Chemistry Reference
In-Depth Information
with low oxygen content. Palladium on carbon provides higher oil yields
than ruthenium supported on carbon, but with a slightly higher oxygen
content and a higher hydrogen consumption because of enhanced
methane formation.
The mechanism of hydrogenation over supported noble metal systems
is still debated. Generally it is acknowledged that the metals activate the
dihydrogen, but the activation of oxygen containing compound has
been proposed to be either facilitated on the metal sites
80,83
or at the
metal-support interface.
83,85,86
This indicates that these catalytic systems
potentially could have the a
nity for two different reaction pathways,
since many of the noble metal catalysts are active for hydrotreating. An
advantage of using noble metal catalyst is that the H
2
that is used in
hydrotreating reactions can be generated in situ by steam reforming.
13,79
This approach is attractive as the expense for H
2
is considered as one of
the disadvantages of the hydrotreating technology.
As alternatives to the expensive noble metal catalysts a series of
investigations of cheaper transition metal (nickel, copper, and cobalt)
catalysts have been performed.
Figure 9 compares the degree of hydrodeoxygenation (HDO) of anisole
over several nickel-based catalysts. The conditions are isothermal within
the interval 523-673 K and total pressure between 0.5 and 2.0 MPa in the
flow fixed-bed reactor.
77
Nickel-copper catalysts are more attractive for
the HDO than mono metal nickel catalysts. On the one hand, copper
facilitates nickel oxide reduction already below 523 K. On the other hand,
copper prevents methanization of oxygenated hydrocarbons at higher
temperature. Furthermore, the catalyst support plays an important role
in the hydrotreating of oxygen-containing compounds. Screening of
various catalyst supports showed that cerium and zirconium oxides are
100
90
80
70
60
50
40
30
20
10
0
Ni/SiO
2
Ni-Cu
/ZrO
2
Ni-Cu
/CeO
2
Ni/Al
2
O
3
Ni-Cu
/Al
2
O
3
Ni/Cr
2
O
3
Ni/ZrO
2
Fig. 9 Catalytic activity of nickel and nickel-copper based catalyst for hydrodeoxygenation
of anisole. Reaction conditions: 573 K, 1 MPa, and 4 h (Data from Yakovlev et al.
77
).
