Chemistry Reference
In-Depth Information
activities for all three reactions were related to molybdenum dispersion
and variation of anion vacancies. The promotion effect of Co and Ni was
observed as activity increased with promoter concentration up to 5 wt%
for all tested reactions.
In conclusion, bifunctional metal catalysts supported on mesoporous
silica materials are promising potential materials for the preparation of
novel HDO catalysts for upgrading of bio-oils. However, further research
on catalyst optimization with respect to the thermal stability, porosity,
acidity, metal loading as well us an understanding of promotion effects is
necessary in order to improve the activity and selectivity of applied
catalysts to attain high quality upgraded bio-oils.
3.4 Noble metal catalysts
Noble metals have well proven and often excellent catalytic properties
and are established commercially for purposes within various fields
including conventional oil refining and petrochemistry. Examples are
Pt-based catalysts (with Re or Sn promoters) for catalytic naphtha
reforming,
220,221
one of the major octane processes, Pt/Sn for de-
hydrogenation of light alkanes (C
3
and C
4
),
222
and Pd-based catalysts for
selective hydrogenation.
223
However, the high cost of noble metals, due
to limited availability, provides a major motivation to minimize their
levels in applications or to find well-performing alternative systems.
A particularly well-defined low temperature activity and often a high
resistance towards deactivation are key properties that are highly ap-
preciated. It is well known that the noble metals typically are more active
than the traditional HDS catalysts.
224
Catalytic activation of both hydro-
gen as well as hydrocarbons and oxygenates can be achieved. Major
challenges for implementing the use of noble metal-based catalysts in
bio-oil upgrading are related to the control of the degree of hydrogen-
ation of unsaturated bonds and the extent of breaking of (for example)
C-O and C-C bonds. Both factors constrain the obtainable oil yield and
quality, as well as determine the H
2
consumption, which is often a key for
eciency and economy. Noble metals can be attractive if aromatic sat-
uration and ring opening reactions are targeted. However, it is also well
known that noble metals are active for breaking carbon-carbon bonds
through cracking- or hydrogenolysis-type reactions. This can be both an
advantage and a disadvantage, since it is important to avoid formation of
extensive amounts of light gases (causing lower oil yield, and eciency
loss via excessive H
2
consumption). This must be achieved in part by
optimizing the operating conditions, but also by careful control of the
composition-structure-performance relationships. Careful selection and
control of the equally important support properties are an important
aspect for the final catalytic performance.
Noble metals have attracted a lot of attention within fast pyrolysis bio-
oil upgrading research, including HDO, since interest had already started
in the 1980s. The use of noble metal catalysts in HDO has recently been
reviewed by Wang et al.
13
It is well known that noble metals possess
beneficial hydrogenation activity for both oxygenates and also un-
saturated hydrocarbons, and can thus be effective catalysts
for
