor of Technical Sciences
presented by
ADELINE VALERIE NIQUILLE-ROETHLISBERGER
Ing.
Chim. Dipl.
EPF, Swiss Federal Institute of Technology Lausanne
bornon May 8, 1979
Citizen of Langnau im Emmental (BE)
accepted
on
the recommendation of
Prof. Dr. R. Prins, examiner
Prof. Dr. J.A.R. van Veen, co-examiner
Zürich, 2005
Abstract
Because of more
stringent
environmental
legislation regarding
the sulfur
process heavier feedstocks, the
role of hydrodesulfurization(HDS) has grown in importance in the last decade.
Problematic over conventional sulfided catalysts is the removal of sulfur from the socontent of
transportation
fuels and the
refractory S-compounds,
necessity to
alkylated dibenzothiophenes. An alternative
is the use of very active noble metal catalysts, such as Pt and Pd. Moreover, under
deep HDS conditions, sulfur and nitrogen components are present in comparable
amounts and may infiuence each other by competitive adsorption. For these reasons,
our research focused on the HDS of 4,6-dimethyl-dibenzothiophene (4,6-DM-DBT)
over different Pt, Pd, and Pt-Pd catalysts and on the effect of N-containing molecules,
such as piperidine and Pyridine.
The HDS reaction network of 4,6-DM-DBT was investigated in detail at 300°C
called
such
as
and 5 MPa total pressure over in situ reduced Pd/y-AI203 in a continuous mode in a
fixed-bed high-pressure unit. This model S-compound reacts via two pathways : 1%
by
3,3'-dimethyl-biphenyl and 99%
transforms through hydrogenation yielding the S-intermediates 4,6-dimethyltetrahydro-dibenzothiophene, 4,6-dimethyl-hexahydro-dibenzothiophene, and 4,6dimethyl-perhydro-dibenzothiophene which subsequentlydesulfurize to 3,3'-dimethylconverts
direct desulfurization
leading
to
cyclohexylbenzene and 3,3'-dimethyl-bicyclohexyl. The three S-intermediates were
synthesized, so that their separate HDS could be studied. It showed that
hydrogenation and dehydrogenation between the three S-intermediates are easy on
the catalyst surface. 4,6-Dimethyl-tetrahydro-dibenzothiophene and 4,6-dimethylhexahydro-dibenzothiophenewere in equilibrium in the HDS of 4,6-DM-DBT and
dose to
was
equilibrium
in the reactions
easier from the
more
starting from the S-intermediates.Desulfurization
saturated molecules, confirming that the molecular
planarity has to be removed to decrease the steric hindrance of the substituents and
to provide better access to the S-atom. Temperature promoted the sulfur-removal
steps more than the hydrogenation reactions.
VII
The desulfurization of
than over Pd
dibenzothiophene
and 4,6-DM-DBT was easier over Pt
alumina. On the other hand, Pd had very good
supported on
hydrogenation ability. The bimetallic Pt-Pd catalysts were extremely active in both
reaction pathways, with a small relative enhancementof the hydrogenation route. This
validates the existence of a synergetic effect between the noble metals and suggests
the formation of a new type of active sites with improved hydrogenation properties.
Moreover, the surface of the metal particles is enriched in Pd, as indicated by the
similar product distributionsobtained with the bimetallic and Pd catalysts. Because the
methyl groups sterically hinder the direct access to the S-atom, 4,6-DM-DBT reacts
almost exclusively via hydrogenation, while dibenzothiophene reacts mainly by direct
desulfurization.
In the HDS of 4,6-DM-DBT, the noble metal
catalysts
were
much
more
active
supported on amorphous silica-alumina (ASA) than on alumina. This is because
the ASA support makes the metal particles electron-deficient by partial Charge transfer
from the metal Clusters to the acidicSites of the support. These electron-deficient metal
particles are more tolerant to H2S. Cracking and isomerization also contributed to this
higher activity. A similar infiuence of the support acidity was found for all the catalysts.
Furthermore, the presence of large metal particles selectively enhanced the
hydrogenation pathway, probably because of the increased proportion of large
ensembles of active Sites, such as terraces, required for adsorption in the Ti-mode.
The HDS of 4,6-DM-DBT over the ASA- and alumina-supported noble metal
catalysts was strongly inhibited by piperidine and pyridine. At low initial concentration
of the amine, the inhibition was stronger by pyridine and at high initial concentrationit
was stronger by piperidine, probably because of partial hydrogenation of pyridine to
piperidine at low initial pyridine concentration. Both reaction routes were inhibited but
hydrogenation to a greater extent; piperidine had the stronger effect. All hydrogenation
steps of the HDS network were slowed down more than the sulfur-removal reactions.
Moreover, piperidine and pyridine suppressed the cracking and further hydrogenation
of the desulfurized products completely and decreased the isomerization drasticallyby
neutralizing the acidic Sites of the support and reducing the amount of highly active
electron-deficient noble metal particles. However, despite their high sensitivity to Ncontaining molecules, the ASA-supported noble metal catalysts remained more active
when
than the
alumina-supported catalysts.
VIII
Resume
A
cause
de la
legislation plus rigoureuse
relative ä la teneur
en
soufre des
carburants et de la necessit.6 de traiter des matieres premieres plus lourdes, le röle
de l'hydrodesulfuration (HDS) a pris de l'importance dans la derniere decennie.
L'elimination du soufre des soi-disant composes soufres refractaires tels que les
dibenzothiophenes alkyles
est
problematique
conventionnels. Une alternative est l'utilisation de
metaux nobles tels que Pt et Pd. En
sur
les
catalyseurs
catalyseurs tres
sulfures
actifs ä base de
outre, dans des conditions d'HDS profonde, les
composes soufres et nitres sont presents en quantites comparables et peuvent
s'influencer mutuellement par adsorption competitive. Pour ces raisons, notre
recherche s'est concentree sur l'HDS du 4,6-dimethyl-dibenzothiophene (4,6-DM-
DBT) sur differents catalyseurs ä base de Pt, Pd et Pt-Pd, et sur l'effet de molecules
nitrees telles que la piperidine et la pyridine.
Le reseau de reaction de l'HDS du 4,6-DM-DBT a ete etudie en detail en
mode continu ä 300°C et 5 MPa de pression totale sur Pd/Y-Al203 reduit in situ dans
une unite haute pression ä lit fixe. Ce compose soufre modele
reagit selon deux
voies : 1% est converti par desulfuration directe donnant le 3,3'-dimethyl-biphenyle,
et 99% sont transformes par hydrogenation produisant les intermediairessoufres 4,6-
dimethyl-tetrahydro-dibenzothiophene, 4,6-dimethyl-hexahydro-dibenzothiophene et
4,6-dimethyl-perhydro-dibenzothiophene, qui sont par la suite desulfures en 3,3'dimethyl-cyclohexylbenzene
soufres ont ete
3,3'-dimethyl-bicyclohexyle.
et
synthetises,
de facon ä
Les trois intermediaires
que leur HDS puisse etre etudiee
separement. Cela a montre que l'hydrogenation et la deshydrogenationentre ces
trois molecules soufrees sont faciles ä la surface du catalyseur. Le 4,6-dimethyl-
tetrahydro-dibenzothiophene et
le
ce
4,6-dimethyl-hexahydro-dibenzothiophene
etaient
equilibre dans l'HDS du 4,6-DM-DBT et proches de l'equilibre dans les reactions ä
partir des intermediaires soufres. La desulfuration a ete plus facile ä partir des
molecules plus saturees, ce qui confirme que la planarite moleculaire doit etre
eliminöe pour diminuer la gene sterique des substituants et fournir un meilleur acces
en
IX
ä l'atome de soufre. La
temperature a promu davantage les etapes d'elimination du
soufre que les reactions d'hydrogenation.
La desulfuration du dibenzothiopheneet du 4,6-DM-DBT a ete plus facile sur
Pt que sur Pd supporte sur alumine. En revanche, Pd a eu une trös bonne capacite
d'hydrogenation. Les catalyseursbimetalliquesä base de Pt-Pd ont ete extremement
actifs dans les deux voies de reaction, avec une legere augmentation relative de la
voie d'hydrogenation. Ceci valide l'existence d'un effet synergique entre les metaux
nobles et suggere la formation d'un nouveau type de sites actifs ayant des proprietes
d'hydrogenation ameliorees. De plus, la surface des particules metalliques est
enrichie en Pd, comme indique par les distributions de produits similaires obtenues
avec les catalyseurs bimetalliques et ä base de Pd. Du fait que les groupes methyles
genent steriquement l'acces direct ä l'atome de soufre, le 4,6-DM-DBT reagit
presque exclusivement par hydrogenation, alors que le dibenzothiophene reagit
principalement par desulfuration directe.
Dans l'HDS du 4,6-DM-DBT, les catalyseurs ä base de metaux nobles ont ete
beaucoup plus actifs, Supportes sur silice-alumine amorphe (ASA) que sur alumine.
Ceci est du au fait que le support ASA rend les particules metalliques deficientes en
electron, par transfert partiel de charge des ensembles metalliques aux sites acides
du support, Ces particules metalliques deficientes en electron sont plus tolerantes au
H2S. Le craquage et l'isomerisation ont egalement contribue ä cette plus grande
activite. Une infiuence similaire de l'acidite du support a ete trouvee pour tous les
catalyseurs. Par ailleurs, la presence de grandes particules metalliques a
selectivement augmente la voie d'hydrogenation, probablement ä cause de la
Proportion accrue de grands ensembles de sites actifs tels que les terrasses
necessaires ä
l'adsorption en mode n.
L'HDS du 4,6-DM-DBT
Supportes
sur
ASA et alumine
a
sur
les
catalyseurs
ä base de metaux nobles
ete fortement inhibee par la
piperidine
et la
pyridine.
d'amine, l'inhibition etait plus forte par la pyridine tandis
qu'ä forte concentration initiale, eile etait plus forte par la piperidine, probablementä
cause de l'hydrogenation partielle de la pyridine en piperidine ä faible concentration
initiale de pyridine. Les deux voies de reaction ont ete inhibees, mais l'hydrogenation
dans une plus grande mesure; la piperidine a eu un effet plus fort. Toutes les etapes
A faible concentration initiale
d'hydrogenation
du reseau d'HDS ont ete
davantage
ralenties que les reactions
piperidine et la pyridine ont completement
l'hydrogenation supplementaire des produits desulfures et
d'elimination du soufre. De
supprime le
craquage et
ont radicalement diminue
plus,
la
l'isomerisation, en neutralisant les sites acides du support
et reduisant la quantite de particules de metaux nobles deficientes en electron et tres
actives. Cependant, malgre leur grande sensibilite aux molecules nitrees, les
catalyseurs ä base de metaux nobles Supportes sur ASA sont restes plus actifs que
ceux Supportes sur alumine.
XI