Catalysis : an Integrated Approach to Homogeneous ...

56 2 -CATALYTIC PROCESSES IN INDUSTRY
Catalysts
The basis of the commercial success was the development of highly selective
catalysts [lo-121, i.e. the vanadium phosphorus oxide (VPO) catalyst which is
prepared from V205 digested in a refluxing acidic solution (aqueous HC1 or
butanol or benzyl alcohol) and orthophosphoric acid. The precipitate is filtered
off and dried below 200°C. The solid is calcined in air at 400°C. A high mechanical
strength is required for VPO used in a fluid-bed operation and therefore silica is
also incorporated. After calcination a thin layer of silica is located at the periphery
of the particles which are now very durable while the silica layer is porous to
reactants and products. There are many promoters for the VPO catalysts, the
most important being Zn, U, Ti, Co, Mo, Mg an Zr. There seems to be a general
consensus that there is an optimum averaged valence state of vanadium, which
is slightly higher than 4 (the alcohols may act as reductors during the prepara-
tion). Figure 2.21 gives a schematic view of a spray-dried particle prepared from
colloidal silica and polysilicic acid.
50% COLLOIDAL SILICA
CATALYST RLRTICLES
POROUS MICROSPHWS
45-150pm
10% PSA
Fig. 2.21. Spray dried particles prepared from 50% colloidal silica (-20 nm diameter) and 10%
silica derived from polysilicic acid [ 131.
The elucidation of the kinetic network has received a lot of attention. The
following simplified scheme has been proposed:
From the extensive literature it can be concluded that a Mars-van Krevelen
mechanism applies; the oxidation of the hydrocarbons is achieved using the
surface oxygens and the reoxidation of the catalysts is accomplished by reduction
of oxygen at separate sites.