Chemistry Reference
In-Depth Information
adding H
2
S to the hydrogen gas feed during start-up. A common method
used to provide the optimum H
2
S concentration is to utilize liquid sul-
fiding agents. Common examples are carbon disulfide, dimethylsulfide
(DMS), and dimethyldisulfide (DMDS). Several models have been pro-
posed to describe the catalyst structure and the active sites for cata-
lysis.
161
One example is the Co-Mo-S model which states that after
sulfiding the catalytically active phase consists of small domains of MoS
2
-
like structures located inside the pores of the g-Al
2
O
3
. Co-Mo(S)/Al
2
O
3
and Ni-Mo(S)/Al
2
O
3
catalysts have substantially higher activity compared
to Mo(S)/Al
2
O
3
, so Co and Ni are typically considered to be promoters for
the Mo activity. The Co-Mo-S model shows that the promoter metal atoms
are located at the edges of the MoS
2
crystallites.
165
The role of the pro-
moter is to act as an electron donor to the molybdenum atoms, weak-
ening the Mo-S bonds and thus generating a sulfur vacancy site active for
heteroatom removal.
161
Because of the resemblance between hydroprocessing of petroleum
and bio-oils, several studies involving conventional CoMo and NiMo-
catalysts have been conducted. Early work has been comprehensively
reviewed.
2-6,166
Compared to hydroprocessing of petroleum, upgrading of
bio-oils seems to require lower Liquid-Hourly-Space Velocity (LHSV) and
higher pressure, and the H
2
-consumption is considerably higher, and the
catalyst stability poorer.
HT-catalysts typically possess their highest activity for HDO in their
sulfide form, but bio-oils generally contain insucient levels of sulfur to
maintain the catalyst structure in the desired sulfided state during op-
eration. As was stated earlier, the catalytically active sites for hydro-
genolysis are coordinately unsaturated sulfur vacancy sites where both
H
2
S and oxygenates adsorb. The concentration of vacancies is a function
of the H
2
S and H
2
concentrations, and thus the absence of S-containing
species in the feed can alter the catalyst structure. Addition of a sulfiding
agent to the bio-oil feed is a way to remediate the loss of activity. Adding
H
2
S to the gas feed during hydrotreating of aromatic compounds like
phenol and anisole strongly decreases the HDO activity of a sulfided
CoMo catalyst.
167
The dominant hydrogenolysis route is more affected,
altering the product distribution. Sulfur addition did not seem to prevent
catalyst deactivation due to coke formation.
168
On the other hand, HDO
of aliphatic oxygenates is promoted by H
2
S.
169
The acid-catalyzed re-
actions are favored with increasing amounts of H
2
S. Forming H
2
S via
addition of CS
2
to the reaction mixture did not have the same effect, but
introduced coke on the acidic sites instead. Both sulfur-compounds do
not prevent catalyst deactivation. The self-inhibiting effect of water
formed during HDO can be effectively compensated by the presence of
H
2
S.
170
The catalyst deactivation rate during processing of rapeseed oil
could be significantly reduced by continuously adding DMDS to the
vegetable oil.
171
Senol et al.
172
compared the reactivity of aromatic and
aliphatic oxygenates and found that H
2
S inhibits HDO of phenol and
promotes the HDO of methyl heptanoate. The NiMo-catalyst was most
active for HDO of methyl heptanoate,
173,174
while the conversion of
phenol was highest for the CoMo-catalyst.
