Page 47 - Principles of Catalyst Development
P. 47
STRUCTURE OF CATALYSTS 33
resistance is present. (52) The best supports are those that are easily manipu-
lated to produce optimum texture properties. Alumina and silica are good
in this regard. (53.54) Much experience has been accumulated on the hydro-
thermal treatment of hydrous alumina and silica gels. Catalyst manufac-
turers put this to good use in producing supports with controlled pore
shapes and sizes. Examples are given in Chapter 6.
Mechanical strength and thermal stability of catalyst particles are
always of concern to process designers. In some cases it may be the most
critical feature. This was emphasized, for example, in steam reforming.
Strong pellets with good thermal resistance are required. Catalyst designers
use mixed oxides fired at high temperatures to form ceramic compounds.
Particles must be preformed and active components added later.
Ideally, support materials should have no catalytic activity leading to
undesirable side reactions. This is usually true for high melting oxides fired
to give low surface area supports. However, colloidal hydrous oxides are
usually acidic in nature. Alumina, for example, is dehydrated during prepar-
ation:
OH OH 0-
I I + I
-O-AI-O-AI - O-AI -O-AI-O- (2.2)
-H,O
Lewis Basic
acid site
site
However, there is always sufficient adsorbed water present on the
catalyst surface to give
0- OH 0-
+ I +H,O I I
-O-AI -O-Al-O - O-Al-O-Al- (2.3)
Bronsted
acid
site
Bronsted sites, that initiate carbonium ion reactions, and Lewis sites, giving
ion radical reactions, coexist, althouth it appears that in practical usage the
Bronsted acidity predominates. (55) When using y-A120 3 as a support,
undesirable side reactions such as cracking, isomerization and "coke"
formation always exist. These give unwanted products and lead to catalyst
deactivation.