Environmental Engineering Reference
In-Depth Information
Overwhelming information confirms that during hydroprocessing operation, most of the
catalytically active metals are in a sulfided form
[53-55]
. Thus, the active phase comprises
Co(Ni)-Mo-S structure, although other sulfided species, such as MoS
2
,WS
2
,Co
9
S
8
,Ni
3
S
2
,
and partially sulfided oxides of these metals, can also be present. The oxidation of these metal
sulfides to corresponding oxides is illustrated using the following general reactions:
MoS
2
+
3.5O
2
=
MoO
3
+
2SO
2
WS
2
+
3.5O
2
=
WO
3
+
2SO
2
Co
9
S
8
+
12.5O
2
=
9CoO
+
8SO
2
Ni
3
S
2
+
3.5O
2
=
3NiO
+
2SO
2
It is believed that the oxidation of these sulfides may proceed to the completion because the
latter stages of regeneration are usually conducted in air and at elevated temperatures (e.g.,
∼
450
◦
C). However, conversion of the metal sulfides may proceed through intermediate
oxosulfidic species, to metal sulfate structures, before this stage of oxidation is attained. In this
regard, some support is provided by
Fig. 6.8 [381]
showing the TPO profiles of the
fresh-sulfided catalysts. Thus, for a commercial catalyst A (CoMo/Al
2
O
3
), a small SO
2
peak
occurring at about 700 K approached the temperature range in which the sulfates of transition
metals usually decompose, giving the corresponding metal oxide and SO
2
. Iwamoto and
Kagami
[382]
reported that the formation of Al
2
(SO
4
)
3
was also possible particularly in the
presence of V. They suggested that the V oxides catalyzed the oxidation of SO
2
to SO
3
leading
Figure 6.8: Formation of SO
2
and weight loss during temperature programmed oxidation (TPO)
of presulfided NiMo/Al
2
O
3
catalyst [From ref.
381
. Reprinted with permission].
