Chemistry Reference
In-Depth Information
Other characteristics of bio-oils
242
are their high oxygen contents
(35-40 wt%), high levels of water (15-30 wt%), and moderate contents of
hydrogen (5.5 - 7 wt%), suggesting that there is a potential for oxidation
of active catalystic phases and disintegration of supports like Al
2
O
3
due to
hydrothermal processes. Surface-modified aluminas could be attractive
in this context. Cecilia et al.
203
claim that the resistance of Ni
2
P towards
H
2
O is high when preparing the catalyst at conditions of high P/Ni ratios.
Fast pyrolysis bio-oils also contain a number of potentially fouling and
poisonous contaminants, with levels varying by source. Some typical
elements present are S, Al, Fe, Zn and K. Levels of nitrogen and sulfur are
typically lower in bio-oils than in fuel oils. While sulfur is a requirement
for conventional HT catalysts, it has also been observed that S has a
positive effect of active sites of phosphide based HDO catalysts,
158
thus
demonstrating an important aspect of intrinsic sulfur tolerance.
The degree of regenerability is highly dependent upon the actual
deactivation mechanism. Carbonaceous deposits can in principle be
removed by oxidation. Metal-based catalysts can then be reactivated by
reduction. However, the stability of the active phases of carbide-, ni-
tride- and phosphide-based catalysts will certainly be endangered by
such a treatment, but severity can possibly be tuned and optimized.
Alternative procedures could be developed, for example by using
hydrogen for partial regeneration - removal of deposits and reduction
of oxidized phases. Other deactivation mechanisms like leaching or
hydrothermal destabilization are less likely to be reversed by in-situ
treatments. Given an inexpensive catalyst designed for an acceptable
lifetime, it can be more attractive to replace it than to perform partial
regeneration.
3.5 Unconventional materials and concepts
Chen et al.
240
have studied the development and performance of zeolite-
encapsulated Pt, with the main purpose of improving the resistance to
sulfur poisoning by establishing shape selectivity. It is claimed that the
system behaved more like the traditional base metal systems, facilitating
direct heteroatom removal rather than primary hydrogenation. A good
hydrogenation activity was obtained at rather high sulfur levels.
The system reported by Tang et al.
232
can also be considered as un-
conventional. One-step upgrading was performed in supercritical ethanol
(improving solvation of lignin monomers) by using a Pd/SO
4
2
/ZrO
2
/SBA-
15 catalyst for hydrotreatment, esterification and cracking.
Carbon-covered alumina (CCA) was synthesized from a mesoporous
starting material and used for supporting molybdenum carbide, nitride
and phosphide. Zheng et al.
241
studied this material for hydrazine de-
composition and claimed that it was a superior support. A monolayer of
carbon was found to be optimal for catalytic activity.
4 Conclusions
In general, although some biomass conversion processes have reached
demonstration level, most of the catalytic processing of the future
