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In-Depth Information
Fig. 6 Transformation of guaiacol over supported cobalt-molybdenum sulphide catalysts
due to a heterolytic cleavage of the O-methyl bond. Reaction conditions: 573 K and 4 MPa
of H
2
(Adapted from Bui et al.
54
).
catalytic system, part of guaiacol is converted directly to phenol by the
demethoxylation step with formation of methanol. Then the phenol
intermediate mostly transforms to benzene via a direct deoxygenation
(DDO) pathway involving direct C-O hydrogenolysis. Similarly to the direct
desulfurization (DDS) pathway in hydrodesulfurization, the main advan-
tage of the DDO pathway compared to the hydrogenation route is its fa-
vorable hydrogen economy. The use of g-alumina as support for this
promoted CoMoS phase leads to demethylation/methyl-substitution re-
actions after C-O cleavage and the formation of methyl-substituted prod-
ucts does not facilitate the deoxygenation process. Clearly, supports play a
significant role in the hydrotreating process, which will be discussed later
(Section 5).
The problem of using sulphide catalysts is that desulphurization may
occur during reaction, which decreases the catalytic activity. To avoid
this, H
2
S must be co-fed into the system to regenerate the sulphide
phase.
69,70
Although adding H
2
S stabilizes the selectivity and improves
the conversion, the total conversion shows a similar decreasing trend as a
function of time as in the absence of H
2
S. The studies of Senol et al. show
that trace amounts of thiols and sulphides were formed during hydro-
treating of 3 wt.% methyl heptanoate in m-xylene at 1.5 MPa and 523 K
over Co-MoS
2
/Al
2
O
3
co-fed with up to 1000 ppm H
2
S.
69,70
These studies
indicate that sulphur contamination of the sulphur free oil can occur
when using sulphide type catalysts. Thus, the influence of the sulphur on
this catalyst is dicult to evaluate and needs further attention. Future
work should focus on the development of a stable process and evaluate
the functioning of non-sulfur-based catalysts.
4.2 Transition metal catalysts
Transition metal catalysts are also frequently tested in the hydrotreating of
bio-oil. These catalysts include supported noble metal (platinum, pal-
ladium, rhodium, and ruthenium) and transition metal (nickel, copper,
and cobalt) catalysts.
45,51,66,73-88
Mechanistic speculations for these systems
indicate that the catalysts should be bifunctional. The bifunctionality of the
catalyst implies two aspects. On the one hand, activation of oxy-compounds
is needed, which is achieved through the valence of an oxide of the tran-
sition metal or on an exposed cation, often associated with the catalyst
support. On the other hand, hydrotreating of the activated compound be-
comes possible in the presence of transition metals, which activate H
2
.
Figure 7 shows an example of the hydrotreating of model compound
vanillin over the bifunctional catalyst, Pt/Al
2
O
3
. Among the possible
